Biomass Gasification in a 100 kWth Steam-oxygen

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Biomass gasification in a 100 kWth steam-oxygen blown

circulating fluidized bed gasifier: Effects of operational

conditions on product gas distribution and tar formation

Xiangmei Meng*, Wiebren de Jong, Ningjie Fu, Adrian H.M. Verkooijen

Faculty of Mechanical, Maritime and Materials Engineering, Process and Energy Department, Energy Technology section,

Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands

a r t i c l e i n f o

Article history:

Received 8 December 2010

Received in revised form

9 March 2011

Accepted 11 March 2011

Available online 9 April 2011

Keywords:

Dry distiller’s grains with solubles

(DDGS)

Circulating fluidized bedSteam-oxygen blown gasification

Tar formation

Sulphur

Carbon conversion efficiency

a b s t r a c t

Biomass gasification is one of the most promising technologies for converting biomass,

a renewable source, into an easily transportable and usable fuel. Two woody biomass fuels

Agrolandwillow,andoneagricultureresidueDryDistiller’sGrainswithSolubles(DDGS),have

been tested using an atmospheric pressure 100 kWth steam-oxygen blown circulating fluid-

ized bedgasifier (CFB).The effects of operational conditions (e.g. steam to biomass ratio (SBR),

oxygen to biomass stoichiometric ratio (ER) and gasification temperature) and bed materials

on the composition distribution of the product gas and tar formation from these fuels were

investigated. Experimental results show that there is a significant variation in the composi-

tion of the product gas produced. Among all the experiments, the averaged concentration of

H2 obtained from Agrol, willow and DDGS overthe temperature range from800 to 820C was

around 24 vol.%,28 vol.% and20 vol.% ona N2 freebasis, respectively.A fairlyhigh amount of H2S (w2300 ppmv), COS (w200 ppmv) and trace amounts of methyl mercaptan (<3 ppmv) on

a N2 freebasis wereobtained fromDDGS.Due toa relativelyhighcontentof K and Clin DDGS

fuel,an alkali-getter (e.g.kaolin) was addedto avoidagglomerationduringgasification. Higher

temperatures and SBRvalues were favorable forincreasing themole ratio of H2 toCOandthe

tar decomposition but less advantageous for the formation of CH4. Meanwhile, higher

temperatures and SBR values also led to higher gas yields, whereas a higher SBR caused

a lower carbonconversion efficiency(CCE%), cold gas efficiency (CGE%) andheating valuesof

the product gas due to a high steam content in the product gas. From solid phase adsorption

(SPA) results,the total tar content obtained from Agrol was thehighest at around12.4 g/Nm3,

followed by that from DDGS and willow gasification. The lowest tar content produced from

Agrol, willow and DDGS using Austrian olivine (Bed 1) as bed materials was 5.7, 4.4 and 7.3 g/

Nm3, values which were obtained at a temperature of 730, 820and 730C,SBRof 1.52, 1.14and

1.10, and ER of 0.36, 0.39 and 0.37, respectively.ª 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Due to the limited supply of conventional fossil fuels, the

steep rise of energy consumption and rapid economic

developments, alternative renewable energy sources need

to be widely explored in order to renew the energy sources

and to comply with a sustainable development. Biomass,

referring to all organic materials that originate from plants or

* Corresponding author. Tel.: þ31 0152786987.E-mail address: [email protected] (X. Meng).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / b i o m b i o e

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 9 1 0e2 9 2 4

0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biombioe.2011.03.028

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animals, is traditionally being used as energy source for

cooking and heating, particularly in the developing countries.

Biomass also plays an essential role in the energy supply of

some European counties. For examples, 11% of the national

energy supplies in Austria, 17% of the national energy supplies

in Sweden and 20% of the primary energy in Finland were

obtained from biomass [1]. Currently the interest in biomass

utilization such as power generation and syngas productionhas continuously increased due to its significant environ-

mental benefits. Transformation of biomass fuels into high

quality energy carriers and other commercial products can be

efficiently achieved via either biological or thermochemical

processes [2]. Among various biomass conversion technolo-

gies within thermochemical and biochemical platforms,

biomass gasification has received the highest interest since it

offers high conversion efficiency and increases options for

combination with various high efficiency power generation

systems using gas engines, gas turbines and fuel cells [3].

During biomass gasification, several parameters such as

gasifier type, reaction temperature, biomass fuels properties,

bed materials and gasifying agent have a substantial influenceon product gas composition, carbon conversion efficiency and

tar formation. The reactors used for biomass gasification are

quite diverse of which the design ultimately determines other

parameters such as gasification temperature and the proper-

ties of the biomass used. Two of the most frequently used

reactor types for biomass gasification are the fixed bed and

fluidized bed (FB) reactor [4]. Current development activities

on large scale biomass gasification have been mainly devoted

to fluidized bed technologies, since FB gasifiers have excellent

heat and mass transfer between the gas and solid phases with

the best temperature distribution and they meet the chal-

lenges of wide variations in fuel quality with a broad particle

size distribution. The gasifying agents for FB biomass gasifi-cation can be either air, steam, pure oxygen or their combi-

nation [5]. Air is the mostly used agent nowadays at

demonstration or commercial scale because of its extensive

low-cost availability. However, the product gas from FB air-

blown biomass gasification normally containing 50 vol.% N2

has a low heating value (4e7 MJ/Nm3) and it can be used only

for electricity production or heat generation [6]. Steam gasifi-

cation can produce a medium calorific value gaseous fuel

(10e14 MJ/Nm3) with a relatively high H2 content and this has

been deeply studied experimentally by many researchers

[7e9]. However, steam gasification is a more complex endo-

thermic process, which requires heat input as the heat

necessary to the process is not directly supplied by the partialcombustion of the feedstock during the gasification process at

temperatures in excess of 800 C. One easy way for supplying

heat is to add some air or oxygen to the gasifier, so that an

autothermal gasification process can be achieved [5,10e13].

However, due to the high price of the oxygen, the product gas

distribution under different operation conditions for steam/

oxygen mixtures as oxidizer has been less studied. Gil et al. [5]

pointed out that the concentration of H2 in the raw gas ranges

between 14 and 30 vol.% depending on the operational

conditions. Lv et al. [11] generally reported that a higher

reactor temperature, proper oxygen to biomass stoichiometric

ratio (ER), steam to biomass ratio (SBR) and smaller biomass

particle size will contribute to increase the H2 production.

Siedlecki et al. [13] reported that the use of magnesite as bed

material can significantly increase the concentration of H2

compared with normal sand as bed material. Besides, biomass

gasification in the entrained-flow gasifier using oxygen [14]

and air [15] has been also studied. Zhou et al. [14] reported

that higher temperature favored H2 and CO production and an

introduction of O2 to the gasifier improved the carbon

conversion, but lowered the H2 /CO ratio of the syngas.However, according to Higman and Van der Burgt [16],

entrained-flow gasifiers generally operated at high tempera-

ture therefore requiring a very small biomass particle size of

100 mm or less to promote mass transfer and high oxygen for

maintaining operation temperature.

Furthermore, as one of most problematic by-product from

biomass gasification, tar is highly dependent on the opera-

tional conditions such as reaction temperature, ER and gasi-

fying agent used etc. To avoid various problems associated

with tar condensation, formation of tar aerosols and poly-

merization to more complex structures, tar reduction is

essentially important before the final utilization of product

gas. Several reviews of the current knowledge on the elimi-nation of tars have been published [17e19]. According to Devi

et al. [17], tar removal technologies can broadly be divided into

two approaches: treatments inside the gasifier (primary

methods) and hot product gas cleaning after the gasifier

(secondary methods). The primary options include: (a) the

proper selection of the operating conditions; (b) the use of

a proper bed additives or a catalyst during gasification; and (c)

a proper gasifier design. Secondary methods include physical

removal, thermal conversion and catalytic destruction of tars.

Corella et al. [20] made an excellent comparison between the

two methods and found no significant difference in their

effectiveness concerning tar reduction. The effectiveness of

dolomite in a second reactor was only slightly higher than theeffectiveness of the in-bed location. Sutton et al. [19] reported

that a suitable combination of different primary and

secondary treatments is likely to improve gasifier perfor-

mance and produce a syngas with minimum tar concentra-

tion. Primary methods are gaining much attention as these

may eliminate or strongly reduce the need for downstream

cleanup.

Since extensive studies have been published on biomass

gasification with air, steam and air steam as gasifying agent,

this paper is mainly focused on understanding how different

operational conditions with an emphasis on SBR affect the

product gas distribution and tar formation produced during

steam-oxygen blown biomass gasification. In this work, threedifferent fuels: Agrol, willow and DDGS (Dry Distiller’s Grains

with Solubles) have been tested on an atmospheric 100 kWth

steam-oxygen blown CFB gasifier. Among these three fuels,

willow is a common woody biomass, Agrol is a commercial

solid as a kind of wood pellets which are made from pure

sawdust and shavings from sawmills using 100% virgin timber

from sustainably managed plantation forestry, while DDGS is

a by-product of the so-called dry-grind process to produce

ethanol from wheat. During past years, the rapid growth in

the production of ethanol led to a parallel rise in DDGS

production. To avoid the adverse effect on ethanol production

which could be caused by saturated DDGS market [21], the

potential applications of DDGS need to be explored and

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 9 1 0e2 9 2 4 2911







therefore its potentiality for gasification to produce a gaseous

fuel is investigated in this work.

2. Gasification experiments

2.1. Biomass fuels, bed materials and additive

Agrol, willow, and DDGS used for the experiments were

obtained from Lantma ¨ nnen, Sweden. Chemical compositions

of these fuels and their ash (obtained at 550 C) were analyzed

by Forschungszentrum Ju ¨ lich GmbH, Germany, and are pre-

sented in Table 1. The following analytical methods were used

for chemical analysis: Inorganic main components of the fuel

wereanalyzedbyInductivelyCoupledPlasmaOpticalEmission

Spectroscopy (ICP-OES). C, H, N, O and S were analyzed by

CHNSO-IR spectrometry (LECO) analyzer, where C, H and S

oxide formed were measured by the IR-Absorption of the

combustion gasesCO2, H2OandSO2, N wasquantified based on

the thermal conductivity of N2 and O2 was measured sepa-

rately. Chlorine was analyzed by Cl-Ion chromatography (IC)

after Wickbold combustion. The water content was measured

after drying the milled DDGS in a vacuum oven at 105 C for

60 h. It can be seen in Table 1 that DDGS sample ashes are rich

in K, Na and Mg, but the total quantified ash composition only

sums up to 72%,whichcouldbe dueto theunavailableanalysis

of P2O5. For all fuels, except for C, H, O, N, S, Cl main elements,

some amounts of K, Na, Ca Si and so on were also detected.

Giuntoli et al. [21] reported that there is ca. 29%P2O5 in the ash

of another, similar DDGS sample. Agrol and willow are fairly

clean and contain almost no sulphur and relatively low ash

content. The proximate analysis of three fuels was carried out

by using SDT Q600 from TA Instrument. This instrument is

capable of simultaneously performing both differential

scanning calorimetry (DSC) and thermo gravimetric analysis

(TGA).Some technical details of this instrumentare: Platinum/

PlatinumeRhodium (Type R) thermocouple, heating rate from

ambient to 1000Cat0.1e

105 C/min andsample pans made of platinum of 40 mL, alumina of 110 mL, 40 mL, and 90 mL.

Four different bed materials have been used during gasifi-

cation. One bed material is untreated Scandinavian olivine

(Bed 4), which contains 44.16 wt.% MgO, 43.9 wt.% SiO2,

9.44 wt.% Fe2O3 and trace NiO, MnO and Al2O3 etc. The other

two bed materials are Austrian olivines which were received

from Biomasse Kraftwerk Gu ¨ ssing GmbH & Co KG, Austria.

The difference between these two Austrian olivines is that one

was pre-treated (Bed 1) by about 1000 redox cycles in a fast

internally circulating fluidized bed (FICFB) real gasification

process and the other one was untreated natural olivine (Bed

2). The untreated natural olivine (Bed 2) contains 48e50 wt.%

MgO, 39e

42 wt.% SiO2, 8e

10.5 wt.% Fe2O3 and traces of Cr2O3,Mn3O3, CaO and Al2O3 etc. Another bed material (Bed 3) is

a mixture of quartz sand and pre-treated olivine with a mass

ratio around 50:50. The quartz sand consists of more than 99%

SiO2, with trace amounts of Fe2O3, CaO, K2O, Na2O and MgO.

The reason olivine has been chosen as bed material is due to

its high attrition resistance and its attractive chemical

composition (being a natural mineral containing magnesium,

iron and silica). Since DDGS fuel has a relatively high K and Cl

content, kaolin was chosen as an alkali-getting additive to

prevent agglomeration. Kaolin can be transformed to meta-

kaolinite particles which potentially adsorb potassium

species. The main constituent of the kaolin is the mineral

kaolinite (Al2Si2O5(OH)4), with a small amount of halloysite(Al2Si2O5(OH)4(H2O)2) [13,22]. Kaolin used in this work is a fine

powder with a mean particle size of 12 mm. Around 50% of the

particles is in the size range of 0.063e63 mm. The composition

is 57.5% SiO2, 37.5% Al2O3, 3.1% K2O, 0.9% Fe2O3, and trace

amounts of CaO, Na2O, MgO and TiO2. Kaolin was continu-

ously added together with biomass with an amount of 3e10%

of total feeding during DDGS gasification.

2.2. Gasification setup and experimental procedure

The gasification experiments have been carried out using an

atmospheric pressure 100 kWth steam-oxygen blown CFB

gasifier which is located in the laboratory of the EnergyTechnology Section of the Process and Energy Department at

Delft University of Technology. Some technical details of this

facility are available in a few papers which have been pub-

lished elsewhere [13,23]. The schematic diagram of the facility

is presented in Fig. 1 and its main characteristics are: a riser

length of 5.5 m, a riser inner diameter of 83 mm, a downcomer

inner diameter of 54 mm, fluidization medium pre-heater

(6 kW), electrical trace heating of the whole rig (15 kW).The

feeding system can supply biomass at a maximum rate of

20 kg/h. The measurement equipment consists of flow meters

(Endress and Hauser AT70 thermal flow meters for all primary

flows, except for steam, where an Endress and Hauser Prowirl

72 vortex flow meter is used), thermocouples (K types),

Table 1e

Chemical composition of biomasses and ashesof biomasses obtained at 550 C.

Types of fuel Agrol Willow DDGS

Moisture (wt.% a.r.) 8 8 12

Proximate Analysis (wt.%, dry)

Volatile matters 74.7 69.8 67.2

Fixed carbon 16.0 20.1 15.5

Ash content [%] 0.14 2.52 4.82

Ultimate Analysis (wt.%, dry)

C 51.0 50.3 48.2

H 6.26 6.17 6.54

O 38.2 37.4 31.2

N 0.15 0.69 5.52

S 0.002 0.002 0.76

Cl 0.01 0.01 0.21

Elemental Ash Analysis (wt.% of Ash at 550 C)

Al2O3 2.68 4.23 0.05

BaO 0.41 0.06 0.01

CaO 33.30 26.31 2.87

Fe2O3 1.32 1.54 0.26

K2O 16.50 13.13 38.07

MgO 7.79 2.89 8.87

MnO 3.96 0.30 0.15

Na2O 1.00 1.67 10.04

SiO2 6.46 33.80 1.86

TiO2 0.06 0.18 e

C 5.24 3.61 0.33

Cl 0.07 0.11 0.02

SO3

2.46 2.27 10.04








differential pressure meters and weighing devices. There are

two high temperature filters ceramic tissue candle filter (BWF,

Germany) and a SieSiC ceramic candle filter (Pall Filter sys-

temseWerk Schumacher, Germany) connected in parallel

downstream of the cyclone which can be switched during

operation. To investigate the effects of different operational

parameters (SBR, ER and the temperature) and bed materials

on product gas composition and tar formation, severaldifferent tests have been carried out with Agrol, willow and

DDGS and the ranges of experimental conditions (e.g. SBR, ER,

temperature and N2 purging flow) are presented in Table 2.

From Table 2, it can be seen that DDGS using Bed 1 and 2 has

been testedonlywithin a lower temperature range from 700 to

760 C, as it is very difficult to reach a higher temperature

during operation because of bed agglomeration risks.

2.3. Product gas and tar measurement

The product gas composition was analyzed by using different

analytical instruments which include HartmannBraun

Uras10PNDIR (online CO2, CO), HartmannBraunMagnos6G PM

(online O2), Varian CP4900 m-GC (semi-online CO, CO2, H2, CH4,

benzene, toluene and xylenes (BTX)), Varian GC 450 (semi-

online CO, CO2, H2, CH4,BTX,H2S, COS and methyl mercaptan)

and Fourier transform infrared (FTIR) spectrophotometer

from ThermoElectron Nicolet 5700 (semi-online CO2, CO, COS,

CH4, C2H4, C2H2, NH3, H2O). The schematic drawing of the gas

sampling line downstream of the CFB gasifier is shown in

Fig. 2. To avoid the coarsest particles from penetrating the

sampling line, the gas analysis probe point is located into the

direction of main gas flow. During the experiment, a sample

flow of the product gas is continuously extracted from the

main stream downstream of the gas outlet of the cyclone. The

gas sample is then led through a primary condenser (w25 C)to remove the condensables with the highest boiling point,

predominately heavy tar and some water, before leading the

gas via a gas pump to the gas analysis equipments. After

passing through a secondary condensation unit which

included a condenser surrounded by ice and then followed by

two flasks filled with silica gel, the dried gas is characterized

with several analysis methods. The gas sampling line is

heated by using a trace heating cable (Horst, type HSS-450 C).

Additionally, the particle filter vessel is heated by using

a heating jacket (Tyco IJ-GL glass silk heating jacket). The

temperature of both the sampling line and particle filter vessel

is maintained at 300 C using temperature controllers.

Tar measurements have been done using the solid phaseadsorption (SPA) method developed by KTH [24] and the

sampling point is located immediately downstream of the

filter outlet. Tar components which were quantitatively

analyzed by KTH are presented in Table 3. In Table 3, tar

components were divided into class 2e5 based on tar classi-

fication suggested by Kiel et al. [25]. The non-identified peaks

(unknown) were quantified using an internal standard.For the

analysis of the wet gas, both condensers were removed and

a heated line (170 C) was connected, leading the gas to the

FTIR analyzer. Water content was measured both gravimet-

rically and by FTIR.

2.4. Investigating variables definition

To characterize the gasification conditions different ratios are

applied. The ER (oxygen to biomass stoichiometric ratio) was

calculated as the ratio of oxygen supplied to the oxygen

required for the complete stoichiometric combustion of the

biomass on a daf (dry ash free) basis. SBR (steam to biomass

mass ratio) was calculated as the ratio of steam supplied to

biomass supplied on an a.r. (as received) basis. In the

following equations, mi, daf is the mass fraction of i on a daf

basis and Fm, i is mass flow rate of i [kg/h]. mO2, air is the mass

fraction of oxygen in the air.

ER ¼

ÂFm;oxygen=Fm;fuelðdaf Þ

ÃActualÂ

Fm;oxygen=Fm;fuelðdaf Þ

ÃStoich

where

Two additional important parameters are the carbon conver-

sion efficiency(CCE%) and the cold gas efficiency(CGE%). CCE

Fig. 1 e Schematic diagram of the 100 kWth steam-oxygen

blown CFB gasifier test-rig.

Fm;oxygen

Fm;fuelðdaf Þ

!Stioch

¼

nmC;daf

MWCMWO2

þ

mH;daf

4MWHMWO2

þ

mN;daf

2MWNMWO2

þ

mS;daf

MWSMWO2

À mO;daf

'

mO2 ;air








% is defined as the ratio of carbon which is converted from the

added fuel into gaseous carbon components (gas þ tar) to the

carbon in the added fuel [26].

CCE% ¼

mC; gasþtarFm; gasþtar

mC; fuelðdryÞFm; fuelðdryÞ

!Â 100%

CGE% ofthe gasification is defined as the ratio ofthe sum ofthe

energy in productgases to the energy of biomass input (biomassenergy). The CGE% applied in this work is based on the lower

heating value (LHV) of the product gas and is defined as [13]:

CGE% ¼

LHVgas

LHVfuelðdryÞ

Fm;gas

Fm; fuelðdryÞ

!Â 100%

The higher heating value (HHV) of biomass fuels on a d.b. (dry

basis) was calculated using a unified formula suggested by

Channiwala and Parikh [27,28], and the lower heating value

(LHV) of the fuel can be estimated by:

where C, H, O, N, S and A are percentages of mass fraction

carbon, hydrogen, oxygen, nitrogen, sulphur and ash in the

dry fuel. mH is the mass fraction of hydrogen in the fuel and

hfg is the enthalpy of vaporization of water (w2.26 MJ/kg).

3. Results and discussion

To obtain a clean bio-syngas and to increase the net energy

conversion efficiency, the gasification operating conditionsneed to be optimized. The effects of reaction temperature, ER,

SBR and bed materials on the product gas composition and tar

formation produced from Agrol, willow and DDGS are pre-

sented in the following parts. The changing trends of the

product gas composition, reactor temperature and dp cell

during experiment as measured versus time during a day’s

experiment (23rd April, 2010) is presented in Figs. 3 and 4.

T_Aver_Riser and T_Aver_Reactor represent the average

temperature of the riser, and of riser and downcomer,

respectively. From these two Figures, some clear and impor-tant points can be concluded:

1) A relatively stable temperature profile was established in

the whole reactor, which can be seen from the value of

T_Aver_Riser and T_Aver_Reactor. The standard deviation

of T_Aver_Riser and T_Aver_Reactor during stable opera-

tion condition was within 7 C;

2) The concentrations of CO and H2 obtained from Agrol,

willow and DDGS were significantly different. No signifi-

cant difference was observed for the concentration of CH4

for all the fuels and that of CO2 for Agrol and willow;

3) The reactor temperature remained fairly stable and similar

during Agrol and willow gasification, while a sharply

decreasing trend was observed by using DDGS fuel, which

indicates of different gasification behavior;

4) No significant fluctuation was observed in the measured

values of all dp cells during Agrol gasification. A gradually

increasing trend of dp 7 and 9 values (downcomer) was

found during willow gasification, and almost all dp-cellsstarted to behave differently by changing from willow to

DDGS.

HHV ¼ 0:3491Cþ 1:1783H þ 0:1005SÀ 0:1034O À 0:0151N À 0:0211A½MJ=kg �LHV ¼ HHV À 9mH Â hfg ½MJ=kg �

Table 2 e Overview of process conditions setting for Agrol, willow and DDGS gasification.

Exp. NO D1 D2 D3 A1 A2 A3 A4 W1 W2 W3

Date 15-10-09 2 1-04-10 23-04-10 2 5-03-10 13-04-10 1 5-04-10 2 3-04-10 25-03-10 19-04-10 2 3-04-10

Time start 12:00 12:00 16:30 16:20 12:50 11:50 11:00 12:20 10:50 13:50

Time end 14:00 15:40 18:10 18:20 17:10 17:00 13:30 16:20 17:10 16:30

Duration 2:00 3:40 1:40 2:00 4:20 5:10 2:30 4:00 6:20 2:40

Oxidant Steam-O2

Fuel DDGS DDGS DDGS Agrol Agrol Agrol Agrol Willow Willow Willow

Additive None Kaolin Kaolin None None None None Kaolin Kaolin KaolinBed materials Bed 4a Bed 1 Bed 2 Bed 3 Bed 1c Bed 1 Bed 2d Bed 3b Bed 1 Bed 2

Temperature (C) 780e830 700e760 700e760 800e830 700e830 700e830 700e830 700e830 700e830 700e830

Steam flow (kg/h) 10.6e10.9 14.8e16.8 14.5e16.8 11.1e11.6 14.7e15.4 13.6e14.7 11.5e14.6 11.6e15.9 10.9e14.8 11.5e14.5

Biomass flow(kg/h) 13.0e13.6 15.27 15.27 10.1e11.1 10.1e10.8 10.1e11.7 11.72 11.1 11.7e12.7 12.71

Oxygen flow (kg/h) 4.9e5.0 5.5e2.7 5.49e5.5 5.0e5.5 4.5e5.7 4.7e5.2 4.80e4.90 5.0e6.0 5e5.6 4.95e5.0

L-valve flow (kg/h) 1.4 1.85 1.77 1.4 1.4 1.4 1.4 1.4 1.4 1.4

Purging N2 flow(kg/h) 2.47 2.7 2.7 2.7 2.34 2.7 2.7 2.7 2.7 2.7

Fluidized velocity (m/s) 2.8e3.3 3.3e3.9 3.3e3.9 2.9e3.3 3.5e4.0 2.9e3.8 3e3.6 2.9e3.9 2.9e3.8 3e3.6

SBR (À) 0.81e0.83 0.97e1.10 0.95e1.08 1.00e1.15 1.36e1.52 0.97e1.45 0.98e1.25 1.00e1.15 0.90e1.20 0.9e1.14

ER (À) 0.38 0.37 0.36 0.40 0.36e0.42 0.35e0.38 0.33 0.40e0.47 0.38e0.39 0.34

a Untreated, used Scandinavian olivine.

b Mixed sand and used treated Austrian olivine.

c Used, treated Austrian olivine.

d Fresh Austrian olivine.








3.1. Effects of different operational parameters on

product gas

3.1.1. Effect on product gas composition

Effects of temperature, SBR, ER and bed materials on the

composition of product gas obtained from Agrol, willow andDDGS is shown in Figs. 5e7, respectively. The concentration of

N2 for Agrol bed 1 and willow-bed 1 at ca.780 C was 25 vol.%

and 21 vol.% on a dry basis, respectively. From Figs. 5e7 it can

be seen that there is a significant variation among the product

gas composition produced from different fuels as a function of

temperature. Generally high temperatures favored the

formation of H2 from all fuels. The concentration of H2

obtained from willow was higher than that from Agrol, while

CO, CH4, C2H2 and C2H4 had an opposite trend. The formations

of CO, CH4, C2H2 and C2H4 behaved differently over different

temperature ranges. The averaged H2 to CO mole ratio

obtained from Agrol, willow and DDGS over the temperature

range from 800 to 820 C was around 1.0, 1.4 and 0.7, respec-tively. With increasing SBR, the concentrations of CO, CH 4,

C2H4 and C2H2 all gradually decreased, while the concentra-

tion of H2 increased with a varying degree depending on fuel

types.The concentration of CO2 obtained from willow was the

highest, followed by that from Agrol and DDGS, but no

significant change was observed with a variation of SBR.

However, when SBR was increased from 0.98 to 1.16 at

a temperature of 800 C, the concentrations of CO, CH4 and

C2H2 obtained from Agrol Bed 2 all slightly increased. Higher

ER values increased the concentration of CO2 in the product

gas but decreased the concentrations of CO, CH4 and H2. The

effect of different bed materials on the product gas composi-

tion can hardly be determined due to the difference of SBR, ERand the temperature. At a similar operational conditions (e.g.

820 C, SBRw1.15 and ERw0.4), the concentration of H2

obtained from willow using Bed 1 (w30 vol.%, dnf.) was higher

than using Bed 3 (w26 vol.%, dnf.) indicating that olivine can

improve H2 production. Without feeding any kaolin into the

gasifier, a severe agglomeration was observed during DDGS

gasification using Bed 4. Most experimental results agreed

well with those reported by other researchers.Turn et al. [29]

reported that higher hydrocarbon (C2H2, C2H4, C2H6) concen-

trations decreased as reactor temperature increased but no

noticeable conversion of C2H2 and C2H4 was observed at lower

reactor temperatures from 750 to 800 C. Gil et al. [5], Kumar

et al. [30] and Franco et al. [31] all reported that the

concentration of H2 increased with increasing the tempera-

ture because higher temperature favored endothermic char

gasification reactions and steam reforming and cracking of

light hydrocarbons and tars. The different behavior of various

gaseous species at lower temperatures could be due to the

variation in reactivities of chars produced during the pyrolysis

step [32] and at higher temperatures could be due to the

intensified effect of steam on the decomposition of highermolecular mass components [33].Giletal. [34] reported that as

ER or SBR was increasedthe concentrations of CO, CH4 and C2-

hydrocarbons decreased due to partial oxidation and steam

reforming reactions. Wang and Kinoshita [35] reported that by

varying SBR from 0.4 to 1.0, the concentration of H2 increased

and that of CO2 remained roughly the same, while the

concentration of CO, CH4 and other light hydrocarbons

produced decrease slightly.

3.1.2. Effect on sulphur species

Because Agrol and willow both contain less than 0.002%

sulphur, almost no sulphur species were detected during

experiments. However, a high content of H2S and wellmeasurable amounts of COS and methyl mercaptan were

determined during DDGS gasification. With increasing

temperature, the concentration of H2S remained fairly stable,

whereas the concentrations of COS and methyl mercaptan

slightly decreased. Since steam-oxygen blown CFB gasifica-

tion of DDGS fuel no reported results are available in the

literature, here a simple thermodynamic equilibrium model

established using Factsage software has been used to predict

the concentration of H2S, COS and methyl mercaptan. A

detailed description about the modeling using Factsage can be

found in reference [36]. The predicted concentration of H2S

was slightly higher than the measured value, while COS and

methyl mercaptan concentrations were lower. The measuredaverage concentrations for H2S, COS and methyl mercaptan

are around 2300, 200 and 0.8 ppmv, respectively, while the

predicted ones are 2600, 40 and 0 ppmv, respectively. Thus

during experiments, less H2S will be produced than predicted

by chemical equilibrium calculations, which probably resul-

ted in higher concentrations of COS, methyl mercaptan and

other sulphur species. Attar and Dupuil [37] reported that part

of the sulphur is retained in the solid as alkali sulphide due to

the reaction of H2S with the alkali minerals which prevents

Table 3 e Tar components quantitatively analyzed fromSPA.

Class Name Tar components

Class 2 Heterocyclic

aromatics

phenol, o-cresol, m-cresol and p-cresol

Class 3 Aromatics

(1 ring)

benzene, toluene, m/p-xylene, o-xylene

Class 4 Light PAHs

(2, 3 ring)

indan, indene, naphthalene,

2-methylnaphthalene,

1-methylnaphthalene, biphenyl,

acenaphthylene, acenaphthene,

fluorene, phenanthrene, anthracene

Class 5 Heavy PAHs

(>3-ring)

Fluoranthene, pyrene

Fig. 2 e Schematic drawing of the dry gas sampling line

downstream of the 100 kWth CFB gasifier.








them from entering the gaseous phase. The mineral elements

such as Fe, Ni, Si, Al, Fe, Na, K, Ti, Mg, Ca present in the fuel

may act as catalysts and affect the concentrations of sulphur

species during gasification. The presence of high levels of Ca,

K and Nain the biomass could leadto a high retention ofS and

Cl in the ash [36]. Therefore, the lower measured H2S could be

attributed to the retention of sulphur in the ash because of the

presence of bed materials and minerals in DDGS fuel.

However, due to the low sulphur content in biomass fuel andlimited measurement capacity and accuracy, equilibrium

modeling is already a valuable tool to get an insight into the

behavior of sulphur species during biomass gasification. With

a variation of SBR, the concentration of H 2S and COS obtained

from DDGS using Bed 1 and Bed 2 showed a similar change

trend. For instance, an increase of SBR from around 1.0 to 1.1

led to a significantly decrease in the concentrations of H 2S

obtained from DDGS using Bed 1 from 2700 to 1800 ppmv

accompanied with an slightly increase in COS concentration

from 116 to 125 ppmv. Predicted results from the Factsage

equilibrium model for DDGS gasification using Bed 1 showedthat when SBR was increased from 0.98 to 1.09, a slight

decrease in the concentrations of H2S and COS from 2640 to

Fig. 4e

Several dp-cell trend lines as measured during the experiment on 23 April 2010.

Fig. 3 e Gas composition and reactor temperature trend lines as measured during the experiment on 23 April 2010.








2600 ppmv and 28 to 25 ppmv was observedwhich partlyagree

with experimental results. A similar predicted result has been

achieved for DDGS gasification using Bed 2. A higher ER led to

a decrease in H2S concentration but an increase in the

concentration of COS. These experimental results only partly

agreed with predicted results from Factsage equilibrium

model. However, these predicted trends agree with experi-

mental and predicted values reported by Dias and Gulyurtlu

[38]. The authors reported that the concentration of H2S

increased from 672 ppmv up to 1204 ppmv with increasing ER

from 0 to 0.4, and the concentration of sulphur oxidizes such

as SO2 and COS also increased in different proportions.

3.1.3. Effect on CCE%, CGE% and heating values of product

gas

The operational conditions also largely influence CCE%, CGE%

and LHV of the product gas. Generally a higher temperature

and a higher SBR value led to a higher gas yield. The effects of

operational parameters on CCE%, CGE% and heating values of

product gas obtained from some Agrol, willow and DDGS

testing are given in Table 4. From Table 4 itcan beseen thatan

increase in SBR led to a higher yield of the product gas, but

simultaneously also caused a lower CCE%, CGE% and LHV of

the product gas. However, higher temperature largely

improved CCE%, CGE% and LHV of the product gas. These

0

5

10

15

20

25

30

35

40

45

7 7 5 º

C , E R

= 0 . 3 8 , S B

R = 1 . 1 3

7 7 0 º C , E R

= 0 . 3 8 , S B

R = 1 . 4 5

8 1 5 º

C , E R

= 0 . 3 5 ,

S B R =

0 . 9 7

8 1 0 º C , E R

= 0 . 3 5 ,

S B R =

1 . 2 5

7 8 0 º C , E R

= 0 . 3 8 , S B

R = 0 . 9 3

7 8 0 º C , E R

= 0 . 3 8 , S B

R = 1 . 2 2

8

2 0 º C

, E R = 0

. 3 9 ,

S B R = 0

. 9

8 2 0 º C , E R

= 0 . 3 9 ,

S B R =

1 . 0 4

O t h e r g a s c o m p o s i t i o n ( v o l . d n f . % )

0

2

4

6

8

10Operation conditions (-)

H2 CO CO2 CH4 C2H2+C2H4

Agrol-Bed1 Willow-Bed1

Fig. 5 e Effects of different operational conditions on the composition of product gas from Agrol and willow using bed 1.

0

5

10

15

20

25

30

35

40

45

50

8 0 0 º C , E R

= 0 . 3 3 , S B

R = 0 . 9 8

8 0 0 º C , E R

= 0 . 3 3 , S B

R = 1 . 1 6

8 0 0 º C , E R

= 0 . 4 , S B R = 1

. 1 5

8 0 0 º C , E R

= 0 . 3 4 , S B

R = 0 . 9 1

8 0 0 º C , E R

= 0 . 3 4 , S B

R = 1 . 1 4

8 1 5 º

C , E R

= 0 . 4 3 , S B

R = 1 . 0 7

O t h e r g a s c o m p o

s i t i o n ( v o l . d n f . % )

0

2

4

6

8

10Operation conditions (-)

H2 CO CO2 CH4 C2H2+C2H4

Agrol-Bed1 Willow-Bed2 Willow-Bed3Agrol-Bed3

Fig. 6 e Effects of different operational conditions on the composition of product gas from Agrol and willow using bed 2

and 3.








results are in agreement with those reported by other

researchers. Gil et al. [5] reported that by increasing GR (the

ratio of steam and oxygen supplied to biomass fuel supplied

on a.r. basis) or decreasing SOR (the mole ratio of steam

supplied to oxygen supplied), the LHV of product gas

decreased. The gas yield on a dry basis increased with an

increment of GR and bed temperature. A higher temperature

in the gasifier bed or lower GR led to higher thermal efficien-

cies. Wei et al. [7] reported that the gas yield increased with

reactor temperature due to enhanced endothermic steam

reforming, cracking reactions of the tar and char gasificationat elevated temperatures. Turn et al. [29] reported that

increasing the temperature of the gasifying agents led to an

increase in the heating value of the product gas and reduced

the tars, soot and char residues. Kinoshita et al. [39] reported

that the product gas yield and CCE% increase as temperature

increases since higher temperatures facilitate tar conversion.

Boateng et al. [40] reported that the yield and LHV of the

product gas, CCE% and CGE% increased with increasing gasi-

fication temperature from 700 to 800 C. From Table 4, it can

also be seen that CCE%, CGE%, the yield and heating values of

the product gas obtained from willow were slightly higher

than those obtained from Agrol. For instance, under similar

operational conditions (e.g. SBR ¼ 1.2, ER ¼ 0.38, 780

C, Bed 1),

the yield of product gas produced from willow and Agrol was

2.63 and 2.48 Nm3 /kg fuel on a daf. basis, respectively.

Furthermore, it seemed that the product gas yield produced

from DDGS was much higher than those from Agrol and wil-

low. For different bed materials, it seemed Bed 2 was better

than Bed 1 on the enhancement of the product gas yield. That

could be due to some accumulation of ash in the V-cone after

several time measurements leading to a slight higher

measured pressure drop, since the experiments using Bed 2

was carried out at the last day.

3.2. Effects of different operational parameters on tar

formation/reduction

3.2.1. Effect of temperature on tar formation

A summary of process parameters (SBR, ER, temperature and

bed materials) for all SPA samples from Agrol, willow and

DDGS is presented in Table 5. The analyzed tar SPA results are

shown in Figs. 5e8, where Total Tar/2, Total Unknown,

Sum > C10H8 and Sum > C10H8UN represent a half of the total

tar content, the total unknown tar content, the total tar

content heavier than naphthalene and the total unknown tar

content heavier than naphthalene, respectively.

0

5

10

15

20

25

30

35

40

45

790ºC,ER=0.38,SBR=0.81 800ºC,ER=0.38,SBR=0.81 810ºC,ER=0.38,SBR=0.82 820ºC,ER=0.38,SBR=0.81

O t h e r G a s C o n c e n t r a t i o n ( v o l . d n f . % )

0

50

100

150

200

250

H 2 S , C

O S , C

H 3 H S C o n c e n t r a t i o n ( p p m v , d n f . )

H2 CO CO2 CH4 COS (ppmv) H2S(ppmv) /10 CH3HS (ppmv)×100

Fig. 7e

Effects of different operational conditions on the composition of product gas from DDGS using bed 4.

Table 4 e CCE%, CGE% and HHV, LHV of the product gas produced from some Agrol, willow and DDGS gasification tests.

Fuel Bed types T(C) ER SBR CCE% CGE% HHV_gas LHV_gas Product gas/Biomass

(À) (À) (C) (À) (À) (%) (%) MJ/kg MJ/kg (Nm3 /kg daf.)

Agrol Bed 1 770 0.38 1.21 84.2 52.5 3.9 3.6 2.48

Agrol Bed 1 770 0.38 1.45 74.0 47.0 2.9 2.7 3.08

Willow Bed 1 780 0.38 1.13 90.5 56.2 4.2 3.8 2.55

Willow Bed 1 780 0.38 1.22 86.9 55.2 4.0 3.6 2.63

Willow Bed 2 800 0.34 1.14 91.8 62.1 4.3 4.0 2.81

Willow Bed 3 820 0.43 1.04 93.5 55.7 4.0 3.7 2.50

DDGS Bed 2 750 0.36 1.08 96.6 71.5 4.6 4.3 2.82








From Fig. 8 (Agrol gasification using Bed 1), we can see that

with increasing temperature the total tar content produced

decreased. The total amounts of class 2 and 4 tars were much

higher than those of class 3 and 5 tars. There observations

agreed well with the results reported by other researchers. Gil

et al. [34] and Kinoshita et al. [39] all reported that phenol and

cresol were predominant only at temperature below 800 C,

while naphthalene and indene were the major components at

900 C. Narvaez et al. [6] reported that the temperature not

only influenced tar formation but also the tar properties by

affecting the chemical reactions involved in the whole gasifi-cation network. Brage et al. [41] reported that with increasing

temperature the differences in molecular thermal stability

lead to a progressive accumulation of non-oxygenated

aromatics and favor the formation C2H2 and C2H4 at the

expense of phenols. Van Paasen and Kiel [42] reported that tar

concentration decreased with temperature varying from 750

to 950 C, and simultaneously tar compositions shifted from

alkyl-substituted poly-aromatic hydrocarbons (PAHs) to non-

substituted PAHs. Furthermore, from samples 0413C and D, it

seemed that a higher temperature favored the decomposition

of the heavier tar fraction class 5. When the temperature was

increased from 780 to 820

C, more than 40% of class 5 tar and60% of Sum > C10H8UN was reduced, and 26% increase in the

Table 5 e Process parameters setting for all SPA samples from Agrol, willow and DDGS gasification.

SPA Sample 0413A 0413B 0413C 0413D 0415A 0415B 0415C 0415D 0415E 0415F

Fuel Agrol Agrol Agrol Agrol Agrol Agrol Agrol Agrol Agrol Agrol

SBR (À) 1.52 1.42 1.35 1.35 1.45 1.21 1.13 0.97 1.16 1.25

ER(À) 0.36 0.33 0.42 0.42 0.38 0.38 0.38 0.35 0.35 0.35

Temperature(C) 730 730 780 820 770 770 775 815 810 810

Bed materials Bed 1 Bed 1 Bed 1 Bed 1 Bed 1 Bed 1 Bed 1 Bed 1 Bed 1 Bed 1

0419A 0419B 0419C 0419D 0419E 0419F 0419G 0419H 0419I 0325W

Fuel Willow Willow Willow Willow Willow Willow Willow Willow Willow Willow

SBR (À) 1.19 0.99 1.27 0.93 1.13 1.22 0.90 1.04 1.14 1.04

ER(À) 0.38 0.38 0.38 0.38 0.38 0.38 0.39 0.39 0.39 0.43

Temperature(C) 740 740 740 780 780 780 820 820 820 820


0325 A 0423A 0423B 0423C 0423D 0423E 0421A 0421B 0423F 0423G

Fuel Agrol Agrol Agrol Agrol Willow Willow DDGS DDGS DDGS DDGS

SBR (À) 1.15 1.25 0.98 1.15 0.90 1.14 1.10 0.98 0.95 1.08

ER (À) 0.40 0.33 0.33 0.33 0.34 0.34 0.37 0.37 0.36 0.36

Temperature (C) 820 770 800 800 800 800 730 740 750 750


0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0413A 0413B 0413C 0413D 0415A 0415B 0415C 0415D 0415E 0415F

Sample Names(-)

T a r c o n t e n t ( g / m 3 )

0

50

100

150

200

250

300

350

400

C l a s s 3 , 5

t a r

( m g / m 3 )

Class 2 Class 4 Total Unknown Sum>C10H8 Sum>C10H8UN Class 3 Class5 Total Tar/2

Decrease SBR&ER Decrease SBR (780 °C) Increase SBR (810 °C)Increase T

Fig. 8e

The analyzed SPA results of tar samples from Agrol gasification using bed 1.








fraction of class 3 tar was also observed. These observations

indicate that a higher temperature converts higher molecule-

weight compounds into smaller molecules. These results

seem to partly agree with those reported by Han and Kim [43].

They reported that an increase in the temperature had

a positive effect on the decomposition of class 1 and 2 tars,

while the concentrations of class 3 and 5 tars increased with

temperature enhancement. Kiel et al. [25] reported that byvarying the temperature no clear change trend of the class

5 tar was found. The concentration of class 4 tar increased

with increasing gasification temperature at ER ¼ 0.4, which

was largely due to the simultaneous occurrence of many

effects such as polymerization reactions. Class 4 tar

comprises of a mixture of alkyl-substituted tars and poly-

aromatic hydrocarbons which shown a different behavior.

Without considering the effects of other operational param-

eters, a higher temperature led to higher proportions of class 4

and 5 tars. With increasing temperature from 770 to 810 C

(sample 0415A to F), the proportional contributions of class 4

and 5 tars increased from 40 to 55% on the average. For Agrol

gasification using Bed 1, the lowest tar content of 5.7 g/Nm3

and highest one of 12.4 g/Nm3 were measured at the

temperature of 730 C with SBR ¼ 1.52 and ER ¼ 0.36, and at

780 C withSB ¼ 1.35 and ER ¼ 0.42, respectively. These results

were surprising because normally at a higher temperature the

tar content should be lower. However, as it is well-known, the

total tar content produced from biomass gasification does not

depend on only the temperature, but also other parameters

such as SBR, ER etc. Somehow lower tar content can be ach-

ieved at lower temperature with a suitable combination of

other critical operational parameters. Rapagna et al. [44] also

observed a much lower tar concentration of around 2.4 g/Nm3

at the exit of the gasifier at a temperature of 770 C using

olivine as bed material. Li et al. [45] reported that the tar yield

from biomass gasification decreased when the temperature

increased from 700 to 815 C.

Similar trends were also observed during willow gasification

usingBed1.FromFig.9 (willowgasification using Bed1), it canbe

seenthat the total tarcontentproduced from willowgasification

decreased with increasing temperature from 740 to 820 C. Due

todifferentfuelproperties(e.g.ashcontent),thetotaltarcontent

produced from willow was significantly lower than that fromAgrol. The lowest and highest tar contents produced from wil-

low were 4.4 and 6.7 g/Nm3, which were measured at a temper-

ature of 820 C withSBR¼ 1.14 and ER¼ 0.39 (sample 0419I), and

at 740 C with SBR ¼ 1.19 and ER ¼ 0.38 (sample 0419A), respec-

tively. During Agrol and willow gasification, a higher tempera-

ture led to a higher concentration of class 5 tar components.

Regarding this point, Van Paasen and Kiel [43] pointed out that

two opposite mechanisms can determine the production of

class 5 tar with increasing temperature. Class 5 tar compounds

could be produced either fromthe decomposition of heavy large

class 1 tar compounds, or from lighter tar compounds due to

PAH growth reactions. The influence of temperature on tar

formation produced from DDGS was difficult to be determinedfrom Fig. 10. However, the total tarcontentproducedfrom DDGS

gasification using Bed 1 was 7.3 g/Nm3 (sample 0421A) which

was comparatively lower, because at the similar operational

conditions the total tar content produced fromAgrol and willow

gasification was around 10.2 (sample 0415C) and 6.7 g/m3

(sample 0419A). Furthermore, at a fairly low temperature of

730 C, the contents of class 2 and 4 tars were only 1 and 1.85 g/

Nm3, respectively, which is below 2 g/Nm3 being considered as

an important limit for many downstream applications [46].

3.2.2. Effect of SBR and ER on tar formation

From Fig. 8, it can be clearly seen that an increment of SBR

significantly promoted tar decomposition during Agrol

0.0

1.0

2.0

3.0

4.0

0419B 0419A 0419C 0419D 0419E 0419F 0419G 0419H 0419I

Sample Names(-)

T a r c o n t e n t ( g / m 3 )

0

50

100

150

200

250

300

350

400

450

C l a s s 3 , 5

t a r ( m g / m 3 )


Increase SBR (740 °C) Increase SBR (780 °C) Increase SBR (820 °C)

Fig. 9e

The analyzed SPA results of tar samples from willow gasification using bed 1.








gasification using Bed 1. With increasing SBR from 1.13 to 1.45

at770 C (sample 0415C to A)and SBR from0.97to 1.25 at810 C

(sample 0415D to F), the total tar content both decreased from

around 10.2 to 7.2 g/Nm3. Generally with increasing SBR the

concentrations of class 3, 4 and 5 tars all decreased, while

the concentration of class 2 increased. However, at 770 C, the

lowest content of class 5 tar was found at an SBR value of 1.21

instead of 1.45. Furthermore, a slight increase in the concen-

trationsofclass3and5tarswasalsoobservedwhenincreasing

SBR from0.97 to1.16 at810 C (sample 0415Dto E). The highest

tar content obtained from Agrol was around 12.4 g/Nm3

(sample 0413C) which was measured at 780 C and SBR ¼ 1.35

and ER ¼ 0.42. However, under a less favorable tar decompo-

sition circumstance, a much lower tar content of 8.7 g/Nm 3

(sample 0415B) was also observed. These two samples were

taken at different days and there may be slight difference

inside the gasifier (e.g. char content). Similar trends were

observedforwillowandDDGSgasificationusingBed1.InFig.9,

withincreasingSBRfrom0.93to1.22at780 C(sample0419Dto

F), the total tar content produced from willow decreased from

6.6 to 4.7 g/Nm3. With increasing SBR from 780 to 820 C

(samples 0419D to I), the concentrations of class 2 to 5 tars

decreased with a varying reduction degree from 20 to 66%, but

at 740 C (samples 0419B to C) the concentration of class 2 tarincreased with a variation of SBR from 0.99 to 1.27. Further-

more, at 820 C, the lowest concentrations of class 3 and 5 tars

were observed at SBR of 1.04 (sample 0419H) instead of at 1.14

(sample 0419I). In Fig. 10, at a temperature of 730 C when SBR

was increasedfrom 0.98 to 1.1 (sample 0421B to A), the total tar

content produced from DDGSdecreased from 9.1 to 7.3 g/Nm3.

More than 45% of class 4 and 60% of class 5 tar were reduced,

but the concentration of class 2 largely increased from 0.6 to

1.0 g/Nm3 and almost no change in the content of class 3 tar.

Similar results were observed for tar compounds produced

from Agrol, willow and DDGS gasification using Bed 2 (see in

Figs. 10 and 11). However, a fairly dramatic change was

observed about the class 2 produced from Agrol and willow.

With increasing SBR from around 0.9 to 1.15 (sample 0423B to

G), the total tar content and the concentrations of class 4 and

5 tars produced from three fuels all decreased. The lowest tar

concentrations obtained from Agrol, willow and DDGS were

9.7, 5.3 and 6.7 g/Nm3, respectively (sample 0423 A, E and G).

Furthermore, it seemed that the increment of SBR from 0.98 to

1.15 at a temperature of 800 C had negligible influence on the

decomposition of tar produced from Agrol. The total tar

content remained around 10 g/Nm3, and similar results were

also observed when using Bed 1. The effect of ER on tar

formation can not be concluded from the SPA results. Fromabove-stated results, we can see that the compositions and

content of tar produced from three fuels were fairly different,

which can be attributed to the difference among their prop-

erties. The results reported in this work are similar to those

reported by other researchers. Aznar et al. [47] reported that

with varying GR from 0.7 to 1.2 more than 85% reduction in

total tar was achieved. The different behavior of class 2 to

5 tars produced from Agrol, willow and DDGS can be attrib-

uted to their different properties such as the distribution of

cellulose, hemicellulose and lignin and ash contents.

According to Paasen and Kiel [43], the final tar composition

depends on the lignocelluloses composition of biomass fuels.

The difference in theconcentration of class 2 tar can be largelydue to the difference in structure between cellulose and

lignin, while the difference in the concentrations of class 4

and 5 tars might be attributed to differences in lignocelluloses

composition of the feedstock. Since class 4 and 5 tars are

mostly PAH compounds which come from aromatic func-

tional group in the molecular structure of lignin.

3.2.3. Effect of the types of bed materials on tar formation

From Figs.8e11,itisdifficulttoconcludewhetherBed1orBed2

was better on tar decomposition. For instance, the total tar

content obtained from willow using Bed 2 was 5.3 g/Nm3 at

800 CwithSBR ¼ 1.14 and ER¼ 0.34 (sample 0423E),which was

slightly lower than that of 5.4 g/Nm3 using Bed 1 at 780 C with

0.0

1.0

2.0

3.0

4.0

5.0

0421A 0421B 0423F 0423G

Sample Names(-)

T a r c o n t e n t ( g / m 3 )

0

50

100

150

200

250

300

C l a s s 3 ,

5 t a r ( m g / m

3 )

Class 2 Class 4 Total Unknown Sum>C10H8

Sum>C10H8UN Class 3 Class5 Total Tar/2

Decrease SBR (bed 1) Increase SBR (bed 2)

Fig. 10 e The analyzed SPA results of tar samples from DDGS gasification using bed 1 and 2.








SBR¼ 1.13 and ER¼ 0.38(sample 0419E),whilea bithigherthan

thatof4.4g/Nm3 at820 CwithSBR¼ 1.14andER¼ 0.39 (sample

0419I). Furthermore, from these three tar SPA samples, it can

also be seen that the concentration of class 5 tar using Bed 2

was much higher than that using Bed 1, while the class 3 tar

showed an opposite trend. The concentrations of class 3 and

5 tarsin samples 0423E, 0419Eand 0419I were 72, 64 and 65 mg/Nm3, and 150, 219 and157 mg/Nm3, respectively. From these

results, it can be concluded that Bed 1 showed considerable

catalytic reactivity toward the decomposition of heavy PAH

compounds (class 5 tar). Some fairly interesting observations

were found during Agrol gasification using different bed

materials. From SPA samples 0415D and E and samples 0423B

andC,itcanbeseenthatwhenthetemperaturewaslowerthan

820 C and ER lower than 0.35 the increment of SBR from 0.9 to

1.16 had negligible influence on tar decomposition and

compositions. Under these conditions, the averaged total tar

content was around 10 g/Nm3. A combination of higher SBR

(>1.4) with lower temperature (w730 C) and intermediate ER

(0.33e

0.36) could lead to a lower tar content produced fromAgrol (samples 0413A to B).A fairlylow tarcontent of 7.7g/Nm3

wasalso obtained from Agrol using Bed 3 under a combination

of operational conditions of a lower SBR (w1.15) with a higher

temperature (>820 C)andahigherER(w0.4).TheeffectsofBed

1 and 2 on the compositions of tar can be seen. These two

similar olivine bed materials(Bed 1 and 2) have also been

studied by Pecho et al. [48] for the optimization of the biomass

gasification under sulphur-free (S-free) and H2S enriched

conditions. They reported that the catalytic activity of Bed 1

increased significantlyby redox-type pre-treatment and it also

acted as an oxygen carrier from combustion zone to the gasi-

fication zone, whereas fresh olivine Bed 2 had almost no

catalyticactivity. Their findingscan be a good reference forthis

work. Rauchet al. [49] compared different olivines for biomass

steam gasification and found that olivine had high attrition

resistance as bed material for fluidized beds and the catalytic

activity for tar reforming. Bed 3 had less positive influence on

tardecomposition than Bed1. Forinstance, at the temperature

of 820 C with SBR of 1.04 and ER of 0.4, the total tar content

produced from willow gasification using Bed 1 and 3 was 4.5and 6.9 g/Nm3, respectively. The total amount of class 4 and

5 tarswas reduced from4.6 to3.1 g/Nm3 and153to62mg/Nm3,

respectively. Regarding to the reactivity of different bed

materials on tar decomposition, more experiments need to be

carried out in order to get a better insight into their influence.

4. Conclusion

The effects of operational conditions (SBR, ER and tempera-

ture) and bed materials on the product gas distribution and tar

formation produced from Agrol, willow and DDGS gasification

have been investigated on an atmospheric pressure 100 kWthsteam-oxygen blown CFB gasifier. The compositions of the

product gas and tar obtained from these fuels are fairly

different,but it is difficult to make a clear comparisonbetween

tem due to some differences in SBR, ER and the temperature

used. Under similar operational conditions the concentration

of H2 obtained from willow was much higher than that

obtainedfrom Agroland DDGS.Amongallthe experiments,the

product gas composition obtained from willow over the

temperature range from 800 to 820 C consisted of the highest

H2 concentration (28 vol.%), followed by Agrol (24 vol.%) and

DDGS (20 vol.%) on a N2 free basis, respectively. A fairly high

amount of H2S (w2300 ppmv), COS (w200 ppmv) and trace

amounts of methyl mercaptan (<3 ppmv) on a N2 free basis

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0325 A 0325W 0423A 0423B 0423C 0423D 0423E

Sample Names(-)

T a r c o n t e n t ( g / m 3 )

0

50

100

150

200

250

300

350

400

450

C l a s s 3 , 5

t a r ( m g / m 3 )


Bed3 (Agrol & willow) Increase SBR (Agrol) Increase SBR (willow)Higher SBR

Bed2

Fig. 11 e The analyzed SPA results of tar samples from Agrol and willow gasification using bed 2 and 3.








were also obtained from DDGS. A highertemperaturesand SBR

were more favorable for H2 production but less advantageous

for theformation of CO and CH4, whereas a higher SBR also led

to a lower CCE%, CGE% and heating values of the product gas

due to high steam content in product gas. Although DDGS fuel

has a relatively high K and Cl content, continually adding

3e10% kaolin (based on total feeding rate) into the reactor can

successfullyavoid agglomeration. Tar produced fromthe threefuels mainlycontains phenol,cresol, naphthalene, indene and

pyrene. The total tar content obtained from Agrol is at

maximum w12.4 g/Nm3, followed by that from DDGS and

willow. A higher temperature and higher SBR were favorable

forthe tardecomposition.However,it seems that anincrement

of SBR from 0.9 to 1.16 had negligible influence on tar obtained

from Agrol fuel at a lower temperature (<820 C)andalowerER

(<0.35). The content of class 5 tar obtained using Bed 1 was

lower than that obtained using Bed 2, which prove Bed 1 was

more reactive on the decomposition of heavy tar compounds.

Although at a fairly low temperature of 730 C, the total tar

contentproducedfrom DDGS using Bed1 was7.3 g/Nm3 where

the contents of class 2 and 4 tars were only 1 and 1.85 g/Nm 3,respectively, which is near 2 g/Nm3 being considered as an

important limit for many downstream applications. However,

DDGSfuelhasahighsulphurcontentwhichalsoleadstoahigh

concentration of H2S in the product gas. So far, the effects of

SBR, ER and temperature on the distribution of H2S and COS

from DDGS are still not entirely clear. Additional experiments

are necessary in order to get a better insight in the reactivity

and influence of different bed materials on tar decomposition

in combination with reduction of other contaminants e.g. H2S.

Acknowledgments

The authors thank Lantma ¨ nnen, Sweden for supplying Agrol,

willow and DDGS fuels, Markus Koch from Biomasse Kraft-

werk Gu ¨ ssing GmbH & CoKG for delivering olivine bed mate-

rials, Michael Mu ¨ ller from Institute of Energy Research,

Forschungszentrum Ju ¨ lich GmbH for performing fuels char-

acterization analysis, Claes Brage from KTH for analyzing all

SPA tar samples. The European Commission is acknowledged

for co-financing the 7th Framework Project, related to this

research: “GreenSyngas” (Project NO. 213628).

Nomenclature

a.r. as received

BTX benzene, toluene, xylenes

CCE% carbon conversion efficiency

CGE% cold gas efficiency

CFB circulating fluidized bed

CHP combined heat and power

DDGS dry distiller’s grains with solubles

DSC differential scanning calorimetry

daf dry ash free

dnf dry nitrogen free

d.b. dry basis

ER the ratio of oxygen supplied to the oxygen required

for the complete stoichiometric combustion of fuel

on a daf basis

FB fluidized bed

FTIR fourier transform infrared

FICFB fast internally circulating fluidized bed

GC gas chromatograph

GHG greenhouse gas emissionsGR the ratio of steam and oxygen supplied to biomass

fuel supplied on a.r. basis

HHV higher heating value [MJ/kg]

LHV lower heating value [MJ/kg]

NDIR non dispersive infrared analyzer

PM paramagnetic analyzer

PAHs polycyclic aromatic hydrocarbons

SPA solid phase absorption

SBR the ratio of steam supplied to biomass fuel supplied

on a.r. basis

SOR the mole ratio of steam supplied to oxygen supplied

TGA thermogravimetric analyzer

Fm, oxygen oxygen supplied mass flow rate [kg/h]Fm, fuel (daf) biomass fuel supplied mass flow rate on a daf

basis [kg/h]

Fm, gas gas mass flow rate [kg/h]

hfg the enthalpy of vaporization of water [MJ/kg]

mi.daf mass fraction of different element (C, H, O, N, S) in

fuel on a daf basis

mO2, air mass fraction of O2 in air

MWi Molar weight of different element (C, H, O, N, S)

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