Methane combustion in Oxygen
Combustion is a chemical reaction in which complex molecules are broken down into smaller and more stable molecules through the rearrangement of links between atoms.
The chemistry of combustion is one of the major parts of high temperature chemistry, which mostly implies radical reactions.
However, it is possible to handle combustion through a unique global reaction.
Example:
Methane combustion in Oxygen
Carbon dioxide CO2 and water H2O are more stable than the oxygen and propane.
Combustion is an oxydo-reduction reaction, meaning oxidization of a combustible by an oxidizer:
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As the combustible is oxidized during combustion, it is a reducer and loses electrons.
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The oxidizer is the part being reduced; it oxidizes as it gains electrons.
As for any chemical reaction, a catalyst eases combustion and as it has a high activation energy level, the use of a catalyst enables work at lower temperatures.
This allows a complete combustion as in the catalyst of the exhaust of a vehicule, where catalytic metals burn residues contained in the exhaust smoke at lower temperatures than in the engine.
Concerning solid combustibles, activation energy will allow vaporization or pyrolysis of the combustible.
The gas produced will then mix with an oxidizer resulting in a combustible mixture.
If the energy produced by the combustion is higher or equal to the quantity of energy required for the combustion, the reaction is self-sustainable.
Produced energy and calorific power
The quantity of energy produced by the reaction is higher to the quantity of energy required to start it.
The quantity of energy produced by the combustion is given in Joules (J); it is the enthalpy of the reaction.
In the application domains (oven, burner, engine with internal combustion, fire-fighting), we use the notion of calorific power,
which is basically the enthalpy of the chemical reaction per unit of weight of combustible or the obtained energy given by the combustion
of one kilogram of combustible, expressed in kilojoules per kilogram (kJ/kg or kJ·kg-1).
Combustion of hydrocarbon produces water in its vapor form; this water vapor contains huge amounts of energy and this parameter has to be taken into account in a specific
way to evaluate correctly the calorific power. We define:
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Superior Calorific Power (SCP): « Quantity of energy produced during a complete combustion of a combustible unit, water vapor is said to be condensed and heat collected ».
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Inferior Calorific Power (ICP): « Quantity of energy produced during a complete combustion of a combustible unit, Water vapor is said non-condensed and heat not collected ».
Difference between ICP and SCP is the latent heat of water vaporization (Lv) multiplied by the quantity of produced vapor (m), that equals +/- 2 250 kJ·kg-1
(this value is influenced by pressure and temperature).
We have the relation SCP = ICP + m·Lv
1 - Fire
2 - Combustive agent
3 - Fire extinction
