ECCJ/AOTS Philippines training course

ECCJ / AOTS Philippines training course

Philippines

Energy Conservation of Combustion
- air ratio and air preheating

Content
2 Optimization of air ratio
3 The air ratio is calculated from the O² content (O²%) in the exhaust gas.
4 Larger air ratio leads to larger heat loss with the exhaust gas.
5 Fuel saving by air ratio improvement
6 Effect of air ratio reduction
7 Criteria for air ratio (standard values)
8 Criteria for air ratio (target value)
9 Energy conservation by air preheating
10 Utilization of waste heat of exhaust gas - air preheating
11 Criteria for exhaust gas temperature
12 Criteria for factories

Optimization of air ratio

The excess air for combustion such as that in a boiler contributes only to an increase in heat loss with the exhaust gas.
The degree of excess air is expressed by air ratio.
The air ratio at 1 (one) represents the theoretical combustion.
The air ratio is expressed by 21/(21-O²%) where O² is oxygen content in the exhaust gas.
The Criteria of the Law for Rational Use of Energy set both standard and target values of air ratios for boilers and industrial furnaces.

The "air ratio" is calculated from the O²% of the exhaust gas.

The simplified equation for obtaining the air ratio is: Air ratio = 21/(21 - O²% value) where O² is the oxygen content obtained by analysis (dray base value).
Normally, the air ratio may be calculated by this equation. The criteria of the Law Concerning Rational Use of Energy specify this equation.
As the air ratio becomes large, the difference between the result obtained by the simplified equation and the real value becomes large. (See the graph.)
The air ratio is defined as "real air value/theoretical air volume."
The air ratio of 1 (one) represents an ideal state in which no more than required air is used for combustion (theoretical combustion).
The air in excess of air ratio at 1 (one) is called "excess air" which causes heat loss by carrying out air from the stack at heated state.
The air ratio short of 1 (one) causes incomplete combustion, associated with the incomplete combustion heat loss.

Larger air ratio leads to larger heat loss with the exhaust gas.

Fuel saving by air ratio improvement - with graph

Reduction of the air ratio (reduction of O²% in the exhaust gas) reduces heat loss with the exhaust gas.
This in turn leads to fuel saving.
The graph illustrates result of air ratio reduction.
In fuel saving rate, 13A gas and fuel oil A differ only very little.
At lower exhaust gas temperatures, the effect is small.
(Example)
If air ratio can be reduced from the present 1.6 (O² = 8.4%) to 1.2 (O² = 3%), a fuel saving of 2.8% is realizable.

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