Storage and Handling Requirements for Biomass Richard Farnish, CEng MIMechE
Storage and Handling Requirements for Biomass The storage and handling of biomass materials need not present major problems if the equipment is designed to suit the properties of the bulk material to be handled. Key Requirements (in most situations): First-in first out stock rotation (reducing risk of self heating) Minimal dust generation Reliability of operation Effective fire fighting procedures
Storage equipment - core flow first in last out discharge dead regions of product erratic discharge caused by product on product shear during emptying central discharge channel exaggerates segregation effects of particles hopper half angle typically greater than 25 degrees from vertical poor stock rotation high storage capacity for a given headroom Flow from top of material
First in - last out strategy Discharge options can include: Multiple outlets feeding by gravity to belt conveyors (though a conical transition vessel) Gravityfeedthroughaflatbottomedsilotomultipleoutletswith final reclamation achieved using a sweep auger. Stockpile formation in bays with reclamation by front end loaders (feeding into discharge pits or direct to conveyor buffer hoppers)
Storage equipment - mass flow first in, first out discharge all storage capacity is live consistent discharge encouraged by the reduced levels of shear generated as the product discharges against relatively smooth wall material - not static product degree of remixing during discharge minimises segregation effects hopper half angle typically less than 25 degrees from vertical relatively low storage volume for a given headroom - but all the product can be retrieved
First in - first out strategy The attainment of first in first out stock rotation can be achieved in different ways: Small scale: Gravity discharge convergent vessels (expensive and practical limitations on size) Flat bottom vessels with orbital discharge (more complex) For both options careful design taking into account bulk properties is required
First in - first out strategy Large scale: Acceptance of core flow operation in multiple silos, but complete series of silos drawn down to empty in the same sequence that they were filled. Thus silos offer economy of core flow designs, but scheme operates using a controlled residence period for the stored material (mass flow in principle).
Multiple small storage versus large store? Key risks for any storage scheme handling biomass: Loading of contaminated shipment Fire Potentially the use of multiple (smaller) stores presents a mitigation strategy for maintaining supplies should an event remove a store from use. The use of larger stores gives economy at the build stage, but risks the loss of a larger proportion of material.
Interfacing of silos to main conveyors Consideration should be given to the interface of the silo outlets to the main belt conveyor. Direct interface: Requires appropriate width of belt (and hence foot print) Increased energy requirements Correct tensioning must be maintained Cheap / simple Feeding from outlet to belt (use of vibratory tray) Reduces wear on main conveyor Main conveyor can be narrower and run at higher speed Reduced energy consumption More expensive
Feeding of pellets Ideally, the cross-sectional area of the vessel should be fully activated during discharge. Methods for achieving this: Multiple screw sets Orbital screw From these mechanical techniques, screw dischargers can often have issues.
Screw feeders Ideally, the cross-sectional area of the vessel should be fully activated during discharge. Problems can arise from the use of screws that develop inadequate capacity along their length. Such designs result in poor control of residency and often poor transfer rates. Standard practice screw design (single screw)
Screw feeders The use of screws that are designed to develop an increase in capacity along their extraction section will provide: Controlled residence time Improved transfer rates (by virtue of improved packing) Concept of even draw down (single screw)
Screw feeders Screw proportions for obtaining an even draw down of product
Fixed Screw feeders Problems can arise where the product to be discharged is poorly defined or changed from that originally anticipated: Poor transfer rates Increased occurrences of hang-up or flow stoppages Failure to take into account the bulk behaviour and particle shape issues are normally to blame An example of a screw featuring insufficient increase in capacity An example of an optimised screw featuring an increase in pitch and a decrease in shaft
Orbital screw feeders In orbital reclaim storage systems offer the potential for full cross sectional activation and therefore a control of residency. However, problems can develop if the screw is not optimised. The most common design issue comes from the adoption of a design of screw offering an increase in capacity along its length but not fully taking into account the ratio of transfer capacity with respect to angle of sweep.
Orbital screw feeders (non-optimised capacity increase)
Orbital screw feeders (optimised capacity increase)
Dust control - traditional Bateman Image c/o Bateman
Hood and Spoon Design Gentle transfer to avoid knocking dust out of flow Design curve to suit particle trajectory Adjustment needed on site Definitely NO wet sprays! Image c/o Martin Engineering
Summary Most problems arise from: Failure to correctly characterise the bulk properties of irregularly shaped material at the design stage for new build projects. Imbalance between equipment functionality, purchase cost, life cycle costs and correct evaluation of risk. As for any bulk handling scheme, it is essential to invest time in understanding what the properties of the materials being passed through the process and to use this information in assessing the adequacy of any designs or plant modifications.
Storage and Handling Requirements for Biomass Richard Farnish, CEng MIMechE www.bulksolids.com