Aeroderivative gas turbine back-up capability with compressed air injection

Abstract

The transition to more renewable energy sources of power generation is associated with grid instability and the need for backup power, due to their intermittency. This provides an opportunity for gas turbine engines, especially the aeroderivative (AD) types that generally have higher ramp rates than heavy-duty engines. Nonetheless, higher ramp rates are still necessary to meet more stringent grid requirements, with increased renewables subscription. The study examines ramp rate improvements and performance enhancement through compressed air injection at the back of the high-pressure compressor (HPC). Two configurations of AD engines are considered in the investigation. In-house gas turbine performance simulation software has been used to simulate the steady-state and transient operations for design and off-design performance. Compressed air injection in the study is facilitated by an assumed compressed air storage or an external compressor. The steady-state analysis for power augmentation shows that for the two-spool engine with fixed speed low-pressure compressor (LPC), a 16% increase in power is obtained with 8% of flow injection. The other engine that is intercooled and consists of a variable speed LPC with power turbine shows a 21% increase in power for the same injection amount. Above 8% injection, the HPC of both engines tends towards an adverse rise in pressure ratio. However, up to 15% of flow injection is allowed before the surge point. It is seen generally that the operating point of the LPC moves away from surge, while the opposite is the case for the HPC. For transient simulations focused on ramp rates, the better improvements are shown for the intercooled engine that runs at variable speed. This is a ramp rate improvement of 100% with air injection, while that of the other engine increases by 85%.

Aerodynamic limits air injection for heavy-duty gas turbine: Compressor aerodynamic limits for power augmentation and ramp-up capabilities

“…The engine model of a heavy-duty gas turbine was developed with TURBOMATCH software, an inhouse Cranfield University gas turbine performance simulation code. It has been proven in a variety of cases, including jet engines [13,14] and other stationary gas turbines configurations [5,15]. The tool uses the Newton-Raphson method in solving the non-linearity problem of various rotating components (compressor and turbine) matching.…”

“…It has been proven in a variety of cases, including jet engines 13,14 and other stationary gas turbines configurations. 5,15 The tool uses the Newton-Raphson method in solving the non-linearity problem of various rotating components (compressor and turbine) matching. The turbomachinery components are defined by generalised/standard component maps that are embedded in the software.…”

“…In the analytical work by Quintero et al [2], air injection was deployed at lower density conditions, to restore a GT power through compressed air injection. The works of Abudu et al [3,5] shows an analytical engine-level analysis of air injection behind the compressor, for heavyduty GT and aero-derivative engines respectively. The former shows that up to 25% power augmentation is allowed without infringing on the surge margin.…”

“…As such, obtained a range of potential ramp-up rates that forms the basis of the ramp-up rate schedule in this study. In the comparable work [5] on the aero-derivative engine-level analysis, the multi-spool configuration brings about different directions of change in the surge margin of the low-pressure compressor (LPC) when compared to the high-pressure compressor (HPC) in the LM6000-type arrangement (fixed LPC rotational speed and variable HPC speed). However, when both shaft rotational speeds vary (by design), the change in surge margin during air injection was in the same direction (reduced).…”

“…The CAES system provides stored higher-pressure air than that of the GT maximum, to allow for air injection for power output augmentation or ramp-up rate increase. Existing research specifically on GT air injection includes [1][2][3] that covers power augmentation, and [3][4][5] that addresses the improvement of engine ramp-up rates. The previous study by Igie et al 1 indicated that it is possible to elevate the GT power output by 36% through 17.5% air injection at the back of the compressor, keeping the combustor outlet temperature (COT) fixed.…”