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PHOTO: KHAMBEKAR & BARNUM
In general, when flow problems such as the aforementioned occur, valuable generation capacity and production time are lost, excessive maintenance and housekeeping costs are incurred, and health and safety issues arise. Flow problems and their solutions can be best understood by first learning how bulk solids flow.
RATH OF BIOMASS: For materials such as milled biomass and sawdust, ratholing may occur as a result of flow channeling.
of flow channeling. During this condition, material moves toward the outlet through a steep, funnel-shaped flow channel surrounded by stagnant material. Hopefully, as the level of material in the flow channel drops, layers of material from the top surface of the stagnant region will slide into this active channel. If this fails to occur, the flow channel empties and a stable rathole forms, resulting in a no-flow condition. Whenever stagnant material is present inside a bin or silo, it will result in limited live storage capacity during flow. Such stagnant material can be a result of ratholing or steep drawdown angles; this material will not discharge by gravity, thus occupying valuable storage space. Another common flow problem is the tripping of feeder and bottom reclaim drive motors. When tripping occurs, the device cannot operate, resulting in the inability to discharge material from storage facility. The only way to address this problem, which typically occurs if the material consolidation pressures acting on the feeder or reclaimer are not properly calculated, may be to vacuum out the entire contents of the silo. Other flow problems that could occur include transfer chute pluggages, attrition and dusting of pellets, and particle segregation. Equipment wear can also be an important issue due to abrasive nature of woody biomass, particularly if it contains bark.
Flow Patterns, Properties
age time at rest. Flow property tests must be run at representative handling conditions and include the following tests: cohesive strength, interlocking strength, wall friction, compressibility, permeability, chute angle, angle of repose, and drawdown angle (see sidebar).
Achieving Reliable Material Flow
When properly designed, the compoAs bulk solids discharge by gravity nents of a handling system will be able to from a storage system, two types of flow store and reliably feed difficult-to-flow mapatterns can develop: funnel flow and mass terials, whereas poorly designed equipment flow. In funnel flow, only a portion of the may not be able to do the same, even with material is in motion during discharge, which flows toward the outlet through a channel that forms within the stagnant material. Funnel flow occurs when a hopper is not sufficiently steep and smooth to ensure flow along the walls, or when the hopper outlet is not fully activated. In mass flow, all of the material is in motion whenever any is discharged; there is no stagnant material. Mass flow occurs when the hopper is sufficiently WITH THE FLOW: As bulk solids discharge by gravity from steep and smooth to en- GO a storage system, two types of flow patterns can develop: funnel sure flow along the hopper flow and mass flow. walls. Shallow valleys—at SOURCE: KHAMBEKAR & BARNUM the intersection of adjacent hopper sidewalls—cannot be present, free-flowing materials. and the outlet must be fully active. For gravity discharge systems such as The flow pattern in which a bulk solid boiler-feed bins, selecting the appropridischarges from a storage system strongly ate flow pattern is critical for reliable perinfluences the flow problems that may oc- formance. Mass flow is recommended for cur. fine, cohesive materials such as sawdust and For the reliable storage and feeding milled woodchips; funnel flow is suitable for of biomass, the type of handling system coarse, free-flowing, non-interlocking, nonused— including its geometry and materials degrading materials in cases where particle of construction— must be designed to suit segregation is not important. the flowability of the material. Whether it is For mechanical reclaim systems such mechanical-reclaim or gravity-reclaim, char- as silos using bottom screw reclaimers, the acterization of flow behavior is necessary in discharge pattern is almost always funnel order to design features of the system. Vari- flow. The key for designing a mechanical ous flow property tests can be performed to reclaim system is selecting the right type determine flowability, which is influenced of mechanism that will work reliably for by moisture content, fines content and stor- the particular application. There are variSEPTEMBER 2012 | BIOMASS MAGAZINE 43