Title of Invention

"PROCESS FOR REMOVING SULPHUR DIOXIDE FROM FLUE GASES , IN PARTICULAR FROM POWER STATION EXHAUST GASES AND EXHAUST GASES FROM WASTE INCINERATING PLANTS"

Abstract The invention relates to a process for removing SO2 from flue gas, wherein the flue gas is treated with an absorption fluid in the washing zone of a washer-scrubber in the countercurrent. In accordance with the invention, the absorption fluid consists of a mixture of ammonia water, a ammonium sulphite-containing fluid which is drawn off from the washer-scrubber below the washing zone and returned without intermediate treatment, and an ammonium sulphate solution which is returned from an oxidation device. The mixture ratio between the ammonium sulphite and the ammonium sulphate is in the range between 15 : 1 and 3:1.
Full Text - 2-
The invention relates to a process for removing sulphur dioxide from flue gases, in particular from power station exhaust gases and exhaust gases from waste incinerating plants, wherein:
the flue gas is directed from the bottom to the top through a washer-scrubber comprising at least one washing zone and is treated in the countercurrent with an absorption fluid which is added at the upperside of the washing zone and drawn off out of the washer-scrubber below the washing zone, and
the absorption fluid is returned in a washing fluid circuit to the upperside of the washing zone,
wherein ammonia water is supplied to the washing fluid circuit and a fiuid flow branches off from the washing fluid circuit and is directed to an oxidation device in which ammonium sulphite ((NH4)2SO3) and ammonium hydrogen sulphite (NH4HSO3) present in the fluid are oxidized to become ammonium sulphate (NH4)2SO4).
When removing sulphur'dioxide from flue gases by means of a gas washing process using an aqueous ammonia solution as the absorption fluid, essentially the following reactions occur:
SO2 + NH3 + H20 =NH4HSO3 NH4HSO3 + NH3 =(NH4)2SO3 (NH4)2SO3 + SO2 + H2O = 2NH4HSO3 (HH4) SO3 + 1/2 O2 = (NH4) 2SO4.
Ammonium sulphate, which is a marketable by-product and which can be used as fertilizer, is produced during the process.
A process comprising the features described in the introduction is known from DE-C 37 33 319. A washing fluid circuit is connected to the middle washing zone of a three stage washer-scrubber, ammonia water is added as an absorber to the washing fiuid circuit and water is added to compensate the material balance. A partial flow is drawn off from the washing fluid circuit and directed to an oxidation

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device, where oxidation occurs to form an ammonium sulphate solution which is further processed by concentration and granulation to form a product which can be marketed as fertilizer. The absorption fluid guided in the circuit includes a high concentration of dissolved salts, in particular (NH4) 2SO3 and NH4HSO3. The absorption fluid used in the washing zone comprises a high NH3-partiai pressure which encourages the formation of impure aerosols, in the case of the known process a third washing zone is connected downstream, in which washing zone the ammonia present in the gas is washed out to a permissible limit value using a washing fluid which has been rendered acidic by the addition of sulphuric acid. The additional washing zone to separate out the NH3 is costiy. Also, the formation of aerosols has not yet been avoided in a fully satisfactory manner.
In the case of a process known from EP-A 0 778 067 ammonium sulphate solution is
used as the absojption fluid which is drawn off from an aerated settling basin of the
washer-scrubber and directed to nozzle devices at the top of the washer-scrubber,
it is endeavoured to achieve as complete as possible an oxidation of the absorption
fluid in the washer settling basin. So much absorption fluid is locked out of the
washing fluid circuit that the salt content of the absorption fluid amounts to approx.
25 to 40 %-weight. The absorption of SO2 by the ammonium sulphate solution
used in the process is unsatisfactory. \i is necessary to provide a longer washing
zone in order to remove the SO2 from the flue gas until the permissible limit value
has been achieved. Furthermore, the level of aerosol formation established is no
longer acceptable, if the smoke to be cleaned contains a high SO2 concentration of
more than 2700 mg/Nm3. Similar processes are known from EP-A 0 212 523 and
EP-A 0 620 187. An ammonium sulphate solution is also used in the case of these
processes as the absorption fluid. j
The object of the invention is to improve the process described in the introduction to such an extent that it is possible to separate out SO2 effectively whilst keeping the aerosol formation as low as possible.
In order to achieve this object, the invention teaches that the washing fluid added to
the upperside of the washing zone consists of a mixture of:
a) ammonia water,
b) ammonium sulphite-containing fluid which is drawn off out of the
washer-scrubber below the washing zone and is returned without
intermediate treatment, and
c) ammonium sulphate solution which is returned from the
oxidation device,

p
wherein a mixture ratio between the ammonium sulphite ({NH4) 2SO3) and ammonium sulphate ((NH4) 2SO4) in the range of 15 : 1 to 3 : 1 is adjusted. The ammonium sulphite/ammonium sulphate mixture ratio preferably lies in the range between 10:1 to "5 : 1.
it was unexpectedly established that ammonium sulphite-containing washing fluid in the mixture with ammonia water and an ammonium sulphate solution produces an absorption fluid which is characterised by a high capability to absorb SO2 and does not demonstrate a tendency for aerosols to form. SO2 is very rapidly absorbed by ammonium sulphite, which is present in a high concentration in the absorption fluid in accordance with the invention, to form NH4HSO3. An equilibrium is formed between SO2 from the flue gas, NH3 added as ammonia water and (NH4) 2SO3. SOp is absorbed more rapidly by the absorption agent used in accordance with the invention than the absorption by ammonia water alone or by a mixture of ammonia and ammonium sulphate solution. It is possible by means of the more rapid and more effective absorption of SO2 to increase the through-flow rate of the flue gas and/or to operate with a short washing zone. With the aid of the process in accordance with the invention, it is possible to ciean flue gases comprising a high sulphur oxide content. The aerosol content in the cleaned flue gas can be maintained constantly below 15 mg/Nm3 and in most cases can even be reduced to below 10 mg/Nm3. The proportion of the ammonium sulphate solution contributes to the iow potential aerosol formation of the absorption fluid used in accordance with the invention. This can be due to the fact that the ammonia partial pressure of the ammonium sulphate is noticeably lower with respect to ammonium sulphite/ammonium hydrogen sulphite.
The ammonia water used generally comprises an ammonia content between 10 and 30 %-weight. It can be formed directly in situ by mixing ammonia with water, but other commercially available products can also be used. The addition quantity of ammonia water is controlled by the pH-value of the ab3orption fluid. The pH-value can be adjusted in the range between pH 4, 5 and 7. The pH-vafue of the absorption fluid on the addition side is adjusted preferably between pH 5 and pH 6.5, wherein the pH-value is adjusted by adding ammonia water.
The quantity flow of the ammonium sulphite-containing fluid returned directly to the washing zone whilst by-passing the oxidation stage is adjusted so that the absorption fluid added to the washing zone contains a hyperstoichiometric quantity of (NH4) 2SO3 corresponding to the reaction equation:

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(NH4) 2SO3 + S02 + H2O = 2NH4HSO3
for the absorption of the quantity of SO2 introduced with the flue gas into the washing zone. In an expedient manner the flow quantity of the ammonium sulphite-containing washing fluid is controlled such that the absorption fluid contains an ammonium sulphite concentration which is greater by approx. 25% than the stoichiometric quantity necessary according to the aforementioned reaction equation for a stoichiometric conversion.
in accordance with a preferred embodiment of the invention, the washer-scrubber comprises in the washing zone a packing bed which is sprayed on the upper side with the absorption fluid. The addition quantity of the absorption fluid is determined according to a value empircally obtained for the packing bed. The fluid flow quantity of ammonium sulphate solution returned from the oxidation device into the washing zone is produced from the following material balance of the fluid flows:
mOX = m - mWF - mNH3 where:
mOX- flow quantity of the ammonium sulphate solution returned from the
oxidation device to the washing zone;
m: addition flow quantity of the absorption fluid
mWF; quantity flow of the ammonium sulphite-containing washing fluid
returned to the washing zone;
mNH3: flow quantity of freshly supplied ammonia water.
A partial flow of ammonium sulphate solution is locked out of the oxidation device. By virtue of the locked-out flow quantity, it is possible to control the salt content of the absorption fluid. Preferably, the process proceeds such that the ammonium sulphate solution comprising a (NH4) 2SO4 concentration of 25 %-weight to 30 %-weight is drawn off from the oxidation device.
The flue gas to be cleaned is generally produced by burning hard coal, brown coal, petroleum coke, oil, refinery residues, waste and similar combustible substances in

- 6-power stations. The gas generally has a temperature of approx. 135 to 280X and is directed to the apparatus by way of corresponding conduits. Upon entering the apparatus, the gas, which frequently contains other toxic substances hydrogen halides such as HCL, is sprayed in the cocurrent with lye, e.g. NaOH or KOH in order to neutralize the hydrogen halides. In accordance with a preferred embodiment of the invention, the flue gas is cooled prior to entering the washing zone by spraying with quenching water and/or ammonium sulphate solution and in so doing is simultaneously saturated with water.
In a further preferred embodiment of the process in accordance with the invention it is provided that the absorption fluid is drawn off from the washer-scrubber on a gas-permeable base below the washing zone and that the ammonium sulphate solution used for quenching purposes is separated out in the washer settling basin. Ammonium sulphate solution is removed again from the washer settling basin and sprayed as a quenching fluid into the flue gas current. The ammonium sulphate solution is concentrated by means of water evaporation. By controlling an incoming flow of ammonium sulphate solution, which is drawn off from the oxidation device, and drawing off a partial flow of concentrated ammonium sulphate solution from the washer settling basin, the (NH4) 2SO4 concentration can be adjusted in the washer settling basin. In this manner, it is possible to obtain the ammonium sulphate solutions of a concentration up to approx. 42 %-weight. Undesired crystallisation occurs at a higher concentration. It was established, that the (NH4) 2SO4 crystals, which form in the case of high concentrations, are not absorbed in the washer-scrubber and form aerosols which can only be separated out using costly filtering apparatus. At least a part of the concentrated ammonium sulphate solution, which is drawn off from the washer settling basin, can be directed to the ammonium sulphate recovery plant and reprocessed to form fertilizers.
The invention is explained hereinunder with reference to only one exemplified embodiment illustrated in the accompanying single figure shows the schematic illustration of apparatus for the purpose of implementing the process in accordance with the invention.
The flue gas directed to the apparatus is sprayed in a quenching section 1 in the cocurrent with a NaOH- or KOH-solution, which is supplied by way of conduit 2, in order to separate out any hydrogen halides present. Moreover, water is [sic] to the flue gas by way of a supply device .3 and/or ammonium sulphate solution by way of a conduit 4 cooled [sic] and saturated with water vapour. The fluid temperature is

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maintained in the region between 45°C and 60°C, wherein the saturation temperature of the gas determines the temperature of the quenching fluid. Preferably, ammonium sulphate solution is used as the quenching fluid and is drawn off from an oxidation device 5 at a concentration of from 25 to 30 %-weight and supplied to the settling basin 6 of a washer-scrubber 7.
The flue gas which has been cooled, saturated with water vapour and is hydrogen halide-free subsequently flows into the washing zone 8 of the washer-scrubber 7. The flue gas which is substantially charged with SO2 is brought into contact with an absorption fluid in the washing zone 8, which absorption fluid in accordance with the invention consists of a mixture of ammonia water, aqueous ammonium sulphate solution and freshly formed ammonium sulphite solution. A partial flow of the absorption fluid drawn off on a gas-permeable base 9 below the washing zone 8 is supplied by way of the conduit 10 to the oxidation device. Oxidation of the ammonium sulphite and ammonium hydrogen sulphite present in the fluid to form ammonium sulphate occurs in the oxidation device by adding an oxygenous gas. The other portion of the absorption fluid is directed by way of the conduit 11 in the circuit, mixed with ammonia water from the conduit 12 and with the ammonium sulphate solution supplied by way of the conduit 13 out of the oxidation device 5 and supplied to the spraying installation 14 above the washing zone 8.
The washing zone 8 contains a packing bed, which increases the contact surface between the flue gas and the absorption fluid and enhances the absorption of SO2. it is possible by using packing to reduce the height of the washing zone 8 and thus to reduce the installation height of the washer-scrubber 7 overalf. Commercially available packing consisting of a material which is inert with respect to acidic gases can be used in the packing bed.
In order to achieve an effective absorption of SO2, the pH-value of the absorption fluid on the addition side is adjusted to between pH 5 and pH 6.5. The pH adjustment is performed by adding ammonia water.
An air line 15 comprising a blower is connected to the oxidation device. The air supplied by way of the line 15 is brought into contact intensively with the fluid containing NH4HSO3 / (NH4) 2SO3. The ammonium sulphate solution produced collects in the settling basin of the oxidation device 5. The ammonium sulphate concentration is determined, for example, by measuring the density. With an ammonium sulphate concentration of from 25 to 30 %-weight the ammonium sulphate solution is drawn off from the settling basin of the oxidation device 5. The

partial flow drawn off by way of the conduit 16 can be directed to an ammonium sulphate recovery plant in which pure ammonium sulphate is produced by removing the water. A partial flow drawn off by way of the conduit 17 is directed to the washer setting basin and is used in a fluid circuit 18, which is connected to the washer settling basin, as a quenching fluid for the purpose of cooling the flue gas, wherein the ammonium sulphate solution is concentrated to form ammonium sulphate concentrations of up to 42 %-weight.
I ha fluid quantify in the settling basin of the oxidation device 5 can be maintained constant by way of a corresponding supply conduit by supplying water or washing fiuid from a buffer container 20.
In the exemplified embodiment illustrated in the figure, the washer-scrubber 7 comprises 3 second washing zone 21, likewise comprising a packing bed. A washing solution is added with a pH-value between pH 4 and pH 0 by way of a fiuid distributor 2.2 to the bed of the second washing zone 21 and brought into contact with the cleaned gas. If lies within the scope of the invention thai the flue gas which has had its acid components removed is directed through an aerosol separating device [not illustrated], although preferabiy such a filter installation is omitted. A wet electro filter disposed above the second washing zone can be used as an aerosol separating filter
The charged washing fluid of the second washing zone is drawn off on a fluid
collecting tray 23 disposed below the second washing zone 21 and directed by way of a conduit 24 to the buffer container. The pH-va!ue of the washing fluid is adjusted by supplying ammonia water by way of the conduit 12. The fluid level in the settling basin of the buffer container 20 is controlled by adding water by way of the conduit 25,
The cleaned flue gas is subsequently directed to the chimney and discharged into the environment.

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WE CLAIM:
1. Process of removing sulphur dioxide from flue gases, in particular from power station exhaust gases and exhaust gases from waste incinerating plants wherein:
the flue gas is directed from the bottom to the top through a washer-scrubber comprising at least one washing zone and is treated in the countercurrent with an absorption fluid which is added at the upperside of the washing zone and drawn off out of the washer-scrubber below the washing zone, and
the absorption fluid is returned in a washing fluid circuit to the upperside of the washing zone,
whe re i n ammoni a water is supp1i ed to the wash ing fluid circuit and a fluid flow branches off from the washing fluid circuit and is directed to an oxidation device in which ammonium
sulphite (NH ) SO ) and ammonium hydrogen sulphite
4 2 3 (NH HSO ) present in the fluid are oxidized to become ammonium
4 3 sulphate (NH > SO ), characterised in that the absorption
4 2 4 fluid added at the upper side of the washing zone consists of a
mixture of:
a) ammonia water,
b) ammonium sulphite-containing washing fluid,which
is drawn off out of the washer-scrubber below the
washing zone and is returned without intermediate
treatment, and

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c) ammonium sulphate solution which is returned from
the oxidation device,
wherein a mixture ratio between the ammonium sulphite and ammonium sulphate is adjusted in the range from 15s 1 to 3 i 1, 2. Process as claimed in claim i wherein the mixture ratio between ammonium sulphite sntf ammonium sulphate in the absorption solution is adjusted in the range between 10:1 and 5:1. 3- Process as claimed in any one of claims 1 or 2, wherein pH-value of the absorption fluid is adjusted on the addition side between pH 5 and pH 6.5, wherein the adjustment of the pH-value is performed by the addition of ammonia water.
4. Process as claimed in any one of claims 1 to 3, wherein
the flow quantity of the ammonium sulphite- containing washing
fluid returned directly to the washing zone whilst by-passing the
oxidation stage is adjusted in such a manner that the absorption
fluid added to the washing zone comprises a hyperstoichiometric
quantity of ammonium sulphite corresponding to the reaction
equation
£NH ) SO + SO + H 0 = NH HSO
423 2 2 243 for absorbing the SO quantity supplied with the flue gas to
2 the washing zone.
5. Process as claimed in any one of claims 1 to 4, wherein
the washer-scrubber contains in the washing zarte a packing bed

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which is sprayed on the upper side by the absorption fluid, wherein the addition quantity of the absorption fluid is determined according to a value empiracally obtained for the packing bed, and that the quantity flow of the ammonium sulphate solution returned to the washing zone from the oxidation device is adjusted according to the value of the predetermined addition quantity.
6. Process as claimed in any one of claims 1 to 5 wherein the ammonium sulphate solution is drawn off with an ammonium sulphate concentration of 25^-weight to ZL0%-weight from the oxidation device.
7- Process as claimed in any one of claims 1 to 6 wherein the flue gas is cooled prior to entry into the washing zone by being sprayed with quenching water and/or ammonium sulphate solution and said flue gas is saturated with water.
S- Process as claimed in claim 7f wherein the absorption fluid is drawn off out of the washer-scrubber on a gas-permeable base below the washing zone and that the ammonium sulphate solution used for quenching purposes is separated out in the washer settling basin, wherein ammonium sulphate solution is removed again from the washing settling basin and sprayed into

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the flue gas flow as quenching fluid and wherein in the washer settling basin an ammonium sulphate concentration of maximum 4255 weight is adjsuted.
The invention relates to a process for removing SO2 from flue gas, wherein the flue gas is treated with an absorption fluid in the washing zone of a washer-scrubber in the countercurrent. In accordance with the invention, the absorption fluid consists of a mixture of ammonia water, a ammonium sulphite-containing fluid which is drawn off from the washer-scrubber below the washing zone and returned without intermediate treatment, and an ammonium sulphate solution which is returned from an oxidation device. The mixture ratio between the ammonium sulphite and the ammonium sulphate is in the range between 15 : 1 and 3:1.

Documents:


Patent Number 208957
Indian Patent Application Number 01241/CAL/1998
PG Journal Number 33/2007
Publication Date 17-Aug-2007
Grant Date 16-Aug-2007
Date of Filing 16-Jul-1998
Name of Patentee LURGI LENTJES BISCHOFF GMBH
Applicant Address BONSIEPEN 13,D-45136,ESSEN,GERMANY
Inventors:
# Inventor's Name Inventor's Address
1 LOURENCO FERRAO BEETHOVENSTRASSE 15,D-46535 DINSLAKEN,
2 THEO RISSE FRIEDRICH-HEBEL-STRASSE 18,D-59368 WERNE,GERMANY
PCT International Classification Number BOLD 53/14,COLC 1/24
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 19731062 1997-07-19 Germany