Study of Natural Circulation Heat Recovery Steam Generator Transient Behavior Using One Dimensional Model for Evaporator Loop

Heat Recovery Steam Generators (HRSGs) as a vital part of cogeneration plants play a fundamental role in energy recovery processes. Recovering of combustion products heat from a gas turbine is done by the HRSGs. In energy systems, sudden changes due to upstream and downstream cycles, affect important parameters of the HRSG, such as wall temperature of risers and steam drum pressure as well as its performance. According to a necessity of accurate analysis of HRSG's parameters variation rate to take a decision on HRSG processes such as start-up and shut-down, the present study aims to investigate the transient behavior of HRSG using dynamic simulation. In this respect, a one-dimensional (nodal) model of evaporator natural circulation loop along with HRSG’s drum and heat transfer models are considered for simulation. Transient study scenarios include changes in the input heat to tube banks due to change in gas turbine load (upstream cycle), feedwater flow rate and steam demand of downstream cycle. A computer code has been developed to solve governing equations of a one-dimensional model and to demonstrate the response of HRSG’s key parameters to different scenarios. Dynamic simulation results showed that a 5% increase in heat input to risers leads to an increase of 15% of the drum pressure as well as an increase of about 10 degrees of the tubes wall temperature. In addition, an increase of 20% in the heat input to HRSG due to the change in gas turbine load would increase wall temperature of tubes by 35 degrees.