Composition of inorganic elements and the leaching behavior of biomass combustion ashes discharged from wood pellet boilers in Japan

To evaluate the characteristics of wood ash as fertilizer, composition of inorganic elements and the leaching behavior were studied. The leaching behavior was evaluated by three leaching methods using different solvents (water, hydrochloric acid, and acetic acid: Japanese leaching test Nos. 18 and 19 and modified toxicity characteristic leaching procedure prescribed by the US Environmental Protection Agency). The ash composition varied according to raw materials for fuel and ash collection systems. Concentration of Na, Al, Si, and V became higher in bark ashes while that of K became higher in wood ashes. Pb, Cd, Se, and Zn were more volatile and enriched in fly ashes. Of the nutrient elements, K showed high water solubility and its phytoavailability also appeared high, although the amount of water-soluble K was low in bark ash. Ca and Mg had intermediate solubility while P was less soluble in both water and acetic acid. The amount of hazardous substances having leached was low for most, but not all the ashes, in terms of Pb, Cr and As. Nevertheless, the phytoavailability of these hazardous substances after their application to the forest appeared low, due to their amphoteric leaching behavior and the acidity of Japanese forest soils.