Gas turbine development emphasizes improved efficiency

Recent moves toward larger and more efficient gas turbines, coupled with lower emissions, have led to feverish activity from manufacturers

By Kevin Dodman, European Editor

During 1995, Siemens announced its series “3A” gas turbines, capable of combined-cycle efficiencies up to 58.5 percent. GE then unveiled its “H” technology turbines and claimed to break the 60-percent efficiency barrier for the first time. Meanwhile, several manufacturers have been working on emissions reduction, including Rolls Royce with its staged-combustion dry low emissions (DLE) combustor on an industrial RB211 and on the new industrial Trent.

POWER-GEN Americas(TM) gave both Siemens and ABB the chance to propose solutions that might make their machines cost-competitive when factors apart from combined-cycle efficiency are taken into account. Their responses highlight the benefit to be gained from being the first to reach the previously elusive 60-percent target. Even though the GE machine is several years from commercial operation–according to the company, the first 9H gas turbine will be tested next year–GE`s rivals have been prompted to respond.

Lower emissions

As we went to press, the Rolls Royce industrial Trent, a 50-MW three-spool variant of the Trent 800 aero engine, had just reached 40-MW output on the test bed in Canada. This unit has a thermal efficiency of approximately 42 percent and uses the company`s latest DLE combustion system proven on the smaller industrial RB211.

The first industrial Trent is expected to enter service later this year in a paper mill cogeneration application in Canada. Meanwhile, the industrial RB211, known as the Coberra 6000, has been operating with the DLE system since 1994 at the Pacific Gas Transmission Station 7 in Washington, USA, where it has consistently demonstrated 25 vppm NOx and CO emissions over a wide operating range.

Describing the development of its pre-mix, lean-burn DLE system, Rolls Royce commented, “It is generally agreed that the most practical method of limiting the production of NOx and CO is by controlling the combustion temperature to around 1,800 K. It can be seen that there is a narrow temperature band over which both NOx and CO are at a minimum 1.” This is shown in Figure 1.

The company developed a series-staged, pre-mix, lean-burn arrangement that is able to achieve low emissions over a wide operating range without using mechanical air controls or bleeds.

One drawback of a series-staged combustor is its length, which demands an increase in film cooling air. However, the cooling air used in the upstream stage becomes combustion air downstream so this effect is minimized, and only the air used for film cooling of the final stage cannot be re-used in the pre-mix stage.

Figures 2 and 3 show the levels of NOx and CO achieved by the DLE system fitted to an industrial RB211 at a range of power outputs. For the industrial Trent, the target emission levels are 9 vppm NOx and 10 vppm CO. This will be a dual-fuel system, with water injection used initially for NOx reduction when burning liquid fuel. However, longer-term developments aim to eliminate water injection completely.

Reference plants

At POWER-GEN Americas(TM), both Westinghouse2 and Siemens3 described combined-cycle gas turbine reference plant concepts. According to Westinghouse, the reference plant approach focuses on customer flexibility while maximizing the use of proven designs. It allows basic system designs to be modified in a modular fashion to meet the differing demands of both utility and non-utility customers.

The company stressed that the concept is not a “standard plant” approach, which would be to adapt a fixed design to different applications in the hope of obtaining replication advantages. Rather, it focuses on identifying a common core to which application-specific elements are added.

Westinghouse said, “The design is configured so that approximately 60 percent of the base plant can be replicated from project to project. This percentage consists primarily of the power block equipment such as the steam turbine, combustion turbine and heat recovery steam generator or boiler. Approximately 15 percent of the design will need to be modified for each plant. This 15 percent typically includes the plant cooling system, the power electrical system and the major plant vessels. The remaining 25 percent of the design is unique to each site. These items usually include the utility intertie, the below-ground foundations, the yard piping and electrical design, the boiler makeup water-treatment system and the plant wastewater treatment system.”

Lothar Balling of Siemens also described the development of customer-oriented combined-cycle concepts. In his paper, he said there has been a significant increase in the proportion of purely commercially oriented developers due to the liberalization of power generating markets over recent years3.

“The more traditional utilities are likewise re-orienting themselves and are viewing power generation with much more of an eye to cost than in the past because of the pressure of competition. Thus, the analysis shows that in the year 2000, the largest group of customers–amounting to around 35 percent–will also accept standard concepts for plants and components that have been optimized on the basis of cost-benefit analysis,” he said. “Whereas yesterday`s power plants were closely tailored to individual customers` requirements, the power plant concepts that will be wanted tomorrow will be those featuring minimal power-generating costs and developed solely according to economic aspects.”

He commented that the increasing pressure to build plants quickly has also led to restructuring within Siemens` manufacturing operation, characterized by closer relationships with outside suppliers, thus enabling the company to achieve much shorter delivery times. Like Westinghouse, Balling stressed that the latest concepts are not simply standard plants held in stock and available for delivery at short notice. A proportion of each plant is project-specific, so competitive costs and delivery times depend on rapid supplier responses.

Regarding costs, Balling`s paper explained that for a combined-cycle gas turbine plant with a net efficiency of 57.2 percent and operating for 6,000 hours/year with a fuel price of (US)$4/GJ, 17 percent of the cost is financing, 13 percent is fixed operating costs and about 70 percent is fuel cost. The decline in capital and maintenance costs, plus the rise in efficiency between 1992 and 1995, led to a drop of about 15 percent in power generation costs, although the relative proportions of the cost elements remained about the same.

Projects under way

At POWER-GEN Americas, Siemens announced that it had secured orders for all three models in the 3A gas turbine series. Construction is under way on Tapada do Outeiro, Portugal`s first combined-cycle plant, which will comprise three 330-MW blocks, each with a 240-MW V94.3A gas turbine. This plant is being built as a build-operate-transfer project with Siemens as consortium leader. British utility PowerGen is the project`s principal partner, with 59-percent ownership. The first block is scheduled to go on-line in March 1998, the final one in May 1999.

Two other V94.3A units will be used at National Power`s Didcot plant in the UK, where commercial operation is scheduled to start in October. Another unit has been ordered by Energieversorgung Nieder-osterreich AG (EVN) of Austria for its Theiss 455-MW combined-cycle plant near Krems.

In the USA, Kansas City Power & Light was the first customer for a V84.3A, which is scheduled for shipment in May to the utility`s Hawthorne plant. In Europe, the first order for a 70-MW V64.3A was placed by Neckarwerke of Essinglen, Germany, for its Altbach-Deizisau facility. According to Siemens, Neckarwerke will use this unit alongside a 350-MW steam turbine in the first parallel-powered combined-cycle plant. Commercial operation is scheduled for June 1997.

High marginal efficiency

The concept of parallel powering has been taken up by ABB, who suggests that it can lead to efficiencies of more than 60 percent. Representatives explained that a gas turbine and heat-recovery steam generator can be installed in parallel to a conventional steam power plant to give greater fuel flexibility, high efficiency and high availability4.

ABB calls the concept High Efficiency-Coal and Gas (HE-C&G) and claims that net efficiencies of more than 60 percent on the gas burned in the gas turbine can be achieved at an investment cost of less than (US)$350/kW for the additional power.

Repowering conventional steam power plants with gas turbines is not a new concept, of course. Two common approaches include hot wind box repowering, where gas turbine exhaust gases become combustion air for the fired boiler, and condensate and feedwater preheating, where a gas turbine and heat recovery economizers preheat the condensate and feedwater of a conventional steam power plant.

The HE-C&G concept differs from these approaches in that a gas turbine and heat recovery steam generator (HRSG) are added to the conventional plant, with the HRSG generating additional live steam from a portion of the feedwater. The remaining exhaust gas energy is used to preheat part of the condensate and the feedwater. A bypass in the reheat loop of the steam turbine allows operation of the steam turbine while the conventional boiler is out of service.

The concept could have a number of advantages. In addition to flexibility with regard to main boiler fuel and operation modes, the marginal efficiency of fuel burned in the gas turbine can exceed 60 percent because advantage is taken of the high live-steam conditions and the optimized cold end of the steam turbine plant. Such a system should have high reliability because outage of the conventional boiler or the gas turbine results in only a partial plant outage. Furthermore, HE-C&G may be attractive to users considering conversion or refurbishment of old steam power plants because it looks like an efficient and cost-effective way of adding capacity. It has the additional advantages of short downtime and the ability to continue burning the existing fuel. Also, the technology is available now.

Considered in the context of GE`s announcement of 60-percent combined-cycle efficiency for its H-technology machines, the HE-C&G concept could be considered a useful response, as it combines the achievement of 60-percent efficiency (albeit marginal efficiency of fuel burned in the gas turbine) with immediate availability.

It is likely that a more robust response will be needed however, ideally along the lines of 60-percent combined-cycle efficiency combined with lower life-cycle costs. ABB is currently in a good position to achieve that goal, with the GT24 and GT26 units already capable of 58.5 percent efficiency. It also seems likely that the sequential combustion technique used on those units has yet to reach its development limits.

The first GT24, at Jersey Central Power & Light Company`s Gilbert Station in the USA, is currently undergoing commissioning. After the turbine is commissioned, it will be interesting to see what comes next in the gas turbine development race. The GT24 and GT26 were announced in 1993, whereas the Siemens and GE machines are barely a year old. ABB could be the company to watch for further developments.

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Kansas City Power & Light was the first customer for Siemens` 60-Hz, 170-MW class V84.3A gas turbine.

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1 “Development of low emission combustion system for high efficiency gas turbines,” D.A. Owen, Chief Engineer, Dry Low Emissions and New Projects, Rolls Royce Industrial and Marine Gas Turbines.

2 “Combined cycle design flexibility in today`s market,” W. Nick DeRidder and Steven J. Knott, Westinghouse Electric Corporation, POWER-GEN `95 Americas, Anaheim, Calif., 5-7 December 1995.

3 “Development of Customer-Oriented, State-of-the-Art Single-Shaft Combined-Cycle Power plant Concepts for 50 and 60 Hz Applications,” L. Balling, H. Schutz, E. Wolt, Siemens AG, Power Generation Group, POWER-GEN `95 Americas, Anaheim, Calif., 5-7 December 1995.

4 “More than 60% Efficiency by Combining Advanced Gas Turbines and Conventional Steam Power Plants,” Rolf Bachmann, Mircea Fetescu and Henrik Nielsen, ABB Power Generation Ltd., POWER-GEN `95 Americas, Anaheim, Calif., 5-7 December 1995.