EFFECTS OF H₂O AND CO₂ DILUTION ON THE CATALYTIC AND GAS-PHASE COMBUSTION OF METHANE OVER PLATINUM AT ELEVATED PRESSURES

Abstract

The impact of large exhaust gas dilution (up to 59.5% H₂O and 30.3% CO₂ per vol.) on the heterogeneous (catalytic) and homogeneous (gas-phase) steady combustion of fuel-lean CH₄/O₂/N₂ mixtures over platinum has been investigated experimentally and numerically at pressures of 5 to 14 bar. In situ, one-dimensional Raman measurements of major gas-phase species concentrations and planar laser induced fluorescence (LIF) of the OH radical were used to assess the heterogeneous and homogeneous combustion processes, respectively. Comparisons between measurements and predictions have yielded catalytic and gas-phase chemical reaction schemes suitable for the combustion of methane with large H₂O and CO₂ dilution. The addition of water inhibits the catalytic reactivity due to the increase in the surface coverage of OH(s) that, in turn, reduces the available free platinum sites. This inhibition has been quantified in terms of operating pressure, temperature, and amount of water dilution through an appropriate one-step catalytic reaction for the complete oxidation of methane. Water promotes chemically the onset of homogeneous ignition, as it directly impacts the radical pool buildup. On the other hand, the chemical effect of CO₂ on either reaction pathway is negligible. The influence of H₂O and CO₂ dilution to the coupling between the two reaction pathways is elucidated and implications for the application of catalytically stabilized combustion to new power generation cycles with large exhaust gas dilution are addressed.

Keywords:

• exhaust gas dilution
• heterogeneous and homogeneous combustion of methane
• in situ Raman
• LIF measurements
• platinum catalyst

Support was provided by the Swiss Federal Office of Energy (BFE), Swiss Federal Office of Education and Science (BBW) through the European project Advanced Zero Emissions Power and ALSTOM of Switzerland. The contribution of AWTEC in the construction of the steam generator is gratefully acknowledged.

Notes

^(a)pressure, % volumetric composition, equivalence ratio, inlet velocity, inlet temperature, and inlet Reynolds number. Homogeneous ignition is attained in Cases 10 to 14.

^(a)From Deutschmann et al. (2000). In the surface and desorption reactions, the reaction rate coefficient is k = A T ^b exp(− E/RT), A(sec⁻¹) and E (kJ/mol). In all adsorption reactions, except S3, A denotes a sticking coefficient (γ). Reactions S2 and S3 are duplicate. Reactions S1, S4 and S14 have a Pt-order of 2.3, 1 and 2, respectively. The suffix (s) denotes a surface species.