Mobility of Elements in Ashes from a Wood-Fired Heat and Power Plant with a Grate-Fired Furnace

The established practice of wood ash disposal in landfills removes valuable elements, such as metals and plant nutrients, from the utilization cycle. In order to use the residual material wood ash as a secondary raw material and to conserve important natural resources (landfill space, mineral, and metallic raw materials), a treatment process must be developed. As a basis for such a process, fundamental knowledge of element solubility is required. Therefore, a sequential extraction process describing the mobility of the ash-forming elements was carried out for three ash fractions from a wood-fired heat and power plant. This work describes the extraction of 24 elements from ash by four sequentially applied extractants. As an aqueous solvent, bidest. water was used, acetic acid was used as the acidic solvent, hydroxylamine hydrochloride was used as the reducing solvent, and ammonium acetate with hydrogen peroxide was used as the oxidizing solvent. Element concentrations in the individual extractants were determined by ICP–OES. We found that the extraction is influenced by the ash fraction, the particle size, and the element-specific behavior during ash formation. Extractability is higher from filter and cyclone ash fractions compared to grate ash as well as from smaller particle size fractions within grate ash compared to coarse grate ash particles. The majority of the metals were acid-soluble. In parameter studies, we found that extractability can be increased by using stronger solvents, grinding the ash, and a longer extraction time. The results provide information on (I) the environmental mobility of the ash-forming elements and (II) suitable solvents and process parameters for the processing of ashes, with the aim of a consistent recycling of valuable substances and nutrients.