Fly ashes from biomass incineration - Analyses of their composition with IC, ICP-OES, SEM-EDX and XRD

Motivation: During the incineration process of biomass (e.g. wood) the organic material is decomposed and ashes remain as inorganic residues. Coarse ash is collected at the bottom of the incinerator, fly ash in the gas cleaning system. The ash fractions contain valuable components for plants like calcium, magnesium, potassium, manganese, zinc, nitrate and phosphate. Ashes from biomass incineration can therefore be valuable as fertilizing agents for soils if the relevant elements are set free by humidity and precipitation and harmful elements (as for example Cd and As) stay in the ashes and are not mobilized. In this way the cycle of nutrients for biomass growth can be closed. In order to evaluate the leachable components of the ashes, representing the plant-available part, ashes can be extracted with distilled water or salt-solutions at different pH-values. Different extraction procedures for soils are described in the literature, but only few of them have been applied to biomass ashes. In this study the mobility of metals and ions important for plant growth, as well as the mobility of harmful elements limited in concentration in the Austrian regulation for the use of biomass ashes on agricultural used lands and on grasslands from 1998 (namely As, Cd, Co, Cr, Cu, Mo, Ni, Pb, Zn) has been investigated. Results: It was found, that high amounts of the plant relevant ions were already extracted in the first extraction step – the highest amounts being found for potassium, calcium, chlorine and sulfate. This result was expected as these components are usually water soluble. The environmental harmful elements which are listed with limited concen-trations in the Austrian regulation for the use of biomass ashes on agricultural used lands and on grasslands (As, Cd, Co, Cr, Cu, Mo, Ni, Pb, Zn), showed to be not very leachable and mobile. Therefore, the take up of these elements by the plants after use from biomass ashes as soil enhancers should be limited. Different extraction times did not have a significant influence on the results of the leached elements concentra-tions. As the change in concentrations in the extraction solutions is mostly below 10% and not significant rise with longer extraction time it can be noted, that shorter extraction times already give good results for the determination of bio-available components. No noticeable differences could be found between the sequential extraction and the single extraction steps. Therefore, the use of a three single step extraction procedure, which can be done in parallel avoids the time con-suming extraction sequence. Discussion and conclusions: Based on the results, in further test the three single step extraction procedure will be used, reducing the neces-sary effort and time of the extraction experiments. In this way, long term extraction tests using extraction times of several weeks can be done in reasonable time. The essential components for plant growth are mostly extracted already in the first extraction steps (water or NaAc as extraction media) and the harmful metals are only released in small concentrations in the three extraction steps. Therefore, the use of biomass ash for agricultural used lands seems to be reasonable. However, further investigations have to be carried out to directly measure the uptake of metals out of the ash by various plants.