Abstract:
A method and apparatus for ultra-high particulate collection of sub-micron aerosols in a fuel gas conveys the fuel gas to a first venturi scrubber for removing a relatively large amount of particulates and leaving a smaller particulate load which is not removable in the first venturi scrubber. The fuel gas with the smaller particulate load is then conveyed to an electrostatic agglomerator for agglomerating the remaining smaller particles in the smaller particle load into larger particles. The fuel gas with the agglomerated larger particles is then conveyed to a second venturi scrubber for removing the agglomerated larger particles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to the U.S. patent application of Jerry D. Blue, William Downs, Timothy A. Fuller, and Christopher L. Verrill, titled SULFUR RECOVERY FROM SPENT LIQUOR GASIFICATION PROCESS, U.S. Ser. No. 09/298,974, filed Apr. 23, 1999 and the U.S. patent application of Jerry D. Blue, William Downs, Timothy A. Fuller, Christopher L. Verrill, Paul S. Weitzel, and Phung H. M. Chan, titled GASIFICATION PROCESS FOR SPENT LIQUOR AT HIGH TEMPERATURE AND HIGH PRESSURE, U.S. Ser. No. 09/298,533, filed Apr. 23, 1999, the text of which are hereby incorporated by reference as though fully set forth herein. Unless otherwise stated, definitions of terms these applications are valid for this disclosure also. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates in general to removing particles from gases, and in particular to a new and useful ultra-high particulate collection apparatus and method. 
     Venturi scrubbers have been used for particulate collection for at least 60 years. The use of venturi scrubbers for particulate collection from coal fired gasifiers is known and has been proposed for use on black liquor gasifiers. Electrostatic precipitation is also well established for fine particulate control. Electrostatic agglomerators are less well known. A form of electrostatic agglomeration is used in the carbon black industry to facilitate the collection of soot sized particles on fabric filters. The use of electrostatic precipitation on fuel gas or synthesis gas from gasifiers has been proposed in the literature but has not actually been commercialized. No prior art is known which proposes use of two venturi scrubbers in combination with an electrostatic agglomerator for the cleanup of any gas borne particulate. 
     An electrostatic agglomerator/venturi scrubber combination was developed in the 1960&#39;s. In the 1960&#39;s a Kraft process was operated in the United States with a combination of an electrostatic agglomerator upstream of a single venturi scrubber. The Kraft process is under much less pressure and has much lower particle loading than the output of a black liquor gasifier and would provide the person having ordinary skill in this art with no motivation to include a second upstream venturi scrubber for any purpose. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved ultra-high particulate collection system and method which utilizes two venturi scrubbers connected in series with an electrostatic agglomerator connected there between. 
     A further object of the present invention is to provide a method and apparatus for ultra-high particulate collection which is simple in design, rugged in construction and economical to manufacture. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic diagram of the present invention; 
     FIG. 2 is a schematic sectional view of an electrostatic agglomerator used in accordance with the present invention; 
     FIG. 3 is a schematic view showing a rapping arrangement according to the present invention; 
     FIG. 4 is a schematic perspective view showing further electrostatic agglomerator according to the present invention; and 
     FIG. 5 is a side elevational view showing the agglomerator of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings generally, wherein like reference numerals designate the same or functionally similar elements through the several drawings, FIG. 1 illustrates a schematic representation of the invention (generally designated  5 ); an apparatus and method for ultra-high particulate collection of sub-micron aerosols from a fuel gas. The system  5  comprises a first venturi scrubber  10  followed by an electrostatic agglomerator or ESA  12  which, in turn, is followed by a second venturi scrubber  14 . Gas from a black liquor gasifier or BLG  11  is supplied to the venturi scrubber  10 . The heart of the system  5  is the electrostatic agglomerator  12 . With some notable exceptions, the principle of operation of this electrostatic agglomerator is similar to that of an electrostatic precipitator. The method of charging particles in an electrostatic field with a corona current is the same. In both cases the particles are accumulated on the walls of the grounded surface, e.g. the collection plate. In the case of the electrostatic precipitator, the accumulated dust on the walls of the grounded surfaces are typically rapped in such a way as to slide the dust vertically into hoppers situated below the collection plates. Extreme effort is made to prevent the dust from being re-entrained into the gas stream. The gas velocity is maintained below about 5 ft/sec and the collection surface is designed to discourage mechanical interactions between the gas and the dust layer. In contrast, in this electrostatic agglomerator  12 , complete re-entrainment of the dust is required. The actual agglomeration occurs on the collection surface as sub-micron particles come into mechanical contact with one another. The forces holding these agglomerates of sub-micron particles together are far stronger than the aerodynamic forces to which the particles are subjected by the re-entraining flue/fuel gases. Gas velocity restrictions do not apply to the electrostatic agglomerator  12  in the same way that they do apply to conventional electrostatic precipitation. One schematic representation of the electrostatic agglomerator  12  is depicted in FIG.  2 . 
     Before explaining FIG. 2 in detail, the functions of the two venturi scrubbers  10 ,  14  will be explained. Venturi scrubbers are quite efficient dust collectors for particles greater than about five microns but they are very poor collectors of particles less than about 0.3 microns. Thus, all that is required of the electrostatic agglomerator  12  is that it converts sub-micron fume and aerosol to agglomerates that are at least a few microns in size. Thus, the function of the venturi scrubber  14  following the electrostatic agglomerator  12  is to collect these agglomerates by conventional means. 
     The function of the venturi scrubber  10  preceding the electrostatic agglomerator  12  is less obvious. A phenomenon known as the“space charge effect” has a deleterious influence on electrostatic separation. This phenomenon occurs when too many charged particles are present in the electric field of the precipitator or electrostatic agglomerator. If the concentration of particulate in the electric field is expressed in terms of grains per cubic foot, the maximum concentration that the precipitator or electrostatic agglomerator can handle without invoking a space charge problem is about 10 grains/ft 3 . For a black liquor gasification process, the concentration of particulate (alkali fume and soot) could exceed 800 grains/ft 3 . 
     The function of the first venturi scrubber  10  is therefore to reduce the dust loading to below 10 grains/ft 3  before the fuel gas enters the electrostatic agglomerator  12 . Thus, for example, if the dust loading leaving the gasifier is 800 grains/ft 3  and if the first venturi scrubber is designed to achieve 99% particulate collection, the dust loading entering the electrostatic agglomerator will be 8 grains/ft 3 . That would be sufficiently low to prevent the space charge effect from having a significant impact on the electrostatic agglomerator  12 . At that point, the electrostatic agglomerator  12  and second venturi scrubber  14  working in combination would be required to operate at a collection efficiency of about 99.97% to achieve the alkali removal requirement necessary to meet the alkali limit specification of the gas turbine manufacturers. 
     The design of the electrostatic agglomerator  12  is shown schematically in FIG.  2 . For capacities up to about 1500 actual cubic feet per minute, a single tube  20  can be used. This tube  20  can be up to 18 inches inside diameter ID (preferably 12 inches inside diameter), approximately 10 feet long, and serves as the containment as well as the collection surface. A high voltage electrode  22  is located along a central axis A of this collection tube  20 . The electrode  22  is isolated from the grounded surface by an insulator  26 . The insulator  26  must in turn be protected from dirt and/or condensation by the appropriate application of radiant heaters  28  and clean dry purge gas entering via an inlet  38  (FIG. 3) into tube  20 . A single transformer-rectifier (TR) set  32  (150 Kv, 20 mA) is connected to the electrode  22 . Both the collection surface  20  and the high voltage electrode  22  must be rapped periodically. Rapping is accomplished by rotating hammers  36  as illustrated schematically in FIG. 3, which raps an upper end of the electrode  22  above an outer sleeve or sheath  24  around part of electrode  22 . One or more stabilizing rods  29  shown in FIG. 2 holds the lower end of electrode  22 . 
     If the capacity of the electrostatic agglomerator  12  exceeds about 1500 acfm, the design can be modified as per FIG.  4  and FIG.  5 . Here, the number of tubes  20  and electrodes  22  are increased in a bundled array as illustrated. Each tube  20  will again be between 12 and 18 inches ID and about 10 feet long. Three insulators  26  support the array of electrodes  22 . These insulators are housed in a penthouse  34  that is pressurized with nitrogen N 2  to slightly above the operating pressure of the treated fuel gas entering gas inlet  30 . 
     If the black liquor gasifier operates at an exit gas temperature of 1800° F., the sodium compounds in the smelt will be exposed to about the same temperatures as those in a conventional Kraft Recovery boiler (RB). In conventional RB&#39;s, the total electrostatic precipitator (ESP) dust catch is typically about 6 to 7% of the total black liquor solids. Although most of the particulate that is formed in the furnace could be classified as fume, as much as a third of it is collected on heat transfer surface in the convection pass. This material becomes agglomerated due to the“sticky” property of salt cake; the collection mechanism is probably thermophoresis. In this black liquor gasifier utilizing a quick quench design, there will be little opportunity for collection of alkali fume on heat transfer surface by thermophoresis. For this gasifier operating at an exit gas temperature of 1800° F., the commercial scale unit (for a 1000 ton per day pulp mill) is estimated here to generate up to about 8000 pounds per hour of alkali fume of which about 35% would be sodium (Na) and potassium (K). That equates to about 2800 pounds per hour of Na and K. The total fuel gas flow for this commercial scale gasifier is about 110,000 pounds per hour. The uncontrolled alkali concentration in the fuel gas is therefore about 25,000,000 parts per billion by weight. The allowable limit of alkali in the gas going to the gas turbine is 20 parts per billion. The fuel gas is diluted significantly before entering the gas turbine. Accounting for dilution with combustion air, the allowable alkali in the fuel gas will be about 85 parts per billion by weight. Based on these estimates, an overall alkali removal efficiency of 99.9997% (“five nines”removal efficiency) will be required to meet this performance level. This level of particulate control is extreme. This problem coupled with the high pressure of the fuel gas is the challenge that this combination of venturi scrubbers  10 ,  14  and electrostatic agglomerator  12  is designed to accomplish. 
     The electrostatic agglomerator and venturi scrubber arrangement ( 10 ,  12  and  14 ) of the present invention is designed for 99.9999 + % efficiency, an efficiency that is unparalleled in industrial practice. According to the present invention, most of the particulate is removed by the upstream venturi scrubber  10  and the electrostatic agglomerator  12  is thus used for its unexpected effect on the remaining smaller particles which are particularly advantageous when applied to the smaller particles remaining in the black liquor gasifier fuel gas. In a test of particulate removal with an electrostatic agglomerator and venturi scrubber combination ( 12  and  14  only) behind a Kraft recovery boiler, performance exceeded 99.94%. 
     Because the process operates at high pressure (over 20 bar), compact equipment offers significant cost and engineering design savings. The venturi scrubber operates at throat velocities that are typically greater than 200 feet per second. For this application, however, the gas velocity will exceed 300 ft/sec. Thus, a venturi scrubber with a circular cross-section and throat diameter of 8 inches can handle the full flow of fuel gas from a 1000 ton per day pulp mill. 
     Various alternative designs of the electrostatic agglomerator are feasible. Although a downflow direction of the fuel/flue gas through the electrostatic agglomerator will normally be preferred, upflow and cross flow designs could also be envisioned. The use of a venturi scrubber is also a preferred embodiment of this patent. But, other devices such as cyclone separators or fabric filters could also be used to collect the agglomerates leaving the electrostatic agglomerator  12 . 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.