Carbon-negative biomethane fuel production: Integrating anaerobic digestion with algae-assisted biogas purification and hydrothermal carbonisation of digestate

This paper presents a new integrated process for producing carbon-negative biomethane fuel which combines anaerobic digestion of sewage sludge with biogas purification, algae generation and carbon capture. Biogas CO2 is recovered through absorption into sodium carbonate solution, producing bicarbonate, subsequently used as substrate for algae growth. Anaerobic digestate is hydrothermally carbonised into biochar, stabilising unused biomass carbon for long-term storage. Proof-of-concept studies are provided, together with a system carbon balance to demonstrate its overall carbon capture potential. D. tertiolecta (CCAP 19/30) was cultivated at different bicarbonate concentrations (2.5?90 g L-1), producing equimolar amounts of carbonate and carbon-derived products (algae, gaseous CO2). Above 40 g L-1, carbonate regeneration significantly exceeded algae growth, achieving higher than expected CO2 uptake potential, but lower overall carbon sequestration potential due to the discharge of excess CO2 to the environment. Single-stage CO2 absorption from a model biogas mixture (60% methane) into carbonate solution was rate limited by the reaction of dissolved CO2 with solution hydroxide ions, achieving steady state methane outlet purities of up to 85%. Spent absorbent was successfully used for algae culture, and the regenerated medium showed equivalent CO2 uptake as the fresh carbonate solution, demonstrating the cyclability of the system. Carbon distribution to biochar and algae by-products was estimated as 35.1%, exceeding the expected emissions associated with the process, to render the process carbon negative.

Automated summary

This paper introduces a new integrated process for producing carbon-negative biomethane fuel by combining anaerobic digestion of sewage sludge, biogas purification, algae generation, and carbon capture. The proof-of-concept studies illustrate the carbon capture potential of the system while also highlighting the impact of carbonate regeneration on algae growth and the rate limitation of CO2 absorption in the carbonate solution.