Koreans set a standard for supercritical systems

The technology of standardised 500 MWe class supercritical coal-fired power stations (with 246kg.cm-2/538°C/538°C steam conditions) is now very well established in Korea. Poryong 3-4 (the lead units), Poryong 5-6, Taean 1-5, Tangjin 1-4, Hadong 1-6 and Samchonpo 5-6 are in commercial operation. Taean 6 is due to go commercial in June 2002, while Tangjin 5-6 (steam conditions 246kg.cm-2/566°C/593°C) are under construction.

Building on this experience, a decision was taken some years ago to develop an 800 MWe class standardised supercritical pulverised-coal-fired plant (246kg.cm-2/566°C/566°C steam conditions). This was at a time of rapid growth in Korean electricity demand.

In the period May 1995-July 1998, KOPEC (Korea Power Engineering Company, Inc) carried out the following scope of services in support of this 800 MWe standardised design: conceptual design; basic design; system optimisation studies; and detail design.

Construction of the first of the 800 MWe units started in October 1999 on the island of Yonghungdo, off the west coast of Korea. The owner is KOSEPCO (Korea South East Power Co Ltd). KOPEC is architect/engineer, with Parsons Energy & Chemicals Group Inc in an advisory capacity, while Doosan is lead main equipment supplier.

Yonghung was originally envisaged as a six unit plant and it is expected that recovery from the Korean recession will give birth to two further units in the near future. Completion of Yonghung 1 is now scheduled for June 2004.

Among the key design features of these new 800 MWe units, which are expected to have a plant efficiency of 43.5 per cent, is use of supercritical pressure steam with variable pressure operation.

The plants employ single-reheat once-through boilers with European style HP/LP turbine bypass systems. These shorten start-up times and allow cyclic operation.

The boilers are of the indoor type, with two pass configuration.

The low load recirculation system includes separators, drain vessels and recirculation pump system.

The furnace water walls and superheater operate with variable pressure. Pressure adjustment is by means of the boiler feedwater pump, without boiler throttle valves.

Boiler auxiliaries include: 2 x 50 per cent axial forced draft fans; 2 x 50 per cent axial induced draft fans; 2 x 50 per cent centrifugal primary air fans; 2 x 50 per cent Ljungstrom tri-sector air preheaters; 2 x 50 per cent steam coil air preheaters; six coal silos; six gravimetric coal feeders; and six pulverisers.

The main steam turbine is a 3600 rpm 800 MW (nominal) tandem compound four-flow condensing type, with 40in titanium LSB. There are eight extractions for feedwater heating.

The generator is hydrogen cooled, with water cooled stator conductors.

A light oil system is used for boiler start up and flame stabilisation.

The seawater-cooled condenser is twin shell, single pressure, single pass per shell, with titanium tube material.

The eight stages of feedwater heating consist of: three stages of closed U-tube type horizontal high pressure heaters in two trains; one deaerating open heater (tray type); and four stages of closed U-tube type horizontal low pressure heaters in a single train.

The feedwater and condensate pumps include: 3 x 50 per cent vertical condensate and horizontal condensate booster pumps; 2 x 50 per cent auxiliary turbine driven boiler feed pumps and motor driven boiler feed booster pumps; and 1 x 30 per cent motor driven start up boiler feed pump.

The cooling water circulating pumps are 2 x 50 per cent vertical, variable pitch vane type.

The four electrohydraulically actuated HP turbine bypass valves are located in the boiler building, while the four LP turbine bypass valves (also electrohydraulically actuated) are located in the turbine building.

Emissions control

NOx reduction measures include two-stage combustion, low-NOx burners and selective catalytic reduction. As a result NOx emissions from the plant are expected to be less than 40 ppm.

A wet limestone-gypsum process is used for flue gas desulphurisation. SOx levels are expected to be less than 25 ppm.

Dust emissions will be controlled with high performance dry-type electrostatic precipitators, coupled with the dust removal effects of the desulphurisation equipment. Dust emissions are to be less than 20 mg/Nm3.

To reduce coal dust, the area round the coal yard will forested with 5 m high trees on the mound to act as a wind break. The coal will also be humidified with sprinklers. The ship unloaders are of the fully enclosed continuous type and conveyors are installed inside a dust-tight gallery.

A high-concentration slurry disposal system is used for handling residual fly ash, to minimise the size of the ash pond.

Plant arrangement

The turbine building has four levels, ground, mezzanine, operating and deaerator floor. The turbine-generator and the boiler feedwater pumps are located on the operating floor supported by pedestals. A single pass condenser housed within the turbine-generator pedestals sits on top of the reinforced concrete pedestal foundation mat on the sub ground floor.

All pulverisers and coal silos are located at the front of the boiler between the turbine and boiler rooms. Coal is brought in by ship and delivered to the active storage area from where it is carried by coal conveyor to the coal silos located in front of each boiler.

Each unit has a separate 200 m high reinforced concrete stack with clad steel inner flue receiving flue gases from the induced draft fans.

The second unit duplicates the first unit, employing a "slide along" arrangement.

Integrated control and monitoring

The central control room for units 1 and 2 is located on the turbine operating floor between the two units, with an operator console for each unit. A simulator is included.

A redundant microprocessor-based distributed digital control system provides integrated control and monitoring functions for boiler, turbine-generator and balance of plant such as ESP, CPP, chemical injection, FGD, cooling water and air compressor. The "state controller with observer" philosophy applied to the digital control increases the speed of response of the control adjustment, with maximum plant reliability and efficiency.

The system includes optimisation software for combustion control and steam temperature control, with a wide control range for steam temperature variation, plus or minus 8°C, with load change of 5 per cent nominal rating at above 50 per cent plant load.

The 345 kV switchyard is SF6 gas insulated to minimise land requirements and to provide protection from contamination due to salty sea air.

To save space and minimise cabling the electrical equipment control and monitoring system is CRT based instead of using a mosaic switching board.

Beyond Yonghung

Looking to the future, KOPEC has been developing a design for an ultrasupercritical coal-fired plant, with an efficiency of over 45 per cent. Design work is due to be completed in 2003, but future deployment of this technology will depend on the recovery of the Korean economy as well as on whether such a design meets the requirements of the new players in Korea's restructured and increasingly competitive energy market.