FIELD OF THE INVENTION The present invention relates to a biomass burner. In particular, the present invention relates to a biomass burner comprising a fuel and air supply system to provide a high efficiency burner for a maple syrup evaporator.
BACKGROUND OF THE INVENTION Maple syrup evaporators used for evaporating water contained in maple sap generally employ wood or oil burning heat sources for causing evaporation. One drawback of such burners is the high cost of the wood or oil fuel, their energy conversion inefficiencies, and environment impacts associated with the pollutants emitted during combustion of the fuel such as particulate matter, Nitrogen Oxide (NOx) and Sulphur Dioxide (SOx).
What is therefore needed, and one aspect of the present invention, is a burner for maple sap evaporators that uses biomass combustibles which are less costly, burn more efficiently and which generate the heat necessary to properly evaporate maple sap. Additionally, what is needed is a biomass burner that provides all the benefits of an oil burner, for instance autonomous operation, ease of use, automatic and constant supply of fuel, and easy ignition, which minimizes the carbon dioxide emissions and other pollutants to reduce environmental impacts on the surrounding environment.
SUMMARY OF THE INVENTION According to the present invention, there is provided a biomass burner for a maple sap evaporator comprising: a burner assembly comprising a base and a top for forming a combustion chamber therein, said top defining an entrance and an exit; a burn pit disposed within said base and defining a plurality of holes for allowing air to communicate between said combustion chamber and a primary air plenum formed between said base and an underside of said burn pit; an air supply conduit for supplying a primary air supply to said primary air plenum; a fuel conduit in communication with said combustion chamber for supplying a pellet fuel for combustion within said burn pit; a secondary plenum in pneumatic communication with said primary air plenum for receiving and cooling a portion of said primary air supply; a plurality of injectors in communication with said secondary plenum for injecting said cooled air into said combustion chamber; wherein when said pellet fuel undergoes combustion with said primary air supply, a generated heat is directed towards said exit by said injected cooled air.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a maple sap evaporator, according to a preferred embodiment of the present invention;
FIG. 2 is a cross-sectional side view of the maple sap evaporator of FIG. 1;
FIG. 3 is a rear perspective view of a burner assembly in accordance with an illustrative embodiment of the present invention;
FIG. 4 is a front perspective view of a burn pit insert of the burner assembly of FIG. 3 in accordance with an illustrative embodiment of the present invention;
FIG. 5 is a cross-sectional side view of the burner assembly of FIG. 3;
FIG. 6 is a front perspective view of the burner assembly of FIG. 3; and
FIG. 7 is a cross-sectional view along line 7 of burner assembly of FIG. 3.
DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS The present invention is illustrated in further details by the following non-limiting examples.
Now referring to FIG. 1, there is provided an evaporator for the evaporation of maple sap in the production of maple syrup generally referred to using the reference numeral 10. The evaporator comprises a maple sap evaporating section 12 and a maple syrup forming section 14 both illustratively comprising horizontally extending pans 16, which are shown in FIG. 2, provided with partitions in which maple sap travels. The evaporator 10 further comprises a hood as in 18 for each section 12, 14 and vapour outlets as in 20 for directing the water vapour evaporated from the sap to atmosphere.
Now referring to FIG. 2, in addition to FIG. 1, extending beneath the maple sap evaporating section 12 and the maple syrup forming section 14 is a heating housing 22 comprising a heating chamber 24 for exposing heated air 26 and flames 28 to the undersides of the horizontally extending pans 16. The heating housing 22 is illustratively formed from an enclosed space beneath the horizontally extending pans 16 enclosed by sheets of stainless steel or the like. As the undersides of the horizontally extending pans 16 are heated, heat is transferred to maple sap 30 travelling along its upper surface of the horizontally extending pan 16 to cause its evaporation. In accordance with an illustrative embodiment of the present invention, the heated air 26 and flames 28 are generated by a biomass burner 32 illustratively connected to the front of the evaporator 10.
Still referring to FIG. 2, the biomass burner 32 comprises a housing 34 enclosing a burner assembly 36. A front 38 of the housing 34 comprises an access opening 39 capable of being enclosed by a door 40, and a rear 42 connected to the heating housing 22. Illustratively, the housing 34 and the door 40 are formed from a double layered walled 44 structure formed from stainless steel or the like and lined with or enclosing an insulating material, such as high temperature insulation wool like ceramic fibre, amorphous Alkaline Earth Silicate (AES) wool, Aluminum Silicate Wool (ASW), or the like, and which is capable of resisting high temperatures generated within the burner assembly 36. The access opening 39 in the housing 34 is covered by the hinged door 40 to provide a user access to the burner assembly 36 for cleaning, repair, ignition, and maintenance when opened, and for confining the heat and flames 28 to within the burner assembly 36 when closed. The rear 42 of the housing 34 is in open connection with the heating chamber 24 such that heat and flames 28 generated within the burner assembly 36 are directed to within heating chamber 24, in a manner as will be described hereinbelow, for the heating of the horizontally extending pans 16.
Still referring to FIG. 2, the burner assembly 36 generates heat from the combustion of combustible materials burned therein, such as wood pellets, vegetable matter, agri-pellets or other forms of pellet fuel 46 which may be employed for heating the air 26 in the heating chamber 24 such that an underside 48 of the horizontally extending pans 16 is heated. Illustratively, the pellet fuel 46 is typically a wood fuel generally comprised of saw dust as is known in the art which permits high combustion efficiency. The high density and compact size of the wood pellets allows for space efficient storage and further allows for precise control of the amount of fuel able to be supplied to the burner assembly 36 for combustion therein. A plurality of steps 50 are provided as part of the heating chamber 24 to direct the heated air 26 upwardly and to generate turbulence so that the underside 48 of the horizontally extending pans 16 is evenly heated. In addition to heat, the burner assembly 36 comprises combustion by-products such as carbon monoxide and the like which are directed and expelled from the heating chamber 24 to atmosphere via a chimney 52.
Now referring to FIG. 3, in addition to FIG. 2, the burner assembly 36 illustratively comprises a structure formed from a base 54 and a semi-cylindrical dome 56 connected to the top of the base 54 by welding, rivets, screws or the like. The semi-cylindrical dome 56 comprises an open front end 60 for communication with the access front opening 39 and which is enclosed by the door 40. The semi-cylindrical dome 56 further comprises an open rear 58 in open communication with the heating chamber 24 for providing a direct exhaust channel from the burner assembly 36 into the heating chamber 24. The base 54 is illustratively comprised of a solid box housing 62 for receiving a burner insert 64 at its open top. The space defined above the burner insert 64 and below the dome 56 is the combustion chamber 66 where pellet fuel 46 undergoes combustion in a manner as described hereinbelow. The box housing 62 and the dome 56 are illustratively formed from bent stainless steel sheets or the like and may be heat insulated with a ceramic wool (not shown) tacked to their outer surfaces to help reduce radiated heat loss to the exterior of the biomass burner 32. For instance, the ceramic wool may be affixed to the inner surface of the base 54 and the dome 56 by tack welding at intermediate points along the inner surface or may be contained within a doubled walled layer structure of the base 54 and dome 56 if provided. The provision of insulation ensures a thermal resistance against the deformation of the dome 56 and the base 54 caused by the intense heat generated during a high burn intensity operation of the biomass burner 32. Of note, such construction of the burner assembly 36 also prevents expansion of the burner assembly 36 when subjected to intense heat and remains free of deformation once cooled during inoperation.
Now referring to FIG. 4, in addition to FIG. 2 and FIG. 3, the burner insert 64 comprises a perforated box like structure formed from bent plate sheet metal to form a burn pit 68 for containing the pellet fuel 46 during combustion thereof. Illustratively, the burner insert 64 is fabricated from a heat resistant material such as nickel, stainless steel, cast metal or the like capable of resisting deformation between operation and inoperation of the biomass burner 32. The burn pit 68 is configured and sized sufficiently large enough to hold the pellet fuel 46 required for a maximum heat output given the size of the evaporator 10. Of note, the sizing of the burn pit 68 and the biomass burner 32 may be varied depending on the application.
Now referring to FIG. 5, in addition to FIG. 3 and FIG. 4, a plurality of holes 70 formed in the walls 72 and floor 74 of the bent plate forming the burn pit 68 provide air communication between a primary air plenum 76 formed between the walls 72 and floor 74 of the burn pit 68 and the inner sides of the box housing 62. The holes 70 ensure that a primary air supply 78 from within the primary plenum 76 are preferably evenly distributed across the pellet fuel 46 deposited within the burn pit 68 in a uniform manner so as to create an equal rate of combustion of the pellet fuel 46. With this configuration, the air holes 70 are provided in a patterned configuration to ensure sufficient amounts of oxygen are supplied to the combustion chamber 66 such that an efficient burn of the pellet fuel 46 results.
Still referring to FIG. 4 and FIG. 5, pellet fuel 46 is supplied to the burn pit 68 via a fuel supply inlet 80 provided for in the side wall 72 of the burner insert 64. As pellet fuel 46 is received therein, a hollow divider 82 projecting upwardly from the floor 74 of the burner insert 64 and also comprising a plurality of holes as in 70 acts to distribute the pellet fuel 46 received from the fuel supply inlet 80 into two separate sub-burn pits 84 formed by the hollow divider 82 and adjacent walls 72. This W cross-sectional shape of the burn pit 68 advantageously improves the distribution and surface exposure of the pellet fuel 46 for a maximum exposure to oxygen from the primary air supply 78 required to generate a maximum energy output and rapid combustion. The hollow divider 82 is illustratively formed by a bend in the burner insert 64 plate which also increases the rigidity of the structure to resist deformation under exposure to the intense heat produced during combustion. Those skilled in the art will understand that other configurations besides W cross-sectional shape of the burn pit 68 could be provided For example, further projections having different shapes and configurations may be provided to increase the distribution and surface exposure of the pellet fuel 46. On the other hand, the number and shape of these projections would reduce the amount of pellet fuel 46 that is available in the burn pit 68, therefore these need to be designed according to design requirements.
Still referring to FIG. 4 and FIG. 5, the double combustion burn pit configuration maximises the combustion and the quality of the combustion of the fuel pellets 46 by maximizing the combustion area of the pellets supplied with oxygen. Of note, the combustion efficiency of the burner assembly 36 can be controlled by regulating the amount of primary air supply 78 entering the primary plenum 76 which in turn passes through the air supply holes 70. Additionally, while one hollow divider 82 has been illustratively shown, additional dividers may also be provided for improving combustion. The hollow divider 82 also promotes combustion by ensuring that all points within each sub-burn pit 84 are supplied with oxygen with the primary air supply 78.
Still referring to FIG. 5, in addition to FIG. 2 and FIG. 3, there is further provided a conical cone 86 connected to the open rear 58 for directing heated air 26 and flame 28 from the combustion chamber 66 into the heating chamber 24. Advantageously, the conical cone 86 also acts to increase the air pressure within the combustion chamber 66 which aids in directing or funnelling of the combustion flames 28 to within the heating chamber 24. Illustratively, the cone 86 directs the flames 28 are funnelled at a generally horizontal direction from the combustion chamber 66 and directed into the heating chamber 24 for contact with the underside 48 of the horizontally extending pans 16 and for heating the air 26 contained therein. The increase in pressure also acts to increase the rate at which exhaust and flames 28 are expelled from the burner assembly 36 and into the heating chamber 24. The inner surface 88 of the conical cone 86 is illustratively fabricated from heat resistant reflective material which minimizes the heat retention within the combustion chamber 66 and promotes the transfer of energy to within the heating chamber 24. Additionally, the cone 86 may be of variable dimension so as to modulate the size of its opening as depending on the type and the length of the required flame 28.
Referring back to FIG. 2, in addition to FIG. 1, FIG. 3 and FIG. 5, the base 54 of the burner assembly 36 further comprises a plurality of inlets for receiving air and fuel. Illustratively, the air and fuel are illustratively provided to the burner assembly 36 by two air conduits 90 and a fuel conduit 92, respectively, which originate from an area beneath the heating chamber 24 enclosed within the heating housing 22. Illustratively, air is supplied to the primary plenum 76 via air conduits 90 through inlets 94 in the side of the base 54. Air may be illustratively drawn from the atmosphere exterior to the evaporator 10 by one or more electrically powered air blowers 96 such as a fan or the like connected to an air intake 98 on the side of evaporator 10. Air supply to the primary plenum 76 can be illustratively controlled by controlling the operation of the of the air blowers 96 or by providing a choke valve 100 or the like for controlling the air supply to the combustion chamber 66 and in turn for controlling the burn rate of the pellet fuel 46. The amount of air within each air conduit 90 can be individually controlled via the control of the air blower 96 or the choke valve 100, either manually or automatically by a feedback controlled system (not shown) for example.
Now referring to FIG. 6, in addition to FIG. 2 and FIG. 5, air received from the air conduits 90 in the first plenum 76 also supplies a secondary plenum 102 formed within the door 40 via second air inlets as in 104 (illustratively, four inlets 104 are shown) for pneumatically connecting the primary plenum 76 with the secondary plenum 102. When primary air supply 78 enters the secondary plenum 102 it is cooled therein before being forced or injected into combustion chamber 66 at a point illustratively above burn pit 68 via a plurality of injectors 106 formed from holes provided for within the inner side of the door 40. The injectors 106 may be formed from projecting tubes (not shown) in the inside of the door 40 and projecting into the combustion chamber 66 for directing secondary air 108 directly into the combustion chamber 66 at specific directions. The injectors 106 provide a stream of secondary air supply 108 to further contribute to the combustion process within the combustion chamber 66 and to create the turbulence of the air within the combustion chamber 66 to further complete the combustion of non-combusted gas of the burn process. Illustratively, the injectors 106 are strategically sized and placed within the door 40 so as distribute the secondary air 108 to the areas within the combustion chamber 66 which favour a complete combustion. Of note, additional channels (not shown) carrying secondary air 108 may also be directed over the cone 86 to help cool it and to further provide oxygen for combustion at the flame as it exits the burner assembly 36.
Still referring to FIG. 5 and FIG. 6, prior to the secondary air 108 passing into the combustion chamber 66, the secondary air 108 undergoes a cooling as the door 40 and the secondary plenum 102 is thermally insulated from the combustion chamber 66. Ceramic wool may be provided on the interior of the door 40 to reflect heat from the combustion chamber 66. In addition to providing a source of secondary oxygen for completing the burn process within the combustion chamber 66, the configuration of the injectors 106 aids in controlling the direction of the flame 28 and heated gas towards the open rear 58 through the cone 86 and towards the heating chamber 24. A plurality of windows 110 may be provided for in the door 40 for viewing the combustion chamber 66 for monitoring the combustion of pellet fuel 46.
Now referring back to FIG. 2, FIG. 3 and FIG. 5, the fuel conduit 92 supplies pellet fuel 46 to the burn pit 68 via the fuel supply inlet 80 and a fuel inlet 112 in the side of the base 54. Illustratively, pellet fuel 46 is supplied to the burn pit 68 by an auger screw 114 provided within the fuel conduit 92 to form a screw conveyor in communication with a fuel storage bay 116. The auger screw 114 comprises a pitch spacing so as to allow a precise control of fuel for combustion to be supplied. Fuel pellets 46 may be supplied to the fuel storage bay 116 by a hopper 118 provided for on the side of the evaporator 10. The rotation of the auger screw 114 transports fuel pellets 46 from the fuel storage bay 116 through the fuel conduit 92 and to the burn pit 68 in a manner as is generally known in the art of variable rate feeders. The rotational speed of the auger screw 114 is illustratively controlled by the modulation of an electric motor 120 either by a user manually controlling the RPM of the motor via a control dial (not shown) or the like or by a feedback control system (also not shown). The feedback control system can automatically provide for an accurate control of the heat generation by the biomass burner 32 by monitoring the consumption of the fuel pellets 46, the level of oxygen in the combustion exhaust in the heated air 26, the burn temperature within the heating chamber 24 and other parameters collected at various points of the evaporator 10. Such feedback control also provides an automatic fuel supply and precision control of fuel to the combustion chamber 66. By regulating the supply of air and fuel to the interior of the combustion chamber 66 in such a manner the amount of heat generated by the biomass burner 32 can be accurately controlled.
Now referring back to FIG. 2 and FIG. 5, in operation of the burner assembly 36, the auger screw conveyor 114 feeds fuel pellets 46 from the fuel storage bay 116 to within the burn pit 68 wherein the fuel pellets 46 are divided into two separate sub-fire pits 84 by the hollow divider 82. The air blowers 96 supply air to the primary plenum 76 and secondary plenum 102 for supplying the fuel pellets with the required oxygen for combustion. The fuel pellets 46 may be illustratively ignited by a user accessing the burn pit 68 via the door 40 which is subsequently closed during operation, or by providing a small pilot flame assembly (not shown) therein. As a result of the combustion, a first burn of the fuel pellets 46 occurs within the burn pit 68 by oxygen supplied from the primary air supply 78 passing through the holes 70.
A second burn of the non-combusted gases occurs in the upper portion of the combustion chamber 66 from oxygen supplied from the cooled secondary air 108 supplied via the injectors 106. The forced secondary air 108 additionally directs the flame 28 and gases towards the cone 86 which funnels and accelerates the flame 28 and exhaust gases towards the heating chamber 24 where heated air 26 and flames 28 heat the underside 48 of the horizontally extending pans 16. As a result, intense heat reaching upwards of 2000 degrees Fahrenheit at the exit of the burner assembly 36 is generated. Air and combustion exhaust in the heating chamber 24 is eventually expelled via the chimney 52. Advantageously, the feeding of pellet fuel 46 by the auger 114 and the supply of air by the air blowers 96 provide a controlled and stable source of heat for the evaporator 10. The high intensity burn of the fuel pellets 46 within the combustion chamber 66 advantageously results in a minimum amount of burn residue, a reduced emission of NOx, SOx, and volatile organic compounds, while providing an energy release comparable to that of oil at a fraction of the cost, in addition to a controllability of the burn process that is comparable to that of oil.
Of note, while the biomass burner 32 has been illustrated as a heat source to the maple sap evaporator 10, the biomass burner 32 may also be employed for other furnace use applications. Additionally, while a single biomass burner 32 has been illustrated, the maple sap evaporator 10 can utilize one or more burners to generate the heat needed for different capacity equipment which serves to evaporate water from maple sap.
Although the present invention has been described hereinabove by way of specific embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention as defined by the appended claims.
