Fuels

A substance that can supply energy either alone or by reacting with another substance is known as fuel. Heat produced by fuel is measured in Calories. An ideal fuel should

1. have high calorific value

2. be cheap and easily available

3. be easily stored & transport

4. be regulated and controlled

5. have low ignition temperature

The quantity of fuel is expressed in the form of calorific value.

Calorific value is the total quantity of heat liberated by complete combustion of a unit mass of fuel in air or oxygen.

Calorific value of fuels are expressed in kcal/m³ or British Thermal Unit (B.T.U) per cubic foot.

1 kcal/m³ = 0.107 B.T.U/ft³

Fuel: may be solid (e.g wood, coal etc.)

Liquid (e.g kerosene oil, petroleum, alcohol etc.) or gas (e.g water gas, producer gas, coal gas, oil gas, natural gas, gobar gas, LPG etc.) However, gaseous fuel are considered to be the best fuels.

1. Water gas (syn gas): It is a mixture of carbon monoxide and hydrogen. It is obtained by the action of steam on a red hot coke at 1000° C.

C + H₂O → CO + H₂ — 28 kcal

                       Water gas

Producer gas: It is a mixture of CO and N₂. It is prepared by burning coke in limited supply of air. It is the cheapest gaseous fuel, however its calorific value is not very high because it has a large proportion of nitrogen.

Coal gas: It is a mixture of H₂, CH₄, CO and other; gases like N₂, C₂H₄,O₂ etc. It is obtained by destructive distillation of coal at about 1000°C.

Oil gas : It is a mixture of H₂, CH₄, C₂H₄, CO and other gases like CO₂. It is obtained by thermal cracking of kerosene oil. It is used in laboratories.

Gobar gas: It contains CH₄, CO and H₂. It is produced by fermentation of gobar in absence of air. It is used as a domestic fuel in villages.

Natural gas: It is a mixture of gaseous hydrocarbons viz methane 85%, ethane, propane butane etc. Liquefied petroleum mainly butane and isobutane.

Liquified Petroleum Gas (LPG): The petroleum gas liquified under pressure is called liquified petroleum gas. It is a mixture of butane and isobutane with small amount of propane and is easily compressed under pressure as liquid and stored in iron cylinders. It is used as domestic fuel.

The stench due to leakage from LPG Cylinder is because of Ethyl mercaptan.

Compressed Natural Gas (CNG): The natural gas compressed at very high pressure is called compressed natural gas (CNG). It consists mainly of methane (95%) which is a relatively unreactive hydrocarbons and make it nearly complete combustion possible. The other 5% is made of various gases such that ethane, propane and butane including small amount of other gases N₂, CO₂, H₂S, water vapour etc. The CNG is now being used as a better fuel than gasoline for running buses, cars and three-wheelers in metropolitan cities like Delhi, Mumbai etc, because of its complete combustion and no unburnt carbon is being released in the atmosphere to cause air pollution.