Water electrolysis is a popular energy storage technique used in tandem with many renewable sources to convert the generated energy into storeable hydrogen. The efficiency of water electrolyzers is greatly affected by overpotential losses. Bubble evolution is a unique characteristic of flows in water electrolysis. The evolution of bubbles alter electrokinetics close to the electrodes and vary ionic mass transport. The localized flow features close to a bubble are often attributed to the variation in the current density and subsequently, the electrolyzer efficiency. For the purpose of modelling flows close to a bubble better, a Lagrangian method for the simulation of passive tracers is developed and programmed to be coupled to the flow field of Bluebottle, an open-source particulate multiphase flow solver that uses the Physalis algorithm. The dynamics of the tracers are modelled using a simplified Langevin equation. In the present work, the migration flux is omitted and priority is given to convection and diffusion with the objective of establishing a foundation for the simulation of ionic mass transport. Brownian motion is described using a random displacement term. The coupling with the flow field is achieved using trilinear interpolation. The domain boundaries in regards to tracer dynamics are modelled as either a rigid wall pair or as a periodic boundary pair. Specular reflection is programmed for the former ensuring elastic collision of a tracer with the domain boundary. For the latter, the tracer position is altered so as to place the tracer in the opposite side of the domain in the axis of intrusion. Particles are assumed to be non-penetrative and hence, specular reflection is implemented at the surface of each particle. Since the tracer module is coupled one-way with the flow field and executed after a Bluebottle time-step, a subroutine is developed to push the tracers out of a particle radially if a tracer is located inside a particle after a Bluebottle time-step. To ensure particle interaction is ensured across periodic boundaries, a subroutine is developed that places the particle in an apparent location that enables particle-tracer interaction.  The module execution time is found to be linearly proportional to the number of particles and the number of tracers and consumes roughly 10% of a Bluebottle iteration execution time in nominal tests. The module is tested to ensure the Brownian displacement term obeys diffusion statistics and also to ensure that the trilinear interpolation works as intended. The numerically enforced no-penetration boundary at particle surfaces is also tested and observed to prevent intrusion of tracers.  The tracer module is then used to stochastically simulate mass transport across a particulate suspension in a stagnant and a sheared flow field. The Sherwood number Sh determined from the tracer module is found to agree well with the expected experimental and numerical results of Wang et al. (2009). The tracer module is also compared to a scalar field solver of Bluebottle. The tracer module is observed to capture features of the flow field quite well. However, the transient tracer positions upon conversion to a transient continuous concentration field exhibits noise due to the discrete nature of the tracers. Hence, transient comparisons with a continuous field in terms of absolute magnitude requires a large number of tracers. Recommendations for improvement of the code is provided. The present work is intended to be followed up with the addition of migration flux to the equations of motion for the tracers through the solution of an additional equation for velocity of the tracer using a force equivalence of Coulomb's law and Stokes' drag law. Future challenges that will be encountered in the development of an accurate ionic mass transport solver is briefly discussed.

The decarbonization of the transport sector is a major challenge for the transition towards a sustainable economy. Amongst the different measures to lower the impact of road transportation on the environment is the use of biofuels, especially for heavy duty vehicles. For biofuels to be affordable and well integrated, improvements in the production process in terms of energy performance and economic feasibility need to be made. Consequently, this thesis aims at the techno-economic assessment of the integration of pyrolysis oil gasification with electrolysis and syngas catalytic upgrading. Oxygen, being a co-product of electrolysis, is used as an oxidizer for entrained flow gasification of biomass derived pyrolysis oil. Hydrogen from the electrolyzer is used to enhance the production of biofuel. The process was designed to have a high-temperature solid oxide electrolysis cell (SOEC) and the biofuel was chosen to be compressed natural gas (bio-CNG). The modeling of the process was performed on Aspen Plus software and calculations of parameters for the gasifier and electrolyzer were done using Fortran and Matlab respectively. As aimed by the integration of electrolysis, the hydrogen produced is added to the syngas to obtain a feed ratio of 3, required for an optimum methane production. The increase in the feed ratio at the level of the gasifier is mainly limited by the oxygen equivalence ratio (OER), considering the extent of combustion reactions and the operating temperature. The amount of steam inputted was found to increase the molar ratio of H2 to CO without significant change in the feed ratio due to the increase in the concentration of CO2. However, steam was still necessary for adjusting the temperature of the gasifier to values that are in accordance with experimental ones in literature. The obtained final product stream of bio-CNG at 250 bar and 15℃ has a purity of 98.63 mol% and 99.15 wt%. Moreover, by applying heat integration to the process, the resulting surplus heat was 10.71 MW, which was assumed to be used to generate steam that is inputted to the SOEC for producing hydrogen as an additional product. The process was found to have an overall efficiency of 73.8% (LHV basis) and 78.2% (HHV basis). Heat integration increased the process efficiency by 15% (LHV basis) and 23% (HHV basis), by removing to a great extent the need for external heat and by having hydrogen as an energy output added to CNG. As a complement to the energy performance, the economic feasibility of the process was assessed. The total capital investment was found to be 889.5 Euros/kWinput, the net present value was 6.0342 million Euros and the rate of return on investment 30.02%.

Biochar for horticultural and agricultural applications using high temperature torrefaction technology Pradeep Ravi Supervisors: Prof Dr. D.J.E.M Roekaerts, ir.Bart de Vries, Dr. Luis Cutz, Dr.Lorenzo Botto & Dr.Ralph Lindeboom Biomass currently accounts for less than 10 percentage of the world’s renewable energy production. Currently the major global sustainability issue stems from the sourcing of virgin wood chips from dense forests for pellet production. An alternative is to use residual biomass from agriculture or forestry, which is produced in large volumes, to produce different products that range from biofuels to chemicals via thermochemical conversion technologies. Among thermochemical technologies, torrefaction is a promising route to produce solid biofuel known as biochar. With an increasing potential for biomass production coupled with an increased scrutiny on the use of biomass as a green fuel, the need for alternative clean applications for the biochar is critical. The aim of this study is to investigate new and novel agricultural residues or other waste streams to produce biochar using high temperature (350 °C) torrefaction technology. The obtained biochar is evaluated experimentally to determine the best feedstocks out of the ones that are selected from a performance and cost point of view for horticultural applications. . This research aims to provide a clear and useful analytical tool which will benefit the scientific community to select suitable biomass materials based on material properties and end applications. The efficacy for the various torrefied biomass feedstocks on the soil and its stability are tested. Overall, about 50 different biomass feedstocks were identified and evaluated based on past performances from literature. The top 10 best performing feedstocks were sourced and subjected to various physical and chemical characterization tests with a specific focus on soil remediation. The selected materials were torrefied in a fixed bed pilot reactor Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS), Brunauer–Emmett–Teller (BET) and pH measurements. Ultimately the feedstocks were scored and ranked from best to worst performing biochar for soil remediation and sequestration-based applications. The results of this project indicate potential for biochar production from woody, grassy and other processed materials that could help to remove the dependence on evergreen forests and wood chips. The system proposed in this work could also yield negative emissions since the feedstocks are residual flows and the biochar is going to be used in the soil.

A magnetorheological fluid is a type of smart fluid that can change its rheological properties when a magnetic field is applied to the fluid. The fluid is a suspension of magnetizable particles in a non-magnetic carrier fluid. When a magnetic field is applied to the fluid, the particles will form structures along the magnetic field lines. These structures resist flow, thereby increasing the viscosity of the fluid. In a non-uniform magnetic field that is generated by a permanent magnet, the particles separate from the fluid and aggregate on the surface of the magnet. This phenomenon is of use in hydrodynamic bearings, where textures are used to increase the load capacity of the bearing. Replacing the fluid in the bearing by a magnetorheological fluid and placing permanent magnets at the desired texture locations, results in particles separating from the fluid, aggregating on the permanent magnets and forming textures. These textures are of a self-healing nature, because any particle that is sheared off, returns into the fluid and is replaced by another particle. Currently, not much is known about the formation of these textures and the influence of their formation on the rheological behavior of the magnetorheological fluid. Therefore, several discrete element models are constructed, that can simulate a magnetorheological fluid in non-uniform field. These models use basic physical laws to determine the dynamics of the particles. A single core model is constructed to simulate the behavior of the magnetorheological fluid in small domains using two different methods for the magnetic interaction forces between the particles. A parallel model is made to simulate the magnetorheological fluid in larger domains. Finally, a model that is used to simulate particulate flows, is modified to research whether two-way coupling is required to determine the steady state behavior of the magnetorheological fluid. The models show that the particles do not separate from the fluid due to the attractive force of the magnet by itself, but rather by a combination of the attractive force and the deformation of the structures when a flow is applied. Furthermore, the rheological behavior of the fluid can still be approximated using the standard viscoplastic models. Finally, the modified particulate flow model showed that two-way coupling is not required to determine the steady state behavior of the magnetorheological fluid.

A water electrolyzer is an electrochemical device that produces hydrogen and oxygen from water via electricity. A sustainable way of producing hydrogen with renewable electricity, also known as green hydrogen, could enable its use in decarbonisation of sectors that depend on fossil fuels. However, to scale up the production of green hydrogen, larger water electrolyzers are required, which presents challenges. Water electrolyzers produce hydrogen in the form of bubbles, which are transported away through outlet channels. The geometry of the outlet channel is important for performance and safety. A channel that is too wide can result in unwanted ionic current leakage, reducing efficiency and increasing the risk of explosion. To reduce the leakage current, the electrical resistance in the channel can be increased to make it difficult for ions to flow through. However, a narrow channel can increase the hydraulic resistance, leading to clogging due to bubble build-up. This study focuses on the characterization of the slug flow regimes that eventually lead to clogging of the outlet channel in a novel electrolyzer design. To characterize the flow when blockage develops, electrochemical tests with industrially relevant 6M KOH were carried out at room temperature and ambient pressure in conjunction with high-speed video recording. The examined channel was 2 mm high, 3 mm wide, and 40 mm long. The experiments showed that clogging occurs for certain flow regimes. The hydraulic configuration caused clogging at the hydrogen side, which caused the liquid flow prefer to travel via the anode rather than the cathode, which was reflected in the temperature plots. Clogging did not occur when the superficial gas velocity was greater than 0.3 cm/s, which corresponds to a current density of 140 mA/cm2. The clogging was also not visible at a superficial liquid velocity greater than 0.25 cm/s. Below these values, clogging is expected to take place for the investigated channel dimensions. The bubble diameters in the bubble layer in front of the channel were measured with a microscopic camera. The mean bubble diameter was 0.4 mm, which is 80% smaller than the height of the channel through which they flow. However, exceptional bubble sizes that were 3 to 4 times the channel height, which ultimately led to clogging of the channel, were also present. The bubble layer thickness and velocity at increasing liquid flow rates and current densities were measured using a high-speed camera. At low current densities, the velocity and thickness of the bubble layer were not affected by the liquid flow rate. At higher current densities, a higher liquid flow rate appeared to reduce the thickness of the bubble layer, but not the velocity of the bubble layer. The bubble layer thickness as well as the velocity strongly depend on the applied current density (and thus the gas flow velocity). This knowledge might be, when the flow regimes are scaled to larger electrolyzer dimensions accordingly, essential for future electrolyzer manufacturers and operators for the operation and design of larger-scale electrolyzers. It will give insight into which flow regimes should be avoided in order to avoid clogging and thus explosion risks during operation.

The last decade has seen an increase in the world wide energy demand, it was recorded as the highest spike encountered in the energy demand over a decade. On analyzing the main sources which were able to satisfy this overwhelming demand it was found that the majority was still from non-renewable sources or fossil fuels such as coal, oil and natural gas. The aftermath of the continued use of these fossil fuels are already well known, one can argue that there has still not been a serious attempt to phase out these non-renewable sources even though the concerns regarding the harmful emissions being released with their continued use are subject of numerous discussions and studies. It is paramount that the share of renewable energy technologies in the overall energy mix needs to be increased,various options must be explored such that it is possible to substitute these renewable energy sources in place of fossil fuels particularly in energy and emission intensive industries without a comprise in meeting their energy demand. Refuse derived fuel (RDF), which is the combustible fraction that has been separated from municipal solid waste (MSW) has gained attention due to it being seen as an alternative to convectional fossil fuels as well as a method of sustainable waste management and disposal. The reason for their use as an alternative fuel is mainly attributed to their physical and chemical characteristic such as their low moisture content, high grindability and calorific value. However, the challenges faced in their application as a substitute fuel or feedstock is attributed mainly to the high variability in the properties of the RDF material that is being supplied to the relevant industries. Therefore a standardization in its properties needs to be carried out particularly with regards to their moisture content and calorific value in order to promote their application. Torrefaction, also referred to as mild pyrolysis is a thermal treatment method that usually is carried out in temperature ranges of 200-300°C and residence times of 30-60 min in the absence of oxygen. The material that is being subject to this treatment method partly decomposes to release volatiles,and the left over solid product has been reported to undergone a modification its physical and chemical characteristics when compared to the initial solid material. From a chemical point of view the final solid product is reported to have an increase in the carbon content and decrease in the hydrogen and oxygen content which leads to a higher calorific value than the original material. Several experimental studies have been conducted for the torrefaction of RDF with aim of improving and standardizing the calorific value and have yielded promising results. However there has not yet been a work that has focused on the torrefaction of RDF on large scale, that takes into account the process design and economics associated with their production. The aim of this thesis was to carry out a techno-economic evaluation into the torrefaction of refuse derived fuel. An initial composition of raw RDF was selected from literature which has a high moisture content and low calorific value. The raw material is then subject to several process such as drying, torrefaction, grinding and pellitization with the aim producing torrefied RDF pellets. These process were simulated and optimized through Aspen Plus in order to obtain a better understanding of the influence of process parameters such as moisture content, drying temperature, torrefaction temperature and residence time on the final product as well as to select the optimum route for their production. Economic evaluation was also carried out to determine if the implementation of such a project is feasible. This was done on the basis of certain economic or profitability indicators. Results from simulations show that the optimum torrefaction conditions were 250°C and residence time of 30 min, which resulted in the final torrefied RDF having an increased calorific value. The economic analysis provides positive results particularly when moving towards a higher processing capacity of initial raw RDF. Finally, the effect of substitution of the torrefied RDF in a cement plant was also evaluated and has shown that significant savings can be achieved through the reduction in fossil fuel consumption and carbon dioxide emissions.

The long-distance maritime transport sector plays one of the most relevant roles in the transport sector. Therefore, this project designs a modular solid hydrogen solution, studying the assumptions made in the literature and including new simplifications that reduce the computational costs of its simulation. Moreover, this project compares the viability of this solution with the different prototypes found on the market.