The paper describes an investigation of slagging and Fouling effects when cofiring coal/biomass blends by using a Predictive model for large utility boilers. This model is based on the use a zone computational method to determine the midsection temperature profile throughout a boiler. coupled with a thermo-chemical model, to define and assess the risk of elevated slagging and fouling levels during cofiring of solid fuels. The application of this prediction tool was made for a 618 MW thermal wall-fired pulverized coal boiler, cofired with a typical medium volatile bituminous coal and two substitute fuels, sewage sludge and sawdust. Associated changes in boiler efficiency as well as various heat transfer and thermodynamic parameters of the system were analyzed with slagging and fouling effects for different cofiring ratios. The results of the modeling revealed that, for increased cofiring of sewage sludge, an elevated risk of slagging and high-temperature fouling occurred, in complete contrast to the effects occurring with the utilization of sawdust as a Substitute fuel.