The paper is focused on the characterization of renewable fuel ashes and their impact on fouling and slagging during combustion. Biomass (A1) and sewage sludge (A2, A3) ashes were investigated for characterization of their corrosive properties. The chemical and phasemineral composition of ashes were studied using a variety of analytical techniques including XRF, ICP-MS, XRD, SEM-EDS, and the ash fusion characteristics by using a thermal microscope. The slagging and fouling indices, as well as thermal conductivity, were calculated and the sintering properties were predicted. The studied ashes had different concentrations of the main chemical compounds and mineral phases. The XRF results showed that CaO, SiO2, K2O, MgO, P2O5 and Al2O3 are the main compounds of A1 and A3 ashes, whereas Na2O and SO3 dominated in A2. Consequently, SEM images showed surface differences between ash particles. Phase analyses indicated the existence of corrosive phases containing potassium and sodium K2Ca (CO3) 2, KAlSi2O6, KCl, NaAlSiO4 (A1) and Na2SO4, NaCl (A2). Such compounds can decrease the sintering temperature due to formation of low temperature eutectics. The lowest fusion temperatures below 1000 degrees C were detected from the sewage sludge ash A2 and above 1200 degrees C from A1 and A2 with high Ca, Si and Al contents. (C) 2016 Elsevier Ltd. All rights reserved.