Optimum Design of a Double Reheat Steam System in an Ultra Supercritical Unit

Abstract

Ultra supercritical power generation technology is entering a fast developmental stage. Thus, using the double reheat system is necessary to improve the thermal performance of ultra supercritical units. In this paper, steam system designs were optimized and exergy analysis research on ultra supercritical double reheat units is carried out. The double reheat steam system based on the traditional eight extractions regenerative system is presented. In addition, the regenerative system setting with an outside steam cooler and two additional extractions is put forward. The quantitative energy saving effect of main optimization measures after various systems are analyzed and compared is also given. The present study proved that the setting with outside steam coolers could obviously decrease the superheat degree of extractions for ultra supercritical double reheat units. Moreover, two additional extractions are helpful in optimizing the distribution of the increase in water enthalpy in the regenerative system. Both optimization measures are conducive in improving the unit efficiency and in decreasing the coal consumption rate. The current paper also presents case studies and exergy analysis based on the aforementioned cases. The results show that the coal consumption rate of the optimized double reheat unit is reduced by 2.36 g/kWh compared with the double reheat unit based on the traditional regenerative system with the same initial parameters. The total exergy loss of the regenerative system is reduced by approximately 10% for the setting with outside steam coolers. The total exergy loss of the regenerative system with ten extractions can be further reduced by nearly 18%. Therefore, setting with outside steam coolers and additional appropriate extractions are both conducive in improving the performance of ultra supercritical double reheat units. Thus, a remarkable energy saving effect is observed. The current paper provides a theoretical basis for performance evaluation and design optimization of ultra supercritical double reheat units.