Last edited 14 Oct 2019

Carbon capture processes

Contents

[edit] Introduction

Carbon capture is a process that can remove as much as 90% of the carbon dioxide (CO2) gas emitted from fossil fuel (coal, oil or gas) power stations which are the main source of anthropogenic CO2 emissions. But the process is also used in cement making and other industries.

Carbon capture has been used for decades by the oil and gas sectors, although long-term storage of CO2 is a relatively new activity. It is thought that carbon capture could achieve around 20% of the total greenhouse gas emissions reductions needed by 2050; it is seen as one of the only practical ways to efficiently decarbonise the industrial sector quickly.

[edit] Carbon capture and storage (CCS)

CO2 captured from industrial processes is captured for secure and indefinite storage. In the UK storage of large quantities of CO2 is likley to take place in offshore underground saline formations or depleted oil and gas fields.

Although at the time of writing there are around 17 operating CCS projects globally, there are no commercial-scale CCS projects in the UK, but relevant health and safety regulations exist for future operators to abide by.

There are three stages to carbon capture and storage:

  1. CO2 is trapped and separated from other gases.
  2. CO2 is transported to a storage site.
  3. CO2 is stored away from the atmosphere.

[edit] Trapping and separating CO2

There are three ways to extract CO2:

  • Post-combustion capture - applied mostly to power plants, post-combustion capture involves capturing the CO2 from the flue gases produced by fossil-fuel combustion. Typically, chemical solvents are used to achieve this. The technology can be retrofitted to existing power plants.
  • Pre-combustion - this process involves removing the CO2 before combustion takes place in chemical, fertilizer, gaseous fuels and power production processes. Rather than being combusted, fuel is ‘gassified’ to produce a synthetic gas (syngas). The carbon monoxide (CO) from this is converted to CO2 which, after the action of solvents, is freed from the hydrogen present.
  • Oxy-fuel combustion - fossil fuel is burned in oxygen to produce a steam-CO2 mixture from which the CO2 is separated by cooling and compression. This is a relatively expensive process due to the amounts of oxygen required, however it ensures that only around 10% of a power plant’s emissions enter the atmosphere.

It may also be possible to extract CO2 from ambient air (direct air capture), although it is more difficult than the aforementioned processes due to the lower concentration of the gas in air. Work on this is still at an early stage.

[edit] Transportation of CO2

Once captured, CO2 must be transported to suitable sites for permanent storage. Conveyance is mostly by pipeline as it is one of the cheapest methods for large volumes. CO2 can also be transported – eg by ships – to other locations for industrial useage.

[edit] Storing CO2

The three techniques available for storing CO2 are:

  • In deep geological formations.
  • In deep ocean water.
  • In the form of mineral carbonates (mineral storage).

For more information see: http://www.bgs.ac.uk/discoveringGeology/climateChange/CCS/howcanCO2bestored.html

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