In a fast-paced market, requirements for flexibility, both operational and fuel, are becoming the cornerstone drivers for heavy-duty gas turbine (GT) design and upgrades. In fact, with the increasing portion of electricity produced from renewable energy sources, and the latest environmental requirements (especially in the EU), conventional power plants need to become more and more flexible, reliable, and resilient.
Specific upgrade packages developed by Ansaldo Energia, described below, pave the way for continuing utilisation of gas-turbine-based power plants during the energy transition, providing economic benefits to their owners/operators and presenting opportunities to participate in ancillary services markets.
The packages, especially intended for the existing service fleet of AE/V94.3A and SGT5-4000F gas turbines, meet the demand for flexibility in terms of faster start up time, grid frequency control, extended operational range (from minimum environmental load (MEL) to base load), and fuel flexibility. All while keeping pollutant emissions below local regulation limits.
Figure 1 gives an indication of the improved operational flexibility achieved.
Ansaldo Energia’s AE94.3A is a proven F-class heavy duty gas turbine featuring a 15-stage axial compressor, an annular combustor with 24 dry low NOx burners and a 4-stage turbine with cooling air brought in by a mix of secondary air system lines, external and internal-rotating (the latter employing airflow holes in two of the compressor discs).
The AE94.3A, accounting for more than 125 units sold worldwide, has accumulated more than 5 million equivalent operating hours worldwide in many different operating conditions.
AutoTune
AutoTune is an online optimisation system that automatically adjusts gas turbine operating parameters. It reduces combustion instability problems in every operational phase of the gas turbine, thus increasing GT flexibility with the consequent possibility of improving performance in areas such as ramp rate, minimum load and maximum power output.
AutoTune continuously analyses combustion parameters as well ambient conditions, operational parameters, fuel gas composition and emission level at exhaust, and actively adjusts the GT controller through specific regulation logics for pilot gas flow rate variation, TETC (turbine exhaust temperature control) setpoint variation and load reduction. It outperforms previous adaptive tuning strategies based on a fixed emission setpoint and on protection mechanisms that react by reducing load when critical acceleration levels are experienced in the combustion chamber.
With the latest features, AutoTune increases the flexibility of operation as follows:
- The AT 2.0 Emission Flex package can manage in a flexible way a dynamic emission setpoint (hour limit, daily limit or annual limit) in order to maximise performance taking into account the emission history and calculating the reached mean value.
- AT 3.0 Combustion Proactive, based on machine learning, can detect humming precursors and act in advance, preventing instability.
Starting from the available AE94.3A ramp rate and MEL flexibility packages, the combination with AutoTune enables increased benefits:
- ramp rate up to 50 MW/min;
- MEL up to 35 MW reduction.
Acting on emissions control, AutoTune can optimise maximum load, also allowing for performance degradation recovery. As the system is tuned continuously, manual or seasonal combustion tuning activities are reduced. Moreover, by reducing the risk of unit trip, the system improves unit reliability and parts lifetimes. A safe and reliable stand-alone system, now in operation at 50+ installations, AutoTune is fully retrofittable and compatible with any control systems and requires minimum downtime to install.
MEL reduction
The possibility of steadily operating gas turbines at low load for long periods while keeping the emissions below local regulation limits allows load to be increased quickly in response to fluctuations in power demand with net benefits for power plant owners/operators in the primary and secondary grid services markets.
The lower the MEL, the lower the fuel consumption and, in turn, the operating costs.
MEL flexibility package development has entailed the design and testing of several modular features, including VIGV (variable inlet guide vane) extra-closure, air bypass through compressor blow-off lines and extended use of anti-ice air recirculation.
Figure 2 shows features contributing to MEL reduction.
None of these require major hardware changes, just upgrades within a normal service outage, together with minor control system adaptations.
The readiness and performance levels achieved by these MEL upgrade packages, already in operation on several power plants, are described in Asme paper GT2023-101957. The modularised features allow activation/de-activation during operation, depending on the load demand of the plant. Figure 3 shows differences between the standard operation concept of the AE94.3A (black lines) and the reduced MEL mode (coloured lines).
The area shown in light grey represents the extension of the emissions compliant operating range by MEL reduction measures (combination of VIGV XC (extra-closure) and BO (blow off) opening). The basic principle of MEL is to keep the hot gas temperature in the combustor high enough to avoid excessive carbon monoxide production. AE94.3A engines are controlled via turbine outlet temperature measurements after the turbine blading outlet upstream of the blow-off flow entrance to the turbine exhaust. To keep sufficiently high combustion temperature the turbine outlet temperature follows the operational concept shown above.
Ramp rate package
By adopting GT controller modifications to the VIGV, operational concept and load control functions, it is possible to achieve faster up and down ramps. Due to the architecture of the AE/V94.3A annular combustion chamber and the presence of ceramic tiles, special sessions on combustion tuning and parameter adjustment are fundamental to keeping the combustion instabilities under acceptable limits. In addition, further load rate improvement can be obtained with the implementation of the AutoTune digital package.
Figure 4 shows ramp rate enhancement packages.
Fuel flexibility
Ansaldo Energia is fully committed to addressing the need of power providers to run gas turbines on green fuel and abide by EU CO2 targets.
The most recently developed burner technology, easily retrofittable in existing engines during an extended minor inspection, coupled with the most recent combustor technology improvements and AutoTune, allow combustor dynamics to be kept stabilised at low levels of NOx with green fuels such as HVO (hydrotreated vegetable oil), hydrogen blended with natural gas up to 40% by vol, and also on diesel oil.
As to hydrogen and diesel oil capabilities, Ansaldo Energia has already demonstrated the capability to burn both in real engines at low NOx levels.
On the hydrogen side, Ansaldo has a proven track record with AE94.3A technology at a southern Italy power plant operating since 2006. Here, two combined cycle units burn natural gas blended with hydrogen up to 25% by vol. with NOx emissions < 15 ppm.
In the diesel oil sector, 2024 saw Ansaldo validate and put into commercial operation the first 94.3 gas turbine able to burn fuel oil + water, with guaranteed NOx < 25 ppm.
Based on these fleet experiences, test rigs and taking into account the fuel properties of HVO, Ansaldo has already sold one unit running on HVO as single fuel + water that will be in operation starting in 2027, with NOx < 25 ppm.
Field and rig experience consolidates the flexible fuel capabilities of the existing hardware and paves the way to future development, already underway, to support the investments of power providers in speeding up the transition to a decarbonised energy future.
