WO2014028329A1

WO2014028329A1 - Trihalomethane control in wet flue gas desulfurization

Info

Publication number: WO2014028329A1
Application number: PCT/US2013/054328
Authority: WO
Inventors: Seyed B. Ghorishi; Eric W. Liimatta; Zhong Tang
Original assignee: Albemarle Corporation
Priority date: 2012-08-13
Filing date: 2013-08-09
Publication date: 2014-02-20
Also published as: AR092092A1; TW201412380A

Abstract

This invention provides methods for controlling oxidation of bromide ions in wet flue gas desulfurization operations. The methods comprise scrubbing a flue gas containing bromide ions in a wet flue gas desulfurization operation, in which the wet flue gas desulfurization operation comprises contacting the flue gas with a scrubbing medium. The oxidation-reduction potential of the scrubbing medium is then measured with an oxidation-reduction potential probe. In response to the measured oxidation-reduction potential, a reducing agent is introduced into the wet flue gas desulfurization operation to reduce and/or maintain the oxidation-reduction potential of the scrubbing medium below the value for oxidation of bromide ions.

Description

TRIHALOMETHANE CONTROL IN WET FLUE GAS DESULFURIZATION

TECHNICAL FIELD

[0001] This invention relates to wet flue gas desulfurization.

BACKGROUND

[0002] Wet flue gas desulfurization unit configurations vary, but have in common a scrubbing medium and a gas-liquid contact zone. In the liquid-gas contact zone, the flue gas, which usually contains sulfur dioxide, contacts the aqueous scrubbing medium. The scrubbing medium normally has an alkaline reagent therein, such as lime, limestone, or caustic soda.

[0003] The oxidation-reduction potential of a scrubbing medium can be measured with an oxidation-reduction potential probe. Typical scrubbing media have oxidation-reduction potentials of about 200 to about 800 mV; oxidation-reduction potentials at the higher end of this range indicate more oxidizing scrubber media. There are several reactions that occur in a wet scrubber which contribute to the oxidation-reduction potential of the scrubbing medium. One species formed in the scrubbing medium that can significantly increase the oxidation-reduction potential of the scrubbing medium is persulfate ion, which is a strong oxidizer.

[0004] Addition of bromide salts or hydrogen bromide to coal or flue gases is an effective way to control mercury emissions from combustion processes. The resultant soluble, oxidized mercury is then captured in the wet flue gas desulfurization unit. At the same time, the bromide ion (Br^~) is captured. In oxidizing scrubber liquids with a high enough oxidation-reduction potential, the bromide ions can be oxidized to form Br⁺, which in turn reacts with organic matter present in the scrubbing medium to form trihalomethanes. Trihalomethanes (e.g. , CHBrCl2, CHBr₂Cl, and CHBrs) are known carcinogens, and are strictly regulated. For example, the U.S. Environmental Protection Agency has set an upper limit of 80 ppb for trihalomethanes in drinking water. SUMMARY OF THE INVENTION

[0005] This invention provides methods and systems to reduce and/or maintain the oxidation-reduction potential of scrubbing liquids and slurries which come into contact with flue gas that contains bromide ions, to levels low enough to minimize or prevent formation of Br⁺ from the bromide ions. Minimizing or preventing formation of Br⁺ thereby minimizes the formation of trihalomethanes. This is achieved by introducing a reducing agent into the wet flue gas desulfurization system.

[0006] An embodiment of this invention is a method for controlling oxidation of bromide ions, which method comprises

a) scrubbing a flue gas containing bromide ions in a wet flue gas desulfurization operation, said wet flue gas desulfurization operation comprising contacting said flue gas with a scrubbing medium;

b) measuring an oxidation-reduction potential of the scrubbing medium with an oxidation-reduction potential probe; and

c) characterized by introducing a reducing agent into the wet flue gas desulfurization operation in response to the oxidation-reduction potential measured in b), to reduce and/or maintain the oxidation-reduction potential of the scrubbing medium below the value for oxidation of bromide ions.

[0007] Other embodiments of this invention include a system for controlling oxidation of bromide ions in a flue gas containing bromide ions in a wet flue gas desulfurization operation.

[0008] These and other embodiments and features of this invention will be still further apparent from the ensuing description, drawing, and appended claims. BRIEF DESCRIPTION OF THE DRAWING

[0009] Fig. 1 depicts a schematic diagram of a generalized wet flue gas desulfurization system configuration.

FURTHER DETAILED DESCRIPTION OF THE INVENTION

[0010] As used throughout this document, "flue gas" is a type of combustion gas, and the phrase "combustion gas" refers to a gas (mixture) resulting from combustion. Throughout this document, a flue gas is regarded as having bromide ions (Br^~) therein if one or more bromide salts and/or hydrogen bromide were added to the fuel being burned and/or to the flue gas prior to introducing the flue gas to the wet flue gas desulfurization unit.

[0011] Oxidation-reduction potential is often referred to as redox potential. The abbreviation ORP is also occasionally used, and stands for oxidation-reduction potential.

[0012] Fig. 1 is not intended to be construed as limiting the invention. For example, the present invention applies to wet flue gas desulfurization systems that do not have a solids dewatering unit, as well as to wet flue gas desulfurization systems that do have a solids dewatering unit. In connection with the wet flue gas desulfurization systems, the term "zone" is used to apply to the portion or portions of the system that serve a particular function, regardless of the type of vessel in which that function is carried out, whether the vessel is a tank, section of pipe, reactor, or other container.

[0013] Prior to performing the methods of the invention, the flue gas can be subjected to particulate removal, although particulate removal is not necessary for the methods of the invention to work properly.

[0014] The scrubbing medium in the wet flue gas desulfurization operation or system in this invention is a liquid or slurry, and normally contains an alkaline reagent, such as lime, limestone, or caustic soda.

[0015] The methods of the invention involve introduction of a reducing agent to a wet flue gas desulfurization operation. The methods comprise introducing a reducing agent into a wet flue gas desulfurization operation in response to the measured oxidation- reduction potential of the scrubbing medium, to reduce and/or maintain the oxidation- reduction potential of the scrubbing medium below the value for oxidation of bromide ions. In these methods, the flue gas being scrubbed contains bromide ions.

[0016] Reducing agents that can be used in this invention are effective to reduce the measured oxidation-reduction potential of the scrubbing medium to a value below the oxidation-reduction potential for bromide ion oxidation. The reducing agent should be compatible with water, since wet flue gas desulfurization systems are normally aqueous. Preferably, the reducing agent is soluble in water at the desired concentrations. It is also desirable that the reducing agent does not adversely affect the scrubbing reaction in the wet flue gas desulfurization system, nor the equipment comprising the wet flue gas desulfurization system.

[0017] Suitable reducing agents in the practice of this invention are typically inorganic or organic, and include, but are not limited to, hydrogen sulfide; alkali metal hydrosulfides, such as sodium hydrosulfide (NaHS; available from Albemarle Corporation); tin(II) compounds such as stannous chloride and stannous oxide; iron(II) salts such as ferrous chloride and ferrous sulfate; cuprous salts such as copper(I) chloride; chromium(II) compounds such as chromium(II) chloride and chromium(II) acetate; titanium(III) compounds such as titanium(III) chloride, titanium(III) oxide, and titanium(III) sulfate; elemental metals such as iron, zinc, aluminum, magnesium, silver, nickel, and copper; sulfite compounds such as sodium sulfite and potassium sulfite; bisulfites such as sodium bisulfite and potassium bisulfite; metabisulfites such as sodium metabisulfite and potassium metasbisulfite; thiosulfates such as ammonium thiosulfate, sodium thiosulfate, and potassium thiosulfate; hydrazine; oxalic acid or salts thereof; formic acid or salts thereof; and ascorbic acid or salts thereof. Salts of organic acids are usually alkali metal salts, such as lithium, sodium, or potassium, preferably sodium or potassium. Mixtures of reducing agents can be used if desired. Preferred reducing agents include sodium hydrosulfide and tin(II) compounds such as stannous chloride and stannous oxide; more preferred is sodium hydrosulfide.

[0018] In regard to sulfites and bisulfites as reducing agents, it is known that sulfite and/or bisulfite species are often present in wet flue gas desulfurization operations. However, the sulfites and/or bisulfites are usually present in concentrations too low to decrease the oxidation-reduction potential of the scrubbing medium.

[0019] The reducing agent can be introduced to the wet flue gas desulfurization system in solid form, although this is not recommended. Water-soluble reducing agents are preferably introduced as aqueous solutions, and water-soluble reducing agents are preferred. Reducing agents that are not water-soluble or soluble only to a limited extent in water are preferably introduced as aqueous suspensions or slurries.

[0020] The oxidation-reduction potential of the scrubbing medium is measured by an oxidation-reduction probe, and can be measured either continuously or intermittently. The oxidation-reduction probe can be any device capable of measuring the oxidation-reduction potential of the scrubbing medium in the absorber zone. Suitable oxidation-reduction probes are available commercially, and include those sold by Oakton and Lazar Research Laboratories, Inc., e.g., their sealed double-junction ORP electrode having an epoxy body, and those sold by Cole Parmer, such as their flat surface ORP probe which has a Ag/AgCl reference cell.

[0021] In response to the measured oxidation-reduction potential of the scrubbing medium, when the value is greater than the value at which bromide oxidation occurs, reducing agent is introduced into the wet flue gas desulfurization operation. As discussed above, introducing an amount of reducing agent to the wet flue gas desulfurization operation decreases the oxidation-reduction potential of the scrubbing medium and, when lowered to a value below that for oxidation of bromide ions, thereby minimizes or prevents oxidation of bromide ions. If the measured oxidation-reduction potential is below the threshold for bromide oxidation, no reducing agent needs to be introduced. However, it may preferable to introduce a small amount of reducing agent to maintain the oxidation-reduction potential below the value for bromide oxidation. The reducing agent can be added continuously or intermittently.

[0022] The threshold value at which bromide ion is oxidized may vary from one wet flue gas desulfurization operation or system to another, and depends on conditions including temperature and ionic strength. In addition, the oxidation-reduction potential of the scrubbing medium, and in turn the amount of reducing agent needed, varies depending on several factors, including, but not limited to, the amount of flue gas present in the wet flue gas desulfurization system or operation, the amount and oxidative power of the oxidizing species present in the scrubbing medium, and the design of the system.

[0023] Reducing agent is introduced to the wet flue gas desulfurization operation or system at least as needed to reduce or maintain the oxidation-reduction potential below the value at which bromide oxidation occurs. When the oxidation-reduction potential of the scrubbing medium is higher relative to the value for oxidation of bromide ions, larger amounts of reducing agent are needed to decrease the oxidation-reduction potential below the value for oxidation of bromide ions.

[0024] A typical, generalized wet flue gas desulfurization system configuration 1 is shown in Fig. 1. The system has a flue gas inlet 2 and a gas-liquid contact zone 3 in fluid communication with the flue gas inlet 2. The gas-liquid contact zone is configured to receive at least flue gas, make-up water, and scrubbing medium. The wet flue gas desulfurization system also has an absorber zone 4 in fluid communication with the gas- liquid contact zone 3, with an alkaline reagent preparation zone 5, and a recirculation pump 6, the absorber zone 4 being in fluid communication with the recirculation pump 6 via an absorber zone outlet 7. Absorber zone outlet 7 can also be referred to as a recirculation pump inlet. The flue gas comes into contact with the scrubbing medium the gas-liquid contact zone 3. The alkaline reagent preparation zone 5 is configured to deliver alkaline reagent to the absorber zone 4. Recirculation pump 6 is in fluid communication with the gas-liquid contact zone 3, and also in fluid communication with the absorber zone 4 via the absorber zone outlet 7. Absorber zone outlet 7 is in fluid communication with the absorber zone 4 and the recirculation pump 6.

[0025] Again referring to Fig. 1, the system also comprises a reducing agent inlet 8 in fluid communication with a portion of the wet flue gas desulfurization system. Reducing agent reservoir 9 is in fluid communication with the reducing agent inlet 8. Makeup water inlet 10 is in fluid communication with, and configured to introduce a fluid into, the gas- liquid contact zone 3. There is an oxidation-reduction potential probe (not shown) placed and configured to measure the oxidation-reduction potential of the scrubbing medium in the absorber zone 4. Optional features that may be part of wet flue gas desulfurization systems not shown in Fig. 1 include a solids dewatering zone, which is normally for removal of water from solids produced during scrubbing, and/or a reheater, which is usually downstream of the gas-liquid contact zone.

[0026] Still referring to Fig. 1, in the gas-liquid contact zone 3, the flue gas comes into contact with scrubbing medium. After contact, the scrubbed flue gas exits the gas-liquid contact zone 3 via gas outlet 11, and the liquid or slurry is transported to the absorber zone 4. Alkaline reagent from the alkaline reagent preparation zone 5 is transported to absorber zone 4, forming scrubbing medium. The recirculation pump 6 moves at least a portion of the scrubbing medium from the absorber zone 4 into the gas-liquid contact zone 3.

[0027] The reducing agent can be introduced at any point in the wet flue gas desulfurization system from which it can travel directly or indirectly into the gas-liquid contact zone. Suitable locations at which the reducing agent can be introduced into the system include the flue gas inlet, the gas-liquid contact zone, the absorber zone, the absorber zone outlet, the alkaline reagent preparation zone, and the makeup water inlet. A consideration for choosing the location for introducing the reducing agent is that the reducing agent is less effective with less thorough mixing. Better mixing in turn allows for better control of the oxidation-reduction potential of the scrubbing medium.

[0028] In Fig. 1 a preferred location for the reducing agent inlet 8 and the reducing agent reservoir 9 is shown. This preferred location allows the reducing agent to be introduced into the absorber zone outlet 7, from which scrubbing medium and reducing agent are fed together through the recirculation pump 6, where good mixing occurs. Another way of describing the preferred location to introduce the reducing agent (absorber zone outlet 7) is as the inlet for the recirculation pump 6.

[0029] The following examples are presented for purposes of illustration, and are not intended to impose limitations on the scope of this invention. EXAMPLE 1

[0030] A calculation was performed on an aqueous-phase electrolyte equilibrium solution to demonstrate the effectiveness of using a reducing agent in a scrubbing medium. The calculation was performed using Stream Analyzer (OLI Systems), a software program designed to calculate values in thermodynamic electrolyte systems.

[0031] In the calculation, an aqueous system containing potassium persulfate was simulated (1 μΜ; 0.27 mg K2S2O₈ in 1 liter of deionized water). The calculations predict an oxidation-reduction potential (ORP) of 750 mV for this system. Clearly, even a small amount of S₂0s^2~ in an aqueous solution can significantly increase the oxidation-reduction potential of the solution. For the next part of the calculation, simulated incremental additions of NaHS (0.1 μΜ increments; 0.04 mg in 1 liter of water) to the simulated 1 μΜ potassium persulfate solution were performed, and the ORP was calculated after each simulated addition step (see Table 1). The calculated ORP of the simulated solution noticeably decreased, and reached negative values after as little as 0.3 μΜ of NaHS had been added to the S₂0₈ ^2" solution. At such low ORP values, little, if any, Br^~ will be oxidized.

TABLE 1

EXAMPLE 2

[0032] Another calculation was performed on an aqueous-phase electrolyte equilibrium solution to demonstrate the effectiveness of using a reducing agent in a scrubbing medium, again using Stream Analyzer (OLI Systems), a software program designed to calculate values in thermodynamic electrolyte systems.

[0033] As in Example 1, the calculation was performed on a simulated aqueous system containing potassium persulfate (1 μΜ; 0.27 mg K2S2O₈ in 1 liter of deionized water), and an oxidation-reduction potential (ORP) of 750 mV was again predicted for this system. In this Example, for the next part of the calculation, simulated incremental additions of Na₂S(¾ (0.1 μΜ increments; 0.09 mg in 1 liter of water) to the simulated 1 μΜ potassium persulfate solution were performed, and the ORP was calculated after each simulated addition step (see Table 2). The calculated ORP of the simulated solution decreased, and reached negative values after 1.1 μΜ of Na₂S0₃ had been added to the S₂0₈ ^2" solution. At such low ORP values, little, if any, Br^~ will be oxidized.

TABLE 2

EXAMPLE 3

[0034] A series of laboratory experiments were conducted to assess the capability of NaHS (sodium hydrosulfide) for reducing the ORP of an aqueous alkaline scrubbing medium containing bromide ions. The aqueous alkaline scrubbing medium was from a wet flue gas desulfurization unit of a coal-fired power plant. This scrubbing medium contained chlorides (4200 ppm), bromides (4.5 ppm), and sulfates (4500 ppm), and had a pH of 5.5. Incremental additions of NaHS (50 μΜ increments; 200 mg in 1 liter of water) to the aqueous alkaline scrubbing medium were made, and the ORP was measured after each addition step (see Table 3).

[0035] The observed ORP change as function NaHS concentration in the aqueous alkaline scrubbing medium is shown in Table 3. As can be seen from Table 3, the observed ORP reached negative values after 100 μΜ of NaHS had been added to the aqueous alkaline scrubbing medium. In this series of experiments, the ORP values did not decrease as dramatically as shown in the calculations of Example 1; however, the trend is similar: relatively small amounts of NaHS appear to cause a noticeable reduction of the ORP of the aqueous alkaline scrubbing medium. Also, the experimentally observed values in this Example were obtained from tests on an actual scrubbing medium, while the calculations reported in Example 1 were performed on a solution of K2S2O₈ in deionized water.

TABLE 3

EXAMPLE 4

[0036] Another series of experiments similar to those in Example 3 were performed on an aqueous alkaline scrubbing medium as described in Example 3. In this series of experiments, the reducing agent was stannous chloride (SnCy. Incremental additions of SnCl₂ (22 μΜ increments; 297 mg in 1 liter of water) to the aqueous alkaline scrubbing medium were made, and the ORP was measured after each addition step (see Table 4).

[0037] The observed ORP change as function SnC^ concentration in the aqueous alkaline scrubbing medium is shown in Table 4. As can be seen from Table 4, the observed ORP reached negative values after 22 μΜ of SnC^ had been added to the aqueous alkaline scrubbing medium. From the observed results, relatively small amounts of SnCl₂ appear to cause a noticeable reduction of the ORP of the aqueous alkaline scrubbing medium, and thus SnCl₂ has been shown to be an effective agent in reducing the ORP of the scrubber solution.

TABLE 4

[0038] Components referred to by chemical name or formula anywhere in the specification or claims hereof, whether referred to in the singular or plural, are identified as they exist prior to coming into contact with another substance referred to by chemical name or chemical type (e.g. , another component, a solvent, or etc.). It matters not what chemical changes, transformations and/or reactions, if any, take place in the resulting mixture or solution as such changes, transformations, and/or reactions are the natural result of bringing the specified components together under the conditions called for pursuant to this disclosure. Thus the components are identified as ingredients to be brought together in connection with performing a desired operation or in forming a desired composition.

[0039] The invention may comprise, consist, or consist essentially of the materials and/or procedures recited herein.

[0040] As used herein, the term "about" modifying the quantity of an ingredient in the compositions of the invention or employed in the methods of the invention refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or use solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients employed to make the compositions or carry out the methods; and the like. The term about also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or not modified by the term "about", the claims include equivalents to the quantities.

[0041] Except as may be expressly otherwise indicated, the article "a" or "an" if and as used herein is not intended to limit, and should not be construed as limiting, the description or a claim to a single element to which the article refers. Rather, the article "a" or "an" if and as used herein is intended to cover one or more such elements, unless the text expressly indicates otherwise.

[0100] Each and every patent or other publication or published document referred to in any portion of this specification is incorporated in toto into this disclosure by reference, as if fully set forth herein.

[0101] This invention is susceptible to considerable variation in its practice. Therefore the foregoing description is not intended to limit, and should not be construed as limiting, the invention to the particular exemplifications presented hereinabove.

Claims

THAT WHICH IS CLAIMED IS:

1. A method for controlling oxidation of bromide ions, which method comprises

2. A method as in Claim 1 wherein said operation is conducted in a wet flue gas desulfurization system comprising

a flue gas inlet;

a gas-liquid contact zone in fluid communication with the flue gas inlet and configured to receive at least flue gas, make-up water, and scrubbing medium;

an absorber zone in fluid communication with the gas-liquid contact zone, an alkaline reagent preparation zone, and a recirculation pump, the absorber zone being in fluid communication with the recirculation pump via an absorber zone outlet, said absorber zone containing scrubbing medium;

the alkaline reagent preparation zone configured to deliver alkaline reagent to the absorber zone;

the recirculation pump in fluid communication with the absorber zone via an absorber zone outlet, and also in fluid communication with the gas-liquid contact zone, which recirculation pump moves at least a portion of scrubbing medium from the absorber zone into the gas-liquid contact zone;

the absorber zone outlet in fluid communication with the absorber zone and the recirculation pump;

an oxidation-reduction potential probe placed and configured to measure the oxidation- reduction potential of the scrubbing medium in the absorber zone; a reducing agent inlet in fluid communication with a portion of the wet flue gas desulfurization system;

a reducing agent reservoir in fluid communication with the reducing agent inlet; and a makeup water inlet in fluid communication with, and configured to introduce a fluid into, the gas-liquid contact zone.

3. A method as in Claim 2 wherein the reducing agent is introduced into the flue gas inlet, gas-liquid contact zone, the absorber zone, the absorber zone outlet, the alkaline reagent preparation zone, or the makeup water inlet.

4. A method as in Claim 2 wherein the reducing agent is introduced into the absorber zone outlet.

5. A method as in Claim 1 or 2 wherein the reducing agent is water soluble.

6. A method as in any of Claims 1-4 wherein the reducing agent is sodium hydrosulfide or a tin(II) compound.

7. A method as in any of Claims 2-4 wherein the wet flue gas desulfurization system further comprises at least one of the following:

a reheater;

a solids dewatering zone.

8. A method as in Claim 7 wherein the reducing agent is water soluble.

9. A method as Claim 7 wherein the reducing agent is a tin(II) compound or sodium hydrosulfide.

10. A system for controlling oxidation of bromide ions, which system comprises

i) an absorber zone in a wet flue gas desulfurization operation, in which absorber zone a flue gas containing bromide ions is contacted with a scrubbing medium; ii) an oxidation-reduction potential probe that measures an oxidation-reduction potential of the scrubbing medium; and

iii) an inlet for introducing a reducing agent into the wet flue gas desulfurization operation to reduce and/or maintain the oxidation-reduction potential of the scrubbing medium below the value for oxidation of bromide ions in response to the oxidation-reduction potential measured in ii).

11. A system as in Claim 10 wherein said operation is conducted in a wet flue gas desulfurization system comprising

a flue gas inlet; a gas-liquid contact zone in fluid communication with the flue gas inlet and configured to receive at least flue gas, make-up water, and scrubbing medium;

an oxidation-reduction potential probe placed and configured to measure the oxidation- reduction potential of the scrubbing medium in the absorber zone;

a reducing agent inlet in fluid communication with a portion of the wet flue gas desulfurization system;

a reducing agent reservoir in fluid communication with the reducing agent inlet; and a makeup water inlet in fluid communication with, and configured to introduce a fluid into, the gas -liquid contact zone.

12. A system as in Claim 11 wherein the reducing agent is introduced into the flue gas inlet, gas-liquid contact zone, the absorber zone, the absorber zone outlet, the alkaline reagent preparation zone, or the makeup water inlet.

13. A system as in Claim 11 wherein the reducing agent is introduced into the absorber zone outlet.

14. A system as in Claim 10 or 11 wherein the reducing agent is water soluble.

15. A system as in any of Claims 10-13 wherein the reducing agent is sodium hydrosulfide or a tin(II) compound.

16. A system as in any of Claims 11-13 wherein the wet flue gas desulfurization system further comprises at least one of the following:

a reheater;

a solids dewatering zone.

17. A system as in Claim 16 wherein the reducing agent is water soluble.

18. A system as Claim 16 wherein the reducing agent is sodium hydrosulfide a tin(II) compound.