The level of carbon dioxide in the atmosphere has been shown to have a significant effect on climate change. Using carbon capture and storage (CCS), it is hoped that, by reducing the amount of carbon dioxide released into the atmosphere, it will be possible to mitigate this effect. CCS involves removing carbon dioxide from a process stream before it enters the atmosphere and transporting it to a suitable facility where it can be stored indefinitely.
Carbon dioxide can be removed directly from a process stream using a variety of technologies, including absorption, adsorption, chemical looping, membrane gas separation, or gas hydration . CCS is a relatively expensive process and because of its low intrinsic value, there needs to be a financial incentive to capturing carbon dioxide. Currently, only about 0.1% of global carbon dioxide emissions are captured by CCS.
Economically, carbon capture makes more sense when combined with a process that uses carbon dioxide to produce high-value chemicals. Carbon capture and utilisation (CCU) is sometimes discussed in parallel with CCS as a means of offsetting the high costs of capture operations . CCU and CCS are known collectively as carbon capture, utilisation, and sequestration (CCUS).
This article is intended to provide an overview of the operations involved in CCUS; namely, capture, transportation, utilisation, and storage. From a design and materials selection viewpoint, there are interesting challenges involved in each of these operations, these are not covered in any detail in this article, but it is planned to cover them in future articles.
Carbon capture is really only cost-effective where there is a concentrated source of carbon dioxide. Often impurities will be present which need to be removed before the carbon dioxide can be processed. Not only may these impurities affect the physical behaviour of the gas stream, but they may also present a significant threat of corrosion, particularly to pipelines and associated plant. In instances where impurities are present, gas-scrubbing equipment will almost certainly be needed to remove them .
Sources of carbon dioxide can be roughly split into three categories: pre-combustion, post-combustion, and oxyfuel combustion. The technology required for each category differs.
- Pre-combustion capture applies to processes where fossil-fuels are partially oxidised. An example is the steam-reforming process, where syngas containing carbon monoxide and hydrogen is produced. The carbon monoxide in the syngas can be converted to carbon dioxide and hydrogen by adding steam through the water-shift reaction. It is then relatively easy to capture the carbon dioxide from the resultant gas stream. A major advantage in comparison to post-combustion capture is that the carbon dioxide can be removed whilst the gas stream is still under pressure.
- Post-combustion capture applies where the carbon dioxide is removed after combustion of the fossil-fuel. It would normally be the technology used in large fossil-fuel power plants. The technology is well understood and is currently used in other industrial applications, although on a smaller scale. Post-combustion capture is ideally suited to fossil-fuel power plants as it can be retrofitted to include CCS technology .
- Oxy-fuel combustion capture applies where the fuel is burned in pure oxygen rather than air. To control the temperature in the combustion chamber, cooled exhaust gas is recirculated and injected into the chamber. The exhaust gas consists of mainly carbon dioxide and water vapour. The water vapour can be easily removed such that the resulting gas stream is almost pure carbon dioxide. Whilst not entirely correct, power plants that use oxy-fuel combustion capture are sometimes referred to as “zero emission”, because the carbon dioxide stored is not a component of the exhaust gas but the exhaust gas itself.
Various technologies exist for actually capturing the carbon dioxide . Absorption, or carbon scrubbing with amines, is the dominant carbon capture technology and is, thus far, the only carbon capture technology to have been used industrially . Another option is adsorption. This uses activated carbon , activated alumina , zeolite , or polymeric adsorbents  to strip the carbon dioxide from the gas stream.
With both absorption and adsorption, the carbon dioxide must be removed from the sorbent or solution so it can be reused. This process is known as regeneration. A high working capacity and high selectivity are desirable in order to maximise the volume of carbon dioxide captured. However, a trade off needs to be made between selectivity and energy expenditure . As the volume of carbon dioxide captured increases, the energy, and therefore cost, required in the regeneration process increases.
Capture accounts for approximately two thirds of the cost of CCS. As such, developments in capture technology are key to the success of CCS. By comparison with capture, the transport and storage steps are far more established .
Once captured, carbon dioxide must be transported to a suitable storage site. Where viable, pipelines are the cheapest form of transport. Where pipelines are unviable and for long enough distances, ships may represent an economical alternative . The cost of road and rail transport is roughly twice that of pipelines and ships .
In the US, as of 2008, there were approximately 5,800 km of carbon dioxide pipelines. There is a 160 km pipeline in Norway . These are being used to transport carbon dioxide to oil production sites where it is injected into wells to enhance oil recovery. A number of pilot schemes are currently under development to evaluate long-term storage of carbon dioxide in non-oil producing geologic formations.
A number of approaches have been proposed regarding the permanent storage of carbon dioxide. There are two principal methods of storage. These are geological storage, where carbon dioxide is stored as a gas in deep geological formations, and mineral storage, where stable carbonates are produced by reacting carbon dioxide with metal oxides.
4.1. Geological Storage
Geological storage involves injecting carbon dioxide into under-ground geological formations. In this respect, unmineable coal seams and saline aquifers appear particularly attractive. In case of unmineable coal seams, methane may be released during carbon dioxide absorption. This can be recovered and provides a useful source of energy that can be used to partially offset the cost of storage. Although burning this methane produces additional carbon dioxide, this can be captured and stored. Saline aquifers have the advantage of a large storage capacity. However, relatively little is known about them, and, unlike unmineable coal seams, they produce no by-products that could be used to offset the cost of storage.
Physical and geochemical trapping mechanisms, (structural trapping, residual trapping, solubility trapping, and mineral trapping) may be used to immobilise the carbon dioxide underground and prevent it from escaping to the surface .
4.2. Mineral Storage
Mineral storage takes advantage of the fact that carbon dioxide reacts exothermically with certain metal oxides to produce stable carbonates (e.g. calcite, magnesite). The reaction rate can be accelerated using a suitable catalyst , as well as by increasing the temperature and/or the pressure, or by mineral pre-treatment (although this can require additional energy).
Olivine is one such metal oxide . It reacts readily with carbon dioxide. Depending on the conditions, it has been reported that one tonne of olivine sand can absorb up to one tonne of carbon dioxide. The European climate initiative Climate-KIC estimates that olivine could be used to store up to 850,000 tonnes of carbon dioxide if it was used in small-scale projects in Rotterdam alone.
A major advantage of mineral storage is that the carbonates produced have practical applications. For example, as a replacement for sand and gravel in landscaping projects.
A significant factor affecting the success of CCS is cost. In this respect, it is necessary to consider both the initial capital expenditure and operating costs. For a CCS project to be considered economically viable, its cost must be less than the perceived cost of emitting the carbon dioxide (i.e. the carbon price).
As already discussed, capturing carbon dioxide requires energy. CCS technology can require anywhere between 10 and 40% of the energy produced by a power plant . In the case of a gas-fired power plant, the additional energy required for CCS would increase the fuel requirement by about 15% . It is estimated that the cost of this extra fuel, together with storage and other system costs, would increase the costs of energy from a power plant with CCS by 30-60%.
If there is not an economic argument for CCS, then the decision has to be political and state subsidies need to cover any shortfall in its cost. As of 2018, a carbon price of at least €100 per tonne of carbon dioxide was estimated to make industrial CCS viable  together with carbon tariffs . EU Allowances (EUAs) are expected to average 55.88 euros a tonne in 2021 increasing to 69.87 euros in 2022 . In addition, several countries are currently considering a Carbon Border Adjustment Mechanism to incentivise use of products with low embedded emissions, or carbon footprint  .
Globally, there has been a steady increase in CCS. In 2020, the global capacity of CCS in operation was around 40 million tons per year and around 50 million tons per year in development . When one considers that the world emits almost 38 billion tonnes of carbon dioxide each year , in 2020, CCS captured about 0.1% of the carbon dioxide emissions.
Even with a move to clean energy, industry is still likely to produce enormous volumes of carbon dioxide. If a reduction in carbon dioxide levels in the atmosphere is to be achieved, then much greater investment in new CCS/CCU projects is required.
One such project is the Moomba carbon capture and storage (CCS) project in South Australia. It is a joint venture between Santos and their partner Beach Energy, with an expected start-up in 2024. Located in the South Australian outback, it will be one of the biggest and lowest cost CCS projects in the world. It will safely and permanently store 1.7 million tonnes of carbon dioxide per year in the same reservoirs that held oil and gas in place for tens of millions of years. LFF is proud to have been chosen to supply approximately 55km of FBE coated pipe for the CO2 transmission pipeline and a significant quantity of piping and fittings for the associated processing plant.
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