Effect of liquid product coverage on mass transfer and model development for gas-solid reaction in later reaction stage

•In-situ gasification demonstrated the slag coverage on residual carbon.•Molten slag on particle surface increase diffusion resistance of gasifying agent.•Residual carbon was found both on particle surface and inside fine slag from gasifier.•Critical point was found in the differential curve of reaction rate.•Ash content and reactivity of coal char affect the critical carbon conversion.