Abstract:
A method and apparatus for sulfur trioxide conditioning of flue gas, to assist in the removal of flyash from flue gas exhaust streams containing such flyash. More particularly, such a method and apparatus, which introduces an improvement in the transportation of dry sulfur and the melting and storage of resultant melted sulfur. The melted sulfur is subsequently burned and thereafter catalytically converted to sulfur trioxide for injection into a flue gas for such aforesaid assistance in the removal of flyash therefrom.

Description:
FIELD OF THE INVENTION  
         [0001]    Known systems for removing fly ash from the flue gases of fossil fuel combustion, for example coal, often include an electrostatic precipitator. The fly ash arresting capability of an electrostatic precipitator may depend upon numerous variables, not the least of which is the surface resistivity of the ash. It is well known that a high sulfur content in coal favorably influences fly ash precipitation by reducing the surface resistivity of the ash. However, when coal with a sulfur content of less than 1% is burned in a boiler, the combustion often does not form sufficient sulfur trioxide to reduce the resistivity of the fly ash to a level at which an electrostatic precipitator can function efficiently (approximately 5 times 10 to the 10th power ohm-cm).  
           [0002]    As an amplification of the above, substantially the entire sulfur content of coal, which may vary from less than 1% to approximately 6%, oxidizes to sulfur dioxide during combustion of the coal, and from 1% and 5% of such sulfur dioxide further oxidizes to sulfur trioxide. Typically, after combustion, the sulfur trioxide component of the flue gases combines with entrained moisture to form sulfuric acid which then condenses on the fly ash particulate as the flue gases cool. The sulfuric acid which condenses on the fly ash particulate generally dictates the electrical resistivity of such particulate. Thus, in instances where low sulfur coal is burned in the boiler, only relatively small quantities of sulfuric acid are generated and, hence, the electrical resistivity of the fly ash is relatively high. Accordingly, when burning low sulfur coal, collecting efficiency of the electrostatic precipitator may be degraded considerably, particularly in instances where the precipitator is designed to receive flue gases at temperatures corresponding generally to normal stack exit temperatures (i.e., 250 degree F. to 320 degree F.).  
           [0003]    Various systems have been developed in attempts to rectify the problems of high resistivity fly ash removal by electrostatic precipitators. Examples of such alternative systems have included utilizing: hot-side precipitators; enlarged cold side precipitators; bag houses; or flue gas conditioning in conjunction with smaller ESPs. In many instances experience has shown that the use of flue gas conditioning is the most satisfactory solution in terms of reliability, efficiency, cost, space requirements and versatility.  
           [0004]    In those instances of flue gas conditioning systems, the gas conditioning means and the method of the type illustrated in U.S. Pat. No. 3,993,429 has proved to be an overwhelming success. For purposes of background the entire content of said U.S. Pat. No. 3,993,429 is hereby incorporated herein and made a part hereof by reference. In these and other systems, a controlled trace amount of sulfur trioxide is injected into the flue gas stream intermediate the boiler and the electrostatic precipitator to thus bring the surface resistivity of the fly ash into the desired range for efficient collection thereof by the precipitator.  
           [0005]    Sulfur trioxide is an extremely dangerous and expensive chemical to purchase, transport and store. As such, the sulfur trioxide systems used for flue gas conditioning (FGC) manufacture gaseous sulfur trioxide on site, starting with either a source of sulfur (which is burned to produce sulfur dioxide), or a source of stored sulfur dioxide. The sulfur dioxide is then passed through a catalytic converter in a known manner to produce the required gaseous sulfur trioxide conditioning agent. From a practical standpoint, FGC systems which start with an on-site sulfur dioxide source are used very little because of the expense of the sulfur dioxide, and the precautions necessary in handling and storing large quantities of this chemical. Accordingly, the vast majority of FGC systems of the type discussed herein all start with a basic sulfur feedstock.  
           [0006]    Sulfur feedstock for use in FGC systems was generally universally provided in molten sulfur form. The molten sulfur was transported via truck or rail, and then unloaded into on-site heated storage facilities. The molten sulfur was metered into a sulfur burner portion of the FGC system on a demand basis, and burned to form sulfur dioxide, which was then catalytically converted into sulfur trioxide to provide the requisite conditioning agent. While a molten sulfur feedstock was used in the majority of the prior art FGC systems, subsequent considerations indicated that, in some circumstances, the sulfur feedstock might be better supplied in a dry sulfur form. In such latter instances it was felt that: dry sulfur is easier and safer to handle; the difficulties and expenses associated with unloading of molten sulfur could be avoided; and in certain circumstances (i.e. seasonal usage, small requirements for FGC, spot market fuel purchases which might vary the need for FGC, and the like) it would be less expensive to provide means for using dry sulfur than heating of the entire feed stock supply, as it is with molten sulfur.  
           [0007]    One of the first dry feed FGC systems to be used as a substitute for providing molten sulfur basic feed stock, simply conveyed dry sulfur to the sulfur burner for direct combustion in the presence of air to produce sulfur dioxide (“dry to direct burn”). History has proved this dry to direct burn feed system to have significant problems at various parts of the feed, for example: accurate conveying and metering of the dry sulfur is extremely difficult, and subject to a number of consistency problems; and the sulfur fed to the burner would have a tendency to melt and “puddle”, thereby resulting in inconsistent and unreliable sulfur dioxide generation. A second dry system which has been utilized heretofore, metered solid sulfur to a melter and thence directly to the burner (“dry to melt to direct burn”). While this latter system alleviated the problem of puddling and inconsistent sulfur dioxide production, it still had metering problems because of the direct dry transfer to the melter and, further, continuous open communication was established between the dry sulfur and the melter, thereby increasing the risk of moisture and gaseous contamination in the metering of the dry sulfur and resultant clogging.  
           [0008]    A still further attempt to arrive at a dry sulfur feed to a melter and thence to a sulfur burner used a manual loading, in conjunction with disposable bags containing dry sulfur. With such an arrangement a worker would position the bags over an opening into a melter and open them to dump their contents into the melter. Such an arrangement has proven to be inefficient, very dangerous to the individual in close proximity to the melter opening, and expensive.  
           [0009]    The invention herein relates to FGC systems, and more particularly, to an improved method and apparatus for handling FGC systems which use a dry sulfur feed stock, and overcomes or, in the least, substantially alleviates the problems discussed hereinabove with respect to the prior systems.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention concerns a sulfur trioxide flue gas conditioning system such as one for conditioning the flue gas from a boiler means and which may be similar in many respects to that described in the hereinabove referenced U.S. patent. According to the present invention such a flue gas conditioning system is provided with an improved means for melting dry sulfur on site and then metering and feeding the resultant molten sulfur to the sulfur burner for combustion and the production of sulfur trioxide. This improved means includes a dry storage silo and a batch feed from the silo to a sulfur melter. After the batch feed, any open connection between the dry storage and the melter is disengaged or closed. The sulfur melter includes an accumulator or molten sulfur storage facility. Molten sulfur is metered on a continuous demand basis to the sulfur burner from the accumulator. Thus the present invention permits using a dry sulfur feed stock and provides an arrangement for melting, storing, dosing and transporting the molten sulfur, while alleviating the disadvantages of the prior art systems described hereinabove. Furthermore, the system of the present invention incorporates a large capacity dry sulfur storage facility and does not rely on manual loading of sulfur into the melter/accunulator but, rather, the dry sulfur is batched fed from the dry storage facility, into the melter/accumulator and then the connection between dry storage and wet storage is discontinued.  
           [0011]    In view of the above, it can be seen that it is one object and advantage of the present invention to provide a safe and efficient means of storing dry sulfur and feeding such dry sulfur to a melter for the selective and metered supply of molten sulfur to a sulfur burner. This is accomplished without constant communication between the dry source of sulfur and the sulfur burner and/or the melter.  
           [0012]    Another object and advantage of the present invention is to reduce the potential of moisture and contamination in the stored dry sulfur, thereby significantly alleviating clogging of the dry sulfur feed.  
           [0013]    A still further object and advantage of the present invention is to increase operator safety.  
           [0014]    These and other objects and advantages of the invention will become more readily apparent upon a reading of the following description with reference to the accompanying drawings in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic illustration of a flue gas conditioning system which is constructed and is operative in accordance with the principles of the present invention;  
         [0016]    [0016]FIG. 2 is a block diagram of a prior art sulfur feed portion of an FGC system, wherein the sulfur is stored and supplied in a molten form to the sulfur burner;  
         [0017]    [0017]FIG. 3 is a block diagram of a prior art sulfur feed portion of an FGC system, wherein the sulfur is stored in dry form and is metered and supplied in a dry form to the sulfur burner;  
         [0018]    [0018]FIG. 4 is a block diagram of a prior art sulfur feed portion of an FGC system, wherein the sulfur is stored in dry form and is then metered and transferred to a melter for conversion to molten sulfur which is then sent to the burner. The latter two transfers are on a continuous basis.  
         [0019]    [0019]FIG. 5 is a block diagram of a sulfur feed portion of an FGC system which is built in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    In FIG. 1 there is generally indicated at  10  a flue gas conditioning system which is constructed and is operative in accordance with the principles of the present invention. Those skilled in the art will appreciate that in general a flue gas conditioning system comprises a highly complex assembly which is ordinarily adapted for, but is not necessarily limited to, sulfur trioxide conditioning of fly ash particulates entrained in flue gas streams which emerge from fossil fuel, primarily coal, burning boilers such as boiler  12 . The sulfur trioxide conditioning is completed prior to the flue gas stream entering an electrostatic precipitator and, hence, enhances removal of the fly ash by the electrostatic precipitator by conventional electrostatic precipitation techniques. For purposes of the description herein, the embodiment described is directed to sulfur trioxide conditioning of a gas stream emerging from coal burning boilers; however, this specific descriptive embodiment is not intended to unduly limit the scope of the invention described.  
         [0021]    The flue gas conditioning system  10  comprises, in part: an air intake fan  14 , the inlet of which communicates with ambient air via an inlet conduit  16 ; a conduit  18  which communicates between the fan  14  and a sulfur burner  20 ; a variable temperature primary heater  22  which is disposed within conduit  18 ; and, as shown, a conduit  24  which communicates between the sulfur burner  20  and a catalytic converter  26 . It is to be noted that in practice sulfur burner  20  and converter  26  may be combined in a unitary staged assembly; however, for purposes of clarity FIG. 1 illustrates a conduit  24  communicating between sulfur burner  20  and converter  26 .  
         [0022]    The portion of the system  10  which is described hereinabove is generally well known in the art and is fully described in the referenced U.S. Pat. No. 3,993,429. Broadly, such portion is operative by energizing fan  14  to provide ambient air via conduit  18  to heater  22  whereat, during start-up of burner  20 , the air is heated to a temperature of approximately 800 degree. to 850 degree F. The hot air is then directed to the sulfur burner  20  to heat up the interior thereof to thereby result in the ignition of the liquid sulfur being delivered to burner  20  by the pump  32 . The ignited sulfur rapidly oxidizes to form a sulfur dioxide and air mixture containing, for example, 5% sulfur dioxide by volume. This sulfur dioxide—air mixture then passes to the catalytic converter  26  via conduit  24  for the production of sulfur trioxide for subsequent injection via conduit  28  into the gas stream flowing within boiler flue  13 . The specific means for injection of the sulfur trioxide into the boiler flue gas streams may be any suitable arrangement; for example, the industrial sulfur trioxide gas injection probe not shown herein, but which is fully illustrated and described in U.S. Pat. No. 4,179,071. In the flue, the injected sulfur trioxide combines with water vapor to form sulfuric acid, which then condenses on the fly ash particles to advantageously influence the surface resistivity of the ash. This resistivity adjustment assists precipitator  30 , of a known type, in the removal of fly ash from the flue gas prior to the flue gas exiting the power plant through a suitable exhaust stack (not shown). The system  10  additionally includes a sulfur storage/delivery portion of the present invention, which is schematically illustrated by block  32  in FIG. 1, and is described hereinafter in detail with respect to FIG. 5. Inasmuch as the invention herein is primarily directed to the apparatus and method of the storage/delivery portion  32  of FGC system  10 , and the balance of system  10  is well known in the art, a further description of such known portions of system  10  is not deemed necessary for one skilled in the art to achieve a full understanding of the invention herein. Accordingly, for a further description of the elements described hereinabove, other than portion  32 , and the operation and interaction thereof, reference is hereby specifically made to U.S. Pat. Nos. 3,993,429, 4,179,071 and 4,333,746.  
         [0023]    [0023]FIGS. 2, 3 and  4  are representative, in schematic form, of prior art sulfur storage/delivery portions of systems  10 .  
         [0024]    [0024]FIG. 2 illustrates the first, and most popular, of a prior art sulfur storage/delivery system  32  (a) used heretofore and, as shown in block format comprises: a molten sulfur storage facility  34 , such as a tank, or in-ground pit; a continuous metered molten sulfur transfer facility  38 ; and a sulfur burner/converter  20  and  26 , respectively. The storage system  32 ( a ) has generally performed quite well in the past; however, for a number of reasons, including: safety; economics; lack of heating steam; inability to readily obtain molten sulfur; in instances where the FGC system  10  was only occasionally used; and the like, users preferred to have an on-site dry sulfur source as the basic feed stock to the FGC system  10 .  
         [0025]    [0025]FIG. 3 illustrates the first attempt of a mechanical (as differentiated from a manual) prior art dry sulfur storage/delivery system  32 ( b ) which, as shown in block format comprises: a dry sulfur storage facility  36 , such as a silo; a continuous metered dry sulfur transfer facility  40 ; and a sulfur burner/converter  21  and  26 , respectively. It is noted that the burner  21  differs from burner  20  to the extent prior art teachings concerning the differences in known construction when directly burning from a molten sulfur feedstock, as differentiated from the direct supply to the sulfur burner of a dry sulfur feedstock. Experience has shown that dry sulfur storage/delivery systems, such as systems  32 ( b ) which convey a dry sulfur feedstock directly into the burner  21 , have considerable problems, including, but not limited to: inconsistent production of sulfur dioxide at the burner due to puddling adjacent the initial deposit points of the dry sulfur in the burner  21 ; inconsistent metering of the dry sulfur; and because of the constant open communication between the meter/transfer facility  38  and the burner  21 , a tendency to accumulate heat, moisture and impurities and clogging in the facility  38  or storage facility  24 .  
         [0026]    [0026]FIG. 4 illustrates a prior art dry sulfur storage/delivery system  32  (c) which, recognized the deficiencies of system  32 ( b ) and, as shown in block format, comprises: a dry sulfur storage facility  36 , such as a silo; a continuous metered dry sulfur transfer facility  40 ; and a melter  44 , which melts the dry sulfur into a molten sulfur for immediate delivery of molten sulfur to the burner  20 . It is noted that the melter  44  may be of any type suitable for and the necessary heat therefore may be provided by steam, electric heating coils, or the like. While experience has shown that dry sulfur storage/delivery systems, such as systems  32 ( c ) have advantages over systems such as system  32 ( b ), system  32 ( c ) still suffers from operational problems, including, but not limited to: inconsistent metering of the dry sulfur; and because of the constant open communication between the meter/transfer facility  40  and the melter  40 , a tendency to accumulate heat, moisture and impurities and clogging in the facility  38  or storage facility  24 .  
         [0027]    [0027]FIG. 5 illustrates the dry sulfur storage/delivery system  32  of the present invention, which, recognizes the deficiencies of prior art systems  32 ( b ) and  32 ( c ) and, as shown in block format, comprises: a dry sulfur storage facility  36 , such as a silo; a dry sulfur batch transfer facility  42 ; a melter/accumulator  46 , which melts the dry sulfur into a molten sulfur, and accumulates it for demand delivery of molten sulfur to the burner  20 . It is noted that the melter  46  may be of any type suitable for and the necessary heat therefore may be provided by steam, electric heating coils, or the like. As differentiated from melter  44 , melter/accumulator  46  contains sufficient volume therein to retain, in a molten state, a selected quantity of molten sulfur therewithin (i.e. one or two days supply). Furthermore, the batch transfer facility is not subject to critical metering of dry material, as is the case with meter transfer assemblies  40 . Still further, the system  32  is designed such that after a selected batch of dry sulfur is conveyed from facility  42  to melter/accumulator  46 , the communication between such elements is discontinued. Thus with an arrangement of elements such as is specified in hereinabove with respect to system  32 , the hereinabove mentioned problems with respect to systems  32 ( b ) and  32 ( c ) are overcome, or in the least, greatly alleviated. For example, since there is no critical metering of dry sulfur, any problems associated with a finely measured transfer of dry sulfur is overcome. Furthermore, inasmuch as there is no constant communication between the stored dry sulfur and the molten sulfur, the problems associated with such communication are alleviated.