The continuously growing demand for fuels in the transportation sector has led to the increased use of liquid biofuels, with a corresponding increase in the amounts of production residues. With energy recovery, these residues could become a source of biomass fuels. The current work investigated the behaviour of four potential biomass fuels during combustion. Fermented sewage sludge (FSS) is the residue from biogas production via anaerobic fermentation; distillers' dried grains with solubles (DDGS) are a residue from barley bioethanol production; and rapeseed cake (RC) and palm kernel cake (PKC) are both oil extraction residues from first generation biodiesel production plants. Since the main challenges during biomass combustion are related to ash behaviour, this paper focuses on the ash-forming matter in the fuels and its effect on the ash sintering tendency. Three different types of tests were applied to study ash sintering: compression strength, microsample sintering, and standard ash sintering. The latter was carried out using hot stage microscopy and computer aided image analysis, and the results were compared with those from the compression strength and microsample tests. To obtain additional information about the nature and extent of the sintering, ash samples before and after sintering were examined with a scanning electron microscope equipped with energy dispersive X-ray (SEM/EDX) technology and a stereomicroscope incorporating a digital camera. To make it possible to study the ash-forming matter, chemical fractionation were performed accompanied by fuel proximate analyses by means of thermogravimetric analyzer. The results clearly showed that the lowest initial sintering temperatures were below 800 degrees C for RC and DDGS in all the sintering tests, while PKC and FSS did not exhibit any significant sintering below 900 degrees C. This could be interpreted to mean that when the combustion temperature is below 900 degrees C. no significant ash-related problems will occur. The chemistry of the fuel ashes investigated is dominated by phosphorus, and the shift between alkali metal and alkaline earth metal phosphates governs the sintering behaviour. The FSS sintering tendency is dominated by the iron oxide chemistry. (C) 2011 Elsevier B.V. All rights reserved.