Varin's Blog

CDR Overview

Description of Methods

There are five general methods of CDR, although within each method there may be significant variation in materials and processes used. These are:

1. Carbon Capture and Storage (CCS)
   1. Post-Combustion CCS
   2. Pre-Combustion CCS
   3. Oxyfuel Combustion CCS
2. Bioenergy Carbon Capture and Storage (BECCS)
3. Direct Air Capture (DAC)

CCS generally refers to the process of capturing carbon dioxide directly from streams of gasses emitted by industrial processes. CO₂ is far more concentrated in these streams called “flue gas streams” (Global CCS Institute, see Table 4) compared to the average atmospheric concentration of CO₂, which is currently ~418 parts per million (NOAA, climate.gov). CCS is sometimes used interchangeably with the term CDR, although they do not have the same meaning; the latter generally refers to the process of removing carbon dioxide from the atmosphere regardless of specific method.

As the list above shows, CCS breaks down into three general types: post-combustion, pre-combustion, and oxyfuel combustion CCS. The first method, post-combustion CCS, is the most widely used and developed, having first been patented in 1930. In the past, it has been used for other gasses such as carbon monoxide and hydrogen sulfide (the latter of which was the main component of acid rain) as a process generally called “gas sweetening”, and recently for CO₂. In this method, flue gas streams pass through an “absorber column” containing a liquid that gets saturated with the target. Once the liquid is fully saturated, superheated steam is passed through the absorber column, which releases the CO₂ for transport elsewhere.

Pre-combustion CCS is generally used with a type of power plant called an integrated coal gasification combined cycle (IGCC) power plant. In IGCC power plants, coal is turned into “syngas” which consists mainly of hydrogen and carbon monoxide. These react with water to form CO₂ and more hydrogen gas. The hydrogen gas is used for energy production, and the CO₂ is sequestered then transported or stored.

Oxyfuel combustion typically refers to a process used in the steel industry and welding, however this process can easily be combined with CCS. In this method, oxygen is separated out of other atmospheric gasses; in welding and the steel industry, this highly concentrated oxygen is useful because it creates a very hot flame. Since it is the oxygen in air that reacts to form CO₂ and water vapor when combusted, the resulting flue gas will only contain CO₂ and water vapor. The proportions of the amounts of the two gasses in the resulting mixture depends on the hydrocarbon used for combustion. (In the oxyfuel process used for welding mentioned in the link above, acetylene (C₂H₂) is combusted with oxygen. This particular reaction is 2(C₂H₂)+5(O₂) -> 2(H₂O)+4(CO₂) , thus ⅔ of this reaction’s products are CO₂). Since most of this gas is CO₂ (and water vapor is itself a mild greenhouse gas), the whole mixture can be sequestered.

The two other methods do not necessarily capture CO2 directly from industrial contexts. Bioenergy Carbon Capture and Storage (BECCS) is a method that utilizes organic material (i.e. biomass) in industrial processes (e.g. ethanol production) and captures the carbon produced from it (typically using the same technologies as those used to trap the CO2 in CCS). This actually leads to a net-CO2 removal from the atmosphere, since the CO2 in the biomass was captured by natural processes. Direct Air Capture (DAC) is somewhat functionally similar to CCS in that air is typically channeled through some kind of material that separates the CO2 from everything else. However, DAC differs in the key aspect that it captures ambient air directly from the atmosphere rather than from industrial processes.

In addition to the carbon capture methods mentioned above, there are also many methods of storing and using the carbon dioxide. Methods of storage and utilization include:

1. Enhanced oil/gas recovery (EOR/EGR)
2. Enhanced coal bed methane recovery (ECBM)
3. Depleted oil/gas field storage
4. Saline formation storage
5. CO2 mineralization
6. Ocean storage

In EOR, captured carbon dioxide is used to extract crude oil and natural gas that may otherwise have not been extracted. The CO2 is channeled into seemingly depleted reservoirs, where it displaces the remaining oil or gas in the reservoir, allowing for its extraction. A similar method is used for ECBM, in which CO2 is channeled into a coal bed, displacing the methane within.

In the other 4 methods, CO2 is stored rather than used. It may be stored in depleted oil/gas fields (not necessarily with the intent of extracting additional oil or gas, as with EOR/EGR). It can also be stored in saline aquifers, which are underground porous rock formations containing brine, or salty water. It can also be stored in the ocean, which naturally traps CO2 as part of the carbon cycle, or in basalt rock formations in volcanic areas underground, which react with CO2.

Analysis of Methods

As mentioned in the previous section, post-combustion CCS is currently the most well-developed CDR method, having been used for decades. This makes it easier to implement as the industries implementing it would be familiar with this method of CDR. However, as the primary aspect of all CCS methods is that it captures CO2 directly from industrial processes, it is not able to sequester CO2 emitted from any other sectors such as transportation. Additionally, CCS is carbon net-zero but not net-negative, since it merely prevents certain CO2 emissions from entering the atmosphere but does not remove additional emissions. Nonetheless, it is currently the most effective and widely utilized method of CDR today.

Pre-combustion CCS, although novel and interesting, is currently in scant use, and it has not been shown to be as effective in capturing carbon or in cost. For example, the Kemper County IGCC power plant began construction in June 2010 and was set to be completed in 2014, but after many delays and hikes in cost of billions of dollars (from an estimated $2.2 billion in 2004 to $7.5 billion in 2017), was eventually made a natural gas plant. In addition, this method of CCS only works with the specific type of coal plant that separates hydrogen and CO2 in coal syngas, using them for energy production and storage respectively. The most common method of energy generation from coal involves pulverizing, then burning it without further processing. So, this method cannot be applied to existing coal plants, and of course, nor can it be applied to any other industrial process besides coal power generation. The few plants that were ever operational (such as the one in Puertollano, Spain, a test plant active from September 2010 to June 2011) do not absorb as much carbon dioxide; where as the IGCC power station in Puertollano absorbs 100 tons CO2/day (therefore around 36,500 tons/year), whereas the Sleipner post-combustion CCS gas p rocessing station in the North Sea, constructed in 1996, absorbs 900,000 tons/year and is still active today. Since Sleipner was constructed, many more post-combustion CCS plants have become operational, some of which absorb far more CO2 than Sleipner (such as the Alberta Carbon Trunk Line, which is absorbing 14.6 million tons/year since June 2020). If pre-combustion CCS will ever become feasible, we are a very long way from it becoming such.

The oxyfuel combustion process, as mentioned earlier, is already widely used in cutting metal and welding, but is not used so much outside of these industries. So, while oxyfuel combustion CCS can easily be implemented in these existing systems, such is not the case for any fuel combustion processes that do not currently use oxyfuels. In addition, they consume a lot of energy; separating just one ton of oxygen from air uses 192 Megajoules (51.3 kilowatt-hours) of energy. The average household in the US uses 10,632 kWh of energy per year, which is around 29 kWh per day — around half the energy consumed to separate just one ton of oxygen from air.

BECCS is, in a sense, the most well-developed of all methods (excluding the CCS part). After all, we have been using biomass to generate heat by burning firewood for hundreds of thousands of years. Today, there are multiple applications of the use of organic material, such as medicine, but the primary industrial one is in the production of ethanol. BECCS may also greatly limit the amount of waste we produce, particularly food wastage, since if any organic material can have a potential usage in industry, it limits the impact of how much we might waste. Furthermore, integrating CDR into these processes to birth industrial BECCS, unlike the CCS approaches, is actually net-carbon negative, even though the details of absorbing the carbon may be similar to those used in CCS.

This is because fossil fuels combusted in other industrial processes are not really involved in the carbon cycle, because they are supposed to act purely as carbon “reservoirs” for excess carbon from which it does not exit. Therefore, when we extract and combust fossil fuels (as opposed to anything else), we are combusting material that should never have reentered the carbon cycle, and by depleting such reservoirs making it more difficult for such excess carbon to reaccumulate underground. Thus, sequestering the atmospheric emissions of this carbon merely sequesters emissions that were never meant to be emitted again, causing neither net harm nor net gain to the environment. Biomass (such as firewood or residual agricultural biomass such as corn husks) also contains carbon, however it was generated partly due to the constant passage of carbon around the carbon cycle (for example, trees grow using photosynthesis, which requires carbon dioxide). Such carbon differs from CO2 entering the atmosphere from fossil fuel combustion in that the CO2 within was going to be dispersed through the carbon cycle eventually anyway. For example, when trees die after absorbing CO2 from the atmosphere for years, their carbon is released into the atmosphere as CO2 when they decompose. Similarly, when we eat food (which is organic and therefore also biomass), the carbon within it either accumulates in our bodies or is released when we exhale it as CO2. As a result, such processes are much less harmful. Therefore, this means that using biogenic sources of fuel (biofuels) in industrial processes is more-or-less net-zero compared to using fossil fuels, so it follows that if we were to sequester the CO2 emissions from combustion of biofuels, this would lead to a net-CO2 removal. For an explanation of the importance of net-carbon negativity versus net-zero, see Policy Recommendations.

DAC is a very novel and promising method, and has received a lot of hype and funding from other organizations. DAC is technically also carbon net-negative, since it extracts ambient air from the atmosphere, which may include naturally present CO2, past anthropogenic emissions, or current ones, and is therefore not simply preventative. However, while DAC may be similar to post-combustion CCS at a very high level, it is currently far less effective than CCS. This is because the air being captured by a DAC facility has a far lower concentration of CO2 than the flue gas coming out of a smokestack (the atmosphere has a CO2 concentration of around 417 parts per million (ppm), whereas as the CO2 concentration in flues can range from 1-33% (see Table 2), which is far higher than 417 ppm though it may seem small). With the funding and hype DAC has received, as well as new advances in its ability to absorb CO2 (see next section), DAC nonetheless seems like a promising approach to CDR.

While EOR, EGR, and ECBM are certainly creative ways of utilizing sequestered CO2 (and have already been used for decades in some areas), they will accelerate the development and usage of fossil fuels in industry by making it easier to extract larger amounts of oil, gas, and coal. As described later in the Policy Recommendations section, we must begin phasing out these technologies rather than maintaining them, let alone developing them, so as to become net-carbon negative. Simply storing CO2 in depleted oil/gas fields, however, is most likely far simpler and far more sustainable. After all, since most anthropogenic CO2 emissions originated from oil and gas reservoirs, it may seem natural to return them to these same reservoirs. Two likely similarly promising methods mentioned earlier are saline formation storage and mineralization. Saline formation storage can be combined with desalination plants to draw water from these formations and make it potable, while also filling the porous formations there with CO2. CO2 mineralization reacts CO2 with volcanic basalt rock deep underground, and also provides a permanent solution to CO2 storage. Although this can only happen in regions where basalt can be found (such as volcanic regions), such areas are plentiful around the world, and it is estimated that there is enough basalt beneath Europe to store 4,000 billion (4 trillion) tons of CO2.

The feasibility of ocean storage is less clear. In order for it to be executed, DAC facilities must be along the coast, otherwise unnecessary energy will be spent on transporting CO2 to the ocean. Furthermore, while the ocean is indeed part of the carbon cycle as a sink, if too much CO2 accumulates in the ocean it will lead to ocean acidification, in which carbonic acid is formed from a reaction between CO2 and water, eventually leading to an increase in the ocean’s hydrogen ion concentration. Ocean acidification stunts the growth of certain marine organisms and disrupts the marine food web. In a pre-Industrial Revolution carbon cycle, carbon would disperse into the deeper parts of the ocean, into seafloor sediments, and perhaps back into the atmosphere at roughly the same rate at which it enters, so ocean acidification would not be an issue. However, fossil fuel combustion pours far too much CO2 into the atmosphere, and consequently into the ocean, for those processes to keep up, causing ocean acidification. Placing the rest of our excess CO2 emissions in the ocean will only accelerate this, thereby making oceanic storage a very unsustainable method of storing CO2.

Recent Developments

Post-combustion CCS, as mentioned before, has received plenty of attention over the past many decades. The largest CDR facility in the world so far, known as the Alberta Carbon Trunk Line (ACTL), became operational in June 2020 and absorbs 14.6 million tonnes of CO2 per year. The ACTL utilizes its CO2 for EOR and permanent storage in depleted oil and gas reservoirs. Other smaller facilities include the Boundary Dam project in Saskatchewan absorbing 1 million tonnes per year at 90% efficiency. Outside of Canada, some large plants include the Sleipner CCS project in the North Sea, absorbing 0.9 million tons/year since 1996, and the Petrobras Lula project sequestering 1 million tons/year since 2013. One plant operated by ExxonMobil in Shute Creek, Wyoming, captured around 7 million tons/year, however rather than capturing it from flues, the CO2 is captured from the natural gas itself, which is unusually rich in CO2 in Shute Creek and not useful for energy production. The plant became operational in 1986, however shamefully it was recently found that they sell nearly half of their CO2 for EOR and release the other half back into the atmosphere, thus although this plant otherwise shows great advancement in CCS as far back as 1986, it certainly can not be considered sustainable or effective.

Pre-combustion CCS, as mentioned before, has seen little recent developments and proper application. The few plants that have successfully demonstrated it (such as the one in Puertollano mentioned earlier) have since been discontinued. One other demonstration plant at Buggenum, Netherlands absorbed 4,478 tons of CO2 over 2 years, which was followed by the development of the Nuon Magnum plant by the same company, Vattenfall, before being sold to the company RWE. However, although it was an IGCC plant, it is not known if it sequestered CO2 emissions as well.

Oxyfuel combustion in general has seen widespread application, as mentioned earlier. Oxyfuel combustion has received quite a bit of attention, although not nearly as much as post-combustion CCS. One major oxyfuel CCS plant called FutureGen 2.0 was planned to absorb 1.1 million tons of CO2/year, but didn’t receive adequate funding from the US government, so it was discontinued. A plant that uses oxyfuel combustion CCS with a novel method that recycles the CO2 for more effective combustion called the NET Power station became operational in 2018. The process effectively produces high-CO2 concentration flue gasses, but it is not known how much CO2 it captures each year.

BECCS has seen a few breakthroughs recently, as has the use of biomass in industry itself. Biomass has seen wide use in co-firing in a kiln for cement production, firing in a furnace for steel production, hydrogen production from biogas and steam, biomass production for heat, usage for bioethanol, and more. A major bioethanol plant in Decatur, Illinois aims to capture around 2 Mt/year over a period of 2.5 years (although it is linked to a second facility that does not use CCS, which will emit more carbon than the neighboring BECCS facility will absorb). On a much larger scale, Summit Carbon Solutions aims to implement CCS and storage in 30 ethanol facilities across five states (North Dakota, South Dakota, Nebraska, Iowa, Minnesota). An Ørsted bioenergy plant in Denmark is promised to absorb 430,000 tonnes of CO2 per year. Biofuels are to be used with CCS at a cement plant in Slite, Sweden, which will capture 1.8 million tonnes of CO2 per year, and a pulp mill that uses BECCS is absorbing 1.3 million tonnes per year. While there are certainly many plants, some of these (such as the integration of CCS at the biocement plant in Slite) have not yet been fully implemented. However, if other promises for BECCS made by other organizations (such as those made recently by the UK to absorb 5 MtCO2/yr from BECCS by 2030) are included, BECCS shows rapid development and provides another feasible avenue for CDR across sectors.

DAC, as mentioned earlier, is currently one of the most hyped CDR methods. Companies, particularly new startups working on it have received a lot of funding. Swiss startup Climeworks, for example, raised $650M in a recent equity round. The innovations themselves, however, are far behind the absorption rates of CCS and BECCS, in part due to the fairly recent beginning of development on DAC projects. The largest DAC facility in the world today, called Orca and built by Climeworks in Iceland, went operational in September 2021 and absorbs 4,000 tonnes of CO2/year. A plant in Texas constructed by Carbon Engineering/1PointFive is set to be completed in 2025 and to absorb ~500,000 tonnes of CO2/year. This means that, currently, the absorption rates of DAC technology are dwarfed by those of CCS and BECCS, although they have shown a rapid ability to scale up. Orca was an improvement upon Climeworks’ first facility built in 2017, which absorbed only 50 tonnes of CO2/year, and another one that absorbed “several hundred” tonnes. A new one called “Mammoth” is set to be completed “within 18 to 24 months [as of June 2022]”, therefore in early 2024, and will absorb 36,000 tonnes/year. Nonetheless, it is most likely too early to say whether DAC will become as effective as technologies like CCS and BECCS are now, but with more development and support we can know this for certain soon.

EOR/EGR is used by many CDR projects. For example, the Alberta Carbon Trunk Line and Boundary Dam projects mentioned earlier use some of their captured CO2 for EOR, and Carbon Engineering’s DAC facility will likely use its captured CO2 for EOR as well. In addition, the ACTL uses its leftover CO2 for storage in depleted oil fields. Project Greensand also stores CO2 captured from its associated facilities in a depleted oil field beneath the North Sea. Multiple CCS ventures store their CO2 in underground saline formations as well, such as the Sleipner project and the Petrobras Lula project. The company Northern Lights is developing a project called Longship which stores CO2 in saline formations beneath the sea, and it is already working with HeidelbergCement (the company behind the aforementioned biocement plant in Slite), and has integrated CCS and saline formation storage with a cement plant in Brevik, Norway. Companies such as Carbfix and 44.01 are focusing on CO2 mineralization as well, and Carbfix in particular has already begun mineralizing CO2 with basalt in conjunction with Climeworks’ Orca project. The only storage method that has not received attention so far is ocean storage, though this is likely for the best given this method’s potential to cause ocean acidification. In addition, given how well developed, it is safe to say that once CDR methods are scaled up, finding methods to store or utilize CO2 will be less of an issue due to the development in tandem of such methods.

Policy: Recommendations and Examples

The bottom line of the sea of complex terms, analyses, and statistics that constituted the previous section is that we must do a lot of work to rapidly develop new ideas and scale existing ones with regards to carbon dioxide removal and storage. While it may seem impressive that, for example, the ACTL captures 14.6 million tonnes of CO2 each year (and it is certainly impressive), here’s one last statistic: in 2022, humans emitted 36.8 billion tonnes of CO2 into the atmosphere, and that number is only increasing each year. The ACTL would only have absorbed that much 2,500 years from now. For reference, 2,500 years ago the Roman Republic was founded, and Egypt had been conquered by the Persians, and paper had not yet been invented. Therefore, support from organizations around the world for CDR is imperative immediately, and especially support from world governments.

One thing that is clear is that all approaches of CDR and CO2 storage/utilization will be needed, not only to maximize the scope of innovation in these fields and advance these technologies further faster, but recall how post-combustion CCS, despite being the most well-developed CDR method, has the problem of being CO2 net-zero, and not net-negative unlike BECCS and DAC. Becoming net-negative is important because it is not the fact that we are adding greenhouse gasses to the atmosphere that is harming the environment, but the fact that an excess of it is there to begin with. This means that in order to lower global temperatures to normal levels again, we will also need to remove all of the excess CO2 that we have added to the atmosphere, and to this end post-combustion CCS is not helpful. This means that world governments must support the development of all CDR technologies.

People have proposed that CDR is not necessary, because decarbonization of our industry is a much better idea to achieve a net-zero reality, whereas others have proposed that CDR is the only viable route to this end because decarbonization is simply not feasible in the limited amount of time we have to act. In reality, these two things are not mutually exclusive, and are in fact both necessary to become CO2 net-negative as soon as possible. CDR is needed to remove current emissions and past anthropogenic emissions from the atmosphere, and decarbonization of our industries is needed to ensure we do not add further emissions to our atmosphere and that human lifestyles can truly become more sustainable. So, for example, investments in renewable energy projects such as new geothermal plants can and should go hand-in-hand with investments in DAC facilities. In fact, this scenario is already happening; Climeworks’ Orca facility runs purely on energy from a nearby geothermal plant.

In the end, with every second we spend not acting to reduce our carbon emissions, we lose something. Perhaps we lose some land to rising sea levels, and perhaps a home, a village, a picturesque beach, a fertile delta, or even a whole culture is lost too. Perhaps we lose some forested land to a wildfire, and perhaps some buildings or maybe even a whole historic town too. Perhaps a city gets inundated, and perhaps a few people are killed by falling trees. Perhaps temperatures become so record-shatteringly hot that even the floor can cause you third-degree burns. With every loss, people try to adapt. But wildfires will keep getting bigger, heat waves will get hotter, storms will become more intense, and therefore human suffering and quality of life will only deteriorate. Yet we still have enough time that, with the right combination of policies, investments, and innovation to decarbonize our world through CDR and renewable energy, we can truly steer away from this future.