POWER May 2010 - 100

BOILER DESIGN
8. Before acid cleaning. These photos show the firing side and nonfiring side of furnace
wall tubes before the July 2008 acid cleaning. The firing side of the tube (left) clearly has deposit
buildup on the ribs of the tube. Buildup on the nonfiring side (right) has more uniform distribution.
Source: Doosan Babcock
start-up and shutdown-a " combined treatment. "
Combined treatment reduces the
speed of deposit buildup and increases the
time between acid cleaning.
As the ribbed tubes have an optimized
Table 1. Operating within limits. Ribbed boiler wall tube deposit weight density results
for outages from 2003 to 2008 for Yaomeng Unit 1. Doosan Babcock operating limits are
400 g/m2. Source: Doosan Babcock
Month of outage
6/2003
3/2004
6/2005
6/2007
7/2008
Weight density of deposits removed (g/m2)
Firing side
204.3
256.1
310.9
411.1
530.9
176.6
221.9
181.4
217.3
9. After acid cleaning. These photos show the firing side and nonfiring side of furnace
wall tubes after the July 2008 acid cleaning. No oxidation deposits are visible on the ribbed tubing.
Source: Doosan Babcock
Nonfiring side
161.1
profile, YPGL questioned how the positive
flow response would be affected by potential
deposit buildup. It can be inferred that
positive flow response is still active if it
can be shown that metal temperatures have
not risen above material limits and tube
failures caused by adjacent tube temperature
differences do not occur even if there
is a high deposit weight density.
YPGL made the decision to acid clean
the unit during the 2008 outage. Tube
samples containing internal deposits were
removed from the high heat flux areas of
the furnace to determine deposit thickness
in July 2008. The tubes were then arranged
by firing side and nonfiring side.
The firing side of the tube clearly shows
deposit buildup on the ribs of the furnace
wall tubes before acid cleaning (Figure 8).
The buildup on the nonfiring side on the
right shows a more uniform distribution of
deposit.
This internal tube deposit was analyzed
during every outage to determine the deposit
weight density.
Table 1 quantifies the increase over time
of the firing side deposit buildup in Unit 1.
The increase in deposit weight density is
also apparent on the nonfiring side, but the
increase is less uniform over time.
Figure 9 shows the results of the acid
cleaning. No oxidation deposit is visible
on any part of the ribbed tubing.
Doosan Babcock currently recommends
acid cleaning a subcritical boiler
at a deposit weight density of 400 g/m2
.
The Yaomeng Unit 1 results after six years
of service show that the ribbed tube metal
temperatures were within design limits
despite a deposit weight density above
recommendations. Also, no furnace tubes
failed, so it can be inferred that a positive
flow response was still active.
furnace, it evaporates entirely into steam.
At Yaomeng this transition occurs in the
low heat flux region, where the tube metal
temperature increase is minimized. The
water purity has to be high to avoid corrosion,
deposition, and carryover of dissolved
solids.
Yaomeng Unit 1 uses a hydrazine, allvolatile
water treatment system. An all-vol100
atile
water treatment is not normally used
for once-through boilers, but because of the
presence of copper alloys in the Yaomeng
condensate system, other options were
not available. For new-build once-through
projects without copper in the condensate
system, Doosan Babcock would use oxygenated
treatment during normal operations
and an all-volatile treatment during
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More Posiflow Projects Pending
China's large resource of anthracite coal ensures
this coal's place in China's fuel mix for
power generation for many years to come.
The next development step for Doosan Babcock
was to prove that the Posiflow furnace
design was equally applicable when firing
this coal. Previously, downshot boilers were
limited to burning anthracite at subcritical
cycles because the octagonal lower furnace
and burner arches would have been difficult
to construct with a spiral furnace arrangement.
With Posiflow, supercritical downshot
boilers are now possible.
POWER | May 2010

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POWER May 2010

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