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.

As the second task of this thesis, we shall propose some mathematical model to simulate the movement of a floating bridge under some moving loadings. The floating bridge system consists of three parts, i.e. river (fluid), floating bridge (multibody system) and vehicles (load) which pass the bridge. Our objective is to find the motion and dynamical responses of the floating bridge with a truck or tracklayer passing on it. The floating bridge is a system of steel rectangular boxes which can be seen as rigid bodies connected by some kinematic joints. In fact, such system is a fluid-structure coupled system and one must treat the governing equations for the floating bridge and fluid, i.e. Euler-Lagrange equations and Navier-Stokes equations, simultaneously. In our work, we apply the one-leg method and operator splitting arbitrary Lagrangian-Eulerian method to solve the coupled system. / When performing dynamical analysis of a constrained mechanical system, a set of index-3 differential algebraic equations, i.e. Euler-Lagrange equations, are often needed to describe the time evolution of the mechanical system. In this thesis, we apply one-leg multi-step methods to integrate the DAEs directly. To overcome some difficulties leading to certain numerical instabilities, a velocity elimination technique is applied to generate a framework that the position and velocity profiles can be obtained in two separate stages: only the position variables and Lagrange multipliers take part in the convergent nonlinear iterations at each time step while the velocity is calculated by the multi-step formula directly without any iteration. The framework is constructed in a manner such that it satisfies all the constraints at the position level and involves variables as few as possible during the iteration. Some convergence analysis are presented and good stability and high efficiency can be seen through the experiments of some benchmark problems. / Zhao Yubo. / "July 2005." / Adviser: Zou Jun. / Source: Dissertation Abstracts International, Volume: 67-01, Section: B, page: 0310. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (p. 244-276). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Polymer materials stand on a very significant position in the materials industry area. The presence of organoclay nanocomposites reinforces polymer materials on many properties like strength, tensile and so on. Most previous studies on the characteristics of organoclays and polymer nanocomposites were based on the experimental approaches such as XRD (X-ray Diffraction) and NMR (Nuclear Magnetic Resonance). These methods have achieved successfully on the basic analysis of chains and layering structures of polymer nanocomposites. However, information on the molecular level cannot be provided by those approaches. MD (Molecular Dynamic) simulation method could be employed to develop further information on the molecular level about organoclays and interlayer structure polymer nanocomposites. In the research of ODTMA-MMT (Octadecyltrimethylammonium-Montmorillonite) organoclay simulation, we find that the strong layering behaviour of interlayer ODTMA molecules is present with the same minimum distance between nitrogen atoms and MMT surface in different T/O (Tetrahedral vs. Octahedral) ratio cases. Nitrogen atoms sit right above the corresponding hexagonal cavities, which is in agreement with the previous research. The interaction energy between surfactants and MMT clay will reach the lowest point when substitution ratio of tetrahedral and octahedral (T/O) is equal to 1:1. Moreover, MSD (Mean Square Displacement) and diffusion coefficient of different models under same CEC (Charge Exchange Capacity) condition are inverse ratio to the T/O proportion. In nylon6 polymer nanocomposites, sodium cations which exist originally in ensemble as charge balancer are absorbed much closer to MMT surface than the organic components in the nylon 6 ODTMA-MMT ensemble. Sodium atoms or nitrogen atoms in surfactants both have higher MSD and coefficients than those atoms in the organic-modified clays. In the exfoliated nylon 6 ODTMA-MMT nanocomposites, pair correlation has been analysed instead of density profile. Layering packing structure is also shown through this analysis, which is also consistent with previous work.

Graduation date: 1989

Graduation date: 1995

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