The first step in carbon capture and storage (CCS) is separating the carbon dioxide from other gases in the exhaust stream and, in the process, capturing the carbon dioxide (CO2).
CO2 capture can be applied to a variety of stationary sources of CO2, using a variety of CO2 capture technologies. The CO2CRC Capture Program researches, develops and demonstrates technologies that can reduce capture costs by 75 to 80 per cent.
CO2 capture applications
In a few instances, industrial processes emit fairly pure carbon dioxide which can be captured and separated relatively cheaply. Such processes include the manufacture of some fertilisers, natural-gas processing (where any carbon dioxide has to be separated from the natural gas to make it marketable) and cement manufacturing (the exhaust stream from modern cement plants have carbon dioxide concentrations of up to 50 per cent).
Most of modern industrial society’s emissions of carbon dioxide come from coal-fired power stations, which can have flue-gas carbon-dioxide concentrations as low as 10 to 15 per cent. Separating such low concentrations of carbon dioxide from an exhaust stream is a complex and, with today’s technology, expensive process. CO2CRC is conducting extensive research into CO2 capture technologies to reduce this cost.
Capturing the carbon dioxide from a typical existing power plant is referred to as post-combustion capture, in which the low-pressure exhaust gases (currently emitted to the atmosphere) are passed through a separation process. Post-combustion facilities can be retrofitted to existing power plants or provided as a feature of new plants in the future, but there is a need to bring down costs considerably. Despite the existing cost barrier, post-combustion capture is receiving increased attention because of the realisation that many existing coal-fired power stations will continue to operate for 30 years or more.
An alternative approach, pre-combustion capture, involves Integrated Gasification Combined Cycle (IGCC). In this type of plant, the fuel is not burnt, but is reacted at high pressure and temperature to form a synthesis gas containing carbon monoxide, carbon dioxide, and hydrogen. This gas stream is then reacted further with water to convert the residual carbon monoxide to carbon dioxide and hydrogen, allowing the carbon dioxide to be captured and sent to storage. The hydrogen is burnt to produce power, leaving water vapour as the main exhaust to the atmosphere.
Oxyfiring or oxyfuels
Another approach is oxyfiring combustion, sometimes called oxyfuel combustion. This technology is similar to that used in existing power plants, except that rather than burning the fuels in air, they are burnt in an artificially created oxygen atmosphere. Without the nitrogen which makes up about 78 per cent of the Earth’s atmosphere, this results in a flue gas with high carbon-dioxide concentrations (greater than 80 per cent by volume). The water vapour is then removed by cooling and compressing the gas stream. Changes are required to the boiler and associated flue-gas handling system to accommodate the higher flame temperatures resulting from combustion with oxygen.