hrsg_process_steam.rst

Heat Recovery Steam Generator for Process Steam Supply

We provide the full script presented in this tutorial here: :download:`stepwise.py </../tutorial/advanced/hrsg.py>`

Figure: Topology of the heat recovery steam generator

Figure: Topology of the heat recovery steam generator

Overview

In this tutorial we will build a combined heat and power plant utilizing flue gas (e.g. from a gas turbine) to generate process steam and electricity. The figure above shows the structure of the system and the 4 steps we take to build the system. The challenge here lies in the complexity of the system. With a well-structured approach the problem can be tackled.

It is a simplified version of an actual plant with some unique features:

• The economizer bypass (connection 28) actually represents a performance downgrade for the boiler. The technical reason is to allow it to produce less than 30 % of the nominal steam mass flow. With this measure the CHP plant can produce almost any steam flow between 40 t/h and 210 t/h, which is a unusual wide range.
• The real economizer has eight stages. To simplify, we only model there here.
• Interestingly, the boiler is operated with what could be called a "negative approach" (the temperature coming out of the economizer was higher than that of the HP drum). This issue is managed by increasing the economizer pressure. Although this solution is inefficient from an energy standpoint, it was chosen for operational safety, likely to reduce the risk of economizer steaming.
• The sweetwater condenser is a solution for HP steam desuperheating.

The table below indicate the most important boundary conditions.

Location	Parameter	Value	Unit
process steam	m	160	t/h
	Td_bp	10	°C
	p	13	bar
live steam	T	480	°C
	p	88.5	bar
gas turbine exhaust	T	720	°C

And the most important component parameters:

Component	Parameter	Value	Unit
turbines	eta_s	90	%
pumps	eta_s	70	%
heat exchangers	pr	100	%
feed water preheater	pr	90	%

Steam supply and turbine section

Figure: First part of the system

Figure: First part of the system

In the first part we will model the steam supply part and the turbine. We start by importing the Network class and creating an instance of it.

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Components and Connections

For the components, we need to import the respective classes as shown in this part of the flowsheet and create the respective instances.

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The same applies for the connections. The extraction steam turbine is connected to the live steam source. At a later stage, this connection will be replaced by one that directly links the high pressure superheater 2. Similarly, the connection 15 connects to a sink, which will be replaced by the low pressure feed pump later.

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Parametrization & solving

For the live steam at the turbine inlet we have to assume a steam mass flow. In the completed model it will be governed from the flue gas mass flow. On top of that, we need temperature and pressure information. For the steam supply a pressure of 13 bar and 10 °C of superheating is required. There also is a specific mass flow to be provided, which however cannot be directly as this specification would cause convergence issues. Instead, massflow 3 is fixed initially. The outlet of the condenser is saturated liquid at a temperature of 35 °C. The condensate pump pumps the pressure back to ambient pressure. It is assumed that 75 % of the process steam leaving the plant returns in the form of condensate. The remaining mass flow is replaced by the makeup water.

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For the components we can specify the isentropic efficiencies of the turbines and the pumps as well as the pressure ratio of the condensing turbine inlet valve as well as the condenser pressure loss. Then we solve the system and with the first solution, the extraction mass flow can be unset and instead, the actual process steam mass flow is specified.

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Low pressure preheating and high pressure economizer

Figure: Second part of the system

Figure: Second part of the system

Completing the Rankine cycle

Figure: Third part of the system

Figure: Third part of the system

Adding the exhaust gas

Figure: Complete system

Figure: Complete system