Design and techno-economical analysis of polygeneration systems for combined heat and power applications

  • Athanasios Pitsas

Student thesis: Master's Thesis

Abstract

Carbon dioxide is the primary greenhouse gas emitted through human activities. The combustion of fossil fuels to generate electricity constitutes the major single source of CO2. In recent years, the need for environmental friendly power generation has led to the development of highly efficient power plants. Solid Oxide Fuel Cells (SOFCs) constitute a promising technology for power generation since they can convert the electrochemical energy directly to electricity. The objective of this research work is to design, simulate and assess the economic feasibility of different polygeneration systems for Combined Heat and Power (CHP) generation. Four different integrated systems are considered in this study: in two integrated CHP systems a conventional Water Gas Shift (WGS) reactor is employed (Pathway I & II), while in the other two CHP systems (Pathway III & IV) a Palladium Membrane WGS Reactor (PMR) is used. The studied systems consist of a SOFC stack, a steam reformer, a WGS reactor, a network of heat exchangers, gas turbines, and compressors. Two different feedstocks, namely methane and heptane, are examined. The proposed pathways are numerically simulated with MATLABĀ® R2011b. Initially, a parametric study of the simulated steam reformer and conventional WGS reactors is carried out to determine the optimal operating conditions in order to maximize the fuel and carbon monoxide conversions. The parametric study reveals that the steam reformer and WGS reactors operating temperatures have a significant effect on the overall performance of the CHP plants as well as on CHP systems economics. Afterwards, a sensitivity analysis of the simulated CHP systems is outlined in order to investigate and evaluate the effects of key operating parameters (e.g., pressure, steam reformer temperature, fuel utilization factor and S/C ratio). The analysis indicates that operation at high pressure, high steam reformer temperature, and high fuel utilization factor seems to be the best possible combination to improve the overall efficiencies of the proposed CHP systems. In addition, two WGS reactor technologies are compared in terms of overall CHP efficiency and economic feasibility. The analysis reveals that the membrane WGS reactor can improve the overall efficiency of the systems by 9% and 9.8% in case of methane- and heptanefuelled system, respectively. From the economic point of view, membrane WGS reactor shows the potential and the economic feasibility for integration into CHP systems. Finally, as the complexity of the pathways is increased using gas turbine and compressor units, the capital and operating costs are evaluated. Although the overall performances of the CHP systems are improved at high pressure operation, the capital expenditure is increased significantly up to 40%. However, the operating cost will decrease slightly, as smaller reactors would be required. The results of this study provide useful information and understanding of the design and operational conditions of the proposed systems, which can further improve the efficiencies of power plants. In addition to this, these results can contribute and guide potentially the commercialization efforts for CHP plants, either for commercial or residential applications.
Date of Award2014
Original languageAmerican English
SupervisorAli Almansoori (Supervisor)

Keywords

  • Applied sciences
  • Optimization
  • Polygeneration
  • Chemical engineering
  • 0542:Chemical engineering

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