A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Chemical Engineering), 25 May 2016 / The Integrated Gasification Combined Cycle (IGCC) is a promising technology in the power generation industry to increase efficiency and reduce environmental emissions associated with fossil fuels. The performance of the gasifier and its economic feasibility largely depends on the gasifier island and many problems experienced during gasification are associated with extreme operating conditions. There is, however, no evidence that the extreme operating conditions in the gasifier yield the maximum possible fuel gas heating value. The main objective of this research was, therefore, to develop a mathematical model to simulate and optimize the performance of the IGCC, particularly focusing on maximizing the fuel gas heating value. The work carried out in this thesis was divided into three parts. The first part presented a 1-D simulation model for a dry-fed entrained flow gasifier with oxygen and steam used as oxidizing agents. The model was then validated against published models for a similar reactor configuration and then extended to an existing entrained flow gasifier of Elcogas IGCC power plant in Puertollano, Spain. The second part presented the optimization model in which the objective function was to maximize the fuel gas heating value. The last part combined gasifier and the gas turbine models and evaluated the overall performance of the gas path. The formulated mathematical model which consisted of mass and energy balances of the system was solved in gPROMS platform in order to determine the optimum conditions of the gasifier. Multiflash for Windows was used to obtain the thermodynamic properties of gas phase. The model was first used to replicate three published simulation models, particularly focusing on the carbon conversion, cold gas efficiency, gasification peak temperature and gasifier exit gas temperature. The results obtained during optimization of the Elcogas entrained flow gasifier showed a 14% increase in fuel gas heating value was realized with a decrease of 519K in operating temperature. The pressure did not have a significant impact on the fuel gas heating value, with only less than 2% increase in heating value being achieved by changing the pressure from 2MPa to 5MPa. Owing to a decrease in operating temperature, the conversion was reduced from 97% to about 63% and that led to a decrease of almost 60% in O2 and 50% in steam used in the gasifier. The results also indicate an almost 2% increase in the efficiency of the gas turbine when burning the gas of the higher heating value. This was mainly due to the increase in the expander inlet temperature. The gas turbine exhaust temperature and the exhaust gas heat capacity also iii increased, thereby, increasing the amount of heat available in the heat recovery steam generator. There was also a 7% notable increase of the overall gas path efficiency. A reduction in operating temperature and pressure of the gasifier, therefore, guarantee an extended operating cycle of the gasifier, thereby, improving commercial attractiveness and competitiveness of the technology compared to other available power generation technologies. These new proposed operating conditions, which are less severe, therefore, signify a possible improvement availability and reliability of the IGCC power plant.

The aerospace industry is constantly looking for methods to reduce the cost of flying their airplanes. These savings can come in many forms, one of them being cost savings attributed to fuel savings by either reducing the weight of the airplane or reducing the drag. Friction stir welding (FSW) was introduced as a means of joining previously unweldable Al 7075, a high-strength aluminum alloy commonly used in aerospace for its high specific strength. This eliminated the need for costly and time consuming rivets to be installed, firstly reducing the production cost of the airplane and secondly reducing the overall weight of the airplane therefore improving fuel consumption. There are many factors at play in the process of producing FSW Al 7075, but the result of this process creates a weld joint that is more susceptible to corrosion than the rest of the panel. For this reason, FSW Al 7075 panel fail prematurely and must be replaced too often. The main goal of this project is, using cold gas dynamic spray, to create a metallic layer on top of an Al 7075 FSW joint to protect it against corrosion. A series of 3 corrosion tests indicated that pure Al, among coatings of pure Al, Al 5038 and Al 7075, offered the best protection against corrosion. Al 5083 would also be a suitable material and should be used in applications where high bond strengths are required. Al 7075, although of the same alloy as the parent material, is not recommended for corrosion protection as it offered little advantage over the parent material. In order to better understand the interaction of creating a coating after a hot welding process, several analyses were performed. These included deposition at multiple substrate temperatures as well as hardness and velocity measurements. Results indicate that some aluminum alloys are very sensitive to temperature, yielding better coatings at high substrate temperatures. Individual particle deposition tests reveal that these improvements do not occur at the substrate-coating interface. Another portion of this project was dedicated to creating tensile specimens composed entirely of pure Al cold sprayed coatings. Several sets of samples were produced. Results indicate that pulling in the direction of nozzle travel direction yields UTS values 50% higher than pulling in the direction perpendicular to the direction of nozzle travel during coating deposition. Results after annealing seem to converge towards the same value. Finally, a new nozzle design was performed which should create a more efficient spraying process, resulting in cost savings for the industry.

Understanding the processes involved in the transport of dissolved gas plumes in groundwater aquifers is essential for comprehending the effect that these transport processes can have on site characterization and remedial design applications. Previous laboratory and field studies have indicated that dissolved gas transport in groundwater can be greatly affected by the presence of even small amounts of trapped gas in the pore space of an aquifer. Recently, Fry et al. (1995) reported an increase in retardation factors R (where R=pore water velocity/dissolved gas velocity) for dissolved oxygen with increasing amounts of trapped gas. Fry showed that the retardation factor for a dissolved gas can be predicted using a relationship between the dimensionless Henry's Law constant for the dissolved gas, the volumetric gas content (i.e., the fraction of the total volume occupied by trapped gas), and the volumetric water content (i.e., the fraction of total volume occupied by water). In their experiments, Fry et al. (1995) found this relationship in an equilibrium model accurately predicted observed retardation factors for dissolved oxygen when the volumetric gas content was small, but underpredicted retardation factors for larger volumetric gas contents. Also, predicted breakthrough curves for dissolved oxygen obtained by incorporating this relationship into the advection-dispersion equation did not match the shape of experimentally observed breakthrough curves. The experimental curves were asymmetrical with long tails indicating that the local equilibrium assumption is inaccurate and suggesting that mass transfer of oxygen between the aqueous and trapped gas phases is diffusion limited. In an effort to gain further understanding of this process, a kinetic model was developed for dissolved gas transport that includes a diffusion type expression for the rate of gas transfer between the mobile aqueous and trapped gas phases. The model was tested in a series of transport experiments conducted in sand packed columns with varying amounts and composition of trapped gas. The kinetic model was found to better fit the shape of dissolved oxygen breakthrough and elution curves than the equilibrium model of Fry et al. (1995). This model was then extended to the case of two-dimensions to simulate dissolved gas transport in the presence of trapped gas under conditions that approximate injection and extraction wells used to distribute dissolved gases in an aquifer (e.g. to promote in situ bioremediation processes or to perform a dissolved gas tracer test). We then compared these predicted concentrations with measured concentrations obtained in a series of dissolved gas transport experiments in a large-scale physical aquifer model using two dissolved gases (oxygen and hydrogen) with very different physical properties. The model could accurately fit the development and movement of these plumes providing that key parameters, the amount of trapped gas and the effective mass transfer coefficient, were adjusted between the injection and drift stages. / Graduation date: 1997

Tunable diode laser absorption spectroscopy using an external cavity diode laser operating in the infra-red has been developed to monitor CO2 in the freestream of the Longshot hypervelocity facility at the Von Karman Institute for Fluid Dynamics. The Longshot facility offers a unique European facility for ground testing and numerical validation applications, however, some of the traditional data rebuilding aspects are in question. A non-intrusive absorption sensor could significantly aid in improving the knowledge of freestream static values thereby improving the models used in data rebuilding and numerical simulation. The design of such a sensor also expands the spectroscopic capabilities of the Von Karman Institute. The absorption sensor is designed around the single P12 (00001)-(30013) rovibrational transition near 1.6µm (6218.09cm-1 specifically) which yields relatively weak direct absorption levels at about 3.5% per meter for typical Longshot freestream conditions. However, when handled carefully, adequate signal-to-noise can be acquired to exploit significant flow information. By being able to operate in this range, total sensor cost can be easily an a factor of two or more cheaper than sensors designed for the deeper infrared. All sensor elements were mounted to a compact portable optics bench utilizing single-mode optical fibers to allow for quick installation at different facilities by eliminating tedious optical realigning. Scans at 600Hz were performed over 20ms of the 40ms test time to extract core static temperature, pressure and velocity. These results are compared with the current state of the Longshot data rebuild method. The non-uniform flow properties of the shear layer and test cabin rested gas accumulation was of an initial concern. The temperature and density gradients along with significant radial velocity components could result in DLAS temperature, pressure and velocity that are significantly different than that of the target freestream inviscid core values. Fortunately, with the proper selection of the P12 rotational number, this effect could be more or less ignored as the higher temperature and lower density gas of this region is relatively transparent. Ultimately, acquired temperature and density were moderately accurate when compared to Longshot rebuilt results owing primarily to the baseline extraction which poses issues for such low absorption signals. However, the extracted velocity data are quite accurate. This is a definite puls for the sensor as the freestream enthalpy of cold hypersonic facilities is dictated primarily by the kinetic energy contribution. Being able to compare velocity gives insight to the level of vibration non-equilibrium in the flow. The velocity of the DLAS and the Longshot rebuild are quite close. This adds more weight to the argument that vibrational excitation is very low (if present at all) in the free stream and that the van de derWaals equation of state usage and constant specific heat assumption might be an adequate model for the data rebuild after all.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 114-120). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 112-113). Also available on the Internet.

Thesis (Ph. D.)--University of Oregon, 2003. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 190-205). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 114-120). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 112-113). Also available on the Internet.

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