Engineering Chemistry S Datta

FUELS AND COMBUSTION 399
Important Petroleum Products
The main products of refinery are motor and aviation gasoline, kerosene, diesel fuel,
fuel oils; other products include lubricating oils, wax, bitumen, coke, LPG etc.
Gasolines. It is used in reciprocating spark-ignition internal combustion engines.
Motor gasoline is a mixture of low boiling hydrocarbons (215°C) and made by blending—
(i) Straight run gasoline (direct distillation of crude oil), (ii) cracked gasoline, (iii) reformed
naphtha and other fractions, 10% butane for easy starting of engines, TEL and other additives.
Colours are added to identify different grades.
Aviation gasolines have higher volatility and higher percentages of TEL and isoparaffin.
Kerosene. Kerosene is used for domestic purpose, space heaters for jet engines. It is
obtained as a straight-run distillate from crude oils. No additives are required. Purification
involves removal of sulphur.
Diesel fuel. Diesel fuels are prepared from heavy distillate obtained from catalytic
cracking units. It is rich in aromatics and iso-paraffins, the low cetane number is improved by
additives. Volatility is improved by blending with light fractions. In a high-speed diesel engine,
the cetane numbers should be more than 45 whereas for low speed engines it should be in the
range of 25 to 35.
Fuel oils. Wide range of liquid fuels used in boilers and furnaces are covered by this
term. Cycle gas oil is the light variety and needs a number of treatments for purification. The
heavy fuel oil produced from the cracking units are the heavy variety.
LPG. Liquefied petroleum gas is obtained as a byproduct during the cracking of heavy
oils or from natural gas. LPG contains hydrocarbons which are gaseous under atmospheric
pressure but can be liquefied under pressure. They are n-butane, isobutane, butylene, propane
etc. It is used as domestic and industrial fuels and in future it may be used as motor fuel.
Non-petroleum Fuels
1. Benzol. Refined benzol is essentially a mixture of 70% benzene, 18% toluene and 6%
xylenes. Its octane rating ranges between 87 and 90. It is a good fuel for internal combustion
engines but its initial boiling point is high (80°C), so it can be used only when blended with
gasolines. The other defect is its high freezing point (55°C), so it cannot be used in cold countries.
Alcohols. Methyl and ethyl alcohols can be used in gasoline engines. Cost considerations
prohibit their use. Octane number is high as 114 and 99, respectively. But their calorific values
are low. Ethyl alcohol is not used as a prime fuel but used as blends with gasoline upto 25%
alcohol and the mixture is called “gasohol”. It serves to increase the octane rating.
The importance of ethyl alcohol as a fuel lies in the fact that it can be produced from
naturally available carbohydrates by fermentation. It requires about 36 hrs. to complete whereby
the fermented liquid contains 18-20% alcohol which is then fractionally distilled to recified
spirit containing 90-95% alcohol, from which absolute alcohol can be obtained after treatment
with lime and distillation.
GASEOUS FUELS
(a) Fuel gas from nature:
(i) Natural gas
(ii) Methane from coal mines.
(b) Fuel gas from solid fuels:
(i) Producer gas
(ii) Water gas
(iii) Coal gas
(iv) Blast furnace gas
(c) Fuel gas from petroleum:
(i) Refinery gases (ii) LPG (iii) Gases from oil gasification.
(d) Fuel gas made by fermentation of organic wastes.