Structure And Parameter Optimization Of Reheat System Of Ultra-Supercritical Double Reheat Unit

Since the double reheat technology can improve the cycle thermal efficiency of the unit without increasing the main parameters,it is considered to be the main development direction of coal-fired power generation in the future.The current supercritical double reheat technology is still not perfect,and the problem of excessive superheat of regenerative extraction steam has hindered the improvement of the thermal economy of the unit.In addition,the optimal thermal system structure and parameters of the double reheat unit have yet to be further studied.The Ebsilon is used to simulate the calculation of a double reheat unit.The exergy analysis method is used as energy consumption evaluation method of the system.The optimal regenerative steam turbine(RT)arrangement of the double reheat unit is obtained by comparing different arrangements.The solution and the optimal outer steam cooler(OSC)layout were compared and compared their thermal economy.The thermal parameter optimization model of the secondary reheat unit was established by deriving the calculation formula of the cycle thermal efficiency of the secondary reheat unit.The effects of water supply soaring,feed water temperature,reheating pressure,regenerative series and main steam parameters on the cycle heat efficiency of the unit were analyzed.The results show that optimizing the parameters of the thermal system can greatly improve the thermal efficiency of the unit;the reheating pressure has a great influence on the efficiency of the unit,and the second reheating pressure has a greater impact;increasing the number of low-pressure heaters can significantly improve the thermal efficiency of the unit;The optimum feed water temperature increases as the initial pressure rises.In order to explore the influence of thermal structure and parameters on the efficiency of MC(Master Cycle)system,a 1000 MW ultra-supercritical secondary reheat unit was used as the reference unit.The arrangement of the regenerative steam turbine(RT),the thermal parameters,and the position of the deaerator are optimized.And the best arrangement is obtained by comparing the variable load characteristics of each scheme.The reason for the difference in the load characteristics of each scheme was analyzed by the enthalpy method.The results show that the best solution is RT extracts steam from hot spot of the first reheat,and the RT provides extraction steam for six heaters.The thermal efficiency of the best solution under THA is increased by 0.709% compared with the reference unit,and the effect of reducing the consumption is more obvious when the load is reduced.After the parameter optimization,the two reheating pressures and the feed water temperature were significantly increased.Most of the exergy loss of the unit comes from the boiler.The change of the thermal efficiency of the unit after the introduction of RT is the result of the comprehensive effect of the energy consumption of each thermal equipment.When the isentropic efficiency of RT is less than 70%,the improvement scheme no longer has an efficiency advantage over conventional units.The thermal economy of the unit using different outer steam cooler(OSC)arrangements was compared.And by optimizing the thermal parameters,the best OSC layout scheme is obtained.Compare the thermal economy,variable load characteristics and technical economy of the best OSC solution and the best RT solution.The results show that when two OSCs are introduced into the system,it is best to arrange the OSCs in the second and fourth extraction stages.The thermal efficiency of the best OSC scheme is increased by 0.466% compared with the reference unit.After the addition of OSC,the total exergy loss of the heaters has not decreased due to the new heat exchange loss introduced by the OSC.However,the increase in feed water temperature leads to a reduction in exergy loss of the boiler.At THA,the thermal efficiency of the best RT scheme is 0.243% higher than that of the best OSC scheme,and the variable load characteristics and technical economy of the RT scheme are also superior to the OSC scheme.