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.

It is standard practice in one-dimensional gasdynamic simulations of high performance two-stroke engines to model the exhaust tail pipe entry as an area change using an algorithm similar to the area change of the reverse cone. In the reverse cone the area continually steps down while at the tail pipe entry it changes from stepping down to constant area. At this point a vena contracta can form that effects the flow resistance of the tail pipe. In an effort to improve the accuracy of the gasdynamic simulations the area change algorithm at the tail pipe entry was replaced with a restriction algorithm that incorporates a coefficient of discharge and allows an increase in entropy on the expansion side. The coefficient of discharge is defined as the actual measured mass flow divided by the mass flow predicted by the restriction algorithm. An experimental set up was designed and constructed to measure mass flows for a variety of tail pipe entry geometries at a range of pressures covering the pressure ratios encountered in a real engine. From the mass flow results the coefficients of discharge for a range of pressure and area ratios and reverse cone angles could be calculated and arranged into matrix form to define Cd-maps. The Cd-maps were incorporated into the simulation software and tested to ensure that it functioned correctly. <p<Finally, the simulation results with and without the Cd-maps were compared to measured results and it was shown that incorporating this refinement improves the accuracy of the simulation results on the “over run” part of the power curve. This is the part of the power curve after maximum power and very important in the development of high performance two-stroke engines. These maps can be used for all future simulations on any engine size that uses the same tail pipe geometry. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted