Fuel flexibility and future proofing in the era of renewables

Renewable energy has disrupted the energy market place. Fuel is free for renewables, and coupled typically with “must run” governmental requirements, renewable generation units are the first to be dispatched on the power grid. Wind and solar are a function of the weather and can experience rapid swings in load. The result of this type of highly variable power demand is that gas turbine power plants must effectively respond to the load swings and capture periods of profitability. It’s called “chasing renewables” and is highlighting operational limitations of the installed base of gas turbine power plants at a time when reducing maintenance costs is critical to maintaining profitability.

As well as operational flexibility, there is also an increasing need for fuel flexibility. Running on alternative fuels can be a way to lower costs. Some of these fuels, such as those produced as byproducts in petrochemical plants and refineries, are readily available, and would otherwise be flared, but are high in hydrogen and require robust gas turbine combustion systems if they are to be used effectively.

In addition there is growing interest in using hydrogen in gas turbines because of its potential as a ‘battery fuel’, with excess power produced by wind and solar employed to produce hydrogen via electrolysis.

However, in the context of gas turbine combustion, hydrogen is a highly reactive fuel and presents challenges for industry-standard dry low NOx combustors when it comes to switching between natural gas and hydrogen fuel blends while remaining stable and keeping NOx emissions always below stringent emission limits.

Significant concerns regarding emissions compliance, combustion dynamics and stability must be addressed prior to operation on these alternative fuels.

Future proofing a mature gas turbine

For a gas turbine operating with high levels of hydrogen, PSM’s LEC-III® ultra-low- emissions combustion system platform is worth consideration. The LEC-III® patented technology was first incorporated into GE Frame 7E gas turbines in 1998. This can- annular, reverse-flow combustion system, shown in Figure 1, was designed to be a direct replacement into an existing gas turbine outfitted with the General Electric DLN-1/1+ system. The LEC-III® combustion platform is a lean premixed system and includes a combination of fuel nozzle assemblies, transition pieces, flow sleeves and combustion liners designed (initially) to achieve less than 25ppm NOx (corrected to 15% O2) at baseload conditions.

To date, the fleet of PSM LEC systems installed on five different types of machines (GE Frames 6B, 7B/E/EA and 9E, as well as Siemens Westinghouse 501B6 and 501D5) has amassed over 1 000 000 hours of operation on over 70 gas turbines.

The LEC-III® platform incorporates a number of technology improvements relative to previous state of the art systems, see Figure 2.

The key mixing features of the LEC-III® include: cooled venturi; increased dilution air to the head-end premixer enabled by the enhanced cooling efficiency of effusion cooling; and a fully-premixed “Fin Mixer” secondary fuel nozzle.

The secondary fuel nozzle is a key contributor to the demonstrated stability of the OEM DLN-1 combustion system. The secondary fuel nozzle sets up a central ‘pilot’ zone of reaction and recirculation that acts as the continual ignition source for the surrounding reaction zones of premixed primary fuel. By design, this secondary reaction zone is a richer mixture, burning hotter to provide excellent combustion stability. In the conventional DLN-1 system, this secondary fuel flow is actually channeled through two separate circuits. The majority of the fuel is discharged from ‘pegs’ near the nozzle’s mid-section.

This fuel premixes with air as it travels along the length of the nozzle, and it goes through swirler vanes for final premixing prior to discharge into the reaction zone. The second circuit within the nozzle has a small amount of fuel discharging at the tip (extreme aft end), which is not premixed at all. It burns in a ‘diffusion’ mode of combustion. This region has some areas where reaction temperatures rise above 3500°F and associated NOx formation is substantial. It is only a small amount of the total fuel flow, but its contribution to the system’s total NOx formation can be significant.

Elimination of this ‘diffusion’ burning aspect of the conventional nozzle has been the focus of the LEC-III’s secondary fuel nozzle design evolution (Figure 3), and a large amount of rig and engine development testing has been conducted as part of the development of this nozzle design.

PSM’s fully evolved, current production secondary fuel nozzle offering is known as the “Fin Mixer” secondary fuel nozzle (Figure 4). As demonstrated in the course of engine verification and validation, this design has demonstrated the ability to significantly reduce NOx.

This improvement is simple in concept and implementation, but it provides a step change in emissions reduction in an already low emissions combustion system. In fact, many of these systems are operating at or below 5ppm NOx across their operation load ranges.

Hydrogen fuel application

A recent hydrogen fuel application of PSM’s LEC-III® combustion system with the Fin Mixer secondary fuel nozzle was a retrofit project carried out over the past ten months for a power plant owner in the Netherlands, on all three of their Frame 9E gas turbines, which are operated in cogeneration configuration.

In addition to generating electricity for the grid, the plant also supplies steam for a chemical plant next door. It is the chemical plant that provides the hydrogen, replacing natural gas up to a level of 25% H2 by volume. Extensive alternative fuel testing has been completed in a test rig environment and is documented in ASME TurboExpo paper GT2018-75553. This testing provided the confidence that implementation would be successful. Today, all three units have the ability to burn up to 25% hydrogen (set by supply limits) and are confirmed stable up to 35% hydrogen. At 25% hydrogen mixing, this equates to about 9% of the energy in the fuel mixture not contributing to the formation of CO2, so the carbon footprint is reduced also by 9%. Additionally, a simple first order payback, assuming the fuel is a non-value byproduct of the chemical refinery and with the price of natural gas about €4.75/GJ, a customer operating a Frame 9E at baseload could save €3M/year.

Automated tuning

The fuel injection retrofit also included installation of PSM’s AutoTune system to ensure high reliability with variation in fuel composition. AutoTune provides the ability to continuously monitor gas turbine emissions and dynamics and perform reactive fuel split adjustments to ensure these are maintained at acceptable levels. It will also apply proactive split adjustments with changes to ambient temperature, machine load, and fuel composition, based on optimal engine performance for each operating point.

Changes in the hydrogen content of the gas turbine’s fuel gas supply will influence combustor behaviour and subsequently affect both combustion dynamics and gas turbine emissions. Furthermore, the effect of changing the hydrogen content varies depending on the engine’s operating point. As such, AutoTune measures changes in fuel composition and reacts to these accordingly.

AutoTune’s effectiveness in actively tuning the gas turbine is greatly dependent on the speed with which it can react to changes in hydrogen content. While gas chromatographs can accurately determine gas composition, this is generally at the expense of a long delay in obtaining measurements. The ability to proactively set fuel split adjustments for changing operating points is greatly hindered when such a time delay is experienced, causing the engine to be susceptible to high combustion dynamics or emissions permit excursions. In order to ensure rapid response to changes in fuel, AutoTune uses an internally calculated (and proprietary) measure, Fuel Property Parameter (FPP), to determine variations in gas composition.

Future-proofing the operator’s assets

Power plant operators, especially in Europe, face challenges stemming from renewables and their variability. Equipping the installed fleet of gas turbines with combustion enhancements – such as PSM’s LEC-III® – that increase fuel flexibility, while at the same time reducing carbon footprint and increasing reliability, enables them to remain a viable option for stabilising the grid and is a cost effective way to future-proof power generation assets. 

* Ansaldo Energia Group