Thermo-economic evaluation, optimization and synthesis of large-scale coal-fired power plants

A key lever to cope with the well-known issues related to today’s extensive use of fossil fuels, e. g., global warming, is to design highly energy-efficient and cost-effective pulverized coal-fired power plants. Future coal-fired power plants will be with high temperature and pressure levels, multiple heat sources, multiple products, and many available technologies integrated to efficiently utilize different-grade heat. This pinpoints the significance of the system improvement and optimization. This thesis investigates progressively the methodologies for the improvement and optimization of system configurations of pulverized-coal power plants, i. e., the employed components and their interconnections. Structural improvement can be effectively proposed by combining exergy-based analysis with engineers’ judgments. The structural optimization (optimal synthesis) is not a trivial task and can be best addressed by mathematical programming (i. e., superstructure-based and -free optimization methods), since automatic generation and identification of structural alternatives is usually involved. This thesis involves further development of the exergy-based analysis, superstructure-based and -free synthesis approaches and their applications to pulverized-coal power plants. Conventional exergy-based analysis can only identify the location and magnitude of inefficiencies, while an advanced analysis further reveals their source and avoidability by splitting each inefficiency into endogenous/exogenous and avoidable/unavoidable parts and their combinations. In the thesis, a new approach of calculating endogenous exergy destructions is introduced and a modern ultra-supercritical coal-fired power plant is evaluated in detail and comprehensively. The results show that over half of the avoidable inefficiencies within most components are endogenous, and the portion differs significantly for different components. Only nearly 10% of the costs of the whole system could be avoided for modern industrial designs at present. In addition, moving convection-leading heating surfaces into the furnace and increasing air preheating temperature are suggested for performance enhancement. The superstructure-based synthesis approach requires the user to manually define a priori the potential solution space through the modeling of a superstructure. In this thesis, superstructure-based synthesis is applied to investigate classical considerations for future plant design, e. g., numbers of reheating and feedwater preheating, reheating pressure and temperature, enthalpy-rise distribution of feedwater in feedwater preheaters. These promising structural alternatives are less likely excluded from the well-defined graphical superstructure built by a simulator. Structural alternatives and other continuous decision variables are continuously manipulated and optimized by specially-designed mutation and crossover operations, while each alternative is evaluated by system simulations. The matches of steam conditions are discussed with optimal reheating ratios. It is found that only when the throttle pressure reaches a specific value corresponding to each temperature level could the benefits from increasing steam temperature levels be fully obtained. Moreover, increasing steam conditions