Patent Publication Number: US-9896739-B2

Title: System and method for melting metal chips

Description:
This application is a continuation of U.S. Pat. No. 9,145,596 issued on Sep. 29, 2015, which claims the benefit of U.S. Provisional Application Ser. No. 61/667,953, filed Jul. 4, 2012 and 61/675,489, filed Jul. 25, 2013, which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments of the invention generally relate to systems and methods for melting metal chips wherein by-product of the process is utilized to create heat that is utilized within the process. In an exemplary system and process, hydrocarbon material is captured from a vortex feed well that supplies a melting furnace, and the captured hydrocarbons are combusted to create heat that is used, at least in part, to dry the metal chips prior to delivery to the vortex well. 
     BACKGROUND INFORMATION 
     Metal chips, such as aluminum chips that are the by-product of machining aluminum castings used in the automotive industry, have the unique benefit of being able to be re-melted back into the same metal as they started. For example, casted aluminum automobile wheels are machined into final products requiring the use of machining coolant, and as a result of this process, a significant percentage of the casted aluminum is converted to scrap chips. A manufacturing company that produces thousands of aluminum wheels thus creates thousands of pounds of scrap metal that can be reused if properly melted. 
     Efficient re-melting processes require properly sized chips that are essentially free of moisture. Drying the chips is important because moisture can interfere with magnetic separation processes that are required to remove undesirable levels of contaminant metal such as iron. And, chips that are essentially free of moisture must be fed to the molten bath for at least safety reasons. 
     While dry unoxidized chips are desirable, drying the chips to remove moisture can be an expensive process. For example, in many commercial processes, aluminum chips are dried in a direct-fired rotary drum to remove the moisture prior to being introduced into the feed system of melt furnace. Direct-fired rotary drum drying, however, requires large quantities of heat energy. 
     And, it has been observed that some of the chips may be destroyed, prior to melting, by converting a significant percentage of the chips to aluminum oxides that must be removed from the furnace as waste. In other words, it is known that aluminum chips resulting from direct-fired processes produce lower yields of usable molten metal due to the formation of high percentages of aluminum oxides. Thus, some drying processes can cause an economic loss. Also, it has also been observed that direct-fired drying substantially removes residual oils from the chips prior to delivery to the feed system of the furnace. 
     One known system and method for more efficiently drying metal chips prior to melting them is disclosed in U.S. Pat. No. 4,721,457, which issued on Jan. 26, 1988 and which is incorporated herein by reference. This system employs the use of a centrifuge that uses centrifugal force to remove 80 to 90% of the moisture and oil from the chips before indirectly heating the chips to a level high enough to vaporize the remaining moisture while leaving an appreciable amount of the oil on the chips until they are charged into the feed system of a furnace where the oil is at least partially combusted. It is believed that the presence of these oils creates a reducing atmosphere within the furnace feed system, thereby reducing the formation of aluminum oxides. 
     SUMMARY OF THE INVENTION 
     One or more embodiments of the present invention provides a process for melting metal chips, the process being of the type that uses a charge system for delivering the metal chips to a melt furnace, and of the type that uses a pre-treatment system for drying the metal chips, the improvement comprising the step of capturing an exhaust stream from the charge system, where the exhaust stream includes hydrocarbons, and combusting the hydrocarbons to produce heat that is utilized in drying the chips. 
     One or more embodiments of the present invention further provides a process comprising the steps of (i) providing metal chips having water and hydrocarbon on the surface of one or more of the chips; (ii) removing at least a portion of the water from the chips; (iii) delivering the chips to a charge system in fluid communication with a melt furnace, where the metal chips are exposed to heat within the charge system, and where the heat is sufficient to vaporize or at least partially combust the hydrocarbons and thereby create an exhaust stream including hydrocarbon therein; (iv) capturing the exhaust stream; (v) combusting the hydrocarbons within the exhaust stream to thereby create heat; and (vi) transferring the heat to metal chips having water and hydrocarbon on one or more of the chips. 
     One or more embodiments of the present invention further provides a process for melting metal chips, the process comprising the steps of: (i) charging metal chips to a melt furnace; (ii) capturing an exhaust stream generated by said step of charging, where said exhaust stream includes hydrocarbons; (iii) combusting the hydrocarbons to form heat; and (iv) transferring the heat to additional metal chips prior to charging the additional metal chips to the melt furnace. 
     One or more embodiments of the present invention further provides a process for charging aluminum chips having oil residue into a melt furnace through a vortex well while reducing the amount of aluminum oxide formed when the chips are added to the vortex well; the process comprising the steps of: (i) feeding the chips into the vortex well by dropping the aluminum chips through a portion of the atmosphere inside a fume hood that covers the vortex well, wherein the temperature inside the fume hood is high enough to at least vaporize the oil on the chips; (ii) controlling the volume of air input to the fume hood; (iii) capturing an exhaust stream generated by said step of feeding, where said exhaust stream includes hydrocarbons. 
     One or more embodiments of the present invention further provides a process for melting chips, the process being of the type that uses a charge system for delivering the metal chips to a melt furnace, the improvement comprising the step of automatically controlling the delivery of the chips into the furnace at a feed rate determined by the temperature of the molten metal in the furnace. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of one exemplary configuration of the system for a single vortex well. 
         FIG. 2  is a schematic of another exemplary configuration of the system for multiple vortex wells. 
         FIG. 3  is a legend for the abbreviations used in  FIGS. 5-11 . 
         FIG. 4  is a legend for the symbols used in  FIGS. 5-11 . 
         FIG. 5  is an enlarged view of the encircled section of  FIG. 2  labeled with the  FIG. 5  indicator. 
         FIG. 6  is an enlarged view of the encircled section of  FIG. 2  labeled with the  FIG. 6  indicator. 
         FIG. 7  is an enlarged view of the encircled section of  FIG. 2  labeled with the  FIG. 7  indicator. 
         FIG. 8  is an enlarged view of the encircled section of  FIG. 2  labeled with the  FIG. 8  indicator. 
         FIG. 9  is an enlarged view of the encircled section of  FIG. 2  labeled with the  FIG. 9  indicator. 
         FIG. 10  is a side elevation view of the fume hood. 
         FIG. 11  is a front elevation view of the fume hood. 
     
    
    
     Similar reference numbers refer to similar parts throughout the specification. 
     DETAILED DESCRIPTION 
     Embodiments of the invention generally provide a system and method for drying and introducing metal chips into a mechanism for melting the metal chips. In an exemplary configuration, the system and method introduces the metal chips into a melt furnace through a charge system, such as those known charge systems that include a vortex well. The system and method captures exhaust gases, and optionally heat energy, escaping from the charge system (e.g. vortex well). The exhaust gases are believed to include hydrocarbons or partially combusted hydrocarbons that can, according to certain embodiments, be used as a fuel source to produce heat that can be used to dry the metal chips by using, for example, indirect drying techniques. Also, it has unexpectedly been observed that the process of capturing the heat and exhaust gases creates an environment within the charge system that produces fewer metal oxides than would otherwise be expected within a similar charge system operating without the capture mechanism. 
     In one exemplary method, the present disclosure provides one or more of (1) at least one drying step, which may also be referred to as a pre-treatment step, whereby one or more drying elements apply indirect heat to metal chips carrying water and oil to thereby drive off or vaporize at least some of the water from the metal chips; (2) a feeding step whereby the metal chips dried in the drying step are conveyed to a charging mechanism; (3) a charging step whereby substantially dried metal chips are introduced to a melt furnace through the charging mechanism, which may include a vortex well; (4) an exhaust capturing step whereby exhaust gases emitted from the charging mechanism are captured; (5) an exhaust combusting step whereby partially combusted hydrocarbons captured in the exhaust capturing step are further combusted to produce heat energy; and (6) a heat transfer step whereby heat produced by the exhaust combusting step is transferred to the one or more drying elements. 
     In one exemplary system, the present disclosure incorporates one or more of (1) at least one drying mechanisms that are adapted to apply indirect heat to metal chips that carry water and oil and thereby drive off an appreciable amount of the water while leaving an appreciable amount of oil on the chips; (2) a feed system to transfer the metal chips from the drying mechanisms; (3) a melt furnace adapted to receive the metal chips and process the same for the collection of usable materials, such as aluminum; (4) a furnace charge system in communication with the feed system and the furnace, the charge system adapted to receive the metal chips and deliver the metal chips into the furnace; (5) a capture device proximate to the charge system adapted to capture exhaust gases escaping from the feed system; (6) a pressure control system adapted to provide a negative pressure within the capture device to thereby allow air entering near the bottom of the capture device to route exhaust fumes leaving the charge system to an exit port within the capture device; (7) a combustion device for combusting uncombusted hydrocarbons captured by the capture device and convert the same to heat; and (8) one or more transfer elements transferring heat generated by the afterburner to the one or more drying mechanisms. 
     For example, in certain embodiments, the system may include one or more of (1) an indirectly heated reservoir, (2) a jacketed, indirectly-heated furnace charge conveyor or conveyors, (3) a combustion device for the combustion of captured exhaust gases, (4) a heat transport system to utilize the exhaust gases from the afterburner to transfer heat to one or more elements of the system, (5) a close-capture fume hood to contain and capture the flame, smoke, and fume that ignites above the molten bath, (6) a pressure control system that provides a slight negative pressure within the fume hood to allow only enough air to enter near or at the bottom of the fume hood to adequately capture all smoke and fume while providing air for incomplete combustion of the oils contained on the metal chips, (7) a device or devices that produce a controlled atmosphere within the fume hood that eliminates or greatly reduces the formation of oxidized chips that enter the fume hood from a sealed hole in the top or side of a fume hood, (8) a side well attached to a melt furnace with the side well having an open top for the introduction of metal chips, and (9) a molten metal circulation device that promotes the movement of molten metal from the main heat chamber of the furnace to the side well where within a circulating well, often referred to as a vortex well, the chips are charged for submerged melting. 
     In one or more embodiments, the feed system includes heated screws and/or one or more heated hoppers. In one or more embodiments, the feed system carries chips from the drying system to the charge system. In one or more embodiments, the charge system includes a circulation system that is adapted to move molten metal from the furnace into a charge vessel where the metal chips are introduced with molten metal. Additionally, the circulation system adapted to carry the chips, which are introduced with the molten metal, into the furnace. In other words, the charge system may include a circulation loop in communication with the furnace and a charge location, such as a vortex well, where molten metal and chips are first introduced. Various charge systems are known in the art, and practice of the present invention is not necessarily limited by the type of charge system employed. 
     In one or more embodiments, the combustion device is an afterburner. Conventionally, afterburners include several layers of refractory brick and include an internal volume that is in fluid communication with an inlet and an outlet. The temperature within the internal volume is at least 1200 F, or in some embodiments, at least 1400 F, and is sized accommodate the captured exhaust gases for a time sufficient to fully combust or substantially combust the hydrocarbon material within the captured exhaust stream. In other embodiments, the combustion device may be the exhaust system of the melt furnace if the exhaust system is suitably sized for the purpose of taking the place of an afterburner. 
     Another exemplary configuration of the system and method increases the capacity of the system allowing it to serve two melt furnaces. In this configuration, the jacketed heated hopper is designed to discharge into two jacketed heated screws. Each screw independently heats up and feeds chips to a melt furnace. Each hot screw is operated automatically by control signals received by the furnace controls that it serves. The hot screws are “turned on” delivering chips to the vortex wells of each furnace as long as three parameters are being met: (1) less than high molten metal level in the furnace; (2) temperature of the molten metal is above a minimum set point; and (3) the furnace combustion system is on. In this configuration, the afterburner receives fumes from two fume hoods and both fume hoods provide a controlled reducing environment to reduce undesirable oxidation. 
     One embodiment of the present disclosure, which can be used in conjunction with those aspects of the invention that capture exhaust fumes from the charge system, or which may be used alone in any melting process (e.g., aluminum melting process), includes an automatic feed system. This automatic feed system advantageously delivers chips, using mechanisms know in the art and/or described here, to the charge system (e.g., a vortex charge system) as a function of the furnace temperature. More specifically, delivery of heated chips to the furnace charge well is accomplished at a feed rate that is proportional to the molten metal temperature within the furnace. Where the temperature of the metal within the furnace is at a higher temperature, the feed rate is automatically increased. Where the temperature of the metal within the furnace is at a lower temperature, the feed rate is automatically reduced. For example, a slaved control that can obtain a signal (e.g., 4 to 20 milliamp signal) from the furnace&#39;s temperature controller can be provided. This control can provide a proportional variable feed rate from a minimum setting where the furnace&#39;s molten metal temperature is, for example, 737° C. to a maximum setting or speed when the furnace temperature is, for example, 760° C. 
     Another configuration interposes a magnetic separator operatively between a screening apparatus and a heated hopper. The screening apparatus is downstream of an initial hot cyclone chamber that initially dries the chips. Locating the magnetic separator upstream of the heated hopper increases the effectiveness of the magnetic separation as most magnetic separation is not effective at elevated temperatures. Separating the drying and heating operations allows the magnetic separation to occur before the temperature of the chips is significantly raised. 
     In one or more embodiments, the negative pressure in the fume hood or hoods may be controlled and maintained by using a sensing device that receives the negative pressure signal, converts it to an output signal and controls an opposed blade outlet damper on the main system exhaust fan. 
     In one or more embodiments, the system and method provides for the drying of metal chips so that they do not require independent drying prior to being remelted. An indirectly heated hopper stores and elevates the temperature of the chips. The final drying of chips is effected in a jacketed rotary drum dryer or jacketed screw conveyor heated by products of combustion from the exhaust of the afterburner, which exhaust is routed to the heated cyclone. 
     In one or more embodiments, the overall process of the present invention dries the metal chips in a manner that minimizes destruction, such as may occur through the vaporization of the oil, during the drying process. For example, in one or more embodiments, aluminum chips are delivered to the centrifuge process together with about 10% by weight liquid (based upon the total weight of the metal chips and the liquid); about 1-2% by weight of the total chip is oil and about 8-9% by weight of the total chip is water. After the chips are dewatered in the centrifuge, the total water and oil content (i.e. total liquid) is reduced to about 1-2% by weight of the total chip, of which about 25%-50% of the liquid is oil (i.e. 0.25 to 0.5% by weight of the total chip is oil). After complete drying, which substantially removes all of the water, the chips contain about 0.25 to 0.5% by weight oil. 
     In one or more embodiments, the system and method provides for the use of a close-capture fume hood that provides for controlled burning of the oil. It is believed that the conditions within the hood provide a reducing atmosphere that significantly reduces the formation of aluminum oxide (AL2O3) in the vortex well. The controlled burning reduces the amount of oxygen available for the oxidation of the metal. A pressure control system is employed to maintain the reducing condition in the fume hood. The features above may be used alone and in combination to define unique configurations. 
     In one or more embodiments, the conditions within the close-capture fume hood are maintained at an air to oil ratio of less than 10 parts air to 1 part oil (which is carried by the chips), in other embodiments less than 9 parts air to 1 part oil, and in other embodiments less than 8 parts air to 1 part oil. In these or other embodiments, the conditions within the close-capture fume hood are maintained at an air to oxygen weight ratio of more than 6 parts air to 1 part oil, in other embodiments more than 7 parts air to 1 part oil, and in other embodiments more than 7.5 parts air to 1 part oil. In one or more embodiments, the conditions within the close-capture fume hood are maintained at an air to oil ratio from about 6 to about 10 parts air, in other embodiments from about 7 to about 9 parts air, and in other embodiments from about 7.5 to about 8 parts air to 1 part oil. As those skilled in the art will appreciate after reading this disclosure, the air to oil ratio can be adjusted by adjusting the opening between the charging device (e.g. the well) and the capture device. For example, in one embodiment, where a hood having a diameter of about 20 inches is positioned over a vortex well having about the same diameter, a gap of about 0.5 inch is provided between the well and hood. Where greater volumes of air are desired, the gap can be increased. Where lower volumes of air are desired, the gap can be decreased. Those skilled in the art will appreciate that the oil carried by the chips is at least partially converted to smaller hydrocarbons and those smaller hydrocarbons are carried into the fume hood together with vaporized oils, and therefore reference to the air to oil ratio within the hood will be understood to include, as the oil component, all hydrocarbon materials within the exhaust stream that derive from the oil carried by the chips. 
     In one or more embodiments, the air permitted within the close-capture hood is regulated based upon the amount of oil introduced into the well, which oil is introduced into the well by the metal chips. As should be appreciated, the amount of oxygen permitted into the hood is regulated to achieve the air to fuel ratio set forth above. As those skilled in the art appreciate, the partial combustion of the oil within the vortex well and/or within the capture device consumes oxygen. Where greater amounts of oil are present on the chips, greater amounts of oxygen are required. Conversely, where the chips carry less oil, lesser amounts of oxygen are required. 
     In one or more embodiments, the close-capture hood, or other duct work extending from the close-capture hood, may include an oxygen sensor to monitor the oxygen content of the exhaust gases exiting the hood. Based upon information obtained from this sensor, the air allowed into the hood, which air enters through the gap between the vortex well and the hood, can be adjusted to provide the desired amount of oxygen above the vortex well. 
     In one or more embodiments, the velocity of the of the air entering the hood (which air may enter between the gap between the capture hood and the vortex well itself) enters at a velocity of at least 150 feet per minute, in other embodiments at least 155 feet per minute, and in other embodiments at least 160 feet per minute. 
     Turning now to the drawings, the different configurations of the system for introducing metal chips for melting are generally indicated by the reference numeral  10 . The methods of the disclosure are described with respect to the operation of systems  10 . One exemplary configuration for a single vortex well is depicted in  FIG. 1  with another exemplary configuration for multiple vortex wells is depicted in  FIG. 2 . System  10  and the method of introducing metal chips for melting described below at least partially utilized energy created from the combustions of captured exhaust gases to at least partially dry the metal chips prior to charging the chips to the furnace. This energy can also be used to heat the chips prior to charging the chips into the furnace. In certain embodiments, heat captured from the melting of the chips is used to pre-treat the chips prior to charging the chips to furnace (e.g. the charge system of the furnace). System  10  may be used with different types of metal chips including, but not limited to, brass and aluminum chips. The exemplary configuration described below use aluminum chips. 
     The raw chips, which may carry water and oil residues from one or more various machining processes, are introduced to into system  10  and do not require prewashing or pre-cleaning. Raw chips are fed into a centrifuge  20  to remove an initial amount of liquid. A conveyor  22  delivers chips from centrifuge  20  into a heated cyclone assembly  24 . Centrifuge  20  removes 80 to 90 percent of the moisture by weight from the metal chips. Conveyor  22  may be a pneumatic conveyor that supplies the chips from a centrifuge  24 . 
     Heated cyclone assembly  24  includes a cyclone  28  surrounded by a hot cyclone chamber  30 . Heated cyclone assembly  24  initially heats and dries the chips with the heat delivered to the hot cyclone chamber from an afterburner  40  that is fed from a fume hood  42 . Walls of cyclone are heated up to 550 degrees Fahrenheit. The temperature is intended to be low enough as to not burn the oil carried on the chips. The evaporation of the water on the chips cools the chips such that they exit assembly  24  relatively cool. 
     In one configuration, the afterburner gases are provided at about 1400 degrees Fahrenheit and these are cooled and delivered at about 550 degrees Fahrenheit to chamber  30  through an inlet  44  that communicates with the outlet  46  of afterburner  40  through a refractory lined duct  48  that rated at 4410 actual cubic foot per minute (ACFM) and may be 12×15 inches. As shown in  FIG. 5 , dilution air  54  may be provided to duct  48  by a dilution air controller  50  that monitors the temperature of the gas near the outlet  52  of chamber  30 . A second passive source  56  of fresh air also may be provided in direct communication with duct  48 . Outlet  52  is connected with an insulated outlet pipe  60  rated at 4410 ACFM with the outlet gases being about 520 degrees Fahrenheit. Outlet pipe  60  delivers the heated gases to further dry and heat the chips at a location downstream of cyclone  26  as described below. The flow through outlet pipe  60  may be split with a butterfly heat balance damper  62  as described below. 
     The heated chips exit cyclone  20  and pass through a diverter gate  70  that allows chips to be diverted into a chip bypass tub  72  or continue into a screening apparatus  74 . Diverter gate  70  is controlled with a pneumatic cylinder  76 . 
     One configuration of screening apparatus  74  is a vibratory screen driven by a motor. Screening apparatus  74  uses a screen that separates the fine metal chips from larger metal chips depending on the desired input size for the aluminum melt furnace or the desired fine size to be collected by the user of system  10 . A fines tub  78  collects the separated fine chips. 
     The chips exit screening apparatus  74  and enter a magnetic separator  80  which removes ferrous chips to a ferrous tub  82  while allowing the non-ferrous chips to continue to a jacketed heated chip surge hopper  100 . 
     The chips are held in hopper  100  until they are delivered into one or more jacket hot screw conveyors  102  for the final drying and heating before being delivered to a vortex well  104  connected to a melt furnace  106 . 
     Hopper  100  holds the heated chips prior to their final drying and heating which occurs in the screw conveyors. A plurality of level sensors  110  are provided to monitor the height of the chips in hopper  100 . A heated jacket  112  is disposed about hopper  100 . Jacket  112  is heated with the outlet flow from conveyor(s)  102  and then delivers the used heated gases to an exhaust fan  114  through an exhaust duct  116  that is also connected with damper  62 . Exhaust fan  114  sends the gas flow to the stack  118  of melt furnace  106 . A heat balance damper controller  120  controls damper  62  based on temperature readings from duct  116 . A blower damper controller  122  monitors the pressure and/or temperature of the gas flow into exhaust fan  114 . In certain embodiments, the monitoring of pressure and/or temperature takes place within duct  116 . In certain embodiments, the pressure in fume hood  42  controls a blade damper  124  disposed in line with exhaust fan  114  to maintain a negative pressure in fume hood  42 . Controlling the pressure and the oxygen inside hood  42  provides for controlled burning of the oil vapor above the surface of the molten metal which maintains under most conditions a reducing atmosphere that significantly reduces the formation for aluminum oxide on aluminum chips in the vortex well. 
     As noted above, the exhaust gas flow from chamber  30  is delivered through duct  60  to further heat and dry the chips. This heating and drying is performed in jacket hot screw conveyor(s)  102  which are fed from hopper  100 . An inlet duct  120  delivers the hot gases around the screw of the conveyor  102  and the exhausted gases from each conveyor  102  are delivered to hopper  100  as noted above. Each screw  102  is activated by signals from the furnace  106  it serves. Screw  102  is activated when there is less than high molten level in furnace  106 , the temperature of the molten metal in furnace  106  is above a minimum set point, and the furnace combustion system is on. 
     Fume collection hood  42  provides the gases and at least partially burned oils to inlet  58  of afterburner  40 . Fume collection hood  42  has a lower section  130  sized to cover the top of vortex well  104 , an angled intermediate section  132  that receives the pretreated chips, and an upper section  134  that exhausts the fumes to the afterburner supply piping  136 . The pretreated metal chips are delivered from screw conveyor  102  through a chip supply pipe  140  that passes through hood  42  such that the outlet  142  of pipe  140  is directly above the centerline of the inlet to vortex well  104 . Pipe  140  passes through the angled wall of angled section  132  such that the metal chips are delivered to vortex well  104  through a portion of hood  42 . An access door  144  is provided in one wall of fume collection hood  42 . Aside from the gap that allows the introduction of air, hood  42  is otherwise sealed with continuous welds to prevent fumes from leaking out of hood  42 . This configuration of fume hood  42  causes the metal chips and any oil still carried on the metal chips to fall from a height through the atmosphere inside fume hood  42  which is heated by vortex well  104  to a temperature that allows for combustion of the oil when oxygen is available. The atmosphere inside fume collection hood  42  is controlled with exhaust fan  114  and blade damper  124 . The percentage of oxygen inside fume hood  42  may be less than the percentage of oxygen found in the air. The oxygen content may be reduced to 16 to 18 percent of the air inside fume hood  42 . A fixed air gap  126  is maintained between hood  42  and vortex well  104  so that air may be drawn into fume collection hood  42  at a controlled rate and such that the atmosphere inside fume hood  42  is maintained at a negative pressure. This limits the amount of oxygen inside the fume hood. Some of the oxygen available inside fume hood  42  is readily used to burn or partially burn the oil carried by the metal chips and thus leave less oxygen inside fume hood  42  to oxidize the metal chips. 
     The burned and partially burned oil materials, which may be referred to as hydrocarbons or partially combusted hydrocarbons, are delivered as fuel to afterburner  40  which reduces the amount of external fuel that must be added to afterburner  40 . Afterburner  40  is fueled with natural gas or another suitable fuel through a fuel supply system  150  that is controlled with a burner controller  152 . In one exemplary configuration, afterburner  40  includes a 1,500,000 BTU burner. Burner controller  152  monitors the temperature of the gas flowing out of outlet  46  with a pair of spaced thermocouples  154 . Controller  152  is capable of increasing and decreasing the fuel and air supplied to afterburner  40  to control the flow in duct  48 . Combustion air is supplied to afterburner  40  from a combustion blower  156  through a combustion air duct  158  that also communicates with fuel supply  150  through a duct  160 . 
     Various modifications and alterations that do not depart from the scope and spirit of this invention will become apparent to those skilled in the art. This invention is not to be duly limited to the illustrative embodiments set forth herein.