Ammonium bisulphate

Ammonium bisulphate, often abbreviated ABS, is a sticky salt formed when ammonia reacts with SO3 and water in cooling flue gas. In boiler language it is one of the most troublesome by-products of SCR operation because it deposits in the same cold-end region where ash is already trying to accumulate.

ABS is soft and adhesive at common air-heater temperatures. It can wet fly ash, glue particles into baskets, foul catalyst outlet ducts and form tenacious deposits that do not behave like ordinary dry dust. Once mixed with ash, it increases pressure drop, reduces heat transfer and can force air-heater washing or load reduction.

Where it appears

The highest-risk areas are downstream of SCR systems: air heaters, economiser exits, ductwork, ESP inlets and baghouses. Risk rises with high ammonia slip, high SO3, low gas temperature, high ash loading and uneven flow. Units burning higher-sulphur fuels or using catalysts with higher SO2 to SO3 conversion need particular attention.

Operational controls

Controls include AIG tuning, catalyst maintenance, low-slip operation, SO3 mitigation, air-heater temperature management, sootblowing, basket material selection and periodic washing. Operators watch air-heater differential pressure, outlet temperature, fan current, ash stickiness and stack opacity or emissions changes.

Acoustic-cleaning relevance

Sonic horns can help with dry fly ash around the ABS temperature window, but they are less effective once a sticky salt film has formed. Their best role is upstream prevention: reduce loose ash residence, keep passages open and delay the ash-plus-ABS mixture from consolidating. They should be specified with realistic expectations when ABS is the dominant foulant.

Formation variables

Ammonium bisulphate formation depends on ammonia slip, SO3 concentration, moisture, gas temperature and residence time. SCR catalyst can increase SO2 to SO3 conversion, while poor ammonia distribution can create local zones with excess NH3. When those streams cool into the deposition range, ABS can condense as a sticky salt that binds ash into a dense fouling layer.

The problem is rarely uniform. It often appears in colder air-heater baskets, leakage zones, low-load operation, duct corners and areas with poor gas mixing. A plant may run acceptably at high load and then plug rapidly during extended low-load operation because the temperature profile has shifted.

Field symptoms

Operators see rising air-heater differential pressure, fan-power increase, declining heat transfer, sticky brown or grey deposits, ammonia odour, higher cleaning frequency and sometimes increased opacity after disturbed deposits release. Laboratory analysis of deposits can confirm ammonium salts and distinguish them from ordinary fly ash or acid dew-point corrosion products.

Maintenance and cleaning

ABS deposits can be difficult to remove online because the sticky phase absorbs and cements ash. Acoustic cleaning helps most before the salt layer matures, by reducing loose ash inventory and limiting the thickness available for binding. Once baskets are plugged, washing, temperature management, reagent retuning and catalyst evaluation become more important than horn output.

Inspection should connect deposit location with ammonia injection settings, catalyst age, SO3 measurements, seal leakage and cold-end temperatures. Treating the air heater alone may clear the symptom without removing the chemistry that created it.

Trend interpretation

ABS problems should be reviewed against load and temperature history. A short period below the deposition window can seed sticky material that later captures dry ash, so the root event may occur before the pressure-drop rise becomes obvious. Comparing SCR outlet temperature, ammonia slip, SO3 indicators and air-heater pressure drop helps locate the cause.

A horn that slows loose ash accumulation may still be overwhelmed if ABS chemistry is uncontrolled. The practical target is to keep deposits thin enough that washing intervals and fan power remain acceptable while the reagent and temperature causes are corrected.

Inspection note

Deposit sampling should distinguish ABS-rich sticky layers from ordinary fly ash. The distinction matters because mechanical cleaning can remove loose ash while leaving the salt chemistry untouched. The corrective action then belongs to slip, SO3 and temperature control.

Related terms

• Acid dew point
• Air heater
• Ammonia slip