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.

Keywords:

• Catalytic ignition and extinction
• Catalytic partial oxidation of methane
• Exhaust gas dilution
• Transient reacting flow modeling

Acknowledgments

Support was provided by the Swiss Federal Office of Energy (BFE), Swiss Federal Office of Education and Science (BBW) through the European Union project Advanced Zero Emissions Power and ALSTOM of Switzerland. The help of Mr. Rolf Schaeren in the experiments is gratefully acknowledged.

Notes

^(a)Type of test (ignition, extinction-related), equivalence ratio, pressure, inlet conditions and volumetric composition, rhodium loading in catalyst, ratio of active-to-geometrical surface area.

^(b)The ratio B (active to geometrical area) corresponds to a catalyst dispersion of 25.9 m²/g-Rh for the 1% wt Rh loading and to 34.5 m²/g-Rh for the 0.5% wt Rh loading.

^(a)Thermal conductivity, density, heat capacity at two selected temperatures.

^(b)In the range 600–1200 K, c _FeCr = b ₀ + b ₁ T + b ₂ T ², b ₀ = 580, b ₁ = 0.394 and b ₂ = 6.57x10⁻⁴.

^(c)The properties of ZrO₂ have been corrected for porosity (37%, assessed from the physisorption tests).

^(a)From Schwiedernoch et al. (2003). The reaction rate coefficient is k = AT ^b exp(-E/RT), A (mol-cm-sec) and E (kJ/mol). In the adsorption reactions, A denotes a sticking coefficient (γ). The suffix (s) designates a surface species. The surface site density is Γ = 2.72 × 10⁻⁹mol/cm².