The effect of fuel composition on the fate of chlorine, sulfur, and potassium was investigated during an extensive measurement campaign in a 107 MWth bubbling fluidized bed (BFB) combustor. Bark, sludge, and solid recovered fuel (SRF) were co-combusted in different proportions during the campaign. The elemental composition of the fuel and outgoing ashes was determined, supplemented with gas composition measurements, to obtain the distribution of chlorine, sulfur, and potassium. Additionally, chemical fractionation was carried out for the pure fuels to study the leachability of the ash-forming elements. When firing bark and bark + sludge, potassium, sulfur, and chlorine ended up mainly in the fly ash stream. When SRF was a part of the fuel mixture a considerable amount of SO2 was measured in the second pass. Most of the chlorine entering with the fuel was found as gaseous HCl in the second pass, which indicates that sulfation reactions took place in the furnace. Most of the HCl and a part of the SO2 were captured in the baghouse filter ash and the emissions of these gases were low. This work showed the positive effects of co-firing challenging fuels. (C) 2011 Elsevier B.V. All rights reserved.