CFD ANALYSIS OF AIR STAGING AND FLUE GAS RECIRCULATION IN BIOMASS GRATE FURNACES

Computational Fluid Dynamics (CFD) is increasingly being used for the optimisation of industrial furnaces. Due to the high complexity of thermal decomposition and combustion processes in biomass fuel beds, only few research work has been performed so far in order to introduce CFD as a cost-efficient tool in the optimisation of biomass grate furnaces. In the present case study the influence of air staging and flue gas recirculation on the flue gas burnout as well as on mixing conditions and temperature distribution was investigated. Simulation results for a Low-NOx biomass furnace with two different grate systems – travelling grate and horizontally moving grate - are presented. The CFD calculations reveal a considerable potential to optimise the mixing of unburned flue gas with air by appropriate air staging and flue gas recirculation. Additionally, the temperature distribution and control in the combustion chambers can be improved in order to prevent slagging, fouling and material stress. The results of the numerical investigations can be taken as guidelines for an improved design and process control of biomass grate furnaces.