‘Carbon Capture and Storage’ (CCS) is a technological process to prevent the release of CO2 from industrial activities into the atmosphere. Let us examine the steps involved, the types of capture systems and the latest innovations.
The objective of Carbon Capture and Storage (CCS) is to reduce greenhouse gas emissions and thus mitigate climate change. It plays a key role in the decarbonisation of ‘hard-to-abate’ sectors, for which no other equally effective solutions exist to date. This is why the IPCC (United Nations Intergovernmental Panel on Climate Change) in its 2023 report defined CCS as an essential technology for meeting global climate targets. In turn, the IEA (International Energy Agency) estimates that CCS will contribute to a 10 per cent reduction in total emissions to be abated over the next 30 years, reaching a value of 6.2 billion tonnes ‘captured’ in 2050.
As of 2022, about one thousandth of global CO2 emissions are captured through CCS, and most of the projects concern fossil gas processing. The technology generally has an efficiency rate of between 50 and 68% of the carbon captured, but some projects have exceeded 95% efficiency.
The terms CCS and CCUS (‘Carbon Capture, Utilisation, and Storage’) are often used interchangeably. The difference between the two is the specified use of captured CO2 for other applications, such as enhanced oil recovery (EOR), the potential production of liquid fuel, or the production of useful consumer goods, such as plastics. Since both approaches capture the emitted CO2 and store it, the two terms are often treated in the same way.
The three stages of the process: capture, transport and storage
In greater detail, in the CCS process, a relatively pure stream of carbon dioxide from industrial sources is separated, treated and transported to a long-term storage site. The CO2 can be captured directly from an industrial source, such as a cement factory, steel mill, chemical plant or biomass plant, using a variety of technologies, including absorption, adsorption, chemical circuit, membrane gas separation or gas hydration. The CO2 is then stored in an underground geological formation.
The CCS technique consists of three stages: capture, transport and storage. In the capture phase, CO2 is separated with the help of technologies placed before or after combustion (such as, post-combustion capture by chemical absorption, pre-combustion capture by fuel gasification, capture by condensation from waste gas after oxyfuel combustion). The transport of CO2 , if the confinement site is not already in the vicinity of the potential release site, can take place in the supercritical state through high-pressure pipelines or in liquid form by means of ships. Finally, storage is achieved by injection and confinement of gas within suitable and safe underground geological formations (depleting hydrocarbon fields, saline aquifers, hydrocarbon reservoirs).
The US National Energy Technology Laboratory (NETL) has reported that, at current production rates, North America has sufficient CO2 storage capacity for more than 900 years. A general problem is that long-term predictions on the safety of underwater or underground storage are very difficult and uncertain, and there is still a risk that some of the CO2 could escape into the atmosphere. Despite this, a recent assessment estimates the risk of substantial leakage to be rather low.
Capture technologies in industry
There are basically three different types of CO2 capture systems at the industrial level. The first is post-combustion, in which CO2 is captured from the spent combustion fumes by absorption into a chemical solvent. The CO2 is then separated from the solvent and compressed for transport and storage. Other methods of post-combustion separation are by high-pressure membrane filtration, or cryogenic separation.
In the pre-combustion system, the fuel is converted before combustion into a mixture of hydrogen and carbon dioxide using a gasification process. The CO2 can then be transported and stored, while the hydrogen, mixed with air, can be used as fuel for electricity generation and, potentially, to power hydrogen-powered cars. A typical example of this process is an Integrated Gasification Combined Cycles (IGCC) plant in which coal is converted into synthesis gas prior to combustion.
Finally, in the oxyfuel system, or oxygen combustion, pure oxygen, or highly enriched air, is used in the combustion chamber. This type of combustion produces mainly steam and concentrated carbon dioxide, which is easier to process and send for storage.
These are complemented by systems for capturing and sequestering carbon dioxide in the environment, known as Carbon Dioxide Removal (CDR).
Finding a valid alternative to the use of amine solvents
With regard to CO2 capture, the challenge is to develop an innovative technology offering an alternative to conventional technologies based on the use of amine solvents (that is, aqueous solutions of specific amines of different nature). For example, the technology under development at Eni’s laboratories is based on the use of innovative solvent mixtures containing ionic liquids. The hallmarks of this innovation are high flexibility in dealing with gases of different composition (CO2 content), high solvent stability, a capture principle that exploits both the chemistry and physical characteristics of CO2, and low toxicity.
Carbon dioxide captured in any of the previous ways can be transported and injected into a suitable confinement site, such as, a geological trap that can contain this gas for a period of time in the order of hundreds of years. The captured CO2 can also be used for the assisted recovery of quantities of hydrocarbons, which otherwise could not be recovered.
In this case, it is injected into an oil field (instead of water or natural gas) repressurising the field, allowing the hydrocarbons to rise to the surface while CO2 remains trapped in the field.
Developments and opportunities
Thanks to CCS and CCU technologies, CO2 can become the basis for the creation of new production chains, and this is especially true for the energy industry. Regarding Italy, ‘hard-to-abate’ industries contribute about 20 per cent of total emissions.
To date, there are no viable technological alternatives to reduce their emissions. Still in the area of storage, a further positive aspect is the possibility of reusing depleted gas fields and decommissioned assets, such as Eni UK’s Hamilton, North Hamilton and Lennox fields, which will be covered by the HyNet project. Or those in the Ravenna offshore, also owned by Eni. As for HyNet, CCS activities will have an initial capacity of 4.5 million tons of CO2 per year (Mton/a), with the possibility of expanding it to 10 Mton/a by 2030.
Emissions will come from industries in the North West of England and North Wales, captured directly at smokestacks and transported to depleted fields. In addition to CCS, a major hydrogen production site will also be built.
Regarding the use of CO2, Eni is working on mineralisation technology, a project based on the reaction between CO2 and some mineral phases, mainly magnesium and/or calcium silicates. This reaction, which occurs spontaneously in nature but takes a very long time, is the basis for industrial processes capable of permanently fixing large quantities of CO2 in the form of inert, stable and non-toxic products.
A further area of research concerns methods for using CO2 in the production of methanol, an energy vector with great potential. A larger project, on the other hand, aims to capture CO2 directly in vehicles.
Source: Controllo e Misura by Publitec