Catalyzed NOx formation under fluidized-bed combustion conditions

Catalyzed NOx formation under fluidized-bed combustion conditions

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Pergamon

Chemical En.qineerinO Science, Vol. 50, No. 15, pp. 2489 2490, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509/95 $9.50 + 0.00

0009-2509(95)00096-8

Catalyzed NO x formation under fluidized-bed combustion conditions (Received 19 November 1994)

Recently much effort has been made to elucidate the interaction between desulphurization and NO~ formation in fluidized-bed combustion (FBC). Sulphur retention in FBC is carried out by adding a Ca-based sorbent (usually natural limestone CaCO3) directly to the fluid bed. Due to the strong catalytic activity of calcium species derived from limestone, it often leads to an increase of NOx (NO and NO2) emissions. There is a difference, however, between circulating FBC (CFBC) and stationary FBC (SFBC). It has been well documented that NO emission in C F B C increases with limestone addition (Leckner and/~mand, 1987; Mj6rnell et al., 1991; Amand and Leckner, 1991; Shimizu et al., 1993). On the contrary, Leckner and /~mand (1987) and Lyngfelt and Leckner (1989a, b) noted no increase of NO emission when limestone was added to SFBC, or even a decrease, in the case of a low-volatile fuel. Retention of SO2 by limestone consists of successive calcination and sulphation of the limestone particles: CaCO3 (s) ~ CaO (s) + CO2 (g)

(1)

CaO (s) + SO2 (g) + 1/202 (g) ~ CaSO4 (s).

(2)

The emission of NO;, in systems with considerable limestone addition can generally be considered to result from two competitive catalytic reactions: the oxidation of volatile-nitrogen (mainly NH3) to NO and the reduction of NO to N2 at the surfaces of the solids involved (Johnsson, 1989; Johnsson and Dam-Johansen, 1991): catalyst NH 3 + 5//402

,

NO + 3/2H20

_ ~ catalyst . . . . NO + tgt9 -------* l/ZN2 + CO2.

(3) (4)

Oxidizing conditions, which are necessary for the sulphation reaction (2), occur in both CFBC and SFBC. In this case, the principal catalyst for the NH3 oxidation reaction (3) is CaO (Furusawa et al., 1985; Hansen et al., 1992; Linet al., 1993). However, in the dense zone of SFBC, the reaction atmosphere changes from oxidizing near the bubble interface and freeboard to reducing next to the burning coal particles. In a reducing atmosphere, the sulphation product CaSO4 is converted to CaS (Lyngfelt and Leckner, 1989a, b). Because of the fast and chaotic movement of limestone particles between air bubbles and burning char particles, the decomposition of CaSO4 to CaS does not proceed to completion. The fluctuation of reducing/oxidizing atmosphere in a particle neighbourhood has been shown to have a characteristic time of about 0.1 s (Sahena etal., 1992}--much shorter than the assumed periodicity of 60 to 600 s (Hansen etal., 1992; Hansen and Dam-Johansen, 1993). Whether sulphur retention results in CaS or CaSO4 and the related

catalytic action of calcium-containing species on NOx formation, would depend on the time-averaged condition in the emulsion phase. Makarytchev et al. (1995) compared the experimentally observed desulphurization trends in an actual emulsion phase of SFBC with the results of multicomponent equilibrium calculations for conditions varying from reducing to oxidizing. It has been shown that with a global air ratio in the combustor of 1.4 the time-averaged atmosphere in the emulsion phase is close to reducing with an effective local air ratio of about 0.8, and that desulphurization essentially involves sulphur capture in the form of CaS. The present work uses the same approach to analyse limestone-catalyzed NOx formation by comparing experimental NOx emissions as observed in an 8-cm i.d. bench-scale SFBC with multicomponent (50 gas-phase, 7 solid-phase species) equilibrium calculations for different constant conditions with an air ratio ranging from 0.6 to 1.4. Figure l(a) and (b) show the measured values of NO~ content (dashed curves) in the flue gas during limestone addition and the calculated amounts (solid curves) of CaS, CaSO4 and CaO in the solid phase. The experimental NOx data are the same for both figures and correspond to a global oxidizing atmosphere in the combustor with an overall air ratio of 1.4 (6% 02 in flue gas) and to no air addition with zero air ratio. The calculated curves reflect different local conditions in the emulsion phase and correspond to a timeaveraged oxidizing environment with an air ratio of 1.4 [Fig. l(a)] and reducing environment with an air ratio of 0.8 [Fig. l(b)]. Both figures show that NO/formation does not follow the rapidly increasing CaO curve, indicating therefore no catalytic effect of CaO. The observed NOx emissions, however, follow both the CaSO4 [Fig. l(a)] and the CaS [Fig. l(b)] curves. As to CaSO4, there is consensus among researchers that its catalytic action on NOx formation is poor or none [-e.g. L i n e t al. (1993) and Hansen and DamJohansen (I 993)]. Therefore, the actual catalyst in the emulsion phase could only be CaS, and NO~ formation and reduction are likely to be driven by the following reactions: NH3 + 5/402 CaS NO + 3/2H20

(3a)

NO + CO CaS 1/2N2 + CO2.

(4a)

The observed increase of NOx emissions with limestone addition, approximately twofold (from 100 to 240 ppm at 900°C), is the result of competition between reactions (3a) and (4a). In the absence of oxygen, reaction (3a) does not proceed and NOx formation is insensitive to limestone addition, i.e. Ca/S ratio. This is shown as the base line NO=,

2489

Shorter Communications

2490 a ) oxidizing 300

3.0

6

air ratio 1.4 air ratio 1.4 exp., air ratio 0

- - t a l c . , - -0- - exp.,

--I--

v

900°C

. , .... /zz z

200

.~,

._-NO*Y-'__/

,,,¢

*

3.0 '0

¢:

o 100

1.0 o K)

o

o

by Lyngfelt and Leckner (1989a, b). It should be noted, that the catalytic action of CaS on NO reduction by CO [reaction (4a)] has been indicated by several researchers (Furusawa et al., 1985; Hansen et al., 1992). However, the importance of CaS-catalyzed NH 3 oxidation to NO [reaction (3a)] has not been mentioned in the literature. S. V. MAKARYTCHEV t K. F. CEN Z. Y. LUO X. T. LI

Institute of Thermal Power Engineering Zhejiang University 310027 Hangzhou, China

O

z 1

2

3

REFERENCES

O.O

4

Ca/S molar ratio b} r e d u c i n g 300

3.0 A

air ratio 0 /

--I-exp.,

900°C/,..O .... ~

- -N-'~O.... q

200

2.0

100

1.0

2

o

m (,2

0 ¢.,

~6

Z

0

1

2

Ca/S

3

4

5

D.0

%9

molar ratio

Fig. 1. Measured NO~ content in the flue gas and calculated amounts of Ca-containing solid-phase species under timeaveraged oxidizing (a) and reducing (b) conditions. emissions of about 100 ppm for zero air ratio, as observed in a pure nitrogen atmosphere which probably result from the slow reactions of char-bound nitrogen and oxygen (shown in parentheses): (CNO)

,NO

+ (C)

(CN) + (CO) - - ~ NO + 2(C).

(5)

(6)

The above results can be interpreted in the light of a particle structure with a sulphided (CaS) core and a sulphated-sulphided (CaSO4-CaS) shell (Makarytchev et al., 1995) under fluctuating reducing/oxidizing conditions. Because of the poor catalytic action of CaSO4, the limestonecatalyzed NOx formation would therefore only depend on CaS in the solid phase, in accordance with Fig. l(b). As to the difference in NO emissions behaviour between CFBC and SFBC, in CFBC the oxygen-rich atmosphere above the secondary air level in the dilute region of the combustor provides a time-averaged oxidizing environment for limestone sulphation, leading to a particle structure consisting of a sulphated (CaSO4) shell and an unreacted (CaO) core. Excessive limestone addition with Ca/S ratios greater than three, however, causes the appearance of exposed CaO surfaces, which catalytically increase NO formation by reaction (3). In contrast, in the emulsion phase of SFBC with a timeaveraged reducing atmosphere sulphur is captured primarily as CaS, and NO emission would follow the CaS content in the solid phase [Fig. l(b)]. In the case of a low-volatile fuel, reaction (3a) is less important and CaS catalyzes only the reduction of NO, while the formation of NO is more likely to proceed according to the slow solid-phase reactions (5) and (6). The net result is a decrease of NO emission, as observed

/~mand, L.-E. and Leckner, B., 1991, Oxidation of volatile nitrogen compounds during combustion in circulating fluidized bed boilers. Energy & Fuels 5, 809-815. Furusawa, T., Kojama, M. and Tsujimura, M., 1985, Nitric oxide reduction by carbon monoxide over calcined limestone enhanced by simultaneous sulphur retention. Fuel 64, 413-415. Hansen, P. F. B. and Dam-Johansen, K., 1993, Limestone catalyzed reduction of NO and N20 under fluidized bed combustion conditions. Proceedings of the 12th International Conference FBC, pp. 779-787, ASME. Hansen, P. F. B., Dam-Johansen, K., Johnsson, J.-E. and Hulgaard, T., 1992, Catalytic reduction of NO and N20 on limestone during sulfur capture under fluidized bed combustion conditions. Chem. Engng Sci. 47, 2419-2424. Johnsson, J. E., 1989, A kinetic model for NOx formation in fluidized bed combustion. Proceedings of the lOth International Conference FBC, pp. 1111-1118, ASME. Johnsson, J. E. and Dam-Johansen, K., 1991, Formation and reduction of NOx in a fluidized bed combustor. Proceedings of the llth International Conference FBC, pp. 1389-1396, ASME. Leckner, B. and Amand, L., 1987, Emission from a circulating and a stationary fluidized bed boiler: a comparison. Proceedings of the 9th International Conference FBC, pp. 891-897, ASME. Lin, W., Johnsson, J. E., Dam-Johansen, K. and van den Bleek, C. M., 1993, Interactions between NO, emission and desulphurization in FBC--a laboratory study of catalytic NH 3 oxidation over CaO based sorbents during sulphation. Proceedings of the 12th International Conference FBC, pp. 1093-1100, ASME. Lyngfelt, A. and Leckner, B., 1989a, Sulphur capture in fluidized bed boilers--the effect of reductive decomposition of CaSO4. Chem. Engng J. 40, 59-69. Lyngfelt, A. and Leckner, B., 1989b, SO2 capture in fluidized bed boilers: re-emission of SO2 due to reduction of CaSO4. Chem. Engng Sci. 44, 207-213. Makarytchev, S. V., Cen, K. F., Lug, Z. Y. and Li, X. T., 1995, High-temperature sulphur removal under fluidized bed combustion conditions--a chemical interpretation. Chem. Engng Sci. (accepted). MjSrnell, M., Leckner, B., Karlsson, M. and Lyngfelt, A., 1991, Emission control with additives in CFB coal combustion. Proceedings of the l lth International Conference FBC, pp. 655-664, ASME. Sahena, S. C., Rag, N. S., Rag, V. G. and Koganti, R. R., 1992, Coal combustion studies in a fluidized-bed test facility. Energy 17, 579-591. Shimizu, T., Fujita, D., Ishizu, K., Kobayashi, S. and Inagaki, M., 1993, Effect of limestone feed on emissions of NO, and N20 from a circulating fluidized bed combustor. Proceedings of the 12th International Conference FBC, pp. 611-617, ASME. tAuthor to whom correspondence should be addressed. Visiting research professor (formerly Moscow University, Russia).