SCR and SNCR
Popcorn ash
Also known as popcorn fly ash, low-density ash.
Popcorn ash is a category of large-particle ash (LPA) consisting of porous, low-density particles 5–25 mm in size that resemble a kernel of popped corn. The particles form during incomplete coal combustion or low-temperature slagging, particularly on sub-bituminous coal and on units operating at reduced load. The low density means the particles are easily carried by flue gas into the SCR.
Why popcorn ash matters
Once a popcorn-ash particle enters a honeycomb catalyst channel, the channel is essentially blocked: the particle is too soft to break up under gas flow, too large to pass through, and too irregular to dislodge with typical sonic-horn energy. The result is a long-lived dead channel that reduces SCR efficiency.
Mitigation
- Coal blending or fuel switching to reduce popcorn-ash formation
- Combustion-tuning to raise furnace temperature and reduce porous-ash output
- LPA screens upstream of the catalyst
- Guard layers as first catalyst layer
Related terms
Related terms
- Large-particle ashLPA is fly ash larger than typical (>1 mm), produced by slag fragmentation and agglomeration in the boiler. It is the leading cause of SCR catalyst channel pluggage.
- 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.
- 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.