Boilers
Acid dew point
Also known as sulphuric acid dew point, SADT, SO3 dew point.
The acid dew point is the temperature at which sulphuric acid (H₂SO₄) begins to condense from flue gas containing SO₃ and water vapour. For typical coal-fired flue gas the acid dew point sits in the 120–160 °C range, depending on SO₃ concentration and moisture content. Higher SO₃ raises the dew point; in extreme cases it reaches 180 °C.
Why it matters
Operating any cold-end surface — air heater baskets, economiser tubes, ducting — below the acid dew point allows condensed sulphuric acid to attack the metal, causing cold-end corrosion. The dew point sets the practical floor on cold-end metal temperature.
SCR-related complication
Boilers with upstream SCR face a double challenge: the SCR catalyst converts a fraction of SO₂ to SO₃ (SO₂/SO₃ conversion), raising the dew point, and unreacted ammonia slip combines with SO₃ to form ammonium bisulphate that condenses and fouls cold-end surfaces in the same temperature window.
Operational implications
- Cold-end air-heater inlet temperature is normally controlled at least 10–15 °C above the calculated dew point
- Steam coil air heaters or hot-air recirculation raise inlet air temperature during low-load operation
- Periodic dew-point measurement campaigns confirm the calculated value
Related terms
Related terms
- Cold-end corrosion and dew-point corrosionCold-end corrosion is the attack on air-heater and economiser surfaces below the acid dew point, where SO3 condenses as sulphuric acid. The leading cold-end failure mechanism.
- Air heaterAn air heater (also air preheater, APH) recovers low-grade heat from flue gas to preheat combustion air. Cold-end fouling and corrosion are the dominant operational challenges.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.