In this paper, we, for the first time report the properties of ash deposits collected from the combustion of lignite mixed with silica additive, in a 30 MWth pulverised-fuel boiler with sub-critical steam conditions. The combustion test was carried out continuously over ten months, and ash deposits were sampled from various locations in the boiler. As has been confirmed, the ash-related slagging and fouling have been controlled effectively. Apart from capturing volatile alkali and alkaline earth metals, silica added in an excess amount is beneficial in mixing with molten liquidus to promote their agglomeration, rather than adhering to the vicinity of coal burners for slagging. Compared to the formation of abundant Fe2+-slag that is prone to stick to coal burner vicinity in the raw coal case, the formation of friable calcium sulfate in fireside deposit is favored upon the mixing of silica with coal, which is even predominant in the fireside ash deposit. The calcium sulfate formed is highly layered in a plerospheric structure in which Ca-silicate is present as the core with calcium sulfate being the shell. The physical adsorption of free Ca on silicate surface is supposed to enhance its specific surface area for a quicker sulfation. The capture of Na by silica is also efficient, leading to the transformation of a remarkable portion of Na into coarse bottom ash with less being sulfated. This in turn alleviates the sulfate - relating fouling propensity on the water tube surface in the convection zone. A noticeable portion of Fe was noticed to remain as discrete grains outside the Ca-rich slag matrix. In contrast, the majority of Mg is highly crystallised as sub-micrometer Ca/Mg-silicate and oxide particles residing within slag matrix, which in turn increase the viscosity and decrease the sticking propensity of the molten slag. (C) 2014 Elsevier Ltd. All rights reserved.