# Biological carbon capture and utilization

How gas-fermenting acetogens build ethanol from carbon monoxide and CO2, why utilisation postpones rather than reverses emissions, and why hydrogen supply and gas–liquid transfer set the limits.

CO2 is fully oxidised carbon, so every biological conversion must repay the energy — and unless the product is durable, the carbon is delayed, not removed.

Source: https://en.bioecon.ru/docs/bioenergy-climate/cdr-carbon-management/biological-carbon-capture-utilization/
Updated: 2026-09-07



Turning CO2 into products sounds like getting carbon for free. The catch is the oxidation state: carbon in CO2 is already fully oxidised, the bottom of the energy ladder, and every molecule with fuel or food value sits above it. Biology cannot waive this. Any bio-CCU route is really an energy route that uses microbes as its construction crew.

## What the microbes actually do

The workhorses of [gas fermentation](../../biofuels-bioenergy/gas-fermentation-chemicals/) are anaerobic acetogens — Clostridium-type bacteria running the Wood–Ljungdahl pathway: one carbon is reduced on a methyl branch, one on a carbonyl branch, and the two meet as acetyl-CoA, the universal cellular building block. From there the cell exports acetate, reduces further to ethanol, or builds biomass. Carbon monoxide is the convenient substrate: oxidising it to CO2 hands the cell the electrons needed to reduce the next carbon, so CO-rich industrial off-gas or syngas powers its own conversion. Pure CO2 does not — it needs an external reductant, hydrogen — and the stoichiometry is unforgiving: reducing CO2 to ethanol or to protein consumes hydrogen in quantities that dominate the energy account. If that hydrogen comes from electrolysis, most of the value in the product is embodied electricity and the CO2 is the skeleton, not the fuel. The protein route is the same chemistry aimed at growth: hydrogenotrophs fed CO2 and H2 build cells that, dried, are largely protein, displacing fishmeal or soy in feed.

## Where the carbon ends up

Utilisation and removal are different claims, and the distinction is mechanical, not rhetorical. A fuel returns its carbon to the atmosphere the moment it burns — weeks after leaving the plant. Feed protein is oxidised by the animal that eats it. A polyhydroxyalkanoate polymer holds its carbon for years to decades: delay with a usefully long half-life, but not storage. Only a product that keeps carbon out of the atmosphere on geological timescales — essentially burial or mineralisation — supports a removal claim. Carbon that returns to the atmosphere is not removal; utilisation displaces fossil production and postpones the return, which is worth doing but belongs to the avoided-emissions ledger. This is exactly why certification frameworks pay more for permanence than for use.

## What is hard

Three things. Mass transfer: hydrogen and CO2 dissolve poorly in water — hydrogen at only milligrams per litre — so feeding cells becomes gas–liquid engineering, with fine sparging, pressurised reactors or novel contactors to raise the transfer rate; usually it, not the biology, limits productivity. Thermodynamics: nothing about the conversion is energetically free, and the electron donor — H2 or CO — is the dominant operating cost. Recovery: dilute broths mean separation energy, the familiar tax of every fermentation industry. The limiting quantity across all of it is reducing power — electrons available per tonne of carbon converted — and a bio-CCU project lives or dies on where those electrons come from.