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) oil 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 NO, 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 NO, 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 NO., 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 NO, emission. The quantitative differences in NO, 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 NO, emission index in the triple flame regime. (C) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.