In this work chemical and physical characteristics of fine ash emissions generated from the combustion of solid biomasses to those from pulverized coals are compared.  Comparisons show that combustion of biomasses generates higher emissions of submicron ash particles than combustion of coals. The high submicron emissions of biomass are problematic, as conventional particulate control devices have low collection efficiencies for such small particles. Moreover, the chemical composition of submicron particles of biomass typically contain large amounts of alkalis (potassium and sodium), chlorine, sulphur and, often, phosphorous, whereas those collected from combustion of coal contain large amounts of silicon, aluminium, iron and sulphur. The high alkali content of biomass increases the tendency of the deposition on the heat transfer surfaces, which is calculated with the empirical correlations in the literature. These calculations, as well as experiences elsewhere, indicate that the slagging and, particularly, the fouling deposition prospects of most biomasses are significantly higher than those of coals.