An advanced ash fusion study on the melting behaviour of coal, oil shale and blends under gasification conditions using picture analysis and graphing method

•Ash fusion behaviour was investigated via an image-based characterisation method.•Oil shale ash is an effective refractory additive to delay ash fusion of coal.•Low melting aluminosilicates reduces sintering temperature of ash samples.•Sinter strength of ash pellet increases with dilatometric shrinkage.•Oil shale improves ash chemistry without adversely affecting reaction quality.