Acid dew point

The acid dew point is the metal-surface temperature at which sulphuric acid starts to condense from flue gas that contains SO3 and water vapour. It is not the same as the water dew point. In coal, heavy-fuel-oil, biomass and waste-fired units, small changes in SO3 can move the sulphuric acid dew point by many deg C, so the risk is set by fuel sulphur, catalyst SO2 to SO3 conversion, moisture, excess air and local gas mixing.

In practical boiler work, the term is used for the cold end of the gas path: economiser outlets, rotary or tubular air heaters, ducts, expansion joints, stack liners and CEMS sample lines. A surface can be safe at full load and then become vulnerable during start-up, low-load operation, wet fuel firing or air-heater leakage, because gas temperature, air ingress and metal temperature do not move together.

Operating mechanism

SO2 formed in combustion is partly oxidised to SO3 in the furnace, across metal oxides in fly ash, and across SCR catalyst. When cooling flue gas reaches the acid dew-point window, SO3 hydrates and condenses as H2SO4 on the first sufficiently cold surface. The liquid film is highly corrosive and also makes dry ash sticky, so corrosion and fouling often appear together.

Design and maintenance implications

Cold-end design normally keeps metal temperature above the calculated dew point with margin. Operators use steam coil air heaters, hot-air recirculation, minimum load rules, sootblower sequencing and outlet temperature limits to protect the air heater. If temperatures are pushed too low to recover efficiency, the gain can be lost through basket plugging, corrosion, fan power and outages.

The SCR complication is ammonia slip. Slipped NH3 reacts with SO3 to form ammonium bisulphate, which deposits in a similar temperature range and can bind fly ash into tenacious air-heater fouling. Acoustic cleaning cannot change the acid dew point, but low-frequency horns can help keep dry ash and early deposits mobile before sticky condensates mature into hard blockage.

Plant checks and warning signs

Operators usually track acid-dew-point risk indirectly. Useful indicators include economiser outlet gas temperature, air-heater gas outlet temperature, cold-end metal temperature, sulphur in fuel, SCR catalyst age, SO3 test results, opacity trends, air-heater differential pressure and corrosion coupon results. A single bulk flue-gas temperature can be misleading because the first damage often occurs at cold corners, leakage paths, casing stiffeners, expansion joints and baskets that see colder metal than the average gas stream.

Common field symptoms are wet-looking ash at the air-heater cold end, rising fan amps, recurring basket plugging, acidic drain liquor, flaking stack liner corrosion, low pH in wash water and brittle deposits that become sticky after load changes. During outages, inspectors look for grooving, pitting under deposits, perforated air-heater elements, acid tracks around seals and localised wastage near areas of air ingress.

Acoustic cleaning decisions should be tied to this temperature map. Horns are best placed where they keep dry ash moving upstream of the condensation zone or prevent deposits from insulating metal and creating colder local surfaces. Once a deposit is acid-wetted, removal becomes a materials and washing problem rather than a sound-transmission problem, and aggressive online cleaning can expose already thinned metal. Good practice is to combine acoustic cleaning records with fuel, load and temperature history so repeated fouling can be separated from true dew-point corrosion.

Measurement context

Dew-point estimates should be treated as operating guidance, not exact boundaries. Correlations, SO3 tests and temperature readings all carry uncertainty, while metal temperature can differ from gas temperature. The safest review combines calculation with inspection evidence from the coldest exposed surfaces.

Operating note

Low-load operation deserves special attention because gas temperature, air leakage and metal temperature can diverge. A unit that avoids acid condensation at full load may cross the dew-point window during overnight turndown, start-up or wet-fuel firing. Those periods should be visible in cleaning and inspection records.

Field interpretation

Acid dew point is not a single fixed number for a plant. It moves with sulphur in fuel, SO3 formation, moisture, excess air, ammonia slip, metal temperature and local mixing. A duct average temperature can look acceptable while a cold corner, leaking expansion joint or uninsulated casing falls below the dew point and begins to corrode. Operators therefore compare gas temperature, metal temperature, fuel sulphur, air-heater cold-end condition, drain evidence and deposit chemistry. Acoustic cleaning can reduce insulating ash layers that create cold spots, but it cannot remove the acid-dew-point risk if the surface temperature remains too low. The durable fix is temperature margin, chemistry control and materials selection.