Numerical modelling of biomass grate furnaces

2 BIOS - Bioenergy Systems, Sandgasse 47/13, A-8010 Graz, Austria Abstract Computational Fluid Dynamics (CFD) is increasingly being used for the optimisation of industrial coal furnaces and gas burners. Due to the high complexity of heterogeneous combustion of fixed or moving biomass fuel beds, only few research projects have so far dealt with the introduction of CFD as a cost-efficient tool in the optimisation of biomass grate furnaces. The present work covers the application and evaluation of the commercial CFD code FLUENT™ focusing on its capability of treating the specific problem of modelling biomass grate-furnaces and the necessity of implementing additional sub-models. A case study regarding the optimisation of the geometry of the combustion chambers and secondary air nozzles was performed. The major goal of CFD modelling is a techno-economic optimisation of the furnace (reduction of furnace volume, optimisation of the mixing of flue gas and air, reduction of erosion by fly ash as well as of emissions by primary measures). Simulation results for a pilot plant equipped with a travelling grate furnace (nominal boiler capacity 440 kWth) and for a newly designed plant, also equipped with a travelling grate (nominal boiler capacity 550 kWth), are presented. An empirically derived model was used for the calculation of mass and energy fluxes entering from the fuel bed on the grate into the gas phase, thus forming the boundary conditions for subsequent CFD calculations. For the modelling of turbulent reacting flows in the furnace, the Realizable k-e Turbulence Model, the Discrete Ordinates Radiation Model and a modified Eddy Dissipation Combustion Model with a 3-step global reaction mechanism considering 6 species (CmHn, H2O, H2, CO, CO2, O2) were applied. The parameters of the empirical model describing the release of energy and mass from the fuel bed will be determined by applying a specially developed FT-IR measurement technique that makes hot gas in-situ measurements of CO, CO2, H2O and CH4 in the furnace possible. Plausibility of results achieved by CFD calculations is checked by test runs at the furnace simulated. The results of the CFD calculations revealed a considerable potential for the optimisation of furnace geometry and secondary air nozzles regarding the mixing of fuel and air. Even with isothermal flow calculations the volume of the furnace geometry can be minimised by visualisation of dead zones. Moreover, erosion can be reduced by estimation of erosion rates with a particle tracing procedure. The Eddy Dissipation Model used is reasonably accurate for most industrial flow problems but cannot properly describe strong coupling between turbulence and multi-step chemistry (e.g. NOx calculations). Therefore, an advanced Eddy Dissipation Concept (e.g. for NOx calculations with a post-processor) is being implemented. Taking the results already achieved into consideration it can be stated that CFD calculations generally represent an efficient tool for the techno-economic optimisation of biomass fixed- bed combustion systems.