Ignition and Extinction in Catalytic Partial Oxidation of Methane-Oxygen Mixtures with Large H2O and CO2 Dilution

Abstract

The ignition and extinction in catalytic partial oxidation (CPO) of CH4/O2 mixtures with large exhaust gas dilution (46.3% H2O and 23.1% CO2 vol.) has been investigated experimentally and numerically at 5 bar. Experiments were carried out in a short contact time Rh-coated honeycomb reactor and involved temperature measurements along the reactor and exhaust gas analysis. Numerical predictions were performed with a 2-D transient elliptic code that included detailed chemical reaction schemes and relevant heat transfer mechanisms in the solid. The employed heterogeneous reaction scheme reproduced the measured minimum inlet temperatures required for catalytic ignition (light-off), the elapsed times for the propagation of the reaction front, and the steady-state exhaust gas compositions at a fuel-to-air equivalence ratio of ϕ = 4.0. The chemical impact of the added H2O, although important already at the early light-off stages, was minimal on the ignition delay times because the latter were dominated by total oxidation and not by partial oxidation or reforming reactions. The key reaction controlling catalytic ignition was the surface oxidation of CO to CO2, which was the main exothermic heat release step in the induction zone. Measurements and predictions indicated that vigorous combustion could be sustained at inlet temperatures at least as low as 473 K and 298 K in CPO with and without exhaust gas dilution, respectively. The extended stability limits of CPO combustion were due to a shift from partial to total oxidation products, and hence to higher exothermicity, with decreasing inlet temperature. The key parameter controlling extinction was the CO(s) coverage, which led to catalyst poisoning. Finally, operation at non-optimal stoichiometries (ϕ = 2.5) was shown to be beneficial in CPO of power generation systems with large exhaust dilution, due to the moderating effect of dilution on the maximum reactor temperature.