Methane auto-ignition delay times and oxidation regimes in MILD combustion at atmospheric pressure

MILD/oxy-fuel combustion is proving to be a reliable technology for clean and efficient energy production systems. It is by now established that the related operative conditions drastically change the combustion kinetics and the relative weight of kinetics and fluid-dynamics involved in the process. Thus a fundamental analysis of single sub-processes involved in fuel oxidation under MILD combustion conditions is needed to make easier the tuning and control of real systems in a wider range of working conditions, thus allowing for the spread of such a technology over a broader industrial scale. Such a work is devoted to the characterization of combustion regimes and the evaluation of auto-ignition delay times of methane in highly diluted and pre-heated conditions at atmospheric pressure in a one-dimensional flow reactor. Experimental tests have been carried out changing the mixture inlet temperature from 1100 up to 1400 K, as well as the mixture carbon/oxygen feed ratios from fuel ultra-lean up to rich conditions. Mixtures are diluted in nitrogen from 85% up to 95%. At the same time, numerical simulations have been performed by means of a commercial software and a kinetic mechanism available in literature to value its reliability in predicting experimental results under MILD oxidation conditions. Results clearly show the occurrence of some discrepancies between numerical and experimental auto-ignition delay time data for fuel rich mixtures at high inlet temperatures. In addition, a richness of combustion regimes (slow combustion, pyrolysis, dynamic behavior, transitional combustion, combustion) was experimentally found. These unique behaviors as well as the dependence of ignition process on environmental parameters outline that modest temperature gradients due to high mixture dilution levels typical of MILD oxidation conditions, coupled with heat exchange to the surroundings, slow down the changeover among kinetic routes (namely oxidation and recombination channels) promoted by temperature thus stressing their competition. Such an aspect is generally hidden in traditional systems by a strong heat release that implies high temperature gradients. Finally, the data reported in this paper have an intrinsic value for the identification of stable operative conditions and for either tuning or reducing detailed kinetic schemes because few data are present in literature on methane ignition and oxidation in a simple reactor at atmospheric pressure. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.