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N2O formation during fluidized bed combustion of chars
N20 formation during combustion of chars I. Gulyurtlu,
H. Esparteiro
fluidized
bed
and I. Cabrita
Departamento de Energias Convencionais, Laboratdrio National Tecnologia Industrial, Edificio J, 1699 Lisboa Codex, Portugal (Received 14 June 1993; revised 25 October 1993)
de Engenharia
e
Experimental studies were carried out with an 80mm i.d. fluidized bed combustor over the temperature range 700-950°C to determine the amounts of N,O formed during the combustion of different chars as well as the heterogeneous reactions involving char and N,O leading to the reduction of the latter. The results suggest that the combustion of volatiles contributes more to the formation of N,O, as the N,O levels measured were -40% of that observed during the combustion of the parent coal. The study also demonstrated the significant reactivity of char with respect to N,O, as >90% of the inlet N,O concentration was adsorbed. (Keywords: fluidized bed combustion; nitrous oxide; char)
In fluidized bed combustion of coals, fuel-N appears to be the main source of the formation not only of NO, but also of N,O,. It has been shown that the combustion temperature is the principal factor controlling the amount of N,O formed, and that its formation is favoured at temperatures < 850°C. As much as 200 ppmv of N,O has been measured in flue gases from fluidized bed coal combustorsip4. In fact, the formation of N,O appears to be of particular concern in fluidized bed combustion systems, in which the temperature is usually maintained at -850°C to maximize SO, removal by limestone. Although both fuel-NO, and N,O have the same origin, fuel-N, and are formed through the same main intermediate (HCN), the reactions leading to the formation of N,O require a lower hydrogen concentration and a lower combustion temperature5. The oxidation of fuel-N remaining in the char resulting in the formation of NO or N,O occurs by heterogeneous reactions involving both physical and chemical adsorption of oxygen on the carbon surface. It is claimed6 that the conversion to N,O takes place faster than the formation of NO. This still needs to be verified. Part of the N,O formed is then reduced by char, and it has been suggested6 that char is highly reactive towards N,O. However, the following aspects of the char reaction with N,O need to be studied to establish a comprehensive reaction mechanism:
(1) whether the same surface sites are responsible
for the formation of NO and N,O from fuel-N; (2) whether NO and N,O are reduced on the same surface complexes of char; of both NO and (3) what is the nature of adsorption N,O on the carbon surface (whether physical or chemical); what are the adsorption products; what is the nature of the reaction between the products of adsorption and NO and N,O; what is the rate of desorption of surface products. 0016-2361,94jO7/1098-05 (’ 1994 Butterworth-Heinemann
1098
Fuel 1994
Volume
Ltd
73 Number
7
This paper reports a study of the levels of N,O formed during the combustion of various chars in a fluidized bed. The aim was to establish whether char combustion or volatile combustion is primarily responsible for the formation of N,O during coal combustion. Work was also undertaken to understand better the N,O reduction on the char surface.
N,O FORMATION MECHANISMS
AND
DESTRUCTION
The mechanism of gas-phase reactions involving N,O is currently well understood, and is in line with the hypothesis of Kramlich et al.‘: formation
of N,O
from NH, and HCN
species:
NH+NO+N,O+H NCO+NO+N,O+CO destruction
of N,O:
N,O+H+N,+OH Both fuel-NO and fuel-N,0 have a common origin, i.e. fuel-N. Three different types of mechanism have been suggested for N,O formation from coal combustion’? (1) heterogeneous char-N oxidation to N,O; (2) homogeneous oxidation of volatile nitrogen compounds (HCN) after devolatilization or char gasification: HCN+O+NCO+H NC0
+ NO-N20
(3) heterogeneous or adsorption surface which NO molecule.
+ CO
reaction between NO and char-N, of an NO molecule on the char subsequently reacts with a second
N,O
Figure 1 Simplified destruction
mechanism
of NO,
and
N,O
formation
formation
and
during fluidized
N,O destruction ways:
can also be explained
(1) homogeneous (2) heterogeneous
reduction reduction
in two different
of N,O to N,; of N,O on char particles.
The N,O can therefore be generally assumed to be formed from HCN, from char-N or even from NO reduction, depending on the combustion system and the operating conditions. The scheme in Figure I briefly illustrates these reaction mechanisms.
I. Gulyurtlu
et al.
oxygen present, using a specially prepared N&argon mixture. The temperature in the combustor was measured within the bed and in the freeboard near the exit. The gases leaving the combustor were collected in bags for N,O determination by gas chromatography. Special measures were taken during collection of the sample to ensure that SO,, H,O and NO, were removed from the gas mixture, to avoid artificial formation of N,O during storage. For CO, CO,, 0, and NO,, on-line analysers were used. Table 1 summarizes the operating conditions. For the preparation of chars, the temperature in a fluidized bed reactor was previously brought to a preset value and then samples of coal were introduced. N, was used as inert carrier gas. During the devolatilization, both CO and CO, were monitored, and when the analysers could no longer detect these gases, the heating was switched off, with the flow of N, maintained. The reactor was cooled to _ 15°C and the flow of N, was turned off. The char thus prepared was sealed in plastic bags. Samples were analysed to determine the amount of fuel-N remaining in char. RESULTS
Figure 2 Experimental set-up: 1, electric furnace; 2, fluidized bed combustor; 3, thermocouple; 4, screw-feeder; 5, cooling system; 6, condenser; 7, collector; 8, filter; 9, flow meter; 10, three-way valve; II, recorder; 12-15, CO, CO,, H, and 0, analysers
bed combustion:
AND
DISCUSSION
Char-N20 reaction Chars produced from different coals were subjected to N,O under different conditions to determine their reactivities. The results suggested that the nature of the parent coal did not have a significant influence on the affinity of the char towards N,O, as shown in Figure 3. All the chars were highly reactive, and N,O adsorption
Table 1
Operating
conditions
Coal feed rate (g mini ‘) Char batch weight (g) Gas velocity (m SK’) Bed temperature (“C) Oxygen concentration (vol.%) N,O inlet concentration (ppmv) Particle size (mm) Sand bed weight (g)
0.5-5 5 0.24.5 700-l 100 2.5-7.5 lOG500 0.1254.0 500
20
EXPERIMENTAL Figure 2 represents schematically the experimental facility. The fluidized bed combustor is placed inside an electrically heated furnace and has an internal diameter of 60 mm and a height of 500 mm. The upper part of the combustor is heavily insulated to prevent heat loss. The distributor plate is of inverted cone type with 10 standpipes, each containing 10 nozzles. An electrical coil in the windbox preheats incoming gas before introduction into the combustor through the distributor plate. For combustion studies, the char was supplied through a small screw-feeder at the top of the combustor. In the case of char-N,0 reactions, the char samples were introduced in batches from the top of the combustor through two-valve lock system. The samples could be introduced at intervals varying from 20 s to 1 min. Reactions of char with N,O were carried out in the absence of oxygen and also with different amounts of
700
800
900
1 000
TEMPERATURE
(“C)
Figure 3 Effect of reaction temperature on residual N,O content of gas after reduction by chars prepared at 1100°C from different coals. N,O inlet concentration: 250 ppmv
Fuel 1994
Volume
73 Number
7
1099
N20
formation
during fluidized
bed combustion:
i. Gulyurtlu
CHAR PREPARED AT 1 000 “C 1 100°C 1 200 “C
700
800
900
tem~rature was varied from 1000 to 1200°C. The chars produced were then subjected to N,O. Figure 4 shows that the devolatilization temperature can have an important influence on the char reactivity. Over the range 700-85O”C, the char prepared at 1200°C showed the greatest adsorption capacity for N,O. This could be due to the fact that this char was of higher porosity, and also to the release at 1200°C of greater amounts of the C-O surface complexes, leaving a greater active surface area for N,O adsorption. Above 900°C the char reactivity to N,O became independent of the temperature of preparation, but this again could be a result of the increased degree of N,O dissociation. The char reactivity to N,O was not influenced by the presence of oxygen in the gas mixture introduced to the combustor. The char prepared at 1100°C was subjected to a gas mixture which contained oxygen varying from 2.5 to 7.5 vol.%. The level of N,O reduction was about the same as in the absence of oxygen, and the amount of oxygen present did not appear to interfere with the extent of N,O adsorption on the surface, as shown by Figure 5.
1 000
TEMPERATURE
et al.
(“C)
Figure 4 Effect of reaction temperature on residual N,O content of gas after reduction by chars prepared at different tem~ratllres from coal of 32.5 wt% volatile matter. N,O inlet concentration: 250 ppmv
r
The influence of pretreatment of the char surface before reaction with N,O was also investigated. Chars prepared at different temperatures were first subjected to a flow of oxygen in a reactor at room temperature. The results (Figure 6) suggest that the char reactivity to N,O was not influenced significantly by such pretreatment. There are two possible reasons for this: (1) the oxygen is adsorbed at different sites on the char surface; (2) the surface oxygen complexes provide sites for N,O adsorption, unlike NO adsorption. In all studies on char-N,0 reactions, very little NO formation was observed. This suggests that the N,O reduction did not lead to the formation of NO through
OXYGEN LNLET CONCENTRATION
CHAR PREPARED FROM ~~~~D~~~
1 200 “C
0
0 800
900
6
7 000
TEMPERATURE
(“C)
Figure5 Effect of reaction temperature on residual N,O content of gas after reduction by char in the presence of different oxygen concentrations. Char prepared at 1100°Cfrom coal of 32.5 wt% volatile matter. N,O inlet concentration: 250 ppmv
was almost complete even at temperatures as low as 700°C. Above 900°C very little N,O was measured in the gases leaving the combustor; this could be due to both N,O adsorption on the char surface and N,O dissociation, as N,O is unstable above 880°C. The effect of the temperature at which the char was prepared on the char reactivity was investigated. Coal of 32.5 wt% volatile matter was used and the devolatilization
1100
Fuel 1994 Volume 73 Number 7
0
700
800
/
900
1 000
TEMPERATURE
(“C)
Figure 6 Effect of reaction temperature on residual N,O content of gas after reduction by chars prepared at different temperatures from coal of 32.5 wt% volatile matter and pretreated with oxygen. N,O inlet concentration: 2.50 ppmv
NzO formation
% 8 @;
b
60_
B z 3
40
2 ? d
‘70 _
0 ‘0 ‘0
--COALS
WITH VOLATILE CONTENT - 22.5 % by weighl - 2X % by weight - 32.5 % by wright F
2 8 @
I
I 700
800
900
1 100
1 000
TEMPERATURE Figure 7 Effect fuel-N remaining
of devolatilization in chars prepared
temperature from different
(“C)
on proportion coals
of
during
fluidized
bed combustion:
1. Gulyurtlu
et al.
devolatilization temperatures were considered for that reason. The nature of the coal did not appear to have a large influence on the level of devolatilization and the extent of fuel-N release in the volatiles. Studies were carried out to determine the contribution of char combustion to the overall N,O formed. Chars prepared at different temperatures were burnt in the fluidized bed and the N,O in the combustion gases was measured. Figures 8 and 9 illustrate the N,O content as a function of bed temperature for different char preparation temperatures and excess air levels. It was found that char-N contributed -40% to the overall N,O formed during the combustion of the parent coal. The greater release of volatiles at higher char preparation temperatures resulted in smaller amounts of fuel-N remaining in the char; consequently the combustion of the char resulted in N,O levels ~40% of those formed in the combustion of the parent coal, as shown in Figure 9. Figure IO compares the N,O levels obtained from the combustion of chars produced from different coals at 1000 and 1300°C. The N contents of the parent coals were different. It is clear that the nitrogen remaining in the char was proportional to the N content of the parent coal. As a result, the N,O formed varied with the coal
Excess air levels
800 TEMPERATURE
700
900 (‘C)
Figure 8 Effect of bed temperature on amount of N,O formed during combustion at 50% excess air of chars prepared at different temperatures from coal of 32.5 wt% volatile matter and 1.4 wt% N
the following possible reactions: N,O + (C-)+NO
+ (C-O)
N,O + (C-O)+NO The end-products observed were CO and CO,. The CO content increased with temperature, as thermodynamically expected. The possible routes for N,O reduction could be the following: N,O+(C)+N,+CO 2N,O + (C-)-t2N,
+ CO,
700
800 TEMPERATURE
900 (“C)
Figure 9 Effect of bed temperature on amount of N,O formed during combustion at different excess air levels of coal of 32.5 wt% volatile matter and char prepared from it at 1100°C
N20+(C-O)+N2+C02 2N,O +(C-0)+2Nz
+ 2C0
The presence of N, in the gases leaving the combustor confirms in fact that most of the N,O was reduced directly to N,.
z 80 B 2 5 60 Y 2
Char combustion
Chars were prepared from different coals at different temperatures and the amount of fuel-N remaining in the char was determined. The N content of the chars depended greatly on the devolatilization temperature, and the fraction of the coal-N retained decreased from - 70 to - 40% when the temperature was increased from 700 to 1lOO”C,as shown in Figure 7. During the fluidized bed combustion of coal, the particle temperature could be as much as 300 K above that of the gas, and higher
Y 40 9 : 0 2 20 700
900
800 TEMPERATURE
($C)
Figure 10 Effect of bed temperature on amount ofN,O formed during combustion at 50% excess air of chars prepared at different temperatures from coals of N content (A) 1.1 wt%, (B) 1.4 wt%
Fuel 1994
Volume
73 Number
7
1101
N20 formation during fluidized bed combustion: /. Gulyurtiu et al.
type and with the temperature at which the char was produced. The chars produced at 1300°C contained much less fuel-N for both coals, and their combustion appeared to produce much lower N,O emissions, particularly in the case of coal B. This again confirms that the amount of N,O formed appears to be independent of the coal or char reactivity but depend on the fuel-N content and the temperature of combustion.
to be influenced by the presence of oxygen or NO. Even pretreatment of the surface with oxygen does not influence the N,O adsorption. Char appears to be highly reactive towards N,O.
ACKNOWLEDGEMENT The authors thank the European financing this project.
Community
for
CONCLUSIONS 1. In fluidized bed combustion of coal, if no control steps
2. 3.
4.
5.
are taken, the amount of N,O released can be as high as 120 ppmv. The formation and destruction of N,O involve both heterogeneous and homogeneous reactions. Most of the N,O formed appears to result from the combustion of volatiles, as the char combustion yields less than half of N,O produced when the parent coal is burnt. Most of the N,O reduction results in the formation of N, through heterogeneous reaction with char; however, a very small amount is also converted to NO. N,O adsorption on the char surface does not appear
1102
Fuel 1994 Volume 73 Number 7
REFERENCES Andersson, S., Amand, L. E. and Leckner, B. Paper to IEA AFBC Technical Meeting, Amsterdam, November 1988 Amand, L. E. and Andersson, S. Paper to 10th International Conference on FBC, San Francisco, 1989, pp. 49-56 Gulyurtlu, I., Costa, M. R., Esparteiro, H., Monteiro, A. and Cabrita, I. Paper to 7th Annual International Pittsburgh Coal Conference, September 1990, pp. 177-186 Robv. R. J. and Bowman. C. T. Combust. Flame 1987, 70. 119 Hao:‘W. M., Wofsy, S. C:, McElroy, M. B., Farmayan, W. F. and Togan, M. A. Cornbust. Sci. Technol. 1987, 55, 23 De Soete, G. G. Paper to 23rd Symposium (International) on Combustion, 1990, pp. 1257-1264 Kramhch, J. C., Cole, J. A., McCarthy, J. M., Lanier, W. S. and McSorley, J. A. Cornbust. Flame 1989,77, 375
H. Esparteiro
fluidized
bed
and I. Cabrita
Departamento de Energias Convencionais, Laboratdrio National Tecnologia Industrial, Edificio J, 1699 Lisboa Codex, Portugal (Received 14 June 1993; revised 25 October 1993)
de Engenharia
e
Experimental studies were carried out with an 80mm i.d. fluidized bed combustor over the temperature range 700-950°C to determine the amounts of N,O formed during the combustion of different chars as well as the heterogeneous reactions involving char and N,O leading to the reduction of the latter. The results suggest that the combustion of volatiles contributes more to the formation of N,O, as the N,O levels measured were -40% of that observed during the combustion of the parent coal. The study also demonstrated the significant reactivity of char with respect to N,O, as >90% of the inlet N,O concentration was adsorbed. (Keywords: fluidized bed combustion; nitrous oxide; char)
In fluidized bed combustion of coals, fuel-N appears to be the main source of the formation not only of NO, but also of N,O,. It has been shown that the combustion temperature is the principal factor controlling the amount of N,O formed, and that its formation is favoured at temperatures < 850°C. As much as 200 ppmv of N,O has been measured in flue gases from fluidized bed coal combustorsip4. In fact, the formation of N,O appears to be of particular concern in fluidized bed combustion systems, in which the temperature is usually maintained at -850°C to maximize SO, removal by limestone. Although both fuel-NO, and N,O have the same origin, fuel-N, and are formed through the same main intermediate (HCN), the reactions leading to the formation of N,O require a lower hydrogen concentration and a lower combustion temperature5. The oxidation of fuel-N remaining in the char resulting in the formation of NO or N,O occurs by heterogeneous reactions involving both physical and chemical adsorption of oxygen on the carbon surface. It is claimed6 that the conversion to N,O takes place faster than the formation of NO. This still needs to be verified. Part of the N,O formed is then reduced by char, and it has been suggested6 that char is highly reactive towards N,O. However, the following aspects of the char reaction with N,O need to be studied to establish a comprehensive reaction mechanism:
(1) whether the same surface sites are responsible
for the formation of NO and N,O from fuel-N; (2) whether NO and N,O are reduced on the same surface complexes of char; of both NO and (3) what is the nature of adsorption N,O on the carbon surface (whether physical or chemical); what are the adsorption products; what is the nature of the reaction between the products of adsorption and NO and N,O; what is the rate of desorption of surface products. 0016-2361,94jO7/1098-05 (’ 1994 Butterworth-Heinemann
1098
Fuel 1994
Volume
Ltd
73 Number
7
This paper reports a study of the levels of N,O formed during the combustion of various chars in a fluidized bed. The aim was to establish whether char combustion or volatile combustion is primarily responsible for the formation of N,O during coal combustion. Work was also undertaken to understand better the N,O reduction on the char surface.
N,O FORMATION MECHANISMS
AND
DESTRUCTION
The mechanism of gas-phase reactions involving N,O is currently well understood, and is in line with the hypothesis of Kramlich et al.‘: formation
of N,O
from NH, and HCN
species:
NH+NO+N,O+H NCO+NO+N,O+CO destruction
of N,O:
N,O+H+N,+OH Both fuel-NO and fuel-N,0 have a common origin, i.e. fuel-N. Three different types of mechanism have been suggested for N,O formation from coal combustion’? (1) heterogeneous char-N oxidation to N,O; (2) homogeneous oxidation of volatile nitrogen compounds (HCN) after devolatilization or char gasification: HCN+O+NCO+H NC0
+ NO-N20
(3) heterogeneous or adsorption surface which NO molecule.
+ CO
reaction between NO and char-N, of an NO molecule on the char subsequently reacts with a second
N,O
Figure 1 Simplified destruction
mechanism
of NO,
and
N,O
formation
formation
and
during fluidized
N,O destruction ways:
can also be explained
(1) homogeneous (2) heterogeneous
reduction reduction
in two different
of N,O to N,; of N,O on char particles.
The N,O can therefore be generally assumed to be formed from HCN, from char-N or even from NO reduction, depending on the combustion system and the operating conditions. The scheme in Figure I briefly illustrates these reaction mechanisms.
I. Gulyurtlu
et al.
oxygen present, using a specially prepared N&argon mixture. The temperature in the combustor was measured within the bed and in the freeboard near the exit. The gases leaving the combustor were collected in bags for N,O determination by gas chromatography. Special measures were taken during collection of the sample to ensure that SO,, H,O and NO, were removed from the gas mixture, to avoid artificial formation of N,O during storage. For CO, CO,, 0, and NO,, on-line analysers were used. Table 1 summarizes the operating conditions. For the preparation of chars, the temperature in a fluidized bed reactor was previously brought to a preset value and then samples of coal were introduced. N, was used as inert carrier gas. During the devolatilization, both CO and CO, were monitored, and when the analysers could no longer detect these gases, the heating was switched off, with the flow of N, maintained. The reactor was cooled to _ 15°C and the flow of N, was turned off. The char thus prepared was sealed in plastic bags. Samples were analysed to determine the amount of fuel-N remaining in char. RESULTS
Figure 2 Experimental set-up: 1, electric furnace; 2, fluidized bed combustor; 3, thermocouple; 4, screw-feeder; 5, cooling system; 6, condenser; 7, collector; 8, filter; 9, flow meter; 10, three-way valve; II, recorder; 12-15, CO, CO,, H, and 0, analysers
bed combustion:
AND
DISCUSSION
Char-N20 reaction Chars produced from different coals were subjected to N,O under different conditions to determine their reactivities. The results suggested that the nature of the parent coal did not have a significant influence on the affinity of the char towards N,O, as shown in Figure 3. All the chars were highly reactive, and N,O adsorption
Table 1
Operating
conditions
Coal feed rate (g mini ‘) Char batch weight (g) Gas velocity (m SK’) Bed temperature (“C) Oxygen concentration (vol.%) N,O inlet concentration (ppmv) Particle size (mm) Sand bed weight (g)
0.5-5 5 0.24.5 700-l 100 2.5-7.5 lOG500 0.1254.0 500
20
EXPERIMENTAL Figure 2 represents schematically the experimental facility. The fluidized bed combustor is placed inside an electrically heated furnace and has an internal diameter of 60 mm and a height of 500 mm. The upper part of the combustor is heavily insulated to prevent heat loss. The distributor plate is of inverted cone type with 10 standpipes, each containing 10 nozzles. An electrical coil in the windbox preheats incoming gas before introduction into the combustor through the distributor plate. For combustion studies, the char was supplied through a small screw-feeder at the top of the combustor. In the case of char-N,0 reactions, the char samples were introduced in batches from the top of the combustor through two-valve lock system. The samples could be introduced at intervals varying from 20 s to 1 min. Reactions of char with N,O were carried out in the absence of oxygen and also with different amounts of
700
800
900
1 000
TEMPERATURE
(“C)
Figure 3 Effect of reaction temperature on residual N,O content of gas after reduction by chars prepared at 1100°C from different coals. N,O inlet concentration: 250 ppmv
Fuel 1994
Volume
73 Number
7
1099
N20
formation
during fluidized
bed combustion:
i. Gulyurtlu
CHAR PREPARED AT 1 000 “C 1 100°C 1 200 “C
700
800
900
tem~rature was varied from 1000 to 1200°C. The chars produced were then subjected to N,O. Figure 4 shows that the devolatilization temperature can have an important influence on the char reactivity. Over the range 700-85O”C, the char prepared at 1200°C showed the greatest adsorption capacity for N,O. This could be due to the fact that this char was of higher porosity, and also to the release at 1200°C of greater amounts of the C-O surface complexes, leaving a greater active surface area for N,O adsorption. Above 900°C the char reactivity to N,O became independent of the temperature of preparation, but this again could be a result of the increased degree of N,O dissociation. The char reactivity to N,O was not influenced by the presence of oxygen in the gas mixture introduced to the combustor. The char prepared at 1100°C was subjected to a gas mixture which contained oxygen varying from 2.5 to 7.5 vol.%. The level of N,O reduction was about the same as in the absence of oxygen, and the amount of oxygen present did not appear to interfere with the extent of N,O adsorption on the surface, as shown by Figure 5.
1 000
TEMPERATURE
et al.
(“C)
Figure 4 Effect of reaction temperature on residual N,O content of gas after reduction by chars prepared at different tem~ratllres from coal of 32.5 wt% volatile matter. N,O inlet concentration: 250 ppmv
r
The influence of pretreatment of the char surface before reaction with N,O was also investigated. Chars prepared at different temperatures were first subjected to a flow of oxygen in a reactor at room temperature. The results (Figure 6) suggest that the char reactivity to N,O was not influenced significantly by such pretreatment. There are two possible reasons for this: (1) the oxygen is adsorbed at different sites on the char surface; (2) the surface oxygen complexes provide sites for N,O adsorption, unlike NO adsorption. In all studies on char-N,0 reactions, very little NO formation was observed. This suggests that the N,O reduction did not lead to the formation of NO through
OXYGEN LNLET CONCENTRATION
CHAR PREPARED FROM ~~~~D~~~
1 200 “C
0
0 800
900
6
7 000
TEMPERATURE
(“C)
Figure5 Effect of reaction temperature on residual N,O content of gas after reduction by char in the presence of different oxygen concentrations. Char prepared at 1100°Cfrom coal of 32.5 wt% volatile matter. N,O inlet concentration: 250 ppmv
was almost complete even at temperatures as low as 700°C. Above 900°C very little N,O was measured in the gases leaving the combustor; this could be due to both N,O adsorption on the char surface and N,O dissociation, as N,O is unstable above 880°C. The effect of the temperature at which the char was prepared on the char reactivity was investigated. Coal of 32.5 wt% volatile matter was used and the devolatilization
1100
Fuel 1994 Volume 73 Number 7
0
700
800
/
900
1 000
TEMPERATURE
(“C)
Figure 6 Effect of reaction temperature on residual N,O content of gas after reduction by chars prepared at different temperatures from coal of 32.5 wt% volatile matter and pretreated with oxygen. N,O inlet concentration: 2.50 ppmv
NzO formation
% 8 @;
b
60_
B z 3
40
2 ? d
‘70 _
0 ‘0 ‘0
--COALS
WITH VOLATILE CONTENT - 22.5 % by weighl - 2X % by weight - 32.5 % by wright F
2 8 @
I
I 700
800
900
1 100
1 000
TEMPERATURE Figure 7 Effect fuel-N remaining
of devolatilization in chars prepared
temperature from different
(“C)
on proportion coals
of
during
fluidized
bed combustion:
1. Gulyurtlu
et al.
devolatilization temperatures were considered for that reason. The nature of the coal did not appear to have a large influence on the level of devolatilization and the extent of fuel-N release in the volatiles. Studies were carried out to determine the contribution of char combustion to the overall N,O formed. Chars prepared at different temperatures were burnt in the fluidized bed and the N,O in the combustion gases was measured. Figures 8 and 9 illustrate the N,O content as a function of bed temperature for different char preparation temperatures and excess air levels. It was found that char-N contributed -40% to the overall N,O formed during the combustion of the parent coal. The greater release of volatiles at higher char preparation temperatures resulted in smaller amounts of fuel-N remaining in the char; consequently the combustion of the char resulted in N,O levels ~40% of those formed in the combustion of the parent coal, as shown in Figure 9. Figure IO compares the N,O levels obtained from the combustion of chars produced from different coals at 1000 and 1300°C. The N contents of the parent coals were different. It is clear that the nitrogen remaining in the char was proportional to the N content of the parent coal. As a result, the N,O formed varied with the coal
Excess air levels
800 TEMPERATURE
700
900 (‘C)
Figure 8 Effect of bed temperature on amount of N,O formed during combustion at 50% excess air of chars prepared at different temperatures from coal of 32.5 wt% volatile matter and 1.4 wt% N
the following possible reactions: N,O + (C-)+NO
+ (C-O)
N,O + (C-O)+NO The end-products observed were CO and CO,. The CO content increased with temperature, as thermodynamically expected. The possible routes for N,O reduction could be the following: N,O+(C)+N,+CO 2N,O + (C-)-t2N,
+ CO,
700
800 TEMPERATURE
900 (“C)
Figure 9 Effect of bed temperature on amount of N,O formed during combustion at different excess air levels of coal of 32.5 wt% volatile matter and char prepared from it at 1100°C
N20+(C-O)+N2+C02 2N,O +(C-0)+2Nz
+ 2C0
The presence of N, in the gases leaving the combustor confirms in fact that most of the N,O was reduced directly to N,.
z 80 B 2 5 60 Y 2
Char combustion
Chars were prepared from different coals at different temperatures and the amount of fuel-N remaining in the char was determined. The N content of the chars depended greatly on the devolatilization temperature, and the fraction of the coal-N retained decreased from - 70 to - 40% when the temperature was increased from 700 to 1lOO”C,as shown in Figure 7. During the fluidized bed combustion of coal, the particle temperature could be as much as 300 K above that of the gas, and higher
Y 40 9 : 0 2 20 700
900
800 TEMPERATURE
($C)
Figure 10 Effect of bed temperature on amount ofN,O formed during combustion at 50% excess air of chars prepared at different temperatures from coals of N content (A) 1.1 wt%, (B) 1.4 wt%
Fuel 1994
Volume
73 Number
7
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N20 formation during fluidized bed combustion: /. Gulyurtiu et al.
type and with the temperature at which the char was produced. The chars produced at 1300°C contained much less fuel-N for both coals, and their combustion appeared to produce much lower N,O emissions, particularly in the case of coal B. This again confirms that the amount of N,O formed appears to be independent of the coal or char reactivity but depend on the fuel-N content and the temperature of combustion.
to be influenced by the presence of oxygen or NO. Even pretreatment of the surface with oxygen does not influence the N,O adsorption. Char appears to be highly reactive towards N,O.
ACKNOWLEDGEMENT The authors thank the European financing this project.
Community
for
CONCLUSIONS 1. In fluidized bed combustion of coal, if no control steps
2. 3.
4.
5.
are taken, the amount of N,O released can be as high as 120 ppmv. The formation and destruction of N,O involve both heterogeneous and homogeneous reactions. Most of the N,O formed appears to result from the combustion of volatiles, as the char combustion yields less than half of N,O produced when the parent coal is burnt. Most of the N,O reduction results in the formation of N, through heterogeneous reaction with char; however, a very small amount is also converted to NO. N,O adsorption on the char surface does not appear
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REFERENCES Andersson, S., Amand, L. E. and Leckner, B. Paper to IEA AFBC Technical Meeting, Amsterdam, November 1988 Amand, L. E. and Andersson, S. Paper to 10th International Conference on FBC, San Francisco, 1989, pp. 49-56 Gulyurtlu, I., Costa, M. R., Esparteiro, H., Monteiro, A. and Cabrita, I. Paper to 7th Annual International Pittsburgh Coal Conference, September 1990, pp. 177-186 Robv. R. J. and Bowman. C. T. Combust. Flame 1987, 70. 119 Hao:‘W. M., Wofsy, S. C:, McElroy, M. B., Farmayan, W. F. and Togan, M. A. Cornbust. Sci. Technol. 1987, 55, 23 De Soete, G. G. Paper to 23rd Symposium (International) on Combustion, 1990, pp. 1257-1264 Kramhch, J. C., Cole, J. A., McCarthy, J. M., Lanier, W. S. and McSorley, J. A. Cornbust. Flame 1989,77, 375