Combustion Efficiency and Excess Air
Optimizing
a boilers efficiency is important to minimize fuel consumption and unwanted
excess to the environment
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To ensure
complete combustion of the fuel used, combustion chambers are supplied with
excess air. Excess air increase the amount of oxygen and the probability of
combustion of all fuel.
- when fuel and oxygen in the air are in perfectly balance - the combustion is said to be stoichiometric
The
combustion efficiency will increase with increased excess air, until the heat
loss in the excess air is larger than than the heat provided by more efficient
combustion.
Typical
excess air to achieve highest efficiency for different fuels are
- 5 - 10% for natural gas
- 5 - 20% for fuel oil
- 15 - 60% for coal
Carbon
dioxide - CO2 - is a product of the combustion and the
content in the flue gas is an important indication of the combustion
efficiency.
An optimal
content of carbon dioxide - CO2 - after combustion is
approximately 10% for natural gas and approximately 13% for
lighter oils.
Normal
combustion efficiencies for natural gas at different amounts of excess air and
flue gas temperatures are indicated below:
Combustion Efficiency (%)
|
||||||
Excess %
|
Net Stack Temperature1)
(oF)
|
|||||
Air
|
Oxygen
|
200
|
300
|
400
|
500
|
600
|
9.5
|
2.0
|
85.4
|
83.1
|
80.8
|
78.4
|
76.0
|
15
|
3.0
|
85.2
|
82.8
|
80.4
|
77.9
|
75.4
|
28.1
|
5.0
|
84.7
|
82.1
|
79.5
|
76.7
|
74.0
|
44.9
|
7.0
|
84.1
|
81.2
|
78.2
|
75.2
|
72.1
|
81.6
|
10.0
|
82.8
|
79.3
|
75.6
|
71.9
|
68.2
|
1) The "net stack
temperature" is the difference between the flue gas inside the chimney and
the room temperature outside the burner.
Flue Gas Loss Combustion Oil
The
relationship between temperature difference flue gas and supply air, CO2
concentration in the flue gas, and the efficiency loss in the flue gas
combustion oil, is expressed in the diagram below.
Example - Heat Loss Flue Gas
If the
temperature difference between the flue gas leaving a boiler and the ambient supply
temperature is 300 oC and the carbon dioxide measured in the
flue gas is 10% - then, from the diagram above, the flue gas loss can be
estimated to approximately 16%.
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