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The combustion of fossil fuels produces carbon dioxide (CO2), which is the main greenhouse gas that causes climate change. Carbon capture and storage (CCS) refers to a range of technologies that capture CO2 emissions and then store them permanently so that they do not enter the atmosphere. Many stakeholders, including the UK Government, now refer to Carbon capture, use and storage (CCUS), to emphasise the potential to use as much captured CO2 as possible, in line with the principles of a circular economy.
ICE’s Energy Expert Panel has published a series of status reports concerned with various forms of energy such as wind, hydro, nuclear and energy from waste. Designed to be both informative and contemporary, the reports are updated regularly and are intended to provide accurate information to a varied audience. This report focuses on carbon capture and storage.
The Department of Business, Energy and Industrial Strategy (BEIS) has a useful webpage https://www.gov.uk/guidance/uk-carbon-capture-and-storage-government-funding-and-support introducing CCS and the issues that it raises. The UK Carbon Capture and Storage Association also provides material on all aspects of CCS. The Global CCS Institute provides an international perspective on CCS https://www.globalccsinstitute.com/.
In summary, CCS is a three-step process which includes:
CO2, when combined with water, can also be utilised for enhanced oil recovery (EOR) 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 on a commercial scale. Consequently, CCS is still an emerging option that needs further demonstration before it can be considered viable. It is also essential that the safety of both CCS operations and long-term CO2 storage can be assured with a very high level of confidence. CCS is applicable to both gas-fired combined cycle gas turbine (CCGT) plant and coal-fired stations, but with significantly different techniques.
There are two approaches to capturing CO2:
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 integrated gasification combined cycle (IGCC) power station.
Existing post-combustion techniques use solvents to capture 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.
While 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, existing solvents are patented to major chemical producers who have no direct interest in power generation, whereas IGCC technology is widely available. To deliver solvent capture technique commercially, there are three key issues that need to be addressed:
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 it has the potential to be a major technological growth area.
Based on the performance of existing (but smaller scale) carbon capture industrial processes, CCS has the potential to reduce CO2 emissions from power plants by up to 90% (Folger, 2013). This offers the opportunity to continue to use 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 the additional costs of CCS, coal generation could still be a competitive source of electricity.
The Stern Review of the economics of climate change estimated that CCS has the potential to contribute up to 20% of global carbon dioxide mitigation by 2050. A report by the Intergovernmental Panel on Climate Change (IPCC) on limiting global temperature rise to 1.5 degrees (IPCC, 2018) concluded that reaching a 1.5 degree target would require the extensive use of “carbon dioxide removal” (CDR) in almost all scenarios. CCS is one of the key potential techniques for CDR and it is clear that CCS could play an important role in making the challenging transition to a low carbon economy.
However, as of 2018, according to the Global CCS Institute, there were only 18 large-scale facilities in commercial operation, with a further 5 under construction. In 2014 there was only one facility in operation, so there has been significant growth in CCS capacity. However, the total of 40 Mtpa of CO2 captured by these plants is a tiny fraction of the 100–1000 Gt of CDR that modelling by the IPCC indicates could be required in future years.
In addition, a number of these existing CCS facilities are made economical by the sale and disposal of CO2 into nearby oil fields to enhance oil recovery. The full costs of CCS, when developed on a large industrial scale, would include reduced efficiency in the process, additional costs for pipeline transport over larger distances and the safe compression and disposal of CO2 into dormant underground reservoirs such as gas fields. These factors could lessen the overall reduction in CO2 for the combined process to significantly below 90%.
The Royal Academy of Engineering has drawn on a number of studies to compare the costs of low carbon generation, giving the range of levelised costs of CCS in coal stations as £105-140/MWh, and for gas, £105-115/MWh (RAEng, 2014). These figures can be compared with conventional CCGT costs of £60-100/MWh. However, a 2016 report by the Energy Technologies Institute (ETI, 2016) highlighted the potential for future cost reductions, down to £70 – 80/MWhr for a gas turbine with CCS, through scale, scope, shared infrastructure and risk reduction. This lower value is much closer to the current lowest cost low carbon sources (such as onshore and offshore wind). The challenge is to achieve these reductions in practice.
Carbon capture and storage raises a number of serious safety concerns. CO2 is not toxic but it is heavier than air – consequently any escape 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.
Storage of CO2, whether underground or subsea, has a risk of a major release associated with it. Every proposal for long-term storage will need to be fully characterised and risk-assessed.
The transit storage and transport of 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 has carried out a number of studies of safety issues arising from carbon capture and storage and these are available on its website.
The British Geological Survey is researching safety and other aspects of geological storage of carbon dioxide and provides an extensive range of reference material on its website.
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:
These areas are the focus of continuing research. A comprehensive report published by the International Energy Agency (IEA), Environmental Assessment for CO2 Capture and Storage, sets out all the potential environmental impacts of CCS in detail.
The IEA also provides regulatory reviews of CCS. with the most recent published in 2016.
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.
In order for CCS to be widely adopted, a number of key regulatory requirements must be met including:
As well as transparent regulatory arrangements, development of public confidence will require successful demonstration projects and mechanisms for public consultation for individual project proposals.
In November 2018, the UK Government launched an action plan on CCUS to set out “the next steps government and industry should take in partnership in order to achieve the government’s ambition of having the option to deploy CCUS at scale during the 2030s, subject to costs coming down sufficiently”.
The initial actions include detailed engagement with industry on the critical challenges to delivering CCUS in the UK, in particular the cost structures, risk sharing arrangements and the necessary market-based frameworks.
There are a number of other introductions to and discussions of CCS:
Folger, P (2013) Carbon Capture: A Technology Assessment. Congressional Research Service, Washington DC, USA.
Royal Academy of Engineering (2014) Wind Energy – Implications of Large-Scale Deployment on the GB Electricity System. London, UK.
Energy Technologies Institute (2016) Reducing the cost of CCS – developments in capture plant technology
IPCC (2018) Global Warming of 1.5 ºC