In this paper, a model that couples a reduced alkali kinetic mechanism for alkali sulfate formation during biomass combustion with an ash deposition model using computational fluid dynamics (CFD) techniques is presented. Starting with a, detailed gas phase kinetic mechanism for alkali chemistry, a systematic reduction procedure has been performed using sensitivity analysis to reduce the reaction mechanism to a level that can be implemented into a CFD, calculation. This reduced mechanism has been validated against the full reaction mechanism under the temperature conditions' that typically occur in biomass combustion furnaces. Both mechanisms are compared to the experimental data obtained using a plug flow reactor reported in the literature. An ash deposition model taking into considerations the ash sticking probability and the condensation of potassium salts has been developed. The reduced mechanism and the deposition model developed are implemented into a CFD model to predict ash depositions in a 10 MWth biomass grate furnace. The model has adequately reproduced the deposit position and shape as observed in the test furnace.