'Clean' coal

'Clean' coal

There are a number of processes that have been used for many years in coal-fired power stations that improve the efficiency and environmental acceptability of coal extraction, preparation and use, and many more are under development. These processes are collectively known as 'clean coal technologies'. Designation of a technology as a 'clean coal' technology does not imply that it reduces emissions to zero or near zero. For this reason, the term has been criticised as being misleading; it might be more appropriate to refer to 'cleaner coal'.

Clean coal technologies reduce emissions of several pollutants, reduce waste and increase the amount of energy gained from each tonne of coal. They include various chemical and physical treatments applied pre- or post-combustion. They may be broadly divided into processes relating either to combustion efficiency or pollution control.

An example is gasification of coal by burning it in oxygen to produce a cleaner gaseous fuel known as 'syngas' (a mixture mainly of hydrogen and carbon monoxide), which is comparable in its combustion efficiency to natural gas. This reduces the emissions of sulphur, nitrogen oxides, and mercury, resulting in a much cleaner fuel. The resulting hydrogen gas can be used for electricity generation or as a transport fuel. The gasification process also facilitates capture of CO2 emissions from the combustion effluent (see discussion of carbon capture and storage below).

Integrated gasification combined cycle (IGCC) systems combine gasification with a heat recovery system that feeds a secondary steam-powered generator, thereby increasing the power generated from a given amount of coal. These systems are currently being employed in many new coal-fired power plants worldwide.

Carbon capture and storage (CCS) is a technology under development that offers much higher prospects of emissions reductions than other clean coal technologies. CCS involves capture of CO2 either before or after combustion of the fuel; transport of the captured CO2 to the site of storage; and injection of the CO2 in deep underground reservoirs for long-term storage (known as geosequestration). CCS is proposed as a means of reducing to near-zero the greenhouse gas emissions of fossil fuel burning in power generation and CO2 production from other industrial processes such as cement manufacturing and purification of natural gas. Many clean coal technologies are being developed with CCS in mind, for example concentrating CO2 in the combustion exhaust to ease the separation and capture of CO2. The majority of CCS effort is being invested in incorporating CCS into new power generation plant designs—current figures indicate that it is cheaper to build a new IGCC plant that produces a pure CO2 exhaust stream than to retrofit an existing plant with post-combustion capture technology.

Most of Australia's current coal-fired electricity-generating plants are of a conventional design, with typical efficiencies of about 33–35 per cent. This means that only about 35 per cent of the usable energy in the coal is actually converted into electricity. Much of the rest is waste heat. Plants with greater energy conversion efficiency (up to about 42 per cent) are possible with combined cycles that recycle heat using very high temperature steam. Such plants are in commercial use in many developed countries and are being installed in greater numbers in developing countries such as China. Even higher efficiencies are expected from plants that utilise additional heat-capturing cycles, technological developments in turbine efficiencies, and higher process temperatures. With these combined improvements, efficiencies of more than 60 per cent are being targeted. Of Australia’s approximately 30 coal-fired power plants, two-thirds are older than 20 years, and only four employ clean coal technology. Victoria's coal-fired power industry is based on brown coal, which produces more emissions than black coal because it has higher water content and burns less efficiently. While brown coal has been a relatively cheap source of energy in Australia until now, the proposed emissions trading scheme is expected to lead to much higher penalties for brown coal than black coal, so there is added incentive to utilise clean coal technology to maintain market competitiveness.

Plants utilising clean coal technology are under construction in Queensland and Victoria, and several more are proposed in Victoria, NSW, Queensland and WA. Projects proposed or in progress include: retrofitting of an existing power station for combustion in oxygen and CCS (2010, Callide); post-combustion capture from an existing power station (in progress, Hazelwood); construction of an integrated gasification combined cycle power plant with CCS (2012, ZeroGen); construction of a capture-ready power station (2011–12, Coolimba); a commercial scale Syngas underground oxygen-injected coal gasification demonstration trial (in progress, Carbon Energy); and construction of an integrated drying gasification combined cycle power plant in the Latrobe Valley (2009, HRL).

Further reading:

Coal21, a collaborative program between Federal and State Governments, the coal and electricity generation industries, the research community and unions, to facilitate development and uptake of clean coal technologies.

NewGenCoal, an Australian Coal Association website designed to disseminate information about the clean coal industry and its role in reducing greenhouse gas emissions.

Department of Resources, Energy and Tourism, ‘Clean Energy Technologies’, Australian Government.

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), a research organisation focussed on carbon dioxide capture and storage in Australia.

International Energy Agency (IEA) Clean Coal Centre, an international resource for information on the sustainable use of coal.

Coal Online, a free resource providing information on clean coal technology based on published reports from the IEA Clean Coal Centre.


22 October, 2010

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