--- title: "Coal bunker hang-ups and arching: restoring reliable flow without entry" description: "Coal bunker hang-ups can starve mills and expose crews to engulfment. Learn the causes, no-entry options and narrow role of acoustic cleaning." canonical_url: "https://sylio.co/resources/blog/coal-bunker-hang-ups-and-arching" last_updated: "2026-07-16" --- ## When a coal bunker stops flowing **Coal bunker hang-ups** are not just awkward maintenance events. In a power station, a blocked [coal bunker](/glossary/bunker-coal-bunker) interrupts the controlled fuel path from conveyor to feeder and mill. Feeder output becomes erratic or stops, a mill can be starved, oil support may rise, and the unit may have to derate. What looks like a local flow problem can quickly become a generation and safety problem. The dangerous instinct is to treat the bunker as empty because nothing is leaving it. The opposite may be true. A large inventory can be held above an outlet by a stable arch, or left around a central flow channel as stagnant coal. That inventory contains stored mechanical energy and may release suddenly when disturbed. The objective is therefore not merely to make coal move. It is to restore predictable flow from outside the vessel, without trading a production interruption for an engulfment, dust or fire event. ## Arching, rat-holing and wall hang-up are different failures [Bridging](/glossary/bridging), also called arching, occurs when cohesive coal forms a load-bearing structure across the outlet or converging section. Material below the arch discharges, then flow stops while the bunker above remains charged. [Rat-holing](/glossary/rat-holing) is different. Coal moves through a channel above the outlet while material at the sides remains stationary. The channel can empty and leave substantial hidden inventory behind. Wall hang-up is coal adhering to shoulders, liners, ledges or transitions, gradually reducing live capacity and disturbing the draw pattern. One bunker can alternate between all three. The distinction matters because an outlet bridge, a stagnant annulus and a sticky wall layer do not respond to the same intervention. A temporary surge after poking proves only that some material moved. It does not show that the underlying [mass-flow or funnel-flow](/glossary/mass-flow-vs-funnel-flow) condition has changed. ## Why coal hangs up Surface moisture is often the swing factor. Water at particle contacts can form capillary bridges, particularly among fines, increasing the coal's cohesive strength. Rain exposure, wet dust suppression, condensation or a change in delivered fuel can turn a previously manageable blend into a sticky one. There is no universal moisture percentage at which every coal stops flowing. Rank, mineral matter, particle distribution, packing and degree of saturation all influence the result, so the plant needs data from its own credible worst case. The fines fraction matters for the same reason. Small particles have greater specific surface area and more particle contacts per unit mass, although their actual contact behaviour depends on packing and load. They can dominate the cohesive behaviour of a mixed coal. Handling and breakage also change the distribution between delivery and the bunker. Segregation during filling can then place different material at the centre and walls, producing an uneven flow pattern even when a composite sample looks acceptable. Time at rest is not passive. Coal under the head of material above it consolidates, particles rearrange and moisture migrates. A blend that discharges after continuous operation may form a stronger arch after a weekend stop, a maintenance hold or an idle reserve period. This is why time-consolidated shear testing is more informative than judging fresh coal by appearance or angle of repose alone. Geometry decides whether that strength becomes a stable blockage. Outlet width, wall angle, the shape of the [discharge cone](/glossary/discharge-cone), transitions and feeder interface all matter. A centre-drawing feeder or partly restricted gate can impose funnel flow even where the upper bunker was intended to empty more uniformly. An outlet sized for an older, coarser fuel may be too small for a wetter, finer replacement. The working wall surface matters as well. Liner material and smoothness determine wall friction, but the installed condition is what coal sees. Worn plates, lifted edges, rough repairs, exposed fasteners, corrosion and deposits can create attachment points. A low-friction liner is not a universal cure, and a liner change cannot compensate for an undersized outlet or unsuitable geometry. Flow testing should use representative wall samples, followed by an inspection of the real bunker. ## Why manual clearing is a poor normal A coal bridge is not a working platform. It can conceal a void and collapse without warning. Coal stuck to a wall can also release as one mass, burying a person below or overloading the feeder and conveyor when the blockage gives way. Hammering, poking or uncontrolled air lancing may change the load path without revealing what remains above. A bunker may also meet OSHA's definition of a permit-required confined space because it has restricted access, converging walls, flowable material, a possible hazardous atmosphere or other serious hazards. Other jurisdictions use their own tests. Dust, low oxygen, toxic gases, moving feeders and incoming conveyors must all be considered. Under OSHA's definition, entry begins when any part of the body crosses the plane of the opening. If a hand, arm or head crosses that plane, the task is entry, even when most of the person remains outside. The first control is to avoid entry and do the work from outside. Nobody should enter onto, below or within the release path of bridged or wall-hung coal. An outage is not isolation. Any interior cleaning can proceed only after stored material has been removed or positively controlled, all material and energy sources have been isolated, and the applicable confined-space system is in force. That system includes a formal entry permit where required, a designated attendant outside the space, atmospheric testing and monitoring, competent supervision, communications and a workable rescue arrangement. Routine production pressure does not relax those requirements. Stagnant coal adds a separate concern: spontaneous heating. Coal oxidises at low temperature, and if the heat generated is not removed, the temperature can rise towards thermal runaway. Coal type, particle size, residence time, temperature and air movement interact, so a hang-up does not automatically mean a fire. But an abnormal temperature or rising carbon monoxide trend changes the event. Routine flow restoration should stop and the plant's coal-fire and emergency procedure should take control. A horn, air cannon or moving feeder is not a fire response. ## Diagnose from outside before choosing a device Start with the operating evidence. Compare bunker level, rate of level change, feeder demand and output, feeder current, mill response and any high or low level alarms. Use fixed CCTV, sight glasses or remotely positioned inspection equipment under the site's procedure, without placing a person at or across an opening. A high indicated level with no feeder output suggests a bridge or blocked gate. Intermittent surges may indicate alternating arch collapse and rat-hole formation. Repeated failure at the same indicated level points towards a geometry change, ledge or liner seam. Add coal condition and time. Record source, blend, surface moisture, size distribution, recent weather, dust-suppression state and how long the bunker sat before the event. Compare the pattern across bunkers and shifts. Temperature and gas trends belong in the same review because abnormal or uncertain indications should move the event out of routine flow troubleshooting and into the plant's fire procedure. Normal readings do not certify that a hot spot is absent or make entry safe. For recurring [bunker flow problems](/glossary/material-flow-promotion), commission bulk-solids tests on representative coal at realistic moisture, temperature and storage-time conditions. Cohesive strength, time consolidation and wall friction can then be used to assess outlet size and wall slope. This evidence separates a material excursion, a maintenance defect and a design limit, which is essential before money is spent on another flow aid. ## No-entry options compared No-entry does not mean no risk. Any remote intervention can release a large mass, create dust or load downstream equipment. The bunker, feeder and receiving conveyor need a controlled state, an exclusion zone and a method-specific assessment. Within that boundary, the options serve different duties.
Option Best fit Main limit
Operating and feeder correction A gate, feeder draw pattern or downstream restriction is causing funnel flow Does not correct cohesive coal or poor bunker geometry
Air cannon A diagnosed localised cohesive build-up may respond to an engineered impulse Cannot correct poor geometry, and blast loads, dust, liners and downstream surge must be assessed
Bin vibrator Selected coarser material in a structurally suitable smaller vessel Can compact cohesive fines or fatigue the structure if misapplied
Fluidisation pad Fine, dry material that has been shown to aerate predictably Wet coal resists aeration, while added air can affect oxidation and promote dust or flooding
Liner repair or replacement Wall friction, damage or rough transitions are the controlling defect Requires material testing and an outage, and cannot enlarge the outlet
Geometry, outlet or mechanical extraction change Persistent arching is built into the vessel and feeder interface Higher capital and outage scope, but often the durable answer
Acoustic horn Dry, dusty coal needs preventive agitation in a gas-filled dead zone Weak or useless on wet, compacted or strongly cohesive coal
Mechanical arch breakers, live bottoms and remotely operated cleaning tools are further [anti-bridging devices](/glossary/anti-bridging-device) where the duty justifies them. Selection should follow the diagnosed failure, structural review and combustible-dust controls. The detailed device trade-offs are covered in the [sonic horn, air cannon and bin vibrator comparison](/resources/blog/sonic-horn-vs-air-cannon-vs-bin-vibrator). ## Where acoustic cleaning fits, narrowly A compressed-air-powered [sonic horn](/glossary/sonic-horn) sends repeated pressure oscillations through the bunker gas space. Its useful role here is preventive: help limit loose, dry accumulation in gas-filled shoulders or dead zones. It works without striking the shell and can operate online, provided the mounting, controls, noise and [compressed-air](/glossary/compressed-air) supply suit the coal-handling area. That is a narrow application. Wet or highly cohesive coal defeats acoustic energy. A horn is not a dependable recovery tool for a mature compacted arch, a sticky mass, frozen coal or a bunker packed so fully that there is no useful gas path. Depending on the diagnosis, recovery may require an approved air cannon or mechanical method for a localised blockage. Preventing recurrence may instead require lower-friction walls, a larger outlet or corrected geometry. Acoustic cleaning is one preventive layer, not the answer to coal silo arching. The starting condition also matters. If a bunker already contains a heavy hang-up, it may need controlled external clearing or a properly isolated outage clean after the stored material has been removed or positively controlled. Only then can a preventive system be judged. A trial should track discharge reliability under known coal conditions, not whether the horn can perform a heroic recovery. The broader [acoustic cleaning system guide](/resources/blog/acoustic-cleaning-system) explains the system-level questions without changing this material limit. ## A practical decision sequence 1. Assess for indications of heating. Check temperature, gas trends and site alarms first, and escalate any abnormal or uncertain condition to the plant's fire procedure. Normal readings do not clear entry or flow-aid activation. 2. Name the failure from external evidence. Distinguish an outlet arch, rat-hole, wall adhesion, feeder restriction and instrument fault. 3. Characterise the worst coal. Use surface moisture, fines, storage time and blend history, supported by representative flow testing. 4. Inspect the flow path. Check liners, seams, transitions, gate opening, outlet geometry and whether the feeder withdraws across the intended area. 5. Choose the least intrusive effective correction. Fix operating or maintenance defects first, then match any flow aid to the material and failure mode. 6. Prove prevention. Track feeder stability, usable bunker capacity, intervention count and fuel condition over a representative campaign. These records turn repeat emergencies into [predictive maintenance](/glossary/predictive-maintenance). The useful outcome is not one successful release. It is fewer feeder interruptions, no routine vessel entry, stable mill feed and a lower chance that a local hang-up becomes a [forced outage](/glossary/forced-outage). ## The bottom line Reliable coal flow comes from matching the bunker to the coal. Surface moisture, fines, time under load, wall friction, outlet geometry and feeder draw interact. When that system is marginal, clearing one arch without correcting the cause only resets the clock. The safe hierarchy starts outside the vessel: identify the failure, assess for indications of abnormal heating and escalate uncertainty, use operating evidence and material testing, repair defects, then choose a remote aid or structural correction that fits. Acoustic horns can help limit loose, dry accumulation in gas-filled dead zones. They are weak or useless on wet, highly cohesive or heavily compacted hang-ups. Depending on the diagnosis, credible options include an engineered air cannon or mechanical method for a localised blockage, and liners, outlet enlargement or geometry changes to prevent recurrence. No flow target justifies treating manual entry as routine.