U.S. patent application number 13/145654 was filed with the patent office on 2011-11-17 for staged biomass pyrolysis process and apparatus.
Invention is credited to Andreas Apfelbacher, Andreas Hornung.
Application Number | 20110278149 13/145654 |
Document ID | / |
Family ID | 40833812 |
Filed Date | 2011-11-17 |
United States Patent
Application |
20110278149 |
Kind Code |
A1 |
Hornung; Andreas ; et
al. |
November 17, 2011 |
STAGED BIOMASS PYROLYSIS PROCESS AND APPARATUS
Abstract
A biomass pyrolysis process in which biomass feedstock particles
(having a d5o mean particle size of at least 1 mm) are thermally
treated substantially in the absence of oxygen to pyrolyse the
biomass feedstock particles. The thermal treatment of the biomass
feedstock particles includes a heat treatment drying step at a
temperature in the range 100.degree. C. to 250.degree. C. Then, at
a pre-pyrolysis heating location in a heat treatment system, the
biomass feedstock particles are heated to a temperature in the
range 280.degree. C. to 350.degree. C., held within the same
temperature range for a time of at least 5 seconds and then moved
from the pre-pyrolysis heating location to a pyrolysis heating
location by a conveyor system for heating to a temperature above
350.degree. C. for pyrolysis.
Inventors: |
Hornung; Andreas;
(Karlsruhe, DE) ; Apfelbacher; Andreas; (Freihung,
DE) |
Family ID: |
40833812 |
Appl. No.: |
13/145654 |
Filed: |
May 11, 2010 |
PCT Filed: |
May 11, 2010 |
PCT NO: |
PCT/GB2010/000939 |
371 Date: |
July 21, 2011 |
Current U.S.
Class: |
201/12 ;
202/211 |
Current CPC
Class: |
Y02E 50/10 20130101;
C10B 49/22 20130101; C10B 53/02 20130101; Y02E 50/15 20130101; C10B
1/10 20130101; C10B 57/02 20130101; Y02P 20/145 20151101; C10B
49/16 20130101; Y02E 50/14 20130101 |
Class at
Publication: |
201/12 ;
202/211 |
International
Class: |
C10B 57/02 20060101
C10B057/02; C10B 49/16 20060101 C10B049/16; C10B 53/02 20060101
C10B053/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 11, 2009 |
GB |
0908082.1 |
Claims
1. A biomass pyrolysis process in which biomass feedstock particles
are thermally treated substantially in the absence of oxygen to
pyrolyse the biomass feedstock particles, wherein the thermal
treatment of the biomass feedstock particles includes a step in
which at least 50% of the mass of the biomass feedstock particles
is heated to a temperature in the range 280.degree. C. to
350.degree. C., held within the same temperature range for a time
of at least 5 seconds and then heated to a temperature above
350.degree. C. for pyrolysis, the biomass feedstock particles
having a d.sub.50 mean particle size of at least 1 mm, wherein
d.sub.50 denotes a number length mean particle size such that 50%
of particles have volume smaller than a sphere of diameter d.sub.50
and 50% of particles have volume larger than a sphere of diameter
d.sub.50.
2. A biomass pyrolysis process according to claim 1 wherein the
biomass feedstock particles are subjected to a heat treatment
drying step at a temperature in the range 100.degree. C. to
250.degree. C., before being heated to the temperature in the range
280.degree. C. to 350.degree. C.
3. A biomass pyrolysis process according to claim 1 wherein when
the biomass feedstock particles are subjected to pyrolysis at a
temperature in the range 350.degree. C. to 550.degree. C.
4. A biomass pyrolysis process according to claim 1 wherein in the
step in which at least 50% of the mass of the biomass feedstock
particles is heated to a temperature in the range 280.degree. C. to
350.degree. C. and held within the same temperature range for a
time t.sub.280-350, the following relationship is satisfied:
t.sub.280-350.gtoreq.Cd.sub.50 in which d.sub.50 is the number
length mean particle size in metres and C is a constant and is at
least 5000 seconds per metre.
5. A biomass pyrolysis process according to claim 1 wherein the
biomass feedstock particles are heated to the temperature in the
range 280.degree. C. to 350.degree. C. at a pre-pyrolysis heating
location in a heat treatment system and subsequently the biomass
feedstock particles are heated to the temperature above 350.degree.
C. for pyrolysis at a pyrolysis heating location in the heat
treatment system.
6. A biomass pyrolysis process according to claim 5 wherein the
biomass feedstock particles are moved between the pre-pyrolysis
heating location and the pyrolysis heating location by a conveyor
system.
7. A biomass pyrolysis process according to claim 5 wherein the
biomass feedstock particles are moved between the pre-pyrolysis
heating location and the pyrolysis heating location substantially
without reduction in temperature.
8. A biomass pyrolysis process according to claim 5 wherein the
conveyor system is a screw or auger.
9. A biomass pyrolysis process according to claim 5 wherein the
pre-pyrolysis heating location and the pyrolysis heating location
are parts of a heat treatment kiln, at different temperatures.
10. A biomass pyrolysis process according to claim 5 wherein the
conveyor system conveys the biomass feedstock particles at a
substantially uniform rate, the residence time of the biomass
feedstock particles in the pre-pyrolysis heating location and the
pyrolysis heating location being determined by the spatial extent
of the pre-pyrolysis heating location and the spatial extent of the
pyrolysis heating location, and on the rate of conveyance of the
biomass feedstock particles.
11. A biomass pyrolysis process according to claim 1 wherein in the
step in which at least 50% of the mass of the biomass feedstock
particles is held within the same temperature range for a time of
at least 5 seconds, the temperature range is 280.degree. C. to
320.degree. C.
12. A biomass pyrolysis apparatus for thermal treatment of biomass
feedstock particles substantially in the absence of oxygen to
pyrolyse the biomass feedstock particles, the apparatus providing a
pre-pyrolysis heating location for heating the biomass feedstock
particles to a temperature in the range 280.degree. C. to
350.degree. C. and for holding the temperature of the biomass
feedstock particles in the range 280.degree. C. to 350.degree. C.
for at least 5 seconds, the apparatus further providing a pyrolysis
heating location for subsequently heating the biomass feedstock
particles to a temperature above 350.degree. C. for pyrolysis.
13. A biomass pyrolysis apparatus according to claim 12 wherein the
pre-pyrolysis heating location and the pyrolysis heating location
are parts of a heat treatment kiln, at different temperatures.
14. A biomass pyrolysis apparatus according to claim 13 further
providing a conveyor system for moving the biomass feedstock
particles between the pre-pyrolysis heating location and the
pyrolysis heating location.
15. A biomass pyrolysis apparatus according to claim 14 wherein the
conveyor system is a screw or auger.
16. A biomass pyrolysis apparatus according to claim 14 wherein the
biomass feedstock particles are moved between the pre-pyrolysis
heating location and the pyrolysis heating location substantially
without reduction in temperature.
17. A biomass pyrolysis apparatus according to claim 14 wherein the
conveyor system conveys the biomass feedstock particles at a
substantially uniform rate, the residence time of the biomass
feedstock particles in the pre-pyrolysis heating location and the
pyrolysis heating location being determined by the spatial extent
of the pre-pyrolysis heating location and the spatial extent of the
pyrolysis heating location, and on the rate of conveyance of the
biomass feedstock particles.
18. A biomass pyrolysis apparatus according to claim 12 further
providing a heat carrier recycling system for at least partial
recycling of heat carrier material from the pre-pyrolysis heating
stage and/or from the pyrolysis heating stage to one or more of the
pre-pyrolysis heating stage and the pyrolysis heating stage.
Description
BACKGROUND TO THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to biomass pyrolysis. It has
particular, but not exclusive, application to biomass pyrolysis
processes for the production of renewable liquid, gaseous and solid
fuels.
[0003] 2. Related Art
[0004] Biomass pyrolysis is the thermal decomposition of biomass
(e.g. plant material such as wood and wood bark) substantially in
the absence of oxygen. Biomass is typically a mixture of
hemicellulose, cellulose, lignin and small amounts of other
organics. These components typically pyrolyse or degrade at
different rates and by different mechanisms and pathways.
[0005] One traditional example of biomass pyrolysis is the
production of charcoal, where the main product of the pyrolysis is
char. Alternative biomass pyrolysis techniques provide a product
which, after cooling, includes a substantial proportion of liquid.
This liquid is typically a dark brown liquid having a heating value
that is around one half the heating value of conventional fuel oil.
The liquid is typically referred to as bio-oil. In many
circumstances, it is the bio-oil which is the most valuable product
of the pyrolysis reaction, since bio-oil can be easily stored for
later use, e.g. for heat and/or electricity generation. Bio-oil
typically is a homogenous hydrophilic mixture of polar organics and
water.
[0006] The rate and extent of decomposition of the components of
biomass depends on the process parameters of the pyrolysis reactor,
e.g. the rate of heating of the biomass, the mode of heating of the
biomass and the residence time of the subsequent products. In turn,
these process parameters may also have an effect on the subsequent
behaviour of the product, e.g. by secondary reactions such as
cracking (of higher molecular mass products) or condensation
reactions (of lower molecular mass products).
[0007] Biomass pyrolysis can be carried out using fast heating
rates and short vapour residence times. Such "fast" pyrolysis
processes are reviewed by Bridgwater et al (A. V. Bridgwater, D.
Meierb and D. Radlein, "An overview of fast pyrolysis of biomass"
Organic Geochemistry Volume 30, Issue 12, December 1999, Pages
1479-1493). It is considered in that disclosure that optimum levels
of organics in the bio-oil may be achieved by fast heating of the
biomass to a reaction temperature of around 500.degree. C. and
vapour residence times of less than around 1 second.
[0008] There are several different options for achieving heating of
the biomass in a fast pyrolysis reactor. For example, ablative
pyrolysis requires the biomass particles to be pressed against a
heated surface and rapidly moved. This allows the use of relatively
large biomass particles. Alternatively, fluid bed and circulating
fluid bed pyrolysis reactors transfer heat from a heat source to
the biomass particles by a mixture of convention and conduction.
Since heat transfer must typically occur quickly, fluid bed
pyrolysis reactor require the use of small biomass particles, e.g.
not more than 3 mm. A further alternative is vacuum pyrolysis, in
which heating rates may be relatively low, but the application of a
vacuum quickly extracts the pyrolysis products and thus simulates
some effects of fast pyrolysis.
[0009] Further, more recent, reviews of biomass pyrolysis have been
conducted by A. V. Bridgwater ("Renewable fuels and chemicals by
thermal processing of biomass" Chemical Engineering Journal Volume
91, Issues 2-3, 15 Mar. 2003, pages 87-102; and "Biomass fast
pyrolysis", Thermal Science Vol. 8 (2004), No. 2, pages 21-49) in
which it is explained that lower process temperatures and longer
residence times in the pyrolysis reactor favours the production of
char. It is further explained that the critical issue in biomass
pyrolysis is to bring the reacting biomass particle to the optimum
process temperature and minimise its exposure to intermediate,
lower temperatures that favour the formation of char. Typical
product yields, stated as a weight percentage of dry wood
feedstock, for fast pyrolysis are given as 75% liquid, 12% char and
13% gas.
SUMMARY OF THE INVENTION
[0010] The present inventors have realised that certain factors in
the pyrolysis process may encourage the formation of high organics
compounds such as tar, which is present in the resultant bio-oil.
This is considered undesirable. Typical compounds that may be
classified as tar in this technical field may be considered to be
organic compounds of molecular weight 300 a.m.u. (atomic mass unit,
1 a.m.u.=1.66.times.10.sup.-27 kilograms) or greater. In
particular, the present inventors consider that the rate of heating
or, more generally, the heat treatment schedule, of the biomass
feedstock particles is a critical parameter.
[0011] Accordingly, in a first aspect, the present invention
provides a biomass pyrolysis process in which biomass feedstock
particles are thermally treated substantially in the absence of
oxygen to pyrolyse the biomass feedstock particles, wherein the
thermal treatment of the biomass feedstock particles includes a
step in which at least 50% of the mass of the biomass feedstock
particles is heated to a temperature in the range 280.degree. C. to
350.degree. C., held within the same temperature range for a time
of at least 5 seconds and then heated to a temperature above
350.degree. C. for pyrolysis, the biomass feedstock particles
having a d.sub.50 mean particle size of at least 1 mm, wherein
d.sub.50 denotes a number length mean particle size such that 50%
of particles have volume smaller than a sphere of diameter d.sub.50
and 50% of particles have volume larger than a sphere of diameter
d.sub.50.
[0012] In this way, the tar content of the pyrolysis products can
be reduced, which is advantageous. Although not wishing to be bound
by theory, the present inventors speculate that this is due to a
modification of the complex reaction pathways that occur in the
degradation of lignin in the biomass, favouring the production of
lower organics compounds in the final pyrolysis products. It is
considered that most tar formed in pyrolysis processes is derived,
at least in part, from lignin, in particular during condensation
reactions and uncontrolled fragmentation of large molecules derived
from the degradation products of lignin.
[0013] In a second aspect, the present invention provides a biomass
pyrolysis process in which biomass feedstock particles are
thermally treated substantially in the absence of oxygen to
pyrolyse the biomass feedstock particles, wherein the thermal
treatment of the biomass feedstock particles includes a step in
which at least a part of each of the biomass feedstock particles is
heated to a temperature in the range 280.degree. C. to 350.degree.
C., held within the same temperature range for a time of at least 5
seconds and then heated to a temperature above 350.degree. C. for
pyrolysis, the biomass feedstock particles having a d.sub.50 mean
particle size of at least 1 mm.
[0014] Preferably, in this second aspect, the part of each particle
whose temperature is controlled in the range 280.degree. C. to
350.degree. C. is an outer part of each particle. The depth of this
outer part of each particle will typically vary depending on the
time for which the particle is held within the 280.degree. C. to
350.degree. C. temperature range. Preferably, this depth extends
substantially to the centre of the particle, so that substantially
the whole particle is within the required temperature range.
[0015] In a third aspect, the present invention provides a biomass
pyrolysis apparatus for thermal treatment of biomass feedstock
particles substantially in the absence of oxygen to pyrolyse the
biomass feedstock particles, the apparatus providing a
pre-pyrolysis heating location for heating the biomass feedstock
particles to a temperature in the range 280.degree. C. to
350.degree. C. and for holding the temperature of the biomass
feedstock particles in the range 280.degree. C. to 350.degree. C.
for at least 5 seconds, the apparatus further providing a pyrolysis
heating location for subsequently heating the biomass feedstock
particles to a temperature above 350.degree. C. for pyrolysis.
[0016] Preferred and/or optional features of the invention will now
be set out. These are applicable either singly or in any
combination with any aspect of the invention, unless the context
demands otherwise.
[0017] Preferably, the biomass feedstock particles are subjected to
a heat treatment drying step at a temperature in the range
100.degree. C. to 250.degree. C., before being heated to the
temperature in the range 280.degree. C. to 350.degree. C. Thus, it
is preferred that there are at least three distinct stages to the
heat treatment schedule: a first pre-pyrolysis heating stage, a
second pre-pyrolysis heating stage and a pyrolysis heating stage.
Preferably the biomass feedstock particles are subjected to a heat
treatment drying step at a temperature in the range 100.degree. C.
to 250.degree. C. for a time sufficient to allow at least 50% of
the mass of the particles subjected to the heat treatment drying
step (and preferably at least 60%, at least 70%, at least 80% or at
least 90% of the mass of the particles subjected to the heat
treatment drying step) to reach a temperature of at least
100.degree. C. (and more preferably at least 110.degree. C., at
least 120.degree. C., at least 130.degree. C., at least 140.degree.
C., at least 150.degree. C., at least 160.degree. C., at least
170.degree. C., at least 180.degree. C., at least 190.degree. C.,
or at least 200.degree. C.). This, of course, is dependent on the
particle size. However, preferably this time is at least 1 second,
more preferably at least 5 seconds, more preferably at least 10
seconds, at least 20 seconds or at least 30 seconds.
[0018] Whilst it is possible for the biomass feedstock particles to
be temperature soaked at each, fixed temperature stage, it is also
possible for the average temperature of the mass of the biomass to
vary during each stage of heating. Thus, during the pre-pyrolysis
heating stage(s), for example, the average temperature of the mass
of the biomass may increase, provided that it remains within the
required range for the required amount of time. However, it is
preferred that between the heat treatment drying stage and the
pre-pyrolysis heating stage the average temperature of the mass of
the biomass has a faster rate of change of temperature than during
either the heat treatment drying stage and the pre-pyrolysis
heating stage. Similarly, it is preferred that between the
pre-pyrolysis heating stage and the pyrolysis stage the average
temperature of the mass of the biomass has a faster rate of change
of temperature than during either the pre-pyrolysis heating stage
and the pyrolysis stage. In this way, a stepped heat treatment
schedule is provided for the biomass. This provides the benefit of
relatively long residence times for the biomass in temperature
ranges that provide technical benefits but relatively short
residence times at temperatures that provide technical drawbacks,
as identified by the present inventors.
[0019] Preferably, in the step in which at least 50% of the mass of
the biomass feedstock particles is held within the same temperature
range for a time of at least 5 seconds (i.e. the pre-pyrolysis
heating stage), the temperature range is 280.degree. C. to
320.degree. C.
[0020] The biomass feedstock particles are typically pyrolysed at a
temperature in the range 350.degree. C. to 550.degree. C. Such
temperatures are lower than temperatures typically used for
gasification. Thus, it is preferred that the main product of the
pyrolysis step is bio-oil. Preferably the pyrolysis step is for at
least 1 second, more preferably at least 5 seconds, more preferably
at least 10 seconds, at least 20 seconds or at least 30 seconds. As
the skilled person will appreciate, the preferred time for the
pyrolysis step depends on the properties of the biomass particles,
and in particular on the dimensions of the biomass particles. For
example, the time for the pyrolysis step may be at least one
minute. The time for the pyrolysis step may preferably be at most
10 minutes. For smaller particles of dimensions of about 1 mm (e.g.
rape seed), the time for the pyrolysis step may be relatively
short. For larger particles of dimensions (in at least one
direction) of up to about 4 cm (e.g. wood chips) the time for the
pyrolysis step may be relatively longer. The d.sub.50 particle size
for the biomass feedstock may be, for example, at least 2 mm, at
least 3 mm, at least 4 mm or at least 5 mm.
[0021] More generally, as the skilled person understands, the
optimum time for each stage depends on the biomass particle size.
Thus, in the step in which at least 50% of the mass of the biomass
feedstock particles is heated to a temperature in the range
280.degree. C. to 350.degree. C. and held within the same
temperature range for a time t.sub.280-350, it is preferred that
the following relationship is satisfied:
t.sub.280-350.gtoreq.Cd.sub.50
in which d.sub.50 is the number length mean particle size in metres
and C is a constant and is at least 5000 seconds per metre.
Preferably C is at least 6000 seconds per metre, and C may be at
least 7000 seconds per metre. For example, for d.sub.50 particle
size of 4 cm, time t.sub.280-350 may be about 5 minutes (300
seconds), corresponding to C of 7500 seconds per metre. C therefore
is in effect an empirical parameter based on the rate of increase
of temperature of central regions of the biomass particle when the
biomass particles are subjected to the heat treatment in the
temperature range 280.degree. C. to 350.degree. C.
[0022] Although not wishing to be bound by theory, the present
inventors consider that the flow of heat energy per unit time to
the biomass feedstock particles (and thus the rate of increase of
temperature of the biomass feedstock particles) has an important
effect on the products of each stage of the process. In particular,
it is considered important that the required temperature for each
stage is not overshot. In turn, this requires that the process is
carried out relatively slowly in order to avoid such an
overshooting effect.
[0023] It is therefore preferred that the heat carrier medium at
each stage has a maximum temperature which is at most 200.degree.
C. higher than the temperature of the biomass feedstock on entry
into that stage of the process. This therefore provides a limit on
the absolute increase in temperature for the biomass feedstock
particles at that stage.
[0024] Similarly, it is preferred that the amount of heat carrier
medium, in proportion to the amount of biomass, is limited at each
stage. Preferably the amount of heat carrier medium is three to
five times the amount of biomass feedstock, by mass.
[0025] Preferably, the heat carrier medium has a relatively high
specific heat capacity, e.g. at least 0.4 kJ kg.sup.-1 K.sup.-1.
Preferably, the heat carrier medium is a plurality of solid heat
carrier particles or objects. For example, steel balls may be used
as the heat carrier medium in at least one stage of the process.
Alloy filled steel balls may have an even higher heat capacity, and
therefore may be of particular utility.
[0026] Preferably, the biomass feedstock particles are heated to
the temperature in the range 280.degree. C. to 350.degree. C. at a
pre-pyrolysis heating location in a heat treatment system and
subsequently the biomass feedstock particles are heated to the
temperature above 350.degree. C. for pyrolysis at a pyrolysis
heating location in the heat treatment system. The biomass
feedstock particles may be moved between the pre-pyrolysis heating
location and the pyrolysis heating location by a conveyor system.
It is particularly preferred that the biomass feedstock particles
are moved between the pre-pyrolysis heating location and the
pyrolysis heating location substantially without reduction in
temperature.
[0027] Preferably the conveyor system is a screw or auger. However,
the conveyor system may be gravity-assisted, e.g. controlled by a
valve or other closure means.
[0028] The process is preferably a continuous process. This is in
preference to a batch process, for example. Thus, there is
preferably a substantially continuous conveyance of biomass
feedstock particles from the pre-pyrolysis heating stage to the
pyrolysis heating stage.
[0029] It is possible for the pre-pyrolysis heating location and
the pyrolysis heating location to be parts of the same heat
treatment kiln, at different temperatures.
[0030] Preferably, the conveyor system conveys the biomass
feedstock particles at a substantially uniform rate. In this way,
the residence time of the biomass feedstock particles in the
pre-pyrolysis heating location and the pyrolysis heating location
are typically determined by the spatial extent of the pre-pyrolysis
heating location and the spatial extent of the pyrolysis heating
location, and on the rate of conveyance of the biomass feedstock
particles.
[0031] It is preferred that the heat treatment stages use a heat
carrier to transfer heat to the biomass feedstock particles. For
example, where a fluidized bed system is used, the heat carrier may
be sand. Where a screw kiln is used, the heat carrier may be steel
balls and/or ceramic balls. It is preferred that at least a portion
of the heat carrier is recycled in the process. In particular, heat
carrier used in a hotter stage of the process may be recycled for
used in a lower temperature stage of the process, optionally
without reheating. Where reheating is required in order to provide
heat carrier at a suitable temperature, heating means may be
provided. Optionally, one or more products of the overall process
(e.g. char and/or waste gas) may be combusted in order to provide
heat for the process.
[0032] In the apparatus, it is preferred that the pre-pyrolysis
heating location and the pyrolysis heating location are parts of a
heat treatment kiln, at different temperatures. Similar preferences
apply where there are first and second pre-pyrolysis heating stages
and/or first and second pyrolysis heating stages.
[0033] The apparatus preferably has a conveyor system for moving
the biomass feedstock particles between the pre-pyrolysis heating
location and the pyrolysis heating location. The conveyor system
may be a screw or auger, for example.
[0034] The apparatus may further provide a heat carrier recycling
system for at least partial recycling of heat carrier material from
the pre-pyrolysis heating stage and/or from the pyrolysis heating
stage to one or more of the pre-pyrolysis heating stage and the
pyrolysis heating stage.
[0035] The plateau in the heat treatment schedule, at an elevated
temperature but below the temperature at which significant
pyrolysis will occur, is used in order to control the steepness of
the temperature gradient at the biomass feedstock particles during
the subsequent transition into the pyrolysis regime. The inventors
consider that the lower the heating rate in this transition, then
the lower the concentration in the pyrolysis products of fragments
from high tar lignin-based components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] Preferred embodiments of the invention will now be
described, by way of example, with reference to the accompanying
drawings, in which:
[0037] FIG. 1 shows the schematic layout of a pyrolysis reactor
apparatus according to one embodiment of the invention.
[0038] FIG. 2 shows the schematic layout of a pyrolysis reactor
apparatus according to another embodiment of the invention.
[0039] FIG. 3 shows the schematic layout of a pyrolysis reactor
apparatus according to another embodiment of the invention.
[0040] FIG. 4 shows a modification to the embodiment of FIG. 3.
[0041] FIG. 5 shows the product distribution for pyrolysis of wheat
straw, at different temperatures.
[0042] FIG. 6 shows the product distribution for pyrolysis of wheat
straw pellets at different temperatures.
[0043] FIGS. 7a-7i illustrate the production of the main products
released during pyrolysis of lignin, and provides corresponding
numerical simulations.
[0044] FIG. 8 shows an enlarged annotated version of FIG. 7a.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, FURTHER OPTIONAL
FEATURES
[0045] FIG. 1 shows the schematic layout of a pyrolysis reactor
apparatus 10 according to one embodiment of the invention.
[0046] In this apparatus, there is provided a first heating stage
with a first fluidized bed reactor 12 with heat carrier (not shown)
heated to 200.degree. C. This heating stage amounts to a first
pre-pyrolysis heating stage. Biomass feed material (not shown) is
fed into first fluidized bed reactor 12 via inlet 14. Gaseous
products (i.e. products, gaseous at or near 200.degree. C., of
heating the biomass feed material at 200.degree. C.) from the first
fluidized bed reactor 12 are extracted at first reactor gas product
outlet 16. The gas product of the first reactor may be water-rich,
but may still have a useful heating value. In this case, the gas
product of the first reactor may be used to heat the heat carrier
used in one or more of the reactors.
[0047] At the base of the first reactor 12 is outlet 18, for
conveying the processed biomass feed material from the first
reactor 12 to a second reactor 20. The input of material into the
second reactor 20 is controlled by second reactor inlet 22. The
material may be conveyed from the first reactor 12 to the second
reactor 20 by gravity alone, or may be conveyed additionally or
alternatively by screw conveyor or auger.
[0048] The heat carrier used in the first reactor is preferably
recycled for further use in the apparatus, most preferably for
further use in the first reactor in order that the amount of
reheating required is minimised. However, it is possible for some
of the heat carrier used in the first reactor to be conveyed into
the second reactor.
[0049] The heat carrier may be, for example, char, sand and/or
steel. Some or all of the heat carrier may be recycled or carried
forward, depending on the effort needed in order to separate it
from the feedstock material.
[0050] The second reactor 20 is also a fluidized bed reactor, but
differs from the first reactor 12 in that it operates at
300.degree. C. This therefore amounts to a second pre-pyrolysis
heating stage. The biomass feed material is held in second reactor
20 for a time sufficient to allow at least 50% of the mass of the
biomass feed material particles to reach about 300.degree. C. The
time required for this depends on the particle size of the feed
material. Typically, the use of a fluidized bed reactor requires
that the biomass feed material particle size is relatively small,
e.g. at least 1 mm or about 2 mm. Thus, the biomass feedstock
particles have a d.sub.50 mean particle size of at least 1 mm or
about 2 mm. As the skilled person understands, d.sub.50 denotes a
number length mean particle size such that 50% of particles have
volume smaller than a sphere of diameter d.sub.50 and 50% of
particles have volume larger than a sphere of diameter d.sub.50.
More preferably, the biomass feedstock particles have a d.sub.90
mean particle size of at least 1 mm, where d.sub.90 denotes a
number length mean particle size such that 10% of particles have
volume smaller than a sphere of diameter d.sub.90 and 90% of
particles have volume larger than a sphere of diameter d.sub.90.
Typically the biomass feed particles are held in the second reactor
for at least 5 seconds. For larger particle sizes, longer residence
times are needed in order to ensure suitable uniformity in
temperature through the diameter of the particles.
[0051] Gaseous products (i.e. products, gaseous at or near
300.degree. C., of heating the biomass feed material at 300.degree.
C.) from the second fluidized bed reactor 20 are extracted at
second reactor gas product outlet 24. The gas product of the second
reactor may be water-rich, but may still have a useful heating
value. In this case, the gas product of the second reactor may be
used to heat the heat carrier used in one or more of the reactors.
Alternatively, this gas product may have a suitably high heating
value to be used as fuel for alternative purposes (e.g. consumer
heat generation, electricity generation, etc.).
[0052] At the base of the second reactor 20 is outlet 26, for
conveying the processed biomass feed material from the second
reactor 20 to a third reactor 28. The input of material into the
third reactor 28 is controlled by third reactor inlet 30. The
material may be conveyed from the second reactor 20 to the third
reactor 28 by gravity alone, or may be conveyed additionally or
alternatively by screw conveyor or auger.
[0053] The heat carrier used in the second reactor is preferably
recycled for further use in the apparatus, most preferably for
further use in the second reactor, in order that the amount of
reheating required is minimised. However, it is possible for some
of the heat carrier used in the second reactor to be conveyed into
the third reactor.
[0054] The third reactor 28 is also a fluidized bed reactor, but
differs from the first reactor 12 and second reactor 20 in that it
operates at 400.degree. C. This therefore amounts to a first
pyrolysis heating stage. As discussed in more detail below, the
result of heating at about 400.degree. C. is to promote pyrolysis,
the products of pyrolysis depending on the temperature of the first
pyrolysis heating stage (400.degree. C. in this case) and the
nature of the heating schedule prior to the first pyrolysis heating
stage. Since in the second reactor the biomass is heated
substantially uniformly to about 300.degree. C., the rate of
heating as the biomass is transferred from the second pre-pyrolysis
heating stage to the first pyrolysis heating stage is not
particularly high.
[0055] The effect of this is that the amount of tar in the products
of the first pyrolysis heating stage is reduced. The gaseous
products of the first pyrolysis heating stage are extracted from
third reactor 28 along line 32. These gaseous products are
condensed (not shown) to give bio-oil and permanent gases (e.g.
syngas (mixture of H.sub.2 and CO)). The gaseous products may be
subjected to a char removal step (not shown), e.g. via
electrostatic precipitation and/or cyclonic separation.
[0056] At the base of the third reactor 28 is outlet 34, for
conveying partially pyrolysed biomass feed material from the third
reactor 28 to a fourth reactor 36. The input of material into the
fourth reactor 36 is controlled by fourth reactor inlet 38. The
material may be conveyed from the third reactor 28 to the fourth
reactor 36 by gravity alone, or may be conveyed additionally or
alternatively by screw conveyor or auger.
[0057] The heat carrier used in the third reactor is preferably
recycled for further use in the apparatus, most preferably for
further use in the third reactor, in order that the amount of
reheating required is minimised. However, it is possible for some
of the heat carrier used in the third reactor to be conveyed into
the fourth reactor.
[0058] The fourth reactor 36 is also a fluidized bed reactor, but
differs from the first reactor 12, second reactor 20 and third
reactor 28 in that it operates at 500.degree. C. This therefore
amounts to a second pyrolysis heating stage. The result of heating
partially pyrolysed biomass material at about 500.degree. C. is to
further pyrolysis, in particular permanent gas formation, since oil
production is concentrated in particular in the first pyrolysis
stage.
[0059] The gaseous products of the second pyrolysis heating stage
are extracted from fourth reactor 36 along line 40. In this
embodiment, line 40 joins with line 32 at a manifold connection 42.
Thus, the gaseous products of both the first and second pyrolysis
stages are condensed together (not shown) to give bio-oil and
permanent gases (e.g. syngas (mixture of H.sub.2 and CO)). In view
of other gas-borne products that may be entrained with the gaseous
products, a char removal step (not shown) may be deployed, e.g. via
electrostatic precipitation and/or cyclonic separation.
[0060] At the base of the fourth reactor 36 is outlet 44, for
conveying the remaining solid material from the fourth reactor 36
to chamber 46. Within chamber 46, the solid char is separated from
the heat carrier. The heat carrier may be recycled for use in the
fourth reactor again, or it may be used to heat one or more of the
other reactors. The collected char may be used as a fuel to provide
heat where required in the apparatus, or may be used as a fuel
elsewhere, e.g. for consumer heat generation, or for electricity
generation.
[0061] FIG. 1 illustrates an embodiment of the invention in which a
series of fluidized bed reactors are placed in series, with
increasing reactor temperature, in order to provide to the biomass
feed material a suitable heat treatment profile in order to obtain
some of the advantages of the invention. However, as will be clear
to the skilled person, other types of reactor may be used. For
example, a mixing drum type of reactor may be used for each heating
stage, with appropriate conveying means between them. These are
particularly useful for biomass feed material that has a relatively
large (e.g. up to about 4 cm d.sub.50) particle size, and may be
suitable where the particle size is non-uniform. Alternatively,
screw reactors may be used. Still further, steel ball reactors or
cycled spheres reactors may be used. Furthermore, it is not
necessary for each heating stage to be carried out using the same
type of reactor. For example, one heating stage may be carried out
in a screw reactor, whereas another stage may be carried out in a
fluidized bed reactor.
[0062] FIG. 2 shows an alternative embodiment of the invention.
Here a mixing drum reactor apparatus 50 is provided. Mixing paddles
52, 54, 56, 58 provide mixing of the biomass feed material 53 and
heat carrier 55. These are rotated about axis 51. In FIG. 2, the
biomass feed material is conveyed in a direction generally left to
right. This may be assisted by a slight declination of the axis of
the reactor, but this is not essential. As can be seen, there are
provided four heating stages, at 200.degree. C., 300.degree. C.,
400.degree. C. and 500.degree. C. These correspond to the first
(200.degree. C.) pre-pyrolysis stage, the second (300.degree. C.)
pre-pyrolysis stage, the first (400.degree. C.) pyrolysis stage and
the second (500.degree. C.) pyrolysis stage described with respect
to FIG. 1.
[0063] In cases where the speed of travel of the biomass along the
apparatus 50 is fixed, the residence time of the biomass in each
heating stage may be varied by varying the length of each stage.
Conveniently, this may be done by varying the axial extent of the
heating means 60, 62, 64, 66.
[0064] Gaseous product removal lines (not shown) and a char removal
conduit (not shown) are also provided.
[0065] FIG. 3 shows an alternative embodiment of the invention.
Here, two screw reactors 70, 72 are provided. Biomass feed material
and heat carrier material at temperature t1 are introduced into
first screw reactor 70 at inlet 71. Thus, the temperature at the
left hand end of the first screw reactor 70 is t1, as illustrated.
The heat carrier is heated to temperature t1 by heater 74. Thus,
the left hand side of the first screw reactor 70 provides the first
pre-pyrolysis heating stage, the biomass feed material and the heat
carrier being conveyed by the screw and the residence time of the
biomass in the first pre-pyrolysis heating stage being determined
by the length of the first pre-pyrolysis heating stage and the
speed of conveyance.
[0066] Heat carrier at temperature t2 is introduced at inlet 76. t2
is greater than t1. Therefore, the right hand side of the first
screw reactor corresponds to the second pre-pyrolysis heating
stage. By the time the biomass reaches the right hand end of the
first screw reactor, it is at a temperature of t2*, where t2* is
greater than t1, but less than t2. Accordingly, t2* is in the range
280-350.degree. C., e.g. about 300-320.degree. C. The pre-heated
biomass, any char and the heat carrier is discharged at outlet 78
into separation chamber 80 where the heat carrier is at least
partially separated from the feed, so that some heat carrier (and
preferably no biomass feed) is recycled to inlet 71 at temperature
t1 via heater 74.
[0067] Typically, if the amount of heat carrier inserted at inlet
71 is 1 (arbitrary units), then the amount of heat carrier inserted
at inlet 76 at temperature t2 is 0.5. Thus, in separation chamber
80, 1 unit of heat carrier is recycled back to inlet 71 and 0.5
units is allowed to proceed into the second screw reactor 72.
[0068] The pre-heated biomass feed particles are passed into the
left hand end of the second screw reactor 72. Additional heated
heat carrier (0.5 units) is supplied via inlet 82 at a temperature
sufficient to provide a temperature t3 at the left hand side of the
second screw reactor, where t3 is greater than t2. Thus, 1 unit of
heat carrier in total is provided to the pre-heated biomass. At
temperature t3, pyrolysis of the pre-heated biomass takes place,
generating pyrolysis gas and vapours that are extracted from the
second screw reaction 72 along an extraction line (not shown). The
left hand side of the second screw reactor therefore corresponds to
the first pyrolysis heating stage.
[0069] Further additional heat carrier (0.5 units) is provided to
the second screw reactor towards the right hand side of the second
screw reactor. This heat carrier is at temperature t4, which is a
higher temperature than t3, but is a temperature at which useful
pyrolysis still occurs. The right hand side of the second screw
reactor therefore corresponds to the second pyrolysis heating
stage. Vapour and gaseous pyrolysis products from the second
pyrolysis heating stage may be extracted along the same extraction
line as for the first pyrolysis heating stage.
[0070] In an alternative embodiment, t3 may be lower than the
temperature required for pyrolysis. In this case, there may be
three pre-pyrolysis heating stages and only a single pyrolysis
stage at temperature t4.
[0071] In either embodiment, the temperature of the combination of
the remains of the biomass feed and the heat carrier may be reduced
to about t2 at the right hand end of the second screw reactor.
These materials exit the second screw reactor at outlet 86, being
discharged into separation chamber 88. Here the char is separated
from the heat carrier (1.5 units). 0.5 units of heat carrier are
recycled to inlet 84 at temperature t4 via heater 90. 1 unit of
heat carrier is recycled to inlet 82 at temperature t3, via conduit
92 and via additional heating means (not shown) if required.
[0072] FIG. 4 shows a modified embodiment compared with FIG. 3.
Identical features are not described or numbered again. In FIG. 4,
conduit 92 does not lead only to inlet 82, but also leads, via a
manifold, to inlet 76.
[0073] The particle size distribution of the biomass feedstock can
be determined, for example, by microscopic examination, based on a
representative sample of the biomass feedstock particles.
[0074] The embodiments of the invention described here are
principally concerned with intermediate pyrolysis processes.
[0075] Such processes allow various types of feed material to be
used, e.g. different types of biomass, different shapes of biomass
particles and also allow for some inhomogeneity in the shapes
and/or types of biomass particles. For this reason, intermediate
pyrolysis is of particular interest for intermediate scale
applications where the quality, type and pre-processing capability
of biomass is limited.
[0076] Intermediate pyrolysis processes can produce high quality
pyrolysis products. Variable relative yields of products (coke
(char), bio-oil and gas) are possible by varying the process
parameters, as explained below.
[0077] It is considered that some intermediate pyrolysis processes
become economically viable at biomass throughput rates of about
12,000 tonnes per year to about 20,000 tonnes per year (based on
dry biomass).
[0078] FIG. 5 shows the product distribution for pyrolysis of wheat
straw at different temperatures, for a 500 kg/hour feedrate.
[0079] FIG. 6 shows the corresponding product distribution for
pyrolysis of wheat straw pellets (i.e. pressed particles of straw)
at different temperatures. Note that the temperatures used in FIG.
6 are different to those used in FIG. 5. It is important to note
that the dimensions and shape of the feedstock does not have a
significant effect on the products of pyrolysis. FIGS. 5 and 6 show
that similar good results can be achieved for both milled straw
(FIG. 5) and for straw pellets (FIG. 6), provided that the
provision of heat is well-controlled.
[0080] Elemental analysis of the composition of the initial wheat
straw and the char formed at 325.degree. C., 350.degree. C.,
375.degree. C., 400.degree. C. and 450.degree. C. shows the
following trends with increasing temperature: N content increases
from 0.350 wt % to 0.615 wt %; C content increases from 42.09 wt %
to 63.02 wt %; H content decreases from 5.67 wt % to 3.26 wt %; S
content increases from 0.105 wt % to 0.251 wt %; ash content
increases from 5.60 wt % to 16.70 wt %; oxygen content decreases
from 46.185 wt % to 16.152 wt %. The C:O ratio increases from 0.9
to 3.9.
[0081] It is of interest to consider how tar forms during typical
pyrolysis processes. Large amounts of tar in the bio-oil are
considered to be undesirable. Typical high organics compounds that
may be classified as tar in this technical field may be considered
to be organic compounds of molecular weight 300 a.m.u. or
greater.
[0082] The biomass feed material is in general composed of
cellulose, glucan, xylan and lignin, in differing proportions
depending on the nature of the feed material. For lignin-rich
biomass feed materials, it is of particular interest to consider
the thermal decomposition of lignin and how the heat treatment
schedule of the biomass may affect the formation of tar from
lignin. Lignin encompasses a class of complex, high molecular
weight polymers whose exact structure varies.
[0083] During thermal decomposition, lignin may decompose by a
variety of mechanisms, such as decomposition into oligomers, the
separation of side groups, and the formation of modified chains.
Side groups in general may form permanent gases. The remaining
components form useful bio-oil, tar and char.
[0084] In more detail, certain reactions can affect the principal
polymer backbone to provide a molecular weight decrease in the
sample and volatile products such as monomers, dimers, etc.
Reactions may affect the principal polymer backbone to provide tars
and waxes (typically from oligomers and polymer chain
fragments).
[0085] Furthermore, certain reactions can affect the polymer
repeated units. For example, eliminations can provide volatile
products and char residue. Cyclization can modify the chains, e.g.
by unsaturation or cross linking, again producing volatile products
and char residue. Cyclization can alternatively form volatile
products by chain scission.
[0086] Thus, the propagation of the reactions can affect both the
molecular weight distribution and the chemical structure of the
polymer and thus of the pyrolysis products.
[0087] FIG. 7 (FIGS. 7a-7i) shows the main production of the
products released during pyrolysis of lignin. FIG. 7 shows that
with a higher heating rate, the amounts of fragments from high
molecular weight species increases. In FIG. 7, the differential
thermogravimetric analysis was carried out on fine powder biomass
feedstock material.
[0088] In FIGS. 7a, 7b and 7c, the graphs show an overall
thermogravimetric envelope (OTE) for the analysis. The heating rate
for each of FIGS. 7a, 7b and 7c was 25.degree. C./min. These graphs
also show (using solid lines) numerical simulation results of the
likely contribution to the overall thermogravimetric envelope from
low temperature decomposition processes, medium temperature
decomposition processes and high temperature decomposition
processes. It is noted that the overall envelope and the numerical
simulation results in FIGS. 7a, 7b and 7c are identical. FIGS. 7a,
7b and 7c further show individual curves (corresponding to mass
spectrometry results) for individual species from the decomposition
of lignin. In FIG. 7b, water has a molecular weight (MW) of 18 and
carbon dioxide has a molecular weight of 44. In FIG. 7c, carbon
monoxide has a molecular weight (MW) of 28 and methane has a
molecular weight of 16.
[0089] FIGS. 7d, 7e and 7f show the relative proportions of the
production of water, carbon dioxide and carbon monoxide,
respectively, at different heating rates.
[0090] FIGS. 7g, 7h and 7i show the relative production of certain
alcohols during the decomposition of lignin, based on the heating
rate. These alcohols can be considered to be useful indicators of
the likelihood of tar formation. Thus, as the heating rate
increases, the amount of tar expected also increases.
(Oxy-allyl)guaiacol has a molecular weight of 151.
4-(hydroxy-prop-2-enyl)guiaiacol has a molecular weight of 136.
Syringol has a molecular weight of 155.
[0091] FIG. 8 shows a marked-up version of FIG. 7a. Here the main
calculated differential thermogravimetric (DTG) curve is marked as
100 for the pyrolysis reaction at a heating rate of 25.degree. C.
per minute. A low temperature step 102 peaks at about 300.degree.
C., corresponding to a low temperature decomposition step for
lignin, produced by numerical simulation. The main decomposition
step 104 peaks at about 410.degree. C., corresponding to a medium
temperature decomposition step for lignin, produced by numerical
simulation. A high temperature step 106 has a much broader
temperature distribution than either the low temperature step of
the main decomposition step, but peaks at about 440.degree. C., and
corresponding to a high temperature decomposition step for lignin,
produced by numerical simulation.
[0092] In FIG. 8, as the temperature increases, there is notable
evolution of (oxy-allyl)guaiacol (curve 108), syringol (curve 110)
and 4-(hydroxy-prop-2-enyl)guiaiacol (curve 112).
[0093] As illustrated in FIGS. 7 and 8, the low temperature step
102 peaks at about 300.degree. C., but the main decomposition step
does not peak until 400.degree. C. The embodiments of the present
invention utilise this gap by heating the biomass feed material to
a temperature in the range 280.degree. C. to 350.degree. C. and
holding the biomass feed material within the same temperature range
for a time of at least 5 seconds. As shown in FIGS. 7 and 8, the
result is that the low temperature step is allowed to start but the
main decomposition step is not allowed to take place. The result is
that the evolved products from the low temperature stage are
water-rich. These can be extracted from the reactor if desired. The
biomass feed material is then heated to a temperature above
350.degree. C. for pyrolysis.
[0094] Allowing the biomass feed material to be held at a
temperature in the range 280.degree. C. to 350.degree. C. allows a
large proportion of the material, by mass, to reach a temperature
within this range. Since typically the feed material particles are
relatively large (which is advantageous in that it reduces
pre-processing requirements of the feed material), the present
inventors consider that not only the surface of the particles
should be allowed to reach this temperature range, but also a
significant proportion (and preferably all) of the internal mass of
the feed particles should be allowed to reach this temperature
range. However, this stage of heating does not result in
substantial pyrolysis.
[0095] The subsequent stage of heating, to temperatures above
350.degree. C. for pyrolysis, requires only a small absolute
increase in temperature of the feedstock particles. The present
inventors consider that the result of a small increase in
temperature to the pyrolysis stage is that the proportion of tar
present in the pyrolysis products is reduced.
[0096] Test with oils from non woody biomass have been made on dual
fuel engines working properly due to the low tar content. Normally
from fast pyrolysis only woody material pyrolysis oil can be
applied but with very high difficulties, everything else fails. In
the present case, the inventors consider that the use of a staged
intermediate pyrolysis process allows the production of useful
fuels with relatively low tar content using, for example, woody
biomass, which is far more abundant than biomass with an inherently
low tar content.
[0097] Preferred embodiments of the invention have been described
by way of example. On reading this disclosure, modifications of
these embodiments, further embodiments and modifications thereof
will be apparent to the skilled person and as such are within the
scope of the invention.
* * * * *