NO formation and reduction during methane/hydrogen MILD combustion over a wide range of hydrogen-blending ratios in a well-stirred reactor

Abstract

Hydrogen-blending methane or even hydrogen moderate or intense low-oxygen dilution (MILD) combustion has attracted much interest due to its advantages of low CO2 and NOx emissions. This paper reports a comprehensive parametric study through kinetic modeling with the Glarborg2018 mechanism in a well-stirred reactor to evaluate the formation and reduction of NO during MILD combustion for CH4/H2 co-firing fuels over a wide range of hydrogen-blending ratios from no hydrogen to pure hydrogen. In particular, NO formation via thermal, prompt, NNH, and N2O-intermediate routes as well as NO reduction not only by hydrocarbon radicals but also by non-hydrocarbon radicals are separately investigated. The results show that, at a fixed residence time of 1 s, hydrogen addition cannot change the dominant role of the pathway via N2O under fuel-lean conditions, whereas it can change the major NO formation pathway from prompt to NNH under fuel-rich conditions. In hydrogen MILD combustion, the predominant pathway taken to convert N2 to NO is via N2O when the equivalence ratio is below 0.8, and with increasing equivalence ratio, the NNH pathway eventually prevails over the N2O-intermediate pathway to become the dominant one under reducing conditions. The NNH pathway at lower mixture temperatures and short residence times contributes more to NO production. On the other hand, NO reduction by non-hydrocarbon radicals, which proceeds primarily through the route H + NO → HNO → NH → N2, is effective in high-percentage hydrogen co-firing, while that by hydrocarbon radicals is weakened with hydrogen addition.