Engineering task and calculation objective
The TSO3 module calculates the acid dew point of a flue gas containing SO3 and H2O. Sulfur trioxide combines with water vapor to form sulfuric acid, which condenses as highly concentrated H2SO4 well above the water dew point — typically at 110 to 160 °C. The calculation is based on the correlation published by R. Pierce in "Chemical Engineering" (April 1977), which determines the dew point from the partial pressures of water vapor and SO3.
Being able to calculate the acid dew point is decisive wherever sulfur-bearing fuels are fired or SO3-laden process gases are handled: in the design of economizers, air preheaters, flue gas heat exchangers, stacks, and flue gas ducts. If the wall temperature falls below the acid dew point, low-temperature corrosion of steel and fouling by acid soot occur — one of the most common damage mechanisms on the cold end of boiler plants.
From the barometric pressure, the SO3 content, and the H2O content of the flue gas, the module determines the partial pressures of both components and from them the acid dew point temperature — the lower limit for permissible wall and flue gas temperatures in heat recovery.
Standard and calculation basis: R. Pierce: "Chemical Engineering", April 2 / 1977
Calculation workflow
- Record the gas composition: The inputs are the barometric pressure together with the SO3 content (as volume fraction or ppm) and the H2O content of the flue gas. The SO3 content follows from the sulfur content of the fuel and the SO2-to-SO3 conversion rate, which is typically a few percent depending on the firing system and catalytic effects.
- Form the partial pressures: From the volume fractions and the total pressure, the H2O vapor pressure and the SO3 vapor pressure in the flue gas are calculated — the two input quantities of the dew point correlation.
- Calculate the acid dew point: Using the Pierce correlation (corrected form of the Verhoff-Banchero relation), the acid dew point temperature is determined from the two partial pressures. The dependence is logarithmic: even a few ppm of SO3 raise the dew point by more than 100 K above the pure water dew point.
- Evaluate the result: The calculated acid dew point temperature is compared with the coldest wall temperatures along the flue gas path. For safe operation, heating surface and stack wall temperatures should stay well above the dew point, or corrosion-resistant materials or coatings should be provided.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| ⇒ Acid dew point temperature | tSTP | °C |
| Barometer pressure | pB | hPa |
| SO3 vapour pressure | pSO3 | bar |
| H2O vapour pressure | pH2O | bar |
| SO3 fraction of flue gas | SO3 | mg/m³ |
| SO3 fraction of flue gas | SO3 | ppm |
| H2O fraction of flue gas | H2O | Vol-% |
Worked example
For the flue gas of an oil-fired boiler plant, the acid dew point temperature is to be calculated — a worked example of an acid dew point calculation. The flue gas contains 10 vol% water vapor and 10 ppm (by volume) SO3; the barometric pressure is 1.013 bar (760 mmHg).
Given values
| Barometric pressure p | 760 mmHg (1.013 bar) |
| H2O content | 10 vol% |
| SO3 content | 10 ppm (vol.) |
Solution
Partial pressures of the components
pH2O = 0.10 · 760 mmHg = 76 mmHg
pSO3 = 10 · 10−6 · 760 mmHg = 0.0076 mmHg
Dew point correlation after Pierce (Verhoff-Banchero, corrected)
1000/TS = 2.276 − 0.02943 · ln pH2O − 0.0858 · ln pSO3 + 0.0062 · ln pH2O · ln pSO3 (p in mmHg, T in K)
With ln 76 = 4.3307 and ln 0.0076 = −4.8796:
1000/TS = 2.276 − 0.1275 + 0.4187 − 0.1310 = 2.4362 1/K
Acid dew point temperature
TS = 1000 / 2.4362 = 410.5 K = 137.3 °C
Although the pure water dew point at 10 vol% H2O would be only about 46 °C, a mere 10 ppm of SO3 raise the dew point to about 137 °C. Wall temperatures along the flue gas path should stay above this value with a safety margin.
Result
| Acid dew point temperature TS | ≈ 137 °C |
All values are illustrative. The applicable standard and project-specific boundary conditions remain authoritative.
Frequently asked questions
Why is the acid dew point so far above the water dew point?
Sulfuric acid and water form a strongly non-ideal mixture with a pronounced vapor pressure minimum. Even traces of SO3 (a few ppm) are enough for highly concentrated sulfuric acid (70 to 85 wt%) to condense well above 100 °C, whereas pure water vapor in the flue gas would not condense until 40 to 60 °C. That is why the governing limit for cooling sulfur-bearing flue gases is the acid dew point, not the water dew point.
How do I obtain the SO3 content if only the SO2 content is known?
SO3 is formed by oxidation of a small fraction of the SO2. As a guide value, conversion rates of about 1 to 5% of the SO2 are assumed for oil and coal firing; catalytically active surfaces (vanadium in oil ashes, SCR catalysts) can increase the fraction. Because of the logarithmic dependence, the uncertainty of the conversion rate affects the dew point only in a damped way, but it should nevertheless be assumed conservatively (high).
For which range is the Pierce correlation valid?
The correlation was established for typical combustion flue gases at atmospheric pressure, i.e. for H2O contents from a few up to about 20 vol% and SO3 contents from below 1 to a few hundred ppm. Outside this range — for example in highly pressurized processes or at extreme acid concentrations — it should be understood only as an estimate. It yields the dew point, not the condensation rate or the acid concentration of the resulting condensate.
Is it sufficient to keep the mean flue gas temperature above the dew point?
No. What governs is the coldest wall temperature, not the gas temperature. At uninsulated duct walls, cold-air inlet zones of air preheaters, or air in-leakage points, the wall temperature can locally lie well below the gas temperature, so that acid condenses there despite a sufficiently high flue gas temperature.