A numerical investigation on the formation of NO2 and N2O in laminar counterflow methane/n-heptane dual fuel flames

Abstract

To understand the fundamental mechanisms of nitrogen dioxide (NO2) and nitrous oxide (N2O) formation in natural gas-diesel dual fuel combustion, a numerical study on NO2 and N2O formation in laminar counterflow methane (CH4)/n-heptane (n-C7H16) dual fuel flames is conducted. The results indicate that NO2 accounts for a small part of total NOx formation. The NO2 emission index first increases and then decreases with increasing CH4 addition and increases monotonically with increasing flame strain rate. The NO2 emission indices by different reactions are identified and analyzed. It is revealed that a small amount of CH4 addition increases the NO2 emission index because of increased HO2 generation while a larger amount of CH4 addition decreases the NO2 emission index due to the reduced NO production. Besides, it is concluded that decreasing flame temperature is beneficial to the formation of NO2, which explains the monotonic increase of NO2 emission index with increasing flame strain rate. Little N2O is formed in CH4/n-C7H16 dual fuel flames compared to NO and NO2. The N2O emission index decreases with increasing CH4 addition and flame strain rate. However, the relatively change of N2O emission index is quite small compared to that of NO2 emission index.