Pilot Links Membrane CO2 Capture to Microbes That Make Grid-Quality Methane

Key Findings · At a Glance

• ▸The two-stage membrane unit reached 94.3% CO2 purity with 71% recovery from a 27% CO2/73% N2 feed.
• ▸At a 15% CO2 feed the same configuration delivered only 70.4% purity, below the 90% needed for methanation.
• ▸Optimal stage pressures were identified as 5.5 bar for the first stage and 1.9 bar for the second.
• ▸Biomethanation reactors fed at a 4:1 H2:CO2 ratio produced gas above 95% methane at a two-hour retention time.
• ▸At two hours, a stable methane production rate of 2.28 m3 CH4 per m3 reactor per day was achieved.
• ▸Falling iron and nickel levels coincided with volatile fatty acid build-up; correcting nutrient supply restored stability.
• ▸Preliminary pilot capture cost was estimated at 150–200 €/t CO2, dominated by electricity for gas compression.

Opportunity

Lignite-fired power plants remain among the largest contributors to greenhouse gas emissions, and the authors frame post-combustion capture as particularly relevant to such hard-to-abate sources. The incumbent route, amine absorption, is the most mature but carries a solvent regeneration penalty the authors cite as 3–4 GJ per tonne of CO2, alongside corrosion, degradation and footprint. Membranes are less mature, at an estimated technology readiness of 6–7, but are compact, modular, solvent-free and can be retrofitted without major plant redesign.

Capture alone leaves the question of what to do with the CO2. Biological methanation offers one answer: hydrogenotrophic microbes combine CO2 with hydrogen to make methane under mild conditions, and the authors note the process tolerates common flue gas impurities. The product can substitute for natural gas in existing grids. The catch is that the microbes need a concentrated, steady CO2 supply, while flue gas typically carries between 7% and nearly 30% CO2.

The team, led from Aristotle University of Thessaloniki with the Hellenic Agricultural Organisation Dimitra and Public Power Corporation of Greece, set out to close that gap. After single-stage laboratory trials, they built a pilot of four hollow-fibre polyimide modules in two stages and connected it at the Agios Dimitrios power plant to a proton exchange membrane electrolyser and two 0.10 m3 trickle-bed reactors running at 55°C. The authors describe the continuous on-site coupling as a first for Greece.

Results

The Practical Applications

Assessment

The authors are explicit that their cost figures are preliminary and that further techno-economic work is needed. Future work will scale the membrane unit to industrial flue gas capacities, add detailed techno-economic and sensitivity analysis, and complete a full life cycle assessment. Savings from scaling up by 10–100 times are left for future work, and they flag investigation of flue gas pre-treatment options.

Readers should note that the separation trials used synthetic CO2/N2 mixtures designed to imitate flue gas rather than reporting performance on raw flue gas, and binary mixtures omit the oxygen and secondary pollutants the authors themselves say real treatment must handle. The 90% purity target was met only at 24% CO2 and above, well over the 10–15% the authors describe as typical of low-concentration industrial flue gas. Recovery of 71% also leaves 29% of the CO2 in the feed uncaptured. The biological side is the more encouraging result: stable grid-quality methane at a two-hour retention time, provided trace metals are actively managed.