Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Method and system for delivering heat through gasification of biomass
FIELD OF THE INVENTION
[0001] The present invention relates to biomass use in thermal
applications. More
specifically, the present invention is concerned with a method and a system
for delivering heat
through gasification of biomass.
BACKGROUND OF THE INVENTION
[0002] Biomass is organic matter that is generally sourced from
waste streams in the
forestry and agricultural industries. As such, biomass is considerably less
expensive, in energy
terms, than fossil fuels. Furthermore, its use in a combustion process is
considered carbon-neutral.
At least for these reasons, biomass is increasingly used in thermal
applications in the residential,
institutional and industrial sectors, delivering a wide array of heating
capacities.
[0003] Because of the multiplicity of biomass sources, the
characteristics of this bio-
fuel vary in chemical composition, moisture, ash content, particle size etc.
For instance, biomass
sourced from residual wood shavings in a saw mill has very low moisture and
ash content, as well as
a small particle size. In contrast, biomass sourced from raw animal manure has
very high moisture
and ash content, as well as a larger particle size. Biomass may also be
sourced from corn stalk, rice
husk, peanut shell, conditioned corn cob, saw dust, wood shavings, paper mill
residues etc.
[0004] In its raw form, biomass generally has poor combustion
characteristics, due to
high moisture, ash content and particle size. For industrial applications, the
biomass is usually
grinded and dried, and as a result has improved combustion characteristics.
However, such "high-
quality" biomass is more expensive and is in much shorter supply than "low-
quality" biomass.
[0005] A number of combustion methods currently exist to burn
various forms of
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biomass, i.e. having various combustion characteristics. Methods using raw
biomass have poor
results in terms of thermal efficiency and ash release in the combustion
chamber.
[0006] A dust burning method uses "high-quality" biomass, i.e.
typically with moisture
content less than 10% and particle size of less than 1 mm. Such biomass has
undergone an
elaborate and expensive conditioning process, including for example grinding
and drying, to
drastically reduce its moisture content and size.
[0007] Given the very low volumetric density of high-quality
biomass, its use also
implies high transportation and storage costs. Consequently, the dust burning
technology often
suffers from uneconomical or insufficient sources of high-quality biomass.
[0008] In addition, the combustion of biomass in conventional
biomass burners
produces residual ashes that are directly admitted to the inside of the
boiler, furnace, or other type of
heat-demanding equipment installed downstream of the burner. The addition of
an emission control
and ash removal system is then needed to eliminate or reduce the presence of
ashes in the heat-
demanding equipment.
[0009] There is still a need in the art for a method and a
system for delivering heat
through gasification of medium-quality biomass.
SUMMARY OF THE INVENTION
[0010] More specifically, in accordance with the present
invention, there is provided a
method for providing thermal energy to a heat demanding equipment, comprising
partial combustion
of biomass in a gasification chamber; and combustion of syngas generated by
the partial combustion
of the biomass.
[0011] There is further provided a system for providing thermal
energy to a heat
demanding equipment, comprising a gasification chamber provided with a fire-
tube; a temperature
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sensor monitoring the temperature within the gasification chamber; a
controlled-speed dosing unit
conveying biomass powder or pellets to the gasification chamber; an air blower
injecting a sub-
stoichiometric quantity of air within the gasification chamber with the
biomass powder or pellets; a
syngas burner receiving hot syngas generated by gasification of the biomass
powder or pellets within
the gasification chamber, from the fire-tube of the gasification chamber, for
combustion; and a control
unit monitoring the temperature and oxygen conditions in the gasification
chamber, and adjusting the
dosing unit according to at least one of: i) the temperature within the
gasification chamber and ii)
thermal heat demand of the heat demanding equipment.
[0012] Other objects, advantages and features of the present
invention will become
more apparent upon reading of the following non-restrictive description of
specific embodiments
thereof, given by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the appended drawings:
[0014] Figure 1 is a schematic representation of a system
according to an
embodiment of an aspect of the present invention; and
[0015] Figure 2 is a schematic representation of a system according to
another
embodiment of an aspect of the present invention.
0012], DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0016] In a nutshell, there is generally provided a system and a
method for burning
medium-quality biomass, including for example biomass powder, i.e. biomass
particles having a
particle size of at most 3 mm and a moisture content between about 15% and
about 20%, and
biomass pellets, i.e. biomass particles having a particle size of about 8 mm
with a moisture content of
at most about 15%.
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[0017] The biomass may originate from wood, such as waste or by-product
of the
forestry industry, or from other sources, such as agricultural and animal
wastes, or pulp and paper
industries and wastewater sludge.
[0018] Biomass pellets may be crushed upstream of the system to
obtain a desired
particle size.
[0019] The present method and system will be described in relation to
Figures 1 and
2.
[0020] Figures 1 and 2 illustrate embodiments of a system of the
present invention,
comprising a storage hopper 1 with an airlock valve or trap door 13, a dosing
conveyor 11, a
pneumatic conveyor 3 and a pneumatic conveyor blower 2 in case of biomass
powder (Figure 1), or
a blower 2' in case of biomass pellets (Figure 2), a gasification chamber 6
with an ash removal
system 9 separated therefrom by a screen 16, an auxiliary fuel tank 5
connected to an auxiliary fuel
burner 4, a temperature sensor 10 monitoring the temperature within the
gasification chamber 6, an
igniter 8 and burner 12 of syngas.
[0021] The biomass is first injected in the gasification chamber
6 placed upstream of
the syngas burner 12, which can be installed onto a heat-demanding eqUipment
(not shown), such as
a boiler, a dryer or a furnace for example. The process of gasification in the
gasification chamber 6 is
a partial combustion, by pyrolysis, which releases an important quantity of
synthesis gases, referred
to as "syngas". In the present system and method, gasification is optimized
and produces small
quantities of residual matter. Ashes are evacuated from the bottom of the
gasification chamber,
through a screen and ash removal system. Thermal energy is produced by
combustion of the
syngas.
[0022] The method comprises feeding biomass into the
gasification chamber 6, in
which a feeding air provides the source of oxygen for gasification. Sensors
connected to a control
unit (not shown) are used to regulate the temperature and oxygen conditions in
the gasification
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producing hot syngas, which is
injected, together with cooling/combustion air using an air blower 7, to the
syngas burner 12, for a
clean and efficient combustion of the syngas. Ash matter is evacuated from the
gasification chamber
6 through a screen 16 and ash removal system 9.
[0023] To start-up the system, the temperature within the
gasification chamber 6 is
first raised to about 600 C, for example between about 580 C and about 620 C.
This may be
achieved by activating an auxiliary fuel burner 4, which operates with a
fossil fuel that is fed from a
dedicated fuel tank 5.
[0024] The gasification chamber 6 has a generally circular or
cylindrical shape so as
to create a swirl effect for the particles suspension entering it as described
hereinbelow.
[0025] The walls of the gasification chamber 6 may be insulated using
refractory
material.
[0026] The walls of the gasification chamber 6 may also be
cooled as part of a heat
recovery process, using a cooling system using air, water or a thermal fluid
for heat transfer and
recovery. In the case of a cooling system using water or a thermal fluid, the
gasification chamber 6
may be constructed using a double walled container, allowing circulation of a
cooling fluid inside the
double wall. The cooling fluid recovering heat from the walls of the
gasification chamber 6 may then
be recycled.
[0027] A temperature sensor 10 connected to the gasification
chamber 6 is used to
monitor the temperature changes inside the gasification chamber 6. When the
temperature in the
gasification chamber 6 falls within a target range between about 600 C and
about 800 C, the
temperature sensor 10 causes the control system to activate the feed of
biomass to the gasification
chamber 6 and to lower the heat load of the auxiliary burner 4. If the
temperature within the
gasification chamber 6 falls outside of this target range, for example by
about 5%, the temperature
sensor 10 triggers the control system to effect adjustments in the biomass
feed rate and the feeding
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airflow, as well as the heat load of the auxiliary burner 4, so as to return
the temperature within the
gasification chamber 6 back to the target temperature range.
[0028] Figure 1 shows a system according to an embodiment of a
system of the
invention, for biomass powder, i.e. a particle size of at most 3 mm.
[0029] The biomass powder is first accumulated in a storage
hopper 1, equipped with
an airlock 13 at the bottom thereof. When the feed of biomass powder is
activated, the airlock 13 is
opened, causing the biomass powder to fall onto a dosing conveyor 11 driven
with a variable speed
motor M, which carries the biomass powder to a pneumatic conveyor 3. Under
action of a pneumatic
conveyor blower 2, a mixture of biomass powder and feeding air is fed through
the pneumatic
conveyor 3 to the gasification chamber 6, tangentially to the inner surface of
the gasification chamber
6. Thus, following this inner surface, the mixture of biomass powder and
feeding air creates a vortex,
which insures an homogeneous mixture of air and biomass within the
gasification chamber 6, which
is found to accelerate the gasification process and facilitates the separation
of ash particles from
combustible matter within the gasification chamber 6.
[0030] The speed of the dosing conveyor 11 is controlled by the
temperature sensor
10. For instance, if the temperature inside the gasification chamber 6 rises
by 10%, the speed of the
dosing conveyor 11 is reduced, causing a reduction of the biomass flow rate
entering the pneumatic
conveyor 3 and, therefore, of the biomass flow rate entering the gasification
chamber 6. Higher
temperature, for example 800, inside the gasification chamber 6 causes the
dosing conveyor 11 to
stop completely.
[0031] The speed of the conveyor 11 may also be controlled by the demand in
thermal energy from the heat demanding equipment, since if there is a smaller
demand, less
biomass has to be fed to the gasification chamber 6. For example, when the
demand of steam/ hot
water in case of a boiler, or of hot air in case of for a dryer, increases,
the consumption of the burner
12 increases, so that the dosing conveyor 11 needs to run faster to provide
the necessary biomass
within the gasification chamber 6.
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[0032] By using a pneumatic conveyer 3 for feeding of the biomass powder
into the
gasification chamber 6, some of the air necessary for the gasification process
and for spreading the
biomass powder inside the gasification chamber, in an homogeneous mixture of
air and powder
within the gasification chamber 6, is provided.
[0033] The dosing conveyer 11 may be a screw conveyor, a single
screw model or a
multiple screw model depending on the application, with a variable speed motor
(M) for modulating
the rate of the feed by modulating the speed of rotation. Alternatively, a
chain conveyor could also be
used.
[0034] To ensure that the biomass powder entering the
gasification chamber 6 has a
particle size of less than 3 mm, a 7 MESH screen (not shown) may be positioned
at the entrance of
the pneumatic conveyor 3. The pneumatic conveyor 3 provides simultaneously the
biomass particles
and air needed for the gasification process. The feeding airflow is controlled
to be sub-stoichiometric
and to keep the biomass particles in suspension in the gasification chamber 6.
[0035] The high temperature, the sub-stoichiometric quantity of
air and the
suspension of biomass particles inside the gasification chamber 6 allow the
gasification, i.e. the
partial combustion of the biomass and, therefore, the generation of syngas.
This gasification also
generates heat in the gasification chamber 6.
[0036] Figure 2 shows a system according to an embodiment of a
system of the
invention, for biomass pellets: i.e. a size up to about 8 mm.
[0037] In this case, the pellets are directly handled from the
hopper 1 to the
gasification chamber 6 by the dozing conveyor 11. As people in the art will
appreciate, a pneumatic
conveyer for pellets would require pipes of a large dimension and the pellets
would break inside the
chamber of the pneumatic conveyer upon impact.
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[0038] A thermocouple or a limit switch may be used to control feeding of
the screw
conveyor 11 in case the temperature inside the screw conveyor 11 increases too
much, for example
in case of backfiring, i.e. presence of a flame that would circulate from the
gasification chamber 6 to
the hopper 1 by propagating through the dosing conveyor 11. Another way to
prevent backfiring is to
install the dosing conveyor 11 with an upward inclination, so that the
discharge of the dosing
conveyor 11, at the gasification chamber 6, is higher than its inlet, at the
hopper 1. Since a flame has
always the tendency to go upwards, in such an arrangement propagation of a
flame through the
dosing conveyor 11 back to the hopper 1 is hindered.
[0039] The sub-stoichiometric feeding air provided by the blower
2' is directly injected
from below the screen 16 into the gasification chamber 6, in an upward and
swirling trajectory, and
keeps the pellets in suspension during the gasification phase. Moreover, the
control unit takes into
account the higher residence time of pellets, by lowering their feed rate into
the gasification chamber
6 correspondingly.
[0040] During the gasification process, ashes fall to the bottom
of the gasification
chamber 6. The bottom of the gasification chamber 6 is equipped with a screen
16, under which an
ash removal system 9 is installed to collect and evacuate the ashes away. The
ashes may be
periodically or continuously removed, depending on the heating capacity of the
system, using a
mechanical automated system, such as, for example, a simple screw conveyor, a
chain conveyor or
a reciprocating container, i.e. an automatic drawer that opens upon command.
The ashes may be
also removed manually, provided a temporary stoppage of the blowers when the
de-ashing doors are
open. The removal of ashes from the gasification chamber 6 allows the system
to maintain high
combustion efficiency.
[0041] The syngas generated by combustion of the biomass rises
to the top of the
gasification chamber 6 and exits through a syngas duct 14, towards a syngas
burner 12. A large
portion of the feeding air inside the gasification chamber 6 is consumed by
the gasification process.
A blower 7 connected to the syngas duct 14 supplies additional air to the
syngas burner 12. The air
flow provided by the air blower 7 allows controlling the temperature in the
syngas burner 12. Being
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equal to or above the stoichiometric level, this air flow also ensures a
complete combustion of the
syngas.
[0042] At the end of the syngas duct 14, the mixture of syngas
and
cooling/combustion air crosses a flame starter 8, which creates a spark to
ignite the combustion
process. Once the mixture is ignited, a flame is created inside the syngas
burner 12, in which the
syngas is completely consumed.
[0043] Thus, according to an embodiment of the present method, a
supply of
biomass powder or biomass pellets is stored inside a storage hopper, with an
airlock valve or trap,
which, when activated, allows the biomass to fall on a dosing unit. With a
speed controlled manually
or automatically, the dosing unit transports the biomass to the entrance of a
pneumatic conveyor in
case of powder, or to the gasification chamber in case of pellets. The biomass
powder is pushed,
through the circuit of the pneumatic conveyor, to the inside of the
gasification chamber with the help
of an air blower (Figure 1). The biomass pellets are injected within the
gasification chamber (Figure
2). An auxiliary fuel burner is used to start the combustion of a wood powder
already inside. The
biomass inside the gasification chamber is quickly heated so its partial
combustion can start. This
partial combustion produces syngas. The difference in pressure makes the
syngas move toward a
fire-tube of the gasification chamber, used as a syngas duct. The syngas are
ignited, producing a
flame that provides the desired thermal energy to a heat demanding equipment.
The ashes inside the
gasification chamber is evacuated manually or using an automatic system. A
cooling fluid circulates
in an outer layer of the wall of the gasification chamber, so it might cool
down.
[0044] The present method and system allow achieving good combustion
performances, in terms of maximizing the combustion's thermal efficiency and
minimizing ashes
release in the combustion chamber, while using biomass that does not require
an elaborate and
expensive conditioning process.
[0045] The present method and system allow and optimized
combustion of medium-
quality biomass in a gasification-combustion burner in order to produce heat
in a manner that
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[0046] The scope of the claims should not be limited by the
embodiments set forth in
the examples, but should be given the broadest interpretation consistent with
the description as a
whole.
