Oxidative steam reforming of glycerol to synthesis gas in a microchannel reactor

Abstract

Oxidative steam reforming (OSR) of glycerol to synthesis gas (syngas) was studied in a microchannel reactor that involved a thin layer of Rh/Al2O3 catalyst coated to the inner wall of a rectangular channel. The effects of reaction temperature (550–700 °C), molar inlet ratios of carbon-to-oxygen (C/O = 0.75, 1.125; C and O: moles of carbon and oxygen atoms at the inlet, respectively) and steam-to-carbon (S/C = 3-5) and microchannel reactor configuration on product distribution and syngas composition were investigated. In all experiments, complete, coke-free conversion of glycerol to gaseous carbon containing species was observed. The only exception was seen at 550 °C and C/O = 1.125 that led to 93% glycerol conversion and solid carbon deposition, the latter being verified by XPS and Raman spectroscopy analyses. Decrease in C/O from 1.125 to 0.75 promoted oxidation of H2, CO, CH4, C2H4, C2H6 and other carbon containing species on the catalyst surface, with the magnitude of promotion being increased notably with temperature. Despite O2-enriched feeding, Rh remained at metallic state as revealed by XPS characterization of the spent samples. Lowering C/O also elevated yields of CO2 and steam, the latter which drived water-gas shift (WGS), an important side reaction that affected product distribution on Rh/Al2O3 in favor of H2. Yields of C1-C2 hydrocarbons decreased at higher temperatures and at lower C/O and S/C ratios due to their consumption via total oxidation and steam reforming routes, respectively. Microchannel reactor packed with particulate form of catalyst gave yields of CO2 and H2 lower and yields of CO and C1-C2 hydrocarbons higher than those obtained in the coated configuration under identical conditions. These findings seemed to correlate with the extents of oxidation reactions and WGS, which were believed to vary mainly with different heat transport characteristics of the reactor schemes. H2/CO ratios close to the ideal value of 1 for Fischer-Tropsch synthesis were obtained at 700 °C, S/C = 3–4 and C/O = 1.125.