Getting into the basics of combustion, the ideal amount of air required to burn a given quantity of fuel is referred to, as stoichiometric air. This is calculated based on the fuel composition. The level of mixing of air and fuel, that is required for complete combustion, is practically never achieved. If we supply the stoichiometric quantity of air for combustion, we would almost ensure that the combustion is incomplete. Hence the burners are designed with a particular level of excess air that ensures proper combustion.
Normally boilers are designed with excess air levels at around 20% for oil and gas fired unit at full load conditions.
The extra air that is supplied carries with it some heat which is not recoverable. So higher the excess air, higher are dry flue gas losses. Though a boiler is designed for a particular level of excess air, when a unit is commissioned, it is tuned to a particular load. Over a period of operation of the unit, there is a tendency for deviations from the tuned values, due to:-
- Variations in the fuel quality like density, viscosity, flash point etc
- Variations in the fuel pump outlet pressure, firing rate etc
- Normal wear and tear of fuel system components
Conventionally, there is no feedback mechanism to regulate the combustion air inflow, to account for the variations and the firing rates. Due to the above reason, we end up with high fuel consumption.
With undesired high excess air, one may end up burning extra fuel. Also the monitoring of oxygen levels is also equally important. Too low quantity of air supply will lead to unburnt/incomplete combustion and subsequently soot formation in the boiler. Thus, it is very important to optimize the air flow for efficient combustion.
The losses in a boiler are:
Stack loss or Heat loss (highest 8 to 10% in a well tuned burner)
Moisture and Hydrogen loss (depends on the fuel and hydrogen & moisture content in it), Radiation loss (about 0.5%)
Unburnt fuel loss (negligible in liquid and gaseous fuels), etc.
When total of all the losses is subtracted from 100, we get efficiency. Typical boiler efficiency for liquid/gaseous fuels is in the range of 88 to 90%. Thus all the losses put together will be in the range of 10 to 12% of which the stack loss is around 8%.
Thus stack loss is 67% of total losses put together, and most important to monitor.
For combustion efficiency optimization , it is always O2, CO, and CO2 that are monitored. O2 tells us the type of combustion, either complete or incomplete. The CO levels tell us how much fine tuning is required for the air to fuel ratio.
Hence, for all small boilers, testo 320 and testo 330-LL is the best solution. While for large power plants and bigger boilers & furnaces, testo 340 and testo 350 are the right solutions. Offcourse with O2 and CO sensors, CO2 is always calculated.