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 ethylenediaminetetraacetic acid (EDTA) or other similar compounds like HEDTA, DTPA and/or NTA, to the flue gas being scrubbed in a wet scrubber used in the industrial process. The chelating agent prevents the reduction of oxidized mercury to elemental mercury, thereby increasing the mercury removal efficiency of the wet scrubber. Exemplary 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%. The invention may be readily adapted to known wet scrubber systems and it specifically provides for the removal of unwanted mercury both by supplying S 2−  ions to convert Hg 2+  ions into mercuric sulfide (HgS) and by supplying a chelating agent to sequester other ions, including but not limited to Fe 2+  ions, which could otherwise induce the unwanted reduction of Hg 2+  to the form, Hg 0 .

Description:
The subject matter of the present invention was developed under a research contract with the U.S. Department of Energy (DOE), Contract No. DE-FC22-94PC94251, and under a grant agreement with the Ohio Coal Development Office (OCDO), Grant Agreement No. CDO/D-922-13. The governments of the United States and Ohio have certain rights in the invention. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of flue gas cleanup methods and apparatus and, in particular, to a method for removing mercury from the flue gas generated during the combustion of fossil fuels or solid wastes, through the use of a chelating agent. 
     In recent years, the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have supported research to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most of the other metals, most of the mercury remains in the vapor phase and does not condense onto fly ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with fly ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg +2 ) or elemental (Hg 0 ) form and each is affected differently by subsequent downstream pollution control equipment. In a conventional wet scrubber Hg +2  is relatively easy to capture while capturing Hg 0  is difficult. The relative amount of each species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and several other factors. As for the type of particulate collector installed, it has been shown that an electrostatic precipitator (ESP), as is used in the majority of utility applications, affects the process chemistry so that Hg +2  is converted to Hg 0  within a downstream wet scrubber, also commonly used in utility applications to reduce SO 2  emissions. The Hg 0  is then emitted with the flue gas. 
     Most of the recent efforts to capture and remove mercury from flue gas have concentrated on gas-phase reactions with introduced reagents such as activated carbon. 
     The subject of mercury emissions by the utility industry is a new area being investigated by both the DOE and EPA. 
     SUMMARY OF THE INVENTION 
     The present invention is a method to adjust wet scrubber chemistry to prevent the reduction of Hg +2  to Hg 0  and thereby increase the mercury removal efficiency of wet scrubber 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 for receiving an industrial gas containing mercury, a method for reducing the mercury content in the industrial gas exiting from the wet scrubber, comprising: adding a chelating agent to the industrial gas; and scrubbing the industrial gas in the wet scrubber with the chelating agent. 
     Advantageously, the chelating agent comprises at least one of ethylenediaminetetraacetic acid (EDTA), hydroxyethylenediaminetetraacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA or pentetic acid), and nitrilotriacetic acid (NTA). Pilot-scale testing employed EDTA, and an amount of about twice the stoichiometric ratio of chelating agent to the transition metals (presumed to be iron, Fe) was shown to be effective. 
     Another aspect of the present invention is drawn to a method for reducing elemental mercury concentration in industrial gases exiting from a wet scrubber which scrubs the industrial gases with a slurry, the industrial gases containing mercury in oxidized (Hg +2 ) and elemental (Hg 0 ) forms, the wet scrubber containing at least one transition metal which converts the Hg +2  into the Hg 0  form, comprising the steps of: supplying a chelating agent in the slurry an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg +2  into the Hg 0  form; and scrubbing the industrial gases with the slurry containing the chelating agent. 
     In certain aspects of the present invention, the method may comprise determining the amount of the at least one transition metal in the wet scrubber slurry and supplying the chelating agent into the slurry in an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg +2  into the Hg 0  form based upon such determination. 
     In other aspects of the present invention, the method may comprise determining the concentration of oxidized (Hg +2 ) and elemental (Hg 0 ) forms of mercury in the industrial gases entering and exiting from the wet scrubber and supplying the chelating agent into the slurry in an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg +2  into the Hg 0  form based upon such determination. 
     Yet still another aspect of the present invention is drawn to a method of operating a wet scrubber to reduce gaseous emissions of oxidized (Hg +2 ) and elemental (Hg 0 ) mercury in industrial gases exiting from the wet scrubber, comprising: scrubbing the industrial gases within the wet scrubber with a slurry containing an amount of chelating agent sufficient to reduce the degree to which the at least one transition metal in the wet scrubber slurry converts the Hg +2  into the Hg 0  form. 
     A still further aspect of the present invention is to improve removal of mercury from flue gas in a process which burns pulverized coal. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained 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: 
     FIG. 1 is an illustration of a coal-fired utility boiler installation of the type used by utilities in the generation of electric power; 
     FIG. 2 is a bar chart plotting mercury concentration at a wet scrubber inlet and outlet, at two power levels, in a pilot facility using an ESP; and 
     FIG. 3 is a bar chart similar to FIG. 2, but showing the improved results achieved through the use of the method according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In February and April of 1998, McDermott Technology, Inc. conducted tests, sponsored by the Ohio Coal Development Office (OCDO) and the U.S. Department of Energy (DOE), at its pilot combustion and wet scrubber facility. The purpose of the tests was to study how mercury was affected by conventional pollution control equipment and to investigate various means of improving mercury capture with such conventional equipment. The research focused on the combination of an ESP followed by a wet scrubber because this is the system most commonly employed by utilities. 
     Referring to the drawings generally, wherein like reference numerals designate the same or functionally similar parts throughout the several drawings, FIG. 1 illustrates a coal-fired utility boiler installation of the type used by utilities in the generation of electric power, generally designated  10 , and which represents one 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 flue gas generated during the combustion of fossil fuels or solid wastes through the use of a chelating agent. Of course, while the aforementioned coal-fired utility boiler installations are but one example, and the method of the present invention will likely first find commercial application to the removal of mercury from the flue gases produced by such utility boiler installations which combust such fossil fuels, any industrial process using a wet scrubber type of absorber module to purify such flue gases may benefit. Such processes could include incineration plants, waste to energy plants, or other industrial processes which generate gaseous products containing mercury. Thus for the sake of convenience, the terms industrial gas, flue gas, or just gas will be used in the following discussion to refer to any gas from an industrial process and from which an objectionable component, such as mercury, is to be removed. 
     As illustrated in FIG. 1, and proceeding in the direction of flue gas flow generated during the combustion process, the boiler installation  10  includes a furnace  12  having a gas outlet  14  which conveys flue gases, generally designated  16 , to an air heater  18  used to preheat incoming air  20  for combustion. Pulverizers  22  grind a fossil fuel  24  (e.g., coal) to a desired fineness and the pulverized coal  24  is conveyed via burners  25  into the furnace  12  where it is burned to release heat used to generate steam for use by a steam turbine-electric generator (not shown). Flue gas  16  produced by the combustion process are conveyed through the gas outlet  14  to the air heater  18  and thence to various types of downstream flue gas cleanup equipment. The flue gas cleanup equipment may comprise a fabric filter or, as shown, an electrostatic precipitator (ESP)  26  which removes particulates from the flue gas  16 . A flue  28  downstream of the ESP  26  conveys the flue gas  16  to a wet scrubber absorber module  30  which is used to remove sulfur dioxide and other contaminants from the flue gas  16 . Flue gas  16  exiting from the wet scrubber absorber module or, simply, the wet scrubber  30 , is conveyed to a stack  32  and exhausted to atmosphere. Forced draft fans  34  and induced draft fans  36  are used to propel the air  20 , fuel  24 , and flue gases  16  through the installation  10 . For further details of various aspects of such installations  10 , the reader is referred to  STEAM its generation and use,  40th Ed., Stultz and Kitto, Eds., Copyright © 1992 The Babcock &amp; Wilcox Company, particularly to Chapter 35—Sulfur Dioxide Control, the text of which is hereby incorporated by reference as though fully set forth herein. While the aforementioned  STEAM  reference contains a description of one form of wet scrubber  30  produced by The Babcock &amp; Wilcox Company (B&amp;W) and to which the present invention is applicable, the present invention is not limited to such B&amp;W wet scrubber designs. Persons skilled in the art will appreciate that the principles of the present invention apply equally well to other types of wet scrubber designs, available from other manufacturers. 
     Referring again generally to FIG. 1, and to FIGS. 2 and 3 in particular, it has been found that an ESP affects the process chemistry so that Hg +2  is converted to Hg 0  within a downstream wet scrubber. FIG. 2 shows the vapor-phase mercury concentration of both Hg +2  and Hg 0  measured at the inlet of a pilot wet scrubber (not shown) at the McDermott Technology, Inc. Alliance Research Center in Alliance, Ohio for the cases when the ESP was operated normally (“ESP Baseline Test”) and when it was operated at high voltage levels (“ESP High Power Test”). In each bar graph of FIGS. 2 and 3, the Hg 0  concentration is designated  200 , while the Hg +2  concentration is designated  400 . FIG. 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 mercury speciation of the flue 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 0  greatly increases across the wet scrubber for the high power test. This indicates that the electric field affects some component of the flue gas which, in turn, has a negative impact on the wet scrubber chemistry. Since Hg is present in such small quantities, it is likely that the affected component is also present in small quantities. 
     A possible mechanism that explains the observed results is presented below. In this scenario, the electric field within the ESP creates ozone (this is known to occur). The ozone then destroys hydrogen sulfide (H 2 S), which is present in small quantities, and is thus unavailable to capture Hg +2  as mercuric sulfide (HgS). Hg +2  is subsequently converted to Hg 0  by some transition metal. For example, in the case of iron (Fe): 
     In the ESP: 
     H 2 S+O 3 →H 2 O+SO 2  Ozone created by the strong electrical field destroys H 2 S 
     In the Wet Scrubber: 
     H 2 S→2H + +S −2  H 2 S dissociates in the wet scrubber 
     S −2 +Hg +2 →HgS H 2 S contributes to Hg removal 
     2Fe +2 +Hg +2 →2Fe +3 +Hg 0  Fe +2  reduces Hg +2  to Hg 0 . 
     The present invention is believed to block the action of transition metals by the use of a chelating agent, particularly ethylenediaminetetraacetic acid (EDTA). As set forth below, one possible mechanism could be as follows: 
     2Fe +2 +EDTA −4 →[2Fe(EDTA)] sequesters Fe +2  species and prevents it from reducing Hg +2 . 
     The use and effect of chelating reagents are well known; however, to the inventor&#39;s knowledge they have never been applied in this industry for the purpose of improving mercury capture within conventional wet scrubbers  30 . The present invention involves the discovery that chelating agents can be used to unexpectedly improve mercury capture in conventional wet scrubbers  30  located downstream (with respect to a direction of flue gas flow  16 ) of an ESP  26 . The exact mechanism by which an ESP  26  affects the process chemistry and causes additional Hg +2  to be converted to Hg 0  within the wet scrubber  30  is not important. Indeed, it is possible that the chelating agent may be acting directly on the mercury species, as well, alone or in combination with actions on the transition metals as postulated above. What is important, however, is that, by some mechanism, Hg +2  is being converted to Hg 0  and that a chelating agent can be used to prevent it. The most likely mechanism by which this occurs probably involves a transition metal and, most likely, iron. 
     EXAMPLE 
     Chelating agents are known to sequester transition metals, however, in the thick chemical soup that describes wet scrubber slurry, the action of a chelating agent could not be predicted. The chemistry of limestone scrubbing is very complicated due to the many species present at equilibrium. The flue gas and limestone, plus fly ash from coal-burning boilers, each contribute several constituents that affect the chemical makeup of the system. SO 2 , SO 3 , CO 2 , O 2 , NO and NO 2  originate from the flue gas; K, Cl, Fe, and other chemicals arrive with the fly ash; and the limestone contains Ca, Mg and several other minor constituents such as Na and K. Therefore, a chelating agent, ethylenediaminetetraacetic acid (EDTA), was added to the reaction tank of a pilot-scale wet scrubber, and mercury concentration was measured at the inlet and outlet of the wet scrubber. 
     The test was conducted at the McDermott Technology, Inc., pilot-scale, Clean Environment Development Facility (CEDF) operated at a nominal heat input of approximately 100 million Btu/hr. Pulverized coal, ground to approximately 75% less than 200 mesh, was burned in a B&amp;W low-NOx, plug-in burner at a coal flow rate of approximately 4 tons per hour to generate flue gas for the test. 
     After passing through an ESP, the flue gas flowed through a wet scrubber comprising a slurry recirculation tank, a reagent feed system, and a mist eliminator wash system all of known design. Pulverized limestone was mixed with make-up water in a reagent feed tank to maintain a solids content of the recirculating slurry at about 12-15%. 
     Ten pounds of a chelating agent, EDTA, were added to the wet scrubber slurry recirculation tank, containing 1,200 gallons of slurry, to produce a solution containing approximately 2 moles chelating agent per estimated mole of transition metal, such as iron, in the wet scrubber slurry or approximately twice the amount needed based on stoichiometry. This solution was introduced into the scrubber at a rate of about 120 gallons per minute per 1000 actual cubic feet per minute of mercury-containing flue gas. 
     FIG. 3 shows how EDTA affected the wet scrubber chemistry. Before EDTA was added, a large portion of Hg +2  was being converted to Hg 0 . After EDTA was added, the concentration of Hg 0  at the wet scrubber outlet was reduced to levels similar to that at the wet scrubber inlet that indicates no new Hg 0  was formed in the wet scrubber. FIG. 3 also shows that EDTA did not affect the normal removal efficiency of Hg +2 . The net result was that total mercury removal across the wet scrubber improved from 46% to 73% with the introduction of EDTA. 
     Referring again to FIG. 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: hydroxyethylenediaminetetraacetic acid (HEDTA), diethylenetriaminepentaacetic 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  could be provided from a chelating feed system, generally designated  52 , via a line  54  into the wet scrubber slurry  56  contained in a lower portion of the wet scrubber  30 . Recirculating pumps  59  continuously pump the wet scrubber slurry  56  from the lower portion to upper headers  57  located within an upper portion of the wet scrubber  30  which spray slurry  56  into the flue gas  16  being treated by the wet scrubber  30 . 
     If desired, the method according to the present invention may comprise determining the concentration of oxidized (Hg +2 ) and elemental (Hg 0 ) forms of mercury in the flue gases  16  entering and exiting from the wet scrubber  30  and supplying the chelating agent  50  into the slurry  56  being recirculated therein in an amount sufficient to reduce the degree to which at least one transition metal converts the Hg +2  into the Hg 0  form based upon such determination. Measurements from mercury concentration sensors  58  and  60  located at the exit, and inlet, respectively, of the wet scrubber  30  may be employed for this purpose, and to determine the effectiveness of chelating agent  50  addition; signals indicative of these measurements could be sent via the dashed lines as shown to the chelating feed system  52  to automatically control the amount of chelating agent  50  supplied. Alternatively, instead of a system employing sensors, batch sampling at the exit and/or inlet of the wet scrubber  30  could be used to determine the Hg levels, and the amount of chelating agent  50  supplied would be based on such batch samples. Still further, it might be desirable to merely 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 the at least one transition metal in the wet scrubber slurry and supplying the chelating agent  50  into the slurry in an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg +2  into the Hg 0  form based upon such determination. Alternatively, a set point could be established based on calculated or historical data and set manually. For all three cases described immediately above, operator control means  64  associated with the chelating feed system  52  could be used to establish setpoints  66 , mode of operation, or perform manual control of the chelating feed system  52  as desired. The signals indicative of the measurements from the aforementioned mercury concentration sensors located at the exit  58  and inlet  60  of the wet scrubber  30  may also be sent directly to the operator control means  64  (via dashed lines as shown) which could then be used to communicate with and/or control the chelating feed system  52  via lines  68 . Chelating agent  50  may be added directly to slurry  56  and/or slurry  56  may be drawn out of scrubber  30  via line  62  with agent  50  being mixed in chelating feed system  52  with the mixture of slurry  56  and chelating agent  50  subsequently reintroduced to scrubber  30  via line  54 . 
     This invention thus generally applies to the process whereby a chelating agent  50  is added to a wet scrubber system  30  for the purpose of facilitating the removal of mercury. As described above, there are a wide variety of chelating agents and methods to introduce them into the wet scrubber  30 . A person skilled in this art can determine the most effective and economical agent, as well as what quantities to use, and the most effective means of delivery. In any application, the critical feature is to ensure supplying the chelating agent into the slurry or liquid used to scrub the flue gases  16  in an amount sufficient to at least reduce the degree to which the at least one transition metal converts the Hg 0  into the Hg +2  form. Similarly, for example, the particular means by which the chelating agent  50  is provided to the wet scrubber  30  is relatively unimportant, so long as some consistent and measurable means are employed so the process can be employed. The chelating agent  50  may be conveyed to the wet scrubber via pneumatic, liquid, or gravity means and introduced continuously or in batch form at desired intervals. Alternatively, the chelating agent  50  could be injected upstream of the wet scrubber  30 . One or more chelating agents  50  may be employed as desired, depending upon the relative economics and the particular transition metal which is determined to be of interest and which is to be sequestered by the chelating agent. The above-identified pilot-scale testing demonstrates that a method and system for implementing same according to the present invention is feasible, effective, and practical. 
     To the inventor&#39;s knowledge, no prior art exists for enhanced mercury removal across wet scrubber systems using chemical additives. 
     FIG. 3 shows that a chelating agent  50  can be used to improve mercury removal efficiency across a wet scrubber  30  for those systems that use an ESP  26  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  30  can be used. That is, no new pollution control equipment need be installed to control mercury, except a small chemical feed system  52  (as illustrated in FIG. 1) for introducing the chelating agent  50 . 
     3. The invention may improve the SO 2  removal efficiency of the wet scrubber  30  as well. In the tests described above, SO 2  removal increased from 95.6% to 97.9% when EDTA was added. Although this may not seem like a big improvement when presented in terms of percent removal, it 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 would make the invention even more attractive to potential customers. 
     4. This invention applies to the majority of flue gas desulfurization systems used by electric utilities and is not limited in application to any particular flue gas desulfurization system or wet scrubber design. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. Accordingly, all such embodiments and applications of the present invention properly fall within the scope and equivalents of the following claims.