Catalytic and gas-phase combustion of SOFC off-gases over platinum surfaces: an experimental and numerical investigation at pressures up to 8 bar

Abstract

The combustion of low calorific value Solid Oxide Fuel Cell (SOFC) off-gases was investigated in a platinum-coated channel-flow reactor, at pressures 1–8 bar and surface temperatures 700–1060 K. H₂/CO/H₂O/CO₂/Air mixtures were used at a global equivalence ratio φ = 0.90, with compositions relevant to either high- or low-FUR (Fuel Utilization Rate) operation of the SOFC. Spatially resolved, in situ Raman measurements of main gas-phase species concentrations evaluated the catalytic (heterogeneous) reactivity, while Planar Laser Induced Fluorescence of the OH radical monitored gas-phase (homogeneous) combustion. Two-dimensional numerical simulations were carried out with detailed heterogeneous and homogeneous chemical reaction mechanisms. Under high-FUR operation, the lower contents of H₂ and CO favored catalytic ignition as they diminished the chemical self-inhibition that both fuels exhibited on Pt. High pressures were beneficial due to the positive pressure dependence of both H₂ and CO reactivities on Pt. For the typically high temperatures of the SOFC off-gases (> 700 K), catalytic ignition was readily achieved, whereas gaseous chemistry was negligible in all high-FUR cases. For the low-FUR cases with much higher H₂ contents, the H₂ preferential diffusion rendered O₂ locally the deficient surface reactant, despite the globally fuel-lean stoichiometry, resulting in reduced H₂ and CO conversions. By increasing the surface temperatures in the low-FUR cases to ∼1100 K, homogeneous combustion was ignited for pressures p ≥ 3 bar. Upon homogeneous ignition, the flames did not stabilize inside the reactor but propagated upstream and anchored at the channel entry. This resulted in an inverse catalytically stabilized hybrid combustion concept, wherein a downstream catalytic section served as an igniter and stabilizer for an upstream homogeneous combustion zone. This concept was advantageous, as it permitted complete consumption of H₂ and CO via homogeneous reactions at appreciably shorter reactor lengths.