(211e) Design and Advanced Optimization of a Natural Gas Combined Cycle Power Plant with CO2 Capture

As the penetration of intermittent renewable energy sources into the electrical grid keeps increasing, traditional fossil-fired power plants would be expected to cycle their load much more frequently than in the past. However, current fossil-fueled generation plants are not optimally designed for cycling. With this motivation, a rigorous model of a natural gas combined cycle (NGCC) plant with CO₂ capture is developed. The plant is then designed for optimal load-following characteristics.

First, a steady-state model of the NGCC plant with a triple-pressure reheat cycle steam generator is developed in Aspen Plus. A stage-by-stage model of the steam turbine is developed in Fortran and implemented as a user model in Aspen Plus for estimating the performance of the triple-pressure steam turbine with multiple steam addition and extraction points including the large LP steam extraction for the CO₂ capture process. A mixture of piperazine-promoted methyldiethanolamine is used as the solvent for CO₂ capture. A rate-based model for this solvent system is developed and validated using available experimental data. While traditional optimization approaches in this area have looked mainly into maximizing efficiency, this work is focused on optimizing a cost function that considers both the operating costs as well as the capital costs required for improved load-following characteristics. As an example, even though increased steam temperature and pressure increases the overall efficiency, it increases steam-side oxidation of the boiler tubes especially the high pressure superheater tubes resulting in lower thermal conductivity, which leads to higher skin temperatures. This effect not only leads to higher corrosion for the tubes but also results in lower strength of the tube materials thereby necessitating a design with greater tube thickness. The oxide scale also has a different thermal expansion coefficient compared to the base material leading to stress buildup that can result in spalling especially during startup and shutdown of the plant. Furthermore, creep and fatigue characteristics of the plant under design conditions, during operations under elevated conditions, and under thermal cycling should be optimally designed for load-following. An evolutionary optimization technique is setup by bridging optimization software with Aspen Plus to maximize the net present value. Pareto curves in the presence and absence of the CO₂ capture process are generated showing the trade-offs between the operating and capital costs.