Coal is a fossil fuel extracted from the ground either by underground mining, open-pit mining or strip mining. It is a readily combustible black or brownish-black sedimentary rock. It is composed primarily of carbon and hydrocarbons, along with assorted other elements, including sulfur. Often associated with the Industrial Revolution, coal remains an enormously important fuel and is the most common source of electricity world-wide.
The largest single use of coal in the steel industry is as a fuel for the blast furnace, either for the production of metallurgical coke or for injection with the hot blast. Other less commonly thought of uses of coal is for making steam and electricity, as sources of carbon addition in steel making processes, and in direct smelting of iron processes. Furthermore, electricity purchased from outside sources is largely generated from pulverized coal combustion and therefore has an indirect influence on steel making operations.
Except for coke making, the requirements for a quality coal product are fairly simple. For pulverized coal combustion, whether taking place in a combustion unit or in the blast furnace, the coal must deliver a known and consistent calorific value, be reasonably low in ash yield or have relatively benign ash chemistry and meet environmental standards for sulfur and nitrogen oxide emissions. In addition, it must be relatively easy to grind and to handle.
As geological processes apply pressure to peat over time, it is transformed successively into:
(i) Coking Coal
The requirements of coals purchased for coke making are much different from those used in other processes. Only a certain class of coals possessing very specific properties and composition are suitable for the making of a quality coke for blast furnace use.
Coal, which, because of its characteristics, is suitable for carbonising to produce blast furnace coke. Important properties required technically to produce good coke are good coking and caking properties (such as fluidity, dilation, crucible swelling number etc.) allied with appropriate rank (indicated by reflectance values around 0.9 - 1.5%). Other properties which are important for commercial reasons are ash and moisture (should be as low as possible), and properties which are important because of the deleterious effects on pig iron are sulphur and phosphorus.
(ii) Non-Coking Coal
A wide variety of non-coking coals have been tested for injection, rangin from lignite to bituminous coals and anthracites. The choice depends on price and availability rather than attaining the highest injection rates or coke replacement. In light of the diversity of coals that have been used, if all other considerations were equal (raw materials, transportation cost, and availability of tonnage), the level of ash yield, followed closely by volatile matter and moisture content and a coal's grindability are the most important properties.
(iii) Anthracite Coal
Anthracite is a hard, compact variety of mineral coal that has a high luster. It has the highest carbon count and is the best burning of all coals. Anthracite differs from bituminous coal in that it contains little or no bitumen, therefore it burns with an almost invisible flame. The purer specimens consist almost wholly of carbon.
It is hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter. The moisture content of fresh-mined anthracite generally is less than 15 percent. The heat content of anthracite ranges from 22 to 28 million Btu per short ton (26 to 33 MJ/kg) on a moist, mineral-matter-free basis. The heat content of anthracite coal consumed in the United States averages 25 million Btu/ton (29 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).
(iv) Wood Charcoal
Charcoal is the blackish residue consisting of impure carbon obtained by removing water and other volatile constituents of animal and vegetable substances. It is usually produced by heating wood in the absence of oxygen, but sugar charcoal, bone charcoal (which contains a great amount of calcium phosphate), and others can be produced as well. The light, black, porous material is 85% to 98% carbon, and resembles coal.