Definition
Ammonia injection grid
An AIG is the array of nozzles that distributes ammonia evenly into flue gas upstream of an SCR catalyst bed. Poor AIG performance is the leading cause of high ammonia slip.
- Subject
- SCR and SNCR
- Also known as
- AIG, ammonia injection grids
An ammonia injection grid, or AIG, is the array of lances and nozzles that injects ammonia reagent into flue gas upstream of an SCR catalyst. Its job is not simply to add ammonia; it must distribute the right ammonia-to-NOx ratio across the full duct cross-section before the gas reaches the catalyst face.
AIGs are used in utility boilers, HRSGs, industrial boilers, WtE plants and other DeNOx systems. The reagent may be anhydrous ammonia, aqueous ammonia or urea-derived ammonia, depending on plant design and local safety rules. Static mixers, turning vanes or sufficient duct length are often needed to finish mixing after injection.
Why distribution matters
If one part of the duct is under-dosed, NOx passes through untreated. If another part is over-dosed, ammonia slip rises. The result can be a plant that meets average NOx only by wasting reagent and creating downstream fouling. Local maldistribution also accelerates catalyst ageing because some areas do more chemical work than others.
Maintenance and tuning
AIG performance depends on nozzle condition, lance plugging, reagent pressure, vaporisation, temperature, gas velocity profile, upstream bends and catalyst blockage. Tuning normally uses grid testing, outlet NOx traverses, ammonia-slip measurement and damper or valve adjustments. Seasonal load changes can require retuning.
Acoustic-cleaning relevance
AIGs sit upstream of equipment that is sensitive to dust and sticky salts. Sonic horns do not tune ammonia distribution, but they can help keep catalyst faces, upstream screens and downstream hoppers freer of ash where deposit growth worsens maldistribution. Stable acoustic cleaning is one layer in controlling the fouling consequences of poor slip.
Design variables
An ammonia injection grid is designed around gas flow distribution, NOx profile, catalyst layout, residence time, mixing length, temperature, reagent concentration and acceptable ammonia slip. The grid may use lances, nozzles, orifices and control zones to shape ammonia distribution across the duct. A good average injection rate is not enough; the reagent must meet the NOx in the right places before the gas reaches the catalyst or SNCR reaction zone.
Common design inputs include duct velocity maps, turning-vane effects, stratification from burners or mills, boiler load range, ash loading and access for lance removal. Computational modelling and field tuning are often used together because ash deposits, damper positions and load changes can alter the real flow pattern after commissioning.
Failure modes
Poor distribution creates local under-injection and over-injection at the same time. Under-injected zones leave high NOx. Over-injected zones create ammonia slip, ammonium bisulphate, plume issues, odour and downstream fouling. Nozzles can plug with ash or reagent salts, lances can erode or crack, and control valves can drift. Temperature excursions can also move the reaction outside the intended window.
Maintenance and cleaning context
Inspection checks include nozzle condition, lance alignment, plugging, corrosion, leaks, control-valve response, reagent pressure, flow calibration and downstream deposit patterns. Acoustic cleaning can be relevant near the grid if dry ash deposits disturb mixing or block access ports, but it does not tune chemistry. The cleaning system should avoid damaging lances or driving deposits into nozzles. In SCR service, horn timing is usually assessed alongside catalyst pressure drop and air-heater fouling because ammonia slip and ash behaviour are linked.
Field evidence
Grid tuning should be revisited after catalyst replacement, burner changes, major duct repairs or persistent fouling. Deposit patterns downstream can reveal maldistribution that instruments miss. Cleaning systems near the grid should be checked so they do not mask a plugged lance or create false confidence in reagent mixing.
Acceptance evidence
AIG acceptance should include both mechanical and process checks. Mechanical checks confirm lance alignment, nozzle orientation, valve response, reagent pressure, purge flow, insulation condition and access for removal. Process checks compare NOx and ammonia profiles at several loads, not just a single average at full load. If the grid is segmented, each zone should be tunable and traceable to the duct area it affects. Fouling evidence downstream is part of the record because ash build-up can disturb the same profiles used for reagent tuning. Acoustic cleaning may support stable profiles by controlling removable ash, but the AIG still needs periodic retesting after catalyst work, burner changes or duct modifications.
Related terms
Explore the subject
Related terms
4 terms
- Selective Catalytic ReductionSCR reduces NOx by injecting ammonia upstream of a catalyst. Temperature, mixing, catalyst condition and ash control determine performance.
- 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 delta P rise and gas-flow maldistribution.
- Sonic hornA sonic horn is a pneumatic low-frequency sound emitter used to dislodge particulate fouling from boilers, ESPs, baghouses, ducts and silos while the plant stays online.
References