Coal as fuel to generate electricity
Refined coal is the product of a coal-upgrading technology that removes moisture and certain pollutants from lower-rank coals such as sub-bituminous and lignite (brown) coals.
It is one form of several precombustion treatments and processes for coal that alter coal's characteristics before it is burned.
Thermal efficiency improvements are achievable by improved pre-drying (especially relevant with high-moisture fuel such as lignite or biomass).
The goals of precombustion coal technologies are to increase efficiency and reduce emissions when the coal is burned.
Precombustion technology can sometimes be used as a supplement to postcombustion technologies to control emissions from coal-fueled boilers.
Power plant combustion
Castle Gate Power Plant near Helper, Utah, US
Coal rail cars
Bulldozer pushing coal in Ljubljana Power Station
Coal burnt as a solid fuel in coal power stations to generate electricity is called thermal coal.
Coal is also used to produce very high temperatures through combustion.
Efforts around the world to reduce the use of coal have led some regions to switch to natural gas and electricity from lower carbon sources.
When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler.
The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity.
The thermodynamic efficiency of this process varies between about 25% and 50% depending on the pre-combustion treatment, turbine technology (e.g. supercritical steam generator) and the age of the plant.
A few integrated gasification combined cycle (IGCC) power plants have been built, which burn coal more efficiently.
Instead of pulverizing the coal and burning it directly as fuel in the steam-generating boiler, the coal is gasified to create syngas, which is burned in a gas turbine to produce electricity (just like natural gas is burned in a turbine).
Hot exhaust gases from the turbine are used to raise steam in a heat recovery steam generator which powers a supplemental steam turbine.
The overall plant efficiency when used to provide combined heat and power can reach as much as 94%.
IGCC power plants emit less local pollution than conventional pulverized coal-fueled plants; however the technology for carbon capture and storage after gasification and before burning has so far proved to be too expensive to use with coal.
Other ways to use coal are as coal-water slurry fuel (CWS), which was developed in the Soviet Union, or in an MHD topping cycle. However these are not widely used due to lack of profit.
In 2017 38% of the world's electricity came from coal, the same percentage as 30 years previously.
In 2018 global installed capacity was 2TW (of which 1TW is in China) which was 30% of total electricity generation capacity.
The most dependent major country is South Africa, with over 80% of its electricity generated by coal.
The total known deposits recoverable by current technologies, including highly polluting, low-energy content types of coal (i.e., lignite, bituminous), is sufficient for many years.
On the other hand, much may have to be left in the ground to avoid climate change, so maximum use could be reached sometime in the 2020s.