Coal is a mineral deposit that is rich in carbon content. This carbon is important, as it is what gives the coal the greater majority of its energy content. When coal is burned in the presence of air or oxygen, heat energy is released. This energy can then be converted to other forms of useful energy.
Coal is classified into ranks, depending on hardness and energy content:
- Graphite can technically be considered the highest rank but as it is difficult to burn it is not commonly used in combustion processes.
- Anthracite is hard, glossy and black. It is the hardest form of coal and has high energy content. It is commonly used for residential and commercial space heating.
- Bituminous coal is dense, black, or sometimes dark brown. It is used as a coking coal for steel making or to provide the heat for steam electric power generation.
- Sub-bituminous coal has properties ranging from those of bituminous to lignite. It is used is for steam electric power generation.
- Lignite - sometimes referred to as brown coal - is the lowest rank of coal, has high inherent moisture content, and has the lowest energy content. It is used is for steam electric power generation.
For deposits of coal that are deep in the earth, vertical or horizontal shafts from the surface provide men and equipment access to the coal and allow removal of the coal to the surface. In Nova Scotia, Canada, this is the method used for mining the local coal. For coal deposits that are relatively close to the surface, open pit or strip mining techniques are used. In Saskatchewan and Alberta, Canada, open pit mines are predominant.
Approximately 67 megatonnes (Mt) of coal were mined in Canada in 2005. About 27 Mt of this was coking coal (for steel making), the remainder was “thermal” coal – that is, coal used as fuel for heat and power applications.
Coal use in Canada
Coal is used in industrial processes, such as cement manufacturing, steel production and electricity generation. Of the approximately 60 megatonnes (Mt) of coal consumed in Canada in 2005, about 56 Mt were used to generate electricity. This amount of coal generated about 92 gigawatt-hours (GWh), or ~ 17% of Canada’s total electricity generation.
How is coal used to produce electricity?
In today's typical coal-fired electric power plant, the coal is first crushed into small lumps and then further ground to a fine powder in a pulverizer. Thus, this type of plant is sometime referred to as a pulverized coal (PC) plant. The pulverized coal is fed, with air as the oxidant, into the furnace of a boiler.
The coal burns at around 1400°C, and the resulting hot gases and radiant heat convert water in tubes, which line the furnace walls, into high-temperature, high-pressure steam.
The high-pressure steam is forced through a steam turbine causing the turbine blades to rotate at very high speed. The turbine shaft is coupled to the shaft of an electricity generator which also then rotates. The generator shaft has wire coils wound around it. As these coils spin rapidly in a strong magnetic field, electricity is generated.
This electricity is transformed into high voltages (e.g. 400 kilovolts) so it can be fed to power grids via transmission lines. At various distribution points (or sub-stations), the electricity is transformed to lower voltages as required by the end user. For instance, large industries, such as steel mills may require electricity supplied at the tens of thousands of volts level, whereas commercial operations, institutions and residences in North America commonly use 110-220 volts systems.
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