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

NO_x 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. NO_x emission levels as a function of air supply mode and fuel nitrogen content are reported, showing a large NO_x reduction potential, up to 91% and corresponding to less than 20 ppm NO_x 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 NO_x reduction. The effect of fuel mixing and fuel nitrogen content on the conversion of fuel nitrogen to NO_x is also reported and discussed. Fuel mixing has a positive influence on the NO_x emission level, but a negative influence on the overall conversion factor for fuel-N to NO_x and N₂O.