Extraction of nutrients from biomass ashes with varying extraction times

During the incineration of biomass (e.g. wood) the organic material is decomposed whereas the inorganic components remain as ashes. The coarse fraction of the ash remains at the bottom of the furnace (bottom ash) and the fine fraction is collected from the flue gas (fly ash). Both ashes contain valuable plant nutrients like N, P, K, Ca, Mg, S, and other trace elements. Therefore these ashes can be used as a kind of fertilizing agents for soil. The plant availability of the nutritive components (which are absorbed in form of ions) depends on their solubility at the environmental conditions in the soil. In this study bottom ash and fly ash from an Upper Austrian biomass incineration plant has been used to investigate the leaching behaviour of the contained nutritive components. Leaching experiments were performed in a sequential extraction procedure starting with distilled water and followed by salt-solutions (ammonium acetate solutions) at different pH values simulating different environmental soil conditions. Also the length of the leaching period was varied from eight to forty eight hours (namely 8, 16 and 48 hours) in order to optimize the extraction time. All leachates were analyzed by ion chromatography IC. The experiments showed that high portions of plant relevant components were already dissolved in the first leaching step with deionized water, especially K+, Ca2+, Cl-, SO42- and also NO3-. Increasing the leaching time did not significantly influence the results. Therefore shorter extraction procedures can be used for the determination of nutritive components in biomass ashes. Compared to distilled water salt solutions released more Ca2+ and Cl- but less K+ and SO42-. From the performed experiments it can be summarised that the nutritive components contained in biomass ashes are quite soluble and biomass ashes can therefore be used to reduce the necessary amount of synthetic fertilizers.