Rick Gould describes how carbon capture and storage (CCS) works, why it could be effective and how the Government plans to proceed. He also summarises the main challenges confronting CCS implementation.
In April 2012, the Department of Energy and Climate Change (DECC) published the Government’s latest plans for CCS. The CCS Roadmap: Supporting Deployment of Carbon Capture and Storage in the UK describes the Government’s aims for CCS, and how it intends to achieve them.
CCS entails filtering CO2 emissions from fossil-fuelled sources of power, and then storing the captured CO2 underground. CCS theoretically allows the creation of low-carbon power from high-carbon fuels.
According to the International Energy Agency (IEA), CCS could contribute 20% of the required global reductions in CO2 emissions by 2050, while creating a new multi-billion pound industry, with economic as well as environmental benefits. DECC estimates that the UK’s expertise and advantages in CCS could generate between £3 and £6.5 billion for the UK’s economy annually by the late 2020s.
However, if the UK is to realise these benefits, then the Government points out that co-ordinated efforts and significant investments are needed both nationally and internationally. For example, the IEA predicts that the world will need around 3000 CCS installations to achieve the potential 20% reduction in global CO2 emissions.
Around the same time as the Government published the CCS Roadmap, the Energy Research Centre at the University of Sussex published a report called Carbon Capture and Storage — Realising the Potential?. This report describes the obstacles and uncertainties as the UK is faced with significant political, financial and technological challenges in producing full-scale CCS plants.
Three promising technologies
Although researchers are looking at several techniques for carbon capture, so far the most promising are post-combustion carbon capture (PCCC), oxy-fuel combustion, and pre-combustion. The techniques work as follows.
Post-combustion carbon capture (PCCC): The stack-gases from a combustion source pass through an absorber column containing a solution of amines or ammonia; although researchers are currently experimenting with different types of amine mixtures, the foundation for PCCC is typically methyl-ethylamine (MEA). The amines react with CO2 to form amine salts in solution. The solution is continually cycled through the absorber column and a second, attached regenerating column that heats the CO2-rich amine solution, which in turn reverses the reaction to liberate CO2 gas and a refreshed amine solution, which is then cycled back to the absorber column.
Oxy-fuel combustion: In this process, a fuel is burnt in an oxygen-rich atmosphere which produces stack gases which are mostly moisture and CO2. These are separated using condensation to form two separated streams of water and CO2. Before the fuel is burnt, the oxygen has to be separated from air to produce a concentrated stream of oxygen. The process of combustion typically entails recycling part of the exhaust gases to ensure near complete combustion and stable temperatures.
Pre-combustion: This technique includes a process called an Integrated Gasification Combined Cycle (IGCC), which converts a solid fuel into a fuel-gas by burning it in a small amount of oxygen. This then produces a gas which is rich in hydrogen and carbon monoxide; further treatment converts the carbon monoxide to CO2, while the hydrogen is burnt as a fuel.
The CO2 captured in all of these techniques can be stored underground. Of the three techniques, industry and research has most experience with PCCC. The technique has been used in the oil and gas sector for decades, with patents going back to the 1930s; for example, amine-based carbon capture was first used for removing CO2 and hydrogen sulphide from natural gas.
Furthermore, in several locations worldwide, the separated CO2 has been compressed and then transported using pipelines to underground oil reservoirs, to help extract crude oil. This technique is known as enhanced oil recovery (EOR). The experience in both carbon capture and the subsequent underground storage will benefit future CCS installations.
PCCC in California
Although a lot of attention has been focused on pilot-plants at power stations, there is a small coal-fired power station at a chemical plant in California which has used PCCC since 1978 to capture CO2 for use in manufacturing commodity chemicals. This suggests that the technology works, and must at least have a potential commercial viability.
However, this is a relatively small power plant, and it is not yet known how well the technology can be adapted to work on a larger scale, and include the transportation and long-term underground storage of CO2 for the largest power stations. So, there are a growing number of pilot-plants worldwide to characterise the operation of carbon capture, and improve efficiency. The UK currently has two pilot-plants and several small-scale experimental plants.
The challenge of high efficiency
Capturing CO2 in a solution of amines is relatively unproblematic, but maintaining a high efficiency is not as easy, due to two specific challenges.
First, in the gas-refining sector, natural gas contains relatively few chemicals when compared with stack gases; the latter contains some particularly challenging substances, such as nitrogen oxides, sulphur dioxide and hydrogen chloride. These gases are acidic and can react with amines to produce foaming and degradation by-products. Foaming decreases capture efficiency and can also damage the capture units, while amine-degradation not only decreases the efficiency of capture, but can also produce toxic by-products such as nitrosamines.
There is published, scientific literature which has shown the presence of nitrosamines in the gases that pass through PCCC plants, as well as ammonia and some MEA in a process known as amine slip. In other words, some of the amines can be lost.
Research to date does not suggest that the levels of by-products and amines released could be problematic, but there is no conclusive evidence to say that by-products are definitely not a potential hazard.
So, much current research is examining this area, but if by-products do prove to be a potentially hazardous release, there are techniques and technologies available to control them. Researchers have already found that an extra level of emissions control is beneficial between the exhaust ducts of a power station and the PCCC plant. Known as flue-gas polishing, this is effectively a second set of acid-gas scrubbers to further reduce the concentrations of acidic gases in the stack-gases, which in turn reduces the formation of amine-degradation by-products.
However, the irony is that one new form of pollution control could itself need pollution control devices, which could increase the energy demand for carbon capture. This would impact on the holy grail of carbon capture technologies — reducing energy consumption.
PCCC, for example, requires a significant amount of energy to release CO2 from the CO2-rich amine-solution. So, current, intense research is looking at different mixtures of amines, together with different process conditions to produce a complete system which captures CO2 efficiently, with an insignificant amount of amine slip and degradation, and then requires the minimum amount of energy to release the captured CO2 for subsequent compression, transportation and storage.
Similarly, the challenge for oxy-fuel combustion is to minimise the energy required to separate oxygen from air.
A question of scale, planning and maps
The current CCS plants at oil and gas refineries and the pilot-plants at power stations are small compared with the scale of plants that will be needed on large power-stations. Scaling up the technology and pipeline infrastructure will be a major challenge. In April, a research report from the UK Energy Research Centre3 at the University of Sussex explored the different uncertainties and challenges that face the full-scale implementation of CCS.
The report’s authors acknowledge that CCS could be a key technology to tackle climate change, but there are many technological, financial and political uncertainties. However, the authors do state that not all the uncertainties need to be resolved before CCS can be implemented effectively.
Furthermore, the authors draw several parallels between CCS and other major technological challenges that have been overcome in the past, such as the implementation of flue-gas desulphurisation at power stations in the USA and UK from the 1960s to the 1980s, the challenges of nuclear waste, and the development of the infrastructure for extracting, processing and distributing natural gas.
The report provides several recommendations, such as a comprehensive policy and plan of action from the Government to address these uncertainties. Ironically, many of these uncertainties are addressed in the Government’s CCS Roadmap, which was also published in April.
Three main objectives
Within the CCS Roadmap, the Government describes its three main objectives for CCS. These are:
removing the main obstacles to the deployment of CCS
lowering the risks and costs connected with technology
creating the regulatory and market frameworks that provide the foundations for the private sector to deploy CCS in a cost effective and competitive way.
The Government intends to do this through five co-ordinated actions (see Table 1).
Many see the UK as having four distinct advantages in this rapidly developing area. First, there are many clusters of high-energy industries and especially power stations, which are close to potential transport and storage networks for CO2. This means that such industries could share the infrastructure for transporting the captured gas.
Second, the UK has extensive offshore sites which could serve as long-term storage reservoirs for CO2, and third, the oil and gas sector has significant experience in working with the transport and storage of gases, including captured CO2.
Finally, the UK has several centres of CCS excellence in academia and industry; for example, several universities including those in Edinburgh, Southampton Nottingham, Leeds, and London already have many years of experience and experimental rigs for CCS. Furthermore, all have close links to DECC and other government bodies, such as the British Geological Survey (www.bgs.ac.uk), which, in turn, is playing a leading role in developing the potential for underground storage-sites.
Imperial College London recently commissioned a four-storey carbon capture research plant using MEA, which continually captures and regenerates 1.2 tonnes of CO2 per day. This new facility contains the latest and most advanced technologies for carbon capture and monitoring, and during its expected 25-year lifetime the university expects that well over 8000 students and researchers will have worked on the experimental plant.
Electricity market reform
Removing the main barriers to CSS
Research, development and innovation
One of the key challenges is a regime of effective, accurate and precise monitoring for all parts of the CCS chain. A critical success factor for CCS will be the price of carbon. In simple terms, under the EU Emissions Trading Scheme, process and power station operators will get a discount for every tonne of CO2 captured and stored. This means that operators will save money by not having to buy allowances for captured CO2.
At the same time, operators will have to be able to provide the evidence to demonstrate capture, which means having the equipment and mechanisms in place to accurately measure or determine how much CO2 is captured, how much is lost during capture, transportation and storage, and whether there is any significant leakage from underground storage reservoirs. Therefore, one of the current research themes is to develop cost-effective and accurate means of measuring CO2 at all the required stages of the CCS chain.
Overcoming the obstacles to CCS appears likely, but the path to full implementation could be measured in decades, rather than years.