Fly ashes from (i) lignite, (ii) bituminous coal, (iii) biomass and a mixture of bituminous coal and biomass were studied for d., unburned C content and particle size distribution. Si02 content in the fly ash was 36.5-43.0% by mass. The alkali content (Na2O + K2O + CaO) were: 28.3 (i), 12.07 (ii), 40.97 (iii) and 22.75%, resp. and strong influenced fly ash resistivity and its removal by electrofiltation.