Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
123

Development of Analysis Method for Iron Tracing inBoiler Feedwater Using Filter Concentration Method
AYAKO URESHINO*1 TETSUYA SAWATSUBASHI*1

AKIHIRO NOZAKI*1

HIROSHI AKAMINE*2

SENICHI TSUBAKIZAKI*3

Boilers that use an oxygen treatment method (OT) for water treatment have the risk where
the growth of powder-like scale consisting of eluted iron in feedwater equipment causes heattransfer inhibition and leads to problems such as tube leakage. For this reason, it is necessary tocontrol the iron concentration of boiler feedwater to 2 μg/L or less to maintain stable boiler
operation. However, the conventional analysis technique of minute amounts of iron is subject to theenvironmental effects and has the problem of ensuring analysis accuracy. This time, we developed
a double filter concentration method that simultaneously uses a membrane filter and an
iminodiacetate type chelate filter as a method to analyze minute amounts of iron easily andinexpensively. This paper presents the developed method. This method is adopted in the JISB8224
Boiler Feed Water and Boiler Water Testing Methods revised and published in March 2016.

|1. Introduction
In recent years, an increase of the amount of iron brought into boilers and the adhesion ofhematite scale on the inner surface of furnace wall pipes have been occurring and causing anincrease in the furnace wall pipe metal temperature in plants that use an oxygen treatment method(OT)(1).
The adhered hematite scale has a low thermal conductivity and a porous shape with a small
particle diameter, and is called “powder scale.” In plants that experienced the adhesion of powder
scale on the inner surface of furnace wall pipes after the application of OT, the iron concentrationin the drain system of the low-pressure feedwater heater increased. It is believed that problems
caused by the adhesion of powder scale have a high possibility of occurring if the ironconcentration at the boiler inlet exceeds 2 μg/L, and therefore the iron concentration at the boiler
inlet should be controlled to 2 μg/L or less.

|2. Iron analysis method used in thermal power plants
Table 1 shows examples of the methods adopted as analysis methods of the ironconcentration in boiler feedwater. In many power plants, a 2,4,6-tris-2-pyridyl-1,3,5-triazine
absorptiometric method (TPTZ method) is used because it does not require expensive analysis
devices and is easy to operate.
Figure 1 shows a schematic diagram of the reaction between TPTZ and iron. The TPTZ
method is a method to quantitatively determine iron by adding hydroxylammonium chloride and2,4,6-tris-2-pyridyl-1,3,5-triazine into an acid solution, adding ammonium acetate to adjust the pH
value to 4 to 5, and then measuring the absorbance of generated blue iron (II) complex. The ironquantitative determination range of the TPTZ method using a 20-mm absorption cell is 1 to 30 μg,
and the repeated analysis accuracy ranges from 2% to 10%. For this reason, the quantitative
determination of low-concentration iron requires analyzing a concentrated sample using a cell witha long light path length. For the quantitative determination of iron in boiler feedwater containing 2*1

Chemical Research Department, Research & Innovation Center*2

Plant Engineering Department, Plant Engineering Division, Mitsubishi Hitachi Power Systems, Ltd.

*3

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

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
124

μg/L of iron using a 50-mm absorption cell, it is theoretically necessary to concentrate the sample
by at least 4 times.
Table 1

Quantitative determination method of iron in boiler feedwater(extracted from page 506 of Thermal and Nuclear Power Plant Handbook)Analysis method
Phenanthroline
molecular absorption
spectrometryFrame atomic
absorption method
TPTZ method
ICP emission spectral
analysis method

Quantitative
Analysis accuracy
determination range[%]

Density lower limitof quantitative
determination[mg/L]

Sampling amount[mL]

20 - 500μg

2 - 100.1

200

0.3 - 6 mg/L

2 - 100.3―

0.001 - 0.03 mg

2 - 100.005

200

20 - 5000μg/L

2 - 100.02―

Figure 1 Reaction of TPTZ and iron

|3. Problem with application of heating concentration and TPTZ
method to boiler feedwater(1)

Problem with operation and analysis time period
As described above, one method to measure the 2 μg/L of iron accurately in a stablemanner is to increase the sampling amount and perform concentration. Generally, concentrationis performed by heating the sample to evaporate the moisture. However, the heating
concentration method is not realistic in terms of operation because the method requires time. It
is also considered that heating concentration becomes more subject to contamination when theconcentration time period is longer, and thus there is a limit on the usable sample amount.
(2) Problem with characteristics of sample
It is believed that in plants that use OT, the use of high-purity water containing amoderate amount of dissolved oxygen generates less soluble-in-water hematite on steel surfaces
and results in various effects(1). Therefore, it is assumed that iron in boiler water exists ashematite particles.
The quantitative determination, using a plasma emission spectrometric analysis device, ofiron concentration in a sample collected from a low-pressure heater drain in a plant that uses
OT was performed before and after filtration using a 0.45 μm Omniopore membrane filter.
Table 2 shows the quantitative determination results. It was confirmed that about 78% of theiron contained in the sample consists of particles 0.45 μm or larger and the amount of dissolved
iron is about 22% or less. The suspended iron to be measured by the TPTZ method needs to besoluble iron, and it may be necessary to take measures such as the extension of the dissolving
treatment time in the case of less soluble hematite.
Table 2

Classification of iron in low-pressure heater drain water
SampleBefore filtering
Filtrate (<0.45 μm)
Filtration residue (>0.45 μm)(Calculated value)

Iron density[μg/L]
7.6
1.7Proportion[%]
100225.978

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
125|4. Filter concentration and TPTZ method
As a method to concentrate a large amount of the sample in a short period of time, wefocused on solid phase extraction, and considered a concentration method using a filter (filterconcentration method) as the analysis method for minute amounts of iron in the feedwater of aplant that uses OT. Iron in the feedwater of a plant that uses OT exists as iron particles that consist
mainly of hematite particles and ionic dissolved iron that is dissolved in water. Thus, we designed adouble filter method that collects iron particles on a membrane filter and dissolved iron on a chelate
filter. Figure 2 shows a schematic view of the double filter concentration method.
Iron particles are collected by the membrane filter with a pore diameter of 0.45 μm
depending on the particle size. On the other hand, dissolved iron is collected as a complex by resinwith an iminodiacetate group and then recovered by eluting with acid. The iminodiacetate group
has a high selectivity against transition metal and thus can be used for the collection of minuteamounts of iron in boiler feedwater. Figure 3 shows a schematic diagram of iron collection using achelate filter.

Figure 2

Schematic diagram of double filter concentration method

Figure 3

Schematic diagram of iron collection using a chelate filter|5. Comparison with conventional method(1)

Analysis time period
Figure 4 compares the analysis flow between the heating concentration method and thefilter concentration method. The heating concentration method takes approximately 180
minutes to concentrate 300 mL of sample (six-times concentrated) using a hot plate. On theother hand, the filter concentration method takes 70 minutes in total (20 minutes for filter
concentration and 50 minutes for heating concentration) to concentrate 2000 mL of sample(forty-times concentrated). In this way, the filter concentration method can complete
concentration in about 1/3 of the time in the case of heating concentration. In addition, the filter
concentration method has the advantage where the sampling amount can be increased
arbitrarily within the iron collection capacity of the filter regardless of the sampling amount.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
126

Figure 4 Comparison of analysis flow between heating
concentration method and filter concentration method(2)

Analysis accuracy
Table 3 shows the results of quantitative determination using a standard iron solution.
The recovery rate was favorably within the range of 100+/-5% excluding samples of 4 μg/L orless recovered by the heating concentration method. Table 4 shows the average value, thestandard deviation (σ) and the coefficient of variation (C.V.) obtained as a result of fiverepeated analyses of distilled water. The lower limit of the quantitative determination calculated
based on 10σ is 4.1 μg/L for the heat concentration method and 0.8 μg/L for the filter
concentration method. This indicates that the use of the filter concentration method allows
higher-accuracy analysis in comparison with the heating concentration method.
Table 3

Evaluation of recovery rate using standard iron solutionMethod

Heating concentration method

Filter concentration method

Densityμg/L
2468
10246810

Recovery rate%
116.6
110.4
104.8
103.2
99.7
100.7
101.7
99.6
97.4
97.2

C.V.
%
19.1
10.1
6.6
6.3
2.9
8.1
3.5
2.3
7.8
1.9

C.V.: Coefficient of variation

Table 4Lower limit of quantitative determination obtained by repeated analysis (n=5)
of distilled waterμg/L―%

Heating concentration
method10.41
39Filter concentration
method0.75
0.08
11μg/L

4.1

0.8

Concentration method

Unit

Iron density average valueStandard deviation (σ)
Coefficient of variation (C.V.)
Density lower limit ofquantitative determination (10σ)

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
127

(3)

Application to real sample
The developed filter concentration method was applied to an ultra-supercritical thermal
power plant that uses OT. Figure 5 shows a schematic system diagram of the plant. Table 5shows the results of analysis of time variation in the iron concentration in feedwater performed
using the heating concentration method and the filter concentration method, respectively. Thesampling was performed in a period when the load was in a static state. The sampling wasperformed from the sampling rack of each point in the system when the load was in a staticstate after three hours or more from the moment the load became static, and the ironconcentration of each point in the system water at the same time was determined. At theeconomizer inlet, which was the main evaluation target, the sample amount used for the filter
concentration method was 2000 mL. At the low-pressure heater drain and the high-pressure
heater drain, analysis was performed using the heating concentration method only. Because ironconcentration in the feedwater varies depending on the boiler state, samples collected atdifferent times were handled as separate samples.
The quantitative values of certain areas, which could not be obtained by the heating
concentration method, could be obtained by the filter concentration method. Thus the massbalance evaluation of the system became possible.

Figure 5 Schematic diagram of water and steam system inultra-supercritical thermal power plants
Table 5 Time variation in iron concentration in ultra-supercritical thermal power plant
Sampling location

Concentration Sample amountmethod(mL)

Iron density (μg/L)
9:00

11:00

13:00

15:00

17:00

5.0

N/A

N/A

N/A

N/A

Condensate pumpoutlet

Heating300

Filter

1000

4.9

3.6

3.7

3.7

4.9

Condensate booster
pump outlet

Heating

300

N/A

N/A

N/A

N/A

N/A

Filter

1000

2.5

1.4

1.3

1.4

2.7

Heating

300

N/A

5.1

4.5

4.2

N/A

Filter

1000

6.2

4.7

4.4

4.8

4.6

Heating

300

N/A

4.1

N/A

N/A

N/A

Filter

1000

5.2

4.1

3.2

4.0

4.5

Heating300

N/A

N/A

N/A

N/A

N/A

Filter

2000

2.7

2.4

1.8

1.8

2.0

Heating

300

N/A

N/A

N/A

N/A

N/A

Filter

1000

1.3

N/A

1.1

1.2

1.3

Heating

300

2924212118

Heating

300

1515151415

Deaerator inlet
Deaerator outlet
Economizer inlet
Main steam
Low-pressure
feedwater heater drain
High-pressure
feedwater heater drainMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
128|6. Countermeasure for powder scale
For reduction in the risk of damage to heat transfer tubes caused by the adhesion of powder
scale, it is necessary to reduce the iron amount brought to the boiler. Methods for this include theadjustment of the feedwater quality (change in pH), the installation of an iron removal
high-temperature filter (Figure 6) in the low-pressure feedwater heater drain system that isassumed as the main iron generation source, etc. For the evaluation of the risk of powder scale
deposition, on the other hand, it is necessary to perform daily management of the mass balance ofiron in the plant system in order to understand the elution and deposition behaviors of iron, andtherefore evaluation using the filter concentration method described above is important.

Figure 6 External view of iron removal device for low-pressure feedwater
heater drain system|7. Conclusion
As a result of the application of the filter concentration method that simultaneously uses amembrane filter and a chelate filter, as an analysis method for minute amounts of iron, instead of
the conventional heating concentration and TPTZ method and the ICP-MS method for the ironconcentration analysis of boiler feedwater, it was confirmed that the filter concentration method
can more accurately analyze minute amounts of 2 μg/L of iron or less by the application of thefilter concentration method, and also that the filter concentration method allows a reduction inanalysis time and is suitable for operational sites.
It is believed that the optimization of analysis conditions and improvement in operability willallow even workers that are unfamiliar with the analysis method to minimize the error and performthe analysis of minute amounts of iron accurately, because the analysis of minute amounts of asubstance is greatly influenced by contamination. In the future, we will apply this method to theanalysis of minute amounts of iron in various power plants in order to continue to accumulate andevaluate data.

References
1.
2.
3.

Takada, Consideration of Countermeasures for Powder Scale Deposit Problems in CWT Operation Plants,
Conference Proceedings of Thermal and Nuclear Power, (2012)
Ureshino ， Double Filter Concentration Method as Pretreatment for Determination of Iron in
Ultra-supercritical Pressure Boiler Feedwater，BUNSEKI KAGAKU vol.64，No.8，p.609-615
JISB8224-2016，Boiler Feed water and Boiler Water – Testing Methods