Firing Biomass
Transcript of Firing Biomass
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 1/30
Grate-firing of biomass for heat and power production
Chungen Yin , Lasse A. Rosendahl, Søren K. Kær
Institute of Energy Technology, Aalborg University, DK-9220 Aalborg East, Denmark
a r t i c l e i n f o
Article history:
Received 20 December 2007
Accepted 9 May 2008
Available online 27 June 2008
Keywords:
Biomass
Grate-fired boiler
Pollutant emission
Particulate matter
Deposit formation
Corrosion
CFD
Fluidized bed
a b s t r a c t
As a renewable and environmentally friendly energy source, biomass (i.e., any organic non-fossil fuel)
and its utilization are gaining an increasingly important role worldwide. Grate-firing is one of the main
competing technologies in biomass combustion for heat and power production, because it can fire awide range of fuels of varying moisture content, and requires less fuel preparation and handling. The
basic objective of this paper is to review the state-of-the-art knowledge on grate-fired boilers burning
biomass: the key elements in the firing system and the development, the important combustion
mechanism, the recent breakthrough in the technology, the most pressing issues, the current research
and development activities, and the critical future problems to be resolved. The grate assembly (the
most characteristic element in grate-fired boilers), the key combustion mechanism in the fuel bed on
the grate, and the advanced secondary air supply (a real breakthrough in this technology) are
highlighted for grate-firing systems. Amongst all the issues or problems associated with grate-fired
boilers burning biomass, primary pollutant formation and control, deposition formation and corrosion,
modelling and computational fluid dynamics (CFD) simulations are discussed in detail. The literature
survey and discussions are primarily pertaining to grate-fired boilers burning biomass, though these
issues are more or less general. Other technologies (e.g., fluidized bed combustion or suspension
combustion) are also mentioned or discussed, to some extent, mainly for comparison and to better
illustrate the special characteristics of grate-firing of biomass. Based on these, some critical problems,
which may not be sufficiently resolved by the existing efforts and have to be addressed by future
research and development, are outlined.& 2008 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726
2. Biomass as a fuel for grate-fired boilers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727
3. Grate-firing: a suitable technology for biomass combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728
3.1. Fuel-feeding system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729
3.2. Grate assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729
3.3. Primary air and traditional combustion mechanism in the fuel bed on the grate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 730
3.4. Advanced secondary air supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 731
3.5. A comparison between grate-firing and FBC for biomass combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7324. Key issues associated with grate-firing biomass: R&D in progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733
4.1. Primary pollutant formation and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733
4.1.1. Pollutants from incomplete combustion and the control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733
4.1.2. NO x emissions and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734
4.1.3. HCl and SO x emissions and control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735
4.1.4. PCDD/PCDF emissions and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736
4.1.5. Particulate matter and heavy metals emissions and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736
4.2. Deposit formation and corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738
4.2.1. Deposition indices based on fuel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738
4.2.2. Mechanisms of deposit formation and high temperature corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739
ARTICLE IN PRESS
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/pecs
Progress in Energy and Combustion Science
0360-1285/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.pecs.2008.05.002
Corresponding author. Tel.: +459940 9279; fax: +45 98151411.
E-mail address: [email protected] (C. Yin).
Progress in Energy and Combustion Science 34 (2008) 725– 754
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 2/30
4.2.3. Possible solutions to the problems of deposition and high-temperature corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740
4.3. Modelling and CFD simulations for diagnosis, optimization, and new design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742
4.3.1. Modelling of biomass conversion in the fuel bed on the grate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742
4.3.2. CFD modelling of the mixing and combustion in the freeboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747
4.3.3. Modelling of NO x formation and emissions from grate-fired boilers burning biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
4.3.4. Modelling of deposit formation in grate-fired boilers burning biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
4.3.5. Modelling or assessing of the discontinuous effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
5. Future R&D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
5.1. Mechanism study of combustion chemistry for grate-firing of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7485.2. Advanced monitoring, testing, and experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
5.3. General and comprehensive model for biomass conversion in the fuel bed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
5.4. Advanced CFD modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
5.5. Optimization and modernization for better performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751
1. Introduction
The worldwide concern with global warming, because of the
emission of CO2 and other greenhouse gases and the limited
availability of fossil fuels, has spurred interest in using biomass asa fuel for energy production. The agreements, signed up by the
European Union (EU) in March 2007, to a binding EU-wide target
to source 20% of the energy needs from renewables such as
biomass, hydro, wind, and solar power by 2020 (8.5% in 2007) [1],
will further boost the use of biomass in power production.
Grate firing is one of the main technologies that are currently
used in biomass combustion for heat and power production.
Grate-fired boilers can fire a wide range of fuels of varying
moisture content and show great potential in biomass combustion
[2–4]. Though grate firing of biomass has been tried and tested
over many years, there are still some problems to be further
studied, for instance, conversion of biomass in the fuel bed on the
grate, mixing in both the fuel bed and the freeboard, deposit
formation and corrosion and their control, pollutant formation
and control, modelling and simulations for a better understanding
of the details in grate-fired boilers.
There are some review papers in the literature in which
biomass combustion is covered to different extents. For example, a
comparison was made of the combustion of coal and municipal
solid waste (MSW), in terms of fuel characteristics, combustion
technology, emissions, and ash utilization and disposal [5]. The
combustion of sewage sludge [6] and agricultural residuals [7]
was reviewed, and the important issues of fuel processing,
combustion, and emission characteristics, as well as the handling
of solid by-products, were discussed. Knowledge, mainly on
suspension co-firing coal with biomass, was summarized [8].
The understanding of the combustion of pulverized coal and
biomass from the viewpoint of computer modelling was reviewed
[9]. The chlorine-associated, high-temperature corrosion and thepotential corrosion problems associated with burning biomass
fuels were discussed [10]. The understanding of fuel nitrogen
conversion in solid fuel (mainly coal, but also biomass) fired
systems was reviewed. The effect of parameters that possibly
affect the oxidation selectivity towards NO and N2 was empha-
sized [11]. Combustion characteristics of different biomass fuels,
the potential applications of renewable energy sources as the
prime energy sources in various countries, and the problems
associated with biomass combustion in boiler systems were
discussed [12,13].
The objective of this paper is to provide a state-of-the-art
overview of the grate-firing of biomass for heat and power
production. Grate-fired boilers are often labelled ‘‘high carbon-in-
ash, low efficiency and high emissions’’. Firing of biomass couldalso bring new problems to the combustion systems, e.g.,
deposition and corrosion. The efforts in the area of grate-firing
of biomass may be grouped as follows:
Pollutant emissions: The incomplete combustion gives rise to
higher emissions of CO, hydrocarbons (C xH y), tar, poly aromatichydrocarbons (PAH) and incompletely burned char from
biomass combustion in grate-fired boilers. The relatively high
contents of specific elements (e.g., Cl, S, and heavy metals) in
some biomass fuels may aggravate the pollutant emissions by
emitting HCl, SO x, polychlorinated dibenzo-dioxins (PCDD),
polychlorinated dibenzofurans (PCDF), and heavy metals. Fuel
NO x is the major source of NO x from biomass combustion in
grate-fired boilers. The important NO x precursors (e.g., NH3,
HCN, and NO) released from the fuel bed on the grate, which
are directly related to the atmosphere and the propagation
speed of the ignition front in the fuel bed, play a vital role in
NO x emissions from grate-fired boilers.
Deposit formation and corrosion: Grate-firing of some biomass
fuels with a high Cl content (e.g., straw) may suffer from severe
deposition and corrosion problems. Deposits reduce both the
heat transfer ability of combustor surfaces and the overall
process efficiency, while corrosion reduces the lifetime of the
equipment. Deposition and corrosion depend not only on fuel
properties but also on combustion environments (e.g., atmo-
sphere, temperatures, and mixing).
Modelling and computational fluid dynamics (CFD) simula-
tions, which could represent the majority of the design efforts
devoted to grate-firing of biomass: Basically, the modelling can
be split into two parts: modelling of biomass conversion in the
fuel bed on the grate and modelling of the mixing, combustion,
deposit, and pollutant formation in the freeboard. In the fuel
bed, the propagation of the flame fronts is of practical interest,
as it determines the releases of volatiles, and affects the heat
output from a given grate area and the stability of combustion.The flame propagation also plays an important role in the
release of NO x precursors, particulate matter formation pre-
cursors, and other pollutant formation precursors. So knowl-
edge of the combustion in the fuel bed is important for
optimizing the gas-phase combustion above the bed. The
modelling efforts on the combustion in the freeboard have
mainly focused on how to optimize the mixing in order to
improve burnout and lower the emissions.
Experimental work is another important contributor to the
study of biomass combustion: Experimental results provide
not only valuable insights into the combustion process, but
also the necessary input for modelling, as well as the data for
the validation of models. Comparatively, the experimental
facilities or techniques are more general for any kind of combustors. Moreover, the experimental studies are spread
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754726
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 3/30
throughout all different issues or problems. Therefore, the
experimental efforts are not highlighted explicitly as a separate
subject in this review. However, it never means there is a lack
of experimental efforts in the grate-firing of biomass. In fact,
the three issues to be highlighted in this review, i.e., pollutant
emissions, deposition and corrosion, modelling and CFD
simulations, are all related to experimental efforts to different
extents. Pollutant formation and control, particularly therelease of the inorganic elements under grate-firing conditions,
the particulate matter and heavy metals emissions from grate-
fired boilers, and some of the emission control techniques
applicable for grate-fired boilers, are heavily based on experi-
mental studies. Deposit formation and corrosion in grate-fired
boilers, as well as their control, are almost exclusively derived
from experimental results. Some measures, which have been
successfully tested for other combustion technology (e.g.,
fluidized bed combustors) to mitigate deposition and corro-
sion, are not necessarily applicable to grate-fired boilers, due to
the substantially different mixing, stoichiometric conditions
and time–temperature inside the fuel bed. Modelling and CFD
simulations are also closely related to experiments, for the
purpose of validation. Almost all the modelling works onbiomass conversion in the fuel bed on the grate are validated,
to different degrees, by experiments.
This review paper is organized as follows. Firstly, biomass fuels
are briefly introduced: the advantages of firing biomass, the
properties of biomass, the considerations for biomass power
projects, as well as a list of reported examples of biomass fuels
fired in grate boilers. Then, grate-firing technology is presented.
Amongst the key elements of a modern grate-fired boiler, the
grate assembly and advanced secondary air system are high-
lighted. The traditional combustion mechanism in the fuel bed on
the grate is also highlighted, followed by a comparison with
fluidized bed combustion (FBC) of biomass. After that, the three
issues are surveyed and discussed, primarily pertaining to grate-fired boilers burning biomass: (1) primary pollutant formation
and control, (2) deposit formation and corrosion, and (3)
modelling and CFD simulations for the diagnosis and optimization
of existing grate-fired boilers and design of new grate-fired
boilers. Combustion chemistry and physics constitute the founda-
tions of all the issues or problems. Pollutant emissions, as well as
deposit formation and corrosion, are more related to combustion
chemistry, whilst modelling and CFD simulations may be more
related to combustion physics, particularly the mixing (fluid
mechanics), thermodynamics, and heat transfer in the freeboard,
and development of physical models (including sub-grid model).
Both combustion chemistry and physics will be discussed
throughout the different sections rather than highlighted as a
separate issue. Experimentation, more combustion physics-based,
is also discussed throughout the different sections. Finally, a few
critical problems in grate-firing of biomass, which may be
addressed by further research and development (R&D), are
suggested on the basis of the discussions.
2. Biomass as a fuel for grate-fired boilers
In general, any organic non-fossil fuel can be considered a
biomass fuel. Biomass for grate-firing can be mainly grouped into
waste products and dedicated energy crops [8,14]. Waste products
include wood materials (e.g., saw dust, wood chips, wood logs,
and bark), crop residues (e.g., wheat straw, rice straw, corn husks),
and municipal and industrial wastes of plant origin (e.g., MSW,
refuse-derived fuel (RDF), manure). Dedicated energy crops areagricultural crops that are solely grown for use as biomass fuels,
e.g., short-rotation woody crops like hard-wood trees and
herbaceous crops like switchgrass. These crops have very fast
growth rates and can therefore be used as a regular supply of fuel.
Biomass fuels are one of the most important energy resources.
Biomass constitutes 14% of the global primary energy, the fourth
largest following coal, oil, and natural gas. Biomass is the most
important source of energy in developing countries, providing
about 35% of their energy demand [12]. The potential, on a globalbasis, to supply biomass for the production of renewable energy is
assessed and reviewed in [15].
Biomass fuels are also considered environmentally friendly.
Firstly, there is no net increase in CO2 for combustion of biomass if
it is replanted. Biomass consumes the same amount of CO 2 from
the atmosphere during growth as is released during combustion.
Secondly, firing biomass brings additional greenhouse gas mitiga-
tion, by avoiding CH4 release from the otherwise landfilled
biomass. CH4 is 21 times more potent than CO2 in terms of global
warming, based on mass and a 100-year period [8,16]. Thirdly,
most biomass fuels have very little or no sulphur and, therefore,
net SO2 emissions can be reduced if high-sulphur coal is replaced
with low-sulphur biomass. When co-firing with high-sulphur
coals, the alkaline ash from biomass can also capture some of theSO2 produced during combustion [17,18]. Fourthly, some of the
biomass fuels, e.g., wood and paper, typically contain much less
nitrogen on mass basis as compared to coal, and for biomass,
significant amounts of NH3 may be released directly from the
solid matrix during devolatilization [11]. Ammonia helps to
reduce NO to N2, which essentially provides an in-situ thermal
DeNO x source. Lastly, soil and water contamination due to
landfilled or stockpiled biomass can be mitigated by firing
biomass fuels.
The physical and chemical properties of biomass span over a
very broad range. All biomass is composed of three main
components (i.e., cellulose, hemicellulose, and lignin) [9], and a
number of minor components (e.g., lipids, proteins, simple sugar,
starches, water, ash) [19]. The fractions of each class of component
vary depending on species, type of plant tissue, stage of growth,
and growing conditions. The fuel properties (e.g., proximate
analysis, ultimate analysis, ash analysis, and trace elements) of
different biomass have been widely reported or reviewed in the
literature, for example, [7,8,12,18–28]. Biomass contains carbon
(C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S), and
chlorine (Cl), as well as major ash-forming elements (Al, Ca, Fe, K,
Mg, Na, P, Si, Ti) and minor ash-forming elements (As, Ba, Cd, Co,
Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Tl, V, Zn). Among the ash-forming
elements, Ca and Mg usually increase the ash melting point, while
K decreases it significantly. Chlorides and low melting alkali- and
aluminosilicates may also significantly decrease the ash melting
point [27–29]. The fractions of the different elements vary from
one biomass fuel to another. Generally speaking:
(1) the C contents of wood fuels (including bark) are higher than
those of herbaceous biomass;
(2) coniferous and deciduous wood, and paper have the lowest N
contents, while grains and grasses usually contain the highest
N contents;
(3) the Cl content of wood is generally very low, while
significantly higher amounts of Cl are present in herbaceous
biomass, grains, and fruit residues;
(4) straw, cereal, grass, and grain have low contents of Ca and
high contents of K and Si in ash.
The different properties of biomass fuels in comparison with
solid fossil fuels result in different reactivities, combustion,deposition and emission behaviours, as studied and concluded
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 727
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 4/30
in, for example, [30–37]. The differences may be summarized as
follows:
Biomass fuels generally have higher volatile contents, less
carbon and more oxygen, and lower specific heating value in
kJ/kg than coal.
Pyrolysis starts at a lower temperature for biomass fuels.
The fractional heat contribution by volatiles in biomass is of the order of 70% compared to 36% for coal.
Biomass fuels, for example, straw, have more free alkali in ash,
which may aggravate the slagging and fouling problems.
Compared to coal chars, biomass chars have higher oxidation
reactivity, probably as a result of the presence of alkalis
(catalytically active) in the char matrix [32].
All these have significant influence on the thermal utilization of
biomass fuels and the choice of the appropriate combustion
technology.
For biomass conversion, a correct evaluation of available
combustion technology options is not sufficient to ensure a
successful biomass power project. A number of other considera-
tions, some of which may be even more crucial, are required toturn biomass into productive heat and/or power, such as
evaluating the availability of suitable biomass resources and
determining the economics of collection, storage, and transporta-
tion. Some biomass fuels may be available for a few weeks per
year, and therefore have to be stored for use throughout the year,
which is clearly different from fossil fuels. Some biomass fuels
may need pre-handling before being fed to combustors for heat
and power production, e.g., leaching, drying, or pelletization.
Moreover, biomass fuels are generally bulky and therefore the
availability of biomass feedstocks in close proximity to a biomass
power project is a critical factor in their efficient utilization. The
primary reasons for the failure of biomass power projects are
changes in fuel supply or demand (wrongly estimated during the
planning stage) and changes in fuel quality [38].
Amongst the numerous biomass fuels given in literature,Table 1 lists some of those which are reported to have been fired
in grate boilers or tested for grate-firing. The original data
collected from the literature, which are not necessarily complete,
are converted into the same basis to ease the comparison, i.e., as-
received basis for proximate analysis and dry-ash-free basis for
ultimate analysis. Not surprisingly, they show great variations in
their chemical properties. Even for the same type of biomass fuels,
their chemical properties may also differ greatly, depending on
the growing conditions (e.g., the place and the season). One can
also observe from the table the diversity of the biomass fuels
currently fired in grate boilers.
3. Grate-firing: a suitable technology for biomass combustion
The two most common types of boilers for biomass combus-
tion are grate-firing systems and fluidized bed combustors, both
of which have good fuel flexibility and can be fuelled entirely by
biomass or co-fired with coal. Suspension burners are often used
to co-fire milled biomass pellets or raw biomass with pulverized
coal or natural gas, in which the air-dried biomass fuels (with
ARTICLE IN PRESS
Table 1
Compositions and heating values for selected biomass fuels fired in grate boilers or tested for grate-firing
Type of biomass fuels (country) Reference Proximate analysis (wt%, as received) Ultimate analysis (wt%, dry and ash-free) Calorific value (MJ/kg)
Moisture VM FC Ash C H O N S Cl Gross CV LCV
Bark (Sweden) [39] 50.00 1.70 51.88 6.10 41.71 0.31 9.00
Bark (the Netherlands) [40] 52.16 6.0 0 41.5 9 0.2 5
Brassia carinata (Spain) [41] 8.88 4.75 48.23 6.32 44.01 1.12 0.32
Coconut shell (India) [42] 6.50 48.15 38.85 6.50 53.21 6.20 39.25 1.28 0.05
Dried sewage sludge (Poland) [43] 3.00 50.00 10.00
Fibreboard (Austria) [44] 10.60 1.48 47.54 6.15 43.19 3.11 15.52
Grape waste (Spain) [45] 11.45 62.51 20.74 5.30 49.87 6.68 40.80 2.50 0.15 16.37
Miscanthus (UK) [46] 6.10 67.90 13.10 12.90 49.26 7.78 42.96 15.40
MSW (UK) [47,48] 36.00 32.00 8 .20 23.80 5 0.20 5.80 42.30 0.97 0.73 7.66
MSW (75%) and RDF (25%) (Germany) [49] 29.00 39.41 4.97 26.63 41.75 5.80 52.45 8.30
Olive husk (Spain) [45] 12.20 64.77 15.87 7.16 50.80 6.05 38.14 4.83 0.18 15.58
Pine (UK) [46] 5.50 81.20 12.10 1.20 53.38 8.68 37.94 18.30
Pine wood (Korea) [50] 25.00 10.00 48.04 7.13 44.84 10.03
RDF (Italy) [51] 20.00 60.77 8.18 11.05 58.28 5.07 33.04 1.42 0.88 1.31 15.00
RDF (UK) [46] 1.90 69.60 9.80 18.70 55.79 7.93 36.27 22.30
Reed canary grass (UK) [52] 8.05 83.87 3.75 4.33 50.45 6.52 42.24 0.80 0.00 16.41Rice (sic) straw (Denmark) [41] 7.40 7.04 47.46 6.36 45.31 0.68 0.18
Straw (the Netherlands) [40] 4 8.52 5.7 0 45.17 0.61
Straw (Poland) [43] 10.70 4.30 15.25
Straw (UK) [52] 7.88 80.08 6.76 5.28 50.18 6.31 42.38 0.69 0.44 16.36
Sugarcane trash (India) [42] 4.00 55.98 38.27 1.75 49.87 5.99 44.13
Switchgrass (UK) [52] 6.43 82.84 7.24 3.49 48.33 6.07 44.55 0.48 0.57 17.26
Waste wood (the Netherlands) [40] 49.90 5.73 42.93 1.45
Wheat straw 2000 (Spain) [41] 7.48 4.70 49.08 6.48 43.60 0.63 0.20
Wheat straw 2005 (Denmark) [53] 12.00 69.52 14.39 4.09 49.24 6.40 43.90 0.46 15.21
Wheat straw (UK) [54] 16.00 63.50 15.00 5.50 49.17 6 .50 42.93 0.76 0.13 0.51 14.58
Willow (UK) [46] 7.20 78.10 13.70 1.00 50.00 7.19 42.81 17.8
Wood (Poland) [43] 35.00 4.00 10.90
Wood (Sweden) [55] 10.00 76.23 13.50 0.27 49.30 6.30 44.40 16.51
Wood chips (Austria) [44] 38.70 0.53 49.07 6.09 44.33 0.50 10.21
Wood chips (Finland) [56] 45.00 46.75 7.15 1.10 50.00 6.12 43.88 10.45
Wood chips (India) [42] 7.00 54.52 38.11 0.37 49.01 6.40 4 4.59
Wood chips (the Netherlands) [40] 50.87 5.9 6 43.05 0.12
Wood chips (Sweden) [57] 30–40 52.00 6.00 41.00 0.60
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754728
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 5/30
relatively low moisture content, e.g., o25wt%) must be finely
pulverized (e.g., particle feed size o6 mm) and have a relatively
low share (e.g., less than 25% on energy basis) [3,4,7,38].
Suspension-fired burners are also more sensitive to changes in
fuel quality: firing off-specification fuels can significantly influ-
ence the performance of the boilers [58]. Moreover, suspension-
fired burners require more fuel-handling and preparation equip-
ment, especially when firing pulverized biomass pellets.
Grate-firing is the first combustion system used for solid fuels.
Now it is used mainly for burning biomass, but also for smaller
coal furnaces. Capacities of grate-fired boilers range from 4 to
300MWe (many in the range of 20–50 MWe) in biomass-fired
combined heat and power (CHP) plants. The heat release rate per
grate area may be up to about 4MWth/m2 as a result of high
volatile and low ash characteristics of typical biomass fuels [38].
Fig.1 shows two modern grate-fired boilers, giving an overall view
of the typical arrangement of grate-fired boilers. Basically, modern
grate-fired boilers consist of four key elements: a fuel feeding
system, a grate assembly, a secondary air (including over-fire air
or OFA) system and an ash discharge system.
Here, the key elements in grate-fired boilers are discussed first,
and the grate assembly and the advanced secondary air system are
highlighted. The former represents the most specific component
in grate-fired boilers, whilst the latter is one of the real
breakthroughs in grate-firing technology. The traditional combus-
tion mechanism in the fuel bed on the grate is also highlighted.
This is followed by a comparison between grate-firing and FBC of
biomass, and then a conclusion.
3.1. Fuel-feeding system
Typical fuel-feeding systems used in biomass-fired grate
boilers are mechanical stokers, as shown in Fig. 1. For biomass
fuels that are very heterogeneous in size and contain a relatively
big mass fraction (30% or higher) of fine particles (i.e., a few
millimetres and smaller), a spreader is needed to reduce the
tendency for fuel size segregation since the grate is typically only
suitable for coarse particles. The finer biomass particles combustin suspension when they fall against the upwardly flowing
primary air. The remaining heavier and bigger pieces fall and
burn on the grate surface [60].
3.2. Grate assembly
The grate, which is at the bottom of the combustion chamber
in a grate-fired boiler, has two main functions: lengthwise
transport of the fuel, and distribution of the primary air entering
from beneath the grate. The grate may be air-cooled or water-
cooled. The water-cooled grate requires little air to cool (primary
air confined to combustion requirement) and is flexible with the
use of an advanced secondary air system. The grates are mainly
classified into stationary sloping grates, travelling grates, recipro-
cating grates, and vibrating grates, the major characteristics of
which are summarized in Table 2.
Table 3 lists a few examples of grate-fired boilers equippedwith the different types of grates for biomass combustion, from
ARTICLE IN PRESS
Table 2
Different types of grates and their main characteristics
Type of grate The major features
Stationary sloping
grate
The grate does not move. The fuel burns as it slides down the
slope under gravity. The degree of sloping is an important
characteristic of this kind of boiler. Disadvantages: (1) difficult
control of the combustion process; (2) risk of avalanching of
the fuel.
Travelling grate The fuel is fed on one side of the grate and is burned when the
grate transports it to the ash pit. Compared to stationary
sloping grate, it has improved control and better carbon
burnout efficiency (due to the small layer of fuel on the grate).
Reciprocating
grate
The grate tumbles and transports fuel by reciprocating
(forward or reverse) movements of the grate rods as
combustion proceeds. Finally, the solids are transported to the
ash pit at the end of the grate. Carbon burnout is further
improved due to better mixing.
Vibrating grate The grate has a kind of shaking movement that spreads the fuel
evenly. This type of grate has less moving parts than other
moveable grates (and thus lower maintenance and higher
reliability). Carbon burnout efficiency is also further improved.
Fig. 1. Examples of grate-fired boilers burning biomass. (a) MSW-fired reciprocating-grate boiler [59]. (b) Straw-fired vibrating-grate boiler [53].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 729
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 6/30
which one can also see the capacities of the grate-fired boilers as
well as the biomass fuels fired.
Amongst the different types of grates, vibrating gates may have
the longest life and highest availability. Fig. 2 shows a laboratory-
scale water-cooled vibrating grate [59]. The grate is made of panel
walls with drilled holes in the fins for the primary air. The grate is
part of the boiler pressure system and connected to furnacewalls by flexible connection pipes, designed for the vibrations.
Instead of a continuous ash discharge in a travelling grate, a
vibrating grate utilizes an intermittent ash removal system
where the grate surface vibrates at high frequency and low
amplitudes for about 2% of the time to move the solids forward
and discharge the ash at the end of the grate. As can be seen from
the figure, this type of grate uses very few moving parts and
the drive mechanism is outside the heat and flame, which
increases grate life, reduces maintenance costs and results in
high equipment availability.
The grate assembly can be optimized to significantly improve
the boiler performance, for instance, by allowing a better under-
grate primary air distribution, by enhancing the mixing of
biomass fuels on the grate, and by improving fabric seal of thesystem to lower air leakage. Fig. 3 shows a close view of a modern
grate system, which was built into a brand new grate-fired boiler
burning MSW in 2006 [72].
3.3. Primary air and traditional combustion mechanism in the fuel
bed on the grate
The design of air supply system (primary air and secondary air)
plays a very important role in the efficient and complete
combustion of biomass. For grate-firing, the overall excess air
for most biomass fuels is normally set to 25% or above. The split
ratio of primary air to secondary air tends to be 40/60 in modern
grate-fired boilers burning biomass, instead of 80/20 in older
units, which leaves much more freedom to advanced secondary
air supply.
The primary air distribution, together with the movement of
the grate, affects significantly the mixing and biomass conversion
in the fuel bed. Though some grate-fired boilers burning biomass
may have a low buildup of materials on the bed, the majority of
the biomass-fired grate furnaces in the literature, for example
those as listed in Table 3, may be interpreted as a cross-flowreactor, where biomass is fed in a thick layer perpendicular to the
ARTICLE IN PRESS
Table 3
Grate-firing of biomass: some examples
Type of grate Fuel fired Grate-fired boilera Reference
Stationary sloping grate Wet wood chips 40 MW industrial boiler [61]
Martin-type moving grate MSW 12 T/h MSW incinerator [47,48]
Inclined moving grate Waste 4 T/h industrial boiler [62]
Travelling grate Coal/biomass blends (bagasse, wood chips, sugarcane trash, coconut
shell)
18.68 MWe power plant boiler [42]
Travelling grate Bagasse, wood chips, rice husk 120 T/h, 87 kg/cm2, 515 1C industrial boiler [4]
Travelling grate Wood chip, rice husk, bark, sugar cane trash, cotton stalks, groundnut
shells
35T/h, 66 kg/cm2, 495 1C industrial boiler [4]
Travelling grate Bagasse 90 T/h, 45 kg/cm2, 480 1C industrial boiler [4]
Travelling grate Wood VU-40 industrial boiler [63]
Air-cooled travelling grate Blends of urban waste, natural wood waste, agricultural waste 20MWe industrial boiler [3]
Recipr ocat ing grate Wood chips, b ark , sawdus t, p ellets of s awdust 15 0 r eciprocating grate b oilers (45 MWth each) [64]
Forward reciprocating
grate
MSW 25 MWe utility boiler [65–67]
Vibrating grate Wheat straw 33 MWe utility boiler [54]
Water-cooled vibrating
grate
Wood pellets 500 kW laboratory-scale [68]
Water-cooled vibrating
grate
Straw 105 MWth boiler [69,70]
Water-cooled vibrating
grate
20% wood chips and 80% straw 33 MWfuel CHP unit (producing 8.3 MWe and 20.8MJ/s
heat)
[71]
Water-cooled vibratinggrate
Wheat straw 108 MWfuel CHP unit (producing 35 MWe and 50MJ/sheat)
[53]
a A unified description of the boiler capacity would be more helpful. Different descriptions (e.g., main steam parameters, feeding rate of biomass, thermal megawatts or
electrical megawatts) are used here due to the lack of the unified information in the references.
Fig. 2. A laboratory-scale water-cooled vibrating grate [59].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754730
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 7/30
primary air flow. The bottom of the biomass bed is exposed to the
preheated primary air while the top of the bed is within the
furnace. The fuel bed consists of a huge number of solid particles
that are piled up on the grate with a characteristic porosity. The
fuel bed is heated by over-bed radiation from flames and
refractory furnace walls until it ignites on the top surface of thefuel bed. The propagation of the ignition front in the bed is of
practical interest, as it determines the releases of volatiles, and
affects the heat output from a given grate area and the stability of
combustion. It is also directly related to the release of volatile
nitrogen species (NH3 and HCN) and NO formed from volatiles. So
knowledge of the factors influencing the speed of the ignition
front is important for optimizing gas-phase combustion in the
freeboard.
The generally accepted combustion mechanism of cross-
current units may be described as follows [73–76]. After ignition,
a reaction front propagates from the surface of the bed down-
wards to the grate against the direction of the primary air.
The heat, generated in the reaction front, is transported against
the combustion air flow and dries and devolatilizes the raw
fuel. This allows the reaction front to propagate. Due to the
opposing directions of the heat flow and the air flow, the
heat is not transported downwards far from the position
where it is released, and the reaction front is narrow. The heat
generated in the reaction front originates from oxidation
of fuel and, if not all oxygen is consumed in the narrow reaction
front, a char layer will be formed above the reaction front.
When the reaction front reaches the surface of the grate, a
secondary reaction front (i.e., a char burnout front), propagating
upwards to the surface of the bed, burns the char layer previously
formed.
This traditional combustion behaviour in the fuel bed on the
grate may not always be observed. Fuel properties (e.g., moisture,
heating value, and particle size) and operating conditions (e.g.,
primary air flowrate) have significant influence on the combustionbehaviour in the fuel bed [74–78]: the main findings will be
summarized later in the modelling and simulation section. In
some extreme cases, different combustion behaviour may be
observed. For instance, in the combustion process of wet biofuels
(50 wt% moisture content, which was 5–15% beyond the limiting
moisture content corresponding to the primary air flowrate) in a
31 MW reciprocating grate furnace (a cross-current flow combus-
tor), the ignition was found to occur close to the grate, followed by
a reaction front propagating from the grate upwards to the surface
of the bed, which is opposite to the traditional mechanism [75].
Depending on the primary air supply rate, three modes of
combustion in the biomass bed were identified [79]: oxygen-
limited combustion under low air supply rate, reaction-limited
combustion when increasing primary air supply, and extinction byconvection if the primary air flowrate is increased further. In
modern grate-fired boilers burning biomass, the biomass combus-
tion in the fuel bed is more likely under substoichiometric
conditions (i.e., fuel-rich), because biomass fuels have typically
higher volatiles content on a dry basis. Moreover, the multiple
zones of under-grate primary air are often used, which can help to
achieve a more favourable temperature distribution, high ashburnout, and low emissions.
3.4. Advanced secondary air supply
Advanced secondary air supply system is one of the most
important elements in the optimization of the gas combustion in
the freeboard, for complete burnout and lower emissions, e.g., by
forming local recirculation zones or rotating flows and by forming
different local combustion environments (e.g., fuel-rich or oxy-
gen-rich). It is probably the most flexible way to retrofit the old
grate-fired boilers for a better burnout and lower pollutant
emissions [63]. The gases released from biomass conversion on
the grate and a small amount of entrained fuel particles continue
to combust in the freeboard, in which the secondary air supply
plays a significant role in the mixing, burnout, and emissions.
Advanced secondary air-staging is also often used in modern
biomass-fired grate boilers. The basic idea of air-staging is to
reduce NO x formation by reducing oxygen availability in the flame
and by lowering flame temperature peaks. In air-staged combus-
tion process, the first air-deficient (i.e., fuel-rich) zone reduces
NO x formation, and the complete combustion is achieved only
after the addition of OFA in the second zone (i.e., the burnout
zone).
As an example, Fig. 4(a) shows the advanced secondary air
supply in the straw-fired vibrating-grate boiler [53], from which
one can see the enhanced air-staging in the lower furnace. Fig.
4(b) gives a close-up view of the secondary air nozzles on the
front wall in the lower furnace. The nozzles have differentdiameters, spacing, and orientations. The eight secondary air
nozzles on the top level (i.e., OFA), four on the front wall and four
on the rear wall, are staggered. The staggered arrangement of OFA
jets can provide an effective curtain of combustion air, and can
also form a double rotating flow on the horizontal cross-sections
in the burnout zone, as shown in Fig. 4(c), which prolongs the
residence time of the combustibles, distributes the temperature
more evenly, and leads to a better burnout. The characteristics of
the combustion air supply and the combustion zone in such a
grate-fired boiler is sketched in Fig. 5. The majority of the
combustibles are released into the freeboard from the first half
grate and the enhanced air-staging forms a local fuel-rich
combustion environment in the front-bottom part. The air jets,
located on the rear wall in the lower furnace, form a local air-richenvironment and a stable recirculation zone in the rear-bottom
ARTICLE IN PRESS
Fig. 3. A brand new grate-fired boiler burning MSW [72].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 731
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 8/30
corner, both of which help stabilize the combustion on the last
section of the grate and reduce the incompletely burned char in
the bottom ash.
During the advanced air-staged combustion process, the
mixing, temperature, residence time, and local-stoichiometry
play key roles. Besides the optimization of the SA jets (e.g., in
terms of amount, momentum, diameter, location, spacing,
orientation), the static mixing devices with or without air
injections, as shown in Fig. 6(a), may be an effective option to
enhance the mixing [2] in biomass-fired grate boilers. Actually,
the ‘Eco’-tube air system is just such a device with air injection,which generates a considerable improvement in efficiency and a
big NO x reduction in different grate-fired boilers burning biomass
[39,80]. Tangential arrangement of SA jets, as shown in Fig. 6(b),
may also be a good option for grate-fired boilers. The tangentially
arranged air jets form a strong rotating flow on the horizontal
cross-sections, which can not only result in a good burnout but
also mitigate the deposit formation and corrosion on the furnacewalls as a result of the local oxidative conditions and the air
curtain formed close to the furnace walls [81].
3.5. A comparison between grate-firing and FBC for biomass
combustion
FBC is also a competing technology in biomass combustion for
heat and power production. Examples of biomass-fired FBC
projects can be seen in, for example, [82], including both
circulating fluidized bed combustion (CFBC) and bubbling
fluidized bed combustion (BFBC). A general evaluation of BFBC
and CFBC for biomass combustion is that BFBC is good enough
for firing biomass alone; when biomass is co-fired with coal,
CFBC may be needed to guarantee proper char burnout.Table 4 gives a brief comparison of the main characteristics of
ARTICLE IN PRESS
Fig. 4. The advanced secondary-air staging system in a biomass-fired grate boiler [53].
Fig. 5. Sketch of the air supply and the resulted different zones in a grate-fired
boiler burning biomass.
Fig. 6. Different options of advanced secondary air supply in grate-fired boilers. (a)
Static mixing devices with or without air injections. (b) Tangentially arranged SA
jets.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754732
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 9/30
grate-fired boilers and fluidized bed combustors when they are
used for biomass combustion. Some of these points may be found
in [7].
In conclusion, grate-firing systems, particularly modern grate-
fired boilers, are one of the combustion technologies suitable for
biomass combustion for heat and power production. Grate-firing
systems are concluded as a good or even the preferred technology
for biomass-to-energy conversion in many applications, e.g.,
[2,45].
4. Key issues associated with grate-firing biomass: R&D
in progress
R&D activities on biomass combustion are progressing rapidly,
partly because of the similarities with coal combustion and the
many years’ experience and knowledge in coal combustion. For
instance, the mechanisms of pollutant formation, ash formation,
deposition, and corrosion during biomass combustion can all be
traced to some extent to the corresponding mechanistic study in
coal combustion. However, biomass fuels fall over a very broad
range and have quite different chemical and physical properties,
which not only result in different combustion and emission
characteristics but also cause some practical problems during
combustion in different plants. In this section, the three key
issues, primary pollutant emissions and control, deposit forma-
tion and corrosion, and modelling efforts for design and
troubleshooting, are highlighted and discussed, primarily relatedto grate-fired boilers burning biomass.
4.1. Primary pollutant formation and control
Primary pollutants from combustion include NO x, SO x, CO,
C xH y, tar, HCl/Cl2, PAH, PCDD/PCDF, heavy metals, particulate
matter, and incompletely burned char particles [27,91,92]. These
pollutants can be classified into two groups: one from the
incomplete combustion or oxidation and the other from the
inorganic species in the biomass fuel, as listed in Table 5.
4.1.1. Pollutants from incomplete combustion and the control
Incomplete combustion is, most likely, a big problem or
challenge for grate-fired boilers, particularly old units, comparedto FBC or suspension-fired boilers. So the pollutant emissions due
ARTICLE IN PRESS
Table 4
A comparison of grate-firing systems and fluidized bed combustors
Water-coole d mo ving -grate systems Fluidized b ed combustors
Fu el flexibility They can fire heterogeneous fu els wit h large p art icle
sizes, and high moisture contents (up to 65%).
Comparatively, a grate needs more adjustments to fit a
particular fuel than an FBC.
They can combust a wide range of fuels having different sizes,
shapes, moisture (up to 65%) and heating values [3].
Solid mixing and combustionintensity in the fuel bed
Average for travelling grates; good for reciprocating orvibrating grates. Very common combustion instabilities
in the fuel bed [53,64].
Very intense solid (bed materials and solid fuels) mixing, leading tovery uniform temperature distribution and higher combustion
intensity in the fuel bed.
Bed agglomeration Quite many biomass fuels have the low melting point
characteristics (because of the high content of
potassium in ash) [83].
Grate-firing systems are insensitive to fuel bed
agglomeration.
Very sensitive to bed agglomeration, leading to de-fluidization and
unscheduled shut-down [84]. Using silica sand as bed materials, bed
defluidization is experienced when firing coffee husks, sunflower
husks, cotton husks/stalk, mustard stalk, soya husks, pepper waste,
groundnut shell and coconut shell [85], wheat straw [83]. The
problem may be mitigated by special additives or bed materials
[86]. No such problem exists in firing rice husk [87,88], wood chips
[86], wood waste [89], palm fibre [85].
Wear to bed components Little Extensive wear to bed components due to high solid velocity [3].
Emissions They can achieve low NO x emission by using advanced
SA (including OFA) system.
Very low NO x emission from CFBC, mainly due to the char inventory
in the circulating bed materials: char efficiently reduces NO x. In
BFBC, NO x is more difficult to control and requires sophisticated airsystems and also often selective non-catalytic reduction (SNCR).
Very low SO x emission from both CFBC and BFBC due to sulphur
capture by addition of limestone into bed materials.
Fly ash Very low dust load in flue gas; high levels of
incompletely burned carbon in fly ash [13,90].
Solid load in CFBC is at least 100 times higher than in grate-fired
systems because the bed materials follow the flue gases to a large
extent. Solid separation equipment is required. Good burnout of fly
ash. Some toxic materials are bound in fly ash, which is one of the
main challenges in fluidized bed combustion of some biomass fuels,
e.g., MSW.
Partial load operation Good operation is possible at partial loads. Partial load operation requires special technology.
Capital cost Medium to low High for CFBC. BFBC is less expensive than CFBC.
Operation & maintenance
costs
Medium for travelling grates, and very low for vibrating
grates.
CFBC has high operating costs due to higher pressure drop over the
dense bed, and high maintenance costs due to, e.g., the extensive
wear (erosion).
Table 5
Pollutant emissions from biomass combustion
Orig in P ollutant emissio ns Typical bio mass fuels
Group 1— from combustion process
Incomplete
combustion
CO, C xH y, tar, PAH,
unburnt char
All biomass fuels
Oxidization NO x, N2O All biomass fuels
Group 2— from inorganic species in biomass fuel
Ash Particulate matter All biomass fuels
Cl and S HCl, SO x, salts (KCl etc.) Waste wood, straw, grasses, fruit
residues
High Cl f raction PCDD, PCDF MSW, wast e wood, st raw, cer eals
etc.
Heavy metals Pb, Zn, Cd, Cu, Cr, Hg etc. Urban waste wood, sludge
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 733
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 10/30
to incomplete combustion tend to be a more prominent topic
associated with grate-fired boilers. These emissions may also
aggravate the formation of other pollutants. For instance, the
incompletely burned char in fly ash, together with the relatively
high excess air in grate-fired boilers, may also lead to high
emissions of PCDD/PCDF when firing some biomass fuels (e.g.,
wastes or wood).
The comparatively poor mixing, both in the fuel bed and in thefreeboard, is the main reason for the incomplete combustion in
grate-fired boilers. Advanced air supply systems and optimized
grate systems can significantly enhance the mixing, reduce the
excess air, improve the combustion process, and lower the
pollutants. For instance, a 1975 vintage travelling grate stoker
boiler in a paper mill in Louisiana, USA, was retrofitted, mainly by
improving air supply system and fabric stoker seal. CO levels were
reduced from over 1000 ppm to 260 ppm after the retrofit as a
result of the improved mixing and combustion. A net reduction of
60% in the pre-retrofit fly ash disposal rates was also observed
[63], which could be due to the reduced char particles elutriated
from the fuel bed, or the reduced unburnt char in the fly ash, or a
combination of the both.
Sufficient residence time of the combustibles in the combus-tion zone is also an important factor for complete combustion in
grate-fired boilers. A 20 MWe air-cooled travelling grate combus-
tion system was commissioned in 2003 in Germany, which
utilizes a combination of urban waste, natural wood waste, and
agricultural waste, with a combined fuel moisture content of
35–40%. Due to the relatively high contaminants contained in the
fuel(s), the regulators directed that considerations of incineration
and reduction of ash disposal take a higher priority, compared to
the gaseous emissions. The design for the boiler, grate, secondary,
and primary air should be capable of meeting the required CO
regulations and, more importantly, should ensure all particles
would obtain 8501C for a minimum of 2s for the design
composite fuel [3]. These requirements are exactly the same as
the EU directive on waste incineration [2,93].
Since grate-firing systems have relatively low combustion
temperatures, good mixing and sufficient residence time of the
combustibles at high temperatures are particularly crucial to
improve the combustion. In water-cooled grate boilers, the grate
requires little primary air to cool. As a result, the flue gas leaves
the fuel bed at a lower superficial velocity and carries away less
combustible particles from the fuel bed. Water-cooled grate
boilers are also flexible with the use of advanced secondary air
system, which can be optimized to enhance the mixing and
improve the combustion in the freeboard and, therefore, lower the
pollutants from incomplete combustion. For instance, a measure-
ment campaign was done on a 108 MWfuel straw-fired water-
cooled vibrating grate boiler built in 1999. The CO measured in the
flue gas at the boiler exit and the boiler efficiency are about
150ppm and 91.7%, respectively, at 100% load. The performance of this boiler could have been further improved to achieve even
lower pollutant emissions due to incomplete combustion and a
higher efficiency [53].
So the pollutant emissions due to incomplete combustion from
grate-fired boilers can be effectively controlled by an optimized
combustion process, i.e., enhanced mixing, sufficient residence
time (at least 41.5 s) at high temperatures (4850 1C), and low
total excess air [27], as well as the appropriate choice of grate
assembly.
4.1.2. NO x emissions and control
NO x emissions from combustion systems can be formed from
different mechanisms [11,94]. As a result of the comparatively lowcombustion temperatures in grate-fired boilers burning biomass,
the yields of thermal NO x can be considered to be small or
negligible, and fuel NO x is the major source of NO x [11,95].
Fuel-N in biomass or coal is released in three stages
[94,96–99], as depicted in Fig. 7. In the first stage, the volatile-N
is released in the primary pyrolysis together with the majority of
volatiles. The major NO x precursors during biomass pyrolysis
include NH3, HCN [100], and HNCO [101,102]. Due to the high O/N
ratio in biomass fuels, part of fuel-N is found to be directlyconverted to NO during primary pyrolysis stage [103]. In the
second stage, the thermal cracking and combustion of volatiles
(mainly tar) provides additional sources of HCN and NH3. In the
third stage, during combustion of the char residue the char-N
mainly forms NO while the rest is converted to N2. The NO formed
may be effectively reduced to N2 over biomass char as a result of
its catalytic effect on NO formation and reduction [104,105].
Therefore, the partitioning of fuel-N between the volatiles and
the remaining char during devolatilization is potentially impor-
tant for final NO x formation. The split between the volatile-N and
char-N is roughly proportional to the volatile matter in the fuel
[7]. Because biomass fuels have higher yields of both light gases
and tar, and lower char yields, a comparatively larger fraction of
the fuel-N may be released with the volatiles. Latest experiments,for a broad range of woody biomass fuels (sawdust, bark, waste
wood, and MDF board) in a lab-scale packed bed batch reactor,
indicates the fraction of the volatile-N increased with increasing
fuel-N content [95].
The mass ratio of the released NO x precursors (e.g., NH3, HCN,
HNCO, NO, N2O, and NO2) from the biomass bed under grate-firing
conditions depends on, for example, the type of the biomass fuels
[95,101], pyrolysis temperatures [102], fuel N content, stoichio-
metric air ratio, particle sizes, and moisture contents [95]. The
most relevant NO x precursors may include NH3, HCN, HNCO, and
NO, amongst which NO is the major NO x precursor from the fuel
bed under air-rich conditions, while NH3 is the most important
NO x precursor under fuel-rich conditions [11,95,106]. Table 6 lists
some of the latest efforts involving NO x precursors released frombiomass bed on the grate: either experimental study of the release
characteristics of NO x precursors from the biomass bed, or
modelling of NO x formation and emissions from fixed-bed or
grate-fired boilers on the basis of a certain NO x precursors. For the
latter, good agreements between the prediction and the measure-
ment were reported [54,57,107].
The experimental results showed the conversion of fuel-N to
gaseous N species was strongly dependent on the stoichiometric
air ratio in the fuel bed lfuel bed, the fuel-N content, and the kind of
biomass fuels. In the experimental data, conversion rates of NO,
NH3 and HCN were correlated to different values of lfuel bed [95]:
ui ¼ kilfuel bed þ di ½ (1)
in which u i represents the conversion rate of i-th N species (e.g.,NO, NH3 and HCN). ki is the slope, and di the y-intercept. lfuel bed is
ARTICLE IN PRESS
Fig. 7. The fuel-N conversion pathways.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754734
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 11/30
the local stoichiometric air ratio in the fuel bed and is given by
lfuel bed ¼ nO;available
nO;stoichiometric½ (2)
where nO,available and nO,stoichiometric represent the local amount of
total oxygen available in the fuel bed and the oxygen needed for a
complete combustion of the fuel, respectively. The coefficients (ki
and di) for the NO x precursors NO, NH3, and HCN, together with
the fuel-N content and the mean particle size of the biomass fuels
investigated, are given in Table 7.
These kinds of models are useful in providing the subsequent
freeboard CFD modelling with the boundary conditions required
(i.e., the profiles of the NO x formation precursors, e.g., NO, NH3,
and HCN) for the predictions of NO x emission from industrial
grate-fired boilers burning biomass as well as for the developmentof design guidelines for low-NO x biomass grate furnaces.
The primary measure for NO x reduction is air-staged combus-
tion, which has also proven to be useful for grate-fired boilers
burning biomass [108]. Unlike char-N, volatile-N is amenable to
reduction to N2 through inexpensive techniques such as burner
and air flow configuration modifications, which can reduce NO x
emission by 50–80%. The most economical combustion modifica-
tion, to reduce NO x, is air staging. Staged combustion limits the O 2
availability in the flame and reduces peak flame temperatures to
some extent to reduce NO x formation, and produces a fuel-rich/
fuel-lean sequence which is favourable to the conversion of fuel-N
to N2. The secondary measures for NO x control, e.g., selective
catalytic reduction (SCR), could be less attractive for grate-firing of
some biomass fuels, because of the accelerated deactivation of thecatalysts caused mainly by the potassium salts that are present in
the submicron ash (dpE100nm) [71,109–111]. Co-firing biomass
(bagasse, wood chips, sugarcane trash, and coconut) with
bituminous coal in a 18.68 MWe travelling grate boiler was found
to have the capability to reduce both NO x and SO2 from the
existing coal-fired power plants [42,112], in which the biomass
ash could play some roles in the NO x reduction. The spraying of an
aqueous solution of sulphate, (NH4)2SO4, into the hot flue gases
upstream of the superheaters, which was originally proposed to
reduce the deposition rates and the corrosion rates for super-
heater tubes, can also drastically reduce the NO x emissions
[113–116].
4.1.3. HCl and SO x emissions and control
During grate firing conditions, the Cl contained in the biomass
mainly forms gaseous HCl, or alkali chlorides (e.g., KCl and NaCl),
and S forms mainly gaseous SO2 and alkali as well as alkali earth
sulphates [27,117]. Due to the subsequent cooling of the flue gas in
the boiler, a large part of the Cl condenses as salts on the heat
exchanger surfaces or on fly ash particles in the flue gas. The main
effects of Cl are the corrosive effect of chloride salts and HCl on
the metal surfaces in the boiler [10,118–120], acidic pollutant
emissions (e.g., HCl) and particulate emissions, and the effect of
HCl on the formation of PCCD/PCDF [121]. SO x forms sulphates
and condenses also on the heat transfer surfaces or forms fly ash
particles, or reacts directly with fly ash particles deposited on heatexchanger surfaces (sulphation) [27]. In short, the release of the
relevant inorganic elements (e.g., S, Cl, and K) is not only
important for direct pollutant emissions (e.g., HCl and SO x), but
also very closely related to the deposition, corrosion, and erosion
problems. The experimental studies, conducted at conditions that
resemble the local conditions on the grate in biomass-fired grate
boilers [117,122–127], indicated that the detailed release char-
acteristics of the inorganic elements depended on the type of the
biomass, the combustion temperatures, the ash composition, the
biomass char matrix, and so on.
The emissions of HCl and SO2 from biomass grate-firing can be
controlled by different means. The most common measure used in
power plants is the post-combustion flue gas cleaning systems,
e.g., by scrubbing the flue gas with limestone or by dry-sorptionwith Ca(OH)2. HCl and SO2 emissions can also be reduced, to a
ARTICLE IN PRESS
Table 6
NO x precursors released from biomass bed under grate-firing conditions
Subject NOx precursors released from the fuel bed on a grate
Modelling of NO x formation from fix-bed combustion of straw [99]. (1) The fuel-N released into volatiles, char and directly converted to NO was
estimated to be 71/25/4 based on [101].
(2) The volatile-N was assumed to form NH3/HCN/HNCO with the mass ratio of
90/5/5 based on [100]. (3) All the char-N was converted to NO.
Modelling of NO x formation from an industrial wet wood chips-fired grate boiler
[57] and a small-scale wood pellet grate boiler [107].
(1) All the fuel-N was assumed to be released into the volatiles. None of it was in
char or directly converted to NO.
(2) The volatile-N was assumed to be released in the form of NH3 and HCN with
a molar ratio of 50/50.
Modelling of straw combustion in a 38 MWe grate boiler, with NO emission as a
main issue [54].
(1) Fuel N was split into volatiles and char.
(2) NH3 was assumed to be the only precursor for the volatile-N.
(3) Char-N was oxidized to NO.
Experimental study of N species release from a broad range of woody biomass
fuels in a lab-scale packed bed batch reactor, in order to provide a
subsequent CFD gas phase combustion model with the boundary conditions
required for NO x emission prediction and reduction [95].
(1) NO2 and N2O had very low concentrations. HNCO was not studied due to the
lack of reference data for the FTIR equipment. The main NO x precursors were
NH3, HCN and NO.
(2) A release model was derived as shown in Eq (1).
(3) The total conversion rates of the different N species were predicted
accurately by the model. Most importantly, the major release zone (pyrolysis
& gasification) of the biomass fuel bed was well predicted.
Table 7
Derived parameters for the release functions of the relevant N species [95]
Fuel Fuel-N (wt%
d.b.)
Mean dp
[mm]
kNO dNO kNH3 dNH3
kHCN dHCN
Sawdust 0.06 0.3 3.30 1.88 0.62 0.91 0.49 0.49
Bark 0.27 3.0 0.85 0.44 2.12 1.94 0.15 0.12
Waste
wood
1.00 2.8 0.90 0.70 0.91 1.20 0.03 0.04
MDF board 6.87 2.7 0.15 0.10 0.94 0.94 0.06 0.03
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 735
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 12/30
smaller extent, by dry-sorption with fly ash on baghouse filter
surfaces.
Pre-treatment of the biomass fuels before being fed to the
boiler, for instance removal of most Cl and K and some S by
aqueous leaching process [128], can reduce HCl and SO2 emissions
and mitigate the deposition and Cl-associated corrosion. However,
leaching process would add moisture to the biomass fuels, which
may cause feeding or other operating problems that could beequally onerous. Such a process will also increase overall costs for
fuel handling and fuel preparation.
The sulphur-retention promotion by in situ sorbent addition
for biomass combustion was found to be possible: placing the
sorbent on top of the fuel bed on the grate, in the path of the
evolving SO2, was sufficient to get much of the effect obtained
when the fuel and sorbent were well-mixed. However, the rule of
thumb developed for optimum sorbent addition for in situ coal
combustion desulphurization, Casorbent/Sfuel ¼ 2, was found to be
insufficient for biomass fuels [41]. This high Casorbent/Sfuel required
may be expected. Most probably, it is because of the reducing
conditions in the fuel bed on the grate, which do not really favour
calcium-sulphur reactions. A fine control of the pyrolysis
temperatures on the grate may also reduce the Cl and S releasedinto the gas phase [122].
4.1.4. PCDD/PCDF emissions and control
PCDD/PCDF can be formed in very small amounts from all
biomass fuels containing Cl. An estimation of the emissions of
PCDD/PCDF into the air for Austria, Germany, Japan, the Nether-
lands, UK, and USA shows that the major contributors are
incineration of municipal, hazardous, and hospital wastes [129].
The formation mechanisms and the control of PCDD/PCDF
from biomass combustion are well elaborated elsewhere [130].
There are probably three primary routes for PCDD/PCDF forma-
tion: gas-phase reactions involving chlorinated precursors; con-
densation reactions involving gas-phase precursors and fly ash;
and solid-phase reactions on the surface of fly ash involving metal
chlorides and fly ash carbon (i.e., the so-called de NOVO
synthesis). However, PCDD/PCDF formation is predominantly
associated with heterogeneous reactions involving fly ash. These
low-temperature synthesis reactions can occur downstream of the
combustor at temperatures ranging from approximately
250–600 1C [130], or only in an even narrower temperature
window of 250–350 1C since the dioxins can be destroyed at
higher temperatures [131,132]. In addition to chlorine (Cl),
activated carbon, oxygen, and catalysts (e.g., CuCl2, CuO, CuSO4,
Fe2O3, ZnO, NiO, Al2O3, amongst which CuCl2 is the most reactive
one) are necessary for PCDD/PCDF formation [27,130].
The primary measures to control PCDD/PCDF emissions are
from combustion technology effects. Combustion conditions and
the time/temperature profile in the cooling zones downstream of the combustor determine the potential for PCDD/PCDF formation
within the devices. The PCDD/PCDF emissions can be significantly
lowered by reducing the entrainment of incompletely burned fly
ash particles, by ensuring a complete combustion of the flue gas
and a complete burnout of the fly ash particles (o0.5% carbon
content preferred) at low excess air ratios, as well as by lowering
heavy metal contents in the fly ash particles. For instance, a linear
increase in the grate/lower furnace temperature was found to
produce an exponential decrease in PCDD/PCDF emissions from
salt-laden wood waste-fired grate boilers [121], which could
partly be attributed to the effects of the increased combustion
temperatures on flue gas combustion and fly ash particle burnout.
Other measures include dry sorption with activated char or
catalytic converters [6,27]. The field experience in the MSW,cement, and hazardous waste industries shows that the most
effective mode of control may be to limit the temperature
entering the particulate control device to values lower than the
optimal formation window [131,133,134].
4.1.5. Particulate matter and heavy metals emissions and control
Formation and emissions of particulate matter from biomass
combustion have drawn considerable attention because of theconcern that they may contain toxic elements or species of
relatively high concentrations, and have been a key research area
in biomass combustion; for example, under the EU-supported
projects BIOASH and BIOAEROSOLS [126,135]. The main focus of
the current R&D activities on this issue includes the following
aspects: (1) characterization of aerosols and ash particles (e.g.,
chemical composition, size distribution and morphology, concen-
tration in flue gas), which may depend on the type of biomass
fuels, firing technology, operation conditions; (2) fundamental
mechanisms of the formation and growth of fly ash particles and
aerosols including the release behaviour of ash-forming elements,
and how fly ash particles and aerosols contribute to deposition
and corrosion; (3) how to effectively reduce their emissions, for
instance, by improving aerosols separation efficiencies of flue gascleaning devices, and by using certain additives into biomass-fired
boilers; and (4) the partition and distribution of metals in bottom
ash and fly ash.
The existing data, concerning ash formation and emissions
from biomass combustion under grate-firing conditions or fixed-
bed conditions, are comparatively limited [110,126,135–143]. Most
of the knowledge is retrieved from measurements in large-scale
circulating fluidized bed (CFB) boilers, some of which are briefly
reviewed in [110]. The biomass fuels fired in a CFB boiler and a
grate-fired boiler have substantially different time-temperature
histories. Because of the intensive mixing in the dense bed in a
CFB boiler, the biomass fuels will be quickly dried, heated, and
combusted. The bed temperature in CFB combustion is relatively
low, about 800–900 1C. This is very different with biomass
conversion process in a grate-fired boiler, where biomass fuels
are slowly heated, dried, and pyrolysed as they are transported on
the grate, and finally the char will be combusted at high
temperatures, about 1000–1100 1C (see Fig. 5 for this process).
These differences will significantly affect the volatilization of
particle-forming precursors.
The ash formation during biomass combustion under grate-
firing conditions is illustrated in Fig. 8. Part of the inorganic
elements contained in the fuel may be released and form
inorganic gas species and particulate matter (fly ash particles
and aerosols), whilst the remaining part forms bottom ash. It can
be seen, from the figure, that the particulate matter can be formed
by two different modes, which lead to a characteristic bimodal
particle size distribution [110,136,137,142,144–146]. One is the fine
mode, in which the main route of particle formation is nucleationand condensation from the gas phase. The fine mode usually
consists of aerosols with particle diameters between 30 and
300 nm. The other is the coarse mode, which mainly consists of
non-volatilized ash residuals and results in fly ash particles with
diameters of 1 mmodpo10mm. Similar ash formation mechanisms
could also be found in [135,145–151].
Amongst all the ash streams, bottom ash represents the
majority, in terms of mass fractions, for grate-fired boilers burning
biomass. The mass fraction of the bottom ash varies with different
factors, e.g., the type of grate-fired boilers, the operation, and the
fuel properties. In a 10.7 MWfuel grate-fired boiler burning straw
which produces 2.3 MWe and 7 MJ/s heat, it was estimated that
approximately 80% of the ash ended up as bottom ash [152]. In a
108 MWfuel grate-fired boiler burning wheat straw equipped withwater-cooled vibrating grates, the bottom ash amounted about
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754736
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 13/30
85–90% of the total ash as a result of reduced primary air flowrate
[53]. Bottom ash and fly ash were sampled from 19 MSW-fired
grate boilers, whose capacities ranged from 60 2 to 500 3ton/
day. The mean mass fraction of bottom ash for the 19 boilers is
about 77% (with 60% as the minimum and 88% as the maximum)
[138]. Concerning the mass split between the aerosols and fly ash,
there are also some investigations, for instance [110,150]. The
experiments on two moist forest residue-fired grate boilers (1 and
6 MWth in capacity, respectively) show that small changes in the
boiler operating parameters can have a large influence on the
mass split between aerosols and fly ash. For instance, PM110 (i.e.,
particulate matter with diameters in the range of 1–10 mm) mass
concentrations increased by more than one order of magnitude
when the boiler load was increased from 50% to 75%. The coarse
(41mm) and fine mode (o1mm) masses were of equal magnitude
downstream of the cyclone when the boilers were operated at
higher load. At lower load, PM1 (o1mm) dominated PM10
(o10mm) [110].
Under grate-firing conditions, the most volatile metals con-
tained in biomass fuels (e.g., Hg, Th, and Se) are completely
vaporized and then may be released into the atmosphere with the
flue gas or condensed on the surfaces of aerosols and fly ash
particles. Measurement data from 19 grate-fired incinerators
burning MSW showed that lithophilic metals such as Fe, Cu, Cr,
and Al remained mainly in bottom ash whilst volatile Cd
transferred to fly ash. Two-thirds of Pb and Zn remained inbottom ash despite their relatively high volatility [138]. The
generally non-volatile compounds (consisting mainly of refractory
species such as Ca, Mg and Si) and some bound volatile
compounds (e.g., K, Na) usually remain on the grate and form
the usable bottom ash [26,27]. So, the bottom ashes from grate-
fired boilers burning biomass are comparatively less problematic
in terms of the leaching concentration of toxic elements and may
be directly used on agricultural land or in forests as secondary raw
materials with fertilizing and liming effects, or landfilled [27].
Compared to the bottom ash, the aerosols and fly ash particles
are more likely to cause some problems. Firstly, they have a
considerable influence on the formation of slagging and fouling
deposits on heat transfer surfaces in grate-fired boilers, which
lead to reduced heat transfer and corrosion. Secondly, the aerosolscontribute to the ambient air pollution level of particles, which
are shown to have adverse health effects on humans. For instance,
it was found that inhaling aerosols emitted from biomass
combustion causes significantly more lung damage in mice than
do aerosols from coal, probably because of the zinc content in the
aerosols from biomass combustion, from which it was even
suggested the environmental advantages of biomass combustion
may be tempered by the risk to respiratory health [153]. The
formation mechanism of aerosols and fly ash particles under
grate-firing conditions may be referred to Fig. 8. The character-
izations of the aerosols and fly ash particles from grate-fired
boilers burning representative biomass fuels, e.g., woody biomass,
agricultural residues (straw), and MSW, are given in Table 8. It
should be emphasized that these measured data from field studies
are just for reference. These data are quite dependent on the
capacity, type and operation of the boiler, the physical and
chemical properties of the biomass fuel fired, and the detailed
combustion control technology (e.g., temperature, turbulence, and
residence time).
There are also some modelling efforts in examining possible
practical influences on aerosol formation in biomass-fired boilers
and determining the amount and chemical composition of
particle emissions, such as the model proposed for the formation
of gaseous alkali sulphates [154] or model for aerosol formation
[135,139].
Since aerosols contribute to the ambient air pollution level of
particles and may contain toxic elements, the effective reductionin aerosols emission is necessary in order to boost biomass
combustion. There are some commercially available dust-pre-
cipitation technologies, as seen, for example, in [155], which
include cyclones, multi-cyclones, flue gas condensation units,
electrostatic precipitators (ESP) (including wet ESP and dry ESP),
and baghouse filters. In general, baghouse filters have the best
precipitation efficiency for aerosols; wet ESP and dry ESP also
have good precipitation efficiencies; scrubber condensers are less
efficient in capturing aerosols; and cyclones are suitable only for
coarse fly ash particles. Besides the dust precipitation, reduction
in aerosols emission may also be achieved from their formation
routines. The influence of six sorbents on the formation of
aerosols during straw combustion in a 100 MWfuel grate-fired
boiler was studied in full-scale. The observations show asignificant reduction in the aerosols formation by using five of
ARTICLE IN PRESS
Fig. 8. Schematic illustration of ash formation routines during grate-firing of biomass.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 737
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 14/30
the six sorbents studied, i.e., bentonite, ICA5000, clay, mono-
calcium phosphate, and ammonium sulphate [143].
4.2. Deposit formation and corrosion
Deposition (i.e., slagging and fouling) and corrosion problem is
one of the major issues that play an important role in the design
and operation of a combustion system. In solid fuel combustion,
the particulate matter formed during combustion may be
deposited on furnace walls and heat-exchanger tubes, which will
reduce the heat transfer and could also give rise to corrosion
problem. Biomass-fired furnaces, in particular straw-fired fur-
naces, are often reported to have severe deposition and corrosion
problems compared to coal-fired boilers (see Fig. 9 for examples).
4.2.1. Deposition indices based on fuel properties
Deposition indices are always an important concept whenevaluating the deposition potential of a solid fuel or handling
deposition problems during combustion of solid fuels. There are
many deposition indices for the prediction or estimation of the
ash deposition tendency, which are based on the fuel properties,
particularly the fuel ash chemistry. The commonly used deposi-tion indices include base/acid ratio (B/ A ¼ (Fe2O3+CaO+MgO+
K2O+Na2O)/(SiO2+Al2O3+TiO2)), iron index (Fe2O3 (B/ A)), slagging
factor (dry S% (B/ A)), silica ratio (100SiO2/(SiO2+Fe2O3+CaO+
MgO)), critical viscosity at 1426 1C (CV1426 1C in the unit of poise),
estimated ash temperature corresponding to an ash viscosity
of 250 poise (T 250 in the unit of 1C), alkali index—the ratio
of the total amount of sodium and potassium expressed as
their corresponding oxides to the heating value of the fuel
(ðY aK2O þ Y aNa2OÞ=Q F in the unit of kg/GJ), chloride–sulphate molar
ratio ((Cl+2S)/(K+Na)), multi-fuel fouling index—the percentage of
water-soluble alkali and alkaline-earth metals (given as oxides),
ash melting points (initial deformation temperature, hemisphe-
rical temperature and flow temperature) [157]. Most of these
indices were originally proposed for coals, and different limitsare suggested for coal quality parameters relating to deposition
ARTICLE IN PRESS
Fig. 9. Deposits on super-heaters during firing straw or straw/coal in boilers. (a) Deposits on superheater in upper furnace during firing straw at Masnedø CHP plant [146].
(b) Deposit build-up on superheaters after 1 week of co-firing coal and straw at Amager power plant [156].
Table 8
Characterization of aerosol particles (o1mm) in flue gas or fly ash particles
Grate-fired boilers Sampling point Concentration of
aerosols in flue gas
Particle size distribution of
aerosols (or fly ash particles)
Composition of aerosol particles
(or fly ash particles)
25MWfuel wheat straw-
fired grate boiler
[136]
The flue gas particles are
sampled upstream the
electrostatic precipitator
(ESP), where gastemperature is 120 1C
and O2 concentration
fluctuates 8%.
The concentration of the
aerosols is 300–480 mg/
N m3 (cascade impactor)
or2 106–1.8 107 N cm3
(scanning mobility
particle sizer).
The number geometric mean of the
submicron peak is 0.19–0.30mm.
Small particles (0.1–0.3mm) are
close to spherical. Large particles(0.3–0.8mm) vary from almost
spherical to aggregates consisting
of 5–10 distinct primary units with
dp 0.1–0.2mm.
The aerosol particles mainly
consist of KCl and K2SO4 with
minor amounts of P. Elements Ca,
Mg and Si are detected in largerparticles only (41 mm).
6 MWth moist forest
residue-fired grate
boiler at 85% load
(The dominant
species of the fuel
are spruce and pine.
The moisture content
is 40%.) [110,137]
The flue gas particles
were sampled
downstream the cyclone
and upstream the ESP,
where the gas
temperature is about
1901C.
The concentration of the
aerosol particles is close
to those of the coarse
particles (1–10mm): both
are 79mg/Nm3
(related to 13% CO2 dry
gas).
Bimodal particle size distribution is
detected: aerosol particles have a
peak at about 0.2 mm; while coarse
particles are peaked at 2 mm.
The dominant species (mass ratio
410%) in the aerosol particles are
K, S, and Cl. Minor elements are Zn
and Ca. Cr, Fe, Ag, Cd, Mn, P, etc. are
present as traces. The dominant
species in the coarse particles are
Ca, K, and S.
Waste wood fired grate
boiler (nominal
capacity 40 MWth)
[135]
From the flue gas duct
behind the economizer
(mean flue gas
temperature 1781C).
The mean concentration
of aerosols is 84.2mg/
N m3 (related to 13 vol.%
O2 and dry gas).
The aerosol particles are mainly in
the range of 0.088–0.707mm with
the peak at 0.354mm.
The aerosols mainly consist of Cl, K,
and Pb. Ca, Na, S, Si, and Zn are
present in minor amounts. Fe, Mn,
and P are present as traces.
450 2 tonnes/day MSW(90% household
waste and 10%
business waste)
grate incinerator
[140]
Fly ash was sampledfrom the ash pit under
the bag filter, which is
located just before the
stack.
No particleconcentration data.
Over 95% of the fly ash particleswere o149mm, in which 49.4%
were in between 53–75mm. The fly
ash is characterized by a more
uniform distribution of concaves
and agglomeration on its surface.
The fly ash has highly complexmineralogy. The main crystalline
compounds detected include KCl,
NaCl, and SiO2. The fly ash requires
further treatment before final
disposal since the leaching
concentration of Pb exceeds the
regulatory level.
In this table, N m3 and Ncm3 mean m3 and cm3 under normal conditions (1 atm and 273K), respectively.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754738
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 15/30
potential. As an example, the coal slagging potentials can be
evaluated online by calculating some of the above deposition
indices from its ash analysis [158]. Care must be taken with these
indices and their application.
The capability and accuracy in predicting ash deposition
potential. The fuel properties represent only half the depositionstory, and the boiler design also plays a large part in
the occurrence of ash deposition problems. Generally, if the
combustion temperatures in the furnace are high, or the
aerodynamics encourage flame impingement, then the potential
for ash deposition problems will be high. On the other hand, if
the fuel ash chemistry is conducive to the formation of sticky
compounds at the prevailing furnace temperatures, then the
potential for deposition problems will also be high [157]. So the
different indices only indicate an ash deposition tendency. In
addition, the limits for the deposition indices, suggested in the
literature, are not necessarily valid for all fuels. For a specific fuel,
one may have to experimentally determine the limits.
The different indices could lead to inconsistent deposition
tendencies for a same fuel. The alkali index and multi-fuelfouling index, both of which have been suggested to be also
applicable to biomass fuels and biomass mixture, were
calculated for rice husk, rice straw, and eucalyptus bark
samples, as well as for some other husks and hulls, straws,
and Scandinavian barks. The calculated indices showed
different deposition risks, in particular for rice husk: the alkali
index indicated a certain risk of deposition whilst the multi-
fuel index showed a low or non-fouling risk. The measure-
ments showed that rice husk was a non-fouling fuel [159].
The correct understanding and use of the ash melting points.
Biomass fuels contain a relatively high content of K and Na,
which are believed to significantly lower the melting points of
ash, and hence may increase ash deposition and fouling
tendency [13,23]. However, in many cases the standard melting
point measurements have resulted in findings that do not
directly correlate with the ash behaviour in full-scale combus-
tors [159]. One of the possible reasons could be the measure-
ments or the understanding of the melting point. Inorganic
mixtures such as fuel ashes do not have one sharp melting
point. Instead, they melt stepwise in a temperature range
where the difference between the temperature of the first
appearance of melt (T 0) and the temperature of complete
melting (T 100) can be several hundred degrees. It was found
that the amount of a melt present in the condensed phases,
rather than the composition of the ash itself, was the main
reason for ash deposition problems. In order to deposit on a
surface, an ash particle should contain a certain amount of
liquid melt. Based on experience with different kinds of boilers
or combustors, it was suggested that an amount of 15 wt% of the condensed phases molten at a certain temperature enabled
deposit formation in the flue gas channel [160–162]. The
melting curves (i.e., the melt portion as a function of the
temperatures) can be used to determine the characteristic
temperatures: the sticky temperature (T 15), i.e., the tempera-
ture below which the salt mixture contains less than 15 wt%
molten phase, and the flow temperature (T 70), i.e., the
temperature above which the share of the liquid phase is
larger than 70 wt%. With this approach, it was assumed that
the material did not stick on surfaces at flue gas temperatures
below the sticky temperature of the depositing material [163].
This approach was used to predict the fly ash deposition (bed
sintering tendencies as well) for different solid fuels (coal and
biomass), and good agreement between the predictions andthe measurements was found [162].
4.2.2. Mechanisms of deposit formation and high temperature
corrosion
To probe into the fundamental mechanisms of fly ash anddeposit formation and corrosion in grate-fired boilers burning
typical biomass fuels (straw, as well as woody biofuels and MSW),
a significant amount of full-scale measuring campaigns, as well as
a few lab- or pilot-scale studies have been conducted, for example,
[69,70,164–171]. Generally speaking, both the volatile (alkali)
inorganic vapours and the inert non-volatile mineral matter can
contribute to the formation of deposit on heat transfer surfaces in
the boiler, in which they may play different roles in the build-up
of the deposit, to be discussed later. The corrosion mechanisms in
combustion systems include gaseous Cl-species induced corro-
sion, solid-phase reactions involving Cl-species in deposits,
reactions involving molten Cl-species, molten sulphate corrosion,
and so on, as reviewed in [10]. Which mechanism dominates will
depend on the combustion environment, combustion tempera-ture, metal temperature, and also the presence of elements such
as alkali metals, sulphur, silicon, and aluminum. Fig. 10 sketches
the in-deposit sulphation corrosion mechanism occurring in a
waste incineration plant [172]. Potassium species are supposed to
behave similarly to the sodium species. This mechanism was also
believed to be responsible for the most severe corrosion problems
on the superheater tubes in grate-fired boilers burning some other
biomass fuels (e.g., straw) [10,119]. Molten phases increase the
corrosion rate because of the faster chemical reactions in a liquid
phase and the electrolyte or pathway provided by the liquid phase
for the electrochemical attack [173].
The following sections briefly describe the deposition forma-
tion and high-temperature corrosion in grate-fired boilers burning
a few representative biomass fuels, i.e., straw, woody biofuels, andMSW, respectively.
Straw-fired grate boilers: The findings are summarized in [119].
Composition of fly ash and deposits: The fly ash and deposits
are rich in KCl (40–80 wt%).
Deposit formation: Initially on the clean tube, inorganic
vapours and fine particles that arrive at tube surface in a sticky
condition contribute to the initial deposit formation, and they
deposit on the entire circumference of the tube. After that, inertial
impaction of big fly ash particles contributes to the deposit build-
up, but mainly on the upstream side of the tube. So the deposit
formed consists generally of a white dense inner layer covering
the entire circumference of the tube and an ellipse-shaped depositon the upstream side, as sketched in principal in Fig. 11. The main
ARTICLE IN PRESS
Fig. 10. Schematic presentation of solid-phase reactions involving Cl-species in
deposits [172].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 739
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 16/30
deposit on the upstream side consists of fly ash particles and large
amounts of condensable salts, forming a matrix that glues the fly
ash particles together. The deposit is quite porous, which makes it
an effective insulator [156]. In the deposit the fly ash particles are
dominated by K and K–Ca silicates.
High-temperature corrosion: the relatively high partial pres-sure of HCl in the bulk gas in a furnace will probably not cause
severe gas-phase corrosion attacks [10] under oxidative condition.
Instead, the corrosion of superheater tubes is closely connected to
the deposit chemical composition, in particular the composition
of the inner part of the deposit. A characteristic feature found in
deposits is the presence of a dense inner layer of KCl or K2SO4 with
a structure of iron oxide (Fe xO y) in it. A suggested corrosion
mechanism for chlorine corrosion is based on gaseous Cl attack,
where Fe and Cr in the metal react with gaseous Cl, forming
volatile metal chlorides. The high partial pressure of chlorine close
to the metal is believed to be caused by a rapid sulphation of KCl
to K2SO4 in a melt, formed adjacent to the metal surface, as
depicted in Fig. 10, by replacing the sodium species with the
potassium species. This mechanism can explain the shift in
corrosion behaviour with temperature observed in full-scale
corrosion tests (i.e., selective corrosion: negligible at steam
temperatures of 4501C, significant if the metal temperature is
raised above 520 1C). At low metal temperatures, the solid-phase
sulphation is slow, and the metal suffers only from general
oxidation. When the KCl/K2SO4/Fe system becomes molten, the
KCl sulphates quickly in the melt, thereby generating a high
partial pressure of Cl2/HCl. Subsequently, this causes accelerated
oxidation and possible internal corrosion of the metal. The K2SO4
found in deposits in straw-fired boilers is believed to originate
mainly from deposition of gaseous KCl, followed by a subsequent
slow sulphation of KCl in the solid phase [119].
However, this suggested corrosion mechanism may not be
universal, particularly for the initiation of the corrosion. There are
quite some recent results indicating that sulphation is not needed
for the chloride corrosion to be initiated in the super-heaters and
that pure KCl will also cause heavy corrosion even in a gas
atmosphere with no SO2. For example, the influence of KCl on the
oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel
at 600 1C in 5%O2 and in 5%O2+40%H2O is investigated in the
laboratory. Based on the observations, it was proposed that the
rapid corrosion was initiated by the reaction of potassium chloride
with chromium oxide in the scale, forming potassium chromate
and Cl2 or HCl [174]:
Cr2O3ðsÞ þ 4KClðsÞ þ 52O2ðgÞ32K2CrO4ðsÞ þ 2Cl2ðgÞ (3)
Cr2O3ðsÞ þ 4KClðsÞ þ 2H2OðgÞ þ
3
2O2ðgÞ32K2CrO4ðsÞ þ 4HClðgÞ (4)
Woody biofuels-fired grate boilers: three different aerosol
formation processes are active, depending on the woody fuels
fired [119,150], which can also be seen in Fig. 8.
When firing chemically untreated wood chips, aerosol forma-
tion is comparable with aerosol formation in straw combustion:
Nucleation and condensation of alkali-salts and subsequent
coagulation of particles are the dominating mechanisms.When firing waste wood, Zn becomes more important. Under
reducing conditions in the fuel bed, Zn is released to the gas phase
and oxidized to ZnO, which then forms the first nuclei. This is
supposed to happen right after the flue gas leaves the fuel bed.
Subsequently, further nucleation of alkali metal and heavy metal-
salts is suppressed by condensation on the large specific surface
provided by a high number of ZnO nuclei, and particles grow due
to condensation and coagulation.
When firing bark, the aerosol formation process is somewhere
in between the two mechanisms mentioned above. The sub-
micron Ca-particles entrained from the fuel and ZnO-particles
formed according to the mechanism mentioned above normally
do not provide the specific surface area needed to suppress
nucleation, so that K-salts still form new particles. Subsequently,the particles grow by condensation and coagulation.
MSW-fired grate boilers: The findings of mature deposits in
MSW-fired grate boilers are summarized [119]. When going
from the outer to the inner deposit layers, a decrease in [Ca],
[Cl] and [Si] and a simultaneous increase in [S], [K] and [Zn] are
observed. The chlorine is responsible for the most serious
corrosion, and the decrease in Cl in the inner layer is because
the chlorides are converted to sulphates. The in-deposit
corrosion mechanism [172] suggested is sketched in Fig. 10.
Stronger sintering in the outer deposit layers is observed and
significant porosity is found in the inner deposit layer.
4.2.3. Possible solutions to the problems of deposition and high-
temperature corrosion
Both high-temperature corrosion and deposit formation during
biomass combustion can be mitigated by using additives, or by co-
firing with, for instance, coal, peat or sludge. The high-tempera-
ture corrosion can also be mitigated by using new alloys or new
forms of ceramic composite coating, or by reducing the surface
temperatures of super-heaters.
The first solution is the use of additives, to raise the melting
temperatures of the ash formed during biomass combustion, or to
prevent the release of gaseous KCl, or react with KCl to form less
corrosive components, or to combine the effects. The lower the
melting temperatures of the ash and/or the more vaporizable the
alkali or chlorine compounds, the higher will be the risk for ash-
related problems. Raising the ash melting temperatures canlargely increase the potentials for the use of the biomass fuels.
The materials that have been found to raise the melting
temperatures of ash, to temperatures higher than those normally
encountered in boiler furnaces, include Al2O3, CaO, MgO,
CaCO3 MgCO3 and kaolin [7,175]. The effect of the additives is
to enrich the ash formed during combustion with non-potassium/
sodium compounds. For example, addition of 3 wt% of kaolin to
chopped oats straw can raise the deformation temperature of the
ash from 700 to 1200–1280 1C [176]. However, one could argue in
a different way, as to how the additives raise the melting or
deformation temperatures. Additives do not change the first
melting temperature (T 0) at all. They may dilute the ash and thus
decrease the percentage of the molten phase in the mixture,
which could show as an increase in the measured empiricaltemperature for the radical deformation of a standard body
ARTICLE IN PRESS
Fig. 11. Principal sketch of the wheat straw-derived deposits formed on top of the
probe [69,119].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754740
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 17/30
pressed from the ash particles. Different additives have been
applied, either to the flue gas or with the fuel, and proven useful in
commercial biomass-fired boilers. For instance,
ChlorOut (ammonium sulphate), a concept developed and
patented by Vattenfall AB [177], may be one of the most
attractive methods for different combustion technologies,
since the additive is added to the flue gas in a certaintemperature window. An aqueous solution of ammonium
sulphate, (NH4)2SO4, is sprayed into the combustion zone at
temperatures around 800–900 1C upstream of the superhea-
ters. It effectively converts alkali chlorides (e.g., KCl) into alkali
sulphates (e.g., K2SO4). These sulphates are much less corrosive
than the chlorides and therefore the overall corrosion rate is
reduced. The spraying of ammonium sulphate can also reduce
NO x formation. The main reactions involved in ChlorOut
process include
ðNH4Þ2SO4ðaqÞ ! 2NH3ðgÞ þ SO3ðgÞ þ H2OðgÞ (5)
2KClðgÞ þ SO3ðgÞ þ H2OðgÞ ! K2SO4ðsÞ þ 2HClðgÞ (6)
4NH3ðgÞ þ 4NOðgÞ þ O2ðgÞ ! 4N2ðgÞ þ 6H2OðgÞ (7)
ChlorOut has been successfully tested in a number of biomass-
fired fluidized bed boilers. The results showed that ammonium
sulphate reduced the KCl levels in the flue gases, removed the
chlorides from the deposits and the metal/oxide interfaces, greatly
reduced the deposition rates and halved the corrosion rates for
superheater materials [114,115]. ChlorOut has also been tested in
grate-fired boilers burning biomass [113,116]. The tests in the
grate-fired boiler at the waste incineration plant Mullverwertung
Borsigstraße (MVB) in Hamburg, within an EU co-financed project
NextGenBioWaste, showed a marked reduction of the corrosivealkali chlorides when ammonium sulphate was injected. The
amount of deposit build-up was halved and the corrosiveness of
the deposits was reduced. There were also some environmental
benefits to using the additive: the level of NO x in the flue gas was
drastically reduced and a reduction in the amount of dioxins in
the fly-ash was also detected. The results from the tests were used
for designing a permanent ChlorOut system in the waste-fired
grate boiler at MVB. The permanent installation was planned for
the end of 2007 [116]. In the light of the idea of ChlorOut, two
different reagents, Al2(SO4)3 and Fe2(SO4)3, were tested as
slagging and corrosion control techniques for biomass firing
[178]. It was concluded that in the following reaction (where M
stands for K or Na),
2MClðgÞ þ SO3ðgÞ þ H2OðgÞ ! M2SO4ðsÞ þ 2HClðgÞ (8)
the effectiveness of the reagents with constant sulphur mass flow was
Al2ðSO4Þ34Fe2ðSO4Þ3bðNH4Þ2SO4 (9)
This order is directly proportional to the temperature needed
for thermal destruction. (NH4)2SO4 probably decomposes faster
than the other sulphates, which then produce fresh SO3 over a
longer furnace zone. Complete decomposition to SO3 over a short
furnace zone leading to high local SO3 concentration may enhance
other SO3-consuming reactions than sulphation and reduce alkali
sulphation selectivity to SO3. With the new reagents, sulphation of
alkali chlorides dominates strongly over the other SO3-consuming
reactions, which is a significant advantage: chemicals can be
used with low dosages with small or insignificant increase of SO2
emissions [178].
The additives can also be added with the fuel onto the fuel bed.
However, the effects may be less general: they are more
dependent on combustion technologies. Different combustion
technologies, e.g., FBC and grate-firing, have very different
mixing, temperatures and combustion environments (i.e.,
oxidizing or reducing) in the fuel beds, which significantly
affect the effectiveness of the additives. One has to keep this in
mind when trying an additive in grate-fired boilers which hasbeen tested and proven efficient in other combustion technol-
ogies (e.g., FBC or suspension combustion). A test programme
in a grate-fired boiler at a CHP plant showed that it was
possible, from an operational, corrosion, and depositional
point-of-view, to co-fire Danish wheat straw with shea nuts,
wood chips, and olive stones on a grate, at the actual energy
shares, i.e., 25–30% on an energy base. In the short-term, full-
scale experiments, no serious operational problems related to
ash and deposit formation or corrosion of superheater tubes
were observed [119]. Adding kaolin to biomass particle seal
under CFB combustion conditions was found to have the
following effects: Kaolin could capture potassium and form
potassium aluminium silicates which have high melting points
and were unlikely to become liquid and stick to the surface of super-heaters; less potassium would be available for the
reaction in the furnace [86]. Since the contact between the
additive particles and flue gas potassium compounds in grate-
fired boilers is less efficient than in a CFB, one may not expect
the same effects if kaolin was used under grate-firing
conditions. When firing pulp sludge with pine bark or with
pine bark/agricultural waste mixture under FBC conditions,
aluminium silicate formation was found to dominate over
sulphation in the reduction of Cl concentration in deposits,
which then reduced the chlorine-induced corrosion of heat
transfer surfaces [118]. Again, one has to be aware of the
difference between FBC and grate-firing if the same method
was used in grate-firing boilers. Coal ash could also be used as
a kind of additives. It was found that during co-firing of straw
and coal there seemed to be a significant capture of K from the
straw, by the coal ash, which was observed in different utility
boilers. Due to the capture of K in the coal ash, only low
concentrations of KCl (o5 wt%) were observed, in the fly ash
and deposits, from the plants. It was concluded that ash
deposition and/or corrosion would not, most likely, be the
major problems during coal-straw co-firing in suspension-fired
boilers provided that a high-quality coal was applied for the
co-firing [179]. Under grate-firing conditions, the capture of K
from the straw, by the coal ash, may also occur to some extent.
The second solution is to use new alloys or ceramic tiles that are
resistant towards chlorine corrosion [166,180], especially for
modern large-scale biomass-fired grate boilers. There are somepreliminary tests on this subject. For instance, various austenitic
and ferritic steels are exposed in a water-cooled probe in the
superheater area of a straw-fired CHP plant. The temperature of
the probe ranges from 450 to 600 1C and the period of exposure is
1400 h. The rate of corrosion is assessed based on unattacked
metal remaining. A clear trend has been observed that selective
corrosion increases with respect to the chromium content of the
alloy [166]. Various forms of coating aimed at preventing
corrosion have been tested at the Siekierki CHP station in
Warsaw, Poland. A new form of ceramic composite coating is
effective in preventing corrosion and has been installed in
different boilers [180].
The third solution is to keep the surface temperatures low.
New biomass-fired grate boilers are often characterized by highsteam parameters (temperature and pressure) for high plant
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 741
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 18/30
efficiencies. The increased steam temperatures raise concerns of
high-temperature corrosion as a result of increased tube surfacetemperatures. A modified Rankine steam cycle is presented as a
solution for a biomass-fired boiler of small or medium sizes, to
prevent the chlorine-induced high-temperature corrosion without
loss of efficiency in the steam cycle by fully utilizing the
permissible wall temperature limits [120].
Other measures against corrosion include co-flow superheaters
(i.e., superheater tubes are placed with axis parallel to particle-
laden gas flow in order to minimize the possibilities of particle
accumulation) and optimized combustion and process-control
technologies. Automatic heat exchanger cleaning systems, e.g.,
soot blowers, are often used to mitigate the deposits on super-
heater tubes. However, they could make the high-temperature
corrosion even worse by effectively removing the corrosion
products from the tubes while exposing them to new corrosive
fly ash deposits.
4.3. Modelling and CFD simulations for diagnosis, optimization, and
new design
Not all the relevant phenomena in a combustion system are
described and understood in full details, but CFD calculations give
an impression of reciprocal relationships. Mathematical model-
ling and CFD simulations form a helpful tool to improve the
understanding of the details, probe the problems, and optimize
the plant operation, as well as aid in a new design. Compared to
modelling of pulverized coal boilers, CFD modelling of biomass-
fired grate furnaces is inherently more difficult due to the complex
biomass conversion in the fuel bed on the grate, the turbulentreacting flow in the freeboard, and the intensive interaction
between them. Fig.12(a) sketches the sub-processes in grate-fired
boilers, i.e., thermal conversion of biomass in the fuel bed, and
primary combustion and burnout in the freeboard, which interact
with each other. Fig. 12(b) shows the most popular methodology
in modelling of grate-firing biomass, in which a separate model
is used to solve the thermal conversion of biomass in the fuel
bed, CFD is used for the freeboard simulation, and they are
coupled by the heat and mass transfer at the interface (i.e., the top
surface of the fuel bed). More precisely, the bed model provides
the inlet conditions (e.g., distribution of gas species concentration,
velocity, and temperature along the grate) for freeboard simula-
tion, whilst the freeboard simulation returns the heat flux
released from the flame and furnace walls onto the fuel layer tothe bed model.
However, there are different modelling methodologies for
grate-firing systems, for instance [3,63,181], in which the wholefuel bed is included as a part of the CFD simulation domain and
there is no need for a separate bed model to provide inlet
conditions. In this methodology, the precise size distribution of
the biomass particles fed into the boiler plays a decisive role in
final CFD simulation results.
Because it is difficult and expensive to carry out comprehen-
sive experimental studies on grate-fired boilers, many modelling
and CFD simulation efforts have been made instead. Table 9 lists
some of the representative modelling works on grate-firing of
biomass in the literature, in which the main purposes and findings
of the works are highlighted. As shown in the table, the modelling
and simulation works can be classified into five groups and the
first two groups’ efforts dominate. Most of the modelling and CFD
works have been validated, to different extents, by experimental
results, particularly the modelling works on biomass conversion
in the fuel bed on the grate.
4.3.1. Modelling of biomass conversion in the fuel bed on the grate
The processes in the biomass bed on the grate may be the most
grate-specific area for grate-fired boilers. The behaviour of
biomass conversion on the grate significantly affects the incom-
pletely burned char in the bottom ash, the distribution of the
combustibles released into the freeboard, and the precursors of
NO x, SO x, PCDD/PCDF, and particle formation. Therefore, biomass
conversion on the grate affects not only the combustion efficiency
of the grate-fired boiler but also the deposition and corrosion
tendency and pollutant emissions from the boiler. As seen in Table
9, there are quite some efforts on this issue, mainly by modelling.Three different approaches on how to model the fuel bed may be
found in literature.
Approach I . Fluent’s porous-media model is used to investigate
the solid-refuse bed on top of a roller grate. The results obtained
from this modelling are used as the inlet conditions for the
modelling of the freeboard region [201,202].
Approach II . More typically, freeboard modelling treats the fuel
bed by using inlet conditions based on experience or measure-
ments. When the combustion rate is prescribed as a function of
the position on the grate, inlet conditions (e.g., temperature,
velocity and individual species concentration) can be calculated
from the overall heat and mass balances of fuel components and
primary air, see, for example, [39,57,61,80,95,106,192,194,195,203].
The experience- or measurements-based bed models have beenproven to be quite robust and useful in studying biomass
ARTICLE IN PRESS
Fig. 12. Grate-firing of biomass and modelling methodology. (a) Biomass conversion in fuel bed, gas combustion in freeboard and their interaction. (b) Modelling concept:
coupling CFD and bed model.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754742
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 19/30
ARTICLE IN PRESS
Table 9
Summary of the modelling of grate-firing of biomass: the main purpose and the main findings
Purpose of work Main conclusions or findings Reference
Group 1—Modelling of biomass conversion in the fuel bed on the grate
Aerodynamics study of chain link stoker mats by CFD,
visualization and tests to improve the under-grate primary air
(PA) distribution.
CFD is used to aid in redesign of a traditional link design which shows
improvement in PA distribution. Pollutant emission from chain grate
furnaces may be mainly attributed to poor PA distribution.
[182]
To develop a model to characterize and quantify the mixing of biomass fuels on a grate.
(1) The existing grate systems do not mix the refuse sufficiently. (2) Modelpredictions show good agreement with measurements from three 1/15
scaled models of industrial grates.
[183]
To derive methods on a statistical basis to describe quantitatively
the mixing process of a packed bed on a forward acting grate.
(1) Two methods are introduced, based on particles’ velocities and
trajectories, to quantify the mixing process. (2) The trajectory-based method
is believed to be more accurate and suitable.
[184]
To study the effect of fuel mixing on waste bed combustion in
MSW grate incinerators by model development and
experiments.
(1) Improvement of the combustion intensity by fuel mixing is observed. (2)
Control of the primary air supply could further enhance the waste
combustion.
[185]
To study the effect of particle mixing caused by grate movement
on the ignition, burning rate, unburnt carbon (UBC) in ash,
and so on.
(1) Increasing bed mixing from low to medium level significantly increases
the burning rate and reduces UBC in ash. (2) Excessive mixing may cause
significant delay in ignition or even extinction.
[186]
To evaluate the residence time of a moving bed on a forward-
acting grate by numerical approaches.
Discrete element method is applied to describe the motion of a moving bed
(e.g., its particles). Tracking particles can obtain better predictions of the
residence time of a moving bed.
[187]
To understand MSW combustion in a grate incinerator by
modelling and tests of wooden particles combustion in a
fixed bed testrig.
(1) A 1D bed model is developed. (2) Radiation in fuel bed is important in
initiating flame front and transferring heat to the cold bed. (3) Air supply
rate, LHV, and particle size are important parameters.
[79]
To develop a fuel bed model and incorporate it in CFD for
modelling of coal combustion in 60 MWfuel grate boiler.
(1) The properties of different bed zones determine the conditions in the gas
phase above the bed. (2) The channelled flows in furnace do not seem to be
sensitive to the details of bed model.
[188]
To develop a bed model on a travelling grate by incorporating
sub-process models and solving governing equations for gas
and solid.
(1) A 2D bed model (FLIC) is developed. (2) Model predictions without
considering channelling effect agree with experiments in total mass loss but
show big discrepancy in temperature and gas composition.
[189]
To study the effect of air preheating on fuels combustion on a
grate by pot fixed-bed experiments and real-scale MSW plant
observations.
(1) Preheating of PA acts as a catalyst for the ignition on a grate rather than
only drying of the waste; (2) pot furnace experiments have only a limited
value in studying grate boiler combustion.
[190]
To study numerically the effects of fuel properties (i.e., LHV,
density, particle size, and packed bed porosity) on biomass
combustion characteristics on a grate.
(1) Average burning rate is mostly affected by fuel particle size. (2)
Combustion stoichiometry is equally affected by LHV and particle size. (3)
Density has the strongest effect on solid temperature. (4) LHV and particle
size have the strongest effect on CO; LHV and density have the dominant
effect on CH4; and particle size has the greatest effect on H 2 concentration at
the bed top.
[78]
To investigate the importance of particle size and density of fuel
on biomass conversion in a packed bed by a 1D model andtests.
(1) Particle density has a very small effect on the conversion rate in a packed
bed. (2) Particle size has a significant influence on the conversion of a packedbed. In a bed of large particles (30mm 30mm 30 mm), a clear
temperature difference exists between gas and solid. In a bed of small
particles (3 mm 3 mm 3 mm), the temperature difference is small and
could be neglected in modelling.
[55]
To study the conversion of biomass on the grate of a 25 MWe
forward reciprocating grate boiler by modelling and
experiments.
(1) By varying grate speed to obtain constant bed height, the furnace can
achieve 499% of conversion efficiency with 40–50% of normal PA supply. (2)
Char conversion is much slower than devolatilization.
[65]
A sensitivity analysis of the uncertainty of model parameters
related to heat and mass transport, reaction rates, and
composition of volatiles.
(1) Prediction of ignition rate and temperature peak is relatively insensitive
to the uncertainty in the parameters; (2) composition of volatiles affects
greatly gas concentration in the bed and gas ignition.
[191]
Group 2—CFD modelling of mixing and combustion in the freeboard
To study the effect of advanced secondary/over-fire air Ecotube
system in a 15 WMth biomass waste-fired grate boiler using
CFD.
New Ecotube air system generates a considerable improvement in efficiency
for biomass combustion in the grate boiler.
[80]
To model and validate a 50 MWth wood chips-fired grate boiler,
and then to predict the effect of an ‘ECO’ air system on NO x
emissions.
With an improved SA supply (by ‘ECO’-tubes), 30% NO x reduction can be
achieved.
[57]
To evaluate the effect of an ‘Ecotube’ air supply system on
combustion and emission from a 25 MWth biomass-fired
grate boiler using CFD.
The new SA system can result in a higher furnace flame occupation
coefficient, a more uniform heat release, a longer life of combustion
chamber, a lower level of pollutant emissions, and combustion noise.
[39]
To investigate the effect of air staging and flue gas recirculation
on flue gas burnout, mixing, and temperature distribution
using CFD.
Appropriate air staging and flue gas recirculation have a considerable
potential to optimize the mixing and improve the temperature distribution
and control to prevent slagging in biomass grate boilers.
[192]
Combined simulation of a 1 MWth wood chips-fired sloping grate
hot-water boiler by interactively employing a 1D bed model
and CFD.
(1) The predicted results of the fuel bed model are rather insensitive to the
freeboard conditions and more likely to be affected by fuel bed properties.
(2) Minor changes in SA largely reduce CO emission.
[56]
To study a 12T/h MSW-fired grate boiler by FLIC/fluent combined
simulation, as well as measurements.
(1) MSW on the grate is ignited at 1.8–2.0 m away from the waste entrance;
the measured maximum bed temperature is 1000–11281C with big
fluctuations up to 800 1C. (2) Improving SA is necessary to reduce particle
carry-over to boiler tubes and to increase the heat transfer.
[48]
To evaluate MSW combustion process in a pilot grate furnace
using CFD and experiments, under centre-flow operation.
(1) CFD and experiments show good agreement on flue gas burnout in
combustion chamber. (2) CFD is a valuable tool for further study on the
effect of PA supply, fuel mass flow, and grate movement.
[49]
To present a CFD analysis of a 33 MWfuel straw-fired grate boiler. (1) Model predictions show a good agreement with available measurements(temperature and species). (2) Poor mixing between bulk flow and SA jets is
partly responsible for high CO and UBC in fly ash.
[193]
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 743
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 20/30
ARTICLE IN PRESS
Table 9 (continued )
Purpose of work Main conclusions or findings Reference
CFD study of the dynamics of particles (spatial distribution,
residence time, momentum at outlet) in a 40 MW wood
chips-fired grate furnace.
By changing the air supply configuration (to form stronger recirculation
zones and higher level turbulence), the residence time of particles could be
prolonged.
[61]
Combined simulation of a 150 tons/day waste-fired grate boiler. (1) A strategy of combining CFD of gas flow field with waste bed combustion
is presented and tested. (2) More realistic submodels of waste bed
combustion would be helpful in this method.
[50]
To model and validate biomass combustion in a semi-industrial
grate boiler (firing 0.2 T/h biomass) and a full-scale grate
boiler (firing 13T/h biomass), and then to optimize them
through modelling.
(1) By modifying the heat release profile over the grate and the split
assumption for C and H in the calculation of the species released from the
fuel bed, the waste incineration model is very capable of predicting biomass
combustion in a grate boiler. (2) Boiler performance can be improved by
optimizing the ratio of PA/SA and the ratio of front wall SA to rear wall SA.
[194]
To model and optimize a 25 MWe MSW-fired reciprocating-grate
boiler using CFD coupled with fuel bed model (FLIC).
In the design case, there exists a large flow recirculation zone in the
radiation pass, which is not good for mixing and combustion, and can be
avoided by changing the distribution of SA jets.
[67]
Using CFD as a tool to upgrade grate-firing systems for improved
boiler operation and reduced emissions.
(1) Multiple rows of small OFA ports result in poor mixing with channelling
of gas up the boiler centre. (2) Poor combustion is overcome by upgrading
OFA system, a consistent conclusion from 30 boilers.
[181]
To investigate by using CFD the important parameters relevant
for design of a multi-fuel low-NO x grate furnace and to derive
guidelines for design of grate furnaces of 0.5–10 MWth
capacity.
(1) The designs of flue gas recirculation nozzles, SA nozzles, air staging and
combustion chamber geometry are investigated. (2) Injection of recycled
flue gas above the fuel bed is good for lower CO and NOx. (3) SA staging is
good for CO reduction. (4) The size and location of recycled flue gas nozzles
and SA nozzles and the jet speeds are important for mixing, combustion and
emissions.
[195]
To retrofit a 1975 vintage travelling grate stoker boiler in a paper
mill in Louisiana, USA, using CFD.
Simulations suggested the new air system and fabric stoker seal be installed.
The boiler was finally retrofitted on the basis of CFD indications.
[63]
To study how to improve air/gas mixing in a waste incinerator to
reduce incomplete combustion and lower emissions.
SA nozzle configuration is important: (1) mixing can be improved by
selecting larger inter-jet spacing, stronger jet speed; (2) staggered
arrangement of two opposite nozzle arrays is more effective.
[196]
To model a biomass grate boiler and locate the reasons for the
always over-predicted temperature in primary combustion
zone.
(1) Turbulence at grate inlet makes no difference to CFD results; (2) over-
predicted temperature is not due to the bed model; (3) mixture fraction/PDF
is better than eddy-dissipation but still over-predicts gas temperatures in
the primary combustion zone.
[197]
To introduce grate firing and also to present a CFD modelling of a
20MWe renewable energy, air-cooled travelling grate boiler.
(1) Care must be taken with SA systems to ensure radial and axial speeds of
SA jets match bulk gas velocity in furnace for proper mixing. (2) Balancing of
SA & PA is also important.
[3]
To establish a reliable baseline CFD model for a 108 MWfuel straw-
fired grate boiler for the purpose of optimizing design and
operation, using a thorough sensitivity analysis in CFD and a
2-day measuring campaign on the boiler.
(1) Raw input data and mesh are important in modelling of grate boilers,
even more important than the bed model in terms of overall CFD results. (2)
Mainly due to the raw inputs (e.g., the conditions of the walls and air-jets
under irregular deposits, the non-continuous biomass feeding and grate
movement and the combustion instabilities in the fuel bed), it is not thateasy to establish a reliable baseline model. (3) Staggered OFA jets are
favourable for forming double rotating flow in horizontal cross-sections in
burnout zone. However, the jets momentum and spacing could be further
optimized.
[53]
Group 3—Modelling of NO x formation and emissions from biomass-fired grate boilers
To develop a model for NO x emissions from biomass grate
furnaces.
(1) A new flamelet combustion model is developed. (2) The preliminary
results by applying the model to a 2D biomass-fired grate boiler are
encouraging.
[198]
To improve the understanding of combustion and NO emission
characteristics by studying numerically and experimentally
the related processes in an 8–11kW updraft wood pellets-
fired furnace.
(1) Biomass combustion in grate boiler can be effectively controlled by SA
and OFA jets. (2) The burnout zone is not sensitive to the bed combustion
process due to the high-speed flow from SA and OFA jets. (3) The rather high
flame temperature (1800 K) in this furnace leads to high NO emission from
thermal-NO mechanism. (4) In air-rich burnout zone, N2O-intermediate
mechanism dominates.
[107]
Combined simulation of a 38 MWe straw-fired grate boiler to
study its performance and the effect of variation in operation
conditions.
(1) Most of the NO is formed in the downstream combustion chamber. (2)
Fuel moisture content is limited to below 25% to prevent excessive CO
emission without compromising the plant performance.
[54]
To model SNCR application to a full-scale MSW-fired grate boiler. (1) The model is successfully evaluated against operational data
(temperature, NO and gas velocity). (2) The appropriate injection port of
reduction material for maximum NO reduction could be determined.
[62]
To propose a thermochemical model for the simulation of the flue
gas cleaning system of an RDF incineration plant.
The model simulates operation of flue-gas treatment section (NO x reduction,
SO2 and HCl scrubbing) and combustion section (grate incinerator, post-
combustion chamber) by using a simplified approach. The simulation results
are validated with operating data, indicating the model can be a practical
tool.
[51]
Group 4—Modelling of deposit formation in biomass-fired grate boilers
To examine possible practical influences on aerosol formation in
biomass-fired boilers and to determine the amount and
chemical composition of particle emissions.
A plug flow model is developed, considering gas-phase modelling by means
of thermodynamic equilibrium calculations and a kinetic approach for
modelling gaseous sulphate formation. The main particle formation and
precipitation mechanisms are considered.
[135,139]
To model the gaseous sulphation of alkali hydroxide and alkali
chloride since the formation of gaseous alkali sulphates may
yield aerosols and also contribute to deposition and
corrosion.
(1) A reaction mechanism for sulphation of alkali metals is proposed. (2)
Modelling predictions are not sensitive to the estimated properties in the
alkali subset. (3) The predicted degree of sulphation is affected mainly by
the rate of SO2 oxidation and the production of chain carrier in the system.
[154]
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754744
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 21/30
combustion in industrial grate boilers, provided that the correct
amount of mass, elements, and heat is released from the fuel bed
into the freeboard.
Approach III . Recently, separate bed models have been devel-
oped to study biomass conversion in the fuel bed on the grate, for
example, in the Sheffield University Waste Incineration Centre
(SUWIC) chaired by Prof. Swithenbank and the group headed by
Prof. Choi at the Korea Advanced Institute of Science and
Technology (KAIST). The ignition front and the combustion front
in fuel beds are tracked, and the temperatures, species, and
velocity at the fuel bed top are also solved, which are used as inlet
conditions for the freeboard modelling, see, for example,
[48,50,54,56,67,193]. The sensitivities of biomass properties (e.g.,
moisture content, particle size and density, solid conductivity,
heating value) and process parameters (e.g., heat and mass
transfer rates, bed porosity, devolatilization rate, primary air flow
rate, heat capacity of both the gas and solid phases) on the
conversion rate, temperature, and gas compositions are also
studied by using separate bed models [55,76–79,191].
Basically, approach III is to solve mass, momentum, energy, and
species balance equations for gas and solid phases, with necessary
process rate equations and empirical correlations/sub-models
used for the closure of the balance equations. However, one may
find huge inconsistencies in the sets of equations solved in the
different works, not only in forms (e.g., in divergence operators, inEinstein convention, or in algebraic forms) but also in some of the
terms (e.g., some terms may be solved in one application but
neglected in the other) or even in the inclusion of some transport
equations (e.g., the inclusion of momentum equations or not). The
large diversity in the sets of equations could be understood due to
the complexity of the fuel bed on the grate or due to the specific
subjects under different studies. For instance, quite some model-
ling efforts were done for fixed-bed combustion of biomass, in
which one-dimensional unsteady heterogeneous models were
solved for the fixed bed and then the time elapsed since ignition in
the fixed bed was mapped to the horizontal distance away from
the start point on the travelling grate in industrial grate boilers.
This kind of approximation may be acceptable for travelling grate
combustion as a result of the relatively small horizontal gradientsin temperatures and species concentration in some industrial
grate boilers. Some efforts were done directly for travelling grate,
in which 2D bed models were developed.
To have a better overview of the governing equations and to
develop a more general computer code for biomass conversion in
the fuel bed on the grate, MFIX (Multiphase Flow with Interphase
eX changes) [204] could be one of the most useful sources. MFIX is
a general-purpose computer code developed at the National
Energy Technology Laboratory (NETL) for describing the hydro-
dynamics, heat transfer, and chemical reactions in fluid–solid
systems. MFIX code is based on a generally accepted set of
multiphase flow equations, which are summarized in [205], and
the source code is available through its website, http://
www.mfix.org [204]. MFIX calculations give transient data on
the three-dimensional distribution of pressure, velocity, tempera-
ture, and species mass fractions. Though MFIX is mainly used for
describing BFBs and CFBs and spouted beds, which are different
from the fuel bed on a grate, the basic governing equations and
the programming techniques are still the same and useful for the
development of bed models for grate-fired boilers.
The biomass bed on the grate can be viewed as a reacting
gas–solid system, which includes one gas phase and one solid
phase. The gas phase has 6 or more different species, e.g., O2, N2,
CO, CO2, CmHn, and H2O vapour. The solid phase has 4
components, i.e., moisture, volatile matter, fixed carbon, and inert
ash. For such an air–solid system, the general governing equationsfor modelling can be summarized as follows.
Continuity equations: The continuity equation for the gas phase
is
qðfrgÞ
qt þ r ðfrgugÞ ¼ S g ðfor gas phaseÞ (10)
where f, rg, ug and S g represent the volume fraction of the gas
phase (i.e., void fraction in fuel bed), the material density of the
gas phase [kg/m3], the gas-phase velocity vector [m/s], and the
conversion rate from solid to gases due to evaporation, devola-
tilization, and heterogeneous reactions [kg/(m3 s)], respectively.
The process rate equations for evaporation, devolatilization, and
char oxidation can be found in the relevant literature (see Table 9)or in MFIX.
ARTICLE IN PRESS
Table 9 (continued )
Purpose of work Main conclusions or findings Reference
To develop a model for predicting the formation of ash deposits in
biomass-fired plants and implement it in CFD.
(1) It covers the release of fly ash particles and ash-forming vapours from
fuel bed, their transport and deposition on boiler walls. (2) The application
in a 440kWth boiler shows plausible results.
[199]
To develop a model for predicting the formation of ash deposits in
biomass-fired plants and demonstrate it in an industrial grate
boiler.
A deposition model is established and demonstrated, in which the deposits
are built up from fly ash particles (2–250mm) by inertial, turbulent, and
thermophoretic mechanisms and KCl vapours-formed particles (0.5 mm) by
diffusion, turbulent, and thermophoretic mechanisms.
[146]
To develop and validate a dynamic model of ash deposit growth
and shedding on a horizontal probe in a straw-fired grate
boiler.
(1) The model covers deposit growth, and also shedding by deposit surface
melting. (2) Model predictions agree with the measured evolutions of the
deposit weight, heat uptake and deposit shape. (3) KCl condensation
initiates the deposit formation and fly ash particle inertial impaction is the
main contribution to the deposit growth. (4) The deposit growth rate is
balanced by the shedding rate after 285 h in this study.
[69]
Group 5—Modelling or assessing of the discontinuous effects
To assess the effect of grate movement and waste feeding cycles
in full-scale MSW incinerator by modelling and tests.
Transient process is observed due to MSW feeding cycles and grate
moving–rest cycles. In flue gas, O2 in flue gas varies in 6–12%, temperature in
850–10101C. Combustion in fuel bed is also dominated by big fluctuations in
temperature and O2: about 600 1C and 12% from the spikes to dips,
respectively.
[47]
To model the discontinuous incineration process in reciprocating
grate boilers for control purposes with the aim of reducing
oscillations through a model-based control system.
Discontinuous features (e.g., discontinuous feeding of MSW, discontinuous
movement of the burning waste by reciprocating grates) are mainly
responsible for the high oscillations of process variables. The modeldeveloped show good agreement with experiments and is used for the
specified control purposes.
[200]
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 745
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 22/30
The continuity equation for the solid phase can be expressed as
qðrsÞ
qt þ r ðrsusÞ ¼ S g ðfor solid phaseÞ (11)
where e, rs, and us are the volume fraction of the solid phase,
e ¼ 1f, the material density of the solid phase [kg/m3], and the
solid-phase velocity vector [m/s], respectively.
Momentum equations: In principle, momentum equations
represent, ‘‘mass times acceleration per volume equals to the
sum of all the external forces per volume’’. Therefore, all the
terms should have the unit N/m3. If neglecting the inter-
phase momentum transfer due to the inter-phase mass
transfer (e.g., heterogeneous reactions), which is accounted
for in MFIX, the momentum equation for the gas phase can be
expressed as
qðfrgugÞ=qt þ r ðfrgugugÞ |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Densityacceleration
¼ fr pg |fflfflfflffl{zfflfflfflffl} Pressure
force
þ r ðfsÞ |fflfflfflffl{zfflfflfflffl} Viscous
force
þ frgg
|ffl ffl{zfflffl} Gravity
bgsðug usÞ
|fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} Momentum
transfer due to
interphase forces
þ f g |{z} Resistance
due to poroussurfaces
(12)
where pg and s represent the pressure in gas phase [Pa] and the
viscous stress tensor for gas phase [Pa], respectively. bgs is the
coefficient for the inter-phase forces, mainly the drag force in
most cases, [kg/(m3 s)]. Different correlations can be found in
MFIX for the calculation of the gas–solid momentum inter-phase
exchange. f g is the gas flow resistance due to the porous surfaces
[N/m3], which is usually calculated by a porous media model, for
example as is done in MFIX.
The momentum equation for the solid phase can be written as
qðrsusÞ=qt þ r ðrsususÞ |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Densityacceleration
¼ r pg |fflfflffl{zfflfflffl} Pressure
force
þ r ðssÞ |fflfflfflffl{zfflfflfflffl} Solid
stresses
þ rgg |ffl{zffl} Gravity
þbgsðug usÞ |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} Momentum
transfer due tointerphase forces
þ f g |{z} Momentum
gain from gratemovement
(13)
where ss and f grate represent the solid-phase stress (or granular
stress) tensor [Pa], and the momentum transfer from the
mechanical movement of grate (e.g., travelling, vibrating, or
reciprocating movement) [N/m3], respectively. In MFIX, the
granular stress equations are calculated on the basis of kinetic
theory and frictional flow theory, and the resulting constitutive
equations can be seen in detail in [205]. Since the constitutiverelations contain granular temperature, a separate transport
equation for granular temperature or an algebraic granular energy
equation is used in MFIX to solve the granular temperature.
Actually, the momentum equation for the solid phase has not
really been solved in any modelling effort for biomass conversion
on the grate, probably due to the difficulty with the solid-phase
stress tensor, or probably due to insignificant movement of the
solids in a fixed bed, or the assumption hereof. In some modelling
efforts which are directly devoted to travelling grates, the
horizontal solid-phase velocity is pre-defined, whilst the vertical
component of the particle velocity in the fuel bed is calculated
from the solid-phase continuity equation [65,189].
Species transport equations: The transport equation for then-th species (e.g., O2, N2, CO, CO2, CmHn, and H2O vapour) in the
gas phase is
qðfrgY g;nÞ
qt þ r ðfrgugY g;nÞ
¼ r ðfrgDg;nr Y g;nÞ þ Rg;n ðfor gas phaseÞ (14)
where Y g,n, Dg,n and Rg,n represent the mass fraction of the n-th gas
species, the diffusion coefficient of the n-th gas species [m2/s], and
the rate of production of the n-th gas species due to evaporation,devolatilization, and combustion [kg/(m3 s)], respectively.
The conversion equation for the i-th solid-phase components
(i.e., moisture, volatiles, fixed carbon, ash) can be expressed as
qðrsY s;iÞ
qt þ r ðrsusY s;iÞ ¼ r ðrsDsr Y s;iÞ
þ Rs;i ðfor solid phaseÞ (15)
where Y s,i, Ds and Rs,i are the mass fractions of i-th particle
compositions, the diffusion coefficient of the solid phase [m2/s],
and the rate of conversion of the i-th solid species due to
evaporation, devolatilization, and heterogeneous combustion
[kg/(m3 s)].
There are some uncertainties with the diffusion coefficients, in
particular, the solid-phase diffusion coefficient, Ds. The movementof a grate enhances the solid mixing in the fuel bed on the grate
and may have important influence on biomass conversion on the
grate. Therefore, the solid-phase diffusion on different types of
grates has been studied experimentally and correlations for the
solid-phase diffusion coefficients have been proposed, see
[186,206].
Energy equations: The energy equation for the gas phase can be
expressed as
frgC pgqT gqt
þ ug r T g
¼ r ðlgr T gÞ
þ hgsðT s T gÞ
þ DH g þ _Q rad;g (16)
where C pg, T g, lg, hgs, DH g and _Q rad;g represent the specific heat of
the gas phase [J/(kg K)], gas-phase temperature [K], gas-phase
conductivity [W/(m K)], gas–solid heat transfer coefficient cor-
rected for inter-phase mass transfer phase [W/(m3 K)], heat of
reaction in gas phase [W/m3], and radiation heat source to the gas
phase [W/m3], respectively. The calculations of these terms are
relatively simple though different correlations may exist and be
used in different efforts. One of the arguments could be on the
role of the radiation heat source. The radiation certainly plays an
important role in initializing the ignition flame on the top of the
fuel bed. However, the heat of reaction could dominate over
radiation in the propagation of the flame in the fuel bed.
Similarly, the energy equation for the solid phase can bewritten as
rsC psqT sqt
þ us r T s
¼ r ðlsr T sÞ
hgsðT s T gÞ
þ DH s þ _Q rad;s (17)
where C ps, T s, ls, DH s and _Q rad;s represent the specific heat of the
solid phase [J/(kg K)], solid-phase temperature [K], solid-phase
conductivity [W/(m K)], heat of reaction in solid phase (including
the heat loss due to moisture evaporation and heat generation due
to char oxidation) [W/m3], and radiation heat source to the solid
phase [W/m3], respectively. In this modelling framework, the
isothermal condition (i.e., Biot number less than 0.1) is assumed
for biomass particles. However, some tests show that thetemperature gradients may not be neglected for big particles, for
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754746
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 23/30
instance, thermally thick biomass fuels over 35 mm can develop a
temperature gradient over 400 1C in the particles at the flame
front under ordinary combustion conditions [207].
All the equations can be expressed in the standard form solved
in finite volume method,
qðrFÞ
qt |fflffl{zfflffl} Transient
þ r ðrFuÞ |fflfflfflfflfflffl{zfflfflfflfflfflffl} Convective
¼ r ðGr FÞ |fflfflfflfflfflffl{zfflfflfflfflfflffl} Diffusion
þ S F |{z} Source
(18)
With basic programming skills and using the finite volume
method, the modelling of biomass conversion on the grate may be
done. It would be better to programme on the basis of the
complete equations, in which the uncertain process parameters
(e.g., the inter-phase heat transfer coefficient) may be calculated
by separate subroutines or functions. 1D or 3D, and inclusion of
part of the contributions or all the contributions in the source
terms do not really add big difficulties to the programming. With a
general and structured computer code, it would be much easier
and more reliable to investigate the effects of some sub-models or
correlations (i.e., the sensitivity analysis) by simply replacing the
corresponding subroutine or function. Comparatively, there are
more uncertainties with the process parameters in the fuel bed ingrate-fired boilers, due to, for example, the relatively poor mixing
and the channel formation [208] and the very common combus-
tion instabilities [64] in the fuel bed on the grate. This also makes
it necessary to conduct the sensitivity analysis on the basis of a
reliable set of equations and a general computer code for the
modelling of biomass conversion in the fuel bed on the grate.
As an example, the second and the third approach are
demonstrated for the straw-fired water-cooled grate boiler shown
in Fig. 1(b) to show the differences. Fig. 13 shows the experience-
based, straw conversion rates along the grate and the primary air
distribution measured. For instance, biomass evaporation rates
85%, 15%, 0% and 0% along the grate length mean that in this boiler
85% of the moisture in the biomass is released in the first 14.6%
of the grate length, the remaining 15% of the moisture is released
in the second 20% of the grate length, and no moisture is released
in the last two grate sections (33.1% and 32.3% in length,
respectively). Based on the conversion rates and the primaryair distribution, heat and elements balance are used to derive
the species, velocity, and temperature of the combustion gas at
the bed top: the results are shown in Fig. 14. Fig. 15 shows the
counterpart calculated by the third approach based on the same
data of straw and primary air. Some differences are observed from
Figs. 14 and 15. However, they should result in the same amount
of elements, mass, and heat flow into the freeboard. Please be
aware that the gas velocities, shown in Figs. 14 and 15, are not
the velocities of the primary air. The superficial velocity of the
primary air at the bottom of the fuel bed is much lower, in the
range of 0.21–0.65 m/s in this grate-fired boiler.
Besides the models for the biomass conversion on the grate,
some modelling work has also been done to investigate the effects
of grate components themselves or to characterize fuel particlesmixing and residence time on the grate, as can be seen in Table 9.
4.3.2. CFD modelling of the mixing and combustion in the freeboard
Compared to pollutant formation, deposition and corrosion,
and biomass conversion in the fuel bed on the grate, the
combustion in the freeboard may be more related to combustion
physics. When the gases are released from the fuel bed, they mix
with the secondary air and combust in the freeboard. The gaseous
combustion is generally fast compared to the rate of mixing.
Therefore, fluid mechanics (i.e., mixing) plays a very important
role in the combustion and pollutant formation in the freeboard,
particularly for grate-fired boilers which have relatively low
combustion temperatures.
The majority of the existing CFD modelling of grate-firedboilers focuses on the mixing and the optimization, as can be seen
in Table 9. The poor mixing in grate-fired boilers could be in the
form of, for example, insufficient mixing between the bulk flow
and the SA jets, or insufficient occupation of the flue gas or flame
in the volume of the freeboard, or the formation of channelling
flow in the freeboard. The CFD modelling results show that the
mixing in the freeboard can be improved by, for example,
advanced air supply systems, optimized secondary air jets
(momentum, configuration, location, and spacing), or adjusted
split between secondary air and primary air (see Table 9 for
details). The basic ideas are to improve the momentum or
penetration of SA jets, form local recirculation zones, or form
rotating flows on horizontal cross-sections in the freeboard. The
mixing can also be improved through grate systems, for instance,
using good fabric stoker seal for low excess air or designing new
grates for improved distribution of primary air.
ARTICLE IN PRESS
Fig. 13. The experience-based conversion rate as a function of position on the
grate.
Fig. 14. Approach II: the gas species (left), gas temperature, and velocity (right) at the fuel bed top.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 747
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 24/30
4.3.3. Modelling of NO x formation and emissions from grate-fired
boilers burning biomass
The major source of NO x from biomass-fired grate boilers is
fuel NO x. Most of the existing understanding of fuel nitrogen
conversion in solid-fuel-fired systems involves coal as a fuel.
Comparatively little is known about other fuels such as biomass,
which limits the modelling capability of NO x formation and
emissions from grate-fired boilers burning biomass. The more
crucial point may be the release of the NO x precursors from the
fuel bed on the grate under different environments for different
biomass fuels. As shown in Table 9, there are some, but still
limited, efforts in the modelling of both NO x precursors released
from the fuel bed and NO x formation in the freeboard. For the CFD
modelling of NO x formation in the freeboard, the NO x precursors
released from the fuel bed on the grate, which are used as the
grate inlet boundary conditions, are mainly based on assumptions
(as seen in Table 6). This could be improved for a better modelling
of NO x emissions from grate-fired boilers burning biomass.
4.3.4. Modelling of deposit formation in grate-fired boilers
burning biomass
The modelling of deposit formation in grate-fired boilersburning biomass is very complicated, as a result of the complex
mechanism of deposit growth and shedding. Alkali vapours, fly
ash particles, and aerosols all contribute to the deposit formation.
Modelling of deposit formation involves the release of the
precursors of formation of alkali vapours and particulate matter
from the fuel bed, the reaction and transport of the vapours and
particulate matter in the freeboard, the transport of the vapour
and particulate matter to the furnace walls, and their sticking
propensities. The existing efforts show an encouraging potential
in estimating the deposit formation in grate-fired boilers burning
biomass; however, these efforts are still quite preliminary.
4.3.5. Modelling or assessing of the discontinuous effects
Quite often, grate-firing of biomass is characterized by somediscontinuous effects, e.g., the discontinuous biomass feeding and
the discontinuous grate movement. These discontinuous pro-
cesses certainly affect the plant operation and control, e.g., big
fluctuations or oscillations in process variables. However, very
little work has been done on this aspect.
5. Future R&D
Based on the knowledge and achievements already acquired,
more efforts in biomass combustion are still needed to resolve the
existing problems, particularly when we have to face the
increasing price of fossil fuels and the more stringent target to
utilize renewables. Further R&D related to grate-firing of biomassmay be suggested as follows.
5.1. Mechanism study of combustion chemistry for grate-firing
of biomass
Combustion chemistry and combustion physics are the two
foundations of any combustion technology. Combustion physics,
for instance, how to improve the mixing in the combustion
chamber, how to develop or improve the physical models to
extend their prediction capabilities, and how to make use of
experimental facilities or techniques (e.g., electric probes, optics,
acoustics, spectroscopy and pyrometry) to measure or monitor the
combustion processes, may be more general for combustion
processes. For grate-fired boilers burning biomass, some of these
have been extensively covered by current efforts, such as, mixing;
some of these are in a great need of enhancement, such as,
comprehensive experimentation; some of these may be less
pertaining to grate-fired boilers burning biomass, for instance,
improvement of some physical models including suitable sub-grid
models. Combustion physics has not been explicitly highlighted as
a separate issue throughout this paper.
Comparatively, combustion chemistry is more dependent on
the fuel and the combustion technology. Combustion of biomass
in grate-fired boilers has substantially different conversion
characteristics than those in other combustion technologies(FBC or suspension combustion). So far, the majority of the
mechanism studies of combustion chemistry in grate-fired boilers
burning biomass go to the combustion characteristics, e.g.,
ignition, devolatilization, char oxidation, reactivity of the released
volatiles, and char, as well as fuel nitrogen conversion. Some
efforts have also been made in the transformation of inorganic
elements, e.g., K, Cl, and S, in grate-firing conditions, in order to
facilitate the study on the ash formation, deposit formation,
and Cl-induced corrosion. The speciation and transformation of
heavy metals during grate-firing of biomass have also been
involved to a small extent, since they significantly affect the
formation of some toxic pollutants (e.g., PCDD/PCDF and heavy
metals emissions) and the utilization and disposal of ash. These
existing mechanism studies are still far from being sufficient andmay have to be strengthened by different extents. Without
sufficient and detailed knowledge of the combustion chemistry,
it is less likely to build up a reliable CFD model, to correctly
optimize/design the combustion system, and to propose efficient
measures to control pollutant emissions. For instance, a greater
part of heavy metals and chlorinated organic compounds (e.g.,
PCDD/PCDF) is bound in the fly ash during the combustion of
some biomass fuels, e.g., MSW. In order to propose efficient
measures to control the concentration of the toxic components in
the fly ash and to dispose of the fly ash, the transformation and
conversion characteristics of all the relevant elements (e.g.,
carbon, heavy metals and Cl) during biomass combustion, as well
as more detailed gas-phase chemistry and how it participates in
the heterogeneous chemistry, should be studied and well under-stood.
ARTICLE IN PRESS
Fig. 15. Approach III: the gas species (left), gas temperature, and velocity (right) at the fuel bed top, calculated by the model [76].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754748
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 25/30
To meet the increasingly stringent target to utilize the
renewables, more and more new biomass fuels will be exploited
and fed into grate-firing systems for heat and power production.
This also demands a significant amount of mechanism studies of
their combustion chemistry, in order to fire them efficiently and
cleanly.
The effects of different additives used in biomass-fired grate
boilers are also an important aspect in the mechanism study of combustion chemistry which needs improvement. Additives could
be used in grate-firing of biomass to achieve different purposes,
for instance, to improve combustion or to mitigate some problems
(e.g., emissions, deposition, and corrosion).
5.2. Advanced monitoring, testing, and experimentation
Some details of the grate-firing systems are not readily
accessible. Older boilers are particularly troublesome since they
even lack good flow measurement devices and monitoring
equipment. As discussed in [53], the uncertainties with the
details in biomass-fired grate boilers challenge the modelling,
operation, and optimization, for instance:
In large-scale grate-firing systems, there is often more than one
grate between the two side-walls at the bottom of the furnace.
Quite normally, the feeds onto the different grates are not
identical. Moreover, the feeding of biomass is not necessarily in
a continuous manner. The biomass feeding cycles, together
with the grate moving–rest cycles, may cause significant
fluctuations in the combustion process in the grate boilers [47].
The combustion instabilities in the fuel bed on the grate, e.g.,
local burnouts, channelling formation, and spatially uneven
fuel-bed thickness [64].
The uncertainties with the process parameters and physical
properties of biomass in the fuel bed on the grate, e.g., the
mixing rate, the heat transfer and mass transfer rates, the size
and shape distribution of biomass particles, the porosity of thefuel bed, and so on.
The uncertainties with wall conditions. The deposit formed on
the furnace walls as well as on heat-exchangers makes it
difficult to estimate the right wall conditions.
The uncertainties with air distribution and air-jet conditions. Air
jets play a very important role in the mixing in the freeboard. Quite
often, in large-scale grate-fired boilers, there are a few different
groups of air nozzles, and amongst each group there are several or
even many individual nozzles. Normally, only the total air flowrate
to each group or even to a few groups is monitored in the plant.
The air flow through different individual nozzles in the same
group may be unevenly distributed and in some cases big
deviations could exist. The deposit formed on the air nozzles
may give rise to an even bigger uncertainty with the conditions of the air jets: the irregular deposit on the air nozzles not only
deflects the direction of the air-jets but also re-distributes the air
flowrates through individual nozzles [53].
Without correct inputs, it is not possible to generate a reliable
model or CFD representation, from which suggestions/measures
on how to guide/improve the operation/design are often derived.
So, advanced monitoring, tests, and experimentation are needed
in order to obtain the necessary raw data as reliably as possible.
5.3. General and comprehensive model for biomass conversion in the
fuel bed
Grate and biomass combustion in the fuel bed on the grate arethe most specific topics in grate-fired boilers burning biomass.
Biomass combustion on the grate determines not only the main
combustibles but also the formation precursors (e.g., of particu-
late matter, pollutants, deposition, and corrosion) released into
the freeboard. So, grate and biomass combustion on the grate play
an important role in the overall performance of a grate-fired
boiler.
As discussed earlier, some efforts have been successfully made
to develop models for the biomass conversion in the fuel bed onthe grate, mainly to study the ignition and combustion character-
istics of biomass in the fuel bed. However, the models need to be
generalized, on the basis of a well-accepted set of multiphase flow
equations and without introducing too many assumptions or
simplifications at the very beginning stage of the development of
the model and the code. The models need also to be extended to
more topics, e.g., fuel nitrogen, fuel inorganic elements, and heavy
metals, to study the conversion and release characteristics of the
relevant species in the fuel bed, as well as to provide reliable
precursors for freeboard modelling. These demand substantial
knowledge in combustion chemistry of biomass conversion under
grate-firing conditions.
The models should satisfy some basic requirements. One of the
necessary conditions for the models is that the combustion gasmust carry the correct amount of mass, heat, and elements into
the freeboard. Knowing the data of the biomass fuel, primary air,
and external heat flux incident onto the fuel bed, the models will
output the profiles of species concentration, temperature, and
velocity of the combustion gas, leaving the bed top into the
freeboard (see Fig. 14 or 15 for examples). An integral analysis
must be carried out for the profiles of the species, temperature,
and velocity at the top of the fuel bed, to ensure the model itself
conserves the elements, mass, and heat.
5.4. Advanced CFD modelling
A reliable baseline CFD model is vital in CFD analysis, for
instance, for the diagnosis and optimization of the existing grate-firing plants, or for the design of new grate-firing systems.
Different from modelling of suspension-firing systems, CFD
modelling of grate-fired boilers may have a separate bed model
for biomass conversion in the fuel bed, as well as large gradients
in species concentrations at the bed top. The reliability of a CFD
model depends heavily on the quality of the raw input data, the
mesh, the models (including the separate bed model), and the
numerical methods. However, these aspects may not be consid-
ered equally in some of the existing CFD work. It is less likely to
generate a reasonable CFD representation for grate-firing systems
if over-highlighting one factor while neglecting others. As shown
in [53], in modelling of biomass-fired grate boilers, the main
effects of the models for biomass conversion in the fuel bed may
only be restricted to the vicinity of the fuel bed. In terms of theoverall flow and combustion patterns in the freeboard, the mesh
could play an even more important role. In CFD modelling of
grate-fired boilers, the raw input data could also be a challenge,
which demands advanced monitoring, tests, and experimentation
for the fuel properties in the fuel bed and the process parameters
both in the fuel bed and in the freeboard.
Advanced CFD modelling of grate-fired boilers should also be
extended from the mixing and combustion in the freeboard to
other important topics. For instance:
Modelling of pollutant emissions (e.g., NO x, PCDD/PCDF, heavy
metal, size and compositions of fly ash) will be helpful to
develop efficient emission control measures. However, it needs
sophisticated fundamental knowledge, for example, on theconversion of biomass fuel nitrogen, inorganic elements, and
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 749
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 26/30
heavy metals, more detailed gas phase chemistry, and how it
participates in the heterogeneous chemistry.
Modelling of deposit formation has gained some concerns. In
biomass-fired grate boilers, the fly ash particles may dominate
in the total mass of the deposit on the tubes due to inertial
impaction mechanism. Biomass particles fired in grate boilers
are quite big and irregular (see [209] for reference). A reliable
estimate of the particles entrained into the freeboard (includ-ing size, composition, and flow rate), the trajectories and
heterogeneous reactions of the entrained particles in the
freeboard, the ‘‘stickness’’ of the particles when they are
transported to the wall surfaces, and the wall conditions all
play important roles in the prediction of deposit formation. Big
irregular particles of relatively low density have different fates
than small (point) heavy particles, which may demand
advanced tracking methodology [210]. Big irregular particles
are thermally thick, which could lead to different particle
conversion process due to the big internal temperature
gradients [207,211]. So, the modelling of deposit formation
may have to take these factors into account.
5.5. Optimization and modernization for better performance
Better performance includes higher combustion efficiency,
lower emissions (both gaseous and particulate pollutants), and
better reliability and availability.
Advanced combustion air system is highly required in modern
grate-fired boilers burning biomass. The multiple zones of
under-grate primary air can help to achieve an optimum
temperature distribution and good ash burnout. Advanced
secondary air supply, for instance, the air staging, staggered SA
jets on the opposite furnace walls, static mixing devices, and
tangentially arranged SA jets as discussed earlier, could be used
to optimize the mixing and combustion in the freeboard and
then to improve the efficiency, reduce emissions, as well as
mitigate the deposition and corrosion in the boiler.
Improved fuel-handling and feeding systems and advanced
combustion grates can enhance the gas–solid mixing on the
grate and reduce the excess air. For instance, the reciprocating
movements in a reciprocating grate or vibration frequencies/
amplitudes in a vibrating grate may be optimized to achieve a
good mixing in the fuel bed. Improved fabric seal of the grate
system (including stokers) will result in a low excess air ratio.
Pre-combustion or post-combustion measures may also be used
to lower pollutant emissions and mitigate the deposition or
corrosion problems, as discussed earlier.
6. Conclusions
Biomass combustion for heat and power production is
progressing relatively fast, not only in research but also in
commercialization. This review paper focuses on grate-firing of
biomass: the main R&D activities, the progress, and the problems,
all of which are primarily pertaining to grate-fired boilers burning
biomass. Fluidized bed combustion of biomass is also discussed to
some extent, mainly for comparison and for a better illustration of
grate-firing of biomass. Both the technologies show great
competence in biomass combustion because they have good fuel
flexibility and can fire a wide range of fuels of varying moisture
and ash contents. The differences between the two technologies
for biomass combustion are summarized.
Amongst the key elements of grate-fired boilers, the grateassembly and the advanced secondary air supply are highlighted.
The former represents the most specific component in grate-fired
boilers whilst the latter is one of the real breakthroughs in grate-
firing technology. The grate assemble plays an important role in
the gas–fuel mixing (and therefore biomass conversion) in the
fuel bed as well as the control of overall excess air in the boiler.
The key combustion mechanism (i.e., propagation of flame fronts)
in the fuel bed on the grate is also discussed, which not only
determines the release of heat and combustibles into thefreeboard but also affects the release characteristics of the
formation precursors of NO x, aerosol and ash particles, and
PCDD/PCDF. Advanced secondary air systems are widely used in
modern grate-fired boilers in order to enhance the mixing and
combustion in the freeboard, lower the pollutant emissions, and
mitigate other operational problems (e.g., deposition and corro-
sion). Advanced secondary air systems may include air-staging for
favourable combustion environment sequences and optimized SA
jets for enhanced mixing, for example, using staggered SA jets,
tangentially arranged SA jets, or static mixing devices to form
local recirculation zones or rotating flow or to increase the jet
penetration into the centre of the freeboard. Compared to air-
cooled grates, water-cooled grates are more flexible with the use
of advanced secondary air systems.Amongst the issues associated with grate-fired boilers burning
biomass, primary pollutant formation and control, deposit
formation and corrosion, modelling and simulations for diagnosis,
optimization, and new design are highlighted. Based on these, the
critical problems that may be addressed by further research and
development are outlined. Combustion chemistry and combustion
physics, the two foundations of all the issues or problems, are
discussed throughout the different issues or problems.
Primary pollutants formation and control: The pollutant emis-
sions due to incomplete combustion can be controlled by
improved combustion, in grate-fired boilers, which mainly
means by improved mixing in the freeboard as well as
increased residence time in the combustion zones. The
pollutant emissions from fuel properties (e.g., ash, heavy
metals, Cl, and S) can be reduced by pre-treatment of the
biomass, well-controlled combustion process, or post-combus-
tion systems. In order to develop efficient measures to control
the pollutant emissions, it is crucial to understand their
formation routines or mechanisms under grate-firing condi-
tions. More efforts need to be made in the fundamental
combustion chemistry to study the transformation, speciation,
conversion, and reaction of the relevant elements (e.g.,
inorganic elements and heavy metals) in biomass fuels under
grate-firing conditions.
Deposit formation and corrosion: In grate-fired boilers burning
biomass, the volatile alkali inorganic vapours and fine particles
may contribute to the initial deposit formation while the inertnon-volatile ash particles contribute to the build-up of the
deposit. The most severe corrosion is associated with the
deposits containing alkali chlorides mainly on the super-heater
tubes in the boiler. However, different opinions may exist on
the role of sulphation in the corrosion mechanism, as
discussed in the paper. Amongst all the measures to mitigate
or even eliminate the deposition and corrosion problems, the
use of additives gets most of the current concern, which can
raise the melting temperatures of the ash formed during
biomass combustion, or prevent the release of gaseous KCl or
react with KCl to form less corrosive components, or a
combination of these effects. Injection of additives into the
combustion gases in a certain temperature window upstream
of the superheaters, e.g., ChlorOut (ammonium sulphate), maybe more attractive since it is less dependent on the combustion
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754750
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 27/30
technology. Due to the poor mixing and the reducing condition
in the fuel bed in grate-fired boilers, adding additives to the
fuel bed may be a less attractive option for grate-fired boilers,
at least compared to fluidized bed combustors. The funda-
mental chemistry on the transformation of the relevant
elements is required, if for instance the formation of the
inorganic vapours and fine particles and the relevant homo-
geneous and heterogeneous reactions is to be studied. Modelling and CFD simulations for diagnosis, optimization, and
new design: Modelling and simulation represent one of the
main efforts devoted to biomass combustion in grate-fired
boilers. However, a more general and comprehensive model for
biomass conversion in the fuel bed on the grate is yet to be
developed. Biomass conversion on the grate is the most
specific area and also plays a key role in the overall
performance of grate-fired boilers (e.g., efficiency, pollutant
emissions, deposition, and corrosion). For this purpose, a
generally accepted set of multiphase flow equations, which is
sufficient to describe the aerodynamics, heat and mass
transfer, and chemical reaction in the fuel bed on the grate,
is summarized. CFD modelling of the mixing and combustion
in the freeboard provides useful details, based on which someefforts of mixing optimization have been achieved. To develop
a reliable baseline CFD model for grate-fired boilers burning
biomass, the quality of the raw input data, the mesh, the
models (including the separate bed model for biomass
conversion in the fuel bed, as well as stand-alone sub-models
for particle tracking, particle conversion, ash deposition, etc.)
and the numerical methods all play important roles and must
be correctly accounted for. Advanced CFD modelling also needs
to be extended to more topics of interest in grate-fired boilers
burning biomass, e.g., formation of aerosol particles, which
covers at least sizes and compositions and formation of PCDD/
PCDF. Advanced CFD modelling also calls for more compre-
hensive experimentation for providing reliable data as inputs
or for validation.
Acknowledgements
The research on grate-firing of biomass was financially
supported by Grant PSO 4792, ‘‘Grate firing of biomass—Measure-
ments, validation and demonstration’’. The authors would like to
thank other project partners, Søren Lovmand Hvid, Torben Hille,
Torben Kvist Jensen, Ejvind Larsen, Marius Kildsig, and Bo Sander
(DONG Energy), Peter Glarborg, Peter Arendt Jensen, Haosheng
Zhou (DTU), Sønnik Clausen (Risø National Laboratory), and
Kenneth Jørgensen (BWV), for their helpful discussions during
all the project meetings. The authors are also grateful to seven
anonymous reviewers and Thomas Condra (AAU) for their
valuable comments that helped improve the quality of this paper.
References
[1] EU renewable energy policy. /http://www.euractiv.com/en/energy/eu-rene-
wable-energy-policy/article-117536S. March 2007.
[2] Goerner K. Waste incineration: European state-of-the-art and new develop-
ment. IFRF Combustion Journal 2003, Article No. 200303. 32pp.
[3] Morrow RS. Renewable fuel grate firing combustion technology—the
European experience. 2005; /http://www.mass.gov/doer/rps/mor-rpt.pdf S.
[4] Subramanian AK, Marwaha Y. Use of bagasse and other biomass fuels in high
pressure travelling grate boilers. Int Sugar J 2006;108:6–9.
[5] Ruth LA. Energy from municipal solid waste: a comparison with coal
combustion technology. Progr Energy Combust Sci 1998;24:545–64.
[6] Werther J, Ogada T. Sewage sludge combustion. Progr Energy Combust Sci
1999;25:55–116.
[7] Werther J, Saenger M, Hartge EU, Ogada T, Siagi Z. Combustion of agriculturalresidues. Progr Energy Combust Sci 2000;26:1–27.
[8] Sami M, Annamalai K, Wooldridge M. Co-firing of coal and biomass fuelblends. Progr Energy Combust Sci 2001;27:171–214.
[9] Williams A, Pourkashanian M, Jones JM. Combustion of pulverised coal andbiomass. Progr Energy Combust Sci 2001;27:587–610.
[10] Nielsen HP, Frandsen FJ, Dam-Johansen K, Baxter LL. The implications of chlorine-associated corrosion on the operation of biomass-fired boilers.Progr Energy Combust Sci 2000;26:283–98.
[11] Glarborg P, Jensen AD, Johnsson JE. Fuel-N conversion in solid fuel firedsystems. Progr Energy Combust Sci 2003;29:89–113.
[12] Demirbas A. Combustion characteristics of different biomass fuels. ProgrEnergy Combust Sci 2004;30:219–30.[13] Demirbas A. Potential applications of renewable energy sources, biomass
combustion problems in boiler power systems and combustion relatedenvironmental issues. Progr Energy Combust Sci 2005;31:171–92.
[14] Easterly JL, Burnham M. Overview of biomass and waste fuel resources forpower production. Biomass Bioenergy 1996;10:79–92.
[15] Smeets EMW, Faaij APC, Lewandowski IM, Turkenburg WC. A bottom-upassessment and review of global bio-energy potentials to 2050. Progr EnergyCombust Sci 2007;33:56–106.
[16] DMU (National Environmental Research Institute, Denmark). Greenhousegases. /http://www2.dmu.dk/1_Viden/2_miljoe-tilstand/3_luft/4_adaei/greenhouse_gases_en.aspS.
[17] Hein KRG, Bemtgen JM. EU clean coal technology—co-combustion of coaland biomass. Fuel Processing Technol 1998;54:159–69.
[18] Spliethoff H, Hein KRG. Effect of co-combustion of biomass on emissions inpulverized fuel furnaces. Fuel Processing Technol 1998;54:189–205.
[19] Jenkins BM, Baxter LL, Miles Jr TR, Miles TR. Combustion properties of biomass. Fuel Processing Technol 1998;54:17–46.
[20] Olanders B, Steenari BM. Characterization of ashes from wood and straw.Biomass Bioenergy 1995;8:105–15.
[21] Blander M, Pelton AD. The inorganic chemistry of the combustion of wheatstraw. Biomass Bioenergy 1997;12:295–8.
[22] Obernberger I. Decentralized biomass combustion: state of the art andfuture development. Biomass Bioenergy 1998;14:33–56.
[23] Tillman DA. Biomass cofiring: the technology, the experience, the combus-tion consequences. Biomass Bioenergy 2000;19:365–84.
[24] Allica JH, Mitre AJ, Bustamante JAG, Itoiz C, Blanco F, Alkorta I, et al. Strawquality for its combustion in a straw-fired power plant. Biomass Bioenergy2001;21:249–58.
[25] McKendry P. Energy production from biomass (part 1): overview of biomass.Bioresour Technol 2002;83:37–46.
[26] Demirbas A. Heavy metal contents of fly ashes from selected biomasssamples. Energy Sources 2005;27:1269–76.
[27] Obernberger I, Brunner T, Barnthaler G. Chemical properties of solidbiofuels—significance and impact. Biomass Bioenergy 2006;30:973–82.
[28] Tortosa Masia AA, Buhre BJP, Gupta RP, Wall TF. Characterising ash of biomass and waste. Fuel Processing Technol 2007;88:1071–81.
[29] Miles TR, Miles Jr TR, Baxter LL, Bryers RW, Jenkins BM, Oden LL. Boilerdeposits from firing biomass fuels. Biomass Bioenergy 1996;10:125–38.
[30] Henrich E, Burkle S, Meza-Renken ZI, Rumpel S. Combustion and gasificationkinetics of pyrolysis chars from waste and biomass. J Anal Appl Pyrol1999;49:221–41.
[31] Winter F, Wartha C, Hofbauer H. NO and N2O formation during thecombustion of wood, straw, malt waste and peat. Bioresour Technol1999;70:39–49.
[32] Blasi CD, Buonanno F, Branca C. Reactivities of some biomass chars in air.Carbon 1999;37:1227–38.
[33] Hansen HK, Pedersen AJ, Ottosen LM, Villumsen A. Speciation and mobilityof cadmium in straw and wood combustion fly ash. Chemosphere2001;45:123–8.
[34] Miller BB, Dugwell DR, Kandiyoti R. Partitioning of trace elements during thecombustion of coal and biomass in a suspension-firing reactor. Fuel 2002;81:159–71.
[35] Backreedy RI, Jones JM, Pourkashanian M, Williams A. Burn-out of pulverized coal and biomass chars. Fuel 2003;82:2097–105.
[36] Vamvuka D, Pasadakis N, Kastanaki E. Kinetic modeling of coal/agriculturalby-product blends. Energy Fuels 2003;17:549–58.
[37] Montgomery M, Sander B, Larsen OH. Biomass firing: Danish experiences.Energy Mater 2006;1:17–9.
[38] US Environmental Protection Agency. Biomass combined heat and powercatalog of technologies. September 2007. /http://www.epa.gov/chp/documents/biomass_chp_catalog.pdf S.
[39] Blasiak W, Yang WH, Dong W. Combustion performance improvement of grate fired furnaces using Ecotube system. J Energy Inst 2006;79:67–74.
[40] Obernberger I. Ash related problems in biomass combustion plants.Inaugural lecture presented on May 20, 2005 at Technische UniversiteitEindhoven.
[41] Lang T, Jensen PA, Knudsen JN. The effects of Ca-based sorbents on sulphurretention in bottom ash from grate-fired annual biomass. Energy Fuels2006;20:796–806.
[42] Narayanan KV, Natarajan E. Experimental studies on cofiring of coal andbiomass blends in India. Renew Energy 2007;32:2548–58.
[43] Pronobis M. The influence of biomass co-combustion on boiler fouling andefficiency. Fuel 2006;85:474–80.
[44] Scharler R, Fleckl T, Obernberger I. Modification of a Magnussen constant of the eddy dissipation model for biomass grate furnaces by means of hot gas
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 751
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 28/30
in-situ FT-IR absorption spectroscopy. Progr Comput Fluid Dyn 2003;3:102–11.
[45] Celma AR, Rojas S, Lopez-Rodrıguez F. Waste-to-energy possibilities forindustrial olive and grape by-products in Extremadura. Biomass Bioenergy2007;32:522–34.
[46] Ryu C, Yang YB, Khor A, Yates NE, Sharifi VN, Swithenbank J. Effect of fuelproperties on biomass combustion: Part I. Experiments—fuel type, equiva-lence ratio and particle size. Fuel 2006;85:1039–46.
[47] Yang YB, Goodfellow J, Nasserzadeh V, Swithenbank J. Study on the transient
process of waste fuel incineration in a full-scale moving-bed furnace.Combust Sci Technol 2005;177:127–50.[48] Ryu C, Yang YB, Nasserzadeh V, Swithenbank J. Thermal reaction modelling
of a large municipal solid waste incinerator. Combust Sci Technol 2004;176:1891–907.
[49] Frey HH, Peters B, Hunsinger H, Vehlow J. Characterization of municipalsolid waste combustion in a grate furnace. Waste Manage 2003;23:689–701.
[50] Ryu C, Shin D, Choi S. Combined simulation of combustion and gas flow in agrate-type incinerator. J Air Waste Manage Assoc 2002;52:174–85.
[51] Jannelli E, Minutillo M. Simulation of the flue gas cleaning system of an RDFincineration power plant. Waste Manage 2007;27:684–90.
[52] Khor A, Ryu C, Yang YB, Sharifi VN, Swithenbank J. Straw combustion in afixed bed combustor. Fuel 2007;86:152–60.
[53] Yin C, Rosendahl L, Kær SK, Clausen S, Hvid SL, Hille T. Mathematicalmodelling and experimental study of biomass combustion in a thermal108MW grate-fired boiler. Energy Fuels 2008;22:1380–90.
[54] Yang YB, Newman R, Sharifi V, Swithenbank J Ariss J. Mathematicalmodelling of straw combustion in a 38 MWe power plant furnace and effectof operating conditions. Fuel 2007;86:129–42.
[55] Thunman H, Leckner B. Influence of size and density of fuel on combustionin a packed bed. Proc Combust Inst 2005;30:2939–46.
[56] Huttunen M, Kjaldman L, Saastamoinen J. Analysis of grate firing of woodwith numerical flow simulation. IFRF Combustion Journal 2004, Article No.200401. 18pp.
[57] Klason T, Bai XS. Combustion process in a biomass grate fired industryfurnace: a CFD study. Progr Comput Fluid Dyn 2006;6:278–82.
[58] Yin C, Luo Z, Zhou J, Cen K. A novel non-linear programming-based coalblending technology for power plants. Chem Eng Res Design 2000;78:118–24.
[59] Jørgensen K. Biomass combustion technology at B&W Vølund A/S,/www.volund.dkS. Lecture presented on 14.05.2007 in Aalborg University.
[60] Babcock. In: Stultz SC, Kitto JB, editors. Steam—its generation and use. 40thed; 1992, ISBN 0-9634570-0-4.
[61] Griselin N, Bai XS. Particle dynamics in a biomass-fired furnace—predictionsof solid residence changes with operation. IFRF Combust J 2000, Article No.200009. 30pp.
[62] Kim HS, Shin MS, Jang DS, Ohm TI. Numerical study of SNCR application to afull-scale stoker incinerator at Daejon 4th industrial complex. Appl ThermalEng 2004;24:2117–29.
[63] Alstom Power Inc. CFD guides design of biomass boiler retrofit to increasecapacity by 25% and decrease ash. J Articles by FLUENT Software Users2004;JA147. 5pp.
[64] Hermansson S, Olausson C, Thunman H, Ronnback M, Leckner B. Combustiondisturbances in the fuel bed of grate furnaces. In: Proceedings of the seventhEuropean conference on industrial furnaces and boilers, Porto, April 18–21,2006. 9pp.
[65] Yang YB, Sharifi VN, Swithenbank J. Substoichiometric conversion of biomass and solid waste to energy in packed beds. AIChE J 2006;52:809–17.
[66] Yang YB, Sharifi VN, Swithenbank J. Converting moving-grate incinerationfrom combustion to gasification—numerical simulation of the burningcharacteristics. Waste Manage 2007;27:645–55.
[67] Goddard CD, Yang YB, Goodfellow J, Sharifi VN, Swithenbank J, Chartier J, etal. Optimisation study of a large waste-to-energy plant using computationalmodelling and experimental measurements. J Energy Inst 2005;78:106–16.
[68] Rosendahl LA, Kær SK, Jørgensen K. Design of 500 kW grate fired test facilityusing CFD. In: Proceedings of the 30th international technical conference on
coal utilization & fuel systems, Florida, USA, April 2005.[69] Zhou H, Jensen PA, Frandsen FJ. Dynamic mechanistic model of superheater
deposit growth and shedding in a biomass fired grate boiler. Fuel2007;86:1519–33.
[70] Zbogar A, Jensen PA, Frandsen FJ, Hansen J, Glarborg P. Experimentalinvestigation of ash deposit shedding in a straw-fired boiler. Energy Fuels2006;20:512–9.
[71] Zheng Y, Jensen AD, Johnsson JE. Deactivation of V2O5–WO3–TiO2 SCR catalyst at a biomass-fired combined heat and power plant. Appl Catal B:Environ 2005;60:253–64.
[72] Reno Nord. MSW incineration at Reno Nord, /www.renonord.dkS. Lecturepresented on 14.03.2007 at Aalborg University.
[73] van der Lans RP, Pedersen LT, Jensen A, Glarborg P, Dam-Johansen K.Modelling and experiments of straw combustion in a grate furnace. BiomassBioenergy 2000;19:199–208.
[74] Saastamoinen JJ, Taipale R, Horttanainen M, Sarkomaa P. Propagation of theignition front in beds of wood particles. Combust Flame 2000;123:214–26.
[75] Thunman H, Leckner B. Ignition and propagation of a reaction front in cross-current bed combustion of wet biofuels. Fuel 2001;80:473–81.
[76] Zhou H, Jensen AD, Glarborg P, Jensen PA, Kavaliauskas A. Numericalmodeling of straw combustion in a fixed bed. Fuel 2005;84:389–403.
[77] Yang YB, Sharifi VN, Swithenbank J. Effect of air flow rate and fuel moistureon the burning behaviours of biomass and simulated municipal solid wastesin packed beds. Fuel 2004;83:1553–62.
[78] Yang YB, Ryu C, Khor A, Yates NE, Sharifi VN, Swithenbank J. Effect of fuelproperties on biomass combustion. Part II. Modelling approach—identificationof the controlling factors. Fuel 2005;84:2116–30.
[79] Shin D, Choi S. The combustion of simulated waste particles in a fixed bed.Combust Flame 2000;121:167–80.
[80] Dong W, Blasiak W. CFD modeling of ecotube system in coal and waste grate
combustion. Energy Convers Manage 2001;42:1887–96.[81] Yin C, Caillat S, Harion JL, Baudoin B, Perez E. Investigation of the flow,combustion, heat-transfer and emissions from a 609MW utility tangentiallyfired pulverized-coal boiler. Fuel 2002;81:997–1006.
[82] Hupa M. Interaction of fuels in co-firing in FBC. Fuel 2005;84:1312–9.[83] Lin W, Dam-Johansen K, Frandsen F. Agglomeration in bio-fuel fired fluidized
bed combustors. Chem Eng J 2003;96:171–85.[84] Skrifvars BJ, Ohman M, Nordin A, Hupa M. Predicting bed agglomeration
tendencies for biomass fuels fired in FBC boilers: a comparison of threedifferent prediction methods. Energy Fuels 1999;13:359–63.
[85] Bapat DW, Kulkarni SV, Bhandarkar VP. Design and operating experience onfluidized bed boiler burning biomass fuels with high alkali ash. In: PretoFDS, editor. Proceedings of the 14th international conference on fluidizedbed combustion. Vancouver, New York, NY: ASME; 1997. p. 165–74.
[86] Davidsson KO, Steenari BM, Eskilsson D. Kaolin addition during biomasscombustion in a 35 MW circulating fluidized bed boiler. Energy Fuels2007;21:1959–66.
[87] Natarajan E, Nordin A, Rao AN. Overview of combustion and gasification of rice husk in fluidized bed reactors. Biomass Bioenergy 1998;14:533–46.
[88] Rao KVNS, Reddy GV. Effect of distributor design on temperature profiles influidized bed during the combustion of rice husk. Combust Sci Technol2007;179:1589–603.
[89] Duo W, Karidio I, Cross L, Ericksen B. Combustion and emission performanceof a hog fuel fluidized bed boiler with addition of tire derived fuel. J EnergyResour Technol 2007;129:42–9.
[90] Chen WY, Gathitu BB. Design of mixed fuel for heterogeneous reburning.Fuel 2006;85:1781–93.
[91] Nussbaumer T, Hustad JE. Overview of biomass combustion. In: BridgewaterAV, Boocock DGB, editors. Developments in thermochemical biomassconversion. London: Chapman and Hall; 1997. p. 1229–46.
[92] Meij R, Te Winkel H. The emissions of heavy metals and persistent organicpollutants from modern coal-fired power stations. Atmos Environ 2007;41:9262–72.
[93] Directive 2000/76/EC of the European Parliament and of the Council of 4December2000 on the incinerationof waste./http://ec.europa.eu/environment/wasteinc/newdir/2000-76_en.pdf S.
[94] Hill SC, Smoot LD. Modeling of nitrogen oxides formation and destruction incombustion systems. Progr Energy Combust Sci 2000;26:417–58.
[95] Stubenberger G, Scharler R, Zahirovic S, Obernberger I. Experimentalinvestigation of nitrogen species release from different solid biomass fuelsas a basis for release models. Fuel 2008;87:793–806.
[96] Tan LL, Li CZ. Formation of NO x and SO x precursors during the pyrolysis of coal and biomass. Part I. Effects of reactor configuration on the determinedyields of HCN and NH3 during pyrolysis. Fuel 2000;79:1883–9.
[97] Zhang H, Fletcher TH. Nitrogen transformation during secondary coalpyrolysis. Energy Fuels 2001;15:1512–22.
[98 ] Kontt in en J , Hu pa M, K allio S, Wint er F, Sam uelss on J. NOformation tendency characterization for biomass fuels. In: Proceedings of the 18th international conference on fluidized bed combustion, 2005.p. 225–35.
[99] Zhou H, Jensen AD, Glarborg P, Kavaliauskas A. Formation and reduction of nitric oxide in fixed-bed combustion of straw. Fuel 2006;85:705–16.
[100] Aho MJ, Hamalainen JP, Tummavuori JL. Importance of solid fuel propertiesto nitrogen oxide formation through HCN and NH3 in small particlecombustion. Combust Flame 1993;95:22–30.
[101] Bassilakis R, Carangelo RM, Wo jtowicz MA. TG-FTIR analysis of biomass
pyrolysis. Fuel 2001;80:1765–86.[102] Hansson KM, Samuelsson J, Tullin C, A mand LE. Formation of HNCO, HCN,
and NH3 from the pyrolysis of bark and nitrogen-containing modelcompounds. Combust Flame 2004;137:265–77.
[103] Im H, Rasouli F, Hajaligol M. Formation of nitric oxide during tobaccooxidation. J Agric Food Chem 2003;51:7366–72.
[104] Sørensen CO, Johnsson JE, Jensen A. Reduction of NO over wheat straw char.Energy Fuels 2001;15:1359–68.
[105] Garijo EG, Jensen AD, Glarborg P. Kinetic study of NO reduction over biomasschar under dynamic conditions. Energy Fuels 2003;17:1429–36.
[106] Weissinger A, Fleckl T, Obernberger I. In situ FT-IR spectroscopic investiga-tions of species from biomass fuels in a laboratory-scale combustor: therelease of nitrogenous species. Combust Flame 2004;137:403–17.
[107] Klason T, Bai XS. Computational study of the combustion process and NOformation in a small-scale wood pellet furnace. Fuel 2007;86:1465–74.
[108] Staiger B, Unterberger S, Berger R, Hein KRG. Development of an air stagingtechnology to reduce NOx emissions in grate fired boilers. Energy2005;30:1429–38.
[109] Khodayari R, Odenbrand CUI. Regeneration of commercial SCR catalysts bywashing and sulphation: effect of sulphate groups on the activity. Appl CatalB: Environ 2001;33:277–91.
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754752
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 29/30
[110] Pagels J, Strand M, Rissler J, Szpila A, Gudmundsson A, Bohgard M, et al.Characteristics of aerosol particles formed during grate combustion of moistforest residue. J Aerosol Sci 2003;34:1043–59.
[111] Larsson AC, Einvall J, Andersson A, Sanati M. Targeting by comparison withlaboratory experiments the SCR catalyst deactivation process by potassiumand zinc salts in a large-scale biomass combustion boiler. Energy Fuels2006;20:1398–405.
[112] Narayanan KV, Natarajan E. Cofiring of coal and biomass in a travelling grateboiler in India. J Appl Sci 2006;6:1924–8.
[113] Kassman H, Berg M. Ash related problems in wood fired boilers and effectof additives. /http://www.ieabcc.nl/meetings/task32_Glasgow_ws_ash/05_Kassman.pdf S. In: Workshop on ash deposition and corrosion, Glasgow,September 2006.
[114] Henderson P, Szakalos P, Pettersson R, Andersson C, Hogberg J. Reducingsuperheater corrosion in wood-fired boilers. Mater Corrosion 2006;57:128–34.
[115] Brostrom M, Kassman H, Helgesson A, Berg M, Andersson C, Backman R, et al.Sulfation of corrosive alkali chlorides by ammonium sulfate in a biomassfired CFB boiler. Fuel Processing Technol 2007;88:1171–7.
[116] NextGenBioWaste. NextGenBioWaste is up and running. Newsletter No. 1,2007. /http://www.nextgenbiowaste.com/Publications/Newsletters/news-letter_1_2007.pdf S.
[117] Knudsen JN, Jensen PA, Dam-Johansen K. Transformation and release to thegas phase of Cl, K, and S during combustion of annual biomass. Energy Fuels2004;18:1385–99.
[118] Aho M, Silvennoinen J. Preventing chlorine deposition on heat transfersurfaces with aluminium–silicon rich biomass residue and additive. Fuel2004;83:1299–305.
[119] Frandsen FJ. Utilizing biomass and waste for power production—
a decade of contributing to the understanding, interpretation and analysis of depositsand corrosion products. Fuel 2005;84:1277–94.
[120] Miltner A, Beckmann G, Friedl A. Preventing the chlorine-induced hightemperature corrosion in power boilers without loss of electrical efficiencyin steam cycles. Appl Thermal Eng 2006;26:2005–11.
[121] Leclerc D, Duo WL, Vessey M. Effects of combustion and operatingconditions on PCDD/PCDF emissions from power boilers burning salt-ladenwood waste. Chemosphere 2006;63:676–89.
[122] Jensen PA, Frandsen FJ, Dam-Johansen K, Sander B. Experimental investiga-tion of the transformation and release to gas phase of potassium andchlorine during straw pyrolysis. Energy Fuels 2000;14:1280–5.
[123] Knudsen JN, Jensen PA, Lin W, Frandsen FJ, Dam-Johansen K. Sulfurtransformations during thermal conversion of herbaceous biomass. EnergyFuels 2004;18:810–9.
[124] Knudsen JN, Jensen PA, Lin W, Dam-Johansen K. Secondary capture of chlorine and sulfur during thermal conversion of biomass. Energy Fuels2005;19:606–17.
[125] van Lith SC, Alonso-Ramırez V, Jensen PA, Frandsen FJ, Glarborg P. Release tothe gas phase of inorganic elements during wood combustion. Part 1:development and evaluation of quantification methods. Energy Fuels2006;20:964–78.
[126] Frandsen FJ, van Lith SC, Korbee R, Yrjas P, Backman R, Obernberger I, et al.Quantification of the release of inorganic elements from biofuels. FuelProcessing Technol 20 07;88:1118–28.
[127] Strand M. Particulate and CO emissions from a moving-grate boiler firedwith sulfur-doped woody fuel. Energy Fuels 2007;21:3653–9.
[128] Jensen PA, Sander B, Dan-Johansen K. Removal of K and Cl by leaching of straw char. Biomass Bioenergy 2001;20:447–57.
[129] Liem AKD, van Zorge JA. Dioxins and related compounds: status andregulatory aspects. Environ Sci Pollut Res 1995;2:46–56.
[130] Kilgroe JD. Control of dioxin, furan, and mercury emissions from municipalwaste combustors. J Hazard Mater 1996;47:163–94.
[131] Eddings EG. The formation and control of polychlorinated dibenzo- p-dioxins(PCDD) and dibenzofuran (PCDF) emissions. Lecture presented on08.10.2007 in University of Miskolc (Hungary). /http://www.combustion.uni-miskolc.hu/oktatas/jegyzetek/2007-Dioxin_review.pdf S.
[132] Gullett BK, Bruce KR, Beach LO. The effect of metal catalysts on theformation of polychlorinated dibenzo- p-dioxin and polychlorinated diben-zofuran precursors. Chemosphere 1990;20:1945–52.
[133] von Alten TR, Lanier WS, Nelson LP. Combustion assessment of test resultsfrom Montgomery County municipal waste combustor. Report submitted tothe US EPA Office of Research and Development under contract number 68-03-3365 by Energy and Environmental Research Corporation, Irvine, CA,September 1992.
[134] Hansen E, Pershing DW, Sarofim AF, Heap MP, Owens WD. An evaluation of dioxin and furan emissions from a cement kiln co-firing waste. In: Wastecombustion in boilers and industrial furnaces. Air & Waste ManagementAssociation; 1995.
[135] Joller M, Brunner T, Obernberger I. Modeling of aerosol formation duringbiomass combustion for various furnace and boiler types. Fuel ProcessingTechnol 2007;88:1136–47.
[136] Christensen KA, Livbjerg H. A field study of submicron particles from thecombustion of straw. Aerosol Sci Technol 1996;25:185–99.
[137] Strand M, Pagels J, Szpila A, Gudmundsson A, Swietlicki E, Bohgard M,et al. Fly ash penetration through electrostatic precipitator and fluegas condenser in a 6 MW biomass fired boiler. Energy Fuels 2002;16:1499–506.
[138] Jung CH, Matsuto T, Tanaka N, Okada T. Metal distribution in incinerationresidues of municipal solid waste (MSW) in Japan. Waste Manage2004;24:381–91.
[139] Joller M, Brunner T, Obernberger I. Modeling of aerosol formation duringbiomass combustion in grate furnaces and comparison with measurements.Energy Fuels 2005;19:311–23.
[140] Chang FY, Wey MY. Comparison of the characteristics of bottom and fly ashesgenerated from various incineration processes. J Hazard Mater 2006;B138:594–603.
[141] Wiinikka H, Gebart R, Boman C, Bostrom D, Nordin A, O¨
hman M. High-temperature aerosol formation in wood pellets flames: spatially resolvedmeasurements. Combust Flame 2006;147:278–93.
[142] Wiinikka H, Gebart R, Boman C, Bostrom D, Ohman M. Influence of fuel ashcomposition on high temperature aerosol formation in fixed bed combus-tion of woody biomass pellets. Fuel 2007;86:181–93.
[143] Zeuthen JH, Jensen PA, Jensen JP, Livbjerg H. Aerosol formation during thecombustion of straw with addition of sorbents. Energy Fuels 2007;21:699–709.
[144] Baxter LL. The behaviour of inorganic material in biomass-fired powerboilers: fields and laboratory experiences. Fuel Processing Technol1998;54:47–78.
[145] Lighty JS, Veranth JM, Sarofim AF. Combustion aerosols: factors governingtheir size and composition and implications to human health. J Air WasteManage Assoc 2000;50:1565–618.
[146] Kær SK, Rosendahl LA, Baxter LL. Towards a CFD-based mechanistic depositformation model for straw-fired boilers. Fuel 2006;85:833–48.
[147] Haynes BS, Neville M, Quann RJ, Sarofim AF. Factors governing the surfaceenrichment of fly ash in volatile trace species. J Colloid Interface Sci
1982;87:266–78.[148] Linak WP, Wendt JOL. Trace metal transformation mechanisms during coal
combustion. Fuel Processing Technol 1994;39:173–98.[149] Christensen KA. The formation of submicron particles from the combustion
of straw. PhD thesis. Department of Chemical Engineering, TechnicalUniversity of Denmark, 1995.
[150] Obernberger I, Brunner T, Joller M. Characterization and formation of aerosols and fly-ashes from fixed-bed biomass combustion. In: NussbaumerT, editor. Aerosols from biomass combustion. Switzerland: Verenum; 2001.p. 69–74.
[151] Frandsen FJ. Fundamentals of aerosol and nano technology. Lyngby: TheNordic Graduate School of Biofuel Science and Technology; 2006 August23–25.
[152] Michelsen HP, Frandsen F, Dam-Johansen K, Larsen OH. Deposition and hightemperature corrosion in a 10 MW straw fired boiler. Fuel ProcessingTechnol 1998;54:95–108.
[153] Fernandez A, Davis SB, Wendt JOL, Cennit R, Young RS, Witten ML.Particulate emission from biomass combustion. Nature 2001;409:998.
[154] Glarborg P, Marshall P. Mechanism and modeling of the formation of gaseous alkali sulfates. Combust Flame 2005;141:22–39.
[155] Scheuch Technology for Clean Air. /http://www.scheuch.comS.[156] Zbogar A, Frandsen FJ, Jensen PA, Glarborg P. Heat transfer in ash deposits: a
modeling tool-box. Progr Energy Combust Sci 2005;31:371–421.[157] Juniper LA. Ash deposition indices revisited. In: Workshop on impact of coal
quality on thermal Brisbane. CRC for Black Coal Utilization; 1996.[158] Ultra-Systems Technology. /http://www.ultrasys.com.au/coalcal.cfmS.[159] Skrifvars BJ, Yrjas P, Kinni J, Siefen P, Hupa M. The fouling behaviour of rice
husk ash in fluidized-bed combustion. 1. Fuel characteristics. Energy Fuels2005;19:1503–11.
[160] Backman R, Hupa M, Uppstu E. Fouling and corrosion mechanisms in therecovery boiler superheater area. Tappi J 1987;70:123–7.
[161] Skrifvars BJ, Backman R, Hupa M. Characterization of the sinteringtendency of ten biomass ashes in FBC conditions by a laboratory testand by phase equilibrium calculations. Fuel Processing Technol 1998;56:55–67.
[162] Zevenhoven-Onderwater M, Blomquist JP, Skrifvars BJ, Backman R, Hupa M.The prediction of behaviour of ashes from five different solid fuels in
fluidized bed combustion. Fuel 2000;79:1353–61.[163] Backman R, Hupa M, Skrifvars BJ. Predicting superheater deposit formation
in boilers burning biomass. In: Gupta A, et al., editors. Impact of mineralimpurities in solid fuel combustion. New York: Kluwer Academic/PlenumPublishers; 1999. p. 405–16.
[164] Jensen PA, Stenholm M, Hald P. Deposition investigation in straw-firedboilers. Energy Fuels 1997;11:1048–55.
[165] Michelsen HP, Frandsen F, Dam-Johansen K, Larsen OH. Deposition and hightemperature corrosion in a 10MW straw fired boiler. Fuel ProcessingTechnol 1998;54:95–108.
[166] Montgomery M, Karlsson A. In-situ corrosion investigation at Masnedø CHPplant—a straw-fired power plant. Mater Corrosion 1999;50:579–84.
[167] Nielsen HP, Frandsen FJ, Dam-Johansen K. Lab-scale investigations of high-temperature corrosion phenomena in straw-fired boilers. Energy Fuels1999;13:1114–21.
[168] Nielsen HP, Baxter LL, Sclippab G, Morey C, Frandsen FJ, Dam-Johansen K.Deposition of potassium salts on heat transfer surfaces in straw-firedboilers: a pilot-scale study. Fuel 2000;79:131–9.
[169] Hansen LA, Nielsen HP, Frandsen FJ, Dam-Johansen K, Hørlyck S, Karlsson A.Influence of deposit formation on corrosion at a straw-fired boiler. FuelProcessing Technol 2000;64:189–209.
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 753
8/13/2019 Firing Biomass
http://slidepdf.com/reader/full/firing-biomass 30/30
[170] Mo ntgomery M, Karlsson A, Larsen OH. Field test corrosion experiments inDenmark with biomass fuels. Part 1: Straw-firing. Mater Corrosion 2002;53:121–31.
[171] Jensen PA, Frandsen FJ, Hansen J, Dam-Johansen K, Henriksen N, Horlyck S.SEM investigation of superheater deposits from biomass-fired boilers.Energy Fuels 2004;18:378–84.
[172] Vaugham DA, Krause HH, Boyd WD. Chloride corrosion and its inhibitionin refuse firing. In: Proceedings of the international conference on ashdeposits and corrosion from impurities in combustion gases, Henniker, New
Hampshire, June 26–July 1, 1997. p. 473.[173] Kofstad P. High temperature corrosion. New York: Elsevier Applied Science;1988.
[174] Pettersson J, Asteman H, Svensson JE, Johansson LG. KCl induced corrosion of a 304-type Austenitic stainless steel at 6001C; The role of potassium.Oxidation Met 2005;64:23–41.
[175] Khullar C. The use of ‘combustion additives’ to improve heat transfer andreduce combustion emissions in package boilers. In: Proceedings of thesecond international conference on combustion and emission control.London, UK: Institute of Energy; 1995. p. 168–77. 4–5 December 1995.
[176] Wilen C, Stahlberg P, Sipila K, Ahokas J. Pelletization and combustion of straw. In: Klass DL, editor. Energy from biomass and wastes 10. London:Elsevier Applied Sciences; 1987. p. 469–84.
[177] ChlorOut. European Patent EP 1354167. International Patent Application:PCT/SE 02/00129; 2002.
[178] Aho M. New and improved slagging and corrosion control techniques forbiomass firing. /http://www.tut.fi/units/me/ener/IFRF/Liekkipaiva2008_Aho_Corr.pdf S, Presented on IV Liekkipaiva, 23.01.2008
[179] Andersen KH, Frandsen FJ, Hansen PFB, Wieck-Hansen K, Rasmussen I,
Overgaard P, et al. Deposit formation in a 150MWe utility PF-boiler duringco-combustion of coal and straw. Energy Fuels 2000;14:765–80.
[180] Vattenfall AB. Good examples: Vattenfall’s R&D, 2007. /http://www.vattenfall.com/www/vf_com/vf_com/Gemeinsame_Inhalte/DOCUMENT/360168vatt/5965811xou/631489rxd/P02.pdf S.
[181] Walsh AR. CFD modeling of waste-fuel boiler combustion systems. In: Reis A,Ward J, Leuckel W, editors. Proceedings of the seventh European conferenceon industrial furnaces and boilers, Porto, 18–21 April 2006. ISBN:972-99309-1-0.
[182] Green AS, Waite ML. The redesign of chain grate stoker links to reducepollutant emissions: aerodynamic design. Fuel 2004;83:1391–5.
[183] Lim CN, Goh YR, Nasserzadeh V, Swithenbank J, Riccius O. The modelling of solid mixing in municipal waste incinerators. Powder Technol2001;114:89–95.
[184] Peters B, Dziugys A, Hunsinger H, Krebs L. An approach to qualify theintensity of mixing on a forward acting grate. Chem Eng Sci 2005;60:1649–59.
[185] Ryu C, Shin D, Choi S. Effect of fuel layer mixing in waste bed combustion.Adv Environ Res 2001;5:259–67.
[186] Yang YB, Lim CN, Goodfellow J, Sharifi VN, Swithenbank J. A diffusion modelfor particle mixing in a packed bed of burning solids. Fuel 2005;84:213–25.
[187] Dziugys A, Peters B, Hunsinger H, Krebs L. Evaluation of the residence timeof a moving fuel bed on a forward acting grate. Granular Matter2006;8:125–35.
[188] Pattikangas TJH, Saastamoinen JJ, Sutinen J, Kjaldman L. Numericalsimulations of grate firing of coal. In: Proceedings of fifth Europeanconference on industrial furnaces and boilers, Porto, April 11–14, 2000.
[189] Yang YB, Goh YR, Zakaria R, Nasserzadeh V, Swithenbank J. Mathematicalmodelling of MSW incineration on a travelling bed. Waste Manage2002;22:369–80.
[190] van Kessel LBM, Arendsen ARJ, de Boer-Meulman PDM, Brem G. The effect of air preheating on the combustion of solid fuels on a grate. Fuel2004;83:1123–31.
[191] Johansson R, Thunman H, Leckner B. Sensitivity analysis of a fixed bedcombustion model. Energy Fuels 2007;21:1493–503.
[192] Scharler R, Obernberger I, Langle G, Heinzle J. CFD analysis of air staging andflue gas recirculation in biomass grate furnaces. In: Proceedings of the firstWorld conference and exhibition on biomass for energy and industry, June2000, Seville, Spain.
[193] Kær SK. Numerical modelling of a straw-fired grate boiler. Fuel 2004;83:1183–90.[194] Goerner K, KlasenTh. Modelling, simulation and validation of the solid biomass
combustion in different plants. Progr Comput Fluid Dyn 2006;6:225–34.[195] Scharler R, Obernberger I. Deriving guidelines for the design of biomass
grate furnaces with CFD analysis—a new multifuel-low-NO x furnace asexample. In: Proceedings of sixth European conference on industrialfurnaces and boilers, vol. IV, Estoril-Lisboa, Portugal, April 02–05, 2002.p. 227–41.
[196] Ryu CK, Choi SM. 3-dimensional simulation of air mixing in the MSWincinerators. Combust Sci Technol 1996;119:155–70.
[197] Ghirelli F, Leckner B, Thunman H, A mand LE. Presumed PDF modelling of gasphase combustion in grate furnaces. In: Proceedings of sixth Europeanconference on industrial furnaces and boilers, vol. IV, Estoril-Lisboa,Portugal, April 2–5, 2002. p. 27–38.
[198] Albrecht BA, Bastiaans RJM, van Oijen JA, de Goey LPH. NO x emissionsmodeling in biomass combustion grate furnaces. In: Reis A, Ward J, LeuckelW, editors. Proceedings of the seventh European conference on industrialfurnaces and boilers. Porto, 18–21 April 2006. ISBN:972-99309-1-0.
[199] Forstner M, Hofmeister G, Joller M, Dahl J, Braun M, Kleditzsch S, et al. CFDsimulation of ash deposit formation in fixed bed biomass furnaces andboilers. Progr Comput Fluid Dyn 2006;6:248–61.
[200] Manca D, Rovaglio M. Numerical modeling of a discontinuous incinerationprocess with on-line validation. Ind Eng Chem Res 2005;44:3159–77.
[201] Nasserzadeh V, Swithenbank J, Jones B. Three-dimensional modelling of amunicipal solid-waste incinerator. J Inst Energy 1991;64:166–75.
[202] Nasserzadeh V, Swithenbank J, Jones B. Effect of high speed secondary air jets on the overall performance of a large MSW incinerator with a verticalshaft. Combust Sci Technol 1993;92:389–422.
[203] Kim S, Shin D, Choi S. Comparative evaluation of municipal solid wasteincinerator designs by flow simulation. Combust and Flame 1996;106:241–51.
[204] MFIX. /http://www.mfix.orgS.[205] Benyahia S, Syamlal M, O’Brien TJ. Summary of MFIX Equations 2005-4. From
URL /http://www.mfix.org/documentation/MfixEquations2005-4-3.pdf S, July2007.
[206] Sabelstrom H. Diffusion of solid fuel on a vibrating grate. PhD thesis, AalborgUniversity, 2007.
[207] Yang YB, Sharifi VN, Swithenbank J. Numerical simulation of the burningcharacteristics of thermally-thick biomass fuels in packed-beds. TransIChemE Part B Process Saf Environ Prot 2005;83:549–58.
[208] Yang YB, Nasserzadeh V, Goodfellow J, Swithenbank J. Simulation of channelgrowth in a burning bed of solids. Trans IChemE Part A 2003;81:221–32.
[209] Rosendahl LA, Yin C, Kær SK, Friborg K, Overgaard P. Physical characteriza-tion of biomass fuels prepared for suspension firing in utility boilers for CFDmodeling. Biomass Bioenergy 2007;31:318–25.
[210] Yin C, Rosendahl L, Kær SK, Condra TJ. Use of numerical modeling in designfor co-firing biomass in wall-fired burners. Chem Eng Sci 2004;59:3281–92.
[211] Johansson R, Thunman H, Leckner B. Influence of intraparticle gradients inmodeling of fixed bed combustion. Combust Flame 2007;149:49–62.
ARTICLE IN PRESS
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754754