(679d) New Operating Strategy for a Combined Cycle Gas Turbine Power Plant

Gas turbines are widely used in the power industry due to their high thermal efficiency, super flexibility, short delivery time, and excellent regulation capacity. The high-temperature exhaust from a gas turbine is normally utilized to produce extra power in a steam cycle that comprises a heat recovery steam generator (HRSG) and several steam turbines. This serial coupling of a gas turbine followed by a steam cycle is called a combined cycle gas turbine (CCGT) power plant.

CCGT plants run under part-load conditions in most of their lifetime. Several factors are responsible for this part-load operation. First, the power demand is hardly steady and rarely equals the design capacity. Second, a power plant is required to maintain spinning reserves (surplus capacity) to meet unforeseen peaks in demands. Third, a power plant may often be overdesigned to buffer against demand uncertainties. Hence, the study of CCGT plants under part-load conditions is of practical importance. Under part-load conditions, the plant operation drifts away from its design conditions and thus the plant thermal efficiency decreases. This wastes non-renewable fossil fuels and increases CO₂ emissions. Therefore, a proper operating strategy is needed for the part-load operation of a CCGT plant.

The operating strategies for gas turbine and power plants were studied in the literature. Kim and Hwang¹ investigated the part-load operation of recuperative gas turbines. They studied several operating strategies such as fuel flow control (FFC), variable speed control (VSC), and inlet guide vane control (IGVC) for the single-shaft configuration, and FFC and variable area nozzle control (VANC) for the two-shaft configuration. They found VSC and VANC gave better performance. Kim² analyzed the part-load performance of a combined cycle with different gas turbine design parameters under FFC and IGVC. They showed that the combined cycle exhibited superior performance under both operating strategies when the gas turbine pressure ratio and temperature were higher. Haglind^3,4 investigated the effects of variable geometry on the part-load performance of gas turbines and combined cycles, and found the use of IGVs and VANs improved their performance. Jimenez-Espadafor Auilar et al.⁵ and Cheng et al.⁶ studied several operating strategies for a combined heat and power (CHP) plant, and showed IGVC held the best regulation capacity and energy saving potential. Variny and Mieka⁷ found that preheating the condensate and changing condensing pressure regulation strategy reduced the fuel consumption based on online monitoring data. Liu and Karimi⁸ obtained an optimal operating strategy using a simulation-based optimization method to maximize the plant thermal efficiency at any part-load. Although these studies investigated different operating strategies, new strategy is needed to improve the plant part-load performance.

In this work, a new operating strategy called EGR-IGVC is proposed to improve the plant part-load performance. The new strategy manipulates exhaust gas recycle (EGR) ratio and inlet guide vane (IGV) sequentially to control gas turbine inlet and exhaust temperatures. It allows the CCGT plant to set its own acceptable gas turbine temperature limits at any time and thus maximize the plant performance. The plant performance under EGR-IGVC is evaluated and compared with that under the conventional inlet guide vane control (IGVC). The results show that EGR-IGVC improves the plant part-load performance. Specifically, the plant efficiency is increased by as much as 1.2% (actual) while the CO₂ emissions are reduced by as much as 13.8 kg MW^-1 h^-1. Furthermore, EGR-IGVC enables the effective low-temperature heat utilization and alleviates the NO_x emissions from the power plant. Therefore, EGR-IGVC promises to a better strategy for the power plant part-load operation.

References

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