# Air Carbon Capture’s Scale Problem: 1.1 Astrodomes For A Ton Of CO2

Air carbon capture continues to get written about as if it is an interesting technology that will play a significant role in reducing global warming. Even CleanTechnica publishes regular articles on the subject. But most articles fail miserably to put the technology in context. A new study or press release comes out, and a bunch of sites publish articles which make it sound as if global warming is practically solved.

But the scale of the problem matters. A lot.

### You would need to filter 1.1 Houston Astrodomes of air to get a single ton of CO2 with 100% effective technology

For the most part, carbon capture is a fig leaf funded by fossil fuel money to allow them to continue to mine fossil fuels and sell them. In some cases, it’s funded by them to provide a source of CO2 to pump into existing tapped out oil wells so that the sludge liquefies and can be pumped out and sold. And many researchers keep plugging away because it’s an interesting scientific challenge to them.

There are three problems with carbon capture and sequestration — capture, shipping and long-term disposal — and air carbon capture only deals poorly with the first of the three. While CO2 has been increasing in the atmosphere, we’re still only at 412 parts per million. That’s enough that it would be about 41 meters (130 ft) thick if it were a single layer in the Earth’s atmosphere, but the Earth’s atmosphere is 100 kilometers or 60 miles deep. That’s more than enough to warm the atmosphere, but it means that there aren’t that many molecules of CO2 present in any given volume of air.

An earlier version of this article contained an erroneous calculation of the mass of CO2 in a cubic meter of air. Thankfully, Geza Gyuk, Director of Astronomy at the Adler Planetarium & Astronomy Museum, Ph.D., Physics, University of Chicago, came to my rescue with a better methodology to achieve the mass and a better result.

The atomic weight of CO2 is about 44. The average atomic weight of air is about 29 (N2 is about 28 and O2 is about 32 so this makes sense). The 410ppm current concentration of CO2 in the air is per volume or equivalently “per molecule”. So for each million air molecules 410 are CO2. So to convert to a mass fraction we simply have to scale 410 by 44/29 which gives 622 ppmm (parts per million mass) or 0.0622%

The density of dry air is about 1.2 kg/m^3. So the CO2 component is 1.2kg/m^3 * 0.0622% = 0.00075 kg/m^3 = 0.75 grams/m^3.

The smallest unit of measure of CO2 for useful discussions of carbon capture and sequestration is the metric ton. That’s a 1,000 kilograms or about 2,200 lbs. A kilogram is a 1,000 grams. That means to get a ton of CO2, we’d need to filter it out of about 1.3 million cubic meters of air, if we were 100% efficient at capturing CO2 molecules from the air (and a bunch of other nuances).

Let’s try some analogies. An Olympic-sized swimming pool contains about 2,500 cubic meters of water. So you’d need to strain about 525 Olympic pools worth of air to get a ton of CO2. The Houston Astrodome is about 1.2 million cubic meters, so you’d need to filter the air from about 1.1 Astrodomes to get a ton of CO2. You’d need about half a Great Pyramid of Giza. If you filtered all of the air in the Grand Canyon, you’d get about 1,270 tons of CO2.

That’s why all air carbon capture devices end up looking like a massive wall of fans.

The above is a visualization of a future air carbon capture device by Carbon Engineering, a Squamish, BC company that does a fairly good job of getting press for its technology. A rather absurd amount of electricity is required to suck 1.3 million cubic meters of air through this kind of device. Imagine a fan that could suck the air out of 1.1 Astrodomes.

Let’s take an industrial 48″ fan with a cubic feet per minute rating of 17,500 to 19,500. This is very generous as the sorbent creates significant back pressure, but let’s be generous. The fan draws 1,080 Watts for operation, but let’s round down to 1 kW to be generous again.

There are about 35 cubic feet in a cubic meter. That means the fan in question can move about 500-560 cubic meters per minutes. Let’s use 500 for convenience, still generous considering back pressure from the sorbent. When you buy lots of these fans at once, the price is around US\$500 per unit.

Pushing all of the air in 1.1 Astrodomes through this industrial blower would take about 44 hours or just over 1.8 days of continuous operation. That’s 440 kWH of electricity required just for moving the air to get a single ton of CO2.

To get a ton of CO2 out of 11 Astrodomes in an hour would take 44 fans drawing 0.44 MW of electricity (about as much as 30 homes). The fans would have a capital cost of about US\$23,000 and the electricity would cost about \$5.30 per ton of CO2 at average US electricity rates.

But that’s just the starting point for the problem. That’s just the cost to move the air containing a ton of CO2. To actually push it through sorbents significantly lowers the rate of air movement. And sorbents don’t capture 100% of CO2 unless you push the air through a large column of sorbents, reducing the air flow further. You can move more air for less electricity, but you also capture less of the CO2. It’s more likely 10x the electricity just to move the air through the device.

Most technologies use live steam to separate the CO2 from the sorbent after it’s ‘full’. And live steam is water that is first heated to 100 degrees Celsius and then through the state change which requires more energy to become steam. It takes a lot more energy to create the steam than it does to move the air. You have a few says to do this, but either you have twice the sorbents which mechanically are removed from the air flow or you stop the air flow to run steam through in the capture cycle. More electricity for all of that and more electricity for creating the steam.

Then you have very large volume of gas. A ton of CO2 takes up about 556 cubic meters uncompressed at one atmosphere and room temperature. You then have to compress and cool the CO2 for any storage or distribution, which entails more energy. And then you have to sequester it in some way, all methods which require more energy.

And then we get to the next problem of scale.

### There are about 3,200 billion tons of excess CO2 in the air that we’ve added since before the Industrial Revolution

If we wanted to get just 10% of that out, we’d need to filter the air from 352 billion Houston Astrodomes or 2.5 billion Grand Canyons. That’s perhaps a bit too many. What about just the CO2 we emit annually?

### About 40 billion tons of CO2 a year are added to the atmosphere

If we wanted to just deal with 10% of our annual increase in CO2, we’d need to filter the air out of 44 billion Houston Astrodomes or 32 million Grand Canyons.

And think of all the electricity we’d need for the fans and heating the water.

But you’ll remember that there are three problems in carbon capture and sequestration: capture, shipping, and long-term disposal. All of the above is only about getting it out of the air in the first place. All of those billions of tons of CO2 are absurdly beyond all of our possible uses of CO2 or the carbon in it for a million years. The global infrastructure to deal with it would be orders of magnitude larger than the entire global oil and gas infrastructure that’s been built over the past 100 years. If we turned it into solids, we’d be burying mountain ranges of carbon, or creating new mountain ranges. Not that we have anywhere on Earth where people wouldn’t likely object to having everywhere they live covered in new mountains. Ugly mountains too.

There are tiny niches where this technology can do some good. Global Thermostat, a firm started by the architect of the carbon market in the Kyoto Protocol, has a solution which uses waste industrial heat from plants which require CO2 as a feedstock. The plant could then generate at least some of its own CO2 and the huge energy cost of creating steam could come from waste heat from the plant. Industrial-scale greenhouses with sensitive crops typically have a heat problem because sunlight comes in but the resultant heat doesn’t leave easily, and they also use CO2 in 1,000–2,000 ppm concentrations to speed plant growth and that CO2 is generated from fossil fuels for the most part right now, either coming compressed in canisters or generated on site. Big cannabis greenhouses could bolt an air carbon capture device on, use a heat pump to pull the excess heat out to make the steam to get the CO2. Some new formulations of cement use CO2 as a feedstock to lock away more carbon permanently in physical form.

But really, the only machine remotely big enough to deal with the excess carbon we’ve put into the air is the entire atmosphere and its carbon cycle. And that takes about 300 years to remove a net carbon atom from the air permanently. We can speed that up a bit with some soil carbon capture techniques, but by a trivial amount.

### The solution is to stop putting CO2 into the air

Stop burning fossil fuels. As rapidly as possible. And let the huge machine that is Mother Nature do the rest. We’ve created an enormous problem over 250 years. We aren’t going to address it in 20.

References:

Note: as always, anyone reading this who spots an error, let me know and I’ll fix it and thank you.

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