Lignite has lower energy content and is only used in power plants located adjacent to lignite quarries. A hard coal-fired plant is slightly more efficient. But in terms of heat value, lignite is less expensive than hard coal per gigajoule. With the use of new technologies such as pressurised steam fluidised bed drying, lignite can reach an efficiency rate comparable to hard coal.
Coal powder burns hot and fast
Coal is usually ground to a fine powder and dried so that it burns hotter and faster. It is then blown into a combustion chamber and burned at a very high temperature. The generated thermal energy heats water, creating steam which is then transferred to a set of turbines that have propeller-like blades.
The steam drives the blades, causing a turbine shaft to rotate at high speed. A generator is placed at one end of the turbine shaft. As the shaft rotates it produces electricity. After passing the turbine, the steam is re-condensed and returned to the boiler to be heated again. In some power plants, the generated heat is also used for district heating.
New plants more efficient
Many plants in countries such as China and India are, however, outdated. In 2008 there were over 8,000 small coal-fired power plants in China, many with low efficiency and high emission levels. Most plants in the US, South Africa and Europe need to be replaced as well. The average efficiency of the world's hard coal-fired power plants is currently 28 per cent, compared to more than 46 per cent for modern plants. The most modern lignite-fired plants have a similar efficiency rate.
New technology reduce emissions
Large quantities of particulates are refined out of the combustion gases that were previously emitted, unfiltered, into the air. Well-developed technologies reduce emissions of sulphur, nitrogen oxides, complex hydrocarbons, dust and heavy metals. Flue gas washing, for example, is used to reduce emissions of sulphur. Effective particulate filters can prevent over 99.9 per cent of dust emissions from escaping into the atmosphere.
There are two basic methods for extracting coal: underground and opencast mining. Underground mining accounts for around 60 per cent of global coal production, though this figure varies by area. In Australia, for example, opencast mining accounts for 80 per cent of total coal production.
Intensive re-cultivation of mining areas
Surface extraction is used where coal lies close to the surface. The coal, mostly lignite, is reached by digging up layers of soil, sand and rock. Former mining areas therefore require intensive re-cultivation. Among other things, soil is used to reconstruct forests, pasturelands and different types of cultivation.
Underground coal extraction is used when the coal is stored deep in the earth. This is more risky than surface mining and requires additional planning measures, such as advanced drainage and ventilation systems.