Method for operating boiler

Abstract

Supplementary water softened by a water softening device 16 and deoxidized by a deoxidizing device 17 is stored in a water supply tank 11, and is supplied from the water supply tank 11 to a boiler 20 through a water supply path 13. The boiler (20) stores water supplied from the water supply path (13) as boiler water, and generates steam by heating the boiler water through a water pipe. At this time, a water treatment agent containing a pH regulator, sodium silicate, and sodium polyacrylate having a mass-average molecular weight of 2,000-40,000 is added from a chemical supply device 40 to the supplied water from the water supply path 13 to the boiler 20, whereby the pH of the boiler water is controlled in an alkaline range, and the sodium silicate and a polyacrylic acid compound having a mass-average molecular weight within the range are made to coexist in the boiler water.

Description

Technical Field

[0001] The present invention relates to a method for operating a boiler, and more particularly to a method for operating a boiler that generates steam by heating boiler water. This application claims priority based on Japanese Patent Application No. 2022-168129 filed in Japan on October 20, 2022, and the contents of which are incorporated herein by reference. Background Art

[0002] A general boiler device for supplying steam to a load device such as a heat exchanger generates steam by heating the water supplied to the boiler as boiler water, and supplies the steam to the load device through a steam path. Since the boiler is in a high temperature and high pressure environment inside, scale or corrosion occurs on the heat transfer surface of the water pipe that generates steam due to the influence of various dissolved components in the supply water. The scale attached to the heat transfer surface hinders heat conduction, thus impairing the operating efficiency of the boiler. In addition, if the corrosion of the heat transfer surface progresses, the water pipe will be damaged, thus hindering the stable and continuous operation of the boiler.

[0003] Therefore, during operation of a boiler device, calcium ions and magnesium ions that cause scale are usually removed from the water supplied to the boiler, dissolved oxygen that causes corrosion is removed, and a water treatment agent is added to the water supply, thereby suppressing scale formation and corrosion in the boiler.

[0004] As a water treatment agent added to the water supply, as described in Patent Documents 1 and 2, there is known a water treatment agent that uses a silicic acid compound such as silicon dioxide or silicate and a scale inhibitor in combination. The silicic acid compound forms a film on the heat transfer surface, and the film inhibits corrosion. The scale inhibitor is a chelating agent such as ethylenediaminetetraacetic acid or a water-soluble polymer such as a polyacrylic acid compound. The chelating agent inhibits the formation of scale by chelating calcium ions and magnesium ions in the boiler water. In contrast, the water-soluble polymer inhibits the formation of scale by dispersing scale substances from calcium ions and magnesium ions in the boiler water and inhibiting the growth of scale crystal nuclei.

[0005] However, it is known that the corrosion generated on the heat transfer surface of the boiler includes general corrosion that is uniformly carried out on the entire heat transfer surface and pitting corrosion that is carried out in a local hole shape along the thickness direction of the heat transfer surface. General corrosion is corrosion that is carried out by a short-circuit battery that generates a microscopic anode and cathode pair on the entire heat transfer surface, while pitting corrosion is corrosion that is carried out by a macroscopic anode and cathode that is locally produced on the heat transfer surface. According to Patent Documents 1 and 2, although the silicate compounds used in these Patent Documents are effective in inhibiting pitting corrosion, the effect of inhibiting general corrosion cannot be expected.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2002-18487,

[0009] Patent document 2: Japanese Patent Application Publication No. 2003-160889. Summary of the invention

[0010] Problems to be solved by the invention

[0011] The present invention uses a silicate compound and a polyacrylic acid compound in combination to suppress the generation of scale in the boiler and simultaneously suppress both general corrosion and pitting corrosion in the boiler.

[0012] Means of solving problems

[0013] The present invention relates to a method for operating a boiler that generates steam by heating boiler water, wherein the pH of the boiler water is controlled in an alkaline region and a polyacrylic acid compound having a mass average molecular weight of 2,000 to 40,000 is allowed to coexist in the boiler water in an environment where a silicate compound exists.

[0014] In one embodiment of the operation method of the present invention, the silicic acid compound is derived from a natural component contained in water supplied to the boiler.

[0015] In another embodiment of the operation method of the present invention, a chemical containing a silicic acid compound and the polyacrylic acid compound is added to the feed water to the boiler.

[0016] In the operation method involved in this embodiment, for example, by adjusting the supply amount of the above-mentioned reagent to the feed water and adjusting the concentration ratio of the boiler water, the pH of the boiler water is maintained in the alkaline region, and the concentration of the silicate compound and the concentration of the polyacrylic acid compound in the boiler water are controlled to target concentrations of 100 to 600 mg / L in terms of silica and 1 to 500 mg / L in terms of salt, respectively. It is preferred to use softened water as the feed water, and when the ratio of the acid consumption (pH 4.8) to the conductivity (acid consumption (pH 4.8) / conductivity) of the feed water shows a tendency for the boiler to corrode, the concentration of the silicate compound and the concentration of the polyacrylic acid compound in the boiler water are controlled to the target concentrations, respectively.

[0017] The present invention according to another aspect is directed to a boiler water treatment agent comprising a silicic acid compound and a polyacrylic acid compound having a mass average molecular weight of 2,000 to 40,000.

[0018] The boiler water treatment agent of the present invention sets the amount ratio (Y / X) of the salt conversion content (Y mass %) of the polyacrylic acid compound to the silica conversion content (X mass %) of the silicic acid compound to 0.001 to 500.

[0019] The boiler water treatment agent of the present invention preferably contains ethylenediaminetetraacetic acid salt and its alkali metal salt in a total amount of less than 1% by mass.

[0020] Effects of the Invention

[0021] The boiler operation method involved in the present invention maintains the pH of boiler water in the alkaline region and allows a polyacrylic acid compound with a mass average molecular weight in a specific range to coexist in an environment where a silicate compound is present. Therefore, the formation of scale in the boiler can be suppressed, and both general corrosion and pitting corrosion in the boiler can be suppressed.

[0022] Since the boiler water treatment agent of the present invention contains a silicate compound and a polyacrylic acid compound having a mass average molecular weight within a specific range, it can suppress the formation of scale in the boiler and suppress both general corrosion and pitting corrosion in the boiler by using it in the boiler operation method involved in the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] [ Figure 1 ]A schematic diagram of an embodiment of a boiler device capable of implementing the operating method involved in the present invention.

[0024] [ Figure 2 ] Based on the results shown in Table 1, a graph showing the relationship between the mass average molecular weight of sodium polyacrylate and the maximum pitting depth for Experimental Examples 2 to 17 and 19 to 33.

[0025] [ Figure 3 ] Based on the results shown in Table 1, a graph showing the relationship between the mass average molecular weight of sodium polyacrylate and the corrosion rate for Experimental Examples 2 to 17 and 19 to 33. DETAILED DESCRIPTION

[0026] Reference Figure 1 , an embodiment of a boiler device capable of implementing the boiler operation method of the present invention is described. Figure 1 In the figure, the boiler device 1 is used to supply steam to a load device 2 as a steam-using device such as a heat exchanger, a steam boiler, a reboiler or an autoclave, and mainly includes a water supply device 10, a boiler 20, a steam condensate pipe 30 and a chemical supply device 40.

[0027] The water supply device 10 is used to supply water used as boiler water in the boiler 20, and mainly includes a water supply tank 11 for storing supply water, and a supply path 12 for supplying make-up water used as supply water to the water supply tank 11. The water supply tank 11 has a water supply path 13 extending from the bottom thereof to the boiler 20. The water supply path 13 is connected to the boiler 20 and has a water supply pump 14 for sending the supply water stored in the water supply tank 11 to the boiler 20.

[0028] The replenishment path 12 has a water injection path 15. The water injection path 15 is used to supply replenishment water from a raw water tank (not shown) storing raw water supplied from a water source such as tap water, industrial water or groundwater to the water supply tank 11, and has a softening device 16 and a deoxygenation device 17 in sequence toward the water supply tank 11.

[0029] The softening device 16 is used to treat the feed water from the raw water tank with a sodium-type cation exchange resin, and replace calcium ions and magnesium ions as hardness components contained in the feed water with sodium ions to convert the feed water into softened water.

[0030] The deoxygenator 17 is used to remove dissolved oxygen in the feed water treated in the softener 16. Various types of deoxygenators may be used, such as removing dissolved oxygen using a separation membrane, removing dissolved oxygen in treated water under reduced pressure, or removing dissolved oxygen by heating treated water.

[0031] The boiler 20 has a plurality of vertical water pipes (not shown) inside, and the water supplied from the water supply path 13 is stored as boiler water at the bottom, and the boiler water is heated by the heat transfer surface of the water pipe, thereby generating steam. The water pipe is formed of a metal that does not naturally passivate in a neutral aqueous solution, such as carbon steel, cast iron, copper or copper alloy. In addition, the boiler 20 has a discharge path 21 for discharging boiler water, and the discharge path 21 has a control valve 22 for adjusting the discharge amount of boiler water. In addition, a steam supply pipe 23 connected to the load device 2 extends above the boiler 20.

[0032] The steam condensate pipe 30 extends from the load device 2 to the water supply tank 11 and has a steam trap 31. The steam trap 31 is used to separate steam and condensate.

[0033] The chemical supply device 40 is used to supply a water treatment agent to the water supply supplied from the water supply tank 11 to the boiler 20, and includes a chemical tank 41 for storing the water treatment agent, a supply path 42 extending from the chemical tank 41 to the water supply path 13, and a supply pump 43 provided on the supply path 42. The supply pump 43 delivers the water treatment agent stored in the chemical tank 41 to the water supply path 13 through the supply path 42, and the supply amount of the water treatment agent can be controlled.

[0034] The water treatment agent stored in the agent tank 41 of the agent supply device 40 contains a silicic acid compound and a polyacrylic acid compound, and is preferably an aqueous solution containing the silicic acid compound and the polyacrylic acid compound. The water used in the aqueous solution is usually purified water such as distilled water or ion exchange water.

[0035] The silicic acid compound contained in the water treatment agent is silicic acid or silicate, which is a component that forms a film on the surface of the water pipes in the boiler 20 to inhibit corrosion of the water pipes. X (OH)4-2X ] n A silicon compound represented by the chemical formula, usually silicon dioxide (SiO 2 ), orthosilicic acid (H 4 SiO 4 ), metasilicic acid (H 2 SiO 3 ) or disilicate (H 2 Si 2 O 5 ). In addition, silicate is, for example, nSiO 2 ·(n+1)M 2 O orthosilicate or hydrate thereof, or nSiO 2 nM 2 O、nSiO 2 ·(n-1)M 2 O or nSiO 2 ·(n-2)M 2 O or a hydrate thereof. In the chemical formula of the silicate, n is an integer greater than 2, and M represents a metal element such as an alkali metal such as sodium or potassium or an alkaline earth metal such as calcium or magnesium. When the metal element is divalent, the number of molecules of M becomes half. The water treatment agent may contain two or more silicate compounds.

[0036] The polyacrylic acid compound contained in the water treatment agent is a component for inhibiting the formation of scale in the boiler 20, and inhibiting both the overall corrosion and local corrosion of the water pipes in the boiler 20 by interacting with the silicate compound. As the polyacrylic acid compound, a water-soluble polymer compound containing a carboxyl group or a salt thereof from at least one of acrylic acid and methacrylic acid is generally used. For example, polyacrylic acid, polymethacrylic acid, a copolymer or terpolymer using at least one of acrylic acid and methacrylic acid as a monomer, or their salts can be used. As a salt, for example, an alkali metal salt such as a sodium salt or a potassium salt can be used. The polyacrylic acid compound is preferably polyacrylic acid and its salt. The polyacrylic acid compound selectively uses a compound with a mass average molecular weight in a specific range that can exert an anti-corrosion effect by interacting with the silicate compound, specifically, a compound with a mass average molecular weight of 2,000 to 40,000 is selectively used, preferably a compound with a mass average molecular weight of 3,000 to 20,000, and more preferably a compound with a mass average molecular weight of 4,000 to 10,000. The water treatment agent may contain two or more polyacrylic acid compounds.

[0037] The water treatment agent may contain other components such as a pH adjuster or a deoxidizer in addition to the silicate compound and the polyacrylic acid compound. The pH adjuster suppresses corrosion in the boiler 20 by adjusting the pH of the boiler water to an alkaline region. For example, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide may be used. Two or more pH adjusters may be used in combination. The deoxidizer suppresses corrosion in the boiler 20 by removing dissolved oxygen in the boiler water. For example, ascorbic acid or its salt, tannin, a sugar deoxidizer, isoascorbic acid or its salt, or sulfite may be used. Two or more deoxidizers may be used in combination.

[0038] It should be noted that common water treatment agents for boiler water often contain ethylenediaminetetraacetic acid (EDTA) or its alkali metal salts as scale inhibitors, but these may promote corrosion in the boiler 20. Therefore, the water treatment agent used in this embodiment preferably does not substantially contain EDTA and its alkali metal salts, and for example, their total content is preferably controlled to less than 1% by mass.

[0039] Regarding the content of the silicate compound and the polyacrylic acid compound in the water treatment agent, since it is easy to control the concentration of the silicate compound and the polyacrylic acid compound in the boiler water to the target concentration described below, it is preferred to set the amount ratio (Y / X) of the silicate compound content (X mass%) to the salt-converted content (Y mass%) of the polyacrylic acid compound to 0.001 to 500, and more preferably to set the amount ratio to 0.1 to 50. Here, the salt-converted content of the polyacrylic acid compound is a content converted by regarding the polyacrylic acid compound as a salt in which all carboxyl groups in the molecule form sodium salts.

[0040] Next, a method for operating the above-mentioned boiler device 1 will be described.

[0041] During operation of the steam boiler device 1 , first, make-up water is supplied from a raw water tank (not shown) to the water supply tank 11 through the water injection path 15 , and the make-up water is stored in the water supply tank 11 as boiler feed water.

[0042] At this time, the feed water from the raw water tank is first treated in the water softening device 16 to remove hardness components and become softened water. The feed water softened in the water softening device 16 is then deoxygenated in the deoxygenation device 17. Thus, dissolved oxygen that promotes corrosion of water pipes and the like in the steam boiler 20 is removed from the feed water. As a result, the deoxygenated softened water is stored in the water supply tank 11 as supply water.

[0043] When the water supply pump 14 is started while the make-up water is stored in the water supply tank 11, the water supply stored in the water supply tank 11 is supplied to the boiler 20 through the water supply path 13. The water supply supplied to the boiler 20 is stored as boiler water, and the boiler water rises in the water pipe while being heated through the water pipe to become steam. Then, the steam generated in the water pipe is supplied to the load device 2 through the steam supply pipe 23. With the generation of such steam, the boiler water is concentrated.

[0044] The steam supplied to the load device 2 flows into the steam condensate pipe 30 through the load device 2, where it loses latent heat and partially turns into condensate. The steam and condensate are separated in the steam trap 31 to become high-temperature steam condensate. The steam condensate generated in this way is recovered to the supply water tank 11 through the steam condensate pipe 30 and reused as supply water. At this time, since the supply water stored in the supply water tank 11 is heated by the high-temperature steam condensate, the heating burden in the boiler 20 can be reduced.

[0045] During the operation of the boiler device 1 as described above, the control valve 22 is adjusted to control the discharge amount of boiler water from the discharge path 21, and the supply amount of water from the water supply tank 11 to the boiler 20 is adjusted by controlling the water supply pump 14, thereby controlling the concentration ratio of the boiler water. The pH of the boiler water is maintained in an alkaline region where corrosion is unlikely to occur by adjusting the acid consumption of the boiler water (pH 4.8), preferably maintaining the pH at 11 to 12.5.

[0046] It should be noted that the acid consumption (pH 4.8) is the amount of hydrogen ions (amount of acid) required to neutralize alkali such as bicarbonate, carbonate, hydroxide, etc. dissolved in the water supply to a specified pH (here 4.8) converted into the amount of calcium carbonate equivalent to the hydrogen ions (acid), and is expressed in mg relative to 1 liter of water supply and can be measured according to the method specified in JIS K 0101.

[0047] In addition, during the operation of the boiler device 1, a water treatment agent is appropriately supplied from the chemical supply device 40 to the water supply flowing to the boiler 20 in the water supply path 13. As a result, the polyacrylic acid compound having a mass average molecular weight in a specific range coexists with the silicic acid compound in the boiler water. As a result, the polyacrylic acid compound suppresses the growth of crystal nuclei of scale in the water pipes of the boiler 20, thereby suppressing the formation of scale, and at the same time, the silicic acid compound forms an anti-corrosion film on the surface of the water pipes, etc., and the interaction between the silicic acid compound and the polyacrylic acid compound suppresses corrosion, especially suppressing both general corrosion and local corrosion.

[0048] Here, from the viewpoint of improving the inhibitory effect of scale and the anti-corrosion effect of both general corrosion and local corrosion, it is preferred that the boiler water is supplied with water treatment agent from the chemical supply device 40 to the water supply by controlling the supply pump 43, and the concentration ratio is adjusted by the above method, thereby maintaining the pH in the alkaline region, especially maintaining it in the range of 11 to 12.5, and controlling the concentration of the silicate compound and the concentration of the polyacrylic acid compound having a mass average molecular weight in a specific range to a predetermined target concentration. Specifically, it is preferred that the concentration of the silicate compound is in the form of silicon dioxide (SiO 2 ) conversion concentration, is controlled to 100 to 600 mg / L, particularly to 200 to 500 mg / L, and the concentration of the polyacrylic acid compound having a mass average molecular weight in a specific range is controlled to 1 to 500 mg / L, particularly to 10 to 200 mg / L, as a salt conversion concentration. Here, the salt conversion concentration is a concentration converted by regarding the polyacrylic acid compound as a salt in which all carboxyl groups in the molecule form sodium salts.

[0049] The addition of the water treatment agent to the water supply is preferably carried out when the water quality of the water supply supplied from the water supply tank 11 to the boiler 20 has a tendency to easily cause corrosion to the water pipes of the boiler 20. This tendency can be evaluated, for example, according to the water quality determination method described in Japanese Patent No. 4033667. Specifically, the acid consumption (pH 4.8) and the conductivity of the water supply supplied from the water supply tank 11 to the boiler 20 through the water supply path 13 are continuously measured, and the index represented by the following formula (1) is obtained. When the index is 2.5 or more, it can be determined that the water supply is a water quality that is difficult to corrode the water pipe. On the other hand, when the index is less than 2.5, it can be determined that the water supply is a water quality that is prone to corrode the water pipe, especially prone to pitting as local corrosion of the water pipe. Therefore, during the operation of the boiler device 1, when the index is less than 2.5, it is preferred to supply the water treatment agent to the water supply from the chemical supply device 40 to control the concentration of the silicate compound and the concentration of the polyacrylic acid compound having a mass average molecular weight in a specific range in the boiler water to the above-mentioned target concentration.

[0050] [Mathematical formula 1]

[0051]

[0052] Here, the acid consumption (pH 4.8) is as described above. In addition, the conductivity is equivalent to the reciprocal of the resistivity (Ω·m) of the supply water at 25°C, and is expressed in units of mS / m (milliSiemens per meter), and can be measured by the method specified in JISK 0101, similar to the acid consumption (pH 4.8).

[0053] In the above-mentioned embodiment, a water treatment agent containing silicate compounds is used as the water treatment agent. However, when the water supplied to the boiler 20 contains a large amount of silicate compounds from the raw water and the silica concentration in the boiler water is easily increased by increasing the concentration ratio of the boiler water, a water treatment agent in which the content of silicate compounds is suppressed or a water treatment agent that does not contain silicate compounds may be used as the water treatment agent.

[0054] Example

[0055] The boiler device 1 according to the above embodiment was operated to investigate the progress of corrosion on the heat transfer surface of the water pipe (STPG (carbon steel pipe for pressure piping) listed in JIS G 3454) of the boiler 20. Here, test water simulating tap water in the suburbs of Osaka City was supplied from the water injection path 15 to the water supply tank 11 and stored, and water was supplied from the water supply tank 11 to the boiler 20 through the water supply path 13. The tap water was considered to be highly corrosive due to the high concentrations of both chloride ions and sulfate ions. The water quality of the test water is as follows.

[0056] (Test water quality)

[0057] pH: 7.5

[0058] Conductivity: 25mS / m

[0059] Acid consumption (pH 4.8): 20 mg CaCO 3 / L

[0060] Silica concentration: 7mgSiO 2 / L

[0061] In this experimental example, the chemical supply device 40 was arranged with respect to the water supply path 13, and the following water treatment agent was appropriately added to the supply water. The operating conditions of the boiler 20 were set such that the operating pressure and the concentration ratio were 0.3 MPa and 10 times, respectively.

[0062] During the operation of the boiler device 1, the dissolved oxygen concentration of the supply water supplied from the water supply tank 11 to the boiler 20 is adjusted to 4.0 mg / L by treating the test water in the deoxygenation device 17. Furthermore, for the supply water to the boiler 20, a water treatment agent is appropriately added from the agent supply device 40, and the discharge amount of the boiler water is adjusted by controlling the control valve 22, thereby controlling the concentration ratio of the boiler water in the boiler 20 to 10 times, and controlling the water quality of the boiler water as shown in Table 1. The water treatment agent added in each experimental example is obtained by appropriately adjusting the concentrations of sodium hydroxide, sodium silicate and sodium polyacrylate so that the water quality of the boiler water is as shown in Table 1. For example, when the pH of the boiler water cannot be sufficiently increased by the increase in pH associated with the decomposition of the acid consumption (pH 4.8), the sodium hydroxide concentration in the water treatment agent is increased. It should be noted that in Table 1, the silica concentration of Experimental Examples 1 to 17 is 7 mgSiO 2 / L of test water is concentrated 10 times in the boiler.

[0063] The operation of the boiler 20 was stopped 48 hours after the start of operation, and the water pipe for evaluation was taken out from the boiler 20. The progress of corrosion on the surface was evaluated for both local corrosion and general corrosion. In this evaluation, for local corrosion, the depth of each pitting generated on the heat transfer surface was investigated, and the maximum depth was calculated. The greater the maximum depth, the more the pitting progressed. In addition, for general corrosion, the mdd (mg / dm 2 / day (day)). It should be noted that mdd represents the unit surface area of ​​the contact surface with water (1dm 2 ) is the mass reduction (mg) of the water pipe material per day. The larger the mdd, the higher the corrosion rate and the more the overall corrosion progresses.

[0064] The evaluation results are shown in Table 1. The evaluation results shown in Table 1 are relative values ​​when the results of the maximum pitting depth (μm) and the corrosion rate (mdd) in Experimental Example 1 are taken as 100%. When the boiler 20 is operated using test water simulating tap water in the suburbs of Osaka City, when the relative value is suppressed to less than 50%, it can be evaluated that the corrosion is sufficiently suppressed in practical terms by comparing with the applicant's empirical rule.

[0065] [Table 1]

[0066] Table 1

[0067]

[0068] The results of Table 1 are plotted and shown in Figure 2 , 3 In. According to Figure 2For Experiments 19 to 33 in which the silica concentration in boiler water was increased by adding sodium silicate, it was confirmed that there was a correlation between the mass average molecular weight of sodium polyacrylate and the maximum pitting depth. Specifically, as the mass average molecular weight of sodium polyacrylate increased, it was confirmed that the maximum pitting depth tended to decrease. In addition, according to Figure 3 For Experiments 19 to 33 in which the silica concentration in boiler water was increased by adding sodium silicate, it was confirmed that there was also a correlation between the mass average molecular weight of sodium polyacrylate and the corrosion rate. Specifically, as the mass average molecular weight of sodium polyacrylate decreased, it was confirmed that the corrosion rate tended to slow down. Figure 2 , 3 The results show that when the silica concentration is increased by adding sodium silicate to boiler water and sodium polyacrylate having a mass average molecular weight in the range of about 2,000 to 40,000 is coexistent, the progress of both pitting corrosion and general corrosion is easily suppressed.

[0069] In addition, according to Figure 2 , 3 When the silica concentration in boiler water is increased, the maximum pitting depth and corrosion rate can be controlled by selecting the mass average molecular weight of the coexisting sodium polyacrylate in accordance with the above correlation, thereby achieving fine corrosion management of the water pipes.

Claims

1. A method for operating a boiler, which is a method for operating a boiler that generates steam by heating boiler water, in, The pH of the boiler water is controlled in an alkaline region, and a polyacrylic acid-based compound having a mass average molecular weight of 2,000 to 40,000 is allowed to coexist in the boiler water in an environment where a silicic acid-based compound exists.

2. The method for operating a boiler according to claim 1, in, The silicic acid compound is derived from a natural component contained in the water supplied to the boiler.

3. The method for operating a boiler according to claim 1, in, A reagent containing a silicic acid compound and the polyacrylic acid compound is added to the supply to the boiler.

4. The method for operating a boiler according to claim 3, in, By adjusting the supply rate of the reagent to the water supply and adjusting the concentration ratio of the boiler water, the pH of the boiler water is maintained in the alkaline region, and the concentrations of silicate compounds and polyacrylic acid compounds in the boiler water are controlled at target concentrations of 100 to 600 mg / L in terms of silica and 1 to 500 mg / L in terms of salt, respectively.

5. The method for operating a boiler according to claim 4, in, Softened water is used as the supply water, and when the ratio of acid consumption (pH 4.8) to conductivity (acid consumption (pH 4.8) / conductivity) shows a tendency for the boiler to corrode, the silicate compound concentration and the polyacrylic acid compound concentration of the boiler water are controlled to the target concentrations, respectively.

6. A boiler water treatment agent comprising: Silicic acid compounds, and A polyacrylic acid compound having a mass average molecular weight of 2,000 to 40,000.

7. The boiler water treatment agent according to claim 6, in, The quantitative ratio (Y / X) of the salt-equivalent content (Y mass %) of the polyacrylic acid-based compound to the silicon dioxide-equivalent content (X mass %) of the silicic acid-based compound is set to 0.001 to 500.

8. The boiler water treatment agent according to claim 6 or 7, in, The total content of ethylenediaminetetraacetic acid and its alkali metal salt is less than 1% by mass.

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