POWER May 2010 - 98

Boiler design
6. Optimized tube selection. The
Posiflow furnace design uses internally ribbed
tubes in the lower portion of the furnace, where
departure from nucleate boiling can form, and
more conventional smooth bore tubes in the
upper furnace. Source: Doosan Babcock
7. After the boiler upgrades. This is the Yaomeng Power Plant after the second boiler
replacement was completed in October 2009. Courtesy: Doosan Babcock
Smooth
bore
tubing
Optimized
ribbed
bore
tubing
Posiflow vertical tube low-mass-flux furnace
pump power is reduced and cycle efficiency
is correspondingly increased.
In the high heat flux areas of the boiler
a condition called departure from nucleate
boiling (DNB) is possible. DNB occurs
when a steam film separates the water
from the tube wall, as shown in Figure 4.
The result is an undesirable spike in tube
metal temperatures because steam has a
lower thermal conductivity than water.
The optimized ribbed tubes of a Posiflow
boiler either limit the potential for DNB
occurring or reduce the peak metal temperature
if it does occur.
Another form of boiling crisis is tube
dryout. For a ribbed tube, dryout of the
tube wall occurs at a higher steam quality.
The example in Figure 5 shows that for
a ribbed tube, dryout does not occur until
the steam quality reaches approximately
0.9, whereas the smooth tube dries out
at a steam quality of 0.6. This occurs because
the swirl generated by the spiral ribs
forces the water to the tube wall, which
maintains the water film up to the higher
steam quality.
Given these fundamental furnace design
features, the Doosan Babcock furnace design
for a typical 800-MW Posiflow boiler
is lighter by 386,000 kg, has about 4,000
fewer welds, and will require close to
50,000 fewer construction hours to erect
98
than a typical 800-MW universal pressure
boiler (Figure 6). The vertical wall panels
are also much easier to fabricate, thereby
reducing manufacturing cost. Another advantage:
The Benson load can be lower for
a Posiflow boiler, which leads to smaller
start-up systems that could eliminate the
need for a recirculation pump.
Posiflow at Yaomeng 1
Yaomeng Power Plant is situated in the
central southern area of Henan Province,
China. The plant consists of four 300-MW
units. Units 1 and 2 entered service in the
mid 1970s and were the first domestically
designed, coal-fired boilers of this size to
operate in China. Units 3 and 4 followed
during the mid-1980s.
The original boilers were universal pressure
type, once-through units designed for
baseload operation. They were designed to
generate 935 metric tons/hour main steam
at 570C. Beginning in 1992, output was reduced
to between 230 MW and 270 MW.
The upper limit was reduced due to overheating
of superheater surfaces and turbine
constraints. The lower load limit was
imposed because temperature differences
between adjacent tubes at low loads caused
pressure part failures and regular outages.
These operating restrictions and the expectation
of new emissions regulations led
the owners, Yaomeng Power Generation
Ltd. (YPGL), into discussions with Doosan
Babcock about a potential boiler retrofit.
The unit's concrete structure and the
tight plant site limited the available retrofit
options. YPGL elected to retrofit Unit 1
with the world's first commercial low mass
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flux vertical tube Benson boiler. Yaomeng
1 was successfully recommissioned in
May 2002. YPGL has since ordered a second
Posiflow boiler for retrofit of Unit 2
that began commercial service in October
2009 (Figure 7).
The success of the Yaomeng 1 retrofit
can clearly be seen through the improvement
in unit availability and the unit's
ability to rapidly change load. Before
the retrofit in 2000, unit availability was
88.7%. Average availability since 2002 has
been 95%, with the majority of the downtime
attributable to planned outages. Boiler
ramp rates improved from 1%/minute to
3%/minute.
Even though Yaomeng Unit 1 was the
first Posiflow unit in China, there have
been no ribbed tube failures caused by
overheating or high temperature differentials
of adjacent tubes in seven years of
operation. Furnace temperature data are
recorded using 106 thermocouples that
are strategically located on the furnace
walls. The metal temperatures recorded
are within design and will provide full
tube life. Also, the temperature difference
between adjacent tubes has been reduced
from 70C to between 20C and 30C. As a
result, no boiler wall tubes have failed, and
the unit can now reduce load to 40% without
oil-firing assistance-something never
achieved with the original boiler.
Boiler Tube Deposits and
Chemical Cleaning
In a once-through subcritical boiler like
that used on Yaomeng Unit 1, there is no
steam drum. As water passes through the
POWER | May 2010
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POWER May 2010

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