Over 90% of the CO2 produced by fossil fuels at large fixed installations can be captured and prevented from reaching the atmosphere. There are three main technology types for CO2 capture - pre-combustion, post-combustion and oxy-firing - allowing CO2 to be captured from industrial processes such as power generation, oil refining and cement manufacture.

  • Pre-combustion capture

A pre-combustion system involves first converting solid, liquid or gaseous fuel into a mixture of hydrogen and carbon dioxide using one of a number of processes such as ‘gasification’ or ‘reforming’.

Reforming of gas is well-established and already used at scale at refineries and chemical plants around the world. Gasification is widely practiced around the world and is similar in some respects to that used for many years to make town gas.

The hydrogen produced by these processes may be used, not only to fuel our electricity production, but also in the future to power our cars and heat our homes with near zero emissions.

  • Post-combustion capture

CO2 can be captured from the exhaust of a combustion process by absorbing it in a suitable solvent. This is called post-combustion capture. The absorbed CO2 is liberated from the solvent and is compressed for transportation and storage. Other methods for separating CO2 include high pressure membrane filtration, adsorption/desorption processes and cryogenic separation.

  • Oxy-fuel combustion systems

In the process of oxy-fuel combustion the oxygen required is separated from air prior to combustion and the fuel is combusted in oxygen diluted with recycled flue-gas rather than by air. This oxygen-rich, nigtrogen-free atmosphere results in final flue-gases consisting mainly of CO2 and H2O (water), so producing a more concentrated CO2 stream for easier purification.

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Today CO2 transport is carried out by truck, ship or pipeline. However, to transport the large amounts of COfrom power plant emissions, pipelines are the only practical solution. This pipeline CO2 transportation process is well understood as CO2 pipelines have been used since the 1970s, transporting large volumes of CO2 to oil fields for enhanced oil recovery (EOR). For example, US pipeline infrastructure has the capacity to safely and reliably carry 50 million tons of CO2 a year.

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The oil and gas industry has years of experience injecting CO2 underground into geological formations in a process used to enhance oil recovery (EOR). Millions of tonnes of CO2 are injected annually under regulations which protect local communities and the environment. As oil and gas has become more difficult to access, the industry has rapidly developed precise drilling practices to meet the challenge. This technology is being deployed to ensure CO2 storage takes place safely and securely.

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Natural Trapping Mechanisms

Oil and gas have remained underground for millions of years. The same natural conditions allow injected CO2 to be stored securely. Once CO2 is injected deep underground (typically more than 800 meters) it is absorbed and then trapped in minute pores or spaces in the rock structure. Impermeable caprock acts as a final seal to ensure safe CO2 storage for millions of years.

Structural trapping - at the storage site the CO2 is injected under pressure deep down into the ground until it reaches the geological storage formation. The rocks of the storage formation are like a rigid sponge; they are both porous and permeable. Fluid CO2 tends to rise towards the top of the formation until it reaches an impermeable layer of rock overlying the CO2 storage site. This layer, known as the caprock, securely traps the CO2 in the storage formation. Structural trapping is the same mechanism that has kept oil and gas securely stored under the ground for millions of years.

Residual trapping - another natural process further traps the CO2. As the injected CO2 moves up through the geological storage site towards the caprock some is left behind, trapped in the microscopic pore spaces of the rock. This process is similar to air becoming trapped in a sponge.

Dissolution and mineral trapping – two additional mechanisms also trap CO2. Over time the CO2 stored in a geological formation will begin to dissolve in the surrounding salty water. The salty water combined with the CO2becomes heavier and sinks towards the bottom of the formation over time. This is known as dissolution storage. Mineral storage occurs when the CO2 held within the storage site binds chemically and permanently with the surrounding rock.

Depleted hydrocarbon reservoirs, such as oil and gas fields, are highly suited to such geological CO2 storage. Other potential storage sites are saline formations – permeable rock formations, which contain salty water in their pore spaces - and unminable coal beds. According to the Intergovernmental Panel on Climate Change (IPCC), such geological formations could provide storage space for at least 2,000Gt (billion metric tonnes) of CO2.

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A wide array of monitoring technologies has been used by the oil and gas industry to track fluid movement in the subsurface. These techniques are readily adaptable to CO2 storage to monitor the behavior of CO2 underground. For example, seismic surveying provides an image of the subsurface, often allowing the behavior of stored CO2 to be mapped and predicted. Other monitoring technologies include down hole and surface CO2 sensors. New technologies such as satellite imaging, which can detect movements of less than 1mm in the Earth's surface are also being developed.

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