A fundamental consideration for handling and discharge arrangements for many types of bulk solids products is that relating to controlling the generation of fugitive material (dust) from processes or handling operations. The need to control dust levels is driven a number of factors such as ATEX/DSEAR and plant hygiene.
High levels of fugitive particles in suspension can sometime be a result of the stripping away of ‘piggy back’ fines (i.e. dust that has attached to the outside of larger particles and is transported with them through the process) by impact or counter directional air flows.
Alternatively, increased fines/dust can be generated through the breakage of larger particles (i.e. impact or shear plane damage) in bulk materials that may be considered fines deficient in their pre-processed form.
Handling operations that have the potential for removal and evolution into the atmosphere of ‘piggy back’ fines could include the filling of bulk storage schemes by either gravity of pneumatics – where significant free-fall distances or high velocity impacts exist (both aspects serving to evolve fugitive particles and establish air turbulent condition to transport this fine material).
The direct generation of dust through particle damage induced by shear forces can occur in large stores where the discharging flow channel expands and flows through static regions of product (often referred to as core flow or internal mass flow) or where constant capacity feeders such as augers, drag links or belts extract from long outlets without optimised design or interface considerations – in which case the feeder will tend to drag conveyed product beneath a non-activated region of product.
Any fines/dust generated through either of these common handling operations may not necessarily become a problem in later stages of operations. For example if fines/dust are generically considered to be represented by sub 3.15mm (as is the case for biomass pellets for example) and constitutes 2% by weight per cubic metre of product, there may be no issues of excessive dust mobilisation through subsequent handling provided that that size fraction remains homogenously distributed within each volume of product.
However, problems often result when products are loaded into storage schemes where many cubic metres are held. In such instances the simple operation of filling has a strong likelihood of mobilising the resident fines and redistributing them within the bulk – with the result that some volumes of material may contain less than the nominal 2% wt, whilst others could contain in excess of 10%wt (instances of up to 25% in some extreme cases are not unknown).
Thus, during the discharge or load out operations from the store/vessel, the dust levels may fluctuate considerably depending upon the redistribution of fines within the mass of material being extracted at any given time depending upon the method of extraction and the inventory level within the store.
The implication of this variance in dust content lies in how the specification for any locally applied dust extraction system has be developed. Using the 2%wt fines model again, it can be understood that this value should be used to size the filter area for an extraction system and the type of filter operation. On first consideration, the use of the ‘specification’ fines content would seem a prudent basis for design.
Economic considerations tend to dictate that the specification of the filter would be closely matched to the application requirement (i.e. there may be physical restrictions that limit the size of filter housing / extraction unit – or, of course, it may that the budget does not adequate provision for a filter house that is any larger than absolutely “necessary”).
However, once installed and commissioned intermitted extreme fluctuations in fines/dust content have the potential to overwhelm the ability of the dust extraction system to operate under stable conditions. Such overload conditions could result in blinding of the filters or excessive air consumption (these potential problems being linked to many different factors ranging from media specification, filter design, reverse pulse layout, cleaning control methodology and pressure settings).
An assessment of the potential dustiness or brittleness of the material being handled can provide the first element of a structured approach to pre-empting fugitive particle levels or the scope for variations in fines/dust released in handling operations and, in turn, developing an effective specification for associated plant.
The assessment to characterise bulk particulate materials for dustiness or breakage behaviour can be undertaken at laboratory scale. The evaluation of dustiness (i.e. surface attached ‘piggy back’ particles) can be effected through the use of a Warren Spring type tester which takes the form of a closed drum that has internal ‘lifting’ slats attached at six points around its circumference that extend down the length of the chamber.
The test sample is tumbles inside the drum whilst an air flow is allowed to pass from one end of the chamber to an exhaust port at the opposite end which delivers dust laden air onto a filter element. The gain weight of the filter is taken as a benchmark indication of the ease with which dust can be liberated from the parent particles.
Breakage tests can be undertaken that impact particulate material at controlled velocity against impact targets that are arranged around a centrifugal accelerator. Assessment of breakage is simply undertaken by the comparison of particle size distribution shifts (with most interest being focussed on increasing fines content in most cases) in response to increasing impact velocities.
Both of these methods represent useful techniques for obtaining information for comparison of the likely dust and breakage behaviour for different bulk materials. The tests can also accommodate variables such as temperature or moisture content variation (both of which can be influential factors relating to dustiness).
In terms of counter measures to deal with excessive dust mobility, one of the key aspects of best practice is to avoid the establishment of high velocity movement of particles or dispersion at loading points (whether at large scale such a silo/bunker – or at smaller scales with bag filling/dosing systems) – both being factors that facilitate the penetration of air into the bulk material and the stripping out of fines/dust.
Finer material having a lower mass and correspondingly relatively large surface area, has a high drag factor – which endows it with a substantially higher mobility in air compared to coarser particles. Belt transfer points are a prime example of where dispersed particulate movements occur, which means that not only can a degree of breakage occur, but also that an existing or additional fines can be easily dispersed to atmosphere or dragged by air flows along transport tunnels.
In conclusion, it is hoped that it can be appreciated that a problem such as dust emissions at certain points in a process can often be the end result of a cumulative breakage and concentrating of fines/dust that has occurred through the plant. Methods exist to design out a wide range of the causes of dust and variable dust loading, but also to reduce the magnitude of problem caused by dusty materials.
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