# Gas fermentation to chemicals

The Wood-Ljungdahl pathway as the metabolic core of steel-off-gas fermentation, why acetogens beat Fischer-Tropsch on selectivity and lose on speed, and why gas-liquid transfer sets the reactor design.

Acetogenic bacteria fix CO, CO2 and H2 into ethanol through the only carbon-fixation pathway that spends no ATP — and then the bottleneck moves from metabolism to physics: the gases barely dissolve in water.

Source: https://en.bioecon.ru/docs/bioenergy-climate/biofuels-bioenergy/gas-fermentation-chemicals/
Updated: 2026-09-07



Steel mills and refineries vent enormous streams of carbon monoxide and carbon dioxide laced with hydrogen — gases too dilute and dirty for most chemistry but perfectly formed for one guild of anaerobic bacteria. Acetogens eat carbon monoxide. A reactor full of Clostridium autoethanogenum turns blast-furnace gas into ethanol at ambient temperature and pressure, which is why gas fermentation exists as an industry. The interesting parts are, first, how cheap the metabolism is and, second, how expensive the physics around it becomes.

## The Wood-Ljungdahl pathway: fixation without ATP

Every other autotrophic lifestyle pays for carbon fixation. The Calvin cycle spends several ATP per CO2; the photosynthetic apparatus spends sunlight. Acetogens run the Wood-Ljungdahl pathway, in which two one-carbon units — one carried on a folate-like cofactor as the methyl branch, one bound to a nickel-iron enzyme as the carbonyl — are joined by the acetyl-CoA synthase into acetyl-CoA, the universal cellular building block. That C–C bond-forming step consumes no ATP; the pathway is the most energy-frugal carbon fixation known. The energetic trick is that CO itself is a superb electron donor: an acetogen can live on carbon monoxide alone, using it as both its carbon and its reducing power. In bookkeeping form, ethanol from CO alone: 6CO + 3H2O into ethanol + 4CO2; ethanol from the steel-deficient reverse: 2CO2 + 6H2 into ethanol + 3H2O. Because the cell reduces only what it must to balance electrons, the natural product is a mix of acetate and ethanol, skewed toward ethanol by low pH and high gas reduction — the levers operators hold. What the metabolism never produces is a spectrum: no chain-growth distribution as in [Fischer-Tropsch synthesis](../gasification-fischer-tropsch-saf/), just the one molecule, at selectivity the catalyst route cannot match.

## The bottleneck that is not biological

The limiting quantity sits outside the cell. CO, CO2 and H2 are among the least soluble gases in water — roughly a millimole per litre at atmospheric pressure for hydrogen and carbon monoxide. A bacterium can only eat what dissolves into its membrane, so reactor productivity is set by the volumetric mass-transfer coefficient, the rate at which gas hops into the liquid. Everything expensive about gas fermentation follows: fine-bubble sparging, aggressive stirring, pressurized vessels to shove more gas into solution, cell-retention schemes that let the liquid stay while the gas races through. This is also why the biology's tolerance advantages matter less than they promise: acetogens forgive trace sulfur that would kill a metal catalyst and repair themselves, but no tolerance cancels Henry's law.

## Downstream, the old fermentation economics

The product leaves dilute — a few percent alcohol in water — so recovery by distillation is a major energy sink, exactly as in grain ethanol. And the product itself joins the [drop-in fuels chain](../advanced-biofuels/): recycled-carbon ethanol feeds the same alcohol-to-jet catalysis as plant ethanol. The route's frontier extends to feeding acetogens electrons directly, the subject of [microbial electrosynthesis](../power-to-x-with-biological-step-microbial-electrosynthesis/). The chemistry, then, is settled and elegant; the engineering quantity — how fast a gas dissolves — is what decides whether a steel mill becomes a refinery.