Coke
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.

production:
Volatile constituents of the coal—including water, coal-gas, and coal-tar—are driven off by baking in an airless furnace or oven at temperatures as high as 2,000 degrees Celsius. This fuses together the fixed carbon and residual ash. Most modern facilities have "by-product" coking ovens. Today, the volatile hydrocarbons are mainly used, after purification, in a separate combustion process to generate energy. Non by-product coking furnaces or coke furnaces (ovens) burn the hydrocarbon gases produced by the coke-making process to drive the carbonization process.

Bituminous coal must meet a set of criteria for use as coking coal, determined by particular coal assay techniques. These include moisture content, ash content, sulfur content, volatile content, tar, and plasticity. This blending is targeted at producing a coke of appropriate strength (generally measured by Coke strength after reaction (CSR), while losing an appropriate amount of mass. Other blending considerations include ensuring the coke doesn't swell too much during production and destroy the coke oven through excessive wall pressures.

The greater the volatile matter in coal, the more by-product can be produced. It is generally considered that levels of 26-29 percent of volatile matter in the coal blend is good for coking purposes. Thus different types of coal are proportionally blended to reach acceptable levels of volatility before the coking process begins.

Natural coke is formed when a coal seam is intersected by a volcanic intrusion. These intrusions heat the surrounding coal in an anoxic atmosphere producing coke in a zone (usually several meters) around the intrusion. However, the coke is of wildly varying strength and ash content and is generally considered unsaleable except in some cases as a thermal product -- as it has been lost it's volatile matter it has lost the ability to be coked again.
Uses
Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace.

Since smoke-producing constituents are driven off during the coking of coal, coke forms a desirable fuel for stoves and furnaces in which conditions are not suitable for the complete burning of bituminous coal itself. Coke may be burned with little or no smoke under combustion conditions, while bituminous coal would produce much smoke.

Coke was widely used as a substitute for coal in domestic heating following the creation of Smokeless zones in the United Kingdom when the government paid for new grates suitable for the fuel. Ironically the grates required the use of a gas fired poker to light the fuel so the new installation included installation of a gas line adjacent to the fireplace. Most consumers converted to a gas fire designed to work with an existing chimney within a few years.
Gas byproductsCoke may be used to make synthesis gas, a mixture of carbon monoxide and hydrogen.

Syngas; Water gas: a mixture of carbon monoxide and hydrogen, made by passing steam over red-hot coke (or any carbon based char)
Producer gas; wood gas; generator gas; synthetic gas; suction gas: a mixture of carbon monoxide, hydrogen and nitrogen, made by passing air over red-hot coke (or any carbon based char)