An Approach Using Oxidative Coupling of Methane for Converting Biogas and Acid Natural Gas into High-Calorific Fuels

Biogas is a renewable resource with prospects in production of commodity chemicals or for power generation. The latter implies co-usage of biogas and natural gas in gas distribution nets but puts a restriction on the higher heating value (HHV) of all gases pumped into the net. In this contribution, we analyzed and experimentally validated the potential of the oxidative coupling of methane (OCM) as a method for upgrading biogas HHV. The effects of the reaction conditions, i.e., reaction temperature, feed CH<sub>4</sub>/O<sub>2</sub> ratio, and co-feeding of CO<sub>2</sub> or H<sub>2</sub>O, on HHV of the resulting gas mixtures were studied. The highest HHV of 11.48 kW·h·m<sup>–3</sup> after water condensation was obtained when performing the OCM reaction over Mn–Na–WO<sub><i>x</i></sub>/SiO<sub>2</sub> catalyst at 800 °C using a feed with CH<sub>4</sub>/O<sub>2</sub> ratio of 12 and 30 vol% water. This performance was possible thanks to 86% selectivity to C<sub>2</sub> hydrocarbons (C<sub>2</sub>H<sub>4</sub> and C<sub>2</sub>H<sub>6</sub>) at 15.2% methane conversion. It was also possible to increase the selectivity to 100% when carbon oxides formed in the OCM reaction were hydrogenated to methane in a separate reactor containing a Fe<sub>2</sub>O<sub>3</sub> catalyst and thus to obtain a gas mixture with HHV of 12.0 kW·h·m<sup>–3</sup>. Such combined approach can open an environmentally friendly way for upgrading biogas or natural gas with high CO<sub>2</sub> content for direct applications in gas distribution nets when using green source of energy for H<sub>2</sub>O splitting to H<sub>2</sub> and O<sub>2</sub> required for the OCM and hydrogenation reactions, respectively.