Note: Descriptions are shown in the official language in which they were submitted.
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STEAM GASIFICATION PROCESS
FIELD OF THE INVENTION
[00011 The present invention relates to low temperature catalytic gasification
of
carbonaceous material. More particularly, the present invention relates to an
improved
process for gasifying carbonaceous material that achieves high carbon
conversion to
methane at mild temperatures.
BACKGROUND - DESCRIPTION OF RELATED ART
[0002] This application claims priority under 35 U.S.C. 1I9(e) to provisional
application 60/695,994 which is hereby incorporated by reference.
[00031 The world-wide availability of petroleum is predicted to peak and then
decline rapidly. Rapid economic, technological and industrial growth of
populous
countries such as China and India serves to increase this demand, making the
need for
alternative sources of energy even more severe. To meet this growing demand it
has
been suggested to convert coal into more useful and t{ansportable forms. One
such
technique is to gasify coal into combustible gases. A coal gasification
process for
producing pipeline grade fuel, such as methane, would be especially desirable
because of
the existing infrastructure adapted to transport methane as natural gas.
[00041 In typical coal gasification systems, coal or other carbonaceous
materials
and steam are reacted with oxygen (or air) to produce a syngas, comprised
primarily of
hydrogen and carbon monoxide. Commercial, non-catalyzed, coal gasification
systems
and designs face a number of economic and technical challenges. These
processes are
expensive to operate since, in order to drive the endothermic non-catalytic
gasification of
carbonaceous materials, they utilize severe temperatures (2400 to 2600 F) and
can
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consume high levels of oxygen. Slagging and corrosion also can preseilt
operating and
maintenance issues which reduce economic viability and increase product cost.
[0005] A current concept of an Integrated Gasification Combined Cycle (IGCC)
system incorporates a non-catalyzed coal gasification system to produce syngas
as an
intermediate and burns the syngas to produce electricity. The capital cost of
an IGCC
system is estimated to range from about $1,250 to $1,400 per KW, depending
upon the
design and process integration. One way to reduce the cost significantly would
be to
develop a process that enables one to gasify coal at lower temperature and
without added
oxygen. Toward this end, it is useful to consider the thermodynamics of
gasifying coal.
[0006] The gasification of coal and similar materials generally involves the
following reactions: -
C + H20 - CO + H2 (Endothermic) (1)
C + 2H2 - CH4 (Exothermic) (2)
C + CO2 - 2C0 (Endothermic) (3)
CO + HZO = CO2 + H2 (Exothermic) (4)
The reaction kinetics during conventional (i.e. thennal) gasification
generally produce
only small amounts of methane. Direct hydrogenation/gasification of carbon
such as
depicted by equation (2) above is very slow compared to the endothermic
reactions of
steam and carbon dioxide with carbon, as depicted in equations (1) and (3).
The
gasification of coal and similar materials thus normally produces a synthesis
gas
composed primarily of hydrogen and carbon monoxide.
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[00071 Addition of alkali metal catalysts enables steam gasification to
proceed at
lower temperatures and can enhance the production of methane through the
following
exothermic reactions:
2C0 + 2H2 -~ CO2 + CH4 (Exothermic) (5)
CO + 3H2 - H20 + CH4 (Exothermic) (6)
CO2 + 4HZ - 2HZ0 + CH4 (Exothermic) (7)
[0008] One such catalytic steam gasification process is disclosed in U.S.
Patent
No. 4,094,650 to Koh et al. ("the '650 process"). The preferred temperature
and pressure
ranges disclosed therein are around 1300 F (700 C) and 500 psia (34 atm).
Potassium
carbonate is disclosed as a preferred catalyst. Though, the temperature is
lower than in
non-catalyzed gasification, the main raw products are still H2 and CO. In
order to
suppress the formation of H2 and CO, and drive the carbon conversion to
methane, the
'650 process teaches recycling the H2 and CO from the raw product. A catalyst
makeup
stream is also required in the '650 process because, at the temperatures
therein, the alkali
metal catalyst can volatilize and/or react with ash constituents of the coal
causing a
substantial decrease in catalyst activity.
[0009] Various combinations of compounds have been investigated to find less
expensive coal gasification catalysts. For example, U.S. Patent No. 4,336,034
to Lang et
al. discloses that at catalyst loadings up to 12% by weight, the relatively
inexpensive
combination of K2SO4 and calcium compounds such as CaO, Ca(OH)2, or CaC03 can
provide gasification rates comparable to relatively expensive K2C03. The '034
patent
reports better performance for mixtures having a K/Ca ratio of 2.0 (i.e., 1/3
calcium) than
for mixtures with more calcium. Lang reports the use of small amounts of
calcium to
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enhance the activity of a relatively poor catalyst such as K2S04. There is no
suggestion
in Lang that higher quantities of calcium can influence the catalytic activity
of potassium
hydroxide or potassium carbonate, or that calcium salts can be used to enhance
product
yield, change the reaction kinetics, or enable gasification to proceed at
lower operating
temperatures. There is also no suggestion that the presence of calcium can
improve
catalyst recovery.
[0010] Other modifications have been proposed to attain more complete carbon
conversion in a catalytic coal gasification process, examples being U.S.
Patent No.
4,558,027 to McKee et al. which discloses using eutectic alkali catalyst
mixtures, and
U.S. Patents No. 4,077,778 and 6,955,695 to Nahas which disclose,
respectively, using
two reactors, or a two-stage reactor. These processes, like that of the '650
patent, report
recycling substantial quantities of H2 and CO from the raw product gases to
the gasifier
to maximize the production of methane.
[00111 Thermodynamically, methane generation is favored at mild temperatures
below about 540 C and high pressures, but catalytic coal gasification
processes typically
operate hotter, i.e., at temperatures between about 700 C to about 820 C,
because the
gasification rate, and yield, are low in conventional catalytic coal
gasification processes
at lower temperatures.
[0012] Mild temperature coal gasification can achieve higher direct conversion
of
carbon to methane and can reduce or avoid catalyst losses which can occur at
higher
temperatures due to binding with mineral matter in the carbonaceous feed or
volatilization. Mild temperature coal gasification can also minimize the
conversion of
coal to significantly less reactive char. However, catalysts have not
heretofore been
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identified that can catalyze mild temperature gasification at acceptably high
reaction
rates.
[0013] Several metals (other than alkali metals) have been identified that can
catalyze steam/coal gasification, but have not shown promise for mild
temperature
gasification. Transition metals such as iron or nickel'can catalyze coal
gasification, but
are subject to being deactivated rapidly, after only about 10 or 15% carbon
conversion,
(D. Tandon, Low Temperature and Elevated Pressure Steam Gasification of
Illinois Coal
(1996) (Ph.D. dissertation, Southern Illinois University at Carbondale)).
Research has
found that unsupported Raney Ni can be severely deactivated by H2S, possibly
due to the
formation of NiAl2S~ on the surface of the catalyst, but can be less affected
when
supported by Zr02 and A1203.
[0014] Catalytic metals, in combination, can be less vulnerable to
deactivation
than single-metal catalysts. For example, eutectic catalyst mixtures can
maintain
catalytic activity longer than one constituent of the mixture. Similarly,
Tandon reported
that potassium combined with nickel or iron as a steam/graphite gasification
catalyst can
remain active longer than iron or nickel alone. It is possible that highly
dispersed alkali
metal salts can provide a reducing atmosphere for transition metal salts and
thus sustain
their catalytic activity.
[0015] A catalytic effect from highly dispersed calcium has also been
observed.
For example, the article by Yasuo Ohtsuka and Kenji Asami, "Highly active
catalysts
from inexpensive raw materials for coal gasification", Catalysis Today 39:111
(1997)
reports that calcium salts, such as CaCO3 or Ca(OH)2, that have been "kneaded"
with
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coal particles, can promote steam gasification of lignite at about 550 C, but
are
reportedly not effective with low-oxygen containing higher rank coals.
[0016] CaO or lime can also be used with coal conversion processes to absorb
CO2. For example, U.S. Patent No. 4,747,938 to Khan, which is directed to coal
pyrolysis at about 550 C, discloses that using particulate CaO at up to 25 wt%
loading
can yield a product stream with less H2S and CO2. Neither the Khan nor the
Ohtsuka and
Asami processes utilize alkali catalysts.
[0017J Though coal gasificatioii catalysis has been extensively researched, it
is
still not completely understood. Without intending to limit this invention to
any
particular theory, it is believed that transition metals that can catalyze
coal gasification
are those which can oscillate between two oxidation states and participate in
oxidation-
reduction cycles on the carbon surface, and that gasification with alkali
metal catalysts
involves the alkali metals donating electrons to the carbon lattice, or
forming
alkali/carbon complexes, thereby increasing the number of active CO complexes
on the
carbon surface. It is also believed that combinations of such catalysts
exhibit sustained
activity because different types of active sites on the carbon surface can be
activated by
different catalytic moieties, making more reaction sites available and
reducing the impact
of the deactivation of any particular type of reaction site or reaction
mechanism.
[00181 It is further believed that transition metals and alkali metals are
catalytically inactive wlien they are oxidized, and that they can be oxidized
by
components of the gasification environment such as H20, C02, CO and H2S. The
alkali
metal catalysts can also become iiiactive or ineffective by volatilizing
and/or binding with
mineral constituents of coal.
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[00191 It would be highly desirable to develop a catalytic coal gasification
process that could sustain high reactivity with high carbon conversion, and
even more
desirable to develop a catalytic process capable of high carbon conversion to
methane
without recycling from the raw product (or feeding) a substantial H2 and CO
stream. It
would be further desirable if such a process could operate at mild
temperatures where
catalyst losses by vaporization or deactivation by interaction with mineral
constituents of
the carbonaceous feed could be minimized. These and other objects are the
subject of the
process disclosed herein.
SUMMARY OF THE INVENTION
[00201 It has been found that using calcium salts to remove or "trap" carbon
dioxide and other oxidizing agents from a catalytic coal gasification
environment can
shift the kinetics towards greater carbon conversion to methane, and can also
drive the
conversion of CO to CO2 such that the process can yield a dry raw gaseous
product
comprised mainly of H2 and CH4 and substantially free of carbon oxides. The
overall
coal/carbon conversion can be at least 50% but conversions greater than 95%
are also
obtainable. The process disclosed herein can directly produce a dry raw
gaseous product
comprised of about 40% methane or more, by volume, without the need for
substantial
recycling or feeding H2 and CO to the environment. Advantageously, the dry raw
gaseous product can be used as a fuel without further enrichment, and can
provide
pipeline quality methane with little additional treatment.
100211 Calcium salts and other compounds can react with CO2 and H2S and form
solids which can be withdrawn in a solid purge, thereby eliminating or greatly
reducing
the need to treat the raw gaseous product for acid gas removal. According to
the present
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invention, calcium salts can also bind with, and render inert or relatively
inert, mineral
constituents of the carbonaceous feed so the alkali metal salt catalysts can
remain active
longer. By preventing such minerals from reacting with and deactivating the
alkali metal
}
catalysts, greater catalyst recovery from the solid purge can be achieved and
catalyst
losses can be reduced. The process can allow for up to -90% catalyst recovery.
[0022) While the invention is not limited to any theory, it is believed that
CO2 in
the gasifier causes the catalyst to deactivate, so that by eliminating the
C02, high catalytic
activity can be sustained and more complete conversion can be achieved. In
addition,
removal of CO2 from the gas phase can substantially alter the ratio of
hydroxide to
carbonate forms of the catalyst. Eliminating CO2 effectively increases the
activity of the
catalyst and enables a high rate of gasification to occur at mild operating
temperatures.
At mild temperatures, the kinetics favor greater direct conversion of coal (or
other
carbonaceous materials) to methane, and the coal, which can convert to less
reactive char
at conventional catalytic coal gasification temperatures, can remain more
reactive. Mild
teinperature operation can also reduce catalyst losses and corrosion of system
components caused by volatilization of the catalyst and hazardous trace
elements in the
carbonaceous feed.
[0023] The catalytic gasification processes of the present invention can also
be
simpler and less costly to build and operate than kno -qm prior processes, and
can be less
prone to overheating, corrosion, char build-up and other problems long
associated with
other gasification processes and systems. The estimated Btu in, versus Btu
out,
efficiency can be on the order of 80 to 85% overall.
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[0024] In one embodiment of the invention there is provided a method for
direct
catalytic gasification of carbonaceous material to methane comprising causing
a reaction
of the carbonaceous material in an environment including steam and an alkali
metal salt
catalyst at mild temperatures in the range from about 300 to about 700 C and
a pressure
from about 15 to about 100 atmospheres, and removing CO2 (and H20) from the
products
of the reaction in the environment so as to produce a dry raw gaseous product
consisting
of from about 30% to about 90% methane. Thus, the dry raw gaseous product can
include at least about 40% methane, or at least about 50%, or at least about
60%, or even
at least about 70% methane by volume. This embodiment can be carried out in
the
absence of or without extensive added or recycled H2 or CO.
[0025] Another embodiment provides an improved method for direct catalytic
gasification of carbonaceous material to combustible gases, wliich can be
carried out in
the absence of added or recycled Ha or CO, wherein the gasification reaction
occurs at a
temperature range from about 300 to about 700 C and a pressure from about 15
to about
100 atmospheres in an environment including steam, an alkali catalyst, and a
mineral
binder material, and wherein said carbonaceous material includes silica and/or
alumina,
and other mineral constituents. The mineral binder material can combine with
at least a
portion of these mineral constituents to inhibit the silica and/or alumina,
and other
mineral constituents from combining with the alkali catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
[00261 The foregoing features of the invention will be more readily understood
by
reference to the following detailed description. Figure 1 is a general Flow
Diagram of a
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Mild Catalytic Coal Gasification (MCCG) Process in,accordance with an
embodiment of
the present invention.
DETAILED DESCRIPTION
[00271 As used in this description and the accompanying claims, the following
terins shall have the meanings indicated, unless the context otherwise
requires:
[00281 The term "catalyst" refers to compositions that are introduced to the
process to facilitate the gasification reactions. The term is not meant to be
limited to the
specific chemical moiety or moieties that activate the carbon surface or
otherwise
actually participate in the gasification reactions.
[0029] "Mild temperature gasification" as used herein, means steam
gasification
of carbonaceous material at about 550 C or lower.
[00301 "Syngas" as used herein, means synthetically produced fuel gas,
typically
produced from standard coal gasification processes, comprising mostly CO and
H2 by
volume.
[0031] "Dry raw gaseous product" as used herein means non-steam or
substantially non-steam products of direct catalytic steam gasification.
Although steam
can be a component of the raw gaseous reaction products from direct catalytic
steam
gasification of carbonaceous materials, reference to 'dry raw gaseous product'
herein
means the gaseous products, other than steam, that flow from the gasification
reactor and
have not been further purified.
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[0032] "CO2 trap material" as used herein can be CaO, Ca(OH)2, dolomite,
limestone, Trona, or other compounds effective for regeneratively combining
with CO2 to
form solid carbonates or bicarbonates, and combinations thereof.
[0033] "Mineral binder material" as used herein can be a calcium salt, such as
CaO, Ca(OH)2, CaCO3, or any other alkaline earth metal salts which can react
with and
tie up silica, alumina, and other mineral constituents of the carbonaceous
feed so as to
inhibit such constituents from reacting with and deactivating the catalyst.
[00341 The present invention provides a catalytic steam gasification process
for
converting carbonaceous materials to gases substantially comprising methane or
other
combustible gases. The process can operate at mild temperatures and produce a
dry raw
gaseous product that can be used either directly as fuel or purified to
pipeline quality
methane without the need to remove therefrom substantial quantities of carbon
monoxide
or acid gases. The process can include a feed preparation zone, a gasification
reactor, a
catalyst recovery system, and a CO2 trap regeneration zone.
[0035] In the gasification reactor operating at between about 3 00 C to about
700 C and with pressure in the range from about 10 atm to about 100 atm,
carbonaceous
material can be reacted with oxidizing agents such as steam and/or oxygen in
the
presence of CO2 trap material, and one or more alkali metal salt catalysts, to
produce
predominantly methane as the raw product gas. In preferred embodiments, the
operating
temperature in the reactor is below about 550 C, and the pressure is in the
range from
about 12 to about 40 atm. The gasification reactor can have a moving bed or a
fluidized
bed. Mineral binder material can also be present in the reactor, and can bind
with silica,
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alumina, and other mineral constituents of the carbonaceous feed and thereby
prevent or
inhibit such constituents from reacting with and deactivating the catalyst.
[00361 The feed preparation zone can include one or more mixers for combining
the carbonaceous material, the alkali metal catalyst, the mineral binder
material, and the
CO2 trap material, and a feed system for introducing the catalyst/carbon/CO2
trap mixture
to the gasification reactor as dry solids or as a liquid slurry. The feed
system can be a star
feeder, screw feeder, or other mechanism effective in maintaining required
temperature,
pressure and flow rate of the materials to be introduced to the gasification
reactor.
[0037] The carbonaceous material can be coal, heavy oils, petroleum coke,
other
petroleum products, residua, or byproducts, biomass, garbage, animal,
agricultural, or
biological wastes and other carbonaceous waste materials, etc., or mixtures
thereof. The
coal or other carbonaceous material can be ground or pulverized to an average
particle
size of about 30 to 100 mesh before its delivery for use in the gasification
process. Such
particles can be impregnated with alkali catalyst in aqueous solution and
dried by known
methods. The impregnated and dried particles can be mixed with the CO2 trap
material
and/or mineral binder material and introduced to the gasifier as a single
stream, or such
streams can be fed separately, or in combination, as convenient.
[00381 In a preferred embodiment, however, the carbonaceous materials for use
in
the process can be more coarse, with an average particle size of about 1-2 mm.
Such
coarse particles can be combined and ground with an aqueous slurry of finely
divided
mineral binder material. The resulting paste can be ground with alkali
catalyst, dried at
about 100 C with superheated steam to recover a fine powder of carbonaceous
material
with highly dispersed mineral binder and alkali catalyst having an average
particle size of
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less than roughly 0.02 mm, pelletized to a particle size of about 30 - 100
mesh, and fed to
the gasification reactor. The CO2 trap material can be combined and fed with
the
prepared carbonaceous material, or can be fed separately.
[00391 The CO2 trap material can be CaO or Ca(OH)2, or any other compound
that can react with CO2 to form solid carbonates or bicarbonates, so as to
shift the
kinetics in the direction of increased methane concentration in the raw gas
product. In
particular embodiments the CO2 trap material is CaO. Sufficient CO2 trap
material can
be used so as to remove substantially all the CO2 from the products of the
reaction to
yield a dry raw gaseous product containing less than about 2% CO2 by volume.
The
molar ratio of CO2 trap material to carbon in the reactor can be in the range
of about 0.1:1
to about 1:1, or more particularly in the range of about 0.3:1 to about 0.7:1,
and more
particularly about 0.5:1. On a weight basis, if CaO is used as the CO2 trap
material, the
CaO to carbon ratio fed to the reactor can be in the range of about 0.5:1 to
about 4:1, or
more particularly in the range of about 1:1 to about 3:1, and more
particularly about 2:1.
The CO2 trap material can be effective without being highly dispersed on the
carbon
surface. Thus operating convenience can dictate whether the CO2 trap material
and the
carbonaceous feed are mixed and then fed or introduced separately to the
gasifier.
[0040] The alkali catalyst can comprise any ofNa2CO3, K2CO3, Rb2CO3, Li2CO3,
Cs2CO3, KNO3, K2SO4, LiOH, NaOH, KOH, or any suitable alkali metal salts, or
naturally occuring minerals containing alkali metal salts such as Troria, or
mixtures
thereof. The catalyst can be a single compound or a combination of alkali
metal salts,
which can be binary or ternary salt mixtures. The alkali catalyst loading can
be from 1 to
50 weight percent based on the carbonaceous feed on a dry, ash-free basis.
Preferably,
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the alkali loading is in the range of about 1 to 30 wt%. The alkali catalyst
can be
effective without the presence of any fluorinated compounds.
[00411 The alkali metal salt catalyst can comprise a eutectic salt mixture of
Li2CO3, Na2CO3, K2C03, Rb2CO3, and Cs2CO3 or mixtures thereof. In one
embodiment,
the eutectic salt mixture can be a binary salt mixture of about 29% Na2CO3 and
about
71% K2C03, mole percent. In other embodiments the eutectic salt mixture can be
a
ternary composition of about 43.5% Li2CO3, 31.5% Na2CO3 and 25% K2C03, mole
percent, or a ternary salt mixture of about 39% Li2CO3, 3 8.5% Na2CO3 and
22.5%
Rb2CO3, mole percent.
[0042] The mineral binder material can be a compound or a mixture of
compounds selected from the group consisting of CaO, Ca(OH)2, CaCO3, and other
alkaline earth metal salts. The mineral binder can be kneaded or otherwise
dispersed on
the carbonaceous feed particles in a feed pretreatment step before the alkali
catalyst is
contacted with the carbonaceous feed. In the present invention, kneading
calcium salts
with the carbonaceous feed particles can be used to help prevent mineral
interactions with
the alkali metal catalyst. In other embodiments, the carbonaceous feed, the
mineral
binder, and alkali catalyst can be mixed together simultaneously by
conventional
methods. In still further embodiments, the mineral binder material can be fed
separately
to the gasifier and/or mineral binder material can forin in the gasifier,
wherein such
mineral binder material (e.g., CaCO3) can react with silica, alumina, and
other mineral
constituents present in the carbonaceous feed and prevent or inhibit some
alkali catalyst
loss and deactivation.
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[00431 The mineral binder can combine with at least a portion of any reactive
mineral constituents in the carbonaceous feed such as aluminum and silicon
constituents,
and thereby prevent or inhibit such reactive mineral constituents from
reacting with the
alkali catalysts. The mineral binder material can thus be effective at
stoichiometric
quantities about equal to that of the reactive mineral constituents in the
carbonaceous
feed. Tllus, for example, if the carbonaceous feed material is Illinois #6
coal which
contains on a dry basis about 10 to 11 wt% ash of which silica comprises about
51 wt%
and alumina comprises about 18 wt%, then 7.1 tons of CaCO3 or the equivalent
amount
of another mineral (e.g., about 4.0 tons of CaO) would be enough to react with
all the
silica and alumina in 100 tons of Illinois #6 coal. It rt7ay be preferable to
use a higher or
lower than stoichiometric amount, e.g. in the range of about 0.5 to about 1.5.
Higher
amounts of mineral binder can promote more complete material binding,
particularly at
higher operating temperatures. Lower amounts can be sufficient at milder
operating
temperatures.
[00441 It may be desired to process carbonaceous feeds according to this
invention promoting mineral binding or CO2 trapping or both. Thus, for the
example of
using CaO for either purpose with Illinois #6 coal, to promote only mineral
binding, the
amount of CaO utilized can be in the range of about 2 to 6 wt% and is
preferably highly
dispersed with the feed; whereas to promote CO2 trapping, higher amounts in
the range of
50 to 200 wt%, which need not be highly dispersed with the feed can be
utilized. It is
expected that substantial amounts in the range of at least 60% to about 90% of
the CO2
trap material can be recovered in the CO2 trap regenerator and recycled within
the
process, such that the amount of fresh CO2 trap material can be about 5 to 80
wt% CaO.
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To promote both mineral binding and CO2 trapping, the feed can include about
5% CaO
highly dispersed within the feed and the balance as a separate stream.
100451 The reactor is designed so that a solid purge can be periodically or
continuously withdrawn. The CO2 trap material reacts with CO2 in the reactor
and is
withdrawn in the "carbonated" form with the solid purge. If the CO2 trap
material is CaO
or Ca(OH)2, the solid purge can include particles of CaCO3, as well as
particles of
unreacted carbon, the ash or mineral constituents of the carbonaceous feed,
and some
alkali catalyst in various forms.
[00461 The process of the invention can include a regeneration process of
conventional design to recover and recycle active CO2 trap material, if
desired or
necessary. If the CO2 trap material is CaO, for example, the CO2 trap material
regenerator can be a calciner. In such case, CaCO3 particles can be separated
from said
withdrawn solids by passing through a coarse sieve, or by elutriation of fine
particles or
other techniques, and can be directed to the calciner to recover the CaO. If
necessary, the
recovered CaO can be activated or its surface area increased by steam
treatment or
similar treatment, during or after calcination and prio.- to recycling. The
regenerated CaO
recycled to the gasifier can constitute as much as about 90% of the calcium
value
withdrawn in the solid purge. The calcined off-gas, mostly CO2 and possibly
some
CaCO3, CaS and CaSO4, as well as H2S and possibly SO2 and 02, can be
sequestered or
otherwise properly disposed.
[00471 The solid purge fraction that passes through the sieve can include
soluble
alkali metal salts, and can also include insoluble alkali and/or calcium
aluminosilicates.
These can be treated in a catalyst recovery system for recovery and recycle of
the
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catalyst. The catalyst recovery system can comprise a water wash system and
optionally
can comprise a lime digestion system. In one embodiment, the hot carbon/ash
particles
can be contacted with water and soluble catalyst constituents of the particles
can dissolve
into solution. If the particles contain small amounts of alkali
aluminosilicates, then the
water contacting step can be sufficient to accomplish essentially complete
catalyst
recovery. If the washed solids contain appreciable amounts of insoluble alkali
components, the washed solids can be digested in an alkaline solution or
slurry to recover
insoluble alkali moieties. The washed solids can contain sufficient calcium or
other
alkaline compounds such that little or no additional lime or other alkaline
solution is
necessary for digestion.
[00481 The partial pressure and/or concentration of steam can be monitored and
controlled to maximize conversion rates and maximize overall conversion to
methane or
other desired gaseous product such as syngas. In some embodiments, causing the
reaction includes maintaining a molar ratio of steam to carbon in the range of
about 1.5 to
3 and/or controlling partial pressure of the steam by addition of a non-
reactive gas to the
gasification environment.
[0049] The catalytic steam gasification process can produce a dry raw gaseous
product that includes at least about 40% methane and can include at least
about 50%, or
at least about 60%, or even at least about 70% or higher, methane by volume,
without the
need for H2 and CO recycling, or extensive recycling, and without the need for
separate
stage water-gas shift reactions. Other embodiments produce a dry raw gaseous
product
that can include about 80% methane or higher.
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[00501 The overall direct carbon conversion of carbonaceous material to
methane
can be at least about 50%, more particularly at least about 65%, still more
particularly at
least about 80%, and still more particularly at least about 90%. In still more
particular
embodiments, the carbon conversion of the carbonaceous material can be at
least about
50% or at least about 65% at less than about 550 C.
[0051] The invention and specific embodiments are described more fully in the
following examples:
Example 1. Low Temperature Steam Gasification Results
[0052] Steam gasification of Illinois #6 coal was studied at elevated
pressures and
low temperatures. In the absence of a catalyst at 500 C and elevated pressure
(500-1000
psig - i.e. -34 to 68 atm), no coal conversion was observed. When the
temperature was
increased to 700 C, a significant amount of conversion was observed.
Apparently the
lower temperature is insufficient to overcome the activation energy barrier.
Gas analyses
at 700 C showed no or substantially no methane formation for de-mineralized
coal
samples. A small amount of methane was detected for the raw coal gasification.
These
observations are in agreement with the findings that significant amounts of
methane
cannot be generated in the absence of catalysts, and that minerals in coal can
contribute to
catalysis.
100531 The catalytic effects of iron, nickel and potassium in steam
gasification
were also studied. In the presence of these catalysts a substantial amount of
Illinois #6
coal was gasified at 500 C. With single catalysts at about 10 wt% loading,
almost 30-
35 wt% coal was gasified and most was gasified in the first 5 minutes. Methane
and
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carbon dioxide were the main product gases, with little to no carbon monoxide
produced.
Thus, at low temperatures and elevated pressures the equilibrium is shifted to
methane
formation and syngas formation is minimized. Higher coal conversion and
methane
formation were observed at 1000 psig (-68 atm) as expected. Iron- and nickel-
catalyzed
reactions were reactive for about 15 minutes, after which a sharp drop in
reactivity was
observed. Overall, higher conversions were obtained for de-mineralized coal
samples but
the methane concentrations were slightly higher for the raw coal gasification.
[00541 When a potassium salt was used with either iron or nickel salts as a
catalyst for raw coal gasification, synergistic effects were observed. At 500
C and
500 psig (-34 atm) a potassium/iron salt catalyst system (5 wt% each) resulted
in 42 wt%
carbon conversion. The conversion went up to 53 wt% when the catalyst loading
was
increased to 10 wt% each. The gas analyses showed that with this catalyst, a
combination of methane and hydrogen production was favored. A nickel/potassium
mixture 5 wt% each did not show significant synergistic effects (39%
conversion), which
may be attributed to mineral interactions with these salts. At 10 wt% each,
however, a
conversion of 55 wt % was achieved. When the pressure was increased from 500 (-
34
atm) to 1000 psig (-68 atm) the conversion increased to 58 wt%. The gas
analyses for
the potassium/nickel catalyst were comparable to the potassium/iron system
under similar
conditions.
[00551 These conversions indicate that alkali metal salts compliment
transition
metal salts in that they keep them active for longer reaction times. The
active catalyst
state may actually contain three metals (two outside catalysts and one from
the mineral in
coal). Studies also indicated that at 500 C and in the pressure range of 500
to 1000 psig,
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sodium salts were more effective than potassium salts or any transition metal -
potassium
salt mixture. Conversions as high as 70% were obtained with Illinois coal.
Transition
metal - sodium salt mixtures were not investigated.
[00561 The results indicate that coal can be gasified at low temperatures and
elevated pressures to produce methane, and that lower temperatures help to
minimize
syngas formation.
Example 2. MCCG in Accordance with an Embodiment of the Invention
[00571 A process flow diagram for the envisioned low temperature steam
gasification process, mild catalytic coal gasification (MCCG), is shown in
Figure 1.
Among the advantages for this process is, as discussed above, that it is a
simple process.
Particulate coal or other carbonaceous material, particles of CO2 trap
material and/or
mineral binder material, and an alkali metal catalyst solution, can be
combined and mixed
in mixer 100 to form a feed stream and fed to one or more lock hoppers shown
generally
as lock hopper 200. Said particulate streams can be fed separately to mixer
100 or
combined (not shown) before being fed to mixer 100. From lock hopper 200, the
feed
stream can be fed to gasifier 300 by a screw feeder 250, which alternatively
can be a star
feeder, or a mechanism that feeds the carbonaceous material as a liquid
slurry, or any
other feed mechanism known in the art which allows carbonaceous material to be
fed to a
gasifier at a rate, temperature and pressure necessary to achieve the desired
gasification
result.
[0058] Gasifier 300 can be operated in a fluid bed 400A or a moving bed 400B
mode. Advantages of fluid bed mode 400A include ease of design and easy tar
control.
One disadvantage of the fluid bed is that fresh feed particles of coal and the
CO2 trap
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material may be removed with converted residue (solid purge). Also the steam
concentration in the outlet gas will be higher than in the moving bed. In
contrast, the
moving bed mode is more complex because solid recycle is needed to move
partially
gasified coal to the top of the bed to prevent tar from leaving the reactor
with the product
gas. Still, an advantage of the moving bed is that the steam concentration in
the outlet
gas will be substantially reduced and attrition of the CO2 trap material is
minimized. This
mode also maximizes coal conversion.
[0059] When CaO, Ca(OH)2, CaCO3, or another alkaline earth metal salt is
present in gasifier 300, such compounds can react with and tie up minerals in
the coal or
other carbonaceous material, preventing or inhibiting the minerals from
reacting with the
alkali metal salt catalysts so the alkali metal salt catalysts will remain
active longer,
increasing the carbon conversion efficiency and carbon conversion rate and
improving
catalytic recovery. For example, such compounds can react with alumina,
silica, or other
mineral constituents of the coal. The coal or other carbonaceous feed can also
be
pretreated with CaO, Ca(OH)2, CaCO3, or other alkaline earth metal salts to
tie up the
minerals/ash in the coal.
[0060] Gasifier 300 is operated at about 550 C or less and at an operating
pressure of less than about 1000 psig (68 atm). CaO, Ca(OH)2, or other
compounds
effective for regeneratively combining with CO2 can be used as a trap for CO2
and sulfur
gases. This will enhance catalytic activity by driving the reaction forward
and will also
enhance production of methane by shifting the reaction kinetics toward
increased
production of methane.
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[0061] Steam is fed to the bottom of gasifier 300. It can be beneficial to add
a
small quantity of 02/air to the steam to activate the catalyst. In such
embodiments,
between about 0.1% to 3% oxygen or air is added to the steam to provide
oxidized sites
on the coal surface and provide complexes where catalyst can interact with the
coal to
produce higher gasification rates and carbon conversion. Product gases will
leave the top
of gasifier 300 and pass through a condenser 500 to remove steam. The
condensed water
can be used within the catalyst recovery system 600. The product gases, mostly
CH4,
with lesser amounts of H2 and NH3 can be diverted for separation (not shown)
using
traditional methods, as needed. Gas separation will be dependent on target
product end
use. If desired, syngas is produced by lowering pressure and reducing CaO feed
(or other
CO2 trap) to control the H2/CO ratio.
[0062] Spent residue leaves the bottom of reactor/gasifier 300 and is
separated
(700) by a screen or other device to separate the larger sized CaCO3
particles, which form
when CaO or Ca(OH)2 is used as the CO2 trap material. The smaller sized
residue is fed
to extractor/catalyst recovery system, shown generally as 600, where the
catalyst is
dissolved, concentrated (if necessary) and recycled. Residue from extractor
600 then
goes to waste, perhaps landfill, or for by-product utilization after
determination of hazard
waste potential. The calcium carbonate is calcined in calciner 800 and
recycled to mixer
100. The calcined off gas, (mostly C02 and possibly some particulate CaCO3,
CaS and
CaSO4 as well as H2S and possibly S02 and 02) is ready for sequestration if
the system is
operated under pressure.
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Example 3. MCCG in Accordance with Another Embodiment
[00631 In other particular embodiments, coal or other carbonaceous material; a
CO2 trap material such as CaO or Ca(OH)2 particles; and an alkali metal
catalyst solution,
are mixed in mixer 100, fed to lock hopper 200, and fed to gasifier 300 as
described
above. Mixer 100 can comprise an impeller and means to heat the contents such
that the
carbonaceous particles can become impregnated with alkali catalyst therein.
[00641 Gasifier 300 can be operated in a fluid bed 400A or a moving bed 400B
mode, as described, and is operated at a temperature between about 300 C about
700 C
and a pressure from about 12 to about 40 atm. As described in Example 2, CaO
or
Ca(OH)2 can be used as a trap for CO2 and sulfur gases, and CaO, Ca(OH)2,
CaCO3, or
other alkaline earth metal salts can react with alumina, silica, or other
mineral
constituents of the coal.
[0065] The remainder of the process follows that described for Example 2.
Example 4. Test Results of Steam Gasification using KOH and CaO.
[0066] Carbon conversion rate for steam gasification of Powder River Basin
coal
(PRB) was studied in the temperature range of 500 C and 700 C. Carbon
conversion
without catalyst was about 60% after 15 minutes at 700 C and increased to
about 75%
after 30 minutes. With KOH catalyst, the conversion increased to about 65% and
85%
respectively. Interestingly, with KOH catalyst and CaO/C loading of 1:2 molar
(about
2:1 weight ratio), conversion increased to about 95% irrespective of the
reaction time.
Thus, with the CaO trap material, the coal conversion is essentially complete
in just about
15 minutes, showing that a gasification reactor for these conditions can be
designed for a
short residence time and achieve good conversion.
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[0067] The effect of temperature can be shown by comparing the conversion
after
30 minutes at 650 C, 600 C, and 550 C. The uncatalyzed conversion was about
70% at
650 C, and decreased to about 50% at lower temperatures. With KOH catalyst,
the
conversion was about 80% at 650 C and decreased to about 65% and about 60% at
600 C and 550 C respectively. With KOH and CaO (loaded at CaO/C of 1:2 molar
as
above), the conversion at 650 C was nearly 100% and decreased only slightly at
the
lower temperatures to about 90%. (The conversion at 650 C was better than that
observed at 700 C, which was about 95%.)
[0068) The conversion at 500 C after just 20 minutes, using the same KOH and
CaO loading, was at least 90%, and increased slightly after 50 or 60 minutes.
This
demonstrates that the CO2 trap enables the use of lower gasification
temperatures (where
methane formation is favored) and small residence times without unduly
sacrificing
conversion.
[0069] Carbon conversion rate for steam gasification of petroleum coke was
studied at 700 C and 650 C. Carbon conversion without catalyst at 700 C was
about
35% after 15 minutes and only increased to about 45% after 60 minutes and
about 55%
after 90 minutes. With KOH catalyst, the conversion increased to about 45%
after 15
minutes, and to about 55%, 60%, and 80% after 30, 60, and 90 minutes
respectively.
With KOH catalyst and CaO (again loaded at 1:2 molar CaO/C), conversion
increased to
about 85% after 15 or 30 minutes and to about 95% after 60 or 90 minutes. The
corresponding conversions at 650 C and 60 minutes, were 15% for uncatalyzed
petroleum coke, about 50% with KOH, and about 80% with the CO2 trap. The
increase
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in conversion with the CO2 trap at 650 C indicates that steam gasification of
petroleum
coke at 650 C can be economically feasible.
Example 5. Test Results of Steam Gasification using KOH, LiOH, NaOH, and
Ca(OH)2.
[0070] Carbon conversion for catalytic steam gasification in the temperature
range of 500 C and 700 C of Powder River Basin coal (PRB) in the presence of
KOH,
LiOH, NaOH, and Ca(OH)2 was measured. The conversion with KOH was about 90%
and 85% respectively at 700 C and 650 C, and decreased to about 70% at 600 C
and to
about 60% at 550 C and 500 C. Surprisingly, NaOH showed significantly better
performance of about 80% conversion at 600 C and 70% at 550 C, and performed
about
the same as KOH at 700 C, 650 C and 500 C. This suggests NaOH as a preferred
low
cost catalyst for low temperature steam gasification.
[00711 The conversion with LiOH was 5 to 10% lower than with NaOH, except at
500 C where LiOH gave about 65% conversion compared to about 60% conversion
with
NaOH, KOH, or Ca(OH)2. The conversion with Ca(OH)2 was about 70% at
temperatures
from 700 C to 550 C, and dropped below 60% at 500 C.
Example 6. Steam Gasification of Coal and Residua in Accordance with the
Invention
[0072] In other particular embodiments, coal is mixed with an alkali metal
catalyst, and calcium salts selected from CaO, Ca(OH)2, CaCO3 and other
alkaline earth
metal salts as described above, and then mixed with petroleum residua. The
coal/residua
mixture is heated to about 400 to 500 C for about 3 to 30 minutes to disperse
the catalyst,
and then is gasified and further processed as described above.
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[0073] Dispersing the catalyst allows for bettcr catalyst contact, allowing
temperatures to be dropped to about 300 C to about 550 C. Such dispersal also
provides
better contact and reaction between reactive mineral components of the
carbonaceous
feed and such alkaline earth salts, thereby avoiding mineral/catalyst
interactions and
enhancing catalyst recovery. Dispersal also allows for more efficient sulfur
removal,
reduced catalyst quantities, and enables extensive gasification to result in
only unreactive
solids and minerals remaining after gasification is complete. In particular
embodiments,
the coal to residua weight ratio is in the range of about 1:1 to about 1:10.
100741 The four feed components can be combined first into two streams, one
comprising coal and calcium salts, and the other comprising alkali catalyst
and residua;
and such combined streams can then be mixed together and heated, as above, to
about
400 to 500 C for about 3 to 30 minutes to disperse the alkali catalyst onto
the coal. This
advantageously allows catalyst to be removed from potential poisons more
quickly and
leaves the coal mixture exposed to the catalyst only in a dilute phase. Again,
such
dispersal allows more complete gasification of the carbon/residue mixture to
gaseous
product.
[0075] Alternatively, a small amount of residua can be combined with the coal,
blended with the catalyst, and then blended with the balance of the residua.
The
coal/residua/catalyst mixture is then introduced into a reactor at the
dispersing
temperature described above (400 to 500 C) for about 3 to 30 minutes as
described, and
the dispersed mixture is introduced into another reactor where steam is added.
Gasification is then done with steam to produce gases (methane, ethane,
propane and
butane) and light distillate C5 to C 10 fraction (gasoline fraction). In this
embodiment,
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catalyst remains dispersed in the liquid phase and only a small amount is
removed with
unreacted material, allowing for better catalyst recovery and recycling,
enhancing
economics.
[0076] While the invention has been described in conjunction with a particular
flow diagram, operating conditions and examples, various modifications and
substitutions
can be made thereto without departing from the spirit and scope of the present
invention.
No limitation should be imposed other than those indicated by the following
claims.
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