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Move in the right direction

Two Saxlund chain conveyors

While the bucket loader still plays an important role in industry, large scale and automated bulk material handling solutions to receive, store, convey, dose and feed material are increasingly important to a growing number of sectors, especially energy from waste, gasification projects, biomass and other alternative fuel applications.

You would think it’s all very straightforward: moving something from A to B and perhaps storing it for a time, before discharging it with a chain conveyor or screw to feed a downstream process.

But materials such as woodchip and other biomass products are notoriously tricky to handle, often sticky and abrasive, non-free-flowing with a tendency to bridge and compact. It means the flow can easily clog and block, disrupting dependent processes up and down the supply chain.

So understanding material behaviour is crucial to system design and plant reliability. This includes particle size, mass flow properties, density variations, moisture content and compressibility. No two materials are alike, and composition or source may change over time, which needs to be factored into the design to ensure an enduring solution.

All this can be challenging for mechanical and process engineers unless they are working with these systems on a continuous basis. Nothing beats experience.

A typical system will involve a reception and storage solution, with discharge and reclaim conveyors, screening to remove contaminants and non-spec material, weighing and dosing systems may also be required, certainly for waste to energy plants.

Silos and storage bunkers

Storage and process feed silos and open-air bunkers need to be designed to provide sufficient on-site storage between deliveries, so that operation of other plant processes is uninterrupted. A silo that is never more than half full is a wasted resource and will take up more space than it needs and adds cost. Getting these calculations right is important, especially where space is at a premium.

For woodchip, wood pellets and other biomass products, truck reception units, which allow the vehicle to discharge contents directly into the storage bunker, are the norm, but other infeed and storage solutions are widely used.

First in-first out

Aside from a tendency to cake and compact, many materials have a limited shelf life and lose calorific value over time by degrading and sweating. So discharge and extraction of material should be based on ‘first in, first out’ principles.

There are a number of options here including push floor or sliding frame technology, continuously developed by Saxlund since the 1960s. A reciprocating action back and forth across the floor of the silo breaks up the material, preventing any build up and ensuring consistent delivery to the discharge point.

Saxlund push floors at Fiddlers Ferry power station

Saxlund push floors at Fiddlers Ferry power station

Saxlund’s TubeFeeder reclaimer technology is for the continuous reclaim of bulk materials from circular and rectangular bunkers and large-diameter silos. It is especially suited to processes that require a higher level of accuracy and large storage volume.

From this point an arrangement of conveyors, bucket elevators, screening and weighing solutions will be required to deliver the material consistently to the next process. Stringing this through and across existing infrastructure might also be required.

The key here is designing-out pinch points and anywhere that material can build up. Transfer points between conveyors need careful attention.

Preventing unplanned shutdown

Minimising wear and maintenance issues are also crucial to avoid the frequently hidden and high cost of unplanned shutdowns. Upfront investment in quality will always pay dividends.

Material choices are important, along with externally accessible maintenance areas, platforms and walkways, combined with cleaning and preventive maintenance regimes. The goal is to deliver the lowest cost of ownership for the lifetime of the plant and design-out premature failure.

Matt Drew is managing director of Saxlund International

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