Abstract:
A coal gasification feed injector is disclosed having an oxidation-resistant insert which prevents oxidative corrosion of the shield, and the subsequent damage to the underlying face of the feed injector. The life of the feed injector, and thus the length of a single gasification campaign, is thereby extended.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to an improved feed injector, or burner, for use in a coal gasification apparatus for producing synthesis gas. More particularly, the invention relates to a feed injector having a heat shield with an insert that is resistant to oxidative corrosion, thus lengthening the service life of the feed injector.  
         BACKGROUND OF THE INVENTION  
         [0002]    Synthesis gas mixtures essentially comprising carbon monoxide and hydrogen are important commercially as a source of hydrogen for hydrogenation reactions, and as a source of feed gas for the synthesis of hydrocarbons, oxygen-containing organic compounds, and ammonia. One method of producing synthesis gas is by the gasification of coal, which involves the partial combustion of this sulfur-containing hydrocarbon fuel with oxygen-enriched air. In the slagging-type gasifier, a coal-water slurry and oxygen are used as fuel. These two streams are fed to the gasifier through a feed injector, sometimes called a burner, that is inserted in the top of the refractory-lined reaction chamber. The feed injector uses two oxygen streams and one coal slurry stream, all concentric, which are fed into the reaction chamber through a water-cooled head. The reaction chamber is operated at much higher pressure than the injector water jacket.  
           [0003]    In this process, the reaction components are sprayed under significant pressure, such as about 80 bar, into the synthesis gas combustion chamber. A hot gas stream is produced in the combustion chamber at a temperature in the range of about 700° C. to about 2500° C., and at a pressure in the range of about 1 to about 300 atmospheres, and more particularly, about 10 to about 100 atmospheres. The effluent raw gas stream from the gas generator typically includes hydrogen, carbon monoxide, and carbon dioxide, and can additionally include methane, hydrogen sulfide, and nitrogen, depending on fuel source and reaction conditions.  
           [0004]    This partial combustion of sulfur-containing hydrocarbon fuels with oxygen-enriched air presents problems not normally encountered in the burner art. It is necessary, for example, to effect very rapid and complete mixing of the reactants, as well as to take special precautions to protect the burner or mixer from overheating. Because of the tendency for the oxygen and sulfur contaminants in coal to react with the metal from which a suitable burner may be fabricated, it is necessary to prevent the burner elements from reaching temperatures at which rapid oxidation and corrosion takes place. It is therefore essential that the reaction between the hydrocarbon and oxygen take place entirely outside the burner proper, and that the localized concentration of combustible mixtures at or near the surfaces of the burner elements be prevented.  
           [0005]    Even though the reaction takes place beyond the point of discharge from the burner, the burner elements are subject to radiative heating from the combustion zone, and by turbulent recirculation of the burning gases. For these and other reasons, the burners are subject to failure due to metal corrosion about the burner tips, even though these elements are water-cooled, and though the reactants are premixed and ejected from the burner at rates of flow in excess of the rate of flame propagation. Typically, after a short period of operation, thermal corrosion fatigue cracks develop in the part of the jacket that faces the reaction chamber. Eventually these cracks penetrate the jacket allowing process gas to leak into the cooling water stream. When leaks occur, gasifier operation must be terminated to replace the feed injector.  
           [0006]    Attempts have been made in the past, with varying levels of success, to minimize this resulting corrosion. For example, U.S. Pat. No. 5,273,212 discloses a shielded burner clad with individual ceramic tiles, or platelets, arranged adjacent each other so as to cover the burner in the manner of a mosaic.  
           [0007]    U.S. Pat. Nos. 5,934,206 and 6,152,052 describe multiple shield segments attached to the face of the feed injector by brazing. These shield segments are typically ceramic tiles, though other high melting point materials can also be used. Each of these tiles forms an angular segment of a tile annulus around the nozzle, the tiles being overlapped at the radial joints to form stepped, or scarfed, lap joints. The individual tiles are secured to the coolant jacket end face by a high temperature brazing compound.  
           [0008]    U.S. Pat. No. 5,954,491 describes a wire-locked shield face for a burner nozzle. In this patent, a single piece ceramic heat shield is attached to the feed injector by passing high temperature alloy wires through the shield and a series of interlocking tabs. The shield is thus mechanically secured over the water jacket end-face of the injector nozzle, and is formed as an integral ring or annulus around the nozzle orifice.  
           [0009]    U.S. Pat. No. 5,947,716 describes a breech lock heat shield face for a burner nozzle. The heat shield is comprised of an inner and an outer ring, each of which forms a full annulus about the nozzle axis, shielding only a radial portion of the entire water jacket face. The inner ring is mechanically secured to the metallic nozzle structure by meshing with lugs projecting from the external cone surface of the nozzle lip. The internal perimeter of the inner ring is formed with a channel having a number of cuts equal to the number of lugs provided, so as to receive the respective external lug element. When assembled, the inner ring is secured against rotation by a spot-welded rod of metal applied to the nozzle cooling jacket face within a notch in the outer perimeter of the inner ring.  
           [0010]    The outer perimeter of the inner ring is formed with a step ledge, or lap, approximately half the total thickness of the ring, that overlaps a corresponding step ledge on the internal perimeter of the outer ring. The outer ring is also secured to the water jacket face by a set of external lug elements, projecting from the outer perimeter of the water jacket face. A cuff bracket around the perimeter of the outer ring provides a structural channel for receiving the outer set of water jacket lugs. The outer heat shield ring is also held in place by a tack-welded rod or bar.  
           [0011]    U.S. Pat. No. 5,941,459 describes a fuel injector nozzle with an annular refractory insert interlocked with the nozzle at the downstream end, proximate the nozzle outlet. A recess formed in the downstream end of the fuel injector nozzle accommodates the annular refractory insert.  
           [0012]    U.S. Pat. No. 6,010,330 describes a burner nozzle having a faired lip protuberance, a modification to the shape of the burner face that alters the flow of process gas in the vicinity of the face. This modification results in improved feed injector life. A smooth transition of recirculated gas flow across the nozzle face into the reactive material discharge column is believed to promote a static or laminar flowing boundary layer of cooled gas that insulates the nozzle face, to some extent, from the emissive heat of the combustion reaction.  
           [0013]    U.S. Pat. No. 6,284,324 describes a coating that can be applied to the shields previously described, to thereby reduce high temperature corrosion of the shield material.  
           [0014]    U.S. Pat. No. 6,358,041, the disclosure of which is incorporated herein by reference, describes a threaded heat shield for a burner nozzle face. The heat shield is attached to the feed injector by means of a threaded projection that engages a threaded recess machined in the back of the shield. The threaded projection can be a continuous member or a plurality of spaced-apart, individual members provided with at least one arcuate surface. This threaded method of attachment is a reliable way to attach the heat shield to the feed injector. It provides greater strength, and is more easily fabricated than other shield attachments. This is especially true when the shield is made of a metal that is easily machined.  
           [0015]    Although the heat shield just described is a significant advance in the art, permitting extended operation times, the operational life is nonetheless limited by the corrosion that occurs at the center of the shield. Operating experience using the threaded attachment method has revealed that a local zone of high oxygen activity causes corrosion of the molybdenum shield. This local zone of high oxygen activity is caused by the gas flow dynamics of the oxygen stream as it exits the feed injector. An area of low pressure exists just outside the lip on the face of the injector. This low pressure zone draws in oxygen, causing corrosion of the molybdenum shield.  
           [0016]    While molybdenum has extremely good resistance to corrosion by reducing gases, it is not so resistant to high temperature oxidation. As the shield corrodes, the protection it provides to the face of the injector is gradually lost, shortening the life of the injector. When this occurs, corrosion of both the back of the shield and the face of the injector results. This corrosion is particularly severe at the base of the threaded attachment ring that protrudes from the face of the injector. In extreme cases, the corrosion has been known to cause the threaded ring to fail and the shield to depart.  
           [0017]    Although the addition of a coated molybdenum shield to the face of the feed injector has doubled the maximum run length of the feed injector, run length is still limited by oxidation of the shield which occurs near the center of the shield, leading to corrosion and cracking of the injector face. As the condition of the shield further deteriorates, more corrosive material accumulates between the shield and the injector face. This causes failure of the attachment ring, and eventual loss of the shield.  
           [0018]    There remains a need to provide a heat shield design for a burner for synthesis gas generation which is an improvement over the shortcomings of the prior art in terms of operational life expectancy, which is simple in construction, and which is economical in operation.  
           [0019]    It is therefore an object of the invention to further extend the operational life expectancy of the gas generation burner nozzle just described.  
           [0020]    Another object of the invention is to provide a gas generation burner nozzle for synthesis gas generation having a reduced rate of corrosion.  
           [0021]    A further object is to provide a burner nozzle heat shield to protect metallic elements of the nozzle from the effects of corrosion caused by combustion gases.  
           [0022]    Yet another object is to provide a ceramic insert that is specifically resistant to the effects of oxygen in removing the molybdenum from the oxidizing zone, thereby protecting the threads that attach the shield to the injector from the effects of corrosion caused by combustion gases.  
         SUMMARY OF THE INVENTION  
         [0023]    These and other objects of the invention are attained by the present invention, which relates to a nozzle having a threaded heat shield, provided with an oxidation-resistant material in place of the portion of the heat shield that is most typically lost to corrosion. The oxidation-resistant insert is preferably separate from the shield, conical in shape, and held in place by the shield itself. This insert occupies the oxidizing zone and prevents oxidation of the shield, thus further prolonging the life of the burner.  
           [0024]    The present invention is accomplished by increasing the diameter of the center hole of the shield, by removing a conically shaped portion of the shield. The basic shape and size of the shield are otherwise retained. The oxidation-resistant material, typically a ceramic, is conical in shape, and is placed over the lip on the face of the feed injector. The heat shield is then screwed into place on the face of the injector in the usual manner, causing the insert to be held in place. The design provides a small amount of clearance between the insert, the injector face, and the shield, to prevent cracking of the brittle ceramic. When assembled in this fashion, the insert occupies the oxidation zone, and the molybdenum is subjected only to reducing conditions, thereby preventing corrosion of the shield and the injector face that is covered by the insert. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a partial sectional view of a synthesis gas generation combustion chamber and burner;  
         [0026]    [0026]FIG. 2 is a detail of the combustion chamber gas dynamics at the burner nozzle face;  
         [0027]    [0027]FIG. 3 is a partial sectional view of a synthesizing gas burner nozzle constructed according to a preferred embodiment of the invention;  
         [0028]    [0028]FIG. 3A is an enlarged, exploded cross-sectional view of a portion of FIG. 3 taken along axis  3 A; and  
         [0029]    [0029]FIG. 3B is a duplicate of the enlarged, exploded cross-sectional view of FIG. 3A, provided so as to clearly label further features according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Referring now to FIG. 1, a partial cut-away view of a synthesis gas generation vessel  10  is illustrated. The vessel  10  includes a structural shell  12  and an internal refractory liner  14  around an enclosed combustion chamber  16 . Projecting outwardly from the shell wall is a burner mounting neck  18  that supports an elongated fuel injection burner assembly  20  within the reactor vessel. The burner assembly  20  is aligned and positioned so that the face  22  of the burner is approximately flush with the inner surface of the refractory liner  14 . A burner mounting flange  24  secures the burner assembly  20  to a mounting neck flange  19  of the vessel  10  to prevent the burner assembly  20  from becoming ejected during operation.  
         [0031]    Although not wishing to be bound by any theory, it is believed that FIGS. 1 and 2 represent a portion of the internal gas circulation pattern within the combustion chamber. The gas flow depicted as arrows  26  is driven by the high temperature and combustion conditions within the combustion chamber  16 . Depending on the fuel and induced reaction rate, temperatures along the reactor core  28  may reach as high as 2,500° C. As the reaction gas cools toward the end of the synthesis gas generation chamber  16 , most of the gas is drawn into a quench chamber similar to that of the synthesis gas process described in U.S. Pat. No. 2,809,104, which is incorporated herein by reference. However, a minor percentage of the gas spreads radially from the core  28  to cool against the reaction chamber enclosure walls. The recirculation gas layer is pushed upward to the top center of the reaction chamber where it is drawn into the turbulent downflow of the combustion column. With respect to the model depicted in FIG. 2, at the confluence of the recirculation gas with the high velocity core  28 , a toroidal eddy flow  27  is believed to be produced, that turbulently scrubs the burner head face  22 , thereby enhancing the opportunity for chemical reactivity between the burner head face material and the highly reactive, corrosive compounds carried in the combustion product recirculation stream.  
         [0032]    Referring to FIGS. 1 and 3, the burner assembly  20  includes an injector nozzle assembly  30  comprising three concentric nozzle shells and an outer cooling water jacket  60 . The inner nozzle shell  32  discharges the oxidizer gas that is delivered along upper assembly axis conduit  42  from axial bore opening  33 . Intermediate nozzle shell  34  guides the coal slurry delivered to the upper assembly port  44  into the combustion chamber  16 . As a fluidized solid, this coal slurry is extruded from the annular space  36  defined by the inner nozzle shell wall  32  and the intermediate nozzle shell wall  34 . The outer, oxidizer gas nozzle shell  46  surrounds the outer nozzle discharge annulus  48 . The upper assembly port  45  supplies the outer nozzle discharge annulus  48  with an additional stream of oxidizing gas.  
         [0033]    Centralizing fins  50  and  52  extend laterally from the outer surface of the inner and intermediate nozzle shell walls  32  and  34 , respectively, to keep their respective shells coaxially centered relative to the longitudinal axis of the burner assembly  20 . The structure of the fins  50  and  52  form discontinuous bands about the inner and intermediate shells, thus offering little resistance to the fluid flow within the respective annular spaces.  
         [0034]    As described in greater detail in U.S. Pat. No. 4,502,633, the entire disclosure of which is incorporated herein by reference, the inner nozzle shell  32  and the intermediate nozzle shell  34  are both axially adjustable relative to the outer nozzle shell  46  for the purpose of flow capacity variation. As intermediate nozzle  34  is axially displaced from the conically-tapered internal surface of outer nozzle  46 , the outer discharge annulus  48  is enlarged to permit a greater oxygen gas flow. Similarly, as the outer tapered surface of the internal nozzle  32  is axially drawn toward the internally conical surface of the intermediate nozzle  34 , the annular space  36 , which defines the coal slurry discharge area, is reduced.  
         [0035]    Surrounding the outer nozzle shell  46  is a coolant fluid jacket  60  having an annular end closure  62 . A coolant fluid conduit  64  delivers a coolant, such as water, from the upper assembly coolant supply port  54  directly to the inside surface of the end closure plate  62 . Flow channeling baffles  66  control the path of coolant flow around the outer nozzle shell, to assure a substantially uniform heat extraction, and to prevent the coolant from channeling and producing localized hot spots. The end closure  62  includes a nozzle lip  70 , such as that described in U.S. Pat. No. 6,010,330, which is incorporated by reference herein, that defines generally an exit orifice or discharge opening for the feeding of reaction materials into the injection burner assembly  20 .  
         [0036]    Referring now to FIGS. 3, 3A, and  3 B, the planar end of the cooling jacket  62  includes an annular surface  72 , forming the face of the injector, which is disposed facing the combustion chamber  16 . Typically, the annular surface  72  of the cooling jacket  62  is comprised of a cobalt base metal alloy material, such as alloy  188 , designed for use at elevated temperatures in both oxidizing and sulfidizing environments. Alloy  188  includes chromium, lanthanum, and silicon, provided to enhance corrosion resistance; and tungsten, to improve strength at elevated temperatures. Other cobalt base alloys such as alloy  25  or alloy  556  might also be advantageously used. One problem with this type of material is that when high sulfur coal is used, the sulfur compounds that are present in the coal tend to react with the cobalt base metal alloy materials, causing corrosion. A self-consumptive corrosion is sustained, that ultimately terminates with failure of the burner assembly  20 . Although cobalt is generally the preferred material of construction for the nozzle assembly  30 , other high temperature melting point alloys, such as alloys of molybdenum or tantalum, may also be used.  
         [0037]    Projecting from the annular surface  72  is a threaded projection  74  for affixing a heat shield  76  to the burner nozzle injector assembly  30 . The heat shield  76  can be constructed from any of several high temperature materials, including ceramics, cermets, and refractory metals such as molybdenum, tantalum, or niobium, that are suitable for use in a reducing gasification environment. The heat shield  76  typically is comprised of molybdenum.  
         [0038]    The threaded projection  74  can be integral to the annular surface  72 ; i.e., the threaded projection can be machined from a solid metal piece comprising the annular surface  72 . Alternatively, the retaining means can be a separate member secured to the annular surface  72 , in which case the projection  74  can be affixed to the annular surface  72  using methods known to those skilled in the art, such as by welding, screwing on, brazing, and the like. The threaded projection  74  extending from the annular surface  72  can be a continuous member, such as a ring, or a plurality of spaced-apart, individual members, each of which may be cylindrical or crescent-shaped. The threaded projection  74  includes an inner surface  78  and an outer surface  80 , either or both of which may be threaded. FIG. 3B depicts threads  82  provided on the outer surface  80  of the threaded projection  74 . An annular channel  88  is provided in an upper surface  84  of the heat shield  76 . The annular channel  88  is threaded on at least one of an inner surface  90  and an outer surface  92  of the annular channel  88 , and is adapted to receive the threaded projection  74 .  
         [0039]    Also projecting from the annular surface  72 , and interior to the threaded retaining means  74  with respect to the axial bore opening  33 , is an annular barrier  94 , or dam, that is integral with the annular surface  72 . The annular barrier  94  is received by an annular groove  95  which is provided in the upper surface  84  of the heat shield  76 . At least a portion  97 , or perhaps a face, of the annular barrier  94  is in contact with the bottom of the groove  95  that is cut in the upper surface  84  of the heat shield  76  to accommodate the projection. The purpose of this annular projection/groove arrangement is to create a barrier to the passage of corrosive species, thus serving as a labyrinth seal, to thereby prevent corrosion and failure of the threaded attachment of the shield. This annular barrier  94  is the subject of a copending patent application, assigned to the present assignee, filed on the same date as the present application.  
         [0040]    Interior to the barrier  94 , with respect to the axial bore opening  33 , is provided an annular, or conical, oxidation-resistant insert  96  according to the present invention, positioned so as to functionally replace the portion of the heat shield  76  that is most likely to be lost to corrosion. This oxidation-resistant insert  96  is separate from the shield, conical in shape, and held in place by the heat shield  76 . The insert  96  is typically fabricated from an oxidation-resistant ceramic that is machinable.  
         [0041]    The oxidation-resistant insert  96  is accommodated by increasing the diameter of the center hole of the shield, by removing a conically-shaped portion of the shield. The oxidation-resistant insert  96  is typically a ceramic, and is positioned by being placed concentrically over the nozzle lip  70  on the face of the feed injector  72 . The heat shield  76  is then screwed into place on the face of the injector  72  in the usual manner, thus holding the insert in place. The design provides a small amount of clearance between the insert  96 , the annular surface  72  of the injector face, and the heat shield  76 , to prevent cracking of the brittle ceramic. When assembled in this fashion, the insert occupies the oxidation zone, and the heat shield  76 , typically comprising molybdenum, is subjected primarily to reducing conditions, thereby preventing corrosion of the shield and the injector face  72  that is covered by the insert.  
         [0042]    The heat shield  76  is formed from a high temperature melting point material such as silicon nitride, silicon carbide, zirconia, molybdenum, tungsten, or tantalum. Representative proprietary materials include the Zirconia TZP and Zirconia ZDY products of the Coors Corp. of Golden, Colo. Characteristically, these high temperature materials tolerate temperatures up to about 1,400° C., include a high coefficient of expansion, and remain substantially inert within a high temperature, highly reducing/sulfidizing environment. Preferably, the heat shield  76  contains molybdenum.  
         [0043]    The heat shield  76  can include a high temperature, corrosion resistant coating  98 , such as that described in U.S. Pat. No. 6,284,324, which is incorporated herein by reference. Such a coating  98  is applied to the lower surface  86  of the heat shield  76  facing the combustion chamber, to a thickness of from about 0.002 to about 0.020 of an inch (0.05 mm to about 0.508 mm), and especially from about 0.005 to about 0.015 of an inch (0.127 to about 0.381 mm). To assist in the application of the coating  98  to the heat shield  76 , a portion of the heat shield  76  proximate the nozzle lip  70  can have a small radius of from about 0.001 inch to about 0.50 inch (0.0254 mm to about 12.7 mm).  
         [0044]    The coating  98  is an alloy having the general formula of MCrAlY, wherein M is selected from iron, nickel, and cobalt. The coating composition can include from about 5-40 weight % Cr, 0.8-35 weight % Al, up to about 1 weight % of the rare earth element yttrium, and 15- 25  weight % Co with the balance containing Ni, Si, Ta, Hf, Pt, Rh and mixtures thereof as an alloying ingredient. A preferred alloy includes from about 20-40 weight % Co, 5-35 weight % Cr, 5-10 weight % Ta, 0.810 weight % Al, 0.5-0.8 weight % Y, 1-5 weight % Si and 5-15 weight % Al 2 O 3 . Such a coating is available from Praxair and others.  
         [0045]    The coating  98  can be applied to the lower surface  86  of the heat shield  76  using various methods known to those skilled in the powder coating art. For example, the coating  98  can be applied as a fine powder by a plasma spray process. The particular method of applying the coating material is not particularly critical as long as a dense, uniform, continuous adherent coating is achieved. Other coating deposition techniques such as sputtering or electron beam may also be employed.  
         [0046]    Having described the invention in detail, those skilled in the art will appreciate that modifications may be made to the various aspects of the invention without departing from the scope and spirit of the invention disclosed and described herein. It is, therefore, not intended that the scope of the invention be limited to the specific embodiments illustrated and described, but rather, it is intended that the scope of the present invention be determined by the appended claims and their equivalents.