Electrostatic precipitators
Fly-ash hopper
Also known as fly ash hopper, ash hopper.
A fly-ash hopper is any inverted-pyramid or trough-shaped vessel that collects particulate ash from a combustion plant's flue-gas-cleaning equipment — ESPs, baghouses, economiser hoppers, air-heater hoppers, duct dropouts. Fly-ash hoppers across the gas-path system are notorious for bridging, rat-holing and pluggage.
Why fly ash bridges
Dry fly ash is a Geldart-C type powder — fine, cohesive, and prone to forming stable arches across narrowing geometries. Cohesion increases with moisture pickup, condensation at the cold end, residual unburnt carbon and chemical composition (high CaO ashes from biomass and lime are especially sticky). Once an arch forms, it tends to consolidate under continued dust accumulation above it.
Sonic horns vs air cannons on fly-ash hoppers
The two technologies compete head-to-head:
| Attribute | Sonic horn | Air cannon |
|---|---|---|
| Mechanism | Continuous low-amplitude vibration | Periodic high-pressure blast |
| Coverage | Whole hopper volume from one unit | Localised to the cannon nozzle |
| Structural stress | None | Significant; fatigue cracking documented |
| Air consumption | Continuous, low | Episodic, high |
| Retrofit complexity | Single roof or wall mounting | Multiple wall mountings + reservoirs |
| Best suited to | Most ash types, retrofit-friendly | Hardest-packed deposits, large silos |
Related terms
Related terms
- ESP hopperAn ESP hopper is the inverted-pyramid vessel below each ESP field that collects rapped-down fly ash. Bridging and rat-holing are common failures; sonic horns are the standard mitigation.
- HopperA hopper is an inverted-pyramid or conical vessel for storing and discharging bulk solids. Bridging and rat-holing are the universal failure modes; sonic horns are a clean, low-maintenance remedy.
- Bridging (bulk-solids)Bridging (also arching) is the formation of a stable arch of bulk solids above the discharge outlet of a hopper or silo, stopping material flow. The universal failure mode of bulk-solids storage.
- Rat-holingRat-holing is a flow pattern in which material discharges through a narrow vertical channel above the outlet, while the surrounding material remains stagnant and consolidates.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.
- Air cannonAn air cannon is a pressure-vessel and quick-release-valve assembly that fires a brief high-pressure air pulse into a hopper or silo to break material bridges. Effective but causes structural stress.