in order to show the influence of co-firing biomass with bituminous coal on ash properties, calculations of fusibility correlations have been carried out. Two Upper Silesian coals (with lower-LS and higher-HS slagging inclination) were chosen for emphasizing the influence of basic fuel. Four kinds of biomass were considered: straw, wood, dried sewage sludge and bone meal. Chemical constitutions of the mineral matter as well as the results of calculations are presented in Tables 2-5. The general conclusion is that co-firing biomass increases the fireside slagging hazard. The most difficult additional fuels are sludge and bone meal. (c) 2004 Elsevier Ltd. All rights reserved.