The first step in carbon capture and storage (CCUS) is separating the carbon dioxide from other gases in the exhaust stream and, in the process, capturing the carbon dioxide (CO2)

What is capture

Combustion of fossil fuels – coal, oil and natural gas, releases carbon mainly in the form of carbon dioxide (CO2), a greenhouse gas. This CO2 becomes part of exhaust gases that are emitted from the smokestacks of power plants and factories. CO2 is also released to the atmosphere from natural gas during its separation process after it is produced from the well. The idea behind CCUS is to “capture” the CO2 before it is released to the atmosphere where it contributes to the risk of climate change. This is achieved by attaching a capture plant to a stationary source like an industrial process, power station or natural gas separation facility. The capture plant uses various technologies to capture the CO2 instead of releasing it to the atmosphere.

For the vast majority of CCUS applications, the cost of CO2 capture is the largest contributor to overall CCUS system cost.  The cost of capture depends in large measure on the pressure and concentration of CO2 in the flue gas or process stream from which the CO2 is being separated.  CO2 capture costs vary considerably across various types of large CO2 point sources and as a general rule, it is cheaper to capture CO2 from a purer and higher pressure CO2 stream.

The Otway capture research skid
The Otway capture research skid (2016- 2019)
Capture research facility at the former Hazelwood power station.

Capture processes

There are three main processes to separate carbon dioxide from other gases in electricity generation: post-combustion, pre-combustion and oxyfuel with post-combustion. Various capture technologies including solvent absorption, membrane, adsorption, and cryogenics can be applied to each type of capture. Similar methods are also used for other industrial processes, including the separation of CO2 from natural gas.

Post-combustion capture

Separates CO2 from combustion exhaust gases, which contain a mixture of 10-15% CO2 and the rest, mainly nitrogen (N2). CO2 can be separated using liquid solvent absorption or other methods such as membrane and adsorption capture technologies.The captured CO2 can then be compressed and transported for storage.

Absorption technology is the most advanced of all capture technologies. Once absorbed by the solvent, CO2 is released by heating, forming a high purity CO2 stream.

Post-combustion capture is applicable to most industries and existing power plants.

post-combustion-capture

Post-combustion carbon capture

Pre-combustion carbon capture

Pre-combustion capture

Processes deal with syngas, a gaseous mixture created by the conversion of solid, liquid or gaseous fuel consisting of hydrogen (H2), carbon monoxide (CO) and CO2. The CO is converted to CO2 and the hydrogen is separated and can be burnt without producing any emissions. The CO2 is captured using various technologies and can then be compressed and transported for storage.

The fuel conversion steps required for pre-combustion are complex involving high pressure gases produced by gasification of fuels, making the technology applicable only to those specific facilities or power plants. However, the reforming of gas is well established and has been used for decades at refineries and chemical plants around the world.

Oxyfuel combustion

Processes use oxygen rather than air for combustion of fuel. This produces exhaust gas that is mainly water vapour and CO2 that can be easily separated to produce a high purity CO2 stream.

Oxyfuel carbon capture

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