| It is an effectve apporach to increasing the energy efficiency of coal-fired power plants by recovering waste heat resources. This paper foucused on the the efficient utilization of the low-grade waste heat resources in coal-fired power plants and performed a thermodynamic evaluation of the main waste heat resouceses, i.e., waste heat from flue gas and exhuast steam, based based on the first and second law of thermodynamics. In addition, according to the principles of energy cascade utilization, this paper studied the mechanism of energy savings by incoporating waste heat recovery from the perspective of integrated physical and chemical energy level upgrade, and proposed new system integration concepts of recovering flue gas and steam waste heat using various coal types and different cooling methods.Based on the analysis of the thermodynamic characteristics of waste heat resources in power plants, this study pointed out that the energy saving effect of waste heat recovery is not only determined by the quantity of the waste heat recovered but also depends on the quality of the waste heat returned to the thermal cycle. This study proposed an optimized flue gas waste heat recovered system by dividing the air pre-heating system in two parts (optimized waste heat recovery system I) to improve the temperature range of the flue gas utilized by the low temperature economizer. Thus, the low temperature economizer could heat the higher-temperature condensed water and can save higher stage steam bleeds from the turbines, which could achieve the energy level upgrading of the flue gas waste heat returned to the thermal cycle. To further increase the thermodynamic performance of the flue gas waste heat recovery, the optimized waste heat recovery system II was then presented to increase the temperature of the outlet air of air preheater by further reasonable arrangement of the temperature range of the heating surfaces at the tail of boiler, which could further achieve the energy level upgrade of the flue gas waste heat. In addition, graphic exergy analysis was conducted to illustrate the energy saving mechanism of the optimized waste heat utilization system from the perspective of the second law of thermodynamics. This paper carried out a thermodynamic and techno-economic analysis of waste heat recovery system in a 1000MWe ultra-supercritical coal-fired power plant. The results showed that:the net efficiency of the two optimized waste heat utilization systems was increased by 0.2 and 0.4 percentage points compared to the conventional waste heat utilization system, and the net annual benefit of the two optimized waste heat utilization systems was increased by 10 and 12.5 million CNY, respectively.This study proposed a widely used concept of energy level upgraded waste heat recovery system, whereby part of the moisture from the high and medium moisture coals could be removed. Furthermore, this study illustrated the energy saving mechanism by using coal pre-drying technology from the perspective of the integrated physical and chemical energy level upgrade, revealed the basic thermodynamic laws of low-grade energy utilization and expanded the application of thermodynamics in waste heat recovery in coal-fired power plants. Likewise, this study performed a thermodynamic and economic analysis of the power plants incorporating the coal pre-drying using flue gas waste heat. The results showed that, the energy efficiency could be increased by 0.9 percentage points and the net economic benefits could be increased by 28 million CNY compared to the reference plant. This study also analysed the variations of thermodynamic performance of the main components after incorporating coal pre-drying and achieved the relationship between the pre-drying degree and the lower heating value, boiler efficiency and power plant auxiliary.Considering the energy utilization and transfer characteristics of coal pre-drying system in coal-fired power plants, this study conducted the thermodynamic analysis of the coal pre-drying using steam bleed from the turbines and optimized the steam source of the dryer by incorporating a supplementary steam cycle, which could significantly reduce the temperature of the steam bleed of the dryer and several regenerative heaters and the corresponding exergy destruction.A thermodynamic and techno-economic analysis of a 600 MWe lignite pre-drying power plant using a supplementary steam cycle was conducted. The results showed that the exergy destruction of the dryer was decreased from 14.23MWth to 13.25MWth. The net energy efficiency could be increased by 0.3 percentage point with enhanced net economic benefit of 6.25 million CNY compared to the power plant incorporating the conventional coal pre-drying system using steam bleed.The exhaust steam pressure and the corresponding condensation temperature in direct air-cooled power plants is relative high as compared to the water-cooled power plant. Against this backdrop, this study proposed a coal pre-drying system using the waste heat of the exhaust steam from the air condenser. In this system, part of the energy of exhaust steam could be efficiently recovered by incorporating the process of coal pre-drying. The thermodynamic and techno-economic analysis of a 600MWe sub-critical air cooling coal pre-drying power plant showed that, the net electric power output increased by 19.7MWe after incorporating coal pre-drying with the net economic benefits increasing of 4.2 million CNY. compared to the reference power plant. |
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