Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1Current Status and New Technologiesof Water Treatment in Thermal Power Plants(Non-Toxic Oxygen Scavenger, Japan Industrial
Standard Revision, Water Quality Diagnosis)

HIROSHI AKAMINE*1 SHINGO TAMURA*2MITSURU NAKAMOTO*3 SENICHI TSUBAKIZAKI*4YUTA NAKATSUCHI*5 TETSUYA SAWATSUBASHI*5Plant water treatment for thermal power plants is conducted to prevent problems such ascorrosion and scale formation/deposition in the boiler and turbine systems, as well as carryover tothe turbine. The required water quality for boilers and turbines is stipulated by the JapaneseIndustrial Standard (JIS) B8223 and this standard is regularly amended based on new knowledge.
Mitsubishi Power, Ltd. (Mitsubishi Power) has been participating in this amendment. Mitsubishi
Power has also developed the "TOMONI Water Quality Diagnosis" system as an initiative
contributing to the early detection of serious problems signs and the improvement of plant
efficiency and is expanding its application to actual equipment.

|1. Introduction
Plant water treatment for thermal power plants is conducted to prevent problems such ascorrosion and scale formation/deposition in the boiler and turbine systems, as well as carryover tothe turbine. The Mechanical Engineering Division for “Specialists Group for Boiler Feedwater andBoiler Water” established in the Japanese Industrial Standards Committee deliberated on water
quality standards, and in February 1961, JIS B8223 “Conditioning for boiler feedwater and boiler
water” was established. The water quality required for boilers and turbines in thermal power plants
stipulated by JIS B8223 is amended regularly based on operational results, technologicalinnovation, equipment nonconformities, etc. We have also been participating in the JIS revision(1).
A water quality standard that eliminates the use of hydrazine, which has been restricted inuse overseas and carries concerns about carcinogenicity, was established in 2015 in the JIS
revision, and has become widely applied in Japan. In addition, High-AVT (LO) water treatment,
which we have applied to Japanese and overseas plants as a measure against FAC (flow accelerated
corrosion), phosphate corrosion and scale adhesion to control valves, feedwater heaters, etc., hasalso been adopted by JIS and its application is currently being expanded.
In recent years, it has become apparent that the cause of water quality abnormalities exists
not only in the main system, but also in the make-up water (pure water) or raw water supplied tothe plant, and a method to detect signs of serious problems at an early stage using a wide range ofwater quality controls is required. Against such a background, we have developed the "plant water
quality monitoring and diagnosis system (TOMONI Water Quality Diagnosis)" that has a wider
water quality monitoring range. The TOMONI water quality diagnosis is expected to not only
contribute to the maintenance of the long-term integrity and performance of the plant, but also tobring about economic benefits from its adoption, such as a reduction of the environmental burden*1

Nagasaki Plant Engineering Department, Project Management Division, Mitsubishi Power, Ltd.

*2

Hitachi Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Power, Ltd.

*3

Chief Staff Manager, Kure Plant Engineering Department, Project Management Division, Mitsubishi Power, Ltd.

*4

Chief Staff Manager, Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi*5

Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Power, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
2by reducing the amount of chemical usage and wastewater, shortening of the plant start-up time,
lowering of operating costs, etc.

|2. Water quality control of thermal power plants(2)A thermal power plant consists of a water-steam cycle, as shown in Figure 1, and has theimportant issue of dealing with corrosion problems that are caused between water-steam and plantcomponent materials. Corrosion products generated in the condensate and feedwater systems arebrought into the boiler system and are deposited and accumulated as scale, causing various types ofcorrosion. In the boiler system, impurities in the boiler water are carried over into the steam andbrought into the turbine system. Then in the turbine system, they are deposited and concentrated on
the rotor blades, etc., causing a decrease in turbine efficiency and corrosion damage such as stresscorrosion cracking, corrosion fatigue, etc. In the steam system, when scale generated by steam
oxidation grows thick and peels off, it scatters in the turbine system and causes erosion.
In recent years, there has been a trend toward dealing with the overall environmental
embrittlement problems of materials such as corrosion, stress corrosion cracking, flow-acceleratedcorrosion, etc., by having the person in charge of chemical management of the power plant takeresponsibility for the entire water-steam cycle of the condensate system, feedwater system, boilersystem, steam system and turbine system, and properly manage the water quality.

Figure 1 Concept of water-steam cycle|3. New technology for water treatment of thermal power plants
Table 1 lists the main feedwater and boiler water treatment methods in thermal power plants.
The water treatment methods used include AVT (all volatile treatment) and OT (oxygen treatment)
for feedwater treatment, and AVT, PT (phosphate treatment) and OT for boiler water treatment. We
are developing new technologies for water treatment methods and diagnostic systems in order todeal with problems caused by water, higher temperature and pressure of the boiler andenvironmental measures such as the elimination of hydrazine, as well as an increase in plant load
fluctuations and start/stop behaviors due to the introduction of renewable energy (water quality
fluctuations).

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
3

Table 1 Major treatment methods for boiler feedwater and boiler water in thermal power plants
Power generation
systemSteam power
generation

Combined cycle
power generation

3.1

Pressureclassification
Subcriticalpressure
Supercriticalpressure
Ultra-supercri
tical pressure
Subcriticalpressure
Supercriticalpressure

Boiler typeFeedwater
treatment method

Circulation boiler

All volatiletreatment

Boiler water
treatment method
All volatile treatment
Phosphate treatment

All volatile treatment
Once-through boiler
Oxygen treatment
Waste heat recovery
boiler (circulation type)
Waste heat recovery
boiler (once-throughtype)

All volatiletreatment

All volatile treatment
Phosphate treatment

All volatile treatment

Elimination of hydrazine (JIS B 8223 revision)(3)Figure 2 is an example of GHS (Globally Harmonized System of Classification andLabeling of Chemicals) labeling of hydrazine tanks. Hydrazine has been indicated as carcinogenic
and it is being discontinued and replaced with alternative technologies progressively mainly in newoverseas projects.

Figure 2

GHS labelling for hydrazine tank

Table 2 compares AVT (R), AVT (LO), AVT (O) and OT (JIS B 8223). The JIS revision in2015 changed the conventional AVT to AVT (R) (Reducing condition) (R) following overseas
standards, and additionally stipulated AVT (LO) and AVT (O), which are water treatment methods
using no reducing agent (hydrazine, etc.). Hydrazine-free low-oxidizing-type AVT (LO),oxidizing-type AVT (O) and OT are effective options for the elimination of hydrazine.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4

Table 2 Comparison of AVT (R), AVT (LO), AVT (O) and OT (JIS B8223)
Water
treatment

Water
conditions

Reducing
agent

AVT (R)

Reducing

Used

AVT (LO)

AVT (O)

Informative
OT (oxygentreatment)Lowoxidizing

Oxidizing

OxidizingNot used

Dissolved
oxygenconcentration
μg/L
7 or under

7 or underForm of iron oxides

NoteFe3O4
When the pH value andconcentration of dissolved
oxygen are low, onlyFe3O4 film is formed inmany cases. When the pH
value and concentration ofdissolved oxygen are high,very thin Fe2O3 film isformed on Fe3O4 film.

If hydrazine
injection issuspended inJapan, this water
quality becomes
AVT (LO).

Not usedHigher than 7and 20 orunder

Thin Fe2O3 film is formedon Fe3O4 film.

If hydrazine
injection issuspended in
power plants inoverseas countries,
this quality
becomes AVT (O).

Not usedHigher than
20 and 200 orunderIn feedwater and boiler
whole systems, stable anddense Fe2O3 film isformed on Fe3O4 film.

Oxygen injection
control facility isrequired.

R：Reducing，LO：Low Oxidizing，O：Oxidizing

3.2

OT (oxygen treatment) and high temperature filter for iron removal (once-through boiler plant)(4)Figure 3 illustrates the difference in scale on the steam generating tube inner surface
between AVT and OT for a once-through boiler. The effects of applying OT include (1) a reductionof the chemical cleaning frequency due to the suppression of the scale growth rate, (2) a reductionof the increase in the boiler differential pressure due to the suppression of liquid scale generation,(3) a reduction of scale adhesion to components and (4) the elimination of hydrazine. In Japan, asof October 2020, OT has been applied to 56 plants.

Figure 3 Difference of scale on the inner surface of boiler steam generating tube
between AVT (all volatile treatment) and OT (oxygen treatment)

In recent years, in some plants to which OT has been applied, an increase in the amount ofiron brought into the boiler and the adhesion of small-particle porous hematite scale (powder scale)to the inner surface of the boiler furnace wall tube have been observed, which are factors in thetemperature increase of the furnace wall tube metal. In order to reduce powder scale, it isMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
5

considered essential to reduce the iron concentration in the drain system of the low-pressure
feedwater heater, which is presumed to be its main generation source. We optimized the water
quality and developed an iron removal device (Figure 4) using a high-temperature filter. As aresult of applying these technologies to actual plants, good results have been obtained.

Figure 4

3.3

External view of iron removal device in low-pressure feedwater heater drain system

High-AVT (LO) (GTCC)(5)

Table 3 compares the conventional GTCC plant water treatment method (AVT for feedwaterand PT for boiler water) with High-AVT (LO). By setting the pH of the feedwater higher thanbefore (max. 9.7), the pH of the boiler water can be maintained without feeding phosphate. Due tothe use of a water treatment method that does not use hydrazine (AVT (LO)), ammonia can betreated independently and hydrazine feeding equipment can be omitted. In addition, by making thewater quality conditions during operation and storage the same, it is possible to omit the blow andwater filling steps for storage and shorten the plant start-up time.
Table 3 Comparison of PT (phosphate treatment) and High-AVT (LO)
Chemical used
Feedwater

AVT (R) + PT
High-AVT (LO)Ammonia

Hydrazine

Used
Used

Used
Not used

Boiler water
Sodiumphosphate
Used
Not used

pH of Feedwater
8.5 to 9.7
9.8 to 10.3

Benefits of introducing High-AVT (LO)- No sodium phosphate used: Phosphate corrosion does not occur.
- Storable under water quality conditions for operation: Easy storage and simple start-up processes- No hydrazine used: Little impact on humans and the environment- AVT operation at high pH (9.8 to 10.3): Suppression of FAC generation

GTCC plants have experienced tube wall thinning due to FAC (flow accelerated corrosion)in the HRSG economizer. In a system where the tendency of wall thinning was observed, thematerial was changed to low alloy steel, which has better FAC resistance than carbon steel, as acountermeasure against tube wall thinning. The effect of suppressing tube wall thinning was
obtained also by changing the water treatment conditions (applying High-AVT). Figure 5 presents
an external view and the installation of the heat-resistant thin film UT sensor used for ourdeveloped tube wall thinning monitoring. Figure 6 gives an example of the tube wall thinningpreventive effect after applying High-AVT (R) observed using the thin film UT sensor. It was
found that the tube wall thinning rate after applying High-AVT (R) was reduced to about 50% orless compared with that before application. By using the thin film UT, monitoring tube wall
thinning during operation of the unit was made possible, and the preventive effect of tube wall
thinning after applying High-AVT (R) could be directly confirmed. (In this case, the FAC
inhibitory effect of pH adjustment was confirmed and hydrazine was used without change.)Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
6Figure 5

External view and installation of heat-resistant thin film UT sensor

Figure 6 Example of preventive effect of High-AVT (R)application against tube wall thinning

Figure 7 is an example of improvement after applying High-AVT (LO) to a
subcritical-pressure drum boiler. After applying High-AVT (LO), the increase in the differential
pressure of the high-pressure feedwater heater due to scale adhesion was prevented.

Figure 7 Example of improvement of High-AVT (LO) application(suppression of differential pressure increase of high-pressure
feedwater heater)

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
7|4. TOMONI Water Quality Diagnosis(6)Figure 8 depicts an example of the screen image of TOMONI Water Quality Diagnosis andFigure 9 gives an example of TOMONI Water Quality Diagnosis service content. In order toprevent problems of thermal power plants caused by or related to water, TOMONI Water Quality
Diagnosis focuses on the boiler condensate and feedwater systems, steam system, make-up watersystem (pure water production facility), circulating water system, wastewater treatment system,
etc., and collects and analyzes water quality monitoring instruments, operation information (flowrate, pressure and temperature), equipment information (valve opening and equipment differentialpressure), etc., to carry out water quality diagnosis. TOMONI Water Quality Diagnosis is basicallya cloud service that provides (1) real-time monitoring (automatic diagnosis, 24 hours a day, 365
days a year), (2) regular reports (automatic issue, once a month) and (3) expert diagnosis reports(comprehensive evaluation, twice a year). Items (2) and (3) are optional.
The diagnosis report (3) lists serious potential problems in the future based on the observed
water quality abnormality signs and makes recommendations for additional investigations to study
countermeasures and inspection items at the next regular inspection.

Figure 8

TOMONI Water Quality Diagnosis screenshot

Figure 9

Example of TOMONI Water Quality Diagnosis service content|6. Conclusion
Water treatment technology for thermal power plants is being studied and improved as acountermeasure against damage to equipment such as corrosion and scale adhesion. We developedand are currently providing new technologies such as the elimination of the use of hydrazine,
high-temperature filter for iron removal, OT, High-AVT (LO) and TOMONI Water Quality
Diagnosis. We will continue to participate in and contribute to new knowledge-based regularrevisions of the required water quality for boilers and turbines stipulated by JIS B8223.
We believe that TOMONI Water Quality Diagnosis not only prevents the occurrence ofserious problems, but also maintains long-term integrity and performance, reduces the burden on
the environment by reducing the amount of chemicals and wastewater used and brings greatMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
8

economic benefits such as shortened plant start-up time and reduced operating costs. We will
expand the application range of TOMONI Water Quality Diagnosis.

References
(1)
(2)
(3)
(4)
(5)

(6)

JIS B8223-2015 Water Conditioning for Boiler Feed Water and Boiler Water
The Handbook of Water Quality Control for Thermal Power Plants (2018), Thermal and Nuclear PowerEngineering Society
Kido, H. et al., Alternatives to Hydrazine in Water Treatment at Thermal Power Plants, Power Plant
Chemistry, 2011, 13(10), p623~627.
Tsubakizaki et al., Water Quality Control Technology for Thermal Power Plants (Current Situation andFuture Prospects), Mitsubishi Heavy Industries Technical Review Vol.50 No.3 (2013)
Urata et al., Development of Thickness Trend Monitoring Technology (Thin-Film UT Sensor) and Field
Verification of Water Treatment Technology (High-AVT) for HRSG FAC Control, Mitsubishi Heavy
Industries Technical Review Vol.56 No.1 (2019)
Tsubakizaki et al., Development of Water Quality Diagnostic System Contributed to Reduce
Environmental Impact and Operational Costs, Mitsubishi Heavy Industries Technical Review Vol.55 No.1(2018)