To investigate the cause of the increase in the unburned fraction when sub-bituminous coal is mixed with bituminous coal observed in previous experimental research using a 100 kg-coal/h-class coal combustion test furnace, numerical simulations of the mixed combustion of sub-bituminous coal and bituminous coal are performed. To take into account the effect of the particle heating rate on the devolatilization parameters, the tabulated-devolatilization-process model (TDP model) is employed. The results show that the simulation could qualitatively reproduce the experimental results, which are an increase in flame lift-off with increasing sub-bituminous coal mixing ratio and a maximum value of the unburned fraction at a sub-bituminous coal mixing ratio of 25%. Furthermore, it is clarified from the simulation results that the nonlinear increase in the overall unburned fraction in the case of sub-bituminous coal mixing with bituminous coal is caused by the steep increase in the unburned fraction of bituminous coal particles with increasing sub-bituminous coal mixing ratio. (C) 2014 Elsevier Ltd. All rights reserved.