Pyrolysis
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Simple sketch of pyrolysis chemistry |
Pyrolysis usually means the
chemical decomposition of
organic materials by heating in the absence of
oxygen or any other reagents, except possibly
steam.
Extreme pyrolysis, that leaves only
carbon as the residue, is called
carbonization. Pyrolysis is a special case of
thermolysis.
Pyrolysis is a means of reusing scrap tires. Pyrolysis in this context is the degradation of the rubber of the tire using heat in the absence of oxygen. It is used to
recycle car tires rather than burn or bury them, which can have a detrimental effect on the environment.
Pyrolysis is usually understood to be anhydrous (without water).
This phenomenon commonly occurs whenever solid
organic material is sufficiently heated, e.g. when
frying,
roasting,
baking,
toasting. (Even though such processes are carried out in a normal atmosphere, the outer layers of the material keep its interior oxygen-free.)
The process also occurs when
burning compact solid fuel, like wood. In fact, the flames of a wood fire are due to combustion of gases released by pyrolysis, not combustion of the wood itself. Thus, the pyrolysis of common materials like wood, plastic and clothing is extremely important for
fire safety and
fire fighting.
An ancient industrial use of anhydrous pyrolysis is the production of
charcoal through the pyrolysis of
wood. More recently, pyrolysis has been used on a massive scale to turn
coal into
coke for
metallurgy, especially
steelmaking.
Anhydrous pyrolysis has been assumed to take place during
catagenesis, the conversion of
kerogen to
fossil fuels.
In many industrial applications the process is done under pressure and at operating temperatures above 430
°C (800
°F).
Anhydrous pyrolysis can also be used to produce liquid fuel similar to
diesel from solid
biomass. The most common technique uses very low residence times (<2 seconds) and high heating rates using a temperature between 350-500 °C and is called either fast or
flash pyrolysis.
Pyrolysis also can be used to degrade wastes, as a form of
incineration.
The term pyrolysis is sometimes used to encompass also thermolysis in the presence of water, such as
steam cracking of oil, or more generally
hydrous pyrolysis. An example of the latter is
thermal depolymerizationof organic waste into light
crude oil.
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Gasification*
Karrick process
