Catalytic oxidation of methane in a microchannel reactor under oxygen-lean conditions

Abstract

The aim of this study is to investigate methane oxidation in a microchannel reactor under the effect of Pt, Rh and Ru catalysts supported on δ-Al2O3. Methane oxidation is an integrated part of the oxidative steam reforming (OSR) process which is based on coupling of exothermic oxidation and endothermic steam reforming of methane. As applied in OSR, experiments are conducted under fuel rich conditions that involved methane-to-oxygen (CH4/O2) ratios greater than the stoichiometric value of 0.50. In all experiments, amount of catalyst (Pt/δ-Al2O3, Rh/δ-Al2O3, Ru/δ-Al2O3) that is coated on a FeCrAl plate stayed in the range of 29 mg to 35 mg. The catalysts are prepared by the incipient-to-wetness impregnation method. Catalyst effect on methane conversion is determined according to two different experimental methods. In the first method four different inlet molar CH4/O2 ratios, 1.32, 1.59, 1.85 and 2.13, are applied at a constant total feed flow 100 ml/min by increasing reaction temperature from 623 K to 923 K for all three catalysts. The other method consisted of keeping CH4/O2 ratio at a constant value as 1.85 while changing feed flow stream to 75, 100 and 125 ml/min through increasing reaction temperature from 623 K to 923 K. Micro structural and elemental characterization of the fresh and used catalysts are achieved by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses. The catalysts performances are investigated in terms of CH4 conversion. Methane conversion is found to increase with temperature regardless of the feed composition. Catalyst activity is found to follow the order of Pt/δ-Al2O3 > Rh/ δ-Al2O3 > Ru δ-Al2O3 with respect to their light-off temperature which is expressed as the value at which 10 per cent of methane conversion is achieved. For all catalysts, light-off temperatures are found to decrease and then increase with increasing values of CH4/O2 ratio. Higher methane conversion is achieved on Pt and Ru catalysts for increased contact time at CH4/O2 = 1.85. However Rh catalyst favors better conversion performance even at reduced contact time because of its high catalytic activity.