We report on numerical analysis of the effects of inflow conditions on sub-optimal combustion in a coal-dust utility boiler of 230 MW_e, believed to be the cause of the observed high-temperature corrosion on fireside membrane walls in the furnace diffuser. To this purpose, a precursor simulation of coal-air mixture in a real double-swirl burner in its full complexity was carried out, and the results extrapolated to 24 burner exits for the simulation of combustion in the boiler. The simulations were performed using Ansys Fluent CFD software for solving RANS equations for turbulent flow, combustion of dispersed coal-dust particles, species transport and reactions with the standard chemical kinetics and radiation models. The comparison of combustion results obtained with precursor burner simulations and those with the commonly imposed uniform burner-exit properties revealed considerable differences in the furnace diffuser. Especially notable are different distributions of CO and O₂ concentrations in the burner-exits near field, which are suspected to be the precursors of the membrane wall corrosion. The simulation of the burner reveals also a sub-optimal and incomplete coal - air mixing with a consequent non-uniform and asymmetric particles distribution in the mixture entering the furnace.