Numerical Simulation of NO Formation in Methane, Methanol and Methyl Formate in a Homogeneous System

Abstract

In this study, methane/air, methanol/air, and methyl formate/air stoichiometric mixtures have been numerically simulated at constant volume, low pressure of 2.7 atm, and temperature ranging from 1000 K to 1950 K with an aim to establish the impact of fuel oxygenation on NO formation. These conditions represent those behind a reflected shock in a shock tube, which is modeled as adiabatic homogeneous mixture with constant internal energy and constant volume. Various chemical kinetic mechanisms have been employed and extensively tested so as to ensure validity of the results. A comparison of NO profiles and other radicals- CH, HCN, N, and N2- that are dominant in its formation have been done. Since the initial temperatures are high, the flame temperatures attained by all the mixtures are also high; from approximately 2800 to 3100 K for initial temperatures of 1000 and 1950 K respectively. Therefore, NO are formed mostly through thermal NO mechanism with prompt NO being less significant. It has been observed that at very high temperatures the difference in N and NO formation in the three fuels is not very significant (same order of magnitude) as compared to that observed in relatively low temperatures attained by freely propagating and diffusion flames. At high temperatures the major rate-limiting steps for NO formation, involving high activation energy are N2 + O =NO + N (318.4 KJ/mol), CH2 + N2 =HCN + NH (309.69 KJ/mol) and N2 + C = CN + N (187.90 KJ/mol).