- Combustion Air
- Combustion air is the air that is used to actually burn the fuel. Without combustion air, which is normally forced into the furnace, combustion is impossible.
- Infiltration Air
- Infiltration air is the outdoor air that is not deliberately in the boiler. Sources of infiltration air maybe cracks or leaks.
- Dilution Air
- Dilution air is the air that combines with the flue gases and lowers the oncentration of the emissions. There are two types of dilution air, natural and induced (artificially created).
Time, Temperature and Turbulence
The combustion process is extremely dependent on time, temperature, and turbulence. Time is important to combustion because if a fuel is not given a sufficient amount of time to burn, a significant amount of energy will be left in the fuel. Too much time to burn on the other hand will produce very long flames, which can be a function of bad mixing. The correct balance of time and mixing will achieve complete combustion, minimize flame impingement (boiler maintenance hazard), and improve combustion safety. In addition, a properly controlled combustion process strives to provide the highest combustion efficiency while maintaining low emissions of harmful gases.
In order to ensure complete combustion, combustion chambers are fired with excess air. Excess air increases the amount of oxygen and nitrogen entering the flame increasing the probability that oxygen will find and react with the fuel. The addition of excess air also increases turbulence, which increases mixing in the combustion chamber. Increased mixing of the air and fuel will further improve combustion efficiency by giving these components a better chance to react. As more excess air enters the combustion chamber, more of the fuel is burned until it finally reaches complete combustion. Greater amounts of excess air create lower amounts of CO but also cause more heat losses. Because the levels of both CO and heat losses affect the combustion efficiency, it is important to control and monitor excess air and the CO levels to ensure the highest combustion efficiency possible.
Calculating Excess Air
As discussed earlier, under stoichiometric (theoretical) conditions, the amount of oxygen in the air used for combustion is completely depleted in the combustion process. Therefore, by measuring the amount of oxygen in the exhaust gases leaving the stack we should be able to calculate the percentage of excess air being supplied to the process.
The following formula is normally used to calculate the excess air:
|Fuel||Type of Furnace||Excess Air %|
|Pulverized Coal||Partially Water Cooled Furnace||15-40%|
|Fuel Oil||Oil Burners, register type||5-10%|
|Fuel Oil||Multifuel burners & flat-flame||10-20%|
|Natural Gas||Register type Burners||5-10%|
What is Draft?
The pressure of the gases in the stack must be carefully controlled to insure that all the gases of combustion are removed from the combustion zone at the correct rate. This draft pressure can be positive or negative depending of the boiler design; natural draft, balance draft, and forced draft boilers are the most commonly used in the industry.
Monitoring draft is important not only to increase combustion efficiency, but also to maintain safe conditions. Low draft pressures create build-ups of highly toxic gases such as carbon monoxide and highly explosive gases. These build ups may take place in the combustion chamber or may even be ventilated indoors creating the risk of injury and death. Conversely, extremely high draft pressures can cause unwanted turbulences in the system preventing complete combustion. Unwanted high draft pressures tend to damage the combustion chamber and heat exchanger material by causing flame impingement.