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Impacts of chemical structure transformation in the plastic layer on the microtexture development during coking

Technical Market Support » Metallurgical Coal

Published: December 24Project Number: C35045

Get ReportAuthor: Soonho Lee, Jangho Jo, Arash Tahmasebi | University of Newcastle

In this project anisotropy quotient (AQ) color-coded reflectance mapping was applied to observe microtexture transitions from the plastic layer to the coke/semi-coke regions, using samples from a 4kg double heated wall coke oven. Additionally, we used Synchrotron Infrared Microspectroscopy (IRM) to explore the chemistry behind microtexture evolution. These techniques were employed to achieve the following objectives:

  • Characterise microtexture development from the plastic layer to coke via AQ mapping.
  • Analyse chemical structure transitions in macerals, especially carbonising vitrinite across the plastic layer using Synchrotron micro-FTIR chemical mapping.
  • Correlate chemical changes in the plastic layer with microtexture development in coke/semi-coke regions, using parameters from both AQ and IRM techniques.
  • Examine how coal properties (rank, vitrinite content, fluidity) impact microtexture evolution during coking.

To achieve the first two goals, plastic layer samples were obtained from interrupted coking tests in a 4kg coke oven, capturing the plastic, semi-coke, and coke regions. The samples underwent AQ bireflectance scanning and Synchrotron IRM chemical mapping. AQ mapping across these samples was segmented and analysed to observe microtexture composition changes along the distance from the plastic layer to the coke region. Selected spots at characteristic regions were mapped with IRM to visualise colour coded chemical composition changes, including aromaticity, aliphatic branching degree and degree of carbon structure condensation.

For the third objective, results from AQ and IRM analyses were aligned to reveal underlying chemistry in microtexture evolution-a first in the relevant research literature. The alignment was enabled by high-resolution maps (AQ at ~1 micron, IRM at 1.75 microns). Ratios representing the formation of anisotropic and long carbon structures were calculated as a function of distance from the plastic layer to coke, illustrating microtexture changes from isotropic to ordered carbon structures. Different microtexture evolutions were explained by the IRM results, showing changes in spatial functional group distributions, aligned with the microtexture changes.

For the fourth objective, five Australian coking coals, varying in rank, fluidity, and vitrinite content, were selected for the developed analytical techniques:

  • We correlated these coal properties to microtexture evolution trends and relevant chemical transitions, improving the interpretation of final coke microtextures and their relation to the coke strength index (particularly Coke Reactivity Index (CRI)).

Key findings from the analyses can be summarised as follows:

  • Rank Effects: Higher rank coals formed more ribbon and lenticular structures from the plastic layer to semi-coke. Delayed removal of methyl groups in lower-rank coals inhibited carbon structure growth, while higher-rank coals showed earlier cross-linking and alignment, leading to longer and more ordered carbon structures.
  • Fluidity and Vitrinite Content Effects: The coal sample with highest fluidity and vitrinite content produced more anisotropic and longer carbon structures during the plastic layer formation. This coal displayed moderate increases in aromaticity but significant condensation during resolidification, contributing to the development of highly ordered carbon layers.
  • Correlation of Coal Properties to Microtexture Development and CRI: Microtexture changes were strongly linked to coal rank, fluidity, and vitrinite content. Higher fluidity and rank promoted more anisotropic and larger carbon structures such as ribbon and lenticular, resulting in lower reactivity with CO2 and thus lower CRI. IRM results help better interpret how functional group transitions during thermoplastic stages influence microtexture evolution during coking and coke quality.

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