WO1991006618A1

WO1991006618A1 - Coal beneficiation and utilization process

Info

Publication number: WO1991006618A1
Application number: PCT/US1990/006224
Authority: WO
Inventors: Richard Edmond Hucko
Original assignee: United States Department Of Energy
Priority date: 1989-11-02
Filing date: 1990-11-02
Publication date: 1991-05-16
Also published as: EP0451265A4; EP0451265A1

Abstract

A method of beneficiating a particulate carbonaceous material such as coal, including formation of a slurry of the material in an aqueous salt solution. The slurry is separated in a cyclone into a low-density fraction and a high-density fraction with the low-density fraction having particulate carbonaceous material with a lower sulfur content and lower ash content than the particulate carbonaceous material in the high-density fraction. The particulate carbonaceous material with a lower sulfur content and lower ash content is separated from the low-density fraction but a residue of the aqueous salt solution is left thereon. The separated particulate carbonaceous material with a residual portion of the salt is used as a fuel in a process producing carbonaceous gases with at least a portion of the lower sulfur content gettered in solid form by the residual salt or further lowered in a process producing superclean coal. Potassium salts as well as calcium and nickel salts are disclosed.

Description

COAL BENEFICIATION AND UTILIZATION PROCESS

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant to the employer-employee relationship of the Government to the inventor, an employee of the U.S.

Department of Energy and Pittsburgh Energy Technology

Center.

Background Of The Invention

This invention relates to coal beneficiation and particularly relates to a process of coal beneficiation wherein an agent used in connection with reducing the sulfur and mineral content of coal is also beneficial to later processes involving combustion, gasification or molten caustic leaching.

Coal beneficiation has been useful to make coal a more flexible and desirable fuel for use in coal-water mixtures for utility or industrial boilers and for use as dry

particulate fuels for diesel or gas turbine applications. Beneficiation is also helpful to reduce both sulfur and particulate emissions from all sources of coal utilization.

Producing such low-ash and low-sulfur fuels for these applications requires complete or near-complete liberation of mineral matter from the coal. The coal, when mined, contains various amounts of sulfur and mineral or stone matter, and by mineral matter, it is intended to include ash as well as pyrites which also contain sulfur. For advanced applications, cleaning includes grinding the coal to small particle sizes, since it has been found that in the grinding operation, relatively clean coal is separated from

particulates which contain higher quantities of sulfur in the form of pyrites as well as other minerals.

It has also been recently found that heavy-liquid cyclone separators are useful in the beneficiation of coal. This has been found to be a satisfactory method of cleaning because clean coal is less dense than the mineral or rock associated with the coal. For instance, clean coal has a specific gravity in the area of about 1.2 to 1.3 whereas the minerals contained in the coal or with the coal have

specific gravities in the area of 2.5 or greater. Pyrites have a specific gravity in the neighborhood of about 5. If a heavy liquid having a specific gravity in the broad general range of greater than about 1.3 to about 1.8 is used as the separating medium in a cyclone, clean coal reports to the overflow and mineral matter reports to the underflow, thereby separating the lighter, cleaner coal from the heavier material having greater mineral or ash content.

In order to clean coal to a greater extent, coal must be ground to finer and fine sizes. This is because the finer the coal is ground, the greater the separation that is possible of the clean coal from the pyrites and ash. In the past, magnetite (Fe₃O₄) has been used in a water suspension as a dense medium, with the magnetite particles being much finer than the coal particles to facilitate the cyclonic separation of the coal. As the coal is ground finer and finer, the coal particle size approaches the magnetite particle size, resulting in the magnetite being less useful as a separating medium for "deep cleaning" of coal.

Magnetite is a preferred medium because it is easily

recovered by magnetic separation, since in order for a cleaning process to be economic, the heavy liquid or the magnetite particles which make up the dense medium must be recovered for reuse.

In order to effectively separate very fine coal in a cyclone, heavy liquids have been used as substitutes for the aqueous magnetite slurries. Typical heavy liquids are freon and halogenated hydrocarbons such as methylene. chloride. Freon has come under attack lately due to environmental concerns resulting from its adverse affect on the ozone layer and methylene chloride has certain alleged

carcinogenic properties.

In any event, to date, all heavy-liquid cleaning of coal has included steps to recover the heavy liquid from the coal in order to have an economically feasible process. In the case of magnetite particles in an aqueous solution, magnetic means are used to recover the magnetite particles, whereas freon or methylene chloride are removed via

filtration and thermal separation. One of the most

expensive steps in the beneficiation of coal with heavy liquids is the recovery of the liquid.

Summary Of The Invention

Accordingly, it is an object of the invention to provide a method of beneficiating coal wherein a residue of the heavy liquid may be left behind on the clean coal without deleterious effect in the later processing of the coal, and more specifically selecting a heavy liquid which actually enhances an additional or later operation of the coal such as combustion, gasification or molten-caustic cleaning of coal.

Another object of the invention is to provide an alternative heavy liquid for the cyclonic beneficiation of coal which reduces the costs of the subsequent liquid recovery step.

Another object of the invention is to provide a method of beneficiating a particulate carbonaceous material comprising: forming a slurry of the material in an aqueous salt solution, separating the slurry into a low-density fraction and a high-density fraction with the low-density fraction having particulate carbonaceous material with a lower-sulfur content and lower ash content than the

particulate carbonaceous material in the high-density fraction, separating the particulate carbonaceous material with a lower-sulfur content and lower ash content from the low-density fraction and leaving a residue of the aqueous salt solution thereon, and beneficially using the separated particulate carbonaceous material with a residual portion of the salt thereon in a process producing a combustion process with at least a portion of the lower sulfur content gettered in solid form by the residual salt or further lowered in a process producing superclean coal.

Another object of the invention is to provide a method of beneficiating a particulate carbonaceous material

comprising: forming a slurry of the material in a liquid modified in density by the inclusion of a sulfur-gettering metal compound, separating the slurry into a low-density fraction and a high-density fraction with said low-density fraction having particulate carbonaceous material with a lower sulfur content and lower ash content than the

particulate carbonaceous material in the high- density fraction, separating the particulate carbonaceous material with a lower sulfur content and a lower ash content from the low-density fraction and leaving a residue of the sulfur- gettering metal compound thereon; and beneficially using the separated material in the presence of the sulfur-gettering metal compound in a combustion process with at least a portion of the lower sulfur content gettered in solid form by the metal compound. A final object of the invention is to provide a method of cleaning particulate coal comprising: forming a coal slurry in aqueous potassium carbonate or potassium hydroxide solution; separating the coal slurry by density into first and second fractions with the first fraction containing coal particulates of lower mineral ash content and lower sulfur content than the coal particles in the second fraction;

filtering the coal particles from the first fraction leaving residual potassium carbonate or potassium hydroxide on the particles; and processing the coal particles to produce a carbon-containing gas leaving mineral ash containing at least a portion of the coal sulfur content is combined with potassium in solid form.

The invention consists of certain novel features and a combination of parts hereinafter fully described, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

Description of the Preferred Embodiment

The reactivities of six coal samples were studied with a thermogravimetric analyzer (TGA) facility in a steam- hydrogen-nitrogen atmosphere for gasification and a oxygen- nitrogen atmosphere for combustion applications. Duplicate tests were performed for most of the coal samples and the average of the results was used in characterizing the coal. The TGA results as well as the assessments of beneficiation on the activity are reported.

The TGA consisted of a DuPont 951 thermogravimetric balance capable of operation up to temperatures of 2200°F, a 9000 series thermoanalysis system, and a data acquisition system connected to a compact C1PAQ deskpro microcomputer for data reduction. The test facility had a horizontal balance that eliminated buoyancy effects and prevented the collection of any condensibles on hand-down wires. Purge gas was introduced axially to minimize torque perpendicular to the sample surface, the samples were placed in a platinum mesh screen shaped in the form of a basket and suspended from the balance arm.

A coal/char sample of about 10-20 milligrams was introduced on a platinum mesh basket and heated at the rate of about 20°C per minute to the operating temperature in a mixture of nitrogen and hydrogen atmosphere and held at this temperature until the sample weight remained constant. A mixture of water, nitrogen and hydrogen was introduced over the sample to conduct gasification. The sample weight was recorded as a function of time. When the gasification rate became very slow, the gasification was terminated and air introduced over the sample to combust the remaining carbon for analysis of the sample. The data reported hereafter were converted to a reaction rate as a function of carbon conversion. Basically, this invention resides in the recognition that coal cleaning by heavy liquids in cyclones may be combined with agents that are useful in later process steps involving any of gasification, combustion or molten-caustic leaching, see Tables 1-4. The recognition that the heavy liquid in the cyclone may be a salt solution, some of which could be left as a residue on the coal to provide a

beneficial agent for a later processing step is a

fundamental improvement in the beneficiation and utilization of coal because it reduces costs of cleaning and recovering the heavy liquid now thought to be necessary to an economic process. Since the recovery of the heavy liquid often is the most expensive step in the coal beneficiation process as heretofore used, this recognition that that residue may be advantageously left on the coal provides a significant advance in the art.

The procedure for characterizing coals in. the oxidation atmosphere was the same as that employed for reactivity runs in the reducing atmosphere with the following exceptions:

1. The pyrolysis atmosphere was nitrogen. Air was not used during pyrolysis since it is difficult to distinguish carbon conversion during pyrolysis and the subsequent combustion.

2. Char combustion was performed in air at a

temperature of 420°C to maintain the combustion reactions at a rate that are accurately measured on the TGA.

3. The size distribution of particles used in the

reactivity runs were the same as that of the samples supplied.

The reactivities of the coal samples measured in both the gasification and combustion atmospheres are presented in Table 5. The Pittsburgh seam coal samples from various beneficiation tests are ranked in Table 6 according to reactivity.

There is a large increase in reactivity in the

gasification atmosphere for both coal samples due to the catalytic effect of potassium resulting from beneficiation in an aqueous solution of K₂CO₃. The reactivity of

Pittsburgh seam coal from test A-23 is the highest of all the samples tested. The reactivities of raw Pittsburgh and Illinois No. 6 coal samples agree with the reactivities of coal samples tested previously.

There is a significant increase in reactivity for

Pittsburgh seam coal in the oxidation atmosphere with beneficiation from the K₂CO₃ solution, but much less than the enhancement observed in the gasification atmosphere. This can be expected due to the inherent difference in the nature of the gasification and the combustion reactions. For Illinois No. 6 coal, however, there is a decrease in reactivity with beneficiation in K₂CO₃ solution. It is not clear why the reactivity decreased in this particular case.

As has been seen, therefore, coal beneficiation in a heavy liquid wherein the heavy liquid is not completely removed but left as a residue to later act as an agent in the coal produced from the cleaning operation is the novel feature of the present invention. Where potassium residues are left, the potassium acts as a sulfur getter and as a catalyst in a later gasification or a later combustion reaction. In the gasification situation, the gas produced from coal is useful as a gaseous fuel, whereas in a

combustion situation, the products are carbon dioxide and water vapor. It has been found that potassium is

advantageously present as a residue in the range of from about 5% by weight to about 20% by weight. It may be that potassium present in an amount less than about 5% by weight may be adequate as a sulfur getter and as a catalyst where the coal used is particularly clean. On the other hand, for the coal used herein, 5% by weight is necessary to provide the appropriate amount of sulfur gettering in the later combustion or gasification reaction. If the coal is

particularly high in sulfur content, it may be that

potassium may be needed in an amount greater than about 20% by weight but at the present time, it is not deemed

necessary.

Calcium acetate may be present for the later combustion step as a sulfur getterer. In the combustion reaction with calcium acetate present, the calcium combines with the available free sulfur to form a solid calcium sulfide, whereas in the potassium reaction, the solid potassium sulfide is formed. Calcium acetate is not a catalyst for gasification. Similarly, nickel acetate functions as a catalyst in the production of superclean coal by the molten- caustic leaching method, but nickel acetate does not function as a catalyst for either gasification or combustion nor does it function as a sulfur getter.

While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

Claims

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

1. A method of beneficiating a particulate

carbonaceous material comprising: forming a slurry of said material in an aqueous salt solution, separating said slurry into a low-density fraction and a high-density fraction with said low-density fraction having particulate carbonaceous material with a lower-sulfur content and lower ash content than the particulate carbonaceous material in said high- density fraction, separating said particulate carbonaceous material with a lower-sulfur content and lower ash content from said low-density fraction and leaving a residue of the aqueous salt solution thereon, and beneficially using said separated particulate carbonaceous material with a residual portion of said salt thereon in a process producing a combustion process with at least a portion of said lower sulfur content gettered in solid form by said residual salt or further lowered in a process producing superclean coal.

2. The method of claim 1, wherein the aqueous salt solution has a specific gravity of greater than about 1.2.

3. The method of claim 1, wherein the aqueous salt solution has a specific gravity of from about 1.3 to about 1.8.

4. The method of claim 1, wherein the aqueous salt solution contains one or more of K₂CO₃, KOH, Ni acetate or Ca acetate.

5. The method of claim 1, wherein the aqueous salt solution contains ion of K or Ca.

6. The method of claim 5, wherein the process

producing carbonaceous gases is combustion.

7. The method of claim 5, wherein the process

producing carbonaceous gases is gasification.

8. The method of claim 1, wherein the aqueous salt solution contains ions.

9. The method of claim 8, wherein the process

producing superclean coal includes molten-caustic leaching of the particulate carbonaceous material.

10. A method of beneficiating a particulate

carbonaceous material comprising: forming a slurry of said material in a liquid modified in density by the inclusion of a sulfur-gettering metal compound, separating said slurry into a low-density fraction and a high-density fraction with said low-density fraction having particulate carbonaceous material with a lower sulfur content and lower ash content than the particulate carbonaceous material in said high- density fraction, separating said particulate carbonaceous material with a lower sulfur content and a lower ash content from said low-density fraction and leaving a residue of said sulfur-gettering metal compound thereon; and beneficially using said separated material in the presence of said sulfur-gettering metal compound in a combustion process with at least a portion of said lower sulfur content gettered in solid form by said metal compound.

11. The method of claim 10, wherein the metal compound is a salt of K or Ca.

12. The method of claim 10, wherein the metal compound is present in an amount sufficient to be an effective sulfur getter.

13. The method of claim 12, wherein the metal compound also acts as a catalyst during the production of

carbonaceous gases.

14. The method of claim 13, wherein the metal compound is K₂CO₃.

15. The method of claim 13, wherein the metal compound is KOH.

16. The method of claim 13, wherein the metal compound is present in an amount of from about 5% to about 20% by weight of the carbonaceous material.

17. The method of claim 10, wherein the metal compound is calcium acetate.

18. A method of cleaning particulate coal comprising: forming a coal slurry in aqueous potassium carbonate or potassium hydroxide solution; separating the coal slurry by density into first and second fractions with said first fraction containing coal particulates of lower mineral ash content and lower sulfur content than the coal particles in said second fraction; filtering the coal particles from said first fraction leaving residual potassium carbonate or potassium hydroxide on the particles; and processing said coal particles to produce a carbon-containing gas leaving mineral ash containing at least a portion of the coal sulfur content is combined with potassium in solid form.

19. The method of claim 18, wherein the gas produced is principally CO₂ and water vapor.

20. The method of claim 18, wherein the gas produced contains substantial quantities of methane.