An alternative on-line method for chloride determination, ion chromatography, discussed in Section 13 of this manual, uses a specific conductivity detector [13,14]. The trace anion method utilizes a sample pre-concentration column where the anions of interest are collected on an ion exchange column. After a suitable volume of sample has been concentrated, eluent is pumped through the concentrator column to remove the trapped anions. Eluent then flows through the analytical column where the anions are separated based on the retention characteristic of each anion relative to the eluent used. The eluent stream containing the anions of interest then passes through a suppressor device where the cations from the eluent are exchanged for hydrogen ions, converting the anions to their acid form. After the suppressor device, the eluent solution passes through a conductivity detector where the separated anions are detected. Detection limits for the anions are enhanced because the anions are in the acid form rather than the sodium salt. This method reports a single operator detection limit of 0.8 µg/L (ppb). The common practical range of the method is listed as 1-100 µg/L (ppb) for chloride.
At least one instrument manufacturer has commercially available on-line ion chromatography equipment operating in the electric utility setting [15].
10.8 End User Considerations
The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line sodium instrument. In general,
manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application.
Other on-line chloride instrument considerations include:
• Appropriate pH adjustment for analysis range of interest to minimize OH- ion interference with chloride measurements.
• Ease and robustness of calibration for the intended use.
• Ease and robustness of calibration verification.
• Appropriate sensing electrode linearity characteristics in the analytical range of interest.
• Reference electrode stability.
10.9 References
1. Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.
2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.
3. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.
4. Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.
5. ASTM D5542-04, “Standard Test Method for Trace Anions in High Purity Water by Ion Chromatography”. 2004 Annual Book of ASTM Standards, Vol. 11.02 Water. American Society for Testing and Materials, Philadelphia, PA.
6. ASTM D5996-05, “Standard Test Method for Measuring Anionic Contaminants in High-Purity Water by On-Line Ion Chromatography”. 2005 Annual Book of ASTM Standards, Vol. 11.02 Water. American Society for Testing and Materials, Philadelphia, PA.
7. ASTM D512-04, “Standard Test Method for Chloride Ion in Water”. 2004 Annual Book of ASTM Standards, Vol. 11.01 Water. American Society for Testing and Materials,
Philadelphia, PA.
8. The Volatility of Impurities in Water/Steam Cycles. EPRI Palo Alto, CA: 2001. 1001042.
9. Model 1817LL Chloride Analyzer Instruction Manual, Thermo Scientific Corporation, Beverly, MA. 2003 S-1817LL-E-0506 RevB.
11. ASTM D512-04, “Standard Test Method for Chloride Ion in Water”. 2004 Annual Book of ASTM Standards, Vol. 11.01 Water, American Society for Testing and Materials,
Philadelphia, PA.
12. Standard Methods for the Examination of Water and Wastewater, 21st Edition. Baltimore, MD. 2005.
13. ASTM D 5996. “Standard Test Method for Measuring Anionic Contaminants in High-Purity Water by On-Line Ion Chromatography”. ASTM International, West Conshohocken, PA. 2005.
14. ASTM D 5542. “Standard Test Method for Trace Anions in High Purity Water by Ion Chromatography”. ASTM International, West Conshohocken, PA. 2004.
15. Dionex Process Analytical DX-800 Product Information, Dionex Corporation, 1228 Titan Way, Sunnyvale, CA 94088-3603. 2004.
16. Fossil Power Plant Chemistry, Scientech, LLC, 1060 Keene Road, Dunedin, FL 34698-6300, 2001.
11
HYDRAZINE
11.1 Purpose and Use
Hydrazine is a reducing agent that is frequently used in boiler feedwater treatment to promote oxygen removal. Hydrazine reacts with oxygen to form water and nitrogen and, under certain conditions (e.g. temperatures > 270°C (518°F)), it can also decompose to form ammonia and nitrogen. Hydrazine is also instrumental in promoting magnetite (Fe3O4) formation which provides an iron oxide surface that consumes dissolved oxygen. Reducing agents such as
hydrazine are critical in mixed metallurgy systems (systems containing copper alloys) to prevent copper corrosion due to amines in the presence of dissolved oxygen.
While hydrazine is not listed as an EPRI Core Monitoring Parameter [1-3], it is often monitored in mixed metallurgy feedwater cycles using reducing All Volatile Treatment—AVT(R). This chemical treatment scheme refers to the feedwater treatment and is not influenced by further addition of phosphate or caustic in the boiler drum. The prescribed location for this sample point is the deaerator inlet, although many plants sample the economizer inlet as well. Proper control of hydrazine as a reducing agent is necessary to protect the copper alloys in the mixed metallurgy system from oxygen attack. Overfeeding hydrazine can lead to excess ammonia in the system, which can also be damaging to the copper alloys. Hydrazine feedrate can be indirectly controlled by measurement of the oxidation reduction potential (ORP) of the system—discussed in detail in Chapter 6.
Hydrazine is continually monitored on-line in the plant for the following reasons, listed in order of importance:
• To check the accuracy of water chemistry control, so ensuring that corrosion rates are kept at acceptably low levels.
• To evaluation of other chemistry parameters (i.e., ORP and dissolved oxygen).
• To provide feedback stimulus for automated process control.
The data generated by continuous on-line monitoring of hydrazine is used by plant chemistry and operations personnel. The goal for plant personnel is to maintain hydrazine within prescribed limits.
11.2 Description of Methods
Several methods are available for monitoring hydrazine concentration on-line: ion
chromatography, colorimetry, amperometry, and the ion selective electrode method. Since ion chromatography is not typically used for on-line surveillance and control of hydrazine, it will be discussed in subsection 11.7, Alternative Methods of Analysis.
The colorimetric method is an automated version of the ASTMD 1385 [4] grab sample
technique, based on the reaction of hydrazine with p-dimethylaminobenzaldehyde to produce a yellow reaction product in an acidic environment. The intensity of the yellow color, which is proportional to the hydrazine concentration in the sample, is measured with a spectrophotometer at a wavelength of 460 nm.
The amperometric methods fall into two variants: a 2-electrode system in which an anode and a cathode are made from dissimilar metals, and a 3-electrode system in which anode, cathode and reference electrodes are connected to a potentiostatic circuit. In each case, the anode contacts the water sample containing hydrazine, and the potential difference between the anode and cathode promotes the oxidation of hydrazine at the anode surface. The current that flows between the anode and cathode as a result of the electrochemical reactions is proportional to the hydrazine concentration.
The ion selective electrode (ISE) method involves adding iodine to the water sample which reacts with the hydrazine to produce iodide. The concentration of iodide produced, which is proportional to the concentration of hydrazine in the water sample, is measured with an iodide ion selective electrode (sometimes called specific ion electrode).
11.3 Technical Considerations
11.3.1 Colorimetry
Typically, the water flow rate to the analyzer inlet exceeds the minimum flow required and the excess flows to waste. This arrangement ensures that a fresh sample is always available to the analyzer. Often, the sample is also conditioned to regulate the inlet water pressure. At the beginning of each analysis cycle, the measurement cell is flushed thoroughly with a fresh sample. An initial sample blank absorbance reading is taken with the spectrophotometer at a wavelength of 458 to 460 nm to provide the zero reference measurement. Then a solution of p-dimethylaminobenzaldehyde in methyl alcohol and hydrochloric acid is added to the sample.
A yellow color characteristic of p-dimethylaminobenzalazine is formed (Eq. 11-1), the intensity of which depends on the amount of hydrazine present. A second absorbance measurement is made at 458 to 460 nm and compared with the zero reference measurement. The difference between the two measurements, which is proportional to the hydrazine concentration in the sample, is displayed directly in units of hydrazine concentration.
(
CH)
NC H CHO N H(
CH)
NC H CHN HCHC H(
CH)
2H O2ρ− 3 2 6 4 + 2 4→ 3 2 6 4 − 6 4 3 2+ 2
Equation 11-1
11.3.1.1 Colorimetric Limitations
All colorimetric tests such as hydrazine rely on the principle of Beer’s Law (also known as the Beer-Lambert Law) which states that the amount of light absorbed by a sample (A) is
proportional to some absorption constant (a), the path-length of light (b), and the concentration of the analyte species(c).
A = a x b x c Equation 11-2
The absorption constant is determined by the color species and test conditions of the method and remains constant as long as the reagents, test conditions, and wavelength of light do not change.
The path length (the amount of sample through which the light passes) is also constant for a given sample cell. The mathematical formula then infers that the amount of light absorbed by a sample (i.e. the intensity of the color developed in the test) is directly proportional to the
concentration of the analyte—in this case, hydrazine. As the hydrazine concentration increases, the intensity of the yellow color will increase and this color change can be quantified.
Limitations on this analytical principle arise at both the low end and high end of the useable range. The low end limit is caused by the detection limit of the test—this is primarily a function of the photo-multiplier (detector) sensitivity and stability. How small a change can be detected by the sensor and how stable is the baseline (or the zero reading)? When trying to quantify a test right at the detection limit, negative readings are sometimes observed or duplicate readings are seen that have large relative errors although the absolute error may only be a few µg/L (ppb).
As a result, most hydrazine analyzers will list a detection limit, an accuracy limit and a precision limit. This accuracy limit is typically an absolute value or some percentage of the reading. The colorimetric analyzers reviewed did not list an absolute accuracy value. A commonly used analyzer has the following specifications which demonstrate this technical consideration [5].
• Minimum Detection Limit: 1 µg/L (ppb).
• Accuracy: ±2% of reading.
• Precision: ±1 µg/L (ppb) or ±2% of reading whichever is greater.
Evaluation of this specification shows that the uncertainty of the measurement is large relative to the measurement itself at or near the minimum detection limit. At lower concentrations (1–5 µg/L (ppb)) end users are cautioned about using data for critical decision making since the inaccuracy is large relative to the measurement itself. At concentrations >50 µg/L (ppb) the precision limit of ± 5% is used since it becomes larger than ± 1 µg/L (ppb). A different analyzer may have a different specification with a lower detection limit as a result of using another analytical method (changing the absorption constant) and using a longer path length sample.
Limitations on the high end of the analytical range (Figure 11-1) arise from a property called self-absorption. As the color intensity becomes more and more yellow, the amount of light coming into the sample cell is almost totally absorbed.
Figure 11-1
Absorption vs. Concentration
At some point there is 100 percent absorption, the line flattens out, and the analytical test is no longer usable. However, the area where linearity first starts to deviate is also an area of concern.
The molecules imparting color (absorbing light) are so numerous that some molecules are shaded from the incident light and not accounted for. The subsequently curved line can no longer be used for accurate analytical determination. The ionic solution strength at higher concentrations also starts to affect the color developing molecules and leads to non-linear response. Most instruments therefore have a maximum analytical range which corresponds to the top portion of the straight-line relationship. Analytical readings above this linear region are disallowed and the analyzer produces an “over range” alarm.
Turbidity or color in the water sample may interfere with this analysis, but this is not normally of concern in the sample streams that are typically analyzed in power plants.
11.3.2 Amperometry
11.3.2.1 Two Electrode Method
As indicated above, there are two types of amperometric methods: the 2-electrode and the 3-electrode methods, and details of both methods vary somewhat from one manufacturer to the next. For instance, in one manifestation of the 2-electrode technique, the water sample flows at a constant rate through a tube made from a porous ceramic [6]; other manufacturers make the tube
this tube in direct contact with the water sample, and a silver wire cathode is wound around the outside of the tube. The electrodes and ceramic tube are surrounded by a non-porous outer jacket. The space between the jacket and tube is filled with a mixture of gel and silver oxide.
The silver oxide, being in intimate contact with the spiral wound silver wire, becomes part of a composite cathode. Electrolytic contact between the platinum anode and silver/silver oxide cathode occurs because ionic transport is possible through the porous ceramic tube. The potential difference between the anode and cathode (essentially a galvanic couple) stimulates oxidation of hydrazine at the anode and reduction of silver oxide and plating of silver at the cathode. The resultant current depends on how rapidly hydrazine is transported to the anode and, for a given flow rate, flow geometry, pH, and temperature, the current is proportional to the concentration of hydrazine in solution. A thermistor or equivalent device may be incorporated into the measurement cell to provide automatic temperature compensation. Some manufacturers suggest that, for optimum performance, the sample should be made highly alkaline by adding sodium hydroxide to the water sample upstream of the measurement cell. The anode reaction is as follows:
N2H4 + 4OH- → N2 + 4 H2O + 4 e- Equation 11-3
The following reaction simultaneously occurs at the cathode:
Ag2O + H2O + 2e- → 2Ag + 2 OH- Equation 11-4
11.3.2.2 Three Electrode Method
Like the 2-electrode method, the 3-electrode method is configured to generate a mass transport limited current that is proportional to the hydrazine concentration. Design details vary from one manufacturer to the next, but in one manifestation, the sample must be pre-conditioned if the conductivity of the water sample is < 8 µS/cm. If pre-conditioning is necessary, the water must flow through a bed of granulated marble to increase the conductivity and adjust the pH. The water sample then flows at a constant rate through a stainless steel tube that also serves as the counter-electrode (cathode) of the measurement cell. A gold-plated stainless steel sensing electrode (anode) is positioned along the axis of this tube; a silver/silver chloride reference electrode is mounted nearby, and brought into electrolytic contact with the water sample by means of an electrolyte bridge. The bridge is typically a plastic tube, filled with potassium chloride solution, and fitted at its end with a porous ceramic diaphragm to minimize mixing of the potassium chloride and the water sample. Another manufacturer pre-conditions (increases the conductivity of) the flowing water sample by injecting potassium nitrate; the water then flows to a specially designed flow cell in which a silver counter-electrode (cathode), a platinum sensing electrode (anode), and a calomel reference electrode are mounted.
In all 3-electrode designs, the sensing electrode, counter-electrode and reference electrode are connected to a potentiostat. The potentiostat controls the electrochemical potential difference between the sensing electrode (anode) and reference electrode by supplying, and automatically adjusting, the flow of direct current between the counter- and sensing electrodes (cathode and
anode). Thus, the potentiostat replaces the stimulus of the galvanic couple in the 2-electrode method, and ensures that the electrochemical conditions prevailing at the anode surface lead to a mass transport limited current. One manufacturer that uses a platinum anode (working electrode) and stainless steel cathode (counter-electrode) reports a linear relationship between cell current and hydrazine concentration in the range of 0 to 500 µg/L (ppb) when the anode potential is controlled at +480 mV versus the Ag/AgCl reference electrode [5]. Following are the reactions at the two electrodes:
Anode: N2H4 + 4 OH- → N2 + 4 H2O + 4e- Equation 11-5
Cathode: 4 H2O + 4 e- → 2 H2 + 4 OH- Equation 11-6
The reducing agent is oxidized on the platinum electrode into nitrogen and water.
Simultaneously the counter electrode decomposes the water. For a given flow rate, flow geometry, pH, and temperature, the measured current is proportional to the concentration of hydrazine in solution (just as it is in the 2-electrode method). One advantage of the 3-electrode system is that the cathode needs very little maintenance whereas, in the 2-electrode system, the silver oxide gel mixture must be replaced when it has been consumed by the cathodic reaction.
In addition, the 3-electrode system theoretically has a faster response because both anode and cathode are in direct contact with the water sample. Teflon® beads, driven by the sample flow, circulate on the surface of the platinum anode to prevent deposition.
In all amperometric methods, the accuracy of measurements can be degraded by the presence of other oxidizable species in the water. Sulfite, for instance, must be removed by passing the sample through an anion bed before the hydrazine concentration can be measured. In addition, the concentrations of morpholine, cyclohexylamine, and ferrous ion should each be maintained below 1 mg/L (ppm) to avoid significant errors. Amperometric methods also respond to carbohydrazide (a hydrazine substitute) which may be of analytical interest or may be an interference depending on the type of reducing agent being used. The reaction of hydrazine in amperometric determinations is enhanced by an elevated pH. Amines such as ammonia, diisopropylamine (DIPPA), diethylamine (DEA), or monoethylamine (MEA) are used to condition the sample to a pH >10.2 before it enters the measuring cell.