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Clean Coal Technologies
Coal is considered a highly polluting energy source - for many it is the worst. This is attributed to the fact that it is the most carbon-intensive fossil fuel available. The burning of coal results in dangerous emissions including carbon dioxide that is responsible for global warming, sulfur dioxide, the main responsible for acid rain and a number of other dangerous toxins and cancer-causing dioxins.
The newly introduced clean coal operation technologies are aiming at reducing the amounts of these pollutants escaping into the atmosphere. They are targeted mostly toward the reduction of carbon dioxide since global warming is the most severe environmental impact. The first clean coal energy plant, actually a pilot plant using US FutureGen technology and operated by a Swedish firm called Vattenfall, started operating in 2008 in Spremberg, Germany. The facility uses the technique of carbon capture and storage to clean coal. More specifically, carbon dioxide is captured and then compressed into a liquid state and stored. This way the CO2 does not escape into the atmosphere. The technique is described in more detail below, along with other clean coal technologies that are currently under development.
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Carbon Capture and Storage - The Most Popular Techniques
Carbon capture and storage or sequestration (CCS) is currently in the forefront of 'clean' carbon operation techniques, with more than 80 CCS projects underway in the United States already. This group of techniques involve the capturing of carbon dioxide before it escapes into the atmosphere and placing it in undergound storage.
More specifically, the idea is based on capturing carbon dioxide from fossil fuels, including coal and natural gas (methane). As soon as the CO2 is separated from the rest of the fuel, it has to be safely stored. Storage or sequestration take place in deep geological formations or deep in the ocean. Although leaks are a major problem in both cases, geological formation storage is considered to be safer since storage in the oceans runs a greater risk of acidification.
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Other Clean Coal Operation Techniques
A variety of other clean coal techniques are under investigation. Some of them are:
Coal washing: When the coal arrives at a power plant, it contains minerals that have to be removed before the process of burning takes place. First, the coal has to be ground into small pieces and then exclude/separate the unwanted material through gravity separation. A method involves the use of barrels that contain fluid. The coal floats and the rest of the material sinks into the bottom. The pulverization of coal follows, and then it is ready to be used for power production.
Gasification: Plants that use gasification techniques have high efficiencies and low emissions, but there are yet to be proven in a large commercial scale. In this technique coal reacts with oxygen and produces syngas. The syngas is cooled and cleaned, and then burned inside a combustor. The heat generates steam that eventually powers steam turbines. A well-known method of gasification is the Integrated Gasification Combined Cycle or IGCC.
Removal of SO2 and NOx pollutants: The burning of coal results in the emission of sulphur dioxide and nitrogen oxides. Several methods are used to remove these pollutants, for instance : Sulphur dioxide - Flue gas desulphurization (FGD) systems use limestone and water, a mixture that reacts with the SO2 of the flue gas to form gypsum, a very popular material used in the construction industry.
Nitrogen oxides - The specially designed low NOx burners prevent the formation of the gas by reducing the amount of oxygen available in the hottest part of the coal combustion chamber.
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There is a great deal of concern regarding the future of these technologies. Greenpeace suggests that no coal-fired power plant is ever truly clean. All that clean coal techniques manage is to circulate the pollutants and finally release them back into the environment. Apart from that, the economic viability of these technologies is in question. Research on clean coal technologies is taking place "for over 10 years and $5.2 billion have already been spent in the US alone" (Greenpeace). It is expected that it will take hundreds of millions of dollars and many more years before they are commercially available.
Despite the concerns, clean coal technology is now shifting toward a newly modified coal gasification technique that may provide what is called "zero emissions" or actually low emissions of carbon dioxide and other pollutants. Although the US DOE planned to have commercial designs for power plants using the technology by 2012, the project was postponed.
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- "Clean Coal" Technologies, Carbon Capture & Sequestration by world-nuclear.org
- Clean Coal Technology by Greenpeace
- Clean coal technology: How it works by BBC