ne of the greatest advancements in the engineering of R. Todd Swinderman, P.E., coal handling systems is the increased use of bulk material science. This field is focused on the testing and and Andy Marti, analysis of both the bulk solid and the construction Omaterials that the bulk solids will move on, over or through. Bulk Martin Engineering, US, material science is an interdisciplinary field, centred on determining the physical properties of bulk materials and applying those discuss how bulk material properties to various problems in the design of bulk material handling systems and components. science is improving Combined with computer-based engineering and modelling systems, bulk materials science offers several potential improvements power plant coal for coal handling operations in power plants. These include helping to manage flow, reduce bottlenecks, minimise dust, spillage and handling. carryback, extend equipment life and reduce maintenance expenses – all improving a plant’s availability, efficiency and profitability. Since the first conveyor was designed, the basic properties of bulk materials, such as bulk density and angle of repose, have been used to size the system and calculate its power requirements. Modern bulk

January 2012 | World Coal | 51 material science traces its roots to Improved testing and computerised horizontal. The surcharge angle is work at the University of Utah, which engineering now make it possible to useful in conveyor design for determined the critical dimensions of improve designs of specific systems to determining the profile of the load on mass-flow bins, based on the strength perform as expected with identified the belt for various belt widths and of the bulk material under various materials. Serious errors can be made trough angles to calculate the conditions. These methods are used to if a material handling system is theoretical carrying capacity. determine the internal strength of bulk designed without determining the solids and the friction between them appropriate basic and advanced Lump or particle size and the belt or chute. These properties properties of the specific bulk material The size of a bulk material is are used to predict the behaviour and being stored, conveyed or otherwise described in two ways – as the flow of the bulk solids from silos and handled. maximum lump size, or as the percent storage vessels and, later – with of particles that will pass a series of increasing success – through chutes Standards standard screens (or sieves). Size is and onto belt conveyors. Many basic properties and tests for often listed as the maximum lump bulk solids are outlined in the width and breadth. A material with a Conveyor Equipment Manufacturers maximum lump width and breadth of Association (CEMA) Standard 550. 50 x 50 mm (2 x 2 in.) would be The properties most often used (and described as “50 mm minus” (or 2 in. sometimes misused) in the design of minus) material. This means the belt conveyor systems are: largest lump is 50 x 50 mm and the rest of the particles are smaller. It is Bulk density common practice to assume the The bulk density of a material is the length of the lump can be as much as weight per unit of volume (kg/m3 or three times larger than its width, or in lb/ft3), measured when the sample is the above example 150 mm (6 in.) in a compacted condition. This long. This information is useful in vibrated or settled density is used sizing various components, as well as with the surcharge angle for the width and height of chutes and determining the volume of material skirtboards. conveyed on the belt. A screen analysis gives the most complete representation of the size of Loose bulk density the bulk solid. The particle size CFI ring shear tester: the ring shear tester Loose bulk density is the weight per distribution is a tabulation of the is used to measure the flow properties of unit of volume measured when the percent represented in each size range powders and bulk solids. sample is in a loose or non-compacted as part of the total sample, usually condition. The loose bulk density shown as passing a given screen size must always be used when designing and being retained on the next the load zone chutes and the height smaller screen. This information is and width of the skirtboards, or the useful for analysing airflow in chutes chute may not be able to handle its and the potential for the creation of specified design capacity due to the airborne dust. increased volume of the material. Flow properties Angle of repose The basic flow properties of a bulk The angle of repose for bulk materials solid can be derived from shearing is the angle between a horizontal line the bulk solid (using a shear cell) and and the sloping line from the top of a measuring the force required. Usually freely-formed pile of bulk material to the fines from the bulk solid are the base of the pile. This angle of tested, since they are the portion that repose for a given material may vary, changes strength with moisture and depending on how the pile is created, pressure and acts as the “glue” that as well as the density, particle shape, retards flow. size consistency and moisture content The shear cell method is also used of the material. to measure the friction between the bulk solid and the belt or chute This Instron provides the basis for Surcharge angle construction materials. For conveyors, a variety of physical property tests, The surcharge angle is the angle of the important conditions are the including rigidity, tensile strength, and the load cross section, measured by variation in strength with different compression. the inclination in degrees to the moisture contents and consolidating

52 | World Coal | January 2012 can be determined from shear cell tests and is very useful in determining how bulk solids will flow. Applying test information This empirical knowledge is certainly useful, but what are the implications for material handling systems and the design of equipment? What can material science help determine more accurately? Conveyor capacity Conveyor capacity (tph) is one of the basic design parameters directly calculated by knowing the density of the bulk material. Density refers to solids such as steel or concrete (particle density). However, in conveyor design, there are several densities to consider that are typically fractions of the material’s particle density. Settled bulk density is used with the cross-sectional area of the load on a belt to determine the nominal carrying capacity of the conveyor. Loose bulk density can be as little as half the settled bulk density. The settled bulk density is the state of the material as it is normally carried on the belt. If a conveyor transfer point is designed using settled bulk density, it will likely plug at less than its rated capacity, because the material flowing from one belt to the other takes up The tensile strength test is performed by stretching an “hour-glass” shaped specimen more room. In this case, the plant is until it fails (breaks). trying to get 10 lb of loose material through a space designed for 10 lb of pressures. Shear cell tests are Adhesion condensed material. particularly time-consuming, because Adhesion can be thought of as the If an inexperienced designer looks in of the number of tests run at different stickiness of the material to surfaces, a general engineering handbook for a moisture contents and consolidating such as chutes and belts. Surface material’s density, the value listed will pressures. condition, moisture and impurities probably be the particle density. The (such as clay) are the principle particle density or true density of a Interface friction variables that affect the level of particulate solid or powder, is the Two values of friction are important in adhesive stress in a bulk solid. density of the particles that make up chute design – the co-efficient of Adhesive stress can be determined the powder, in contrast to the bulk friction between the bulk solid and the from shear cell tests and is very useful density, which measures the average chute wall, and between the bulk solid in determining the likelihood of the density of a large volume of the powder and the belt. Bulk solids, particularly material to stick or cling to surfaces. in a specific medium (usually air). the fines, have the ability to cling Using the particle density, the upside down on horizontal surfaces Cohesion inexperienced designer can oversize the and exhibit strength, even under Cohesion is the ability of the particles conveyor by afactor of two to three. negative consolidating forces greater to stick to each other. Cohesion in a This error will have serious than gravity. The shear force of bulk solid is affected by three consequences in unecessary costs. negative consolidating forces is of conditions: moisture content, particular interest in chute design in electrostatic attraction and Chute design determining adhesion and cohesion agglomeration (the tendency to lump Chute design is more than a matter of values. together into a mass). Cohesive stress having the correct cross-sectional area

January 2012 | World Coal | 53 based on the loose bulk density. The forces on a micro level than those of reliable flow of bulk solids through a particles typically used for testing chute depends, among other factors, bulk solid flow. The stresses in bulk upon the friction between the bulk materials are mainly created by solid and the chute walls and wear mechanical interlocking and surface liners. If the friction is too great, the moisture of larger particles on a material will slow in its passage macro level. Applying bulk solid through the chute. This decreased properties to carryback can flow leads to bottlenecks, buildups significantly underestimate the and blockages. projected carryback levels and The design of the new generation cleaning performance. At Martin Engineering’s Center for Innovation, the process simulation area of flow-engineered chutes depends on Adhesion and cohesion of incorporates a three-belt recirculating knowing the properties of the bulk carryback sized particles are conveyor loop for observation of solid in relation to the flow surfaces. important properties used in this material flow and component life. Most often, when a handbook value prediction. Knowing the material’s of chute angle based on the angle of critical moisture content – where its repose or surcharge is used, buildups adhesion and cohesion change leading to blockages are the result. dramatically – allows a designer to For example, lignite has a calculate the volume of water needed significantly higher co-efficient of to reduce the carryback material’s friction on stainless steel than strength or to wash the belt. bituminous coal, but the co-efficients of friction are similar when ultra-high Safety molecular weight (UHMW) Testing a bulk solid for its properties polyethylene is the liner. Serious flow allows the designer to develop safe problems can result from not testing storage and conveyance for bulk the actual bulk material transported solids. For example, flowing bulk and the actual lining considered for solids can create unequal wall the design. pressures on silos. Without testing the The direct shear tester determines a The data from the testing of the specific materials under the expected material’s friction, cohesion and adhesion specific coal being used and the conditions for storage and haulage, a to a substrate such as a liner material. specific construction materials, such designer is only guessing at the forces as stainless steel or ceramic liners, is involved. There have been many critical. It will help predict the flow of instances in which a worker has been the material through the chutes, injured by falling material when reduce wear on components, and trying to remove buildups from a eliminate the escape of fugitive chute. Less catastrophic, but just as material like spillage and airborne damaging to productivity, are dust. systems designed using typical or average values to design conveyors. Belt cleaning Many a conveyor designed without The properties of a bulk solid can the specific knowledge of the material only be used as a general guide to properties has failed to deliver its predict the nature of the belt cleaning design capacity. challenge the operation will face. By testing the properties of the The benefit – or A dry abrasion tester checks three-body carryback, predictions can be made as consequence – of material abrasive wear, as between a conveyor to how much the fines will adhere to testing: an example belt, a metal chute liner and the bulk the belt past the discharge and how The numerous varieties of coal can material. changes in conditions – such as an have very different characteristics. increase in moisture level from a The CEMA 550-2003 publication rainstorm – can affect carryback Classification and Definitions of Bulk The 6th edition of CEMA’s levels and cleaning performance. The Materials lists nine different Belt Conveyors for Bulk Materials gives fines used for testing bulk solids are classifications for coal, from 0.5 in. detailed equations for calculating the on the order of 2000 times larger than minus anthracite to ROM bituminous. capacity of a conveyor based on the the average carryback particle. This reference lists from 45 – 60 lb/ft3 trough angle and the surcharge angle. Carryback acts like a powder, where as the loose bulk densities for various To demonstrate the value of basic the physical properties are more coals, with surcharge angles listed material data, the cross sectional related to surface tension and nuclear from 20 – 30˚. areas found by using the values of

54 | World Coal | January 2012 two different coals from either end of The quantity of material conveyed cost of testing is a minor part of the the published list can be compared. at 500 ft/minute then ranges from overall cost of a material handling 1218 – 1890 tph (81.2 lb/ft x or conveying system. Having this Givens 500 ft/minute x 60 min/hr/2000 lb/t data is one of the most important to 126 lb/ft x 500 ft/minute x tools for trouble-shooting the ll Loose bulk density: 45 – 60 lb/ft3 60 min/hr/2000 lb/t). conveyor in the future, when (720 – 960 kg/m3). processes or raw materials change. ll Angle of repose: 30 – 40˚. Summary of example If an existing material handling ll Angle of surcharge: 20 – 30˚. The differences in loose bulk density system works now, it should ll Belt width: 48 in. (1200 mm). and surcharge angle yield a shortfall continue to work as long as the ll Trough angle: 35˚. in conveyor capacity of more than material stays the same and the ll Edge distance: standard CEMA 600 tph. This error would have a equipment does not suffer wear or edge distance. major effect on the ability of the coal abuse that changes its performance. ll Belt speed: 500 fpm (2.5 m/s). handling system to achieve its But changes in material – from production rate, and therefore hinder changes in source or increased The publication specifies the the ability of the entire plant to achieve moisture from rain, or from changes cross-sectional areas: its operational goals. If this plant relies in the process or in the equipment, on 1800 tph to fill its day bins or coal like increasing the speed of the belts ll 20˚ surcharge angle: 1.804 ft2 bunkers and only gets 1200 tph to move more material or changing (0.168 m3). through the system, the plant will a liner inside a chute – can have ll 30˚ surcharge angle: 2.100 ft2 need to operate its conveyors on a dramatic consequences on the (0.195 m3). longer schedule, increasing hours for performance of a coal handling both personnel and equipment, or system. Therefore, the range of the quantity downrate its generating capacity. And when a material handling of material/ft of the conveyor is: system is being engineered, whether What it all means designed from the ground up, or ll Cross sectional area x loose bulk No two bulk materials are the same, substantially rebuilt, then the density/ft3 x ft of belt no matter what type or classification materials it will carry need to be they are. This is the main reason carefully tested to achieve the In this example, there is a range of why physical testing of a bulk solid overall performance required and 81.2 – 126 lb/ft (1.804 ft2 x 45 lb/ft3 is so important to proper design of receive the maximum return on x 1 ft to 2.1 ft2 x 60 lb/ft3 x 1 ft). bulk material handling systems. The investment.