Abstract:
Disclosed is a furnace adapted for burning solid materials, including biomass fuels. The furnace comprises an igniter having a heating element carried by a ceramic core and disposed within a ceramic cover tube for directing air at fuel disposed within the furnace for the purpose of igniting the fuel. Also disclosed is an igniter having a heating element carried by a ceramic core and disposed within a ceramic cover tube for directing air at fuel disposed within the furnace for the purpose of igniting the fuel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 11/273,589, filed Nov. 14, 2005, which application is incorporated herein in their entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to furnaces, in particular furnaces for burning biomass and to igniters for such furnaces. 
     BACKGROUND OF THE INVENTION 
     Biomass is gaining popularity as a replacement fuel for fossil fuels such as coal, natural gas and petroleum-based products such as fuel oil. The energy stored in a biomass fuel ultimately comes from the same source as fossil fuels, solar energy. The process of photosynthesis captures the solar energy and stores it by creating carbon-carbon bonds. This stored energy can be released by burning or oxidation, breaking the bonds and generating gaseous carbon typically in the form of carbon dioxide. The burning of fossil fuels, therefore, releases carbon into the atmosphere that has otherwise been stored under the earth&#39;s surface for millions of years whereas burning of biomass such as wood, corn and other plant material releases gaseous carbon into the atmosphere that was removed only recently through the photosynthetic process. 
     A number of hurdles exist to utilizing biomass fuels on a widespread basis. For example, storage and conveyance of biomass fuels to the furnace can be a burden that may put off many potential users of biomass fuels. However, a number of biomass fuels, such as most cereal grains, fruit pits, weed seeds, wood pellets, plastic pellets and other pelletized fuels, are easily stored and conveyed. 
     Dried, shelled corn is often used because of its availability. In addition, dried corn is often much cheaper on a British Thermal Unit (“btu”) basis for generating heat when compared to generating heat using electricity, LP gas, fuel oil and coal. This is especially true where the corn to be burned is not desirable for use in food or feed applications and can be obtained at a discount relative to other higher grade corn. Dried, shelled corn can also be conveyed and transported in a manner that is straightforward and routine due to its use in agricultural settings. 
     The burning of biomass fuels typically leaves ash and residues in amounts that are greater than fossil fuel burning. Fuels such as corn also leave a slag or clinkers after burning. Mechanisms for removal of these residual materials has been largely operated manually by the user, however newer units are becoming available that make the removal of these residuals more automatic. 
     Furnaces for burning of biomass and, in particular, corn are known and have been disclosed previously in US20040200394 and US20050208445, the disclosures of which are both incorporated by reference in their entirety. Such corn stoves are available, for example from Nesco, Inc. (Cookeville, Tenn.) under the AMAIZABLAZE trademark. Another such corn stove may be obtained EvenTemp, Inc. (Waco, Nebr.) under the SaintCroix trademark. Yet another such corn stove may be obtained from Bixby Energy Systems (Rogers, Minn.). These corn stoves incorporate features that make corn burning more convenient and reliable, overcoming many of the previously described difficulties associated with burning corn. 
     Consistent and reliable components for fuel ignition are also important in biomass fuel burning. The fuel must be rapidly and reliably brought to a temperature where the fuel burns, thereby releasing a greater amount of heat energy. One such ignition system that can be employed is an air or gas ignition system in which ambient or pre-warmed air is passed over or brought into contact with a heating element, thereby warming the air to a sufficient temperature to ignite the fuel. The elements are typically disposed within a cover tube. Prior art igniters have used materials that do not provide for optimum durability and conveyance of heat to the fuel. Durability of the tube is especially important where air flow through the tube may be interrupted. 
     While attempts have been made to overcome the problems described, it would be desirable to have a furnace for burning of biomass materials with an igniter optimized for ignition of such biomass materials. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward an apparatus for burning solid fuel, wherein the apparatus has a burning chamber for receiving fuel in communication with a fuel inlet, an air inlet, an exhaust outlet and at least one igniter and the at least one igniter includes at least i) an inlet block defining a channel therethrough, the inlet block including structure defining first, second and third orifices in communication with the channel; ii) a seal disposed within the second orifice; iii) a ceramic cover tube having first and second ends, the first end of the ceramic cover tube operably secured in the first orifice to the inlet block and the second end in communication with the burning chamber; a ceramic core disposed within the ceramic cover tube, v) a heating element carried by the core, and vi) electrical leads in electrical communication with first and second ends of the heating element, the electrical leads passing through the seal. The apparatus further includes a control circuit connected to the electrical leads, a gas source in communication with the third orifice for forcing a gas through the channel, into the ceramic cover tube, and out through the second end of the ceramic cover tube into the burn pot assembly. The apparatus may also have a fuel feed mechanism in communication with the fuel inlet. The fuel to be burned in the apparatus may be a biomass fuel and may be dried, shelled corn. In another embodiment, the ceramic cover tube of the at least one igniter is constructed from a ceramic such as alumina, mullite or corderite. In yet another embodiment, the ceramic core of the at least one igniter is constructed from a ceramic such as alumina, mullite or corderite. In some embodiments, the ceramic cover tube has an outer diameter of about 0.5 inches. In other embodiments, the heating element of the at least one igniter is rated between 300 and 600 watts at 120 volts AC and may be rated at 500 watts at 120 volts AC. In yet another embodiment, the gas source delivers a gas flow of 25-30 SLPM at 25-35 IN-WC to the inlet box. It will be understood that the descriptions various embodiments of the apparatus for burning solid fuel presented in this Summary of the Invention are not intended to be mutually exclusive. 
     The present invention is also directed toward an igniter, wherein the igniter includes an inlet block defining a channel therethrough, the inlet block including structure defining first, second and third orifices in communication with the channel, and a ceramic cover tube having first and second ends, the first end of the ceramic cover tube secured in the first orifice to the inlet block and a ceramic core disposed within the ceramic cover tube, and a heating element carried by the core, and at least one electrical lead in electrical communication with a first end of the heating element, the at least one electrical lead passing through the second orifice. In many embodiments, the igniter may also include a second electrical lead is connected to a second end of the heating element, the second electrical lead passing through the second orifice and the second orifice may sealed against airflow. In some embodiments, the ceramic cover tube is constructed from a ceramic such as alumina, mullite or corderite. Also, in some embodiments, the ceramic core may be constructed from a ceramic such as alumina, mullite or corderite. The ceramic core may also be hollow. In some embodiments, the heating element of the at least one igniter is rated between 300 and 600 watts at 120 volts AC and may be rated at about 500 watts at 120 volts AC. In some embodiments, the ceramic cover tube has an outer diameter of about 0.5 inches. Finally, in some embodiments, the temperature of the gas exiting the ceramic cover tube is about 1100°-1300° C. when the heating element is connected to a 120 volt AC power source and gas is delivered at 25-30 SLPM at 25-35 IN-WC. It will be understood that the descriptions various embodiments of the igniter presented in this Summary of the Invention are not intended to be mutually exclusive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exploded view of a combustion chamber and burn pot of a prior art corn burning furnace. 
         FIG. 2   a  shows a top view of an inlet block for an igniter of the present invention. 
         FIG. 2   b  shows an end view of an inlet block for an igniter of the present invention. 
         FIG. 2   c  shows a side view of an inlet block for an igniter of the present invention. 
         FIG. 3  shows a side view of an igniter. 
         FIG. 4  shows a side view of a core carrying a heating element. 
         FIG. 5  show a perspective view of an igniter of the present invention with the cover tube removed showing the heating element carried by the core. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is an exploded perspective view of a portion of the combustion chamber  110  and the burn pot  300  of the furnace  100 , according to an embodiment of the invention disclosed in US 20050208445. The combustion chamber  110  is bounded by a top burner plate assembly  210  and a bottom plate  220 . The combustion chamber also includes a back wall  212 . Attached to the bottom plate  220  is a first pin  222  and a second pin  224 . The burn pot assembly  300  includes a first burn pot portion  310  and a second burn pot portion  320 . The first burn pot portion includes a side wall  312 . The side wall  312  has openings, such as opening  314  therein, for directing combustion air around the burn pot assembly  300 . The second portion of the burn pot  320  also has a side wall  322 . The sidewall  322  also includes openings, such as opening  324 , for directing air entering from outside the burn pot assembly  300  to within the burn pot assembly. Also attached to the side wall  322  of the second burn pot portion  320  is a mounting wing  326 . The mounting wing  326  includes openings that allow the mounting wing  326  to fit over the first pin  222  and the second pin  224  attached to the bottom plate  220  of the combustion chamber  110 . Attached to the side wall  312  of the first burn pot portion is another mounting wing  316 , which has opening therein so that the mounting wing  316  also fits over the first pin  222  and the second pin  224  of the bottom plate  220  of the combustion chamber  110 . 
     Also located within the combustion chamber is a movable floor  240  and a translating plate  250 . The movable floor includes a grill  242  and an opening  244 . The movable floor  240  is attached to a pivot pin  245  so that the moving floor  240  can pivot around the pivot pin  245 . The translating plate  250  also has an opening  254  therein. The translating plate  250  also includes a solid surface area  252 . The translating plate  250  also is pivotally attached to the pivot pin  245 . An actuator rod  400  is attached to the movable floor  240  as well as the translating plate  250 . The actuator rod  400  is used to move the movable floor  240  and the translating plate  250  between a first position and a second position. In some embodiments, separate actuator rods are used to move the movable floor  240  and the translating plate  250 . 
     Also attached to the burn pot assembly  300 , and specifically to the second portion of the burn pot  320 , is an igniter  260  and an igniter  262 . The igniters  260 ,  262  place heated air into the burn pot assembly  300 . The igniters  260 ,  262  are in fluid communication with the interior portion of the burn pot assembly. The igniters  260 ,  262  are used to initially fire the furnace or to initially ignite biomass fuel added to the burn pot assembly  300 . Once the biomass fuel within the burn pot has been started, the igniters  260 ,  262  no longer place heated air into the burn pot assembly  300 . 
     Improved igniter  500  may be constructed as follows. A heating element  532 , prepared from nichrome wire, is disposed along the surface of a ceramic core  530  between a first end and a second end of the ceramic core. The length and thickness of the nichrome wire used in the heating element may be determined by one of skill in the based on the desired wattage of the element. For example, an element having a wattage of 300 watts would require thinner wire and possibly less total wire than an element having a wattage of 600 watts. 
     In one embodiment, a first electrical lead  511  is attached to a first end  546  of heating element  532  at the first end  542  of ceramic core  530  either directly or by a connecting wire  538  with optional connector  540  and a second electrical lead  513  may be attached to a second end  548  of heating element  532 . Electrical leads  511 ,  513  may be connected directly to a control circuit within the furnace or may terminate within a connector  515  that allows for straightforward connection and removal of the electrical leads with the furnace. The control circuit controls flow of power to the heating element  532  and usually will be used at the beginning of a burn operation. The ignition, or the time the heating element is on and gas is flowing into the burn pot assembly, may be from five to fifteen minutes and may be about ten minutes. 
       FIG. 4  shows another embodiment of the ceramic core in which heating element  532  is wound around ceramic core  530  between first  542  and second  544  ends of ceramic core  530  and first electrical lead  511  is attached directly or through an electrical connection (including, for example, end wire  538  and connector  540 ) to first end  546  of heating element  532  at or near first end  542  of ceramic core  530  as above. However, in this embodiment ceramic core  530  is hollow and an unsheathed wire  536  is attached to the second end  548  of heating element  532  at or near the second end  544  of ceramic core  530  and unsheathed wire  536  passes through the ceramic core to the first end  542  of ceramic core  530  where unsheathed wire  536  is attached to the second electrical lead  513  directly or through an electrical connection. Where ceramic core  530  is hollow, either of first end  542  or second end  544  of ceramic core  530  may be sealed with application of inorganic ceramic cement. 
     The attachment of heating element  532  to electrical leads  511 ,  513  and of electrical leads  511 ,  513  to wire connectors may be by direct mechanical contact, by weld, solder or other type of connection. In both embodiments, the electrical leads are passed into inlet block  504  through first orifice  514  and exit inlet block  504  through second orifice  516 . Second orifice  516  may then sealed in a manner that prevents air or gas flow from the channel through second orifice  516 . 
     First end  518  of ceramic cover tube  502  may then be inserted over ceramic core  530  and heating element  532  and into first orifice  514 . Ceramic cover tube  502  may then be secured in place by application of an inorganic ceramic cement or other heat resistant material to the junction of ceramic cover tube  502  and inlet block  504 . Ceramic cover tube  502  may be secured within first orifice  514  to be parallel in both X and Y axes relative to inlet block  504  with plus or minus one-sixteenth of an inch. Ceramic cover tube may have a outer diameter of 0.50 inches +/−0.015 inches. The length of ceramic cover tube  502  from inlet block  504  to second end  520  of ceramic cover tube  502  may be approximately 7.7 inches and inlet block  504  and may be approximately 1.5 inches along the side and approximately 0.75 inches square on the ends. When ceramic cover tube  502  is fully inserted into inlet block  504 , the second end of ceramic core  530  should be set back from second end  520  of ceramic cover tube  502 . This setback may be 0.5 to 2 inches and may be one inch. 
       FIG. 2   a ,  FIG. 2   b  and  FIG. 2   c  show top, end, and side views, respectively of an inlet block  504  according to the present invention. Inlet block  504  may be constructed from stainless steel, such as 304SS, or from plated steel, such as nickel plated steel. Inlet block  504  defines a channel  512  in communication with three orifices. Channel  512  and the orifices may be constructed by drilling to various depths at the various widths required for each orifice. For example, channel  512  may be constructed drilling five holes to form the first, second and the third orifices to form channel  512  as shown, for example, in  FIG. 2   c . A first orifice receives a first end the cover tube of the igniter and therefore must be wide enough and deep enough to securely receive the cover tube. Second orifice  516  must be of sufficient diameter to accommodate one or more electrical leads. Third orifice  518  provides fluid communication between a gas source [not shown] and channel  512  and may be threaded to receive a fitting  508  or adapter that can facilitate connection of inlet block  504  to the gas source. It will be understood that the placement of third orifice  518  need not be adjacent or on the same surface as second orifice  516 . Fitting  508  may be a brass fitting having ⅛″ NPT×¼″ Hose Barb. Seal  506 , such as a seal constructed from TEFLON material may be interposed between fitting  508  and inlet block  504  to ensure the connection between these two elements is able to withstand the pressures generated by the gas source. In embodiments where two electrical leads are connected to the heating element, inlet block  504  may also be grounded. Inlet block  504  may further define hole  528  which may in turn be threaded to enable acceptance of a screw or other connection for a grounding wire. 
     The gas source may be a pump to deliver ambient air at a pre-defined pressure; alternatively the gas source may be a tank containing a pressurized gas, such as oxygen. Suitable pumps include the GAST-30B pump available from Gast Manufacturing, Inc. (Benton Harbor, Mich.), the Thomas-5030 available from Thompson Pump &amp; Machinery (Slidell, La.), the AL-30B and the Alita 15B both available from Alita Industries (Arcadia, Calif.). Gas flow may be in the range of 20-35 Standard Liters per minute (SLPM) at 25-35 inches of water column (IN-WC) and may be in the range of 25-30 SLPM at 25-35 IN-WC. Excessive gas flow may cause localized rapid burning of fuel, potentially leading to rapid outgassing of trapped moisture. The effect may appear to be similar to popping of corn 
     Inlet block  504  may further define a channel comprising two chambers: a first chamber  522  in communication with the first and second orifice and a second chamber  524  in communication with third orifice  518 , the first and second chambers in communication each in communication with a fourth orifice  526  defined by inlet block  504  and disposed between the first and second chambers. Fourth orifice  526  may be sized to regulate the flow of gas from second chamber  524  to the first chamber  522  to achieve a desired pressure to be applied to first end  518  of ceramic cover tube  502 . 
     The ceramic material used in the construction of the ceramic core and ceramic cover tubes may be alumina or mullite or other aluminum-containing ceramics. These materials have low thermal expansion, good strength including at the temperatures achieved within the igniter and interlocking grain structure and therefore have desirable thermal shock and thermal stress qualities. Corderite may also be used in the construction of the ceramic core and ceramic cover tubes. Electrical leads should be able to withstand the temperatures generated within the inlet block and may be 20 gauge, 600 volt UL 1659 wire with insulation rated to 200° C. or 250° C. Inorganic ceramic cements should able to withstand the high temperatures and pressures generated within the igniter. Such cements are commercially available from a number of sources including Sauereisen, Inc. (Pittsburgh, Pa.). 
     In operation, the air source will be engaged to direct air into the third orifice through the channel exiting via the first orifice and passing through the ceramic cover tube. Once the flow of air or gas has been established, electrical power (e.g. 120 VAC) may be applied through electrical leads to the heating element. As the element heats, the air or gas passing through the cover tube will be warmed. The temperature of the air or gas exiting the ceramic cover tube may be 900°-1500° or 1100°-1300°. 
     The present invention has been described with respect to particular illustrative embodiments. It is to be understood that the invention is not limited to the above-described embodiments and modifications thereto, and that various changes and modifications may be made by those of ordinary skill in the art without departing from the spirit and scope of the appended claims.