Performance analysis of partially recuperative gas turbine combined cycle under off-design conditions

A partially recuperative gas turbine combined cycle (GTCC(Pr)) is proposed in this paper to improve the off-design performance of power plants. The performance coupling with a new operation strategy is also analyzed. In the proposed GTCC(Pr), the gas flow at the outlet of the turbine is divided into two streams, and only a part of exhaust gas from the turbine is used to preheat the outlet air of the compressor. Exhaust gas streams from the recuperator are mixed with flue from the second reheater at the inlet of the first reheater to achieve similar temperature level. Specifically, the fraction of exhaust gas that preheats air can be changed to adjust the load of the combined cycle. The PG9351FA gas turbine unit is selected as the reference unit for comparison. Then, the design/off-design characteristics of the two cycles are compared and analyzed with the same turbine initial temperature. Results indicate that the partially recuperative GTCC(Pr) running with the new operation has better off-design performances than the reference unit with approximately 1.7% maximum difference of efficiency. At the design point, the exhaust gas used for partial recuperation only accounts for 10% of the total mass, and the effect of recuperation is not evident. Therefore, the two cycles possess nearly similar output and efficiency. The efficiency of partially recuperative GTCC(Pr) is nearly unchanged with the increasing recuperative mass fraction, which changes its specific power to adjust the load. At low loads, with decreasing turbine exhaust gas temperature, the efficiency improvement also decreases due to the partial recuperation. A comprehensive comparison with the reference unit revealed that GTCC(Pr) with the proposed "recuperation adjustment" method can maintain a higher off-design efficiency and slightly lower design power output compared with the conventional cycle. Specific power adjustment of recuperation is one of the efficient methods for load adjustment, and GTCC(Pr) is one of the best options.