Integrated Design And Multi-objective Optimization Of Supercritical CO₂ Cycle Based Coal-fired Power Generation Systems

Supercritical carbon dioxide（s CO₂）Brayton cycle（SCBC）is widely regarded as one of the most promising coal-fired power generation technologies because of its advantages of high efficiency,compact system,low noise and quick response under different operating conditions.At present,the application and research of s CO₂ power generation are in the initial stage,and there is a large gap in the research of s CO₂ coal-fired generation system,especially the demonstration small-scale coal-fired power generation system.In addition,due to the complex system structure,boiler and recuperator equipment design involves a large number of equipment parameters,physical properties and thermal integrated calculation,the synchronous design of the whole system is faced with a huge amount of computing,solving difficulties and other challenges.In addition,the current system design has the problems of high temperature of CO₂ into the boiler and cooler,and large amount of heat is lost to the environment with flue gas emission and working fluid cooling,and the energy utilization efficiency needs to be further improved.Finally,the comprehensive evaluation and analysis of s CO₂ power generation system is still lacking,and the correlation between energy,economic and environmental performance of the system is not clear.Therefore,the research of small-scale s CO₂ coal-fired power generation system,and the development of a high performance system synchronous optimization model and algorithm,the proposed system design and waste heat utilization strategy,build system of comprehensive evaluation and optimization method,to promote small-scale coal-fired power generation system performance,guide the construction of demonstration project and promote s CO₂ coal-fired power generation technology is of great significance.Aiming at the above problems,this thesis firstly establishes the global design model of each equipment of 50 MW s CO₂ coal-fired power generation system,and designs the efficient solution strategy.Then,the influence of different cycle structures on the system is discussed.In order to further improve the efficiency of the system and utilize the low temperature waste heat,two waste heat absorption strategies were proposed,and the system was optimized with the thermal efficiency as the objective.Finally,a s CO₂ coal-fired power generation system with bottom cycle was optimized with energy-economic-environmental multi-objective,and a multi-index balanced solution for each system was obtained.In this thesis,a global design model of cycle-boiler-heat exchanger for s CO₂ coal-fired power generation system is constructed.By analyzing the coupling relationship between system s CO₂ Brayton cycle,boiler and heat exchanger,an efficient solution algorithm is customized for the model.It is found that the average relative error of cycle model is 0.01%,and the average relative error of temperature and pressure distribution of printed circuit heat exchanger（PCHE）model is 1.27%and 0.01%,respectively.It is found that the thermal efficiency of the recompression cycle system increases by 2.76 percentage points compared with the simple heat recuperated cycle under the same operating parameters,and the recompression-intercooling cycle increases by 1.50 percentage points compared with the recompression cycle.Another significant feature is that the pressure drop and heat load of the recuperators account for a large proportion,and the low pressure side pressure drop is much larger than the high pressure side pressure drop.We proposed three SCBC-Organic Rankine cycle（SCBC-ORC）combinations（Design A-C）and one SCBC-Li Br/H₂O absorption refrigeration cycle（SCBC-AR）combination（Improved B）to solve the waste heat cascade utilization problem of 50 MW coal-fired power generation system.In order to reduce the complexity of system integration design,the grey box model of neural network is introduced to reduce the calculation time of integration design by 50-60%.Sensitivity analysis shows that flue gas temperature at the cooling wall（CW）outlet has a marginal effect on the system performance,while the main compressor inlet pressure and the evaporation state of ORC working fluid need to be paid attention to,which are critical to the thermal efficiency of SCBC-ORC and SCBC-AR coal-fired power generation systems.Optimization results show that Design A and Improved B have the most effective efficiency improvement,achieving the highest thermal efficiency of 45.73%and45.72%,respectively,which are 2.75 and 2.74 percentage points higher than the standalone SCBC coal-fired power generation system.We also constructed an energy-economic-environmental model with the thermal efficiency(η_th),the levelized cost of electricity（LCOE）and the environmental impact load（EIL）as objectives.Multi-index analysis and optimization were carried out for original system（Original）,Design A（Improved A）and Improved B.Analysis shows that Original,Improved A and B have the optimal inlet pressure of the main compressor.If the isentropic efficiency of compressors and turbines are too large,the cost of the three systems will rise sharply but the efficiency will be limited.In multi-objective optimization,η_th-EIL has synergistic relationship,whileη_th-LCOE-EIL,η_th-LCOE and LCOE-EIL are mutually contradictory.Forη_th-LCOE-EIL balanced scheme,compared with Orignial,theη_th of Improved A increased by 1.40%,LCOE decreased by 0.56%and EIL decreased by 1.16%.For Improved B,η_th increased by 1.70%,LCOE increased by 3.66%and EIL decreased by 1.59%.