Abstract:
An improved solid fuel combustion apparatus intended for use in residential or light commercial settings capable of sustaining a controlled, continuous blue flame burn, resulting in high efficiency heat output with low emissions and low ash. The combustion apparatus is further capable of being thermostatically controlled, turning off combustion of the fuel when a desired temperature is reached and automatically re-igniting when more heat is called for, and also comprises improved safety features.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The invention relates generally to devices suitable for burning solid fuel for heat and more particularly to wood burning combustion apparatuses intended for use in residential or light commercial settings. Such combustion apparatuses include wood stoves, wood fired boilers, and the like. 
         [0003]    2. Description of Prior Art 
         [0004]    Solid fuel burning combustion apparatuses are well-known in the art. Common examples include wood stoves and wood fired boilers. All such apparatuses operate on the following basic principles: solid fuel and combustion air (comprising oxygen) are ignited; some degree of gasification of the solid fuel occurs, resulting in the release of gases which mix with the combustion air to achieve further combustion; and the output of the combustion process is heat, unburned combustion gases, and particulates. Devices to achieve combustion of solid fuel may comprise one or more chambers in which the combustion occurs. They may utilize the natural flow of combustion air into the combustion chamber(s), or they may use forced introduction of combustion air into the combustion chamber(s) by electric fans, vacuum pumps, or other mechanical devices. They may involve the burning of the solid fuel with an upward oriented flame, or with a downward oriented flame (“down drafting”). They may employ heat exchangers to more efficiently utilize the heat generated by combustion in building heating systems. 
         [0005]    Known solid fuel combustion apparatuses typically burn at temperatures between 900° F. and 1500° F. This is known as a “yellow flame” burn, as that is the primary color of the flame evident during combustion. At those temperatures, however, complete combustion is not possible, resulting in emissions of combustion gases and particulates, often in the form of visible smoke. These emissions are considered pollutants and are undesirable. Combustion involving a stabilized, continuous blue flame burn, which occurs at temperatures exceeding 2000° F., leads to much more complete combustion with a minimum of emissions. However, solid fuel combustion apparatuses intended for use in residential or light commercial settings known in the art cannot achieve consistent continuous blue flame burns. 
         [0006]    Known solid fuel combustion apparatuses are also typically difficult to control, in that they need direct user attention to begin combustion, to monitor combustion, to end combustion, and to re-start combustion. This does not lend itself to thermostatic control, whereby the generation of heat is called for by an automatic device, such as a thermostat, in response to environmental conditions. Those devices that do employ thermostatic control typically do so with poor emissions and efficiency characteristics. 
         [0007]    Known solid fuel combustion apparatuses are also typically less safe than other heating systems, due to the difficult to control combustion. This may lead to dangerous overheating with deleterious effect on any heating system to which the apparatus is connected. 
         [0008]    Notwithstanding the above-described shortcomings of known solid fuel combustion apparatuses, there remains a need for alternatives to the burning of fossil fuels for heat. Wood burning combustion apparatuses use a renewable resource that often is less costly than oil, natural gas, or coal. Many individuals prefer the ability to obtain fuel domestically, often from their own land. There is thus a need to overcome the deficiencies of known solid fuel combustion apparatuses to improve their efficiency, safety, ease of use, and environmental impact. 
         [0009]    It is therefore an objective of this invention to provide an improved combustion apparatus which achieves a consistent, continuous blue flame burn. 
         [0010]    It is a further objective of this invention to provide an improved combustion apparatus which minimizes harmful emissions generated by the combustion process. 
         [0011]    It is yet a further objective of this invention to provide an improved combustion apparatus which allows the automatic re-ignition of partially burned solid fuel. 
         [0012]    It is yet a further objective of this invention to provide an improved combustion apparatus which may be thermostatically controlled by use of an automatic device such as a thermostat which causes a complete halt of combustion when heat is not called for and which causes a re-start of combustion when heat is called for. 
         [0013]    It is yet a further objective of this invention to provide an improved combustion apparatus which comprises safety features to prevent damage to either itself or a heating system to which the apparatus is connected. 
         [0014]    It is yet a further objective of this invention to provide an improved combustion apparatus which is easy and cost efficient to manufacture. 
         [0015]    Other objectives of this invention will be evident from the following disclosure. 
       SUMMARY 
       [0016]    The present invention is directed to an improved combustion apparatus for burning solid fuel, namely wood, for purposes of providing heat. The present invention is suitable for residential or light commercial use, and indoor or outdoor use. It is intended to be integrated with existing heating systems and is capable of safe automatic shutdown and automatic re-ignition after an extended shutdown period. 
         [0017]    The apparatus employs initial combustion and gasification of solid fuel in a primary combustion chamber and a secondary combustion in a secondary combustion chamber. Measured amounts and distribution of pre-heated primary and secondary combustion air is provided via an oxygen introduction mechanism and a pressurizing mechanism, resulting in a controlled, self-sustaining continuous blue flame secondary combustion in the secondary combustion chamber. The resulting superheated exhaust gases have extremely low particulate emissions and a very limited amount of granular ash is produced. 
         [0018]    The primary combustion chamber of the combustion apparatus includes a fuel loading, storage, and drying area that lies above a primary combustion zone. The primary combustion zone is found in the lower portion of the primary combustion chamber and is divided into a gasification region having substoichiometric air and a char fuel bed located at the bottom of the primary combustion chamber. The char fuel bed is created from an initial preparatory burn of the solid fuel and builds up from the floor of the primary combustion chamber to a height proximate to the point of introduction of oxygen into the primary combustion chamber by the oxygen introduction mechanism. 
         [0019]    Once the char fuel bed is created, the combustion gases generated by gasification of the solid fuel, in combination with ongoing combustion of the char fuel, are mixed with a measured amount of pre-heated combustion air introduced by the oxygen introduction mechanism and pressurized by the pressurizing mechanism, resulting in a stabilized initial combustion in the primary combustion chamber. Then the gases, under continuous positive pressure, pass through a permeable divider into the secondary combustion chamber, where they undergo turbulent high temperature mixing with an additional amount of measured pre-heated combustion air introduced by the oxygen introduction mechanism into the secondary combustion chamber. The resultant automatic ignition and secondary combustion is a high temperature stabilized continuous blue flame, generating very high heat energy and minimal particulates. 
         [0020]    The heated gases then pass out of the secondary combustion chamber via an exhaust structure and across a heat exchanger. The heat exchanger extends along the bottom and back of the insulated housing of the combustion apparatus, below and behind the combustion chambers, such that the heated gases must move in a downward direction for at least a portion of the distance between the secondary chamber and the heat exchanger. The heat exchanger extracts the maximal amount of heat energy from the heated combustion gases before the gases are exhausted via the exhaust structure. 
         [0021]    A combination of high temperatures and an extended transit time of the combustion gases from the primary combustion chamber to the secondary combustion chamber, together with measured amounts of supplemental oxygen in the secondary combustion chamber, allow for virtually complete combustion of the solid fuel, yielding an extremely clean, smokeless burn. The extended transit time is provided by the depth and density of the char fuel bed and the gas flow resistance of the divider between the primary and secondary combustion chambers. A limit on the absolute air flow into the primary and secondary combustion chambers also controls the overall combustion rate and the transit time of the combustion gases. The rate of fuel combustion is substantially constant, dictated by the fixed amount of combustion airflow and the ratio of primary and secondary combustion airflows. 
         [0022]    The combustion chambers and divider are constructed at least in part of a durable material having the properties of absorbing and retaining high levels of heat energy. This allows the combustion apparatus to be thermostatically controlled. By controlling the amounts of combustion air allowed into the combustion chambers, combustion can be initiated and halted. When the supply of combustion air is discontinued, combustion of the solid fuel ceases completely. However, the heat energy absorbed by the material comprising the combustion chambers and divider maintains the internal temperature of the combustion chambers well above the flash point temperature of the solid fuel for an extended period of time. Upon the reintroduction of combustion air into the super-heated combustion chambers the solid fuel and combustion gases automatically re-ignite and combustion continues. The introduction and discontinuation of combustion air into the combustion chambers may be controlled by a thermostat, thus allowing the combustion apparatus to maintain an ease of operability similar to oil or gas fueled furnaces and boilers. 
         [0023]    The present invention also employs several design features to improve the safety of its operation. A typical danger with wood fired heating systems is the uncontrolled provision of heat to a heat exchanger. If too much heat is provided to a heat exchanger, for example during a power failure whereby the heat exchange medium within the heat exchanger (typically a liquid) stops circulating, the heat exchanger may overheat and the heat exchange medium contained therein may pressurize beyond the capability of the heat exchanger to contain it, causing a rupture of the system and a potential danger to any bystanders. The present invention minimizes the potential of overheating the heat exchanger by setting the default state of the apparatus to prevent introduction of combustion air into the combustion chambers. Thus, if there is a power interruption, the apparatus is designed to automatically cut off combustion air from the combustion chambers, causing further combustion to cease. As no further heat energy is added into the system, the heat exchanger does not receive additional heat energy and further dangerous pressurization of the heat exchange medium does not occur. Another design feature requires the heated exhaust gases to travel at least in part in a downward direction from the combustion chambers to the heat exchanger. Upon a power failure, the apparatus will cease providing the forces necessary to move the exhaust gases downward, so the hot gases will naturally rise and thus be prevented from reaching the heat exchanger. An additional feature of the present invention is a direct communication between the primary combustion chamber and the exhaust structure, controlled by a bypass damper. The bypass damper is designed to be opened when the access door to the primary combustion chamber is opened and to be closed when the access door to the primary combustion chamber is closed, thereby automatically releasing hot combustion gases out of the combustion chambers when a user accesses the primary combustion chamber, preventing a flash ignition in the primary combustion chamber. These safety features represent improvements over the known art in the safe operation of the apparatus. 
         [0024]    Other features and advantages of the invention are described below. 
     
     
       DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a perspective front view of the present invention. 
           [0026]      FIG. 2  is an identical view of the present invention as shown in  FIG. 1 , with the outer walls of the housing and insulation removed to depict the underlying structures. 
           [0027]      FIG. 3  is a cut-away perspective view of the present invention, depicting portions of the combustion chambers. 
           [0028]      FIG. 4  is an identical view of the present invention as shown in  FIG. 3 , with portions of the outer walls of the combustion chambers removed to depict the interior structures thereof. 
           [0029]      FIG. 5  is a perspective rear view of the present invention. 
           [0030]      FIG. 6  is an identical view of the present invention as shown in  FIG. 5 , with the outer walls of the housing and insulation removed to depict the underlying structures. 
           [0031]      FIG. 7  is a cut-away view of the present invention as shown in  FIG. 6 , depicting the relationship of the heat exchanger to the combustion chambers. 
           [0032]      FIG. 8  is side section view of the present invention. 
           [0033]      FIG. 9  is a perspective section view of the present invention. 
           [0034]      FIG. 10  is a side plan view of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    The combustion apparatus  1  of the present invention is comprised of an insulated housing  10 , a primary combustion chamber  20 , a secondary combustion chamber  22 , a divider  30  disposed between and separating the primary and secondary combustion chambers  20 , 22 , an oxygen introduction mechanism  40 , a pressurizing mechanism  50 , and an exhaust structure  60 . 
         [0036]    The housing  10  of the present invention is constructed of a heavy-duty metallic material, such as steel or cast iron, though other materials may also be used. It comprises insulation within its interior sufficient to allow for safe contact of the outer surface during operation of the combustion apparatus  1 . The housing  10  defines an internal space in which the other elements are located, though portions of the pressurizing mechanism  50  and the exhaust structure  60  extend outside the housing  10 . See  FIGS. 1 ,  5 ,  8 ,  9 , and  10 . One or more access doors  80  may be provided to allow access into the interior of the housing  10 . See  FIGS. 1 ,  8 , and  10 . 
         [0037]    The primary combustion chamber  20  is a contained space located within the housing  10 , suitably adapted to contain a quantity of solid fuel  5  and gases and to allow for the combustion of the fuel  5  and gases. See  FIGS. 2 ,  3 ,  7 ,  8  and  9 . The primary combustion chamber  20  may have separate walls defining the contained space, or the contained space may be defined by the walls of the housing  10 , or by a combination thereof. In the preferred embodiment the primary combustion chamber  20  occupies most of the interior of the housing  10 . The secondary combustion chamber  22  is a contained space located within the housing  10 , suitably adapted to contain gases and to allow for the combustion of gases. See  FIGS. 2 ,  3 ,  4 ,  8 , and  9 . The secondary combustion chamber  22  may have separate walls defining the contained space, or the contained space may be defined by the walls of the housing  10 , or by a combination thereof. In the preferred embodiment the secondary combustion chamber  22  has a substantially smaller interior than that of the primary combustion chamber  20 . See  FIGS. 3 and 8 . The secondary combustion chamber  22  is located adjacent to the primary combustion chamber  20  and is separated from the primary combustion chamber  20  by the divider  30 . See  FIG. 8 . While the primary and secondary combustion chambers  20 , 22  may have any orientation with respect to each other, for example, they may be placed side by side, in the preferred embodiment the primary combustion chamber  20  is oriented above the secondary combustion chamber  22 , with the divider  30  forming the floor of the primary combustion chamber  20  and the roof of the secondary combustion chamber  22 . The region of the secondary combustion chamber  22  adjacent to the divider  30  is the burn region  24 . See  FIG. 8 . In the preferred embodiment having a vertical orientation of the combustion chambers  20 , 22 , the burn region  24  of the secondary combustion chamber  22  is located directly beneath the divider  30 . The divider  30  is gas permeable, so that gases may pass from the primary combustion chamber  20  into the secondary combustion chamber  22 . The primary and secondary combustion chambers  20 , 22 , in combination, contain all combustion activities during operation of the combustion apparatus  1 . 
         [0038]    One or more of the housing&#39;s  10  access doors  80  allow access into the primary combustion chamber  20 , and one or more of the housing&#39;s  10  access doors  80  allow access into the secondary combustion chamber  22 . Fuel  5  may be loaded into the primary combustion chamber  20  through the access doors  80 , and residue from combustion, such as fine ash, may be removed from the combustion chambers  20 , 22  through the access doors  80 . 
         [0039]    The divider  30  between the primary and secondary combustion chambers  20 , 22  has a first surface  32  located within the primary combustion chamber  20 , a second surface  34  located within the secondary combustion chamber  22 , and at least one aperture  36  passing completely through it from the first surface  32  to the second surface  34 , resulting in the primary combustion chamber  20  and the secondary combustion chamber  22  being in communication with each other through the aperture or apertures  36 . See  FIGS. 4 ,  8 , and  9 . The divider  30  must be constructed of a material suitably adapted to absorb and retain heat energy at temperatures exceeding the flash point of the fuel  5 . Exemplary materials for the divider  30  may be cast iron or stainless steel. However, in the preferred embodiment, the divider  30  is constructed of a castable refractory ceramic material. Castable refractory ceramic is both durable and has the ability to retain heat energy for an extended period of time. The longer that the temperature within the combustion chambers  20 , 22  is maintained above the flash point of the fuel  5 , the longer the period of time the combustion apparatus  1  can re-ignite the fuel  5  after cessation of combustion. 
         [0040]    The divider  30  may have any shape or configuration suitable for separating the primary and secondary combustion chambers  20 , 22 , as described above. In one embodiment the divider  30  may be formed of a plurality of intersecting rods, forming a grate. In another embodiment the divider  30  may be a single planar member having one or more apertures formed through it. In the preferred embodiment of the present invention, the divider  30  is comprised of multiple elongated rectangular bricks  38  placed side by side, each brick  38  placed proximate to another brick  38  such that a gap exists between the pair, with each gap representing an aperture  36  of the divider  30 . See  FIGS. 4 ,  8 , and  9 . The gaps between the bricks  38  should be relatively narrow, between one eighth inch and two inches. The bricks  38  themselves may be of any suitable thickness, though it is recommended that they have a thickness of between two and ten inches. The thicknesses across all bricks  38  should be substantially uniform. In the preferred embodiment the sides of the bricks  38  are somewhat tapered in a downward direction, so that the gaps between pairs of bricks  38  widen from the first surface  32  of the divider  30  to the second surface  34  of the divider  30 . In the most preferred embodiment the bricks  38  are constructed of castable refractory ceramic. 
         [0041]    Both the primary combustion chamber  20  and the secondary combustion chamber  22  may be lined with castable refractory ceramic material. In the preferred embodiment the primary combustion chamber  20  is lined along one or more of its vertical walls for a portion of the height of those walls, beginning from the bottom of the chamber  20  and extending upwards, and the secondary combustion chamber  22  is lined along one or more of its vertical walls, beginning from the top of the chamber  22  and extending downwards. See  FIGS. 3 ,  4 ,  7 , and  8 . Use of castable refractory ceramic liners  26  in the combustion chambers  20 , 22  increases the overall amount of heat energy which may be absorbed and retained by the combustion apparatus  1 , thereby extending the amount of time that automatic re-ignition can occur. In the most preferred embodiment of the present invention, using a divider  30  and chamber liners  26  constructed of castable refractory ceramic, re-ignition of the fuel  5  has been achieved more than forty-eight hours after cessation of combustion. 
         [0042]    The oxygen introduction mechanism  40  of the present invention is suitably adapted to direct oxygen into the primary and secondary combustion chambers  20 , 22  in a controlled manner. See  FIGS. 6 ,  9  and  10 . In the preferred embodiment the oxygen introduced into the combustion chambers  20 , 22  is a component of ambient air, though it is contemplated that pure oxygen or oxygen mixed with other gases could also be used. The oxygen introduction mechanism  40  comprises duct work  48  and at least one primary inlet  42  and at least one secondary inlet  44  attached to the duct work  48 . See  FIGS. 2 ,  6 ,  8 ,  9 , and  10 . In the preferred embodiment the duct work  48  is located substantially exterior to the primary combustion chamber  20 , with a portion of the duct work  48  located within the secondary combustion chamber  22 . In the most preferred embodiment, at least a portion of the duct work  48  is situated proximate to each access door  80 . See  FIG. 2 . This causes combustion air passing through the duct work  48  to absorb heat energy from the access doors  80 , preheating the combustion air for improved combustion efficiency, while also cooling the access doors  80 . 
         [0043]    A controlled amount of combustion air (comprised at least in part of oxygen) travels through the duct work  48  and through the primary and secondary inlets  42 , 44 . In the preferred embodiment the amount of combustion air passing through the primary inlets  42  and the secondary inlets  44 , respectively, is controlled by the length of duct work  48  the combustion air must pass through before arriving at the respective inlets  42 , 44 , in combination with the gas flow resistance of said ductwork  48  and inlets  42 , 44 . An additional or alternate mechanism for controlling the combustion air passing through the oxygen introduction mechanism  40  involves the use of one or more dampers  46  situated within the duct work  48  of the oxygen introduction mechanism  40 . See  FIGS. 6 and 10 . Each damper  46  is suitably adapted to be positioned in either an open position or a closed position, such that the open position of a damper  46  permits combustion air to pass through one or more of the inlets  42 , 44  and the closed position of a damper  46  prevents combustion air from passing through one or more of the inlets  42 , 44 . In the most preferred embodiment a single damper  46  is adapted to control the passage of combustion air through all of the inlets  42 , 44 . When the damper  46  in this most preferred embodiment is in the closed position no additional combustion air is permitted into either the primary combustion chamber  20  or the secondary combustion chamber  22 , thereby causing combustion to completely cease. When the damper  46  in this most preferred embodiment is in the open position combustion air is introduced into the combustion chambers  20 , 22  permitting initial ignition or re-ignition of the solid fuel  5  and continued combustion thereof. 
         [0044]    In the preferred embodiment the damper or dampers  46  of the oxygen introduction mechanism  40  are suitably adapted to automatically be positioned to the closed position when operation of the combustion apparatus  1  must cease due to unsuitable environmental conditions. This stops combustion, thereby preventing the buildup of heat in the system. Each damper  46  comprises a gravity-based mechanism which disposes the damper  46  to the closed position. Alternatively, a spring mechanism may be used to dispose the damper  46  to the closed position. The flow of combustion air, controlled by the pressurizing mechanism  50 , is sufficient to overcome the force of gravity on the damper  46  and disposes the damper  46  to the open position. During adverse environmental conditions, such as overheating, the pressurizing mechanism  50  can be stopped to completely halt the flow of combustion air into the duct work  48 , whereby gravity returns the damper  46  to the closed position. A loss of power to the combustion apparatus  1 , which could cause damage to the heating system if heat were to continue to be generated, would also stop the pressurizing mechanism  50  from causing air to flow into the duct work  48 , achieving the same result and a complete cessation of combustion. Alternatively, the damper or dampers  46  can be mechanically positioned to the closed position, by a manual lever or by an actuator. 
         [0045]    The primary inlet or inlets  42  are located in the primary combustion chamber  20  and are oriented to direct oxygen onto fuel  5  placed into the primary combustion chamber  20 . In the preferred embodiment there are multiple primary inlets  42 , with the primary inlets  42  disposed along at least two of the side walls of the primary combustion chamber  20 . See  FIGS. 3 and 4 . In the most preferred embodiment the primary inlets  42  are apertures formed through the liners  26  of the primary combustion chamber  20 , the apertures of the primary inlets  42  being in communication with the duct work  48 . See  FIGS. 2 ,  3 ,  4 ,  7 , and  9 . In other embodiments the primary inlets  42  may be metal nozzles depending from the duct work  48  and extending into the interior of the primary combustion chamber  20 . The primary inlets  42  should all be substantially the same distance from the floor of the primary combustion chamber  20 , with the preferred distance being between one inch and fourteen inches from the floor. 
         [0046]    The secondary inlet or inlets  44  are located in the secondary combustion chamber  22  and are oriented to direct oxygen into the burn region  24  of the secondary combustion chamber  22 . In the preferred embodiment there are multiple secondary inlets  44 , and in the most preferred embodiment the secondary inlets  44  are apertures formed bi-laterally into a horizontal extension of the duct work  48  located within the secondary combustion chamber  22  proximate to the divider  30 . See  FIG. 8 . The preferred distance between the secondary inlets  44  and the second surface  34  of the divider  30  is between one and three inches. Other embodiments of the secondary inlets  44  are also contemplated, for example metal nozzles depending from the duct work  48  and extending into the interior of the secondary combustion chamber  22 . 
         [0047]    The pressurizing mechanism  50  of the present invention is suitably adapted to supply positive pressure to the primary combustion chamber  20  to create a pressure differential between the gases contained in the primary combustion chamber  20  and the gases contained in the secondary combustion chamber  22 , such that the gases contained in the primary combustion chamber  20  are at a higher pressure relative to the gases contained in the secondary combustion chamber  22 . The pressure differential must be sufficient to cause gases contained in the primary combustion chamber  20  to flow through the aperture or apertures  36  of the divider  30  into the secondary combustion chamber  22 . In the most preferred embodiment, the pressure differential is between 0.005 and 0.030 inches of mercury as measured by the difference in pressures between the maximal values of the primary inlet or inlets  42  and secondary inlet or inlets  44 . During combustion this pressure differential causes the flow of combustion gases through the divider  30  in a downward direction, resulting in a downward burn in the burn region  24  of the secondary combustion chamber  22 . 
         [0048]    The pressurizing mechanism  50  may comprise any means for generating positive pressure to create the required pressure differential. In the preferred embodiment the pressurizing mechanism  50  comprises a low power electric fan. See  FIG. 10 . It is contemplated that such a fan would operate on standard alternating current. The combustion apparatus  1  may comprise a backup power source to provide electricity to the pressurizing mechanism  50  in the event of an interruption in a primary source of power to the pressurizing mechanism  50 , for example, a battery. In the most preferred embodiment the pressuring mechanism is integrated with the oxygen introduction mechanism  40 , whereby the electric fan supplies air directly into the duct work  48  of the oxygen introduction mechanism  40 . A shorter length of duct work  48  supplying the primary inlets  42  than the length of duct work  48  supplying the secondary inlets  44 , in combination with the gas flow resistance of said ductwork  48  and inlets  42 , 44 , allows a single fan to create the required differential pressure. In other embodiments, multiple fans may be used, with a more powerful fan supplying combustion air to the primary combustion chamber  20  and a less powerful fan supplying combustion air to the secondary combustion chamber  22 . In yet other embodiments, multiple fans may supply combustion air to each chamber. In any of the foregoing embodiments, the required pressurization is achieved by the one or more fans causing a greater quantity of combustion air to be introduced into the primary combustion chamber  20  than into the secondary combustion chamber  22  for a given period of time. Other embodiments of the pressurizing mechanism  50  are also contemplated, such as air pumps in connection with the primary combustion chamber  20 . In yet other embodiments the pressurizing mechanism  50  may be separate from the oxygen introduction mechanism  40 . 
         [0049]    The combustion apparatus  1  may comprise a thermostatic control device, such as a thermostat. The thermostatic control device must be suitably adapted to control the combustion of fuel  5  and gases contained within the combustion apparatus  1 , such that when the thermostatic control device calls for heat fuel  5  and gases within the combustion apparatus  1  are burned and when the thermostatic control device does not call for heat the burning of fuel  5  and gases within the combustion apparatus  1  ceases. This thermostatic control of combustion makes the combustion apparatus  1  more convenient to use and better regulates the ability of the combustion apparatus  1  to provide only desired amounts of heat to a heating system. This in turn yields better fuel efficiency. In the preferred embodiment, the thermostatic control device controls the operation of the oxygen introduction mechanism  40 , such that thermostatic control is achieved by depriving oxygen to extinguish combustion, and re-introducing oxygen to re-ignite combustion. No separate energy source is needed to re-ignite combustion, because the temperature within the combustion chambers  20 , 22  is above the flash point of the fuel  5 . In this embodiment, when the thermostatic control device calls for heat the thermostatic control device positions the damper or dampers  46  of the oxygen introduction mechanism  40  to the open position, allowing combustion air to be introduced into the combustion chambers  20 , 22 , and when the thermostatic control device does not call for heat the thermostatic control device positions the damper or dampers  46  of the oxygen introduction mechanism  40  to the closed position, preventing further introduction of combustion air into the combustion chambers  20 , 22 . In the most preferred embodiment, the use of a thermostatic control device in conjunction with the use of castable refractory ceramic material for the divider  30  and combustion chamber liners  26  ensures efficient extinguishment and re-ignition of combustion on an as-needed basis. 
         [0050]    The exhaust structure  60  of the present invention is suitably adapted to remove heated gases from the secondary combustion chamber  22 . It has a connection end  62  in communication with the secondary combustion chamber  22 , and a chimney which vents outside the housing  10  of the combustion apparatus  1 . See  FIGS. 3 ,  4 ,  5 ,  6 ,  7 ,  8 , and  10 . 
         [0051]    The combustion apparatus  1  may further comprise a bypass damper  90 . See  FIGS. 3 ,  4 ,  7 ,  8 , and  9 . The bypass damper  90  forms a closable communication between the primary combustion chamber  20  and the exhaust structure  60 . The bypass damper  90  is suitably adapted to be positioned in either an open position or a closed position, such that when in the open position the bypass damper  90  permits gases to pass from the primary combustion chamber  20  directly into the exhaust structure  60  and when in the closed position the bypass damper  90  prevents direct communication between the primary combustion chamber  20  and the exhaust structure  60 . The bypass damper  90  is further adapted to be positioned in its open position whenever an access door  80  to the primary combustion chamber  20  is opened. This safety feature allows the venting of combustion gases to reduce the risk of a flash ignition when an access door  80  is opened. 
         [0052]    The combustion apparatus  1  may also comprise a heat exchanger  70 . See  FIGS. 4 ,  7 ,  8 , and  9 . In the preferred embodiment the heat exchanger  70  is in communication with the exhaust structure  60 . In the most preferred embodiment the heat exchanger  70  is contained within the exhaust structure. High temperature gases exiting the secondary combustion chamber  22  via the exhaust structure  60  pass over the heat exchanger  70 , which in turn removes heat energy from the gases and delivers that heat energy to a heating system. Heat exchangers are well known in the art, and any style heat exchanger may be used with the present invention. In the preferred embodiment the heat exchanger  70  comprises closed loops of piping which contain a heat exchange medium, preferably a liquid, though a gaseous heat exchange medium is also contemplated. The heat exchange medium absorbs heat energy from the heated gases, then is circulated through the closed loop piping where it releases heat energy into a heating system. The heat exchange substance may be circulated by means of a circulating pump, as is well known in the art. 
         [0053]    In the preferred embodiment the heat exchanger  70  is located within the housing  10  but remotely from the secondary combustion chamber  22  in such a manner that heated gases moving from the secondary combustion chamber  22  via the exhaust structure  60  to the heat exchanger  70  must move at least partially in a downward direction prior to reaching the heat exchanger  70 . This is a safety feature, since the failure of a circulating pump could prevent the heat exchange medium from releasing heat energy, and an uncontrolled buildup of heat energy in the heat exchange medium could lead to a rupture of the closed loop piping. Moving the heated gases in a downward direction can only be achieved by applying a force to the gases, for example, the pressure differential created by the pressurizing mechanism  50  is sufficient to move the gases downward. However, upon a power failure, which would disable the circulating pump, the pressurizing mechanism  50  would also cease pressurizing the system, causing the damper or dampers  46  of the oxygen introduction mechanism  40  to return to the closed position, completely halting combustion. This prevents more heat energy from being created, and the heated gases already in the combustion apparatus  1  will naturally rise, moving away from the heat exchanger  70 . This dual safety design greatly reduces the risk of an undesirable overheating of the heat exchange medium of the heat exchanger  70 . 
         [0054]    In the most preferred embodiment the heat exchanger  70  runs horizontally along the bottom of the housing  10 , beneath the secondary combustion chamber  22 , and then extends vertically along the rear of the housing  10 . See  FIGS. 7 and 8 . The extended length of the heat exchanger  70  permits the heated gases to give up more heat energy as they pass over the heat exchanger  70  and are vented. This improves the overall efficiency of the system, allowing more heat to be extracted and less to be wasted as heated exhaust gases. 
         [0055]    The initial combustion of the solid fuel  5  occurs in the lower portions of the primary combustion chamber  20 . Combustion air combines with combustion gases released from the fuel  5  to feed the combustion process. A char fuel bed  7  is created from an initial preparatory combustion. The char fuel bed  7  is created from the lowest portions of the fuel  5  located at the bottom of the primary combustion chamber  20 . The top of the char fuel bed  7  is proximate to the primary inlets  42  of the oxygen introduction mechanism  40 . Gasification of the solid fuel  5  occurs above the char fuel bed  7 , releasing combustion gases which are burned in the initial combustion; these gases are then forced downward by the pressure differential through the char fuel bed  7  and through the apertures  36  of the divider  30  into the secondary combustion chamber  22 , becoming superheated in the process. The addition of a controlled amount of combustion air into the burn region  24  of the secondary combustion chamber  22  causes the superheated combustion gases to re-ignite in the burn region  24  of the secondary combustion chamber  22 , resulting in secondary burn achieving an extremely hot, clean, continuous blue flame. As a result of the two burns, the exhaust gases are extremely hot, reaching temperatures in excess of 2000° F., with very little particulate matter remaining. The gases then move past the heat exchanger  70  where heat energy is released into the heating system, and are then exhausted. 
         [0056]    Modifications and variations may be made to the disclosed embodiments of the present invention without departing from the subject or spirit of the present invention as defined in the following claims.