6,664,302 Method of forming a feed for coal gasification

Patent Number/Link:

111111111111111111111111111111111111111111111111111111111111111111111111111

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

Air

Air Separation

Unit

Nitrogen

Ash

Steam

Diesel Fuel

Electricity

Fischer-Tropsch

Reactor,

Hydrocracker

and Product

Recovery Plant

f-----4-----~ Carbon Dioxide

and Sulfur

Synthesis Gas

Treatment and

Recovery Unit

Gasifier

Water

Raw Lowrank

Coal

Wax

. __ .. - - - - - - -- - __ - -- - __ - - __ - - - -- - - ._.- --:1..-----,/-+ Naphtha

d•

'JJ.

•

~.....

Nitrogen =.....

Ash ~

'""'" Carbon Dioxide ~~

and Sulfur NCC~

Electricity

22 'JJ. =- ~

~....

'""'" 0....,

Diesel Fuel ~

Steam

20 e Naphtha \Jl

-..CJ\

II CJ\

CJ\

Fischer-Tropsch

Reactor,

Hydrocracker

and Product

Recovery Plant

Integrated Gas

Combined Cycle I I ~

Unit

Wax

Synthesis Gas

Treatment and

Recovery Unit

Figure 1

Gasifier

Feedstock

Preparation

System

Air Separation

Unit

Water

Air

Raw LowrankCoal

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

Open Storage Pile ~ Water Vapor

,.----ll..----.., 'v)

Thennal Dryer --4

Water Vapor

Fischer-Tropsch Wax (9)

Pug Mill

Briquetter

-53

Solid Particulate Feedstock

Figure 2

u.s. Patent Dec. 16,2003 Sheet 3 of 3 US 6,664,302 B2

Raw Low-rank Coal (7)

Crusher

Figure 3

US 6,664,302 B2

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

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

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

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.

US 6,664,302 B2

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°

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:

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).