Chemical fractionation and SEM-EDX was used for characterisation of ashes and deposits from different combustion tests in a commercial 20 MW bubbling fluidized bed (BFB) boiler. The fuel combusted was a mix of sorted MSW (Municipal Solid Waste) and industrial waste often referred to as RDF (Refuse Derived Fuel) mostly containing combustible material as paper, plastics and wood. This fuel type often contains a lot of alkali and chlorine and is therefore considered as a risk fuel prone to cause bed agglomeration, deposit formation and corrosion. In order to investigate the impact of the bed temperature on the alkali and chlorine distribution in the boiler combustion tests were performed. The bed temperature for this boiler is designed to be in the range 850-900°C. In this investigation however the bed temperature was reduced to 700-750°C. Two deposit probes, each carrying two rings made of high alloy steel, were used for collection of deposits during combustion. In addition, samples taken on the bed ash, return sand, return shaft ash, cyclone ash and textile filter ash were analysed. By reducing the bed temperature the need for fresh bed sand was reduced and the fly ash flow decreased. In addition, the agglomerates found in the tests with the normal bed temperature disappeared totally when the bed temperature was reduced. The deposits formed on the bed ash and on the return sand particles were found to consist of compounds with melting temperatures between 675 and 801C, which could explain the difference in agglomeration tendency.