Combustion is the chemical union
of a combustible substance with oxygen, resulting in the production
of heat. The substances used to produce heat by their combustion
in the air are called fuels. They are either carbon or compounds
The process of combustion results in the transformation of fuel
into carbon dioxide, water, and so on, by reactions with atmospheric
oxygen, heat being evolved during the oxidation.
Fuels may be divided into solid, liquid and gaseous. Solid and liquid
fuels are frequently transformed into gaseous state for convenience
in use. Indeed, the modern tendency is to employ gaseous fuels whenever
it is possible because of the cheaper cost of transport and greater
flexibility of application.
Solid fuels cover the different kinds
of wood, peat, vegetable refuse, coal, charcoal and so on.
Wood has low calorific value. It
is still in demand in those parts of the world where its cost is
not great. Wood is the raw material for the production of charcoal.
Charcoal, which is not only used
for fuel but also in some mechanic, artistic and chemical processes,
is obtained by burning firewood.
It is very porous and can absorb
big quantities of gases as well as solid and liquid bodies. It is
used in refining sugar to remove the brown colouring matter and
in filters to purify water.
Coal is a natural product consisting
essentially of carbon. It is black or blackish, hard and opaque.
It is formed by the decay of vegetable matter and solidified by
great pressure. Peat is the first stage in this transformation;
then brown coal or lignite which preserves its ligneous structure;
then ordinary house coal or bituminous coal, and finally, anthracite
which has a large proportion of carbon, but it is not easy to ignite.
Coke, which is a valuable fuel, is
the solid left when the volatile parts are distilled from coal.
It contains more than 50 % of carbon and it is largely used in the
manufacture of iron and other metals, also for cooking and other
purposes. It is hard, porous and gray and burns with an intense
heat giving no smoke.
Most of the liquid fuels are called
hydrocarbon oils because they are distilled from petroleum which
contains hydrogen and carbon.
Mineral oils are obtained by the
distillation of coal or in a natural form in oil fields.
At first petroleum undergoes a general
refining process called the separation into groups in which the
more volatile parts of it, like gasoline and kerosene, are separated
from the heavy oils, such as fuel oil, diesel oil, paraffin, etc.
Gasoline, a volatile distillate of
crude petroleum, is a refined inflammable fuel that can be easily
atomized to form an explosive mixture. When the heat in the refining
retort gets to a temperature about 37° C to 100° C, the
product obtained is termed kerosene, formerly called illuminating
oil as it was used for lighting. Another name for this substance
is paraffin oil.
From 100° C on the heavy oils
obtained are known with different names according with their degree
of density, viscosity and flashing point.
Gas oil and fuel oil are used in
heavy oil engines, while a special oil called Diesel oil is employed
in Diesel motors.
Alcohol is made by the distillation
of vegetable matter such as corn, Indian corn, potatoes, beetroot,
wood and by the fermentation of sugar.
Natalite and Discol are the trade
names for two new fuels made of alcohol mixed with other substances.
Who knows what scientists will invent or discover in the field of
Petroleum is a thick, greenish-black, unpleasant-smelling liquid
occurring in large underground deposits in different parts of the
Petroleum is a complex mixture of
hydrocarbons together with small amounts of nitrogen and sulphur
compounds. It is extracted by drilling holes or wells through the
soil until layers of oil are reached. Sometimes it is under great
pressure and springs up from the well as a gusher, but after a time
the flow stops and the gusher is changed into an ordinary well from
which the fuel must be lifted by pumps. Crude petroleum is refined
by distillation which can be done due to the fact that the different
constituents of the oil have different boiling points.
The main substances into which natural
petroleum is thus separated are: light naphtha, gasoline, heavy
naphtha, paraffin oil, lubricating oil, vaseline and paraffin wax.
The residue is a black, pitch-like substance known as petroleum
pitch used in the making of roads.
There are different kinds of gaseous
fuels: natural gas, producer gas, coal gas, blast furnace gas, acetylene,
Natural gas, used for domestic purposes
and in internal combustion engines, consists largely of methane
or marsh gas. It also contains some hydrocarbons such as ethylene.
Coal gas is a popular domestic and
industrial fuel made in horizontal or vertical retorts in the absence
of air. Its composition varies according to the nature of the coal
and the temperature of decomposition. Before it leaves the holder
it must be submitted to a process of cooling and cleaning.
Water gas is a mixture of carbon
monoxide and hydrogen. It is obtained by passing an air blast through
incandescent coal. When the coke is white-hot, a steam blast is
sent to the coal by a nozzle. Then the gas is cooled and filtered.
The calorific value of this fuel is considerably less than that
of coal gas.
Producer gas, an inflammable mixture
of carbon monoxide and nitrogen, is produced by passing steam over
red-hot coke. It is not much used except in gas retorts, coke ovens,
Blast-furnace gas is a by-product
of iron-smelting used mainly for steam-raising in factories.
Acetylene, a colourless gas with
an unpleasant smell when impure, is obtained by the action of water
on calcium carbide and it is used principally for cutting and welding
steel. It produces a very hot, luminous, and smoky flame. In special
burners which supply it with plenty of air, its flame is extremely
bright and non-smoky.
Thermodynamics is the study of the
general laws governing processes which involve heat changes.
James Prescot Joule was the first
who made quantitative experiments as regards heat.
The first law of Thermodynamics is
stated as follows: When heat is converted into mechanical energy,
or the reverse, the total quantity remains unchanged.
The second law states: It is impossible
for any self acting machine unaided by an external agency, to convey
heat from one body to another at a higher temperature.
One consequence of the second law
is that although the total amount of energy is always the same,
less and less of it becomes available to our use, since in all natural
processes, energy of other kinds is being converted into heat energy.
This heat energy only raises the temperature of the surroundings
and cannot then be used again.
The third law states: The absolute
zero of temperature can never be attained.
By absolute zero we understand the
lowest temperature theoretically possible. It is zero on the absolute
temperature scale. On this scale the temperature interval between
the ice point and the steam point is defined to be 100 degrees,
so that the magnitude of the degrees is the same as on the centigrade