Homogeneous ignition of H₂/CO/O₂/N₂ mixtures over palladium at pressures up to 8 bar

The catalytic and gas-phase combustion of fuel-lean H₂/CO/O₂/N₂ mixtures over palladium was studied experimentally and numerically at a global equivalence ratio Φ = 0.285, H₂/CO volumetric ratios of 1-4, pressures of 1-8 bar and catalyst surface temperatures of 950-1200 K. In situ planar laser induced fluorescence (PLIF) of the OH radical was used to monitor homogeneous combustion inside a channel-flow catalytic reactor, while 1-D Raman measurements of main gas-phase species concentrations across the channel boundary layer were conducted to evaluate the heterogeneous processes. Simulations were carried out with a 2-D numerical code using detailed heterogeneous and homogeneous chemical reaction mechanisms and realistic transport. The simulated and measured transverse species profiles attested to a transport-limited catalytic conversion of H₂ and CO at all operating conditions. The OH-PLIF measurements and the simulations confirmed the establishment of appreciable homogeneous combustion only for p < 4 bar, with progressively diminishing gas-phase contribution as the pressure increased from 4 to 8 bar. This strong pressure dependence reflected the complex pressure/temperature dependence of the homogeneous ignition chemistry as well as the competition between the catalytic and gaseous reaction pathways for H₂ and CO consumption. Over the gaseous induction zones, the wall temperatures were below the pressure-dependent upper temperature limit for the decomposition of PdO to metallic Pd. Although palladium catalysts exhibited a “self-regulating” temperature effect due to PdO decomposition, the attained temperatures were still sufficient to ignite homogeneous combustion of the H₂/CO/O₂/N₂ mixtures. This was contrary to hydrocarbon fuels for which gas-phase combustion was largely suppressed over PdO in the pressure range of 1-8 bar. The results suggested that for the elevated pressures and preheats of syngas-fueled heterogeneous/homogeneous combustion power systems, gas-phase chemistry cannot be ignored during reactor design.