Blending coal as feedstock is a potential cost-effective way to reduce the gasifier operation cost. Slag properties of blending coal from an industrial Opposed Multi-Burner (OMB) coal water slurry entrained-flow gasifier was investigated in this paper. Experimental data from an OMB entrained-flow gasifier using a blend of high quality coal with a relatively high ash content coal as feedstock were analyzed. The transformation behaviors of the slag from an industrial gasifier were investigated by viscosity analysis and thermodynamic calculation with assistance of FactSage software and validated by the industrial data. The results show that the slag properties were diversified and differ based on the coal blending ratios. It was discovered that the optimized blending ratio (high quality coal/high ash content coal) was in the range of 0.3-0.5. Most of the mineral matter was transformed into Ca aluminosilicates with relatively high liquidus temperatures around 1500 degrees C. As the proportion of coat with higher ash and SiO2 content increases, the slag shows a trend of transformation to mullite. The liquidus temperature decreased at first and then increased gradually as the ratio was increased, which provided a minimum blending ratio of similar to 0.3, consequently with about 8% reduction of feeding cost. Meanwhile, the viscosity of the slag also increased as the blending ratio of higher ash and SiO2 content coal increased. At this point it should be noted that the loading and operating temperature of the gasifier had to be adjusted as the blending ratio fluctuated in order to maintain proper operation. (C) 2014 Elsevier Ltd. All rights reserved.