Pyrolysis

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Simple sketch of pyrolysis chemistry

Pyrolysis is 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.

1 Anhydrous pyrolysis
2 Pyrolysis and waste management
3 Hydrous pyrolysis
4 Vacuum pyrolysis
5 See also
6 External links
7 References

[edit] Anhydrous pyrolysis

Pyrolysis is usually understood to be anhydrous (without water).

This phenomenon commonly occurs whenever solid organic material is heated strongly in absence of oxygen, 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 or plastics . 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.

[edit] Pyrolysis and waste management

The application of pyrolysis to waste management has been gaining acceptance along with other advanced waste treatment technologies. Pyrolysis also can be used as a form of thermal treatment to reduce waste volumes and produce fuels as a byproduct. Low temperature pyrolysis can also be used to produce a synthetic diesel fuel from waste film plastic, through systems such as Thermofuel.<ref>Thermofuel Cynar plc (2006) Converting waste plastic into diesel fuel</ref>

[edit] Hydrous pyrolysis

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 depolymerization of organic waste into light crude oil.

[edit] Vacuum pyrolysis

In vacuum pyrolysis organic material is heated in a vacuum in order to decrease boiling point and avoid adverse chemical reactions. It is used in organic chemistry as a synthetic tool. In flash vacuum thermolysis or FVT the residence time of the substrate at the working temperature is limited as much as possible again in order to minimise secondary reactions.