Abstract: Aiming at the problem of slag plugging in gasifier caused by Xinjiang high alkali coal as gasification raw material,the slag produced in the actual operation of four-nozzle gasifier was taken as the research object and the physical and chemical characteristics of ash and slag were analyzed by means of scanning electron microscopy,X-ray electron spectroscopy( SEM-EDX),X-ray fluorescence spectroscopy( XRF) and ash melting point tester.In addition,the ash transformation behaviors of Beishan( BS) coal,Muchang( MC) coal and their mixture were examined in a high temperature furnace.The results show that the distribution of minerals in Xinjiang coal are heterogeneous.During the process of thermal transformation at high temperature,a series of complex physical and chemical reactions occur among minerals,which result in the diversity of mineral ash.When Xinjiang coal is used as gasification material,a large amount of ash accumulates at the slag mouth of the gasifier and form layered slagwhich is composed of silicate or alumina silicate eutectic of Na,Ca,Mg and Fe and Na-Al-Si-O.The contents of Na in the different forms of slags after coal gasification are quite different,which leads to different viscoustemperature characteristics and curing temperature of mineral slag.The thermodynamic simulation shows that the initial liquid phase forming temperature of Na-Si-O system is ca.800 ℃,and it varies slightly with the increase of Na_2O.Nevertheless,the liquid phase forming temperature rapidly increases to 1 050 ℃ when a certain amount of Al_2Si_2O_7 is mixed. The minerals melt to form complex eutectics at high temperatures with the decrease of temperature,the minerals in liquid ash with high molten point would form crystal nucleus,and then the crystal nucleus grows rapidly and precipitates out into crystalline particles.In the process of thermal transformation at high temperature,the active Na can react with quartz and clay minerals to form Na Al SiO_4 with low melting point.However,Ca,Mg and Fe react with acid minerals to form silica-aluminate minerals substances with high melting point,which are firstly precipitated from slag during the cooling process,resulting in slag blocking.The coal ash slagging propensity could be prevented by selecting or blending the coals with viscosity and melting temperature approaching to the gasification operation parameter.