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Improving performance of discharge equipment for coals with poor handling characteristics

Improving performance of discharge equipment for coals with poor handling characteristics

Ariza-Zafra, Karol (2015) Improving performance of discharge equipment for coals with poor handling characteristics. PhD thesis, University of Greenwich.

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The accepted design techniques for bulk solids handling equipment are frequently overlooked during the installation of industrial process plants. As a result, flow unreliability is often observed in silos in the form of flow stoppages, product bulk density variations, formation of rat-holes, flushing, flooding and product segregation. In most cases when these problems are detected in silos, they are the result of a discharge pattern known as core flow, where localised flow channels promote the preferential draw of material from certain zones of the silo, with the rest of the material remaining stagnant. Coal handling is not the exception and process equipment is not always designed to cope with the often variable characteristics of the coal, which is frequently processed in a large variety of forms, with particle sizes ranging from fine dust to a top size several inches and other parameter like moisture content varying from completely dry to dripping wet.

In order to solve the problems caused by core flow, the discharge behaviour of the silo needs to be modified to produce a uniform movement of all the material, to achieve a flow pattern known as mass flow. Static inserts have been proved to be an effective method of modifying the discharge patterns in silos, but their use and design procedures are not well understood or are well hidden behind patents and trade secrecy. This research project aims to produce practical guidelines for the design and positioning of static insert to improve flow in silos.

The work presented in this thesis follows an experimental approach where the performance of an insert is first evaluated at bench scale in a 3 litre model silo and then validated at semi-industrial scale in a 400 litre test rig. The bench scale model allows the evaluation of numerous changes in insert morphology and positioning at relatively low costs, facilitating the development of practical rules for their design. Following this approach a design procedure for inverted cone inserts is proposed as a modification of a method developed by J. Johanson [Johanson, 1965]. A performance comparison was undertaken both at bench and semi-industrial scales with inserts designed following Johanson’s method and the modified method proposed by the author. The results showed that both inserts were capable of producing mass flow in an otherwise core flow silo, but the modified insert produced more consistent results, particularly with lower heights of powder bed.

This experimental approach was also followed to develop a novel type of insert called open double cone which maximizes the area of influence of the insert inside the silo facilitating flow. For this insert, three design procedures where proposed with each of them producing inserts capable of achieving mass flow in the bench scale model. The main difference between the inserts produced by the three procedures, was the size of the insert in relation to the volume of the silo hopper. In a similar way, two procedures were also proposed for the design of double cone inserts, with the resulting inserts capable of achieving mass flow in the bench scale silo. Then, a prototype of an inverted cone designed with the modified method, a prototype of the open double cone and a prototype of a double cone were tested in the semi-industrial scale test rig. The results at both scales showed that the open double cone and the double cone inserts outperformed the inverted coned, by producing more uniform velocity profiles across the silo and also producing more consistent flow rates. Although the performance between the open double cone and the double cone was very similar, the open double cone was more consistent in producing flatter velocity profiles and also the double cone was more prone to produce slightly off centre discharges.

The procedures proposed for insert design provide the tools needed to apply insert technology to industrial processes. This is demonstrated with the design of double cones which successfully eliminated rat-holing problems in conical silos from an industrial pneumatic conveying system. The bench scale methodology is also employed to try to solve flow unreliability issues experienced in an industrial coal silo. For this case, a bespoke type of insert was developed to respond to the complex geometry and mode of operation of the silo. The proposed insert produced very positive results at bench and semi-industrial scales, laying the bases for a solution for the full scale silo.

Item Type: Thesis (PhD)
Uncontrolled Keywords: Bulk solids handling, silos, coals
Subjects: T Technology > TN Mining engineering. Metallurgy
Faculty / School / Research Centre / Research Group: Faculty of Engineering & Science
Last Modified: 20 Sep 2018 11:51

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