Mechanisms of NO formation in MILD combustion of CH4/H2 fuel blends

The mechanisms of formation and destruction of NO in MILD combustion of CH4/H2 fuels blends are investigated both experimentally and numerically. Experiments are carried out at a lab-scale furnace with the mass fraction of hydrogen in fuel ranging from 0% to 15%; furnace temperature, extracted heat and exhaust NOx emissions are measured. Detailed chemical kinetics calculations utilizing computational fluid dynamics (CFD) and well-stirred reactor (WSR) are performed to better analyze and isolate the different mechanisms. When the MILD combustion of the CH4/H2 fuel is established in experiments, the thermal field is quasi uniform and the high temperature zone is located at the junction of the fuel and air jets. As the mass fraction of hydrogen in fuel is increased from 5.7% to 14.4%, although the furnace average temperature is increased, the NOx emission remains unchanged. This cannot be explained by the thermal NO mechanism. CFD and WSR simulations both suggest that, when equivalence ratio ≤0.8, the N2O-intermediate route controls the NO formation and the NO-reburning reaction is also strong. With the hydrogen addition, the importance of the NNH route is increased but that of the prompt route is decreased, consequently non-affecting the NOx emission as measured. Chemical kinetics calculations indicate that the conversion from NO to NOx becomes significant and thus the relative importance of NOx is increased in the total NOx emission under low temperature MILD conditions. As the reactor temperature is increased from 1100 K to 1600 K, the importance of N2O route decreases while that of thermal route increases. In contrast, as the initial mass fraction of oxygen is increased from 3% to 9%, the importance of N2O route increases but that of the prompt and NNH routes decreases. Likewise, as the equivalence ratio increases, the NO-reburning reaction becomes strong. Worth noting is that the N2O-intermediate route controls the NO production under fuel lean conditions whereas the prompt route is dominant in rich ones.SCOPUS: ar.j