U.S. patent application number 12/988193 was filed with the patent office on 2011-03-10 for process for converting biomass to coal-like material using hydrothermal carbonisation.
This patent application is currently assigned to CSL CARBON SOLUTIONS LTD.. Invention is credited to Markus Antonietti.
Application Number | 20110056125 12/988193 |
Document ID | / |
Family ID | 39494401 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110056125 |
Kind Code |
A1 |
Antonietti; Markus |
March 10, 2011 |
PROCESS FOR CONVERTING BIOMASS TO COAL-LIKE MATERIAL USING
HYDROTHERMAL CARBONISATION
Abstract
The present invention relates to a hydro thermal carbonization
process for the preparation of coal-like material using biomass.
The process comprises a step (i) of heating a reaction mixture
comprising water and biomass to obtain a reaction mixture
comprising activated biomass; and a step (ii) of adding a
polymerization initiator to the reaction mixture obtained in step
(i) to polymerize the activated biomass and to obtain a reaction
mixture comprising coal-like material. The process is beneficial in
terms of product control, and process engineering.
Inventors: |
Antonietti; Markus;
(Bergholz-Rehbruecke, DE) |
Assignee: |
CSL CARBON SOLUTIONS LTD.
St. Helier, Jersey
JE
|
Family ID: |
39494401 |
Appl. No.: |
12/988193 |
Filed: |
April 17, 2009 |
PCT Filed: |
April 17, 2009 |
PCT NO: |
PCT/EP2009/054602 |
371 Date: |
November 9, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61045833 |
Apr 17, 2008 |
|
|
|
Current U.S.
Class: |
44/605 |
Current CPC
Class: |
C10B 53/02 20130101;
C10L 5/44 20130101; Y02P 20/145 20151101; C10L 9/086 20130101; Y02E
50/30 20130101; C10L 9/00 20130101; C10L 9/02 20130101; Y02E 50/14
20130101; Y02E 50/10 20130101 |
Class at
Publication: |
44/605 |
International
Class: |
C10L 5/44 20060101
C10L005/44 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2008 |
EP |
EP 08 007 516.1 |
Claims
1. A hydrothermal carbonization process for the preparation of
coal-like material from biomass, characterized by comprising at
least the following steps: heating a reaction mixture comprising
water and biomass to obtain a reaction mixture comprising activated
biomass; and (ii) adding a polymerization initiator to the reaction
mixture obtained in step (i) to polymerize the activated biomass
and to obtain a reaction mixture comprising coal-like material.
2. The process of claim 1, wherein the polymerization initiator is
selected from oxygen, peroxides, azo compounds and redox
initiators.
3. The process of claim 1, wherein the amount of the polymerization
initiator is 0.01 to 2 wt %, in terms of the reaction mixture.
4. The process of claim 1, wherein the reaction mixture is heated
to a temperature in the range of 210 to 250.degree. C. during step
(i).
5. The process of claim 1, wherein the reaction mixture has a pH in
the range of 3 to 7, preferably 5 to 7, in step (i).
6. The process of claim 1, wherein an acid is further added to the
reaction mixture prior to or during step (i).
7. The process according to claim 1, wherein step (ii) is carried
out at a temperature in the range of 140 to 220.degree. C.
8. The process according to claim 7, wherein the temperature in
step (i) is above the temperature in step (ii).
9. The process according to claim 1, wherein the coal-like material
is the product of the polymerization in step (ii).
10. The process according to claim 1, wherein the coal-like
material has, in terms of the dry mass thereof, at least one of the
following features (a) and/or (b): (a) a carbon content of >50
wt %; (b) a calorific value of >23 MJ/kg, preferably >24
MJ/kg.
11. The process according to claim 1, further comprising the step
of separating the reaction mixture obtained in step (ii) into a
solid phase of the coal-like material, and a liquid phase.
12. The process according to claim 11, wherein the liquid phase
obtained in the separation step is oxidized to obtain an oxidized
liquid phase.
13. The process according to claim 12, wherein the oxidation is
carried out by contacting the liquid phase with an
oxygen-containing gas.
14. The process of claim 13, wherein the oxygen-containing gas is
air.
15. The process according to claim 12, wherein the, optionally
oxidized, liquid phase is recycled to step (i) and/or (ii).
16. A coal-like material obtainable by a hydrothermal carbonization
process according to claim 1.
17. The coal-like material of claim 16, which has in terms of the
dry mass thereof, a carbon contact of >50 wt % and a calorific
value of >23 MJ/kg.
18. The process of claim 3, wherein the amount of polymerization
initiator is 0.05 to 0.2 wt %, in terms of the reaction
mixture.
19. The process of claim 5, wherein the reaction mixture has a pH
in the range of 5 to 7, in step (i).
20. The process according to claim 7, wherein step (ii) is carried
out at a temperature in the range of 170 to 210.degree. C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to a process for the
preparation of coal-like material from biomass, more particularly
to an improved and quicker process yielding the coal-like material
in enhanced space-time yield, and which, moreover, allows for
enhanced quality control of the final product, as well as improved
reproducibility. Also, the process of the invention is cheaper in
terms of the necessary equipment, and it is a very safe
process.
BACKGROUND
[0002] In the past, many efforts have been made to imitate the
natural coalification of biomass, which takes place on a time scale
of some hundred (peat) to hundred million (black coal) years.
Besides the formation of charcoal by pyrolysis of dry biomass, the
so-called hydrothermal carbonization (HTC) process for the
manufacture of coal or coal-like materials has recently attracted
increasing attention. The first experiments were carried out
already in 1913 by Bergius, who described the hydrothermal
transformation of cellulose into coal-like materials. More
systematic investigations were later performed by E. Berl et al.
(Ann. Chem. 493 (1932), pp. 97-123; Angew. Chemie 45 (1932), pp.
517-529) and by J. P. Schumacher et al. (Fuel, 39 (1960), pp.
223-234). Recently, the hydrothermal carbonization has seen a
renaissance starting with reports on the low temperature
hydrothermal synthesis of carbon spheres using sugar or glucose as
precursors (Q. Wang et al., Carbon 39 (2001), pp. 2211-2214 and X.
Sun and Y. Li, Angew. Chem. Int. Ed. 43 (2004), pp. 597-601).
Furthermore, metal/carbon hybrid nanostructures, such as nanocables
prepared by a hydrothermal carbonization co-reduction process using
starch and noble metal salts such as AgNO.sub.3 as starting
materials were described by S. H. Yu in Adv. Mater 16 (2004), pp.
1636-1640. H. S. Qian et al., in Chem. Mater 18 (2006), pp.
2102-2108 reported the synthesis of Te@carbon-rich composite
nanocables and carbonaceous nanofibers by the hydrothermal
carbonization of glucose.
[0003] M. M. Titirici et al., in. New J. Chem., 31 (2007), pp.
787-789 described the catalyzed HTC as an attractive alternative
for the sequestration of carbon from biomass to treat the CO.sub.2
problem. According to the paper, the optimum reaction conditions
involve a heating of a biomass dispersion under weakly acidic
conditions in a closed reaction vessel for 4-24 h to temperatures
of around 200.degree. C.
[0004] Generally, the HTC of biomass to afford carbon-like products
was carried out as a one-step process.
[0005] US 2008/0006518 A1 relates to a process for reforming a
biomass by heating the biomass in pressurized hot water to
carbonize the biomass. According to a specific embodiment, the
process comprises performing a primary heating of the biomass
gradually at a temperature ranging from 200 to 260.degree. C., and
then performing a secondary heating of the biomass at a temperature
ranging from 270 to 330.degree. C. The products of the process are
referred to as carbides. As mentioned in the document, the obtained
carbides contain approximately 75 wt % of carbon. Owing to the
temperatures close to the critical range which are used in the U.S.
patent application, the process is complex and requires elaborate
and thus expensive equipment to be carried out with safety.
[0006] The known processes for the preparation of coal-like
materials left still room for optimization in terms of yield,
efficiency and quality control of the final coal-like material.
Accordingly, it is an object of the present invention to provide a
process for the preparation of coal-like material from biomass
which is quicker, more efficient and gives a higher yield of
product, and which moreover allows an enhanced control and
reproducibility of the quality of the final product in comparison
to the methods of the prior art. A still further object resides in
a process which requires less elaborate and expensive equipment and
can nevertheless be carried out with high safety.
SUMMARY OF THE INVENTION
[0007] It has been surprisingly found by the present inventor that
the above objects can be attained by a process for the preparation
of coal-like material from biomass as recited in claim 1, which is
a process comprising a first activation step, and a second
polymerization step, with the second step being initiated by the
addition of a polymerization initiator to the reaction mixture
obtained in the first, i.e. the activation step.
[0008] Preferred embodiments are subject of the dependent
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The appended FIG. 1 provides a schematic flow diagram
showing a preferred mode of carrying out the process of the
invention in a continuous mode.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The process for the preparation of coal-like material
according to the present invention can be referred to as a
hydrothermal process, in particular as a hydrothermal carbonisation
process. This terminology is intended to show that the process
involves the heating of a reaction mixture comprising water and
will yield carbonized coal-like material.
[0011] The term "biomass" as used herein is broadly understood as
encompassing all kinds of plant and animal material and material
derived from the same. According to a preferred embodiment, biomass
as meant in the present specification shall not include petroleum
or petroleum derived products.
[0012] The biomass for use in the present invention may comprise
macromolecular compounds, examples of which are lignin and
polysaccharides, such as starch, cellulose, and glycogen. As used
herein, the term "cellulose" is intended to encompass
hemicelluloses commonly also referred to as polyoses.
[0013] As will be appreciated, certain kinds of biomass may include
both, plant and animal-derived material. As examples, manure
(dung), night soil and sewage sludge can be mentioned. While the
biomass for use in the present invention is preferably plant
biomass, i.e. biomass of or derived from plants, certain contents
of animal biomass (i.e. biomass of or derived from animals) may be
present therein. For instance, the biomass may contain up to 30% of
animal biomass.
[0014] According to a preferred embodiment, the biomass for use in
the present invention, which is preferably plant biomass, contains
more than 70 wt %, most preferably >90 wt %, of polysaccharides
and lignines in terms of the solid contents of the biomass.
[0015] For instance, the plant biomass may be agricultural plant
material (e.g. agricultural wastes) or all kinds of wood
material.
[0016] Without limitation, examples of biomass are crop,
agricultural food and waste, feed crop residues, wood (such as wood
flour, wood waste, scrap wood, sawdust, chips and discards), straw
(including rice straw), grass, leaves, chaff, and bagasse.
Furthermore, industrial and municipal wastes, including waste paper
can be exemplified.
[0017] The term "biomass" as used herein preferably also includes
monosaccharides such as glucose, ribose, xylose, arabinose,
mannose, galactose, fructose, sorbose, fucose and rhamnose, as well
as oligosaccharides.
[0018] As is known to one of average skill in the art, "coal-like
material", as used herein, refers to a material, which is similar
to natural coal in terms of property and texture. Owing to the
method of the preparation thereof, it may also be referred to as
hydrothermal coal. It is a product, more precisely a carbonized
product that is obtained or obtainable by the hydrothermal
carbonization process of the invention.
[0019] As such, the coal-like material can be distinguished from
synthetic resins, including those synthetic resins, which have been
prepared using monomers obtained from lignocellulosic material,
such as phenolic resins, and in particular novolac-type phenolic
resins. Such synthetic resins are preferably not encompassed by the
expression "coal-like material" as used herein.
[0020] The coal-like material obtained in the process of the
present invention typically comprises, without limitation, <70
wt % of carbon, for example 60 to 65 wt % of carbon. Also, the
coal-like material obtained in the process of the invention can
have a carbon content of >50 wt % of carbon, which is
significantly above the carbon content of dried (raw) biomass,
which is in the order of 40 wt %. For instance, the coal-like
material of the invention may comprise 65 to 72 wt % of carbon. As
will be appreciated, the above carbon contents refer to the dry
mass of the coal-like materials. In the present invention, the
carbon content of the coal-like materials can be determined by
elemental analysis (combustion). For instance, the carbon content
of the materials can be measured using a Vario Micro analyzer
(Elementar Analysensysteme, Hanau, Germany).
[0021] Moreover, as can be ascertained by way of solid state
.sup.13C-NMR spectroscopy, the coal-like material as meant herein
is more aliphatic than e.g. the product presumably obtained in US
2008/0006518, which is more aromatic.
[0022] The coal-like material obtained in the hydrothermal
carbonisation process of the present invention has a high calorific
value, for instance >23 MJ/kg, preferably 24 to 38 MJ/kg, and
more preferably 24 to 32 MJ/kg. The calorific value of the
coal-like material can be determined by standard calorimetry. More
specifically, in the present invention, the calorific values of the
materials can be determined in accordance with DIN 51900 or BGS
RAL-GZ 724. As will be appreciated, the above calorific values of
the coal-like material are expressed in terms of the dry mass of
the coal-like material (in kg).
[0023] Both, the carbon content and the calorific value of the
coal-like material are indicative of the high degree of
carbonization of the coal-like material of the invention that can
be achieved in the process of the present invention. According to a
preferred embodiment, the coal-like material of the invention that
is obtained by polymerization of the activated biomass initiated by
the addition of a polymerization initiator in step (ii), has a
carbon content of >50 wt % (in terms of the dry mass of the
coal-like material) and a calorific value of >23 MJ/kg (in terms
of the dry mass of the coal-like material). As meant herein, the
"dry mass" of the coal-like material refers to the mass thereof
after drying under such drying conditions that no further loss of
water is observed. For instance, the dry mass can refer to the mass
of the coal-like material after drying at a temperature of
80.degree. C. for at least 24 h.
[0024] In the step (i), i.e. the first step, the biomass is
activated for the subsequent polymerization to coal-like material
in the second step.
[0025] Without wishing to be bound to theory, the mechanism of the
activation in step (i) is assumed to be as follows. Upon heating,
the macromolecular species, e.g. the polysaccharides contained in
or constituting the biomass may be molten or dissolved in water.
For instance, cellulose contained in the biomass, which is a
crystalline material, will be molten in water. Moreover, the
polysaccharides can be disintegrated or broken down to smaller
fragments, such as monosaccharides and oligosaccharides. Those
fragments will undergo consecutive rapid dehydration to more
reactive intermediates, which are capable of undergoing rapid
conversion to coal-like material, i.e. coalification, in the second
step. Due to this capability, the reactive intermediates can also
be referred to as "coal monomers". The dehydration of glucose to
hydroxymethylfurfural is an example for such a dehydration
reaction. These "coal monomers" are typically characterized by
increased chemical reactivity towards intermolecular reactions, as
compared to the raw biomass, e.g. via vinylic subunits, reactive
aldehyde side groups, or activated hydroxymethyl groups onto furane
moieties. For this reason, the first step of the hydrothermal
carbonization process of the invention can also be referred to as
the activation step, and the intermediate product of that step as
activated biomass.
[0026] The water being present in the reaction mixture of the first
step may be water adhering or bound to the original biomass, which
can also be referred to as "raw" biomass. In this specification,
the biomass to be subjected to step (i) is also referred to as raw
biomass. As meant herein, raw biomass may for instance be the
biomass obtained as waste (e.g. wood, agricultural, municipal
waste) from the provider, without further treatment, or as
collected from natural sources. In the case of wood, the raw wood
biomass may be the wood collected in the forest (as the natural
source), or sawdust from the wood processing industry. The water
content of raw biomass may for instance be up to 80 wt %. As will
be appreciated from the above, the raw biomass can be used as such
and with water contents as mentioned above. Though drying is not
excluded, e.g. in order to reduce the weight and consequently the
transportation costs, the (raw) biomass to be subjected to the
process of the invention is preferably not dried. Consequently, the
present invention allows avoiding the energy-consuming drying of
the biomass.
[0027] The presence of water in the process of the invention
distinguishes this from e.g. pyrolytic processes for the conversion
of biomass to coal-like materials by simple heating, typically in
the absence of oxygen (carbonization).
[0028] In addition to the water present in, e.g. bound to the raw
biomass such as obtained from natural sources, water may be added
to the wet or dry biomass to adjust the water content in the
reaction mixture of step (i). The total amount of water, i.e. the
water bound to or contained in the as-obtained biomass and the
additional water is not specifically limited. Preferably, the
weight ratio of water to biomass (water/biomass) in the reaction
mixture of the first step is in the range of 0.3 to 10. For the
ease of transportability, especially in a continuous process, the
solid contents of the reaction mixture to be subjected to the step
(i) is preferably 5 to 35%, more preferably 10 to 30%, especially
15 to 25% by weight. The reaction mixture having such solid
contents is preferably in the form of a slurry.
[0029] The reaction mixture comprising water and biomass to be
subjected to heating in the first step may comprise, without
limitation, further ingredients as long as these will not inhibit
the activation of the biomass.
[0030] The hydrothermal carbonisation process of the present
invention can be carried out in water alone. Organic solvents such
as ketones are unnecessary, and they are preferably omitted.
According to a preferred embodiment, the reaction mixture of step
(i) contains water as a single solvent, with other solvents such as
ethanol only incidentially brought in by the biomass, e.g. by
fermentation. Consequently, preferably at least 95 wt %, more
preferably at least 98 wt % of the solvent present in the reaction
mixture of the first step is water.
[0031] The present inventor found that an acidic pH in step (i) is
advantageous. The pH is preferably in the range of 3 to 7, more
preferably 4 to 6. By adjusting the pH to the acidic range, the
disintegration, in particular of polymeric compounds in the
biomass, e.g. by hydrolysis can be accelerated, and the yield of
activated biomass, e.g. smaller fragments can be increased. There
are kinds of biomass, which are more difficult to activate than
others. Wood is an example of biomass, which is quite difficult to
activate. In the case of biomass, which is more difficult to
activate, the pH is adjusted to lie within the acidic range with
particular benefit.
[0032] The desired pH of the reaction mixture in the first step can
be controlled to lie within the above ranges by adding suitable
acids, which do not interfere with the activation of the biomass.
The acid is preferably a strong acid, e.g. having a pK.sub.a of
less than 4.5. Both, anorganic acids, e.g. mineral acids, and
organic acids can be used. An example of a suitable mineral acid is
phosphoric acid. Citric acid, lactic acid and pyruvic acid are
examples of (strong) organic acids. According to a preferred
embodiment, an acid such as those exemplified above is added to the
reaction mixture prior to or during step (i) for the adjustment of
the pH of the reaction mixture to lie in the range of 3 to 7,
especially 4 to 6.
[0033] The reaction mixture to be subjected to the step (i), which
may e.g. comprise an acid in addition to the (raw) biomass and
water, can be prepared in a suitable mixer.
[0034] Dependent on which type of biomass is used as a starting
material, the particular reaction conditions in step (i) may be
selected appropriately. In particular, for biomass which can be
activated relatively easily in step (i), such as monosaccharides,
the duration of the activation step may be shorter and the pH less
acidic than for polymeric biomass starting material.
[0035] The heating temperature (or the reaction temperature) in
step (i) is not particularly limited, as long as it is sufficient
to convert at least larger parts of the (raw) biomass subjected to
the process to activated biomass as defined herein. Preferably, the
heating temperature is such that at least 80 wt % of the raw
biomass are converted to activated biomass. The heating temperature
(or the reaction temperature) may be in the range of 190 to
270.degree. C., and it preferably is 210 to 250.degree. C., more
preferably 230 to 240.degree. C.
[0036] In this specification, the reaction temperature in step (i)
is occasionally denoted T.sub.1.
[0037] According to a particularly preferred embodiment, the
temperature is 210 to 250.degree. C., and the pH value is acidic,
especially 3 to 7.
[0038] In the process of the invention, in particular under the
above preferred reaction conditions in terms of temperature and pH,
the reaction time of step (i) (i.e. until at least 80 wt % of the
biomass have been converted to activated biomass as meant herein)
can be reduced to 5 to 15 minutes, preferably 5 to 10 minutes. As
will be appreciated, the duration of the first step will depend on
the kind of biomass used.
[0039] The (raw) biomass to be subjected to the first step may be
used in any form. Preferably, however it is divided into an
appropriate particle size prior to use, e.g. in the range of 0.1 to
20 mm, more preferably 0.3 to 10 mm, especially 0.5 to 5 mm.
Suitable particle sizes such as those exemplified above can be
obtained by methods such as grinding, chopping or sawing.
[0040] The activated biomass present in the reaction mixture
obtained in step (i) comprises the products of the disintegration
and/or dehydration of the starting "raw" biomass as detailed above,
collectively referred to as "activated biomass" in the present
specification.
[0041] In step (ii), the activated biomass is subjected to
polymerization to give coal-like material as defined above. To
account for the fact that the "activated biomass" obtained in step
(i) will be polymerized in step (ii), the "activated biomass" may
in the alternative be referred to as "polymerizable biomass".
[0042] The polymerization in the second step is initiated by
addition a polymerization initiator to the reaction mixture. The
"polymerization" which takes place in the second step (i.e. in step
(ii)) is to be construed broadly and means any reaction of
molecules of the activated biomass resulting in the built-up of
larger molecules eventually yielding coal-like material. The
polymerization may include chain-growth of the monomers and
inter-chain crosslinking. In the process of the invention, the
polymerization will yield coal-like material, which preferably has,
in terms of the dry mass thereof, a carbon content of >50 wt %
and a calorific value of >23 MJ/kg. In the present invention,
the polymerization initiator is added in step (ii) as a reactant to
initiate the polymerization resulting in coal-like material, which
preferably has a carbon content and calorific value as indicated
above.
[0043] According to a particularly preferred embodiment, the
reaction mixture obtained in step (i) is directly subjected to step
(ii), i.e. without any intermediate treatment. However, the
reaction mixture obtained in step (i) may be cooled by allowing to
stand or by active cooling, prior to adding the polymerization
initiator to start step (ii).
[0044] It may be noted that some polymerization of components
contained in the activated biomass may take place already in step
(i). The polymerization step (ii) as such is initiated by adding
the polymerization initiator.
[0045] The polymerization initiator for use in the present
invention is not specifically limited in kind, as long as it is
suitable to initiate the polymerization of the activated biomass to
the carbon-like material in the second step of the hydrothermal
carbonization process of the present invention. At the reaction
conditions of the second step, the polymerization initiator is
usually capable of generating radicals which will start the
polymerization of the activated biomass to coal-like material.
[0046] Useful polymerization initiators are for instance azo
compounds, peroxides, oxygen and redox initiators, as well as
mixtures thereof. The azo compound may be azobisisobutyronitrile.
Useful peroxides are inorganic peroxides, e.g. persulfates such as
potassium persulfate and ammonium persulfate; metal peroxides such
as (C.sub.2H.sub.5).sub.2BOOC.sub.2H.sub.5 and compounds obtained
by replacing the boron atom of
(C.sub.2H.sub.5).sub.2BOOC.sub.2H.sub.5 with Al or Zn; organic
peroxides, e.g., acyl peroxides such as benzoyl peroxide, alkyl
peroxides such as t-butyl peroxide and cumyl peroxide, peroxy acid
esters such as t-butyl peroxalate, or hydrogen peroxide. As the
redox initiator, there may be used hydrogen peroxide-Fe.sup.2+
(Fenton's reagent), persulfate and sulfite and cumene
hydroperoxide-amine-based compounds. In addition, copper salts such
as CuCl.sub.2 can be used. In the alternative and most preferably,
FeCl.sub.3 and H.sub.2O.sub.2 are used as the redox initiator.
[0047] More specifically, polymerization initiators known to be
useful for the hardening of unsaturated polyesters are also useful
to initiate the polymerization in the second step of the present
process.
[0048] One specific type of polymerization initiators useful in the
present invention is commonly referred to as warm hardeners. Warm
hardeners are typically peroxides which will be decomposed at their
decomposition temperature to form a radical which will start the
polymerization, i.e. carbonization, to yield the target coal-like
material. Examples of such peroxides are benzoyl peroxide,
cumolhydroperoxide, methylisobutylketonperoxide, and
tert.-butylperoxybenzoate. As will be appreciated, the reaction
mixture in the second step is preferably heated to or above the
decomposition temperature of the peroxide to generate radicals,
when warm hardeners are used as polymerization initiators.
[0049] The second type of polymerization initiators which are
likewise useful in the process of the present invention are often
referred to as cold hardeners, e.g. in the field of unsaturated
polyesters. Cold hardeners representing suitable polymerization
initiators for use in the present invention generally comprise an
accelerator compound and a peroxide, the peroxide being added with
preference after the accelerator compound. Examples of the
accelerator compound are iron salts. Suitable peroxides are e.g.
acetylacetone peroxide, methylethylketone peroxide and
cyclohexanone peroxide. Further examples of the cold hardeners are
amine-based accelerators, such as dimethyl aniline and
diethylenaminetetracetate, in combination with benzoyl peroxide. As
suggested by the name, cold hardeners do not require any heating to
form radicals to start the polymerization in the second step.
[0050] In order to avoid any contamination of the product, i.e. the
coal-like material obtained in step (ii), the polymerization
initiator does preferably not contain any metal. If the initiator
contains any metal, the metal content in the coal-like material is
adjusted to preferably not more than 0.5 wt %, more preferably not
more than 0.1 wt %.
[0051] Due to the addition of the polymerization initiator, the
reaction temperature can be kept much lower in the second step of
the process of the invention in comparison to the prior art such as
US 2008/0006518 A1. In particular, the reaction temperature in the
second step can be lower than in the first step. This allows for
carrying out the process in less elaborate equipment, e.g. in a
simple autoclave and has also significant benefits in terms of the
reproducibility of the process and the quality control of the end
product. Furthermore, in comparison to the one-step processes of
the prior art, the polymerization, i.e. carbonization to the
coal-like material will proceed much quicker, e.g. by a factor of 3
to 10.
[0052] For instance, the reaction temperature in the second step
(occasionally denoted T.sub.2 in this specification) can be in the
range of 140 to 220.degree. C., preferably it is 170 to 210.degree.
C., and more preferably 180 to 200.degree. C. According to a
particularly preferred embodiment, the temperature in the second
step is below the temperature in the first step. For instance, the
reaction mixture may be heated to a temperature in the range of 210
to 250.degree. C. in the first step, and to a temperature in the
range of 170 to below (i.e. not including) 210.degree. C.,
especially to 180 to 200.degree. C. in the second step. This is a
particularly preferred embodiment. According to another preferred
embodiment, the reaction temperature in the first step may be 220
to 250.degree. C., and in the second step to 170 to 210.degree.
C.
[0053] Without limitation, the polymerization in the second step
may be finished for instance within 1 to 3 hours. That is, within
that time frame, a coal-like material of reproducible quality can
be obtained. Of course, the reaction can be carried out longer or
shorter if desired.
[0054] The process according to the present invention is preferably
carried out in a pressure resistant reactor, e.g. an autoclave or
an extruder. Due to the water in the reaction mixture, there will
be a pressure increase upon heating. As the hydrothermal
carbonization reaction is exothermic, external heating may no
longer be necessary, once the reaction, e.g. in the first step, has
started, provided the thermal insulation of the reactor or reactors
is sufficient.
[0055] For the purpose of this specification, the reaction
temperature is meant to refer to the temperature, more specifically
the average temperature, inside the reaction mixture, which can be
measured with a thermocouple. Consequently, it is readily possible
to control the reaction temperature to lie within the desired range
by heating or cooling the reactor, as appropriate.
[0056] Subsequent to the second step, the solid phase comprised or
consisting of the coal-like material can be separated from the
reaction mixture, e.g. by filtration or decantation, preferably by
filtration, while a liquid phase will remain. Without restriction,
this separation, in particular the filtration can take place at the
elevated temperatures of the second reactor, thus allowing an
advantageous heat management of the reaction system. Then, the
coal-like material can be dried.
[0057] As the present inventor found out, the residual liquid phase
obtained in the separation subsequent to step (ii) can be reused in
the hydrothermal carbonisation process of the invention with
particular benefit. For instance, the above liquid phase, which
preferably contains >80 wt %, more preferably >90 wt % of
water can be oxidized to obtain an oxidized liquid phase. This can
be done with any oxidizing agent, as long as this has a suitable
oxidation potential to effect the oxidations as outlined
hereinafter, and as long as the oxidizing agent or the reaction
products thereof does not interfere with the further uses of the
(oxidized) liquid phase as detailed below. Examples of useful
oxidizing agents are, without limitation, oxygen, hydrogen
peroxide, percarbonate, and percarbonic acids. Preferably, the
oxidizing agent is an oxygen-containing gas, which is preferably
air. In the case of the oxygen-containing gas, such as air, the
oxidation of the liquid phase can be effected by bubbling the gas
through the liquid phase, stirring the liquid phase in an
atmosphere of the gas or by allowing the liquid phase to stand in
the presence of the gas.
[0058] According to an alternative embodiment, the reaction mixture
obtained in the second step which contains the liquid (aqueous)
phase, rather than the (separated) liquid phase as such is
oxidized, and the oxidized liquid phase is subsequently separated
from the solid phase of the coal-like material in the reaction
mixture.
[0059] The present inventor has discovered that the liquid phase in
the mixture obtained in the second step, as a result of the
hydrothermal carbonization process, contains e.g. ethers and
ketones (such as laevulic acid), which can be converted into the
corresponding peroxides through the oxidation. Examples of the
peroxides are ketone peroxides, such as hydroxyacetone peroxides.
These (hydro)peroxides can be recycled to the second step of the
reaction, and be used as a polymerization initiator.
[0060] Moreover, it was found out by the present inventor that
strong organic acids such as lactic acid or pyruvic acid will be
generated during the hydrothermal carbonization process of the
present invention. Hence the liquid phase obtained in the second
step can also be recycled to the first step, where these acids will
be effective in accelerating the activation, in particular the
disintegration as detailed above. If desired, the liquid phase
obtained in the second step can be subjected to oxidation, as
explained above, prior to recycling to the first step.
[0061] As meant herein, the recycling of the liquid phase to the
first step also covers the embodiment where it is recycled to the
mixing unit (to be further explained hereinafter) from which it
will be transferred to the reactor in which the step (i) is to be
carried out.
[0062] With an eye on the heat management, the optionally oxidized
liquid phase preferably is still hot (e.g. it has a temperature of
e.g. 50-220.degree. C., preferably 90-220.degree. C., most
preferably 140-180.degree. C.) when it is introduced in the
reaction mixture of the first step and/or the second step.
[0063] Using the separated liquid (aqueous) phase from the second
step (with or without oxidation) to admix it to the biomass prior
to or during the first step of the hydrothermal carbonization
process of the invention has a number of benefits. For instance,
the water content (and hence also the viscosity) of the reaction
mixture in the first step can be optimally adjusted. Moreover, the
heat included in the separated liquid phase can be reused in the
process.
[0064] According to a preferred embodiment, the liquid phase of the
reaction mixture in the second step is divided into two parts, one
of which is recycled (with or without oxidation) to the first step
for pH management, and the second of which is recycled (after
oxidation) to the second step to make up for consumed
polymerization initiator. As will be appreciated, the liquid phase
from the second step may be subjected to the oxidation prior to
dividing this into the above two parts. In addition, the caloric
heat can be handled efficiently, when the respective parts of the
liquid phase are recycled when still hot. As will be appreciated,
the above recycling is particularly advantageous in a continuous
flow system. Generally speaking, the above recycling allows for a
minimization of side products and an optimization of the carbon
yield in the hydrothermal carbonization process of the
invention.
[0065] Using the aqueous liquid phase of the second step for
process reasons (both, as a carrier of heat and of chemically
active components) is also beneficial from the point of view of
sustainability. Side products in the water phase (i.e. the aqueous
liquid phase) only cumulate up to their solubility limit while
afterwards the equilibrium reactions under HTC conditions (in
particular in the second step) ensure that the yield of coal-like
material is increased. For these reasons, bringing the reaction
water (i.e. the aqueous liquid phase) into the reaction multiple
times has a beneficial influence on both the reaction kinetics as
well as the carbonization yield, i.e. the yield of the coal-like
material.
[0066] In relation to the one-step hydrothermal carbonization
processes of the prior art, the two-step process of the present
invention, in which the first step is preferably characterized by
higher temperature, shorter residence time, higher reaction
viscosity (due to the raw biomass), and higher technical demands,
and in which the second step is preferably characterized by
comparably longer residence time, lower temperatures and pressures,
and a more passive reaction handling also exhibits many processing
advantages.
[0067] Specifically, the reactor, in which the first step is to
take place, which step, due to the preferably higher temperature
and pressure, is more demanding from a process engineering point of
view than the second, can be kept rather small, which is safer and
moreover gives better heat transfer.
[0068] For instance, the reactor for carrying out the first step
can be an extruder. Extruders allow very high pressures of up to
700 bars, temperatures of up to 400.degree. C., are made for
viscous starting materials, are regarded as very reliable and safe.
Moreover, extruders allow effective and rapid heating and the
injection of additional reactants at desired positions.
[0069] In a most preferable version, the temperature of the
reaction mixture containing the activated biomass is reduced
downstream, e.g. at the end of the extruder to the desired reaction
temperature of the second step, making use of an efficient heat
exchanger, and the heat is used for the preheating of the biomass
to be subjected to the first step.
[0070] As lower temperature and pressure can be employed in the
second step due to the addition of the polymerization initiator,
less demanding and safer constructions of the reactor(s) to be used
in the second step are possible in the hydrothermal carbonization
process of the invention. As heat is produced throughout the
hydrothermal carbonization process (it is a highly exothermic
process) the reactor(s) for use in the second step--sufficient
insulation provided--may not require further external heating and
may be heated by recirculation of the cooling liquid of the first
step.
[0071] Without restriction of generality, the second reaction step
can be carried out in a cascade of smaller reactors, which improves
the residence time within the second step and gives an improved
coal-like material.
[0072] By reference to the appended FIG. 1, a preferred mode of
carrying out the process for the preparation of coal-like material
according to the present invention will be illustrated. As can be
seen, the process schematically shown in the flow diagram provided
in the figure is a continuous process.
[0073] Reference numeral 1 denotes a (raw) biomass storage vessel.
Preferably, the biomass contained in the vessel 1 has a suitable
particle size, e.g. in the range of 0.1 to 20 mm. From the storage
vessel 1, the biomass is fed to a mixing unit 2 where it is mixed
with further ingredients such as water and acid to give a reaction
mixture. A stream 11 of the (aqueous) liquid phase separated in the
separating unit 6 is recycled to the mixing unit 2 to provide at
least part of the water and acid. According to a preferred
embodiment, the stream of the aqueous liquid 11 (comprising acids)
is mixed with the biomass in the mixing unit 2 without adding any
further ingredients. Moreover, the separated liquid phase 11
preferably has a temperature of 140 to 220.degree. C. The reaction
mixture obtained in the mixing unit 2 is then transferred to the
reactor 3 in which the step (i) is carried out. The reactor 3 is
preferably an extruder as detailed above. The reaction temperature
T.sub.1 may be between 190 and 270.degree. C. The reaction mixture
comprising activated biomass obtained in the reactor 3 is then
transported through a heat exchanger 4, from which the heat energy
8 can be recycled to the reactor 3. The reaction mixture comprising
activated biomass leaving the heat exchanger 4 is transferred to
the reactor 5 for carrying out the polymerization step (ii). The
reactor 5 is a pressure vessel, e.g. an autoclave. Preferably, it
is provided with stirring means, which are schematically shown in
the figure. The internal temperature in the reactor 5 may be
between 140 and 220.degree. C. To the reactor 5, a polymerization
initiator may be added from the external (not shown). In the
alternative or in addition, oxidized aqueous liquid phase 10
separated in the separating unit 6 and containing suitable
peroxides is fed along with the reaction mixture comprising
activating biomass to the reactor 5. The reaction mixture
comprising coal-like material obtained in the step (ii) is
transferred from the reactor 5 to the separating unit (e.g.
filtration unit) 6, in which it is separated into a solid phase of
the coal-like material 12, and an (aqueous) liquid phase 9. In the
continuous process illustrated in the figure, the liquid phase 9 is
split into two parts. The first part is recycled as stream 11
(preferably having a temperature between 140 and 220.degree. C.) to
the mixing unit 2. The second part is oxidized by adding a suitable
oxidizing agent from the oxidizing unit 7 to obtain species in the
oxidized liquid phase 10 which can act as a polymerization
initiator and can as such be recycled to the reactor 5 as detailed
above.
[0074] While the above description was focussed on the hydrothermal
carbonization process, the present invention, according to a
preferred embodiment, relates to the coal-like material obtained or
obtainable by the process of the invention as such which is
preferably characterized by the physical characteristics, in
particular the carbon content and the calorific value as detailed
above.
[0075] The present invention will be further understood from the
following examples, which are given by way of illustration and must
not be construed in a limiting sense. In the examples, the carbon
content of the materials was determined with a Vario Micro analyzer
(Elementar Analysensysteme, Hanau, Germany), and the calorific
value of the materials was measured in accordance with BGS RAL-GZ
724.
EXAMPLES
Example 1
[0076] 6.0 g glucose is dissolved in 24.0 g water, and citric acid
is added to adjust the pH to 5. A 40 ml stainless steel autoclave
is charged with the mixture and heated to 230.degree. C. for 5 min.
After cooling, the autoclave is opened, and 30 mg benzoyl peroxide
is added under stirring. Then, the autoclave is sealed again, and
heated to 180.degree. C. After 90 min, the reaction is terminated
by quenching. 2.9 g of hydrothermal coal containing 64 wt % carbon
is obtained. The calorific value of the coal is 26 kJ/g.
Example 2
[0077] 200 g wood flour (sawdust) and 600 g water are formed into a
slurry, and phosphoric acid is added to adjust the pH to 4. A 1 l
stainless steel autoclave (manufactured by Paar, Germany), equipped
with an internal thermoelement, is charged with the mixture and
heated to 250.degree. C. After exceeding the internal temperature
of 240.degree. C., the reaction is quenched after 10 min.
Subsequently, 100 mg FeCl.sub.3.6 H.sub.2O and 200 mg 30% aqueous
H.sub.2O.sub.2 are added at a temperature of about 100.degree. C.
The reaction mixture is heated again to 195.degree. C., and further
reacted for 3 h. After cooling and opening the autoclave, the
product is separated into a solid phase (containing the
hydrothermal coal) and a liquid phase. There is obtained 76 g of
hydrothermal coal which contains 68 wt % carbon and can be
pulverized by hand. The calorific value of the coal is 25 kJ/g.
Example 3
[0078] The liquid phase separated from the reaction mixture in
Example 2, while still warm (60.degree. C.), is bubbled with air to
obtain an oxidized liquid phase. Then, Example 2 is repeated except
for using 200 ml of the thus-obtained oxidized liquid phase in
place of the FeCl.sub.3.6 H.sub.2O and H.sub.2O.sub.2. A product is
obtained which is, within the measurement accuracy, the same as
that obtained in Example 2.
* * * * *