Tonghae becomes Asia`s largest CFB

Tonghae Unit 2 reached full-load in May this year. Combined with the 200 MW Unit 1, this made the plant possibly the biggest circulating fluidized bed power station in operation in Asia.

While the Republic of Korea`s state-run electric utility, Korea Electric Power Corporation (Kepco), has now begun a restructuring programme that will ultimately lead to the sale of most of its non-nuclear generating assets, the completion of its newest coal-fired power plant – the 400 MW Tonghae circulating fluidized bed combustion (CFB) power station – remains right on schedule.

In May, Kepco announced Tonghae Unit 2 reached full-load, coal-only operation and joined its counterpart – the already operating Tonghae Unit 1 – as Asia`s largest and highest pressure CFB steam generator. Unit 2 was scheduled to reach full, commercial operation this month.

Located on Korea`s east coast, some 125 miles east of Seoul, the Tonghae plant`s CFBs were designed by Combustion Engineering, Inc., Windsor, Conn., USA, a unit of ABB Alstom Power. The components were manufactured locally by Korea Heavy Industries and Construction Co. Ltd. (Hanjung).

The twin CFBs provide Kepco with a power plant capable of: burning a variety of anthracites taken from several local mines and; a plant capable of operating at 50 percent nominal rating without support fuel (No. 2 fuel oil).

Since it began operating commercially last October, Tonghae Unit 1 has achieved lower-than-guarantee emissions (SOx: 150 ppm at 6 per cent O2; NOx: 250 ppm at 6 per cent O2) and has operated at near 100 per cent availability (other than during a planned outage in January, 1999). Since May, Tonghae Unit 2 has provided similar performance.

Under the restructuring programme, Kepco says it will allot 42 power generation facilities – including the Tonghae plant – into five power companies. The five companies are then expected to be offered to potential domestic and foreign buyers next year.

Regardless of who ultimately owns the facility, Kepco`s original bid specifications – which called for a cutting-edge CFB boiler design that would provide Tonghae power station with optimal fuel and operational flexibility – should enhance its economic standing in a competitive environment. After all, the new CFBs are designed to produce clean, economical electric power by burning local, rather than imported, coals.

Design considerations

In order to meet all of Kepco`s specifications, the Tonghae CFB design has several unique features. For example, the plant`s low-grade anthracite fuels (which differ slightly with regard to ash and volatile content, heating value and alkali content) are highly erosive due to the high silica concentration in the ash. As a result, a number of measures were taken to minimize erosion of the unit`s natural circulation waterwall-lined furnace, including:

Designing the furnace for an appropriate velocity. While most current CFB designs employ velocities of 6.1 m/s (20 ft/s) or more, the Tonghae units operate at a lower (proprietary) velocity rate. This lower velocity results in lower solids impact velocities and lower fluidization rates, both of which will significantly decrease the potential for furnace erosion.

Designing the furnace for optimum combustion. The Tonghae furnace features a rectangular footprint of 60 ft by 23 ft (18.3 m x 7.0 m) that incorporates an aspect ratio of more than 2:1. With the lower velocities and a maximum fuel mixing length of less than 25 ft (7.62m), this configuration enhances fuel mixing, fuel/limestone mixing, fuel burning efficiencies, and sulphur capture. Fuel is introduced at six locations along the front furnace wall. Limestone is injected with the fuel in the fuel feed chutes and is also introduced in two injection ports along the rear wall.

Designing a hard-face metal spray system to apply to the bare waterwall tubes at the lower furnace refractory interface to protect against erosion. While the material will have to be replaced over time, its application is a relatively simple, routine maintenance procedure that is much less expensive than replacing waterwall tubes. In addition, the lower section of the furnace waterwall has been covered with an erosion-resistant plastic refractory.

In a CFB, ash flows up the centre of the combustor and some returns, or slides, down the unit`s waterwalls. In first generation CFBs, when the ash reached the lower furnace refractory/waterwall interface, it created a flow “eddy” and gouged out the waterwall, resulting in tube leaks and unplanned downtime. At Tonghae, steps have been taken at this point to eliminate the refractory “shelf” or disturbance, minimizing the solids flow eddies and the subsequent tube erosion.

Due again to the erosive fuel, the Tonghae unit is provided with an external fluid bed heat exchanger arrangement, instead of a hanging or pendant heat exchange surface within the furnace. ABB Alstom CE has offered primary loop heating surface, or hanging surface, on some of its other CFB units, but none within potentially erosive furnaces like Tonghae.

Primary air is introduced to the furnace via the fluidization grate, which is composed of ABB Alstom CE`s patented T-style fluidized nozzles. These nozzles have relatively large openings to reduce the potential for plugging – a common problem with many nozzle designs and a potentially particular concern at Tonghae – while maintaining a pressure drop to preclude backsifting. These nozzles are also used in the unit`s seal pots and external heat exchangers.

Since the design coal or the range coal has an ash split of 50 per cent bottom and 50 per cent fly ash, the bottom ash is removed from the furnace via two ash control valves. A fluidized bed ash cooler contains economizer and cooling water heat transfer surface, which improves boiler efficiency by returning the heat to the furnace via four front-wall locations.

A newly developed three-layer refractory system lines the inside of the unit`s three steel plate cyclone separators and seal pots. The cyclones receive a flue gas and solids mixture from the furnace, remove the entrained particles from the gas, and send them, through the seal pots, to the heat exchangers. This refractory system – designed to better meet the refractory expansion/contraction requirements common to these CFB components – consists of an insulating layer, a firebrick layer, and an erosion resistant brick refractory located within a carbon steel shell.

Turndown capability

In addition to addressing fuel concerns, steps were also taken to enhance the unit`s turndown ability. The most important step involved the use of a particular arrangement (patent pending) of three external fluid bed heat exchangers (FBHE).

After the entrained solids/particles are removed from the gas stream in the cyclone separators, they are sent through the seal pots (via an ash control valve) to the heat exchangers. Each heat exchanger is a refractory-lined bubbling bed containing a single type of heat transfer surface – either a natural circulation evaporator, a superheater or reheater.

As the solids flow over and through the FBHE heat transfer surfaces, the ash is cooled and then returned to the furnace. Each surface is arranged within the refractory-lined boxes such that the solids flow perpendicular to the flow of the steam or water through the tubes. By placing superheat and reheat heat transfer surfaces in separate heat exchangers, and introducing the solids at a controlled rate, optimum turndown control is achieved.

In addition to optimum turndown control, marginal heat duties can be made up without altering the unit`s backpass performance or using costly desuperheating spray for either the superheater or reheater circuits. This arrangement virtually eliminates the need for desuperheating spray for the superheater and reheater circuits.

The convective backpass – which contains the first stages of superheater and reheater heat transfer surfaces – is steam-cooled as is typically done on ABB Alstom CE utility boiler designs.

The steam-cooled backpass walls are considered the first section of superheat surface while the final stages of superheater and reheater surface are in the FBHEs. The spray desuperheater is installed between superheater tube banks in the backpass. Since the backpass is wide (aspect ratio of approximately 3 to 1), two desuperheater stations are installed in parallel to minimize the possibilities of steam imbalance.

The lower section of the steam-cooled backpass contains the bare tube economizer surface, which accepts feedwater preheated in the fluid bed ash cooler economizer tube assemblies. The economizer tubes terminate in junction headers located just above the top economizer tube bank. Tubes from this junction header extend straight through the top of the convective backpass. These hanger tubes are used to support the superheater and reheater tube assemblies. Retractable sootblowers are located between each tube bank in the back pass.

Startup experience

After passing a number of Korean government tests, Unit 1 achieved commercial operation in October, 1998. Preliminary performance tests were conducted on the unit at the end of October, 1998. Several positive results were observed, including the ability to achieve lower-than-guarantee emissions (SOx: 150 ppm at 6 per cent O2; NOx: 250 ppm at 6 per cent O2, both at maximum boiler load) and power consumption values.

However, while commercial operation with the fine, low reactivity fuel was deemed acceptable, it was still not considered optimal. Due to the inherent fineness of the fuel, ABB Alstom CE, Kepco and Hanjung decided to make proprietary cyclone modifications to make the cyclone more efficient in capturing fine particles. Towards that end, an outage was taken during January 1999 to modify the cyclone inlet duct.

On restart, the modifications revealed beneficial results during preliminary operation. Furnace temperatures were more consistent from the lower furnace to the furnace outlet. Cyclone and seal pot temperatures had decreased dramatically from the previous operation due to the increased combustion within the furnace. Whereas additional combustion optimization will be made to decrease carbon in the fly ash and to conform to the guaranteed load change rate and unit startup time. The unit will undergo its first inspection and maintenance outage shortly.

Unit 2 construction was completed in February 1999. Initial turbine-generator synchronization was achieved in April, and the unit began firing on coal only with no support fuel for the first time in early May. The unit was shut down for turbine bearing inspection at the end of May, and then restarted in June.

July and August activities focused on preliminary testing and tuning of the boiler, turbine and BOP control system. Initial full-load testing and a 240-hour reliability test are both complete, with commercial operation scheduled for early October. Lessons learned from firing this very difficult fuel on Unit 1 eliminated potential cyclone pluggage problems greatly attributed to the success of the Unit 2 startup.

The Tonghae power station demonstrates the strength of the CFB design, which can efficiently burn a wide variety of fuels – from biomass and sludge, to natural gas and anthracite – while meeting ever-tightening emission standards around the globe.

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Figure 1. Tonghae Unit 1 began operation in October

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Figure 2. Tonghae is located on Korea`s east coast, roughly 125 miles from Seoul

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Figure 3. Cross-section of one of the Tonghae units

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Figure 4. The coal feed system is common for both units

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Figure 5. Hammer type fuel crushers are used to grind the coal