In this study, the influence of various factors on nitrogen oxides (NO_x) emissions of a low NO_x burner is investigated using a central composite design (CCD) approach to an experimental matrix in order to show the applicability of design of experiments methodology to the combustion field. Four factors have been analyzed in terms of their impact on NO_x formation: hydrogen fraction in the fuel (0%–15% mass fraction in hydrogen-enriched methane), amount of excess air (5%–30%), burner head position (20–25 mm from the burner throat) and secondary fuel fraction provided to the burner (0%–6%). The measurements were performed at a constant thermal load equal to 25 kW (calculated based on lower heating value). Response surface methodology and CCD were used to develop a second-degree polynomial regression model of the burner NO_x emissions. The significance of the tested factors over their respective ranges has been evaluated using the analysis of variance and by the consideration of the coefficients of the model equation. Results show that hydrogen addition to methane leads to increased NO_x emissions in comparison to emissions from pure methane combustion. Hydrogen content in a fuel is the strongest factor affecting NO_x emissions among all the factors tested. Lower NO_x formation because of increased excess air was observed when the burner was fuelled by pure methane, but this effect diminished for hydrogen-rich fuel mixtures. NO_x emissions were slightly reduced when the burner head was shifted closer to the burner outer tube, whereas a secondary fuel stream provided to the burner was found to have no impact on NO_x emissions over the investigated range of factors.

An experimental investigation was carried out to study the NO_x 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_x 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_xH_y, 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_x reduction of 50–80% was achieved, where the highest reduction represents the case with the highest fuel-N content. The NO_x 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_x 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_x 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_x reduction and low fuel-N conversion to NO_x. Tops and branches did not show desirable NO_x 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.