A comparative experimental and numerical investigation of the heterogeneous and homogeneous combustion characteristics of fuel-rich methane mixtures over rhodium and platinum

Abstract

The heterogeneous and homogeneous combustion of fuel-rich CH4/O2/N2/CO2 mixtures (equivalence ratios φ=1.8–3.5) was investigated experimentally and numerically at 5bar. Experiments were carried out in an optically accessible channel-flow reactor, which was coated with either rhodium or platinum, and involved in situ spatially-resolved Raman measurements of major gas-phase species concentrations for the evaluation of the heterogeneous processes and planar laser induced fluorescence (LIF) of formaldehyde for the assessment of homogeneous combustion. Simulations were performed with an elliptic 2-D code that included detailed heterogeneous and homogeneous chemical reaction mechanisms. The surface reaction mechanism for Rh modestly overpredicted the formation of partial oxidation products (H2/CO) and underpredicted the total oxidation products (H2O/CO2) at φ≥3.0. Rhodium was shown superior to platinum in syngas production and, furthermore, it maintained a good catalytic partial oxidation (CPO) capacity even at the low φ=1.8 where Pt showed minimal H2/CO yields. The higher syngas production in Rh, and in particular of the highly reactive hydrogen, had a drastic impact on the ensuing gas-phase combustion characteristics. While vigorous homogeneous combustion was always established in Rh, it was altogether suppressed in Pt despite the higher attained surface temperatures in Pt. The agreement between LIF-measured and predicted flame anchoring positions and flame lengths in Rh was particularly good. The strong gaseous combustion in Rh had profound implications, as it considerably reduced the length of the oxidation zone in CPO reactors such that the reforming zone could be initiated farther upstream. It was also shown that homogeneous combustion did not affect the reactor thermal management and that it promoted the syngas yields at the reactor outlet.