Experimental and Numerical Investigation of NOx-Emissions by a Biomass Combustion With Oscillating Second Air

With the Paris Climate Agreement EU aims to reduce the emissions of greenhouse gases by at least 40% by 2030 compared to 1990. In order to achieve this goal the importance of biomass as a solid, lumpy fuel increases steadily. In Germany the substitution of fossil fuels by biomass in heat generation saves 15.7 Mio.t CO2-equivalent yearly. However, the combustion of biomass results in emissions of NOx, fine particles and CO. Since these emissions equally harm humans and environment, their limit values are regularly tightened by the legislature. Hence, in order to minimize the emissions, new and innovative solutions are needed.
One possible primary measure for NOx-emissions reduction is the oscillating supply of the secondary air in a staged combustion, which was applied in this work.
With the help of this technological approach the study targets the reduction of NOx-emissions down to concentrations below the required maximum permissible values according the German emission standard for clean air.
At the Institute for Technical Chemistry (ITC) experiments in a batch-operated reactor have been carried out, in which the combustion behavior of solid fuel discharges as well as the conversion of the formed flue gas under different oxidizing conditions were investigated. ... mehrIf the fuel is burnt sub-stoichiometric in the primary stage, through oscillation of the secondary air supplied to the post-combustion chamber above the fuel bed, the NOx-concentration can be reduced by as much as 40% compared to the non-oscillating operation mode. However, this requires homogeneous conversion of the solid fuel during the primary stage.
Complementary to the experiments 3D-combustion modelling of the fuel transformation and nitrogen oxide formation after the secondary air supply to the post-combustion chamber of the reactor have been carried out at LEAT. For the NOx-formation modelling the Eddy-Dissipation-Concept (EDC) combined with In-Situ-Tabulation (ISAT) and a detailed kinetics were applied. The NOx-formation in the lab-reactor was successfully simulated and the results overlap well with the experiments.