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What is carbon capture?

Refineries — in a sense, energy facilities because they produce the transportation fuels that account for a large and growing share of the world’s energy-related CO₂ emissions — would be ideal candidates for carbon capture because they are so energy-intensive themselves in terms of both electricity and thermal energy. But the costs would be huge. One study has shown that post-combustion CO₂ capture for a large refinery was technically feasible, but put the costs for CO₂ capture at $50–60 per metric ton of CO₂ captured. That would put the bill for CO₂ capture at that hypothetical refinery at more than $100 million per year. That same study considered subsequent geologic sequestration of the captured CO₂ for enhanced oil recovery, which could bring in new revenues to help offset some of the costs of CO₂ capture and storage.

What is carbon capture?


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Carbon capture refers to the separation and capture of CO₂ from emissions point sources or the atmosphere and the recovery of a concentrated stream of that CO₂ that can be feasibly stored (sequestered) or converted in such a way as to mitigate its impact as a greenhouse gas. For all practical purposes, it entails the capture of CO₂ from stationary sources, such as fossil fuel-fired power plants and industrial facilities. Research efforts are focused on systems for capturing CO₂ from coal-fired power plants because they are the largest stationary sources of CO₂. Although current R&D emphasizes CO₂ capture in coal-fired power plants, the carbon capture technologies to be developed will apply to natural gas-fired power plants and industrial CO₂ sources as well. CO₂ capture has been happening for many years in the petroleum, chemical, and power industries, for a variety of reasons relevant to those industrial processes. However, in those cases, only a small portion of the CO₂ produced is captured. Capturing all, or even just three-fourths, of the CO₂ in a typical power plant with current technology would require equipment many orders of magnitude larger-a very expensive and highly energy-intensive option. In addition, without feasible, cost-effective ways to transport and store the captured CO₂, there is no point to capturing it from power plants. There are three types of CO₂ capture: post-combustion, pre-combustion, and oxy-combustion. Post-combustion CO₂ capture applies mainly to conventional coal-fired power generation but can also apply to combustion turbines fired by natural gas. In this case, the CO₂ is captured from flue gases after the fossil fuel has been burned. This technology is well-known and used to a limited degree. Pre-combustion entails a technology, widely used in chemical and some power plants, in which the fossil fuel is gasified instead of directly combusted, and the CO₂ can be readily captured from the gasification exhaust stream. With oxy-combustion, coal is burned in oxygen instead of in air, with resulting exhaust containing only CO₂ and water vapor. Because it yields an almost 100% CO₂ stream that is readily transportable, the process has strong potential but is extremely energy-intensive.

What is the regulatory environment for carbon capture?

In general, carbon capture projects are often initiated in response to government regulations, mandates, and incentives. However, there are currently no federal regulations related to carbon dioxide emission in the U.S. Other factors influencing a decision to initiate a carbon capture project may include a desire to increase familiarization with new technology, to evoke positive public relations, and/or to mitigate concern about environmental impacts of greenhouse gas emissions. The primary reason listed by carbon sequestration project developers is the presence of a regulatory emission reduction, cap and trading programs, or a need to avoid financial risks associated with the possibility of future regulations.

As national and international deliberations move ahead, state, local, and even corporate groups in the United States have already acted to impose regional CO₂ emissions limits. In 2002, some of the nation’s largest greenhouse gas emitters joined with the City of Chicago to form the Chicago Climate Exchange based on a pledge to reduce CO₂ emissions below their 1998–2001 levels. In 2003, New York State obtained commitments from nine Northeast states to form a cap-and-trade CO₂ emissions program called the Regional Greenhouse Gas Initiative or RGGI. In 2007, the California legislature passed a law aimed at curbing CO₂ emissions through an array of project offsets. California has also led the development of the Western Climate Initiative in which six states and two Canadian provinces recently agreed to cut greenhouse gas emissions to 15% below 2005 levels by 2020. Wishing to avoid the uneven consequences of regional regulation, various corporate alliances have formed to petition Congress to level the playing field with uniform national standards for CO₂ emissions.

Once you capture the carbon dioxide from a power plant, what can you do with it?

Generally speaking there are three possibilities: (1) Use the carbon dioxide as a value-added commodity, (2) store the carbon dioxide, such as in underground formations, or (3) convert the carbon dioxide to methane, biomass, mineral carbonates, or other substances. Some of the uses for commodity carbon dioxide result in a portion of the carbon dioxide being sequestered, which is an added benefit. A common example of this is enhanced oil recovery. Oil companies currently inject over 30 million tons of carbon dioxide per year in depleting oil formations to enhance the production of crude oil. A portion of this carbon dioxide remains underground. A similar carbon dioxide use/storage application is the enhancement of methane production from coal seams that are too deep to be mined. Concepts for converting carbon dioxide to other chemicals, especially fuels, are in the very early stages of research.

What is the difference between carbon capture at power plants and at other industrial facilities?

Carbon capture — or more specifically, carbon dioxide (CO₂) capture — is a process that entails separating CO₂ from stationary energy or other industrial point sources to be readied for transportation, followed by value-added uses or disposal to avoid emission to the atmosphere.

CO₂ separation has been practiced for decades in certain industries. CO₂ is used extensively in the food and beverage industry, such as to carbonate beverages — making them fizzy. It also is used as a cryogenic fluid to quickly freeze food or to transport food (as dry ice, also known as CO₂ “snow”). As with CO₂ separation processes used in other industries, CO₂ is captured in food and beverage operations via amine scrubbing. Here, an exhaust gas stream containing CO₂, typically from fermentation processes, is bubbled through an amine solution that serves as an absorbent in towers that can be more than 120 feet tall. The liquid stream containing the CO₂ is then sent to a desorption tower, where the liquid is heated to free the COamine scrubbing.. When used in products for human consumption, the CO₂ must be of the highest purity to avoid contamination, and extensive filtration is required (beverage-quality CO₂ can sell for more than $100 per ton). This is a complex and expensive process, one that has never been done at the large scale required of a power plant.

In all, the U.S. commercial CO₂ industry supplies only about 25,000 tons of CO₂ per day to the food and beverage, medical, agricultural, and other industries. Of this total, 95% of the CO₂ is a by-product sourced from processes such as fermentation or air separation. The remainder comes from natural sources. Given the high cost of producing and transporting CO₂, an economically and technically feasible method for capturing and upgrading CO₂ at the large industrial point sources could prove a significant source of new revenue for that industrial operation.

In the natural gas processing industry, amine-based processes are used commercially to remove (again, via chemical absorption) corrosive impurities known as acid gases (typically CO₂ and hydrogen sulfide, or H₂S) from process gas streams. The acid gases removed through amine treating are then sent to a sulfur recovery unit, which converts the H₂S in the acid gas stream into elemental sulfur. The gas residue from this step undergoes further treating, and this treated residual gas — including the CO₂ — is burned, and finally is vented through the flue gas stack. In recent years, natural gas processors have been implementing a new process involving the use of polymeric membranes to dehydrate and separate the CO₂ and H₂S from the natural gas stream.

CO₂ separation also occurs in a similar fashion in the production of synthesis gas that is used to manufacture ammonia, alcohols, and synthetic liquid fuels (produced via the Fischer-Tropsch process). In almost all such industrial processes, if the CO₂ captured this way is not sold or used for another process, it is then vented to the atmosphere.

CO₂ emissions are also generated by industrial facilities other than power plants, such as refineries, petrochemical facilities, smelters, and cement plants, in the course of burning fossil fuels to produce heat and process steam. The CO₂ is emitted as part of the flue gas stream from the boilers and burners that produces the needed thermal energy in such industrial operations. In these instances, the technology options for capturing CO₂ are essentially the same as they are for power plants that burn fossil fuels: Pre-combustion, post-combustion, and oxy-combustion.

What is important to remember is how the total volume of CO₂ emissions from power plants compares to all other industrial point sources. When looking at the energy sector as a whole, CO₂ emissions from petroleum exceed those of coal, even though coal use emits the most CO₂ per unit of energy. That is because the transportation sector accounts for the largest portion of petroleum-related CO₂ emissions. However, transportation sector carbon mitigation research and prospective policies focus on fuel economy/efficiency and fuel substitution, not carbon capture, which would be impractical for hundreds of millions of mobile sources.

Because the technologies suitable for carbon capture at power plants would be adaptable to other industries, the importance of NETL's research into reducing costs and improving effectiveness of carbon capture at fossil fuel-fired power plants is even more critical.

What types of energy facilities can participate in carbon capture?

In the United States, 98% of anthropogenic CO₂ emissions come from combustion of fossil fuels; consequently, CO₂ emissions and energy use are highly correlated. All forms of fossil fuel electric power plants would be candidates for carbon capture, as the power sector accounts for 40% of total U.S. energy-related CO₂ emissions. Coal-fired plants account for more than 80% of the power sector’s CO₂ emissions.

Other power plant sources of CO₂ emissions include those that burn municipal solid waste, tires, or other carbon-containing materials to generate electricity. Non-fossil energy facilities that would not be candidates for carbon capture include nuclear, hydroelectric, solar, wind, geothermal, and other fuels or energy sources with zero or near-zero CO₂ emissions. Although geothermal contributes a small amount of CO₂ emissions, it is typically categorized with non-fossil energy sources.

Natural gas processing plants must separate CO₂ from natural gas wellhead production streams in order to “sweeten” the production stream, as part of a process to treat and condition the natural gas so that it can be shipped via pipeline to energy consumers including power plants, and industrial, commercial, and residential users. Gas processing plants typically vent this CO₂, as there is little economic incentive to use it for any other purpose. At present, the gas processing industry’s focus on reducing greenhouse gas emissions is on methane, through its Natural Gas STAR program partnership with the U.S. Environmental Protection Agency.

In a much broader sense, the industries that extract fossil energy resources — coal mining and transport, oil and gas drilling-production and transport, etc. — in and of themselves are sources of CO₂ emissions, as they typically burn liquid fuels and often consume fossil fuel-fired electricity to carry out their operations. In the greater scheme of things, however, CO₂ emissions from such energy “facilities” as coal mines and pipelines are inconsequential and not feasible for carbon capture.

In sum, the greatest potential impact for mitigating CO₂ emissions through carbon capture in any energy-related endeavor is to focus R&D efforts on carbon capture at coal-fired power plants — where much of the National Energy Technology Laboratory’s research is focused.

How is CO₂ transported from the capture facility to the sequestration site?


	CO₂ pipeline from Dakota Gasification Plant in Beulah, ND to Weyburn, Canada.

Transporting CO₂ from a capture facility to a sequestration site is the easiest part of the CO₂ emissions mitigation challenge. CO₂ pipeline transportation has been in existence for more than 30 years in the United States. The first long-distance CO₂ pipeline started up in the early 1970s. More than 1,550 miles of pipeline transports over 40 million tons per year of CO₂ from natural and anthropogenic sources. For the most part, this CO₂ is shipped to oil fields in the Permian Basin of West Texas and New Mexico. There it is injected into the subsurface oil-bearing formations, or reservoirs, to enhance oil recovery (EOR).

Shipping CO₂ via pipeline involves compressing gaseous CO₂ to a pressure above 1,160 pounds per square inch (psi), to increase CO₂ density and make it easier and less expensive to ship. The long pipelines moving CO₂ to the EOR projects operate in the dense phase mode, at ambient temperatures and at high pressures provided by compressors at the upstream end and occasionally at points in between the source and the injection site. CO₂ also can be transported as a liquid in seagoing vessels or via tankers on roads or railways. In these instances, the CO₂ is held in insulated tanks at low temperatures and relatively low pressures. Road and rail tankers transport CO₂ at a temperature of 20°C and 290 psi. Because of the small capacities available, road and rail transport of the huge volumes of CO₂ targeted for sequestration would not be economically feasible. When large volumes or great distances overseas are involved, it makes better economic sense to transport the CO₂ by marine tankers that carry the CO₂ at low temperatures and pressures (about 100 psi). There is a small volume of CO₂ shipped this way because of limited demand. However, CO₂ properties are similar to those of liquefied petroleum gases (LPG), which are shipped overseas in large volumes. Consequently, CO₂ marine tankers could readily be scaled up to the size of large commercial LPG carriers if significant overseas demand for CO₂ manifested itself.

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