Coke (fuel)

Coke is a hard, grey, and porous fuel with a very high carbon content.

It is a vital industrial product used primarily for the smelting of iron ore. Smelting is the chemical process used to extract pure metal from raw ore. Because coke burns with very little smoke, it is also used in stoves and forges. While most people think of coke as a coal product, it can also be made from petroleum. This version is known as petroleum coke, or pet coke. Some coke may even form naturally through geologic processes deep within the Earth.

The production of coke from coal is a process called coking. It involves heating coal in an airless kiln, often called a coke furnace or coking oven.

During this process, temperatures typically reach between 1,000 and 1,100 degrees Celsius, though they can climb as high as 2,000 degrees Celsius. This intense heat causes the organic substances in the coal to decompose or vaporize. Water, coal gas, and coal tar are driven off as volatile products. The coke that remains is the non-volatile residue. It is a hard, somewhat glassy solid made of cemented-together carbon and mineral residue.

To make high-quality coke, workers must carefully select and blend specific types of bituminous coal. They use coal assay techniques to check for moisture, ash, sulphur, and volatile content. A successful blend usually contains between 26% and 29% volatile matter. This balance ensures the coke is strong enough for industrial use without causing too much pressure in the ovens. The resulting coke is graded based on its ash percentage-by-weight after it burns. For example, Steel Grade I contains no more than 15% ash. In contrast, Washery Grade IV can contain up to 35% ash.

Historically, the way coke was made has changed significantly. In the early 19th century, people used the "hearth" process. This involved making a large heap of lump coal and covering it with coke dust.

This method was very slow and lengthy. Two major inventions changed this. In 1828, a man named Neilson introduced the hot blast in Scotland. This allowed for hot air to be used in smelting furnaces instead of cold air. Later, the beehive coke oven became a common tool for production.

A beehive oven is a firebrick chamber shaped like a dome.

These ovens are usually about 4 meters wide and 2.5 meters high. Coal is loaded through a hole in the roof to create a layer 60 to 90 centimeters deep. The process of carbonization starts at the top and moves downward. It takes about two to three days to complete. In older beehive ovens, the volatile gases were simply burned to provide heat, meaning no by-products were recovered. In modern "by-product" ovens, these gases are collected and purified for use in other industries.

Coke is incredibly useful in many different chemical and industrial ways. In a blast furnace, coke acts as both a fuel and a reducing agent. When coke burns, it produces carbon monoxide. This gas reacts with iron oxide, specifically hematite, to produce pure iron.

Coke can also be used to create synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. By passing steam over red-hot coke, engineers can produce water gas. By passing air over red-hot coke, they can produce producer gas. These gases are essential for various chemical processes and fuel needs.

Beyond heavy industry, coke has many specific applications. Petroleum coke is used to manufacture electrodes for welding and dry cells. In foundries, finely ground coal called "sea coal" is added to sand molds. When the metal is molten, the coal burns slowly to release gases. These gases prevent the liquid metal from soaking into the pores of the sand. Even the waste from the process has uses. The slag, which is the accumulation of impurities, can be used to make bricks, cement, or fertilizer.