Carbon Capture and Storage (or CCS) refers to a range of technologies that capture Carbon Dioxide (CO2) emissions and then store them permanently so that they do not enter the atmosphere.

CCS is a 3-step process which includes:

• capturing the CO2 from power plants and other industrial sources; transporting the CO2 (usually via pipelines); and
• storing the CO2 in geological sites such as deep saline formations or depleted oil and gas fields.

The CO2, when combined with water, can also be utilised for EOR (Enhanced Oil Recovery) by lowering the viscosity of the oil in the well and improving flow rates through the production well bore.

The processes required for CCS already exist, but they have not yet been used together at a commercial scale. Consequently, CCS is still an emerging option that needs further demonstration before it can be considered viable. In addition, it is essential that the safety of both the CCS operations and the long-term CO2 storage can be assured with a very high level of confidence.

Further introductory guides to CCS technology can be found at:

• Department of Energy and Climate Change (DECC)
• The UK Carbon Capture and Storage Association

Capture Techniques and their status

There are two approaches to capturing CO2:

• Pre-combustion; and
• Post-combustion.

Pre-combustion solutions use coal gasification and reforming of the fuel into hydrogen and CO2. The CO2 is removed before the hydrogen is burnt. This is a proven technology and would be used as part of an IGCC (Integrated Gasification Combined Cycle) power station.

The existing post-combustion techniques use solvents to capture the CO2 from flue gases. Water-based amines (organic compounds that contain nitrogen as the key atom, similar to ammonia) are already widely used in industry to capture CO2. However, the scale is much smaller than a typical coal power station, so further development is needed to confirm that the technique can be scaled up.

Whilst capital costs are estimated to be similar for these two options, the IGCC approach is less flexible, as it requires a more complex set of integrated processes, compared to a modern supercritical boiler with flue gas capture.

Conversely, the existing solvents are patented to major chemical producers, who have no direct interest in power generation, whereas IGCC technology is widely available. To deliver the solvent capture technique commercially, there are three key issues that need to be addressed:

1) The capture of relatively low concentrations (10-15%) of CO2 in flue gases,
2) The impact of impurities in this flue gas on the chemical process, and,
3) The high energy costs associated with compression of the captured CO2

A third option is “oxyfuel” combustion. In this technique the fuel is burnt in pure oxygen and a portion of the flue gas is returned back to the furnace. This produces a flue gas that is almost entirely composed of CO2, requiring only simple, low cost purification before storage. Oxyfuel is still at an early demonstration stage.

At current electricity and CO2 prices, there is no commercially available solution for carbon capture. Governments are keen to promote national CCS champions who will develop economic solutions, as this has the potential to be a major technological growth area.

Benefits

CCS has the potential to reduce CO2 emissions from power plants by more than 85%. This offers the opportunity to continue to use the extensive world coal reserves for power generation for decades to come. Continued coal generation could maintain security and diversity of electricity supply while other low carbon sources of generation are developed to fully commercial status. Even with CCS, coal generation could still be a competitive source of electricity.

CCS has significant potential in the UK and worldwide to reduce CO2 emissions. The Stern Review of the economics of climate change estimated that it has the potential to contribute up to 20% of global carbon dioxide mitigation by 2050. CCS could play an important role in making the challenging transition to a low carbon economy.

Safety

CCS raises a number of serious safety concerns. CO2 is not toxic but it is heavier than air. Consequently any escape of CO2 has the potential to displace air and pose an asphyxiation hazard to people and the environment. There are also risks from any escape into the marine environment. The storage of CO2, whether underground or subsea, presents the risk of a major release of CO2. Every proposal for long-term storage will need to be fully characterised and risk-assessed.

The transit storage and transport of the captured CO2 to the ultimate disposal site is most likely to use pressurised vessels and pipelines, deploying a similar technology to liquid natural gas operations. Appropriate guidance on the routing and design rules for pipelines will be essential.

The UK Health and Safety Executive carried out an initial assessment of the risks presented by CCS as part of their response to the UK Energy Review in 2006.

Their conclusion is that, in principle, the risks are manageable, but a substantial amount of work remains to be done ensure that the residual risk is at an acceptably low level, comparable to other industrial processes.

This work should include:-

• further research on the properties of supercritical CO2,
• the development of analytical tools to carry out soundly based risk assessments of CCS installations,
• the development of international codes of practice and standards for CCS,
• the development of a global regulatory regime for CCS.

Since 2006, HSE has carried out further research and reports that it has a better, although still incomplete, understanding of the potential hazards from a large-scale high-pressure carbon dioxide release.

Environmental Impacts

The environmental impacts of CO2 releases direct to air are similar to the impacts on human safety and reasonably straightforward to quantify. There is less understanding of:

• the effects on vegetation due to CO2 releases through the underlying soil, and,
• the effects on marine organisms and ecology due to CO2 releases through the underlying seabed.

These are the focus of current research. The International Energy Agency has published a comprehensive report “Environmental Assessment for CO2 capture and storage” (IEA, 2007), which sets out all the potential environmental impacts of CCS in detail. It provides regulatory reviews of CCS with the most recent published in 2012.

The extraction, handling and underground storage of natural gas has many similarities to the CCS process and has been managed largely without incident for many decades, providing an immediate source of data and information on the mechanisms for uncontrolled releases.

Regulatory Challenges

Key regulatory challenges for CCS include:

• international agreement that CCS projects, both with and without Enhanced Oil Recovery, are legally permissible.
• the reduction in CO2 emissions when placing CO2 into storage must be recognised
• when calculating the emissions of fossil plant or other CO2 emitting industrial installations under the EU Emissions Trading Scheme and equivalents.
• there must be clarification of the extent and duration of liability of the storage operator for CO2 emission releases, before and after a facility is formally declared closed and successfully sealed.
• criteria for certifying the successful sealing of a storage facility must be developed and the appropriate certifying body identified.
• an overall health and safety framework and environmental regulatory framework for CCS must be developed.

As well as transparent regulatory arrangements, the development of public confidence will require successful demonstration projects and mechanisms for public involvement in decision making on individual project proposals.

UK Activities

The processes involved in CCS are not novel, but have yet to be demonstrated together at scale. The Government made clear in the Energy Review in 2006 that the next logical step for CCS would therefore be the construction of a full-scale demonstration plant, subject to this being cost effective. Following the 2006 Energy Review a competition was held to select and deliver a first demonstration project in the UK. However, this was ultimately abandoned and a new competition was launched in 2012. Fourteen bids were submitted and the winner was expected to be announced in October 2012. The latest news can be found on the DECC website.

Further Reading

As well as the DECC and CCSA websites, there are a number of other introductions and discussions of CCS.

The Intergovernmental Panel on Climate Change (IPCC) report on carbon capture provides a definitive overview of the subject.

The UK CCS Research Centre has a wide range of information on the latest developments in CCS R&D.