JPS6328485A - Treatment of waste water from stack gas desulfurization stage - Google Patents

Abstract

PURPOSE:To prevent sticking of combustion ashes, corrosion of furnace materials, etc., by adding an iron compd. directly to the supernatant water discharged from a gypsum sepn. stage, or adding an ammonium bicarbonate thereto to settle and separate the Ca component, then adding the iron compd. to said water, and feeding such water to a boiler. CONSTITUTION:The iron compd. 25 is directly added to the supernatant water 12' discharged from the gypsum sepn. stage 4 in a method for treating the waste water from a desulfurization stage which treats exhaust combustion gases of fossil fuel contg. sulfur by a lime gypsum method or after the ammonium bicarbonate 23 is added to said water, the pH of the water is adjusted by a pH controlling agent 24 such as slaked lime and the Ca is removed in a settling tank 7, then the iron compd. 25 is added to the water. Such water is fed to the boiler and is incinerated. As a result, the melting and sticking of the combustion ashes, the corrosion of the furnace materials, etc., are prevented.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is applicable to the treatment of exhaust gas from boilers, gas turbines, diesels, etc. that use fossil fuels containing sulfur, particularly wastewater treatment in flue gas desulfurization treatment using the lime-gypsum method. Regarding the method.

[Conventional technology]

Coal fuel containing sulfur compounds 9 Heavy oil 1 By-product oil from petroleum refining, residual oil 1 Gas fuels such as petroleum coke, asphalt and coke oven gas, blast furnace gas (coal-based 1 petroleum-based and gas fuels are abbreviated), etc. BACKGROUND OF THE INVENTION Fire couplers such as combustion boilers and gas turbines are equipped with flue gas desulfurization equipment to remove sulfur oxides (Box) contained in the flue gas. Various methods have been developed for flue gas desulfurization, but among them, the lime-gypsum method is adopted by many power plants because the desulfurization process is simple, efficient, and economical. has been done. However, the treated wastewater contains persistent nithionic acids (H, S, Os) and S-N compounds (NHlSO,H), which cause high COD wastewater. Furthermore, this wastewater contains M2 compounds which are contained as impurities in the lime and fuel used, and various attempts have been made to treat them.

First, as a method for treating nithionic acid and S--N compounds in wastewater, the following methods have been conventionally used.

(1) Wastewater containing nithionic acid and S-N compounds,
The above compound is concentrated using an ion exchange resin (referred to as a concentrated regeneration solution), and sulfuric acid and steam are added to this solution to obtain 100%
A method of oxidation and thermal decomposition at temperatures below ℃.

12) A method in which wastewater containing nithionic acid and S-N compounds is oxidized and thermally decomposed with sulfuric acid and steam in its original state (without using an ion exchange resin).

f3) When the concentrated regenerated liquid of nithionic acid and S-N compound obtained by the method of (1) was directly injected into the combustion area of the boiler furnace, υ mixed into the fuel, and in the case of solid fuel, The method used is to spray it on top of it and then burn it in a boiler furnace.

Whichever method is adopted, it is possible to decompose and remove nithionic acid and S-N compounds, which cause COD, but there are advantages such as the simplification of wastewater treatment equipment and low operating costs. , method (3), that is, the method of injecting into the boiler furnace, is attracting attention.

An outline of this method will be explained below using FIG. 2.

The combustion exhaust gas of coal-based 9 petroleum-based and gas fuels is cooled in the cooling process 1.
The clean gas is cooled and dust removed, and then led to absorption step 2, where the absorption liquid absorbs sulfur oxides (SO 803, etc.), and then is released outside the system (flue, chimney, etc.). (emitted to the outside through the process). At that time, various heavy substances such as chlorine and fluorine that were present in the fuel are removed from the cooling process 1.

Cooling tower wastewater 11 containing light metal elements and unburned carbon (all components of soot and dust) is discharged to the outside as is, or after solid-liquid separation of soot and dust.

On the other hand, the treated water that has passed through absorption process 2 is sent to oxidation process 3, where air is blown into the water to convert sulfite into sulfate (SO, → 504
) and precipitates in the liquid as sparingly soluble gypsum (Canon), which is then sent to a gypsum separation step 4, and the generated gypsum is discharged to the outside as by-product gypsum. 5. The water after noble separation (supernatant water 12) contains COD components such as persistent nithionic acid and S-N compounds, and cannot be discharged into rivers or the sea. Therefore, the above R water 12 and the cooling tower waste water 11 are mixed, and the pT (adjusting agent 21) such as caustic soda or slaked lime is added in the neutralization tank 5 to reach a predetermined pH, and then a reprinted polymer flocculant 22 is added. Then, soot, dust, fluorine compounds, heavy metals, etc. are removed in the coagulation sedimentation tank 16.In order to prevent the Ca contained in the supernatant water 13 separated and removed in the coagulation sedimentation tank 6 and scaling in the ion exchange column 9, Calcium carbonate (CaCO3) is separated and removed in the precipitation tank 7 by reaction with sodium carbonate 27 in the presence of a pH adjuster 26 such as caustic soda. Upper U water 14, nithionic acid and S-N in ion exchange tower 9
Adsorbs and removes compounds, etc. and discharges them.

The ion exchange resin that has been saturated with adsorption by K in this way is sulfuric acid 2
8 and caustic soda 29 for reuse. However, the recycled waste liquid 15 containing concentrated nithionic acid and S-N compounds along with the sulfuric acid and caustic soda used during the regeneration process is stored in the recycled waste liquid storage tank 10. , mixed with fuel in small quantities, dispersed, and burned in a boiler.

[Problem that the invention seeks to solve]

Although the conventional method described above, in which ion exchange resin concentrated regenerated wastewater is evaporated and decomposed in a boiler furnace, has the following economic advantages: Due to inherent problems, practical application has been delayed.

(1) Since the ion-exchange resin regenerating liquid contains sulfuric acid and caustic soda at high concentrations, if this is directly injected into the boiler furnace, there is a risk of corrosive wear and tear on the boiler heat exchanger tubes.

(2) In particular, Na compounds react with other corrosive components in the fuel (V, S for petroleum-based, S9 for gas fuel-based, S, K for coal-based) to reduce It forms a highly corrosive compound at its melting point, promoting contamination of heat transfer surfaces and accelerating corrosive action.

(3) Furthermore, when waste liquid is injected into the boiler, white compounds such as Ca and MS' contained in it adhere to the surface of the furnace wall tubes, reducing the rate of heat absorption by radiation and reducing the combustion environment. As a result of the high temperature (flame temperature) continuing for a long time, Samar N
It promotes the generation of OX (NOx produced by high-temperature oxidation of nitrogen in the air), disrupts the heat absorption balance of each part of the boiler, contaminates the heat transfer surface, and shortens the life of the denitrification catalyst installed in the furnace. Failures such as

The present invention aims to provide a method for treating waste water from a gypsum separation process that eliminates the above-mentioned drawbacks.

[Failure to solve the problem]

The present invention provides a method for treating wastewater from a desulfurization process in which sulfur-containing fossil fuel combustion exhaust gas is treated using a lime-gypsum method.
Add iron compounds directly to the supernatant water discharged from the gypsum separation process, or add ammonium bicarbonate to the supernatant water to precipitate and separate calcium components, then add iron compounds and inject into the boiler. This is a wastewater treatment method characterized by:

The features of the present invention are summarized as follows.

(1) In the conventional method, sulfuric acid and Na are brought into the boiler furnace, so it was decided to inject supernatant water from the gypsum separation process, which contains few such components, into the boiler furnace. This supernatant water contains about 1000 ppm each of Ca and MP, so in boilers using petroleum fuel,
It has the effect of suppressing corrosion of heat exchanger tubes in high-temperature areas caused by compounds, and also reacts with Sox in combustion gas to reduce sulfuric acid dew point corrosion in low-temperature areas.

(2) Nithionic acid and S contained in the supernatant water
-N compounds can be completely decomposed in a boiler furnace.

(3) The above method can be used as is for coal-fired boilers, but in oil-based boilers, the calcium component in the supernatant water reacts with 30x in the combustion gas in the furnace, causing hard carbon.
a S 04 is generated and firmly adheres to the heat transfer tube, causing contamination of the heat transfer surface, so it is preferable that the calcium component in the supernatant water is as low as possible.bo Therefore, ammonium bicarbonate is added to the supernatant water. Then, C
A is precipitated and removed as CaCO3, and is injected into the boiler furnace as clean water with a high MP content.

(Since Na1CO3 is not used, Na is not brought into the boiler furnace.) (4) Even with this treatment, the supernatant water containing a large amount of M may not be injected into the boiler furnace for a long time. When I was there,
White MP compound (MPO.

As a result of MPSO4) adhering to the surface of the furnace wall tube, the heat absorption rate in that area decreases, thereby increasing the flame temperature and promoting the generation of thermal NOx.

In addition, the rise in flame temperature disrupts the heat absorption balance of the superheater, reheater, energy saver, etc. installed on the downstream side, causing many new problems such as an increase in the exhaust gas temperature at the boiler outlet. do.

As a countermeasure against this, Fe compounds are added to the supernatant water or to the fuel to prevent whitening of the furnace wall tubes and to prevent the problems associated with this. This method can also be used for coal-fired and gas-fired boilers.

[Effect]

In order to solve the problems of the prior art, namely, the melting and fixation of combustion ash caused by boiler combustion of water containing a large amount of chemicals and sodium ions, and the corrosion of door materials, the present invention provides a first method.
The flow shown in the figure was adopted. The present invention will be explained below based on FIG.

The present invention provides cooling tower waste water 11 containing soot, CL, F, heavy metals, etc. caused by fuel discharged from the cooling process 1, and discharged from the gypsum separation process 4 after passing through the absorption process 2 and the oxidation process 5, The objective is to separate and separately treat the supernatant water 12 containing COD components such as hard-to-decompose nithionic acid and S--N compounds originating from the smoke exhaust system.

The cooling tower wastewater 11 in the cooling process 1 is neutralized 5, after adjusting the pg with a pH adjuster 21 such as caustic soda and slaked lime, a flocculant 22 such as a polymer flocculant is added, and it flows into coagulation sedimentation 16. Here, dust, F compounds, heavy metals, etc. are precipitated, and # is separated. 1-1 The supernatant water that does not contain these components is discharged into the tank. On the other hand, the supernatant water 12 containing nithionic acid and S-N compounds is either led to the oak mixing storage tank 8, added with a predetermined amount of iron compound 25, and burned in a boiler (indicated by 12' in the figure). ,or,
Heavy carbon [after adding 23% ammonium, ammonia aqueous solution,
Adjust the pH with a pH adjuster such as slaked lime, and precipitate mW.
In step 17, Ca is removed, the supernatant water 15 flows into the mixing storage tank 8, a predetermined amount of iron compound 25 is added thereto, and the water is sent to the boiler for incineration.

The effects of the method of the present invention are as follows.

(1) By injecting the supernatant water from the gypsum separation process into the boiler furnace, the content of chemicals and sodium ions can be substantially ignored, so no problems occur as in the prior art.

12) Since only the supernatant water from the gypsum separation process is injected into the boiler furnace, the amount of water to be treated is smaller than the conventional method of mixing it with cooling tower waste water and burning it in the boiler. -N compounds can be burned and disposed of efficiently.

(3) The necessity of calcium removal 1i differs depending on whether petroleum-based fuel or gas fuel is used or when coal is used as fuel.

When oil-based or gas fuels are used, the calcium that flows into the boiler is predominantly brought in from the water to be burned, so calcium removal is important. In view of the characteristics of the collected soot and dust, the calcium content in the water to be burned is preferably maintained at 500 η/ or less, more preferably 500 η/ or less. On the other hand, when coal is used as fuel, the calcium flowing into the boiler is predominantly brought in from the fuel, so there is no need to remove calcium from the water to be burned, and the supernatant water 12' is directly injected into the boiler. Since it can be combusted, it can be operated economically.

(4) Use a pH adjuster when removing calcium.
An ammonia aqueous solution or slaked lime that does not contain IJium ions is preferable. In addition, the adjusted pg is preferably pH 8 to 9 based on the efficiency of heavy carbon, ammonium, and pH regulator, and preferably p) TEL 5 to ? It is.

(5) Conventional technology is applied to the treatment of cooling tower wastewater, and the effects are achieved as previously achieved. When petroleum-based natural materials and gas fuels are used, caustic soda slaked lime can be used as the pH adjuster, but it is preferable to use caustic soda to reduce sludge generation during coagulation and sedimentation. The adjusted pH is preferably 7.5 to a5, more preferably pH8 to a5. When coal is used as a fuel, it is preferable to use slaked lime as a pH adjuster from the perspective of pretreatment, and the adjusted pH is approximately pH 8 to a5, as is the case with petroleum fuels.

t6) If a large amount of supernatant water containing MP or M2 and Ca is injected into the boiler furnace for a long time, the furnace wall/tube surface (
'Pale magnesium, calcified metals (e.g.
MoriO, MfSO4, Cab, CaSO4) are attached and the color gradually becomes white. Therefore, the side radiation absorption rate of the furnace wall tube decreases, and as a result, the combustion gas flows to the rear of the boiler in a state higher than the designed value. As a result, the heat absorption in the superheater, regenerator, economizer, etc. increases, causing the entire boiler to become unbalanced, making it impossible to operate in a stable condition.

Further, it causes serious problems such as a rise in combustion gas temperature, the generation of thermal Noyc, and a decrease in thermal efficiency. In this regard, even if iron compounds are added to the supernatant water, colored iron oxides (for example, F, 08sF,
e30. ) and coexists with magnesium and calcium compounds to prevent their whitening development, so new intermediaries do not occur.

The whitening of the furnace wall tubes can be prevented by adding ferrite not only to the supernatant water but also to the fuel.

Furthermore, if an Fe compound is added, it will also adhere to the surface of the denitrification catalyst installed in the rear flue of the boiler. The denitrification catalyst has an oxidation catalytic effect, and removes alkali metal compounds such as Na and K (Na2SO2,
It is known that the function of FetO is lost by adhesion of Fe (K4SO4), but it has oxidizing power by implanting Fe compounds.

adheres to the surface of the catalyst, promoting catalyst function and extending its life.

The denitration catalyst is operated by injecting NH and gas into its upstream side, but due to the oxidizing action of the catalyst, SO! A side reaction occurs in which part of the oxidized to SO3.

It is known that So combines moisture in the gas to become H, So, which in turn reacts with NH3 gas to produce ammonium bisulfate (NH4H8O,). This compound has a low melting point (147°C) and is corrosive, so it may adhere to the air preheater element installed on the downstream side and increase ventilation resistance or cause corrosion. In this respect, iron oxide also reacts with ammonium bisulfate, converting it into ammonium sulfate C (NpJzs"a)K, which is non-corrosive and has a high melting point.
It is also effective in preventing failures caused by 4HS 04K.

Examples of iron compounds 25 that can be used in the present invention include the following, but other compounds may also be used as long as they eventually become colored oxides such as Fed○3+ Fe3O4 in the boiler furnace. It is possible.

(1) Iron oxide: Fed, Fe, On, Fe0O
H, FplOl (2) Inorganic iron compounds: iron sulfate, iron chloride,
Iron nitrate (3) Organic iron compounds niokuchi, iron 9, iron naphthenate, iron complex iron stearate, iron formate, iron methacrylate, and as a surface treatment agent for iron compound 25 used in the present invention, hydrophilic or lipophilic surfactants, examples of which are as follows.

Lipophilic surfactant; nonylphenyl (E-O)xcat
~, 4 Secondary alcohol (E-0) engineering X is 6 or less. (E.0) is an abbreviation for ethylene oxide.

Hydrophilic surfactant;
o [Example] Example 1 (Example of application to a boiler burning C heavy oil) Release! Output 4110MW, S content in fuel 2.6%, ■70
The process of the present invention (the process shown in FIG. 1) was applied to a large boiler operating by burning ~75 ppm C heavy oil. As a result, in the conventional method (Fig. 2), the amounts of sulfur d and caustic soda injected into the boiler furnace from the recycled waste liquid storage tank 10 reached 970 yu and 1,200 kg/da'7', respectively, but in the present invention Since the method does not use any ion exchange resin, no waste liquid containing this type of corrosive chemical is generated. (The corrosive components H, So4, and NaOH undergo a neutralization reaction in the recycled waste liquid tank 10 shown in Figure 2 to produce Na□SO4, but this compound is figure,
It is a causative agent that induces heat transfer surface contamination and corrosion. ) Next, an example in which the method of the present invention was applied over a long period of time (about 5 months) will be shown below.

In the supernatant water 12 of the present invention: Ca (as CaO) 600 to 1200 HI/l, M (as Mr○) 3
It contains 50-560M9/l, but NH4HC0
, 23, the supernatant water 13, Ca (
(as CaO) 200~350η/1°M? (M?O
) 1,000 to 2. s o o yq / t
Therefore, when injected into the boiler furnace, the wastewater contained a large amount of M20 components. After continuing to inject the upper δ water 13 into the boiler furnace, the boiler operation was stopped, and the amount of deposits on the surface of the heat exchanger tubes was investigated. As a result, the amount of deposits was approximately 65 times that of the conventional method. The MPO content in it had increased 2-5 times. The reduction in the amount of deposits is thought to be due to the increase in the Mho component, which made the deposits mainly composed of fuel ash brittle and easily peeled off from the heat exchanger tubes when the soot blower was operated. Furthermore, no corrosion was observed in the heat exchanger tubes.

However, when this method is applied to a boiler (supernatant water 13
(injected into the furnace), and from about 3 weeks later the N in the exhaust gas
0xff (increased, increasing by approximately 20% after the second month of injection and by 26% after the third month.

Therefore, the iron compound 25, which is one of the present invention, was added to the supernatant water 13.
When injection into the furnace was started (the addition of iron compound 25 was carried out in the mixed storage tank 8), NOX it in the flue gas began to decrease after 5 days, and completely disappeared after 20 days. The situation has become. The iron compound 25 used was F+403 powder whose surface was treated with a hydrophilic surfactant, and its addition amount was 2% of the total amount of tV'i,)#O+Ca,0.

The boiler used in this example was equipped with a denitrification catalyst mainly composed of Tie and VIOs, but an iron compound 25
When using supernatant water 16 with added
There was no decline at all even after 8 months of continuous operation. iron compound 2
In the case where No. 5 was not added, better results were obtained than in the case where the catalytic action decreased after about 6 months. In addition, the addition of iron compound 25 is Fe! whose surface is coated with carboxylic acid iron and a lipophilic surfactant! O, MrO+ powder in fuel
Even when 2 to 3% of the total amount of CaO was added, the same effect as above was obtained. Furthermore, when supernatant water 13 containing FezO1 was injected into the furnace, corrosion of the blockage element of the air preheater frequently occurred due to generation and adhesion of acidic ammonium sulfate when no injection was applied. After that, this problem did not occur.

Example 2 (Example in which petroleum coke is applied to a boiler during combustion) When petroleum coke (sulfur content: 1 to 57%, 140 to 280 ppm) is used as fuel in an industrial boiler with an evaporation capacity of 1120 tons and l/h. , when only U water 13 was injected into the reactor, the same phenomenon as in Example 1 (rise in NOx) occurred; however,
This could be prevented by injection of iron compound 25. In this case, the iron compound is injected into the supernatant water 13.
In addition, it was added to C heavy oil for combustion aid, petroleum coke powder, etc., but the effect did not change and the object of the present invention could be achieved.

Example! + (Example in which the present invention is applied to a boiler that burns coal) An example will be described in which the present invention is applied to a boiler that burns coal and has a power generation output of 500 MW and a composition as shown in Table 1. As is clear from the coal composition in Table 1, the ash contains a large amount of CaO, so the amount of CaO contained in the upper water is
There is no need to remove components. Therefore, in this supernatant water
The FA compound was added and injected into the boiler furnace. There are a lot of coal ash spears, most of which are Att03.5i02.
Cab.

Since it is a white component such as M2O, the amount of iron compound added was about 5 times the total amount of MrO-)-CaO contained in the commercially available water. In addition, injection of iron compounds
It was added not only to the supernatant water but also to the coal; in the latter case, taking into account that the coal is lipophilic, Fe, O, and powder coated with a lipophilic surfactant were used. The results are shown in Table 2.

The effects are shown in Table 2. In this example, the measured values were set to 100 when commercially available water was injected into the boiler as it was (containing Ca and My) and when no lipophilic surfactant was added to the coal powder, and an Fe compound was added. The measured values in each case are expressed as a ratio. The iron compound added to the supernatant water was prepared by adding 5% Fe, O, and powder to iron lead acid. dissolved into it.

Due to this effect, the pH of the iron complex acid is close to neutral).

As is clear from Table 2, when Fe compounds were added to the supernatant water of poop, the combustibility of coal was improved, resulting in less generation of unburned matter and lower amounts of NOx and So. Furthermore, the performance of the denitrification catalyst installed in the furnace did not deteriorate, but was actually improved. It has been found that these effects increase as the amount of FA compound added increases, but the efficiency of the increase is gradual.

On the other hand, the same effects as those added to supernatant water were observed when added to coal powder, and it was confirmed that the effects of the present invention can also be applied to coal-fired boilers.

Table 1 Example 4 (Example of applying coke oven gas to a boiler during combustion) Evaporation meter 120t/h, Conea 3, C mountain 30%, H2
The main component is S compound (HtstSo,
The present invention was applied to a boiler operating using coke oven gas containing 2300 ppm of

There is almost no ash in the blast furnace gas, so there is no heat transfer, contamination or corrosion of f.
Injection of iP'ff caused the surface of the iP'ff to turn yellow, resulting in an increase in 'cTOX'rl in the exhaust gas and difficulty in stable operation due to the loss of heat absorption balance; Examples 1 to 3.
The same thing happened.

(7) However, by adding iron compound 25 to supernatant water 12 or 13, these 4- and other problems could be prevented.

[Brief Description of the Drawings] Figure 1 shows the flow of the wastewater treatment process in the flue gas desulfurization treatment Vζ according to the present invention, and Figure 2 shows the flow of the conventional lime-stone A method Qζ process. . Fig. 2 heavy (open moat) Fig. 1 (fire pit)

Claims (1)

[Claims] In a method for treating wastewater from a desulfurization process in which sulfur-containing fossil fuel combustion exhaust gas is treated by a lime-gypsum method, an iron compound is directly added to the supernatant water discharged from the gypsum separation process, or the supernatant water is A method for treating wastewater, which comprises adding ammonium bicarbonate to precipitate and separate calcium components, and then adding an iron compound and injecting the mixture into a boiler.