Pyrolysis is a thermochemical
decomposition
of organic
material at elevated temperatures in the absence of oxygen (or any halogen). It
involves the simultaneous change of chemical composition and physical phase, and
is irreversible. The word is coined from the Greek-derived
elements pyro
"fire" and lysis "separating".
Pyrolysis is a type of thermolysis,
and is most commonly observed in organic
materials exposed to high temperatures. It is one of the processes involved in charring wood,
starting at 200–300 °C (390–570 °F). It also occurs in fires where
solid fuels are burning 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, char. Extreme
pyrolysis, which leaves mostly carbon as the residue, is called carbonization.
The process is used heavily 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 and biochar, 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 is also used in the
creation of nanoparticles, zirconia and oxides utilizing an ultrasonic
nozzle in a process called ultrasonic spray pyrolysis (USP).
Pyrolysis also plays an important role in several cooking
procedures, such as baking,
frying, grilling, and caramelizing.
In addition, it is 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.
In their embalming process, the ancient Egyptians used a mixture of substances,
including methanol, which they obtained from the pyrolysis of wood.
Pyrolysis differs from other high-temperature processes like
combustion
and hydrolysis
in that it usually does not involve reactions with oxygen, water, or
any other reagents. In practice, it is not possible to achieve a completely
oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system,
a small amount of oxidation occurs.
The term has also been applied to the decomposition 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 a solid,
called smouldering,
is the combustion of the solid residue (char or charcoal) left
behind by pyrolysis. Thus, the pyrolysis of common materials like wood,
plastic, and clothing is extremely important for fire safety
and firefighting.
Cooking
Pyrolysis occurs whenever food is exposed to high enough
temperatures in a dry environment, such as roasting, baking, toasting, or
grilling. 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 undergo
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, in general, avoided in normal cooking. It may occur, however, when
grilling 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 requires temperatures
substantially higher than 100 °C (212 °F), 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 begins only 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 170 °C
(338 °F) produces caramel, a beige to brown water-soluble product 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 on 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) occurs, 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. 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
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
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 animal manure, 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. The
solid, carbon-containing char produced can be sequestered in the ground, where
it will remain for several hundred to a few thousand years.
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 of
heating the material in closed vessels to very high temperatures (up to
2,000 °C or 3,600 °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.
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 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.
Pyrolytic carbon
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.
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. 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. Crops and plant
material wastes provide biomass feedstock on the basis of their lignocellulose portions.
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, in which finely divided feedstock
is quickly heated to between 350 and 500 °C (660 and 930 °F) for less
than 2 seconds.
Fuel bio-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, with a higher cetane value
and lower sulphur content than traditional diesel. Using pyrolysis to extract
fuel from end-of-life plastic is a second-best option after recycling, is
environmentally preferable to landfill, and can help reduce dependency on
foreign fossil fuels and geo-extraction. Pilot Jeremy Roswell plans to make the
first flight from Sydney to London using diesel fuel from recycled plastic
waste manufactured by Cynar PLC.
Waste tire disposal
Pyrolysis of scrap or waste tires (WT) can separate solids
in the tire, such as steel and carbon
black, from volatile liquid and gaseous compounds that can be used as fuel.
Although the pyrolysis of WT has been widely developed throughout the world,
there are legislative, economic, and marketing obstacles to widespread
adoption.
Processes
In many industrial applications, the process is done under
pressure and at operating temperatures above 430 °C (806 °F). For
agricultural waste, for example, typical temperatures are 450 to 550 °C
(840 to 1,000 °F).
Processes
Since pyrolysis is endothermic,
various methods to provide heat to the reacting biomass particles have been
proposed:
- Partial combustion of the biomass products through air injection. This results in poor-quality products.
- Direct heat transfer with a hot gas, the ideal one being 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 used for the traditional production of charcoal. Poor, slow heat transfer result 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 a particle's 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. The process is offered by BTG-BTL, a subsidiary from BTG Biomass Technology Group B.V. in The Netherlands. Like other shallow transported-bed reactors relatively fine particles (several mm) are required to obtain a liquid yield of around 70 wt.%. Larger-scale commercial implementation (up to 5 t/h input) is underway.
- 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 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 a 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.
- Chain Grate: Dry biomass is fed onto a hot (500C) heavy cast metal grate or apron which forms a continuous loop. A small amount of air aids in heat transfer and in primary reactions for drying and carbonization. Volatile products are combusted for process and boiler heating.
Use of vacuum
In vacuum pyrolysis, organic material is heated in a vacuum in order to
decrease its 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. Thus, a synthesis of 2-Furonitrile
has been described employing the dehydration of 2-furoic acid amide or oxime
via flash vacuum pyrolysis over molecular sieves in the gas phase.
Industrial sources
Many sources of organic
matter can be used as feedstock for pyrolysis. Suitable plant material
includes greenwaste, sawdust, waste wood, woody weeds; and agricultural sources
including nut shells, straw, cotton trash, rice hulls, switch grass; and animal
waste including poultry litter, dairy manure, and potentially other manures.
Pyrolysis is used as a form of thermal
treatment to reduce waste volumes of domestic refuse. Some
industrial byproducts are also suitable feedstock including paper sludge and
distillers grain.
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 provide both the energy needed for pyrolysis and some excess production
- solid char that can either be burned for energy or be 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.
Chemistry
Current research examines the multiple reaction pathways of
pyrolysis to understand how to manipulate the formation of pyrolysis' multiple
products (oil, gas, char, and miscellaneous chemicals) to enhance the economic
value of pyrolysis; identifying catalysts to manipulate pyrolysis reactions is
also a goal of some pyrolysis research. Published research suggests that
pyrolysis reactions have some dependence upon the structural composition of
feedstocks (e.g. lignocellulosic biomass), with
contributions from some minerals present in the feedstocks; some minerals
present in feedstock are thought to increase the cost of operation of pyrolysis
or decrease the value of oil produced from pyrolysis, through corrosive
reactions.
SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
For affiliation:
No comments:
Post a Comment