“The single most effective way to tackle these [greenhouse] gases is to capture them and store them safely underground”

Shell Website

Carbon capture and storage (CCS) involves trapping the CO₂ we emit and storing it in order to prevent it entering the atmosphere. It is frequently flouted as the answer to our climate change problems and is particularly attractive to policy makers because it allows us to continue burning fossil fuels, which, unlike wind turbines, provide a reliable energy supply. Technically, CCS could even be used in conjunction with biofuels to achieve a net removal of CO₂ from the atmosphere. Meanwhile, Greenpeace argues that “[CCS] technology is largely unproven and will not be ready in time to save the climate.” So is CCS the answer or not?

The first stage in the CCS process is to capture the CO₂. This is only realistically possible for large point sources like power plants. Next, the CO₂ must be compressed and cooled for transportation via either tanker or pipeline and then stored.

Various storage sites have been suggested. CO₂ could be stored deep in the ocean, but this has not yet progressed beyond the demonstration phase and there are concerns about increasing ocean acidity. Another option is to react the CO₂ with naturally occurring metal oxides to form carbonate minerals. Unfortunately, this requires such extensive quarrying of metal oxides that any given power plant would need to use more energy capturing and storing its CO₂ than it produced in the first place. Given current technology, the only suitable CO₂ storage option is underground.

Storage space exists underground in depleted oil and gas reservoirs – CO₂ can even be injected into dwindling oil reservoirs to help recover the last dregs of oil, a process known as enhanced oil recovery (EOR). Deep saline aquifers, rocks whose pore spaces are filled with salty water, are another possibility, but their potential is only likely to be realised if the water is removed. The pore spaces in un-mineable coal seams are a third alternative.

The European Association of Geoscientists and Engineers estimates that the EU has room for 20,000Mt of CO₂ in depleted oil and gas fields, 95,000Mt in deep saline aquifers and 1,000Mt in coal seams. The EU currently emits 2,000Mt of CO₂ each year – if deep saline aquifers prove difficult to use, we only have space for ten years’ emissions.

CCS has been put to the test, but to date there are only four industrial-scale examples. At Statoil’s Sleipner gas field off the coast of Norway the natural gas contains 9% CO₂ – more than the allowed maximum for gas entering the pipeline network. Statoil therefore use amine solvents to remove most of the CO₂ and, since carbon taxes are high in Norway, they pump it into deep saline aquifers rather than simply venting it to the atmosphere. Overall, 1Mt of CO₂ is sequestered each year. Other similar projects at Snohvit, Norway, and In Salah, Algeria, also sequester around 1Mt of CO₂ per year. At Weyburn-Midale, in North America, 1.5Mt of CO₂ is captured each year from a gasification plant in North Dakota and piped to southeast Saskatchewan, where it is used for EOR.

However, by 2014 the Gorgon Project in Australia, a joint venture between Chevron, Exxon Mobil and Shell, will be the world’s largest CO₂ sequestration facility. The large reserves of natural gas at Barrow Island also contain more CO₂ than is allowed in the network so a plant will be built to remove and sequester the CO₂ at a rate of 3.5Mt per year.

But these are small numbers. According to the Intergovernmental Panel on Climate Change (IPCC), by 2020 global annual CO₂ emissions are predicted to be anywhere between 29,000Mt and 44,000Mt per year. Furthermore, Greenpeace accuse the coal industry of using CCS as a justification for the construction of new coal-fired power plants. These plants are said to be “capture-ready”, meaning that they can be retrofitted with CO₂ capture, but no large-scale, coal-fired power plants currently do so.

The IPCC estimates that globally we have space for at least 675,000Mt of CO₂ in depleted oil and gas fields and approximately 1,000,000Mt in deep saline aquifers. CCS has a bright future but it is clearly only a small part of the solution to climate change. We need to be honest about how much it can realistically contribute.

Featured image credit: CC BY-NC 2.0 by emanuel balanzategui

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Tim Middleton

Tim is currently an undergraduate in the Department of Earth Sciences at Cambridge University. He has a keen interest in science communication, in particular the public perception of science, and to that end spends an increasingly large amount of his time indulging in student journalism. He is President of BlueSci, Cambridge University’s science magazine, and also writes for the student newspaper Varsity.

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