Coke (fuel)
Coke is a grey, hard, and porous coal-based fuel with a high carbon content. It is made by heating coal or petroleum in the absence of air. Coke is an important industrial product, used mainly in the smelting of iron ore, but also as a fuel in stoves and forges.
The unqualified term "coke" usually refers to the product derived from low-ash and low-sulphur bituminous coal by a process called coking. A similar product called petroleum coke, or pet coke, is obtained from crude petroleum in petroleum refineries. Coke may also be formed naturally by geologic processes. It is the residue of a destructive distillation process.
Production
Industrial coke furnaces
The industrial production of coke from coal is called coking. The coal is baked in an airless kiln, a coke furnace or coking oven, at temperatures as high as but usually around. This process vaporises or decomposes organic substances in the coal, driving off water and other volatile and liquid products such as coal gas and coal tar. Coke is the non-volatile residue of the decomposition, the cemented-together carbon and mineral residue of the original coal particles in the form of a hard and somewhat glassy solid.Additional byproducts of the coking are coal tar pitch, ammonia, hydrogen sulphide, pyridine, hydrogen cyanide and carbon based material. Some facilities have "by-product" coking ovens in which the volatile decomposition products are collected, purified and separated for use in other industries, as fuel or chemical feedstocks. Otherwise the volatile byproducts are burned to heat the coking ovens. This is an older method, but is still being used for new construction.
The "hearth" process
The "hearth" process of coke-making, using lump coal, was akin to that of charcoal-burning; instead of a heap of prepared wood, covered with twigs, leaves and earth, there was a heap of coal, covered with coke dust. The hearth process continued to be used in many areas during the first half of the 19th century, but two events greatly lessened its importance. These were the invention of the hot blast in iron-smelting and the introduction of the beehive coke oven. The use of a blast of hot air, instead of cold air, in the smelting furnace was first introduced by Neilson in Scotland in 1828.The hearth process of making coke from coal is a very lengthy process.
Beehive coke oven
A fire brick chamber shaped like a dome is used, commonly known as a beehive oven. It is typically about wide and high. The roof has a hole for charging the coal or other kindling from the top. A discharging hole is provided in the circumference of the lower part of the wall. In a coke oven battery, a number of ovens are built in a row with common walls between neighboring ovens. A battery consisted of a great many ovens, sometimes hundreds, in a row.Coal is introduced from the top to produce an even layer of about deep. Air is supplied initially, to ignite the coal. Carbonization starts and produces volatile matter, which burns inside the partially closed side door. Carbonization proceeds from top to bottom and is completed in two to three days. The heat required for the process is supplied by the burning volatile matter, so no by-products are recovered. The exhaust gases are allowed to escape to the atmosphere. The hot coke is quenched with water, and is discharged manually through the side door. When the oven is used on a continuous basis, the walls and roof retain enough heat to initiate carbonization of the next charge.
When coal was burned in a coke oven, the impurities of the coal that were not driven off as gases accumulated in the oven as slag – effectively a conglomeration of the removed impurities. Since this slag was not the desired product, it was initially just discarded. Later, however, coke oven slag was found to be useful, and has since been used as an ingredient in brick-making, mixed cement, granule-covered shingles, and even as a fertilizer.
Occupational safety
People can be exposed to coke oven emissions in the workplace by inhalation, skin contact, or eye contact. For the United States, the Occupational Safety and Health Administration has set the legal limit for coke oven emissions exposure in the workplace as 0.150 mg/m3 benzene-soluble fraction over an eight-hour workday. The US National Institute for Occupational Safety and Health has set a recommended exposure limit of 0.2 mg/m3 benzene-soluble fraction over an eight-hour workday.Uses
Coke can be used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. The carbon monoxide produced by combustion of coke reduces iron oxide to produce iron:Coke is commonly used as fuel for blacksmithing.
Coke was used in Australia in the 1960s and early 1970s for house heating, and was incentivized for home use in the UK after the 1956 Clean Air Act, which was passed in response to the Great Smog of London in 1952.
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 combusted producing little or no smoke, while bituminous coal would produce much smoke. Coke was widely used as a smokeless fuel substitute for coal in domestic heating following the creation of "smokeless zones" in the United Kingdom.
Coke 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. Hydrocarbonate is identical, although it emerged in the late eighteenth century as an inhalation therapeutic developed by Thomas Beddoes and James Watt categorized under factitious airs
- Producer gas; wood gas; generator gas; synthetic gas: a mixture of carbon monoxide, hydrogen, and nitrogen, made by passing air over red-hot coke
- Coke oven gas generated from coke ovens is similar to syngas with 60% hydrogen by volume. The hydrogen can be extracted from the coke oven gas economically for various uses.
In foundry components
Phenolic byproducts
Wastewater from coking is highly toxic and carcinogenic. It contains phenolic, aromatic, heterocyclic, and polycyclic organics, and inorganics including cyanides, sulfides, ammonium and ammonia. Various methods for its treatment have been studied in recent years. The white rot fungus Phanerochaete chrysosporium can remove up to 80% of phenols from coking waste water.Properties
Before bituminous coal is used as coking coal, it must meet a set of criteria determined by particular coal assay techniques.The bulk specific gravity of coke is typically around 0.77. It is highly porous. Both the chemical composition and physical properties are important to the usefulness of coke in blast furnaces. In terms of composition, low ash and sulphur content are desirable. Other important characteristics are the M10, M25, and M40 test crush indexes, which convey the strength of coke during transportation into the blast furnaces; depending on the blast furnace's size, finely crushed coke pieces must not be allowed into the furnace because they would impede the flow of gas through the charge of iron and coke. A related characteristic is the Coke Strength After Reaction index; it represents coke's ability to withstand the violent conditions inside the blast furnace before turning into fine particles. Pieces of coke are denoted with the following terminology: "bell coke", "nut coke", "coke breeze".
The water content in coke is practically zero at the end of the coking process, but it is often water quenched so that it can be transported to the blast furnaces. The porous structure of coke absorbs some water, usually 3–6% of its mass. In more modern coke plants an advanced method of coke cooling uses air quenching.
Other 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 and of electrolytic and welding electrodes.Gas works manufacturing syngas also produce coke as an end product, called gas house coke.
Fluid coking is a process which converts heavy residual crude into lighter products such as naphtha, kerosene, heating oil, and hydrocarbon gases. The "fluid" term refers to the fact that solid coke particles behave as a fluid solid in the continuous fluid coking process versus the older batch delayed-coking process where a solid mass of coke builds up in the coke drum over time.
Due to a lack of oil or high-quality coals in East Germany, scientists developed a process to turn low-quality lignite into coke called high temperature lignite coke.