Combustion and Flame

(1) Fuel — a substance that can burn (wood, LPG); (2) Oxygen — from air (21%); (3) Heat — enough to reach the ignition temperature (provided by the rough side strip of a matchbox). If even one condition is absent, burning will not occur (for example, a candle in water, or a candle under a bell jar with no air).

Ignition temperature is the minimum temperature at which a substance starts to burn. Below this temperature, the substance will not catch fire (so always be careful with hot objects). Inflammable substances have a very low ignition temperature (petrol, alcohol, LPG) — they catch fire very easily — always keep them away from fire.

(1) Dark zone — innermost — contains unburnt wax vapour — least hot; (2) Luminous zone — middle — yellow — incomplete combustion — carbon particles produce soot; (3) Non-luminous zone — outermost — blue — complete combustion — hottest zone. A goldsmith works with the blue non-luminous zone because it is the hottest.

CO binds to haemoglobin in the blood much more strongly than oxygen does — as a result, the blood cannot carry oxygen to the body — this causes suffocation and can even lead to death. This is why a closed kitchen, a closed car with the engine running, or a leaking geyser can be very dangerous. Proper ventilation is essential.

Calorific value is the amount of heat energy (in kJ/kg) produced when 1 kg of a fuel is completely burned. LPG has a calorific value of about 50,000 kJ/kg (excellent), kerosene is about 45,000 kJ/kg, coal is lower (about 25,000–30,000 kJ/kg), and dung cake has the lowest. A high calorific value combined with less pollution makes a fuel a good fuel.

Coal and petrol contain sulphur and nitrogen compounds — when they burn, they release SO2 and nitrogen oxides into the air — these gases mix with rainwater and form acids — this is acid rain. Its effects include: damage to buildings (especially marble), harm to trees, soil, and rivers. The solution is to use clean fuels (such as CNG) and to install scrubbers in factories.