Petroleum and coal are formed from the fossilized remains of animals and plants, hence they are known as fossil fuels.
As they are used up much more rapidly than they are replenished by nature, thus it might ultimately result in fuel shortage.
Coal :
Coal is defined as stratified rock, consisting of organic matter of fuel value, derived from the partial decay and alteration of accumulated plant materials by the action of heat and pressure over million of years.
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Coal is of following four types:
Increase of
(a) Carbon content,
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(b) Hardness,
(c) Calorific value
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Increase of
(a) Moisture content,
(b) H. O, N and S content,
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(c) Volatile matter content
Advantages :
The factors which are in fovour of usage of coals in huge tonnage quantities are availability, low cost, least risk of fire hazards and easy storage.
Disadvantages :
(i) Combustion of coal is a slow process.
(ii) Combustion control is not easy.
(iii) After combustion, ash is always produced so its disposal is a problem. Smoke is invariably produced.
(iv) Use of coal in internal combustion engine is not possible.
(v) Calorific value and thermal efficiency is least.
Petroleum:
The word petroleum is derived from Latin Petra which means rock and oleum which means oil.
Petroleum is a complex mixture of paraffinic, olefinic and aromatic hydrocarbons with small quantities of organic compounds containing oxygen, nitrogen and sulphur. It is also called mineral oil because it occurs beneath the earth.
Petroleum refining of crude oil or petroleum provides many liquid fuels that are in current use. A brief description of important liquid fuels viz. gasoline, diesel and kerosene oil is given below.
Gasoline or petrol:
It is a mixture of hydrocarbons from pentane to octane. It is highly volatile and inflammable. It is used as a fuel for internal combustion engines. Its calorific valve is about 11,250 cal/gm.
Diesel oil :
It is a mixture of higher hydrocarbons (C15 to C18). It is used as a fuel for diesel engine and its calorific value is about11,000 cal/gm
Kerosene oil :
It is a mixture of hydrocarbons (C10 to C16). Its calorific value is about 11,100 cal/gm It is used as domestic fuel in stoves.
Gaseous fuels natural gas :
It is obtained from well dug in the oil bearing regions. It is mainly composed of methane, ethane and other hydrocarbons. It is also called ‘Marsh gas’ because it majority contains methane (about 88.5%). It is used as a domestic and industrial fuel, because of its high calorific value (8000-14000) kcal/m3.
Compressed natural gas (CNG) :
The natural gas compressed at very high pressure of about 1000 atmosphere is called CNG. The calorific value of CNG is 31400 – 37700 kJ/m3. The use of CNG as a fuel for automobiles has reduced pollution in urban cities, as it undergoes complete combustion in CNG engine so there is nil possibility of release of CO in the atmosphere. Further, CNG is much safer fuel with lower operating cost.
Liquefied petroleum gas (LPG) :
The main constituents of LPG are n-butane, iso-butane, butylene and propane. The calorific value of LPG is about 27,800 kcal/m3. It is mainly used as domestic fuel. To help in the detection of gas leakage, a strong smelling substance viz. ethyl mercaptan is added to the LPG gas cylinders. LPG is also used as motor fuel because it easily mixes with air and burns cleanly without residue and without knocking.