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

• Experiments and kinetic simulations on combined NO x reduction measures. • Analysis of NH 3 slip reduction with different CO levels at urea injection point. • Synergy effects of the combined DeNO x concept led to ultra-low NO x emissions. • 17 mg∙m −3 NO x at 13 vol% O 2 achieved for coffee pellets (>2 % N d.b.) • Suggestions for implementation in practical applications. 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.