Note: Descriptions are shown in the official language in which they were submitted.
CA 02788471 2012-08-27
APPARATUS AND METHOD FOR UPGRADING COAL
TECHNICAL FIELD
[0001] This document relates to an apparatus and method for upgrading
coal.
BACKGROUND
[0002] Coal is the world's most abundant and widely distributed fossil
fuel. The
current proven world coal reserve is estimated at 1000 billion tons, and
contains more energy
than that the combined known oil and natural gas reserves. Lignite, often
referred to as
brown coal, or Rosebud coal by Northern Pacific Railroad, is a soft brown fuel
with
characteristics that put it somewhere between coal and peat. It is considered
the lowest rank
of coal. Lignite is mined in Greece, Germany, Poland, Serbia, Russia, China,
the United
States, India, Australia and many other parts of Europe and it is used almost
exclusively as a
fuel for steam-electric power generation. Canada holds close to 10 billion
tons of proven coal
reserves. Efforts to upgrade low rank coal include thermal drying using waste
heat, steam
drying, hydrothermal drying, hydrothermal dewatering, and hydrothermal-
mechanical
drying. Upgraded low rank coal from thermal treatment can be slurried into a
coal water
slurry (CWS) for applications in coal power plant.
SUMMARY
[0003] A method of upgrading coal is disclosed, the method comprising:
subjecting
the coal to a hydrothermal dewatering process at a temperature and a pressure
above ambient
conditions to produce dewatered coal; removing ash tailings from the dewatered
coal to
produce reduced ash dewatered coal; and producing a coal water slurry with the
reduced ash
dewatered coal.
[0004] An apparatus for upgrading coal is also disclosed, the apparatus
comprising: a
hydrothermal dewatering reactor connected to receive coal and output dewatered
coal; an ash
separator connected to receive dewatered coal from the hydrothermal dewatering
reactor and
output reduced ash dewatered coal; a slurrifier connected to receive reduced
ash dewatered
coal from the ash separator and output a coal water slurry.
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[0005] In various embodiments, there may be included any one or more of the
following features: The coal water slurry is subject to gasification to
produce syngas. The
coal water slurry is reacted with oxygen in a gasifier and cooling the
produced syngas in a
quench cooler. The importance of slurry feed to the gasifier is the capability
of its feeding in
to the gasifier operating at higher pressures, which is believed to be not
possible to achieve
by using dry feed.
[0006] In other embodiments, the coal may comprise low rank lignite coals
such as
the ones from- Inner Mongolian coal in China or Boundary Dam coal in Canada.
The coal
may be subjected to hydrothermal dewatering at between 200 and 300 degrees
Celsius. The
coal in the thermal reactor may have particle size less than 1 mm, or less
than 0.5 mm,
preferably in 0.5 ¨ 0.1 mm range. The coal may be subjected to hydrothermal
dewatering at
between 2.0 and 8.0 MPa. The gas from the hydrothermal dewatering process may
be
removed and supplied to a heat exchanger for maximizing the energy efficiency.
[0007] Grinding of the dewatered coal to a particle size less than 0.5 mm
may be
carried out before the thermal reactor, or after the thermal reactor and
before removing ash
tailings. Removing ash from dewatered coal may be carried out in one or more
flotation
cells. The coal surface becomes hydrophobic after the hydrothermal treatment.
This
hydrophobic feature improves the flotation performance of the treated coal.
Coal being
treated in the thermal reactor preferably has a size less than 0.5 mm, which
may be achieved
by for example high intensity agitation within the thermal reactor or grinding
in a separate
grinder before introduction to the thermal reactor.
[0008] Water produced at one or more stages of the method may be supplied
into one
or more stages of the method that use water. A gasifier may be connected to
receive coal
water slurry from the slurrifier and the output from gasifier is syngas. A
quench cooler may
be connected to receive syngas from the gasifier. These and other aspects of
the device and
method are set out in the claims, which are incorporated here by reference.
BRIEF DESCRIPTION OF THE FIGURES
[0009] Embodiments will now be described with reference to the figures, in
which
like reference characters denote like elements, by way of example, and in
which:
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[0010] Fig. 1 is a line graph illustrating the energy consumption
comparison of
hydrothermal dewatering with flue gas drying.
[0011] Fig. 2 is a line graph illustrating the flotation recovery of Inner
Mongolia
(IM) coal after hydrothermal dewatering at different temperatures.
[0012] Fig. 3 is a bar graph illustrating the ash content comparison of
concentrate
and tailing for IM Coal after flotation following hydrothermal dewatering at
different
temperatures.
[0013] Fig. 4 is a line graph illustrating the flotation recovery of
Boundary Dam
(BD) coal after hydrothermal dewatering at different temperatures.
[0014] Fig. 5 is a bar graph illustrating the ash content comparison of
concentrate
and tailing for BD Coal after flotation following hydrothermal dewatering at
different
temperatures.
[0015] Fig. 6 is a bar graph illustrating the maximum solid content of IM
coal water
slurry (CWS), where the y-axis is the weight fraction, after hydrothermal
dewatering at
different temperatures, flotation and addition of a surfactant.
[0016] Fig. 7 is a bar graph illustrating the maximum solid content of BD
CWS,
where the y-axis is the weight fraction, after hydrothermal dewatering at
different
temperatures, flotation, and addition of a surfactant of PCE.
[0017] Fig. 8 is a flow diagram illustrating an embodiment of the
disclosed apparatus
and method.
DETAILED DESCRIPTION
[0018] Immaterial modifications may be made to the embodiments described
here
without departing from what is covered by the claims.
[0019] The kinds of coal, in increasing order of alteration, are lignite
(brown coal-
immature), sub-bituminous, bituminous, and anthracite (mature). Coal generally
starts off as
peat. After a considerable amount of time, heat, and burial pressure, it is
metamorphosed
from peat to lignite. Lignite is considered to be "immature" coal at this
stage of development
because it is still somewhat light in color and it remains soft. As time
passes, lignite
increases in maturity by becoming darker and harder and is then classified as
sub-bituminous
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coal. As this process of burial and alteration continues, more chemical and
physical changes
occur and the coal is eventually classified as bituminous. At this point the
coal is dark and
hard. Anthracite is the last of the classifications, and this terminology is
used when the coal
has reached ultimate maturation. Anthracite coal is very hard and shiny.
[0020] The degree of alteration (or metamorphism) that occurs as a coal
matures
from peat to anthracite is referred to as the "rank" of the coal. Low-rank
coals include lignite
and sub-bituminous coals. These coals have a lower energy content because they
have a low
carbon content. They are lighter (earthier) and have higher moisture levels.
As time, heat,
and burial pressure all increase, the rank does as well. High-rank coals,
including bituminous
and anthracite coals, contain more carbon than lower-rank coals which results
in a much
higher energy content. They have a more vitreous (shiny) appearance and lower
moisture
content then lower-rank coals.
[0021] Lignite is brownish-black in color and has a carbon content of
around 25-
35%, a high inherent moisture content sometimes as high as 66% (usually 30-60
wt%), and
an ash content ranging from 6% to 19% compared with 6% to 12% for bituminous
coal.
[0022] The energy content of lignite ranges from 10 - 20 MJ/kg (9-17
million BTU
per short ton) on a moist, mineral-matter-free basis. Because of this range of
energy content,
lignite is considered to be of low heating value. The energy content of
lignite consumed in
the United States averages 15 MJ/kg (13 million BTU/ton), on an as-received
basis (i.e.,
containing both inherent moisture and mineral matter). The energy content of
lignite
consumed in Victoria, Australia averages 8.4 MJ/kg (6.5 million BTU/ton). When
reacted
with quaternary amine, amine treated lignite (ATL) forms. ATL is used in
drilling mud to
reduce fluid loss.
[0023] Lignite has a high content of volatile matter (usually 35-50%)
which makes it
easier to convert into gas and liquid petroleum products than higher ranking
coals. However,
its high moisture content and susceptibility to spontaneous combustion may
cause problems
in transportation and storage. It is known that efficient processes that
remove latent moisture
locked within the structure of brown coal will increase the risk of
spontaneous combustion to
the same level as black coal, will transform the calorific value of brown coal
to a black coal
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equivalent fuel while significantly reducing the emissions profile of
'densified' brown coal to
a level similar to or better than most black coals.
[0024] It is estimated that for the same energy value of bituminous coal,
the CO2
emission of lignite combustion is 20% more. In addition, the susceptibility of
lignite to
spontaneous combustion is high and lignite is thus very dangerous during
transportation and
storage. Therefore, it is helpful to develop cost-effective processes to
upgrade low rank coal.
[0025] There is a recent interest in using low grade coals such as
lignite. However,
these coals have several limitations as mentioned above, such as higher
moisture content and
mineral matters, lower energy content due to their lower carbon content as
well as lower
calorific values. High water content is the most important factor in
discouraging the use of
lignite. Removing moisture efficiently from lignite is a significant issue in
the upgrading of
coals. Most of the water contained in lignite is inherent moisture, which is
very difficult to
separate and consumes a lot of energy for removal due to the low evaporation
rate of this
moisture. Furthermore, lignite that has merely been dried may re-adsorb
moisture from air
and turn itself into waterish coal, unless the pore structure and organic
functional groups in
lignite are changed or destroyed during the drying process.
[0026] Referring to Fig. 8, an exemplary method and apparatus 10 for
upgrading coal
is disclosed. This upgrading process includes several stages. First, coal is
subjected to a
hydrothermal dewatering process, for example in reactor 11, at a temperature
and a pressure
above ambient conditions to produce dewatered coal. Ash tailings are removed
from the
dewatered coal, for example in flotation cell 14, to produce reduced ash
dewatered coal. This
processing results in a product with lower ash and moisture content. Finally,
a coal water
slurry is produced, for example in slurrifier 16, with the reduced ash
dewatered coal.
[0027] Low rank coal, for example lignite may be used, although other
forms of coal
may be used. In the tests performed, lignite (particle size distribution ( 50
mm with moisture
content 20-70%) added to reactor 11 via line 28 was hydrothermally dewatered
at between
200-300 C, and between 2.0-8.0MPa. Steam was also added to reactor 11 from
line 30. Coal
particles were discharged from the bottom of the reactor 11 through line 36
and gas was
released through lines 32 and 34 from the top of reactor 11. Gas removed from
reactor 11
may be supplied to a heat exchanger 26 for energy recovery. Fluid such as
water may be
CA 02788471 2012-08-27
passed through the heat exchanger 26 from line 44 and removed via line 46 as
heated water.
After passing through heat exchanger 26, the fluid may be passed into a gas-
water separator
24 from line 48 where off gas is removed via line 58 and water is removed via
line 62 and
passed into one or more treatment or recycling stage, for example by passage
into a
circulating reservoir 20 in the example shown.
[0028] Common lignite dewatering processes includes flue gas drying, steam
drying,
hydrothermal drying and hydrothermal-mechanical drying. The general flue gas
drier used is
the drum roller drier for lignite drying. In addition, components such as a
rotary dryer,
fluidized bed dryer, microwave dryer, screw conveyor dryer, and integrated
drying and
solvent displacement have also been used. However, the dry coal product
obtained from
these methods may not be used to make coal water slurry as such methods will
result in
dewatered coal that will reabsorb moisture.
[0029] Hydrothermal dewatering treatment is carried out in a high
temperature and
high pressure (such as 200-300 degrees Celsius, 2.0-8.0MPa for example,
although other
suitable conditions may be used such as higher temperatures and pressures)
water system to
remove water in lignite as a liquid phase. During this process little or no
evaporation occurs
and the energy consumption is thus much lower than that of the ordinary water
evaporation
of drying method, due to the large heat of evaporation of water. Fig. 1 is a
line graph
illustrating the energy consumption comparison of hydrothermal dewatering with
flue gas
drying; it can be seen from Fig. 1 that the energy consumption in the
hydrothermal
dewatering treatment disclosed here is roughly 1/4 of that used in flue gas
drying.
[0030] In a hydrothermal dewatering process, many of the carboxyl and
hydroxyl
groups in the coal may be removed as H20, CO or CO2, thus improving the
hydrophobicity
of the coal and favoring flotation for ash separation. Fig. 2 is a line graph
illustrating the
flotation recovery of Inner Mongolia (IM) coal after hydrothermal dewatering
at different
temperatures ranging from 180-300 C, and with initial pressure 1 bar and
going up to a final
autogenous pressure of 40-80 bar depending on the process temperature. Fig. 3
is a bar graph
illustrating the ash content comparison of concentrate and tailing for IM Coal
after flotation
following hydrothermal dewatering at different temperatures and pressures
(about 40-80
bar).
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[0031] Fig. 4 is a line graph illustrating the flotation recovery of
Boundary Dam
(BD) coal after hydrothermal dewatering at different temperatures ranging
from180-300 C,
initial pressure 1 bar and going up to a final autogenous pressure of 40-80
bar depending on
the process temperature. Fig. 5 is a bar graph illustrating the ash content
comparison of
concentrate and tailing for BD Coal after flotation following hydrothermal
dewatering at
different temperatures.
[0032] From Figs. 2 and 4, it can be seen that the treated IM and BD coals
have
higher flotation rate and recovery ratio. Figs. 3 and 5 show the difference in
the ash content
of concentrate and tailing for coals treated in the disclosed methods and
apparatuses, thus
also indicating the better floatability for hydrothermal treated coals.
[0033] Before or after reactor 11, dewatered coal may be ground in grinder
12
(which may be placed after the reactor as shown, or before, or both), for
example in one
embodiment to an average particle size of less than 1 mm, or in another
embodiment to an
average particle size of less than 0.5 mm, for example 0.5 mm to 0.1 mm, and
then passed
via line 38 to flotation cell 14. Particle size less than 0.5 mm increases the
surface area and
so improves the thermal treatment. Particle size less than 0.5 mm also aids in
flotation, so if
the particles are not already less than 0.5 mm average particle size, then
grinder 12 after the
reactor 11 may be used. In another embodiment the dewatered coal may be ground
within
the thermal reactor 11 by high intensity agitation. In cell 14 ash tailings
separate out from
concentrate, with the ash tailings being removed at or near the base of cell
14 via line 42, and
coal concentrate being skimmed or removed at or near a concentrate level (not
shown) in cell
14 via line 50. Other suitable ash removal stages may be used, for example
centrifugation.
Removal of ash makes the process suitable for both low and high ash containing
low grade
coals.
[0034] The reduced ash dewatered coal is then passed into slurrifier 16.
In this state,
the reduced ash dewatered coal may contain some water, although additional
water may then
be added via line 54. In addition, surfactant may be added to complete the
slurry as desired.
In this way, the concentrate may be collected to prepare a high solid loading
slurry (in the
tests performed, solids content were >58 wt.%).
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[0035] Coal water slurry (CWS) formation in slurrifier 16 is an efficient
technology
for clean coal utilization. Coal-water slurry fuel is a fuel which consists of
fine coal particles
suspended in water for example using one or more surfactants. The presence of
water in
CWS reduces harmful emissions into the atmosphere, makes the coal explosion-
proof, makes
use of a coal equivalent to use as liquid fuel (e.g. heating oil), and gives
other benefits. CWS
generally has 55-70% of fine dispersed coal particles and 30-45% of water,
although other
ranges may be used. CWS may be used as a fuel replacement, for example as a
diesel fuel
replacement. CWS may be used in place of oil and gas in any size of heating
and power
station. CWS is also suitable for existing gas, oil, and coal boilers. During
the last 30 years
the US Department of Energy has been researching the use of coal water fuels
in boilers, gas
turbines and diesel engines. When used in low speed diesels CWS has a thermal
efficiency
rating that rivals combined cycle gas turbines that burn natural gas as their
primary fuel. It
has been suggested that slightly modified modular diesel engine power plants
that burn CWS
are economically competitive with natural gas fired peaking electric plants in
the 10 MWe to
100 MWe range of power supply.
[0036] Low rank coal is not considered to be suitable for CWS preparation
due to its
high inherent moisture content. According to the literature, the maximum solid
loading for
lignite CWS is only 50% (1200 mPa.s, 1000. However, hydrothermally dewatered
coals
have the advantage of having less inherent moisture content due at least
partially to the fact
that during pyrolysis of the coal in reactor 11, tar forms and heals the pore
of the coal
particles thus preventing reabsorption of moisture. As shown in Fig. 6 and 7,
the treated IM
coal and BD coal at 300 C can prepare solid loading up to 58% and 62%. Fig. 6
is a bar
graph illustrating the maximum solid content of IM coal water slurry (CWS),
where the y-
axis is the weight fraction, after hydrothermal dewatering at different
temperatures, flotation
and addition of a surfactant. 1% surfactant was used for CWS preparation and
the surfactant
is a polymeric material with commercial name of Melflux 2651F (from BASF
Germany).
The chemical name is polycarboxylate ether (PCE). Fig. 7 is a bar graph
illustrating the
maximum solid content of BD CWS, where the y-axis is the weight fraction,
after
hydrothermal dewatering at different temperatures, flotation, and addition of
a surfactant of
PCE.
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[0037] Other surfactants that can be used for this purpose include:
(i) Sodium Polystyrene Sulfonate
(ii) Sodium Naphthalene Sulfonane Formaldehyde Condensate
(iii) alkyl mononaphthalene sulfonic acid and its sodium and ammonium salts
(iv) 2-ethylhexyl poly-phosphoric ester acid anhydride and its potassium
salt.
[0038] The coal water slurry formed in slurrifier 16 may then be passed via
line 52
into gasifier 18 where the CWS is subjected to gasification to produce syngas.
Oxygen may
be added via line 60, and slag removed via line 66. The high temperature gas
removed from
gasifier 18 via line 64 may be cooled using quench cooler 22 with cooled water
supplied in
the tower via line 74 and heated water discharged from the tower via line 68.
The heated
water may be reused, for example in the hydrothermal treatment or pumped into
the
circulating reservoir 20 for storage. Ash water removed via line 72 may be
treated or
disposed of, and syngas removed via line 70 may be used as desired.
[0039] Syngas (synthesis gas) is the name given to a gas mixture that
contains
varying amounts of carbon monoxide and hydrogen. The name comes from the use
of syngas
as intermediates in creating synthetic natural gas and for producing ammonia
or methanol.
Syngas is also used as an intermediate in producing synthetic petroleum for
use as a fuel or
lubricant via the Fischer - Tropsch process and previously the Mobil methanol
to gasoline
process. Syngas consists primarily of hydrogen, carbon monoxide, and very
often some
carbon dioxide, and has less than half the energy density of natural gas.
[0040] Gasification is a process that converts organic or fossil based
carbonaceous
materials into carbon monoxide, hydrogen and carbon dioxide. This is achieved
by reacting
the material at high temperatures (for example >700 C), without combustion,
with a
controlled amount of one or more of oxygen and steam. The resulting gas
mixture is called
syngas (from synthesis gas or synthetic gas) or producer gas and is itself a
fuel.
[0041] The advantage of gasification is that using syngas is potentially
more efficient
than direct combustion of the original fuel because the syngas may be
combusted at higher
temperatures or even in fuel cells, so that the thermodynamic upper limit to
the efficiency
defined by Carnot's rule is higher or not applicable. The high-temperature
process refines out
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corrosive ash elements (slag, line 66) such as chloride and potassium,
allowing clean gas
production from otherwise problematic fuels. Gasification of fossil fuels is
currently widely
used on industrial scales to generate electricity.
[0042] In general, water produced at one or more stages of the method or
apparatus
may be re-supplied into one or more stages of the method that use water.
Recycling of
water and heat increases the efficiency of the process and conserves material.
Thus, reservoir
may be connected to one or more of reactor 11, quench cooler 22, heat
exchanger 26,
flotation cell 14 (via line 56), and other components as shown. Water in
reservoir 20 may
also be sent via line to waste water treatment.
[0043] Each stage of the process may be carried out with one or more
undisclosed
additional components as desired or required. For example, more than one
flotation cell 14
may be used to de-ash the dewatered coal. Also, other components may be used
such as a
screen prior to the reactor stage 11, or one or more pumps on one or more of
the lines in Fig.
8. All % concentrations herein are weight percentages unless indicated
otherwise. Although
the methods and apparatuses are disclosed in connection with syngas
production, the CWS
produced may be used in other fashions as desired. Each line used in Fig. 8
denotes a transfer
step that may be carried out using suitable infrastructure, such as a pipe or
conveyor belt, as
desired.
[0044] In the claims, the word "comprising" is used in its inclusive sense
and does
not exclude other elements being present. The indefinite articles "a" and "an"
before a claim
feature do not exclude more than one of the feature being present. Each one of
the individual
features described here may be used in one or more embodiments and is not, by
virtue only
of being described here, to be construed as essential to all embodiments as
defined by the
claims.
