US3640016A

US3640016A - Desulfurization of coal

Info

Publication number: US3640016A
Application number: US811654A
Authority: US
Inventors: Bernard Shing-Shu Lee; Frank C Schora Jr
Original assignee: GTI Energy
Current assignee: GTI Energy
Priority date: 1969-03-28
Filing date: 1969-03-28
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

A method for desulfurizing coal by reacting sulfur-containing bituminous coal with hydrogen in the presence of a hydrogen sulfide ''''getter'''' at a temperature about 600-800* F.

Description

v I United States Patent [151 3,640,016 Lee et al. Feb. 8, 1972 [54] DESULFURIZATION 0F COAL [56] References Cited [72] Inventors: Bernard Shing-shu Lee, Lincolnwood; UNITED STATES PATENTS Frank C. Schora, Jr., Palatine, both of ill. 2,726,148 12/1955 McKinley et al ..44/1 1 Asslsnw Institute of Gas Techlmhgy 2,824,047 2/1958 Gorin et al ..201/17 22 Filed; 2 19 9 3,130,133 4/1964 Loevenstein .23/209.9 X PP 811,654 OTHER PUBLICATIONS Guntermann et al. Chemical Abstracts" Vol 62, Apr. Related US. Application Data Q JVQ 89172 d [63] Continuation-impart of Ser. No. 579,923, Sept. I6,

1966, abandoned, Primary ExaminerEdward J. Meros Attorney-Molinare, Allegretti, Newitt and Witcoff [52] US. Cl ..44/1, 23/209.9, 23/225,

2 [57] ABSTRACT [51] Int. Cl. ..Cl0l 5/00 5s 1 Field of Search ..23/209.9, 66, 131, 225; A method for desulfuflzmg coal by reactmg sulfur-contammg 201/17; 44/1 bituminous coal with hydrogen in the presence of a hydrogen sulfide getter at a temperature about 600-800 F.

7 Claims, 1 Drawing Figure DESULFURIZATION or COAL CROSS-REFERENCES TO RELATED APPLICATION This is a continuation-in-part of our parent application, Ser. No. 579,923, filed Sept. 16, 1966, for Desulfurization of Coal," now abandoned. BACKGROUND OF THE INVEN- TION This invention relates to a novel method for desulfurizing coal at low temperatures to produce desulfurized coal useful as fuel, and, in addition, elemental sulfur as byproduct. Desulfurized coal made by our method provides a fuel product which greatly reduces the emission of air pollutants and increases the market for coal, especially high-sulfur coal.

By way of background, the sulfur content in coal can be broadly divided into three classes: pyritic, organic and sulfate. The last named occurs only in weathered coal, as CaSO and can be neglected essentially when dealing with freshly mined coal. As total sulfur content of the coal increases, the percentage of sulfur as pyritic sulfur also increases. In high-sulfur coals, pyritic sulfur constitutes the bulk of the total sulfur. In a desulfurization scheme by hydrogenation such as ours, chemical equilibrium calculations indicate that the organic sulfur in the coal should be more readily hydrogenated than the pyritic sulfur. Thus when dealing with high-sulfur coals as candidates for desulfurization treatment, one must cope with pyritic sulfur as the chief contributor to total sulfur.

Pyrites are typically represented by the formula FeS x being positive. The removal of the x portion of the sulfur is relatively easy. In fact, certain pyrite ore gives off the first sulfur directly upon heating as elemental sulfur vapor. Thus for process considerations, attention is focused on the FeS portion.

Various thermodynamic studies of the reaction:

FeS+H Fe+H S 1 indicate that even at temperatures as high as 1,300 E, the equilibrium partial pressure ratio of hydrogen sulfide to hydrogen are still so low that a tremendous recycle of hydrogen is necessary. At equilibrium conversion some 400,000 s.c.f. of hydrogen have to be recirculated to reduce the sulfur in one ton of coal from 4 to 0.5 percent. Subsequent removal of H 8 from the hydrogen stream is uneconomical even when considering byproduct sulfur credit. To go to higher temperatures would completely devolatilize the coal without reducing the hydrogen recycle appreciably.

There have been attempts to desulfurize coal by oxidation. However, such processes consume a substantial portion of the coal without reducing the sulfur content sufficiently. Altematively, desulfurization by hydrogen reduction alone, as pointed out above, has consistently been limited by equilibrium conversion even at high temperatures as well as by tremendous hydrogen recycle problems.

There have also been proposed desulfurization processes wherein ammonia is added to sulfur-containing coal and the sulfur converted to gaseous products by nascent hydrogen produced in the decomposition of the ammonia. However, such processes still suffer from the adverse H -H S equilibrium considerations above noted.

This problem has been further complicated by the fact that most of the coal having a high sulfur content, which therefore requires some treatment to remove a portion of the sulfur therefrom, is a bituminous variety which is also known as the caking coal. As indicated in an American Society of Mechanical Engineers publication, No. 66-PWR-3, entitled The Search for Low-Sulfur Coal by Harry Perry and Joseph A. DeCarlo, engineers with the Bureau of Mines, U.S. Department of Interior, on the coal reserves in the United States, practically all of the coal containing more than 1.5 percent sulfur by weight are of the bituminous or caking coal variety. Unfortunately, some of the available methods for desulfurization of carbonaceous solid fuels are specifically not applicable to the caking type of coal. Thus, there is a present need for a process which is capable of desulfurizing the type of coal which contains large amounts of sulfur.

SUMMARY OF THE INVENTION AND DESCRIPTION OF THE PREFERRED EMBODIMENT We have now invented a new method for desulfurizing caking coal which specifically removes the equilibrium limitation in reaction (1) by hydrogenating in the presence of a getter. The getter chemically combines with the hydrogen sulfide formed. Introducing the getter" strongly shifts the equilibrium conversion toward greater hydrogen utilization by per-. mitting more desulfurization to take place. Examples of cheap and effective getter" is lime, calcined dolomite, or other alkaline earth metal oxides. In the following discussions lime is used as the example with the understanding that the others are just as suitable.

It is thus the object of this invention to provide a novel process for desulfurizing coal by hydrogcnating sulfur in the coal in the presence of a getter for H 8 whereby the unfavorable equilibrium condition in converting FeS to H 5 is overcome.

Broadly speaking, our invention is based on the fact that the hydrogen reduction of FeS by itself proceeds according to reaction (I) which at 800 F., for example, permits a ratio of hydrogen sulfide to hydrogen of 0.00004 at equilibrium, requiring a recycle of 99.996 percent of the hydrogen. With a getter such as CaO, the desulfurization proceeds according to:

CaO-H'I l-FeS rt CaS+Fe+H O 2 which at 800 F., permits a ratio of steam to hydrogen of 3 at equilibrium, requiring a recycle of only 25 percent of the hydrogen.

In the 600800 F. range, at the higher temperatures by colrtrolling the partial pressure of water vapor, operation is according to equation (2) and no hydrated lime is formed according to (3).

At low temperatures, equations (2) and (3 )yield:

2CaO+FeS+H CaS-l-Ca( OH) +Fe 4 At intermediate temperatures, the surface of the lime particles may be reacted with water vapor to form hydrated lime. Then further reaction is by:

Ca(OH) -l-FeS+H CaS+Fel-2H O s; This equation permits a steam-to-hydrogen partial pressure ratio of 2.5 at 800 F. Thus, in the 600 to 800 F. range, it is possible to achieve good steam-to-hydrogen partial pressure ratios so that the hydrogen can be recycled after simple condensation of the water. In this temperature range, devolatilization of the coal would be limited, and the desulfurized product would have similar combustion characteristics as the raw coal.

From an economic standpoint, it is necessary to reuse the lime or other alkaline earth oxide getter material. To regenerate lime, the CaS produced may by hydrolyzed with steam as follows:

CaS+2H O Ca(OH) +H S (6) With steam, the above reaction proceeds sufi'rciently to the right at low temperatures for feasible operations. To increase the rate of reaction Equation (6), the preferred method of hydrolysis of CaScan be accomplished in the presence of carbon dioxide according to:

CaS+H O+CO CaCO +H S 7 In either case, the H 8 produced can be reacted to form elemental sul'fur by processes well known in the art. For example, the Claus process operates on the overall reaction:

2H S-l0 2S+2H O (3) The Ca(OH) or Ca CO produced can be dehydrated or calcined, respectively, to produce lime for recycle.

The drawing shows a schematic flow diagram of one embodiment of our method. Other arrangements of the equipment shown in the drawing will be apparent to those skilled in the art. Raw coal from storage is dried, crushed and sized in suitable particle size reduction equipment 1 to obtain a particle size suitable for fluidization, typically minus one-fourth inch. Particle size is not critical but should be chosen depending upon reactor volume and configuration and gas flow rates. Limestone or other source of alkaline earth metal oxide material, such as dolomite, is also dried, crushed and sized in suitable apparatus 2 to approximately the same particle size as the coal and fed to the calciner 3 to produce the oxide. The particle size reduction equipment shown diagrammatically at l and 2 and calciner 3 form no part of the invention per se and may be conventional equipment well in the art.

The coal and lime are then fed into the desulfurizer 4 which may be a reactor of any conventional type having a reaction chamber and means to inject gas into it.

A fluidized bed may be used and would be particularly advantageous for good temperature control. The material in the fluidized bed may comprise fine coal particles with coarse particles of lime raining down through the bed, thus permitting subsequent separation of lime. and coal particles by screening. An altemative is a fluidized bed of coal and lime of essentially the same size, the mixture being separated later by differences in density. A third possible arrangement is a fluidized packed bed" wherein small coal particles are fluidized in the interstices of a fixed bed of large pieces of lime. Broadly speaking, any reactor arrangement wherein the coal and lime particles are placed in a relatively close degree of juxtaposition permitting hydrogen gas to flow therethrough can be used in our method. The process is effected at substantially atmospheric pressure and at temperatures between about 600 and 800 F.

Steam-laden hydrogen gas exiting from the reactor is directed through heat exchanger 5, and condenser 6, to remove water vapor and then recycled with makeup hydrogen from supply to the desulfurizer.

The reacted lime-coal mixture is removed from the desulfurizer and the coal and lime fractions are separated in a separator device 7 by techniques, well known in the art, which utilize either the size or the density difference between the materials. The unreacted lime-CaS mixture is then regenerated in a regenerator 8, wherein steam and CO are fed through the mixture to effect reactions (6) or (7) as above described. The resultant H 8 gas is then sent to sulfur recovery 9, where e.g. elemental sulfur is produced by well-known techniques such as the Claus process.

The regenerated calcium carbonate from 8 is then fed to the calciner 3, along with makeup limestone feed. The carbon dioxide from 3 is used as feed for the lime regenerator.

Although the above description of our method discusses use of lime or calcium oxide, it should be understood that other alkaline earth metal oxides or oresv containing alkaline earth metal oxides are equally effective. For example, dolomite which, as well known in the art, is a mixture of calcium carbonate and. magnesium carbonate is a particularly useful material when calcined to produce CaO-MgO.

The following pilot plant data shows a specific embodiment of our invention which is intended in no way to limit the scope thereof:

EXAMPLE 1 A mixture containing equal volumes of coal and calcined limestone in the size range of minus 16 to plus 80 mesh was reacted at 750 F. with hydrogen in a fluidized bed reactor. The reactor was a 2-inch diameter stainless steel cylinder about 6 feet long. Flow rate of hydrogen through the coallimestone mixture was 53 s.c.f./hour (1.5 linear feet/sec). The coal employed as the starting material contained 1.92 percent pyritic sulfur, 1.78 percent organic sulfur and 0.18 percent sulfate sulfur. All percentages are based on the weight of coal. Analysis of the coal after the treatment showed substantially no pyritic sulfur. The organic sulfur content was reduced by about 34 percent. Although the sulfate content was also substantially reduced, this item is not particularly significant in view of the low percentage present in the starting material. No hydrogen sulfide was detected in the effluent gas, indicating that substantially all hydrogen sulfide has reacted with the lime. As indicated at the beginning of this application, the pyritic sulfur is the chief contributor to the total sulfur content of coals which are to be treated for desulfurization. ln that respect, the method of the presentinvention is especially effective.

EXAMPLE 2 A sample of calcined limestone which had previously been reacted with hydrogen sulfide until it contained 15.6 percent sulfur by weight was regenerated as follows: a sample of such sulfur-containing limestone was placed in an Erlenmeyer flask. The sample was then covered with water. Carbon dioxide was bubbled through the mixture for 3 hours and H 8 was evolved in accordance with reaction (7) above.

EXAMPLE 3 A sample of a caking bituminous coal containing 3.51 percent by weight of sulfur was treated by passing hydrogen over the sample at atmospheric pressure for 1 hour at a temperature between 700 and 800 F., and then for 2 hours at 800 F. The sulfur content of the coal after the treatment was found to be about 3.06 percent by weight. This example shows that the presence of the getter is necessary in order to reduce the sulfur content of the coal to an acceptable level.

The invention has been described in detail with reference to particular and preferred embodiments thereof, but it will be understood that variations and modifications can be made within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What is claimed is:

l. A method of desulfurizing caking coal without substantial caking, devolatilization and conversion to coke or char, which comprises the steps of:

a. mixing a solid getter material with sulfur-containing caking coal, said getter and said caking coal being in particulate form,

b. passing hydrogen gas through said particulate getter-caking coal mixture to form a fluidized bed of said particulate mixtureand to maintain the particulate integrity thereof,

c. maintaining said mixture at a temperature in the range of from about 600 to 800 F. and at substantially atmospheric pressure, to form a nongaseous sulfide by reactions with said getter and said hydrogen, and

d. separating said nongaseous sulfide and unreacted getter material from said mixture to recover therefrom particulate, uncaked coal having a substantial amount of sulfur removed therefrom without substantial devolatilization.

2. Method of claim 1 wherein said solid getter material is an alkaline earth metal oxide or a compound of alkaline earth metal oxides.

3. Method of claim 2 wherein said solid getter material is calcined dolomite.

4. Method of claim 1 wherein coarse particles of solid getter material are dropped through a fluidized bed of coal particles, thereby keeping the solid material and coal particles separate.

5. Method of claim 1 wherein said fluidized bed is a fluidized packed bed with small coal particles being fluidized in the interstices of a fixed bed of larger solid getter material particles.

6. Method of claim 1 which includes the added steps of:

a. regenerating getter material from said nongaseous sulfide and recovering a sulfur-containing byproduct, and

b. recycling said regenerated getter material to said mixing step.

7. Method of claim 6 wherein said solid getter material is an alkaline earth metal oxide or a compound of alkaline earth metal oxides, and wherein said regeneration includes the steps of:

a. reacting said nongaseous sulfide with carbon dioxide and water to produce hydrogen sulfide-rich gas and regenerated solid getter material,

b. recovering elemental sulfur by oxidation of said hydrogen sulfide-rich gas,

c. calcining said regenerated solid getter material to produce oxide and carbon dioxide, and

d. cycling said carlzion dioxide to said step of reacting said nongaseous sulfide.

Claims

2. Method of claim 1 wherein said solid getter material is an alkaline earth metal oxide or a compound of alkaline earth metal oxides.
3. Method of claim 2 wherein said solid getter material is calcined dolomite.
4. Method of claim 1 wherein coarse particles of solid getter material are dropped through a fluidized bed of coal particles, thereby keeping the solid material and coal particles separate.
5. Method of claim 1 wherein said fluidized bed is a fluidized packed bed with small coal particles being fluidized in the interstices of a fixed bed of larger solid getter material particles.
6. Method of claim 1 which includes the added steps of: a. regenerating getter material from said nongaseous sulfide and recovering a sulfur-containing byproduct, and b. recycling said regenerated getter material to said mixing step.
7. Method of claim 6 wherein said solid getter material is an alkaline earth metal oxide or a compound of alkaline earth metal oxides, and wherein said regeneration includes the steps of: a. reacting said nongaseous sulfide with carbon dioxide and water to produce hydrogen sulfide-rich gas and regenerated solid getter material, b. recovering elemental sulfur by oxidation of said hydrogen sulfide-rich gas, c. calcining said regenerated solid getter material to produce oxide and carbon dioxide, and d. cycling said carbon dioxide to said step of reacting said nongaseous sulfide.