JP2014104430A - Corrosion prevention method of sea water desulfurizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion prevention technique where the material cost of the throat member of a prescrubber is reduced and high corrosion resistance is kept.SOLUTION: Corrosion at the throat part 4 of a prescrubber is suppressed by the oxidation of SOor a sulfurous acid contained in a liquid to SOdue to the addition of an oxidant to a prescrubber cleaning liquid consisting of a limestone slurry and then significant cost reduction can be achieved. It is also possible that obtaining the necessary oxidant and pH adjustment are performed by the electrolysis of sea water.

Description

  The present invention relates to a corrosion prevention technique for a seawater desulfurization apparatus, and particularly suitable for a member constituting a press clubber which is a pre-absorption tower for cooling exhaust gas to 50 to 60 ° C. for exhaust gas dust removal and heavy metal removal in a seawater-based desulfurization apparatus. Related to anti-corrosion technology.

  With the tightening of environmental regulations, the installation of seawater desulfurization equipment is expected to increase. In addition, the installation of press clubbers is also expected to increase for projects that consider the effects of marine pollution from heavy metals.

  Conventionally, there is no example in which a press clubber is installed in a seawater desulfurization apparatus, but a similar condition includes a press clubber in a two-column desulfurization apparatus. In this press clubber, a high corrosion resistance material such as Hastelloy is applied as a countermeasure against corrosion of the throat member, and no countermeasure by sulfite oxidation has been adopted.

  However, as an equipment material such as a sulfur oxide (SOx) absorption tower instead of a press clubber in a wet flue gas desulfurization apparatus using seawater, an absorption tower tank installed at the lower part of the SOx absorption tower, an expensive high Cr high Stainless steel is used by using relatively inexpensive stainless steel without using Hastelloy, which is a Ni-based alloy of Mo, and as a measure against corrosion, by supplying oxygen-containing gas or oxidizing agent to increase the dissolved oxygen amount in the absorbing solution. A method of forming a stable passive film on the surface is described in Patent Document 1 (Japanese Patent Laid-Open No. 3-52623). In Patent Document 1, as a method of forming a stable passive film on the surface of stainless steel, oxygen is used so that the natural potential of stainless steel is between the passivation potential and the pitting corrosion potential or crevice corrosion potential. It is described that a gas or oxidant is supplied to the absorbent.

  Furthermore, Patent Document 2 (Japanese Patent Laid-Open No. 2008-36554) discloses that a treatment liquid containing limestone slurry is sprayed from a spray nozzle to an absorption tower of an exhaust gas desulfurization apparatus to remove sulfur oxides in the exhaust gas and at the same time included in the limestone slurry. A method of separating mercury from exhaust gas by converting hardly soluble mercury in exhaust gas into water-soluble mercury by using a mercury oxidizing agent.

  In this method of separating mercury from exhaust gas, it is disclosed that iodine having an oxidation reduction potential (700 to 750 mV) or less at which corrosion occurs on the inner wall of a stainless steel absorption tower is used as an oxidizing agent.

  In Patent Document 3 (Japanese Patent Laid-Open No. 4-59026), after exhaust gas containing SOx and nitrogen oxides (NOx) is brought into contact with exhaust gas in an absorption tower of limestone slurry, a part of the absorption slurry is removed from the absorption tower. Extracted and solid-liquid separated, hypochlorous acid for decomposing NS compound while suppressing pH of filtrate to 4 or less in order to promote decomposition of NS compound contained in separated filtrate Mix the salt. At this time, in order not to inhibit the SOx absorption reaction in the exhaust gas by the limestone slurry, the filtrate is adjusted so that the pH of the filtrate can be maintained at 3 to 4 or more, and re-supplied to the absorption tower as an absorption liquid that is made up again. A method is disclosed.

Japanese Patent Laid-Open No. 3-52623 JP 2008-36554 A JP-A-4-59026

The throat part of the press clubber installed for the purpose of dust removal and heavy metal removal in the exhaust gas flow path on the upstream side of the wet flue gas desulfurization device dissolves SO _{2 as} an exhaust gas component in the cleaning liquid used for dust removal and scaling prevention. And it becomes a reducing environment. Therefore, the passive state of stainless steel is destroyed and an environment in which severe corrosion occurs is obtained.

  However, in the conventional press clubber, there is no example in which countermeasures against corrosion other than the application of a high corrosion resistance material have been implemented. The apparatus with the anticorrosion measures described in Patent Documents 1 to 3 is an absorption tower for absorbing sulfur oxide in exhaust gas, not a press clubber on the upstream side of the absorption tower.

The absorption tower and press clubber differ in the following points.
(1) In the press clubber, the pH is lowered by reusing the absorption liquid, but the absorption tower is not reused.
(2) The amount of sulfurous acid mixed is different.

  The behavior of the absorbing liquid in the absorption tower and the behavior of the cleaning liquid in the press clubber are different, and the cleaning liquid in the press clubber tends to deteriorate the cleaning performance, and therefore the corrosion of the press clubber may be easily accelerated.

  An object of the present invention is to propose an anticorrosion technique for reducing the material cost of a press clubber / throat member and maintaining high corrosion resistance.

The problems of the present invention are solved by the following means.
The invention according to claim 1 is a wet type exhaust gas that absorbs and removes sulfur oxides in the exhaust gas by introducing the exhaust gas discharged from the combustion device and bringing it into contact with an absorption liquid mainly composed of limestone slurry sprayed in the absorption tower. In an anticorrosion method for a press clubber (dust removal tower) installed to remove exhaust gas dust and heavy metals with a cleaning liquid composed of the same component as the absorption liquid in the exhaust gas flow path upstream of the absorption tower in the smoke desulfurization apparatus, The press clubber anticorrosion method is characterized in that the cleaning liquid used in is oxidized in advance to clean the wall surface of the press clubber.

  The invention according to claim 2 is the anticorrosion method for press clubber according to claim 1, wherein an oxidizing agent is added to the cleaning liquid used in the press clubber so that the cleaning liquid has a pH of 1 or more and less than 7.

  The invention according to claim 3 is characterized in that hydrogen peroxide, ozone, chlorine, sodium hypochlorite (NaClO) or hypochlorous acid (HClO) is used as an oxidizing agent added to the cleaning liquid. This is a corrosion prevention method for press clubbers.

  The invention according to claim 4 is the anticorrosion method for a press clubber according to claim 1, characterized in that a potential is applied to the cleaning liquid so that the cleaning liquid has a pH of 1 or more and less than 7.

  The invention according to claim 5 is an anticorrosion method for a press clubber according to claim 1, wherein the potential is controlled to the potential range of the passivating region of the structural member of the press clubber by applying a potential to the cleaning liquid. is there.

  In the invention according to claim 6, a potential is applied to the cleaning liquid in the cleaning liquid tank, and the potential is in the potential range of −200 to −100 mV with respect to the saturated calomel electrode, which is a passive formation region of the structural member of the press clubber made of SUS316L. The method for preventing corrosion of a press clubber according to claim 1, wherein the potential is controlled.

The invention according to claim 7 adjusts the pH of the cleaning liquid using sodium hydroxide generated by electrolysis of seawater to 5 to 6 and simultaneously oxidizes sulfur oxides in the exhaust gas using the generated chlorine gas, thereby cleaning the cleaning liquid. The potential in the liquid is controlled to a potential range of -200 to -100 mV with respect to a saturated calomel electrode which is a passive formation region of the structural member of SUS316L. It is an anticorrosion method of the described press clubber.
(Function)
By adding an oxidizing agent to the press clubber cleaning liquid, oxidation of SO ₂ or sulfurous acid contained in the liquid to SO ₄ ²⁻ is performed. Thereby, corrosion of the throat part of the press clubber is suppressed, and a significant cost reduction is possible. In addition, it is possible to obtain necessary oxidant and adjust pH by electrolysis of seawater.

Corrosion of the stainless steel used as the throat portion of the press clubber is caused by the reduction of the oxide film on the surface of the stainless steel by the reducing action of sulfurous acid generated by dissolving SO _{2 in} the exhaust gas in the cleaning liquid.

In the present invention, it is possible to remove the sulfurous acid generated by dissolving SO _{2 in the} press clubber cleaning solution by adding an oxidizing agent, and to mitigate the corrosion of the press clubber.

  According to the present invention, it is possible to reduce the grade of high corrosion resistance material such as Hastelloy applied to press clubbers, and it is possible to provide a pollution-free desulfurization apparatus that is inexpensive and considers marine pollution by heavy metals. Become.

According to the first aspect of the present invention, the sulfite ion (SO ₃ ⁻ ) contained therein is converted to sulfate ion (SO ₄ ²⁻ ) by previously oxidizing the cleaning liquid used in the press clubber and cleaning the wall surface of the press clubber. The corrosion of the press clubber structural member can be suppressed.

  According to the second and third aspects of the invention, an oxidizing agent is added to the cleaning liquid used in the press clubber, and the cleaning liquid is set to a pH of 1 or more and less than 7 to oxidize sulfite ions in the liquid to sulfate ions, thereby causing corrosion of the structural member. Can be suppressed.

  According to the fourth aspect of the present invention, it is possible to suppress the corrosion of the press clubber structure member by applying a potential to the cleaning liquid, setting the cleaning liquid to pH 1 or more and less than 7, and oxidizing the sulfite ions in the cleaning liquid to sulfate ions.

According to the fifth aspect of the present invention, the potential in the cleaning liquid can be measured and applied to the cleaning liquid to control the potential within the potential range of the passive formation region of the structural member of the press clubber.
According to the sixth aspect of the present invention, a potential range of −200 to −100 mV (vs. saturated calomel electrode) which is a passive formation region of the structural member of the press clubber made of SUS316L by applying a potential to the cleaning solution in the cleaning solution tank. It is possible to prevent corrosion of the press clubber by controlling the potential.

  According to the seventh aspect of the present invention, the pH of the cleaning liquid using sodium hydroxide generated by electrolysis of seawater is adjusted to 5-6, and at the same time, sulfur oxides in the cleaning liquid are oxidized using the generated chlorine gas. By applying a potential to the cleaning liquid and controlling the potential within the cleaning liquid to a potential range of −200 to −100 mV (vs. saturated calomel electrode), which is a passivating region of the structural member of SUS316L, anticorrosion of the press clubber It can be performed.

It is a figure which shows the structural example of the seawater desulfurization apparatus of a press clubber installation type. It is the figure which simulated the cross-section of the press clubber throat part of FIG. It is the figure which observed the press clubber throat part of FIG. 2 from upper direction. is a diagram illustrating the effects of SO ₂ content on the corrosion of various stainless steel material at pH1 environment. On the anode polarization curve of SUS316L steel is a diagram showing the effect of SO ₂ content. It is a figure showing the structural example of the washing | cleaning-liquid oxidation system shown by embodiment by this invention. It is a figure which shows the detailed structure of a throat part among the structural examples of the washing | cleaning-liquid oxidation system shown by embodiment by this invention. It is a figure showing the structural example of the absorption liquid oxidation system using the electrolysis of seawater shown by embodiment by this invention. It is a figure which shows the example of a press clubber integrated desulfurization apparatus structure shown by embodiment by this invention.

The anticorrosion technique of the press clubber / throat member for seawater desulfurization according to the embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 shows an example of the configuration of a seawater desulfurization apparatus. The seawater desulfurization apparatus is composed of a press clubber 1 into which exhaust gas discharged from a combustion apparatus such as a boiler is introduced, a cleaning liquid tank 2 and a flue gas desulfurization apparatus 3. ing. The press clubber 1 is provided with a throat structure section (press clubber / throat section) 4.

  The main component of the cleaning liquid in this embodiment is limestone slurry, which is the same liquid as the desulfurization reaction absorbing liquid used in the desulfurization apparatus 3, but is used for cleaning the wall surface of the press clubber / throat section 4. In order to distinguish this liquid from the absorption liquid for the desulfurization reaction used in the flue gas desulfurization apparatus 3, it is referred to as a cleaning liquid.

  2 and 3 are views of the press clubber / throat portion 4 as viewed from the side and from above. The press clubber / throat section 4 is provided with a cleaning nozzle 5 for removing dust and heavy metals in the exhaust gas and a wall cleaning nozzle 6 for preventing scaling of the throat section 4. The arrows in the figure indicate the spray direction of the cleaning liquid sprayed from the cleaning nozzle 5 and the spray nozzle 6.

Seawater is used as the cleaning liquid sprayed from each cleaning nozzle 5 and spray nozzle 6 and is circulated through the cleaning liquid tank 2. As the cleaning liquid is circulated and used, SO ₂ contained in the exhaust gas is absorbed in the liquid, resulting in an environment containing sulfurous acid.

FIG. 4 shows the results of actual measurements of the corrosion amounts of various materials constituting the press clubber / throat section 4 at various SO ₂ -containing concentrations of the cleaning liquid in an environment containing pH 1 and chloride ions. As apparent from FIG. 4, the corrosion amount increases due to the inclusion of SO _{2 in} the cleaning liquid, and the corrosion amount of SUS316L is about 20 mm per year at a concentration of about 500 ppm assumed in the press clubber / press clubber cleaning liquid.

Below, the corrosion mechanism of the stainless steel assumed in the environment containing the cleaning solution SO ₂ is shown.
Sulfurous acid in the cleaning solution acts as a reducing agent by the reaction of the formulas (1) and (2) and destroys the passive film formed on the surface of the stainless steel, which causes severe corrosion.

SO ₂ + H ₂ O → H ₂ SO ₃ → H ⁺ + HSO ₃ ⁻ (1)
HSO ₃ ⁻ + H ₂ O → HSO ₄ ⁻ + 2H ⁺ + 2e ⁻ (2)

FIG. 5 shows the results of anodic polarization measurement in various concentrations of SO ₂ -containing environment in SUS316L steel cleaning solution. The vertical axis represents the current density, and the horizontal axis represents the potential with respect to the saturated calomel electrode. From also this figure, in an environment where SO ₂ is not contained in the washing solution, unlike the case containing SO _2, SUS316L any passivation areas are formed, it is found to have sufficient corrosion resistance. Therefore, it is desirable as a countermeasure against corrosion to remove SO ₂ dissolved in the solution or sulfurous acid (HSO ₃ ⁻ ) generated by dissolution by some method.

Formula (3) shows the oxidation-reduction reaction of sulfurous acid and the standard electrode potential.
H ₂ SO ₃ + H ₂ O═SO ₄ ²⁻ + 4H ⁺ + 2e ⁻ (3)
(Standard electrode potential: -0.17V vs SHE)
The above equation (3) indicates that sulfurous acid is oxidized and changed to sulfuric acid by adding an oxidizing agent or applying a potential. By this oxidation reaction, sulfurous acid in the cleaning liquid can be efficiently removed. However, it is known from past knowledge that the oxidation reaction of SO ₂ by oxygen or air bubbling hardly proceeds in a low pH environment. Therefore, in order to promote the oxidation reaction, it is necessary to accelerate the oxidation reaction by adding an oxidizing agent.

Next, an example of an apparatus configuration according to the present invention will be shown.
FIG. 6 shows an apparatus configuration example of the press clubber 1 to which the anticorrosion technique using the sulfite oxidation system is applied.

  In this embodiment, an oxidizing agent such as hydrogen peroxide, ozone, chlorine, NaClO, or HClO is added in the cleaning liquid tank 2 to promote the oxidation reaction of sulfurous acid. The addition amount of the oxidizing agent is determined by measuring natural potential or redox potential, and is controlled to an appropriate amount. The potential is measured by, for example, measuring a sample made of the same material as the structural member in the cleaning liquid tank 2.

  When the oxidant is excessively added, it is assumed that a potential at which pitting corrosion occurs in the stainless steel, and therefore, control is performed so that the pitting corrosion potential is not more than. For example, when SUS316L is used as a structural member, control is performed in a potential range of −200 to −100 mV with respect to the saturated calomel electrode that is the passive potential shown in FIG. Moreover, pH is controlled so that it may be maintained in the range of 1-7.

  When a material other than SUS316L, such as 23Cr25Ni5Mo steel, which has a higher corrosion resistance than SUS316L, is used as the material for the press clubber / throat part 4, the passivating potential range is widened, so -200 to 150 mV The purpose can be achieved by controlling the potential within the range of. For other metal materials and thermal spray materials, the purpose can be achieved by obtaining an anodic polarization curve in advance by electrochemical measurement and controlling the potential within the passivating potential range.

  FIG. 7 shows an example of the structure of the press clubber / throat section 4. The structure of the press clubber / throat portion 4 in the embodiment shown in FIG. 7 is that the wall surface is uniformly immersed in the cleaning liquid by adding a baffle plate 7 to the wall surface portion to properly classify the spray area per spray unit. Structure.

Moreover, in this system using seawater, it is possible to generate chlorine as an oxidizing agent and sodium hydroxide capable of adjusting pH by electrolysis of seawater.
FIG. 8 shows an apparatus configuration example in the case of using seawater electrolysis.

First, the electrolysis apparatus 8 performs the electrolysis reaction of seawater shown in Formula (4) and Formula (5) to generate sodium hydroxide and chlorine gas, respectively.
2Na ⁺ + 2H ₂ O + 2e ⁻ → 2NaOH + H ₂ (4)
2Cl ⁻ → Cl ₂ + 2e ⁻ (3)
Subsequently, in the pH adjustment tank 9, first, pH adjustment using sodium hydroxide is performed to control the pH to 5-6. Further, chlorine gas is added to the cleaning liquid in the oxidation tank 10, and the sulfurous acid in the cleaning liquid is oxidized by a reduction reaction of chlorine gas represented by the formula (6).

Cl ₂ (g) + 2e ⁻ = 2Cl ⁻ (aq) (6)
(Standard electrode potential: 1.358V)
The amount of chlorine added to the cleaning solution is determined by natural potential measurement or oxidation-reduction potential measurement as in the other examples, and must be controlled to an appropriate amount, but the objective can be achieved at a concentration of several hundred ppm or less. It is.

  As described above, the material cost of the press clubber member 4 can be reduced by using the anticorrosion technique based on sulfite oxidation in the press clubber cleaning liquid shown in the embodiment of the present invention.

  In addition to the case where the press clubber 4 and the desulfurization tower 3 for desulfurizing exhaust gas are separated as shown in FIG. 1, the press clubber portion 11 and the desulfurization device portion 12 are integrated as shown in FIG. The present invention can also be applied to a seawater desulfurization apparatus.

  The present invention can be applied not only to a seawater desulfurization apparatus but also to a dust removal tower in a desulfurization apparatus using a limestone-gypsum method.

DESCRIPTION OF SYMBOLS 1 Press clubber 2 Cleaning liquid tank 3 Flue gas desulfurization apparatus 4 Press clubber throat part 5 Cleaning nozzle 6 Wall surface cleaning nozzle 7 Current plate 8 Electrolysis apparatus 9 pH adjustment tank 10 Oxidation tank

Claims (7)

Same as the absorption liquid component in the exhaust gas flow path on the upstream side of the flue gas desulfurization apparatus that introduces the exhaust gas discharged from the combustion apparatus and makes it contact with the absorption liquid made of sprayed limestone slurry to absorb and remove sulfur oxides in the exhaust gas. In the anti-corrosion method for press clubbers installed to remove exhaust gas dust and heavy metals with cleaning liquid consisting of components,
An anticorrosion method for a press clubber, wherein the cleaning liquid used in the press clubber is previously oxidized to clean the wall surface of the press clubber.   The anticorrosion method for a press clubber according to claim 1, wherein an oxidizing agent is added to the cleaning liquid used in the press clubber so that the cleaning liquid has a pH of 1 or more and less than 7.   The method for preventing corrosion of a press clubber according to claim 2, wherein hydrogen peroxide, ozone, chlorine, sodium hypochlorite (NaClO) or hypochlorous acid (HClO) is used as an oxidizing agent added to the cleaning liquid.   The method for preventing corrosion of a press clubber according to claim 1, wherein a potential is applied to the cleaning liquid so that the cleaning liquid has a pH of 1 or more and less than 7.   2. The press clubber anticorrosion method according to claim 1, wherein the potential in the cleaning liquid is measured and applied to the cleaning liquid to control the potential within the potential range of the passivating region of the structural member of the press clubber.   A potential is applied to the cleaning liquid in the cleaning liquid tank, and the potential is controlled in a potential range of −200 to −100 mV with respect to the saturated calomel electrode, which is a passive formation region of the structural member of the press clubber made of SUS316L. The method for preventing corrosion of a press clubber according to claim 1.   At the same time as adjusting the pH of the cleaning liquid using sodium hydroxide generated by electrolysis of seawater to 5 to 6, the generated chlorine gas is used to oxidize sulfur oxides in the exhaust gas, and a potential is applied to the cleaning liquid. The corrosion prevention method for a press clubber according to claim 1, wherein the potential is controlled to a potential range of −200 to −100 mV with respect to a saturated calomel electrode which is a passive formation region of the structural member of SUS316L.

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