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Combustion
Efficiency and Excess Air
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:
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%.
