Incorrect amount of air in fuel combustion
accounts for the largest losses in combustion systems.
All fuels consist mostly of atomic
Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), minerals (ash) and
water (H2O).
In practice, mixing is never perfect,
a certain amount of excess air is needed to complete combustion and ensure that
release of the entire heat contained in fuel oil.
One kg of fuel requires a certain minimum of ambient air to be fully combusted. We call this minimum amount of air the stoichiometric air. When fuel and oxygen in the air are in perfectly balance - the combustion is said to be stoichiometric.
One kg of fuel requires a certain minimum of ambient air to be fully combusted. We call this minimum amount of air the stoichiometric air. When fuel and oxygen in the air are in perfectly balance - the combustion is said to be stoichiometric.
If an insufficient amount of air is
supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide
exhausts from the boiler - resulting in heat transfer surface fouling,
pollution, lower combustion efficiency, flame instability and a potential for
explosion.
If too much air than what is required for
completing combustion were allowed to enter, additional heat would be lost in
heating the surplus air to the chimney temperature. This would result in
increased stack losses. Less air would lead to the incomplete combustion and
smoke. Hence, there is an optimum excess air level for each type of fuel.
if air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean.
if air content is less than the stoichiometric ratio - the mixture is fuel-rich.
Fuel combustion means to let the
molecular Oxygen (O2) in air react with the combustible components of a fuel.
As an example the fuel Carbon (C) reacts with O2 of the air to generate Carbon
Dioxide (CO2). If the reaction is incomplete Carbon Monoxide (CO), a deadly
gas, is generated.
It is worthwhile to point out that all
combustion products such as CO2, CO, NOx, SO2, SO3, except for the water
generated by combustion of H to H2O, are harmful. There is essentially nothing
benign in stack gas, except the water vapor. And even the water vapor, because
it reacts with SO3 to sulfuric acid (H2SO4) is not really harmless.
This environmental and health threat
of stack gas is one more reason to reduce energy consumption per unit of
product output.
Excess Air, EA = mass of air in kg to
combust one kg of fuel/stoichiometric air(Kg air/Kg fuel)
For complete combustion of every one
kg of fuel oil 14.1 kg of air is needed. In practice, mixing is never perfect;
a certain amount of excess air is needed to complete combustion.
1 Kg of coal will require 7 – 8 kg of
air depending upon the hydrogen, carbon, oxygen, nitrogen and sulphur content
for complete combustion.
1 Kg of Natural gas will require 9 –
10 kg of air.
Recommended excess air level at full
boiler load is given below:
For fuel oil - 10 - 20%
For Natural gas - 10 - 20%
For stoker coal - 35 – 40%
For pulverized coal - 20
– 25%
For optimum combustion of fuel oil,
the CO2 or O2 in flue gases should be maintained at 14 – 15.5 % in case of CO2 and
2 – 3.5 % in case of O2.