What happens to the CO2 after you capture it: utilisation pathways and their energy cost

The capture step is the half everyone photographs

A direct-air-capture unit is a satisfying thing to point at. Air goes in, CO2 is separated, a counter somewhere increments. But a capture system that has nowhere to send its output is not a carbon system. It is a concentration system with a storage problem attached, and the storage problem is where most of the cost and nearly all of the engineering difficulty actually live.

So this piece takes the question that follows the one I wrote about last week: once the CO2 is off the sorbent and in a tank, what happens to it, and what does each answer cost in energy?

Four destinations, and only four

Captured CO2 can go to geological storage, to mineralisation, to a chemical or fuel product, or to a direct use that consumes it without transforming it. Everything else is a variation on one of those.

Geological storage puts it into a suitable formation and leaves it there. Together with mineralisation, it is one of the two routes that are genuine sinks rather than delays, and its engineering burden is compression, transport and monitoring rather than chemistry.

Mineralisation reacts CO2 with alkaline material, often industrial residues such as steel slag or concrete waste, to form stable carbonates. The reaction is thermodynamically downhill, which is the whole attraction: it wants to happen. The engineering problem is kinetics and mass transport, not driving force.

Direct use covers beverage carbonation, greenhouse enrichment and similar. These are real markets, they are small, and the CO2 is released again within weeks. Useful revenue; not a sink.

Conversion to a chemical or fuel is the route that attracts the most attention, and it is the one that deserves the most careful accounting.

Electroreduction, and the number that governs it

CO2 electroreduction uses electricity to drive CO2 and a proton source to products: carbon monoxide, formate, ethylene, methanol, methane, depending on catalyst, potential and electrolyte.

The physics to hold on to is this. CO2 is the bottom of the energy hill. It is what carbon looks like after the energy has been extracted. Turning it back into something combustible means putting that energy back, plus the losses. No catalyst changes that. What a good catalyst changes is how much of the energy you put in ends up in the product you wanted rather than in hydrogen you did not.

Which gives the governing metric, and it is the same shape as the one from the capture side: energy consumed per unit of product, divided by the energy content of that product.

If that ratio is well above one, and for a fuel it always is, then CO2-to-fuel is not an energy source. It is an energy carrier: a way of moving low-carbon electricity into a liquid or gas that an existing engine, aircraft or furnace can burn. That is a legitimate and sometimes valuable thing to do. It is not the same claim as turning emissions back into fuel, and the two get conflated constantly.

Where the honest opportunity sits

Three conditions make conversion worth the energy it consumes.

The electricity is genuinely surplus. Curtailed renewable generation has a low or negative marginal value. A process that can start and stop with availability, and that is not penalised for running at a low capacity factor, is a good match for it. A process that needs steady baseload is not.

The product is one that is hard to electrify directly. Aviation fuel, shipping fuel, and chemical feedstocks such as ethylene are the honest targets. Making methane to burn in a boiler that a heat pump could have served is a long way round.

The CO2 source is concentrated. This is where the capture and utilisation questions meet. Ambient air is roughly 420 parts per million. A cement or steel flue stream is a different problem by orders of magnitude in concentration, which is why capture from a point source costs materially less energy per unit than capture from air. Any utilisation scheme that assumes cheap CO2 needs to say which source it means.

Mineralisation is underrated, and the reason is unglamorous

Of the four destinations, mineralisation asks least of the energy system. It runs downhill thermodynamically, it produces a stable solid rather than something that needs monitoring for decades, and its feedstock is frequently a waste stream that a producer is already paying to manage.

It is underrated because the output is aggregate rather than fuel, and aggregate does not photograph well. But for an urban platform of the kind I described last week, with modest capture rates, distributed sites and no pipeline nearby, a route that ends in a stable solid handled by ordinary logistics is a considerably better fit than one that ends in a gas needing compression and transport.

What a credible utilisation claim contains

The same four-layer test applies here as everywhere else. A claim about CO2 utilisation should carry the source and its concentration, stated as ambient air, flue gas or process stream with the figure; the full energy balance, including capture, compression, conversion and any separation of the product from unreacted feed; faradaic efficiency to the named product rather than to products in aggregate, because a cell that is 90 per cent efficient across a mixture may be 30 per cent efficient to the one molecule you can sell; catalyst durability in hours at the stated current density, because selectivity that decays in a fortnight is a laboratory result; and the counterfactual, meaning what that same electricity would have displaced if it had gone somewhere else.

That last one is the one most often left out, and it is frequently the one that decides whether a project makes sense.

The system view

Capture and utilisation are usually presented as two industries. Engineered properly they are one system with a single energy budget, and the budget is dominated by two terms: the energy to separate CO2 from whatever it is mixed with, and the energy to put chemical potential back into it.

Anyone can lower the first term by choosing a concentrated source. Nobody lowers the second term below what thermodynamics requires. Designing around that constraint honestly, picking destinations that suit the energy available rather than the destination that sounds most impressive, is what separates a system that will still be running in ten years from one that produces a good launch photograph.

Measure everything. Validate everything. Improve continuously.

Cite this note
A referenced edition of this note, with a permanent DOI, is deposited on Zenodo: Rohera, H. K. (2026). What happens to CO₂ after capture: utilisation pathways and their energy cost [Technical note]. Zenodo. https://doi.org/10.5281/zenodo.22700061

Hemant K. Rohera is an independent inventor and engineer in Pune, India, sole named inventor on three granted patent families: bioelectronic medical devices, hybrid energy storage and vehicle power electronics, with grants in force in India, the United States, Japan, South Korea, Canada, Mexico, Australia, Vietnam, Saudi Arabia, South Africa, Russia, Kazakhstan, Germany, the United Kingdom and Switzerland. All three families are available for licensing, assignment or joint development.

Technologies: emedica.in · nextgenpowerpack.com · roheraindustries.com

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Hemant Rohera — Bioelectronics, Energy Storage and Power Electronics