Experimental Investigation on NO<em>_x</em> Reduction by Primary Measures in Biomass Combustion: Straw, Peat, Sewage Sludge, Forest Residues and Wood Pellets

An experimental investigation was carried out to study the NO<em>_x</em> formation and reduction by primary measures for five types of biomass (straw, peat, sewage sludge, forest residues/Grot, and wood pellets) and their mixtures. To minimize the NO<em>_x</em> level in biomass-fired boilers, combustion experiments were performed in a laboratory scale multifuel fixed grate reactor using staged air combustion. Flue gas was extracted to measure final levels of CO, CO₂, C<em>_x</em>H<em>_y</em>, O₂, NO, NO₂, N₂O, and other species. The fuel gas compositions between the first and second stage were also monitored. The experiments showed good combustion quality with very low concentrations of unburnt species in the flue gas. Under optimum conditions, a NO<em>_x</em> reduction of 50–80% was achieved, where the highest reduction represents the case with the highest fuel-N content. The NO<em>_x</em> emission levels were very sensitive to the primary excess air ratio and an optimum value for primary excess air ratio was seen at about 0.9. Conversion of fuel nitrogen to NO<em>_x</em> showed great dependency on the initial fuel-N content, where the blend with the highest nitrogen content had lowest conversion rate. Between 1–25% of the fuel-N content is converted to NO<em>_x</em> depending on the fuel blend and excess air ratio. Sewage sludge is suggested as a favorable fuel to be blended with straw. It resulted in a higher NO<em>_x</em> reduction and low fuel-N conversion to NO<em>_x</em>. Tops and branches did not show desirable NO<em>_x</em> reduction and made the combustion also more unstable. N₂O emissions were very low, typically below 5 ppm at 11% O₂ in the dry flue gas, except for mixtures with high nitrogen content, where values up to 20 ppm were observed. The presented results are part of a larger study on problematic fuels, also considering ash content and corrosive compounds which have been discussed elsewhere.