DeNOx

DeNOx is the collective term for post-combustion NOx-reduction technologies on industrial flue gas. The two dominant options are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). Both rely on a reagent - ammonia or urea - that reacts with NOx to produce nitrogen and water.

Why DeNOx is mandatory

NOx is a regulated pollutant under the Industrial Emissions Directive (IED), MATS, EPA NSPS, TA Luft 2021 and most national emission codes. Limits for coal-fired power stations and large WtE plants are usually 100-200 mg/Nm3 on a 30-day average, with stricter site-specific BAT-AEL values from BREF revisions.

Choice of technology

Factor	Favours SCR	Favours SNCR
Reduction efficiency required	> 70%	30-60%
Plant size	Large	Small / medium
Capital available	Higher	Lower
Space available	More	Less
Catalyst cost tolerance	Yes	Avoid
Fuel chemistry	Predictable	Variable

Many plants run combined systems: SNCR provides bulk reduction, SCR polishes to meet permit limits.

Process chemistry

DeNOx systems reduce nitrogen oxides by reacting them with ammonia or urea-derived ammonia to form nitrogen and water. SNCR injects reagent directly into a hot furnace or flue-gas zone, so it depends on a narrow temperature window and good mixing. SCR passes the reagent and flue gas over catalyst, allowing higher removal efficiency at lower temperature.

The chemistry is simple in headline form but sensitive in practice. Too little reagent leaves high outlet NOx. Too much reagent creates ammonia slip. Poor mixing creates both at the same time: some gas lanes are under-treated while others carry excess ammonia downstream.

Operating and regulatory context

Permit limits are usually expressed as concentration over an averaging period, often corrected to reference oxygen and dry gas basis. Plants must meet both normal-load and low-load conditions, which is difficult because temperature, residence time and gas distribution change with load.

SCR systems add catalyst management, pressure drop, sulphur oxidation and ammonium-salt fouling to the operating burden. SNCR systems add injection-lance wear, reagent storage, furnace temperature mapping and slip control. Combined systems use SNCR for bulk reduction and SCR for polishing when strict limits require both cost control and high removal.

Acoustic-cleaning context

Sonic horns support DeNOx mainly in SCR systems by keeping catalyst faces, turning vanes and ash screens clear. Cleaner catalyst gives lower pressure drop, better reagent contact and less ammonia slip. Acoustic cleaning does not replace ammonia tuning or catalyst activity management, but it helps the reactor maintain the flow distribution assumed by those controls.

Field checks

DeNOx performance is tracked through inlet NOx, outlet NOx, ammonia or urea flow, oxygen, temperature, load, ammonia slip and catalyst or furnace condition. The control problem changes with the technology. SCR needs the catalyst in the right temperature window with open flow channels. SNCR needs reagent to mix with flue gas in a narrow temperature zone without hitting walls or creating excess slip.

Maintenance and compliance teams look at CEMS data, reagent skids, injection lances, dilution air, static mixers, catalyst layers, sootblowers and deposit patterns. Failure modes include plugged catalyst, poisoned catalyst, poor reagent distribution, low temperature, high slip, ammonium salt fouling and catalyst erosion. Acoustic cleaning is most relevant to SCR systems where ash or ammonium salts mask or plug the catalyst face. It supports DeNOx reliability by preserving access to active catalyst sites, but it does not solve incorrect reagent chemistry or a furnace temperature window that is outside design.