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Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam.
It is used in chemical analysis to break down complex matter into simpler molecules for identification, for example by pyrolysis gas chromatography mass spectrometry.
In industry, it may be used to convert one single chemical; for example, ethylene dichloride is pyrolysed to vinyl chloride to make PVC. It may also be used to convert complex materials such as biomass or waste into substances that are either desirable or less harmful (e.g. - syngas).
Extreme pyrolysis, which leaves only carbon as the residue, is called carbonization. Pyrolysis is a special case of thermolysis.
Additional recommended knowledge
Some pyrolysis processes are 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 (which is why the outer layer oxidizes (burns), but the inside does not).
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. In more recent times, pyrolysis has been used on a massive scale to turn coal into coke for metallurgy, especially steel-making.
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 (806°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.
The term pyrolysis is sometimes used to encompass 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.
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 minimize secondary reactions.
Processes for biomass pyrolysis
Fast pyrolysis of biomass feedstocks is required to achieve high yields of liquids. It is characterized by rapid heating of the biomass particles and a short residence time of product vapors (0.5 to 2 s). Rapid heating means that the biomass must be ground into fine particles and that the insulating char layer that forms at the surface of the reacting particles must be continuously removed.
Since pyrolysis is slightly endothermic, various methods have been proposed to provide heat to the reacting biomass particles:
The following technologies have been proposed for biomass pyrolysis:
There is also the possibility of integrating with other processes such as mechanical biological treatment and anaerobic digestion.
Biochar used as fertilizer is very attractive since it ameliorates the soil texture and releases fertilizer slowly. When compared to chemical fertilizers, it contains oligoelements, such as selenium, which help achieve higher crop yields. When compared to other “natural” fertilizers such as manure or sewage, it is safer since it has been disinfected at high temperature and, being a solid, greatly reduces any risk of water table contamination (inhalation risks remain). Pyrolytic char is thought to be a major component in the formation of ancient terra preta soils. Efforts are underway to recreate these soils through the production of biochar, which is designed to promote nutrient retention and enhance soil ecology. Biochar is also being considered for carbon sequestration in the mitigation_of_global_warming.  
Destructive fires in buildings will often burn with limited oxygen supply, resulting in pyrolysis reactions. Thus, pyrolysis reaction mechanisms and the pyrolysis properties of materials are important in fire protection engineering for passive fire protection. Pyrolytic carbon is also important to fire investigators as a tool for discovering origin and cause of fires.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pyrolysis". A list of authors is available in Wikipedia.|