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Chapter – 6 Combustion and Flame Combustion: Theprocess of burning a substance inthe presence ofair (oxygen) andundergoes a chemical reaction to produce heat and light. The substances which burn in air are called combustible. Oxygen (inair) is essential for combustion. During the process of combustion, heatand light aregiven out. Ignition temperature is the lowest temperature at whicha combustible substance catches fire. Types of combustion: The type of combustion differs depending on the typeof fuel. Based on nature andintensity combustions areclassified into threetypes. They are: (i) Rapid combustion (ii) Spontaneous combustion (iii) Explosion Flame: Itis a zoneor burning vapour. The substances whichvaporise during burning give flames. Example: Kerosene oil andmolten wax. Inflammable substances havevery low ignition temperature. Fire can becontrolled by removing one or morerequirements essential forproducing fire. Water is commonly used to control fires. Water cannot be used to control fires involving electrical equipments or oils. There arethree different zonesof a flame- dark zone,luminous zone andnon- luminous zone. Fuel isany material thatis burned toobtain energy thatcan be usedto heat ormove another object. A good fuelmust: Be readily available. Be cheap. Burn easily ata moderate rate.
• Produce a large amount of heat. • Not leave behind anyundesirable substances. Fuels differ in their efficiency and cost. Fuel efficiency is expressed in terms ofits calorific valuewhich is expressed in units ofkilo joule perkg. Types ofFuels: (i) Solid Fuels: Combustible substances which are solid at room temperature. Example: coal, coke, wood, charcoal, etc. (ii) Liquid fuels: Volatile liquids which produce combustible vapour. Example: Petrol, kerosene, alcohol, diesel, etc. (iii) Gaseous fuels: Combustible gasesor mixture of combustible gases. Example: Natural gas,LPG, biogas, coalgas, etc. Effects o fBurning of Fuels: (i) Carbon fuels likewood, coal petroleum release un burntcarbon particles. Theseare dangerous pollutants causing respiratory diseases, such as asthma. (ii) Incomplete combustion ofcarbon fuels gives carbon monoxide which is a poisonous gas. (iii) Increased concentration of carbon dioxide in the airis believed tocause global warming. (iv) Oxides ofSulphur and nitrogen dissolve in rainwater and formacids. Such rainis called acidrain. It is very harmful for crops, buildings and soil. Un burnt carbon particles in airare dangerous pollutants causing respiratory problems. Incomplete combustion of a fuelgives poisonous carbon monoxide gas. Increased percentage of carbon dioxide in airhas been linked to global warming. Oxides ofsulphur and nitrogen produced by theburning of coal,diesel and petrol cause Acid rain which is harmful forcrops, buildings andsoil.
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