Effect of Heterogeneous Reactions of Coal Char on Nitrous Oxide

N2O emission from circulating fluidized-bed coal combustion is even higher. ...... Alejandro Molina , Eric G. Eddings , David W. Pershing , Adel F. Sa...
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Energy & Fuels 2001, 15, 696-701

Effect of Heterogeneous Reactions of Coal Char on Nitrous Oxide Formation and Reduction in a Circulating Fluidized Bed Hao Liu,†,‡ Toshinori Kojima,*,‡ Bo Feng,† Dechang Liu,† and Jidong Lu† National Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, P. R. China, and Department of Industrial Chemistry, Seikei University, 3-3-1 Kichijoji Kita-machi, Musashino-shi, Tokyo 180-8633, Japan Received October 23, 2000. Revised Manuscript Received February 8, 2001

Fluidized-bed combustion has been recognized as a clean coal combustion technology. However, it has been discovered that the nitrous oxide (N2O) emission from fluidized-bed coal combustion is much higher than that from pulverized coal combustion. N2O emission from circulating fluidized-bed coal combustion is even higher. Heterogeneous reactions of char play an important role in N2O formation and reduction in circulating fluidized-bed coal combustion but there exist many unknowns. In this work the effects of heterogeneous reactions of char on N2O formation and reduction were examined in a bench-scale circulating fluidized bed. The experiments revealed that during circulating fluidized-bed coal combustion, N2O formation from oxidization of char-N has the same order as N2O formation from NO reduction by char, and neither of them is negligible (inlet O2 concentration: 21%, temperature: 1135-1225 K). Absorption of oxygen on the surface of char is not an indispensable condition for N2O formation from NO reduction, i.e., NO can also be reduced into N2O on the surface of char in absence of O2. N2O formation from NO reduction in the presence of O2 contributes about 70% and is the main path of N2O formation from NO reduction. N2O formation from NO reduction in the absence of O2 contributes about 30%, and the relative importance of these two mechanisms of N2O formation does not change significantly with NO concentration. Reduction of N2O by char is a first-order reaction with an Arrhenius dependence on temperature. The kinetics of N2O reduction by various chars were obtained (9741223 K). Different chars have different effects on N2O reduction, suggesting that N2O reduction ability of char has a dependence on the property of the char itself. These results may partially account for the diversities in N2O emissions during combustion of different coals. On the basis of the results of this work, char-related reaction paths of N2O formation and reduction were proposed.

Introduction Fluidized-bed combustion technology has gained extensive application owing to its excellent fuel flexibility, high combustion efficiency, and environmentally favorable performance. It has been recognized as a clean coal combustion technology. However, it has been discovered that the nitrous oxide (N2O) emission from fluidizedbed coal combustion is much higher than that from pulverized coal combustion. N2O emission from circulating fluidized-bed coal combustion is even higher than that from bubbling fluidized-bed coal combustion. Both homogeneous and heterogeneous reactions contribute to N2O formation, with their relative contributions depending on the combustion system arrangement, temperature, solid particle concentration, and on the particle catalytic properties in formation and destruction of N2O. The main and the most important heterogeneous reduction reactions of N2O in practical com* Author to whom correspondence should be addressed. Tel: +81422-37-3750.Fax: +81-422-37-3871.E-mail: [email protected]. † Huazhong University of Science and Technology. ‡ Seikei University.

bustion system are reactions with char-C, CO, H2, CxHy, Fe3O4, and CaS.1 The oxidation reactions of volatiles may be homogeneous and/or heterogeneous, catalyzed by solid particles and metallic surfaces. In systems with high particle concentrations (FBC) and lower temperatures the heterogeneous, catalyzed reactions are prevailing.1 N2O formation from volatile during coal combustion was investigated by some researchers. Boavida2 studied the relative importance of the contribution of volatile-N to the formation of NO and N2O during the combustion of coal in a FBC. They concluded that the main source of both N2O and NO is the combustion of char; however, the volatile significantly contributes to the amount of N2O formed. Bramer3 et al. examined N2O emission from a 1 MWth FBC pilot plant and found for their two investigated coal types, about two-thirds of the N2O was (1) Svoboda, K.; Cermak, J.; Hartman, M. Chem. Pap. 2000, 54, 118-130. (2) Boavida, D. H.; Lobo, L. S.; Guyurtlu, I. K.; Cabrita, I. A. Proceedings of the 12th International Conference on Fluidized Bed Combustion, ASME, New York, 1993; pp 977-982. (3) Bramer, E. A.; Valk, M. Proceedings of the International Conference on fluidized Bed Combustion, ASME, New York, Vol. 2, pp 701707.

10.1021/ef000233b CCC: $20.00 © 2001 American Chemical Society Published on Web 04/13/2001

N2O Formation and Reduction in a Fluidized Bed

produced by the volatile and one-third by the char from the coal. Hayhurst et al.4 studied N2O and NOx formation in a fluidized bed and argued that for a low rank coal, 70-90% of all the N2O produced appears while the coal is undergoing devolatilization. The fraction drops to 40% for a coal with a low volatile content. During char burning, there are also two fundamental steps, i.e., N2O formation and N2O destruction. Mechanism of N2O and NO formation from char-N during combustion is still subject of discussions. As it was suggested in the literature and proved experimentally, there are three possible mechanisms for char-N conversion into N2O:5-7 (1) direct oxidation of char nitrogen; (2) reaction of gaseous NO with char nitrogen (both in the presence and absence of oxygen in gas phase); (3) the devolatilization or gasification of char nitrogen leads to formation of HCN and NH3 as nitrogen intermediates followed by gas phase, mostly homogeneous, oxidation through reactive radicals to N2, NO, and N2O. Conversion of char-nitrogen into NO and N2O is influenced by many factors such as temperature, oxygen content of char, etc., the complexity of which coming from the coupling of formation with immediate destruction of N2O. Destruction of N2O is influenced by specific surface area of char, by catalytic effects of alkai compounds, and by catalytic decomposition of N2O by CaO, FeO, and other ash components. Char and CaO are in practical combustion systems the most effective catalysts for thermal decomposition of N2O. Heterogeneous reactions of char play an important role in N2O formation and reduction in circulating fluidized-bed coal combustion owing to the high particle concentration, particularly the much higher particle concentration in the upper part of a CFBC than in the freeboard of a BFBC. De Soete8 has studied the conversion of char-N in a fixed bed reactor in the temperature range of 750-1050 K. He found that the fraction of char-N transformed into NO and N2O during combustion is roughly proportional to the degree of carbon burnout. He proposed that the mechanism of N2O formation from char combustion is the reaction of (-CN) and (-CNO) sites. Many investigations on N2O and NOx formation during circulating fluidized-bed combustion revealed that with increase in residence time (or height along the combustor), N2O increases but NOx decreases, suggesting the significant effect of char on formation/ destruction of N2O and NOx.9-11 Mochizuki et al.12 measured N2O concentrations from the combustion of three types of char at 1073 K. The conversion ratio of char-N to N2O was 1-9%, dependent on coal type and reaction conditions. They detected no (4) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1996, 341357. (5) Miettinen, H. Energy Fuels 1996, 10, 197. (6) Mallet, C.; Aho, M.; Hamalainen, J.; Rouan, J. P.; Richard, J. R. Energy Fuels 1997, 11, 792. (7) Croistet, E.; Heurtenbise, C.; Rouan, J. P.; Richard, J. R. Combust. Flame 1998, 112, 33. (8) De Soete, G. G. Symp. (Int.) Combust., [Proc.] 1990, 23, 12571264. (9) Amand, L.-E.; Leckner, B.; Andersson, S. Energy Fuels 1991, 5, 815-823. (10) Diego, L. F. D.; Londonot, C. A.; Wang, X. S.; Gibbs, B. M. Fuel 1996, 75, 971-978. (11) Feng, B.; Liu, H.; Yuan, J. W.; Lin, Z. J.; Liu, D. C. Int. J. Energy Res. 1996, 20, 1015-1025. (12) Mochizuki, M.; Koike, J.; Horio, M. 5th International Workshop on Nitrous Oxide Emissions, NIRE, Tsukuba, Japan, 1992.

Energy & Fuels, Vol. 15, No. 3, 2001 697

N2O in the absence of either NO or O2 as reactant. They suggested that the pathway of N2O formation is the reaction of NO with char-N. Suzuki et al.13 studied the effect of devolatilization temperature, reactor type, and combustion temperature on N2O and NO formation from char combustion. They argued the possibility of N2O formation from the heterogeneous reaction of NO and char. Tullin et al.14 also studied the mechanism of N2O formation from char combustion in a small-scale fluidized-bed reactor. They proposed that the principal path for N2O formation is from the reaction of NO with char nitrogen in the presence of oxygen. The importance of oxygen is in consuming the carbon to provide a mechanism for the release of nitrogen which is firmly bound in coal mostly in heterocyclic structures. Moreover, Jones et al.15 studied NOx and N2O release during coal char combustion using carbon-13 materials as models and concluded that char having a high heteroatom content gave N2 as the major product rather than N2O. The relative amounts of N2, N2O, and NO formed during combustion appear to have a complex dependence upon temperature of combustion, rate of combustion, oxygen content of the char, nitrogen content, and functionality in the char. Hayhurst et al.4 studied NOx and N2O formed in a fluidized-bed combustor during the burning of coal volatiles and char. They found that coals of higher rank give more N2O and also more NOx from both stages of combustion, i.e., the burning of the volatiles and also of char. On the other hand, lower rank coals give a greater fraction of the NOx and N2O during devolatilization, rather than the subsequent stage of char combustion. Matos et al.16 developed a method of calculating internal surface areas of particles from kinetic rate data and obtained the rate constants for the NO/char reaction. Krammer and Sarofim17 examined the reaction of char nitrogen during fluidized-bed coal combustion and found that N2O was formed in amounts that increased with increasing NO concentration, showing the importance of NO for N2O formation. Teng et al.18 studied the global kinetics of N2O-carbon reaction with a TGA and argued that the reaction order with respect to N2O partial pressure is not constant, which was considered to be due to the influence of pore resistance variation with the carbon burnoff level. Rodriguez-Mirasol et al.19 studied N2Ochar reaction in a fixed bed and obtained reaction kinetics. They also concluded that the kinetic parameters for N2O decomposition on a char surface depend on char structure. Furthermore, in our previous work, we studied the mechanisms of N2O formation from char combustion in (13) Suzuki, Y.; Moritomi, H.; Kido, N.; Ikeda, M.; Suzuki, K.; Torikai, K. 5th International Workshop on Nitrous Oxide Emissions, NIRE, Tsukuba, Japan, 1992. (14) Tullin, C. L.; Sarofim, A. F.; Beer, J. M. Proceedings of the 12th International Conference on Fluidized Bed Combustion, ASME, New York, 1993; pp 599-609. (15) Jones, J. M.; Thomas, K. M. Carbon 1995, 33, 1129-1139. (16) Matos, M. A.; Pereira, F. J. M. A.; Ventura, M. P. Fuel 1991, 70, 38-43. (17) Krammer, G. F.; Sarofim, A. F. Combust. Flame 1994, 97, 118124. (18) Teng, H.; Lin, H. C.; Hsieh, Y. S. Ind. Eng. Chem. Res. 1997, 36, 523-529. (19) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijin, J. A. Combust. Flame 1994, 99, 499-507.

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Liu et al. Table 1. Ultimate and Proximate Analyses of Coals and Chars ultimate analyses (daf, wt %)

proximate analyses (wt %)

fuel

C

H

N

S

O

FC

VM

Ash

Coal A Char A Coal B Char B Coal C Char C

79.5 95.4 83.7 90.0 89.4 91.6

4.9 1.8 5.4 2.1 4.1 1.8

1.0 0.7 1.3 1.5 1.2 1.3

0.6 0.6 1.0 1.1 0.3 1.5

14.0 1.5 8.6 5.3 5.0 3.8

62.9 88.8 47.5 50.6 73.7 81.8

30.1 4.5 16.3 4.8 13.3 3.1

4.0 5.8 31.3 43.8 12.2 14.7

Table 2. Microscopic Characteristics of Char

Figure 1. Schematic of the bench-scale circulating fluidizedbed reactor.

a bubbling fluidized bed and nitrogen oxide emission from a circulating fluidized-bed combustor, with possible pathways of N2O formation from char combustion in a bubbling fluidized bed proposed.11-20 Despite the above-mentioned research works, study concerning the effect of heterogeneous reaction of coal char on N2O formation and reduction is limited and there are still many unknowns to be clarified, particularly in circulating fluidized-bed coal combustion, such as the relative importance of N2O formed from char-N and N2O converted from NO, the mechanism of N2O formation from NO reduction by char, kinetics of N2O reduction by char, the effect of char type (or coal type) on N2O formation/reduction during circulating fluidizedbed coal combustion, and so on. Besides that, research under conditions close to that of practical circulating fluidized-bed coal combustion is also needed because experimental conditions, such as heating rate and so on, may influence the results. The main objective of this paper is to clarify, through experiments under conditions close to a practical circulating fluidized bed, the mechanism and effect of charrelated heterogeneous reactions on N2O formation and N2O reduction and to derive the kinetics of N2O reduction by char during circulating fluidized-bed coal combustion. Experimental Section The experiments were conducted in a bench-scale circulating fluidized-bed as shown in Figure 1. The reactor was made of a quartz tube with an inner diameter of 0.02 m and height of 0.6 m. The reactor was heated with an electric furnace and the temperature was measured with a thermocouple. To avoid the potential catalytic effect of the thermocouple on the reaction system, the thermocouple was located outside the tube near the wall. The measurements showed that the difference between the inside and the outside was less than 10 K. The reactant gases were well mixed through a mixer and entered the reactor from the bottom. Silicon sand, with size of 1.82.3 × 10-4 m, was used as bed material. Char particles, also with size of 1.8-2.3 × 10-4 m, were fed into the reactor with a very small fluidized-bed feeder. A fraction of the gas mixture was also fed into the small fluidized-bed feeder as fluidization medium and char particles were fed into the reactor through a thin stainless steel pipe which connects the side wall of the small fluidized-bed feeder and the reactor. The total gas flow rate was 6.3 × 10-5 Nm3/s. For experiments on N2O formation, inlet O2 concentration ) 21 vol %, temperature ) 1135-1225

type of char

Char A

Char B

Char C

specific surface area (103 m2/kg) pore volume (10-6 m3/kg)

10.94 13.19

9.29 13.29

1.61 6.19

K. For experiments on NO reduction, inlet NO concentration ) 0-1000 ppm and temperature was 1133 K. For experiments on N2O reduction, inlet N2O concentration ) 1020 ppm; temperature ) 974-1223 K. According to the particle size, bed temperature, and so on, the terminal velocity of particles was estimated; and accordingly, the mean residence time of char in the riser was estimated to be in the order of 0.4 s. In all experiments, Ar was set as balance of gas concentration. A cyclone was used to separate solids from flue gas and the separated particles were fed back to the reactor through a L-valve. The flue gases were cooled and removed of moisture and particulates. Then the concentrations of NOx, O2, CO, and CO2 were measured on-line. NOx was analyzed with a chemiluminescent analyzer, CO2 and CO with an NDIR analyzer, and O2 with a paramagnetic analyzer, respectively. The concentration of N2O in the exhausted gas was measured through a gas chromatograph (GC) equipped with an ECD detector. A gas sample was obtained and stored in a glass container for measurements after passing through an ice bath and a NaOH solution. The concentration of N2O in each gas sample was measured three times, and the average value was taken as the N2O concentration. Experiments showed that the difference among the measurements was within 10%. To separate char-N converted N2O from NO-converted N2O, we conducted experiments in the presence and absence of O2, respectively. The coal chars used in the experiments were prepared through devolatilizing the coals in a nitrogen stream at a temperature of 1173 K for 15 minutes. Table 1 specifies their properties. Table 2 lists the specific surface area and pore volume of those chars. No CO was detected during our experiments even though we tried to measure it. We did not include CO2 concentrations in the text considering the effect of CO2 on NO and N2O is negligible.

Experimental Results N2O Formation from Char-N. There are mainly two char-related paths of N2O formation during circulation fluidized-bed coal combustion, i.e., N2O formation from char-N and from NO reduction on char surface. First, we studied N2O formation from char-N. Coal char was burned in the reactor and N2O formation from char-N was investigated by measuring N2O concentration at the outlet of the reactor. Inlet O2 concentration was 21% and the oxygen-fuel stoichiometric ratio was 1.2. Figure 2 shows N2O formation versus temperature. It can be seen that char combustion can also produce some N2O. And N2O concentration decreases with temperature, which is similar to the case of coal combustion.

N2O Formation and Reduction in a Fluidized Bed

Energy & Fuels, Vol. 15, No. 3, 2001 699

Figure 2. N2O formation from char-N, inlet O2 concentration ) 21%, and oxygen-fuel stoichiometric ratio ) 1.2 (char A).

N2O Formation from NO Reduction on Char Surface. NO reduction on char surface is another important char-related N2O formation path during circulating fluidized-bed coal combustion. To clarify the contribution of NO reduction on char surface to N2O formation, experiments were conducted at 1133 K with NO of various concentrations fed into the reactor from the bottom and N2O concentration at the outlet was measured. We studied the cases in the absence and presence of O2, respectively. In the presence of O2, inlet O2 concentration was 21% and oxygen-fuel stoichiometric ratio was 1.2. To clarify the mechanism of N2O formation from NO reduction, N2O formation from NO was obtained by subtracting the char-N converted N2O from the total N2O formation in the presence of both O2 and NO. Here char-N converted N2O refers to the N2O formed from char-N during char combustion. Figure 3 shows the results (with N2O formed from char-N excluded). It was demonstrated that even in absence of O2, a small amount of N2O was also detected, which suggested that NO can be converted into N2O on the char surface even in absence of O2 (mechanism A). It mainly includes the following reactions concerning free sites of (-C), (-N), (-CN), and (-CNO):4,20

NO + (-CN) f N2O + (-C)

(1)

NO + (-C) f (-CNO)

(2)

(-N) + NO f N2O

(3)

NO + (-CNO) f N2O + (-CO)

(4)

In our previous work, we conducted similar experiments with a fixed bed reactor and we also found that N2O can be generated from the reduction of NO on char surface in the absence of O2.20 The results of this work and our previous work are in agreement with that of Gulyurtlu et al.21 as well. On the other hand, when O2 was added into the reactor, N2O concentration at the outlet of the reactor increased significantly. This occurred not only because fuel-N2O (N2O formed from char-N) was produced, but also because the presence of O2 changed the mechanisms of N2O formation from NO reduction, i.e., in addition to the above-mentioned path of N2O formation (20) Feng, B.; Liu, H.; Yuan, J. W.; Lin, Z. J.; Liu, D. C. Energy Fuels 1996, 10, 203-208. (21) Gulyurtlu, I.; Esparteiro, H.; Cabrita, I. 5th International Workshop on Nitrous Oxide Emissions, NIRE, Tsukuba, Japan, 1992.

Figure 3. N2O formation from NO reduction on char surface (with N2O formed from char-N excluded), T ) 1133 K (char A)

from NO reduction in absence of O2 (mechanism A), N2O formation from NO reduction in the presence of O2 (mechanism B) also takes place. In this case, O2 reacts with active sites on the char surface and forms (-CNO), and then (-CNO) reacts with NO to produce N2O (mechanism B): 1

/2O2 + (-CN) f (-CNO)

(5)

NO + (-CNO) f N2O + (-CO)

(6)

Besides that, the following reactions concerning free sites of (-N) and (-CNO)4,17 may also proceed in mechanism B:

(-CNO) f NO + (-C)

(7)

2(-CNO) f N2O + (-CO) + (-C)

(8)

(-CN) + 1/2O2 f CO + (-N)

(9)

2(-N) + O2 f 2NO

(10)

Therefore in the presence of O2, more N2O was converted from NO. Furthermore, it was revealed that although both mechanisms enhance N2O formation with the increase of inlet NO concentration, the relative contributions of these two mechanisms are approximately constant. Mechanism A (N2O formation from NO in absence of O2) contributes about 30% and mechanism B (N2O formation from NO in the presence of O2) contributes about 70% to the overall N2O formation from NO reduction. These results suggested that mechanism B, i.e., N2O formation from NO in the presence of O2, is the main path of N2O formation from NO reduction on char surface. The contribution of mechanism A is smaller because in the absence of oxygen, the (-CN) site in char is difficult to release. If we compare Figure 2 and Figure 3, it can be found that during char combustion, N2O formation from oxidization of char-N has the same order as N2O formation from NO reduction by char and neither of them is negligible. Reduction of N2O by Char. Experiments were conducted with three types of char prepared from three coals to examine the reduction of N2O on char surface. An inlet N2O (1020 ppm) was fed into the reactor from the inlet, with Ar (argon) as balance, so as to derive the pure effect of N2O reduction by char. Experiments were

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Liu et al.

Figure 4. N2O reduction ratio by char, inlet N2O ) 1020 ppm, Ar as balance

conducted at various temperatures and the results were shown in Figure 4. Here N2O reduction ratio was defined as the ratio of the reduced N2O amount to the inlet N2O amount. It was revealed that different chars have different N2O reduction ability, but N2O reduction ratio for the three chars all increases with temperature. The different ability of N2O reduction demonstrated by different chars suggested that NO reduction ability is d?ependent on the property of char itself. Besides that, we conducted experiments at various inlet N2O concentrations and found that the N2O amount destructed was almost linear to the inlet N2O concentration, which suggested that N2O reduction on char surface is almost a one-order reaction with respect to N2O concentration. This result was in accordance of those of Madley22 and Smith.23 The mechanism of N2O reduction on char surface is probably the effect of active sites on char surface, such as (-C), (-CO), and so on. The possible heterogeneous reactions include:8

N2O + (-C) f N2 + (-CO)

(11)

N2O + (-CO) f N2 + (-CO2)

(12)

By using the experimental data in Figure 4, the reaction rate constant on apparent area basis, k, can be obtained for N2O reduction by char. According to the overall mass balance of N2O, in unit time, the amount of N2O decomposed by char is equal to the difference of N2O mass flow between inlet and outlet:

QOut - QIn ) -kCS

(13)

where QIn and QOut refer to inlet and outlet N2O mass flow rate respectively (mol/s); k is reaction rate constant of N2O reduction by char on apparent area basis (m/s). C refers to the average of inlet and outlet N2O concentrations (mol/m3), and S represents the total apparent surface area of all char particles in the bed material (m2). Figure 5 shows an Arrhenius plot of reaction rate constant versus temperature. It was demonstrated that the reaction rate constant has an Arrhenius relationship with temperature. The kinetics of N2O reduction by various chars was listed in Table 3. It can be seen that different chars have different ability of N2O reduction, (22) Madley, D. G.; Strickland-Constable, R. F. Trans. Faraday Soc. 1953, 49, 1312-1324. (23) Smith, R. N.; Lesnini, D.; Mooi, J. Phys. Chem. 1957, 61, 8186.

Figure 5. Arrhenius plot of reaction rate constant of N2O reduction by char versus temperature, inlet N2O ) 1020 ppm, Ar as balance Table 3. Kinetics of N2O Destruction by Char type of char

reaction rate constant k (m/s)

Char A Char B Char C

5.8 × 101 exp(-30600/RT) 7.1 × 101 exp(-33400/RT) 2.7 × 103 exp(-69400/RT)

very probably owing to the property of char itself. In fact, Rodriguez et al.19 also found diverse kinetics of N2O reduction by different chars. In this work, char A demonstrated the strongest N2O reduction ability. Char A was made from a coal of high VM content, which may account for the strong ability of N2O reduction of char A. Anyway, further research is needed to clarify the mechanism. Discussion The reaction mechanisms of N2O formation and reduction from char combustion have been proposed by many researchers. De Soete8 argued that the reaction process of N2O formation should be as follows: first oxygen is adsorbed to the surface of char and reacts with the bound carbon to form (-CO), (-C), and (-CN) sites. The further reaction of oxygen with (-C) and (-CN) produces (-CNO), which reacts with (-CN) to release N2O. Meanwhile, N2O can be decomposed by (-CO). Thus the formation of NO and N2O is dependent on the adsorption of oxygen. On the other hand, Mochizuki et al.12 suggested that the main pathway to produce N2O is through the heterogeneous reactions of NO with char-N in the presence of oxygen. De Soete8 and Mochizuki12 agree that oxygen adsorption is the first step to produce N2O, but they disagree on the effect of NO. However, the present study and our previous results obtained with a fixed bed reactor both showed that not only the burning of char can produce N2O, N2O can also be formed through NO reduction on char surface in absence of O2. On the basis of the results of this work, we tentatively proposed the char-related reaction paths of N2O formation and reduction as shown in Figure 6. Besides that, our experiments with a pilot-scale circulating fluidized-bed coal combustor revealed that N2O concentration increased with the height of the combustor but NO concentration decreased with the height,11 which suggested that N2O formation via NO reduction on char surface is very probably an important reason to account for the increase of N2O concentration and decrease of NO concentration along the height of the combustor.

N2O Formation and Reduction in a Fluidized Bed

Figure 6. Char-related reaction paths of N2O formation and reduction.

From Table 1, Table 2, and Figure 5, it is known that those chars made from coals with high volatile matter content have great pore volume and specific surface area, and also demonstrated stronger N2O reduction ability. Therefore, we may attribute the different N2O formation and reduction characteristics of various chars to the difference in their internal pore structures developed from devolatilization of different coals. The reaction rate constant of N2O reduction, k, defined in eq 13 is a global one and further work is needed to investigate the intrinsic reaction rate constant with chars derived from various coal types. Conclusions The effects of heterogeneous reactions of coal char on N2O formation and reduction during circulating fluidized-bed coal combustion were examined in a benchscale circulating fluidized-bed. The following conclusions were reached for circulating fluidized-bed coal combustion:

Energy & Fuels, Vol. 15, No. 3, 2001 701

1. N2O formation from oxidization of char-N has the same order as N2O formation from NO reduction by char and neither of them is negligible. 2. It was found that absorption of oxygen on the surface of char is not an indispensable condition for N2O formation from NO reduction, i.e., NO can also be reduced into N2O on the surface of char in absence of O2. N2O formation from NO reduction in the presence of O2 contributions about 70% and is the main path of N2O formation from NO reduction. N2O formation from NO reduction in absence of O2 contributes about 30%, and the relative importance of these two mechanisms of N2O formation does not change significantly with NO concentration. 3. Studies on the three types of chars suggested that the reduction of N2O by char is a first-order reaction with an Arrhenius dependence on temperature. The kinetics of N2O reduction by various chars were obtained. 4. Different chars have different effects on N2O reduction, which suggests that N2O reduction ability of char depends on the property of the char itself. These results may partially account for the diversities in N2O emissions during combustion of different coals in circulating fluidized beds. 5. Based on the results of this work, char-related reaction paths of N2O formation and reduction were proposed. Nomenclature C ) N2O molar concentration (mol/m3) QIn ) inlet N2O mass flow rate (mol/s) QOut ) outlet N2O mass flow rate (mol/s) k ) reaction rate constant of N2O reduction by char on apparent area basis (m/s) S ) total apparent surface area of all char particles (m2)

Acknowledgment. The authors are grateful to the National Natural Science Foundation of China for financial support of this work. EF000233B