Hoppers and silos
Bunker (coal bunker)
Also known as coal bunker, bunkers.
A bunker (in industrial usage almost always a coal bunker) is an intermediate coal-storage vessel located above each pulveriser mill on a PC boiler. Coal arrives from the main coal-handling system, is held in the bunker for short-term buffering, and is metered by gravimetric feeders into the mill below. A typical utility unit has 4–8 bunkers, one per pulveriser.
Why coal bunkers bridge
- Sub-bituminous and lignite coals are particularly prone to cohesion under self-weight
- Wet coal from rain-exposed yards consolidates rapidly
- Long residence times in lightly-loaded bunkers harden surface material
- Vibration from operation gradually compacts the mass
A bridged bunker interrupts mill feed; the mill trips on low coal flow, the burner loses fuel, and the unit derates or trips. On a 600 MW utility unit, a single bunker pluggage can mean an hour or more of lost generation.
Mitigation
Sonic horns installed at the discharge cone keep the coal mobile. They are usually rated for ATEX Zone 22 dust-area service and feature stainless-steel construction to handle the abrasive-and-corrosive environment of coal storage.
Related terms
Related terms
- SiloA silo is a large vertical bulk-solids storage vessel. Cement, fly-ash, lime, biomass, fertilizer and food-powder silos all bridge and rat-hole; sonic horns are the leading flow aid.
- 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.
- Raw mill, cement mill and coal millCement plants run three principal mills: raw mill (limestone+clay→raw meal), cement mill (clinker→cement), coal mill (raw coal→pulverised fuel for the kiln burner).
- 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.