Ammonia slip

Ammonia slip is unreacted NH3 leaving an SCR or SNCR system with the flue gas. It is caused by excess reagent, poor mixing, low catalyst activity, temperature outside the reaction window, blocked catalyst passages or uneven gas flow. Slip wastes reagent and can create serious downstream fouling.

In SCR systems, the target is a controlled ammonia-to-NOx ratio at the catalyst face. If reagent distribution is uneven, some lanes remain NOx-rich while others carry surplus NH3. The average stack NOx may look acceptable while local ammonia concentrations are high enough to form deposits in the air heater or particulate-control equipment.

Consequences

The major cold-end consequence is ammonium bisulphate. NH3 reacts with SO3 in cooling flue gas to form sticky salts that plug air-heater baskets, bind fly ash and raise fan power. Slip can also contaminate fly ash, create odour, interfere with continuous monitoring and complicate ash reuse.

Measurement and control

Slip is measured with extractive analysers, tunable diode laser systems, sorbent traps or periodic testing, depending on the plant. Operators control it through AIG tuning, reagent flow control, catalyst management, sootblowing, temperature control and flow distribution. A sudden slip increase can indicate catalyst masking or plugging rather than a reagent-control problem.

Acoustic-cleaning relevance

Acoustic cleaning does not remove ammonia from flue gas. Its relevance is deposit control around SCR catalysts, screens, ducts, hoppers and air heaters, where ash build-up worsens maldistribution and ABS accumulation. In Sylio-style applications, sonic horns support stable flow paths so the DeNOx system can be tuned with less hidden fouling bias.

Operating variables

Ammonia slip is controlled by reagent flow, NOx measurement accuracy, mixing quality, catalyst activity, gas temperature, load changes and the time delay between injection and analyser response. A low stack NOx number can hide local over-injection if the ammonia is not distributed evenly. That is why operators tune by load, burner pattern, catalyst condition and grid zone rather than by a single total ammonia flow.

The risk rises when catalyst activity declines, when ash plugs catalyst channels, when the ammonia injection grid is out of balance, or when the control system chases fast NOx changes with too much reagent. In SNCR systems, poor temperature window control is a common cause because reagent injected into gas that is too cold can pass through unreacted.

Plant consequences

Ammonia slip can cause odour, visible plume reactions, contaminated fly ash, corrosion concerns and ammonium salt deposition. In coal and waste-fired units with SO3 present, the most important fouling product is often ammonium bisulphate. It can form sticky deposits in air heaters, ducts, catalyst exits and low-temperature surfaces, raising pressure drop and fan power.

Measurement and maintenance

Slip is measured with extractive systems, tunable diode laser instruments, periodic manual tests or inferred from downstream symptoms. Sampling is difficult because ammonia can adsorb, react or condense in lines. Maintenance teams check analyser conditioning, calibration, heated lines, grid plugging, catalyst pressure drop and air-heater deposit chemistry.

Acoustic cleaning can help manage dry ash deposits that worsen maldistribution or air-heater fouling, but it cannot correct over-injection. If sticky ammonium salts dominate, the primary controls are reagent tuning, catalyst management, SO3 control, temperature control and washing strategy.

Control evidence

Good slip control uses both upstream and downstream evidence. NOx analyser data, reagent valve position, grid-zone balance, catalyst pressure drop, air-heater pressure drop and deposit chemistry should be reviewed together. A low reported slip value is less convincing if the air heater is rapidly forming ammonium salts.

Cleaning-system data can help separate causes. If acoustic horns are operating normally but sticky deposits continue to grow, reagent distribution or SO3 chemistry is probably the controlling issue. If dry ash accumulation increases after horns are disabled, the cleaning system may be contributing to deposit management even though it does not control ammonia chemistry.

Diagnostic pattern

A useful ammonia-slip investigation separates chemistry, distribution and measurement faults. If outlet NOx is high and slip is low, the system may be under-injecting or operating outside the reaction window. If outlet NOx is low and slip is high, reagent may be over-injected or poorly mixed. If both NOx and slip vary across the duct, the AIG, mixers, catalyst pluggage or gas-flow profile need attention. Downstream symptoms also matter: sticky air-heater deposits, fly-ash odour, visible plume behaviour and sudden pressure-drop rise can reveal slip problems before a single analyser trend is convincing. Acoustic cleaning data should be reviewed as supporting evidence for ash distribution, not as proof that reagent tuning is correct.