Introduction to carbon capture utilisation and storage
27 February 2014
Author: Kirsten Foy, BA BAI PhD CEng MIEI, principal engineer, Parsons Brinckerhoff
The devastating effects of extreme weather conditions are increasingly noticeable with every season. Most experts in the scientific community now accept that climate change is the key factor here.
The prevailing wisdom is that the worst of the damage can be avoided if the global temperature does not rise by more than 2°C, which would be achievable if the level of carbon dioxide (CO2) in the atmosphere does not increase beyond 450ppm . But this requires a massive global reduction in CO2 emissions, including a fall in European Union-country emissions to 20% below 1990 levels by 2020.
We could carry on regardless and allow climate change to continue unabated, or we could take drastic measures and shut down 20% of industry, transport and power production, severely impacting the economy and our way of life. So, how to maintain the economic recovery and a high standard of living without emitting vast amounts of CO2?
CCUS stands for carbon capture, utilisation and storage. CO2 from combustion of a fuel or industrial process is captured, compressed or cooled to liquid, then transported and used in other products or stored away from the atmosphere. Other methods seek to reduce the amount of CO2 produced.
With CCUS, CO2 continues to be produced before being captured and reused or stored. Coal and natural gas can still be used as energy sources and we can continue to produce CO2 in industrial processes, but without contributing to climate change.
CCUS VERSUS RENEWABLES
Many believe that renewables are the answer to global warming, so why not forget fossil fuels altogether? Firstly, even if all power were to be generated by renewables, it would still be worthwhile applying CCUS to the production of steel, cement, chemicals, fertiliser, paper and other products.
Secondly, most renewables are an intermittent source of power, requiring combustion plants as backup. Thirdly, many forms of renewable energy are currently more expensive than coal and gas plants with CCUS. Finally, there is a limit to how much renewable energy we can produce and how quickly we can access it, so fossil fuels will remain a dominant form of energy.
There are many ways of capturing CO2. Something as simple as pumping cleaned flue gas from a power plant through a greenhouse will trap some of the carbon in vegetative matter. This concept can be applied on an industrial scale to convert the CO2 in flue gas to biofuel using algal growth. However, once the vegetables are digested or the biofuel is combusted, the CO2 will be released again to atmosphere.
Extending forests or increasing ocean algae are also forms of carbon storage. Although this carbon will be released when the plant dies and decomposes, provided the overall mass of plant life is continually replenished, the carbon remains stored. These forms of CCUS collect and store CO2 from the atmosphere after it has been emitted. For large-point sources of CO2, such as power plants, steel foundries or cement works, it is more efficient to capture at the source, where the CO2 concentration is highest.
There are many carbon-capture technologies in development. The three closest to commercialisation, which can be applied to large-point sources of CO2, are: post-combustion capture; reforming of fuel with pre-combustion capture; and oxy-combustion technology. Although the term ‘combustion’ is used, most forms of capture can also be applied to CO2 emissions from sources other than combustion of a fuel; e.g., CO2 released from a chemical reaction in an industrial process such as cement production.
- Post-combustion capture
Post-combustion capture is the removal of CO2 from flue gas after combustion of the fuel. The most common process is temperature-swing absorption using a liquid chemical solvent. The power plant or industrial process continues as normal, but instead of going up a chimney, the flue gas is cooled and blown through an absorber. In the absorber, a solvent that absorbs CO2 is sprayed into the flue gas. The cleaned flue gas can then be reheated before being released into the atmosphere. The CO2-loaded solvent is reheated in a stripper to release the CO2. Typically 90% of CO2 can be captured using this process.
- Pre-combustion carbon capture
Pre-combustion capture involves removal of CO2 prior to combustion. A hydrocarbon fuel is converted to H2 and CO2 using steam and a chemical reaction. CO2 is then removed, typically using liquid solvent, and the hydrogen used as a fuel. There will be some CO and CO2 remaining in the fuel, but up to 90% of CO2 can be captured.
Many types of solvents are being developed, as well as solid sorbents and selective membranes for both post-combustion and pre-combustion capture.
Oxy-combustion refers to burning a fuel with pure oxygen instead of air. Assuming the fuel is a hydrocarbon, the combustion products are overwhelmingly composed of water vapour and carbon dioxide, with little or no nitrogen. This greatly simplifies the separation process, requiring only cooling to separate the water from the CO2. To prevent extreme temperatures, some flue gas is recycled to the combustion chamber.
Oxy-combustion can be applied to boilers and combustion turbines;l e.g., combined cycle gas plants, as well as to some industrial processes that produce CO2. Oxy-combustion has the highest capture rate of the three technologies, allowing capture of up to 98% of the CO2.
Future developments are typically related to reducing the power requirement for the oxygen production. Oxygen-transport membranes and chemical-looping combustion are two technologies under development for oxy-combustion capture.
TRANSPORT, UTILISATION AND STORAGE
Once the CO2 is captured, it must be transported to the permanent storage site. There are thousands of kilometres of CO2 pipelines in operation in the US. Clustering projects and storage sites allows the use of shared trunk pipelines to reduce costs; a CO2 pipeline network could be feasible if there are large numbers of CCUS projects. CO2 shipping and trucking is also in operation, using tanks similar to those used to transport liquefied petroleum gas.
CO2 can be used in many ways, the most common being enhanced oil or gas recovery, in which CO2 is pumped into an oil or gas field at low pressure to increase fuel production. CO2 can also be used to produce biofuels, methanol, urea (to produce fertiliser), formic acid and other chemicals. CO2 can be converted via chemical reaction into a number of minerals, some of which are useful and can be sold.
The most common permanent storage site is a porous rock formation covered with an impermeable caprock. It must be a well-mapped site with no faults or areas of thin caprock through which the CO2 could burst or leak, and any existing wells must be plugged and monitored.
Luckily, in Europe there are many depleted oil and gas wells meeting these exact requirements, many of them offshore. The geology of these sites is well understood and they have stored liquid and gas, often containing CO2, at high pressures for thousands of years.
CO2 injection into geological storage sites has been ongoing for decades as part of enhanced oil and gas-recovery programmes. In the absence of sufficient depleted oil and gas fields, saline aquifers meet the same requirements for storage sites.
Storage of CO2 in rock formations requires long-term monitoring and laws have already been agreed in many countries regarding responsibility for this activity and liability for any potential leakage. CO2 will become irreversibly trapped in porous rock, eventually becoming mineralised to form part of the solid structure of the storage site.
PERFORMANCE AND COST
Currently, the costs of CCUS technology are quite high. A supercritical coal plant using first-generation liquid solvent to capture all its CO2 would see its electricity output reduced by roughly 20%. A recent report indicated that oxy-combustion would become economic for coal plant at a carbon price of US$60 per tonne of CO2. One reason for utilisation rather than storage is to reduce the price of the project. Second-generation CCUS technologies will be more efficient.
Today, CCUS is in operation at large scale in the oil and gas industry. There are a number of projects capturing two million tonnes or more of CO2 per annum from natural gas purification and using this CO2 for enhanced oil recovery. To date, no large-scale carbon capture plants have been permitted in other industries.
There are a few pilot-scale plants (up to 40 MJ/s) in operation for both power and industrial processes, and one demonstration-scale plant (110 MW of net electrical power) under construction, expected to become operational in 2014. A number of demonstration plants are at advanced stages of design and awaiting final investment decision; however, the recession has reduced the funding available and many projects are struggling to fill funding gaps.
Utilisation of CO2 is key for the projects that have the best chance of success in the current economic climate: a plant in the US will capture CO2 from a power plant and use it to produce fertiliser, selling the excess CO2 for enhanced oil recovery. This ‘tri-gen’ plant will produce power, CO2 and fertiliser, with the majority of the income expected from the fertiliser production.
While fertiliser production essentially means the CO2 is recycled rather than permanently stored, this type of plant is an important stepping stone to a long-term CCUS industry.
CCUS is expected to be applied widely and become a multi-billion euro industry. Many countries, including EU countries, already have legal and commercial drivers in place or planned, such as carbon credits/taxes, levies, and laws on CCUS. However, storage of CO2 is currently illegal in Ireland, even offshore.
CO2 can smother or poison humans and animals, and natural CO2 releases have been fatal. A large leak from a storage site or pipeline would be extremely dangerous, and safety concerns are similar to those for natural gas pipelines. This can be handled through robust laws, careful planning and public interaction. Good design, including block valves and automatic shut-down of pipelines, and the proper mapping and monitoring of storage sites, also play an important role.
Cost is another concern, but utilisation of CO2 and improved design for next-generation technologies will reduce costs significantly. It is estimated that at least 10 demonstration-scale plants each costing approximately one billion euro are required to reduce the costs of the technology to the level needed for widespread rollout.
To have any real impact on global emissions, CCUS must be rolled out within 10 years or we will be too late to meet the 2°C target.
CCUS offers the possibility of continuing to use cheap high-energy fuels while giving us time to develop sustainable sources of energy. People in emerging economies have the right to use their resources to improve their quality of life. People in developed countries want their children to have a high standard of living. With CCUS, we can do both of these things, while responding to the threat of climate change.
Dr Kirsten Foy is a chartered mechanical engineer with a doctorate in carbon capture technology. She has 10 years of experience in both research and application of carbon capture and storage (CCS) technologies. Dr Foy is a principal engineer with Parsons Brinckerhoff and is currently project manager for the UK Government’s Industrial 2050 Carbon Reduction and Energy Efficiency Roadmaps.
Parsons Brinckerhoff has a strong CCS team offering technical advice on all aspects of CCS, from planning to design to policy. It has advised a number of governments and international institutions as well as developers of CCUS projects and has published a number of reports on carbon capture. See www.pbworld.com.
 The World Energy Outlook report published by the International Energy Agency in 2008 found that limiting the CO2-equivalent concentration of greenhouse gases in the atmosphere to 450ppm would limit the global temperature increase to 2°C.