Simple sketch of pyrolysis chemistry.
is the
chemical decomposition of condensed organic substances by heating. The word is coined from the
Greek-derived
elements
pyro "fire" and
lysys "decomposition".
Pyrolysis is a special case of
thermolysis related to the chemical process of
charring, and is most commonly used for
organic materials. It occurs spontaneously at high temperatures (e.g., above 300 °C for wood, it varies for other material), for example in
fires or when vegetation comes into contact with
lava in
volcanic eruptions. In general, it 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. It does not involve reactions with
oxygen or any other reagents but can take place in their presence.
Pyrolysis is heavily used 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 the
cracking of medium-weight
hydrocarbons from
oil to produce
lighter ones like
gasoline.
It is an important chemical process in several
cooking procedures such as
baking,
frying,
grilling, and
caramelizing. Pyrolysis is also a tool of
chemical analysis, for example by
pyrolysis gas chromatography mass spectrometry and in
carbon-14 dating. Indeed, many important chemical substances, such as
phosphorus and
sulfuric acid, were first obtained by this process. It has been assumed to take place during
catagenesis, the conversion of
buried organic matter to
fossil fuels. Pyrolysis is also the basis of
pyrography.
Although
water is normally excluded along with other reagents, the term has also been applied to the decompositon of organic material in the presence of
superheated water or
steam (
hydrous pyrolysis), for example in the
steam cracking of oil.
Occurrence and uses
Fire
Pyrolysis is usually the first chemical reaction that occurs in the
burning of many solid organic fuels, like wood, cloth, and paper, and also of some kinds of
plastic. In a wood fire, the visible flames are not due to combustion of the wood itself, but rather of the gases released by its pyrolysis; whereas the flame-less burning of
embers is the combustion of the solid residue (charcoal) left behind by it. Thus, the pyrolysis of common materials like wood, plastic, and clothing is extremely important for
fire safety and
fire-fighting.
Cooking
Pyrolysis occurs whenever food is exposed to high enough temperatures in a dry environment, such as roasting, baking, toasting, grilling, etc.. It is the chemical process responsible for the formation of the golden-brown crust in foods prepared by those methods.
In normal cooking, the main food components that suffer pyrolysis are
carbohydrates (including
sugars,
starch, and
fiber) and
proteins. Pyrolysis of
fats requires a much higher temperature, and since it produces toxic and flammable products (such as
acrolein), it is generally avoided in normal cooking. It may occur, however, when barbecuing fatty meats over hot coals.
Even though cooking is normally carried out in
air, the temperatures and environmental conditions are such that there is little or no combustion of the original substances or their decomposition products. In particular, the pyrolysis of proteins and carbohydrates begins at temperatures much lower than the
ignition temperature of the solid residue, and the volatile subproducts are too diluted in air to ignite. (In
flambé dishes, the flame is due mostly to combustion of the
alcohol, while the crust is formed by pyrolysis as in baking.)
Pyrolysis of carbohydrates and proteins require temperatures substantially higher than , so pyrolysis does not occur as long as free water is present, e.g. in
boiling food — not even in a
pressure cooker. When heated in the presence of water, carbohydrates and proteins suffer gradual
hydrolysis rather than pyrolysis. Indeed, for most foods, pyrolysis is usually confined to the outer layers of food, and only begins after those layers have dried out.
Food pyrolysis temperatures are however lower than the
boiling point of
lipids, so pyrolysis occurs when frying in
vegetable oil or
suet, or
basting meat in its own
fat.
Pyrolysis also plays an essential role in the production of
barley tea,
coffee, and roasted nuts such as
peanuts and
almonds. As these consist mostly of dry materials, the process of pyrolysis is not limited to the outermost layers but extends throughout the materials. In all these cases, pyrolysis creates or releases many of the substances that contribute to the flavor, color, and
biological properties of the final product. It may also destroy some substances that are toxic, unpleasant in taste, or those that may contribute to
spoilage.
Controlled pyrolysis of
sugars starting at produces
caramel, a beige to brown water-soluble product which is widely used in
confectionery and (in the form of
caramel coloring) as a
coloring agent for
soft drinks and other industrialized food products.
Solid residue from the pyrolysis of spilled and splattered food creates the brown-black encrustation often seen on cooking vessels, stove tops, and the interior surfaces of ovens.
Charcoal
Pyrolysis has been used since ancient times for turning wood into charcoal in an industrial scale. Besides wood, the process can also use
sawdust and other wood waste products.
Charcoal is obtained by heating wood until its complete pyrolysis (carbonization), leaving only carbon and inorganic
ash. In many parts of the world, charcoal is still produced semi-industrially, by burning a pile of wood that has been mostly covered with mud or bricks. The heat generated by burning part of the wood and the volatile byproducts pyrolyzes the rest of the pile. The limited supply of oxygen prevents the charcoal from burning too. A more modern alternative is to heat the wood in an airtight metal vessel, which is much less
polluting and allows the volatile products to be condensed.
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 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
Residues of incomplete organic pyrolysis, e.g. from cooking fires, are thought to be the key component of the
terra preta soils associated with ancient
indigenous communities of the
Amazon basin.
Terra preta is much sought by local farmers for its superior fertility compared to the natural 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.
Biochar improves the
soil texture and
ecology, increasing its ability to retain fertilizers and release them slowly. It naturally contains many of the
micronutrients needed by plants, such as
selenium. It is also safer than other "natural" fertilizers such as
manure or
sewage since it has been disinfected at high temperature, and since it releases its nutrients at a slow rate, it greatly reduces the risk of
water table contamination.
Biochar is also being considered for
carbon sequestration, with the aim of
mitigation of global warming.
Coke
Pyrolysis is used on a massive scale to turn
coal into
coke for
metallurgy, especially
steelmaking.
Coke can also be produced from the solid residue left from petroleum refining.
Those starting materials typically contain hydrogen, nitrogen or oxygen atoms combined with carbon into molecules of medium to high molecular weight. The coke-making or "coking" process consists in heating the material in closed vessels to very high temperatures (up to ), 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.
Carbon fiber
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 1500 C to 3000 C).
The first carbon fibres 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.
Biofuel
Pyrolysis is the basis of several methods that are being developed for producing fuel from
biomass, which may include either crops grown for the purpose or biological waste products from other industries.
Although synthetic
diesel fuel cannot yet be produced directly by pyrolysis of organic materials, there is a way to produce similar liquid ("bio-oil") that can be used as a fuel, after the removal of valuable bio-chemicals that can be used as food additives or pharmaceuticals.
Higher efficiency is achieved by the so-called
flash pyrolysis where finely divided feedstock is quickly heated to between 350 and 500 C for less than 2 seconds.
Fuel bio-oil resembling light
crude oil can also be produced by hydrous pyrolysis from many kinds of feedstock, including waste from
pig and
turkey farming, by a process called
thermal depolymerization (which may however include other reactions besides pyrolysis).
Plastic waste disposal
Anhydrous pyrolysis can also be used to produce liquid fuel similar to diesel from plastic waste.
Processes
In many industrial applications, the process is done under pressure and at operating temperatures above . For agricultural waste, for example, typical temperatures are 450 to 550 °C.
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
Since pyrolysis is
endothermic, 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.
For flash pyrolysis the biomass must be ground into fine particles and the insulating char layer that forms at the surface of the reacting particles must be continuously removed. 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. 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.
*
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 has been scaled up by companies such as
Dynamotive and Agri-Therm. The main challenges are 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
There is also the possibility of integrating with other processes such as
mechanical biological treatment and
anaerobic digestion.
Industrial products
- 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
- solid char that can either be burned for energy or recycled as a fertilizer (biochar).
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
