Carbon Capture: The Technology That Works and Rarely Runs
Carbon capture attracts two opposite errors: that it is speculative technology, and that it is a licence to carry on burning. Neither survives contact with the numbers. The chemistry is a century old and reliable. What it costs, in energy and money, is what has kept deployment far below the plans.
How Capture Actually Works
Post-combustion capture is the most common approach and the easiest to retrofit. Flue gas passes up a tower against a descending stream of amine solution, typically monoethanolamine, which binds carbon dioxide chemically. The loaded solvent is then pumped to a second tower and heated to around 120 degrees, which reverses the reaction and releases a concentrated stream of carbon dioxide for compression and transport. The regenerated solvent returns to the first tower.
Nothing in that description is new. Amine scrubbing was patented in 1930 and has been used ever since to remove carbon dioxide from natural gas and from the air in submarines and spacecraft. The novelty of applying it to a power station is one of scale, not of principle.
Two alternatives change the problem rather than solving it downstream. Pre-combustion capture converts fuel into hydrogen and carbon dioxide before burning, so the separation happens at high pressure and concentration where it is easier, at the cost of a more complex plant. Oxy-fuel combustion burns the fuel in nearly pure oxygen rather than air, producing a flue gas that is mostly carbon dioxide and water - separation becomes trivial, but producing the oxygen consumes a great deal of energy.
Once captured, the gas is compressed to a supercritical fluid and moved by pipeline to storage, usually a deep saline aquifer or a depleted oil or gas field at more than 800 metres depth, where pressure keeps it dense and overlying rock keeps it in place. Norway's Sleipner project has been injecting about a million tonnes a year since 1996, which is the longest-running demonstration that storage works.
The Energy Penalty
The reason capture is hard is not that it fails but that it costs energy. Regenerating amine solvent requires heat, and that heat is steam that would otherwise have made electricity. A coal plant fitted with capture loses 20 to 30 percent of its net output; a gas plant loses 15 to 20.
This has a consequence that is often missed. A plant that captures 90 percent of its carbon dioxide but produces 25 percent less electricity must burn roughly a third more fuel to deliver the same output to the grid - which means a third more mining, transport and upstream emissions, and a third more of everything else that comes out of the chimney.
The penalty is why capture is more attractive on processes that are not producing electricity in the first place. In cement making, roughly 60 percent of the carbon dioxide comes from decomposing limestone rather than from burning fuel, and no change of fuel touches it. The same is true of parts of steel and chemical production. For those, capture is not an alternative to electrification; it is the only route currently visible.
Research on the penalty is active and has produced results. Newer solvents, solid sorbents and membrane systems reduce the regeneration heat, and second-generation plants report penalties nearer the bottom of the ranges above. It is an engineering improvement rather than a step change, because the thermodynamic work of separating a dilute gas has a floor.
Direct Air Capture
Pulling carbon dioxide from a power station flue gas means working with a stream that is 4 to 15 percent carbon dioxide. Pulling it from the open atmosphere means working with 420 parts per million, which is 0.042 percent. That difference of two orders of magnitude is the whole difficulty.
The thermodynamic minimum work rises as concentration falls, so direct air capture needs far more energy per tonne - currently 1,000 to 2,000 kilowatt-hours, against 200 to 500 for capture at a chimney. Large volumes of air must also be moved across the contactor, which costs further energy in fans.
The compensating advantage is that it can be built anywhere, which means next to cheap clean energy and directly above a storage site, with no pipeline. It is also the only approach that addresses emissions that have already happened or that come from sources too dispersed to capture individually, such as aviation and agriculture.
Current global direct air capture capacity is in the tens of thousands of tonnes per year. Annual emissions are above 37 billion. The gap is six orders of magnitude, which is the honest context for any discussion of the technology - it is real, it works, and it is very small.
Why So Little Is Running
Global operating capture capacity is around 50 million tonnes a year. Announced projects total several times that, and the gap between announced and operating has been a persistent feature of the field for two decades. Several flagship power-sector projects have been cancelled or mothballed after construction.
The reason is structural rather than technical. Capture costs money and produces a product almost nobody wants to buy. Without a carbon price above the cost of capture, or a direct subsidy, the operator pays to reduce output. That is not a business, and it explains why the projects that do run are mostly ones where the carbon dioxide has a buyer.
That buyer has historically been the oil industry. Enhanced oil recovery injects carbon dioxide into ageing fields to push out more oil, and it accounts for a large share of capture capacity in operation. The carbon accounting of that arrangement is contested for obvious reasons, and it is why capture statistics need reading carefully.
The realistic assessment is narrower than either the advocates or the critics usually allow. Capture is expensive, energy-hungry and slow to deploy, which makes it a poor substitute for not emitting - efficiency, renewables and electrification are cheaper per tonne avoided. It is also the only available answer for cement chemistry and a handful of industrial processes, where those alternatives do not apply at all. Both things are true, and most disagreement about carbon capture comes from arguing one while meaning the other.
Frequently asked questions
Does carbon capture actually work?
Yes. Amine scrubbing has separated carbon dioxide from gas streams industrially since the 1930s, and capture rates of 90 percent or more are routinely achieved. Norway's Sleipner project has stored about a million tonnes a year since 1996. The obstacles are cost and energy consumption, not whether the chemistry functions.
What is the energy penalty?
The share of a plant's output consumed by the capture process, mostly as heat to regenerate the solvent. It runs 20 to 30 percent for coal and 15 to 20 for gas. A plant with capture must therefore burn roughly a third more fuel to deliver the same electricity, which increases everything upstream proportionally.
Why is direct air capture so much harder?
Concentration. A flue gas is 4 to 15 percent carbon dioxide; the atmosphere is 0.042 percent. The thermodynamic work of separation rises sharply as concentration falls, so direct air capture needs 1,000 to 2,000 kilowatt-hours per tonne against 200 to 500 at a chimney, plus the energy to move enormous volumes of air.
Where does the captured carbon dioxide go?
It is compressed to a supercritical fluid and injected below about 800 metres into deep saline aquifers or depleted oil and gas fields, where pressure keeps it dense and impermeable rock above keeps it contained. A substantial share of today's captured carbon dioxide is instead used for enhanced oil recovery, which complicates the accounting.
Is carbon capture a reason to keep burning fossil fuels?
It is cheaper per tonne avoided to not emit in the first place, through efficiency, renewables and electrification, and capture's energy penalty means more fuel burned for the same electricity. Its strongest case is industrial process emissions - cement, steel, chemicals - where the carbon dioxide comes from the chemistry itself and changing fuel does not help.
