The expanding market for biomass fuels seems set to increase in the foreseeable future and the importation and supply chain are rising to the challenges in the form of infrastructure developments. One of the key challenges associated with biomass management lies in the combination of handling a time sensitive material in very large volumes which require a relatively rapid onward transportation. These characteristics have required a review of how to best store these types of materials and to interface these storage systems with the rail network.
Storage considerations
In general the ‘few/large’ – ‘many/small’ storage silo dichotomy seems generally to resolve itself towards the ‘few/large’ option. This has been mainly dictated by capital expenditure limitations driving ports towards lower construction cost – but potentially at the risk of larger inventory write-offs if/when a serious contamination or combustion event occurs. The sheer mass of material required for storage sees the use of ground storage silos (typically slip cast construction) and the use of gravity discharge to obtain a feed to material onto conveyor belts passing through subterranean galleries. The use of sweep augers to deliver the primary means of material extraction is encountered less often, and where such devices are considered, in the context of inventory removal, is more often for reclaim of stagnant regions of material resident outside the natural flow channel zone for gravity discharge installations.
One area of installation design that can be found to vary considerably is the point at which sampling of the incoming material is undertaken and, more particularly, the proximity of this point to the main storage silos. Why this feature should be noteworthy is determined by what the samples obtained will be assessed for. If the primary function of the sampler is to provide material for a calorific measurement, then the location of the sampler is not of such importance. However, if a moisture analysis (ie. dust content) or temperature measurement is taken, it could be prudent to position the sample point as far away from the silos as practical. In this way an early detection of material that may present a hazard if introduced into storage amongst good quality pellet can give the maximum response time to divert incoming contamination away from the silos and into an open spoil storage at ground level.
The fact that the vast majority of silo storage schemes operate on a ‘first in – last out’ basis means that if contaminant material is not actively prevented from entering into the silo, it will be drawn though material already within the store and partially or wholly out loaded during the next bulk transfer operation – if the silo is in active use. If the silo is being replenished following a complete or partial emptying (ie. multiple silos are be operated in rotation to obtain some degree of control over residence times) then the material could present a hazard (spoiling, cementation, combustion, etc.) if retained for too long. If such an issue arose, the implications in terms of storage volume taken out of commission for dealing with the situation would put significant pressure on the duty of the remaining available installation capacity. Of course this type of scenario serves well to illustrate how a ‘many/small’ storage strategy can minimise operational risk.
Volume transfers
Aside from the storage aspects of biomass pellets, the importance of conveying and transferring these in large volumes has also presented some interesting challenges. An important, but often poorly considered aspect of bulk pellet handling is the effect of fines/dust content. Clearly the dust evolved from the attrition of pellets through the multiple handling operations associated with cargo unloading, conveying, silo loading, discharge and subsequent conveying to rail loading must be managed. The main issue is linked to an accurate assessment of what dust levels can be anticipated to develop – since this has major implications for the specification of dust extraction plant (ie. duct sizing, air mover, and filter area in the bag house). Most specifications for pellets suggest that a fines content of up to eight per cent can be anticipated (ex-mill). However, irrespective of whether this is assumed to mass or volume, this does not mean that for any sample size taken from a process that a comparable quantity of fines/dust will be found. A more likely scenario is that fines/dust may vary from one to 20 per cent by weight (ie. up to 40 percent by volume) through the mass of pellets unloaded from a ship and handled through the port – simply through a combination of segregation and, to a lesser extent, particle attrition.
Presenting such a potentially variable bulk material the opportunity to mobilise dust into the environment is clearly not an option for any responsible plant design and in this respect the basic principles applied in other bulk industries (notably the minerals sector) for reducing dust emissions are often applied. These embody ensuring that the mass of material is maintained in a high density condition through transfer points. To achieve this effect ‘hood and spoon’ technology is applied at belt transfer points such that the trajectory of material leaving the belt is intercepted by a downward curving overhead plate which serves to deny the material the opportunity to disperse to a loose form. A second plate collects the densified flight of material and performs a change of direction such that the material is brought into alignment with the direction of movement for the belt onto which it is effectively laid. By preventing a dilation of the material the interaction with air is reduced to the outer boundary only and hence the scope for dust entrainment and transportation is kept to a minimum. This approach is particularly beneficial since, the use of misting atomisers to keep dust down cannot be readily applied to pelletised material due to the absorbent nature of the pellets (unless torrified of course).
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