NOx emission reduction by staged combustion in grate combustion of biomass fuels and fuel mixtures

NOx and N2O emissions have been investigated for different pelletized biomass fuels and fuel mixtures thereof both with and without air staging in a grate fired multi-fuel reactor. The fuels investigated are wood, demolition wood and coffee waste, and selected mixtures of these. The multi-fuel reactor was operated at close to constant operating conditions due to impactor (ELPI) measurements, with a total excess air ratio of about 1.6, and a primary excess air ratio of about 0.8 in the air staging experiments. The reactor set point temperature was held constant at 850 °C. NOx emission levels as a function of air supply mode and fuel nitrogen content are reported, showing a large NOx reduction potential, up to 91% and corresponding to less than 20 ppm NOx at 11% O2 for a fuel containing about 3 wt.% fuel-N, using air staging. The effect on N2O, however, is adverse at the selected set point temperature and optimum primary excess air ratio for NOx reduction. The effect of fuel mixing and fuel nitrogen content on the conversion of fuel nitrogen to NOx is also reported and discussed. Fuel mixing has a positive influence on the NOx emission level, but a negative influence on the overall conversion factor for fuel-N to NOx and N2O.