A combined primary and secondary DeNOx concept to achieve ultra-low NOx emissions in small-scale multi-fuel biomass grate furnaces

High NO _x emissions are still a major problem for biomass furnaces that use low-grade fuels with a high nitrogen content. The aim of this work was therefore to achieve extremely low NO _x emissions with a combined concept of optimised primary and secondary NO _x reduction measures. Experiments were conducted in a 70-kW grate furnace that uses double air staging and urea-based SNCR. In addition, a kinetic simulation applying ideal reactors in conjunction with a detailed nitrogen chemistry mechanism was performed to gain deeper insights into the process. The results showed a synergistic effect for double air staging with a long reduction zone in combination with a novel SNCR injection strategy in the oxidation zone, resulting in an extremely efficient NO _x reduction of up to 94% compared to conventional single air staging. In this optimised configuration, the urea was injected shortly after the last air nozzles, at a point where high CO and OH radical concentrations were still present. This significantly reduced the NH ₃ slip, allowing more urea injection until the NH ₃ legal limit was reached. The kinetic simulation confirmed the experimental trends. Lastly, suggestions are given for the implementation of the presented combined DeNO _x concept in future low-NO _x small-scale furnaces, with which NO _x emissions below 20 mg∙m ⁻³ at 13 vol% O ₂ with minimal NH ₃ slip could be achieved for biogenic residues with an extremely high nitrogen content of above 2 % d.b.