Abstract: The combustion residues of Xinwen coal water slurry (CWS) and black liquor coal slurry (CS) were studied. The typical combustion ashes sampled from different locations such as furnace wall, silicon carbide rod, heat flux probe et al. were analyzed in detail, including the chemical composition, mineral component, element distribution and microstructure. The results show that the content of compound and mineral phase comprising Al, Fe, Ca in the ash from combustion of CWS is higher than that of CS, while the case of Na, K, S and Cl is opposite. The enrichment of Fe in the ash is the reason of fouling and slagging during combustion of CWS, but Na and Fe are the essential mineral elements for the ash deposition of CS. Na plays more important role than Fe in the forming and developing of ash deposit in combustion of CS. The ash samples of CS are composed of many sodium rich and fusing temperature low minerals, such as nepheline, thenardite and hauyne. Hence, Na is the key element giving rise to severe fouling and slagging when black liquor coal slurry is fired in furnace, which is confirmed by microscopic analysis and practical burning test. The ash deposition trend for CS is much stronger than that for CWS, and their deposition mechanisms are different because of the existence of Na2SO4 and other S, Cl elements in the black liquor.