GB2106489A - Process for removal of sulphur oxides from waste gases - Google Patents

Abstract

The waste gas is contacted with a sorbent comprising one or more of trona, sodium bicarbonate and activated sodium carbonate and then the spent sorbent is dissolved in alkaline ammonia liquor. The liquor is then carbonated to precipitate trona or sodium bicarbonate which is then separated and heat activated. The resulting liquor is then treated with an alkaline earth metal oxide or hydroxide to precipitate the sulphur as sulphates and sulphites. <IMAGE>

Description

SPECIFICATION Process for removal of sulphur oxides from waste gases This invention relates to a process for absorbing sulphur oxides from industrial waste gases with a solid sorbent and regenerating the solid sorbent for reuse.

In the combustion of fossil fuels, and in many industrial processes, a serious problem is presented by the combustion of the sulphurcontaining components therein. The noxious sulphur oxides produced are an environmental pollutant and in recent years considerable effort has been made to remove the sulphur oxides from the combustion gases exhausted to the atmosphere. Several methods for removing such oxides are known (see, for example, U.S. Patent Specification No. 3,852,410 and U.S. Patent Specification No. 3,846,535), but it is believed that all prior art processes have certain disadvantages.

According to the present invention there is provided a method for the removal of sulphur oxides from industrial waste gas containing the same, which method comprises: (a) contacting said gas with a solid sorbent which is one or a mixture of two or more of activated sodium carbonate, sodium bicarbonate and trona, in an amount sufficient to react with substantially all of the sulphur oxides present in said gas to form solids of unreacted sorbent, sodium sulphite, sulphate, and mixtures thereof, and a waste gas substantially free of sulphur oxides; (b) dissolving said solids in an alkaline ammonia liquor to form soluble sodium compounds; (c) carbonating the resultant alkaline sodium liquor from step (b) and cooling to a temperature sufficient to form crystals of sodium bicarbonate or trona;; (d) separating the crystals from the liquor of step (c) and recycling said crystals to step (a) or heating said crystals for a time and at a temperature sufficient to form activated sodium carbonate and recycling said activated sodium carbonate to step (a); and (e) removing carbon dioxide from the cooled liquor of step (c), adding a precipitant selected from the alkaline earth metal hydroxides, oxides and mixtures thereof to the resultant liquor to render it alkaline and form insoluble solids comprising alkaline earth metal sulphates, sulphites and mixtures thereof, separating said insoluble solids and recycling the resultant alkaline liquorto step (b).

Briefly, the method of the invention comprises treating the waste gas containing sulphur oxides (hereinafter for convenience referred to as sulphur dioxide) with a solid sorbent which is one of or a mixture of two or more of activated sodium carbonate, sodium bicarbonate and trona which can remove 90 percent or more of the sulphur dioxide. Trona is the mineral name for Na2CO3. NaHCO3 . 2H20 and activated sodium carbonate can be formed from sodium bicarbonate or trona or a mixture of the two, by calcining at a temperature ranging from about 700C to about 2000C.For example, activated sodium carbonate can be prepared from sodium bicarbonate having a characteristic particle dimension of about 50 microns by calcination over a period of about 10 to about 30 minutes at a temperature of about 1 500C. The clean gas is vented and the unreacted solids, sodium sulphites, sulphates and mixtures thereof, are dissolved in a basic ammonia liquor that is alkaline enough to convert carbonic acid to bicarbonate, to form soluble sodium compounds.

Carbonation of the resultant liquor forms a sodium precipitate containing bicarbonate or trona or mixtures thereof.

The precipitate is separated from the carbonated liquor and the liquor treated with a precipitant selected from the alkaline earth metal hydroxides, oxides and mixtures thereof to form insoluble alkaline earth metal sulphates, sulphites and mixtures thereof. Suitable alkaline earth metals include calcium, barium and strontium.

After removing the solids, the liquor is recycled to treat spent sorbent.

The presence of ammonia in the method provides several distinct advantages. For one, it permits the use of lower flow rates in the regeneration loop of the process. Another important advantage is that since it does not degrade chemically or biologically to any significant extent, there is little loss of ammonia in the system, which accordingly reduces the amount of materials utilized in the process.

Moreover, since the ammonia does not act as a reducing agent in the regeneration loop or in the solid waste disposal area the sulphites and/or sulphates present are not reduced to noxious sulphur compounds as, for example, hydrogen sulphide which can present serious health and disposal problems.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing which is a schematic flow diagram of the method of the invention.

Referring to the drawing, a flue gas containing sulphur dioxide is fed via a conduit 2 to a gassolid contactor 4. Contactor 4, which can take many forms (e.g. fixed bed, moving bed, fluidized bed, etc.), is suitably a baghouse collector employing tube type fabric filter dust collecting surfaces preloaded with a suitable sorbent which is introduced into contactor 4 via conduit 6.

Alternatively, the sorbent may be introduced into the gas stream upstream of the contactor 4. On passing through the contactor 4, the sulphur dioxide in the flue gas reacts with the sodium containing sorbent to produce sodium sulphite and sulphate, leaving a flue gas substantially free of any sulphur dioxide and which is vented from contactor 4 via a conduit 8.

A solids product is removed from contactor 4 via a conduit 10 and transferred to a spent sorbent storage vessel 12. At this point, the solids product will comprise unused sorbent initially in gas-solid contactor 4 plus soluble (sodium) sulphite and/or sulphate resulting from the reaction of the sorbent with the sulphur dioxide in the flue gas. The solids produce is transferred to mixing tank 16 via a conduit 14 where it is admixed with an alkaline recycle liquor containing ammonia from a line 18 and makeup chemicals which can include NH4C I, (NH4)2SO4, Na2CO3, Na2SO4, NaCI or various mixtures thereof. In the mixing tank 16, the soluble sulphite and/or sulphate, which were formed by the reaction of the sulphur dioxide with the sorbent, are dissolved.The liquor from mixing tank 16 is transferred via a conduit 22 to a fly ash filter 24 where any fly ash is removed and disposed of via a a conduit 26. Conduit 26 may go to a reaction tank 64 when not all of the sodium sulphite or sulphate from the spent sorbent storage vessel 12 dissolves in mixing tank 1 6. The liquor free of fly ash leaves filter 24 via a conduit 28, and is introduced into a carbonator 30 where it is reacted with CO2 which is introduced into carbonator 30 via conduit 32. Bicarbonate ions are formed which are transferred via conduit 36 to crystallizer 38 where they are converted to solid sodium bicarbonate which crystallizes out of solution. Excess CO2 leaves carbonator 30 via conduit 34 for venting to the atmosphere or, if preferred, to the clean gas stack via conduit 8.

The sodium bicarbonate and/or trona crystallized in crystallizer 38 is transferred via conduit 40 to sodium bicarbonate filter 42. Carbon dioxide may also be added to the crystallizer 38 to drive the crystallization of sodium bicarbonate toward completion. The sodium bicarbonate recovered from filter 42 is transferred via a conduit 46 to drier/calciner 48 where it is dried and calcined to an active form of sodium bicarbonate and transferred via a conduit 52 to regenerated sorbent storage vessel 54. Alternatively, trona may be formed in crystallizer 38. It may be dried or dried and calcined to an active form of sodium carbonate in drier/calciner 48 and transferred via conduit 52 to regenerated sorbent storage vessel 54.

The liquid from filter 42 passes via conduit 44 into carbon dioxide stripper vessel 56 where it is contacted countercurrently with a stripper (e.g., steam) introduced in the lower portion of stripper vessel 56 via conduit 58. A portion of the CO2, other undissolved gases and any remaining stripper is vented from stripper vessel 56 via conduit 60. This CO2 containing gas may be added to carbonator 30 or crystallizer 38. The CO2 stripped liquor from stripper vessel 56 is introduced, via conduit 62, into reaction tank 64 where it is contacted with an alkaline precipitant, preferably lime, introduced via line 66. In reaction tank 64, the precipitant reacts with the soluble sodium sulphite and/or sulphate to produce, for example, insoluble calcium sulphate and/or calcium sulphite and regenerate the alkaline liquor.The mixture in reaction tank 64 is transferred via conduit 68 to a sludge dewatering vessel 70 where the insoluble calcium sulphate and/or sulphite is disposed of via conduit 72, the liquid from vessel 70 being recycled, as noted above, to mixing tank 16 via conduit 18.

As can be seen from the drawing, the method comprises two basic steps: viz, a sorption step and a regeneration step. In the sorption step, the sulphur dioxide in the flue gas is contacted with the sorbent and converted into soluble sulphate and/or sulphite compounds. In the regeneration step, or loop, the sulphur species is ultimately purged from the process as an insoluble sulphur compound and the sorbent is regenerated for reuse in the sorption step.

The sorbent is preferably a sodium carbonate obtained by calcining a sodium-containing compound, such as sodium bicarbonate or trona or a mixture thereof, at a temperature of from about 70 to about 2000C. It has been found that while sodium carbonate which has been produced by crystallization directly from solution does not act as an effective sorbent in the process of the present invention, calcined sodium carbonate produced by calcining sodium bicarbonate or trona makes an excellent sorbent and is easily obtained by calcining the precipitated sodium bicarbonate produced in crystallizer 38.

To remove the soluble sulphides and/or sulphates from the system, a precipitant comprising an alkaline earth metal hydroxide, oxide or mixture thereof is employed. Thus, for example, the process can employ an oxide or hydroxide of calcium, barium or strontium or mixtures. The preferred alkaline earth metal is calcium.

As noted above in connection with the description of the drawing, the process, with advantage, employs a carbon dioxide stripper. The stripper, which can be any gas-liquid countercurrent contactor, serves to remove excess CO2 which would otherwise be precipitated as calcium carbonate in vessel 64, thereby increasing the use of lime in the process. The CO2 stripper gas can include steam or any oxygen-containing gaseous medium such as, for example, air.

As pointed out above, the method of the present invention ultilizes ammonia in the liquor in mixing tank 16. The ultimate source of alkalinity in the process is supplied by the precipitant (hereinafter for convenience referred to as lime) added to the reaction tank 64.

However, without the use of some medium to transfer alkalinity from the solid phase (lime) to the liquid phase, the alkalinity of the solution would be rapidly depleted during the carbonation step. Accordingly, for a given circulation rate in the system, production of sodium bicarbonate in the carbonator would be greatly reduced. This would necessitate an increased pumping or recirculation rate in the system to the point where the process could become economically not feasible.

The ammonia serves the function of effecting the alkalinity transfer from the line to the liquid phase and can thus be considered as "alkalinity carrier".

This alkalinity carrier has an acid form (ammonium ion) and a base form, (ammonia), being in the base form as it leaves reaction tank 64. The clear liquid which is removed from ash filter 24 and which is used to dissolve the gassolid contactor solids from contactor 4 is pumped to the carbonator 30 where the liquid phase alkalinity of the carrier is now exchanged for liquid phase bicarbonate alkalinity. This liquid phase bicarbonate alkalinity is now converted to the solid phase alkalinity of the sodium bicarbonate in the crystallizer. The alkalinity carrier in the clear liquid from crystallizer 38 is now in the acid form, i.e. ammonium ion. Upon entering reaction tank 64, the ammonium ion once again contacts the solid phase alkalinity provided by the lime, and is converted into the basic form (ammonia) and the cycle repeated.

It will be apparent that the alkalinity carrier can be addded as the base, as ammonia, or in the acid form of its respective salts. Thus, for example, the ammonia can be added in the form of ammonium sulphate, ammonium chloride or the like. The limits for ammonia may be determined from the following considerations. As an alkalinity carrier, it is desirable to maximize its concentration. The limitation on ammonia concentration is the vapour pressure of ammonia. This is greatest at high temperature and pH. An upper limit is the total solution vapour pressure (water, ammonia, and CO2) of the solution equal to five atmosphere absolute (60 psig).

The following Examples serve to illustrate preferred embodiments of the invention.

Example I Flue gas containing 700 lb. mole/hr. of SO2 was treated with 760 lb. mole/hr. of activated sodium carbonate and reacted with 90 percent of sulphur dioxide in the flue gas. The resulting solids were collected in a bag-house. The solids from the bag-house were dissolved using 1350 gal./min. of a recirculated liquor containing 2.6 m ammonia and 6.5 m sodium, and other dissolved species such as chlorides, sulphites, sulphates, carbonates, calcium, etc. Also, makeup soda ash was dissolved into the liquor at the rate of 35.0 lb.

mole/hr. The resulting liquor was then carbonated with 760 Ib. mole/hr. of CO2 from a combination of clean flue gas, and CO2 recycled from other parts of the process. Excess CO2 was combined with the cleaned flue gas. Makeup ammonia was also added to the liquor at a rate of 25.4 Ib. mole/hr. in the carbonator tower. The resulting hot, carbonated liquor was cooled to 950F to precipitate 1520 lb. mole/hr. of sodium bicarbonate. The sodium bicarbonate solids were separated from the liquor, dried and calcined at 3000F to form an activated sodium carbonate which was recycled to the baghouse to treat the flue gas. The separated liquor was passed through a CO2 stripping column to remove 90 Ib. mole/hr.

of carbon dioxide from the liquor. The liquor leaving the CO2 stripping column was treated with 660 Ib. mole/hr. of lime in a reaction tank to precipitate calcium sulphite and/or calcium sulphate solids. These solids were separated from the slurry leaving the reaction tank and constituted the waste product. The separated liquor was recycled as noted above to dissolve the baghouse solids.

Example II The Example was run in accordance with the general procedure of Example I, but in the absence of an alkalinity carrier as indicated in the following Table. The results obtained demonstrate that the circulation rate would be increased at least a thousandfold (if even technically feasible) over the 1350 gpm circulation rate of Example I.

molallty, moles/KgH2O Circulation Rate Ammonia gpm 0 1,350,000+

Claims (9)

Claims

1. A method for the removal of sulphur oxides from industrial waste gas containing the same, which method comprises: (a) contacting said gas with a solid sorbent which is one or a mixture of two or more of activated sodium carbonate, sodium bicarbonate and trona, in an amount sufficient to react with substantially all of the sulphur oxides present in said gas to form solids of unreacted sorbent, sodium sulphite, sulphate, and mixtures thereof, and a waste gas substantially free of sulphur oxides (b) dissolving said solids in an alkaline ammonia liquor to form soluble sodium compounds; (c) carbonating the resultant alkaline sodium liquor from step (b) and cooling to a temperature sufficient to form crystals of sodium bicarbonate or trona;; (d) separating the crystals from the liquor of step (c) and recycling said crystals to step (a) or heating said crystals for a time and at a temperature sufficient to form activated sodium carbonate and recycling said activated sodium carbonate to step (a); and (e) removing carbon dioxide from the cooled liquor of step (c), adding a precipitant selected from the alkaline earth metal hydroxides, oxides and mixtures thereof to the resultant liquor to render it alkaline and form insoluble solids comprising alkaline earth metal sulphates, sulphites and mixtures thereof, separating said insoluble solids and recycling the resultant alkaline liquor to step (b).

2. A method according to claim 1, wherein said sorbent comprises trona.

3. A method according to claim 1 or 2, wherein said sorbent comprises activated sodium carbonate.

4. A method according to claim 1,2, or 3, wherein said precipitant comprises calcium oxide.

5. A method according to any one of claims 1 to 4, wherein said precipitant comprises calcium hydroxide.

6. A method according to any one of claims 1 to 5, wherein ammonium sulphate of ammonium chloride is employed to provide the ammonia.

7. A method according to claim 1 wherein an activated sodium carbonate sorbent is employed with a lime precipitant.

8. A method according to claim 2, wherein in step (c) CO2 is added to drive the crystallization of the sodium bicarbonate to completion.

9. A method according to claim 1, substantially as hereinbefore described with reference to the accompanying drawings.