Carbon capture and storage (CCS) or carbon capture, utilization and storage (CCUS) refers to the process of removing CO2 from a fossil fuel either before (pre-combustion) or after (post-combustion) it is burned to prevent the CO2 from entering the atmosphere. The CO2 can then be stored temporarily and used for industrial purposes, or it can be pressurized into a liquid and injected underground where it is intended to remain indefinitely. Figure 1 shows a high level process schematic for CCUS.
Two forms of pre-combustion CCS are underground coal gasification (UCG) and the integrated gasification combined cycle (IGCC) process. In both cases coal is converted to gas using chemicals, the CO2 removed, and the remaining gas burned to produce power. Post-combustion CCS involves removing the CO2 from the flue gas of a coal or other fossil-fueled power plant, similar to the way the more conventional air pollutants are scrubbed from power plant exhaust. CO2 scrubbing uses chemical solvents called amines, which bind to and absorb CO2. Heat is then used to strip the CO2 from the amine solution, and the CO2 stream is compressed into a liquid and used for industrial applications or stored underground. Most of the key components of this CCS process are well-established technologies and techniques. For example, amine solvents were developed over 75 years ago to remove acid gases such as CO2 and hydrogen sulfide from natural gas. And the US oil and gas industry has decades of experience piping and storing gases underground in large volumes, albeit natural gas rather than CO2, as well as using CO2 for injecting into wells for Enhanced Oil Recovery (EOR).
Until fairly recently CCS has been primarily used to supply CO2 to oil compaies for EOR. However, the technology is now seen more broadly as a tool to be used to reduce CO2 emissions from the burning of fossil fuels for power generation and, even more recently, to support the generation of "blue hydrogen"
. Worldwide, per the International Energy Agency (IEA) as of 2020
, there are 21 CCS projects in operation, 3 under construction and 41 more in some stage of planning or development, comprising a wide range of different CCS technologies (Figure 2). Of particular note:
SaskPower's Boundary Dam carbon capture project, which went online in Saskatchewan, Canada in October 2014 (see Figure 3), retrofitted the plant's 110-megawatt Number 3 boiler with CCS. It is designed to capture up to one million metric tons of CO2 each year which can either be used for enhanced oil recovery or stored underground. At this time it is the only operating CCS project that uses post-combustion capture at a power plant. While this is a major step forward towards commercializing CCS, the Boundary Dam project is still a relatively small application at only 110 MW. And at a cost of CAD$1.5 billion (approx $1.2 billion) or about $11,000 / KW for the retrofit only, it is too expensive to be profitable without government assistance. And in 2018 SaskPower and the provincial government announced that they would not be expanding the technology on any further coal-fired plants in the near future due to lack of a business case.
Petra Nova is a joint venture between NRG Energy and JX Nippon Oil & Gas Exploration that became operational in early January 2017 as the largest post-combustion CCS project installed on an existing coal-fired power plant in the world. The project is designed to capture approximately 90 percent, or 1.6 million tons annually, of the CO2 from NRG Energy's WA Parish 240 MW generating station southwest of Houston, Texas. The new CCS facilities at WA Parish can be seen in Figure 4, and the CCS process is illustrated in Figure 5. The captured CO2 is used to increase oil production at the West Ranch oilfield from around 500 barrels per day to approximately 15,000 barrels per day through EOR. The project's final cost is approximately $1 billion, or about $4200 / KW for the retrofit alone. Although the project appeared successful in capturing up to 92% of CO2 emissions, the facility was placed in "reserve shutdown" mode in 2020 due to economic forces in the oil industry.
The Kemper County 582 MW power plant located in Kemper County, Mississippi, is a poster child for the problems surrounding "clean coal". Kemper began construction in 2010 and was supposed to be operational in 2014 (see Figure 6). It was billed by its owner, Southern Company, as a demonstration plant that would show the world how to burn coal without CO2 emissions. However, its reliance on multiple first-of-its-kind technologies applied at commercial scale led to major problems. Kemper includes both IGCC technology to convert lignite coal into synthetic gas and underground sequestration. Kemper construction costs increased from its original budget of $2.4 billion to approximately $7 billion, and problems with critical pieces of equipment were identified in June 2017. At that point, based on updated economic analyses, Southern Company decided to abandon the gassification portion of the plant and run fully on natural gas. According to a Sierra Club analysis, Kemper will be the most expensive power plant ever built on a per-kilowatt basis at about $12,000 / KW.
What is the future of CCS? Three central challenges to commercializing CCS remain: 1) capture systems must be scaled up in size, 2) costs must be reduced, and 3) lenders must be convinced that carbon capture projects can be reliable, profitable investments. There is also pressure from environmental groups like Greenpeace and WWF who advocate for 100 percent renewable energy, such as wind and solar power, rather than attempting to save the coal power industry. During the Obama administration, the US Department of Energy (DOE) invested $4.8 billion in carbon capture and storage technologies under the Clean Coal Power Initiative, including six major demonstration projects. Five of the projects have now been canceled or suspended, with the remaining demonstration project being Petra Nova. In 2020 the IEA published a new report
outlining the role of CCUS in the clean energy transition which suggests that in order to meet the Paris Agreement goals $160 billion (USD) of investment will be required in CCUS prior to 2030, representing a ten-fold increase on the investment for the period 2010 to 2020.
See OWOE Topic: How much does it cost to build a new power plant?
to see how these CCS costs compare to other power generation facilities.