Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACXGROUMD OF T~IE INVE~TION
1. Field of the Invention
The field of this invention relates to a process for
reducing the sulfur content of coal.
2. Prior Art
The problem of air pollution due to the emission of
sulfur oxides when sulfur-containing fuels are burned has
received increasing attention in recent years. It is now widely
recognized that sulfur oxides can be particularly harmful pollu-
tants since they can combine with moisture to form corrosiveacidic compositions which can be harmful and/or toxic to living
organisms in very low concentrations.
Coal is an important fuel, and large amounts are burned
in thermal generating plants primarily for conversion into
electrical energy. One of the principal drawbacks in the use of
coal as a fuel is that many coals contain amounts of sulfur which
generate unacceptable amounts of sulfur oxides on burning.
For example, coal combustion is by far the largest single source
of sulfur dioxide pollution in the United States at present,
and currently accounts for 60 to 65~ of the total sulfur oxide
emlsslons .
The sulfur content of coal, nearly all of which is
emitted as sulfur oxides during combustion, is present in essen-
tially two forms: inorganic, primarily metal pyrites, and
organic sulfur. The inorganic sulfur compounds are m~inly iron
pyrites, with lesser amounts of other metal pyrites and metal
sulfates. The orqanic sulfur may be in the form of thiols, disulfide,
sulfides and thiophenes (substituted, terminal and sandwiched
forms) chemically associated with the coaI itself. Depending
on the particular coal, the sulfur content can be primarily in
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the form of either inorganic sulfur or organic sulfur. Distri-
bution between the two forms varies widely among various coals.
In the United States, except for Western coals, the
bulk of the coal produced is known to be high in pyrite. soth
Appalachian and Eastern interior coals have been analyzed to be
rich in pyritic and organic sulfur. Generally the pyritic sul*ur
represents from about 25% to 70% of the total sulfur content in these
coals.
Heretofore, it was recognized that it would be highly
desirable to remove (or at least lower) the sulfur content of coal
prior to combustion. A number of processes, for example, have
been suggested for removing the inorganic (pyritic) sulfur from
~oal.
For example, it is known that at least some pyritic sulfur
can be physically removed from coal by grinding the coal,
and subjecting the ground coal to froth flotation or washing
processes. While such processes can remove some pyritic sulfur,
these processes are not fully satisfactory because a large portion
of the pyritic sulfur is not removed. Attempts to increase the
portion of pyritic sulfur removed have not been successful because
these processes are not sufficiently selective. Because the process
is not sufficiently effective, a large portion of coal can be
discarded along with ash and pyrite.
There have also been suggestions heretofore to
chemically remove sulfur from coal. For example, U.S. Patent
3,768,988 to Meyers, issued October 30, 1973, discloses a process
for reducing the pyritic sulfur content of coal involving exposing
coal particles to a solution of ferric chloride. The patent
suggests that in this process ferric chloride reacts with pyritic
sulfur to provide free sulfur according to the following reaction
process: 2FeC13+FeS2 -~ 3FeC12+S
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While this process is of interest, a disadvantage of this process
is that the liberated sulfur solids must then be separated from
the coal solids. Processes involving froth flotation, and
vaporization are proposed to separate the sulfur solids. All
of these proposals, however, inherently represent a second
discrete process step with its attendant problems and cost which
must be employed to remove the sulfur from coal.
In another approach, U.S. Patent 3,824,084 to Dillon
issued July 16, 1974, discloses a process involving grinding
coal containing pyritic sulfur in the presence of water to form
a slurry, and then heating the slurry under pressure in the
presence of oxygen. The patent discloses that under these
conditions the pyritic sulfur (for example, FeS2) can react to
form ferrous sulfate and sulfuric acid which can further react
to form ferric sulfate. The patent discloses that typical reaction
equations for the process at the conditions specified are
as follows:
- FeS2+H20+7/202 t FeS04+H2S4
2FeSO4+H2sO4~l/2o2 ~Fe2 (SO4) + H2O
These reaction equations indicate that in this parti-
cular process the pyritic sulfur content continues to be associated
with the iron as sulfate. While it apparently does not always
occur, a disadvantage of this is that insoluble material,
basic ferric sulfate, can be formed. When this occurs, a discrete
separate separation procedure must be employed to remove this
solid material from the coal solids to adequately reduce sulfur
content. Several other factors detract from the desirability of this
process. The oxidation of sulfur in the process does not
proceed at a rapid rate, thereby limiting output for a given
processing capacity. In addition, the oxidation process is not
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highly selective such that considerable amounts of coal itself
can be oxidized. This is undesirable, of course, since the amount
of coal recovered from the process is decreased.
Numerous other methods have been proposed for reducing
the sulfur content of coal. For example, U.S. Patent 3,93~,966,
to Kindig et al issued February 17, 1976, discloses treating
coal with iron carbonyl to enhance the magnetic susceptibility
of iron pyrites to permit removal with magnets. In summary,
while the problem of reducing the sulfur content of coal has
received much attention, there still exists a present need for
a practical method to more ef~ectively reduce the sulfur content
of coal.
SUMM~RY OF THE INVENTION
This invention provides a practical method for more
effectively reducing the sulfur content of coal. In its broad
aspect, this invention presents a process for reducing the pyritic
sulfur content of coal comprising:
- 1) contacting an aqueous slurry of water and coal
particles at elevated temperature with oxygen;
2) maintaining the aqueous slurry at a pH of from
about 5.5 to 12.0; and
3~ recovering coal particles of reduced pyritic sulfur
content.
A particularly important aspect of this invention is
that the aqueous slurry be maintained at a pH in the range of
from about 5.5 to 12.0 during the process. It has been discovered
that maintaining the pH in this range provides faster reaction
rates (reducing processing time), more selective oxidation of
sulfur compounds, and some organic sulfur removal. These
desirable attributes are important, and are made available in
the process of this invention.
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DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED EMBODI~NTS
This invention provides a method for reducing the
pyritic sulfur content of coal by a process comprising:
l) contacting an aqueous slurry of water and coal
particles at elevated temperature with oxygen;
2) maintaining the aqueous slurry at a pH of from
about 5.5 to 12.0; and
3) recovering coal particles of reduced pyritic
sulfur content.
The novel process of this invention is especially
effective for reducing the pyritic sulfur content of coal. An
advantage of the process is that it can also provide a reduction
in the organic sulfur content of some coals.
~uitable coals which can be employed in the process
of this invention include brown coal, lignite, subbituminous,
bituminous (high volatile, medium volatile, and low volatile),
semi-anthracite, and anthracite. Regardless of the rank of feed
- coal, excellent pyri~e removal can be achieved by the process of
this invention.
The coal particles employed in this invention can
be provided by a variety of known processes, for example, grinding.
The particle size of the coal can vary over wide
ranges and in general the particles need only be sufficiently
small to enhance contacting with the aqueous medium. For instance,
the coal may have an average particle size of one-fourth inch
in diameter or larger in some instances, and as small as minus
200 mesh tTyler Screen) or smaller. The most practical particle
size is often minus 5 mesh, preferably minus 18 mesh, as less
energy is required for grinding and yet the particles are suffi-
0 ciently small to achieve an optimum rate of pyrite removal~
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The manner of forming the aqueous slurry of water and
coal particles is not critical. The aqueous slurry of water and
coal can be formed, for example, by grinding coal in the presence
of water or water can be added to coal particles of a suitable size.
Preferably, the aqueous slurry contains from about 5 to about
50~, by weight, coal particles and more preferably from about
10 to about 30%, by weight, coal particles and the balance water.
From about 0.01 to 1~, by weight of coal, of a wetting
agent can be a useful addition to the slurry. Suitable wetting
agents include anionic, nonionic and amphoteric surfactants.
This aqueous slurry of coal is contacted, in a suitable
vessel, for example, an autoclave,at elevated temperatures in the
presence of oxygen, preferably at pressures above atmospheric, such
that pyritic sulfur is preferentially oxidized without significant
adverse oxidation of the coal substrate. For example, temperatures
of from about 150 to 350F., more preferably from about 175 to about
270~F. can be suitably employed. The oxygen can be present as
pure oxygen gas or it can be mixed with other inert gases. For
example, air or air enriched with oxygen can be suitably employed as
a source of gaseous oxygen. Preferably, the gaseous oxygen is above
atmospheric pressure, for example, pressures of from about 50 to
500 psig., and more preferably from about 100 to 400 psig. If the
oxygen is mixed with other gases, the partial pressure of oxygen
is most suitably within the pressure ranges mentioned hereinbefore.
Under these conditions, the oxygen gas and water readily
remove pyritic sulfur from the coal. This removal involves
oxidation of the pyrltic sulfur to sulfate, thionate and thio
; sulfate forms. As the reaction proceeds, oxygen is consumed.
Additional oxygen can be added to the system to maintain the
partial pressure of oxygen.
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The coal should be held under these conditions for
a period of time sufficient to effect a significant reduction
in the pyritic sulfur content, i.e., a reduction of 50%, and more
preferably, a reduction of from 70% to 95% or more, by weight, of
pyritic sulfur. Generally, a time period in the range of from about
5 minutes to 2 hours can be satisfactorily employed. Preferably,
a time period of from 10 minutes to 1 hour is employed. During
this time, it can be desirable to agitate the aqueous slurry of
coal and water. Known mechanical mixers, for example, can be
employed to agitate the slurry.
When coal containing pyritic sulfur is held under
these reaction conditions, the pH of the aqueous slurry falls
since sulfuric acid is formed in the reaction. The final pH
will be greatly dependent on the level of pyritic sulfur in the
feed coal. Often the final pH is quite low, for example, the
pH of the reaction slurry can fall to a pH of from about 1 to 3,
or less. It has been found that if the pH of the aqueous slurry
is maintained at from 5.5 to 12.0, preferably 6.5 to 10.0 that
certain very distinct advantages are obtained. (As used herein,
"maintain" means keeping the pH within the required limits for at least
a period of time sufficient to substantially obtain the ad~antages
of the invention). As noted hereinbefore, these advantages
include faster reaction rates, and more selective oxidation. Just
why these advantages are achieved is not fully understood. While
not wishing to be bound to any particular theory, it is suggested
that one reason for the advantage may be that the sulfur oxidized
does not remain associated with the iron. The following chemical
equation, employing, for example, ammonium bicarbonate to maintain
pH, could be representative of the reaction course:
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~0~4fl~3~
FeS2 + 2 + H20 + NH4HC03 ~ Fe(OH)
or Fe2(CO)3 + (NH4)2 SO~
(wherein the II2S04 is neutralized sufficiently to maintain the
appropriate pH range required in the process of this invention).
It will be recognized by those skilled in the art
that there are many ways to maintain the pH of the aqueous slurry
within the desired range. For example, the pH of slurry can
be continuously monitored using commercially available pH meter~,
and a suitable quantity of basic material can be metered to the
slurry as needed to maintain the desired pH. Another suitable
method for maintaining the pH in the desired range involves
adding an appropriate amount of basic material to the aqueous
slurry of coal and water prior to subjecting the slurry to the
reaction conditions involving increased temperature and pressure.
Examples of suitable basic materials include alkali and
alkaline earth metal hydroxides such as sodium hydroxide, potassium
hydroxide, calcium hydroxide, magnesium hydroxide and their
~ corresponding oxides. Other suitable basic materials include
alkali and alkaline earth carbonates, such as sodium carbonate,
sodium bicarbonate, potassium bicarbonate~ ammonia, ammonium
bicarbonate and ammonium carbonate. Among these basic materials,
sodium hydroxide, sodium bicarbonate, potassium bicarbonate and
ammonium bicarbonate are preferred. Suitable basic materials
include suitable buffering agents, generally the salts of weak
acids and strong bases.
Such buffering agents added to the aqueous slurry can
be a very useful aid in maintaining the desired pH. An example of
a suitable buffering agent is sodium acetate~ As oxidation of
the pyritic sulfur proceeds to generate sulfuric acid, part of the
sodium acetate is converted to acetic acid to yield a buffer
mixture, sodium acetate and acetic acid, in situ in the reactor.
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~.0$~1F30
Control of pH within a very narrow range ean be
achieved using such a buffering agent. The most suitable basie
materials for maintaining the pH as required in this process
are those having cations which form soluble salts with sulfur-
oxygen anions such as thiosulfate, sulfate and thionate. The
most suitable basic materials have anions comprising sodium,
ammonium and/or potassium since such materials are readily avail-
able and form water soluble materials with sulfate.
After holding the aqueous slurry of coal partieles and
water under these reaction conditions for a suffieient time, the
pyritic sulfur is substantially oxidized to water separable
sulfur compounds, for example, water soluble sulfate salts.
This water, containing dissolved sulfur compounds,
is separated from the coal particles. Such a liquids-solids
separation is relatively simple, and can be effected in a variety
of ways, Filtering with bar sieves or screens, or centrifuging,
for example, can be employed to separate the coal and water.
The resulting coal product has a substantially reduced
pyritic sulfur eontent and ean exhibit a diminished organic sulfur
eontent. Preferably, the coal is dried prior to use or storage.
The water separated from the coal, containing dissolved
sulfate compounds, can be discarded or more preferably, is
treated to remove the sulfate eontent. The sulfate content can
be removed, for example, by treating the water with compounds
which form insoluble compounds with sulfate. Preferably, the
sulfate content is concentrated prior to such treatment, for
example, by evaporating a portion of the water. For example,
ealcium hydroxide added to eoncentrated sulfate water solutions
will form insoluble calcium sulfate which will precipitate from
the water solution. ~he precipitate and water can be separated
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by conventional methods, such that the resulting water is
substantially free of sulfate content.
The following specific embodiments are provided to
more specifically illustrate the invention described herein.
EXA~LE_I
Portions of Illinois #6 coal were ground and screened
to provide quantities of coal having a particle size of less than
100 mesh. Each of these portions (feed coal) was analyzed to
determine its sulfur content and sulfur typeO ~ach of the ground
coal portions were then treated in the following manner. The
portion of coal particles and water was added to an autoclave to
form a slurry. In accordance with the invention, a quantity of
alkali material (pX control a~ent) was added to the slurry
to maintain a desired pH. The autoclave was sealed and
heated to the indicated temperature, oxygen was then introduced
to the autoclave and maintained at the indicated oxygen pressure.
The coal was held under these conditions for a period of time,
- and then filtered to separate the coal and water.
The particular reaction conditions employed, and the
reduction in sulfur content obtained are shown in Table I below.
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~a
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0~ 80
Run 1 is not an example of the invention, but is
provided for comparative purposes to illustrate the advantage of
the ~nvention. Run 2 is an example of the invention. As can
be seen, the reaction conditions of Run 2 provide greater
pyritic sulfur removal than those employed in Run 1.
The process disclosed herein can be conducted on a
batch, semi-continuous or continuous basis. All parts and
percentages herein are based on weight unless otherwise specified.
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