U.S. patent number 3,847,567 [Application Number 05/391,082] was granted by the patent office on 1974-11-12 for catalytic coal hydrogasification process.
This patent grant is currently assigned to Exxon Research and Engineering Company. Invention is credited to Theodore Kalina, Roger E. Moore.
United States Patent |
3,847,567 |
Kalina , et al. |
November 12, 1974 |
CATALYTIC COAL HYDROGASIFICATION PROCESS
Abstract
Methane is produced by the thermoneutral reaction of steam with
coal or other carbonaceous material in a hydrogasification zone
containing an alkali metal catalyst and sufficient hydrogen to
suppress competing endothermic reactions. The gas taken overhead
from the gasifier is subjected to steam reforming and then
processed for the removal of acid constituents, hydrogen which is
recycled to the hydrogasification zone, and carbon monoxide which
is used as fuel for the steam reformer.
Inventors: |
Kalina; Theodore (Morris
Plains, NJ), Moore; Roger E. (El Paso, TX) |
Assignee: |
Exxon Research and Engineering
Company (Linden, NJ)
|
Family
ID: |
23545154 |
Appl.
No.: |
05/391,082 |
Filed: |
August 27, 1973 |
Current U.S.
Class: |
585/733;
48/127.7; 585/943; 48/202 |
Current CPC
Class: |
C07C
9/04 (20130101); C07C 1/00 (20130101); C10K
1/005 (20130101); C10J 3/54 (20130101); C10K
1/004 (20130101); C10L 3/08 (20130101); C10L
3/106 (20130101); C10L 3/102 (20130101); C07C
1/00 (20130101); C10J 2300/0973 (20130101); C10J
2300/1823 (20130101); C10J 2300/1807 (20130101); C10J
2300/1853 (20130101); C10J 2300/0986 (20130101); C10J
2300/0959 (20130101); C10J 2300/0966 (20130101); C07C
2527/232 (20130101); C10J 2300/093 (20130101); Y10S
585/943 (20130101); C10J 2300/1892 (20130101); C10J
2300/1884 (20130101); C10J 2300/1662 (20130101) |
Current International
Class: |
C10J
3/54 (20060101); C10J 3/46 (20060101); C07C
1/00 (20060101); C10j 003/06 () |
Field of
Search: |
;48/196R,202,210,197R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bashore; S. Leon
Assistant Examiner: Kratz; Peter F.
Attorney, Agent or Firm: Reed; J. E.
Claims
We claim:
1. A process for the manufacture of methane which comprises:
a. reacting finely divided carbonaceous solids with steam and
hydrogen in the presence of an alkali metal catalyst in a
hydrogasifier and withdrawing a methanerich gas overhead from said
hydrogasifier;
b. reacting said methane-rich gas with steam in a catalytic steam
reforming furnace and withdrawing a methane-rich gas of increased
hydrogen content from said reforming furnace;
c. treating said gas of increased hydrogen content for the removal
of acidic constituents;
d. separating the treated gas into a product methane stream, a
hydrogen stream, and a carbon monoxide stream;
e. recycling at least a portion of said hydrogen stream to said
hydrogasifier; and
f. burning at least a portion of said carbon monoxide stream as
fuel in said steam reforming furnace.
2. A process as defined by claim 1 wherein carbonaceous solids are
reacted with said steam and hydrogen in said hydrogasifier at a
temperature in the range between about 1,200.degree. F. and about
1,500.degree. F. and at a pressure in the range between about 100
and about 5,000 psig.
3. A process as defined by claim 1 wherein said alkali metal
catalyst is an alkali metal carbonate.
4. A process as defined in claim 1 wherein said methane-rich gas is
contacted with said steam in said catalytic steam reforming furnace
at a temperature in the range between about 1,300.degree. F. and
about 1,700.degree. F. and at a pressure in the range between about
100 and about 5,000 psig.
5. A process as defined by claim 1 wherein said steam reforming
catalyst is an alkali metal.
6. A process as defined by claim 1 wherein said separation is
performed cryogenically.
7. A process as defined by claim 1 wherein said catalyst in said
hydrogasifier comprises from about 5 to about 30 weight percent, on
a moisture and ash-free basis, of the total solids in the
hydrogasifier.
8. A process as defined by claim 1 wherein said carbonaceous solids
comprise a bituminous or lower rank coal.
9. A process as defined by claim 2 wherein said solids are reacted
with said steam and hydrogen in said hydrogasifier at a pressure
between about 500 and about 1,000 psig.
10. A process as defined by claim 3 wherein said catalyst comprises
potassium carbonate.
11. A process as defined by claim 4 wherein said gas is reacted
with said steam in said reforming furnace at a pressure between
about 500 and about 1,000 psig.
12. A process as defined by claim 5 wherein said catalyst comprises
potassium carbonate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the gasification of coal and other
carbonaceous materials and is particularly concerned with
gasification processes carried out over alkali metal catalysts for
the production of methane.
2. Description of the Prior Art
Efforts to develop processes for the conversion of coal and similar
carbonaceous materials into high Btu synthetic gases suitable for
use as fuels have focused attention on the hydrogasification
reaction: C + 2H.sub.2 .revreaction. CH.sub.4. Because this
reaction is highly exothermic and requires the presence of
hydrogen, it has been suggested that it be integrated with
endothermic hydrogen-producing reactions such as the steam/carbon
gasification reaction, C + H.sub.2 O .revreaction. CO + H.sub.2,
and the methane/steam reforming reaction, CH.sub.4 + H.sub.2 O
.revreaction. CO + 3H.sub.2, in order to conserve heat and reduce
the amount of hydrogen which must be provided. It has been found
that this can be done by reacting the coal or other carbonaceous
material with steam and hydrogen in a hydrogasification zone to
produce a methane-rich gas, passing at least a portion of this gas
stream through a methane reforming zone where it is contacted with
steam to reduce part of the methane and form hydrogen, and then
recycling hydrogen and carbon monoxide recovered from the steam
reformer overhead gas to the hydrogasification zone. Coal char or
other carbonaceous solids are circulated between the
hydrogasification and reforming zones to provide heat integration.
This system results in a substantially thermoneutral process which
has numerous advantages over other processes suggested in the
past.
SUMMARY OF THE INVENTION
This invention provides an improvement over the process referred to
above which permits significant savings in operating costs and has
other advantages. In accordance with the invention, a high Btu gas
suitable for pipeline purposes is produced by reacting finely
divided coal or similar carbonaceous material with steam and
hydrogen in the presence of an alkali metal catalyst, passing the
resultant gas to a catalytic reforming unit where steam and methane
react to form hydrogen and carbon monoxide, treating the reformer
gas product for the removal of carbon dioxide and other acidic
gases, and then separating the treated gas into a hydrogen stream
which is recycled to the gasifier, a carbon monoxide stream which
is used to fuel the reformer and provide heat for the process, and
a product gas stream composed primarily of methane. The hydrogen
and reactant steam concentrations in the gasifier are controlled so
that the exothermic hydrogasification reactions provide sufficient
heat for the endothermic steam reactions, reactant preheat and
reactor heat losses. This results in a substantially thermoneutral
process which has economic advantages over processes employed in
the past.
The hydrogasification reaction is preferably carried out in a
fluidized bed containing from about 1 to about 50 percent by
weight, based on the carbonaceous feed material, of an alkali metal
catalyst such as potassium carbonate, sodium carbonate, or the
like. The use of such a catalyst permits operation of the gasifier
at temperatures within the range of about 1,200.degree. to about
1,500.degree. F., which in turn favors the production of methane.
The net hydrogen required for reaction with the carbonaceous
material in the hydrogasification zone is generated by steam
reforming the overhead gas, preferably in the presence of an alkali
metal or similar methane reduction catalyst at a temperature in the
range between about 1,300.degree. and about 1,700.degree. F., in a
steam reformer furnace. Here from about 15 to about 50 percent of
the methane produced in the hydrogasification step is reduced to
carbon monoxide and hydrogen. Some additional carbon dioxide is
also formed. The reformer gases are treated with triethanolamine,
hot potassium carbonate, or a similar solvent for the removal of
carbon dioxide and hydrogen sulfide and then passed to a cryogenic
separation unit where hydrogen, carbon monoxide, and methane are
separately recovered. The hydrogen is recycled to the
hydrogasification zone where it reacts with the carbonaceous
material in the methane-forming reaction. The recovered carbon
monoxide is not recycled as in earlier processes and instead is
burned with ambient pressure air to fire the steam reformer
furnace. This provides essentially all of the heat required in the
process and eliminates the necessity for burning either solid fuels
under conditions which may result in pollution problems or more
costly product methane which results in the removal of hydrogen
from the system as water vapor, rather than as a valuable product.
The methane from the cryogenic unit is of high purity and can be
employed as a pipeline gas without further treatment.
The invention will be further understood by referring to the
drawing and the description of the preferred embodiments set forth
below.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE in the drawing is a schematic flow sheet of a
hydrogasification process carried out in accordance with the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process depicted in the drawing is one for the production of
methane by the thermoneutral reaction of coal or a similar
carbonaceous solid with steam and hydrogen in a fluidized bed
reactor. The solid feed material, which will normally be a
bituminous or lower rank coal but may be oil shale, petroleum coke,
char, charcoal or other carbonaceous material, is introduced into
the system in finely divided form through line 10 from a suitable
storage facility or feed preparation plant which does not appear in
the drawing. The use of a feed material which has been crushed and
screened to a particle size less than about 8 mesh on the Tyler
Screen Scale is normally preferred. The particulate feed material
is passed from line 10 into hopper 11 where it is combined with a
finely divided alkali metal catalyst introduced through line 12.
Suitable alkali metal catalysts include the oxides and salts of
cesium, potassium, sodium and lithium. The alkali metal carbonates
are particularly effective. Cesium carbonate normally exhibits the
greatest catalytic activity, potassium carbonate and sodium
carbonate being somewhat less active and lithium carbonate normally
being the least active material. Because of its relative high
activity, widespread availability and low cost, potassium carbonate
is normally the preferred catalyst for use in the process. Other
alkali metal compounds which can also be used but are less
effective include the chlorides, hydroxides, sulfates, silicates,
sulfides and the like. The catalyst employed may be introduced into
the system as a finely divided powder or granular material which is
mixed with the feed material in hopper 11 to obtain relatively
uniform distribution of the catalyst particles. In lieu of this,
the carbonaceous solids employed as feed material can be
impregnated with the catalyst during or following the feed
preparation step or particles of coal char or a similar
carbonaceous material having a high surface area can be treated
with aqueous solutions of the catalyst and thereafter introduced
with the feed material.
The amount of catalyst employed in the gasifier will normally range
from about 1 to about 50 weight percent, preferably from about 5 to
about 30 weight percent, of the combined total weight of the
carbonaceous solids and catalyst on a moisture and ash-free basis.
The use of from about 15 to 25 weight percent of potassium has been
found to be particularly effective. The optimum amount of catalyst
for a particular operation will depend upon the particular alkali
metal compound selected and hence it may be preferable to employ
somewhat greater amounts of sodium carbonate or lithium carbonate
than would be used if cesium carbonate or potassium carbonate were
employed.
The hydrogasifier employed in the process of the invention is
operated at elevated pressure and hence the feed material and
catalyst in hopper 11 must be pressurized prior to introduction
into the hydrogasifier. The system depicted in the drawing involves
the use of a star wheel feeder or similar device 13 through which
the solid particles are discharged into line 14 containing steam,
product methane, recycle hydrogen or a similar carrier gas at a
pressure sufficiently high to permit entrainment and injection of
the solids into the hydrogasifier 15. In lieu of or in addition to
an arrangement of this type, parallel lock hoppers, pressurized
hoppers, or aerated standpipes operating in series may be employed
to raise the input coal stream to the system operating pressure.
The use of such devices for handling coal and other finely divided
solids at elevated pressures has been described in the patent and
technical literature and will be familiar to those skilled in the
art.
The overall reaction which takes place in the process of the
invention can be represented by the equation: 2H.sub.2 O + 2C
.fwdarw. CH.sub.4 + CO.sub.2. This overall reaction is carried out
in two separate steps, an initial step for the formation of methane
in the hydrogasifier in accordance with the equation: 2C + 4H.sub.2
.fwdarw. 2CH.sub.4, and a second step for the formation of hydrogen
in the steam reformer in accordance with the equation: 2H.sub.2 O +
CH.sub.4 .fwdarw. 4H.sub.2 + CO.sub.2. Concurrently, in the first
reactor the reaction C + H.sub.2 O .fwdarw. CO + H.sub.2 is carried
out to balance the exothermic hydrogasification and produce carbon
monoxide for use as fuel. The hydrogasifier is operated under
conditions which are thermodynamically favorable to the production
of methane in high concentrations, normally at relatively high
pressure and a temperature as low as possible without sacrificing
high reaction rates. The temperature in hydrogasifier 15 will
therefore generally be maintained in the range between about
1,200.degree. F. and about 1,500.degree. F. and the pressure will
generally range from about 100 psig to about 5,000 psig, preferably
from about 500 psig to about 1,000 psig.
The coal or other carbonaceous solids and catalyst injected through
line 14 into hydrogasifier 15 form a fluidized bed in which the
solid particles are supported by up-flowing steam introduced into
the lower end of the reaction vessel through line 16 and by recycle
hydrogen admitted by way of line 17. The steam and hydrogen react
with the carbon in the fluidized bed to produce a methane-rich
synthesis gas containing from about 30 to about 60 mole percent
methane. This gas is taken overhead through one or more cyclone
separators or similar devices 18 by means of which entrained solids
are removed from the gas stream and returned to the fluidized bed.
A solids stream composed primarily of ash and catalyst, but also
containing some char, is withdrawn from the fluidized bed through
line 19 near the lower end of the hydrogasifier and passed to
catalyst recovery zone 20. Here the hot solids are quenched with
water introduced through line 21 in order to permit heat recovery
and leaching out of the catalyst. The resulting aqueous catalyst
solution can then be concentrated and recycled through line 22 for
use with the fresh make-up catalyst introduced into the system
through line 12. Alternatively, the catalyst solution can be
sprayed on the incoming coal introduced into the system through
line 10 or used to pretreat coal or other carbonaceous solids in
the feed preparation plant. The ash containing a small amount of
char is withdrawn from the catalyst recovery unit as indicated by
line 23 and may be employed for the manufacture of building
materials or used for other purposes.
The gas stream taken overhead from hydrogasifier 15 is passed
through line 24 into the tubes 25 of steam reformer furnace 26.
This furnace also contains steam coils 27 from which water
introduced through line 28 is withdrawn as steam by means of line
29. A portion of the steam thus generated is injected through line
31 into the gas stream from the hydrogasifier for reaction with
methane in the reformer furnace tubes. The remaining steam is
passed through lines 32 and 16 to the lower end of the
hydrogasifier for reaction with the coal feed.
The steam-methane reaction carried out in reformer furnace 26 is
conducted in the presence of a methane reduction catalyst at a
temperature in the range between about 1,300.degree. F. and about
1,700.degree. F. and at a pressure between about 100 and about
5,000 psig, preferably between about 500 and about 1,000 psig. The
catalyst may be a transition metal of Group VIII of the Periodic
Table such as iron, nickel or cobalt or an oxide, carbonate,
nitrate, carbide, chloride, sulfate or other compound of such a
metal. Preferably, however, an alkali or alkaline earth metal
catalyst similar to that employed in the hydrogasifier will be
used. The catalyst selected, in the form of metal shot, metallic
turnings, ore particles, nitrited steel wool, or similar particles,
is disposed within the tubes of the reformer furnace. The use of
potassium carbonate particles or particles of alumina, kieselguhr
or a similar inert high surface material on which potassium
carbonate has been deposited is generally preferred. In the
presence of such a catalyst, a portion of the methane in the gas
stream from the hydrogasifier reacts with the steam in the furnace
tube to produce hydrogen, carbon monoxide and carbon dioxide. The
gas discharged from the reformer furnace will thus contain methane,
hydrogen, carbon monoxide, carbon dioxide, unreacted steam, and
small amounts of nitrogen and hydrogen sulfide derived from
impurities in the coal.
The gas discharged from the reformer furnace 26 is passed through
line 33 to heat exchanger 34 where it is cooled and the steam
initially present is recovered as condensate by means of line 35.
Cooling water supplied to vessel 34 through line 36 is withdrawn as
steam through line 37. The overhead gas stream from which steam has
been removed is passed through line 38 to vessel 39 for the removal
of acid gases, principally carbon dioxide and small amounts of
hydrogen sulfide. Here the gas stream is scrubbed with a
conventional carbon dioxide-absorbing solvent introduced through
line 40. The solvents which may be employed for this purpose
include n-methyl pyrrolidone, triethanolamine, propylene carbonate,
and hot potassium carbonate. Spent solvent containing absorbed
carbon dioxide and hydrogen sulfide is withdrawn from the lower end
of the scrubber by means of line 41 and may be treated before
removal of the acidic constituents and then recycled. Most of the
carbon dioxide and hydrogen sulfide can generally be removed from
the solvent by flashing and the rest can usually be taken out by
stripping with an inert gas. Cryogenic separation or other
conventional methods for removal of the acidic constituents from
the gas stream can also be employed. The overhead gas from vessel
39 is passed through line 42 to cryogenic separation unit 43 where
it is fractionated to produce a product methane stream that is
withdrawn through line 44, a hydrogen stream which is taken off
overhead and recycled through lines 45 and 17 to the hydrogasifier,
and a carbon monoxide stream which is passed through line 46 to
reformer furnace 26. Here the carbon monoxide is injected into the
furnace burners 47 with low pressure air and employed as fuel for
the furnace.
The process of the invention can be further illustrated by
considering a plant for the production of 250 million standard
cubic feet of methane per stream day. The feed to this plant will
consist of about 500 tons of coal per hour and about 59 tons
potassium carbonate as a catalyst. The solids feed stream to the
hydrogasifier will thus comprise about 89 weight percent of finely
divided coal screened to less than about 8 mesh on the Tyler Screen
Scale and about 11 weight percent potassium carbonate. Following
the initial startup, substantially all of the catalyst withdrawn
from the system will be separated from the ash in the catalyst
recovery unit and recycled so that only make-up quantities of
potassium carbonate need be added. About 77 tons of ash per hour
containing about 14.6 percent char is withdrawn from the catalyst
recovery unit for use in the manufacture of building materials or
for other purposes.
The coal fed to the hydrogasifier is supported in a fluidized bed
by steam introduced at the rate of about 468,300 pounds per hour
and recycle hydrogen injected into the lower end of the gasifier at
a rate of about 31.5 million standard cubic feet per hour. At the
gasifier temperature of about 1,200.degree. F. and pressure of
about 100 psig, these materials react to form about 41.2 million
standard cubic feet per hour of a methane-rich synthesis gas having
the following composition:
TABLE I ______________________________________ Synthesis Gas
Composition Constituent Mole %
______________________________________ CO 9.5 CO.sub.2 4.7 H.sub.2
29.7 H.sub.2 O 12.2 CH.sub.4 42.4 N.sub.2 0.4 H.sub.2 S 1.2
______________________________________
The methane-rich gas taken overhead from the hydrogasifier at a
temperature of about 1,200.degree. F. is combined with 990,810
pounds of steam per hour from the steam reformer furnace. The
resulting mixture of steam and methane-rich synthesis gas is passed
through the reformer furnace tubes where it is contacted with a
potassium carbonate on alumina catalyst. At the furnace temperature
of about 1,300.degree. F., the steam and synthesis gas react to
reduce a portion of the methane and produce additional carbon
monoxide, carbon dioxide and hydrogen. This results in about 74
million standard cubic feet per hour of gas having the following
composition:
TABLE II ______________________________________ Reformed Gas
Composition Constituent Mole %
______________________________________ CO 10.4 CO.sub.2 5.6 H.sub.2
43.6 H.sub.2 O 24.1 CH.sub.4 15.5 N.sub.2 0.2 H.sub.2 S 0.6
______________________________________
The reformed gas is cooled to permit the removal of 844,182 pounds
of water of condensate per hour and then scrubbed with
triethanolamine for the removal of acid gases. This results in the
separation from the gas of about 4.1 million standard cubic feet
per hour of carbon dioxide and about 0.4 million standard cubic
feet per hour of hydrogen sulfide. The scrubbed gas, about 51.6
million standard cubic feet per hour, contains about 62.6 percent
hydrogen, 22.3 percent methane, 14.9 percent carbon monoxide and
about 0.2 percent nitrogen. This gas stream is passed to the
cryogenic unit where about 250 million standard cubic feet of
methane per stream is recovered as product gas. About 31.5 million
standard cubic feet of hydrogen per hour is recycled to the
hydrogasifier. The remaining gas stream, about 9.7 million standard
cubic feet per hour of a mixture of 79.1 percent carbon monoxide,
8.3 percent hydrogen, 11.1 percent methane and 1.5 percent
nitrogen, is transmitted to the steam reformer furnace for use a
fuel. This burning of carbon monoxide, together with lesser
quantities of hydrogen and methane not recovered in the cryogenic
separation unit, provides all of the heat which is required for the
process served to generate the by-product stream required for the
hydrogasification and reforming steps.
It will be apparent from the foregoing that the process of the
invention has numerous advantages over processes for the production
of methane which have been employed in the past. The use of a steam
reformer furnace to simultaneously carry out the steam reforming
reaction and generate process steam eliminates the need for
supplying heat to the process from an external source and does away
with the need for the air compressors or high pressure oxygen
facilities which are normally needed for carrying out endothermic
gasification reactions requiring high heat input levels. Because
the process is substantially thermoneutral, it does not require
removal of the large quantity of heat that would otherwise have to
be recovered. Moreover, the process maximizes the quantity of
carbon dioxide which is rejected from the system with the flue
gases and minimizes the quantity which must be taken out by
scrubbing of the gas. The use of the carbon monoxide as fuel,
rather than recycling it to the hydrogasifier to suppress carbon
monoxide production as in earlier processes, eliminates the need
for burning solid fuel or product methane to generate process steam
and satisfy the other heat requirements of earlier processes. This
in turn simplifies pollution control problems and avoids the
removal of large quantities of hydrogen as water vaor in the flue
gas. These and other advantages make the process of the invention
attractive for a variety of applications.
* * * * *