Towards a CFD-based mechanistic deposit formation model for straw-fired boilers

This paper discusses the application of FLUENT™ in the analysis of grate-fired biomass boilers. A short description of the concept used to model fuel conversion on the grate and the coupling to the CFD code is offered. The development and implementation of a CFD-based deposition model is presented in the reminder of the paper. The growth of deposits on furnace walls and super heater tubes is treated including the impact on heat transfer rates determined by the CFD code. Based on the commercial CFD code FLUENT™, the overall model is fully implemented through the User Defined Functions. The model is configured entirely through a graphical user interface integrated in the standard FLUENT™ interface. The model considers fine and coarse mode ash deposition and sticking mechanisms for the complete deposit growth, as well as an influence on the local boundary conditions for heat transfer due to thermal resistance changes. The model is applied to the straw-fired Masnedø boiler. Results are in good qualitative agreement with both measurements and observations at the plants.