Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1057~)Z8
This application is a divisional of Serial Number 198,928 filed
3 May, 1974 and is directed to a process for removing sulfur oxides from a
gas stream containing same using ammonia as absorbent and also as reducing
agent. t
BACKGROUND_OF THE INVENTION
This invention relates to a process for the reduction of sulfur
dioxide with ammonia and to a method of separating sulfur dioxide from a
gaseous mixture containing the same wherein the separated sulfur dioxide is
subsequently reduced with ammonia.
Processes for the reduction of sulfur dioxide are well known. For
example, it is known to desulfurize flue gas by contacting it with a sorbent
of copper oxide on alumina, regenerating the sorbent with hydrogen or other
reducing gas, and reducing the sulfur dioxide in the regenerating off-gas to
elemental sulfur. It is also known to reduce sulfur dioxide to H2S
catalytically with hydrogen, a hydrogen-containing gas or a hydrocarbon such
as methane.
It is additionally known to subject sulfur dioxide in smelter gas to
thermal reduction with methane or natural gas and then convert the thermal
reactor effluent to elemental sulfur. Gases containing S02, such as smelter
gas, can also be passed through a bed of incandescent solid carbonaceous
fuel, and the resultant effluent gas can be reacted with additional S02 to
produce elemental sulfur.
As a general proposition, it must be concluded that prior art
reducing agents are effective to convert sulfur dioxide to hydrogen sulfide,
elemental sulfur or to a mixture of both. The use of a carbonaceous reducing
agent is, however, accompanied with an undesirable tendency to form soot,
carbon oxysulfide and carbon disulfide as by-products of the reducing reaction.
Such by-products are undesirable in that all will, to some extent, adversely
affect the purity of the more desirable sulfur product or products. More-
over, soot will, generally, discolor the elemental sulfur product whilethe carbon oxysulfide and
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1057028
1 carbon disulfide, both of which are not easily con~erted to
2 elemental sulfur, will decrease the yield thereof. Also,
3 since the more preferred of the prior art reducing agents
4 are gases, availability at the site and storage have created
some problems with respect to use thereof.
6 Absorption processes for the separation of sulfur
7 dioxide from various gaseous mixture comprising the same are
8 also well known in the prior art. Generally, such prior art
9 proce~ses involve first contacting the gaseous mixture with
an absorbent which combines with the sulfur oxide to form
11 either a solid or liquid and thereby separates them from the
l2 gaseous mixture~ followed either by disposal of the solid or
13 liquid product or by regeneration of the absorbent with the
14 release of absorbed sulfur oxide. Such sulfur oxide removal
processes have been classified as either "throw-away" pro-
16 cesses or regenerative processes. Throw-away processes in-
17 clude, for exa~ple, processes in which sulfur oxides in a
18 gas stream are reacted with calcium oxide (or calcium carbon-
19 ate) and the resultant calcium sulfite and sulfate are dis-
carded. Regenerative processes include both those processes21 that use a dry solid sorbent or acceptor and those that use
22 a liquid, and usually aqueous, scrubbing medium which is
23 capable of reacting with S02 (and SO3 when present), follow-
24 ed by regeneration of the scrubbing medium. The use of
ammonia in an aqueous medium for removing sulfur oxides from
26 gases is well known. The sulfur dioxide stream liberated on
27 regeneration of the absorbent solution has a much higher con-
28 centration of S02 than the original flue gas stream. This
29 sulfur dioxide is then converted to sulfurlc acid by known
means, although, as indicated previously, processes have been
31 sugge~ted to reduce such S02 to H2S or elemental sulfur.
32 The spent scrubbing solution is regenerated and reused for
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~S7~3Z8
1 scrubbing further quantities of sulfur dioxide containing
2 gas.
3 A principal problem associated with the prior art
4 processes, however, is that generally a multiplicity of
treating agents are required. For example, in some pro-
6 cesses one treating agent, e.g. CuO, might be used as the
7 absorbent, a second treating agent~ e.g. hydrogen, might be
8 used to regenerate the absorbent, and a third treating agent,
9 e.g. methane, might be used to reduce the sulfur dioxide
liberated in the regeneration step. Use of such multiple
11 agents further complicates those problems associated with
l2 availability at the site as well as those problems associa-
13 ted with storage at the site once the treating agents are
14 there available.
The need, then, for a reducing agent which, when
l6 not readily available at the site of intended use, might be
17 conveniently transferred and stored thereat is believed to
18 be readily apparent. Similarly, the need for a process
19 which will effectively permit the separation of sulfur di-
oxide from a gaseous mixture containing the same and the
21 ~ubsequent reduction thereof to either elemental sulfur,
22 hydrogen sulfide or a mixture of both with a single treat-
23 ing agent is also believed to be readily apparent.
24 It has now been surprisingly discovered that the
foregoing and other disadvantages associated with the prior
26 art reducing agents as well as the disadvantages of the
27 prior art methods for separating sulfur dioxide wherein a
28 multiplicity of treating agents are used can be avoided with -
29 the reducing agent and the method of separation of this in-
vention. The reducing agent can be conveniently transport-
31 e~ and stored as a liquid. A single treating agent can be
32 used to accomplish both the separation of sulfur dioxide
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1 from a gaseous mixture comprising the same and the subsequent
2 reduction thereof.
3 In one embodiment of this invention sulfur dioxide
4 is reduced with ammonia, and, in a second embodiment ammonia
is used as the sole treating agent. In the first embodimen~
6 the reduction will be accomplished by contacting the ammonia
7 and sulfur dioxide in a suitable manner and effecting the
8 desired reaction either thermally or catalytically. In the
9 second embodiment, ammonia is used as the absorbent and
various ammonium salts are formed during the absorption step.
11 These ammonium salts can then be decomposed so as to yield
12 sulfur and/or hydrogen sulfide or into a mixture comprising
13 ammonia and sulfur dioxide. This mixture can then be reac-
14 ted in the same manner as in the first embodiment, i.e.
either thermally or catalytically, to yield sulfur and~or
l6 hydrogen sulfide.
17 Broadly, the process for reducing sulfur dioxide
18 in this invention can be used to reduce sulfur dioxide from
19 essentially any source to sulfur and/or hydrogen sulfide.
Such sources include, but are not necessarily limited to,
21 flue gases, gases obtained from acid sludge decomposition,
22 ore roasting or smelter gases and the like. Generally, how-
23 ever, the sulfur dioxide will first be separated by known
24 methods from these gases, which gases will usually contain
oxygeh and/or other materials that might interfere with the
26 reduction, and the gases will then be contacted with the
27 ammonia reducing agent. Generally, then, the reduction will
28 be accomplished with a feed gas stream or gaseous mixture
29 comprising sulfur dioxide, generally at a higher concentra-
tion than in the source gas and, generally, free of undesir-
31 able components such as oxygen and the like. The actual
32 concentration of sulfur dioxide is, of course, not critical.
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l Notwithstanding this, however, it is desirable to have the
s~J~f~,r
A fulfur dioxide concentration as high as possible so as to
3 minimize heat requirements. For this reason, then, the sul-
4 fur dioxide concentration in the gaseous mixture to be treat-
ed should be at least lO mol %, preferably at least 20 mol %
6 and most preferably at least 50 mol %.
7 The reducing method of this invention is particu-
8 larly useful in the reduction of sulfur dioxide to sulfur
9 and/or hydrogen sulfide contained in a sulfur dioxide-rich
regeneration off gas streams obtained by the regeneration of
11 a solid sulfur dioxide removal sorbent. Flue gas desulfur-
12 ization processes wherein sulfur oxides are removed from
13 flue gas by employing solid sorbents, e.g. copper oxide on
14 alumina, which are regenerated with reducing gas are well
known. The regeneration off gas typically contains at least
l6 abou~, 10~ by volume of sulfur dioxide, plus small amounts of
17 reducing components. Other sulfur-dioxide-rich gas streams
18 can also be treated according to this invention. The sulfu~
19 dioxide-rich stream preferably is substantially devoid of
free oxygen.
21 The reducing gas in the process of this invention
22 is an ammonia-rich gas which may be either substantially
23 pure ammonia or a gas mixture comprising a ma~or proportion
24 Of ammonia. A preferred ammonia-rich gas stream will con-
tain no more than minor amounts of carbon monoxide and/or
26 hydrocarbons. Moreover, the ammonia-rich gas stream will
27 preferably be substantially free of any hydrocarbons higher
28 than methane.
29 The chemical reactions involved in the reduction
of sulfur dioxide with ammonia and which are of interest
31 wlth respect to the present invention are the following:
32 (1) 4NH3 + 2S02 > 2N2 + 2H2S + 4H20;
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1 (2) 2H2S + S02 ?3S + 2H20; and
2 (3) 4NH3 + 3S02 > 2N2 + 3S + H20
3 Equation (3) is, of course, simply the sum of Equations (1)
4 and (2).
As in the case of other sulfur dioxide reduction
6 reactionsJ the reduction with ammonia is highly exothermic.
7 It is thus possible to effect the reduction thermally and
8 autogenously (autothermally) through a proper control of re-
9 actant concentrations. Generally, the reduction will pro-
ceed, thermally, at a temperature above about 1200F. and
11 preferably at a temperature within the range of about
12 1500F. to about 3000F. and the same will proceed autogen-
13 ously with feed at a lower temperature when the heat of re-
14 action is equal to the sum of any heat loss from the reactor
plus any change in the sensible heat of the components fed
î6 to the reactor. The compositions and inlet temperatures
17 may then be varied to achieve any reaction temperature with-
18 in the stated range and, as already indicated, the tempera-
19 ture in the reduction reactor will be-determined, primarily,
by the concentrations of ammonia and sulfur dioxide in the
21 feed gas streams, and by the respective inlet temperatures
22 of these streams. Preferably, the feed streams will have
23 sufficient concentrations of ammonia and sulfur dioxide such
24 that the reactor does not require extraneous heat, e.g., hot
combustion gases or external fuel and air, except during
26 startup, in order to maintain a self-sustaining reaction.
27 Moreover, the process will not depend upon molecular oxygen
28 contained in either feed stream to supply heat, since ordin-
29 arily both feed streams, i.e. the ammonia stream and the
S02 stream, are essentially devoid of molecular oxygen.
31 It will, of course, be appreciated that the auto-
32 thermal reaction can be carried out at any desired pressure
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1 ranging from a slight superatmospheric pressure (e.g., about
2 5-10 psig in the thermal reactor) sufficient to overcome the
3 pressure drop in the process to several atmospheres. It will
4 also be appreciated that the process gas stream produced in
the reduction reactor can be cooled, preferably in a waste
6 heat boiler, to condense elemental sulfur.
7 When dilute feed gas streams, i.e. either the
8 ammonia-containing gas stream or the sulfur-dioxide-contain-
9 ing gas stream or both, are used, difficulty may be encount-
ered in maintaining a self-sustaining reaction in the ther-
11 mal reactor. In these situations, thermal reduction can
l2 still be achieved, but extraneous heat will be required so
13 as to maintain the reduction temperature with the afore-
14 specified ranges; viz., above about 1200F. and preferably
within the range from about 1500F. to about 3000F. Alter-
l6 natively, such dilute streams could be contacted wlth any of
17 the catalyst known in the prior art to be effective in the
18 reduction of S02 with hydrogen. These include, but are not
19 necessarily limited to, the transition metals or transition
metal oxides such as chromium, cobalt, nickel, molybdenum,
21 vanadium, manganese, tungsten, palladium and zinc and the
22 oxides thereof. These catalysts, when used will generally
2~ be supported on a suitable carrier such as alumina. Still
24 further examples of suitable catalysts are activated bauxite,
other silica based catalyst and activated carbon catalyst.
26 Co/Mo on alumina is particularly effective and is widely
27 used commercially in various reduction type reactions.
28 It will be appreciated that concentrated feed
29 streams COUld also be contacted in the presence of any one
3a or more of the aforementioned catalysts and the reduction
3t accomplished catalytically therewith. Because of the exo-
32 thermic nature of the reaction, however, staging and/or
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1 cooling will be required so as to avoid temperature runa-
2 ways which might adversely affect the particular catalyst
3 actually used. In either case, i.e. in catalytically treat-
4 ing either dilute or concentrated reactant streams, the re-
duction temperature will be controlled, generally within the
6 range from about 500~'. to about 2000F. and preferably
7 within the range of from about 900F. to about 1500F.
8 Generally, the reaction between the ammonia-con-
9 tain~ng gas and the sulfur-dioxide-containing gas according
to the present invention produces a process gas stream which
11 contains elemental sulfur, hydrogen sulfide, unconverted su~
l2 fur dioxide, and water vapor, and which may also contain
13 minor amounts of other constituents, such as carbon oxysul-
14 fide and carbon disulfide. This process gas stream may,
then, be cooled in order to condense water and elemental
l6 sulfur, which sulfur may conveniently be separated as a li-
17 quid from the gas stream. The uncondensed portion of the
18 process stream, which will comprise principally the H2S and
19 S02 may then be fed to a conventional type Claus plant. As
is well known, the feed to a Claus plant will generally con-
21 tain H2S and S02 in a mol ratio of 2:l. Any deviation from
22 this ratio in the uncondensed portion of the process gas
23 should desirably be corrected with additional H2S and/or S02,
24 as required. Alternatively, the relative concentrations of
ammonia and sulfur dioxide to the reduction reaction and/or
26 the temperature thereof can be controlled so as to yield
27 the proper H2S/S02 ratio in the process stream.
28 When the uncondensed portion of the process gas
2~ stream is fed to a conventional Claus plant, the same will
generally be treated in a plurality of catalytic conversion
31 ~tag~s in order to convert the hydrogen sulfide and sulfur
32 dioxide contents thereof to elemental sulfur. Each cataly-
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1~57~8
1 tic conversion stage includes a catalytic converter which
2 will generally contain a known Claus conversion catalyst,
3 such as alumina or bauxite, followed by a condenser in which
4 the gas stream is cooled and the elemental sulfur in the
stream is condensed as liquid sulfur and removed. Each con-
6 version stage will also generally include a reheater which
7 precedes the catalytic converter, in order to bring the
8 inlet temperature of the gas stream as it enters the cataly-
9 tic converter up to the desired temperature. It is import-
ant to maintain the gas temperature above the condensation
11 point of sulfur in the catalytic converters and in all
12 parts of the system except the sulfur condensers.
13 The effluent gas from the last stage will also be
14 cooled to condense elemental sulfur, which is in turn re-
moved. The remaining gas, i.e., the tail gas, may be incin-
16 erated to convert all sulfur species (i.e., hydrogen sulfide,
17 elemental sulfur, carbon oxysulfide and carbon disulfide) to
18 sulfur dioxide. In flue gas desulfurization installations,
19 the incinerated gas is preferably recycled to the flue gas
stream. Alternatively, the incinerated gas may be dis-
21 charged into the atmosphere after treatment, if necessary,
22 to reduce the sulfur content to a level acceptable from the
23 air pollution standpoint.
24 As indicated previously, the sulfur dioxide
treated in accordance with the present invention may be de-
26 rived from essentially any source. Moreover, the same may
27 contain sulfur trioxide, which trioxide will be reduced to
28 either elemental sulfur or H2S or both in substantially the
29 same manner as the sulfur dioxlde. In a most preferred
embodiment of the present invention, however, the sulfur
31 dioxide (and sulfur trioxide when present) will be obtained
32 by separation from a flue gas stream, and ammonia will be
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1~S7~Z8
1 used as the absorbent in the separation of the S02 from the
2 flue gas. The ammonium salts thus obtained will then be
3 decomposed directly to sulfur and/or hydrogen sulfide or to
4 a mixture comprising S02 and NH3, which mixture will then be
reacted either thermally or catalytically to yield elemental
6 sulfur and/or hydrogen sulfide.
7 In this most preferred ~mbodiment of the invention,
8 sulfur oxides, (sulfur dioxide, sulfur trioxide, or mixtures
9 of both) in a gas stream such as flue gas or smelter gas,
are converted into elemental sulfur by an oxidation-reduc-
11 tion reaction with ammonia5 This embodiment includes two
12 ma~or operations. First the gas stream containing sulfur
13 oxides is treated with ammonia in order to remove the sulfur
14 oxides therefrom, and the resulting ammonium salts (ammonium
sulfite, ammonium bisulfite, ammonium sulfate, ammonium bi-
16 sulfate, or a mlxture thereof) are separated from the gas
17 stream. Second, the separated ammonium salts are decomposed
18 via an oxidation-reduction reaction into products comprising
19 elemental sulfur and elemental nitrogen.
There is no known reference in the literature in-
21 dicating that the high temperature decomposition of ammonium
22 sulfite or s~lfate salts leads to the production of elemen-
23 tal sulfur, H2S, water and nitrogen. While many reducing t
24 agents for S02 have been suggested and are reported in the
literature (H2, C0, CH4, H2S for example), NH3 has never
26 been tried.
27 When using ammonia as the absorbent, the initial
28 gas stream containing sulfur oxides is first treated with
29 ammonia either by scrubbing the gas stream with an aqueous
medium (solution or slurry) containing ammonia, by in~ecting
31 ammonia into the gas stream, or by a combination of the two.
32 The ammonia in the scrubbing solution or slurry may be in
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1 the form of ammonium hydroxide (or dissolved ammonia),
2 ammonium sulfite, a mixture of ammonium sulfite and bisul-
3 fite, or other ammonium salt or salt mixture which is cap-
4 able of reacting with sulfur dioxide. Ammonia that is in-jected into the gas stream is preferably gaseous ammonia,
6 but may be an aqueous ammoniacal solution similar to the
7 above-described scrubbing solution. Both ammonia injection
8 and scrubbing with an aqueous ammonia solution are known
9 methods in the art for removing sulfur oxides from flue gas.
Treatment with ammonia is ordinarily carried out at a tem-
11 perature below about 400F., and the initial gas stream may
12 be cooled by conventional means, such as quenching with an
13 aqueous medium, where necessary.
14 The basic process of this embodiment may also in-
clude the step of quenching the hot flue gas and removing
16 fly ash therefrom prior to scrubbing with aqueous ammonia.
17 These steps can, however, also be carried out according to
18 known techniques. Fly ash removal can be carried out by
19 electrostatic precipitation, cyclone separation, or bag fil-
tering of the essentially dry solid from the gas. Alterna-
21 tively, fly ash can be removed simultaneously with Sx re-
22 moval by the liquid scrubbing medium.
23 Treatment with ammonia removes most of the sulfur
24 oxides and part of the nitrogen oxides (when present) in the
initial gas stream. The treated gas stream is generally dis-
26 charged into the atmosphere and typically contains 10~ or27 less of the S02 initially present, and little or no S03 or
28 ammonia. Ammonia treatment results in the formation of am-
29 monium salts of sulfur acids, i.e., at least one salt of the
group consisting of ammonium sulfite, ammonium bisulfite,
31 ammonium sulfate, and ammonium bisulfate. Sulfur dioxide in
32 the gas stream leads to the formation of sulfites, with some
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l sulfate formation when oxygen is also present; sulfur triox-
2 ide leads to the formation of sulfates, as is well known.
3 Some ammonium nitrate and/or ammonium nitrite may also be
~ formed due to the partial removal of nitrogen oxides from
the gas stream. These ammonium salts are separated from the
6 gas stream, either in the dry state or in an aqueous liquid
7 medium, such as the aqueous solution or slurry resulting
8 from the above-described gas scrubbing operation. When the
9 separated ammonium salts are in an aqueous medium, it is
desirable to remove at least a substantial portion of the
11 water associated with the ammonium salts prior to decomposi-
l2 tion thereof.
13 The NH3/SOX mol ratio in the separated ammonium
14 salts should be at least equal to the stoichiometric quan- !
tity for conversion of the chemically combined sulfur in the
l6 separated ammonium salts to elemental sulfur. The NH3/S0x
17 mol ratio should be at least 4/3~ which is the stoichiome-
18 tric ratio when all of the combined sulfur in the salts is
l9 present as sulfites (i.e., ammonium sulfite or a mixture of
ammonium sulfite and bisulfite). When all of this sulfur
21 in the separated salts is present as sulfate, the stoichio-
22 metric ratio is 2/1. When treating flue gas (in which
23 approximately 3-10% of the Sx content is S03, the rest
2~ S02), an NH3/S0x mol ratio in the separated salts of about
1.5 is preferred. Greater than stoichiometric quantities of
26 ammonia can be present without harmful effect. For example,
27 when it is desired to remove substantial quantities of ni-
28 trogen oxides in addition to sulfur oxides from the gas
29 stream, additional ammonia must be added to react with N0x,
and the NH3/SOX mol ratio in the separated salts may be as
31 h~gh a~ 2/l or even higher.
32 Separated ammonium salts are decomposed via a high
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''' 1057~Z~3 ,
1 temperature oxidation-reduction reaction into a reaction
2 product mixture comprising elemental nitrogen and at least
3 one member of the group consisting of sulfur and hydrogen
4 sulfide. Decomposition of the ammonium salts can be carried
S out in either one or two steps. In the one step method, the
6 ammonium salts are fed directly into a high temperature re-
7 action zone whlch is maintained at a temperature of about
8 500 to about 3000~., preferably about 900 to about 1500F.
9 This reaction zone preferably contains a catalyst, which may
be an activated bauxite. Alternatively, other silica-based
11 catalysts or activated carbon catalysts may be used. Th~
l2 catalyst may be dispensed with at temperatures toward the
13 upper end of the broad range.
14 The two step decomposition, which is ordinarily
preferred, differs from the one step decomposition in that
l6 the ammonium salts are first thermally decomposed into a
17 gaseous mixture comprising ammonia, sulfur oxides (S02 an~6r
18 S03), and water vapor. This gas mixture (or the ammonia
19 and sulfur oxides therein, after removal of the water vapor,
if desired) is then introduced into a high temperature re-
21 action zone where the reduction is accomplished either ther-
22 mally or catalytically in a manner heretofore described.
23 When sulfates (which decompose at higher temperatures than
24 sulfites) are present in the separated ammonium salts, the
thermal decomposition can be operated at temperatures suffi-
26 ciently high to cause decomposition of both sulfites and sul-
27 fates, or alternatively these salts can be decomposed at a
28 somewhat lower temperature sufficiently high to cause de-
29 composition of the æulfites while leaving a residue compris-
ing the sulfates.
31 The reaction product of the oxidation-reduction
32 reaction zone comprises elemental nitrogen and at least one
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1 member of the group consisting of elemental sulfur and hy-
2 drogen sulfide. Even when the stoichiometric quantity of
3 ammonia is present in the oxidation-reduction zone feed, the
4 reaction product mixture contains elemental sulfur, hydrogen
sulfide, and unreacted sulfur dioxide, with an H2S/S02 mol
6 ratio of approximately 2/1. When an excess of ammonia is
7 used, more hydrogen sulfide and less unreacted sulfur diox-
8 ide (or none) are present in the reaction product mixture;
9 this reaction product mixture may contain elemental sulfur
lQ and H2S, with little or no SO2. The use of low temperatures
11 within the broad operating range in the presence of a catal-
12 yst (and with stoichiometric or near stoichiometric quanti-
13 ties of ammonia) favors the formation of elemental sulfur
14 and lessens the amount of hydrogen sulfide formed.
To enhance the yield of elemental sulfur, the hy-
16 drogen sulfide in the reaction produce mixture from the oxi-
17 dation-reduction zone may be catalytically reacted with sul-
18 fur dioxide via the Claus reaction in one or more stages to
19 produce further quantities of elemental sulfur. The SO2 for
this reaction may be unconverted SO2 from the decomposition
21 of the original ammonium salt, or extraneous S02 added to
22 the reaction product mixture to adjust the H2S/S02 ratio to
23 the desired value of 2/1. Addition of an oxygen-containing
24 gas to the reaction product mixture or decomposer will have
an effect similar to S02 addition, as it will convert some
26 of the H2S and sulfur formed to SO2.
27 The Claus reaction is exothermic, since S8 is pro-
28 duced, and is carried out in the presence of a catalyst,
29 preferably activated bauxite, as is well known. Suitable
reaction temperatures for this step are about 4O0 to about
31 750~. The high temperature reaction and the subsequent
32 catalytic Claus conversion can be carried out either in
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1 separate reactors or in a single reaction vessel, as desired.
2 Even when a single reaction vessel having no physical divi-
3 der between the high temperature zone and the subsequent
4 lower temperature catalytic conversion zone is used, two re-
S action zones will be established, even though there may be
6 no precise boundary between them. In the first zone, the
7 reaction between NH3 and the sulfur oxide occurs, while the
8 second zone is essentially a clean-up step to drive the Claus
9 reaction as much as possible to completion. Intermediate
removal of elemental sulfur, and water, may be advisable be-
11 fore the final, low temperature Claus reaction step to en-
12 sure as complete a conversion of sulfur oxides and H2S as
t3 possible.
14 When the oxidation-reduction reaction is carried
1S out at low temperatures which favor the direct formation of
16 elemental sulfur, it may be pGssible to dispense with the
17 Claus conversion stages.
18 While the process of the present invention is gen-
19 erally applicable to the treatment of gas streams containing
sulfur dioxide and/or sulfur trioxide, the invention will be
21 described with particular reference to the treatment of flue
22 gas. The flue gas referred to here is a gas produced by
23 combustion of a sulfur-containing fuel (e.g., coal or fuel
24 oil) and ordinarily contains, besides nitrogen, water vapor,
and carbon dioxlde, some molecular oxygen (due to use of ex-
26 cess air) plus small amounts (typically about 0.2-0.3~ by
27 volume, the quantity depending on the quantity of sulfur in
28 the fuel) of sulfur oxides, of which not more than about 10%
29 (typically approximately 3-lO~) is in the form of S03, the
rest as S02. Flue gas may also contain fly ash, particular-
31 ly when coal is used as the fuel.
32 As previously stated, it is desirable to remove
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1 water from the solution or slurry of ammonium salts prior to
2 the oxidation-red~ction reaction between ammonia and sulfur
3 oxides. In the Claus equilibrium between S02 and H2S, water
4 is an end product and, therefore, suppresses the formation
S of elemental sulfur according to the equation
6 S2 ~ 2H2S ~ ~ 2H20 + 3S
7 Accordingly, some water removal is desirable before going
8 into the oxidation-reduction converter. A large part of
9 this water can be evaporated and removed prior to decomposi-
tion of the ammonium salts, if desired. As much water as
11 possible is removed by evaporation at low temperature,
12 e,g., about 250F.
13 The ammonium salts are then thermally decomposed
14 to yield a gas mixture comprising ammonia, sulfur oxides,
and water vapor. This vapor mixture, containing principally
16 SO2J ammonia, and water vapor, is heated and passed into a
17 high temperature reaction zone, preferably in the presence
18 of a catalyst, such as activated bauxite, thereby causing an
19 oxidation-reduction reaction between ammonia and sulfur
oxides to take place with the formation of a reaction pro-
21 duct gas mixture containing elemental sulfur, nitrogen, and
22 water vapor. As indicated previously, the temperature in
23 this high temperature reaction zone is about 500-3000F.
24 and preferably about gooo-l5oooF. when a catalyst is em-
ployed. This reac~ion product gas mixture will usually also
26 contain hydrogen sulfide and may contain small amounts of
27 sulfur dioxide. This reaction product gas mixture may be
28 passed into contact with a Claus conversion catalyst (e.g.,
29 activated bauxite) in one or more stages (typically about
400-750F.) at lower temperatures than that prevailing in
31 the high temperature reaction zone in order to cause reac-
32 tion of hydrogen sulfide and sulfur dioxide to form further
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1 quantities of elemental sulfur. Use of more than one stage
2 will insure a high degree of conversion to elemental sulfur.
3 The above-described high temperature reaction zone
4 preferably contains a catalystJ which may be activated baux-
ite, other silica-based catalyst, or activated carbon. An
6 oxidation-reduction reaction takes place in this high tem-
7 perature reaction zone, at temperatures in the range of about
8 500 to 2000F., but preferably 900 to about 1500F., re-
9 sulting in the formation of elemental sulfur and hydrogen
sulfide. Even when the stoichiometric quantity of ammonia
11 for conversion of all S02 present to elemental sulfur is
i2 present, some elemental sulfur and hydrogen sulfide are
13 formed, and some unreacted sulfur dioxide remains. The re-
14 maining S02 will amount to half the H2S formed in the NH3-
S02 interaction, and can be converted to elemental sulfur by
16 the Claus reaction previously cited. The principal reactions
17 taking place in the high temperature reaction zone are given
18 by equations 19 2 and 3 below:
(1) 3S02 + 4NH3 ~ 6H20 + 2N2 ~ 3S
(2) S02 + 2NH3 ~ 2H20 + N2 + H2S
21 (3) S3 + 2NH3 ~ 3H20 + N2 + S
22 and, of course, the Claus equation between H2S and S02 to
23 form elemental sulfur.
24 Various modifications of the basic process can be
made without departing from the scope of this invention. It
26 has already been stated that the treatment of the sulfur
27 oxide-containing gas stream with ammonia can be accomplished
28 by in~ecting ammonia into the gas stream or by a combination
29 of ammonia in~ection and scrubbing with ammonia in an aqueous
medium, rather than merely by scrubbing with ammonia in an
31 aqueous medium as described above. For example, sulfur ox-
32 ides can be removed from the gas stream by in~ecting ammonia
-18-
1057~Z8
1 into the gas stream and by reducing the temperature of the
2 gas stream, e.g., by quenching with water, to a temperature
3 at which ammonium sulfite and/or bisulfite will form. The
4 gas stream is preferably, but not necessarily, quenched to
below about 150F. and preferably to about 130F. It is
6 preferred to in~ect the ammonia upstream of quenching, in
7 order to minimize the formation of acids (sulfuric or sul-
8 furous acid) in quenching and thereby minimize corrosion.
9 An alternative mode of operation is to use a combination of
ammonia in~ection and scrubbing in order to remove sulfur
11 oxides. In such a mode of operation, one may inject ammonia
12 into the gas stream and then in~ect the scrubber effluent
13 medium (solution or slurry) containing ammonium salts (e.g.
14 ammonium sulfite and bisulfite) into the flue gas stream
lS ahead of the scrubber, thereby quenching the hot flue gas to
16 a temperature below about 150~. and evaporating large quan-
17 tities of water in the effluent solution or slurry. This
18 will form solid ammonium sulfite and bisulfite in granular
19 form or a highly concentrated slurry of the salts in aqueous
medium which are easily recovered by conventional means.
21 The solid granular ammonium salts or thickened slurry are
22 separated from the gas stream before the gas enters the
23 scrubber zone, and the separated salts are then decomposed
24 lnto ammonia, sulfur dioxide, and water as has been pre-
vlously described. When operating according to this mode of
26 operation, fly ash may be recovered either by conventional
27 means (e.g., water spraying followed by cyclone and/or elec-
28 trostatic precipitators or bag fllters) ahead of the ammonia
29 in~ection, or alternatively, the fly ash and the ammonium
salts can be separated together, e.g., in a cyclone, spray
31 dryer, gas filter, electrostatic precipitator or other con-
32 ventional means of solid/gas separation.
-19-
. -
~05 ,~8
1 The appended drawings illustrate particularly pre-
2 ferred embodiments of the invention. Figure 1 is a flow
3 diagram illustrating both the first and second embo~iments
4 of the invention, and Figures 2 and 3 are flow diagrams of
two respective modified forms of the second embodiment. Re-
6 ferring to Figure 1, a flue gas stream 10, containing sulfur
7 oxides (S02 and S03, principally for former) at a temperature
8 of approximately 350F. is passed through a fly ash removal
9 and quench unit 11. The flue gas is quenched and fly ash
removed in this unit by conventional means, as for example,
11 by injecting a water spray sufficient to lower the tempera-
12 ture to approximately 130F., and by separating the fly ash
13 by any of the conventional means already disclosed, either
14 before or after quenching. The separated fly ash is removed.
The quenched flue gas stream 13 is passed to scrubber ]4.
16 The flue gas is first countercurrently contacted with an
17 aqueous ammonia solution in Sx removal section 16 and then
18 with water, admitted via inlet 17, in ammonia removal section
19 18. The term "aqueous ammonia solution" refers broadly to
a solution which contains unneutralized ammonia, and encom-
21 passes aqueous ammonium hydroxide, aqueous ammonium sulfite,
22 aqueous ammonium sulfite-bisulfite solutions, etc., all of
23 which are known in the art. The aqueous medium admitted via
24 line 15 is preferably a solution but may be a slurry contain-
ing dissolved as well as undissolved ammonium salts. Gaseous
26 ammonia may also be injected via line 15. Scrubbed flue gas
27 leaves the top of scrubber 14 via overhead line 19, and may
28 then be reheated in a conventional reheater 20 and discharged
29 to the atmosphere through stack 21. This flue gas contains
only a small fraction, e.g., 10% or less of its original Sx
31 content, and will ordinarily also have an N0x content lower
32 than that o~ the entering flue gas. The overhead gas stream
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`; ~057~)Z8
1 19 is essentially free of S03 and ammoniaO
2 The aqueous scrubber effluent medium, which may be
3 either a solution or a slurry containing ammonium sulfite
4 and ammonium bisulfite, and which may also contain ammonium
sulfate, is separated from the flue gas stream and removed ~;
6 from the bottom of the scrubber through line 22. This
7 aqueous scrubber effluent medium is fed into a heated rotary
8 kiln 23. Preferably a large part of the water in the solu-
9 tion or slurry is evaporated at temperatures ranging from
150F. to 300F., preferably 175-255F., and taken off
11 separately via line 24 before decomposition of the ammonium
12 salts takes place. This water vapor leaving via line 24
13 will contain some NH3 and S02 which it is desirable to re-
14 cover. This is done by combining this vapor with feed to
scrubber 14 where said recovery is accomplished. The am-
l6 monium salts are decomposed in the latter portion of kiln
17 23 at temperatures ranging from 225F. to 800F., preferably
18 250-700F., to yield a vapor mixture containing sulfur
19 dioxide, ammonia, and water vapor. Sulfur trioxide may also
be present in small amounts due to the presence of ammonium
21 sulfate. In the process here illustrated, ammonium sulfite -
22 and ammonium sulfate are essentially decomposed completely
23 lnto gaseous products, i.e., ammonia, the corresponding sul-
24 fur oxide, and water vapor. As explained earlier, the de-
composition may be carried out at lower temperatures, say
26 225 to 500F., so that some residue from the ammonium sul-
27 fate remains. The gas containing ammonia, S02, and water
28 vapor is removed from kiln 23 via line 25, preheated in pre-
29 heater 26 to a temperature of approximately 1200F.,and then
passed into the high temperature reactor 27. It will, of
31 cours~, be appreciated that the S02 (and S03 when present)
32 which is fed to reactor or reduction unit 27 could be de-
-21-
1 rived from any of the sources heretofore noted and that the
2 same need not be mixed w th ammonia at this point or contain
3 ammonia at the desired concentration. In this regard, it
4 will be appreciated that "make-up" ammonia could be added to
the S02 or S02-ammonia mixture at essentially any point prior
6 to the reactor 27 even though this is not illustrated in the
7 Figure. It should be noted, however, that even when the S02
8 is not derived in the manner presently described, i.e. by
9 absorption with ammonia, the same may already be mixed with
sufficient ammonia to effect the desired reduction. For
11 example, when the S02 is absorbed with a supported metal
12 oxide sorbent such as CuO, the supported sorbent could be
13 regenerated with an excess of ammonia and, indeed, this would
14 be a preferred mode of operation with sorbents of this type.
Preferably, the reactor 27 will contain a catalyst
16 such as cobalt-molybdenum on alumina or activated bauxite,
17 especially when dilute feed streams are treated, and the tem-
18 perature will be ma~ntained within a range from about 500F.
19 to about 1500F. When concentrated feed streams are treated,
on the other hand, the reduction will, preferably, be effec-
21 ted thermally at a temperature within the range of about
22 1500F. to about 3000F. In either case, an oxidation-reduc-
23 tion reaction will take place in the reactor with the for-
24 mation of a reaction product mixture containing elemental
sulfur, hydro~en sulfide, unreacted sulfur dioxide, nitrogen,
26 and water vapor. This reaction i5 exothermic. The reaction
27 product nixture may then be cooled in cooler 28 to a suitable
28 Claus reactor inlet temperature, e.g., about 500-600F.,
29 anA then may be passed through one or more Claus reactors 29
containing a suitable catalyst such as activated bauxite.
31 Hydrogen sulfide and sulfur dioxide react in a 2/l mol ratio
32 in the Claus converters 29 to form further quantities of
lOS70Z8
1 elemental sulfur. The Claus reactor effluent stream 30 is
2 cooled in cooler 31 and passed to separator drum 32. Liquid
3 elemental sulfur 33 and the Claus unit tail gas stream 34
4 are taken off separately from drum 32. This tail gas stream
34 may be returned to the scrubber 14 if desired in order to
6 scrub out S02 as well as H2S prior to discharging into the
7 atmosphere.
8 Turning now to Figure 2 there is illustrated a
9 modified form of the present invention in which sulfur oxides
are removed from flue gas by a combination of ammonia injec-
11 tion and scrubbing. Like parts have the same reference nu-
l2 merals as in Figure l.
l3 As illustrated, then, a flue gas stream 10, typi-
14 cally at a temperature of about 350F., is passed through fly
ash removal unit 11 where fly ash is removed by conventional
l6 means. The discharged fly ash 12 is separated. A stream of
17 anhydrous ammonia 41 is in;ected into the flue gas stream 13.
18 Downstream of the ammonia in~ection, the aqueous scrubber
l9 effluent medium, which is a solution or slurry containing
ammonium bisulfite and ammonium sulfite (and, in some cases,
2l ammonium sulfate as well) is in~ected into the flue gas
22 stream via line 42. The water content of this aqueous
23 medium is substantially evaporated. This also cools the
24 flue gas stream to a suitable temperature for reaction be-
tween ammonia and sulfur oxide, e.g., approximately 130F.
26 Ammonium salts, i.e., ammonium sulfite, ammonium bisulfite,
27 and ammonium sulfate when present, are separated from the
28 flue gas stream in cyclone 43. The flue gas, from which a
29 large portion of the sulfur oxides have already been removed,
passes via overhead line 44 to scrubber 14. This scrubber is
31 similar to the scrubber 14 of Figure 1, and the detailed
32 description of its operation will not be repeated. Desulfur-
~, 105~
1 ized gas l9 is removed overhead from the scrubber. The
2 liquid scrubber effluent 42 is injected into the flue gas
3 stream upstream from the scrubber as has already been des-
4 cribed. The solid ammonium salts which are separated in
cyclone 43 are removed therefrom through line 44. These
6 solid salts are fed into rotary kiln 239 where they are de-
7 composed into a gaseous mixture containing S02, ammonia and
8 water vapor, which mixture is discharged from the kiln via
9 line 25. The operation of kiln 23 in Figure 2 differs from
that in Figure l in that there is little or no water of
11 solution associated with the salts to be decomposed; hence,
12 water overhead line 24 can be omitted, or the quantity of
13 water taken off via such a line can be sharply reduced, ;
14 limited solely to water of hydration of the salts and some
residual water adhering to the separated crystals. The gas
16 mixture in line 25 may be preheated in preheater 26, and
17 passed into reactor 45 which, again, will preferably con-
18 tain a catalyst, especially when treating relatively dilute
19 streams, but which will, preferably be operated without a
catalyst when treating relatively concentrated streams ! In
21 either case, the temperatures will be as heretofore indica-
22 ted. The concentrations at which the break in preference
23 between catalytic and thermal reduction occurs is, of
24 course, not clear cut since the relative advantages of both
methods of operation depend upon such factors as the heat
26 capacity of any other components in the feed stream and the
27 maximum temperature and/or temperature variation which can
28 be tolerated by the catalyst actually employed and the
29 equipment in which the reduction is accomplished. Nonethe-
less, it can be generally concluded that any feed compri-
31 9~ng 50 mol % or more S02 would be advantageously reduced
32 thermally. In the embodiment illustrated, both the
-24-
.
lOS ~Z8
1 previously described oxidation-reduction reaction and the
2 Claus reaction take place in reactor 45, giving an effluent
3 stream 30 which contains elemental sulfur, nitrogen, and
4 water vapor, but only small amounts of H2S and SO2. Any
hydrogen sulfide formed as a product of the oxidation-re-
6 duction reaction in reactor 45 may be converted to elemental
7 sulfur by the Claus reaction in the latter portion of
8 reactor 45, which may be maintained at a temperature of
9 400-750F. Liquid elemental sulfur 33 and a tail gas stream
34 are obtained as described in Figure l.
11 The ammonia-sulfur oxide-water vapor mixture 25
12 in Figure 2 can be further treated as shown in Figure l if
13 desired. Likewise, the gaseous mixture 25 in Figure l can
14 be treated as shown in Figure 2 if desired.
Turning now to Figure 3, there is illustrated a
l6 still further modified form of the present invention in
17 which fly ash and ammonium sulfites are recovered together
18 rather than separately as in Figures l and 2. Also, Figure
19 3 illustrates a modified procedure for converting the gas
stream containing SO2, ammonia, and water vapor, obtained by
21 decomposition of the ammonium sulfites, into elemental sul-
22 fur. As illustrated in Figure 3, then, ammonia is injected
23 via line 50 into a flue gas stream 51 which is at a tempera-
24 ture of approximately 350-400F. This flue gas stream with
added ammonia is fed into spray dryer 52. A portion of the
26 scrubber effluent solution from scrubber 14, containing
27 ammonium sulite and ammonium bisulfite (and optionally
28 ammonium sulfate) in an aqueous solution or slurry, is in-
29 ~ected into the flue gas stream in spray dryer 52 via line
53. In~ection of the aqueous solution or slurry into the
31 hot, dust-laden gas stream will act to coagulate the fine
32 dust particles, a salt matrix will stay behind essentially
-25-
. .
1 gluing the fine dust particles together into larger agglo- -
2 merates. This makes fly ash recovery much easier than
3 attempting to recover unagglomerated fines.
4 In addition, any fly ash particles that escape
this recovery step will be picked up on the subsequent
6 liquid scrubbing step and will become part of the circula-
7 ting liquor and in turn will become incorporated into the
8 above-mentioned agglomerates when the water evaporates from
9 the spray dryer particles. Some fly ash and solid ammonium
salts are separated from spray dryer 52, and are conveyed
11 via line 54 to a collecting bin 55. The flue gas stream 56
12 containing some entrained fly ash and ammonium sulfites is
13 passed to a cyclone separator 579 where most of the remain-
14 der of the solids (fly ash and ammonium sulfites) are sepa-
rated out and conveyed via line 5~ to collector 55. The flue
l6 gas stream 59 is withdrawn overhead from the cyclone sepa-
17 rator 57, and flows into the lower zone of scrubber 14. As
18 in the embodiment of Figure 2, a substantial portion of the
19 sulfur oxides have been removed from the flue gas stream
before it reaches the scrubber 14. The scrubber 14 in
21 Figure 3 is essentially the same in its operation as its
22 counterpart in Figures 1 and 2. As in the previous figures,
23 the scrubber will not only remove the sulfur oxides, but
24 also any particles of fly ash that have escaped previous
removal steps. Some nitrogen oxides in the flue gas will
26 also be removed by the scrubbing medium in the form of
27 ammonium salts. In this case, however, a portion of the
28 scrubber effluent solution or slurry 22, which contains
29 ammonium sulfite and bisulfite, is pumped around and rein-
troduced at the top of the SOx scrubbing section 16 via
31 pump around line 60. (A similar pump-around line may also
32 be used in the embodiment of Figures 1 and 2; pump-arounds
-26-
' 1~57~8
1 of this type are known in the art.)
2 The mixture of fly ash and ammonium salts which is
3 collected in collector 55 is fed into rotary kiln 23. The
4 ammonium salts are decomposed into ammonia, sulfur dioxide,
and water vapor, with sulfur trioxide also being formed when
6 the ammonium salt mixture contains aMmonium sulfate. This
7 reactor product gas is withdrawn from the kiln via line 25.
8 A residue which is principally fly ash is discharged via
9 line 61 from kiln 23. This residue may also contain
ammonium sulfate9 calcium sulfate, sodium sulfate, andtor
11 magnesium sulfate~ etc. depending on the temperature at
12 which the kiln 23 is operated and on the chemical composi-
13 tion of the fly ash.
14 The gases containing SO~ ammonia9 and water
vapor, which again could be derived from other sources, in
16 line 25 are passed to a high temperature reactor 62, which
17 in this embodimen~ includes a plurali~y of reaction sections.
18 The gas stream 25 is split into two portions 63 and 64. The
19 first portion 63 is preheated to the desired reaction tem-
perature such as by the addition of air9 and combustion of
21 some of the contained NH3 in burner 65. This raises the
22 temperature of this stream to approximately 1300-1400F., at
23 which temperature it enters the reactor 6'J. Reactor 62 con-
24 tains a plurality of reaction sections (three are shown
here). Since the reaction between ammonia and sulfur di-
26 oxide to make nitrogen and sulfur and hydrogen sulfide is
27 exothermic, the portion of the reaction gas 64 is split
28 again into two portions and added as a quench gas to reactor
29 62 after the first and second sections thereof. In this way
the inlet temperature to each section is approximately
31 1300-1400F. The reaction product gas discharged from
32 reactor 62 via line 689 containing sulfur dioxide, hydrogen
'
l~ Z8
1 sulfide, and elemental sulfur, is cooled in heat exchanger
2 69 to approximately 400 -650Fo ~ and is introduced at this
3 temperature into the low temperature Claus reactor 70. One
4 or more Claus reaction stages may be used. As in the pre-
vious embodiments, this Claus reactor may contain a catalyst
6 such as activated bauxite. The Claus reactor effluent is
7 passed to condenser 71, liquid elemental sulfur is withdrawn
8 via line 72, and Claus unit tail gas is withdrawn via line
9 73.
In place of kiln drying and decomposing, alternate
l1 methods of achieving the same results can be employed. One
12 of several ~luid beds can be used for this purpose, with
l3 separate or combined drying steps. The simplest operation
14 is a sing~e high temperature fluid bed maintained at high
lS temperature reaction conditions into which the ammonium sul-
l6 fite-containing liquid, slurry or solid mixture is injected.
17 Drylng, decomposing9 and S2 reduction will take place side
l8 by side, resulting in the release of a reaction product gas
19 containing, besides nitrogen and water vapor, a mixture of
sulfur, sulfur dioxide, and hydrogen sulfide. The fluidized
21 solids may be a captive bed of reaction catalyst such as
22 activated bauxite, or fly ash, or both. The fluid bed may
23 be operated between 500 and 2000F. but preferably between
24 900 and 1500F. Fly ash may be withdrawn as needed.
Further passage of the reaction product gas through subse-
26 quent fixed bed Claus reactors is within the scope of this
27 invention.
28 Alternatively, the resultant slurry can be dried
29 by injection into a fluidized bed maintained at about 150-
300F., preferably 175-255F., follGwed by decomposition of
31 the dried salt in a second fluid bed operating at about 225
32 to ~00F. The vapors from the second bed are taken to the
?
1057~Z8
1 previously described high temperature reaction zone.
2 Various combinations of drying, decomposition, and
3 reaction steps can be visualized without departing from the
4 essence of this embodiment of the present invention. Batch
operation is, of course, possible with an initial drying
6 step, followed by decomposition and even a high temperature
7 oxidation-reduction step. A high temperature fluidized bed ~ -
8 is also within the scope of this embodiment of the invention
9 where salt decomposition and the oxidation-reduction reaction
lo take place essentially simultaneously with the release of -~
11 elemental sulfur, water, nitrogen and hydrogen sulfide to-
12 gether with unconverted sulfur dioxide. Clean-up of this
13 gas stream by passage through lower temperature Claus reactor
1~ ætages is similar to the techniques described in connection
with Figures 1-3.
16 The foregoing disclosure describes the application
17 of this invention to the removal of sulfur oxides from gases
18 containing them and the reduction of such oxides using
19 ammonia as a reducing agent. Moreover, when ammonia is
used as an absorbent some nitrogen oxides may also combine
21 with the ammonia to form salts, such as ammonium nitrate and
22 ammonium nitrite, leading to some NOX removal from the gas.
23 The resultant ammonium salts will decompose along with the
24 salts of the sulfur acids and, after passage through the
high temperature reaction zone, will substantially decom-
26 pose into nitrogen and water. The subject process is there-
27 fore a method of desulfurization of gases to form elemental
28 sulfur and a partial nitrogen oxide removal method.
29 EXAMPLE 1
This example describes four laboratory runs in
31 which sulfur dioxide was thermally reduced with ammonia. In
32 each run, a feed gas containing sulfur dioxide and gaseous
-29-
~L~5~7~Z8
am~nonia, was passed through a heated reactor. The NH3/S02
2 mole ratio in the feed gas and the temperature were varied
3 in each run as indicated in Table I below. The effluent gas
4 from each run was an~lyzed and the analysis from each run is
also indicated in Table I below. In each of the runs, the
6 residence time in the reactor was between about 1 and 1.5
7 seconds.
8 TABLE I
9 Results of Runs in Example 1
Run No. 1 2 3 4
11 Reactor Temp.C. 943 920 1040 1120
12 NH3/S02 mol ratio 5.6/3 5.8/3 5/3 4/3
13Effluent ComPosition,
mo 1 ~/0 .
14 H20 58.7 63.4 44.9 58.5
H2S 11.6 39.0 21.9 4.7
l6 S2 1.5 4.6 Nil 5.6
17 N2 24.7 20.1 28.9 24.7
18 S2 3.4 7.1 2.1 6.0
19 NH3 03 Nil 1.8 0.2
The contents of nitrogen, elemental sulfur and
21 hydrogen sulfide in the effluent demonstrate that the oxida-
22 tion-reduction reaction took place to a substantial extent.
23 EXAMPLE 2
24 This example describes two laboratory runs in
25 which sulfur dioxide was reduced by ammonia over a catalyst.
26 In each run, a feed gas containing sulfur dioxide, ammonia,
27 water vapor, and argon (as an inert tracer) was passed
28 through a heated bed of activated bauxite catalyst. The
29 NH3/S02 mole ratio in the feed gas was about 1.5/1; mole
30 rates of each component will be given in Table II below.
31 The effluent gas was cooled to remove condensable substances.
32 The remaining gas was analyzed and analysis indicated a
-30-
~ 3L0~7~Z8
1 complete disappearance of both ammonia and sulfur dioxide,
2 and the appearance of hydrogen sulfide, nitrogen, and in one
3 run, hydrogen. When the apparatus was taken apart, after
4 the second run, a large quantity of elemental sulfur was -
found deposited on the lower part of the catalyst bed. A
6 white soluble solid, possibly ammonium sulfite, was also
7 found deposited on the catalyst.
8 The contents of nitrogen, elemental sulfur and
9 hydrogen sulfide in the effluent demonstrate that the
oxidation-reduction reaction took place to a substantial
11 extent.
12 Data and results are in Table II below.
lOS7~i28
1TABLE II
2Experimental Resuits
3(782 ~_En~elhard Activated Bauxite CatalYst)
4 Run No. A B
Reactor Temp.,F. 1300-1530 1275-1375
6 Feed Gas,cc/min.S.T.P.
7 A 920 635
8 H20 1244 1244
9 NH3 590 590
S02 396 396
11 Effluent Gas, mol %
l2 A 70.7 67-73
13 H2 1.0
14 N2 21.2 17-24
NH3 0.1 0.1-0.2
l6 S02 0.8 0-0.1
l7 H2S 6.2 5.9-8.6
l8 Liquid Analysis~ wt.%
l9 Elemental S 1.6
NH4+ 1.7
2l S04- 2.3 t
22 S03~ 8.3
23 S' Tr.
24 Solid AnalYsis
Elemental S,% of Feed 19%
26 EXAMPLE 3
27This example describes three experiments showing
28 that it is feasible to decompose ammonium sulfite, or ammon-
~9 ium sulfite-bisulfite mixtures, by heat either in the pres-
ence or in the absence of fly ash.
3lExperiment 1. Thirty grams of ammonium sulfite
32 was heated in a round bottom flask for a period of 1.5 hours.
.
-32-
.. . . , - -
-
.. ~ ~ ~ . - . - -
1~i7~Z~
1 About 90% disappeared as the temperature was gradually
2 raised to 340F.
3 Experiment 2. Twenty grams of fly ash, 59 grams
4 of 45% aqueous ammonium bisulfite solution, 36 grams of
ammonium sulfite, 1.5 grams of ammonium sulfate, and 10 cc
6 of water were mixed into a paste and heated over a period of
7 1.5 hours to a final temperature of 520F. in a round bottom
8 flask. The final residue weighed 23 grams, indicating a
9 nearly complete disappearance of the sulfites.
lo Experiment 3. This experiment, using an identical
11 charge as above, followed the procedure of Experiment 2,
12 except that the pot temperature was maintained below 400F.
13 over a period of about 3 hours, and was not allowed to climb
14 above this level. A loss of approximately 75% of the salt
content of the charge was observed.
16 The process of this invention provides an efficient
17 means for converting the sulfur dioxide content in an S02-
18 rich gas stream to elemental sulfur. This process is parti-
19 cularly applicable in locations where hydrogen-rich gas
streams are utilized in a flue gas desulfurization process.
21 The advantages of hydrogen over carbonaceous gaseous redu-
22 cing agents, e.g., carbon monoxide and low molecular weight
23 hydrocarbons, include minimal formation of soot, COS and
24 CS2 with attendant high quality product sulfur and minimum
quantities of sulfur (elemental or combined) in the tail gas
26 stream. The use of a thermal reactor upstream of the cata-
27 lytic conversion stages is desirable because a higher opera-
28 ting temperature and greater temperature rise can be toler-
29 ated in a thermal reactor than in a catalytic converter.
This makes it possible to replace several catalytic conver-
31 te~ stages and the coolers following each stage with a
32 single thermal reactor, resulting in considerable heat
-33-
.
5~Z ~
1 economy. Reduction of S02 to a mixture of elemental sulfur,
2 H2S, and unreacted S02, rather than entirely to H2S in the
3 thermal reactor ls desirable because this results in a
4 smaller load on the Claus catalytic conversion unit, thereby
increaslng the sulfur recovery or reducing the number of
6 catalytic converter stages required for the reaction of H2S
7 and S2 to form sulfur.
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: , - .. ~ - - . ,
,. ~ .
