Pollutant emissions in a bubbling fluidized bed combustor...
Transcript of Pollutant emissions in a bubbling fluidized bed combustor...
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Submitted, accepted and published by: Applied Energy 102 (2013) 860–867 Pollutant emissions in a bubbling fluidized bed combustor working in
oxy-fuel operating conditions. Effect of flue gas recirculation.
L.F. de Diego, M. de las Obras-Loscertales, A. Rufas, F. García-Labiano, P. Gayán, A.
Abad, J. Adánez.
Dept. Energy and Environment, Instituto de Carboquímica (ICB-CSIC)
Miguel Luesma Castán 4, 50018 Zaragoza. Spain
Corresponding author: [email protected]
Abstract
In this work, pollutant emissions in a continuous bubbling fluidized bed combustor (~3
kWth) at oxy-firing conditions were measured. An anthracite coal was used as fuel and a
limestone was added for sulfur retention. Flue gas recirculation was simulated by
mixing different gases (CO2, SO2, steam, and NO) and the effect of varying the gas
composition of the recycled flow on the pollutant emissions was analyzed. It was
observed that the most important effect of CO2 recirculation was the increase of the
optimum temperature for SO2 retention from ~ 850 ºC (conventional air combustion) to
900-925 ºC. SO2 recirculation increased the Ca-based sorbent utilization and did not
affect the N2O emissions at any temperature. In addition, at 850 ºC the CO emission
increased and the NO emission decreased, however, at 925 ºC the CO variation was
negligible and the NO reduction was low. About 60-70% of the recycled NO was
reduced to N2 and N2O, being the NO converted to N2O lower than 5%. Steam
recirculation mainly led to a sharp decrease in NO emission. A synergetic effect among
the different recycled gases was not found in any case.
Keywords: Oxy-fuel combustion; fluidized bed; pollutant emission; gas recirculation.
1. Introduction.
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Nowadays, there is an important public awareness in relation to the hazard of pollutant
gas emissions into the atmosphere from fossil fuel combustion to obtain energy. On the
one hand, CO2 gas has been recognized as one of the major contributors to the build-up
of greenhouse gases and, on the other hand, sulfur and nitrogen content in coal are
oxidized to SO2 and NOx respectively, which contribute to acid rain formation.
According to the IPCC 2005 [1], carbon dioxide capture and storage technologies would
be needed to mitigate CO2 emissions from large power plants into the atmosphere for
stabilization of atmospheric greenhouse gas concentrations. Oxy-fuel combustion is a
CO2 capture technology which is characterized by the use of a mixture of pure O2 and
CO2-rich recycled flue gas (instead of air which is used in conventional combustion) to
perform the combustion process. As a consequence, with this technology, the CO2
concentration in the flue gas may be enriched up to 95 vol% (dry), and therefore it is
possible to achieve an easy CO2 recovery.
There are mainly two types of boilers for coal combustion, pulverized coal (PC) boilers
and fluidized bed boilers. Although the majority of the researches in oxy-fuel
combustion have been developed for PC boilers [2,3], fluidized bed combustors (FBC),
and specially circulating fluidized bed (CFB) combustors, are also very appropriate for
this combustion system. This technology has the advantage that external solid heat
exchangers can be used to extract heat from the combustion process [4]. This will allow
the use of high oxygen concentrations in the combustor reducing the amount of recycled
flue gas and the area of the CFB combustor. Moreover, in situ desulfurization of
combustion gases, by feeding a low cost sorbent such as limestone [4,5], and relatively
low NOx emissions can be achieved [6,7].
Currently, the oxy-fuel combustion technology in CFB combustors is growing. Alstom
[8], VTT and Foster Wheeler [6], Metso [9], Czestochowa University of Technology
[7], and Canmet Energy [4,5,10,11] have performed oxy-fuel combustion experiments
with CFB combustors at scales up to 4 MWth. The Fundación Ciuden [12] in Spain has
built two plants able to operate at both conventional air and oxy-fuel combustion
conditions. The first plant is a 20 MWth PC boiler and the second plant is a CFB
combustor of 15 MWth operating in air-mode and 30 MWth operating in oxy-mode.
However, there are few works published which deal with the effect of the flue gas
recycle on the pollutant gas emission and thus there is still some lack of knowledge. The
research group of Canmet Energy has successfully operated two CFB combustors of
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100 kWth [4,5,13] and 0.8 MWth [10,11] with flue gas recycle. At an operating
temperature of ~850 ºC, they found lower SO2 retention by calcium sorbents in oxy-fuel
combustion than in air combustion conditions, but the SO2 retention improved in oxy-
fuel combustion by increasing the operation temperature, that is, when the conditions
changed from direct to indirect sulfation. In addition, mainly due to flue gas
recirculation, lower NOx emissions in oxy-firing operation were observed compared to
air operation.
Our research group has recently carried out several tests in a continuous bubbling
fluidized bed (BFB) combustor (~3 kWth) where the effect of the temperature on the
sulfur retention by limestone addition was analyzed [14]. It was concluded that the
optimum operating temperature from the point of view of SO2 retention shifted from
850-870 ºC in combustion with enriched air to 900-925 °C in oxy-fuel combustion
mode.
In this work, pollutant gas emissions at different operating temperatures, mainly
working at oxy-fuel combustion mode, were measured in a continuous BFB combustor
(~3 kWth). However, the main objective of this work was to analyze the effect of the
flue gas recycled into the reactor on the pollutant emissions working in oxy-firing
mode. The gas inflow recycled was simulated by mixing different gases (CO2, SO2,
H2O, and NO), which allowed us to observe the influence of the different gases, both
separately and combined with each other.
2. Experimental section
2.1. Materials
A Spanish anthracite coal was selected as fuel for this study. The coal was crushed and
sieved, and the particle size in the range of 0.2-1.2 mm was used. Table 1 gives the
proximate and ultimate analyses of the coal. A high purity Spanish limestone
“Granicarb” (97.1 wt.% CaCO3) was used as calcium-based sorbent for sulfur retention.
Table 2 gives the analysis of the Granicarb limestone. The particle size of the limestone
was in the range of 0.3-0.5 mm. The porosities of the raw and after calcination sorbent
were 3.7 and 49 vol.%, respectively. Inert silica sand of size 0.2-0.6 mm was fed
together with the coal and the limestone during all the tests to control the residence time
of the sorbent in the fluidized bed reactor.
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2.2. Experimental installation
The experimental installation consisted of a fluidized bed combustor (~3 kWth) and
different auxiliary systems for gas supply, solid feeding, solid recovering, and gas
analysis. Figure 1 shows a schematic diagram of the installation.
The combustor consisted of a stainless steel reactor of 9.5 cm i.d. and 60 cm height and
a freeboard of 15 cm i.d. and 50 cm height. The height of solid in the BFB was
maintained constant at 40 cm. A heat exchanger located inside the bed allowed the
perfect control of temperature. This heat exchanger could be moved vertically through
the reactor to modify the contact surface inside the bed, which allowed the extraction of
the needed heat from the combustor to reach the desired temperature.
The reactant gases, air, CO2, and O2, were supplied from cylinders by means of
electronic mass-flow controllers to simulate typical gas compositions entering into the
reactor in oxy-firing mode. The gases were fed into the reactor through a gas distributor
plate. An air pre-heater allowed the introduction of hot air inside the reactor during the
start-up of the plant. In the tests where the recycle of H2O, SO2, and/or NO was
simulated, the gas feeding system allowed the introduction of different gas mixtures in
order to analyze the influence of each gas in the process. H2O was supplied by a
peristaltic pump, subsequently evaporated and fed into the reactor through the gas
distributor plate together with the CO2 and O2. SO2 and NO were supplied from
cylinders by means of electronic mass-flow controllers just above the gas distribution
plate to avoid its corrosion.
The solids were fed to the reactor by means of water-cooled screw feeders located just
above the distributor plate. To ensure a constant Ca/S molar ratio, coal and sorbent were
fed together by means of two screw feeders in series: the first one controlled the
coal/sorbent feeding rate and the second one introduced the solid mixture as quick as
possible into the bed to avoid coal pyrolysis and plugging of the pipe. Other screw
feeder controlled the silica sand fed to the combustor. Table 3 shows the feeding rates of
solids and flow rate of gases used in the tests.
The flue gas stream leaving the combustor passed through a high efficiency cyclone to
recover the elutriated solids and it was afterwards sent to the stack. The gas composition
at the exit of the combustor was continuously analyzed after water condensation by on-
line gas analyzers. CO2, CO, and SO2 concentrations were measured in a Non-
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Dispersive Infra-Red analyzer (NDIR, Siemens/Ultramat 23), O2 concentration in a
paramagnetic analyzer (Siemens/Oxymat 6), NO, N2O, and NO2 concentrations in an
infrared gas analyzer, via Fourier transform infrared (FTIR, Temet CX 4000).
The facility was equipped with pressure sensors and thermocouples to know the
pressure drops and temperatures in different locations of the installation. General
process data (temperatures, pressures, flow of gases, gas composition, etc.) were
continuously recorded by a computer.
2.3. Procedure
To start-up, the bed was filled with ~1.8 kg of silica sand and hot air was introduced
through the gas pre-heater to heat the bed up to the ignition temperature of coal. At this
temperature, the coal feeding started and the bed temperature rose rapidly due to the
coal combustion. When the desired temperature was reached, the preheating system was
turned off, the air was replaced by the desired gas mixture, sand and coal/limestone
mixture were fed into the bed, and the heat exchanger was introduced into the bed to
control the temperature. Once stable operating condition was reached, it was maintained
until steady state operation for SO2 retention was attained. An important feature of the
tests carried out in the continuous unit was the certainty that the results were obtained
under steady state conditions. This aspect was commented in detail in a previous paper
[14].
The SO2 retention (SR) was calculated by equation (1) as the molar fraction of sulfur
retained by the bed solids with respect to the sulfur contained in the coal feeding.
100.
/
/(%)
,,0
,2,,0
ScoalScoal
outSOoutScoalScoal
MxF
CQMxFSR
(1)
being F0,coal the coal feeding rate, xS,coal the coal sulfur content, MS the molecular weight
of S, CSO2,out the SO2 concentration in the flue gas at the exit of the reactor, and Qout the
gas flow rate at the reactor exit. Qout was calculated by means of a mass balance,
considering the coal and gas feeding flow rates and the flue gas composition. CSO2,out
was considered as an average value of the measurements taken during the whole test
duration in steady state conditions. The average concentrations of the other gases along
the test were also taken into account and were calculated in the same way as for SO2.
Steady state was maintained for at least 1 h for each experimental condition.
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3. Results and discussion.
3.1. Influence of the combustion temperature.
First, combustion tests without feeding SO2, NO or steam were performed at different
temperatures between 800 and 950ºC. In all cases, the experimental conditions showed
in Table 3 with an inlet gas composition of O2/CO2 =35/65 were used. The coal feeding
rate was controlled to keep the O2 concentration at 3.75±0.75 vol% at the combustor
exit (dry basis).
Figure 2 shows the effect of the combustion temperature on the SO2, CO, NO, and N2O
emissions. NO2 emission was never detected (the lowest concentration reliably
detectable was 10 ppm). The SO2 emission decreased with increasing temperature up to
900-925 ºC and then, a further increase in temperature caused an increase in SO2
emission. In a previous work [14] it was found that calcium sorbent calcination had a
strong effect on SO2 retention under oxy-fuel combustion conditions. The SO2 retention
was higher working at calcining (indirect sulfation) than at non-calcining (direct
sulfation) operating conditions. So, the optimum combustor temperature from the point
of view of SO2 retention was 900-925 °C in oxy-fuel combustion mode.
The CO concentration decreased exponentially with rising temperature because the
overall coal combustion process improved as the temperature increased.
Figure 2 also shows a clear dependence of NO and N2O emissions with temperature.
The NO emission increased with temperature up to 900 ºC and then it was almost
constant until 950 ºC. On the contrary, the N2O emission decreased when the combustor
temperature increased.
A number of researchers have attempted to elucidate the effect of temperature on the
N2O emission in coal combustion. Some researchers [15,16] believed that
heterogeneous NO reduction by the char was the major source of N2O formation in the
fluidized bed combustion of coal. Thereby, the emission of N2O would decrease with
decreasing char concentration in the bed as the temperature increased. On the other
hand, other researchers [17,18] suggested that the homogeneous gas-phase reactions of
NCO played an important role in N2O formation. They concluded that the decrease in
the N2O emission at a higher temperature was the result of the NCO oxidation to NO
promoted by the higher temperature, decreasing the amount of NCO available for N2O
formation. In addition, homogeneous reduction of N2O by H or OH radicals was also
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shown to be important [18,19]. The increasing NO emission with increasing bed
temperature may be due to the fact that a higher bed temperature reduced the char and
CO concentrations in the combustor, decreasing the heterogeneous reduction of NO on
the char surface. In addition, higher temperatures promoted the NCO oxidation to NO.
3.2. Effect of the reaction atmosphere.
To analyze the effect of reacting atmosphere, a series of experimental tests where N2
(O2/N2 =35/65) was fed in the system instead of CO2 was performed. The experimental
operating conditions showed in Table 3 and the temperature range (800-950 ºC) were
kept constants.
As can be seen in Figure 2, comparing the results obtained when working with O2/CO2
(oxy-fuel combustion) and O2/N2 (enriched air combustion), the most important effect
of the CO2 was to shift the optimum temperature for SO2 retention from ~860 ºC to 900-
925 ºC. The CO emission was slightly higher when working under oxy-fuel combustion
conditions than in enriched air combustion conditions. The NO and N2O emissions were
almost the same in both reacting atmospheres, which suggests that nitrogen oxides are
hardly produced from the N2 in air in the experimental conditions used (800-950ºC).
Hosoda and Hirama [20] burned a bituminous coal in a BFB and found the same
influence for N2O emission. However, they observed a slight decrease on NO emission
with increasing CO2 concentration.
3.3. Effect of the gases recycled with the CO2.
In the CO2-rich recycled flue gas, besides CO2 and O2, there are other gases in a lower
concentration that can affect to the pollutant emissions of the combustor. In this section,
a detailed study of the effect of the recycle of different gases, such as SO2, steam, and
NO, both separately and combined, was carried out at two different temperatures, 925
and 850 ºC. The first one was chosen because at this temperature the calcium sorbent
calcines (indirect sulfation) and, in addition, it was the optimum temperature for SO2
retention. The second one was chosen for comparison purposes because at 850 ºC the
calcium sorbent does not calcine (direct sulfation). All other operating conditions were
kept constants.
3.3.1. Effect of SO2 recirculation.
To simulate the flue gas recirculation of SO2, a flow of this gas was injected just above
the gas distribution plate. The pure SO2 flow rate introduced at each operating condition
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was estimated by a mass balance of the system and it was modified until the feeding
flow had the same SO2 fraction as the recycled flow taking into account the SO2
concentration at the exit of the combustor. Thereby, a flow of SO2 of 2.0 ln/h in the test
carried out at 925 ºC and a flow of SO2 of 7.8 ln/h in the test carried out at 850 ºC were
injected. This corresponded to SO2 concentrations of 900 vppm and 3500 vppm,
respectively, in the inlet gas flow rate.
Figure 3 shows the part of the tests just before and after SO2 injection. Obviously, at
both temperatures, the SO2 concentration in the outlet gas stream increased after its
injection. In a real oxy-fuel combustion system, SO2 is enriched in furnace owing to the
flue gas recirculation and consequently a good sulfation condition is provided. In this
work the SO2 recirculation was simulated by injecting SO2. The SO2 retention with
respect to the sulfur fed by the coal was calculated by means of the following
expression:
100.
/
/(%)
,,0
,2,2,,0
ScoalScoal
outSOoutinSOinScoalScoal
MxF
CQCQMxFSR
(2)
being F0,coal, xS,coal, MS, CSO2,out. and Qout the same as in equation (1), CSO2,in the SO2
concentration in the flue gas coming into the reactor, and Qin the gas flow rate at the
reactor inlet.
It was found that the SO2 retention increased from ~69% to ~81% at 925 ºC and from
~34% to ~39% at 850 ºC because of the higher SO2 concentration in the combustor. It
means that the Ca utilization was higher after SO2 injection. Therefore, although the
SO2 concentration at the outlet of the combustor increases due to the recirculation, the
total SO2 emission is much lower.
In Figure 3 it can be seen that the effect of the SO2 injection on the emissions of the
other gases depended on the temperature. On the one hand, at 850 ºC after SO2 injection
there was an important reduction in the NO concentration and an important increase in
the CO concentration at the outlet of the reactor. However, the NO reduction was lower
and the CO increase was negligible at 925 ºC. On the other hand, the SO2 recirculation
did not affect the N2O emission at both temperatures.
Amand et al. [21] worked at conventional air combustion in a FBC and found a
decrease in the NO emission and an increase in the N2O emission when SO2 was
injected into a sand bed (without limestone). The decrease in the NO emission was
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explained by an increase in CO concentration, since the CO oxidation was moderately
inhibited by the addition of SO2 [22] and the increase in CO concentration improves the
reduction of NO on the char surfaces [23-25]. Some researchers [26-27] attributed the
inhibition of CO oxidation caused by the presence of SO2 to the decrease in the radical
inventory. Recently, Giménez-López et al. [27] observed that the presence of SO2
inhibited the oxidation of CO for both CO2 and N2 atmospheres and for all
stoichiometries.
3.3.2. Effect of NO recirculation
The effect of NO recirculation was investigated at two different temperatures, 850 and
925 ºC, by injecting different flow rates of NO (2 vol% NO in Ar), which were
calculated to be equivalent to 200, 350, 500 and 625 vppm with respect to the inlet gas
flow rate.
Figure 4 plots the gas distribution before and after the different injections of NO at 925
ºC, and Figure 5 shows the concentrations of NO measured at the combustor exit at both
temperatures. The amount of NO reduced (c in Fig. 5) was calculated by equation (3) as
the difference between the NO introduced in the inlet gas (a in Fig. 5) and the increase
in the NO concentration in the outlet gas due to NO injection (b in Fig. 5).
100.)(
(%),
,,,
recinNOin
outNOrecoutNOoutrecinNOinreduction CQ
CCQCQNO
(3)
being CNO,in rec the inlet NO concentration recirculated into the combustor, CNO,out rec the
the NO concentration in the flue gas at the exit of the reactor in the tests with NO
recirculation, and CNO,out the NO concentration obtained in the flue gas at the exit of the
reactor in the tests without NO recirculation. CNO,out rec and CNO,out were considered as
an average value of the measurements taken during the whole test duration in steady
state conditions.
It was observed that about 60-70% of the NO introduced was reduced to N2 and N2O,
and the reduction rate was almost independent of the initial concentration of NO and
temperature. The formation of N2O was very small, less than 5% of the NO introduced
(see Figures 6 and 7). In addition, neither SO2 nor CO emissions were affected by the
NO injection, as can be observed in Figure 4.
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Guerrero et al. [28] found that if the amount of char is kept constant, an increase in the
reaction temperature leads to an increase in NO reduction. This behavior was explained
by the fact that thermal desorption is an activated phenomenon and increases with
increasing temperature. However, this phenomenon could not be observed in our
experimental tests because an increase in the temperature led to a decrease in char and
CO concentrations and as a consequence a decrease in the NO reduction. Therefore, it
could be that the opposite effects of the increase of the temperature and the decrease of
the char and CO concentrations compensate each other.
Hosoda and Hirama [20], in a similar test carried out in a BFB at 850 ºC using a
bituminous coal, found that about 80% of the added NO and N2O were reduced to N2
and 4-5% of the added NO was reduced to N2O. These results and the results found in
our tests seem to indicate that heterogeneous NO reduction on the char surface was not
the major source of N2O formation in the fluidized bed coal combustion.
3.3.3. Effect of steam recirculation.
Wet and dry gas recirculations are possible in oxy-fuel combustors. In this section the
effect of H2O(v) (steam) recirculation on the emission of gases was analyzed. The effect
of steam recirculation was simulated by feeding H2O instead of CO2 in the inlet gas
flow. The gas composition fed to the combustor in these tests was 35 vol% O2, 45-50
vol% CO2, and 15-20 vol% H2O.
In all the tests the gas composition was measured after steam condensation. So, to
compare the emissions, the measured gas concentrations for the tests with steam feeding
were corrected to the same outlet gas flow as in the tests without steam addition.
As it can be seen in Figure 8, when steam was introduced a sharp decrease in NO
emission was observed. Therefore, it could be inferred that wet recirculation would
produce a great decrease in the NO concentration in comparison with dry recirculation
in a fluidized bed furnace. Similar results have been observed by other researchers using
different coals [13,20]. It can be also observed in the figure that the change in the CO
concentration was negligible and the N2O concentration slightly increased. Hosoda and
Hirama [20] also observed a slight increase in the N2O concentration with H2O addition.
Regarding to SO2 concentration and thus the limestone sulfation conversion, Wang et al.
[29] found in TGA tests that the presence of steam improved the capacity of sulfation of
the limestones, in some cases up to a relative increase of 86.7%. Similar results were
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found in a TGA and in a tube furnace by Stewart et al. [30], who added steam and
achieved limestone sulfation conversions up to 98% with sorbent particle sizes between
75 and 115 m and a residence time of 10 h. They suggested that the effect of H2O was
negligible in the kinetic-controlled stage but it was especially important in the diffusion-
controlled stage with an optimum of water concentration around 15-30 vol%. However,
these researchers [13] burnt petcoke in a CFB combustor and found an increase in the
sulfur capture in air-firing mode and a decrease in oxy-fuel firing mode when steam was
added. In our tests the effect of H2O addition (20 vol% at 850 ºC and 15 vol% at 925 ºC
+ 5 vol% generated by the coal combustion) on sulfation conversion was very small,
both at 850 ºC (direct sulfation) and 925 ºC (indirect sulfation). However, it must be
remarked that in this work the tests were carried out with a mean residence time of
solids of ≈ 2 hours, which could be not long enough to notice that effect.
3.3.4. Effect of combined NO and H2O recirculation.
As it was above showed, the H2O recirculation produced an important decrease in the
NO concentration in the outlet gas stream. Therefore, it would be expected a higher NO
reduction when H2O and NO are recycled together in comparison to the NO recycled
without H2O. Figure 6 shows the NO and N2O emissions and Figure 7 shows the
increase of NO and N2O emissions measured with and without H2O recirculation (wet
and dry gas recirculation) as a function of the recirculated NO concentration. It is
observed in these Figures that although the NO emission was lower with H2O
recirculation, the reduction of the NO recycled was almost the same with H2O and
without H2O recirculation. Therefore, it can be concluded that H2O recirculation has
only influence during the stage of NO formation but once NO is formed, the
contribution of the H2O to minimize the NO emission is very small.
3.3.5. Effect of combined H2O/SO2, SO2/NO, and H2O/SO2/NO recirculations.
In an oxy-fuel boiler, the recycled stream, besides CO2 and H2O, contains different
amount of minor gases, which could affect each other. The effect of the addition of
different gas combinations, H2O/SO2, SO2/NO and H2O/SO2/NO, in the inlet gas stream
was analyzed. The global effect on the pollutant emissions was approximately the same
to the tests with the addition of each gas separately and no a synergetic effect among
them was observed.
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4. Conclusions
Combustion tests at oxy-firing conditions were carried out in a ~3 kWth continuous BFB
combustor with an anthracite coal as fuel and a limestone for sulfur retention. The effect
of the flue gas recirculation on pollutant emissions was analyzed.
A comparison of the results obtained feeding O2/CO2 (oxy-fuel combustion) and O2/N2
(enriched air combustion) as reactant gases showed that the most important effect of
CO2 was that the optimum temperature for SO2 retention shifted from ~860 ºC to 900-
925 ºC. The NO and N2O emissions were almost the same in both reacting atmospheres,
and the CO emission was slightly higher in oxy-fuel combustion than in enriched air
combustion conditions.
In oxy-firing operating conditions it was observed that:
- SO2 recirculation increased the Ca-based sorbent utilization and did not affect N2O
emission. At 850 ºC the CO emission increased and the NO emission decreased,
however, at 925 ºC the CO variation was negligible and the NO reduction was low.
- About 60-70% of the recycled NO was reduced to N2 and N2O. The reduction rate was
almost independent of the initial concentration of NO and temperature, and the
formation of N2O was very small. Less than 5% of the recycled NO was converted to
N2O. Neither SO2 nor CO emissions were affected by the NO recirculation.
- Steam recirculation produced a sharp decrease in NO emission, a slight increase in
N2O emission, and did not affect the CO emission. Steam affected the formation of NO
but once formed it did not have any influence in the NO reduction. In our tests, with a
mean residence time of solids in the combustor of ~2 hours, the effect of steam on
sulfation conversion was very small, at both 850 ºC (direct sulfation) and 925 ºC
(indirect sulfation).
- The combined effect of the recirculation of different gases was almost the same as the
sum of the effects of the recirculation of each gas separately and a synergetic effect
among them was not observed.
Acknowledgements
This work has been supported by Spanish Ministry of Science and Innovation
(MICINN, Project: CTQ2008-05399/PPQ) and by FEDER. M. de las Obras thanks
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MICINN for a FPI fellowship grant and A. Rufas thanks CSIC for a JAE fellowship
(grant co-funded by the European Social Fund).
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16
Captions for Tables and Figures
Table 1. Analysis of anthracite coal.
Table 2. Granicarb limestone characteristics.
Table 3. Feeding rates of solids and flow rate of gases used in the tests.
Figure 1. Oxy-fuel Bubbling Fluidized Bed Combustor at ICB-CSIC. Measurements of
temperature (T) and pressure (P).
Figure 2. Effect of temperature on SO2, CO, NO, and N2O emissions. Empty symbols:
enriched air conditions (O2/N2= 35/65). Filled symbols: oxy-fuel conditions (O2/CO2=
35/65)
Figure 3. Effect of SO2 recirculation on SO2, CO, NO, and N2O emissions.
Figure 4. Typical test showing the effect of NO recirculation on the SO2, CO, NO, and
N2O emissions. T=925ºC.
Figure 5. NO emission and reduction as a function of the recirculated NO
concentration. a= recirculated NO (vppm), b= increase in NO concentration at the outlet
due to NO recirculation (vppm), c= decrease of the recirculated NO (vppm)
Figure 6. NO and N2O emissions as a function of the recirculated NO concentration for
different inlet gas composition. Empty symbols: 850 ºC. Filled symbols: 925 ºC.
Figure 7. Increase of NO and N2O emissions as a function of the recirculated NO
concentration for different inlet gas compositions. Empty symbols: 850 ºC. Filled
symbols: 925 ºC.
Figure 8. Effect of steam recirculation on SO2, CO, NO, and N2O emissions.
Continuous lines = measured values (dry basis). Dotted lines = corrected values (wet
basis), Ci,out(wet)=Ci,out(dry)/(1-CH2O,in)
17
Table 1. Analysis of anthracite coal.
Proximate analysis (wt %)Moisture 2.3 Ash 31.7 Volatiles 5.6 Fixed C 60.4 Ultimate analysis (wt %, daf)C 90.36 H 2.53 N 1.41 S 2.30 LHVa (kJ/kg) 21807
a Lower Heating Value.
Table 2. Granicarb limestone characteristics.
Composition (wt %) CaCO3 97.1 MgCO3 0.2 Na2O 1.1 SiO2 <0.1 Al2O3 <0.1 Fe2O3 <0.1 Porosity (%)
Raw 3.7 Calcinedb 49
b Calcined in N2 atmosphere at 900 ºC for 10 minutes.
Table 3. Feeding rates of solids and flow rate of gases used in the tests.
Qin (lN/h) 2230 F0,coal (g/h) 576 ± 14 F0,limestone (g/h) 82 ± 2 F0,sand (g/h) 900 ± 20
18
Coal +
LimestoneSilicasand
Bubbling fluidized bed
Freeboard
Drainage deposit
Cyclone
Heat exchanger
Water-cooled screw-feeders
O2
T3
T2
T4
T5P3
AnalyzerCO2, CO, SO2, O2
Stack
Cool water
Hot water
T1
P1
Preheater
Air
CO2
NO
H2O
Analyzer (FTIR) NO, N2O, NO2
Evaporator
SO2
P2
Coal +
LimestoneSilicasand
Bubbling fluidized bed
Freeboard
Drainage deposit
Cyclone
Heat exchanger
Water-cooled screw-feeders
O2
T3
T2
T4
T5P3
AnalyzerCO2, CO, SO2, O2
Stack
Cool water
Hot water
T1
P1
Preheater
Air
CO2
NO
H2O
Analyzer (FTIR) NO, N2O, NO2
Evaporator
SO2
P2
Figure 1. Oxy-fuel Bubbling Fluidized Bed Combustor at ICB-CSIC. Measurements of
temperature (T) and pressure (P).
19
Temperature (ºC)
800 825 850 875 900 925 950 975 1000
SO
2 (v
pp
m,
dry
bas
is)
0
500
1000
1500
2000
2500
Temperature (ºC)
800 825 850 875 900 925 950 975 1000
CO
(vp
pm
, d
ry b
as
is)
0
200
400
600
800
1000
Temperature (ºC)
800 825 850 875 900 925 950 975 1000
N2O
(vp
pm
, d
ry b
asis
)
0
50
100
150
200
Temperature (ºC)
800 825 850 875 900 925 950 975 1000
NO
(vp
pm
, d
ry b
as
is)
0
200
400
600
800
Figure 2. Effect of temperature on SO2, CO, NO, and N2O emissions. Empty symbols:
enriched air conditions (O2/N2= 35/65). Filled symbols: oxy-fuel conditions (O2/CO2=
35/65)
20
Time (min)
0 20 40 60 80 100 120 140 160
SO
2 (
vpp
m,
dry
bas
is)
0
1000
2000
3000
4000
5000
6000
CO
, N
O,
N2O
(vp
pm
, d
ry b
asis
)
0
200
400
600
800
1000
NO
N2O
SO2
CO
NO
SO2
CO
SO2 recirculation850 ºC
Time (min)
0 20 40 60 80 100 120 140 160 180 200
CO
, N
O,
N2O
(vp
pm
, d
ry b
asis
)
0
100
200
300
400
500
600
700
800
SO
2 (
vpp
m,
dry
ba
sis
)
0
300
600
900
1200
1500
1800
SO2
NO
CO
N2O
SO2 recirculation 925 ºC
Figure 3. Effect of SO2 recirculation on SO2, CO, NO, and N2O emissions.
21
Time (min)
0 20 40 60 80 100
Ga
s co
nc
entr
ati
on
(vp
pm
, d
ry b
asis
)
0
200
400
600
800
1000
1200
SO2
NO
CO
N2O
recirculation 0 vppm
200350
500625
Figure 4. Typical test showing the effect of NO recirculation on the SO2, CO, NO, and
N2O emissions. T=925ºC.
22
NO
co
nce
ntr
atio
n (
vpp
m)
0
200
400
600
800
1000
1200
1400850 ºC
6255003502000
NO recirculated (vppm)
NO decreaseNO measured
925 ºC
6255003502000
NO recirculated (vppm)
a
b
c
Figure 5. NO emission and reduction as a function of the recirculated NO
concentration. a= recirculated NO (vppm), b= increase in NO concentration at the outlet
due to NO recirculation (vppm), c= decrease of the recirculated NO (vppm)
23
NO recirculated (vppm)
0 100 200 300 400 500 600 700
NO
mea
sure
d (
vpp
m)
0
200
400
600
800
1000
1200
NO
N2O
Non NO recirculation NO + SO2
NO + H2O
NO + SO2 + H2O
Figure 6. NO and N2O emissions as a function of the recirculated NO concentration for
different inlet gas compositions. Empty symbols: 850 ºC. Filled symbols: 925 ºC.
24
NO recirculated (vppm)
0 100 200 300 400 500 600 7000
50
100
150
200
250
300
NO
N2O
Non NO recirculationNO + SO2
NO + H2O
NO + SO2 + H2O
N
O a
nd
N
2O(v
pp
m)
Figure 7. Increase of NO and N2O emissions as a function of the recirculated NO
concentration for different inlet gas compositions. Empty symbols: 850 ºC. Filled
symbols: 925 ºC.
25
Time (min)
0 20 40 60 80 100 120
Ga
s c
on
cen
tra
tio
n (
pp
mv)
0
500
1000
1500
2000
2500
SO2
CO
NON2O
H2O recirculation 850 ºC
Time (min)
0 20 40 60 80 100 120
Ga
s c
on
cen
tra
tio
n (
vpp
m)
0
200
400
600
800
1000
1200
SO2
NO
CO
N2O
H2O recirculation 925 ºC
Figure 8. Effect of steam recirculation on SO2, CO, NO, and N2O emissions.
Continuous lines = measured values (dry basis). Dotted lines = corrected values (wet
basis), Ci,out(wet)=Ci,out(dry)/(1-CH2O,in)