Release of NOx precursors from biomass fuel beds and application for CFD-based NOx postprocessing with detailed chemistry

In this work the development and test of a combined model for NOx formation in biomass fixed bed furnaces is presented. Pot furnace experiments were performed with the fuels bark, waste wood and fibreboard in order to measure the composition of the flue gas above the fuel bed. Conversion rates from fuel nitrogen to the NOx precursor species NH3, HCN and NO were derived from this experiments and implemented in an empirical model for the combustion of the biomass fuel layer in a fixed bed furnace. The results of this model are used as boundary conditions for subsequent calculations with a newly developed CFD-based NOx postprocessor. This technology allows for spatially resolved calculation of NOx formation with detailed reaction kinetics. Simulations of a pilot-plant were performed in order to validate the combined model and the results showed good qualitative and quantitative agreement with measurements. It is concluded, that the presented combined model of NOx formation allows to investigate NOx reduction in biomass grate furnaces by primary measures in detail and is therefore a powerful tool for the optimisation of furnace designs and process control.