One of the key challenges in the production of synthetic fuel is to find a source of carbon dioxide that is easily accessed at low cost and is reliable. While it might appear attractive to look to the relatively high concentrations of CO2 in exhaust plume from other production processes, there is a risk that these processes will change or cease as pressure comes on businesses to reduce all forms of carbon emission.
A better option might be to look at the much lower concentrations of CO2 present in the air, which is all around us and can be freely accessed. This universal availability will allow the extraction plant to be situated at any convenient location, as the only input required is electricity.
There are a number of different approaches to direct air capture for carbon dioxide, but they all involve passing air over or through a solid or liquid chemical agent that reacts with the CO2 to bind it and remove it from the air. The resulting agent is then treated, usually with heat, to release the captured CO2 and restore the agent to its original state for reuse. The different processes have varying requirements for operation (such as high temperatures or pressures during the carbon release phase), but they all require energy to drive them.
The energy to drive the processes is expected to be provided by electricity generated from renewable sources to avoid any further carbon emissions Depending on the process, there may be heat generated during some steps which can be recovered and used in other steps of the process.
There are reported to be about 20 direct air capture plants operating in America and Europe at present, most being small scale or pilot operations. However, Carbon Engineering in Canada currently has plans to produce a large scale plant capturing a million tonnes of CO2 per year, with additional large scale plants to follow.
The captured carbon dioxide is able to be used in industrial processes, from carbonating drinks through to the manufacture of synthetic fuel, or it can be sequestered for long term storage to assist in removal of carbon from the atmosphere. The largest plant using direct air capture for sequestration is currently in Iceland.
For more details on the current state of direct air capture technology and use, refer to the International Energy Agency report on Direct Air Capture.