SCR Reducing Agent Systems: Anhydrous Ammonia vs Urea

Part of the SCR DeNOx: The Complete Guide — this article is one of the detailed pages in the guide.

Direct answer: SCR needs NH₃ as the reducing agent. It can be supplied as anhydrous ammonia, aqueous ammonia, or urea (hydrolyzed or pyrolyzed to NH₃ on site). Anhydrous ammonia is cheapest per kilogram of NH₃ but carries storage and safety burdens; urea is safer to handle and increasingly required in urban sites, at the cost of conversion energy and response lag.

The three supply routes

Route	NH₃ content	Safety profile	Typical energy input
Anhydrous ammonia	100%	Toxic gas, pressurized storage, regulated	Low (vaporization only)
Aqueous ammonia (19–25%)	19–25%	Corrosive liquid, lower vapor hazard	Moderate (vaporization)
Urea hydrolysis / pyrolysis	56% (as urea) → NH₃ on site	Solid/water solution, benign handling	High (hydrolysis heat)

Anhydrous ammonia

Advantages — lowest unit cost, instant response, no conversion equipment.
Disadvantages — toxic-release hazard, site permitting, inspection regimes, and transport logistics.
Best for — large power plants with established ammonia infrastructure and rural siting.

Urea systems

Urea must be converted to NH₃ before injection. Two routes exist:

Hydrolysis — urea solution (40–50%) is heated under pressure to produce NH₃ gas. Reliable, lower corrosion, but needs ~20–40 minutes to respond to load changes.
Pyrolysis — urea solution sprayed into a hot gas stream decomposes to NH₃ + CO₂. Faster response than hydrolysis, but requires hot air and risks deposit formation if temperatures drift.

Both routes add energy cost and response lag; the AIG control loop must compensate.

Aqueous ammonia

Middle ground: no urea conversion step, lower hazard than anhydrous. Suits mid-size installations, district heating and industrial boilers in urban areas.

Selection criteria

Safety and permitting — urban siting often rules out anhydrous ammonia.
NOx load and variability — high, fast-changing NOx favors ammonia's instant response.
Energy cost — urea conversion consumes steam or electricity every hour of operation.
Footprint — ammonia storage needs a dedicated, separated area.
Logistics — delivery frequency and supplier proximity for each reagent.

Manufacturer perspective

The reagent system and the catalyst system are designed together: ammonia distribution quality at the reactor inlet determines how much of the installed catalyst volume is actually used. We recommend selecting the reagent route on total lifecycle cost — reagent price plus conversion energy plus safety compliance — not on reagent price alone.

← Back to the SCR DeNOx: The Complete Guide