Research On The Dynamic Characteristics Of Gas-steam Combined Cycle Heat Recovery Steam Generators

The problem of energy environment between China and the world is serious,and the proportion of renewable energy is increasing,which brings the problem of stable operation of power grid and flexible load regulation of power plants.Gas-steam combined cycle is an ideal peak-load regulation and power generation system with high efficiency and fast load regulation.The installed capacity of combined cycle power plants is increasing rapidly in China.How to improve the off-design efficiency of combined cycle system to meet the demand of peak load regulation and the study of its dynamic characteristics have been paid more and more attention.As one of the three major components of the gas-steam combined cycle system,the waste heat boiler has great thermal inertia relative to the gas engine and steam engine power components,and when the system load changes,the temperature and flow rate of exhaust gas of gas turbine and inlet gas of waste heat boiler also change greatly,and the steam which produces corresponding parameters has great hysteresis,which is one of the focuses of dynamic research on combined cycle.In this paper,the dynamic thermodynamic characteristics of the waste heat boiler are studied in two aspects.First,the start-up and operation data of the superheater and reheater of the waste heat boiler in a gas turbine power plant are analyzed,the changing trend of temperature and heat transfer during the start-up process of different heat exchangers is analyzed,and the simplified equations of transient heat transfer process for waste heat boiler are established by lumped parameter method,and their analytical solutions are obtained by solving equations,the physical meaning,Law and application of the analytical solution are analyzed.First of all,based on the operation data of a waste heat boiler during the start-up process,for superheater and reheater,based on the changes of inlet and outlet flue gas temperature,inlet and outlet steam temperature and flow rate of each heat exchange surface,based on the energy balance and the assumption that the heat storage capacity of the heat exchanger is proportional to the heat exchange capacity,the changing trend of flue gas and steam temperature and heat transfer with time in the starting process of Secondary Reheater,high-pressure secondary superheater,primary reheater and high-pressure primary reheater are calculated and analyzed.The results show that the heat transfer temperature difference formed by the inlet temperature is the fundamental driving force for heat transfer.In addition,the change of heat transfer is closely related to the change of steam flow rate,and the change of steam flow rate can significantly change the heat transfer coefficient,during the start-up phase,considering the thermal fatigue and thermodynamic properties of the metal,and meeting the demand of steam temperature,the control of the rising velocity of the average metal temperature of the heat exchanger should consider the control of the rising velocity of the flue gas and the rising steam flow.Then,an ideal model of the dynamic heat transfer process of the single-phase heat exchanger is established.The heat exchanger is simplified as a single-phase and single-tube heat exchanger,the physical model is simplified reasonably to ignore the effect of pressure change and temperature change on heat transfer coefficient.According to the heat transfer characteristics and energy conservation of single-phase heat exchanger,the heat balance equations of the cold fluid and the hot fluid of the heat exchanger,the heat transfer equations of the hot fluid to the metal,the heat transfer equations of the cold fluid to the metal and the heat absorption equations of the metal are listed,the physical meaning of analytic solution of key physical quantity is discussed.The constant of the analytical solution is the difference between the current temperature of the metal and the equilibrium temperature of the metal under the current conditions,which is balanced by an exponential change,the time is related to the total thermal resistance between the heat flow and the metal,the total thermal resistance between the metal and the cold fluid,and the heat capacity of the metal.Finally,based on the analytical solution and some basic heat transfer data of superheater and reheater,the analysis and prediction of dynamic heat transfer characteristics are carried out.The analysis found that,the equilibrium time of high pressure secondary superheater,secondary reheater,high pressure primary superheater and primary reheater is 2590s,2190s,2640s and 6520s respectively,assuming the difference between heat release of hot fluid and heat absorption of cold fluid is less than 1%The heat transfer characteristics of heat exchanger basically determine the dynamic characteristics of heat exchanger,the smaller the metal heat capacity,the larger the heat transfer coefficient,the faster the equilibrium time,when the heat transfer Coefficient is the same,the flat time is proportional to the metal heat capacity(the metal consumption of heat exchanger).The bigger the metal heat capacity is,the longer the equilibrium time is The Analytical Solution can also be used to predict the dynamic characteristics of the heat exchanger.Based on the inlet temperature and flow rate of flue gas and steam,the heat transfer coefficient can be corrected reasonably,the outlet temperature parameters of flue gas,steam and metal can be estimated accurately.In a word,this paper studies the superheater and reheater of hrsg from the point of view of changing characteristic and analytic solution of start-up data.