Abstract:
The present invention provides a burner which uses solid fuels, especially waste plastic fuels. Burner size is minimized by having multiple combustion chambers concentrically located around a rotating screw conveyor. Heat efficiency is improved by having air passages disposed around the combustion chambers, thus preheating air for the combustion prior to its delivery to the combustion chambers, while simultaneously thermally insulating the combustion chambers against the environment. Waste plastic is transported from a fuel hopper to the combustion chambers by a rotating screw conveyor having the spiraling auger blades. Speed of screw conveyor rotation controls the consumption of waste plastic and, thus, the amount of thermal energy generated in the burner.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to solid fuel burners, especially burners for waste plastic. Considerable research effort has been invested toward finding methods of converting waste plastics to usable solid fuels as a means of plastic recycling. Waste plastics are burned to generate heat, which may be used for water heating, industrial heat, or other purposes. Important considerations related to waste plastics as fuel sources are: maximizing energy by burning the solid fuel completely, minimizing heat losses to the environment, compactness of the burner, and minimizing soot and harmful gases emission. 
     Some existing waste fuel burners have multiple combustion chambers, which improves the completeness of the burning, but the combustion chambers are arranged one after another, therefore resulting in a long burner and significant heat losses due to the exposed outer surfaces. 
     Other existing waste fuel burners accumulate ash, soil, and sand during the burning process. These burners have to be periodically stopped for the removal of accumulated non-combustible material. 
     There is therefore a need for solid waste burners that minimize burner size and heat losses, while maximizing the completeness of fuel burning. The burner should also minimize soot and harmful gases emission, while reducing the accumulation of the non-combustible material inside the burner. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates generally to burners that use solid fuels, especially waste plastic fuels. Burner size is minimized by having multiple combustion chambers arranged concentrically around a rotating screw conveyor. Heat efficiency is improved by having an air chamber disposed around the combustion chambers, because the air for the combustion is preheated prior to being delivered to the combustion chambers, while the air chamber at the same time thermally insulates the combustion chambers against the environment. Waste plastic is transported from a fuel hopper to the combustion chambers by a rotating screw conveyor having spiraling auger blades. Speed of the screw conveyor rotation controls the consumption of waste plastic and, hence, the amount of thermal energy generated in the burner. Parts of the combustion chambers can also rotate to auger waste plastic for better oxidation, therefore enhancing the combustion process. 
     In one embodiment, a burner for waste plastics has a rotatable feed mechanism for directing the waste plastic to a combustion unit. The burner has first, second and third combustion chambers substantially coaxially arranged with respect to the rotatable feed mechanism, the first combustion chamber being arranged to receive waste plastic from the feed mechanism. The burner has an outlet for discharging exhaust materials from the third combustion chamber. 
     In one aspect, the combustion chambers have approximately like axial extents, and the first through third combustion chambers are arranged radially one after the other so that the combustion unit has an overall axial length in the direction of the feed conveyor approximately equal to the lengths of an individual combustion chamber. 
     In another aspect, the burner includes radially oriented end walls arranged between adjacent combustion chambers and spaced apart from respective ends of the tubular walls for generating an S-shaped flow of combustible materials, combustion air, smoke and particulates from the first through the third combustion chambers. 
     In yet another aspect, the burner includes a screw conveyor having a hollow interior extending axially along the conveyor and into the combustion unit for directing combustion air to the combustion unit, and one or more orifices disposed radially from the longitudinal hollow interior and axially located on the screw conveyor so that the orifices discharge air to at least one combustion chamber. 
     In another aspect, the burner includes a housing surrounding the feed mechanism and the combustion unit including an air flow passage extending over substantially the full extent of the housing, thus insulating the burner against the thermal losses to the environment. 
     In yet another aspect, the apparatus for solid waste burning further has an auxiliary burner configured to start burning of waste fuel. 
     In another aspect, the apparatus for solid waste burning further has a fuel hopper configured to provide waste fuel to the auger shaft. 
     For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show partial sectional plan and right views, respectively, of one embodiment of the invention. 
         FIG. 2  shows a side sectional view of another embodiment of the invention. 
         FIG. 3  shows a detail sectional view of the fuel supply unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1A ,  1 B, and  2 , A burner  1  for waste material, particularly plastic waste, has a feed mechanism  2  defined by an elongated transport or conveyor screw  4  provided with a central lumen  6  extending substantially through the entire length of the transport screw. The screw is situated below an intake opening  8  of the feed mechanism and extends forwardly (to the right as seen in  FIGS. 1A and 2 ) to a combustion unit  10  defined by a plurality of combustion chambers, for example, first, second and third combustion chambers  12 ,  14  and  16  that are coaxially disposed about conveyor screw  4  at an increasing radial distance from the conveyor screw. The downstream end of the third combustion chamber is in fluid communication with a discharge section  18  of the burner, which receives smoke and incombustible particulates from the combustion chambers and discharges them from the burner into the atmosphere. A double walled outer housing  20  defines an air passages  48 , which surrounds a portion of the transport screw  4 , the combustion unit  10  and the discharge section  18 . 
     Feed mechanism  2  of burner  1  includes the earlier mentioned conveyor screw  4  with spiral windings  5  and a generally tubular, double walled housing portion  22 , which surrounds the rotating screw  4 . A motor  24  drives a shaft  28  of the screw  4  via a chain  26 . Other suitable drives such as a gear drive, a belt drive or the like can be employed. 
     Intake opening  8  is arranged proximate to an upstream end of the screw (on the left as seen in  FIG. 2 ) through which plastic waste or other material is entrained for conveyance in a downstream direction (to the right as seen in  FIG. 2 ) towards combustion unit  10 . The downstream end of shaft  28  of screw  4  is free of spiral windings and extends into the combustion unit where it is suitably journaled. 
     Combustion unit  10  is formed by the three concentric combustion chambers  12 ,  14 ,  16 , each of which has inner and outer radial boundaries that are concentric with the axis of shaft  28  and interconnected by radially extending walls. In particular, the inside of first combustion chamber  12  is defined by the circumference of conveyor shaft  28  and its outside is defined by an extension  30  of the tubular housing portion  20  surrounding the conveyor screw. The inside of second combustion chamber  14  is defined by the exterior of the extension  30 , and the outside of the second chamber is defined by a tubular wall  34  that is coaxial with and spaced apart from extension  30 . An end wall  32  connected to and substantially perpendicular to tubular wall  34  is fixed to conveyor shaft  28 , is axially spaced from a downstream end of extension  30 , and forms a transition space between the first and second combustion chambers  12 ,  14 . Finally, an exterior surface of tubular wall  34  defines the inside of third combustion chamber  16 . The outside of the third combustion chamber is formed by the inside of housing  20 . The transition space between the second and third chambers is provided by radial air passage  47  of housing  20 . The downstream end of the third combustion chamber opens to discharge section  18  of the burner. 
     As is illustrated in  FIG. 1A , gaseous material, particulates and the like from first combustion chamber  12  move along an S-shaped line  38 , past the second and third combustion chambers, and into discharge section  18 . To facilitate movement of the materials through the combustion chamber, auger plates  40 , which are preferably inclined relative to the axis of shaft  28  to help advance the materials in a downstream direction, are suitably arranged on the inner radial surfaces of the first through third combustion chambers. In the embodiment illustrated in  FIG. 2 , the auger plates for the first and third combustion chambers rotate with shaft  28 , while the plates for the second chamber are stationary. Alternatively, the plates for the second chamber can be mounted on the inside of tubular wall  34  so that they, too, rotate with the shaft. Waste plastic, in particular, plastic waste, introduced through intake opening  8 , is moved in a downstream direction (to the right as seen in  FIGS. 1A and 2 ) and it enters first combustion chamber  12 . Auger plates  40  in the first chamber distribute the material relatively evenly where it is liquefied, gasified and ignited by heat generated by friction and heat transfer via tubular wall  34 . The resulting partially combusted waste plastic together with flames, smoke and other particulates generated in the first combustion chamber propagates in a downstream direction through second and third combustion chambers  14 ,  16  where the waste plastic burns so that substantially only smoke, gaseous matter and non-combustible particulates are then discharged into the discharge section  18  of the burner. Rotational discharge blades  19  swirl the exhaust gas flow, thus improving a flush-out of the incombustible materials from the burner. The discharge blades which provide a sufficient swirling of the incombustibles may be made in different shapes. One example is a substantially propeller shaped discharge blade. 
     A particular advantage provided by the waste burner of the present invention is that fresh combustion air is provided just upstream of each of the combustion chambers. Complete incineration of all the waste plastic takes time, thus feeding just sufficient air at the upstream end of each chamber helps to sustain optimal combustion therein. This, in turn, helps to maintain maximum temperature in each chamber, because combustion air that is needed further downstream in the process, namely in the second and third combustion chambers, does not travel through the chambers where it is not needed and need not be heated. In addition, the flow of relatively cool combustion air along the outside of the housing enhances energy efficiency because the air flow reduces heat losses from the combustion unit to the atmosphere, while at the same time preheating the air needed for the combustion in the combustion chamber. 
     Referring now to  FIG. 2 , air for incinerating waste plastics is supplied from a suitable source (or sources) at an air inlet  44 , like, for example, a fan or a blower used to enhance the air intake. Air next enters inner air passage  46  defined by tubular double-walled housing portion  22 . Some of the air in passage  46  is released into the space for conveyor screw  4  from an orifice  50 , enters shaft lumen  6  via inlets  52 , and continues to flow in the direction of combustion unit  10 , while simultaneously cooling the transport screw, thus increasing the reliability of the screw and its bearings. The remainder of the air in the annular inner air passage  46  continues in a downstream direction and partially encircles first combustion chamber  12 . A radial air passage  47  fluidically connects axially extending inner air passage  46  with axially extending outer air passage  48 , which surrounds combustion unit  10  and discharge section  18  of the burner. 
     As shown in  FIG. 2 , air from the lumen is discharged via first, second and third sets of orifices  54  arranged, respectively, in the transition space between the first and second combustion chambers  12  and  14  and into discharge section  18  of the burner, as is further described below. Additionally, the air needed for burning the waste plastic is separately introduced into each of the three combustion chambers. Air flowing along air passage  46  is discharged into an upstream portion of first combustion chamber  12  via orifices  56 . A further set of housing orifices  58  is arranged upstream of third combustion chamber  16  and extends from air passage  46  into the transition space between the second and third combustion chambers  14 ,  16 . Instead of or in addition to orifices  58 , radial air passage  47  can be provided with additional orifices such as, for example, orifices  60  located just upstream of third combustion chamber  16 , as shown in  FIG. 1 . Air for the second combustion chamber  14  is introduced by the first set of orifices  54  (located on screw  4 ) into the transition space between the first and second combustion chambers and therefore also upstream of the second combustion chamber. The air flow through orifices  54 ,  56 ,  58  and  60  is suitably modulated to match the air flow rate to the amount of waste plastics introduced through intake opening  8 . 
     To facilitate the incineration of waste plastic, particularly during the startup operations, an auxiliary burner  62  in the transition space between the second and third combustion chambers  14 ,  16  for heating all three chambers, either directly (chamber  16 ) or indirectly (chamber  12  via housing section  30  extending into the combustion unit and chamber  14  via tubular wall  34 ). The auxiliary burner  62  may be oil burner, gas burner, solid fuel burner, or electrical heater. The inventors have found that using the auxiliary burner for about 5 minutes preheats the waste plastic sufficiently to efficiently start the combustion. 
     Referring now to  FIG. 3 , a waste plastic supply unit  150  can be attached to the intake opening  8 . Waste plastic is deposited in a fuel hopper  151 , wherefrom it is gravitationally fed into rotator housing  153 . A granular waste plastic is illustrated in the fuel hopper  151 , but other constitutions of the waste plastic are possible. Rotation of a rotator  154  directs waste plastic towards the intake opening, and further toward the transport screw (not shown). The inventors have found that the rotator protrusions  155  having a triangle or a semi-circular shape work wall, but other rotator protrusion shapes can also be used. The inventors have also found that inclining the rotator protrusions  155  in the direction opposite from the direction of their rotation minimizes sticking of the waste plastic against the rotator housing  153 . 
     The pollution emission of one embodiment of the invention was tested by the KTL (Korean Testing Laboratory) by measuring harmful gas emissions during the waste fuel burning. According to the tests, the dioxin level was 0.119 ng-TEQ/Sm3, the hydrogen chloride level was 0.78 ppm, and the sulfur oxides level was 6.60 ppm. Thus, these harmful gas emission levels were significantly below the Korean emission standard levels (dioxine: 5 ng-TEQ/Sm3, hydrogen chloride: 50 ppm, and sulfur oxides: 6.60 ppm), rendering the invention environmentally friendly. 
     The above description is illustrative and is not restrictive, and, as it will become apparent to those skilled in the art upon review of the disclosure, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. For example, while the above invention is described in conjunction with plastic waste fuel, the embodiments of the present invention can also be used with other solid fuels, waste or not, like, for example, coal, saw dust, wood chips, or a mixture of solid fuels. Furthermore, while three combustion chambers are described, a different number of combustion chambers may be used. These other embodiments are intended to be included within the spirit and scope of the present invention. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the following and pending claims along with their full scope of equivalents.