JPS58205590A - Prevention of scaling in cooling and dust removing column - Google Patents

Abstract

PURPOSE:To improve the effect of preventing scaling, by supplying hydrogen chloride, etc. until the concn. of Cl<-> ions attains >=4.6 times mole of the amt. exceeding the concn. of the Ca compd. to be introduced into a cooling and dust removing column at which the deposition of gypsum initiates. CONSTITUTION:A waste gas of combustion contg. at least dust and SOx is introduced into a cooling and dust removing column, where the waste gas is brought into contact with cooling and dust removing liquid so as to be humidified and cooled; at the same time, the dust in the waste gas of combustion is removed. Hydrogen chloride, water soluble chloride, etc. are supplied into the cooling and dust removing column until Cl<-> ions attain >=4.6 times mole of the increased amt. of the Ca compd. in the dust contg. the Ca compd. in excess as much as of the concn. at which deposition of gypsum initiates. The waste gas including SOx discharged from the cooling and dust removing column is introdued into an absorption column where the gas is brought into contact with absrobent liquid and the SOx is removed. The crystallization of the gypsum dihyfrate crystal in the cooling and dust removing liquid is prevented by the above-mentioned method.

Description

[Detailed Description of the Invention] Exhaust gas from burning coal or oil contains environmental pollutants such as dust and sulfur oxides, and wet flue gas desulfurization equipment has been widely put into practical use as a method for cleaning this. ing.

Particularly in the widely used wet lime method flue gas desulfurization equipment, one issue in its practical application is that the absorption tower handles calcium compounds with low solubility, so gypsum precipitates as a result of the absorption reaction, and some of this is scaled. It was important to prevent problems such as sticking to the surface of the device material and narrowing or even clogging the gas and liquid flow paths.

As a countermeasure to this problem, a method of using slurry containing seed crystal gypsum has been adopted, and its effectiveness has been recognized.On the other hand, in recent years, coal has been increasingly used due to the need for fuel conversion, and With the strengthening of measures to prevent secondary pollution caused by wastewater from cleaning equipment, the present inventors have come to realize that new scale countermeasures are required in wet lime method flue gas desulfurization equipment.

In this process, combustion exhaust gas containing at least dusty sulfur oxides, such as coal-fired exhaust gas, is introduced into a cooling and dust removal tower, and brought into contact with a cooling and dust removal solution to humidify and cool the exhaust gas, as well as remove dust from the exhaust gas. The substance is guided to an absorption tower where it is absorbed and removed, and calcium compounds are used as an absorbent.
In wet calcareous flue gas desulfurization equipment that collects sulfur oxides in flue gas, calcium compounds contained in the dust may precipitate as gypsum scale in the cooling removal tower; By adopting a method to reduce the amount of wastewater by reusing excess wastewater from the absorption tower as part of the water supply to the evaporation water bucket in the cooling and dust removal tower, the calcium compounds contained in the excess wastewater are concentrated and the scale is reduced. It may precipitate as follows.

In other words, the calcified metals produced by using the surplus waste water from the absorption tower as make-up water for the cooling and dust removal tower and by collecting the dust,
We have come to recognize the need for measures to prevent gypsum scale from solidifying in the cooling and dust removal tower and narrowing the flow paths for gas and liquid.

Conventionally, the methods used to prevent gypsum scale are (1) adding seed crystals, (2) chemically removing calcium ions by softening, and (3) reducing concentration by increasing drainage volume. However, when handling the seed crystal slurry in (1), it is necessary to take measures to prevent wear and blockage caused by the slurry, which is troublesome, and in method (2), the process is complicated. In addition, method (3) requires a large amount of drainage and requires countermeasures against secondary pollution, which impairs economic efficiency.The present invention provides a method for preventing gypsum scale that can solve all of these problems. This method is characterized in that hydrogen chloride or its equivalent is added to the cooling desalting solution of the cooling dessicating tower in proportion to the amount of calcium supplied to the cooling dessicating tower. Moreover, as a result of various studies on the chemical reaction equilibrium, we found that the amount of hydrogen chloride to be added is 4.6 times the molar amount or more compared to the concentration at which calcium precipitates as gypsum, and the scale prevention effect becomes significant. It is known that hydrogen chloride has a strong chemical effect and that gypsum dissolves well in hydrogen chloride water. Furthermore, in the cooling dust removal tower, some of the sulfur oxides in the exhaust gas are absorbed by the cooling dust removal solution, which is further oxidized by 02 in the exhaust gas to produce sulfuric acid, so that the gypsum can be dissolved even better. The reaction formula for these is as follows.

HO4+ CaSO4-2) (20 (solid) -> C
a” 10H8O4+ 01-+ 2H20-・1llH
2S04+CaSO4・2H20 (solid) −+ Oa”
+ 2H8O4-+ 2H20-...t21 However, OaO, MgO, and Na2O contained in the dust.

Alkaline components such as K2O, NH4OH, etc. dissolve in water and generate OH- ions, and the OH- generates H804- into SO42-', which in turn causes the precipitation of gypsum. ) In a cooling desaturation tower where the gypsum dissolution reaction in (2) and the gypsum precipitation reactions in (3) to (7) occur simultaneously, it is difficult to judge whether gypsum will precipitate or dissolve based on simple stoichiometric relationships. According to the research conducted by the present inventors, the conditions for adding hydrogen chloride to the cooling removal solution in proportion to the amount of calcium supplied to the cooling removal tower and preventing gypsum from precipitating are as follows: It was found that the correlation shown in Figure 1 was obtained.

That is, if 4.6 times the mole of hydrogen chloride is supplied to increase the amount of calcium supplied beyond the point at which the calcium compound starts to precipitate gypsum by itself, the gypsum saturation state is maintained exactly, and the increase in calcium supply exceeds the point where the calcium compound starts to precipitate gypsum by itself. If hydrogen chloride is supplied, the gypsum becomes unsaturated and gypsum scale can be prevented.On the other hand, if less than 46 times the mole of hydrogen chloride is supplied, the gypsum becomes supersaturated and gypsum precipitates, causing scale problems. Figure 1 shows that this induces

To explain in more detail, FIG. 1 shows a gas amount of 1.000. O
Dust concentration 275 in D Om3N/H (dry basis)
Exhaust gas containing 7 m3N and SOx concentration of 800 ppm f is brought into contact with a cooling desalinating solution in a cooling scavenging tower, while surplus wastewater containing calcium compounds from the absorption tower is supplied to the cooling scavenging tower. 100% saturation
This figure shows a correlation diagram of the results of determining the amount of hydrogen chloride added, where the temperature is 48° C. and the amount of H2SO4 produced is 77 mol/H.

When no hydrogen chloride is added, the molar supply rate of calcium compounds resulting from the dust and surplus water is 12.
5 mol/H indicates the start point of gypsum precipitation, and if 12.5 mol/H or more of calcium is supplied, 4.6 times the mol of hydrogen chloride relative to the increased amount of calcium supply. It is shown that the saturation of gypsum is maintained at 100% by supplying . In other words, the slope of the gypsum saturation line in the figure is 1
It is 74.6. As is clear from this figure, simply supplying hydrogen chloride in an amount equivalent to calcium is insufficient to completely prevent gypsum scaling, and gypsum dissolution and precipitation cannot be determined based on simple stoichiometric relationships. This shows that.

When the amount of hydrogen chloride supplied in FIG. 1 is zero, the amount of calcium supplied to saturate the gypsum is 1.2.5 mol/H. This varies depending on increases and decreases in dust concentration and changes in the amount of alkaline components other than calcium in the dust.
A result was obtained in which the slope of the gypsum saturation line satisfies the condition 46.

The results shown in Figure 1 indicate that the liquid extracted from the cooling dust removal tower is 1
It was determined from steady-state mass balance and equilibrium calculations when 0 m37H.

Furthermore, as a method for achieving a desired concentration of C1- ions in the cooled desalting solution, in addition to hydrogen chloride, water-soluble chlorides other than calcium chloride (e.g., NaCt, KCl) can be used.

A7Ct3. In an aqueous solution, hydrogen chloride dissociates into H+ ions and C
7- ions are generated, and each of these ions is effective in the present invention, so even if Cl- ions brought from other chlorides are supplied separately from H+4 ions, the same effect will be achieved in the cooling descaling solution. It is also possible to obtain this. However, calcium chloride supplies CZ- ions and at the same time Ca2
Since + ions are also supplied in an equivalent amount, the effects of the present invention could not be obtained because Cl - ions could not be adjusted in any equivalent ratio to Ca 2+ ions provided from a separately supplied calcium compound.

Using seawater as a source of chloride was also effective. If seawater is used, the Cl concentration will become too high due to evaporation concentration in the cooling tower, so tap water is used as evaporation make-up water, and the seawater is suitable for the amount of calcium compounds supplied to the cooling tower. I supplied just enough. Hydrogen chloride contains an equivalent amount of H+ as well as CZ--, whereas seawater contains less H+. However, since some of the sulfur oxides in the exhaust gas are absorbed by the cooling dust remover, H'' is naturally supplied from the sulfur oxides, so the same scale prevention effect as when hydrogen chloride is supplied cannot be obtained. Ta.

In other words, it is thought that H+ produced from CZ- contained in seawater and sulfur oxide contained in exhaust gas exerted the same effect as when all hydrogen chloride was supplied.

According to the method of the present invention, for example, when dust containing calcium compounds is collected together with a part of sulfur oxides in a cooling tower and gypsum is precipitated, or when excess wastewater containing calcium compounds from an absorption tower is cooled. When gypsum precipitation occurs due to concentration when used as make-up water for a dust removal tower, gypsum precipitation can be prevented by adding at least one selected from hydrogen chloride, water-soluble chlorides other than calcium chloride, and seawater. 〈〈, 1 for each increase in calcium supply
By adding hydrogen chloride or its equivalent to 4.6 times the mole or more of atomic ions, gypsum precipitation can be completely prevented, eliminating the need for troublesome seed crystal adjustment and softening as in conventional methods. , slurry handling.

ID-John, blockage, increase in sludge waste.

In addition to eliminating the complexity of the process, it also provides excellent scale prevention effects.

Example 1 In a wet limestone injection flue gas desulfurization pilot plant,
Coal combustion ash 275 mf/m3N% Sulfur oxides 8
Coal-fired exhaust gas containing 00 ppm f 2000m3N/
It was introduced into a cooling dust removal tower that sprays a cooling dust removal solution at a H'i liquid-gas ratio of 21/m3N. Gypsum separation supernatant liquid F40 is used to replenish water that evaporates as exhaust gas is humidified and cooled.
While supplying at a rate of t/h, a portion of the cooling desalting solution was continuously drawn out of the system at a rate of 20 t/h, maintaining steady state operation.

The gypsum separation supernatant liquid is a liquid in which gypsum is dissolved in a saturated state in a trench, and as it is concentrated by water evaporation in a cooling tower, gypsum crystals are precipitated. Dissolved calcium at this time is 125
It was mmol/l.

Next, when the calcium compounds contained in the dust were determined by analysis, Ca = Q, 54 mmol/g, and the calcium in the gypsum separation supernatant was 18 mmol/l, so the supply of 11, Total calculation: 1015
mmol/I ('e obtained, by subtracting 250 mmol/Hk of the amount of dissolved gypsum extracted from the saturated dissolved concentration of gypsum without adding hydrogen chloride and the flow rate extracted from the system, an increase in calcium supply of 765 mmol/H is obtained. 5519 mmol/corresponding to 4.6 times this value by mole
When hydrogen chloride (H) was added and normal operation was maintained, the gypsum crystals in the cooled desalting solution disappeared, and operation without scale problems was achieved.

Since hydrogen chloride may be contained in exhaust gas depending on the type of fuel, in such a case, the amount of hydrogen chloride added can be reduced by that amount.

In addition, the method of adding hydrogen chloride is to adjust the CZ- ions to the desired concentration in the cooling descaling solution. In addition to the method of adding hydrogen chloride to the gypsum separation supernatant when calcium compounds are not supplied together with the exhaust gas, hydrogen chloride is added to the gypsum separation supernatant while adjusting the concentration of about 4 pT of the calcium compound concentration contained in the gypsum separation supernatant. .The method of replenishing the hydrogen chloride to a 6-fold molar concentration and then replenishing it to the cooling and desaturation tower was convenient and easy to adjust.

[Brief explanation of drawings]

FIG. 1 shows the saturated dissolution line of dihydrate gypsum crystals in the cooling destaining solution, which is the key to the structure of the present invention, determined from the correlation between the amount of hydrogen chloride supplied and the amount of calcium compound supplied. Sub-agents 1) Meifuku agent Ryo Hagiwara -

Claims (1)

[Claims] Combustion exhaust gas containing at least a small amount of dust and sulfur oxides is brought into contact with a cooling dust removal liquid to humidify and cool the exhaust gas and remove dust from the combustion exhaust gas, and then discharged from the cooling dust removal tower. In a wet exhaust gas treatment device consisting of an absorption tower that removes sulfur oxides by bringing the flue gas containing sulfur oxides into contact with an absorption liquid, gypsum precipitation of calcium compounds brought to the nuclear cooling removal tower begins. At least one selected from hydrogen chloride, water-soluble chlorides other than calcium chloride, and seawater is supplied so that the CZ-ion concentration is 4.6 times the mole or more with respect to the increased amount exceeding the concentration, and the cooling removal is performed. A method for preventing scale in a cooling and dust removal tower, characterized by preventing crystallization of dihydrate gypsum crystals in a solution.