With increasing energy demand globally and, in particular, in South Africa coupled with depletion of the earth’s fossil energy resources and growing problem of disposal of nonbiodegradable waste such as waste tyres, there is a need and effort globally to find alternative energy from waste material including waste tyres. One possible way of exploiting waste tyre for energy or chemicals recovery is through gasification for the production of syngas, and this is what was investigated in this study. The possibility of gasification of waste tyre blended with coal after pyrolysis was investigated and two Bituminous coals were selected for blending with the waste tyre in co-gasification. A sample of ground waste tyre / waste tire, WT, a high vitrinite coal from the Waterberg coalfield (GG coal) and a high inertinite coal from the Highveld coalfield (SF coal) were used in this investigation. The waste tyre sample had the highest volatile matter content of 63.8%, followed by GG coal with 27% and SF coal with 23.8%. SF coal had the highest ash content of 21.6%, GG coal had 12.6% and waste tyre had the lowest of 6.6%. For the chars, SF char still had the highest ash of 24.8%, but WT char had higher ash, 14.7%, when compared to GG char with 13.9% ash. The vitrinite content in GG coal was 86.3%, whilst in SF coal it was 25% and SF coal had a higher inertinite content of 71% when compared to GG coal with 7.7%. SF char had the highest BET surface area of 126m2/g, followed by GG char with 113m2/g, and WT had the lowest value of 35.09m2/g. The alkali indices of the SF, WT and GG chars were calculated to be 8.2, 4.2 and 1.7 respectively. Coal samples were prepared by crushing and milling to particle sizes less than 75μm before charring in a packed bed balance reactor at temperatures up to 1000oC.Waste tyre samples were charred at the same conditions before milling to < 75μm particle size. Coal and WT chars were blended in ratios of 75:25, 50:50 and 25:75 before gasification experimentation. Carbon dioxide gasification was conducted on the blends and the pure coal and WT chars in a Thermogravimetric analyser (TGA) at 900oC, 925oC, 950oC and 975oC and ambient pressure. 100% CO2 was used at a flow rate of 2L/min. Reactivity of the pure char samples was found to be in the order SF > GG > WT, and the relationship between the coal chars’ reactivities could be explained by the high ash content of the SF char and low reactivity of the WT char corresponds to its low BET surface area. In general, the coal/WT char mixtures were less reactive than the respective coal, but more reactive than the pure WT char, the only exception being the 75% GG char blend which was initially more reactive than the GG char, and reactivity decreased with increasing WT content. For all samples reactivity increased with increasing temperature. The relationship between the reactivities of the GG char and its blends and that of the SF char and its blends was found to be affected by the amount of WT char added, especially at the lower temperatures 900oC and 925oC. SF coal is more reactive than GG coal, but at 900oC and 925oC, the reactivity of GG/WT blends improves in relation to the SF/WT blends with an increase in the ratio of WT in the blends, i.e. the 25% GG char blend is more reactive than the 25% SF char blend. The reactivity of the coal/WT blends was also checked against predicted conversion rates based on the conversion rates of the pure WT and coal samples. At 900oC and 925oC, the reactivities of the blends of both coal chars with WT char were found to be greater than the predicted conversion rates, and for the GG/WT blends the deviation increased with increasing WT ratios, while for the SF/WT blends the deviation increased with increasing SF ratios. These findings suggest the presence of synergism or enhancement between the coal chars and WT char in gasification reactions. The random pore model (RPM) was used to model the gasification results and it was found to adequately describe the experimental data. Activation energies determined with the RPM were found to be 205.4kJ/mol, 189.9kJ/mol and 173.9kJ/mol for SF char, WT char and GG char respectively. The activation energies of the coal/WT blends were found to be lower than those of both the pure coal and the pure WT chars. For the GG/WT blends the activation energy decreased with increasing WT char ratio, while for the SF/WT blends the activation energy decreased with increasing SF char ratio. The trends of the activation energies and conversion rates of the blends point to synergism or enhancement between the coal and WT chars in CO2 gasification reactions, and in the GG/WT blends this enhancement is driven more by the WT char, while in SF/WT blends it is driven by SF chars. It is possible that enhancement of the reactions is caused by mineral matter catalysis of the gasification reactions. The ash contents and alkali indices of the pure samples follow the order SF > WT > GG. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

An investigation was undertaken to compare the kinetics of combustion and gasification reactions of chars prepared from two South African coals in different reaction atmospheres: air, steam, and carbon dioxide. The two original coals were characterised as vitrinite-rich (Greenside) and inertinite-rich (Inyanda) coals with relatively low ash content (12.5-16.7 wt. %, adb). Chars were prepared from the parent coals under nitrogen atmosphere at 900 °C. Characterisation results show that the volatiles and moisture were almost completely driven off from the parent coals, indicating that the pyrolysis process was efficient. Physicalstructural properties such as porosity and surface area generally increased from the parent coals to the subsequent chars. The heterogeneous char-gas reactions were conducted isothermally in a TGA on ~1 mm size particles. To ensure that the reactions are under chemical reaction kinetic control regime, different temperatures zones were selected for the three different reaction atmospheres. Combustion reactivity experiments were carried out with air in the temperature range of 387 °C to 425 °C; gasification reactivity with pure steam were conducted at higher temperatures (775 °C - 850 °C) and within 825 °C to 900 °C with carbon dioxide. Experimental results show differences in the specific reaction rate with carbon conversion in different reaction atmospheres and char types. Reaction rates in all three reaction atmospheres were strongly dependent on temperature, and follow the Arrhenius type kinetics. All the investigated reactions (combustion with air and gasification with CO2 and steam) were found to be under chemical reaction control regime (Regime I) for both chars. The inertinite-rich coals exhibit longer burn-out time than chars produced from vitrinite-rich coals, as higher specific reaction rate were observed for the vitrinite-rich coals in the three different reaction atmospheres. The determined random pore model (RPM) structural parameters did not show any significant difference during steam gasification of Greenside and Inyanda chars, whereas higher structural parameter values were observed for Greenside chars during air combustion and CO2 gasification (ψ > 2). However a negative ψ value was determined during CO2 gasification and air combustion of Inyanda chars. The RPM predictions was validated with the experimental data and exhibited adequate fitting to the specific rate of reaction versus carbon conversion plots of the char samples at the different reaction conditions chosen for this study. The activation energy determined was minimal for air and maximum for CO2 for both coals; and ranged from 127-175 kJ·mol-1 for combustion, 214-228 kJ·mol-1 and 210-240 kJ·mol-1 for steam and CO2 gasification respectively. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

With increasing energy demand globally and, in particular, in South Africa coupled with depletion of the earth’s fossil energy resources and growing problem of disposal of nonbiodegradable waste such as waste tyres, there is a need and effort globally to find alternative energy from waste material including waste tyres. One possible way of exploiting waste tyre for energy or chemicals recovery is through gasification for the production of syngas, and this is what was investigated in this study. The possibility of gasification of waste tyre blended with coal after pyrolysis was investigated and two Bituminous coals were selected for blending with the waste tyre in co-gasification. A sample of ground waste tyre / waste tire, WT, a high vitrinite coal from the Waterberg coalfield (GG coal) and a high inertinite coal from the Highveld coalfield (SF coal) were used in this investigation. The waste tyre sample had the highest volatile matter content of 63.8%, followed by GG coal with 27% and SF coal with 23.8%. SF coal had the highest ash content of 21.6%, GG coal had 12.6% and waste tyre had the lowest of 6.6%. For the chars, SF char still had the highest ash of 24.8%, but WT char had higher ash, 14.7%, when compared to GG char with 13.9% ash. The vitrinite content in GG coal was 86.3%, whilst in SF coal it was 25% and SF coal had a higher inertinite content of 71% when compared to GG coal with 7.7%. SF char had the highest BET surface area of 126m2/g, followed by GG char with 113m2/g, and WT had the lowest value of 35.09m2/g. The alkali indices of the SF, WT and GG chars were calculated to be 8.2, 4.2 and 1.7 respectively. Coal samples were prepared by crushing and milling to particle sizes less than 75μm before charring in a packed bed balance reactor at temperatures up to 1000oC.Waste tyre samples were charred at the same conditions before milling to < 75μm particle size. Coal and WT chars were blended in ratios of 75:25, 50:50 and 25:75 before gasification experimentation. Carbon dioxide gasification was conducted on the blends and the pure coal and WT chars in a Thermogravimetric analyser (TGA) at 900oC, 925oC, 950oC and 975oC and ambient pressure. 100% CO2 was used at a flow rate of 2L/min. Reactivity of the pure char samples was found to be in the order SF > GG > WT, and the relationship between the coal chars’ reactivities could be explained by the high ash content of the SF char and low reactivity of the WT char corresponds to its low BET surface area. In general, the coal/WT char mixtures were less reactive than the respective coal, but more reactive than the pure WT char, the only exception being the 75% GG char blend which was initially more reactive than the GG char, and reactivity decreased with increasing WT content. For all samples reactivity increased with increasing temperature. The relationship between the reactivities of the GG char and its blends and that of the SF char and its blends was found to be affected by the amount of WT char added, especially at the lower temperatures 900oC and 925oC. SF coal is more reactive than GG coal, but at 900oC and 925oC, the reactivity of GG/WT blends improves in relation to the SF/WT blends with an increase in the ratio of WT in the blends, i.e. the 25% GG char blend is more reactive than the 25% SF char blend. The reactivity of the coal/WT blends was also checked against predicted conversion rates based on the conversion rates of the pure WT and coal samples. At 900oC and 925oC, the reactivities of the blends of both coal chars with WT char were found to be greater than the predicted conversion rates, and for the GG/WT blends the deviation increased with increasing WT ratios, while for the SF/WT blends the deviation increased with increasing SF ratios. These findings suggest the presence of synergism or enhancement between the coal chars and WT char in gasification reactions. The random pore model (RPM) was used to model the gasification results and it was found to adequately describe the experimental data. Activation energies determined with the RPM were found to be 205.4kJ/mol, 189.9kJ/mol and 173.9kJ/mol for SF char, WT char and GG char respectively. The activation energies of the coal/WT blends were found to be lower than those of both the pure coal and the pure WT chars. For the GG/WT blends the activation energy decreased with increasing WT char ratio, while for the SF/WT blends the activation energy decreased with increasing SF char ratio. The trends of the activation energies and conversion rates of the blends point to synergism or enhancement between the coal and WT chars in CO2 gasification reactions, and in the GG/WT blends this enhancement is driven more by the WT char, while in SF/WT blends it is driven by SF chars. It is possible that enhancement of the reactions is caused by mineral matter catalysis of the gasification reactions. The ash contents and alkali indices of the pure samples follow the order SF > WT > GG. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

An investigation was undertaken to compare the kinetics of combustion and gasification reactions of chars prepared from two South African coals in different reaction atmospheres: air, steam, and carbon dioxide. The two original coals were characterised as vitrinite-rich (Greenside) and inertinite-rich (Inyanda) coals with relatively low ash content (12.5-16.7 wt. %, adb). Chars were prepared from the parent coals under nitrogen atmosphere at 900 °C. Characterisation results show that the volatiles and moisture were almost completely driven off from the parent coals, indicating that the pyrolysis process was efficient. Physicalstructural properties such as porosity and surface area generally increased from the parent coals to the subsequent chars. The heterogeneous char-gas reactions were conducted isothermally in a TGA on ~1 mm size particles. To ensure that the reactions are under chemical reaction kinetic control regime, different temperatures zones were selected for the three different reaction atmospheres. Combustion reactivity experiments were carried out with air in the temperature range of 387 °C to 425 °C; gasification reactivity with pure steam were conducted at higher temperatures (775 °C - 850 °C) and within 825 °C to 900 °C with carbon dioxide. Experimental results show differences in the specific reaction rate with carbon conversion in different reaction atmospheres and char types. Reaction rates in all three reaction atmospheres were strongly dependent on temperature, and follow the Arrhenius type kinetics. All the investigated reactions (combustion with air and gasification with CO2 and steam) were found to be under chemical reaction control regime (Regime I) for both chars. The inertinite-rich coals exhibit longer burn-out time than chars produced from vitrinite-rich coals, as higher specific reaction rate were observed for the vitrinite-rich coals in the three different reaction atmospheres. The determined random pore model (RPM) structural parameters did not show any significant difference during steam gasification of Greenside and Inyanda chars, whereas higher structural parameter values were observed for Greenside chars during air combustion and CO2 gasification (ψ > 2). However a negative ψ value was determined during CO2 gasification and air combustion of Inyanda chars. The RPM predictions was validated with the experimental data and exhibited adequate fitting to the specific rate of reaction versus carbon conversion plots of the char samples at the different reaction conditions chosen for this study. The activation energy determined was minimal for air and maximum for CO2 for both coals; and ranged from 127-175 kJ·mol-1 for combustion, 214-228 kJ·mol-1 and 210-240 kJ·mol-1 for steam and CO2 gasification respectively. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

Advances in the knowledge of chemical structure of coal and development of high performance computational techniques led to more than hundred and thirty four proposed molecular level representations (models) of coal between 1942 and 2010. These models were virtually on the carboniferous coals from the northern hemisphere. There are only two molecular models based on the inertinite- and vitrinite-rich coals from the southern hemisphere. The current investigation is based on the chars derived from the Permian-aged coals in two major South African coalfields, Witbank #4 seam and Waterberg Upper Ecca. The two coals were upgraded to 85 and 93% inertinite- and vitrinite-rich concentrates, on visible mineral matter free basis. The coals were slow heated in inert atmosphere at 20 ℃ min-1 to 450, 700 and 1000 ℃ and held at that temperature for an hour. After the HCl-HF treatment technique at ambient temperatures, the characteristics of the coals and chars were examined with proximate, ultimate, helium density, porosity, surface area, petrographic, solid-state 13C NMR, XRD and HRTEM analytical techniques. The results largely showed that substantial transitions occurred at 700-1000 ℃, where the chars became physically different but chemically similar. Consequently, the chars at the highest temperature (1000 ℃) drew attention to the detailed study of the atomistic properties that may give rise to different reactivity behaviours with CO2 gas. The H/C atomic ratios for the inertinite- and vitrinite-rich chars were respectively 0.31 and 0.49 at 450 ℃ and 0.10 and 0.12 at 1000 ℃. The true density was respectively 1.48 and 1.38 g.cm-3 at 450 ℃ and 1.87 and 1.81 g.cm-3 at 1000 ℃. The char form results from the petrographic analysis technique indicated that the 700-1000 ℃ inertinite-rich chars have lower proportions of thick-walled isotropic coke derived from pure vitrinites (5-8%) compared with the vitrinite-rich chars (91-95%). This property leads to the creation of pores and increases of volume and surface area as the softening walls expand. It was found that the average crystallite diameter, La, and the mean length of the aromatic carbon fringes from the XRD and HRTEM techniques, respectively, were in good agreement and made a definite distinction between the 1000 ℃ inertinite- and vitrinite-rich chars. The crystallite diameter on peak (10) approximations, La(10), of 37.6Å for the 1000 ℃ inertinite-rich chars fell within the HRTEM’s range of minimummaximum length boundary of 11x11 aromatic fringes (27-45Å). The La (10) of 30.7Å for the vitrinite-rich chars fell nearly on the minimum-maximum length range of 7x7 aromatic fringes (17-28Å.) The HRTEM results showed that the 1000 ℃ inertinite-rich chars comprised a higher distribution of larger aromatic fringes (11x11 parallelogram catenations) compared with a higher distribution of smaller aromatic fringes (7x7 parallelogram catenations). The mechanism for the similarity between the 700-1000 ℃ inertinite- and vitrinite-rich chars was the greater transition occurring in the vitrinite-rich coal to match the more resistant inertinite-rich coal. This emphasised that the transitions in the properties of vitrinite-rich coals were more thermally accelerated than those of the inertinite-rich coals. The similarity between the inertinite- and vitrinite-rich chars was shown by the total maceral reflectance, proximate, ultimate, skeletal density and aromaticity results. Evidence for this was the carbon content by mass for the inertinite- and vitrinite-rich chars of respectively 90.5 and 85.3% at 450 ℃ and 95.9 and 94.1% at 1000 ℃. The aromaticity from the XRD technique was respectively 87 and 77% at 450 ℃ and 98 and 96% at 1000 ℃. A similar pattern was found in the hydrogen and oxygen contents, the atomic O/C ratios and the aromaticity from the NMR technique. The subsequent construction of large-scale molecular structures for the 1000 ℃ inertinite-rich chars comprised 106 molecules constructed from a total of 42929 atoms, while the vitrinite-rich char model was made up of 185 molecules consisting of a total of 44315 atoms. The difference between the number of molecules was due to the inertinite-rich char model comprising a higher distribution of larger molecules compared with the vitrinite-rich char model, in agreement with the XRD and HRTEM results. These char structures were used to examine the behaviour on the basis of gasification reactivity with CO2. The density functional theory (DFT) was used to evaluate the interactions between CO2 and the atomistic representations of coal char derived from the inertinite- and vitrinite rich South African coals. The construction of char models used the modal aromatic fringes (fringes of highest frequencies in size distributions) from the HRTEM, for the inertinite- and vitrinite-rich chars, respectively (11x11 and 7x7 parallelogram-shaped aromatic carbon rings). The structures were DFT geometrically optimized and used to measure reactivity with the Fukui function, f+(r) and to depict a representative reactive carbon edge for the simulations of coal gasification reaction mechanism with CO2 gas. The f+(r) reactivity indices of the reactive edge follows the sequence: zigzag C remote from the tip C (Czi = 0.266) > first armchair C (Cr1 = 0.087) > tip C (Ct = 0.075) > second armchair C (Cr2 = 0.029) > zigzag C proximate to the tip C (Cz = 0.027). The DFT simulated mean activation energy, ΔEb, for the gasification reaction mechanism (formation of second CO gas molecule) was 233 kJ mol-1. The reaction for the formation of second CO molecule is defines gasification in essence. The experimental activation energy determined with the TGA and random pore model to account essentially for the pore variation in addition to the gasification chemical reaction were found to be very similar: 191 ± 25 kJ mol-1 and 210 ± 8 kJ mol-1; and in good agreement with the atomistic results. The investigation gave promise towards the utility of molecular representations of coal char within the context of fundamental coal gasification reaction mechanism with CO2. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

Advances in the knowledge of chemical structure of coal and development of high performance computational techniques led to more than hundred and thirty four proposed molecular level representations (models) of coal between 1942 and 2010. These models were virtually on the carboniferous coals from the northern hemisphere. There are only two molecular models based on the inertinite- and vitrinite-rich coals from the southern hemisphere. The current investigation is based on the chars derived from the Permian-aged coals in two major South African coalfields, Witbank #4 seam and Waterberg Upper Ecca. The two coals were upgraded to 85 and 93% inertinite- and vitrinite-rich concentrates, on visible mineral matter free basis. The coals were slow heated in inert atmosphere at 20 ℃ min-1 to 450, 700 and 1000 ℃ and held at that temperature for an hour. After the HCl-HF treatment technique at ambient temperatures, the characteristics of the coals and chars were examined with proximate, ultimate, helium density, porosity, surface area, petrographic, solid-state 13C NMR, XRD and HRTEM analytical techniques. The results largely showed that substantial transitions occurred at 700-1000 ℃, where the chars became physically different but chemically similar. Consequently, the chars at the highest temperature (1000 ℃) drew attention to the detailed study of the atomistic properties that may give rise to different reactivity behaviours with CO2 gas. The H/C atomic ratios for the inertinite- and vitrinite-rich chars were respectively 0.31 and 0.49 at 450 ℃ and 0.10 and 0.12 at 1000 ℃. The true density was respectively 1.48 and 1.38 g.cm-3 at 450 ℃ and 1.87 and 1.81 g.cm-3 at 1000 ℃. The char form results from the petrographic analysis technique indicated that the 700-1000 ℃ inertinite-rich chars have lower proportions of thick-walled isotropic coke derived from pure vitrinites (5-8%) compared with the vitrinite-rich chars (91-95%). This property leads to the creation of pores and increases of volume and surface area as the softening walls expand. It was found that the average crystallite diameter, La, and the mean length of the aromatic carbon fringes from the XRD and HRTEM techniques, respectively, were in good agreement and made a definite distinction between the 1000 ℃ inertinite- and vitrinite-rich chars. The crystallite diameter on peak (10) approximations, La(10), of 37.6Å for the 1000 ℃ inertinite-rich chars fell within the HRTEM’s range of minimummaximum length boundary of 11x11 aromatic fringes (27-45Å). The La (10) of 30.7Å for the vitrinite-rich chars fell nearly on the minimum-maximum length range of 7x7 aromatic fringes (17-28Å.) The HRTEM results showed that the 1000 ℃ inertinite-rich chars comprised a higher distribution of larger aromatic fringes (11x11 parallelogram catenations) compared with a higher distribution of smaller aromatic fringes (7x7 parallelogram catenations). The mechanism for the similarity between the 700-1000 ℃ inertinite- and vitrinite-rich chars was the greater transition occurring in the vitrinite-rich coal to match the more resistant inertinite-rich coal. This emphasised that the transitions in the properties of vitrinite-rich coals were more thermally accelerated than those of the inertinite-rich coals. The similarity between the inertinite- and vitrinite-rich chars was shown by the total maceral reflectance, proximate, ultimate, skeletal density and aromaticity results. Evidence for this was the carbon content by mass for the inertinite- and vitrinite-rich chars of respectively 90.5 and 85.3% at 450 ℃ and 95.9 and 94.1% at 1000 ℃. The aromaticity from the XRD technique was respectively 87 and 77% at 450 ℃ and 98 and 96% at 1000 ℃. A similar pattern was found in the hydrogen and oxygen contents, the atomic O/C ratios and the aromaticity from the NMR technique. The subsequent construction of large-scale molecular structures for the 1000 ℃ inertinite-rich chars comprised 106 molecules constructed from a total of 42929 atoms, while the vitrinite-rich char model was made up of 185 molecules consisting of a total of 44315 atoms. The difference between the number of molecules was due to the inertinite-rich char model comprising a higher distribution of larger molecules compared with the vitrinite-rich char model, in agreement with the XRD and HRTEM results. These char structures were used to examine the behaviour on the basis of gasification reactivity with CO2. The density functional theory (DFT) was used to evaluate the interactions between CO2 and the atomistic representations of coal char derived from the inertinite- and vitrinite rich South African coals. The construction of char models used the modal aromatic fringes (fringes of highest frequencies in size distributions) from the HRTEM, for the inertinite- and vitrinite-rich chars, respectively (11x11 and 7x7 parallelogram-shaped aromatic carbon rings). The structures were DFT geometrically optimized and used to measure reactivity with the Fukui function, f+(r) and to depict a representative reactive carbon edge for the simulations of coal gasification reaction mechanism with CO2 gas. The f+(r) reactivity indices of the reactive edge follows the sequence: zigzag C remote from the tip C (Czi = 0.266) > first armchair C (Cr1 = 0.087) > tip C (Ct = 0.075) > second armchair C (Cr2 = 0.029) > zigzag C proximate to the tip C (Cz = 0.027). The DFT simulated mean activation energy, ΔEb, for the gasification reaction mechanism (formation of second CO gas molecule) was 233 kJ mol-1. The reaction for the formation of second CO molecule is defines gasification in essence. The experimental activation energy determined with the TGA and random pore model to account essentially for the pore variation in addition to the gasification chemical reaction were found to be very similar: 191 ± 25 kJ mol-1 and 210 ± 8 kJ mol-1; and in good agreement with the atomistic results. The investigation gave promise towards the utility of molecular representations of coal char within the context of fundamental coal gasification reaction mechanism with CO2. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015