Pyrolysis

 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.

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Anhydrous pyrolysis

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.^[1] 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.

Hydrous pyrolysis

Main article: Hydrous 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.

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 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,^[2] various methods have been proposed to provide heat to the reacting biomass particles:

• Partial combustion of the biomass products through air injection. This results in poor-quality products.
• Direct heat transfer with a hot gas, ideally product gas that is reheated and recycled. The problem is to provide enough heat with reasonable gas flow-rates.
• Indirect heat transfer with exchange surfaces (wall, tubes). It is difficult to achieve good heat transfer on both sides of the heat exchange surface.
• Direct heat transfer with circulating solids: Solids transfer heat between a burner and a pyrolysis reactor. This is an effective but complex technology.

The following technologies have been proposed for biomass pyrolysis:

• Fixed beds were used for the traditional production of charcoal. Poor, slow heat transfer resulted in very low liquid yields.
• Augers: This technology is adapted from a Lurgi process for coal gasification. Hot sand and biomass particles are fed at one end of a screw. The screw mixes the sand and biomass and conveys them along. It provides a good control of the biomass residence time. It does not dilute the pyrolysis products with a carrier or fluidizing gas. However, sand must be reheated in a separate vessel, and mechanical reliability is a concern. There is no large-scale commercial implementation.
• Ablative processes: Biomass particles are moved at high speed against a hot metal surface. Ablation of any char forming at the particles surface maintains a high rate of heat transfer. This can be achieved by using a metal surface spinning at high speed within a bed of biomass particles, which may present mechanical reliability problems but prevents any dilution of the products. As an alternative, the particles may be suspended in a carrier gas and introduced at high speed through a cyclone whose wall is heated; the products are diluted with the carrier gas.^[3] A problem shared with all ablative processes is that scale-up is made difficult since the ratio of the wall surface to the reactor volume decreases as the reactor size is increased. There is no large-scale commercial implementation.
• Rotating cone: Pre-heated hot sand and biomass particles are introduced into a rotating cone. Due to the rotation of the cone, the mixture of sand and biomass is transported across the cone surface by centrifugal force. Like other shallow transported-bed reactors relatively fine particles are required to obtain a good liquid yield. There is no large scale commercial implementation.^[4]
• Fluidized beds: Biomass particles are introduced into a bed of hot sand fluidized by a gas, which is usually a recirculated product gas. High heat transfer rates from fluidized sand result in rapid heating of biomass particles. There is some ablation by attrition with the sand particles, but it is not as effective as in the ablative processes. Heat is usually provided by heat exchanger tubes through which hot combustion gas flows. There is some dilution of the products, which makes it more difficult to condense and then remove the bio-oil mist from the gas exiting the condensers. This process can not be easily scaled up, it is not yet applied commercially .See Brigdwater: There are technical and economic challenges. Technical challenges lie in scaling up the endothermic pyrolysis reactor, particularly concerning heat transfer, and in improving the quality and consistency of the bio-oil.
• Circulating fluidized beds: Biomass particles are introduced into a circulating fluidized bed of hot sand. Gas, sand and biomass particles move together, with the transport gas usually being a recirculated product gas, although it may also be a combustion gas. High heat transfer rates from sand ensure rapid heating of biomass particles and ablation is stronger than with regular fluidized beds. A fast separator separates the product gases and vapors from the sand and char particles. The sand particles are reheated in fluidized burner vessel and recycled to the reactor. Although this process can be easily scaled up, it is rather complex and the products are much diluted, which greatly complicates the recovery of the liquid products.

Industrial sources

• domestic refuse: Pyrolysis is used as a form of thermal treatment to reduce waste volumes
• waste film plastic: to produce a synthetic diesel fuel^[5]
• agricultural waste: temperature of 450 to 550 °C
• sawdust and waste wood

There is also the possibility of integrating with other processes such as mechanical biological treatment and anaerobic digestion.^[6]

Industrial products

• synthetic diesel fuel
• bio-oil: can be used as a fuel, after removal of valuable bio-chemicals that can be used as food additives or pharmaceuticals. The bio-oil cannot be used directly in most car engines.
• syngas (flammable mixture of carbon monoxide and hydrogen): can be produced in sufficient quantities to both provide the energy needed for pyrolysis and some excess production ^{[7] [8]}
• solid char that can either be burned for energy or recycled as a fertilizer, also known as biochar.

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)^[9]. 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. ^{[10] [7]}

Fire protection

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.

See also

• Cracking (chemistry)
• Destructive distillation
• Dry distillation
• Gasification
• Karrick process
• Pyrolytic coating
• Woodgas

References