Gas turbine selection for feedwater repowering

The paper presents the concept of using hot exhaust gases from gas turbines with different power output to heat up feedwater in a supercritical power plant unit. The gas turbine is connected to the system, bypassing a high pressure regenerative heat exchanger. The benefits of this solution are discussed and the factors to be taken into account are listed. The criteria to be met by the gas turbine to ensure safe and optimal connection to the steam system are discussed. A reference unit model with 800 MW electric power (an existing super-critical power unit in Poland—Belchatow II) was created and presented in a previous paper by the same authors. This model was later supplemented with a gas turbine (three different models with different levels of power production are taken into consideration). The system with a gas turbine enjoys greater power and efficiency over the steam cycle alone. The power increase is due to the extra power generated by the gas turbine and the higher output of the steam system caused by increasing the steam flow through the turbine (closed extraction to the "bypassed" high-pressure heat exchanger). System power is changed linearly with the steam flow and reaches the nominal point 40..50% higher than without an added gas turbine (depending on gas turbine power and efficiency). The efficiency characteristics of the whole system are flatter, with higher values.