Energy cost: CO2 capturing from air vs CO2 capture at source

Summary

Capturing CO2 from air at 0.04% may seem absurd given after two decades of research and development there are no full-scale power plants with CO2 capture for which CO2 gas concentration > 10%. The fundamental thermodynamics and physics of CO2 capture suggest that it need not be much more difficult than post-combustion CO2 capture from power plants. Nonetheless, it appears clear that if an air capture plant and a post-combustion facility at a large power plant are designed and operated under the same economic conditions, i.e., the same cost of construction and energy, as well as the same cost of capital, the cost of air capture will always be significantly higher than the cost of post-combustion capture.

According to current estimates, the cost of carbon capture ranges between $200 and $1,000 per tonne of CO2. To put that figure into perspective, it would cost the entire US GDP (give or take a factor of 2) to capture one year's worth of carbon emissions using current technology. Even if we assume a 2-5-fold reduction in the energy cost of carbon capture, recapturing our current annual CO2 emissions will cost trillions of dollars.

General

If you've been following the climate crisis, you've probably heard that we need to get to zero carbon emissions. Simply put, the more CO2 there is in the air, the warmer the planet becomes, and the only way to stop the warming is to stop emitting carbon.

This is a difficult task. It's so difficult that many experts believe we won't get to zero without some form of 'negative carbon emissions,' and one big idea here is to develop new technologies that suck carbon dioxide out of the air on a global scale.

However, any future technology that extracts carbon from the air will never be a replacement for reducing our carbon emissions. That's because there's a fundamental cost to capturing carbon dioxide - a cost imposed by physical laws. The bad news is, it's expensive. While DAC [ direct air capture] appears to be a promising technology, it faces numerous challenges. Due to the low concentration of CO2 in atmospheric air, DAC necessitates massive amounts of energy per unit of CO2 captured, resulting in significant energy costs in processing large volumes of air and regeneration of the sorbent material. As a result of the high energy demand, it becomes a rather costly process. As of 2022, it is yet to be profitable because the cost per tonne of carbon dioxide is several times the carbon price.

What exactly is direct air capture (DAC)?

Direct air capture (DAC) is the process of capturing carbon dioxide (CO2) directly from the ambient air (rather than from a point source, such as a cement factory or a biomass power plant) and producing a concentrated stream of CO2 for sequestration, utilization, or the production of carbon-neutral fuel. When ambient air comes into contact with chemical media, typically an aqueous alkaline solvent or sorbents, carbon dioxide is removed. These chemical media are then stripped of CO2 using energy (specifically heat), resulting in a CO2 stream that can be dehydrated and compressed while also regenerating the chemical media for reuse.

Thermodynamics

An examination of thermodynamic constraints reveals that the low concentration of carbon dioxide in ambient air does not impose strict limits on the economics of air capture. Even when using an irreversible sorbent-based process, the thermodynamic energy requirement is low. When compared to flue gas scrubbing, the additional energy requirement appears to be minor and can be met with low-cost energy. Because the absorption is usually irreversible, the free energy expended in regenerating a sorbent will usually exceed the free energy of mixing. The irreversibility, which increases with scrubbing depth, tends to affect flue gas scrubbing more than air capture, which can operate successfully while extracting only a small fraction of the carbon dioxide available in the air. This results in a significantly lower theoretical thermodynamic efficiency for a single-stage flue gas scrubber than for an air capture device, but a lower carbon dioxide concentration in the air still results in higher energy demand for air capture.

Process

To absorb CO2 from a gas, most commercial techniques employ a liquid solvent, usually amine-based or caustic. A common caustic solvent, sodium hydroxide, for example, reacts with CO2 to form stable sodium carbonate. This carbonate is heated to produce a stream of highly pure gaseous CO2. Causticizing can be used to recycle sodium hydroxide from sodium carbonate. Alternatively, in the chemisorption process, CO2 binds to the solid sorbent. The CO2 is then desorbed from the solid using heat and vacuum.

Cost

Cost of carbon capture

The main takeaway is that as the CO? concentration decreases, the cost of extracting it blows up! The concentration of CO? in the air is approximately 400 parts per million, or 0.0004 when expressed as a decimal fraction.

Because CO? is a trace gas in the atmosphere, extracting carbon from the air is like panning for gold — to extract 1 tonne of CO?, you need to sift through over 2,000 tonnes of air — and this makes carbon capture unavoidably expensive & inefficient.

Theoretical minimum energy cost to extract 1 tonne of CO2 from air is equivalent to 500 million Joules = 140 kWh.

(As a check on the math, this number is within 10% of most published estimates, and within a factor of 2 of other estimates. kWh is short for kilo Watt-hour, which is the standard unit of energy on your electricity bill.)

This number tells us what’s theoretically possible, not what’s technologically feasible. Any real-world machine has unavoidable energy losses which cause it to consume more energy than this.

So how close can we get to this theoretical limit? There’s a lot of debate on this subject, but many estimates suggest the best we can realistically achieve is 10 times the theoretical estimate (aka 10% thermodynamic efficiency).

Practical minimum energy cost to extract 1 tonne of CO2 from air is equivalent to 5000 million Joules = 1400 kWh

Let’s think about this number in a few ways.

Let us assume that US industries pay ~7 cents for 1 kWh of electricity. Using this as a ballpark conversion rate from energy to money, the practical minimum cost of extracting CO? comes out to about ($0.07 / kWh) ? (1,400 kWh / tonne) ≈ $100 per tonne of CO?.

So, physics teaches us two things about the cost of pulling CO? from the air.

It can go down somewhat.

It’s still really expensive.

According to current estimates, the cost of carbon capture ranges between $200 and $1,000 per tonne of CO2. To put that figure into perspective, it would cost the entire US GDP (give or take a factor of 2) to capture one year's worth of carbon emissions using current technology.

The physics suggests that the cost could be reduced further, but not significantly. Even if we assume a 2-5-fold reduction in the energy cost of carbon capture, recapturing our current annual CO2 emissions will cost trillions of dollars.

And that's just for the energy. It excludes the cost of carbon storage, as well as the costs of land, infrastructure, maintenance, labor, and so on. There is no getting around the fact that carbon capture is inherently costly.

Credit: Google