Abstract:
A screw feeder can be used to transport a high temperature particulate material. A housing contains the material within the screw feeder which includes an inlet and an outlet port. A screw is rotatably positioned within the housing to advance the material from the inlet port to the outlet port, which rotates axially. A labyrinth seal can be formed around and in communication with the screw to eliminate reverse movement of the material. A cooling medium can be directed into contact with at least the housing. A fluid can be injected through the screw to prevent blockage of the particulate material. A pressure differential is created from the inlet to the outlet port of at least 0.069 MPad.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to hydrogen generation by steam reforming of natural gas and more specifically to a device and method for pumping solid materials used in such a reforming process. 
     BACKGROUND OF THE INVENTION 
     The generation of hydrogen from natural gas via steam reforming is a well established commercial process. One drawback is that commercial units tend to be extremely large in volume and subject to significant amounts of methane slip, identified as methane feedstock which passes through the reformer un-reacted. 
     To reduce the size and increase conversion efficiency of the units, a process has been developed which uses calcium oxide to improve hydrogen yield by removing carbon dioxide generated in the reforming process. See U.S. patent application Ser. No. 10/271,406 entitled “HYDROGEN GENERATION APPARATUS AND METHOD”, filed Oct. 15, 2002, commonly owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference. The calcium oxide reacts with the CO 2  in a separation reaction, producing a solid calcium carbonate (CaCO 3 ) and absorbing the CO 2 . 
     Limitations on the calcium carbonate reuse process include that as the calcium carbonate, in either CaCO 3  or CaO (solid) forms are circulated, both pumping and metering are required at the inlets to the hydrogen generator and a calcination reactor (where the CO 2  is removed). Accurate metering of this high temperature granular material while pumping it into higher pressure regions has commonly been performed using very tall stand pipes and riser systems (such as developed by KBR Inc. of Houston, Tex.). These stand pipe transfer systems are large, some exceeding 15.24 m (50 ft) in height, and do not provide for downstream solids flow splitting required for the calcination reactor. With the large size of these units, gas leakage from or into the system is also an issue. 
     SUMMARY OF THE INVENTION 
     According to a preferred embodiment of the present invention, a screw feeder to transport a particulate material from a low pressure to a high pressure environment includes a housing to contain the material while it is within the screw feeder. The housing includes an inlet port to receive the particulate material and an outlet port to allow the particulate material to exit the housing. A screw is rotatably mounted within the housing to advance the particulate material from the inlet port to the outlet port. A jet port is disposed in the screw, the jet port operably assisting in moving the particulate material to the outlet port. A material of the screw is selected from one of a high temperature compatible metal and a ceramic material. 
     In another preferred embodiment of the present invention, a screw feeder to transport a particulate material from a low pressure to a high pressure environment includes a housing to contain the material while it is within the screw feeder. The housing defines an inlet port and an outlet port. A screw is rotatably disposed within the housing to advance the material from the inlet port to the outlet port, and adapted to rotate axially. A labyrinth seal is formed around and in communication with the screw to substantially eliminate reverse movement of the material. A cooling medium is in operable contact with at least the housing. A pressure differential from the inlet port to the outlet port is at least 0.069 MPad (10 psia). 
     In still another preferred embodiment of the present invention, a method for transporting a high temperature particulate material using a screw pump, the screw pump including a screw, and a housing, includes: rotatably mounting the screw within the housing; feeding the particulate material at an elevated temperature into the housing to operably contact the screw; axially rotating the screw to operably advance the particulate material; creating a labyrinth seal around and in communication with the screw to substantially eliminate reverse movement of the material; cooling at least the housing; and generating a differential pressure across the screw pump of at least 0.069 MPad (10 psia). 
     Advantages of the present invention include a screw pump having materials compatible with a high temperature particulate material, operable to transfer the particulate material by axial rotation of a screw. A fluid source can be connected to a jet of the screw to reduce clogging of the particulate material in the screw. A cooling source can be connected to the screw pump to cool portions of the screw pump not directly exposed to the elevated temperature of the particulate material. 
     The features, functions, and advantages can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a diagram of a reformation system having a plurality of hot rotary screw pumps according to a preferred embodiment of the present invention; 
         FIG. 2  is an elevational cross sectional view of a hot rotary screw pump of the present invention; 
         FIG. 3  is a cross sectional elevation view taken at area  3  of  FIG. 2 ; 
         FIG. 4  is a cross sectional view taken at section  4 - 4  of  FIG. 3 ; and 
         FIG. 5  is a cross sectional view taken at section  5 - 5  of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring generally to  FIG. 1 , according to a preferred embodiment of the present invention, a reformation system  10  can include a hydrogen generator  12  which receives reaction products from a calciner  14  via a generator feed line  16 . Discharge from the hydrogen generator  12  can be provided via a generator discharge line  18  to a hydrogen cyclone separator  20 . Hydrogen gas  22  is largely removed from hydrogen cyclone separator  20  via a hydrogen discharge line  24 . A plurality of calcium carbonate (CaCO 3 ) particles  25 , which are entrained in a flow that can contain hydrogen, steam and nitrogen gases from hydrogen generator  12 , can be separated and collected for discharge at a discharge end  27  of hydrogen cyclone separator  20 . The calcium carbonate particles  25  are transferred via a hot rotary screw pump (hereinafter screw pump)  26  of the present invention via a return line  28  back to calciner  14 . 
     In return line  28  a flow splitter  30  can be disposed having at least one feed tube  32  discharging the calcium carbonate particles  25  into a calciner injector  34 . An exemplary calciner injector  34  is disclosed in United States patent application entitled “DRY, LOW NITROUS OXIDE CALCINER INJECTOR, concurrently filed herewith, commonly assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. Calciner injector  34  can be connected to a calciner inlet  36  of calciner  14 . A hot, vitiated air volume  38  can be introduced via a vitiated air generator  40  into calciner injector  34 . Details of vitiated air generator  40  are provided in U.S. patent application entitled “NON-SWIRL DRY LOW NOx (DLN) COMBUSTOR” filed Feb. 26, 2004, commonly owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference. 
     Calciner inlet  36  can receive a mixture  42  including the calcium carbonate particles  25  and the hot vitiated air volume  38  discharged into calciner inlet  36  upstream of a cyclone separator  44  within calciner  14 . Regeneration of the calcium carbonate particles  25  back to calcium oxide occurs primarily within calciner inlet  36 . As a result of the regeneration process, as well as the addition of steam and methane as noted below, a calcium oxide/nitrogen/carbon dioxide/oxygen mixture  46  can be created within cyclone separator  44 . A plurality of relatively heavier calcium oxide particles  48  can be separated within cyclone separator  44  and fall into a hopper  50  within calciner  14 . A gas volume  52  that can contain nitrogen, excess oxygen and carbon dioxide gases, together with a small carryover volume of calcium oxide particles  48 , can be discharged from cyclone separator  44  via a gas discharge line  54  to a cyclone separator  56 . Gas volume  52  can be discharged from cyclone separator  56 , leaving the carryover volume of calcium oxide particles  48  to collect in a bottom hopper area  58  of cyclone separator  56 . A screw pump  60  can return the carryover volume of calcium oxide particles  48  via a calciner input line  62  to hopper  50  of calciner  14 . A steam supply  64  and a methane supply  66  can be connected to calciner  14  and a steam/methane mixture  68  together with the regenerated calcium oxide particles  48  can be transferred via a screw pump  70  to hydrogen generator  12  to repeat the process. 
     During operation of reformation system  10 , hydrogen generator  12  reacts steam from steam supply  64  and methane from methane supply  66  to generate hydrogen and carbon dioxide. The carbon dioxide is removed from hydrogen generator  12  by reaction with the calcium oxide particles  48  entrained with steam/methane mixture  68 . The hydrogen  22  is removed via hydrogen cyclone separator  20  as previously discussed. As the calcium oxide particles  48  absorb the carbon dioxide, calcium carbonate particles  25  are formed which are transferred by screw pump  26  in particulate form out of hydrogen cyclone separator  20 , as previously discussed, and into calciner injector  34 . Hot, vitiated air volume  38  can impinge and react with the calcium carbonate particles  25  in calciner inlet  36  to reform calcium oxide particles  48  from mixture  42 , which subsequently enter cyclone separator  44  of calciner  14 . Within calciner  14 , the calcium oxide particles  48  can be separated from mixture  46 . During operation of reformation system  10 , calcium carbonate particles  25  are continuously reformed to calcium oxide particles  48  and calcium oxide particles  48  are returned in particulate form with steam/methane mixture  68  using screw pump  70  to hydrogen generator  12 . 
     In one embodiment, system conditions adjacent each of the screw pumps  26 ,  60  and  70  are approximately as follows:
         a) Screw pump  26  inlet: pressure approximately 0.793 MPa (115 psia), temperature approximately 649° C. (1200° F.);   b) Screw pump  26  outlet: pressure approximately 0.931 MPa (135 psia), temperature approximately 649° C. (1200° F.);   c) Screw pump  60  inlet: pressure approximately 0.655 MPa (95 psia), temperature of calcium oxide particles approximately 982° C. (1800° F.);   d) Screw pump  60  outlet: pressure approximately 0.793 MPa (115 psia), temperature of calcium-oxide-particles approximately 982° C. (1800° F.);   e) Screw pump  70  inlet: pressure approximately 0.793 MPa (115 psia), temperature of calcium oxide particles approximately 760° C. (1400° F.); and   f) Screw pump  70  outlet: pressure approximately 0.931 MPa (135 psia), temperature of calcium oxide particles approximately 760° C. (1400° F.).       

     Referring generally to  FIGS. 2 through 5 , an exemplary one (screw pump  26 ) of screw pumps  26 ,  60  and  70  used in reformation system  10  is shown in greater detail. Screw pumps  60  and  70  are similar in design to screw pump  26  but can operate at different temperatures and/or pressures as discussed above. Any of screw pumps  26 ,  60  or  70  generate approximately a 0.138 MPad (20 psid) pressure differential between the screw pump inlet to the outlet. This pressure differential can vary between a low of about 0.069 MPad (10 psid) to a high of about 0.689 MPad (100 psid) differential. Screw pumps  26 ,  60  and  70  use one or more components of one or more ceramic matrix composite (CMC) materials. CMC material for the various component parts of hot rotary screw pumps  26 ,  60  and  70  of the present invention is disclosed in U.S. Pat. No. 6,418,973, to Cox et al., issued Jul. 16, 2002, commonly assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. 
     Screw pump  26  can receive a mixture of hydrogen, steam, oxygen and nitrogen gases with the calcium carbonate particles  25  at discharge end  27  of hydrogen cyclone separator  20 . The gas mixture and calcium carbonate particles  25  at discharge end  27  is fed into an inlet end  72  of a screw barrel  74 . The inlet end  72  of screw barrel  74  includes a feed sleeve  76 . The remainder of inlet end  72  of screw barrel  74  is defined by a stationary sleeve  78  which substantially surrounds and seals the remainder of inlet end  72 . Turning within screw barrel  74  is a screw  80  generally including a central shaft  82  and a screw thread  84  surrounding central shaft  82 . Between each turn of screw thread  84  is defined a threaded space  86  where material is held and moved. The calcium carbonate particles  25  entrained in the hydrogen/steam/nitrogen stream from discharge end  27  are driven from inlet end  72  to a high pressure end  88  where the calcium carbonate particles  25  and the hydrogen/steam/nitrogen stream are discharged via a conduit  90  into return line  28  via a fitting  92 . The pressure at high pressure end  88  is higher by approximately 0.138 MPad (20 psid) than the pressure at inlet end  72 . 
     Screw  80  is rotated through an interconnection of a screw gear  94  and a drive gear  96 . Drive gear  96  is driven by a drive motor  98 . The drive motor  98  may be any appropriate motor that may be powered by electricity or other fuels. An interconnecting gear  100  allows the direction of rotation of drive gear  96  to be the same as the screw gear  94 . Drive motor  98  also drives a second or sleeve drive gear  102  which interconnects with splines formed on the exterior of a rotating sleeve  104 . Drive motor  98  therefore directly drives rotating sleeve  104  while it drives screw  80  with interconnecting gear  100 . Therefore, screw  80  rotates in a direction opposite the angular rotation of rotating sleeve  104 . When geared correctly, this allows screw  80  to rotate substantially freely relative to rotating sleeve  104  even if screw  80  interacts with rotating sleeve  104 , as discussed further herein. 
     Near inlet end  72  is a fluid delivery mechanism  106 . Fluid delivery mechanism  106  delivers a fluid such as CO 2  gas or steam through a fluid feed line  108  from a fluid supply  110 . The fluid from fluid supply  110  is used to increase the pressure differential across screw pump  26  of calcium carbonate particles  25 , while reducing the transfer/loss of hydrogen through screw pump  26 . The fluid from fluid supply  110  may be any suitable fluid, but in one form for application to screw pump  26  includes gaseous CO 2 . Fluid feed line  108  enters a housing  112  through a sealant nipple  114 . Within housing  112  is defined a sealed space  116  which is defined by housing  112  and a seal  118 . Once the fluid fills sealed space  116 , it is forced down a bore  120  defined within central shaft  82  of screw  80 . Although bore  120  is defined substantially as the center of central shaft  82 , it will be understood that bore  120  may be positioned radially on central shaft  82 . Bore  120  allows the fluid from fluid supply  110  to be provided to any portion of screw  80 . It will be understood that bore  120  may be defined along an entire length of central shaft  82  or may only be defined to a stopping point  122  to limit the volume of fluid required to fill bore  120 . 
     Also formed within housing  112  is a first or first shaft bearing  124 . First shaft bearing  124  allows central shaft  82  to rotate substantially freely. In addition, seal  118  allows central shaft  82  to also rotate within seal  118  while maintaining sealed space  116 . 
     Between housing  112  and screw gear  94  there does not need to be a substantial seal. Although it may be desired to include tight tolerances to ensure a smooth operation of screw pump  26 , there is no leakage of either calcium carbonate particles  25  or hydrogen/steam/nitrogen from discharge end  27  or fluid from housing  112  which may occur between housing  112  and screw gear  94 . It may be desirable, however, to provide a very tight tolerance or seal to seal discharge end  27  with screw barrel  74  of screw pump  26 . Either tight tolerances or a seal  126  may be provided between appropriate portions of the discharge end  27  and screw barrel  74 . It will also be understood that although discharge end  27  is illustrated to be in contact with both rotating sleeve  104  and screw gear  94 , it does not necessarily need to be in contact with these moving parts. It will also be understood that appropriate designs may be included in the present invention which provide that discharge end  27  be in contact with stationary portions of screw pump  26  and provide a seal therebetween. In addition, the areas between stationary sleeve  78  and both screw gear  94  and rotating sleeve  104  are also sealed with an appropriate high temperature seal member  128 , acceptable for use at an operating temperature of at least 982° C. (1800° F.). Therefore, material fed into inlet end  72  of screw barrel  74  is not able to freely pass through screw barrel  74  and escape along screw  80  to possibly interfere with the mechanism of screw pump  26 . Any such material is kept within screw barrel  74  itself. 
     Surrounding high pressure end  88  and rotating sleeve  104  is a body  130 . Body  130  is generally immobile relative to rotating sleeve  104 . Therefore, a first sleeve bearing  132  and a second sleeve bearing  134  are provided to allow a substantially easy rotation of rotating sleeve  104  relative to body  130 . Also, a seal member  136  is provided between rotating sleeve  104  and high pressure conduit  90 . This is because high pressure conduit  90  is at a pressure higher than the area surrounding rotating sleeve  104 , which may be sealed or open to ambient conditions. Therefore, to reduce the possibility or eliminate material blow back into other areas of screw pump  26 , seal member  136  is provided. Seal member  136  is adapted to allow substantially free rotation of rotating sleeve  104  regardless of the presence of seal member  136 . In addition, a second shaft bearing  138  is provided to receive a second end of central shaft  82 . Therefore, first shaft bearing  124  and second shaft bearing  138  substantially hold central shaft  82  in a selected position while allowing its substantially free rotation powered by drive motor  98 . 
     The calcium carbonate particles  25  entrained within the hydrogen/steam/nitrogen stream from discharge end  27  of hydrogen cyclone separator  20  is moved from inlet end  72  to high pressure end  88  by the motion of screw thread  84  of screw  80 . As screw  80  rotates, the motion of screw thread  84  moves the calcium carbonate particles  25  entrained within the hydrogen/steam/nitrogen stream from inlet end  72  to high pressure end  88  because screw  80  remains stationary. As the calcium carbonate particles  25  entrained within the hydrogen/steam/nitrogen stream moves from inlet end  72  to high pressure end  88 , compressive forces at the interfaces of touching calcium carbonate particles  25  are increased along with the gas density within the interstices of calcium carbonate particles  25 . 
     Without adding additional fluid into a threaded space  86 , increased fluid density would develop by back-flowing high pressure fluid from high pressure conduit  90  into threaded space  86 . This back flowing fluid further increases the compressive forces acting at the interfaces of the touching calcium carbonate particles  25 . Eventually, these interface compressive forces will stop the flow of calcium carbonate particles  25  through screw pump  26 . If this were to occur, screw  80  and the compacted calcium carbonate particles  25  would simply rotate as a solid cylinder rather than moving from inlet end  72  and ejecting out high pressure end  88 . 
     To minimize the possibility of the calcium carbonate particles  25  being compacted by compressive forces into a single solid plug, central shaft  82  defines bore  120  through which a fluid may be pumped. The fluid from fluid supply  110  is provided to bore  120 . With reference to  FIGS. 3  and  4 , the fluid provided through bore  120  is then ejected out a fluid nozzle  142  formed in individual screw threads  84  of screw  80 . The screw threads  84  each define a plane “A”. Fluid nozzle  142  is formed about a central axis “B” and central axis “B” is formed at an angle θ from plane “A” of screw thread  84 . Angle θ may be any appropriate angle to move the material along rotating sleeve  104  but is generally about 15° to about 30°. The angle θ is generally acute relative to the direction of rotation of screw  80 . The fluid is provided within bore  120  at a pressure higher than screw pump  26  discharge pressure. Although the pressure may be regulated and selected, if screw pump  26  is included in reformation system  10 , the pressure provided to bore  120  is preferably approximately 1.586 MPa (230 psia). Therefore, the fluid would flow through bore  120  into a nozzle bore  144  and be ejected out of fluid nozzle  142 . 
     The rotating sleeve  104  includes a female notch groove  146  to receive screw thread  84  of screw  80 . The female notch groove  146  may be formed in rotating sleeve  104  to substantially cooperate with the helical shape of screw thread  84 . Therefore, as rotating sleeve  104  rotates in a first direction, and screw thread  84  of screw  80  rotates in a second direction, screw  80  is able to rotate freely within rotating sleeve  104 . This provides a labyrinth seal between screw  80  and rotating sleeve  104 . Therefore, the material provided in threaded spaces  86  and the fluid ejected out of fluid nozzle  142  is not able to move towards inlet end  72  of tube  126 , but rather is always directed towards high pressure end  88  due to the motion of screw  80 . 
     As best seen in  FIG. 3 , the angle θ of fluid nozzle  142  relative to plane “A” of screw thread  84  allows for a substantially continuous directional movement of the calcium carbonate particles  25  within the threaded spaces  86 . Each fluid nozzle  142  is generally aimed in the rotational (material transport) direction “C” of screw thread  84 . Therefore, the jet of fluid being emitted by fluid nozzle  142  substantially forces the calcium carbonate particles  25  in the threaded spaces  86  towards high pressure end  88 . Not only does the fluid emitted from fluid nozzle  142  provide additional momentum to the calcium carbonate particles  25  within threaded spaces  86  to ensure that the material does not agglomerate or become a solid mass, but the fluid ejected from fluid nozzle  142  also helps counteract the compressive forces within the calcium carbonate particles  25 . Because the calcium carbonate particles  25  include fluids (including, but not limited to steam/nitrogen/oxygen/carbon dioxide/hydrogen gases) in the interstitial spaces, between the individual particles of the calcium carbonate particles  25  these fluids become compressed as the calcium carbonate particles  25  are forced toward outlet  138 . Therefore, the inclusion of a volume of fluid ejected through fluid nozzle  142  accommodates the compression of the initial volume of interstitial fluid by providing a make-up volume of fluid. Even though the calcium carbonate particles  25  are moved towards a higher pressure head, the introduction of additional fluid through fluid nozzle  142  allows the compression of the original interstitial fluids. 
     Although the rotational speed of screw  80  may depend upon the material from which screw  80  is formed, it may generally be formed of a CMC material. A preferable CMC material includes a matrix formed of a silicon carbide and a fiber formed of either a carbon or a silicon carbide material. CMC material is also preferably used for central shaft  82 . It will also be understood, however, that screw  80  and central shaft  82  may be formed of other appropriate materials such as a high temperature compatible metal (including metal alloys), defined as a metal or metal alloy adaptable for use at or above a temperature of approximately 538° C. (1000° F.) and up to a temperature of at least 982° C. (1800° F.). CMC or high temperature compatible metal or metal alloy material can also be used for many of the high temperature parts of the screw pump of the present invention, including the bearing balls and races of first and second sleeve bearings  132 ,  134 , and first and second shaft bearings  124 ,  138 . Screw  80  is sized to maintain its angular tip speed at about 61 m/sec (200 ft/sec) using rotational speeds between 3,600 and 20,000 rpm. When calcium carbonate particles  25  are the material being moved with screw  80 , keeping the angular tip speed of screw  80  at about 61 m/sec or less, together with the use of silicon carbide in the CMC material, ensures that no substantial erosion or corrosion of screw  80  occurs. Furthermore, screw  80  may be any appropriate diameter, but is generally about 2.54 cm to about 12.7 cm (one inch to about five inches) in diameter. This provides the ability to move up to 50 kg/sec (4,760 tons per day) out high pressure side  138 . 
     The fluid (for example CO 2 ) generally exits fluid nozzles  142  above sonic speed in a range of about mach 3.0 or more. This provides a substantial force against the calcium carbonate particles  25  becoming fixed in any one position within threaded space  86 . Therefore, the material is free to be forced along by the rotational movement of screw  80  towards high pressure end  88 . It will be understood that screw pumps  26 ,  60  or  70  may also be used to pressurize other solid materials besides calcium carbonate or calcium oxide. 
     Referring back to  FIG. 2 , due to the elevated temperature range that a screw pump of the present invention operates within, approximately 538° C. to approximately 982° C. (1000° F. to approximately 1800° F.), additional or forced cooling can also be provided.  FIG. 2  identifies two possible types of external cooling, although it should be recognized that alternate forms of cooling known in the art can be used. A flow of cooling air  148 , provided from a cooling air source  150  can be directed toward the screw pump. A cooling jacket  152  (only partially shown) can be constructed to surround selected components such as body  130 . A cooling fluid such as steam or similar cooling fluid can be directed into cooling jacket  152  from a cooling fluid source  154 . Cooling flow can also be directed to ports and/or channels (not shown) formed within body  130 . A first or low pressure end  156  of screw pump  26  is substantially bounded by stationary sleeve  78 . A second or high pressure end  158  of screw pump  26  substantially includes rotating sleeve  104  and conduit  90  of body  130 . 
     Referring back to  FIGS. 1 and 2 , because CO 2  is actively being removed adjacent the locations of screw pump  60  and/or  70  of reformation system  10 , an alternate injection fluid is desirable for screw pump  60  and/or  70 . Fluid supplied by fluid supply  110  can be steam for the application of screw pump  60  and/or  70  to eliminate further injection of CO 2  at these locations and to advantageously use the steam available from steam supply  64 . Other fluids including air or other gases can be supplied by fluid supply  110 . 
     A screw pump of the present invention provides several advantages. By enclosing a rotatable screw within a sleeve of the present invention, the screw can advance a particulate material. By using a high temperature compatible metal or ceramic material for the screw and/or the sleeve, the particulate material transferred by a rotatable screw of the present invention can be at elevated temperature, for example at a temperature ranging from about 538° C. to about 982° C., suitable for transporting calcium carbonate particles used in a hydrogen reformation process such as reformation system  10 . By providing for a cooling medium, a rotatable screw of the present invention can also use lower temperature materials for components not directly exposed to the elevated temperature of the particulate material. By injecting a fluid into the flow stream of the screw and particulate material, blockages of the particulate material can be reduced or eliminated. 
     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.