Membrane-integrated process for simultaneous biogas upgrading and hydrogen storage via methanol

This work presents the design of a membrane-integrated process for biogas valorisation and renewable hydrogen storage via CO2-to-methanol conversion. The process maximizes CO2 utilisation by incorporating H2 from renewable sources, while simultaneously separating methane from biogas to produce a stream suitable for direct injection into the natural gas grid. Membrane units are integrated upstream and downstream of the methanol synthesis reactor: upstream membranes allow to obtain a CO2-rich stream for methanol production and a CH4-rich stream compliant with grid specifications, while downstream membranes recover unreacted CO2 and H2 for recycling, minimizing emissions and hydrogen losses. The system is analysed in a step/stage configuration using performance maps from a validated one-dimensional model, accounting for the selectivity and permeance of a polyimide membrane. Results show that biogas can be fully valorised, achieving 98.5% CH4 recovery with molar purity ≥97.5% and ∼97% CO2 conversion to methanol, with nearly complete utilisation of renewable hydrogen. This membrane-integrated approach provides an effective strategy for coupling biogas upgrading with renewable hydrogen storage, enabling sustainable energy storage in the form of methanol e-fuels and contributing to carbon-neutral energy pathways.