Effect of multistage combustion on NOx emissions in methane–air flames

Coflow and counterflow methane–air flames are simulated over a complete partially premixed regime in order to characterize the effects of dominant combustion modes (i.e., single-, two-, and three-stage combustion) on NOx emissions. Simulations employ a comprehensive numerical model that uses detailed descriptions of transport and chemistry (GRI-2.11 mechanism) and includes radiation effects. It is demonstrated that a complete partially premixed regime, which extends from premixed flames to triple flames and then to double flames, can be simulated by suitably varying the equivalence ratios in the fuel-rich and fuel-lean streams, while maintaining the global equivalence ratio fixed. Both counterflow and coflow simulations show that NOx emissions decrease significantly from the premixed to the triple flame regime, and then increase from the triple to the double flame regime. Therefore, triple flames not only extend the rich and the lean flammability limits, but also exhibit superior NOx characteristics compared to the corresponding premixed flames and double flames, with thermal, prompt, NNH-intermediate, and N2O-intermediate routes being the important contributors (in descending order) to NOx formation. Coflow and counterflow flames exhibit qualitatively similar NOx characteristics in the entire partially premixed regime and an optimum level of partial premixing that yields the lowest NOx emission. The quantitative differences in NOx emissions between the two configurations can be attributed to geometry-dependent effects. In particular, compared to counterflow flames, the coflow flames have significantly larger flame volume and therefore lower peak temperature and NOx emission index in the triple flame regime.