Simultaneous Reduction of SO2, NOx, and N2O Emissions from a Two

The two-stage BFBC was found to decrease SO2 and N2O emissions ..... (25) Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.;, Lanier, W. M.;. McSorley, J...
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Energy & Fuels 2000, 14, 862-868

Simultaneous Reduction of SO2, NOx, and N2O Emissions from a Two-Stage Bubbling Fluidized Bed Combustor Tadaaki Shimizu,* Masato Satoh, Tomoyasu Fujikawa, Masaru Tonsho, and Makoto Inagaki Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi, Niigata, 950-2181, Japan Received October 25, 1999

The effect of limestone feed during bubbling fluidized bed combustion of coal on emissions of SO2, NOx and N2O was investigated. Two types of bubbling fluidized bed combustors (BFBCs) were employed; One was a conventional single-stage BFBC in which limestone was fed to the bed where coal was burnt, and the other was a two-stage BFBC in which coal was burnt in the lower bed and the desulfurization of the flue gas from the lower bed was conducted in the upper bed. For the single-stage BFBC, limestone feed into the bed decreased SO2 and N2O emissions but it increased NOx emission. The two-stage BFBC was found to decrease SO2 and N2O emissions without increasing NOx emission. The results of SO2 capture and N2O reduction in the upper bed of two-stage BFBC were analyzed using Kunii-Levenspiel model. The results of the calculation agreed fairly well with the experimental results.

Introduction An advantage of fluidized bed combustors (FBCs), both bubbling and circulating, is SO2 capture without external wet scrubbing systems. However, SO2 capture by sorbent (limestone) feed into FBCs is known to increase NOx emission.1-3 The increase in NOx emission is explained by the catalytic activity of calcined limestone (CaO) to oxidize nitrogen-containing volatile matter to mainly NO; NH3 and HCN are released from the fuel during devolatilization and their oxidation is catalyzed by calcined limestone.4-7 For bubbling FBC (BFBC), staged air feed is known to be an effective modification for NOx reduction. However, reduction of primary air is known to increases SO2 emission1,8 due to the reduced SO2 capture rate under O2 lean conditions8,9 and decomposition of CaSO4 in reducing atmosphere.10 * 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, 6th International Conference on Fluidized Bed Combustion (Atlanta, GA); US Department of Energy, Office of Coal Utilization: Washington, DC, 1980; pp 986-995. (2) Gavin, D. G.; Dorrington, M. A. Proceedings, 1991 International Conference on Coal Science (Newcastle, UK); International Energy Agency Coal Research Ltd., Butterworth-Heinemann: Oxford, UK, 1991; pp 347-350. (3) Shimizu, T.; Tachiyama, Y.; Kuroda, A.; Inagaki, M. Fuel 1992, 71, 841-845 (4) Hirama, T., Kochiyama, Y., Chiba, T.; Kobayashi, H. NenryoKyokai-Shi (J. Fuel Soc. Jpn.) 1982, 61, 268-274. (5) Furusawa, T.; Tsujimura, M.; Yasunaga, K.; Kojima, T. Proceedings, 8th International Conference on Fluidized Bed Combustion (Houston, TX); DOE/METC-85/6021, US Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; pp 1095-1104. (6) Lee, Y. Y.; Sekthira, A.; Wong, C. M. Proceedings, 8th International Conference on Fluidized Bed Combustion (Houston, TX), DOE/ METC-85/6021, US Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; pp 1208-1218. (7) Shimizu, T.; Tachiyama, Y.; Fujita, D.; Kumazawa, K.; Wakayama, O.; Ishizu, K.; Kobayashi, S.; Shikada, S.; Inagaki, M., Energy Fuels 1992, 6, 753-757

Another problem of FBCs is the emission of a large amount of N2O. Recently, 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.7,11-14 Indeed, N2O emission from BFBC has been reported to decrease with sorbent feed.2,3 An approach to reduce both NOx emission and SO2 emission is two-stage BFBC, 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.15,16 Since a negligible amount of HCN and NH3 is usually found in flue gas, the increase in NOx through oxidation of HCN and NH3 can be avoided for SO2 capture from flue gas. In addition, SO2 capture is conducted in flue gas, which contains about 4% O2, and thus the decomposition of CaSO4 can be avoided even when staged air (8) Shimizu, T.; Take, K.; Kojima, T.; Furusawa, T.; Kunii, D. Fluidization ‘85 Science and Technology Kwauk, M., Kunii, D., Zheng, J., Hasatani, M., Eds.; Elsevier: Amsterdam, The Netherland, pp 330341, 1985. (9) Kojima, T., Take, K, Furusawa, T. and Kunii, D. J. Chem. Eng. Jpn. 1985, 18, 432-438. (10) Hansen, P. F. B.; Dam-Johansen, K.; Bank, L. H.; Ostergaard, K., Proceedings, 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 73-82. (11) Iisa, K.; Salokoski, P.; Hupa, M. Proceedings, 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 1027-1034. (12) Miettinen, H.; Stro¨mberg; D., Lindquist, O. Proceedings, 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 999-1004. (13) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings, 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; pp 1005-1012 (14) Shimizu, T.; Inagaki, M. Energy Fuels 1993, 7, 648-654. (15) Tomita, M.; Hirama, T.; Adachi, T.; Yamaguchi, H.; Horio, M. Proceedings, 6th International Conference on Fluidized Bed Combustion (Atlanta, GA); US Department of Energy, Office of Coal Utilization: Washington, DC, 1980; pp 623-631. (16) Wormser, A.; Beckwith, W. Proceedings, 7th International Conference on Fluidized Bed Combustion (Philadelphia, PA), DOE/ METC/83-48, USA, 1983; pp 406-419.

10.1021/ef9902202 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/23/2000

Reduction of Emissions from a Two-Stage BFBC

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Figure 1. Two-stage bubbling fluidized bed combustor experimental apparatus. Table 1. Analyses of Fuel ultimate analysis (daf, wt %) fuel

C

H

Oa

high-volatile bituminous coal

80.1

6.1

1.7

a

By difference. b Combustible sulfur. c Volatile matter.

d

proximate analysis (dry, wt %)

N

Sb

VMc

FCd

ash

1.5

0.6

38.3

47.5

14.2

Fixed carbon.

feed is conducted in the lower bed. However, the ffect of two-stage BFBC on N2O emission has not yet been known. On the basis of concept of two-stage BFBC, Shimizu et al.17 conducted “simulated” two-stage BFBC experiments. Namely, flue gas was sampled from a circulating fluidized bed combustor, then after ash removal they led the flue gas to an external fluidized bed reactor in which limestone was fed. The results of “simulated” twostage BFBC experiments showed that an increase in NOx emission during limestone feed was suppressed and better N2O reduction was achieved. However, “simulated” two-stage BFBC experiments employed flue gas after fly ash removal. It is not yet certain if two-stage BFBC is effective for SO2 capture and N2O reduction of flue gas that contains ash and unburnt carbon. The objective of this work is to evaluate the effect of sorbent feed on emissions of NOx and N2O from a twostage BFBC. In this work, a bench-scale two-stage fluidized bed combustor was operated. The changes in NOx and N2O emissions by SO2 removal in the upper bed was compared with those for conventional singlestage FBC with in situ SO2 capture. A mathematical model was developed to analyze SO2 capture and N2O decomposition in the upper bed of the two-stage BFBC. Experimental Section Two-stage bubbling fluidized bed combustion (BFBC) experiments were conducted to evaluate the effects of air staging and limestone feed on the emissions of NOx, N2O, and SO2. A bench-scale BFBC was employed, the schematic diagram of which is shown in Figure 1. The total height was 1.3 m. The (17) Shimizu, T.; Miura, M.; Togashi, T.; Tonsyo, M.; Inagaki, M.; Matsukata, M. Proceedings, 13th International Conference on Fluidized Bed Combusiton (Kissimee, FL); ASME: New York, 1995; pp 10831090.

inner diameter of the lower bed was 5.4 cm. The upper bed was made of a quartz tube of 4.5 cm in inner diameter. A quartz perforated plate gas distributor was installed at the bottom of the quartz tube. By inserting the upper bed from the top of the reactor, the BFBC reactor worked as a two-stage BFBC. Single-stage BFBC experiments were conducted by removing the upper bed. The reactor was heated by electric heaters and the temperature in the bed was maintained at 1123 K. Air feed staging was also conducted. Primary air was fed through the distributor at the bottom of the lower bed. Secondary air was injected at 0.24 m above the primary air distributor through a nozzle of 16 mm inner diameter. For the two-stage BFBC, the location of the secondary air nozzle corresponded to 44 cm below the distributor of the upper bed. The total air feed rate was fixed at 4.7 × 10-3 mol/s, i.e., the superficial gas velocity of air in the lower bed above the secondary air inlet and in the upper bed was 20 and 28 cm/s at 1123 K, respectively. By taking account of the change in composition, the superficial gas velocity of flue gas in the lower bed above the secondary air inlet and in the upper bed was 22 and 31 cm/s at 1123 K, respectively. Quartz sand having an average size of 0.27 mm was employed as bed material of the lower bed. For the bed material of the upper bed, quartz sand having an average size of 0.40 mm was employed. The minimum fluidizing velocity at 1123 K was calculated to be 2.6 cm/s for the sand in the lower bed and 5.5 cm/s for the sand in the upper bed. 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. The fuel was continuously fed through a vibration feeder, conveyed pneumatically in air stream, and then injected into the bottom of the bed about 1 cm above the distributor. The feed rate of coal was controlled so that the desired O2 concentration in the flue gas was attained. Chichibu limestone was used as the sorbent. Its composition is shown in Table 2. Its particle size was between 0.42 and 0.59 mm. Batch feed of limestone was conducted.

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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

Concentrations of O2, total NOx (NO and NO2), 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 adsorption analyzers for 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. Concentration of SO2 in the gas was measured by use of Kitagawa detector tubes.

Results and Discussion (1) Effect of Upper Bed without Limestone on NOx and N2O Emissions. Figure 2 shows the effect of oxygen concentration in dry flue gas on emissions of NOx and N2O from single-stage BFBC. With decreasing flue gas oxygen concentration, i.e., decreasing air/fuel ratio, both NOx and N2O emissions decreased. The emissions of NOx and N2O during staged air feed were less than those during single-staged air feed. The emissions of NOx and N2O from the two-stage BFBC without limestone feed are also shown in Figure 2 at a dry flue gas O2 concentration of 3.5-4.5 vol %. The emissions of NOx and N2O from two-stage BFBC were nearly the same as those from the single-stage BFBC. Therefore, the distributor, reactor wall, and the bed material of the upper bed were found to have no effect on the emissions of NOx and N2O. In the following discussion, data obtained at a dry flue gas O2 concentration of 3.5-4.5 vol % are shown. (2) SO2 Capture in Single-Stage BFBC and TwoStage BFBC. Figure 3 shows the emission of SO2 before and after batch feed of limestone. Just after the batch feed, SO2 emission became a minimum, then the emission gradually increased with time due to deactivation of the sorbent. When the same amount of limestone was fed, the minimum SO2 emission from the two-stage BFBC was found to be less than that from the singlestage BFBC. Figure 4 shows the effect of limestone batch amount on the maximum SO2 capture efficiency observed just after the limestone feed. The maximum SO2 capture efficiency was calculated from the minimum emission of SO2 ([SO2]MIN) and the SO2 emission before limestone feed ([SO2]Ca/S)0) as follows:

maximum SO2 capture [%] ) 100 ([SO2]MIN - [SO2]Ca/S)0)/[SO2]Ca/S)0 (1) For the single-stage BFBC without air feed staging, the maximum SO2 capture efficiency increased with increasing limestone feed and a maximum SO2 capture of 80% was attained with a limestone feed of 10 g. However, with air feed staging, a maximum SO2 capture of only 50% was attained under the same limestone feed. Even when the amount of limestone batch was increased up to 30 g, no improvement of the maximum SO2 capture was observed. Decreased SO2 capture efficiency with reducing primary air ratio is attributable to reduced SO2 capture rate under oxygen lean conditions8,9 and decomposition of CaSO4 under reducing

atmosphere.10 Although air feed staging was also effective for reduction of both NOx and N2O (Figure 2), air feed staging inhibited SO2 capture within the singlestage BFBC. As shown in Figure 4, a maximum SO2 capture of 80% was attained for the two-stage BFBC when only 2 g of limestone was fed in the bed. In addition, the air staging did not affect the maximum SO2 capture for the twostage BFBC. Since even under air staging conditions the flue gas at the inlet of the upper bed contains about 4% oxygen, neither the reduction of SO2 capture rate under oxygen lean condition nor decomposition of CaSO4 by reducing gases takes place in the upper bed. Thus, one of the advantages of the two-stage BFBC is the staged air feed without hindering SO2 capture. Comparing the results obtained under single stage air feed condition with limestone batch feed of 2 g, the maximum SO2 capture for the two-stage BFBC was better than that of the single-staged BFBC (Figure 4). The reason for the better maximum SO2 capture efficiency for the two-stage BFBC is attributable to several reasons: (1) The oxygen concentration in the flue gas fed into the upper bed is higher than that in the emulsion phase of the lower bed; thus, the SO2 capture rate is higher in the upper bed due to the higher oxygen concentration. (2) The flue gas contains only a small amount of reducing gases; thus, the decomposition of CaSO4 by reducing gases does not take place. (3) The contact efficiency between SO2 and limestone is better in the upper bed. In the lower bed, limestone particles in the bottom of the dense bed cannot contact with the SO2 released in the upper part of the dense bed. By integrating the amount of captured SO2 after batch feed of limestone (Figure 3), it was found that the total amount of SO2 captured by 2 g of limestone under single-stage air feed condition was more for the singlestage BFBC than for the two-stage BFBC. One possible explanation for the greater total amount of SO2 captured for the single-stage BFBC is the movement of limestone between the oxidizing atmosphere zone and the reducing atmosphere zone in the bed. Okamoto18 reported that limestone conversion was improved by alternating an oxidizing and reducing atmosphere, in comparison with the SO2 capture in only an oxidizing atmosphere. In the lower bed, the reducing atmosphere is considered to be formed in the vicinity of the fuel particle by volatile matter evolution and CO formation during char combustion in the emulsion phase, and gas in the bubble gas is oxidizing. In contrast, the atmosphere in the upper bed is considered to be oxidizing since only small amount of combustibles exists in the flue gas. Therefore, limestone conversion was better for SO2 capture in the lower bed, since limestone particles are exposed to alternating oxidizing and reducing atmospheres. However, if the reason for better SO2 capture for the singlestage BFBC is attributable to the alternating oxidizing/ reducing atmosphere, it is necessary to take account of CaS formation in the reducing atmosphere.18 Because CaS forms harmful H2S when ash is disposed of in the (18) Okamoto, T.; Sakaue, T.; Nakamichi, J.; Suzuki, Y.; Nishimura, M.; Moritomi, H. Proceedings, 2nd SCEJ Symposium on Fluidization (Tokyo, Japan), SCEJ (Soc. Chem. Engrs., Jpn), 1996; pp 398-405.

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Energy & Fuels, Vol. 14, No. 4, 2000 865

Figure 2. Effect of flue gas O2 concentration on emissions of NOx and N2O from single-stage BFBC and two-stage BFBC under no limestone feed conditions. (PA/TA, Primary air feed/total air feed; w/o UB, without upper bed; empty UB, empty upper bed; UB+QS, upper bed with quartz sand).

Figure 4. The effect of air feed staging and limestone batch amount on maximum SO2 capture efficiency observed just after limestone feed (PA/TA, primary air feed/total air feed).

addition, a decrease in N2O just after limestone feed was more for the two-stage BFBC than for the single-stage BFBC when the same amount of limestone was fed. Since the objective of limestone feed into FBCs is to reduce SO2 emission to meet the environmental regulation, the difference in the effect of limestone feed on NOx and N2O emission between the single-stage BFBC and the two-stage BFBC should be evaluated at the same SO2 emission level. Figure 6 shows the relationship between SO2 emission and emissions of NOx and N2O. For the single-stage BFBC, NOx increased and N2O decreased with decreasing SO2 emission. By employing the two-staged BFBC, the increase in NOx with SO2 capture could be avoided. In addition, the decrease in N2O with SO2 capture was slightly more than that for the single-stage BFBC. By employing air feed staging for two-stage BFBC, less NOx and N2O emissions were attained than without air feed staging. The increase in NOx emission with limestone feed into the single-staged BFBC can be explained by the formation of NO through limestone-catalyzed oxidation of HCN and NH3, which are released as volatile matter during pyrolysis, as schematically shown in Figure 7.4-7 It should also be noted that volatile nitrogen species are released from fuel not only during devolatilization but also during char combustion and gasification.19-22 Indeed, NOx emission during fluidized bed combustion of char particles became higher when catalysts such as CaO and MgO were employed as bed materials instead of inert sand particles.23,24 The increase in NOx with

environment, neither fly ash nor bottom ash (bed material withdrawn form the bottom) should contain H2S. However, CaS could not be analyzed by the present experimental apparatus and these problems, elucidation of the mechanism of better limestone conversion for in situ SO2 capture and observation whether CaS is formed or not, are the subjects of the future work. (3) Emissions of NOx and N2O after Limestone Feed. A problem of limestone feed into conventional single-stage BFBC is the increase in NOx emission. Figure 5 shows the change in NOx and N2O emissions after limestone feed. Considerable increase in NOx emission after limestone batch feed was observed for the single-stage BFBC, whereas no change in NOx emission was observed for the two-stage BFBC. In

(19) Kasaoka, S.; Sasaoka, E.; Ozaki, A. Nenryo-kyokai-shi (J. Fuel Soc. Jpn.) 1983, 62, 53-62. (20) Moritomi, H.; Suzuki, Y.; Ikeda, M.; Suzuki, K.; Torigai, K. Kagaku-Kogaku-Ronbunsyu 1994, 20, 849-856. (21) Winter, F.; Wartha, C.; Hofbauer, H.; Anthony, E. J.; Preto, F.; Gogolek, P. Circulating Fluidized Bed Technology V (Proceedings, the 5th International Conference on Circulating Fluidized Beds (Beijing, China); Kwauk, M., Li, J., Eds.; 1996; pp 333-337. (22) Winter, F.; Lo¨ffler, G., International Energy Agency Workshop on NOx, N2O formation and destruction in Fluidized Bed Combustion (38th IEA Fluidized Bed Conversion Meeting, Savannah, GA, May 16, 1999); Winter, F., Anthony, E. J., Eds.; VTWS-99-PB-25, University of Technology, Vienna, 1999. (23) Klein, M.; Rotzoll, G., Proceedings of 6th International Workshop on Nitrous Oxide Emissions, (Turku/A° bo, Finland), Combustion Chemistry Research Group, A° bo Akademi University, Finland, 1994; pp 239-253. (24) Liu, H.; Gibbs, B. M., Proceedings, 15th International Conference on Fluidized Bed Combustion (Savannah, GA) (on CD-ROM), paper number FBC99-0114, 1999.

Figure 3. Emission of SO2 after limestone batch feed into the single-stage BFBC and two-stage BFBC (PA/TA, Primary air feed/total air feed).

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Figure 5. Change in NOx and N2O emissions after limestone batch feed without air staging (PA/TA, primary air feed/total air feed; SS, single-stage BFBC; TS, two-stage BFBC).

Figure 6. Relationship between SO2 emission and emissions of NOx and N2O (PA/TA, primary air feed/total air feed; SS, singlestage BFBC; TS, two-stage BFBC).

Figure 7. Reaction pathways of HCN and NH3 to NOx and N2O for homogeneous oxidation and CaO-catalyzed oxidation in the presence of char.

limestone feed into the single-staged BFBC is considered to be partly attributable to this reaction pathway. The decrease in N2O emission by limestone feed is explained partly by the decomposition of N2O over calcined limestone.7,11-13 In addition, for the single-stage BFBC, HCN oxidation over limestone is also considered to contribute to the decrease in N2O emission;7 A large amount of N2O is formed through gas-phase HCN oxidation,25 whereas the conversion of HCN to N2O through limestone-catalyzed oxidation is far lower than that for homogeneous oxidation.7 Therefore, the shift from homogeneous oxidation to heterogeneous oxidation over limestone results in a decrease in N2O emission, though it increases NOx emission. (25) Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.;, Lanier, W. M.; McSorley, J. A. Mechanism of nitrous oxide formation in coal flames. Combust. Flame 1989, 77, 375-384.

However, there is still an unresolved question about the decrease in N2O by limestone feed. The decrease in N2O for the two-staged BFBC was more than that for the single-staged BFBC. For the two-staged BFBC, it is considered that limestone behaves as a catalyst for only N2O decomposition. On the other hand, for the single-staged BFBC, limestone behaves as a catalyst for not only N2O decomposition but also for the shift from homogeneous HCN oxidation to heterogeneous HCN oxidation; indeed, NOx emission increased after limestone feed. Therefore, it is likely that the decrease in N2O is more for the single-stage BFBC than the twostage BFBC. However, the experimental results are the opposite. One hypothesis is that N2O is formed through reduction of NO over char in the presence of O2.26-28 NO is formed through oxidation of HCN and NH3 over limestone and then a part of NO is reduced to N2O over char as schematically shown in Figure 7. Thus the decease in N2O formation from HCN due to the shift from homogeneous to heterogeneous oxidation is canceled out by the secondary formation of N2O from char-N and NO. The elucidation of the mechanism is a subject of future works. Modeling of SO2 Capture and N2O Decomposition in the Upper Bed. Modeling of SO2 removal and N2O decomposition by limestone in the upper bed of the (26) Mochizuki, M.; Koike, J.; Horio, M. Proceedings, 5th Int. Workshop on N2O Emissions (Tsukuba), NIRE/IFP/EPA/SCEJ, 1992; pp 237-244. (27) Dam-Johansen, K.; Åmand, L.-E.; Leckner, B. Fuel 1993, 72, 565-571. (28) Tullin, C. J.; Goel, S.; Morihara, A.; Sarofim, A. F.; Beer, J. M. Energy Fuels 1993, 7, 796-802.

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Energy & Fuels, Vol. 14, No. 4, 2000 867

Figure 8. Transient change in concentrations of SO2 and N2O at the outlet of the two-stage BFBC (PA/TA, primary air feed/total air feed).

Kbe ) 4.5(umf/db)

(7)

two-stage BFBC were conducted. The bubbling bed model for intermediate size particle proposed by Kunii and Levenspiel29 was employed as a reactor model. The fluidized bed reactor consists of two regions, bubble and emulsion. Both N2O decomposition and SO2 capture are first-order reactions;14 thus, the change in concentration in the bubble (Cb,i) and that in the emulsion (Ce,i) with increasing height (z) are expressed as follows

Bubble diameter is assumed to be db ) 2 cm. The rise velocity of bubble gas (ub*) is given by bubble rise velocity (ub), superficial gas velocity (u0), and umf as follows:

ub* ) ub + 3umf

(8)

-δub* dCb,i/dz ) δKbe(Cb,i - Ce,i) + δγbkav,iCb,i

ub ) u0 - umf + 0.711(gdb)1/2

(9)

(2)

and

-(1 - δ)umf dCe,i/dz ) -δKbe(Cb,i - Ce,i) + (1 - δ)(1 - mf)kav,iCe,i (3)

where g is the acceleration of gravity. The volume of bubbles per unit volume of bed (δ) is given as

δ ) (u0 - umf)/(ub + 2umf)

(10)

where the subscript i denotes the gaseous component (i ) N2O and SO2). The concentrations of N2O and SO2 at the bottom (z ) 0) of the upper bed are assumed to be the same as the concentrations in the wet flue gas observed before limestone feed. kav,i is the average first-order rate constant. In the present fluidized bed experimental apparatus, the limestone was diluted with quartz sand, thus the average rate constant is given as follows

mf and γb are the void fraction in the bed at the minimum fluidizing condition and the volume of solids per unit volume of bubble, respectively. γb and mf are assumed to be γb ) 0 and mf ) 0.5, respectively. The height of the bed (z0) is given by the inventory of the solids, cross sectional area of the reactor (A), mf, and δ as

kav,i ) ki(WL/FL)/(WL/FL + WQ/FQ)

The concentrations of each component at the outlet of the reactor (COUT) are given by the concentrations in the bubble phase and emulsion phase at the upper surface of the bed (z ) z0):

(4)

ki is the rate constant of SO2 capture (kSO2) or N2O decomposition (kN2O). The reaction rates of the present limestone were measured in the previous study14 and they are expressed as eqs 5 and 6

log(kSO2) ) 4.06 - 8.31XCaSO4

(5)

kN2O/kSO2 ) 0.133 + 3.07XCaSO4

(6)

where XCaSO4 is the conversion of CaO to CaSO4 given by eq 13. WL, WQ, FL, and FQ are the inventory of limestone, inventory of quartz sand, density of limestone, and density of quartz sand, respectively. The gas interchange coefficient between bubble and emulsion (Kbe) is given by the minimum fluidizing velocity of the bed material (umf) and bubble diameter (db). (29) Kunii, D.; Levenspiel, O. Fluidization Engineering 2nd ed.; Butterworth-Heinemann: Stoneham, MA, 1991; Chapter 12.

z0 ) (WL/FL + WQ/FQ)/{A(1 - δ)(1 - mf)}

u0COUT,i ) δub*Cb,i + (1 - δ)umfCe,i

(11)

(12)

The change in the conversion of CaO to CaSO4 (XCaSO4) with time is calculated from the difference in the concentration of SO2 between the inlet and the outlet as

dXCaSO4/dt ) u0A(CIN,SO2 - COUT,SO2)/ (RCaCO3WL/MCaCO3) (13) where RCaCO3 and MCaCO3 are the CaCO3 content of the limestone and molecular weight of CaCO3, respectively. At t ) 0, XCaSO4 is assumed to be zero. The calculated outlet concentrations of SO2 and N2O agreed fairly well with the experimental results, as shown in Figures 8 and 9. Thus the behavior of the

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N2O reduction behavior in the upper bed of the twostage FBC than in conventional FBCs. Conclusion

Figure 9. Relationship between SO2 emission and N2O emission after limestone feed (PA/TA, primary air feed/total air feed).

upper bed (SO2 capture stage) of the two-stage fluidized bed combustor during limestone feed can be analyzed using the kinetic data and a fluidized bed model. In contrast, it is difficult to analyze the behavior of conventional bubbling FBCs during limestone feed; Formation pathways of N2O have not been fully clarified, limestone feed increased NOx formation, and it affects N2O formation, and SO2 capture is hindered by air feed staging. Thus it seems easier to predict the SO2/

Emissions of SO2, NOx, and N2O from a two-stage BFBC and a single-stage BFBC were measured under sorbent (limestone) feed conditions. SO2 capture for the two-stage BFBC was not hindered by the staged air feed, whereas it was hindered by staged air feed for the single-stage BFBC. NOx emission increased with the limestone feed for the single-stage BFBC, whereas NOx emission was not affected by sorbent feed for the twostage BFBC. N2O emission decreased with increasing SO2 capture efficiency for both of the single-stage BFBC and the two-stage BFBC. SO2 capture and N2O decomposition in the upper bed of the two-stage BFBC were analyzed by use of the Kunii-Levenspiel bubbling bed model. The results of the modeling agreed fairly well with the experimental results. Acknowledgment. T.S. thanks Hatakeyama Foundation for financial aid. The authors thank Idemitsu Kosan Co. Ltd. for their cooperation. EF9902202