Computational investigation of NOx emission in biogas combustion.

Abstract

Landfills and anaerobic digesters in the waste treatment processes generate biogas. Biogas can be used as a fuel and excess biogas is typically burned in a flare to reduce the greenhouse effect. However, burning biogas produces several pollutants, including CO2, NOx, and SO2. To minimize these emissions, the amount of excess air used in the combustion process needs to be considered, which has a significant impact on NOx emissions. This study developed a Computational Fluid Dynamics (CFD) model to simulate a small-scale biogas combustion system and analyses the effect of excess air on heat output and NOx emissions during biogas combustion. The GRI-Mech reaction mechanism was used to simulate reactions, and the model was validated by comparing it to experimental data from the DLR-Stuttgart CH4/H2/N2 Jet Flame. To reduce computational costs, a Tabulation of Dynamic Adaptive Chemistry (TDAC) algorithm was used to dynamically adapt the reaction mechanism in real time. Turbulence in the DLR flame was simulated using Reynolds-Averaged Navier-Stokes (RANS). The CFD model used a co-flow of a natural draft to provide additional air, while the air was premixed with fuel. The CFD model was used to simulate various premixed equivalent ratios, and the resulting emissions and heat outputs were compared. The study found that the optimal premixed equivalent ratio for the studied system was between 0.85 and 1.1, as this range produced the highest temperature and lowest NOx emissions. This model facilitates emission analysis of gas-phase combustion systems.