What do you mean by dry flue gas loss in Boilers?
Dry flue gas heat loss refers to the energy carried away by exhaust gases (excluding moisture content) leaving the boiler stack.
What are the gases associated with dry flue gas?
Gases mainly consist of nitrogen (N₂), Carbon dioxide (CO₂), Oxygen (O₂), and traces of other gases.
On what factors dry flue gas loss depends?
- Dry flue gas loss in Boilers is mainly depends on;
- Flue gas temperature
- Flue gas quantity
- Excess air
- Moisture in the fuel
- Efficiency of Economiser and Air pre-heaters
What is the significance of calculating heat loss due to dry flue gas in Boilers?
In most industrial boilers, dry flue gas loss can account for 5% to 15% of total heat loss, making it the largest single loss component.
Calculating heat loss due to dry flue gas in Boilers is utmost important because;
Indicates Boiler Efficiency
Dry flue gas carries a significant amount of heat energy out of the boiler stack. By calculating this loss, you can determine how much heat is not being utilized for steam generation.
Helps Reduce Fuel Consumption
- When flue gas heat loss is high, it means fuel energy is being wasted.
- Identifying this loss allows operators to:
- Optimize combustion
- Adjust excess air levels
- Improve burner performance
Optimizes Flue Gas Temperature
Stack temperature directly affects dry flue gas loss.
High stack temperature = More heat loss
Proper control ensures:
- Maximum heat transfer in boiler
- Minimal energy escaping through chimney
Improves Combustion Efficiency
Dry flue gas loss depends on:
- Excess air supplied
- Fuel composition (carbon, hydrogen, etc.)
- Flue gas temperature
By analysing this loss, you can:
- Maintain ideal air-fuel ratio
- Avoid incomplete combustion
- Prevent energy wastage
Reduces Environmental Impact
Lower heat loss means:
- Less fuel burned
- Reduced emissions (CO₂, NOx)
Calculating heat loss due to dry flue gas is crucial because it:
- Directly reflects boiler efficiency
- Helps save fuel and cost
- Enables better combustion control
- Supports energy conservation and environmental compliance
Heat loss due to dry flue is calculates as below;
Heat loss% = Mass of flue gas (Mf) X Cp X (Tf-Ta) X 100 / GCV of fuel
Where,
Mf = Mass of dry flue gas (kg/kg of fuel)
Cp = Specific heat of flue gas (kcal/kg°C)
Tf = Flue gas temperature (°C)
Ta = Ambient temperature (°C)
Dry flue gas loss increases as the mass of flue gas increases
This mass of flue gas increases due to;
- Type of the fuel
- More excess air
- Over loading of the Boiler
- High moisture content in the fuel: High moisture content fuel drag more air for combustion
Also dry flue gas loss increases as the temperature of the flue gas at APH outlet increases, this is mainly due to poor heat transfer in Economiser and APHs.
Let’s calculate heat loss due to dry flue gas in boilers with sample values:
Mf = 9 kg/kg fuel
Cp = 0.24 kcal/kg°C
Tf = 150°C
Ta = 28°C
GCV = 46000 kcal/kg
Heat loss% = Mass of flue gas (Mf) X Cp X (Tf-Ta) X 100 / GCV of fuel
Heat loss% = 9 X 0.24 X (150-28) X 100 / 4600
Heat loss% = 5.2%
A boiler of capacity 150 TPH, consumes coal of GCV 3900 kcal/kg at the rate of 45 TPH, a total combustion air of 350 TPH is being supplied to ensure complete combustion. Calculate the heat loss due to dry flue gas. Assume APH outlet flue gas temperature 150 deg C and Ambient air temperature 30 deg C.
Coal consumption; 45 TPH
Total combustion air = 350 TPH
Total flue gas generated = 350 + 45 = 395 TPH = 395/45 = 8.77 kg/kg of fuel
Note: Neglected ash percentage in coal
Heat loss% = 8.77 X 0.24 X (150-30) X 100 / 3900
Heat loss% = 6.47%
