During the combustion of coal and biomass blends, ash particles are deposited on the furnace wall, or on the heat transfer surfaces, and gradually they become sintered and harder. This makes it difficult to remove the deposits by soot blowing, and therefore manual cleaning is often required. In this paper, a computational fluid dynamics (CFD) model that predicts the deposition rates in boilers has been developed. This deposition model is based on the combined sticking probabilities of the ash particle viscosity and the melting behaviour of the ash particles. A numerical slagging index (NSI) is also employed to estimate the degree of the sintering of the deposits. The NSI is based on the ash viscosity, ash fusibility, ash chemistry and ash loading and the experimental data from the entrained flow reactor (EFR) at Imperial College, London, have been used to validate the models. We have found that the predicted results from the ash deposition model on the coal fired with short rotation crop (SRC) and miscanthus are reasonably consistent with the experimental measurements. Further, the NSI has been used to rank the investigated coal-biomass mixtures according to their degree of sintering.