JPH07194938A - Flue gas desulfurization method for heavy oil boiler - Google Patents

Abstract

PURPOSE:To desulfurize exhaust gas from a heavy oil boiler with magnesium hydroxide. CONSTITUTION:Magnesium hydroxide is recovered from ash-dust from a dust collector installed in a flue for exhaust gas from a heavy oil boiler. The recovered magnesium hydroxide is used as a part or the whole of magnesium hydroxide necessary for the desulfurization of the exhaust gas. In this way, the waste ash-dust can be utilized effectively, and the desulfurization can be conducted using recycled raw material within the same perocess.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for desulfurizing exhaust gas of a boiler using heavy oil as a fuel with magnesium hydroxide.

[0002]

2. Description of the Related Art As one of methods for treating combustion exhaust gas discharged from a boiler facility that burns fossil fuels such as coal and heavy oil such as a thermal power plant, there is a desulfurization treatment method using magnesium hydroxide shown in FIG. In this method, the exhaust gas a discharged from the boiler is first cooled and humidified in the cooling / dust removing step 10 while being in contact with the absorbing liquid q ′ which is a part of the absorbing liquid q. By this operation, the dust in the exhaust gas is removed. The evaporated water is replenished with industrial water c. The processing gas n ₁ from which soot and dust have been removed and the cleaning liquid o containing soot and dust flow into the next absorption step 6. In the absorption step 6, the treatment gas n ₁ and the absorption liquid q come into contact with each other to perform desulfurization. The chemical reaction at that time is represented by the following formulas (1) to (4). Magnesium hydroxide p is injected to promote the above reaction. As the magnesium hydroxide p, a commercially available product or a product obtained by coagulating and precipitating seawater with calcium hydroxide is used. After the mist component of the processing gas n ₂ is removed by the demister 7, the processing gas n ₂ is released into the atmosphere as the processing gas n ₃ . Most of the absorption liquid q from the absorption process 6 flows into the oxidation process 8 and contacts the air r. Thereby, the magnesium sulfite salt (MgSO ₃ ) in the absorbing solution q is oxidized according to the formula (5). Oxidizing solution from oxidation step 8
Contains magnesium sulfate (MgSO ₄ ) and dust (suspended substance). Magnesium sulfate in the oxidizing solution s is dissolved because it has high solubility, but since dust particles are contained as a suspended substance, it is discharged out of the system as a dehydrated cake u in the next dehydration step 9.

[0003]

Embedded image SO ₂ + H ₂ O → H ₂ SO ₃ (1) H ₂ SO ₃ + Mg (OH) ₂ → MgSO ₃ + 2H ₂ O (2) MgSO ₃ + SO ₂ + H ₂ O → Mg (HSO ₃ ) ₂ ( 3) Mg (HSO ₃ ) ₂ + Mg (OH) ₂ → MgSO ₃ + 2H ₂ O (4) MgSO ₃ + 1 / 2O ₂ → MgSO ₄ (5)

In recent years, various diversification of fuels has been studied in thermal power plants and the like, and use of heavy oil such as orinoco oil, petroleum coke, and asphalt has been studied. As one of them, a heavy oil / water emulsion (hereinafter referred to as “orinocotal”) that can be handled at room temperature by mixing orinoco oil with about 30% by weight of water and a trace amount of a surfactant to form an emulsion. ) Is attracting attention as a new fuel. Hereinafter, in the present specification, heavy oil includes heavy oil / water emulsion.

[0005]

Conventionally, the dust ash generated in a thermal power plant or the like has been disposed of in landfill after being subjected to a simple treatment for preventing scattering and the like. However, in recent years, from the viewpoint of environmental protection, in order to prevent the leaching of heavy metals and the like, a reliable treatment for preventing elution before landfilling has been required, which causes a considerable economical burden.
Further, with respect to drainage and drainage, more advanced measures are required to satisfy the discharge standard. In particular, heavy oil such as orinocotal has a higher content of ash, nitrogen and sulfur oxides than heavy oil, so that the amount of dust, SOx and NOx generated in the exhaust gas is large. Furthermore, it is common to add magnesium compounds to the heavy oil to prevent corrosion when the heavy oil is burned in a boiler, and the combustion exhaust gas contains NH4 as a material corrosion countermeasure in ducts and exhaust gas treatment equipment. ₃
Since gas is added, the properties of dust ash and wastewater discharged from the exhaust gas treatment process of boilers that use heavy oil as fuel are significantly different from the conventional ones, and new measures are needed. There is. Examples of components contained in the fly ash of the heavy oil combustion exhaust gas ^{_{V, NH 4 +, Mg,}} SO 4 2 - There are such conventionally, V and NH ₄ ⁺ such as that contained in these current Chirihai No valuables have been collected. The object of the present invention is to provide vanadium (V) and ammonia (NH ₃ ).
It is an object of the present invention to provide a flue gas desulfurization method for a heavy oil fuel fired boiler, which can recover valuable materials such as the above and can supply a part or all of the desulfurization agent in the same system.

[0006]

DISCLOSURE OF THE INVENTION The present inventors have found that the dust ash collected from the exhaust gas of a heavy oil fuel fired boiler contains V
Focusing on the fact that a large amount of Mg compounds are contained in addition to valuable materials such as NH ₄ ⁺ and NH ₄ ^+, as a result of intensive research, an efficient flue gas desulfurization process was constructed by combining it with the desulfurization technology of the magnesium hydroxide method. The present invention has been completed by finding out what can be done. That is, the present invention relates to (1) a method of desulfurizing exhaust gas of a boiler using heavy oil as a fuel, using magnesium hydroxide, to remove magnesium hydroxide from dust ash emitted from a dust collector provided in an exhaust gas flue of the boiler. And a recovered magnesium hydroxide is used as a part or all of the magnesium hydroxide used in the desulfurization treatment, and a flue gas desulfurization method for a heavy oil fueled boiler, and (2) heavy fuel In a method of desulfurizing the exhaust gas of a boiler using oil as a fuel with magnesium hydroxide, dust ash emitted from a dust collector provided in the exhaust gas flue of the boiler is mixed with water, and the resulting mixed liquid is divalent. After the addition of the iron compound, the coagulation-precipitation treatment is performed at a pH of 5 or more and less than 10 to separate the first precipitate and the first supernatant liquid, and the valuable material is recovered from the first precipitate, and the first supernatant liquid is collected. The liquid is adjusted to pH 10 or more and subjected to coagulation-sedimentation treatment to separate into a second precipitate and a second supernatant liquid, and the second precipitate containing magnesium hydroxide as a main component is used for the desulfurization treatment. It is a flue gas desulfurization method for heavy oil fuel-fired boilers, which is used as a part or all and the second supernatant is subjected to a distillation treatment to recover ammonia.

The method of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic flow chart showing an example of a flue gas desulfurization process according to the method of the present invention. Exhaust gas a from the heavy oil fuel fired boiler is removed in the dust collecting step 1. The collected dust ash b is sent to the mixing step 2 and the dedusted processing gas n ₁ is sent to the absorption step (desulfurization step) 6. In addition, dust collection process 1
Most of the dust collecting ash b in (1) is dust collecting ash by the dust collecting device, but in addition, ash captured by an air heater or the like can be treated together. The amount of dust ash b generated and the components contained vary depending on the type of fuel, operating conditions of the boiler, etc.
7,000 when treating 1 million m ³ N / h of exhaust gas
A dust collection ash of about kg / h is generated, and vanadium (V), fixed ammonium and magnesium are contained in an amount of 2 to 5% by weight, 5 to 10% by weight and 4 to 13% by weight, respectively.
In addition, 50-70% by weight of SO ₄ ^2- , unburned carbon content 1
.About.20% by weight and 0.1 to 1% by weight of calcium.

In the mixing step 2, the dust ash b and the industrial water c are mixed. You may use the treated water in another process as some or all of the industrial water. The mixing ratio of the collected ash b and the industrial water c is 100 parts by weight of the industrial water c and the collected ash b is 5 parts.
-40 parts by weight (50-400 for 1 liter of water)
g). This ratio is preferably 5 to 10 parts by weight in order to improve the treatment efficiency in the next coagulation and precipitation step. Normal,
The pH of the mixed solution d is 2-3. In this process, most of the components other than unburned carbon content in the collected ash are dissolved.

The pH of the mixed solution d obtained in the mixing step 2 is adjusted to 5 to 10, preferably 6 to 8 by adding the ferrous salt e and the alkaline agent f in the first flocculation and precipitation step 3. Then
Polymer flocculant g is added to carry out flocculation and precipitation treatment. Ferrous chloride or ferrous sulfate is preferable as the ferrous salt e used here. As the alkaline agent f, caustic soda (NaO
H), sodium carbonate (Na ₂ CO ₃ ), slaked lime (Ca (0
H) ₂ ) and quick lime (CaO) can be used. The addition amount of the ferrous salt may theoretically be an equimolar ratio with respect to the pentavalent vanadium (V) of the mixed solution d, but in consideration of the influence of coexisting ions, the addition amount is not less than the equimolar ratio. preferable. As the polymer coagulant, a cationic or nonionic polymer coagulant used in an ordinary waste water treatment apparatus can be used. By the treatment in the first coagulation-precipitation step 3, pentavalent vanadium is reduced to tetravalent in accordance with the following formulas (6) and (7), and V
Since it precipitates in the form of O (OH) ₂ , it is separated into a supernatant liquid i and a precipitate h. At this time, the first precipitate h contains 2% by weight or more of vanadium and gypsum, iron hydroxide or unburned carbon, and the purity can be further increased by an appropriate means to make effective use. When slaked lime or quick lime is used as the alkaline agent, a large amount of gypsum is formed and the vanadium content is low, but when sodium carbonate, caustic soda, etc. are used, the vanadium content is high. The supernatant liquid i separated here contains almost no vanadium, and a part thereof is returned to the mixing step 2 as a return liquid i ′ if necessary, but most of the following second coagulation-precipitation step 4 Sent to.

The supernatant liquid i separated in the first coagulation-sedimentation step 3 is added with an alkali agent f in the second coagulation-sedimentation step 4 to obtain p.
Polymer coagulant g after being adjusted to H10 or more
Is added. By this operation, magnesium hydroxide is precipitated and separated as the second precipitate k as represented by the formula (8). The pH here is preferably in the range of 11 to 12 in order to sufficiently precipitate magnesium hydroxide and to improve the efficiency in the next distillation step. The second supernatant liquid j separated in the second flocculation-precipitation step 4 is sent to the distillation step 5, and the second precipitate k is sent to the absorption step 6. Distillation process 5
Then, the ammonia gas is separated and recovered from the second supernatant liquid j by a distillation process. As the distillation means here, in addition to steam stripping, ordinary distillation means such as vacuum distillation can be applied, but steam stripping is preferred. By this operation, the ammonia gas m is separated from the liquid, and further subjected to an appropriate condensation operation to be concentrated and recovered as ammonia water. Meanwhile, ammonia is separated,
The removed liquid is discharged as treated water 1, further subjected to post-treatment such as COD treatment, and then discharged to the outside of the system.

[0011]

Embedded image VO ₂ ⁺ (pentavalent) → VO ²⁺ (tetravalent) (6) Fe ²⁺ (divalent) → Fe ³⁺ (trivalent) (7) Mg ²⁺ + 2OH ⁻ → Mg (OH) ₂ ↓ (8)

On the other hand, in the absorption step 6, desulfurization is performed by the reactions of the formulas (1) to (4). The second precipitate k is effectively used as magnesium hydroxide as a desulfurizing agent, and the amount of water is appropriately adjusted to obtain an absorbing liquid. At the start of operation or when the amount of the second precipitate k to be recovered is insufficient, magnesium hydroxide is replenished from the outside, but normally, the amount recovered from the dust ash is sufficient during steady operation. The processing gas n ₂ in the absorption step 6 is released into the atmosphere as the processing gas n ₃ after the mist component is removed by the demister 7.

Most of the absorption liquid q from the absorption step 6 flows into the oxidation step 8 and is brought into contact with the air r. As a result, the magnesium sulfite salt (MgSO ₃ ) in the absorbing solution q is oxidized according to the formula (5) to become MgSO ₄ . Oxidation process 8
The oxidizing solution s from 1 contains magnesium sulfate (MgSO ₄ ) and dust (suspended substance). Magnesium sulfate in the oxidizing solution s is dissolved because it has high solubility, but since dust is contained as a suspended substance, it is sent to the next dehydration step 9 and separated into the dehydrated cake u and the desorbed liquid t and then outside the system. Is discharged to.

[0014]

EXAMPLES The method of the present invention will be described in more detail with reference to the following examples. Exhaust gas from a boiler using orinocotar as a fuel was treated by the process of FIG. 1 to test the flue gas desulfurization method of the present invention. Table 1 shows the composition of the exhaust gas and the collected dust ash at this time.

[0015]

[Table 1]

When 6.0 kg of the dust ash and 54 liters of industrial water were mixed and 400 g of ferrous chloride was added, the pH was adjusted to 8 with slaked lime, and 850 g of slaked lime was required. A polymeric flocculant was added to this solution in an amount equivalent to 10 mg / liter, and the generated precipitate (first precipitate) was separated to obtain 970 g of a solid matter by dry weight. This solid contains 18 wt.% Vanadium, 22 wt.% Gypsum and 6 wt.
It contained wt% unburned C. The liquid from which the solid matter was separated (first supernatant liquid) was adjusted to pH 11 by further adding NaOH, and 1850 g of NaOH was required. A polymer flocculant was added to this solution in an amount of 30 mg / liter, and the resulting precipitate (second precipitate) was separated and dried to a dry weight of 1
750 g of solid was obtained. This solid was magnesium hydroxide having a purity of 99% by weight, and the whole amount was used in the absorption step (desulfurization step). The liquid (second supernatant liquid) from which the second precipitate has been separated is blown with steam to be stripped,
₃ 5540 g of ammonia water having a content of 10% by weight was recovered. The main component contained in the stripping residual liquid was Na ₂ SO ₃ , and it was possible to discharge the product as it was. on the other hand,
Magnesium hydroxide (equivalent to 1750 g dry weight equivalent) separated as the second precipitate is supplied to the absorber (desulfurization device) in the slurry state to adjust the absorbing liquid, and the amount corresponding to the amount of dust collecting ash used here. When desulfurization treatment was performed through the boiler exhaust gas from the dedusting zone, the SO ₂ at the treated gas outlet was
The concentration was 50 ppm, and a desulfurization rate of 93.8% could be achieved.

[0017]

The method of the present invention has the following effects. (1) With the conventional technology, it is possible to perform collective treatment of dust ash and wastewater that have been treated in different processes.
The processing efficiency and the economical efficiency of the apparatus are significantly improved. (2) In the past, it had to be discarded as it was, and furthermore, vanadium (V), ammonia (NH ₃ ) and magnesium (M
Valuable materials such as g) can be recovered and resources can be reused. In addition, the amount of waste as a whole is reduced, which is a great environmental advantage. (3) Magnesium is recovered in the form of magnesium hydroxide, so it can be used as it is for desulfurization of exhaust gas, effective use of dust ash is possible, and desulfurization is performed only by the recovered material in the same process. be able to. (4) The method of the present invention does not require seawater or commercially available magnesium hydroxide, so that it is not restricted by site conditions. (5) In the method of the present invention, the second supernatant liquid from which magnesium hydroxide has been recovered has a pH at which ammonia can be distilled and recovered as it is.
It does not require H adjustment and is an efficient process.

[Brief description of drawings]

FIG. 1 is a schematic flow chart showing an example of a flue gas desulfurization process according to the method of the present invention.

FIG. 2 is a schematic flowchart showing an outline of a desulfurization treatment process using magnesium hydroxide.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location F23J 15/00 6908-3K F23J 15/00 A (72) Inventor Atsushi Tani Nishi-ku, Hiroshima City, Hiroshima Prefecture 4-6-22 Kannon Shinmachi, Hiroshima Research Laboratory, Mitsubishi Heavy Industries, Ltd. (72) Inventor Naohiko Ugawa 4-6-22, Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture (72) Inventor, Susumu Okino Hiroshima 4-22 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Prefecture Mitsubishi Heavy Industries, Ltd. Hiroshima Research Institute (72) Inventor Hiroshi Tanaka 4-6-22 Kannon-shinmachi, Nishi-ku, Hiroshima City, Hiroshima (72) Invention Satoru Nakamura 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries Ltd. Hiroshima Research Laboratory (72) Inventor Hideki Kamiyoshi Hyogo Prefecture Toshi Hyogo-ku KomatsuTsu 5 chome No. 16 Co., Ltd. Kamihishi in the Hi-Tech (72) inventor 揖場 Satoshi Hyogo Prefecture, Kobe, Hyogo-ku, KomatsuTsu 5 chome No. 16 Co., Ltd. Kamihishi in the High-Tech

Claims (2)

[Claims] 1. A method for desulfurizing exhaust gas of a boiler using heavy oil as a fuel with magnesium hydroxide, wherein magnesium hydroxide is recovered from dust ash emitted from a dust collector provided in an exhaust gas flue of the boiler. Then, the recovered magnesium hydroxide is used as a part or all of the magnesium hydroxide used in the desulfurization treatment, and a flue gas desulfurization method for a heavy oil fuel fired boiler. 2. A method for desulfurizing exhaust gas of a boiler using heavy oil as a fuel with magnesium hydroxide, wherein dust ash from a dust collector provided in an exhaust gas flue of the boiler is mixed with water, After adding a divalent iron compound to the resulting mixed solution, a coagulation-precipitation treatment is performed at a pH of 5 or more and less than 10 to separate the first precipitate and the first supernatant liquid, and the valuable material is recovered from the first precipitate. Then, the first supernatant is adjusted to pH 10 or more and subjected to coagulation precipitation treatment to separate into a second precipitate and a second supernatant, and the second precipitate containing magnesium hydroxide as a main component is used in the desulfurization treatment. A method for flue gas desulfurization of a heavy oil fuel-fired boiler, wherein the second supernatant liquid is subjected to a distillation treatment to recover ammonia, which is used as a part or all of magnesium hydroxide.