SULFATE REDUCTION IN FLUE GAS DESULFURIZATION SYSTEM BY BARIUM PRECIPITATION

Abstract

A process for treating a flue gas desulfurization discharge stream containing dissolved sulfates is presented. Soluble barium compounds, such as barium chloride or barium carbonate are added to the stream in lieu of the traditional two-step lime/carbon dioxide process. The barium compounds cause the sulfate to precipitate as insoluble barium sulfate. The barium sulfate solids settle out of the discharge stream and can be filtered from the process water. The use of soluble barium compounds eliminates the need for subsequent pH adjustment, results in lowering calcium and magnesium concentrations in the discharge stream, and decreases scaling potential in downstream equipment.

Claims

1. A method for reducing sulfate concentration of a flue gas desulfurization discharge stream, comprising: processing a discharge stream from a flue gas desulfurization system, wherein the discharge stream contains dissolved sulfate; adding one or more barium compounds to the discharge stream; and reacting the barium compounds with the dissolved sulfate to form barium sulfate, thereby reducing concentration of the dissolved sulfate in the discharge stream.

2. The method of claim 1, wherein the one or more barium compounds include barium chloride or barium carbonate.

3. The method of claim 1, wherein the barium sulfate is removed from the discharge stream by filtration.

4. The method of claim 1, wherein the barium sulfate is removed from the discharge stream by a combination of settling and filtering.

5. The method of claim 1, wherein after the barium sulfate is removed from the discharge stream, the acidity of the discharge stream is adjusted to be between a pH of 6 and a pH of 9.

6. The method of claim 1, wherein flocculants, polymers, or coagulants are added to the discharge stream to assist in solids separation, filtration and removal from the discharge stream.

7. The method of claim 1, wherein the barium sulfate is recovered for recycle, reuse, or sale.

8. The method of claim 1, wherein the barium sulfate is combined with gypsum for disposal or sale.

9. A system for reducing sulfate concentration of a flue gas desulfurization discharge stream, comprising: a reaction vessel receiving a discharge stream having dissolved sulfate from a flue gas desulfurization system and receiving a barium reagent feed stream; and a solids removal device located downstream from the reaction vessel, wherein the barium reagent reacts with the dissolved sulfate in the reaction vessel to form barium sulfate, thereby reducing concentration of the dissolved sulfate, and the discharge stream passes through the solids removal device to remove the barium sulfate.

10. The system of claim 9, wherein the barium reagent includes barium chloride or barium carbonate.

11. The system of claim 9, wherein the solids removal device is a primary cyclone or rotary filter.

12. The system of claim 9, wherein after wherein the barium sulfate is removed from the discharge stream, the acidity of the discharge stream is adjusted to be between a pH of 6 and a pH of 9.

13. The system of claim 9, wherein flocculants, polymers, or coagulants are added to the discharge stream to assist in solids separation, filtration and removal of barium sulfate from the discharge stream.

14. The system of claim 9, wherein the barium sulfate is recovered for recycle, reuse, or sale.

15. The system of claim 9, wherein the barium sulfate is combined with gypsum for disposal or sale.

Description

DESCRIPTION OF THE DRAWINGS

[0020] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0021] FIG. 1 provides a table with an analysis of scrubber liquor using the two-step process known in the prior art;

[0022] FIG. 2 depicts one embodiment of the sulfate removal process of the present invention having a barium reagent storage and feed system, a mixing vessel for the reaction to occur, and a solids separation device to remove the precipitated barium sulfate solids from the purge stream; and

[0023] FIG. 3 provides a table with an analysis of scrubber liquor using one embodiment of the sulfate removal process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The present invention is directed to improved methods and systems for, among other things, sulfate reduction in a flue gas desulfurization discharge stream by the addition of barium. The configuration and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of contexts other than sulfate reduction in a flue gas desulfurization system by barium precipitation. Accordingly, the specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. In addition, the following terms shall have the associated meaning when used herein:

[0025] “flue gas desulfurization” or “FGD” means any process in which sulfur dioxide is reduced in, or removed from, exhaust flue gases of fossil-fuel power plants or from the emissions of other sulfur oxide emitting processes;

[0026] “lime,” when referring to the addition of lime to a flue gas desulfurization discharge stream, can include limestone, magnesium compounds, sodium compounds or other chemicals or compounds that react with sulfur dioxide in the stream to create a sulfite compound;

[0027] “purge stream” or “discharge stream” means any fluid stream emitted from a flue gas desulfurization process.

[0028] Although lime and carbon dioxide are traditionally used to lower sulfate concentrations in FGD system purge waters, it is possible to use other chemicals as well. Embodiments of the present invention include the addition of soluble barium compounds such as barium chloride (BaCl.sub.2) or barium carbonate (BaCO.sub.3), both of which result in equimolar reduction of sulfate concentrations. The addition of BaCl.sub.2 or BaCO.sub.3 precipitates the sulfate anions as the very insoluble barium sulfate (BaSO.sub.4). The BaSO.sub.4 solids settle and are filtered from the process water and recycle. In this process, calcium carbonate (CaCO.sub.3) scaling resulting from the addition of carbon dioxide (CO.sub.2) for pH control can be avoided.

[0029] Moreover, the use of BaCl.sub.2 does not require any subsequent pH adjustment. The use of BaCO.sub.3 results in lowering calcium and magnesium concentrations, decreasing scaling potential in downstream equipment. The incremental capability of reducing the sulfate concentration makes this attractive to any subsequent processing step that produces a concentrated waste stream (e.g. reverse osmosis, ion exchange, ultrafiltration, brine concentration) that experiences gypsum or calcite scale formation.

[0030] Referring now to FIG. 2, embodiments of the present invention include a barium reagent feed system 201 which could include a silo for storage of dry reagent, and/or a tank for slurry or liquid solution, a feed system that may be manually or automatically controlled that adds the desired amount of reagent to the high sulfate purge stream 204, a mixed reactor vessel 202 that allows sufficient time for reaction between barium and sulfate to occur, and a solids removal device 203 that will separate the low sulfate purge stream 206 from the barium sulfate solids 205. Additional reagents may be added to control pH or other substances in the purge water as is known in the art. In some instances, flocculants, polymers, or coagulants are added to the discharge stream to assist in the solids separation, filtration and removal from the process discharge water stream.

[0031] Other embodiments of the process shown in FIG. 2 include the combined use of the barium reagent with other chemical reactions, so that existing waste water treatment equipment can be used with minimal modifications. The precipitation of barium sulfate is essentially equimolar to the barium dose added when an excess of sulfate is present. Since the other common cations (e.g., magnesium, sodium, potassium, and calcium) present in FGD purge water do not have a low solubility product, the sulfate reduction occurs independently of other reactions (hydroxide precipitation, mercury complexation, selenium reduction, etc.). This permits sulfate reduction to be employed at a minimal additional capital cost with existing purge water treatment systems.

[0032] Those skilled in the art will appreciate that embodiments of the present invention are particularly effective when used with FGD purge streams that do not contain gypsum solids. In those instances in which the FGD purge stream does contain gypsum solids, a reduction in the liquid phase sulfate level by barium precipitation would lower the relative saturation of the stream due to the common ion effect, thereby causing solid phase gypsum to dissolve to re-attain equilibrium.

[0033] The precipitated barium sulfate solids will produce slurry that can be independently dewatered and recycled or disposed. Alternately, the solids are at chemical equilibrium and will not re-dissolve if returned to a gypsum containing slurry. This offers an option for disposal with the bulk gypsum/FGD waste.

[0034] In an example case for one embodiment of the process of the present invention, rather than adding lime to the high-sulfate containing purge stream, BaCl.sub.2 was added to the stream. The results are shown in the table included in FIG. 3. The chemical makeup of the purge stream is shown in column 1 of the table. The first column of the table identifies the chemical species present in the purge stream and the second column shows the amount in mg/liter of each of the species present. The third column of the table labeled “Using BaCl.sub.2” shows the results in mg/liter for BaCl.sub.2 addition to the purge stream. The results show an increase in the level of chloride and a reduction in the level of sulfate while maintaining a pH of 6.8. As a result, this process does not require acidification for neutralization while reducing the sulfate to low levels which avoids scaling in the downstream process equipment.

[0035] The fourth, fifth and sixth columns of the table in FIG. 3 labeled “Using BaCO.sub.3” show a two-step process in which BaCO.sub.3 is added to the purge stream which reduces the magnesium, calcium and sulfate, but increases the pH to 9.52. The addition of HCl in the second step brings the pH down to a neutral 7.16 while slightly increasing the chloride level. As an alternative to the addition of HCl, CO.sub.2 can be added to the stream which also effectively reduces the pH to a neutral 7.34 without a significant increase in the chloride level or the formation of calcite scale.

[0036] Referring now to the seventh column of the table in FIG. 3 labeled “Using Ba(OH).sub.2” where Ba(OH).sub.2 is added to the purge water stream. This process lowers the sulfate concentration significantly, thus avoiding scaling in downstream equipment, and it also substantially lowers the level of chloride in the stream, but the pH is increased to 12.8. In order to reduce the pH, either HCl or CO.sub.2 could be added. However, the addition of HCl would have the undesirable side effect of increasing the chloride concentration and the addition of CO.sub.2 would result in the precipitation of calcite. Therefore, it does not appear that barium hydroxide would be suitable for general use in this particular embodiment of the process, but it may be applicable in certain industrial conditions.

[0037] While the present system and method has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions “in one embodiment” or “in another embodiment” are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms “a”, “an” and “the” mean “one or more” unless expressly specified otherwise. The term “connected” means “communicatively connected” unless otherwise defined.

[0038] When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be readily apparent that a single embodiment may be substituted for that one device.

[0039] In light of the wide variety of methods for sulfate reduction in a flue gas desulfurization system known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto.

[0040] None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims.