# data[].attributes.attachment_link.readable_text
Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Bodies of living organisms begin to decompose shortly after death. Animals, such as worms, also help decompose the organic materials. Organisms that do this are known as decomposers. Although no two organisms decompose in the same way, they all undergo the same sequential stages of decomposition. The science which studies decomposition is generally referred to as from the Greek word taphos, meaning tomb. Decomposition can also be a gradual process for organisms that have extended periods of dormancy.

One can differentiate abiotic from biotic substance ( biodegradation). The former means "degradation of a substance by chemical or physical processes, e.g., hydrolysis. The latter means "the metabolic breakdown of materials into simpler components by living organisms", typically by microorganisms.

Decomposition begins at the moment of death, caused by two factors: 1.) autolysis, the breaking down of tissues by the body's own internal chemicals and enzymes, and 2.) putrefaction, the breakdown of tissues by bacteria. These processes release compounds such as cadaverine and putrescine, that are the chief source of the unmistakably putrid odor of decaying animal tissue.

Prime decomposers are bacteria or fungi, though larger scavengers also play an important role in decomposition if the body is accessible to insects, mites and other animals. The most important arthropods that are involved in the process include carrion beetles, mites, the flesh-flies (Sarcophagidae) and blow-flies ( Calliphoridae), such as the green bottle flies seen in the summer. In North America, the most important non-insect animals that are typically involved in the process include mammal and bird scavengers, such as coyotes, dogs, wolves, foxes, rats, crows and vultures.[citation needed ] Some of these scavengers also remove and scatter bones, which they ingest at a later time. Aquatic and marine environments have break-down agents that include bacteria, fish, crustaceans, fly larvae and other carrion scavengers.

Five general stages are used to describe the process of decomposition in vertebrate animals: fresh, bloat, active decay, advanced decay, and dry/remains. The general stages of decomposition are coupled with two stages of chemical decomposition: autolysis and putrefaction. These two stages contribute to the chemical process of decomposition, which breaks down the main components of the body. With death the microbiome of the living organism collapses and is followed by the necrobiome that undergoes predictable changes over time.

Among those animals that have a heart, the "fresh" stage begins immediately after the heart stops beating. From the moment of death, the body begins cooling or warming to match the temperature of the ambient environment, during a stage called algor mortis. Shortly after death, within three to six hours, the muscular tissues become rigid and incapable of relaxing, during a stage called rigor mortis. Since blood is no longer being pumped through the body, gravity causes it to drain to the dependent portions of the body, creating an overall bluish-purple discolouration termed livor mortis or, more commonly, lividity. Depending on the position of the body, these parts would vary. For instance, if the person was flat on their back when they died, the blood would collect in the parts that are touching the ground. If the person was hanging, it would collect in their fingertips, toes, and earlobes.

Once the heart stops, the blood can no longer supply oxygen or remove carbon dioxide from the tissues. The resulting decrease in pH and other chemical changes causes cells to lose their structural integrity, bringing about the release of cellular enzymes capable of initiating the breakdown of surrounding cells and tissues. This process is known as autolysis.

Visible changes caused by decomposition are limited during the fresh stage, although autolysis may cause blisters to appear at the surface of the skin.

The small amount of oxygen remaining in the body is quickly depleted by cellular metabolism and aerobic microbes naturally present in respiratory and gastrointestinal tracts, creating an ideal environment for the proliferation of anaerobic organisms. These multiply, consuming the body's carbohydrates, lipids, and proteins, to produce a variety of substances including propionic acid, lactic acid, methane, hydrogen sulfide, and ammonia. The process of microbial proliferation within a body is referred to as putrefaction and leads to the second stage of decomposition, known as bloat.

Blowflies and flesh flies are the first carrion insects to arrive, and they seek a suitable oviposition site.

The bloat stage provides the first clear visual sign that microbial proliferation is underway. In this stage, anaerobic metabolism takes place, leading to the accumulation of gases, such as hydrogen sulfide, carbon dioxide, methane, and nitrogen. The accumulation of gases within the bodily cavity causes the distention of the abdomen and gives a cadaver its overall bloated appearance. The gases produced also cause natural liquids and liquefying tissues to become frothy. As the pressure of the gases within the body increases, fluids are forced to escape from natural orifices, such as the nose, mouth, and anus, and enter the surrounding environment. The buildup of pressure combined with the loss of integrity of the skin may also cause the body to rupture.

Intestinal anaerobic bacteria transform haemoglobin into sulfhemoglobin and other colored pigments. The associated gases which accumulate within the body at this time aid in the transport of sulfhemoglobin throughout the body via the circulatory and lymphatic systems, giving the body an overall marbled appearance.

If insects have access, maggots hatch and begin to feed on the body's tissues. Maggot activity, typically confined to natural orifices, and masses under the skin, causes the skin to slip, and hair to detach from the skin. Maggot feeding, and the accumulation of gases within the body, eventually leads to post-mortem skin ruptures which will then further allow purging of gases and fluids into the surrounding environment. Ruptures in the skin allow oxygen to re-enter the body and provide more surface area for the development of fly larvae and the activity of aerobic microorganisms. The purging of gases and fluids results in the strong distinctive odors associated with decay.

Active decay

Active decay is characterized by the period of greatest mass loss. This loss occurs as a result of both the voracious feeding of maggots and the purging of decomposition fluids into the surrounding environment. The purged fluids accumulate around the body and create a cadaver decomposition island (CDI). Liquefaction of tissues and disintegration become apparent during this time and strong odors persist. The end of active decay is signaled by the migration of maggots away from the body to pupate.

Advanced decay

Decomposition is largely inhibited during advanced decay due to the loss of readily available cadaveric material. Insect activity is also reduced during this stage. When the carcass is located on soil, the area surrounding it will show evidence of vegetation death. The CDI surrounding the carcass will display an increase in soil carbon and nutrients, such as phosphorus, potassium, calcium, and magnesium; changes in pH; and a significant increase in soil nitrogen.

Dry/remains

During the dry/remains stage, the resurgence of plant growth around the CDI may occur and is a sign that the nutrients present in the surrounding soil have not yet returned to their normal levels. All that remains of the cadaver at this stage is dry skin, cartilage, and bones, which will become dry and bleached if exposed to the elements. If all soft tissue is removed from the cadaver, it is referred to as completely skeletonized, but if only portions of the bones are exposed, it is referred to as partially skeletonised.

A dead body that has been exposed to the open elements, such as water and air, will decompose more quickly and attract much more insect activity than a body that is buried or confined in special protective gear or artifacts. This is due, in part, to the limited number of insects that can penetrate a coffin and the lower temperatures under the soil.

The rate and manner of decomposition in an animal body are strongly affected by several factors. In roughly descending degrees of importance, they are:

The speed at which decomposition occurs varies greatly. Factors such as temperature, humidity, and the season of death all determine how fast a fresh body will skeletonize or mummify. A basic guide for the effect of environment on decomposition is given as Casper's Law (or Ratio): if all other factors are equal, then, when there is free access of air a body decomposes twice as fast as if immersed in water and eight times faster than if buried in the earth. Ultimately, the rate of bacterial decomposition acting on the tissue will depend upon the temperature of the surroundings. Colder temperatures decrease the rate of decomposition while warmer temperatures increase it. A dry body will not decompose efficiently. Moisture helps the growth of microorganisms that decompose the organic matter, but too much moisture could lead to anaerobic conditions slowing down the decomposition process.

The most important variable is the body's accessibility to insects, particularly flies. On the surface in tropical areas, invertebrates alone can easily reduce a fully fleshed corpse to clean bones in under two weeks. The skeleton itself is not permanent; acids in soils can reduce it to unrecognizable components. This is one reason given for the lack of human remains found in the wreckage of the Titanic, even in parts of the ship considered inaccessible to scavengers. Freshly skeletonized bone is often called "green" bone and has a characteristic greasy feel. Under certain conditions (normally cool, damp soil), bodies may undergo saponification and develop a waxy substance called adipocere, caused by the action of soil chemicals on the body's proteins and fats. The formation of adipocere slows decomposition by inhibiting the bacteria that cause putrefaction.

In extremely dry or cold conditions, the normal process of decomposition is halted - by either lack of moisture or temperature controls on bacterial and enzymatic action - causing the body to be preserved as a mummy. Frozen mummies commonly restart the decomposition process when thawed (see Ötzi the Iceman), whilst heat-desiccated mummies remain so unless exposed to moisture.

The bodies of newborns who never ingested food are an important exception to the normal process of decomposition. They lack the internal microbial flora that produces much of decomposition and quite commonly mummify if kept in even moderately dry conditions.

Aerobic decomposition takes place in the presence of oxygen. This is most common to occur in nature. Living organisms that use oxygen to survive feed on the body. Anaerobic decomposition takes place in the absence of oxygen. This could be a place where the body is buried in organic material and oxygen can not reach it. This process of putrefaction has a bad odor accompanied by it due to the hydrogen sulfide and organic matter containing sulfur.

Embalming is the practice of delaying the decomposition of human and animal remains. Embalming slows decomposition somewhat but does not forestall it indefinitely. Embalmers typically pay great attention to parts of the body seen by mourners, such as the face and hands. The chemicals used in embalming repel most insects and slow down bacterial putrefaction by either killing existing bacteria in or on the body themselves or by "fixing" cellular proteins, which means that they cannot act as a nutrient source for subsequent bacterial infections. In sufficiently dry environments, an embalmed body may end up mummified and it is not uncommon for bodies to remain preserved to a viewable extent after decades. Notable viewable embalmed bodies include those of:

A body buried in a sufficiently dry environment may be well preserved for decades. This was observed in the case for murdered civil rights activist Medgar Evers, who was found to be almost perfectly preserved over 30 years after his death, permitting an accurate autopsy when the case of his murder was re-opened in the 1990s.

Bodies submerged in a peat bog may become naturally "embalmed", arresting decomposition and resulting in a preserved specimen known as a bog body. The generally cool and anoxic conditions in these environments limits the rate of microbial activity, thus limiting the potential for decomposition. The time for an embalmed body to be reduced to a skeleton varies greatly. Even when a body is decomposed, embalming treatment can still be achieved (the arterial system decays more slowly) but would not restore a natural appearance without extensive reconstruction and cosmetic work, and is largely used to control the foul odors due to decomposition.

An animal can be preserved almost perfectly, for millions of years in a resin such as amber.

There are some examples where bodies have been inexplicably preserved (with no human intervention) for decades or centuries and appear almost the same as when they died. In some religious groups, this is known as incorruptibility. It is not known whether or for how long a body can stay free of decay without artificial preservation.

Various sciences study the decomposition of bodies under the general rubric of forensic science because the usual motive for such studies is to determine the time and cause of death for legal purposes:

Forensic taphonomy specifically studies the processes of decomposition to apply the biological and chemical principles to forensic cases to determine post-mortem interval (PMI), post-burial interval as well as to locate clandestine graves.

Forensic pathology studies the clues to the cause of death found in the corpse as a medical phenomenon.

Forensic entomology studies the insects and other vermin found in corpses; the sequence in which they appear, the kinds of insects, and where they are found in their life cycle are clues that can shed light on the time of death, the length of a corpse's exposure, and whether the corpse was moved.

Forensic anthropology is the medico-legal branch of physical anthropology that studies skeletons and human remains, usually to seek clues as to the identity, age, sex, height and ethnicity of their former owner.

The University of Tennessee Anthropological Research Facility (better known as the Body Farm) in Knoxville, Tennessee has several bodies laid out in various situations in a fenced-in plot near the medical center. Scientists at the Body Farm study how the human body decays in various circumstances to gain a better understanding of decomposition.

Decomposition of plant matter occurs in many stages. It begins with leaching by water; the most easily lost and soluble carbon compounds are liberated in this process. Another early process is physical breakup or fragmentation of the plant material into smaller bits which have greater surface area for microbial colonization and attack. In smaller dead plants, this process is largely carried out by the soil invertebrate fauna, whereas in the larger plants, primarily parasitic life-forms such as insects and fungi play a major breakdown role and are not assisted by numerous detritivore species.

Following this, the plant detritus (consisting of cellulose, hemicellulose, microbial products, and lignin) undergoes chemical alteration by microbes. Different types of compounds decompose at different rates. This is dependent on their chemical structure.

For instance, lignin is a component of wood, which is relatively resistant to decomposition and can in fact only be decomposed by certain fungi, such as the black-rot fungi. Wood decomposition is a complex process involving fungi which transport nutrients to the nutritionally scarce wood from outside environment. Because of this nutritional enrichment the fauna of saproxylic insects may develop and in turn affect dead wood, contributing to wood decomposition and nutrient cycling in the forest floor. Lignin is one such remaining product of decomposing plants with a very complex chemical structure causing the rate of microbial breakdown to slow. Warmth increases the speed of plant decay, by the same amount regardless of the composition of the plant

In most grassland ecosystems, natural damage from fire, insects that feed on decaying matter, termites, grazing mammals, and the physical movement of animals through the grass are the primary agents of breakdown and nutrient cycling, while bacteria and fungi play the main roles in further decomposition.

The chemical aspects of plant decomposition always involve the release of carbon dioxide. In fact, decomposition contributes over 90 percent of carbon dioxide released each year.

This section needs expansion with: information from the main article. You can help by adding to it.

The decomposition of food, either plant or animal, called spoilage in this context, is an important field of study within food science. Food decomposition can be slowed down by conservation. The spoilage of meat occurs, if the meat is untreated, in a matter of hours or days and results in the meat becoming unappetizing, poisonous or infectious. Spoilage is caused by the practically unavoidable infection and subsequent decomposition of meat by bacteria and fungi, which are borne by the animal itself, by the people handling the meat, and by their implements. Meat can be kept edible for a much longer time - though not indefinitely - if proper hygiene is observed during production and processing, and if appropriate food safety, food preservation and food storage procedures are applied.

Spoilage of food is attributed to contamination from microorganisms such as bacteria, molds, and yeasts, along with natural decay of the food. These decomposition bacteria reproduce at rapid rates under conditions of moisture and preferred temperatures. When the proper conditions are lacking the bacteria may form spores which lurk until suitable conditions arise to continue reproduction.

The rate of decomposition is governed by three sets of factors-the physical environment (temperature, moisture and soil properties), the quantity and quality of the dead material available to decomposers, and the nature of the microbial community itself.

Decomposition rates are low under very wet or very dry conditions. Decomposition rates are highest in damp, moist conditions with adequate levels of oxygen. Wet soils tend to become deficient in oxygen (this is especially true in wetlands), which slows microbial growth. In dry soils, decomposition slows as well, but bacteria continue to grow (albeit at a slower rate) even after soils become too dry to support plant growth. When the rains return and soils become wet, the osmotic gradient between the bacterial cells and the soil water causes the cells to gain water quickly. Under these conditions, many bacterial cells burst, releasing a pulse of nutrients. Decomposition rates also tend to be slower in acidic soils. Soils which are rich in clay minerals tend to have lower decomposition rates, and thus, higher levels of organic matter. The smaller particles of clay result in a larger surface area that can hold water. The higher the water content of a soil, the lower the oxygen content and consequently, the lower the rate of decomposition. Clay minerals also bind particles of organic material to their surface, making them less accessible to microbes. Soil disturbance like tilling increases decomposition by increasing the amount of oxygen in the soil and by exposing new organic matter to soil microbes.

The quality and quantity of the material available to decomposers is another major factor that influences the rate of decomposition. Substances like sugars and amino acids decompose readily and are considered labile. Cellulose and hemicellulose, which are broken down more slowly, are "moderately labile". Compounds which are more resistant to decay, like lignin or cutin, are considered recalcitrant. Litter with a higher proportion of labile compounds decomposes much more rapidly than does litter with a higher proportion of recalcitrant material. Consequently, dead animals decompose more rapidly than dead leaves, which themselves decompose more rapidly than fallen branches. As organic material in the soil ages, its quality decreases. The more labile compounds decompose quickly, leaving an increasing proportion of recalcitrant material. Microbial cell walls also contain recalcitrant materials like chitin, and these also accumulate as the microbes die, further reducing the quality of older soil organic matter.

1. ^ Lynch, Michael D. J.; Neufeld, Josh D. (2015). "Ecology and exploration of the rare biosphere". Nature Reviews Microbiology. 13 (4): 217-29. doi:10.1038/nrmicro3400. PMID 25730701. S2CID 23683614.

2. ^ González Medina A, González Herrera L, Perotti MA, Jiménez Ríos G (2013). "Occurrence of Poecilochirus austroasiaticus (Acari: Parasitidae) in forensic autopsies and its application on postmortem interval estimation". Exp. Appl. Acarol. 59 (3): 297-305. doi:10.1007/s10493-012-9606-1. PMID 22914911. S2CID 16228053.

3. ^ Braig, Henk R.; Perotti, M. Alejandra (2009). "Carcases and mites". Experimental and Applied Acarology. 49 (1-2): 45-84. doi:10.1007/s10493-009-9287-6. PMID 19629724. S2CID 8377711.

4. ^ González Medina A, Soriano Hernando Ó, Jiménez Ríos G (2015). "The Use of the Developmental Rate of the Aquatic Midge Chironomus riparius (Diptera, Chironomidae) in the Assessment of the Postsubmersion Interval". J. Forensic Sci. 60 (3): 822-826. doi:10.1111/1556-4029.12707. hdl:10261/123473. PMID 25613586. S2CID 7167656.

5. ^ Carter D.O., Yellowlees; D., Tibbett M. (2007). "Cadaver decomposition in terrestrial ecosystems". Naturwissenschaften. 94 (1): 12-24. Bibcode:2007NW.....94...12C. doi:10.1007/s00114-006-0159-1. PMID 17091303. S2CID 13518728.

6. ^ Dash, HR; Das, S (November 2020). "Thanatomicrobiome and epinecrotic community signatures for estimation of post-mortem time interval in human cadaver". Applied Microbiology and Biotechnology. 104 (22): 9497-9512. doi:10.1007/s00253-020-10922-3. PMID 33001249.

7. "Chapter 1, The Decomposition Process | Earth-Kind® Landscaping". aggie-horticulture.tamu.edu.

8. ^ Quigley, C. (1998). Modern Mummies: The Preservation of the Human Body in the Twentieth Century. McFarland. pp. 213-214. ISBN 978-0-7864-0492-6.

9. ^ Moore, Tim; Basiliko, Nate (2006), Wieder, R. Kelman; Vitt, Dale H. (eds.), "Decomposition in Boreal Peatlands", Boreal Peatland Ecosystems, Ecological Studies, Springer, pp. 125-143, doi:10.1007/978-3-540-31913-9_7, ISBN 978-3-540-31913-9

10. ^ "Effects of soil macro- and mesofauna on litter decomposition and soil organic matter stabilization". Geoderma. 332: 161-172. 2018-12-15. doi:10.1016/j.geoderma.2017.08.039. ISSN 0016-7061.

11. ^ "Do soil fauna really hasten litter decomposition? A meta-analysis of enclosure studies". European Journal of Soil Biology. 68: 18-24. 2015-05-01. doi:10.1016/j.ejsobi.2015.03.002. ISSN 1164-5563.

12. ^ Filipiak, Michał; Sobczyk, Łukasz; Weiner, January (2016-04-09). "Fungal Transformation of Tree Stumps into a Suitable Resource for Xylophagous Beetles via Changes in Elemental Ratios". Insects. 7 (2): 13. doi:10.3390/insects7020013. PMC .

13. Filipiak, Michał; Weiner, January (2016-09-01). "Nutritional dynamics during the development of xylophagous beetles related to changes in the stoichiometry of 11 elements". Physiological Entomology. 42: 73-84. doi:10.1111/phen.12168. ISSN 1365-3032.

14. Chu, Jennifer. "MIT News". The mathematics of leaf decay. MIT News Office.

15. Anita, Tull (1997). Food and nutrition. Oxford University Press. pp. 154, 155. ISBN 978-0-19-832766-0.

# data[].attributes.attachment_link.description
Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere.

# data[].attributes.body
Coal- Found in the ground    in coal beds
Natural gas- Can be found as deep as  4,500 meters (15,000 feet) in the ground
Petroleum- Found in under ground reservoirs

# data[].attributes.attachment_link.readable_text
The flames caused as a result of a fuel undergoing combustion (burning)

Air pollution abatement equipment provides combustion control for industrial processes.

Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. While activation energy must be supplied to initiate combustion (e.g., using a lit match to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining.

Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction which is commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure):

Uncatalyzed combustion in air requires relatively high temperatures. Complete combustion is stoichiometric concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, the chemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as carbon monoxide, hydrogen and even carbon ( soot or ash). Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures in atmospheric air, which is 78 percent nitrogen, will also create small amounts of several nitrogen oxides, commonly referred to as NOx, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, fuel gas cleaning or catalytic converters may be required by law.

Fires occur naturally, ignited by lightning strikes or by volcanic products. Combustion ( fire) was the first controlled chemical reaction discovered by humans, in the form of campfires and bonfires, and continues to be the main method to produce energy for humanity. Usually, the fuel is carbon, hydrocarbons, or more complicated mixtures such as wood that contain partially oxidized hydrocarbons. The thermal energy produced from the combustion of either fossil fuels such as coal or oil, or from renewable fuels such as firewood, is harvested for diverse uses such as cooking, production of electricity or industrial or domestic heating. Combustion is also currently the only reaction used to power rockets. Combustion is also used to destroy ( incinerate) waste, both nonhazardous and hazardous.

Oxidants for combustion have high oxidation potential and include atmospheric or pure oxygen, chlorine, fluorine, chlorine trifluoride, nitrous oxide and nitric acid. For instance, hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion. Although usually not catalyzed, combustion can be catalyzed by platinum or vanadium, as in the contact process.

The combustion of methane, a hydrocarbon.

In complete combustion, the reactant burns in oxygen and produces a limited number of products. When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide. Nitrogen is not considered to be a combustible substance when oxygen is the oxidant. Still, small amounts of various nitrogen oxides (commonly designated species) form when the air is the oxidative.

Combustion is not necessarily favorable to the maximum degree of oxidation, and it can be temperature-dependent. For example, sulfur trioxide is not produced quantitatively by the combustion of sulfur. species appear in significant amounts above about 2,800 °F (1,540 °C), and more is produced at higher temperatures. The amount of is also a function of oxygen excess.

In most industrial applications and in fires, air is the source of oxygen (). In the air, each mole of oxygen is mixed with approximately of nitrogen. Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to (mostly , with much smaller amounts of ). On the other hand, when there is insufficient oxygen to combust the fuel completely, some fuel carbon is converted to carbon monoxide, and some of the hydrogens remain unreacted. A complete set of equations for the combustion of a hydrocarbon in the air, therefore, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel.

The amount of air required for complete combustion is known as the "theoretical air" or "stoichiometric air". The amount of air above this value actually needed for optimal combustion is known as the "excess air", and can vary from 5% for a natural gas boiler, to 40% for anthracite coal, to 300% for a gas turbine.

Incomplete

Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however, carbon and carbon monoxide are produced instead of carbon dioxide.

For most fuels, such as diesel oil, coal, or wood, pyrolysis occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.

The designs of combustion devices can improve the quality of combustion, such as burners and internal combustion engines. Further improvements are achievable by catalytic after-burning devices (such as catalytic converters) or by the simple partial return of the exhaust gases into the combustion process. Such devices are required by environmental legislation for cars in most countries. They may be necessary to enable large combustion devices, such as thermal power stations, to reach legal emission standards.

The degree of combustion can be measured and analyzed with test equipment. HVAC contractors, firefighters and engineers use combustion analyzers to test the efficiency of a burner during the combustion process. Also, the efficiency of an internal combustion engine can be measured in this way, and some U.S. states and local municipalities use combustion analysis to define and rate the efficiency of vehicles on the road today.

Carbon monoxide is one of the products from incomplete combustion. The formation of carbon monoxide produces less heat than formation of carbon dioxide so complete combustion is greatly preferred especially as carbon monoxide is a poisonous gas. When breathed, carbon monoxide takes the place of oxygen and combines with some of the hemoglobin in the blood, rendering it unable to transport oxygen.

Problems associated with incomplete combustion

Environmental problems

These oxides combine with water and oxygen in the atmosphere, creating nitric acid and sulfuric acids, which return to Earth's surface as acid deposition, or "acid rain." Acid deposition harms aquatic organisms and kills trees. Due to its formation of certain nutrients that are less available to plants such as calcium and phosphorus, it reduces the productivity of the ecosystem and farms. An additional problem associated with nitrogen oxides is that they, along with hydrocarbon pollutants, contribute to the formation of ground level ozone, a major component of smog.

Human health problems

Breathing carbon monoxide causes headache, dizziness, vomiting, and nausea. If carbon monoxide levels are high enough, humans become unconscious or die. Exposure to moderate and high levels of carbon monoxide over long periods is positively correlated with the risk of heart disease. People who survive severe carbon monoxide poisoning may suffer long-term health problems. Carbon monoxide from the air is absorbed in the lungs which then binds with hemoglobin in human's red blood cells. This reduces the capacity of red blood cells that carry oxygen throughout the body.

Smoldering is the slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typically incomplete combustion reaction. Solid materials that can sustain a smoldering reaction include coal, cellulose, wood, cotton, tobacco, peat, duff, humus, synthetic foams, charring polymers (including polyurethane foam) and dust. Common examples of smoldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire) and the persistent combustion of biomass behind the flaming fronts of wildfires.

Spontaneous combustion is a type of combustion that occurs by self-heating (increase in temperature due to exothermic internal reactions), followed by thermal runaway (self-heating which rapidly accelerates to high temperatures) and finally, ignition. For example, phosphorus self-ignites at room temperature without the application of heat. Organic materials undergoing bacterial composting can generate enough heat to reach the point of combustion.

Combustion resulting in a turbulent flame is the most used for industrial applications (e.g. gas turbines, gasoline engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.

Colourized gray-scale composite image of the individual frames from a video of a backlit fuel droplet burning in microgravity.

The term 'micro' gravity refers to a gravitational state that is 'low' (i.e., 'micro' in the sense of 'small' and not necessarily a millionth of Earth's normal gravity) such that the influence of buoyancy on physical processes may be considered small relative to other flow processes that would be present at normal gravity. In such an environment, the thermal and flow transport dynamics can behave quite differently than in normal gravity conditions (e.g., a candle's flame takes the shape of a sphere.). Microgravity combustion research contributes to the understanding of a wide variety of aspects that are relevant to both the environment of a spacecraft (e.g., fire dynamics relevant to crew safety on the International Space Station) and terrestrial (Earth-based) conditions (e.g., droplet combustion dynamics to assist developing new fuel blends for improved combustion, materials fabrication processes, thermal management of electronic systems, multiphase flow boiling dynamics, and many others).

Combustion processes that happen in very small volumes are considered micro-combustion. The high surface-to-volume ratio increases specific heat loss. Quenching distance plays a vital role in stabilizing the flame in such combustion chambers.

Generally, the chemical equation for stoichiometric combustion of a hydrocarbon in oxygen is:

where .

For example, the stoichiometric burning of propane in oxygen is:

If the stoichiometric combustion takes place using air as the oxygen source, the nitrogen present in the air ( Atmosphere of Earth) can be added to the equation (although it does not react) to show the stoichiometric composition of the fuel in air and the composition of the resultant flue gas. Treating all non-oxygen components in air as nitrogen gives a 'nitrogen' to oxygen ratio of 3.77, i.e. (100% - O2%) / O2% where O2% is 20.95% vol:

where .

For example, the stoichiometric combustion of propane ( ) in air is:

The stoichiometric composition of propane in air is 1 / (1 + 5 + 18.87) = 4.02% vol.

The stoichiometric combustion reaction for C αH βO γ in air:

The stoichiometric combustion reaction for C αH βO γS δ:

The stoichiometric combustion reaction for C αH βO γF δ:

Various other substances begin to appear in significant amounts in combustion products when the flame temperature is above about . When excess air is used, nitrogen may oxidize to and, to a much lesser extent, to . forms by disproportionation of CO ₂, and and form by disproportionation of .

For example, when of propane is burned with of air (120% of the stoichiometric amount), the combustion products contain 3.3% . At , the equilibrium combustion products contain 0.03% NO and 0.002% OH. At , the combustion products contain 0.17% NO, 0.05% OH, 0.01% CO, and 0.004% .

Diesel engines are run with an excess of oxygen to combust small particles that tend to form with only a stoichiometric amount of oxygen, necessarily producing nitrogen oxide emissions. Both the United States and European Union enforce limits to vehicle nitrogen oxide emissions, which necessitate the use of special catalytic converters or treatment of the exhaust with urea (see Diesel exhaust fluid).

The incomplete (partial) combustion of a hydrocarbon with oxygen produces a gas mixture containing mainly , CO, , and . Such gas mixtures are commonly prepared for use as protective atmospheres for the heat-treatment of metals and for gas carburizing. The general reaction equation for incomplete combustion of one mole of a hydrocarbon in oxygen is:

When z falls below roughly 50% of the stoichiometric value, can become an important combustion product; when z falls below roughly 35% of the stoichiometric value, elemental carbon may become stable.

The products of incomplete combustion can be calculated with the aid of a material balance, together with the assumption that the combustion products reach equilibrium. For example, in the combustion of one mole of propane (CH) with four moles of , seven moles of combustion gas are formed, and z is 80% of the stoichiometric value. The three elemental balance equations are:

Carbon:

Hydrogen:

Oxygen:

These three equations are insufficient in themselves to calculate the combustion gas composition. However, at the equilibrium position, the water-gas shift reaction gives another equation:

;

For example, at the value of is 0.728. Solving, the combustion gas consists of 42.4% , 29.0% CO ₂, 14.7% , and 13.9% CO. Carbon becomes a stable phase at and pressure when z is less than 30% of the stoichiometric value, at which point the combustion products contain more than 98% and CO and about 0.5% .

Substances or materials which undergo combustion are called fuels. The most common examples are natural gas, propane, kerosene, diesel, petrol, charcoal, coal, wood, etc.

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapor that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its flash point. The flash point of liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

Combustion of gaseous fuels may occur through one of four distinctive types of burning: diffusion flame, premixed flame, autoignitive reaction front, or as a detonation. The type of burning that actually occurs depends on the degree to which the fuel and oxidizer are mixed prior to heating: for example, a diffusion flame is formed if the fuel and oxidizer are separated initially, whereas a premixed flame is formed otherwise. Similarly, the type of burning also depends on the pressure: a detonation, for example, is an autoignitive reaction front coupled to a strong shock wave giving it its characteristic high-pressure peak and high detonation velocity.

A general scheme of polymer combustion

The act of combustion consists of three relatively distinct but overlapping phases:

Preheating phase, when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.

Distillation phase or gaseous phase, when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light. Flames are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.

Charcoal phase or solid phase, when the output of flammable gases from the material is too low for the persistent presence of flame and the charred fuel does not burn rapidly and just glows and later only smoulders.

Efficient process heating requires recovery of the largest possible part of a fuel's heat of combustion into the material being processed. There are many avenues of loss in the operation of a heating process. Typically, the dominant loss is sensible heat leaving with the offgas (i.e., the flue gas). The temperature and quantity of offgas indicates its heat content ( enthalpy), so keeping its quantity low minimizes heat loss.

In a perfect furnace, the combustion air flow would be matched to the fuel flow to give each fuel molecule the exact amount of oxygen needed to cause complete combustion. However, in the real world, combustion does not proceed in a perfect manner. Unburned fuel (usually CO and ) discharged from the system represents a heating value loss (as well as a safety hazard). Since combustibles are undesirable in the offgas, while the presence of unreacted oxygen there presents minimal safety and environmental concerns, the first principle of combustion management is to provide more oxygen than is theoretically needed to ensure that all the fuel burns. For methane () combustion, for example, slightly more than two molecules of oxygen are required.

The second principle of combustion management, however, is to not use too much oxygen. The correct amount of oxygen requires three types of measurement: first, active control of air and fuel flow; second, offgas oxygen measurement; and third, measurement of offgas combustibles. For each heating process, there exists an optimum condition of minimal offgas heat loss with acceptable levels of combustibles concentration. Minimizing excess oxygen pays an additional benefit: for a given offgas temperature, the NOx level is lowest when excess oxygen is kept lowest.

Adherence to these two principles is furthered by making material and heat balances on the combustion process. The material balance directly relates the air/fuel ratio to the percentage of in the combustion gas. The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion. Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air, or enriching it in oxygen.

Combustion in oxygen is a chain reaction in which many distinct radical intermediates participate. The high energy required for initiation is explained by the unusual structure of the dioxygen molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in a triplet spin state. Bonding can be described with three bonding electron pairs and two antibonding electrons, with spins aligned, such that the molecule has nonzero total angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spins and zero total angular momentum. Interaction between the two is quantum mechanically a " forbidden transition", i.e. possible with a very low probability. To initiate combustion, energy is required to force dioxygen into a spin-paired state, or singlet oxygen. This intermediate is extremely reactive. The energy is supplied as heat, and the reaction then produces additional heat, which allows it to continue.

Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not proton abstraction) from the fuel to oxygen, to give a hydroperoxide radical (HOO). This reacts further to give hydroperoxides, which break up to give hydroxyl radicals. There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. Such intermediates are short-lived and cannot be isolated. However, non-radical intermediates are stable and are produced in incomplete combustion. An example is acetaldehyde produced in the combustion of ethanol. An intermediate in the combustion of carbon and hydrocarbons, carbon monoxide, is of special importance because it is a poisonous gas, but also economically useful for the production of syngas.

Solid and heavy liquid fuels also undergo a great number of pyrolysis reactions that give more easily oxidized, gaseous fuels. These reactions are endothermic and require constant energy input from the ongoing combustion reactions. A lack of oxygen or other improperly designed conditions result in these noxious and carcinogenic pyrolysis products being emitted as thick, black smoke.

The rate of combustion is the amount of a material that undergoes combustion over a period of time. It can be expressed in grams per second (g/s) or kilograms per second (kg/s).

Detailed descriptions of combustion processes, from the chemical kinetics perspective, require the formulation of large and intricate webs of elementary reactions. For instance, combustion of hydrocarbon fuels typically involve hundreds of chemical species reacting according to thousands of reactions.

The inclusion of such mechanisms within computational flow solvers still represents a pretty challenging task mainly in two aspects. First, the number of degrees of freedom (proportional to the number of chemical species) can be dramatically large; second, the source term due to reactions introduces a disparate number of time scales which makes the whole dynamical system stiff. As a result, the direct numerical simulation of turbulent reactive flows with heavy fuels soon becomes intractable even for modern supercomputers.

Therefore, a plethora of methodologies have been devised for reducing the complexity of combustion mechanisms without resorting to high detail levels. Examples are provided by:

The kinetic modelling may be explored for insight into the reaction mechanisms of thermal decomposition in the combustion of different materials by using for instance Thermogravimetric analysis.

Antoine Lavoisier conducting an experiment related to combustion generated by amplified sunlight.

Assuming perfect combustion conditions, such as complete combustion under adiabatic conditions (i.e., no heat loss or gain), the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as the ).

In the case of fossil fuels burnt in air, the combustion temperature depends on all of the following:

The adiabatic combustion temperature (also known as the adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.

Most commonly, the adiabatic combustion temperatures for coals are around 2,200 °C (3,992 °F) (for inlet air and fuel at ambient temperatures and for ), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) for natural gas.[50][51]

In industrial fired heaters, power station steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used.

Combustion instabilities are typically violent pressure oscillations in a combustion chamber. These pressure oscillations can be as high as 180dB, and long-term exposure to these cyclic pressure and thermal loads reduces the life of engine components. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage to the combustion chamber and surrounding components. This problem was solved by re-designing the fuel injector. In liquid jet engines, the droplet size and distribution can be used to attenuate the instabilities. Combustion instabilities are a major concern in ground-based gas turbine engines because of emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce the emissions; however, running the combustion lean makes it very susceptible to combustion instability.

The Rayleigh Criterion is the basis for analysis of thermoacoustic combustion instability and is evaluated using the Rayleigh Index over one cycle of instability

where q' is the heat release rate perturbation and p' is the pressure fluctuation. When the heat release oscillations are in phase with the pressure oscillations, the Rayleigh Index is positive and the magnitude of the thermoacoustic instability is maximised. On the other hand, if the Rayleigh Index is negative, then thermoacoustic damping occurs. The Rayleigh Criterion implies that thermoacoustic instability can be optimally controlled by having heat release oscillations 180 degrees out of phase with pressure oscillations at the same frequency. This minimizes the Rayleigh Index.

1. ^ colloquial meaning of burning is combustion accompanied by flames

2. ASM Committee on Furnace Atmospheres, ^ Furnace atmospheres and carbon control, Metals Park, OH [1964].

3. ^ Goussis, D.; Maas, U. (2011). Turbulent Combustion Modeling. Springer Science. pp. 193-220.

4. ^ Kooshkbaghi, Mahdi; Frouzakis, E. Christos; Chiavazzo, Eliodoro; Boulouchos, Konstantinos; Karlin, Ilya (2014). "The global relaxation redistribution method for reduction of combustion kinetics" (PDF). The Journal of Chemical Physics. 141 (4): 044102. Bibcode:2014JChPh.141d4102K. doi:10.1063/1.4890368. PMID 25084876. S2CID 1784716. Archived (PDF) from the original on 2022-10-09.

5. ^ Maas, U.; Pope, S.B. (1992). "Simplifying chemical kinetics: intrinsic low-dimensional manifolds in composition space". Combust. Flame. 88 (3-4): 239-264. doi:10.1016/0010-2180(92)90034-m.

6. ^ Bykov, V.; Maas, U (2007). "The extension of the ILDM concept to reaction-diffusion manifolds". Combust. Theory Model. 11 (6): 839-862. Bibcode:2007CTM....11..839B. doi:10.1080/13647830701242531. S2CID 120624915.

7. ^ Nafe, J.; Maas, U. (2002). "A general algorithm for improving ILDMs". Combust. Theory Model. 6 (4): 697-709. Bibcode:2002CTM.....6..697N. doi:10.1088/1364-7830/6/4/308. S2CID 120269918.

8. ^ Ren, Z.; Pope, S.B.; Vladimirsky, A.; Guckenheimer, J.M. (2006). "The invariant constrained equilibrium edge preimage curve method for the dimension reduction of chemical kinetics". J. Chem. Phys. 124 (11): 114111. Bibcode:2006JChPh.124k4111R. doi:10.1063/1.2177243. PMID 16555878.

9. ^ Lebiedz, D (2010). "Entropy-related extremum principles for model reduction of dissipative dynamical systems". Entropy. 12 (4): 706-719. Bibcode:2010Entrp..12..706L. doi:10.3390/e12040706.

10. ^ Reinhardt, V.; Winckler, M.; Lebiedz, D. (112). "Approximation of slow attracting manifolds in chemical kinetics by tra trjectory-based optimization approaches" (PDF). J. Phys. Chem. A. 112 (8): 1712-1718. Bibcode:2008JPCA..112.1712R. doi:10.1021/jp0739925. PMID 18247506. Archived (PDF) from the original on 2022-10-09.

11. ^ Lam, S.H.; Goussis, D. (1991). Conventional Asymptotic and Computational Singular Perturbation for Symplified Kinetics Modelling. Berlin: Springer.

12. ^ Keck, J.C.; Gillespie, D. (1971). "Rate-controlled partial-equilibrium method for treating reacting gas mixtures". Combust. Flame. 17 (2): 237-241. doi:10.1016/S0010-2180(71)80166-9.

13. ^ Chiavazzo, Eliodoro; Karlin, Ilya; Gorban, Alexander (2010). "The role of thermodynamics in model reduction when using invariant grids" (PDF). Commun. Comput. Phys. 8 (4): 701-734. Bibcode:2010CCoPh...8..701C. CiteSeerX 10.1.1.302.9316. doi:10.4208/cicp.030709.210110a. Archived (PDF) from the original on 2022-10-09.

14. ^ Chiavazzo, Eliodoro; Karlin, Ilya; Frouzakis, Christos E.; Boulouchos, Konstantinos (2009). "Method of invariant grid for model reduction of hydrogen combustion". Proceedings of the Combustion Institute. 32: 519-526. arXiv:. doi:10.1016/j.proci.2008.05.014. S2CID 118484479.

15. ^ Chiavazzo, Eliodoro; Karlin, Ilya; Gorban, Alexander; Boulouchos, Konstantinos (2010). "Coupling of the model reduction technique with the lattice Boltzmann method for combustion simulations". Combust. Flame. 157 (10): 1833-1849. doi:10.1016/j.combustflame.2010.06.009.

16. ^ Reyes, J.A.; Conesa, J.A.; Marcilla, A. (2001). "Pyrolysis and combustion of polycoated cartons recycling. kinetic model and ms analysis". Journal of Analytical and Applied Pyrolysis. 58-59: 747-763. doi:10.1016/S0165-2370(00)00123-6.

17. ^ John William Strutt, 3rd Baron Rayleigh, Sc.D., F.R.S.., Honorary Fellow of Trinity College, Cambridge; "The Theory of Sound", §322h, 1878:

18. A. A. Putnam and W. C. Dennis (1953) "Organ-pipe oscillations in a flame-filled tube", ^ Fourth Symposium (International) on Combustion, The Combustion Institute, pp. 566-574.

19. E. C. Fernandes and M. V. Heitor, ^ "Unsteady flames and the Rayleigh criterion" in F. Culick, M. V. Heitor, and J. H. Whitelaw, ed.s, Unsteady Combustion (Dordrecht, the Netherlands: Kluwer Academic Publishers, 1996), p. 4

20. Dowling, A. P. (2000a). "Vortices, sound and flame - a damaging combination". ^ The Aeronautical Journal of the RaeS

Poinsot, Thierry; Veynante, Denis (2012). Theoretical and Numerical Combustion (3rd ed.). European Centre for Research and Advanced Training in Scientific Computation.

Lackner, Maximilian; Winter, Franz; Agarwal, Avinash K., eds. (2010). Handbook of Combustion, 5 volume set. Wiley-VCH. ISBN 978-3-527-32449-1.

Baukal, Charles E., ed. (1998). Oxygen-Enhanced Combustion. CRC Press.

Glassman, Irvin; Yetter, Richard. Combustion (Fourth ed.).

Turns, Stephen (2011). An Introduction to Combustion: Concepts and Applications.

Ragland, Kenneth W; Bryden, Kenneth M. (2011). Combustion Engineering (Second ed.).

Baukal, Charles E. Jr, ed. (2013). "Industrial Combustion". The John Zink Hamworthy Combustion Handbook: Three-Volume Set (Second ed.).

Gardiner, W. C. Jr (2000). Gas-Phase Combustion Chemistry (Revised ed.).

# data[].attributes.attachment_link.description
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction.