---
title: "SO₂/SO₃ conversion"
description: "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:"
canonical_url: "https://sylio.co/glossary/so2-so3-conversion"
last_updated: "2026-06-28T02:29:37.724Z"
---

**SO₂/SO₃ conversion** refers to the unwanted side reaction whereby [SCR catalyst](/glossary/selective-catalytic-reduction) 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](/glossary/air-heater)
- **Sulphuric-acid dew-point excursion** in [economisers](/glossary/economiser) and ducting, driving [cold-end corrosion](/glossary/cold-end-corrosion-dew-point-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](/glossary/air-heater) fouling and benefit more from [sonic-horn](/glossary/sonic-horn) installation on the cold end.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)
- [Cold-end corrosion / dew-point corrosion](/glossary/cold-end-corrosion-dew-point-corrosion)