U.S. patent application number 12/047641 was filed with the patent office on 2008-09-18 for method to improve the efficiency of removal of liquid water from solid bulk fuel materials.
Invention is credited to Robert R. French, Robert A. Reeves.
Application Number | 20080222947 12/047641 |
Document ID | / |
Family ID | 39760045 |
Filed Date | 2008-09-18 |
United States Patent
Application |
20080222947 |
Kind Code |
A1 |
French; Robert R. ; et
al. |
September 18, 2008 |
Method To Improve The Efficiency Of Removal Of Liquid Water From
Solid Bulk Fuel Materials
Abstract
The invention provides methods to efficiently reduce the water
concentration of raw solid fuels, including low rank coals such as
brown coal, lignite, subbituminous coal, and other carbonaceous
solids. Efficiently drying these materials at low temperatures
significantly reduces greenhouse gas emissions and allows the
production of low-rank coals for gasification and liquifaction.
Inventors: |
French; Robert R.;
(Wellington, CO) ; Reeves; Robert A.; (Arvada,
CO) |
Correspondence
Address: |
SHERIDAN ROSS PC
1560 BROADWAY, SUITE 1200
DENVER
CO
80202
US
|
Family ID: |
39760045 |
Appl. No.: |
12/047641 |
Filed: |
March 13, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60980780 |
Oct 17, 2007 |
|
|
|
60894591 |
Mar 13, 2007 |
|
|
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Current U.S.
Class: |
44/500 |
Current CPC
Class: |
C10L 5/04 20130101; B30B
3/00 20130101; F26B 21/14 20130101; F26B 15/00 20130101; C10J
2300/0906 20130101; F26B 9/10 20130101; F26B 3/04 20130101; F26B
1/00 20130101; C10F 5/04 20130101; C10L 2290/08 20130101; C10L
5/361 20130101; F26B 25/007 20130101; C10F 7/06 20130101; C10L 5/08
20130101; C10L 5/06 20130101; F26B 2200/08 20130101; C10L 2290/30
20130101; F26B 7/00 20130101; C10J 2300/0909 20130101; F26B 11/028
20130101; F26B 11/02 20130101; C10J 2300/093 20130101; C10J 3/72
20130101 |
Class at
Publication: |
44/500 |
International
Class: |
C10L 5/00 20060101
C10L005/00 |
Claims
1. A method of treating a solid carbonaceous material comprising:
compacting a solid carbonaceous material under a force of at least
about 5000 lb/in.sup.2 to form a compacted carbonaceous material;
and, contacting the compacted carbonaceous material with a working
fluid, wherein the working fluid causes evaporative drying of the
compacted carbonaceous material to form a dried carbonaceous
material.
2. The method of claim 1, wherein the solid carbonaceous material
is selected from the group consisting of brown coal, lignite,
subbituminous coal and mixtures of these materials.
3. The method of claim 1, wherein the solid carbonaceous material
has a top size between about 0.1 mm and about 6 mm.
4. The method of claim 1, wherein the solid carbonaceous material
has a moisture content between about 15 weight percent and about 65
weight percent.
5. The method of claim 1, wherein the solid carbonaceous material
has a temperature between about 17.degree. C. and about 66.degree.
C.
6. The method of claim 1, wherein the force is between about 5000
lb/in.sup.2 and about 50000 lb/in.sup.2.
7. The method of claim 1, wherein the working fluid is selected
from the group consisting of unsaturated air, nitrogen, inert gas,
flue gas, superheated steam and mixtures of these fluids.
8. The method of claim 1, wherein the contacting comprises applying
the working fluid to the compacted carbonaceous material in at
least one of a stockpile of the compacted carbonaceous material and
a rotary dryer containing the carbonaceous material.
9. The method of claim 1, wherein the contacting comprises
contacting the compacted carbonaceous material on a conveyor with a
working fluid flowing past the conveyed material.
10. The method of claim 1, further comprising collecting dust
created in the contacting step and returning the collected dust to
a solid carbonaceous material for the compacting step.
11. The method of claim 1, further comprising compacting the dried
carbonaceous material to form a shaped compacted material.
12. The method of claim 11, wherein the shaped compacted material
is a briquette of ovoid shape with a minor dimension between about
6 mm and about 100 mm.
13. The method of claim 11, wherein the shaped compacted material
has a moisture content between about 7 weight percent and about 17
weight percent.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119(e) to U.S. Provisional Patent Application No.
60/894,591 filed Mar. 13, 2007, and to U.S. Provisional Patent
Application No. 60/980,780 filed Oct. 17, 2007, both of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention describes a method to efficiently reduce the
moisture content of solid carbonaceous materials including brown
coal, lignite and subbituminous coal to produce premium-quality
fuels.
BACKGROUND OF THE INVENTION
[0003] Low-rank coals (LRCs) are abundant in the United States and
elsewhere and have the potential to provide an economic feedstock
for gasification. LRCs typically contain between 25 and 45 wt %
moisture in the United States, and can be as great as 65 wt % in
other countries. The high moisture content of LRCs has impeded
their use as gasification feedstock because the gasification
industry has identified an optimum moisture content not exceeding
15 wt %. If the feedstock moisture is greater than 15 wt %, plant
efficiency is impaired and economics may not be viable. A LRC with
high-moisture content emits more carbon dioxide during utilization,
on an equal energy basis, than low-moisture bituminous coal because
the extra energy consumed evaporating moisture contained in the LRC
is not available for useful work. Efficiency is reduced and
emissions are increased.
[0004] Coal gasification systems produce clean burning gas and
liquid fuels from solid fuels including coal and lignite. This
technology is especially attractive from an environmental
standpoint because carbon dioxide, believed to be an agent of
global climate change, can be concentrated and removed during
processing. Clean burning synthetic natural gas (SNG) is available
for residential and industrial use. However, the yield of liquids
such as diesel and naphtha that are produced by gasification and
liquefaction processes is severely impaired as the moisture of the
feedstock increases above 12 wt %. The costs imposed by limiting
feedstock to less than 12 wt % moisture severely reduce the ability
to use LRCs to produce petroleum liquids and gases.
[0005] Various methods have been employed by commercial entities
and evaluated by research laboratories to reduce the moisture
content of LRCs. Virtually all require thermal energy to
sufficiently heat the LRC to evaporate water and remove the
superheated vapor from the dried solids. Some methods use an
autoclave or pressure vessel to remove a portion of the water as a
high-pressure superheated liquid.
[0006] The high-temperature (above 200.degree. C.) thermal process
uses an external heat source to produce a working fluid such as
air, combustion flue gas, steam, or other inert gases. Steam
generators, combustors, stoker furnaces, or gas or oil burners are
required to heat the working fluid to the high temperature. The
cost of these external energy sources can be great, especially when
environmental equipment is included to treat flue gases created
during combustion.
[0007] In an attempt to reduce the cost of high-temperature thermal
energy from external heat sources, experiments have been conducted
to extract low-temperature (less than 100.degree. C.) thermal
energy from waste heat sources such as power plant condenser
circuits, low-pressure steam cycles, flares, and thermal oxidizer
flue gases.
[0008] The low-temperature drying methods require more time to
evaporate a given amount of water than the high-temperature
methods. Therefore a substantially larger drying vessel is required
to provide the residence time to evaporate the water. The expense
of the larger drying vessel and ancillary equipment is often
greater than the benefit gained from using a low-temperature waste
energy source to heat the working fluid.
[0009] Certain LRC's are heat sensitive and are easily oxidized
during drying. Oxidation also reduces the useful energy contained
in the dried product, and therefore reduces its commercial value.
The rate of oxidation can be reduced by drying at relatively low
temperature, preferably less than 100.degree. C.
[0010] Commercial drying systems require substantially more energy
to evaporate the water contained in LRCs than that required to
evaporate an equal amount of liquid water held in direct contact
with the working fluid. This fact can be explained by considering
how liquid water is held by the LRC: [0011] 1. Water residing on
the surface of the LRC particle. [0012] 2. Water held in the
interior pores of the LRC particle.
[0013] Water that is chemically bound to organic and inorganic
molecules is not relevant to the present invention because this
form of water can only be removed by thermal energy.
[0014] Water residing on the surface of the LRC particle is the
easiest to evaporate because it comes into direct contact with the
working fluid. Because little heat is transferred into the solid
LRC particle during this operation, evaporation of this surface
moisture is efficient and rapid. More energy is required to remove
water held in pores of the bulk material because a sufficient
amount of thermal energy must be imparted by the working fluid to
both evaporate water and heat the porous solid material to
evaporation temperatures.
[0015] Therefore the total energy required to evaporate liquid
water held by LRC is the sum of the energy required to evaporate
and remove water residing on the surface and within the pores of
the material. The sum is always greater using existing thermal
drying systems than that required to evaporate the equivalent
amount of liquid water from the surface of a solid in contact with
a working fluid.
SUMMARY OF THE INVENTION
[0016] The present invention improves the efficiency of thermal
drying methods by evaporating liquid water that was transferred to
the surface of the particle from interior pores during compaction
by mechanical forces. Increased efficiencies result because water
residing on the surface that is direct contact with the working
fluid can be evaporated with less time and energy than water
residing in the material's internal pores. The present invention
transforms LRC to remove moisture, and in a gasification
application, improves the gasification characteristics of raw LRC
feedstock.
[0017] Most LRCs have porous structures that contain liquid water
and other tightly held materials. The process described in U.S.
patent application Ser. No. 11/380,884, filed Apr. 28, 2006 (U.S.
Patent Publication No. 2007-0023549 A1), which is incorporated
herein by reference, compresses raw material to reduce pore volume
and express a significant proportion of the water contained in the
pore volume. The high compaction forces permanently deform the raw
feed to produce a nearly solid impermeable product that is less
susceptible to reabsorb water and oxygen. This transformation, when
conducted on finely sized raw feed materials, has proven useful to
both reduce the moisture content of feedstock and modify the
texture of the material.
[0018] In the present invention, high compaction forces are
continuously imparted at ambient temperature to the feed material.
Sufficient force is used to collapse the material's porous
structure and force the expelled water to the surface of the
compacted material. The wet compacted material is then fed to a
low-temperature or ambient temperature-drying device where a
substantial proportion of the water is evaporated from the surface
of the material. As an additional benefit, the present invention,
by being more efficient, can dry materials at ambient temperatures
that are too low to be economically practical with conventional
thermal drying systems that do not treat the feed prior to drying.
Operating the present invention at ambient temperatures will
provide additional desirable cost advantages to the utility and
gasification industries, among others, by allowing production and
use of low cost dried LRC products. Benefits include, via increased
drying efficiencies, reducing the amount of carbon dioxide and
other gaseous pollutants such as sulfur dioxide and nitrous oxides
released during production and utilization. Providing the
opportunity to economically use domestic LRC resources to produce
motor fuels will substantially reduce use of foreign oil. Thus the
present invention proves beneficial in three ways: economically
reducing moisture content below 15 wt %, forming a briquette that
has predictable reaction kinetics with steam and oxygen, and
providing a strong material that can support the weight of burden
held in the gasification reactor.
[0019] The present invention provides processing methods to
efficiently process raw bulk materials into low-moisture content
products. The present invention includes the following subsystems:
[0020] 1. Raw solid fuel preparation. [0021] 2. Material
compaction. [0022] 3. Working fluid management. [0023] 4. Drying.
[0024] 5. Dust collection. [0025] 6. Optional secondary compaction
means to form the material into desired shapes, as may be required
by specific applications
[0026] Raw materials, such as LRC's are often mined and crushed to
50 mm top size, a size typically traded worldwide. The raw
materials are typically carbonaceous materials and particularly
carbonaceous fuels that may include brown coal, lignite,
subbituminous coal, waste coals and mixtures of these materials.
The present invention receives this carbonaceous material and
crushes it to pass a 5 mm screen or other similar size, depending
on the application. Preferably, the feedstock is crushed to reduce
its nominal top size to between 0.1 mm to 6 mm, and more preferably
to a nominal top size of about 0.5 mm. The present invention
processes all of the feed material, thus achieving greater recovery
of resources than other drying techniques that must remove and
potentially discard finely sized materials prior to processing.
[0027] The feed material is then compacted using an applied
mechanical force sufficient to deform the feedstock to reduce its
pore volume. Preferably, the force applied is in the range of
between 5,000 lb/in.sup.2 and 50,000 lb/in.sup.2, and more
preferably the applied force is about 30,000 lb/in.sup.2. The
prepared feed material may be fed to compactors, such as roll
presses, that exert high pressures on the material. The pressures
exerted by the roll presses may range as high as 275,000 kPa per cm
of roll width. The material is physically transformed under the
pressure to collapse the porous structures that are present in most
LRC'S. The pores contain water, which collapse under pressure,
forcing the water from the pores to the surface of the material. In
some cases, sufficient water is present in the bulk starting
material to be removed from the compressed material as a liquid and
be carried away from the processing stream. Separating liquid water
from the material prior to drying reduces the thermal load on the
system.
[0028] The wet compacted material is transported to low-temperature
processing, such as an indirect rotary dryer to evaporate the
liquid water present on or near the surface of the compressed
particles. Drying rates of compacted materials can be many times
greater than drying rates of the raw material before compaction.
The reason for the increased drying rate is the water expressed
from the pores is in direct contact with unsaturated gas ("working
fluid" as defined below) passing over the material. Increasing
drying rates at low temperatures provides the operator with several
benefits not offered by traditional processes. For example, smaller
and less costly equipment can be used to achieve the desired
capacity. If costs do not constrain the operation, greater capacity
can be achieved with compaction. Lower working temperatures can be
used to dry heat-sensitive materials, thereby avoiding or
substantially reducing oxidation and product deterioration.
[0029] In another embodiment of the invention, a covered open
stockpile can be used to gently but efficiently dry compacted
material. Experiments reveal that the stockpiled material can be
well managed because oxidation rates of LRC's can be greatly
reduced by compaction.
[0030] The low-temperature drying process requires a source of
unsaturated gas (working fluid) to heat the compacted material and
transport the superheated water vapor away from the dried material.
Heat sources can range from ambient air to gas supplied from
electric heaters, gas and other fossil-fired combustors, and waste
heat available from existing industrial processes such as power
plants. Management of the heat source can be affected by
readily-available commercial equipment.
[0031] Spent working fluid, containing the water removed by
evaporation, often contains dust that must be collected and
processed to meet environmental regulations. Experiments by the
present inventors have confirmed that the spent working fluid
produced during low-temperature drying does not contain significant
organic vapors to require additional collection or thermal
treatment. Substantial cost savings result. In one embodiment,
collected dust can be introduced, or re-introduced, to the
compaction operation to increase product yield.
[0032] Dried product may be transformed into desired shapes, such
as briquettes, that can be readily handled, stored, and transported
by rail or ship to distant customers. The formed shapes may be
desired to provide favorable material handling properties including
acceptable bulk density, reduced breakage and dust generation, and
resistance to oxidation during storage.
[0033] Numerous gasification processes have been identified and
developed. One such process that has commercial application
processes solid feedstock to produce carbon monoxide and hydrogen
(referred to as syngas) and slag as a waste product. The process
vessel resembles a tall vertical tank that accepts feed at the top
of the vessel. Oxygen and steam are injected near the bottom of the
vessel (reaction zone) to create exothermic reactions that produce
syngas. The feed slowly descends the vessel as material is consumed
in the reaction zone. New feed is continuously added to make up
volume consumed. The efficiency of the reactions depends on the
feed material maintaining sufficient mechanical strength to support
its bulk weight and porosity to allow gases to flow upward and out
of the reaction vessel. An ideal feed therefore contains an optimum
moisture content (less than 15 wt %), and produce a carbonized
material (coke) with exceptional mechanical strength and stability.
In addition, the texture (grain size) of the feed material is
specified to provide the desired reaction rate between the coke,
oxygen, and steam.
[0034] Briquettes produced from LRC by the processes of the present
invention have proven to be beneficial as a gasifier feedstock
because of its ideal moisture content (8-15 wt %), mechanical
strength after coking (greater than 600 lb/in2 compressive strength
at ambient temperature), and moderate rate of reaction with steam
at high temperature.
[0035] The operating conditions operating conditions of the
processes of the present invention can be adjusted to provide
briquette products with the specified moisture content, strength,
and texture.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 shows a schematic drawing of a low-temperature drying
process integrated into a typical fossil-fired power plant
operation in which a source of waste heat is available to heat the
working fluid.
[0037] FIG. 2 shows a schematic drawing of low-temperature drying
process that can be independently sited where no waste heat is
available. Ambient air provides the working medium.
[0038] FIG. 3 shows a schematic drawing of low-temperature drying
process that can be independently sited and uses an external heat
source to provide warm air for drying.
[0039] FIG. 4 shows a schematic drawing of an ambient-temperature
drying process that can be independently sited where material is
stored in a covered stockpile. The stockpile is managed to accept
compacted material on a continuous basis and be reclaimed as
required.
[0040] FIG. 5 is a graph showing the results of a study comparing
the relative drying rates of raw lignite and compacted lignite.
DETAILED DESCRIPTION OF THE INVENTION
[0041] The present invention provides a novel method to treat solid
carbonaceous materials such as lignite and subbituminous coal used
to fire boilers, combustors, stokers, and to feed coal gasifiers.
This method takes advantage of the fact that a significant
proportion of the water contained in pores of low-rank coal (often
as much as 74% of the total water) can be efficiently evaporated
without the difficulties of the conventional thermal drying
systems. Conventional drying operations must heat the solids to
evaporation temperatures to remove water held in the interior of
the material. Because the rate of drying is greater with the
present invention, lower temperatures can be efficiently used,
significantly reducing material oxidation.
[0042] The method continuously compacts and collapses the porous
material to express water held in pores, and transfers the
expressed water to the surface of the processed material. Water
residing on the surface is efficiently evaporated in the presence
of the working fluid. By avoiding the difficulties associated with
heat and mass transfer into and out of a particle, the present
invention provides the superior heat and mass transfer only
available when water is placed in direct contact with a working
fluid. The efficiency gains can make both ambient-temperature and
elevated-temperature drying systems practical in many
applications.
[0043] The present invention includes the following subsystems:
[0044] 1. Raw solid fuel preparation. [0045] 2. Material
compaction. [0046] 3. Working fluid management. [0047] 4. Drying.
[0048] 5. Dust collection. [0049] 6. Optional secondary compaction
to form the dried product into desired shapes, as may be required
for certain commercial or industrial applications.
[0050] Each of these subsystems is described in detail below.
Raw Solid Fuel Preparation
[0051] The raw solid fuel preparation subsystem receives crushed
material of traditional trade top size, typically about 50 mm. In
one preferred embodiment, the minus -50 mm raw solid fuel is
comminuted by a hammer mill, roll crusher, or other appropriate
device to produce a product of approximately 5 mm top size. The
optimum particle size required to provide the desired compaction
properties is experimentally determined for a particular
application and feed source. However, feed to the compactor may
have a top size that typically varies between about 0.1 mm and
about 19 mm. Preferably, the top size is about 0.5 mm. The crushed
material may include carbonaceous materials such as brown coal,
lignite, subbituminous coal, waste coals and mixtures of these
materials. Preferably, this raw feedstock contains between about 15
weight percent moisture and about 65 weight percent moisture, and
more preferably about 35 weight percent moisture. Preferably, the
temperature of this raw feedstock is between about 17.degree. C.
and about 66.degree. C., and more preferably the feedstock is at
ambient temperature.
Primary Material Compaction
[0052] The prepared bulk raw material is compacted with sufficient
force to mobilize and transfer waters held in fractures, voids, and
pores from the interior of the solid particle to the surface of the
solid particle. Preferably, the compaction of the pre pared bulk
raw material is conducted in a continuous manner. The compaction
force produces a pressure of between about 5000 lb/in.sup.2 and
about 50000 lb/in.sup.2, and more preferably the compaction force
produced is about 30000 lb/in.sup.2.
[0053] In the preferred embodiment, a roller press is used to
compact the feed material using a specific roll force between about
5 kN/cm and about 150 kN/cm of roll width. Water driven from the
interior to the surface of particles by these compaction forces
therefore becomes readily available for contact with a working
fluid.
Working Fluid Management
[0054] The working fluid can be unsaturated air, nitrogen, inert
gas, flue gas, superheated steam, or other substances that are
compatible with the dried material. The working fluid management
system generates a substance containing less than 100% relative
humidity. In a preferred embodiment, the substance is air
containing less 100% relative humidity that is collected and
contacted with the wet material using natural convection, fans, or
blowers. In these cases, the entire drying system is independent of
external heat sources. The material can contact the working fluid
in stockpiles and drying vessels such as a rotary dryer. In another
embodiment, the working fluid can be heated by an external source.
Supplied heat may be transferred to the working fluid by a heat
exchanger. The heat exchanger is configured to suit the
application. Sources of external heat may include, for example,
condenser cooling water, flue gas desulfurization sludge, gasifier
cooling water, syngas cooling water, heat recovery steam generator,
or other forms of heat that would otherwise be rejected to the
environment. In yet another embodiment, the working fluid generated
by an external heat source can be hot flue gas that is tempered
with air, or other material that is at a lower temperature than the
combustion gas. In yet another related embodiment, a purpose-built
boiler or combustor can be used to heat the working fluid.
Drying
[0055] The compacted product, usually in flake or pellet form, is
transferred to a vessel where feed particles can be efficiently
contacted with the unsaturated working fluid. In a preferred
embodiment that works with coal or other heat-sensitive
applications, the drying vessel is an indirect rotary dryer.
Indirect rotary dryers transfer heat into the wet compressed
material in two ways. First, heat is transferred by convection.
This is accomplished by passing hot working fluid over the wet
material. Second, heat is transferred by conduction by contacting
the wet compressed material with a hot surface (shell of the rotary
dryer). Both sources of heat evaporate water. In other applications
that work with materials that are not heat sensitive, a direct
rotary dryer may be used. Direct dryers use heat supplied by the
hot working fluid alone, and do not heat the material by
conduction. The working fluid used in a direct dryer is typically
hotter than the working fluid used in an indirect rotary dryer.
[0056] In another embodiment, unsaturated air can be directed
across a stockpile of compacted material. Fans or natural
convection can used to accelerate the air to increase the rate of
drying.
[0057] In another embodiment, material can be conveyed on a
vibrating pan conveyor fitted with a perforated screen deck.
Working fluid enters upward though the perforations and flows past
the conveyed material. The conveyor device is sufficiently long to
provide the required residence time to dry the material. Saturated
vapor is removed from the top of the conveyer. Additional methods
including fluid bed dryers and other vessels of commercial
configuration are available. The present invention is not limited
to the type of style of drying vessel as long as it is compatible
with the process material.
Dust Collection
[0058] Vapors emanated from the dryer often contain dust that must
be removed before venting to the atmosphere. Standard dust
separation and collection devices such as electrostatic
precipitators, bag houses or wet scrubbers may be used to separate
fine particles from water vapors as dictated by the application.
Collected fine particles may be recycled to the compaction
subsystem as desired, so that all, or nearly all, feed material is
processed without waste.
Secondary Material Compaction
[0059] Applications that require the finished product to be of a
specified shape, such as a briquette, can be accommodated by
compacting the dried product as described above. Formed products
are typically used where the dried material is transported, or used
in stoker furnaces where a coarse particle size distribution is
required.
[0060] In instances in which a shaped final product is desired, the
product is preferably a briquette of ovoid shape with a minor
dimension of at least about 6 mm, but less than about 100 mm, and
more preferably having a minor dimension of about 50 mm. These
shaped products are preferably formed bulk materials having a void
space of between about 12 volume percent and about 60 volume
percent, and more preferably having a void space of about 30 volume
percent void space. Preferably, the shaped products are formed such
that upon being subjected to coking conditions, they form coke that
has a compressive strength between about 100 lb/in.sup.2 and about
2,000 lb/in.sup.2, and more preferably a compressive strength of
about 800 lb/in.sup.2. Preferably, the shaped products have a total
moisture content between about 7 weight percent and about 17 weight
percent, and more preferably a total moisture content of about 12
wt %.
[0061] FIG. 1 shows a schematic of the overall system of a
preferred embodiment of the invention. A source of raw solid fuel
(1) supplies material (2) to the raw solid fuel comminution circuit
(3) where the feed is crushed and sized. The prepared raw feed (4)
and collected dust (12) are feed to the compaction circuit (5)
where they are compressed under high pressure to force water from
its internal pores to produce a flake product with water adhering
to the surface of the compacted material (6). The compacted
material is fed to the dryer (7) where it is mixed with heated air
(19), evaporating the water residing on the surface of the
compacted material. The resulting vapors and dust (8) are passed to
a dust collection circuit (9) where the dust and water vapor are
separated. Dust-free vapor (10) is vented to the atmosphere (11).
Dust (12) is conveyed to the compaction circuit. Dryer product (13)
containing substantially less moisture than the feed, but within
the application product specifications, is conveyed to a dried
product storage point (14) where it is available for use or
additional processing. A source of waste heat (15) capable of
supplying sufficient power to satisfy the evaporative load provides
a hot flow input (16) and accepts a hot flow return (17). A
sufficient temperature drop exists between the input and return
flows to impart the required energy to the working fluid (21). An
ambient air source (20) provides the cool working fluid (21) to the
heat exchanger (18) where it is heated to a specified temperature
by the circulating hot in and hot out flows. The heated working
fluid (19) passes to the dryer where it contacts the wet feed
material.
[0062] FIG. 2 shows a schematic of the overall system of a
preferred embodiment of the present invention. A source of raw
solid fuel (21) supplies material (22) to the raw solid fuel
comminution circuit (23) where the feed is crushed and sized. The
prepared raw feed (24) and collected dust (212) is feed to the
compaction circuit (25) where it is compressed under high pressure
to force water from the feed's internal pores to produce a flake
product with water adhering to the surface of the compacted
material (26). The compacted material is fed to the dryer (27)
where it is mixed with unsaturated ambient-temperature air (215)
thus evaporating the water residing on the surface of the compacted
material. The resulting vapors and dust (28) are passed to a dust
collection circuit (29) where dust and water vapor is separated.
Dust-free vapor (210) is vented to the atmosphere (211). Dust (212)
is conveyed to the compaction circuit. Dryer product (213)
containing substantially less moisture than the feed, but within
the application product specifications, is conveyed to a dried
product storage point (214) where it is available for use or
additional processing. A source of ambient-temperature air (215) is
fed into the dryer.
[0063] FIG. 3 shows a schematic of the overall system of a
preferred embodiment of the invention. A source of raw solid fuel
(31) supplies material (32) to the raw solid fuel comminution
circuit (33) where the feed is crushed and sized. The prepared raw
feed (34) and collected dust (312) is feed to the compaction
circuit (35) where it is compressed under high pressure to force
water from the feed's internal pores to produce a flake product
with water adhering to the surface of the compacted material (36).
The compacted material is fed to the dryer (37) where it is mixed
with heated air and flue gas (322) thus evaporating the water
residing on the surface of the compacted material. The resulting
vapors and dust (38) are passed to a dust collection circuit (39)
where dust and water vapor are separated. Dust-free vapor (310) is
vented to the atmosphere (311). Dust (312) is conveyed to the
compaction circuit. Dryer product (313) containing substantially
less moisture than the feed, but within the application product
specifications, is conveyed to a dried product storage point (314)
where it is available for use or additional processing. A source of
ambient air (319) provides combustion air (320) and tempering air
(321) to the process. A source of fuel (315) is supplied (16) to a
furnace (317) where it is combusted to provide hot flue gas (318).
The flue gas is mixed with tempering air (321) to provide a warm
gas (322) of the specified temperature for drying purposes.
[0064] FIG. 4 shows a schematic of a preferred embodiment of the
invention. A source of raw solid fuel (41) supplies material (42)
to the raw solid fuel comminution circuit (43) where the feed is
crushed and sized. The prepared raw feed (44) is feed to the
compaction circuit (45) where it is compressed under high pressure
to force water from the feed's internal pores to produce a flake
product with water adhering to the surface of the compacted
material (46). The compacted material is stacked out in a covered
stockpile (47). A source of ambient, unsaturated air (48) is
available to sweep (49) over the stockpiled material thus
evaporating the water residing on the surface of the compacted
material. The resulting vapors (410) are released as a gas to the
atmosphere (411). Dried product (412) containing substantially less
moisture than the feed, but within the application product
specifications, is reclaimed to a dried product storage point (413)
where it is available for use or additional processing.
[0065] Additional objects, advantages, and novel features of this
invention will become apparent to those skilled in the art upon
examination of the following examples thereof, which are not
intended to be limiting.
EXAMPLES
Example 1
[0066] A detailed study of lignite (high-moisture lignite from
North Dakota) was undertaken to assess the relative drying rates of
raw material and compacted product. Experiments were conducted on
materials spread out on a flat tarpaulin at ambient conditions.
Samples of raw lignite and compacted lignite were taken
periodically from the spread out material and assayed for total
moisture. The measured moisture values were normalized as percent
of the total water evaporated to compare the results on an equal
basis. Drying conditions were 31.degree. C., and 23% relative
humidity. Results are plotted in FIG. 5.
[0067] These results demonstrate the increased drying rates
possible by compacting the raw lignite. Table 1 summarizes the
ratio of drying rates between raw and compacted lignite processed
at ambient conditions of 31.degree. C., 23% relative humidity.
TABLE-US-00001 TABLE 1 Ratio of Relative Drying Rates for North
Dakota Lignite % Moisture Drying Removed Time, hr Raw Compacted
Ratio 0.5 4 16 4.0 1.0 9 22 2.4 1.5 13 26 2.0 2.0 16 28 1.8
Example 2
[0068] A detailed study of one LRC (high-moisture lignite from
South East Asia) was undertaken to assess the relative drying rates
of raw material and compacted product processed by an indirect
rotary dryer (180 mm diameter.times.3000 mm long). The heated
portion of the drying tube was 2000 mm long, the cooling zone was
600 mm long, and the feed zone was 400 mm long. Test conditions
were identical for the compacted and raw materials. The results are
summarized in Table 2.
TABLE-US-00002 TABLE 2 Rotary Indirect Dryer Test Results Compacted
Raw Test Parameter Material Material Feed moisture, wt % 45 46
Product moisture, wt % 16 38 Residence time, min 20 20 Sweep gas
temperature, at 130 130 feed point, .degree. C. Shell temperature,
.degree. C. Less than 110 Less than 110 Maximum material Less than
100 Less than 100 temperature, .degree. C. Sweep gas flow rate,
L/min 700 700 Material feed rate, kg/min 10 10
[0069] These data show the compacted material dries to a lower
moisture content that is about half that of the raw material under
identical drying conditions. The increased drying rate afforded by
compaction effectively doubles the capacity of the drying
equipment. If cost savings are more important than capacity, the
cost of the drying equipment will be half.
Example 3
[0070] An experiment was conducted to measure the relative drying
rates of South East Asia lignite held under warm, moist conditions
that would be expected in a covered stockpile located in
non-condensing, warm, humid tropical climates. Table 3 lists
results.
TABLE-US-00003 TABLE 3 Moisture Content of Lignite Stored in Warm,
Humid Atmosphere 30.degree. C., >60% Relative Humidity Raw
Lignite Compacted Moisture, Lignite Moisture, Time, hrs wt % wt % 0
40 43 48 36 37
[0071] These test data show that the compacted material lost 50%
more moisture than the raw material in 48 hours.
Example 4
[0072] Samples of lignite produced from North Dakota were processed
by the GTLE process to form briquettes of low moisture content.
These briquettes were then processed at high temperature and gas
conditions that are typical of those found in the reaction zone of
a solid feed gasifier.
[0073] The compressive strength of the briquettes and associated
coke produced from the gasification conditions were measured. Tests
were also conducted on briquettes formed from North Dakota lignite
by standard industrial processes typically used to form home
heating fuels and other industrial products. Test results are
listed in Table 4.
TABLE-US-00004 TABLE 4 Compressive Strength of Briquettes and Coke
Produced from LRCs Briquette Compressive Moisture, Strength,
lb/in.sup.2 Sample Description wt % Briquette Coke Process Method
North Dakota Lignite 16.3 1,405 711 Present Invention #5 A North
Dakota Lignite 10.9 944 676 Present Invention #5 B North Dakota
Lignite 12.0 1,211 898 Present Invention #6 A North Dakota Lignite
9.98 767 755 Present Invention #6 B North Dakota Lignite 8.24 735
506 Present Invention #6 C German Lignite 10.81 n/a 247 Industrial
Process
[0074] These results demonstrate the increase of compressive
strength from 247 lb/in.sup.2 for coke produced from briquettes
formed from industrial processes to over 500 lb/in.sup.2 for coke
produced from briquettes formed by the present invention.
[0075] The foregoing description of the present invention has been
presented for purposes of illustration and description.
Furthermore, the description is not intended to limit the invention
to the form disclosed herein. Consequently, variations and
modifications commensurate with the above teachings, and the skill
or knowledge of the relevant art, are within the scope of the
present invention. The embodiment described hereinabove is further
intended to explain the best mode known for practicing the
invention and to enable others skilled in the art to utilize the
invention in such, or other, embodiments and with various
modifications required by the particular applications or uses of
the present invention. It is intended that the appended claims be
construed to include alternative embodiments to the extent
permitted by the prior art.
* * * * *