Glossary
SCR and SNCR
SO₂/SO₃ conversion (in SCR)
Also known as SO2 to SO3, SCR SO3 generation, sulphur oxidation in SCR.
SO₂/SO₃ conversion refers to the unwanted side reaction whereby SCR catalyst oxidises a fraction of the flue-gas SO₂ to SO₃ — typically 0.3–1.5% across a high-dust SCR. The newly-formed SO₃ has three downstream consequences, all undesirable:
- Ammonium bisulphate (ABS) formation in cooler downstream zones, plugging air heaters
- Sulphuric-acid dew-point excursion in economisers and ducting, driving cold-end corrosion
- Visible blue plume from sulphate aerosol at the stack
Minimising conversion
- Catalyst formulation tuned for low V₂O₅ content (V is the conversion driver)
- Lower SCR operating temperature where the catalyst window allows
- Reduced excess air at the burner to limit SO₃ formation
- Fuel sulphur control where economically possible
Why it matters for cleaning
Higher SO₃ means more ABS in the cold end. Plants with significant SO₂/SO₃ conversion face heavier air-heater fouling and benefit more from sonic-horn installation on the cold end.
Related terms
Related terms
- Selective Catalytic ReductionSCR is the dominant NOx-control technology on industrial combustion plant. Ammonia is injected upstream of a catalyst that converts NOx to nitrogen and water.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
- Cold-end corrosion and dew-point corrosionCold-end corrosion is the attack on air-heater and economiser surfaces below the acid dew point, where SO3 condenses as sulphuric acid. The leading cold-end failure mechanism.