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US006664302B2
(12) United States Patent
French et al.
(10) Patent No.:
(45) Date of Patent:
US 6,664,302 B2
Dec. 16,2003
(73) Assignee: GTL Energy, Wellington, CO (US)
(54) METHOD OF FORMING A FEED FOR COAL
GASIFICATION
(75) Inventors: Robert French, Wellington, CO (US);
Robert A. Reeves, Arvada, CO (US);
Charles B. Benham, Littleton, CO
(US)
( *) Notice:
7/1980 Kromrey 44/6
5/1982 Burns 44/6
5/1982 Draper et al. 44/24
6/1983 Bergmann et al. 44/6
6/1983 Pike 44/6
11/1983 Yaghmaie et al. 44/51
6/1987 Mark 106/283
11/1988 Hueschen 44/51
2/1990 Najjar et al. 44/51
7/1991 Kennepohl et al. 44/502
12/1991 Koppelman 44/621
6/1994 Child 44/608
10/1998 Dean 34/340
5/2000 Benham et al. 208/950
11/2001 Waycuilis 565/314
4,214,875 A
4,331,445 A
4,331,446 A
4,389,216 A
4,389,218 A
4,417,902 A
4,670,058 A
4,783,198 A
4,904,277 A
5,033,230 A
5,071,447 A
5,324,336 A
5,815,946 A
6,068,760 A
6,313,361 B1
Primary Examiner-J. Parsa
(74) Attorney, Agent, or Firm---8heridan Ross Pc.
Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.c. 154(b) by 62 days.
Appl. No.: 10/121,972
Filed: Apr. 12, 2002
Prior Publication Data
(21)
(22)
(65)
The invention provides a method by which low-rank coal
may be processed to provide a high-energy feedstock for
coal gasification and synthesis gas production. Preliminary
coal, preparation, which may include washing and drying, is
followed by wax-impregnation to produce a high-energy,
low-moisture, stable feedstock. The wax is preferably
obtained from an on-site Fischer-Tropsch reactor that also
produces diesel fuel and naptha.
US 2003/0192235 A1 Oct. 16, 2003
(51) Int. CI? C07C 27/00; C07C 1/02;
C10J 3/00; F02G 3/00
(52) U.S. Cl. 518/700; 252/373; 48/210;
60/39.02
(58) Field of Search 518/700; 252/373;
48/210; 60/39.02
(56) References Cited
U.S. PATENT DOCUMENTS
(57) ABSTRACT
3,996,026 A 12/1976 Cole 48/197 24 Claims, 3 Drawing Sheets
14
Air
Air Separation
Unit
Nitrogen
16
Ash
Steam
Diesel Fuel
Electricity
19
22
Fischer-Tropsch
Reactor,
Hydrocracker
and Product
Recovery Plant
17
f-----4-----~ Carbon Dioxide
and Sulfur
Synthesis Gas
Treatment and
Recovery Unit
Gasifier
12
Water
Raw Lowrank
Coal
Wax
. __ .. - - - - - - -- - __ - -- - __ - - __ - - - -- - - ._.- --:1..-----,/-+ Naphtha
II
d•
'JJ.
•
~
~.....
~
Nitrogen =.....
Ash ~
~
~
'""'" Carbon Dioxide ~~
and Sulfur NCC~
Electricity
22 'JJ. =- ~
~....
'""'" 0....,
Diesel Fuel ~
19
Steam
I
20 e Naphtha \Jl
-..CJ\
II CJ\
CJ\
~
~
Q
N
~N
Fischer-Tropsch
Reactor,
Hydrocracker
and Product
Recovery Plant
Integrated Gas
Combined Cycle I I ~
Unit
17
5
6
16
18
Wax
9
Synthesis Gas
Treatment and
Recovery Unit
Figure 1
15
4
14
Gasifier
10
3
Feedstock
Preparation
System
2
Air Separation
Unit
Water
7
12
Air
Raw LowrankCoal
~
8
u.s. Patent Dec. 16,2003 Sheet 2 of 3
Raw Low-rank Coal (7)
US 6,664,302 B2
54 55
Crusher and
Water Washer Effluent
50
56
57
5
Open Storage Pile ~ Water Vapor
58
52
59
,.----ll..----.., 'v)
Thennal Dryer --4
60
Water Vapor
Fischer-Tropsch Wax (9)
5
61
62
Pug Mill
Briquetter
-53
Solid Particulate Feedstock
Figure 2
u.s. Patent Dec. 16,2003 Sheet 3 of 3 US 6,664,302 B2
8
Raw Low-rank Coal (7)
Crusher
Figure 3
US 6,664,302 B2
2
SUMMARY OF THE INVENTION
The present invention is a method of beneficiating lowrank
coal to produce a relatively high-energy, cohesive,
low-moisture, stable feedstock for coal gasification. One
embodiment of the invention comprises contacting partially
or completely dried low-rank coal with wax at defined
temperatures and pressures, thereby forming a waximpregnated
coal. The wax-impregnated coal may be either
slurried or formed into briquettes for coal gasification.
Gasification produces synthesis gas that can be used to
co-produce electricity and liquified Fischer-Tropsch
products, including diesel fuel, naptha, and wax. A fraction
of the wax can then be recycled to the coal preparation
section to aid in materials handling, agglomeration, reducing
moisture levels, and increasing the specific energy of the
feedstock operation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic of a coal gasification system
operated in conjunction with a Fischer-Tropsch reactor and
an IGCC (Integrated Gasification/Combined-Cycle) gas turbine
unit.
slurry to decrease the viscosity, thereby lowering the water
content necessary to maintain the pumpability. This effectively
increases the concentration of the coal in the slurry,
thereby raising the Btu value. Unfortunately, the addition of
5 these chemicals is often expensive and the chemicals themselves
can further decrease the efficiency of the gasification
process.
U.S. Pat. No. 3,996,026 teaches a method of using organic
liquids as additives to the coal-water slurry, which can then
10 be successfully pumped from the source of the slurry to the
gasifier. Immediately prior to entering the gasifier, the slurry
is fed through a separator where the organic liquids are
removed and the coal-water mixture is injected into the
gasification zone. In this method, the coal is ground and
mixed with water to form a slurry having a water content
15 between 35 and 55% by weight. An organic liquid such as
kerosene, hexane, or light vacuum gas oil is then added to
the coal-water slurry to improve the pumpability. These
chemicals have the added advantage of increasing the Btu
value of the lower-rank coals. Unfortunately, these organic
20 liquids are quite valuable and must be recovered, to the
extent possible, before the slurry enters the gasifier. For this
reason, the slurry is pumped through the machinery to a
modified gasifier having a distillation apparatus that recovers
the expensive organic chemicals from the slurry before
25 the slurry is added to the gasifier. The organic liquids are
removed from the slurry as a super-critical liquid or dense
gas and recycled to once again act as an aid to the pumping
of the coal-water mixture. The method is limited to organic
liquids ranging from four to twenty carbons in length so that
30 they can be successfully removed in the separator before the
coal is injected into the gasifier. The method suffers from the
greatly increased costs of running the distillation apparatus
to recover the expensive organic liquid from the coal-water
slurry, as well as the costs associated with continual losses
of the expensive organics resulting from incomplete removal
35 from the coal.
Therefore, there still exists a need for an improved
method of preparing a feedstock for a coal gasifier that
allows the use of lower-rank coals in an economically
feasible manner. The method should result in a feedstock
40 having a sufficient Btu value and a restricted water content
to ensure economically efficient conversion to a synthesis
gas. Preferably, the feedstock should be capable of being
conveyed to the gasifier by a means that allows control over
the amount and rate of solid fuel entering the gas generator
45 while avoiding potential backflash.
FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
1
METHOD OF FORMING A FEED FOR COAL
GASIFICATION
The invention relates to improved methods of forming a
feedstock, comprising a wax-impregnated coal, for coal
gasification.
The gasification of solid fuels such as coal is well known.
Several methods have been proposed for feeding the coal
into the gasifier. In one method, the coal is ground to a fine
powder and fed to the gas generator as a suspension in steam
or a free oxygen-containing gas. This method is unsatisfactory
as it is difficult to control the amount and rate of the coal
fed to the gas generator and, in the case of a free oxygencontaining
gas, care must be taken to maintain the velocity
of the suspension above the rate of flame propagation to
avoid a dangerous and damaging backflash.
Newer methods have been developed to overcome the
drawbacks of the dry, ground coal feedstock. One method is
the production of a coal-water slurry in which the coal is
ground to a particle size, mixed in water or organic liquids,
and injected into the gasifier. The coal is ground to a fine
particle size to ensure that almost complete conversion of
carbon to oxides takes place during the residence time in the
gasification zone of the gasifier. To properly feed such a
slurry into the gasification zone, the slurry must be conveyed
from the point at which it is generated to the gasifier. The
slurry must not be too viscous to be pumped from its starting
point to its destination but, simultaneously, cannot be diluted
to a level that will cause incomplete or inefficient conversion
to gas in a gasifier. The total water content of the slurry must
therefore be kept, preferably, close to 30-40%.
This restriction on the water content of a coal-water slurry
is readily attained by using high-rank solid coal sources such
as anthracite and bituminous coal. However, many coal
sources contain varying amounts of inherent water, and in
many instances the water content may be as high as 30
weight percent; it may be higher in the case of lower rank
coals such as sub-bituminous coal, lignite, and brown coal.
The water is present as surface water on the face of the coal,
as inherent water found in the smaller pores of the coal, and
as chemically bound water within the carbon lattice. This
higher water content has made these fuel sources largely
useless for the production of a slurry feedstock for a gasifier.
Different approaches have been taken to render these
low-rank coals useful as a feedstock for the coal gasification 50
process. For example, the coal may be dried at an elevated
temperature. The drying process successfully removes surface
and inherent water but is typically incomplete, or too
energy intensive, to economically remove chemically bound
water in the low-rank coals. Moreover, such dried coals, 55
when formed into a slurry, tend to take up a significant
amount of water from the slurry. Mixing of lower-rank coals
with a smaller percentage of finely ground, higher-rank coals
has also been used to make a less costly fuel although the
improvement in cost is minor after providing the means 60
necessary to precisely grind and mix the higher- and lowerrank
coals.
Other methods have focused on various chemical treatments
to decrease the water content of the coal slurry and
thereby boost the (British thermal unit) value of the slurry. 65
Chemicals such as surfactants, detergents, suspension
stabilizers, and amines have been used as additives to the
US 6,664,302 B2
3
FIG. 2 shows a schematic of a method of preparing a
wax-impregnated coal feedstock for a fixed-bed gasifier.
FIG. 3 shows a schematic of another method of preparing
a wax-impregnated coal feedstock for a slurry-fed gasifier.
DETAILED DESCRIPTION OF IRE
INVENTION
There is an enormous amount of coal available as an
energy supply. Indeed, it is estimated that world wide there
is more energy available in coal than in petroleum, natural
gas, and oil shale combined. Coal is a useful energy source
for gasification. High-grade coals have historically been
preferred because of their high energy content, which makes
the gasification process economically attractive. Considerable
lower-grade coal reserves exist, which have a higher
water content and lower energy content. Therefore, one
embodiment of the present invention provides a new method
of producing a feedstock for coal gasification in which
low-rank coals are mixed with a wax that has been heated to
a temperature above its melting temperature to form a
wax-impregnated feedstock. The wax-impregnated coal is
then introduced as briquettes to a fixed-bed gasifier or as a
slurry to a conventional gasifier. This method of preparing a
feedstock has the advantage of increasing the energy content
of the coal prior to introduction into the gasifier. The energy
content is increased two ways. First, the wax reduces the
amount of water necessary to produce a pumpable slurry.
Second, the relatively high energy content of the wax
(typically greater than 19,000 Btullb) increases the overall
energy content of the feedstock.
Low-rank coals generally have an energy content of less
than 7,000 Btullb, making them unattractive as a feedstock
for conversion to synthesis gas via gasification using conventional
technology. While the present invention is useful
with any type of coal, it is particularly beneficial with
low-rank coals such as sub-bituminous, lignite, and brown
coal.
One aspect of the present invention involves mixing coal
with a wax. Waxes are relatively heavy paraffinic hydrocarbon
compounds, typically having a carbon number in excess
of twenty. These waxes exist as solids at ambient temperatures.
Preferred waxes for the present invention include
waxes produced within the mixture of paraffinic hydrocarbon
compounds produced in the conversion of hydrogen and
carbon monoxide on a powdered catalyst to liquid hydrocarbons
(the Fischer-Tropsch reaction described below). The
wax formed by the Fischer-Tropsch reactor is a
polymethylene-type wax formed by the polymerization of
carbon monoxide. The Fischer-Tropsch wax typically has a
melting point ranging from 45° C. to 104° C. The hydrocarbons
drawn from a Fischer-Tropsch reactor must be
maintained above the wax melting temperature to prevent
solidification of the wax, which results in a heavy solid that
fouls and plugs the separation and transport machinery. This
is an energy-intensive solution, and therefore it is imperative
to use the wax near its source to prevent its transport from
becoming prohibitively expensive. Depending on the catalyst
and the Fischer-Tropsch reaction conditions, the liquid
hydrocarbon phase drawn from a Fischer-Tropsch reactor
typically has a composition resembling a highly paraffinic
crude oil containing, for example, ranges of 10 to 40%
naphtha, 20 to 40% distillate, and 20 to 60% wax compounds
by volume. The naphtha recovered in this process
may be mixed with or separated from the wax.
The temperature required for contacting or mixing the
wax with the coal is a temperature sufficient to maintain the
4
wax in a liquid state. The temperature should be at least
about 5° C. greater than the temperature at which a significant
portion of the heavier paraffinic wax compounds
solidify. For the preferred wax generated in the Fischer-
5 Tropsch reaction, this temperature is above about 100° C.
Preferably, the temperature is maintained above about about
110° c., and more preferably the temperature is maintained
between about 120° C. and about 140° C.
The wax-impregnated coal feedstock is produced by
10 contacting the liquid wax with the coal. Preferably, the coal
is thoroughly mixed with the melted wax so that individual
coal particles or coal fines are coated with the wax. The wax
becomes absorbed in pores of the coal through hydrophobic
interactions, thereby displacing any water present. In addi-
15 tion to boosting the energy content of the coal, the wax
prevents re-absorption of water by the coal after drying.
Excess water in the coal feedstock, beyond the water necessary
to form a pumpable slurry, is deleterious because of
the high heat of vaporization of water, which is about 1,000
20 Btullb. The presence of excess water may cause incomplete
or inefficient conversion of the feedstock to gas in the
gasifier. The wax has a heat of vaporization of about 150
Btullb and therefore does not significantly decrease the
efficiency of conversion of the feedstock in the gasifier.
25 Thus, the incorporation of wax into the coal to form a
feedstock acts to boost the energy content of the feedstock
while excluding excess water that can decrease the efficiency
of the gasification process.
The amount of wax to be mixed with the coal is deter-
30 mined in part by the composition of the coal used in forming
the feedstock. For example, lower-rank, high-water content
coals may need to be combined with higher amounts of wax
to sufficiently boost the energy content of the wax to a
suitable level for use in the gasification process. Indeed, as
35 discussed below, some low-rank coals may have a water
content requiring the use of drying methods to remove some
water prior to combining with the wax. Typically, wax is
combined with the coal in a wax-to-coal ratio (by weight) of
about 1:7 to about 1:13. Preferably, the wax is combined
40 with the coal in a wax-to-coal ratio of about 1:8 to about
1:12, and more preferably is combined in a wax-to-coal ratio
of about 1:9 to about 1: II.
The wax-impregnated coal feedstock can be produced by
a number of suitable mixing methods known in the art,
45 including those described below. The wax and coal can be
combined by physical admixture. For example, the wax can
be milled with the coal in a pug mill to blend the coal and
the wax to the desired degree or consistency. For this
operation, the wax and coal are combined and gently mixed
50 or kneaded at the desired ratio in a pug mill maintained at a
temperature above the melting point of the wax. Typically
the temperature is maintained in a range between about 5° C.
and about 30° C. greater than the melting point of the wax.
The wax and coal may also be combined by briquetting
55 the coal in the presence of a wax under increased temperature
and pressure. For example, in suitable briquetting
operations, the mixed coal and wax are subjected to temperatures
between about 5° C. and about 30° C. greater than
the melting point of the wax, more preferably between about
60 8° C. and about 20° C. greater than the melting point of the
wax, and most preferably between about 10° C. and about
15° C. greater than the melting point of the wax. Mixtures
of coal and wax can also be subjected to pressures between
about 2,000 psi and about 14,000 psi, more preferably
65 between about 5,000 psi and about 12,000 psi, and most
preferably between about 8,000 psi and about 11,000 psi for
briquetting.
5
US 6,664,302 B2
6
The wax may also be combined with the coal in an
autoclave. This process serves to maintain the wax in a
liquid form and allows for a partial purification of the
low-rank or other coals. The coal can be autoclaved initially
to drive off excess water, carbon dioxide, sulfur gases, or
other impurities prior to mixing with the wax in the autoclave
or treated initially by other means. Additionally, the
coal may be partially purified and mixed with the wax in the
autoclave in a single step. In suitable autoclaving operations,
the mixed coal and wax are subjected to temperatures
between about 90° C. and about 310° c., more preferably
between about 175° C. and about 260° c., and most preferably
between about 200° C. and about 230° C. The coal
and wax mixtures are also subjected to pressures between
about 300 psi and about 1,500 psi, and more preferably
between about 400 psi and about 700 psi. Autoclaving has
the advantage of producing a cleaner coal feedstock for
gasification and is therefore desirable as a preliminary step
when a low-rank coal source requiring purification is used.
In various embodiments of the present invention, the coal
is dried before it is mixed with the wax. The drying
procedures can be either active or passive. For example, if
the drying is conducted in a hot, dry environment, passive
drying by exposure to the environmental conditions may be
sufficient. Alternatively, active drying may include subjecting
the coal to heat, vacuum, or other dehumidifying conditions.
The drying is typically conducted before the coal is
contacted with a wax. Typically, the coal is dried to a water
content of less than about 15 weight percent, preferably less
than about 10 weight percent, and more preferably less than
about 5 weight percent. This drying process is usually
sufficient, for example, for low-rank coals that have an
undesirably high water content. In instances when drying is
used, the coal can be air-dried prior to impregnation with the
wax, by other means such as autoclaving, briquetting, or pug
milling. Drying in this manner allows for the use of coals
having an initial water content in excess of 40%. Coals
particularly well suited for this embodiment of the present
invention have high water contents, including up to about
60%.
The synthesis gas generated by the gasification of the
wax-impregnated coal can be used to generate electricity
and/or be directed to a Fischer-Tropsch reactor to generate
diesel fuels while recovering wax and/or naphtha for recycling
to generate more wax-impregnated coal feedstock. The
synthesis gas exiting the gasification operation may be
cleaned first to condense water and then to remove sulfur
and carbon dioxide contaminants from the gas stream from
the gasifier. Such cleaning and water removal steps are
conventional and well known in the art. As noted above, in
one embodiment of the present invention, a portion of the
cleaned gas from the gasifier is directed to a combustion
turbine-generator set to produce electricity. Additionally, tail
gases exiting the Fischer-Tropsch reactor can be directed to
the combustion turbine-generator set to produce electricity
after condensing out the liquid hydrocarbons and water. In
some instances, it is also desirable to use the naphtha
produced in the Fischer-Tropsch reactor as fuel for the gas
turbine. The present invention involves any process suitable
for combustion of a synthesis gas to produce electricity and
Fischer-Tropsch liquids. For example, a preferred process
for electricity generation is an IGCC process. In the IGCC
technology, the hot combustion gases exiting the gas turbine
are fed to a boiler to generate steam, which is fed to a steam
turbine-generator set to produce additional electrical power.
IGCC technology utilizing waste heat from the Brayton
cycle to provide energy to a Rankine cycle is well known
and provides efficient, clean, and low-cost energy. Additional
energy in the form of steam can be obtained from
cooling the gases exiting the gasifier, from cooling the gases
exiting the Fischer-Tropsch reactor, and from removing the
5 heat generated within the Fischer-Tropsch reactor to maintain
a constant temperature. This steam can be used within
the plant as steam or in a steam turbine for power generation.
In another embodiment of the present invention, the gas
from the gasifier is directed to a Fischer-Tropsch reactor to
10 produce diesel fuel, naphtha, and wax. In this process, the
synthesis gas is reacted in a slurry reactor on a powdered
catalyst to form liquid hydrocarbons and waxes. The
Fischer-Tropsch process is described in U.S. Pat. Nos.
5,324,335; 5,500,449; 5,504,118; 5,506,272; 5,543,437;
15 5,620,670; 5,621,155; 5,645,613; 5,763,716; and 6,068,760,
which are incorporated herein by reference. The product
stream from the reactor contains naphtha, diesel fuel, and
waxes. The slurry is maintained in the reactor at a constant
level by continuously or intermittently removing wax from
20 the reactor while separating the catalyst from the removed
wax and returning the catalyst to the reactor. This wax can
then be collected and used as a wax source for the formation
of a wax-impregnated coal feedstock for gasification. The
diesel fuel product can be collected and sold as an end
25 product.
By monitoring the production of the synthesis gas and the
need for electricity, diesel fuel, and additional wax, the
synthesis gas can be diverted to electricity production, to the
Fischer-Tropsch process reaction, or split between the two
30 processes. For example, in some areas of the world the price
of electricity fluctuates significantly between peak and offpeak
times. In view of such price fluctuations, the present
invention includes a method to regulate the proportion of
synthesis gas that is dedicated to electricity generation and
35 to the Fischer-Tropsch reaction. The price of electricity is
monitored and the synthesis gas is controlled to divert more
gas into electricity generation when the price of electricity is
sufficiently high to make the combustion of the synthesis gas
economically more favorable than the production of diesel
40 fuel, naptha, and wax. Alternatively, when the price of
electricity drops below this level, the synthesis gas can be
diverted more to the production of diesel fuel, wax, and
naphtha. As the price of electricity fluctuates between these
points, the synthesis gas can be split between the electricity
45 generation and the Fischer-Tropsch processes. In this way,
the price of electricity can be used to determine how to split
the use of the synthesis gas. It should be recognized,
however, that even at periods of peak electricity demand,
there is a need to maintain some production of wax from the
50 Fischer-Tropsch process for mixing with the coal. Similarly,
at times of off-peak electricity demand, it may be beneficial
to maintain some production of electricity to maintain
continuous operation of the electricity generation facility.
It is also possible to use other products generated in the
55 Fischer-Tropsch process to generate electricity. For
example, the naphtha and diesel fuel products can be combusted
to produce electricity in addition to the electricity
generated by the combustion of the synthesis gas.
Additionally, the Fischer-Tropsch reaction gives rise to a tail
60 gas that can be captured and diverted to electricity generation.
The tail gas comprises nitrogen, carbon monoxide,
hydrogen, water vapor, and hydrocarbons. This tail gas can
also be diverted to the production of electricity by burning
the hydrocarbons. Optionally, the tail gas can be purified to
65 remove carbon dioxide, nitrogen, or other components
present from the hydrocarbons prior to diverting the tail gas
to electricity generation, in an IGCC unit for example. This
US 6,664,302 B2
7 8
A feedstock preparation system used to provide waximpregnated
solid particulate feedstock for a fixed-bed gasifier
is shown in FIG. 2. The principle unit operations are a
crusher and washer (50 open storage pile (51) thermal dryer
5 (52), pug mill (53), and briquetter (54).
Raw low-rank coal (7) is crushed to the desired top size
and washed by the crusher and washer (50) and stored for a
pre-determined time in open storage pile (51). Effluent
containing soluble ash (55) is discarded. The crushed and
10 washed coal (56) is partially dried while in storage, releasing
water vapor (57) that reports to the atmosphere. Partially
dried raw coal (58) is dried to a lower moisture level by the
thermal dryer (52), releasing water vapor (59). Hot, dried
product (60) is mixed with wax (9) produced by the FischerTropsch
reactor and product recovery plant (reference FIG.
15 1, item 5). The temperature of the thermal dryer product and
wax is maintained at a predetermined level to melt the wax
to form a wax-impregnated product (61).
The wax-impregnated product (61) is compressed by the
briquetter (54) to form a stable, durable, particulate feed20
stock (62) for a fixed-bed gasifier.
A feedstock preparation system used to provide waximpregnated
slurry feedstock for a conventional gasifier is
shown in FIG. 3. The principle unit operations are a crusher
(1), slurry preparation unit (2), autoclave (3), and thickener
25 (4).
Raw low-rank coal (7) is crushed to the desired top size
by the crusher (80). The crushed coal (84) feeds the slurry
preparation unit (81). Wax (86) produced by the FischerTropsch
reactor and product recovery plant (reference FIG.
30 1, item 5) and clarified process water (90) feed the slurry
preparation unit and are mixed with the crushed coal (84).
The wax, coal, and water slurry (85) is pumped under
pressure to an autoclave (82). The autoclave conditions
maintain the slurry at the desired temperature and pressure
35 for sufficient time to release a portion of the inherent
moisture, carbon dioxide, and sulfur-containing compounds.
The product (87) feeds a thickener (83) to separate a portion
of the water from the solids. The thickener is operated to
produce clarified process water (90) and a high-solids con-
40 centration slurry feedstock (89) for a conventional slurry-fed
gasifier. A water balance is maintained by releasing or
adding water (88) to the circuit as necessary. A separate
bleed stream containing soluble ash (91) is diverted from the
clarified process water (90) to limit the concentration of
45 dissolved solids.
What is claimed is:
1. A method of producing a feedstock for coal
gasification, comprising:
a) contacting coal with a Fischer-Tropsch wax at a temperature
between about 5° C. and about 30° C. greater
than the melting point of said Fischer-Tropsch wax to
form a Fischer-Tropsch wax-impregnated coal; and,
b) introducing said Fischer-Tropsch wax-impregnated
coal to a coal gasification operation.
2. The method of claim 1, wherein said coal is selected
from the group consisting of sub-bituminous coal, lignite,
and brown coal.
3. The method of claim 1, wherein said coal has a Btu
content of less than about 7,000 Btullb.
4. The method of claim 1, further comprising drying said
coal prior to said contacting step.
5. The method of claim 4, further comprising drying said
coal to less than about 15% weight percent water prior to
said contacting step.
6. The method of claim 1, wherein said contacting step
comprises pug milling said coal with said Fischer-Tropsch
wax.
use of the naphtha, diesel fuel, and tail gas to produce
electricity is useful when the price of electricity fluctuates to
its higher levels, making the electricity economically more
valuable than storing the energy in the form of diesel fuel
and naphtha. Thus, at times of high electricity prices, it will
be economically desirable to divert the products of the
Fischer-Tropsch reaction, including naphtha, diesel fuel, and
tail gas, to electricity generation while the wax is continu0usly
recycled to the feedstock for the gasifier. When the
price of electricity falls, the products of the Fischer-Tropsch
reactor may become more valuable and the synthesis gas
may be diverted to the Fischer-Tropsch reactor to produce
diesel fuel and naphtha, and wax for use in the coal
feedstock.
In addition to mixing wax with the coal to form the
wax-impregnated coal feedstock, the naphtha generated by
the Fischer-Tropsch reaction may also be added to the coal,
further boosting the energy content of the coal feedstock.
This is the preferable use of the naphtha when the price of
electricity falls below the point where it is economically
efficient to divert the naphtha to combustion for electricity
production.
Another aspect of the present invention provides the
compositions described above, including a waximpregnated
coal. Typically, wax is combined with the coal
in a wax-to-coal ratio of about 1:7 to about 1: 13. Preferably
the wax is combined with the coal in a wax-to-coal ratio of
about 1:8 to about 1:12, and more preferably is combined in
a wax-to-coal ratio of about 1:9 to about 1:11. Preferably, the
wax used to produce the wax-impregnated coal is the
product of a Fischer-Tropsch reactor.
The coal gasification system can be operated in conjunction
with a Fischer-Tropsch reactor and IGCC unit as shown
in FIG. 1. The principle unit operations are the feedstock
preparation system (1), air-separation unit (2), gasifier (3),
synthesis gas treatment and recovery unit (4), FischerTropsch
reactor, hydrocracker and product recovery plant
(5), and IGCC unit (6).
Raw low-rank coal (7) feeds the feedstock preparation
system (1). Water (8) feeds the system as necessary. Wax (9)
produced by the plant (5) is mixed with the coal to form a
wax-impregnated gasifier feedstock (10). Naphtha (11) produced
by the plant (5) may be optionally mixed with the
wax-impregnated feedstock (10) to increase the specific
energy of the feedstock.
Air (12) feeds a air separation unit (2) to provide oxygen
(13) for the gasifier (3). The by-product nitrogen (14) may
be sold or used by other unit operations shown in FIG. 1.
Wax-impregnated feedstock (10) and oxygen (13) feed 50
the gasifier (3) producing raw synthesis gas (15) and ash
(16). Ash may be sold as a building material or sent to
landfill for final disposal.
Raw synthesis gas (15) feeds a synthesis gas treatment
and recovery unit (4) producing sulfur, and carbon dioxide 55
(17), and clean synthesis gas (18).
Clean synthesis gas (18) normally feeds the FischerTropsch
reactor, hydrocracker and product recovery plant
(5). Optionally, the clean synthesis gas (18) may be burned
by the IGCC unit to produce electricity (22) when the value 60
of electrical energy is high or when the demand for diesel
fuel is low.
The Fischer-Tropsch reactor, hydrocracker and product
recovery plant (5) produces distillate fuels including diesel
fuel (19), wax (9), naphtha (11), steam (20), and tail gas (21). 65
Tail gas (21) is burnt by the IGCC unit (6) to produce
electricity (22).
US 6,664,302 B2
9 10
* * * * *
14. The method of claim 13, wherein a portion of said
synthesis gas is combusted to generate electricity.
15. The method of claim 14, further comprising monitoring
a current price of electricity and increasing the portion
5 of said synthesis gas being combusted to generate electricity
when the current price of electricity rises.
16. The method of claim 13, wherein said FischerTropsch
synthesis further forms naphtha.
17. The method of claim 16, wherein said naphtha is
10 mixed with said Fischer-Tropsch wax-impregnated coal for
coal gasification.
18. The method of claim 16, wherein a portion of said
naphtha is combusted to generate electricity.
19. The method of claim 13, wherein said FischerTropsch
synthesis further forms a tail gas and a portion of
15 said tail gas is combusted to generate electricity.
20. The method of claim 13, wherein said coal has a Btu
content of less than about 7,000 Btullb.
21. The method of claim 13, further comprising drying
said coal to less than about 15 weight percent water prior to
20 said contacting step.
22. The method of claim 13, wherein said contacting step
comprises:
a. autoclaving said coal in the presence of said FischerTropsch
wax to form said Fischer-Tropsch waximpregnated
coal; and,
b. separating water displaced from said coal during said
step of autoclaving from said Fischer-Tropsch waximpregnated
coal.
23. The method of claim 13, wherein said Fischer30
Tropsch wax-impregnated coal is mixed with water to form
a slurry feedstock for coal gasification.
24. The method of claim 13, wherein said coal gasification
is fixed bed coal gasification.
7. The method of claim 1, wherein said contacting step is
conducted at a pressure between about 2,000 psi and about
14,000 psi.
8. The method of claim 1, further comprising autoclaving
said coal to remove impurities prior to said contacting step.
9. The method of claim 1, wherein said contacting step
comprises:
a) autoclaving said coal in the presence of said FischerTropsch
wax to form said Fischer-Tropsch waximpregnated
coal; and,
b) separating water displaced from said coal from said
Fischer-Tropsch wax-impregnated coal during said step
of autoclaving.
10. The method of claim 9, wherein said autoclaving step
is conducted at a temperature between about 90° C. and 310°
C.
11. The method of claim 9 wherein said autoclaving step
is conducted at a pressure between about 300 psi and about
1,500 psi.
12. The method of claim 1, wherein said step of introducing
comprises: mixing said Fischer-Tropsch waximpregnated
coal with water to form a slurry feedstock for
coal gasification.
13. A method for utilizing synthesis gas, comprising:
25
a) contacting coal with Ficher-Tropsch wax at a temperature
between about 5° C. and about 30° C. greater than
the melting point of said Fischer-Tropsch wax to form
a Fischer-Tropsch wax-impregnated coal,
b) subjecting said Fischer-Tropsch wax-impregnated coal
to coal gasification to produce a synthesis gas;
c) liquefying said synthesis gas by Fischer-Tropsch synthesis
to form products comprising diesel fuel and
Fischer-Tropsch wax, wherein said Fischer-Tropsch
wax is used in said contacting step of (a).