Retrofitting for greater flexibility

by Karin Lindvall, Alstom Thermal Services, Baden, Switzerland (karin.lindvall@power.alstom.com)

With the rise of renewables and increasingly volatile energy markets, transmission system operators (TSOs) are looking for electricity generators who can help stabilise the grid.

Today, the role of providing flexible generation to complement intermittent renewables is primarily being filled by simple cycle and combined cycle gas fired units. New gas plants are being designed for greater operational flexibility while owners of existing plants are faced with challenge of meeting new operating requirements and at the same time maintaining profitability.

In markets such as the USA and even more so in Europe, gas power plants originally designed to run at mainly base load now have to operate at intermediate load or shutdown/start up on a daily or weekly basis to balance the availability of renewables and fluctuating demand.

This difficult market environment has had a damaging effect on the profits of several major utilities, while the shift towards more cyclic operation has a negative impact on main component lifetimes and environmental footprints.

In response to these market conditions Alstom, building on the latest technologies employed in new equipment, is offering a portfolio of hardware and software packages suitable for retrofit. These enhance operational and maintenance flexibility in the existing E- and F-class combined cycle fleet, leveraging particular features of the Alstom gas turbine architecture.

The portfolio of flexibility products, known as FLEX SUITE (Figure 1), consists of five packages: 'Start' for reliable fast start-up; 'Perform' for improved plant performance; 'Respond' for fast response to changing operating requirements; 'Reserve' for economic turndown; and 'Care' focusing on smart maintenance for optimised costs and planning.

The Start, Perform and Reserve packages are specifically aimed at giving plant owners the opportunity to be competitive in a market where operational flexibility is valued.

Faster start-up

A key requirement for operators in today's market is to get back online faster to meet grid demands for energy and stability.

Flex Start is a series of packages that allow a plant operator to reduce the start-up time and cost for hot, warm and cold start of a combined cycle power plant, starting up in eithercombinedoropencyclemode.Toget the full benefit of the modules, optimisation of software as well as hardware is required.

Start-up times for a KA26 power plant can be typically reduced by up to 25, 35 and 60 minutes for hot start, warm and cold start conditions, respectively. This also results in substantial fuel savings and reduction of start-up emissions. Importantly, these improvements can be realised without negatively impacting plant lifetime. The full lifetime of the plant can be maintained even in high cyclic operation.

Flex Start consists of several modules related to the plant start-up sequence.

The start-up sequence of a combined cycle plant can be divided into three main steps: (1) GT synchronisation, (2) combined cycle start up and (3) GT and ST loading (see Figure 2). The Flex Start packages (FS1, FS2, FS3) apply to eachofthesesequencingsteps.Anadditional package (FS4) is designed to conserve heat when plants have to be shut down.

Table 1 summarises the benefits and time savings arising from the four packages for hot, warm and cold starts.

The FS1 package is designed to reduce the time for the GT synchronisation phase by applying 'purge credit' so operators can avoid purging before start-up as well as faster release for GT load-up. Optimising GT synchronisation can typically save about eight minutes on a KA26 plant start-up.

Following synchronisation of the gas turbine, the loading of the entire plant begins. The FS2 package saves time by optimising the warm-up of the water steam cycle as well as the release steps of the ST and combined cycle load-up.

FS2 is a combination of software and hardware modifications that allows steam to be fed into the steam turbine faster through water chemistry measurement optimisation and water chemistry preservation. It also enables faster combined cycle release by optimising: hold points and loading for the gas turbine; steam parameters; steam turbine logic and sequencer; and steam turbine run-up and loading rates.

The prototypes of the FS1 and FS2 packages have already been installed on a few units in Europe.

In Germany the owner of a KA26-2 unit needed to deliver, from any condition (cold/ warm/hot), the whole 2xGT capacity within 45 mins. In April last year, the two GTs went from 0 to 100% load in 38 minutes from warm and in 43 min from cold (see Figure 3).

Meanwhile, in Spain, the operator of two KA26-1 combined cycle plants wanted to deliver between 140 and 180 MW within 30 min, for so called tertiary reserve, again from any condition (cold, warm or hot), and to keep the power steady for a certain interval. In July 2014, 180 MW was achieved within 30 min (cold start, GT only), 100% combined cycle power output was reached within 110 min. See Figure 4.

The FS3 package increases the GT and ST ramp rates. To maintain plant lifetime, certain component improvements may be needed.

FS4 is designed to conserve heat during shut down and stand still. This means cold - and in some cases warm-start conditions can be avoided since the main components, ie, gas turbine, steam turbine and HRSG will remain in a pre-warmed state for faster loading on restart.

FS4 goes beyond the usual methods operators can use to keep components warm. Both hardware and software modifications and protection aspects are considered. Whereas an operator can typically implement techniques such as shutting off air inlets and outlets to avoid drafts or using heaters, each FS4 package, following a plant assessment, is tailor-made for Alstom components within the plant. The heat conservation techniques for all components are based on active control of air circulation to either prevent cool down or to actively warm up by using air or steam.

FS3 and FS4 are now in the final stages of development, becoming ready for first implementation on customer units very shortly.

Higher peak load

To be competitive in a deregulated energy market, the ability to take advantage of peaks in electricity market prices is key. However, maximising output and efficiency as well as extended lifetime are often conflicting objectives.

Alstom's well established MXL family of upgrades provides the operator with increased flexibility via its online operation- mode switch function. This allows the gas turbine to be instantly switched to the power-optimised operation mode (M-mode) for maximum power output when market demand and electricity price is high. At times of lower demand the unit is switched online to the lifetime-optimised operation mode (XL-mode). In M-mode a GT26 can deliver up to 25 MW more combined cycle power and up to 0.8 % higher efficiency.

To enable full utilisation of M and XL mode operation and benefit from minimum lifetime impact in M-mode and lifetime extension in XL-mode, a turbine and/or compressor upgrade is needed. The currently available upgrade packages are known as GT24/GT26 MXL2 and GT13E2 MXL2.

Another possibility for increasing maximum plant power capacity for a limited time, in order to profit from peak electricity price without hardware modifications, is what is called the Peak Load Concept, which is part of the Perform module.

The Peak Load Concept (Figure 5) allows operators to increase power by combining an increase in firing temperature with an increase in mass flow (by opening up the inlet guide vanes).

Depending on the amount of temperature increase, a lifetime counting mechanism is also included in the concept. Up to 12 MW more combined cycle power can be achieved without hardware changes while maintaining frequency response capability across the increased load range.

The first Peak Load installation was carried out in the US in 2012 on a GT24-based combined cycle unit. Since the first installation, Peak Load has been implemented on 13 other GT24 machines in the US. The concept enables the operators to commit higher capacity to the grid to become eligible for capacity payments.

Alternatively, it can be used in the so-called "intra-day" market, where plants bid on an hourly basis to provide peak capacity. If the
peak price is high enough they will use the Peak Load concept, as the revenue from the power sales outweighs the economic impact on lifetime and increased fuel cost.

Improved turndown capability

During times when the demand and spark spread is low, plant owners may want to optimise operation costs yet still be available to get up to high load quickly in order to profit during peak demand times. Here, reducing the minimum load at which the GT can operate within emission limits - known as Minimum Environmental Load (MEL) - is the main aim of the operator.

In recent years, Alstom has developed turndown concepts for retrofit application on the E and F-fleet units enabling economically favourable operation at full emission and frequency response compliance.

There are two different concepts for turndown: Low Part Load (LPL) operation, for the GT13E2 and GT24/GT26; and Low Load Operation (LLO), for GT24/GT26 only.

LPL and LLO (see Figure 6) offer an alternative to a start-stop cycle, whereby the turbine can be 'parked' avoiding start- up cycling stress and cost in the form of equivalent operation hours (EOH) and reducing cumulative start up emissions. With both concepts an operator can choose optimal turndown conditions depending on spark spread or response time required by the grid operators.

Keeping a unit on-line at low load enables an operator to react instantly at the first sign of increasing power demand. Furthermore, additional income may be generated from the ability to offer a wider load range (head room) flexibility and ancillary services.

For KA13E2 units the turndown capability was significantly enhanced by the development of the Advanced EnVironmental (AEV) burner and the ability to switch off selected AEV burners as power is reduced. The burner enables the LPL concept to achieve turndown to about 30% GTCC relative load at approximately 45% GTCC efficiency for KA13E2 at full emission compliance.

The AEV uses a continuously variable fuel staging system, which completely removes the need for switchover and switchback between pilot and premix gas, which was typical for Alstom's previous EV burner technology (see Figure 7). The AEV burner permits a wider operational range for the GT13E2 with acceptably low emissions and dual fuel flexibility.

The first GT13E2 AEV burner LPL upgrade was installed in August 2014 at the Berlin Mitte combined heat and power (district heating) plant in Germany (see Figure 8).

At this 440 MW KA13E2-2 facility it has enabled operation at less than 30% GT relative load while achieving less than 25 ppm NOx and less than 80 ppm CO emissions on natural gas. This meant the owner could profit by running in LPL mode with emission compliance during the night when the spark spread is unfavourable, while still providing heat to the city of Berlin, and providing base load power during the daytime.

The owner of the plant believes that the upgrade will help the Berlin Mitte gas turbines remain "among the best performing and flexible units in the GT13E2 fleet" and will enable the plant to "continue to operate successfully in the future energy market of our city."

The implementation of LPL requires modification of the fuel distribution system and control concept to allow stepwise shut off of individual burner groups (Figure 9).

At Berlin Mitte, the test programme included mapping of combustor temperatures at different mass flows with a varying number of burners in operation. During tests the temperatures and pressures in the combustor as well as the fuel distribution system were measured by standard instrumentation but also additional temperature and pressure measurements were taken for the specific purpose of validation.

Emissions and pulsations were also recorded and evaluated. The purpose of the mapping was to find the lowest load within pulsation, emission and temperature limits to determine the MEL for LPL operation.

Parking at even lower loads is possible with the KA24/KA26 fleet due to the unique configuration of the sequential combustion system used in GT24/GT26 turbines. Sequential combustion employs two combustor stages: EV ((EnVironmental) combustor followed by Sequential EV (SEV) combustor.

When operating at base load the first step is to de-load with all EV and SEV burners inoperation,thenstepwiseshut-offofSEV burners while the water steam cycle (WSC) is in operation, thus keeping efficiency at a competitive level. With all EV burners and 1/3 of the SEV burners in operation, a MEL of 30% GTCC load is reached. This is the LPL operation concept.

The next step is to shut-off all SEV burners with only EV burners in operation, thereby reaching the Low Load Operation (LLO) point of about 15% combined cycle load, hence reducing the fuel consumption even further. As the WSC is in operation for both LPL and LLO, fast load up to full combined cycleloadisstillpossible.TheLLOallowsa KA26 power plant to offer a spinning reserve of > 300 MW for KA26-1 in 30 min. The LLO concept was introduced with the KA26 rating 2006 and since then installed on seven units.

The first GT26 gas turbine to demonstrate LPL operation has been running in the Netherlands since the summer of 2013 in a 2 x KA26-1 plant.

With one unit operating with LPL (unit 1) and the other unit without LPL typically shut down at times of low spark spread, the owner could optimise the plant operation costs by still being able to react fast to changing demand through the spinning reserve offered by the LPL unit.

At the time of the first A-inspection, after 8000 hours of operation, there was no noticeable impact on the hot gas parts and water steam cycle in unit 1 relative to unit 2 (see Figure 10).

The LPL concept has now also been installed on unit 2 and has been available to the operator since autumn 2014.

To date, eight LPL upgrades have been installed on GT26 units. In addition to the units in the Netherlands two were installed in the UK in the autumn of 2014 to allow the plant owner take part in the balancing market. Around the same time, one more began operation in Spain. This year, three more were installed, in Spain, UK and Chile.

Future development

These early installations demonstrate the effectiveness of the flexibility packages and will form the basis for further development of FLEX SUITE.

One direction for future development is to further extend operational load ranges to even lower MEL levels. It is also planned to develop automated operation systems to further enhance the existing products.

Looking to the future, it is necessary to closely monitor evolving grid requirements and to respond quickly with the technologies needed to support flexible operation.

In terms of maintenance, there will be further development of component lifetime management tools and methods to enhance lifetime monitoring and prediction capabilities.

The continuing development of retrofit technologies will allow plants built before flexibility was a market requirement to regain competitiveness in an increasingly volatile energy business.