VOLUME 18, NUMBER 4
JULY/AUGUST 2004
© Copyright 2004 American Chemical Society
Articles Critical Parameters for Particle Emissions in Small-Scale Fixed-Bed Combustion of Wood Pellets Henrik Wiinikka* and Rikard Gebart Energy Technology Centre (ETC), Box 726, S-941 28, Piteå, Sweden Received October 23, 2003. Revised Manuscript Received April 2, 2004
In this study, laboratory experiments in a small-scale (10 kW) reactor have been performed to investigate the particle formation mechanisms and the influence of different operating parameters on the particle emissions from combustion of wood pellets under fixed-bed conditions. The results presented herein show that the particles from fixed-bed combustion are formed from three different mechanisms: coarse fly ash particles (>10 µm) are released by mechanical ejection from the fuel bed, submicrometer-sized fly ash particles are produced from the vaporization and nucleation of ash minerals, and, finally, submicrometer-sized soot particles are produced from incomplete combustion. Significant effects on the particle emissions have been observed for the combustor wall temperature and the flow pattern in the combustion zone. Increasing the combustor wall temperature yields a decrease in the emissions of coarse fly ash and soot particles; however, the emissions of submicrometer-sized fly ash particles increase simultaneously. For example, the emissions of soot are reduced by a factor of ∼5 and the emissions of fly ash are increased by a factor of ∼2 when the wall temperature increases from 400 °C to 950 °C. Increasing the mixing rate in the combustion chamber will also decrease the emissions of soot particles. An important conclusion from this study is that the total emissions of particles can be minimized in fixed-bed combustion of a solid biomass by minimizing the combustion temperature in the burning char particle and maximizing the temperature in the secondary combustion zone.
Introduction Fixed-bed combustion of solid biomass fuels is a wellestablished technique in both large- and small-scale furnaces. However, the combustion of biomass will generate particulate emissions ranging from millimeters to ultrafine nanoparticles.1 The largest particles are removed in the combustion zone as bottom ash or wall deposits or are collected in the post-combustor gas cleaning devices. The smallest particles in the flue gas are so small that they are very difficult to separate in * Author to whom correspondence should be addressed. Telephone: +46-911-232380. Fax: +46-911-232399. E-mail address: henrik@ etcpitea.se. (1) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. J. Air Waste Manage. Assoc. 2000, 50, 1565-1618.
ordinary gas cleaning devices.2 Therefore, they contribute to the ambient air pollution. Contemporary epidemiological studies have shown that fine particles are a health risk to the general public.3,4 It is therefore essential to find ways to minimize particulate formation already in the combustion process. The formation mechanisms of particles from pulverized coal combustion have been studied extensively and (2) McElroy, M. W.; Carr, R. C.; Ensor, D. S.; Markowski, G. R. Science 1982, 215, 13-19. (3) Dockery, D. W.; Pope, C. A.; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M. E.; Ferris, B. G. J.; Speizer, F. E. N. Engl. J. Med. 1993, 329, 1753-1759. (4) Pope, C. A.; Thun, M. J.; Namboodiri, M. M.; Dockery. D. W.; Evans, J. S., Speizer, F. E.; Heath, C. W. J. Am. J. Respir. Crit. Care Med. 1995, 151, 669-674.
10.1021/ef030173k CCC: $27.50 © 2004 American Chemical Society Published on Web 05/07/2004
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are well-understood.5-10 It is therefore of interest to review these results to determine if they can be applied to biomass combustion. Field and laboratory studies have shown that ash from coal combustion is produced from two mechanisms, resulting in a bimodal size distribution of the emitted particles. The larger ash particles (1-10 µm in diameter) are produced by fusion of nonvolatile ash-forming species on the surface of burning char particles. If the combustion is complete, the fine particles (
