- Title
- Mechanisms of metaplast formation during coal pyrolysis
- Creator
- Tran, Quang Anh
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Coke is a solid carbon residue essential to the production of steel via the blast furnace route due to its ability to act as a high-strength permeable support for the blast furnace charges. The solid coherent coke is produced from coking coal via the high temperature carbonization process. Such coal-to-coke transformation is governed by the coal’s thermoplastic behaviour which includes softening, bubble formation, swelling, and resolidification during pyrolysis. Only coals of a certain rank are able to exhibit thermoplastic development and are termed as coking coals. Non-coking coals, on the other hand, do not soften or melt and are not suitable for coke-making purposes. The thermoplastic development of a coking coal within the plastic range (400-600 °C) is attributable to the present of materials either existing in the raw coal (here called the mobile phase and extracted with tetrahydrofuran, THF) or being generated on heating coal (here called the metaplast and also extracted by THF). The physical changes during coking coal pyrolysis are well described and can be examined to empirically determine coal plastic properties by a number of standard tests. However, these physical phenomena are induced by underlying chemical changes, particularly the chemical description of the metaplast formation, which are poorly understood. Further, coal thermoplasticity is heavily influenced by the concentration of its organic components which can be classified into vitrinite, liptinte, and inertinite groups. During pyrolysis, only the vitrinites and liptintes exhibit thermoplasticity while components in the inertinite group are expected to remain inactive. For that reason, investigation on the metaplast formation in individual maceral groups is of interest for a better understanding of coal thermoplasticity. This research studied mechanisms of the metaplast formation by conducting the thermoplastic investigation on two coals of different types, one was a coking coal and the other was a non-coking coal. Such investigation was also extended to their maceral concentrates acquired by using a novel water-based separation technique. The utilization of water allowed studies to be conducted without concerns of negative impact imposing on the thermoplastic behaviour of the concentrates as observed when other conventional organic and inorganic solvents were used as separation media. The thermoplastic properties of samples during pyrolysis were examined by using conventional and novel thermal analytical techniques to describe their thermoswelling and volatiles release. Broad chemical changes were assessed via molecular weight distributions of the metaplast (THF extracts) and volatile tars (dissolved in acetone) by using a soft ionization mass spectrometry technique. Finally, physical properties of the resultant cokes and residues were evaluated via their morphological structure by using scanning electron microscopy (SEM). This work confirmed the contribution of both the mobile phase and the metaplast to the thermoplastic development of the investigated coking coal. The mobile phase accounted for ~2% of the raw coal mass but contributed to ~20% of the coal swelling. The metaplast yield, acquired at temperature prior to softening, was ~20% and its removal from the heated coal via solvent extraction has shown to eliminate the coal thermoswelling. Further investigation on the molecular weight distribution of the mobile phase and the metaplast revealed that they shared similar molecular characteristics with the presence of two classes of materials, one containing 12-14 Da repeating structures at <600 Da and the other consisting of 24 Da reoccurring units at >600 Da. However, while the metaplast possessed a very broad molecular weight distribution which could extend to ~3000 Da, the upper molecular weight limit in the mobile phase was found to be ~1000 Da. Therefore, the difference in contributing to coal swelling between the mobile phase (materials pre-existing in the raw coal) and the metaplast (heat generated materials) was attributable to the difference in their molecular weight limit. The molecular characteristics of the metaplast were found to vary with pyrolysis temperatures and closely followed the coal thermoplastic development. The upper limit in its molecular weight distribution extended from ~1800 Da at temperatures below the plastic range to ~3000 Da at temperatures prior to the softening onset and decreased back to only ~600 Da at the end of the plastic range. Such significant variation in the metaplast molecular weight was also accompanied by the absence the 24 Da reoccurring units at high temperatures, indicating the important role of these materials to coal thermopalsticity. The variation in vitrinite content imposed a great impact on the thermoplastic behaviour of coking samples. Vitrinite-rich concentrates exhibited extensive thermoswelling during pyrolysis, while negligible volumetric expansion was recorded in inertinite-rich samples despite showing evidence of particles softening at micro level. The alteration of the vitrinite content via coal maceral concentration also changed the yields of the metaplast, tar, and light gases with vitrinite-rich samples generating higher yields of these pyrolysis products than inertinite-rich samples. The molecular features of the metaplast were insignificantly altered despite the wide vitrnite variation in the maceral concentrates. Specifically, coking concentrates all produced the metaplast with a bimodal molecular weight distribution, similar to that of their parent coal. Non-coking concentrates, on the contrary, generated solvent extracts with unimodal molecular weight distribution containing only the 12-14 Da repeating structures. Such absence of the second material class with 24 Da reoccurring units was also found in solvent extracts isolated from the non-coking coal at multiple temperatures covering the plastic range, confirming the importance of these heavier materials to the thermoplastic development of a coal sample. A focused study of coal extracts revealed that their pyrolysis products also consisted of tar, light gases, and solid residue, similar to the pyrolysis products obtained from coal. Their molecular characteristics were shown to have a critical influence to their devolatilization and product distribution. Coking coal extracts with a relatively more complex structure (e.g., the metaplast with molecular weight limit of ~3000 Da) generated higher yield of solid residue and lower yield of volatile matter than non-coking extracts with relatively less complex structure (e.g., molecular weight limit of ~1000 Da). In addition, as coking extracts were found to be active in the plastic range, it was speculated that these metaplastic materials might linger inside coal particles when formed and contribute to the coal thermoplastic behaviour. By comparison, as solvent extracts acquired from non-coking samples were less active in the plastic range, these materials were suggested to not retain inside coal particles. They might vaporize as soon as they were formed and not contribute to the formation of the liquid phase.
- Subject
- coal pyrolysis; metaplast; coal macerals
- Identifier
- http://hdl.handle.net/1959.13/1383998
- Identifier
- uon:32002
- Rights
- Copyright 2018 Quang Anh Tran
- Language
- eng
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