Coal is the most widely used fuel source used to produce electricity. Air is necessary to provide oxygen for burning coal. It is the chemical reaction between carbon in coal and the oxygen in the air that produces heat energy.
Elements in Coal
There are three elements in coal that combine with oxygen in the air duing the combustion process. The most important element is carbon. This makes up about 30% to 60% of the substance. Hydrogen makes up about 1% to 3% and Sulphur, 0.3% to 3%.
The other elements found in coal are nitrogen, which can make up about 1% to 2% of the substance, and oxygen, 5 % to 12%. Oxygen is also used up in the combustion process. The exact amounts of each element depends on the type and rank of coal, how the coal was formed millions of years ago and the location or mines from which the coal is sourced.
Stoichiometric or Theoretical Air Quantity
Theoretical Air Quality is calculated based on the chemical reaction between the elements and oxygen.
- Carbon combines with oxygen to form carbon-dioxide and heat.
C+ O2 > CO2
1 C +32/12 O > 44/12 CO2
1 kg Carbon +2.67 kg Oxygen > 3.67 kg Carbon Dioxide
- Hydrogen combines with oxygen to form water and heat
2 H2+ O2> 2H2O
1 H +32/4 O >36/4 H2O
1 kg Hydrogen +8 kg Oxygen >9 kg Water
- Sulphur combines with oxygen to form sulphur dioxide
S + O2 > SO2
1 S +32 /32 O >64 /32 SO2
1 kg Sulphur +1 kg Oxygen >2 kg Sulphur Dioxide.
For this example, we are considering coal having a makeup of 57.2 % Carbon, 2.2 % Hydrogen , 0.5 % Sulphur and 6.9 % Oxygen.
- The theoretical oxygen required to burn this coal is then
2.67 x C % + 8 x H% + 1 x S % – O% = 1.64 kg of Oxygen for 1 kg of Coal.
Air contains 23.2% by weight of oxygen.
- The theoretical air required to burn the coal is
= 1.64 / 23.2% = 7.1 kg of Air for 1 kg of Coal.
This is the theoretical air required to burn the coal.
The heating value of coal also depends on the elemental carbon and hydrogen. This means that the air required and the heating value have an almost fixed relationship. The theoretical air required for a unit heating value is practically a fixed value. This is around 0.332 kg of air for one MJ of heat input. This is true for a wide range of coals used in power plants.
By knowing the calorific value of coal, the theoretical air quantity can be directly calculated using this factor.