The Combustion Process - Gas Analysis Techniques

Combustion Optimization

Major contributions to combustion optimization are made by :  

• Composition of fuel and combustion air
• Ignition procedure and combustion temperature
• Details of burner and combustion chamber design
• The fuel/air ratio
For a given plant and a given fuel the optimum fuel/combustion air ratio (ex.air value) can be determined from gas analysis results using the combustion diagram, see fig. 7. In this diagram the concentration of the gas components CO, CO2 and O2 are displayed in function of the excess excess air value. The line representing ideal combustion without any excess air (ex.air=1) is in the center of the diagram; to the right the excess air value increases; air deficiency (ex.air<1)
exists to the left. Air deficiency also means deficiency of oxygen.

The combustion diagram provides the following information:
Left area with ex.air<1 (deficiency of air)
• CO exist because there is not enough oxygen available to oxidize all CO to CO2.
Note: CO may be dangerous to people when it escapes through leaks.
• With increasing oxygen content the CO concentration decreases through oxidation to CO2; CO2 concentration increases accordingly. This reaction will be stopped at or slightly above ex.air=1, CO will be zero and CO2 reach its
maximum value.
• Oxygen is not or almost not present in this area because all oxygen supplied to the system is consumed immediately to oxidize CO to CO2.
Right area with ex.air>1 (excess of air)
• Here O2 increases because the amount of oxygen supplied as part of the increasing volume of combustion air is no longer consumed for oxidation (CO is almost zero). Practically, however, some amount of air (oxygen) excess is
required for complete combustion because of the inhomogeneous distribution of air (oxygen) in the combustion chamber. Furthermore the fuel particle size influences combustion: the smaller the particles, the more contact between fuel and oxygen will be and the less excess air (oxygen) will be required.
• CO2 will decrease again relatively to the maximum value at ex.air=1 because of the dilution effect caused by the increasing volume of combustion air which itself carries almost no CO2.

Conclusion
Optimum combustion is achieved if:
• excess air and thus oxygen volume is high enough to burn all CO completely and at the same time
• the excess air volume is limited in order to minimize the energy loss through the hot flue gas emission to the atmosphere

The optimum range of excess air for a particular combustion plant can be determined from the concentration values of CO2 and CO (CO2 alone is not definite due to the curve maximum). Currently the O2-method is more often used.
Sampling point locations may differ from plant to plant depending on the plant design and plant operator. The functions shown in the combustion diagram are substantiated for the combustion of hard coal in table 8:

Economic relevance
Optimization of a combustion process through plant operation at the most effective excess air level has, besides reduction of emission levels, the objective of saving fuel costs. Based on experience and documented in the literature is the fact, that reduction of oxygen excess of 1%-point, e.g. from 4,5% to 3,5%, will improve the efficiency of the combustion plant by 1%. With fuel costs of $ 15 Mio. per month for a middle sized power station this results in monthly cost savings of $ 30.000 if, by means of reliable gas analysis, the plant can be operated at only 0,2%-point closer to the optimal excess air value than before! Similar savings are possible if short time deviations from optimum operation conditions are recognized and eliminated early by using gas analysis continuously.