Ash deposition is one of the main challenges that needs to be tackled in response to increased percentage of biomass co-firing in pulverized fuel boilers. In this study, a model has been developed to investigate the slagging behavior of meat and bone meal (MBM) at higher co-firing rates in the Maasvlakte boiler operated by E.ON Benelux. The model includes the combustion of solid fuels in a tangentially fired boiler and post-processing of ash deposition on the heat exchange surfaces. The deposition tendency of the impacting ash particles is predicted on the basis of ash viscosity, which is calculated with the Urbain viscosity model. Thermodynamic equilibrium is used to calculate the various fuel ash properties for both oxidizing and reducing conditions. On the basis of the thermal heat input, solid fuel combustion is modeled and evaluated for various co-firing rates which consists of 0%, 12.5%, 25% and 40% of MBM. The calculation results show that the deposition propensity is the highest for a co-firing ratio of 25% MBM. The preferred deposition locations in the boiler calculated by the CFD model are in line with observations in operational practice.