Abstract
In this paper, an improved system to efficiently utilize the low-temperature waste heat from the flue gas of coal-fired power plants is proposed based on heat cascade theory. The essence of the proposed system is that the waste heat of exhausted flue gas is not only used to preheat air for assisting coal combustion as usual but also to heat up feedwater and for low-pressure steam extraction. Air preheating is performed by both the exhaust flue gas in the boiler island and the low-pressure steam extraction in the turbine island; thereby part of the flue gas heat originally exchanged in the air preheater can be saved and introduced to heat the feedwater and the high-temperature condensed water. Consequently, part of the high-pressure steam is saved for further expansion in the steam turbine, which results in additional net power output. Based on the design data of a typical 1000 MW ultra-supercritical coal-fired power plant in China, an in-depth analysis of the energy-saving characteristics of the improved waste heat utilization system (WHUS) and the conventional WHUS is conducted. When the improved WHUS is adopted in a typical 1000 MW unit, net power output increases by 19.51 MW, exergy efficiency improves to 45.46%, and net annual revenue reaches USD 4.741 million while for the conventional WHUS, these performance parameters are 5.83 MW, 44.80% and USD 1.244 million, respectively. The research described in this paper provides a feasible energy-saving option for coal-fired power plants.
Highlights
Power generation in China depends highly on coal-fired power plants, which contribute around70% of the total installed power capacity (1.25 billion kW at the end of 2013) and approximately 78%of the total electricity generation (5.25 trillion kWh) [1,2]
With the above context and following our previous research on low-grade waste heat for power plants, we propose a novel waste heat recovery system and improve the original system based on a comprehensive understanding of the performance relevance between the air preheating process in the boiler island and the feedwater preheating process in the turbine island
The gross work output of the improved waste heat utilization system (WHUS) increases by 22.01 MW. This increase is mainly because the temperature of the flue improved WHUS increases by 22.01 MW. This increase is mainly because the temperature of the flue gas used to heat the feedwater and the condensed water is from 372 °C down to 131 °C in the gas used to heat the feedwater and the condensed water is from 372 ◦ C down to 131 ◦ C in the improved improved WHUS, which is much higher than that of the conventional WHUS
Summary
Power generation in China depends highly on coal-fired power plants, which contribute around. Espatolero et al [23] explored the effects of the temperature of exhausted flue gas and the heat-exchanger performance on waste heat recovery, and evaluated the energy-saving effect for the boiler cold-end. The German Schwarze Pumpe power plant (2×800 MW lignite generation units) implemented a flue gas division system after the electrostatic precipitator and recovered low-grade heat to heat up the condensed feedwater. With the above context and following our previous research on low-grade waste heat for power plants, we propose a novel waste heat recovery system and improve the original system based on a comprehensive understanding of the performance relevance between the air preheating process in the boiler island and the feedwater preheating process in the turbine island.
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