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What is Lignite?
Wood and other vegetable matter decayed and transformed under pressure millions of years ago to form coal. Lignite has a Lower Rank in the coalification process, meaning this more recent than bituminous and anthracite coals.
Called Brown Coal, vast reserves of lignite are available in limited areas of the world. Australia, US and China have the major reserve of Lignite. Germany leads the pack with the largest number of power plants burning Lignite. In US, most of the reserves are located in the North Dakota province.
In India, the Lignite reserves are in Neyveli in the south and in Rajasthan.
Low calorific value and resulting low cost is what makes it attractive. Because of the lower energy density, transportation is not attractive. Lignite power plants are located near to the mines.
Around 17 % of the world’s coal reserves are lignite. As the world’s oil and gas reserves decline, other sources become attractive. That is why there is a sustained interest in the use of Lignite.
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Characteristics of Lignite
Lignite has some special characteristics.These characteristics vary from mine to mine.
- The most important is that the calorific value is low. The calorific value ranges from 8000 kJ/kg to 15000 kJ/kg. Compare this to bituminous coal that has calorific value of 12000 to 20000 kJ/kg.
- Lignite has high moisture content in the range of 45 % to 55 %.
- The Volatile matter content is also high. On an ash and moisture, free basis the Volatile Matter percentage in the range of 45% to 55 %.
- High amounts of Sulphur especially in the form of Ferrous Sulphide FeS2, reduce the ash fusion temperature to low levels like 900 °C. This gives the coal high potential for slagging.
- The lignite is softer than bituminous coal, with a HGI in the range of One Hundred making it easier to pulverize.
These characteristics make Lignite burning different from burning bituminous coals. Germany with large number of Power plants burning Lignite is the forerunner in the technology for Lignite fired boilers.
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Mining of Lignite also raises many environmental issues.
- Because of the lower Calorific value, the quantity of Coal required is high.
- This has a higher CO2 emission than plants firing bituminous coal.
- The overburden to coal ratio is higher than other types of coal. This results in destruction of agricultural land and displacement of people.
- Lignite mining involves the removal of enormous quantities of water. Depending on the quality of the water, it could be a boon for agriculture or an environmental issue.
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Even with all the drawbacks, availability of large reserves of lignite makes it feasible to use this energy source effectively. Large capacity Lignite fired boilers in the 800 MW to 1100 MW range supercritical parameters are in service today.