- Title
- A mechanistic study of the formation of plastic layers during the heating of coking coal
- Creator
- Lee, Soonho
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The pyrolysis of coking coals in a thermoplastic range involves not just chemical changes but also physical changes, which are referred to as thermoplasticity. With increasing temperature in the thermoplastic range, coking coals become softened and coalesce into a plastic mass and are then resolidified into a solid porous coke. These stages take place layer by layer in a coke oven due to the temperature gradients. A coal charge in an industrial coke oven undergoes a plastic transformation, which corresponds to the formation of a plastic layer. The plastic layer is a foam like material consisting of molten vitrinite and liptinite, solid coal components (inertinite macerals) and the gases generated by the decomposition of the coal. It forms near the heating walls and migrates toward the centre of the oven. The migration is attributed to a decrease in the temperature gradient in the coke oven as the coal charge adjacent to the heating wall is heated rapidly and the heat is then transmitted to the rest of the coal charge. As the temperature exceeds the resolidification temperature, the plastic layer gradually converts into a layer of viscoelastic semi-coke, having a typical cellular coke structure. With a further temperature increase, the order and size of the aromatic layers of the semi-coke increase, followed by condensation reactions which correspond to the formation of the solid porous residue, coke. When the plastic layers merge in the centre of the oven and the centre temperature reaches a desirable temperature (900-1100 °C), followed by a period of soaking time, the whole coal charge is converted into coke. The formation of the plastic layer is considered as a critical stage in the coking process as the pore structures, which are closely related to the coke quality, are mainly determined during this process. The generation of significant amounts of volatile matters (gases) in the thermoplastic phase builds up the internal gas pressure (IGP). The transmission of IGP to the coke oven walls causes coking pressure against the walls, which is referred to as the oven wall pressure (OWP). It is known that an excessive OWP causes serious oven wall damage, shortening the battery life and an increased pushing force when the coke is discharged. The physical and chemical changes that occur in a thermoplastic temperature range have been widely investigated due to their impacts on the coal-coke transformation and the phenomenon attributed to the formation of the plastic layer. Pre-heated samples prepared in the thermoplastic range were analysed by a variety of analytical techniques, such as Fourier transform infrared spectroscopy (FTIR), solid state 13C nuclear magnetic resonance (SSNMR), X-ray powder diffraction (XRD). A two-stage solvent extraction was used to acquire the light and heavy extracts formed in the thermoplastic range. The extracts were analysed using laser desorption time of flight imaging mass spectrometry (LDI-TOF-IMS), 1H nuclear magnetic resonance (NMR) spectrometry and gas chromatography-mass spectrometry (GC-MS). An in-situ FTIR was employed to measure the functional group changes while a single coking coal was heated in the thermoplastic range. These studies provide an understanding of the changes in the chemical structures that occur in the thermoplastic stage and proposes explanations for its mechanisms. However, what happens in real coking conditions, in particular heating conditions, is far more complicated than in the above experimental conditions. Therefore, another way of studying the plastic layers is by direct observation of the process and in-situ measurement of the thermoplastic properties, while they are happening, although this is extremely difficult for coal heated in an oven. However, a few attempts have been made by different researchers who have used sole-heated ovens combined with micro-CT and X-ray filming techniques. These techniques were employed to observe the pore structures inside the plastic layer, which provided a better understanding of the transitions in the physical structures of coal to coke during the coking process. However, these techniques used rather small amounts of coal samples (several hundred grams scale) which were heated from one side. This means that the heat transfer conditions, including the temperature gradients, may vary greatly from those in a practical coke oven. Under such conditions, it is questionable whether the characteristics of the plastic layer would be equivalent to those in large scale ovens. In addition, the previous research activities have largely focused on the changes in the physical structures of the plastic layer, rather than the changes in the chemical structures. Therefore, the main objectives of this study are to investigate the changes in the chemical and physical structures inside the plastic layer, under practical heating conditions, in order to provide insights into the underlying mechanisms of the plastic layer. In particular, this research is focused on the: (a) development of a 4 kg laboratory-scale coke oven which enables the sampling of plastic layer samples and the in-situ measurement of the temperature profiles and internal gas pressures at different locations under practical heating conditions; (b) characterization of the thermoplastic behaviours of six Australian coking coals with varying petrographic properties and Gieseler maximum fluidities using the 4 kg coke oven; (c) in-situ study of the formation of the plastic layers regarding the changes in the thicknesses of the plastic layers and the internal gas pressures during coking; (d) characterization of the changes in the physical and chemical structures inside the plastic layers formed from the six Australian coking coals through a combination of analytical techniques; and (e) identifying the effects of the petrographic properties and Gieseler maximum fluidities on the characteristics of the plastic layers. To achieve these objectives, a 4 kg laboratory scale double-heated coke oven was employed for the in situ measurement of the temperature histories and internal gas pressures in different locations of the coal charge and the sampling of plastic layer samples. A noticeable geometric feature of the 4 kg coke oven is that the length of the laboratory coke oven (400 mm) is similar to the width of a practical coke oven. Additionally, insulation enveloped the surface of the coal charge, except for the heating wall sides, in order to simulate the one-dimensional heat transfer from the heating walls. Six Australian coking coals with different petrographic properties and Gieseler maximum fluidities were selected for the coking experiments. The crushed coals were charged in the coke reactor with a packing density of 825kg/m³ and were placed in the coke oven for heating. The pressure probes, together with the thermocouples, were placed across the width of the coal charge to measure the temperature profiles and IGPs at multiple points throughout the coking cycle. For the plastic layer sampling, the sampling probe located in the coke reactor was used to acquire semi-coke samples that consisted of loose coal, thermoplastic regions and coke/semi cokes. The pore structures of the semi-coke samples were analysed using the Synchrotron micro-CT in a non-destructive method before the extraction of the plastic layer samples. The plastic layer samples were then sectioned from the thermoplastic regions of the semi-coke samples and they were analysed using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and a thermogravimetric analyser coupled with a Fourier transform infrared spectrometer (TG-FTIR). The ATR-FTIR was used to analyse the chemical structures of the parent coals and the sectioned plastic layer samples regarding their structural parameters, such as Aar/Aal, CH₂/CH₃, aliphatic CH and its components (CH₂ and CH₃). The TG-FTIR was employed to investigate the pyrolysis behaviours of the sectioned plastic layer samples during the TGA heating, including their volatile matter yields and the changes in the aliphatic and aromatic CH structures of the volatile matter. Based on the in-situ study of the formation of the plastic layers, the new findings of the present study are as follows: (a) the thicknesses of the plastic layers were estimated using a data exploration of the measured temperature profiles and the characteristic temperatures obtained from the Gieseler fluidity test. It was observed that the thicknesses of the plastic layers were consistent with the Gieseler thermoplastic temperature ranges of the parent coals; (b) the changes in the IGP termination temperatures measured in different locations of the coal charges indicated the different extents of the fissuring at the coke/semi-coke sides, which influenced the maximum IGPs. These results were attributed to the properties of the coals, such as their mean maximum vitrinite reflectance, maceral concentrates, Gieseler maximum fluidities and volatile matter contents. The characterization of the physical and chemical changes inside the plastic layers using the analytical techniques provided the following insights: (a) there were correlations between the Gieseler maximum fluidities, the chemical structures analyzed by structural parameters and the petrographic properties (Mean maximum vitrinite reflectance and vitrinite contents). The selected coals with the lower mean maximum vitrinite reflectance and the higher vitrinite contents showed lower Aar/Aal ratios, which indicated lower contents of the aliphatic CH in the coal structures. It seemed that this appeared to cause higher levels of decomposition in the thermoplastic ranges, resulting in the higher Gieseler maximum fluidities. These correlations impacted on the different physical and chemical characteristics of the plastic layers; (b) the ATR-FTIR results showed that the normalized aliphatic CH decreased in the initial softening regions. This was followed by a relatively constant stage, and finally a drastic decrease on the coke sides of the plastic layers. These changes were consistent with a loss of bonds throughout the devolatilization process, which were analysed by the TG-FTIR. However, the levels of response appeared to vary with the Gieseler maximum fluidities derived from the chemical structures of the six coals; (c) the higher fluidity coals that appeared to have higher aliphatic CH contents showed a lower decrease in their aliphatic CH at the earlier stages of the thermoplastic regions due to the aliphatic hydrocarbons in the volatile matter. This may have potentially caused further depolymerization in the intermediate plastic regions, which corresponded to the higher fluid levels of the plastic layers. In this context, the higher fluidity coals showed higher maximum porosities in the thermoplastic regions due to the higher degrees of gas entrapment in the highly viscous plastic regions. This suggested that the aliphatic functionalities of a coal may play a significant role in the development of its plastic properties and the subsequent semi-coke formation during the coking process.
- Subject
- coking coal; plastic layer; structural transition; carbonization behavior; thermoplasticity; coke-making
- Identifier
- http://hdl.handle.net/1959.13/1405227
- Identifier
- uon:35458
- Rights
- Copyright 2019 Soonho Lee
- Language
- eng
- Full Text
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