Fire Engineering - Chapter 03. Combustion

1. Combustion

Ordinary combustion is a chemical reaction between a fuel and oxygen in presence of heat. Or it may be stated that three things are essential for combustion which can be represented by the tree arms of a triangle, viz. heat, combustible substance and oxygen.

2. Type of Combustion

i) Slow or incipient combustion

One in which the amount of heat and light emitted is feeble.

ii) Rapid Combustion

One in which a considerable amount of heat and light are emitted within a short time.

iii) Deflagration Combustion

A case of combustion which takes place with considerable rapidity evolving heat and light.

iv) Explosive Combustion

A very rapid combustion accompanied by a loud report within an extremely short time, with generation of very high pressure and temperature.

3. Oxygen content in air by weight and volume

Consider we have 100 g of air.

Mass of oxygen (O₂) in 100 g of air = 23 g

Mass of nitrogen (N₂) in 100 g of air = 77 g

Molar mass of Oxygen = 32 g1 mole of oxygen or 32 g of Oxygen occupies 22.4 L volume.

Therefore, volume of 23 g of oxygen will occupy 22.4/32 x 23 = 16.1 L similarly; 28 g of Nitrogen occupies 22.4 L volume.

Therefore, 77 g of Nitrogen will occupy 22.4/28 x 77 = 61.6 L

Total volume of air = 16.1 + 61.6 = 77.7 L% by volume of Oxygen = 16.1/77.7 x 100 = 20.7 %

4. Exothermic and Endothermic Reactions.

Sr. No	Exothermic Reactions	Endothermic Reactions.
1	A reaction that releases energy from the system in the form of heat.	A reaction that the system absorbs energy from its surrounding in the form of heat.
2	The energy is released from the system to its environment.	The energy is absorbed from the surround into the reaction
3	Energy is released as heat, electricity, light or sound.	Energy in the form of heat
4	Rusting iron, settling, chemical bonds, explosions, nuclear fission are a few examples.	Melting ice, evaporation, cooking, gas molecules, photosynthesis are few examples

5. Jet Fire and Flash Fire

A) Jet Fire

i) A jet fire occurs when a flammable liquid or gas is ignited after its release from a pressurized, punctured vessel or pipe.

ii) The pressure of release generates a long flame, which is stable under most conditions.

iii) A flash flame may take the form of jet flame on reaching the spill point.

iv) The duration of the jet fire is determined by the release rate and the capacity of the source.

B) Flash Fire

i) A flash fire occurs when a cloud of flammable gas and air is ignited.

ii) The speed of burning is function of the concentration of the flammable component in the cloud and also the wind speed.

iii) Within a few second of ignition the flame spreads both upwind and downwind of the ignition source.

iv) Initially the flame is contained within the cloud due to premixed burning of the regions within the flammable limits.