CA1250410A - Removal of sulfur oxides from gas streams with ammonium sulfite - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process is disclosed to produce a commercially-valuable, concentrated nitrogen-sulfur solution by scrubbing sulfur dioxide from flue and other gases using ammonium sulfite solution as a scrubbing media in a two-stage process. The process also has the capability of removing the sulfur dioxide in the form of an insoluble zinc salt or calcium salt from the scrubbing solution without having to regenerate, recover and recirculate gaseous ammonia.

Description

~S()410 This invention relates to the treatment of gas streams containing sulfur oxides with ammonium sulfite in a manner that minimizes air pollution and produces a useful product while conserving both the ammonia and the sulfur.
Sulfur oxides are gaseous effluents which are a major source of air pollutants. Such sulfur oxides may be produced by the combustion of sulfur-containing fossil fuels or they may result from various erocesse~ where sulfur-containing compounds are processed. When the Sll Lfur content of the materials being processed is high enough, it is usually recovered either as elemental sulfur or in the form of sulfuric acid.
Many industrial processes and fossil uel combustion plants, however, produce gaseous effluent wherein the sulfur dioxide content is a maximum of approximately 2.0% and wherein very small amounts of sulfur trioxide are present. It is with gases of this type that the present invention finds particular utility.
Prior workers in the art have developed numerous types of erocesses to remove these sulfur oxides. At the present state of development, the most popular process in use employs lime as the active ingredient and produces a throw-away calcium sulfite-sulfate mixture. Other processes that have been designed with some success employ ammonia and the present invention is concerned with an improvement over such processes. Historically, ammonia scrubbing processes have either processed the scrubber liquid to solids to recover the ammonia and sulfur for fertilizers or other industrial uses, or have decomposed the ammonium salts to liberate the ammonia for reuse and the sulfur dioxide for sulfuric acid production.

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The Tennessee Valley ~uthority has investigated ammonia scrubbing at great length. Their publication "Sulfuc Oxide Removal From Power Plant Stack Gas, Ammonia Scrubbing Conceptual Design ~ost Study Series", Report ~PTD-0615, PB 196 804, Sept. 1970 contains both process and cost data on ammonia scrubbing and the production of ammonium sulfate and its use as an intermediate in phosphate fertilizer manufacture. Three processes for tceating the scrubber liquid are described. TVA
publication ~'Pilot-plant Study of an Ammonia Absorption-Ammonium Bisulfate Regeneration Process, Topical Report Phases 1 & 2", June 1974, PB-237-170 describes extensively the treatment of scrubber liquor with ammonium bisulfate to ~roduce ammonium sulfate and sulfur dioxide.
It is an object of a broad aseect of the present invention to erovide a process for treating gaseous streams containing sulfur oxides to remove at least ninety percent of the sulfur oxides from the effluent gases and to produce a useful fertilizer comeonent with no additional scrubber liquid treatment.
Since the use of fertilizer is seasonal, during certain times of the year, the fertilizer might not be saleable, and sufficient storage capacity may not be available. In such a case, the invention in another aspect teaches how the ammonium salts can be converted to calcium sulfite in the form of a neutral sludge that is not hazardous to the environment as defined by the Environmental Protection Agency. The absorbent ammonium sulfite is substantially regenerated and recycled without the necessity of liberating ammonia as a gas, collecting and recycling it, in the manner 30 described in U.S. Patents Nos. 3,579,296 and 3,929,967.

. 1~5~410 The entire ope~ation. with oc without the pcoduction of calcium sulfite is controlled with simple instrumentation, and is capable of handling large changes of sulfur o~ides cvncentration with a minimum of attention.
By one broad aspect of this invention, a process is provided for the removal of sulfur oxides from sulfur oxide-containing flue gases comprising scrubbing the flue gases in the first stage scrubber with an ammonium sulfite solution to remove most of the sulfur oxides and thereby to produce a liquor comerising ammonium sulfite, bisulfite and sulfate salts: recycling the liquor in the first stage sceubber and thereby to produce a 40% - 50% solution of dissolved ammonium sulfite, bisulfite and sulfate salts in a first stage reservoir while maintaining the pH between 5 and 5.8 maintaining the ratio of bisulfite to sulfite between 2.3 to 1 and 1 to 1 without liberating ammonia; removing portions of the 40% to 50%
solution o dissolved salts from the first stage reservoir;
scrubbing the flue gases in a second stage scrubber with an ammonium sulfite solution of pH between 6.2 and 6.8 to remove additional sulfur oxides and thereby to produce a dilute liquoe of 1% to 10% solids comerising ammonium sulfite, bisulfite and sulfate salts in a second stage ceservoir: and pumping the dilute liquor from the second stage reservoir to the first stage reservoir in amounts sufficient to replace the removed portions.
In more general terms, gases containing up to 2% S02 and minor amounts of SO, after running through a suitable device to remove particulates, e.g. a fabric filter or precipi-tator, are run through two series of ducts, each with ~25~

its own set of spray nozzles entrainment separation and recirculation tank and pump. Each tank is provided with a pH
meter, a specific gravity meter, a level controller, a temperature indicator and means for introduction of liquid or gaseous ammonia, which means are operated by the pH meter.
The pH stage 1 is maintained between 5.2 and 6.0 and in stage 2 between 6.2 and 6.8, e.g., by adding a pH-maintaining agent selected from the group consisting of ammonia and ammonium sulfite solution. at this pH, the formation of an ammonia salt plume is virtually eliminated, as is the loss of ammonia. If the ammonia salt solution is the desired end product, recirculation in both stages with continuous pH ad~ustment is run until the specific gravity in the first stage reaches a desired level, usually between 1.25 and 1.3, at which point the sulfur content of the solution is over 12% and the nitrogen content is over 8%.
This solution is then f:iltered to remove any particulates and can be used in the production of liquid fertilizers with no further preparation.
If there is no market for fertilizer, as during the late fall and early winter months, and the tank storage is full, the system is operated substantially as described except that a continuous stream of liquid is removed from the first stage, treated with an inexpensive metal oxide or hydroxide, e.g.
calcium hydroxide or zinc oxide, thereby to remove sufficient sulfur dioxide to regenerate the ammonium sulfite by producing an ,q ,, ~2S~

insoluble metal sulfite a~ a pH of 6 to 7.5, e.g. 6.5 to 7Ø
(Since the reported pH of pure ammonium sulfite solution is 8.0, maintaining the system below this pH insures minimum free ammonia in the filter cake). The resultant calcium or zinc sulfite slurry is then pumped into a thickener, the clear overflow is returned -to the second stage and the heavy underflow is filtered and washed. The solids are either hauled to a disposal site, particularly if calcium hydroxide is used, or stored for use in another process if zinc oxide is employed. This process may be carried on without the release of free ammonia, whatever the treatment agent may be. However, if the treating agent is calcium hydroxide, this process may be carried out with the step of recovering the ammonia for reuse. Additionally, if the treating agent is zinc oxide, it may be carried on with the additional step of removing the sulfur oxides as an insoluble zinc salt, A typical reaction is:
2Ca(011)2 + ~NH411S03 -~2CaS03 -~ 1/2 H20 + 2(NH4)2S03 ~ 3H20, wherein no free ammonia is liberated.
According to an aspect of the present invention, sulfur oxides are removed from flue or stack gas by contacting a gas stream containing the sulfur oxides in an absorber with an aqueous absorbent solution that is primarily ammonium sulfite-bisulfite. In substance, and in accordance with an aspect of this invention, sulfur oxide-containing effluent gases at elevated temperatures are initially contacted with ammonium .. . .. ... . . ..

~L2Sa4~0 - 5 a -sulfite-bisulfite solution in a scrubber having a series of spray nozzles along its length so as to permit effective contact with the rapidly moving gas stream. In both stages the following main reactions take place:

1- ((NH4)2S03 + S2 + H20_~2(NH4)Hso3

2. NH4HSD3 + NH3-~(NH4)2so3

3, 2(NH4)2S03 + 02-~2(NH4)2S4 In practicing an embodiment of the present invention, in effect, two multi-spray scrubbers are employed. The liquid from the first stage reaction flows into a reservoir wherein ~S~iO

the liquo~. containing mainly ammonium sulfite, bisulfite and sulfate is held while a portion of the liquor, after contcolling the pH by reaction 2, is recycled to the spray nozzles. The recycling is continued until the concentration of ammonium salts is measured to be 40% to 50%, an economical level permitting it to b2 removed from the first stage scrubbeL
for use as a fertilizer or as a liquid fertilizer component.
Gases passing through the first stage scrubber then enter the second stage scrubber which includes a series o~

spray nozzles through which a more dilute ammonium sulfite-bisulfite solution is pumped. The second stage scrubber treats gas that has been stripped of the majority of the sulfur oxides in the gas.
When the liquid in the first stage reaches the desired concentration, (some 60%), it is removed and replaced by liquid from the second stage. ~ level in the second stage is maintained by adding water.
In order to reduce the plume to an acceptable level, means are provided so as to control the pH of the scrubber liquid in the second stage system to 6.2 to 6.8.
The use of a series of sprays in each of the scrubbers is essential in carrying out aspects of this invention since minimum portions of the sulfur oxides are oxidized to the sulfate form. The plurality of sprays functions also to remove a substantial por~ion of any particles that may pass through the initial filter. ~dditionally, the arrangement of sprays in the scrubber permits the treatment of high velocity gases, i.e.
twenty to fifty feet per second, and the treatment of uniform ~2~410 or non-llniform sulfur oxide-~ontaining gases by continuous treatment with ammonium sulfite along the length of the scrubber.
The invention will be more fully explained and exemplified in the following description and claims of a preferred embodiment. taken in conjunction with the accompanying drawings, wherein like reference characters refer to similar parts throughout the several views, and in which:
Fig. 1 is a schematic flo~ sheet of a broad aspect of the process of this invention;
Fig. ~ is a schematic reeresentation exemplifying one embodiment of the system used in carrying out the process of an aseect of this invention; and Fig 3 is a schematic representation exemplifying a Eurther embodiment of a system used in carrying out the erocess of another aspect of this invention.

'"'-` 1~5~10 A'though specific terms are used in the following description ~or the sake of clarity, these terms are intended to refer onlv to the particular structure of the invention selected for illustration in the drawings and are not i.ntended to define or limit the scope of the invention.
Fig. 1 is a schematic flow sheet of the process occurring in each stage of the system shown in Fig. 2. Generally, hot filtered flue or stack gases which have been filtered substantially free of particulates enter into the first stage scrubber 11. There are no temperature limitations, anrl if the enter.ing gas is over 300 degrees Fahrenheit, the water sprays 19, which are positioned ahead of the scrubbing sprays 14 will be operated to cool the gases to 300 degrees Fahrenheit. The filtered gases are sprayed with ammonium sulfite liquor by means of a series of spray nozzles 14 which are positioned along the length of the scrubber. The gas velocities within the duct may range from 10 to 80 feet per second. The liquor containing the dissolved sulfite, bi-sulfite, and sulfate salts enters into a reservoir.
In the rese.rvoir, the pH is m,onitored and controlled to 5 to 5.8 by the addition of ammonia, and a portion of the liquid is recycled back to the spray nozzles 14. The pH of the liquid is maintained by the pH controller 2~. ~7hich controls the valve feeding ammonia into the vessel 16 as required.
The speciEic gravity of the liquid is continuously monitored by the meter 34 and the temperature could be monitored by a pyrometer.
The recycling step takes place until the concentration of the dissolved salts in the liquor in the reservoir 16 reaches 40~ to 50~, preferably 45%.
It is at this concentration that the ammonium salts in the solution are considered to he of such concentration as to warrant removal. ~.i.~lor may be added into the ~svstem as r~quired to m~intain fluid level along with ammonia so as to regenerate the ammonium sulfite. The pH selected, 5 to 5.8 maintains ratios of bisulfite to sulfite of 2.3 to 1 to 1 to 1. The higher ratios of bisulfite to sulfite are desirable to prevent crystallization upon cooling, since the bi~llfite is much more soluhle than the sulfite.

~25~ 10 The trcate~ gas enters from the first stage scrubber 11 into the second stage scrubher ?8 wherein the sr~me process which has just been described also takes place. ~owever, the gas in the second stage contains only dilute amounts of sulfur oxides. The ammonium salts liquor is used to rnaintain the level in the first stage recirculation tank 16. Jmhe liquid level in the second stage reservoir 32 is rnaintained with water. A two stage scrubber system as illustrated is considered to be adequate for most treating operations, although, it should be noted that it is possible to add additional scrubbirlg stages if desired.

As illustrated in Fig. 2, the apparatus is sh~wn with a gas entry pipe 10 wherein inccmmg flue or stack gases which have been passed through a kn~n filtering apparatus (not shcwn) pass by a sulfur oxide concentration indicator 15 and into the first stage scrubber 11. The first stage scrubber 11 ccmprises a scrubbing colurnn 12, which scrubbing colurnn is equipped with a ternperature indicator and temperature controller 13, 17 associated with an initic~l spray nozzle or nozzles 19. The nozzle 19 is controlled in kncwn manner to spray sufficient quantities of water so as to lower the incoming gas ternperature to 300 degrees Fahrenheit, i necessary. There is also provided in the scrubbing colurnn 12 a series of spray nozzles 14 through which an ammonium sulfite-bisulfite solution at a pH of 5 to 5.8 is sprayed onto the incorning gas by employing a system pur~? 48 of conventional design for the p~pose in known manne The ammonium sulfite solution immediately reacts with the sulfur oxides of the flue gas and the liquids flow downwardly by gravity into the reservoir 16.
At the bottcm of the re.servoir 16 is positioned a valved removal line 18 fron which an ammonium salts-rich liquor can be removed for subsequent resale or recirculation. A recycle line 20 extends from the pump 48 and transfers from the reservoir 16 a portion of the liquid for recycling to the scmb}~ing colunln 12 through the spray nozzles 14. A p~ indicator and controller 22 is provided in cooperation with a source for ammonia and a valve 24 so as to rraintain the pH in the system in the desired range. The ammonium sulfite-bisulfite solution generate~ may be continucusly recycled _ 9 _ ~5~

until the concentration has reached the desired level (40~ to 50%), at which time, the ammonium salt rich liquid can be removed. To maintain the desired liquid level, a controller 21 is provided, which allows liquid from the second stage reservoir 32 to he transferred to the first stage reservoir 16 through the recirculating line 26.
Gases flow.ing through the first stage duct 12 to the vessel or reservoir 16 discharge their entrained solution droplets into the bottom of the reservoir 16 b~ gravity and flow about a baffle 31 into the second stage duet system or serubbing column 29 through a conneetor 27 where ano'cher series of sprav nozzles 30 are positioned. The liquid from the second stage reservoir 32 is sprayed into the gas stream and through the spray nozzles 30 and is collee'ced by means of a eyelone separator 45. The cyclone separator 45 funetions to return the liquid to the second stage reservoir 32 frcm which the second stage pump 50 can again pump it back to the spray nozzles 30. m is liquid is a more dilute solution of ammonium sulfite-bisulfite having a specific gravity between ll and ~1.2. The pH is maintainecl at a point between ¦6.2 and 6.8 by the pH meter 36 which is provided to feed ammonia through the control valve 38. By reeirculating the liquid at a rate that provides much more ammonium sulfite than the stoichiometric ratio, high scrubbinq efficieney is maintained over wide .ranges of SO2 concentrations.
The liquid level in the second stage reservoir 32 is maintained by a conventional level controller 33, which functions to open a valve in the f.eed water line 41 until the desired liquid level is reached. The specific gravity and temperature of the liquid are continuously monitored by suitable instr~ments, including a conventional pyrometer 37. The gases, which are now separated fr~m the liquid drops, have the sulfur oxide content reduced by a least ninety percent are discharged to atmosphere through the stack 40.
The sulfur oxide content of the cleaned gas stream can be continuously monitored at the exit stack 40 by employing a suitable SO meter 46 and recorder 49.

~2SV~10 ~ 'hen the liauid in the first stage reservoir ]6 has reached the ~esired ~pecific gravity, the liquid is withdrawn, filtered and ~irected into a storage tanlc (not shc~n). ~dditionally, when the level in the reservoir 16 has reached the halfway point, the level controller 21 functions in knewn manner to c~en a valve in line 26, allc~;ng the reservoir 16 to fill up with liquid from the second stage reservoir 32. This is done at a rate slcw enough not to seriously reduce line pressure in the reeirculating ]ine 26 to thereby not reduee the effieieney of the spray nozzles 30. The liquid level in the second stage reser~oir or tank 32 is control]ed bv the level controller 33 which functions to open a water valve in the feed water line 41 to brina the reservoir up to its desired liquid level.
Gases leaving the first stage scrubber 11 are then direeted to pass through the top of the first stage reservoir 16 into the seeond stage serubber 28 which comprises a scrubbing duct 29. The gases ent2r '-he serubbing cluct 29 and are eontaeted with sprays of ammonium sulfite solution form the plurality of spray nozzles 30. The ser~bing solution then enters the seeond reservoir 32 wherein a portion of the solution is recyeled to the spray nozzles 30 for continuous treatment of the incoming gases in the seeoncd stage scrubber 28. The pH of the liauid in reservoir 32 is constantly monitored by a pll indicator and controller 36 at the desired level.
The liquid flc~s in each p~p diseharge line 20, 26 are sensed by suitable instru~ents in knc~n manner, which instruments send electronic or other signals to suitcible recorder-controllers (not shc~n). ~lthough the second stage scrubber 28 is shc~n to ~e horizontally disposed in the described embodiment, a vertical scrubber may be used instead without affecting the operation of the syst~m.
In the first illustrated embodi~,ent, two stages of treatment wlth the ammoniaeal liquid are indieated, but it will be understood that the operation need not be limited to this number. The S0 eontaining aas in the first stage of absorption contacts am~,onium sulfite, and continues its descent throuah the ~succeeding staaes of S0 absorp-tion -to discharge. In l;~S~ ~O

each successive stage of treatment, the pH of the liquid is maintained and adjud~ed independently. The cyclone separator 45 has been found to be advantageous for removal of liouid.
Thus, it is apparent that there has been provided, according to aspects of this invention, an outstanding, simple system for treating flue or stack gases containing sulfur oxides for the efficient removal of these contaminants and for the generation of ammonium salts solution which can be utilized or industrial or agricultural purposes.
In accordance with a further feature of the present invention, if the desired end product is a calcium or zinc sulfite salt, a continuous stream of solution from the first stage reservoir J6 is passed through a oonventional filter 60 as illustrated in Fig. 3, and enters a s~ge tank 61.
The liquid frcm the t~nk 61 is them passed into a pH adjustment container 63 here a pH controller 65 operates a lime feeder 66 and storage bin 67 to control the pH to the desired range, typically at approximately 6.8. A
mixer 62 may be provided in the container 63 to insure a complete reaction in a known manner. By maintaining the pH within the desired range, substantially no free ammonia will be liberated and the filter cake will be odorless. Accordingly, there will be a resultant savings of energy since it is not necessary to remove free ammonia or to provide protective means for personnel which may be handling and ultimately disposing of the product.
The liq~lid from the pH adjustment container 63 along with the suspended metal sulfites are continuously pumped into a thickening means 68 wherein the solids will settle to the bottom and can be removed by means of slurry pump 69, or the like, as a slurry containing approximately twenty percent to thirty percent solids. The slurry i5 them passed to a filter means 71, e.g.
rotary vacuum filter. A clear overflow liquid from the settler or thickening means 68, as controlled by a level controller 76 goes to the regenerated ammonium sulfite solution storage tank 74 through operation of the liquor controller pump 70. The filter cake frcm the filter can be conveyed into a disposal conveying means 73 such as a truck in known nanner.
Liquid from the filter 71 is pumped by means of a pump 72 back into the 25V~10 thickening means 68 for further clarification. Wash water 77 that is equalapproximately in amount to the dry metal sulfite cake weight may be us0d to wash the filter cake so as to minimize loss of ammonia salts. The wash liquid is then combined with the filtrate for use in the system.
The thickening means 68 is preferablv provided with a liquor sump level controller pump 70 which removes the clean (or clear) liquid and passes it to a regenerated ~mmonium sulfite storage tank 74. The liquid in the storage tank 74 c~ontains ammonium sulfite solution which is returned by a suitable pump 75 to the first stage scr~bber reservoir 16 at approximatel~

the same volumetric rate as that at which the liquor is removed. ~ny pH
adjustment which may be required in the first stage reservoir 16 is acccmplished by adding ammonia as required by the pH meter 22, or if too high, by temporarily stopping the return of the regenerated liquor.

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for removal of sulfur oxides from sulfur oxide-containing flue gases comprising:
scrubbing the flue gases in a first stage scrubber with an ammonium sulfite solution to remove most of the sulfur oxides and thereby to produce a liquor comprising ammonium sulfite, bisulfite and sulfate salts;
recycling said liquor in said first stage scrubber and thereby to produce a 40% - 50% solution of dissolved ammonium sulfite, bisulfite and sulfate salts in a first stage reservoir while maintaining the pH between 5 and 5.8:
maintaining the ratio of bisulfite to sulfite between approximately 2.3 to 1 and 1 to 1 without liberating ammonia:
removing portions of said 40% to 50% solution of dissolved salts from the stage reservoir scrubbing the flue gases in a second stage scrubber with an ammonium sulfite solution of pH between 6.2 and 6.8 to remove additional sulfur oxides and thereby to produce a dilute liquor of 1% to 10% solids comprising ammonium sulfite, bisulfite and sulfate salts in a second stage reservoir: and pumping said dilute liquor from said second stage reservoir to said first stage reservoir in amounts sufficient to replace said removed portions.

2. The process of claim 1 wherein maintaining the pH is accomplished by adding a pH-maintaining agent selected from the group consisting of ammonia and ammonium sulfite solution.

3. The process of claim 2 further comprising treating said removed portions of the solution with zinc oxide to remove sufficient sulfur dioxides to regenerate the ammonium sulfite by producing an insoluble metal sulfite at pH between 6.5 and 7Ø

4. The process of claim 3 and including the step of raising the pH to a value of 6.5 to 7.

5. The process of claim 3 wherein the treating with zinc oxide is carried on without the release of free ammonia.

6. The process of claim 5 including the additional seep of removing the sulfur oxides as an insoluble zinc salt.

7. The process of claim 2 further comprising treating the removed portions of the solution with calcium hydroxide and recovering the ammonia for reuse.

8. The process of claim 7 and including the step of raising the pH to a value of 6.5 to 7.

9. The process of claim 7 wherein the treating with calcium hydroxide is carried on without the release of free ammonia.