The "Invisible Killer" of Flue Gas Waste Heat Recovery: Dew Point Corrosion
In flue gas waste-heat recovery systems, when the heat transfer surface temperature falls below the flue gas acid dew point, SO₃ and water vapor in the flue gas condense to form a sulfuric acid film that strongly corrodes the metal surface. According to industry statistics, perforation, leakage, and scrapping of heat exchangers caused by dew-point corrosion account for over 40% of failures in waste-heat recovery systems, making it the primary factor affecting system reliability and service life.
I. Formation Mechanism of the Acid Dew Point
The flue gas acid dew point depends on the partial pressures of SO₃ and H₂O in the flue gas. The empirical formula for the dew point temperature is:
Tdp = 125 × S^(1/3) + 105 (°C)
where S is the converted sulfur content of the fuel (%). For example, coal with a sulfur content of 1.5% gives a converted sulfur content S≈0.15, so Tdp≈125×0.15^(1/3)+105≈125×0.531+105≈171℃.
Boiler exhaust gas temperature is typically 120~180℃, while the acid dew point is mostly 100~170℃, so the exhaust temperature falls right within the dangerous corrosion zone. Once the wall temperature is below the dew point temperature, sulfuric acid condensation begins immediately.
II. Key Factors Affecting Dew Point Corrosion
| Factor | Effect Direction | Control Recommendation |
|---|---|---|
| Fuel sulfur content | Higher sulfur → higher dew point → aggravated corrosion | Prefer low-sulfur fuel or desulfurization |
| Excess air coefficient | Higher excess air → increased SO₂→SO₃ conversion | Control at 1.1~1.2 |
| Flue gas moisture content | Higher moisture → more condensate → intensified corrosion | Control combustion conditions |
| Heat transfer surface wall temperature | Wall temp below dew point → immediate condensation corrosion | Wall temp > dew point + 15℃ |
| Fly ash content | More fly ash → erosion + deposition → localized corrosion | Appropriately increase flue gas flow velocity |
III. Protective Measures System
Layer 1: Material Protection
- For low-temperature-section heat exchange tubes, use ND steel (09CrCuSb): its corrosion resistance in sulfuric acid dew-point corrosion environments is 5–8 times that of carbon steel, offering the best cost-performance ratio
- For more severe conditions, use 2205 duplex stainless steel or 316L
- Use acid-resistant gaskets (PTFE-coated) at tube sheet joints
- Apply acid-resistant anti-corrosion coating (epoxy phenolic) on the inner shell wall
Layer 2: Process Protection
- Control the heat exchanger outlet flue gas temperature above acid dew point + 10~15℃ (e.g., if dew point is 145℃, exhaust temperature ≥160℃)
- Use a preheater bypass to raise the inlet flue gas temperature under low-temperature operating conditions
- Install a condensate collection and drainage system to avoid prolonged soaking by accumulated liquid
Layer 3: Operation and Maintenance
- Immediately blow-dry the heat transfer surface after each boiler shutdown to avoid wet-state shutdown corrosion
- Measure heat exchanger tube wall thickness every quarter, focusing on the low-temperature tube bundle
- Monitor exhaust temperature and inlet/outlet differential pressure changes, and investigate promptly when anomalies occur