Energy & Fuels 1999, 13, 1203-1211
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Effect of Coal Minerals on Chlorine and Alkali Metals Released during Biomass/Coal Cofiring David C. Dayton* and Deirdre Belle-Oudry National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401-3393
Anders Nordin Energy Technology Center, Department of Inorganic Chemistry, University of Umea˚ , S-901 87 Umea˚ , Sweden Received May 10, 1999
The combustion behavior, gaseous emissions, and alkali metals released during the combustion of several biomass/coal blends were investigated using a direct sampling, molecular beam mass spectrometer (MBMS) system in conjunction with a high-temperature alumina-tube flow reactor. Pittsburgh No. 8 and Eastern Kentucky coals were blended with various biomass samples such as red oak wood chips, Imperial wheat straw, and Danish wheat straw. The coal/biomass blends were 15% biomass on an energy input basis. All pure fuels and blends were subjected to combustion in 20% oxygen in helium at a furnace temperature of 1100 °C, and the products were monitored with the MBMS. The amounts of NO(g) and SO2(g) detected during the combustion of the coal/biomass blends suggested that any change was the result of diluting the nitrogen and sulfur present in the fuel blend. The amount of HCl(g) detected during the combustion of the coal/wheat straw blends was higher than expected based on the combustion results for the pure fuels. Conversely, the amounts of KCl(g) and NaCl(g) detected during the combustion of the coal/ wheat straw blends was lower than expected. Chemical equilibrium calculations indicated that the amounts of condensed alkali species were significantly enhanced, usually in the form of Sanidine (K2O*Al2O3*6SiO2) and Albite (Na2O*Al2O3*6SiO2). Therefore, the lower-than-expected amounts of gas-phase potassium chloride detected during combustion of the wheat straw/coal blends was likely the result of potassium being sequestered in the condensed phase by the clay minerals in the coal.
Introduction The threat of increased global warming has subjected the use of fossil fuels to increasing scrutiny in terms of greenhouse gas and pollutant emissions. As a result, the use of renewable and sustainable energy resources, such as biomass, for electricity production has become increasingly attractive. The use of dedicated biomass feedstocks for electricity generation could help reduce the accumulation of greenhouse gases because carbon dioxide is consumed during plant growth. The agricultural and wood products industries generate large quantities of biomass residues that could also provide fuel for electricity production. Increasing the use of such fuels could alleviate the burdens of waste disposal in the agricultural and wood products industries. One solution to increasing the use of biomass to produce electricity is to build dedicated biomass power plants that use 100% biomass fuel. As of July 1995, there were 360 biomass plants and approximately 1% of the grid-connected electricity in the United States was generated from renewables.1 The initial capital investment to build new biomass power plants to increase this * Author to whom correspondence should be addressed. (1) Cogen Database, Utility Data Institute, July 1995.
percentage is high. Another scenario for increasing the use of biomass to produce electricity is to cofire biomass and coal in coal-fired power plants. Coal-fired power plants are used to produce a large fraction of the electricity in the United States. If biomass were cofired at low percentages in these plants, the use of biomass for power production could dramatically increase. Cofiring biomass and coal increases the use of sustainable fuels without large capital investments and takes advantage of the high efficiencies obtainable in coal-fired power plants. Fuel diversity is another advantage of biomass/coal cofiring which reduces the need for a constant supply of biomass that would be required in a biomass power plant. Cofiring biomass and coal is also a viable way to manage the increasing emissions of greenhouse gases and other pollutants from power generating facilities. Biomass and coal have fundamentally different fuel properties. For instance, biomass is a more volatile fuel and has a higher oxygen content than coal. In general, biomass contains less sulfur than coal, which translates into lower sulfur emissions as higher blending ratios of biomass are used. Wood fuels generally contain very little ash (1% or less); consequently, increasing the ratio of wood in biomass/coal blends can reduce the amount
10.1021/ef9900841 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/21/1999
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of ash that needs to be disposed. A negative aspect of biomass (especially some grasses and straws) is that it can contain more reactive potassium and chlorine than coal. Several U.S. utilities have tested biomass/coal cofiring in utility boilers,2-7 and cofiring tests have been conducted in Europe.8-14 A comprehensive, multi-laboratory study known as the European Commission’s APAS Clean Coal Technology Program15 was formed to study the feasibility and technical challenges associated with partially replacing coal by biomass and converting wastes (such as sewage sludge) to energy in coal-fired power plants. Analyses of the results from many laboratory- and large-scale demonstrations of biomass/coal cofiring have led to the publication of various recommendations and guidelines for cofiring biomass and coal.16,17 Nevertheless, several issues regarding how blending biomass and coal will affect combustion performance, emissions, fouling and slagging propensities, corrosion, and ash saleability still remain.4 (2) Boylan, D. M. Southern Company Tests of Wood/Coal Cofiring in Pulverized Coal Units. Proceedings from the conference on Strategic Benefits of Biomass and Waste Fuels, March 30-April 1, 1993, in Washington, DC. EPRI Technical Report (TR-103146), 1993, pp 4-33. (3) Gold, B. A.; Tillman, D. A. Wood Cofiring Evaluation at TVA Power Plants. Biomass Bioenergy 1996, 10(2-3), 71-78. (4) Tillman, D. A.; Prinzing, D. E. Fundamental Biofuel Characteristics Impacting Coal-Biomass Cofiring. Proceedings from the Fourth International Conference on the Effects of Coal Quality on Power Plants, August 17-19, 1994, in Charleston, SC. EPRI Technical Report (TR-104982) 1995, pp 2-19. (5) Aerts, D. J.; Ragland, K. W. Co-Firing Switchgrass and Coal in a 50 MW Pulverized Coal Utility Boiler. Proceedings of Bioenergy ′98: Expanding BioEnergy Partnerships, held October 4-8, 1998, in Madison, WI, 1998, pp 295-305. (6) Hunt, E. F.; Prinzing, D. E.; Battista, J. J.; Hughes, E. The Shawville Coal/Biomass Cofiring Test: A Coal/Power Industry Cooperative Test of Direct Fossil-Fuel CO2 Mitigation. Energy Conserv. Mgmt. 1997, 38, S551-S556. (7) Benjamin, W. Biomass Development and Waste Wood CoFiring. Energy Conserv. Mgmt. 1997, 38, S545-S549. (8) Hughes, E. E.; Tillman, D. A. Biomass Cofiring: Status and Prospects 1996. Fuel Process. Technol. 1998, 54 (1-3), 129-142. (9) Ru¨diger, H.; Kicherer, A.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Investigations in Combined Combustion of Biomass and Coal in Power Plant Technology. Energy Fuels 1996, 10, 789-796. (10) Pedersen, L. S.; Morgan, D. J.; van de Kamp, W. L.; Christensen, J.; Jespersen, P.; Dam-Johansen, K. Effects on SOx and NOx Emissions by Co-Firing Straw and Pulverized Coal.” Energy Fuels 1997, 11, 439-446. (11) Knoef, H. A. M.; Leenders, M. E. T.; Stassen, H. E. M. CoFiring of Powder Coal and Pulverized Wood Waste in a Power Station: Solution of an Environmental Problem? In Biomass for Energy, Environment, Agriculture and Industry, Proceedings of the 8th European Biomass Conference in Vienna, Austria 3-5 October 1994. Chartier, Ph., Beenackers, A. A. C. M, and Grassi, G., Eds.; Pergamon Press, Elmsford, NY, 1995; pp 1697-1709. (12) Hansen, P. F. B.; Andersen, K. H.; Wieck-Hansen, K.; Overgaaard, P.; Rasmussen, I.; Fransden, F.; Hansen, L. A.; Dam-Johansen, K. Cofiring straw and Coal in a 150-MWe Utility Boiler: In-situ Measurements. Fuel Process. Technol. 1998, 54 (1-3), 207-225. (13) Van de Kamp, W. L.; Morgan, D. J. The Co-Firing of Pulverised Bituminous Coals with Straw, Waste Paper and Municipal Sewage Sludge. Combust. Sci. Technol. 1996, 121, 317-332. (14) Henriksen, N.; Larsen, O. H. Fouling and Corrosion in Straw and Coal-Straw Fired USC Plants. In Biomass for Energy and Environment. Proceedings of the 9th European Bioenergy Conference in Copenhagen, Denmark 24-27, June 1996. Chartier, Ph., Ferrero, G. L., Henius, U. M., Hultberg, S., Sachau, J., Wiinblad, M., Eds.; Pergamon Press, Elmsford, NY, 1996; pp 192-197. (15) Bemtgen, J. M.; Hein, K. R. G.; Minchener, A. J. APAS Clean Coal Technology; Vol. 1: Co-Utilization of Coal, Biomass, and Waste; Vol. 2: Combined Combustion of Biomass/Sewage Sludge and Coals; Vol. 3: Co-Gasification of Coal/Biomass and Coal/Waste Mixtures. Published at the Institute for Process Engineering and Power Plant Technology, University of Stuttgart, 1995. (16) Tillman, D. 1997. Biomass Cofiring Guidelines. Final Report prepared for the Electric Power Research Institute, Palo Alto, CA. September 1997, Report EPRI/TR-108952.
Dayton et al.
The objective of the present work was to investigate the interactions between various biomass feedstocks and coal in a high-temperature, oxidizing environment to address the issues of gaseous emissions and ash deposit chemistry when cocombusting these two solid fuels. In an idealized, controlled laboratory flow reactor, the combustion products from small samples of biomass, coal, and biomass/coal blends were analyzed to determine whether there were any fundamental interactions between the two fuels during combustion of the blends or whether the two fuels behave independently when cofired. The results reveal the synergistic effects of cofiring biomass and coal. Experimental Approach The coals were prepared as utility grind fuels (70% through 200 mesh). The biomass samples were prepared using a tub grinder followed by a micropulverizer, resulting in a final product that passed through a 1-mm screen. For this study, results are presented for blends of Pittsburgh No. 8 and Eastern Kentucky coals with red oak, Danish wheat straw, and Imperial wheat straw (from California). The results from the proximate, ultimate, and elemental ash analyses for each pure fuel are presented in Table 1. The blends investigated during this study consisted of 15% biomass on an energy input basis, based on the higher heating value of the fuel as received. The Pittsburgh No. 8 and Eastern Kentucky coals are both high-volatile A bituminous coals with similar heating values. Both have similar ash contents; however, the Pittsburgh No. 8 coal contains more iron and has a higher sulfur content than the Eastern Kentucky coal. The biomass samples represent fuels with a wide range of ash, alkali, and chlorine contents. The Imperial wheat straw sample contains high levels of ash, potassium, sodium, and chlorine compared to a range of biomass fuels.18,19 This wheat straw sample represents an upper limit of the amount of potassium, sodium, and chlorine that can be found in biomass fuels, and would be considered a problematic fuel in terms of ash deposition and corrosion in combustion systems.20-23 The (17) Robinson, A.; Baxter, L.; Junker, H.; Shaddix, C.; Freeman, M.; James, R.; Dayton, D. Fireside Issues Associated with Coal-Biomass Cofiring. Proceedings of Bioenergy ’98: Expanding BioEnergy Partnerships, held October 4-8, 1998, in Madison, WI, pp 275-284. (18) Dayton, D. C.; Milne, T. A. Laboratory Measurements of Alkali Metal Containing Vapors Released During Biomass Combustion. Application of Advanced Technologies to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996; pp 161-185. (19) Nordin, A. Chemical Elemental Characteristics of Biomass Fuels. Biomass Bionergy 1994, 6, 339-347. (20) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Dayton, D.; Milne, T.; Bryers, R. W.; Oden, L. L. The Behavior of Inorganic Material in Biomass-Fired Power BoilerssField and Laboratory Experiences: Alkali Deposits Found in Biomass Power Plants, Vol. 1 and 2; Technical Report No. SAND96-8225, NREL/TP-433-8142). Sandia National Laboratories, National Renewable Energy Laboratory, 1996. (21) Baxter, L. L.; Miles, T. R., Jr.; T. R. Miles; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The Behavior of Inorganic Material in Biomass-Fired Power Boilers: An Overview of the Alkali Deposits Project. Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; 1997; Blackie Academic and Professional: London, pp 1424-1444. (22) Baxter, L. L.; Nielsen, H. P. The Effects of Fuel-bound Chlorine and Alkali on Corrosion Initiation. In the Fuel Division Preprints from the 214th American Chemical Society National Meeting, Las Vegas, NV; 1997, 3, 1089-1095.
Release of Cl, K, and Na during Biomass/Coal Cofiring
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Table 1. Results for the Analyses of the Solid Fuels Used in the Cofiring Studies Pittsburgh No. 8 coal moisture ash volatile fixed carbon heating value (HHV as received - Btu/lb) moisture C H N S Ash Oa Clb SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P2O5 SO3 a
Eastern Kentucky coal
proximate (wt % as received) 1.14 1.63 7.90 7.43 36.80 35.44 54.16 55.50 13691 13351 ultimate (wt % as received) 1.14 1.63 78.02 76.83 4.87 4.88 1.36 1.47 2.78 0.88 7.90 7.43 3.93 6.88 0.09 0.17
red oak
Danish wheat straw
Imperial wheat straw
4.76 1.15 82.88 11.21 7800
5.41 7.33 73.39 13.87 7355
7.99 13.49 65.14 13.38 6318
4.76 49.02 5.34 0.19 0.02 1.15 39.52 98%) and a small but significant fraction of the sodium vapors released during Imperial wheat straw combustion are predicted to form gaseous chlorides. Other gaseous alkali species have low predicted equilibrium mole fractions. Most of the sodium and the remainder of the potassium form stable condensed-phase alkali silicates. When Imperial wheat straw is cocombusted with coal, 50-80% of the potassium is predicted to form Sanidine and 40-75% of the sodium is found in either Albite at higher temperatures or as sulfate and silicate at lower temperatures. Conclusions The MBMS results for the relative amounts of NO(g) and SO2(g) detected during the combustion of the coal/ biomass blends suggested that any decrease in the
Release of Cl, K, and Na during Biomass/Coal Cofiring
amount of NO(g) or SO2(g) observed because of blending coal and biomass was the result of diluting the nitrogen and sulfur present in the fuel blend. The chlorine released during the combustion of the coal/biomass blends, however, appeared to be affected by blending the two fuels beyond just a dilution effect. The amount of HCl(g) detected during the combustion of the coal/ wheat straw blends was higher than expected based on the combustion results for the pure fuels. Conversely, the amount of KCl(g) and NaCl(g) detected during the combustion of the coal/wheat straw blends was lower than expected. Blending coal and high-chlorine- and -alkali-containing fuels apparently affects the chlorine equilibrium in such a way that cannot be explained on the basis of just mixing of the pure fuels. Some other chemical interactions between the two blended fuels affect the partitioning of chlorine in the gas phase between alkali and hydrogen chlorides. The results of the equilibrium calculations qualitatively help explain the repartitioning of the gas-phase chlorine inferred from the MBMS results. The high alkali and chlorine contents and normally low silica content of certain biomass fuels can result in high gasphase alkali metal concentrations during combustion. This has been observed experimentally and inferred from the equilibrium calculations presented here. Actual gas-phase alkali metal concentrations are generally assumed to be significantly higher than predicted by chemical equilibrium because of kinetic restrictions. However, gas-phase alkali metal concentrations can be affected by introducing mineral matter into a fuel blend, either as clay or mineral additives or in the form of coal. Potentially favorable reaction kinetics and other driving forces may result in lower gas-phase alkali metal concentrations when alkali metals are sequestered by added mineral matter. The equilibrium results showed that when mixing the biomass fuels with coal, the amounts of condensed alkali species are significantly enhanced, preferably in the form of Sanidine (K2O*Al2O3*6SiO2) and Albite (Na2O*Al2O3*6SiO2). Solid forms of SiO2, Al2O3, most silicates, and aluminosilicates, however, are generally recognized for slow, diffusion-controlled reactions and small surface areas. Several minerals associated with coal (kaolinite (K2O*Al2O3) and other clay minerals) have, on the other hand, proven to react readily with gaseous alkali species. Therefore, the lower-thanexpected amounts of gas-phase KCl detected during combustion of the wheat straw/coal blends are likely the result of potassium being sequestered in the condensed phase by the clay minerals in the coal. With less potassium available for the formation of gas-phase KCl, more HCl(g) forms when the wheat straws and coals are cocombusted. Alkali metals are sequestered via chemical reaction to form new mineral matter or via physisorption of alkali vapors by the clay minerals. The implications for NOx and SOx emissions and potential fouling and slagging propensities in com-
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mercial-scale systems cofiring biomass and coal are mixed based on the results presented earlier. These idealized combustion experiments in no way reproduce the complexity of the combustion processes in largerscale systems. Adding biomass to the fuel blend in largescale systems not only changes the amount of fuel-bound nitrogen in the fuel mix (assuming the biomass contains less nitrogen than does the coal), but also the flame characteristics in the burner, the temperature of the flame, the amount of air needed for combustion, and other engineering factors that affect NOx formation. The SO2 emissions should decrease by the amount of biomass added to the fuel blend because biomass typically contains much less sulfur than coal. Any advantages of larger-than-expected SO2 reductions because of sulfur capture by the biomass ash constituents were not observed in the present study but may occur in largescale systems. The potassium sequestration by the coal minerals may be beneficial in terms of reducing ash deposition in cofiring situations. Cofiring high-alkali-metal-containing biomass fuels with coal would certainly be an advantage in terms of ash deposition compared to firing the biomass independently. Reducing the gas-phase alkali metal concentration should decrease the ash deposition rate. How this changes the fouling and slagging propensity in a commercial-scale system can be inferred by the ash fusion temperatures of the biomass fuel, coal, and blend. If the biomass fuel contains a high percentage of chlorine and a large amount of potassium, the reduction in gas-phase KCl comes at the expense of an increase in the amount of HCl(g) formed, which could increase the rate and severity of corrosion in a commercial boiler. The present study has identified several issues that face successful commercial-scale biomass cofiring. This and similar studies, however, cannot take the place of full-scale cofiring demonstration tests that address these issues under actual boiler conditions. The effects of cofiring biomass and coal on emissions and ash deposition rates are also likely to be very sensitive to the type of combustor in which the cofiring is conducted. Acknowledgment. The authors acknowledge support from the Solar Thermal, Biomass Power, and Hydrogen Technologies Division of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. Special thanks go to Richard L. Bain and Thomas A. Milne for programmatic and technical support and guidance. Dr. Larry Baxter, Sandia National Laboratories, supplied the biomass and coal samples. Dr. Anders Nordin acknowledges the support of Teknikvetenskapliga Forsknings Ra˚det (TFR), the Swedish Research Council for Engineering Sciences, while at the National Renewable Energy Laboratory. EF9900841
