SCR Catalyst Poisoning: Arsenic, Alkali Metals and SO₂ Effects

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

Direct answer: SCR catalysts lose activity mainly through three mechanisms: arsenic deposition (gas-phase As₂O₃ plugging pores), alkali metal neutralization of acid sites (K, Na from fly ash), and SO₂/SO₃ effects including ammonium bisulfate fouling. Each mechanism has different prevention and recovery options.

Arsenic poisoning

As₂O₃ vapor in coal-derived flue gas condenses inside catalyst pores and physically blocks them. Poisoning is progressive and mostly irreversible by washing.

Detection: declining NOx conversion at constant NH₃ slip, rising pressure drop.
Prevention: high-arsenic coals need higher catalyst volume margins or arsenic-tolerant formulations.
Recovery: limited; regeneration can partially restore surface, replacement is often the final step.

Alkali metal poisoning

K and Na in fly ash neutralize the acidic sites that adsorb ammonia — a chemical deactivation, not a physical blockage.

Sources: biomass co-firing, cement kiln dust, some coals.
Effect: faster deactivation than arsenic at equal concentration.
Prevention: dust-tolerant structures (large pitch), formulation adjustments, positioning after dust removal where feasible.

SO₂ and SO₃ effects

The catalyst necessarily oxidizes a small fraction of SO₂ to SO₃ (we document ≤1% as the target in our product data, subject to verification).

SO₃ + NH₃ + H₂O forms ammonium bisulfate below ~280–320°C, fouling the catalyst and downstream equipment.
SO₃ + H₂O forms sulfuric acid mist — a corrosion and plume opacity issue.
Control: keep the operating temperature above the bisulfate dew point, limit SO₃ conversion by formulation, and manage NH₃ slip.

Water and dust

Water vapor itself is not a poison at normal levels but lowers the bisulfate formation threshold. High dust accelerates erosion and can mask poisoning symptoms.

Monitoring practice

Track NOx conversion, NH₃ slip and pressure drop monthly. A simultaneous rise in slip and drop in conversion usually indicates chemical deactivation; a pressure drop rise alone indicates plugging.

Manufacturer perspective

We ask for the full coal/fuel analysis (arsenic, alkali, sulfur) before sizing, because the same NOx removal target can require 20–40% more catalyst volume on a high-poisoning fuel. This is a material-selection decision, not a marketing one.

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FAQ

Can a poisoned SCR catalyst be regenerated? Partially — washing restores some activity for ash/alkali fouling; arsenic and thermal sintering damage are largely irreversible. We provide used-catalyst recycling guidance and replacement planning.