Energy-saving Performance Analysis Of The In-depth Optimized Waste Heat Utilization System For Large-scale Coal-fired Units

In recent years, as a consequence of the rapidly increasing energy price and more and more stringent energy-saving and emission reduction policies, corresponding measures are taken by the majority of the coal-fired plants in China, including improving the power plant efficiency, reducing the unit standard coal consumption rate as well as CO2 emissions.During the development period of coal-fired power plants, plenty of energy-saving methods are available. Among them, utilizing the flue gas waste heat at the boiler rear area to heat the condensed water in the regenerative system is recognized as a simple but efficient scheme. In this paper, a low-temperature flue gas waste heat recycling analytic model was constructed based on the relevant theories on equivalent enthalpy drop, heat transfer and fluid flow characteristics. Subsequently, from the perspective of system safety and stability, assuming that the conventional waste heat utilization system is applied to the active power plant, its connecting modes in the flue gas side and steam-water cycle side, heat transfer tube type and material are analyzed to determine the reasonable retrofitting scheme. Combined with the actual operation data of a 1000 MW coal-fired power unit, the energy-saving benefit of the conventional waste heat utilization system is firstly calculated, and its drawbacks in reducing the exhausted flue gas temperature are analyzed afterwards based on the comprehensive heat exchanging curves.Based on the above completed work and the energy-saving constraints of the conventional waste heat utilization system, with consideration of the "energy grade mismatch" issue which is common during the heat and mass transfer processes in the boiler and turbine side, an in-depth optimized waste heat utilization system is proposed which combines three heating processes comprehensively, including the regenerative heating process in the turbine side, the air preheating process in the boiler side and the waste heat utilization process. Essentially speaking, the proposed system creatively combines the heat release and absorption processes in the boiler and turbine side together, by integrating the heat transfer process of various working mediums such as flue gas, steam, feeding water and the air, energy grade of the flue gas introduced into the turbine side is improved significantly, which can further enhance the heat to work conversion efficiency of the power unit. Moreover, thermodynamic, exergetic and techno-economic analysis are conducted to deeply explore the energy-saving characteristics of the proposed system.The research result of this paper can lay a firm foundation for the optimization design and efficient operation of the large-scale coal-fired units in China, and it provides key technical support for further reducing the unit energy consumption rate under various working conditions.