Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
1Reliability Improvement by Applying Hydrazine Free
Operation and High-AVT(LO) to Sub-critical PressureDrum Type Steam Power Plant

SO TERADA*1

HIDEAKI TAKAHASHI*2

JUNYA IWASAKI*3

SAYAKA NAKAMURA*4

KAEWMANEE NETRKAEW*5

BLCP Power plant Unit 1 and Unit 2 in Thailand are sub-critical pressure drum type steam
power plants. Both the Units changed their water treatment method from the conventional AVT(R)(All Volatile Treatment (Reducing)) and PT (Phosphate Treatment) to the latest
environmentally-friendly High-AVT(LO)(All Volatile Treatment (Low Oxygen)), which is free fromhydrazine and sodium phosphate. This paper presents this change, which resulted in good
operational achievements.

|1. Introduction
Hydrazine, which is used to remove oxygen that causes corrosion in steam power plants, isdesignated as a mutagenic (suspected carcinogenic) chemical. Therefore, safer oxygen scavengers
and water treatment without oxygen scavengers are being adopted overseas.
In general, sub-critical pressure drum type steam power plants adopt AVT(R) using ammonia
and hydrazine for the feedwater system and PT using sodium phosphate for the boiler water. BLCPpower plant Unit 1 and Unit 2 in Thailand also adopt AVT(R) and PT. And then, they changed
water treatment method to High-AVT(R), which has a higher feedwater pH than conventional
AVT(R), to suppress FAC (flow-accelerated corrosion). However they have stopped hydrazine
injection from the perspective of the hydrazine free operation, and changed the water treatment
method to High-AVT(LO). After this change, good operational results have been achieved and arepresented in this paper.

|2. Overview of High-AVT(LO)
Table 1 compares conventional AVT(R) and PT with High-AVT(LO).
AVT (All Volatile Treatment) is roughly divided into AVT(R) and AVT(O). Conventionally,
All Volatile Treatment is called AVT(R) ((Reducing)), which uses ammonia for pH adjustment andhydrazine for oxygen scavenging (as a reducing agent), has been adopted in Japan. On the otherhand, AVT(O) is a hydrazine free water treatment method that uses only mechanical deaeration
through devices such as a condenser and deaerator to remove oxygen from feedwater. Furthermore,
High-AVT(LO) is a treatment method that exceeds the feedwater quality management standard(upper limit: pH 9.7) defined in the existing JIS B8223(1) standards and the dissolved oxygen
concentration is low. High-AVT(LO) has been applied to domestic and overseas combined cycle
plants and has achieved good operational results.(2)*1Manager, Nagasaki Plant Engineering Department, Project Management Division, Mitsubishi Hitachi Power Systems,Ltd.

*2

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

*3

Chief Staff Manager, Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi*4Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems,Ltd.

*5

BLCP POWER LIMITED Senior Internal Audit

Power Systems,Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
2Table 1 Compares conventional AVT(R) and PT with High-AVT(LO)
Chemical used
Water treatment
methodFeedwater

AVT(R)+PT

Used

Used

Boiler water
Sodiumphosphate
Used

High-AVT(LO)

Used

UnusedCan be unused*

Ammonia

Hydrazine

Feed water pH

9.8 to 10.3

8.5 to 9.7*Note: Used when seawater leakage in some cases.
AVT (O) ：All Volatile Treatment (Oxidizing)
AVT (LO) ：All Volatile Treatment (Low Oxidizing)
AVT (R) ：All Volatile Treatment (Reducing)
PT：Phosphate Treatment

At the 2013 JIS revision committee, in which Mitsubishi Hitachi Power Systems, Ltd.
participated, the standardization of High-AVT water treatment was promoted and adopted as arevision plan. High-AVT water treatment method, which is the latest environmentally friendly
water treatment technology, can maintain higher pH simply by injecting ammonia into thefeedwater, so the use of hydrazine and sodium phosphate under normal operation can be stopped(2).
Furthermore, since the feedwater pH of this water treatment is higher, pipe thinning caused by FAC
is relieved compared with conventional AVT(R) and PT. The dissolution of iron is also reduced, so
the increase in the iron concentration in the feedwater can be suppressed and a reduction in scaleadhesion can be expected.
Figure 1 shows the iron potential-pH diagram. For both AVT(R) and High-AVT(LO), theoxide, in which iron is stable, is considered to be magnetite (Fe3O4).

Figure 1

Iron potential-pH diagram

AVT in the figure indicates High-AVT.

|3. Overview of Power Plant
The BLCP power plant is located in the Map Ta Phut Industrial Park in Rayong Province insoutheastern Thailand, about 2.5 hours by car from Bangkok, the capital, as shown in Figure 2.
Table 2 lists the main facilities of BLCP power plant Unit 1 and Unit 2. The rated output per unit is717 MW and each unit has a subcritical pressure drum type boiler.
The commercial operation of Unit 1 was started in October 2006 and that of Unit 2commenced in February 2007(3). Thereafter, pipe thinning caused by FAC had occurred in eachpart of the boiler feedwater pump (BFP) and surrounding piping from sometime around September
2012. In March 2013, the water treatment was changed to High-AVT(R), the feedwater pH ofwhich is made higher with use of ammonia, to suppress FAC. The water treatment of Unit 1 andUnit 2 was then again changed to High-AVT(LO) in September and October, respectively, 2017,
which was accompanied by the stop of hydrazine injection. Sodium phosphate injection has been
also stopped.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
3

Figure 2

Location of BLCP power plant
Table 2 The main facilities of BLCP power plant Unit 1 and Unit 2
Specifications

Item
Start of commercial operation
BoilerFuelUnit 1

October 2006
February 2007
Pulverized coal fired, forced circulation single-drum radiantreheat type boiler
Coal (bituminous coal), Diesel oil (for starting up)
2285t/hMaximum continuous evaporation
Boiler outlet steam condition
TurbineRated output

Unit 2

17.26MPa ×541/541°C
Tandem-compound quadruple exhaust flow reheat regeneratingcondensing turbine
717 MW (at generator terminal)|4. Evaluation of Operation
Figure 3 depicts the water quality trend before and after stopping hydrazine injection. The
hydrazine injection was stopped with the pH maintained at 9.8. The dissolved oxygen concentrationin the feedwater after stopping hydrazine injection was about 5 ppb, which indicated that theoperation was as planned without hydrazine.

Figure 3 Water quality trend before and after
stopping hydrazine injection.
AVT in the figure indicates High-AVT

Figure 4 X-ray diffraction results of theinner surface scale of a boiler walltube and the peak trend of theidentified substance（After about 10months of operation after stopping
hydrazine injection）

Figure 4 presents the X-ray diffraction results and the peak trend of the substance of theinner surface scale of a furnace wall tube after about 10 months of operation after stopping
hydrazine injection. The oxide film was composed of a single phase of magnetite (Fe3O4), and noMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
4

scale modification or abnormal growth was observed. After the change to the High-AVT water
treatment method, the scale adhesion amount tended to be saturated shown in Figure 5, and thespeed of the pressure drop increase of the high pressure feedwater heater decreased shown in
Figure 6. Accordingly, it is presumed that the dissolution amount of iron from the system piping
such as the low pressure feedwater heater becomes lower and the amount of bringing in andadhering to the high pressure feedwater heater and the boiler decreases.

Figure 5

Amount of scale adhesion after the change to the High-AVT

Figure 6

Pressure drop of high-pressure heater|5. Conclusion
BLCP Power Station Unit 1 and Unit 2 in Thailand are sub-critical pressure drum type steam
power plants. Both the Units changed their water treatment method of the feedwater and boiler
water from the conventional AVT(R) and PT to High-AVT(LO), and they have been operating
without any problems up to now. High-AVT(LO) provides a water treatment option especially foroverseas sub-critical pressure drum type steam power plants where hydrazine is tightly regulated.
Furthermore, in the case of this water treatment method, since the feedwater pH is higher
compared with conventional AVT(R) and PT, the dissolution of iron is reduced, so a reduction inscale adhesion can be expected.

References
(1)
(2)
(3)

JIS B8223-2015 Water conditioning for boiler feed water and boiler water (2015 Edition)
Tsubakizaki et al, Improved Reliability of High-AVT (High-pH Water Treatment) Application toCombined Cycle Plants, Mitsubishi Heavy Industries Technical Review Vol.50 No.1 (2013)
Asada et al, Design and Construction Overview of the Biggest Coal Fired Power Plant in Thailand (BLCP
Power Plant), Developed by Private Companies as IPP Business, Mitsubishi Heavy Industries TechnicalReview Vol.44 No.4 (2007)