(190f) Dynamic Modeling and Control of a Natural Gas Combined Cycle (NGCC) Power Plant Integrated with CO2 Capture
A steady-state model of the NGCC plant with a triple-pressure heat recovery steam generator (HRSG) is established in Aspen Plus and then converted to a pressure-driven dynamic model in Aspen Plus Dynamics. A thermo-hydraulic model of the HRSG is developed and used in a two-stage optimization approach to maximize the compactness for the required heat duty within the operational constraints. Thus a detailed design of the HRSG that includes the number of rows and tubes, dimensions of the tubes arrangement of the tubes, fin geometry and dimensions in each section of the HRSG is obtained. A stage-by-stage model of the steam turbine is developed to estimate the performance of the triple-pressure steam turbine with multiple steam addition and extraction points under nominal and off-design operations.
The steady-state model of the CO2 capture process is developed using rate-based absorber and stripper columns. This model is used to obtain an optimal design of the CO2 capture process. Since the rate-based model is not supported in Aspen Plus Dynamics, an equilibrium-based method is used in the dynamic simulation. A model for the CO2 Murphree efficiency as a function of various operational variables is obtained by using results from the rate-based Aspen Plus model. This efficiency model is then implemented in Aspen Plus Dynamics.
For the NGCC power plant being modeled, five parallel CO2 capture trains are needed in order to achieve 90% CO2 capture. In the integrated NGCC-CO2 capture plant, a significant amount of steam is extracted from the low pressure turbine (LP)-intermediate pressure (IP) turbine crossover for use in the reboiler of the regenerator of the CO2 capture system. Not only is this system strongly coupled, but there are significant differences in the time scales of the gas turbine (GT), steam turbine (ST), HRSG, and the CO2 capture unit. Dynamics and control performance of this highly coupled system are studied under various operating scenarios such as variable and fixed CO2 capture with varying load ramp rates.
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