---
title: "Ammonia injection grid (AIG)"
description: "An ammonia injection grid (AIG) is an array of injector nozzles that distributes ammonia (or vaporised aqueous-ammonia / urea) evenly across the flue-gas duct upstream of an SCR catalyst bed. The quality of the NH₃/NOx mixing at the catalyst inlet is the single biggest determinant of NOx reduction efficiency and ammonia slip: under-mixing leaves NOx-rich zones unreacted and causes locally over-stoichiometric ammonia in other zones."
canonical_url: "https://sylio.co/glossary/ammonia-injection-grid"
last_updated: "2026-06-28T02:29:36.821Z"
---

An **ammonia injection grid (AIG)** is an array of injector nozzles that distributes ammonia (or vaporised aqueous-ammonia / urea) evenly across the flue-gas duct upstream of an [SCR](/glossary/selective-catalytic-reduction) catalyst bed. The quality of the NH₃/NOx mixing at the catalyst inlet is the single biggest determinant of [NOx reduction efficiency](/glossary/nox-reduction-efficiency) and [ammonia slip](/glossary/ammonia-slip): under-mixing leaves NOx-rich zones unreacted *and* causes locally over-stoichiometric ammonia in other zones.

## Common failure modes

- **Nozzle plugging** — ash, ammonium-salt deposits or carbon block individual nozzles
- **Lance fouling** — deposits accumulate on lance bodies and disturb spray patterns
- **Erosion** — abrasive ash wears injector tips, distorting the spray pattern
- **Maldistribution** — uneven gas flow at the AIG inlet means even a perfect AIG delivers uneven mixing

## Sonic horns on the AIG deck

[Sonic horns](/glossary/sonic-horn) mounted near the AIG deck keep ash from accumulating on the injection lances, on the inlet duct walls and on the gas-distribution turning vanes upstream. Maintaining clean lances preserves the design spray pattern and the NH₃/NOx mixing quality on which the entire SCR depends.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Ammonia slip](/glossary/ammonia-slip)
- [Catalyst pluggage](/glossary/catalyst-pluggage)
- [Sonic horn](/glossary/sonic-horn)