Energy & Fuels 1994,8, 1083-1094
1083
Empirical Modeling of N20 Emissions from Circulating Fluidized-Bed Combustion Michael E. Callings* and Michael D. Mann Energy and Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202-9018 Received April 7, 1994. Revised Manuscript Received June 20, 1994@
This paper describes the development of a first-generation empirical model for predicting N2O emissions as a function of significant circulating fluidized-bed combustion (CFBC) operating parameters and fuel properties. The relative magnitude of the parameter estimates for the significant operating parameters are ranked temperature > excess air > SOdsorbent, which is consistent with data reported in literature. The SO2 concentration in the flue gas was substituted for the limestone feed rate as a main operating parameter for modeling. Unlike the limestone feed, the SO2 concentration was shown to be a significant modeling variable. The reason for the improved correlation is probably the result of (1) the SO2 concentration reflecting the level of sorbent utilization and (2) the homogeneous chemistry of N2O being dependent on the magnitude of SO2 emissions in the flue gas. Ranked in order of importance, the most significant fuel properties affecting the net formation of N2O emissions were char-N > moisture > oxygen > total-N. The stability of the char-N may be important for mass transport; i.e., the char may act as a vehicle for transporting fuel-N out of the highly reducing dense-bed region of the CFBC. This mechanism may account for the greater N2O emissions observed in CFBCs over their bubbling fluidized-bed combustion (BFBC)counterparts. The total coal-N flux was also included in the model and is believed to be representative of the volatile-N fuel component in the model. The effect of fuel-0 on N2O emissions is uncertain but may be attributed to rank effects when considered in concert with fuel moisture (H20). The parameter estimate for the fuel moisture variable showed that H2O decreased N2O emissions. The moisture from the fuels is believed to be a source for H and OH radicals that may effectively destroy N2O through homogeneous gasphase reactions.
Introduction Fluidized-bed combustion (FBC) has successfully emerged as an advanced coal combustion technology as a result of the favorable emissions and fuel flexibility characteristics. First-generation FBCs have evolved from the simple bubbling-bed combustors used for process steam production in manufacturing, to highefficiency, pressurized FBCs utilized for electrical power generation. Circulating fluidized-bed combustion (CFBC), a refinement over early bubbling-bed technology, has become the most popular of the atmospheric FBCs. This popularity originates from improved carbon burnout and sorbent utilization (sulfur capture), reduced NO, emissions, and smaller overall “footprint”resulting from the higher flue gas velocities of 5-10 d s . Since the advent of FBC technology in the 1970s, these combustors have dramatically increased in scale,l with present expectations of utility CFBCs producing a generating capacity of 600 MWe by the year 2000.2 Currently, the largest CFBCs in service are the twin 165-MWeboilers operated by Texas-New Mexico Power C O . , ~although a larger 250-MWe is scheduled for operation in France in 1995.2 Abstract published in Advance ACS Abstracts, August 1, 1994. (1)Dry, R. J.; La Nauze, R. D. Chem. Eng. Prog. 1990,86, 31-47. (2)Lucat, P.;Morin, J-X.; Semedard, J-C1. InProceedings of the 12th International Conference on Fluidized-Bed Combustion, Sun Diego, CA; ASME: New York, 1993;p 9. (3)Cavanaugh, H. A.Electrical World 1993,207,23.
Atmospheric FBCs operate at temperatures near 1125
K t o optimize the capture of SO2 by limestone and prevent the formation of agglomerates from ash fusion. At these low temperatures, the NO, emission from fuel-N conversion is reduced, and NO, formation through air-N fxation is insignificant. In addition, a considerable quantity of the fuel-N is partially oxidized to nitrous oxide (N20), resulting in a net decrease in NO, emissions by splitting the fuel-N pool with N2O. The formation of N2O in FBCs has led to considerable concern about the future of this technology because of the potential role of N2O as a greenhouse gas and a source of stratospheric NO, a catalytic ozone destroyer. Early studies implicating fossil fuel combustion as a cause for the rising levels of atmospheric N204v5were exaggerated as a result of gas-sampling artifacts. Recent measurements of N2O emissions from pulverized coal-fired boilers were an order of magnitude below the previous estimates,6 with values typically less than 5 ppmv. Nitrous oxide emissions from FBCs, however, are at levels initially thought to be of concern for impacting the global inventory. At present, these levels do not present a threat to the environment because of the relatively small share of the energy market captured
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(4)Weiss, R. F.;Craig, H. Geophys. Res. Lett. 1976,3, 751. (5)Hao, W. M.; Wofsy, S. C.; McElroy, M. B.; Beer, J. M.; Toqan, M. A. J. Geophys. Res. 1987,92, 3098. ( 6 )Linak, W. P.; McSorley, J. A.; Hall, R. E.; Ryan, J. V.; Srivastava, R. K.;Wendt, J.O.L.; Mereb, J. B. J. Geol. Res. 1990,95,7533.
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by FBCs. In addition, it should be emphasized that the postulated harmful effects from the “global warming” and “ozone destruction” theories may be premature or nonexistent, even though they are based on sound physical principles. Nevertheless, the future of FBC development and utilization could be enhanced if voluntary N20 control were demonstrated before regulations are deemed necessary and promulgated. Extensive research has been conducted in the past decade to assess the effects of operating parameters, sorbents, and fuels on N20 emissions from FBCs. Research by the authors of this paper has specifically addressed the impact of fuel selection and operating parameters on N2O emissions from a 1-Mwth pilot-scale CFBC.7,8 The results from many such studies have been compiled in several reviewsg-ll and embody the present state of knowledge on N20 emissions. Most of the available data in the literature are in agreement concerning the significant parameters affecting N20 emissions in FBCs. These include temperature, excess air, limestone feed, and coal rank. With respect t o coal rank, general qualitative trends characterizing the effect of volatile matter, oxygen-to-nitrogenratio, and the fuel ratio (fixed carbodvolatile matter) on NzO emissions have been noted.12-14 However, very little is known about the quantitative effects of specific fuel properties on N2O emissions. Preliminary work concerningparametrization of fuels for the modeling of N20 emissions has shown good correlations between select fuel properties and the temperature dependence of N20 emissions.15J6 Simple relations utilizing the observed slopes and intercepts of the N20 temperature curves for the various fuels were used as dependent variables for identifying significant fuel properties or ratios of fuel properties. In the present study, a more rigorous approach to modeling is taken, using both operating and fuel property factors to model the N20 emission response. The main fuel parameters, operating parameters, and the interaction between fuel and operating parameters were used as the independent variables. Key variables were identified through statistical inference, while models were selected by maximizing the correlation coefficient using a multiple linear regression analysis. This paper describes the development of a firstgeneration empirical model for predicting N2O emissions as a function of significant CFBC operating parameters (7)Mann, M. D.; Collings, M. E.; Botros, P. E. In Proceedings of the 8th International Pittsburgh Coal Conference, Pittsburgh, PA; University of Pittsburgh: Pittsburgh, 1991;p 1047. (8) Collings, M. E.; Mann, M. D.; Young, B. C. Energy Fuels 1993, 7,554. (9)Mann, M. D.; Collings, M. E.; Botros, P. E. Prog. Energy Combust. Sci. 1992,18, 447. (10)Hayhurst, A. H.; Lawrence, A. D. Prog. Energy Combust. Sci. 1992, 18, 529. (11)Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1. (12) Wojtowicz, M. A.; Oude Lohuis, J. A.; Tromp, P. J. J.;Moulijn, J. A. In Proceedings of the 11th International Conference on FluidizedBed Combustion, Montreal, Canada; ASME: New York, 1991;p 1013. (13)Aho, M. J.;Rantanen, J. T.Fuel 1989, 68,586. (14)Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. In Proceedings of the 11th International Conference on Fluidized-Bed Combustion, Montreal, Canada; ASME: New York, 1991;p 1005. (15)Mann, M. D.; Collings, M. E.; Young, B. C. In Proceedings of the 9th Annual International Pittsburgh Coal Conference, Pittsburgh, PA; University of Pittsburgh: Pittsburgh, 1992;p 1006. (16)Collings, M. E.; Mann, M. D.; Young, B. C.; Botros, P. E. In Proceedings of the 12th International Conference on Fluidized-Bed Combustion, Sun Diego, CA; ASME: New York, 1993;p 619.
Collings and Mann
and fuel properties. This model may facilitate preliminary CFBC design by estimating prospective N2O emissions based upon the fuel selected and the operating conditions employed. Alternatively, because of the difficulty in accurately measuring NzO emissions without the required sampling and treatment protocol,17the model may be applied to existing facilities for estimating N20 emissions where the necessary equipment is unavailable. In addition, the fuel properties selected in the model on the basis of statistical inference may lend insight into the mechanisms of N2O formation and destruction.
Experimental Section Equipment. Parametric testing was performed using a 1-Mwth pilot-scale CFBC. The test rig has an internal diameter of 0.51 m and a height of 12.8 m. The combustor contains a series of refractory-lined sections bolted together and has been designed to operate over a temperature range from 1000 to 1200 K, excess air range of 0-loo%, superficial gas velocity range of 3.6-7.0 mJs, and a top coal size of 3.2 to 12.7 mm (120-35 mesh). A complete description of the pilotscale CFBC design, test program, operation, and performance characteristics can be found elsewhere.ls A typical steady-state period of 6-8 h was used for each CFBC combustion test. At the end of each test, a nominal 8-h period was used for transition to the next test condition. All process data, including the on-line gas analyzers and the system temperatures, pressures, feed rates, and flows, were logged at 5-min intervals using a computerized data acquisition and process control system. Continuous extractive sampling of the flue gas from a location downstream of the hot cyclone was utilized for analysis of the major gas components. These components included COz, CO, 0 2 , NzO, NO,, and SOz. NzO emissions were measured with a Siemens nondispersive infrared analyzer (NDIR). This continuous analyzer utilizes the 7.2-pm wavelength NzO absorption bands (and higher) for detection. At these wavelengths, the main interferences come from HzO, SOz, and NO, which were eliminated by passing the flue gas through a 0.6 M NaZC03 solution to remove ,502, and Drierite (CaS04) to remove residual moisture prior to analysis. NO, emissions from the CFBC were typically too low (-
E Q
6 z
100
0 1060
1120
1140
Mean Combustor Temperature, K Figure 10. Actual and predicted NzO emissions as a function of mean combustor temperature for the EVT 72-MWth utility CFBC.
EVT (Ahlstrom) and operated by Stadtwerke in Pforzheim. This CFBC was chosen because the data published by Boemer and BraunZ2showed effects from both temperature and excess air and provided the necessary modeling information. The actual NzO emissions and the model predictions for these two cases are presented in Figures 9 and 10, respectively. As seen from Figure 9, the prediction of the magnitude of the NzO emissions as well as the slope of the temperature effect shows excellent agreement with the actual data acquired from firing an Ohio bituminous coal in the MSFB. In addition, the model compensated for the differences in excess air and SO2 concentration between the individual test conditions. The prediction of Nz0 emissions for the full-scale CFBC showed considerable deviation from the actual values as the amount of excess air was decreased, as evidenced in Figure 10. At the highest excess air conditions, the predicted N20 emissions were very close to the actual values in magnitude and temperature dependence. As the excess air was decreased, the model
1094 Energy &Fuels, Vol. 8, No. 5, 1994
greatly over protected the magnitude of the N2O emissions while the temperature dependence was in good agreement. The loss in the ability of the model to predict the magnitude of N2O emissions is believed to be design-related and is presently not accounted for in the model. The fact that the slopes are predicted adequately may be evidence that this effect is not fuel property-related. The two most obvious design differences that may be responsible for the observed deviation are residence time and the distance between primary and secondary air inlets. Combustor residence time is believed to be the dominant factor contributing to the lower N2O emissions in the full-scale CFBC. The reduction in the excess oxygen from 5.6 to 0.7% resulted in an increase in residence time from 7 to 10 s for the EVT boiler because of reduction in the flux of combustion air.22 On the other hand, the combustor gas velocity (residence time) was held constant when the excess air was changed for the EERC pilot-scale combustion tests used to create the model. The differences denoted in Figure 10 for the predicted N2O values are, therefore, strictly related t o excess air effects, since the residence time was held constant. The EVT test results can then be explained by the progression of residence time; i.e, the linear offset of the displaced curves for 0.7 and 3.8% 0 2 would be corrected by applying a factor (possibly nonlinear) to account for residence time. The other design parameter, the distance from the primary air (distributor plate) to the secondary air ports, may be equally important, but also strongly interdependent upon the combustor residence time. By increasing the distance between the air ports, Amand and L e ~ k n e showed r~~ that a significant decrease in N2O emissions will occur. This trend was verified in our pilot-scale CFBC when a highly volatile sewage sludge (C) was fired, where a 60% reduction in N20 emissions (1115 K, 20%excess air) was observed after the secondary air port was moved from 1.75 t o 3.20 m above the distributor plate. Dimensionally, moving the secondary air ports represented a change from 14 t o 25% of the total combustion chamber height. These tests, however, were excluded from the modeling database, since they were performed in a nonstandard configuration. Because of this dramatic air-staging effect, it is easy t o rationalize that an increase in residence time between the primary and secondary air ports by reducing fluidizing velocity could create a similar effect. Unfortunately, the velocity would have to be reduced nearly 50% to provide the residence time equivalent to moving the secondary air port, which would drastically affect the fluidization character. The combined effects of combustor residence time and air port location (and their interaction) may ultimately explain the differences commonly seen between the N2O emissions from fullscale and pilot-scale CFBCs.
Collings and Mann The magnitudes of the operating parameter estimates are ranked temperature > excess air > SOdsorbent, consistent with data reported in literature acquired over normal CFBC operating conditions. This relation validates the model by predicting the effects of operating parameters on N2O emissions. The SO2 concentration in the flue gas was substituted for the limestone feed rate as a main operating parameter for modeling. Unlike the limestone feed, the SO2 concentration was shown to be a significant modeling variable. The reason for the improved correlation is probably the result of (1)the SO2 concentration reflecting the level of sorbent utilization and (2) the homogeneous chemistry of N2O being dependent upon the magnitude of SO2 emissions in the flue gas. Ranked in order of importance, the most significant fuel properties affecting the net formation of N20 emissions were N , > H20 > 0 > N. The stability of the char-N (N,) may be important for mass transport; i.e., the char may act as a vehicle for transporting fuel-N out of the highly reducing dense-bed region of the CFBC. In the dilute phase, where N20 survivability is improved, the char-N can then be liberated in the form of volatile compounds or react directly through char oxidation reactions. This mechanism may account for the higher N2O emissions observed in CFBCs relative to BFBCs. The total coal-N flux (N) was included in the model in addition t o the char-N variable and is believed to be representative of the volatile-N fuel component in the model. The effect of fuel-0 on N2O emissions is uncertain, but may be attributed to rank effects when considered in concert with fuel moisture. The parameter estimate for the H2O variable showed that fuelH2O decreases N2O emissions. The moisture from the fuels is believed to be a source for producing H and OH radicals that may effectively destroy N20 through homogeneous gas-phase reactions. The trend of increasing N2O emissions with increasing rank noted by many researchers may be related to the quantity of nitrogen retained by the char. The char-N was the only significant coal property that followed this qualitative trend. The application of this model will result in a conservative estimate of the N20 emission factor for CFBCs. The predictions were shown to be biased high for a fullscale facility, probably as a result of combustor residence time and design-related parameters. Besides including more fuels in the database, a second-generation model for predicting N20 emissions should include dimensionless factors to account for the effects of scaleup, namely, combustor residence time, reducing zone residence time, and air port locations.
Conclusions Based upon the results of this empirical modeling effort, the following conclusions and observations were drawn: (34)Amand, L. E.; Lecher, B. In Proceedings of the 24th Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 1407.
Acknowledgment. This research has been supported by the Morgantown Energy Technology Center under Contract No. DE-FC21-86MC10637, with Dr. Peter E. Botros as the technical monitor. We also thank Dr. John Erjavec for his suggestions regarding the model selection procedures.
