Experimental and numerical investigation of high-pressure methane catalytic synthesis from H2 and CO2

Abstract

The catalytic synthesis of methane from H2 and CO2, which is an efficient way for the production of synthetic natural gas (SNG) by utilizing the H2 originating from carbon-free renewable power sources and the greenhouse gas CO2, was investigated in a non-isothermal channel-flow reactor coated with Ni/Al2O3 at pressures 1.5–10 bar, H2:CO2 volumetric ratios 4–6, and surface temperatures 450–760 K. In situ, spatially resolved Raman measurements of gas-phase species concentrations across the channel height were complemented by gas chromatography analysis of the exhaust gas. Two-dimensional simulations were carried out, using elementary catalytic and gaseous chemical reaction mechanisms and detailed transport. Comparisons of the Raman data and simulations revealed that the preferential diffusion of H2 drastically altered the local H2:CO2 ratios at the catalytic walls, which attained values up to 8 times higher than the corresponding inlet values. This behavior exemplified the caution needed when performing kinetic studies in the mixed kinetically/transport controlled regime. The production of methane increased with increasing pressure and decreasing H2:CO2 ratio. At the highest ratio H2:CO2 = 6, the simulations underpredicted (overpredicted) the measured reactant (product) mole fractions, however, as H2:CO2 decreased, the trend was reversed. The discrepancies reduced with increasing pressure, resulting in a model overprediction of methane by 7.4% at 10 bar. Methane was produced from C(s) via successive hydrogenation. C(s) was in turn produced from COOH(s), followed by dissociation of COOH(s) to CO(s), and finally H-assisted dissociation of CO(s) to C(s). Gas-phase chemistry was negligible over the investigated conditions. The in-channel Raman measurements in conjunction with the exhaust gas analysis facilitated the construction of a global step for the studied reaction. The methane synthesis was assessed to follow a ∼p0.41 pressure dependence over the temperature range 520–680 K.