A comprehensive evaluation of slagging and fouling indicators for solid fuel combustion

•Slagging and fouling are two significant challenges associated with solid fuel combustion.•Prediction indices based on theoretical calculation are widely developed and preferred.•Experimental tests and ash observation were employed to evaluate slagging-fouling propensity indices.•New criteria were proposed for better prediction accuracy of slagging and fouling indices.•The developed approach can be adopted for other solid fuels, including biomass.