Coking is the heating of
coal in the absence of
oxygen 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 content called
coke. Coke consists almost entirely of
hydrocarbons. 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
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...
. In a continuous process, coke,
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
, and
limestone are mixed together and placed in the top of the blast furnace, and at the bottom liquid
iron and waste
slag
Slag is a by-product of smelting (pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/base metals (by-prod ...
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, the process is undesirable because the
clinker
Clinker may refer to:
*Clinker (boat building), construction method for wooden boats
*Clinker (waste), waste from industrial processes
*Clinker (cement), a kilned then quenched cement product
* ''Clinkers'' (album), a 1978 album by saxophonist St ...
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
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene i ...
:
: 3 C
2H
4 → 2 C ("coke") + 2 C
2H
6
A more realistic but complex view involves the
alkylation of an aromatic ring of a coke nucleus.
Acidic catalysts are thus especially prone to coking because they are effective at generating
carbocation
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
s (i.e., alkylating agents).
Coking is one of several mechanisms for the deactivation of a
heterogeneous catalyst. Other mechanisms include
sintering,
poisoning, 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