Abstract:
A burner for fuel in pellet or granular form in which the fuel has a high ash content, for example grass or hay pellets in which the ash content is 5% or more. The burner has a combustion chamber with a grate at its lower end, into which fuel and air are fed, the combustion chamber having a hot gas outlet for exhausting combustion gases. The grate is provided with a rotary member having upstanding vanes which is adapted to continually move ash or clinker on the grate through outlet apertures spaced around the grate. The grate is a generally planar, horizontal surface which is imperforate apart from the outlet apertures; air inlet ports are arranged in a circumferential wall surrounding the grate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a burner for combustion of fuel in pellet or granular form, and may be referred to as a “pellet burner” or “pellet furnace”. The invention is especially designed to operate with pellet or granular fuel of high ash content, for example grass or hay pellets. While the invention is particularly described in relation to a water heater or boiler, the invention may equally be applied to other combustion devices, e.g. a stove or space heater. 
     2. Prior Art 
     Pellet burners are known both for space heating and water heating. Usually, the pellets used are formed from wood, e.g. sawdust, and have an ash content usually less than 1%. When such pellets are burnt the ash is very light, and the combustion air fed to the burner, which is usually forced air provided by a fan, blows the ashes out of the top of the combustion chamber through a chimney. 
     It would be desirable to use pellets from other sources, e.g. made from plant material which is otherwise waste. Such materials however usually have an ash content of over 1%. For example, grass or hay pellets usually have over 5% ash content, and may be contaminated with sand and potassium chloride which produce glassy materials, in the form of clinkers or slag, when it burns. Such combustion products, which are usually heavier than the light hay or grass pellets, build up on a conventional burner grate and may put out the fire after a few hours. The ashes would have to be cleaned out too often for such operation to be practical. 
     It has been proposed, for example in U.S. Pat. No. 6,336,449 to Drisdelle et al., to make a pellet burner with a special combustion chamber, and restricted air supply, and it is claimed that this burner is capable of efficiently burning fuel containing 10% ash without the formation of slag and clinkers. This patent also suggests the use of a special grate; this has no moving parts. 
     U.S. Pat. No. 5,893,358 and U.S. Patent Application No. 2007/0125282 both show pellet burners in which clinkers or slag are periodically removed by reciprocating means associated with the grate. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pellet burner capable of burning fuel containing at least 5%, and in some cases up to 7% of ash, including grasses, without the grate becoming clogged with clinkers or slag. 
     In accordance with the present invention, a burner for fuel in pellet or granular form comprises, a combustion chamber having a grate at its lower end; means for feeding the fuel into the combustion chamber; means for supplying air to the combustion chamber; a hot gas outlet for combustion gases from the combustion chamber; and means for transferring heat from the combustion chamber and/or said hot gas outlet to air or other fluid to be heated; wherein the grate is provided with a rotary member adapted to continually move ash or clinker on said grate through outlet apertures spaced around said grate. 
     The term “grate” is used in the sense of a surface which holds the burning fuel, and does not imply that this has openings for combustion air as is usual with grates. In fact, the grate of this invention may be unperforated except for the outlet apertures, and the combustion air enters the combustion chamber through air inlets spaced around a lower part of the combustion chamber which is termed the “burn pot”, these air inlets being above the level of the grate. 
     Preferably, the grate includes a generally planar surface provided with recesses over which the rotary member passes, the rotary member and recesses being such that ash or clinker particles may be broken into smaller particles by engagement of said particles in said recesses as the rotary member passes over the recesses and fragments the particles. 
     The planar surface of the grate is usually horizontal, and the rotary member rotates on a vertical shaft. The grate is preferably surrounded by a cylindrical wall which defines the burn pot, and the outlet apertures of the grate are spaced around outer edges of the grate near the cylindrical wall. The rotary member may include a generally horizontal bar mounted on a vertical shaft, and vanes projecting upwardly from an upper surface of the bar and angled to the bar in such way as to move ash outwardly towards the outlet apertures as the bar rotates. 
     The cylindrical wall of the burn pot may be provided with a spring-biased door through which large ash or clinker particles can be expelled by the rotary member. The vertical shaft may be rotated by a horizontal-axis motor via a bevel gear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which; 
         FIG. 1  is a longitudinal sectional elevation through a burner in accordance with the invention; 
         FIG. 2  is an enlarged sectional elevation through the lower part of the combustion chamber of the burner; 
         FIG. 3  is a plan view of the lower part of the combustion chamber taken on lines  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a view similar to  FIG. 1  of a modified burner. 
         FIG. 5  is a view similar to  FIG. 2  of the bottom portion of a slightly modified combustion chamber, and 
         FIG. 6  is a plan view of the combustion chamber part of  FIG. 5 ; 
         FIG. 7  is a view similar to  FIG. 1  of a further modified burner. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1 to 3 , the pellet burner, which is in the form of a water heater, has a support frame  10 , including a central part with legs  10   a  which support a casing  12  surrounding a combustion chamber  14 . The lower part of chamber  14  is in the form of a burn pot  16  having a cylindrical wall  18  of reduced diameter compared to the upper part of chamber  14 . Wall  18  is connected to the upper part of the combustion chamber  14  by an upwardly diverging wall portion  20  which forms the top of an annular inlet air chamber  22 , which chamber communicates with the burn pot  16  via air inlet ports  24  spaced around the wall  18 . Annular chamber  22  receives forced air for combustion from an air blower  26 . The bottom of the burn pot  16  is formed by a grate  28 , having a special ash removal mechanism described below. 
     This embodiment of burner being in the form of a water heater, an upper portion of the combustion chamber  14  is surrounded by a lower part of a water chamber  30 . A chimney  36  leading upwardly from the combustion chamber  14  passes up through the center of an upper part of the water chamber  30  and conducts hot flue gases out of the combustion chamber while heating the water in chamber  30 . The flue gases then pass through a radial pipe  38  and then down a downpipe  40 , at the bottom of which the flue gases pass out of a side outlet  42 , while fly ash is collected in collector pot  44 . 
     The burner also includes a pellet hopper  50 , the lower end of which feeds pellets  51  into an auger  52  which delivers the pellets via a pipe  54  into a central area of the combustion chamber  14 , the auger being driven by motor  56 , these parts being generally conventional. 
       FIGS. 2 and 3  show details of the burner pot  16  and the associated grate  28 , these parts being central features of this invention. As shown in these drawings, the grate  28  at the base of the burn pot is a generally planar and generally circular plate fixed to the base of the cylindrical wall  18  around most of its circumference, but provided with four narrow, circumferentially extending apertures  60  located at points where the edges of the plate  28  are cut away and do not meet the wall  18 . Also, the annular plate  28  is provided with a number (about  6  or  8 ) recesses in the form of circular holes  62  which are blind, i.e. do not extend through the thickness of the plate. 
     The lower part of wall  18  is also provided with a larger, side outlet  64  normally closed by a door  66  pivoted at its upper edge by pivots  68 , and urged into the closed position (as illustrated in  FIG. 2 ) by a tension spring  70  acting on a radial extension  66 a of the door. This allows removal of clinkers as described below. 
     Immediately above the plate  28  is a bar  72  in the form of an elongated horizontal plate having a flat lower surface overlying the plate and extending out to the apertures  60 . The bar  72  is fixed to a vertical shaft  74  extending upwardly through a bore in the center of the grate and supported by a bearing  76  attached to the lower surface of the grate. The lower end of shaft  74  is fixed to a first bevel gear  78  which meshes with a second bevel gear  80  fixed to a horizontal shaft  82  driven by a low r.p.m. geared electric motor  84  mounted on the outer side of one of the legs  10   a.    
     As shown in  FIGS. 2 and 3 , the upper surface of the bar  72  is provided with several, for example three, upstanding vanes  86  which (as seen in  FIG. 3 ) lie at an angle of about 30 to 45° to the circumferential direction of movement of the respective vane. The vanes also lie at an angle of about 25 to 30° to the vertical, as seen in  FIG. 2 . The vanes are angled so that as the bar  72  rotates in a clockwise direction, as seen in  FIG. 3 , the vanes push outwardly the ash which has collected on the grate, eventually causing at least some of this ash to drop through the apertures  60  into an ash pan  88 , shown in  FIG. 1 , below the grate. The grass or hay pellets, which are relatively light compared to wood pellets, burn while floating on top of the ash, so that few of the unburnt pellets are expelled with the ash. 
     Some of the ash particles, e.g. clinkers, may be too large to fall through the apertures  60 . Some of these will be moved around the grate by bar  72  and fall into one of the recesses  62 , whereupon continued movement of the bar  72  breaks off parts of any of such particle lying above the upper surface of the grate. This fragments the particles so that most of them can later drop through the apertures  60 . In addition, even larger pieces of slag or clinker, which may be too large to be held by recesses  62 , can be pushed by the vanes  86  radially out of the spring biased door  66  (the door being openable in the direction illustrated by the arrow  71 ), the spring  70  yielding to the radial force provided by the bar  72 . This mechanism prevents undue accumulation of ash or clinkers on the grate, such as would interfere with air being admitted through air inlet ports  24 . The slag, clinker and ash pushed through the opened door  66  falls (as illustrated by the arrow  73 ) into the ash pan  88  for subsequent removal and disposal. 
       FIG. 4  shows a modified version of the first embodiment having an additional heat exchanger  90  in the form of a water jacket surrounding the down pipe  40  and connected to the upper and lower portions of the water chamber  30  by upper and lower water tubes  92   a  and  92   b  respectively. This improves efficiency by transferring additional heat from the chimney into the water. 
       FIGS. 5 and 6  are views showing parts similar to those shown in  FIGS. 2 and 3 , and the similar parts are shown by the same reference numerals, increased by  100 . The main differences are that the recesses  162  are more numerous than recesses  62 , the vanes  186  are thinner than vanes  86 , and the air inlet ports  124  are more numerous, although smaller, than the air inlet ports  24 . For convenience, the cone connected to the upper end of the burn pot is not shown in  FIG. 5 . 
     With reference to  FIG. 7 , a further embodiment of the present invention is illustrated similar to the previously described embodiments of the present invention, in this embodiment, a chimney  36  leading upwardly from the combustion chamber  14  passes up through the center of a water chamber  30  and conducts hot flue gases out of the combustion chamber while heating the water in chamber  30 . The flue gases then pass a deflector plate  41  which redirects and distributes the flue gases into an exhaust jacket  37  surrounding the outside of the water chamber  30 , the flue gases within the exhaust jacket  37  further heating the water in the chamber  30  before the flue gases pass out of a side outlet  42 , the fly ash being downwardly directed as it passes through the side outlet  42  by a deflector plate  45 , the fly ash thereafter exiting through a fly ash passage  43  for collection and disposal. Water may be added or removed (or pressure relieved) from the water chamber  30  by way of an opening  29  in the top of the water chamber  30 . An access doorway  15  is provided to light and monitor the fire. 
     A further embodiment of the burner, not shown, may be in the form of a space heater. In this case, either air may be blown over the exterior of the water chamber  30 , or the air may be blown through a chamber, similar to chamber  30 , with appropriate venting in both cases. 
     It is understood that combustion catalysts, such as COMATE® (available, for example from ATLANTIC COMBUSTION TECHNOLOGIES, INC. in Mulgrave, Nova Scotia, Canada) may be added to the hay pellets to reduce the formation of clinkers, or alternatively, to soften the clinkers that may be formed during the combustion process. 
     The present invention has been described herein with regard to preferred embodiments. However, it will be obvious to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as described herein.