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Coke is a solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Coke from coal is gray, hard, and porous.
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Coking (coke-making)
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 2,000 degrees Celsius. This fuses together the fixed carbon and residual ash. Most coke in modern facilities is produced in "by-product" coke ovens 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 burn the hydrocarbon off gasses produced by the coke making process to drive the carbonization process.
Properties and usage
The specific gravity of coke is typically around 0.77. It is highly porous.
The most important properties of coke are ash and sulfur content, which are linearly dependent on the coal used for production. Coke with less ash and sulfur content is highly priced on the market. Other important characteristics of coke include M10, M25, and M40 test crush indexes as they convey the strength of coke during transportation into the blast furnaces (BF); depending on BF size, there are certain requirements for coke size before entering the blast furnace, finely crushed coke pieces must not be allowed into the BF because gas dynamics would be impeded inside. Coke Strength After Reaction or CSR index is another important characteristics of coke as it represents coke's ability to withstand the violent conditions inside the blast furnace before turning into fine particles.
The volatility of coke reaches minimum levels at the end of the coking process. Volatility is an important property index for bituminous coal used in coke production, the greater volatile matter inside coal the more by-product could be produced, but there are some limitations on acceptable volatile content depending on the technical specifications of the coke batteries. Coal is being blended in proportions among different types of coal to reach acceptable levels of volatility before the coking process begins. Too low or too high levels of volatile matter in the coal blend results in inferior coke produced in respect to coke quality properties, it is generally considered that levels of 26-29 % of volatile matter in the coal blend is good for coking purposes.
The water content in coke is practically zero at the end of the coking process, but coke is often water quenched to reduce its temperature so that it can be transported inside the BF. The porous structure of coke absorbs some water, usually to 3-6 % of its mass. In modern coke plants an advanced method of coke cooling is by air quenching.
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.
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.citation needed
History
The use of coke as a fuel was pioneered by the Chinese in the 11th century during the Song Dynasty (960–1279 AD). In Europe, it was innovated during the 17th century in 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 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 beer. In 1709 Abraham Darby established 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.
In England in the first years of steam railway locomotives, coke was the normal fuel. This resulted from an early piece of environmental legislation; any proposed locomotive had to "consume its own smoke".citation needed This was not technically possible to achieve, but burning coke, with its low smoke emissions, was considered to meet the requirement. This rule was quietly dropped, and cheaper coal became the normal fuel, as railways gained acceptance among the general public.
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, electrodes, etc. 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 coke particles are in a continuous system versus older batch-coking technology.
Gases produced
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
See also
Wikipedia content modification information:
- This page was last modified on 5 October 2008, at 06:03.
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