Pyrolysis is the
chemical decomposition of condensed substances by heating, that occurs spontaneously at high enough temperatures.The word is coined from the
Greek-derived
elements
pyro "fire" and
lysys "decomposition".
Pyrolysis is a special case of
thermolysis, and is most commonly used for
organic materials, being then one of the processes involved in
charring. The pyrolysis of wood, which starts at ,
Burning of wood,
InnoFireWood's website. Accessed on 2010-02-06.
occurs for example in fires or when vegetation comes into contact with lava in
volcanic eruptions. In general, pyrolysis of organic substances 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.
This chemical process 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 transforming medium-weight
hydrocarbons from
oil into
lighter ones like
gasoline. These specialized uses of pyrolysis may be called various names, such as
dry distillation,
destructive distillation, or
cracking.
Pyrolysis also plays an important role in several
cooking procedures, such as
baking,
frying,
grilling, and
caramelizing. It is also a tool of
chemical analysis, for example in
mass spectrometry and in
carbon-14 dating. Indeed, many important chemical substances, such as
phosphorus and
sulfuric acid, were first obtained by this process. Pyrolysis has been assumed to take place during
catagenesis, the conversion of
buried organic matter to
fossil fuels. It is also the basis of
pyrography.
Pyrolysis differs from other high-temperature processes like
combustion and
hydrolysis in that it does not involve reactions with
oxygen, water, or any other reagents. However, 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
fibre) 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.
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 ).
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.Evans, G.
"Liquid Transport Biofuels - Technology Status Report", "
National Non-Food Crops Centre", 14-04-08. Retrieved on 2009-05-05.
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.