Investigations on Dynamic Behaviour of Heat Recovery Steam Generators Carried Out With a Commercial Software Program

Abstract

Heat recovery steam generators (HRSGs) downstream of gas turbines are often used in combination with process steam applications. A HRSG trip is more severe in applications with highly important process steam than in applications producing only electricity. HRSGs in important process steam applications can be equipped with a supplementary and fresh air firing system having the capacity of replacing at least the total waste heat coming from the gas turbine. A fresh air firing system provides the capability of keeping the HRSG in operation without the gas turbine running. If the HRSG is required to stay in operation even after a gas turbine trip, a change over from waste heat firing to fresh air firing has to follow immediately. The gas turbine speed rundown after a trip occurs very rapidly, so the change over procedure has to be carried out within a few seconds to avoid a HRSG shut down. Enormous gradients of heat input and steam mass flow extracted from the HRSG occur in such cases. This will cause the HRSG parameters to deviate considerably from the steady state, thus conventional HRSG calculations cannot be used in such cases. The thermal inertia of the HRSG needs to be considered, which requires the use of special software programs. A commercial boiler software program with a dynamic calculation module for unsteady calculations was utilized and a comparison with data gathered from the operation of the HRSG was performed. The boiler performance parameters during change over procedures were investigated in detail using extensive measurements at an Austrian steel mill (VOESTALPINE). The parameters of this investigation were compared with calculation results gained from a commercial software program for validation purposes. This comparison will enable predictions to be made for future projects with sufficient accuracy, which will allow the risk to be reduced when offering guarantees in this regard.