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Reduction of SO2 and N2O Emissions without Increasing NOx Emission from a Fluidized Bed Combustor by Using Fine Limestone Particles Tadaaki Shimizu,* Masato Satoh, Kazuna Sato, Masaru Tonsho, and Makoto Inagaki Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181, Japan Received June 18, 2001. Revised Manuscript Received October 17, 2001
A modification of in-situ SO2 removal was proposed to suppress the increase in NOx with limestone feed to a bubbling fluidized bed combustor (BFBC). A laboratory-scale BFBC was operated. Coarse coal was burnt in the dense bed and fine limestone particles were employed so that only sorbent particles were entrained to the freeboard, thus SO2 removal by the sorbent was separated from the combustion of coal. By separating SO2 removal from combustion, an increase in NOx due to contact between sorbent and volatile-N could be avoided. Also N2O reduction by SO2 removal was observed.
Introduction An advantage of fluidized bed combustors (FBCs), both bubbling FBC (BFBC, Figure 1) and circulating FBC (CFBC), is SO2 capture by sorbent (limestone) feed into the bed without external wet scrubbing systems. However, SO2 capture by limestone feed into atmospheric FBCs is known to increase NOx emission.1-6 The increase in NOx emission is explained by the catalytic activity of calcined limestone (CaO) to oxidize nitrogencontaining volatile matter (VM-N) to mainly NO; NH3 and HCN are released from the fuel during devolatilization and oxidation of VM-N is catalyzed by calcined limestone.6-9 * Corresponding author. Tel/Fax: (Int’l) +81-25-262-6783 (direct). E-mail:
[email protected]. (1) Tatebayashi, J.; Okada, Y.; Yano, K.; Ikeda, S. Proceedings of the 6th International Conference on Fluidized Bed Combustion (Atlanta, GA); U.S. Department of Energy, Office of Coal Utilization: Washington, DC, 1980; pp 986-995. (2) Hirama, T.; Takeuchi, H.; Horio, M. Proceedings of the 9th International Conference on Fluidized Bed Combustion (Boston, MA); ASME: New York, 1987; p 898. (3) Åmand L.-E.; Leckner, B.; Andersson, S.; Gustavsson, L. European workshop on N2O emissions LNETI/EPA/IFP (Lisbon, Portugal), 1990; p 171. (4) Gavin, D. G.; Dorrington, M. A. Proceedings of the 1991 International Conference on Coal Science (Newcastle, U.K.), International Energy Agency Coal Research Ltd.; Butterworth-Heinemann: Oxford, U.K., 1991; pp 347-350. (5) Shimizu, T.; Tachiyama, Y.; Kuroda, A.; Inagaki, M. Fuel 1992a, 71, 841-845. (6) Shimizu, T.; Tachiyama, Y.; Fujita, D.; Kumazawa, K.; Wakayama, O.; Ishizu, K.; Kobayashi, S.; Shikada, S.; Inagaki, M. Energy Fuels 1992b, 6, 753-757. (7) Hirama, T.; Kochiyama, Y.; Chiba, T.; Kobayashi, H. NenryoKyokai-Shi (J. Fuel Soc. Jpn.) 1982, 61, 268-274. (8) Furusawa, T.; Tsujimura, M.; Yasunaga, K.; Kojima, T. Proceedings of the 8th International Conference on Fluidized Bed Combustion (Houston, TX), DOE/METC-85/6021; U.S. Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; pp 1095-1104. (9) Lee, Y. Y.; Sekthira, A.; Wong, C. M. Proceedings of the 8th International Conference on Fluidized Bed Combustion (Houston, TX), DOE/METC-85/6021; U.S. Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; pp 1208-1218.
Another problem of FBCs is the emission of a large amount of N2O. N2O has become a focus of attention not only as a greenhouse gas but also as an agent of ozone destruction in the stratosphere. Calcined limestone (CaO) is known to be a catalyst to decompose N2O.6,10-13 Indeed, N2O emission from FBC has been reported to decrease with sorbent feed.4,5 As an approach to reduce SO2 emission without increasing NOx emission, two-stage BFBC has been proposed, in which combustion of fuel is conducted in the lower bed and SO2 capture from the flue gas is carried out in the upper bed (Figure 1b).14-16 By separating the combustion zone and the desulfurization zone, the increase in NOx with SO2 capture by limestone can be suppressed. Since a negligible amount of HCN and NH3 is usually found in the flue gas, the increase in NOx through oxidation of HCN and NH3 can be avoided for SO2 capture from flue gas. In addition, Shimizu16 found that the N2O reduction by limestone from a two-stage FBC was nearly the same as that from a conventional single-stage FBC. However, a problem of the two-stage FBC is the complex structure of the combustor; it is necessary to install two gas distributors. (10) Iisa, K.; Salokoski, P.; Hupa, M. Proceedingsof the 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 1027-1034. (11) Miettinen, H.; Stro¨mberg, D.; Lindquist, O. Proceedingsof the 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 999-1004. (12) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 1005-1012. (13) Shimizu, T.; Inagaki, M. Energy Fuels 1993, 7, 648-654. (14) Tomita, M.; Hirama, T.; Adachi, T.; Yamaguchi, H.; Horio, M. Proceedings of the 6th International Conference on Fluidized Bed Combustion (Atlanta, GA); U.S. Department of Energy, Office of Coal Utilization: Washington, DC, 1980; pp 623-631. (15) Wormser, A.; Beckwith, W. Proceedings of the 7th International Conference on Fluidized Bed Combustion (Philadelphia, PA), DOE/ METC/83-48; 1983; pp 406-419. (16) Shimizu, T.; Satoh, M.; Fujikawa, T.; Tonsho, M.; Inagaki, M. Energy Fuels 2000, 14 (4), pp 862-868 .
10.1021/ef010132t CCC: $22.00 © 2002 American Chemical Society Published on Web 12/19/2001
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Figure 1. Concept of the separation of combustion zone and desulfurization zone (c) by employing fine limestone and (b) by employing two-stage BFBC in comparison to (a) conventional in-bed SO2 capture BFBC without separation.
Another approach proposed in this work to separate the combustion zone and the desulfurization zone in BFBC is to employ fine limestone particles. This concept is illustrated in Figure 1c. By decreasing the particle size of limestone, only limestone is carried over by the gas stream and the desulfurization takes place in the freeboard, while combustion of coarse fuel particles is conducted in the dense bed. Thus the contact between limestone and volatile matter from fuel can be avoided. By recycling the elutriated fine limestone particles, the concentration of limestone in the freeboard can be sufficiently high to achieve desired SO2 removal efficiency. The objective of this work is to demonstrate if the reduction of limestone size can suppress the increase in NOx emission and to evaluate how the reduction of N2O emission by limestone feed is affected by the size of limestone. In this work, a bench scale bubbling fluidized bed combustor was operated. The change in NOx and N2O emissions by SO2 removal was evaluated for the limestone with different size. The results of the present concept were compared with those of a circulating fluidized bed combustor in which both fuel and limestone were carried over by a gas stream.
Table 1. Analyses of Coal ultimate analysis (daf, wt%) fuel
C
H
Oa
N
proximate analysis (dry, wt%) Sb
high-volatile 80.1 6.1 11.7 1.5 0.6 bituminous
V.M.c
F.C.d
ash
38.3
47.5
14.2
a By difference. b Combustible sulfur. c Volatile matter. carbon.
d
Fixed
Experimental Section (1) Bubbling Fluidized Bed Combustion. A bench-scale BFBC of 1.3 m in height and 5.4 cm in inner diameter was employed. The reactor was heated by electric heaters and the temperature in the bed and the freeboard was maintained at 1123 K. Primary air was fed through the distributor at the bottom of the bed. Secondary air was not injected. The air feed rate was fixed at 4.7 × 10-3 mol/s, i.e., the superficial gas velocity was 20 cm/s at 1123 K. Quartz sand having an average size of 0.27 mm was employed as bed material. The minimum fluidizing velocity of the sand at 1123 K was calculated to be 2.6 cm/s. The static bed height was 10 cm. One kind of bituminous coal was employed as a fuel. The analyses of the fuel are shown in Table 1. The size of the fuel was between 0.3 and 1.0 mm. As shown in Figure 2a, the terminal velocity of the char particle was higher than the superficial gas velocity of BFBC, thus the fuel particles were considered to stay in the dense bed. The fuel was continuously fed through a vibration feeder, conveyed pneumatically in the air stream, and then injected into the bottom of the bed about 1 cm above the distributor. The feed rate of coal was controlled
Figure 2. Estimated terminal velocity of (a) char and (b) calcined limestone particles compared with the superficial gas velocity in the present BFBC and CFBC test apparatus (assumed particle density: 1000 kg/m3 for char, 1500 kg/m3 for calcined limestone). so that the desired O2 concentration in the flue gas (3.5-4.5%) was attained. Chichibu limestone from Japan was used as a sorbent. The sorbent composition is shown in Table 2. The limestone was crushed and sieved to obtain desired particle size fractions. As coarse limestone for conventional in-bed SO2 capture, fractions between 0.420 and 0.590 mm (fraction A) and
Reduction of SO2 and N2O Emissions from a FBC
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Table 2. Analysis of Limestone (wt %) limestone
CaCO3
MgCO3
SiO2
Al2O3
Fe2O3
Chichibu
96.9
1.4
0.6
0.8
0.3
between 0.145 and 0.250 mm (fraction B) were used. As fine limestone, a fraction between 0.053 and 0.105 mm was used (fraction C). Figure 2b shows the estimated terminal velocity of calcined limestone particles. For fractions A and B, the superficial gas velocity was lower than the terminal velocity of calcined limestone, thus the limestone particles were considered to remain in the dense bed. For fraction C, the superficial gas velocity was higher than the terminal velocity, thus the fine particles were considered to be entrained by the gas stream and exist in the freeboard. The limestone was mixed with the fuel prior to fuel feed to meet the desired Ca/S ratio. In the present study, the recirculation of elutriated fine limestone was not conducted due to the complexity during operation. Instead, Ca/S ratio was controlled in order to achieve desired SO2 capture. Even when solid recirculation is not conducted, the increase in NOx with sorbent feed can be evaluated, thus the present concept can be tested. Concentrations of O2, total NOx (dNO + NO2), SO2, CO2, and CO in the flue gas were continuously measured by a magnetic oxygen analyzer for O2, a chemical luminescence analyzer for NOx, and NDIR absorption analyzers for SO2, CO2, and CO, respectively. A portion of the sample gas was dried using an ice bath gas cooler and stored in Tedler gas bags. Concentrations of O2, N2, and N2O in the gas were measured by gas chromatography with a thermal conductivity detector. (2) Circulating Fluidized Bed Combustion. In this work, reduction of limestone particle size was proposed to separate the desulfurization zone from the combustion zone by the elutriation of fine limestone particles in BFBCs. However, there is another possibility that the limestone particle size affects the emissions of NOx and N2O; It is possible that the size of limestone affects the effectiveness factor. The intrinsic reaction rate per unit volume of limestone differs among the reactions (SO2 capture, oxidation of NH3 and HCN, and N2O decomposition), thus the change in the particle size may change the overall reaction rate in a different manner among the reactions. The change in particle size may favor the reduction of VM-N oxidation rate relative to SO2 capture rate. This hypothesis can be tested by employing a circulating fluidized bed combustor (CFBC) in which all the limestone particles and fuel particles are entrained to the upper part of the reactor. In this work, a small-scale CFBC of 4.3 m in height and 5.3 cm in inner diameter was employed. The detail of the experimental apparatus is described elsewhere.6 The temperature in the reactor was maintained at 1123 K. The bed material was quartz sand of average size of 0.177 mm. Secondary air was injected through a nozzle at 1.3 m above the primary air distributor. The superficial gas velocity below the secondary air inlet and that above the secondary air inlet were 3 and 6 m/s (at 1123 K), respectively. The same type of fuel and limestone as employed for the present BFBC experiments were employed for CFBC study. The maximum size of the coal and limestone were 1.0 mm and 0.59 mm, respectively. The terminal velocity of char and calcined limestone were lower than the superficial gas velocity above the secondary air inlet (Figure 2), thus all the fuel and limestone particles were considered to be carried over to the upper part of the combustor. The flue gas samples were analyzed by the same procedure as the BFBC experiments.
Results and Discussion (1) Bubbling Fluidized Bed Combustion. Figure 3 shows the transient behavior of emissions of SO2, NOx, and N2O after starting limestone feed. After starting
limestone feed, SO2 and N2O decreased with time not only for both coarse limestone fractions (Figure 3a and b) but also for the fine limestone fraction (Figure 3c). The change in NOx emission after limestone feed was different between fine limestone and coarse limestone; for coarse limestone fractions, NOx emission clearly increased after limestone feed, whereas the increase in NOx emission was suppressed for fine limestone. In this work, the steady-state operation could not be attained, thus the change in emissions of NOx and N2O with the change in SO2 emission was evaluated. Figure 4 shows the relationship between SO2 removal efficiency (ηS) and change in NOx and N2O. SO2 removal efficiency (ηS) was defined as follows:
ηS ) 1 - (SO2 emission during SO2 capture)/(SO2 emission without SO2 capture) (1) Emissions of NOx and N2O at SO2 capture efficiency of ηS (NOx(ηS) and N2O(ηS)) were normalized by the emissions without SO2 capture (NOx(ηS ) 0) and N2O(ηS ) 0)), respectively. For the coarse particles, the increase in NOx with SO2 capture was nearly the same between limestone size fraction of 0.42-0.59 mm and that of 0.147-0.25 mm. The results of batch limestone feed, which employed the same apparatus, fuel, and limestone,16 also agreed with the present results of continuous coarse limestone feed. These results indicate that the increase in NOx emission is governed by the overall activity of limestone for SO2 capture if the limestone particles stay in the dense bed. The increase in NOx with SO2 capture for coarse limestone particles is attributable to the formation of NOx through oxidation of volatile-N such as NH3 and HCN; The selectivity to NOx for limestone-catalyzed oxidation of volatile-N is known to be higher than that of gas-phase oxidation,6-9 thus the addition of limestone increases the formation of NOx through the shift from gas-phase oxidation to limestone-catalyzed oxidation. As shown in Figure 4a, the increase in NOx with SO2 capture was suppressed by employing fine limestone particles. The results of the present fine limestone feed were quite similar to the results of the two-stage fluidized bed.16 This suggests that the separation between combustion zone and desulfurization zone can be done not only by employing two-stage FBC but also by employing fine limestone particles. Figure 4b shows the effect of SO2 capture on N2O emission. With decreasing SO2 emission, N2O emission decreased. There are two mechanisms of the decrease in N2O emission with limestone feed. One is the catalytic decomposition of N2O over calcined limestone.6,10-13 The other mechanism is the shift from homogeneous oxidation to limestone-catalyzed oxidation of HCN. Shimizu et al.6 compared the selectivity to N2O for limestone-catalyzed HCN oxidation with the literature value for gas-phase HCN oxidation and found that the selectivity to N2O was lower for the limestonecatalyzed reaction than the gas-phase reaction. Thus the shift from gas-phase reaction to limestone-catalyzed reaction may be an explanation of N2O reduction by limestone feed to FBCs. To clarify which is the dominant mechanism of N2O reduction by limestone feed, catalytic decomposition or
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Figure 3. Change in concentrations of SO2, NOx, and N2O in the flue gas after starting limestone feed. (Concentrations corrected to dry flue gas, containing 6% O2.)
Figure 4. Relationship between SO2 emission and emissions of NOx and N2O. (Results for batch feed with DS ) 0.49-0.59 mm and two-stage BFBC. (DS ) 0.49-0.59 mm) were from literature.16)
shift of HCN oxidation pathway, the relationship between the increase in NOx and decrease in N2O is discussed. Figure 5 shows the change in NOx emission (∆NOx) and the change in N2O emission (∆N2O) with increasing SO2 removal efficiency (ηS) from 0% to 80%. ∆NOx and ∆N2O were defined as follows:
∆NOx ) NOx(ηS ) 0.8) - NOx(ηS ) 0)
(2)
∆N2O ) N2O(ηS ) 0.8) - N2O(ηS ) 0)
(3)
NOx(ηS ) 0.8) and N2O(ηS ) 0.8) were obtained by
approximating the relationship between emissions of NOx and N2O and SO2 removal efficiency (Figure 4) as follows:
NOx(ηS)/NOx(ηS ) 0) ) 1 + aNOx log(1 - ηS)
(4)
N2O(ηS)/N2O(ηS ) 0) ) 1 + aN2O log(1 - ηS)
(5)
With increasing ∆NOx, decrease in N2O became more remarkable. This suggests that the shift from homogeneous HCN oxidation to limestone-catalyzed HCN oxidation contributes the decrease in N2O. Also the cata-
Reduction of SO2 and N2O Emissions from a FBC
Figure 5. Relation between the change in NOx emission and that in N2O emission with increasing SO2 capture efficiency from 0% to 80%. (Coarse limestone: fractions A and B; fine limestone: fraction C.)
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results shown in Figure 6 indicate that the change in limestone particle size gave no effect on the relative reaction rate among desulfurization and NOx formation. By comparing the results of BFBC and CFBC, the suppression of the increase in NOx with SO2 capture observed for BFBC during fine limestone feed is explained by the separation of desulfurization zone from combustion zone; in the BFBC, the gas velocity was lower than the terminal velocity of fuel but higher than the terminal velocity of fine limestone, thus the devolatilization and combustion took place in the dense bed and desulfurization took place in the freeboard, that is, the contact between limestone and volatile matter could be avoided. In CFBC, the gas velocity was higher than the terminal velocity of both fuel and limestone, thus the fuel as well as limestone were entrained to the upper part of the combustor and the contact between limestone and volatile matter took place throughout the reactor. The results of CFBC indicate that the change in effectiveness factor of limestone with reducing particle size plays only a minor role in determining the increase in NOx with SO2 capture. Conclusion
Figure 6. Effect of limestone particle size on the relationship between SO2 emission and NOx emission during circulating fluidized bed combustion.
lytic decomposition is considered to contribute to the reduction of N2O since the decrease in N2O was observed even when only slight increase in NOx was observed for fine limestone feed. Therefore, the results of the present study indicate that both of the mechanisms contribute to the decrease in N2O. (2) Circulating Fluidized Bed Combustion. Figure 6 shows the results of CFBC experiments. The relationship between SO2 emission and emission of NOx was not affected by the particle size. In this CFBC, the limestone particles as well as fuel particles were entrained to the upper part of the reactor and the particles existed throughout the reactor, thus the separation of combustion zone from SO2 removal zone by changing limestone particle size was not attained. Thus, the
By employing fine limestone which was entrained to the freeboard, and by employing coarse fuel which burned in the dense bed in BFBC, the desulfurization zone was separated from the combustion zone. Thus the increase in NOx with SO2 capture due to contact between calcined limestone and volatile matter could be suppressed. N2O emission was also reduced by fine limestone feed but the reduction was less effective than that for coarse limestone feed. Acknowledgment. T. Shimizu thanks The Ministry of Education, Culture, Sports, Science and Technology for Grant-in-Aids (No.11218204) and Idemitsu Kosan Co., Ltd for cooperation. The authors thank Mr. Yasuo Takano, Ms. Satoko Hori, Mr. Akira Sakagami, Mr. Jun Asadzuma, and Mr. Masayuki Shinkai, students of Niigata University, for their assistance during the experiments. Nomenclature DS ) sorbent (limestone) particle size EF010132T
