CO₂ Capture and Storage

1. What is carbon dioxide capture and storage?

• 1.1 What is CO₂ capture and storage and what could its applications be?
• 1.2 What role could CO₂ capture and storage play in the fight against climate change?

1.1 What is CO₂ capture and storage and what could its applications be?

The source document for this Digest states:

Possible CCS systems

Carbon dioxide capture and storage (CCS), the subject of this Special Report, is considered as one of the options for reducing atmospheric emissions of CO₂ from human activities. The purpose of this Special Report is to assess the current state of knowledge regarding the technical, scientific, environmental, economic and societal dimensions of CCS and to place CCS in the context of other options in the portfolio of potential climate change mitigation measures.

The structure of this Technical Summary follows that of the Special Report. This introductory section presents the general framework for the assessment together with a brief overview of CCS systems. Section 2 then describes the major sources of CO₂, a step needed to assess the feasibility of CCS on a global scale. Technological options for CO₂ capture are then discussed in Section 3, while Section 4 focuses on methods of CO₂ transport. Following this, each of the storage options is addressed. Section 5 focuses on geological storage, Section 6 on ocean storage, and Section 7 on mineral carbonation and industrial uses of CO₂. The overall costs and economic potential of CCS are then discussed in Section 8, followed by an examination in Section 9 of the implications of CCS for greenhouse gas emissions inventories and accounting. The Technical Summary concludes with a discussion of gaps in knowledge, especially those critical for policy considerations.

Overview of CO₂ capture and storag

CO₂ is emitted principally from the burning of fossil fuels, both in large combustion units such as those used for electric power generation and in smaller, distributed sources such as automobile engines and furnaces used in residential and commercial buildings. CO₂ emissions also result from some industrial and resource extraction processes, as well as from the burning of forests during land clearance. CCS would most likely be applied to large point sources of CO₂, such as power plants or large industrial processes. Some of these sources could supply decarbonized fuel such as hydrogen to the transportation, industrial and building sectors, and thus reduce emissions from those distributed sources.

CCS involves the use of technology, first to collect and concentrate the CO₂ produced in industrial and energy- related sources, transport it to a suitable storage location, and then store it away from the atmosphere for a long period of time. CCS would thus allow fossil fuels to be used with low emissions of greenhouse gases. Application of CCS to biomass energy sources could result in the net removal of CO₂ from the atmosphere (often referred to as ‘negative emissions’) by capturing and storing the atmospheric CO₂ taken up by the biomass, provided the biomass is not harvested at an unsustainable rate.

Figure TS.1 illustrates the three main components of the CCS process: capture, transport and storage. All three components are found in industrial operations today, although mostly not for the purpose of CO₂ storage. The capture step involves separating CO₂ from other gaseous products. For fuel- burning processes such as those in power plants, separation technologies can be used to capture CO₂ after combustion or to decarbonize the fuel before combustion. The transport step may be required to carry captured CO₂ to a suitable storage site located at a distance from the CO₂ source. To facilitate both transport and storage, the captured CO₂ gas is typically compressed to a high density at the capture facility. Potential storage methods include injection into underground geological formations, injection into the deep ocean, or industrial fixation in inorganic carbonates. Some industrial processes also might utilize and store small amounts of captured CO₂ in manufactured products.

The technical maturity of specific CCS system components varies greatly. Some technologies are extensively deployed in mature markets, primarily in the oil and gas industry, while others are still in the research, development or demonstration phase. Table TS.1 provides an overview of the current status of all CCS components. As of mid-2005, there have been three commercial projects linking CO₂ capture and geological storage: the offshore Sleipner natural gas processing project in Norway, the Weyburn Enhanced Oil Recovery (EOR) project in Canada (which stores CO₂ captured in the United States) and the In Salah natural gas project in Algeria. Each captures and stores 1–2 MtCO₂ per year. It should be noted, however, that CCS has not yet been applied at a large (e.g.,500 MW) fossil-fuel power plant, and that the overall system may not be as mature as some of its components.

Table TS.1. Current maturity of CCS system components

Source & ©: IPCC (WGI)  Intergovernmental Panel on Climate Change IPCC
Carbon Dioxide Capture and Storage: Technical Summary (2005)
1. Introduction and framework of this report, p. 19

1.2 What role could CO₂ capture and storage play in the fight against climate change?

The source document for this Digest states:

In 1992, international concern about climate change led to the United Nations Framework Convention on Climate Change (UNFCCC). The ultimate objective of that Convention is the “stabilization of greenhouse gas concentrations in the atmosphere at a level that prevents dangerous anthropogenic interference with the climate system”. From this perspective, the context for considering CCS (and other mitigation options) is that of a world constrained in CO₂ emissions, consistent with the international goal of stabilizing atmospheric greenhouse gas concentrations. Most scenarios for global energy use project a substantial increase of CO₂ emissions throughout this century in the absence of specific actions to mitigate climate change. They also suggest that the supply of primary energy will continue to be dominated by fossil fuels until at least the middle of the century (see Section 8). The magnitude of the emissions reduction needed to stabilize the atmospheric concentration of CO₂ will depend on both the level of future emissions (the baseline) and the desired target for long-term CO₂ concentration: the lower the stabilization target and the higher the baseline emissions, the larger the required reduction in CO₂ emissions. IPCC’s Third Assessment Report (TAR) states that, depending on the scenario considered, cumulative emissions of hundreds or even thousands of gigatonnes of CO₂ would need to be prevented during this century to stabilize the CO₂ concentration at 450 to 750 ppmv. The TAR also finds that, “most model results indicate that known technological options could achieve a broad range of atmospheric CO₂ stabilization levels”, but that “no single technology option will provide all of the emissions reductions needed”. Rather, a combination of mitigation measures will be needed to achieve stabilization. These known technological options are available for stabilization, although the TAR cautions that, “implementation would require associated socio-economic and institutional changes

In this context, the availability of CCS in the portfolio of options for reducing greenhouse gas emissions could facilitate the achievement of stabilization goals. Other technological options, which have been examined more extensively in previous IPCC assessments, include: (1) reducing energy demand by increasing the efficiency of energy conversion and/or utilization devices; (2) decarbonizing energy supplies (either by switching to less carbon-intensive fuels (coal to natural gas, for example), and/or by increasing the use of renewable energy sources and/or nuclear energy (each of which, on balance, emit little or no CO₂); (3) sequestering CO₂ through the enhancement of natural sinks by biological fixation; and (4) reducing non-CO₂ greenhouse gases.

Model results presented later in this report suggest that use of CCS in conjunction with other measures could significantly reduce the cost of achieving stabilization and would increase flexibility in achieving these reductions . The heavy worldwide reliance on fossil fuels today (approximately 80% of global energy use), the potential for CCS to reduce CO₂ emissions over the next century, and the compatibility of CCS systems with current energy infrastructures explain the interest in this technology.

Major issues for this assessment

There are a number of issues that need to be addressed in trying to understand the role that CCS could play in mitigating climate change. Questions that arise, and that are addressed in different sections of this Technical Summary, include the following:

• What is the current status of CCS technology?
• What is the potential for capturing and storing CO₂?
• What are the costs of implementation?
• How long should CO₂ be stored in order to achieve significant climate change mitigation?
• What are the health, safety and environment risks of CCS?
• What can be said about the public perception of CCS?
• What are the legal issues for implementing CO₂ storage?
• What are the implications for emission inventories and accounting?
• What is the potential for the diffusion and transfer of CCS technology?

When analyzing CCS as an option for climate change mitigation, it is of central importance that all resulting emissions from the system, especially emissions of CO₂, be identified and assessed in a transparent way. The importance of taking a “systems” view of CCS is therefore stressed, as the selection of an appropriate system boundary is essential for proper analysis. Given the energy requirements associated with capture and some storage and utilization options, and the possibility of leaking storage reservoirs, it is vital to assess the CCS chain as a whole.

From the perspectives of both atmospheric stabilization and long-term sustainable development, CO₂ storage must extend over time scales that are long enough to contribute significantly to climate change mitigation. This report expresses the duration of CO₂ storage in terms of the‘fraction retained’, defined as the fraction of the cumulative mass of CO₂ injected that is retained in a storage reservoir over a specified period of time. Estimates of such fractions for different time periods and storage options are presented later. Questions arise not only about how long CO₂ will remain stored, but also what constitutes acceptable amounts of slow, continuous leakage from storage. Different approaches to this question are discussed in Section 8.

CCS would be an option for countries that have significant sources of CO₂ suitable for capture, that have access to storage sites and experience with oil or gas operations, and that need to satisfy their development aspirations in a carbon-constrained environment. Literature assessed in the IPCC Special Report ‘Methodological and Technological Issues and Technology Transfer’ indicates that there are many potential barriers that could inhibit deployment in developing countries, even of technologies that are mature in industrialized countries. Addressing these barriers and creating conditions that would facilitate diffusion of the technology to developing countries would be a major issue for the adoption of CCS worldwide.

Source & ©: IPCC (WGI)  Intergovernmental Panel on Climate Change IPCC
Carbon Dioxide Capture and Storage: Technical Summary (2005)
1. Introduction and framework of this report, p. 20