Abstract:
A refractory vessel comprising a metal shell having a generally cylindrical portion and an upper hemispherical dome. A refractory liner has a cylindrical portion spaced inwardly from the cylindrical portion of the metal shell and a hemispherical portion spaced inwardly from the dome. The hemispherical portion has a plurality of layers of refractory brick forming successively higher and lesser diameter rings. At least a portion of the rings have mating keys and keyways to restrain the layers of bricks in a vertical direction.

Description:
FIELD OF THE INVENTION 
     The present invention relates to refractory vessels and more particularly to a hemispherical dome design for a refractory liner in such a vessel. 
     BACKGROUND OF THE INVENTION 
     Black liquor is a by-product of the wood pulping process. Black liquor is a mixture of hydrocarbon, caustic, chlorine and other corrosive chemicals. It is normally completely combusted in a recovery boiler. Inorganic chemicals including sodium sulfate and sodium sulfide are recovered for reuse in the pulping process. Heat produced by the complete combustion is converted to steam, which in turn is used to produce process heat and/or electrical power. An alternative device proposed for recovering inorganic chemicals from black liquor is a gasifier. In a gasifier, the black liquor is burned in a sub stoichiometric atmosphere to produce a combustible gas. Inorganic salts are recovered in the process. The combustible gases can be used directly to fuel a gas turbine, or combusted in a power boiler. 
     Low pressure gasification requires an insulated environment, which is obtained through a refractory lined vessel. Refractory vessels of current design for use as gasifiers employ a stainless steel jacket and a fused-cast alumina liner. The alumina liner normally has a first inner layer of blocks comprising both alpha and beta alumina and a second outer layer of blocks comprising beta alumina. A small expansion allowance is provided between the outer layer of beta alumina blocks and the stainless steel jacket. 
     After vessels of this design are operated for a few months, it has been found that the refractory materials react with the soda in the liquor and expand to completely consume the normal expansion allowance provided between the refractory and the stainless steel jacket. At this point, the refractory layers begin to press against the inside of the stainless steel jacket. This situation causes early failure in the refractory materials themselves and plastic deformation of the stainless steel jacket. As a consequence, refractory linings of a conventional design have been unsatisfactory for use in a black liquor gasifier. 
     SUMMARY OF THE INVENTION 
     The inventors have found that alumina refractories not only are subject to thermal expansion as is in the prior art, but are also subject to chemical expansion. Sodium in the black liquor combines with the refractory material to produce sodium aluminate. Sodium aluminate expands on the order of 130% relative to alumina. This causes not only radial expansion but expansion in the vertical direction of the refractory liner. Prior torispherical domes associated with refractories used in gasifiers required so-called skew blocks supported directly against the shell. This practice causes two problems with refractory linings that have very large expansion: a) The dome is overly constrained from expansion along the radial direction, which causes development of high stress both in the refractory and in the shell, and b) These stresses are difficult to quantify in the design of the refractory shell system. The present invention addresses these problems by utilizing a hemispherical dome with unique layers of blocks forming the hemisphere. The hemispherical dome is backed by a layer of material that has a controlled crushability that resists expansion in a measured way. 
     The present invention thus provides a refractory vessel including a generally cylindrical metal shell having an upper hemispherical dome. A refractory liner has a cylindrical portion spaced inwardly from the shell and a hemispherical portion spaced inwardly from the hemispherical dome. The hemispherical portion includes a plurality of circular layers of refractory bricks, each layer having a lesser diameter than the immediately preceding layer. Each layer is composed of a plurality of blocks having tops and bottoms and sides shaped to form a ring. At least one of the successive layers and the next preceding layer has blocks with interlocking keys and keyways. The keyways are preferably positioned on the next preceding layer adjacent the outer end of each of the blocks. The keys are positioned on the successive layer adjacent the outer end of the block and extend downwardly and into mating relationship with the keyways on the next preceding layer. This keyed system is required to ensure stability of the upper layers of the dome bricks, in case they do not expand as much as the lower layers (because the upper layers are not exposed to as much alkali as the lower layers). 
     Another feature of the hemispherical dome is that the center of curvature of the hemispherical dome comprised of the refractory is at a lower elevation than the center of curvature of the hemispherical dome comprised of the metal shell. This provides an expansion gap which increases in thickness along the curvature of the dome. This “crescent shaped” gap in the dome allows for radial expansion of the dome as well as axial expansion of the cylindrical section. The entire refractory dome rises in the vertical direction as the cylindrical section expands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an isometric view of a refractory vessel constructed in accordance with the present invention having a vertical pie-shaped segment removed therefrom to expose the interior and the wall structure; and 
     FIG. 2 is an enlarged cross-sectional view of one-half of the hemispherical dome constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, the refractory vessel  10  has an outer metal shell  12 . The outer metal shell is preferably comprised of carbon steel but can be composed of any other suitable material with adequate strength and corrosion resistance. The upper portion of the metal shell comprises a dome  14  that terminates in an upper opening  15 . The bottom portion of the metal shell  12  merges into a support cone  16  having a central bottom opening  17 . A refractory liner  20  has a cylindrical portion  22  positioned radially inward from the shell  12  and also has a dome portion  24  and a bottom cone portion  26 . A cylindrical expansion gap  27  is provided between the metal shell  12  and the cylindrical portion  22  of the refractory liner  20 . The dome portion of the refractory liner is positioned inwardly and below the dome  14  of the metal shell. 
     Referring to FIGS. 1 and 2, in a preferred embodiment, the upper portion  24  of the refractory liner  20  is hemispherical in shape. The center of curvature of the hemispherical dome  24  of the refractory liner  20  is at a lower elevation than the center of curvature of the hemispherical dome portion  14  of the metal shell  12 . This provides an expansion gap  28  which increases in thickness as the two hemispherical portions  14  and  16  extend upwardly and inwardly toward the opening  15 . Expansion gap  28  connects with the cylindrical expansion gap  27 . A selectively crushable layer  70  is positioned between the refractory liner  30  and the outer shell  12 . The crushable layer  70  is described in more detail below. 
     The refractory liner  20  has an inner layer of blocks  34  and an outer layer of blocks  30 . The outer layer of blocks  30  are stacked on each other to form an outer refractory shell and the inner layer of blocks are stacked on each other to form an inner refractory shell. The blocks in the inner layer are preferably comprised of alumina and most preferably of alpha and beta alumina. The blocks in the outer layer are positioned in intimate contact with the outside of the inner layer of blocks and are preferably composed of beta alumina. However, other refractory materials with suitable strength and resistance to chemical attack could be used. The crushable layer  70  is positioned between the outer surface of the outer layer of blocks  30  and the interior surface of the metal shell  12 . The width of the gaps  27  and  28  are adjusted based on the measured or expected expansion of the refractory material. 
     Referring to FIGS. 1 and 2, the hemispherical dome  24  of the refractory liner is formed by a plurality of rings of blocks  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54  and  56  positioned on the blocks  30  and  34  forming the inner and outer cylindrical shell. Blocks  40  form a first horizontal ring comprising the base of the hemispherical refractory dome. Successive layers of blocks  42 ,  44  and  46  are formed into rings of lesser diameter to form the bottom portion of the inwardly and upwardly sloping dome. Each of the successive layers have flat upper and lower surfaces that are appropriately angled relative to each other to form the dome shape. The next successive layer of blocks  48  also has a lesser diameter than the previous layer of blocks  46 . Blocks  48  have a flat bottom surface formed to contact the flat top of the blocks  46  of the previous layer. However, the upper surface of the layer blocks  48  has a downwardly extending circular keyway  48   a  positioned in upper surface of the blocks  48  adjacent their outer edges. The next successive layer of blocks  50  has a lesser diameter than the layer of blocks  48  and has a downwardly extending circular key  50   b  positioned adjacent the lower outer edges of the blocks  50 . Downwardly extending key  50   b  extends into and mates with the keyway  48   a  in blocks  48 . Similarly, the next set of blocks  52  also forms a ring of lesser diameter than that of the layer formed by blocks  50 . Blocks  52  have a downwardly extending circular key  52   b  that similarly engages a corresponding keyway  50   a  in the preceding layer formed by blocks  50 . The next successive layer of blocks  54  have a circular key  54   b  that similarly mates with a circular keyway  52   a  in blocks  52 . The final layer of blocks  56  is positioned upwardly and inwardly from the layer of blocks  54 . Blocks  54  have a horizontal bevel  54   a  on their upper surface. Blocks  56  have an outwardly extending flange portion  56   b  that overlies the bevel  54   a.  Thus, each successive layer of blocks from the layer formed by blocks  48  through the layer formed by blocks  56  are keyed into the next preceding layer and restrained from falling downwardly or inwardly as differential expansion of the refractory materials occur. 
     A second hemispherical layer of blocks  60  may be positioned outwardly from blocks  40  to  56 . These blocks are conventional in design that have slightly beveled edges to mate to form the hemispherical curve. 
     Based on studies of the prior failure in refractory vessels used for gasifiers, it has been found that the refractory liner  20  must be allowed to expand outwardly and upwardly a certain distance, otherwise the inner surface of the refractory will fail due to excess spalling and cracking caused by the vertical and radial expansion. On the other hand, the refractory liner cannot be allowed to expand too quickly, or the growth rate will exceed the structural limitations of the liner and will ultimately lead to structural failure. It has been postulated for the alumina-type refractory materials that if a predetermined resistance to expansion is provided, the thermal expansion rate can be inhibited in a controlled manner while still allowing sufficient expansion to eliminate excess spalling from the inner surface of the refractory. This internal compression stress (ICS), that is resistance against expansion, may be defined by the formula (for the cylindrical section)        ICS   =       2   ×   yield                 stress   ×   shell                 thickness       shell                 diameter                              
     wherein the yield stress is yield stress of a stainless steel metal shell used in a prior art, thickness is the thickness of the metal shell used in a prior art, and D is the diameter of the metal shell used in a prior art. For a typical refractory vessel used in a gasifier, this will result in an internal compression stress of about 2 MPa. This internal compression stress can be provided by a crushable liner  40  that has a yield stress of about 2 MPa at 65% strain, defined as 
     
       
         (initial thickness−final thickness)/initial thickness. 
       
     
     When that yield stress is exceeded, the crushable liner will irreversibly compress but will still resist radial expansion of the refractory liner  20  with a force equivalent to the internal compression stress. 
     The yield stress of the crushable layer may be varied, depending upon the composition of the refractory material, the composition of the outer shell, as well as the dimensions of the vessel. In practice the yield stress is maintained in the range of from 0.5 to 4.0 MPa, more preferably from 1.0 to 3.0 MPa, and most preferably from 1.5 to 2.5 MPa. 
     One material that will function in this environment is foam material available under the trademark Fecralloy™FeCrAlY, which is an iron-chromium-aluminum-yttrium alloy. This material is an alloy with nominal composition by weight %, respectively, of  — 72.8_% iron,  — 22_% chromium,  — 5_% aluminum, and  — 0.1_% yttrium and 0.1% zirconium. This metal foam is produced commercially by Porvair Fuel Cell Technology, 700 Shepherd Streel, Hendersonville, N.C. It has further been found that the yield stress of this metal foam, that is the compression stress at which the material will irreversibly begin to compress, can be varied depending upon the density of the foam. For example, a foam having a density on the order of 3-4% relative density will have a yield strength of about 1 MPa. A material having a relative density of about 4.5-6% will have a yield strength of approximately 2 MPa, while a material having a relative density greater than about 6% will have a yield strength of about 3 MPa or greater. Thus, a material having a yield strength of about 2 MPa has been found to be most desirable for use as a crushable liner  40  for refractory vessels used in the gasifier environment. Other metal foams composed of stainless steel, carbon steel, and other suitable metals and metal alloys that have the foregoing properties can also be used. 
     As the alumina refractory material is exposed to process conditions, over time the typical refractory liner will expand about 1 inch in the radial direction per year. It is therefore desirable to provide a crushable liner  40  that has an original thickness which allows a compression of 1 inch while providing a yield strength of less than or equal to 2MPa. 
     Another desired characteristic of the crushable liner  40  is that it must be sufficiently conductive so as to maintain the temperature of the crushable liner under approximately 600° C. It has been posutlated that below this temperature, certain species produced in the gasifier will condense to a solid. If such condensation is allowed to occur in the foam lining, it will fill with solid over time and lose its crushability, therefore becoming ineffective to selectively resist expansion of the refractory liner. It has been found that the composite metal foams just described have an adequate thermal conductivity on the order of 0.5 W/mK to maintain the outer surface of the brick at a temperature under 600° C. Thus, any gaseous species will condense in the refractory itself, as opposed to the metal foam, thus allowing the metal foam to retain its selective crushability. 
     The metal from which the shell  12  is made can be carbon steel, stainless steel, or any other suitable alloy. One of ordinary skill will be able to choose other crushable materials that will exhibit the controlled crushability characteristics of the metal foam after understanding the requirements for controlled crushability and substantially constant resistance to expansion over the limited distance between the refractory material and the outer shell of the vessel, as outlined above. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.