Engineering task and calculation objective
Module ABGV calculates the flue gas losses of combustion plants based on the Energietechnische Arbeitsmappe (14th edition, 1995), a German energy engineering reference handbook. The flue gas loss is the share of the supplied fuel energy that escapes unused through the stack with the hot combustion gases — for boilers, process furnaces and thermal oil heaters it is usually the largest single loss and largely determines the combustion efficiency.
For the calculation, the user selects the fuel (e.g. natural gas or heavy fuel oil); built-in fuel-specific coefficients then link the flue gas temperature and the residual oxygen or CO2 content of the dry flue gas to the percentage heat loss. To calculate flue gas losses, you therefore only need the usual measured quantities of a flue gas analysis.
In practice, the module is used to evaluate the efficiency of existing combustion plants, to check emission measurements, to estimate the savings potential of economizers or burner tuning, and to compare different operating points and fuels.
Standard and calculation basis: Energietechnische Arbeitsmappe 14. Auflage: 1995
Calculation workflow
- Select the fuel: The fuel selection sets the fuel-specific coefficients that represent the composition and heating value of the fuel — for example for natural gas or heavy fuel oil. These coefficients determine how strongly excess air and flue gas temperature affect the loss.
- Enter the flue gas analysis readings: Required are the flue gas temperature at the boiler outlet, the combustion air or ambient temperature, and the measured oxygen or carbon dioxide content in the dry flue gas. The excess air of the combustion follows from the O2 or CO2 content.
- Calculate the flue gas loss: From the temperature difference between flue gas and combustion air and the fuel-dependent coefficients, the flue gas loss is determined as a percentage of the supplied fuel heat (Siegert approach). The higher the flue gas temperature and the excess air, the larger the loss.
- Evaluate the result: The calculated loss is compared with typical reference values and, where applicable, the requirements of the German 1. BImSchV emissions ordinance. Measures can be derived from this: burner tuning (less excess air), cleaning of heating surfaces, or flue gas heat recovery to lower the flue gas temperature.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Measured carbon dioxide content | yCO2,T | – |
| Measured carbon monoxide content | yCO,T | – |
| Flue gas temperature | ϑ | °C |
| Absolute humidity of the combustion air | xH2O,L | kg/kg |
| Calorific value of the fuel oil | Hu | J/kg |
| Measured O2-content of the dry flue gas | yO2,T | – |
| Fuel selection | Brennstoff | – |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Mean specific heat capacity (flue gas) | cp,G | J/(kg·K) |
| Mean specific heat capacity (air) | cp,LT | J/(kg·K) |
| Specific enthalpy of the flue gas | hG | J/kg |
| Specific enthalpy of the dry air | hLT | J/kg |
| Waste gas loss of the fuel | lAB | – |
| Reference flue gas volume (dry) | VGT | m³/kg |
| Loss due to incomplete combustion | ICO | – |
| Reference CO2-content of the flue gas | μCO2 | kg/kg |
| CO2-content of the wet flue gas | xCO2 | – |
| Reference steam content of the flue gas | μH2O | kg/kg |
| Reference steam content (without humidity) | μH2O,B | kg/kg |
| H2O-content of the wet flue gas | xH2O | – |
| Reference dry combustion air mass | μLT | kg/kg |
| Reference dry flue gas mass | μGB | kg/kg |
| Reference flue gas mass | μG | kg/kg |
| Reference CO2-volume | VCO2 | m³/kg |
| Reference stoichiometric dry flue gas volume | VGoT | m³/kg |
Calculation options
Fuel selection
fuel · fuel oil · natural gas
Worked example
A natural gas fired steam boiler is operated at a flue gas temperature of 180 °C; the flue gas analysis shows 3.0 vol% O2 in the dry flue gas. The combustion air is drawn in at 20 °C. This worked example calculates the flue gas loss with the Siegert formula.
Given values
| Fuel | Natural gas |
| Flue gas temperature tA | 180 °C |
| Combustion air temperature tL | 20 °C |
| O2 content (dry flue gas) | 3.0 vol% |
| Siegert coefficients, natural gas | A2 = 0.66; B = 0.009 |
Solution
Set up the Siegert formula
qA = (tA − tL) · [A2/(21 − O2) + B]
Evaluate the fuel term
A2/(21 − O2) + B = 0.66/(21 − 3.0) + 0.009 = 0.03667 + 0.009 = 0.04567 1/K
Calculate the flue gas loss
qA = (180 − 20) · 0.04567 ≈ 7.31 %
The combustion efficiency is thus about 100 − 7.31 ≈ 92.7 %. Lowering the flue gas temperature by 20 K (e.g. with an economizer) would reduce the loss to about 6.4 %.
Result
| Flue gas loss qA | ≈ 7.31 % |
| Combustion efficiency | ≈ 92.7 % |
All values are illustrative. The applicable standard and project-specific boundary conditions remain authoritative.
Frequently asked questions
Why does the flue gas loss increase with excess air?
Every cubic meter of excess combustion air is heated up to flue gas temperature and leaves the plant unused. A high residual oxygen content in the flue gas indicates high excess air: the denominator (21 − O2) in the loss formula becomes small and the loss becomes large. Optimal is the lowest excess air at which complete combustion without CO formation is still ensured — for gas burners typically 2 to 3 vol% O2.
Does the flue gas loss also cover the water vapor in the flue gas (condensing effect)?
The classical Siegert approach refers to the net calorific value (lower heating value) and evaluates only the sensible heat of the flue gas. The latent heat of vaporization bound in the water vapor is considered non-recoverable and does not appear as a loss. For condensing boilers that cool the flue gas below the water dew point, calculated efficiencies above 100 % (referred to the net calorific value) can therefore result.
Which measured values are the most common source of error?
The flue gas temperature must be measured in the core stream at the boiler outlet — a measurement near the duct wall or downstream of air in-leakage yields values that are too low. Likewise, tramp air between boiler and measuring point distorts the O2 reading upwards and simulates a higher excess air. The reference for the temperature difference is the combustion air temperature, not a blanket 20 °C.
Is the flue gas loss equal to the total boiler loss?
No. The combustion efficiency (100 % minus flue gas loss) accounts neither for radiation and standby losses through the boiler surface nor for losses from unburned constituents (CO, soot) or blowdown. The boiler efficiency is therefore always below the combustion efficiency.