Coke (fuel)

Lore & Background

Coke is a grey, hard, and porous fuel derived from coal, with a high carbon content. It is produced by heating coal in the absence of air, a process that drives off water and volatile substances like coal gas and coal tar, leaving a non-volatile, somewhat glassy solid residue. The defining characteristic of coke is its cemented-together carbon and mineral residue from the original coal particles. Its appearance is that of a hard, porous, grey material. The raw material for coke is typically low-ash and low-sulphur bituminous coal, which must meet specific criteria for moisture, ash, sulphur, volatile content, tar, and plasticity. Different coal types are blended to achieve a volatile matter level of 26–29%, ensuring the resulting coke has appropriate strength and does not swell excessively during production. Coke is graded by its ash content after burning, ranging from Steel Grade I (ash not exceeding 15%) to Washery Grade IV (28–35%). Historically, coke was made using a hearth process similar to charcoal-burning, where a coal heap was covered with coke dust. Later, beehive ovens—dome-shaped firebrick chambers—were used, where coal was carbonized from top to bottom over two to three days, with volatile matter burning to provide heat and no by-products recovered. In modern by-product ovens, volatile decomposition products such as coal-tar pitch, ammonia, hydrogen sulphide, pyridine, and hydrogen cyanide are collected, purified, and used as fuel or chemical feedstocks. Coke is primarily used in smelting iron ore, but also as a fuel in stoves and forges. A similar product, petroleum coke, is obtained from crude petroleum.

Reader's Guide

Coke's significance lies in its role as a smokeless fuel and reducing agent in iron smelting, enabling the production of iron from hematite via carbon monoxide. Coke is also used to make synthesis gas (a mixture of carbon monoxide and hydrogen) by passing steam or air over red-hot coke. The coking process yields valuable byproducts, and coke oven gas contains about 60% hydrogen, which can be extracted economically.

Did You Know?

• Coke can be formed naturally by geologic processes as the residue of destructive distillation.
• The 'hearth' process of coke-making, using lump coal, was akin to charcoal-burning and continued into the 19th century.
• Coke oven slag, initially discarded, has since been used as an ingredient in brick-making, mixed cement, granule-covered shingles, and fertilizer.

What Coke Is and Why Industry Depends on It

Coke is a grey, hard, and porous solid fuel derived from coal, distinguished by its exceptionally high carbon content. It is produced by subjecting coal or petroleum to intense heat in the complete absence of air, a process that strips away volatile components and leaves behind a dense carbon-rich residue. In everyday industrial parlance, the unqualified word "coke" most often refers to the product obtained from low-ash, low-sulphur bituminous coal through coking. A closely related material, petroleum coke or pet coke, is instead generated from crude petroleum within refineries. Coke can even arise naturally through geologic processes over vast stretches of time. Its primary industrial role is in the smelting of iron ore, making it a cornerstone of heavy manufacturing. Beyond the smelting furnace, coke also serves as a practical fuel in stoves and forges. Fundamentally, coke is the non-volatile residue left after destructive distillation has broken down the original organic material, cementing together carbon and mineral particles into a hard, slightly glassy solid. Because it has already shed its volatile matter during production, a given piece of coke cannot be coked a second time, making each batch a final product of the transformation.

The Coking Process: Transforming Coal Under Extreme Heat

The industrial conversion of coal into coke, known as coking, takes place in sealed kilns called coke furnaces or coking ovens. The coal is baked at temperatures reaching as high as 2,000 °C, though the typical operating range sits around 1,000 to 1,100 °C. Under these extreme conditions in the absence of air, organic substances within the coal are vaporized or decomposed. Water, coal gas, coal tar, and other volatile or liquid compounds are driven off, leaving behind the non-volatile residue: a cemented mass of carbon and mineral matter that forms the hard, somewhat glassy structure characteristic of finished coke. The coking process also yields a range of chemical byproducts, including coal-tar pitch, ammonia, hydrogen sulphide, pyridine, hydrogen cyanide, and various carbon-based materials. Modern "by-product" coking ovens are designed to capture, purify, and separate these volatiles so they can be sold as fuel or chemical feedstocks to other industries. An older approach, still encountered in some new construction, simply burns the volatile byproducts to supply the heat needed to run the ovens, forgoing any recovery of those materials.

From Hearth Piles to Beehive Domes: Evolving Coke-Making Methods

Before industrial ovens, coke was made through a hearth process akin to charcoal-burning. A heap of lump coal, covered with coke dust, was left to carbonize over a lengthy period. This method persisted through the first half of the nineteenth century until two innovations reduced its importance: the hot blast in iron-smelting, introduced by Neilson in Scotland in 1828, and the beehive coke oven. The beehive oven is a firebrick dome about four meters wide and 2.5 meters high, with a charging hole in the roof and a discharging opening in the lower wall. In a battery, hundreds of such domes share common walls in a row. Coal is layered to a depth of sixty to ninety centimeters, ignited with initial air, and allowed to carbonize from top to bottom over two to three days. Heat comes entirely from burning volatile matter, so no byproducts are recovered and exhaust gases escape freely. Finished coke is quenched with water and removed by hand. In continuous operation, retained wall and roof heat helps start the next charge. Impurities not driven off accumulate as slag, initially discarded but later put to use in brick-making, mixed cement, granule-covered shingles, and even as fertilizer.

The Coal Behind the Coke: Sourcing, Blending, and Grading

Not all coal is suitable for coking. Bituminous coal intended for this purpose must satisfy specific criteria determined through coal assay techniques, including moisture content, ash content, sulphur content, volatile matter, tar, and plasticity. The aim is to produce a coke of appropriate strength, generally measured by coke strength after reaction, while losing an appropriate amount of mass. Blending must also ensure the coke will not swell excessively during production and damage the oven through extreme wall pressures. Volatile matter levels of 26 to 29 percent in the coal blend are generally considered ideal for coking. Different coal types are therefore proportionally blended to hit that target. If the range of coal types is too wide, the resulting coke shows widely varying strength and ash content, rendering it unsaleable, though in some cases it may be sold merely as an ordinary heating fuel. Coking coal differs from thermal coal in its maceral composition, arising from different plant species and formation conditions. Coking coal is further graded by ash percentage after burning: Steel Grade I (not exceeding 15%), Steel Grade II (15–18%), and four Washery grades extending to 35%.