Upgrading of biogas into gas fuels: pathways towards energy with net zero carbon emissions

Various methodologies exist for upgrading biogas, each differing in their capacity to valorise CO 2 . Certain processes merely separate CO 2 from CH 4 without converting it. Conversely, other pathways transform biogas into synthesis gas, thereby valorizing CO 2 . Synthesis gas is not the final product, requiring further conversion to liquid fuels by methanol or Fischer-Tropsch synthesis, which can account for some CO 2 emissions. The conversion of CO 2 in biogas to biomethane using electrolytic H 2 represents an efficient CO 2 valorisation pathway. This process enriches the product gas with CH 4 through CO 2 separation, which is subsequently transformed to yield additional CH 4 and mitigate CO 2 emissions. The production of CH 4 is further incentivized by the possibility to enrich CH 4 to concentrations that can enable the storage within the natural gas grid, which use to be highly accessible to most biogas treatment facilities. Additional reductions in CO 2 emissions can be achieved by minimizing the number of process steps or by intensifying the processes. This is exemplified by the recently proposed integrated CO 2 capture and methanation, which employs a single reactor. This review examines biogas upgrading technologies, encompassing both commercially mature solutions and emerging concepts. The evaluation of these technologies is conducted with a critical emphasis on achieving net-zero carbon emissions throughout the transformation process of biogas to the final product.