boiler feedwater oxygenated treatment in power

PPChem

PowerPlant Chemistry 2014, 16(5)294

© 2014 by Waesseri GmbH. All rights reserved.

Boiler Feedwater Oxygenated Treatment in Power Plantsin China

ABSTRACT

This paper presents the development of the oxygenated treatment (OT) technique application in power plants in China.The oxide morphologies of boiler tubes (economizer and water wall) under three different feedwater treatment techniques – all-volatile treatment under reducing conditions (AVT(R)), all-volatile treatment under oxidizing conditions(AVT(O)) and oxygenated treatment (OT) – were analyzed, and it was found that the reddish brown Fe2O3 coating layerformed by oxygen in feedwater only extended as far as the economizer inlet section. This paper also has a detaileddiscussion about the CrO4

2– release phenomenon and demonstrates that the CrO42– detected in the steam cycle

comes from sampling tubing and apparently does not originate from the boiler tube material.

INTRODUCTION

The Status Quo of Oxygenated Treatment in China

In the 1980s, the Thermal Power Research Institute (TPRI)realized the value of the oxygenated treatment (OT) tech-nique, which was first proposed and implemented inGermany, and researched the technology both in the laboratory and by field testing. In 1988, TPRI successfullyconducted OT field tests for the first time in the Wang Tingpower plant subcritical boiler and achieved satisfactoryresults. Subsequently, the application of OT has becomean effective solution to the problem of iron oxide deposi-tion on economizer inlet orifices of 300 MW subcriticalunits made in China. In the 1990s, as China importedmany 300 to 600 MW supercritical units and put them intooperation one after another, OT was applied successfullyto solve the frequent problems of high boiler heating surface corrosion rates and rapid increases in the boilerpressure differential. In 1995, TPRI first applied the OTtechnique to the Shanghai Shidongkou Power Plant 2 x600 MW supercritical boilers, and achieved the desiredresults [1]. The units have also become the longest OT-operating supercritical units in China at present. In 2002,TPRI formulated a "Once-through Boiler FeedwaterOxygenated Treatment Guideline" to regulate the feed -water oxygenated treatment mode of boilers in domesticpower plants [2]. In the 21st century, with a large numberof domestic 600 MW supercritical units and 1 000 MWultra-supercritical units having been put into operation inChina, high corrosion rates of boiler heating surfaces,deposits on water wall tube orifices and deposits plugginghigh-pressure (HP) heater drain control valves havebecome more prominent (Figures 1 to 4). Corrosion and

deposition of downstream equipment caused by flow-accelerated corrosion (FAC) is particularly prominent.These units originally applied the all-volatile treatmentunder oxidizing conditions (AVT(O)), but to solve theabovementioned problems, OT has been more activelyutilized in these supercritical units [3]. At the same time,when the feedwater is in the AVT condition, mixed bedcondensate polishing has a heavy operational burden, andmany units are forced to operate in ammonium form,which causes severe leakage of chlorine and sodium intothe mixed bed effluent. The deposition and corrosion of

Zhigang Li, Wanqi Huang, Songyan Cao, and Hongbo Zhang

Figure 1:

Example of an economizer tube with a scaling rate of260 g ·m–2 per year.

Boiler Feedwater Oxygenated Treatment in Power Plants in China

Boiler Feedwater Oxygenated Treatment in Power Plants in China PPChem

PowerPlant Chemistry 2014, 16(5) 295

steam turbine blades is also very common (Figures 5 and 6). The advantages and effects of the OT techniquefor solving these problems are obvious, and thereforehave become the driving force for the development of thetechnology.

As of the end of 2011, there are more than 100 units withOT in China, of which more than 80 % are ultra-super -critical or supercritical units. All the 1 000 MW ultra-super-critical units in operation are using OT for feedwater treat-ment. Statistics of the units with OT and unit capacity areshown in Figure 7.

According to the "Once-through Boiler FeedwaterOxygenated Treatment Guideline," which was mainly prepared by TPRI , the dissolved oxygen in feedwater islimited within the range of 30 to 150 µg · L–1 for units onOT. In practice, the oxygen content in the feedwater ofoperating units with OT is in the range of 30 to 80 µg · L–1.The merit of OT is fully reflected in these units, as demon-strated by data showing that the iron content in the econ-omizer inlet and HP heater drain is less than 1.0 µg · L–1

(Figure 8), and the average deposit rate in the tubing of theeconomizer and water wall after running for six years inthis mode is less than 30 g · cm–2 per year. In addition, the

Figure 2:

Oxide deposition in the last stage of HP heater tube inlets.

Figure 3:

Water wall tube orifice deposits.

Figure 4:

Deposits on a drain control valve.

Figure 5:

Deposits on the blade of a HP rotor.

296 PowerPlant Chemistry 2014, 16(5)

PPChem Boiler Feedwater Oxygenated Treatment in Power Plants in China

regeneration cycle for the condensate polishing systemhas been extended 4 to 5 times compared with AVT oper-ation, and the rate of boiler differential pressure increase issignificantly reduced (Figure 9). At the same time, thedeposits on the water wall orifice and oxide blockage inthe HP heater drain regulating valve have been signifi-cantly mitigated.

However, the use of OT is still limited in many generatingunits in China due to the concern about high-temperaturesteam oxidation. To reduce the possible risk of introducingincreased oxygen concentration into the steam system,hypoxia (low-oxygen) treatment in feedwater, also calledweak oxygenated treatment (WOT), has been adopted in

some units, and the dissolved oxygen content in the econ-omizer inlet is generally controlled at 5 to 20 µg · L–1. It isevident that hypoxia treatment cannot protect the HPheater drain system. High iron content in the drainscaused by two-phase flow-accelerated corrosion (FAC)and drain valve blockage still cannot be avoided with low-oxygen treatment. In addition, the frequent regeneration ofthe condensate polishing mixed beds is a difficult problemfor power plant chemistry workers.

Oxygen injection to feedwater systems often uses theauto-adjustment mode. An automatic oxygen feed devicedeveloped by TPRI for units with OT is popular in China.Normally, the dissolved oxygen content in the feedwater

Figure 6:

Corrosion on the blades of a low-pressure rotor.

300 MW subcriticalonce-through units

8 %

300 MW–600 MWsubcritical drum units

10 %

1 000 MW ultra-subcritical units

24 %

600 MW subcritical units58 %

Figure 7:

Statistics on unit capacity of thermal power plants using OT inChina.

CPD

10.0

8.0

6.0

4.0

2.0

0

Iron

Concentr

ation

[µg

kg

]·

–1

MS DI EI HPD

AVT(O)

pH: 9.2~9.6

OT

pH: 8.8~9.0

CPD

MS

DI

EI

HPD

Condensate pump discharge

Main steam

Deaerator inlet

Economizer inlet

High-pressure heater drains

Figure 8:

Iron concentration of AVT(O) vs. OT in a 1 000 MW unit.

CPD condensate pump dischargeMS main steamDI deaerator inletEI economizer inletHPD high-pressure heater drains

297PowerPlant Chemistry 2014, 16(5)

PPChemBoiler Feedwater Oxygenated Treatment in Power Plants in China

can be controlled within the setting range ± 20 µg · L–1

even in the case of unit load fluctuations, because thisunique oxygen feed device is equipped with a trace gasregulatory agent and a differential pressure stabilizationadjusting device, as shown in Figure 10.

THE INVESTIGATIONS

Change in the Metal Oxide Film in the Low-Tempera -ture Zone of the Boiler Heating Surface

At the water temperature of the condensate low-pressureheaters and the first high-pressure heater, the magnetite

film has higher solubility and it is in an active state.Magnetite solubility reaches the highest point at about150 °C. When the local flow conditions deteriorate, disso-lution of the oxide film can cause FAC at local metal surfaces in the feedwater system under reducing condi-tions, and that is why the iron content is higher (average 8to 10 µg · L–1) in the feedwater systems of boilers treatedwith AVT(R) (all-volatile treatment, reducing conditions).When the feedwater treatment was changed from AVT(R)to AVT(O) (all-volatile treatment, oxidizing conditions), theFAC was only partly mitigated, and the iron content of thefeedwater was only lowered to an average of 4 to 6 µg · L–1

due to less dissolved oxygen in the feedwater. The corro-sion products deposit at various locations in the system,

3

3.5

3.2

2.9

2.6

2.3

2.0

Pre

ssure

Dro

p[M

Pa

]

3 4 5 9 10 15 18 20 23 27 27 28 29 29 30 30

Month

O + NH conditioning2 3

O + NH conditioning2 3 Pressure drop of Unit No. 1

Pressure drop of Unit No. 2

Figure 9:

Pressure drop from the feed pump to the steam-water separator section in a 1 000 MW unit.

2 000

1 600

1 200

800

400

0

Fe

ed

wa

ter

Flo

w[t

h]

·–

1

400

300

200

100

0

Dis

so

lve

dO

xyg

en

Co

nte

nt

[µg

L]

·–

1

Time [h]

Oxygen flow

Feedwater flow

DO in feedwater

Set value

0 3 6 9 12 15 18 21 24 27 30

Figure 10:

The control effect of the automatic oxygen feed device in running a unit on OT.

DO dissolved oxygen



as previously mentioned, and this deposition is a wide-spread problem in supercritical boilers in China.

Metal Oxide Film Forming Mechanism

According to the oxide film formation mechanism, themetal oxide film in pure water without oxygen below300 °C consists of a compact Fe3O4 topotactic layer and aporous Fe3O4 epitactic layer due to the insufficient oxidiz-ing effect of water. Oxide film forming reactions can bedivided into the following steps [4]: carbon steel is dis-solved to form ferrous hydroxide and the magnetite andhydrogen is released. The third step is the slowest.

Fe + 2H2O = Fe2+ + 2OH– + H2� (1)

Fe2+ + 2OH– = Fe(OH)2� (2)

3Fe(OH)2 = Fe3O4 + 2H2O + H2� (3)

As the water temperature increases above 200 °C, thethird step of the Fe3O4 oxide film formation mechanism isaccelerated, but the presence of ferrous hydroxide is stillan intermediate link in the corrosion cell reaction. AsFe(OH)2 has high solubility, suppression of Fe

2+ will occurif the pH of the water is elevated.

When oxygen is added to the water, the oxygen moleculesaccept electrons to form OH– in the corrosion cell cath-ode. While water as an oxidizer has insufficient oxidizingstrength to transform Fe2+ to Fe3+, oxygen molecules sup-port the reduction reaction at the cathode and provide therequired energy for Fe2+ to convert to Fe3+. This increasesthe phase interface reaction speed, and eventually accel-erates the conversion of ferrous hydroxide to ferric oxide.

2Fe(OH)2 + ½O2 = Fe2O3 + 2 H2O (4)

Because oxygen is continuously added to the feedwater,the Fe2+ diffused through the Fe3O4 topotactic layer is oxidized to form FeOOH or Fe2O3, as shown in Eq. (4),which then is deposited in holes and surface pores in theFe3O4 epitactic layer. The Fe2O3 seals the porous Fe3O4film, resulting in a dense and stable "double layer protec-tive film" formed on the steel surface [5], as shown inFigure 11a. The iron content in the feedwater is thendecreased to less than 1 µg · L–1.

The Surface Difference on the Inlet Tube and OutletTube of the Economizer

The status of the metal oxide films on the surfaces of thefeedwater system cannot be directly analyzed and com-pared. This is because the sampling is difficult, and it canonly be judged by means of the analysis of economizertubes. Before 2008 in China, all feedwater oxygenatedtreatment was directly converted from AVT(O) while oper-ating the boilers without first carrying out chemical clean-ing. After the boiler treatment had been converted to OT(the oxygen concentration in the feedwater was about 50to 70 µg · L–1) for a period of time, tubes were sampled andmetal surfaces were observed. It was found that the surface color of all tubes from the economizer to the waterwall appeared brown-red due to heavy powdery ironoxides, which resulted from performing the conversion to OT without a prior chemical clean. This observed phenomenon led to some confusion regarding the theoret-ical effective oxygen range for OT.

After 2008, power plants in China began to focus more onsaving energy and reducing the cost of running the unit. Inorder to completely clear out the iron oxide deposits in thefeedwater system and boiler, chemical cleaning was carried out prior to OT conversions. Some tubes from

Figure 11:

Surface appearance of boiler economizer tubes.

a) Brown-red color on economizer inlet zone tube surface of a 600 MW boiler operating under OT

b) Economizer outlet zone tube surface without brown-red color of a 600 MW boiler operating under OT

c) Economizer inlet zone tube surface of a 660 MW boiler operating under AVT(O)

a b c



economizers of 600 MW supercritical boilers which weretreated with the abovementioned method were analyzedand studied. It was discovered that with this approach,when economizer and water wall tubes were sampledfrom 600 MW boilers which were treated with OT, theiroxide film color and appearance characteristics wereobviously different – the inner surface of the economizerinlet section tube sample had a reddish brown layer, andthat of the economizer outlet section tube had a steel-greycolored layer with very weak brown, as shown inFigure 11b. The water wall tube also had a light steel-greycolored layer on the surface. The color of the economizerinlet zone tube surface in a boiler operating under AVT(O)is dark steel-grey (Figure 11c).

The product on the metal surface has been analyzed by X-ray diffraction (XRD), showing that the oxidation filmformed in hot water with O2 is different from that formed inhot air. The product on the economizer inlet and outletzone tube surfaces in a boiler operating under AVT(O) is apure Fe3O4. The product on the economizer inlet zonetube surface in a boiler operating under OT is �-Fe2O3 andsome Fe2O3 with cubic crystals as well as a small amountof Fe3O4. The product on the tube surface of the econo-mizer outlet zone in a boiler operating under OT is Fe3O4and a small amount of �-Fe2O3.

These tube sample surface appearances were magnified500 times by scanning electron microscopy (SEM) and theimages are shown in Figure 12. The reddish brown layeron the surface of the inlet section tube is a dense coveringlayer. This may suggest that a certain amount of ferroushydroxide still exists in the film, which is the driving forcefor the oxygen reduction at the cathode. Due to theincreased oxygen concentration in the water, Fe2+ from themetal matrix was oxidized to Fe3+ by oxygen and sub -

sequently a Fe2O3 oxide coating in the oxide layer wasformed under the conditions of the water temperature atthe inlet section of the economizer, which is very importantfor inhibiting FAC at the economizer tube entrance.

This kind of Fe2O3 film is different from the �-Fe2O3 formedat high temperatures and is unstable at the temperature offeedwater [6] and the high flow rate of feed water; itrequires continuous oxygen feed (above 30 µg · L–1) tomaintain its stability.

The oxide film color and appearance characteristics of theeconomizer outlet tube samples are close to those ofwater wall tube samples, showing a "clean" surface com-pared with that of the inlet section tube although there is avery weak brown color on the tube surface. This phenom-enon may also suggest that the effect of oxygen promot-ing the formation of a double protective layer on the metalsurface by means of cathodic reduction in a corrosion cellhas visibly begun to weaken. Thus, the Fe2O3 oxide coat-ing layer formed by oxygen feed only extends as far as theeconomizer inlet section where the water temperature isstill about 300 °C (water temperatures of the economizerinlet and outlet are 290 °C and 320 °C, respectively).However, the oxidation-reduction potential (ORP) of thewater here has been kept at a positive value in the boilerwater solution due to oxygen feeding, until oxygen in thewater enters into the steam in the water wall evaporatingsection of the boiler.

When the water temperature of the economizer outletexceeds 300 °C, redox reactions occurring on the metalsurface may directly oxidize ferrous ions to ferric ionsinstantly, as illustrated in the chemical reaction Eq. (5),which also suggests that bivalent iron ion migrating out-ward from the metal matrix may be converted to ferric ions

Figure 12:

SEM surface appearance (magnified 500 times) of 600 MW unit boiler tubes under OT.

a) Economizer inlet tube surface with a dense covering layer

b) Economizer outlet tube surface

c) Water wall tube surface

a b c



in an instant. The formation of dense magnetite oxide filmshas no ferrous hydroxide as a transitional product here(Eq. (5)):

3Fe + 4H2O = Fe3O4 + 4H2� (5)

As long as the ferric iron concentration is above a very lowthreshold, it precipitates and becomes a stable solidphase, because the solubility of magnetite is much lowerthan that of ferrous hydroxide.

In this process, Eq. (5) is in balance with no excess biva-lent ferrous ions or electrons, so the oxygen molecule herehas no way to participate in the formation process of theoxide film reaction.

In other words, the metal oxide film is in a passive statedue to the temperature (above 300 °C) and ORP (above100 mV of the water, and an excess of oxygen in the waterand elevation of water pH are not important in this purewater system. The scaling rate on the tube surface herealso indicates that the oxidation film is very thin in theeconomizer outlet in Table 1. Among 4 boilers, the scalingrate of the economizer of the boiler operating under OT isthe lowest.

This phenomenon indicates that in metal oxidation there isa competitive relationship between hot water and a smallamount of oxygen at the temperatures in the thermal sys-tem. In the lower temperature zone, oxygen in the watercan promote the formation of a double protective coatinglayer on the metal surface. In the middle temperaturezone, the oxidizing strength of the water is enhanced dueto higher temperature and positive ORP of the water;water and oxygen may contribute jointly to metal oxida-tion. In the steam zone, the contribution of water steamand oxygen to metal high-temperature oxidation mainlydepends on the gas partial pressure.

Deposition on the Water Wall Tube Surface

The temperature of water or steam in supercritical boilerwater wall tubes is high (above 350 °C), so the metal sur-

face can be spontaneously oxidized to form a fine anddense film by water and steam with sufficient oxidizingstrength. However, there are different surface appear-ances due to iron corrosion products depositing on tubesurfaces when boiler feedwater treatments are different;Figure 13 illustrates boiler water wall tube surfaces withfeedwater treatments of AVT(R), AVT(O) and OT, respec-tively. The outside layer of the oxide film on a water walltube treated with AVT(R) (Figure 13a) has an oxide that hasa coarse granular particle size, since the iron oxide particlecomes from the upstream thermal system by corrosionproduct transport. The outside layer of the oxide film on awater wall tube treated with OT (Figure 13c) has a finegranular particle size since the oxide film was formed insitu and few iron oxide particles originate from theupstream thermal system water. The outer layer of theoxide film on the water wall tube treated with AVT(O)(Figure 13b) has iron oxide particles of a size between theabove two scenarios; nevertheless, the oxide layer still hasobvious rippled characteristics.

The products on the metal surfaces have been tested byXRD. The product on the water wall tube surface in aboiler with AVT(O) is a Fe3O4. The product on the waterwall tube surface, including the economizer outlet section,in a boiler with OT is Fe3O4 and a small amount of �-Fe2O3, because hot water is strongly oxidizing underconditions of higher temperature and positive ORP; corro-sive product migration and deposition can also impact theproducts on the water wall tube surfaces.

Change in the Metal Oxide Film in the High-Tempera -ture Zone of the Boiler Heating Surface

It has been a concern to chemistry personnel whetherfeedwater oxygenated treatment (OT) affects the oxidationbehavior of the internal surfaces of superheater andreheater tubes. All collected data from field samples showthat OT did not increase the oxide growth rate of super-heater and reheater tube interiors. There is almost no con-troversy regarding this result. With regard to the oxide layerpeeling off, it has been confirmed that oxide layer exfolia-tion of low-chromium alloy steel (T23), martensitic steel

Unit #1 #2 #3 #4

TreatmentAVT(O) 1 year AVT(O) AVT(O) OTWOT 0.4 years 0.83 years 1 year 1.17 years

Measuring point EIS EOS EIS EOS EIS EOS EIS EOS

Scaling rate (g ·m–2 per year) 60.6 46.9 42.9 33.5 79.5 51.3 35.3 29.2

Table 1:

Scaling rate (g ·m–2 per year) at the tube surface of the economizers in one power plant.

WOT weak OTEIS economizer inlet sectionEOS economizer outlet section



(T91) and high-chromium alloy steel (HR3C) has not beenaffected by oxygen concentrations in steam. The focus ofthe argument is whether oxygen in the steam promotes thespallation of the metal oxide layer of 18 Cr series austeniticstainless steel in high-temperature zones [7,8].

It was found from the investigations and surveys in Chinathat the issue of oxide removal from 18 Cr series austeniticstainless steel is more prominent in boilers treated withOT. The extent of serious oxide layer exfoliation is quitedifferent when the design of the unit includes the samepipe material in boilers with or without OT. In some units,exfoliation took place in the superheater, while in otherunits exfoliation took place in the reheater, and in someunits, the exfoliation occurred in both the superheater andthe reheater, or in neither. After careful research and com-parison, it was shown that temperature is the key factor inaddition to the specific metal alloy. The influential factorsfor boiler tube temperature are the boiler model, type ofcoal, combustion mode, heating surface design, arrange-ment of superheater and reheater, etc.

Chromium Acid Ion (CrO42–) Release during OT

Conversion

An increase in cation conductivity in water and steam is acommon phenomenon during OT conversion. This isbecause some anionic impurities are released from theoxide film as phase transition occurs. These anionic impu-rities commonly include Cl–, F–, SO4

2–, formic acid andacetate ions as well as carbonate ions. As the concentra-tion is low and release time is short, they do not jeopardizethe water and steam quality. TPRI has found anotheranionic impurity, the chromium acid ion (CrO4

2–) [9]released in later phases of OT conversion, as shown in

Table 2, which has a higher concentration than commonanionic impurities, and also contributes to the cation con-ductivity increase.

The source of chromium acid ion is a special concernbecause boiler section materials contain the element Cr.According to the data compiled after converting to OT indozens of units, TPRI has developed a summary of thecharacteristic release of chromium acid ion as follows:

1. Chromium acid ion mainly appears in economizer inletand superheated steam samples (see Table 3 below). Itis usually accompanied by an oxygen content increasein the feedwater and superheated steam samples afteroxygen is added to the feedwater, and the CrO4

2– con-tent peaks in a short time. The higher the dissolvedoxygen (DO) content, the larger the peak value is. Thislevel gradually decreases, but this rate of decrease isnot related to DO. In fact, CrO4

2– release does notappear in all units during the OT conversion process,which may be associated with the sample tubing mate-rial.

2. There is no evidence of chromate ions in the water/steam cycle. CrO4

2– is never detected in the conden-sate, low-pressure feedwater, boiler water, and high-pressure drains, or reheat steam in either the OT conversion process or under normal operating condi-tions of a unit. Chromium content is also not detectedin deposits in steam systems and turbines. When running on normal OT, DO is 30 to 80 µg · L–1, andCrO4

2– is not detected in any sampling points of thewater/steam cycle. If the DO concentration is increasedintentionally for testing, CrO4

2– will clearly increase inthe economizer inlet sample. This reveals that the DOconcentration plays an important role in CrO4

2– release.

Figure 13:

Surface appearance (magnified 2 000 times) of 600 MW boiler tubes with different feedwater treatments.

a) AVT(R), operation for 4.5 years

b) AVT(O), operation for 1 year

c) OT, operation for 1 year

a b c



3. In superheated steam sample tubing, CrO42– release is

only found when dry steam in the end part of the pipesegment is gradually converted to wet steam or evenwater. There is only a trace of CrO4

2– in superheatedsteam sampling of some supercritical units. For oneunit, it has been confirmed that the sample tubing insu-

lation is good [10]. However, CrO42– has not been found

in low-pressure and low-temperature feedwater sam-ples, even though the low-pressure heater tubing ismade of stainless steel. This also confirms that tem-perature plays an important role in CrO4

2– release.

Unit Unit Capacity

Economizer Inlet Steam(µg · L–1) (µg · L–1)

DO CrO42– DO CrO4

2–

A600 MW

supercritical once-through boiler 150–200 < 0.3 < 10 < 0.3

B500 MW

subcritical once-through boiler 200–300 < 0.3 50–100 5.1

C300 MW

subcritical drum boiler 300–350 4.6 150–250 0.5

D600 MW

supercritical once-through boiler 150–200 18.4 70–60 < 0.3

E600 MW

supercritical once-through boiler 150–200 211 ~ 100 1.4

F300 MW

subcritical once-through boiler 200–300 17 50–100 26

G600 MW

supercritical once-through boiler 100–200 7 50–150 < 0.3

H1 000 MW

ultra-supercritical once-through boiler 100–150 23 30–40 1.2

Table 2:

Dissolved oxygen (DO) and chromium acid ion content at some sample points during OT conversion.

DataDeaerator Inlet Economizer Inlet Steam HP Heater Reheater Condensate

Mode(µg · L–1) (µg · L–1) (µg · kg–1) (µg · L–1) (µg · kg–1) (µg · kg–1)

6-27 < 0.3 < 0.2 < 0.3 < 0.3 < 0.3 < 0.3

6-28 < 0.3 < 0.2 < 0.3 < 0.3 < 0.3 < 0.3 AVT(O)

6-30 < 0.3 < 0.2 < 0.3 < 0.3 < 0.3 < 0.3

7-1 < 0.3 < 0.2 < 0.3 < 0.3 < 0.3 < 0.3

7-3 < 0.3 < 0.2 < 0.3 < 0.3 < 0.3 < 0.3

7-5 < 0.3 2.82 < 0.3 < 0.3 < 0.3 < 0.3

7-8 < 0.3 90.8 < 0.3 < 0.3 < 0.3 < 0.3 OT

7-17 < 0.3 148.5 1.4 < 0.3 < 0.3 < 0.3 convertion

7-18 < 0.3 211.0 < 0.3 < 0.3 < 0.3 < 0.3

7-20 < 0.3 171.1 < 0.3 < 0.3 < 0.3 < 0.3

7-23 < 0.3 54.5 < 0.3 < 0.3 < 0.3 < 0.3

Table 3:

CrO42– content at cycle sampling point in Unit E during OT conversion.

Note: < 0.3 indicates that the CrO42– content is less than the detection limit (0.3 µg · L–1) for the chromatograph.



The analysis above reveals that CrO42– only comes from

the sample tubing surface film. One possible mechanismis as follows:

In the stainless steel sample tubing of the economizerinlet, the water temperature is at 270 °C to 290 °C; thereare Cr2O3, Crx FeyO3, Cr(OH)2 and Cr(OH)3 present, fromthe inside to outside of the metal oxide film under AVTconditions. The higher the temperature, the higher theratio of chromium oxide in the oxide film, due to the accel-erating diffusion of chromium ions in the material. Duringthe unit conversion to OT, as oxygen passes through thesampling tubes, the redox potential rises, and the waterchanges to a more oxidizing state. The metal oxide film onthe surface of the sampling tube changes accordingly.Low-valence oxides in the oxide film are oxidized to high-valence oxides, such as Fe2+, Cr2+ and Cr3+ being con-verted to Fe3+ and Cr6+. Part of the high-valence chromiumcompounds dissolve and combine with water to formCrO4

2–. Under a certain water temperature and fluid flowvelocity, if the oxygen concentration is beyond a certainrange, the higher the oxygen concentration, the faster thechromium ion release from the metal [11]. Therefore, forfeedwater sample tubing made of stainless steel material,specific high temperatures, water flow rate and DO con-centration prove to be three key conditions for chromiumacid ion release; without any of these three conditions,chromium acid ion release will not happen.

CONCLUSION

The application of feedwater oxygenated treatment tech-nology in China is successful and has achieved technicaland economic benefits. Feedwater oxygenated treatmentfor the pre-boiler system can significantly reduce the ironcontent in the feedwater system and then reduce theboiler differential pressure drop and deposit rate on boilertubing, which eventually results in the improvement ofboiler efficiency and energy savings.

Feeding oxygen to the water makes the metal surfaceform a reddish brown Fe2O3 coating layer. This layerrequires continuous oxygen feed to maintain its stability.Research has confirmed that oxygen promotes the forma-tion of a double protective film on the metal surfacethrough the cathodic reduction effect in the corrosion cell.This mechanism is closely related to water temperature,and its effective temperature range is from room tempera-ture to about 300 °C.

The CrO42– detected at some water sampling points dur-

ing OT conversion and/or normal operation with OT doesnot come from the thermal equipment material but mostlikely is released from stainless steel sample line tubing.

REFERENCES

[1] Li, Z., Shen, B., Thermal Power Generation 1998,27(6), 42 [in Chinese].

[2] Guideline for Cycle Chemistry in Fossil Power Plant:Part One, Oxygenated Treatment of Once-ThroughBoiler, 2002. Thermal Power Research Institute,Xi'an, P. R. China, DL-T 850.1 [in Chinese].

[3] Li, Z., Chen, R., China Electric Power 2004, 37(11),47 [in Chinese].

[4] VGB Guidelines for Boiler Feed Water, Boiler Water,and Steam of Steam Generators with a PermissibleOperating Pressure >68 bar, 1988. VGB Kraftwerks -technik GmbH, Essen, Germany, VGB-R 450 Le.

[5] Li, Z., Thermal Power Generation 2002, 31(6), 26 [inChinese].

[6] Evans, U. R., Corrosion and Oxidation on Metal,1976. China Mechanical Industry Press [in Chinese].

[7] Jia, J., Li, Z., China Electric Power 2008, 41(5), 37 [inChinese].

[8] Dooley, R. B., Bursik, A., PowerPlant Chemistry2011, 13(4), 236.

[9] Li, Z., Shen, B., Huang, W., Thermal PowerGeneration 2012, 41(2), 47 [in Chinese].

[10] Shanghai Shidongkou Power Plant Report, 2004.Thermal Power Research Institute, Xi'an, P. R. China[in Chinese].

[11] Effertz, P.-H., Die kombinierte Ammoniak-, Sauer -stoff konditionierung von Wasser-, Dampfkreis lä�ufenin Kraftwerken: Tagungsbericht d. Allianz-Work shopsim Allianz-Zentrum fü� r Technik Ismaning beiMü�nchen, 8./9. November 1982 (Editor P.-H. Effertz).Allianz-Berichte fu� r Betriebstechnik und Schaden -verhü� tung, 1985, 23. Allianz-Versicherungs-AG,München, Germany.

ACKNOWLEDGMENTS

We acknowledge Mr. Andrew Howell from Xcel Energy andMr. Peigang Cao from Ontario Power Generation forreviewing and revising this paper.

THE AUTHORS

Zhigang Li (B.S., Metal Corrosion Protection, DalianUniversity, China) is a senior committee member of theChinese Society of Electrical Engineering and has beenresponsible for research and development of power plant



chemistry for fossil plants in China since 1980 as a engi-neer of the Thermal Power Research Institute, China. Hehas been a director engineer of the Chemical EngineeringResearch Department of Xi'an Thermal Power ResearchInstitute since 2011. He was a researcher and deputydirector of the Chemical Engineering Research Depart -ment of Xi'an Thermal Power Research Institute, P. R.China, from 1999 to 2010.

Wanqi Huang (B.S., Power Plant Chemistry, WuhanUniversity of Hydraulic and Electrical Engineering, China)is a senior engineer and has been working at the ThermalPower Research Institute (TPRI) Co. LTD. since 1999. Heis a director of a department of TPRI which is responsiblefor the technology of metal corrosion and protection ofthermal equipment and water treatment for power plants.

Songyan Cao (M.S., Applied Chemistry, University ofNortheast China Institute of Electric Power, M.E., ThermalPower Engineering, Xi'an Thermal Power ResearchInstitute) started her career in 2004 in a power plant inJiangsu province in China. Since 2006, she has been

working at TPRI Co., LTD. with emphases on water chem-istry treatment, corrosion and protection of thermal equip-ment. In 2011 she obtained a Master's degree from TPRI.

Hongbo Zhang (M.S., Environmental Engineering, M.E.,Chemical Engineering, University of Northeast ChinaInstitute of Electric Power) has been working at TPRI Co.,LTD. in China with emphases on water chemistry treat-ment, corrosion and protection of thermal equipmentsince 2011.

CONTACT

Zhigang LiThermal Power Research Institute32, Xiying RoadXi'an, Shaanxi ProvinceP. R. China, 710043

E-mail: [email protected]

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