SO2/SO3 conversion (in SCR)

SO2/SO3 conversion is the unwanted oxidation of sulphur dioxide to sulphur trioxide across a selective catalytic reduction catalyst. SCR is installed to reduce NOx, but vanadium-based catalyst formulations can also promote sulphur oxidation when the flue gas contains SO2 and excess oxygen. The conversion fraction is usually small, often a fraction of one percent to a few percent, but the operational impact can be large because SO3 readily forms sulphuric acid vapour and ammonium salts downstream.

Conversion depends on catalyst formulation, gas temperature, residence time, sulphur content, oxygen level and catalyst age. Higher vanadium activity, higher temperature and high-sulphur fuel generally increase the risk. Low-SO2 fuels and low-conversion catalyst formulations reduce it, but the design has to maintain enough NOx activity for the emissions duty.

Downstream consequences

SO3 is important because it moves the plant into a different fouling and corrosion regime:

• It reacts with water vapour to form sulphuric acid vapour and fine acid mist.
• It raises the acid dew point, increasing cold-end corrosion risk in air heaters, ducts and stacks.
• It reacts with ammonia slip to form ammonium bisulphate, a sticky salt that plugs air-heater baskets, catalyst pores and cold-end surfaces.
• It can contribute to blue plume visibility when sulphate aerosol forms at the stack.

The SCR may therefore meet its NOx target while worsening air-heater pressure drop, fan power, wash frequency and plume management.

Operating and design controls

Control measures include specifying low-SO2-oxidation catalyst, limiting unnecessary catalyst volume, controlling ammonia slip, keeping the SCR within its intended temperature window, managing excess oxygen, and considering fuel sulphur variability during catalyst selection. Plants with high sulphur or tight opacity limits may also use sorbent injection, air-heater washing strategy, improved temperature control or catalyst-layer management to keep the cold end stable.

Maintenance teams should track SO3-related symptoms together rather than separately: rising air-heater differential pressure, sticky deposits, increased washwater acidity, cold-end metal loss, visible plume and changes in ammonia slip. A single chemistry shift can drive all of them.

Relevance to acoustic cleaning

Acoustic cleaning can help with the physical fouling consequences, especially where ammonium bisulphate and fly ash begin to bridge or blind air-heater passages, SCR inlet screens or downstream hoppers. It does not change the SO2-to-SO3 chemistry. The best Sylio-style approach is to treat sonic horns as part of a wider fouling-control package: keep deposits mobile, while the plant controls catalyst choice, ammonia slip and cold-end temperature.

Measurement and troubleshooting context

SO3 is difficult to manage because it is present at low concentration but has outsized effects once the gas cools. Plants may infer conversion from controlled SO3 sampling, acid dew point checks, ammonia slip data, air-heater deposit analysis, opacity behaviour and changes in stack aerosol. A single grab sample rarely tells the whole story because conversion varies with load, catalyst temperature, sulphur input, sootblowing pattern and which catalyst layer is most active.

The practical question is usually not only "how much SO3 is formed", but where it becomes a sticky or corrosive compound. A high-dust SCR outlet, a Ljungstrom air heater, a cold duct expansion, an induced-draught fan casing and a wet stack can all see different symptoms from the same chemistry. During troubleshooting, operators compare deposit colour and texture, washwater pH, basket plugging pattern, temperature margins above acid dew point and whether pressure drop recovers after washing or immediately returns. That distinction separates a cleaning-frequency problem from a chemistry problem.

For acoustic-cleaning selection, SO3 conversion is a warning that deposits may be adhesive rather than free-flowing. Horns can limit early bridging if ash is still friable, but they cannot reliably remove aged ammonium bisulphate glaze or acid-wetted material. The best design practice is to place acoustic devices where deposits are newly formed, accessible to the sound field and still able to move to hoppers or drains.

Related terms

• Selective Catalytic Reduction (SCR)
• Ammonium bisulphate
• Cold-end corrosion / dew-point corrosion