Combustion, a chemical reaction between substances, usually including oxygen and usually accompanied by the generation of heat and light.
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Before you put the jar over the burning candle, you had all the ingredients necessary for combustion; heat from the match, fuel in the candle and oxygen from the air. This time, take a pair of scissors and cut off the wick below the flame and remove the candle. Again, the fire will go out after a short period when the rest of the wick that was left on the scissors is consumed.
This time you had plenty of oxygen in the air but you removed the fuel. The same principle is used in fighting wildfires. Remove heat, oxygen or fuel and the fire goes out. In suppression of a wildfire, the objective is to stop combustion by removing or altering one or more sides of the triangle.
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Transition — fuel is partially consumed by combustion while flaming continues in portions of the fuel resulting in initiation of smoldering and smoke generation. Smoldering — combustion of the fuel is essentially complete where oxygen is available and smoldering continues resulting in smoke generation. Suppressing fire and smoke generation segment on Suppression.
When a wildfire has started, we try to remove the oxygen side of the triangle by smothering the fire with a fire retardant, foam, dirt or water in a fine spray or fog. They will replace the oxygen around the fuel affecting one side of the fire triangle. They also absorb heat and thus also alter the heat side of the triangle. Retardants will coat the fuel and protect it from the heat even after the water has evaporated. They also inhibit the flaming combustion by chemical action. Foams also coat the fuel and last longer than water.
They reduce heat as well as supply of oxygen to the fuel. They will adhere to vertical fuel and can be easily applied by ground units. Water absorbs vast amounts of heat, especially when applied as a fog. Each droplet absorbs a large amount of heat which turns the water into a hot gas or vapor steam. The hot steam is then dispersed by the wind into the atmosphere. However, water is heavy and it is difficult to deliver it to the fireline in inaccessible areas. Such a combustion is frequently called an explosion , though for an internal combustion engine this is inaccurate.
Combustion
When the fuel-air mixture in an internal combustion engine explodes, that is known as detonation. Spontaneous combustion is a type of combustion which 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 application e. The term 'micro' gravity refers to a gravitational state that is 'low' i. In such an environment, the thermal and flow transport dynamics can behave quite differently than in normal gravity conditions e. Microgravity combustion research contributes to the understanding of a wide variety of aspects that are relevant to both the environment of a spacecraft e.
Combustion processes which 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:.
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.
Note that treating all non-oxygen components in air as nitrogen gives a 'nitrogen' to oxygen ratio of 3. When excess air is used, nitrogen may oxidize to NO and, to a much lesser extent, to NO 2.
combustion
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.
Such gas mixtures are commonly prepared for use as protective atmospheres for the heat-treatment of metals and for gas carburizing. 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. The three elemental balance equations are:. 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:. 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: The flash point of a 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: Similarly, the type of burning also depends on the pressure: Typically, the dominant loss is sensible heat leaving with the offgas i.
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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 H 2 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 CH 4 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: 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: Adherence to these two principles is furthered by making material and heat balances on the combustion process. The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion. Combustion in oxygen is a chain reaction in which many distinct radical intermediates participate.
The Fire Triangle
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, whose spins are 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.
combustion - Wiktionary
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. Combustion for electricity generation by utilities is the end use for 86 percent of the coal…. The oxygen theory of combustion resulted from a demanding and sustained campaign to construct an experimentally grounded chemical theory of combustion, respiration, and calcination. The theory that emerged was in many respects a mirror image of the phlogiston theory, but gaining evidence to support…. The theory explained that air was simply the receptacle for phlogiston, and any combustible or calcinable substance contained phlogiston as a principle or element and thus could not itself be elemental.
Iron, in rusting, was considered to lose its compound nature…. The chemical revolution was as…. More About Combustion 18 references found in Britannica articles Assorted References carbon sources and carbon sequestration In carbon sequestration: Carbon sources and carbon sinks fossil fuel effect on urban climate In urban climate genetic disease In human genetic disease: Combustion products chemical reactions In chemical reaction: Energy considerations ammonia In ammonia: Chemical reactivity of ammonia hydrocarbons In hydrocarbon: Chemical reactions oxygen compounds In oxygen group element: Natural occurrence and uses redox reactions In oxidation-reduction reaction: Historical origins of the redox concept property of coal In coal utilization In coal utilization: Coal type View More.
Articles from Britannica Encyclopedias for elementary and high school students. Help us improve this article! Contact our editors with your feedback. Introduction History of the study of combustion Physical and chemical aspects of combustion The chemical reactions Special combustion reactions Physical processes Combustion phenomena and classification Premixed flames Diffusion flames Oxidizing and reducing flame Explosions Thermal explosions Chain-branch reactions Detonation Special aspects Applications In heating devices In explosives In internal-combustion engines In rocket propulsion In chemical reactions.
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