With efficiency and environmental performance high on the agenda for utilities and power generators, and with a growing demand for distributed generation solutions, the small gas turbine market is a competitive one. Responding to these changing market demands, Rolls-Royce has updated its Trent 60 package, and will see the first units enter operation in 2006.

Siân Green, Managing Editor

In today’s competitive power markets, equipment manufacturers must have products that not only meet regulatory requirements, but also meet users’ needs in terms of cost, reliability, maintainability and more. The small gas turbine market is no exception, and could even be described as one of the most demanding market segments.

As well as meeting regulatory, environmental and reliability needs, power generators specifying small gas turbines require units to be efficient, quick to install and straightforward to operate and maintain. Gas turbines designed for this sector must also be versatile and flexible, able to operate in a variety of applications in different markets around the world.

Figure 1. Small gas turbines such as the Trent 60 face high levels of competition
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In many power generation markets, regulatory or economic drivers are encouraging the development of decentralized generation solutions, leading to a growth in demand for small gas turbines as well as other, new technologies. It is a crowded market, with key industry names such as GE, Rolls-Royce, Siemens and Pratt & Whitney all offering a range of products in the 20-60 MW output range. In addition to this, small gas turbine manufacturers now face an increasing level of competition from reciprocating engines with manufacturers such as Wärtsilä and GE Jenbacher making a concerted effort to sell into this market.

Recognising this demanding and evolving market, Rolls-Royce took the decision to update its industrial Trent 60 gas turbine – an aeroderivative product with a 51-58 MW output serving baseload and peaking application markets around the world.

Trent 60

The Trent 60 is a high-efficiency unit developed from Rolls-Royce’s aircraft engine lineage. It is the highest power (non-intercooled) aeroderivative gas turbine available on the market and also has the highest simple cycle efficiency of any of its competitors.

The Trent 60 gas turbine is equipped with a combustion system that guarantees NOx to 25 ppm. Two different configurations are available: wet low emissions (WLE) combustor or dry low emissions (DLE) combustor. The DLE-equipped Trent 60 uses an eight canular staged combustion system which allows the engine to operate in part load while still maintaining NOx and CO compliance. The engine produces 52 MWe at ISO conditions and is flat rated at 58 MWe at temperatures below 2°C. The WLE Trent 60 uses an annular combustor with water injection to reduce emissions and boost performance. At temperatures below 19°C, the engine is flat rated at 58 MWe.

Development of the DLE combustor took place largely in Montreal, Canada and the first units were commissioned in the field in December 2002. The Trent has gained some 20 000 hours of emission compliant operation and over 108 000 hours fleet-wide.

Development of the WLE system began in late 1999 and the first unit was commissioned at the Derby cogeneration plant in the UK in late 2001. Derby is a baseload combined cycle cogeneration plant and the installed Trent 60 units clock up around 5000-6000 operating hours per year.

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The design and higher output of the WLE system means that it is usually the best choice for utilities as the canular DLE system is harder to operate and maintain. “With the WLE system, you get about 15 per cent more power output compared to the DLE system and it is a lower cost system because you don’t have all the additional cans around the outside of the engine,” explains Scott Nolen, product marketing manager for Rolls-Royce.

The DLE Trent 60 comes into play where regulations influence technology choices – for example where water injection is not allowed, and in regions where water is expensive or scarce. “We are currently building six Trent 60 DLE units for the Dolphin pipeline project in the Middle East,” says Nolen. “Water is scarce and the customer had a requirement of 29 ppm NOx so the DLE system was the obvious choice.”

The six units for the Dolphin natural gas pipeline project will start to arrive on site in mid-2005. These will be the first Trent 60 units to operate in a mechanical drive application; today there are ten Trent 60 units operating around the world in power generation applications, with the 11th unit due to be commissioned in December this year.

Figure 2. Trent 60 WLE nominal performance
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A new package

While initial uptake of the Trent 60 was good, Rolls-Royce became aware of the need to improve on the overall footprint of the package as well as the installation time and cost in order to meet market needs. The company therefore spent around 14 months redeveloping the Trent 60 genset package so that key components such as the air, fuel and oil systems were standard and could be deployed regardless of application.

“There is no question that the package that we had before was not up to the level that was available on the market at the time. Before [the redesign], it took 180 days to install whereas now we believe that we can get this down to about 45 days; it’s a phase shift in the installation time,” notes Nolen. “The footprint size has also been reduced to less than half of what it was before,” he adds.

According to Nolen, few design changes to major engine components were made in the development of the new package, which was officially launched in late 2002. However, the oil systems were completely revised to improve them from a cost, maintenance and footprint standpoint, and the enclosure is completely different. The engine discharge volute has been improved and is now similar to that used on the Rolls-Royce RB211 gas turbine, says Nolen. “The original discharge volute was actually a Westinghouse design and we didn’t feel we could keep on taking it to market.”

The overall result of the modularization programme is that Rolls-Royce is now able to assemble, test and de-bug each unit in the shop before it is shipped out to site. “In order to do that we have one single base plate for the gas turbine that contains the oil systems (lubricating, hydraulic or start-up), and the fuel gas systems,” says Nolen. “The control system for the gas turbine is also pre-wired – including the PLC – on the base plate so that it can be commissioned and tested in the shop before shipping.

“The generator is on a separate base plate purely because of weight and size. The generator base plate is assembled and designed with the same philosophy as the gas turbine baseplate. All the oil systems are built in, and the controls are linked in to the control panel on the gas turbine baseplate.”

The package is designed so that the entire engine can be removed and replaced in just 24 working hours. “The ability to ‘swap’ the engine core is probably one of the biggest advantages of an aero engine from a maintenance perspective compared to a heavy industrial machine,” notes Nolen.

Figure 3. Trent 60 DLE nominal performance
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Rapid start

The development of a standardized package will help to give a competitive edge to the Trent 60, which is already a versatile product, says Nolen. “The beauty of this is that it is a very flexible product. It is very good for the peaking market because it gives you the most bang for your buck with the highest simple cycle efficiency of any non-intercooled machine out there today. It also has a ten minute start time, so a DLE unit gives you 52 MW and a WLE unit gives you 58 MW on the bars in ten minutes which is a pretty good pick up for a lot of markets around the world.”

The unit is also suited to combined cycle applications, with Trent 60-based CCGT plants achieving efficiencies of 51-52 per cent. Of all the Trent 60 units in operation today, half are installed as combined cycle units.

In addition, the Trent 60 is also suited to mechanical drive applications such as the Dolphin gas pipeline project. “These will be good units for Dolphin and other similar projects because they have the ability to do between 70 and 105 per cent speed range and still match up to the power demand from a compressor in that range,” says Nolen. “The Dolphin units will be on a waste heat recovery system as well so they’ll also be generating steam”.

The six units for the Dolphin project will enter operation in mid-2006, and will be the first Trent 60s in the field with the new standardized package design. While the US and European markets are relatively quiet at present, the Middle East is offering a lot of opportunities for small gas turbines. “There is no question that the Middle East is starting to become a very key peaking market – of anywhere in the world they probably have the largest peaks and valleys in their power demand in any given day. Efficiency is also becoming more important in this region.”