By PRAGYANSHARMA POLAVARAPU (Vrs.Somanchi)
(1) PYROLYSIS
When hard fuels like coal and wood
are burnt in an insufficient supply of oxygen ,slow combustion called
“pyrolysis” takes place. Pyrolysis leads to many
chemical decomposition reactions within the fuel. Several organic
chemical compounds are released. The word pyrolysis means “separation by
fire”.
The charring of wood gives
charcoal. It is a type of pyrolysis. The pyrolysis of big pieces of
organic substances releases volatile substances which
are gases or oily fluids.Finally a black solid residues
remains. The residue is rich in carbon.
There is an industrial process called
“ gasification” which uses the technology of pyrolysis to produce
fuel gases. Pyrolysis is used in production of ethylene and
other important carbon compounds from coal and petroleum. Coal is converted as
coke using the process of pyrolysis.
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In large gas tanks used in combustion
type reactions in industry and inside the internal combustion engines in
vehicles unburnt solid substances may accumulate over long use . They
have to be cleaned periodically. In some machines there may be lot of smoke due
to defective designing and lack of maintenance. Good designing of combustion
devices should be ensured.
There is environmental legislation in
many countries about automobile engines, motor cycle engines regarding
the exhaust emissions. Thermal power stations have to strictly adhere to high
standards prescribed for emission of polluting gases, industrial water ,
mountainous quantities of ash.
(2) UNDERTANDING BASIC THEORY
OF FLAME
Oxygen gas exists all around us
in seemingly limitless quantities. In all combustion reactions in nature, from
smallest candle flame and match stick flame to the tongues of flame in
huge forest fires oxygen gas is the “oxidant” the fire creator.
At the very high temperatures of
around 1000 degrees centigrade existing in every flame (including flame of of a
small candle or a burning match stick) the hydro carbon molecules
of the fuel breakup soon after they come into contact with oxygen molecules(oxident).
The hydrogen atoms and carbon atoms inside the hydrocarbon molecule
come out. The oxygen molecule too breaks up and the two atoms of
oxygen come out and immediately at almost explosive speed strongly bind
themselves (a) to atoms of hydrogen to form molecules of H2O(hot water
vapor at 1000 degrees)(b)to atoms of carbon to form carbon dioxide gas
.(Molecules of other elements like nitrogen, and sulphur may
also be present and oxygen reacts with their atoms also in small quantities ).
All these reactions take place at very high temperature of the order of 1000
degrees. This is the heat we get in all kinds of combustion reactions and
utilize for various purpose like cooking food, heating houses (in cold
countries) and for running the wheels in industry.
(3) BINDING ENERGY OF MOLECULES
We should note that to create any
molecule, great energy is absorbed by the molecule to tightly bind atoms
within molecule structure. Binding means one single pair of electrons
starts revolving around the nuclei of two or three atoms at
terrific speeds of billions of times per second. The electron pair
appears to be present everywhere every moment and behaves like a bond.
Molecules are broken in chemical
reactions and atoms come out and again new molecules with new combinations of
same atoms are formed. The biding energy in the molecules before reaction
is first released and in a moment for the new bonds formed in
the new molecules some energy is utilized back. The net unused energy is
released as heat.
We should also note that atoms
of gaseous elements cannot exist independently . Two atoms of the gas form a
molecule. Here too one and the same electron pair revolves around both
the atoms and binds them strongly.
When there is reaction between
hydrocarbon molecule and the oxygen molecule, the bond between the two
oxygen atoms in O2 molecule break and enormous energy is released . H2O
molecules and CO2 molecules form. The energy released when O2(the oxygen
molecule ) breaks into two oxygen atoms is far in excess of energy
existing in the old hydrocarbon bond , the newly formed H2O(water molecule)
bond and the newly formed CO2(carbon dioxide molecule). This excess energy
released is what we experience as the enormous heat energy released during
combustion of hydrocarbons in oxygen.
(4) GENERAL
An important point is to be
noted. Even the highly inflammable hydrocarbons will not
react by themselves with oxygen when simply kept in oxygen atmosphere at
room temperature. Hydrocarbon molecules and oxygen molecules reacts with
each other only when there is availability of a 1000 degree centigrade heat–
created at least by a small flame of a burning match or burning flame of a
candle or a burning oil soaked cloth. Then only the main combustion
reaction starts and soon becomes self-sustaining as heat energy is
continuously produced. The initial small combustion reaction
grows thousands of times larger . The big tongues of flame we
see in combustion can be treated as the directly visible reaction
areas.
The H2O
molecules(water) are formed at 1000 degrees and are very hot and appear as
steam and instantly disappear into atmosphere. In the big
fires (as in burning big logs of wood)we see lot of pure black highly
brittle substance called carbon. Though we think of carbon as cheap and
unimportant substance , it is one of the purest substances in universe
and is a fundamental and important chemical element. When a small candle
burns this same elemental carbon is released in the
form of soot. The candle is nothing but a mass of pure solid hydro
carbon. The terrific heat of the candle flame converts the paraffin into an
oily form . The wick of a lighted candle helps in easy creation of a flame.
In the times 300 or 400 years ago
scientists had no idea about existence of substances called “
hydrocarbons”. They did not also know about the nature and existence of
gaseous element “oxygen” . They did not know that oxygen is
only 21% of air and that only oxygen sustains burning.They could not properly
understand chemistry of combustion or what constitutes fire. Some scientists
wrongly believed that every substance contains a weightless substance
called phlogiston and that when a substance is burnt the phlogiston escapes.
Now we correctly know
that combustion is a series of endothermic chemical reactions
between hydrocarbons and atmospheric oxygen at around 1000 degrees centigrade
and that the excess energy remaining after all the reactions are completed
appears as the heat of combustion and heat radiated from the flame..
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