Determination of temperature and water-concentration in fuel-rich oxy-fuel methane flames applying TDLAS

Abstract Combustion processes with pure oxygen (oxy-fuel) instead of air as oxidant are attractive for high temperature thermal or thermochemical and gasification processes. The absence of nitrogen in such applications leads to higher temperature and species concentrations, which can stabilize even extremely rich flames. Despite their benefits, there is lack of knowledge concerning the internal structure of rich oxy-fuel flames, which feature reactions with largely diverging chemical time scales, namely, the fast oxidation reactions and the slow endothermic formation of synthesis gas. In order to get a better insight, the scope of this study was to determine axial H2O- and temperature profiles of flat, fuel-rich methane-oxygen flames with equivalence ratios from 2.5 ≤ ϕ ≤ 2.9. A Heat-Flux-burner was used to stabilize quasi-adiabatic one-dimensional flames. The inlet temperature of the gas mixture was kept constant at TP = 300 K and the inlet velocity equal to the laminar burning velocity, which was determined in a preceding experimental study. The in-situ temperature and H2O-concentration measurements were performed using Tunable Diode Laser Absorption Spectroscopy (TDLAS). Laser measurements were carried out with three different diode lasers at center wavelengths λcw = 1344.5 nm, 1392.3 nm and 1853.5 nm, respectively, where multiple absorption peaks of the water molecule were investigated. Additionally, one-dimensional calculations with detailed chemistry were performed using the PREMIX code together with the GRI3.0 and CalTec2.3 mechanisms and compared with the experimental data. The results of the temperature measurements showed temperature peaks in the flame zone and a temperature decrease in the endothermic post flame zone, where synthesis gas is formed. The measured peak temperatures exceed the calculated equilibrium temperatures by approximately 100–400 K indicating super-adiabatic flame temperatures (SAFT). Both reaction mechanisms showed similar trends with respect to the decrease of the temperature in the post flame zone and were in line with the measured temperature. In contrast, the calculated decomposition of water in the post flame zone highly depends on the applied chemistry scheme. Here, the CalTech2.3 mechanism showed excellent performance in comparison to the experimental data for ϕ > 2.5. For ϕ = 2.5 the GRI3.0 performed better.