Every on-site power plant has its own particular operational and performance needs; more so for cogeneration applications. Here, Hartmut Hähnle, Klaus Döbbeling and Bernard Tripod explain the options for plant based on a particular gas turbine now installed at many industrial sites.
In addition to high performance and high efficiency, operational flexibility is a key requirement for gas turbine plants in many regions across the globe. The demand to minimize the cost of electricity is very pressurized for combined heat and power (CHP) plants – this is also the case for industrial plants. So the right balance between power plant efficiency, reliability and flexibility is the key for optimizing the whole CHP plant or industrial power plant operation.
This article describes the technical differences offered by the GT13E2 gas turbines, produced by Alstom, and their value in providing the overall best compromise between the low cost of electricity and higher than standard flexibility and reliability levels – as demonstrated in CHP plant and industrial power plant applications.
THE GT13E2 GAS TURBINE
The GT13E2, which was first introduced in August 1993 and of which there are 139 units, has accumulated more than 5,200,000 operating hours and 37,000 starts. In 2002, after years of successful operation, an upgrade was introduced while maintaining/improving the reliability and availability profile of the engine. The upgrade introduced a significant reduction in maintenance costs by extending the inspection intervals from 24,000 to 36,000 operating hours, and an improved engine performance to expand the economic envelope of the gas turbine into higher fuel price ranges. All engine parameters such as mass flow and hot gas temperature remained unchanged; the proven ‘hot zone’ hardware design was retained while receiving additional protection to enhance parts life. The well-proven turbine blade was retained but with the addition of Thermal Barrier Coating (TBC), while the last stages of the turbine were fine-tuned to take advantage of fluid mechanics, cooling, and sealing technology advances.
In 2005, a further upgrade was introduced with a performance increase, in terms of power. This was achieved by increasing the mass flow through the engine by 4% – by re-staggering a number of the front compressor stages and associated aerodynamic improvements of the turbine blade.
Table 1. KA13E2 combined-cycle output and efficiency
In parallel to the performance upgrade described, the improved rating of the GT13E2 also incorporates a modified combustor zone and an advanced pulsation control system to achieve a wider operation regime for lower NOx emissions on gas operation.
Thus, these three measures – a further compressor mass flow increase, an improved turbine blade, and a wider operation regime for lower NOx emissions – are the basis for the current rating on the GT13E2.
Figure 1. GT13E2 gas turbine performance data
All design changes are retrofitable to the GT13E2 baseline configuration. The performance upgrade has been achieved without increasing the turbine inlet temperature.
All the results received so far, prove not only the incremental and robust character of the upgrade, but they also show the stability and robustness of the development and validation for the processes used.
FLEXIBILITY IN POWER AND LIFETIME MODES
The flexible GT13E2 can be operated in a ‘power’ or ‘lifetime’ optimized operation mode. The ISO ratings and outage intervals in the two different operation modes are shown in Figure 1. The operator has the option to select an on-line switch-over (i.e. while operating) according to the actual operational requirements and how he wishes to run the engine. Thus, he can maximize the performance of the engine according to changing demands – all with the same hardware and with on-line selection.
OPERATIONAL FLEXIBILITY ACROSS APPLICATIONS
Major challenges ahead
Today, gas-fired power plant planners and operators are facing great challenges, as they cope with an unprecedented tough business environment characterized by some major uncertainties concerning fuel price volatility and security of supply, with the trend towards ever more stringent environmental regulations and electrical grid requirements, and, in some regions, the de-regulation of the power and fuel markets. All this contributes to making any economic evaluation a challenge.
The GT13E2-based combined-cycle power plants (known as KA13E2) with their very flexible design and operation, can more easily adapt to the changing conditions and therefore help mitigate the risks associated with this new environment.
Configurations and performance
Alstom offers a wide range of simple cycle and combined-cycle power plant using the GT13E2 gas turbine. Reference combined-cycle plants configurations use one, two or three gas turbines per block. An overview of the plant performance which can be achieved in ‘power’ optimized mode, at ISO conditions, is shown in Table 1. These performances are based on a water steam cycle concept of two pressure levels, without reheating.
High degree of customization
This basic ‘reference’ concept has proven to be the best solution for most projects. However, flexibility remains a prime design consideration, allowing for a very high degree of customization. A project specific optimization process is in place to ensure the best fit to any given requirements. In addition, the concept is easily adaptable with a number of pre-engineered variants and options, such as:
- power augmentation (gas turbine peak firing, gas turbine inlet cooling, supplementary firing)
- cogeneration configurations
- arrangement concepts
- phased construction concept.
Combined cycle plants based on the gas turbine GT13E2 have been installed on all five continents, under all climate conditions, and used to generate power in very different operating regimes, including base load and cycling operation.
Applications dedicated to the supply of peaking power operation have also delivered. These are normally based on GT13E2 plants in simple cycle configuration.
Complete plant supply
Although the GT13E2-type gas turbines can be offered as equipment, most of the gas turbine-based projects being delivered come with a larger scope of supply and services.
Overall, Alstom has installed around 200 turnkey combined-cycle plants around the world, totalling more than 100 GW, with Alstom taking the full responsibility of the supply, including overall plant performance guarantees.
Table 2. GT13E2s in industrial grids
This method is typical of Alstom and uses the unique ‘plant integrator’ approach, in which the customer advantage is systematically analysed and measured to ensure the right optimization – technically, economically and ecologically – with the objective to design solutions which best match customer interest.
WIDE RANGE OF APPLICATIONS
Apart from pure power generation, KA13E2 configurations have proved to be very versatile and have been used in numerous other applications:
Aluminium smelters and industrial grids
Highest operational flexibility requirements are set by industrial applications, like aluminium smelter plants, where it is essential that the power plant is capable of independently maintaining the power supply of the sometimes quite small utility grid. Numerous examples testify the GT13E2 capabilities in such industrial grids (see Table 2).
To comply with the topology of industrial grids, Alstom has developed customer-tailored features, like:
- frequency response capability – keeping the industrial grid stable while covering large consumer load variations or grid disturbances, this, without additional lifetime factors.
- disconnection from the utility grid and switch to ‘island’ grid with the gas turbine control system – effectively stabilizing the gas turbine within about 20 seconds, at a frequency excursion far below the protection limit of 55 Hz.
- partial load rejection, with the plant management system co-ordinating the gas turbine and steam turbines, and their connection to the grid is coping with the largest considerable load jumps.
- improved start-up concept combining high compressor stability at the full range of site boundary conditions, including hot restart. The acceleration-controlled run-up of the engine ensures constant start-up times. A start-up reliability of 98% has been achieved on the three front runner units (the units with the highest number of starts executed with the improved concept) in a period during which each has performed more than 200 starts. More than 1500 starts in total have been performed with the improved start-up concept.
- on-line (i.e. while operating) fuel switch-over capability from fuel gas to fuel oil within less than three minutes, at uninterrupted stable power supply.
Several cogeneration projects using the GT13E2 are in operation in the chemical, aluminium and mining industries, or for heat supply to a desalination process or district heating system. These specific applications all require a high degree of customization to meet the specific requirements of power and heat production, as well as a high level of operation flexibility to cope with the variations in the processes, plus a very high availability and reliability to guarantee discontinued operation.
At the Al Hidd 2 project operating in Bahrain, the combined -cycle KA13E2-3 delivers 675 MW electrical power and has the interconnection for a desalination plant of 60 million gallons (272 million litres) per day of fresh water production capacity.
The Keppel Merlimau KA13E2-2 project in Singapore generates 500 MW of electrical power to the Singaporean network, as well as steam to future consumers of the Jurong Island industrial park
This block was delivered turnkey and entered into full commercial operation in 2004. It is installed next to two GT13E2 simple cycle cogen units, commissioned in 2000, providing medium pressure steam to the four MSF (Multiple Stage Flash) desalination units which are rated 7.5 million gallons (34 million litres) per day each.
The Keppel Merlimau KA13E2-2 project was supplied turnkey to Singapore. It generates 500 MW of electrical power to the Singaporean network, as well as steam to the future consumers of the Jurong Island industrial park. In this plant, the GT13E2s are fitted with power augmentation devices consisting of combined evaporative cooling and high fogging systems to maximize the power output.
The Berlin Mitte combined heat and power plant is another good example for a KA13E2 application with high operational flexibility. The total plant power is 1130 MW – with a split of 460 MW electrical, 430 MW heat, and 240 MW hot water, for a total thermal efficiency (fuel utilization) of 90%.
The electricity is fed into the 380 kV overland grid and into the local 110 kV grid. A district heating network is operated by Vattenfall, with the Berlin Mitte plant having a significant share in the overall heat production.
The plant and process layout is designed around two GT13E2s, each with a generator and a waste heat boiler module used for high-pressure (HP) and low-pressure (LP) steam generation and hot water pre-heating. The steam drives a steam turbine and the steam turbine generator. The heat sink of the back-pressure steam turbine consists of heat condensers that supply the district heating system, together with hot water pre-heated in the waste heat boiler module.
When the old plant was replaced in the heart of Germany’s capital city Berlin in 1996 with two GT13E2s, a back-pressure steam turbine (avoiding a cooling tower) and two boilers, for recovering the waste heat from the gas turbine exhaust, the electrical power output of the plant nearly tripled from about 350 GWh/year to about 1100 GWh/year while heat production remained at around 1200 GWh/year. The plant is heat driven and electricity production depended at that time on commanded heat output.
In 2000, the operating profile was changed and the annual operating hours continually increased to about 3000 per gas turbine and more than 5000 operating hours for the steam turbine in 2005.
In 2005–2006 an external cooler was set in operation. This helped to decouple the generation of electrical power and heat to a significant extent, making up for the absence of a cooling tower – plumes would not be tolerable in the centre of Berlin. By means of this, the generation of electricity is possible at times when the market is profitable – independent from the heat production. As a consequence, the annual operating hours increased to more than 4000 per gas turbine with only about 100 operating hours in simple cycle operation (i.e. without the steam turbine).
The operational flexibility is exploited to the maximum extent. The air-preheater is not only used for anti-icing reasons in winter time when ambient humidity is high. It is also operated to increase the gas turbine exhaust temperature in order to fully exploit the heat output.
The Alstom development philosophy for the GT13E2 is to attain a balance between providing improvements to the existing platform, while maintaining the proven levels of availability and reliability. This has resulted in a versatile engine that is flexible in terms of operation by offering the choice between a ‘lifetime’ versus a ‘power’ optimized operation mode (on-line switchable).
A high degree of customization allows the company to meet project and site-specific requirements. The robust start-up procedure, the high load flexibility (even in island grid conditions with frequency response operation), and the fuel changeover capability from fuel gas to back-up fuel, even at full load, have been demonstrated in numerous applications. The experience with combined heat and power generation applications has shown that the GT13E2 is fulfilling a high yearly output of both electrical and heat energy.
The GT13E2, with its reliable performance in all configurations, the high operational adaptability and the superior fuel flexibility, is designed to meet the current and future power market requirements.
Hartmut Hähnle, Klaus Döbbeling and Bernard Tripod are with Alstom, Baden, Switzerland.