Abstract:
The illustrated embodiment of the invention is a method for mitigating mercury emissions in an exhaust gas. The method includes the steps of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas. The deposited compound of mercury is sublimed back into the exhaust gas as volatile mercury dichloride from the surface without any substantial production of elemental mercury. The sublimed mercury dichloride is removed from the exhaust gas by scrubbing.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is related to U.S. patent application Ser. No. 10/429,114, filed on May 1, 2003, which is incorporated herein by reference and to which priority is claimed pursuant to 35 USC 120. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to methods for controlling undesirable emission of mercury, primarily in coal combustion flue gases.  
         [0004]     2. Description of the Prior Art  
         [0005]     There are ever increasing concerns about the levels of mercury in our normally encountered environment. As a result many governmental and state agencies are legislating for potentially severe controls on the currently non-regulated emissions of mercury from primarily coal combustors and incinerators. Likewise, the use of mercury in such items as switches, thermostats, thermometers, vaccines and industrial processes, such as the chlor-alkali method for chlorine and caustic soda production, are all being gradually phased out. In particular, there are significant concerns over the combined effects of the organic form of mercury used in medical vaccines, and the bioaccumulation that occurs in fish, with many believing that these were the base trigger that has caused a significant increase since the early 1990&#39;s in the occurrence of autism in children.  
         [0006]     Although mercury is present in all coals at levels of below 1 ppmw (part per million by weight), the United States currently consumes about 1.7 billion tons per year, and China now 2.7 billion per year with a total global use of at least 5 billion tons per year. As a result, although only a trace, its contribution to the global background is no longer negligible. Likewise, arguments that it is of little use for the U.S. to unilaterally control mercury are in fact meaningful at present unless a reasonable-cost solution can be realized. Consequently although there now are numerous control methods being implemented these are generally complex and very expensive. Other than the presently discovered method outlined below, there is no technique that can be globally embraced at this time.  
         [0007]     Current control methods are based on classical techniques and have been previously discussed in some detail. The favored current effort suggests using activated charcoal, as is, or additionally pre-treated with sulfur and/or halogens, or fly ash with a high carbon content. The absorption that occurs is not an efficient process, requiring large quantities of particulate injection into the cooling flue gases to be effective. Generally, such usage also requires a second-particle-removing device if the major fly ash is to continue to be sold for concrete use. Otherwise, too much carbon in the ash eliminates such usage and so also removes a valuable income stream. The process can work but is labor intensive and expensive.  
         [0008]     Other methods tend to be based on attempts to oxidize the elemental mercury in the gaseous form to its oxide or dichloride using injected oxidizing agents, ultraviolet radiation, or by using a recognized catalytic material. These can all be made to work to some degree. However, not knowing the chemistry of the system, they are generally regarded as engineering solutions that have been arrived at by trial and error. At present, one such scheme is an endeavor to use a selective catalytic reactor (SCR) to control both the nitric oxide (NO) emissions for which it is designed and also mercury. This technique controls NO emissions by adding traces of ammonia to the flow and utilizing catalysts at temperatures of about 370° C. Results do show some additional benefits for also controlling mercury. Such a multiple use device would be a God send for the industry, but invariably the operating temperatures are rather high for mercury oxidation, and elevated levels of chlorine are also required that can additionally interfere with the ammonia additions. There is no doubt that recognized catalysts can oxidize mercury, but they also encounter the problems of surface blinding by ash or by solid sodium sulfate formation resulting in a need for frequent cleaning. This requires using a catalyst that can withstand the practicalities of air blasting methods generally used for cleaning, which are far from gentle.  
         [0009]     Methods based on additives generally are fraught with introducing their own possible problems of, for example, corrosion, collection and disposal that can all add to additional costs.  
         [0010]     Mercury&#39;s somewhat unique property of amalgamation with various metals is well known and found an initial major use in dentistry. However, it amalgamates most efficiently only with gold and silver, and has a much smaller interaction with other metals. To catch small quantities of mercury, gold can be very quantitative and useful. It has even been tried as a control method in large scale application. However, a general application of amalgamation has been found not to be practical and the concept is now discarded.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The illustrated embodiment of the invention is a method for mitigating mercury emissions in an exhaust gas, the method comprising the steps of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas without use of a chemical catalyst; subliming the deposited compound of mercury back into the exhaust gas as mercury dichloride from the surface without any substantial production of elemental mercury, and removing the sublimed mercury dichloride from the exhaust gas by scrubbing.  
         [0012]     The illustrated embodiment also includes the method for mitigating mercury emissions in an exhaust gas, the method comprising the steps of providing a surface which is in contact with the exhaust gas; chemi-depositing a substantial portion of the total amount of mercury in the exhaust gas upon a noncatalytic surface to form a stable compound of mercury; subliming the deposited compound of mercury back into the exhaust gas as the volatile mercury dichloride without substantial production of elemental mercury; and scrubbing the sublimed compound of mercury from the exhaust gas by conventional methods.  
         [0013]     The step of chemi-depositing a substantial portion of the total amount of mercury in the exhaust gas upon a noncatalytic surface comprises chemi-depositing elemental mercury in the exhaust gas. upon the surface.  
         [0014]     The step of chemi-depositing a substantial portion of the total amount of mercury in the exhaust gas upon a noncatalytic surface comprises chemi-depositing-at least one sulfur compound in the exhaust gas upon the surface.  
         [0015]     The step of chemi-depositing a substantial portion of the total amount of mercury in the exhaust gas upon a noncatalytic surface comprises chemi-depositing with a form of chlorine in the exhaust gas selected from the group consisting of: hydrogen chloride; atomic chlorine; and molecular chlorine.  
         [0016]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a chemically deposited compound of mercury by utilizing a flow modifying spoiler configured to enhance gas-surface collisions.  
         [0017]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a chemically deposited compound of mercury on a flow modifying spoiler disposed at a location within the flow of exhaust gases which enhances deposition of mercury thereon.  
         [0018]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a chemically deposited compound of mercury on a flow modifying spoiler disposed at a location within the flow of exhaust gases where the exhaust gases have a temperature of between approximately 150° C and 300° C.  
         [0019]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a chemically deposited compound of mercury on a flow modifying spoiler disposed within the flow of exhaust gases, and further comprises the step of heating the surface, if necessary, to a temperature between approximately 150° C. and 300° C.  
         [0020]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a chemically deposited compound of mercury on a surface composed at least in part of a base-metal and/or a ceramic material.  
         [0021]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a mercury oxide, which is subsequently converted into mercury dichloride.  
         [0022]     The step of noncatalytically forming a chemically deposited compound of mercury on a surface from a substantial portion of the total amount of mercury in the exhaust gas comprises noncatalytically forming a mercury sulfate, which is subsequently converted into mercury dichloride.  
         [0023]     In another embodiment, the method further comprises the step of adding, if necessary to a sulfur poor exhaust gas, sulfur dioxide to the exhaust gas prior to the exhaust gas contacting the surface, so that it is in excess of the mercury concentration and to enhance a formation of mercury sulfate and hence dichloride upon the surface.  
         [0024]     In still another embodiment the method further comprises adding, if necessary in a chlorine poor exhaust gas, at least one of atomic chlorine, molecular chlorine and hydrogen chloride to the exhaust gas prior to the exhaust gas contacting the surface, so as to ensure and enhance a formation of mercury dichloride upon the surface.  
         [0025]     The step of removing the sublimed mercury dichloride from the exhaust gas by scrubbing comprises the step of water scrubbing or chemical dry scrubbing.  
         [0026]     The illustrated embodiment of the invention is also an apparatus for mitigating mercury emissions in an exhaust gas. The apparatus comprises a surface configured to be disposed within an exhaust gas stream so as to facilitate formation of a mercury compound thereupon and configured so as to facilitate sublimation of the mercury compound therefrom.  
         [0027]     The surface is configured to facilitate deposition of mercury sulfate thereon. In one embodiment the surface is configured to facilitate deposition of mercury sulfate thereon and subsequent conversion of the deposited mercury sulfate into mercury dichloride. In another embodiment the surface is configured to facilitate deposition of mercury oxide thereon and subsequent conversion of the deposited mercury oxide into mercury dichloride. In yet another embodiment the surface is at least partially defined by a flow spoiler, the flow spoiler being configured to enhance a gas-surface collision frequency of mercury atoms in the flow. For example, the surface comprises a base-metal or ceramic surface.  
         [0028]     The apparatus in it various embodiments further comprises a chlorine and/or sulfur injection system configured to inject molecular or atomic chlorine or hydrogen chloride of sulfur dioxide gases into the exhaust gas prior to the exhaust gas contacting the surface.  
         [0029]     The apparatus is understood to include in some embodiments a scrubber for removing sublimed mercury dichloride from the exhaust gas. The apparatus thus further comprises a water scrubber or a dry chemical scrubber for removing sublimed mercury dichloride from the exhaust gas in some embodiments.  
         [0030]     The illustrated embodiment of the invention is also characterized as an exhaust system for mitigating mercury in an exhaust gas. The exhaust system comprises a smokestack, and one or more surfaces within the smokestack configured to facilitate deposition of a mercury compound thereon. The surface is configured to facilitate sublimation of mercury dichloride therefrom. A scrubber is configured to facilitate scrubbing of sublimed mercury from the exhaust gas.  
         [0031]     The illustrated embodiment includes a method for mitigating mercury emissions in an exhaust gas which prior to mitigation comprises at least one sulfur compound or a form of chlorine selected from the group consisting of hydrogen chloride, atomic chlorine and molecular chlorine, the method comprising the steps of providing a flow modifying spoiler disposed at a location within the flow of exhaust gases which enhances deposition of mercury thereon at a location within the flow of exhaust gases where the exhaust gases have a temperature sufficient to maintain the surface at approximately 150° C. to 300° C., so that mercury deposited upon the surface forms mercury oxide or mercury sulfate, which is subsequently converted into mercury dichloride; by chemi-depositing noncatalytically a substantial portion of the total amount of mercury in the exhaust gas upon the spoiler to form a stable compound of mercury; subliming the deposited compound of mercury as mercury dichloride back into the exhaust gas without substantial production of elemental mercury; and water scrubbing or chemical dry scrubbing the sublimed mercury dichloride from the exhaust gas without substantial production of elemental mercury; and adding sulfur dioxide or a form of chlorine to the exhaust gas prior to the exhaust gas contacting the spoiler, so as lo enhance a formation of mercury sulfate and hence dichloride upon the spoiler.  
         [0032]     While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is a photograph of an experimental setup in which the chemi-deposition of a molecular form of mercury is measured.  
         [0034]      FIG. 2  is an x-ray spectrum of a deposit collected at 300° C. from the burned gases of a C 3 H 8 /O 2 /N 2  (0.9/5/20) flame containing 25 ppmv Hg and 100 ppmv SO 2 .  
         [0035]      FIG. 3  is a corresponding FT-Raman spectra of the same deposit as in  FIG. 2 .  
         [0036]      FIG. 4  is a graph of the corresponding rates of deposition from flame gases of mercury or sodium compounds onto a steel surface as a function of collection probe temperature.  
         [0037]      FIG. 5  is a graph of a multitude of individual experiments examining the nature of chemi-deposition from flame gases as a function of probe temperature.  
         [0038]      FIG. 6  is a diagram illustrating the heterogeneous chemi-deposition mechanism of mercury from fossil fuel combustion exhaust gases onto relatively cooler surfaces (150-450° C.). 
     
    
       [0039]     The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     In most cases it is often the general lack of understanding of the chemical interactions in combustors that leaves a high degree of dissatisfaction with most of the prior art approaches. The illustrated invention is the first to build on a characterized combustion chemistry of mercury. This long overlooked and surprising chemistry of mercury has been discussed in general terms in a recent paper entitled “Mercury Emission Chemistry: The Similarities or are they Generalities of Mercury and Alkali Combustion Deposition Processes” (K. Schofield,  Proceedings of the Combustion Institute,  Volume 30 (2005) pp. 1263-1271); and with some early results in an initial publication (“Let them Eat Fish: Hold the Mercury,” K. Schofield,  Chemical Physics Letters,  Volume 386 (2004) pp. 65-69). The remarkable similar deposition behavior of alkali metals and mercury from flame gases has illustrated a commonality between these very different elements. It, along with other results to presented below, show that these elements can be deposited from flame gases onto any type of material surface that might be encountered in spite of their very different chemical natures. This is seen to result mainly from the combined effects of going from 3-, to 2-dimensional space with the additional change of gas to condensed phases that modifies the thermodynamic and kinetic properties that can be involved. As a result, the present method uses base materials that are neither catalytic nor amalgamating with mercury. An inexpensive new concept based on a noncatalytic process for mercury control is disclosed.  
         [0041]     The illustrated embodiment of the invention relates to an original, simple low-cost method for controlling the undesirable emission of mercury, primarily in coal combustion flue gases, which is currently of major global concern. However, it is appropriate in general for mitigating mercury emitted from any high temperature combustion or processing method. The mercury, which is always in an atomic form in such flows, is intercepted by a base metal surface at an appropriate temperature. It momentarily condenses by chemi-deposition as a mercury sulfate or oxide, but is then rapidly converted to volatile mercury dichloride that sublimes back into the flue gases. The latter is soluble in scrubber fluids and readily removed for adequate disposal. The process is efficient and is neither catalytic nor amalgamating in nature. The surface serves solely to facilitate this conversion and requires minimal maintenance.  
         [0042]     As most clearly seen in the photograph of  FIG. 1 , mercury has a pronounced propensity for heterogeneous chemical deposition. Chemi-deposition of a molecular form of mercury (mercuric sulfate) from the flame gases of a fuel-lean propane/oxygen/nitrogen flame seeded with 25 ppmv (parts per million by volume) of mercury and traces of sulfur is shown in the discolored underside of a probe collected in  FIG. 1  on an air-cooled platinum-clad probe with a surface temperature of 250° C. Similar performance is seen on steel and other base materials.  
         [0043]     This is the first ever clearly defined observation of this behavior. In combustion, because mercury has no molecular compounds that are thermally stable at flame temperatures, any mercury present in a fuel is necessarily atomized to gaseous elemental mercury. In practical coal combustors, what is observed is that such mercury passes through the combustor and generally remains predominantly unchanged.  
         [0044]     However, to varying degrees a small fraction of the mercury does convert and become gaseous HgCl 2 . Also, some is adsorbed onto fly ash particles to an extent that depends on the ash carbon content. This partitioning has never been understood until the present work. Many attempts over the years that have invoked gas phase chemistry as an explanation have failed and more recent studies of kinetic rates in fact now indicate that any gas phase component is totally unlikely, even though some advocates still persist in pursuing such modeling.  
         [0045]     As seen in the spectra  FIGS. 2 and 3 , X-Ray and Raman spectroscopic analyses of such deposits immediately indicated, if sulfur was present in the exhaust gases, that the deposits were comprised mainly of mercuric sulfate, HgSO 4 , mixed with some contribution of its basic sulfate that is now known after its discoverer as Schuetteite, HgSO 4 .2HgO.  FIG. 2  is an X-ray spectrum of a deposit collected at 300° C. from the burned gases of a C 3 H 8 /O 2 /N 2  (0.9/5/20) flame containing 25 ppmv Hg and 100 ppmv SO 2 . This is compared to samples of pure HgSO 4 , HgSO 4 .2HgO (Schuetteite) and the Pt substrate of the analyzing instrument. The deposit Hg:S ratio, as determined by inductively coupled plasma (ICP) analysis, is 1.32 implying a 16% Schuetteite contribution to the predominantly HgSO 4  formation.  FIG. 3  is a corresponding FT-Raman spectra of the same deposit as in  FIG. 2  confirming it to be solely a mixture of mercury sulfate and the basic sulfate, Schuetteite.  
         [0046]     If no sulfur was present in the flame gases, a dark brown deposit resulted instead that was pure HgO. What was especially intriguing was that I noted that these deposits behaved in a very similar manner to those observed in previous unrelated studies of alkali sulfates that pertain to high temperature corrosion in gas turbines (“The Controlling Chemistry in Flame Generated Surface Deposition of Na 2 SO 4  and the Effects of Chlorine,” K. Schofield and M. Steinberg,  Symp.  ( Int .)  on Combustion,  Volume 26 (1996) pp. 1835-1843; “Method for the Prevention of High Temperature Corrosion Due to Alkali Sulfates and Chlorides and Composition for Use of the Same,” K. Schofield, U.S. Pat. No. 6,328,911 (2001)).  
         [0047]     With alkalis there was also a propensity for sulfate formation, for example Na 2 SO 4  or K 2 SO 4 . Moreover if no sulfur was available the alkali turned to carbon and made an equivalent amount of carbonate, Na 2 CO 3 . With mercury it was sulfate if it could, but if not then the oxide.  
         [0048]     Such deposition of alkalis is well known and remains the reason why gas turbines are forced to use natural gas. The alkali and sulfur content of alternate fuels is such that it corrodes the turbine blades and only now are methods finally emerging to control the alkalis down to the parts per billion levels needed by hot gas cleanup methods. Moreover, the alkali sulfates (sodium and potassium) are known to deposit on any surface very efficiently.  
         [0049]      FIG. 4  is a graph of the corresponding rates of deposition from flame gases of mercury or sodium compounds onto a steel surface as a function of collection probe temperature. This is for similar element atomic inputs in either oxygen-rich hydrocarbon or hydrogen/air flames in the presence or absence of sulfur. As seen in  FIG. 4 , when rates of deposition of mercury sulfate or oxide are compared to those of the alkalis they are seen to reflect essentially the same efficiencies. Also, all are measured to be first order in the concentration of the metal. It is the rate at which the metal gets to the surface that is the major controlling parameter at the lower surface temperatures and reflects such a linear dependence. The fact that it is HgSO 4  forming in one case and Na 2 SO 4  in the other appears to be irrelevant. Based on the number of metal atoms forming a deposit on the surface, for similar concentrations in the unburned fuel gases, the same rates are observed at the lower temperature where data are common to all of them. Such a phenomena is now being referred to as chemi-deposition as it appears to be independent of the nature of the surface and uses the latter solely to permit condensed phase chemistry to occur on a two dimensional surface instead of the three dimensions of the gas phase. The modification of three to two-dimensional space with a resulting different phase totally changes the thermodynamic and kinetic scenarios. Moreover, concepts such as dew point are seen to lose all meaning in these processes.  
         [0050]     The similar behavior between the alkalis and mercury is a powerful indication that this is not a unique mechanism requiring special conditions of surface and catalyst but rather a new general surface phenomenon.  
         [0051]     The fall-off regions to higher temperatures reflect the thermal stabilities of each of the deposits. In the case of alkali deposition, one interesting point shown in  FIG. 4  is that there is no discontinuity in the rates on passing through the indicated melting points of sodium sulfate or in sulfur free flames that deposit sodium carbonate. The rate of deposition is the same whether onto an existing solid phase surface deposit or on to one that is a liquid phase. The mercury salts dissociate before melting and their fall-off is consistent with thermal dissociation. In the case of mercury sulfate, deposits are seen up to about 425° C. However, elemental mercury has a boiling point of 357° C. so the initial deposition chemistry is sufficiently fast that it retains the mercury on a surface even though this is hotter than mercury&#39;s boiling point.  
         [0052]     Experiments that confirmed the chemi-deposition nature of these observations involved measuring the rates of deposition from differing compositions onto different probe materials. Of major importance were experiments that confirmed exactly similar rates of deposition, whether they be onto a platinum or a base metal surface such as steel. Platinum generally is known to be a highly catalytic metal whereas steel is not. Moreover, any role of amalgamation in these deposits also was eliminated as steel in particular is reluctant to form an amalgamation with mercury. Consequently, the observations were indicative of neither catalytic nor amalgamation contributions.  
         [0053]     Experiments such as the arrangement indicated in  FIG. 1  were aimed at resolving the detailed chemistry of the deposition process. However, the cooled collection probe was still located in the high temperature burned gases and so not directly pertinent to much cooler flue gases. Nevertheless, such experiments proved that the nature of the flame gases were somewhat irrelevant provided that they had an oxygen rich equivalence ratio. The observed chemistry is only observed under oxidizing conditions and deposition is not possible under reducing conditions such as with fuel rich flames. For oxygen rich flames that reflect practical combustors, behavior was independent of flame composition and not affected by highly non-equilibrated conditions, nor by flame temperature or equivalence ratio or even fuel type. Behavior in hydrogen flames was no different from that observed using propane.  
         [0054]     Once established, additional experiments were then designed to relate to downstream cooling flue gases. Probes were moved downstream and internally heated if necessary. Similar behavior was observed except at the lower temperatures a low temperature onset now became observable. As a result, deposits were obtained from flue gases as cool as 200° C. onto metal probes. Optimum conditions were around 250 to 300° C. probe surface temperatures. The temperature of the surface is seen to be more important than the gas temperature. In fact the phenomenon is totally controlled by the surface that solely uses the flue gases as its provenience.  
         [0055]     In coal combustors it is general that Hg&lt;&lt;Cl&lt;S. Sulfur is present in the burned gases predominantly as SO 2  and chlorine as HCl. At higher temperatures, mercury is totally elemental and none is retained in the bottom ash nor the initially formed fly ash. Consequently, the sulfate and basic sulfate deposits that ultimately will form on surfaces at acceptable temperatures will be in an environment containing HCl.  
         [0056]      FIG. 5  is a graph which provides a summary of a multitude of individual experiments examining the nature of chemi-deposition from flame gases as a function of probe temperature. The top curve represents deposits of HgSO 4  and reflects its thermal stability. When the Hg to Cl ratio is 1:1 in the flame, the deposit rate is halved and its intrinsic Hg:S content modified slightly. With more chlorine no deposits are observed. This reflects the rapid conversion of deposited HgSO 4  by gaseous HCl to volatile HgCl 2 . The numbers on the curves refer to the exact Hg:S ratio content of the deposit. A value of unity for this ratio implies pure HgSO 4  and a value of two would indicate a 50/50 molecular mix of the sulfate and its basic sulfate, HgSO 4 .2HgO. Consequently, what is seen to higher temperatures is a gradually increasing contribution of the basic sulfate that increases more so at dissociation. It is known on thermal dissociation that the sulfate does pass through the basic salt as it falls apart (“Thermal Analysis of Mercurous and Mercuric Sulfates”, S. A. Tariq and J. O. Hill,  Journal of Thermal Analysis,  Volume 21 (1981) pp. 277-281).  
         [0057]     If chlorine is present in the flame with exactly the same concentration as the mercury the measured deposit on the probe is smaller as seen in the  FIG. 5 . In the 200-300° C. range the rate of deposit is exactly halved. This together with other evidence indicates that half of the initially formed sulfate is being rapidly and quantitatively converted to the dichloride. This has volatility almost the same as elemental mercury and so sublimes off the surface. The horizontal lines on the curve are the levels to be expected for a 50% removal rate. Moreover, indications are that Hg and HCl cannot react directly on a surface. The sulfate has to be initially formed and then converted for HgCl 2  to be produced. Rather interesting are the corresponding values for Hg:S content of the deposit that remains. With chlorine there are indications that if the basic sulfate is present that it is preferentially converted to a larger degree before the HgSO 4 . If chlorine is present in the flame gases in amounts twice or more that of mercury then no deposits are observed.  
         [0058]     It is known that molecular chlorine, Cl 2 , can react with elemental mercury on even a quartz surface to form HgCl 2  (“Surface Catalyzed Reaction of Hg+Cl 2 ,” A. K. Medhekar et al.  Chemical Physics Letters,  Volume 65 (1979) pp. 600-604). However, there is a negligible amount of Cl 2  in combustion gases. The observed efficiencies and the halving of the deposit rates when Hg and HCl are equal define a specific role for HCl. Consequently, this work establishes without question that there is a very efficient deposition mechanism involving atomic mercury, SO 2 , and HCl. Whether it is the major dominant mechanism that can occur in coal combustors may be debatable. But as will be indicated, there appears to be full-scale coal combustor data that is consistent with this mechanism explaining present observations therein.  
         [0059]      FIG. 6  is a diagram which illustrates the heterogeneous chemi-deposition mechanism of mercury from fossil fuel combustion exhaust gases onto relatively cooler surfaces (150-450° C.). HgSO 4 (s) is formed but momentarily as it is rapidly converted by HCl to the dichloride. The high cut off temperature for this mechanism is controlled by the competition between the rates of thermal dissociation of the HgSO 4 (s) back to elemental Hg and rate of the conversion reaction to HgCl 2  that is specifically controlled by the chlorine concentration levels. The onset lowest temperature is controlled by a low activation energy of formation of the sulfate.  
         [0060]     Consequently,  FIG. 6 , pictorially indicates what can happen on a surface that interfaces with flue gases. The atomic mercury and SO 2  reach the surface and if the temperature is in the functioning window will rapidly produce the sulfate. The influx appears to be the rate controller and not any kinetic or thermodynamic limiter. This then generally will rapidly react with HCl, be converted to the dichloride that will sublime back into the flow. In current combustors, the surface temperature constraint, the concentrations of HCl, and the generally minor interactions between gas flows and surface ducts will determine the fraction of gaseous mercury that manages to convert to HgCl 2 (g). This is the desirable thermodynamic goal of the mercury in such systems but it is currently too constrained to achieve it to any significant degree. Plant-to-plant variations that have been observed obviously arise from these several mechanistic constraints the extent of which can differ between combustor systems. The flue gas flow rate also introduces an additional time constraint as it controls how quickly the gases pass through the current region where oxidation of the mercury can occur. Consequently, providing more surface area with which the flows can interact at the optimal surface temperature, and at a specific fixed location, would seem to be the solution suggested by this research. It forms the basis of this Patent Application and would not have been a considered approach by the previous state of the art. It is based solely on enhancing to completion a natural process that mercury is seen to be currently struggling to accomplish.  
         [0061]     All the laboratory results that have been obtained in the above experiments necessarily have studied mercury in flame gases at concentrations that are on a parts per million scale. The levels in full scale coal combustors are more on a 10 parts per billion scale. Consequently, having established an efficient heterogeneous deposition mechanism in laboratory flame gases it is imperative to analyze the expected consequences of such a thousand-fold scaling down of the mercury concentrations. The concentration levels of SO 2  and HCl remain similarly on ppm levels.  
         [0062]     The influx of mercury to a surface has been shown to be a linear first-order dependence. This implies a normal scalability and rates of deposition will therefore decrease by 1000-fold. However, in a current combustor, once on the surface as conveyed by  FIG. 6 , the full extent of conversion will depend on the relative rates of reaction with HCl and thermal dissociation of the deposit. The upper fall-off curve reported in  FIG. 5  reflects the competition between deposition and thermal dissociation. Consequently, at about 415° C., the rate of thermal dissociation is seen to be one-half of the rate of influx to the surface. In other words, rates of dissociation are also operating on a ppm scale. Consequently, it may be questioned if the influx is decreased by 1000-fold whether the rate of thermal dissociation scale down similarly.  
         [0063]     Dissociation of solid phases is complex. When studied in macro-size quantities the solid consists of many molecular layers and dissociation is mainly at the surface. In such cases, kinetic rates can exhibit a zero order dependence on concentration and are controlled by a dissociation activation energy. This corresponds, for example, to evaporation from a bucket filled with water. Whether the bucket is full or fractionally filled the rate of evaporation is independent of the quantity but at various different temperatures the rate of loss will reflect the heat of evaporation. From supplementary experiments in a thermogravimetric analyzer and also from the shape of the fall-off curve in  FIG. 5 , it has been possible to derive a value for the heat of dissociation of HgSO 4 . This is of the order of 243 kJ mol −1 . Due to the rapid onset of dissociation of 400° C. with HgSO 4 , rates of dissociation of deposited thin films could not be directly studied accurately. However, corresponding data for HgO deposits show a more gradual fall-off. Several experiments with this were undertaken, initially obtaining a known amount of deposit of HgO at 220° C. where it is stable, and then raising the temperature to 350° C. to measure the thermal loss rate in a specific time. This was repeated numerous times with a differing initial amount of deposit. In this way, the mass dependence of thermal dissociation was examined for very small quantities. As somewhat expected, for larger deposit masses the loss rate was constant and showed no dependence on quantity. However, as deposits got smaller in thickness this changed and began to also show a first order mass dependence. In full scale combustors one can expect that deposits of mercury on any surface will always be sparse and never even form a total monomolecular layer. Consequently, as mercury concentrations are scaled down from ppm to ppb levels the thermal dissociation is expected to remain constant for a certain range but then also will fall away in a linear manner. Its overall rate on scaling down may be less than 1000-fold but the importance of the dissociation step will also decrease significantly. If it is somewhat less than the scaling down of mercury it will potentially have the effect of pulling the fall-off curve in  FIG. 5  to slightly lower temperatures for ppbv quantities and so slightly narrow the operating temperature window for this mechanism. However, in competition with this thermal dissociation is the rate of conversion by HCl. All the evidence obtained so far implies that this is a highly efficient reaction with a negligible activation energy and as witnessed in FIGS.  5  can remove mercury quantitatively from a deposit even when Cl:Hg=2:1.  
         [0064]     Measurements of the specific parameters involved in the removal rates of HgSO 4  by HCl flows indication a significant efficiency. Its importance increases for higher HCl concentrations. These increased conversion rates with higher chlorine levels therefore can compensate for the scaling down effects on thermal dissociation and result in leaving the operating window at ppb levels of mercury not too different from that shown in  FIG. 5 . Such arguments relate to the upper limit of the window where this competition exists. The ultimate conclusion suggests that scaling down to the ppbv levels of coal combustors will have a functioning window that initiates at about 150° C. and is similar to  FIG. 5  at its upper end. There would appear to be no reason why this mechanism cannot proceed in combustors if the mercury can locate adequate surfaces. Moreover, there is no reason to discount this as being the predominant mechanism currently being observed in combustion but at reduced efficiencies.  
         [0065]     At present, coals with high chlorine content are favored for use as half of the mercury can be found to be gaseous HgCl 2  in the exhaust gases. This is desirable as it can then be controlled by the now ever present sulfur scrubbers in use, as it is water soluble. However, low chlorine coals show hardly any conversion and the mercury emissions remain atomic. This results from the limited surfaces currently available at the right temperatures for the mechanism identified herein to occur. Any trace deposition as mercury sulfate that may occur will be converted very slowly and cannot dissociate. It can go unnoticed except when parts of the system are blown clean. Then spikes of mercury emissions that are reported will be observed.  
         [0066]     The implications never seem to be discussed but it is also well known that it is not possible to spike elemental mercury directly into the flue gases and sample it via a slipstream flow and sampling probe without getting high levels of conversion to HgCl 2  (W. J. O&#39;Dowd et al., “Recent Advances in Mercury Removal Technology at the National Energy Technology Laboratory,”  Fuel Processing Technology,  Volume 85 (2004) pp. 533-548). This is indicative that if surface is provided then conversion will be effective.  
         [0067]     As a result there is significant supporting evidence from full scale monitoring that the presently suggested method to control mercury emissions is feasible. It is simple and inexpensive which is important for a wide spread implementation. It is nothing more than maximizing the natural mechanism available to the mercury and a mechanism that is endeavoring to occur already in combustors. Unknown until recently, gaseous mercury has a great affinity for heterogeneous chemistry. It is relatively inert in the gas phase but its condensed phase offers specific chemistry whereby the driving thermodynamics can achieve their goal without constraints. Consequently by inserting tubular or honeycomb steel or other base material surfaces at a suited stationary location in the flue gases, the required surface can attain the optimal surface temperature naturally from that of the flue gas flows. Standard methods to spoil the gas flows can be used to introduce some turbulence to ensure enhanced gas/surface interactions and so minimize the surface area that will be required for the desired level of effectiveness. Being a passive surface that remains unaffected by the process, minimal maintenance will be needed. Occasional blowing clean of particulate matter should be all that is required. The converted mercury will be captured by conventional scrubber technology and treated for disposal preferably as its original insoluble and quite stable natural ore, cinnabar HgS.  
         [0068]     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments.  
         [0069]     Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. A teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other, but may be used alone or combined in other combinations. The excision of any disclosed element of the invention is explicitly contemplated as within the scope of the invention.  
         [0070]     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.  
         [0071]     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it Is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.  
         [0072]     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.  
         [0073]     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.