Numerical Study of Nitrogen Oxides (NOx) Formation in Homogenous System of Methane, Methanol and Methyl Formate at High Pressures

Abstract

—The main aim of this study is to determine the effect of equivalence ratio and pressure on the formation of nitrogen oxides (NOx ) in homogenous ignition of methane/air, methanol/air and methyl formate/air. A constant volume reactor of 200 cm3 , at initial temperature of 1300 K and at pressure, P, ranging between 1-50 atmospheres has been considered. The equivalence ratios of the test mixtures have been varied from 0.7 to 1.3. This represents the lean-to-rich region which is most relevant to the conditions in an internal combustion engine. CH4 , CH3OH and CH3OCHO flames have been modelled with different detailed reactions mechanisms, which have been modified and extended to incorporate high pressure oxidation reactions. Flame structures, minor species and NOx time histories have been plotted for the three fuels under different conditions. The results obtained show that the formation of NOx vary with pressure and equivalence ratios. NO mole fraction profiles and other radicals; N2 , N, O, OH, CH, HCN, and N2O that are dominant in formation of NO have been compared. It is established that in homogenous system, NO formation is high in CH4 at lean and stoichiometric conditions while CH3OCHO has high NO at rich conditions for all pressures. At fuel lean and stoichiometric conditions, as pressure increase from 1 to 50 atm peak NO formed in all the three fuels increases. At fuel rich condition, as pressure increase from 1 to 50 atm peak NO formed in all the three fuels decrease. High concentration of N2 , O, OH and high temperatures observed in all flames indicate that Zel’dovich mechanism is the main NO formation route in a homogenous reactor.