Environ. Sci. Technol. 1997, 31, 2499-2503
Theoretical Feasibility for Ecological Biomass Ash Recirculation: Chemical Equilibrium Behavior of Nutrient Elements and Heavy Metals during Combustion ANDERS LJUNG* AND ANDERS NORDIN Energy Technology Center, Department of Inorganic Chemistry, Umeå University, S-941 28 Pitea, Sweden
To obtain a sustainable increased use of the CO2-neutral biomass fuels, the nutrient elements in the ashes formed have to be recirculated back to the forest and farm lands. During their growth, plants accumulate significant amounts of heavy metals of anthropogenic origin, normally enriched in the ashes during the energy conversion processes. If some kind of heavy metal separation technique could be applied during or after the processes, a more ecologically safe ash fraction may be produced for the recirculation of the nutrients. In addition, contaminated soils could be efficiently cleaned by controlled cultivation and combustion of biomass fuels. Previous experimental results from fullscale combustion plants have indicated that a significant fraction of a heavy metal-free ash may be obtained at high temperatures due to the lower volatilization temperatures of these metals. In the present work, the theoretical feasibility of a proposed high-temperature cyclone separation technique was evaluated by means of chemical equilibrium model calculations. The equilibrium behavior of both nutrient elements (Ca, Mg, K, Na, P) and heavy metals (Cd, Cu, Cr, Pb, Ni, Zn, As, V) as functions of temperature was determined. The results indicate that Cd, Cu, Pb, and possibly As and Cr may be volatilized, and thus separated, through a hot cyclone (800-850 °C), still keeping all Ca, Mg, and P and 75% of K and Na in a condensed form in the cyclone ash.
Introduction Energy production from biomass fuels does not contribute to any increase of the atmospheric CO2 concentrations. A significant shift from the use of fossil to biomass fuels could, therefore, be an effective measure to reduce the greenhouse effect. In Sweden, about 80 TWh was derived from biomass fuels during 1994, corresponding to almost 20% of the total energy supply (1). An increase to 200 TWh/yr within the next 3-4 decades is both desirable and possible (2). However, the losses of nutrient elements from productive lands by the total out-take of biomass raw materials and by acid rain leaching presently exceeds the natural weathering capacity (3). Thus, to obtain a sustainable utilization of biomass fuels, the nutrient-containing ashes have to be recycled back to the soils. During the combustion processes, the nutrient elements (Ca, Mg, K, Na, P) are normally enriched in the largest fraction (fly ash) of the total ash formed, but so are also Cd and other heavy metals (4). Historically, the heavy metal concentrations and accumulation rates in the biosphere have shown a dramatic increase due to anthropogenic emissions (5, 6). Thus, the heavy metal content in biomass has become significant,
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1997 American Chemical Society
FIGURE 1. Schematic illustration of the separation process concept. and some form of separation technique for the metals prior to the recirculation would be desirable. Many of the heavy metals volatilize during the hightemperature process, and previous works (7, 8) have indicated that by utilizing for example a hot cyclone prior to the condensation of the heavy metals in the heat exchangers, it could be possible to separate a heavy metal-free ash for recirculation (see Figure 1). The total fly ash is then fractionated into one large (>90%) heavy metal-depleted fraction (cyclone ash) and one smaller (10% of S and Cl), the volatilization temperature of CuCl(g) may be as low as 900 °C (Figure 3d). This is probably the most realistic case in
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FIGURE 4. Distribution diagrams showing species formed in the temperature range 100-1600 °C for the elements Cr, Pb, Ni, and Zn. normal combustion operation. At the lowest temperatures, the sulfates CuSO4(c) and CuO‚CuSO4(c) are stable. Chromium. By using only the data contained in the SGTE database, Cr will form CrSO4(c) and Cr2O3(c) up to 1000 °C if S is available. At this temperature, CrO3(g) will start to form, and at 1250 °C it is the predominant species, although the amount of CrO2(g) increases with increasing temperature. By considering some recent thermodynamic data (29) on gaseous chromium hydroxides and oxohydroxides, the distribution as depicted in Figure 4a was obtained, showing that Cr may be volatilized at a temperature as low as 500 °C. The new data suggest that CrO2(OH)2(g) and other oxohydroxides will form in the temperature range of 500-1600 °C. The only difference between 0% or 100% sulfur and chlorine available is that Cr2(SO4)3(c) is stable up to 350 °C if sulfur is available. No chromium chlorides were formed at any of the studied Cl concentrations. Lead. Lead is highly influenced by the amounts of S and Cl available. Without any S and Cl, solid carbonate and oxide are stable at lower temperatures, and PbO(g) starts volatalizing at about 550 °C. In Figure 4b, the distribution for 1% available S and Cl is shown, where the only difference is that PbSO4(c) will form at lower temperatures instead of PbCO3(c) and Pb3O4(c). For higher amounts of S and Cl, a gaseous chloride was unexpectedly formed at temperatures even below 100 °C, probably due to some inaccuracy of thermochemical data of PbCl4(g). If S and Cl concentrations higher than 10% were used, PbCl4(g) was formed around 700 °C. Nickel. Ni will form NiO(c) at temperatures as high as 1500 °C, irrespective of the concentrations of Cl. If S is available, NiSO4(c) will form at the low temperatures (Figure 4c). Nickel will thus be condensed at temperatures up to about 1450 °C. Zinc. The behavior of Zn is quite similar to Ni, although the volatilization temperature is somewhat lower (1170 °C),
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see Figure 4d. No significant effect of the Cl content was observed. Arsenic. The speciation of As is independent of both sulfur and chlorine content. The initial calculations utilizing only the SGTE database indicated a volatilization temperature of about 550 °C for As4O6(g). Below this temperature, As2O5(c) was stable and AsO(g) was formed at temperature higher than 850 °C. However, by adding the thermochemical data (28) for Ca3(AsO4)2(c), this species becomes stable at temperatures up to 1050 °C. Vanadium. V2O5(c) is the stable species up to 1200 °C, where VO2(g) will form. No influence of Cl was observed, and the only influence by higher amounts of available S is that VOSO4(c) is stable up to 275 °C. In conclusion, the relatively low volatilization temperatures of Cd, Cu, and Pb indicate that hot cyclone separation of an ash almost free from these elements could be possible. The species that we theoretically find hard to separate through the proposed cyclone process are Ni, V, and possibly Zn, which have too high volatilization temperatures. As and Cr may be separated, but the calculations also point out the importance of the availability of accurate thermochemical data. The findings reported from a previous field study using different fuels, including Salix, in a circulating fluidized bed combustor were in good agreement with the calculated results. Zn, Cd, and Pb were separated and enriched in the filter fly ash, whereas V and Ni were not. The fate of Cu, Cr, and As was however reported to be different in the different studies (8, 31). Thus, the effects of inaccuracies in thermochemical data, variations in fuel characteristics, and operating conditions have to be further studied. Our future work will therefore include both experimental validation of calculated results and a sensitivity study of the effects of inaccuracy in
thermochemical data, fuel, and process variables on the equilibrium relations.
Acknowledgments The financial support from the European Community through the EU-Joule Project JOR3-CT95-0001 is gratefully acknowledged.
Literature Cited (1) Hillring, B. Energy in Sweden 1995; Swedish National Board for Industrial and Technical Development: Stockholm, 1996. (2) Biomass fuels for the future; SOU 1992:90; ISBN 91-38-13162-5; 1992; in Swedish. (3) Eriksson, J.; Bo¨rjesson, P. Vattenfall report 1991/49, 1991. (4) Obernberger, I.; Biedermann, F.; Widmann, W.; Riedl, R. Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions. Biomass Bioenergy 1997, 12, 211-223. (5) Renberg, I.; Wik Persson, M.; Emteryd, O. Nature 1994, 386, 323-326. (6) Cole, K. L.; Engstrom, D. R.; Futyma, R. P.; Stottlemyer, R. Environ. Sci. Technol. 1990, 24, 543-549. (7) Obernberger, I.; Narodoslawsky, M. Material fluxes of elements in biomass combustion plants and their characteristics. In Proceedings of the 3rd European conference on industrial furnaces and boilers, April 18-21, 1995; INFUB: Lisbon, Portugal, 1996. (8) Obernberger, I. Characterisation and utilization of ashes from biomass combustion plants. In Proceedings of the Earth Conference on Biomass for Energy, Development and the Environment, Jan 10-13, 1995; Eurosolar: Bonn, Germany, In press. (9) Jakob, A.; Stucki, S.; Kuhn, P. Environ. Sci. Technol. 1995, 29, 2429-2436. (10) Gadd, G. Metal Tolerance. In Microbiology of extreme environments; Open University Press: Oxford, 1990. (11) Kumar, N. P. A.; Dushenkov, V.; Motto, H.; Raskin, I. Environ. Sci. Technol. 1995, 29, 1232-1238. (12) Johansson, L. Vattenfall report 1995/5, 1995. (13) Riddell-Black, D. M.; Rowlands, C.; Snelson, A. The take up of heavy metals by wood fuel cropssimplications for emissions and economics. Presented at 9th European Bioenergy Conference June 24-27, 1996. (14) Nordin, A. Ph.D. Thesis, Umeå University, 1993. (15) Nordin, A. Biomass and Bioenergy 1994, 6, 339-347. (16) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Prog. Energy Combust. Sci. 1994, 20, 115-138. (17) Mojtahedi, W.; Backman, R.; Larjava, K. Fate of some trace elements in fluidised-bed combustion and gasification processes; Publication 42; Technical Research Centre of Finland: Espoo, 1987.
(18) Ericsson, G.; Spencer, P. J.; Buekens, A.; Verhulst, D. Environ. Sci. Technol. 1996, 30, 50-60. (19) Wochel, J.; Stucki, S. Fate of heavy metals during refuse incineration. Annual Report 1994, Annex V, PSI General Energy Technology, Newsletter 1994. (20) Vogg, H.; Braun, M.; Scheider, J. Waste Manage. Res. 1986, 4, 65-74. (21) Nordin, A.; Forsberg, S.; Backman, R.; Rose´n, E. Application of extensive equilibrium calculations to the study of ash formation and sulfur capture during combustion and gasification of biomass fuels. Proceedings of the 8th European Conference pn Biomass for Energy, Environment, Agricultue and Industry, Wienna, Oct 3-5, 1994; Pergamon: Oxford, 1995. (22) Kallner, P.; Nordin, A.; Backman, R. Fate of ash forming elements in gas turbine combustion of pulverized woodschemical equilibrium model calculations. Proceeding of ASME TURBO ASIA’96, Djakarta, 1996; in press. (23) Nordin, A.; Schager, P.; Hall, B. Mercury speciation in flue gases; A comparison of results from equilibrium calculations with results from laboratory experiments. Proceedings of the FinnishSwedish Flame Days 1990, Sept 4-5 Åbo, Finland, 1990; A° bo Ahademi: A° bo, Finland, 1990. (24) Eriksson, G.; Hack, K. Met. Trans. 1990, 21B, 1013-1023. (25) SGTE thermochemical database 1994. Scientific Group Thermodata Europe. (26) Joint Army and Navy Air Forces (JANAF). JANAF Thermochemical Tables; National Institute of Standards and Technology, Standard reference database 13; 1985. (27) Barin, I. Thermochemical data of pure substances; VCH Verlagsgesellschaft; Weinheim, 1989; Vols I + II. (28) HSC Chemistry for Windows. Chemical reaction and equilibrium software with extensive Thermochemical database; Outokumpu Research: Finland, 1994. (29) Ebbinghaus, B. B. Combust. Flame 1993, 93, 119-137. (30) Nordin, A.; O ¨ hman, M. Sulphur capture by co-combustion with biomass fuels-gathered experiences of process optimization and emission minimization. Proceedings of 3rd Nordic Conference on SOx and NOx from Heat and Power Generation, Lyngby, March 13-14, 1996; Technical University of Denmark: Lyngby, 1996. (31) NUTEK-Report Termisk Kadmiumrening av tra¨dbra¨nsleaskor. Report 1996:30, 1996.
Received for review October 7, 1996. Revised manuscript received April 1, 1997. Accepted April 14, 1997.X ES960856X X
Abstract published in Advance ACS Abstracts, June 15, 1997.
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