Flow properties and design procedures for coal storage bins

The handling and storage of black coal has always presented industry with problems of erratic or spasmodic feed, partial reclamation of the total contents of bins and flow blockages at hopper outlets. These problems can lead to extreme cost penalties for all users, from the coal producers and export market loading facilities to the secondary industries using coal for process energy requirements. Any reduction in the occurrence of these handling problems and the subsequent increase in efficiency would be of benefit. The aim of this work was to investigate two major aspects in the design of coal storage bins to ensure reliable and predictable operation, particularly in regard to gravity assisted discharge. First an experimental study investigated the flow properties of black coal and the influence on these flow properties of variations in the physical characteristics of the test samples. Variables considered included moisture content, particle top size of test samples, coal particle shape, time consolidation at rest and ash content. Samples for the test program were obtained from the six collieries located in the Southern Coalfields (Illawarra Measures) of the Sydney Basin of New South Wales. The coals ranged in rank from sub-bituminous to semi-anthracite. The study highlighted the most influential variables to be moisture content, sample particle size and time consolidation at rest. Other factors such as particle shape, coal rank and ash content were minor considerations. Often a variation of variable affected other properties and led to decreased sample flowability. A common example was that of coals with a high friability; this leads to greater particle degradation and generation of fines with handling operations, which then leads to higher moisture retention capabilities and significantly large critical arching dimensions, particularly with time storage. The flow property testing program utilised a Jenike - type Direct Shear Tester for the coal sample shear testing. To improve the consistency of this instrument, and eliminate operator and test data interpretation related errors a standardised testing procedure was developed. The second aspect of investigation dealt with the design procedures for the determination of mass flow hopper geometries based on the coal flow properties and utilising the well accepted theories of Jenike. A novel method of design data presentation was developed which links the flow properties and the hopper geometry parameters. This was achieved by presenting all parameters as a function of a common independent variable, the major consolidation stress. This approach has advantages in accounting for experimental error in the flow properties and for the determination of hopper geometries that have design constraints. The hopper design procedures were further advanced by the development of an alternate presentation of the original Jenike flow factor charts .These alternative charts have been abbreviated to display only the critical design values in the border region between mass flow and funnel flow. The charts eliminate the need for imprecise parameter interpolations by displaying the required design parameters in the form of contours of constant wall slope and flow factor as a function of the effective angle of internal friction and kinematic angle of wall friction. These new concepts were combined to allow the generation of manual hopper geometry design nomograms or worksheets. This design presentation represents a compact and rapid method for the determination of mass flow hopper geometry parameters for axisymmetric and plane flow outlets. The influence and sensitivity of the coal sample variations was explored further by determining the hopper geometry parameters of wall slope and outlet dimension based on the respective flow properties. This has allowed standardised hopper design guidelines to be formulated. An important aspect highlighted by this study was the significant role of wall friction in achieving a successful design. In consideration of the design procedures for bulk solid storage, computer software was developed and implemented for the computer aided design of storage bins. Two programs were developed, the first, to aid in the rapid processing and analysis of experimental flow property data, describing the flow properties by empirical equations and graphically. The second program utilised the empirical flow property equations for the determination of critical hopper geometry parameters and the generation of other design graphs. The programs operate both on a mainframe computer and a microcomputer, and utilise interactive execution and high resolution graphics