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Ammonia Slip Control in SCR Systems

Part of the SCR DeNOx: The Complete Guide — this article is one of the detailed pages in the guide.

Direct answer: Ammonia slip — unreacted NH₃ leaving the SCR reactor — is controlled by correct NH₃/NOx ratio management, even ammonia distribution across the duct, and adequate catalyst activity. It is the key constraint that balances NOx removal against downstream fouling: more ammonia removes more NOx, up to the point where slip costs exceed the removal benefit.

Why slip matters

Unreacted ammonia is not inert. Downstream of the reactor it can:

  • React with SO₃ and water to form ammonium bisulfate, fouling air preheaters and ductwork.
  • React with HCl to form ammonium chloride, a hard deposit in cold-end equipment.
  • Appear as ammonia in stack emissions, where many permits set explicit limits.
  • Contaminate fly ash, reducing its sale value in some markets.

The stoichiometry constraint

The SCR reaction consumes NH₃ and NOx in roughly 1:1 molar ratio. Every ppm of slip represents ammonia that did not find an active site. The usual engineering limit is around 2–3 ppm slip at design conversion, subject to local permit values — but the economic optimum is often lower than the permit allows because downstream fouling has real maintenance costs.

Root causes of high slip

Cause Symptom Fix
Catalyst deactivation Slip rises while conversion falls Regeneration or replacement
Uneven ammonia distribution Local zones with high slip AIG tuning, mixing improvement
Flow maldistribution Local high-velocity channels Flow straightening, reactor design
Excess NH₃ injection Slip high while conversion adequate Reduce ratio, verify analyzer
Temperature below window Low activity, high slip Raise temperature or manage load

Ammonia injection grid (AIG)

The AIG distributes NH₃ across the duct. Tuning means measuring the NOx profile at the reactor inlet and adjusting individual lances or zones so that each channel receives NH₃ proportional to its local NOx load. A well-tuned AIG can cut slip in half at the same conversion; a badly tuned one wastes ammonia and fouls equipment regardless of catalyst quality.

Measurement practice

  • Measure NOx and NH₃ at multiple points across the duct, not one traverse point.
  • Cross-check slip analyzers against wet chemistry periodically.
  • Track the NH₃/NOx ratio at which conversion plateaus — the ratio where extra ammonia buys no more conversion marks the onset of slip.

Manufacturer perspective

When a plant reports high slip, we first ask for the inlet NOx distribution and the AIG tuning history — catalyst replacement is the last resort, not the first. A new catalyst behind a badly tuned AIG will slip ammonia from day one.

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