A BETTER WAY TO STORE CARBON

All CO2 from fossil fuels can be recovered by speeding up Earth’s natural CO2 mineralization cycle according to Cleantech Croup

“We’ve got all these great solutions capturing carbon, typically from air or industrial emitters. But capture is only half of the problem. What do we do with all that captured carbon?”

-- Buff López, Cleantech Group Associate Analyst, Materials & Chemicals

Interview with Buff López

By Suzanne Forcese

WT: What are the recognized solutions for capturing carbon?

López: The leading capture solutions are Point Source Capture (PSC), Direct Air Capture (DAC), and nature-based solutions.

PSC captures emissions from the point of emission through concentrated CO2 streams, typically above 90% CO2 purity.

DAC captures CO2 from air with large volumes of airflow or from the ocean (Direct Ocean Capture).

Nature-based solutions include forestation and soil amendments.

WT: What is done with the captured carbon? Are synthetic fuels a viable solution?

López: We've captured all this carbon, so now how do we utilize and/or store it? We can categorize utilization and storage in the short-term and long-term.

Short-term is when carbon is utilized in fuels and chemicals where there's potential for re-entry back into the atmosphere. For example, carbon waste streams may be used to produce e-fuels that release CO2 during combustion.

Long-term storage is when carbon is permanently sequestered underground or in solid form as an aggregate, such as low-carbon concrete and biochar.

In the case of synthetic fuels, we're creating efficiency by reusing carbon sources that are present in the air rather than drilling for new fossil resources. This route doesn't result in carbon removal, but it does allow for greater carbon efficiency.

The duration of carbon storage is critical to assessing the life cycle of CO₂-derived products and storage capabilities. Synthetic fuels derived from captured CO₂ are typically recombusted within one year of being captured. Therefore, synthetic fuels are not a viable long-term storage solution.

WT: Can you speak more about the duration of carbon storage and alternatives to synthetic fuels that are not a long-term storage solution?

López: Geologic carbon storage is a widely accepted long-term solution which has a global technical potential of at least 2,000 Gt of CO₂, IPCC 2018. Here, carbon is injected into underground reservoirs, mostly for enhanced oil recovery for oil and gas.

WT: What are the issues with current carbon storage methods?

López: Currently, the incumbent method is to store carbon underground as a supercritical fluid. This method is effective with small leakage percentages. But over time, even the smallest leakages spread out across global storage sites can result in combined millions of tons of carbon leaked back into the atmosphere.

WT: And the geologic solution you just mentioned – are there leakage issues?

López: The leading innovation is to mineralize carbon where CO2 is stored as a solid whether underground (GreenOre) or as a building material (CarbonUpcycling, and CarbonClean). This completely eliminates the leakage issues associated with supercritical fluids.

WT: What is sub-surface mineralization? How does the technology work? What is significant about the duration of storage time and the possibility of leakage?

López: Subsurface mineralization stores CO2 in solid form, generally as a carbonate mineral in either in-situ, ex-situ, or surficial operations. 

For in-situ operations, CO2 and water are injected underground to create a calcium carbonate derivative that stores the CO2 when reacted with calcium-/magnesium-bearing minerals like mafic and ultramafic rocks that are globally abundant.

Water is sourced from the same reservoir in which the injection takes place or seawater may be used.

Solidification can take up to two years to form a stable mineral before CO2 is permanently sequestered for millennia.

Leakage is so low that it’s essentially eliminated with instant solubility of dissolving CO2 in water. But lack of effective monitoring techniques for subsurface and surface systems exist to keep track of gas and water leakage while the CO2 solidifies. Still, no long-term monitoring is needed.

WT: What is the potential storage capacity?

López: Storage has the most potential with mafic or ultramafic (basalt, igneous, or magma) rocks because they are highly reactive and contain the metals needed to permanently immobilize CO2.

The theoretical storage capacity exceeds the total CO2 stemming from the burning of all fossil fuel-derived carbon on Earth.

Globally, the discovered storage capacity is upwards of 250 GT of CO2 in on-land basalts and up to 100 GT in submarine basalts (National Academies of Sciences Engineering Medicine, 2019).

WT: Are there regional differences that need to be taken into consideration?

López: Other critical factors such as the availability of water or permeability of the bedrock can vary greatly between regions.

Basaltic rocks vary in terms of how fractured and porous they are, which can impact the total storage space for the mineralized CO2. For example, many basalts in the U.S. do not have potential for storage due to their shallow depth, closed fractures, and high probability of fault reactivation.

Other reactive rocks such as andesites, peridotites, breccias and sedimentary formations containing calcium, magnesium, and iron-rich silicate minerals may also be feasible.

WT: Is the research ready for the field or are there still gaps that need to be addressed? What are the challenges?

López: It’s difficult to estimate the storage capacity of a well in the long-term partly because there exists a maximum rate of injectivity for a given reservoir.

The rate of mineralization depends on the amount of dissolved CO2, the presence of divalent ions in the host rock, and the alkalinity of the solution it’s dissolved in.

This step is perhaps the most limiting as researchers are trying to achieve more rapid carbonation acceleration. Moreover, utilization of heat that is generated during the process is of interest.

Still, in-situ mineralization does not require additional facilities, mining, or transportation of reactants or minerals.

Basalts are of main consideration since over 90% mineralizes within just a few months.

Researchers believe basalt systems may be self-sealing where mineralization is common at “dead-ends” thus containing itself.

WT: Can you comment on the use of water in relationship to cost please.

López: At 30 bar pressure and 20°C, approximately 22 mt of water is required per ton of CO2 that costs $10-$40 per ton.

Carbfix’s pilot facilities cost approximately $10M-$20M per year or $25 per ton of soluble gas stored using existing infrastructure at a large geothermal facility.

Free-phase CO2-based mineralization typically runs $5M per well. Costs are strongly correlated with permeability, where low permeability incurs higher costs due to larger water volume requirements. But there’s a negative correlation between cost and CO2 content thus carbon capture is attractive to increase CO2 purity.

 WT: Are there any start-ups with pilot projects that you can describe?

López: We've got 55 startups on i3 with mineralization technologies. Atmosfuture, combines its fan-less, cryogenic-based REVFRACC system (REVerse FRActionation Carbon Capture), a Direct Air Capture (DAC) solution, with CO2 utilization.

Once captured, CO2 is then mixed with calcium hydroxide to create chalk in an exothermic reaction. The resulting chalk suspension can be used to pump into depleted oil and gas wells. The chalk method can be used to reconstitute open chalk mines which are depleted or sold as part of a circular economy in building.

This draws on the momentum of leading innovators like Carbfix, known for its subsurface mineralization solution that captures carbon from point source emitters or by DAC near promising rock formations, like for geothermal projects.

Climeworks launched its largest project, Mammoth, in Hellisheiði, Iceland in 2022. It’s a DAC plant that will have an annual capture capacity of 36,000 tons of which Carbfix will be responsible for storing the CO2 underground in basaltic rocks. It’s expected to begin operations this year.

WT: In your opinion, can sub-surface mineralization recover all the CO2 from fossil fuels?

López: The theoretical storage capacity exceeds the total CO2 stemming from the burning of all fossil fuel-derived carbon on Earth.

Globally, the discovered storage capacity is upwards of 250 GT of CO2 in on-land basalts and up to 100 GT in submarine basalts (National Academies of Sciences Engineering Medicine, 2019).

WT: What perceptions and regulations will be required to pivot away from the current status quo, in your opinion?

López: Regulations will need to be relaxed once it’s understood that subsurface mineralization is more secure than those systems utilized by the oil and gas industry with supercritical (liquid) CO₂.

Likewise, education is needed to ensure the public that these systems will not harm local environments — the most significant concerns being human-induced tremors.

Despite these hurdles, the potential of subsurface mineralization to provide a safe and permanent solution for carbon storage only warrants rapid deployment.