CA1131149A - Process for removing sulfur from coal - Google Patents

Abstract

PROCESS FOR REMOVING SULFUR FROM COAL
ABSTRACT OF THE DISCLOSURE
A process for reducing the sulfur content of coal comprising the steps of:
1) contacting an aqueous slurry of water and sulfur-containing coal particles at elevated temperature with oxygen under conditions such that the resulting coal can form coal-oil agglomerates;
2) contacting the slurry of coal particles with hydrocarbon oil to form coal-oil agglomerates; and 3) recovering coal-oil agglomerates wherein the coal has reduced sulfur content.

Description

BACKGF~ THE INV~2~TION
~ c the Invention .- ..... ,.. , .. _ Ihe 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 with moisture 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 oxides during combustion, is present in essentially two forms: inorganic, primarily metal pyrites, and organic sulfur. The inorganic sulfur com-pounds are mainly iron pyrites, with lesser amounts of other metalpyrites and metal sulfates. The organic sulfur may be in the form of thiols, disulfide, sulfides and ~hiophenes (substitu~ed, terminal and sandwiched forms~

~,~ -~, ! 2 chemically acsocia.ed with the coal itself. Depending on the partic;~lar roal, the sulfur ~ontent 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 coals, the bulk of the coal produced is known to be high in pyrite. Both Appalachian and Eastern interior coals have been analyzed to the rich in pyritic and organic sulfur.
Generally the pyritic sulfur 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 coal.
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 ~s~l fur content 9~

of coal involving exposing coal particles to a solution of ferric chloride. The patent suggests thai ia this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process:

2FeC13+FeS2 3FeC12+S
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+H2o+7~2o2 FeS04+H2S04 ,.
2FeSO4+H2so4+l/2o2 Fe2(So4)3+H2 ` These reaction equations indicate that in this particular process 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, hasiic ferric sulfate, can be formed.

. _ _ . _ _ .. . , _ .. . ~ , . .. . . _ . . . , . , ~ ... . . ... . . . .. .

When this occurs, a d1screte ~eparate separation procedure must be employed to remove this solid material ~om the coal solids to adequately reduce sulfur content.
Heretofore it was known that coal particles could be agglomerated with hydrocarbon oils. See, for example, U.S. Patent 3,856,668 to Shubert issued December 24, 1974, and U.S. Patent 3,665,066 to Capes et al issued May 25, 1972. It was not known heretofore, however, that coal particles which have been subjected to oxidation conditions to remove pyritic sulfur can be agglomerated with hydrocarbon oil to effect an overall coal recovery and provide sulfur and ash removal.
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 process for reducing the sulfur content of coal comprising the steps of:
l) contacting an aqueous slurry of water and sulfur-containing coal particles at elevated temperature with oxygen under conditions such that the resulting coal can form coal-oil agglomerates;
2) contacting the slurry of coal particles with hydrocarbon oil to form coal-oil agglomerates; and

3) recovering coal-oil agglomerates wherein the coal has reduced sulfur content.
These recovered coal-oil agglomerates can be used as a fuel exhibiting reduced sulfur content. Alternatively, ~he hydroca~bon oil can be removed from the recovered coal-oil agglomerates to provlde coal particles of significantly reduced sulfur content:.
In another aspect of this invention, it has been discovered that this process provides coal particles significantly reduced in ash content.

DETAILED DESCRIPTION OF THE INVENTION
AND ITS PREFERRED EMBODIMENTS
This invention provides a process for reducing the sulfur content of coal comprising the steps of:
1) contacting an aqueous slurry of water - and sulfur-containing coal particles at elevated temperature with oxygen under conditions such that the resulting coal can form coal-oil agglomerates;
2) contacting the slurry of coal particles with hydrocarbon oil to form coal-oil agglomerates and 3) recovering coal-oil agglomerates wherein the coal has reduced 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.
Another advantage of the process is that it can provide a reduction in the ash content of coal. Another advantage is that since the coal can be agglomerated it can be more readily separated other solids (e.g. ash) and water.
Suitable 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 pyrite removal can be achieved by the process of this lnrv~en~ ,a~
The coal particles employed in this invenrior can be provided by a variety of kno~n processes, for example, grinding.
The particle size of the coal can vary over wide ranges. For instance, the coal may have a particle size of minus 10 mesh and as small as minus 200 mesh (Tyler Screen) or smaller. The most practical particle size is often minus 100 mesh, preferably minus 80 mesh.
The first step of the process of this invention involves contacting an aqueous slurry of water and sulfur-containing coal particles at elevated temperature with oxygen.
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.
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~ In particular, the coal substrate must not be oxidized to an extent such that the coal becomes hydrophillic to such an extent that it cannot be readily agglomerated with oil.

In this regard, the parameters of time, temperature and oxygen pressure must be adjusted sucn that the resulting coal can form coal-oil agglomerates. For example, i high temperatures and/or high pressures are employed, the time should be short. Moderate temperatures and pressure with corresponding longer times are preferred.
For example, temperatures of from about 150 to 500F., more preferably from about 175 to about 375F., and most preferably from about 225 to 325F. are 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 10 to 500 psig., and more preferably from about 100 to 400 psig., although depend-ing on the sulfur present in the coal feed atmospheric pres-sure may be utilized. If the oxygen is mixed with other gases, the partial pressure of oxygen is most suitabl 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, poly-thionates 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 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 90% or more, by weight, of pyritic sulfur. Generally, a time period in the range of from about 5 minutes to ~
hours can be satisfactorily employed. Preferab]y, a time period of from 10 minutes to 1 hour is employed~

... . . .

Vuring this time, it can be desirable to agicate the aqueous slurry of coal and water. Known mechanical mixers, for example, can be employed to agitate the slurry.
~ Ihen coal containiny pyritic sulfur is held under these oxida-tion conditions, the pH of the aqueous slurry Ealls, since sulEuric acid is formed in the reaction. The final pH will be grea-tly dependent on the level of pyritic sulfur in the feed coal. Often the final pE~ is quite low, for example, the pll of the reaction slurry can fall to a pH of from about 1 to 3, or less. ~ihile good sulEur removal is obtained without regulating this pE-I, it has been found that if the pH of the aqueous slurry is maintained at a pH
from 6.5 to 12.0, preferably 8 to 10.0 for a time sufficient to convert at least a majority, preferably a substantial majority, of the pyrite to a removable specie, sulfur removal is enhanced.
Examples of suitable basic materials which can be employed to regulate the pH of the aqueous slurry are alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and their corresponding oxides. Other suitable basic materials include alkali carbonates, such as sodium carbonate, sodium bicarbonate, potassium bicarbonate, ammonia, ammonium bicarbonate and ammonium carbonate.
Particularly suitable are alkaline earth metal hydroxides, their corresponding oxides and carbonates, for example, cal^lllm hydroxides, lime and limest:one. ~mong these basic materials, sodium bicarbona-te, potasslum bicarbonate and ammonium bicarbonate are preferr~d. Suitable basic materials include suitable buffering agents, generally the salts of weak acids and strong bases~ A preferred specific example of a basic buffering agent is NaHCO3 and/or Na2CO3 with boric acid.
After holding the aqueous slurry of coal particles and water under these reaction conditions for a sufficient time, the pyritic sulfur is substantially oxidized to water separable sulfur compounds, for example, water soluble sulfate salts may be formed. The resulting aqueous slurry of coal particles and water separable sulfur compounds can be employed in the second step of the invention, or the coal particles can be separated and employed in the second step of the invention.
As noted hereinbefore, in the second step of the process of this invention a hydrocarbon oil is added to an aqueous slurry of coal particles which have been treated in accordance with step 1. The aqueous slurry employed in this step suitably contains from about 5~ to 50%, and preferably from about 10% to 30%, by weight, coal particles.
Suitable hydrocarbon oils for use herein are derived from petroleum, shale oil, tar sand and coal.
Suitable hydrocarbon oils include light and heavy refined petroleum fractions, for example, light cycle oil, clari-fied oil, heavy cycle oil, heavy vacuum gas oil, vacuum gas oil, residual oil/ coal tar and solvent refined coal oil. Mix~ures of the various hydrocarbon oils can also be employed~ particularlY when one of the materials is very viscou~.

T~e most suitable hydrocarbon oils are light cyc~;~ o- he1vy cycle oil, hea~y gas oil, coker oil and residual oil.
The hydrocarbon oils employed in this invention are hydrophobic and will wet the coal particles which have been carefully oxidized such their hydrophobic characteristic has not been adversely changed. When the aqueous slurry of coal particles is contacted with the hydrocarbon oil and in accordance with the second step of the invention, 10 preferably with agitation, the hydrocarbon wet coal particles collide with one another forming agglomerates (hereinafter referred toascoal-oil agglomerates). In general the size of the coal-oil agglomerate is generally 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.
- 20 An amount of hydrocarbon oil is employed which is sufficient to agglomerate the coal particles. The optimum amount of hydrocarbon oil employed will depend upon the particular hydrocarbon oil employed, the size of the coal particles, and the agglomeration conditions.
Generally, the amount of hydrocarbon oil will be from about 5% to 60%, preferably 10% to 30%, by weight, of coal.
Generally,it will not be desirable to employ more oil than can associate with the coal as coal-oil agglomerates.

:

11~1149 Ir~ -cordance with the process of this inventio~i~ coal~oil agglomerates wherein the coal has reduced sulfur and/or ash content are recovered. These coal-oil agglomerates can be recovexed in a variety of ways.
For example, filtering with bar sievesor screens can be employed separate to coal-oil agglomerates.
The recovered coal-oil agglomerates are coal-oil agglomerates wherein the coal portion is significantly reduced in sulfur content and ash content. These coal-oil 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.
For example, agglomerates can be washed with an organic fluid, for example, hexane or toluene, in which the hydrocarbon oil is soluble, and separating the resulting solution from the coal particles.
The resulting coal product has a substantially reduced pyritic sulfur and ash content and can exhibit a diminished organic sulfur content, for example, up to about 30% by weight. Preferably, the coal is reduced in moisture prior to use or storage.
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.

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This coal was divided into two portions. Two runs were then made to treat each of th~ ~oal portions respectively to reduce the sulfur and asii content in accordance with the invention. The pxocedure employed is described below.
Sixteen parts, by weight, of this feed coal was slurried with 84 parts, by weight, water in an autoclave.
The autoclave was sealed and heated to 300F. ~xygen was then introduced, and maintained at 300 psig. The coal was held under these conditions for one hour. The auto-clave was then cooled, and the contents 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. These agglomer-ates were washed several times with fresh water.
The resulting agglomerates were then dried, ~ de-oiled and analyzed. Two separate runs under different pH conditions during oxidation were made employing the above procedure. In run 1 the pH was uncontrolled; in run 2 the pH was adjusted by addition of a mixture sodium car-onate and sodium bicarbonate.
The results obtained are shown in Table I. In Table I, the sulfur and ash content, and the sùlfur content by sulfur type are presented for the feed and coal after treatment. In both runs, the su~fur content of the coal . . , ., . , . ~ _, . , , ,, _ . ~ ., ~ , . , , . . . , . _ , _. _ _ _ _. _ _ ____ _,, _ _ . _ .. , _ _ . . .
. .

1~1149 after oxidation is presented (line i), and the sulfur content of the oxidized C.i~? ~ter agglomeration i5 prese~ted ~line ii). 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 ash reduction obtained in the process disclosed herein is an important aspect of the process of this invention as ash concentration can effect the combustion characteristics of coal.
EXAMPLE II
When in Example I, Run 2 sufficient limestone is added to the coal slurry instead of sodium carbonate and bicarbonate to provide an initial pH of 7.80 and a final pH of 5.75, similar results were obtained in that good sulfur and ash reduction were obtained. An advantage of using limestone is that sulfur species removed from the coal can react with the limestone to form more environmentally acceptable compounds, e.g., gypsum. The gypsum solids which are very small, do not associate with the coal-oil agglomerates and are easily separated therefrom with the 40 mesh screen employed in Example I.

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~a EXAMPLE III
When in EX~QP1e I the following hydrocarbon. v.i.'i s are used in place of light cycle oil, the same or similar results are obtained in that coal of reduced sulfur and ash content is obtained: coker oil, solvent refined coal oil, clarified oil, heavy cycle oil, heavy vacuum gas oil and vacuum gas oil.
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 excess of 90~ even up to in excess of 95%.
While this invention has 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.

,

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1.) A process for reducing the sulfur content of coal comprising the steps of:
a) contacting an aqueous slurry of water and sulfur-containing coal particles at elevated temperature with oxygen under conditions such that the resulting coal can form coal-oil agglomerates;
b) contacting the slurry of coal particles with hydrocarbon oil to form coal-oil agglomerates; and c) recovering coal-oil agglomerates wherein the coal has reduced sulfur content.

2.) The process of claim 1 wherein the temperature is from about 150°F to about 350°F.

3.) The process of claim 1 wherein the oxygen is at a pressure of from about 10 to 500 psig.

4.) The process of claim 3 wherein oxygen gas is mixed with inert gas.

5.) The process of claim 2 wherein the pH is maintained at 6.5 to 12.0 by adding an alkali material to the aqueous slurry.

6.) The process of claim 5 wherein the alkali material is selected from the group consisting of potassium hydroxide, sodium hydroxide, potassium bicarbonate, sodium bicarbonate, ammonium bicarbonate and mixtures thereof.

7.) The process of claim 5 wherein the alkali material is selected from the group consisting of calcium hydroxide, lime, limestone and mixtures thereof.

8.) The process of claim 2 wherein the aqueous slurry of water and coal particles contains from about 5 to 50%, by weight, coal particles.

9.) The process of claim 7 wherein the aqueous slurry of water and coal particles contains from about 10 to 30%, by weight, coal particles.

10.) The process of claim 2 wherein the coal particles have a particle size of less than 10 mesh.

11.) The process of claim 1 wherein from about 5% to 60%, hydrocarbon oil, by weight, of coal particles, is added to the aqueous slurry.

12.) The process of claim 10 wherein the hydrocarbon oil is selected from the group consisting of light and heavy refined petroleum fractions, solvent refined coal oil.

13.) The process of claim 10 wherein the hydrocarbon oil is a refined petroleum fraction selected from the group consisting of light cycle oil, clarified oil, heavy cycle oil, heavy vacuum gas oil and vacuum gas oil.