Know where your towel is

Machines that suck CO₂ from the air might be cheaper than we thought

Operators of pilot plant publish their design and costs for scaling up.

Scott K. Johnson – Jun 7, 2018 5:10 PM | 198

Carbon Engineering's pilot plant, which captures CO₂ from the atmosphere. Credit: Stephen Hui, Pembina Institute

When you spill a drink, you don’t say, “Oh well, the only thing we can do is spill fewer drinks in the future.” You grab a towel. So there’s also a natural attraction to the idea that we should develop a towel that can remove CO₂ from the atmosphere. That isn’t as simple as grabbing one from a Home Goods store, however, and cost estimates have not fueled optimism for most methods of doing this.

Reforestation is an obvious option, but its potential impact is probably smaller than you think. Other biological schemes could include growing biofuels to burn in power plants that capture emissions and store them underground. Recently, we’ve also seen a couple of working pilot projects that look like a power plant run in reverse—they suck in air and harvest concentrated CO₂, ready for storage.

One of those plants, located an hour north of Vancouver, British Columbia, is the brainchild of a company called Carbon Engineering. One of the founders of Carbon Engineering is Harvard’s David Keith, a researcher studying this and other conceivable methods of “geoengineering” our planet’s climate. This week, the Carbon Engineering team has published a nuts-and-bolts breakdown of its design, providing the first cost analysis of a working carbon capture plant.

Carbon Engineering’s pilot plant and another pilot plant built by Climeworks in Switzerland diverge at the substance used to grab CO₂ from the air. Climeworks opted for solid granules of amine, while Carbon Engineering is using a water-based solution of potassium hydroxide.

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Cycling components

In their paper, the researchers explain their design choices. While their end task is obviously different, they were able to adapt existing machinery for each step in the process. By getting a little creative and optimizing components, they were able to bring the cost down significantly from previous estimates. All of this is laid out in great detail.

detailed diagram of the components of the design plant — Building a carbon capture plant probably isn't a good weekend project. Credit: Keith et al/Joule

The process starts with what is basically a cooling tower, where a fan pulls air in through a sort of filter that has a continuous flow of water trickling downward through it. But instead of cooling the water, the goal here is to get air in contact with the potassium dissolved in the water, which forsakes its hydroxide partner to grab molecules of CO₂. The team designed a different geometry for the parts the water trickles through in order to maximize the chemical reaction, but otherwise this machine is essentially stock.

The potassium carbonate that forms enters something called a “pellet reactor,” more commonly used for special water treatment systems. The solution flows upward through tiny pellets of calcium carbonate, keeping them suspended. Add in a little calcium hydroxide, and the disloyal potassium trades partners again. The calcium carbonate that results causes the pellets to grow until they’re large enough to sink downward despite the flow. Potassium hydroxide flows out the other side, ready to get sent back to the first stage and grab another CO₂ dance partner.

The enlarged calcium carbonate pellets now get sent to the third and final stage: heating that separates them into pure CO₂ gas and calcium hydroxide that can go back to stage two. The heat comes from a natural gas furnace that produces steam. The steam travels through loops that efficiently dump heat into the calcium carbonate before reaching a generator turbine that provides the plant’s electricity.

I know what you’re thinking—they’re burning fossil fuel to capture CO₂? While it’s possible to redesign this to run all-electric, in this configuration the CO₂ emissions from burning natural gas are also captured, so this is not self-defeating from a climate standpoint.

Carbon neutral fuels

At this point, the pure CO₂ could be compressed into a pipeline and sent off to a facility that pumps it underground for storage, reducing the amount in the atmosphere. But Carbon Engineering also has some other ideas. If you combine this with hydrogen gas split from water, you can synthesize conventional fuels.

Fossil fuels are made of carbon pulled from the atmosphere by plants millions of years ago and slowly cooked deep underground—carbon that we return to the atmosphere when fuels are burned. If you take your carbon from the atmosphere today and put it back tomorrow, you have no effect on the concentration of greenhouse gas. It’s the industrial equivalent of growing biodiesel. For transportation that resists electrification—notably air travel—that could provide a way to eliminate the greenhouse gas emissions.

Unless you’re aiming to build your own hobbyist carbon-capture plant, the biggest thing here is probably the cost estimate. While this pilot plant can currently only capture about 200 tons of CO₂ per year, Carbon Engineering's estimate is based on a larger “commercial-scale” plant design that would handle about 1 million tons per year—equal to the emissions of roughly 200,000 US cars. Scaling up is simple for the modular components (just stack ‘em), but a couple of pieces would become more cost-effective when supersized.

The end result is a cost of $94 to $232 per ton of CO₂ captured by a plant of that size. Past estimates for capture plants have been all over the place but are generally much higher than that. A major 2011 report, for example, estimated a plant like this would cost at least $550 per ton.

For comparison, British Columbia’s tax on emissions is currently CAN$35 per ton of CO₂, rising to CAN$50 in 2021. That’s generally the range of existing carbon prices, though there have certainly been arguments for higher numbers. If a carbon capture plant could come in below $100 per ton—and the Carbon Engineering folks certainly don’t think they’re done cutting costs—it could enter the conversation with other plausibly economically viable technologies for countering emissions.

Of course, not spilling a drink is still cheaper than buying a towel to clean it up.

Open access at Joule, 2018. DOI: 10.1016/j.joule.2018.05.006 (About DOIs).

Listing image: Stephen Hui, Pembina Institute

Scott K. Johnson Associate Writer

Scott has written about geoscience and energy at Ars as a freelancer since 2011.

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