Tangjin 5 and 6: Korea's first ultrasupercritical units

5 October 2002

Two 500 MWe units under construction 75 km from Seoul will have steam conditions of 3500 psi/1050°F/1100°F (242 bar/566°C/593°C), setting the standard for ultrasupercritical coal-fired plants in Korea and elsewhere. Thomas M Logan, GE Power Systems, Schenectady, NY, USA and Un-Hak Nah, Doosan Heavy Industries, Changwon, Gyeongnam, Korea

Doosan Heavy Industries & Construction Co Ltd. has started site work on units 5 and 6 of the Tangjin thermal power plant, a milestone project that will feature the first two GE-designed ultrasupercritical (USC) steam turbines to be co-manufactured by Doosan & GE Power Systems and the first two USC units to be installed in Korea. This project is an expansion of an existing coal-fired power plant (Figure 1) located about 75 km southwest of Seoul. Tangjin 5 and 6 are rated at over 500 MWe each, with unit 5 scheduled to enter operation by the end of 2005 and unit 6 to follow in mid-2006.

The Tangjin facility is owned by Korea East West Power Co Ltd, an affiliate of the Korea Electric Power Co (KEPCO). The expansion project is among the first to be implemented since the restructuring of KEPCO as part of the privatisation of Korea's government-run electric power industry, the goal of the privatisation effort being to increase the efficiency of the country's power plants through open competition.

KEPCO subsidiary KOPEC (Korea Power Engineering Company) is architect-engineer.

Improved efficiency was a key factor in the selection of GE USC steam turbine technology for Tangjin. The technology offers improved cycle efficiency due to the increased temperature and pressure at which steam is admitted to the turbine. The Tangjin units will operate at steam conditions of 3500 psi/1050°F/1100°F (242 bar/566°C/ 593°C).

In addition, Tangjin 5 and 6 will feature the latest once-through USC steam generator technology, also to be supplied by Doosan. These steam generators will have improved heat efficiency to further increase the plant's capability to compete in the new Korean power market environment.

The GE-Doosan relationship

In early 2002, Doosan won a turnkey contract for the Tangjin expansion, including equipment supply, mechanical and electrical engineering, construction and coal handling. In addition, this project marks the first time Doosan will carry out civil engineering and architectural work for a power project, and is a major step towards the company's goal to be recognised as a world-leading, turnkey builder of power plants.

GE will supply the design and basic engineering for the two steam turbines, along with manufacture of critical turbine components including the combined high pressure/intermediate pressure bucketed rotor, last stage buckets, generator excitation system and Mark VI steam turbine generator and control systems. Doosan will complete the manufacture and assembly of the steam turbines and ship them to the project site.

Tangjin is the latest in a long series of projects jointly developed and implemented by GE and Doosan. For 25 years, GE has had a licensing agreement with Doosan, formerly known as Korea Heavy Industries Company (KHIC), to develop steam turbine technology for Korea. During this period, the two companies have designed and developed 16.1 GW of coal-fired steam turbine generating capacity, 2.4 GW of combined cycle steam turbine generating capacity and 10.4 GW of nuclear steam turbine generating capacity.

To date, there are 70 GE-technology steam turbines ordered or installed for projects in Korea, including combined-cycle, nuclear, subcritical coal-fired and supercritical coal-fired steam turbine applications. Fifty-one of these steam turbines were co-produced with Doosan.

Advanced technology

The USC steam turbines to be supplied by Doosan & GEPS for Tangjin represent the most advanced steam turbine design yet manufactured for Korea. The machines are rated at 518.8 MW; however at valves wide open (VWO) the turbines will generate 550 MW.

The steam turbine is a single reheat design, consisting of a combined, opposed flow high pressure (HP) / intermediate pressure (IP) section with a four flow low-pressure section that uses GE's 40-inch titanium last stage bucket (LSB) (not GE's 40-inch steel LSB that will be used in future USC steam turbines to improve efficiency and reduce cost). The low pressure section will use integral cover buckets on the first three stages and will have an aerodynamically optimised steam path.

The fundamental steam path design for the HP/IP section will employ GE's Dense Pack technology, which incorporates the following improvements over prior GE designs:

• 360-degree nozzle box;

• increased number of stages;

• reduced wheel spacing;

• decreased inner ring diameters;

• increased reaction levels;

• integral cover buckets (ICB);

• advanced tip seals and root radial spill strips;

• optimised steam balance holes;

• customised bucket vane profiles;

•larger pitch-to-width ratios;

• smooth wall contour.

The steam turbine will also feature solid particle erosion (SPE) protection for the first and second HP and IP stages. SPE is a primary cause of steam path efficiency degradation in units with high temperature stages. More than two decades ago, GE launched a multi-faceted programme to find cost-effective solutions to this industry problem, which resulted in the development of a SPE-resistant control stage nozzle. It was first applied to two 650 MW super critical, double reheat steam turbines with well-established histories of severe control stage SPE.

The technology has been confirmed on numerous supercritical units since that first application and today, the GE SPE-resistant control stage nozzle has become standard equipment on new units.

In the past, material and corrosion problems with steam generators have limited the application of ultrasupercritical technology. Today, however, improved steam generator technology has overcome these problems.

Evolution of USC technology

The history of steam turbine development is basically an evolutionary advancement towards greater power density and efficiency.

Improvements in the power density of steam turbines have been driven largely by the development of improved rotor and bucket alloys capable of sustaining higher stresses and enabling the construction of longer last stage buckets for increased exhaust area per exhaust flow. (GE and Toshiba, for example, recently introduced 48-inch steel, last-stage buckets which, in terms of annulus area, are the largest buckets of their type in the world. These will be covered in a subsequent article.)

Increases in efficiency have been achieved largely through two kinds of advancements:

• improving mechanical efficiency by reducing aerodynamic and leakage losses as the steam expands through the turbine; and

• improving thermodynamic efficiency by increasing the temperature and pressure at which heat is added to the power cycle.

The latter improvement is the core of USC technology.

Efforts to increase the efficiency of the Rankine cycle by raising steam pressures and temperatures are not new. Steam turbines produced early in the 20th century were designed for inlet pressures and temperatures of approximately 200 psi and 500°F (13.7 bar and 260°C) respectively. As average unit size increased, main steam temperatures and pressures also increased. The 1950s was a period of rapid growth in average power plant size. During this period, the reheat cycle became well established commercially and maximum steam conditions were raised.

This effort led to several large capacity, cross-compound units entering service in 1960 with modest, but for the times challenging, steam conditions. By 1969, a simpler tandem-compound double reheat design entered service, combining 3500 psi, 1000°F (242 bar, 538° C) high pressure and 1025°F (552°C) first reheat turbine sections into a single opposed flow casing.

In addition to units with double reheat design, during the 1960s and 1970s GE placed into service numerous supercritical units with single reheat and nominal steam conditions of 3500 psi, 1000°F (241 bar, 538°C).

The combination of experience with single and double reheat units, along with the knowledge gained about advanced steam condition designs, served as the basis for several Electric Power Research Institute studies in the 1980s. These studies centred on double reheat turbines designed for operation at the advanced steam conditions of 4500 psi, 1100°F/ 1100°F/ 1100°F (310 bar, 593°C/593°C/593°C).

Such advanced designs have been offered for a number of years and while there has been limited interest in the US in advanced steam conditions, countries in Asia and northern Europe have pursued this option.

GE and Doosan, for example, worked on two supercritical projects in Korea, shipping two units with 3500 psi/1000°F/1000°F (242 bar/538°C/538°C) steam conditions to Poryong in 1990 and two units with 3500 psi/1050°F/1050°F (242 bar/566°C/566°C) steam conditions to Yong Hung in 2001 and 2002.

While Tangjin 5 and 6 will be the first USC installation in Korea, USC units also are being considered for a future expansion of the Yong Hung site.

Optimising the steam cycle

As the first step in optimising cycle steam conditions, the potential cycle efficiency gain from elevating steam pressures and temperatures must be considered. Starting with the traditional 2400 psi/1000°F (165 bar/538°C) single reheat cycle, dramatic improvements in power plant performance can be achieved by raising inlet steam conditions to levels up to 4500 psi (310 bar) and temperatures to levels in excess of 1112° F (600°C). Figure 3 illustrates the relative heat rate gain for a variety of main steam and reheat steam conditions for single reheat units compared to the base cycle.

To maximise the heat rate gain possible with USC steam conditions, the feedwater heater arrangement also needs to be optimised. In general, the selection of higher steam conditions will result in additional feedwater heaters and an economically optimal, higher final feedwater temperature. In many cases, the selection of a heater above the reheat point (HARP) also is warranted. The use of a separate de-superheater ahead of the top heater for units with a HARP can result in additional gains in unit performance.

For supercritical and ultrasupercritical reheat units, eight feedwater heaters will be used, resulting in a typical heat rate benefit of 0.6 per cent.

The selection of the cold reheat pressure is an integral part of any power plant optimisation, but becomes even more important for plants with advanced steam conditions. Comparing the heat rate at the thermodynamic optimum, the improvement resulting from the use of a HARP can be about 0.6 per cent. However, economic considerations of the steam generator design without a HARP tend to favour a lower reheater pressure at the expense of a slight decrease in cycle performance. The resulting net heat rate gain is usually larger, approaching 0.6-0.7 per cent.

The use of advanced reheat steam conditions strongly affects the inlet temperature to the low-pressure turbine section. An increase in hot reheat temperature translates into an almost equal increase in crossover temperature for a given crossover pressure. However, the maximum allowable low-pressure inlet temperature is limited by material considerations associated with the rotor, crossover and hood stationary components.

Two basic parameters can be varied to adjust the low-pressure inlet temperature for a given hot reheat temperature: reheater pressure and crossover pressure. There is a direct correlation between reheat pressure and unit performance. Since the use of a HARP is likely to be the economic choice for most USC cycles, the reheater pressure will be lower to maximise the heat rate gain from the HARP. However, this will result in increased crossover temperatures. This can be offset by lowering the crossover pressure by an equivalent pressure ratio.

Steam turbine configurations

The appropriate steam turbine configuration for a given USC application is largely a function of the number of reheats selected, the unit rating, the site backpressure characteristics and any special requirements such as district heating extractions.

Available configurations for single-reheat applications are shown in Figure 4.

The use of a combined high pressure / low pressure section such as the one selected for Tangjin allows a smaller overall power island, which leads to savings in turbine building, foundation and maintenance costs. Supercritical units with this type of construction have operated successfully at ratings above 600 MW for many years.

Improved materials

The design of high temperature steam turbines is strongly influenced by the development of improved materials and by the use of more effective cooling steam arrangements for critical components.

GE has extensive experience with two rotor alloy steels in high-pressure rotor applications: CrMoV and 12 CrMoVCbN. The 12Cr steel is generally used when a higher rupture strength is required at elevated temperatures. Sixty-three rotors have been built with 12Cr forgings, and have successfully operated in some of the most challenging applications in units rated between 500 and 1000 MW.

Buckets for early high pressure and reheat stages of steam turbines must have good high temperature strength and low thermal expansion to minimise thermal stress. For USC applications, GE has developed a 10CrMoVCbN bucket alloy which possesses a rupture strength nearly 50 per cent higher at 1050°F (566° C) than the AISI 422 alloy traditionally used in applications up to those temperature levels.

Low alloy CrMoV materials generally suitable for stationary components in turbines designed for conventional steam conditions are not suitable for the higher temperature regions of USC turbines. GE developed high strength martensitic stainless steel casting alloys (10CrMoVCb) for valve bodies and nozzle boxes in applications with 1050°F (566°C) and 1100°F (593°C) inlet temperatures.

The primary low pressure section design issue associated with USC turbines is the elevated crossover temperature frequently encountered with these power cycles. Conventional NiCrMoV rotor materials have a tendency to embrittle at low pressure bowl temperatures above 660-710°F (350-375°C). Recent advances in chemistry, combined with other enhancements such as raising the nickel content and gashing between the wheels prior to quenching, have resulted in rotor forgings with superior fracture toughness and tensile ductility properties.

This improvement provides additional freedom to optimise the cycle parameters to achieve higher efficiency levels without performance losses associated with previously used cooling schemes.

Advanced steam path design

Using recent advances in computational fluid dynamics (CFD), turbine components today can be optimised for reduced flow losses. The performance of steam path components such as nozzles, buckets and seals has been significantly enhanced as a result of applying this new technology.

In addition to the steam path improvements, performance gains also can be achieved by optimising stationary components such as valves, inlets and exhausts, using the same CFD tools.

All USC designs today, including, of course, Tangjin, incorporate these CFD-based design enhancements.

Setting the standard

Increased fuel costs, improved technology and a heightened focus on reducing power plant emissions have combined to revitalise power industry interest in coal-fired power plants using USC steam conditions. To achieve an economically optimised plant, cycle conditions need to be carefully evaluated, taking into account such parameters as the number of reheats employed, inlet steam conditions and feedwater heater arrangement.

A variety of steam turbine configurations for USC applications are available. Each of these configurations utilises materials and design features to ensure long turbine life with reliability levels comparable to conventional designs.

The Tangjin 5 and 6 project will set the standard for future USC applications in Korea, as well as elsewhere in the world.

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