AU674822B1

AU674822B1 - Exhaust gas desulfurization process

Info

Publication number: AU674822B1
Application number: AU44448/96A
Authority: AU
Inventors: Kenichi Nakagawa
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1995-08-22
Filing date: 1996-02-12
Publication date: 1997-01-09
Anticipated expiration: 2016-02-12
Also published as: PL183184B1; KR970009863A; CN1143535A; CZ56196A3; KR0174794B1; JPH0957053A; MY112500A; CN1089022C; PL313006A1; TW326404B; JP3751340B2

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: *5

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Name of Applicant: Toyo Engineering Corporation Actual Inventor(s): Kenichi Nakagawa Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: EXHAUST GAS DESULFURIZATION PROCESS Our Ref: 441100 POF Code: 1566/52259 The following statement is a full description of this invention, including best method of performing it known to applicant(s): -el 1A-

SPECIFICATION

Title of the Invention EXHAUST GAS DESULFURIZATION PROCESS Background of the Invention 1. Field of the Invention This invention relates to an exhaust gas desulfurization process for treating various types of exhaust gas including exhaust gas containing sulfur oxides exhaust gas resulting from the combustion of oil or coal) and exhaust gas containing both sulfur oxides and hydrogen chloride.

2. Description of the Prior Art The lime-gypsum process is known as a typical wet desulfur. ',ation method for treating various types of exhaust gas. In this process, an absorbent comprising calcium carbonate or calcium hydroxide is directly added S. S to the desulfurization tower and, therefore, calcium ions are dissolved in the treating fluid. Accordingly, when "0 these calcium ions react with sulfur oxides or the like in the desulfurization tower, scale comprising precipitates of gypsum dihydrate, calcium sulfite dihydrate, calcium carbonate and the like is deposited in the desulfurization tower and piping. This makes it difficult to maintain smooth operation and requires much labor to remove such 2 scale. Moreover, calcium hydroxide is a desulfurizing agent which is inherently capable of absorbing two molecules of sulfur dioxide, but calcium sulfite that is a desulfurizing agent having absorbed one molecule of sulfur dioxide has significantly lower solubility than magnesium sulfite that is an analogous desulfurizing agent.

Accordingly, this process is also disadvantageous from an economic point of view, in that the treating fluid has low absorption efficiency for sulfur oxides and hence requires an increase in the size of equipment such as the desulfurization tower and circulating pumps.

On the other hand, the double alkali process is also known in which the absorption of sulfur oxides in the desulfurization tower is carried out by using a basic desulfurizing agent such as a basic sodium compound, eno :ammonia or a basic magnesium compound and the desulfurizing agent is regenerated by effecting double decomposition with the aid of quick lime outside the desulfurization tower. This double alkali process is less S" liable to the deposition of scale. In particular, the process using a basic magnesium compound as the desulfurizing agent is characterized by high absorption efficiency for sulfur oxides, high solubility of the resulting magnesium sulfite, and less scale deposition in the absorption tower. However, this process using a basic desulfurizing agent has the problem that two types of 3 crystals gypsum dihydrate and magnesium hydroxide) are precipitated in the double decomposition step and difficulties in separating them necessitates the use of intricate equipment.

In addition, the Kawasaki magnesium-gypsum process is known as a compromise between the lime-gypsum process and the double alkali process [A Collection of Environmental Pollution Control Techniques for Practical Use Kagaku Kogyosha, p. 14), According to this process, sulfur oxides are absorbed in the desulfurization step by using a mixed slurry of magnesium hydroxide and calcium hydroxide as the desulfurizing agent. Then, while being adjusted to pH 2.0-4.0 with sulfuric acid, this treating fluid is oxidized by air or the like to form magnesium sulfate and gypsum dihydrate. Subsequently, the treating fluid is subjected to a settling separation step and a centrifugal separator and thereby separated into gypsum dihydrate and an aqueous magnesium sulfate solution. The separated aqueous magnesium sulfate solution is recycled to a desulfurizing agent regeneration step to which a mixed slurry of magnesium hydroxide and calcium hydroxide is added. Thus, the magnesium sulfate undergoes a double decomposition reaction with some of the calcium hydroxide present in the mixed slurry to form magnesium hydroxide and gypsum dihydrate. The fluid mixture containing these compounds and the remaining calcium hydroxide is recycled

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to the absorption step and used as the desulfurizing agent. However, this process is similar to the lime-gypsum process in that calcium hydroxide and gypsum dihydrate are introduced into the desulfurization tower. Accordingly, the problem of liability to scale deposition in the desulfurization tower, circulating pumps and piping has not been solved.

Summary of the Invention It would be desirable to provide an exhaust gas desulfurization process which permits the treating fluid to exhibit high absorption efficiency for sulfur oxides and can hence be carried out in simple and small-sized equipment.

It would further be desirabe to provide an exhaust gas desulfurization process which can minimise the calcium ion concentration in the treating fluid present in the desulfurization tower, thus making it to prevent scale deposition and blockage in the desulfurization tower, circulating pumps and piping and to maintain smooth operation.

It would still further be desirable to provide an exhaust gas desulfurization process which can accomplish the above objects even when the exhaust gas contains not only sulfur oxides but also hydrogen chloride.

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As a result of intensive investigations on the simplification of a process using a magnesium-based desulfurizing agent according to the double alkali technique, the present inventor has discovered that, in contrast to the prior art in which the gypsum dihydrate and magnesium hydroxide formed in the double decomposition step are separated and only the magnesium hydroxide is returned to the desulfurization tower, the process can be carried out without separating the gypsum dihydrate and magnesium hydroxide, if only the calcium ions dissolved in the treating fluid is not brought into the desulfurization tower. The present invention has been completed on the basis of this discovery.

Thro'sghout the description and claims of this specification, the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives or components or integers or steps.

Specifically, according to one aspect of the present invention, there is provided an exhaust gas desulfuru.ation process which comprises the desulfurization step of bringing exhaust gas containing sulfur oxides into contact with a treating fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxides contained in the exhaust gas to be absorbed into the treating fluid, the oxidation step of bringing the desulfurization step treating fluid into contact with an oxygen-containing gas and thereby converting the magnesium salts present in the treating fluid into magnesium sulfate, and the double decomposition step of adding a basic calcium compound to the oxidation *oo o i* oe *e *e 6 step treating fluid and thereby decomposing the magnesium sulfate present in the treating fluid to magnesium hydroxide and gypsum dihydrate, and in which the mixed slurry obtained in the double decomposition step is returned to the desulfurization step and the gypsum dihydrate present in the treating fluid is removed from the system, ch r c e i e n t a prior to returning the mixed slurry obtained in the double decomposition step to the desulfurization step, a portion of the desulfurization step treating fluid is added to the mixed slurry so as to convert the calcium ions present therein into calcium sulfite.

.According to another aspect of the present invention, there is provided an exhaust gas desulfurization process which comprises the desulfurization step of bringing fee: exhaust gas containing sulfur oxides and hydrogen chloride .00. into contact with a treating fluid containing a too* magnesium-based desulfurizing agent and thereby causing eo the sulfur oxides and hydrogen chloride contained in the exhaust gas to be absorbed into the treating fluid, the oxidation step of bringing the desulfurization step ".itreating fluid into contact with an oxygen-containing gas and thereby converting the magnesium salts present in the treating fluid into magnesium sulfate, and the double decomposition step of adding a basic calcium compound to the oxidation step treating fluid and thereby decomposing 7 the magnesium sulfate present in the treating fluid to magnesium hydroxide and gypsum dihydrate, and in which the mixed slurry obtained in the double decompositvo, step is returned to the desulfurization step, the gypsum dihydrate present in the treating fluid is removed from the system, and the magnesium chloride accumulated in the treating z> eve fluid is discharged from the system, haractgrized in- -h-lk prior to returning the mixed slurry obtained in the double decomposition step to the desulfurization step, a portion of the oxidation step treating fluid is added to the mixed slurry so as to convert the calcium ions present therein into gypsum dihydrate, and a portion of the desulfurization step treating fluid is then added to the mixed slurry so as to convert the calcium ions present therein into calcium sulfite.

According to still another aspect of the present invention, there is provided an exhaust gas desulfurization process which comprises the desulfurization step of bringing exhaust gas containing sulfur oxides and hydrogen chloride into contact with a treating fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxides and hydrogen chloride contained in the exhaust gas to be absorbed into the treating fluid, the oxidation step of bringing the desulfurization step treating fluid into contact with an oxygen-containing gas and thereby converting the magnesium 8 salts present in the treating fluid into magnesium sulfate, and the double decomposition step of adding a basic calcium compound to the oxidation step treating fluid and thereby decomposing the magnesium sulfate present in the treating fluid to magnesium hydroxide and gypsum dihydrate, and in which the mixed slurry obtained in the double decomposition step is returned to the desulfurization step, the gypsum dihydrate present in the treating fluid is removed from the system, and the magnesium chloride accumulated in the treating fluid is LWkeYQ-4'-i' discharged from the system, -hao-i rioej ie in that, prior to returning the mixed slurry obtained in the double decomposition step to the desulfurization step, a portion of the desulfurization step treating fluid is added to the mixed slurry so as to convert the calcium ions present therein into calcium sulfite, and ba-tkaL the chloride ion concentration in the mixed slurry present in the double decomposition step is determined and the amount of basic calcium compound added in the double decomposition step is regulated on the basis of the chloride ion concentration so determined.

The term "magnesium-based desulfurizing agent" as se** used herein comprehends basic magnesium compounds consisting essentially of magnesium oxide or magnesium hydroxide, such as magnesium hydroxide produced by using seawater magnesium as the raw material, magnesium oxide A W \tFo 9 obtained by burning magnesite ore, and magresium hydroxide obtained by slaking this magnesium oxide.

Brief Description of the Drawings Fig. 1 is a graph showing the relationship between the concentration of magnesium sulfate dissolved in a solution and the solubility of calcium sulfate therein; Fig. 2 is a graph showing the relationship between the amount of basic calcium compound added and pH in the double decomposition step of the desulfurization process of the present invention; Fig. 3 is a schematic view illustrating the construction of an exemplary system used to carry out the desulfurization process of the present invention; Fig. 4 is a schematic view illustrating the construction of another exemplary system used to carry out the desulfurization process of the present invention; Fig. 5 is a schematic view illustrating the construction of still another exemplary system used to 00 S• carry out the desulfurization process of the present invention, and Fig. 6 is a schematic view illustrating the method for controlling the amount of basic calcium compound added in the double decomposition step of the desulfurization process of the present invention.

10 Detailed Description of the Preferred Embodiments In the exhaust gas desulfurization process of the present invention, exhaust gas containing sulfur oxides, or both sulfur oxides and hydrogen chloride, is brought into contact with a treating fluid comprising an aqueous solution containing a magnesium-based desulfurizing agent in the desulfurization step, so that the sulfur oxides and hydrogen chloride are absorbed into the treating fluid.

Since the treating fluid returned from the double decomposition step to the desulfurization step is a mixed slurry containing magnesium hydroxide, gypsum dihydrate and a slight amount of calcium sulfite, the treating fluid in the desulfurization step becomes a suspension tontaining coarse particles of gypsum dihydrate and a slight amount of calcium sulfite. Since the magnesium hydroxide in the treating fluid is consumed as a desulfurizing and hydrogen chloride-removing agent, the fine particles of magnesium hydroxide disappear in the desulfurization step.

An apparatus suitable for use in the desulfurization step comprises a tower having a structure which permits S* efficient contact between such gas and liquid. This tower may be of the type in which an aqueous solution containing a desulfurizing agent is sprayed from a nozzle while a gas is passed therethrough in the counter flow or parallel flow mode. Since the treating fluid contains coarse 11 particles of gypsum dihydrate, the nozzle must be designed so that it will not be clogged therewith. In order to improve the efficiency of the gas-liquid contact, the tower may be provided with packing, plates or other means.

The treating fluid leaving the desulfurization step (hereinafter referred to briefly as "the desulfurization step treating fluid") is an aqueous solution containing a mixture of magnesium sulfite, magnesium hydrogen sulfite, magnesium sulfate and magnesium chloride which are formed by the reaction of the aqueous solution of the magnesiumbased desulfurizing agent with sulfur oxides, and also containing gypsum dihydrate and a slight amount of calcium sulfite as suspended matter.

In the desulfurization step, the temperature of the treating fluid is usually in the range of 50 to 60'C. The pH of the desulfurization step treating fluid is preferably in the range of 5.0 to 7.5 and more preferably in the range of 5.5 to 6.5. In the desulfurization step, it is necessary to take a proper measure for preventing the precipitation of magnesium sulfite having low

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solubility in water. Generally, this can be accomplished by blowing air or the like into the treating fluid to oxidize magnesium sulfite to magnesium sulfate having higher solubility in water and thus control the magnesium sulfite concentration below its solubility. As another method for controlling the magnesium sulfite concentration 12 in the desulfurization step below its solubility without blowing air or the like into the treating fluid, it is known to recycle a portion of the treating fluid leaving the oxidation step described hereinbelow to the desulfurization step and thereby reduce the magnesium sulfite concentration in the desulfurization step.

In the process of the present invention, the gypsum dihydrate contained in the treating fluid is removed from the system by subjecting at least one of the desulfurization step treating fluid and the oxidation step treating fluid, which will be described later, to solidliquid separation. These treating fluids contain little solid matter other than gypsum dihydrate and a slight amount of calcium sulfite, so that the gypsum dihydrate can be separated easily. With the removal of gy-sum dihydrate, calcium sulfite is also removed from the system.

Alternatively, the separation and removal of gypsum .dihydrate from the desulfurization step treating fluid may be carried out by withdrawing the desulfurization step treating fluid from the desulfurization tower by other means, subjecting it to solid-liquid separation, and then returning the residual fluid to the desulfurization tower, or by subjecting the desulfurization step treating fluid to solid-liquid separation before feeding it to the succeeding oxidation step. For the separation and removal 13 of gypsum dihydrate, there can be used a wet classifier such as a wet cyclone, centrifugal settler or Dorr thickener. Among others, a wet cyclone is preferred. The separated gypsum dihydrate is withdrawn from the system and can be widely used for the manufacture of cement, gypsum boards and the like.

The desulfurization step treating fluid is then fed to an oxidation step. In the oxidation step, the treating fluid is brought into contact with an oxygen-containing gas, so that the magnesium sulfite and magnesium hydrogen sulfite present in the treating fluid are oxidized to form magnesium sulfate and sulfuric acid. Usually, the concentration of magnesium sulfate in the treating fluid is in the range of 3 to 10% by weight and the pH thereof is in the range of 2 to 3. In the oxidation step, a tank reactor is usually used and the treating fluid may or may not be stirred.

In the oxygen-containing gas fed to the oxidation step, the types of gaseous components other than oxygen are not critical, so long as they are inert to the desulfurization step treating fluid. Usually, air is used as the oxygen-containing gas.

Again, the treating fluid leaving the oxidation step *(hereinafter referred to briefly as "the oxidation step treating fluid") contains no solid matter other than gypsum dihydrate. Accordingly, the removal of the gypsum 14 dihydrate present in the treating fluid from the system can also be carried out with respect to the oxidation step treating fluid.

The oxidation step treating fluid is then fed to a double decomposition step. In this double decomposition step, a basic calcium compound is added to the treating fluid containing the magnesium sulfate and sulfuric acid formed in the oxidation step as principal components and also containing magnesium chloride in certain cases, and the resulting mixture is stirred. Thus, the sulfuric acid reacts with the basic calcium compound to form gypsum dihydrate, and the magnesium sulfate reacts with the basic calcium compound to form gypsum dihydrate and magnesium hydroxide. Subsequently to the formation of gypsum dihydrate, the basic calcium compound added in excess is consumed to form magnesium hydroxide and calcium chloride by reaction with the magnesium chloride. However, since calcium chloride has high solubility, it usually produces no precipitate.

Usually, a tank reactor is used in the double decomposition step. Although higher reaction temperatures .00. are preferred, it is advisable from an operational point of view to employ a reaction temperature similar to that employed in the desulfurization step. The residence time should preferably be in the range of 4 to 5 hours or more.

Thus, the resulting gypsum dihydrate grows to coarse 15 particles generally having an average diameter (major diameter) of 70 Am or greater and usually up to 200 Am.

On the other hand, magnesium hydroxide is formed as fine particles having a size of 1 Am or less and usually about 0.3 to 1 gm, and these particles aggregate together to an apparent size of about 10 to 20 im.

As the basic calcium compound used in the double decomposition step, calcium hydroxide, calcium oxide or a mixture thereof is preferred. Although this basic calcium compound may be fed to the reaction tank in the form of powder, an aqueous slurry thereof is most suitable from the viewpoint of workability In order to increase the particle diameter of gypsum dihydrate, the feed rate of the basic calcium compound is preferably regulated so that the fluid mixture present in the double decomposition step will have a pH of about 11i. However, where the treating oeoe fluid contains magnesium chloride, the feed rate of the basic calcium compound should be regulated as will be described later.

In the process of the present invention, the mixed slurry of gypsum dihydrate and magnesium hydroxide obtained in the double decomposition step is returned to be..

the desulfurization step without separating the two solid components. However, it is necessary to minimize the amount of calcium ions dissolved in the mixed slurry returned to the desulfurization step. The reason for this 16 is that the solubility of gypsum dihydrate (as calcium sulfate) varies according to the concentration of coexisting magnesium sulfate as shown in Fig. 1. In the double decomposition step, magnesium sulfate is converted into gypsum dihydrate and magnesium hydroxide by reaction with the basic calcium compound, so that the concentration of magnesium sulfate is approximately zero. Consequently, the solubility of calcium sulfate therein is relatively high, about 2,000 to 2,200 ppm. However, since the double decomposition step is a stage in which gypsum dihydrate is precipitating, calcium sulfate is dissolved at a concentration higher than its solubility. Thus, its degree of supersaturation is considered to be Cbout 1.4 to 1.8, and the actual concentration of calcium in the mixed slurry is estimated to be 2,800 to 3,960 ppm.

In contrast, the concentration of magnesium sulfate in the desulfurization step is in the range of about 3 to 10% by weight, the solubility of calcium sulfate therein is about 1,500 ppm. Accordingly, if the mixed slurry obtained in the double decomposition step is directly fed to the desulfurization step, the degree of supersaturation of

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calcium sulfate will have a considerably high value of 1.87 to 2.64. This will cause gypsum dihydrate to form S" and grow in the desulfurization step and deposit in the C 0 form of scale.

Consequently, in order to decrease the amount of 17 calcium ions dissolved in the mixed slurry, the process of the present invention includes the calcium ion conversion step of adding a portion of the desulfurization step treating fluid to the mixed slurry and fixing the calcium ions present in the mixed slurry by converting them into solid calcium sulfite.

As described above, 0.3 to 0.4% by weight of calcium sulfate is dissolved in the mixed slurry and, therefore, about 0.1% by weight of calcium ions are dissolved therein. When the desulfurization step treating fluid containing magnesium sulfite, magnesium hydrogen sulfite and magnesium hydroxide is added to the mixed slurry and the resulting mixture is stirred, the calcium ions undergo the reactions represented by the equations to (3) given below. Thus, the calcium ions are fixed by the formation of insoluble calcium sulfite (solubility: 0.0051 **e g/100 g aqueous solution), so that the calcium ion concentration in the slurry is markedly reduced. At a pH of less than 6, however, magnesium hydrogen sulfite will react with not only the dissolved calcium ions but also the coexisting magnesium hydroxide. Accordingly, the pH should preferably be 6 or greater and more preferably in the range of 6 to 11. The reaction temperature is Spreferably 80"C or below and more preferably 60'C or below.

Ca MgSO 3 2H 2 0 CaSO 3 -2H 2 0 Mg" (1) 18 Ca++ Mg(HS0 3 2 Mg(OH) 2 CaSO 3 *2H 2 0 Mg" (2) Ca MgSO 4 2H20 CaSO 4 -2H 2 0 Mg (3) Of the above reactions taking place in the calcium ion conversion step, the reactions and have considerably high reaction rates, so that the residence time of the mixed slurry in this step may be as short as minutes or so. Accordingly, this step can be carried out by using a small-sized tank reactor. That is, the significance of the process of the present invention lies in the fact that the above reactions to can be effected not in the closed desulfurization tower in the desulfurization step but in a small-sized tank reactor.

If scale is deposited on the inner wall of this reactor, it is advisable to use two reactors alternately.

Where the exhaust gas to be desulfurized contains principally sulfur oxides, as in the case of exhaust gas resulting from the combustion of fuel oil or the like, the process of the present invention is carried out in the above-described manner. However, where the exhaust gas contains not only sulfur oxides but also hydrogen chloride, as in the case of exhaust gas resulting from the combustion of coal or the like, it is necessary to prevent the mixed slurry fed from the double decomposition step to the calcium ion conversion step and further to the desulfurization step from containing calcium ions in the form of highly soluble calcium chloride.

i 19 Specifically, a basic calciuml compound is added to the treating fluid in the double decomposition step.

Where the treating fluid contains a chloride, the reactions taking place in the double decomposition step are represented by the equations and given below.

That is, the addition of a basic calcium compound first brings about the formation of gypsum dihydrate and magnesium hydroxide. Subsequently to the formation of gypsum dihydrate, the basic calcium compound added in excess is consumed to form magnesium hydroxide and calcium chloride by reaction with magnesium chloride.

MgSO 4 Ca(OH) 2 2H20 CaS0 4 2H 2 0 Mg(OH) 2 (4) MgC12 Ca(OH) 2 CaCl2 Mg(OH) 2 Fig. 2 is a graph showing the relationship between the amount of basic calcium compound added and pH in the 8 *double decomposition step. This indicates that the change in pH is slight in the vicinity of the end point of the reaction of equation Accordingly, in this double decomposition step, it is very difficult to stop the addition of the basic calcium compound upon completion of the reaction of equation in response to a change in a pH. 'If the basic calcium compound is added in excess and, therefore, the reaction of equation proceeds, the resulting calcium chloride has high solubility and hence dissolves in the mixed slurry. As a result, the mixed slurry fed to the calcium ion conversion step contains a

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20 relatively large amount of calcium ions, so that an unduly high treatment load is imposed on the calcium ion conversion step.

One method for preventing the mixed slurry leaving the double decomposition step from containing calcium ions from calcium chloride is to add a portion of the oxidation step treating fluid to the mixed slurry obtained in the double decomposition step so as to convert the calcium ions present in the mixed slurry into gypsum dihydrate and thereby reduce the calcium ion concentration in the mixed slurry to the solubility level of gypsum dihydrate, and then feed the mixed slurry to the calcium ion conversion step.

As described above, it is difficult to confine the reactions taking place in the double decomposition step to the reaction of the above equation and, therefore, an adequate amount of a basic calcium compound is added to promote the growth of crystalline particles of gypsum dihydrate. Accordingly, the reaction of equation proceeds to some degree in the double decomposition step of this process. Then, a portion of the oxidation step .*se treating fluid is added to the mixed slurry in which some calcium chloride is dissolved, so that the calcium ions 'are reacted with the sulfate ions present in the oxidation step treating fluid to precipitate them in the form of gypsum dihydrate. That is, the oxidation step treating 21 fluid is added so that the basic calcium compound added in excess will undergo the reaction represented by the equation given below. Similarly to the double decomposition step, the step of converting the calcium chloride dissolved in the mixed slurry into magnesium chloride (hereinafter referred to as "the calcium chloride conversion step") is preferably carried out by using a tank reactor.

CaCl2 MgS0 4 2H 2 0 MgCl 2 CaSO 4 -2H 2 O (6) Since the relationship between the amount of oxidation step treating fluid added and pH in the calcium chloride conversion step traces the graph of Fig. 2 in the reverse direction (because an increase in the amount added causes the value of the abscissa to change from the right to the left), the end point of the reaction can readily be detected by pH measurement and, therefore, the feed rate of the oxidation step treating fluid can be properly regulated with ease. This calcium chloride conversion step causes all of the calcium chloride formed in the double decomposition step and dissolved in the mixed slurry to be converted into magnesium chloride. Thus, the calcium ions are crystallized as gypsum dihydrate, so that the calcium ion concentration in the mixed slurry is reduced to the solubility level of gypsum dthydrate.

Another preventive method is to stop the addition of the basic calcium compound upon completion of the reaction 22 of the above equation so that the mixed slurry leaving the double decomposition step will contain no calcium ion.

Specifically, the chloride ion concentration in the mi.ed slurry present in the double decomposition step is measured first of all. Although various methods may be employed to measure the chloride ion concentration, the method in which a portion of the mixed slurry is introduced into a measuring tank and its chloride ion concentration is determined on the basis of specific gravity measurement is described herein. More specifically, a portion of the mixed slurry is introduced into a measuring tank and an aqueous solution of a basic calcium compound is added thereto until the reaction of equation is completed the pH of the slurry reaches a value of 10-11). At this stage, it is practically calcium chloride alone that is dissolved in the liquid fraction of the mixed slurry. Accordingly, the Go*" concentration of calcium chloride in the mixed slurry present in the measuring tank can be determined by measuring the specific gravity of the liquid fraction of the mixed slurry and comparing it with the concentration- @0 specific gravity curve for aqueous calcium chloride solutions. Since there is no r-ir' of chloride ions between the double decomposition tank and the measuring tank, the chloride ion concentration in the mixed slurry present in the double decomposition tank can be determined 23 by correcting the calcium chloride 'concentration in the measuring tank for dilution by the addition of the aqueous solution of the basic calcium compound to the measuring tank, On the basis of the chloride ion concentration so determined, the amount of basic calcium compound added in the double decomposition step is regulated so as to be just enough to complete the reaction of equation This regulation of the amount added may be carried out, for example, in the following m~inner: If the basic calcium compound is added to this! double decomposition tank in an ideal amount which is just nough to complete the reaction of equation but insufficient to initiate the reaction of equation it will practically be magnegium chloride alone that is dissolved in the ±iquid fraction of p. the mixed slurry present in the doubl.e decomposition tank.

Accordingly, the magnesium chloride concentration in the double d'ocomposition tank at the time of completion of the reaction of equation can be calculated on the basis of e.,the previously determined chloride ion concentration.

:Then, the specific gravity of the aqueous solution fraction of the slurry present in the double decompoo'3tion tank is measured and the amount of basic calcium compound added to the double decomposition tank is regulated so that the measured specilic gravity will become equal to the specif$c gravity of an aqueous magnesium chloride 24 solution having the above-calculated concentration. In this manner, the reactions taking place in the double decomposition step can be confined to the reaction of equation so that the calcium ion concentration in the mixed slurry is reduced to the solubility level of gypsum dihydrate.

According to the process of the present invention, the chloride ions absorbed into the treating fluid in the desulfurization step are fixed in solid form and not discharged, so that they are circulated through the system together with the treating fluid. In order to prevent the accumulation of chloride ions above a predetermined concentration, it is common practice to suitably discharge the chloride ions from the system in the form of blow water comprising an aqueous magnesium chloride solution.

Preferably, the discharge of blow water from the system is oe* carried out with respect to the liquid fraction of the mixed slurry returned to the desulfurization step because this mixed slurry gives the lowest magnesium ion concentration in the treating fluid.

Various modifications may be made in the process of the present invention. For example, a portion of the oxidation step treating fluid may be returned to the desulfurization step (desulfurization tower). If the S oxidation step treating fluid is added to the St desulfurization tower, the proportion of magnesium sulfate 25 in the treating fluid present in the desulfurization tower will increase and the proportion of sulfurous acid salts will decrease, so that the precipitation of magnesium sulfite can be minimized.

In the following examples, the exhaust gas desulfurization process of the present invention is more specifically explained with reference to the accompanying drawings. However, these examples are not to be construed to limit the scope of the present invention.

Example 1 This example is concerned with the desulfurization treatment of exhaust gas (containing no hydrogen chloride) from an oil-fired boiler. An outline of this process is illustrated in Fig. 3.

A treating fluid having a magnesium-based desulfurizing agent dissolved therein and containing *too coarse particles of gypsum dihydrate as suspended matter too: was made to flow down from the upper part of a desulfurization tower 1 in the form of a shower, and S. brought into gas-liquid contact with sulfur oxidescontaining exhaust gas Gl introduced thereinto from below.

Thus, the sulfur oxides were absorbed into the treating fluid and fixed in the form of magnesium sulfite, magnesium hydrogen sulfite and the like, while the exhaust gas G2 freed of sulfur oxides was discharged from the top of the tower.

26 Since the exhaust gas fed to the desulfurization tower 1 hot, it was cooled by spraying process water from a nozzle. The exhaust gas had a flow rate of 105 Nm 3 /hr and a SO2 concentration of 2,000 ppm.

The treating fluid having absorbed sulfur oxides and fallen to the bottom of the desulfurization tower 1, together with a fresh treating fluid fed from a magnesium hydroxide slurry supply tank 7, was conveyed to the upper part of the tower by means of a pump P1 and a pipeline L1 and made to flow down. Thus, the treating fluid was continuously circulated through the desulfurization tower 1. In order to prevent the precipitation of magnesium sulfite, air was blown into the bottom of the tower.

Moreover, a portion of the treating fluid was withdrawn from the pipeline L1 at a rate of 30 t/hr and fed to a gypsum separator 2 where the gypsum dihydrate suspended in the treating fluid was separated. The separated gypsum dihydrate was discharged from the system at a rate of 1.6 t/hr, while the residual fluid was returned to the desulfurization tower 1. The salt concentration in the treating fluid present in the desulfurization tower 1 w-s S. 7.50% by weight as expressed in terms of magnesium sulfate, the combined concentration of magnesium sulfite and magnesium hydrogen sulfite therein was 1.50% by weight as expressed in terms of magnesium sulfate, and the pH thereof was 5.8-6.0. The SO 2 concentration in the exhaust 27 gas G2 was 100 ppm and the degree of desulfurization was The desulfurization step treating fluid was withdrawn from the desulfurization tower 1 by means of a pump P2 and a pipeline L2, and fed to an oxidation tank 3 at a rate of 11 t/hr. In this oxidation tank 3, the desulfurization step treating fluid was oxidized by exposure to air and thus converted into an aqueous solution of magnesium sulfate and sulfuric acid. This oxidation step treating fluid was fed to a double decomposition tank 4 by means of a pump P3 and a pipeline L3. To this double decomposition tank 4, an aqueous slurry containing 30% by weight of calcium hydroxide was added from a calcium hydroxide supply tank 5 through a pipeline L4 at a rate of 1.8 t/hr.

While the contents thereof were mixed by means of a 'stirrer, magnesium sulfate and sulfuric acid were reacted with calcium hydroxide to form solid particles of gypsum dihydrate and magnesium hydroxide. The reaction temperature was The resulting mixed slurry was then introduced into a ee* e calcium ion conversion tank 6 through a pipeline L5. At the same time, a portion of the desulfurization step treating fluid having absorbed sulfur oxides was withdrawn from the desulfurization tower 1 through the pipelines L1 a a.

and L6, and fed to the calcium ion conversion tank 6 at a rate of 1.3 t/hr. While the contents thereof were 28 intimately mixed by means of a stirrer, the calcium ions dissolved in water at the solubility level of gypsum dihydrate were reacted with the magnesium sulfite and magnesium hydrogen sulfite present in the aforesaid treating fluid to form a precipitate of calcium sulfite.

Then, the aqueous slurry containing solid particles of gypsum dihydrate, magnesium hydroxide and calcium sulfite was recycled to the desulfurization tower 1 through a pipeline L7.

Example 2 This example is concerned with the desulfurization treatment of exhaust gas containing hydrogen chloride, and involves the addition of a calcium chloride conversion step. An outline of this process is illustrated in Fig.

4.

In the course extending from the desulfurization tower to the double decomposition tank, this example was exactly the same as Example 1. In this example, the mixed slurry obtained in the double decomposition tank was introduced into a calcium chloride conversion tank 8 through a pipeline L8. At the same time, a portion of the oxidation step treating fluid was fed to the calcium chloride conversion tank 8 through a pipeline L3. Thus, calcium ions from the calcium chloride dissolved in the mixed slurry were reacted with sulfate ions to precipitate them in the form of gypsum dihydrate. The feed rate of 29 the oxidation step treating fluid was regulated on the basis of pH control.

The mixed slurry treated in the calcium chloride conversion tank 8 was then introduced into a calcium ion conversion tank 6 through a pipeline L9.' In the same manner as in Example 1, the mixed slurry was mixed with a portion of the desulfurization step treating fluid by means of a stirrer, so that the calcium ions dissolved in water at the solubility level of gypsum dihydrate were precipitated in the form of calcium sulfite.

Then, the mixed slurry containing solid particles of gypsum dihydrate, magnesium hydroxide and calcium sulfite was recycled to the desulfurization tower through a pipeline L7. At the same time, in order to remove the magnesium chloride accumulated in the treating fluid, a portion of the liquid fraction was discharged from the system as blow water.

Example 3 This example is also concerned with the desulfurization treatment of exhaust gas containing hydrogen chloride. An outline of this process is illustrated in Fig. 5. This example is substantially the satre as Example 1, except that the amount of calcium hydroxide slurry added to the double decomposition tank 4 was controlled in the manner described below.

Specifically, in order to control the amount of Specifically, in order to control the amount of 30 calcium hydroxide slurry added, this example includes a measuring tank 9. As shown in Fig. 6, a portion of the mixed slurry within the double decomposition tank was introduced into the measuring tank 9, and a calcium hydroxide slurry was added to the measuring tank 9 until the pH of the mixed slurry reached a value of 10-i. At this time, the concentration of calcium chloride in the liquid fraction, which was regarded as an aqueous calcium chloride solution, was determined by measuring the specific gravity of the liquid fraction with a gravimeter 12 and comparing it with the specific gravityconcentration curve for aqueous calcium chloride solutions. This concentration was corrected for dilution by the addition of the calcium chloride slurry to determine the chloride ion concentration in the double decomposition tank 4. Then, the specific gravity of an o s i. aqueous magnesium chloride solution having the same concentration as the chloride ion concentration so *4@s determined was obtained by reference to the specific gravity-concentration curve for aqueous magnesium chloride solutions. Finally, while the specific gravity of the aqueous solution fraction of the slurry within the double decomposition tank 4 was being measured with a gravimeter 11, the amount of calcium chloride added to the double decomposition tank 4 was regulated so Lhat the measured specific gravity would become equal to the above- 31 calculated specific gravity.

Also in this example, in order to remove the magnesium chloride accumulated in the treating fluid, a portion of the liquid fraction of the mixed slurry returned to the desulfurization tower through the pipeline L7 was discharged from the system as blow water in the same manner as in Example 2.

The present invention enables an exhaust gas desulfurization process using a magnesium-based desulfurizing agent according to the double alkali technique to be carried out in simple and small-sized equipment. Although gypsum dihydrate, together with the treating fluid, is circulated through the system, this may be regarded as an inert SS and will cause no scale deposition in the desulfurization tower, piping and the like.

9SO* Moreover, even in the desulfurization of exhaust gas e(u k containing hydrogen chloride, the treating fluid returned to the desulfurization tower can be prevented from containing calcium ions. As a result, the accumulation of residues which may cause scale deposition and/or blockage in the circulation system can be perfectly prevented, 9o stable operation can be maintained, and efficient exhaust gas desulfurization can be achieved.

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Claims

1. An exhaust gas desulfurization process which comprises the desulfurization step of bringing exhaust gas containing sulfur oxides into contact with a treating fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxides contained in the exhaust gas to be absorbed into the treating fluid, the oxidation step of bringing the desulfurization step treating fluid into contact with an oxygen-containing gas and thereby converting the magnesium salts present in the treating fluid into magnesium sulfate, and the double decomposition step of adding a basic calcium compound to the oxidation step treating fluid and thereby decomposing the magnesium sulfate present in the treating fluid to magnesium hydroxide and gypsum dihydrate, and in which the mixed slurry obtained in the double decomposition step is returned to the desulfurization step and the gypsum dihydrate present in the treating fluid is removed from 3 C- the system, .ha.r tori 'd in th t, prior to returning the mixed slurry obtained in the double decomposition step to 0090 •the desulfurization step, a portion of the desulfurization step treating fluid is added to the mixed slurry so as to convert the calcium ions present therein into calcium sulfite.

2. A process as claimed in claim 1 wherein the 7 0Lz 33 gypsum dihydrate is removed from the system by subjecting at least one of the desulfurization step treating fluid and the oxidation step treating fluid to solid-liquid separation.

3. A process as claimed in claim 1 wherein the step of converting the calcium ions present in the mixed slurry obtained in the double decomposition step into calcium sulfite is carried out at a pH of 6 or greater.

4. A process as claimed in claim 1 wherein a portion of the oxidation step treating fluid is returned to the desulfurization step. An exhaust gas desulfurization process which *comprises the desulfurization step of bringing exhaust gas containing sulfur oxides and hydrogen chloride into contact with a treating fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxides and hydrogen chloride contained in the exhaust gas to be absorbed into the treating fluid, the oxidation step of bringing the desulfurization step treating fluid into contact with an oxygen-containing gas and thereby converting the magnesium salts present in the treating fluid into magnesium sulfate, and the double decomposition step of adding a basic calcium compound to the oxidation

34- step treating fluid and thereby decomposing the magnesium sulfate present in the treating fluid to magnesium hydroxide and gypsum dihydrate, and in which the mixed slurry obtained in the double decomposition step is returned to the desulfurization step, the gypsum dihydrate present in the treating fluid is removed from the system, and the magnesium chloride accumulated in the treating fluid is discharged from the system, oharaCterizds in -tht prior to returning the mixed slurry obtained in the double decomposition step to the desulfurization step, a portion of the oxidation step treating fluid is added to the mixed slurry so as to convert the calcium ions present therein into gypsum dihydrate, and a portion of the desulfurization step treating fluid is then added to the mixed slurry so as to convert the calcium ions present therein into calcium sulfite. ee *:Go 6. A process as claimed in claim 5 wherein the **s gypsum dihydrate is removed from the system by subjecting at least one of the desulfurization step treating fluid and the oxidation step treating fluid to solid-liquid .separation. 7. A process as claimed in claim 5 wherein the step of converting the calcium ions present in the mixed slurry S into calcium sulfite is carried out at a pH of 6 or 35 greater. 8. A process as claimed in claim 5 wherein a portion of the liquid fraction of the mixed slurry returned to the desulfurization step is discharged from the system as blow water. 9. A process as claimed in claim 5 wherein a portion of the oxidation step treating fluid is returned to the desulfurization step. A process as claimed in claim 5 wherein the amount of oxidation step treating fluid added to the mixed slurry obtained in the double decomposition step is regulated on the basis of pH measurement. 11. An exhaust gas desulfurization process which comprises the desulfurization step of bringing exhaust gas 9*c* containing sulfur oxides and hydrogen chloride into contact with a treating fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxides and hydrogen chloride contained in the exhaust gas to be absorbed into the treating fluid, the oxidation step of bringing the desulfurization step treating fluid into contact with an oxygen-containing gas and thereby converting the magnesium salts present in the treating 36 fluid into magnesium sulfate, and the double decomposition step of adding a basic calcium compound to the oxidation step treating fluid and thereby decomposing the magnesium sulfate present in the treating fluid to magnesium hydroxide and gypsum dihydrate, and in which the mixed slurry obtained in the double decomposition step is returned to the desulfurization step, the gypsum dihydrate present in the treating fluid is removed from the system, and the magnesium chloride accumulated in the treating fluid is discharged from the system, charactoariad i-i -t4a.4, prior to returning the mixed slurry obtained in the double decomposition step to the desulfurization step, a portion of the desulfurization step treating fluid is added to the mixed slurry so as to convert the calcium ions present therein into calcium sulfite, and 44, -tAa the chloride ion concentration in the mixed slurry present in the double decomposition step is determined and the amount of basic calcium compound added in the double decomposition step is regulated on the basis of the chloride ion concentration so determined. 12. A process as claimed in claim 11 wherein th. gypsum dihydrate is removed from the system by subjecting at least one of the desulfurization step treating fluid and the oxidation step treating fluid to solid-liquid separation. 0 37 13. A process as claimed in claim 11 wherein the step of converting the calcium ions present in the mixed slurry obtained in the double decomposition step into calcium sulfite is carried out at a pH of 6 or greater. 14. A process as claimed in claim 11 wherein a portion of the liquid fraction of the mixed slurry returned to the desulfurization step is discharged from the system as blow water. A proc;ss as claimed in claim 11 wherein a portion of the oxidation step treating fluid is returned to the desulfurization step. 16. A process as cla.imed in claim 11 wherein the *chloride ion concentration in the mixed slurry is 9* determined by introducing a portion of the mixed slurry into a measuring tank, addinq thereto an aqueous solution of a basic calcium compound; and then measuring the specific gravity of the liquid fraction thereof. 17. A process as claimed in claim 11 wherein the specific gravity of the aqueous solution fraction, of the slurry present in the double decomposition tank is determined and the amount of basic calcium compound added to v.he double decomposition tank is regulated so that the measured specific gravity will become equal to the specific gravity of an aqueous magnesium chloride solution having a desired concentration. 18, A process according to any one of claims 1 to 17, substantially as hereinbefore described with reference to any one of the examples. DATED: 14 May, 1996 TOYO ENGINEERING CORPORATION By their Patent Attorneys PHILLIPS ORMONDE FITZPATRICK S ooo *l -I 39 Abstract of the Disclosure In an exhaust gas desulfurization process wherein sulfur oxides and HCl present in exhaust gas are absorbed into a treating fluid containing a Mg-based desulfurizing agent, and the treating fluid is circulated through desulfurization, oxidation and double decomposition steps to fix and remove the sulfur oxides in the form of gypsum dihydrate by adding calcium hydroxide to the treating fluid and, at the same time, to regeneratt the treating fluid, a portion of the desulfurization step treating 6*66 fluid is added to the treating fluid obtained in the double decomposition step so as to remove calcium ions therefrom, and the treating fluid thus obtained is then recycled to the desulfurization step. This process makes it possible to effect exhaust gas desulfurization according to the double alkali technique while using 6.9g simple and small-sized equipment and to maintain stable *oo operation without scale deposition in the desulfurization tower. 0 S