Is Mitsubishi’s J-Class just the job for 60 per cent efficiency?

Takashi Kishine of Mitsubishi Heavy Industries’ gas turbine division allows PEi readers a sneak preview of the Japanese firm’s new J-Class gas turbine, which is set to feature the world’s largest power generation capacity and highest thermal efficiency.

Takashi Kishine, Mitsubishi Heavy Industries, Japan

Concerns about global warming have generated controversy about carbon dioxide (CO2) emissions resulting from the use of hydrocarbon fuels. Compared to conventional power generation facilities based on coal or fuel oil, high efficiency combined-cycles, based on natural gas firing turbines, can reduce significantly CO2 emissions.

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Earlier this year Mitsubishi Heavy Industries (MHI) announced the introduction of a new gas turbine flame called the J-Series with a turbine inlet temperature level of 1600 à‚ºC. This new frame is expected to achieve higher combined-cycle efficiency and will contribute to reducing CO2 emissions, which are approximately 50 per cent lower than conventional coal fired power plants.

The new engine incorporates a high pressure ratio compressor based on the advanced M501H compressor, which was verified during the M501H development in 1999 and 2001; a steam-cooled combustor, which has accumulated extensive experience in the MHI G-series engine à‚— with more than 824 000 actual operating hours à‚— and state-of-the-art turbine designs developed through a Japanese National Project (see below) to develop a 1700 à‚°C gas turbine component technology.

The new technologies applied to the turbine section in the J-class engine include advanced thermal barrier coatings, enhanced cooling technology and turbine aerodynamics improvement. The target combined-cycle efficiency of the ‘J’ is well above 60 per cent and the one-on-one combined-cycle output will reach 460 MW for 60 Hz engines and 670 MW for 50Hz engines.

J-class Design Features

This new machine features the world’s largest power generation capacity and highest thermal efficiency. The ‘J’ was designed to operate at a turbine inlet temperature of 1600 à‚°C, which is 100 à‚°C higher than the predecessor the G class.

Increasing turbine inlet temperature is a key technology to achieve high combined-cycle efficiency but involves difficult technical challenges that should be overcome through exhaustive research and development efforts and verification testing.

The J-series will operate at an intermediate turbine inlet temperature between the current 1500 à‚ºC (i.e. the G-series gas turbine) technology and the 1700 à‚ºC turbine inlet temperature involved in the Japanese National Project. This approach allows early application of the technology developed for the 1700 à‚ºC turbine inlet temperature class and provides initial validation at a lower temperature exposure.

Target Performance for CCGTs

MHI normally introduces a new series of turbines for power generation every ten years. We expect the J-series to be the company’s main turbine for the next ten years. The company has demonstrated incremental improvements in efficiency with its G-class combined-cycle.

For instance, its G2 achieves more than 59 per cent efficiency in site condition at a Tokyo Electric Power Company plant. Our target for the J-class is to achieve well above 60 per cent efficiencies (lower heating value) in combined-cycle. The power generation capacity of the J combined-cycle will be about 1.2 times that of the G-class.

The high exhaust gas temperature of the J-series gas turbine is effectively used in combined-cycle application (Figure 1). Combined-cycles based on the J series gas turbines are expected to exceed the 60 per cent efficiency barrier which has limited existing combined-cycle technology.

Figure 1: A bird’s eye view of the new J-series combined-cycle (one-on-one)
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One of the biggest challenges posed by the J-class involves the combustor development. MHI is levering its long steam cooled combustor experience in the MHI ‘G’ gas turbine, where excellent durability has been achieved to realize higher turbine inlet temperatures.

The steam flow rate can be adjusted to maintain the metal temperature of combustor hardware. Improvements can be realized by MHI’s extensive steam-cooled combustion experience with the G-class. In addition, the state-of-the-art Japanese National Project technologies originally developed for a 1700 à‚ºC turbine inlet temperature gas turbine are used so that a smooth transition to this 100 à‚ºC higher design is achieved.

The Japanese National project

The Kyoto Protocol came into effect in February 2005 as part of a global effort to reduce global warming. Japan is committed to cutting by six per cent the total CO2 emission released by the country in1990 between 2008 to 2012. Under this ambitious target, it becomes fundamentally important to improve the thermal efficiency of gas turbines used in combined-cycle power plants.

Several new technologies applied in the ‘J’ are derived from a government supported project called the Japanese National Project.

Based on Japan’s Basic Act on Energy Policy, which was enacted in 2002, an energy plan was approved by the Japanese Cabinet in 2003. In response to this new plan, the Japanese Ministry of Economy, Trade and Industry began to promote the development of a high-efficiency gas turbine for power generation. This project pursued an aggressive efficiency target through the development of a 1700 à‚°C gas turbine.

The combined-cycle efficiencies for this ambitious project targets 62 per cent to 65 per cent (Figure 2). As part of this gas turbine project, core technologies are being developed and commercialized.

Figure 2: The Japanese National Project is developing a 1700 à‚°C gas turbine, with a combined-cycle efficiency target of between 62-65 per cent
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The component development phase of this government project took four years and was completed in 2007. During the first phase, MHI carried out several R&D efforts as key technologies for 1700 à‚°C gas turbines in the areas of exhaust gas recirculation combustor for lower emissions; higher turbine cooling efficiency; advanced thermal barrier coating; higher pressure ratio compressor; advanced heat resistant turbine materials and turbine aerodynamics.

Higher Turbine Cooling Technology

Gas turbine blades in the 1700 à‚ºC class are exposed to high heat load and thermal stress. High performance cooling schemes with small amounts of cooling air are required in the cooled turbine blades to prevent damage without significant penalizing thermal efficiency.Therefore, a hybrid cooling system combining closed-circuit steam cooling and air cooling is considered.

In this study, newly developed cooling methods, which are high performance film cooling, a 180à‚°-turn structure with low-pressure drop, semi-transpiration cooling and high performance turbulence promoter, are investigated experimentally.

Several types of shape of film cooling hole were considered and manufactured experimentally, and detailed film cooling effectiveness was measured with an infrared camera and laser-induced fluorescence. The film cooling effectiveness of the shaped hole with a rib was approximately 25 per cent higher than that of the shaped hole.

Advanced Thermal Barrier Coating Technology

The application of advanced thermal barrier coating is essential for high temperature gas turbines. In general, the cooled metal parts such as turbine blade are first coated with MCrAlY (M:Alloy elements such as Co, Ni, CoNi) as a bond coat material, which has superior oxidation resistance, and then coated with ZrO2 type ceramics (YSZ à‚— yttria (partially)-stabilized zirconia) as a top coat, which has low thermal conductivity.

As a result, thermal barrier coating has been used in recent high temperature gas turbines to reduce metal surface temperature of cooled metal parts. Based on the evaluation results of the short-term properties of the thermal barrier coating, it was confirmed that new developed topcoat materials had about 20 per cent lower thermal conductivity than conventional YSZ, with the same durability. Further, long time durability characteristics are also under evaluation. Once developed, the same technology can be applied effectively to the lower turbine inlet temperature of J-series.

The F-series and G-series gas turbines have undergone several upgrades for better performance and durability. The upgrading process continues even after new frames are developed because some of the new designs can be retrofitted to preceding frames. To date, MHI’s F-series and G-series gas turbines have accumulated over 5.2 million and 824 000 actual operating hours, respectively.

During the upgrading process, the basic structural design remains unchanged from previous MHI large industrial gas turbines, e.g. cold-end drive, two-bearings support, cannular type combustor and four-stage turbines to keep reliability high. The J-series gas turbine has been developed with the same philosophy based on the verified G-series gas turbine technologies, especially in regards to the steam cooled type combustor.

One approach which can be used to improve combined-cycle efficiency is increasing the turbine inlet temperature. However, higher temperatures induce increased nitric oxides (NOx) emissions. In an air cooled combustor, cooling air flows into the combustor inducing a reduction in combustion gas temperature. As an example, in case of a flame temperature of 1500 à‚°C or 1600 à‚°C the turbine inlet temperature would be 100 à‚°C lower.

By applying closed loop steam cooling from the bottoming cycle the inlet temperature is not diluted and therefore remains high. NOx is directly related with the flame temperature, while the combined-cycle performance is affected by the turbine inlet temperature, therefore the steam cooled combustor provides a cycle efficiency improvement with the same NOx emissions.

Higher pressure ratio

The J-series pressure ratio is higher than in the G-series (21:1). Therefore, the J-series compressor is based on MHI’s H-class compressor that has a pressure ratio of 25:1. Levering the extensive steam cooling experience accumulated in the 64-unit MHI G fleet, the combustion system in the J-design is similar to the G-design.

Redundant steam supply sources are provided to maintain a steam cooling back-up source at all stages of operation. The steam cooling application will help maintain the NOx emission within acceptable levels. And units in its fleet with close to 70 000 actual operating hours and 1000 start cycles confirm that the integration can be designed reliably. MHI’s G-class operating fleet has grown to 37 units (64 units sold) and, as of July 2009, has accumulated more than 824 000 actual operating hours and 9200 starts.

Restriction in CO2 emissions are generating controversy around the use of hydrocarbon fuels. Compared with conventional coal plants, high efficiency natural gas fired combined-cycle power plants can reduce CO2 emissions by 50 per cent, therefore high efficiency benefit goes far beyond energy conservation.

MHI has introduced upgrade to existing gas turbine frames targeting higher efficiency and durability, and new developments are pursued to produce a leap-change in combined-cycle efficiency. With a combined-cycle efficiency in excess of 60 per cent, the J-series will achieve lower carbon emissions and contribute to worldwide efforts to reduce global warming. The first M501J (60Hz) will be ready to operate in 2011, with the M701J (50Hz) scheduled for assemble in 2014.

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