Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the upgrading of solid
fuels. More particularly, it is concerned with the benefi-
ciation of low rank solid fuels such as sub-bituminous coal
and lignite.
Millions of tons of low rank solid fuels exist in
this country and, although many of these deposits may be
readily mined, they are not used extensively as fuels because
for the most part, they are located at a great distance from
the point of ultimate use and in addition they have several
characteristics wbich make them less valuable as fuels~ For
example, these low rank solid fuels, although generally they
have a relatively low sulfur content, still contain too much
sulfur to permit their use as a fuel and yet meet the current
regulations with respect to S02 emissions. In addition, to
make these coals economically attractive, means must be
found for separating the components of the coal having
little or no heating value from those components having a
high heating value. Thus, inorganic mineral matter, water
and carbon dioxide are desirabl~ removed from quch fuels to
produce a fuel having a higher B~U per pound value and
thereby produce a fuel which is more economic to transport
either by rail or pipeline.
The removal of ash-forming minerals from coal is
difficult and ordinary beneficiation techniques such as
jigging, tabling and sink and float techniques are not
particularly efficient with the lower rank coals. Ash-
forming minerals generally occur in mined coals either as
"segregated impurities" or as an inherent part of the coal.
The segregated ash forming impurities are those that exist
as individual discrete particles when the coal has been
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broken down. They are composed, for the most part, of
shale, clay, sand, stone and other mineral material derived
either from strada interbedded with the coal or from the
roof and floor of the coal bed. Ordinarily, they are remov-
able by mechani~al means. On the other hand, the term
"inherent" or "fixed" ash is used to distinguish that part
of the impurity i.n the coal which cannot be separated by
mechanical means. For economic and practical reasons, there-
fore, it is deQirable to reduce the ash content of the fuel
but conventional procedure~ have little effect on the fixed
ash.
Another undesirable characteristic, particularly
in the case of low quality fuels such as sub-bituminous coal
and lignite, is that these fuels contain a considerable
amount of combined or bound water. This is a most undesir-
able ingredient in that although bound water is present in
the solid fuel it does not play any part in the formation of
the slurry vehicle and consequently it has little effect on
the viscosity or pumpability of a solid fuel-water slurry.
This means that if the fuel is to be transported by pipeline
additional energy is consumed in the movement of the slurry.
Additionally, the bound water affects the use of the fuel as
its presence in the combustion zone results ~n a reduced
flame temperature. It is therefore desirable to remove as
much combined or bound moicture a conveniently practical
from the solid fuel prior to its transportation or use.
It is also desirable to reduce the oxygen content
of the solid fuel and as a result, increase its BTU value.
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It is therefore, an object of this invention to reduce the sulfur
content of solid fuels. Another object is to reduce the bound water content
of solid fuels. Still another object is to increase the heating value of
solid fuels by reducing the ash and oxygen content thereof. These and other
objects will be obvious to those skilled in the art from the following
disclosure.
These objects can be fulfilled by subjecting the solid fuel to
hydrothermal treatment in the presence of a decarboxylation catalyst.
According to the invention, there is provided a process for the
beneficiation of a solid fuel which comprises forming a mixture of
particulate solid fuel and water, heating the mixture to a temperature
between about 300F. and 706F. at a pressure sufficient to maintain
substantially all of the water in liquid state in the presence of an added
catalytic amount of a decarboxylation catalyst comprising a soluble salt of
copper, nickel or vanadium.
The feed used in the process of our invention includes any solid
carbonaceous combustible material containing ash-forming ingredients and/or
sulfur compounds and/or bound water and carboxylates and mixtures thereof.
Such materials include bituminous coal, sub-b;tuminous coal, lignite,
biomass, organic waste and the like. The feed solid fuel should be ground
so ~hat at least 80% and preferably 100% passes through a U.S. Standard 14
mesh sieve, the finer the grind, the faster and more complete the reaction
for a given set of reaction conditions.
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The particulate solid fuel and water are mixed in
an amount to provide a mixture cont:aining from about 30 to
65 weight % solids with 40 to 60% solids being preferred.
If the process is of the batch type, the solid fuel and
water may be charged separately to the reaction zone such as
an autoc]ave or they may be charged together as a slurry.
In such latter event, the water excluding bound water should
be present in the slurry in an amount between about 40 and
60 weight % as, if the water content is less than about 40
percent, the slurry may be difficult to pump. Such a slurry
is also used when the process is of the continuous type
where the slurry is, for example, passed through an
elongated tubular hydrothermal reaction zone.
The hydrothermal or beneficiation trea-tment is
well known and is disclosed, for example, in U. S. Patents
4,018,571 and 4,104,035 to Cole et al. As practiced in the
process of the invention, it may be effected under either
static or dynamic conditions. In one embodiment of the
invention, the slurry is introduced into a pressure vessel
which is then sealed and heated under autogenous pressure to
a temperature between about 300 and 706F, preferably
between 400 and 650F, with the pressure being such that
water in the liquid state is maintained in the reaction
vessel. After a period of time between about one minute
and two hours, the vessel is vented and the slurry removed
therefrom.
In another embodiment of the invention, the solid
fuel-water slurry is passed under conditions of turbulent
flow through an elongated tubular reaction ~one. The con-
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tinuous reaction conditions are substantially the same asthose recited above for the batch I:ype of operatlon.
To assist in the removal of oxygen, the hydro-
thermal treatment is carried out in the presence of a decar-
~oxylation catalyst comprising a soluble salt of copper,
nickel or vanadium such as the chloride, benzoate, ~arbonate,
acetate and the like, i.e. any salt that is soluble to at
least 0.1 wt. % in the reaction mixture.
The catalyst may be present in the slurry in an
amount between about 1 and 15 percent based on the total
weight of the slurry with concentrations of from 4 to 8
weight percent being preferred.
When the catalyst is reduced to a lower valence
state, it may be regenerated by contact with a free oxygen
containing gas such as air, oxygen enriched air or substan-
tially pure oxygen. In the batch type of process, the
oxygen-containing gas may be pressured into the pressure
vessel or introduced into the reaction zone before the run
is ~tarted and the reaction vessel sealed. Similarly, in
the continuous process the oxygen-containing gas may be
introduced with the slurry feed or at one or variou~ stages
of the reaction.
The reaction may be promoted by the presence of
salts of magnesium, cobalt and barium present in an amount
between about 0.001 and 0.2 moles per lOOg of solid fuel.
The following examples are submitted for illustra-
tive purposes only and it should not be construed that the
invention is restricted thereto.
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EXAMPLE I
The feed in this example is a sub-bituminous coal
having the following analysis:
TAsLE 1
Ultimate Moisture and
Analysis As Received Ash Free
Moisture % 23.9
Carbon ~ 45.5 71.21
Hydrogen % 5.9 5-05
Nitrogen % 0.69 1.08
Sulfur % 0.71 1.11
Ash ~ 12.2
Oxygen % (By difference) 11.1 21.57
Calorific Value, BTU/lb. 7637 12088
The coal was air dried at room temperature and
pulverized in a hammer mill to minus 60 mesh. Several 200
gram portions of coal were then riffled from the large batch
of coal to use for a serieR of hydrcthermal treatment
experiments .
To a two liter steel bomb equipped for operation
at high pressure were added 200 grams of coal, 400 grams of
water and an amount of cupric acetate listed below in Table
2. The bomb was then sealed, flushed with nitrogen and
pressured except in Run 1 to 200 psig with nitrogen. The
bomb was then heated with rocking to the desired temperature.
After 30 minutes at this temperature the apparatus is cooled
to 300F and the water and gas is vented. After cooling to
room temperature, the coal is removed from the bomb and
washed with 200 ml. of water on a Buchner funnel to remove
the residual catalyst from the coal. The product coal is
air dried, ground ~o minus 60 mesh and sampled for analysis.
The results of these runs are summarized in Table 2.
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TABLE 2
Run Temp. Max Pre~sure Cupric Acetate %02(MAF)* BTU/lb.
No. F PSIG ~ in--coal (MAF)*
_,
** - - - 21.57 12088
1 500 700 4 21.24 12019
2 500 1150 8 19.18 12440
3 500 1200 0 22.30 12159
4 400 620 8 20.30 12343
400 630 0 21.93 11813
* Moisture and ash free
** Untreated
It is clear from the results presented in Table 2
that the addition of cupric acetate benefically decreases
the oxygen content of the coal. When the higher concentra-
tion of catalyst is used, the loss of oxygen from the coal
is reflected in an increase in the BTU content relative to
control runs and feed coal. At low catalyst concentrations,
the change in BTU appears to be small and within experi-
mental error.
EXAMPLE II
The runs in thi~ example are substantial dupli-
cates of the runs in Example I, the differences being that
the bomb was initially flushed with oxygen, pressured with
oxygen to the pressure designated in Table 3 at room temp-
erature and then pressured to a 200 psig total pressure with
nitrogen. Data on these runs appear below in Table 3.
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The results in Table 3 clearly demonstrate the
beneficial effect of oxygen when used in conjunction with
cupric acetate in the hydrothermal treatment of a low qual-
ity coal over the runs in Table 2 which were carried out in
an inert atmosphere in the absence of added hydrogen. In
Runs 7, 8 and 9 the average sulfur reduction was about 10%.
Similar results are obtainable with the soluble salts of
nickel and vanadium.
From the foregoing, it can be seen that the use of
a decarboxylation catalyst permits the hydrothermal treat-
ment to be run at lower temperatures and pressures than con-
ventional hydrothermal treatments. Since the catal~stc are
water-soluble, they can be recovered and re-used. Additional
benefits by way of lower temperatureq and pressures for
equivalent decarboxylation are obtained when the water used
in the hydrothermal treatment contains from 1 to 5 wt. %
alkali metal compound such as sodium carbonate or sodium
hydroxide.
Various modifications of the invention as herein-
before set forth may be made without departing from the
spirit and scope thereof and therefore only such limitations
should be made as are indicated in the appended claims.
