CA1092521A - Process for treatment of exhaust gas - Google Patents
Process for treatment of exhaust gasInfo
- Publication number
- CA1092521A CA1092521A CA299,476A CA299476A CA1092521A CA 1092521 A CA1092521 A CA 1092521A CA 299476 A CA299476 A CA 299476A CA 1092521 A CA1092521 A CA 1092521A
- Authority
- CA
- Canada
- Prior art keywords
- exhaust gas
- aqueous solution
- dust collector
- spray column
- ppm
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- 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
Landscapes
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Health & Medical Sciences (AREA)
- Biomedical Technology (AREA)
- Environmental & Geological Engineering (AREA)
- Analytical Chemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Treating Waste Gases (AREA)
- Electrostatic Separation (AREA)
- Catalysts (AREA)
Abstract
PROCESS FOR TREATMENT OF EXHAUST GAS
Abstract of the Disclosure Disclosed is a process for the treatment of an exhaust gas containing sulfur oxides and oxygen by the use of the aqueous solution of an alkali metal or alkaline earth metal salt. This process includes bringing the exhaust gas, within a specified spray column, into contact with finely divided droplets of the aqueous solution of the salt prepared to a specific pH value and subsequently introducing the exhaust gas which has undergone the contact into an electric dust collector. In this way, the disadvantageous formation of dithionic acid, which reduces the effectiveness of the process and can itself result in pollution, can be substantially avoided.
Abstract of the Disclosure Disclosed is a process for the treatment of an exhaust gas containing sulfur oxides and oxygen by the use of the aqueous solution of an alkali metal or alkaline earth metal salt. This process includes bringing the exhaust gas, within a specified spray column, into contact with finely divided droplets of the aqueous solution of the salt prepared to a specific pH value and subsequently introducing the exhaust gas which has undergone the contact into an electric dust collector. In this way, the disadvantageous formation of dithionic acid, which reduces the effectiveness of the process and can itself result in pollution, can be substantially avoided.
Description
~09~5Zl This invention relates to a process for reducing pollutants in exhaust gases containing sulfur oxides (herein-after referred to as ''Sox'') and oxygen.
Generally, exhaust gases resulting from the combustion of fuels, such as those discharged from boilers and other combustion devices, con~ain SO and oxygen. Some, if not all, of these exhaust gases contain nitrogen oxides (hereinafter referred to as "NOX") in concentrations in the order of from 100 to a few hundred ppm in addition to SO which axe present in concentrations in the order of from 100 to 3,000 ppm.
Heretofore, the treatment of such exhaust gases has been carried out by a process which includes bringing the exhaust gas into contact with an aqueous solution of an alkali metal or alkaline earth metal sulfite or a slurry of slaked lime, -thereby causing the Sx present in the exhaust gas to be absorbed in the aqueous solution or the slurry and thus freeing ~he exhaust gas from the Sox, for example. Where the exhaust gas is treated by such a so-called wet type process, however, it is not unusual that the Sx survives to a certain degree (50 to 300 ppm, for example) in the treated exhaust gas. More-over, the absorption of Sx gives rise to dithionic acid in the aqueous solution or the slurry. This dithionic acid occurs as a by-product when the Sx are absorbed in the form of sulfites by the aqueous solution or the slurry and the sulfites are ~-then oxidized into sulfates by the oxygen present in the exhaust gas. The dithionic acid thus formed accumulates in the form of ~ -a salt in the aqueous solution or the slurry and consequently -brings about a decline in the concentration of the component which functions effectively in the removal of Sox. When the aqueous solution or the slurry containing this dithionic acid ; is discarded in its unaltered form into a nearby body of ;
~ .
.~
... . . . .
-.
~09Z521 natural water, it increases the COD (chemical oxygen demand) value, possibly inducing a serious issue concerning the disposal of effluent.
In ~he meantime, the NOX present in the exhaust gas is removed by bringing the exhaust gas into contact with ammonia in the presence of a catalyst. This method, however, has a disadvantage that the catalyst employed has its activity reduced by the Sx present in the exhaust gas. In order that the catalytic reaction of NO by ammonia may be stably carried 7 out for a long time, therefore, the exhaust gas to be treated is desirably freed from the Sx in advance of the NOX treatment.
It is known that, of the various sulfur oxides contained in the ~ -exhaust gas, SO3 can be removed to a fairly high extent by means of a wet type electric dust collector. This wet type ~ -electric dust collector, however, is entirely ineffective with respect to SO2. `~
This means that even a gas which has been treated by the electric dust collector cannot avoid poisoning the ~e catalyst used for the NOX reduction. Although the SO2 can be removed substantially completely from the exhaust gas when it is thoroughly brought into contact with an aqueous solution of an alkali salt, the gas-llquid contact effected by ordinary methods requires the use of a large volume of the solution and inevitably entails the production of the dithionic acid.
It is, therefore, an object of the present invention to provide a process for the wet type treatment of an exhaust gas containing Sx and oxygen, which process provides effective , ~ , removal of Sx from the exhaust gas without involving any - secondary production of dithionic acid.
We have made various studies with a view to ~ accomplishing the object described above and have, consequently, ;~ _ 3 _ ~ '' ' ;;, .. . . .. ..
,., ~ , , ~09252~
acquired a knowledge that if the aqueous solution of an alkali metal or alkaline earth metal salt to be used in the wet type treatment of the exhaust gas happens to have heavy metals such as iron, cobalt, nickel and vanadium dissolved in the form of ions therein, these heavy metal ions catalytically function in the by-production of dithionic acid and that, among these heavy metal ions, iron ions have a catalytic activity more than ten times that of any other metal ion in terms of concentration. We have further found that if such heavy metals are present in the aqueous solution of the alkali metal or alkaline earth metal salt, the amount of the heavy metals dissolved (namely, the amount of heavy metal ions) in the aqueous solution, particularly the amount of iron ion in the solution increases and, consequently, the amount of.by- ~ -produced dithionic acid increases in proportion to the pH
status of the aqueous solution as it shifts from neutrality -to acidity. We have additionally learned that the occurrence of this dithionic acid can be repressed where the oxidation, which occurs in the conversion into sulfates of the sulfites 20 produced in consequence of the absorption of the Sx present - :-in the exhaust gas by the aqueous solution of an alkali metal or alkaline earth metal salt, is caused to proceed rapidly. ~:
On the basis of these findings described above, we have made an experiment on a process including the steps of introducing the exhaust gas into a spray column of which at least the part destined for direct contact with the aqueous solution of said salt is made of a non-metallic material, ; bringing the introduced exhaust gas within the column into contact with finely di~ided droplets of the aqueous solution of said salt having a pH value of not less than 8, and then introducing the exhaust gas thus contacted into an electric . - - : ~ , -`-` 109252~
dust collector thereby removing the finely diYided particles contained in the exhaust gas. As a result, we have found that this process provides effective removal of Sx from the exhaust ~;
gas while entailing substantially no by-production of dithionic acid. The present invention has been accomplished on the basis of this finding.
Thus, according to the present invention, there is provided a process for the treatment of an exhaust gas con-taining sulfur oxides and oxygen by bringing said exhaust gas :
into contact with the aqueous solution of an alkali metal oralkaline earth metal salt thereby causing the sulfur oxides present in said exhaust gas to be absorbed by said aqueous solution, which process comprises the steps of: (a) introducing said exhaust gas into a spray column of which at least the part destined for direct contact with the aqueous solution is made of a non-metallic material; (b) bringing the introduced exhaust gas, within said spray column, into contact with finely divided droplets of the aqueous solution of an alkali metal or alkaline earth metal salt having a pH value of not less than 8;
and (c) introducing the exhaust gas thus contacted into an " ~ .
electric dust collector thereby removing from said exhaust gas the finely divided particles contained in said.exhaust gas.
The accompanying drawing is a graph showing the : relationship between the pH value of the aqueous solution of ~ sodium sulfite containing dissolved iron (namely, iron ions) ., in various concentrations and the amount of sodium dithionate produced in the aqueous solution, when determined by blowing - air into the aqueous solution.
The present invention is initiated by the step of introducing an exhaust gas containing Sx and oxygen into a spray column of which at least the part destined for direct '~ , ,~ .. - . . . . . , : . . .
.: . . .
~ 1092S21 contact with the aqueous solution of an alkali salt is made of a non-metallic material and bringing the introduced exhaust gas, within the spray column, into contact with finely divided droplets of the aqueous solution of an alkali salt having a pH
value of not less than 8.
The spray column to be used in this case is required to be of a type such that, for the purpose of repressing the possible occurrence of dithionic acid in the aqueous solution of the alkali salt, at least the part of the inner wall of the column destined for direct contact with the aqueous solution is made of a non-metallic material. A spray column of which the inner wall is coated with a synthetic resin typifies the -spray columns which fulfil the requirement described above. ~`
Where there is used a spray column of which at least the part - destined for direct contact with the solution is not made of a non-metallic material, namely a spray column which is wholly made of iron, for example, the iron is gradually dissolved out in the form of ions into the aqueous solution of an alkali salt and the iron ions inevitably accelerate the formation of dithionic acid in the solution.
For use in the present invention, the aqueous solution of an alkali salt may be any one of those which have won general acceptance for application to the wet type treatment of exhaust gases according to the prior art. Specifically, these are aqueous solutions of at least one salt selected from the group consisting of the salts of alkali metals and those of alkaline earth metals. The aqueous solution of sodium ; hydroxide and that of sodium carbonate are preferred choices.
In order that the amount of heavy metals dissolved in this aqueous solution of an alkali salt may be prevented from increasing, the aqueous solution is required to have its pH
';~ ' ~
,. . ,. .. . ., ,.. . . . - :
~ lO9ZS21 value maintained above the level of 8. Further to ensure acceleration of the oxidation into sulfates of those sulfites which have been formed by the absorption of SOx, the aqeuous solution is required to be in the form of finely divided droplets. The heavy metals which are present in the aqueous solution of an alkali salt possibly have originated in those heavy metals entrained by the exhaust gas subjected to the treatment and those heavy metals contained as impurities from the beginning in the alkali salt. The aqueous solution of an alkali salt is preferred to avoid containing iron therein in an amount more than 1 ppm as dissolved iron. The adjustment ~. .
- ~, .~ - .
~. ~
.. .~
:, .
.' ' ~
- ' -, :, , ~: :
: :~
., .
_ 7 _ 10~ZS21 !
ll il of the pH value of the a~ueous solution of an alkali salt may ¦ be carried out by an ordinary manner.
The finely divided droplets of the aqueous solution of ¦ an alkali salt can be obtained by spurting the aqueous solution !¦ through an ultrasonic nozzle or two-fluid nozzle. This spurting is carried out by forcing out a compressed gas (e.g. an jl oxygen-containing gas, steam) through the nozzle in conjunction with the aqueous solution of an alkali salt. Upon departure fro~
I the nozzle, the aqueous solution of an alkali salt now in the form ¦ of finely divided droplets is thoroughly mixed with the exhaust ¦ gas within the spray column, then suspended in the exhaust gas ¦ and allowed to absorb the SO2 efficiently, with the result that - ¦¦ the absorbed SO2 is oxidized, with added rapidity due to the ¦ increased surface area for contact, by the oxygen present in the ~-! 15 I exhaust gas. For the purpose of increasing the velocity of the absorption of SO2 and that of the oxidation of the absorbed SO2 and improving the operational efficiency of the spray column, the ¦¦ droplets produced by spraying the aqueous solution through the i nozzle are preferred to have as small a particle diameter as ¦ possible. The droplets are notably effective when their particle diameter is below 100 ~, preferably in the range of from 1 to 30 ~, ¦ more preferably from 20 to 30 ~. In consideration of the capacity for collecting these droplets with respect to an electric dust Il collector to be used in the subsequent step of process, the li particle diameter of the droplets is preferred to be greater than _ ~ _ 109Z5Zl 1l 1 :: ~
The aqueous solution of an alkali salt with which the llexhaust gas is brought into contact is required to have a pH value Ijexceeding the level of 8. ~ven after the aqueous solution has 5 1l adsorbed the SO2 from the exhaust gas, it is still preferred to retain its pH value above this level 8. Any drop of the pH
I;value below the level of 7.7 proves unpreferable for the purpose ; of thorough removal of SO2, because at the lowered pH value, ` I there occurs a bisulfite in the aqueous solution which has ¦absorbed the SO2 and this bisulfite eventually comes to hold a :`: ¦partial pressure of SO2. Particularly, even if the droplets have I ~ I a high pH value at the time of their charge through the spray ` ¦¦ nozzle, the pH value of the droplets nevertheless is liable :
l¦ to drop sharply owing to the absorption of SO2 and the subsequent 15 l¦ solution of SO3. The aqueous 501ution of an alkali salt to be used for the spraying in the column, therefore, is preferred to have a sufficient high concentration. Where the exhaust gas subjected to the treatment contains Sx in a concentration of 100 j to 200 ppm, for example, the aqueous solution of an alkali salt ~: 20 ¦ is preferred to have a concentration of from 2 to 10%, specificall~-about: 5~. In this case, the amount of the aqueous solution of ¦ an alkali salt to be used is sufficient in the~range of from 0.5 ¦ to 5 Q/hour, preferably:from 1 to 3 ~/hour, per 100 Nm3/hour Of the exhaust gas under treatment. This amount is only several percent, :25 Ij or even.less, of the~amount required by the ordin~ry method of ! ¦
.:':: 1 Ii . I
, .
1l.
lO9Z5Zl Il I
absorption.
, Where the aqueous solution in such a small amount is ¦I sprayed in the form of finely divided droplets, the pressure loss 1i within the spray column is so small as to make it possible to ,~increase the flow rate of the gas to the order of from 5 to 20 jlm/sec, preferably from 8 to 15 m/sec, a range notably large as compared with the range of from 2 to 3 m/sec which is usual with ¦ the ordinary absorption column. Better still, in this increased ¦range, any slight variation in the flow rate has no appreciable effect on the overall operation. At such a flow rate of the gas, a retention time of about 0.1 second suffices for desirable absorption of SO2.
The part of the spray column which is not destined for direct contact with those droplets of the aqueous solution which have already come into contact with SO2 may be made of ¦ an iron type material because the pH value of the aqueous solution is 8 or over. Practically it is, therefore, permissible ¦for the spray nozzle of the column to be made of stainless steel, ¦ -¦ for example.
As desoribed above,-the present invention accomplished the absorption of SO2 from the exhaust gas by bringing the exhaust ¦ gas into ~ontact with the droplets of the aqueous solution of an alkali salt of a concentration of 1 to 100 ~ by means of a 1¦ spray column. This operation not merely permits the absorption of SO2 and the oxidation of the absorbed SO2 to proceed at-a-' ~' ; ' , il /_ i'' lO9Z5Zl ll ¦ very high efficiency but also offers a notably high economici advantage in other respects as compared with the operation to be il performed by means of an ordinary packed column, plate column, i wetted-wall column or Venturi scrubber. For example, the spray ¦ column of the present invention involves a pressure loss of the order of from lO to 20 mm.aq, whereas the operation using a i Venturi scrubber involves a pressure Ioss about lO times as great.
¦¦ Compared with the operations using other absorption devices, the ; 1¦ amount of absorption per unit inner volume of absorption column obtained by this operation is decisively large and the amount of the absorbent to be consumed by the present operation is very small on the order of a minus third power of 10. Consequently, the present operation enjoys an advantage that~the spray column is small in size and the effluent is also smali in volume.
! :~ 15 Further, the operation using the spray column enjoys high . efficiency of gas-li~uid contact and high veloaity of absorption, ~ I only has a very short reaction time of the order of O.l second j I and-entails a small pressure loss and, therefore, permits the exhaust gas to be fed in at a high rate of flow. A small column height, therefore, sufficies for the absorptlon column to function effectively in the operation. It follows that the spray colum-can readily be improvised out of a duct or some other similar utensil available on hand and there is no need for particular installation of any regular absorption column. In this case, the ¦ spray column is preferred to be disposed in a vertical arrangement.
.
.
~ j .
109Z5Zl ` ~ I
¦¦ If it is disposed horizontally, it is still capable of thoroughly absorbing SO2 from the exhaust gas, because the droplets of the ! aqueous solution are so fine that most of them can be carried alonc ¦¦ in the current of the exhaust gas through the interior column.
5 ¦¦ Subsequently, in the present invention, the exhaust gas ¦I whose SO2 content has been lowered below 1 ppm, preferably below ¦¦ 0.1 ppm owing to the treatment described above is introduced into an electric dust collector, wherein the exhaust gas is freed from the fine particles (such as, for example, SO3 mist and the droplets of aqueous solution entrained by the exhaust gas) contained therein. Even after the exhaust gas has entered the electric dust collector, the absorption of SO2 and the oxidation of the absorbed SO2 continue to proceed. In the gas -i emanating from the electric dust collector, the Sx (S2 and SO3 combined) content is below 1 ppm. Preferably, a mist , arrester may be interposed between the spray column and the ,1 electric dust collector and operated so as to remove from the ¦I current of the exhaust gas those floating droplets capable of being removed by a mechanical means, namely, those floating droplets having diameters exceeding the level of 30 ~, preferably of I00 ~ and, consequantly, lower the frequency with which short circuits are possibly formed between the opposite poles of the ~1 ~ electric dust collector and alleviate the load exerted upon the dust collector.
Through the treatments described above, the Sx content . , .
.~ 1 '' .
: I
lO9Z5Zl of the exhaust gas can be lowered to below the level of 0.1 ppm.
At the time of the absorption of SOx, there is a possibility I that part of the absorbed Sx will be converted into dithionic ¦l acid. As already described, dithionic acid is a substance 1! which adds to the COD value of the effluent and rejects decompositon or detoxication to the utmost extent, possibly I giving rise to a serious problem concerning the disposal of ¦¦ effluent. Thus, formation of this by-product must be avoided ¦I by all means.
1 With a view to repressing the formation of dithionic acid, I the present invention imposes two essential conditions; one ¦ ¦ being that the pH value of the aqueous solution used for the ~
. .
absorption of Sx should be maintained above the level of 8 and the other being that at least the part of the spray column which is destined for direct contact with the aqueous solution should be made of a non-metallic material such as, for example, ' I fiber-reinforced plastic ~hereinafter referred to as "FRP") ; 1¦ or synthetic resin rubber so as to preclude the possibility of any metal ion, particularly iron ion, being dissolved out 20 ¦¦ of the inner wall of the spray column. If the absorption of
Generally, exhaust gases resulting from the combustion of fuels, such as those discharged from boilers and other combustion devices, con~ain SO and oxygen. Some, if not all, of these exhaust gases contain nitrogen oxides (hereinafter referred to as "NOX") in concentrations in the order of from 100 to a few hundred ppm in addition to SO which axe present in concentrations in the order of from 100 to 3,000 ppm.
Heretofore, the treatment of such exhaust gases has been carried out by a process which includes bringing the exhaust gas into contact with an aqueous solution of an alkali metal or alkaline earth metal sulfite or a slurry of slaked lime, -thereby causing the Sx present in the exhaust gas to be absorbed in the aqueous solution or the slurry and thus freeing ~he exhaust gas from the Sox, for example. Where the exhaust gas is treated by such a so-called wet type process, however, it is not unusual that the Sx survives to a certain degree (50 to 300 ppm, for example) in the treated exhaust gas. More-over, the absorption of Sx gives rise to dithionic acid in the aqueous solution or the slurry. This dithionic acid occurs as a by-product when the Sx are absorbed in the form of sulfites by the aqueous solution or the slurry and the sulfites are ~-then oxidized into sulfates by the oxygen present in the exhaust gas. The dithionic acid thus formed accumulates in the form of ~ -a salt in the aqueous solution or the slurry and consequently -brings about a decline in the concentration of the component which functions effectively in the removal of Sox. When the aqueous solution or the slurry containing this dithionic acid ; is discarded in its unaltered form into a nearby body of ;
~ .
.~
... . . . .
-.
~09Z521 natural water, it increases the COD (chemical oxygen demand) value, possibly inducing a serious issue concerning the disposal of effluent.
In ~he meantime, the NOX present in the exhaust gas is removed by bringing the exhaust gas into contact with ammonia in the presence of a catalyst. This method, however, has a disadvantage that the catalyst employed has its activity reduced by the Sx present in the exhaust gas. In order that the catalytic reaction of NO by ammonia may be stably carried 7 out for a long time, therefore, the exhaust gas to be treated is desirably freed from the Sx in advance of the NOX treatment.
It is known that, of the various sulfur oxides contained in the ~ -exhaust gas, SO3 can be removed to a fairly high extent by means of a wet type electric dust collector. This wet type ~ -electric dust collector, however, is entirely ineffective with respect to SO2. `~
This means that even a gas which has been treated by the electric dust collector cannot avoid poisoning the ~e catalyst used for the NOX reduction. Although the SO2 can be removed substantially completely from the exhaust gas when it is thoroughly brought into contact with an aqueous solution of an alkali salt, the gas-llquid contact effected by ordinary methods requires the use of a large volume of the solution and inevitably entails the production of the dithionic acid.
It is, therefore, an object of the present invention to provide a process for the wet type treatment of an exhaust gas containing Sx and oxygen, which process provides effective , ~ , removal of Sx from the exhaust gas without involving any - secondary production of dithionic acid.
We have made various studies with a view to ~ accomplishing the object described above and have, consequently, ;~ _ 3 _ ~ '' ' ;;, .. . . .. ..
,., ~ , , ~09252~
acquired a knowledge that if the aqueous solution of an alkali metal or alkaline earth metal salt to be used in the wet type treatment of the exhaust gas happens to have heavy metals such as iron, cobalt, nickel and vanadium dissolved in the form of ions therein, these heavy metal ions catalytically function in the by-production of dithionic acid and that, among these heavy metal ions, iron ions have a catalytic activity more than ten times that of any other metal ion in terms of concentration. We have further found that if such heavy metals are present in the aqueous solution of the alkali metal or alkaline earth metal salt, the amount of the heavy metals dissolved (namely, the amount of heavy metal ions) in the aqueous solution, particularly the amount of iron ion in the solution increases and, consequently, the amount of.by- ~ -produced dithionic acid increases in proportion to the pH
status of the aqueous solution as it shifts from neutrality -to acidity. We have additionally learned that the occurrence of this dithionic acid can be repressed where the oxidation, which occurs in the conversion into sulfates of the sulfites 20 produced in consequence of the absorption of the Sx present - :-in the exhaust gas by the aqueous solution of an alkali metal or alkaline earth metal salt, is caused to proceed rapidly. ~:
On the basis of these findings described above, we have made an experiment on a process including the steps of introducing the exhaust gas into a spray column of which at least the part destined for direct contact with the aqueous solution of said salt is made of a non-metallic material, ; bringing the introduced exhaust gas within the column into contact with finely di~ided droplets of the aqueous solution of said salt having a pH value of not less than 8, and then introducing the exhaust gas thus contacted into an electric . - - : ~ , -`-` 109252~
dust collector thereby removing the finely diYided particles contained in the exhaust gas. As a result, we have found that this process provides effective removal of Sx from the exhaust ~;
gas while entailing substantially no by-production of dithionic acid. The present invention has been accomplished on the basis of this finding.
Thus, according to the present invention, there is provided a process for the treatment of an exhaust gas con-taining sulfur oxides and oxygen by bringing said exhaust gas :
into contact with the aqueous solution of an alkali metal oralkaline earth metal salt thereby causing the sulfur oxides present in said exhaust gas to be absorbed by said aqueous solution, which process comprises the steps of: (a) introducing said exhaust gas into a spray column of which at least the part destined for direct contact with the aqueous solution is made of a non-metallic material; (b) bringing the introduced exhaust gas, within said spray column, into contact with finely divided droplets of the aqueous solution of an alkali metal or alkaline earth metal salt having a pH value of not less than 8;
and (c) introducing the exhaust gas thus contacted into an " ~ .
electric dust collector thereby removing from said exhaust gas the finely divided particles contained in said.exhaust gas.
The accompanying drawing is a graph showing the : relationship between the pH value of the aqueous solution of ~ sodium sulfite containing dissolved iron (namely, iron ions) ., in various concentrations and the amount of sodium dithionate produced in the aqueous solution, when determined by blowing - air into the aqueous solution.
The present invention is initiated by the step of introducing an exhaust gas containing Sx and oxygen into a spray column of which at least the part destined for direct '~ , ,~ .. - . . . . . , : . . .
.: . . .
~ 1092S21 contact with the aqueous solution of an alkali salt is made of a non-metallic material and bringing the introduced exhaust gas, within the spray column, into contact with finely divided droplets of the aqueous solution of an alkali salt having a pH
value of not less than 8.
The spray column to be used in this case is required to be of a type such that, for the purpose of repressing the possible occurrence of dithionic acid in the aqueous solution of the alkali salt, at least the part of the inner wall of the column destined for direct contact with the aqueous solution is made of a non-metallic material. A spray column of which the inner wall is coated with a synthetic resin typifies the -spray columns which fulfil the requirement described above. ~`
Where there is used a spray column of which at least the part - destined for direct contact with the solution is not made of a non-metallic material, namely a spray column which is wholly made of iron, for example, the iron is gradually dissolved out in the form of ions into the aqueous solution of an alkali salt and the iron ions inevitably accelerate the formation of dithionic acid in the solution.
For use in the present invention, the aqueous solution of an alkali salt may be any one of those which have won general acceptance for application to the wet type treatment of exhaust gases according to the prior art. Specifically, these are aqueous solutions of at least one salt selected from the group consisting of the salts of alkali metals and those of alkaline earth metals. The aqueous solution of sodium ; hydroxide and that of sodium carbonate are preferred choices.
In order that the amount of heavy metals dissolved in this aqueous solution of an alkali salt may be prevented from increasing, the aqueous solution is required to have its pH
';~ ' ~
,. . ,. .. . ., ,.. . . . - :
~ lO9ZS21 value maintained above the level of 8. Further to ensure acceleration of the oxidation into sulfates of those sulfites which have been formed by the absorption of SOx, the aqeuous solution is required to be in the form of finely divided droplets. The heavy metals which are present in the aqueous solution of an alkali salt possibly have originated in those heavy metals entrained by the exhaust gas subjected to the treatment and those heavy metals contained as impurities from the beginning in the alkali salt. The aqueous solution of an alkali salt is preferred to avoid containing iron therein in an amount more than 1 ppm as dissolved iron. The adjustment ~. .
- ~, .~ - .
~. ~
.. .~
:, .
.' ' ~
- ' -, :, , ~: :
: :~
., .
_ 7 _ 10~ZS21 !
ll il of the pH value of the a~ueous solution of an alkali salt may ¦ be carried out by an ordinary manner.
The finely divided droplets of the aqueous solution of ¦ an alkali salt can be obtained by spurting the aqueous solution !¦ through an ultrasonic nozzle or two-fluid nozzle. This spurting is carried out by forcing out a compressed gas (e.g. an jl oxygen-containing gas, steam) through the nozzle in conjunction with the aqueous solution of an alkali salt. Upon departure fro~
I the nozzle, the aqueous solution of an alkali salt now in the form ¦ of finely divided droplets is thoroughly mixed with the exhaust ¦ gas within the spray column, then suspended in the exhaust gas ¦ and allowed to absorb the SO2 efficiently, with the result that - ¦¦ the absorbed SO2 is oxidized, with added rapidity due to the ¦ increased surface area for contact, by the oxygen present in the ~-! 15 I exhaust gas. For the purpose of increasing the velocity of the absorption of SO2 and that of the oxidation of the absorbed SO2 and improving the operational efficiency of the spray column, the ¦¦ droplets produced by spraying the aqueous solution through the i nozzle are preferred to have as small a particle diameter as ¦ possible. The droplets are notably effective when their particle diameter is below 100 ~, preferably in the range of from 1 to 30 ~, ¦ more preferably from 20 to 30 ~. In consideration of the capacity for collecting these droplets with respect to an electric dust Il collector to be used in the subsequent step of process, the li particle diameter of the droplets is preferred to be greater than _ ~ _ 109Z5Zl 1l 1 :: ~
The aqueous solution of an alkali salt with which the llexhaust gas is brought into contact is required to have a pH value Ijexceeding the level of 8. ~ven after the aqueous solution has 5 1l adsorbed the SO2 from the exhaust gas, it is still preferred to retain its pH value above this level 8. Any drop of the pH
I;value below the level of 7.7 proves unpreferable for the purpose ; of thorough removal of SO2, because at the lowered pH value, ` I there occurs a bisulfite in the aqueous solution which has ¦absorbed the SO2 and this bisulfite eventually comes to hold a :`: ¦partial pressure of SO2. Particularly, even if the droplets have I ~ I a high pH value at the time of their charge through the spray ` ¦¦ nozzle, the pH value of the droplets nevertheless is liable :
l¦ to drop sharply owing to the absorption of SO2 and the subsequent 15 l¦ solution of SO3. The aqueous 501ution of an alkali salt to be used for the spraying in the column, therefore, is preferred to have a sufficient high concentration. Where the exhaust gas subjected to the treatment contains Sx in a concentration of 100 j to 200 ppm, for example, the aqueous solution of an alkali salt ~: 20 ¦ is preferred to have a concentration of from 2 to 10%, specificall~-about: 5~. In this case, the amount of the aqueous solution of ¦ an alkali salt to be used is sufficient in the~range of from 0.5 ¦ to 5 Q/hour, preferably:from 1 to 3 ~/hour, per 100 Nm3/hour Of the exhaust gas under treatment. This amount is only several percent, :25 Ij or even.less, of the~amount required by the ordin~ry method of ! ¦
.:':: 1 Ii . I
, .
1l.
lO9Z5Zl Il I
absorption.
, Where the aqueous solution in such a small amount is ¦I sprayed in the form of finely divided droplets, the pressure loss 1i within the spray column is so small as to make it possible to ,~increase the flow rate of the gas to the order of from 5 to 20 jlm/sec, preferably from 8 to 15 m/sec, a range notably large as compared with the range of from 2 to 3 m/sec which is usual with ¦ the ordinary absorption column. Better still, in this increased ¦range, any slight variation in the flow rate has no appreciable effect on the overall operation. At such a flow rate of the gas, a retention time of about 0.1 second suffices for desirable absorption of SO2.
The part of the spray column which is not destined for direct contact with those droplets of the aqueous solution which have already come into contact with SO2 may be made of ¦ an iron type material because the pH value of the aqueous solution is 8 or over. Practically it is, therefore, permissible ¦for the spray nozzle of the column to be made of stainless steel, ¦ -¦ for example.
As desoribed above,-the present invention accomplished the absorption of SO2 from the exhaust gas by bringing the exhaust ¦ gas into ~ontact with the droplets of the aqueous solution of an alkali salt of a concentration of 1 to 100 ~ by means of a 1¦ spray column. This operation not merely permits the absorption of SO2 and the oxidation of the absorbed SO2 to proceed at-a-' ~' ; ' , il /_ i'' lO9Z5Zl ll ¦ very high efficiency but also offers a notably high economici advantage in other respects as compared with the operation to be il performed by means of an ordinary packed column, plate column, i wetted-wall column or Venturi scrubber. For example, the spray ¦ column of the present invention involves a pressure loss of the order of from lO to 20 mm.aq, whereas the operation using a i Venturi scrubber involves a pressure Ioss about lO times as great.
¦¦ Compared with the operations using other absorption devices, the ; 1¦ amount of absorption per unit inner volume of absorption column obtained by this operation is decisively large and the amount of the absorbent to be consumed by the present operation is very small on the order of a minus third power of 10. Consequently, the present operation enjoys an advantage that~the spray column is small in size and the effluent is also smali in volume.
! :~ 15 Further, the operation using the spray column enjoys high . efficiency of gas-li~uid contact and high veloaity of absorption, ~ I only has a very short reaction time of the order of O.l second j I and-entails a small pressure loss and, therefore, permits the exhaust gas to be fed in at a high rate of flow. A small column height, therefore, sufficies for the absorptlon column to function effectively in the operation. It follows that the spray colum-can readily be improvised out of a duct or some other similar utensil available on hand and there is no need for particular installation of any regular absorption column. In this case, the ¦ spray column is preferred to be disposed in a vertical arrangement.
.
.
~ j .
109Z5Zl ` ~ I
¦¦ If it is disposed horizontally, it is still capable of thoroughly absorbing SO2 from the exhaust gas, because the droplets of the ! aqueous solution are so fine that most of them can be carried alonc ¦¦ in the current of the exhaust gas through the interior column.
5 ¦¦ Subsequently, in the present invention, the exhaust gas ¦I whose SO2 content has been lowered below 1 ppm, preferably below ¦¦ 0.1 ppm owing to the treatment described above is introduced into an electric dust collector, wherein the exhaust gas is freed from the fine particles (such as, for example, SO3 mist and the droplets of aqueous solution entrained by the exhaust gas) contained therein. Even after the exhaust gas has entered the electric dust collector, the absorption of SO2 and the oxidation of the absorbed SO2 continue to proceed. In the gas -i emanating from the electric dust collector, the Sx (S2 and SO3 combined) content is below 1 ppm. Preferably, a mist , arrester may be interposed between the spray column and the ,1 electric dust collector and operated so as to remove from the ¦I current of the exhaust gas those floating droplets capable of being removed by a mechanical means, namely, those floating droplets having diameters exceeding the level of 30 ~, preferably of I00 ~ and, consequantly, lower the frequency with which short circuits are possibly formed between the opposite poles of the ~1 ~ electric dust collector and alleviate the load exerted upon the dust collector.
Through the treatments described above, the Sx content . , .
.~ 1 '' .
: I
lO9Z5Zl of the exhaust gas can be lowered to below the level of 0.1 ppm.
At the time of the absorption of SOx, there is a possibility I that part of the absorbed Sx will be converted into dithionic ¦l acid. As already described, dithionic acid is a substance 1! which adds to the COD value of the effluent and rejects decompositon or detoxication to the utmost extent, possibly I giving rise to a serious problem concerning the disposal of ¦¦ effluent. Thus, formation of this by-product must be avoided ¦I by all means.
1 With a view to repressing the formation of dithionic acid, I the present invention imposes two essential conditions; one ¦ ¦ being that the pH value of the aqueous solution used for the ~
. .
absorption of Sx should be maintained above the level of 8 and the other being that at least the part of the spray column which is destined for direct contact with the aqueous solution should be made of a non-metallic material such as, for example, ' I fiber-reinforced plastic ~hereinafter referred to as "FRP") ; 1¦ or synthetic resin rubber so as to preclude the possibility of any metal ion, particularly iron ion, being dissolved out 20 ¦¦ of the inner wall of the spray column. If the absorption of
2 and the oxidation of sulfite ions into sulfate ions have been completed within the spray column, then there is no possibility of any dithionic acid being formed within the electric dust collector. If the absorption of S02 present in the exhaust gas and/or the oxidation of the absorbed S02 now present in the ' 1i , ' .
1~
:'. ill .
109ZSZl 1 !
droplets has not been completed within the spray column and the exhaust gas and/or the droplets still containing SO2 is suffered I -to enter the electric dust collector, there is a fair possibility Il that dithionic acid will be formed within the electric dust 5 1I collector. It is, therefore, preferable that the part of the electric dust collector destined for direct contact with the Il aqueous solution is formed of a non-metallic material. It is ¦l likewise preferable that the part of the electric dust collector ¦I required to have high electroconductivity is made of a material 1l selected from the group consisting of mixtures of carbonaceous ¦ substances with synthetic resins, carbonaceous substances (inclusive of both carbonic and graphitic substances), lead, titanium and tantalum. From the economic point of view, use of ,¦ a composite material made up of a carbonaceous substance and a I synthetic resin proves to be most advantageous.
jl Where the removal of SO2 from the exhaust gas within the spray column is not sufficient, there is a possibility that the amount of dithionic acid to be formed within the electric ¦ dust collector will increase. When So2 is injected in a 1l concentration of lO0 ppm, by way of model, into the line laid ¦~ to interconnect the spray column and the electric dust collector, formation of dithionic acid definitely ensues within the electric dust collector, no matter whether the part of the ¦¦ electric dust collector destined for direct contact with ; 25 the aqu~ous solution may be made of a non-metallic material. The ~,' "
. Il I
i, 1 li 109Z5Zl ,, production of dithionic acid is ascribable to the heavy metal - ~I components such as Fe, V and Ni which are collected within the elec~ric dust collector. Where the collection of SO2 and/or Il the oxidation of the collected SO2 has not been thoroughly I completéd within the spray column, there is a possibility ,i that dithionic acid will occur within the electric dust collector. Of course, the amount of dithionic acid thus produced will decrease with the decreasing amount of SO2 I which is suffered to enter the electric dust collector. It ll has been observed that the ratio of the amount of dithionic ¦ acid formed to the amount fo SO2 injected into the intervening ¦ lead line sharply decreases as the SO2 content decreases to j a certain extent and that the amount of dithionic acid produced greatly decreases especially when the SO2 content of the exhaust 1 gas at the outlet of the electric dust collector is lower . ~ i, ,¦ than 1 ppm. For the purpose of repressing possible formation - l¦ of dithionic acid due to the heavy metals originating in the exhaust gas, therefore, it islpreferable that the absorption of ! S2 and/or the oxidation of the absorbed SO2 is thoroughly ~20 ~j! completed before the exhaust gas entraining the droplets enters I I the electric dust collector. When this i~ fulfilled, formation !
¦ of dlthionic acid can no longer be observed within the electric ~ dust~collector. Since-there~is undeniably a possibility that ; l, the exhaust gas will be led into the electric dust collector -~1 25 I before the absorption of SO2 and/or the oxidation of the absorbed `'' 11 ," ~
'''~ `~' 1' - a~-, , .
. , 1, lO9Z5Zl I
~¦ S2 has been completed, the amount of the aqueous solution of an alkali salt which is drifted along in the current of the exhaust ¦gas from the spray column into the dust collector is preferred to I, be enough to provide thorough neutralization of SO3 and at least 1! avoid shifting the overall pH level toward the acidic side. This ¦ condition is preferred to be filled for the reaction system within ¦¦the spray column. If this condition is satisfied within the ~ electric dust collectorl it means that the condition is fully ; met also within the spray column.
When the reaction time within the spray column is fixed abo~e the level of 0.1 second, preferably 0.2 second, the absorption of SO2 and the oxidation of sulfite ions produced in ~-consequence of the absorption are completed within the spray column In this case, therefore, the part of the electric dust collector 1, 15 destined for direct contact With the aqueou5 solution may be not ¦¦formed of a non-metallic material.
¦ The solution which collects in the bottom of the spray column may be replenished with an alkali salt and put to cyclic ¦luse, while at least a part of this solution is forwarded to the -20 ¦ liquid collection unit of the electric dust collector and : ¦ subjected to a neutralizing treatment. ~enerally, the exhaust i gas from any ordinary boiler contains SO3 approximately in an '~ amount of from 20 to 50 ppm where the exhaust gas happens to be rich in SOx. The greater part of the alkali salt is used up in absorbing SO2 in the spray column and the solutlon collecting in j lO9Z5Zl the bottom of the electric dust collector assumes acidity. Thus, ' the solution from this electric dust collector is combined with the solution collecting in the bottom of the spray column, so j¦ that the resultant mixture has a neutral to alkaline pH value 5 ~! when taken out.
il Although the possibility of the oxidation of sulfites ! inside the electric dust collector is far less than in the absorption column, the droplets of the aqueous solution within the ele~tric dust collector tend to assume an acidic pH value.
It is, therefore, advantageous that the droplets is caused to be drifted along in the current of the exhaust gas from the spray column into the electric dust collector, there to be used for neutralizing the collected SO3. It is preferable that for the saXe of precautions, the absorbate collecting in the bottom of l the spray column should be injected into the bottom of the electri dust collector. It is important that the solution discharged from ! the bottom is not allowed to be acidic at least when it is dischar I ed from the bottom.
In the case of an exhaust gas which contains NOX in addition to Sx and oxygen, the exhaust gas is first treated as . described above to be freed substantially from Sx and the exhaust gas still containing NOX is then passed, in conjunction with I ammonia, through a catalyst bed. By this operation, the reduction of NOx can be continued for a long time without entailing any :: ; , ,,., , 11 109Z5Zl degradation in the activity of the catalyst. For the purpose f¦ of this catalytic reduction of NOX, there can be used any of the I various catalysts for NOx-reduction which have been known to ¦l the art. The catalytic reduction of NOX can be carried out at a I temperature in the wide range of from lO0 to 500C. ~rom the viewpoint of heat economy, however, the catalytic reduction is ¦~ preferred to proceed at the lowest possible temperature. For I this reason, it is preferable that the reaction is carried out in ¦ the presence of a catalyst which manifests its activity at a relatively low temperature such as, for example, a catalyst ¦ made up of manganese oxides alone or in conjunction with Fe, ¦~ Ni, Co, V, W, Cr, Cu, Sn, Ti and Zn at a low temperature in ¦ the range of from 100 to 220C. Among the various manganese 11 oxides available, manganese carbonate and/or calcined rhodochrosit ! is the-best choice as the catalyst.
The known catalysts such as those using noble metals like ¦I Pt, Ru and Rh, those using Co as a basic component, those made predominantly of activated carbon and those consisting of ¦ activated carbon coated with various metal oxides and with ammonium salts can be adapted for the reduction of NOX on condition ¦ that the exhaust gas has been treated so as to decrease its Sx ¦ content to an extremely low level. ~
¦I The present invention can be practiced even on an exhaust 1I gas which contains Sx in an amount exceeding 1000 ppm.
109Z5Zl Preferably, however, the present invention may be applied to the exhaust gas whose Sx content is not more than 500 ppm, more ¦preferably not more than about 100 ppm. The removal of NOx from llthe exhaust gas may be advantageously accomplished by preliminarily ¦I subjecting the exhaust gas to the treatment of SOx-removal so as to have its Sx content lowered below about 100 ppm.
As described above, *he present invention is directed to la process which includes the gas-liquid contact reaction performed ¦in the spray column for ~he absorption of S02 and the removal of ¦S03 mist performed in the electric dust collector.
¦ Since the absorption and oxidation effected by means of ¦finely divided droplets of the aqueous solution are completed ¦lin a very short time, the possibility of the formation of ¦ dithionic acid is notably decreased. Moreover, the possibility llthat iron and other heavy metals will be dissolved out of the l ~linner wall and bottom of the spray column can be substantially ¦Iprevented by proper adjustment of the pH value of the aqueous solution and proper selection of the material for the column.
!
20 1l Thus, the present invention eliminates the difficult problem . ~¦otherwise possibly involved in the disposal of dithionic acid ¦¦and, at the same time, offers a perfect solution to the problem ¦encountered in removing the N~x at a low temperature from ~; ¦ the exhaust gas.
25 1~ Now, the present invention will be described more .' 11 .
: I
.
109Z~iZl I¦ specifically herein below with reference to working examples of ¦¦ the invention. It should be noted that the present invention is ¦I not limited in any way to these working examples.
¦¦ Example 1:
5 ~¦ The exhaust gas from a boiler using as its fuel a high-¦ sulfur hèavy oil containing 2.8% by weight of sulfur was subjected to a wet type treatment for SOx-removal by using the sodium sulfite, to afford an exhaust gas containing 44 ppm of SO2, 28 ppm of SO3, 175 ppm of NOX, 4.5% of 2~ 66 mg/Nm3 of soot and about 15% of water and having a temperature of 58C.
This exhaust gas was fed downwardly into a cylindrical duct of FRP about 80 cm in diameter at a flow rate of 10,000 Nm3/
hour ~with the flow rate of the gas fixed at about 7 m/sec.).
Simultaneously and parallelly with the flow of the exhaust gas, an aqueous caustic soda solution of pH ll was spurted into ~7 Ithe duct interior through an ultrasonic nozzle at a rate of 150 Q/hour to produce finely divided droplets about 1 to 30 ¦¦in particle diameter.
The exhaust gas departing from the duct was led into another duct disposed horizontally at a distance of about 4.5m from the nozzle, then sent through a mist arrester and led into an electroc dust collector. With the gas-liquid contact effected in a very short timé of about 0.6 second, the absorption and oxLdation of SO2 were substantially complete. In the gas :.~ -' .
.' 1i ', ^' . I
¦~emanating from the mist arrester, the SO2 content was found to have been lowered to the order of 0.01 to 0.02 ppm. While the ¦igreater part of the sprayed droplets was collected in the bottom ¦lof the FRP duct, about one third of the droplets (particularly lilthose of relatively small particle diameters) were entrained by ¦Ithe current of exhaust gas into the electric dust collector.
il The electric dust collector having an interpole space ¦of 100 mm was operated at 40 KV and 32 m~ to free the exhaust gas from SO3, sootand sprayed droplets. The defiling matter thus removed was allowed to flow down the collector interior and was collected in the bottom of the collector.
I After the treatment given in the electric dust collector, ¦the exhaust gas was found to contain less than 0.01 ppm of SO2 and less than 0.02 ppm of SO3 and an amount of soot incapable !f detection by the method specified by JIS (Japanese Industrial IStandard) and had a temperature of 56C.
¦~ The absorbate which had been collected in the bottom ¦¦of the FRP duct was found to contain by-produced Glauber's salt ¦lin addition to caustic soda and was confirmed to contain no l¦discernible amount of sodium bisulfite, sodium sulfite or ¦jdithionic acid. The pH value of the absorbate was about 8.8 and the Na2S2O6/Na2SO4 ratio was less than 1/10000. The jabsorbate in the bottom of the spray duct was wholly transferred Ilinto the bottom of the electric dust collector, neutralized I with the SO3 collected by the dust collector. The neutralized Il, i! ~
~ D /
~ 1' ",,',' ' ' ~.
I! !
lO!J;~5Zl I
.
effluent wa= fed to the side of the main SOx-removing unit of the sodium sulfite process. The effluent was found to contain dithionic acid, a substance liable to increase the COD
j value of the final effluent, less than l/10000 in terms of ¦~ the Na2S26/Na2S4 molar ratio-In this case, the spray duct and the mist arrester were wholly made of a synthetic resin. In the electric dust collector, the main body was made of FRP, the dust collector unit and other parts required to have high electroconductivity were formed of an anticorrosive unsaturated polyester and carbonaceous fibers and other parts required to have particularly high electroconductivity were made of titanium material.
The exhaust gas which had been treated as described above was heated and then fed at a space velocity (SV) of 7,500 hr l, i lS in conjunction with NH3 in a concentration of 170 ppm, to a catalyst bed packed with cylindrical catalysts 2mm in diameter ¦and 15 mm in length and prepared from what had been obtained by baking rhodochrosite at 400C, so as to induce a catalytic reaction at 160C. Consequently, the NOX content fell to i 20 2 ppm, indicating that the NOx-removal was effected at a rate of 98.8%. ~
¦ The operation was continued for a period of 850 hours.
¦During this operation, the rate of NOx-removal and the pressure ¦¦loss in the catalyst bed were found to involve variations ¦ which invariably fell within the ranges of allowable errors.
I, .. I I . . ~ .. ..
10925Zl 1' ~ he gas which had undergone the ~Ox-removing treatment and was released into the atmopshere was found to contain Il extremely small amounts of harmful substances, i.e. not more jl than 0.~3 ppm of SOx, 2 ppm of NOx and less than 1 mg/Nm3 of ¦ soot. Tt has, thus, been ascertained that, despite the use of a fuel of very inferior quality, the rate of air pollution by the gas resulting from the treatment by this invention was extremely small.
Example 2:
The same exhaust gas as used in Example 1 was fed at a flow rate of 300 Nm3/hour into a spray column of transparent PVC (Polyvinyl chloride) pipe about 10 cm in diameter and 2.3 m in length so that it flowed at a velocity of about 11 Nm/sec inside the column and, through the two ultrasonic nozzles disposed }5 one at the gas inlet of the duct and the other at a point 1 m belo~
¦ the gas inlet, the absorbent was sprayed downwardly in the same direction as that of the current of exhaust gas to free the gas ¦¦ from its defiling matter.
An aqueous sodium carbonate solution of about pH 12 was ~0 used as the absorbent. Into the raw exhaust gas containing 44 ppm of S02, this absorbent was sprayed through the lower nozzle at a rate of 5 ~/hour so that most of the droplets ¦ resulting from the spraying would have particle diameters in the range of from 1 to 30 ~. Most of the droplets were mixed in the form of mist with the current of exhaust gas and entrained .
~' .
~ ~ ~3 1~ 1 lO9Z5Zl by the current. Under the conditions described above, the gas emanating from the outlet of the electric dust collector was found to contain less than 0.1 ppm of SO2, less than 0.1 ppm of SO3 and less than 1 mg/Nm3 of soot.
5 I Via the line leading to the gas inlet of the spray column, !, SO2 was added to the raw exhaust gas in amounts such as to give-l SO2 concentrations of 50, 100 and 150 ppm. In all the test runs, the exhaust gas at the outlet was found to contain SO3 1 and soot in the same amounts as above, except the SO2 content 1, rose to 0.8 ppm in the case of the test run using the SO2 1l concentration of 150 ppm.
; ¦ At this time, the same absorbent as used above was sprayed llthrough the upper nozzle at a rate of 5 Q/hour. In all the test ¦,runs involving the SO2 concentrations of 150, 200 and 300 ppm, ! ~ 15 ¦I the SO2 content in the exhaust gas at the outlet was less than ,lO.1 ppm.
In another test run in which the SO2 concentration was increased to 500 ppm, the SO2 content in the exhaust gas at the outlet rose to 1.2 ppm. So, the feed rate of the absorbent l¦through the upper nozzle was increased to 8 Q/hour. Then, the ~S2 content of the exhaust gas at the outlet fell below 0.1 ppm ~in the test run involving the SO2 concentration of 500 ppm and ~ below 0.5 ppm in the other test run with the SO2 concentration ;' I of 1000 ppm. When the feed rate of the absorbent was further lincreased to 12 Q/hour, the SO2 content of the exhaust gas at ' 11 ~., ,, , !, I
lO9Z5Zl the outlet did not exceed 1 ppm in any of test runs using SO2 ¦ concentrations of up to 2500 ppm. Under these conditions, the SO3 content and the soot content continued to be below 0.1 ppm and Il 1 mg/Nm3~respectively in the exhaust gas at the outlet.
I The gas was further sent through a mist arrester made of ,~ a synthetic fiber and then led into a vertically disposed, I cylindrical electric dust collector which was lined with a ¦I synthetic resin applied by the flake-lining method. The ¦¦ discharge pole was made of a carbonaceous material. The ¦ collector pole was designed so that the electroconductive coat deposited on the wall surface thereof was protected against harm by having a film of water formed on the coat. -The suspenders for the poles were made of channels of¦stainless steel (of grade SUS-304) coated with an electro-- 15 ~¦ conductive fluorine rubber containing carbonaceous fibers.
¦IThe interior of the insulator chamber was not given any ¦I specific coating treatment.
¦I To the electric dust collector was applied an electric potential of 70 KV and 2.5 mA. The outlet and inlet temperatures ~1 20 of the dust collector were 57C and 55C, respectively.
; Although the pH value of the absorbate collected in the bottom of the spray column was variable with the SO2 concentration at the gas inlet and the amount of the absorbent sprayed, it was ¦ maintained at above the level of 8.
The liquid collected in the bottom of the spray oolumn . 1~
~ _S_ lO9Z5Zl Il .
I
¦¦ and that collected in the bottom of the electric dust collector ¦i were combined, and the resultant mixture was poured downwardly il into the dust collector through the top and used for the ¦¦ formation of water film on the collector pole to provide 1¦ protection for the collector pole.
~¦ The li~uid collected at the bottom of the collector pole was freed from soot by filtration and released into the drainage. Despite variations in the conditions described above, the dithionic acid content was extremely small and the Na2S2O6/Na2SO4 molar ratio was less than 1/lO000 in all the test runs insofar as the SO2 content of the exhaust gas was less than 0.1 ppm. When the SO2 content was less than 0.5 ppm, the molar ratio of Na2S2O6/Na2SO4 was found to be about 1/10000.
In the absorbate which had been collected in the bottom of the l spray column, the Na2SO3 which had been absorbed but had not thoroughly undergone the subsequent oxidation was hardly detectable.
Example 3: ~
With the same apparatus and under the same conditions as used in Example 2, the raw exhaust gas was fed to the spray column at various feed rates so that the gas was permitted to flow at velocities of 5, 8, 15 and 20 Nm~sec within the spray column.
The rawexhaust gas was treated with the same absorbent sprayed ¦ throush the first-step nozzle at a rate of 5 Q/hour. When the ~ f~ow rate of the exhaust gas was 300 Nm3/hour or over, a part : , : !
. I . I
., ~ . I
~ 2 ~ ~
l i ; ~ .
10925Zl !l ¦ of the exhaust gas was separated and treated by the electric dust ji collector in the same manner as above. In all the test runs, 1,l the exhaust gas at the outlet was found to contain less than i! o .1 ppm of SO2~ less than 0.1 ppm of SO3 and less than 1 mg/Nm3 5 ¦1 of soot. The liquid finally released into the drainage was found to contain dithionic acid in an amount of less than I¦ 1/10000 in terms of Na2S2O6~Na2SO4 molar ratio.
Example ~:
With the same apparatus and under the same conditions as used in Example 3, excepting the SO2 concentration in the raw exhaust gas which was fixed at 150 ppm and the flow velocity of the gas inside the spray column whlch was fixed at 11 Nm/sec., the treatment o~ the exhaust gas with the absorbent was carried ¦out with test pieces of a various material placed at prescribed 15 1¦ positions in the reaction system, to determine the effect of the presence of the material in the system upon possible by-¦production of dithionic acid.
Each test run was continued for the fixed period of five ~hours and the data of the test run were those obtained over Ithe entire period of five hours. On completion of each testrun, the interior of the entire system was cleaned and the absorbate which had been collected therein was thoroughly removed before the subsequent test run was started.
I All the test pieces used in the test runs were prepared 25 ¦¦ each in the shape of a plate 3 cm in width and 30 cm in length.
lO~Z5Zl 1~
In the spray column, one test piece was hung do~m from the upper portion along the wall in the longitudinal direction.
,1 In the dust collector, one test piece was hung down along the ¦ wall extending from the gas inlet on the spray column side to 5 1l the discharge zone. -In the test runs using test pieces made of carbon, titanium, tantalum and FRP incorporating carbonaceous fibers, no increase in the dithionic acid content was observed in any of the absorbates.
a) In the test run wherein test pieces of ordinary steel were placed in the spray column and the dust collector, the ¦ molar ratio f Na2S26/Na254 was found to be 4/10000.
.
b) In the test run wherein a test piece of stainiess steel I (SUS-304) was placed only ln the dust collector, the molar~
I ratio was found to be 2/10000. cj The result was the same ~
: ' ' , when only the discharge pole of the dust collectar was made ~ f the same stainless steel ~SUS-304). In all these test runs, ; ; ,i the conditions on the exhaust gas side were kept on similar I levels to those of Example 2.
Example 5~
A various heavy metal believed to function catalytically in the formation of dithionic acid within the spray oolumn and the dust collector was dissolved in concentrations ranging ¦ from pH 3 to pH 8 and subjected to a flask test, to determine ~ ¦~the effeot of the dissolved heavy metal upon the by-production : ~
I
1 ~ .
. ~
:
, 1i lO~Z5Zl ~1 of dithionic acid.
Flasks were charged each with 400 mQ of an aqueous solution containing 10% by weight of sodium sulfite and a prescribe d ll amount o`f a given heavy metal. The liquids in the flasks were jl raised in temperature to 58C, adjusted in pH value by use of , caustic soda and, while under continued stirring, bubbled with air , which was blown upwardly from the bottom at a rate of 15 Q/hour.
¦¦ The contents of the flasks were thus allowed to undergo a reaction ¦¦ for about one hour. At the end of the treatment, the contents , were analyzed.
The accompanying drawing graphically represents the relationship between the amount of iron dlssolved in the liquid and the amount of dithionic acid produced in the liquid possibly as a function of the pH status of~the liquid. In the ¦I drawing, the vertical axis shows the amount of produced Na2S2O6 ¦l (X10-3 mol/R h), the holizontal axis PH of the aqueous solution.
It is clea~ly seen from this graph that, at pH 5-5. 5 or below, the presence of iron accelerated the formation of dithionic acid. The I amount of dissolved iron sharply decreased as the pH value of the I liquid rose. At pH 6 or over, the solubility of~iron decreased j below l ppm and substantially no production of dithionic acid was jobserved.
¦ Where the pH value of the liquid is low, particularly i ¦ where it falls in the neighborhood of 4, a treatment which is effective in rendering the structural material of the system I I
'~ I . I
I , .
" _ ;~_ ll ! I
- I -lO9Z5Zl incapable of releasing iron into the liquid attains a deeper significance. Where the pH value is 6 or over, the amount of li dissolved iron is so small that special attention may not ji always b~e required to be directed to the selection of 5 i¦ structural material of the system. In due consideration of l~ the possibility that the nature of the li~uid will be locally ¦ varied or temporarily affected by a variation in the operational status, it is nevertheless preferable to pay due attention to the selection of the structural material of the system, especially of the dust collector. The test was also csnducted with respect to Mn, Cu, V, Ni, Ca and Mg besides iron, to ~scertain that the effects of these metals on the by-production of dithionic acid were less than 1/10 of that by iron.
~ Example 6:
15 ! With the same appratus as used in Example 3, the raw ¦ exhaust gas fed at a flow velocity of 11 Nm/sec was treated with ,I the absorbent sprayed into droplets of various particle diameters.
¦ ! The~treatment was performed under various conditions such that most of the sprayed droplets had particles falling (a) in the range of from 1 to 30 ~ similarly to Example 3, (b) in the range of from 5~ to 100 ~i (by use of an ultrasonic nozzle) and (c) ¦ in the range of from 70 to 300 ~ (by use of an ordinary nozzle).
I ~In the test run (b), the exhaust gas at the outlet was found to have a SO2 content of 4.8 ppm and a Na2S2O6/Na2SO4 molar ~I ratio of 9/1000. In the test run (c), the SO2 content was 11.~2 pp !¦
~ ' _~_ .
Il . I
and the Na25~O6/~a~504 molar ratio was 24/1~00. The SO3 and ¦ soot contents in the test runs (b) were the same as those in the test run (a). In the test run (c), however, the soot I was sl1ghtly greater (though not determinable quantitatively) I and the SO3 content was 0.4 ppm. These results suggest that the finely divided droplets of the absorbent contribute to the aggregation of SO3 mist, etc. to some extent. In the , test run (c), the amount of sprayed droplets which were -entrained by the current of exhaust gas and drifted in the form of fog into the dust collector was smaller than in the test runs (a) and (b).
Example 7:
With the same apparatus as used in Example 3, the raw exhaust gas was fed at a flow velocity of 8 Nm/sec. and the SO3 originating in fuming sulfuric acid was added to the exhaust gas so much as to increase the SO3 concentratlon in the exhaust gas to 80 ppm. The exhaust gas at the outlet and the liquid finally released into the drainage were found ~ to have properties of the same tolerable level as those obtained in Example 3.
Example 8: ~
At the entrance to the catalyst bed in the system used in Example 1, the exhaust gas (containing NH3 gas) was ¦ dlverted and a plurality of various catalysts indicated in ~ Table 1 below were fed at a fixed feed rate of 300 Qfhour, , ~1 . I ~
jl ~
.. ' '''"'' ~' . ' 10925Zl one each into as many test catalytic reactors. The results were as shown in Table 1 below.
Tab1e 1 ._ . _ l eaction SV Percentage 1l Catalyst empera- h-l of NO -ll ure (C ( ) removXal(%) Ij , _._ . __ a Obtained by adding 10% by weight 140 7,000 96 of CuO to what had been produced by baking rhodochrosite at 400C
and baking the resultant mixtur at 400C.
b Obtained by adding 10% by weigh 140 7,000 96 of ZnO to what had been produced by baking rhodochrosite at 400C
and baking the resùltant mixtur I at 400C.
c Obtained by baking manganese 130 5, 000 97 carbonate at 350C. ~
d Manganese dioxide obtained by 150 5,000 92 electrolysis technique.
, e Obtained by baking manganese 200 5,000 72 dioxide ore at 400C.
f Obtained by coating activated 110 5,000 93 carbon with platinum.
g Obtained by coating activated 120 2,500 84 carbon with ammonium bromide.
h Obt ned by bakinq Co3O~ at 180 ~ 88
1~
:'. ill .
109ZSZl 1 !
droplets has not been completed within the spray column and the exhaust gas and/or the droplets still containing SO2 is suffered I -to enter the electric dust collector, there is a fair possibility Il that dithionic acid will be formed within the electric dust 5 1I collector. It is, therefore, preferable that the part of the electric dust collector destined for direct contact with the Il aqueous solution is formed of a non-metallic material. It is ¦l likewise preferable that the part of the electric dust collector ¦I required to have high electroconductivity is made of a material 1l selected from the group consisting of mixtures of carbonaceous ¦ substances with synthetic resins, carbonaceous substances (inclusive of both carbonic and graphitic substances), lead, titanium and tantalum. From the economic point of view, use of ,¦ a composite material made up of a carbonaceous substance and a I synthetic resin proves to be most advantageous.
jl Where the removal of SO2 from the exhaust gas within the spray column is not sufficient, there is a possibility that the amount of dithionic acid to be formed within the electric ¦ dust collector will increase. When So2 is injected in a 1l concentration of lO0 ppm, by way of model, into the line laid ¦~ to interconnect the spray column and the electric dust collector, formation of dithionic acid definitely ensues within the electric dust collector, no matter whether the part of the ¦¦ electric dust collector destined for direct contact with ; 25 the aqu~ous solution may be made of a non-metallic material. The ~,' "
. Il I
i, 1 li 109Z5Zl ,, production of dithionic acid is ascribable to the heavy metal - ~I components such as Fe, V and Ni which are collected within the elec~ric dust collector. Where the collection of SO2 and/or Il the oxidation of the collected SO2 has not been thoroughly I completéd within the spray column, there is a possibility ,i that dithionic acid will occur within the electric dust collector. Of course, the amount of dithionic acid thus produced will decrease with the decreasing amount of SO2 I which is suffered to enter the electric dust collector. It ll has been observed that the ratio of the amount of dithionic ¦ acid formed to the amount fo SO2 injected into the intervening ¦ lead line sharply decreases as the SO2 content decreases to j a certain extent and that the amount of dithionic acid produced greatly decreases especially when the SO2 content of the exhaust 1 gas at the outlet of the electric dust collector is lower . ~ i, ,¦ than 1 ppm. For the purpose of repressing possible formation - l¦ of dithionic acid due to the heavy metals originating in the exhaust gas, therefore, it islpreferable that the absorption of ! S2 and/or the oxidation of the absorbed SO2 is thoroughly ~20 ~j! completed before the exhaust gas entraining the droplets enters I I the electric dust collector. When this i~ fulfilled, formation !
¦ of dlthionic acid can no longer be observed within the electric ~ dust~collector. Since-there~is undeniably a possibility that ; l, the exhaust gas will be led into the electric dust collector -~1 25 I before the absorption of SO2 and/or the oxidation of the absorbed `'' 11 ," ~
'''~ `~' 1' - a~-, , .
. , 1, lO9Z5Zl I
~¦ S2 has been completed, the amount of the aqueous solution of an alkali salt which is drifted along in the current of the exhaust ¦gas from the spray column into the dust collector is preferred to I, be enough to provide thorough neutralization of SO3 and at least 1! avoid shifting the overall pH level toward the acidic side. This ¦ condition is preferred to be filled for the reaction system within ¦¦the spray column. If this condition is satisfied within the ~ electric dust collectorl it means that the condition is fully ; met also within the spray column.
When the reaction time within the spray column is fixed abo~e the level of 0.1 second, preferably 0.2 second, the absorption of SO2 and the oxidation of sulfite ions produced in ~-consequence of the absorption are completed within the spray column In this case, therefore, the part of the electric dust collector 1, 15 destined for direct contact With the aqueou5 solution may be not ¦¦formed of a non-metallic material.
¦ The solution which collects in the bottom of the spray column may be replenished with an alkali salt and put to cyclic ¦luse, while at least a part of this solution is forwarded to the -20 ¦ liquid collection unit of the electric dust collector and : ¦ subjected to a neutralizing treatment. ~enerally, the exhaust i gas from any ordinary boiler contains SO3 approximately in an '~ amount of from 20 to 50 ppm where the exhaust gas happens to be rich in SOx. The greater part of the alkali salt is used up in absorbing SO2 in the spray column and the solutlon collecting in j lO9Z5Zl the bottom of the electric dust collector assumes acidity. Thus, ' the solution from this electric dust collector is combined with the solution collecting in the bottom of the spray column, so j¦ that the resultant mixture has a neutral to alkaline pH value 5 ~! when taken out.
il Although the possibility of the oxidation of sulfites ! inside the electric dust collector is far less than in the absorption column, the droplets of the aqueous solution within the ele~tric dust collector tend to assume an acidic pH value.
It is, therefore, advantageous that the droplets is caused to be drifted along in the current of the exhaust gas from the spray column into the electric dust collector, there to be used for neutralizing the collected SO3. It is preferable that for the saXe of precautions, the absorbate collecting in the bottom of l the spray column should be injected into the bottom of the electri dust collector. It is important that the solution discharged from ! the bottom is not allowed to be acidic at least when it is dischar I ed from the bottom.
In the case of an exhaust gas which contains NOX in addition to Sx and oxygen, the exhaust gas is first treated as . described above to be freed substantially from Sx and the exhaust gas still containing NOX is then passed, in conjunction with I ammonia, through a catalyst bed. By this operation, the reduction of NOx can be continued for a long time without entailing any :: ; , ,,., , 11 109Z5Zl degradation in the activity of the catalyst. For the purpose f¦ of this catalytic reduction of NOX, there can be used any of the I various catalysts for NOx-reduction which have been known to ¦l the art. The catalytic reduction of NOX can be carried out at a I temperature in the wide range of from lO0 to 500C. ~rom the viewpoint of heat economy, however, the catalytic reduction is ¦~ preferred to proceed at the lowest possible temperature. For I this reason, it is preferable that the reaction is carried out in ¦ the presence of a catalyst which manifests its activity at a relatively low temperature such as, for example, a catalyst ¦ made up of manganese oxides alone or in conjunction with Fe, ¦~ Ni, Co, V, W, Cr, Cu, Sn, Ti and Zn at a low temperature in ¦ the range of from 100 to 220C. Among the various manganese 11 oxides available, manganese carbonate and/or calcined rhodochrosit ! is the-best choice as the catalyst.
The known catalysts such as those using noble metals like ¦I Pt, Ru and Rh, those using Co as a basic component, those made predominantly of activated carbon and those consisting of ¦ activated carbon coated with various metal oxides and with ammonium salts can be adapted for the reduction of NOX on condition ¦ that the exhaust gas has been treated so as to decrease its Sx ¦ content to an extremely low level. ~
¦I The present invention can be practiced even on an exhaust 1I gas which contains Sx in an amount exceeding 1000 ppm.
109Z5Zl Preferably, however, the present invention may be applied to the exhaust gas whose Sx content is not more than 500 ppm, more ¦preferably not more than about 100 ppm. The removal of NOx from llthe exhaust gas may be advantageously accomplished by preliminarily ¦I subjecting the exhaust gas to the treatment of SOx-removal so as to have its Sx content lowered below about 100 ppm.
As described above, *he present invention is directed to la process which includes the gas-liquid contact reaction performed ¦in the spray column for ~he absorption of S02 and the removal of ¦S03 mist performed in the electric dust collector.
¦ Since the absorption and oxidation effected by means of ¦finely divided droplets of the aqueous solution are completed ¦lin a very short time, the possibility of the formation of ¦ dithionic acid is notably decreased. Moreover, the possibility llthat iron and other heavy metals will be dissolved out of the l ~linner wall and bottom of the spray column can be substantially ¦Iprevented by proper adjustment of the pH value of the aqueous solution and proper selection of the material for the column.
!
20 1l Thus, the present invention eliminates the difficult problem . ~¦otherwise possibly involved in the disposal of dithionic acid ¦¦and, at the same time, offers a perfect solution to the problem ¦encountered in removing the N~x at a low temperature from ~; ¦ the exhaust gas.
25 1~ Now, the present invention will be described more .' 11 .
: I
.
109Z~iZl I¦ specifically herein below with reference to working examples of ¦¦ the invention. It should be noted that the present invention is ¦I not limited in any way to these working examples.
¦¦ Example 1:
5 ~¦ The exhaust gas from a boiler using as its fuel a high-¦ sulfur hèavy oil containing 2.8% by weight of sulfur was subjected to a wet type treatment for SOx-removal by using the sodium sulfite, to afford an exhaust gas containing 44 ppm of SO2, 28 ppm of SO3, 175 ppm of NOX, 4.5% of 2~ 66 mg/Nm3 of soot and about 15% of water and having a temperature of 58C.
This exhaust gas was fed downwardly into a cylindrical duct of FRP about 80 cm in diameter at a flow rate of 10,000 Nm3/
hour ~with the flow rate of the gas fixed at about 7 m/sec.).
Simultaneously and parallelly with the flow of the exhaust gas, an aqueous caustic soda solution of pH ll was spurted into ~7 Ithe duct interior through an ultrasonic nozzle at a rate of 150 Q/hour to produce finely divided droplets about 1 to 30 ¦¦in particle diameter.
The exhaust gas departing from the duct was led into another duct disposed horizontally at a distance of about 4.5m from the nozzle, then sent through a mist arrester and led into an electroc dust collector. With the gas-liquid contact effected in a very short timé of about 0.6 second, the absorption and oxLdation of SO2 were substantially complete. In the gas :.~ -' .
.' 1i ', ^' . I
¦~emanating from the mist arrester, the SO2 content was found to have been lowered to the order of 0.01 to 0.02 ppm. While the ¦igreater part of the sprayed droplets was collected in the bottom ¦lof the FRP duct, about one third of the droplets (particularly lilthose of relatively small particle diameters) were entrained by ¦Ithe current of exhaust gas into the electric dust collector.
il The electric dust collector having an interpole space ¦of 100 mm was operated at 40 KV and 32 m~ to free the exhaust gas from SO3, sootand sprayed droplets. The defiling matter thus removed was allowed to flow down the collector interior and was collected in the bottom of the collector.
I After the treatment given in the electric dust collector, ¦the exhaust gas was found to contain less than 0.01 ppm of SO2 and less than 0.02 ppm of SO3 and an amount of soot incapable !f detection by the method specified by JIS (Japanese Industrial IStandard) and had a temperature of 56C.
¦~ The absorbate which had been collected in the bottom ¦¦of the FRP duct was found to contain by-produced Glauber's salt ¦lin addition to caustic soda and was confirmed to contain no l¦discernible amount of sodium bisulfite, sodium sulfite or ¦jdithionic acid. The pH value of the absorbate was about 8.8 and the Na2S2O6/Na2SO4 ratio was less than 1/10000. The jabsorbate in the bottom of the spray duct was wholly transferred Ilinto the bottom of the electric dust collector, neutralized I with the SO3 collected by the dust collector. The neutralized Il, i! ~
~ D /
~ 1' ",,',' ' ' ~.
I! !
lO!J;~5Zl I
.
effluent wa= fed to the side of the main SOx-removing unit of the sodium sulfite process. The effluent was found to contain dithionic acid, a substance liable to increase the COD
j value of the final effluent, less than l/10000 in terms of ¦~ the Na2S26/Na2S4 molar ratio-In this case, the spray duct and the mist arrester were wholly made of a synthetic resin. In the electric dust collector, the main body was made of FRP, the dust collector unit and other parts required to have high electroconductivity were formed of an anticorrosive unsaturated polyester and carbonaceous fibers and other parts required to have particularly high electroconductivity were made of titanium material.
The exhaust gas which had been treated as described above was heated and then fed at a space velocity (SV) of 7,500 hr l, i lS in conjunction with NH3 in a concentration of 170 ppm, to a catalyst bed packed with cylindrical catalysts 2mm in diameter ¦and 15 mm in length and prepared from what had been obtained by baking rhodochrosite at 400C, so as to induce a catalytic reaction at 160C. Consequently, the NOX content fell to i 20 2 ppm, indicating that the NOx-removal was effected at a rate of 98.8%. ~
¦ The operation was continued for a period of 850 hours.
¦During this operation, the rate of NOx-removal and the pressure ¦¦loss in the catalyst bed were found to involve variations ¦ which invariably fell within the ranges of allowable errors.
I, .. I I . . ~ .. ..
10925Zl 1' ~ he gas which had undergone the ~Ox-removing treatment and was released into the atmopshere was found to contain Il extremely small amounts of harmful substances, i.e. not more jl than 0.~3 ppm of SOx, 2 ppm of NOx and less than 1 mg/Nm3 of ¦ soot. Tt has, thus, been ascertained that, despite the use of a fuel of very inferior quality, the rate of air pollution by the gas resulting from the treatment by this invention was extremely small.
Example 2:
The same exhaust gas as used in Example 1 was fed at a flow rate of 300 Nm3/hour into a spray column of transparent PVC (Polyvinyl chloride) pipe about 10 cm in diameter and 2.3 m in length so that it flowed at a velocity of about 11 Nm/sec inside the column and, through the two ultrasonic nozzles disposed }5 one at the gas inlet of the duct and the other at a point 1 m belo~
¦ the gas inlet, the absorbent was sprayed downwardly in the same direction as that of the current of exhaust gas to free the gas ¦¦ from its defiling matter.
An aqueous sodium carbonate solution of about pH 12 was ~0 used as the absorbent. Into the raw exhaust gas containing 44 ppm of S02, this absorbent was sprayed through the lower nozzle at a rate of 5 ~/hour so that most of the droplets ¦ resulting from the spraying would have particle diameters in the range of from 1 to 30 ~. Most of the droplets were mixed in the form of mist with the current of exhaust gas and entrained .
~' .
~ ~ ~3 1~ 1 lO9Z5Zl by the current. Under the conditions described above, the gas emanating from the outlet of the electric dust collector was found to contain less than 0.1 ppm of SO2, less than 0.1 ppm of SO3 and less than 1 mg/Nm3 of soot.
5 I Via the line leading to the gas inlet of the spray column, !, SO2 was added to the raw exhaust gas in amounts such as to give-l SO2 concentrations of 50, 100 and 150 ppm. In all the test runs, the exhaust gas at the outlet was found to contain SO3 1 and soot in the same amounts as above, except the SO2 content 1, rose to 0.8 ppm in the case of the test run using the SO2 1l concentration of 150 ppm.
; ¦ At this time, the same absorbent as used above was sprayed llthrough the upper nozzle at a rate of 5 Q/hour. In all the test ¦,runs involving the SO2 concentrations of 150, 200 and 300 ppm, ! ~ 15 ¦I the SO2 content in the exhaust gas at the outlet was less than ,lO.1 ppm.
In another test run in which the SO2 concentration was increased to 500 ppm, the SO2 content in the exhaust gas at the outlet rose to 1.2 ppm. So, the feed rate of the absorbent l¦through the upper nozzle was increased to 8 Q/hour. Then, the ~S2 content of the exhaust gas at the outlet fell below 0.1 ppm ~in the test run involving the SO2 concentration of 500 ppm and ~ below 0.5 ppm in the other test run with the SO2 concentration ;' I of 1000 ppm. When the feed rate of the absorbent was further lincreased to 12 Q/hour, the SO2 content of the exhaust gas at ' 11 ~., ,, , !, I
lO9Z5Zl the outlet did not exceed 1 ppm in any of test runs using SO2 ¦ concentrations of up to 2500 ppm. Under these conditions, the SO3 content and the soot content continued to be below 0.1 ppm and Il 1 mg/Nm3~respectively in the exhaust gas at the outlet.
I The gas was further sent through a mist arrester made of ,~ a synthetic fiber and then led into a vertically disposed, I cylindrical electric dust collector which was lined with a ¦I synthetic resin applied by the flake-lining method. The ¦¦ discharge pole was made of a carbonaceous material. The ¦ collector pole was designed so that the electroconductive coat deposited on the wall surface thereof was protected against harm by having a film of water formed on the coat. -The suspenders for the poles were made of channels of¦stainless steel (of grade SUS-304) coated with an electro-- 15 ~¦ conductive fluorine rubber containing carbonaceous fibers.
¦IThe interior of the insulator chamber was not given any ¦I specific coating treatment.
¦I To the electric dust collector was applied an electric potential of 70 KV and 2.5 mA. The outlet and inlet temperatures ~1 20 of the dust collector were 57C and 55C, respectively.
; Although the pH value of the absorbate collected in the bottom of the spray column was variable with the SO2 concentration at the gas inlet and the amount of the absorbent sprayed, it was ¦ maintained at above the level of 8.
The liquid collected in the bottom of the spray oolumn . 1~
~ _S_ lO9Z5Zl Il .
I
¦¦ and that collected in the bottom of the electric dust collector ¦i were combined, and the resultant mixture was poured downwardly il into the dust collector through the top and used for the ¦¦ formation of water film on the collector pole to provide 1¦ protection for the collector pole.
~¦ The li~uid collected at the bottom of the collector pole was freed from soot by filtration and released into the drainage. Despite variations in the conditions described above, the dithionic acid content was extremely small and the Na2S2O6/Na2SO4 molar ratio was less than 1/lO000 in all the test runs insofar as the SO2 content of the exhaust gas was less than 0.1 ppm. When the SO2 content was less than 0.5 ppm, the molar ratio of Na2S2O6/Na2SO4 was found to be about 1/10000.
In the absorbate which had been collected in the bottom of the l spray column, the Na2SO3 which had been absorbed but had not thoroughly undergone the subsequent oxidation was hardly detectable.
Example 3: ~
With the same apparatus and under the same conditions as used in Example 2, the raw exhaust gas was fed to the spray column at various feed rates so that the gas was permitted to flow at velocities of 5, 8, 15 and 20 Nm~sec within the spray column.
The rawexhaust gas was treated with the same absorbent sprayed ¦ throush the first-step nozzle at a rate of 5 Q/hour. When the ~ f~ow rate of the exhaust gas was 300 Nm3/hour or over, a part : , : !
. I . I
., ~ . I
~ 2 ~ ~
l i ; ~ .
10925Zl !l ¦ of the exhaust gas was separated and treated by the electric dust ji collector in the same manner as above. In all the test runs, 1,l the exhaust gas at the outlet was found to contain less than i! o .1 ppm of SO2~ less than 0.1 ppm of SO3 and less than 1 mg/Nm3 5 ¦1 of soot. The liquid finally released into the drainage was found to contain dithionic acid in an amount of less than I¦ 1/10000 in terms of Na2S2O6~Na2SO4 molar ratio.
Example ~:
With the same apparatus and under the same conditions as used in Example 3, excepting the SO2 concentration in the raw exhaust gas which was fixed at 150 ppm and the flow velocity of the gas inside the spray column whlch was fixed at 11 Nm/sec., the treatment o~ the exhaust gas with the absorbent was carried ¦out with test pieces of a various material placed at prescribed 15 1¦ positions in the reaction system, to determine the effect of the presence of the material in the system upon possible by-¦production of dithionic acid.
Each test run was continued for the fixed period of five ~hours and the data of the test run were those obtained over Ithe entire period of five hours. On completion of each testrun, the interior of the entire system was cleaned and the absorbate which had been collected therein was thoroughly removed before the subsequent test run was started.
I All the test pieces used in the test runs were prepared 25 ¦¦ each in the shape of a plate 3 cm in width and 30 cm in length.
lO~Z5Zl 1~
In the spray column, one test piece was hung do~m from the upper portion along the wall in the longitudinal direction.
,1 In the dust collector, one test piece was hung down along the ¦ wall extending from the gas inlet on the spray column side to 5 1l the discharge zone. -In the test runs using test pieces made of carbon, titanium, tantalum and FRP incorporating carbonaceous fibers, no increase in the dithionic acid content was observed in any of the absorbates.
a) In the test run wherein test pieces of ordinary steel were placed in the spray column and the dust collector, the ¦ molar ratio f Na2S26/Na254 was found to be 4/10000.
.
b) In the test run wherein a test piece of stainiess steel I (SUS-304) was placed only ln the dust collector, the molar~
I ratio was found to be 2/10000. cj The result was the same ~
: ' ' , when only the discharge pole of the dust collectar was made ~ f the same stainless steel ~SUS-304). In all these test runs, ; ; ,i the conditions on the exhaust gas side were kept on similar I levels to those of Example 2.
Example 5~
A various heavy metal believed to function catalytically in the formation of dithionic acid within the spray oolumn and the dust collector was dissolved in concentrations ranging ¦ from pH 3 to pH 8 and subjected to a flask test, to determine ~ ¦~the effeot of the dissolved heavy metal upon the by-production : ~
I
1 ~ .
. ~
:
, 1i lO~Z5Zl ~1 of dithionic acid.
Flasks were charged each with 400 mQ of an aqueous solution containing 10% by weight of sodium sulfite and a prescribe d ll amount o`f a given heavy metal. The liquids in the flasks were jl raised in temperature to 58C, adjusted in pH value by use of , caustic soda and, while under continued stirring, bubbled with air , which was blown upwardly from the bottom at a rate of 15 Q/hour.
¦¦ The contents of the flasks were thus allowed to undergo a reaction ¦¦ for about one hour. At the end of the treatment, the contents , were analyzed.
The accompanying drawing graphically represents the relationship between the amount of iron dlssolved in the liquid and the amount of dithionic acid produced in the liquid possibly as a function of the pH status of~the liquid. In the ¦I drawing, the vertical axis shows the amount of produced Na2S2O6 ¦l (X10-3 mol/R h), the holizontal axis PH of the aqueous solution.
It is clea~ly seen from this graph that, at pH 5-5. 5 or below, the presence of iron accelerated the formation of dithionic acid. The I amount of dissolved iron sharply decreased as the pH value of the I liquid rose. At pH 6 or over, the solubility of~iron decreased j below l ppm and substantially no production of dithionic acid was jobserved.
¦ Where the pH value of the liquid is low, particularly i ¦ where it falls in the neighborhood of 4, a treatment which is effective in rendering the structural material of the system I I
'~ I . I
I , .
" _ ;~_ ll ! I
- I -lO9Z5Zl incapable of releasing iron into the liquid attains a deeper significance. Where the pH value is 6 or over, the amount of li dissolved iron is so small that special attention may not ji always b~e required to be directed to the selection of 5 i¦ structural material of the system. In due consideration of l~ the possibility that the nature of the li~uid will be locally ¦ varied or temporarily affected by a variation in the operational status, it is nevertheless preferable to pay due attention to the selection of the structural material of the system, especially of the dust collector. The test was also csnducted with respect to Mn, Cu, V, Ni, Ca and Mg besides iron, to ~scertain that the effects of these metals on the by-production of dithionic acid were less than 1/10 of that by iron.
~ Example 6:
15 ! With the same appratus as used in Example 3, the raw ¦ exhaust gas fed at a flow velocity of 11 Nm/sec was treated with ,I the absorbent sprayed into droplets of various particle diameters.
¦ ! The~treatment was performed under various conditions such that most of the sprayed droplets had particles falling (a) in the range of from 1 to 30 ~ similarly to Example 3, (b) in the range of from 5~ to 100 ~i (by use of an ultrasonic nozzle) and (c) ¦ in the range of from 70 to 300 ~ (by use of an ordinary nozzle).
I ~In the test run (b), the exhaust gas at the outlet was found to have a SO2 content of 4.8 ppm and a Na2S2O6/Na2SO4 molar ~I ratio of 9/1000. In the test run (c), the SO2 content was 11.~2 pp !¦
~ ' _~_ .
Il . I
and the Na25~O6/~a~504 molar ratio was 24/1~00. The SO3 and ¦ soot contents in the test runs (b) were the same as those in the test run (a). In the test run (c), however, the soot I was sl1ghtly greater (though not determinable quantitatively) I and the SO3 content was 0.4 ppm. These results suggest that the finely divided droplets of the absorbent contribute to the aggregation of SO3 mist, etc. to some extent. In the , test run (c), the amount of sprayed droplets which were -entrained by the current of exhaust gas and drifted in the form of fog into the dust collector was smaller than in the test runs (a) and (b).
Example 7:
With the same apparatus as used in Example 3, the raw exhaust gas was fed at a flow velocity of 8 Nm/sec. and the SO3 originating in fuming sulfuric acid was added to the exhaust gas so much as to increase the SO3 concentratlon in the exhaust gas to 80 ppm. The exhaust gas at the outlet and the liquid finally released into the drainage were found ~ to have properties of the same tolerable level as those obtained in Example 3.
Example 8: ~
At the entrance to the catalyst bed in the system used in Example 1, the exhaust gas (containing NH3 gas) was ¦ dlverted and a plurality of various catalysts indicated in ~ Table 1 below were fed at a fixed feed rate of 300 Qfhour, , ~1 . I ~
jl ~
.. ' '''"'' ~' . ' 10925Zl one each into as many test catalytic reactors. The results were as shown in Table 1 below.
Tab1e 1 ._ . _ l eaction SV Percentage 1l Catalyst empera- h-l of NO -ll ure (C ( ) removXal(%) Ij , _._ . __ a Obtained by adding 10% by weight 140 7,000 96 of CuO to what had been produced by baking rhodochrosite at 400C
and baking the resultant mixtur at 400C.
b Obtained by adding 10% by weigh 140 7,000 96 of ZnO to what had been produced by baking rhodochrosite at 400C
and baking the resùltant mixtur I at 400C.
c Obtained by baking manganese 130 5, 000 97 carbonate at 350C. ~
d Manganese dioxide obtained by 150 5,000 92 electrolysis technique.
, e Obtained by baking manganese 200 5,000 72 dioxide ore at 400C.
f Obtained by coating activated 110 5,000 93 carbon with platinum.
g Obtained by coating activated 120 2,500 84 carbon with ammonium bromide.
h Obt ned by bakinq Co3O~ at 180 ~ 88
3~
_ lO9Z5Zl 11 1l I All these test runs were continued over a period of 850 hours. It was only in the test runs of f and g that the percentage of NOx-removal showed a variation in excess of 1 2~. In the test runs of f and g, since the catalytic activity ~ :
¦ was impeded owing to decomposition of the formed ammonium nitrate, the temperature of the exhaust gas had to be temporarily i raised to about 200C for the treatment to restore the original ¦ catalytic activity. This meant that the catalysts in these tes~ ~uns were not p~rman-n~ly po ~oned.
~ : ; ~
:~
i ' :
I , . , ' 33 I :~
J, ~
,
_ lO9Z5Zl 11 1l I All these test runs were continued over a period of 850 hours. It was only in the test runs of f and g that the percentage of NOx-removal showed a variation in excess of 1 2~. In the test runs of f and g, since the catalytic activity ~ :
¦ was impeded owing to decomposition of the formed ammonium nitrate, the temperature of the exhaust gas had to be temporarily i raised to about 200C for the treatment to restore the original ¦ catalytic activity. This meant that the catalysts in these tes~ ~uns were not p~rman-n~ly po ~oned.
~ : ; ~
:~
i ' :
I , . , ' 33 I :~
J, ~
,
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for the treatment of an exhaust gas containing sulfur oxides and oxygen by bringing said exhaust gas into contact with the aqueous solution of an alkali metal or alkaline earth metal salt thereby causing the sulfur oxides present in said exhaust gas to be absorbed by said aqueous solution, which process comprises the steps of:
(a) introducing said exhaust gas into a spray column of which at least the part destined for direct contact with the aqueous solution is made of a non-metallic material;
(b) bringing the introduced exhaust gas, within said spray column, into contact with finely divided droplets of the aqueous solution of an alkali metal or alkaline earth metal salt having a pH value of not less than 8; and (c) introducing the exhaust gas thus contacted into an electric dust collector thereby removing from said exhaust gas the finely divided particles contained in said exhaust gas.
(a) introducing said exhaust gas into a spray column of which at least the part destined for direct contact with the aqueous solution is made of a non-metallic material;
(b) bringing the introduced exhaust gas, within said spray column, into contact with finely divided droplets of the aqueous solution of an alkali metal or alkaline earth metal salt having a pH value of not less than 8; and (c) introducing the exhaust gas thus contacted into an electric dust collector thereby removing from said exhaust gas the finely divided particles contained in said exhaust gas.
2. The process according to Claim 1, wherein said finely divided droplets have particle diameters in the range of from 1 to 100 µ.
3. The process according to Claim 1, wherein the flow velocity of the exhaust gas within the spray column is in the range of from 5 to 20 m/sec.
4. The process according to Claim 1, wherein the exhaust gas fed at a feed rate of 100 Nm3/hour and the aqueous solution of an alkali salt fed at a feed rate of from 0.5 to 5 ?/hour are brought into contact with each other.
5. The process according to Claim 1, wherein the part of said electric dust collector destined for direct contact with the aqueous solution is formed of a substantially non-metallic material.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP3499077A JPS53119270A (en) | 1977-03-29 | 1977-03-29 | Treating method for exhaust gas |
| JP34990/1977 | 1977-03-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1092521A true CA1092521A (en) | 1980-12-30 |
Family
ID=12429575
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA299,476A Expired CA1092521A (en) | 1977-03-29 | 1978-03-22 | Process for treatment of exhaust gas |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS53119270A (en) |
| CA (1) | CA1092521A (en) |
| DE (1) | DE2813125C2 (en) |
| FR (1) | FR2385435A1 (en) |
| GB (1) | GB1596809A (en) |
| NL (1) | NL7803244A (en) |
| PL (1) | PL109740B1 (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2820357A1 (en) * | 1978-05-10 | 1979-11-15 | Metallgesellschaft Ag | METHOD FOR REMOVING SULFUR OXIDS FROM COMBUSTION EXHAUST GASES |
| WO2008100317A1 (en) * | 2007-02-16 | 2008-08-21 | Mecs, Inc. | Scrubber system for the desulfurization of gaseous streams |
| EP2411154B1 (en) * | 2009-03-24 | 2017-08-09 | MEGTEC TurboSonic Inc. | Wet electrostatic precipitator |
| GB2487895B (en) | 2010-07-19 | 2012-12-26 | Kraft Foods R & D Inc | Improvements in containers |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3607037A (en) * | 1968-12-20 | 1971-09-21 | Wellman Lord Inc | Separation of sulfur dioxide from gases |
| JPS5312902B2 (en) * | 1972-03-27 | 1978-05-06 | ||
| DE2607715A1 (en) * | 1976-02-25 | 1977-09-01 | Radian Corp | Sulphur dioxide removal with scrubber scaling prevention - by controlled soln seeding to control level of supersatd |
-
1977
- 1977-03-29 JP JP3499077A patent/JPS53119270A/en active Pending
-
1978
- 1978-03-22 CA CA299,476A patent/CA1092521A/en not_active Expired
- 1978-03-25 DE DE19782813125 patent/DE2813125C2/en not_active Expired
- 1978-03-28 PL PL20563578A patent/PL109740B1/en unknown
- 1978-03-28 NL NL7803244A patent/NL7803244A/en not_active Application Discontinuation
- 1978-03-28 GB GB1211278A patent/GB1596809A/en not_active Expired
- 1978-03-29 FR FR7809082A patent/FR2385435A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| GB1596809A (en) | 1981-09-03 |
| FR2385435B1 (en) | 1982-12-03 |
| DE2813125A1 (en) | 1978-10-12 |
| JPS53119270A (en) | 1978-10-18 |
| FR2385435A1 (en) | 1978-10-27 |
| DE2813125C2 (en) | 1983-08-25 |
| PL205635A1 (en) | 1979-01-02 |
| NL7803244A (en) | 1978-10-03 |
| PL109740B1 (en) | 1980-06-30 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4696804A (en) | Method for treating SO2, SO3 and dust simultaneously | |
| CA2343640C (en) | Process for controlling ammonia slip in the reduction of sulfur dioxide emission | |
| CN102755823B (en) | Method for desulfurizing high-sulphur boiler flue gas to prepare high-purity ammonium bisulfite | |
| US20100074828A1 (en) | Method and Apparatus for the Removal of Carbon Dioxide from a Gas Stream | |
| GB1589222A (en) | Process and apparatus for combustion gas scrubbing | |
| US20170029343A1 (en) | Sulfur enhanced nitrogen production from emission scrubbing | |
| CN103949144B (en) | A kind of method and device purifying the flue gas containing sulfur dioxide | |
| CN110152476B (en) | Device and method for synchronous desulfurization and denitrification of flue gas | |
| CN105233687B (en) | A kind of method of elemental mercury in removing coal-fired flue-gas | |
| US4026992A (en) | Method for removing nitrogen oxides from combustion exhaust gases | |
| DE4039213C2 (en) | Method and device for dedusting, desulfurizing and denitrifying combustion gases | |
| CN106621766A (en) | Method and system for removing SO3 from boiler flue gas by using desulfurization waste water | |
| CN104162359A (en) | Method for synchronous desulfurization, denitrification, mercury removal and dust removal in bag type dust collector | |
| CN106630269A (en) | Method and system for desorbing SO3 in flue gas of boiler by using desulfurization waste water | |
| CN111514716A (en) | Flue gas desulfurization, denitrification and demercuration purification method and equipment | |
| CN110064293B (en) | Method for desulfurization, denitrification and demercuration of flue gas | |
| US5366710A (en) | Process for removing nitrogen oxides and sulfur oxides from gaseous stream | |
| CA1092521A (en) | Process for treatment of exhaust gas | |
| CN109550338A (en) | A kind of flue gas desulfurization denitration dust-removing technique | |
| DE2342861A1 (en) | METHOD AND DEVICE FOR WET DESULFURIZING OF SMOKE GASES | |
| CN212492355U (en) | Ozone oxidation is SOx/NOx control system of ammonia process in coordination | |
| CN104138701A (en) | Flue gas purifying device of multi-functional coal-fired boiler | |
| CN107413181A (en) | A kind of regeneration fume from catalytic cracking dry type desulfurizing denitration dust removal system | |
| CN205570069U (en) | While SOx/NOx control's processing apparatus | |
| JPS62225226A (en) | Wet stack-gas desulfurization facility |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |