Four types of pellets and three agro-fuels were chemically characterised and burned in a pellet stove. To assess the influence of the material composing the firebox and the combustion efficiency of distinct biomass heating devices in the composition of the bottom ashes, three of the pellets were also burned in a conventional woodstove and in a fireplace. Ashes were analysed for their C, H and N contents by an elemental analyser, whilst major and trace elements were quantified by inductively coupled plasma atomic-emission spectrometry and inductively coupled plasma mass spectrometry, respectively. The mineralogy of ashes was determined by X-ray powder diffraction. The European standard test was applied to samples to determine the leaching potential of major, minor and trace elements. The contents of major and trace elements in the different types of biomass presented enormous variations, which are reflected in dissimilar mineralogical and chemical compositions of the respective ashes. The leachable potential of several elements of environmental concern present in oxy-anionic form at the alkaline pH of biomass ashes were generally high in all samples. Concentrations of some elements in the leachates were in the range of values with classification of "hazardous materials" by the European legislation in what respects the acceptance of these wastes at landfills. Pellets made up of wood wastes and containing preservatives (chromated copper arsenate and ammoniacal copper arsenate) are of concern. Due to lower combustion efficiencies, the leachable potential for most of the trace elements in ashes from the woodstove, and especially from the fireplace, was lower than that of the pellet stove.
Keywords: Agro-fuels; Ashes; Leaching potential; Major and trace elements; Pellets; Residential combustion.