Energy & Fuels 1989,3, 278-283
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Art ic 1es Decomposition of NH3 over Calcined and Sulfated Limestone at 725-950 "C D.A. Cooper,* S. Ghardashkani, and E. B. Ljungstrom Department of Inorganic Chemistry, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden Received August 15, 1988. Revised Manuscript Received November 21, 1988
With use of a conventional fixed-bed reactor, calcined and sulfated Ignaberga limestone particles have been shown to catalyze the decomposition of NH,, under conditions relevant to fluidized-bed combustion. The reaction on the calcined limestone surface was found to be inhibited by the production of H2 a t temperatures above ca. 800 OC, while a reduction of the sulfated limestone was observed at temperatures above ca.850 "C. Although no single rate expression could be used to analyze the entire data set, estimates of the first-order kinetic parameters gave the intrinsic rate constants k' = 0.69 X lo4 and 1.20 X lo4 s-' cm-2 and the activation energies E, = 98 and 146 kJ mol-' for calcined and sulfated limestone, respectively. Similar experiments using pure CaO and CaS04samples confirmed that the catalytic activity was not a property specific to the limestone impurities.
Introduction In view of the more stringent environmental legislation concerning pollutants, fluidized-bed coal combustion has been deemed an attractive and realistic source of power generation from solid fuels.' Continued efforts are aimed at optimizing this combustion system even further, especially to reduce the emissions of the main pollutant species, SO2and NO,, as much as possible. A detailed description and understanding of the formation and removal reactions of these species is a key step in order for improvements to be implemented. The chemistry and control of SO2 using limestone addition into the fluidized-bed boiler has received considerable attention to the point where several reasonable chemical models e ~ i s t . ~As * ~far as NO, is concerned however, a recent review highlighted the lack of experimental data needed if a complete NO, model is to be developed! Such a model would be a valuable asset in optimizing future boiler designs6i6and also for theoretical predictions concerning the effect of NO,-reducing additives.' It has been well established that the predominant route for NO formation in fluidized-bed combustion originates from the organically bound nitrogen in the coal (fuel N); ca. 10% is estimated to arise from atmospheric N2 (thermal NO).6** Coal particles injected into the bed undergo (1) La Nauze, R.D.; Davidson, J. F.; Cliit, R.; Harrison, D. Fluidization, 2nd ed.; Academic Press: London, 1985; pp 631-674. (2) Lee, D.;Georgakis, C. AZChE J. 1981,27,472-481. (3)Dennis, J. S.; Hayhurst, A. N. J. Znst. Energy 1988, 61, 98-109. (4) Johnsson, J. E. Presented at the IEA AFBC Meeting on Mathematical Modelling of FBC, Siegen, West Germany, Oct 26-27, 1987. (5) Furusawa, T.;Tsujimura, M.; Yasunga, K.; Kojima, T. h o c . Znt. Conf. Fluid. Bed Combust. 1985,8th, 1095-1104. (6) Be&, J. M.; Sarofim, A. F.; Lee, Y. Y. J. Inst. Energy 1981, 54, 38-47. (7) Lyon, R. K.;Hardy, J. E. 2nd. Eng. Chem. Fundam. 1986, 25, 19-24. (8)Hampartaoumian, E.;Gibbs, B. M. J. Znst. Energy 1984, 57, 402-410.
Table I. Scope of Experiment flow rate (at 25 OC) 170-3320 mL/min bed temp reacn time NH3 concn bed vol bed voidage fraction
725-950 "C 5-112 ms 4730-5050 ppm 1.7-2.8 cm3 0.284.39
thermal decomposition yielding gaseous volatile species and a residual solid char. Depending on the type of coal used and bed conditions, both the char and volatiles contain fuel N in varying degrees, although a greater fraction of fuel N is believed to be released within the volatiles? Combustion with oxygen of these two fuel N components thus leads to the fuel-derived NO formation. The volatile nitrogenous compounds, consisting largely of amines and cyanides, can have an additional role, however, since they are reducing agents capable of converting NO to N2. Most of the attempts to model the fate of the nitrogen-containingvolatiles in fluidized-bed combustion have relied on these species being approximated by NH3,5,sps-13 although the importance of HCN has also been noted.14 In the homogeneous gas phase, the chemical role of NH3 with respect to the formation and reduction of NO has been studied closely and an application patented under Exxon's "Thermal De-NO," process.'Js Similarly under heterogeneous conditions, both the oxidation of NH, to (9) De Soete, G. G. Fifteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1976; pp 1093-1102. (IO) Huama,T.;Kochiyama, Y.; Chiba, T.;Kobayaahi, H. J.Fuel SOC. Jpn. 1982, 61, 268-275. (11) Lee, Y. Y.; Sekthira, A.; Wong, C. M. h o c . 2nt. Conf. Fluid. Bed Combust. 1985, Bth, 1208-1218. (12) Lee, Y. Y.; Soares, S. M. S.; Sekthira, A. h o c . Znt. Conf.Fluid. Bed Combust. 1987,9th, 1184-1187. (13) Bose, A. C.; Dannecker, K. M.; Wendt, J. 0.L. Energy Fuels 1988, 2, 301-308. (14) Homer, T.J.; McCarville, M. E.; Gu Zhuo-Ying. Fuel 1988, 67, 642-650. (15) Lyon, R. K. U.S. Patent 3,900,544, 1975.
088'7-0624/89/2503-0278$01.50/00 1989 American Chemical Society
Energy & Fuels, Vol. 3, No. 3, 1989 279
Decomposition of NH3 over Limestone Table 11. Bed Materials BET area, porosity, approx ma/g cms/g diam,rcm w t , ~ Ignaberga limestone 0.96 0.0041 187 2.00 187 1.18 calcined Ignaberga 7.5 0.15 limestone 187 2.04 sulfated Ignaberga 0.86 0.0068 limestone 4.0 0.011 2.5 0.79 calcium oxide 5.5 0.033 1.5 1.07 calcium sulfate (anhydrous) 0 500 2.00 0.018 quartz sand Table 111. Chemical Composition (wt % ) of Ipaberga Limestone A1 Zn Ba Sr CaCOS MgCOS SiOz Fe 91.1
1.0
7.3
0.39
