The goal of this test case is to find the gas turbine necessary to produce 58 MW of total net power for the conversion of a steam turbine to a combined gas-steam cycle while providing the highest level of cycle thermal efficiency.
The exhaust gases from the gas turbine are used to heat up steam through three HRSGs (Heat Recovery Steam Generators) in series. The steam is then used in the studied steam turbine which is comprised of two “cylinders” in series.
- The first heat exchanger gives the steam conditions for the turbine inlet.
- The second HRSG is used between the two cylinders, as an induction, to increase the mass flow going through the last cylinder and also to superheat the fluid to ensure appropriate fluid quality before the condenser, therefore maximizing turbine reliability.
- Finally, the third heat exchanger acts as an intermediate before the first recuperator. This way the steam entering it is already preheated and the steam temperature coming out of the second boiler can be higher, therefore providing extra enthalpy to the flow after the mixing.
The gas turbines chosen for this test case were selected based on their heat rate, exhaust temperature and mass flow and produced power among the library elements embedded inside AxCYCLE™. The results of this test case are presented in the table below.
From the results presented one can see that the cycles with the GE 6581B, the Siemens SGT-900 Westinghouse 251 B10A and B12 as well as the Alstom GT8 are the only ones that meet the required net power production of 58 MW. The cycles featuring the B12 and the GT8 produce significant extra power due to the larger capacity and this implies that the gas turbine price and cost of operation will be increased beyond necessary. We are then left with the other two cycles, the one with the GE 6581B which provides the highest combined cycle thermal efficiency due to its high exhaust gas temperature and mass flow rate and the one featuring the B10A which falls only shortly behind.
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