CN1143535A - Exhaust gas desulfurization process - Google Patents
Exhaust gas desulfurization process Download PDFInfo
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- CN1143535A CN1143535A CN96100008A CN96100008A CN1143535A CN 1143535 A CN1143535 A CN 1143535A CN 96100008 A CN96100008 A CN 96100008A CN 96100008 A CN96100008 A CN 96100008A CN 1143535 A CN1143535 A CN 1143535A
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/48—Sulfur compounds
- B01D53/50—Sulfur oxides
- B01D53/501—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
- B01D53/502—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound characterised by a specific solution or suspension
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/68—Halogens or halogen compounds
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2251/00—Reactants
- B01D2251/40—Alkaline earth metal or magnesium compounds
- B01D2251/402—Alkaline earth metal or magnesium compounds of magnesium
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2251/00—Reactants
- B01D2251/40—Alkaline earth metal or magnesium compounds
- B01D2251/404—Alkaline earth metal or magnesium compounds of calcium
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2251/00—Reactants
- B01D2251/60—Inorganic bases or salts
- B01D2251/604—Hydroxides
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Abstract
In exhaust gas desulfurization process, sulfur oxide and hydrogen chloride contained in the exhaust gas are absorbed by a processing fluid which contains magnesium desulphuration agents; the processing fluid is recycled after desulfurization, oxidation and double decomposition; calcium hydroxide is added into the fluid to fix up and remove sulfur oxide which is in the form of dihydrate gypsum and to re-produce processing fluid; part of the processing fluid of the desulfurization process is added into the processing fluid obtained in the double decomposition process to remove the calcium ion therein, then the processing fluid circulates in desulfurization process. The method makes it possible to desulfurize the exhaust gas based on double alkali method with a simple and small-sized device; the operation is stable and no sedimentation exists in the desulfurization tower.
Description
The present invention relates to a flue gas desulfurization method for treating various types of flue gas containing sulfur oxide (for example, flue gas generated by burning oil or coal) and flue gas containing sulfur oxide and hydrogen chloride.
The lime-gypsum process is a typical wet desulfurization process known for various types of exhaust gases. In this process, an absorbent consisting of calcium carbonate or calcium hydroxide is directly added to the desulfurization tower, and thus calcium ions are dissolved in the treated fluid. Thus, when these calcium ions react with sulfur oxides or the like in the desulfurization tower, fouls consisting of precipitates of gypsum dihydrate, calcium sulfite dihydrate and the like are deposited in the desulfurization tower and the piping. This makes smooth operation difficult and requires much labor to remove such dirt. In addition, calcium hydroxide is a desulfurizing agent that naturally absorbs two molecules of sulfur dioxide, but calcium sulfite is a desulfurizing agent that has absorbed one molecule of sulfur dioxide and has much lower solubility than a similar desulfurizing agent, magnesium sulfite. Therefore, this method is also uneconomical because the treated fluid has a low sulfur oxide absorption rate and thus requires an increase in the size of equipment such as a desulfurization tower and a circulation pump.
On the other hand, a double alkali process (double alkali process) is also known in which absorption of sulfur oxide in a desulfurization tower is performed by using a basic desulfurizing agent such as a basic sodium compound, ammonia, or a basic magnesium compound, and the desulfurizing agent is produced by a metathesis reaction with quick lime, which is an auxiliary agent outside the desulfurization tower. The double alkali method is not easy to deposit dirt. In particular, the method uses a basic magnesium compound as a desulfurizing agent and has the following characteristics: high absorption of sulfur oxides, high solubility of the magnesium sulfite produced, and less fouling in the absorber. However, this method has a problem that two types of crystals (i.e., gypsum dihydrate and magnesium hydroxide) are precipitated in the metathesis step, and it is difficult to separate them, necessitating the use of complicated equipment.
Furthermore, the magnesium-gypsum process of Kawasakiis a so-called compromise between the lime-gypsum process and the double alkali process [ A Collection of environmental pollution Control technologies for Practical Use (1), Kagaku Kogyosha, p.14]. According to this method, sulfur oxide is absorbed in the desulfurization step by using a mixed slurry of magnesium hydroxide and calcium hydroxide as a desulfurizing agent. Then, while adjusting the pH to 2.0 to 4.0 with sulfuric acid, the treated fluid is oxidized by air or the like to form magnesium sulfate and gypsum dihydrate. This treated fluid is then subjected to a settling separation step and centrifugation and is thereby separated into gypsum dihydrate and an aqueous magnesium sulfate solution. The separated aqueous magnesium sulfate solution is recycled to the desulfurizing agent regenerating step to which a mixed slurry of magnesium hydroxide and calcium hydroxide is added. The magnesium sulfate then undergoes a metathesis 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 to the absorption step and used as a desulfurizing agent. However, this process is similar to the lime-gypsum process in that calcium hydroxide and gypsum dihydrate are introduced into a desulfurization tower. Therefore, the problem of easy deposition of scale in the desulfurization tower, the circulation pump and the piping has not been solved.
It is an object of the present invention to provide a flue gas desulfurization method which allows the treated fluid to exhibit a high sulfur oxide absorption rate and thus to be carried out in a simple, small-sized apparatus.
It is another object of the present invention to provide an offgas defluidization process which can reduce the calcium ion concentration in the treated fluid present in the desulfurization tower to a minimum level, thus preventing the occurrence of scale deposits and clogging thereof in the desulfurization tower, circulation pump and piping to maintain smooth operation.
It is still another object of the present invention to provide a method for desulfurizing exhaust gas, which can achieve the above object even in the case where the exhaust gas contains not only sulfur oxide but also hydrogen chloride.
As a result of intensive studies to simplify the method of using a magnesium-based desulfurizing agent according to the double alkali method, the present inventors have now found that, unlike the prior art, which separates gypsum dihydrate and magnesium hydroxide formed in the metathesis step and returns only magnesium hydroxide to the desulfurizing tower, the desulfurization process can be carried out without separating gypsum dihydrate and magnesium hydroxide as long as calcium ions dissolved in the treated fluid are not taken into the desulfurizing tower. The present invention has been completed based on this finding.
Specifically, according to one aspect of the present invention, there is provided a flue gas desulfurization method comprising a desulfurization step of bringing a sulfur oxide-containing flue gas into contact with a treatment fluid containing a magnesium-based desulfurizing agent and thereby causing sulfur oxide contained in the flue gas to be absorbed into the treatment fluid, an oxidation step of bringing the desulfurization step treatment fluid into contact with an oxygen-containing gas and thereby converting magnesium salts present in the treatment fluid into magnesium sulfate, an alkaline calcium compound being added to the oxidation step treatment fluid and thereby decomposing magnesium sulfate present in the treatment fluid into magnesium hydroxide and gypsum dihydrate, and wherein the mixed slurry obtained in the metathesis step is returned to the desulfurization step and gypsum dihydrate present in the treatment fluid is removed from the system, characterized in that, before the mixed slurry obtained in the metathesis step is returned to the desulfurization step, a part of the desulfurization step treatment fluid is added to the mixed slurry so as to convert calcium ions present therein into sulfurous acid ions Calcium.
According to another aspect of the present invention, there is provided a flue gas desulfurization method comprising a desulfurization step of bringing a flue gas containing sulfur oxide and hydrogen chloride into contact with a treatment fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxide and hydrogen chloride contained in the flue gas to be absorbed into the treatment fluid, an oxidation step of bringing the treatment fluid of the desulfurization step into contact with an oxygen-containing gas and thereby converting magnesium salts present in the treatment fluid into magnesium sulfate, and a double decomposition step of adding a basic calcium compound to the treatment fluid of the oxidation step and thereby decomposing the magnesium sulfate present in the treatment fluid into magnesium hydroxide and a gypsum dihydrate, and wherein the mixed slurry obtained in the double decomposition step is returned to the desulfurization step, the gypsum dihydrate present in the treatment fluid is removed from the system, and the magnesium chloride accumulated in the treatment fluid is discharged from the system, characterized in that, before returning the mixed slurry obtained in the metathesis step to the desulfurization step, part of the treating fluid of the oxidation step is added to the mixed slurry so as to convert calcium ions present therein into gypsum dihydrate, and then part of the treating fluid of the desulfurization step is added to the mixed slurry so as to convert calcium ions present therein into calcium sulfite.
According to still another aspect of the present invention, there is provided a flue gas desulfurization method comprising a desulfurization step of bringing a flue gas containing sulfur oxide and hydrogen chloride into contact with a treatment fluid containing a magnesium-based desulfurizing agent and thereby causing absorption of the sulfur oxide and hydrogen chloride contained in the flue gas into the treatment fluid, an oxidation step of bringing the treatment fluid of the desulfurization step into contact with an oxygen-containing gas and thereby converting magnesium salts present in the treatment fluid into magnesium sulfate, and a double decomposition step of adding a basic calcium compound to the treatment fluid of the oxidation step and thereby decomposing the magnesium sulfate present in the treatment fluid into magnesium hydroxide and a gypsum dihydrate, and wherein the mixed slurry obtained in the double decomposition step is returned to the desulfurization step, the gypsum dihydrate present in the treatment fluid is removed from the system, and the magnesium chloride accumulated in the treatment fluid is discharged from the system, characterized in that, before returning the mixed slurry obtained in the metathesis step to the desulfurization step, part of the desulfurization step treatment fluid is added to the mixed slurry so as to convert calcium ions present therein into calcium sulfite, and the chloride ion concentration in the mixed slurry present in the metathesis step is determined and the amount of the basic calcium compound added in the metathesis step is adjusted on the basis of the chloride ion concentration thus determined.
The term "magnesium-based desulfurizing agent" as used herein includes basic magnesium compounds mainly composed of magnesium oxide or magnesium hydroxide, such as magnesium hydroxide produced from using marine magnesium as a raw material, magnesium oxide obtained by burning magnesite, and magnesium hydroxide obtained by aging such magnesium hydroxide.
FIG. 1 is a graph showing the relationship between the concentration of magnesium sulfate dissolved in a solution and the solubility of calcium sulfate in the solution;
FIG. 2 is a graph showing the relationship between the amount of basic calcium compound added and the pH in the metathesis step of the desulfurization process of the present invention;
FIG. 3 is a schematic diagram illustrating an exemplary system configuration used to practice the desulfurization process of the present invention;
FIG. 4 is a schematic diagram illustrating another exemplary system configuration used to practice the desulfurization process of the present invention;
FIG. 5 is a schematic diagram illustrating the composition of yet another exemplary system used to practice the desulfurization process of the present invention; and
fig. 6 is a schematic view illustrating a method of controlling the amount of the basic calcium compound to be added in the metathesis step of the desulfurization method of the present invention.
In the flue gas desulfurization method of the present invention, the flue gas containing sulfur oxide, or sulfur oxide and hydrogen chloride is brought into contact with a treatment fluid comprising an aqueous solution containing a magnesium-based desulfurizing agent at the time of the desulfurization step, so that the sulfur oxide and hydrogen chloride are absorbed into the treatment fluid.
Since the treating fluid returned from the metathesis step to the desulfurization step is a mixed slurry containing magnesium hydroxide, gypsum dihydrate, and asmall amount of calcium sulfite, the treating fluid becomes a suspension containing large particles of gypsum dihydrate and a small amount of calcium sulfite in the desulfurization step. Since the magnesium hydroxide in the process fluid is consumed as a desulfurizing and dehydrochlorinating agent, the fine particulate magnesium hydroxide disappears in the desulfurization step.
Suitable apparatus for the desulfurization step include columns having a structure that allows for efficient contact between such gases and liquids. The column may be of the type in which the aqueous solution containing the desulphurating agent is sprayed from a nozzle and the gas is passed in countercurrent or cocurrent. Since the treatment fluid contains large particle gypsum dihydrate, the nozzle must be designed so that it is not clogged by large particle gypsum dihydrate. To increase the efficiency of the gas-liquid contact, the column may be equipped with packing, trays or other devices.
The treated fluid leaving the desulfurization step (hereinafter simply referred to as "desulfurization step treated fluid") is an aqueous solution containing a mixture of magnesium sulfite, magnesium bisulfite, magnesium sulfate and magnesium chloride formed by the reaction of an aqueous solution of magnesium-based desulfurization agent with sulfur oxide, and further contains gypsum dihydrate as suspended matter with a small amount of calcium sulfite
In the desulfurization step, the temperature of the treatment fluid is usually in the range of 50 to 60 ℃. The pH of the fluid treated in the desulfurization step is preferably 5.0 to 7.5 and particularly preferably 5.5 to 6.5. In the desulfurization step, corrective measures must be taken in order to prevent the deposition of magnesium sulfite having low solubility in water. In general, this can be achieved by blowing air or the like into the treatment fluid to oxidize the magnesium sulfite to magnesium sulfate having a high solubility in water and thereby control the concentration of magnesium sulfite below its solubility.
As another method for controlling the concentration of magnesium sulfite below its solubility in the desulfurization step without blowing air or the like into the treating fluid, there is known a method of recycling a part of the treating fluid leaving the below-mentioned oxidation step to the desulfurization step and thereby reducing the concentration of magnesium sulfite in the desulfurization step.
In the method of the present invention, the gypsum dihydrate contained in the treating fluid is treated by subjecting at least one of the desulfurization step treating fluid and the oxidation step treating fluid described later to solid-liquid separation. These process streams contain some solids that are not gypsum dihydrate and a small amount of calcium sulfite so that the gypsum dihydrate can be conveniently separated. Calcium sulfite is also removed from the system at the same time as the gypsum dihydrate is removed.
Alternatively, the separation and removal of gypsum from the desulfurization step process stream may be accomplished by either: first, a desulfurization step treatment fluid is withdrawn from the desulfurization tower by other means, subjected to solid-liquid separation treatment, and then the remaining fluid is returned to the desulfurization tower; or subjecting the desulfurization step treatment fluid to a solid-liquid separation treatment before it is sent to the subsequent oxidation step. For separating and removing the gypsum dihydrate, a wet classifier such as a hydrocyclone, a centrifugal settler or a Dol thickener can be used. Among these wet classifiers, a hydrocyclone is preferred. The separated gypsum dihydrate is withdrawn from the system and can be used extensively in the manufacture of cement, gypsum board and the like.
The desulfurization step treated fluid is then sent to the oxidation step. Contacting the process fluid with an oxygen-containing gas in an oxidation step; so as to oxidize the magnesium sulfite and magnesium bisulfite present in the treatment fluid to magnesium sulfate and sulfuric acid. Typically, the magnesium sulfate concentration in the treatment fluid is 3 to 10 wt% and the pH is 2 to 3. In the oxidation step, a tank reactor is generally used and the treatment fluid may or may not be stirred.
The type of gas phase components other than oxygen is not critical when the oxygen-containing gas is fed to the oxidation step, as long as they are inert to the desulfurization step process fluid. Typically, air is used as the oxygen-containing gas.
Also, the process fluid exiting the oxidation step (hereinafter referred to simply as the "oxidation step process fluid") is solids free except for gypsum dihydrate. Thus, removal of gypsum dihydrate present in the treatment fluid from the system can also be performed with respect to the oxidation step treatment fluid.
The oxidation step treatment stream is then sent to the metathesis step. In this metathesis step, a basic calcium compound is added to a treatment fluid containing, as main components, magnesium sulfate and sulfuric acid formed in the oxidation step, and in some cases also magnesium chloride. Thus, sulfuric acid reacts with the basic calcium compound to form gypsum dihydrate, and magnesium sulfate reacts with the basic calcium compound to form gypsum dihydrate and magnesium hydroxide. After the formation of gypsum dihydrate, the excess added alkaline calcium compound is consumed by reaction with magnesium chloride to form magnesium hydroxide and calcium chloride. However, since calcium chloride has high solubility, it does not generally form a precipitate.
Typically, a tank reactor is used in the metathesis step. Although a higher reaction temperature is preferred, it is preferable from the viewpoint of operation to use a reaction temperature similar to that in the desulfurization step. The residence time is preferably 4 to 5 hours or more. The gypsum dihydrate thus produced grows into particles which generally have an average particle diameter (main particle diameter) of 70 μm or more and usually up to 200 μm. On the other hand, magnesium hydroxide is formed as fine particles having a size of 1 μm or less and usually about 0.3 to 1 μm, and these particles are aggregated to form particles having an apparent size of about 10 to 20 μm.
As the basic calcium compound used in the metathesis step, calcium hydroxide, calcium oxide or a mixture thereof is preferable. Although such alkaline calcium compounds may be added to the reaction tank in the form of powder, their water slurry is most suitable from the viewpoint of workability. In order to increase the particle size of the gypsum dihydrate, the feed rate of the basic calcium compound is preferably controlled to such an extent that the fluid mixture present in the metathesis step has a pH of about 11. However, in the case where the treatment fluid contains magnesium chloride, the feed rate of the basic calcium compound should be controlled as described below.
In the process of the present invention, the mixed slurry of gypsum dihydrate and magnesium hydroxide obtained in the metathesis step is returned to the desulfurization step without separating the two solid components. However, at this time, it is necessary to minimize the amount of calcium ions dissolved in the mixed slurry returned to the desulfurization step. The reason for this is that the solubility of gypsum dihydrate (like calcium sulfate) is a function of the concentration of magnesium sulfate coexisting, as shown in figure 1. In the metathesis step, magnesium sulfate is converted to gypsum dihydrate and magnesium hydroxide by reaction with a basic calcium compound, so that the concentration of magnesium sulfate is near zero. Subsequently, the solubility of calcium sulfate therein is relatively high, i.e., about 2000-2200 ppm. However, since the metathesis step is a step in which gypsum dihydrate is precipitating, calcium sulfate is dissolved at a concentration higher than its solubility. Thus, the supersaturation degree thereof is considered to be about 1.4 to 1.8, and the actual concentration of calcium sulfate in the mixed slurry is estimated to be 2800 to 3960 ppm. Conversely, the magnesium sulfate concentration in the desulfurization step is about 3 to 10 wt%, wherein the dissolution of calcium sulfate is about 1500 ppm. Therefore, if the mixed slurry obtained in the metathesis step is directly fed to the desulfurization step, the supersaturation degree of calcium sulfate will have an ultra-high value of 1.87 to 2.64. This causes the formation and growth of gypsum dihydrate and deposits in the form of scale during the desulfurization step.
Therefore, in order to reduce the amount of calcium ions dissolved in the mixed slurry, the method of the present invention includes a calcium ion conversion step of adding a part of the desulfurization step treatment 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 a desulfurization step treatment fluid containing magnesium sulfite, magnesium bisulfite, and magnesium hydroxide is added to the mixed slurry and the resultant mixture is stirred, calcium ions undergo reactions represented by reaction formulas (1) to (3) given below. Then, calcium ions were fixed by forming insoluble calcium sulfite (solubility: 0.0051g/100g aqueous solution), and as a result, the concentration of calcium ions in the slurry was remarkably reduced. However, at a pH of less than 6, the magnesium bisulfite will react not only with dissolved calcium ions but also with the coexisting magnesium hydroxide. Therefore, the pH is preferably 6 or more, and more preferably 6 to 11. The reaction temperature is preferably 80 ℃ or less, and more preferably 60 ℃ or less.
Among the above-mentioned reactions occurring in the calcium ion conversion step, the reactions (1) and (2) have a particularly high reaction rate, so that the residence time of the mixed slurry in this step can be as short as about 10 seconds. Therefore, this step can be carried out by using a tank reactor of small size. That is, the important point of the process of the present invention is in fact that the above reactions (1) to (3) can be carried out not in a closed desulfurization tower of the desulfurization step but in a tank reactor of small size. If fouling is deposited on the inner walls of such a reactor, it is preferable to use two alternating reactors.
When the exhaust gas to be desulfurized is a gas containing sulfur oxides, as in the case of an exhaust gas produced by burning fuel oil or the like, the method of the present invention is carried out in the manner described above. However, in the case where the off-gas contains not only sulfur oxide but also hydrogen chloride, as in the case of off-gas generated by burning coal or the like, it is necessary to prevent the mixed slurry fed from the metathesis step to the calcium ion-converting step and further fed to the desulfurizationstep from containing no calcium ions in the form of highly soluble calcium chloride.
Specifically, a basic calcium compound is added to the process fluid in the metathesis step. When the treatment fluid contains chloride, the reactions occurring in the metathesis step are represented by equations (4) and (5) given below. That is, the addition of the alkaline calcium compound first entails the formation of gypsum dihydrate and magnesium hydroxide. After the formation of gypsum dihydrate, the excess added alkaline calcium compound is consumed by reaction with magnesium chloride to form magnesium hydroxide and calcium chloride.
Figure 2 is a graph showing the relationship between the amount of basic calcium compound added and the pH in the metathesis step. FIG. 2 shows that the change in pH is slight in the vicinity of the end point of the formula (4). Therefore, it is very difficult to stop the addition of the basic compound according to the pH change when the reaction formula (4) is completed. If the alkaline calcium compound is added in excess and thus the reaction of the reaction formula (5) continues, the calcium chloride produced has high solubility and is thus dissolved in the mixed slurry. Therefore, the mixed slurry fed to the calcium ion converting step contains a considerable amount of calcium ions, so that an excessively high processing load is imposed on the calcium ion converting step.
One way to prevent the mixed slurry leaving the metathesis step from being free of calcium ions from calcium chloride is to add a partial oxidation step treatment fluid to the mixed slurry obtained in the metathesis step to convert calcium ions present in the mixed slurry to gypsum dihydrate and thereby reduce the calcium ion concentration in the mixed slurry to the solubility level of the gypsum dihydrate, and then send the mixed slurry to a calcium ion conversion step.
As described above, the reaction occurring in the metathesis step is limited to the reaction of the above reaction formula (4), and therefore it is difficult to add an appropriate amount of the basic calcium compound to promote the growth of the crystal particles of the gypsum dihydrate. Thus, the reaction of formula (5) will continue to some extent in the metathesis step of the process. The partial oxidation step treatment fluid is then added to the mixed slurry in which some of the calcium chloride has been dissolved so that the calcium ions react with the sulfate ions present in the oxidation step treatment fluid to precipitate them out as gypsum dihydrate. That is, the oxidation step treatment fluid is added so as to cause the alkali calcium compound added in excess to undergo a reaction represented by reaction formula (6) given below. Similar to the metathesis step, the step of converting calcium chloride dissolved in the mixed slurry into magnesium chloride (hereinafter referred to as "calcium chloride conversion step") is preferably carried out by using a tank reactor.
Since the relationship between the amount of the treating fluid added for the oxidation step and the pH in the calcium chloride conversion step follows the curve of FIG. 2 in the reverse direction (since the increase in the added amount causes the change of the horizontal seat value from right to left), the end point of this reaction can be easily found from the results of pH measurement, and therefore the feed rate of the treating fluid for the oxidation step can be easily and accurately controlled. This calcium chloride conversion step causes all of the calcium chloride formed in the metathesis step and the calcium chloride dissolved in the mixed slurry to be converted to magnesium chloride. The calcium ions are then crystallized into gypsum dihydrate, so that the calcium ion concentration in the mixed slurry is reduced to the solubility level of the gypsum dihydrate.
Another method of prevention is to stop the addition of the basic calcium compound upon completion of the reaction of equation (4) above so that the mixed slurry leaving the metathesis step does not contain calcium ions. Specifically, the chloride ion concentration in the mixed slurry present in the metathesis step is first determined. Although various methods can be used to determine the chloride ion concentration, the method described herein is to introduce a portion of the mixed slurry into a measurement tank and determine the chloride ion concentration thereof according to gravimetric determination. More specifically, a part of the mixed slurry is introduced into a measuring tank and an aqueous solution of an alkali chloride is added thereto until the reaction of the reaction formula (5) is completed (i.e., the pH of the slurry reaches 10 to 11). At this point, only the calcium chloride is actually dissolved in the liquid portion of the mixed slurry. Therefore, the concentration of calcium chloride in the mixed slurry present in the measuring tank can be determined by measuring the specific gravity of the liquid portion of the mixed slurry and comparing it with the concentration-specific gravity curve of the calcium chloride aqueous solution. Since there is no chloride ion migration between the metathesis tank and the measurement tank, the chloride ion concentration in the mixed slurry present in the metathesis tank can be determined by correcting the calcium chloride concentration in the measurement tank in terms of dilution by adding the basic calcium compound to the measurement tank.
The adjustment of the amount ofthe basic calcium compound added to the metathesis step so as to be sufficient to complete the reaction of the formula (4) may be performed, for example, in the following manner based on the chloride ion concentration thus determined: if the amount of basic compound added to the metathesis tank is the ideal amount just sufficient to complete the reaction of equation (4) but insufficient to initiate the reaction of equation (5), only the magnesium chloride is actually dissolved in the liquid fraction present in the metathesis tank. The concentration of magnesium chloride in the metathesis tank at the completion of the reaction of equation (4) can therefore be calculated from the previously determined chloride ion concentration. Then, the specific gravity of the portion of the aqueous slurry solution present in the metathesis tank is measured and the amount of basic calcium compound added to the metathesis tank is adjusted so as to equalize the measured specific gravity to the specific gravity of the aqueous magnesium chloride solution having the calculated concentration as described above. In this manner, the reaction energy occurring in the metathesis step is limited to that of equation (4), with the result 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 treatment fluid of the desulfurization step are fixed in solid form and are not discharged, so that they circulate in the system together with the treatment fluid. In order to prevent the accumulated chloride ions from being higher than a predetermined concentration, it is common practice to appropriately discharge the chloride ions in the form of effluent water including an aqueous magnesium chloride solution from the system. Preferably, the discharge of blowdown water from the system should be carried out relative to the liquid portion of the mixed slurry returned to the desulfurization step, since such mixed slurry results in the lowest concentration of magnesium ions in the treatment fluid.
Various modifications can be made to the process of the present invention. For example, part of the oxidation step process fluid may be returned to the desulfurization step (desulfurization tower) if the oxidation step process fluid is added to a desulfurization tower, the magnesium sulfate fraction in the process fluid will increase and the acid salt fraction of the sulfur will decrease in the desulfurization tower, with the result that precipitation of magnesium sulfite can be minimized.
In the following examples, the flue gas desulfurization method of the present invention will be described in more detail with reference to the accompanying drawings. However, these examples should not be construed as limiting the scope of the present invention.
Example 1
This example relates to the desulfurization of oil-fired boiler exhaust gas (without hydrogen chloride). The outline of this method is shown in FIG. 3.
The treating fluid having the magnesium-based desulfurizing agent dissolved therein and containing large particles of gypsum dihydrate as suspended matter is passed down in a shower form from the upper portion of the desulfurizing tower 1 and brought into gas-liquid contact with the sulfur oxide-containing exhaust gas G1 introduced into the tower from below. Then, the sulfur oxides are absorbed into the treatment fluid and fixed in the form of magnesium sulfite, magnesium bisulfite and the like, and the exhaust gas G2 free of sulfur oxides is discharged from the top of the tower.
Since the off-gas fed to the desulfurization tower 1 is hot, it is cooled by the sprayed operating water from the spray nozzle. Flow rate ofexhaust gas 105Nm3Hr and SO2The concentration was 2,000 ppm.
The treated fluid containing the absorbed sulfur oxide falls to the bottom of the desulfurization tower 1, is fed to the upper part of the tower through a pump P1 and a line L1 together with a new treated fluid fed from the magnesium hydroxide slurry supply tank 7, and flows downward. The treatment fluid is then continuously circulated through the desulfurization tower. To prevent the magnesium sulfite from settling, air is sucked into the bottom of the tower. In addition, a part of the treatment fluid was withdrawn from the line L1 at a flow rate of 30t/hr and fed to the gypsum separator 2, and gypsum dihydrate suspended in the treatment fluid was separated in the separator 2. Discharging the separated gypsum dihydrate from the system at a rate of 1.6t/hr while removing the remaining liquidThe body is returned to the desulfurization tower 1. The salt concentration of the treatment fluid in the desulfurizing tower 1 is 7.5 wt% expressed by magnesium sulfate, the mixed concentration of the magnesium sulfite and the magnesium bisulfite is 1.5 wt% expressed by magnesium sulfate, and the pH value is 5.8-6.0. SO in exhaust G22The concentration was 100ppm and the degree of desulfurization was 95%.
The desulfurization step treating fluid was withdrawn from the desulfurization tower 1 through a pump P2 and a line L2 and fed to the oxidation tank 3 at a rate of 11 t/hr. In this oxidation tank 3, the desulfurization step treatment fluid is oxidized by exposure to air and thereby converted into an aqueous solution of magnesium sulfate and sulfuric acid. This oxidation step treatment stream is fed to metathesis tank 4 via pump P3 and line L3. An aqueous slurry containing 30% by weight of calcium hydroxide from the calcium hydroxide supply tank 5 via line L4 was added to the metathesis tank 4 at a flow rate of 1.8 t/hr. Upon mixing of the contents of the metathesis tank by the agitator, the magnesium sulfate and sulfuric acid react with the calcium hydroxide to form gypsum dihydrate as solid particles and magnesium hydroxide. The reaction temperature was 50 ℃.
The resulting mixed slurry is then introduced into the calcium ion conversion tank 6 through a line L5. At the same time, part of the desulfurization step treatment fluid containing the absorbed sulfur oxide was withdrawn from the desulfurization tower 1 through the lines L1 and L6 and fed into the calcium ion-converting tank 6 at a flow rate of 1.3 t/hr. Upon intimate mixing of the contents by the agitator, calcium ions dissolved in the water at the solubility level of the gypsum dihydrate react with the magnesium sulfite and magnesium bisulfite present in the treatment fluid described above to form a calcium sulfite precipitate.
The aqueous slurry containing solid particles of gypsum dihydrate, magnesium hydroxide, is then recycled to desulfurization tower 1 via line L7.
Example 2
This example relates to the desulfurization treatment of a waste gas containing hydrogen chloride and to the addition of a calcium chloride conversion step. The outline of this method is shown in fig. 4.
This example is exactly the same as example 1, from the desulfurization column up to the metathesis tank. In this case, the mixed slurry obtained in the metathesis tank is introduced into the calcium chloride conversion tank 8 through line L8. At the same time, part of the oxidation step treatment fluid is sent to the calcium chloride conversion tank 8 through line L3. Thus, calcium ions from the calcium chloride dissolved in the mixed slurry react with sulfate ions to precipitate them as gypsum dihydrate. The feed rate of the treatment fluid to the oxidation step is adjusted in response to the pH control.
The treated mixed slurry in the calcium chloride conversion tank 8 is then introduced into the calcium ion conversion tank 6 through the line L9. The mixed slurry was mixed with a part of the desulfurization step treatment fluid by means of a stirrer in the same manner as in example 1, with the result that 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 the solid particles of gypsum dihydrate, magnesium hydroxide and calcium sulfite is recycled to the desulfurization tower through a line L7. At the same time, a portion of the liquid fraction is discharged from the system as bleed water in order to remove the magnesium chloride that has accumulated in the process fluid.
Example 3
This example also relates to the desulfurization treatment of a hydrogen chloride-containing exhaust gas. The outline of this example is shown in fig. 5. This example is essentially the same as example 1 except that the amount of calcium hydroxide slurry added to the metathesis tank 4 is controlled in the manner described below.
Specifically, to control the amount of calcium hydroxide slurry added, this example includes a measuring tank 9. As shown in fig. 6, a part of the mixed slurry in the metathesis tank was introduced into the measurement tank 9, and the calcium hydroxide slurry was added to the measurement tank 9 until the pH of the mixed slurry reached 10 to 11. At this time, the calcium chloride concentration in the liquid portion regarded as the calcium chloride aqueous solution is determined by measuring the liquid portion specific gravity with the weight difference meter 12 and comparing it with the specific gravity-concentration curve of the calcium chloride aqueous solution. This concentration is corrected for dilution by addition of calcium chloride slurry to determine the chloride ion concentration in the metathesis tank 4. Then, the specific gravity of the aqueous magnesium chloride solution having the same concentration as the chloride ion concentration thus determined is obtained by referring to the specific gravity-concentration curve of the aqueous magnesium chloride solution. Finally, just as the specific gravity of the aqueous solution portion of the slurry in the metathesis tank 4 is measured with the gravimeter 12, the amount of calcium chloride added to the metathesis tank 4 is adjusted so that the measured specific gravity is equal to the specific gravity calculated above.
Also, in this example, in order to remove magnesium chloride accumulated in the treatment fluid, a part of the liquid portion of the mixed slurry returned to the desulfurization tower through the line L7 was discharged from the system as drain water in the same manner as in example 2.
The present invention enables the flue gas desulfurization method using a magnesium-based desulfurizing agent according to the double alkali method to be performed in a simple and small-sized apparatus. Although the gypsum dihydrate is circulated in the system with the treatment fluid, this can be considered as an inert SS and does not cause scale-like deposits in the desulfurization tower, piping, etc.
Further, even when the hydrogen chloride-containing exhaust gas is desulfurized, the treatment fluid returned to the desulfurizing tower can be prevented from containing calcium ions. Therefore, the accumulation of residues which may cause scale-like deposition and/or clogging in the circulation system can be completely prevented, stable operation can be maintained, and effective desulfurization of exhaust gas can be achieved.
Claims (17)
1. A flue gas desulfurization method comprising a desulfurization step of bringing a sulfur oxide-containing flue gas into contact with a treatment fluid containing a magnesium-based desulfurizing agent and thereby causing sulfur oxide contained in the flue gas to be absorbed into the treatment fluid, an oxidation step of bringing the desulfurization step treatment fluid into contact with an oxygen-containing gas and thereby converting magnesium salts present in the treatment fluid into magnesium sulfate, an alkaline calcium compound being added to the oxidation step treatment fluid and thereby decomposing the magnesium sulfate present in the treatment fluid into magnesium hydroxide and gypsum dihydrate, and wherein the mixed slurry obtained in the metathesis step is returned to the desulfurization step and gypsum dihydrate present in the treating fluid is removed from the system, characterized in that part of the treating fluid of the desulphurisation step is added to the mixed slurry to convert calcium ions present therein to calcium sulphite before returning the mixed slurry obtained in the metathesis step to the desulphurisation step.
2. The method as set forth in claim 1 wherein gypsum dihydrate is removed from the system by subjecting at least one of the desulfurization step treatment fluid and the oxidation step treatment fluid to a solid-liquid separation treatment.
3. The process as claimed in claim 1, wherein the step of converting calcium ions present in the mixed slurry obtained in the metathesis step into calcium sulfite is carried out at a pH of 6 or more.
4. The method as set forth in claim 1 wherein a portion of the oxidation step treatment fluid is returned to the desulfurization step.
5. A flue gas desulfurization method comprising a desulfurization step of bringing a flue gas containing sulfur oxide and hydrogen chloride into contact with a treatment fluid containing a magnesium-based desulfurizing agent and thereby causing the sulfur oxide and hydrogen chloride contained in the flue gas to be absorbed into the treatment fluid, an oxidation step of bringing the treatment fluid of the desulfurization step into contact with an oxygen-containing gas and thereby converting magnesium salts present in the treatment fluid into magnesium sulfate, and a metathesis step of adding a basic calcium compound to the treatment fluid of the oxidation step and thereby decomposing the magnesium sulfate present in the treatment fluid into magnesium hydroxide and gypsum dihydrate, and wherein the mixed slurry obtained in the metathesis step is returned to the desulfurization step, the gypsum dihydrate present in the treatment fluid is removed from the system, and magnesium chloride accumulated in the treatment fluid is discharged from the system, characterized in that, before returning the mixed slurry obtained in the metathesis step to the desulfurization step, a portion of the oxidation step treatment fluid is added to the mixed slurry to convert calcium ions present therein to gypsum dihydrate, and then a portion of the desulfurization step treatment fluid is added to the mixed slurry to convert calcium ions present therein to calcium sulfite.
6. A process as set forth in claim 5 wherein gypsum dihydrate is removed from the system by subjecting at least one of the desulfurization step treatment fluid and the oxidation step treatment fluid to a solid-liquid separation treatment.
7. A method as set forth in claim 5 wherein the conversion of calcium ions present in the mixed slurry to calcium sulfite is carried out at a pH of 6 or greater.
8. The method as set forth in claim 5, wherein a part of the liquid portion of the mixed slurry returned to the desulfurization step is discharged from the system as effluent.
9. A process as claimed in claim 5, wherein part of the oxidation step treatment fluid is returned to the desulphurisation step.
10. A process as claimed in claim 5, wherein the amount of the oxidation step treatment fluid added to the mixed slurry obtained in the metathesis step is adjusted on the basis of the pH measurement.
11. A flue gas desulfurization method comprising a desulfurization step of bringing a waste gas containing sulfur oxide and hydrogen chloride into contact with a treatment fluid containing a magnesium-based desulfurizing agent and thereby causing absorption of the sulfur oxide and hydrogen chloride contained in the waste gas into the treatment fluid, an oxidation step of bringing the treatment fluid of the desulfurization step into contact with an oxygen-containing gas and thereby converting magnesium salts present in the treatment fluid into magnesium sulfate, and a step of adding a basic calcium compound to the treatment fluid of the oxidation step and thereby decomposing the magnesium sulfate present in the treatment fluid into magnesium hydroxide and gypsum dihydrate, wherein the metathesis step, and wherein the mixed slurry obtained in the metathesis step is returned to the desulfurization step, the gypsum dihydrate present in the treatment fluid is removed from the system, and magnesium chloride accumulated in the treatment fluid is discharged from the system, characterized in that, before the mixed slurry obtained in the metathesis step is returned to the desulfurization step, adding part ofthe desulphurisation step treatment fluid to the mixed slurry to convert calcium ions present therein to calcium sulphite, and determining the chloride ion concentration in the mixed slurry present in the metathesis step and adjusting the amount of basic calcium compound added in the metathesis step in dependence on the chloride ion concentration so determined.
12. The method as set forth in claim 11 wherein gypsum dihydrate is removed from the system by subjecting at least one of the desulfurization step treatment fluid and the oxidation step treatment fluid to a solid-liquid separation treatment.
13. The process as claimed in claim 11, wherein the step of converting calcium ions present in the mixed slurry obtained in the metathesis step into calcium sulfite is carried out at a pH of 6 or more.
14. The method as set forth in claim 11, wherein a part of the liquid portion of the mixed slurry returned to the desulfurization step is discharged from the system as effluent.
15. The method as set forth in claim 11 wherein a portion of the oxidation step treatment fluid is returned to the desulfurization step.
16. The method as set forth in claim 11, wherein the chloride ion concentration in the mixed slurry is measured by introducing a part of the mixed slurry into a measuring tank, adding an aqueous solution of an alkaline chlorine compound thereto, and then measuring the specific gravity of the liquid part thereof.
17. A process as claimed in claim 11, wherein the specific gravity of the aqueous solution of the slurry present in the metathesis tank is determined and the amount of basic calcium compound added to the metathesis tank is adjusted so as to make the determined specific gravity the same as the specific gravity of the aqueous solution of magnesium chloride having the desired concentration.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP21329695A JP3751340B2 (en) | 1995-08-22 | 1995-08-22 | Exhaust gas desulfurization method |
| JP213296/95 | 1995-08-22 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CN1143535A true CN1143535A (en) | 1997-02-26 |
| CN1089022C CN1089022C (en) | 2002-08-14 |
Family
ID=16636779
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN96100008A Expired - Fee Related CN1089022C (en) | 1995-08-22 | 1996-02-28 | Exhaust gas desulfurization process |
Country Status (8)
| Country | Link |
|---|---|
| JP (1) | JP3751340B2 (en) |
| KR (1) | KR0174794B1 (en) |
| CN (1) | CN1089022C (en) |
| AU (1) | AU674822B1 (en) |
| CZ (1) | CZ56196A3 (en) |
| MY (1) | MY112500A (en) |
| PL (1) | PL183184B1 (en) |
| TW (1) | TW326404B (en) |
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN1085088C (en) * | 1998-03-11 | 2002-05-22 | 安乐仁 | Medicine for treating psoriasis |
| CN1118315C (en) * | 1997-10-06 | 2003-08-20 | 日立造船株式会社 | Process and apparatus for removing acid gases from exhaust gases |
| CN100335154C (en) * | 2005-09-09 | 2007-09-05 | 清华大学 | Magnesium oxide flue gas desulfurization and outgrowth thick-slurry method oxidation reclaim process |
| CN100404108C (en) * | 2005-12-08 | 2008-07-23 | 六合天融（北京）环保科技有限公司 | Method for preparing desulfurizer magnesium oxide and sulfur dioxide utilizing desulfurizing by-product magnesium sulfite by magnesium process |
| CN100435910C (en) * | 2006-04-10 | 2008-11-26 | 广州市天赐三和环保工程有限公司 | Fume desulfurizing process employing multiple circulation and stable double alkali method and apparatus |
| CN101879406A (en) * | 2010-07-15 | 2010-11-10 | 煤炭工业济南设计研究院有限公司 | Dual-alkali flue gas desulfurization device and desulfurization method |
| CN102172472A (en) * | 2011-02-14 | 2011-09-07 | 彭振超 | Method for smoke desulphurization by utilizing dolomitic lime |
| TWI457167B (en) * | 2011-11-18 | 2014-10-21 | Long Chen Paper Co Ltd | Apparatus for flue gas desulfurization with zero emission using system for recovery and recycle of magnesium hydroxide |
| CN104959012A (en) * | 2015-04-17 | 2015-10-07 | 熊天渝 | Magnesium-calcium based wet method desulphurization system and method for removing sulfur dioxide in flue gas and producing gypsum |
| CN107042060A (en) * | 2017-04-05 | 2017-08-15 | 中冶焦耐(大连)工程技术有限公司 | A kind of method of resource of the sour gas containing ammonia |
| CN110605011A (en) * | 2019-08-08 | 2019-12-24 | 陆泳凯 | Magnesium-calcium double-alkali flue gas desulfurization method |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| KR100303388B1 (en) * | 1999-06-02 | 2001-09-24 | 세 영 모 | Aaaaa |
| JP6223654B2 (en) | 2011-10-31 | 2017-11-01 | 三菱日立パワーシステムズ株式会社 | Flue gas desulfurization equipment |
| CN107185385A (en) * | 2017-07-07 | 2017-09-22 | 金川集团股份有限公司 | A kind of spuious flue gas during smelting wet dedusting of atm number, sulfur method |
| JP6958682B2 (en) * | 2018-08-08 | 2021-11-02 | 宇部興産株式会社 | Cement composition manufacturing method and cement composition manufacturing system |
| CN116124939B (en) * | 2023-01-10 | 2024-03-26 | 华北电力科学研究院有限责任公司 | Desulfurization slurry gypsum supersaturation degree and gypsum scaling risk evaluation method and system |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5190978A (en) * | 1975-02-04 | 1976-08-10 | HAIGASUCHUNOIOSANKABUTSUOJOKYOSURUHOHO | |
| JPS5488870A (en) * | 1978-08-08 | 1979-07-14 | Idemitsu Kosan Co Ltd | Wet exhaust gas desulfurization method |
| CA1310807C (en) * | 1986-05-29 | 1992-12-01 | Roderick Beittel | Method for reduction of sulfur products from flue gases by injection of powdered alkali sorbent at intermediate temperatures |
| JPH03143527A (en) * | 1989-01-10 | 1991-06-19 | Kenichi Nakagawa | Method for desulfurizing waste gas |
| CA2147059C (en) * | 1994-04-28 | 1999-08-10 | John W. College | Magnesium-enhanced sulfur dioxide scrubbing with gypsum formation |
| CA2159521C (en) * | 1994-10-07 | 1999-11-09 | Shigeo Iiyama | Method for desulfurizing exhaust gas |
-
1995
- 1995-08-22 JP JP21329695A patent/JP3751340B2/en not_active Expired - Fee Related
-
1996
- 1996-01-06 TW TW085100125A patent/TW326404B/en active
- 1996-02-09 MY MYPI96000488A patent/MY112500A/en unknown
- 1996-02-12 AU AU44448/96A patent/AU674822B1/en not_active Ceased
- 1996-02-23 KR KR1019960004396A patent/KR0174794B1/en not_active IP Right Cessation
- 1996-02-23 CZ CZ96561A patent/CZ56196A3/en unknown
- 1996-02-28 CN CN96100008A patent/CN1089022C/en not_active Expired - Fee Related
- 1996-02-29 PL PL96313006A patent/PL183184B1/en unknown
Cited By (12)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN1118315C (en) * | 1997-10-06 | 2003-08-20 | 日立造船株式会社 | Process and apparatus for removing acid gases from exhaust gases |
| CN1085088C (en) * | 1998-03-11 | 2002-05-22 | 安乐仁 | Medicine for treating psoriasis |
| CN100335154C (en) * | 2005-09-09 | 2007-09-05 | 清华大学 | Magnesium oxide flue gas desulfurization and outgrowth thick-slurry method oxidation reclaim process |
| CN100404108C (en) * | 2005-12-08 | 2008-07-23 | 六合天融（北京）环保科技有限公司 | Method for preparing desulfurizer magnesium oxide and sulfur dioxide utilizing desulfurizing by-product magnesium sulfite by magnesium process |
| CN100435910C (en) * | 2006-04-10 | 2008-11-26 | 广州市天赐三和环保工程有限公司 | Fume desulfurizing process employing multiple circulation and stable double alkali method and apparatus |
| CN101879406A (en) * | 2010-07-15 | 2010-11-10 | 煤炭工业济南设计研究院有限公司 | Dual-alkali flue gas desulfurization device and desulfurization method |
| CN102172472A (en) * | 2011-02-14 | 2011-09-07 | 彭振超 | Method for smoke desulphurization by utilizing dolomitic lime |
| TWI457167B (en) * | 2011-11-18 | 2014-10-21 | Long Chen Paper Co Ltd | Apparatus for flue gas desulfurization with zero emission using system for recovery and recycle of magnesium hydroxide |
| CN104959012A (en) * | 2015-04-17 | 2015-10-07 | 熊天渝 | Magnesium-calcium based wet method desulphurization system and method for removing sulfur dioxide in flue gas and producing gypsum |
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| CN110605011A (en) * | 2019-08-08 | 2019-12-24 | 陆泳凯 | Magnesium-calcium double-alkali flue gas desulfurization method |
Also Published As
| Publication number | Publication date |
|---|---|
| AU674822B1 (en) | 1997-01-09 |
| MY112500A (en) | 2001-06-30 |
| CN1089022C (en) | 2002-08-14 |
| PL183184B1 (en) | 2002-06-28 |
| KR0174794B1 (en) | 1999-02-18 |
| TW326404B (en) | 1998-02-11 |
| PL313006A1 (en) | 1997-03-03 |
| JPH0957053A (en) | 1997-03-04 |
| CZ56196A3 (en) | 1997-07-16 |
| KR970009863A (en) | 1997-03-27 |
| JP3751340B2 (en) | 2006-03-01 |
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