Engineering task and calculation objective
The RGAS module calculates the thermophysical properties of flue gases from the combustion of solid fuels, fuel oils or natural gases. Using the methods of statistical combustion calculation, the flue gas composition is first determined from the net calorific value Hu and the measured CO2 content; building on this, the module delivers the thermophysical properties of the flue gas — density, gas constant, molar mass, specific heat capacity, enthalpy, thermal conductivity, viscosity, thermal diffusivity and Prandtl number. The technical basis are the standard German references by Brandt (Wärmeübertragung in Dampferzeugern und Wärmetauschern; Brennstoffe und Verbrennungsrechnung) and Günter (Verbrennung und Feuerung).
Being able to calculate flue gas properties is the key to the thermal design and rating of steam generators, waste heat boilers, economizers, air preheaters and flue gas heat exchangers: heat transfer coefficients and pressure drops on the flue gas side depend directly on cp, thermal conductivity, viscosity and Prandtl number at the respective flue gas temperature.
The approach via Hu and measured CO2 content is practical: both quantities are available anyway in power plant and boiler operation, so the flue gas composition can be reconstructed even without a complete fuel analysis.
Standard and calculation basis: Brandt: Wärmeübertragung in Dampferzeugern und Wärmetauschern; Brandt: Brennstoffe und Verbrennungsrechnung; Günter: Verbrennung und Feuerung
Calculation workflow
- Specify fuel type and characteristics: The user selects whether the flue gas originates from solid fuel, fuel oil or natural gas; the characteristic inputs are the net calorific value Hu and the measured CO2 content of the flue gas.
- Reconstruct the flue gas composition: Using the statistical correlations of the combustion calculation after Brandt, the excess air ratio and composition are inferred from Hu and CO2 content: fractions of CO2, H2O, N2 and O2 in the wet flue gas.
- Form the mixture properties: From the composition follow the molar mass, gas constant and standard density of the flue gas as mole-fraction averages of the components.
- Temperature-dependent properties: For the desired flue gas temperature, specific heat capacity, enthalpy, thermal conductivity and viscosity are mixed from the component properties; from these follow the thermal diffusivity, thermal expansion coefficient and Prandtl number for the heat transfer calculation.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Lower calorific value of the fuel | Hu | J/kg |
| CO2-volume fraction of dry flue gas | yCO2T | -- |
| O2-volume fraction of dry flue gas | yO2T | -- |
| Air factor | n | - |
| Relative atmospheric humidity | (0 ≤ φ ≤ 1) φ | -- |
| Total pressure | p | Pa |
| Saturation vapour pressure | ps | Pa |
| Absolute atmospheric humidity kg H2O/kg air | xH2OL | kg/kg |
| Flue gas mass kg flue gas/kg fuel | μG | kg/kg |
| Temperature of the flue gas | ϑ | °C |
| Actual specific heat capacity | cpG | J/(kg·K) |
| Specific enthalpy | h | J/kg |
| Standard density (0°C, 1.01325 bar) | ρN | kg/m³ |
| Dynamic viscosity | η | mPa·s |
| Thermal conductivity | λ | W/(m·K) |
| Prandtl number | Pr | - |
| Specific gas constant | R | J/(kg·K) |
| Molar mass of the flue gas | M | kg/kmol |
| Density (actual) | ρ | kg/m³ |
| Kinematic viscosity | ν | m²/s |
| Thermal diffusivity | a | m²/s |
| Specific volume | v | m³/kg |
| Content wet | CO2 y(CO2) | Vol-% |
| Content wet | H2O y(H2O) | Vol-% |
Calculation options
Flue gas from
Coal · Fuel oils · Natural gas · Known composition
Frequently asked questions
Why are Hu and the CO2 content sufficient to determine the flue gas composition?
For the usual fuel groups, close statistical correlations exist between calorific value, minimum air requirement and minimum flue gas quantity. The measured CO2 content additionally fixes the excess air ratio, since excess air dilutes the flue gas and lowers the CO2 content below the fuel's maximum value CO2,max. This effectively determines the composition of the flue gas — without a complete ultimate analysis.
For which fuels is the statistical approach suitable — and where is it not?
It is suitable for standard fuels whose composition matches the underlying statistics: hard coals and lignites, light and heavy fuel oil, natural gases. For special and refuse-derived fuels, biomass with strongly varying water and ash content, or hydrogen-rich process gases, the correlations can deviate significantly; there, the composition should be calculated directly from the fuel analysis, for instance with a gas mixture property module.
Is the water vapor fraction in the flue gas taken into account?
Yes, the properties apply to the wet flue gas including the water vapor from fuel hydrogen, fuel moisture and air humidity. This is important because the water vapor fraction noticeably raises cp and thermal conductivity. Below the dew point (partial condensation), however, the single-phase gas properties no longer apply.
Why must the CO2 content match the correct reference basis?
Flue gas analyzers usually measure on the dried gas. If a CO2 value measured on the dry gas is interpreted as a wet value (or vice versa), the result is a wrong excess air ratio and hence a wrong composition. Before entering the value, it must therefore be clarified which basis the measurement refers to.