Energy & Fuels 1993, 7, 554-558
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Effect of Coal Rank and Circulating Fluidized-Bed Operating Parameters on Nitrous Oxide Emissions Michael E. Collings,' Michael D. Mann, and Brian C. Young Energy & Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota, 58202-9018 Received December 9, 1992. Revised Manuscript Received May 3, 1993
The effects of operational parameters and coal type on N2O emissions in a pilot-scale circulating fluidized-bed combustor (CFBC) were examined. Operational parameters included combustor temperature, excess combustion air, limestone feed rate, and primary:secondary combustion air split. Seven coals ranging in rank from lignite to high-volatile A bituminous were used to study coal-related effects. At typical operating conditions (20% excess air, 1120 K), N20 emissions ranged from 30 to 170 ppmv at 3% excess 0 2 or 16 to 100 mg N20/MJ heat input due to variations in coal type. The relative impact of operational parameters on N2O emissions is given for Salt Creek coal by the relation: temperature > excess air, primary/secondary air split > sorbent feed. Increasing combustion temperature was shown to decrease N2O emissions,while increasing excess air was shown to increase N2O emissions. The effect of limestone addition waa shown to be coal-dependent, with both increasing and decreasing trends in N2O emissions observed. The primary air-to-total-air ratio has a minimal impact on N20 emissions but tends to decrease emissions at higher temperatures, while the opposite trend occurs at lower temperatures.
Introduction Fluidized-bed combustion (FBC) has captured a significant portion of the manufacturing market where waste products or low-priced fuels are burned to produce process steam and/or electricity. More recently, FBC technology has been increased in scale to compete in the electric utility market, with the largest of the units capable of generating 160MW of electricity in either the bubbling or circulating mode. The motivation for utilizing fluidized-bed combustors in today's markets has evolved around environmental concerns and fuel flexibility. Since these units operate at considerably lower temperatures (1120K)than do pulverized coal-fired combustors, NO, emissions are greatly reduced and SO, emissions can be controlled in situ by direct combustor sorbent injection. As a consequence of operatingFBCs at low temperatures to enhance the capture of SO, and to reduce NO,, the formation of nitrous oxide (N20) becomes prevalent. This observation has been documented on lab-, pilot-, and fullscale units by numerous researchers.l-l0 Although N2O is nontoxic as a trace pollutant, it is a greenhouse gas and ~~~~
(l)Amand, L. E.; Andersson, S. In Proceedings of the Tenth International Conference on Fluidized-Bed Combustion, San Francisco, CA, 1989; ASME: New York, NY, 1989; p 49. (2) Gulyurtlu, M. C.; Cabrita,I.; Lopes, H.; Reforco, A. In Proceedings of the International Conference on Coal Science, Tokyo, Japan, 1989; p 473. (3)Amand, L. E.;Leckner,B.;Andersson, S.;Gustavsson, L. Presented at 1990 European Workshop on N20 Emissions, Lisbon, Portugal, June 1990. (4) Hiltunen, M.; Kilpinen, P.; Hupa, M.; Lee, Y. Y. In Proceedings of theEleuenthZnternationalConference on Fluidized-BedCombustion, Montreal, Canada, 1991; ASME: New York, NY, 1991; p 687. (5) Amand, L. E.; Leckner, B.; Andersson, S. Energy Fuels 1991, 5, 815. (6) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. In Proceedings of the Eleventh International Conference on Fluidized-Bed Combustion, Montreal, Canada, 1991; ASME: New York, NY, 1991; p 1005. (7) Brown, R.;Muzio,L. InProceedings oftheEleuenthInternational Conference on Fluidized-Bed Combustion, Montreal, Canada, 1991; ASME New York, NY, 1991; p 719.
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a potential source of stratospheric nitric oxide (NO), a catalytic ozone destroyer. Regulations for the control of N20 emissions do not currently exist but may be promulgated as the greenhouse/ozone debate unfolds. Continued progress in development of FBC technology may be contingent on NzO abatement through the understanding of N20 chemistry and the effects of operational parameters. Published N20 emissions data for experimental studies examining large numbers of coals are generally scarce at the pilot-scaleFBC level, although some data are available at the laboratory scale.11J2 The primary objective of this study is to establish the effecta of operating parameters and fuel specifications on N2O formation in a CFBC. This database can be used to facilitate the design of CFBCs for minimizingN2O formation and, with further development, to draft a preliminary empirical model for predicting N2O emissions performance.
Experimental Section Parametric testing was performed using a 1-MWth pilot-scale CFBC. The test rig has an internal diameter of 0.51 m and ie 12.8 m high. The combustor contains a series of refractory-lined sections bolted together and has been designed to operate over ranges in temperature of 1000-1200 K (1350-1700 OF), in excess air of 0-100%,in superficial gas velocity of 3.6-7.0 m/s, and in coal size of -3.2 to -12.7 mm (120-35 mesh). A schematic of this unit is displayed in Figure 1. Favorable comparisons have been (8) Shimizu, T.;Tachiyama, Y.; Souma, M.; Inagaki, M. InProceedings of the Eleuenthlnternational Conference on Fluidized-BedCombuetion, Montreal, Canada, 1991; ASME: New York, NY, 1991; p 695. (9) Mann, M. D.;Collings, M. E.; Botros, P. E. In Proceedings of the EighthInternationalPittsburghCoal Conference,Pittaburgh, PA, 1991; University of Pittaburgh: Pittaburgh, PA, 1991; p 1047. (10) Amand, L. E.; Leckner, B. Combust. Flame 1991,84, 181. (11) Wojtowicz, M. A.;Oude Lohuis, J. A.; Tromp, P. J. J.; Moulijn, J. A. In Proceedings of the Eleventh International Conference on Fluidized-Bed Combustion, Montreal,Canada, 1991;ASME New York, NY, 1991; p 1013. (12)Oude Lohuis, J. A.; Tromp, P. J. J.; Moulijn, J. A. Fuel 1992, 71, 9.
0 1993 American Chemical Society
Energy & Fuels, Vol. 7, No. 4, 1993 556
Effect of Coal Rank on NzO Emissions
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Figure 1. Schematic of the 1-MWth EERC pilobscale CFBC. made between results obtained from this pilot-scale facility and those from the full-scale 110-MWeColoradoUte Nucla Station.ls The effects of combustor scale on the size distribution of recirculating material, heat flux, combustion efficiencies, and flue gas emissions data were among the factors examined. The complete description of the pilot-scale CFBC and its operation can be found elsewhere." During combustion testing, continuous flue gas monitoring of 02, CO, COZ, SOz, NO,, and N20 was performed, with NzO analyzed by a nondispersive infrared spectrometer (NDIR). A previously e s t a b l i ~ h e dsampling ~~ protocol for NzO analysis was used to minimize cross interferences from other gaseous species. Initially, experiments examining the effects of operational parameters on N2O emissions were conducted using an eight-run fractional factorial design. This test matrix was used to screen the main parameters (effects), namely, temperature (1075and 1158 K), excess air (15% and 45%), limestone feed (1 and 3 alkali-to-sulfur ratio), and air split (5050 and 7030 primary: secondary air), for significance. Further experimentation concentrated on parameters shown to influence N20 emissions. The N2O emissions data are expressed on a mg N20/MJ heat input basis. This approach effectivelynormalizes for combustor firing rate and allows fuel-component input rates to be placed on the same basis. Other bases, such as conversion of fuel nitrogen to NzOor ppm N2O at 3 % 0 2 , assume a constant firing rate between tests as well as coals. Since firing rate can fluctuate with coals having high ash or moisture levels, we feel the firing rate basis will produce less error over the wide range of experimental conditions and coal ranks. The distribution of fuel-nitrogen into the char and volatile matter was determined by performing ultimate analyses on coal and coal-derived chars produced using ASTM (D3175) procedures for determining volatile matter. The quantity of nitrogen in the volatile matter was thus determined by difference. In this ASTM procedure, dried coal samples are heated for 7 min a t a temperature of 1225 K in crucibles covered with lids.
Results and Discussion The current test matrix includes two lignites (one from Asia and one from North Dakota), two subbituminous coals (Wyoming), and three bituminous coals (Pennsylvania, Colorado, and New Mexico). The properties of the coals are listed in Table I. As seen in this table, coals were chosen from a wide range of geographical formations and (13) Hajicek, D. R.; Henderson, A. K.; Moe, T. A.; Mann, M. D. In Proceedings of the Effectsof Coal Quality - on Power Plants Conference,
San Diego, CA, 1992:
(14) Henderson, A. K.; Moe, T. A.; Hajicek, D. R.; Mann, M. D. In Proceedims of the Sixteenth Biennial Low-Rank Fuels SymDosium, - ~ & a , MT,i s i ; p 407. (15) Montgomery, T. A.; 39,721.
Samuelsen, G. S.; Muzio, L. J. JAF'CA 1989,
ranks. The partitioning of the coal-nitrogen into the volatile matter and the char is also presented in Table I. The significance of the distribution of nitrogen in the coal is related to the reaction pathway in which fuel-bound nitrogen is converted to N2O. The non-matrix-bound nitrogen released with the volatile matter is free to react early in the combustion process through homogeneous or heterogeneous reaction pathways to form nitrogen or nitrogen oxides (NO, NO2, or N2O). In the case of CFB combustion, matrix-bound char nitrogen will produce nitrogen oxides through heterogeneous oxidation reactions throughout the combustor but is believed to be a minor contributor to N2O formation.16 The char nitrogen may also be gasified and may react through homogeneous mechanisms.16J7 Effect of Temperature and Coal Rank. The effect of temperature and coal rank on N2O emissionsis presented in Figure 2. The pilot-scale results show appreciably different N2O emissions for the seven coals tested. For a given temperature, the N2O emissions were the greatest for the higher-ranked Salt Creek bituminous coal and the least for the Powder River subbituminous coal. Two obvious temperature trends appear intimately related to coal rank. The first is that the absolute NzO emissions from the higher-ranked bituminous coals are greater than those from the lower-ranked coals. The second trend indicates that the temperature dependence (slope of temperature versus emissions curve) of a given coal rank is of similar magnitude. The latter trend probably corresponds to differences in coal composition or physical characteristics loosely associated with rank. As seen in Figure 2, the bituminous, lignite, and subbituminous coals have distinctly different slopes. Further examination of Figure 2 shows that the effect of coal rank on NzO emissions is not a continuous relationship. At lower CFBC operating temperatures (
