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Reduction of N2O emissions from circulating fluidized bed combustors by injection of fuel gases and changing of coal feed point
Energy Convers. Mgmt Vol. 37, Nos 6-8, pp. 1285-1290, 1996
Pergamon
0196-8904(95)00334-7
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/96 $15.00 + 0.00
REDUCTION OF N20 EMISSIONS FROM CIRCULATING FLUIDIZED BED COMBUSTORS BY INJECTION OF FUEL GASES AND CHANGING OF COAL FEED POINT YOSHIZO SUZUKI*, HIROSHI MORITOMI National Institute for Resources and Environment, 16-3, Onogawa, Tsukuba, Ibaraki, 305 Japan and HIROHISA T A N A K A Dept. o f Mechanical Engineering, Science Univ, o f Tokyo, 2641, Yamazaki, Noda, Chiba, 278 Japan Abstract - This paper describes the reduction techniques of N20 emff~.edfrom circulating fluidized bed combustion (CFBC) of coal. Two methods, injecting the fuel gases into the riser and changing the position where coal was supplied, were tried to decrease N20 emission. A small lab-scale CFBC whose whole parts were made of quartz was used. In the first method, methane, propane and hydrogen were used as a fuel gas. By injecting these gases into the riser, N20 emission was decreased remarkably. Reduction rate was almost in proportional to the volumetric flow rate of injected gas. At this time, NO emission did not increase. Required flow rate of injected gas to achieve the same N20 reduction was different among the above gases. N20 emission was able to be decreased remarkably by changing the position where coal was supplied. When coal was fed to the top of the downcommer, devolatilization took place and volatile matters were burnt in the cyclone. As the results, the temperature in the cyclone became high enough to reduce N20.
1. INTRODUCTION Circulating fluidized bed combustion (CFBC) is one of the most advanced coal combustion technologies having an environmentally acceptable manner. NOx emission is very low, and in-situ desulfurization efficiency is high. It is well known, however, that nitrous oxide (N20) emission is relatively high comparing to other coal combustors. Research works on the reduction of N20 have been done since early 90's. In FBC and CFBC, N20 cannot be decreased enough by two staged combustion method which has been effective for the NO reduction. Shimizu et al. [8-9] showed that N20 emission was able to be decreased effectively by using an supplemental fluidized bed of limestone particles to decompose N20. However, such method makes the CFBC or FBC system more complicated one. A simple and sure N20 reduction technique is required. Homogeneous N20 destruction reaction is strongly dependent on temperature according to previous research works [5-6] on the N20 formation mechanisms, and the N20 emission becomes very low in higher temperature than 1150 K. Therefore, the technique to make a part in the combustor a high temperature is effective to decrease N20. In order to make hot zones, natural gas or LPG was injected to the cyclone [12]. However, the increase of NO emission and the decrease of desulfurization efficiency take place. Injection of the fuel gas to some positions of the riser instead of the cyclone seems to be effective to * To whom all correspondence should be addressed. tD137:6/8-X
1285
SUZUKI et
1286
Flue
gas
!
Controklr Mass ROW
7~
Digital
sional
-trr Personal ~ e r
Cylinder Gas
Fig.1
I
-
al.:
REDUCTIONOF N20 EMISSIONS
~
Coal
feeder
l
Rlser,~
• feed
ter ''
:~ ~
~Oowncomer
Mainair Control
air
Schematic diagram of experimental System. Fig. 2
Details of lab-scale CFBC.
decrease N20 emission. It is actually assumed to inject coal particles with high volatile matter contents instead of the fuel gas. To obtain the fundamental data, the reduction of N20 emission by injecting the fuel gas to the riser of CFBC was evaluated. Moreover, reduction of N20 emission was also tried by a modification of combustion by changing the coal feed position from the bottom of the riser to the top of the downcomer. 2. EXPERIMENTAL A lab-scale CFBC system, shown in Fig. 1, was employed in this experiment. A detail schematic diagram of the CFBC is shown in Fig. 2. The main parts of this CFBC (a riser, a downcomer, a cyclone and an L-valve) are made of quartz. The riser is 23 m m in inside diameter and 2300 mm in height. The diameter of the downcomer is equal to that of the riser. Downcomer and riser are connected with an Lvalve. Both riser and downcomer were divided into five sections. In the riser section, all sections are covered with an electric furnace. In the downcomer, lower three sections are covered with an electric furnace, and upper two sections are covered with heat insulators. Power supplied to each electric furnace was controlled independently. Circulation of solid particles was driven by injecting the air into the L-valve from two positions shown in Fig. 2. Silica sand particles, the average diameter of 0.1 mm, were used as a bed material. A typical circulating rate was 20 ~ 30 kg/m2.s. Twenty-three taps are installed in the riser wall at intervals of 100 mm. This taps are utilized for measurement of temperature and static pressure, sampling of gas and particles from the riser, and injection of gases into the riser. In fuel gas injection trials, the fuel gases were injected through the taps shown in Fig. 2. Methane, propane and hydrogen gases used to inject were commercial grade and their purity was up to 99.99%. Flow rates of above gases were controlled by a mass flow controller and were injected into the center of the riser through a thin quartz tube. The coal used was Datong Coal (Chinese coal), and the proximate and ultimate analysis values are listed in Table 1. Mean diameter of coal particles was as same as that of sand particles. Coal particles were Table 1. Anal},ses of Daton~ coal. usually fed to combustor from riser bottom by a specially designed Proximate analysis (dry, woW,) pneumatic transportation type feeder. Limestone particles for V.M. 30.4 desulfurization were not added in these experiments. In the F.C. 61.6 Ash 8.0 experiments with changing the feed point of coal, coal particles were fed to the top of the downcomer by a small screw feeder. Ultimate analysis (dry, wet*/.) For 02, CO 2, CO, NO and N20, on-line analyzers were used. A gas chromatography was also used in N20 measurement to detect the CH4 which interferes NDIR N20 analyzers. Analog output data from gas analyzers and therrnocouples to measure the temperature
C H
74.8 4.3
O
11.6
N S
0.9 0.4
SUZUKI et al.: REDUCTION OF N20 EMISSIONS
1287
distributions in the combustor were converted to the digital data and were recorded on a computer every five second. 3. RESULTS AND DISCUSSION Reduction of N~O emission bv injection of fuel t,ases into the riser NO and N20 emissions may change when the fuel gases are injected into the riser, because oxygen concentration in flue gas changes. Therefore, the influence of oxygen concentration on NO and N20 emissions was checked in the beginning and an appropriate experimental condition was set by preliminary combustion experiments. In general, NO and N20 emissions have an increasing tendency according to the increase of oxygen concentration in flue gas, A similar tendency was observed in the lab-scale CFBC used in this work. However, the dependency to the oxygen concentration of NO and N20 emission was not large in the range from 2 to 8% of oxygen concentration in the flue gas. Because the NO and N20 emissions were able to be considered almost constant in above oxygen concentration range, the experimental condition was set so that the oxygen concentration in flue gas might became within this range when the fuel gases were injected into the riser. Oxygen concentration was typically kept as 8% before injection of fuel gases, and oxygen concentration in flue gas decreased according to fuel gas injection and fmal!y reached to 2%. The reduction rate in NO or N20 emission which decreased as a result of injecting the fuel gases into the riser was defined by the following expression. 1 1 = ( 1 - X/X0) x 100 where: rl: Reduction rate of NO or N20 by fuel gas injection. [%] X0: Conversion of fuel-N to NO or N20 with coal combustion. [ - ] X: Conversion of fuel-N to NO or N20 after fuel gas injection. [ - ] A temperature of 1123K was chosen to be a standard temperature in the riser at the injection of the fuel gases. In this condition, typical NO and N20 concentration were 100 ppm and 380 ppm respectively. The reduction rate of N20 and NO is shown in Figs. 3 and 4 respectively as a function of the volumetric gas injection rate in the standard temperature and pressure condition when propane was injected from the tap shown in Fig. 2. The N20 emission was greatly decreased by injection of propane, and the reduction rate of NzO was almost proportional to the volumetric gas injection rate. Moreover, it is shown that the third injection port is the best position from the results shown in Fig. 3. The third injection port is a position of 1/3 in the upper part of the riser and the effect of the fuel gas injection on the decrease of N20 emission becomes smaller when fuel gas is injected in the upper or lower position than this. The decrease of NO emission was observed at the same time in many cases for injection of propane. However, the NO emission has increased when propane was injected into the third injection port contrary to the case to N20. When methane or hydrogen was injected, N20 emission decreases remarkably as propane was injected.
50 ..~ :::::::::::::::::::::::: Bottom
40 l r~\\xx~ Injection1 tO °--
30 20
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........................... l ..................
. . . . . . .
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10 0 50
100
150
200
C3H 8 injection, cm3/min Fig. 3
N20 reduction by C3Hs injection.
50
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C3H8 injection, cm3/min Fig. 4 NO reduction by C3H8injection.
1288
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OF N20 EMISSIONS
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Fig. 5 Change in temperature distribution by injection of fuel gases. N20 reduction rate was about 35%. (a) Injection of C3Hs from various injection ports. (b) Injection of C3H8 from injection port #3 at various injection rates. (c) Injection of H 2 from injection port #3 at various injection rates. Moreover, the decrease of NO was seen for hydrogen at all injection positions. Absolute amount of gas injected has increased greatly, about six times larger than that of propane for hydrogen, two times larger for methane to obtain the same N20 reduction rate. Both cases of propane and hydrogen injection, N20 reduction up to 35% was achieved in this experiment. Oxygen consumption by the injected fuel gases was about 25% of whole oxygen consumption when fuel gases injected at maximum flow rate. Injection of the fuel gases was extremely effective in the reduction of N20 as shown above. There are two possible mechanisms for the reduction of N20. The first possible mechanism is the decomposition reaction of N20 is promoted by the temperature rising by burning the fuel gas. The second is an effect that N20 emission decreases by the radicals, generated when the fuel gas is burnt, takes part in the destruction reaction of N20. We examine which mechanism is dominant in actual conditions from experimental results. Figure 5 shows the temperature distributions measured by the thermocouples inserted into the center of the riser when the maximum N20 reduction rate was obtained in propane and hydrogen injection trials. When propane was injected into the riser, the temperature in the riser increased compared with the case that no gases were injected. This temperature rise, however, was comparatively small and was 15 K or less. Moreover, when hydrogen was injected, the temperature rise was about 10K and became smaller comparing with propane. By visual observation, when the fuel gas was injected, formation of stable flame was not observed. The reason for this may be that the formation of the flame is disturbed because of existence of a large amount of solid particles in the riser. However, these data can not deny the existence of local hot spots in the riser because measured temperatures were only
50
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Comparison of C3H8 and H 2 in molar injection rate.
.-TkWr-rw, , I - '"+ I 40 ...-.+
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0
0,1
0.2
0.3
Additional heat input, kW
Fig. ?
Comparison of C3Hs and H 2 in heat release rate.
SUZUKI et al.: REDUCTIONOF N20 EMISSIONS
1289
.~verage temperature indicated by thermocouples. To achieve the same N20 reduction rate, as in the above-mentioned, the six times larger amount of gas injection was needed for hydrogen compared with propane. In order to evaluate hydrogen radicals, the volumetric injection rates are converted into the equivalent molar hydrogen flow rate, and the relation between the N20 reduction rate and the injection rate is shown in Fig. 6. One propane molecule equivalently contains four hydrogen molecules. Paying attention to the hydrogen atoms in a propane molecule, the molar injection rates of propane were multiplied four and are shown in black circles in Fig. 6. For methane, molar injection rates of methane were multiplied two. When the flow rate of methane and propane are evaluated with the equivalent molar hydrogen flow rate, the relation between the reduction rate and the injection rate becomes considerably same among three gases. On the other hand, the relationship between the reduction rate and the amount of heat added by combustion of injected fuel gas is shown in Fig. 7 as another approach. The calculation was made by using higher caloric value of 0.890 MJ/mol for methane, 2.218 MJ/mol for propane and 0.286 MJ/mol for hydrogen respectively [3]. As can be seen, when the relation between the N20 reduction rate and the gas injection rate is arranged with the heat addition by the combustion of the fuel gas, there are no differences in methane, propane and hydrogen. When Figs. 6 and 7 are compared, the correlation in Fig. 7 is better. A thermal effect seems to be able to explain the decrease of N20 emission by injection of the fuel gas from the correlation in Fig. 7. However, because small temperature rise is shown in Fig. 5 (c), it is not possible to explain N20 reduction simply by the thermal effect. Hirama [4] carded a similar experiment using a bubbling fluidized bed combustor recently and confirmed the decrease of N20 emission. It was shown that radicals were effective in Hirama's case although the influence of the temperature rise in freeboard on N20 reduction was large. It is not possible to judge whether radical's chemical effezt or simple thermal effect is dominant from above data only. It is necessary to accumulate more data. Reduction of N20 emission by changing the feed point A reduction technique of N20 by changing the coal feed point from the bottom of the riser to the top of the downcomer was tried. /~mand et al. [1] and Gustavsson et al. [2] installed a small gas burner in the cyclone to make a high temperature zone, and showed the considerable destruction of N20. The method in this work is modification of their techniques by utilization of coal only. Figure 8 shows the concept of this method. Devolatilization of coal takes place because of the reductive atmosphere when coal is supplied to the top of the downcomer, and volatile matters formed bums in the cyclone. Therefore, the temperature in the cyclone will become higher and N20 generated in the riser section will be destructed. On the other hand, char particles generated in the downcomer section move to the riser and bum. The order of combustion stages in CFBC, char combustion succeeds to volatile combustion, leads a disadvantageous condition in order to reduce N20 emission. N20 is easily destructed in a flame. However, N20 produced in char combustion will emit directly. In this modified coal feed method, the order of combustion stages is reversed to destruct N20 formed in char combustion. Although there is a possibility that trouble happens in the coal supply by forming the tar at the feed point, no trouble in coal feed was found and coal was supplied successfully. The flame was observed intermittently in the cyclone. NO and N20 emissions when whole quantity of coal was supplied to the top of the downcomer are shown in Fig. 9 by the comparison with the usual way of coal supplying. NzO emission became 1/5 by greatly decreasing when coal was supplied to the top of the downcomer as shown in Fig. 9. However, NO emission increased a_o.dbecame two times or more at the same time. It is shown that distribution in the conversion of fuel-N to NO or N20 can be changed by the changing of the coal feed position in CFBC. When the whole coal had been fed to the top of the downcomer, the increase of NO emission occurred, even though the decrease of N20 was achieved. It is more realistic that coal is supplied both to the top of the downcomer and to the bottom of the riser.
....
oal
iiiii Char
Comboslc~
iiiii~iiii!i!i!l
Char
!jJi3i i i i il 4 Air
Fig. 8
Concept of coal feed to the top of the downcomer.
S U Z U K I et al.: R E D U C T I O N O F N 2 0 E M I S S I O N S
1290
500
Cc~txmlc n of volalllu
~" 400
8 E 300
+ O
200 100
Z
0 Downcomer Top
Fig. 9
Riser Bottom
NO and N20 emission when coal was fed to the top of the downcomer.
500
1000
i 1500 2000 2500 3000
Height, mm Fig. 10 NO and N20 concentration profiles in the riser when coal was fed to the top of the downcomer.
Figure 10 shows the concentration profiles of NO and N20 in the riser when coal was supplied to the top of the downcomer. In general, N20 shows the profile that increases toward the riser height, and NO shows the decreasing profile toward the riser height when coal was supplied to the bottom of the riser [7]. However, monotonously increasing profiles of NO and N20 toward the riser height were obtained in this supplying method. Moreover, it is shown that NO and N20 concentrations do not change consequentially before and after the passage through the cyclone. This means that formation of NO and N20 balances the destruction in the cyclone. The concentration profiles of NO and N20 shown in Fig. 10 are very strange and give important information in understanding the formation of N20 in CFBC. More detailed research is necessary. 4. CONCLUSION Two methods, injecting the fuel gases into the riser and changing the position where coal was supplied, were tried in order to reduce N20 emission from CFBC. Methane, propane and hydrogen gas were used as a fuel gas in the first method. Based on the experiment results, the following findings were obtained: (1) N20 emission was decreased remarkably. Reduction rate was almost in proportional to the amount of injected gas. (2) There was an optimum injection point to achieve maximum reduction rate when amount of injected gas was kept constant. Maximum N20 reduction of 40% was achieved when propane was injected into the riser. At this time, NO emission did not increase. N20 and NO were able to be decreased at the same time in some conditions. (3) When the flow rate of injected gas was evaluated with equivalent hydrogen molar flow rate or heat of combustion, good correlations were obtained among the injected gases to achieve same N20 reduction. (4) N20 emission was able to be decreased remarkably by changing the position where coal was supplied. In this method, however, NO emission increased. REFERENCES 1. L.E. •mand, et al.: European Workshop on N20 Emissions LNETI/EPA/IFP, Lisbon, Pom~gal 1990. 2. L. Gustavsson, et al.: Proc. of the 1lth Int. Conf. on Fluidized Bed Combustion, Montreal, ed. by E. J. Anthony, p. 677 (1991). 3. M. Hasatani and J. Kimura: "Nensho no Kiso to Ouyo (Fundamentals and Applications of Combustion ", p. 18, Kyo-ritsu (1986). 4. T. HJrama, et al.: Preprint of the 27th Autumn Meeting of the SCEJ, H213 (1994). 5. T. Hulgaad, et al.: Proc. of the 1 lth Int. Conf. on Fluidized Bed Combustion, Montreal, ed. by E. J. Anthony, ASME, p. 991 (1991). 6. J.E. Johnsson, et al.: Preprint of the 5th Int. Workshop on Nitrous Oxide Emissions, Tsukuba, .Tapan, 4-5 (1992). 7. H. Moritomi, et al." Circulating Fluidized Bed Tech. III, ed. by P. Basu et al., Pergamon Press, p. 399 (1991). 8. T. Shimizu, et al.: Proe. of the 6th SCEJ Symp. on Circulating Fluizized Beds, p. 129 (1993). 9. T. Shimizu, et al.: Energy & Fuels, 7, p. 645 (1993). 10. Y. Suzuki, et al.: Preprint of the 27th Autumn Meeting of the SCEJ, H214 (1994).
Pergamon
0196-8904(95)00334-7
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/96 $15.00 + 0.00
REDUCTION OF N20 EMISSIONS FROM CIRCULATING FLUIDIZED BED COMBUSTORS BY INJECTION OF FUEL GASES AND CHANGING OF COAL FEED POINT YOSHIZO SUZUKI*, HIROSHI MORITOMI National Institute for Resources and Environment, 16-3, Onogawa, Tsukuba, Ibaraki, 305 Japan and HIROHISA T A N A K A Dept. o f Mechanical Engineering, Science Univ, o f Tokyo, 2641, Yamazaki, Noda, Chiba, 278 Japan Abstract - This paper describes the reduction techniques of N20 emff~.edfrom circulating fluidized bed combustion (CFBC) of coal. Two methods, injecting the fuel gases into the riser and changing the position where coal was supplied, were tried to decrease N20 emission. A small lab-scale CFBC whose whole parts were made of quartz was used. In the first method, methane, propane and hydrogen were used as a fuel gas. By injecting these gases into the riser, N20 emission was decreased remarkably. Reduction rate was almost in proportional to the volumetric flow rate of injected gas. At this time, NO emission did not increase. Required flow rate of injected gas to achieve the same N20 reduction was different among the above gases. N20 emission was able to be decreased remarkably by changing the position where coal was supplied. When coal was fed to the top of the downcommer, devolatilization took place and volatile matters were burnt in the cyclone. As the results, the temperature in the cyclone became high enough to reduce N20.
1. INTRODUCTION Circulating fluidized bed combustion (CFBC) is one of the most advanced coal combustion technologies having an environmentally acceptable manner. NOx emission is very low, and in-situ desulfurization efficiency is high. It is well known, however, that nitrous oxide (N20) emission is relatively high comparing to other coal combustors. Research works on the reduction of N20 have been done since early 90's. In FBC and CFBC, N20 cannot be decreased enough by two staged combustion method which has been effective for the NO reduction. Shimizu et al. [8-9] showed that N20 emission was able to be decreased effectively by using an supplemental fluidized bed of limestone particles to decompose N20. However, such method makes the CFBC or FBC system more complicated one. A simple and sure N20 reduction technique is required. Homogeneous N20 destruction reaction is strongly dependent on temperature according to previous research works [5-6] on the N20 formation mechanisms, and the N20 emission becomes very low in higher temperature than 1150 K. Therefore, the technique to make a part in the combustor a high temperature is effective to decrease N20. In order to make hot zones, natural gas or LPG was injected to the cyclone [12]. However, the increase of NO emission and the decrease of desulfurization efficiency take place. Injection of the fuel gas to some positions of the riser instead of the cyclone seems to be effective to * To whom all correspondence should be addressed. tD137:6/8-X
1285
SUZUKI et
1286
Flue
gas
!
Controklr Mass ROW
7~
Digital
sional
-trr Personal ~ e r
Cylinder Gas
Fig.1
I
-
al.:
REDUCTIONOF N20 EMISSIONS
~
Coal
feeder
l
Rlser,~
• feed
ter ''
:~ ~
~Oowncomer
Mainair Control
air
Schematic diagram of experimental System. Fig. 2
Details of lab-scale CFBC.
decrease N20 emission. It is actually assumed to inject coal particles with high volatile matter contents instead of the fuel gas. To obtain the fundamental data, the reduction of N20 emission by injecting the fuel gas to the riser of CFBC was evaluated. Moreover, reduction of N20 emission was also tried by a modification of combustion by changing the coal feed position from the bottom of the riser to the top of the downcomer. 2. EXPERIMENTAL A lab-scale CFBC system, shown in Fig. 1, was employed in this experiment. A detail schematic diagram of the CFBC is shown in Fig. 2. The main parts of this CFBC (a riser, a downcomer, a cyclone and an L-valve) are made of quartz. The riser is 23 m m in inside diameter and 2300 mm in height. The diameter of the downcomer is equal to that of the riser. Downcomer and riser are connected with an Lvalve. Both riser and downcomer were divided into five sections. In the riser section, all sections are covered with an electric furnace. In the downcomer, lower three sections are covered with an electric furnace, and upper two sections are covered with heat insulators. Power supplied to each electric furnace was controlled independently. Circulation of solid particles was driven by injecting the air into the L-valve from two positions shown in Fig. 2. Silica sand particles, the average diameter of 0.1 mm, were used as a bed material. A typical circulating rate was 20 ~ 30 kg/m2.s. Twenty-three taps are installed in the riser wall at intervals of 100 mm. This taps are utilized for measurement of temperature and static pressure, sampling of gas and particles from the riser, and injection of gases into the riser. In fuel gas injection trials, the fuel gases were injected through the taps shown in Fig. 2. Methane, propane and hydrogen gases used to inject were commercial grade and their purity was up to 99.99%. Flow rates of above gases were controlled by a mass flow controller and were injected into the center of the riser through a thin quartz tube. The coal used was Datong Coal (Chinese coal), and the proximate and ultimate analysis values are listed in Table 1. Mean diameter of coal particles was as same as that of sand particles. Coal particles were Table 1. Anal},ses of Daton~ coal. usually fed to combustor from riser bottom by a specially designed Proximate analysis (dry, woW,) pneumatic transportation type feeder. Limestone particles for V.M. 30.4 desulfurization were not added in these experiments. In the F.C. 61.6 Ash 8.0 experiments with changing the feed point of coal, coal particles were fed to the top of the downcomer by a small screw feeder. Ultimate analysis (dry, wet*/.) For 02, CO 2, CO, NO and N20, on-line analyzers were used. A gas chromatography was also used in N20 measurement to detect the CH4 which interferes NDIR N20 analyzers. Analog output data from gas analyzers and therrnocouples to measure the temperature
C H
74.8 4.3
O
11.6
N S
0.9 0.4
SUZUKI et al.: REDUCTION OF N20 EMISSIONS
1287
distributions in the combustor were converted to the digital data and were recorded on a computer every five second. 3. RESULTS AND DISCUSSION Reduction of N~O emission bv injection of fuel t,ases into the riser NO and N20 emissions may change when the fuel gases are injected into the riser, because oxygen concentration in flue gas changes. Therefore, the influence of oxygen concentration on NO and N20 emissions was checked in the beginning and an appropriate experimental condition was set by preliminary combustion experiments. In general, NO and N20 emissions have an increasing tendency according to the increase of oxygen concentration in flue gas, A similar tendency was observed in the lab-scale CFBC used in this work. However, the dependency to the oxygen concentration of NO and N20 emission was not large in the range from 2 to 8% of oxygen concentration in the flue gas. Because the NO and N20 emissions were able to be considered almost constant in above oxygen concentration range, the experimental condition was set so that the oxygen concentration in flue gas might became within this range when the fuel gases were injected into the riser. Oxygen concentration was typically kept as 8% before injection of fuel gases, and oxygen concentration in flue gas decreased according to fuel gas injection and fmal!y reached to 2%. The reduction rate in NO or N20 emission which decreased as a result of injecting the fuel gases into the riser was defined by the following expression. 1 1 = ( 1 - X/X0) x 100 where: rl: Reduction rate of NO or N20 by fuel gas injection. [%] X0: Conversion of fuel-N to NO or N20 with coal combustion. [ - ] X: Conversion of fuel-N to NO or N20 after fuel gas injection. [ - ] A temperature of 1123K was chosen to be a standard temperature in the riser at the injection of the fuel gases. In this condition, typical NO and N20 concentration were 100 ppm and 380 ppm respectively. The reduction rate of N20 and NO is shown in Figs. 3 and 4 respectively as a function of the volumetric gas injection rate in the standard temperature and pressure condition when propane was injected from the tap shown in Fig. 2. The N20 emission was greatly decreased by injection of propane, and the reduction rate of NzO was almost proportional to the volumetric gas injection rate. Moreover, it is shown that the third injection port is the best position from the results shown in Fig. 3. The third injection port is a position of 1/3 in the upper part of the riser and the effect of the fuel gas injection on the decrease of N20 emission becomes smaller when fuel gas is injected in the upper or lower position than this. The decrease of NO emission was observed at the same time in many cases for injection of propane. However, the NO emission has increased when propane was injected into the third injection port contrary to the case to N20. When methane or hydrogen was injected, N20 emission decreases remarkably as propane was injected.
50 ..~ :::::::::::::::::::::::: Bottom
40 l r~\\xx~ Injection1 tO °--
30 20
004 z
I
I ............. 1°l~°.4
--I
........................... l ..................
. . . . . . .
O Z
10 0 50
100
150
200
C3H 8 injection, cm3/min Fig. 3
N20 reduction by C3Hs injection.
50
100
150
200
C3H8 injection, cm3/min Fig. 4 NO reduction by C3H8injection.
1288
SUZUKI 2500
....
! ....
et
REDUCTION
al.:
2500'''
i ......
''
I
. . . .
OF N20 EMISSIONS
2500 .
' . . . . . . . .
+
2000
. . . .
, . . . . . . . . . .
+oo .............~........................ •
.....
Injection 3
E E
1500
/•~
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Fig. 5 Change in temperature distribution by injection of fuel gases. N20 reduction rate was about 35%. (a) Injection of C3Hs from various injection ports. (b) Injection of C3H8 from injection port #3 at various injection rates. (c) Injection of H 2 from injection port #3 at various injection rates. Moreover, the decrease of NO was seen for hydrogen at all injection positions. Absolute amount of gas injected has increased greatly, about six times larger than that of propane for hydrogen, two times larger for methane to obtain the same N20 reduction rate. Both cases of propane and hydrogen injection, N20 reduction up to 35% was achieved in this experiment. Oxygen consumption by the injected fuel gases was about 25% of whole oxygen consumption when fuel gases injected at maximum flow rate. Injection of the fuel gases was extremely effective in the reduction of N20 as shown above. There are two possible mechanisms for the reduction of N20. The first possible mechanism is the decomposition reaction of N20 is promoted by the temperature rising by burning the fuel gas. The second is an effect that N20 emission decreases by the radicals, generated when the fuel gas is burnt, takes part in the destruction reaction of N20. We examine which mechanism is dominant in actual conditions from experimental results. Figure 5 shows the temperature distributions measured by the thermocouples inserted into the center of the riser when the maximum N20 reduction rate was obtained in propane and hydrogen injection trials. When propane was injected into the riser, the temperature in the riser increased compared with the case that no gases were injected. This temperature rise, however, was comparatively small and was 15 K or less. Moreover, when hydrogen was injected, the temperature rise was about 10K and became smaller comparing with propane. By visual observation, when the fuel gas was injected, formation of stable flame was not observed. The reason for this may be that the formation of the flame is disturbed because of existence of a large amount of solid particles in the riser. However, these data can not deny the existence of local hot spots in the riser because measured temperatures were only
50
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Comparison of C3H8 and H 2 in molar injection rate.
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SUZUKI et al.: REDUCTIONOF N20 EMISSIONS
1289
.~verage temperature indicated by thermocouples. To achieve the same N20 reduction rate, as in the above-mentioned, the six times larger amount of gas injection was needed for hydrogen compared with propane. In order to evaluate hydrogen radicals, the volumetric injection rates are converted into the equivalent molar hydrogen flow rate, and the relation between the N20 reduction rate and the injection rate is shown in Fig. 6. One propane molecule equivalently contains four hydrogen molecules. Paying attention to the hydrogen atoms in a propane molecule, the molar injection rates of propane were multiplied four and are shown in black circles in Fig. 6. For methane, molar injection rates of methane were multiplied two. When the flow rate of methane and propane are evaluated with the equivalent molar hydrogen flow rate, the relation between the reduction rate and the injection rate becomes considerably same among three gases. On the other hand, the relationship between the reduction rate and the amount of heat added by combustion of injected fuel gas is shown in Fig. 7 as another approach. The calculation was made by using higher caloric value of 0.890 MJ/mol for methane, 2.218 MJ/mol for propane and 0.286 MJ/mol for hydrogen respectively [3]. As can be seen, when the relation between the N20 reduction rate and the gas injection rate is arranged with the heat addition by the combustion of the fuel gas, there are no differences in methane, propane and hydrogen. When Figs. 6 and 7 are compared, the correlation in Fig. 7 is better. A thermal effect seems to be able to explain the decrease of N20 emission by injection of the fuel gas from the correlation in Fig. 7. However, because small temperature rise is shown in Fig. 5 (c), it is not possible to explain N20 reduction simply by the thermal effect. Hirama [4] carded a similar experiment using a bubbling fluidized bed combustor recently and confirmed the decrease of N20 emission. It was shown that radicals were effective in Hirama's case although the influence of the temperature rise in freeboard on N20 reduction was large. It is not possible to judge whether radical's chemical effezt or simple thermal effect is dominant from above data only. It is necessary to accumulate more data. Reduction of N20 emission by changing the feed point A reduction technique of N20 by changing the coal feed point from the bottom of the riser to the top of the downcomer was tried. /~mand et al. [1] and Gustavsson et al. [2] installed a small gas burner in the cyclone to make a high temperature zone, and showed the considerable destruction of N20. The method in this work is modification of their techniques by utilization of coal only. Figure 8 shows the concept of this method. Devolatilization of coal takes place because of the reductive atmosphere when coal is supplied to the top of the downcomer, and volatile matters formed bums in the cyclone. Therefore, the temperature in the cyclone will become higher and N20 generated in the riser section will be destructed. On the other hand, char particles generated in the downcomer section move to the riser and bum. The order of combustion stages in CFBC, char combustion succeeds to volatile combustion, leads a disadvantageous condition in order to reduce N20 emission. N20 is easily destructed in a flame. However, N20 produced in char combustion will emit directly. In this modified coal feed method, the order of combustion stages is reversed to destruct N20 formed in char combustion. Although there is a possibility that trouble happens in the coal supply by forming the tar at the feed point, no trouble in coal feed was found and coal was supplied successfully. The flame was observed intermittently in the cyclone. NO and N20 emissions when whole quantity of coal was supplied to the top of the downcomer are shown in Fig. 9 by the comparison with the usual way of coal supplying. NzO emission became 1/5 by greatly decreasing when coal was supplied to the top of the downcomer as shown in Fig. 9. However, NO emission increased a_o.dbecame two times or more at the same time. It is shown that distribution in the conversion of fuel-N to NO or N20 can be changed by the changing of the coal feed position in CFBC. When the whole coal had been fed to the top of the downcomer, the increase of NO emission occurred, even though the decrease of N20 was achieved. It is more realistic that coal is supplied both to the top of the downcomer and to the bottom of the riser.
....
oal
iiiii Char
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Fig. 8
Concept of coal feed to the top of the downcomer.
S U Z U K I et al.: R E D U C T I O N O F N 2 0 E M I S S I O N S
1290
500
Cc~txmlc n of volalllu
~" 400
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200 100
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0 Downcomer Top
Fig. 9
Riser Bottom
NO and N20 emission when coal was fed to the top of the downcomer.
500
1000
i 1500 2000 2500 3000
Height, mm Fig. 10 NO and N20 concentration profiles in the riser when coal was fed to the top of the downcomer.
Figure 10 shows the concentration profiles of NO and N20 in the riser when coal was supplied to the top of the downcomer. In general, N20 shows the profile that increases toward the riser height, and NO shows the decreasing profile toward the riser height when coal was supplied to the bottom of the riser [7]. However, monotonously increasing profiles of NO and N20 toward the riser height were obtained in this supplying method. Moreover, it is shown that NO and N20 concentrations do not change consequentially before and after the passage through the cyclone. This means that formation of NO and N20 balances the destruction in the cyclone. The concentration profiles of NO and N20 shown in Fig. 10 are very strange and give important information in understanding the formation of N20 in CFBC. More detailed research is necessary. 4. CONCLUSION Two methods, injecting the fuel gases into the riser and changing the position where coal was supplied, were tried in order to reduce N20 emission from CFBC. Methane, propane and hydrogen gas were used as a fuel gas in the first method. Based on the experiment results, the following findings were obtained: (1) N20 emission was decreased remarkably. Reduction rate was almost in proportional to the amount of injected gas. (2) There was an optimum injection point to achieve maximum reduction rate when amount of injected gas was kept constant. Maximum N20 reduction of 40% was achieved when propane was injected into the riser. At this time, NO emission did not increase. N20 and NO were able to be decreased at the same time in some conditions. (3) When the flow rate of injected gas was evaluated with equivalent hydrogen molar flow rate or heat of combustion, good correlations were obtained among the injected gases to achieve same N20 reduction. (4) N20 emission was able to be decreased remarkably by changing the position where coal was supplied. In this method, however, NO emission increased. REFERENCES 1. L.E. •mand, et al.: European Workshop on N20 Emissions LNETI/EPA/IFP, Lisbon, Pom~gal 1990. 2. L. Gustavsson, et al.: Proc. of the 1lth Int. Conf. on Fluidized Bed Combustion, Montreal, ed. by E. J. Anthony, p. 677 (1991). 3. M. Hasatani and J. Kimura: "Nensho no Kiso to Ouyo (Fundamentals and Applications of Combustion ", p. 18, Kyo-ritsu (1986). 4. T. HJrama, et al.: Preprint of the 27th Autumn Meeting of the SCEJ, H213 (1994). 5. T. Hulgaad, et al.: Proc. of the 1 lth Int. Conf. on Fluidized Bed Combustion, Montreal, ed. by E. J. Anthony, ASME, p. 991 (1991). 6. J.E. Johnsson, et al.: Preprint of the 5th Int. Workshop on Nitrous Oxide Emissions, Tsukuba, .Tapan, 4-5 (1992). 7. H. Moritomi, et al." Circulating Fluidized Bed Tech. III, ed. by P. Basu et al., Pergamon Press, p. 399 (1991). 8. T. Shimizu, et al.: Proe. of the 6th SCEJ Symp. on Circulating Fluizized Beds, p. 129 (1993). 9. T. Shimizu, et al.: Energy & Fuels, 7, p. 645 (1993). 10. Y. Suzuki, et al.: Preprint of the 27th Autumn Meeting of the SCEJ, H214 (1994).