S9-THE SCIENCE OF COMBUSTION

 

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..