Note: Descriptions are shown in the official language in which they were submitted.
104~ V
The invention herein relates to apparatus for use in treating plant
exhaust gases or so-called stack gases and more particularly relates to appar-
atus for use in wet scrubbing stack gases and thereby removing pollutants con-
tained therein.
Stack gases generate various particulate and gaseous matter which
contribute to air pollution. Of particular concern are the relatively heavy
concentrations of sulfur dioxide found in stack gases created by plants and
power generating stations burning fossil fuel. The use of low sulfur fuels
would substantially reduce the amount of sulfur dioxide emitted, but the
supply of such fuels is lim;ted. Moreover, these stack gases also contain
other objectionable pollutants including oxides of nitrogen, dust and trace
amounts of other materials such as beryllium, arsenic, vanadium and mercury.
Although the nitrogen oxides, primarily as N0, typically exist in stack gases
in rather minute amounts, about 100 to 600 parts per million by volume, they
are still a major component in the formation of photochemical smog. Mercury - -
and the other trace elements, while not posing a photochemical smog problem
are toxic thereby creating a potential health hazard. Therefore, there is a
present need for a system which will effectively reduce sulfur oxide emis-
sions. There is also a need for a process or system which is capable of con
trolling the emissions of nitrogen oxides and other pollutants.
Several methods and systems have been used in the past for removing
and controlling such pollutants, but the effectiveness of each is limited.
Hot or cold electrostatic precipitators, bag or filter houses, and mechanical
precipitators such as cyclone separators are substantially limited to the
removal of particulate matter. Scrubbers, particularly wet scrubbers, pro-
vide the most effective means for control of both particulate and gaseous
poIlutants. However, the wet scrubbers heretofore used are inherent with
various problems.
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1040l~40
The conventional wet scrubhers typically employ a
vertical tower construction. Included withln the wet scrubhers
pre~iously used and more particularly within scrubbin~ zone or
the zone in which the pollutants are removed from the stack gases
is some form of restriction of the cross-sectional flow area of
the stack gases, normally either trays or packed beds of material
or a venturi. This restriction in the flow path through the
scrubber necessary results in an increased pressure drop as the
stack gases move through the scrubber which in turn can be detri-
mental to the operating efficiency of the plant. Thus, it isconvention to use several fans or other means requiring additional
power to assist the flow of stack gases through the scrubber
because of this pressure drop.
The bed or tray type scrubbers generally include trays,
packed beds, sometimes in the form of marble spheres, or mobile
beds, as for example light weight balls like ping pong balls,
with a spray of reagent scrubbing liquor in the tower creating
droplets which move downwardly against the direction of gas flow
or counter-current thereto and onto the trays or beds. In either -
instance, these trays or beds present a geometric configurationon which particulate matter collects to at least partially plug
the scrubber and thereby increase the presqure drop thereacross. ;
This problem of plugging is particularly severe in , -
those scrubber systems employing a lime or calcium hydroxide
reager,t. Such a reagent is particularly desirable for the removal
of s~llfur dioxide because of its reactivity and cost. However,
the product of the reaction of lime and sulfur dioxide is calcium
sulfite/bisulfite which frequently oxidizes to calcium sulfate ~;
commonly known as gypsum in the hydrated form or plaster of paris
when dehydrated. This calcium sulfate when reacted with the
water present and allowed to set up forms a substance which will
clog and plug the system to such an extent that frequent and
.. . .
" 10*184()
extended shut downs of the entire scrubber will be required for
removal of this substance. When fly ash is present in the stack
gases, as occurs for example when coal is burned, a concrete-
like substance is then formed which is even more difficult to
remove.
The plugging problem associated with these bed or tray
type wet scrubbers particularly directed to the control of sulfur
dixoide emissions has been partially solved by using a less
reactive reagent, such as limestone. ~lowever, this change to a
less reactive reagent reduces the pollutant remo~al effectiveness
of the system and in addition has created seversl new problems.
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For example, the limestone slurries are abrasive and cause
erosion of the pumps and spray nozzles used in the system.
Moreover, an external reaction tank or holding tank of sub- -
stantial size is required because of the extended reaction
time. Also, limestone is not readily available in certain
areas and when shipping of the limestone is necessary it
becomes less economical to use than a lime reagent which, in
comparable amounts required for use in the scrubber systems,
can be shipped at a significantly lower cost.
Aside from the increased pressure drop and general
interference with the operation of the system caused by the
plugging of the bed, there are other disadvantages associated;~
generally with the vertical tower wet scrubbers wherein the
scrubbing liquor droplets move counter-current to the stack
gas flow. This counter-current flow factor alone probably contri- -
butes to a slight increase in the pressure drop. However, as
the size of this type wet scrubber increases the nature of the
enlarged vertical tower structure will result in further in- -~
creases in the pressure drop if the pollutant control or collec- -
tion efficiency of the scrubber is to be maintained or increased. - -
This pollutant control or collection efficiency, is expressed
in the logarithmic ratio of inlet concentration to o~tlet-con-
centration of pullutant matter, is:
loge Ci - K (L/G) (E/r) (Z) (vt/vd~ wherein;
Co
Ci = inlet concentration
CO ~ outlet ~oncentrat}on
K ~ a numerical constant containing
unit conversion factors
L/G = ratio 0f liquid to gas flow ~;
E/r - droplet efficiency fractian -~
divided by the radius of the droplet
Z ~ height of tower
'~.
- 3 -
8 ~
vt/vd ~ velocity ratio of the settling
velocity to the velocity of the
droplet relative~to the duct or
tower wall
Thus it can be seen that for a vertical tower counter-
current wet scrubber an increase in the height and decrease in
the diameter (because of the velocity ratio term) results in a
higher logarithmic ratio of the inlet to the outlet concentration.
To the stack gas flow this means, however, that increasing the
collection efficiency ~ill result in an increase in the gas
pressure drop of the scrubber.
These vertical tower counter flow systems also re~uire
a rather sophisticated control arrangement to maintain a proper
raeio of scrubbing liquid flow to stack gas flow. That is, the
removal of the scrubbing liquid in this type scrubber is parti-
ally dependent on the stack gas flow and reduction of the gas
flow could result in a flooding of the scrubber if unaccompanled
by an appropriate reduction in the flow of the scrubbing liquid -
sprayed into the tower. Also, the downward counter-current flow
of the reagent in such vertical tower scrubbe~ precludes the use
of different reagents at different stages without a comminglin~
of the reagents~
In addition, a vertical construction of the tower when
enlarged requires an increase in the thickness of the wall mater-
ial and thus additional construction costs in order to satisfy -
the new structural demands. Lastly, the vertical tower construc-
tion is rather undesirable from an aesthetic standpoint due to
its impact on the skyline.
-The venturi type wet scrubbers are generally less
effective in removing pallutants from the stack gases than the
bed or tray type wet scrubbers. Moreover, greater pressure drops
are often associated with these venturi type scrubbers. In the
scru~er
venturi type ~c~r~Jhor the contaminated stack gas enters the
- 4 -
104~8~()
venturi and is accelerated to a high velocity at the throat where it im-
pringes upon a scrubbing liquid spray generally flowing in the direction of
the gas flow or co-current thereto. This results in the atomization of the
liqu~ The high differential velocity between the gas and atomized droplets
promotes impaction of the particles with the fine droplets. As the gas de-
celerates, further impaction and agglomeration of the droplets take place.
After the pollutants have been trapped within the droplets, the resulting
agglomerates are removed from the gas stream in a separator. The scrubbed
gas enters the separator tangentially, spinning the liquid droplets against
the wall and leaving at the upper part of the unit. The pollutant laden
liquid drains by gravity to the bottom of the tower as the stack gas pro-
gresses upwardly. As indicated previously, the restriction created by the
venturi in the stack flow path results in a significant pressure drop across
the scrubber. Moreover, increased pollutant collection efficiency in this
type scrubber is generally dependent on increased velocity at the throat
thereby necessitating an even greater pressure drop.
Aside from the deficiencies and probles associated with the here-
tofore used scrubber apparatuses and processes, there are specific probles
as~ociated with the removal of certain pollutants, in particular nitric
oxide and other pollutants which exist as minor components of the stack
gas because removal thereof is complicated by the interference of the major --
components of the stack gas which monopolize the reagent. -
Moreover, both the bed type and the venturi type wet scrubbers ~-
heretofore used are, as indicated previously,both substantially limited to
a single type reagent within a particular unit because of the direction of
the spray with respect to the flow of the stack gases. This is considered
a significant limitation because different reagents are more effective in ~ -
removing certain pollutants.
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104~)8~)
According to one aspect of the invention there is provided an
apparatus for wet scrubbing stack gases, comprising: an elongated chamber
having an inlet at one end thereof for receiving the stack gases, an outlet
at said other end discharging the scrubbed stack gases and sidewall means
defining therebetween a substantially horizontally extending flow path for
the stack gases passing therethrough with a substantiaIly unrestricted cross-
sectional flow area throughout the length thereof to avoid any substantial
pressure drop between said inlet and said outlet; means for spraying a liquid
reagent across said chamber substantially perpendicular to said flow path to
substantially cover at least at one stage between said inlet and said outlet
said cross-sectional flow area with spray droplets of said liquid reagent
through which the stack gases pass whereby pollutants contained within the --
stack gases are removed therefrom by said spray droplets; and discharge means
at the bottom of said sidewall means for discharging pollutant laden liquid
which impinges on said sidewall means and is removed therefrom to said dis-
charge means by the influence of gravity and the washing action created by
subsequently impinging liquid droplets.
According to another aspect of the invention there is provided an
apparatus for wet scrubbing stack gases with one or more liquid reagents,
comprising: an elongated chamber having an inlet at one end thereof for
receiving the stack gases, an outlet at said other end for discharging the
scrubbed stack gases and sidewall means defining between said inlet and said
outlet a substantially horizontally extending and substantially unrestricted
flow path for the stack gases passing therethrough which avoids any sub-
stantial pressure drop between said inlet and said outlet; means for spraying
at least one liquid reagent across said chamber including a mixing tank con-
ta;n;ne a liquid reagent, pump means associated therewith, and at least one
spray element in communication with said flow path, said pump means commu- -
nicating said liquid reagent under pressure to said spray element and said
1~4~1B40
spray element spraying said liquid reagent through said flow path and per- -
pendicular thereto with the liquid spray droplets created thereby substan-
tially covering the cross-sectional flow area of said low path at least at
one stage thereof whereby the liquid droplets effect removal of pollutants ~ --
contained within the stack gases passing therethrough; discharge means at
the bottom of said chamber for discharging the pollutant laden liquid which
impinges on said sidewall and is removed therefrom to the discharge means by
reason of gravity and the washing action of subsequently impinging liquid
droplets; and tank means in communication with said discharge means for re-
ceiving the discharged pollutant laden liquid which comprises a liquid slurry.
In use of the apparatus, the stack gases flow through a substantially un-
restricted and substantially horizontally extending chamber which defines a
scrubbing zone. One or more reagents are sprayed, preferably at various
stages, into the chamber with the liquid droplets therefrom moving across
the path of flow of the stack gases. The pollutant laden liquid is washed
from the chamber by the spray and discharges therefrom to be recycled,
sewered or reclaimed. A lime solution is used as a reagent for the removal
of sulfur dioxide. In a process directed particularly to the removal of
nitric oxide and other pollutants which exist as minor components of the
stack gases, such as mercury, beryllium, arsenic and vanadium there are in-
cluded the novel steps of intimately contacting the stack gases with a re- ~ -
agent or liquid scrubbing medium comprising an oxidizing agent selected
from the class consisting of an acid solution of an alkali metal dichromate
and permanganate ion. This latter process may exist separately or as part
of the above described unrestricted horizontal flow process for treating
stack gases.
The invention will be further illustrated with reference to the
accompanying drawings showing, by way of example, embodiments of apparatus
in accordance with the invention and processes which can be carried out in
the apparatus, wherein:
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1041)84()
Figure 1 is a diagramatic side view of the present invention
illustrating the horizontally extending wet scrubber chamber provided
with a multiplicity of stages.
Figure 2 is a sectional end view taken substantially along the
lines 2-2 of Figure 1 and illustrating a preferred arrangement of the
scrubbing liquid spray nozzles in the horizontal chamber.
Figure 3 is a sectional end view similar to Figure 2 illus-
trating another arrangement of the spray nozzles.
Referring now in detail to the drawings, the sometimes referred
to unrestricted horizontal flow process
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- 104~40
invontion-and the apparatus in which this process is preferably
carried out are diagrammatically illustrated by Figure 1 of the
drawings. The scrubbing apparatus generally designated 10,
includes a horizontally extending and cylindrical chamber 12 with
a stack gas inlet 14 at one end thereof and a stack gas outlet 16
at the other end. While the cylindrical design of the chamber is
generally preferable, economics may dictate a different design
for large sc~ubbers, as for example a chamber rectangular or
square in cross-section. The chamber 12 defines a scrubbing zone
where removal of the pollutants from the stack as passing there-
through is effected. As indicated by the drawings, the horizon-
tally extending chamber 12 provides a uniform and unrestricted
cross-sectional flow area throughout its length. The importance
of this feature will become readily apparent from the following ;
discussion. -
The stack gases or plant exhaust gases enter the hori-
zontally extending cylindrical chamber 12 at 14 preferably per- ~-
pendicular to the direction of the path of gas flow defined by
the inner cylindrical sidewall surface 18 of the chamber and indi~-
cated by the directional arrow 20. This necessitates a change in
the direction of the stack gas flow after it enters the chamber
12 and effec~s a partial inertial separation of the particulate
matter conta~ned w~thin-the stack gases at the inlet end 14. As
seen best in Figure 2, the center axis of the inlet is offset
with respect to the center axis of the chamber 12 whereby at
least a portion of the entering stack gases flowing in the dir-
ection indicated by the directional arrow 22 enter tangentially
with respect to the inner cylindrical sidewall 18. This creates
a swirling or spinning axtion o~ the stack gases as they flow
through the horizontal chamber 12 effecting further inertial
separation of particulate matter and droplets from the stack
,
gases flowing ~rougn ~ne chamber lZ. After the stack gases ha~e
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`` 1l~4~840
progressed the length of the chamber 12, they exit the chamber
through the outlet 16 which is also preferably perpendicular
to the direction of gas flow in the chamber thereby effecting
still another directional change of the stack gas flow and
further inertial separation of particulate matter contained
within the stack gases before exiting the chamber 12.
Spraying means, generally designated 30, include a
plurality of spray elements 31 which spray a reagent or scrubb-
ing liquid into the chamber 12 thereby causing liquid droplets
therefrom to cover substantially the entire cross-sectional area
of stack gas flow in the chamber at various stages along the
length thereof. As shown in Figure 1, each spray element 31
includes one or more spray nozzles 32 at the various stages along
the chamber 12. Four stages are shown, but it should be rec~g-
nized that the present invention is not limited to any particular
number of stages. However, it is felt that the use of more than
one spraying stage will reduce the flow rate of scrubbing li4uid
required through the scrubber system to effect a particular pollu-
tant collection or control rate. Moreover, a plurality of spray-
ing stages is considered particularly desirable where the stackgases enter the scrubbing chamber at a relatively high temperature.
Preferably, as shown in Pigure 2, a pair of nozzles :.
32 are located at each stage on the upper side of the cylindrical
sidewall 18 with one nozzle spaced about 45 from each side of the
vertical center line of the chamber 12. It has been ound that
this particular positioning of the nozzles 32 at each stage pro-
vides for a substantially complete curtain or sheet of liquid
droplets across the cross-sectional area of flo~ of the stack
gases in the chamber. Howeve~, more or less nozzles at each
stage or at a particular stage along the chamber 12 may b~ ~refer-
able in certain instances. Figure 3 represents a modified em-
bodiment of the present invention wherein only a single nozzle
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32 is employed at one or more stages. Where a single nozzle 3Z
is used it is generally preerable to locate this nozzle at the
top of the inner cylindrical sidewall 18 along the vertical center
line of the chamber. It should be noted in either instance, the
nozzles 32 preferably direct a relatively narrow sheet of spray,
as indicated by the phantom lines 34, across the chamber 12 sub^
stantially perpendicular to or cross-current to the flow of gas
therein. The cross-current flow ofthe spray is generally desir-
able because it avoids interference with the spray at other stages,
but there may be instances where it would he preferable to adjust ~ -
slightly the direction of the spray in the chamber while still
maintaining a substantially cross-current relationship between the
gas flow and spray. For example, at relatively high volumes of gas
flow it may be desirable to adjust the spray to a direction partly
counter-current to the direction of stack gas flow to minimize the
so-called problem of mist carryover wherein the spray droplets are
carried out of the scrubber chamber by the stack gases. This is -
a problem often associated with the venturi type scrubbers wherein
the spray is co-current to the gas flow.
An important feature of the present invention involves
a self-washing action. A relatively large volume of scrubbing
liquid is sprayed by the nozzle or nozzles at each stage with the
liquid droplets emitted therefrom moving across the chamber 12
and eventually impinging on the side wall 18. As the droplets
of liquid move across the chamber 12 they create the scrubbing -~
action of the stack gases passing therethrough by absorbing
pollutant matter contained therein. This combination of ~.
pollutant matter and liquid reagent produces a liquid slurry
h~hich, due to the influence of gravity, naturally tends to drain
downwardly to the bottom of the chamber. However, any of this
liquid slurry which might otherwise collect on the sidewall is
removed therefrom not only because of the gravitational drainage,
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1~498g~0
but because of the ~ashlng action created by the subsequent
impinging spray droplets.
Drainage means, generally designated 40, extend down-
wardly from the horizontal chamber 12 to remove thercfrom the
liquid slurry drained and washed to the bottom of the chamber.
Preferably, the drainage means comprises a discharge pipe 42
at each stage of the chamber and axially positioned along the
length thereof generally opposite the spray element 31. Each
discharge pipe communicates the liquid slurry from the chamber
12 through a lower open end 44 thereof into an open tank 46,
commonly referred to as a slurry tank. The discharge pipes or
conduits are not absolutely necessary, but they are preferable
because they tend to preclude splashing. As shown in Figures 1
and 2, vertically extending weirs 47 in the bottom of the
chamber 12 separate each stage. The weirs 47 extend upwardly
only a minimal distance into the chamber and in the flow path
and thus have no appreciable effect on the flow of the gases
therethrough. Although such weirs are not necessary and may be
deleted in certain instances, the weirs 47 do tend to control
~0 the relatively high volume of liquid flow within the chamber.
When the present invention comprises a plurality of
sta~es, such as shown in Figure 1, it is generally preferable
to provide separate slurry tanks 46 for each discharge pipe 42.
An overflow line 49 preferably interconnects these various tanks
whereby a more sophisticated control of the required pumps is
avoided as will also be made apparent from the following dis-
cussion. In addition, there is also a discharge line or conduit
49a at the outlet end 16 of the scrubbing chamber 12 which commi-
cates material therefrom to a slurry tank 46. While the chamber ~
and tanks are shown as separate components it is also within the -
scope of the present invention to include the tank or tanks as
an integral part of the chamber, but below the flow path of the
stack gases.
- 11'
0840
Although a plurality of stages are shown, it should
be recognized that in the particular embodiment of the invention
illustrated, these stages are all part of a closed cycle for a
single rea~ent. The following represents a discussion of this
particular cycle, but thereafter, it will also be shown that
the present invention could readily comprise a single unit with
several different reagent cycles. Referring now in detail to the
particular reagent cycle illustrated in Figure 1, it should be
noted that the reagent, in this particular instance, lime, is
introduced into the c~e, generally designated 50, at a mixing
tan~ 5~. Here the lime is put into solution or slurry to form
a scrubbing liquor 54. This scrubbing liquor 54 generally compri-
ses calcium hydroxide or hydrated lime. This scrubbing liquor ~ -
54 is communicated by conduit means 56 through a check valve 58
to a pump 60. The pump 60 communicates the scrubbing liquor
through conduit means 62 to the spray ele~ent 31 or in this par-
ticular instance, the p~r of nozzles 32 at stage 4 of the scr~b- ;
bing chamber 12. Sta~o 4 is generally adjacent the downstream
outlet end 16 of the scrubbing chamber. The scrubbing liquor at ~
20 stage 4 is sprayed by the nozzles 32 in a sheet across the ~ ~-
chamber 12 and thus through the path of any stack gases flowing
therein. As the spray droplets of calcium hydroxide move across
the stack gases progressing through stage 4 of the chamber, they
provide the so-called scrubbing action wherein they absorb par-
ticulate matter contained within the stack gases and also absorb
certain of the pollutant gases and react chemically therewith.
In this particular instance, the calcium hydroxide
scrubbing liquor reacts readily with the sulfur dioxide in the
stack gases to produce calcium sulfite/bisulfite which then
oxidizes to calcium sulfate, commonly known asgypsum or plaster
of paris. This product of th~ rea~c~-ion of the lime scrubbing
liquor and sulfur dioxide together with the particulate matter
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~040~4~
such as fly ash, collected by the spray droplets, form the
liquid slurry. This slurry if allowed to set up fo~ms a con-
crete-like substance and is the substance which clogs and plugs
the bed type wet scrubbers. However, in the wet scrubber of the
present invention, this slurry on the cylindrical sidewall 18 of
the chamber is washed therefrom by subsequent impinging droplets
from the continuing spray and drains along the cylindrical side-
wall 18 to the bottom of the chamber where it discharges through
the discharge conduit 42 in communication with stage 4 of the
chamber.
The discharge line 42 communicates the slurry to a
slurry tank 46. This slurry from stage 4 is then communicated
from the slurry tank by conduit means 64 through check valve 66
to a pump 68. This liquid slurry which still contains a sub-
stantial amount of unreacted calcium hydroxide is communicated
by the pump 68 under pressure through conduit means 70 to the
nozzles 32 of state 3. These nozzles 32 spray this slurry liquid
across stage 3 of the chamber and through the stack gases flowing
therethrough to collect additonal particulate matter and pollu-
tant gases for reaction with the calcium hydroxide. The resulting
slurry formed thereby is washed and drained from stage 3 of the - -
chamber through a discharge conduit 42 in the same manner as
described with respect to stage 4.
After discharge into a slurry tank 46, the slurry is
communicated by conduit means 72 through a check valve 74 to the
pump 76. Thereafter, pump 76 communicates the slurry liquid under ~ ~-
pressure through conduit means 78 to the nozzles 32 of stage 2
wherein the scrubbing, washing, and discharge steps are repeated.
The slurry from stage 2 is discharged into a slurry tank 46 and
~hen communicated by conduit means through check valve 82 $o the
pump 84. The pump 84 then communicates the slurry of stage 2
under pressure through conduit means 86 to the nozzles 32 of the
upstream stage 1 which i$ generally adjacent the inlet end 14 of
~4V~9~0
the scrubbing chamber. Again, the slurry is sprayed across the
chamber at stage 1 and again the steps of scrubbing of the stack
gases passing therethrough, washing of the resultant slurry from
the chamber and subsequent discharge into a slurry tank 46 are
r~peated. The slurry from stage 1 is communicated from the slurry
tank 46 by conduit means 88 through check valve 90 to pump 92.
Thereafter, this slurry is pumped through a re-cycling line 94.
Within the ~ecycling line 94 at 96, conventional means, such as
a centrifuge, filter, or clàr~fier, are provided to separate solids
contained within this slurry. The separated solids are there-
after dischar~ed by conduit means 98 to a settling tank 100 for
subsequent collection and disposal while the liquid continues `
through the re-cycling line 94 into the mixing tank 54 for repeated
spraying at the various stages into the scrubbing chamber 12.
Altho~gh not shown, control means are provided within
the cycle S0, preferably in communication with the liquid slurry
discharged from stage ~ which adjusts the pH of the lime scrub-
bing liquor to maintain a predetermined stoichiometric ratio with
respect to the calcium hydroxide-sulfur dioxide reaction. It has
been found that a stoichiometric ratio lime solution in a range
between l.O and 2.0 and preferably about 1.5 for a liquid flow
rate at each stage between about 10-50 tGPM) and a gas flow rate
of about 1000 (CFM), wherein the lime scrubbing liquid discharged
from the upstream stage is slightly acidic (pH between about 6.0
and 7.0 and preferably about 6.5), is particularly effective for
the removal of the sulfur dioxide from the stack gases.
Briefly summarizing the process of the cycle heretofore
described, it should be noted that the stack gases after enter-
ing the scrubbing chamber 12, pass horizontally therethrough, and
if desired in a swirling motion, toward the outlet end 16. At
the four stages along the chamber, a reagent or scrubbing liquor
in this instance comprising calcium hydroxide, is sprayed in a
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- 1041)~40
sheet through the path of stack gas flow and substantially
perpendicular or cross-current thereto thereby scrubbing the
stack gases for removal of the particulate and gaseous pollu-
tants contained therein. As indicated previously, the spray
droplets of scrubbing liquor absorb pollutants and react chem-
ically with certain of the pollutants. The principle reaction
involves the reaction of the calcium hydroxide with the sulfur
dioxide contained witin the stack gases to produce calcium
sulfite~bisulfite, but it is also felt that there is at least
a partial reaction involving the nitrogen oxides contained with
the stack gases. The preferred process for removing nitric
oxide is however discussed below and gener~lly involves the use
of a different reagent. The combination of the reagent or
scrubbing liquor with the pollutants produces a liquid slurry
which is washed by the spray and ~rained downwardly on the cylin-
drical-sidewall 18 to the bottom of the scrubbing chamber 12.
The slurry is then discharged from the chamber 12 into a slurry
tank and thereafter pumped for spraying at another stage until
it reached the slurry tan~ associated with stage 1 at the inlet
end of the scrubbing chamber. Prom this slurry tank 46 the
liquid slurry is re-cycled back to the mixing tank 54 after
solids have been bled thèrefrom into a settling tank. During ~
the process, the stoichiometric ratio is maintained at a pre- ;
determined level to provide the most effective lime scrubbing ~ -
liquor.
As indicated previously the lime reagent is generally
preferred for wet scrubbing stack gases including heavy concen-
trations of sulfur dioxide. However, the overall process of the
present invention is not intended to be restricted to this par-
ticular reagent and any reagent which will react with or absorbthe pollutant in the gas which is being scrubbed, including
brackish water and a limestone slurry, could also be used in the
above described cycle 50.
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.
` ~049~40
The following specific examples of this above described
cycle are submitted merly to illustrate the present invention
and are not intended to limit it.
Example 1
Exhaust gas from a low sulfur coal-fired furnace or a
furnace fired with a mixture of natural gas and coal containing
water either resulting from the combustion of natural gas or from
feeding a wet slurry of coal into the furnAce or both is passed
through an air pre-heater. It should be recogr.ized that the
exhaust gas will be diluted somewhat with air leakage. This
exhaust gas is then passed through an electrostatic precipitator
which will remove most of the suspended fly ash. This gas,
hereafter called the scrubber inlet gas, is at a temperature of
225F and an absolute pressure of 29.4 in Hg abs. and contains
about 400 ppm SO and 0.125 gr/SCF of suspended fly ash (rang-
ing in particle size from 0.2 to 10 microns in diameter). The
composition of this suspended fly ash is about 53~ SiO2, 21% ;~
A123. 6~ Fe23, with the remaining 20% by weight being oxldes
of other metals in smaller amounts.
Esch minute 68.08414 pounds of this gas enters the ~ -
scrubber containing 0~00152 lbs. of suspended fly ash (i.e.,
0.0125 gr/SCF) and 0.06053 lbs. of SO2 (i.e., 400 ppm by volume)
and 10.402 lbs. of Co2~5.675 lbs. of 2, 4.895 lbs. of H2O,
47.009 lbs. of N2, 0.03511 lbs. of NO and 0.00598 lbs. of NO2. ` ~
At each stage the liquid reagent is sprayed into the cylindri- -
cal scurbber chamber at a rate of 37.5 GPM. Thus, the above
amount of gas as it first progresses through the scrubber cha~ber
contacts 37.5 gallons of aqueous lime solution twhose pH is
adjusted to 6.5 leaving the scrubber) sprayed through a nozzle
at an inlet pressure of lO~si gauge. The gas is contacted a ^
second time with 37.5 gallons per minut¢ of the aqueous lime
- 16 -
- 104~840
solution, then a third time with 37. 5 ~,P~I of lim~ solution, and
finally in the 4th stage, 37.5 GPM of the lime solution.
The pressure drop of the gas through the scrubber is 0.2
inches of water and its temperature at ~he outlet is 120~F.
68.66027 lbs. per minute of the gas leaves thc scrubber at a flow
rate of about 1,000 cu. ft./min. (the diameter of the circular
scrubber is about 22 inches) and thus each minute leaving the
scrubber there are 0.00042 lbs. of suspended fly ash ~articles
(whose diameter is smaller than 0.3 microns) i.e., 0.0034 gr/SCF,
0.00276 lbs of SO2, i.e., 18 ppm (by vol~ as well as 10.402 lbs.
of CO2, 5.67S lbs. of 2~ 5 530 lbs. of H2O, 47.009 lbs. of N2,
0.03511 lbs. of NO and 0.00598 lbs. of NO~.
Thus, it can be seen that 0.635 lbs/minute of H2O is
absorbed by the gas and 0.05777 lbs/minute of SO2 and 0.00110
lbs/minute of fly ash are contained in the scrubbing liquid.
Preliminary data indicates that substantially none of
the CO2, 2 or N2 are removed with this scrubbing liquor. The
solubility of N2 is so slight that the amount dissolved is negli- ~
gible compared to the quantity of N2 in the gas. The solubility '!~"f
of 2 is also slight, but some amount of 2 reacts with the
calcium sulfite in the scrubbing liquor to form calcium sulfate.
However, the maximum amount that this would be is about 0.0144
lbs/minute, which is small compared to the 5.675 lbs/minute of
O~ in the gas so that the overall percentage of 2 in the gas
is not changed appreciably. It is also felt that some portion
of the 0.00598 lbs/minute of NO2 is absorbed by the scrubbing
liquor. Absorption or desorption of CO2 occurs in the scrubbing
liquor depending upon the pH of the scrubbing liquor but a buffer --
zone exists in the pH region found to be optimum for SO2 removal
30 (i.e., pH - 6.5) and hence no appreciable change in the CO2 con- -
tent of the gas occurs. Thus, it can be seen that four stages of
- 17 -
,
4 V ~ 4~
scrubbing at an L/G of 37.5 GPM/1000 CFM result in the lowering
of the SO2 content of the gas from 400 ppm to 18 ppm with the
dust loading being decreased from 0.0125 gr/SCF to 0.0034 gr/
SCF.
The liquid slurry leaYing the scrubber at each stage
is collected in a 50 gallon tank so that a holding time of 45
seconds is realized between each stage. The pll of the liquid
leaving the first stage is measured to be 6.5 in this tank.
After the average holding time of 45 seconds, 98% of this 37.5
GPM of liquid (i.e., 36.75 GPM) is pumped to a mixing tank where
sufficient lime is added so that the pH in the first stage tank
is maintained at 6.5. It has been found that stoichiometric
quantities ~i.e., about 0.051 lbs/minute of CaO) are sufficient.
At this time, sufficient water is added, about 6.864 lbs/minute,
to compensate for the bleed-off ~6.2 lbs/min), the liquid solution
including the water added to the gas (0.635 lbs/min.) and the
water removed with the calcium sulfite/sulfate mixture (about
0.029 lbs/min). This is then pumped at 37.5 GPM from the mixing
tank to the 4th stage nozzles. Leaving this stage, the liquid
passes into the 50 gallon vertical tank below this stage. The
liquid from this tank is then pumped at a rate of 37.5 GPM to the
nozzles of the second stage. Leaving this stage, it drains into
the vertical tank'below the 2nd gas stage, The liquid from this
tank is then pumped at a rate of 37.5 GPM into the nozzles of the
first stage. It draws from this stage into the vertical tank -
below this stage where the pH is measured to be 6.5 as previously
described.
The process thus operates in a continuous fashion with
37.5 gallons of liquid passing through the scrubber each minute,
through four stages in a counter-current manner with 36.75 gallons
being re-cycled and 0.75 gallons per minute being bled off which
contains 73~ of the fly ash entering the scrubber in the scrubber -
inlet gas and 95.5~ of the SO2 in the form of calcium sulfite and
5ul fate.
- 18 -
---` ` 104V841~
This solid ma~erial (predominantly CaSO4) is then
separated from the liquid in a conventional manner and the remain-
ing liquor is returned to the mixing tank along with the necessary
make up water so that 37.5 GPM of liquor can be circulated. The
amount of this solid material would be about 0.0011 lb/min of fly
ash and 0.123 lb/min as CaSO4. Water of hydration 0.029 lb/min
as well as that included in the sludge (20%) would also be an
additional amount so that about 0.18 lb/min of sludge would be
leaving the system.
Thus, in summary, very low absolute levels of SO2 (18 ppm)
and fly ash (0.0034 gr/SCF) were obtained utilizing this invention
with a gas pressure drop of 0.2 inches of water and an L/G per
stage for four stages of 37.5 GPM/1000 CFM. Plugging of the gas
passage or the liquid lines did not occur after a six month
testing period. -
Example 2
Exhaust gas resulting from the combustion of 3.0~ ~-
sulfur oil in a utility boiler is fed directly ti.e., no electro- -
static precipitator is present) to a 5.5 ft. diameter cylindrical ~ -
horizontal scrubber at a gas flow rate of 33,200 SCFM (standard
cubic feet - at 1 atmosphere pressure and 60F - per minute)
where it is scrubbed with 664 gpm (gallons per minute of a lime
slurry (i.e. at an L/G ratio per stage of 20 gpm/1000 SCFM).
This scrubbing results in lowering the amount of various harmful -~
constituents in the gas notably, SO2 from g98 parts per million
(ppm) to 105 ppm, SO3 and sulfuric acid mist from 18 ppm to 9 ppm
(expressed as H2S04 2H2O) as well as lowering the amount of
"trace elements" contained in the exhaust gas.
Specifically in one hour of operation at these condi-
tions, the weight of various harmful constituents entering and
leaving the scrubber with the gas is as follows: -
- 19 - . .
:
~04~)~40
Scrubber Scrubber
Inlet Gas Exit Gas
lbs. lbs. Remoral
S2 332.4 35.4 89.4
SO3tas H2so4.2H20) 12.78 6.42 50
As 2.2 x 10-4 1.0 x 10-4 55
B 7.8 x 10 3 1.4 x 10-3 82
Be 1.4 x 10 5 6.1 x 10-6 57
Cd 8.2 x 10-5 3,2 x 10-5 61
Cr 1.2 x 10-3 6~ x 10-4 70
Hg 3.8 x 10-4 3.2 x 10-4 16
Mn 7.6 x 1~-4 4,6 x 10-4 40
Ni 1.7 x 10-3 1.1 x lo-l 35
Pb 3.6 x 10-3 2.6 x 10-3 28
V 1.9 x 10~1 1.6 x lo-l 16
There are, of course, other constituents of the stack gas,N2,
2, CO2, sometimes CO, H2O, NO and NO2. As discussed in the
previous example, the amounts of these are relatively unchanged
except for a reduction in NO2 and an increase in the H2O vapor
contént (in the present example from 7980 lbs to 10,260 lbs).
To achieve this removal, 4640 lbs o water contain-
ing 421 lbs of Ca (OH)2 are added to the scrubbing system and `
2290 lbs of sludge are removed as a filter cake. This contains
about 1550 lbs of water, 730 lbs of a mixture of calcium sulfate
.~ld calcium sulfite (in the ratio of 70~ sulfite to 30~ sulfate)
and about 1.6 lbs of particulate matter containing the trace
materials removed from the gas as well as other oil combustion
ash constituents.
Thus, it can be seen that this process results in the
remoral of many undesirable constituents in the stack gas resul~
ting from the combustion of high sulfur oil.
- 20 -
~04084(~ -
Example 3
The gas resulting from the combustion of 0.4% sulfur
coal in a utility boiler is passed through an electrostatic
precipitator to a rectangular horizontal scurbber 30 ft. wide
by 15 ft deep at a flow rate of 480,000 SCFM tstandard cubic
feet - at 1 atmosphere pressure and 60F - per minute). It is
scrubbed in each of four (4) successive scrubbing stages with
10,000 gpm (gallons per minute) with a lime slurry. The inlet
concentration of SO2 of 200 parts per million ~ppm) is reduced
to 6 ppm to achieve a reduction of 97% of the SO2 in the exhaust
gas. The opacity of the gas is reduced from 49~ (as measured by
a light transmittance meter) to 27% at the exit. This reduction -
in opacity is due to the rèduction in concentration of fine parti- -
culate matter or "fly ash" contained in the exhaust gas resulting
from the combustion of coal with 10~ ash. Specifically, at the
conditions outlined above, the gas leaving the electrostatic pre-
cipitator and entering the scrubber contains 0.0678 grains/SCF
of fly ash, 68% ~or 0.046 grains/SCF) of which is smaller in
diameter than two ~2) microns with 17% by weight ~0.0114 grains/
SCF) of the particles being smaller in diameter than 0.5 microns.
The total "grain loading" of the gas leaving the scrubber is
0.0074 grains/SCF so that 89 weight percent of the particles of
all sizes are removed. However, 79~ by weight of these particles
(p.00581 gr/SCF) are smaller in diameter than two (2) microns
with 6~ by weight (0.000445 grains/SCF being smaller in diameter
than 0.5 microns. Thus, operating the scrubber at an L/G ratio
per state of 21 gpm/1000 SCFM not only results in removal of 97%
of the S02 and 89~ of the total particulate matter of all sizes, --
but also results in the removal of 87~ by weight of all of the ;
particles smaller than two (2) microns in diameter and 96% by
weight of all of the particles smaller in diameter ~han 0.5 micron.
- 21 -
.
.
4~4(~
The electrical power required to achieve this reduction (i.e.
for forced draft fans to overcome the gas pressure drop, for
pumps to pump the liquid slurry through the scrubber, thickener,
and process tanks, and to operate instruments and controls), 3
megawatts, is less than 2~ of the power ~enerated by the electric
generating unit which produced the 480,000 SCFM of exhaust gas.
Of particular note with respect to the overall scrub-
bing method of the present invention as described, is the sub-
stantially unrestricted scrubbing zone defined by the chamber
through which the stack gases flow. That is, the stack gases
flow substantially unin~rrupted through the scrubbing zone
except for the spray of scrubbing liquor. As shown by the above
example, the present invention provides for a significantly
high removal of pollutants from the stack gases, but the
pressure drop occuring during the scrubbing operation is sub-
stantially minimized because of this unrestricted flow through
the scrubbing zone. Moreover, it can be shown that pollutant
removal is maintained or in some instances increased in a
system for handling increased volumes of stack gas flow without
increasing the pressure drop. Referring now to the pollutant
control efficiency for this system, if expressed in the logar-
ithmic ratio of inlet concentration to outlet concentration of
pollutant matter, it is: ;
loge Ci ~ K tL/G (E/r) tD) wherein;
Ci # inlet concentration - ~ -
CO ~ outlet concentration ;,~
K - a numerical constant containing
unit conversion factors
L/G - ratio of liquid to gas flow
Ejr ~ droplet efficiency fraction divided
by the radius of the droplet
D ~ Diameter of the scrubbing chamber
or duct
- 22 -
- . . . . . . .
: ,, . : ' .
iO4~)840
Thus, to increase the collection efficiency the
diameter of the scrubbing chamber should be increased assuming
that all of the other factors remain the same. However, increas-
ing the diameter of the scrubbing chamber would also decrease
the pressure drop of the stack gas so that for the same collec,
tion efficiency the wet scrubber of the present invention would
have a lower pressure drop. This is in direct contrast to the
heretofore used wet scrubbers where an increase in pollutant
collection efficiency represented a similar incr~ase in pressure
drop.
Of similar importance is the self-discharging or wash-
ing aspects of the present invention. This is particularly
important when a lime reagent is used for the removal of sulfur
dioxide from the stack gases since the resulting product if
allowed to set up forms a concrete-like substance which would
clog and plug the scrubber. However, because the slurry is
washed and removed from the scrubber chamber, there is suffi-
cient time for such set up and thus plugging is avoided. More-
over, the rate at which the liquids are pumped through the
cycle svoids a premature set up within the slurry tanks.
With respect to the use of a calcium hydroxide scrub-
binB liquor for the removal of sulfur dioxide, other advantages
of the overall process of the present invention to be noted
aside from the lack of plugging, include efficient pollutant
control by maintaining a particular stoichiometric ratio, the
reduction of equipment fatique associated with ~he use of an
abrasive liquor, and the solution of an air pollution problem
without creating a water pollution problem. This latter advan-
tage is achieved because the by-product material produced as a
result of the calcium hydroxide and sulfur dioxide reaction is
in solid form which is in contrast to the by-products when
soluble reagents such as sodium compounds are used.
- 23 -
, ' .
,
10408~0
Moreover, as discussed more fully below, the horizontal
cross-flow scrubber of the present invention also permits the use
of several reagents in a single unit. In addition, the horizontal
structure proYides for lower construction costs since it is not
necessary to support its own weight in the manner of a vertical
structure and provides for a system which is less objectionable
from an aesthetic standpoint.
The following relates to a process directed ~articularly
to the purification of a gas stream contsining minute amounts of
nitric oxide and other trace ~ements, such as meIcury vapor, which
is in itself novel, but which is also particularly suitable as a
part of the above described unrestricted horizontal flow process.
With regard to the prior art directed to this specific process as
distinguished from the previously described general oYerall process
for treating stack gases, several technical articles dealing with
analytical chemical techniques teach that various nitrogen oxides
can be absorbed in sulfuric acid solutions of alkali metal dichro-
mates. See for example, Ind. Eng. Chem, Anal. Ed., pp 660-3 (1941);
Zhur. Anal, Khim 4, pp 292-7 (1949); Japan Analyst 2, 3-5 (1953).
In addition, Romanian patent 50,557 teaches that sodium dichromate
added to an ammonia synthesis gas washing liquid simultaneously
eliminates nitrogen oxides and carbon dioxide and U.S. Patent
1,765,254 discloses that gases such as N203 are absorbed in a solu-
tion of an alkali metal salt of an -ic acid of the metals of Group
6 of the Periodic Chart. It is also known that gases containing
relatively ~igh amounts of nitrogen oxides can be treated to
produce colorless stack discharges by scrubbing the gas with an ;
oxidant such as potassium permanganate. See Journal of the
Air Pollution Control Association Vol. 4 pp 122-247 (lg71).
. : - .
- 24 -
- ,. ~ .
1040~4()
The use of an aqueous solution of hydrogen peroxide as a
reagent in purifying smoke from steam generating stations
containing sulphur dioxide and possibly nitrogen oxides is
also known. See U.S. Patent 3,733,393.
The prior art further discloses that mercury can be
removed from gas streams employing acidified permanganate
solutions, GIG. SANIT. ll (6), 27-33 (1946), and that metallic
mercury reacts with dichromate solutions, particularly acidified
dichromate solutions to produce various, substantially non-
volatile mercury compounds depending upon the ~ype of acidemployed, Bull. Soc> Chem~ 5 (4) 18n3-11 (1~37). However, the
former reference indicates that the use of permanganate as a
scrubbing medium for removal o mercury is undesirable because
of its relatively short lifetime whereas the latter reference
fails to suggest treatment of gases such as flue gas with dich-
romate solutions to render such gases suitable for release to
the atmosphere.
In essence, none- of the known prior art discloses
the present process wherein a stack gas or flue gas stream con-
taining nitric oxide as a minor component is intimately contac~ed -
with a li~uid scrubbing medium comprised of an oxidizing agent,
selected from the class consisting of an acid solution of an
alkali metal dichromate and permanganate ion, which oxidizes
at least a part of the NO to various other nitrogen containing
species. Mercury vapor present in the flue gas is converted to
less volatile mercury compounds and, accordingly, removed. If
necessary, the effulent gas from this first scrubbing zone is
either passed to a second scrubbing zone to chemically react
with and remove any oxidized NO species such as NO2,HNO3, etc.
~ormed in the first scrubbing zone, or, if economically desirable,
the sc-rubbed gas containing any oxidized NO species can be passed
through a separating and collecting zone for recovery of the oxi-
- 25 -
: . -
. ,
` ` ~0 4 0 ~ 4 0
dized N0 species, e.g. nitric acid. In preferred form, when an
acid-dichromate medium is used, the temperature of the dichromate
scrubbing medium is maintained at a level so as to keep the vapor
pressure of water above the scrubbin~ medium substantially equal
to or greater than the partial pressure of the water in the gas
being scrubbed.
In a flue gas or stack gas typical of those obtained
when a fossil fuel suc}- as coal or a petroleum derivative is
being used as the energy source, the amount of N0X, primarily
N0, will range from approximately 0.01 to 0.15% by volume,
rarely exceeding 0.5% by volume, whereas, the mercury content
will range from about 0.2 to about 20% ppbv as mercury vapor.
In such flue gases, other components such as sulfur oxides,
carbon oxides and water are also present, the amount of water
ranging generally from about 5 to about 13% by volume depending
upon the precise fossil fuel employed.
Referring now in detail to the process, stack gases
obtained as a combustion product of a typical fossil fuel enter
the wet scrubber apparatus 10 which, as described more fully
above, is a horizontal cross flow or cross-current scrubber
wherein the gas to be scrubbed moves horizontally through a
scrubber zone or chamber 12 and is contacted by a reagent or
scrubbing medium introduced into the chamber 12 at least one
stage thereof in a spray through spray nozzles 32. However,
it should be noted that this N0x removal process involving the
novel steps of intimately contacting the gas stream ~ith the
particular medium defined can also be carried out, although less
effectively, in other conventional gas-liquid scrubbing systems.
rhe scrubbing medium, comprising either an alkali metal dichromate
acid solution or a solution containing permanganate ion, such
as a solution of potassium permanganate, eventually discharges
the scrubber 10 in the form of a slurry.
- 26 -
~ V ~ 4 ~
Upon leaving scrubber 10, the scrubbing medium can
be regenerated for re-cycle through the system. For example,
in the case of the use of an acid-dichromate scrubbing medium,
an electrolytic regeneration system can be used wherein any
dichromate which has been reduced to chromium (III) is oxidized
back to dichromate.~
Under the conditions existing in scrubber 10, at least
some of the NO is converted to products wherein the nitrogen is `
in a higher oxidation state, at least a portion of which may pass,
either entrained or as a vapor, with the scrubhed gas via outlet
16 from scrubber 10. However, most of the NO will remain as
oxidized species in the scrubbing medium.
Operating parameters, such as flow rates, both gas
and liquid, pressure, temperature, etc., in the scrubbing zone
or chamber 12 will vary depending upon the amount of pollutants,
particularly NO, in the gas to be scrubbed, the type of scrubber
employed, the composition of the scrubbing medium, the volume -
of gas to be handled, etc. In general, pressure will be atmos- ~
pheric or higher. -
When the scrubbing medium employed is of the acid-
dichromate type, the temperature in the scrubbing zone will be
maintained at approximately within 30C of a level which will
ensure that the vapor pressure of water in the scrubber is sub-
stantially equal to the partial pressure of the water in the gas
to be scrubbed, and, in the preferred case, will be maintained
at a level which ensures that the vapor pressure of water in
the scrubber is substantially equal to the partial pressure of
the water in the gas to be scrubbed. For practical purposes, at l-
any given temperature, the vapor pressure of a concentrated
sulfuric acid solution can be considered to be due to the partial
pressu~e of the water in the sulfuric acid. Thus, for example,
a; ming that an 80% by weight sulfuric acid scrubbing medium
is being employed and the gas to be scrubbed contains 10 mole
- 27 -
.
11)4U~4V
per cent water and further assuming that the scrubber is operated at
a pressure of one atmosphere, the partial pressure of water in the
scrubbed gas would be approximately 76 mm Hg. Thus, in order to
obtain a water vapor pressure of 76 mm Hg above an 80% sulfuric acid
solution, it is necessary to maintain the solution at a temperature
of around 137C. In the table below, equilibrium operating tempera-
tures for this process are shown as a function of the partial pres-
sure of water in the scrubbed gas and the strength of the sulfuric
acid being used.
`` 1040t341)
Thus, to avoid the necessity for the excessive addition of
make-up dichro~ate to the scrubbing medium, it is preferable
that the chromium (III) be oxidized back to dichromate. This
can be conveniently carried out by an electrolytic technique
wherein the dichromate is regenerated from chromium (III), the
regeneration process being carried out in a cell employing a
suitable anodecathode couple which minimized the overall cell
voltage, One such electrode system comprises a lead dioxide
anode and a 316 stainless steel cathode. Preferably, the
electrolytic regeneration is carried out in a two compartment
cell separated by a membrane which substantially keep dichro-
mate out of the cathode compartment and in which the sulfuric
acid concentration ranges from about 55 to about 60~ by weight.
However, the acid concentration can range from about 40 to
about 80~ by weight. It should be noted in this regard that in ~;
the electrolytic regeneration process involving the dichromate
medium, to obtain optimum current efficiences, it may be nec- ~ `
essary to adjust the acid concentration of the feed to the re-
generation zone to the desired level and then readjust follow~
ing regeneration, if necessary.
As in the case of the dichromate, it is possible to
regenerate the permanganate, which, upon ~eaction with NO, is
converted to compounds containing manganese in a lower oxida-
tion state such as manganese dioxide. It is to be understood
that the reaction between either the dichromate iscrubbing medium
or the permanganate scrubbing medium with the mercury vapor ~ -
also results in the conversion of the dichromate or the per-
manganate to compounds in which the chromium or manganese is
in a lower oxidation state. Although only electrolytic regen- -
eration has been discussed abo~e, it will be understood that
other regenerative processes may be employed depending on which
sorbent is employed and the types of reduction products obtained.
- 29 -
, .
,,
.
lQ40~34(~
Althdugh phosphoric and other acids can be employed
when the scrubbing medium is of the acid-dichromate type, it
is preferable that sulfuric acid be employed, the concentration
of the sulfuric acid generally ranging from about 40 to about
90~ by weight and preferably from about 50 to about 85~ by
weight, On a strictly stoichiometric basis, it is desirable
that the scrubbing medium contain at least one mole of dichro-
mate for every two moles of NO in the gas to be scrubbed.
Assuming complete reaction, this ensures that all the NO will
be converted to nitrate, As a practical matter, however, the
reaction does not proceed to completion, and it has been found
that N0 removal increases with increasing dichromate concentra-
tion, However, the decomposition of dichromate in a sulfuric
acid solution is concentration dependent and the rate of decom-
position increases with increasing concentrations of dichromate.
Thus, to minimize raw material cost without sacrificing scrub-
bing efficiency, it is desirable that the scrubbing medium
contain an amount of dichromate which will, within practical
limits,effectively remove the N0 present in the gas stream
but yet will not be so high that the rate of decomposition of
the dichromate exerts a deleterious effect on the economics of
the process. In general, it has been found that when a typical
flue gas is being scrubbed, dichromate concentrations rangi~
from about 0.1 to about 3~ by weight and more preferable frQm
about 0.3 to about 1.5~ by weight of the scrubbing medium can
be used.
As indicated above, operating parameters in the
process of tho prooont invontion and particularly in the oxidi~
zing scrubbing zone will vary depending upon the factors noted
above, Thus, for example, when a permanganate containing
scrubbing medium is employed, the temperature in the scrubber
can range from below ambient up to the boiling point of the
- 30 -
":
`- 104~84(~
scrubbing medium but more generally will range from about
100f to about 210F., pressures ranging from sub-atmospheric
to super-atmospheric.
Unlike the case of the acid-dichromate scrubbing
medium, the permanganate scrubbing medium~ can be acidic, b~icor
~eutralj neutral to basic solutions being preferred. In parti-
cular, a permanganate scrubbing medium wherein the pH is from
about 6 to about 12 is especially preferred, when it is
desired to achieve maximum removàl of the NO from the flue gas
and sulfur oxides are absent. Suitable buffering agents such
as sodium bicarbonate can be employed to maintain the p~ of the
scrubbing medium at the desired level. When an acid permanganate -
solution is employed as a scrubbing medium, virtually any mineral
acid can be employed, although it is preferred to use acids such
as phosphoric or sulfuric, sulfuric being especially desirable.
The acid concentration, when an acid is used in the permanga-
nate scrubbing medium, will range from about 0.05 to about Z0
by weight with the permanganate concentration ranging from
about 1.0 to about 20% by weight.
As noted, in terms of removing NO from flue gas
streams, it is prcferred that the permanganate sc~ubbing medium
be neutral or basic. However, in order to efficiently employ
basic permangsnate solutions for the removal of N0 from flue
gas streams, it is necessary to effect preliminary removal of
any SO2, since SO2 absorption into the alkali permanganate
results in the lowering of pH and a concomitant lowering of the
ability to remove NO. When a neutral or basic solution of
permanganate is employed, the concentration of permanganate
employed in the scrubbing medium will generally range from about ;
0.05 to about 1.0 moles per liter. It is to observed that,
unlike the case of N0 removal, the removal of mercury from the
gas stream i5 as efficient at low as at high pH values.
- 31 -
1(~4'~)~40
The following nonlimiting examples are presented to
more fully demonstrate the NOx removal process both as a sep-
arate process employed in conventional scrubber syste~s and
as part of an overall scrubbing process wherein the stack gases
are passed through a horizontal scrubbing chamber.
Example 4
A typical flue gas such as is obtained as a combus-
tion product of coal is scrubbed in a dichromate scrubbing
system similar to that described above. The flue gas to be
10 scrubbed has the following compositon:
Component Concentration
....
N2 68% mol
H20 11~ mol
C2 13% mol
2 7S mol
S2 250 ppm
NO 400 ppm
Other Trace
The recirculation scrubbing medium employed has the follow-
ing general compositon: ;.
Component Concentration
H20 16.0
H2so4 64.0 -
Na2Cr207 '
Na2S4 5.1 -~
Cr2(so4)3 14.2
The scrubber employed is a four-stage horizontal ~ -~
cross-flow spray scrubber operated under the following condi-
tions:
Sctubber medium flow rate 300 gal/min
- 32 - .
iO4~840
Scrubber temperature 280-290F
Flue gas temperature 260-300F
Plue gas flow rate 500-3,000 SCPM
(STP-60F., lATM)
It is found that under the above conditions, 90% of
the nitric oxide present in the flue gas is converted to nitric
acid.
Example_5
Various gas blends simulating typical power plant flue
gas streams were prepared. These blends were ~hen processed in a
laboratory scale scrubbing system. The system employed a two-
inch diameter countercurrent scrubber containing one sieve tray
equipped with liquid downcomers. Sampling connections were
provided on both the gas and liquid streams entering and leaving
the scrubber. On line vapor measurement of NO and N02 were
supplied using a photometric technique while wet chemical methods
were used to obtain liquid phase analytical results on dichro- ~-
mate, nitrite and nitrate concentrations. Since the overhead
vapor from the dichromate scrubber contained relatively large
amounts of water~(l0~ mol), a quench column was installed to
cool the gas and condense the water before obtaining an effluent
gas suitable for analysis. In order to obtain a more accurate
material balance on the NOX species in the runs, the effluent
liquid ~condensate) from the quench column was collected and
the amount of nitrite and nitrate therein determined. The cooled
effluent ~as was then passed through a caustic bubbler to remove
the oxidized NO species such as nitric acid. The caustic in the ~- -
bubbler was also analyzed for nitrate and nitrite content. The
results of the various runs are shown in Table I below. In the
runs wherein the gas blend contained no S02, only N2 and NO were -
present. For runs in which N02 and SO2 were present, the blends
contained in addition ~2, C02 and 2
- 33 -
,, ' ' : ' :
~ 31 104V840
^l
~ ~ L, ! ~ ~ u')
R 1: ~5) N O
.... . . ,~
U ~ Q ~) ~ `J O O N
U~ tl' _ .
.~ .
. . . . . . I t t t t t
,~ ~ .,~ o o o o ~ o In u~
'r ,1~ 11; ~1 ~ ~ ~1 ~ ~1 ~I N N ~1 ~1
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- 34 -
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1(~4()~40
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3 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
c~ ~ -I
t` G~ ~ ~) ~ ~ ~ ~ r-- ~ N ~ U~ O ~r
~ ~ r~ 0~ N . C~ -1 0
as O ~ ~ N N N ,1 ~ r~ _I rt O O
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` -` 10~01~40
Example 6
The following example shows the results obtained
using a potassium permanganate scrubbing medium for removing
N0. The flue gas scrubbed was obtained from a coal fired
utility boiler and the system employed a 3,000 cfm horizontal
four-stage spray scrubber. The flue gas contained 400-450
ppm N0, 200-225 ppm SO2, 11~ H2O, 7~ oxygen and the balance
C2 and N2. The scrubber temperature was maintained at 122F.
Table II below summarizes the conditions under which the scrub-
bing experiments were carried out and the results.
- 36 -
n^¦ 104084()
Gh Q ~ . . . .
0~1 ~ O O O O O O O O O O O O O O O O O
X a~ ,
X O . `,~,.. .
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r~
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Z O ~ ,1 ~r I r~ ~ ~ ~ ~ ~ _I ~ ~ ~ ~ ~r
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- 37 - ~`
.. . . , ,~:.
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--`` ` 104(~840
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O O O O O O O O O O O O O O O O
O Ir)r~ ~ ~ ~ N~D~r)~~D~r ~00
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1040840
Examl~le_7
To demonstrate the effectiveness of an acid-dichro-
mate solution in removing mercury vapor as per the process of the
present invention, synthetic blends of nitrogen and varying
amounts of mercùry vapor were prepared, the synthetic gas blend
then being scrubbed through a one-inch ID column, seven inches
in length packed with one-quarter inch Berl saddles. The scrub-
bing solution was a 0.03 M ~2Cr2O7 solution in 80% by weight
sulfuric acid. The operating conditons and results are shown
below in Table III.
, . .
''
- 39 - : ~
.: ,
-` ` 104~8~()
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a
O O ~ ~ G~
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a) o
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o
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-40-
.
104~84(~
Example 8
To demonstrate the effectiveness of the use of a
permanganate scrubbing medium in the removal of mercury as
per the process of the present invention, synthetic blends of
mercury vapor and nitrogen were prepared and scrubbed using a
wet electrostatic precipitator as the scrubbing vessel. In
Table IV below is shown the conditions employed and the results
obtained togehter with a comparison of the results obtained
using de-ionized water as the scrubbing medium.
. .
t: ~
~ G) ~ . O ~ ~(~gV840
P' ~
S::
O R Q Q.
e; ~ t!l t!.
S~
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- 42 -
104V84V
From the foregoing, it is apparent that the above
process provides an effective means for removing from stack
gases nitric oxide, mercury and other elements which exist as
minor components in the stack gases. Although most effective
results are obtained when the acid dichromate or permanganate
scrubbing reagent is sprayed cross-current with respect to the
flow of the stack gases with the stack gases passing through a
substantially unrestricted and horizontal flow path, substantial
removal of the nitric oxide and other trace elements is effected
by the acid dichromate or permanganate reagent in conventional
scrubbing systems.
It is also apparent from the above discussion relating
to both the unrestricted horizontal flow process and NOX removal
process that in the removal of nitric oxide or mercury the pro- -
cess involving a scrubbing medium containing an oxidizing agent
selected from the class consisting of acid-alkali metal dichro-
mate and permanganate ion provides for the desired results. On
the other hand, when removal of sulfur dioxide is of primary
importance the use of a calcium hydroxide or lime scrubbing medium
is most suitable for economic reasons although a dichromate and
permanganate reagent could also be used to effect removal of
sulfur dioxide. This then leads to another important aspect of
the apparatus and without deviating from the scope of the inven-
tion, it is readily apparent ~hat the scrubbing zone defined by ~;~
the chamber could be extended to incorporate one or more stages -
of different reagents. These reagents would be added through a
separate cycle similar to those discussed abo~e. That is, the
cycle would include a separate mixing tank and one or more slurry -~
tanks depending on the number of stages used. As before, the ~
rea8ent would be pumped from the mixing tank or a slurry tank to
the nozzle or nozzles at a stage and sprayed across the chamber
and thus across the flow p~h of the stack gases. This would ini-
- 43 -
. .
1040B40
iate at that state the scrubbing, washing, and discharging steps
in substantially the same manner as heretofore described. Pre-
ferably, the liquid slurry discharged from the upstream stage
would be recycled to the mixing tanl~ after solids were bled
therefrom. It should be noted that the horizontal cross-flow
scrubber is particularly adapted for such a multi-reagent pro-
cess because there would be no interference or mixing of the
reagent sprays as there would be in vertical tower scrubber
and because plugging is eliminated, various reagents can be
used.
It ls apparent that such a multi-reagent process and
apparatus for carrying out such a process would be particularly
useful where it is desired to remove both S02 and N0 from the
stack gases. In this instance, a lime solution reagent in one
cycle would be employed for the removal of S02 and the use of
permanganate or dichromate in another cycle would be used for
the removal of N0x.
Thus, with the multi-stage, separate reagent cycle
arrangement in the horizontal cross-flow scrubber of the pre-
sent invention, it is possible to remove some of the~particu-
late matter in earlier stages, remove the S02 in the next stages
and remove the oxides of nitrogen in the latter stages. It is
also possible that scrubbing with a basic solution followed by
scrubbing with an acidic solution will result in removal of
other trace amounts of elements which otherwise would be emitted
with the stack gsses.
_,.,
` 104084V
Having fully described my invention, it is to be
understood that I do not wish to be limited to the details
herein ~et forth but my invention is of the full scope of the
appended claims.
;',' ,
~....
- 45 -
.
