Technology atlas / Biological conversion

Gas fermentation

Acetogenic bacteria convert CO, CO₂ and H₂ from industrial off-gas or electrolysis into ethanol and other chemicals.

Technical reference · Updated 2026-10-10 · No current-news claim

6 CO + 3 H₂O → C₂H₅OH + 4 CO₂ (CO-rich off-gas)

2 CO₂ + 6 H₂ → C₂H₅OH + 3 H₂O (CO₂ + hydrogen)

Where the carbon goes

Depends on the product: ethanol used as fuel re-releases carbon quickly; chemicals and polymers made from it hold carbon for their service life.

Conceptual process flow · left to right

01Gas cleaning
02Bioreactor
03Broth separation
04Distillation
05Product

From chemistry to process

Feedstocks: Steel-mill or other CO-rich off-gas, CO₂ + hydrogen, Nutrients
Products: Ethanol, Acetate and other chemicals, Microbial protein
Catalysts: Biocatalyst: acetogenic bacteria (e.g. Clostridium autoethanogenum)
Conditions: Near-ambient temperature in aqueous broth; performance depends on gas–liquid mass transfer, strain and gas impurities.

Performance & demonstrated scale

Commercial plants operate on CO-rich industrial off-gas; CO₂ + H₂ operation must be assessed separately.

1.911 t CO₂/t ethanol theoretical
Ideal net reaction from CO₂ + H₂. No biomass growth, by-products or losses.

0.263 t H₂/t ethanol theoretical
Ideal net reaction from CO₂ + H₂. Not actual plant consumption; CO-rich gas routes need less or no added hydrogen.

Pathway TRL: not assigned. Project-specific evidence is required.

Economics

Off-gas availability, ethanol prices and separation energy drive the result; CO₂-only routes add hydrogen cost.

Climate considerations

When using fossil-derived off-gas, the climate benefit is avoided production, not removal. Allocation with the host plant must be explicit.

CCU Reality Check · pathway assessment

What remains to be demonstrated?

This initial assessment separates process knowledge from the evidence needed for a commercial conclusion. Missing information is left unresolved.

Dimension	Assessment and evidence gap
CO₂ feedstock availability	No site-specific supplier, volume, purity guarantee or delivery contract has been verified.
Capture and delivered cost	Not quantified. Capture, conditioning, compression and transport need separate boundaries.
Energy and hydrogen demand	Theoretical stoichiometric or thermodynamic values are shown above. Actual process energy and feed losses remain scenario assumptions.
Catalyst and reactor performance	Biocatalyst: acetogenic bacteria (e.g. Clostridium autoethanogenum). No plant lifetime or sustained performance is verified here.
Product separation	The process flow identifies separation stages where relevant; a quantified separation duty and cost remain unavailable.
CAPEX and OPEX	No plant-specific capital estimate or complete operating-cost dataset is attached.
Minimum selling price	Not calculated. Capital recovery, utilization, financing and a complete operating boundary are required.
Fossil benchmark competitiveness	No verified regional benchmark comparison. Calculator benchmark values, where shown, are illustrative.
Climate performance	When using fossil-derived off-gas, the climate benefit is avoided production, not removal. Allocation with the host plant must be explicit.
Scale-up bottlenecks	Low gas solubility and mass transfer; Dilute product and separation energy; Feed-gas variability and contaminants; Hydrogen supply for CO₂-only feeds.
Customer and offtake readiness	No qualified-customer or executed-offtake evidence is attached to this pathway profile.
Policy dependence	Not assessed for a specific project or jurisdiction. Do not assume subsidies or certification eligibility.

Source references

Original diagrams and explanatory text. References support process context; no numerical commercial performance has been extrapolated.