Pellet fuel burner

A burner for burning, granular, pellet or similarly sized solid heating fuel includes a retort in which solid fuel is ignited and burned. A peripheral electrical igniter is included within the retort for initiating self-sustaining combustion of the solid fuel at its uppermost surface. Combustion air is supplied centrally within the retort and is emitted radially outward for burning solid fuel evenly across its uppermost surface. Thermostatic control and safety circuits are included for safely and efficiently operating the burner.

TECHNICAL FIELD 
This invention relates generally to burners for burning granular, pellet or 
similarly sized solid biomass heating fuel. 
BACKGROUND OF THE INVENTION 
Pellet burners or stoves are becoming increasingly popular. Such burners 
typically use a biomass fuel in the form of a small pellet 1/4-3/8 inch in 
diameter and one inch in length which is made from waste of the timber 
industry. Such pellets typically have a heat value of approximately 8500 
BTU's per pound. 
Pellet burners have advantages over conventional wood burning stoves. One 
advantage is that substantially less emissions are produced by pellet 
burners than wood burners. Additionally, almost all pellet burning stoves 
meet environmental emissions standards and can be vented using 
conventional dryer-type exhaust hoses. Pellet burning stoves also 
typically achieve a much higher efficiency than conventional wood stoves 
due to a more precisely controlled air-fuel mixture ratio in the 
combustion chamber. Pelletized fuel is also cleaner and easily handled and 
does not generate the mess created by the handling of wood. 
Pellet burner manufacturers continue to strive to increase the efficiency 
and ease of operation of pellet burning stoves. This invention relates to 
such improvements. 
Although the invention arose primarily out of the needs and concerns in the 
biomass pellet fuel stove art, it will be apparent to those skilled in the 
heating art that the teachings of the invention may have applicability to 
the burning of other granular, pellet or similarly sized solid heating 
fuels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following disclosure of the invention is submitted in compliance with 
the constitutional purpose of the Patent Laws "to promote the progress of 
science and useful arts" (Article 1, Section 8). 
Referring to FIGS. 1 and 2, a burner for burning granular, pellet or 
similarly sized solid heating fuel 40 is indicated generally by reference 
numeral 10. Burner 10 includes a retort 12 for burning solid fuel, and 
feed means 14 for supplying solid fuel thereto. 
Retort 12 includes a hollow tubular base piece 15 which has a horizontally 
extending lower portion 16 and an elbow portion 18 at one end which curves 
upwardly to form an upright central axis 20. The opposite end of 
horizontal portion 16 terminates in a retaining flange 22 for mounting the 
burner relative to a burning chamber in which the retort is adapted to be 
received, as is more fully described below. A vertically oriented hollow 
cylindrical sleeve 36 is received internally atop elbow portion 18 and 
extends upwardly about central axis 20. 
A fuel supply tube 24 extends through flange 22 and into the hollowed 
central portion of horizontal portion 16 of retort base 15. The outer end 
portion of tube 24 includes a vertical, upwardly extending passageway or 
throat 26 which terminates in a mounting flange 28. Flange 28 mounts a 
hopper 94 (See FIG. 5) for retaining solid fuel to be fed through throat 
26 and into retort horizontal portion 16. A screw feeder 30 extends along 
tube 24 from throat portion 26 and terminates adjacent upright central 
axis 20 within horizontal portion 16 of retort base 15. A motor 32 
rotationally drives screw feeder 30 for feeding solid fuel to the retort 
for burning. Tube 24, throat 26, screw 30, and motor 32 comprise feed 
means 14. In the illustrated embodiments, the feed means supplies fuel 
upwardly through the bottom portion of retort 12 for combustion. Feeding 
of fuel through the top or side portion of a retort could alternately be 
employed, although bottom feeding is preferred. 
Retort 12 has a predetermined elevation at which an upper surface 38 of the 
solid fuel is maintained in a burning condition during burner operation. 
Uppermost surface 38 within retort 12 is somewhat arcuate, having a 
marginally higher elevation at the periphery of the retort than at the 
center adjacent upright central axis 20. 
An igniter means is mounted internally within retort 12 for initiating 
self-sustaining combustion of solid fuel 40 at its uppermost surface 38. 
The igniter means is illustrated as being in the form of an annular 
igniter 42 which extends about the internal periphery of retort 12. It 
comprises an electrical resistance element which is positioned adjacent 
the internal surfaces of the retort sidewalls just beneath uppermost 
surface 38 of solid fuel 40. Although an annular electrical resistance 
element is disclosed, alternately shaped, positioned, and powered igniters 
could be used without departing from the principles and scope of the 
invention. However, an annular, peripheral igniter provides the distinct 
advantage of not interfering with rising solid fuel being fed to uppermost 
surface 38 by means of rotating screw feeder 30. 
Combustion air supply means is provided to emit combustion air radially 
outward from upright central axis 20 within retort 12 at the fuel surface 
38. This radially outward emission of combustion air assists in burning 
solid fuel evenly across uppermost surface 38. In the preferred 
embodiment, the combustion air supply means is in the form of a hollow 
tube 44 which extends into retort 12 from a top portion thereof and 
downwardly along upright central axis 20. The lowermost portion of tube 44 
extends into retained solid fuel 40 a small extent and includes a pointed 
end 46. In this manner, the combustion air supply means is coaxially 
spaced radially inward relative to annular peripheral igniter 42. The end 
of tube 44 is pointed to assist in passage of rising solid fuel to 
uppermost surface 38 around the exterior of tube 44. 
Tube 44 includes a plurality of apertures 48 positioned to emit combustion 
air within retort 12 above igniter 42. Apertures 42 extend 
circumferentially around as well as axially along tube 44. This provides 
combustion air which is emitted radially outward within the retort at a 
plurality of axially spaced locations along the upright central axis above 
fuel uppermost surface 38. Supplying combustion air in this manner 
elevationally above fuel surface 38 assists in combustion of gases rising 
upwardly from surface 38. 
A flame deflector 50 is positioned axially above the top of retort 12. It 
comprises a flat circular plate which extends radially outward the same 
peripheral distance as retort 12. Flame deflector 50 functions to 
concentrate heat and promote more complete burning of fuel within retort 
12. Combustion air supply tube 44 extends downwardly through flame 
deflector 50 and into retort 12 along upright central axis 20. A flange 51 
extends outwardly from tube 44 and elevationally retains flame deflector 
50 relative to retort 12. 
Combustion air is preferably supplied to retort 12 entirely through tube 
44. This enables radially outward flowing combustion air to more 
completely and evenly burn fuel across a given elevational cross section 
of the retort. As such, the vertical sidewalls of the retort are 
preferably constructed to be impervious to inward passage of combustion 
air. Were the walls constructed to be somewhat porous, it would be 
preferable to supply a flow rate of combustion air through tube 44 that 
was sufficiently great to prevent inward passage of air through any retort 
pores. Conventional pellet stoves typically provide combustion to the 
retort means by inward passage through apertures or pores in the sidewalls 
of a retort. This tends to produce a very intense burn at the center of 
the retort but not at locations within the retort which are radially 
displaced from the center. The present apparatus achieves a more efficient 
burn by radially emitting all or at least a portion of the combustion air 
outwardly from the center portion of the retort to achieve complete and 
more even burning of the solid fuel across its uppermost surface. 
Reference is made to FIG. 2 for an explanation of a typical burning profile 
of the upper portion of solid fuel 40 produced by the described apparatus. 
For purposes of illustration, retort 12 can be considered as divided into 
three elevational ranges 1, 2, and 3, shown diagrammatically at the left 
in FIG. 2. Elevation range 3 comprises that area within retort 12 which is 
elevationally above uppermost surface 38 of retained solid fuel 40. 
Elevation range 2 is immediately below range 3 and extends downwardly to 
some predetermined point where solid fuel is beginning to heat to the 
point of combustion. Elevation range 1 is immediately below range 2, and 
comprises that level at which fuel is being horizontally fed to the 
vertical burn portion of retort 12. 
After fuel within the burner has been ignited and steady state burning 
conditions have been reached, typical temperatures will be as follows. 
Within elevation 1, the average fuel temperature will fall somewhere 
between 150.degree. and 200.degree. F. This is below the combustion 
temperature of the pellets. Combustion occurs within elevation 2. The 
average temperature within elevation 2 will vary from 300.degree. F. up to 
1100.degree. F. Again for ease of illustration, elevation range 2 is 
divided into three sections comprised of lower section 2A, mid section 2B, 
and upper section 2C. Within section 2A, the temperature of the pellets is 
rapidly increasing elevationally as the fuel reaches section 2B. The 
average temperature within section 2A is approximately 500.degree. F. 
Within section 2B, the fuel will begin to char and form a blackened 
charcoal-like substance. The average temperature within section 2B will be 
approximately 700.degree. F. As fuel continues to flow upwardly to 
uppermost surface 38 within section 2C, the fuel is exposed to combustion 
air being emitted from supply tube 44, which produces a visible flame. The 
average temperature within section 2C is approximately 1100.degree. F. 
Further combustion of rising gases occurs within elevation range 3 above 
uppermost surface 38. Temperatures within this region of retort 12 can 
reach from 2200.degree. to 2500.degree. F. as substantially complete 
combustion of the rising gases occurs. 
As illustrated, the elevational position of electrical igniter 42 is at the 
uppermost portion of section 2B. The first row of combustion apertures 48 
is positioned in section 2C, immediately above igniter 42. In this manner, 
the igniter is completely immersed in solid fuel and is below the hottest 
combustion area of the solid fuel. This preferred positioning increases 
the life of the igniter as it is not directly exposed to the higher 
combustion temperatures. This preferred positioning also enables the 
igniter upon activation, to more quickly start fuel burning within section 
2C than would occur were the igniter to be placed above upper surface 38, 
or within section 2C. However, alternate elevational positioning of an 
igniter could of course be employed without departing from the invention. 
FIGS. 3-6 illustrate a complete pellet burning apparatus 52. Burner 52 
comprises a cabinet 53 which includes aesthetically pleasing front, side, 
rear, top and bottom cabinet walls 54. Cabinet walls 54 define a hollow 
internal cavity 56 which receives the retort, feed means, igniter means, 
combustion air supply means and other various operational components as is 
more fully described below. 
A burn chamber 58 is received within walls 54 and fills the major portion 
of the front half of cavity 56. Burn chamber 58 includes an upper portion 
60 and a lower portion 62. Chamber 58 is hollow throughout defining a burn 
cavity 59. Upper portion 60 is larger than lower portion 62 and connects 
at its lower end therewith. Upper portion 60 is six-sided, having the 
general appearance of a widened honeycomb cell. Lower portion 62 is 
substantially rectangular in cross section. The front wall of upper 
portion 60 includes an outwardly projecting hollow, rectangular extension 
64 which terminates in a glass viewing and access door 66. Door 66 
projects outwardly just past the front cabinet wall 54a of burner 52. The 
upper portion of front cabinet wall 54a is grated to enable outward 
passage of heated convection air. The lower portion of front cabinet wall 
54a also includes slots 63 to enable outward passage of heated convection 
air from the lower portion of cabinet 53. Extension 64 extends through the 
grated portion of front cabinet wall 54a. A pair of vertically elongated 
convection air passageways 68, 70 (FIG. 4) extends through burn chamber 58 
on opposite sides of extension 64 and door 66 in upper portion 60. 
Convection air for heating is forced through these passageways by fans 112 
(FIG. 5) and emitted into the room of the building being heated through 
the grated portion of front cabinet wall 54a. 
A hinged access door 65 extends across the bottom portion of the front wall 
of lower portion 62 of burn chamber 58. Door 65 functions to permit 
removal of ashes which are emitted outwardly from the retort during 
combustion and collect in lower portion 62 of burn chamber 58. A hinged 
door 67, corresponding in position to hinged door 65, is also included in 
front cabinet wall 54a to enable easy access to door 65. 
Opposed flanges 72 extend outwardly from the walls of upper portion 60 of 
burn chamber 58 for mounting of the burn chamber within cabinet cavity 56. 
Flanges 58 are sandwiched between flanges 74 and flanges 76 of an upper 
shroud 78 and a lower shroud 80, respectively (FIG. 4). Lower shroud 80 is 
trough-like, being open at its front and rear ends. Lower portion 62 of 
burn chamber 58 rests on a floor surface 82 of lower shroud 80. 
Accordingly, shroud 80 functions as a positioning device for securing burn 
chamber 58 within cabinet chamber 56. 
Upper shroud 78 includes side, top and rear walls 84, 86, and 88 
respectively. The forward portion of side walls 84 are beveled inwardly, 
and the forward portion of top wall 86 beveled downwardly, to assist in 
directing convection air outwardly through the grate portion of front 
cabinet wall 54a. The lower outer portions of rear wall 88 taper inwardly 
and rearwardly to provide sufficient space for convection air to flow 
through passageway 68, 70, as will be more fully described below 
Retort 12 is centrally positioned within the lower portion of burn chamber 
58. Horizontal portion 16 of retort 12 extends through a hole formed in a 
rear wall of lower portion 62 of burn chamber 58. Mounting flange 22 of 
horizontal portion 16 abuts against the interior portion of the rear wall 
of burn chamber 58 and bolts thereto. Vertical sleeve portion 36 of retort 
12 extends upwardly into upper portion 60 of burn chamber 58 to an 
elevation just below horizontal door extension 64. 
Combustion air supply tube 44 extends inwardly from the rear wall of upper 
portion 60 of burn chamber 58 and angles at ninety degrees to extend into 
retort 12. Combustion air supply tube 44 includes an elbow connecter 90 at 
the ninety degree bend. An air conduit 92 connects to elbow 90 and extends 
in angled upward fashion to a horizontally positioned air supply bar 91 
positioned to emit air downwardly adjacent glass door 66. This downwardly 
emitted air assists in keeping glass door 66 clean and clear, and also 
provides additional combustion air which is emitted from outside of retort 
12 to within cavity 59 to assist in further combustion of gases being 
emitted from retort 12. 
A metal fuel hopper 94 is provided rearwardly adjacent burn chamber 58 and 
upper shroud 78 within rearward portion of cabinet cavity 56. Hopper 94 
includes a flange 97 which bolts to mounting flange 28 of the vertical 
portion of feed means 14, which defines fuel feed passageway 26. Screw 
feeder motor 32 is mounted rearwardly adjacent the lower end of throat or 
passageway 26 for driving screw feeder 30. This configuration supplies 
fuel from hopper 94 to the burn area within retort 12 in a regulated 
manner as is more fully described below. 
The rear portion of top cabinet wall 54b includes a pivoting door which is 
positioned atop hopper 94. A plastic coated magnetic seal 96, similar to 
that employed with refrigerator doors, is secured to door 95 and is 
positioned to coincide with the top periphery of hopper 94. Magnetic seal 
96 provides an airtight seal relative to hopper 94 when door 95 is closed, 
as shown in FIGS. 4 and 5. 
An exhaust system communicates with cavity 59 for exhausting combustion 
gases emitted from retort 12 to within cavity 59. The exhaust system 
comprises a pair of exhaust conduits 98, 100 which extend through the rear 
portion of burn chamber 58 into chamber cavity 56 outwardly adjacent the 
lowermost portion of convection passageways 68, 70 (see FIG. 6). Exhaust 
conduits 98, 100 connect to form a single exhaust conduit 102 which 
extends outwardly through the left rear side of the lowermost portion of 
exterior cabinet walls 54. Exhaust conduit 102 is connected to an exhaust 
fan (not shown) typically placed at the exterior of the building being 
heated by burner 52. 
Combustion air supply tube assembly 44 connects with a rectangular air 
supply conduit 104 which angles downwardly along the right rear portion on 
the exterior of burn chamber 58. A circular conduit 106 communicates with 
conduit 104 and extends to connect at the left rear lower portion of 
cabinet cavity 56 with an air supply box 108. Exhaust conduit 102 
sealingly extends through the central portion of box 108 (FIG. 5). A 
combustion air supply valve assembly 105 is included where conduit 106 
meets supply box 108. An enlarged circular conduit 110 extends rearwardly 
from box 108 out from the rear cabinet wall 54c. Exhaust conduit 102 is 
coaxially inwardly spaced to extend through larger conduit 110 defining an 
annular space through which combustion air flows to box 108. Accordingly, 
exhaust is drawn from cabinet 52 through circular conduit 102 along arrows 
A, while combustion air is supplied through burner 52 to burn chamber 58 
around conduit 102 and through conduit 110 along arrows B. 
Such an exhaust/combustion air supply system is typically referred to as a 
negative draft system, as the exhaust fan connected to conduit 102 creates 
a pressure which is less than atmospheric within burn chamber 58. With 
such systems, combustion air is supplied to burn chamber at substantially 
ambient atmospheric pressure. Alternate exhaust/combustion air supply 
systems could, of course, be employed without departing from the 
principles and scope of the invention. 
A pair of convection fans 112 is mounted internally within cabinet cavity 
56 on opposite sides of and rearwardly adjacent the lower portion of 
hopper 94. Fans 112 functin to provide a rather large volume of convection 
air for passage through passageways 68, 70 for heating and emission 
through the grate portion of front cabinet wall 54a. 
Various operational and safety electronic circuits are retained within 
cabinet cavity 56 for providing safe and efficient, thermostatically 
controlled operation of burner 52. Such systems are preferably configured 
to enable burner 52 to be thermostatically controlled from a conventional 
wall thermostat. A master control circuit 111 is employed for controlling 
the various circuits housed within cabinet 53. Sub-circuits include 
convection fan controller 114, convection air temperature sensor circuit 
116, combustion air controller 118, screw feeder motor controller 120, 
exhaust fan controller 122, igniter controller 123 and burn chamber 
temperature sensor circuit 124. Such sub-circuits are operably connected 
with the master control circuit 111 which coordinates overall operation of 
burner 52. A thermostat (not shown) also preferably connects to controller 
111 and mounts to a wall within the building in which burner 52 is 
operated. 
A starting sequence for igniting burner 52 is described as follows. With 
power being supplied to master control circuit 111, the wall thermostat 
senses a low temperature condition within the building or room being 
heated. Controller 111 thereafter activates igniter controller 123, which 
supplies electricity to electrical resistant igniter element 42 to 
initiate self-sustaining combustion of solid fuel within the retort. In 
the embodiment shown, the igniter is operated for slightly more than three 
minutes which heats the upper portion of the retained solid fuel 
sufficiently to initiate self-sustaining combustion. Thereafter, the 
controller deactivates the igniter. Although a timed sequence is described 
for operation of the igniter, the igniter controller could also include a 
temperature sensor control which deactivates the igniter upon reaching a 
predetermined temperature evidencing self-sustaining combustion. 
At some point during ignition or shortly thereafter, controller 111 
activates exhaust fan controller 122 and combustion air valve controller 
118 to begin supplying combustion air through supply tube assembly 44 to 
solid fuel upper surface 38. Screw motor controller 120 is also activated 
to begin supplying a slow but steady flow of fuel to upper surface 38 
within retort 12 for burning. Convection temperature sensor control 
circuit 116 senses temperature immediately adjacent the upper exterior of 
burn chamber 58. When such temperature reaches approximately 120 degrees 
F., the controller 111 activates convection fan controller 114 to operate 
fans 112. This blows convection air for heating through passageways 68, 
70, and out the front end of the stove. 
When the temperature in the room has reached the thermostat setting, 
controller 111 shuts down the auger motor, exhaust fan, and combustion air 
supply valve to stop the burn within retort 12. Shortly thereafter, the 
temperature adjacent burn chamber 58 falls below 120 degrees F., causing 
the convection air controller circuit to turn convection fans 112 off. 
When temperature in the room or building again falls below the 
predetermined thermostat setting, master controller 111 first accesses 
burn chamber sensor 124 to determine if there is sufficient heat within 
retort 12 to begin self-sustaining combustion. If the temperature is 
sufficient, the exhaust fan, auger motor and combustion air valve are 
caused to be activated, which begins combustion. If the temperature is not 
sufficiently high to initiate self-sustaining combustion, master 
controller 111 activates the igniter circuit and begins the ignition 
sequence described above. 
Burner 52 also has a number of safety features for shutting down the burner 
in the event excessive temperatures are reached to prevent a fire. Each of 
the safety circuits employs a sensor which, upon reaching a predetermined 
condition, signals controller 111 to turn off the combustion air supply 
valve, auger motor, and exhaust fan. One such sensor is a high temperature 
sensor 126 positioned within the upper portion of cabinet cavity 56 
adjacent convection fan temperature sensor circuit 116. Sensor 126 is 
preferably configured to signal controller 111 when a temperature of 185 
degrees F. is reached within chamber 56. 
A second backup high temperature sensor 128 is also positioned adjacent 
convection air temperature circuit 116. It preferably signals master 
controller 111 to shut the system down if a temperature of 200 degrees F. 
is reached. High temperature sensor 128 also preferably employs a fuse or 
circuit breaker mechanism which must be replaced or reset upon activation 
to alert the operator to a potential problem. 
Yet another safety feature is burn-back safety temperature sensor 130. This 
sensor constantly monitors temperature within the horizontal portion of 
the retort and feed means to insure that fuel at this location is not 
heated to the point of ignition. If it were, master controller 111 would 
be signaled to turn off the combustion air supply valve, auger motor, and 
exhaust fan. 
Another safety sensor is low fuel sensor 132. This sensor signals master 
controller 111 when the fuel level within hopper 94 falls below a 
predetermined elevation. The volume of fuel within hopper 94 creates an 
air barrier which prevents air from flowing into feed supply tube 24 and 
subsequently to the retort. The low fuel sensor senses a low fuel 
condition to prevent any air present within cavity 56 from flowing through 
the lower mass of fuel and into the retort. 
Another safety feature is hopper lid switch 134. This switch, upon opening 
of cabinet door 95, signals master controller 111 to shut the system off 
to prevent any air from flowing into the hopper and into the retort to 
prevent a burn-back condition from developing. Upon closing the door, the 
motor, exhaust fan and combustion air valve function in their previous 
operational mode. 
The various components of burner 52 are constructed of conventional 
materials. For example, retort 12 is constructed of cast iron. Deflector 
shields 50, 63 are constructed of stainless steel. Burn chamber 58 is 
constructed of 1/8" plate steel. Heat is removed at such a rate from 
cabinet cavity 56 that cabinet walls 54 can be constructed of wood. 
In compliance with the statute, the invention has been described in 
language more or less specific as to structural features. It is to be 
understood, however, that the invention is not limited to the specific 
features shown, since the means and construction herein disclosed comprise 
a preferred form of putting the invention into effect. The invention is, 
therefore, claimed in any of its forms or modifications within the proper 
scope of the appended claims, appropriately interpreted in accordance with 
the doctrine of equivalents.