Surface mining is performed by removing the soil layer covering the coal seam and excavation the coal bed by highly automated machinery. Coal is then removed in 3-5 meter depths at the time at rates as high as 10,000 tons /hour. Excavator volume capacities can reach 150 cubic meters. The World’s coal mining capacity at the turn of the millennium reached 4.7 billion tons.
Largest producers: China, USA, India, Australia, Union of South Africa and Russia. These countries also have the largest current known coal reserves.
Coal at the end of the 20th century: Coal production, mainly in Europe, declined significantly due to the exhaustion of coal sources and the increase in mining costs. The main users of coal are now the energy production and smelting. The home heating use declined, at was replaced by natural gas use. The switch from coal to natural gas improved environmental conditions also.
Coal utilization: Most coal is used in power plants for electricity generation. Many new power plants are built next to mines to lower coal transportation costs, but this has to be balanced with electrical transmission costs to the final users. Currently next to nuclear power plants, coal fired power plants generate the most electricity. The power generation converts the chemical energy of the coal to heat energy, which is converted to rotational energy and finally in the generator to electrical energy. Modern coal fired steam power plants utilize high efficiency fire tube assemblies and coal feeding assemblies to generate the high pressure steam, which drives the turbines. Modern steam power plants operate at pressures as high as 100 atm.
Steam turbines can be of several different designs, but the modern ones are multi-blade type where the steam drives the turbine by passing over successive blade rows as its pressure is reduced and finally, the exhausted steam leaves the turbine.
Modern power plants recycle as much of the recoverable heat from the used steam as possible and in some cases external to the site utilization is made also, such as “distance” heating. The majority of the steam is condensed in multi-tube condensers, reheated and recycled to the boiler part of the plant. This is done because condensate has been already purified and contains only limited impurities from the steam cycle.
The generator created around 10,000 volt electricity is transformed next to the plant to as high as 400,000 volts for transmission by high voltage transmission lines. Before utilization, the electricity is transformed to lower voltages as close to the final utilization as possible, to lower transmission costs.
The environmental impact of these coal fired power plants would be significant if particulate pollution controlling bag houses and other chemical treatment methods are not utilized.