JP2012193434A - Water treatment method of steam generation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water treatment method of a steam generation device which can suppress corrosion of an economizer made of a steel and a steam condensate system as well as evaporation pipes of a steam generator for generating steam and the inside of can body of a boiler.SOLUTION: The steam generation device 10 uses softened water as feed water and includes the economizer 8 which heats the feed water by the exhaust gas after heating the evaporation pipes. In the steam generation device 10, dissolved oxygen in the water in a water tank 1 is reduced to 2 mg/L or below by an oxygen reduction equipment 4 for reducing the dissolved oxygen in the water by replacing with nitrogen. Moreover, alkaline chemicals are added in the water tank 1 or to the feed water line between the water tank 1 and the economizer 8, in order to enhance a pH value to 9 or more of the feed water supplied to the economizer 8. Furthermore, deoxidizer is added to the feed water line between the water tank 1 and the economizer 8.

Description

  The present invention relates to a water treatment method for steam generating equipment. More specifically, the present invention relates to a water treatment method for a steam generating facility provided with an economizer that uses softened water as makeup water and heats feed water with exhaust gas.

  In steam generation equipment that generates steam using softened water as make-up water, anti-corrosion treatment is performed to prevent corrosion in the steam generator and boiler cans that contact water (for example, in the evaporation pipe). Yes. As the method, while inject | pouring chemical | medical agents, such as a corrosion inhibitor and an oxygen scavenger, an alkaline agent is added as needed so that the pH of boiler water may become predetermined value (usually 11.0-11.8). The method of adding is common.

  However, the treatment using an oxygen scavenger has a low concentration of the oxygen scavenger in the water in the non-concentrating part such as an economizer (feed water preheater) that heats the feed water with the exhaust gas after heating the evaporation tube. Therefore, it is not possible to sufficiently deoxidize, and almost no effect of reducing corrosion is obtained. The same applies to treatments that use anticorrosives, and they are not concentrated by an economizer.Therefore, if the concentration is set to exhibit anticorrosive effect in the steam generator or boiler can, which is the concentrating part, the concentration in the water supply will have anticorrosive effect. The concentration is lower than the concentration obtained, and corrosion reduction cannot be expected. In addition, when the drain condensed with steam is recovered as feed water and reused, the temperature of the feed water rises and the corrosion of the economizer further increases.

  On the other hand, using a deoxygenator, the oxygen in the water is physically removed to reduce the oxygen concentration (several to several tens of μg / L or less with a heated deaerator, countercurrent contact nitrogen displacement deoxygenator And 0.1 mg / L or less with a vacuum degassing device), there is also a method of suppressing corrosion. However, this method has a problem that the scope of application is limited because the deoxygenation apparatus is large in size, high in cost, and complicated to install in existing facilities.

  In addition, JIS B8223 defines the pH of feed water as 5.8 to 9.0, and it is said that pH 7 or higher is preferable for corrosion inhibition. However, with respect to the economizer, a sufficient anticorrosive effect is obtained within this range. I can't. As described above, the economizer is a portion where corrosion is particularly likely to occur among the respective parts constituting the steam generation facility, and the tendency is remarkable when the economizer is formed of a steel material.

  Therefore, various anticorrosive agents or anticorrosion methods have been proposed for the purpose of preventing corrosion of economizers (see, for example, Patent Documents 1 to 5). Specifically, Patent Document 1 discloses an anticorrosive agent for a soft water boiler, which is composed of a product obtained by adding an alkali metal hydroxide to an aqueous solution of a mixture of a monosaccharide and iron gluconate and reacting them. Has been.

  Patent Document 2 discloses an anticorrosion method in which silicate is injected into boiler feed water to form an insoluble anticorrosion coating with silicate on a water pipe. Furthermore, in Patent Document 3, an alkaline agent for adjusting tartaric acid or a salt thereof to 10 to 50 mg / L, citric acid or a salt thereof to 10 to 50 mg / L, and a pH of 8 to 12 is added, and iron-based metal is added. A method for preventing corrosion and pitting corrosion is disclosed.

  On the other hand, in the anticorrosion method described in Patent Document 4, a solution obtained by mixing and reacting hydrolyzed tannin or a neutral salt thereof and an alkali metal hydroxide is used as a deoxidizer, and is provided in the middle of a water supply tank or a makeup water path. Supply. In the technique described in Patent Document 4, the mixing time of the circulating water and the oxygen scavenger is increased to increase the time for the deoxygenation reaction, thereby reducing the dissolved oxygen in the feed water and supplying the water containing the economizer. The anti-corrosion performance of the entire system is improved.

  In addition, Patent Document 5 discloses an alkali metal hydroxide and, if necessary, silicic acid, etc. for the water supply in which hardness components are removed by a water softener using a cation exchange resin and dissolved gas is removed by a deaerator. An operation method of a steam boiler for supplying water to the steam boiler after adding the anticorrosive is disclosed. In this steam boiler device, in the steam boiler, a part of the boiler water which has been concentrated to increase the alkali metal hydroxide concentration and whose pH has been raised is refluxed and mixed with the feed water.

Japanese Patent Laid-Open No. 10-204662 JP 2001-335975 A Japanese Patent Laying-Open No. 2005-220396 (Patent No. 4277072) JP 2001-116207 A (Patent No. 3116042) JP 2010-230183 A

  However, the conventional techniques described above have the following problems. First, the anticorrosion method for adding a drug as described in Patent Documents 1 to 3 has a problem that the amount of the drug added increases when trying to obtain a sufficient anticorrosive effect in a non-concentrated portion such as an economizer. There is. Thus, when a large amount of a chemical is added to the water system, the electrical conductivity of the boiler water is significantly increased, and so-called “carryover” in which the boiler water itself is easily mixed into the steam generated from the boiler is likely to occur. Further, if the blow amount is increased in order to prevent the occurrence of carryover, the energy cost increases and it becomes uneconomical.

  Furthermore, in the method of adding a silicate as described in Patent Document 2, the amount of scale generated when a hardness component leaks from a water softener increases, resulting in energy loss due to heat transfer inhibition and heating of the heat transfer surface. There is a possibility that blasting will occur.

  On the other hand, the water supply tank is usually open at the top or is provided with an air vent (air vent valve) at the top. For this reason, when performing deoxygenation treatment in a water supply tank as in the anticorrosion method described in Patent Document 4, oxygen in the air is re-dissolved in the water supply, so the amount of oxygen scavenger added in accordance with the dissolved oxygen concentration When set, there arises a problem that the dissolved oxygen cannot be sufficiently removed. This problem can be solved by increasing the amount of oxygen scavenger added to the feed water. In this case, however, the amount of chemical added increases, which increases processing costs, causes carryover, and increases the amount of blow. The associated energy cost increases.

  Moreover, since the anticorrosion method described in Patent Document 5 uses a hydrogen ion type cation exchange resin, when the addition of alkali hydroxide is insufficient or stopped, the pH of feed water or boiler water is significantly reduced, There is a risk of causing significant corrosion problems. In addition, as described above, the method of physically removing oxygen to a low concentration by a deoxygenation device to reduce corrosion is large in size and high in cost, complicated to install in existing facilities, and operates. There is a problem that it is restricted.

  Therefore, the present invention provides a water treatment method for a steam generating facility capable of suppressing corrosion of a steel economizer and steam condensate piping as well as the inside of an evaporation pipe of a steam generator for generating steam and the inside of a boiler body. The main purpose is to provide.

A water treatment method for a steam generation facility according to the present invention is a water treatment method for a steam generation facility comprising an economizer that uses softened water as makeup water and heats feed water with exhaust gas after heating the evaporation pipe, A step of reducing the amount of dissolved oxygen in the water supply tank to 2 mg / L or less by a deoxygenation device that reduces dissolved oxygen in the water by replacement, and a water supply line between the water supply tank or the water supply tank and the economizer A step of adding an alkaline agent to the pH of the water supplied to the economizer to 9 or higher, and a step of adding an oxygen scavenger to the water supply line between the water supply tank and the economizer.
In the present invention, the amount of dissolved oxygen in the water supply tank is reduced to 2 mg / L or less, and the oxygen absorber is added in the water supply line. Is reduced. Moreover, since the nitrogen substitution type deoxygenation apparatus is used, removal of dissolved oxygen is easy. Furthermore, since an alkaline agent is added to the water supply so that the pH of the water supplied to the economizer is 9 or more, the solubility of the corrosion products is lowered and an iron oxide film having corrosion resistance is formed on the metal surface. . As a result, the corrosion inhibiting effect in the steam generating equipment system, particularly in the economizer is improved.
In this water treatment method, erythorbic acid and / or ascorbic acid and an alkali are contained as an oxygen scavenger, and the blending amount of the alkali is erythorbic acid and ascorbic acid in terms of P alkalinity (mgCaCO ₃ / L). An aqueous solution of 0.6 times or more the content (mg / L) can be used.
In that case, you may add the deoxidation catalyst which consists of a calcium compound and / or a magnesium compound together with the said deoxidation agent.
On the other hand, it contains tannin and / or tannic acid and an alkali as an oxygen scavenger, and the blending amount of the alkali is a content of tannin and tannic acid (mg / kg) in terms of P alkalinity (mgCaCO ₃ / L). An aqueous solution that is 0.5 times or more of L) can also be used.
In that case, a deoxygenation catalyst composed of a calcium compound may be added together with the deoxidation agent.
When the oxygen scavenger and the oxygen scavenging catalyst are used in combination, the total hardness of the water supplied from the magnesium compound and / or the calcium compound (mgCaCO ₃ / mg) is further added to the water supply line between the water supply tank and the economizer. It is desirable to add an acrylic polymer so that the concentration is equal to or higher than that of L).
Furthermore, for example, sodium hydroxide or potassium hydroxide can be used as the alkali.

  According to the present invention, after the dissolved oxygen contained in the water in the water supply tank is reduced by the nitrogen substitution type oxygen absorber, the oxygen absorber is added in the water supply line, and further supplied to the economizer by the addition of the alkaline agent. Since the pH of the feed water is 9 or higher, not only the inside of the evaporator pipe of the steam generator that generates steam and the inside of the boiler body, but also corrosion of highly corrosive steel economizers and steam condensate piping is suppressed. be able to.

It is a systematic diagram showing an example of composition of steam generation equipment to which a water treatment method concerning an embodiment of the present invention is applied.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments described below. A water treatment method according to an embodiment of the present invention is applied to a steam generation facility including softened water as makeup water and an economizer, and suppresses corrosion of a steam generator, a boiler can, an economizer, a steam condensate piping, and the like. Is.

  FIG. 1 is a system diagram showing a configuration example of steam generation equipment to which the water treatment method of the present embodiment is applied. As shown in FIG. 1, a steam generation facility 10 to which the water treatment method of the present embodiment is applied includes a water supply tank 1, one or a plurality of boilers 2, and a heat exchanger 3. Further, an economizer 8 for heating the feed water with the exhaust gas after heating the evaporation pipe is provided in front of each boiler 2.

  In this steam generation facility 10, the feed water stored in the feed water tank 1 is heated by the economizer 8 and then supplied to the boiler 2 by the feed water pump P 1. For example, a part of the steam generated in the boiler 2 is directly used, and the rest is sent to the heat exchanger 3. Further, the drain (condensate) generated in the heat exchanger 3 is sent to the water supply tank 1 in whole or in part and reused as water supply.

  In addition, the steam generation facility 10 is provided with a circulation line including a pump P4 and piping in the water supply tank 1, so that water in the water supply tank 1 circulates. In the middle of this circulation line, in order from the upstream side, a softened water replenishment section 7 for injecting softened water as make-up water, and a nitrogen substitution type desorption system for replacing dissolved oxygen with nitrogen to reduce the amount of dissolved oxygen in the water. An oxygen device 4 is provided. The reason why the nitrogen substitution type deoxygenation device 4 is arranged downstream of the softened water replenishment unit 7 is to efficiently remove oxygen in the water supply.

  The configuration of the nitrogen substitution type deoxygenation device 4 provided in the steam generation facility 10 is not particularly limited. For example, as shown in FIG. 1, the nitrogen gas injecting nitrogen gas into the water flowing through the pipe What has the injection | pouring part 4c and the nitrogen gas mixing part 4d which mixes the inject | poured nitrogen gas and water can be used. In the case of the nitrogen substitution type deoxygenation device 4 having such a configuration, the flow rate adjustment units 4a and 4b and the pressure gauge PG are further provided, and the pressures of the nitrogen gas injection unit 4c and the nitrogen gas mixing unit 4d are within a specific range ( For example, it is desirable to adjust the flow rate of water flowing through the pipe so that the pressure is 0.1 to 0.6 MPa.

  In addition, in the nitrogen substitution type deoxygenation device 4 shown in FIG. 1, the flow rate adjusting units 4a and 4b are installed at two locations upstream of the nitrogen gas injection unit 4c and downstream of the nitrogen gas mixing unit 4d. However, the present invention is not limited to this, and it is sufficient that at least one flow rate adjusting unit is provided at any position in the nitrogen substitution type deoxygenation apparatus 4. Further, for example, the flow rate adjustment unit 4a may be omitted, and the flow rate may be adjusted by changing the capacity of the pump P4.

  Further, the steam generation facility 10 is provided with a chemical liquid tank (deoxygenating agent tank) 5 for storing an oxygen scavenger and a chemical liquid tank (alkali agent tank) 6 for storing an alkali chemical. The oxygen scavenger stored in the oxygen scavenger tank 5 and the alkali agent stored in the alkali agent tank 6 flow through the water supply pipe via the chemical injection pump P2 or the chemical injection pump P3, respectively. To be added to the water supply.

  In addition, in the steam generation equipment 10 shown in FIG. 1, although it has the structure which adds an alkali agent to the feed water which flows in the feed water piping, this invention is not limited to this, The chemical injection pump P3 is equipped. It is good also as a structure which is added to the water supply in the water supply tank 1 via. Moreover, when there is little fluctuation | variation of water quality conditions, it can also be set as the structure which mixes a deoxidizer and an alkali agent to make one liquid, and adds this to the feed water which flows through a feed water piping.

  Next, a specific water treatment method for the steam generation facility 10 described above will be described. In the water treatment method of the present embodiment, first, the dissolved oxygen concentration of water in the water supply tank 1 is set to 2 mg / L or less by the nitrogen substitution type deoxygenation device 4. This is because if the dissolved oxygen concentration of the water in the water supply tank 1 exceeds 2 mg / L, the anticorrosion effect of the economizer 8 is remarkably lowered and a sufficient treatment effect cannot be expected.

  Next, an alkaline agent is added to the water supply flowing through the water supply tank 1 or the water supply pipe so that the pH of the water supplied to the economizer 8 is 9 or more. Thereby, since the reaction rate of the oxygen scavenger is increased, the concentration of dissolved oxygen in the feed water can be reduced and the anticorrosion effect can be improved. In addition, when the pH of the water supplied to the economizer 8 is less than 9, the anticorrosion effect of the economizer 8 cannot be sufficiently obtained.

  Further, from the viewpoint of the anticorrosive effect, it is preferable that the pH of the feed water is high. However, if the pH of the feed water is too high, the boiler water system is concentrated later. As a result, the alkaline agent is concentrated in the boiler can and the pH and electrical conductivity are excessively increased, which may cause carryover and alkaline corrosion. In order to prevent this, it is necessary to suppress the concentration by increasing the blow rate, resulting in a large loss of energy and water. For these reasons, the upper limit of the pH of the feed water is preferably about pH 11, more preferably about pH 10.5, and particularly preferably about pH 10.

  The alkali agent used in the water treatment method of the present embodiment is not particularly limited, but a caustic alkali such as sodium hydroxide or potassium hydroxide is suitable, and water is particularly preferable from the treatment effect and economical viewpoint. Sodium oxide is preferred. These alkali agents may be used alone or in combination.

  Also, the method for adjusting the addition amount of the alkaline agent is not particularly limited. For example, a necessary addition amount is calculated in advance from the quality of the feed water before adding the alkaline agent, and the addition is performed based on the value. Alternatively, the necessary concentration may be confirmed by adding an alkaline agent to the water supply in advance. Furthermore, the pH of the feed water after adding the alkali agent may be measured continuously or intermittently, and the addition amount of the alkali agent may be adjusted automatically or manually based on the result.

  In the water treatment method of the present embodiment, dissolved oxygen in the feed water is removed by adding an oxygen scavenger to the feed water. The method for adjusting the addition amount of the oxygen scavenger is not particularly limited. For example, for the oxygen scavenger to be used, the oxygen scavenging amount under the operating conditions of the steam generation facility 10 to be treated is obtained in advance. The required amount can be calculated and added from the value and the dissolved oxygen concentration in the feed water. Further, in the steam generation facility 10 to be treated, the dissolved oxygen in the steam condensed water obtained by cooling a small amount of continuously collected steam is measured, or the dissolved oxygen in the drain at the outlet of the heat exchanger is measured. The amount of oxygen scavenger added may be determined and adjusted so that the oxygen that does not react with the oxygen scavenger and moves to the vapor is sufficiently low (<0.5 mg / L).

  The oxygen scavenger used here is not particularly limited. For example, tannin, tannic acid or a salt thereof, gallic acid or a salt thereof, erythorbic acid or a salt thereof, ascorbic acid or a salt thereof, glucose, and gluconic acid And aldonic acids such as glucoheptonic acid or salts thereof. The above-mentioned “tannin” and “tannic acid” may be either hydrolyzed or condensed, and also include a mixture with an alkali and a hydrolyzate such as gallic acid generated by mixing with an alkali. “Glucose” also includes reactants obtained by mixing with alkali.

  Among the oxygen scavengers described above, it is desirable to use tannin, tannic acid or a salt thereof, erythorbic acid or a salt thereof, which can adjust a chemical solution of high concentration, has a high oxygen scavenging effect, and has little influence on the boiler cycle. . These compounds may be used alone or in combination.

  Tannin, tannic acid, erythorbic acid and ascorbic acid produce organic acids in boiler cans. When these are used as oxygen scavengers, the amount of alkali necessary for neutralization of the organic acids to be produced is blended. It is desirable to do. The alkali used here is preferably a caustic alkali such as sodium hydroxide or potassium hydroxide, but is not particularly limited thereto, and potassium carbonate or the like can also be used.

For example, when an aqueous solution containing erythorbic acid and / or ascorbic acid and an alkali is used as the oxygen scavenger, the amount of the alkali is erythorbic acid in terms of P alkalinity (mgCaCO ₃ / L). And / or it is desirable to make it 0.6 times or more of the concentration (mg / L) of ascorbic acid. Thereby, the organic acid which erythorbic acid and ascorbic acid decompose and produce | generate in a boiler can can be neutralized. In addition, when using the sodium salt or potassium salt instead of erythorbic acid and ascorbic acid, the alkali used together may be reduced by that much.

On the other hand, when the amount of alkali added increases, the solubility of erythorbic acid and ascorbic acid in the oxygen scavenger decreases, resulting in a chemical solution having a low concentration, and the pH and electrical conductivity of the boiler water increase too much. It may be necessary to increase the amount. Therefore, when an alkali is blended in the oxygen scavenger, the blending amount of the alkali is less than twice the concentration of erythorbic acid and / or ascorbic acid (mg / L) in terms of P alkalinity (mgCaCO ₃ / L). It is desirable to be

Further, for example, when an aqueous solution containing tannin and / or tannic acid and an alkali is used as the oxygen scavenger, the amount of the alkali is converted to tannin and / or in terms of P alkalinity (mgCaCO ₃ / L). Alternatively, the concentration is 0.5 times or more of the tannic acid concentration (mg / L). Thereby, the organic acid which a tannin or a tannic acid decomposes | disassembles in a boiler can and can produce | generate can be neutralized. In addition, when using the neutralized salt by the sodium or potassium instead of tannin or tannic acid, the alkali used together may be reduced by that much.

In this case as well, when the amount of alkali added increases, the solubility of tannin and tannic acid in the oxygen scavenger decreases, resulting in a chemical solution having a low concentration, and the pH and electrical conductivity of the boiler water increase too much. It may be necessary to increase the amount of blow. For this reason, when blending an alkali, the blending amount of the alkali should be 5 times or less of the tannin and / or tannic acid concentration (mg / L) in terms of P alkalinity (mgCaCO ₃ / L). It is desirable.

  In the water treatment method of the present embodiment, an oxygen catalyst may be added to the water supply line between the water supply tank and the economizer together with the oxygen scavenger described above. For example, when erythorbic acid, ascorbic acid or a salt thereof is used as the oxygen scavenger, a calcium compound or a magnesium compound is suitable as the oxygen scavenging catalyst used in combination.

  At that time, examples of the calcium compound used for the deoxygenation catalyst include water-soluble salts such as calcium chloride and calcium sulfate. Examples of the magnesium compound include water-soluble salts such as magnesium chloride and magnesium sulfate. In addition, a deoxygenation catalyst is not limited to these, What is necessary is just a water-soluble salt, and you may use the calcium complex and magnesium complex of ethylenediaminetetraacetic acid.

  On the other hand, when tannin or tannic acid is used as the oxygen scavenger, a calcium compound is suitable as the oxygen scavenging catalyst used in combination. In this case, the calcium compound used for the deoxygenation catalyst includes, for example, water-soluble salts such as calcium chloride and calcium sulfate, but is not limited thereto, and may be any water-soluble salt such as ethylenediaminetetraacetic acid. The calcium complex may be used.

Further, when the oxygen scavenger and the oxygen scavenging catalyst are used in combination, it is desirable to add an acrylic polymer to the water supply line between the water supply tank and the economizer. In that case, the concentration of the acrylic acid polymer in the feed water is equal to or higher than the total hardness (mgCaCO ₃ / L) of the feed water derived from magnesium and calcium contained in the deoxygenation catalyst, preferably twice or more. It is desirable to make it. Thereby, calcium and magnesium can be prevented from being scaled in the boiler can.

Examples of the acrylic acid-based polymer used here include homopolymers of acrylic acid, copolymers with maleic acid, acrylamide propane sulfonic acid, and the like. The molecular weight is preferably 5.0 × 10 ^{2 to} 1.0 × 10 ⁵ , more preferably 2.0 × 10 ^{3 to} 7.0 × 10 ⁴ , and still more preferably 1.0 × 10 6. ^{4 to} 5.0 × 10 ⁴ . By adding an acrylic acid polymer having a molecular weight within this range, scaling of calcium and magnesium can be effectively suppressed.

  In addition, in FIG. 1, although the structure which adds an oxygen scavenger and an alkali agent separately is shown, this invention is not limited to this, When there are few fluctuation | variations of water quality conditions, An oxygen scavenger and an alkali agent may be mixed and made into one liquid and added to the water supply line. At that time, if the amount of the alkaline agent is not sufficient to raise the pH of the boiler water to a predetermined value, an additional amount may be added. Further, in the water treatment method of the present embodiment, existing water such as a steam condensate anticorrosive agent such as amine, an iron dispersant, and a chelating agent such as EDTA, in addition to the alkali agent, oxygen absorber and oxygen absorber catalyst described above. A treating agent can also be used.

  As described above in detail, in the water treatment method of the present embodiment, corrosion of the inside of the evaporation pipe of the steam generator and the inside of the boiler body can be prevented by adding the oxygen scavenger and the alkali agent. In addition, the dissolved oxygen concentration of the feed water is reduced to 2 mg / L or less, and the pH of the feed water is set to 9 or more. Therefore, near the economizer inlet at a relatively low temperature, the corrosiveness is reduced and the solubility of the corrosion products. Corrosion is suppressed due to the decrease.

  Moreover, in the part which is more likely to corrode at a relatively high temperature at the outlet from the center of the economizer, by setting the pH of the feed water to 9 or more, the reaction rate of the oxygen scavenger is increased and the effect of reducing dissolved oxygen in water can be enhanced. . Furthermore, the synergistic effect that the pH of the feed water is 9 or more forms an iron oxide film having corrosion resistance on the metal surface, thereby suppressing corrosion. As a result, not only the inside of the evaporator pipe of the steam generator that generates steam and the inside of the boiler can, but also the corrosion of highly corrosive steel economizers and steam condensate piping can be operated uneconomically, such as increasing blow. Therefore, it can be suppressed at an economically advantageous cost.

  Hereinafter, the effects of the present invention will be specifically described with reference to Examples and Comparative Examples of the present invention. In the present Example, the water treatment was performed using the experimental apparatus which imitated the steam generator 10 shown in FIG. 1, and the anticorrosive effect was confirmed.

(Operating conditions)
A 50 L capacity water supply tank was supplied with softened Nogimachi water as makeup water, and a heater was installed inside and heated to 45 ° C., assuming drain recovery. In addition, the quality of softened water of Nogi-cho water is pH: 7.4, electrical conductivity: 16 mS / m, acid consumption (pH 4.8): 18 mg CaCO ₃ / L, chloride ion: 11 mg / L, sulfate ion : 12 mg / L, silica: 10 mg / L.

  Then, the water in the water supply tank is passed through a heat exchanger that heats the 45 ° C. water imitating the economizer to 110 ° C. at a flow rate of 50 L / hour with a water supply pump, a part of which is used for an experiment of 5 L capacity. Water was supplied to the test boiler. In the experimental boiler, the feed water heated to 110 ° C. was further heated with an electric heater to generate 0.7 MPa saturated steam at 9.5 L / hour. At that time, the blow amount was 5%, and the test period was 7 days.

(Evaluation method)
As an evaluation of corrosiveness, a removable carbon steel test tube with an inner diameter of 8 mm, a length of 150 mm, and a wall thickness of 3 mm is installed in an economizer (heat exchanger), and the mass before the test and the derusting treatment after the test. The corrosion rate was calculated from the difference from the mass after the treatment. In addition, the test tube was cut out by machining and degreased, and then the mass was measured.

  In addition, a test tube made of detachable carbon steel (outer diameter 30 mm, length 300 mm, wall thickness 5 mm) into which an electric heater is inserted is installed in the test boiler can, and deposits generated on the heat transfer surface after the test are installed. I counted the number. As the test tube, the outer surface that comes into contact with the can water was subjected to # 400 polishing and then degreased.

  Furthermore, a small amount of water supply and boiler blow water at the inlet portion of the economizer were periodically collected, and the pH of the water supply and boiler water was measured.

<Examples 1-7>
The test boiler described above was provided with a circulation line for circulating the water in the water supply tank with a circulation pump, and a soft water replenishment section and a nitrogen substitution type deoxygenation device were installed there. Specifically, the soft water replenishment unit pipes the soft water to be supplied to the suction side of the circulation pump so that the flow rate is 70 L / hour by the level control of the water supply tank, and the ON / OFF control is performed. I tried to replenish it.

  Moreover, as a nitrogen substitution type deoxygenation device, nitrogen gas (purity 99.9%) is injected at 20 NL / hour after the softening water injection point, and further, a nitrogen gas mixing section and a gate valve are installed at the subsequent stage. installed. And by adjusting this gate valve and the valve installed between the circulation pump outlet and the nitrogen gas injection part, the secondary pressure of the valve on the circulation pump outlet side becomes 0.3 MPa, and the amount of circulating water is 80L. / It was time.

  At this time, the dissolved oxygen concentration of the water stored in the water tank changed from 1.4 to 2.0 mg / L. Then, an oxygen scavenger necessary for removing 2.0 mg / L of dissolved oxygen and an alkali agent in an amount such that the pH of the feed water at the economizer inlet was 9.0 or more were added to this feed water. The evaluation results are summarized in Table 1 below. For Examples 3 to 6, in addition to the corrosion rate of the economizer test tube and the number of deposits of the test tube in the test boiler can, the scale adhesion to the test tube in the test boiler can was also confirmed. The results are also shown in Table 1 below.

<Comparative Example 1>
For comparison, the same evaluation as in the example was performed in a system without a circulation line and without a nitrogen substitution type deoxygenation apparatus. First, in Comparative Example 1, an oxygen scavenger in an amount necessary for removing oxygen (6.2 mg / L) dissolved in the 45 ° C. water supply tank is injected into the water supply line between the water supply tank and the economizer. However, it was operated for 7 days under the conditions described above.

  At that time, as the oxygen scavenger, erythorbic acid: 22.5% by mass, 48% by mass of potassium hydroxide aqueous solution: 31.5% by mass, and water mixed at a ratio of 46% by mass were used. Injected so that the density | concentration with respect to feed water might be 150 mg / L. In Comparative Example 1, the pH of the feed water was 8.5, which satisfied the water quality standards 5.8 to 9.0 defined by JIS standards. Moreover, the pH of boiler water was 11.3, and this also satisfied 11.0-11.8 prescribed | regulated by JIS specification.

<Comparative example 2>
In addition to the treatment of Comparative Example 1, a 48% by mass aqueous sodium hydroxide solution was added as an alkaline agent to the water supply line between the water supply tank and the economizer so as to be 15 mg / L with respect to the water supply. As in Comparative Example 1, the operation was continued for 7 days. Thereby, pH of feed water rose to 9.1 and pH of boiler water rose to 11.7.

<Comparative Example 3>
A continuous operation was performed for 7 days under the same method and conditions as in Comparative Example 2 described above, except that the oxygen scavenger was tannin (Kebracho).

<Comparative example 4>
A continuous operation was performed for 7 days under the same method and conditions as in Comparative Example 2 described above except that the oxygen scavenger injection point was changed to a water supply tank.

  The evaluation results of Comparative Examples 1 to 4 are summarized in Table 2 below.

  As shown in Table 2 above, in Comparative Examples 1 to 4, the amount of dissolved oxygen in the water supply tank is not 2 mg / L or less, so the amount of oxygen scavenger added is higher than in Examples 1 to 7. Furthermore, the corrosion rate of the test tube installed in the economizer was also high.

  In particular, it was found that in Comparative Example 1 in which the alkaline agent was not added and the pH of the feed water was less than 9.0, the corrosion rate of the test tube was high and corrosion was likely to occur with an economizer. Moreover, in the comparative example 4 which added the oxygen absorber to the water supply tank, removal of dissolved oxygen was not enough and corrosion also generate | occur | produced also in the test boiler can.

  On the other hand, as shown in Table 1 above, in Examples 1 to 7, the occurrence of corrosion is suppressed for both the economizer and the test boiler can with a smaller amount of chemical addition than Comparative Examples 1 to 4. I was able to. In Examples 3 and 4 to which a deoxygenation catalyst was added, scale adhesion was observed. However, as in Examples 5 and 6, it is possible to prevent scale adhesion by using sodium polyacrylate together. confirmed.

DESCRIPTION OF SYMBOLS 1 Water supply tank 2 Boiler 3 Heat exchanger 4 Nitrogen substitution type deoxygenation apparatus 4a, 4b Flow control part 4c Nitrogen gas injection part 4d Nitrogen gas mixing part 5 Oxygen agent tank 6 Alkaline agent tank 7 Softening water replenishment part 8 Economizer 10 Steam Generation equipment P1 to P4 Pump PG Pressure gauge

Claims (7)

A water treatment method for a steam generation facility comprising an economizer that uses softened water as make-up water and heats feed water with exhaust gas after heating an evaporator tube,
A step of reducing the dissolved oxygen amount of water in the water supply tank to 2 mg / L or less by a deoxygenation device that reduces dissolved oxygen in water by replacing with nitrogen;
Adding an alkaline agent to a water supply tank or a water supply line between the water supply tank and the economizer, and setting the pH of the water supplied to the economizer to 9 or more;
Adding an oxygen scavenger to the water supply line between the water supply tank and the economizer;
A water treatment method for a steam generating facility. As the oxygen scavenger, erythorbic acid and / or ascorbic acid and an alkali are contained, and the blending amount of the alkali is a content of erythorbic acid and ascorbic acid in terms of P alkalinity (mgCaCO ₃ / L) (mg The water treatment method for steam generating equipment according to claim 1, wherein an aqueous solution that is 0.6 times or more of / L) is used.   The water treatment method for a steam generating facility according to claim 2, wherein a deoxygenation catalyst composed of a calcium compound and / or a magnesium compound is added together with the deoxidation agent. As the oxygen scavenger, tannin and / or tannic acid and alkali are contained, and the blending amount of the alkali is tannin and tannic acid content (mg / L) in terms of P alkalinity (mgCaCO ₃ / L). The water treatment method for steam generating equipment according to claim 1, wherein an aqueous solution that is 0.5 times or more of () is used.   The water treatment method for steam generating equipment according to claim 4, wherein a deoxygenation catalyst comprising a calcium compound is added together with the deoxidation agent. Furthermore, an acrylic acid polymer is added to the water supply line between the water supply tank and the economizer so that the total hardness (mgCaCO ₃ / L) of the water supplied from the magnesium compound and / or the calcium compound is equal to or higher. It adds, The water treatment method of the steam generation equipment of Claim 3 or 5 characterized by the above-mentioned.   The water treatment method for steam generating equipment according to any one of claims 2 to 6, wherein the alkali is sodium hydroxide or potassium hydroxide.

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