Hetero-/homogeneous combustion of syngas mixtures over platinum at fuel-rich stoichiometries and pressures up to 14 bar

Abstract

The catalytic and gas-phase combustion of H2/CO/O2/N2 mixtures was investigated in a platinum-coated channel-flow reactor at fuel-rich equivalence ratios 2–7, H2:CO volumetric ratios 1:2 to 5:1, pressures 1–14bar, and wall temperatures 750–1250K. In situ, 1-D Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the catalytic processes. Gas-phase combustion was monitored by planar laser-induced fluorescence (LIF) of the OH radical at pressures p⩽5bar and of hot O2 at p>5bar, wherein OH-LIF was not applicable due to strong quenching. Simulations were performed with a 2-D elliptic code that included detailed heterogeneous and homogeneous chemical reaction mechanisms. The capacity of O2-LIF to assess the flame front positions at elevated pressures and fuel-rich stoichiometries has been demonstrated. The employed reaction schemes reproduced the measured H2, CO and O2 catalytic conversions as well as the flame positions and shapes over the entire investigated parameter range. Even though the catalytic conversion of the limiting O2 reactant was nearly transport-limited, the competition between H2 and CO for O2 consumption allowed for evaluation of the catalytic kinetics. Under rich stoichiometries, the oxidation of CO(s) via the HCOO(s) reaction pathway was significant. The sensitivity of gaseous combustion on catalytic reactions was strong, exemplifying the need of accurate surface chemistry when modeling syngas hetero-/homogeneous combustion. Adsorption of the H radical considerably inhibited gaseous combustion, especially at the lowest H2:CO ratios. The key surface reactions affecting gas-phase combustion were the same for all pressures. For a given stoichiometry, the sensitivity of homogeneous ignition on the inlet concentration of CO was modest, as the preferential catalytic oxidation of CO increased the H2:CO ratio over the gaseous induction zone.