Research On Dynamic Modeling And Response Characteristics Of Bottoming Cycle In Partially Recuperative Gas Turbine Combined Cycle Unit

With the proposal of the dual carbon plan,China will gradually build a renewable energy-based clean low-carbon energy system.Due to the restriction of natural conditions,the fluctuation of renewable energy generation is large and the trend is difficult to control,so it puts forward higher requirements on the peak regulation capacity of the power grid system,the gas turbine combined cycle units will undertake peak load regulation more frequently,thus the units will operate under partial load for a long time,the performance of partial load and the dynamic response characteristics of load fluctuation are of great significance to the safe operation of units and the economy of power plants.The bottoming cycle of a gas turbine combined cycle unit has the characteristics of large thermal inertia and long dynamic response time.Partially recuperative unit is a new method which can be used to improve the performance of combined cycle at partial load,it is an important part of the system feasibility evaluation to study the dynamic characteristics of the bottoming cycle of partially recuperative unit.In this paper,a dynamic simulation model of the bottoming cycle system of a partially recuperative unit is built,the dynamic response characteristics of gas turbine bottoming cycle under the disturbance of exhaust parameter step and recuperative variable load are studied and compared with those of conventional units.Firstly,based on the modular modeling approach,the bottoming cycle is divided into several main components according to typical nodes,including:recuperator,heat recovery steam generator,steam turbines,etc.The dynamic model of the recuperator is built based on the dynamic heat balance of gas,metal wall and work mass;the dynamic models of single-phase heat exchanger and phase-change heat exchanger are built separately,and the dynamic model of heat recovery steam generator is obtained by iterating the heat exchanger models at all levels in series according to the process flow.The general working stage model of the steam turbine is built on the basis of the Flugel formula,and the integral model of the steam turbine is superimposed on each stage of the working stage.The model is integrated and verified by static state and dynamic process,which proves that the model has good simulation precision and meets the requirements of dynamic simulation.Secondly,according to the parameter fluctuation of the outlet of the gas turbine in operation,the dynamic simulation is carried out for the disturbance of the exhaust gas temperature and the exhaust gas flow,the differences and similarities of response trend and speed of conventional unit bottoming cycle are studied and the reasons are analyzed.The disturbance of exhaust gas temperature mainly affects the high-pressure superheated steam and reheated steam,and the influence range is larger.The disturbance of T4 with 5%can reduce the bottoming cycle power by 16.32%.The response speed of the unit is slower,and the time constant of the steam turbine power is about 400 seconds.The disturbance of exhaust gas flow affects the steam of each stage,and the influence range is relatively small,the disturbance of 10%reduces the bottoming cycle power by 9.49%,the response speed of the unit is faster,the time constant of the steam turbine power is about 60 seconds.Thirdly,the same disturbance of exhaust parameters is simulated for the bottoming cycle of partially recuperative units,and the results show that the dynamic response of partially recuperative units under the disturbance of exhaust parameters is basically consistent with that of conventional units When the ratio of exhaust gas participating in recuperation at turbine exit is disturbed,the parameters of high-pressure superheated steam and reheated steam exit appear obvious fluctuation,and the response speed is slow,the dynamic response of the bottoming cycle is similar to that of the exhaust gas temperature disturbance.Finally,the dynamic simulation of the load’s changing process of partially recuperative units is carried out,which makes the conventional units complete the load’s changing process with the same amplitude by using the constant turbine exhaust gas temperature(T4)operation strategy,the parameter and load’s response speed of the bottoming cycle of the two units during the load reduction process are analyzed to explore the applicability of the recuperative regulation strategy.The results show that the operation strategy of recuperative regulation causes all of the load variation of combined cycle borne by the bottoming cycle,the response was delayed by 1100 seconds,which is disadvantageous to the rapid response of the unit,and the load’s response speed of partially recuperative units is slower than that of the conventional unit in the load range of 100%to 95.6%.However,the response speed of the two units is basically the same in the rest of the load range.The strategy of partially recuperative regulation is suitable for long-term operation under partial load and less demand for fast load’s responsing capacity,it can replace the regulation mode of pure fuel quantity in the range of load lower than 51.4%.