Note: Descriptions are shown in the official language in which they were submitted.
~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
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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
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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,
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~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
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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
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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
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~ 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
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~ 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
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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
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109Z5Zl
1l 1
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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
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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-
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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.
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109Z5Zl
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¦¦ 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
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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
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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
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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
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,¦ 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
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~¦ 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
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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
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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
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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
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109Z521
¦~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
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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.
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~ 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
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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
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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
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¦¦ 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
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¦ 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.
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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
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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
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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
!¦
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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,
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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
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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.
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