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 investigated experimentally and numerically at a global equivalence ratio ϕ = 0.285, H2:CO volumetric ratios 1-4, pressures 1-8 bar and catalyst surface temperatures 950-1200 K. In situ planar laser induced fluorescence (PLIF) of the OH radical monitored homogeneous combustion inside a channel-flow catalytic reactor, while 1-D Raman measurements of main gas-phase species concentrations across the channel boundary layer assessed 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 \u3c 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 ( x \u3c xig), the wall temperatures were below the pressure-dependent upper temperature limit for the decomposition of PdO to metallic Pd. Even though palladium catalysts exhibited a self-regulating temperature effect due to the decomposition of PdO, the attained temperatures were still sufficient to ignite homogeneous combustion of the H2/CO/O2/N2 mixtures, in contrast to hydrocarbon fuels for which gas-phase combustion was largely suppressed over PdO in the pressure range 1-8 bar. The results indicated that for the elevated pressures and preheats of syngas-fueled hetero-/homogeneous combustion power systems, gas-phase chemistry cannot be ignored during reactor design