Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUt~) OF THE INVENTION
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 parti-
cularly harmful pollutants since they can combine withmoisture to form corrosive acidic 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 emissions.
The sulfur content of coal, nearly all of which
is emitted as sulfur oxide~ during combustion, is present
in essentially two forms: inorganic, pr~arily metal
pyrites, and organic sulfur. The inorganic sulfur com-
pounds are mainly iron pyrites, with lesser amounts of
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other metalpyrite3 and metal sulfate~. The organic sulfur
may be in the form of thLols, disulfide, sulfides and
thiophenes (substituted, terminal and sandwiched forms)
chemically associated with the coal itself. Depending
on the particular coal, the sulfur content can be pri-
marily in the form of either inorganic sulfur or organic
sulfur. Distribution between the two forms varies widely
among various coals.
In the United States, except for Western coal3,
the bulk of the coal produced is known to be high in
pyrite. Both Appalachian and Eastern interior coals have
been analyzed to be rich in pyritic and organic sulfur.
Generally the pyritic sulfur represent~ from about 25% to
70S of the total sulfur content in these coals.
~eretofore~ 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 coal.
For example, it is kno~n 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 i5 not sufficiently selective, 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,
disclose~ a proces~ for reducing the pyritic sulfur content
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of coal involving expo~ing coal particles to a solution of
ferric chloride. The patent suggests tha~ in thiR process
ferric chloride reacts with pyritic sulfur to provide free
sulfur according to the following reaction proces~:
2FeC13+FeS2 3FeC12+S
While thi~ proceqs is of interest, a disadvantage of thi~
process is that the liberated sulfur solids must then be
- separated from the coal solids. Processes involving froth
flotation, and ~aporization are proposed to separate the
sulfur solids. All of these proposal~, 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
eguations for the process at the conditions specified
are as follows:
FeS2+H20+7/202 FeS04+H2S04
2FeSO4+H2SO4+l/202 Fe2~So4)3+H2o
These reaction eguations indicate that in this
particular proces-c the pyritic sulfur content continues to
be associated with the iron as sulfate. While it apparent-
ly does not always occur, a disadvantage of this is that
insoluble material, basic ferric sulfate, can he formed.
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In addition, elemental sulfur which is also water soluble can
be formed. When these materials are formed, a discrete sepa-
rate 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 desirabil~ty
of th~s 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 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.
In this prior art process, the water separated from
~ the coal contains dissolved acidic sulfur compound$ for example,
- sulfuric acid. This water is not acceptable for disposal
and must be treated, for example, with lime, to remove the
dissolved sulfur compounds. This is a disadvantage in that
i this treatment represent3 a further process step.
SUMMARY 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 proce-s for reducing the
pyritic sulfur content of coal comprising the steps of:
1) contacting an aqueous slurry of water, an
alkaline earth metal base and pyrite-containing
coal at elevated temperature with oxygen, said
alkaline earth metal base being present in an
amount at least equal to the stoichiometric
amount of pyrite, and said aqueous slurry being
maintained at a pH of from about 5.0 to about
12.0; and
2) recovering coal particles of reduced pyritic
sulfur cont-nt.
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A particularly important aspect o~ this invention
is that the aqueous slurry is maintained at a p~ ln the range
of from about 5.5 to 12.0 during the process. It has been
discovered that maintaining the p~ in thi~ range provides
faster reaction rates (reducing processing time), more selective
oxidation of sulfur compounds, and some organic sulfur removal.
It has also been discovered that maintaining this pH range
can substantially reduce elemental sulfur formation.
Another aspect of this invention is that dissolved acid
i 10 sulfur compounds react with alkaline earth metal base during
the process forming insoluble compounds more acceptable for
disposal. These desirable attributes are important, and
are made available in the process of this invention.
DETAILED DESCRIPTION OF THE INVENTION
AND ITS PREFERRED EMBODIMENTS - _
This invention provides a method for reducing the
pyritic sulfur content of coal by a process comprising the
steps of:
1) contacting an aqueous slurry of water, an
alkaline earth metal base and pyrite-containing
coal at elevated temperature with oxygen, said
alkaline earth metal base being present in an
amount at least equal to the stoichiometric
amount of pyrite, and said aqueous slurry being
maintained at a pH of from about 5.0 to about
12.0; and
2) recovering coal particles of reduced pyritic
sulfur content.
~he novel proce~s of this invention is especially
- 30 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 ~ulfur content of some coals.
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Sultable coals which can be employed in the process
of this inven~ion include brown coal, lignite, subbltuminous,
bituminous ~high volatile, medium volatile, and low volatile),
semi-anthracite, and anthracite. Regardless of the rank of
feed coal, excellent pyrite removal can be achieved by the
process of thi~ invention.
The c081 employed in this invention is most suit-
ably in the form of particles, or particles of coal agglomer-
ated with oil into coal-oil agglomerates.
Coal particles 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 (U.S. 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 sufficiently small to achieve an
optimum rate of pyrite removal.
Coal-oil agglomerates are most suitably formed from
coal particles as small as minus 200 mesh or smaller and more
generally minus 80 mesh. Such agglomerates can be formed by
agitating a water slurry of coal particles with from about
5% to 60%, preferably 5% to 30%, and more preferably 5~ to
15%, by weight of coal, of hydrocarbon oil.
Coal-oil agglomerates are most suitably formed
by adding hydrocarbon oil to an aqueous slurry of coal
particles and agitating the slurry.
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Sultable hydrocarbon oll~ for forming coal-oil
agglomerates are derived from petroleum, shals oil, tar sand
and coal. Suitable hydrocarbon oils lnclude light and heavy
refined petroleum fractlons, for example, light cycle oil,
vacuu~ gas oll, residual oil, coal tar and solvent refined
coal oil. Mlxture~ of the various hydrocarbon oils can
also be employed, particularly when one of the materials
is very vlscous.
The most suitable hydrocarbon oils are light
cycle oil, heavy cycle oil, heavy gas o~l, coker o~l and
residual oil.
The hydrocarbon oils are hydrophobic and will
wet the coal particles. When an aqueous slurry of coal
particles is contacted with the hydrocarbon oil and agitated,
the hydrocarbon wet coal particles collide with one another
forming agglomerates. In general the size of the coal-oil
agglomerate is at least about 2 to 3 times the average size of
the coal particles which make up the coal-oil agglomerates.
Agitating the mixture can be suitably accomplished
using stirred tanks or other apparatus. An apparatus which
provides a zone of shearing agitation is preferred for
agitating the mixture.
The term ~coal particulates" will be employed
hereinafter from time to time to refer to coal particle~
and/or coal-oil agglomerates.
In the process of thig invention an aqueous slurry
of water and coal particulates 19 contacted at elevated
temperature~ with oxygen. 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 auitable size. Preferably, the aqueou~ slurry contains
from about 5 to about 50~, by weight, coal particulates and
more preferably from about 10 to about 30~, by weight, coal
particulates and the balance water.
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This aqueous slurry of coal is ~ontacted, in a
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 270Y. 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 pyritic sulfur to sulfate, thionate
and thiosulfate forms. As the reaction proceeds, oxygen is
consumed. Additional oxygen can be added to the system to
maintain a constant partial pressure of oxygen.
' The feed coal should be held under these conditions
for a period of time sufficient to effect a significant reduc-
tion 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.
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- When co~l cont~inlng pyritlc sulfur ~s held under
these reaction conditions, the pR of the aqueo~ slurry falls
since sulfuric acld i~ ~ormed in the reaction as pyr~te i8
oxidized. Thi~ invention contemplates a process lnvolving
~ removing an amount of pyrlte from coal such that without the
- presence of ba~e material the pH would fall below about 5Ø
Thi~ i~ a condit$on whlch would generally occur lf meaningful
pyrite reductlon is obtained. In such a situation, the final
pH can be 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 about 5.0 to about 12.0,preferably 5.5 to 12.0, and most
preferably from about 6.5 to about 10.0 that certain very distinct
advantages are obtained. (As used herein, "maintain" means
keeping the pH within requlred limits for at least a period
of time sufficient to substantially obtain the advantages of
the invention, i.e., a significant reduction in pyritic sulfur.)
As noted hereinbefore, these advantages include faster reaction
rates, more selective oxidation and reduced formation of
elemental sulfur.
A pH of from about 5.0 to about }2.0 during the course
of the reaction is preferably maintained by employing an amount
of alkaline earth metal base material in excess of the stio-
chiometric amount of pyrite in the coal. Preferably, the
amount of alkaline earth metal base material employed is
from about 1.5 to 3 times to the stoichiometric amount of
pyrite. Examples of alkaline earth metal bases include
calclum hydroxide, lime, l~mestone, magne~ium oxide, mag
nesium carbonate and dolomite. The preferred alkaline earth
metal bases mater~al~ are the calcium bases, for example,
calclum hydroxlde, lime and limestone. The most preferred
ba~e materlal i8 llmeston~.
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While an excess of alkaline earth base material is
~- preferred for maintaining the pH, it is within the scope of
the invention to employ other base materials in addition to
- alkaline earth metal base material to maintain desired pH.
Examples of other base materials are alkali metal bases, for
example, sodium hydroxide, potassium hydroxide, and their
corresponding oxides, sodium carbonate, sodium bicarbonate,
potassium bicarbonate. Ammonia, ammonium bicarbonate and
ammonium carbonate are additional examples of other suitable
base materials.
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 meters, 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.
The process of this invention requires at least a
stoichiometric amount of alkaline earth metal base material.
Such a base material not only acts to maintain the desired pH
of the aqueous coal slurry, but it also forms insoluble salts
; with the sulfur species removed from the coal in the course
of the reaction. Heretofore it was assumed that these water
insoluble salts would be difficult or impossible to separate
from the coal particulates. It has been discovered that this
is not the case. Therefore, if the amount of base material
present is at least equal to the stoichiometric amount of
pyrite the desirable re.sult is that water soluble sulfur
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~lZ3773
species removed frcm coal in the process are converted
to water separable insoluble solids. This is ln contrast
to prior suggested practice wherein process water containing
~- soluble sulfur qpecies removed from coal was separated
from the coal; and, the process water was subjected to a
separate step (generally involving addition of lime, etc.
to form insoluble solids) to separate the environmentally
unacceptable acidic sulfur compounds from the process
water.
After holding the aqueous slurry of coal particleq
and alkaline earth metal base material under the reaction
conditions of the process, the pyritic sulfur in the coal is
substantially oxidized to water separable compounds which are
predominately water insoluble salts, for example, water
insoluble sulfate salts.
Water containing the insoluble sulfur salts is
separated from the coal particulates. Such a separation is
conveniently made using bar sieves or screens. For example,
screens which are sized to retain the coal particles, and
pass water and very small insoluble alkaline earth metal sulfur
salts.
If very fine coal particles were employed in the
process, e.g., minus 60 mesh, this separation can be aided
by agglomerating the particles with oil in the manner mentioned
~` herein~efore. Surprisingly, such an agglomeration does not
occlude the insoluble sulfur salts. The resulting separated
particulates have a substantially reduced pyritic sulfur
content and can exhibit a diminished organic sulfur content.
As noted hereinbefore, coal-oil agglomerates are
employed in preferred embodiment~ of the invention.
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The recovered coal-oil agglomerates are coal-oil
agglomerates wherein the coal portion is significantly reduced
in sul~ur content and ash content. These coal-o11 agglomerates
are an excellent low sulfur, low ash fuel and can be used as
such.
If desired the oil can be removed from these coal-
oil agglomerates to provide coal particles reduced in sulfur
and/or ash content. A variety of methods can be employed to
remove the hydrocarbon oil from the coal-oil agglomerates.
Por example, agglomerates can be washed with an organic
fluid, for example, hexane or toluene, in which the hydroc~bon
oil is soluble, and separating the resulting solution from
the co~l particles.
Generally, it will be desirable to separate the
insoluble sulfur salts, for example, gypsum, from the process
water. This can be accomplished in a number of ways. For
example, the process water containing the insoluble sulfur
solids can be placed in settling ponds, and the salts allowed
to precipitate from the water. A variety of alternative
methods of course could be employed.
The following examples are provided to better
illustrate the invention by presenting several specific
embodiments.
EXAMPLE I
Upper Freeport, Kingwood Mine coal was ground and
screened to provide a quantity of feed coal having a particle
size of less than 80 mesh. This feed coal was analyzed to
determine its sulfur content and type.
Sixteen parts, by weight, of this feed coal was
slurried with 84 parts, by weight, water and ~ quantity of
limestone were placed in an autoclave. The quantity of lime-
stone was such that the initial pH was 7.80 and the final
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pH was 5.75. Thi~ quantity of limestone amounted to approxi-
mately 1.5 times the stoichiometric amount of pyrite in the
coal. The autoclave was sealed and heated to 300F. Oxygen
was then introduced, and maintained at 300 psig. The coal
was held under these conditions for one hour. The autoclave
was then cooled.
The contents of the autoclave were transferred to
a beaker equipped with baffles and a stirrer. One hundred
parts of water were added to the beaker. Stirring was
~ commenced, and light cycle oil was slowly added to the
beaker. In the course of the addition of the light cycle
oil, the coal particles began to agglomerate. The amount
of light cycle oil added was 15%, by weight, of coal.
The contents of the beaker were then emptied onto
a 40 mesh screen; substantially all of the coal agglomerates
were retained on the screen. Finely divided gyp~um solids
formed by the reaction of limestone and sulfur products
removed from the coal did not agglomerate with the coal
and passed the screen with the water. The coal agglomerates
were washed several times with fresh water.
The resulting agglomerates were then dried,
de-oiled and analyzed.
The results obtained are shown in Table I. In
Table I, the sulfur and ash content, and the sulfur content
by sulfur type are presented for the feed and coal after
treatment. All results are on a dry, ash-free basis.
It is notable in Table I that significant sulfur
reduction is achieved by the process of the invention
presented. It is also especially notable that very signifi-
cant ash reduction can be obtained. The significant ashreduction obtained in the proce 8 d~sclosed herein is an
important aspect of the proces~ of this invention as ash
concentration can effect thP combustion characteristics
of coal.
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TABLE I
Total % Sulfur Type
Ash Sulfur Sulfate Pyrite Organic
.
Feed Coal 12.7 3.30 0.32 1.96 1.02
Treated Coal 7.98 1~29 0.01 0.45 0.83
~XAMPLE II
Upper Freeport, Xing~ood Mine coal was ground
and screened to provide a quantity of coal ha~ing a
particle size of less then 80 mesh.
~ One part, by weight, of this coal and 10 parts,
by weight, water were added to a beaker equipped with an
electric stirrer. The stirrex was activated, and 15%,
b~ weight of coal, of light cycle oil was added to the
beaker. When the light cycle oil was added, the coal
particles began to agglomerate, forming coal-oil agglomerates.
Stirring was continued until agglomeration was essentially
complete. The contents of the beaker were then poured onto
a 40 mesh screen to recover the coal-oil agglomerates. The
coal-oil aggLomerates were washed with water.
One part by weight coal-oil agglomerate~ and 4
parts by weight water were slurried together and added
to an autoclave. A quantity of limestone was added to the
autoclave to provide an initial pH of 8.30 and a final pH
of 5.75. Thi~ quantity o~ limestone amounted to approximately
1.5 times the stoichiometric amount of pyrite in the coal.
The autoclave was sealed and heated to 300F. Qxygen was
then introduced, and maintained at 300 psig. The coal was
held under these conditions for one hour. The autoclave was
then cooled, and the contents poured onto a 40 mesh screen
to separate the coal-oil agglomerate~ and water. Insoluble
sulfur compounds, for example, gypsum formed by r~ac~ion with
limestone passed the screen with the water.
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The coal-oil agglomerates were de-olled by washing
the coal-oil agglomerate~ with a hydrocarbon oil ~olvent
~toluene and hexane) to remove the hydrocarbon oil and
recover a coal produce of reduced sulfur content.
The sulfur content of the feed coal before
i treatment, and the sulfur content of the coal after treat-
--~ ment are shown in Table IIbelow.
TABLE II
Total Sulfur Type (%Coal)
Sulfur Sulfate Pyrltic Organic %Ash *DAF
Feed Coal 3.30 0.32 1.96 1.02 12.7
Treated Coal 1.09 0.03 0.47 0.79 11.2
*Dry Ash Free Basis
Analysis of oil separated from the de-oiled
coal-oil agglomerates indicated little or no sulfur uptake
from the coal. The treated coal-oil agglomerates formed
in this example are reduced in sulfur and ash content and
- can be very suitably employed as an improved iow sulfur, --
~'.f' low ash fuel. ~-
In the above examples the agglomerates were de-
oiled in order to better illustrate the effectiveness of
the process of the invention in reducing sulfur and ash
in coal. The resulting coal-oil agglomerates, however,
are an excellent fuel exhibiting reduced sulfur and ash
contents and can be used as such or in blends with other coals.
It is also noteworthy that the process of the~
invention provides for enhanced BTU recoveries of coal often
in exces3 of 90% even up to in excess of 95%.
While this invention ha~ been described with
respect to various specific examples and embodiments, it
is to be understood that the invention is not limited thereto
and that it can be variously practiced within the scope of
the following claims.
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