Combustion - SpeedyLook encyclopedia

Triangle of fire

The chemical reaction of combustion can occur only if three elements are joined together: a Combustible , a Combustive , a energy of sufficient activation in quantities. One represents way symbolic system this association by the Triangle of fire .

The combustible can be:

a gas (Butane, Propane, gas for domestic use, Dihydrogène…),
a liquid or liquefiable solid (gasoline, Gas oil, Oil, Kerosene…),
a solid (Wood, Paper, paperboard, fabric, plastic…).

It can be a mixture of various bodies.

The combustive is the other reagent of the chemical reaction. Most of the time, it is of the ambient Air, and more particularly about the one of its principal components, the Dioxygène. By depriving a fire of air, one extinguishes it; for example, if one places a candle heating-dish lit in a bottle of jam and that the bottle is closed, the flame dies out; contrary, if one blows on a wood fire, that activates it (one brings more air). In certain blowtorches, one brings pure dioxygene to improve combustion.

In certain very particular cases (often explosive as with aluminum), the combustive one and the fuel are a single body (for example celebrates it Nitroglycérine, molecule unstable comprising an oxidizing part grafted on a reducing part).

The reaction is started by a energy of activation . The production of heat makes it possible this reaction car-to discuss in the majority the cases, to even develop in a Chain reaction.

The energy of activation is the release of fire. It is generally about Chaleur. For example, it will be the match which one rubs, the electric cable which heats, or another flame (propagation of fire), spark (of the fire lighter, the stone of the Briquet or an electrical appliance which gets under way or stops). But there are other ways of providing the energy of activation: electricity, radiation, pressure… which will always allow an increase in the temperature.

Various types of combustion

Rapid

Fast combustion is a form of combustion during which great quantities of heat and energy in the form of light are slackened, giving rise to fire. It is used in certain machines such as the internal combustion engines or the thermobaric weapons.

Slow

Slow combustion is a reaction being carried out at not very high temperatures. One can quote the case of cellular breathing.

Complete

During a complete combustion, the reagent will burn in the presence of dioxygene, which limits the products of the reaction. When a hydrocarbon burns in dioxygene, the products are thus only carbon dioxide and water.

When elements like carbon, the nitrogen, sulfur, and iron are flarings, one notices the production of the most common oxides.

Heat

Hot combustion is a combustion characterized by heat flows. It is often used in industry (for example, the gas turbines, the diesel engines…) because heat facilitates the operation of mixture between fuel and oxidant.

Incomplete

Incomplete combustion takes place when there is not enough oxygen to make it possible the fuel (often a hydrocarbon) to react completely with dioxygene to produce carbon dioxide and water. When a hydrocarbon burns in the air, the reaction will produce carbon dioxide, water, carbon monoxide, pure carbon (soot or ashes), but also many others composed such as nitrogen oxides. The incomplete reaction of combustion is consequently most common of all, and will thus produce very great quantities of secondary products, and, in the case of the fuel combustion in the automobile engines, these secondary products can be rather bad for health and damage the environment. The quality of combustion can be improved by studying the parts implied in the reaction, like the burners, and the internal combustion engines. Other improvements are realizable by catalytic parts post-combustion (like catalytic mufflers). Such parts are made compulsory by the automobile legislation in the majority of the countries, and should be necessary in large devices of combustion.

Processes of extinction of a combustion

To extinguish a reaction of combustion, it is necessary to remove one of three elements of the triangle of fire:

removal of fuel: closing of a valve or a tap which feeds the combustion, fuel distance near fire, discharge system to drive out the smoke (which contains unburnt residues),…
suppression of combustive (smothering): use of an extinguisher with dry ice, of a cover, water sprinkling on a solid fuel (the formed steam displaces air)…
suppression of the energy of activation (cooling): water spraying in the case of a premixed atmosphere (mixture of gas or combustible particles and combustive gas), grid absorbing heat (Davy lamp), discharge system to drive out the smoke (which is hot),…

It is noted that water can have two different roles:

in the case of a solid fuel, the limiting factor is the contribution into combustive (air), fire producing its own heat; water thus chokes fire by displacing air;
in the case of a premixed atmosphere, one cannot separate fuel from combustive, the only possible action consists in cooling the atmosphere to prevent the flame from being propagated (the steam also plays a part of thinner).

Chemical approach

The reaction of combustion, like all the reactions, is the rupture of the connections between the molecules of two bodies, and the creation of new stable molecules more chemically.

Combustion is a Réaction of oxydoreduction, in fact the Oxydation of a Combustible by a Comburant;

the fuel is the body which is oxidized during combustion; it is a reducer, it loses electrons;
the combustive one is the body which is reduced; it is oxidizing, it gains electrons.

Some chemical compounds, called catalysts, modify by their presence the quantity of energy necessary to activate the reaction, either by reducing it (Activateur), or by increasing it (inhibiting).

In the case of the solid fuels, the energy of activation will allow to vaporize or of Pyrolyse R the fuel. The gases, thus produced, will mix with combustive and will give the combustible mixture. If the energy produced by combustion is higher or equal to the energy of activation necessary, the reaction of combustion car-discusses.

Released energy and calorific value

The quantity of energy produced by combustion is expressed in Joule S (J); it is about the Enthalpie of reaction. In the scopes of application (furnaces, burners, internal combustion engines, fights against fire), one often uses the concept of Calorific value, which is the enthalpy of reaction per unit of fuel mass or the energy obtained by the combustion of one kilogram fuel, expressed in general in kilojoule per kilogram (noted kJ/kg or kJ·kg^-1).

Hydrocarbon combustions release from water in the form of vapor. This steam contains a great quantity of energy. This parameter is thus taken into account of manner specific for the evaluation of the calorific value, and one defines:

the higher calorific value (PCS) : “Quantity of energy released by the complete combustion of a unit of fuel, the steam being supposed condensed and recovered heat”.
the lower calorific value (PCI) : “Quantity of heat released by combustion supplements of a unit of fuel, the steam being supposed non-condensed and not recovered heat”.

The difference between the NCV and the GCV is the Latent heat of vaporization of the water (

L_v

) multiplied by the quantity of produced vapor (

m

), which is worth with little-near 2 250 kJ·kg^-1 (this last value depends on the pressure and the temperature).

One with the relation.

Speed of the face of flame and explosion

Metal fires

The oxidation of metals is in general slow. The heat released is thus low and is slowly dissipated in the environment; it is the field of the Corrosion (for example the rust of the Fer and the Acier).

However, in certain cases, oxidation is violent and thus constitutes a combustion. There exist five notable cases:

combustion in the air of the Magnesium: magnesium burns easily, by emitting a very sharp and white light; it was used before for the photographic flashes;
combustion of the Sodium in water: it is not strictly speaking the sodium which burns; sodium reacts violently with water and causes a release of Dihydrogène, and with the heat produced by the reaction, the dihydrogene ignites in the air;
Aluminothermy: the combustive one is a metallic oxide here, it acts of a chemical reaction between two solids;
combustion at high temperature and strong concentration of dioxygene: when metal is heated very extremely and that one sends pure dioxygene, the reaction is sufficiently fast car-to discuss; this phenomenon is used for the Oxycoupage with the Chalumeau, the thermal Lance, and it is also the accident of the “shot” which can occur with a pressure reducer of a bottle of dioxygene (for example medical oxygen or bottle of blowtorch);
combustion of a metal in the form of Powder or of foam: the chemical reaction is done with the contact between metal and the air, but in the case of a foam or of a powder, this surface of contact (the specific Surface) is very large, the reaction is thus fast and the important heat released; it is a phenomenon comparable with the Dust explosion.

Application in astronautics

The combustion is used in the field of the Astronautique to provide the energy of Propulsion space engines. The corresponding terms in English are burning and combustion .

According to the type of combustion employed in a Propellent , one speaks about:

Internal burning (in English cigarette burning and end burning ) which is a combustion of a Bloc of powder characterized by a plane surface of combustion progressing in the longitudinal direction, forwards or backwards;

erosive Combustion (in English erosive burning ) which is a combustion of a Bloc of powder if the gas flow of combustion causes an erosion of the block;

external transverse Combustion (in English external burning ) which is a combustion of a longitudinally extending Bloc of powder characterized by a surface of combustion and progressing of outside towards the interior;

interior transverse Combustion (in English internal burning ) which is a combustion of a Bloc of powder which is carried out interior towards outside starting from a central channel.

Reference

Right French: decree of February 20th, 1995 relating to the terminology of sciences and space technology.

See too

Combustible Fire
Flame
Combustive
Reaction of oxydoreduction
Explosion
Combustion chamber
Internal combustion engine
Pressure of combustion
Combustion of carbon (chemical reaction)

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