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Keeping a cap on carbon: part one

Hydrocarbon Engineering,

Climate change is widely recognised as being one of the major global threats, with far reaching consequences, including rising sea levels, increasing numbers of severe weather events, disruption to ecological environments and a rising trend for mass population migration as people seek to move away from the worst affected areas.

A major culprit has been identified as increasing production levels and release to the environment of the major greenhouse gas CO2. Highly efficient at capturing atmospheric heat, CO2 is driving the rising trend in global temperatures that is causing climate change.

Capturing the carbon

The pressing need to limit the release of CO2 to the atmosphere has led to increasing interest in carbon capture and storage (CCS) technologies. There are three stages to the process: capture, transport and storage. In the capture stage, the CO2 is removed from the plant’s emissions, using one of three methods: post combustion, pre combustion and oxy fuel combustion.

In the post combustion method, CO2 is captured from the exhaust of a coal or gas power plant by absorbing it in a solvent. The absorbed CO2 is removed from the solvent and compressed for transport and storage. The solvent can then be reused to capture further CO2. Other methods for separating CO2 include high pressure membrane filtration, adsorption/desorption processes and cryogenic separation.

In pre combustion capture, a solid, liquid or gaseous hydrocarbon fuel is converted into a mix of hydrogen and CO2, using well established processes such as gasification or reforming. With the CO2 separated and compressed for transport, the hydrogen produced can then be used as a zero carbon fuel.

For oxy fuel combustion systems, oxygen is separated from air prior to combustion and the coal or gas is then burned in this oxygen. This results in a final flue gas mix consisting mainly of CO2 and water, producing a more concentrated CO2 stream for easier separation.

For the transport stage, the CO2 is compressed and transported to a suitable storage site, most commonly by pipeline. The pipelines will be very similar to those already used for the transport of natural gas and will be subject to the same standards. Transport by ship is also an option for offshore storage.

The storage stage involves injecting the CO2 into a storage site deep below the ground, usually depleted oil and gas fields or deep saline formations. The site must be geologically stable to allow for safe, long term storage.

The UK government has recognised that CCS, combined with CO2 storage in oil reservoirs, could make a significant contribution to meeting its commitment to reducing the country’s production of greenhouse gases.

The oil and gas factor

Companies extracting oil and gas can potentially play a major part in the story. Storage of CO2 in depleted oil and gas fields, either on or offshore, is a major long term option, with two projects in the North Sea currently being the only operational facilities of their kind in Western Europe.

The world’s first large scale CCS facility was established by Statoil at the Sleipner oil and gas field in 1996. The gas recovered at Sleipner contains 9% CO2, which exceeds the market requirement of 2.5%. If emitted, the captured CO2 would have cost nearly US$60/t in carbon taxes and emission quota purchases. By stripping the CO2 from the natural gas, compressing it and injecting it into a storage reservoir 1000 m beneath the seafloor, Statoil is able to meet the market specifications, cut its greenhouse gas emissions and avoid considerable cost. Approximately 1 million t of CO2 is stored at Sleipner every year.

Statoil opened another large scale CCS process at the Snøhvit gas field in the Barents Sea in 2008. There, natural gas with a CO2 content of between 5 - 8% is piped 143 km to an onshore LNG plant on the island of Melkøya. The excess CO2 is stripped at Melkøya and piped 153 km offshore, where it is injected into a saline aquifer 2800 m below the seafloor. The facility captures and stores up to 700 000 tpy of CO2.

Statoil has also invested in a Carbon Dioxide Technology Centre at Mongstad, which will develop carbon capture technologies and further reduce the cost and risks associated with large scale capture and storage.

Part two of the article will be available soon.

Written by Will Leonard, Chemical, Oil and Gas UK, ABB Limited. This is an abridged version of an article taken from the June 2015 issue of Hydrocarbon Engineering.

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