Dynamic Behaviour Analysis of a Single Pressure Heat Recovery Steam Generator during Cycling Operation

In the recent years the flexibility of fossil-fuel power plants has become of primary importance since the competitiveness of the electricity market imposes to such plants to be characterized by short run times and fast cycling, not only to be competitive but also to compensate for fluctuating renewable sources. Cyclic operations guarantee high profits in the short term but cause a reduction in the lifetime due to thermo-mechanical fatigue, creep and corrosion effects in the medium-long time. In this context, combined cycle power plants fed by natural gas are the most concerned in flexible operation problems. Two research groups have developed a procedure to estimate the devices residual life reduction, focusing in particular on the most stressed components, such as steam drums and superheaters. The procedure can be utilized for planning the power production and scheduling maintenance interventions. For estimating the life reduction, due to cycling, it is essential to foresee the trends of the main thermodynamic parameters (such as mass flow rates, temperatures and pressures) that describe the plant behavior. Therefore, the core of the procedure is the power plant dynamic model. At this purpose, in this paper, three different models of the same single pressure combined cycle power plant are presented. The models have been built using three different approaches and, at the present time, they are used by the two research groups to simulate the plant under real operating conditions. To analyze the plant behavior, different transient conditions are simulated. Despite the difference in how the models solve the problem and represent the geometry of the components, the obtained time profiles of the thermodynamic parameters are in good accordance as accurately described in the paper. At last, an evaluation of the residual life of the drum is presented as a function of pressure and temperature variations which cause thermal and mechanical stress, namely thermo-mechanical fatigue.