Abstract:
A method and apparatus for reducing the amount of unused ammonia (“ammonia slip”) in instances where ammonia is provided to SNCR and/or SCR processes for reducing NOx pollution in flue gas produced by a fossil fuel burning facility. Such a facility incorporates a rotary regenerative heat exchanger (i.e. an air preheater and/or a gas to gas preheater) for absorbing heat from the gas flow on one side of the heat exchanger, and releasing such absorbed heat to heat the gas flow on the other side of the heat exchanger. The invention herein includes absorbent/desorbent media carried by at least some of the elements of the heat exchanger. The media is active to adsorb at least a portion of the ammonia slip before the gas flow is exhausted to the environment, and to desorb essentially an equilibrium portion of such adsorbed ammonia into gas flow.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/416,655, filed Oct. 12, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to the reduction of nitrogen oxides from flue gas, and, more particularly, to a method and apparatus for reducing ammonia slip to the atmosphere through adsorption/desorption at the air preheater, while simultaneously relieving the air heater pluggage due to ammonia sulfate and bisulfate formation and, further, to reduce the amount of ammonia attaching to the flyash collected by precipitators or baghouses.  
         BACKGROUND OF THE INVENTION  
         [0003]    Much of the electrical power used in homes and businesses throughout the world is produced in power plants that burn a fossil fuel (i.e. coal, oil, or gas) in a boiler. The resulting hot exhaust gas (also sometimes termed “flue gas”) turns a gas turbine or boils water to produce steam, which turns a steam turbine, and the turbine cooperates with a generator to produce electrical power. The flue gas stream is passed through an air preheater, such as a rotating wheel heat exchanger, that transfers heat from the flue gas to an incoming air stream, that thereafter flows to the combustor. The partially cooled flue gas is directed from the air preheater to the exhaust stack.  
           [0004]    An important consideration for modem power plants is the cleanup of the exhaust gas. The exhaust gas produced in the boiler contains gaseous pollutants such as nitrogen oxides (“NOx”) and sulfur oxides (“SOx”), as well as particulates termed “fly ash”. Environmental laws establish permissible levels of gaseous pollutants and particulates that may be emitted from the exhaust stack of the plant. Various types of pollution control equipment are available to reduce the levels of gaseous pollutants and particulates from the flue gas before it reaches the exhaust stack. For example, among other methods, NOx is often removed by selective catalytic reduction (SCR) and/or selective non-catalytic reduction (SNCR), and fly ash is often removed by an electrostatic precipitator (ESP) and/or a baghouse. The invention herein deals with those particular pollution control systems which utilize ammonia within the process in order to initiate, cause and/or supplement the removal of NOx, and in particular SCR, SNCR and/or staged systems (i.e. systems which include one or more SCR or SNCR systems).  
           [0005]    To remove the NOx, a nitrogenous compound, such as ammonia, is injected into the flue gas stream. The ammonia reacts with the NOx to form nitrogen and water, reducing the NOx content of the flue gas. The reaction of ammonia and NOx may be performed at high temperature without a catalyst, a process termed “selective non-catalytic reduction” (SNCR), or at lower temperature in the presence of a catalyst, a process termed “selective catalytic reduction” (SCR).  
           [0006]    SNCR is accomplished by injecting a nitrogenous compound, such as a source of ammonia, into the hot flue gas, and permitting the reduction reaction to occur in the flue gas. U.S. Pat. Nos. 3,900,554, 4,208,386, and 4,325,924 illustrate known types of SNCR applications. SCR is generally accomplished at lower temperatures than SNCR, and necessitates the use of a catalyst, which is placed onto surfaces of catalyst modules, which are positioned within a flue gas stream. U.S. Pat. No. 5,104,629 illustrates one known type of SCR installations.  
           [0007]    It is important to accomplish the reaction of the ammonia and NOx in an efficient manner, for maximum possible reaction of both the NOx and the ammonia. If the reaction is incomplete, either NOx or ammonia (or both) may pass through to the stack and be emitted to the atmosphere. Both NOx and ammonia are classified as pollutants, and their emission is to be maintained within legal limits. Furthermore, depending upon the temperature at the cold end of the air preheater, excess ammonia slip may cause clogging of the space between adjacent air preheater heating elements because of the formation of ammonium sulfate/bisulfate, and/or agglomerated flyash. In addition, many power plants dispose of the collected flyash by selling it to purchasers who further process the flyash for commercial uses (i.e. lightweight aggregate for concrete mixtures). If the ammonia amount of which adheres to the flyash is relatively high (i.e. in excess of 100 ppm, by weight, or as otherwise mandated by users), the flyash may not be able to be sold, and the utility will have to pay for the disposal.  
           [0008]    The prior art is replete with ways of preventing or alleviating ammonia slip (i.e. larger SCR units, more responsive control arrangements, early replacement of catalyst, and the like); however, in all instances, some ammonia slip will indeed enter the air preheater. It is specifically to the ammonia slip which enters the air preheater for which the instant invention is directed. In this regard, at the air preheater, very little original thinking has been directed for the last several years, for example: efforts to control or alleviate the effects of ammonium sulfate/bisulfate formation by applying an enamel coating to the air preheater elements, coupled with aggressive sootblowing (an expensive and short lived solution); and efforts to alleviate the ammonia slip by catalyzing the air preheater elements, such as is illustrated in U.S. Pat. Nos. 4,602,673 and 4,867,953 (a concept which to date has not proved economical or practical).  
           [0009]    As will be discussed hereinafter in detail, the present invention overcomes, or in the least, greatly alleviates the prior art deficiencies discussed above, in an efficient, cost effective and novel manner.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention includes an adsorbent compound which is integrally formed with, or adhered to, the exposed surfaces of some of the air preheater heating elements. The adsorbent is selected and structured to act as a molecular sieve to adsorb, or capture, molecular ammonia on the gas side of the air preheater, and to desorb, or release, such captured ammonia on the air side of the preheater. The desorbed ammonia is either destroyed in the boiler fire ball or, in the alternative, is used to destroy NOx in an SNCR and/or SCR stage of the NOx reduction system. The preferred pore size of the adsorbent material will be sufficient to adsorb ammonia (i.e 2.8 Å), but less than 4 Å, so as to prevent SO 2  or SO 3  penetration. As will be discussed hereinafter in the detailed description of the preferred embodiment, other critical criteria for the application of the existing invention is the selection of preheater layers or partial layers on which elements having adsorbent surfaces will be positioned, as well as the minimum catalytic activity of such adsorbent (i.e. to alleviate the concern for amplifying the potential of ammonium sulfate/bisulfate problems, or creating additional SO 3  where perhaps none is required).  
           [0011]    In view of the above, it is to be appreciated that an object and advantage of the present invention is to provide a method and apparatus for alleviating the problems of ammonia slip to the environment, by including a means and method for adsorbing excess system ammonia at the gas side of the air preheater and continually releasing an equilibrium portion of such adsorbed ammonia as the heating elements rotate to the air side of the preheater.  
           [0012]    It is another object and advantage of the present invention to lessen air preheater element fouling due to ammonium sulfate/bisulfate deposits.  
           [0013]    It is an additional object and advantage of the present invention to address the ammonia slip problem, without catalytic activity occurring in the area of adsorption/desorption.  
           [0014]    It is still a further object and advantage of the invention as submitted herewith, to use the inventive concepts discussed in other fossil fuel burning systems for producing electrical power and which have ammonia slip from NOx reduction means and includes a rotary regenerative apparatus; for example, but without limitation, in conjunction with: known types of low dust arrangements, also known as “tail end types”, which includes a gas to gas rotary regenerative heat exchanger which exchanges heat from the outlet of the gas flowing from an SCR reactor, to heat the incoming gas flow, which has exited from an FGD system and is being directed to such SCR reactor; and in gas turbine regenerative cycles wherein a regenerative heat exchanger uses heat from the gas flow leaving the SCR unit to heat the high pressure system air exiting from the compressor portion of the cycle, such air being subsequently additionally heated and directed to a turbine portion of the cycle.  
           [0015]    These and other objects and advantages of the present invention will become more readily apparent upon a reviewing and reading of the following drawings and description in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic view of a portion of a power plant flue gas flow system according to the invention;  
         [0017]    [0017]FIG. 2 is a exemplary schematic view of heating elements illustrating a single element from the hot end layer, intermediate layer, and cold end layer of a heat exchanger, showing the distribution of adsorbent materials thereon. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIG. 1 schematically illustrates a fossil fuel burning power plant  10  which utilizes an adsorption/desorption system  12  of the present invention therewithin. A carbonaceous fuel such as coal is combusted with air in a furnace  20  and produces, in addition to power, combustion products contained within the flue gas. The flue gas may contain nitrogen oxides (NOx), fly ash, SO 2 , SO 3 , and other undesirable pollutants. As shown, the plant  10  includes an SNCR system  14  for reducing NOx within the furnace  20 , and an SCR system  16  for reducing NOx downstream from the furnace.  
         [0019]    The combustion flue gas exits from the furnace  20  into an exhaust duct assembly  22 . The exhaust duct assembly  22  extends from the furnace  20  through a known air preheater  24 , typically of the rotary regenerative type. The air preheater  24  is provided to absorb heat on the gas side, and use such absorbed heat to heat incoming air on the air side of the preheater  24 . The combustion air is provided by blower  27 , and is directed to the furnace  20  through the inlet duct assembly  26 . The flue gas passes from the preheater  24 , through a suitable cleaning device, such as a baghouse or, as shown, an electrostatic precipitator, schematically illustrated as  28 , and is exhausted from the power plant  10  through a stack  30 .  
         [0020]    The discussion of the power plant  10  is intended to be highly schematic in nature, and it is not deemed necessary to provide more information and background to understand, practice and enable the present invention. In an operating power plant, there are typically many other systems, as well as alternative systems, that are not shown here. The present invention is compatible with such other systems and may, wherever applicable, be used with them.  
         [0021]    The level or amount of nitrogen oxides in the flue gas is an important consideration because of present day standards, which severely limit NOx emissions from many fossil fuel burning power generation facilities. To this end, regulations may require the usage of an SNCR system  14 , an SCR system  16 , or, on occasion, a combination SNCR/SCR system (a staged system, such as is shown in FIG. 1). SNCR and/or SCR systems  14  and  16 , respectively, are well established, and detailed descriptions thereof are not necessary to one skilled in the art to understand the need for and operation of the instant invention. Furthermore, although the particular schematically illustrated SCR system  16  is of the type which is for limited NOx reduction, the present invention is also compatible with for the more conventional full scale systems commonly used for 90%+reduction.  
         [0022]    A nitrogenous compound, such as urea, in the case of SNCR  14 , or ammonia, in the case of SCR  16 , is injected into the flue gas stream to convert NOx into free nitrogen and water. In most instances when using SNCR and SCR, a certain amount of ammonia added to the process is not utilized. This excess ammonia is commonly known as “ammonia slip” and, as will be described hereinafter, can result in several problems at the air preheater  44 , and downstream thereof. It is particularly to ammonia slip entering the air preheater  24  to which the instant invention is directed.  
         [0023]    The air preheater  24  includes a plurality of heating elements  46 , which are heated by the passage of the hot flue gas. The elements  46  are positioned, in a known manner, within baskets, or partitions, positioned within the wheel  47  of the preheater  24 . Typically, wheel  47  rotatably carries two or more levels of elements  46 . In the embodiment illustrated, there is schematically shown three layers, the layer closest to the entry point of the flue gas into the heat exchanger  24  being designated hot end layer  50 , the layer at the exit end of the flue gas being designated cold end layer  54 , and the middle layer being designated intermediate layer  52 .  
         [0024]    The wheel  47  of the heat exchanger  24  continuously rotates about its axis  49 , which may be oriented either vertically or horizontally, to carry the exchange elements  46  from the flue gas side  42  to an air side  48 . During such rotation, the heat in the heating elements  46  is transferred to a flow of cool incoming air being provided from blower  27 .  
         [0025]    [0025]FIG. 2 schematically illustrates hot end heat exchange element  46 ( a ), intermediate elements  46 ( b ) and cold end elements  46 ( c ), from layers  50 ,  52  and  54 , respectively. Depending upon operational circumstances, as well as physical and chemical considerations, at least some of the heat exchange elements  46  are coated with an adsorbent. The coated elements  46 , shown as elements  46 ( a ) and  46 ( b ) in the embodiment illustrated, carry a suitable adsorbent (shown as shaded) on the exposed surfaces thereof. It is noted that the intermediate element  46 ( b ) is illustrated as having absorbent on only one half the height thereof, as exemplary of conditions which are indicative of not applying adsorbent to the entire height. Other conditions may require all or none of element having adsorbent being applied thereto. The adsorbent, which is preferably selected and applied to specific surfaces in accordance with the guidelines discussed below, is operable to adsorb ammonia slip from the flue gas passing thereby, and to desorb an equilibrium portion of the adsorbed ammonia to the combustion air passing thereby as the wheel  47  rotates from the flue gas side  42  to the air side  48 . Examples of preferred adsorbent is zeolite,, carbon, or a combination of zeolite and carbon.  
         [0026]    Zeolite is presently the preferred adsorbent. Further, because consistency and uniform pore size is necessary, it is anticipated that a manufactured, rather than a natural zeolite will be preferred in most instances. In this regard, a range of contemplated pore size diameter is 2.8 to 3.8 Å for the zeolite is recommended, with a preferred size of approximately 3.0 Å. Note, the lower end of the above range is selected because ammonia gas, which is to be adsorbed, has a molecular size of 2.8 Å, and the upper range is selected because SO 2  and SO 3 , have a molecular size of more than 4.0 Å. It is highly desired to avoid the adsorption of SO 2  or SO 3 , in order to not amplify the potential problem of formation of ammonium sulfites or sulfates, which plug the adsorbent pores, and also have a tendency to clog the flow paths between adjacent heating elements  46 .  
         [0027]    Several other criteria must be addressed when selecting the proper, as well as determining which elements, or layers of elements to preferably apply the adsorbent media. Specifically:  
         [0028]    1. The adsorbent should be substantially catalytically inactive in the temperature zone in which it will operate. The primary reason for this is that any catalytic activity will increase the conversion of SO 2  to SO 3 , which in most instances is not a desirable effect. By way of example, the typical temperature of flue gas passing through an air preheater  24  ranges from up to 700° F. at the hot end to 250° F. at the cold end. Also, typical catalytic activity threasholds for a V-Ti oxide catalyst is 550 to 750° F.  
         [0029]    2. The adsorbent should be applied to elements  46  between the following limits (along the depth of preheater  24 ):  
         [0030]    T min  is the low limit defined by the temperature of formation of ammonium sulfate/bisulfate and their deposition on ash particle surfaces. This temperature is a function of SO 3  and NH 3  concentrations in the flue gas.  
         [0031]    T max  is the high limit defined by the condition that maximum adsorption capability of the adsorbent for adsorbate partial pressure on the “gas” side and maximum “metal” temperature at this longitudinal location is not less than the maximum adsorption capability for adsorbate partial pressure on the “air” side and minimum “metal” temperature at the same longitudinal location.  
         [0032]    It is anticipated that the calculated temperature range, in most instances, will be 750° F. to 300° F., with a preference of 650° F. to 450° F.  
         [0033]    3. Another governing factor concerning the placement of adsorbent is that the surface area to which adsorbent is applied, and the amount of adsorbent, should provide a maximum adsorption capability of the adsorbent exposed to the flue gas at any moment at operating adsorbent temperature and the partial pressure of the ammonia in the flue gas, which should be significantly higher (i.e. at least an order of magnitude higher) than the amount of ammonia that should be removed from the gas stream during the time period when an individual adsorbent element is exposed to flue gas.  
         [0034]    4. The adsorbing layer should be specifically structured to create a molecular sieve with a consistent selected diameter.  
         [0035]    5. The length (depth) of the adsorbing material should be preferably longer than the distance traveled by the concentration wave of the adsorbate in the longitudinal direction during the time of adsorbent exposure to the flue gas to avoid a breakthrough of ammonia to a cooler zone of the air preheater  24 , even through a small angular portion of the gas side sectors.  
         [0036]    6. The adsorbent should preferably be hydrophobic and water resistant to allow for cleaning of the air preheater  42  by water washing and also to withstand the occurrence of occasional tube leaks.  
         [0037]    7. Preferable the adsorbent will be selected from a group of materials whose specific storage capacity for ammonia in the operating temperature and partial pressure range is more sensitive to pressure variations than to temperature variations. In this regard, the adsorbing material should be applied to those layers of the preheater  24  where the effect of the “metal” temperature swing on the specific storage capacity for ammonia is overcompensated by the effect of ammonia partial pressure swing.  
         [0038]    8. The adsorbing material should preferably be selected from a group of adsorbents efficient in the temperature range of 300-700° F., and partial pressure of ammonia of 10 −6   -3×10   −5  bar.  
         [0039]    Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention, for example: alternative adsorbent substrates are anticipated, such as a honeycomb structure; depending upon conditions, perhaps only the hot end or intermediate layer of the elements  46  will be coated with an adsorbent compound, or perhaps only part of the hot end or intermediate layer elements will be coated; if deemed appropriate to reduce the metal temperature swing, and increase the ammonia transfer potential, it may be desirable to consider increasing the heat capacity of the heating element carrying adsorbent, by increasing it thickness; and the like. Furthermore, it is to be understood that the inventive concepts discussed hereinabove is equally applicable in other fossil fuel burning systems for producing electrical power and which have ammonia slip from NOx reduction means and includes a rotary regenerative apparatus. As to this latter point and for example, but without limitation, the inventive concept herein can be used in environments other than that described in detail hereinabove, such as: in conjunction with: known types of low dust arrangements, also known as “tail end types”, which includes a gas to gas rotary regenerative heat exchanger which exchanges heat from the gas flowing from an SCR reactor, to heat the incoming gas flow, which has exited from an FGD system and is being directed to such SCR reactor; and in gas turbine regenerative cycles wherein a regenerative heat exchanger uses heat from the gas flow leaving the SCR unit to heat the high pressure system air exiting from the compressor portion of the cycle, such air being subsequently additionally heated and directed to a turbine portion of the cycle. Inasmuch as the invention herein, as well as the operation of the rotary regenerative heat exchanger is well described hereinabove, there is no need to illustrate and describe in detail, to one skilled in the art, the well known generalities and flow of gas turbine regenerative cycles, as well as tail end type SCR processes.  
         [0040]    The invention is not to be limited except as by the appended claims.