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

by Dr Ioannis Mantzaras

Thursday 12 Mar 2020, 14:00 → 15:00 Europe/Zurich

OHSA/B17 (Paul Scherrer Institut)

The catalytic-rich/gaseous-lean (R/L) concept is a novel hybrid combustion approach, which is currently investigated for gas turbines in power generation. The R/L methodology has the advantages of broader extinction limits, control of catalytic combustion by the O2 availability that in turn prevents flame flashback, enhanced flame stability, and low NOx emissions. In co-operation with ALSTOM Switzerland (now GE) we have recently investigated experimentally and numerically the R/L combustion of syngas fuels with H2:CO volumetric ratios 1:0, 4:1 and 1:2, fuel-air stoichiometries in the catalytic-rich stage rich = 2-10, pressure of 8 bar and air preheat of 673 K.

Experiments were performed in a subscale R/L burner (built by ALSTOM and installed in the high-pressure test‑rig of PSI) with optical access to both catalytic-rich and gaseous-lean combustion stages. OH-PLIF (planar laser induced fluorescence) monitored the turbulent combustion in the gaseous-lean stage, OH* chemiluminescence assessed the propensity for homogeneous ignition within the catalytic-rich stage, while exhaust gas analysis provided NOx and CO emissions from the flue gases of the lean stage. Two-dimensional simulations were carried out for both stages (laminar catalytic simulations for the rich stage and SST k-ω turbulence reactive modeling for the lean stage) while a 1-D opposed jet code modeled NOx emissions. The exothermicity of the catalytic reactions promoted homogeneous ignition and flame anchoring in the upstream parts of the catalytic‑rich stage and allowed for complete consumption of the deficient O2 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 fuel pre-conversions (lowest rich) and highest CO content mixtures. Very stable, two-sided diffusion flames were established in the gaseous-lean stage due to the recirculation of O2 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 rich led to an increased heat transfer from the catalytic rich stage to the bypass air feeding the gaseous-lean stage, which reduced the enthalpy in the fuel stream of the gaseous lean stage and thus lowered the peak flame temperatures (by up to 400 K). The reduction in flame temperatures with decreasing rich resulted in a six-fold drop in NOx emissions.