Energy & Fuels 2006, 20, 993-1000
993
Trace Element Enrichment and Behavior in Wood Pellet Production and Combustion Processes Christoffer Boman,* Marcus O ¨ hman, and Anders Nordin Energy Technology and Thermal Process Chemistry, Umeå UniVersity, SE-901 87 Umea˚ , Sweden ReceiVed NoVember 11, 2005. ReVised Manuscript ReceiVed March 6, 2006
The extensive and well-documented concerns regarding environmental dispersion of toxic trace metals constitute solid motives for a special focus of their fate and forms in fuel treatment and conversion processes. The potential enrichment of trace elements during fuel pellet production processes and behavior during combustion was, therefore, studied in a combined field sampling and chemical equilibrium modeling work. Raw materials, pellet fuels, and particulate matter in the drying gases in two different pelletizing plants were sampled and analyzed. In addition, chemical equilibrium model calculations were performed with variations in the content of trace elements, moisture, sulfur, and chlorine, at both oxidizing and reducing conditions. A significant enrichment of Zn, Cu, Cd, and Pb was documented when using bark combustion gases for direct drying of the sawdust and was also supported by the chemical equilibrium results. This is presumably caused by the volatilization of these elements from the bark fuel during combustion, subsequently forming fine particles in the flue gases and being captured by the sawdust during drying. The magnitude and importance for these trace elements were, however, found to be relatively small, regarding concentrations in different fuels as well as potential increased emissions to air during combustion compared to national total emission estimations. In addition, some alternative measures for prevention of trace metal contamination during fuel pellet production were suggested, including fuel quality aspects, high-temperature particle separation, and indirect drying processes.
Introduction An increasing interest for utilization of biomass as a CO2neutral and sustainable alternative to fossil fuels for production of heat, power, and liquid fuels can today be seen globally. There is further a growing market for upgraded biomass fuels, in particular, the use of wood pellets in small- and medium-sized devices in the Nordic countries as well as in Middle Europe and North America.1,2 Fuel pellets are a dried, homogenized, and densified biomass fuel with several advantages during handling, storage, and combustion compared to unprocessed biomass fuels. Today, the raw material for fuel pellets is mainly stem-wood assortments such as sawdust, planer shavings, or dried chips from sawmills and the wood-working industry, while bark, agricultural residues, and other forest fuels only occasionally occur.3 The development of systems for production and use of biomass fuel pellets has been extensive during the past decade, mainly in Europe (e.g., Sweden, Austria, and Germany) and North America,1,2 with a significant further potential for conversion from heating by oil, wood logs, or electricity to fuel pellets within the residential sector. For example, the production of wood fuel pellets in Sweden has increased from 10 000 tons in 1990 to over 1 million tons (∼5 TWha) in 2005, of which ∼25% presently are used in the residential sector.4 During thermal processing (i.e., combustion), the noncombustible fractions of the biomass fuels will produce a solid * Corresponding author. E-mail:
[email protected]. (1) Fiedler, F. Renewable Sustainable Energy ReV. 2004, 8, 201-221. (2) Vinterba¨ck, J. Wood pellet use in SwedensA systems approach to the residential market. Ph.D. Dissertation. Swedish University of Agricultural Sciences, Uppsala, Sweden, 2000. (3) Hillring, B.; Vinterba¨ck, J. For. Prod. J. 1998, 48, 67-72. (4) Swedish Association of Pellet Producers (PiR). Official statistics 2005. http://www.pelletsindustrin.org, February 2005.
residue of ash. The major part of biomass ashes consists of nutrients such as Ca, K, Mg, Na, and P with minor and varying amounts of trace elements such as Zn, Cu, Cd, Pb, and Cr.5,6 An increasing amount of work concerning the chemical and physical processes during biomass combustion that transform such inorganic elements to be distributed between bottom ash, coarse (>1 µm) fly ash particles, fine (
