Catalyst pluggage

Catalyst pluggage is the physical blockage of SCR catalyst channels by particulate material. Unlike catalyst masking (a thin surface blanket), pluggage fills the catalyst channels themselves, stopping gas flow through affected cells. The result is delta P rise across the SCR, gas-flow maldistribution into the remaining open cells, and channelling effects that reduce overall NOx reduction.

Sources of pluggage material

Large-particle ash (LPA) - slag fragments and agglomerated ash carried over from the boiler
Popcorn ash - porous low-density ash particles that wedge into honeycomb cells
Ammonium-salt deposits - ammonium bisulphate on tail-end SCRs at lower temperatures
Refractory debris - fragments from upstream furnace or duct repairs

Prevention

LPA screens - coarse mesh screens upstream of the catalyst trap large particles
Guard layers - sacrificial top catalyst layer with larger pitch absorbs the initial particulate
Larger pitch on the top layer - wider cell openings on the first catalyst layer pass LPA through to a removable screen below
Periodic sonic-horn cleaning - dislodges accumulating ash before it cements
Steam sootblowing - for harder deposits

How pluggage develops

Pluggage normally starts at the leading face of the first catalyst layer. Large particles bridge across cell openings, then catch finer ash until a plug forms. In honeycomb catalyst, a single blocked channel removes that passage from service and pushes gas to neighbouring channels. Those open channels then carry higher velocity, higher particulate load and less residence time, so NOx reduction becomes uneven.

In plate catalyst, the larger passage can tolerate coarse ash better, but deposits can still lodge at spacers, support grids and layer edges. If the reactor has poor approach flow or a turning vane fault, pluggage follows the flow pattern rather than the theoretical module layout. This is why ash distribution surveys and pressure mapping matter during troubleshooting.

Operating signs

The clearest sign is a rising reactor pressure drop that does not recover after normal cleaning. Outlet NOx may rise at the same reagent flow, while ammonia slip also increases because gas and reagent distribution are no longer matched. In severe cases, the blocked region becomes a bypassed dead zone and the remaining open area is overloaded.

Inspection distinguishes loose ash from cemented deposits. Loose large-particle ash can often be vacuumed or blown out during an outage. Hard ammonium-salt or alkali deposits may require mechanical or specialist cleaning and can damage fragile catalyst if removal is too aggressive.

Design and cleaning implications

Prevention starts upstream: good boiler combustion, effective economiser ash removal, intact turning vanes, working hoppers and correctly located LPA screens. Catalyst pitch should match the ash challenge. A low-pressure-drop fine-pitch layer may perform well on clean gas but fail quickly in high-dust service.

Sonic horns are useful for loose and early-stage deposits because acoustic energy disturbs particles before they wedge deeply in the cells. They are less effective on fully cemented plugs. For that reason, horn installation should be paired with pressure-drop trending and inspection intervals that catch pluggage before a layer loses significant open area.

Field checks

Pluggage is tracked through reactor differential pressure, flue-gas temperature, ash loading, velocity distribution and outage inspection of the catalyst face. A sharp pressure-drop increase after a fuel or load change often points to a deposit event rather than gradual catalyst ageing. In high-dust SCRs, the first layer and the inlet turning region are the usual checkpoints because they receive coarse ash, popcorn ash and large agglomerates before flow has settled.

Maintenance teams look for blocked channels, erosion at open lanes, bypass gaps around module frames and dust piles on support beams. Partial pluggage can be deceptive: gas simply shifts to the remaining open area, increasing local velocity and erosion while average reactor data looks acceptable. Acoustic cleaners are useful when pluggage material is loose or weakly attached and when horn placement reaches the upstream face. They are much less effective against wet ammonium salt masses or deposits that have hardened through repeated thermal cycles. Good prevention combines gas distribution, ash control, sootblower or horn sequencing and planned face inspection.