Fly ash characteristics in co-combustion of wood with coal, oil or peat

Abstract Co-firing of bio fuels with fossil fuels is often applied for environmental, technical and economic reasons. The properties of the solid residues produced from such fuel combinations cannot be predicted based on the properties of the ash from each fuel. In this work, fly ash materials from combustion of wood/oil, wood/coal and wood/peat combinations were characterized. Elemental compositions, mineralogical speciation of major elements as well as leaching properties are reported. The fly ash materials from co-combustion had in many aspects similar properties to those from combustion of pure wood. The differences noted were lower calcium, potassium and chlorine contents and higher contents of aluminium, iron and sulphur when coal or peat was added. The speciation of alkali metals in the ash materials studied here is somewhat different from that of wood ash, which leads to lower salt levels in leachates. The pH of these leachates was not as high as in leachates from pure wood ash because of a lower content of calcium oxide in the ash. Trace metal levels and leaching rates were in the same range as for wood ash materials studied earlier.