1. What do you mean by combustion air?
The amount of air required for complete combustion of fuel
in furnace is called as combustion air. The efficiency of the Boiler or furnace
depends on efficiency of combustion system.
2. On what parameters the requirement of combustion air
depends?
Combustion air requirement depends on;
- Type of fuel burnt
- Type & quantity of its elemental constituents
- Type of Boiler and furnace
- Amount of moisture content in it
3. What is the relation between moisture content in the
fuel & combustion air required?
Combustion air requirement increases as the moisture content
in the fuel increases and vice versa
4. What is the relation between carbon & Hydrogen content
in the fuel & combustion air required?
Combustion air requirement increases as the % of carbon &
Hydrogen content in the fuel increase and vice versa.
5. What is the relation between oxygen content in the
fuel & combustion air required?
Combustion air requirement decreases as the % of oxygen
content in the fuel increases and vice versa.
6. Do content of sulphur & Nitrogen in the fuel
affect combustion air requirement?
Increase and decrease in sulphur & Nitrogen content in
the fuel does not affect much on combustion air requirement.
7. What is meant by total air of combustion?
The total air supplied to the Boiler combustion chamber is
divided into two parts Primary air and secondary air.
Primary air supports the flame and takes part in the initial
combustion process. The second part is called as secondary air. Secondary air
is admitted into the furnace from top to create turbulence in furnace and to
ensure complete combustion of the fuel.
8. What are the functions of Primary and secondary air in
Travelling grate, pulverized coal fired and FBC Boilers?
In case of travelling grate Boilers Primary air is supplied
below the grate to support flame & combustion stabilisation. And secondary
air from top of the furnace as over fired air to create turbulence for complete
combustion. And also secondary air is used to spread the fuel in furnace
In case of Pulverized coal fired Boilers, Primary air is
used to carry the pulverized coal into the furnace.
In case of FBC Boilers Primary air is used to carry fuel and
fluidisation. Secondary air is supplied above the bed to ensure complete
combustion
9-What is meant by theoretical air & excess air in
combustion?
Theoretical air: Amount of air required to burn the fuel. It
is stoichiometric air, it does not ensure complete combustion.
Excess air: Amount of extra air given for complete
combustion
10-Calculate the Theoretical air required to burn imported
coal having carbon 55%, Oxygen 8.2%, Hydrogen 3.3% and sulphur 0.32% in it
Theoretical air is calculated by using below formula
Thair = (11.6 X %C + 34.8 X (%H2-%O2/8) + 4.35 X %S)) / 100
Thair = (11.6 X 55 + 34.8 X (3.3-8.2/8) + 4.35 X 0.32)) /
100
Thair = 7.18 kg/kg of fuel burnt
In the above formula, you can vary the % of Carbon, Hydrogen
& Sulphur to observe changes in air requirement
11-How do you measure % of excess air supplied?
Excess air is generally measured from Oxygen analyser
installed at the out let of Boiler (Economiser)
It is to be noted that, excess air & excess oxygen are
not same. Air has around 21% of oxygen in it by volume. So, 100% excess air is
roughly equals to 10.5% of oxygen.
12-What is the significance of excess air?
For combustion, if less air is
supplied it leads to incomplete combustion forming CO instead of Co2. And if
more excess air is supplied it leads to reduction of combustion efficiency by
cooling the furnace & carrying the heat through flue gas.
So, it is important to adjust the air supply in such a way
that complete combustion will take place without much extra air.
13-Calculate the % of excess air required if oxygen
measured in flue gas at economiser outlet is 5.5%.
Excess air = O2% X 100 / (21-O2%)
Eair = 5.5 X 100 / (21-5.5)
Eair = 35.48%
14-Calculate the total air required for complete combustion
of coal if Theoretical air supplied is 7.1 kg/kg of coal and O2 measured in
flue gas is 6.4%
Actual or total mass of air supplied = (1 + Excess air /
100) X theoretical air
We have
Excess air = O2% X 100 / (21-O2%)
Eair = 6.4 X 100 / (21-6.4)
Eair = 43.84%
Actual or total mass of air supplied = (1 + 43.84 / 100) X 7.1
Actual or total mass of air supplied = 10.21 kg/kg of coal
15-How do you control the excess air?
Excess air is controlled by;
- Optimizing the moisture content in the fuel
- Improving combustion chamber performance
- Auto control of fuel feeding
- Continuously monitoring O2 content in flue gas
- Incorporating auto combustion control
- Incorporating VFD drives to ID, FD, PA & SA fans
16-Among Bagasse, coal and Natural gas, which fuel needs
more excess air?
Bagasse, since it has more moisture content
17-A boiler has supplied 27% excess air, calculate % of
O2 in flue gas
Excess air = O2% X 100 / (21-O2%)
27 = O2% X 100 / (21-O2%)
27 X 21 -27 X %O2 = 100 X O2%
567= 127 O2%
O2% = 4.46
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