What is Coke (fuel)?

Coke is a solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal.

The volatile constituents of the coal—including water, coal-gas, and coal-tar—are driven off by baking in an airless oven at temperatures as high as 1,000 degrees Celsius. This fuses together the fixed carbon and residual ash. Most coke in modern facilities is produced in "by-product" coke ovens, such as in the upper photograph, and the resultant coke is used as the main fuel in iron-making blast furnaces. Today, the hydrocarbons are considered to be by-products of modern coke-making facilities (though they are usually captured and used to produce valuable products). Non by-product coke ovens, such as in the lower photograph, burn hydrocarbon off-gases on site to provide the heat needed to drive the carbonization process.

Properties and usage

Coke typically has a specific density between 1.85 and 1.9. It is highly porous.

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 which would result in a large amount of smoke if bituminous coal were the fuel.

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.

Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is gray, hard, and porous.

Discovered by accident to have superior heat shielding properties when combined with other materials, coke was one of the materials used in the heat shielding on NASA's Apollo program space vehicles. In its final form, this material was called AVCOAT 5026-39. This material has been used most recently as the heat shielding on the Mars PATHFINDER vehicle. Although not used for modern day space shuttles, NASA is utilizing coke and other materials for a new heat shield for its next generation space craft, named Orion, which is due to be completed in 2014.

History

The use of coke as a fuel was pioneered in 17th century England in response to the ever-growing problem of European deforestation. Wood was becoming increasingly scarce and expensive. Coal's fumes, particularly smoke and sulfur compounds, disqualified it from many applications, including cooking and iron smelting. In 1603, Sir Henry Platt suggested that coal might be charred in a manner analogous to the way charcoal is produced from wood. This process was not put into practice, however, until 1642, when coke was used for roasting malt in Derbyshire. Coal cannot be used in brewing, because its sulfurous fumes would impart a foul taste to the resulting beer. Perhaps more significantly, in 1709, Abraham Darby set up a coke-fired blast furnace to produce cast iron. Coke's superior crushing strength allowed blast furnaces to become taller and larger. The ensuing availability of inexpensive iron was one of the factors leading to the Industrial Revolution.

Other coking processes

The solid residue remaining from refinement of petroleum by the "cracking" process is also a form of coke. Petroleum coke has many uses besides being a fuel, such as the manufacture of dry cells, electrodes, etc. Gas works that manufacture syngas also produce coke as an end product, called gas house coke.

Fluid coking is a process by which heavy residual crude is converted into lighter products such as naptha, kerosene, heating oil, and hydrocarbon gases. The "fluid" term refers to the fact that coke particles are in a continuous system versus older batch-coking technology.

Gases from coke

Coke may be used to make fuel gases. It appears that the names have different meanings in the USA and the UK so confusion is possible. The following are UK meanings:

• 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)

These are useful gases but require careful handling because of the risk of carbon monoxide poisoning.

References

• Coking coal manufacturing procedure