Simple Heat Recovery Steam Generator

The following figure shows the interconnection of a gas turbine with a waste heat steam generator. An orange frame surrounds two heat exchangers, which transfer heat energy from the exhaust gas of the gas turbine to the steam generator: The Heat Transfer component template Heat_Output_Air forms the Heat recovery boiler HT, and the Heat Transfer component template Heat_Input_Steam forms the Steam_Generator.

Gas Turbine with Waste Heat Steam Generator

The heat exchanger is parameterized in the two associated components. All heat exchangers are limited by entering the Maximum Heat Transfer. If different upper barriers are specified in the two heat exchanger halves (here Heat recovery boiler HT and Steam_Generator), at most the lower one becomes effective.

In heat exchangers with the heat transfer medium Air, the maximum transferable energy quantity is additionally limited by specifying the Minimum Outlet Temperature \( T_{air\ out,\ min}\) (or the Maximum Outlet Temperature in the case of the Heat_Input_Air component).

The minimum specific enthalpy \( h_{air\ out,\ min}\) is calculated from the given Minimum Outlet Temperature of the heat exchanger (here: of the Heat recovery boiler HT) (see equation 1). From the minimum specific enthalpy \( h_{air\ out,\ min}\) results the thermal enthalpy flow \( \dot{H}_{air\ out,\ min}\) (see equation 2), which limits the actually escaping enthalpy flow \( \dot{H}_{air\ out}\) (see equation 3). This form of restriction depends on the inlet condition (here: the operation of the gas turbine and its outlet temperature). This is the difference between this method for limiting \( \dot{H}_{air\ out}\) and the specification of Maximum Heat Transfer.

\( \begin{equation} \begin{aligned} h_{air\ out,\ min} &=c_{p,air}\cdot (T_{air\ out,\ min}-T_{ref})&& \qquad(1)\\[.3cm] \dot{H}_{air\ out,\ min}&=h_{air\ out,\ min}\cdot \dot{m}_{air\ out}&& \qquad(2)\\[.3cm] \dot{H}_{air\ out} &\geq \dot{H}_{air\ out,\ min}&& \qquad(3)\\[.3cm] \dot{Q}_{air\ out} &\leq \dot{H}_{air\ in}-\dot{H}_{air\ out,\ min}&& \qquad(4)\end{aligned}\end{equation}\\\)

To parameterize the heat exchangers, the minimum (or maximum) outlet temperatures must be specified. These are usually not technical data, but are system-specific variables. They must first be calculated or estimated externally for the given conditions.

Heat Recovery Steam Generator with Low Temperature Heat Extraction

The modeling of a waste heat steam generator with low temperature heat extraction is also carried out by interconnecting the individual components. The following figure shows a possibility of interconnection. Here, the hot exhaust gas emerging from the gas turbine is used to generate steam. The remaining residual heat of the exhaust gas is used to cover a heat demand at a lower temperature level. For this purpose, the scheme is extended by the component templates Heat_Output_Air for the Heat recovery boiler LT and Heat_Input_Water for the LT Heat exchanger.

Heat Recovery Steam Generator with Low Temperature Heat Extraction and Auxiliary Fire

In order to achieve greater independence from gas turbine operation, a waste heat steam generator can be operated with an auxiliary firing system. The additional firing raises the temperature level of the exhaust gas stream used and increases the steam generator output.

The following figure shows the scheme with the interconnected components. The Auxiliary fire is modeled in Top-Energy with the component templates Hot_Water_Boiler (in the scheme shown: Auxiliary firing), Heat_Output_Water and Heat_Input_Air (in the scheme: Auxiliary fire). The heat transfer here is loss-free. There is no energetic difference to the normally practiced direct firing of the exhaust gas without the heat transfer medium water.

It is not possible to consider the effective increase in heat transfer due to the pinch shift. The heat transfer quantity is always calculated from the potential difference between the incoming and outgoing medium. Therefore, the increase in efficiency of auxiliary firing must be represented indirectly by parameterization of the Auxiliary fire component.

Depending on data situation and operating mode, certain input data must be calculated when parameterizing a heat recovery steam generator. A proposal for parameterization in case that the steam generation capacity (the steam mass flow produced) is known without the use of auxiliary firing, is presented below.

The maximum output of the heat exchangers Heat recovery boiler HT and LT differs in the operating modes with and without auxiliary firing. Therefore, the performance of the individual heat exchanger components cannot be limited by specifying a generally valid Maximum Heat Transfer. The outlet temperature must be selected or calculated according to the amount of energy that can be cooled down.

The outlet temperature of the high-temperature part of the waste heat boiler (Heat recovery boiler HT) is determined as in the following equations. GT stands for the exhaust gas flow from the gas turbine, which is at the same time the inlet air flow into the heat input component Auxiliary fire (on top) of the balance scope described by the equations.

\( \begin{equation} \begin{aligned} \dot{H}_{GT}+0 [kW]&= \dot{Q}_{steam}+\dot{H}_{HT,\ out}&& \qquad(5) \\[.3cm]\dot{m}_{GT}\cdot c_{p,\ gas}\cdot({T_{GT}-T_{ref}})&= \dot{Q}_{steam}+\dot{m}_{GT}\cdot c_{p,\ gas}\cdot({T_{HT,\ out}-T_{ref}})&& \qquad(6)\\[.3cm] with\ c_{p,\ gas} &= const.:\\[.3cm] \dot{m}_{GT}\cdot c_{p,\ gas}\cdot({T_{GT}-T_{HT,\ out}})&= \dot{Q}_{steam}&& \qquad(7)\\[.3cm] \dot{m}_{GT}\cdot c_{p,\ gas}\cdot({T_{GT}-T_{HT,\ out}})&= \dot{m}_{steam,\ without\ auxiliary}\cdot \Delta h_{steam}&& \qquad(8)\\[.3cm] T_{HT,\ out}&= T_{GT}- \frac{\dot{m}_{steam,\ without\ auxiliary}\cdot \Delta h_{steam}}{\dot{m}_{GT} \cdot c_{p,\ gas}} && \qquad(9) \end{aligned}\end{equation}\\\)

The equations refer to the nominal operating condition. The energy balance is formed around the upper part of the heat generators (see following figure).

The outlet temperature of the Heat recovery boiler LT can be calculated accordingly. Since this is an actual outlet temperature from the technical component, the values may also be known in advance. In the Auxiliary fire component, the auxiliary firing output in rated operation (Nominal Thermal Capacity) and the losses in the form of Nominal Thermal Efficiency are specified.