Ammonia Slip (from SCR/SNCR) — properties & control

Context: Post-combustion NOₓ control Species: NH₃ in flue gas Typical range: < 2–10 ppm (target), higher if mal-tuned
WARNING — irritating/basic gas; forms ammonium salts (NH₄HSO₄/NH₄₂SO₄) causing fouling & visible plume. Capture with acid scrubbing and high-efficiency demisting; tune SCR/SNCR to cut slip at source.
Impact
Odour, plume visibility (NH₄HSO₄), corrosion/fouling
ATEX
Typically not near LEL in stacks
Solubility
High (absorbs readily)
Monitoring
In-situ TDL/FTIR; extractive analysers
Toxicity
irritant (ppm-levels)
Flammability
not near LEL
Chronic risk
low–moderate
Environmental
plume/PM, deposition
AspectDetail
DefinitionUnreacted NH₃ passing SCR/SNCR due to reagent excess, poor mixing, temp window miss, or catalyst ageing.
ConsequencesFormation of NH₄HSO₄/NH₄₂SO₄ → fouling of air heaters/ducts; odour complaints; visible “blue” plume.
Where seenPower/biomass/waste boilers, glass, refinery/chem plants with SCR/SNCR.

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