(371r) Dynamic Optimization of a Natural Gas Combined Cycle (NGCC) Power Plant for Load-Following Operation

Authors: 
Wang, Y., West Virginia University
Bhattacharyya, D., West Virginia University
Turton, R., West Virginia University
With the increasing penetration of intermittent renewables into the electric grid, conventional power plants are expected to follow their load much more frequently than in the past. Due to the higher efficiency, lower emissions, and lower capital cost compared to coal fired plant, natural gas combined cycle (NGCC) plants are playing a key role in balancing the fluctuating grid demand. Therefore, NGCC plants should be designed/operated to ensure that the operational constraints of the plant are not violated during low-load operation and/or during fast transients when the load is rapidly cycled.

At low loads, the exhaust temperature of the gas turbine (GT) increases as the GT efficiency decreases. Furthermore, the steam flow generated in the heat recovery steam generator (HRSG) also decreases at low load. With the higher GT exhaust temperature and lower steam flowrate, the temperature of the main/reheat steam can increase beyond the desired value since the final superheater (SH) and reheater (RH) are located at the front end of HRSG. In order to maintain the SH/RH steam temperature at the setpoint, the attemperator spray rate needs to be increased significantly leading to saturation (known as ‘spraying to saturation’) at the inlet of the SH/RH immediately following the attemperator. The free water can cause water hammer leading to considerable damage in the SH/RH tube banks since they are not designed for the two-phase operation. While this issue has been reported in the open literature as one of the leading causes of failure of the superheater/reheater tubes (Moelling et al., 2015; Sorge et al., 2017), to the best of our knowledge, there is no study in the literature that has quantitatively investigated this problem.

With this motivation, both hardware design and optimal operation using dynamic optimization are proposed to remedy the issue of ‘spraying to saturation’. A plant-wide dynamic model of a NGCC plant is first developed with detailed equipment level sub-models. A model of the GT for estimating its performance under off-design conditions, a thermo-hydraulic model for the HRSG, and a model of the steam turbine (ST) with moisture detection and correction capability have been developed. Five novel configurations are proposed to address two SH/RH arrangement structures (in series or in parallel) and three attemperation strategies (single-stage, two-stage or damper combined with spray). Dynamic optimizations are performed for each of these configurations for maximizing the plant efficiency while satisfying the operational constraints such as ‘spraying to saturation’ and state transition constraints under the coordinated control strategy. It was observed that while more than one configurations can avoid ‘spraying to saturation’ and maintain the steam temperature within its limits even during very fast load transients, their efficiency can greatly vary. The parallel SH/RH configuration using the damper control combined with the spray attemperation shows the highest efficiency.

References

Moelling, D., Jackson, P., Malloy, J. (2015) Protecting steam cycle components during low-load operation of combined cycle gas turbine plants. Power, 159 (3), 42-45.

Sorge, J., Taft, C., Boohaker, C., Seachman, S. (2017, June). HRSG Damage Reduction through Improved Controls – Phase 1 and 2 Findings. In 60th Annual ISA POWID Symposium, Cleveland, OH.