An experimental and numerical investigation of the catalytic-rich/gaseous-lean combustion of H2/CO/air mixtures at 8 bar

The catalytic-rich/gaseous-lean (R/L) combustion concept was investigated experimentally and numerically for syngas fuels with H-2:CO volumetric ratios 1:0, 4:1 and 1:2, catalytic-rich stoichiometries phi(rich) = 2-10 (including operation without air), pressure of 8 bar and air preheat of 673 K. Experiments were performed in a subscale R/L burner with optical access to both catalytic-rich and gaseous-lean stages. OH-PLIF mon-itored the turbulent combustion in the gaseous-lean stage, OH*-chemiluminescence assessed the propensity for homogeneous ignition in the catalytic-rich stage, and exhaust gas analysis provided the NOx and CO emissions. Two-dimensional simulations were carried out for both stages, while a 1-D opposed-jet code mod-eled the NOx emissions. The exothermicity of the heterogeneous reactions promoted homogeneous ignition and flame anchoring in the upstream parts of the catalytic-rich stage and allowed for complete consump-tion of the deficient O-2 reactant, a process that could not be achieved by the catalytic pathway alone due to transport limitations. Homogeneous combustion in the catalytic-rich stage was beneficial for attaining the highest possible fuel pre-conversion. The catalyst not only initiated gaseous combustion but also mitigated potential NOx emissions from the catalytic-rich stage at the highest pre-conversions (lowest phi(rich)) and high-est CO-content mixtures. Two-sided diffusion flames were established in the gaseous-lean stage due to the recirculation of O-2-rich combustion products, which was advantageous for the burner compactness. It was shown that cardinal to the R/L concept was the fact that a decreasing phi(rich) led to an increased heat transfer from the catalytic-rich stage to the bypass air, which reduced the enthalpy in the fuel stream of the gaseous-lean stage and thus lowered the peak flame temperatures (by 400 K for H-2:CO = 1:0). The reduction in flame temperatures with decreasing.rich led to a six-fold drop in NOx emissions, while CO emissions were less than 5 ppmv. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

The catalytic-rich/gaseous-lean combustion concept was studied for syngas fuels, showing that the exothermicity of heterogeneous reactions promoted homogeneous ignition and complete fuel conversion. The catalyst not only initiated gaseous combustion but also reduced potential NOx emissions, while two-sided diffusion flames were advantageous for burner compactness.