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Technical Market Support

Links Between Microstructure Development in Softening Coal and the Characteristics Controlling Coke Quality

Technical Market Support » Metallurgical Coal

Published: August 15Project Number: C23048

Get ReportAuthor: Karen Steel, Robin Dawson, David Jenkins, Robin Pearce, Merrick Mahoney, Hannah Lomas, Richard Roest, Harold Rogers | University of Queensland, CSIRO, University of Newcastle, University of Wollongong

The goal of this project was to obtain an understanding of the microstructural features in coke responsible for its strength and elucidate the mechanisms involved in the formation of those features. To achieve this goal a number of tools were developed and used in combination. Micro-CT analysis was applied to 5 cokes produced in pilot scale ovens and having various strength indices. CT analysis was also carried out on samples quenched at various stages during coking to capture the process of pore network formation. The samples included rheometry samples to enable the results to be mapped onto the viscoelastic properties and Sole-Heated Oven samples to view all of the major stages of coking in a single image. The pore and wall (solid) networks were characterised by fitting maximum sized spheres into the spaces and then obtaining the distribution of those spheres.

 

CT analysis of the rheometry quenched samples showed that the pore network becomes highly connected very early in the softening phase. This result confirmed Normal Force measurements to be a good indicator of the point of bubble coalescence and the point at which the pore network becomes highly connected. The size distribution of the connected pore clearly showed a contraction in size following bubble coalescence, indicating bubble coalescence to be a key reason for contraction of pore networks. However, rheometry quenched samples also showed a steady increase in pore size up to the point of resolidification, indicating that applied forces also play a crucial role in pore network contraction.

 

CT analysis of SHO semicokes provided insights into the early stages of softening in which bubbles could be seen forming inside large particles. In this early stage bubbles are isolated and the permeability of the plastic layer is very low. A significant amount of coalescence of the bubbles appears to occur during the plastic phase. Contraction of the semicoke could be seen and also banding of high and low porosity within the semicoke for a high vitrinite sample. It is speculated that coal grind in addition to viscoelasticity may influence permeability of the plastic layer and consequently affect volatile release, contraction and ultimately pore size distribution in the coke. Therefore, coal grind could be used to influence pore size in coke and hence strength. The pore network properties of the SHO semicokes mimicked those from the pilot scale cokes although the SHO pores were smaller and walls were thicker.

 

The cokes studied in this project had variable inertinite levels and the weaker cokes tended to have higher inertinite. However, one of the cokes (C158) had high strength and significant inertinite. Finite element modelling was undertaken to obtain the number of high stress points in the cokes, called the critical stress index (CSI). A good correlation was found between the CSI and the M40, which showed that C158 did have a low CSI. CT analysis on the cokes before and after subjecting them to high load to cause them to crack showed the crack for C158 to pass straight through some inertinite, rather than around it. This indicated that the inertinite in C158 was well-bound. Whether the low CSI is due to this or other aspects of the structure is not known at this stage. The reasons for high and low CSI needs to be studied in more detail.

 

CT analysis of matched blast furnace feed and tuyere cokes as well as cokes subjected to heat treatment under neutral and simulated blast furnace gasification conditions was also carried out. Clear structural changes were observed from the feed to the various tuyere coke samples (bosh, raceway, birds next and deadman). Interestingly the CSI for the tuyere cokes were similar or superior to the feed coke values despite an increase in porosity. Assuming material properties had not changed significantly this would imply the remaining lumps low in the furnace would have good strength. The laboratory heat treated samples showed clear changes in mineral size and distribution associated with some high temperature reactions.

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