Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF TE~E INVENTION:
Field of the Invention
_ _ _
The present invention relates to a
plant for the gasification of biomass for production
of fuel gases that may be used directly for combustion,
in the case of low BTU gas, or be treated to form medium
BTU substitute natural gas useful for industrial
processes (for drying, steam production, space heating
and lighting, conversion into electrical energy, etc).
The invention constitutes also a solution to the waste
disposal problem.
The term "biomass", as used in this description
and the appended claims, includes solid wastes, peat,
coal, wood residues such as wood chips and sawdust,
organic and inorganic residues including solid and semi-solid carboneous
materials as well as shale and cellulosic fibers.
Also, the expression "oxygen-containing gas"
is to be understood to mean either air or enriched air or oxygen.
Description_of the prior art
The Applicants are well aware that quite exten-
~` sive research, development and experimental work has
been made to convert waste material into fuel gas useful
directly for combustion or for conversion into industrial
process gases. A typical and very pertinent document,
in this respect, is U.S. patent No. 4,592,762 of June
2nd. 1986. However, the process disclosed in this patent
rsmains applicable only on an experimental stage as
~ many parameters involved do not permit a large scale
; ~ profitable operation. Thus, while it is said that the
moisture content of the biomass may be reduced to 5%
to 50~, it is added that if the reduction is from 5~
to 40~, steam has to be added to the fluidized bed which,
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of course, produces a highly moist output gas which has
to be more heavily treated either for direct combustion
or for use in industrial processes in general. Additionally,
the fluidized bed is fed, according to this patented
process, intermittentl~v which affects its efficiency
and shows that the process is still in the experimental
stage. Notable also is the fact that while the disclosure
of the patent mentions feeding the biomass into the fluidized
bed, the precise location where feeding is to be made
is not given which, again, may be detrimental to the
proper and efficient functionning of the bed.
In general, the litterature known to the Applicants
is unable to teach a process and disclose a plant suitable
to produce low and medium BTU gases on an industrial
scale, that is having a high yield of BTU gases recuperated
at a relatively low cost.
SUMMARY OF THE INVENTION:
The invention proposes a plant
to produce methanol and/or a low to medium BTU gas, which
makes use of a fluidized bed gasifier using air or oxygen.
A specific fluidization grid is provided in
the gasifier which allows a large range of gaslfication
procedures, whatever be the feeding required to reach
the fixed pressure and/or temperature, and which is pro~ected
during the preheat mode from overpressure, that is over
270kPa.
The location of the biomass feed point is or.e
over the more important gasifier design parameter and
the following factors should be considered carefully.
To prevent local defluidization due to an increase
in minimum fluidization velocity (Umf), the biomass concen-
tration in the bed, particularly in the case of wood or similar
matters, should be kept below 12%. Even at this value,
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if the system is operating in a zone of the bed where
the superficial velocity (Uo) is close to Umf, for ins-
tance at the bottom of the b~d, then the wood concentra-
tion should be kept to about 8%.
The rate of feed in volume of the biomass contain-
ing wood, particularly at high pressures, must be quickly
dissipated into the reactor bed. Accoxding to the inven-
tion, the biomass flow is 12T/Hr, at 20% moisture, which
compares with a bed weight of 2.8 T. Thus to prevPnt
fluidization problems, the biomass should be gasified
within 1.6 minutes (l minute for oxygen gasification)
on assumption that the biomass is completely mixed with
; the bed. If not, then the gasification time should be
shorter. The volume flow of the biomass is also a signi-
ficant factor. At an assumed bulk density of 12 lbs/ft3
the volumetric flow is 37ft3/m which is twice the volume
of the bottom section of the bed or half the total bed
volume.
Biomass dissipation rates must be greater than
the rate of biomass flow into a section of the bed other-
wise the biomass will flood that section.
For example, it is assumed that Umf is = 4cm/sec,
at operating conditions, then at twice Umf, the wood
dissipation rate is .023gm/cm2/s = 485Kg/Hr. At lO times
Umf, the rate in the bed becomes equal to 4370 Kg/Hr or about half
the requirement for norrnal oxy-gasifier-flow.
The temperature distribution within the bed
must also be controlLed so that hot spots do not develop
; and that heat produced in the combustion zone dissipates
into the pyrolysis zone. Since the chemical reactions
are not instantaneous, the biomass must be dissipated
into the bed, otherwise, high local concentration and
consequent defluidization will occur.
A pre-reaction stage requires a finite amount of time
to drive the water from the fuel reaction and to bring
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-the particle temperature to 400 C where pyrolysis is
initiated, the pre-reaction times being related to particle
size or to the minimum particle dimension and moisture
content. With small particles, this pre-reaction time
is of the order of 5 to 10 seconds (chips require a greater
interval before reaction, of about 1 minute).
Since the principal method of char conversion
is oxidization, char oxidization is much slower than gas
combustion at 800 C. To maximise char conversion, it
is preferable that pyrolysis gases be absent from the com-
bustion zone at the bottom section of the bed.
Tar is also produced in the reaction but is de-
troyed by reaction in the hot bed. Then, there is only
a small increase in tar yield as pyrolysis takes place
at the battom zone of the bed (less than 10%).
The feed point has to be located well above the
fluidization gas injection grid. Pyrolysis near the grid
would result in very high local temperature since there
already are dead zones in this area (any further defluidi-
zation would be detrimental and would very quickly be self
compounding). In addition, the volume into which dissipa-
tion can occur would be very limited. Biomass introduced
at 2/3 level, permit distribution both up and down. Biomass
introduced at the bottom of the bed permit only dissipation
vertically upwards, not downwards. So, introduction into
the upper part of the bed encourages the oxidation of char
and if any defluidization should occur in this region,
it will not affect the performance of the remainder of
the bed. It is also necessary to ensure that biomass be
.
introduced in the area of maximum velocity to encourage
~ 30 rapid dissipation into the bed and to prevent problems
- of local defluidization.
The pyrolysis reaction takes time to occur and
blomass will dissipate Into the~lower aswell as upper section
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of the bed. Same pyrolysis will occur on the lower section
with its concomitant increase in gas flow thus further
increasing the rate of mixing (Uo - Umf) so that dissipation
rates in this upper area will be 2-3 times greater than
in a zone close to the distributor.
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The feed rate must be controlled to permit
the introduc-tion of the biomass into the fluidized bed
gasifier on a continuous basis so the feeder will auto-
matically compensate for change in quantity, size, composi-
tion, moisture content.
In order to maintain a constant plug density
of biomass, a pressure is applied to the plug which
depends on the power used by the feeder motor. According
to the invention, the compression screw provides a flow
of biomass against a cone actuated by an air cylinder
- facing the feeder's pipe section which chokes the biomass,
thus building a constant pressure against it and thereby
- 15 causing the biomass to compress into a plug.
The process of the invention re~uires an auto-
mated and computerized system to ensure and to thoroughly
monitor logging aswell as processing of the data. The
operation is monitored and controlled by process ins-
truments and controllers. These instruments and control-
lers aswell as recorders and math units are linked to-
gether by control loops. Each loop has a specific func-
tion: some require the operator to manually adjust opera-
ting set points on the controllers, others have pre-
determined set points. Lack o~ operator attention
- can initiate automatic system shutdown if operating
conditions exceed or fall below pre-determined set points.
However, a few control loops are connected to recorders
; and alarms that inform the operator of operating and
alarms conditions on a continu~ous basis.
The temperature in the fluidized bed is con-
trolled by temperature control loop TIC that in turn
set the flow of air (or oxygen) through the fluidized
bed.
As a backup to the temperature control, high
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temperature alarms (TAH) and extra alarms (TAHH) will
notify the operator and shutdown the gasifier if necessary
(or in case of lack of action by the operator); low
temperature alarm (TAL) and extra low alarm (TALL) will
do the same on low temperatures.
Differential pressures (PDT) are measured and
recorded along the gasifier at the following points:
gasifier bed differential pressure (between the head
space and the bottom of the fluidized bed); gasifier
distributor diEferential (between the distributor header
and fluidized bed) and gasifier lower bed differential
pressure (between the bottom of the fluidized bed and
the middle of the bed).
Gasifier pressure (PT) and alarm (PAH) are
also measured and recorded.
During operation, the area within the gasifier
separates into two zoness the fluidized bed portion
and the head space. In the first portion, the biomass
comes into intimate contact with the fluidized sand
of the bed and is gasified. In the upper section, most
of the heavier particles in motion (biomass and sand)
disengage and fall back into the fluidized bed. The
lightest fraction of airborneparticl~s are carried over
into the gas stream from the head space and are separated
in the produced gas cleaning equipment known as "cyclone".
The material collected in the cyclone consisting
mainly of fluid bed sand and unburned wood (char) are
returned t~ the fluidized bed by a dip leg which has
; ~ a check valve preventing back flow should the process
fail one way or the other.
~; Speciically and in accordance with the invention~
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there is provided a plant for the gasification of biomass of
the type comprising:
- a reaction vessel;
- a bed of sand in said vessel;
S - means for feeding biomass as a given rate into
said bed of said in said vessel;
- means at the bottom of said bed for feeding, at
another given rate, and distributing through said bed an
oxygen-containing gas under pressure to hold said bed in
fluidized state while leaving a head space thereof in the
vessel, said gas feeding means including tubular members
having gas orifices directed downwardly for preventing in-
flow of sand in said members;
- said reaction vessel and eeding means designed
to allow the biomass to be quickly dissipated into the
fluidized bed, to form an oxygen-rich, heat-forming
combustion zone in the lower portion of said bed, and a
hydrogen-rich gas-forming pyrolysis zone in the upper
portion of said bed; and to hold the fluidized bed at an
operating temperature of 750 to 860C by means of the heat
formed in the combustion zone; and
- a primary cyclone interconnected to said
reaction vessel to receive the gases and particles released
from the fluidized bed and to separate the same, said
primary cyclone having an inlet conduit in gas communication
: with the head space of said vessel, an outlet conduit and
discharge conduit for the separated particles.
: This plant is characterized in that~it comprises
the following improvements:
- it comprises drying means for predrying the
; : biomass to a moisture content of from 10~ to 35~ by weight
prior to ~eeding it into said fluidized bed;
- said sand bed has a height of sand when measured
: at rest and said means for feeding the biomass in the
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pyrolysis zone of the fluidized bed is located, at a height
within 20~ of the total height of sand, when measured at
rest, on either side fo the center of the height sand
measured at rest;
- said means for feeding said biomass into said
bed are fluid-tight and uninterrupted operating in order to
provide continuous and constant introduction of biomass into
said bed;
- said oxygen-containing gas feeding means is
connected to a source of oxygen gas free of steam;
- said reaction vessel and feeding means are so
adjusted as to keep the biomass concentration in the bed
be-tween 6% and 12~ by weight and to hold the reaction vessel
under an operating pressure of 400 ]cPa to 1,750 kPa;
- said discharge conduit of the primary cyclone
has a disc~arge end opening into the reaction vessel and is
adapted and positioned to allow the separated particles to
return therethrough into the combustion zone of the
fluidized bed;
- check valve means are provided for preventing
back flow from the flu.idized bed into said discharge
conduit;
- said plant further comprises a secondary cyclone
interconnected to said primary cyclone to receive the gases
and very light particles released from the primary cyclone
through the outlet conduit of said primary cyclone and
separate said very light particles from said gases prior to
releasing said gases for subsequent~use;
~: - said means for feeding and distributing the
steam-free oxygen-containing gas comprises a plurality of
horizontal hollow rings vertically spaced from one another,
the diameters, of said rings decreasing from the top to the
bottom rings such that the outer walls of said rings are
~ tangent to a surface defining an inveFted cone, said rings
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being pierced with said downwardly directed gas orifices,
and means for feeding said oxygen-containing gas to said
rings; and
- said vessel has a refractory lining with a
refractory plug at the bottom thereof defining said inverted
cone, said feeding maans being embedded in said plug.
A description of preferred embodiments of the
invention now follows having reference to the appended
drawings.
IN THE DRAWINGS:
Figure 1 is a flow chart of the operation of the
: process and illustrating the main components of a plant made
ot the invention;
Figure 2 is a longitudinal cross-sectional view of
a compressor for feeding biomass into the fluidized bed;
Figure 3 is a top plan view of a grid feeding air
into the fluidized bed;
20Figure 4 appearing on the same sheet of drawings
as F gure 2, is a side view, half in cross-section,
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of an air ejector for the grid of Figure 3;
Figure 5 is a partial cross-sectional view
of the lower part of the gasifier, using an oxygen grid
for feeding into the bed;
Figure 5a showing a detail;
Figures 6 and 6a are, respectively, a side
elevation view and a top plan view of trickle valve
for use in conjunction with and at the lower end of
a discharge conduit of a first cyclone;
Figures 7 and 8 are curves relative to gasifi-
cation with oxygen and air, respectively.
Description of the preferred embodiments
. .
The gasification process utilizes a hot fluidized
sand bed ta convert the biomass and the oxygen-
containing gas into a hot combustible gas.
The oxygen-containing gas is fed at the bottom
of the gasifier vessel and flows through a grid and
into a large bed of sand~ The gas flow produces a bub-
bling action in the sand such that the appearance of
the bed is almost like that of a gently boiling pot
of water. Under such conditions, the sand bed behaves
almost like a liquid or fluid in motion. Hence the bed
may be described as being "fluidized".
When biomassis fed into the hot fluidized bed,
it rapidly decomposes into a hot combustible gas.
This process is known as "gasification". In other words,
the gasification process can be summarized as follows:
The biomass comes into intimate contact with
a bed of hot sand and an oxygen-containing gas;
a small fraction of pyrolyzed biomass burns due to the
air, or oxygen, present and heat is released;
the remainder of the bio-mass is pyrolyzed,
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utilizing the heat released from the burning of the pyrolyzed biomaass.
The process can be represented briefly by the
following chemical reactions~
A first reaction in which a mixture of biomass and char
with air o~ oxygen, in the high temperature bed causes
a fire with a large amount of heat being released, this
reaction being known as burning or combustion:
Combustion = siomass (and char) + air (or oxygen) -~ Co2 + H2C~heat
During the pyrolysis stage, this heat released
from combustion is absorbed by the fluidized sand bed
in intimate contact with the biomass. When these condi-
tions are met, the biomass chars;
Pyrolysis = Biomass + heat -~ Hot combustible gas + char
The biomass char or carbon then reacts further
with carbon dioxide or water vapour to provide more gaseous
products;
Gasification = Char+Co2+H20+HEAT-~ Carbon monoxide +
hydrogen
The operation of the fluidized bed gasifier,
of the invention, is based on the knowledge that the
heat necessary to sustain the pyrolysis and gasification
reactions comes from the combustion reaction. The oxygen-
containing gas is used to keep the combustion reaction
going.
The gasifier, according to the invention, has
been designed to operate under high pressure ~1.9MPA)
; and temperature (915C). It is a cylindrical vessel
- (h=46'X6'0.D.) with a dish bottom and top made of a steel
shell with a 6'' refractory lining and 5'' internal insu-
lation to protect the shell from the heat and minimize
process heat losses.
The gasification of the biomass in the fluidized
bed takes pIace in two stages: an endothermic pyrolysis
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reaction followed by an exo-thermic combustion reaction.
To minimize the production of tar, it is neces-
sary to obtain a fast pyrolysis reaction which in turn
necessitates a rapid rate of heat transfer to the biomass
particles.
Thus, the fluidized bed performs two functions.
It provides a medium in which rapid heat to the biomass
particle can take place and, by adding an inert medium
to the bed it ensures a controlled fluidization environ-
ment over a wide range of biomass feed rates.The latter is accomplished by diluting the biomass material
with readily fluidizable sand.
The paramPters that influence and limit the reactor's design,
are for example, biomass concentration which is detrimental
to fluidization quality above 6% and adverse above 12%
of biomass mixed with sand (240 microns sand size) aswell
as specified velocity which upon increasing can improve
mixing (biomass mixing rate) (Uo-Umf) and finally reactions
which because of increasing gas flow can rearrange normalbed
behaviour, subsequently. The manner in which these para-
meters influence and limit the design and also improve
mixing, minimize the formation of local hot spots and
improve the efficiency of char conversion. The reactor
head space should preferably be located at about 1/3
the height of the vessel.
The parameters that affect the gasifier design
include the pyrolysis which produces a large increase
in the volume of gas present in the reactor by converting
solid fuel into a fuel gas and oxidation which produces
only a slight increase in gas volume~
.
This changing volume tends to disrupt the normal patterns
of mixing and fluidizability within the reactor.
The parameters that affect fluidizability are
; ~ as follows. If the gas flowing upwards through a fine
~ 35 bed of solids is increased, it reaches a velocity known
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as the minimum fluidization velocity (Umf) which is just
sufficient to support the particles, as a resul-t of rag
forces on the particles. Increase in the gas flo~ beyond
this point lifts the particles and separates them very
slightly. In fact, all of the gas flow in excess of that
required to support the particles tends to form bubbles.
If however the gas flow is distributed through high velocity
jets at the bottom of the bed then the jets themselves
tend to break off and form intermediate size bubbles in
this zone. Particles underneath the bubbles tend to be
"sucked" into the void created as the bubbles flow up
through the bed. Thus, particles below the bubbles track
the bubbles upward with a flow rate similar to the bubbles.
It is this movement of the particles upward and sideways
which imparts solids mixing into the bed. As the velocity
is increased still further, the particles blow up the bed.
This gas velocity (terminal velocity) is about 10 to 100
times the minimum fluidization velocity. A Eluidized bed
maintains good fluidizability over a wide range of gas
velocity. At higher concentrations of biomass in the bed,
the minimum fluidization velocity rapidly increases such
that, at 1~% of wood by weight Umf is 4 times that of sand.
It should be noted that minimum fluidization
velocity will decrease with increasing temperature.
It should be noted that as the pressure is in-
~creased, the number and size of the bubbles decrease, thus
reducing mixing and off-setting the temperature effect.
When the gasifier is ~operating at a velocity
considerably in excess of Umf within the bed at pressure
in the neighbourhood of 600 kPa, then a sudden increase
to 800 kPa results in rapid defluidization of the bed.
; Other parameters are factors affecting mixing.
Solid mixing is affected by the passage of bubbles through
the bed. Most of the mixing take place in the vertica~
direction. Because the bubbles also push the solids laterally,
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radial mixing does occur albeit at a much
slower rate. The rate of mixing is also proportional
to the bubble volume which, in turn, is proportional
to the differencial gas velocity, Uo - Umf. To obtain
good mixing rates it is necessary to operate with
ratios of Uo/Umf greater than 2(velocities less than
this can cause stratification rather than mixing). Two
such data po1nts show the effect of superficial velocity
on mixing rates.
If sand size = 240 microns
Uo - Umfbiomass mixing rate
(cm/s)(g/cm2/s)
4.5 .025
18 .103
Under normal conditions of gasification, cold
fluidizing gas enters the bottom of the reactor where
it is rapidly heated to the reactor conditions. This
takes place within the first 10 cm of bed and it results
in an almost four-fold increase in gas flow.
Other factors affecting gas flow are the pyrolysis
and gasification reactions.In air gasifier, the pyrolysis
; reaction results in a doubling of gas flow while the
gasification reactions do not significantly affect gas
flow. In oxygen gasifier the pyrolysis reaction causes
a six-~old increase in gas flow while the gasification
reaction produces two-fold increase in reactant flow
for a total increase of 7:1. Thus Uo - Umf changes signi-
ficantly throughout the reactor as a result of temperature
and reaction.
To resume, fluidization can occur with a wide
range of gas velocities as 10:1 or higher. There are
two restrictions which are imposed by the following factors.
In the case of distribution limitations, there
are created by the need to provide a reasonable pressure
drop to ensure even distribution of gas flow in the bed.
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The process of the invention incorporates a distributor
pressure drop that is about 15kPa.
If the entering gas velocity is increased,
then the distributor pressure drop also increases but
at the square of the velocity (for example: if the gas
velocity is increased 10:1 as above, the distributor
pressure drop would be increased by 100:1 or 1500kPa
which is not tolerable so, for this reason, the entering
design gas velocity is restricted to a 3:1 ratio).
If the velocity in the cyclone is too low it
will not create sufficient centrifugal force to separate
the small particles. Velocities that are too high, tend
to re-entrain the separated particles and also create
very high abrasion rates. As a result, cyclone inlet
velocities are usually maintained at a ratio of 2:1.
Thus, from a design stand point, the outlet
gas velocity must be limited to a 3:1 ratio. It is not
feasible therefore, to utilize the same distributor
for both oxygen and air. Obviously, if cold oxygen is
introduced into the bottom of the bed, then the volume
of gas leaving at the top is 7 to 8 times as much as cold
design flow and bed particles must be carefully ~luidized
during oxygen gasification.
The process according to a preferred embodiment
of the invention is generally carried out as follows,
having reference to Figure 1.
Biomass is fed into a hopper 1, after being
pre-dried, moved by a screw conveyor 3 over an inclined
fleight conveyor 5 to be discharged overa weighing belt
7 suitable for controlling the biomass weight rate prior
to feeding it into the gasifier vessel 9. The biomass
then falls onto a screw conveyor 11 which feeds it into
a plug feeder 13 which compresses the biomass to a pres-
sure in excess of that in the vessel 9 to avoid back-
flow from the vessel. The plug feeder 13 includes a
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blow-back damper 15 suitable to shu-t off the o~tlet of
the plug feeder, in case of failure of the latter to operate
or in case of lack of biomass in the hopper 1. The bulk
density of the plug is kept constant by a feedback motor
amperage control. The controlled flow of biomass coming
out of the plug feeder 13 falls into the conveyor assembly
17 through a conduit passage 19 provided with a shut-off
valve 21 in case of malfunctioning of the blow-back damper
15. The pre-dried biomass is fed, from the conveyor assembly
17, directly at the center of the hot fluidized bed 23
formed in the lower portion of the gasifier vessel 9; the
upper portion defining the head space 25 thereof. Gases,
including light particles of char (incompletely burnt
:biomass), sand and various tar and biomass ashes, form
a stream that passes from the head space 25 to a first
cyclone 27 where most of the light particles are returned
to the hot fluidized bed 23. The stream of useful gases
along, along with some remaining lighter residues, mostly
ashes, which have not been separated from the useful gases
.20 in the first cyclone 27, are brought into a second cyclone
29 where a second and final separation takes place; the
clean useful gases being expelled and collected at 31 while
the ashes are carried, by a screw conveyor 33, into a conven-
tional lock hopper system 35 whence they are collected
in an ash disposal container 37.
: According to the invention, the biomass is pre-
dried to a moisture content of from 10% to 35~ by weight.
Steam~free oxygen~containing gas (oxygen or air as aforeaid)
is fed and distributed through a grid system 39 at the
~::30: ~bottom of the hot sand bed 23 to hold the bed in fluidized
state and to form, in its lower portion, an oxygen-rich
:~heat-forming combustion zone and, in its upper portion,
;~a hydrogen-rich gas-forming pyrolysis zone. The pre-dried
;biomass is uninterruptedly fed in the pyrolysis
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zone at essentially the center of the height of the hot
fluidized bed 23; this center being determined when the
sand bed stands up at rest. The injection of biomass
may take place, in practice, in an area of the pyrolysis
zone comprised between + 20~ of the height of the bed,
relative to the bed center. ~he fluidized bed 23 is held
at an operating temperature of 750C to 860C and at
an operating pressure of 400kPa to 1750kPa by controlling
the feeding rate of the fluidiæing gas aswell as the
feeding rate of the biomass. The gases and biomass residues
released from the hot fluidized bed 23, are removed in
a gas stream from the head space 25 above the bed and
sent to a primary cyclone 27 which separates the useful
gases from most of the biomass residue; the latter being
returned to the combustion zone of the fluidized bed
23. The gases and biomass residue that have remained
in the first cyclon~ 27 are then moved into a second cycle
29 where the useful gases are collected and the biomass
residue discarded.
The biomass should preferably be pre-dried
i to a moisture content of between 13% to 24%, ideally
15%.
As to the fluidized bed, it is best held at
a temperature of between 820C and 830C while the pressure
should preferably range between 435kPa and 720 kPa.
Appropriately, the granulometry of the sand
of the bed 23 should preferably range from 100 to 400 ~m
while the volumetric ratio of air to biomass, should
preferably range from 1~25 to 1.35.
Figure 2 shows the significant portion of
the plug feeder 13 which serves as means for uninterrupted-
ly and fluid-tightly feeding the pre-dried biomass in
the pyrolysis zone of the fluidized bed 23 by strongly
compacting the biomass at a pressure greater than that
in the vessel 9 and by ~suring that the conduit passage
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19 and the conveyor assembly 17 are constantly fully filled
with compacted biomass. The plug feeder 13 has a hollow
elongated conveyor body 41 comprising, in succession: a
biomass cyclindrical feeding section 43, a biomass conical
compression section 45 in which the biomass is compressed,
a compaction biomass plug-forming cylindrical section 47
and a biomass outlet section 49 serving also as a seat
for the blow-back damper 15 (Figure 1).
The compression section 45 may be provided with
up to eight lubricant injections 46 regularly spaced apart
all around the section 45 to inject oil, water or any similar
lubricating substance between the innex wall of the plug feed-
er and the plug as such and thus reduce to a substantial ex-
tent the coefficient of friction between the biomass and the
feeder surface (down to .06 to .10).
~ rotary screw 51 is mounted in the body 41,having a cylindrical portion 53 in the biomass receiving
feeding end section 43; a conical portion 55 in the compres-
sion section 45 where the biomass is first compressed and
a plug-forming elongated nose 57 which projects into the
plug-forming section 47 along a major portion thereof;
nose 57 Eurther flaring slightly outwardly, as shown, to
allow the already compressed biomass to be highly compacted
into a gas-tight plug discharging into the conduit passage
25 19 ~Figure 1) and conveyor assembly 17 to fill them up
tightly so that continuous feeding of biomass into the
fluidized bed, at a constant rate, is ensured so long as
the screw 51 rotates.
Since the biomass, falling into the feed end
section 43, is pre-dried and~to prevent it from rotating
when:formed as a plug in the body section 47, the bore
of the latter should be formed with lengthwise grooves
59 of any appropriate cross-section to receive material
from the biomass plug and hold it stationary in spite of
the screw 51.
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Referring now to Figure 3, there is shown an
air grid system 39 (Figure 1 also) tha-t can be used as a means for
feeding and distributing steam-free air at the bottom of
the fluidiz~d bed 23. It comprises a plurality of nozzles
61, one being shown in detail in Figure 4. Each nozzle
; has an upright hollow cylindrical body 63 defining
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a blind passage 65 opened at its bottom end 67 and fprmed
with downwardly directed orifices 69 at the blind end
71; the inclination of the orifices 65 being preferably
45. The blind end 71 should preferably be conical.
With this arrangement, plugging of orifices 69 and pas-
sage 63 by sand from the bed 23 is avoided.
The lower ends of the nozzle 61 are formed
with radial flanges 73 suitable for them to be removably
fixed, as by bolts and nuts, to a circular base plate
75 made fast with the vessel 9 in any known manner (not
shown). It is seen, from Figure 3 that the nozzles are
evenly and symmetrically distributed within the circular
base plate 75, itself coaxial with the vessel 9. In
Figure 3, 2 U-shaped outer conduits 77 feed-air to the
bottom open ends 67 of two sets of seven nozzles 61
each, while a central conduit 79 feeds air to five
central nozzles. All conduits 77, 79 are appropriately
fed from outside the vessel 9, as at 81, 83. As to the
vessel, it may advantageously be provided with a re-
fractory and insulating lining (not shown) to reduce
heat losses.
Figures5and5~ however showapreferred grid system39'which
is the one reacted in the main claim, for use in feeding and distri
buting steam-free oxygen instead of air. As shown, the grid system
comprises three horizontal hollow rings 85, 87, 89, spaced ver-
tically from one another; their diameter decreasing
Erom top to bottom such that their outer walls are tangent
to a surface 91 defining an inverted cone of about 60.
In fact, this conical surface 91 is one defined by the
center of a refractory plug 93~closing the bottom end
of a cylindrical refractory lining 95 within the gasifier
vessel 9. To be noted also is an oxygen inlet opening
96 at the apex of the plug 93. It may also serve as
~- a drain 98 for the fluidized bed. ~leat insulation material
Inot shown) should of course be inserted between the
~, lining 95 and the vessel shell 9.
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All three rings, 85, 87, 8g, are pierced with downwardly
oxygen ejection orifices 97, best illustrated in Figure
3a, inclined an angle of about 60 with respect to the
vertical, that is toward the apex of the conical surface
91.
Oxygen is fed to the rings 85, B7, 89, by a conduit
system 99, most of which is embedded in the refractory
plug 93, having an auxiliary conduit 101 acting as a central
injector and an inlet 103.
Returning to Figure 1, and as mentioned previously,
gases formed in the head space 25 gather in a stream sent
to a first cyclone 27 by inlet conduit 105. The light
particles separatPd from the gases in the cyclone 27 are
returned to the combustion zone of the fluidized bed 23
by an outlet conduit 107 of which the lower end i5 connected
to the vessel 9, as illustrated in Figures 6 and 6a. The
lower end of the conduit 107 enters slightly into the vessel
; 9 and its opening is normally kept closed by a flap 109
hung to a pair of hingers lll.. The flap thus acts as
a check valve preventing back flow of sand from the fluidized
bed 23 into discharge conduit 107. It is restricted in
its opening movement by an inclined open wall structure
113 fixed to vessel 9. Figure 6a shows the flap 109 in
fully opened condition and resting against to the structure
: 25 113.
i
: Tests have been carried out by the applicants
: at St-Juste de Bretenières, Quebec, Canada, in a plant including
a gasifier vessel having 6' in diameter and 46' in height.
The following operating conditions were applied:
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- 20 -
Temperature range: 750 C to 860C
Pressure range: 400kPa to 1750kPa
Granulometry of sand: 100 to 40G~m
Volumetric ratio,
fluidization air/biomass: 1.25 to 1.35
oxygen/biomass: .25 to .35
The tests have given a useful gas production
of 200 Nm3 to 280 Nm3 per minute and a carbon conversion of 97%.
By way of example, Figure 7 shows the plotted
curves of oxygen flow (upper curve) and biomass feed
rate (lower curve), over time during a test where steam-
free oxygen was used as fluidizing m~dium. The operating
conditions were as follows:
. .
Rate of feed of biomass 3590 kg/H
Moisture content of
biomass 24.0%
Oxygen flow rate 9.8 Nm3/Min.
Pressure of oxygen 434 kPa abs.
Temperature of oxygen 820C
The results obtained were as follows:
Rate of flow of wet gases: 72.0 Nm3/Min.
Composition of dry gases:
Hydrogen : 20.2%
Carbon monoxyde : 30.35%
Carbon dioxyde : 31.82%
Nitrogen : 0.40%
Methane : 13.33%
Ethane : 1.57%
Ethylene : 2.25%
~rgon + Oxygen : 0.00%
Other hycrocarbons : 0.08%
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Figure 8 shows similar curves where air was
used as gasification medium. The operating conditions
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~ were as follows:
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- 21 - ~3~(~88~
Rate of feed of biomass : 4300 kg/H.
Moisture content of biomass : 13.3%
Air flow rate : 62.9Nm /Min
Air pressure : 721kPa abs
Air temperature : 828C
The results obtained were as follows:
Rate of flow of wet gases : 137 Nm3/Min
Composition of dry gases
Hydrogen :9.26%
Carbon monoxyde : 16.15%
Carbon dioxyde : 16.82%
Nitrogen :49.06%
Methane :7.58%
Ethane :0.468%
Argon + Oxygen : 0.525%
Other hydrocarbons :1~051%
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