Waste fuel combustion system

A waste fuel combustion system including a furnace having a grate for supporting waste fuel for burning, the grate including a plurality of movable members and a mechanism for reciprocating the movable members. The furnace also includes a primary gas passageway, a secondary gas passageway, and a throat through which the primary and secondary gas passageways pass in order to accelerate the flow of combustion gases. The waste fuel combustion system also includes recirculation conduits communicating between a source of exhaust gases and the primary and secondary gas passageways in order to recirculate the exhaust gases for additional combustion and degradation.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention relates generally to combustion systems, and particularly to 
combustion systems for burning waste. 
2. Related Prior Art 
As the quantities of INDUSTRIAL municipal, agricultural and municipal waste 
products increase, and the dangers which such waste products can pose 
become more well-recognized, the need for an efficient and effective means 
for disposal of such waste products becomes greater. Often, conventional 
waste storage and disposal facilities, such as landfills or incinerators, 
cannot adequately destroy the waste products. For example, 
non-biodegradable materials, such as various types of plastic and tires, 
cannot be successfully accommodated over the long-term by conventional 
landfills because of the bulk and the extremely long biodegradation 
process of these wastes. 
Similarly, conventional incinerators may not be able to effectively process 
some industrial, chemical or toxic wastes because of incomplete combustion 
which can result in the emission of toxic gases and particulates. In order 
to comply with various emission regulations, various exhaust filtering 
equipment such as flue scrubbers or the like may be required for the 
operation of the incinerator. Such ancillary equipment can significantly 
increase the cost of operation of the incinerator. 
Also, many normally disposable waste products are merely difficult to 
handle and are difficult to burn completely, but could provide an 
additional source of fuel if burned efficiently. For example, agricultural 
and municipal wastes such as waste paper, food and trimmings could, if 
burned completely, provide an additional source of fuel. In light of the 
above-described circumstances, the need for an economical and ecologically 
acceptable means for disposing of such waste has been realized. 
Attention is directed to U.S. Pat. No. 4,543,890 which issued to Johnson on 
Oct. 1, 1985, and which illustrates an example of a known wood fuel 
combustion system. 
SUMMARY OF THE INVENTION 
The invention provides a waste fuel combustion system for generating heat. 
The combustion system includes a furnace having a plurality of aligned and 
communicating combustion chambers defining a plurality of combustion 
zones, means for feeding waste fuel into the furnace, and means for 
completely combusting and degrading the waste fuel. In order to completely 
combust and degrade the waste fuel, the waste fuel combustion system 
includes a reciprocable support for the burning of the fuel, means for 
mixing the flow of combustion gases through the zones for more complete 
combustion, and means for recirculating flue gases from the waste fuel 
system to assure the complete degradation of any uncombusted or otherwise 
toxic emissions. 
More particularly, the reciprocable support for the waste fuel includes a 
fuel grate formed by elongated rods which are supported for relative 
longitudinal movement relative to one another, and a mechanism for 
reciprocating and rotating the rods to shake off ash from the waste fuel 
supported thereon. The mechanism for reciprocating the rods can, in one 
embodiment, be adjusted to vary the rate of reciprocating movement in 
order to accommodate various waste fuels, depending on the rate of 
combustion thereof, to remove ash into an ash collection container and to 
expose uncombusted fuel. 
The means for mixing the flow of gases through the furnace includes, in one 
embodiment, an air flow conduit or passage having a constricted region 
which causes an acceleration of the flow of air due to a venturi effect on 
the flow. More particularly, the furnace includes a combustion chamber, a 
secondary air passage surrounding the combustion chamber and communicating 
with the combustion chamber. The air passage has an end having a 
diminished cross-sectional area forming a throat through which combusted 
gases flow into the combustion chamber. The flow of gases accelerates as 
it passes through the throat, resulting in a greater degree of mixing of 
the gases in the combustion chamber. More complete combustion of the gases 
can be realized by such mixing. 
The means for recirculating the flue gases of the waste fuel combustion 
system includes a conduit for redirecting the flow of exhaust gases from 
the secondary combustion chamber, or from another, similar source of 
exhaust gases, back into the primary combustion chamber and, 
alternatively, for returning the flow of exhaust gases to the secondary 
combustion chamber without passage through the primary combustion chamber. 
Depending on the type and amount of uncombusted exhaust gases, and the 
degree of degradation or pyrolysis of the exhaust, the exhaust can be 
recycled into the waste fuel combustor for additional exposure to heat and 
for additional combustion. Provision of the recirculating means allows 
operation of the combustion system in concert with existing sources of 
toxic or particulate-laden exhaust gases. For example, flue gases from 
municipal waste incinerators and oil or coal-fired boilers can be 
recirculated into the combustion system for additional degradation until 
the exhaust gases are ecologically acceptable. 
The invention thus provides a combustion system which can burn waste fuels 
efficiently and which can "clean-up" flue gases in a cost-effective 
manner. 
Various other features and advantages of the invention will become apparent 
to those skilled in the art upon review of the following detailed 
description, claims and drawings.

Before one embodiment of the invention is explained in detail, it is to be 
understood that the invention is not limited in its application to the 
details of construction and the arrangements of components set forth in 
the following description or illustrated in the drawings. The invention is 
capable of other embodiments and of being practiced or carried out in 
various ways. Also, it is to be understood that the phraseology and 
terminology used herein are for the purpose of description and should not 
be regarded as limiting. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The drawings illustrate a waste fuel combustion system 10 embodying the 
invention. The waste fuel combustion system 10 can operate as a 
stand-alone generator of heat or can operate in association with other, 
known systems to provide heat therefore. For example, the waste fuel 
combustion system 10 can be operated in concert with known municipal waste 
incinerators, oil or coal-fired boilers, or other conventional heating 
systems to provide energy therefore and, as discussed below, to treat 
exhaust gases produced thereby. 
As shown in FIG. 1, the combustion system 10 comprises a furnace 11 
including a first shell or elongated cylinder 12 having an inlet end 14 
and an outlet end 16 and a substantially uniform inner diameter. A back 
grate 18, the details of which are discussed below, partitions the inner 
shell 12 into a first combustion chamber or zone 20 adjacent the inlet end 
14 of inner shell 12 and a second combustion chamber or zone 22 adjacent 
the outlet end 16 of inner shell 12. 
The furnace 11 also includes (FIG. 1) means in the form of a funnel 
assembly 24 for supplying fuel to the first combustion zone 20 of the 
inner shell 12, and means in the form of an ash box 26 for receiving ash 
from the inner shell 12. In operation, the funnel assembly 24 and the ash 
box 26 are closed to the atmosphere to prevent flow of air through the 
funnel assembly 24 and the ash box 26 into the inner shell 12. 
An intermediate or second shell 28 surrounds the inner shell 12 and extends 
substantially the entire length of the inner shell 12. The inner and 
intermediate shells 12, 28 are (FIG. 2) concentrically aligned on the 
longitudinal axis 30 of the inner shell 12 so that the inner and 
intermediate shells 12, 28 are spaced and define therebetween a generally 
annular secondary air passage 32. As discussed more fully below, the 
secondary air passage 32 includes an outlet 34 which extends 
circumferentially around the outlet end 16 of the inner shell 12. The 
intermediate cylinder 28 extends beyond the outlet end 16 of the inner 
shell 12 and defines therein a third combustion chamber or zone 36 which 
communicates with the second combustion zone 22 through the outlet end 16 
of the inner shell 12 and with the secondary air passage outlet 34. 
The furnace 11 also includes a third or outer cylinder 38 which surrounds 
the inner and intermediate cylinders 12, 28 and which extends 
substantially the entire length of the intermediate cylinder 28 so that 
(FIG. 2) the intermediate and outer cylinders 28, 38 also are in generally 
concentric, spaced relation. The end 40 of the outer shell 38 adjacent the 
inlet end 14 of the inner shell 12 supports a housing 42 which defines an 
air cavity 44 communicable with the first combustion zone 20 and the 
secondary air passage 32. The opposite end 46 of the outer cylinder 38 has 
therethrough a plurality of openings 48 spaced circumferentially about the 
outer cylinder 38. 
The intermediate and outer cylinders 28, 38 define therebetween an airspace 
or preheat air passage 50 extending the length of the intermediate and 
outer cylinders 28, 38. The airspace 50 (FIGS. 1 and 5) communicates 
between the plurality of openings 48 and the housing 42. The preheat air 
passage 50 can conduct a flow of gases from the openings 48 to the inlet 
end 14 of the inner shell 12 and is sufficiently long to heat the flow of 
gases passing therethrough. 
An end plate 52 overlies the inlet end 14 of the inner shell 12 and 
separates the housing 42 and the first combustion zone 20. The end plate 
52 has therein a first opening or primary inlet 54 which communicates 
between the air cavity 44 and the first combustion zone 20 and has therein 
a pair of second openings or secondary inlets 56 which communicate between 
the air cavity 44 and the secondary air passage 32. The end plate 52 also 
supports means in the form of door 58 which overlies primary inlet 54 for 
selectively affording communication between the air cavity 44 and the 
first combustion zone 20. The end plate 52 also supports means in the form 
of doors 60 which overlie the secondary inlets 56 for selectively 
affording communication between the air cavity 44 and the secondary air 
passage 32. 
The furnace 11 operates to provide a primary flow of combustion gases from 
the air cavity 44 through primary inlet 54 and into the first combustion 
zone 20. Primary gases burn in the first combustion zone 20 and flow 
through the inner shell 12 through the second combustion zone 22 and into 
the third combustion zone 36. A secondary flow of gases passes from the 
air cavity 44 into openings 56, through the secondary air passage 32 and, 
in manner discussed below, into either the second or third combustion 
zones 22, 36; and a preheat flow of gases from the openings 48 in the 
outer cylinder 38 along the preheat air passage 50 to the air cavity 44. 
The furnace 11 also includes means 62 located in the inner shell 12 for 
supporting waste fuel for burning. As shown in FIGS. 1 and 3, the means 
for supporting the waste fuel includes a generally horizontal fuel grate 
64 located in the first combustion zone 20 generally below the funnel 
assembly 24, a screen 66 extending downwardly from the upper portion of 
the inner shell 12 adjacent the end plate 52 to the fuel grate 64 to 
prevent fuel from falling against the end plate 52, and the back grate 18 
which extends vertically across the interior of the inner shell 12 between 
the first and second combustion zones 20, 22. As shown in FIG. 1, the 
screen 66 and the back grate 18 are preferably arranged to retain fuel 
supplied to the furnace 11 by way of the funnel on the fuel grate 64. 
The fuel grate 64 comprises a frame 68 supported by the inner shell 12 
inside the first combustion zone 20 and (FIG. 3) a plurality of elongated 
rods supported by the frame 68. A first plurality of the rods 70 is fixed 
to the frame 68 so that the rods 70 extend generally parallel to the axis 
30. The frame 68 supports a second plurality 72 of rods so that the rods 
72 are generally parallel to the first plurality of rods 70 and are 
preferably in alternating relation to the rods 70. The frame 68 supports 
the second plurality of rods 72 for longitudinal movement relative to the 
frame 68 and to the first plurality of rods 70 in the direction of axis 
30. The frame 68 also supports the rods 72 for rotation about their 
respective longitudinal axes. While various constructions can be used, in 
the illustrated embodiment, each of the second plurality of rods 72 (FIG. 
3) has a threaded end 74 which extends through the end plate 52 and which 
is housed by the air cavity 44. The end plate 52 supports thereon a 
plurality of blocks 76, each of which has therethrough an internally 
threaded bore 78 surrounding and engaging the threaded ends 74 of the rods 
72. Due to the threaded engagement of the blocks 76 and the rods 72, 
longitudinal movement of the rods 72 causes rotation of the rods 72. 
The threaded ends 74 of the second plurality of rods 72 respectively 
support drive bearing assemblies 80 which drivingly engage the rods 72 for 
reciprocal longitudinal movement and which afford rotation of the rods 72 
about their respective longitudinal axes. FIG. 6 illustrates a preferred 
construction for the bearing assembly 80. Each bearing assembly 80 
includes a plurality of ball bearings 82 housed by a circumferencially 
extending groove 84 in the end 74 of the rod 72 and by a grooved bearing 
end cap 86 fixed on the end 74 of the rod 72. A drive pin 88 extends 
between the bearing caps 86 on the rods 72 so that the moveable rods 72 
are connected and move longitudinally in unison. 
Drive means 90 is also provided for reciprocally moving the second 
plurality of rods 70. The drive means 90 includes a variable speed motor 
92 located under housing 42 (FIG. 1) and a drive arm 94 which is driven by 
the motor 92, and is operably connected to a follower arm 96. The follower 
arm 96 (FIGS. 1-3) has a first end 98 which is pivotally connected to the 
drive arm 94 and a second end 100 which is in the form of a clevis 102. 
Clevis 102 is drivingly connected to the bearing assemblies 80 on the ends 
74 of the movable rods 72 by means of a pivotable connection with the 
drive pin 88. 
The follower arm 96 extends generally vertically upwardly from the drive 
arm 94 through (FIG. 1) a slot 104 in the lower portion of the housing 42 
and into the air cavity 44. A bracket 106 which is supported by the 
intermediate cylinder 28 and which extends into the air cavity 44 
pivotally supports the follower arm 96 so that the ends 98, 100 of the 
follower arm 96 are reciprocally movable in the direction of axis 30. 
Operation of the motor 92 cause reciprocal motion of the second end 100 of 
the follower arm 96 (to the left and right in FIG. 1), causes pivotal 
movement of the arm 96, and causes reciprocating motion of the second 
plurality of elongated rods 72. Operation of the drive means 90 for 
reciprocating the rods 72 acts to shake ash from the fuel supported on the 
fuel grate 64 so that ash from the fuel can fall into the ash collection 
box 26 and so that uncombusted fuel is exposed for burning. Preferably, 
the drive means 90 for reciprocating the rods 72 also includes means, such 
as the variable speed motor 92, for varying the speed of reciprocating 
motion or means, such as a timer mechanism (not shown), for intermittently 
reciprocating the rods 72 in order to accommodate the rate of combustion 
of various waste fuels. 
The furnace 11 includes means for affording a flow of a portion of the 
secondary flow of gases from the secondary air passage 32 into the second 
combustion zone 22. In the illustrated embodiment, the means for affording 
a flow of secondary gases into the second combustion includes the back 
grate 18 which comprises a plurality of hollow tubes 108 extending and 
communicating between the secondary air passage 32 and the interior of the 
inner shell. Preferably, the tubes 108 include an upper portion 110 and a 
lower portion 112 which respectively extend inwardly of the inner shell 12 
and slightly axially toward the outlet end 16 of the inner shell 12. Each 
of the tubes 108 also has therein at least one opening 114 to provide 
means affording a flow of a portion of the secondary gases to enter the 
second combustion zone 22 from the secondary air passage 32. As secondary 
air flows from the secondary air passage 32 into the tubes 108, relatively 
high temperature primary combustion gases heat the secondary air so that 
secondary air introduced to the primary flow through the back grate 108 
has a temperature in the same temperature range as that of the primary 
gases. 
The provision of means affording a flow of secondary air into the second 
combustion zone 22 effects several desirable results. The introduction of 
uncombusted, heated gases into the second combustion zone 22 enhances the 
further combustion of primary combustion gases passing from the first 
combustion zone 20 into the second combustion zone 22. Also, the passage 
of secondary air through the tubes 108 helps cool the back grate 18 which 
extends the operational life of the back grate 108. 
Preferably, the holes in the tubes 108 are located at various radial 
positions so that secondary air injected into the second combustion zone 
22 spirals and mixes with the primary flow of gases. The resultant 
turbulence caused by the injection of secondary gases into the second 
combustion zone 22 also enhances combustion in the second combustion zone 
22. Thus, the furnace 11 provides means for enhancing the combustion of 
gases in the second combustion zone 22. 
The furnace 11 also includes means 116 for enhancing combustion in the 
third combustion zone 36 including means 117 for accelerating the 
secondary flow of gases from the secondary air passage 32 into the third 
combustion zone 36. The accelerating means 117 includes a first portion 
118 of the secondary air passage 32 which has a generally uniform 
cross-sectional area in a plane perpendicular to axis 30, and a second 
portion 120 having a decreasing inner diameter so as to have a diminished 
cross-sectional area in a plane perpendicular to axis 30. As shown in FIG. 
1, the second portion 120 of the secondary air passage 32 diminishes in 
cross-sectional area from a point intermediate the inlet end 14 and the 
outlet end 16 of the inner shell 12 to a throat 122 at the outlet 34 of 
the secondary air passage 32. Preferably, the cross-sectional area of the 
secondary air passage 32 decreases approximately one-third to one-half 
along the second portion 120 to the throat 122. 
As the secondary flow of gases passes from the air cavity 44, along the 
first portion 118 of the secondary air passage 32, some of the secondary 
flow passes the second portion 120 of the secondary air passage 32 and 
through the back grate 18. The remainder of the secondary flow passes into 
the second portion 120 of the secondary air passage 32 and accelerates 
into the throat 122. The secondary gas flow exits the nozzle-like outlet 
34 with increased velocity into the third combustion zone 36 due to the 
venturi-like effect of the reduced cross-sectional area of the throat 122. 
As discussed more fully below, the secondary flow of gases is directed 
generally radially inwardly into the third combustion zone 36 due to the 
radially inward direction of the second portion 120 of the secondary air 
passage 32. 
The means 117 for accelerating the flow of combustion gases also includes 
(FIGS. 1 and 4) a stator 124 for introducing a rotational component into 
the flow of combustion gases. The stator 124 includes (FIG. 4) an inner 
ring 126 which is disposed on the exhaust end of the inner shell 12 and 
which defines the inlet of the third combustion zone 36. The stator 124 
also includes a plurality of spaced-apart fins 128 which are disposed 
circumferentially around the inner ring 126 and which extend radially 
outwardly therefrom. The stator 124 further includes an outer ring 130 
which concentrically surrounds the inner ring 126, is connected to the 
fins 128, and which is disposed on the interior of the intermediate 
cylinder 28 adjacent the outlet 34 of the secondary air passage 32. 
As shown in FIG. 1, the stator 124 is located immediately downstream of the 
outlet end 16 of the inner shell 12 and is constructed so that the primary 
flow of gases flows from the inner shell 12 and through the inner ring 126 
into the third combustion zone 36. The fins 128 extend across the outlet 
34 of the secondary air passage 32 and across the secondary flow of gases. 
Each fin 128 has a surface 132 which is angled relative to axis 30 and to 
the direction of the secondary flow of gas exiting the secondary air 
passage 32. Each fin 128 deflects a portion of the secondary flow so that 
the stator 124 introduces a rotational component to the secondary flow as 
the secondary flow leaves the outlet 34 and enters the third combustion 
zone 36. 
The stator 124 also introduces a rotational component to the primary flow 
of gases. Because of the generally radially inward direction of the 
secondary flow as the secondary flow enters the third combustion zone 36, 
and because the secondary flow is angled, due to the stator 124, relative 
to the generally axial flow path of the primary flow, the secondary flow 
mixes with the primary flow and causes rotation of the primary flow in the 
third combustion zone 36. The combination of acceleration and rotation of 
the primary and secondary flows of gases increases mixing and turbulence 
of the combustion gases in the third combustion zone 36 and enhances 
combustion of unburned gases in the third combustion zone 36. 
Because the secondary flow passes through the stator 124, the accelerated 
flow from the secondary air passage 32 also provides means for cleaning 
the stator 124 by helping to prevent the accumulation of ash which can 
block the stator 124. The accelerated secondary flow tends to blow ash 
away from the fins 128 and thereby maintains the effectiveness of the 
stator 124. 
The construction of the intermediate cylinder 28 also aids in the mixing in 
the third combustion zone 36 of the primary and secondary flows. As shown 
in FIG. 1, the intermediate cylinder 28 includes a portion 134 which is 
located adjacent the outlet end 16 of the inner shell 12 and which, 
adjacent the outlet end 16, has an inner diameter substantially equal to 
the diameter of the outer ring 130 of the stator 124. The interior 
cross-sectional area of the portion 134 of the intermediate cylinder 28 
increases along the length of the intermediate cylinder 28 so that the 
interior cross-sectional area of the portion 134 when viewed in a plane 
generally perpendicular to axis 30 also increases. The provision of an 
increasing cross-sectional area of the third combustion zone 36 adjacent 
the outlet end 16 of the inner shell 12 promotes turbulence and mixing of 
the flow of combustion gases by acting as a diffuser. The accelerated 
gases passing through the throat 122 decrease in velocity because of the 
increasing cross-sectional area of the third combustion zone and become 
more turbulent. 
The waste fuel combustion system 10 also includes (FIG. 5) recirculating 
means 136 for returning exhaust or flue gases from the third combustion 
zone 36 or from other associated sources of exhaust gases to the furnace 
11 for further combustion or degradation. More particularly, and as shown 
in FIGS. 1 and 5, the recirculating means 136 includes means for 
selectively conducting exhaust gases to the inlet 14 of the inner shell 12 
and for selectively conducting exhaust gases to the second and third 
combustion zones without passing through the first combustion zone 20. The 
recirculating means 136 includes a first conduit 140 which communicates 
with (FIG. 5) a source of exhaust gases, such as the exhaust of the third 
combustion zone 36, the exhaust of a boiler or heating system A operated 
in conjunction with the waste fuel combustion system 10, or any other 
source of toxic or uncombusted gases. For example, the first conduit 140 
could communicate with a conventional exhaust stack, such as exhaust stack 
B illustrated in FIG. 6 or the exhaust stack illustrated in the 
aforementioned U.S. Pat. No. 4,543,890. The first conduit 140 also 
communicates, as shown in FIG. 1, with the first portion 118 of the 
secondary air passage 32 adjacent the air cavity 44. Exhaust gases from 
the source of exhaust gases can flow from the source, through the first 
conduit 140, and into the secondary air passage 32. Gases from the first 
conduit 140 are thereby introduced to the secondary flow of gases and pass 
into the second and third combustion zones 22, 36 without passing through 
the first combustion zone 20. 
The recirculating means 136 also includes a second conduit 142 which 
communicates between the source of exhaust gases and the preheat air 
passage 50 in a manner similar to the first conduit 140. Exhaust gases can 
flow through the second conduit 142 between the source of exhaust gases to 
the air cavity 44 through the preheat air passage 50. By controlling the 
flow of gases from the cavity into the secondary air passage 32 or into 
the first combustion zone 20 by the doors 58, 60 the recirculated exhaust 
gases can be directed into the first combustion zone 20 through inlet 54 
or into the secondary air passage 32 through inlet 56. 
Means are also provided for selectively and adjustably regulating the flow 
of gases through the first and second recirculating conduits 140, 142. As 
shown in FIG. 1, each of the first and second conduits 140, 142 house a 
valve 144 which is selectively operable to control the flow of gas 
therethrough. Preferably, the valves 144 are in the form of a butterfly 
valve. 
Depending on the type of exhaust gases produced by the source, additional 
combustion or degradation thereof maybe required. Because the first and 
second conduits 140, 142 lead to different passageways, i.e. the preheat 
air passage 50 and the secondary air passage 32, the recirculated exhaust 
gases can be subjected to various additional amounts of heat for various 
periods of time. For example, if combustion of the recirculated exhaust 
gases requires exposure to relatively intense heat for a relatively short 
duration, the exhaust gases can be recirculated through the first conduit 
140 so that the exhaust gases are recirculated directly into the second 
and third combustion zones 22, 36. As a second example, if further 
degradation or combustion of the exhaust gases requires exposure of the 
gases to lower levels of heat for a longer duration, the exhaust gases can 
be recirculated through the second conduit 142 and through the preheat air 
passageway 50 into the air cavity 44. Once in the air cavity 44, the 
recirculated gases can be mixed with either of the primary flow or 
secondary flow by adjusting the position of doors 58 and 60. 
Various other features of the invention are set for in the following claims 
.