Capture, transport and storage of CO2

Fossil power production and power-intensive industry are associated with substantial CO2 emissions. Capture of CO2 and storage in geological formations has emerged as an important potential measure to reduce global greenhouse gas emissions.

Fossil power production and power-intensive industry are associated with substantial CO2 emissions. Capture of CO2 and storage in geological formations has emerged as an important potential measure to reduce global greenhouse gas emissions.

There has been considerable international interest for a number of years in developing technology for capture and storage of CO2. In Norway, the focus has primarily been on capture and storage of CO2 from gas power plants. Both the IEA and the IPCC note that capture and storage of CO2 will be necessary in order to achieve the two-degree target. In its fifth main report, the IPCC emphasises that the costs of achieving the two-degree target will more than double without CCS (IPCC, 2014b).

We have little experience in operating an entire CCS chain. CCS technology is still expensive, and more testing is needed.

CO2 capture

There are three main concepts that are most relevant for large-scale CO2 capture. They are distinguished according to where CO2 is captured in the process:

  • Capture after combustion
  • Capture before combustion
  • Capture during combustion with pure oxygen

Fossil power generation forms flue gas which contains CO2. The volume of CO2 in flue gas from power plants can vary from 3-4 per cent for gas power plants to 12-15 per cent for coal power plants. As regards industrial emissions, the concentration can vary more. When CO2 is captured after combustion (post-combustion) CO2 is separated from the flue gas after combustion in the power plant using chemical cleaning. There are a number of different technologies for post-combustion capture, including techniques based on amine blends and chilled ammonia. Because CO2 is separated from the flue gas, these capture technologies can be used on existing power plants without major modifications of the actual plant. Pre-combustion capture takes place by converting the natural gas to a hydrogen-rich gas mixture. The gas mixture is treated so that CO2 is captured, and the new fuel is thus "decarbonised", which means that the flue gas then contains very little CO2.

In combustion using pure oxygen (oxy-fuel), the fuel is combusted using pure oxygen instead of air. Oxygen can be produced in a plant which separates oxygen from air.

A challenge shared by all three concepts is reducing costs and energy consumption associated with capturing CO2. With today's technology, coal power plants have a power efficiency of 45-47 per cent. The power efficiency is the ratio between produced power and the energy content in the fuel. Gas power plants with a combination of gas and steam turbines have a comparable efficiency of 58-59 per cent. With CCS, the efficiency for both types of power plants will decline by 10 percentage points, or more. Furthermore, there are considerable investments associated with CO2 capture, which also result in increased operating costs for the power plant.

The challenges surrounding CO2 capture demand targeted research, development and demonstration.

CO2 transport

CO2 can be transported by pipeline or ships to a suitable location for permanent storage. Which alternative is best, depends on the volume of CO2 to be transported, the distance between source and storage, as well as the period of time CO2 transport will be needed from the respective source. Smaller volumes, longer distances and/or need for CO2 transport over a shorter period favours ship transport, while pipeline transport is relevant for larger volumes, short or moderate distances and CO2 capture sources with a long lifetime.

CO2 transport shares many of the same characteristics as transport of oil and gas, both as regards pipeline and ship transport. CO2 in liquid form has unique properties that pose specific challenges, and must therefore be subjected to special analysis. Challenges associated with pipeline transport include corrosion, modelling flows in the CO2 pipeline and dispersal modelling in the event of leaks. Ship transport of CO2 is familiar technology and it has been practiced for around 20 years in small volumes for the food industry. Offloading CO2 from ships to an underground reservoir will require qualification of technology.

CO2 storage

Safe storage of CO2 presumes that CO2 is injected into a suitable type of rock which will ensure that it does not leak out. A number of preconditions must be fulfilled to achieve safe storage. The storage reservoir must have adequate capacity to receive and store CO2 over the entire project lifetime, and there must be a seal rock in place which prevents CO2 from moving upwards to the surface. The location should be deeper than 800 metres below the seabed to achieve sufficient temperature to ensure that the CO2 is in a compressed, liquid form that maximizes the volume of CO2 that can be stored.

There is a significant technical potential for storing CO2 in geological formations around the world. Both oil and gas fields in production as well as old oil and gas fields and other formations may be candidates for such storage.

Norway has extensive experience with storing CO2 in geological structures. Since 1996, one million tonnes of CO2 have been separated each year from the gas production on Sleipner Vest in the North Sea, and stored in Utsira, a geological formation 1000 metres below the seabed. Since 2008, CO2 has been separated from the wellstream on the Snøhvit field at the onshore LNG facility on Melkøya.

CO2 can also be injected into oil fields to enhance recovery (EOR). This has been done on land since the 1970s in the US. Studies have also been conducted of the potential for using CO2 to improve recovery on the Norwegian shelf. Under the current conditions, this is not profitable. When CO2 gas is injected into an oil reservoir, the gas blends with the oil and makes the oil swell. CO2 also reduces the surface tension between oil and water. All this entails that the oil flows more easily, making it easier to produce the oil, and more oil can be recovered from the reservoir. In order to use this method, the reservoir pressure must be above the release pressure for CO2, otherwise the CO2 will not mix with the oil. Another challenge is that some of the CO2 that is injected into the reservoir will be produced along with the oil. This means that the facility must be equipped to recapture the CO2 and return it to the reservoir. This entails substantial additional costs.

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