Coal Burning : Effect of excess air or Less air

Coal Burning : Effect of excess air or Less air

Quantity of air required in burning of the cole is most important. By this we can save the energy losses in coal burning.

As in manufacturing of sponge iron coal is used in different size, and it is required that ever part of the coal should come in air contact for proper combustion. (In case of burning of un-burnt coal in ABC.)  

CASE - I (Air is Less than required)

•  In this case some part of coal goes out with out burning which is a loss of potential energy, which is known as un-burnt carbon loss. 
• in the normal combustion reaction Carbon and Oxygen combine to form Carbon dioxide giving out heat. Actually this takes place in two stages , first is the formation of Carbon monoxide and then the Carbon monoxide reacts to form Carbon-dioxide. If the air is less the second stage does not take place, and Carbon monoxide is formed. This means a part of the potential energy of the coal is not released. This is an energy loss.

CASE - II (Air is More than required)

If the air supplied for combustion is more than required, the air that is in excess of the combustion requirement does not contribute to the combustion process , but takes away a part of the combustion heat then goes up the stack as waste heat. This is an energy loss. This is called the Dry Gas loss.

Burning Coal : How much Air is required ?

Written by: johnzactruba • Edited by: Lamar Stonecypher
Updated Oct 20, 2009 • Related Guides: Carbon

The most important requirement to burn coal is the correct quantity of air. This article explains how to calculate the correct quantity of air .

Coal is the most widely used fuel source used to produce Electricity. Air provides the necessary Oxygen for burning coal. It is the chemical reaction between Carbon in coal and the Oxygen in the air that produces the heat energy.

Elements in Coal

There are 3 elements in coal that combine with Oxygen in the air duing the combustion process. The main and important element is Carbon which could be around 30% - 60 % .Then there is Hydrogen in the range of 1 % to 3 % and Sulphur in the range of 0.3 % to 3 % . Apart from this three there is Nitrogen 1% to 2 % and Oxygen from 5 % to 12 % . The 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.

This 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 kg Carbon +2.67 kg Oxygen > 3.67 kg Carbon Dioxide1C+32/12 O > 44/12 CO₂

• 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 Suphur Dioxide

S + O₂ > SO₂

1 S +32 /32 O >64 /32 SO₂

1 kg Sulphur +1 kg Oxygen >2 kg Sulphur Dioxide.

For the purpose of calculation we consider a coal having 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.

Quick Calculation of Theoretical Air.

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.

Composition of dry atmosphere, by volume

ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, )

GasVolume
Nitrogen (N₂)                        780,840 ppmv (78.084%)
Oxygen (O₂)                          209,460 ppmv (20.946%)
Argon (Ar)9,                          340 ppmv (0.9340%)
Carbon dioxide (CO₂)           390 ppmv (0.039%)
Neon (Ne)                            18.18 ppmv (0.001818%)
Helium (He)                           5.24 ppmv (0.000524%)
Methane (CH₄)                     1.79 ppmv (0.000179%)
Krypton (Kr)                        1.14 ppmv (0.000114%)
Hydrogen (H₂)                      0.55 ppmv (0.000055%)
Nitrous oxide (N₂O)             0.3 ppmv (0.00003%)
Carbon monoxide (CO)        0.1 ppmv (0.00001%)
Xenon (Xe)                           0.09 ppmv (9×10⁻⁶%) (0.000009%)
Ozone (O₃)                           0.0 to 0.07 ppmv (0 to 7×10⁻⁶%)
Nitrogen dioxide (NO₂)         0.02 ppmv (2×10⁻⁶%) (0.000002%)
Iodine (I₂)                              0.01 ppmv (1×10⁻⁶%) (0.000001%)
Ammonia (NH₃)                     trace


Not included in above dry atmosphere:
Water vapor (H₂O)                ~0.40% over full atmosphere, typically 1%-4% at surface