Abstract:
A method of removing mercury from flue gas containing mercury and particulate solids emanating from a fossil-fuel energy conversion plant and passing through a flue gas duct. The method includes (a) contacting the mercury in the flue gas with a solution containing at least one chloride-containing salt dissolved in a solvent by injecting the solution into the flue gas duct at an injection location, in order to oxidize mercury into HgCl 2 , (b) heating the solution prior to or after step (a) to at least about 300° C., and (c) removing oxidized mercury from the flue gas with a device for removing particulate solids from the flue gas.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and an apparatus for removing mercury species, in particular, elemental mercury, from hot flue gas produced in a fossil-fuel energy conversion plant. 
     Exposure to high levels of mercury is associated with serious neurological and developmental effects in human beings. Concentrations of mercury in air are usually low and of little concern, but once mercury enters water, it can accumulate in fish and cause harm to people who eat mercury-contaminated fish. Fossil fuels contain many heavy metals, including mercury. Even if the levels of mercury in coals are low, usually between about 0.05 and 0.2 ppmw, mercury emissions from coal-fired power plants have recently been determined to pose a significant hazard to public health. Thus, the reduction of mercury in the exhaust gases of utility power plants is of great importance. 
     It is known that exhaust gases of fossil-fuel fired power plants may contain mercury in elemental, oxidized, and particulate forms. Elemental mercury in the exhaust gases does not stick to soot and other particles, but tends to remain in vapor form even after the exhaust gases are cooled to about 65° C. Therefore, elemental mercury in the exhaust gases is not recovered by conventional dust removal devices, such as, electrostatic precipitators, fabric filters, or conventional scrubbers, but is, instead, released into the atmosphere. 
     High mercury emissions in the exhaust gases from municipal solid waste incinerators are often regulated with powdered, activated carbon being injected into the exhaust gases upstream of the air pollution control devices. However, the level of mercury emissions per unit volume of flue gases from power plants is about one or two orders of magnitude lower than that emitted from waste incinerators. This makes it very difficult to capture such low mercury concentration levels from power plants by using the current activated carbon technology in a cost-effective manner. 
     Many fuels contain chlorine, which reacts with a portion of the mercury in the flue gases to form mercury chlorides. Gaseous mercury chlorides tend to condense on fly ash particles or on high surface area sorbents, which may effectively be removed from exhaust gases by conventional dust removal devices. Mercury chlorides are also highly soluble in water and, thus, they may be removed from the flue gas by absorption in the aqueous solutions of wet scrubbing units. 
     Early studies on trace elements released from coal combustion systems have shown that an increase in chlorine content in the furnace of the combustion systems leads to an increase in HgCl 2  formation and that a spray dryer is effective in removing HgCl 2  from the flue gas exiting the furnace. More recently, patents have disclosed mercury reduction methods to be used with specific flue gas cleaning equipment, which methods include increasing the Cl-content in the exhaust gas. 
     U.S. Pat. No. 5,435,980 discloses increasing the amount of chloride supplied to a spray dryer when cleaning flue gas that results from combusting coal having a low chloride content in order to convert elemental Hg to HgCl 2 . The chloride increase is performed by incorporating, e.g., an alkaline metal salt solution in the aqueous suspension of basic absorbent in the spray dryer, by supplying chlorine-containing material to the coal in the furnace or by injecting gaseous HCl into the flue gas downstream of the furnace. Alternatively, U.S. Pat. No. 5,900,042 suggests reacting a gas stream with, e.g., a chlorine solution or chloric acid (HClO 3 ) to convert elemental mercury in the gas stream to soluble mercury compounds, and passing the gas stream through a wet scrubber. 
     European patent publication No. 0 860 197 suggests adding a mercury chlorinating agent, e.g., hydrogen chloride (HCl) or ammonium chloride (NH 4 Cl), to exhaust gas upstream of a catalytic NO x  reduction unit to convert elemental mercury into mercury chloride (HgCl 2 ) on the denitrating catalyst. In this method, the water-soluble HgCl 2  is removed in a wet desulfurizing unit with an alkaline absorbing solution. This method is usable only in systems comprising a denitrating catalyst. 
     All the methods discussed in the patents referred to above, however, may suffer from poor mercury removal efficiency at low mercury levels and/or cause corrosion in the exhaust gas duct. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a new and efficient method and apparatus for removing mercury from hot flue gas. 
     Another object of the present invention is to provide a method and an apparatus for effectively removing low levels of mercury from a voluminous flue gas stream. 
     A further object of the present invention is to provide a method and an apparatus for effectively removing mercury from hot flue gas and minimizing corrosion in a flue gas duct. 
     Still a further object of the present invention is to provide a low cost method and apparatus for simultaneously removing mercury and nitrogen oxides from hot flue gas. 
     In order to achieve these and other objects of the present invention, a novel method of removing mercury from flue gas is provided, as described in the independent method claim. Thus, the present invention provides a method of removing mercury from flue gas containing mercury and particulate solids emanating from a fossil-fuel energy conversion plant and passing through a flue gas duct. The method comprises the following steps: (a) contacting the mercury in the flue gas with a solution containing chloride-containing salt dissolved in, for example, water by injecting the solution into the flue gas duct at an injection location in order to oxidize mercury into HgCl 2 , (b) heating the solution prior to or after step (a) to at least about 300° C., and (c) removing oxidized mercury from the flue gas with means for removing particulate solids from the flue gas. 
     Also, the present invention provides a novel apparatus for removing mercury from flue gas, as described in the independent apparatus claims. Thus, the present invention provides an apparatus for removing mercury from flue gas containing mercury and particulate solids emanating from a fossil-fuel energy conversion plant. The apparatus comprises a flue gas duct for conveying exhaust gases; either (i) means for heating a solution of chloride-containing salt dissolved in, for example, water to at least about 300° C. and means for injecting the solution into the flue gas duct, or (ii) means for injecting a solution of chloride-containing salt dissolved in water into an upstream portion of the flue gas duct, for oxidizing mercury in the flue gas to HgCl 2 , and means for removing particulate solids and oxidized mercury condensed on the particulate solids from the flue gas. 
     When the flue gas cools, the oxygen in the flue gas oxidizes at least a portion of the Hg to HgO. A small fraction of the HgO condenses on fly ash particles in the flue gas and, thus, can be removed from the flue gas with means for removing particulate solids from the flue gas, such as an electrostatic precipitator or a fabric filter. 
     A basic idea of the present invention is that elemental mercury in the flue gas is effectively oxidized to mercury chlorides by contacting the mercury with a solution containing chloride-containing salt dissolved in a solvent such as water and heated to at least about 300° C. During heating, the salt in the solution dissociates into molecules and ions. Thus, heating of the solution improves the capability of the salt to convert the mercury in the flue gas to HgCl 2 . 
     According to a preferred embodiment of the present invention, the injection location is selected so that either (i) the flue gas temperature therein is from about 650° C. to about 980° C., causing the solution to be rapidly heated to at least about 300° C. in the flue gas duct, or (ii) the flue gas temperature therein is below about 650° C. and the solution is heated to at least about 300° C. prior to its injection into the flue gas duct. 
     According to a preferred embodiment of the present invention, the chloride-containing salt is ammonium chloride (NH 4 Cl). When a solution of NH 4 Cl in a solvent such as water is injected into the flue gas duct in an injection location at which the flue gas temperature is above about 650° C., the NH 4 Cl in the solution is rapidly heated up and dissociates into many forms, including Cl −  and NH4 +  ions, and Cl 2 , NH 3  and HCl molecules. When the flue gas cools down in the flue gas duct, the chlorine species react with Hg and HgO at and below about 370° C., and mostly HgCl 2  is formed. The injection location is preferably at an upstream portion of the flue gas duct so that the chlorine species formed from the NH 4 Cl have sufficient retention time to convert most of the elemental mercury to HgCl 2 . 
     Preferably, the injection location is such that the temperature of the flue gas is above about 700° C., even more preferably above about 800° C. At these temperatures, the NH 3  formed from the NH 4 Cl reduces the nitrogen oxide level of the flue gas according to a selective non-catalytic reduction (SNCR) process. However, the reaction rate of NH 3  with NOx decreases substantially below about 700° C. 
     When the energy conversion plant comprises a circulating fluidized bed boiler, the NH 4 Cl solution is advantageously injected immediately downstream of the furnace of the boiler, preferably in the channel between the furnace and the hot loop cyclone of the boiler. At this location, the temperature is typically above about 800° C., and the concentration of ash and unburned fuel particles is relatively high. In a plant comprising a pulverized coal combustor, the NH 4 Cl is advantageously injected immediately downstream of the furnace, where the temperature is typically above about 800° C., and the exhaust gas still contains unburned carbon particles. 
     The NH 4 Cl solution is advantageously heated to some extent, e.g., to between about 100° C. and about 200° C., before it is injected into the flue gas duct. The higher initial temperature of the solution speeds up the dissociation of NH 4 Cl into many ions and molecules in the flue gas duct, thus assuring that the desired chlorine compounds and ions are formed before the flue gas is cooled to about 370° C., where significant HgCl 2  formation begins. 
     According to another preferred embodiment of the present invention, the NH 4 Cl solution is first heated to above about 300° C. so that the NH 4 Cl molecules dissociate e.g., into NH 3  and HCl molecules, before the solution is injected into the flue gas duct. In this way, the solution can be injected into flue gas at a lower temperature, because HCl and other chlorine compounds and ions can immediately react with Hg and form HgCl 2 . Simultaneously, the injected NH 3  can be utilized for reducing the NO x  level of the flue gas, e.g., in a selective catalytic reduction (SCR) unit. 
     According to still another preferred embodiment of the present invention, the chloride-containing salt is selected from a group consisting of sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl 2 ). Similar to the other preferred salts, these salts can be injected into a high temperature zone of the flue gas duct and be rapidly heated therein to at least about 300° C., or they are heated at least to a minimum temperature before being injected into a lower temperature zone of the flue gas duct. The minimum heating temperatures vary with the form of the chloride-containing salt, but generally they are between about 300° C. and about 700° C. 
     The HgCl 2  molecules have a much higher tendency to condense on fly ash particles in the flue gas than does elemental mercury. When a sufficient amount of chloride-containing salt is injected into the flue gas as described above, practically all of the elemental mercury in the flue gas is oxidized, and the amount of remaining elemental mercury is reduced to trace levels. Conventional low-temperature dust collectors, advantageously located at a temperature between about 130° C. and about 170° C., can be used to remove more than about 90% of the oxidized or particulate mercury. The dust collector may be, e.g., an electrostatic precipitator or a fabric filter. Between these two alternatives, the fabric filter seems to be more effective. I believe this is because HgCl 2  molecules have a higher probability of condensing on the dust collected on the filter bags. 
     To increase the probability of the HgCl 2  molecules condensing onto the particles in the flue gas, the amount of fly ash can be advantageously increased by circulating a portion of fly ash collected in the particulate removing equipment back to the flue gas duct. Preferably, the portion of the circulated fly ash is selected so that the fly ash content in the flue gas is increased to at least about 1 g/Nm 3 . The solids concentration in the flue gas can rise as high as to about 1000 g/Nm 3 , depending on variables such as the ash surface porosity, sulfur oxides level, chlorine concentration in the input solids, moisture content of flue gas and operating temperature. 
     The circulated fly ash may also be treated before it is injected back to the flue gas duct, thereby improving its ability to remove the HgCl 2  from the flue gas. One method of treating the fly ash entails screening out larger particle from the smallest particles, e.g., by a cyclone, from the fly ash before reinjecting the fly ash into the flue gas duct. Thus, the fine particle fraction increases the mercury chloride removal, especially because of its high surface area and porous surface structure, which is related to its relatively high content of unburnt carbon. Depending on its composition, the fly ash can also catalyze the oxidation of elemental mercury in the presence of HCl in the flue gas. This effect can be enhanced by adding to the recirculated fly ash substances which catalyze the oxidation of mercury, e.g., trace metal oxides such as Fe 2 O 3  or CuO. 
     Mercury removal can be further improved by removing the HgCl 2  molecules, which have not been removed from the flue gas with a dust collector. At least a portion of the remaining HgCl 2  molecules can be removed by the absorbing material or solution in a spray dryer or a wet scrubber located downstream in the flue gas duct. 
     The price of NH 4 Cl is about the same as that of activated carbon. However, while the reaction between Hg and Cl-containing particles, e.g., HCl molecules, is a gas phase reaction, no physical adsorption is required and, thus, for the same mercury reduction effect, the required quantity of NH 4 Cl is less than that of activated carbon. Also, when the use of activated carbon for mercury reduction is avoided, the increase of carbon in the ash is avoided. This improves the beneficial uses of the ash. 
     The quantity of chloride-containing salt used in the injection depends on the type of fuel employed and, especially, on the mercury and chlorine content of the fuel. When there is more chlorine in the fuel, less salt is required for sufficient mercury oxidation. According to a preferred embodiment of the present invention, the quantity of injected chloride-containing salt is such that the level of chlorine in the flue gas is equal to or less than that which would result from combusting fuel having a fuel chlorine content of 0.3% in dry fuel feed. For example, the desired chlorine concentration of the flue gas may correspond to that created by fuel having a 0.1-0.2% chlorine content, i.e., typically about 100 to about 200 ppm chlorine concentration in the flue gas. 
     Advantageously, a molar ratio of at least 100:1 between the HCl and Hg levels in the flue gas is used in oxidizing the Hg to HgCl 2 . When mercury levels are low, the required ratio of the HCl and Hg levels in the flue gas may be much more than 100:1, e.g., 1000:1 or even more up to 50000:1. An upper limit for the quantity of chloride-containing salt used in the injection is determined by a desire to avoid any corrosion of the flue gas duct or the heat recovery surfaces and other equipment therein. 
     The present invention provides a novel method and apparatus for adding chlorine species into mercury-containing flue gas, wherein the method and apparatus improve the use of the injected chlorine. By properly selecting the injection location and the temperatures of the exhaust gas and the chloride-containing salt solution at the injection, more efficient use of the chlorine is obtained. Hence, the amount of excess chlorine and, hence, corrosion of the flue gas duct are minimized. 
     The present invention can be applied to many types of fossil-fuel conversion plants. These include, e.g., circulating and bubbling fluidized bed combustors and gasifiers, pulverized fuel firing and gasifying plants and waste incinerators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings, wherein 
         FIG. 1  shows schematically a boiler plant according to a first preferred embodiment of the present invention. 
         FIG. 2  shows schematically a boiler plant according to a second preferred embodiment of the present invention. 
         FIG. 3  shows schematically a boiler plant according to a third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows schematically a boiler plant  10 , with a circulating fluidized bed combustor  12 . In a circulating fluidized bed combustor, fuel, bed material and possible sorbent material are fluidized in a furnace  14  with fluidizing air, which is introduced to the furnace by combustion air introduction means  16 . Normally, air is introduced to the furnace  14  at multiple levels of the furnace, but for clarity,  FIG. 1  only shows the means  16  for introducing air being located at the bottom of the furnace. Exhaust gases produced in the furnace  14  and bed particles entrained with the exhaust gases are discharged through a channel  18  in the upper part of the furnace  14  to a solids separator  20 . In the solids separator  20 , which is usually a cyclone, most of the bed particles are separated from the exhaust gases and returned to the furnace  14  via a return duct  22 . 
     The exhaust gases are led from the separator  20  to an exhaust gas duct  24 , which comprises heat transfer surfaces  26  and  28  for cooling the exhaust gases and for producing steam and heating the fluidizing air  16 , respectively. The cooled, exhaust gases are conducted to a dust separator  30 , which may be an electrostatic dust separator or a bag filter separator. In dust separator  30 , most fly-ash particles and other small dust particles are removed from the flue gases and discharged through an ash discharge  32 . The flue gases, now cleaned by the dust separator  30 , are led to a stack  34  and released into the environment. 
     The exhaust gas duct  24  may comprise additional gas cleaning equipment, such as a catalyst for reducing NO x  emissions and a wet scrubber or a spray dryer for reducing SO 2  emissions. Such additional gas cleaning equipment is, however, not shown in FIG.  1 . 
     According to a preferred embodiment of the present invention, a solution of chloride-containing salt, dissolved in a solvent such as water, is injected into the channel  18  between the furnace  14  and the particle separator  20  by injection means  36 . In the channel  18 , the temperature of the exhaust gases is typically at least about 700° C. Thus, the chloride-containing salt rapidly heats to a high temperature, at least to above about 300° C., and dissociates into many kinds of molecules and ions. In some applications, it is advantageous to locate the injection means  36  at the upstream end of the exhaust gas duct  24 , but downstream of the separator  20 . Preferably, the injection means  36  is located upstream of the first heat exchanger  26 . 
     According to a preferred embodiment of the present invention, the chloride-containing salt is ammonium chloride (NH 4 Cl), which dissociates in the exhaust gas to at least ammonia (NH 3 ) and chlorine species. When the exhaust gas is cooled with the heat exchangers  26  and  28  to about 370° C., at least a portion of the formed Cl-containing particles, which may include HCl and Cl 2  molecules and Cl −  ions, reacts with Hg atoms and forms HgCl 2  molecules. The HgCl 2  molecules tend to adsorb onto the dust particles remaining in the exhaust gas, and are thus removed from the exhaust with the dust separator  30 . 
     According to a preferred embodiment of the present invention, the ash discharge  32  includes an ash handling system including means  38  for recirculating a portion of the fly ash particles discharged by discharge  32  from the dust collector  30  back to the exhaust gas duct  24 . The recirculated fly ash is, preferably, injected into a downstream portion  40  of the exhaust gas duct  24 . The fly ash recirculation means may include a treatment device  42  for treating the recirculated fly ash. Treatment device  42  for treating the fly ash may be a separator to screen the smallest fly ash particles to be injected into the exhaust gas duct  24 . Also, it is possible to add substances which catalyze elemental mercury oxidation, such as trace metal oxides Fe 2 O 3  or CuO, to the recirculated fly ash. 
     The chloride-containing salt, injected by means  36 , may also be selected from a group consisting of sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl 2 ). When injected into a high temperature zone of the exhaust gas duct  24 , these salts rapidly form molecules and ions, which can react with Hg atoms and form HgCl 2  molecules. The HgCl 2  molecules tend to adsorb onto the fly ash particles and thus, can be collected by the dust separator  30 . 
     When the chloride-containing salt is injected at an early stage of the exhaust gas duct  24 , the high temperature of the exhaust gases causes rapid dissociation of the molecules. This early injection location also guarantees a long retention time for the solution so that all salt dissociation has taken place when the exhaust gases are cooled to the onset temperature of the HgCl 2  formation, which is about 370° C. 
     When NH 4 Cl is used as the chloride-containing salt, the resulting formation of NH 3  molecules can be used for non-catalytic NO x  reduction. Specifically, the NH 3  molecules formed at a sufficiently high temperature, preferably above about 700° C., convert nitrogen oxides to N 2  and H 2 O. Also, the NH 3  may increase the amount of particle-bound mercury in the flue gas. 
     The chloride-containing salt solution injection means  36  may include means (not shown) for heating the solution to some extent, for example, from about 100° C. to about 200° C., prior to its injection into the flue gas duct  24 . Higher initial temperatures of the solution speed up the dissociation of the salt into many ions and molecules in the flue gas duct, thus assuring that the desired chlorine compounds and ions form before the flue gas is cooled to about 370° C., at which significant HgCl 2  formation begins. 
     The reactor  12  does not have to be a circulating fluidized bed combustor. It can also be a bubbling fluidized bed combustor, a fluidized bed gasifier, a pulverized fuel combustor or gasifier, or a waste incinerator. According to the first preferred embodiment of the present invention, the chloride-containing salt solution is injected into the exhaust gas line of any of the above-mentioned, or other suitable, reactors, at a location at which the temperature of the exhaust gas is at least about 650° C. Such location is preferably immediately downstream of the furnace  14 , but, in some applications, may be later in the exhaust gas duct  24 , and is preferably upstream of a first heat exchanger  26 . 
       FIG. 2  shows schematically a boiler plant  10 ′ according to a second preferred embodiment of the present invention. The boiler plant  10 ′ differs from that shown in  FIG. 1  mainly in that the exhaust gas duct  24  comprises a catalyst unit  46  for providing catalytic NO x  reduction, and that there is a wet scrubber  48  for SO 2  reduction downstream of the dust separator  30 . An alternative to the wet scrubber  48  is a spray dryer upstream of a dust separator. Although  FIG. 2  does not show a fly ash recirculation system  38 , as shown in  FIG. 1 , such a system could be incorporated in the boiler plant  10 ′, or in other plants to which the present invention is applied, as well. 
     According to the second preferred embodiment of the present invention, as shown in  FIG. 2 , a solution of chloride-containing salt dissolved in a solvent such as water is injected into the exhaust gas duct  24  by means  36 ′ to a location downstream of the heat exchanger  26 , at which location the temperature of the exhaust gas is below about 650° C., and, preferably, above about 370° C. In order to guarantee that the chloride-containing salt is dissociated into the required molecules and ions before the exhaust gas is cooled to about 370° C., the solution is heated by heat exchanger  44  to a temperature of at least about 300° C., before it is injected into the exhaust gas duct  24 . 
     The chloride-containing salt solution injected into the duct  24  by means  36 ′ may be ammonium chloride (NH 4 Cl). When heated by heater  44 , ammonium chloride dissociates and forms, e.g., NH 3  molecules. Thus, the injection of dissociated ammonium chloride salt solution upstream of a NO x  catyalyst unit provides NH 3  molecules readily available for SCR NO x  reduction. In many applications of the present invention, the chloride-containing salt may also be selected from a group consisting of sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl 2 ). 
       FIG. 3  shows schematically a boiler plant  10 ″ according to a third preferred embodiment of the present invention.  FIG. 3  shows a method of performing mercury reduction in a boiler plant having a dust separator  30 ′ downstream of the first heat exchanger  26 , which is at a higher temperature than that in the embodiments shown in  FIGS. 1 and 2 . Correspondingly, a NO x  catyalyst unit  46 ′ and an air heater  28 ′ are located downstream of the dust separator  30 ′. According to  FIG. 3 , a wet scrubber  48  is located downstream of the NO x  catyalyst unit  46 ′. The wet scrubber  48  could also be replaced by, for example, a spray dryer and an additional particle separator. 
     According to the third preferred embodiment of the present invention, shown in  FIG. 3 , the chloride-containing salt solution is injected by means  36 ″ into the portion of the exhaust gas duct  24 , which is downstream of the dust separator  30 ′ and upstream of the NO x  catyalyst unit  46 ′. When the chloride-containing salt solution is heated by heater  44 ′ to at least about 300° C., the solution dissociates into many types of molecules and ions prior to its injection by means  36 ″ into the above-noted portion of the exhaust gas duct  24 . The Cl-containing particles, including one or more of HCl and Cl 2  molecules and Cl −  ions, formed by dissociation of the salt or salts, are readily available for forming HgCl 2  molecules with the mercury in the exhaust gas. Also, the possibly formed NH 3  is readily available for SCR NO x  reduction in the catalyst  46 ′. 
     While the invention has been herein described by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations and/or modifications of its features and other applications within the scope of the invention as defined in the appended claims.