Capturing the greenhouse gang
Box 2 | Clean technologies for fossil fuels
Of the fossil fuels, coal emits more carbon dioxide than oil or gas and also releases pollutants such as sulfur dioxide, nitrogen oxides and particulates. In climate change forums, the term 'clean coal' technologies generally refers to innovations aimed at reducing carbon dioxide emissions by improving the efficiency of the use of coal for power generation. The technologies fall into three categories: pre-combustion, post-combustion, and oxy-fuel.
Pre-combustion
Integrated gasification combined cycle (IGCC) is an example of lower emission technology, which uses coal to synthesise gas (syngas). The reaction combines the carbon in coal with water to produce a mixture of hydrogen and carbon monoxide. The mixture is then combusted with air or oxygen to drive a gas turbine to generate electricity; the exhaust heat is converted into steam, which then drives a steam turbine which will produce additional electricity.
IGCC improves the efficiency with which coal is converted to electricity and therefore lowers greenhouse gas emissions per unit energy. Further emission reductions can be obtained by removing carbon dioxide from the mix of waste gases.
In a pre-combustion capture plant, the syngas is further reacted with water to convert the carbon monoxide to carbon dioxide and hydrogen. The carbon dioxide is captured and the hydrogen is used as a fuel in a gas turbine to generate electricity. The costs are likely to be high and there no IGCC plants generating electricity with pre-combustion carbon capture currently in operation.
The hydrogen produced in this process could also be used in fuel cells for power generation and in vehicles.
Post-combustion
Post-combustion technologies focus on capturing carbon dioxide after the fuel has burned but before it is released from the power station. Using current technology, the waste gas from the combustion process must first be cleaned of impurities such as sulfur oxides, nitrous oxides and particulate dust. It is then piped to a tall, cylindrical vessel called an absorber and mixed with a solvent at a temperature of about 50ºC. Carbon dioxide molecules bind to the solvent and the cleaned flue gas can then be released into the atmosphere. The carbon dioxide/solvent mixture is sent to a stripper – another tall, cylindrical vessel – and the carbon dioxide removed in the presence of heat. The solvent can be used again, while the carbon dioxide is compressed and sent to long-term storage.
Several post-combustion scrubbing pilot plants are in operation around the world, but the use of the technology at a large scale is held back by its high cost, particularly due to its effect on energy efficiency.
dioxide capture
(©CO2CRC 2008)
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Oxy-fuel combustion
In conventional coal-fired power stations, coal is burned in air, which contains about 20 per cent oxygen. Oxy-fuel combustion uses a gas mix containing about 97 per cent oxygen and recirculated flue gas, which is composed almost entirely of carbon dioxide and water. This process reduces the net volume of flue gas produced while increasing the concentration of carbon dioxide in the flue gas. One advantage of this is that the flue gas is ready for sequestration without the need for post-combustion scrubbing. Another is that the lower volume of flue gas means a lower heat loss and therefore a more efficient process. The disadvantage is the financial and energy cost of producing the oxygen used in the combustion process.
Box
Box 1. Natural carbon capture
Related sites
Clean energy special: A greener goal for coal (New Scientist, 3 September 2005)
Clean coal technologies (Australian Coal Association)
Carbon capture and storage (Australian Coal Association)
Environmental impacts of coal use (World Coal Institute )
Integrated gasification combined cycle (IGCC) (World Coal Institute )
About geosequestration: Capture (CO2CRC, Australia)
Post-combustion carbon capture from coal-fired plants – solvent scrubbing (International Energy Agency Clean Coal Centre, UK)
Callide oxyfuel project (CS Energy, Australia)
Posted July 2008.