MXPA99010903A - Mercury removal in utility wet scrubber using a chelating agent - Google Patents

Abstract

A method for capturing and reducing the mercury content of an industrial flue gas such as that produced in the combustion of a fossil fuel or solid waste adds a chelating agent such as ethylenediaminetetracetic acid (EDTA) to the flue gas being scrubbed in a wet scrubber used in the industrial process. The EDTA prevents the reduction of oxidized mercury to elemental mercury, thereby increasing the mercury removal efficiency of the wet scrubber. Tests on inlet and outlet mercury concentration in an industrial flue gas were performed without and with EDTA addition. Without EDTA, mercury removal totaled 42%. With EDTA, mercury removal increased to 71%.

Description

ELIMINATION OF MERCURY IN WET TREATMENT EQUIPMENT USING A DISSOLVING AGENT DESCRIPTION OF THE INVENTION The subject matter of the present invention is developed under a research contract with the United States Department of Energy (DOE), no. of contract DE-FC22-94PC94251, and under a guarantee agreement with "the Ohio Carbon Development Office (OCDO), Guarantee Agreement No. CDO / D-922-13." The United States and Ohio Governments they have certain rights in the invention Field and background of the invention The present invention relates generally to the field of methods and apparatus for cleaning fluid gas and, in particular, to a method for removing mercury from the fluid gas generated during the combustion of fuels. fossils or solid waste, through the use of a chelating agent In recent years, the Department of Energy of the United States (DOE) and the United States Environmental Protection Agency (EPA) have supported research to measure and control emissions of Hazardous Air Pollutants (HAP) from coal-fired heating equipment The initial results of several research projects show that emissions of heavy metals and carbons Volatile organic compounds (VOC) are very low, except for mercury (Hg). Unlike most other metals, most mercury remains in the vapor phase and does not condense into ash particles that fly at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it can not be collected and disposed along with the ash that fly like the other metals. To complicate matter, mercury can exist in its oxidized (Hg + 2) or elemental (Hg °) form and each is affected differently by the subsequent downstream contamination control equipment. In a conventional wet scrubber the Hg + 2 is relatively easy to capture while it is difficult to capture the Hg °. The relative amount of each species seems to depend on several factors such as the type of fuel, combustion efficiency of the heater, the type of particulate collector installed, and various other factors. It has been demonstrated for the type of particulate collector installed, that an electrostatic precipitator (ESP), as used in most functional applications, affects the chemical process in such a way that Hg + 2 is converted to Hg ° within a downstream wet scrubber, also commonly used in equipment applications to reduce SO2 emissions. The Hg ° is then emitted with the fluid gas. Most recent efforts to capture and remove mercury from the fluid gas have been concentrated in gas phase reactions with introduced reagents such as activated carbon. The issue of mercury emissions by the functional industry is a new area that is investigated by both DOE and EPA. SYNOPSIS The present invention is a method to adjust the chemistry of the wet scrubber to avoid the reduction of Hg + 2 to Hg ° and thereby increase the efficiency of elimination of mercury from wet scrubbing systems. The invention increases the mercury removal efficiency of conventional wet scrubber systems especially those preceded by an ESP. Accordingly, one aspect of the present invention is to provide, in an industrial process using a wet scrubber to receive an industrial gas containing mercury, a method for reducing the mercury content in the industrial gas leaving the wet scrubber, which comprises : add a chelating agent to industrial gas; and purify the industrial gas in the wet scrubber with the chelating agent. Advantageously, the chelating agent comprises at least one of an ethylenediaminetetraacetic acid (EDTA), hydroxyethylene diamine tetraacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA or pentetic acid), and nitrilotriacetic acid (NTA). It is shown that the pilot-scale test using EDTA is effective, and an amount of about twice the stoichiometric ratio of the chelating agent to the transition metals (which is assumed to be iron, Fe). Another aspect of the present invention is to provide a method for reducing elemental mercury concentration in industrial gases leaving a wet scrubber that cleans industrial gases with a coarse slurry, industrial gases contain mercury in oxidized (Hg + 2) and elemental forms (Hg °), the wet scrubber contains at least one transition metal that converts the Hg + 2 form into the Hg ° form, which comprises the steps of: supplying a chelating agent in the coarse slurry in sufficient quantity to reduce the degree of which at least the transition metal converts the Hg + 2 to the Hg ° form; and purify the industrial gases with the thick suspension that contains the chelating agent. In certain aspects of the present invention, the method may comprise determining the amount of at least one transition metal in the coarse slurry of the wet scrubber and supplying the chelating agent in the coarse slurry in an amount sufficient to reduce the degree to which the the least transition metal converts Hg + 2 into the Hg ° form based on such determination. In other aspects of the present invention, the method may comprise determining the concentration of oxidized (Hg) and elemental (Hg °) forms of mercury in industrial gases entering and leaving the wet scrubber and supplying the chelating agent in the coarse slurry in a sufficient amount to reduce the degree to which at least the transition metal converts Hg + 2 to the Hg ° form based on such determination. Yet another aspect of the present invention is to provide a method for operating a wet scrubber to reduce gaseous emissions of oxidized (Hg + 2) and elemental (Hg °) mercury in industrial gases exiting the wet scrubber, which comprises: industrial gases within the wet scrubber with a coarse slurry containing a sufficient amount of chelating agent to reduce the degree to which the at least transition metal converts Hg + 2 to the Hg ° form. Yet another aspect of the present invention is to improve the removal of mercury from fluid gas in a process that burns pulverized coal. The various features of novelty that characterize the invention are indicated with particularity in the claims appended to and forming a part of this description. For a better understanding of the invention, its operating advantages and specific benefits obtained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is an illustration of a heating equipment installation burned with coal of the type used by apparatuses in the generation of electrical energy; Figure 2 is a bar graph that graphs the concentration of mercury in a wet scrubber inlet and outlet, in two energy levels, in a pilot facility that uses an ESP; and Figure 3 is a bar graph similar to Figure 2, but showing the improved results achieved through the use of the method according to the present invention. Description of the preferred modalities In February and April 1998, McDermott Technology, Inc. Performed tests, sponsored by the Office of Development of Carbon of Ohio (OCDO) and the Department of Energy of the United States (DOE), in its installation of pilot combustion and humid scrubber. The purpose of the tests is to study how mercury is affected by conventional pollution control equipment and to investigate various means to improve the capture of mercury with such conventional equipment. The research focuses on the combination of an ESP followed by a wet scrubber since this is the system most commonly used in appliances. With reference to the drawings generally, in which similar reference numbers designate the same parts or functionality to improve in all the various drawings, Figure 1 illustrates a burner installation of charcoal-burned apparatus of the type used by equipments in power generation electrical, generally designated 10, and representing a type of industrial process to which the present invention is applicable. In its broadest form, the present invention comprises a method for removing mercury from the fluid gas generated during the combustion of fossil fuels or solid waste through the use of a chelating agent. Of course, while the aforementioned carbon-fired heater installations are an example, and the method of the present invention will probably first find commercial application to the removal of mercury from the fluid gases produced by such burning apparatus heater installations. Such fossil fuels may be beneficial to any industrial processes that use a type of wet scrubber of the absorber module to purify such fluid gases. Such processes may include incineration plants, waste for power plants, or other industrial processes that generate gaseous products that contain mercury. In this manner for: type of convenience, the terms industrial gas, fluid gas, or gas only will be used in the following discussion to refer any gases from an industrial process and of which an objectionable component, such as mercury, will be eliminated. As illustrated in Figure 1, and proceeding in the direction of a flow of fluid gas generated during the combustion process, the heater installation 10 includes an oven 12 having a gas outlet 14 conveying fluid gases, generally designated 16, to an air heater 18 used to preheat incoming air for combustion. The pulverized ones 22 crush a fossil fuel 24 (for example, coal) to a desired fineness and the pulverized coal 24 is transported via burners 25 in furnace 12 where it is burned to release heat used to generate steam for use by an electric generator. steam turbine (not shown). The fluid gas 16 produced by the combustion process is transported through the gas outlet 14 to the air heater 18 and therefore to various types of downstream fluid gas cleaning equipment. The fluid gas cleaning equipment may comprise a cloth filter or, as shown, an electrostatic precipitator (ESP) 26 which removes particulates from fluid gas 16. Fluid 28 downstream of ESP 26 transports fluid gas 16 to a wet scrubber absorber module 30 which is used to remove sulfur dioxide and other contaminants from the fluid gas 16. The fluid gas 16 exiting the wet scrubber absorber module or simply the wet scrubber 30, is transported to a stack 32 and it goes out into the atmosphere. Forced exhaust fans 34 and induced exhaust fans are used to drive air 20, fuel 24, and fluid gases 16 through facility 10. For further details of various aspects of such facilities 10, reference is made to STEAM its generation and use, 40h Ed., Stultz and Kitto, Eds., copyright ® 1992. The Babcock & Wilcox Company, particularly to Chapter 35 - sulfur dioxide control, the text of which is incorporated herein by reference as fully taught herein. While the aforementioned STEAM reference contains a description of a form of wet scrubber 30 produced by The Babcock &; Wilcox Company (B &W) and to which the present invention is applicable, the present invention is not limited to such designed wet scrubbers B &W. Those skilled in the art will appreciate that the principles of the present invention will apply equally well to other types of wet scrubber designs available from other manufacturers. Referring again generally to Figure 1 and to Figures 2 and 3 in particular, it has been found that an ESP affects the chemical process in such a way that Hg + 2 is converted to Hg ° inside a wet downstream scrubber. Figure 2 shows the gaseous phase mercury concentration of both Hg + 2 and Hg ° measured at the inlet of a pilot wet scrubber (not shown) at McDermott Technology, Inc. Alliance Research Center in Alliance, Ohio, for the cases when the ESP is operated normally ("ESP baseline test") and when operated at high voltage levels ("ESP high energy test"). In each bar graph of Figures 2 and 3, the concentration Hg ° is designated 200, while the concentration of Hg + 2 is designated 400. Figure 2 clearly shows that the electric field in the ESP has a negative impact on the mercury collection efficiency of the wet scrubber, but does not directly affect the mercury speciation of the fluid gas. The relative amount of the different mercury species at the wet scrubber inlet is the same for both cases. However, the amount of Hg ° is greatly increased through the wet scrubber for the high-energy test. This indicates that the electric field affected some component of the fluid gas which, in turn, has a negative impact on the chemistry of the wet scrubber. Since Hg is present in such small amounts, it is likely that the affected component is also present in small amounts. A possible mechanism that explains the results II observed is presented later. In this scenario, the electric field inside the ESP creates ozone (this is known to occur). Ozone then destroys hydrogen sulfide (H2S), which is present in small amounts, and thus is not available to capture Hg + 2 as mercuric sulfide (HgS). The Hg + 2 is subsequently converted to Hg ° by some transition metal. For example, in the case of iron (Fe): In the ESP: H2S + 03? H20 + S02 The ozone created by the strong electric field destroys the H2S In the wet scrubber: H2S? 2H + + S "The H2S dissociates in the wet scrubber H" 2 + Hg + 2? HgS H2S contributes to the elimination of. Hg 2Fe +2 Hg '"-> 2Fe ++ 3J + Fe +" 2 reduces Hg, + "2 to Hg ° Hg < It is believed that the present invention blocks the action of transition metals by the use of a chelating agent, particularly ethylenediaminetetraacetic acid (EDTA).

As indicated below, a possible mechanism may be as follows: 2Fe + 2 + EDTA "4 - [2Fe (EDTA)] sequesters Fe + 2 species and avoids reducing Hg + 2.

The use and affect of chelating reagents are well known; however, to the knowledge of the inventor they have never been applied in this industry for the purpose of improving the capture of mercury within conventional wet scrubbers 30. The present invention involves the discovery that chelating agents can be used to unexpectedly improve the capture of mercury in conventional wet scrubbers 30 located downstream (with respect to a flow direction of fluid gas 16) of an ESP 26. It is not important the exact mechanism by which an ESP 26 affects the chemical process and causes the Hg + Additional 2 is converted to Hg ° inside the wet scrubber. Indeed, it is possible that the chelating agent can act directly in the mercury species, as well as, alone or in combination with actions in the transition metals as previously postulated. What is important, however, is that, by some mechanism, Hg + 2 is converted to Hg ° and that a chelating agent can be used to avoid it. The most probable mechanism by which this occurs probably involves a transition metal and more likely iron. Example Chelating agents are known to sequester transition metals, however, in the thick chemical soup that describes the coarse suspension of the wet scrubber, the action of a chelating agent can not be predicted. The chemistry of gypsum scrubber is very complicated due to the many species present in balance. The fluid gas and the gypsum, more ash that flies the heaters that burn coal, each contribute several constituents that affect the chemical formation of the system. S02, S03, C02, 02, No and N02 originated from the fluid gas; K, Cl, Fe, and other chemicals come from the flying ash; and the gypsum contains Ca, Mg and several other minor constituents such as Na and K. Therefore, a chelating agent, ethylenediaminetetraacetic acid (EDTA), is added to the reaction tank of a wet scrubber at pilot scale, and the mercury concentration at the entrance and exit of the wet scrubber. The test is carried out at the Cleaning Environmental Development Facility (CEDF), at the pilot scale of McDermott Technology, Inc., operated at a nominal heat input of approximately 100 million Btu / hr. The pulverized coal is burned, crushed to approximately 75% less than 200 mesh, in a B & W low NOx burner, with plug at a coal flow rate of approximately 4 tons per hour to generate fluid gas for the test. After passing through an ESP, the fluid gas flows through the wet scrubber comprising a thick suspension recirculation tank, a reagent feed system, and a mist eliminator wash system all of known design. The powdered gypsum is mixed with forming water in a reagent feed tank to maintain a solids content of the recirculating coarse slurry at about 12-15%. Ten pounds of a chelating agent are added, EDTA, to the thick suspension recirculation tank of the wet scrubber, which contains 1,200 gallons of coarse slurry, to produce a solution containing approximately 2 moles of chelating agent per estimated mol of transition metal, such as iron, in the coarse slurry of wet scrubber or approximately twice the amount needed based on the stoichiometry. This solution is introduced into the scrubber at a rate of approximately 120 gallons per minute per 1000 cubic feet per minute of fluid gas containing mercury. Figure 3 shows how EDTA affects the chemistry of the wet scrubber. Before EDTA is added, a large portion of Hg + 2 is converted to Hg °. After EDTA is added, the concentration of Hg ° at the outlet of the wet scrubber is reduced to levels similar to those at the inlet of the wet scrubber which indicates that no new Hg ° has formed in the wet scrubber. Figure 3 also shows that EDTA does not affect the normal clearance efficiency of Hg + 2. The net result is that the elimination of total mercury through the wet scrubber improves from 46% to 73% with the introduction of EDTA. Referring again to Figure 1, the method according to the present invention can be easily adapted to an existing installation 10 using a wet scrubber 30. The preferred chelating agent, generally designated 50, according to the present invention is EDTA . Other suitable chelating agents include, but are not limited to: hydroxyethyldiamite tetraacetic acid (HEDTA), diethylenetriaminpentaacetic acid (DTPA or pentetic acid), or nitrilotriacetic acid (NTA). The salt or acid forms of the chelating agents are suitable for use in the present invention. The chelating agent 50 can be provided from a chelating feed system, generally designated 52, via a line 54 in the coarse slurry of wet scrubber 56 contained in a lower portion of the wet scrubber 30. The recirculating pumps 59 continuously pump the coarse suspension of the wet scrubber 56 from the lower portion to upper heads 57 located within an upper portion of the wet scrubber 30 which sprayed thick suspension 56 in the fluid gas 16 which is treated by the wet scrubber 30. If desired, the method according to present invention may comprise determining the concentration of oxidized (Hg + 2) and elemental (Hg °) mercury forms in the fluid gases 15 entering and exiting the wet scrubber 30 and supplying the chelating agent 50 in the coarse slurry 56 which is recirculates therein in an amount sufficient to reduce the degree to which the at least one transition metal converts Hg + 2 into Hg ° form based on such determination. The measurements of the mercury concentration detectors 58 and 60 located at the outlet, and inlet, respectively, of the wet scrubber 30 can be employed for this purpose, and to determine the efficiency of chelating agent addition.; Indicative signals of these measurements can be sent via the dotted lines as shown for the chelator feed system 52 to automatically control the amount of the chelating agent 50 supplied. Alternatively, instead of systems employing detectors, sampling of batches at the outlet and / or exit of the wet scrubber 30 may be used to determine the Hg levels, and the amount of chelating agent 50 supplied may be based on such batch samples. . Still further, it may be desirable to simply ensure that an excess of chelating agent 50 is provided at all times to ensure that a desired level of Hg removal is obtained. Similarly, the method according to the present invention may comprise determining the amount of at least one transition metal in the coarse slurry of the wet scrubber and supplying the chelating agent 50 in the coarse slurry in an amount sufficient to reduce the degree to which the at least transition metal converts the Hg + 2 into the Hg ° form based on such determination. Alternatively, a point can be established based on the calculated or historical data and manually set. For all three cases described immediately above, the operator control means 64 associated with the chelator feeding system 52 can be used to establish fixed points 66, mode of operation, or perform manual control of the chelating feed system 52 as shown in FIG. want. Signals indicative of the measurements of the aforementioned mercury concentration detectors located at the outlet 58 and inlet 60 of the wet scrubber 30 can also be sent directly to the operator control means 64 (via dotted lines as shown) which can be then be used to communicate with and / or control the chelating feed system 52 via lines 68. In this way this invention generally applies to the process where a chelating agent is added to a wet scrubber system 30 for the purpose of facilitating the removal of the mercury. As described above, there is a wide variety of chelating agents and methods for introducing them into the wet scrubber 30. A person skilled in the art can determine the most effective and economical agent, as well as what amounts to use, and the most effective means of supply. In any application, the critical feature is to ensure supplying the chelating agent in the suspension or liquid used to purify the fluid gases 16 in an amount sufficient to at least reduce the degree to which the at least transition metal converts the Hg + 2 into the Hg ° form. Similarly, for example, the particular means by which the chelating agent 50 is provided to the wet scrubber 30 is relatively unimportant, while using some consistent and measurable means the process may be employed. The chelating agent 50 can be transported to the wet scrubber via tires, liquid or gravity medium and introduced continuously or in batch form at desired intervals. Alternatively, the chelating agent 50 can be injected upstream of the wet scrubber 30. One or more chelating agents 50 can be employed as desired, depending on the relative economy and the particular transition metal that is determined to be of interest and which will be sequestered by the chelating agent. The pilot scale test identified above demonstrates that a method and system for implementing it in accordance with the present invention is feasible, effective, and practical. For the knowledge of the inventor, there is no prior art for the elimination of improved mercury through a wet scrubber system using chemical additives. Figure 3 shows that a chelating agent 50 can be used to improve the mercury removal efficiency through a wet scrubber 30 for those systems that use an ESP 25 for particulate control. The advantages of this invention are several: 1. Chelating agents 50 are well known, widely available, and relatively inexpensive. 2. Conventional wet scrubbers can be used. That is, it is not necessary to install new contamination control equipment to control the mercury, except a chemical feed 52 system 52 (as illustrated in Figure 1) to introduce the chelating agent 50. 3. The invention can improve the efficiency of removal of S02 from the wet scrubber 30 as well. In the tests described above, there is an increased S02 removal from 95.6% to 97.9% when EDTA is added. Although this may not seem similar to a large improvement when presented in terms of percent removal, this represents a 24% increase in transfer units from (3.12 to 3.86) which is very significant. This is an unexpected result, and a result that can make the invention even more attractive to potential customers. 4. This invention applies to most fluid gas desulfurization systems used by electrical installations and is not limited in application to any particular fluid gas desulphurization system or wet scrubber design. While a specific embodiment of the invention has been demonstrated and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention can be exemplified in another way without departing from such principles. Accordingly, such embodiments and applications of the present invention fall appropriately within the scope and equivalents of the following claims.

Claims (21)

1. CLAIMS 1. In an industrial process that uses a wet scrubber to receive industrial gas containing mercury, a method to reduce the mercury content in the industrial gas leaving the wet scrubber, characterized in that it comprises: adding a chelating agent to the industrial gas; and purify the industrial gas in the wet scrubber with the chelating agent.
2. 2. The method according to claim 1, characterized in that it comprises the step of adding the chelating agent to the industrial gas in the wet scrubber.
3. 3. The method according to claim 1, characterized in that it comprises the step of purifying the industrial gas with a suspension containing the chelating agent.
4. 4. The method according to claim 1, characterized in that it comprises removing the particulate matter from the industrial gas before transporting the industrial gas to the wet scrubber.
5. 5. The method of compliance with the claim 4 characterized in that it comprises transporting the industrial gas through an electrostatic precipitator.
6. 6. The method according to claim 1, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA.
7. 7. The method according to claim 1 characterized in that the chelating agent comprises EDTA. The method according to claim 1 wherein the wet scrubber contains a wet scrubber suspension containing at least one transition metal, characterized in that it comprises the step of supplying EDTA in an amount of about twice the stoichiometric ratio of EDTA to at least one transition metal. 9. The method of compliance with the claim 1 characterized in that the wet scrubber contains a wet scrubber suspension containing at least one transition metal, comprising the step of supplying the chelating agent in an amount of about twice the stoichiometric ratio of the chelating agent to at least one metal of Transition. 10. The method according to claim 9, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA. 11. A method to reduce the concentration of elemental mercury in industrial gases leaving a wet scrubber that cleans industrial gases with a slurry, industrial gases containing mercury in oxidized (Hg + 2) and elemental (Hg °) forms, the wet scrubber containing at least one transition metal which converts the Hg + 2 into the Hg ° form, which comprises the steps of: supplying a chelating agent in the suspension in an amount sufficient to reduce the degree to which the less transition metal converts Hg + 2 into the Hg ° form; and debug industrial gases with the suspension containing the chelating agent. The method according to claim 11, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA. The method according to claim 11, characterized in that it comprises the step of: determining the amount of at least one transition metal in the wet scrubber suspension and supplying the chelating agent in the suspension in an amount sufficient to reduce the degree to which the at least transition metal converts Hg + 2 to the Hg ° form based on such a determination. 14. The method according to the claim 13, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA. 15. The method of compliance with the claim 11, characterized in that it comprises the step of: determining the concentration of the oxidized (Hg + 2) and elemental (Hg °) forms of mercury in the industrial gases entering and leaving the wet scrubber and supplying the chelating agent in the suspension in a sufficient amount to reduce the degree to which the at least one transition metal converts Hg + 2 to the Hg ° form based on such determination. 16. The method according to claim 15, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA. 17. A method for operating a wet scrubber to reduce gaseous emissions of oxidized (Hg + 2) and elemental (Hg °) mercury in industrial gases leaving the wet scrubber, characterized in that it comprises: scrubbing industrial gases within the wet scrubber with a suspension containing a sufficient amount of chelating agent to reduce the degree to which the at least transition metal in the wet scrubber suspension converts Hg + 2 to the Hg ° form. 1
8. 8. The method according to claim 17, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA. The method according to claim 17, characterized in that it comprises the step of supplying the chelating agent in an amount of about twice the stoichiometric ratio of the chelating agent to the at least one transition metal. 20. A method to operate a wet scrubber to reduce gaseous emissions of mercury in industrial gases leaving the wet scrubber, which comprises: scrubbing industrial gases within the wet scrubber with a suspension containing a sufficient amount of chelating agent to reduce the amount of mercury in industrial gases leaving the wet scrubber. 21. The method according to claim 20, characterized in that the chelating agent comprises at least one of EDTA, HEDTA, DTPA and NTA.