Engineering task and calculation objective
The RG module provides flue gas data for solid fuels, fuel oils and natural gases according to the Energietechnische Arbeitsmappe (14th edition), a standard German energy engineering reference. From fuel-specific characteristics such as the calorific value and the combustion parameters typical of each fuel group, it delivers the quantities needed for combustion calculations: air and flue gas quantities, flue gas composition including CO2 and water vapor content, density at standard conditions, and the saturation or dew point range of the wet flue gas.
Being able to calculate flue gas data is a prerequisite for the design of steam generators, waste heat boilers, air preheaters, flue gas ducts, induced-draft fans and stacks: without reliable flue gas quantity and density, neither heat transfer surfaces nor pressure drops nor the stack draft can be determined. For this purpose, the Arbeitsmappe uses proven statistical correlations between calorific value and combustion parameters, which deliver good accuracy for the usual standard fuels — coals, light and heavy fuel oil, natural gases — without requiring a complete ultimate analysis.
The module works closely with the flue gas property module (RGAS) and the stack and fan modules, to which composition and density can be passed directly.
Standard and calculation basis: Energietechnische Arbeitsmappe 14. Auflage: 1995
Calculation workflow
- Select the fuel: The fuel is assigned to the category solid fuel, fuel oil or natural gas and characterized by its net calorific value. For each fuel group, the Arbeitsmappe provides its own statistical combustion parameters.
- Air requirement and excess air ratio: The stoichiometric minimum air requirement follows from the calorific value; with the chosen excess air ratio, the actually supplied combustion air quantity is obtained, optionally dry or wet.
- Flue gas quantity and composition: From fuel and air quantity, the specific flue gas quantity as well as the fractions of CO2, water vapor, nitrogen and residual oxygen are determined — separately for dry and wet flue gas.
- Density and saturation state: For the determined composition, the standard density of the flue gas and, via the water vapor partial pressure, the saturation state or water dew point are calculated — decisive for corrosion assessments on cold heating surfaces and in the stack.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Calorific value of the oil | Hu | J/kg |
| Analysed CO2 molar fraction (dry) | YCO2T | – |
| Air ratio | n | – |
| Temperature of the combustion air | TL | °C |
| Saturation pressure at TL | Ps | Pa |
| Total pressure | P | Pa |
| Relative humidity at TL | phi | % |
| Flue gas volume at stoichiometric combustion per kg fuel oil | VGOT | m³/kg |
| CO2-volume per kg fuel oil | VCO2 | m³/kg |
| Dry content of oxygen | YO2T | – |
| max. CO2 content of the dry flue gas | Y^CO2T | – |
| absolute humidity of the combustion air | xH2OL | kg/kg |
| weight of dry combustion air | μLT | kg/kg |
| weight of combustion air | μL | kg/kg |
| volume of dry combustion air | VLT | m³/kg |
| weight of dry combustion air | μLT μLTN | kg/m³ |
| weight of combustion air | μL μLN | kg/m³ |
| volume of dry combustion air | VLT VLTN | m³/m³ |
| weight of dry flue gas | μGB | kg/kg |
| weight of flue gas | μG | kg/kg |
| volume of dry flue gas | VGB | m³/kg |
| volume of flue gas | VG | m³/kg |
| weight of dry flue gas | μGB μGBN | kg/m³ |
| weight of flue gas | μG μGN | kg/m³ |
Calculation options
Option
Solid fuels · Heating oil · Natural gas
Frequently asked questions
How accurate are the statistical combustion parameters compared with a calculation from the ultimate analysis?
For standard fuels (hard coal, lignite, light/heavy fuel oil, natural gas), minimum air and flue gas quantities correlate closely with the net calorific value; the statistical approximations of the Arbeitsmappe then typically lie within a few percent of the exact combustion calculation. For special fuels — refuse-derived fuels, biomass with high water or ash content, process gases — the calculation should instead be based on the complete ultimate analysis.
What does the water dew point of the flue gas mean in practice?
If the wall or flue gas temperature falls below the water vapor dew point, water condenses on heating surfaces and in the stack. For sulfur-containing fuels, the acid dew point caused by SO3/sulfuric acid lies considerably higher still than the pure water dew point. The cold-end temperature of air preheaters and economizers is therefore deliberately kept above these dew points, unless a corrosion-resistant condensing mode of operation is intended.
Why is a distinction made between dry and wet flue gas?
Measuring instruments for CO2 and O2 usually operate on the dried gas, while heat transfer and flow calculations require the real wet flue gas quantity. The two reference states differ considerably depending on the hydrogen and water content of the fuel — for natural gas the water vapor fraction in the wet flue gas is particularly large. Mixing up the reference basis is one of the most common sources of error in combustion calculations.
What role does the excess air ratio play?
The excess air ratio (lambda) determines the residual oxygen and CO2 contents as well as the flue gas quantity: more excess air dilutes the flue gas, lowers the CO2 content and increases the stack loss. Typical values range from just above 1 for gas firing to about 1.2 to 1.4 for pulverized coal and grate firing.