Simulation of Optimizing the Partial Load Performance of a Gas Turbine Combined Cycle Using Exhaust Heat Recuperation and Inlet Bleed Heating

Recently, the penetration of renewable sources, such as wind and solar power, has increased rapidly in power grids worldwide. On the other hand, power generation from these renewable energies is unstable because they generate power according to the time-varying strength of the renewable energy source, which is difficult to predict. The gas turbine combined cycle (GTCC) is considered the most reliable power source that can back up the electric grid rapidly in response to the fluctuating power generation of renewable sources. Accordingly, many GTCCs are needed to operate at a load-following mode. Hence, their partial load operation rates are high. Consequently, gas turbine manufacturers have attempted to improve the operational flexibility of the GTCCs. Operational flexibility includes fast start-up/shut down time, high ramp rate, high partial load performance, and extending the operating range. This paper proposes a method to enhance the partial load performance and extend the operating range of GTCC. Exhaust heat recuperation and inlet bleed heating (IBH) were adopted, and a cycle simulation was conducted to confirm that the research goal could be achieved. A recuperator was installed between the compressor and combustor of the gas turbine, and the degree of heat recuperation was modulated during partial load operation to enhance the cycle efficiency compared to the conventional GTCC plant.

In contrast to the conventional GTCC plant, the recuperation ratio was modulated before control of the variable inlet guide vane (VIGV) began. This means that the recuperation control covers the high partial load regime. The gas turbine power remained almost constant in this regime because the inlet flow rate and turbine inlet temperature were kept constant. In contrast, the power of the bottoming cycle decreased with increasing recuperation ratio due to the decrease in exhaust gas energy. After the recuperation ratio reached a limit, the load control was the same, as in conventional plants: VIGV control followed by fuel only control. The purpose of using IBH was to reduce CO emissions in the low load regime. Some of the compressor discharge air was recirculated to the compressor inlet, and the combustion temperature was maintained at a high level. The simulation showed that both the IBH and recuperation are effective in extending the operating range. The predicted reduction in the turndown ratio was approximately 10%p. The partial load efficiency improvement by the recuperation was sensible. The efficiency remained higher than the full load efficiency over a wide partial load range. The efficiency of the recuperated GTCC was 4.1%p higher at 50% power than that of the conventional GTCC. Applying IBH to recuperated GTCC can enhance the efficiency and operation range slightly. Overall, the heat recuperation possibly assisted by the IBH is the optimal new partial load control method proposed in this study.