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Combustion

Elements of combustion

Combustion is a chemical reaction which needs three essential elements (commonly referred to as the combustion/fire triangle):

  • Heat
  • Oxygen
  • Fuel

The reaction create products of combustion and heat.

Complete combustion of Methane

The ideal proportions of fuel gas and oxygen (air) is called the stoichiometric mixture which are given as:

    CH4       +       O2         =                CO 2           +     2H2O  
Methane    +   Oxygen      =     Carbon dioxide      +   Water Vapour

or with Air which contains Nitrogen which does not take part in the equation

  CH4     + 2(O2 + 3.76N2) =       CO2           +   2H2O   +   7.52N2
Methane +       Air            = Carbon dioxide +   Water  +  Nitrogen

From the second reaction shows for 1 m3 of Methane, about 10 m3 of Air are needed to ensure complete combustion is carried out.

Complete combustion of Propane

The ideal proportions of fuel gas and oxygen (air) is called the stoichiometric mixture which are given as:

    C3H8       +      5O 2         =             3CO 2           +     4H 2O  
Propane      +   Oxygen      =     Carbon dioxide      +   Water Vapour

or with Air which contains Nitrogen which does not take part in the equation

  C3H8     + 5(O2 + 3.76N2) =       3CO2         +   4H2O   +   18.8N2
Propane  +       Air            = Carbon dioxide +   Water  + Nitrogen

From the second reaction shows for 1 m3 of Propane, about 24 m3 of Air are needed to ensure complete combustion is carried out.

Complete combustion - Generic for all Hydrocarbons

The ideal proportions of fuel gas and oxygen (air) is called the stoichiometric mixture which are given as:

CxHy + (x+y/4)O2 + 3.76(x+y/4)N2 = xCO2 + (y/2)H2O + 3.76(x+y/4)N2

Following this for Butane shows that about 30 m3 of Air are needed to ensure complete combustion is carried out.

Incomplete Combustion

Incomplete Combustion is where there is insufficient air for combustion to be completed. As a result Carbon Monoxide is produced. A example of the equation for this process with insufficient oxygen is as follows:

  2C3H8  +     7O2  =     2C    +           2CO         +       2CO2         +  8H2O
Propane + Oxygen = Carbon + Carbon Monoxide + Carbon dioxide + Water

Carbon Monoxide is Poisonious. Its effect on humans gives rise to similar symptoms as those for flu:

  • severe headaches
  • nausea
  • dizziness
  • tiredness
  • general body weakness

The symptoms arise as Carbon Monoxide is able to combine with haemoglobin in human blood instead of oxygen. This stops our bodies from supplying the oxygen we need to function. 

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