Carbon capture and storage
Carbon capture and storage (CCS) is a process in which a relatively pure stream of carbon dioxide (CO2) from industrial sources is separated, treated and transported to a long-term storage location.[2]: 2221 For example, the burning of fossil fuels or biomass results in a stream of CO2 that could be captured and stored by CCS. Usually the CO2 is captured from large point sources, such as a chemical plant or a bioenergy plant, and then stored in a suitable geological formation. The aim is to reduce greenhouse gas emissions and thus mitigate climate change.[3][4] For example, CCS retrofits for existing power plants can be one of the ways to limit emissions from the electricity sector and meet the Paris Agreement goals.[5]: 16
This article is about removing CO2 from industrial flue gas. For removing and sequestering CO2 from the atmosphere, see carbon sequestration.
Carbon dioxide can be captured directly from the gaseous emissions of an industrial source, for example from a cement factory (cement kiln). Several technologies are in use: adsorption, chemical looping, membrane gas separation or gas hydration.[6][7][8] However, as of 2022, only about one thousandth of global CO2 emissions are captured by CCS, and most of those CCS projects are for natural-gas processing.[9]: 32 CCS projects generally aim for 90% capture efficiency,[10] but most of the current installations have failed to meet that goal.[11]
Storage of the captured CO2 is either in deep geological formations or in the form of mineral carbonates. Geological formations are currently the favored option for storage. Pyrogenic carbon capture and storage (PyCCS) is another option.[12] Long-term predictions about submarine or underground storage security are difficult. There is still the risk that some CO2 might leak into the atmosphere.[13][14][15] A 2018 evaluation estimates the risk of substantial leakage to be fairly low.[16][17]
CCS is so far still a relatively expensive process.[18] Carbon capture becomes more economically viable when the carbon price is high, which is the case in much of Europe.[9] Another option is to combine CCS with a utilization process where the captured CO2 is used to produce high-value chemicals to offset the high costs of capture operations.[19]
Some environmental activists and politicians have criticized CCS as a false solution to the climate crisis. They cite the role of the fossil fuel industry in origins of the technology and in lobbying for CCS focused legislation.[20] Critics also argue that CCS is only a justification for indefinite fossil fuel usage and equate to further investments into the environmental and social harms related to the fossil fuel industry.[21][22] With regards to public support, communities who have been negatively affected by an industrial activity in the past are less supportive of CCS.[23] Communities that feel inadequately informed about or excluded from project decision-making may also resist CCS development.[24]
Globally, a number of laws and rules have been issued that either support or mandate the implementation of CCS. In the US, the 2021 Infrastructure Investment and Jobs Act provides support for a variety of CCS projects, and the Inflation Reduction Act of 2022 updates tax credit law to encourage the use of CCS.[25][26] Other countries are also developing programs to support CCS technologies, including Canada, Denmark, China, and the UK.[27][28]
Terminology[edit]
The term carbon capture and storage, (CCS) also known as carbon dioxide capture and storage refers to a process in which a relatively pure stream of carbon dioxide (CO2) is separated (“captured”), compressed and transported to a storage location for long-term isolation from the atmosphere.[2]: 2221 Bioenergy with carbon capture and storage (BECCS), is a related technique that involves the application of CCS to bioenergy in order to reduce atmospheric CO2 over the course of time.
CCS and CCUS (carbon capture, utilization, and storage) are often used interchangeably. The latter involves 'utilization' of the captured carbon for other applications, such as enhanced oil recovery (EOR), liquid fuel production, or the manufacturing of consumer goods, such as plastics. Both approaches capture CO2 and effectively store it, whether in geological formations or in material products.[29]
Purpose[edit]
Early uses[edit]
The natural gas industry has used carbon capture technology for decades. Raw natural gas contains CO2 that needs removal to produce a marketable product. The sale of captured CO2, mainly to oil producers for EOR, has enhanced the economic viability of natural gas development projects.[30] CO2 removal for this purpose first occurred at The Terrell Natural Gas Processing Plant, in Terrell, Texas, US, in 1972.[31] The use of CCS as a means of reducing anthropogenic CO2 emissions is more recent. The Sleipner CCS project, which began in 1996, and the Weyburn-Midale Carbon Dioxide Project, which began in 2000, were the first international demonstrations of the large-scale capture, utilization, and storage of anthropogenic CO2 emissions.[32]
Technology components[edit]
Capture[edit]
Capturing CO2 is most cost-effective at point sources, such as large fossil fuel-based energy facilities, industries with major CO2 emissions (e.g. cement production, steelmaking[39]), natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Extracting CO2 from air is possible,[40] although the lower concentration of CO2 in air compared to combustion sources complicates the engineering and makes the process therefore more expensive.[41]
The net storage efficiency of carbon capture projects is maximally 6–56%.[42]
Impurities in CO2 streams, like sulfurs and water, can have a significant effect on their phase behavior and could cause increased pipeline and well corrosion. In instances where CO2 impurities exist, especially with air capture, a scrubbing separation process is needed to initially clean the flue gas.[43]
A wide variety of separation techniques are being pursued, including gas phase separation, absorption into a liquid, and adsorption on a solid, as well as hybrid processes, such as adsorption/membrane systems.[44] There are three ways that this capturing can be carried out: post-combustion capture, pre-combustion capture, and oxy-combustion:[45]
Cost[edit]
Cost is a significant factor affecting CCS. The cost of CCS, plus any subsidies, must be less than the expected cost of emitting CO2 for a project to be considered economically favorable.
Tests of CCS at the Petra Nova and Boundary Dam coal-fired power plants and has been found to be technically feasible but not economically viable for use with coal, due to reductions in the cost of solar PV technology.[98]
CCS technology is expected to use between 10 and 40 percent of the energy produced by a power station.[99][100] The energy consumed by CCS is called an "energy penalty". It has been estimated that about 60% of the penalty originates from the capture process, 30% comes from compression of the extracted CO2, while the remaining 10% comes from pumps and fans.[101] CCS would increase the fuel requirement of a gas plant with CCS by about 15%.[102] The cost of this extra fuel, as well as storage and other system costs, are estimated to increase the costs of energy from a power plant with CCS by 30–60%. This makes it more difficult for fossil fuel plants with CCS to compete with renewable energy combined with energy storage, especially as the cost of renewable energy and batteries continues to decline.
Constructing CCS units is capital-intensive. The additional costs of a large-scale CCS demonstration project are estimated to be €0.5–1.1 billion per project over the project lifetime. Other applications are possible. CCS trials for coal-fired plants in the early 21st century were economically unviable in most countries,[103] including China,[104] in part because revenue from enhanced oil recovery collapsed with the 2020 oil price collapse.[105] A carbon price of at least 100 euros per tonne CO2 is estimated to be needed to make industrial CCS viable,[106] together with carbon tariffs.[107] But, as of mid-2022, the EU Allowance had never reached that price, and the Carbon Border Adjustment Mechanism had not yet been implemented.[108] However, a company making small modules claims it can get well below that price by mass production by 2022.[109]
According to UK government estimates made in the late 2010s, carbon capture (without storage) is estimated to add 7 GBP per MWh by 2025 to the cost of electricity from a gas-fired power plant. However, the CO2 will need to be stored, so in total the increase in cost for gas or biomass generated electricity is around 50%.[110]
A 2020 study concluded that half as much CCS might be installed in coal-fired plants as in gas-fired; these would be mainly in China and India.[111] However a 2022 study concluded that it would be too expensive for coal power in China.[112]
Bill Gates said in 2023 that in his view CCS was unlikely to be economically viable for mass-scale use in the long term, and that "for most cases, you should use an alternative technique rather than emitting and then paying for capturing.... For everything you can, you want to solve it by never generating the carbon dioxide.”[113][114]
Related impacts[edit]
Since liquid amine solutions are used to capture CO2 in many CCS systems, these types of chemicals can also be released as air pollutants if not adequately controlled. Among the chemicals of concern are volatile nitrosamines, which are carcinogenic when inhaled or drunk in water.[115] CCS systems also reduce the efficiency of the power plants that use them to control CO2. For super-critical pulverized coal (PC) plants, CCS' energy requirements range from 24 to 40%, while for coal-based gasification combined cycle (IGCC) systems it is 14–25%.[116] Using CCS for natural gas combined cycle (NGCC) plants can decrease operating efficiency from 11 to 22%.[116] This in turn could cause a net increase of non-GHG pollutants from those facilities. However, most of these impacts are controlled by the pollution control equipment already installed at these plants to meet air pollution regulations.[117] CCS technology also has operational impacts. These impacts increase as the capacity factor decreases (the plant is used less - for example only for times of highest demand or in emergencies).[9]: 42
Other impacts occur outside the facility. As a result of efficiency losses at coal plants, fuel use and environmental problems arising from coal extraction increase. Plants equipped with flue-gas desulfurization (FGD) systems for sulfur dioxide control require proportionally greater amounts of limestone, and systems equipped with selective catalytic reduction systems for nitrogen oxides produced during combustion require proportionally greater amounts of ammonia.
Analysis of IPCC modeling work shows that mitigation strategies that rely less on CCS would bring about localized, near-term benefits from reduced air and water pollution, human rights violations, and biodiversity loss.[33]
