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Direct Air Capture: Liquid phase sorption systems by Carbon Engineering

Updated: Apr 1, 2023

To accomplish direct air capture, several major changes have to be made to conventional sorption processes to accommodate the extraordinarily high air flows needed to capture meaningful amounts of carbon from atmospheric air. One of the principal engineering challenges is that the system pressure drop must be minimal to avoid huge power consumptions that would otherwise be present with, for example, a bubbler.

There are two leading design solutions for the first generation of direct air capture systems to scale to MT/yr capacity. The first design is a solid sorbent system, where air is passed through a packed bed of solids whose surfaces are tuned to have a high affinity for CO2. This is the approach used by Climeworks, Svante, and a host of others.

The second leading solution is the liquid sorbent system, pioneered by Prof. David Keith's lab at Harvard and demonstrated at pilot scale by his spin-out company Carbon Engineering. Most recently, Carbon Engineering has partnered with 1PointFive, the carbon capture arm of Occidental Petroleum, to build a first scaled plant with a capacity of 0.5 MT/yr.

The primary advantage of the Carbon Engineering approach is that it looks very much like other petrochemical processes. The core technology is a closed-loop process that employs a combination of fans, filters, and chemical reactions to capture CO2 from ambient air. The air is first drawn into the system using large fans and passed through a contactor, where it comes into contact with an aqueous solution containing a chemical sorbent, typically a hydroxide. The CO2 reacts with the sorbent to form stable carbonate compounds, effectively trapping the carbon dioxide. The captured CO2 can then be purified and concentrated, allowing it to be stored or utilized in various industrial applications.

The entire process consumes a considerable amount of energy, about $40-$50 of energy costs alone per ton of CO2 captured, at least assuming something close to today's market cost for natural gas or electricity as inputs. 1PointFive plans to use the technology for enhanced oil recovery, so by siting the plant near sources of gas its costs may be significantly cheaper. The future of this technology is not likely to be limited by energy costs, especially when the company can stack a federal EOR credit of $130/ton on top of a California low carbon fuel credit of $200/ton. There are likely other sources of revenue beyond these headline numbers. Potential from the operation dwarfs the cost of energy.

Of more interest is the cost of capital. The Carbon Engineering solution looks like a chemical plant, which gives Occidental some comfort. But it also only works at scale, so a large amount of money has to be spent on scaling before the plant generates any revenue, or before the process can event be debugged. This creates substantial financial risk. Carbon Engineering estimated capex costs in the range of $85/ton to $130/ton for the first plant at a scale of 1 million tons/year. Occidental expects costs for its smaller first plant to be higher still, saying in its public SEC disclosures that it expects the total cost of capture to be less than $400/ton. The payoff will come over time, as the operation scales and plant economics improve.

This is a very long game. The Carbon Engineering technology is low risk relative to the novel nature of direct air capture, but the potential for capital cost overruns seems high, and it will be a long time before the first plants pay down their investment. Occidental's strategy depends on being able to execute on the project without cost overruns, or schedule slips. The company has deep petrochemical expertise, so they are better positioned than almost anyone in the world to estimate this well. Still, new technology tends to offer surprises when it is scaled. It will be interesting to see if the benefits continue to pay for the costs over the long haul.

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