Benchmarking of primary measures to achieve lowest NOx emissions in small-scale biomass grate furnaces

Despite recent progress, NO x is still a major problem in biomass combustion, especially when using fuels with high nitrogen contents. The aim of this work was to determine the minimum NO x emissions achievable by applying primary measures in small-scale biomass grate furnaces. For this purpose, a new benchmark for furnaces using double air staging was defined on the basis of three different plants. An experimental study with different biomass fuels was conducted in two of these plants to systematically improve this benchmark. The results show that the new NO x benchmark using double air staging is up to 39 % lower than values achieved with conventional single air staging. Furthermore, this new benchmark was improved by extending the reduction zone, additionally reducing the NO x by up to 30 %. Secondary flue gas recirculation and partial load further reduced the NO x emissions. By combining the primary measures investigated in the 70-kW furnace, NO x emissions were achieved that were up to 70 % lower than those from other small-scale furnaces described in the literature. This efficiency also applies to fuels with a high nitrogen content, so that secondary NO x reduction measures can be avoided in most cases in furnaces in the future by taking this approach. • A new benchmark for NO x emissions in double air staged biomass furnaces was set. • Double air staging led to a major reduction in NO x compared to single air staging. • Long residence times in the reduction zone improved the NO x reduction efficiency. • NO x emissions below 100 mg ⸱ m −3 at 13 vol% O 2 were reached using straw pellets. • A novel furnace achieved up to 70 % lower NO x emissions than commercial boilers.