Selective Catalytic Reduction (SCR) - catalytic NOx control for combustion plant

Selective Catalytic Reduction (SCR) is a NOx control process that injects ammonia, aqueous ammonia, or urea-derived ammonia into flue gas upstream of a catalyst. On the catalyst surface, NOx reacts with ammonia and oxygen to form nitrogen and water.

SCR is used on power boilers, waste-to-energy units, cement kilns, refinery heaters, gas turbines, marine engines, and many industrial combustion plants. It can achieve high NOx reduction when temperature, mixing, catalyst activity, and flow distribution are controlled.

Main system elements

An SCR system includes reagent storage and forwarding, vaporisation or urea conversion where required, an ammonia injection grid, static mixers or duct length for mixing, the reactor, catalyst modules, seals, access doors, sootblowers or acoustic cleaners, and downstream monitoring. The catalyst may be honeycomb, plate, or corrugated geometry.

The reactor temperature window is critical. Too cold, reaction rate falls and ammonium salts can form. Too hot, catalyst can sinter or unwanted reactions can increase. Poor ammonia mixing creates zones of low NOx removal and zones of ammonia slip.

Failure modes

Common problems include catalyst pluggage, catalyst masking, erosion, poisoning, thermal deactivation, ammonia slip, reagent distribution imbalance, ash accumulation, and rising pressure drop. High-dust SCR systems are especially exposed because they sit upstream of the particulate collector.

Operators monitor inlet and outlet NOx, ammonia slip, pressure drop, gas temperature, catalyst sample activity, flow distribution, and ash loading. A rise in pressure drop can indicate dust pluggage even while chemical activity remains acceptable. Rising ammonia slip can indicate poor mixing, low activity, blocked catalyst area, or over-injection.

Acoustic cleaning context

Sonic horns are widely used to keep SCR catalyst faces and channels free from dry ash. They are most effective when deposits are loose and removed before they bridge or harden. Horns should be placed to cover the catalyst face without eroding the catalyst or creating bypass paths. They complement, rather than replace, correct ammonia injection, flow modelling, catalyst management, and fuel ash control.

Design variables

SCR performance depends on catalyst volume, pitch, gas temperature, space velocity, ammonia-to-NOx ratio, mixing quality, ash loading, sulphur chemistry, and allowable pressure drop. The reactor may be installed in a high-dust position before the air heater, a low-dust position after particulate control, or a tail-end position with reheating. Each arrangement changes the fouling risk, catalyst life, energy penalty, and maintenance access.

The ammonia injection grid and static mixer are as important as the catalyst itself. Poor distribution creates zones with high ammonia slip and zones with poor NOx reduction. Operators therefore use catalyst inlet temperature, NOx profile, ammonia slip, pressure drop, economiser or air-heater fouling, and stack emissions to understand whether the reactor is chemically active and hydraulically open. A single average outlet reading can hide maldistribution across layers.

Fouling, cleaning and safety context

High-dust SCRs are exposed to fly ash, popcorn ash, ammonium bisulphate, catalyst masking, erosion, and pluggage. Low-load operation can drop temperature into a range where ammonium salts become sticky. Fuels with high alkali, arsenic, phosphorus, or heavy metals can poison catalyst rather than merely cover it. These failure modes require different responses: sootblowing or acoustic cleaning for loose ash, temperature and ammonia control for salt deposition, and regeneration or replacement for poisoned catalyst.

Maintenance checks include reactor seals, layer pressure drop, sample coupons, ammonia grid nozzles, dilution air, access doors, turning vanes, expansion joints, and hopper discharge. Acoustic cleaners are relevant when they prevent loose ash from masking plate, honeycomb, or corrugated catalyst faces, especially before deposits compact. They should be coordinated with sootblowers and hoppers so dislodged ash leaves the reactor instead of settling on the next layer. Safety and regulatory reviews cover ammonia storage, leak detection, confined-space entry, hot surfaces, and compliance with permitted NOx and ammonia-slip limits.

Commissioning and diagnosis

SCR diagnosis usually starts with a profile, not an average. Engineers compare inlet and outlet NOx across the duct, ammonia slip, temperature, oxygen, pressure drop by layer, and ash loading. A reactor that meets the stack limit may still be ageing unevenly if one side receives more ammonia or more fly ash. Conversely, a poor outlet number may come from a failed ammonia valve, a blocked static mixer, or a bypass seal rather than exhausted catalyst.

Cleaning systems should be commissioned against those same profiles. Acoustic cleaners are expected to hold pressure drop and face cleanliness, not to change the chemical activity of the catalyst. The acceptance evidence should include before-and-after visual inspection, layer pressure drop, ash removed to hoppers, and any change in sootblower frequency. Operators also need alarm limits for hopper high level, blocked drains, and high ammonia slip, because dislodged ash and excess reagent can create downstream fouling in air heaters, fabric filters, or wet scrubbers.

For retrofit projects, space and access often decide the cleaning strategy. Catalyst layers need room for lances, horns, inspection doors, lifting beams, and safe module removal. A design that leaves no route to inspect hoppers or seals may look acceptable in pressure-drop calculations but become unreliable after the first ash upset.

Lifecycle management

SCR performance changes over years, not only from one tuning event to the next. Catalyst activity declines through poisoning, masking, thermal sintering, erosion and plugging. Operators manage that decline with layer rotation, replacement planning, catalyst sampling, pressure-drop trending, ammonia tuning and inspection of seals or bypass paths. Cleaning systems are part of that lifecycle when ash masking or pluggage is a known ageing mechanism. The acceptance question is whether the system preserves active catalyst face area and pressure-drop margin between outages. If NOx reduction falls because catalyst chemistry is exhausted or poisoned, acoustic cleaning cannot restore activity; it can only protect accessible surface and flow paths from removable deposits.

Selective Catalytic Reduction