JP2013068341A - Method for preventing corrosion of economizer in boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for effectively preventing corrosion of an economizer in a boiler having the economizer using exhaust gas heat of the boiler for boiler feed-water.SOLUTION: The method for preventing corrosion of an economizer using the exhaust gas heat of the boiler for heating boiler feed-water includes continuously passing water to the economizer by continuously operating a feed-water pump for feeding water to the boiler.

Description

  The present invention relates to a method for preventing corrosion of an economizer in a boiler. More specifically, the present invention relates to a method for effectively preventing an economizer by continuously passing the exhaust gas heat of a boiler through an economizer used for heating boiler feed water.

Conventionally, the main factors that cause boiler system corrosion are (1) high dissolved oxygen concentration, (2) out of the appropriate pH range, (3) chloride ions and sulfuric acid. There are three points such as high concentration of harmful ions such as ions.
Therefore, as a method to prevent corrosion on the water side in the boiler in the boiler water system, generally, a chemical is injected to control the pH within an appropriate range, to remove dissolved oxygen, or to form an anticorrosion film on the steel surface. The method of doing is performed. At this time, in order to inject the drug in consideration of the concentration in the boiler can, in the non-concentrated part, the pH is low and the drug concentration is low, so that the formation of the anticorrosion film becomes insufficient.
Since the economizer of a boiler is generally a non-concentrated part, the concentration is low even when a chemical is added at a low pH, and the formation of an anticorrosive film becomes insufficient. Moreover, although the water temperature rises inside the economizer, the dissolved oxygen concentration is high, so that it is highly corrosive and has many obstacles. In particular, carbon steel economizers for low-pressure boilers have a high frequency of corrosive hole breakage, and can sometimes become broken two to three years after installation.

Examples of the anti-corrosion technology of the economizer include a corrosion prevention method using chemicals and a corrosion prevention method using a deoxidizer.
Examples of the anticorrosion method using a chemical include a method in which the chemical is injected upstream of the economizer, the pH is adjusted, and an anticorrosive film is formed on the surface of the steel material. At this time, if a chemical agent having a concentration sufficient for preventing corrosion of the economizer is added, the pH and electrical conductivity are excessively increased in the concentrating part such as in a boiler can, and carryover and alkali corrosion are likely to occur. On the other hand, when the drug is injected at a low concentration in consideration of concentration in the boiler can, the anti-corrosion film is insufficiently formed in the non-concentrated part such as a water supply pipe or an economizer because the drug concentration is low at a low pH. .

As an anticorrosion method using chemicals, Patent Document 1 discloses a boiler in which a boiler water treatment composition containing an organic polybasic acid having two or more carboxyl groups in the molecule or a salt thereof is added to boiler water during boiler operation. A method for preventing corrosion is disclosed.
Patent Document 2 discloses a corrosion prevention method for boiler systems such as an economizer water pipe and a boiler water pipe, in which a silicate is injected into boiler feed water to form an anticorrosion coating.
In addition, Patent Document 3 describes corrosion and pores in a boiler in which tartaric acid or a salt thereof, citric acid or a salt thereof, and an alkaline agent in an amount capable of adjusting the pH of the boiler feed water to 8 to 12 are added to the boiler feed water. A food prevention method is disclosed.
On the other hand, in the case of anticorrosion by a deoxygenation device such as a nitrogen substitution type, the deoxygenation device having a performance capable of removing oxygen to a low concentration capable of anticorrosion of the economizer becomes large and often requires a large amount of capital investment, The scope of application is limited.

JP-A-4-232285 JP 2001-336701 A Japanese Patent Laid-Open No. 2005-220396

In Patent Documents 1 and 2, only the evaluation in the boiler can is performed, and no investigation is made on the effect on the economizer which is a non-concentrated portion and has greatly different pH conditions.
Further, in Patent Document 3, since only continuous flow conditions are simulated in the desktop evaluation according to Test Example 1, the operation conditions of an actual boiler in which flow and stop are repeatedly performed by ON / OFF of the water supply pump are studied. Absent.
The present invention has been made under such circumstances, and it is an object of the present invention to provide a method of effectively preventing corrosion of an economizer in a boiler equipped with an economizer that uses boiler exhaust gas heat for heating boiler feedwater. To do.

  As a result of intensive studies to solve the above problems, the present inventors have found that the influence of ON / OFF of the water supply pump promotes corrosion and pitting corrosion of the economizer, and the water supply pump supplied as a countermeasure for this The present invention has been completed by finding that the above-mentioned problems can be solved by continuously operating and supplying water to an economizer.

That is, the present invention provides the following [1] to [5].
[1] A corrosion prevention method for an economizer that uses the exhaust gas heat of a boiler for heating boiler feed water, and continuously feeds water to the economizer by continuously operating a feed water pump that supplies feed water to the boiler. An anti-corrosion method for an economizer characterized by:
[2] The economizer anticorrosion method according to [1] above, wherein a flow rate of water supply in the economizer is 5 cm / s or more.
[3] The anticorrosion method for an economizer according to [1] or [2] above, wherein the boiler is a small once-through boiler.
[4] The economizer anticorrosion method according to any one of [1] to [3], wherein the anticorrosive agent is added to the water supply in proportion to the water supply flow rate.
[5] The anticorrosion method for an economizer according to any one of the above [1] to [4], wherein the anticorrosive is an organic acid having a hydroxy group in the molecule.
[6] The economizer anticorrosion method according to any one of the above [1] to [5], wherein the pH of the water supply is adjusted to 8.5 or more by adding an alkaline agent to the water supply.

  ADVANTAGE OF THE INVENTION According to this invention, in the boiler provided with the economizer which utilizes the exhaust gas heat of a boiler for the heating of boiler feed water, the method of preventing corrosion of an economizer effectively can be provided.

It is the schematic of the test apparatus produced imitating the economizer of the small once-through boiler used in the Example and comparative example in this invention.

[Anti-corrosion method]
The economizer anticorrosion method of the present invention (hereinafter, also simply referred to as “the anticorrosion method of the present invention”) is an economizer anticorrosion method that uses the exhaust gas heat of a boiler for heating boiler feedwater, and supplies water to the boiler. The water supply pump is continuously operated to continuously pass water through the economizer.

The target water system in the anticorrosion method of the present invention is a boiler water system, and any of pure water supply, reverse osmosis (RO) water supply, and soft water supply water can be applied as the water supply type.
The type of the boiler is not particularly limited as long as it has an economizer that uses the exhaust gas heat of the boiler for heating the boiler feed water, such as a small once-through boiler, special circulation boiler, water tube boiler, round boiler, exhaust heat recovery boiler, etc. However, it can be suitably used for small once-through boilers that frequently flow and stop when the feed water pump is turned on and off.
The small once-through boiler in the present invention means a special circulation boiler in JIS B8223. That is, it is mainly composed of a pipe, and refers to a water pipe boiler in which water is fed from one end, a brackish water mixture is taken out from the other end, separated by a brackish water separator, and the amount of hot water returning to the heating pipe is 50% or less of the brackish water mixture. . Since the water supply method of the small once-through boiler is generally supplied by ON / OFF control of the water supply pump, the flow and stop of the supply water are repeated inside the heat transfer pipe of the economizer installed in the small once-through boiler.
Although there is no restriction | limiting in particular in the pressure of the boiler to apply, Since it will become easy to thermally decompose the anticorrosive agent etc. which will be added if a pressure becomes high too much, Preferably it is used under the pressure of 3.0 MPa or less, More preferably, 2.0 MPa or less It is desirable to do.

Next, the anticorrosion method of the economizer of the present invention will be specifically described.
In the present invention, a boiler in which an economizer is installed between the feed water pump and the boiler body is used.
The water supply to the boiler is controlled by controlling the water supply pump so that the water supply is maintained above the boiler safe low water level (required minimum water supply) and below the high water level according to the amount of steam generated. It is continuously operated and water is continuously passed through the economizer so that the water supply does not stop.
The feed water pump is preferably controlled by inverter control that corrects the feed water amount to an appropriate value by controlling the motor rotation speed of the feed water pump with an inverter. A command signal is output.

The flow rate of the water supply in the economizer is preferably 4 cm / s or more, more preferably 8 cm / s or more, and still more preferably 15 cm / s or more from the viewpoint of the anticorrosion effect. When the flow rate is less than 4 cm / s, iron rust or the like is deposited, and secondary corrosion tends to occur in the lower part thereof, and the anticorrosion effect may be insufficient when the chemical component is used together.
In the present invention, by continuously supplying water to the economizer by the above water supply method, the following chemical components are added in proportion to the water supply flow rate from the upstream of the economizer as necessary, whereby the economizer and the boiler are added. In addition, the inside of the can is protected against corrosion.

[Additive drug components]
In the anti-corrosion method of the present invention, the anti-corrosion effect is synergistic if water is continuously passed through the economizer by the above-mentioned water supply method, and if necessary, the chemical component is added in proportion to the feed water flow rate from the upstream of the economizer. The frequency of occurrence of oxide film defects is greatly reduced. Thereby, not only an economizer but the inside of a boiler can can be prevented in combination.
Examples of the additive chemical component include existing anticorrosives, alkali agents, scale inhibitors, oxygen scavengers, and neutralizing amines.
These additive drug components may be arbitrarily mixed and added to the boiler water system as long as the object of the present invention is not impaired, or may be added separately and mixed in the boiler water system.
These additive drug components can be used singly or in combination of two or more.

(Anticorrosive)
Although there is no restriction | limiting in particular as an anticorrosive, From the viewpoint of an anticorrosive effect, the organic acids which have a hydroxyl group in a molecule | numerator are preferable.
Examples of organic acids having a hydroxy group in the molecule include citric acid, isocitric acid, gluconic acid, glycolic acid, tartaric acid, erythorbic acid, malic acid, lactic acid, ascorbic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid ( PBTC) and hydroxyphosphonoacetic acid (HPAC), and salts thereof may be used.
Examples of the salt include alkali metal salts such as sodium salt and potassium salt, ammonium salt and the like.
The 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and hydroxyphosphonoacetic acid (HPAC) have the following structures.

Among these organic acids, citric acid, tartaric acid, gluconic acid, erythorbic acid, ascorbic acid and salts thereof are more preferable.
In addition, the addition concentration with respect to boiler feed water of these organic acids becomes like this. Preferably it is 20-60 mg / L in total, More preferably, it is 25-55 mg / L in total.

(Alkaline agent)
Alkaline agents can be used to maintain boiler feed water and boiler water pH at 8.5 or higher, thereby inhibiting metal corrosion.
The higher the pH is, the higher the anticorrosion effect is. However, in an aqueous system in which concentration occurs in the latter stage, such as a boiler aqueous system, if the pH is too high, the operation of the latter stage is affected. 10.5, more preferably pH 8.5 to 10.0, still more preferably pH 8.5 to 9.5.
Examples of the alkali agent include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide, alkali metal carbonates such as sodium carbonate and potassium carbonate, alkali metals such as trisodium phosphate and sodium hydrogen phosphate. A phosphate etc. are mentioned. Among these, alkali metal hydroxides are preferable from the viewpoint of pH adjustment effect and carbon dioxide is not generated by thermal decomposition, and sodium hydroxide, potassium hydroxide and the like are more preferable from the viewpoint of economy.

(Scale inhibitor)
Examples of the scale inhibitor include various phosphates, water-soluble polymer compounds such as polyacrylic acid, polymaleic acid, and sodium salts thereof, phosphonates, and chelating agents.

(Oxygen absorber, oxygen absorber)
Examples of the oxygen scavenger include hydrazine, carbohydrazide, 1-aminopyrrolidine, 1-amino-4-methylpiperazine, N, N-diethylhydroxylamine, tannin (acid) and its salt, erythorbic acid and its salt, ascorbic acid And salts thereof.
Moreover, you may use together with deoxygenation apparatuses, such as nitrogen substitution type, a film | membrane type, and a vacuum type.

(Neutralizing amine)
Examples of neutralizing amines include monoethanolamine (MEA), cyclohexylamine (CHA), morpholine (MOR), diethylethanolamine (DEEA), monoisopropanolamine (MIPA), 3-methoxypropylamine (MOPA), 2-Amino-2-methyl-1-propanol (AMP) or the like can be used.

[Adjustment of water quality]
When the M-alkalinity of boiler feed water and silica are insufficient, the anticorrosion effect can be further improved by adding an alkali agent and a silicate (Na ₂ SiO ₂ or the like).
The concentration of the alkali agent and silicate is such that the M-alkalinity is 5 mg CaCO ₃ / L or more, the silica is 5 mg SiO ₂ / L or more, preferably the M-alkalinity is 10 mg CaCO ₃ / L or more, and the silica is 10 mg SiO ₂ / L. The dosage is preferably adjusted so that the M-alkalinity is 15 mg CaCO ₃ / L or more and the silica is 15 mg SiO ₂ / L or more, more preferably L or more.

  The anticorrosion method of the present invention is an anticorrosion method for continuously supplying water to the economizer, and in combination with chemicals as necessary, by adding in proportion to the water supply flow rate from the upstream of the economizer, the inside of the economizer and boiler can be combined. Can be effectively anticorrosive.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.

Examples 1-10 and Comparative Examples 1-10
Using the test apparatus produced by imitating the economizer of the small once-through boiler shown in FIG. 1, the operation was performed as follows, and the anticorrosive effect of the present invention was confirmed.
The heater 3 installed in the test water tank 2 and the heater 14 installed in the heat medium tank 13 were respectively operated to heat the test water and the heat medium. For the water supply, synthetic water obtained by adding a reagent to pure water to a predetermined concentration was used, and the anticorrosive agent was added from the chemical injection tank 4 to the water supply line via the chemical injection pump 5 linked to the water supply pump 6. .
Thereafter, degreased and weighed test tubes were installed in the test column 8 and the test column 10, respectively, and the water supply pump 6 and the heat medium pump 15 were operated to start the test. The dissolved oxygen was saturated.

The test was conducted at 60 ° C. and 120 ° C., assuming temperature ranges of the water inlet and outlet of the economizer. The water supply method is intermittent water supply (operation where the water supply pump is started 1 minute 40 seconds (startup flow rate 24 cm / s), stop operation 1 minute repeated) and continuous water supply (flow rate is always 15 cm / s) The anti-corrosion effect was confirmed by continuous operation of the pump. Moreover, the amount of water supply per hour was set to 37.5 L / h, respectively. After the predetermined test period, the apparatus was stopped, and after cooling, derusting treatment was performed according to JIS K100, and the test tube corrosion weight loss during the period was calculated.
Tables 1 and 2 show the corrosion rate as a test period of 3 days by subtracting the 3-day corrosion weight loss from the 6-day corrosion weight loss at each temperature condition.

As test water, synthetic water having the following composition was used. Moreover, the anticorrosive agent was added to the water supply line so that it might become the density | concentration shown in Table 1 and Table 2.
(Composition of synthetic water)
Base: Ultrapure water NaHCO ₃ : 40 mg / L as CaCO ₃
HCl and NaCl: Cl ^- as 40 mg / L
Na ₂ SO ₄ : 40 mg / L as SO ₄ ^2-
Na ₂ SiO ₃ : 30 mg / L as SiO ₂
Feed water pH: 25 ° C. 9.0 (adjusted with NaOH or HCl)

  From the comparison between Examples 1 to 5 and Comparative Examples 1 to 5 in Table 1 and the comparison between Examples 6 to 10 and Comparative Examples 6 to 10 in Table 2, any temperature condition is continuous compared to intermittent water supply. It turns out that corrosion rate becomes very small when water is supplied, and the anticorrosion effect is excellent.

Examples 11 to 13 and Comparative Example 11
A simple test device simulating an economizer of a small once-through boiler in which test water heated to 60 ° C. in a water supply tank flows in a test tube wrapped with a ribbon heater was used. As test water, synthetic water prepared in the same manner as in Examples 1 to 10 was used. The dissolved oxygen was saturated.
The test was conducted at a temperature condition of 60 ° C. The water supply method consists of intermittent water supply (repeated operation of 1 minute (starting flow rate 20 cm / s) and stopping 1 minute), continuous water supply (flow rate is always 5 or 10 or 20 cm) The anticorrosive effect was confirmed by continuous operation of the pump so as to be / s. After the predetermined test period, the apparatus was stopped, and after cooling, derusting treatment was performed according to JIS K100, and the test tube corrosion weight loss during the period was calculated.
In Table 3, the corrosion rate is shown as a test period of 7 days by subtracting the 3-day corrosion weight loss from the 10-day corrosion weight loss for each flow rate condition.

  When comparing Examples 11 to 13 and Comparative Example 11 in Table 3, even when the flow rate condition changes, the corrosion rate is much smaller during continuous water supply than during intermittent water supply, and the anticorrosive effect is improved. It turns out that it is excellent.

  According to the corrosion prevention method of the present invention, an economizer that is a non-concentrating part of a boiler can be effectively prevented by a simple operation.

DESCRIPTION OF SYMBOLS 1 Supplementary water 2 Test water tank 3, 14 Heater 4 Chemical injection tank 5 Chemical injection pump 6 Feed water pump 7 Flow meter 8, 10 Test column 9, 11 Thermometer 12 Back pressure valve 13 Heat medium tank 15 Heat medium pump 16 Heat exchanger 17 Flow control valve

Claims (6)

  An anti-corrosion method for an economizer that utilizes boiler exhaust gas heat to heat boiler feed water, wherein continuous operation of a feed water pump for supplying water to the boiler allows continuous water flow to the economizer. A featured economizer anticorrosion method.   The method for preventing corrosion of an economizer according to claim 1, wherein a flow rate of water supply in the economizer is 4 cm / s or more.   The anticorrosion method for an economizer according to claim 1 or 2, wherein the boiler is a small once-through boiler.   The anticorrosion method for an economizer according to any one of claims 1 to 3, wherein an anticorrosive agent is added to the water supply in proportion to the water supply flow rate.   The anticorrosion method for an economizer according to any one of claims 1 to 4, wherein the anticorrosive is an organic acid having a hydroxy group in the molecule.   The anticorrosion method for an economizer according to any one of claims 1 to 5, wherein the pH of the feed water is set to 8.5 or more by adding an alkaline agent to the feed water.

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