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Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere.[1] It involves a change of chemical composition. The word is coined from the Greek-derived elements pyro "fire" and lysis "separating".

Pyrolysis is most commonly used in the treatment of organic materials. It is one of the processes involved in charring wood.[2] In general, pyrolysis of organic substances produces volatile products and leaves a solid residue enriched in carbon, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization. Pyrolysis is considered as the first step in the processes of gasification or combustion.[3][4]

The process is used heavily in the chemical industry, for example, to produce ethylene, many forms of carbon, and other chemicals from petroleum, coal, and even wood, to produce coke from coal. Used also in the Methane Pyrolysis conversion of natural gas (methane) into non-polluting hydrogen gas and non-polluting solid carbon char, initiating production in industrial volume.[5] Aspirational applications of pyrolysis would convert biomass into syngas and biochar, waste plastics back into usable oil, or waste into safely disposable substances.

Terminology

Pyrolysis is one of various types of chemical degradation processes that occur at higher temperatures (above the boiling point of water or other solvents). It differs from other processes like combustion and hydrolysis in that it usually does not involve the addition of other reagents such as oxygen (O2, in combustion) or water (in hydrolysis).[6] Pyrolysis produces solids (char), condensable liquids (tar), and uncondensing/permanent gasses.Pyrolysis is most commonly used in the treatment of organic materials. It is one of the processes involved in charring wood.[2] In general, pyrolysis of organic substances produces volatile products and leaves a solid residue enriched in carbon, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization. Pyrolysis is considered as the first step in the processes of gasification or combustion.[3][4]

The process is used heavily in the chemical industry, for example, to produce ethylene, many forms of carbon, and other chemicals from petroleum, coal, and even wood, to produce coke from coal. Used also in the Methane Pyrolysis conversion of natural gas (methane) into non-polluting hydrogen gas and non-polluting solid carbon char, initiating production in industrial volume.[5] Aspirational applications of pyrolysis would convert biomass into syngas and biochar, waste plastics back into usable oil, or waste into safely disposable substances.

Pyrolysis is one of various types of chemical degradation processes that occur at higher temperatures (above the boiling point of water or other solvents). It differs from other processes like combustion and hydrolysis in that it usually does not involve the addition of other reagents such as oxygen (O2, in combustion) or water (in hydrolysis).[6] Pyrolysis produces solids (char), condensable liquids (tar), and uncondensing/permanent gasses.[7][8][9][10]

Types of pyrolysis

Complete pyrolysis of organic matter usually leaves a solid residue that consists mostly of elemental carbon; the process is then called carbonization. More specific cases of pyrolysis include:

General processes and mechanisms

carbon; the process is then called carbonization. More specific cases of pyrolysis include:

  • methane pyrolysis, in the presence of catalytic molten metals for the direct conversion of methane to non-polluting hydrogen fuel and separable solid carbon
  • decomposition temperature, breaking chemical bonds in its molecules. The fragments usually become smaller molecules, but may combine to produce residues with larger molecular mass, even amorphous covalent solids.

    In many settings, some amounts of oxygen, water, or other substances may be present, so that combustion, hydrolysis, or other chemical processes may occur besides pyrolysis proper. Sometimes those chemical are added intentionally, as in the burning of firewood, in the traditional manufacture of charcoal, and in the steam cracking of crude oil.

    Conversely, the starting material may be heated in a vacuum or in an inert atmosphere to avoid adverse chemical reactions. Pyrolysis in a vacuum also lowers the boiling point of the byproducts, improving their recovery.

    When organic matter is heated at increasing temperatures in open containers, the following processes generally occur, in successive or overlapping stages:

    • Below about 100 °C, volatiles, including some water, evaporate. Heat-sensitive substances, such as vitamin C and proteins, may partially change or decompose already at this stage.
    • At about 100 °C or slightly higher, any remaining water that is merely absorbed in the material is driven off. Water trapped in crystal structure of hydrates may come off at somewhat higher temperatures. This process consumes a lot of energy, so the temperature may stop rising until this stage is complete.
    • Some solid substances, like fats, waxes, and sugars, may melt and separate.
    • Between 100 and 500 °C, many common organic molecules break down. Most sugars start decomposing at 160–180 °C. Cellulose, a major component of wood, paper, and cotton fabrics, decomposes at about 350 °C.[3] Lignin, another major wood component, starts decomposing at about 350 °C, but continues releasing volatile products up to 500 °C.[3] The decomposition products usually include water, carbon monoxide CO and/or carbon dioxide CO
      2
      , as well as a large number of organic compounds.[4][12] Gases and volatile products leave the sample, and some of them may condense again as smoke. Generally, this process also absorbs energy. Some volatiles may ignite and burn, creating a visible flame. The non-volatile residues typically become richer in carbon and form large disordered molecules, with colors ranging between brown and black. At this point the matter is said to have been "charred" or "carbonized".
    • At 200–300 °C, if oxygen has not been excluded, the carbonaceous residue may start to burn, in a highly exothermic reaction, often with no or little visible flame. Once carbon combustion starts, the temperature rises spontaneously, turning the residue into a glowing ember and releasing carbon dioxide and/or monoxide. At this stage, some of the nitrogen still remaining in the residue may be oxidized into nitrogen oxides like NO
      2
      and <

      In many settings, some amounts of oxygen, water, or other substances may be present, so that combustion, hydrolysis, or other chemical processes may occur besides pyrolysis proper. Sometimes those chemical are added intentionally, as in the burning of firewood, in the traditional manufacture of charcoal, and in the steam cracking of crude oil.

      Conversely, the starting material may be heated in a vacuum or in an inert atmosphere to avoid adverse chemical reactions. Pyrolysis in a vacuum also lowers the boiling point of the byproducts, improving their recovery.

      When organic matter is heated at increasing temperatures in open containers, the following processes generally occur, in successive or overlapping stages:

      Pyrolysis has many applications in food preparation.[13] Caramelization is the pyrolysis of sugars in food (often after the sugars have been produced by the breakdown of polysaccharides). The food goes brown and changes flavour. The distinctive flavours are used in many dishes; for instance, caramelized onion is used in French onion soup.[14][15] The temperatures needed for caramelization lie above the boiling point of water.[14] Frying oil can easily rise above boiling point. Putting a lid on the frying pan keeps the water in, and some of it re-condenses, keeping the temperature too cool to brown for longer.

      Pyrolysis of food can also be undesirable, as in the charring of burnt food (at temperatures too low for the oxidative combustion of carbon to produce flames and burn the food to ash).

      Coke, carbon, charcoals, and chars

      Charcoal briquettes, often made from compressed sawdust or similar, in use.

      Carbon and carbon-rich materials have desirable properties but are nonvolatile, even at high temperatures. Consequently, pyrolysis is used to produce many kinds of carbon; these can be used for fuel, as reagents in steelmaking (coke), and as structural materials.

      Charcoal is a less smoky fuel than pyrolyzed wood).Pyrolysis of food can also be undesirable, as in the charring of burnt food (at temperatures too low for the oxidative combustion of carbon to produce flames and burn the food to ash).

      Carbon and carbon-rich materials have desirable properties but are nonvolatile, even at high temperatures. Consequently, pyrolysis is used to produce many kinds of carbon; these can be used for fuel, as reagents in steelmaking (coke), and as structural materials.

      Charcoal is a less smoky fuel than pyrolyzed wood).[16] Some cities ban, or used to ban, wood fires; when residents only use charcoal (and similarly-treated rock coal, called coke) air pollution is significantly reduced. In cities where people do not generally cook or heat with fires, this is not needed. In the mid-20th century, "smokeless" legislation in Europe required cleaner-burning techniques, such as coke fuel[17] and smoke-burning incinerators[18] as an effective measure to reduce air pollution[17]

      A blacksmith's forge, with a blower forcing air through a bed of fuel to raise the temperature of the fire. On the periphery, coal is pyrolyzed, absorbing heat; the coke at the center is almost pure carbon, and releases a lot of heat when the carbon oxidizes.
      Typical organic products obtained by pyrolysis of coal (X = CH, N).

      The coke-making or "coking" process consists of heating the material in "coking ovens" to very high temperatures (up to 900 °C or 1,700 °F) so that those molecules are broken down into lighter volatile substances, which leave the vessel, and a porous but hard residue that is mostly carbon and inorganic ash. The amount of volatiles varies with the source material, but is typically 25–30% of it by weight. High temperature pyrolysis is used on an industrial scale to convert coal into coke. This is useful in metallurgy, where the higher temperatures are necessary for many processes, such as steelmaking. Volatile by-products of this process are also often useful, including benzene and pyridine.[19] Coke can also be produced from the solid residue left from petroleum refining.

      The original vascular structure of the wood and the pores created by escaping gases combine to produce a light and porous material. By starting with a dense wood-like material, such as nutshells or peach stones, one obtains a form of charcoal with particularly fine pores (and hence a much larger pore surface area), called activated carbon, which is used as an adsorbent for a wide range of chemical substances.

      Biochar is the residue of incomplete organic pyrolysis, e.g., from cooking fires. They are a key component of the terra preta soils associated with ancient indigenous communities of the Amazon basin.[20] Terra preta is much sought by local farmers for its superior fertility and capacity to promote and retain an enhanced suite of beneficial microbiota, compared to the typical red soil of the region. Efforts are underway to recreate these soils through biochar, the solid residue of pyrolysis of various materials, mostly organic waste.

      Carbon fibers produced by pyrolyzing a silk cocoon. Electron micrograph, scale bar at bottom left shows 100 μm.

      Carbon fibers are filaments of carbon that can be used to make very strong yarns and textiles. Carbon fiber items are often produced by spinning and weaving the desired item from fibers of a suitable polymer, and then pyrolyzing the material at a high temperature (from 1,500–3,000 °C or 2,730–5,430 °F). The first carbon fibers were made from rayon, but polyacrylonitrile has become the most common starting material. For their first workable electric lamps, Joseph Wilson Swan and Thomas Edison used carbon filaments made by pyrolysis of cotton yarns and bamboo splinters, respectively.

      Pyrolysis is the reaction used to coat a preformed substrate with a layer of pyrolytic carbon. This is typically done in a fluidized bed reactor heated to 1,000–2,000 °C or 1,830–3,630 °F. Pyrolytic carbon coatings are used in many applications, including artificial heart valves.[21] Charcoal is a less smoky fuel than pyrolyzed wood).[16] Some cities ban, or used to ban, wood fires; when residents only use charcoal (and similarly-treated rock coal, called coke) air pollution is significantly reduced. In cities where people do not generally cook or heat with fires, this is not needed. In the mid-20th century, "smokeless" legislation in Europe required cleaner-burning techniques, such as coke fuel[17] and smoke-burning incinerators[18] as an effective measure to reduce air pollution[17]

      The coke-making or "coking" process consists of heating the material in "coking ovens" to very high temperatures (up to 900 °C or 1,700 °F) so that those molecules are broken down into lighter volatile substances, which leave the vessel, and a porous but hard residue that is mostly carbon and inorganic ash. The amount of volatiles varies with the source material, but is typically 25–30% of it by weight. High temperature pyrolysis is used on an industrial scale to convert coal into coke. This is useful in metallurgy, where the higher temperatures are necessary for many processes, such as steelmaking. Volatile by-products of this process are also often useful, including benzene and pyridine.[19] Coke can also be produced from the solid residue left from petroleum refining.

      The original vascular structure of the wood and the pores created by escaping gases combine to produce a light and porous material. By starting with a dense wood-like material, such as nutshells or peach stones, one obtains a form of charcoal with particularly fine pores (and hence a much larger pore surface area), called activated carbon, which is used as an adsorbent for a wide range of chemical substances.

      Biochar is the residue of incomplete organic pyrolysis, e.g., from cooking fires. They are a key component of the terra preta soils associated with ancient indigenous communities of the Amazon basin.[20] Terra preta is much sought by local farmers for its superior fertility and capacity to promote and retain an enhanced suite of beneficial microbiota, compared to the typical red soil of the region. Efforts are underway to recreate these soils through biochar, the solid residue of pyrolysis of various materials, mostly organic waste.

      vascular structure of the wood and the pores created by escaping gases combine to produce a light and porous material. By starting with a dense wood-like material, such as nutshells or peach stones, one obtains a form of charcoal with particularly fine pores (and hence a much larger pore surface area), called activated carbon, which is used as an adsorbent for a wide range of chemical substances.

      Biochar is the residue of incomplete organic pyrolysis, e.g., from cooking fires. They are a key component of the terra preta soils associated with ancient indigenous communities of the Amazon basin.[20] Terra preta is much sought by local farmers for its superior fertility and capacity to promote and retain an enhanced suite of beneficial microbiota, compared to the typical red soil of the region. Efforts are underway to recreate these soils through biochar, the solid residue of pyrolysis of various materials, mostly organic waste.

      Carbon fibers are filaments of carbon that can be used to make very strong yarns and textiles. Carbon fiber items are often produced by spinning and weaving the desired item from fibers of a suitable polymer, and then pyrolyzing the material at a high temperature (from 1,500–3,000 °C or 2,730–5,430 °F). The first carbon fibers were made from rayon, but polyacrylonitrile has become the most common starting material. For their first workable electric lamps, Joseph Wilson Swan and Thomas Edison used carbon filaments made by pyrolysis of cotton yarns and bamboo splinters, respectively.

      Pyrolysis is the reaction used to coat a preformed substrate with a layer of pyrolytic carbon. This is typically done in a fluidized bed reactor heated to 1,000–2,000 °C or 1,830–3,630 °F. Pyrolytic carbon coatings are used in many applications, including artificial heart valves.[21]

      Liquid and gaseous biofuels

      Pyrolysis is the basis

      Pyrolysis is the reaction used to coat a preformed substrate with a layer of pyrolytic carbon. This is typically done in a fluidized bed reactor heated to 1,000–2,000 °C or 1,830–3,630 °F. Pyrolytic carbon coatings are used in many applications, including artificial heart valves.[21]

      Pyrolysis is the basis of several methods for producing fuel from biomass, i.e. lignocellulosic biomass.[22] Crops studied as biomass feedstock for pyrolysis include native North American prairie grasses such as switchgrass and bred versions of other grasses such as Miscantheus giganteus. Other sources of organic matter as feedstock for pyrolysis include greenwaste, sawdust, waste wood, leaves, vegetables, nut shells, straw, cotton trash, rice hulls, and orange peels.[3] Animal waste including poultry litter, dairy manure, and potentially other manures are also under evaluation. Some industrial byproducts are also suitable feedstock including paper sludge, distillers grain,[23] and sewage sludge.[24]

      In the biomass components, the pyrolysis of hemicellulose happens between 210 and 310 °C.[3] The pyrolysis of cellulose starts from 300-315 °C and ends at 360-380 °C, with a peak at 342-354 °C.[3] Lignin starts to decompose at about 200 °C and continues until 1000 °C.<

      In the biomass components, the pyrolysis of hemicellulose happens between 210 and 310 °C.[3] The pyrolysis of cellulose starts from 300-315 °C and ends at 360-380 °C, with a peak at 342-354 °C.[3] Lignin starts to decompose at about 200 °C and continues until 1000 °C.[25]

      Synthetic diesel fuel by pyrolysis of organic materials is not yet economically competitive.[26] Higher efficiency is sometimes achieved by flash pyrolysis, in which finely divided feedstock is quickly heated to between 350 and 500 °C (660 and 930 °F) for less than two seconds.

      Syngas is usually produced by pyrolysis.[13]

      The low quality of oils produced through pyrolysis can be improved by physical and chemical processes,[27] which might drive up production costs, but may make sense economically as circumstances change.

      There is also the possibility of integrating with other processes such as mechanical biological treatment and anaerobic digestion.[28] Fast pyrolysis is also investigated for biomass conversions.[29] Fuel bio-oil can also be produced by hydrous pyrolysis.

      Non-polluting industrial process producing Hydrogen from methane (natural gas), see next section.

      Hydrogen

      Pyrolysis is used to produce ethylene, the chemical compound produced on the largest scale industrially (>110 million tons/year in 2005). In this process, hydrocarbons from petroleum are heated to around 600 °C (1,112 °F) in the presence of steam; this is called steam cracking. The resulting ethylene is used to make antifreeze (ethylene glycol), PVC (via vinyl chloride), and many other polymers, such as polyethylene and polystyrene.[43]

      Semiconductors

      Pyrolysis can also be used to treat municipal solid waste and plastic waste.[4][12][44] The

      Gallium arsenide, another semiconductor, forms upon co-pyrolysis of trimethylgallium and arsine.

      Waste Manage

      Pyrolysis can also be used to treat municipal solid waste and plastic waste.[4][12][44] The main advantage is the reduction in volume of the waste. In principle, pyrolysis will regenerate the monomers (precursors) to the polymers that are treated, but in practice the process is neither a clean nor an economically competitive source of monomers.[45][46][47]

      In tire waste management, tire pyrolysis is well developed technology.[48] Other products from car tire pyrolysis include steel wires, carbon black and bitumen.[49] The area faces legislative, econ

      In tire waste management, tire pyrolysis is well developed technology.[48] Other products from car tire pyrolysis include steel wires, carbon black and bitumen.[49] The area faces legislative, economic, and marketing obstacles.[50] Oil derived from tire rubber pyrolysis contains high sulfur content, which gives it high potential as a pollutant and should be desulfurized.[51][52]

      Alkaline pyrolysis of sewage sludge at low temperature of 500 °C can enhance H2 production with in-situ carbon capture. The use of NaOH as has the potential to produce H2-rich gas that can be used for fuels cells directly.[24][53]

      Pyrolysis is also used for thermal cleaning, an industrial application to remove organic substances such as polymers, plastics and coatings from parts, products or production components like extruder screws, spinnerets[54] and static mixers. During the thermal cleaning process, at temperatures between 310 C° to 540 C° (600 °F to 1000 °F),[55] organic material is converted by pyrolysis and oxidation into volatile organic compounds, hydrocarbons and carbonized gas.[56] Inorganic elements remain.[57]

      Several types of thermal cleaning systems use pyrolysis: