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Thread: Wood Chip Handling

  1. #1
    nick.monether Guest

    wood chip handling

    We are retrofitting small sites (schools) with wood fired boilers and need a pragmatic solution for fuel delivery & storage.

    One route is standard agricultural silos filled by 'big bags'. This will be time consuming and not ideal.

    ro-ro bins appear expensive for this application.

    Does anyone have experience of augur / elevator / other means for rapid delivery of woodchip from bulk lorry to silo?

    woodchip c. 30mm, mc 20%

    Appreciate any advice offered.

  2. #2
    Dennis Hauch - Freeport, TX, USA Guest

    Woodchip Handling

    Nick,

    I would do some feeding / pneumatic conveying tests with the woodchip material. If these prove successful you could consider receiving the woodchip material in a tipping tanker. That would make the unloading / silo filling process very easy indeed.

    Regards,

    Dennis Hauch

  3. #3

    Woodchip handling

    Nick,
    Bulk woodchips are typically handled by open tipper truck, 40 to 50 cu.m (or more) capacity, and delivered to a "walking floor" which feeds the material to a screw conveyor. This does not seem practical in your case. If you are able to arrange for the material to be delivered by pneumatic tanker, that is probably the best solution (unloading by pneumatic conveyor into a silo). Your ideal solution depends on how you are able to arrange transport. Some discussion with transport companies who handle waste products would be a good place to start.
    Don't forget, storing the material in a silo is going to raise other problems for you. The manufacturers who use wood chips for particle board and MDF have to go to a lot of trouble to get the stuff out of silos. There are no simple, inexpensive solutions for this issue.
    I have lots of experience here. Please let me know if you need more info.

    Good luck,

    Michael Reid.

  4. Dear Nick,

    It would be better if you could inform the height and diameter of the storage silo. Also, the unloading capacity in tonnes/hr or time interval for unloading of the truck of specific capacity. What would be the weight of one piece of chip or its thickness and width, so as to judge its suitability for type of conveying.

    Regards,
    Ishwar G Mulani.
    Author of Book : Engineering Science and Application Design for Belt Conveyor
    Email : parimul@pn2.vsnl.net.in

  5. Dear Nick,

    I gather from your description that you have not, as yet, installed silos behind the schools -- and that your primary objective is minimize the unload time from the delivering carrier to storage. If so, a simpler answer may be to use a block (in your case English stone) building that the delivery vehicles can drive up to and dump directly into the top. The wood chips can then be moved to a chute to feed the boiler by a front end loader. Alternatively, a belt system to move the chips from storage to the boiler can be used, if you install a gate system under the storage pile.

    Naturally, there are other alternatives.

    Regards,
    Dave Miller
    ADM Consulting
    10668 Newbury Ave., N.W.,
    Uniontown, Ohio 44685 USA
    Tel: 001 330 265 5881
    FAX: 001 330 494 1704
    E-mail: admconsulting@cs.com

  6. #6
    BrianHRutledge Guest

    Wood chips

    Don't forget the potential of heat generation in storage, especially near a school.

  7. #7
    Stan King Guest
    Listen closely to what Mike R. has to say. As plant engineer I have been dealing with wood waste/chips from plywood, sawmill, OSB for over 30 yrs and will 2cd the fact that storing in a cylcone with a tapered cone at bottom is asking for grief. Is it a homogenious mix or random with sawdust/flakes/chips. Any bark thrown in for good measure? How uniform are the chips? Best bet is no cone, altho manufacturers sell them, there are better ways to go with straight side wall storage. If storage is small enough, even a screw feed out would be much better than tapered cone. Have fun!

    Must be old age, wanted to say silo not cyclone!

  8. Hopper for wood chips

    Re - Feeding Wood chips

    As many of the contributors have said, the awkward flow properties of wood chips can give rise to flow difficulties, with serious arching, piping and rathole formation problems. Cones are just about the worst shape for flow, although convenient for strength and fabrication. The basic design impediment in this application is that the shear strength of this type of material grows very rapidly with compacting stress. There is an associated difficulty in that the description ‘wood chips’ is totally inadequate to define the product and its shear strength is subject to wide variations according to its composition and condition. Both these obstacles to determining a suitable hopper design are compounded by the elastic nature of the material in bulk form.

    The problem is not insuperable, but is not to be taken lightly. The most effective route is to employ a container that exploits Vee section flow channels with Sigma-two relaxation of the end walls and insert technology. This construction allows the flow channel to diverge slightly in one plane whilst converging in the other. The effect is to reduce the confining stresses and facilitate minimum converging wall inclinations to be used and flow to take place through the minimum possible width of opening. Although this technique is not as effective on materials that have a very high value of internal friction, it still counters the end-wall effect of a plane flow channel and offers a weaker region for failure than the normal wedge shape. Measurement of wall friction values with different contact materials and surface finish also permits an optimum selection to be made of the contact material for the construction of the walls.

    At this point it should be recognised that in certain regions it is advantageous to employ contact surfaces that have a high frictional value and flow obstacles, whilst in other locations the lowest friction value possible is needed. This is because high resistance to slip and flow where flow is not a problem will reduce the flow over-pressures lower down in the stored contents, where the span of the flow route is less. The profile of the flow surfaces is established from measured wall friction values, to determine the wall inclination needed for mass flow and that for self-clearance, for where these modes of flow are appropriate.

    The next stage is to find the critical arching span for the material at differing conditions of relatively low consolidation pressures and time storage, if time effect are found to be relevant. It should be noted that a Vee hopper is not able to sustain a stable ‘rathole’ if live flow is generated over the full length of the outlet slot. A key feature of the installation is that inserts are used to control the maximum over-pressure that can develop in the flow channel, and therefore limit the span over which a stable arch can form. The design and placement of inserts can then be evaluated from the determination of arching potential at different heads of storage. The overall design can then be firmed up from this data, to accommodate any cross sectional area required. All that remains it to fix the length of the outlet, in an interactive exercise with the setting of the cross section of the container, to secure the total volume of storage that is required for the installation.

    Well, almost all that remains. It is still necessary to install an extraction device that will guarantee to generate live flow over the total cross section of the outlet slot. A well-designed screw feeder can develop this pattern over an axial length of up to about ten diameters of the flights, but an aspect ratio of eight to one is a more conservative working figure. Two or more screws can be used to give a wider opening than obtained by a single screw, but the use of multiple screws can lead to very high over pressures and large torques on the screw drives.

    The reason for referring earlier to the slope of the wall inclination needed to affect self-clearing is that a mass flow channel is not necessarily the best form of flow channel in these circumstances, in fact in some regions is best avoided. The gain of width of the container within a given height is achieved far better, and reliable flow more effectively achieved, by inserts that form twin unconfined flow channels to an orifice slot, than with a mass flow channel that induces high passive converging stresses on the flowing bulk.

    The design sophistication of this approach should not suggest that the constructional cost is more expensive than a conventional ‘brute force’ approach in fact it should be less. The development of an effective design does involve more thorough and experienced ‘up-front’ attention and costs, but offers considerable more interest and satisfaction, with eventual cost savings, over crude designs such as using vertical or diverging hopper walls with a full ‘live-bottom’ design that employs multiple screws. The nature of the material demands that special attention is inherent in securing a solution and this must cost more than a storage device for a free flowing material. There is nothing more expensive than plant that does not work and the overall viability of the plant must be verified by the total economics of the undertaking. If the installation is to be repeated for many locations, I would suggest that it is well worthwhile undertaking a professional approach of this kind, with start-of-art technology. All you really need to do is find your expert with whom you can work closely.

  9. #9

    Wood chip handling

    Two companies in Europe design and install silos for handling wood chip, and other difficult materials, SHW and Saxlund. Their equipment is used extensively in particle board and MDF plants world-wide.
    If you need to know how to store and retrieve wood fibre, don't try to re-invent the wheel, contact them.

    Michael Reid.

  10. #10
    Join Date
    Mar 03
    Posts
    1,641
    I think the problem here is one of scale. As Michael Reid has said companies like SHW, Saxlund, Redler and many others have established equipment for handling wood chips, sawdust, wood waste and similar products. However such equipment is normally used in particle board plant and now in biomass power stations, operations which are much larger than that which would be required for a school in the UK.

    Because of the well known poor handling and storage properties of these materials the necessary equipment is not cheap. Just because the handling rates and storage capacities for a school would be low it is not easy to shrink down the size of equipment necessary. The capital cost for school installations must be a major consideration in the current UK economic climate.

    Also, there is the question of operation and maintenance. Gas or oil fired boilers are relatively simple to run and maintain as handling and storage problems are minimal. This is not the case firing boilers with wood chips. Even ignoring the ash collection and disposal, the reception, storage and handling of wood chips etc will require significantly more attention. Remember just one piece of tramp material cam wreck havoc with the handling equipment, will the wood chips really be free of tramp? Is the necessary labour to look after the plant available at a school?

    Overall I wonder if such schemes are actually viable in the long term for such small scale installations.

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