In this paper, fly ash and bottom ash produced from the gasification of pine sawdust in a fixed bed gasifier were compared in details to investigate the ash forming characteristics and fouling/slagging behaviours. For this purpose, a detailed comparison of fly ash and bottom ash was carried out. It was found that compared with the ash composition of raw pine sawdust, gasification ashes showed higher contents of alkali metals (K, Na) and Cl. S. Both the bottom ash and fly ash were strongly crystalline, with calcite as the prevailing phase. The ash fusion temperatures for fly ash were higher than those for bottom ash, while the interval between the deformation temperature and softening temperature for bottom ash (88 degrees C) was larger than that for fly ash (73 degrees C), suggesting the higher fouling/slagging propensity of bottom ash. The Si/(Ca + Mg) ratio for the bottom ash (1.10) was lower than that for fly ash (1.29), further indicating the low melting of bottom ash. From a mineralogical standpoint, the major difference between fly ash and bottom ash was that the former possessed a higher content of potassium-rich salts that facilitated the adhesion of small particles to larger particles during biomass gasification process. (C) 2020 Elsevier Ltd. All rights reserved.