Coke fuel

Coke is a grey, hard, and porous fuel with a very high carbon content. It is produced by heating coal or petroleum in the absence of air, a process known as destructive distillation. The fuel is widely used in heavy industry—especially in iron smelting—as well as in domestic heating and various metallurgical applications. In its unqualified form, the term coke generally refers to the product derived from low-ash, low-sulphur bituminous coal. A closely related material, petroleum coke or pet coke, is obtained from crude petroleum during oil refining, and natural coke can also form through geological processes.

Production and Industrial Processes

Industrial coke is produced by coking, the heating of coal in an airless environment within a coke furnace or coking oven. Temperatures typically reach about 1000 °C. At these temperatures, volatile substances in the coal are vaporised, including water, coal gas, and coal tar, leaving behind a solid residue composed mostly of carbon and mineral matter. This solidified, fused material is the coke.
The main by-products of coking include coal tar pitch, ammonia, hydrogen sulphide, pyridine, hydrogen cyanide, and a range of carbon-based compounds. Modern by-product ovens capture and refine these volatile components for industrial fuels and chemical feedstocks. In older or less complex systems, the volatile gases are simply burned to maintain the heat needed for carbonisation.
Coal selection is central to controlling product quality. Coal blends are assessed using parameters such as ash content, sulphur levels, volatile content, and plasticity. An ideal coking blend contains roughly 26–29 per cent volatile matter, ensuring the coke produced is of sufficient strength and has suitable carbon structure. Excessive variability in coal types leads to coke of inconsistent quality, often rendering it unsuitable for metallurgical use.
Coking coal differs from thermal coal despite originating from similar biological material. It contains a distinctive composition of macerals—the fossilised structural components of ancient vegetation. These macerals influence how the coal softens, melts, and resolidifies during coking. Classification of coking coal often uses ash content, which is divided into steel and washery grades depending on percentage by weight.

Historical Methods of Cokemaking

For centuries, coke was produced using the hearth process, which resembled charcoal burning. Piles of coal covered with coke dust were slowly carbonised over long periods. This method persisted into the nineteenth century but declined sharply after the introduction of the hot blast in iron smelting in 1828 and the emergence of the more efficient beehive oven.
The beehive coke oven was a widely used nineteenth- and early twentieth-century installation. Each oven consisted of a dome-shaped firebrick chamber with a charging hole in the roof and a side opening for discharge. Ovens were built in long batteries, each containing dozens or sometimes hundreds of connected units. Coal was charged from above, forming a uniform layer. Carbonisation occurred from top to bottom over two to three days, with volatile matter burning inside the partially closed chamber to generate the necessary heat. Since these ovens were not designed for by-product recovery, exhaust gases were vented to the atmosphere. Once complete, coke was quenched with water and removed manually. Residual impurities accumulated as slag, which was eventually repurposed for brickmaking, cement mixtures, roofing granules, and fertiliser.

Occupational Safety

Workers may encounter coke oven emissions through inhalation or skin and eye contact. Regulatory standards exist to mitigate these hazards. In some regions, occupational exposure limits stipulate strict permissible concentrations of benzene-soluble fractions of coke oven emissions over an eight-hour shift. Recommended exposure limits are often set even lower to ensure long-term worker safety.

Uses and Industrial Applications

Coke has a number of important industrial uses:

• Metallurgical applications: Coke is a major reducing agent in the smelting of iron ore within a blast furnace. Carbon monoxide generated from coke combustion reduces iron(III) oxide to metallic iron.
• Blacksmithing and forge fuel: Its high carbon content and minimal smoke output make it ideal for high-temperature forge operations.
• Domestic heating: Coke became a widely adopted smokeless fuel in the United Kingdom following the Clean Air Act of 1956, which promoted alternatives to smoky bituminous coal. It was also used for household heating in countries such as Australia.
• Whisky production: Some distilleries, particularly in northern Scotland, burn a mixture of coke and peat when kilning malted barley.
• Synthesis gas production: Coke is used to generate water gas or syngas—a mixture of carbon monoxide and hydrogen—when steam or air is passed over red-hot coke.
• Hydrogen production: Coke oven gas contains significant amounts of hydrogen, which can be economically extracted for industrial processes.
• Foundry applications: Finely ground bituminous coal, referred to as sea coal, is used in foundry sand. Its slow combustion releases reducing gases that prevent molten metal from penetrating sand moulds. Sea coal is also used in mould washes and as a component of the clay lining in cupola furnaces.

Sources and Grading of Coking Coal

Metallurgical-grade bituminous coal must satisfy precise criteria to be used as coking feedstock. Coal assays examine physical and chemical attributes such as tar levels, moisture, plasticity, ash percentage, and sulphur content. Coking coal is graded based on ash content:

• Steel Grade I: Ash not exceeding 15%
• Steel Grade II: 15–18%
• Washery Grades I–IV: A sequence from 18% to 35% ash content

Only specific blends yield coke of the required structural integrity. Excessive swelling during coking can damage oven walls, while inadequate volatile content may result in insufficient fusion of carbon particles.

Additional By-Product and Environmental Considerations

Coke production generates wastewater and effluents containing phenolic compounds and other pollutants. Modern installations incorporate treatment systems to reduce environmental impact. Historical facilities lacked such measures, resulting in substantial emissions and contamination.