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Fly Ash, Erosion and SCR Catalyst Mechanical Life

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

Direct answer: Fly ash attacks SCR catalysts mechanically — plugging channels, eroding walls and masking chemical activity — long before chemical deactivation ends the catalyst's useful life in many high-dust applications. Managing ash means choosing the right catalyst geometry, pitch and face velocity, plus inlet flow conditioning.

The high-dust reality

In conventional coal-fired layouts the SCR reactor sits between the economizer and the air preheater — upstream of dust removal. The catalyst therefore sees the full ash load, typically several tens of grams per cubic meter. Ash-related failure is the dominant end-of-life mode in these units.

The three mechanical mechanisms

Plugging. Large ash particles bridge across narrow channels. Once a channel plugs, the local velocity in neighbouring channels rises, accelerating further deposition — a self-reinforcing cycle.

Erosion. Ash at high velocity abrades channel walls, thinning the substrate and removing active surface. Erosion concentrates at the leading edge of elements and in regions of flow maldistribution.

Masking. A surface ash layer blocks gas access to active sites. This looks like chemical deactivation in the data — conversion falls — but is reversible by cleaning.

Geometry choices against ash

  • Pitch is the primary defense: high-dust applications use larger pitches (typically ≥6–7 mm) so single ash particles cannot bridge a channel.
  • Plate-type catalysts have flat, open channels that resist plugging and tolerate erosion better than thin honeycomb walls; they are the standard choice for high-dust, high-ash coals.
  • Honeycomb with adequate pitch serves well at moderate dust loads with proper flow conditioning.
  • Face velocity design (typically 4–6 m/s, subject to verification) balances pressure drop against erosion rate.

Flow conditioning

Catalyst erosion and plugging start upstream. Flow straighteners, turning vanes and hoppers before the reactor reduce maldistribution. Uneven velocity across the catalyst face concentrates ash in slow zones (plugging) and accelerates erosion in fast zones simultaneously.

Monitoring mechanical health

  • ΔP trend across the reactor: step changes indicate sudden plugging events.
  • Visual inspection during outages: channel condition, leading-edge wear, element displacement.
  • Hardness and wall thickness spot checks on aged elements.

Manufacturer perspective

We always ask for the dust concentration, ash analysis (abrasiveness, particle size) and the velocity distribution measurement at the planned reactor location before recommending geometry. Ash management is designed in — a catalyst that is chemically perfect but mechanically wrong will fail early.

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