Glossary
SCR and SNCR
Ammonium bisulphate
Also known as ABS, ammonium bisulfate, ammonium sulphate, NH4HSO4.
Ammonium bisulphate (NH₄HSO₄, ABS) — sometimes written ammonium bisulfate in US technical literature — is a sticky, low-melting deposit formed when slipped ammonia reacts with SO₃ in cooling flue gas. ABS condenses between roughly 150 °C and 250 °C, coating the cold end of any air heater downstream of an SCR.
Why ABS is the most-feared cold-end deposit
ABS is uniquely problematic because it is:
- Sticky — bonds tenaciously to air-heater baskets and economiser tubes
- Hygroscopic — picks up moisture and accelerates cold-end corrosion
- Hard to remove — resists steam sootblowing once consolidated
- Self-reinforcing — coated surfaces trap more ash, accelerating fouling
- Concentrated in a narrow temperature band — predictably plugs the same air-heater rows
Mitigation
- Minimise ammonia slip at the SCR (the single biggest lever)
- Manage SO₃ formation — fuel sulphur control, catalyst formulation
- Avoid the dew-point window — keep cold-end gas temperature above the formation band
- Sonic horns on the cold end — continuous cleaning prevents ABS from consolidating before periodic water-washing
- Water-washing campaigns — periodic offline washes restore air-heater performance
Related terms
Related terms
- Ammonia slipAmmonia slip is unreacted ammonia leaving the DeNOx system in the flue gas. It is regulated, expensive in lost reagent, and causes ammonium-bisulphate fouling downstream.
- Selective Catalytic ReductionSCR is the dominant NOx-control technology on industrial combustion plant. Ammonia is injected upstream of a catalyst that converts NOx to nitrogen and water.
- 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.
- 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.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.