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Thermal vs. Metallurgic
By inspecting the physical properties of collected coal, it is ranked into two main types, thermal and metallurgic. Thermal, or steam coal generally contains less carbon and more water, and is generally considered younger geologically. Though more abundant, it is primarily used as an energy source. Lignite and sub-bituminous coal are examples. The rarer type, metallurgical coal, is in high demand by the steel industry, and is used both for a heat source, and to create coke.
PCI Coal
The steel used to make the bridges, buildings and cars that society depends on is manufactured using two main ingredients: iron and coke made from coal. Metallurgical coal, already in short supply, is split into another three categories based on caking properties and impurities. The first, PCI, is the softest. Though not pure enough to actually be converted into coke, it is invaluable in its production and is used to super heat the blast furnace.
Hard
All metallurgical coals are types of bituminous coals, but not all coal of this type are appropriate for coke making. Aside from carbon and water, coal also contains a variety of other materials, sulfur and ash being the most important to coke manufacturers. Coal with the least amount of impurities is highly desired, drawing high prices by steel manufacturers.
Semi-soft
The remaining coal can be blended in order to make usable and moderately priced coke. In order to create coke, the coal is heated at extreme temperatures until it melts and swells into a plastic-like substance. Undesired materials are released as gases, and as it cools, coke is formed. If the starting material is too high in volatile matter, the resulting product is too weak, but containing too few volatiles does not provide enough air transfer. Manufacturers experiment with blending different types of coal, as well as alternate firing techniques to develop the best end product.