Fire or combustion is a violent exothermal chemical reaction called oxydoreduction. When the combustion is intense, it leads to a flame or an explosion.
The chemical reaction of combustion is only possible when 3 elements are combined: a combustible, an oxidizer, and a sufficient amount of energy. We usually draw this association with the symbolic fire triangle as seen above.
- solid forming embers (class A fire) (wood, paper, carton, textile, PVC, …) ;
- liquid of liquefying solid (class B fire) (benzin, gasoline, oil, polyethylen, polystyren …) ;
- gas (classe C fire) (butane, propane, methane, dihydrogen, …) ;
- metal (classe D fire) (iron, aluminum, sodium, magnesium, …) ;
- fat or grease (class F fire) (cooking oil,…)
The oxidizer is the other reactive element in the chemical reaction. In most cases it is the ambient air, and in particular one of its components: oxygen.
By depriving a fire of air, we can extinguish it; for example, when covering the flame of a small candle with an empty glass, fire stops; to the contrary, if we blow on a wood fire, we can activate it (by providing more air). In certain torches, the presence of oxygen improves combustion.
In very particular cases (often with explosives such as aluminum), the oxidizer and the combustible are the same (as for nitroglycerin, an instable molecule made of one oxidizing part linked to a reducing part).
The reaction is initiated by an activating energy, in most cases, heat. Friction for example, in case of matches, heating electrical wire, flames (propagation of fire), sparks (from a lighter or any electrical device).
There are many other ways to bring sufficient activation energy such as: electricity, radiation, and pressure … which will all lead to a rise in temperature. In most cases, heat production enables self-sustainability of the reaction, or even growth into a chain reaction. The temperature at which a liquid produces a sufficient amount of vapor to form an ignitable mixture in air is called the flash-point.
Methane combustion in Oxygen
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 :
- As the combustible is oxidized during combustion, it is a reducer and loses electrons.
- 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 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).
- 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 ».
- 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.
To stop a combustion reaction, one of the 3 elements of the fire-triangle has to be removed :
- Suppression of the combustible : closing the valve fueling the combustion, sufficient distance between fire and flame, exhaust for hot smoke (containing unburnt elements), …
- Suppression of the oxidizer (choking) : use of a carbon dioxide fire-extinguisher, a blanket, or spraying a sufficient amount of water on a solid combustible (water vapor removes fresh air) …
- Suppression of activation energy (cooling down) : spray water in a mix of air + combustible particles), absorbing the heat (Davy Miners), exhaust to remove hot smoke, …
ROLE OF WATER IN FIRE-FIGHTING
Water can have two different roles :1.In the case of a solid combustible, the limitation factor is the oxidizer (air) intake, as the fire self-produces sufficient heat ; water chokes the fire by vapor production, which removes fresh air and thus oxygen; 2.In the case of a mixed atmosphere, the combustible and the oxidizer cannot be separated, the only possible action therefore consists of cooling down to prevent flames from spreading (water vapor will also dilute the atmosphere).
There are also cases for which the ignition factor is not the source of activation energy. For example, a smoke explosion is a very violent combustion of unburnt gases contained in smoke created by a sudden oxygen input (oxidizer input). The interval in which an air/gas mix can burn is restricted to the explosive limits of the air. This interval can vary from a few percentage points (kerosene) to several dozen percents (acetylene).
ROLE OF WATER ADDITIVES
The role of water in extinguishing a fire can be summarized as follows :
- Cooling down the fire by absorption of heat energy
- Removing Oxygen from the fire
- Diluting the atmosphere by adding vapor inside the flammable air mixture
The limits to the use of water are directly linked to the physio-chemical characteristics of water itself :
> Water cannot be used on certain types of fires :
- Hydrocarbon fires (B class) as it will only spread the fire due to the difference of density.
- Metal fires (D class) as these fires produce huge amounts of energy (up to 7.550 calories/kg for Aluminum) water can create violent chemical reactions with burning metal (by oxidization).
- Fat fires (F class) as vapor will carry and spread burning oil.
> Water (H²O) contains Oxygen (O²), and considering the fact that a water molecule will be modified at +/- 1.500°C (mainly due to the excessive speed of the atoms, which will lead to connection failures), and add highly flammable Hydrogen (H) + Oxygen (O²) to the fire.
Science has well studied and understood the above mentioned reactions, it has thus been possible to create specific water-additives which allow :
- Better heat absorption with a higher density than water.
- Bringing free radical catchers to the fire.
- Bringing foaming agents to enable water to stay on the surface of a liquid fire and prevent gas release.
- Bringing specific reactives which react with and change the nature of the burning material.