The objectives of this study were the evaluation of the effect of MgO addition to coal on the reduction of ash deposition during upgraded brown coal (UBC) combustion, and the elucidation of the mechanisms of the reduction of ash deposition. The melting temperature of UBC ash is 1494 K, which is lower than that of bituminous coal ash. Before the actual ash-deposition experiments, the molten slag fraction in the UBC ash was estimated by means of chemical equilibrium calculations for various mixing mass ratios of MgO to coal ash. The simulation results indicate that MgO addition plays a role in decreasing the molten slag fraction. It was confirmed that Mg formed solid composites with Si, Fe, Al, Ca, and Mn, and played a role in decreasing the molten slag fraction in the ash on the tube. As a next step, ash-deposition tests were conducted using a pilot-scale pulverized coal combustion furnace equipped with a refractory wall. The results showed that MgO addition contributed to decreasing the rate of ash deposition even for UBC. These calculations and experimental results suggested that one of the reduction mechanisms due to MgO addition involved the production of solid-phase aluminosilicates. (C) 2011 Elsevier Ltd. All rights reserved.