---
title: "Ammonium bisulphate (ABS)"
description: "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."
canonical_url: "https://sylio.co/glossary/ammonium-bisulphate"
last_updated: "2026-06-28T02:29:36.646Z"
---

**Ammonium bisulphate (NH₄HSO₄, ABS)** — sometimes written *ammonium bisulfate* in US technical literature — is a sticky, low-melting deposit formed when [slipped ammonia](/glossary/ammonia-slip) reacts with SO₃ in cooling flue gas. ABS condenses between roughly 150 °C and 250 °C, coating the cold end of any [air heater](/glossary/air-heater) downstream of an [SCR](/glossary/selective-catalytic-reduction).

## 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](/glossary/cold-end-corrosion-dew-point-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

- [Ammonia slip](/glossary/ammonia-slip)
- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Air heater](/glossary/air-heater)
- [Cold-end corrosion / dew-point corrosion](/glossary/cold-end-corrosion-dew-point-corrosion)
- [Sonic horn](/glossary/sonic-horn)