22-01-2013, 11:41 AM
Supercritical Boiler Technology Matures
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Introduction
The Hitachi-Naka Thermal Power Station Unit No.1 of the Tokyo Electric Power
Company (TEPCO), whose “Benson” type boiler was designed and built by
Babcock-Hitachi K. K. (BHK is the latest supercritical coal-fired utility plant to commence
commercial operation in Japan. State-of-the-art technologies such as high pressure, high
temperature steam parameters of 3680 psig 1120ºF/1115ºF (604ºC/602ºC) and Hitachi’s
advanced burner system for low NOx combustion were integrated into the new design.
Flexible sliding pressure operation, advanced steam temperature control methods, and
sophisticated computer control technologies make this unit an ideal plant for load
demand following applications. The sliding pressure supercritical Benson boiler
technology has been fully established and has markedly surpassed drum type boilers in
the areas of efficiency, flexibility in operation and availability, as proven by over10 years
operating experience in Japan.
Latest experience in supercritical boiler
Outline of Hitachi-Naka No.1
The Hitachi-Naka Thermal Power Plant Unit 1 (1,000MW), which commenced
commercial operation from December 2003, is the latest supercritical unit to be
placed in operation in Japan. The plant is located in Ibaraki prefecture in Japan,
approximately 60 miles from Tokyo city. The engineering and construction of the
boiler, turbine and generator (BTG) power island was managed by Hitachi Ltd. The
supercritical sliding-pressure Benson boiler was engineered and manufactured by
Babcock-Hitachi K.K. (BHK), a subsidiary company of Hitachi Ltd group.
At the plant rated load, the boiler can supply the turbine generator with 6,327,000
lb/h of steam at supercritical steam conditions. The main steam parameters at the
turbine inlet are 3,550 psig and 1,112ºF, and the reheat steam temperature is 1,112
ºF. The plant was designed for load cycling operation, to follow the changing load
demands throughout the operation day.
Combustion performance
The Hitachi-Naka No.1 boiler is the first unit equipped with the newly developed
Hitachi NR-3 burner. The NR-3 burner is the latest design in the NR series of rapid
ignition low-NOx pulverized coal burners for large-scale commercial plants. The
results of combustion testing are summarized in Figure 3.
Two types of coals, type A coal from Indonesia and type B from Australia, were
tested during the commissioning phase. For both the coals, combustion tests were
made with varying combustion settings and adjustments including air flow ratios. As
shown in Figure 3, a reduction in outlet NOx emission led to less complete
combustion of the fuel, resulting in a higher unburned carbon (UBC) level in the fly
ash. As type B coal has a high fuel ratio (i. e. ratio of fixed carbon to volatile matter,
2.0) and high nitrogen content, it is normally difficult to achieve low levels of both
NOx and UBC in the fly ash simultaneously. However, when burning type B coal, the
observed NOx emission and UBC were much lower than the design point In the
case of type A coal, which has lower fuel ratio and nitrogen content, significantly
lower NOx and UBC emissions levels were measured.
Sliding pressure operation
As the nuclear power has become the primary source for base load generation in
Japan, coal-fired power plant equipment suppliers were challenged to design new
supercritical coal-fired units with flexibility for frequent load cycling. By adopting the
sliding pressure operation with lower boiler pressures at partial loads, the plant heat
rate can be improved at partial loads due to 1) improvement of high pressure (HP)
turbine efficiency, 2) reduced auxiliary power consumption by boiler feed pumps,
and 3) higher steam temperature at the HP turbine outlet. In addition to the plant
efficiency advantages, there are other benefits such as reduction in start-up time,
increase in ramp rate and reduced erosion of bypass valves as described in Section
3 of this paper.
Spiral Waterwall
For sliding pressure boilers, maintaining uniform fluid conditions during low load /
low pressure operation becomes critical to reduce the potential of tube damage
caused by high metal temperatures. The lower part of the Hitachi-Naka boiler
furnace is arranged in a spiral configuration such that the fluid path wraps around
the boiler as it travels up the furnace. A comparison of fluid temperature distribution
between the conventional vertical wall and the spiral waterwall is shown in Figure 5.
As a result of the uniform waterwall fluid temperature profile that is achieved across
the full range of boiler loads, the spiral waterwall system does not require any flow
adjusting devices to be installed at the furnace inlet
Steam Separator
As the Hitachi-Naka boiler is a Benson type unit, a steam separator and a
separator drain tank were installed to separate the steam and the water at the
furnace outlet during a low-load recirculation operation. This design is different from
that of a conventional NC boiler, for which a steam drum is installed to separate the
water from the steam under all operating loads. The steam drum is designed to
have sufficient water storage capacity, and usually contains complicated internal
parts, such as steam cyclones, scrubbers, internal feed pipes, and baffles. Because
of the complex internals, steam drums require a large amount of maintenance work
during outage periods. However, the steam separator design of a Benson boiler is
simple in configuration and has no internal, therefore significantly less maintenance
work is required.
Boiler start-up systems
The Hitachi-Naka Boiler includes fully automatic start-up systems such as the
turbine bypass system and the low load recirculation system. The turbine bypass
system was designed to minimize the start-up time by controlling the main steam
pressure and temperature before turbine rolling, and enabling the steam to flow
through the superheater sections at a short time after light-off. The low-load
recirculation system was designed to recover residual heat during start-up by
circulation of the un-evaporated water from the furnace back to the economizer inlet,
which also can assist in reducing start-up time. As this system is automatically
operated, the start-up process is as simple as with a natural circulation (NC) boiler.
Table 2 shows a comparison of the start-up systems betweem a NC boiler, a
constant pressure once-through boiler and a sliding pressure Benson type boiler
(the Hitachi-Naka boiler).