Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
24Achievement on OT (Oxygenated Feed-Water Treatment)
Application and Introduction of Countermeasuresfor Powdered Scale Deposit
SENICHI TSUBAKIZAKI*1

MASAHARU TAKADA*2

TAKAYUKI SUTO*3

HIROTAKE KAWASHIMA*4

TARO ICHIHARA*2

AKITO YOSHIDA*5

OT (Oxygenated Feed-Water Treatment) using oxygenation methods for boiler feed water
was first introduced at a thermal power utility plant in Japan in 1990 based on the success of OT
applications in Europe. It is now operational at 53 Japanese electric utility thermal power plants,
20 years after its introduction. Recently, some OT plants have experienced increased ironconcentrations in the drain system of the low-pressure feed-water heater. In addition, a powderedscale deposit has been generated and attached to the inside of the furnace wall tubes, contributing
to an increase in the wall tube temperature. As a countermeasure, a high-temperature filter wasused to remove the iron suspended in the drain system; the results validated the effectiveness of thefilter.

|1. Introduction
In domestic thermal power plants, boiler feed-water treatment is conducted to prevent
obstructions such as corrosion in the boiler and turbine systems, scale generation and deposition,and carryover into the turbine.
The history of boiler feed-water treatment for domestic thermal power plants is shown inFigure 1.

Figure 1

History of boiler feed-water treatment for domestic thermal power plants*1

Engineering Manager, Power Systems Service Division, Power Systems*2

Power Systems Service Division, Power Systems*4Power Systems Project Engineering Department, Power Systems Project Management Division, Engineering Headquarters*5Group Manager, Power Systems Service Division, Power Systems*3Boiler Engineering Department, Power Systems

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)25

Domestic verification tests for a combined water treatment (CWT) using oxygenation
methods were conducted based on success in Germany, Russia, and other nations. The tests
resulted in Japanese Industrial Standard (JIS) B8223, the first water chemistry control criterion, in1989. In 1990, the first CWT application in Japan was to a large once-through utility boiler.
Because a condensate polishing plant (CPP) is required, there have been few CWT applications todrum boilers. However, after more than 20 years since its introduction, CWT is operational at 53
thermal power utility plants, resulting in satisfactory achievement records.
In this paper, we present countermeasures against obstruction from powdered scale
deposition, which is an important issue for domestic CWT plants.

|2. CWT Plant Operation
2.1

Boiler feed-water treatments for once-through boilers

Figure 2 shows current boiler feed-water treatments for once-through boilers in domestic
thermal power utility plants.
Boiler typeOnce-through boiler
All volatile treatment
AVTWater treatment type

Oxygenated treatment
NWTCWTpH(at 25 oC)

9.0-9.7

Cation conductivity(mS/m)≦0.025≦0.02

Dissolved oxygen(μg/L)≦720-200

Ammonia
Hydrazine

Chemicals
AVT: all-volatile treatment

Figure 2

8.0-9.3≒7OxygenAmmonia
OxygenNWT: neutral water treatment
CWT: combined water treatment

Boiler feed-water treatments for once-through boilers

In conventional all-volatile treatment (AVT), the water quality is adjusted using ammonia tocontrol pH and hydrazine as a deoxidant. Because dissolved oxygen is thought to be a corrosive
component, its concentration is minimized and the boiler feed-water pH is adjusted to prevent
corrosion.
Oxygenated treatment (OT), on the other hand, is based on the theory that slightly solubleoxides adhered to the surface of steel can prevent steel corrosion and elute corrosion products intowater. OT includes neutral water treatment (NWT), in which dissolved oxygen is allowed tocoexist in neutral water, and CWT, in which dissolved oxygen is allowed to coexist in weakalkaline water adjusted to a range of pH 8.0 to 9.3 by ammonia.
The solubility of hematite, generated during CWT, is approximately a hundred trillion times
lower than that of magnetite, generated during AVT; thus, hematite reduces iron elution from thesystem. CWT applications have been promoted in domestic thermal power utility plants (mainlyonce-through boilers) as a means of preventing scale deposition in equipment and boilers.

2.2

CWT track record and effectivenessIn Japan, CWT has been applied to 57 thermal power utility plants, and is operated in 53
plants as of March 2011.
CWT in Japan has effectively (1) reduced the frequency of chemical cleanings by lowering
the build-up rate of scale, (2) alleviated the increase of boiler pressure loss by preventing rippled
scale build-up, (3) improved environmental conservation by reducing ammonia usage, (4) reduced
scale deposition on equipment surfaces, and (5) reduced flow-accelerated corrosion (FAC).
As shown in Figure 3, CWT has slowed the build-up rate of rippled scale compared with
AVT, alleviating the increase of boiler pressure loss. Thus, the increase in feed-water pressure at
the boiler inlet (i.e., the increase in feed-water pump power) has been curbed.
In addition, the slower build-up rate via CWT allows the boiler chemical cleanings (scaleremoval) to be deferred from 2-4 years to more than 10 years. With this effect and others, it hascontributed in some boilers to a savings of more than 50 million yen per year for a 600-MW plant.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)26

Figure 3 CWT effect of reduced rippled scale build-up

2.3

Example problems at inspected CWT plants

The problems identified at inspected CWT plants are shown in Figure 4. It is not clear
whether all these events have a cause-and-effect relationship with CWT, but all are listed herein.
Almost all the events were addressed during CWT introduction, and there are currently noreports of operational problems.

Figure 4 Example problems at inspected CWT plants

2.4

Powdered scale deposition

In recent years, an increased amount of iron transferred into the boiler and the deposition ofhematite scale on the inside surfaces of the boiler furnace wall tubes in some CWT plants hascaused the temperature of the metal furnace wall tubes to increase.
The deposited hematite scale, called “powered scale”, has low thermal conductivity and aporous structure consisting of small particles. Figure 5 shows powdered scale deposited on theinside surfaces of boiler furnace wall tubes.
The iron concentration in the drain system of the low-pressure feed-water heater has
increased at plants with powdered scale deposition on the inside surfaces of the boiler furnace walltubes after CWT. It is thought that dissolved oxygen in the drain system of the low-pressure
feed-water heater was transferred to the vapor side (the condenser side), shifting the drain to lowMitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)27

oxygen conditions, similar to AVT, and to pH conditions of 8.5 to 9.0, lower than that of AVT.
Thus, the iron concentration in the drain increased because the hematite-formation reaction wasdelayed due to low temperatures.

Figure 5

Powdered scale deposited on the boiler generating tube

|3. Countermeasures against Powdered Scale Deposition
3.1Measures for lowering the iron concentration in low-pressure feed-water heaterdrain systems (measures for normal operation)

Test results of material balance of iron in the plant systems indicated that iron removal andtransfer are primary sources of powdered scale and therefore it is thought that reducing the transferof iron particles into the boiler will reduce powdered scale deposition by decreasing the ironconcentration in the drain system of the low-pressure feed-water heater. The following
countermeasures were studied and introduced in the plants:(1) Replacing the low-pressure feed-water heater material with SUS (reducing iron elution);(2) Shifting the pH value of the feed water (lowering iron solubility);(3) Shifting the pH value in the low-pressure feed-water heater drain system by injecting
ammonia (lowering iron solubility);(4) Injection of oxygen into the low-pressure feed-water heater drain system (acceleratinghematite formation);(5) Closed operation of the vent valve of the low-pressure feed-water heater (acceleratinghematite formation);(6) Installation of a high-temperature filter for removing suspended iron in the low-pressure
feed-water heater drain system (lowering iron concentration).
Figure 6 shows an example in which implementing countermeasures (2)-(4), i.e.,
controlling water quality and lowering the iron concentration of the low-pressure feed-water heaterdrain system, reduced powdered scale deposition. Test results of suspended iron removal in thedrain system via installation of a high-temperature filter are also presented.

Figure 63.2

Powdered scale reduction via optimized water quality during normal operations

Suspended iron removal test in the low-pressure feed-water heater drain system(measures for start-up operation)

Figure 7 shows an external view of the suspended iron removal test unit with ahigh-temperature filter for the low-pressure feed-water heater drain system.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
28oThe high-temperature filter (spec.: 80 C) of the test unit was installed in the sample line,
located in the sample rack, of the drain system.
Because the suspended iron concentration in the drain system showed a strong increasing
trend during start-up operations, the test was conducted during start-up and normal operations. The
tests results for start-up operations are shown in Figure 8. The suspended iron concentrations at thefilter outlet were approximately 50 μg Fe/L, even when the transient iron concentrations were over
90,000 μg Fe/L at the inlet of the filter installed in the drain system, and were maintained at lessthan approximately 10 μg Fe/L.
Because the target water quality in the treated drain was less than 50 μg Fe/L during start-up
operations, the high-temperature filter, when installed, enabled the suspended iron concentration in
the drain to be reduced to less than the target value in the initial stages of the water-feeding
operation (reduction in start-up time). It is also expected to reduce the amount of removed ironparticles flowing into the boiler, which contribute to powered scale deposition.

Figure 8 Suspended iron removal in thelow-pressure heater drain using ahigh-temperature filter during start-up
operations

Figure 7 External view of the suspended
iron removal test unit with ahigh-temperaturefilterforthelow-pressure feed-water heater drainsystem|4. Conclusion
CWT is superior to AVT for a once-through boiler. Although powdered scale deposition
remains a problem, the suspended iron removal unit with a high-temperature filter in thelow-pressure feed-water heater drain system reduced the iron concentration in the feed-water drain
system.
We believe that these data can be used to reduce the amount of iron particles flowing into
boilers during start-up operations. Future tests in commercial plants are necessary to verify theseresults.

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