Simplified depiction of pyrolysis
chemistry.
Pyrolysis is a thermochemical decomposition of
organic material at elevated temperatures in the
absence of oxygen. Pyrolysis typically occurs
under pressure and at operating temperatures
above 430 °C (800 °F). The word is coined from
the Greek-derived elements pyr "fire" and lysis
"separating".
Pyrolysis is a special case of
thermolysis, and is most commonly used for
organic materials, being then one of the
processes involved in charring. The pyrolysis of
wood, which starts at 200–300 °C (390–570
°F),[1] occurs for example in fires or when
vegetation comes into contact with lava in
volcanic eruptions. In general, pyrolysis of
organic substances produces gas and liquid
products and leaves a solid residue richer in
carbon content. Extreme pyrolysis, which leaves
mostly carbon as the residue, is called
carbonization.
The process is used
heavily in the chemical industry, for example,
to produce charcoal, activated carbon, methanol
and other chemicals from wood, to convert
ethylene dichloride into vinyl chloride to make
PVC, to produce coke from coal, to convert
biomass into syngas, to turn waste into safely
disposable substances, and for transforming
medium-weight hydrocarbons from oil into lighter
ones like gasoline. These specialized uses of
pyrolysis may be called various names, such as
dry distillation, destructive distillation, or
cracking.
Pyrolysis also plays an important
role in several cooking procedures, such as
baking, frying, grilling, and caramelizing. And
it is a tool of chemical analysis, for example
in mass spectrometry and in carbon-14 dating.
Indeed, many important chemical substances, such
as phosphorus and sulfuric acid, were first
obtained by this process. Pyrolysis has been
assumed to take place during catagenesis, the
conversion of buried organic matter to fossil
fuels. It is also the basis of pyrography.
In their embalming process, the
ancient Egyptians used a mixture of substances,
including methanol, which they obtained from the
pyrolysis of wood.
Pyrolysis differs from other
high-temperature processes like combustion and
hydrolysis in that it does not involve reactions
with oxygen, water, or any other reagents. In
practice it is not possible to achieve a
completely oxygen-free atmosphere. Because some
oxygen is present in any pyrolysis system, a
small amount of oxidation occurs.
The term has also been applied to
the decomposition of organic material in the
presence of superheated water or steam (hydrous
pyrolysis), for example in the steam cracking of
oil
