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dc.contributor.advisorStrydom, C.A.en_US
dc.contributor.advisorMatjie, R.H.en_US
dc.contributor.advisorBunt, R.J.en_US
dc.contributor.authorUwaoma, R.C.en_US
dc.date.accessioned2020-11-16T10:30:20Z
dc.date.available2020-11-16T10:30:20Z
dc.date.issued2020en_US
dc.identifier.urihttps://orcid.org/0000-0002-3306-9878en_US
dc.identifier.urihttp://hdl.handle.net/10394/36370
dc.descriptionPhD (Chemistry), North-West University, Potchefstroom Campus
dc.description.abstractThe South African coal mining industry produced approximately 307 million tons of low-rank bituminous coal in 2007. The local thermo-chemical processes utilise this coal for oils, petrochemicals and steam generation. South African coal mines and power stations in Secunda Synfuels Operations produce more than 10 million tons of coal fine discards (an unavoidable by-product) per year. This study focuses on the beneficiation of these discards, using the float-sink density separation method and investigates the possible utilisation of the float fractions in thermochemical processes. This investigation was divided into three phases; namely, the density separation and liquefaction phase, pyrolysis of the residues derived from the liquefaction and gasification of the char residues. The first stage of this investigation aimed at beneficiation of two South African coal fine discards using float-sink density separation techniques, followed by liquefaction and analyses of the different products generated from the liquefaction of the float fraction and the discarded coal fines’ samples. The direct liquefaction of South African coal fine discards and their density separated (float) fractions were carried out under moderate conditions in a laboratory autoclave. The liquefaction temperature ranged between 380 and 420 °C, using tetralin as a solvent and an initial nitrogen gas pressure of 3 MPa. Results from the liquefaction tests show that the carbon conversion and oil yields are higher for the float fractions when compared to the coal fine discards. Waterberg and Highveld coal float fractions achieved a high carbon conversion of 50.7 wt.% daf and 52.7 wt.% daf respectively, compared with < 42 wt.% daf carbon conversion for the coal fines wastes. The efficiency of the liquefaction carbon conversion was correlated with the reactive macerals and the surface areas of the individual samples. It was observed that the density separated coal fractions, which have higher surface areas, higher vitrinite contents and higher reactive macerals contents achieved higher extraction efficiencies. The residues and extracts generated during the liquefaction were characterised using nuclear magnetic resonance spectroscopy, proximate and ultimate analyses. The analytical results indicated that the residues showed slight decreases in calorific values and aliphatic components, with lower H/C ratios and higher ash yields. Also, the results showed that using the float fractions of South African coal fine discards at moderate liquefaction temperatures could be beneficial in the production of liquid fuel. The second phase, which was one of the primary aims of this investigation, involved the use of the liquefaction residues (waste) generated from the first stage for pyrolysis experiments. In this phase, float fractions produced from two South African discard coal fines samples and their liquefaction residues generated from phase one were used as feedstock for pyrolysis experiments. Analysis of the pyrolytic products from the liquefaction residues and float fractions was reported. Pyrolysis was carried out in a modified Fischer Assay setup at 750°C and 920°C under an argon atmosphere. Tar samples were analysed using gas chromatography-mass spectrometry (GC-MS), nuclear magnetic resonance (NMR) spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy. The chars produced were analysed using proximate and ultimate analyses, solid-state ¹³C NMR and CO₂ low-pressure gas adsorption (CO₂-LPGA). Results obtained from the pyrolysis experiments showed that the liquefaction residues produced higher char and gas yields when compared to the coal fine discards float fractions. The pyrolysis char yields of the solvent extraction residues ranged from 74–76% and that of float fractions from coal fine discards ranged between 67.0% and 71.5%. Gas pyrolysis yields ranged from 16.0–20.0% (daf) for the residues and 14.5–18.4% for pyrolysis gas yields produced from the coal fines fractions, whilst the pyrolytic water and the tar yields of the coal float fractions were slightly higher compared to that of the liquefaction extraction residues. Proton NMR analysis of the tars produced from the liquefaction residues showed a higher amount of aromatic protons, ranging from 60.5–62.0 ppm, in comparison to < 56.0 ppm for the float fractions of the coal fine discards. The amounts of aliphatic protons of the tars obtained from the float fractions from the coal fine discards were higher when compared to those of the solvent extraction residues. The chars produced after pyrolysis of the liquefaction residues showed a higher porosity when compared to those of chars produced during the pyrolysis of the float fractions from the coal fine discards. The porosity values of the chars produced from the liquefaction residues ranged from 15.5–16.3%, in comparison to < 14% after pyrolysis of the float fractions. The differences in the porosity values were attributed to the opening of pores and extraction of some of the molecular structures from the coal matrix during liquefaction. Further analysis of the pyrolysis products showed that the residues generated after liquefaction of the float fractions from South African bituminous coal fines samples may be utilised for pyrolysis and may offer a means of using some of the liquefaction waste material. The final stage of this investigation was the use of the liquefaction residue chars generated from the pyrolysis of the liquefaction residues for gasification. The char residues generated from tetralin liquefaction of density separated coal float fractions of the coal fines waste products and the coal float fractions, were subjected to CO₂ gasification tests. CO₂ gasification was investigated using a thermogravimetric analyser by heating the samples at isothermal temperatures between 880 and 940°C. Results from the gasification studies showed that the liquefaction residues’ char reactivities values (Ri, Rs, Rtf, Rt/0.5, Rt/0.9) were higher when compared to the reactivity values from the float fraction chars. The higher reactivities of the liquefaction residues were attributed to opening up of pores in the structure of the residues’ chars, after tetralin liquefaction. It was observed that the initial reactivities of the liquefaction residues were doubled compared to that obtained from the coal float fraction chars. The increases in the number of pores, caused by liquefaction, accelerated the reaction process and aided in the reduction of the activation energies needed for the residues’ char gasification reactions. The activation energies of the float fraction chars were found to be 190.5 kJ·mol⁻¹ and 236.7 kJ·mol⁻¹ for the two float fractions samples produced from South African coal fine discards. The activation energies for the liquefaction residues from the same two float fraction samples from the coal fine discards were 145.3 kJ·mol⁻¹ and 196.0 kJ·mol⁻¹. These results showed that tetralin liquefaction affected the coal matrix and aided in enhancing the gasification reactivities of the residues. Gasification of residues (waste material) generated after liquefaction of float fractions produced from density separation of South African coal fines (also waste products) has the potential to be utilised in the gasification process.
dc.language.isoenen_US
dc.publisherNorth-West University (South Africa)en_US
dc.subjectdiscarded coal fines
dc.subjectdensity separation
dc.subjectfloat fraction
dc.subjectresidues
dc.subjectliquefaction
dc.subjectpyrolysis
dc.subjectgasification
dc.titlePyrolysis and gasification of coal residues derived from tetralin liquefaction of South African density separated coalen_US
dc.typeThesisen_US
dc.description.thesistypeDoctoralen_US
dc.contributor.researchID20682972 - Strydom, Christiena Adriana (Supervisor)en_US
dc.contributor.researchID21166625 - Matjie, Ratale Henry (Supervisor)en_US
dc.contributor.researchID20164200 - Bunt, John Reginald (Supervisor)en_US


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