(190f) Dynamic Modeling and Control of a Natural Gas Combined Cycle (NGCC) Power Plant Integrated with CO2 Capture

The high availability of shale gas in the U.S. has provided strong incentive for switching from coal to natural gas. However, for environmentally sustainable operation of natural gas fired power plants, further reduction in CO₂ emission must be achieved. CO₂ capture and storage (CCS) technology can be instrumental in achieving this goal. Since the natural gas combined cycle (NGCC) plants are expected to follow the load due to the high penetration of the renewables in the grid, transient performance of these plants must be optimized so that maximum efficiency can be achieved without violating operational or environmental constraints. However, studies on the transient performance of NGCC with CCS is limited. With this motivation, a dynamic model of a NGCC plant integrated with a methyl diethanolamine (MDEA)-based post-combustion carbon capture (PCC) process is developed and used for control studies under various load-following scenarios.

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.