Combustion and Flame
Substances which are burnt to produce heat energy are known as fuels. Fuels can be in the form of solid, liquid or gas. Gaseous fuels are better than liquid fuels due to the highest calorific value. Liquids in turn have more calorific value than solid fuels. The main gaseous fuels are natural gas, producer gas, water gas, LP.G and biogas. L.P.G is a mixture of two hydrocarbons - butane and isobutane. The advantages of L.P.G are
(i) high calorific value.
(ii) burns with a smokeless flame.
(iii) does not produce any poisonous gases on burning.
The amount of heat produced by burning 1 g of fuel completely is known as its calorific value.
- Characteristics of an ideal fuel
An ideal fuel should be cheap, readily available, combustible, easy to transport and store, safe and should have a high calorific value.
It is an oxidation reaction which is accompanied by the evolution of heat and light. Paper, kerosene, petrol, straw, etc. are combustible substances. Iron, glass and diamond are non- combustible. The lowest temperature at which a substance starts burning is called its ignition temperature.
- Conditions necessary for combustion are
(i) the presence of combustible substance,
(ii) the presence of supporter of combustion (oxygen),
(iii) the attainment of ignition temperature.
(i) If combustion of a substance takes place with high speed, it is known as rapid combustion, e.g., candle starts burning when a burning matchstick is brought near its wick.
(ii) The combustion in which no external heat is supplied to the substance is known as spontaneous combustion, e.g., burning of white phosphorus in air.
(iii) The combustion in which large amounts of gases are evolved with the production of large amounts of heat, light and sound by a substance is called explosion, e.g., burning of crackers.