An increasing range of industrial processes are incorporating waste of biomass materials into their operations. However, in many cases, issues of poor handling or storage characteristics can create considerable problems when these types of materials are handled in loose bulk form. This article will consider the issues that can arise when handling some types of biomass or waste materials (typified by extreme particle shapes – flaky, stringy, or elastic behaviour), and potential techniques for addressing these issues.
Typical handling issues
Most bulk handling facilities are well suited to dealing with a diverse range of conventional materials (such as ore concentrates, grain, etc.). However, when these systems are required to handle biomass type material problems can often arise.
Within bunkers or silos, materials that by visual inspection exhibit free-flowing behaviour when handled in loose form can be capable of forming substantial cliffs of self supporting particles or arching over outlets or feeders. In this respect, the bulk particulates resemble the static storage behaviour of cohesive materials.
The standard approach for the design of storage equipment to achieve reliable discharge consists of the use of measurements of wall friction, bulk density and internal strength measurements obtained from a representative sample of bulk particulate. However, the combination of irregular, large or flaky particle shapes for some types of biomass materials renders existing design approaches of limited value due to the problems of accommodating a statistically significant number of particles within test equipment designed for use with granular or powder forms of particulates.
Biomass, by its very nature, consists of degradable or fermentable materials. It also represents a bulk material that presents a media through which moisture can migrate or be absorbed. Thus, where large volumes of these materials are stored, considerable care must be taken to reduce exposure to the environment (wind erosion and moisture uptake) and to ensure that stock rotation of stores is undertaken.
Mobilisation and migration of moisture through stored bio mass materials is not an unusual phenomenon and is one that can be exacerbated if the storage bunker/silo is located on site where a significant differential between day and night ambient temperatures can occur. In such circumstances moisture liberated by evaporation can condense within the headspace of the storage structure and generate ‘rain’ onto the top surface of the stored material. Depending upon the levels of moisture mobility and location within the inventory, the material can also have the potential to ferment – generating heat in the process and providing an additional mechanism by which moisture can be driven from the bio mass. This mechanism is capable of driving the development of significantly sized concretions that may pose sufficient strength to completely block the outlets of equipment – sometimes requiring manual entry and extraction of the inventory from the top downwards in order to reclaim lost ‘live’ storage capacity. A good example of this type of problem would relate to the storage of pelletised citrus material (or other extruded waste products), which can expand considerably or (in some instances) disintegrate back to its original form depending upon the severity of moisture uptake from the environment.
Most materials that are received and handled in bulk form tend to be at very low moisture content. While this approach minimises the potential for bioactivity through subsequent storage schemes, it also endows the bulk material with a propensity to readily generate fugitive particles when handled. Although the issue of fugitive particles in the form of dust clouds represents an immediate hazard (in terms of not only inhalation by operatives, but also as a potential for dust explosion) it can also generate a longer term health hazard through mould growth/spore generation amongst the spilt and settled particles. The deposition of these fugitive fines can often be dispersed over considerable distances from the original source. Thus, the removal of these deposits can tend to be quite an extensive undertaking if carried out diligently around the site.
Minimising the problems
Improvements on site can be obtained in several areas of handling operations if the biomass material is particularly fibrous or exhibits high fines content (and is therefore prone to generating significant fugitive particles).