Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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GRATE AND METHOD OF BURNING A GRANULAR FUEL MATERIAL
TECHNICAL FIELD
The technical field relates generally to grates and methods of burning
granular fuel materials so
as to produce heat energy with an increased overall thermal efficiency as well
as a reduction of
gas and particle emissions in the atmosphere.
BACKGROUND
Many different models of heat generators have been suggested over the years
for burning fuel,
thereby producing heat energy for a given purpose. Existing heat generators
vary in size,
configuration, shape and efficiency, to name just a few of the differences
between them. The
type of fuel being used to generate the heat and the heat output requirements
are two examples of
factors that generally have an impact on their design.
While maximizing thermal efficiency is almost always one of the goals when
designing a heat
generator, further increasing the thermal efficiency above levels already
obtained using existing
approaches is a continuous challenge since this has a direct impact on the
operational costs.
Goals set for reducing gas and particle emissions in the atmosphere also
prompts designers to
optimize the thermal efficiency, especially in large commercial or industrial
installations. One
way to express the thermal efficiency of a heat generator is to measure the
heat energy output per
given quantity of material burned therein.
Some heat generators are designed for burning one or more granular fuel
materials, for instance a
biomass material such as corn cobs, sawdust, scrap or loose wood fragments,
etc. Many other
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variants exist. Many such biomass materials are often considered waste
byproducts and are often
simply discarded or not used for generating heat. While most such fuels are
not particularly
efficient compared to other possible fuels, they have the advantage of being
generally economical
and widely available in some areas, in particular some rural areas.
The heat generators designed for burning granular fuel materials often include
a grate to support
the burning fuel and promote air circulation through it. A grate generally
includes perforations
and/or spaced-apart bars. Increasing the available air generally increases the
combustion
efficiency, i.e. the capacity to burn all fuel matter. However, increasing the
air feed can also
decrease the overall heat transfer efficiency of the heat generator since the
temperature of the hot
gases from the combustion decreases when the air is in excess. The excess air
is also increasing
the losses at the chimney by increasing the mass of unused heated air released
in the atmosphere.
Minimizing the excess air is thus highly desirable for maximizing the thermal
efficiency. With a
greater thermal efficiency, less fuel is needed and therefore, gas and
particle emissions are
reduced. A reduction of the excess air can also reduce the amount of particles
being carried away
out of the chimney.
While the existing approaches for burning granular fuel materials have been
successful in terms
of heat production, there is still room for many improvements in this area of
technology,
particularly for further increasing the overall thermal efficiency.
SUMMARY
The proposed concept relates to a grate and a method of burning a granular
fuel material in which
the distribution efficiency of the primary air is controlled using the grate
itself. Unlike existing
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grates, the char that forms near the end of the burning process is
concentrated in a conduit located
under the perforated bed floor of the grate, where it burns until only ashes
are left. This way, the
perforated bed floor can always remain covered with granular fuel materials
and the primary air
is substantially prevented from bypassing the grate through uncovered
perforations of the bed
floor.
In one aspect, there is provided a substantially horizontally-disposed grate
for burning a granular
fuel material to be fed onto a loading area of the grate while an air feed is
coming from below the
grate, the grate including: a perforated bed floor having a downwardly-sloping
upper surface
converging towards a discharge opening where char is concentrated as the
granular fuel material
is burned during operation; and an elongated and bottom-perforated char-
receiving conduit
positioned immediately under the bed floor, the char-receiving conduit having
an inlet end
positioned under the discharge opening, and an outlet end that is opposite the
inlet end, the char-
receiving conduit downwardly sloping between the inlet end and the outlet end.
In another aspect, there is provided a method of burning a granular fuel
material, the method
including the concurrent steps of: loading the granular fuel material in a
loading area of a
substantially horizontally-disposed bed floor; vibrating the bed floor to move
the granular fuel
material from the loading area towards a discharge opening located away from
and vertically
below the loading area; feeding primary air across the bed floor, the primary
air coming from a
bottom side and passing through a multitude of spaced-apart perforations made
in the bed floor;
drying, mostly by radiation heat, the granular fuel material immediately after
the granular fuel
material is loaded onto the bed floor; transforming, by pyrolysis, the dried
granular fuel material
into volatile compounds and char, and generating heat above the bed floor;
collecting and
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concentrating the char passing through the discharge opening into an elongated
chamber
extending substantially horizontally underneath the bed floor; and generating
heat by burning the
char inside the chamber as the char is moved from the discharge opening
towards an outlet end of
the chamber by the vibrations of the bed floor.
Further details on these aspects as well as other aspects of the proposed
concept will be apparent
from the following detailed description and the appended figures.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic view illustrating an example of a generic heat generator
for burning a
granular fuel material;
FIG. 2 is a side view illustrating an example of a grate having a construction
based on the
proposed concept;
FIG. 3 is a top isometric view of the grate of FIG. 2;
FIG. 4 is a view similar to FIG. 3, taken from another angle;
FIG. 5 is an enlarged top view of the grate of FIG. 2;
FIG. 6 is a side view illustrating another example of a grate having a
construction based on the
proposed concept;
FIG. 7 is a top isometric view of the grate of FIG. 6; and
FIG. 8 is a bottom isometric view of the grate of FIG. 6.
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DETAILED DESCRIPTION
FIG. 1 is a schematic view illustrating an example of a generic heat generator
10 for burning a
granular fuel material. This heat generator 10 can be used, for instance, as a
furnace or a boiler.
The heat energy can be transferred to a fluid passing through a heat exchanger
or be directly used
5 for another purpose, such as to heat a pressure vessel around which the hot
flue gases circulate.
Other configurations and arrangements are possible as well.
The heat generator 10 includes a casing 12 inside which a grate 14 is
provided. The grate 14
either fills the entire internal width of the casing 12, as schematically
shown, or is either mounted
on a supporting structure preventing air under the grate 14 from bypassing it
around its periphery.
The grate 14 is designed to hold the granular fuel material while it bums
continuously after being
loaded thereon and ignited. Ignition is done using one or more of the possible
ignition methods,
as known to those skilled in the art.
The grate 14 is disposed substantially horizontally, meaning that the grate 14
is acting as a
receptacle over which the granular fuel material is supported by gravity. An
example of granular
fuel material is a biomass material, such as corn cobs, sawdust, scrap or
loose wood fragments,
etc. Coal is another example of a granular fuel material. Many other variants
exist. The granular
fuel material can be a homogenous material or a mix of two or more materials,
regardless
whether the expression refers to "material" or "materials". Also, the term
"granular" as used in
the present context means a particle or a small piece, such as for example but
not limited to,
having a size ranging from about a fragment of a few millimeters in length to
about a coarse
granule of a few centimeters in length, as generally understood by those
skilled in the art. When
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used with "fuel material" or "fuel materials", the term "granular" refers to a
burnable substance
that can be handled in bulk and that is not a gas or a liquid, as generally
understood by those
skilled in the art.
For the sake of simplicity, the granular fuel material will simply be referred
to as "fuel" from this
point onwards.
As schematically shown in FIG. 1, the fuel is fed to the grate 14 from a fuel
source 16. The grate
14 is configured and shaped to hold a given quantity of fuel and fuel will
cover almost the entire
bed floor when the heat generator 10 is in operation. The fuel is loaded onto
the grate 14 at a
loading area located on the upper surface of a perforated bed floor of the
grate 14. The fuel falls
by gravity onto the bed floor, for instance coming from an endless screw
conveyor. Variants are
possible as well.
Air coming from a primary air source 18 is supplied under the grate 14 when
the fuel is burning.
The primary air source 18 is for instance a blower or any other suitable
device. The primary air
source 18 generates a primary air feed 20. The primary air feed 20 reaches the
bottom of the
grate 14 and then passes through perforations provided across the thickness of
the perforated bed
floor because of a pressure differential between both sides thereof. The exact
size, shape and
spacing of the perforations depend on various factors. The size of the
perforations will depend,
among other things, on the size of the fuel pieces. It is of course desirable
to prevent fuel pieces
from falling by gravity through the perforations. Still, the perforations are
not necessarily made
or all made with a circular cross section. For instance, the perforations can
be oblong or can even
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have any other shapes, such as rectangular, octagonal, etc. They can also have
an irregular shape
or even be tapered. Other variants are possible.
The burning process occurring at the grate 14 generates heat (radiant and
convective) as well as
gases, among other things. These gases rising from the grate 14 still contain
inflammable gases
in form of volatile compounds, especially when the primary air feed 20 does
not supply enough
oxygen for a complete combustion. An example of volatile compound is carbon
monoxide (CO).
The combustion is completed in a zone 22 located above the grate 14. This zone
22 receives
additional air from a secondary air source 24. The secondary air source 24
generates a secondary
air feed 26. The secondary air source 24 is for instance a blower or any other
suitable device.
Typically, the primary air feed 20 is about 35 % of the total supplied air and
the secondary air
feed 26 is thus about 65 % of the total supplied air. The relative proportions
of the primary and
the secondary air feed can be controlled manually and/or using an automated
control system. The
control system can also modulate the air flow in function of the amount of
fuel being supplied.
Other configurations, arrangements and proportions are also possible.
In the illustrated example, the hot gases coming from the zone 22 pass through
a heat exchanger
28 where convective heat energy is collected. The heat exchanger 28 also
receives radiant heat
from the burning fuel. This heat exchanger 28 has an internal fluid circuit
connected to an
incoming conduit 30 and an outgoing conduit 32. The outgoing conduit 32 sends
a heated fluid
where or closer to where the heat energy is needed. The incoming conduit 30
and the outgoing
conduit 32 can form a closed-loop circuit and/or an opened-loop circuit,
depending on the needs.
Variants are also possible.
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In some implementations, one or more additional heat exchangers (not shown)
can be provided to
recover more heat energy from the gases downstream the heat exchanger 28. The
gases
eventually exit the casing 12 as flue gases 34. The flue gases 34 can be
discarded through a
chimney and/or used in another process. It should be noted that the flue gases
34 often contain
small particles in suspension. Nevertheless, they will still be referred to as
"gases" for the sake of
simplicity.
The illustrated heat generator 10 is designed to be operated in a continuous
manner, meaning that
the fuel bums continuously for as long as fuel is supplied or unless the
combustion is abruptly
stopped for some reason. Accordingly, fuel is loaded continuously or at given
intervals (regular
or not) while the burning process is ongoing. A portion of the fuel that was
put on the perforated
bed floor will transform into granular char and a portion will transform into
the volatile
compounds to be burned as well in the zone 22. Typically, about 80% of the
carbon from the fuel
will be transformed in volatile compounds and about 20% will become char. The
combustion of
the volatile compounds forms the visible flames in a fire while the char is
seen as glowing red
coals or embers which often burn without the presence of flames. On average,
the volatile
compounds will require about two times more oxygen than char to bum.
The char will eventually form ashes and other solid debris that need be
removed from the grate
14. Debris can be, for instance, fragments or pieces (such small rocks, sand,
metal fragments,
etc.) that cannot burn at the temperatures involved. Other kinds of debris are
also possible. For
the sake of simplicity, the terms "ash" and "ashes" are meant to include
debris present therein, if
any. In FIG. 1, the ashes are removed from the bottom of the grate 14 and out
of the casing 12
using an ash removal system, which system is schematically depicted in FIG. 1
at 36. The ash
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removal system 36 can include, for instance, an endless screw carrying the
ashes outside for
disposal. Other kinds of systems are also possible.
In use, the grate 14 is vibrated to progressively move the fuel over the
perforated bed floor of the
grate 14. The vibrations can be generated using a vibrations generator that is
connected to or
mounted on the grate 14. The vibrations generator is schematically depicted in
FIG. 1 at 40. The
vibration generator 40 of the illustrated example is located outside the
casing 12 and is
mechanically connected to the grate 14 using a link that is schematically
depicted in FIG. 1 at 42.
This vibration generator 40 could also be located inside the casing 12 in some
implementations.
Other configurations and arrangements are also possible. The grate 14 is
supported at its
periphery by a suitable supporting arrangement which, however, is not part of
the proposed
concept and does not need to be described furthermore. The vibrations can be
continuous or
intermittent, depending on the implementations.
The grate 14 can be made of a material such as a metal or a coated metal
capable of withstanding
the temperatures involved over long time periods while the fuel is burning.
These temperatures
can be up to about 800 C, sometimes even more.
If desired, the grate 14 can include an internal cooling circuit, for instance
an internal cooling
circuit having a network of conduits designed to keep some of the parts of the
grate 14 below a
given temperature. The internal cooling circuit and the associated cooling
system located outside
the casing 10 are schematically depicted in FIG. 1 at 50.
The grate 14 can be constructed like the one of the example illustrated in
FIG. 2. In FIG. 2, this
grate is referred to as the grate 100. The grate 100 has a perforated bed
floor 102 that is generally
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conical in shape. Its periphery is also generally circular in shape, as best
shown in FIGS. 3 and 4.
FIGS. 3 and 4 are both top isometric views of the grate 100. FIG. 5 is an
enlarged top view of the
grate 100.
The perforated bed floor 102 of the illustrated grate 100 is made of a
plurality of juxtaposed flat
5 panels 104, for example panels welded together along their edges so as to
form a downwardly-
sloping upper surface. The average inclination of the bed floor 102 can be
generally between 5
and 25 with reference to the horizontal, although other values are possible
as well. The panels
104 form a funnel-like structure that will hold the fuel when the grate 100 is
disposed
substantially horizontally. The illustrated grate 100 also includes a circular
rim 106 located
10 around the periphery of the bed floor 102. The rim 106 has a plurality of
axisymmetric holes 108
for connecting the grate 100 to a supporting arrangement or the like.
It should be noted that the bed floor 102 can be constructed differently. For
instance, one can use
a single panel and shape it as desired in a large press or the like. Other
constructions and ways of
mounting the grate 100 inside the casing 10 are also possible.
The grate 100 has a loading area 110. The upper surface of the bed floor 102
converges towards
a discharge opening 112 that is somewhat located away from the loading area
110. Also, the
discharge opening 112 of the illustrated grate 100 is offset with reference to
a geometric center of
the upper surface of the bed floor 102. This was made to maximize the length
of the path of the
fuel over the grate 100. Nevertheless, using another configuration is also
possible.
The discharge opening 112 of the illustrated grate 100 is located within the
periphery of the upper
surface of the bed floor 102, thus inside the rim 106. Variants are possible
as well. For instance,
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one can design a grate with a discharge opening 112 that is located at the
edge of the periphery of
the bed floor 102.
In use, the various steps of the burning process occur concurrently since the
fuel bums
continuously, unlike for instance a heat generator using a liquid fuel or gas
fuel for which
interrupting the burning process is much easier. Once on the bed floor 102,
the fuel is vibrated
and will progressively move from the loading area 110 towards the discharge
opening 112. The
discharge opening 112 is located away from and vertically below the loading
area 110. Thus,
using the vibrations, the fuel will progressively move towards that location
as it burns.
As best shown in FIG. 2, the grate 100 includes an elongated and bottom-
perforated char-
receiving conduit 120 positioned immediately under the bed floor 102. The
illustrated char-
receiving conduit 120 has an inlet end 120a which includes an upper opening
positioned directly
under the discharge opening 112, and an open-ended outlet end 120b that is
opposite the inlet end
120a. The char-receiving conduit 120 downwardly slopes between the inlet end
120a and the
outlet end 120b. The average inclination can be generally between 5 and 20
with referenced to
the horizontal, although other values are also possible as well.
The char-receiving conduit 120 is entirely supported by the bed floor 102. For
instance, the char-
receiving conduit 120 of the grate 100 can be welded or otherwise attached
underneath the bed
floor 102 around the periphery of the discharge opening 112. One can also use
brackets or the
like, if desired. The bed floor 102, the rim 106 and the char-receiving
conduit 120 form a
compact monolithic unit. Variants are possible as well.
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In the illustrated example, the char-receiving conduit 120 is substantially
tubular in shape.
Nevertheless, other shapes and configurations are possible as well. For
instance, the char-
receiving conduit 120 could have a rectangular cross section or any other
shape (oval, triangular,
etc.) The size and shape of the char-receiving conduit 120 can also vary along
its length. The
inlet end 120a of the char-receiving conduit 120 includes an inclined end wall
panel 122 so that
the char received from the discharge opening 112 can only go towards the
outlet end 120b. The
outlet end 120b, however, is open ended. Alternatively, one can provide a char-
receiving conduit
120 with an end wall panel (not shown) at the end 120b and use a large bottom
opening adjacent
to the end 120b as the ash outlet/air intake.
The cross-sectional area of the char-receiving conduit 120 can be generally
about 2 to 4% of the
area of the upper surface of the bed floor 102. These values should provide
very good results in
most implementations. Nevertheless, other values are possible as well.
The illustrated grate 100 has a vertical plane of symmetry depicted by line
130 in FIG. 5. The
char-receiving conduit 120 of the grate 100 has a longitudinal axis that
extends substantially
parallel to the plane of symmetry 130. This way, the heat generated inside the
char-receiving
conduit 120 will spread evenly on both sides of the bed floor 102.
Nevertheless, one can
construct a grate that is not symmetrical or not entirely symmetrical.
One of the goals of the grate 100 is to maximize the thermal efficiency of the
heat generator by
optimizing the use of the amount of the primary air from the primary air feed
coming from below
the grate 100. The approach involves using different amounts of air for the
different stages of the
burning process occurring on the grate 100. It also involves the fact that the
perforations on the
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bed floor 102 are always covered with a layer of fuel by way of the
concentration of the fuel
matter along the end of the grate 100 and that the concentrated char will burn
under the bed floor
102 in a specially and specifically designed component of the grate 100.
The stages of the burning process can be roughly segmented, for instance, as a
drying stage, a
pyrolysis stage, a fuel combustion stage and a char combustion stage. While
the boundaries
between the various stages are not necessarily clearly visible in practice
within the fuel mass, it is
possible to predict by mathematical models based on the physic of combustion
of fuel where each
stage will approximately happen for a given type of fuel. The present concept
uses this
predictability to better control the amount of the primary air to be supplied
to the fuel and
optimizing the solution through the design of the grate 100 itself. This is
done by selecting one
or more perforation patterns of the grate 100 instead of using a segmented
primary air feed, for
example. A segmented primary air feed generally involves using a plurality of
compartments
directing different streams of the primary air to specific locations on the
underside of a grate.
While this approach may perhaps still be useful in some implementations, it is
more desirable to
use only a single primary air feed to lower both costs and complexity.
The first stage is the drying stage. Not all fuels necessitate a drying stage
but most biomass fuel
materials will require one since they often have relatively high moisture
contents. In the drying
stage, the fuel mostly uses the intense radiant heat coming from the
combustion in the subsequent
stages to evaporate this moisture. Convective heat may also contribute to
drying the fuel but at a
lesser extent. The primary air requirement is the lowest at the drying stage
since there is
essentially no combustion. The drying stage occurs at and around the loading
area 110, generally
at a temperature of about 100 C.
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The next stage is the pyrolysis stage. Pyrolysis can be broadly defined as a
thermochemical
decomposition of organic material at elevated temperatures. Using the oxygen
contained in the
primary air, the carbon material then transforms itself into char (fixed
carbon) and volatile
compounds (volatile carbon). The volatile compounds will generally start
forming at about
250 C. The rate of pyrolysis will increase as the temperature increases in the
combustion
chamber. The temperature in the combustion chamber can even reach as high as
1200 C
depending on the type of fuel used.
The combustion stage occurs after the pyrolysis stage. In the fuel combustion
stage, more air
(thus more oxygen) is generally needed compared to the preceding stages.
Generally, the
primary air feed is calculated so that the entire oxygen content of the
primary air will be used at
the grate 100. The combustion of the volatile compounds will thus be
incomplete. The
combustion will be completed above the grate 100 using the secondary air
provided downstream.
It should be noted that the production of volatile compounds also continues
during the
combustion stage. It will continue until only char is left. The main
differences between the
pyrolysis stage and the combustion stage include the amount of oxygen
available and the amount
of heat being generated.
The grate 100 is designed so that most of the fuel becomes char when it
reaches the discharge
opening 112. The char then slowly sink into the char-receiving conduit 120,
where it is further
concentrated and where it bums right underneath the bed floor 102. The char-
combustion stage
is the last stage of the burning process.
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Ashes are formed as a result of the combustion of the char and, as aforesaid,
exit through the
open-ended outlet end 120b. Ashes generally represent from 1 to 4% of the
total mass of fuel
provided over the grate 100, depending on the fuel grade.
It should be noted that while char is also some fuel by definition, the
skilled reader will
5 understand that the distinction between "fuel" and "char" is only made in
the context of the
transformation of the fuel during the combustion process.
As shown in FIG. 5, the bed floor 102 of the illustrated grate 100 has five
different sets of
perforation to control the amount of the primary air passing there through.
The perforations of
the perforation patterns vary in size, shape, density and/or diversity. Each
perforation pattern on
10 the bed floor 102 forms what is referred to hereafter as a region. The
density refers to the relative
spacing between the perforations while the diversity refers to the possible
combination of two or
more different kinds of perforations. Still, one can use identical
perforations in some or even all
of the regions. One can also design a grate with fewer regions of distinct
perforation patterns.
Using a single region is even possible is some very simple designs.
15 During operation of the heat generator, fuel is loaded on the perforated
bed floor 102 of the grate
100 up to a given level. Because of the vibrations to which the grate 100 is
subjected, the fuel
will be scattered over the entire bed floor 102 and the fuel level will tend
to be leveled on the top
thereof. The perforations of the bed floor 102 are constantly covered by some
fuel and the
thickness of the fuel mass is thus another factor to consider. Constantly
covering the perforations
will create an air restriction, especially if the fuel pieces are relatively
small as they will be more
densely packed than larger ones, thereby preventing the primary air from by-
passing the grate
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100 to create excess air diluting the hot gases above the grate 100. This
approach will greatly
improve the overall thermal efficiency.
In the illustrated grate 100, the first region is adjacent to the periphery of
the upper surface of the
bed floor 102 and includes the loading area 110. This first region corresponds
approximately to
the drying stage for the fuel pieces that are near the upper surface of the
bed floor 102.
It should be noted that during operation, fuel is loaded, as aforesaid, up to
a given level on the
perforated bed floor 102. Therefore, fresh fuel arriving at the loading area
110 over the fuel mass
already present will not necessarily follow a straight line from the loading
area 110 to the
discharge opening 112. For instance, some fuel pieces will rather follow an
arcuate path near the
top of the fuel mass. The flow of fuel on the bed floor 102 is tridimensional
in nature when
considering the fuel mass. The design of the grate 100 takes into account the
time taken by all
fuel pieces to travel from the loading area 110 down to the discharge opening
112. This is the
reason why the first region (which is two-dimensional in nature) relates to
the corresponding
perforation pattern and is only indicative of where the drying stage
approximately occurs for the
fuel pieces that are near the upper surface of the bed floor 102.
The second region of the illustrated grate 100 is located closer to the
discharge opening 112 and
surrounds the periphery of the first region. It corresponds approximately to
the pyrolysis stage
for the fuel pieces that are near the upper surface of the bed floor 102.
As can be seen in FIG. 5, the first region and the second region are wider
along the plane of
symmetry 130 than on their sides. This takes into account the fact that fuel
pieces will tend to
travel more quickly when they are near the upper surface of the bed floor 102
compared to fuel
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pieces at the top of the fuel mass. Another factor that can be taken into
account is the heat. Fuel
pieces receiving more heat than others will dry faster and complete the
pyrolysis stage faster, for
instance.
The third region of the illustrated grate 100 is located between the periphery
of the second region
and the periphery of the discharge opening 112. This third region corresponds
approximately to a
portion of the fuel combustion stage for the fuel that is near the upper
surface. The fourth and
fifth regions are located further away from the loading area 110. They
correspond approximately
to other portions of the combustion stage for the fuel pieces that are near
the upper surface of the
bed floor 102.
As aforesaid, the transformation of the fuel into the volatile compounds and
the char is completed
about the time the fuel (now in the form of char) reaches the discharge
opening 112. Char pieces
will then fill the entire width of the discharge opening 112 and will slowly
progress into the char-
receiving conduit 120.
The char-receiving conduit 120 has bottom perforations to receive primary air.
These
perforations can include one or more different patterns. For instance, the
perforations near the
outlet end 120b can be smaller to prevent the progressively smaller char
pieces and the ashes
from falling through. Air can also enter the char-receiving conduit 120
through the outlet end
120b. Thus, as a skilled reader will understand in the context, the outlet end
120b of the char-
receiving conduit 120 is also a primary air inlet and the discharge opening
112 is also a primary
air and combustion gases outlet.
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In use, the char will progress along the char-receiving conduit 120 because of
the vibrations to
which the grate 100 is subjected. The gases resulting from the combustion of
the char will escape
through the discharge opening 112. The thickness of the char layer inside the
conduit 120 will
diminish progressively from the inlet end 120a to the outlet end 120b. The
perforations of the
char-receiving conduit 120 near the outlet end 120b will only be covered by a
progressively
thinner layer of char. Some perforations may also be completely uncovered.
Nevertheless, the
presence of concentrated char inside the inlet end 120a of the conduit 120
(thus, inside the
discharge opening 112) will prevent the primary air from flowing in large
quantities across the
discharge opening 112. Thus, unlike existing grates, the location where the
char burns will not
generate unused primary air that would only increase the excess air and lower
the overall thermal
efficiency.
Still, providing the char under the bed floor 102 in a concentrated manner
will maximize the heat.
The char will also consume a good amount of the oxygen from the primary air.
The heat
generated therein will be transferred to the bed floor 102 through radiant
heat and also some
convective heat. The primary air coming through the perforations of the bed
floor 102 and from
the discharge opening 112 will already be pre-heated to some extent.
In the example shown in FIGS. 2 to 5, the grate 100 is designed so that the
overall primary air
feed passageway of the first region will be about 6% of the primary air feed.
The second, third,
fourth and fifth regions will provide about 19 %, 25 %, 30 % and 16 % of the
total primary air feed,
respectively. The balance (4%) will come through the discharge opening 112.
Other designs,
configurations and proportions are possible as well.
CA 02771112 2012-03-21
19
FIG. 6 is a side view illustrating another example of a grate incorporating
the proposed concept
and for use in a heat generator such as the heat generator 10 shown in FIG. 1.
This grate referred
to as the grate 200. It includes a perforated bed floor 202 having an upper
surface with a rim 204
around its periphery. The perforated bed floor 202 is generally rectangular in
shape, as best
shown in FIGS. 7 and 8. FIGS. 7 and 8 are a top isometric view and a bottom
isometric view of
the grate 200, respectively. The perforated bed floor 202 is made using a
plurality of flat panels
206 that are welded together. Variants are also possible. The grate 200 has a
loading area 208.
The grate 200 includes two discharge openings 210 and two char-receiving
conduits 212 with
bottom perforations. This feature can also be implemented on another kind of
grate, such as the
grate 100. The discharge openings 210 of the grate 200 are disposed side-by-
side and are located
adjacent to the rim 204 of the perforated bed floor 202.
Still, one can design the grate 200 with only one discharge opening 210 and
only one char-
receiving conduit 212. Other possible configurations and arrangements include
using more than
two discharge openings 210 and more than two char-receiving conduits 212,
and/or using a grate
having two or more spaced-apart loading areas converging towards one or more
discharge
openings 210. Still, one can use two or more char-receiving conduits 212 with
only one
discharge opening 210. Many other combinations are possible as well.
Also shown in FIGS. 6 and 8 are examples of brackets 220 for attaching the
char-receiving
conduits 212 underneath the bed floor 202 of the grate 200. Variants are also
possible.
As can be appreciated, the proposed concept provides a way to better control
the amount of
primary air when generating heat energy using a granular fuel material fed on
a grate. It also
CA 02771112 2012-03-21
provides a way of designing a grate that is very compact. This grate can
improve the overall
thermal efficiency of a heat generator.
The present detailed description and the appended figures are meant to be
exemplary only. A
skilled person will recognize that variants can be made in light of a review
of the present
5 disclosure without departing from the proposed concept.
