Biomaterials challenge

The current political drive behind recycling of waste material and the generation of power from renewable energy sources such as biomass has led to a significant increase in storage and handling problems associated with extreme shape particles, grasses, straws, wood chips & municipal wastes.

The standard classification method for shape draws a cuboid around the particle as shown.  Problem bulk solids are those with particulates that are fibres or flakes i.e. where one particle dimension is either very much larger or very much smaller than the other two.

These types of bulk material have the potential to nest - pack into structures which are very difficult flowing. This is aggravated by the low density of the materials which means that the gravity forces acting to cause flow are very low.

The flow issues with extreme shape materials is best conceptualised by considering what happens when compacting a column of particulate material in tube. 

With a conventional granular material that has approximately “rounded” particles, when it is compacted vertically the particles try to rotate about their contacts leading to the development of a horizontal stress (the stress ratio of the material which is typically 0.4) and a degree of lateral spreading, or flow of the material. 

If this test is repeated with fibrous material, often the particles are elastic and therefore deform and flatten so that the transmitted stress remains vertical. 

The horizontal stress then remains close to zero so the fibrous material does not spread laterally and flow, like a conventional material. 

If the extreme shape particles have a rigid structure, the vertical deformation is reduced, but particles are unable to rotate due to the large number of contacts along their length, so they retain the ability to transmit a vertical load without spreading laterally.

This is illustrated in by the nesting of matchsticks as the confining tube is lifted. Note that when the matchstick column eventually failed it was due to instability or buckling rather the self-weight exceeding the compressive strength.

Problems are likely to occur if these fibrous materials are stored in conventional converging hoppers intended for granular particles, with the material forming stable obstruction over the outlet. 

A recent MPhil research project at The Wolfson Centre investigated the flow extreme shape materials. This indicated a potential route forward but at present there are no reliable design guidelines for fibrous materials.

The current industrial best practice for storage vessel design for reliable discharge of fibrous materials uses the same equipment that would be used for very difficult to handle or “sticky” ordinary bulk solids. 

That is a parallel sided, full-live-bottom discharger, thus removing the converging section that the material struggles to flow through. 

The downside of this approach is that a large feeder or multiple feeders are required to extract the material over the full cross sectional area of the vessel.

The loads acting on the feeder are high so that it needs to ruggedly engineered and therefore expensive.  There are a number of discharger types for achieving a full live bottom which this include a moving floor, multiple screws, orbital screws, vibrating slats and non-consolidating feeders.