Feature Report: Coal, Still a Growing Energy Resource

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Coal in Power Generation, Iron and Steel, and Coal Chemistry

Coal is used for many applications, from producing heat for households to firing industrial boilers and generators to manufacturing cast iron and synthetic fuel. Depending on its quality, coal can be burnt, transformed into coke or gasified. 

Coal is now mainly used in large power plants © THINKSTOCK

Coal-Fired Power Plants 

CoalCoal is ranked by its degree of transformation or maturity, increasing in carbon content from... is still used as a fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... today, though less for home heating and more for firing industrial boilers and generators. It is primarily used for generating electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor..., via coal-burning powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... plants.

The principle is simple. Combusted in a boiler, the coal releases heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... that converts water into steam. This drives a turbine connected to a generator that produces electricity. As it passes through a condenser, the steam cools down, becoming water again. This water is sent back to the boiler.

Today, coal is mainly used to generate power.

Iron and Steel and Other Industries

CokeCoke is a coal derivative obtained through pyrolysis. It consists of almost pure carbon..., which is a fuel with high levels of carbon, usually made from coal, is utilized in the steel industry. It is used in blast and industrial furnaces, where it is mixed with iron ore to manufacture cast iron (an iron and carbon alloy that is a precursor of steel), by smelting reduction of iron oxide.

Manufacturing coke, or cokingCoking is a thermal process (pyrolysis at about 1000°C in the case of coal) used to turn coal and heavy residuals..., involves heating a mixture of high-rank coal, or pitch, to a temperature of 1,000°C. This process requires batteries of several dozens of eight-meter-high ovens.

This pyrolysisThermochemical decomposition of organic material at elevated temperatures... releases gases that are cooled and treated with solvents. A million metric tons of coal produces:

There are also smaller pyrolysis units adapted to various coal types. These manufacture special coke, such as ligniteRock whose properties are somewhere between peat and coal. It has a carbon content of about 70 to 75%... coke, which can be used instead of charcoalCharcoal is carbon produced by the pyrolysis of wood in the absence of oxygen... (see below).

Occurring at about 700°C, pyrolysis preserves heavy aromatic molecules, or phenols. After processing, these are used to manufacture insecticides, fungicides, antioxidants and phenol‑formaldehyde resins used as drying accelerators for varnish and ink.

Coal Chemistry: Manufacturing Hydrocarbons

Coal gasification is increasingly being used in industrial processes, such as at integrated gasification combined cycle (IGCC) power plants (see Feature Report: What Is "Clean Coal"?).

Coal can be gasified in the presence of oxygen and steam to produce synthetic gas, a mixture of carbon monoxideCarbon monoxide is a carbon oxide with a chemical formula of CO. It is produced by the partial oxidation of carbon-containing compounds... and hydrogenThe simplest and lightest atom, the most abundant element in the universe. .

This gas is an intermediate product that can be converted into different products, such as methanolMethanol or methyl alcohol is the simplest alcohol, with a chemical formula of CH3-OH. It is used in the production of methyl tert-butyl ether..., ureaUrea, or carbamide, is an organic compound with the chemical formula (NH2) 2-C=O..., pure hydrogen and dimethyl ether, and above all used to make synthetic fuels.

In the latter case, the product obtained is a wax containing many different types of preservation (hydrocarbons)The final phase in petroleum system formation, after a deposit has accumulated... with varying numbers of carbon atoms. This wax undergoes a series of conversion processes to produce mostly high quality dieselDiesel is the name of an internal combustion engine that works by compression-ignition... and smaller amounts of autogas lpgAutogas refers to liquefied petroleum gas, or LPG (see definition), used as a fuel for cars... and gasoline. 

Barbecue Charcoal The charcoal we use for barbecues isn’t natural coal. It is obtained through wood pyrolysis, a process that requires a temperature of 400°C. Charcoal used to be made in forests by lighting a fire inside a large pile of wood that was protected from the air to prevent it from burning. Today smaller, less polluting, automated production facilities are used where pyrolysis is carried out through contact with hot gases. Charcoal isn’t just used for barbecues — it also serves as filter material, to treat wastewater for example.