Analysis of process parameters for steady operations in methane mild combustion technology

Abstract

The main process parameters affecting combustors of all types are analyzed in the range of interest concerning mild combustion processes for methane oxidation. They are studied by means of direct comparison between experimental measurements made in a Jet Stirred Flow Reactor and numerical predictions based on a kinetic scheme developed for general use. Wide ranges of both inlet temperature (875–1275 K) and oxygen/fuel ratio (C/O from 0.01 to 1.4) as well as narrow ranges of dilution (85–90% of nitrogen content) and residence time (0.35–0.5 s) were covered in relation to the identification of regimes either with multiple operating points (hysteresis), steady–unsteady behavior or stable–unstable evolution. It was assessed that the competition between oxidation and recombination channels is stressed under such conditions. The prevalence of acetylene formation and its stabilization with respect to the oxidation of the recombination products is responsible for exothermicity-damping at temperatures higher than 1175 K in rich conditions. This competition is responsible for temperature oscillation modulation in stoichiometric and lean conditions, even though the prevalence of acetylene formation still inhibits temperature increase at temperatures higher than 1300 K. The main practical conclusion derived from this is that diluted combustion processes cannot be designed without a preliminary, accurate analysis of the auto-ignition process. This has thus far been completely ignored for low mass molecular species of interest in natural gas, such as methane. In that case, relatively high air temperatures of above 1400 K must be reached for stable conditions for all air/fuel ratios, whereas temperatures above 1000 K may be high enough only when the richness is more than twice the stoichiometric value. Decreasing both residence time and dilution level in a narrow range of values is beneficial in suppressing temperature oscillations, according to the analysis in terms of inlet temperature and air/fuel ratio.