Partial catalytic oxidation of methane to synthesis gas over rhodium: in situ Raman experiments and detailed simulations

The partial catalytic oxidation of methane to synthesis gas over Rh/ZrO₂ was investigated experimentally and numerically at fuel-to-air equivalence ratios of 2.5 and 4.0 and pressures of 4 and 6 bar. Experiments were performed in an optically accessible, laboratory-scale, channel-flow catalytic reactor and involved in situ one-dimensional Raman measurements of major species (CH₄, O₂, H₂O, CO₂, H₂, CO, and N₂) concentrations across the reactor boundary layer. The numerical model included a two-dimensional elliptic code with elementary homogeneous (gaseous) and heterogeneous (catalytic) chemical reaction schemes. Homogeneous ignition experiments and numerical predictions have validated the employed gas-phase reaction mechanism and have further delineated the reactor extent over which the contribution of the homogeneous reaction pathway was negligible. Over the reactor extent where oxygen was still available, the employed heterogeneous reaction scheme provided good agreement with the measured species concentrations, overpredicting only to a small degree the partial over the total oxidation route. In the oxygen-depleted zones of the reactor, however, the heterogeneous scheme overpredicted to a greater degree the impact of steam reforming and water gas shift reactions, resulting in higher computed hydrogen yields at the reactor exit. Additional experiments and predictions were carried out in a sub-scale gas-turbine honeycomb reactor, at operating conditions leading to oxygen breakthrough. The predictions again favored the partial over the total oxidation route. A modified heterogeneous scheme was proposed that provided very good agreement with measurements in the honeycomb reactor and in the oxygen-rich zones of the laboratory-scale reactor. The hydrogen produced during partial oxidation was partly re-adsorbed on the catalyst leading to superadiabatic surface temperatures, thus exemplifying the importance of proper thermal management in commercial reactors.