Long-term carbon storage
Long-term carbon storage

Long-term carbon storage

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TL;DR

  • We know how to capture carbon from the atmosphere, now we need to decide what to do with it. The goal is to lock it away permanently, or for as long as possible, all while inputting as little energy as possible.
  • Trapped CO2 itself also has value as it can be transformed into various useful industrial products. In this case, we are locking up the CO2 and displacing the production of new (usually very high emissions) materials.

Carbon storage

There are many ways we can draw carbon out of the atmosphere. From direct air capture to nature-based solutions like reforestation and algae farming, we have the tools and techniques at our disposal. The next question at hand is, what do we do with all of that carbon once we have captured it from the atmosphere? The best case scenario is for long-term, or better yet permanent, utilization and storage of carbon to ensure we don’t release it back into the atmosphere.

Source:
Source: Science, 2018

Permanent storage vs carbon neutral utilization

There are two primary ways to look at what happens to carbon once we have removed it from the atmosphere. We can either lock it up permanently, or we can turn it into something that will eventually re-release that carbon back into the atmosphere. The latter case isn’t necessarily bad because it wouldn’t be adding more carbon to the atmosphere (like burning fossil fuels does), but the longer we can lock it up, the better off we will be.

A very non-scientific generalization of the permanence associated with a variety of carbon storage options. The position of each method on this scale can vary greatly depending on how it is employed and how the material is used.
A very non-scientific generalization of the permanence associated with a variety of carbon storage options. The position of each method on this scale can vary greatly depending on how it is employed and how the material is used.

Methods of carbon storage

Mineralization

This technique uses the natural process of silicate rock weathering that traps CO2 permanently into mineral structures—weathering being the breakdown or deterioration of rock material, in this case, silicates like basalt.

When it rains, some of the CO2 in the atmosphere gets mixed up with water, forming carbonic acid that dissolves rocks. Much of that dissolved material eventually runs into the ocean, where another reaction takes place, and the rock particles react with CO2 in the ocean water to produce carbonate minerals that finally lock up the CO2.

The natural process works very slowly, trapping around 1 gigatonne of CO2 annually, but we can speed this process up by crushing rocks and exposing more of the surface to the atmosphere (even faster if heat is applied).

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Rain + Rocks = trapped carbon AND ⬆ rock surface = ⬆ trapped carbon

Enhanced weathering

Also considered a method of direct air capture, the general idea is to grind up a silicate-rich mineral material and spread it out. Grinding the material exposes more of the surface to the air, allowing CO2 to bond with it and speeding up the extraction process. This is a permanent removal process, locking the carbon up for tens of thousands of years, no matter if the material is distributed on land or in and around the oceans.

One of the proposed methods of enhanced weathering is to spread the ground-up silicates over beaches and shorelines because seawater helps to dissolve the minerals, further enhancing the weathering process. These minerals also increase the alkalinity of the ocean surface, and that can draw down even more atmospheric CO2 by enhancing natural ocean carbon cycles.

  • This process could help reverse the problem of ocean acidification, but if done too rapidly would likely harm ocean ecosystems. More research is needed to understand the potential impacts on ocean ecology.
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Alkalinity refers to water’s ability to stay neutral. Oceans contain alkaline minerals that help remove CO2 from the atmosphere and keep water pH around 8. As CO2 concentrations have increased, that alkalinity has decreased (this is known as ocean acidification).

If the crushed rock material is scattered on croplands, enhanced weathering may have potential agricultural co-benefits. The rock dust can help increase plant growth rates and even rebuild degraded soils. Experiments are ongoing, and caution must be taken around what rock types are used since some contain heavy metals that can be toxic if ingested.

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In the US, organic farmers can supplement organic matter with "other natural products such as mined minerals." So while rock fertilizer itself likely couldn't be sold as organic, farmers wouldn't lose their organic status.

Industrial mineralization

Ground-up silicate-rock material is added to a reactor where heat and pressure speed up CO2 uptake. The process itself actually generates energy that can be harvested to reduce energy demands, and the resulting material can be used to create valuable products (see below).

  • Modular reactors offer the benefit of being able to process CO2 on-site, preventing the need for transporting liquid CO2 over long distances. Three proposed CO2 pipelines in the US have met resistance from local stakeholders, environmental activist groups, and conservative farmers that do not want the pipelines to be built across cropland.
    • Source:
    Source: Paebbl

In-situ weathering

CO2 dissolved in water (carbonated water) can be injected underground, allowing the mineralization process to happen there rather than bringing the rock to the surface. The carbonated water is acidic, which drives the weathering process and locks up the CO2 into carbonate minerals. There is, however, a large amount of water required (35 to 100 tons of water per ton of CO2) for the dissolution and injection processes which would be problematic for water-stressed regions.

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Injecting large amounts of water underground carries a risk of earthquakes, so shouldn’t be done near populated areas.

Mineralization of industrial residuals

Industrial residuals can be used for mineralization (in place of silicate rocks), which also offers a way to manage industrial waste. Since these industrial processes emit CO2, it should be possible to combine mineralization with on-site direct capture.

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Sources: Iron and steel slag, fly ash (a byproduct of burning coal), mining waste, cement waste, gypsum (used in drywall and paper production, for example) waste, etc.

Utilization

There are energy demands when it comes to mineralization (just crushing the rock requires energy input, and then it needs to be transported), but utilization offers a way to minimize its carbon footprint by effectively substituting the need for other carbon-intensive ingredients. The CO2-enriched material created by mineralization processes can be used in the manufacturing of a variety of valuable industrial products, offering additional economic value to storing CO2, as well as displacing would-be emissions.

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The market for CO2-based products is expected to reach between $800 billion and $1 trillion by 2030.

Paper

Lime (calcium oxide) is used heavily during the paper-making process as treatment, filler, and even to neutralize acidic wastewater. It is possible to replace the lime in this process with CO2-enriched mineral materials and still maintain the same quality product.

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Replacing lime is a great example of displacing emissions. Lime is created by heating limestone to extremely high temperatures. In this process, you’re not only using fuel, you’re releasing CO2 trapped within the limestone as a byproduct of the process itself.
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Source: Calix Global

Polymers

Captured CO2 can be converted into polymers for use in plastic and foam production, with the potential to lock up 10 to 50 million tonnes of CO2 by 2050. When used to create polyols (for polyurethane production), every tonne of CO2 used results in 3 avoided tonnes from the alternative production process.

Cement

Cement production is responsible for 8% of global CO2 emissions, and more than half of this comes from making "clinker," the binding agent in cement (this is because clinker is typically made by burning limestone like the lime process above). Clinker can be fully or partially replaced using CO2-enriched minerals, and this substitution could displace up to 1.5 gigatonnes of CO2 per year (about 3% of global GHG emissions).

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Click here to read our open research on climate and cement.

Synthetic fuel

Also known as eFuel, synthetic fuel is made by combining water and carbon. The process of creating eFuel involves many energy-intensive steps, involving higher energy demands than any other application of captured CO2, and the CO2 is re-released as the fuel is burned.

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To power a car: Only 8-18% of the initial energy that goes into producing eFuels will actually go towards moving the car compared to 70% for an electric vehicle. That means up to 92% of the energy that goes into producing eFuels is wasted.

Deep ocean

Our oceans absorb around 25% of the CO2 humans generate, but as emissions increase, so do the negative consequences on ocean life. Additionally, as ocean temperatures continue to rise due to global warming, the amount of CO2 that can be absorbed decreases. Steps can be taken to lock up ocean CO2, improve the health of our oceans, and ensure this vital function carries on.

Grow seaweed and then sink it

Here we are specifically talking about growing seaweed to sink it, not to use it. Seaweed and other algae can also be grown for other purposes: food, cosmetics, fertilizer, etc.

Seaweeds are good at capturing CO2 because they generate more organic material than gets used up within their ecosystems. Globally, wild seaweed sequesters about 173 million tons of carbon yearly, and farmed seaweed could potentially trap another 6.8 million tons of carbon (equivalent to 2.48 million tons of CO2). To permanently remove that carbon from circulation, seaweed needs to be sunk in ocean waters with depths greater than 1,000 meters, and it needs some help to get there. Looking at kelp (which grows in depths of less than 30m), only up to 22% has the potential to reach the deep ocean on its own.

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Source: Harvard University
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Click here to read our open research on climate and seaweed.

Injecting CO2

Another proposed method has been to take compressed CO2 and inject it into the deep ocean (but this has generally been abandoned for the reasons listed below). The general idea is that the high pressure at ocean depths greater than 1000 meters will prevent the gas from traveling back to the surface, and it will eventually form “CO2 lakes” on the seafloor. Alternatively, CO2 can be injected directly into former oil and gas reservoirs under the seafloor, which is still being explored.

  • Increasing the concentration of CO2 at the seafloor can have negative consequences for ocean life, in particular, the microscopic life that makes up the foundation of the ocean food web.
  • There is turnover within the ocean where deep water comes to the surface and vice versa. This means that any CO2 injected into deep ocean waters will come to the surface waters about every 500 years, so this is not a permanent option.

Soil

Soil carbon sequestration is one of the cheaper methods available to us, but permanence is the big question here. For the most part, carbon trapped in soils only stays there as long as it remains undisturbed—something easier said than done.

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Source: University of Nebraska-Lincoln, Institute of Agriculture and Natural Resources

Plants take in CO2 from the atmosphere and use it to grow. If part of the plant is left in the soil when it dies or after harvesting, it can become part of the soil, essentially trapping the carbon. When exposed to oxygen, microorganisms within the soil will break down plant material and release CO2 back into the atmosphere.

Regenerative agriculture

Regenerative agriculture is a departure from standardized industrial food production methods and prioritizes customized, close relationships to the land being farmed. Much of the theory behind regenerative agriculture is around minimizing soil disturbance and limiting soil exposure to the air. Here are some of the methods associated with this land-management practice (more here):

  • Cover cropping: After the primary crop is harvested, a cover crop like clover or radish is planted and allowed to die and decompose into the soil. This can add nutrients and prevent weeds, but you need to be sure to use the right cover crop for the primary crop and environment (otherwise, you might end up depleting nutrients or attracting pests, or even having the cover crop become a weed and hard to control).
  • Silvopasture/Regenerative grazing: Preventing overgrazing by keeping animals in tightly packed herds, and moving them from area to area, allowing the grazed areas time to rest and recover. This method of grazing reduces soil degradation and erosion, and can actually increase the nutrient content of soil if done properly.
  • Agroforestry: Growing crops under the forest canopy. Trees can provide protection from direct sunlight and wind, and more uniform temperature/humidity conditions. Depending on the types of trees grown, they can also provide a secondary source of income (fruits, nuts, wood products, etc.).
  • No-till farming: Leaving plant roots and soil organisms to do what they are designed to do. This naturally aerates and adds nutrients to the soil, reduces weathering, and conserves water. It is important to note that this is not universally applicable, and carbon storage is increased in only certain types of soils and environments—in some cases, it may actually reduce carbon in deeper soil.
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The USDA has found that farmers are more likely to alternate between no-till and till than to practice continuous no-till, and 1/3 of farmers who practice no-till will eventually till the soil after a few years, releasing much of the carbon that has been sequestered.

Biochar

Farms produce enormous amounts of biomass waste that, when left to decompose, releases GHGs. Instead, this biomass can be turned into biochar, a charcoal-like soil amendment that is created by heating the biomass in an oxygen-poor environment. Biochar prevents about 50% of the emissions, locking up the carbon that was sequestered while the plants were growing.

Honorable mention: Mass timber

Not directly made out of captured CO2, but worthy of a mention since the material itself locks up carbon for an extended amount of time (versus using wood for fuel or short-term products like paper). Mass timber products are wooden construction materials that have been engineered for strength and durability to replace steel and concrete in building construction for less emissions-intensive construction projects (up to 77% less).

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Source: Monash University

The timber industry is a large driver of emissions via forest harvesting and land-use change, so responsible forestry will be very important here. Recent research shows that it is possible to have a net-positive benefit when switching over to mass timber in construction, especially with investments in yield-increasing technologies (and avoiding the use of wood as fuel!).

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Click here to read our open research on climate and forestry.

Resources

Last updated: Oct 2022