---
title: "Ammonia slip"
description: "Ammonia slip is the concentration of unreacted ammonia (NH₃) in the flue gas leaving an SCR or SNCR system. It is the single most important operational KPI after NOx reduction itself: slip is regulated (typically capped at 2–10 ppm in permits), represents wasted reagent, and drives downstream fouling."
canonical_url: "https://sylio.co/glossary/ammonia-slip"
last_updated: "2026-06-28T02:29:36.743Z"
---

**Ammonia slip** is the concentration of unreacted ammonia (NH₃) in the flue gas leaving an [SCR](/glossary/selective-catalytic-reduction) or [SNCR](/glossary/selective-non-catalytic-reduction) system. It is the single most important operational KPI after NOx reduction itself: slip is regulated (typically capped at 2–10 ppm in permits), represents wasted reagent, and drives downstream fouling.

## Causes of high ammonia slip

- **Poor NH₃/NOx mixing** at the [AIG](/glossary/ammonia-injection-grid)
- **Catalyst masking** or [pluggage](/glossary/catalyst-pluggage) reducing active surface area
- **Catalyst age and de-activation** towards end of life
- **Operating temperature outside the catalyst window**
- **Over-injection of ammonia** to compensate for falling NOx-reduction efficiency

## Downstream consequences

Slipped ammonia combines with SO₃ in cooling flue gas to form [ammonium bisulphate (ABS)](/glossary/ammonium-bisulphate), a sticky low-melting deposit that fouls [air heaters](/glossary/air-heater), [economisers](/glossary/economiser) and downstream catalysts and filters. Excessive slip can therefore destroy the cold end of a boiler within months.

## Sonic horns and slip reduction

[Sonic horns](/glossary/sonic-horn) reduce slip indirectly by keeping the catalyst face clear of [masking](/glossary/catalyst-masking) deposits, which preserves active surface area, which lets the catalyst convert ammonia to nitrogen instead of letting it slip. They also keep the AIG decks clean, preserving the designed spray pattern.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Selective Non-Catalytic Reduction (SNCR)](/glossary/selective-non-catalytic-reduction)
- [Ammonia injection grid](/glossary/ammonia-injection-grid)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)
- [Catalyst masking](/glossary/catalyst-masking)