Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF TORREFACTION OF A BIOMASS COMPRISING THE STEP
OF COOLING THE TORRE FACTION REACTION
Technical field
The present invention relates to the field of torrefaction of biomass. In
particular, it relates to a method and an arrangement for precise control of
torrefaction temperature, which is crucial for accurate control of the quality
and properties of the torrefied material.
Background
To be able to compete with and replace fossil fuel energy carriers such as
coal, oil and natural gas, lignocellulosic biomass would benefit from some
form of pre-treatment method to overcome inherent drawbacks. The pre-
treatment method torrefaction has been shown to improve biomass fuel
qualities such as energy density, water content and milling, feeding and
hydrophobic properties [1-4]. These improvements establish torrefaction as a
key process in facilitating an expanding market for biomass raw materials.
Torrefaction is a thermal pre-treatment method that normally takes place in a
substantially inert (oxygen free) atmosphere at a temperature of about 220-
600 C. During the process course a combustible gas comprising different
organic compounds is released from the biomass feedstock in addition to the
torrefied biomass.
The process of producing a torrefied material from lignocellulosic biomass
can be said to include four stages:
1) a drying step, wherein free water retained in the biomass is removed;
2) a heating step in which physically bound water is released and the
temperature of the material is elevated to the desired torrefaction
temperature;
3) a torrefaction stage, in which the material is actually torrefied and which
starts when the material temperature reaches about 220 C -230 C. During
this stage, the biomass partly decomposes and relseases different types of
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volatiles, such as hydroxy acetone, methanol, propanal, short carboxylic acids
and other hydro carbons. In particular, the torrefaction stage is
characterised
by decomposition of hem icellulose at temperatures from 220 C -230 C, and
at higher torrefaction temperatures cellulose and lignin also starts to
decompose and release volatiles; cellulose decomposes at a temperature of
305-375 C and lignin gradually decomposes over a temperature range of
250-500 C;
4) a cooling step to terminate the process and facilitate handling. The
torrefaction process is terminated as soon as the material is cooled below
220 C -230 C
Summary of the present disclosure
The requirements for quality and properties of the torrefied products differ
considerably depending of the intended use of the product. The inventors
have realized that it is crucial to be able to precisely control the
torrefaction
temperature in order to generate a torrefied product with the desired
characteristics. The present invention is based on the insight that
exothermal,
temperature-increasing reactions, takes place during the torrefaction process
and that the amount of generated energy differs considerably between
different types of lignocellulosic materials. For example, the inventors have
discovered that the torrefaction of woody biomass from eucalyptus generates
considerably more energy by exothermal reactions than the torrefaction of
woody biomass from spruce. The exothermal reactions in the torrefaction
process thus makes it hard to keep a constant torrefaction temperature and to
obtain a torrefied product of a desired and reproducible quality. Hence, the
inventors have realized a need for improved torrefaction methods which
allows for a precise control of torrefaction temperature and which facilitates
accurate control of the quality and properties of the torrefied material.
The inventors have solved the problem described above with a method
of torrefaction of a dried and heated biomass, comprising the step of cooling
the torrefaction reaction so as to at least partly counteract a temperature
increase derived from the exothermic torrefaction reactions. Another aspect of
3
the invention relates to a torrefaction arrangement comprising at least one
torrefaction
zone wherein the torrefaction zone comprises means for cooling and optionally
also
means for heating and wherein the means for cooling is connected to a cooling
source.
Certain exemplary embodiments can provide a method of torrefaction of a dried
and
heated biomass, comprising the step of cooling the torrefaction reaction in a
torrefaction
zone so as to at least partly counteract a temperature increase in the
torrefaction zone
derived from exothermic torrefaction reactions in the torrefaction zone,
wherein said
biomass is woody biomass from spruce or eucalyptus, and wherein said
torrefaction
zone is a specific region of a compartment in a torrefaction arrangement,
located
.. downstream of a heating zone in relation to a biomass inlet of a
torrefaction
arrangement, comprising means for specifically regulating the temperature in
said
specific region and wherein the temperature of a previously heated biomass is
kept
virtually constant at a desired torrefaction temperature in the range of 220 C
to 600 C for
a desired torrefaction time, which torrefaction arrangement further comprises
at least one
drying zone located in a different compartment than the torrefaction zone.
Certain exemplary embodiments can provide a torrefaction arrangement
comprising a
torrefaction zone and at least one drying zone located in a different
compartment than
the torrefaction zone, said torrefaction zone being a specific region of a
compartment in a
torrefaction arrangement, located downstream of a heating zone in relation to
a biomass
inlet of a torrefaction arrangement, comprising means for specifically
regulating the
temperature in said specific region and wherein the temperature of a
previously heated
biomass is kept virtually constant at a desired torrefaction temperature in
the range of
220 C to 600 C for a desired torrefaction time, wherein the torrefaction zone
comprises
means for cooling wherein the means for cooling is connected to a vessel or
arrangement containing a cooling media, which cooling media is liquid water.
Brief description of the figures
Figure 1 shows a torrefaction arrangement comprising a torrefaction zone
wherein the
torrefaction zone comprises means for cooling.
Figure 2 shows a typical temperature variation in the torrefaction arrangement
shown in
figure 1. Note that the cooling zone is not shown in figure 1.
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Figure 3 shows a typical temperature variation in the torrefaction arrangement
disclosed
in figure 1. Note that the cooling zone is not shown in figure 1.
Definitions:
.. Torrefaction:
A thermal pre-treatment method that takes place in a virtually inert (oxygen
free)
atmosphere at a temperature above 220 C but below 600 C and which produces a
torrefied biomass and combustible gases. During a torrefaction stage, parts of
the
biomass, in particular hemicellulose, decompose and give off different types
of organic
volatiles. In a torrefaction process starting from raw biomass, the actual
torrefaction
stage is preceded by a drying stage wherein free water retained in the biomass
is
removed and by a heating stage wherein the biomass is heated to the desired
torrefaction temperature.
Heating zone:
A specific region of a compartment in a torrefaction arrangement, located
upstream of a
torrefaction zone in relation to a biomass inlet of a torrefaction
arrangement, comprising
means for specifically regulating the temperature in said specific region and
wherein the
temperature of a biomass is increased to a temperature near the desired
torrefaction
temperature prior to torrefaction.
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Torrefaction zone:
A specific region of a compartment in a torrefaction arrangement, located
downstream of a heating zone in relation to a biomass inlet of a torrefaction
arrangement, comprising means for specifically regulating the temperature in
said specific region and wherein the temperature of a previously heated
biomass is kept virtually constant at the desired torrefaction temperature for
a
desired torrefaction time wherein a desired torrefaction temperature is in a
range between 220 C to 600 C.
Drying zone
A specific region of a compartment in a torrefaction arrangement, located
upstream of a heating zone in relation to a biomass inlet of a torrefaction
arrangement, comprising means for regulating the temperature in said
specific region and wherein a biomass is dried to a water content below 10 %
prior to heating.
Cooling zone
A specific region in a torrefaction arrangement, located downstream of a
torrefaction zone in relation to a biomass inlet of a torrefaction
arrangement,
comprising means for regulating the temperature in said specific region and
wherein the biomass is cooled to a temperature below 220 C preferably
below 100 C.
Connecting zone
A specific region in a torrefaction arrangement located immediately upstream
of a heating zone and immediately downstream of a torrefaction zone in
relation to a biomass inlet of said torrefaction arrangement.
Torrefaction time:
The time the temperature of the material is kept virtually constant at the
torrefaction temperature
Transport screw:
Any type of helicoidal material transport devices including discontinuous
helicoidal transport devices. The helicoidal transport device can be fixed to
a
central shaft or to the inner casing of a compartment, such as a drum,
surrounding the transport screw.
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Detailed description
In one aspect the invention relates to a method of torrefaction of a
dried and heated biomass, comprising the step of cooling the torrefaction
5 reaction so as to at least partly counteract a temperature increase
derived
from the exothermic torrefaction reactions. Preferably the temperature in the
torrefaction zone is controlled using means for cooling and optionally also
means for heating. The means for cooling can easily be subjected to fouling,
since gases released from the biomass material in the torrefaction zone will
condense on the said means for cooling. Therefore, in a preferred
embodiment of the invention, the means for cooling and heating are inter
changeable. Thereby heating/cooling means which becomes fouled during
the cooling can be cleaned by heating up the said means for heating/cooling
which leads to evaporation of the said condensed gases. In one additional
embodiment the said means for cooling and heating are represented by heat
exchangers.
In another embodiment the biomass is heated in a heating zone and
thereafter torrefied in a torrefaction zone and preferably the residence time
in
the torrefaction zone is controlled separately from the residence time in the
heating zone.
Cooling of the torrefaction reaction enables precise control of torrefaction
temperature which facilitates accurate control of the quality and properties
of
the torrefied material. Therefore, in a preffered embodiment of the invention
the material temperature of the biomass during the torrefaction stage should
be kept virtually constant such as that the maximum temperature and the
minimum temperature of the biomass in a torrefaction zone deviates with at
most 50 C, preferably with at most 40 C, preferably with at most 30 C
preferably with at most 20 C, preferably with at most 10 C preferably with
at
most 5 C and more preferably with at most 2 C. In another embodiment,
before a dried and heated material reaches a desired torrefaction temperature
an additional heating can take place in the torrefaction zone. Prior to this
short additional heating the temperature can be more than 50 C below the
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desired torrefaction temperature, for example 60 C or 65 C or 70 C or 75
C or even 80 C below the desired torrefaction temperature.
In a preferred embodiment the residence time in the heating zone is
controlled by controlling the rotational speed of a heating zone transport
screw and in another preferred embodiment the residence time in the
torrefaction zone is controlled by controlling the rotational speed of a
torrefaction zone transport screw.
According to another embodiment of the invention the temperature of the
biomass entering a first heating zone is between 90 C and 130 C. According
to another embodiment of the invention the temperature of the biomass
leaving a heating zone deviates from the torrefaction temperature with at
most 80 C, such as 75 C, such as 70 C, such as 60 C, such as 65 C,
such as 60 C, such as 55 C, preferably at most 50 C, preferably with at
most 40 C, preferably with at most 30 C, preferably with at most 20 C,
preferably with at most 15 C, preferably with at most 10 C and more
preferably with at most 5 C.
The preferred torrefaction temperature according to the present invention is
in
the range between 220 C to 600 C, such as 220-500 C, such as 220-450
C, such as 220-400 C, such as 230-600 C, such as 230-500 C, such as
230-450 C, such as 230-400 C, preferably 240-500 C, preferably 240-400
C, preferably 240-350 C most preferably 270-350 C
The preferred torrefaction time according to the present invention is in the
range between 1 and 60 minutes preferably between 1 and 30 minutes,
preferably 2-25 minutes and more preferably 2-20 minutes. The torrefaction
time normally refers to the residence time of the dried and heated biomass in
a torrefaction zone. According to one embodiment, the cooling is performed
during the second half of the torrefaction time or in the downstream half of
the
torrefaction zone. Such an embodiment may be preferred as the heat from
the exothermal reactions may accumulate over the torrefacation reaction
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leading to an increased need for cooling during the later stage of the
torrefaction reaction.
In another embodiment of the invention the material is dried in a drying zone
before the material enters the heating zone and preferably the water content
in the biomass is lower than 10%, preferably lower than 7%, preferably lower
than 5 %, preferably lower than 4 % preferably lower than 3 %, preferably
lower than 2 %, more preferably lower than 1 % when the biomass enters the
heating zone. In another embodiment the torrefied material is cooled in a
cooling zone after the material have been torrefied in the torrefaction zone.
According to another embodiment the material is heated in the heating zone
using the means for heating in the heating zone and the temperature in the
torrefaction zone is regulated using heat generated from the exothermic
energy generated from the biomass during the torrefaction process and
cooling supplied from the means for cooling in the torrefaction zone. External
heating can also be supplied in the torrefaction zone to control the
torrefaction
temperature via the means for heating in the torrefaction zone. According to
another embodiment no external heating is used in the torrefaction zone.
According to a preferred embodiment the biomass is represented by
lignocellulosic biomass.
Another aspect of the invention relates to a torrefaction arrangement
comprising at least one torrefaction zone wherein the torrefaction zone
comprises means for cooling and optionally also means for heating and
wherein the means for cooling is connected to a cooling source. Said cooling
source may be any vessel or arrangement containing a cooling media or a
coolant. The cooling media can be in liquid phase or in gaseous phase. In
one embodiment the cooling media is a liquid such as water or thermal oil and
in another embodiment the cooling media is a gas or a gas mixture such as
air or cold flue gases. In one embodiment the cold flue gases are withdrawn
from a boiler in connection with the torrefaction arrangement. In another
embodiment the cold flue gases are withdrawn from the drying zone in the
torrefaction arrangement. In a preferred embodiment of the invention the
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means for cooling and heating are interchangeable and preferably said
means for heating and/or cooling is represented by heat exchangers. In an
other embodiment the torrefaction arrangement further comprises at least one
heating zone wherein said heating zones comprises means for heating and
wherein the torrefaction arrangement comprises material transport
arrangements such as that the residence time of the material in the
torrefaction zones can be controlled separately from the residence time in the
heating zone(s). In a preferred embodiment the torrefaction arrangement
comprised at least two compartments wherein the material transport in at
least one of the compartment can be controlled separately from the material
transport in the other compartments and in which the torrefaction zone(s) are
located in a different compartment than the heating zone(s). At least one,
preferably at least two of the compartments can be represented by rotatable
drums in which screws may be fixed such that the material therein is
transported when the drum rotates. In another embodiment, the residence
time in the heating zone can be controlled by the rotational speed of a first
rotatable drum and the residence time in the torrefaction zone(s) is
independent of the rotational speed of said first rotatable drum. Preferably
the
residence time in the torrefaction zone is controlled by the rotation speed of
a
second rotatable drum wherein the residence time in the heating zone(s) is
independent of the rotation speed of said second rotatable drum. In one
additional embodiment the at least two compartments are connected with a
connecting zone. The material transport in said connecting zone can be
mediated by gravity or by mechanical measures and the material transport in
the connecting zone is preferably independent of the material transport in the
torrefaction zone. Preferably, the connecting zone comprises means for
measuring the material surface temperature of the material in the connecting
zone, the gas temperature, the oxygen concentration, the pressure, the gas
composition or product parameters. In another embodiment at least one of
the material transport arrangements in the torrefaction arrangement is
represented by a helicoid screw or a flight conveyor and wherein the helicoid
screw preferentially can be represented by a helicoid screw flight or a
helicoid
screw flighting welded on a central pipe or a helicoidal screw feeder. In
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another embodiment the torrefaction arrangement further comprises at least
one drying zone. Said drying zone is preferably located in a different
compartment than the torrefaction zone and the material transport in the
drying zone is preferably independent of the material transport in the
torrefaction zone. The material transport arrangement in the drying zone can
for example be represented by a helicoid screw or a flight conveyor and
wherein the helicoid screw preferentially can be represented by a helicoid
screw flight or a helicoid screw flighting welded on a central pipe or a
helicoidal screw feeder. In another embodiment the material transport
arrangement in the drying zone and the heating zone is represented by a
common transport screw. In a different embodiment the material transport in
the drying zone is separate from the material transport in the heating zone.
The torrefaction arrangement can further comprise at least one cooling zone
and said cooling zone can preferably comprise at least one screw cooler.
Note that the cooling of the cooling zone is different from the cooling of the
torrefaction zone.
Detailed description of exemplary embodiments
Figure 1 shows a torrefaction arrangement having a biomass inlet (1)
wherein the biomass is introduced in the torrefaction arrangement by means
of a feeding screw (2). The biomass is dried in a drying zone (3) wherein heat
is supplied to the drying zone (3) by means of a heating media (e.g. hot
gases) through a drying zone heating media inlet (4) and wherein the heating
media leaves the drying zone through the drying zone heating media outlet
(5). Dried biomass is transported through the drying zone (3) at a speed
regulated by the feeding speed in the biomass inlet (1) and enters the heating
zone (6) where the temperature of the biomass is elevated to a temperature
near the desired torrefaction temperature. The heat is supplied to the heating
zone (6) by means of a heating media through a heating zone heating media
inlet (7) which leaves the heating zone through a heating zone heating media
outlet (8). The heated material enters a first torrefaction zone (9) in which
the
temperature can be controlled by introducing heating media and/or cooling
media in the first torrefaction zone heating/cooling media inlet (10) wherein
said heating/cooling media exits the first torrefaction zone through the
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torrefaction zone heating/cooling media outlets (11). The biomass thereafter
enters a second torrefaction zone (12) wherein the temperature can be
controlled using special means for cooling (18) wherein the means for cooling
(18) is connected to a cooling source. Cooling media can be supplied to the
5 second torrefaction zone via the torrefaction zone cooling media inlet
(13) and
said cooling media exits the torrefaction zone via a torrefaction zone cooling
media outlet (14). The cooling media inlet (13) is connected to cooling
source.
The material transport in the heating zone (6) and torrefaction zones (9, 12)
is
driven by a common transport screw which is attached to a drum enclosing
10 the heating zone (6) and torrefaction zones (9, 12). The said drum can
be
attached to a threading (15). Torrefaction gases from the drying zone (3),
heating zone (6) and torrefaction zones (9, 12) are collected from the
torrefaction gas outlet (16) for combustion or processing. Torrefied biomass
exits the torrefaction arrangement through a torrefied biomass outlet (17) and
is preferably subsequently cooled to a temperature below 100 C.
Figure 2 shows typical temperatures of the biomass in the different zones in
the torrefaction arrangement disclosed in figure 1: Zone 1 represents the
drying zone (3), zone 2 represents the heating zone (6), zone 3 represents
the first torrefaction zone (9) and zone 4 represents the second torrefaction
zone (12)_In the drying zone (3) the biomass is dried, typically to a water
content of 2-10 % (w/w) and the temperature is elevated to about 100 C. In
the heating zone (6), the material is post-dried to about 0 % moisture (w/w)
and the temperature of the material is elevated to close to the desired
torrefaction temperature which in this example is 350 C. In the torrefaction
zones the temperature is kept virtually constant at the desired torrefaction
temperature for a time corresponding to the desired torrefaction time. Cooling
of the torrefaction reaction in the torrefaction zones counteracts a
temperature increase derived from the exothermic torrefaction reactions and
thereby facilitates the constant temperature in the torrefaction zones. In
figure
1 the second torrefaction zone have special means for cooling the torrefaction
reaction (18) but the torrefaction reaction can also be cooled using cooling
media which is introduced to the torrefaction zones via the torrefaction zone
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cooling media inlet (11, 13) Thereafter the temperature is decreased below
100 C in a cooling zone.
Figure 3 shows typical times and temperatures of the biomass in the different
zones in the torrefaction arrangement disclosed in figure 1. In the present
example the torrefaction temperature is 350 C and the torrefaction time is 20
minutes.
REFERENCES
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Energy
2006, 31, (15), 3458-3470.
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Flow Gasification of Biomass; Report C--05-067;
Energy Research Centre of The Netherlands (ECN):
Petten, The Netherlands, July 2005;
[3] K. H6kansson et al. Torrefaction and gasification of
hydrolysis residue. 16th European biomass conference
and exhibition, Valencia, Spain. ETAFlorence, 2008.
[4] A. Nordin, L. Pommer, I. Olofsson, K. H6kansson, M.
Nordwaeger, S. Wiklund Lindstrom, M. BrostrOm, T.
Lestander, H. orberg, G. Kalen, Swedish
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2009-12-18 (2009).
