CO2 can be captured from large sources, such as power plants, natural gas processing facilities and some industrial processes. Capture from the open atmosphere is also possible.
Where fossil fuels are burnt at power plants, there are three techniques to remove or ‘scrub’ CO2:
» post-combustion
» pre-combustion
» oxyfuel combustion
Post-combustion In this process, CO2 is removed after burning the fossil fuel. CO2 is captured (‘scrubbed’) from the exhaust (or ‘flue’) gases. This is the method that would be applied to most conventional power plants as it can be retro-fitted. The technology is well understood and is currently used in other industrial applications.
Pre-combustion This technique traps CO2 before burning the fossil fuel. First, the fossil fuel is partially burned in a ‘gasifier’ to form synthetic gas. CO2 can be captured from this relatively pure exhaust stream. The process also produces hydrogen, which can be separated and used as fuel.
Pre-combustion is used in the production of fertiliser, chemical gas fuel and power production. It is a cheaper option than post-combustion but cannot be retro-fitted to old power plants.
Oxyfuel combustion In oxyfuel combustion, the fossil fuel is burned in oxygen instead of air. The resulting flue gas consists of mainly CO2 and water vapour. The water condenses through cooling and the result is almost pure CO2 that can be transported and stored.
Electricity plant processes based on oxyfuel combustion are sometimes referred to as ‘zero emission’ as nearly all the CO2 is captured. It is possible that some CO2 will dissolve in the condensed water, so the water may have to be treated.
Hydrogen production Another way of reducing atmospheric emissions of CO2 in some areas, such as heavy goods vehicles, where battery-based electric power is unsuitable, and trains, and domestic heating and gas cookers, is to convert to hydrogen.
Whilst hydrogen produced by a process called electrolysis of water using renewable power has very low CO2 emissions, it is currently expensive. A cheaper and more mature option to enable the conversion to hydrogen to be started is to produce hydrogen from natural gas. This process is called methane reformation. However, the CO2 produced in the reforming process must also be captured and stored.
Direct air capture It is possible to capture CO2 directly from the open atmosphere, but this technology is new and not matured yet.
The energy needed for air capture is only slightly more than for capture from large emission sources. The costs may be higher as well, but may be feasible for dealing with emissions from diffuse sources.
