Large-particle ash (LPA) - coarse ash that plugs SCR catalysts

Large-particle ash (LPA) is coarse ash, slag or agglomerated particulate that is much larger than ordinary fly ash. In coal-fired high-dust SCR systems it is a major cause of catalyst pluggage, because particles can be large enough to wedge into the inlet cells of a honeycomb catalyst.

LPA may be above about 1 mm and can be much larger, depending on boiler slagging, ash fusion behaviour, sootblowing, economiser geometry and fuel mineral matter. Related plant language includes popcorn ash, slag debris and coarse ash.

Where it comes from

Slag shedding from furnace walls or superheater tubes.
Agglomeration of fine ash in hot, sticky zones.
Mechanical break-up of deposits during sootblowing.
Re-entrainment from hoppers or ledges.
Refractory or insulation debris entering the gas path.
Abnormal combustion or fuel changes that increase slagging tendency.

LPA is often intermittent. A unit may run normally for days, then release a slug of coarse material after load change, sootblower operation or a deposit collapse.

Why LPA causes pluggage

Normal fly ash is small enough to pass through catalyst channels. LPA can match or exceed the catalyst pitch, lodge at the channel mouth and collect smaller ash around it. The inlet face then develops local blinding, which pushes gas through the remaining open channels. That raises velocity, pressure drop and maldistribution.

The consequences include lower NOx reduction efficiency, higher ammonia slip, local catalyst abrasion, higher reactor differential pressure and earlier catalyst replacement. A small number of plugged cells is tolerable; broad inlet-face coverage is not.

Mitigation

LPA screens upstream of the catalyst to capture oversize particles.
Pop-up grids or traps in economiser hoppers to intercept coarse material.
Larger-pitch guard layers at the top of the reactor.
Improved gas distribution so coarse ash does not concentrate in one lane.
Sootblower optimisation to avoid releasing large slugs at the wrong time.
Regular inspection and vacuuming during outages before deposits harden.
Sonic horns or sootblowers to keep loose ash mobile between maintenance windows.

Acoustic-cleaning relevance

Sonic horns can help when LPA accumulation is loose and sitting on screens, grids or the catalyst inlet face. They are less effective once a particle is physically wedged into a cell or cemented by sticky ash. Acoustic cleaning should therefore be specified as prevention and housekeeping, not as a guarantee that a blinded catalyst can be recovered online.

For Sylio-style designs, the important inputs are particle size distribution, reactor velocity, catalyst pitch, layer spacing, access for inspection and whether the LPA source is continuous or episodic.

Sources and diagnostic detail

Large-particle ash often comes from economiser shedding, furnace slag fracture, sootblower dislodgement, burner-zone agglomerates or upstream duct deposits that break loose during load changes. In SCR service, the harmful fraction is not only large by mass; it is large relative to catalyst pitch. A particle that passes through a coarse screen can still bridge across a fine honeycomb cell or lodge at a module edge.

Plants diagnose LPA by inspecting catalyst faces, screens, turning vanes, hoppers and sample traps. Useful observations include particle size, shape, colour, hardness, magnetic content and whether the particle is porous popcorn ash or dense slag. The time pattern matters too. A continuous rain of particles suggests an upstream generation mechanism, while sudden showers after sootblowing or load swings suggest release events.

Control options include upstream screens, larger catalyst pitch, guard layers, better gas distribution, sootblower tuning, hopper evacuation and online cleaning of ledges where particles accumulate before falling. Sonic horns can reduce secondary accumulation on screens and catalyst faces, but they do not eliminate the upstream ash source. The best design combines prevention of particle formation with enough cleaning and access to keep the reactor from becoming the first collection point.

Maintenance implication

LPA events should trigger an upstream inspection, not only a catalyst-face clean. If the same ledge, screen or economiser pocket keeps shedding particles, the SCR will remain a receiver for the problem. Cleaning the reactor face without removing the source only resets the clock.

Acoustic cleaning can be applied to intermediate ledges or screens where particles collect before falling, provided the mounting can survive the local temperature and ash abrasion.