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 < 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 < 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.


Mechanical and Aerospace Engineering


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Keywords and Phrases

Homogeneous ignition of syngas over palladium at elevated pressures; In situ Raman and OH-PLIF measurements; Self-regulating temperature of palladium; Syngas combustion over palladium

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Article - Journal

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Publication Date

27 Aug 2020