18.3: Storing CO2 Removed from the Air

After we remove CO₂ from the air with the methods described in the previous section, it still needs to be permanently stored. Biochar provides one means of storage, as does mineralization. Biological means such as soil carbon or trees provide other important forms of storage that have to be maintained to keep their integrity—you can’t plow up the soil or cut down the trees in the future without losing some or all of the benefit. Our estimates of the maximum capacity of these systems are in the range of 10 billion tons per year. That is an outstanding start but not enough. And some of the important systems, like bioenergy and direct air capture, generate pure CO₂ that still needs another storage form.

Recycling CO₂ into carbon-based products

Many of the carbon-containing products we use every day are made from petroleum, including carpets, fabrics, and plastics. The availability of carbon from petroleum has made these products easy to make and inexpensive, but there is no fundamental reason that they cannot be made from carbon sourced from carbon dioxide. The important difference is that in general energy must be added to CO₂ in order to make the reduced-carbon chemicals that can be used in things like polymers (Figure 18.3.1). The good news today is that energy is increasingly abundant and inexpensive because of renewable sources. We can expect that in the future the energy to turn carbon dioxide into organic chemicals and make things like carpet fiber will be a small fraction of the total cost of the product.

Figure 18.3.1 CO₂ is a low-energy form of carbon: energy must be added to chemically reduce the carbon molecule in order to make organic chemicals that are used in products and fuels today. Image from French Ministry of Energy and Environment.

Researchers are working today on the catalysts and electrochemical systems required to achieve this chemistry, with promising results. Worldwide, we produce enough chemicals (other than fuels) to take up about 1.4 billion tons of CO₂ (Table 18.3.1) if we completely replace petroleum as the carbon source. In principal those chemicals could all be produced using CO₂ processed with the use of renewable electricity, but the amount of electricity would be gigantic—as much electricity as the world produces today! Clearly we need more efficient methods, which seem very likely, given progress in catalysis and electrochemistry.

Table 18.3.1 Possible chemicals and building materials that can be made from CO₂, and the maximum amount of CO₂ that would be utilized

	Carbonate Materials		Chemicals and Fuels		Durable Carbon Materials
	Binders/ Cement	Aggregates	Commodity Chemicals	Fuels	Fiber, Nanotubes, Graphene
Market size (GT/y)	4 Portland cement (2016)	44 Non-metallic minerals (2017)	0.5 Upstream chemicals (2013)	2.5 Transport fuels (2016)	~0.0001 Carbon fiber (2018 est.)
CO2 demand	0.1–1	1 Waste streams	1.4	7.7	?
CO2 abatement	Unclear— indirect benefits	Reduced emissions likely—negative emissions possible	Reduced emissions possible—negative emissions possible	Reduced emissions possible	Unclear—indirect benefits

Note: The estimates in this table are highly uncertain. GT = gigatons. Source: Prepared by Sean McCoy, University of Calgary, based on analyses in ICEF. 2017. Carbon Dioxide Utilization (CO₂U): ICEF Roadmap 2.0. LLNL Report LLNL-TR-739322. https://www.icef-forum.org/platform/ upload/CO₂U_Roadmap_ICEF2017.pdf. Used by permission.

The lifetime of the materials we make from industrial chemicals is of primary interest when we discuss negative emissions. For instance, it is not realistic to think of fuels as contributors to negative emissions, since they are burned soon after being made. But polymers, fabrics, and plastics have longer lives, although they are still not permanent. Researchers are currently evaluating the benefits of producing these intermediate-life materials and the amounts of CO₂ removal from air they represent.

While we might think of organic chemicals as the principal place CO₂ could be used in our economy, it turns out that construction materials are another large possible sink. Concrete is composed of an aggregate material like gravel, which is held together with cement. That cement is mainly portland cement, which uses calcium hydroxide as the primary binding chemical. When water is added, the calcium hydroxide reacts with sand and fine rock in the aggregate mixture to form new minerals that bind all the material together. Production of calcium hydroxide is a major contributor to emissions, as it involves burning the CO₂ off limestone.

Remember calcium hydroxide from our discussion of carbon mineralization? CO₂ can also be permanently stored in cement and concrete, where instead of reacting all the calcium hydroxide with water and sand, as is currently done, we react some of the calcium hydroxide with carbon dioxide, also forming strong, stable minerals that bind the material together. While this substitution does not reduce the amount of CO₂ in the atmosphere, it does offset some of the emissions from calcium hydroxide production. That CO₂ is still emitted, but the net greenhouse gas total is reduced because some CO₂ is added to the resultant concrete. However, the gravel or other aggregate that goes into the concrete is another story. If that aggregate could be made from calcium carbonate produced from atmospheric CO₂ and calcium from wastes, minerals, or the ocean, that could be a very large contributor to negative emissions (Table 18.3.1).

Figure 18.3.2 An oil pump at sunset. Image by skeeze from Pixabay.

Geologic storage

Figure 18.3.3 (Left) Underground storage of CO₂ as a liquid can be done at depths greater than 3,000 feet (1,000 meters) where the pressure keeps the CO₂ in supercritical state with properties very similar to those of oil. The CO₂ fills the space in between sandstone grains. Image reproduced with permission from Reuben Juanes, MIT News. (Right) The volume taken up by CO₂ gas decreases as you add pressure to it, as happens when it is injected deep into the earth. At depths greater than 800 meters it converts into a supercritical liquid with much higher density than the original gas, taking up a small amount of space in the subsurface. Image from US Department of Energy, National Energy Technology Laboratory.

An important form of storage will be injection of liquified CO₂ deep into the earth in rocks like those that oil was produced from originally (Figure 18.3.3). When it is injected at depths greater than 1,000 meters (about 3,000 feet), the pressure is sufficient to keep the CO₂ in a liquid state. (Chemists will recognize this as a supercritical state where the distinction between liquid and gas is no longer meaningful, but the material’s properties are very much like a liquid.) This is about as dense as oil (a little less dense than water), with about the same viscosity. Thus, if we inject CO₂ into an old oil field, or rocks similar to an oil field, the CO₂ will stay there permanently as the oil did. (Of course, some oil leaks out naturally, as on the beach in Santa Barbara, California, but those oil deposits are very shallow, only a few hundred feet below the surface.) The US Department of Energy has conducted extensive tests of this approach, placing 16 million tons of CO₂ underground in a series of experimental sites that have been carefully monitored. No leaks have been observed in 10 years of experiments in the US, nor in 20 years at the Sleipner site in Norway, which is an offshore platform that injects CO₂ beneath the seabed.

This form of CO₂ storage can put very large amounts of CO₂ safely away from the atmosphere. The US Geological Survey and the National Academies estimate that about 3,000 billion tons can be safely stored in rocks under the United States. This number, of course, needs to be verified for individual sites and projects, but there appears to be more than adequate capacity for the US to store negative emissions in, under, and around old oil fields and similar rocks.

The technical issues associated with storing CO₂ underground are very similar to those of oil production, which involves very similar wells, surface equipment, and safety procedures. This is good news because the skilled workforce required to rapidly scale up geologic CO₂ storage already exists in the oil industry. As the use of oil declines in the future, there is an opportunity to reemploy those workers in the carbon storage workforce, doing very similar jobs and in the same places where they do them today. This is valuable for a just transition, that is, the conversion of jobs in the old economy to jobs in the new economy that are similar in skills, location, and pay to the old jobs. If it is not possible to make these transitions, the workforce ends up suffering while the climate improves. Geologic storage is one opportunity to make a just transition for workers in the oil fields.

As with oil activities, there are safety issues with geologic storage that are being addressed in the ongoing demonstration programs. Leakage is always a concern, but to date it has not been observed (and it has been the primary focus of monitoring science). Earthquakes are also a concern, since changing the pressure on fluids underground (such as CO₂) can change the forces holding faults locked and cause induced seismicity, where the fault slips and an earthquake occurs. This effect is limited by the size of the fault—short faults can only make small earthquakes, while long ones like the San Andreas in California are capable of massive earthquakes. Clearly any CO₂ storage activity needs to take place well clear of large faults. Small faults are common in oil fields, however, and the mechanics of those faults are well understood. They often form barriers to underground fluid flow, trapping oil. They are also relatively easy to locate using seismic methods. This is another area where the expertise and monitoring equipment that was developed for the oil industry can be put to use in safely storing CO₂ underground.

Another concern is that leaking CO₂ might affect groundwater, making it slightly acidic and potentially releasing metals at higher levels than originally present. This could occur in large leaks, but in general the CO₂ is stored much, much deeper than groundwater. All CO₂ is stored below 3,000 feet, while most groundwater is held at depths of only several hundred feet. The US EPA has strictly regulated CO₂ underground storage on the basis of protecting groundwater. They require that wells be constructed to protect from leakage and that CO₂ can only be stored in rocks where the native groundwater is not drinkable (it must have greater than 10,000 ppm total dissolved solids, basically salt, rendering it undrinkable). Leaks that just return CO₂ to the atmosphere are not a safety problem, just a climate problem.

Geologic storage is one of the climate technologies that generate public concern because it is unfamiliar and occurs out of sight. However, it looks like managing the CO₂ content of the atmosphere will require geologic storage, so it is important to develop the safety and monitoring procedures appropriate for public confidence. As with many issues in climate technology, as a society we must balance the possible risk of a new technology against the known hazard of the effects of climate change. Learning about risks and being prepared to control and mitigate them is extremely important.