Carbon Management 2.0: Responsible Demonstration and Deployment

The following discussion includes common questions and misconceptions associated with Carbon Dioxide Removal (CDR) and often conflated concepts with CDR and Carbon Capture and Storage (CCS). CDR and CCS are unique climate tools and both are essential elements of a broader portfolio that should be heavily weighted toward renewables, increased energy efficiency, clean hydrogen for some modes of transportation, and industry decarbonization, and other low-carbon energy technologies. There is no perfect solution when it comes to meeting climate goals globally. Holding out for a perfect solution will only further delay progress, leaving the burden on future generations.

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Q: What is the difference between point-source carbon capture and direct air carbon capture?

Carbon capture refers to separating the carbon dioxide (CO₂) from other gasses in a stream. This can be done to reduce the amount of CO₂ emitted from point-sources or can be done directly from the air. In the literature, point-source carbon capture is often referred to as carbon capture and storage (CCS), while carbon capture directly from the air is referred to as direct air capture (DAC).

Both of these methods require energy to separate out the CO₂ from the other gasses present in the stream. The minimum amount of energy required to do this separation can be calculated using the Minimum Work of Separation equation, which combines both the first and second laws of thermodynamics (Chapter 1 of Carbon Capture, Assessment of reasonable opportunities for direct air capture).

If we assume that CO₂ from a natural gas fired power plant comes out ~5% CO₂ and it needs to be separated, resulting in a 99%+ pure stream, the minimum amount of energy required to do this is 8-10 kJ/mol CO₂. On the other hand, if we look at an even more dilute stream of CO₂, the ambient air, where CO₂ is only 0.04%, to separate it out at 99%+ purity would require ~22kJ/mol CO₂. Two of the most important variables in this analysis are the concentration of CO₂ in the initial stream (i.e., natural gas power plant flue gas or ambient air) and the required purity of the separated CO₂ (i.e., 99%+). In cases where the CO₂ is not going to be transported and/or stored, but rather used as a feedstock in mineralization or as a chemical feedstock, the minimum work is reduced because the required purity is also less. The third variable in this equation that can be changed is the capture efficiency of the capture chemistry. The capture efficiency can be optimized to result in the optimal process time, least cost, least pressure drop, or even most CO₂ captured.

Carbon capture can be applied to more systems than power plants and the atmosphere. CCS can also be used to address the process emissions from industrial processes (Pilorgé et al 2020, Pisciotta et al 2022). Process emissions are the CO₂ emissions that come from the chemistry of the industrial process itself, rather than the combustion of fossil fuels. An example of this would be the CO₂ that is released as a result of calcining limestone in the cement production process. When limestone (CaCO₃) is calcined, it breaks down into calcium oxide (CaO) and CO₂. The CO₂ from this limestone reaction makes up nearly 60% of all emissions associated with cement production, so even if fossil fuels were to be replaced with renewable energy, the cement production process would not be fully decarbonized. Other industrial processes that could benefit from CCS are steelmaking, and chemical refining. In addition, these processes often have a higher concentration of CO₂ in their flue gas streams, so the minimum work to capture is lower than that of power plants and much lower than that of ambient air. For example, the cement production process releases flue gas that is nearly 30% CO₂, steelmaking 20-42% CO₂, and chemical refining 8-20% CO₂.

CCS activities in 2022 increased by 44% growing to 244 MtCO₂/yr across 61 new facilities online bringing today’s total to 30 CCS projects in operation, 11 under construction, and 153 in development. The US Bipartisan Infrastructure Law (BIL) allocated roughly $3B of funding in support of the US government to install 6 integrated CCS demonstrations, with 4 on power plants and 2 on industrial units over the next several years. There’s an additional $1B focused specifically on pilots, which are required for the more nascent CCS industrial emissions applications such as cement, paper, steel, aluminum, and glass.

In addition to the BIL, the Inflation Reduction Act (IRA) provides additional incentive for the private sector to build out 2^nd-of-a-kind to nth-of-a-kind (NOAK) demonstrations that will lead to commercial deployment of CCS. During the Obama Administration, the Recovery Act enabled the successful demonstration of CO₂ separation from ethanol and steam methane reforming for hydrogen production. Recent IRA incentives including the tax credit 45Q provides $85/tCO2 captured and stored. In Spring 2023, DOE published a Commercial Lift-Off report on Carbon Management that discusses the current status of CCS demonstrations from high, medium, to low-purity gas streams containing CO₂. The exhaust streams of power plants are in a different category than industrial streams which contain a mixture of process and combustion emissions. The industrial sources are considered medium-purity in terms of concentration, while power plants are considered dilute or low-purity in concentration, and even more dilute is direct air capture, at very low purity. Ultimately NOAK demonstrations on more concentrated exhaust streams of cement for instance will likely be less costly than the more dilute streams such as that of a natural gas power plant.

The Minimum Work, determined above, for these systems only illustrates the absolute minimum energy required. Unfortunately none of these processes is 100% efficient, so to actually run CCS or DAC, more energy than the minimum is required. The metric used to compare the actual energy required to the minimum energy required is the second law efficiency, which relates to the second law of thermodynamics. As the CO₂ in the initial stream continues to become more dilute, the lower the second law efficiency, and therefore, the higher the actual energy required to run the process (NASEM 2019 report, Chapter 5). This also means that the second law efficiency for CCS is higher than that of a DAC system.

Regardless, avoiding CO₂ entering the atmosphere will always be easier than taking it back out, but to combat climate change, both of these strategies are needed.

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Q: What are the specific roles of CCS and CDR in a net-zero framework? And are they mutually exclusive?

First, it is important to not conflate or interchange CCS with DAC. CCS is a tool for mitigating CO₂ emissions from industrial and power sources, while CDR is a climate tool for removing CO₂ from the accumulated pool in the atmosphere, and DAC associated with storage is one of many carbon dioxide removal (CDR) options.

We should not expect CDR to scale up to the extent of becoming a silver bullet solution. The role of CDR is not to offset emissions that we have the technologies for avoiding emissions, but rather to help in counterbalancing the truly hard to decarbonize sectors (e.g., agriculture, long-haul trucking, shipping, and aviation). This is emphasized in several reports (US Fifth National Climate Assessment released November 14^th, 2023; IPCC AR6 report; DOE’s Fossil Energy and Carbon Management’s Strategic Vision; and the CDR Primer Chapter 1, The Case for Carbon Dioxide Removal: From Science to Justice).

The bottom-up calculation of CDR needs, when thinking about truly hard to decarbonize sectors, results in roughly gigatons of CO₂ (Chapter 1 of the CDR Primer, DOE’s Fossil Energy and Carbon Management Strategic Vision). We are at 1000s of tons of durable CDR today and we need to scale up to millions and then to billions in less than 3 decades. If we do not invest now, then we simply will not have this climate tool in time. Indeed, climate models and reports such as IPCC AR6 make clear that CDR is a vital component for meeting our climate goals. Investing across the broad portfolio of CDR approaches is required if we are to achieve the gigaton scale that IPCC states is required by 2050.

The concept of net-zero requires to balance all CO₂-equivalent (CO₂e) emissions and all CO₂e removals. The oceans and terrestrial biosphere remove roughly half of our anthropogenic emissions each year, but at the expense of our oceans, with increased acidification. In addition, due to rising temperatures across the globe, more regions are experiencing reversal of their carbon stores through forest fires. Forests that we’ve been depending on for carbon removal are becoming significant sources (Harris et al. 2021). Net-zero takes into account these natural sinks, in addition to emissions mitigation efforts and additional CDR. Achieving net-zero occurs by prioritizing deep decarbonization alongside existing ocean and terrestrial biosphere sinks, with CDR playing a small role in counterbalancing the truly hard to decarbonize sectors. In parallel to investing in CDR and CCS, we need to invest in all other aspects of the broader portfolio, including investments in renewables, energy storage, nuclear, hydrogen, and other clean energy technologies.

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Q: How does direct air capture work, and how is doing this today?

There are three leading direct air capture approaches globally, solid-sorbent, liquid-solvent, and DAC via mineralization.

Solid sorbent materials are used similar to filters inside of the air contactor. The capture chemistry is affixed to a high-surface-area structure, similar to a catalytic converter. The air is blown into the contactor using fans, and as the CO₂ in the air makes contact with the solid sorbent material, the CO₂ becomes bound to it, while the other gasses flow out the back end. Once the solid sorbent chemistry is saturated and can no longer capture additional CO₂, the air contactor closes, a vacuum is pulled, and the contactor is heated to temperatures between 80-120ºC. Under these heated conditions, the CO₂ is liberated from the solid sorbent material and compressed for transportation and storage. Climeworks has a notable DAC facility in Iceland, where they have partnered with CarbFix to inject the captured CO₂ into basaltic rock formations underground, where the CO₂ mineralizes, forming carbonates. (Beuttler et al. 2019, Snæbjörnsdóttir et al. 2020). Other companies who use the solid-sorbent approach to DAC include Carbon Capture Inc. (Wyoming), Global Thermostat (Colorado), and Noya (California).

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(Figure from Beuttler et al. 2019)

Another leading form of DAC is the  liquid solvent approach, which is accomplished with a strong base, potassium hydroxide, (KOH) to capture CO₂. One company that is pursuing this method is Carbon Engineering. Their process starts with an air contactor that was inspired by industrial cooling towers. In this contactor, the liquid solvent (KOH dissolved in water) flows downward through structured packing material optimized for exposing it to the ambient air as it is blown into the contactor using fans. As the air passes over the liquid solvent, the CO₂ becomes bound to the KOH, forming potassium carbonate K₂CO₃. This potassium carbonate is then sent to a reactor with calcium hydroxide, where the potassium hydroxide KOH is regenerated and calcium carbonate (CaCO₃) is formed. The regenerated potassium hydroxide is sent back through the air contactor to continue capturing CO₂. The CaCO₃ is taken to a high-temperature (900ºC) reactor, where it is heated, forming CaO and gaseous CO₂. The gaseous CO₂ is removed from the reactor and compressed for transportation and storage. The CaO undergoes a hydration step, regenerating the calcium hydroxide (Ca(OH)₂) to continue supporting the regeneration process (Keith et al. 2018).

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(Figure from Keith et al. 2018)

One of the other DAC approaches is DAC via mineralization. The carbon capture reaction between the calcium hydroxide and ambient CO₂ is a natural process that usually happens over hundreds of years, known as mineralization. Heirloom, which started in the Clean Energy Conversions lab by Noah McQueen, uses calcium hydroxide (Ca(OH)₂) to capture CO₂. In the Heirloom process, the CO₂ capture chemistry is developed by heating (~900ºC) calcium carbonate (CaCO₃) to form CaO and gaseous CO₂. The gaseous CO₂ is removed from the reactor and compressed for transportation and storage. The solid CaO is sent through a hydration step, forming calcium hydroxide (Ca(OH)₂). The calcium hydroxide is then laid out in thin beds in the air contactor, where it naturally binds with CO₂ from the atmosphere, forming CaCO₃ once again. Once there is sufficient CaCO₃ formed in the reactor, the material is then processed again, liberating the CO₂ and regenerating the CaO (McQueen et al. 2023).

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(Figure from McQueen et al. 2023)

More information about Climeworks, Carbon Engineering, and another solid-sorbent DAC company, Global Thermostat can be found in A review of DAC: scaling up commercial technologies and innovating for the future.

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Q: What are the energy requirements for DAC and how can these be met to maximize climate benefit?

The leading DAC technologies today require a mix of heat and power at a roughly 80% and 20% split, respectively. The heat is required primarily for regeneration of the sorbent, while the power is required to run the fans, pumps, compressors, etc.

The solid sorbent (i.e., Climeworks) DAC technologies require low-grade heat for desorption (80-120ºC), while the liquid solvent (i.e., Carbon Engineering) and DAC via mineralization (i.e., Heirloom) approaches require high-grade heat (900ºC). The difference in heat requirements stems from the difference in capture chemistry between the approaches. In the liquid solvent and DAC via mineralization approaches, the capture chemistry are strong bases (potassium hydroxide and calcium hydroxide), while the solid sorbent chemistry is made up of weak bases (amines).

Provided that the goal of DAC is to remove CO₂ from the atmosphere, to maximize the climate benefit of this technology, the CO₂ emitted during the removal process needs to be minimized. This means looking at ways to meet the energy requirements from low- or no-carbon energy sources.

From a first pass, it may be evident that solid-sorbent DAC approaches would couple well with renewable energy because of the low-grade heat required for regeneration, whereas the other DAC approaches may face some challenges. The low-grade heat requirement does indeed provide some flexibility for sourcing this energy. Low-grade heat can be delivered via steam, which is easily produced from electricity sources; think boiling water on an electric stove. So theoretically, any renewable electricity source can be coupled to solid-sorbent DAC approaches to meet this thermal energy requirement. Specifically geothermal energy has become of interest to solid-sorbent DAC developers because of readily available low-grade heat. In addition, it can be co-optimized for sorbent regeneration in addition to electricity production. The work of Max Pisciotta in the Clean Energy Conversions lab at Penn focuses in-part on this in their dissertation work, and one configuration for this has been proposed in a publication from the Clean Energy Conversions lab, Cost Analysis of Direct Air Capture and Sequestration Coupled to Low-Carbon Thermal Energy in the United States.

In the case where renewables may not be available, solid-sorbent DAC could also make use of industrial waste heat. This is heat that is rejected to the environment from an industrial process, such as cement or steel production. However, solid sorbents often require a pre-conditioning step that may filter out co-pollutants from the waste heat stream. Siting DAC systems that have the ability to capture co-pollutants in regions where communities are experiencing air pollution could lead to significant community benefits through improving air quality.

The high-grade heat requirement for liquid solvent and DAC via mineralization approaches requires other innovations to reduce potential CO₂ emissions. In a techno-economic assessment of the liquid solvent approach, developed by Carbon Engineering, the high-grade heat step uses an oxy-fired kiln. In this way, natural gas is fed into the kiln with calcium carbonate and oxygen. Given that nitrogen is not present in the kiln, the separation process involves only CO₂ and water, which can be done easily through condensing. For every molecule of CO₂ captured from air using the solvent-based process, there are an additional 2 molecules produced from natural gas in the kiln, which leads to 3 molecules total to be stored. There has still been further innovation in the area of kiln technology and calcining specifically with Calix’s electric kilns. These kilns are run on electricity and employ a system to capture the CO₂ emissions from the calcium carbonate that is being calcined. Heirloom has since announced a formal partnership with Calix to use their calcination system for their DAC via mineralization process.

This is all to say that DAC should be operated with low- or no-carbon energy sources, but that we must also be mindful that these same renewables need to be deployed for grid decarbonization. There’s ongoing work in the Clean Energy Conversions lab where siting of DAC takes into account prioritizing renewables to first replace fossil fuel-based energy production, with DAC as a secondary priority (CDR Primer Chapter 3, Section 4). On the Federal government side, the recent DOE DAC hub announcements for $1.2B include two projects that are coupling renewables to DAC plants to meet their electricity needs. One of the two is using an oxy-fired natural gas kiln while the other is using an electric kiln technology.

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Q: How can the investment in DAC support the investment and progress in other decarbonization solutions, or are these mutually exclusive?
Strategies to reach net-zero, as outlined in the IPCC AR6 report, include a variety of options that can be classified in three categories and that can all work together to complement each other. First, it is essential to reduce emissions of CO₂, by not producing these emissions in the first place. Second, emissions from point sources that cannot be otherwise reduced, can be captured and stored (CCS). Third, emissions that cannot be addressed otherwise would be captured from the atmosphere using carbon dioxide removal (CDR).

Decarbonization efforts include the electrification of processes (electric kilns, electric vehicles, etc.), it is thus essential to decarbonize the grid, while providing reliable energy to society today. This can be addressed by deploying renewable energy (solar, wind, geothermal, etc.), and by implementing CCS on existing fossil fuel power plants. The goal is not to build new fossil fuel power plants but to retrofit existing ones and to mitigate any leakage related to the natural gas supply chain.

The US continues to produce record amounts of natural gas and hit a record output in 2023. In 2022 EIA reports roughly 2000 natural gas-fired power plants, with cumulative capacity of approximately 500,000 MW. The boiler units of many of these plants are not expected to reach retirement age until mid-century, leaving their emissions committed. These boilers could be targeted in priority for implementing CCS systems to avoid wasting existing and needed infrastructure that currently provides baseload power to the grid. Also, CCS is more efficient when collocated with viable CO₂ storage (Psarras et al. 2020), indicating that boilers with later retirement dates collocated with CO₂ storage should be prioritized for point source capture deployment.

It remains unclear how much methane leakage exists in the natural gas supply chain from production to use. Significant efforts are being made through organizations such as the Environmental Defense Fund and DOE to determine exactly how much is being emitted. In parallel to the quantification efforts, DOE is investing in a number of approaches – from collaborating with the Department of the Interior to locate and plug orphaned and abandoned wells, collaborating with EPA to provide states with the funding to close and properly plug marginally producing wells, to investing in the quantification and mitigation technology to prevent leakage.

Direct air capture systems need energy to operate, and higher TRL (technology readiness level) DAC technologies today need about 80% thermal energy and 20% electricity. The conversion of electricity to heat being quite inefficient, geothermal energy is a good candidate to provide heat directly without having to convert the energy from low-carbon sources of electricity, provided by solar PV and wind for instance, to heat. Temperatures of the geothermal resources are compatible with requirements of solid-sorbent DAC. Previous work at the Clean Energy Conversions Lab, and current work by PhD candidate Max Pisciotta has been investigating the coupling of DAC with geothermal energy. This work shows that the carbon abatement potential when the geothermal heat is used to regenerate DAC sorbents, is increased, rather than when it is used to produce electricity alone, due to the low conversion rate of heat to electricity. However, to generate electricity the geothermal reservoir temperatures  are often higher than temperatures for DAC sorbent regeneration, so this allows for co-optimization of DAC and geothermal power generation.

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Q: As DAC scales, is it going to be complex (like scaling a nuclear power plant) or simple (like scaling solar cells)?

The DAC industry is currently only operating at the 1000s of ton of durable carbon removal today. If we don't start to scale these solutions, we will not reach the gigaton (billions of tons) scale in time to meet climate goals.

As we’ve seen, the costs and performance of new technologies can change as they scale. In two very different cases, nuclear power plants are often quite complex, unique in their design, and fully integrated. Due to these factors, as they scale up, their costs increase. On the other hand, solar panels are quite simple to produce, do not require uniqueness, and are modular in design. Due to these factors, their costs decrease as they scale up.

A question that may face the DAC industry today is which of these two scaling trends is DAC bound to follow? The answer to this question is very difficult to predict because there are multiple approaches to accomplishing DAC, each with their own process considerations, optimization opportunities, and deployment strategies.

In the emerging DAC designs, there are components of the process that are highly modular and others where they are not modular. This makes the evaluation of DAC being aligned with either nuclear or solar PV to be very difficult. The CO₂ capture step of the process is modular in design, often by having multiple air contacting units operating simultaneously and being regenerated at a staggered time. In some cases, the regeneration facilities are more integrated than the modular capture devices. Another consideration when evaluating the complexity, uniqueness, and modularity of DAC is to consider the storage mechanism, which is likely to lag behind in modularity due to the limited areas in which CO₂ can be stored. The US has a large in-situ storage capacity, however, only a couple of wells allow for dedicated underground CO₂ storage today, with a fast growing list of Class VI well permit applications under review by the DOE.  The non-modularity of storage location could be addressed through mechanisms similar to the hub-and-spoke, point-to-point, or hybrid architectures used by some airlines today.

To support the initial scaling of the DAC industry, through the Bipartisan Infrastructure Law (BIL), DOE has launched a Pre-Commercial DAC Technology Prize. There are currently 429 teams registered. The recent award announcement for the $3.5B BIL provision on DAC hubs, made awards to meet industry where the technology is at today. In other words, across three topic areas, from concept, to Front-End Engineering Design (FEED), to demonstration. $1.2B was announced for 2 DAC hub demonstrations and $100M announced across roughly 20 projects spanning concept stage through FEED stage.

In addition, there are opportunities for knowledge, expertise, and in some cases capital to be leveraged from fields associated with the subsurface (e.g., petroleum engineering, geophysics, geochemistry, geology, etc.). As energy companies are aligning their assets along a path to net-zero, it is critical that ahead of demonstration, there needs to be a community engagement process that involves listening and understanding the needs, concerns, and values of a community and integrating community benefits into projects. The dollars invested through BIL at DOE require a community benefits plan. DOE’s Fossil Energy and Carbon Management recruited a world-class social scientist, Holly Jean Buck, to assist in developing materials for applications to develop robust community benefits plans. Engineers and scientists are well-skilled at de-risking the technological aspects of projects, but not well equipped at de-risking the social aspects. Through robust engagement between industry and communities, we may come to agreements that lead to benefits on both sides. In order for the once-in-a-life climate investments from BIL and IRA to be long-lasting, they need to be invested such that they take care of people. Robert Socolow and Chris Greig describe this well in their recent opinion piece in the Washington Post, Hate fossil fuels? Give them a role and get to clean energy quicker.

Overall, it is short-sighted to constrain the scale up projection of this nascent field to either solar PV or nuclear energy, because it is likely to be more DAC process dependent and there may be similarities between the challenges DAC faces and other industries which have proven economic models to solve for them. Furthermore, with the intention for DAC to scale in a way that is justice-centered, which has never been accomplished within industrial processes today, it is not unheard of that it would chart a scaling trend that has never been seen before.

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Q: In theory, DAC could be sited anywhere in the world. What practical constraints and considerations are there for responsible/strategic siting?

As air is the feedstock for DAC, DAC plants could theoretically be located anywhere in the world. However, in the real world, other constraints arise that make certain locations more appropriate for the deployment of such technologies. Here are some of the considerations that are important to take into account:

• The overarching goal is to have a more sustainable environment, and a planet that feeds everyone. DAC should thus not compete with land of high biodiversity and conservation value, with arable land that could be used for food production, or with land that could be used for other CDR strategies like afforestation/reforestation, or responsible BiCRS (biomass carbon removal and storage).
• DAC itself has specific needs in energy as discussed above. The goal being to remove carbon dioxide out of the atmosphere, the energy supply of these systems need to be provided by low-carbon or no-carbon emitters, such as solar, wind, or geothermal energy. All of these resources are geographically constrained and will thus constrain sites for DAC deployment.
• DAC should also be associated with viable storage of CO₂ to effectively remove CO₂ out of the atmosphere. These stores can be underground geologic reservoirs, or long-lived products (i.e., concrete). The collocation of DAC with these storage options minimizes the required transport of CO₂, and maximizes the CDR potential. As stated above, few Class VI wells are operating today for dedicated in-situ storage in underground geologic reservoirs. DOE’s CarbonSAFE program was put in place to facilitate the deployment of class VI wells, and many applications are currently under review at the EPA.
• Different DAC approaches perform better in different ambient environments (i.e., temperature, relative humidity, etc.). The performance of these specific approaches is then also constrained by the regions in which they can perform economically and provide a climate benefit. An example of this constraint can be seen in the water consumption for DAC. In the case of the liquid-solvent approach, in arid regions water from the solvent is lost to evaporation and will need to be supplied via water make up stream (i.e., consumption). However, in the solid sorbent approach, some sorbents co-adsorb water with CO₂, so when the CO₂ is regenerated, water from the air is also liberated and separated out, resulting in water production.
• Also, scholars and the US government are stressing the need for a just and equitable deployment of these technologies. Communities where DAC projects are planned should be able to voice their concerns and needs, and work with the project developer from an early stage in the process to better integrate local knowledge. This dialogue can also influence the land area that a community is ready to dedicate to DAC, and which land they would choose for it.

Overall, DAC has the ability to be sited “anywhere” because air is the primary feedstock. However, for DAC to be deployed responsibly and strategically, there are non-negligible decisions, considerations, and constraints.
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Q: What happens to the CO₂ once it is captured and how is it managed downstream?

As outlined in the previous question, one of the strategies to site DAC is to look at the constraints of the entire capture and storage system, and collocate the DAC plant with viable CO₂ storage. If this might be a possibility at the beginning of the scale up, the gigaton scale at which DAC needs to be deployed might not allow it to meet all the constraints.

Transportation of CO₂ is limited today, with the majority of it being transported by pipelines along the Gulf Coast (Louisiana, Mississippi), around the Permian Basin (Texas, New Mexico, Colorado, Oklahoma), and in Wyoming and North Dakota. Large scale transportation of CO₂ is often envisioned by building new pipelines. Proposed networks (Princeton Net-zero America report, Great Plains Institute) focus mainly on routing the network to accommodate large point sources for CCS and BiCRS. However, if built, these networks could also be used to transport CO₂ captured with DAC. Recently, CO₂ pipeline projects are facing public pushback and deploying an extensive CO₂ pipeline network might be more challenging than expected.

Alternatively, existing infrastructure like rail, roads, and barges could be used to transport CO₂, cutting down on capital costs, increasing the capacity and routing flexibility, and potentially saving and creating jobs. Details on these options can be found in the soon-to-be published Roads to Removal report.

Also, CO₂ transportation could be minimized by pairing sources to nearby storage options instead of planning to transport CO₂ over long distances across the country. CO₂ storage opportunities in the United States are geographically constrained by the geology of the subsurface, but sufficiently distributed to organize local CO₂ transportation networks.

CO₂ storage is mainly thought about in terms of in-situ storage in the pore space of deep sedimentary formations. Other options exist, like in-situ storage in basalts (CarbFix, Wallula, Cella) or in peridotite (44.01), surficial carbon mineralization of mine tailings (Arca), or ex-situ carbon mineralization and integration into long-lived products (CarbonCure). CO₂ storage in sedimentary formation having the highest TRL and the largest capacity in the United States, DOE has been investing in over two decades in the development and demonstration of the tools required for reservoir characterization, CO₂ monitoring, reporting, and verification (MRV). DOE’s CarbonSAFE program walks applicants through 4 phases starting with reservoir characterization in Phase 1 to a Class VI permit for geologic storage in Phase 4. EPA requires MRV plans for all Class VI wells. BIL-funded DOE integrated CCS and DAC demonstration projects have only been coupled to dedicated geologic storage of CO₂ to maximize net carbon return to the geosphere.

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Q: How doe we know that CO₂ is effectively and durably removed from the atmosphere?

From a climate standpoint, it is essential to know if the CO₂ that is claimed to be captured and stored is actually removed from the atmosphere and remains in storage. These claims are used to generate carbon credits, which then allow companies to claim a reduction in emissions or carbon neutrality. This outlines the importance of rigorous and transparent measurement, monitoring, reporting, and verification (MMRV) protocols, to ensure that removals are real, and that net-neutrality goals are truly met.

Approaches like reforestation, afforestation, improved forest management, accelerated weathering of rocks, and increased carbon storage in soils or the ocean require rigorous MMRV for these approaches to be durable and therefore dependable. We need the entire portfolio of CDR – from land, to oceans, to geochemistry and engineering. DOE is investing in the Carbon Negative Shot initiative collaboratively across DOE with a focus on developing robust measurement, monitoring, and verification for durable CDR. The US government is working in a coordinated way to couple real projects on the ground with the hardware and modeling required to assess impact, e.g., NOAA, DOI, EPA, USDA, and DOE.

Microsoft has been a frontrunner among corporations in articulating with MRV framing the distinction between low durability, medium durability, and high durability CDR. Examples from their high durability biomass sourcing principles include avoiding harvest from protected areas, avoiding the use of biomass that would have an otherwise higher-value, e.g., in long-lived wood products, and sourcing only from sustainable harvest of areas with stable or increasing forest carbon stocks. Carbon Direct and Microsoft published a Criteria for High-Quality Carbon Dioxide Removal in 2023. A detailed list of Microsoft’s 1.5MtCO₂ removal contracts is available online. This transparency allows for other corporations to learn from and hopefully adopt Microsoft’s principles for responsible CDR as this market is primarily voluntary today. In September 2023, Microsoft signed a long-term contract to purchase up to 315,000 tons of CO₂ removal with Heirloom, a DAC company that uses limestone as a feedstock. The technology was developed in the Clean Energy Conversions lab at Penn in collaboration with Peter Kelemen at Columbia University, Phil Renforth at Heriot-Watt, and Greg Dipple at the University of British Columbia. Heirloom’s co-founder Noah McQueen is an alumnae of Penn and the Clean Energy Conversions lab.