Analysis of kinetic models for rich to ultra-rich premixed CH4/air weak flames using a micro flow reactor with a controlled temperature profile

Aromatic formation in methane combustion is still not well understood. The first step to build kinetic models for aromatic formation is to understand fuel-rich oxidation and formation of aromatic precursors like acetylene and ethylene in varied temperature and equivalence ratio conditions. This understanding is also useful to optimize partial oxidation of methane in fuel-rich conditions. With this view, the current work presents investigation of fuel-rich but non-sooting premixed methane/air flames for equivalence ratios between 1.7 and 6.0 using a micro flow reactor with a prescribed temperature gradient from 300 K up to maximum temperature of 1200K. Weak flames which represent the ignition behavior of fuel were studied. Concentrations of major stable species, CH4, CO, CO2, C2H6, C2H4 and C2H2 were measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). Computations with 1-D reactive flow model using existing kinetic models, GRI 3.0, SD 2016 (San Diego mech), KAUST (USC II) and ARAMCO 1.3 were performed. Change in weak flame structure with change in equivalence ratio was studied. KAUST (USC II) predicted the weak flame positions very well at both equivalence ratios of 2.0 and 6.0, but species mole fractions were not so well predicted. It was found that models with higher rates of CH3+O-2 double left right arrow CH2O+OH (SD 2016) and CH3 +HO2 double left right arrow CH3O+OH (GRI 3.0) predicted upstream weak flame positions and thus higher reactivity. However, SD 2016 and GRI 3.0 predicted species mole fractions better, particularly at higher equivalence ratios. All the mechanisms used in the current work underpredicted the mole fractions of C2H2 by factors of four to five at all equivalence ratios addressed here. Reaction path analysis showed significant differences among kinetic models. Particularly, consumption pathways of CH3 vary greatly among kinetic models leading to different reactivity. Flexibility and applicability of existing mechanisms to predict the current fuel-rich to ultra-fuel-rich CH4 combustion was discussed. HO2 radicals, which are generally important at high pressures are found to be significant in current fuel-rich conditions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.