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Energy & Fuels 2007, 21, 710-717
Slagging Characteristics during Combustion of Cereal Grains Rich in Phosphorus Erica Lindstro¨m,*,† Malin Sandstro¨m,† Dan Bostro¨m,† and Marcus O ¨ hman†,‡ Energy Technology and Thermal Process Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden, and Energy Technology Centre in Piteå, SE-941 28 Piteå, Sweden ReceiVed August 24, 2006. ReVised Manuscript ReceiVed NoVember 13, 2006
A residential cereal burner (20 kW) was used to study the slagging characteristics of cereal grains with and without lime addition. The deposited bottom ash and slag were analyzed using X-ray diffraction (XRD), to identify the crystalline phases, and environmental scanning electron microscopy, coupled with energy-dispersive X-ray spectroscopy (ESEM/EDS), to study the morphology and elemental composition. Phase-diagram information was utilized to extract qualitative information about the behavior of cereal grain ashes under combustion conditions. Chemical equilibrium model calculations were used to interpret the experimental results. In addition, investigations of the melting behavior of the produced slags were conducted. The results showed significant differences in slagging characteristics between the fuels that were used. The slags consisted of high-temperature melting crystalline phases (calcium/magnesium potassium phosphates) and a potassium-rich phosphate melt for all cereal grains. For oat and barley, cristobalite was also identified in the slag. Furthermore, in these cases, the slags most probably contained a potassium-rich silica melt. The differences in the melting behaviors of the slags had a considerable effect on the performance of the burner. The addition of lime reduced the formation of slag for barley and totally eliminated it for rye and wheat. This occurs because lime contributes to the formation of high-temperature melting calcium potassium phosphates.
Introduction Biomass and biomass-derived waste materials are considered to be sustainable energy sources that have significant potential for replacing fossil fuels. Because of the enhanced concern of global warming and other environmental, economical, and political regulations, energy-producing companies in Sweden have been using biomass and waste materials to an increasing greater extent, rather than fossil fuels, for heat and power production. As a consequence, many new “opportunity fuels” with higher ash content and more-problematic ash-forming elements have become interesting for biofuel producers, retailers, and users. In addition to the traditionally studied major ashforming elements (calcium, potassium, magnesium, silicon, chlorine, sulfur), many of these biomass fuels contain significant amounts of phosphorus. A growing interest has been observed among farmers in Scandinavia in regard to the use of cereal grains (oats, barley, wheat, and rye) for small- and medium-scale heating.1 Studies have been performed to determine the fuel quality of whole grains for combustion purposes.1,2 Results from these investigations showed that the fuel ash from oats had the highest ashmelting temperature, whereas that from wheat had the lowest * Author to whom correspondence should be addressed. Phone: +46 90 786 59 71. Fax: +46 90 786 91 95. E-mail: erica.lindstrom@ chem.umu.se. † Energy Technology and Thermal Process Chemistry, Umeå University. ‡ Energy Technology Centre in Piteå. (1) Hadders, G.; Arshadi, M.; Nilsson, C.; Burvall, J. On the Fuel Quality of Cereal GrainssImpact of Soil Composition, Cereal Species and Variety (in Swed.); JTI-rapport, Lantbruk & Industri, No. 289; JTIsInstitutet fo¨r jordbruks- och miljo¨teknik: Uppsala, Sweden, 2001. (ISSN: 1401-4963.) (2) Ro¨nnba¨ck, M.; Persson, H.; Segerdahl, K. Burners for Firing of GrainsFunction, Safety and Emissions (in Swed.); SP-rapport 2006:18; SP Sveriges Provnings- och Forskningsinstitut: Borås, Sweden, 2005. (ISBN: 91-85533-03-3.)
ash-melting temperature (
