Glossary
SCR and SNCR
Large-particle ash
Also known as LPA, large particle ash.
Large-particle ash (LPA) is fly ash significantly larger than typical particulate (above ~1 mm and sometimes up to 25 mm), produced by fragmentation of waterwall and superheater slag, agglomeration of finer ash, or thermal break-up of refractory. LPA is the dominant cause of SCR catalyst channel pluggage on coal-fired utility boilers.
Why LPA causes pluggage
Normal fly-ash particles are smaller than typical honeycomb catalyst pitch (3.5–7.4 mm) and pass through. LPA particles match or exceed the pitch dimension, wedge into a channel mouth, and progressively block the cell. A single LPA particle can block one channel; clusters of LPA across the top of the catalyst face can block tens of percent of the open area.
Mitigation
- LPA screens — coarse mesh installed upstream of the catalyst, ahead of the AIG or just below it, trapping particles above a set size
- Pop-up grids in the economiser hopper trap LPA before it reaches the SCR inlet
- Larger-pitch top guard layer — first catalyst layer with wider channels admits LPA which then drops through to a screen below
- Sonic horns and sootblowers — dislodge accumulating LPA-driven deposits between maintenance windows
Related terms
Related terms
- Selective Catalytic ReductionSCR is the dominant NOx-control technology on industrial combustion plant. Ammonia is injected upstream of a catalyst that converts NOx to nitrogen and water.
- Popcorn ashPopcorn ash is porous low-density fly-ash particles, typically 5–25 mm, formed during incomplete coal combustion. They wedge into SCR catalyst channels and resist cleaning.
- Catalyst pluggageCatalyst pluggage is the physical blockage of SCR catalyst channels by large-particle ash, popcorn ash or ammonium-salt deposits. It causes ΔP rise and gas-flow maldistribution.
- EconomiserAn economiser is the final tube bank in a boiler's convective pass that recovers heat from the flue gas by preheating feedwater. Ash bridging in the economiser is a routine cleaning challenge.