3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Development of activated carbons from South African coal waste for application in natural gas storage(2019) Abdulsalam, JibrilEvery year, South African coal sector generates over 60 million tons of coal waste, which are landfilled in discard dump and slurry ponds. The stockpile of this waste “resource” poses a severe danger to public health, the environment and the socio-economic development of the coal mining region. Therefore, there is an urgent need for an innovative strategy to coal waste reuse and recovery. In this study, the potential of three South African coal waste samples (run-of-mine fines, discard and flotation slurry) were examined in synthesizing an activated carbon for application in natural gas storage. Activated carbons were prepared by KOH activation, and the impacts of KOH/sample weight ratio and temperature on the activated carbon adsorptive characteristics were examined and optimized using Response Surface Methodology (RSM). The results obtained indicated that with an increased temperature and KOH/sample weight ratio, the surface area and pore volume of the resulting activated carbon increased. The activated carbon with the highest surface area and pore volume from each of the samples were obtained at a temperature of 800 oC and KOH/sample weight ratio of 4:1. The morphology, textural characteristics and elemental composition of the activated carbons produced were compared. The synthesized activated carbons were characterized by nitrogen at 77 K adsorption – desorption isotherms and SEM/EDS characterization. Surface area of 1925.34 m2/g, 1826.41 m2/g, 1484.96 m2/g, pore volume of 1.26 cm3/g, 1.21 cm3/g, 1.03 cm3/g and pore size of 2.90 nm, 2.66 nm and 2.51 nm were obtained for activated carbon from run-of-mine fines, discard and slurry, respectively. The SEM/EDS analysis showed pore development and high carbon content. The XRD evaluation confirms the activated carbons as amorphous. The presence of a hysteresis loop in the nitrogen isotherms and the pore size distribution (PSD) confirms highly porous activated carbons consisting of micropores and mesopores. The characteristics of methane (the major constituent of natural gas) adsorption onto the activated carbons produced are measured for temperatures ranging from 0 to 50 oC, and pressures up to 40 bar. For this measurement, activated carbon with the highest surface area and pore volume from each of the coal waste samples was used. The activated carbon produced from the run-of-mine fines (ACR) offers a greater adsorption capacity due to its higher surface area and pore volume. Three adsorption isotherm models (Langmuir, Toth, and Dubinin-Astakhov) were used to validate the measured adsorption data and the Dubinin-Astakhov isotherm model was found to be the most appropriate. The model described the measured data with an average regression error of less than 1% for all the three activated carbon samples. The impact of diffusion on adsorption kinetics was determined in relation to the time taken to achieve equilibrium for the methane/activated carbon system using a mass balance equation that defines pore and surface diffusion. The findings indicate the relationship between the adsorption kinetics, diffusivity, and temperature. An increase in temperature for the methane/activated carbon system was noted to cause an increase in diffusivity, thus reducing the time taken to attain equilibrium. The study indicated that adsorption characteristics (isotherm and kinetics) are the key information in designing and analysing an adsorbed natural gas storage (ANG) system. An adsorption system using the activated carbons produced as the adsorbent bed was simulated using Aspen Adsorption Software (Adsim). In this study, adsorption capacity was found to be significantly increased by a lower flowrate. This enhances thermal stability and maximizes the quantity of gas adsorbed on the bed. The simulation shows that an ANG storage system's efficiency depends on the suitable selection of adsorbent, inlet flow conditions, and bed geometry.Item Evaluating carbon dioxide storage in a variety of South African coals to estimate the potential for enhanced methane recovery(2019) Premlall, KasturieDue to the energy- and carbon-intensive economic structure of South Africa (SA), the country has become one of the biggest contributors to greenhouse gas emissions, emitting more CO2 than any other African country. The ratio of greenhouse gas emissions compared to per capita economic benefit, the so called carbon intensity of the economy, is amongst the highest in the world. Carbon capture and storage (CCS) seems to be the most immediate form of action that can be implemented with the possibility of instantaneous reduction of CO2. The injection of CO2 into deep-unmineable coal seams, although not commercially viable for coal production, is a possible mitigation option under CCS for permanent underground storage of CO2. As a spin-off, useful coal-bed CH4, referred to as enhanced coal bed CH4 (ECBM), could be extracted from the coal seam following CO2 injection. In SA it has been estimated that approximately 1.2 Gt of CO2 could be stored in the coalfields. Although not currently the preferred option for geological storage, coalfields provide the largest onshore CO2 storage possibility. The current research project aimed to study the fundamental differences in CO2 adsorption in a variety of SA coal samples in order to access the CO2 sorption capacities and secondly to evaluate the potential CH4 characteristics of SA coals. The investigation aimed to identify the fundamental differences around the effects of increased pressure under simulated in-seam conditions including super-critical pressures up to ~90 bar for gaseous and supercritical CO2 injection. The effects on CO2 adsorption with regard to the difference in coal moisture contents, simulated in the range from ~0.5 – 4.4% and the influence of increased temperatures in the range of 35 to 55 ˚C were carried out on ten (10) SA coals taking into consideration differences in coal properties, samples with varying rank, ash and maceral compositions were sourced for this research. Then secondly, to evaluate the desorption potential of CH4 for seven (7) selected SA coals. A High Pressure Volumetric adsorption system (HPVAS) was successfully designed and constructed in order to conduct experimental tests to generate the adsorption isotherms for the various parameters tested. Results presented show comparable results with published literature in terms of the degree of variance in coal properties (with respect to rank, maceral and mineral content, ash contents and the effects of moisture, and temperature variance) and the uptake of CO2. Higher rank coals have a greater CO2 absorption propensity, whereas lower rank bituminous coals tend to exhibit lesser CO2 uptake, however, this is dependent on the coals’ petrographic composition. It was clear that samples in the range greater than a vitrinite reflectance of 0.7% (RoVmr) exhibited increased CO2 uptake due to larger macro, increasing meso porosity and micro-pore volumes. Findings related to coal properties; revealed that coals with a higher ash content exhibited a negating effect with regard to enhanced CO2 adsorption. On average, for a 1% increase in ash content in HRC and MRC coals, a decrease of CO2 adsorption capacity of 1.1 mmol/g and 0.018 mmol/g is observed respectively. While for maceral composition these findings suggest that a specific or ideal ratio between only the maceral components, in similar rank coals, is the controlling factor for best CO2 adsorption required. In terms of addressing the adsorption parameters, such as super-critical pressure, temperature and moisture variations inherent in natural coal seams, etc., it was determined that with increased pressure, more adsorption takes place for most coal types. A very positive correlation was found to exist between adsorption of CO2 and desorption of CH4, with increased pressure injections, ranging from sub-critical to super critical pressures, exhibiting increased sorption results, irrespective of coal moisture or temperature effects. From these findings for simulated conditions regarding the effects of coal seam moisture and temperature variations, it has been concluded that results displayed an obvious decrease in CO2 sorption ranging from sub-critical to supercritical pressures overall. The decrease in CO2 sorption was as much as 77% from dry (0%) to the maximum moisture simulated value of ~4.4%. Sorption decreased almost linearly for every 1% of coal moisture increase, until the maximum coal saturation was approached at around 4%. Sorption results relating to increased temperature also displayed an inverse relationship, and hence lower overall CO2 sorption capacities were calculated. The heats of adsorption for these coals were found to be between 21.9 and 39.9 kJ/mol confirming the nature of adsorption to be physical. Results confirm that the calculated heat of adsorption (KJ/mol) and the adsorption capacity (mmol/g) are positively correlative. For investigations pertaining to CH4 desorption for CH4 saturated simulated coals (CH4 added to and then removed from coal samples due to the unavailability of freshly cored coal samples), it was observed that CO2 uptake by pressurized injection for low - high pressures certainly enhances CH4 desorption rate. Results revealed that incremental CO2 injection pressures yielded higher CH4 desorption rates, for both the HRC and MRC coals. Generally there was an observed increase in the rate of CH4 desorbed for all coals tested at 55 oC as compared to 35 oC. This can as well be attributed to the fact that the increase in temperature causes the adsorbed CH4 molecules to vibrate more due to the increased kinetic energy of the molecules. This consequently leads to ease of desorption when CO2 is pumped under pressure into the coal structure, which clearly favours ECBM potentials. Some very good findings have been highlighted in the thesis from a SA coal perspective, and certainly serve as a very good starting point for further investigations pertaining to CO2, CH4, and coal interactions. However, from the vast literature already published globally, it can be seen that much more needs to be done in terms of addressing coal-CO2-CH4 research from a SA perspective, and indeed CCS in SA in general. It is apparent that the results and sum of the key findings presented in this thesis, are of importance for the selectivity and technical modelling for CO2 onshore coalbed storage and ECBM projects to be implemented in SA in the near future so as to meet the demands required to reduce CO2 emissions in SA as part of the global community.Item Synthesis and performance evaluation of chitosan based adsorbent for CO2 capture(2017) Osler, KerenCarbon dioxide capture is essential to reducing CO2 emissions in an attempt to mitigate climate change. Absorption via amine based solvents is currently the mature technology that is applied for the capture of CO2. However, amines can pose health and environmental risks when emitted into the air from CO2 capture plants. Furthermore, the efficiency penalty caused by CO2 capture via absorption and the huge costs associated with the regeneration of the spent amine based solvent poses a threat to the economic viability of CO2 capture by the absorption process. Adsorption technology is an alternative to absorption technology. Adsorption technology seems promising due to its moderate energy consumption (which stems from the ability to operate at moderate temperatures and pressures) as well as being health and environmentally benign. Recently, extensive research has been conducted on designing adsorbents that have the ability to adsorb large quantities of CO2 with a low energy consumption. The challenge in CO2 adsorption technology is to design an adsorbent that is not only non-toxic, biodegradable and cost effective but also has the ability to selectively and efficiently remove CO2 gas from a mixed gas stream. This study proposes chitosan, a biodegradable, non-toxic polymer, as one such adsorbent. Chitosan has the potential to be a suitable adsorbent for CO2 capture because it contains the desired amine groups which act as CO2 adsorption sites. In this study, chitosan was studied as an adsorbent in order to confirm that it is suitable for CO2 capture. Chitosan and chitosan impregnated carbon nanotubes (CNTs) (chitosan/MWCNTs) composite adsorbents were synthesized. Chitosan was impregnated onto MWCNTs in order to enhance the physical properties (surface area, pore size and pore volume), CO2 adsorption capacity and CO2 affinity of the composite adsorbent. The synthesized materials (chitosan and chitosan/MWCNTs) were characterized and evaluated for CO2 adsorption. Chitosan was successfully synthesised from chitin. This was confirmed using FTIR spectroscopy. The synthesised chitosan had desirable properties for CO2 capture. This was confirmed using TGA and custom built CO2 adsorption equipment. The synthesised chitosan samples were inexpensive, had the desired amine groups and were thermally suitable at industrial CO2 capture operational temperatures. The CO2 adsorption capacity of the synthesised chitosan was generally low when compared with literature. The highest CO2 adsorption capacity achieved by the synthesised chitosan in this study was 11 gCO2/kg adsorbent. However, it is important to consider that the polymer is derived from a waste material and as such it is possible to cost effectively utilize a large amount. The amount of CO2 adsorbed by the synthesised chitosan is dependent on the number of amine groups present. Against this background this study aimed to increase the number of amine groups present. This was done using response surface methodology (RSM) to develop a polynomial regression model. The developed polynomial regression model is able to predict the DDA of the synthesised chitosan based on the synthesis variables used. The polynomial model was validated using chitosan from literature and found to be statistically significant. The polynomial model showed the optimum synthesis conditions to yield the highest DDA. Chitosan was successfully impregnated onto MWCNTs. This was confirmed using FTIR spectroscopy. The synthesised chitosan/MWCNT adsorbent was not suitable for CO2 capture. This was confirmed using Raman spectroscopy, N2 physisorption, SEM TGA and custom built CO2 adsorption equipment. The CO2 adsorption capacity of the synthesised chitosan/MWCNTs was low when compared to literature. This is attributed to the fact that the MWCNTs used in this study are not suiTable as adsorbents for CO2 capture as they showed a low CO2 adsorption capacity before chitosan impregnation. However, the CO2 adsorption capacity of the MWCNTs was improved by 650 % after chitosan impregnation. Reports from literature, where CNTs were impregnated with other amines did not show such a significant increase in CO2 adsorption capacity. It is hypothesized that if chitosan were impregnated onto more suiTable CNTs for CO2 capture is would improve the CO2 adsorption capacity of that CNT by 650 %. Thus, yielding a suitable non-toxic, biodegradable adsorbent for CO2 capture. It is concluded that chitosan possesses properties that make the polymer suitable for use as an adsorbent for CO2 capture.