*6.1.6 Chemical hydrogenation method*

The reduction of CO2 with H2 can be either conducted biologically or chemically, based on Sabatier reaction. Regarding the chemical hydrogenation process, various catalysts, with Nickel and Ruthenium to be the most commonly used in industrial applications have already been tested under elevated temperature (e.g., 300°C) and pressure levels (e.g., 5–20 MPa) [180, 181]. Due to high selectivity, complete conversion of CO2 and H2 can be practically achieved [182]. Nevertheless, despite the high process efficiency, specific drawbacks still remain. For instance, the sustainability is affected by the presence of trace gasses in the biogas, which degenerate the catalysts leading to increased need for periodical replacement [183]. The high cost of energy to preserve the operational conditions, the lack of elements to synthesize effective catalysts, and the need for pure gasses are further technical challenges of the system [11].
