This study assesses the socioeconomic feasibility of an integrated waste tyre pyrolysis and electricity generation system in the Gauteng region of South Africa. The primary aim of the research is to address two of South Africa’s challenges, waste tyre management and electricity generation shortages, by integrating waste tyre pyrolysis with a gas turbine. This research is in direct response to the call for further research into the applicability of pyrolysis from the South African government. Gauteng is a highly populated and economically significant province, which provides a crucial case study for this system due to its substantial waste tyre stockpiles, built-up infrastructure, dense population, and high electricity demand.

The designed system involves the pyrolysis of waste tyres to produce char and oils, which are then used to generate electricity through an external gas turbine. This integrated approach highlights the applicability of using pyrolysis outputs for electricity generation, bypassing the need for uncertain secondary markets for pyrolysis products. The system design relies on the availability of waste tyres, regulatory considerations, and market demands for electricity, sizing the system first for the desired electricity generation capacity and fitting the pyrolysis size to meet the demands for electricity generation.

The economic analysis shows that the combined system is a highly feasible solution, with a Net Present Value (NPV) of approximately R4.85 billion and a payback time of 5.4 years, building a solid investment case. The Levelised Cost of Electricity (LCOE) is calculated at 0.12 R/kWh, making the system more cost-effective than current electricity generation methods in South Africa under the national generation company. The economic model’s outlook and results are supported by sensitivity analyses, which indicate the system remains feasible under changing waste tyre and electricity prices, even without government subsidies, a critical condition to eligibility for the support schemes.

The Social Cost-Benefit Analysis (SCBA) reveals a range of significant societal benefits that the integrated system provides. These include job creation, improved power quality, and increased national economic output through reduced load shedding. The system’s Social Net Present Value (SNPV) is strongly positive at R8.56 billion, with significant social benefits across various different sectors. Further, the results highlight the benefits of pyrolysis for waste tyre management over the direct use of tyres in coal power plants, making a clear case to the government regarding the processing method to pursue. Significantly, these benefits outweigh the additional carbon emissions from the increased electricity generation capacity and the increased tax burden of the subsidies to the processing system. Not all of the societal benefits are evident in the SCBA; the system also plays a role in public health improvements and a reduction in road accidents by addressing the waste tyre stockpile issue. These benefits further highlight the potentially significant gains for society following implementation.

The study concludes that the proposed system is a socio-economically feasible solution for Gauteng’s waste tyre and electricity challenges. And a potential driving force for South Africa’s energy landscape transition, paving the way for renewable energy systems to connect to the grid en masse through the added grid stability. Its positive economic indicators and net societal benefits make it a promising candidate for addressing South Africa’s energy transition and waste tyre management goals, meeting all of the government’s objectives of the Industry Waste Management Plan and fitting into the Renewable Energy Independent Power Producer Role. The study paves the way for future research to focus on environmental impacts, changing system assumptions and long-term electricity grid impacts.

In Europe, 20.7 Mt of Polystyrene (PS) was produced in the year 2021, mostly consisting of packaging, insulation, and food utensils. Like many other kinds of plastics, they take hundreds of years to naturally degrade in the environment. To prevent the buildup of plastics in our environment, recycling technology will have to be employed. Current recycling technologies can be divided into three categories: organic, mechanical and chemical recycling. Organic recycling involves the use of biological organisms or enzymes to breakdown plastic into useful materials. Mechanical recycling technologies include processes such as dissolution and conventional mechanical recycling. Among chemical recycling, baseline technology includes
pyrolysis that involves higher temperatures (>500°C) which produces char, gasses, and complex oils. Hydrothermal Liquefaction (HTL) of PS has shown promising results in tackling the problem plastic waste buildup in the environment. This process utilizes water as solvent and subjecting it to mild temperatures and pressures to produce less complex oils that contain building blocks such as monomeric compounds and high value chemicals (HVC). These compounds can then be used as platform chemicals or as source to manufacture new materials, as well as closing the loop on PS waste.
This thesis focuses on the use of catalysis to increase the selectivity for PS conversion through HTL to increase the yields of monomeric compounds and high-value chemicals (HVCs) at lower temperatures (<370°C) than current approaches. In this work, different catalysts were screened and the best performing catalyst was used to optimize the HTL process. The HTL process was optimized using a design-ofexperience (DOE) approach for the following process conditions: temperature (330-350°C), catalyst loading (0-15%) and reaction time (30-60 minutes). The analytical techniques used to characterize the quality of oil as well as determine the amount of chemicals present are: Bomb Calorimetry, Ultimate analysis (CHN-analysis), Gas Chromatography-Mass Spectrometry with Flame Ionization Detection (GCMS-FID)
and Two-Dimensional Gas Chromatography with Flame Ionization Detection (GCxGC-FID).
Results from a screening campaign indicated high yields of oil (90 wt%). Following this, a simple distillation was conducted to separate the lighter fraction from the heavier ones within 90 wt%. Ultimate analysis of the compounds showed high C,H,and N ratios similar to what was found in literature. Bomb calorimetry of the PS-Crude oil product showed of 40 MJ/kg, indicating that oil product is comparable to what is found in literature. GCMS-FID indicated that 17 wt% of styrene was produced from this process,
along with 5.5 wt% of alpha-methyl styrene and other HVCs. The rest of the oil consists of heavy fractions (C-20) which could be go through another upgrading process.
After the screening campaign MgO was chosen as a catalyst to increase the yield of styrene. The yield of oil remain high with 80-90 wt% yield of oil in most cases. Aqueous phase and gas yields remain low (<5 wt%), while char can vary but also remains quite low (<10 wt%). Analysis of Bomb calorimetry showed similar results to screening with an average of 38 MJ/kg. CHNO also showed similar ratios of C,H, and N ratios. When looking at the GCMS-FID and GCxGC-FID, the amount of styrene remained at a maximum of 16-17 wt% with no significant increase. However, results of the research also showed that catalyst does have effect in increasing yield of styrene. Optimization for oil was conducted and optimum point for oil production is at 340°C, 34 minutes and 8% catalyst loading. The research will contribute to the growing body of research into processes for plastic waste valorization and offers more insight into catalytic
hydrothermal liquefaction as well as experimental methodology to separate the oils into its components.

The rise of CO2 concentration in the atmosphere is a leading cause of global warming. Utilizing CO2 ob- tained from point sources, such as chemical industries, as a feedstock to produce high energy density fuels and chemicals could mitigate the emission of CO2 as well as provide various economic bene- fits. One promising technology is the electrochemical reduction of CO2, however, the presence of contaminants in the industry-supplied feedstock and the separation of products downstream would be challenging in a continuously operated large-scale plant. To address these challenges and indentify the bottlenecks involved, it is important to study which pre-treatment and post-treatment steps are required and how to integrate these in a large-scale electrochemical CO2 reduction process.

The gas and liquid feed streams to the CO2 electrolyzers are first cleaned to the desired levels. The liquid feed stream is water from the river Rhine that is purified so the specifications of the water meet the requirements for type 1 water (ultrapure water). The gas feed stream is the flue gas stream of an average steel-producing plant in Europe and is cleaned to remove sulfur and nitrogen compounds. A two-step electrolysis process is used where CO2 is first reduced to CO, followed by the reduction of CO to C2+ products. The electrolyzer for the first step is a membrane electrode assembly-based flow cell with a current density of 300 𝑚𝐴 𝑐𝑚2− and a faradaic efficiency (FE) of 96% to CO. In the second step, a gas diffusion electrode-based flow cell with a current density of 300 𝑚𝐴 𝑐𝑚2− and a faradaic efficiency of 9.35%, 15.49%, 45.57%, and 16.38% towards acetic acid, ethanol, ethylene, and propanol, respectively, is used. The anolyte and catholyte in the reactors are recycled 3,500 and 2,000 times, respectively, to reduce the size of purification of the liquid feed stream section and increase the liquid product concentration. In both steps, unreacted CO2 and CO are recycled. The gaseous and liquid products are separated and purified to meet the industry standards using established separation techniques.

The total capital investment for a process with an industrial gas feed of 381.678 tons per hour is 4,053.5 million dollars with a daily operating cost of 7.403 million dollars. The daily income from selling the products is 2.363 million dollars, but this could increase if the FE towards acetic acid is increased since this product has the highest income per electron consumed. The net present value (NPV) for the base case, assuming current technological and market conditions, is -19.4 billion dollars after 15 years. To analyze which parameters have the most influence on the NPV, a sensitivity analysis is also per- formed with a better and optimistic scenario. It was found that the economic feasibility of the currently designed process is not limited by the technological progress, but mainly by the market conditions.

The target of this process is to reduce the emission of CO2, however, the operation of the plant itself contributes to some CO2 emissions. Therefore the process should consume more CO2 than it emits. The units that consume most energy and emit the most CO2 are the CO to C2+ products electrolyzer and the first distillation column in the liquid product separation section to remove acetic acid. It was found that the process is only carbon negative when the consumed energy is generated by nuclear, wind, or solar energy. The net CO2 emission is the lowest when nuclear or wind is used as an energy source. Generating all the required energy from these sustainable sources brings another challenge since the total installed capacity of these sources are currently not sufficient to cater to the needs of such a large-scale continuously operating CO2 electrolysis plant.

Keywords: Electrochemical CO2 reduction, large-scale, pre-treatment, post-treatment, technoeconomical analysis, energy analysis