Study of new cascade heating system with multi-heat sources based on exhausted steam waste heat utilization in power plant

Utilizing waste heat of exhausted steam in power plant as a new type of heat source is a key direction to achieve both energy-saving and clean district heating. This paper firstly reviews current mature waste heat utilization technologies, including “absorption heat pump”, “increasing back pressure of turbine” and “decreasing return water temperature of heat network”. Using only one method is not applicable to recovery all the exhausted steam waste heat of multiple turbine units due to its respective limitations. In view of this technical difficulty, this paper proposes a new cascade heating system with multi-heat sources based on waste heat utilization. Extraction steam and exhausted steam are involved to achieve the cascade temperature-rise of heat network water. Corresponding evaluation methods are presented, including the last-stage safety of the large unit and the comprehensive energy efficiency of the new cogeneration heating system. Taking 2 × 300 MW water-cooled units as the example to actualize the new system, the effects of key parameters on energy efficiency and economy are analyzed, and then further researches about integration optimization of the new system are developed. In this study, first keep constant return water temperature of primary heat network, the optimal condition is gained. At this point, all the waste heat of exhausted steam is recovered, the energy consumption of power plant is reduced by 33% and the total cost per heat output ct is reduced by 22.3%, compared with conventional cogeneration system. Then reduce the return water temperature in different level, both the energy consumption and the total cost per heat output ct can be further lowered, the energy efficiency and the economy are better improved.