(737c) Dynamic Performance and Equipment Health Analysis of a Natural Gas Combined Cycle (NGCC) Power Plant for Load-Following Operation

With the rapid deployment of intermittent renewables, such as wind and solar energy, traditional fossil-fueled power plants are being pushed to follow their load much more frequently. Natural gas combined cycle (NGCC) plants are playing a key role in fulfilling the fluctuating grid demand due to their high operational flexibility and rapid load-following capability.

Even though cycling operation could improve the competitiveness of a power plant in the market, it leads to increased equipment wear-and-tear due to thermo-mechanical fatigue, creep and corrosion. During the load-following operation, the high pressure (HP) superheater and intermediate pressure (IP) reheater tube banks, that have the highest operating temperature, are subjected to both fatigue and creep damage, while the HP steam drum and superheater/reheater collectors mainly suffer from fatigue damage due to their thick walls. In addition, the discontinuities at the drum-downcomer junction and collector-tube junction cause adverse stress profiles during load-following. While creep-fatigue damage limitations for cycling operations have been reported in the open literature ^[1,2], there are few studies that give a comprehensive investigation that consider tube banks, collectors and drum damages together with a detailed plant-wide model for load-following operation.

In this work, a dynamic model of a NGCC plant with detailed equipment level sub-models was developed for both creep-fatigue damage identification and load-following strategy development. A model of the gas turbine (GT) was developed to estimate its performance under off-design conditions. In addition, a thermo-hydraulic model for the heat recovery steam generator (HRSG), and a model for the steam turbine (ST) with moisture detection and model adaptation capability were developed. Furthermore, the main steam and reheat steam temperature control with spray attemperation were considered.

In the current study, the temperature distribution in each component is calculated by a detailed dynamic model that takes into account the configurational and design details of the NGCC. The model computes the through-wall temperature transients during load-following. Spatial and temporal thermo-mechanical stress evolutions are calculated based on classic elasticity theory ^[3]. The creep assessment is calculated on the basis of the R5 British procedure and the fatigue evaluation is computed according to the EN 13345 and UNI EN 12952^[4-5] standards. The model is used to investigate the effect of various load-following scenarios. Several scenarios that can cause premature high-temperature failures of boiler components are identified. A number of load-following strategies are proposed for maintaining acceptable component life.

[1] Fontaine, P., and J. F. Golopin. "HRSG Optimization for cycling duty based on Euro Norm EN 12952-3." ETD Conference on Cyclic Operation of Power Plant. Vol. 5. No. 1. 2007.

[2] Benato, A., et al. "LTE: A procedure to predict power plants dynamic behaviour and components lifetime reduction during transient operation." Applied energy 162 (2016): 880-891.

[3] Hetnarski, Richard B., M. Reza Eslami, and G. M. L. Gladwell. Thermal stresses: advanced theory and applications. Vol. 41. New York: Springer, 2009.

[4] British Energy. R5 Assessment procedure for the high temperature response of structures, Issue 3. British Energy; Gloucester, UK; 2003.

[5] European Committee for Standardization. EN 13345 part 3 unfired pressure vessels, clause 17; simplified assessment of fatigue life, and clause 18; detailed assessment of fatigue life; 2002.

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