Coking is the heating of

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

in the absence of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

to a temperature above 600 °C to drive off the volatile components of the raw coal, leaving a hard, strong, porous material of high

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

content called coke. Coke consists almost entirely of

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...

. The porosity gives it a high surface area, which makes it burn faster (as does a sheet of paper versus a wooden log). When a kilogram of coke is burned it releases more heat than a kilogram of the original coal. Coke is used as fuel in a blast furnace. In a continuous process, coke, iron ore, and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

are mixed together and placed in the top of the blast furnace, and at the bottom liquid

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

and waste slag are removed. The raw materials continuously move down the blast furnace. During this continuous process more raw materials are placed at the top, and as the coke moves down, it must withstand the ever-increasing weight of the materials above it. It is the ability to withstand this crushing force, in addition to its high energy content and rapid combustion, that makes coke ideal for use in blast furnaces.

Petroleum coking

“Coking is a refinery unit operation that upgrades material called bottoms from the atmospheric or vacuum distillation column into higher-value products and produces petroleum coke—a coal-like material.”. In

heterogeneous catalysis In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. ...

, the process is undesirable because the clinker blocks the catalytic sites. Coking is characteristic of high temperature reactions involving hydrocarbon feedstocks. Typically coking is reversed by combustion, provided that the catalyst will tolerate such. A simplified equation for coking is shown in the case of ethylene: : 3 C₂H₄ → 2 C ("coke") + 2 C₂H₆ A more realistic but complex view involves the

alkylation Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecti ...

of an aromatic ring of a coke nucleus.

Acidic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...

catalysts are thus especially prone to coking because they are effective at generating carbocations (i.e., alkylating agents). Coking is one of several mechanisms for the deactivation of a

heterogeneous catalyst In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. P ...

. Other mechanisms include

sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

poisoning A poison can be any substance that is harmful to the body. It can be swallowed, inhaled, injected or absorbed through the skin. Poisoning is the harmful effect that occurs when too much of that substance has been taken. Poisoning is not to ...

, and solid-state transformation of the catalyst.{{cite journal, last1=Bartholomew, first1=Calvin H, title=Mechanisms of Catalyst Deactivation, journal=Applied Catalysis A: General, volume=212, issue=1–2, pages=17–60, doi=10.1016/S0926-860X(00)00843-7, year=2001

References

Coking works Catalysis