ARTICLE pubs.acs.org/EF
Process Simulation of Emission and Control for NO and N2O during Coal Combustion in a Circulating Fluidized Bed Combustor Based on Aspen Plus Xuemin Yang,*,† Bing Liu,†,‡ Wenli Song,† and Weigang Lin† †
State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100039, P. R. China ABSTRACT: The profiles of temperature and gas component concentrations along the circulating fluidized-bed (CFB) riser height have been successfully simulated by the developed process simulation model of coal combustion in a 30 kW CFB combustor based on Aspen Plus. The average bed temperature increases from about 1025 to 1160 K along the CFB riser height of 6.63 m. The simulated effects of excess air percentage, the first stage stoichiometry, and the introducing position of secondary air on concentrations of gas components, especially NO and N2O, have good agreement with the experimentally measured results that increasing excess air percentage as well as the first stage stoichiometry can improve emissions of NO and N2O in flue gas, but coal rank also affects the pollutant emissions. The simulated results show that increasing the introducing position of secondary air reasonably can effectively decrease emissions of NO and N2O. The contribution ratios of all possibly existed reactions to the formation and decomposition of NO and N2O have been quantitatively predicted by the developed process simulation model to clarify the formation and emission mechanism of NO and N2O during CFB coal combustion.
1. INTRODUCTION Circulating fluidized bed (CFB) combustion technology has been rapidly developed and widely applied since the 1980s because of its high combustion efficiency, low pollutant emissions, and other advantages.13 However, higher NOx emission, especially higher N2O, from coal combustion in a CFB combustor is an obvious obstacle under the conditions of raising environmental protection consciousness from society as well as more strict legislations of limiting greenhouse gas emissions, although the CFB combustor emits lower NO compared to other combustion technologies. To deplete N2O emission and control NO emission during coal combustion in a CFB combustor, many new technologies,413 such as reverse air stage combustion911 and decoupling combustion,13 have been proposed and developed in the past few years. The emission behavior of NO and N2O during CFB coal combustion has been investigated in a 30 kW CFB combustor under conditions of changing the excess air number, the first stage stoichiometry, introducing the position of secondary air, the position of charging coal, and coal types or ranks.4,5,14,15 To further provide the difficultly obtained information on NO and N2O emissions from common experiments, mathematically simulating the formation and emission of NO and N2O along the CFB riser height is of importance by the newly developed process simulation model of coal combustion in a 30 kW CFB combustor based on Aspen Plus.16 The simulated results of formation and emission for NO and N2O by the developed process simulation model16 have been compared to the measured results during the combustion process of three coals in a 30 kW CFB combustor to verify the validity of the applied coal combustion kinetics model, especially r 2011 American Chemical Society
the macro-combustion reaction kinetics model of transferring char N to NO and N2O,16 and to further reveal the mechanism of formation and emission for NO and N2O along the CFB riser height. The ultimate objective of this study is to develop a universal simulation method to predict formation and emission of NO and N2O during CFB coal combustion and then to provide valuable information of emission and control for NO and N2O during the newly developed CFB decoupling coal combustion process.13
2. SIMULATED CASES BASED ON EXPERIMENTS AND OTHER CASES FOR PREDICTION The applied process simulation model for describing the coal combustion behavior in a CFB combustor based on Aspen Plus with some indispensable assumptions had been described in detail in a previous publication,16 including the gassolid hydrodynamics model and the macro-combustion reaction kinetics model. The chemical compositions of the applied three coals as coals A, B, and C are summarized in Table 1 as that reported elsewhere.16 The detailed parameters of configuration and operation procedures for the simulated 30 kW CFB combustor can be simply summarized as follows:16 (1) The 30 kW CFB combustor mainly consists of a riser with a height of 6.63 m and an inner diameter of 0.086 m, and a downer with a height of 3.0 m and an inner diameter of 0.039 m. (2) A cyclone was installed at the top of the downer to separate solid particles from flue gas, while a U-type valve was installed at the bottom of the downer. (3) A specially designed feeding system containing a screw feeder and a pneumatic feeder was Received: May 17, 2011 Revised: June 28, 2011 Published: June 29, 2011 3718
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Table 1. Chemical Compositions and Size Distributions of Three Applied Coals size distribution (mass %) proximate analysisa (mass %) coal type volatile matter ash fixed carbon
a
ultimate analysis (mass %) C
H
size range (mm)
N
S
Ob
0.600.45 0.450.355 0.3550.28 0.280.20 0.200.16 0.160.154
coal A
36.2
5.5
58.3
72.4
4.6
1.3
0.5
15.7
40.6
9.4
16.6
13.8
coal B
30.7
18.4
50.9
65.8
4.2
1.2
1.6
8.8
32.1
9.8
18.7
9.9
coal C
26.4
13.1
60.5
73.0
3.7
1.2
0.6
8.4
49.7
13.5
16.5
11.2
4.7 15 4.4
14.9 14.5 4.7
On a dry basis. b By difference.
Figure 1. Transformation schemes of key elements from coal into gas pollutants applied in the developed process simulation model of coal combustion in a CFB combustor. installed at the position just below a solid particles storage hopper on the downer for charging silica sands. (4) The silica sands with an average diameter of 0.255 mm were applied as a solid heat carrier. (5) The pulverized coal powders were introduced from the specially designed feeding system into the CFB riser at a position of 0.020 m above the gas distributor. (6) The primary air was preheated to 473 K and charged into the riser through the bottom of a gas distributor to ignite the charged coal powders easily. (7) The secondary air at ambient temperature was tangentially inlet into the riser through a port located at 1.66 or 2.8 m above the gas distributor. The applied chemical reaction equations, expressions of the chemical reaction rate, and corresponding reaction rate constants for describing combustion of pyrolysis products in the CFB riser are also summarized in the developed process simulation model elsewhere.16 To illustrate the combustion transformation of elements C, H, and N in pyrolysis products, the applied 16 chemical reactions are labeled in Figure 1 as that in the developed process simulation model elsewhere.16
The steady process of coal combustion in the CFB combustor can be characterized by the built-in modules in Aspen Plus containing the gassolid hydrodynamics model, equivalent pyrolysis model, and macro-combustion kinetics model of pyrolysis products as described elsewhere16 and represent it again in Figure 2 to easily understand the developed process model.16 Certainly, it can be summarized from Figure 2 as follows: (1) The pyrolysis process of charged coal powders at the CFB riser bottom is represented by the built-in module of the reactor yield (RYield) and stoichiometry reactor (RStoic1) in Aspen Plus with specified reaction contents or conversion ratios, as summarized in Table 2. (2) The oxidation of both generated H2S from coal pyrolysis and the residual sulfur element into SO2 by the oxygen element in coal particles is represented by the module of the stoichiometry reactor (RStoic2). (3) The combustion process of pyrolysis products along the CFB riser height is represented by five modules of the continuous stirred tank reactor (RCSTRi, with i = 1, 2, 3, 4, and 5). (4) The cyclone separation of unburned char particles and silica sands from flue gas is represented by a module of a separator (Sep1). (5) The 3719
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Figure 2. Constructed flow sheet of the CFB coal combustion process based on Aspen Plus.
Table 2. Conversion Ratio of Key Components in Each Combination Reaction of pyrolysis gas for Three Coals at 1023 K conversion ratio (102) combination reaction
key component coal A
coal B
coal C
C + 2H2 f CH4
C(graphite)
1.984
2.378
2.199
C + O2 f CO2
C(graphite)
0.677
0.416
0.257
C + 0.5O2 f CO
C(graphite)
1.851
1.196
0.873
H2 + 0.5O2 f H2O
H2
10.382
7.254
6.546
N2 + 3H2 f 2NH3
H2
0.382
0.419
0.476
0.5N2 + C + 0.5H2 f HCN
H2
0.100
0.110
0.125
H2 + S f H2S
H2
0.677
2.375
1.011
mixing between silica sands and unburned char particles from flue gas is represented by a module of a hopper-type mixer (Hopper). (6) The heat exchange among the existing silica sands, unburned char particles, and the newly charged silica sands at ambient temperature in the CFB downer is represented by a module of a heater (Heater2). (7) The ash discharging operation from the CFB riser bottom is represented by a
module of a separation (Sep2). (8) The heating process of primary air is represented by a module of a heater (Heater 1). The mixing of the heated primary air with solid substances from the CFB downer is represented by a module of a mixer (Mixer1). (9) The mixing of secondary air at ambient temperature and substances from RCSTR2 is presented by a mixer module (Mixer2). The size decrease of silica sands by friction and that of coal powders by combustion and friction were not considered in the developed process simulation model as an assumption.16 Therefore, the elutriation behavior was not included in the developed process simulation model16 for simplification of the model. The process-simulated cases for the investigated three coals can be summarized as follows: (1) The temperature profiles along the CFB riser height are simulated for three coals under the specified CFB operation conditions and compared to the measured temperature profiles under the same CFB conditions. (2) The concentration profiles of major gas components along the CFB riser height are calculated for three coals under a specified CFB operation condition and compared to the measured results at the CFB riser top under the same CFB operation condition. (3) The effects of excess air and the first stage stoichiometry on the concentration of gas components, especially on the emission of NO and N2O for three coals in flue gas have been simulated under the 3720
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specified CFB condition and compared to the measured results under the same CFB operation condition. (4) The effect of changing the introducing position of secondary air on the emission of gas products is simulated and predicted. (5) The emission of NO and N2O under other CFB operation conditions without experimental results is also simulated. (6) The contribution of the specified reactions on the formation and decomposition of NO and N2O in the cascade five subunits along the CFB riser height is quantitatively calculated. To compare the simulated and measured results of the gas concentration to those reported in other references, all gas concentrations in flue gas or at the outlet of the CFB cyclone, except O2, have been converted to those of flue gases containing 6% O2 on a dry basis by yi, 6% ¼
yi, out λO2 , out 21 , λO2 , out ¼ 21 yO2 , out 1:4
ð1Þ
where yi,6% is the converted concentration of component i based on the concentration of O2 as 6% on a dry basis (102 or 10 6), yi,out is the measured concentration of component i in flue gas or at the outlet (102 or 106), λO2,out is the excess air number based on the measured O2 concentration in flue gas or at the outlet, 1.4 is the excess air number based on the measured concentration of O2 as 6%, and yO2,out is the measured O2 concentration in flue gas or at the outlet (102). However, all of the calculated gas concentrations in the CFB riser or concentration profiles along the CFB riser height, such as O2, CO2, NO, N2O, CO, and H2O, do not convert from eq 1.
3. PROCESS SIMULATION RESULTS AND DISCUSSION 3.1. Temperature Profiles along the CFB Riser Height. The simulated and measured13 temperature profiles along the CFB riser height during combustion for three kinds of coals in the 30 kW CFB combustor are shown in Figure 3 under the specified CFB combustor operation conditions listed in Table 3.16 It can be observed from Figure 3 that the simulated temperature profiles have a relative consistency with the measured temperature profiles in a 20 K error range, except at top of the CFB riser for three coals. The temperatures sharply increase with an increase of the CFB riser height in the dense region at the lower CFB riser, and a smooth increase can be observed in the dilute region at the upper CFB riser. The temperature increase along the CFB riser height can be explained as follows: (1) The released heat from coal combustion with the primary air can contribute to the sharp increase of the temperature in the dense region at the lower CFB riser. The released heat from char combustion with secondary air can further improve the temperature in the dilute region at the upper CFB riser. (2) The heat accumulation effect plays a key role to improve the temperature along the CFB riser height because five modules of RCSTRi (i = 1, 2, 3, 4, and 5) as adiabatic reactors have been used to represent combustion reactions in the CFB riser. However, the measured temperature is obviously lower than the simulated temperature at a height of 6.63 m for three coals because the insulation at the CFB riser top is not good enough for reducing heat loss into the ambient atmosphere. 3.2. Gas Component Concentration Profile along the CFB Riser Height. The simulated concentration profiles of O2, CO2, NO, N2O, and CO along the CFB riser height during combustion of three kinds of coals in the 30 kW CFB combustor are illustrated in Figure 4 under the CFB operation conditions listed in Table 3. The measured16 concentrations of O2, CO2, N2O,
Figure 3. Comparison of calculated and measured temperature profiles along the CFB riser height in a 30 kW CFB combustor for three applied coals.
Table 3. Operation Conditions of a 30 kW CFB Combustor for Three Coals coal feed
flow rate of
flow rate of
theoretically
primary air of secondary
flow rate
coal type rate (kg 3 h1) required air (m3 3 h1) (m3 3 h1) air (m3 3 h1) coal A
2.85
22.43
18.40
7.50
coal B coal C
3.15 2.55
23.31 20.22
22.40 20.20
0.00 3.00
NO, and CO in flue gas are also given in Figure 4 for comparison to the simulated results under the same CFB operation conditions for three coals. It should be specially emphasized that all calculated gas concentrations along the CFB riser height in Figure 4 have not been converted to those containing 6% O2 on a dry basis by eq 1, and only the calculated concentrations in Figure 4 are plotted. Although there are obvious differences between the simulated and measured concentrations for some gas components, such as NO and N2O for coal A, O2 and N2O for coal B, and CO and NO for coal C, shown in Figure 4, the reliable variation tendency of concentrations for five gas components along the CFB riser height can be observed as follows: (1) The O2 concentration decreases with the proceeding of coal combustion reactions along the CFB riser height from 21 to about 36%. (2) The CO2 concentration increases from 0 to 1315% with the proceeding of coal combustion reactions along the CFB riser height. (3) The concentrations of NO and N2O increase from 0 106 to the maximum values along the CFB riser height in the dense region at the lower CFB riser, sharply decrease to a value as inletting secondary air, and then fluctuate in a narrow range with further increasing of the CFB riser height for coals A and C. (4) The CO concentration generated from the formation of coal pyrolysis products by stable elementary substances as listed in Table 216 decreases from 15004500 106 to 160230 106 along the CFB riser for three kinds of coals. The generated CO concentration from the formation of coal pyrolysis products by stable elementary substances16 for coal A is much larger than that for coals B and C, as described in Table 2.16 It should be specially emphasized that no sharp change of the concentration for O2, CO2, N2O, NO, and CO can be found in panels ae of Figure 4 at the CFB riser height of 1.67 m for coal B because no secondary air is charged for CFB combustion of coal B, as listed in Table 3. 3721
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Figure 4. Calculated gas concentration profiles of O2, CO2, NO, N2O, and CO along the CFB riser height and measured concentration of corresponding components in flue gas in a 30 kW CFB combustor for three applied coals.
Table 4. Operation Conditions of a 30 kW CFB Combustor for Three Coals at Different Excess Air Percentagesa flow rate of air at 298.15 K (m3 3 h1) coal A
a
coal B
coal C
test number
excess air percentage (102)
Qprimary air
Qsecondary air
Qprimary air
Qsecondary air
Qprimary air
Qsecondary air
1
10
11.215
13.458
15.149
10.488
12.132
10.110
2
20
13.458
13.458
17.480
10.488
14.154
10.110
3
30
15.701
13.458
19.810
10.488
16.176
10.110
4
40
17.944
13.458
22.141
10.488
18.198
10.110
5
50
20.187
13.458
24.472
10.488
20.220
10.110
6
60
22.430
13.458
26.802
10.488
22.242
10.110
7
70
24.264
10.110
8
80
26.286
10.110
The feeding rates of three coals are the same as those listed in Table 3.
3.3. Simulation and Prediction of Excess Air on Gas Components. The simulated relationship between concentra-
tions of CO2, CO, NO, N2O, SO2, and H2O in flue gas and the excess air percentage in an excess air percentage range from 10 to 60% during combustion of three kinds of coals in the 30 kW CFB
combustor under conditions listed in Table 4 is illustrated in Figure 5. The measured13 concentration of NO and N2O in flue gas is illustrated in panels c and d of Figure 5 for comparison. The relation between the excess air percentage and the calculated concentrations in Figure 5 is plotted. Certainly, the basic effect of 3722
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Figure 5. Relation between calculated concentration of CO2, CO, NO, N2O, SO2, and H2O in flue gas and excess air percentage and a comparison between calculated and measured concentrations of NO and N2O under conditions of various excess air percentages in a 30 kW CFB combustor for three applied coals.
the excess air percentage on the concentration of CO2, NO, N2O, CO, SO2, and H2O in flue gas can be reliably simulated by the developed process simulation model.16 The reasonable increase of CO2 concentration can be simulated with an increase of excess air percentage from 10 to 60%, while a slight decrease or stable tendency of the concentration for H2O, CO, and SO2 can be simulated with an increase of excess air percentage for three studied coals. The slight increase tendency of the simulated CO2 concentration in the flue gas is caused by complete combustion with an increase of excess air percentage. To pay more attention to the effect of the excess air percentage on NO and N2O emissions, the effect of the excess air percentage on other concentrations of gas components will be ignored in the following explanation. Figure 5c shows that increasing excess air percentage can lead to an increase of NO emission for three coals. The difference between the simulated and measured results in Figure 5c can be explained as follows: (1) A larger excess air percentage implies a higher O2 concentration in the CFB combustor, hence, a larger reaction probability of char N with O2 can make more NO formed. (2) The oxidation of NH3 by O2 can be intensified to form more NO with a larger excess air percentage. (3) A higher excess air percentage corresponds a lower voidage as well as less
char particle numbers in the CFB riser and a smaller CO concentration in the dilute region at the upper CFB riser. Therefore, the heterogeneous reactions of the reduction of NO on the char particle surface15 represented in eqs 24 will be effectively inhibited as4,5,14,15,17 NO þ char C f NCO
ð2aÞ
NCO þ NO f N2 O þ CO
ð2bÞ
NO þ char N f N2 O
ð3Þ
NO þ char NCO f N2 O þ CO
ð4Þ
Hence, more NO can be generated under the condition of a larger excess air percentage during CFB coal combustion, as shown in Figure 5c. The simulated results in Figure 5d show that increasing excess air percentage can obviously improve N2O emission for three kinds of coals as reported by De Diego et al.1,2,15 and Zhao et al.14,15,18 However, a relatively complex effect of the excess air percentage on N2O emission has been reported13 during combustion for the three kinds of investigated coals because increasing excess air percentage can slightly improve N2O emission for 3723
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Table 5. Operation Conditions of a 30 kW CFB Combustor for Three Coals under Conditions of Various Values of the First Stage Stoichiometry with an Excess Air Percentage of 30%a flow rate of air at 298.15 K (m3 3 h1) coal A
coal B
test number
Qprimary air
Qsecondary air
Qprimary air
Qsecondary air
Qprimary air
Qsecondary air
1
15.701
13.458
16.314
13.984
14.154
12.132
2
17.944
11.215
18.645
11.653
16.176
10.110
3 4
20.187 22.430
8.972 6.729
20.976 23.306
9.322 6.992
18.198 20.220
8.088 6.066
5
24.673
4.486
25.637
4.661
6 a
coal C
22.242
4.044
24.264
2.022
The feeding rates of three coals are the same as those listed in Table 3.
coals A and B and slightly decrease N2O emission for coal C. The results of improving N2O emission for coals A and B by increasing excess air percentage based on the simulated and measured results, which are similar to the results by De Diego et al.1,2,15 and Zhao et al.,14,15,18 can be explained as follows: (1) A higher O2 concentration from a larger excess air percentage is a benefit to the reaction of char N with O2 to form N2O. (2) The higher O2 concentration can intensify the homogeneous oxidation reactions of released HCN and NH i, especially HCN, in volatile matter from coal pyrolysis to form N2O as follows:4,5,14,15,17 HCN þ O2 f NCO þ H2
ð5aÞ
NCO þ O2 f NO þ CO
ð5bÞ
NCO þ NO f N2 O þ CO
ð5cÞ
NH þ O2 f NO þ H2
ð6aÞ
NH2 þ NO f N2 O þ H2
ð6bÞ
NH3 þ NO f N2 O þ CO
ð6cÞ
However, the negative contribution of the heterogeneous reactions of the NO reduction on the char particle surface15 for forming N2O as expressed in eqs 3 and 4 will be weaken by decreasing char particle numbers in the CFB riser under the condition of a higher excess air percentage. Generally speaking, the amount of formed N2O is mainly controlled by the homogeneous reactions expressed in eqs 5c, 6b, and 6c. The contribution of the reverse heterogeneous reactions for converting N2O to NO can counteract or exceed that of forming N2O by the homogeneous oxidation reactions in eqs 5c, 6b, and 6c for coals with lower volatile matter and higher fixed carbon during CFB combustion.5,15 Therefore, decreasing or keeping stable N2O emission by increasing excess air percentage is also possible for some coals with lower volatile matter and higher fixed carbon, such as coal C. The applied coals A and B have higher volatile matter as 36.2 and 30.7 mass %, respectively. Therefore, N2O emission is mainly governed by the homogeneous oxidation reactions in eqs 5c, 6b, and 6c, i.e., the greater excess air percentage, the lower N2O emission. However, the lower volatile matter as 26.4 mass % and higher fixed carbon as 60.5 mass % in coal C can make the reverse
heterogeneous reactions of the NO reduction on the char particle surface in eqs 2 and 3 play an important role in N2O emission, i.e., a greater excess air percentage corresponds to a lower or stable N2O emission. The same treatment method for three kinds of coals has been applied in the developed process simulation model based on Aspen Plus.16 The differences of coal rank for three coals have not been considered in the process simulation model.16 This may be the main reason for not correctly simulating the relationship between the excess air percentage and N2O emission for coal C. Accurately characterizing coal composition by Aspen Plus is a challenging task to be solved in a further study. 3.4. Simulation and Prediction of First Stage Stoichiometry on Gas Components. The simulated relationship between concentrations of CO2, CO, NO, N2O, SO2, and H2O in flue gas and the first stage stoichiometry, i.e., volume ratio of the primary air amount to the theoretically required air amount, in a first stage stoichiometry from 0.721.19 with a stable excess air percentage of 30% during combustion of three kinds of coals in the 30 kW CFB combustor under CFB operation conditions listed in Table 5 is illustrated in Figure 6. The measured13 concentration of NO and N2O is given in panels c and d of Figure 6 for comparison. The relation between the first stage stoichiometry and calculated concentrations in Figure 6 are plotted. Obviously, the basic effects of the first stage stoichiometry on the concentration of CO2, CO, NO, N2O, SO2, and H2O in flue gas can be reliably simulated by the developed process simulation model.16 A higher first stage stoichiometry means to some degree a longer residence time of char particles under the condition of a constant flow rate of the total input air, hence, the complete combustion can lead to a lower O2 and a smaller CO concentration in flue gas. No obvious change of the concentration for H2O and SO2 can be caused by an increase of the first stage stoichiometry, because sulfur in three coals is assumed as equivalent elementary substances of the pyrolysis product16 and can be rapidly converted into SO2 via H2S in the developed simulation model.16 To focus more attention on the effect of the first stage stoichiometry on NO and N2O emissions, the effect of the first stage stoichiometry on other concentrations of gas components will be omitted in the explanation below. It is observed from Figure 6c that decreasing the first stage stoichiometry can effectively result in a decrease of NO emission for three coals, although there is an obvious difference between the simulated and measured emissions of NO and N2O under the condition of the first stage stoichiometry of less than 0.9 for coals 3724
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Figure 6. Relation between calculated concentration of CO2, CO, NO, N2O, SO2, and H2O in flue gas and first stage stoichiometry and a comparison between calculated and measured concentrations of NO and N2O under conditions of various values of first stage stoichiometry in a 30 kW CFB combustor for three applied coals.
A and C. Deceasing the first stage stoichiometry can effectively increase the O2-lean region in the dense region at the lower CFB riser and further decrease the reaction rate of forming NO by R(9) in Figure 1; meanwhile, the higher mass ratio of the charged coal particles or char particles and the greater amount of reducing components, such as CO, in the dense region at the lower CFB riser can promote the reduction reactions of formed NO to N2 by R(12) and R(13) in Figure 1. However, the complex relationships between the first stage stoichiometry and N2O emission can be observed for three studied coals from the simulated and measured13 in Figure 6d. The simulated relationship between the first stage stoichiometry and N2O emission is basically consistent with that of the measured13 for coals A and B, i.e., decreasing the first stage stoichiometry can lead to a limited decrease of N2O emission for coals A and B. The reasons for this result can be summarized as follows: (1) The O2-lean region in the dense region at the lower riser because of the decrease of the first stage stoichiometry is a benefit to promote the decomposition reactions of forming N2O by the reverse homogeneous reactions in eqs 5c, 6b, and 6c and reverse heterogeneous reactions in eqs 24, described in section 3.3.1,5,15,18 (2) More precursors of N2O, such as HCN and NHi, especially HCN, can be generated in the dense region at the lower riser to deplete N2O formation under the condition of
decreasing the first stage stoichiometry. The comprehensive or interactive effects of the above-mentioned two reasons on N2O formation can decrease the effect of decreasing the first stage stoichiometry on N2O emission. Therefore, no obvious or a slight decrease of N2O emission can be found from Figure 6d for coals A and B by decreasing the first stage stoichiometry. The inconsistent results between the simulated and measured13 for coal C shown in Figure 6d can be explained with the same reasons described in section 3.3, i.e., the lower volatile matter and higher fixed carbon in coal C compared to coals A and B can make higher N2O emission under the condition of the smaller first stage stoichiometry because N2O formation is controlled by the heterogeneous reactions in eqs 2415 rather than the homogeneous reactions in eqs 5c, 6b, and 6c, described in section 3.3.1,5,15,18 Therefore, the characteristics of coal composition, including coal-rank differences, should be given more attention in the process simulation model. The comprehensive effects of both the excess air percentage and first stage stoichiometry on the simulated NO and N2O emissions are illustrated in Figure 7 for three kinds of coals. The concentrations of NO and N2O in Figure 7 have been converted to those containing 6% O2 on a dry basis by eq 1. Certainly, the optimal parameters of both excess air and first stage stoichiometry can be obtained from the minimum emission of NO and 3725
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Figure 7. Prediction of the comprehensive effect of both excess air and first stage stoichiometry on NO and N2O concentrations in flue gas in a 30 kW CFB combustor for three applied coals.
N2O during combustion of three kinds of coals in the CFB combustor shown in Figure 7. 3.5. Prediction of the Introducing Position for Secondary Air on Concentration Profiles of Gas Components. The reasonable introducing position of secondary air is a key CFB operation parameter to optimize the effect of the first stage stoichiometry on NO and N2O emissions. Artificially changing
the introducing position of secondary air is certainly limited in a fixed CFB combustor. The process simulation is considered as a cost-saving method to determine the optimal introducing position of secondary air. The simulated concentration profiles of O2, CO2, NO, N2O, and CO along the CFB riser height introducing secondary air at 1.66 and 2.80 m are illustrated in Figure 8 for coal A with an excess air percentage of 30% under CFB operation 3726
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Figure 8. Calculated concentration profiles of O2, CO2, NO, N2O, and CO under the condition of introducing secondary air at 1.66 and 2.80 m along the CFB riser height in a 30 kW CFB combustor for coal A.
conditions as listed in Table 3. The gas concentrations in Figure 8 have not been converted to those containing 6% O2 on a dry basis by eq 1. However, the O2 concentration in flue gas or at the outlet of the CFB cyclone is about 6%, this means that the unconverted gas concentration in Figure 8 can be used to compare to the reported gas concentration from the literatures. As shown in panels a and b of Figure 8, changing the introducing position of secondary air from 1.66 to 2.80 m can make an obvious variation of concentration profiles for O2 and CO2 along the CFB riser height, although there is no obvious change of concentrations of both O2 and CO2 in flue gas at two different introducing positions for secondary air. The simulated results show that increasing the introducing position of secondary air from 1.66 to 2.80 m can lead to a decrease of the molar flow rate of O2 at the outlet of module RCSTR2 in Figure 2 from 1.50 105 to 1.29 105 kmol s1 and an increase of the molar flow rate of CO2 at the outlet of module RCSTR2 from 2.34 105 to 2.55 105 kmol s1 in the dense region at the lower CFB riser simultaneously. The simulated residence time of O2 in the dense region at the lower CFB riser will increase from 0.408 to 0.724 s because the space of the dense region is enlarged by increasing the introducing position of secondary air. The lower CO concentration in flue gas shown in Figure 8e can be
explained by the reaction rates of R(1) and R(2) in Figure 1 in model RCSTR5 in Figure 2 increasing from 3.14 106 to 6.69 106 kmol s1 and from 2.50 106 to 5.20 106 kmol s1, respectively, by increasing the O2 concentration in the dilute region at the upper CFB riser with the increasing introducing position of secondary air, as shown in Figure 8a. As shown in panels c and d of Figure 8, increasing the introducing position of secondary air from 1.66 to 2.80 m can result in decreasing concentrations of both NO and N2O along the CFB riser height for coal A. The reasons for the decreasing NO profile shown in Figure 8c can be explained as follows: (1) Increasing the introducing position of secondary air can expand the O2-lean zone, therefore, gas and solid particles will have a longer residence time in the dense region at the lower CFB riser, where a stronger reducing atmosphere can be formed to benefit NO decomposition via R(12) and R(13) in Figure 1. (2) Formed NO in the expanded O2-lean zone within the dense region at the lower riser can be reduced15 by the accumulated char particles to form N2O by eqs 24. The conflicting conclusions of changing the introducing position of secondary air on N2O emission have been reported by different researchers,19 such as enhancing or weakening or no influence on N2O emission. It is obvious that increasing the 3727
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Table 6. Calculated Reaction Rates of 16 Chemical Reactions in Five RCSTR Modules in a 30 kW CFB Combustor under CFB Operation Conditions Listed in Table 3 for Coal A chemical reaction rate (kmol 3 s1 3 m3) reaction number
chemical reaction equations
RCSTR1
RCSTR2
RCSTR3
RCSTR4
RCSTR5
(1)
C + (1/ϕ)O2 f (2 (2/ϕ))CO + ((2/ϕ) 1)CO2
3.20 103
1.87 103
9.17 104
4.45 104
3.26 104
3
3
4
4
2.61 104
6
1.93 106 4.44 1010
(2)
CO + (1/2)O2 f CO2
2.74 10
6
1.55 10
6
3.60 10
6
(3) (4)
CO2 + C f 2CO CH4 + (3/2)O2 f CO + 2H2O
1.84 10 1.73 104
(5)
H2 + (1/2)O2 f H2O
3.33 103
0.00
0.00
0.00
0.00
(6)
NH3 + (5/4)O2 sf NO + (3/2)H2O
5.47 106
4.05 109
1.21 1011
1.44 1010
4.87 1013
(7)
NH3 + (3/4)O2 sf (1/2)N2 + (3/2)H2O
5.47 106
4.05 109
1.93 1011
3.84 1013
3.89 1013
6
6
(8) (9)
char char
HCN + (3/4)O2 f (1/2)H2 + CO + (1/2)N2O
7.12 10
6
[N]fuel + (1/2)O2 f NO
8.02 10
15
4.73 10 4.42 105
7.32 10
1.16 10
6
1.16 10
15
2.07 10 2.89 106
1.79 10 4.75 108
7
1.28 10
8.88 10
7
15
1.54 10
8
1.83 109
2.29 10
7
9.18 108
15
2.31 1014
(10)
(1/2)N2 + (1/2)O2 f NOthermal
1.05 10
2.72 10
5.18 10
8.34 10
(11) (12)
[N]fuel + (1/4)O2 f (1/2)N2O NO + C f CO + (1/2)N2
1.76 106 1.02 106
6.72 107 1.23 106
3.11 107 3.64 107
1.41 107 2.73 107
7.25 108 2.42 107
(13)
NO + CO f CO2 + (1/2)N2
2.04 1010
2.83 1010
2.91 1010
4.45 1010
4.72 1010
(14)
N2O + C f CO + N2
3.86 107
5.49 107
1.57 107
1.04 107
1.10 107
(15)
N2O + CO f CO2 + N2
3.44 107
2.24 107
8.49 108
4.21 108
2.76 108
(16)
N2O + (1/2)O2 f N2 + O2
1.05 107
1.31 107
1.13 107
1.19 107
1.15 107
Table 7. Contribution Ratios of Related Element N Transformation Reactions to the Formation or Decomposition of NO and N2O in Five RCSTR Modules in a 30 kW CFB Combustor under CFB Conditions Listed in Table 3 for Coal A contribution ratio of the chemical reaction (102) reaction number
NO formation
NO decomposition N2O formation
N2O decomposition
chemical reaction equation
RCSTR1
char
RCSTR2
RCSTR3
RCSTR4
RCSTR5
R(6)
NH3 + (5/4)O2 sf NO + (3/2)H2O
40.52
0.35
0.00
0.06
0.00
R(9)
[N]fuel + (1/2)O2 f NO
59.48
99.65
100.00
99.94
100.00
R(10)
(1/2)N2 + (1/2)O2 f NOthermal
0.00
0.00
0.00
0.00
0.00
R(12)
NO + C f CO + (1/2)N2
99.98
99.98
99.92
99.84
99.80
R(13) R(8)
NO + CO f CO2 + (1/2)N2 HCN + (3/4)O2 f (1/2)H2 + CO + (1/2)N2O
0.02 80.23
0.02 63.25
0.08 29.10
0.16 9.85
0.20 2.46
R(11)
[N]fuel + (1/4)O2 f (1/2)N2O
19.77
36.75
70.90
90.15
97.54
R(14)
N2O + C f CO + N2
46.26
60.67
44.17
39.27
43.45
R(15)
N2O + CO f CO2 + N2
41.20
24.80
23.94
15.88
10.92
R(16)
N2O + (1/2)O2 f N2 + O2
12.54
14.53
31.89
44.85
45.63
introducing position of secondary air can not only expand the O2lean zone but also change the circulating characteristics of solid particles, residence time of solid particles, and temperature profile in the CFB riser.19 The simulated result that N2O emission for coal A can decrease by increasing the introducing position of secondary air shown in Figure 8d is similar to the reported results by Zhao et al.18 The smaller temperature profile as shown in Figure 3 is maybe the major reason for the simulated decrease of N2O emission18 by increasing the introducing position of secondary air for coal A. 3.6. Contribution of Related Reactions to the Formation and Decomposition of NO and N2O. Not only the concentrations of gas components in flue gas but also concentration profiles along the CFB riser height can be quantitatively simulated by the developed process simulation model16 for CFB coal combustion. The calculated reaction rates of 16 applied reactions listed in Figure 1 are summarized in Table 5 under CFB operation conditions listed in Table 3 for coal A. The relative contribution ratios of R(6), R(9), and R(10) on forming NO,
R(12) and R(13) on decomposing NO, R(8) and R(11) on forming N2O, and R(14), R(15), and R(16) on decomposing N2O are summarized in Table 6. It can be concluded from Tables 6 and 7 that NO formation is simultaneously controlled by R(6) and R(9) but NO decomposition is dominated by R(12) in module RCSTR1. Hence, the formation and decomposition of NO in module RCSTRi (i = 2, 3, 4, and 5) are dominated by R(9) and R(12), respectively. This means that fuel N is the main resource of NO along the CFB riser height, while NH3 as the precursor of NO only plays an important role in the dense region at the lower CFB riser, i.e., pyrolysis zone. No quantifiable thermal NO, i.e., NOthermal, can be formed during CFB coal combustion, which is in good agreement with the literatures.20,21 The char particles account for almost the whole contribution of NO decomposition along the CFB riser height during CFB coal combustion by R(12). A negligible contribution of CO to the reduction of NO via R(13) can be quantitatively simulated. 3728
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Energy & Fuels The contribution of R(8) on N2O formation shows a decreasing tendency from 80.23 to 2.46%; however, the contribution of R(11) on N2O formation shows an increasing tendency from 19.77 to 97.54% along the CFB riser height. The importance of R(11) will be 2 times greater than that of R(8) above the introducing position of secondary air. This means that HCN as the precursor of N2O from coal pyrolysis plays an important role in the dense region at the lower CFB riser, while fuel N or char N, i.e., [N]fual, accounts for preponderant contribution to N2O formation in the dilute region at the upper CFB riser. The decomposition of N2O is controlled by char reduction via R(14), CO reduction via R(15), and O2 oxidization via R(16) simultaneously. R(14) has at least 40% contribution along the CFB riser height, and a maximum contribution, i.e., about 60%, can be found in module RCSTR2 for R(14) to the decomposition of N2O. The contribution of R(15) shows a decreasing trend from 41.2 to 10.92% to N2O decomposition along the CFB riser height, while an increasing tendency of R(16) can be obtained from 12.54 to 45.63% on N2O decomposition along the CFB riser height. This means that N2O decomposition is dominated by the reduction of both char and CO in the dense region at the lower riser, while N2O oxidation by O2 will account for at least 31.89% on N2O decomposition in the dilute region at the upper riser.
4. CONCLUSIONS A process simulation model has been developed to simulate the emission behavior of NO and N2O during coal combustion in a 30 kW CFB combustor for three coals coupled with the gassolid hydrodynamics via Aspen Plus in-line FORTRAN codes and the combustion reaction kinetics via some external FORTRAN subroutines simultaneously as described in a previous publication.16 The simulated results by the developed process simulation model of CFB coal combustion based on Aspen Plus16 have been compared to the measured results obtained in the simulated CFB combustor. The good agreement between the simulated and measured results shows that the developed process simulation model of CFB coal combustion based on Aspen Plus16 can be successfully used to describe the basic characteristics of both coal combustion and gas pollutant emission in a CFB combustor. The main summary remarks can be obtained as follows: (1) The temperature profile can be successfully simulated as that the temperature increases sharply from the CFB riser bottom to the introducing position of secondary air and then maintains a relatively stable value above the introducing position of secondary air along the CFB riser height. (2) The concentration profiles of major gas components, such as O2, CO2, NO, N2O, and CO, can be successfully simulated along the CFB riser height during CFB combustion for three investigated coals. The simulated results of concentrations for these gas components in flue gas are in basic agreement with the measured ones. This means that the not easily obtained concentrations of gas components at different CFB riser heights can be reliably predicted by the developed process simulation model.16 (3) The measured results, in which increasing excess air percentage and the first stage stoichiometry can increase emissions of NO and N2O in flue gas for coals A and B, can be successfully predicted by the developed process simulation model.16 However, the influence of the coal rank on the relation between the emissions of NO and N2O and excess air percentage or the first stage stoichiometry cannot be accurately simulated by the developed process simulation model16 for coal C. (4) The
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simulated results show that reasonably raising the introduction position of secondary air can effectively decrease emissions of NO and N2O by taking coal A as an example. (5) The contribution of all possibly existing reactions to the formation and decomposition of NO and N2O can be quantitatively predicted by the developed process simulation model16 by taking coal A as an example. The NO formation along the CFB riser height is predominated by oxidation of fuel N, while the decomposition of NO is controlled by the reduction of char particles in the CFB riser. The N2O formation is controlled by oxidation of HCN as the coal fast pyrolysis product in the region below the introducing position of secondary air, while the oxidation of fuel N will take a dominant role in the region above the introducing position of secondary air. The N2O decomposition is comprehensively controlled by the reduction reactions of N2O with char and CO in the region below the introducing position of secondary air, while the N2O decomposition is comprehensively dominated by the reduction reaction of N2O with char as well as the oxidation reaction of N2O by oxygen in the region above the introducing position of secondary air.
’ AUTHOR INFORMATION Corresponding Author
*Telephone/Fax: 86-10-82622893. E-mail: yangxm71@home. ipe.ac.cn or
[email protected].
’ ACKNOWLEDGMENT The financial support of this work by the Natural Sciences Foundation of China (Project 50576101) is kindly appreciated. ’ REFERENCES (1) de Diego, L. F.; Londono, C. A.; Wang, X. S.; Gibbs, B. M. Influence of operating parameters on NOx and N2O axial profiles in a circulating fluidized bed combustor. Fuel 1996, 75 (8), 971–978. (2) Muzio, L. J.; Quartucy, G. C. Implementing NOx control: Research to application. Prog. Energy Combust. Sci. 1997, 23 (3), 233–266. (3) Basu, P. Combustion of coal in circulating fluidized-bed boilers: A review. Chem. Eng. Sci. 1999, 54 (22), 5547–5557. (4) Xie, J.; Yang, X.; Zhang, L.; Ding, T.; Yao, J.; Song, W.; Lin, W.; Guo, H. Behavior of NO, N2O and SO2 emissions during coal combustion in a circulating fluidiized bed combustor. J. Fuel Chem. Technol. 2006, 34 (2), 9. (5) Zhang, L.; Yang, X.; Xie, J.; Ding, T.; Yao, J.; Song, W.; Lin, W. Investigation progress on release and control of NOx and N2O during coal combustion in circulating fluidized bed combustor. Chin. J. Process Eng. 2006, 6 (6), 104–110. (6) Mahalik, K.; Sahu, J. N.; Patwardhan, A. V.; Meikap, B. C. Kinetic studies on hydrolysis of urea in a semi-batch reactor at atmospheric pressure for safe use of ammonia in a power plant for flue gas conditioning. J. Hazard. Mater. 2010, 175 (13), 629–637. (7) Mahalik, K.; Sahu, J. N.; Patwardhan, A. V.; Meikap, B. C. Statistical modelling and optimization of hydrolysis of urea to generate ammonia for flue gas conditioning. J. Hazard. Mater. 2010, 182 (13), 603–610. (8) Sahu, J. N.; Chava, V. S. R. K.; Hussain, S.; Patwardhan, A. V.; Meikap, B. C. Optimization of ammonia production from urea in continuous process using ASPEN Plus and computational fluid dynamics study of the reactor used for hydrolysis process. J. Ind. Eng. Chem. 2010, 16 (4), 577–586. (9) Desroches-Ducarne, E.; Dolignier, J. C.; Marty, E.; Martin, G.; Delfosse, L. Modelling of gaseous pollutants emissions in circulating 3729
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