Iron and aluminium sulphates are added to processes producing sewage sludge (SWS), and SWS also contains aluminium silicates. These compounds can destroy alkali chlorides and prevent Cl deposition on superheaters. Sulphation of chlorides requires formation of SO(3) in the furnace. Raw and digested SWSs were mixed with a fuel containing bark and recycled fuel (REF). Combustion experiments were conducted with a pilot-scale bubbling bed reactor, and concentrations and deposition of critical elements (Cl, Na, K, S...) in the furnace were measured. Change of the iron and aluminium sulphates added to the SWS to phosphates and hydroxides evidently reduced the power of the sulphur to form SO3. Nevertheless, addition of SWS to the biomass feedstock effectively decreased alkali chloride concentration and Cl deposition, and the alkali sulphates found in the fine fly ash confirmed the contribution of sulphation to alkali capture. The importance of aluminium silicate reaction in alkali capture was indicated. (C) 2010 Elsevier Ltd. All rights reserved.