Analytical Solution and its Application for the Dynamic Characteristics of a Hrsg in Gas–Steam Combined Cycle

Abstract

A study from the perspective of analytical solution is performed in this paper to analyze the dynamic characteristics of a heat recovery steam generator (HRSG), and its application is discussed. The lumped parameter method is used to simplify the processes of a HRSG metal heat storage and temperature changing, and the differential equations based on the law of the conservation of mass and energy are presented for a HRSG dynamic heat transfer process. The analytical solution is then obtained and the physical meaning of the key parameters in the expressions are discussed. Based on the analytical solutions, the dynamic heat transfer processes for heating surfaces in a HRSG with typical starting conditions are analyzed. The results show that the larger heat transfer factor or the smaller heat capacity factor of HRSG metal (or the large of the ratio of the two factors), the faster of the heat transfer process to reach thermal equilibrium. Moreover, a dynamic forecasting model is developed based on the analytical solution. Particle swarm optimization (PSO) algorithm is applied as a parameter identification technique to adjust the model initial parameters, because some necessary initial parameters maybe missing for a real start-up process. Real-time on-site measurement data of a cold start-up of a HRSG is used for the model testing and comparisons. The results show that the model has good predictive capability and the average absolute error ranges from 0.5℃ to 2℃, which demonstrates a high accuracy. From the comparison with the thermal equilibrium model, it can be concluded that in the cold start-up of a HRSG, the heating surfaces always show the dynamic characteristic of a non-steady state for the first half of the time and a quasi-steady state for the rest time.

Keywords: Heat recovery steam generator, Analytical solutions, Dynamic characteristics, Differential equation