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CS-031: Combined Cycle Power Plant

This case study models a Combined Cycle power plant of a High Temperature Gas-Cooled Reactor using Flownex, focusing on the Power Conversion Unit. It predicts steady-state performance and transient behavior as mass flow through the process heat exchanger increases from 0 to 70 kg/s. The simulation demonstrates stable operation with appropriate controls and Flownex’s capability to handle multiple fluid types in a complex network.

 

CHALLENGE

This case study demonstrates the modeling of a Combined Cycle (CC) power plant of a High Temperature Gas-Cooled Reactor (HTGR).

The objective of the example is twofold. Firstly, it is to predict the steady-state performance of the plant, and secondly, it is to investigate the behavior of the plant in a situation where the mass flow through the process heat exchanger changes from 0 to 70 kg/s during a 35 second period.

 

BENEFITS

  • Multiple fluid networks in a single simulation, including pure gasses, gas mixtures, and two-phase fluid.
  • Validate and implement control system philosophies.
  • Interaction between various networks, i.e., Heat Exchangers.

 

SOLUTION

Flownex was used to simulate a complex combined cycle power plant that produces both electricity and process heat. It was shown that with the appropriate control system, the plant will operate stably as the mass flow through the process heat exchanger is increased from 0 to 70 kg/s.

This case study also demonstrated Flownex’s ability to handle a network with different types of fluids i.e. pure gasses, gas mixtures, and two-phase fluids.

 

SYSTEM DESCRIPTION

The layout of the system is shown in Figure 1.

 

Figure 1: Layout of a combined cycle power plant.

 

The plant consists of three closed loops that are connected through two heat exchangers. The first loop is a helium loop that consists of a blower, a High Temperature Gas-Cooled Reactor (HTGR) and an intermediate heat exchanger (IHX).

The second is a Brayton cycle that consists of a compressor, a turbine, an IHX and a steam generator. The working fluid for the Brayton cycle is a mixture of 20 percent helium and 80 percent nitrogen.  In parallel over the IHX is another heat exchanger called the process heat exchanger that can supply heat to e.g. a sulphur iodine hydrogen production plant.

The third is a bottoming Rankine cycle that consists of the following components: a steam generator, three steam turbines (high pressure, intermediate pressure and low pressure), condenser and two condensate pumps.

 

OBJECTIVE OF SIMULATION

The objective of the example is twofold. Firstly, it is to predict the steady-state performance of the plant and secondly, it is to investigate the behavior of the plant in a situation where the mass flow through the process heat exchanger changes from 0 to 70 kg/s during a 35 second period.

 

FLOWNEX MODEL

The reactor is modeled as a pipe with a specified constant exit temperature of 900 ºC. The reason why the reactor is not modeled in detail is that the focus of the simulation, as mentioned earlier, is not on the reactor but on the CC PCU.

Three controllers are used to control the speed of the helium blower and the two condensate pumps that form part of the Rankine cycle. The blower speed controller senses the IHX exit temperature and controls the speed to keep this temperature constant. The controller on the high pressure condensate pump attempts to keep the boiler exit temperature constant, while the controller on the low pressure condensate pump attempts to keep the mass source at node 1 zero.

 

DESCRIPTION OF SIMULATION

Firstly, a steady-state simulation was done, and the results of this simulation were then used as initial conditions for a transient simulation. With the transient simulation, the mass flow through the IHX was ramped up from 0 to 70 kg/s over a 35 second period.

 

Figure 2: Flownex model of the Combined Cycle High Temperature Reactor Gas-Cooled Power Plant.

 

RESULTS

Figure 3 shows the variation in mass flow through the IHX, the helium loop, and in the Rankine cycle with time.

Interesting to note is that the mass flow in the helium loop increases as the IHX mass flow increases. The mass flow in the steam cycle, however, stays constant.

 

Figure 3: Variation in mass flow with time. 

 

Figure 4 shows the variation in temperature at a few positions in the plant with time. The temperature stays mostly constant except at the exit of the IHX where the temperature increases drastically as the mass flow through the IHX increases. This is as expected.

 

Figure 4: Variation in temperature at a few positions in the plant with time.

 

CONCLUSION

Flownex was used to simulate a complex combined cycle power plant that produces both electricity and process heat. It was shown that with the appropriate control system, the plant will operate stably as the mass flow through the process heat exchanger is increased from 0 to 70 kg/s.

This case study also demonstrated Flownex’s ability to handle a network with different types of fluids, i.e., pure gasses, gas mixtures, and two-phase fluids.