Catalyst poisoning

Catalyst poisoning is the chemical deactivation of SCR catalyst active sites by trace species in the flue gas. Unlike masking (physical blanket) or pluggage (channel blockage), poisoning is a chemical process that binds molecules to the catalyst's vanadium, tungsten or titanium active centres. Cleaning cannot reverse it; the affected layer must be regenerated off-site or replaced.

Common poisons

Mitigation

• Fuel selection / blending to control fuel-bound poison content
• Guard layers (sacrificial top catalyst layers protecting layers below)
• Catalyst formulation tuned to expected poisons (e.g. alkali-resistant for biomass)
• Catalyst regeneration vs replacement campaigns to extend catalyst life

How it differs from fouling

Poisoning is a chemistry problem, so pressure drop may remain normal. A layer can look clean and still have poor activity in a laboratory test. This is why visual inspection is not enough for catalyst life assessment. Samples are tested for relative activity, surface area, chemical contamination, sulphur conversion and mechanical strength.

Some poisons are concentrated on the inlet face, especially when carried as aerosols or fine ash. Others penetrate into the catalyst wall with time. Once active sites are neutralised, online cleaning cannot restore them. Cleaning can remove ash that carries the poison and can slow further exposure, but it cannot reverse a completed chemical reaction.

Sources and risk factors

Fuel changes are the main practical trigger. Co-firing biomass, refuse-derived fuel, sewage sludge or high-arsenic coal can shift trace chemistry faster than the original SCR design assumed. Upstream sorbent injection can also change catalyst exposure; for example, calcium-rich dust may mask acid sites while also adding physical fouling.

Temperature matters. Operation below the intended range can promote ammonium-salt deposition, which holds contaminants against the catalyst surface. Operation above the intended range can accelerate thermal sintering and make the catalyst less tolerant of poisons. Repeated low-load cycling therefore affects both fouling and poisoning risk.

Management options

The first defence is fuel and additive control. The second is catalyst management: periodic sampling, spare-layer planning, activity forecasting and decisions about regeneration versus replacement. Guard layers can absorb some exposure but become consumables. If poisoning is severe and irreversible, replacement is the only reliable way to recover permit margin.

Acoustic cleaning still has a role. By keeping the catalyst face free of ash, sonic horns reduce the residence time of poison-bearing dust on the surface and help operators distinguish chemical deactivation from masking. They should be presented as life-extension support, not as a cure for poisoned catalyst.

Field checks

Poisoning is usually diagnosed by performance testing rather than by appearance. A poisoned catalyst may look physically open, with normal pressure drop, while NOx reduction falls or ammonia slip rises. Plants confirm the mechanism by sending cores or elements for laboratory activity testing, chemical analysis and sometimes surface-area measurement. The result is compared with operating history, fuel changes, sorbent use and upstream process upsets.

The main maintenance decision is whether the loss is reversible. Alkali metal contamination, arsenic, phosphorus and some heavy metals can permanently reduce active-site availability. Surface masking by fly ash or ammonium salts may look similar in operation but can be improved by cleaning if the active material is still healthy. Acoustic cleaning helps only with the mechanical deposit part of the problem. It cannot remove a chemical poison from the catalyst structure, but it can prevent operators from compensating for masked catalyst by over-injecting ammonia and creating additional ammonium salt deposits.

Related terms

• Selective Catalytic Reduction (SCR)
• Catalyst masking
• Catalyst regeneration vs replacement