Note: Descriptions are shown in the official language in which they were submitted.
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Compact Fast Pyrolysis System for Conversion of Carbonacous Materials to
Liquid, Solid and Gas
The present invention relates to the field of biomass pyrolysis, and provides
a compact,
substantially self contained fast pyrolysis system and apparatus for
conversion of biomass into
solid, liquid and gas pyrolysis products.
BACKGROUND OF THE INVENTION
Biomass pyrolysis in itself is not a new technology and the literature abounds
with examples of
various types of pyrolysis units. It is generally accepted that biomass
pyrolysis can be carried
out in either a fast mode or a slow mode. The fast mode maximizes liquid
yield, while the slow
mode maximizes solid (charcoal) yield. Lede (2013) published an extensive
review and
description of fast pyrolysis systems. The extensive reference list is
testimony to the volume
of literature available on pyrolysis. The following is a list of fast
pyrolysis processes including
but not restricted to circulating bed (Fred l ), fluid bed (Piskorz et al,)
twin auger ( Brown,
2009 or single auger (Hornung, Fransham). Virtually all fast pyrolysis systems
involve mixing
the biomass with a heat carrier. Silica sand is the most often used, although
steel shot
(Fransham), steel balls (Hornung) or ceramic shot is used in most auger
pyrolysis. When the
heat carrier is circulated with recycled gas, sand is the heat carrier of
choice. The general
object of fast pyrolysis is to drive off volatiles from biomass material and
condense them in a
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matter of seconds. Slow pyrolysis processes include augers (Flottvik),
multiple hearth and
batch systems used in the coking industry. Slow pyrolysis systems do not
generally use a
heat carrier and is generally carried out in a matter of minutes. People
skilled in the art will
recognize that other possible processes also exist for conducting pyrolysis.
Circulating bed type fast pyrolysis systems (Freel) also known as transport
bed processes
involve moving sand vertically in a tube at velocities in the order of 20
m/sec. The
motivating gas is oxygen free recycled gas obtained after all of the
condensable volatiles have
been stripped from the non¨condensing gas. Biomass enters the vertical tube at
a point
above the base of the tube. The biomass mixes with the sand in the tube and
the volatile
matter in the biomass is converted to a hot gas. There are however,
fundamental problems
with transport beds. The first is the use of sand as a heat carrier. Sand has
a low thermal
conductivity and the sand temperature has to be high enough to transfer
sufficient energy to
raise the biomass temperature to approximately 515 C. However, the short
contact time and
the low thermal conductivity of sand mean that only a fraction of the energy
contained in the
sand grain can be transferred to the biomass. Also, the large amount of
recycled gas that is
required for transport is a parasitic load on the system. The gas has to be
heated and cooled
for each cycle. Condensers have to be large enough to handle the heat load in
the gas
stream. The excessive electrical energy required to transport sand several
meters vertically
in the air at velocities of about 20 m/sec. greatly reduces the efficiency of
the process.
Furthermore the sand and biomass are in a dispersed low density mixture of
sand, biomass
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and motivating gas. Heat transfer from the sand to the biomass is predicated
on the random
contact between the sand grains and the biomass particles.
Fluid bed pyrolysis processes have similar short comings to circulating bed
reactors. Fluid
beds have to be shallow to ensure short vapour residence times required to
limit secondary
chemical reactions. Preventing carryover of the sand into the char recovery
circuit requires
balancing of sand size and airflow. A further limitation is the transferring
of heat into the bed.
The only ability to do pyrolysis work is dictated by the mass of gas
multiplied by the
temperature differential between the incoming and exiting recycle gas
multiplied by the
specific heat of the recycle gas. Large blowers are required to move the
recycled fluidizing
gas. The key technical challenge is to scale up the reactor to meet the
demands of short
residence time while maintaining the sand in the reactor. The sand in the
fluid bed is in a low
density medium and contact between the sand particle and the incoming biomass
relies on
rapid mixing before a large bubble of gas rises to the bed surface and expands
outwardly.
There is therefore a considerable technical challenge to feeding biomass into
large reactors
and separating reacted biomass (charcoal) from sand. Attempts have been made
to scale up
fluid bed reactors for pyrolysis. No biofuel pyrolysis plant is currently
operating at a
commercial scale using this technology.
Auger pyrolysis offers a solution to the deficiencies in the fluid bed and
circulating bed
processes. Steel shot has a higher thermal conductivity than sand and hence
more energy can
be transferred at a lower operating temperature from the shot to the biomass
in an equivalent
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period of time as compared to sand reactors. Fransham (2001) developed an
auger system
whereby the shot was recirculated via a bucket elevator. The charcoal and
non¨condensing
gases were burned to provide process heat. The system relied on pressure from
the
expanding raw pyrolysis gas and the volume reduction in the gas at the first
stage condensing
unit to move the gas from the reactor to the condensers with a residence time
similar to that
of the other pyrolysis systems. The advantage of the auger system is the
biomass is in close
contact with the heat carrier and hence high heating rates are achieved. The
use of steel shot
over sand significantly reduces auger wear and heat carrier attrition when
compared to sand
filled reactors.
Hornung used hollow steel balls to rapidly pyrolyze biomass and separated the
charcoal from
the balls in a trommel screen. The balls were heated and circulated in a
system of screw
conveyors. This system has been used for a variety of applications and
numerous papers have
been written and patents filed on results from this process.
SUMMARY OF THE INVENTION
Auger pyrolysis has been shown above to be an improvement on the fluid bed and
circulating
bed pyrolysis systems. This invention is an improvement and simplification of
the auger
pyrolysis system developed by Fransham (2001). There are several unique
features that have
been added to the basic system. In a broad aspect, the present invention
provides a pyrolysis
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system where the system consists of two reactors. The horizontal reactor
removes most of the
volatile matter while the second, inclined auger reactor, allows for a longer
contact time and
removal of the remaining volatile matter under slow pyrolysis conditions.
In a broad aspect, then, the present invention relates to an apparatus for
pyrolysis of organic
material biomass, comprising: i) a first, horizontal auger tube having a first
inlet for a heat carrier
and a second inlet for biomass; and a first outlet for pyrolysis gas and a
second outlet for said
heat carrier and transformed biomass; and ii) a second, inclined auger tube
having an inlet at or
below the second outlet of said first auger tube, to receive the heat carrier
and transformed
biomass from the second outlet of the first auger tube and an outlet at a
level above the inlet
thereof, said outlet communicating with the first inlet of the first auger
tube to deliver heat
carrier thereto, wherein said heat carrier is steel shot.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic of the system of the present invention.
Figure 2a is a schematic of an auger tube with an auger having a fine pitch at
one end and a
coarser pitch at the other end.
Figure 2b is a schematic of an auger tube with an auger having a small
effective carrying volume
at one end, and a large effective carrying volume at the other end.
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Figure 3 is a schematic of a char/shot separator used in a preferred
embodiment of the
present invention.
Figure 4 is a front view of an embodiment of the present invention.
Figure 5 is a rear view of the embodiment of the present invention shown in
Figure 4, with the
outer panels removed.
Referring first to the schematic diagram of Figure 1, the basic components of
the pyrolysis
apparatus of the present invention are shown. A first reactor comprises first
tube 2 containing
a screw conveyor comprising an auger. First tube 2 provided with a first inlet
for steel shot,
and a second inlet for biomass feed, downstream of the first inlet.
Steel shot with a temperature between 350 C and 550 C drops into the reactor
conveyor
tube (2). The level of shot is maintained higher than top elevation of the
auger to ensure the
auger is 100% full. The auger is turning at between 20 and 200 RPM with a
preferred
rotational speed of between 80 and 100 RPM. Feed enters the reactor from
storage and can
be conveyed to the reactor by an auger or series of augers or by any other
means that will
quickly introduce the biomass to the steel shot. The auger flighting pitch
upstream of the
feed entry point is set at 1/4 to 3/4 pitch, preferably 1/2 pitch. At the
point of feed entry
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from (1) the pitch is increased to full or greater than full in such a manner
that the shot level
drops to approximately one¨half full. The increase in pitch assures space is
available for the
biomass in the auger and that there is head space above the shot and biomass
to allow the gas
to flow down the auger and out to a condenser. An alternate means of
controlling the shot
level in the reactor is to increase the auger shaft size upstream of the feed
point and maintain
full pitch throughout the entire auger length. The shaft at the feed entry
point is a smaller
diameter and therefore the shot level will drop. Figure 2a shows the half
pitch auger while
Figure 2b shows the larger shaft auger. Both of these methods have been tested
by the
Applicant and found to perform identically.
The hot pyrolysis gas exits the reactor downstream of the feed entry point and
is conveyed by
a system of pipes to a condensation unit (4) where the gas is cooled and the
condensable
materials are removed. The charcoal and shot continue past the gas exit point
and drop into a
second conveyor tube (3) containing a screw conveyor comprising an auger. The
second
conveyor tube is inclined upwardly relative to the first tube. The inclined
conveyor tube
provides a physical seal between the first reactor (2) and a charcoal recovery
system (5) as
will be discussed. Heat transfer through the shell of the inclined tube
provides a means of
heating the steel shot up to the desired temperature.
The temperature of the recycled steel shot is governed by the temperature
external to the
reactor and inclined auger shells. The mean reaction temperature is a function
of the shot
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temperature and the mass ratio of steel shot to biomass. The temperature
differential between
the external temperature and the desired reaction temperature is a function of
the type of
biomass and its moisture content. Empirical observations have shown the
biomass is rapidly
mixed into the steel shot in two revolutions of the auger. Sixty to one
hundred and twenty
revolutions per minute for the reactor auger appear to be adequate to provide
total
incorporation of the biomass into the steel shot in a matter of 1 second.
Given the high
thermal conductivity of steel shot and the rapid mixing, high liquid yields
can be obtained for
woody biomass within a temperature range of 350 C to 500 C. The preferred
temperature is
a function of the type of biomass, the amount of volatile matter to remain in
the biochar, the
desired liquid yield and the physical properties of the biomass. Circulating
bed type reactors
are known to have an optimum temperature of 515 C for similar biomass. Auger
pyrolysis
systems with a steel shot heat carrier are able to perform the same pyrolysis
activity at a
lower temperature and are hence more thermally efficient.
The heating of the charcoal in the second auger tube to a higher temperature
than the
primary reaction temperature can result in additional gas being produced. The
amount of gas
depends on the temperature of the shot following mixing with the biomass and
the
temperature to which the shot is heated in the inclined auger tube (3). At 400
C primary
reaction temperature, more volatile matter remains in the char and hence more
gas is
produced during heating in the inclined auger. As discussed below, the
charcoal recovery
system (5) is isolated from the reactor system (2), and therefore an increase
in pressure will
occur in the charcoal recovery system if pressure relief isn't available.
Pressure relief is
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possible by allowing the excess gas to permeate through the shot at the entry
point to the
reaction auger tube (2). The size of the steel shot particles therefore is a
factor in controlling
pressure relief. If pyrolysis is conducted at 400 C and more gas is produced
then the shot
size has to be greater to allow for higher gas permeability. Steel shot with a
grain size of 1 mm
was found to be somewhat impervious to the excess gas given the length of the
full section
upstream of the feed entry point when the operating temperature was less than
450 C. Finer
shot (1mm) can be used if the length of the full section is shortened. The
choice of shot size
and the geometry of the reactor can both be used to govern the pressure in the
char recovery
circuit.
The shot exits the inclined tube and drops down a chute to a char/shot
separator (5). The
separator (5) is a simple classifier device whose only moving part is a small
radial fan blower
(7). Recycle gas classification is possible because of the density difference
between the shot
and the char particles. Steel has a density of 7.3 g/cc while charcoal
particles are reported
to have particle densities in the range of 1.5 to 1.7 gm/cc. The steel shot
acts as a ball mill
and post pyrolysis charcoal has a grain size normally less than 1 mm.
Referring to Figure 3,
the shot and char exit the upper end of the inclined auger tube and slide down
an inclined
plane and the combined mass drops over a step. Recycle gas from a blower (7)
enters the
separator from the back of the step and carries the char in the gas stream
while the clean
shot flows down an inclined plane and drops into a small surge bin at the
start of the
horizontal reactor tube (2). The charcoal is separated from the recycle gas
stream via a
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cyclone (6). The charcoal exits the cyclone (6) and is conveyed to storage
(8).
An advantage of the overall design of the system is in its compactness and
simplicity. The
reactor tube (2) and inclined tube (3) are located in parallel planes and the
separator is small
enough to fit between the two tubes. The apparatus has a small footprint per
volume of
biomass converted. Biooil yields of 70% have been measured. This yield is
consistent with
yields obtained by fluid bed and transport bed pyrolysis units. The simplicity
and low capital
cost and low operating cost per litre produced are unique to this invention.
Referring to Figure 5, it will be observed that all essential parts of the
closed loop of the
present apparatus, namely the reactor tube 2, the chute between reactor tube
and the
inclined tube 3, and the separator 5 that connects the top of the inclined
tube with the steel
shot input of the reactor tube, are all contained within a rectangular box-
like structure. For
clarity, the side walls of the box and heat source are not shown, but in
operation, insulated
walls, entirely enclose the box, creating an oven that can be maintained at a
temperature
suitable for conducting a pyrolysis reaction. Other portions of the system,
such as the motors
for turning the augers, and the hopper for biomass are located outside of the
box.
Experimental Results
Three tests were performed to demonstrate the validity of auger pyrolysis. In
theory the
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lower the reactor temperature, the higher the char yield. The system of the
present invention
is designed to produce biooil and char is a by¨product. Normally the plant is
operated with a
reactor temperature between 450 C and 500 'C. In an attempt to maximize char
yield the
first test was run at 400 C.
The results of the three tests are shown in the summary table and graph that
are provided
below.
temperature biochar biooil gas
400 23.2 71.75 5.04
450 24.1 70.97 4.88
485 17.2 69.5 13.2
February 26, 27, 2013
80 - - - --
70 _________________________________________ =
50 biochar
¨ 40 --=-- biooil
1-1-I 30 gas
T-
õ.õ.7
0
390 400 410 420 430 440 450 460 470 480 490
REACTOR TEMPERATURE (c)
The key finding is the potential to conduct fast pyrolysis at temperatures in
the low 400 C
range.
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References
1) J. Lede
Oil & Gas Science and Technology ¨ Rev. IFP Energies nouvelles
Copyright 6 2013, IFP Energies nouvelles
DOI: 10.2516/ogst/2013108
2) Brown, J.N. Development of a lab-scale auger reactor for biomass fast
pyrolysis and process
optimization using response surface methodology. M.S. thesis, Iowa State
University, Ames,
2009.
3) Hornung et al. 2001 PLANT FOR THE THERMAL TREATMENT OF MATERIAL AND
OPERATION PROCESS THEREOF: EP 1354172 B1
3) Piskorz, Jan, Piotr Majerski, and Desmond Radlein, ENERGY EFFICIENT
LIQUEFACTION
OF BIOMATERIALS BY THERMOLYSIS CA 2255922
4) A. Barry Freel RAPID THERMAL CONVERSION OF BIOMASS, CA 2705775 and Method
and apparatus for a circulating bed transport fast pyrolysis reactor system
EP 0513051 B1
5) Fransham, Peter PROCESS FOR THE CONVERSION OF CARBONACEOUS
FEEDSTOCK INTO LIQUID, CHAR AND GAS CA 2351892
