NO x emissions in n-heptane/air partially premixed flames

Abstract

NO x emissions in n-heptane/air partially premixed flames (PPFs) in a counter-flow configuration have been investigated. The flame is computed using a detailed mechanism that combines the Held’s mechanism for n-heptane and the Li and Williams’ mechanism for NO x. The combined mechanism contains 54 species and 327 reactions. Based on a detailed analysis, dominant mechanisms responsible for NOx formation and destruction in PPFs are found to be thermal, prompt, and reburn mechanisms. The dominant reactions associated with these mechanisms are also identified. The effects of strain rate (a s) and equivalence ratio (φ) on NO x emissions are characterized for conditions in which the flame contains two spatially separated reaction zones; a rich premixed zone on the fuel side and a non-premixed zone on the air side. For most conditions, except for relatively high level of partial premixing, the NO formation rate in the non-premixed zone is significantly higher than that in the rich premixed zone. Within the rich premixed zone, the contribution of thermal NO to total NO x is higher than that of prompt NO, while in the non-premixed zone, the prompt NO is the major contributor. The behavior is related to the transport of acetylene from the rich premixed to the non-premixed zone, and higher concentrations of CH, O, and OH radicals in the latter zone. A notable result in this context is that the existence of CH does not automatically imply that prompt NO will form. The existence of O and OH is also necessary, in addition to CH, to form prompt NO. The relative contributions of thermal and prompt mechanisms to total NO x are generally insensitive to variations in a s, but show strong sensitivity to variations in φ. There is a NO x destruction region sandwiched between the rich premixed and the non-premixed reaction zones. The NO x destruction occurs mainly through the reburn mechanism. The NO x emission index (EINOx) is computed as a function of φ and a s. These results are qualitatively in accord with previous numerical and experimental results for methane-air PPFs.