(181b) Optimal Load-Following Operation of Natural Gas Combined Cycle (NGCC) Power Plants with Quantified Uncertainty of Equipment Health

Due to the rapid penetration of renewables into the electrical grid, natural gas combined cycle (NGCC) plants are being forced to cycle their loads more frequently and aggressively. However, impacts of load-following operations on plant efficiency and equipment health are still poorly understood. This work focuses on the optimal operation of NGCC plants by quantifying the impact of load-following on plant efficiency and equipment health.

Under load-following operation, the high pressure (HP) drum and superheater (SH)/reheater (RH) headers that have thick walls suffer from fatigue damage due to the large thermal stress [1, 2]. The SH/RH also experience creep damage caused by high steam temperature excursion in addition to the fatigue damage [3]. At low-load operation of NGCC power plants, efficiency of the gas turbine (GT) decreases leading to an increase in the inlet temperature of the flue gas to the heat recovery steam generator (HRSG). At low load, steam flowrate through the SH/RH also decreases. These two combined effects can lead to an increase in the superheated/reheated steam temperature. To control the steam temperature, the addition of significant water spray may be required in the SH and/or RH attemperators for certain configurations of NGCC plants eventually leading to two-phase flow at the inlet of SH and/or RH [4]. The two-phase flow can cause considerable damage to the RH/SH tube banks. These adverse phenomena are interactive and can trade-off with each other. For example, a lower attemperators spray flowrate can cause creep damage in the exit section of the SH/RH section while a higher attemperator spray can lead to large thermal and mechanical stress in the SH/RH inlet header and/or the inlet sections of the SH/RH tube banks. An optimal load-following operation needs to take these tradeoffs into account while maximizing the plant efficiency.

In the existing literature on optimal operation of NGCC plants, the thermal gradient, instead of creep/fatigue, is considered as an operational constraint [1, 2]. However, the thermal gradient does not provide a true estimate of the resulting overall stress due to complex interaction between mechanical and thermal stress as the thermal stress can be compressive or tensile but the mechanical stress is always tensile and thus they can add to each other or partially negate each other. In addition, the stress amplitude rather than the stress magnitude affects the fatigue damage. One difficulty in developing models of creep/fatigue is the uncertainty in material properties such as Young’s modulus and thermal diffusivity [5]. Furthermore, uncertainties in the creep rupture data would affect the creep damage assessment [3]. Therefore, equipment failure probability should be computed with due consideration of uncertainties in material properties.

In this work, a dynamic model of an NGCC plant is developed. It consists mainly of the GT, HRSG and steam turbine (ST). A parallel SH/RH configuration with two-stage attemperation is considered for the main steam and reheat steam temperature control. For the equipment damage model, through-wall temperature transients of critical components 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. The creep damage is assessed on the basis of the material rupture data [3] and the fatigue evaluation is computed according to the UNI EN 12952 standards [6]. Load-following operation of the NGCC plant under various operational constraints is optimized while accounting for the probability of failure of various components of the HRSG.

[1] Kim, T. S., Lee, D. K., and Ro, S. T. (2000). Analysis of thermal stress evolution in the steam drum during start-up of a heat recovery steam generator. Applied Thermal Engineering, 20(11), 977-992.

[2] Rúa, J., and Nord, L. O. (2020). Optimal control of flexible natural gas combined cycles with stress monitoring: Linear vs nonlinear model predictive control. Applied Energy, 265, 114820.

[3] Bendick, W., Cipolla, L., Gabrel, J., and Hald, J. (2010). New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91). International Journal of Pressure Vessels and Piping, 87(6), 304-309.

[4] 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.

[5] Harris, D. O., Wells, C. H., Grunloh, H. J., Ryder, R. H., Bloom, J. M., Schultz, C. C., and Viswanathan, R. (1993). Bless-boiler life evaluation and simulation system: A computer code for reliability analysis of headers and piping. ASME-PUBLICATIONS-PVP, 251, 17-17.

[6] UNI – Ente Nazionale di Normazione. UNI EN 12952-5:2011. Water-tube Boilers Standards; 2011.