Carbon Capture: Part 5

Technologies for direct air capture

In this section, we turn our attention to technologies to capture CO2 from the air. From Part 3, separating CO2 from the air is much more difficult than capturing CO2 from a concentrated source like a cement plant. The challenges associated with cost-effectively removing the small amount of CO2 in air require process designs and adsorbents that are very different from point-source technologies. This generally leads to higher capital and operating costs to capture CO2 from air. Estimated costs of $90–250/tonne CO2 for DAC systems at commercial scale were published in a 2019 report from the US National Academy of Sciences.

However, direct air capture (DAC) facilities have the advantage that they can be located anywhere, including next to cheap energy sources, CO2 utilization facilities, or sequestration sites. Unlike point-source carbon capture technologies, which reduce industrial emissions (bringing them closer to carbon neutral), DAC technologies are negative emissions technologies (NETs). Analyses from numerous groups, including the International Panel on Climate Change (IPCC), indicate that NETs and specifically carbon capture and storage will be required to limit global temperature rise to <2 degrees C. [1] The capture costs at the low end of the NAS range may be economically viable with a sufficient market price for CO2 coupled with growing public or private incentives.

Solid Adsorbent Systems

Note that because we think that carbon capture with solid adsorbents is a particularly exciting area, this section is longer than the other technology descriptions. Solid adsorbent processes are exciting because they are applicable to removing CO2 from a wide range of sources, from air (0.04% CO2) to cement plants (20–30% CO2). Compared to liquid absorption processes, systems with solid adsorbents are more easily customized, modularized and scaled for different use cases. From an investment standpoint, breakthrough innovations in solid adsorbents and process technology are also more likely to have additional applications beyond carbon capture, for example in other gas separations and catalysis.

What is an adsorbent? Solid materials have long been engineered to reversibly capture and release CO2 or other gases, acting like a selective sponge. These materials are called adsorbents. The little “do not eat” bags of material you find in packaged foods and shoes are examples of adsorbents that really like to capture water out of air. An even more relevant example are the CO2 scrubbers used by NASA and featured in the Apollo 13 movie. If you zoom in on a typical adsorbent, it would look like an endless network of connecting tunnels, or holes. Physical adsorbents rely on the size of holes (called pores) in the material to trap CO2 but not other gases. They include activated carbon, zeolites, and some metal-organic frameworks (MOFs). Chemical adsorbents form chemical bonds with the gas they capture (e.g. CO2). Like liquid solvents, solid materials for CO2 capture that rely on chemical adsorption generally include a type of chemical called an amine. Examples include aminated silicas, polymers with amines, and other families of MOFs.

Designing an effective carbon capture system starts with the adsorbent. A good carbon capture adsorbent has a high CO2 capacity while being very selective, meaning it will take up (“adsorb”) a lot of CO2 but not other gases. [2] The adsorbent needs to be cheap, last a long time, and continue to work if it encounters water, NOx, or other pollutants. The amount of CO2 that an certain adsorbent will hold changes with the amount of CO2 in the gas, the pressure, and the temperature. Because of this, one adsorbent may be good for capturing CO2 from cement plants, while another may be better for capturing CO2 from air.

A good adsorbent alone is not enough. Systems that use solid adsorbents for carbon capture act like filters for CO2. In a Brita water filter, gravity causes water to flow through the filter cartridge. For gases, a fan or gas compressor is needed to push the gas with CO2 through the filter (or through the liquid in absorption systems). The amount of energy it takes to force CO2 through the capture system can be very large, especially for capturing CO2 directly from air, where more than 1650 tonnes of gas must be processed to capture 1 tonne of CO2. [3] A major area of innovation for direct air capture systems is reducing the amount of energy needed to move air through the system. A key metric for this is the change in gas pressure (pressure loss) across the carbon capture system. Reducing the pressure loss across the system reduces the cost of carbon capture. [4]

Based on renderings from Climeworks and Carbon Engineering. A colleague commented that these technologies look like giant HVAC systems, which is exactly what they are! These systems take in air, process it, and return it.

Carbon capture systems with solid adsorbents typically have multiple filter containers or “beds” of the solid material that act like rechargeable batteries. There are two ways that adsorbents filled with CO2 are generally recharged. The first way is to heat the material (a “temperature swing”). The second way is to expose the filter to a lower pressure or vacuum to pull off the CO2 (a “pressure swing”). In a typical process, the gas containing CO2 is pushed through one or several of the beds, while beds that have already been filled with CO2 are recharged offline.

The speed of filling and recharging the adsorbent is important, because the faster you fill and recharge the adsorbent, the less adsorbent and housing you need. Less adsorbent means a smaller system to capture the same amount of CO2, which reduces cost. The total amount of time to fill a bed of adsorbent with CO2 and recharge for the next use is called the cycle time. For capturing CO2 from point-sources (cement and power plants), these times are a few hours or less. [3]

For direct air capture processes, using a bed more than twice a day is an accomplishment. It can take hours to completely fill the adsorbent with CO2 by blowing air over it. The time to fill the adsorbent depends on how fast you blow gas through the bed. However, at high gas velocities the CO2 won’t have time to stick. (Imagine playing putt-putt golf and you putt too hard. The golf ball [CO2] may skip over or bounce out of the hole.) The energy needed to push CO2 through the bed also increases with the speed of the gas, incentivizing moderation.

Decisions about how the CO2 is removed from the adsorbent to recharge it strongly affect cost. Generating enough heat to remove CO2 is generally much cheaper than pulling vacuum to remove it. Heating a solid is much slower than removing CO2 by lowering the pressure, which creates a trade-off with cycle time. The fastest way to heat and regenerate a solid adsorbent is to flow steam through it, but this may cause some adsorbents to adsorb water instead of CO2 or possibly degrade. Electric heaters could in theory be used to take advantage of renewable energy, but heat from electricity is typically more expensive than steam due to conversion losses.

As with liquid absorption, solid adsorbents that require less heat to regenerate reduce the energy requirements and cost, but with the trade-off that they are less selective about capturing CO2 but not other gases. Overall the design of carbon capture systems is a fascinating optimization problem, which is why the cost of capture varies so widely between applications.

Companies actively developing solid adsorbent technologies for carbon capture include:
Point Source: Shell, Svante, Innosepra, Mosaic Materials, TDA Research, GTI
Direct Air Capture: Climeworks, Global Thermostat, Skytree, Svante, Mosaic Materials, Infinitree, Klaus Lackner/ASU

Carbonate Formation

An alternate approach to direct air capture first converts CO2 to carbonate ions (CO32-) by dissolving CO2 in a water-salt solution, similar to the mineralization process discussed in Part 4. However, once the CO2 is dissolved, this process recovers pure CO2 through several additional steps. The first step, carbonate formation, is similar to liquid scrubbing systems, but uses water and potassium hydroxide rather than an amine solution. Potassium hydroxide (KOH) is cheaper, less toxic and less corrosive than amines. Large fans draw air into the absorber, where the gas passes over thin plastic surfaces that have the liquid water+KOH flowing over them. The KOH plus CO2 produces a carbonate salt (KCO3) that stays with the liquid.

The liquid containing carbonate (“CO2 rich solution”) is used as an input into a series of chemical processes to purify and convert it back into pure CO2 gas for use or storage. This involves separating the carbonate salt out from the liquid into small solid pellets and then heating the pellets to 900 degrees C in a third step, a calciner, to release pure CO2 in gas form. This step also leaves behind solid salts that are redissolved in water in a slaker (like sugar in coffee) and recycled back to the absorber.

The main company active in this area is Carbon Engineering, although others use a related process to produce concrete and other products (see discussion in Part 4 on “Mineralization”).

Current State of the Art, Direct Air Capture (DAC)

At present, the direct air capture area ecosystem includes fewer large companies, though this may change if funding from public and private entities increases. Three companies have publicly deployed pilot or demonstration systems to date: Climeworks (Switzerland), Carbon Engineering (Canada), and Global Thermostat (USA). There is significant room for growth and additional technology development if additional resources enter this space.

The previous post in this series discusses point-source carbon capture technologies. This is the final post in this short introduction to carbon capture. I hope it has been helpful, and welcome any comments, feedback or suggestions!

Photo by Jessie Jess on Unsplash

Notes

1. There are many negative emissions technologies, and that many are technically simpler than building large carbon capture facilities. Each has distinct advantages and disadvantages in terms of cost, time for deployment, land use, water use, and energy use to name a few. To have a chance at meeting the 2 deg C goal set by the Paris Accord, “all of the above” will be required. Source: Center for International Climate Research
2. Alameda, CA-based startup Mosaic Materials has developed a MOF adsorbent with a high capacity for CO2 even at the low CO2 content of air.
3. This assumes that the carbon capture plant removes 100% of the CO2 in the incoming gas. In reality, removing 30–90% of the CO2 is more likely Fun fact: to capture 1 tonne of CO2 from air, the volume of air needed is 1.2x the volume of the Empire State Building.
4. Emerging DAC company Global Thermostat has developed a monolith (honeycomb-like) adsorbent that leads to very low pressure losses.
5. A key innovation of one growth stage startup, Svante, is a wheel-shaped bed design that rotates the adsorbent through filling and recharging in 60 seconds (this is an example of a solid adsorbent used for point-source carbon capture).

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