U.S. patent application number 13/458285 was filed with the patent office on 2012-08-23 for biomass pretreatment process.
This patent application is currently assigned to KiOR, Inc.. Invention is credited to Michael Brady, Paul O'Connor, Dennis Stamires.
Application Number | 20120214216 13/458285 |
Document ID | / |
Family ID | 41396350 |
Filed Date | 2012-08-23 |
United States Patent
Application |
20120214216 |
Kind Code |
A1 |
Brady; Michael ; et
al. |
August 23, 2012 |
Biomass Pretreatment Process
Abstract
A process is described for pretreating lignocellulosic biomass.
The process comprises swelling the lignocellulosic biomass with an
aqueous liquid. The pretreated lignocellulosic biomass may be used
as a feedstock for the enzymatic conversion to ethanol, or in a
thermal conversion. process to produce bio-oil. The pretreatment
results in a greater yield and, in the case of a thermal conversion
process, a better quality of the bio-oil. The pretreatment process
may be used to adjust the composition and amount of inorganic
material present in the lignocellulosic biomass material.
Inventors: |
Brady; Michael; (Studio
City, CA) ; O'Connor; Paul; (Hoevelaken, NL) ;
Stamires; Dennis; (Dana Point, CA) |
Assignee: |
KiOR, Inc.
Pasadena
TX
|
Family ID: |
41396350 |
Appl. No.: |
13/458285 |
Filed: |
April 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12921343 |
Jan 26, 2011 |
8168840 |
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PCT/EP2009/057955 |
Jun 25, 2009 |
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13458285 |
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61075563 |
Jun 25, 2008 |
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Current U.S.
Class: |
435/165 ;
554/124; 554/156; 554/158 |
Current CPC
Class: |
Y02P 30/20 20151101;
C10L 1/04 20130101; Y02E 50/10 20130101; Y02E 50/16 20130101; C13K
1/02 20130101; C10G 2300/80 20130101; C10G 1/00 20130101; D21C 1/06
20130101; D21C 1/04 20130101; C10G 2300/44 20130101; C10G 2300/1014
20130101; C08L 97/00 20130101; C10G 1/083 20130101; D21C 11/0007
20130101; C12P 2201/00 20130101 |
Class at
Publication: |
435/165 ;
554/124; 554/156; 554/158 |
International
Class: |
C12P 7/10 20060101
C12P007/10; C07C 51/00 20060101 C07C051/00 |
Claims
1-29. (canceled)
30. A process for converting a solid lignocellulosic biomass
material to a bio-oil, the process comprising: a. pretreating a
solid lignocellulosic biomass material to produce a solid modified
lignocellulosic biomass material having an increased bulk porosity;
and b. subjecting the solid modified lignocellulosic biomass
material to enzymatic hydrolysis, thermoconversion, or any
combination thereof
31. The process of claim 30 wherein step (a) comprises contacting
the solid lignocellulosic biomass material with a swelling
agent.
32. The process of claim 31 wherein the swelling agent is a
solvent.
33. The process of claim 31 wherein the swelling agent is an
aqueous liquid.
34. The process of claim 31 wherein the solid lignocellulosic
biomass material is contacted with a swelling agent at a
temperature between 35.degree. C. and 100.degree. C.
35. The process of claim 31 wherein the solid lignocellulosic
biomass material is contacted with a swelling agent at a
temperature greater than 100.degree. C.
36. The process of claim 31 wherein step (a) further comprises
removing the swelling agent from the solid lignocellulosic biomass
material.
37. The process of claim 36 wherein the swelling agent is removed
by applying mechanical action to the solid lignocellulosic biomass
material.
38. The process of claim 37 wherein the mechanical action results
in a particle size reduction of the solid lignocellulosic biomass
material.
39. The process of claim 30 wherein the ash content of the solid
lignocellulosic biomass material is reduced to less than 5 wt %,
based on dry weight.
40. The process of claim 30 wherein the ash content of the solid
lignocellulosic biomass material is reduced to less than 3 wt %,
based on dry weight.
41. The process of claim 30 wherein the Fe content of the solid
lignocellulosic biomass material is reduced to less than 2,000
mg/kg, based on dry weight.
42. The process of claim 30 wherein the Fe content of the solid
lignocellulosic biomass material is reduced to less than 1,600
mg/kg, based on dry weight.
43. The process of claim 32 wherein the solvent comprises an
inorganic acid or an inorganic base.
44. The process of claim 30 further comprising adding an inorganic
catalytic material before step (b).
45. The process of claim 44 wherein the inorganic catalytic
material is dissolved in a solvent.
46. The process of claim 45 wherein the solvent is an aqueous
liquid.
47. The process of claim 44 wherein the catalytic material is in a
particulate form.
48. The process of claim 44 wherein the catalytic material
comprises an inorganic salt.
49. The process of claim 44 wherein the catalytic material
comprises a cation selected from the group consisting of K,
NH.sub.4, Na, Ba, Mn, Mg, Ca, Li, Zn, and mixtures thereof.
50. The process of claim 44 wherein the catalytic material
comprises an anion selected from the group consisting of OH,
CO.sub.3, SO.sub.4, PO.sub.4, Cl, NO.sub.3, and mixtures
thereof
51. The process of claim 44 wherein the catalytic material
comprises an insoluble inorganic material, in a colloidal or
nanoparticular form.
52. The process of claim 38 wherein the insoluble inorganic
material is selected from the group of refractory oxides, zeolites,
cationic clays, anionic clays, layered hydroxide materials,
hydrotalcites, and any mixture thereof.
53. The process of claim 30 wherein thermoconversion comprises a
process selected from the group consisting of pyrolysis, catalytic
cracking, gasification, steam gasification, hydrotreatment, and any
combination thereof.
54. The process of claim 30 further comprising the step of
subjecting the solid modified lignocellulosic biomass material to
drying, milling, comminuting or any combination thereof before step
(b).
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
12/921,343, filed Jun. 25, 2009, which is a national phase filing
under 35 U.S.C. .sctn.371 of International Application No.
PCT/EP09/057955, filed on Jun. 25, 2009, and claims the benefit of
U.S. Provisional Application No. 61/075,563, filed Jun. 25, 2008,
the entirety of these applications are hereby incorporated herein
by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to a process for pretreating
biomass material prior to conversion to liquid fuel, and more
particularly to a pretreatment process comprising soaking biomass
material in an aqueous solvent, which may sometimes also contain
additive(s). In a preferred embodiment the pretreatment process
comprises removal of inorganic materials from the biomass
material.
[0004] 2. Description of the Related Art
[0005] World energy demand is projected to increase substantially
due to: an increase in the world's population; improvement of the
standard of living in underdeveloped countries; and depletion of
the reserves of fossil fuels.
[0006] Now, generally recognized by major countries, global
climatic changes caused by increasing emissions of greenhouse
gases, such as CO.sub.2, require that newly developed energy
sources must be environmentally compatible and sustainable.
Therefore, greener sources of energy are needed to replace or
reduce the consumption of fossil fuels. Biomass is a sustainable
and renewable source of fuel, with potentially a net zero
greenhouse gas impact.
[0007] Biomass conversion technologies include: biological
processes, such as anaerobic or aerobic fermentation; and thermal
conversion processes, such as direct combustion for heating and
generating electricity; gasification for producing syngas; and
pyrolysis for producing biooils, which can be converted to fuels
and chemicals.
[0008] The thermal conversion processes include hydrothermal
processes, wherein biomass is treated in slurry form in autoclaves
at temperatures above 200.degree. C. and under autogenous
pressures.
[0009] Pyrolysis processes have a high potential for large scale
commercialization, as they provide flexibility in varying process
conditions, such as heating rate, temperature, pressure, contact
time, atmosphere, etc., to optimize yields of liquids (oil), gas
and char. Of particular interest is fast (or flash) pyrolysis
designed to convert the biomass to maximum amounts of oil,
employing a very low residence time, a very high heating rate and
temperatures up to 500.degree. C. The oil produced has a high
energy density that can be directly used in combustion or refined
to fuels and specialty chemicals.
[0010] However, the pyrolysis-derived bio-oils, because of their
high oxygen contents, high viscosity, corrosiveness and low
stability, have limited direct applications as fuels. Intensive
research is being now carried out to upgrade the quality of said
bio-oils to products that are comparable to conventional fuels in
composition, and chemical and physical properties.
[0011] Pretreatment processes of biomass before pyrolysis offer
possible solutions in the form of biomass modification that will
allow the pyrolysis process to be conducted at less severe
conditions (i.e., lower temperatures, shorter contact times), and
more efficiently in that more oil is produced, and the oil produces
is of a better quality.
[0012] In an alternative approach, biomass conversion in large
commercial plants is now carried out to produce ethanol, primarily
using as feeds from renewable sources such as com, sugar cane, and
cereal grains. Because the cost of these raw materials represents
roughly one-half of the total cost of the process to produce the
ethanol, it is of paramount interest to use cheaper biomass raw
materials for conversion to ethanol. Furthermore, it is important
to utilize biomass sources other than grains, in order to minimize
the impact on food prices.
[0013] Consequently, less costly lignocellulosic biomass materials
derived from agricultural and forestry residues are very attractive
for use as biomass sources to be converted to ethanol or other
fuels.
[0014] The use of ethanol in automobile fuels not only reduces the
need for petroleum (crude oil), but also substantially reduces the
carbon dioxide car-exhaust emissions.
[0015] Commercial large scale operations involving the production
of ethanol from cellulosic biomass use biological or non-biological
processes to depolymerize (break down) the cellulose. The most
commonly used biological processes use enzymes, whereas the
nonbiological processes use an acid hydrolysis to convert the
cellulose to sugars, mostly using dilute or concentrated sulphuric
acid. These processes are considered as a pretreatment of the
biomass in the overall bioconversion processes, which are followed
by fermentation and distillation.
[0016] In the prior art one finds descriptions of other kinds of
pretreatments such as steam explosion, which is followed by
enzymatic hydrolysis, fermentation and distillation in the
production of ethanol. (See, C. E. Wyman et al, Bioresource
Technology 96 (2005), 1959-1966.)
[0017] Since the presently known processes for conversion of the
lignocellulosic biomass (derived from agricultural and forestry
residues) are more expensive than the processes used now
commercially to produce ethanol from grains and cereals, there is
strong interest in developing new or improved processes that will
allow a more cost-effective and environmentally acceptable manner
of converting lignocellulosic biomass (from residues derived from
agriculture and forestry materials) to ethanol. (See N. Mosier et
al, Bioresource Technology 96 (2005), 673-686.)
[0018] In general, lignocellulosic biomass from such residues
consists mainly of three components: cellulose, hemicellulose and
lignin. The cellulose component is a polymer of glucose, formed in
long strands. It is associated with the hemicellulose component
layer, and both the crystalline cellulose and hemicellulose are
encapsulated by the lignin cell wall.
[0019] In ethanol production from cellulosic biomass, the cellulose
and hemicellulose are converted to sugars, such as glucose and
xylose, followed by fermentation. Lignin is a 3-dimensional
branched polyaromatic matrix acting as a sheath, or a protective
coating to the cellulose and hemicellulose components of the
biomass.
[0020] As a result, due to the differences in the bonding of the
components, the high crystallinity of the cellulose, and the
presence of the protective sheath of the lignin, the penetration
of, and interaction with, the acids and/or the enzymes is highly
impeded. This problem is much less observed when processing cereal
grains to ethanol by converting the cellulose and hemicellulose to
glucose, using acid or enzymatic hydrolysis.
[0021] However, for biomass from sources other than cereal grain,
the lignin present resists the enzyme attack and hence lower yields
are obtained. To at least partly overcome this problem,
pretreatment of the biomass is necessary prior to subjecting the
biomass to enzymatic hydrolysis. (See T. A. Hsu, et al. Chemtech,
May (1980), 315-319.)
[0022] Since the major cost of the overall conversion process is
due to the biomass feed and enzymes, it is necessary to minimize
the use of enzymes and obtain the maximum conversion of the
carbohydrates to ethanol.
[0023] For these reasons, a considerable amount of R&D work has
been devoted during the last few years for developing means to
pretreat the lignocellulosic biomass in such ways that the
accessible surface area increases, which will allow an increase in
the biomass ethanol conversion. The most popular processes are acid
and enzymatic hydrolysis processes, which are used mostly to
convert the cellulose and hemicellulose to glucose.
[0024] In the prior art there are several versions of the original
acid hydrolysis process. These involve very concentrated acids or
dilute acids, in one or two step treatments, and combinations of
acid treatment with steam treatments, such as steam-explosion.
[0025] Overall, the pretreatment processes utilizing acids such as
sulphuric acid, require specially constructed plant equipment that
must be resistant to acid corrosion. Additionally, the use of acid
requires neutralization by a low-cost base such as calcium
hydroxide, and the formed sulphate salt must be filtered and washed
from the biomass. This creates large waste streams that require
disposal and lead to additional costs. Further, for the use of
highly concentrated acids, the process requires an additional
evaporator to produce/recycle the highly concentrated acid.
[0026] The acid pretreatments of lignocellulosic biomass feeds used
to convert the cellulose and hemicellulose to fermentable sugars
have certain disadvantages relating to higher costs, lower
efficiencies and environmental problems. Specifically, processes
using a high acid concentration have disadvantages relating to
corrosion of equipment, and high cost waste stream disposal,
whereas the dilute (low acid concentration) process produces a low
conversion of the biomass to fermentable sugars.
[0027] Pretreatments using steaming (steam-explosion) as such and
combinations with acid treatments have certain disadvantages.
During steam-explosion pretreatments, the pentoses and hexoses
produced from the hydrolysis of the cellulose material are to some
extent further converted to undesirable by-products, such as
furfural, levullinic acid (4-oxopentanoic acid) and formic acid,
together with other products. (See M. M. Wu et aI, Appl.
Biochemistry and Biotechnology 77 (1999) 47-54.)
[0028] In general, processes involving acid treatments and
steam-explosion produce compounds such as aliphatic acids, phenolic
and furan derivatives. These degradation products act as inhibitors
in subsequent processes using enzymes to convert the sugars to
ethanol. (See V. S. Chang, et al., Appl. Biochemistry and
Biotechnology 84 (2000) 5-37.)
[0029] Further, although high severity steam explosion allows the
enzymes to react more effectively, it does degrade the produced
sugars and reduces the yields, as well as making the lignin less
reactive. Using less severe steaming-acid pretreatments produces
lower glucose yields, since the enzymes cannot react with a major
part of the cellulosic material. (See J. Soderstrom, et al.,
Biomass and Bioenergy 24 (2003), 475-486--U.S. Pat. No. 4,880,473
and U.S. Pat. No. 6,692,578, U.S. Patent Application. No.
US2005/0069998A1, U.S. Pat. No. 5,597,714, WO20061085762 A1.)
[0030] Enzymatic hydrolysis presents a promising process for
large-scale operations using lignocellulosic biomass, as it is
low-energy intensive, environmentally compatible and does not
require the use of corrosive chemicals. The main disadvantage of
this process has been the cost of producing the enzymes, even
though during the most recent years, with new improved processes,
such cost has been reduced. (See V. S. Chang et al., Applied
Biochemistry and Biotechnology 84 (2000) 5-37.)
[0031] Thus, there is need for developing pretreatment processes
that allow the maximum conversion of lignocellulosic biomass to
ethanol via high yield enzymatic hydrolysis, without the use of
corrosive chemicals, waste streams and specialty high-cost
equipment. There is a further need for such pretreatment processes
that allow conversion of lignocellulosic biomass via pyrolysis
process resulting in higher yields of bio-oil, as well as improved
quality of bio-oil.
[0032] In general, any pretreatment of the lignocellulosic biomass
to enhance its conversion must at least increase the micro-and
macro-accessibility to the bulk of the particle allowing
penetration of the enzymes and chemicals.
[0033] In the prior art, the term cellulases is used to describe a
class of enzymes responsible for the biodegradation natural
process. Cellulases are mainly produced by bacteria and fungi. For
the purpose of this discussion, it is noted that the proteinic
conveyors of the complex enzyme groups have molecular weights in
the region of 30,000 to 100,000, and have globular shapes with
hydrodynamic diameters in the range of 3 to 4 nm. Therefore, the
openings of the cannulae, pores, cavities and interfibrillar
interstices, must be large enough to allow molecules, ions,
compounds, and enzymes to penetrate in the bulk of biomass. For an
efficient enzymatic digestion and conversion, the biomass particle
should have the largest possible number of such openings with
diameters at least 3 to 4 nm. (See H. A. Kzassig et al, in Polymer
Monographs, "Cellulose", vol. 11 (1993) p 202.)
BRIEF SUMMARY OF THE INVENTION
[0034] The present invention addresses these problems by providing
a process for pretreating a lignocellulosic biomass material, said
process comprising the step of contacting the lignocellulosic
biomass material with an aqueous fluid, causing the lignocellulosic
biomass material to swell.
[0035] Another aspect of the invention comprises a method for
adjusting the amount and composition of inorganic materials present
in the lignocellulosic biomass material.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0036] The following is a description of certain embodiments of the
invention, given by way of example only.
[0037] This invention is based on optimizing and utilizing the
basic property of lignocellulosic materials, such as wood, which is
the swelling and that is caused by polar liquids, such as water.
Although this property of woods is a disadvantage for applications
in construction, boards or packaging, etc., to the contrary, the
swelling property of woods and other lignocellulosic materials is
very useful for a pretreatment to a subsequent thermal conversion
to bio-oil, or a subsequent enzymatic conversion of such
lignocellulosic materials to ethanol. (See Mentanis G., et al.,
Wood Sci. Technol. (1994), 28, 119-134. F. F. Wangaard, et al.,
Wood Sci. Technol. (1967) 1,253-277.)
[0038] This invention involves optimizing the water swelling
process to affect penetration into the intercrystalline regions,
reached through pores and capillaries leading into the
interfibrillar spaces.
[0039] In particular, the objective of this invention is to provide
materials and process conditions that cause optimum swelling,
thereby that increases the accessibility and reactivity. Such
optimum swelling involves both intrafibrillar and intercrystalline
water penetration.
[0040] To increase the penetration of water to achieve maximum
bulking or swelling, solutions of salts, acids, bases and organic
water soluble compounds can be used, and preferably salts or
inorganic bases. The paths that the water and solute molecules
follow on their way into the bulk of the biomass involve the
existing structural pores, capillaries and voids between fibrillar
elements. As water molecules penetrate into interior of biomass
particles, they cause disruption of fibrillar associations and move
into regions interlinking the crystallite ensembles forming the
fibrils. (See A. Stamm, Ind. Eng. Chem. Vol. 27, No. 4 (1935)
401-406.)
[0041] Deeper penetrations, which require more severe process
conditions and higher solute concentrations, involve the
penetration of water solute molecules into the lattice structure of
the crystallites, causing rupture of the hydrogen bonded layers and
creation of accessible and reactive internal surfaces. The strong
interaction of water and, for example, a strong inorganic base with
the biomass, results in the opening of the intraplanar and
interplanar hydrogen bonded links that cause lattice
transformation, which in turn allows solute molecules and dissolved
ions to diffuse between lattice layers. Usually, the swelling or
bulking of the lignocellulosic materials by liquids causes
corresponding changes in the dimensions of the wood particles.
However, the changes, if any, in the dimensions of the particles do
not necessarily reflect the amount of solvent absorbed in the bulk
of the particle. This is due to the presence of fine and coarse
capillaries within the bulk structure, which allow solvents (i.e.,
water) to fill the available space without causing measurable
changes in the dimensions of the lignocellulosic mass.
[0042] The effectiveness of the solvent to cause swelling depends
primarily on its basicity, hydrogen bonding affinity, and molecular
bonding. The swelling properties of lignocellulosic materials
(wood), as well as the ability of different chemicals to cause
swelling have been studied for over 70 years. (See A. Stamm, Ind.
Eng. Chem. vol. 27, No. 4, 1934.) Briefly, it has been show that
the extent of swelling and solvent sorption can be related to the
hydrogen bonding affinity of the solvent.
[0043] A simple model of the mechanism of the swelling process of
wood with water involves the penetration of water molecules via
capillaries into the bulk structure, whereby the water molecules
first interact with the hydrogen-bonded hydroxyl groups of the
lignocellulosic mass to form an energetically unstable
transition-state. The hydroxyl group forms a water molecule, which
is hydrogen-bonded to the lignocellulosic mass. Thus, this
mechanism is based on a chemically activated process, and obeys the
classical Arrhenius equation for chemical reactions requiring an
activation energy.
[0044] Accordingly, the rate and extent of swelling substantially
increase with increasing temperature. The interaction of water with
the biomass can further be increased by the presence in the water
of certain soluble salts. (See A. Stamm, Ind. Eng. Chem. vol. 27,
No. 4, 1934.)
[0045] The "activity" of certain salts to increase swelling is in
the following order: [0046] Cations:
K<NH4<Na<Ba<Mn<Mg<Ca<Li<Zn and [0047]
Anions:
ClO.sub.3<SO.sub.4<NO.sub.3<Cl<Br<ClO.sub.4<I<CNS
[0048] However, there are exceptions to this order, depending on
concentrations, temperature and kind of biomass used. In general,
much more swelling occurs in alkaline solutions than in acidic
solutions. (See Penn, W. S. (1949) Elec. Manuf. 5, (1), 8.)
[0049] Certain salts (like concentrated ZnCl.sub.2) used in hot
solutions to cause swelling, react much further by splitting
fibrillar aggregates and even dissolving parts of biomass. (See
Penn, W. S. (1949) Elec. Manuf. 5, (1), 8.)
[0050] Bases, both organic and inorganic, have much more of an
affinity to interact with biomass materials. According to one
theory, cellulosic materials can be considered to exhibit chemical
properties similar to mono-basic acids, which can be neutralized by
contacting the biomass with strong bases.
[0051] In general, the affinity of certain bases to cause swelling
for cellulosic materials can classified in the following order:
[0052] LiOH>NaOH>KOH>RbOH>CsOH
(See, K. E. Cabradilla and S. H. Zeronian, "Influence of
Crystallinty on the Thermal Properties" in Thermal Uses and
Properties of Carbohydrates and Lignins, Academic Press
(1976).)
[0053] Briefly, and for the purpose of this invention, the action
of water or other polar solvents and when enhanced by soluble salts
bases or acids, but preferably with strong bases, and conducted at
optimum temperature, concentration and pH result, to different
extents, in the following: [0054] (i) Rupture hydrogen bonds that
hold together fibril aggregates creating more reactive bulk surface
areas; and [0055] (ii) Breaking of intraplanar and interplanar
hydrogen bonds, allowing different biomass components to move,
dissolve or rearrange as well as allow the soluble (salt) ions to
penetrate to the interior of the biomass.
[0056] Overall, the bulk is now transformed to a sponge-like
structure. The swelling widens the pores and capillaries creates
exits to the surface, as well as opening of interfibrillar spaces
now becoming available for reactions with chemical compounds, salt,
acids, bases, as well as enzymes.
[0057] Biomass swollen with polar liquids like water, when it is
dried (at 80-100.degree. C.), does not allow all liquids present in
the swollen regions to escape. This entrapment of the swelling
agents and/or present solutes, like salts, is accompanied by some
shrinkage of the biomass particles. Consequently, the swelling
process followed by a drying step to entrap (encapsulate) chemical
compounds, which may form inclusion complexes within the pores,
voids, capillaries, interfibrillar interstices, provides a means
for activating the biomass in a way that it becomes more reactive
in a subsequent enzymatic conversion, or in a thermal or
hydrothermal conversion for the production of fuels.
[0058] In general, lignocellulosic biomass, depending on its
origin, contains, besides cellulose, hemicellulose and lignin,
other components such as resins, tannins, terpenes, fats, etc.
These materials, referred to as "extractables," can be removed by
organic solvents, including alcohols. Additionally, the
lignocellulosic biomass, depending on its kind and origin, contain
a variety of metals. Mild treatments, like hot water (50-90.degree.
C.) can remove most of these "extractables" without altering the
cellulosic components of the biomass. In general, the removal of
"extractables" results in increasing the rate of diffusion of the
solvent and solutes into the biomass, hence is increasing the size
of the capillaries, disrupting the cell wall structure, and
decreasing the network of secondary hydrogen bonds. Thus, the
internal structure of the cell wall loses stability, which
increases the reactivity of the exposed surfaces towards the
solvent/solute molecules. Therefore, removal of the "extractables"
increases the rate, as well as the extent of swelling. (See G.
Mantanis et al., Holzforschung, 49 (1995) 239-248; WO 00/74909
A1.)
[0059] Lignocellulosic biomass, besides the organic extractable
components, contains also inorganic extractables. About 20 kinds of
metals have been identified in various kinds of lignocellulosic
biomass, which vary not only with the kind of biomass, but also
with its origin.
The Role of Inorganic Matter Present in Biomass during Thermal
Decomposition
[0060] Biomass conversion to fuels using thermal decomposition
processes, such as combustion, carbonization, gasification,
liquefaction, and pyrolysis, have received considerable attention
during the last 30 years and, in particular, during the last 15-20
years. (See Bridgwater, A. V., Appl. Catalysis A 116 (1994) 5-47.
D. Mohan, et al., Energy and Fuels (2006) 20, 848-889.)
[0061] Starting with the very early work on combustion and
gasification processes, it was recognized that inorganic compounds
present or added to the biomass strongly affect the behavior of the
biomass during the thermal decomposition processes (pyrolysis,
combustion, and gasification). However, the objectives of these
three processes are different, specifically, pyrolysis processes
aim towards maximum yields of organic liquids, with minimum yields
of char, and light gases, including CO/CO.sub.2 and water. To the
contrary, combustion and gasification processes aim towards
complete conversion of the biomass carbon to combustible gases.
[0062] Further, there are examples of processes wherein a pyrolysis
process is combined with a combustion process, such that char
formed by the pyrolysis is subsequently combusted or gasified.
[0063] In all three thermal processes, the presence of inorganic
matter plays a major role in determining yields, kind of products,
and product properties. Further, the presence of metals in the
biomass plays also a major role in the thermochemical behavior
during combustion, gasification and pyrolysis.
[0064] In general, depending on the source of the biomass, its
history of growth, location, etc. about 20 inorganic species have
been found to be present in different kinds of biomass, with the
most abundant being Na, K, Ca, Mg, S, Si, Fe, Mn, Al, and P. In
some biomass sources, the total concentration of inorganic species
can reach 25% based on dry weight of the biomass.
[0065] Relating to pyrolysis processes are the flaming and
smoldering combustion processes, wherein the flaming combustion
involves the gas phase oxidation of the pyrolysis products, and
smoldering combustion involves the combustion of residues remaining
after evaporation of the volatile components.
[0066] The kind of pathways, rates of conversion and yields are
strongly affected by the heating conditions and the presence of
inorganic matter.
[0067] Pyrolysis of cellulose involves two alternative pathways,
one at lower temperature involving the decomposition of the
glycosyl units to char, H.sub.2O, CO, CO.sub.2; and another at
higher temperatures involving the depolymerization of the glycosyl
units to volatile oily (tarry) products.
[0068] The addition of flame-retardants to cellulosic materials
promotes reactions that produce CO, CO.sub.2, H.sub.2O and char,
while reducing the amount of volatile organic tarry products.
[0069] Early work on the kinetics and mechanisms of these reactions
using cellulosic materials and inorganic additives as flame
retardants was based on measurements obtained by Thermo-gravimetric
analysis (TGA). (See F. Shafizadeh, J. Appl. Pyrolysis 6 (1984)
217-232. M. J. Antal, Ind. Eng. Chem. Res. (1995) 34, 703-717.) In
general, the kinetics, mechanisms, yields and types of products of
the pyrolytic processes depend strongly on the composition of the
biomass, pretreatments, heating conditions and atmosphere, wherein
the process is conducted and catalyst present.
[0070] In their original pioneer work, Shafizadeh (see Cellulose
Chemistry and Its Application, T. P. Nevell and S. H. Zeronian
(Eds), Chapter 11, "Thermal degradation of cellulose", Ellis
Horwood Ltd (1980s) p 266) and co-workers used Thermogravimetric
analysis (TGA); thermal evolution analysis (TEA); evolved gas
analysis (EGA); and differential thermal analyses (DTA) to
determine the kinetics and mechanisms of the, thermal and
thermo-catalytic conversions of cellulosic biomaterials. They
showed that the main biomass components (cellulose, hemicellulose
and lignin) undergo chemical/physical changes at different
temperatures and at different rates. Furthermore, they showed that
the addition of inorganic components, in general, lower the
temperature of the thermo-catalytic conversion as determined by the
DTA, and EGA analyses.
[0071] Additionally, the presence of inorganic compounds, whether
indigenous or added, selectively promotes the formation of char at
the expense of tarry oils.
[0072] There is a considerable amount of research work published on
the kinetics of biomass decomposition in pyrolysis processes for
the production of organic liquids. (See Antal, M., In Adv. in Solar
Energy Vol. 2, Amer. Solar Energy Soc., NY, 1983.)
[0073] A simplified model proposed originally by Shafizadeh (see F.
Shafizadeh and W.F. DeGroot, "Combustion Characteristics of
Cellulosic Fuels" in Thermal Uses and Properties of Carbohydrates
and Lignins, Academic Press (1976)) describes the decomposition of
biomass taking place first through primary reactions producing gas,
tar (oil) and char, and subsequently the tar (oil) via undergoes
secondary reactions producing gas and char. These reactions take
place in parallel, thus complicating the kinetic model for biomass
thermal degradation.
[0074] Work reported later on (see Linden, et al., Chem. Eng. Comm.
1988, 65, 207-221) showed that, if a high concentration of
inorganic compounds (salts) is present during pyrolysis, the
conversion route follows a mechanism enhancing ring fragmentation,
thereby yielding higher amounts of lower molecular weight
compounds, for example, hydroxyacetaldehyde. When low
concentrations of inorganic salts are present on the biomass during
pyrolysis, the mechanism of degradation involves a depolymerization
producing high molecular weight compounds, such as levoglucosan and
fructose. Accordingly, the maximum degradation temperature depends
on the amount of metals present on the biomass.
[0075] Feldmann, et al. (see Amer. Chem. Soc., Symp. Ser. No. 144,
ACS, Washington, D.C., 1982) showed that adding ash or calcium
oxide to lignocellulosic wood biomass prior to pyrolysis of the
mixture increases the yield of volatile organic liquids. Both the
wood-ash and calcium oxide decreased the yield of the char and
increased the yield of the organic liquids.
[0076] The work of Hsisheng Teng and Yun-Chow Wei (see Ind. Eng.
Chem. Res. 1998, 37, 3806-3811) is of particular interest. These
authors used rice hull biomass in their pyrolysis studies. Using a
high resolution DTG analysis technique, they were able to dissect
the evolution of volatile materials into three individual
components, that is, the lowest temperature (about 350.degree. K)
corresponding to the decomposition involving moisture
volatilization, next at higher temperatures were the decompositions
of the hemicelluloses and cellulose and finally at the highest
temperature was the decomposition due to lignin. Another
interesting finding of Hsisheng was an observed increase in the
organic volatile yields, and a decrease of the char yield, when the
rice-hulls were washed with water before pyrolysis. At the same
time, the maxima of the decomposition peaks occurred at higher
temperatures. The increase in the temperature where the peaks
occurred, as well as the increase in the activation energies of the
volatilization, was attributed to the washing process, which
removed indigenous inorganic salts that are known to decrease the
decomposition peak temperatures when present in the biomass and act
as catalysts in the thermo-degradation process of the biomass.
Further, it was observed that the yield of the volatile organics
was increased, while the yield of the residue char was decreased.
The cause for these changes was due to removal of soluble organic
matter by the washing, which during pyrolysis reacts with other
components to form chars.
[0077] Gray, R., et al., (see Ind. Eng. Chem. Process Dev., Vol.
24, No. 3 (1985) 646-65) in their work they used a wood-derived
material which was pyrolized in a fluidized bed reactor operated in
a nitrogen atmosphere at different temperatures. They acid treated
the samples, as well as ion exchanged them with a calcium salt.
They measured the yield of gases, aqueous and tar (oily) products,
at three different pyrolysis temperatures. Both the acid-treated
and calcium exchange samples gave higher tar yields (organic,
volatile compounds) than the untreated samples, which gave
increased yields of water, char, and gases.
[0078] Zaror, C. A. et al., (see Fuel, (1985), Vol. 64, 990-994)
studied the char formation in the catalytic pyrolysis of biomass
using different kinds of wood, as such, and impregnated with
alkaline salts, which were pyrolized in a Thermo-gravimetric
balance and also in a Gray-King Retort. Their data show that the
salt-impregnation caused the temperature at which the peak of the
maximum mass loss is located to decrease, and all the weight loss
curves were shifted to lower temperatures. This confirmed previous
work indicating the decrease of the decomposition temperature that
was caused by contacting biomass with inorganic salts, while at the
same time enhancing devolatilization and increasing the weight loss
at lower temperature. Further, they observed that these effects are
more pronounced when the biomass is impregnated with sodium and
potassium carbonates. Most interesting are their data showing that
when the carbonates (K and Na) impregnated feed samples (after
impregnation) were washed with water before being subjected to the
pyrolysis, they produced much less char and gases and at lower
decomposition peak-temperatures.
[0079] Varhegyi, G., et al., (see Energy & Fuels (1988) 8,
267-272) have studied the pyrolytic devolatilization of cellulose
and sugar cane bagasse. The samples were impregnated with MgCh,
NaCl, FeSO4, ZnCh catalysts. The authors used a sophisticated
experimental equipment arrangement by combining the
Thermo-gravimetric and Mass Spectrometric instruments, and
operating in a continuous simultaneous mode. In general, their data
show substantial changes in yields and product distribution, as
well as lowering the temperature of the weight-maximum loss
peaks.
[0080] DeGroot, W, and Shafizadeh, F. (see Fuel (1988), Vol. 67,
345-361) reported on the thermal decomposition of different kinds
of wood and the effects of metal salts (K and Ca) additives. In
these studies, the pyrolysis of wood was conducted in a
Thermo-gravimetric apparatus (DTG). The wood samples were treated
with acid, ion-exchanged with K or Ca salts or impregnated with
these salts. One of their observations indicated that the lower
temperature decomposition peak, which is assigned to the
decomposition of hemicellulose, shifted to a lower temperature,
indicating that the presence of indigenous (exchangeable) metals,
or additive metals are interacting with the acidic groups of the
hemicellulose, thus resulting in decreasing the thermal stability
of the hemicellulose component of the wood biomass. Further they
observed that the addition of potassium increases the char yield,
while it reduces the decomposition (devolatilization) temperature.
By contrast, the addition of calcium reduces slightly the char
yield, while increasing the decomposition temperature.
[0081] In these studies the specific tar yields, and gases, were
not measured, but only the complete char residues. However, it is
expected that in, general, thermal biomass degradations in a
pyrolytic process that result in lower char yields, usually such
lower char yields are associated with higher tar (organic liquids)
yields.
[0082] Additionally, it was observed that the results obtained in
the pyrolysis depended on the specific mode of application of the
particular salt to the biomass chars. For example, potassium
carbonate impregnated into the biomass increased its decomposition
temperature, while to the contrary, when same salt is ion-exchanged
it produces the opposite result.
[0083] Related studies were reported by DeGroot, W., and Richards,
G. Wei-Pink Pan and G. Richards (see J. F. Analytical & Applied
Pyrolysis (1989) 16, 117-126) where they conducted similar
investigations involving the ion exchanging of metal ions on wood
and measuring their effect on decomposition temperatures and yields
of volatile products. Their technique of measuring these effects
involved a combination of DTA and gas-phase FTIR spectroscopy. The
ion exchange was done using calcium and potassium acetate salts.
Further, the indigenous metals from the wood were removed by acid
treatments, resulting in reducing the yields of the volatile
products. Potassium ions exchanged into the wood increased
substantially the yield of carbon dioxide, while at the same time
lowering the temperature of the decomposition peak. It was
concluded that at least part of this excessive amount of carbon
dioxide is derived from the thermal decomposition of cellulose.
[0084] In general, the samples which were acid treated first and
subsequently calcium or potassium exchanged gave similar results to
those obtained with the untreated materials. These results indicate
the important role that the indigenous metals present in the wood
play in determining the yields, products and decomposition
temperatures of the wood during pyrolysis.
[0085] Raveendrank, et al, (see Fuel (1995), Vol. 74, No. 12,
1814-1816) conducted extensive work on the influence of mineral
matter on the pyrolysis of different wood biomass. The experimental
apparatus consisted of a dynamic TGA, and a pyrolysis unit. Wood
samples included high lignin and low lignin biomass materials.
Samples were pretreated, (a) by acid-treatment to remove the
natural minerals (indigenous) present in the wood, (b) the
acid-treated samples, subsequently impregnated with metals, and (c)
synthetic biomass samples prepared by mixing lignin, hemicellulose
and cellulose in proportions similar to the compositions found in
the natural biomass samples. In these experiments, the yields of
gas, char volatiles and temperature of maximum decomposition were
measured. The salts used for the impregnation included
Na.sub.2CO.sub.3, K.sub.2CO.sub.3, ZnCO.sub.3, NaCl, KCl,
ZnCl.sub.2, HCl, and NaOH, and were used to treat 13 natural
biomass materials plus several synthetic samples.
[0086] It was observed that all types of biomass, except three
(which contained large amounts of lignin and K), after having been
demineralized (by HCl treatment), when pyrolyzed, produced less
char and gas and more volatiles. In all cases (except for biomass,
from milled husk), the temperature of maximum devolatilization
(peak) was increased by the removal of indigenous inorganic matter
from biomass.
[0087] The authors attributed the exceptional behavior of coir
pith, ground nut shell and rice husk due to their high contents of
lignin and potassium. Potassium is well known for its catalytic
activity in char gasification, yielding large quantities of
CO.sub.2. Further, it was suggested that changes observed with the
use of salt impregnations with different metal salts are due to the
nature of the particular cation (i.e., Na, K, Zn, Ca, and Mg) and
not due to the anion.
[0088] Fahmi, R., et al., (see R. Fahmi et al, Fuel, 86 (2007)
1560-1569) reported on the effect of alkali metals present or added
to two kinds of grasses and two kinds of wood on their catalytic
pyrolysis. They used TGA and pyrolysis-GCMS apparatus to measure
the thermo-catalytic decomposition, product yields and kinds of
products. Acid treatments (HCl) were used to remove the indigenous
metals from the samples, which were milled to below 500 .mu.m
particle size. High resolution DTG measurements showed the
individual peaks for the decomposition of hemicellulose (about
500.degree. K) and the higher temperature (598.degree. K)
corresponded to the cellulose component decomposition, which is
more thermally stable as it is crystalline, whereas the
hemicellulose (semi-crystalline) is less thermally stable. Further,
using this high resolution DTG analysis, the intensity of the
individual peaks provided relative quantitative estimates of the
concentration of the individual components in the biomass samples.
For example, the acid pretreatment of Festuca grass resulted in a
900% increase in the yield of levoglucosan during pyrolysis.
[0089] Further, a good linear correlation was established in this
work between the decrease of the temperature of maximum cellulose
degradation (Tmax peak) with increasing metal content (K+Na),
present in the char/ash.
[0090] Similarly, a good correlation was established showing the
increase in the char production with increasing metal content
during pyrolysis for the acid-treated and untreated samples. It was
concluded that the acid treatment that removed at least most of the
metals resulted in decreasing the amount of char formed and
increased the yield of organic liquids as well as improving its
quality for use as a fuel. Other catalytic biomass pyrolysis
studies, using different kinds of catalysts are described in
publications 26, 27 and 28. (See Lappas, A., et al., Fuel 81 (2002,
2087-2095); Samolada, M. C., and Vasalos, I. A., Fuel 70 (1991)
883-890; and Samolada, M. C., et al, Energy and Fuels, 14, (2000),
1161-1167.)
[0091] U.S. Pat. No. 3,926,947 describes a process in which
cellulosic waste materials such as paper and newsprint are treated
with an acidic fire retardant that enhances the yield of certain
bio-oils during pyrolysis. The fire retardants claimed were:
phosphoric acid, ammonium phosphates, ammonium sulphate, and zinc
sulphate, which were impregnated into the biomass before subjected
to pyrolysis.
[0092] U.S. Pat. No. 5,807,952 teaches a process for converting
lignins to phenolic compounds by pyrolyzing the lignin in the
presence of a strong base like KOH. The lignin and potassium
hydroxide powders were mixed and placed into a single stage quench
reactor heated to 600.degree. C. Vapors were analyzed with a mass
spectrometer.
[0093] U.S. Pat. No. 5,395,455 describes a process for producing
anhydrosugars from lignin and cellulose containing biomass by
pyrolysis, using a strong acid pretreatment of the biomass before
pyrolysis.
[0094] Besides claiming the acid-removal of the natural minerals
from the biomass, it also claims that certain anions, such as
sulphate, sulphite or nitrate can be added to the acid-treated
biomass by impregnation. Further, after the strong acid-treatment
(digestion) of the biomass, soluble organic material can be
separated and the residue then pyrolyzed.
[0095] The disclosures of WO 2007/128799 A1 are incorporated herein
by reference. This patent application discloses methods of
embedding particulate inorganic matter into biomass particles. To
allow the formation of said particles into the bulk of the biomass
particles, swelling of the wood was used to enlarge/create openings
large enough to accommodate the in situ formation of particulate
inorganic matter in the biomass.
[0096] Biomass derived from forestry, agriculture and cellulosic
waste materials, due to its compact strong physical construction
and its chemical nature containing primarily cellulose,
hemicellulose, lignin, mineral matter and other materials, resists
conversion processes such as thermal, hydrothermal, and enzymatic
processes, which are used to convert said biomass to fuels and
chemicals.
[0097] The most abundant and useful components for the conversion,
the cellulose and hemicellulose, are bundled up and sealed by the
protective coating provided by the lignin component. Therefore, a
direct exposure of the cellulose and hemicellulose to chemical
reagents or even to thermal conditions is prevented by the lignin
and other foreign, noncellulosic substances present.
[0098] Additionally, any primary product resulting from the contact
of the biomass with a chemical reagent or during
thermo-decomposition, and derived from one or more of the
components in the biomass substance, is diffusionally restricted
from escaping the reaction zone due to the lack of bulk
accessibility in the biomass particle.
[0099] The reaction products and intermediates being restricted in
the bulk of the biomass, and remaining in contact within themselves
for longer periods, can further interact within themselves, or can
interact with unreacted segments of the biomass or with other
components present, to form secondary products. These secondary
products are not only undesirable, but their presence in the
biomass substrate can alter the reaction pathway, thereby causing
changes in the yields and kinds of products obtained from the
commercial process.
[0100] Further, the three major biomass components (cellulose,
hemicelluloses and lignin) have different reactivities towards
acids and bases, as well as having different thermal stabilities,
and decompose at different rates to different products like organic
volatiles, chars, water and gases, including CO/CO.sub.2.
Unfortunately, the production of chars and gases are produced at
the expense of the yield of organic volatiles, thus making the
known commercial conversion processes inefficient and costly.
[0101] Therefore, there is a need for an improved pretreatment
process that will modify the biomass-feed in such a way that when
subjected to thermo-conversion (pyrolysis) processes, it will yield
more volatile condensable oily products and less char, CO/CO.sub.2,
other gases and water.
[0102] Additionally, such modification of the biomass-feed must
render the feed more easily digested by enzymes to produce high
yields of sugars in the enzymatic conversion to ethanol, while
using lesser amounts of enzymes.
[0103] It has been discovered that by enhancing the natural
tendency of lignocellulosic biomass to swell with water under
thermal and mechanical treatments, the internal compacted bulk of
the biomass particle is disrupted, thus creating voids, openings,
surface area and accessibility to the exterior of the particle.
Additionally, the swelling properties of the biomass are further
enhanced by utilizing the acidic property of the biomass that
causes it to react as a monobasic acid by neutralizing it with a
strong base. Thus this modified biomass, when subjected to thermal
or enzymatic conversions, yields more of the desirable products,
such as oils or ethanol.
[0104] Additionally, by optimizing the swelling conditions under
thermal and mechanical treatment, the water in the swollen biomass
acts as a kind of "chimie-douce" treatment, which results in
dissolving and hydrolyzing, and cleaning up foreign materials in
the bulk of the biomass, such as soluble salts and resins, etc. The
removal of these undesirable materials can be accomplished by
extruding the swollen biomass wherein the swollen biomass is
squeezed through the small orifices under pressure, thus de-sorbing
the water containing the soluble extracted materials.
[0105] The removal of the water soluble extractables "unblocks" the
openings between the cells. This allows penetration of the water
molecules below the external surface of the biomass particle.
[0106] Further, the "cleansing" effectiveness of the "chimie-douce"
process can be further increased by using an acidic solution or a
basic solution to enhance the swelling. The removal of the water
from the swollen biomass containing the extractables, can be
conducted in a high pressure filter press.
[0107] The removal of the extractables is an important process as
it causes, under thermoconversion, higher yields of char.
[0108] Another embodiment of this invention involves, as a first
step, the swelling of the biomass by water, with an optional
dewatering step to remove the extractables, followed by an addition
of soluble salt solution, acid or a base.
[0109] The addition of soluble salts or colloidal suspensions to
the swollen biomass provides another process wherein "additives"
can be incorporated into the bulk of the biomass particles. The
additional porosity created by the water swelling (not accessible
in the raw [untreated] biomass) provides a "hosting" volume for the
accommodation of additive materials, such as catalysts in the
interior bulk of the biomass particle.
[0110] Accordingly, the catalytically active sites (i.e., metal
ions) are now located close to the individual components present in
the biomass (i.e., lignin, hemicellulose and cellulose) and
interact directly with the reactive sites of these components at
the initiation of the thermoconversion process.
[0111] Further, the inorganic salts implanted into the bulk of the
biomass particles, not only act as near next-neighbor catalyst
sites, but also act as a heat-sink, thus increasing the heat
transfer rate into the interior of the biomass particle.
[0112] This process is distinctly different from the known
prior-art processes, wherein the catalytic metals are deposited on
the exterior surface of the biomass particle, such as, for example,
using impregnation techniques, or by physical mixing or by dusting
on the biomass fine powders of such catalyst). In these cases, the
devolatilized oily vapors generated in the bulk, need to travel
throughout the biomass interior volume, and only when they exit the
particle, see (interact) with the catalyst particle, which is
located on the external surface of the biomass particle.
[0113] The devolatilized oily vapors, while traveling throughout
the bulk of the particle, not only react with other reactive
regions of the three main components (lignin, hemicellulose and
cellulose), but also react with themselves, as the residence time
within the particle is increased under (internal) pressure and high
temperature.
[0114] Therefore, the devolatilized condensable oily vapors, in the
case of having the catalyst sitting on the exterior surface of the
biomass particle, after having undergone interactions among
themselves and interactions with other regions of the biomass, then
react with the catalyst, which sits on the surface. In the case of
the process of this invention, the devolatilized oily vapors not
only have increased accessible volume to escape away fast from the
biomass residue char, but they are able to interact with the
catalyst located in the close proximity within the bulk volume, at
their first on-set formation.
[0115] Metal compounds deposited on the exterior surface of the
particles by means of chemical and/or mechanical means, when
subjected to a thermo-conversion like pyrolysis, as the surface of
the particle is first heated this inorganic matter interacts first
with the initial vapors produced in the outer skin area of the
biomass and form chars containing mostly metal oxides, which are
covering the outer surface of the biomass particle. Said
skin-chars, containing most of the inorganic matter, covering the
external surface of the biomass particle, acts further to alter the
mechanisms of the remaining thermo-devolatilization process and to
increase the yields of gases and char at the expense of volatile
oily condensable vapors.
[0116] An effective severe swelling of the biomass causes
disruption of the internal bulk hydrogen bonding links between
components and in general it upsets the compact nature of the bulk
biomass particle.
[0117] However, an even more disruptive effect is caused when the
sorbed water phase in the bulk of the biomass particle, is suddenly
heated up to vaporize within the bulk biomass particle. This
creates an "in-situ" high pressure steaming or "in-situ" steam
explosion within the solid biomass particle, which not only can
cause further hydrolysis, but also causes additional disruption of
the compact interior texture, thus creating more internal open
volume and increased accessibility to the exterior of the particle.
Suitable fast heating sources are: [0118] microwaves, flash drying,
and AC induction heating.
[0119] Another embodiment of this invention involves enclosing the
water swollen biomass in a closed container, which is heated to
temperatures above 100.degree. C. This treatment allows the
interior, as well as the exterior of the biomass particles, to be
exposed to high pressure steam.
[0120] Such treatment increases the destruction of the bonding
between components, causes additional hydrolysis and loosening of
the compact biomass substrate, thus creating more openness to the
interior bulk of the particle, as well as increased accessibility
to the exterior.
[0121] In a different mode of practicing this invention, the
swollen biomass is dried first and then milled. It was observed
that the treated biomass was more brittle than the raw biomass,
which caused the milling to be more effective in producing smaller
particles with a less severe milling.
[0122] In another embodiment the biomass is comminuted first prior
the treatment according to this invention.
[0123] In another mode of operation of this invention, the
comminuting and swelling is conducted simultaneously.
[0124] Process [0113] and [0114] are conducted in the presence of
an additive(s).
[0125] Thus, the invention has been described by reference to
certain embodiments discussed above. It will be recognized that
these embodiments are susceptible to various modifications and
alternative forms well known to those of skill in the art. For
example, the swelling may be modified by the milling process.
[0126] Many modifications in addition to those described above may
be made to the structures and techniques described herein without
departing from the spirit and scope of the invention. Accordingly,
although specific embodiments have been described, these are
examples only and are not limiting upon the scope of the
invention.
* * * * *