Carbon and energy footprint of the hydrate-based biogas upgrading process integrated with CO₂ valorization

The present paper aims at assessing the carbon and energy footprint of an energy process, in which the energy excess from intermittent renewable sources is used to produce hydrogen which reacts with the CO₂ previously separated from an innovative biogas upgrading process. The process integrates a hydrate-based biogas upgrading section and a CO₂ methanation section, to produce biomethane from the biogas enrichment and synthetic methane from the CO₂ methanation. Clathrate hydrates are crystalline compounds, formed by gas enclathrated in cages of water molecules and are applied to the selective separation of CO₂ from biogas mixtures. Data from the experimental setup were analyzed in order to evaluate the green-house gas emissions (carbon footprint CF) and the primary energy consumption (energy footprint EF) associated to the two sections of the process. The biosynthetic methane production during a single-stage process was 0.962Nm³, obtained mixing 0.830Nm³ of methane-enriched biogas and 0.132Nm³ of synthetic methane. The final volume composition was: 73.82% CH₄, 19.47% CO₂, 0.67% H₂, 1.98% O₂, 4.06% N₂ and the energy content was 28.0MJ/Nm³. The functional unit is the unitary amount of produced biosynthetic methane in Nm³. Carbon and energy footprints are 0.7081kgCO_2eq/Nm³ and 28.55MJ/Nm³, respectively, when the electric energy required by the process is provided by photovoltaic panels. In this scenario, the overall energy efficiency is about 0.82, higher than the worldwide average energy efficiency for fossil methane, which is 0.75.