The thesis contains an experimental study of the fusion and sintering of ashes collected during straw and coal/straw co-firing.A laboratory technique for quantitative determination of ash fusion has been developed based on Simultaneous Thermal Analysis (STA). By means of this method the fraction of melt in the investigated ashes has been determined as a function of temperature. Ash fusion results have been correlated to the chemical and mineralogical composition of the ashes, to results from a standard ash fusion test and to results from sintering experiments. Furthermore, the ash fusion results have been employed in a simple model for prediction of ash deposit formation, the results of which have been compared to ash deposition formation rates measured at the respective boilers.The ash fusion results were found to directly reflect the ash compositional data:a) Fly ashes and deposits from straw combustion are characterised by a large fraction of KCl and a smaller fraction of K-, Ca-, Al-silicates and quartz. The salt part of these ashes melt in the temperature range from 600-750°C, whereas the silicate part predominantly melts between 1000 and 1200°C. Increasing salt (KCl) content in the ashes lead to increased melt fractions in the temperature range 600-750°C.b) Bottom ashes from straw combustion consist purely of silicates, with varying ratios of the quite refractory Al-silicates and quartz to the less refractory K- and Ca-silicates. Bottom ashes melt in the temperature range 800-1300°C, and a trend of higher fusion temperatures with increasing contents of Al-silicates and quartz was found.c) Fly ashes, bottom ashes and deposits from coal/straw co-firing were all found to consist mainly of metal-alumina and alumina-silicates. These ashes all melt in the temperature range 1000-1400°C.Comparison of the obtained ash fusion results to results from the commonly used standard ash fusion test revealed that at the Initial Deformation Temperature, IDT, substantive melt formation had occurred. The difference between the melting onset, as determined by the STA, and the IDT varied typically between -300 - -50°C for salt rich ashes and between -150 - +40°C for silicate-rich ashes. Melt fractions present at the IDT varied between 0 - 35% melt for silicate rich ashes and between 10 - 70% melt for salt rich ashes. This emphasises that the IDT does not denote the onset of ash melting, the biggest deviations being found for salt rich (i.e. straw derived) ashes.A simple model assuming proportionality between fly ash fusion and deposit formation was found to be capable of ranking deposition rates for the different straw derived fly ashes, whereas for the fly ashes from coal/straw co-firing, the model only had a qualitative agreement with the measured ash deposit formation rates.Sintering measurements were carried out by means of compression strength testing of ash pellets. This method showed to not be applicable for the salt rich fly ash derived from straw combustion. For the fly ashes obtained during coal/straw co-firing, substantive sintering strength was observed to build up in the ashes below the melting onset. The strength obtained was thus assumed to be due to viscous flow sintering, and the sintering onset was for the four ashes investigated simultaneous to a calculated ash viscosity of 1-3 · 106 Pa·s.In conclusion, the employed laboratory technique to measure ash fusion gives a more detailed and precise melt quantification compared to conventionally used techniques, and since ash fusion is important for ash deposition propensities, the employed method is judged to bring valuable new information. The measurement principle is not especially designed for studying ashes from combustion, so the method is as well applicable for other types of materials, for which a detailed characterisation of the melting behaviour is relevant.