(690g) Multi-Objective Dynamic Optimization of Natural Gas Combined Cycle (NGCC) Plant Load-Following Operation with Equipment Health Constraints

With increasing penetration of renewable energy sources into the electric grid, conventional natural gas combined cycle (NGCC) plants are being forced to load follow more frequently and rapidly. These load-following operations are leading to decreased plant efficiency, increased equipment wear and tear, and higher maintenance cost of NGCC plants [1,2]. Optimal dynamic operation of the NGCC plants with due consideration of equipment health constraints can maximize the equipment life while maximizing plant efficiency.

In this work, a novel multi-objective dynamic optimization approach is developed for optimal load following operation of the NGCC plants. A plant-wide dynamic model of an NGCC plant is developed with detailed equipment level sub-models of gas turbine, heat recovery steam generator and steam turbine. A parallel superheater (SH)/reheater (RH) configuration with two-stage attemperation is considered for the main steam and reheat steam temperature control. The through-wall temperature transients of critical components (e.g., high-press drum, SH, RH) are calculated with consideration of detailed geometries and configurations. Spatial and temporal thermo-mechanical stress evolutions are calculated based on classic elasticity theory. In addition, stress concentrations caused by the discontinuities at the drum-downcomer junction and header-tube junction are considered.

While the thick-walled HP drum has been considered as the equipment health constraints of dynamic optimization in our previous work [1], SH and RH are also the vulnerable components in NGCC plant. Under load-following operation, the creep rupture failure of SH/RH tubes caused by overheating is a major cause of forced outages of power plants [3]. Therefore, it is crucial to maintain the final main/reheat steam temperature to avoid overheating and resulting creep rupture in the SH/RH. To control the steam temperature, the addition of significant water spray is required at the SH/RH attemperators. The water spray at attemperator leads to the possible two-phase flow at the inlet of SH/RH [2]. The two-phase flow can cause hammering and significant mechanical damage to the critical components. During load-following operation, the most stressed location can vary with time as the plant operation changes. Therefore, a model is developed that can capture the spatial variation of stress in SH/RH. In addition, the SH/RH inlet header stress is modeled as that is one of the thickest components in a boiler and therefore in general remain highly stressed under load-following operation.

The dynamic model of the NGCC plant with the health model included is used for dynamic optimization for maximizing plant efficiency under load-following operation. If the desired ramp rate is high, satisfying the equipment stress constraints can be infeasible without relaxing the ramp rate constraint. However, for a load-following power plant responding to the grid demand, following the desired ramp rate as closely as possible is desired and therefore ramp rate relaxation must be minimized. Therefore, a multi-objective dynamic optimization problem is considered for minimizing ramp rate relaxation and maximizing plant efficiency while satisfying equipment health constraints. A lexicographic approach is developed to solve this multi-objective optimization problem through prioritization of the objectives, where following the desired ramp rate as closely as possible is considered to be the most important objective. Optimal load-following operation under various operational scenarios and constraints are investigated. It was observed that either drum stress or the SH stress can be the limiting constraint. In addition, it was observed that there is strong tradeoff between the relaxation in ramp rate and the time-average thermal efficiency.

[1] Wang, Y., Bhattacharyya, D., & Turton, R. (2021). Multi-objective Dynamic Optimization for Optimal Load-Following of Natural Gas Combined Cycle Power Plants under Stress Constraints. Industrial & Engineering Chemistry Research, 60(39), 14251-14270.

[2] Wang, Y., Bhattacharyya, D., and Turton, R. (2019). Evaluation of Novel Configurations of Natural Gas Combined Cycle (NGCC) Power Plants for Load-Following Operation using Dynamic Modeling and Optimization. Energy and Fuels, 34(1), 1053-1070.

[3] Viswanathan, R., Paterson, S. R., Grunloh, H., & Gehl, S. (1994). Life assessment of superheater/reheater tubes in fossil boilers. Journal of Pressure Vessel Technology, 116(1): 1-16.