U.S. patent application number 12/091503 was filed with the patent office on 2009-09-03 for process, system and apparatus for passivating carbonaceous materials.
Invention is credited to James Coleman, David Cork.
Application Number | 20090217574 12/091503 |
Document ID | / |
Family ID | 37967351 |
Filed Date | 2009-09-03 |
United States Patent
Application |
20090217574 |
Kind Code |
A1 |
Coleman; James ; et
al. |
September 3, 2009 |
PROCESS, SYSTEM AND APPARATUS FOR PASSIVATING CARBONACEOUS
MATERIALS
Abstract
A process, system and apparatus is provided for passivating
carbonaceous material against spontaneous combustion. The process
involves drying the carbonaceous material in a low oxygen
environment and pre-conditioning the carbonaceous material by
contacting it with volatile matter contained in a countercurrent
gas stream. The volatile matter coats the particles of dried
carbonaceous material and plugs the micropores of the dried
carbonaceous material, thereby passivating it against adsorption of
water and oxygen, and thus spontaneous combustion. The
pre-conditioned dried material then undergoes devolatilisation at
temperatures at which volatile matter is evolved. The evolved
volatile matter mixes with the countercurrent gas stream and is
used to pre-condition dried carbonaceous material located
upstream.
Inventors: |
Coleman; James; (Perth,
AU) ; Cork; David; (Mayfield, AU) |
Correspondence
Address: |
LADAS & PARRY LLP
224 SOUTH MICHIGAN AVENUE, SUITE 1600
CHICAGO
IL
60604
US
|
Family ID: |
37967351 |
Appl. No.: |
12/091503 |
Filed: |
October 26, 2006 |
PCT Filed: |
October 26, 2006 |
PCT NO: |
PCT/AU2006/001604 |
371 Date: |
September 16, 2008 |
Current U.S.
Class: |
44/501 ; 201/9;
202/100; 202/99; 422/233 |
Current CPC
Class: |
C10L 9/10 20130101; C10L
9/08 20130101; C10L 5/00 20130101 |
Class at
Publication: |
44/501 ; 202/99;
202/100; 201/9; 422/233 |
International
Class: |
C10L 9/02 20060101
C10L009/02; C10L 5/00 20060101 C10L005/00; C10L 9/08 20060101
C10L009/08; C10L 9/10 20060101 C10L009/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 26, 2005 |
AU |
2005905934 |
Dec 5, 2005 |
AU |
2005906808 |
Claims
1. A process for preparing a passivated carbonaceous material
comprising the steps of: a) drying a carbonaceous material feed
stream; b) treating the dried carbonaceous material with volatile
matter, and c) devolatilising the treated dried carbonaceous
material feed stream and forming the passivated carbonaceous
material and volatile matter.
2. The process according to claim 1, wherein the step of
pre-conditioning the dried carbonaceous material feed stream
comprises contacting the dried carbonaceous material feed stream
with a gas stream containing volatile matter.
3. The process according to claim 2, wherein the gas stream
containing volatile matter is directed in a counter current flow
relative to the dried carbonaceous material.
4. The process according to claim 2, wherein the volatile matter
contained in the gas stream comprises volatile matter evolved
during step c).
5. The process according to claim 4, wherein the volatile matter
evolved during devolatilisation of the dried carbonaceous material
is augmented by doping the carbonaceous material feed stream with
materials containing large amounts of hydrophobic aromatic
moieties.
6. The process according to claim 2, wherein the volatile matter
contained in the gas stream comprises volatile matter evolved from
devolatilisation of a volatile matter feedstock distinct and
separate from the carbonaceous material feed stream of the present
process.
7. The process according to claim 6, wherein the volatile matter
feedstock comprises materials containing large amounts of
hydrophobic aromatic moieties.
8. The process according to claim 2, wherein the volatile matter
contained in the gas stream comprises volatile matter evolved
during step a).
9. The process according to claim 1, wherein the step of drying the
carbonaceous material feed stream comprises heating the
carbonaceous material fees stream from about 100.degree. C. to
400.degree. C.
10. The process according to claim 1, wherein the drying step is
conducted under a low oxygen (0-5% O.sub.2 v/v) and high moisture
(up to 50% v/v) atmosphere.
11. The process according to claim 9, wherein the carbonaceous
material feed stream is directly heated.
12. The process according to claim 11, wherein the carbonaceous
material feed stream is directly heated by contacting the
carbonaceous material feed stream with a hot gas stream having a
low oxygen content.
13. The process according to claim 12, wherein the oxygen content
of the hot gas stream is less than 5% v/v.
14. The process according to claim 13, wherein the oxygen content
of the hot gas stream is less than 1% v/v.
15. The process according to claim 12, wherein the hot gas stream
is contacted with the carbonaceous material feed stream in a
countercurrent direction relative to the carbonaceous material feed
stream.
16. The process according to claim 12, wherein volatile matter
evolves at the temperatures at which the carbonaceous material feed
stream is dried.
17. The process according to claim 16, wherein the volatile matter
evolved during step a) mixes with the hot gas stream and is
directed in a countercurrent flow relative to the carbonaceous
material feed stream, and subsequently contact the dried
carbonaceous material thereby treating the dried carbonaceous
material.
18. The process according to claim 12, wherein the step of
contacting the dried carbonaceous material with a gas stream
containing volatile matter is facilitated by directing a
countercurrent flow of the hot gas stream relative to the dried
carbonaceous material, whereby the hot gas stream is used to heat
the carbonaceous material feed stream to temperatures at which step
a) is performed.
19. The process according to claim 1, wherein step a) and step b)
are performed at substantially the same time.
20. The process according to claim 1, wherein the step of
devolatilising the dried carbonaceous material comprises heating
the dried carbonaceous material from about 400.degree. C. to
900.degree. C.
21. The process according to claim 20, wherein the step of
devolatilising the dried carbonaceous material comprises heating
the dried carbonaceous material from about 600.degree. C. to
800.degree. C.
22. The process according to claim 1, wherein the devolatilising
step is conducted under a low oxygen (0-5% O.sub.2 v/v)
atmosphere.
23. The process according to claim 20, wherein the dried
carbonaceous material is directly heated.
24. The process according to claim 23, wherein the dried
carbonaceous material is directly heated by contacting the dried
carbonaceous material with a hot gas stream having a low oxygen
content.
25. The process according to claim 24, wherein the oxygen content
of the hot gas stream is less than 5% v/v.
26. The process according to claim 25, wherein the oxygen content
of the hot gas stream is less than 1% v/v.
27. The process according to claim 24, wherein the same hot gas
stream is contacted with the dried carbonaceous material in a
countercurrent direction relative to the dried carbonaceous
material.
28. The process according to claim 24, wherein the same hot gas
stream is used in step c) and then subsequently in step a).
29. The process according to claim 28, wherein the volatile matter
mixes with the hot gas stream and is directed in a countercurrent
flow relative to the dried carbonaceous material, and subsequently
contacts the dried carbonaceous material located upstream, thereby
treating the dried carbonaceous material before it is
devolatilised.
30. The process according to claim 1, wherein the passivated
carbonaceous material feed stream produced by the process of the
present invention has its moisture content reduced to between 0-20%
moisture and its volatile matter content reduced to 0-25% in
comparison with the moisture and volatile matter content of the
carbonaceous material feed stream.
31. The process according to claim 1, further comprising the step
of quenching the passivated carbonaceous material.
32. The process according to claim 31, wherein the passivated
carbonaceous material is quenched with water and/or cool insert
gas.
33. The process according to claim 31, wherein the passivated
carbonaceous material is quenched with untreated carbonaceous
material, including but not limited to wet screened coal.
34. A system for preparing passivated carbonaceous materials
comprising: a dryer for drying a carbonaceous material feed stream;
a pyrolyser for devolatilising dried carbonaceous material and
forming passivated carbonaceous material and volatile matter; and a
carrier vehicle for facilitating contact of volatile matter with
the dried carbonaceous material.
35. The system according to claim 34, wherein the dryer comprises a
rotary kiln, a multiple hearth furnace (MHF), flash dryer, or a
circulating fluid bed (CFB).
36. The system according to claim 35, wherein the rotary kiln is
configured at an angle of up to 10.degree. above the horizontal to
facilitate passage of the carbonaceous material feed stream through
the rotary kiln under gravity.
37. The system according to claim 35, wherein the rotary kiln is
provided with a means to rotate the rotary kiln about its central
longitudinal axis, and the rotational speed thereof is typically
selected to correspond with the length of the rotary kiln such that
a residence time of the carbonaceous material feed stream in the
rotary kiln is bout 15-40 minutes.
38. The system according to claim 34, wherein the dryer is arranged
to heat the carbonaceous material feed stream to 100.degree. C. to
400.degree. C.
39. The system according to claim 38, wherein the dryer is heated
by a hot gas stream (400.degree. C. to 800.degree. C.) having a low
oxygen content.
40. The system according to claim 39, wherein the oxygen content of
the hot stream of gas is less than 5% v/v and preferably less than
1% v/v.
41. The system according to claim 34, wherein the pyrolyser for
devolatilising the dried carbonaceous material feed stream and
forming the passivated carbonaceous material and volatile matter
comprises any one or more in combination of a rotary kiln, multiple
hearth furnace (MHF), or a circulating fluid bed (CFB).
42. The system according to claim 41, wherein the dried
carbonaceous material feed stream in the pyrolyser is directly
heated with a hot gas stream having a low oxygen content to
temperatures of about 600.degree. C.-800.degree. C.
43. The system according to claim 42, wherein the oxygen content of
the hot gas stream is less than 5% v/v and preferably less than 1%
v/v.
44. The system according to claim 38, wherein the system is further
provided with an external burner to generate the hot gas stream
used for directly heating the pyrolyser and heating the dryer of
the system, respectively.
45. The system according to claim 44, wherein the hot gas stream is
directed in counter current flow against the dried carbonaceous
material in the pyrolyser and the carbonaceous material feed stream
in the dryer.
46. The stream according to claim 45, wherein the hot gas stream
combines with the volatile matter evolved in the gas pyrolyser and
thus acts as the carrier vehicle for the volatile matter.
47. The system according to claim 34, wherein the system further
comprises a means for feeding the dried carbonaceous material to
the pyrolyser.
48. The system according to claim 47, wherein the means for feeding
the dried carbonaceous material to the pyrolyser comprises a closed
pneumatic system.
49. A process for reducing inherent moisture in and/or increasing a
specific energy of a carbonaceous material comprising the steps of:
a) drying a carbonaceous material feed stream; and b) carbonising
the dried carbonaceous material by contacting the dried
carbonaceous material with a counter current gas stream having a
low oxygen content.
50. The process according to claim 49, wherein the gas stream is
contacted with the dried carbonaceous material at a temperature of
between 400.degree. C. and 800.degree. C.
51. The process according to claim 49, wherein the oxygen content
of the gas stream is less than 5%.
52. The process according to claim 51, wherein the oxygen content
of the gas stream is less than 1%.
53. The process according to claim 49, wherein steps a) and b) are
both carried out by contacting the carbonaceous material with the
gas stream having a low oxygen content whereby said gas stream
initially dries the carbonaceous material and then proceeds to
carbonise the carbonaceous material.
54. The process according to claim 49, wherein the gas stream
contains volatile matter.
55. The process according to claim 54, wherein the volatile matter
evolves during step b) and mixes with the gas stream.
56. The process according to claim 54, wherein the volatile matter
evolves during step a) and mixes with the gas stream.
57. A process for improving the coking characteristics of
non-coking carbonaceous material comprising the steps of: a) drying
a non-coking carbonaceous material feed stream; b) treating the
dried non-coking carbonaceous material with volatile matter, and c)
devolatilising the treated dried non-coking carbonaceous material
and forming a carbonaceous material with improving coking
characteristics and volatile matter.
58. A process for quenching hot passivated char comprising
contacting the hot passivated char with a particulate material.
59. The process according to claim 58, contacting the hot
passivated char with the particulate material comprises blending
the hot passivated char with the particulate material and
facilitating a solid-solid head exchange between particles of the
hot passivated char and the particulate material.
60. The process according to claim 58 wherein the particulate
material is at ambient temperature.
61. The process according to claim 58, further comprising mixing
the hot passivated char and the particulate material blend with a
cool insert gas stream to facilitate further quenching thereof.
62. The process according to claim 58, wherein the particulate
material is a carbonaceous material.
63. The process according to claim 62, wherein the particulate
material is wet screened coal.
64. The process according to claim 59, wherein the step of blending
the carbonaceous material and passivated char is conducted in a
substantially oxygen-free atmosphere.
65. An apparatus, for use in a continuous process, for passivating
carbonaceous material, the apparatus comprising: a) an inlet for
receiving a feed stream of carbonaceous material; b) an inlet for
receiving a gas stream containing volatile matter; c) a reaction
portion configured to allow the carbonaceous material to come into
contact with the gas stream containing volatile matter, d) an
outlet for receiving the passivated carbonaceous material after it
has passed through the reaction portion; and e) an outlet for
receiving the gas after it has passed through the reaction
portion.
66. An apparatus for heating oxygen sensitive carbonaceous material
in a controlled oxygen environment, the apparatus comprising: f) an
inlet for receiving a flow of carbonaceous material; g) an inlet
for receiving a flow of gas with a controlled oxygen content
containing volatile matter; h) a reaction portion, configured to
allow the carbonaceous material to come into contact with the gas
with a controlled oxygen content containing volatile matter, i) an
outlet for receiving the carbonaceous material after it has passed
through the reaction portion; and, j) an outlet for receiving the
gas after it has passed through the reaction portion.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a process, system and
apparatus for passivating carbonaceous materials against
spontaneous combustion. An apparatus for heating oxygen sensitive
carbonaceous material in a controlled oxygen environment is also
described.
BACKGROUND OF THE INVENTION
[0002] Coal pyrolysis occurs upon heating coal to produce gases,
liquids, and a solid residue (char or coke). Pyrolysis occurs in
all coal utilisation processes, including combustion, gasification,
liquefaction, and carbonisation. The nature of pyrolysis and of the
resulting products is intimately related to the operating
conditions and to the composition and properties of the coal.
Consequently, control of pyrolysis is important in coal utilisation
processes.
[0003] The principal difference between char and coke is that the
parent coal for char has high oxygen content and a non-aromatic
structure and therefore the char particles do not tend to
agglomerate during pyrolysis. The parent coal for coke has much
lower oxygen content and an aromatic structure. Coke feed stock
undergoes a plastic phase and agglomerates during pyrolysis. Feed
stock that normally produces coke can be used to produce char by
pyrolysis thereof in an atmosphere with a slight to moderate oxygen
content.
[0004] Historically, on an industrial scale, char has been an
undesirable by-product of a smokeless fuel plant, coke works or
coal gasification plant, although smaller scale plants produce char
for activated carbon and micro blast furnaces. One of the least
desirable characteristics of some dried chars is that they can be
pyrophoric when exposed to an oxygen-containing atmosphere.
[0005] This tendency to spontaneously combust is promoted by rapid
adsorption of water vapour and oxygen by dried char. Oxygen
physically adsorbs onto the surface of the dried char and
chemically reacts in an exothermic oxidation reaction with the
organic molecules within the char. The heat release, if not
dissipated, will promote a self-accelerating oxidation process,
causing the temperature of the char to rise progressively until the
char spontaneously ignites. The rise in temperature of the char is
also promoted by latent heat of vaporization released by adsorption
of water onto the char particles.
[0006] Other dried carbonaceous material, in particular dried low
rank coal is also susceptible to spontaneous combustion for similar
reasons as outlined above. Thus the storage of stock piles of
carbonaceous material is closely dependent on control of the
moisture content within the stock pile, and it is desirable that
the carbonaceous material is suitably treated to passivate it
against water adsorption and oxidation.
[0007] One approach to reduce the potential for spontaneous
combustion of dried char is to seal the exterior surface of the
char by using oils, polymers, waxes or other materials to coat the
surface thereof. Examples of such coating processes are U.S. Pat.
Nos. 3,985,516 and 3,985,517, which disclose heating and intimate
mixing of coal with heavy oils to coat the particles. Such coating
procedures are effective in preventing reabsorption of moisture by
the char, however, such coatings are expensive due to the cost of
the hydrocarbon materials added.
[0008] Another approach is to subsequently treat the dried coal or
char particles with oxygen under controlled conditions to
oxidatively passivate the char. U.S. Pat. No. 5,601,692 describes a
continuous process for treating a non-caking coal to form stable
char. The process involves several sequential steps including
drying the coal to remove moisture therefrom and pyrolysing the
dried coal to vaporise and remove low end volatile materials from
the coal to form char and to mobilize at least a portion of high
end volatile materials within the char and at least partially
collapse micropores within the char. The char is then cooled to a
temperature sufficient to demobilize the volatile materials within
the partially collapsed micropores of the char to pyrolytically
passivate the char, and is subsequently treated with an oxygen
containing gas to oxidatively passivate the coal by chemisorption
of oxygen. The oxidatively passivated char is then simultaneously
rehydrated and cooled.
[0009] It would be advantageous to dry coals and process them in
such a manner that the dried coal or char particles are passivated
against spontaneous combustion without the need for externally
supplied coating materials or subsequent multi-step treatment
processes.
[0010] The present invention seeks to overcome at least some of the
aforementioned disadvantages. Advantageously, the invention
increases the specific energy of the treated carbonaceous material
making it a more commercially attractive product, particularly for
shipping. Additionally, the organic sulfur content thereof is also
reduced.
[0011] It is to be understood that, although prior art use and
publications may be referred to herein, such reference does not
constitute an admission that any of these form a part of the common
general knowledge in the art, in Australia or any other
country.
SUMMARY OF THE INVENTION
[0012] It has been demonstrated that a carbonaceous material feed
stream may be dried and carbonised to produce char, both processes
being conducted simultaneously in a single vessel or,
alternatively, the drying and carbonisation processes may be
conducted sequentially in two separate vessels. The volatile matter
evolved from the carbonaceous material feed stream during
carbonisation, often referred to as coal gas, generally has a high
calorific content and contains tars. Previously, attention has been
directed to separating the coal gas from the char and improving the
high calorific content of the coal gas generated during
carbonisation so that its commercial potential can be fully
exploited.
[0013] The present invention is based on the realisation that it is
possible to passivate carbonaceous material by treating the dried
carbonaceous material with the volatile matter evolved during low
temperature and/or medium temperature devolatilisation of the
carbonaceous material.
[0014] Accordingly, in one aspect of the invention there is
provided a process for preparing a passivated carbonaceous material
comprising the steps of: [0015] a) drying a carbonaceous material
feed stream; [0016] b) treating the dried carbonaceous material
with volatile matter; and [0017] c) devolatilising the dried
carbonaceous material feed stream and forming the passivated
carbonaceous material and volatile matter.
[0018] In one embodiment of the invention, the step of treating the
dried carbonaceous material feed stream comprises contacting the
dried carbonaceous material feed stream with a gas stream
containing volatile matter.
[0019] While not wishing to be bound by theory, the inventors opine
that tars and other organic compounds contained in the volatile
matter coat particles of the carbonaceous material, plugging the
micropores of the particles and thereby reducing the adsorption of
water and oxygen. Upon further heating of the carbonaceous material
during step c), the tars and organic compounds undergo pyrolysis.
Pyrolytic rupture of functional groups attached to aromatic and
hydroaromatic moieties on the carbonaceous material leads to
release of low molecular weight reactive free radical fragments and
stabilization of the former fragment sites by hydrogen. They
hydrophilic polar functional groups are thereby converted to, and
replaced by, an hydrophobic aromatic coating, thus passivating the
dried carbonaceous material against spontaneous combustion.
[0020] In one embodiment, the gas stream containing volatile matter
is directed in a counter current flow relative to the dried
carbonaceous material.
[0021] In one embodiment of the invention, the volatile matter
contained in the gas stream comprises volatile matter evolved
during step c). In another embodiment, the volatile matter evolved
during devolatilisation of the dried carbonaceous material is
augmented by doping the carbonaceous material feed stream with
materials containing large amounts of hydrophobic aromatic moieties
that would enrich the volatile matter with these species during
devolatilisation. Suitable examples of such materials containing
large amounts of hydrophobic aromatic moieties will be well known
to those skilled in the art and include, but are not limited to,
waste rubber, in particular vehicle tyres, and plastics
materials.
[0022] In an alternative embodiment, the volatile matter contained
in the gas stream comprises volatile matter evolved from
devolatilisation of a volatile matter feedstock distinct and
separate from the carbonaceous material feed stream of the present
process. Typically, the volatile matter feedstock comprises
materials containing large amounts of hydrophobic aromatic moieties
as described above.
[0023] While the temperature of the onset of thermal decomposition
proper is generally recognized to be approximately 350.degree. C.,
low molecular weight species will evolve at temperatures less than
350.degree. C. during low temperature devolatilisation. For example
at temperatures just in excess of 100.degree. C., a coal such as
lignite, which contains many carboxylic functions as part of the
coal structure, will evolve carbon dioxide by thermal
decarboxylation. As the temperature of the thermal treatment is
increased to the range 200-370.degree. C., coals lose a variety of
lower molecular weight organic species, especially aliphatic
compounds, and some of the lower molecular weight aromatic species
can also be obtained. At higher temperatures e.g. >370.degree.
C., methane, polycyclic aromatics, phenols, and nitrogen compounds
are produced. Thus, volatile matter including, but not limited to,
acidic species, hydrophobic species such as low molecular weight
aromatic species, polycyclic aromatics and phenols, evolves during
step a) of the present invention.
[0024] Accordingly, in another embodiment of the invention, the
volatile matter contained in the gas stream comprises volatile
matter evolved during step a).
[0025] In one embodiment, the step of drying the carbonaceous
material feed stream comprises heating the carbonaceous material
feed stream from about 100.degree. C. to 400.degree. C. Typically,
the drying step is conducted under a low oxygen (0-5% O.sub.2 v/v)
and high moisture (up to 50% v/v) atmosphere.
[0026] The carbonaceous material feed stream may be heated directly
or indirectly to the temperatures at which the drying step is
performed.
[0027] The terms "direct heating" or "heated directly" as used
herein refer to a manner of heating the carbonaceous material feed
stream wherein a hot gas stream, from a local or a remote source,
at a pre-determined temperature is arranged to come into contact
with the particles of carbonaceous material of the carbonaceous
material feed stream to facilitate a gas-solid heat exchange.
[0028] The terms "indirect heating" or "heated indirectly" as used
herein encompass a manner of heating the carbonaceous material feed
stream wherein a gas stream from a local or a remote source, at a
pre-determined temperature is prevented from coming into contact
with the particles of the carbonaceous material feed stream, but is
used instead to heat the vessel containing the carbonaceous
material feed stream.
[0029] Additionally, the terms "indirect heating" or "heated
indirectly" also encompass any manner of heating the vessel
containing the carbonaceous material feed stream so as to heat the
carbonaceous material feed stream to a desired temperature, as
would be understood and known to a person skilled in the art.
[0030] In one embodiment, the carbonaceous material feed stream is
directly heated.
[0031] In one embodiment of the invention, the carbonaceous
material feed stream is directly heated by contacting the
carbonaceous material feed stream with a hot gas stream having a
low oxygen content. Typically, the oxygen content of the hot gas
stream is less than 5% v/v, and preferably less than 1% v/v.
[0032] In one embodiment, the hot gas stream is contacted with the
carbonaceous material feed stream in a countercurrent direction
relative to the carbonaceous material feed stream.
[0033] As described previously, volatile matter can evolve at the
temperatures at which the carbonaceous material feed stream is
dried. The volatile matter evolved during step a) mixes with the
hot gas stream and is directed in a countercurrent flow relative to
the carbonaceous material feed stream, and subsequently contacts
the dried carbonaceous material thereby treating the dried
carbonaceous material. Thus, in one embodiment of the invention,
step a) and step b) are performed at substantially the same
time.
[0034] In another embodiment, the step of contacting the dried
carbonaceous material with a gas stream containing volatile matter
is facilitated by directing a countercurrent flow of the hot gas
stream relative to the dried carbonaceous material, wherein the hot
gas stream is used to heat the carbonaceous material feed stream to
temperatures at which step a) is performed.
[0035] In return, the countercurrent flow of dried particles cools
and scrubs the hot gas stream, thereby improving the thermal
efficiency of the process.
[0036] In one embodiment of the invention, the step of
devolatilising the dried carbonaceous material comprises heating
the dried carbonaceous material from about 400.degree. C. to
900.degree. C., preferably 600.degree. C. to 800.degree. C.
Typically, the devolatilising step is conducted under a low oxygen
(0-5% O.sub.2 v/v) atmosphere.
[0037] In one embodiment, the dried carbonaceous material is
directly heated. Typically, the dried carbonaceous material is
directly heated by contacting the dried carbonaceous material with
a hot gas stream having a low oxygen content. The oxygen content of
the hot gas stream is less than 5% v/v, and preferably less than 1%
v/v.
[0038] In one embodiment, the hot gas stream is contacted with the
dried carbonaceous material in a countercurrent direction relative
to the dried carbonaceous material.
[0039] Volatile matter evolves at the temperatures at which the
dried carbonaceous material undergoes devolatilisation in step c).
In accordance with one embodiment of the invention the same hot gas
stream is used in step c) and then subsequently in step a). The
volatile matter mixes with the hot gas stream and is directed in a
countercurrent flow relative to the dried carbonaceous material,
and subsequently contacts the dried carbonaceous material located
upstream, thereby pre-conditioning the dried carbonaceous material
before it is devolatilised.
[0040] Typically, the passivated carbonaceous material feed stream
produced by the process of the present invention has its moisture
content reduced to between 0-20% moisture and its volatile matter
content reduced to 0-25% in comparison with the moisture and
volatile matter content of the carbonaceous material feed
stream.
[0041] The process further comprises the step of quenching the
passivated carbonaceous material. In one embodiment, the passivated
carbonaceous material is quenched with water and/or cool inert gas.
The quenched passivated carbonaceous material can then be cooled to
ambient temperature, stockpiled and loaded out. In an alternative
embodiment, the passivated carbonaceous material is quenched with
untreated carbonaceous material, including but not limited to wet
screened coal.
[0042] In a second aspect of the present invention there is
provided a system for preparing passivated carbonaceous materials
comprising: [0043] a dryer for drying a carbonaceous material feed
stream; [0044] a pyrolyser for devolatilising dried carbonaceous
material and forming passivated carbonaceous material and volatile
matter; and [0045] a carrier vehicle for facilitating contact of
volatile matter with the dried carbonaceous material.
[0046] Suitable examples of a dryer include, but are not limited
to, a rotary kiln, a multiple hearth furnace (MHF), flash dryer, or
a circulating fluid bed (CFB). In one embodiment, the dryer
comprises a rotary kiln. In an alternative embodiment, the dryer
comprises a circulating fluidized bed, preferably a differentially
circulating fluidized bed.
[0047] Typically, the rotary kiln is configured at an angle of up
to 10.degree., preferably 2-5.degree., above the horizontal to
facilitate passage of the carbonaceous material feed stream through
the rotary kiln under gravity. The rotary kiln is provided with a
means to rotate the rotary kiln about its central longitudinal
axis, and the rotational speed thereof is typically selected to
correspond with the length of the rotary kiln such that a residence
time of the carbonaceous material feed stream in the rotary kiln is
about 15-40 minutes.
[0048] In one embodiment of the present invention the dryer is
arranged to heat the carbonaceous material feed stream to
100.degree. C. to 400.degree. C. Typically, the dryer is heated by
a hot gas stream (400.degree. C. to 800.degree. C.) having a low
oxygen content. The oxygen content of the hot stream of gas is less
than 5% v/v and preferably less than 1% v/v. Oxygen is preferably
substantially excluded from the carbonaceous material feed stream,
or at least at a controlled low concentration, throughout its
residence time within the dryer.
[0049] The pyrolyser for devolatilising the dried carbonaceous
material feed stream and forming the passivated carbonaceous
material and volatile matter comprises any one or more in
combination of a rotary kiln, multiple hearth furnace (MHF), or a
circulating fluid bed (CFB). In the preferred embodiment, the
pyrolyser comprises a multiple hearth furnace.
[0050] In one embodiment, the dried carbonaceous material feed
stream in the multiple hearth furnace is directly heated with a hot
gas stream having a low oxygen content to temperatures of about
600.degree. C.-800.degree. C.
[0051] The oxygen content of the hot gas stream is less than 5% v/v
and preferably less than 1% v/v. Typically, the hot gas stream
comprises combustion gas generated from an external burner. In one
embodiment the system is further provided with an external burner
to generate a hot gas stream used for directly heating the
pyrolyser and heating the dryer of the system, respectively. In
this way, the carbonaceous material feed stream may be heated in
the dryer and/or the pyrolyser at a controlled temperature and
oxygen concentration. Oxygenated hot gas is not unnecessarily mixed
with the carbonaceous material feed stream unless combustion is
required.
[0052] The hot gas stream is directed in counter current flow
against the dried carbonaceous material in the pyrolyser. The hot
gas stream combines with the volatile matter evolved in the
pyrolyser and thus acts as a carrier vehicle for the volatile
matter. In this way, the volatile matter is directed in counter
current flow against the dried carbonaceous material in the
pyroliser to facilitate contact of the volatile matter with the
dried carbonaceous material.
[0053] In another embodiment of the invention, the system further
comprises a means for feeding the dried carbonaceous material from
the dryer to the pyrolyser. Typically, the means for feeding the
dried carbonaceous material to the pyrolyser comprises a closed
pneumatic system.
[0054] In the description of the present invention it will be
evident that the inherent moisture of the carbonaceous materials is
reduced. Additionally, the specific energy of the carbonaceous
materials is also increased by both the removal of oxygen and
sulfur from the carbonaceous material during drying and
carbonisation of the dried carbonaceous material. In this way,
carbonaceous materials treated by the process of the present
invention are able to be transported more economically over long
distances.
[0055] Thus, in a further aspect of the present invention there is
provided a process for reducing inherent moisture in and/or
increasing a specific energy of a carbonaceous material comprising
the steps of: [0056] a) drying a carbonaceous material feed stream;
and [0057] b) carbonising the dried carbonaceous material by
contacting the dried carbonaceous material with a counter current
gas stream having a low oxygen content.
[0058] In one embodiment the oxygen content of the gas stream is
less than 5%. Typically, the oxygen content of the gas stream is
less than 1%. In some embodiments, the gas stream having the low
oxygen content is produced by the combustion of a carbon source.
Typical examples of such carbon sources include, but are not
limited to, coal gas, pulverized coal, char or coke.
[0059] In one embodiment the gas stream having a low oxygen content
is contacted with the dried carbonaceous material at a temperature
of between 400.degree. C. and 800.degree. C.
[0060] In another embodiment, steps a) and b) are both carried out
by contacting the carbonaceous material with the gas stream having
a low oxygen content whereby said gas stream initially dries the
carbonaceous material and then proceeds to carbonise the
carbonaceous material.
[0061] In a further embodiment of the invention, the gas stream
contains volatile matter. In one embodiment the volatile matter
evolves during step b) and mixes with the gas stream. In another
embodiment, the volatile matter evolves during step a) and mixes
with the gas stream. The volatile matter coats the carbonaceous
material and provides the aforementioned advantages.
[0062] One of the most surprising benefits of the process of the
present invention is that the character of some carbonaceous
materials can be significantly changed. For example, low rank coals
have a high inherent moisture content, comparatively low specific
energy, and are unsuitable for production of metallurgical coke.
The suitability of the present invention to reduce inherent
moisture content and increase specific energy of a carbonaceous
material has already been described. Additionally, however, the
process of the present invention produces a carbonaceous material
whose particles are coated with tarry gases, aromatic and other
hydrophobic moieties comprised in the volatile matter. These
species improve the plasticity of the carbonaceous material during
high temperature carbonization. In this way, low rank coals treated
in accordance with the process of the present invention are
converted to a carbonaceous material which has improved
characteristics for inclusion in a coking coal blend.
[0063] Thus, in a yet further aspect of the present invention there
is provided a process for improving the coking characteristics of
non-coking carbonaceous material comprising the steps of: [0064] a)
drying a non-coking carbonaceous material feed stream; [0065] b)
treating the dried non-coking carbonaceous material with volatile
matter; and [0066] c) devolatilising the treated dried non-coking
carbonaceous material and forming a carbonaceous material with
improved coking characteristics and volatile matter.
[0067] In prior art systems, hot char produced in a carbonising
process is typically quenched with water and/or inert gases to
lower the temperature of the particles to below 100.degree. C.
Carefully controlled conditions for the quenching process and
subsequent storage of the quenched char are required because of the
tendency of char to spontaneously combust under conditions where
oxygen and/or water adsorption onto the char particles is allowed
to occur, as described above.
[0068] The present invention is based on the realisation that
passivated char does not tend to spontaneously combust when exposed
to conditions under which water and/or oxygen adsorption occur, and
thus it is possible to quench hot passivated char by contacting the
hot passivated char with a particulate material under ambient
conditions to facilitate solid-solid heat transfer. Precautions
against exposing the char to conditions under which oxygen and/or
water adsorption occur, to prevent spontaneous combustion of the
char, are no longer required.
[0069] Thus, in, an alternative aspect of the present invention
there is provided a process for quenching hot passivated char
comprising contacting the hot passivated char with a particulate
material.
[0070] In one embodiment of the invention the step of contacting
the hot passivated char with the particulate material comprises
mixing the hot passivated char with the particulate material and
facilitating a solid-solid heat exchange between particles of the
hot passivated char and the particulate material. Typically, the
particulate material will be at ambient temperature. The hot
passivated char and the particulate material blend can be further
mixed with a cool inert gas stream to facilitate further quenching
thereof.
[0071] In one embodiment of the invention the particulate material
is a carbonaceous material, in particular wet screened coal.
Advantageously, when the particulate material is wet screened coal,
the temperature of the hot passivated char is lowered by direct
heat transfer to the wet screened coal at ambient temperature.
Additionally, thermal energy contained in the hot passivated char
will also be employed in removing moisture from the wet screened
coal. Preferably, the step of blending the carbonaceous material
and passivated char is conducted in a substantially oxygen-free
atmosphere.
[0072] In this way, it is possible to mix wet screened coal with
hot passivated char to produce a blended carbonaceous material with
desired particle size, fixed carbon content, volatile matter
content, and moisture to meet specific market requirements, the
preferred proportion of passivated char to carbonaceous material
being dependent on the preferred parameter, such as for example
volatile matter content of the blend.
[0073] According to another aspect the present invention provides
an apparatus, for use in a continuous process, for passivating
carbonaceous material, the apparatus comprising: [0074] a) an inlet
for receiving a feed stream of carbonaceous material; [0075] b) an
inlet for receiving a gas stream containing volatile matter; [0076]
c) a reaction portion configured to allow the carbonaceous material
to come into contact with the gas stream containing volatile
matter; [0077] d) an outlet for receiving the passivated
carbonaceous material after it has passed through the reaction
portion; and [0078] e) an outlet for receiving the gas after it has
passed through the reaction portion.
[0079] Additionally, there is provided an apparatus for heating
oxygen sensitive carbonaceous material in a controlled oxygen
environment, the apparatus comprising: [0080] a) an inlet for
receiving a flow of carbonaceous material; [0081] b) an inlet for
receiving a flow of gas with a controlled oxygen content containing
volatile matter; [0082] c) a reaction portion, configured to allow
the carbonaceous material to come into contact with the gas with a
controlled oxygen content containing volatile matter; [0083] d) an
outlet for receiving the carbonaceous material after it has passed
through the reaction portion; and, [0084] e) an outlet for
receiving the gas after it has passed through the reaction
portion.
[0085] In the description of the invention and the claims, except
where the context requires otherwise due to express language or
necessary implication, the words "comprise" or variations such as
"comprises" or "comprising" are used in an inclusive sense, i.e. to
specify the presence of the stated features, but not to preclude
the presence or addition of further features in various embodiments
of the invention.
BRIEF DESCRIPTION OF THE FIGURES
[0086] Preferred embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying figures, in which:
[0087] FIG. 1 shows a block diagram illustrating the steps involved
in a process for preparing passivated carbonaceous material in
accordance with the present invention;
[0088] FIG. 2 shows schematically a process flow diagram in
accordance with a process for preparing passivated carbonaceous
material under low temperature carbonisation conditions;
[0089] FIG. 3 shows a schematic diagram of a dryer comprised in an
apparatus for preparing passivated carbonaceous material in
accordance with the present invention; and,
[0090] FIG. 4 shows schematically a process flow diagram in
accordance with a process for preparing passivated carbonaceous
material under medium temperature carbonisation conditions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0091] Before the preferred embodiment of the present process and
apparatus is described, it is understood that this invention is not
limited to the particular materials described, as these may vary.
It is also to be understood that the terminology used herein is for
the purpose of describing the particular embodiment only, and is
not intended to limit the scope of the present invention in any
way. It must be noted that as used herein, the singular forms "a",
"an", and "the" include plural reference unless the context clearly
dictates otherwise. Unless defined otherwise, all technical and
scientific terms used herein have the same meanings as commonly
understood by one of ordinary skill in the art to which this
invention belongs.
[0092] Additionally, in considering the Figures, it will be
appreciated that for purposes of clarity certain details of
construction are not provided in view of such details being
conventional and well within the skill of the person skilled in the
art once the invention is disclosed and explained. For example, a
hopper, a conveyor, a bag house filter, a cyclone, a multi-hearth
furnace, flues, blowers and valves may be any such known
commercially available components with the exception that such
components may be modified as necessary by one skilled in the art
to be employed in the overall process of the present inventions
discussed herein. In addition, many control devices which are
conventional and standard in chemical processing have been omitted
for clarity of illustrating and describing the invention. For
example, control valves, thermocouples, thermistors, coupled with
suitable servo circuits are readily available and conventionally
used for measuring and controlling temperature and process
flow.
[0093] Referring to the figures in which like numerals refer to
like features, in FIG. 1 there is shown a block diagram of the
steps of a process 10 for preparing a passivated carbonaceous
material.
[0094] The steps of the process 10 include drying 2 a carbonaceous
material feed stream, treating 3 the dried carbonaceous material
with volatile matter, and devolatilising 4 the dried carbonaceous
material and forming passivated carbonaceous material.
[0095] The terms "devolatilisation" or "devolatilising" as used
herein refers to a process that involves thermal decomposition of
carbonaceous material, typically coal, in a controlled oxygen
atmosphere with production of volatile matter, liquor (low
molecular weight liquids), tar (high molecular weight liquids), and
char or coke. There is some variation of the product distribution
with the temperature of thermal decomposition. It will be
appreciated that the process of carbonisation of carbonaceous
materials undertaken at low, medium or high temperature conditions
is encompassed by the terms "devolatilisation" and
"devolatilising".
[0096] The term "volatile matter" as used herein refers to those
products described previously, exclusive of moisture, given off as
gas and vapour. The content of volatile matter in coal can be
determined by definite prescribed methods (ASTM, 1981, D2361-66,
D3761-79, D3175-77, D3175-77, D3176-74, D3178-73, and D3179-73, and
will vary according to the composition of the coal or feedstock
materials for the volatile matter.
[0097] The term "passivated carbonaceous material" refers to
carbonaceous material which has been treated to be resistant to
spontaneous combustion in conditions where carbonaceous material
would be reasonably expected to spontaneously combust.
[0098] The term "carbonaceous material" as used herein is defined
in the broadest terms and includes coal, coal-based products,
charcoal, activated carbon, wood, wood chips, sawdust, biomass,
waste rubber including but not limited to vehicle tyres, waste
plastic materials, contaminated soils, mixtures thereof and
mixtures of said carbonaceous materials with other substances.
[0099] Typically, the carbonaceous material feed stream comprises a
plurality of particles of non-agglomerating coal including but not
limited to lignite, sub-bituminous coal, bituminous coal,
anthracite, and a blend of two or more thereof. Anthracite is a
class of non-agglomerating coals as defined by the American Society
for Testing and Materials having more than 86 percent fixed carbon
and less than 14 percent volatile matter on a dry,
mineral-matter-free basis. Bituminous coal is a class of coals as
defined by the American Society for Testing and Materials high in
carbonaceous matter, having less than 86 percent fixed carbon, more
than 14 percent volatile matter on a dry, mineral-matter-free
basis, a moisture content from 1.5 to 7 percent, and more than
10,500 Btu/lb (29.68 MJ/kg) on a moist, mineral-matter-free basis.
Bituminous coals may be either agglomerating or non-agglomerating
coals. Sub-bituminous coal is a class of non-agglomerating coals
with a carbon content between 71 and 77 percent and a moisture
content to 10 percent, and having a heat value content of more than
8,300 Btu/lb and less than 11,500 Btu/lb on a moist,
mineral-mater-free basis. Lignite is a class of brownish-black,
low-rank coal defined by the American Society for Testing and
Materials as having less than 8,300 Btu/lb (23.46 MJ/kg) on a
moist, mineral-matter-free basis. Typically, lignites or brown coal
have high oxygen content (up to 30 percent), a relatively low
carbon content (60-75 percent on a dry basis) and a high moisture
content (30-70 percent).
[0100] The carbonaceous material feed stream can also comprise a
plurality of particles of agglomerating coal (or coking coal) in
combination with an anti-caking agent to reduce swelling and
agglomeration of the coal particles during carbonization.
[0101] The process of the present invention is particularly suited
in respect of a carbonaceous material feed stream comprising a
plurality of coal particles of low rank coal with a high moisture
content, such as lignite, sub-bituminous coal, and bituminous coal
with a moisture content of 10%-70%.
[0102] For example, the process 10 of the present invention is
particularly suited for sub-bituminous coal from the Ewington mine
in Western Australia having approximately the following composition
by weight: 44% fixed carbon, 6% ash, 25% moisture, and 27% volatile
matter.
[0103] Although the process 10 with reference to FIGS. 1, 2 and 4
refers to performing the invention with respect to sub-bituminous
coal or lignite, it will be appreciated that the process 10 the
present invention may be used to prepare passivated dried coal or
passivated char from other types of coals, biomass, waste rubber
products such as for example tyres, woodchips, and other
carbonaceous materials. Alternatively, the process may be used to
dry other oxygen sensitive, flammable substances, for example
thermal desorption of activated carbon or even contaminated
soil.
[0104] Typically, the particles of carbonaceous material are sized
less than 50 mm, preferably less than 20 mm, and even more
preferably less than 15 mm. Advantageously, with a particle size of
less than 20 mm, the particles of carbonaceous material do not tend
to suffer excessive stress due to shrinkage and subsequent
decrepitation when dried, or at temperatures under which
devolatilisation and/or carbonization occurs. Thus, the percentage
particle breakdown throughout the process is typically <15%.
[0105] Additionally, the particles do not tend to suffer excessive
transient heat transfer. In other words, the temperature at the
centre of the particle is similar to the temperature at the surface
of the particle, and thus each particle can be rapidly heated or
cooled.
[0106] Referring now to the flow diagrams shown in FIGS. 2 and 4,
where like numerals refer to like parts throughout, typically the
carbonaceous material feed stream is prepared by washing, crushing
and classifying to provide coal of suitable quality, quantity and
particle size. The carbonaceous material feed stream is fed into a
dryer 12 at ambient temperature via a screw conveyor 14, typically
at a rate of 90-100 tph.
[0107] The dryer 12 is shown in more details in FIG. 3. The dryer
12 comprises two 20 m long, 3 m diameter co-axial rotary kilns 12a,
12b in fluid communication with one another. However, the dryer 12
is operated as if it were a single rotary kiln, the dual
configuration of the rotary kilns 12a, 12b being arranged merely to
facilitate control of operating conditions within each rotary kiln
12a, 12b and ensure safe operating conditions. The rotary kilns
12a, 12b are disposed on an angle of 0-10.degree., preferably
2-5.degree. above the horizontal which facilitates passage of the
carbonaceous material feed stream therethrough by gravity. Each
rotary kiln 12a, 12b is rotated via its own gearbox and motor. The
dryer running gear has a temperature transmitter and a local
temperature indicator to the monitor lubricant temperature to
ensure the bearings operate properly.
[0108] Additionally, at the end of each rotary kiln 12a, 12b there
is a temperature indicator and temperature transmitter PLC
instrumentation. The dried carbonaceous material feed stream
exiting each rotary kiln 12a, 12b of the dryer 12 is monitored to
ensure that the temperature of the dried carbonaceous material feed
stream does not increase more than 10.degree. C. per second, which
would indicate imminent spontaneous combustion. In the event of a
rapid temperature rise means are provided for water to be sprayed
on the dried carbonaceous material feed stream. Explosion flaps are
also provided at each transfer box to vent the dryer 12 should the
internal pressure exceed 20 kPa (g).
[0109] Each co-axial rotary kiln 12a, 12b houses an internal tube
20a, 20b having a diameter of 1.5 m and 1.8 m, respectively. The
external diameter of each co-axial rotary kiln 12a, 12b is about 3
m. The internal tubes 20a, 20b are provided with a thick wall to
withstand high temperature conditions. For example, the strength of
steel at 650.degree. C. is approximately 30% of its original
strength at ambient temperature. Thus the internal tubes 20a, 20b
are provided with the thick wall to prevent creep in the steel at
temperatures of 650.degree. C. and above.
[0110] The co-axial rotary kilns 12a, 12b are configured to receive
the carbonaceous material feed stream in an outer passage 22
between an outer shell of the co-axial rotary kilns 12a, 12b and
the internal tubes 20a, 20b. Immediately after passing through the
outer passage 22 of rotary kiln 12b, the carbonaceous material feed
stream is fed into the outer passage in rotary kiln 12a by a screw
conveyor (not shown). The carbonaceous material feed stream then
travels through the outer passage 22 of rotary kiln 12a. Typical
residence time of the carbonaceous material feed stream in the
rotary kilns 12b, 12a is about 30 minutes.
[0111] During the passage of the carbonaceous material feed stream
in the outer passage 22 of the rotary kilns 12b, 12a, the
carbonaceous material feed stream is heated so that the temperature
of the carbonaceous material feed stream progressively increases
from ambient temperatures to between 100-400.degree. C., under
which temperature conditions the carbonaceous material feed stream
is dried. At temperatures >100.degree. C. low temperature
devolatilisation commences. Volatile matter is thus evolved from
the carbonaceous material feed stream in the dryer 12.
[0112] The carbonaceous material feed stream is heated to
temperatures of about 100.degree. C.-400.degree. C. by a counter
current flow of a gas stream with low oxygen content at
temperatures of 400.degree. C. to 900.degree. C., preferably
600.degree. C. to 800.degree. C.
[0113] The volatile matter evolved in the dryer 12 mixes with the
gas stream in the outer passage 22 and is directed in a counter
current flow relative to the carbonaceous feed stream. Tar and
other organic compounds contained in the volatile matter coat the
particles of the carbonaceous material in the dryer 10, plugging
the micropores of the particles, thereby passivating the particles
against spontaneous combustion.
[0114] The gas stream has a low oxygen (0-5% O.sub.2 v/v), high
moisture (up to 50% moisture) content. Typically, the gas stream is
generated from an external burner 30 and fed to the outer passage
22 of the rotary kiln 12a via conduit 28, and then from the outer
passage 22 of rotary kiln 12a to the outer passage 22 of rotary
kiln 12b. The gas stream flows at approximately at 33 m.sup.3/s
i.e. 37500 kg/h.
[0115] The heat load capacity of the external burner 30 is selected
according to the moisture content of the carbonaceous material feed
stream. In this particular embodiment the heat load of the external
burner 30 is about 18-20 MW. At the commencement of operation of
the process of the present invention, the external burner 30 may be
fuelled by LPG combustion, and/or pulverised coal can be conveyed
into the combustion chamber thereof, where it mixes with preheated
secondary combustion air and combustion takes place. Later in the
process, it will be understood that coal gas generated during
devolatilisation of the carbonaceous material feed stream can be
diverted and fed to the external burner 30 and combusted for
heating purposes as a replacement fuel for LPG or PCI.
[0116] The gas stream with a low oxygen content is supplied to the
external passages 22b, 22a of the rotary kilns 12a, 12b and
comprises the hot combustion gases produced by the external burner
30.
[0117] Before being directed to the dryer through conduit 28, the
hot combustion gases pass through a heat exchanger 32 which
transfers some of the heat to ambient air to produce a hot air
stream.
[0118] The ambient air that is passed through the heat exchanger 32
is supplied by two fans, a Variable Speed Drive (VSD) fan and a
soft start fan. The hot air stream leaves the heat exchanger at
650.degree. C. and a flow rate of 33 m.sup.3/s through a conduit 26
which is typically a 1.5 m diameter stainless steel duct. Conduit
26 is provided with a butterfly valve to bleed off excess hot
air.
[0119] The hot combustion gases leaves the heat exchanger 32 at
400.degree. C.-800.degree. C., typically 650.degree. C., at a flow
rate of 33 m.sup.3/s. The hot combustion gases travel through
conduit 28 which is typically a 1.8 m diameter refractory lined
steel pipe. A refractory lining is used to protect the pipe because
the hot combustion gas is low in oxygen content and could
potentially decarbonise the steel.
[0120] The hot combustion gas tends to cool as it traverses the
external passage 22 of the dryer 12 in counter current flow to the
carbonaceous material feed stream by virtue of gas-solid heat
exchange.
[0121] In order to maintain the drying capacity of the hot
combustion gas, the hot air stream produced by heat exchanger 32 is
fed simultaneously to the internal tubes 20a, 20b of the dryer 12
via conduit 26. The hot air stream is 400.degree. C. to 800.degree.
C., preferably about 600.degree. C.-700.degree. C., and flows at 33
m.sup.3/s through the internal tubes 20a, 20b in the same direction
as the hot combustion gas. One portion of the hot air stream is fed
into the internal tube 20a of the rotary kiln 12a and the other
portion of the hot air stream is fed into the internal tube 20b of
the rotary kiln 12b.
[0122] Typically, after these two streams pass through rotary
dryers 12a, 12b their temperatures are reduced to about 200.degree.
C. The streams are then recombined and fed as secondary combustion
air into the external burner 30 by two fans, a VSD fan and a soft
start fan via conduit 24.
[0123] Thus it will be understood that the dryer 12 is externally
heated and oxygen is substantially excluded from contact with the
carbonaceous material feed stream and the dried carbonaceous
material feed stream produced in the dryer 12.
[0124] Fine dried coal particles (approximately -2 mm) are
entrained in the hot combustion gas and exit the dryer 12 with this
gas through conduit 36. The hot inert gas and fine dried coal
particles are separated after leaving the dryer 12 with a plurality
of cyclones (not shown). The cyclones have two induced draft fans,
a VSD fan and a soft start fan. The fine dried coal particles are
removed and stored in a fine coal bin, and can be subsequently
briquetted in accordance with a briquetting process described in
WO2004/072212.
[0125] The separated combustion gas is now warm, humid and
corrosive. After passing through the cyclones the hot combustion
gas travels to two heat recovery regenerators where trace amounts
of volatile materials are thermally destroyed in an oxidising
environment. These after burners are designed so that around 70% of
the heat used in the combustion of these volatiles is regenerated;
the remaining heat comes from the air blown through the heat
exchanger that arrives at about 650.degree. C. The combustion gas
then flows to a scrubber by two scrubber fans, a VSD fan and a soft
start fan. The scrubber is chemically active towards acidic gases,
in particular SO.sub.x, NO.sub.x, and PO.sub.x. Thickened sludge
flows out of the bottom of the scrubber which is sent to the
tailings pond.
[0126] Around half of the hot combustion gas is fed back into the
external burner 30 to dilute down the oxygen content of the
pre-heated combustion air.
[0127] After completion of the drying process, the dried
carbonaceous material feed stream can be cooled, stockpiled and
stored, or it can undergo further pyrolysis.
[0128] When the dried carbonaceous material feed stream is
immediately cooled after completion of the drying process, the
dried carbonaceous material feed stream is transferred from the
dryer 12 by a conveyor 16 which takes the dried coal feed stream to
a further two conveyors. The dried coal feed stream travels at a
nominal 100 tph along the conveyors to a coal holding bin 54
disposed above a cooler 50, such as, for example, a multi hearth
fluidised bed cooler. Each of the conveyors is provided with
temperature transmitters to monitor the temperature of the dried
carbonaceous material feed stream. Similarly, at the coal holding
bin 54 disposed above the cooler 50 there are local instruments
consisting of a low level switch and a high level switch. The PLC
instruments include a level high alarm, a high level sensor and a
low level sensor. The dried carbonaceous material feed stream is
then fed into the cooler 50 by a screw conveyor 56.
[0129] The dried carbonaceous material, typically dried low rank
coals, are then cooled by mixing the dried carbonaceous material
with ambient particulate matter, preferably wet screened coal to
produce a solid-solid heat exchange between the two materials. In
this way, the inherent heat of the dried carbonaceous material is
not only transferred to the particulate matter to facilitate
thermal equilibrium, but the thermal energy of the dried
carbonaceous material is also utilized to dry the particulate
matter and therefore facilitate moisture equilibrium. Cool inert
gas, typically comprising N.sub.2, CO.sub.2, and Ar, is also blown
through the fluidised bed cooler to help bring the mixed materials
to thermal and moisture equilibrium.
[0130] The mixed carbonaceous material is eventually discharged
onto a conveyor at a nominal rate of 100 tph, and transported to
stockpiling.
[0131] The cool inert gas which has circulated through the cooler
is then passed through six 1 m diameter cyclones to remove fine
coal particles (-2 mm) entrained in the cool inert gas. The fine
coal particles are stored and then transported in a pneumatic
conveyor. The de-dusted air is returned as exhaust to the
atmosphere via a bag house.
[0132] In an alternative embodiment of the invention, the dried
carbonaceous feed material produced in the dryer 12 may undergo
further devolatilisation, such as medium temperature carbonization,
to produce passivated char.
[0133] Referring to FIG. 3, the dried carbonaceous material feed
stream is fed from the dryer 12 by a screw conveyor 16 onto one or
more pneumatic conveyors 18. Typically, the dried carbonaceous
material feed stream is already heated to 100-400.degree. C., which
reduces the heating load required during carbonization and improves
thermal efficiencies in the system. The dried carbonaceous material
feed stream is transported by the pneumatic conveyor(s) 18 in a
substantially oxygen free atmosphere (0-5% O.sub.2) composed mostly
of N.sub.2, CO.sub.2, and Ar with traces of CO, H.sub.2 and
CH.sub.4, at a temperature of 100.degree. C.-500.degree. C.,
typically 300.degree. C. The inert gas is pressurized via a
compressor before being heated. The transport fans exert high
pressures on both the inlet and the outlet of the pneumatic
conveyor so as only to overcome any pressure drops associated with
transport of the dried carbonaceous material feed stream, line
losses, cyclones and multi-clones.
[0134] Dried carbonaceous material feed stream is then fed into a
large cyclone 34 disposed above a pyrolyser 40, in this instance a
multi-hearth furnace 40. The feed rate of dried carbbnaceous
material into the pneumatic transport system is controlled by the
pressure drop across the cyclone 34. The inert gas is re-circulated
at 20 m/s through the system except in the portion of the system
immediately before the cyclone 34 where the flow rate is 12
m/s.
[0135] The cyclone 34 removes particles -1.5 mm thus preventing
fine particles being fed to the pyrolyser 40. The separated inert
gas is recirculated and the fines are fed to either the external
burner 30 or to a briquette plant.
[0136] In this embodiment, one or more multi-hearth furnaces 40 are
employed as the pyrolyser.
[0137] Further, additional thermal efficiency is obtained in the
process of the present invention by feeding the dried carbonaceous
material feed stream from the dryer 12 to the pyrolyser 40 at a
high temperature, thereby reducing the heat load and gas volume
required in the pyrolyser 40.
[0138] The hot dried carbonaceous material feed stream cascades
down through the multi-hearth furnace 40 against a counter current
flow of hot gases which rise to the top of the multi-hearth furnace
40. The hot gases comprise coal gas evolved from the devolatilised
carbonaceous material feed stream (i.e. volatile matter),
combustion product gas arising from instances of combustion of the
carbonaceous material feed stream that occur in the multi-hearth
furnace 40, and hot inert gases fed from the external burner 30 by
conduit 38. The hot inert gases are preheated in the external
burner 30 to 650.degree. C. before being delivered to the
multi-hearth furnace 40. However, the majority of the heat for
carbonisation will be derived from the combustion of coal gas in
the external burner 30.
[0139] There are significant advantages in feeding a pre-heated
dried carbonaceous material feed stream to the pyrolyser, including
but not limited to: [0140] the water vapour content of the coal gas
generated during devolatilisation is reduced by pre-drying the coal
and venting off the moisture laden gases [0141] the following
undesirable reactions are less likely to occur if the coal gas has
a reduced water vapour content
[0141] C+H.sub.2O->CO+H.sub.2 and
C+2H.sub.2O->CO.sub.2+2H.sub.2 [0142] the formation of carbon
monoxide by the following reaction is less likely to occur as
carbon dioxide is generally evolved in low temperature pyrolysis
and is vented before the dried carbonaceous material is transferred
to the pyrolyser 40
[0142] C+CO.sub.2->2CO [0143] combustion within the pyrolyser 40
is minimized.
[0144] Typically, the dried carbonaceous material feed stream
cascades down through each of the hearths in counter current flow
with a stream of gas with a low oxygen content of less than 5% and
preferably less than 1% comprising hot combustion fuel gases
generated in each of the hearths and/or an external burner. Under
the operating conditions of the multiple hearth furnace, the dried
carbonaceous material feed stream thermally decomposes to form char
and a gas product stream containing volatile matter. The gas
product stream mixes with the hot combustion fuel gases and is
directed in a counter current flow through the multi-hearth
furnace. In this way, volatile matter contained in the gas product
stream is brought into contact with the dried carbonaceous material
feed stream immediately prior to its ingress to the multi-hearth
furnace, as described above, and additionally as the dried
carbonaceous material feed stream cascades through the multi-hearth
furnace and undergoes carbonization. Hydrophobic species in the
volatile matter coat the particles in the dried carbonaceous
material feed stream, plug the micropores of the dried particles as
described above, and passivate the particles of dried carbonaceous
material against adsorption of water and oxygen, and thus
spontaneous combustion.
[0145] As the dried carbonaceous material feed stream traverses the
multi-hearth furnace 40, the dried carbonaceous material feed
stream is heated to temperatures of about 600.degree.
C.-850.degree. C. by a counter current flow of hot gases as
described above, at which temperatures the dried carbonaceous
material feed stream is carbonized and converted to char.
[0146] Volatile matter is also evolved from the dried carbonaceous
material feed stream at these temperatures. The volatile matter
mixes with the hot gases and is subsequently directed in a counter
current flow against the dried carbonaceous material feed stream.
Thus the volatile matter coats the dried particles, plugging the
micropores, and reducing the absorption of water and oxygen. At the
temperatures in the multi-hearth furnace 40 the tar and the
particles undergo pyrolysis. Pyrolytic rupture of functional groups
attached to aromatic and hydro-aromatic units of the coal particle
structure leads to the release of low molecular weight, reactive,
free radicals (fragments) and stabilisation of the former fragment
sites by hydrogen. Thus hydrophilic polar-functional groups are
removed from the coal particles and replaced by hydrophobic
aromatic coating. During low temperature carbonisation this process
can produce a passivated carbonaceous material with similar
properties to a reduced sulphur, pseudo-bituminous coal from low
rank coal. The passivated carbonaceous material may be as
chemically stable as any other naturally occurring bituminous coal.
Upon further heating the tar coating mobilises and in the
subsequent pyrolysis phase the tar provides the hydrogen used to
stabilise more radical sites on the coal particle thus producing a
passivated char with similar properties to a reduced sulphur,
pseudo anthracite coal.
[0147] The mixed coal and combustion gases produced and used in the
multi-hearth furnace can be processed in several ways including but
not limited to combustion to produce electricity, or used to make
fertilizer. Additionally, as described above, the coal gas can be
used as an alternative fuel source for the external burner 30.
Regardless of the method of gas utilization, there is provided a
safe and environmentally acceptable method of flaring the mixed
gases generated in the multi-hearth furnace.
[0148] The passivated char exits the multi-hearth furnace and can
be quenched by water and cool inert gas in a manner well known to a
person skilled in the art, including using a multi-hearth cooler 50
as described above, before being cooled to ambient temperature,
stockpiled and loaded out. The passivated char produced in the
pyroliser is at greater temperatures than the passivated dried
carbonaceous material produced in the dryer 12. Thus, due to the
potential to generate explosive gases and dusts within the cooler
50, as a precautionary measure, liquid water is not introduced to
the cooler 50 as a quenching medium until the temperature of the
passivated char is below 200.degree. C.
[0149] The passivated char produced by the above described process
from the above described coal feedstock has approximately the
following composition by weight: 81.3% fixed carbon, 11.9% ash, 2%
moisture, and 5% volatile matter.
[0150] Alternatively, the hot passivated char is blended with wet
screened coal of the above described composition to produce a
blended carbonaceous material with the following composition by
weight: 64.8% fixed carbon, 9.2% ash, 10% moisture, and 14.7%
volatile matter.
[0151] The process 10 of the present invention can also be
conducted under conditions wherein the carbonaceous feed material
undergoes high temperature carbonisation, as described in the
following example.
EXAMPLE
[0152] Crushed coal from the Ewington mine in Western Australia
(100%-15 mm) at 28% moisture is fed at 49 tph to a 16.5-m long five
cell fluidised bed dryer. The coal in the fluidised bed dryer is
heated with a flow (28,500 kg/h) of waste gas containing 1.6% (m/m)
O.sub.2 at temperatures of 800.degree. C. produced in a 10 MW
burner. The coal feed stream has a residence time of 9 to 10
minutes in the dryer. The last cell of the dryer is heated by the
off gas generated from the pyrolising means in this system.
[0153] Hot (150.degree. C.) dry coal is fed to a 13.2-m long
refractory lined carboniser via a gas lock feeder. The dried coal
is heated to 1,300.degree. C. These high temperatures are generated
by feeding hot air (800.degree. C.) to the carboniser which
combusts with some of the coal gas evolved during pyrolisis. The
coal gas flows in a counter-current direction to the flow of the
coal/char feed stream. The total residence time of the char feed
stream held above 900.degree. C. is between 11 and 12 minutes.
Approximately 24,500 kg/h of dry char is produced. Excess coal gas
and combustion gases are collected.
[0154] The carboniser feeds three 9.9-m long refractory lined
coolers which depress the char temperature from 1,300.degree. C. to
less than 500.degree. C. These coolers are fed a counter-current
flow of coal inert gas. The quenched char is then fed to a further
six steel refractory coolers which reduce the gas temperature to
below 70.degree. C. The moisture content of the char is raised to
about 6% to suppress dust.
[0155] The above-described high temperature carbonisation system
has a nominal capacity of 190,000 tpa of passivated char with 6%
moisture, 2% volatile matter content.
Example 2
Trial Data
[0156] Collie coal, with 24% moisture, was pre-dried in an oven
under nitrogen at 120.degree. C. This coal was then heated at
5.degree. C./min until it reached 300.degree. C. The mass lost was
0.6% and the equilibrium moisture was 6.5%. The same dried coal was
placed in an oven for 10 minutes at 400.degree. C. The resulting
mass loss was 2.6% and the equilibrium moisture was 4.5%. The feed
coal typically has an equilibrium moisture of about 13%.
[0157] A large scale trial was performed using a rotary kiln with
recycled coal gas and air dried coal. The results are presented
below in Table 1. The coal residence time was 20 minutes to heat
the coal to an average discharge temperature of 266.degree. C.
TABLE-US-00001 TABLE 1 Low Temperature Carbonized Coal Total Vola-
Specific Mois- tile Fixed Energy ture Level Ash Carbon Sulfur MJ/kg
feed coal (gar) 25.5% 27.2% 3.60% 43.7% 0.55% 21.28 feed coal (db)
36.5% 4.83% 58.7% 0.74% 28.56 carbonised 8.5% 26.0% 5.5% 60.0%
0.66% 27.4 coal (gar) carbonised 28.4% 6.0% 65.7% 0.72% 29.2 coal
(db)
[0158] The overall yield was 65.6% on wet basis or 82.8% yield on
dry basis. The increase in specific energy was 28.8% and the
reduction in sulfur was 215%. The drying and carbonization energy
was 15.5% of total energy throughput.
[0159] A higher degree of carbonization was trialed using a heating
rate of 10.degree. C. per minute and dry coal pre-heated to
100.degree. C. for 20 minutes. The results are presented below in
Table 2. The coal residence time was 60 minutes to heat the coal to
an average discharge temperature of 700.degree. C. The carbonized
coal was cooled using inert gas.
TABLE-US-00002 TABLE 2 Mid Temperature Carbonized Coal Total Vola-
Specific Mois- tile Fixed Energy ture Level Ash Carbon Sulfur MJ/kg
feed coal (gar) 24.5% 25.75% 3.00% 46.55% 0.20% 20.9 feed coal (db)
33.82% 3.97% 61.90% 0.26% 27.6 carbonised 7.9% 12.0% 5.25% 73.7%
0.22% 32.1 coal (gar) carbonised 13.0% 5.70% 80.0% 0.24% 35.1 coal
(db) C H N S O feed coal (db) 77.87% 4.05% 1.64% 0.32% 17.12%
carbonised 87.0% 5.61% 2.16% 0.24% 5.02% coal (db)
[0160] The overall yield was 57.1% on wet basis or 65.7% yield on
dry basis. The increase in specific energy was 53% and the
reduction in sulfur was 37%. The drying and carbonization energy
was 12.6% of total energy throughput.
[0161] From the above description of the preferred embodiment of
the process of preparing passivated carbonaceous material in
accordance with the invention, it will be evident that the process
has significant advantages compared to the prior art processes,
including the following advantages:
(1) The process does not produce excessive fine particles due to
the deliberate sizing of the feed stock. (2) The economy of this
process is improved by: [0162] drying the carbonaceous material
prior to carbonisation; [0163] feeding the carboniser with hot
dried carbonaceous material; [0164] directing the coal gas
containing volatile matter to flow counter current to the
carbonaceous material feed stream; and [0165] directly heating the
carbonaceous material feed stream by partially combusting the
carbonaceous material feed stream and the coal gas in the
pyroliser. [0166] coal gas containing volatile matter is also burnt
outside the carboniser to provide heat for the process. (3) The
process can use a variety of feed stocks but in particular it is
readily suited to removing oxygen and organic sulphur from lignite,
sub-bituminous and bituminous coals. (4) The process prepares a
passivated char from low rank coals and reduces the risk of
spontaneous combustion. Thus it is expected that these coals will
be the prime feed stock. (5) Drier, pneumatic transport and coal
gases are separated where possible and are controlled in terms of
temperature, flow and pressure independently (6) The process is not
equipment limited as it could employ fluidised beds, multi-hearth
furnaces or rotary kilns or a combination of these unit operations.
(7) The activity of the tarry material in the coal gas is greatly
increased by pre-drying the coal and not combining this drier gas
with the carbonizing gas. Thus the effect of moisture stabilization
can occur at lower temperature if there is a pre-drying step due to
less coal gas being required.
[0167] Numerous variations and modifications will suggest
themselves to persons skilled in the relevant art, in addition to
those already described, without departing from the basic inventive
concepts. All such variations and modifications are to be
considered within the scope of the present invention, the nature of
which is to be determined from the foregoing description.
[0168] For example, the process of the present invention can be
applied to other heat sensitive, flammable substances, for example
thermal desorption of activated carbon or even contaminated
soil.
* * * * *