SCR and SNCR
Ammonia injection grid
Also known as AIG, ammonia injection grids.
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.
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 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
Related terms
- 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.
- 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.
- Catalyst pluggageCatalyst pluggage is the physical blockage of SCR catalyst channels by large-particle ash, popcorn ash or ammonium-salt deposits. It causes ΔP rise and gas-flow maldistribution.
- 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.