Ignition Temperature (it)

IGNITION TEMPERATURE (IT)

It is defined as, "the lowest temperature to which the fuel must be heated, so that it starts burning smoothly”.

Ignition temperature of coal is about 300°C. In the case of liquid fuels, the ignition temperature is called the flash point, which ranges from 200 – 450°C. For gaseous fuels, the ignition temperature is in the order of 800°C.

1. Spontaneous Ignition Temperature (SIT)

It is defined as “the minimum temperature at which the fuel catches fire (ignites) spontaneously without external heating":

If the ignition temperature of a fuel is low it can catch fire very quickly. On the other hand if the ignition temperature is high it is difficult to ignite the fuel. If the heat evolved in a system is unable to escape, temperature of the system goes on increasing and when SIT is reached, the system burns on its own.

EXPLOSIVE RANGE (or) LIMITS OF INFLAMMABILITY

All gaseous fuels have two limits called upper limit and lower limit. These limits represents percentage by volume of fuel present in fuel-air mixture.

1. Lower limit represents the smallest proportion of combustible gas (fuel).

2. Upper limit represents the largest proportion of combustible gas.

The range covered by these limits is termed as explosive range of the fuels. For continuous burning the amount of fuel present in the fuel-air mixture should not go below the lower limit or above the upper limit.

Example

The explosive range of petrol is 2-4.5. This means that when the concentration of petrol vapour in petrol-air mixture is between 2 and 4.5 by volume, the mixture will burn on ignition. When the concentration of petrol vapour in petrol-air mixture is below 2% (lower limit) or above 4.5% (upper limit) by volume, the mixture will not burn on ignition. Some of the limits of inflammability are given in the following table. 6.1.

Table 6.1: The limits of inflammability of hydrogen, acetylene, natural gas

Thus, explosive range (or) explosive limit is the limiting composition of a gas-air mixture beyond which the mixture will not ignite and continue to burn is called explosive range (or) explosive limit.