A Study of Single Shaft Gas Turbine based Cogeneration Plant Mode of Operation for Performance Evaluation

A gas turbine generally runs at off-design conditions due to change of load or ambient conditions.
Therefore, when evaluating the overall performance of the gas turbine, it is important to account for all
operating conditions that can be encountered. The cogeneration plant considered for this study consists of
a single shaft gas turbine and a heat recovery steam generator (HRSG). The gas turbine in turn consists
of an axial compressor that has an inlet guide vane and variable stator vanes (VSVs). At off-design the
VSVs are re-staggered so that it would improve the cogeneration performance. Furthermore, two modes
of operation were identified. The first mode is for part load less than 50% running to meet the power
demand by regulating the fuel mass flow while the VSVs are fully opened. The second mode is for part
load greater than 50% running to meet both the power demand and to maintain the exhaust gas
temperature set value so that it would be used to produce steam. This is done by regulating both the fuel
mass flow and compressor VSVs position. Therefore, this work aims at the study of the mode of operation
of a single shaft gas turbine based cogeneration plant for performance evaluation. To accommodate the
change of the compressor performance map with each VSVs opening, correction coefficients are
introduced. The effect of variation of power (relative load) and ambient temperature on cogeneration
parameters such as fuel consumption, temperatures, pressures, VSVs openings and efficiencies were
studied. This model serves as the basis of an advanced strategy to evaluate a stationary gas turbine’s
overall operation. Comparisons between the field data and simulation results demonstrate a good degree
of agreement. To support the huge amount of calculations required for off-design analysis, a computer
program was developed in MatLAB environment.