Abstract:
An apparatus for thermally debinding a cellular ceramic green body includes a duct preferably defined between a first housing and a second housing. A carrier assembly for the green body is adapted for arrangement within a channel such that the green body is positioned between a first portion of the channel and a second portion of the channel. The apparatus further includes a nozzle positioned to inject gases from the duct into a first portion of the channel and a recirculation fan positioned to draw gases out of a second portion of the channel and discharge the gases into the duct. Also described is a carrier assembly including a base support comprising a plurality of spaced stringer beams having spaces between adjacent ones; and a plurality of ring supports including openings, said ring supports mounted on, and bridging the spaces between, the stringer beams, the ring supports having a surface adapted to support the green body.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/860,382, filed Nov. 21, 2006, entitled “Method and Apparatus for Thermally Debinding a Ceramic Cellular Green Body.” 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to methods and apparatus for firing ceramic cellular bodies. More specifically, the invention relates to a method and apparatus for thermally debinding a ceramic cellular green body. 
         [0003]    Ceramic cellular bodies, otherwise known as ceramic honeycomb substrates, are used in a variety of applications, such as exhaust gas purification applications. In exhaust gas purification applications, the ceramic cellular body may contain an array of longitudinal channels defined by intersecting porous walls, which may be bare or coated with various catalyst(s). The channels and walls are typically bounded by a surrounding skin. For particulate filtration, the channels may be divided into inlet and outlet channels and some may be plugged. Typically, the inlet channels are plugged at an outlet end of the ceramic cellular body, and the outlet channels are plugged at an inlet end of the ceramic cellular body. Exhaust gas enters the ceramic cellular body through the unplugged ends of the inlet channels, passes through the porous walls into the outlet channels, and exits through the unplugged ends of the outlet channels. When the ceramic cellular body is used as a catalyst support, it is typically not necessary to plug the channels in the ceramic cellular body. Typically, in these applications, the ceramic cellular body is made of cordierite or silicon carbide and the channels are unplugged. 
         [0004]    In a process for making ceramic cellular bodies, a ceramic cellular green body is prepared by extruding a plasticized batch of ceramic-forming materials, and processing aids through an extrusion die. The processing aids are typically extrusion and forming aids, such as organic binders (typically methocel), plasticizers, lubricants, and pore formers. After extrusion, the green body is dried and subsequently fired at high temperature to form a ceramic cellular body having a high mechanical strength. The firing process has two main components: thermal debinding and sintering. Thermal debinding involves heating the green body, typically to a temperature less than 650° C., such that carbonaceous materials (such as methocel, pore formers and/or oils, for example) in the green body react with oxygen in the atmosphere to form volatile materials that can be released from the green body. Sintering also involves heating the green body, but to a much higher temperature than used in the thermal debinding process. Typically, this temperature is in a range from 1000° C. to 1600° C., or higher. During sintering, any remaining carbonaceous materials in the green body may also react with oxygen, and the resulting volatile materials may be released. 
         [0005]    Large temperature differentials between the interior and exterior of the green body during thermal debinding can be a major cause of crack formation in the fired ceramic cellular bodies. Therefore, it is desirable to minimize the temperature differential between the interior and exterior of the green body during the thermal debinding step. 
       SUMMARY 
       [0006]    In one aspect, the invention relates to an apparatus for thermally debinding a ceramic cellular green body which comprises a duct defined between a first housing and a second housing, a carrier for the green body arranged within a channel defined by the first housing such that the green body is positioned between a first portion of the channel and a second portion of the channel, a nozzle positioned to inject gases from the duct into the first portion of the channel, and a recirculation fan positioned to draw gases from the second portion of the channel and discharge the gases into the duct. 
         [0007]    In another aspect, the invention relates to a method of thermally debinding a ceramic cellular green body which comprises disposing the green body in a channel, receiving gases from a duct and discharging the gases into a first portion of the channel and allowing the gases in the first portion of the channel to flow into and around the green body into a second portion of the channel, and drawing the gases out of the second portion of the channel and discharging the gases into the duct. 
         [0008]    In yet another aspect, the invention relates to an apparatus for supporting a ceramic cellular green body in a kiln, such as a tunnel kiln, which comprises a base support having spaces for flow of gases, and a ring support mounted on the base support. The ring support may have an annular body and an annular surface upon which the green body rests. The outer diameter of the annular body is selected to be the same as or slightly smaller than an outer dimension of the green body. 
         [0009]    Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
           [0011]      FIG. 1  is a vertical cross-section of a debinding unit. 
           [0012]      FIG. 2  is a block diagram of a tunnel kiln including a plurality of the debinding units of  FIG. 1 . 
           [0013]      FIG. 3A  is a side view of carrier assembly for supporting ceramic cellular green bodies in the debinding unit of  FIG. 1 . 
           [0014]      FIG. 3B  is a top view of the base support of the carrier assembly of  FIG. 3A . 
           [0015]      FIG. 3C  is a cross-sectional view of the support ring of the carrier assembly of  FIG. 3A . 
           [0016]      FIG. 3D  is a partial top view of the support ring mounted on the spaced stringer beams of  FIG. 3A . 
           [0017]      FIG. 3E  is a partial cross-sectional side view of the support ring mounted on the spaced stringer beams of  FIG. 3A . 
           [0018]      FIG. 4  is a top view of the support ring mounted on the spaced stringer beams of and illustrating a high density packing arrangement achievable with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In describing the preferred embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements. 
         [0020]      FIG. 1  depicts a vertical cross-section of a debinding unit  100  for thermally debinding ceramic cellular green bodies  101 . As will be illustrated later, a tunnel kiln may include one or more debinding units  100  for continuous firing of ceramic cellular bodies. The debinding unit  100  provides a homogeneous atmosphere around the green bodies  101  during thermal debinding of the green bodies  101 , particularly when fresh gases are continuously or periodically injected into the atmosphere. One reason for injecting fresh gases into the kiln atmosphere may be to reduce the oxygen content of the atmosphere, or to reduce the concentration of volatile organic compounds (VOCs) in the atmosphere. Providing a homogeneous atmosphere around the green bodies  101  during thermal debinding may have the effect of promoting a uniform temperature distribution around the green bodies  101 , which may, in turn, reduce induced thermal stresses in the green bodies  101  that may otherwise lead to crack formation in the green bodies  101 . The debinding unit  100  induces axial flow of the homogeneous atmosphere through the interior of the green bodies  101 . This induced axial flow may have the effect of reducing the temperature differential between the interior and the exterior of the green bodies  101 , as well as facilitating removal of volatile materials from the interior of the green bodies  101 . 
         [0021]    In  FIG. 1 , the debinding unit  100  includes a duct  102  defined by an inner surface  104  of an outer housing  106  and an outer surface  108  of an inner housing  110 . The outer portion of the outer housing  106  may be made of an insulating material, such as refractory material or other insulating material suitable for making furnace chambers. The inner surface  104  may be lined with stainless steel or other corrosion-resistant, highly-conductive metal, for example. The inner housing  110  may be made of stainless steel or other corrosion-resistant, highly-conductive metal as well, for example. The inner housing  110  defines a longitudinal channel  112  for receiving within, and for passage of the green bodies  101  therethrough. The inner housing  110  may be perforated to allow exchange of gases between the channel  112  and the duct  102 . A burner  114  is inserted through the inner housing  110 , and possibly the outer housing  106 , to deliver heated flow to the channel  112 , as desired. Although only one burner  114  is shown, the debinding unit  100  may include additional burners for delivering heated flow to the channel  112 , as required for the particular ceramic being debindered. Nozzles  116  are inserted through the inner housing  110  or formed in the wall of the inner housing  110  to deliver gases into the channel  112 . For example, where the inner housing  110  is made of a metal, the nozzles  116  could be sheet metal nozzles. 
         [0022]    The green bodies  101  are mounted on, and move along with a moveable kiln car  118 . Typically, several kiln cars  118  are used to convey stacks of green bodies  101  through the channel  112  in a continuous or semi-continuous manner. The kiln car  118  may be conveyed through the channel  112  using a suitable conveyance mechanism, such as a rail or belt conveyor or other motive element. In the example illustrated in  FIG. 1 , the kiln car  118  includes a deck  120  supported on wheels  122 . A base portion  124  of the kiln includes an opening  126  for receiving the deck  120  of the kiln car  118 . The deck  120  may be made of one or more layers of material. For example, the deck  120  may include a base layer  128  made of a durable material, such as a metal (e.g., steel), and a top layer  130  made of an insulating material, such as a high-temperature ceramic fiber insulation. Vertical posts  132  project upwardly from the deck  120  of the kiln car  118 . Carriers  300  for supporting the green bodies  101  extend between the vertical posts  132 , and are coupled to, and mounted upon, the vertical posts  132 . For example, the vertical posts  132  may include lugs on which the carriers  300  may be mounted. A load space  136  is provided in the channel  112  below the stack of carriers  300  and green bodies  101 . A plenum  138  is provided in the channel  112  above the stack of carriers  300  and mounted green bodies  101 . The nozzles  116  are positioned to receive gases from the duct  102  and deliver the gases to the load space  136 . The burner  114  is positioned to receive fuel from an external source and deliver heat to the plenum  138 . Ports  140 ,  141  may extend through the inner and outer housings  110 ,  106  into the channel  112  to allow direct injection of additional gases into the channel  112 , i.e., traverse the duct  102 , and/or to allow direct removal of exhaust gases from the channel  112 . The ports  140 ,  141  may be positioned at the top of the debinding unit  100  as shown, or at the sides or bottom of the debinding unit  100 . 
         [0023]    A high volume recirculation fan  144  is mounted above an opening  142  at the top of the inner housing  110  and at the top of the channel  112 . For illustration purposes, the recirculation fan  144  is coupled to a shaft  146 , which is supported for rotation on bearings  147 . The shaft  146  is in turn coupled to a motor  148  through a system of pulleys  150 . In practice, any suitable system for operating the recirculation fan  144  may be used. The high volume recirculation fan  144  draws gases from the plenum  138  and discharges the gases into the duct  102  as illustrated by arrows labeled “b.” A perforated plate  152  is provided in the plenum  138 , above the stack of green bodies  101 , to allow even drawing of the gases in the plenum  138  by the recirculation fan  144 . The perforated plate  152  assists in a more uniform distribution of gases across the plenum  138 . During debinding, gases in the duct  102  are injected into the load space  136  through the nozzles  116 . The gases are drawn upwardly from the load space  136 , through and around the stack of green bodies  101 , into the plenum  138 , where they mix with the gases in the plenum  138 , which may include burner flow and injected gases, such as low oxygen content (or inert) gases. The gases in the plenum  138  are then drawn into the inlet of the recirculation fan  144 , which pressurizes the gases and returns them to the duct  102 , causing them to be re-circulated where they are again drawn into the load space  136  through the nozzles  116 . 
         [0024]      FIG. 2  is a simplified diagram of a tunnel kiln  200  including an array of debinding units  202   a - 202   f , as described above. The number of debinding units in the tunnel kiln  200  is arbitrary in this figure. Typically, the number of debinding units needed would be determined by the heating rates, the temperature setpoint, and the amount and type of carbonaceous materials in the green bodies. Typically, thermal debinding occurs at temperatures ranging from room temperature to about 650° C., with temperature increasing from the first debinding unit  202   a  to the last debinding unit  202   f . The debinding units  202   a - 202   f  are followed by a higher temperature sintering section  201 , and then a cooling section (not shown). Sintering takes place at temperatures in excess of 650° C., typically in a range from 1000° C. to 1600° C. 
         [0025]    The debinding unit  202   a  is provided with an outside door  204  and an inside door  206  and forms a vestibule section of the tunnel kiln  200 . To load the tunnel kiln  200  with fresh green bodies, the inside door  206  is closed and the outside door  204  is opened. A kiln car  207  carrying green bodies  210  is then allowed to enter the kiln channel  212  of the debinding unit  202   a . The outside door  204  may then be closed, and the channel  212  of the debinding unit  202   a  may be purged with the same oxygen level preheated gas as in debinding unit  202   b  prior to opening the inside door  206  and allowing the kiln car  207  to move into the debinding unit  202   b . As shown, the debinding unit  202   a  is provided with inlet and outlet ports  208 ,  210  for injecting and removing gases from the channel  212 , for example, for the purposes of purging the channel  212 . The gases may be, for example, any VOC cleaned gas, such as air, N 2 , helium, Argon or other inert gas, or even gases re-circulated back from the VOC abatement process (a thermal oxidizer) provided the gas temperature is at or below the set point temperature. In particular, any recirculated gas should be scrubbed of any corrosive acids such as fluorine or chlorine. 
         [0026]    The debinding units  202   b ,  202   c  form a temperature/atmosphere preconditioning section of the tunnel kiln  200 . After moving the kiln car  207  from the debinding unit  202   a  into the debinding unit  202   b , the inside door  206  of the debinding unit  202   a  can be closed, and the debinding units  202   b ,  202   c  can be purged by injecting gases into the debinding unit  202   b  through the inlet port  215  and removing gases from the debinding unit  202   c  through the outlet port  213 . The debinding units  202   b ,  202   c  are purged so that the atmosphere, e.g., oxygen level, in these units is close to the atmosphere, e.g., oxygen level, in the adjacent unit  202   d . The green bodies are also heated to an initial temperature in the debinding units  202   b ,  202   c . Thermal debinding of the green bodies continues in the debinding units  202   d - 202   f . After thermal debinding, the green bodies are moved into the sintering section  201  of the tunnel kiln  200 . After sintering, the green bodies are cooled down. 
         [0027]    As the green bodies are heated in the debinding units  202   a - 202   f , volatile organic compounds (VOCs) are released into the kiln atmosphere. It is important to maintain the VOC level at a safe limit to avoid a possible explosion. In the illustration, lower flammability limit (LFL) detectors  214  are positioned in the debinding units  202   c ,  202   f  to detect the VOC level. The output of the LFL detectors  214  can be used to determine when to inject low oxygen (or inert) gases into the kiln atmosphere in order to control the VOC level. The multiple debinding units  202   a - 202   f  allow the amount of low oxygen content (or inert) gases injected into the kiln atmosphere to be tailored to the VOC level along the tunnel kiln  200 . 
         [0028]      FIG. 3A  shows in greater detail the carrier assembly  300  used in supporting the green bodies  101  on the kiln car ( 118  in  FIG. 1 ). The carrier assembly  300  includes a base support  302  and a plurality of ring supports  304  mounted on the base support  302 . Also shown are cookies or sacrificial disks  306  between the ring supports  304  and the green bodies  101 . The cookies  306  may be made of the same material as the green bodies  101  and help protect the bottom ends of the green bodies  101  from warping and contamination from the material of the ring supports  304 . 
         [0029]      FIG. 3B  shows one example of the base support  302 . The base support  302  may include an array of stringer beams  308 . The stringer beams  308  are spaced apart and arranged in generally parallel relation to each other, and mounted on cross beams  310 , such as at their ends. The stringer beams  308  and cross beams  310  may be made of any suitable high temperature material for kiln furniture, such as silicon carbide. The spaces  311  between the stringer beams  308  allow free flow of gas (indicated by arrows “a”) in the load space ( 136  in  FIG. 1 ) to readily reach the bottom ends of the green bodies ( 101  in  FIG. 3A-3E ) mounted on the ring supports  304 .  FIG. 3B  shows one possible arrangement of the ring supports  304  on the base support  302 . In particular, the ring supports  304  rest on the stringer beams  308  at two locations  308   a ,  308   b  thereby leaving the space  311  between the stringer beams vertically aligned with the opening in the ring support  304  (see  FIG. 3D-3E ). The ring supports  304  may also be staggered on the stringer beams  308  to achieve a different arrangement and higher packing density of the green bodies. The base support  302  may also have alternate configurations. For example, a perforated or slotted plates may also be used as the base support  302  wherein the ring supports rest and are mounted on the base supports. 
         [0030]      FIG. 3C  shows a cross-section of the ring support  304 . The ring support  304  may include an annular body  312  having a planar surface  313 , such as an annular surface, for supporting the cookie and green body mounted thereon. An inner dimension of the opening  314  in the ring support  304  of the annular body  312  is made large enough to allow substantial exposure of the bottom end of the green body to the gases in the load space ( 136  in  FIG. 1 ). In other words, the inner dimension of the opening  314  of the body  312  is selected to substantially match the outer diameter of the green body, but of course being smaller than the outer diameters such that the body is supported. In particular, a minimum amount of overlap is desired. The underside  316  of the annular body  312  may include an undercut  318 , which has the effect of minimizing the contact area between the ring support  304  and the base support ( 302  in  FIG. 3A ) when the ring support  304  is mounted on the base support as shown in  FIG. 3A . The ring support  304  may be made of a high temperature ceramic material, such as silicon carbide, alumina, mullite, and zirconia or other like refractory materials. 
         [0031]    Other modifications are possible to the examples described above. For example, referring to  FIG. 1 , a fan or other suitable device may be used to assist in pushing gases in the load space  136  into and through the arrangement of green bodies  101 . The fan may be used in addition to or in lieu of the nozzles  116 . In the latter case, the fan may draw gases from the duct  102  and discharge the gases into the load space  136  with sufficient pressure to induce axial flow through the green bodies  101 . Suitable ducting may also be used to channel the gases in the load space  136  into the green bodies  101 . 
         [0032]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.