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Small Gas Turbines: New beginnings

By Siàƒ¢n Green

In 2001, the US Department of Energy’s Advanced Turbine Systems (ATS) programme that started almost a decade ago will draw to a close. And while much attention has focussed on the achievements of the large, utility-scale turbine systems, enormous progress has also been made in the industrial-scale ATS programme towards building the future generation of gas turbine-based power systems.

Figure 1. Rolls-Royce will infuse ATS technology into its current 501-K and 601-K product lines
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The $700 million ATS programme is a joint effort between the DOE and the Office of Fossil Energy to complete the development of high efficiency natural gas turbine systems for electric utilities, independent power producers and industrial end-users.

Turbine systems being developed under the programme include 400 MW utility-scale gas turbines and combined cycle systems, and simple cycle industrial gas turbines of 5-15 MW for the distributed generation, industrial and cogeneration markets.

Industrial-scale simple cycle machines currently have efficiencies of around 30 per cent, and one of the main goals of the ATS programme has been to improve that by 15 per cent. Other objectives of the programme are to enhance fuel flexibility, achieve a ten per cent reduction in the cost of electricity and single digit NOx.

The two manufacturers that have been involved in the major systems development part of the industrial ATS programme are Solar Turbines and Rolls-Royce.

Under the industrial ATS programme, Solar has developed the Mercury 50, a compact, recuperated 4.2 MWe single shaft gas turbine. The Mercury 50 takes advantage of Solar’s proprietary primary-surface recuperator and its latest generation of compressors, the ACE compressor, and boasts electrical efficiencies of over 43 per cent.

The Mercury 50 has met the goals of the DOE ATS programme, according to Solar, including single digit NOx. The product underwent testing in 1998 and the first unit was launched towards the end of 1999. Solar now has Mercury 50 units operating at six sites across the US undergoing field evaluation.

A low risk approach

Like Solar, Rolls-Royce has also made some significant achievements in the course of its ATS programme. But instead of developing a complete, new ATS product, Rolls-Royce has opted to enhance and develop new and existing turbine system components to achieve the industrial ATS goals.

When it embarked on the ATS programme in 1995, Rolls-Royce took a similar approach to Solar: to develop an entire ATS product. The company therefore pursued the design and development of a 701-K engine that would meet the goals of the small gas turbine ATS programme.

The 701-K product was to be a 7 MW-class engine featuring advanced cooling and single crystal turbine materials, and by 1999, Rolls-Royce had reached the stage of detailed design. In 1999, however, Rolls-Royce decided to change the strategy and focus of its programme, opting for a minimal risk ‘technology building block’ approach.

One of the main reasons for the strategic change was the fact that the company was on the point of launching a new product, the 601-K. Rolls-Royce felt that it should focus on one advanced product rather than two, and decided to address the market with a 601-K engine core to which it could infuse ATS technology. In addition, development of the 701-K would require significant investment.

“So we decided to change our focus to get away from delivering on the 701K product,” said Frank Macri, ATS programme manager at Rolls-Royce, “and instead use the lessons learned from that experience to look at developing various kinds of technologies – such as materials systems, or combustion or mechanical drive systems – that we could apply to our existing products.”

Since 1999, Rolls-Royce has therefore focused on developing technology building blocks and demonstrating them internally and at field test sites. The company is aiming to apply the demonstrated technologies to its 501-K fleet, totalling around 1500 units worldwide in stationary power applications, and its new 601-K product.

Advanced combustion

Rolls-Royce’s development of advanced, low emissions combustion technology has focused on the design of a lean pre-mix, single-digit NOx system for the 601-K, and more recently on integration with the 501-K product line.

Figure 2. Rolls-Royce believes that the 601-K will benefit from the ATS design enhancements
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The combustor design is a two-stage system where the primary stage is based on the LE4 system used on the Allison 501-K. Ten secondary premix tubes introduce the fuel flow radially into the combustor, which is designed for the 601-K DLE (dry low emissions) engine configuration.

This technology has undergone full-scale testing on a high pressure, high temperature combustion rig using a quarter sector 610-K engine arrangement, and initially Rolls-Royce was successful in achieving single digit NOx attainment up to 75 per cent power conditions, although it experienced problems with combustion noise. Rolls-Royce has since overcome this problem, and has demonstrated single digit NOx at 100 per cent power conditions.

Given the several hundred 501-K DLE units operating in the field, Rolls-Royce is keen to integrate this technology with the 501-K product line, and will then resume efforts to integrate it with the 601-K turbine.

An industry milestone

Rolls-Royce’s development of advanced materials systems has focused on two areas: the development of a ceramic first stage turbine vane; and the development of a metallic high pressure turbine vane.

Ceramic first stage vane: Together with Honeywell, Rolls-Royce has designed and fabricated a ceramic first stage vane composed of silicon nitride. During phase one, the uncoated vane underwent testing in a 501-K engine at an Exxon gas pumping facility in Alabama, USA, and ran for over 800 hours – an industry first.

Figure 3. ATS goals: Rolls-Royce initially achieved single digit NOx at 75 per cent power conditions with the lean pre-mix combustion technology and has more recently achieved this at 100 per cent power conditions
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Rolls-Royce discovered that during the test, significant oxidation and erosion occurred. This led the company to focus on the development of environmental barrier coatings (EBCs) in phase two.

Phase two was a collaborative effort between Rolls-Royce, Honeywell, Kyocera and the government. EBCs were applied to the ceramic vanes, and in April 2001 the system was re-tested at the Exxon facility. According to Macri, over 1200 operating hours have now been accumulated, and the overall goal is to reach 2500 hours. Macri expects that the next inspection will take place at 1850 hours in late July.

Although this testing represents a major step forward in the development of ceramic vane technology, more work is required, says Macri. The vanes need to be able to last for at least 10 000 hours to be competitive, and Rolls-Royce will reduce its focus in this area when the ATS programme finishes.

Metallic HP vane: The objective of this aspect of the programme was to develop and demonstrate an advanced metallic first stage turbine vane with enhanced durability thermal barrier coatings (TBCs). Rolls-Royce has successfully developed a vane cast on MAR-M247 with a platinum aluminide and yttrium TBC for the 501-K product.

Rolls-Royce fabricated two sets of vane hardware and has installed it at two end-user sites using 501-K engines: one in Ireland and one in Japan. The company tried to find sites where conditions would be challenging, according to Macri, and after 6000 hours of operation the vanes appear to be in “very good condition”.

Rolls-Royce has extracted six vanes from each of the field test engines, and will now carry out materials analysis to complete the ATS contract. The remaining vanes are still operating and will continue to accumulate operating hours over their life cycle.

During in-house testing, these vanes showed significant improvement in durability over the current 501-K production vane. During cyclic thermal testing in a burner rig, the new vane showed a 10-15 times improvement over the existing vane.

Mechanical technology

In order to improve durability and reduce engine cost, Rolls-Royce has also looked at design elements of the 601-K engine. It has simplified the bearing configuration and has also designed and evaluated a supercritical shaft. This new design was successfully tested in a 601-K11 engine in January 2001. The technology is now in production for the 601-K series industrial engine.

These design changes will allow easier assembly and balance of the rotor system, and eliminate the need for an intershaft bearing and sump arrangement.

This reduced parts count will result in lower capital costs for the 601-K, and cost studies have shown that this will result in a total engine cost reduction of up to five per cent for the 601-K.