US3444046A

US3444046A - Method for producing coke

Info

Publication number: US3444046A
Application number: US430360A
Authority: US
Inventors: Earl V Harlow
Original assignee: Koppers Co Inc
Current assignee: Raymond Kaiser Engineers Inc
Priority date: 1965-02-04
Filing date: 1965-02-04
Publication date: 1969-05-13
Anticipated expiration: 1986-05-13
Also published as: DE1571651A1; FR1490493A; GB1081781A

Description

May 13, 1969 Filed Feb. 4, 1965 COAL STORAGE PREPARING ELUTRIATING I oxmIzme HEAT TREATING HOT FORMING OXIDIZING CARBONIZING COKING COOLING PRE FORMED COKE INVENTOR. EARL V. HARLOW 3,444,046 METHOD FOR PRODUCING COKE Earl V. Harlow, Towson, Md., assignor to Koppers Company, Inc., a corporation of Delaware Filed Feb. 4, 1965, Ser. No. 430,360 Int. Cl. C101 /02; C!) /02 U.S. Cl. 2016 5 Claims ABSTRACT OF THE DISCLOSURE In accordance with the invention, coal, which may be a low-grade variety, is dried and pulverized to a suitable particle size. The coal is then elutriated to remove un desirable fines and is then partially oxidized. Thereafter the coal is heated to reduce volatile matter, is hot-formed into briquettes or other shapes, is oxidized again to produce a thin surface layer on the shape, is carbonized, and finally is cooled.

This invention relates to the carb'onization of coal and, more particularly, to an improved process for producing formed shapes of metallurgical quality coke.

A good metallurgical coke contains very little volatile matter and is between 85 and 90 percent fixed carbon. It is a cellular, porous chunk which is sufficiently firm and strong to resist shattering by rough handling, and crushing by the pressure exerted by the ore burden in a blast furnace.

Not all bituminous coals will produce metallurgical coke in conventional type ovens. This invention provides a process for treating those coals which do not readily coke, so that they will produce desirable metallurgical coke forms. One desirable metallurgical coke form is a pillow-shaped chunk having dimensions of about 1%" x 1% x /4". Another preformed shape which may be desirable is spherical. One advantage of the preformed coke is uniformity in size. In addition, preformed coke can be produced from coals which otherwise would not be useful for making coke, and, moreover, preformed coke has desirable physical properties that enables it to preform better in a blast furnace and significantly increase the output of hot metal from the furnace.

The method of the present invention is particularly applicable to coals which do not readily produce desirable metallurgical coke in the present type of coke ovens. And, moreover, the method of the present invention is readily carried out continuously, which is to be contrasted with the conventional batch-type processes of the beehive and byproduct coke ovens.

Briefly, in accordance with the present invention, coal, which may be a low grade non-coking variety, is dried and pulverized to a suitable particulate size. The coal is then elutriated to remove undesirable fines and oxidized. Subsequently, the oxidized coal: is heated to reduce the volatile matter; is hot-formed as briquettes or other shapes in a press or other suitable machine; is oxidized again to produce a thin surface layer on the shapes; is carbonized; and is then cooled.

A particular feature of the present invention is that strong, hard, and dense carbonized briquettes or shapes can be made from widely different types of coal without using a binder material or substance.

For a further understanding of the present invention and for other features and advantages thereof, reference may be made to the following description taken in conjunction with the accompanying drawing, forming a part 3,444,% Patented May 13, 1969 of this application, which illustrates schematically a flow process diagram of the steps of the method of the present invention.

From a storage area, the coal moves to a conventional breaker, such as a Bradford breaker for example, and then passes over a magnetic pulley, or other suitable apparatus, to remove tramp iron and other metallic refuse.

Either before or after crushing, the coal must be dried to attain a surface moisture condition of about one percent. This enables the pulverized coal to be free flowing and capable of reasonably sharp particle size fractionation.

Preferably, the crushing and pulverizing apparatus is of the impact type, and, advantageously, the pulverization is carried out with a conventional air classifier working coordinately with the impact pulverizer.

After pulverization, the coal is elutriated to remove fusain and fines smaller than 325 mesh. Such fusain and particles that pass through 325 mesh screen oxidize so rapidly that they become completely nonagglomerating if they are mixed with particles of larger size. Elutriation may be carried out in any suitable manner; it being desirable primarily to separate the fusain and material that passes through a 325 mesh screen from the other particulate matter. In the lower grade noncoking coals, the fusain is very friable and concentrates in the finest fraction; is nonagglomerating; and seriously interferes with bonding of the coal under pressure.

From the elutriator, the coal passes to a station where it is oxidized to a certain and moderately critical degree. This oxidation prevents fusion and random agglomeration when it is heated to a higher temperature for partial devolatilization, as described in a subsequent step of the present process.

The degree of oxidation at this stage is usually judged by the value of the free swelling index. The lower grade noncoking coals have a free swelling index of about 5.0, whereas the better grade coking coals have a free swell ing index in the range of from 8-10. The desired free swelling index is about 2.5, which means that the poorer grades of coal have to absorb only about /5 to /6 as much oxygen as the better grade coking coals.

The particulated coal is now heated to the oxidation temperature in a stream of pure, hot nitrogen with suflicient oxygen added to control the rate and amount of free swelling. The oxidation is carried out in a fluidized bed at a temperature well below the fusion point of the coal, and, in the case of the lower grade coals, a temperature of about 437 F. is suitable. Too high a concentration of oxygen in the fluidizing gas could cause overheating and fusion. Safe amounts of oxygen concentration vary for different kinds of coal. A suitable value of the percentage of oxygen is thought to lie between about 5 and 9 percent.

The average resident time for the coal in oxidation is about 30 minutes. An important reason for oxidizing the coal before the partial devolatilization at a higher temperature is to prevent agglomeration at the higher temperature to which the coal is later subjected. During oxidation, a difficulty fusible shell forms on the surface of the particulated coal which is very effective to prevent welding of the particles in a later forming operation. Low grade coals are generally oxidized to attain a free swelling index value of about 3, which provides sufficient residual fusibility to bond satisfactorily and to form strong green briquettes.

After the coal obtains the desired 2.5 free swelling index in an oxidizing atmosphere, the coal is transferred to a fluidizer. There, the temperature is raised as rapidly as possible, without agglomerating the coal, by a non oxidizing fluidizing gas, such as nitrogen or flue gases, to a temperature of maximum fluidity, which occurs at about 800 F. The maximum temperature of the fluidizing gas, which will not result in agglomeration, is about 1110 F. In some applications, it may be desirable to preheat the coal with a suitable preheater to a temperature of about 660 F., before it enters the final heater. As a result, a lower temperature of fluidizing gas may be used with less chance for agglomeration to occur.

The ideal temperature of the coal for forming briquettes or shapes is the temperature of maximum fluidity, which in the lower grade coals is in the range of about 790 F.-805 F.

From the heat treating stage, the hot coal is fed into a briquetting machine or other apparatus for forming it to a desired size and shape. A desirable briquette is one which has a rounded contour, which resists abrasion, which shrinks uniformly during carbonization, and which resists fissuring.

When the briquettes emerge from the briquetting machine or apparatus, they are conveyed, while still hot, to a carbonizer or carbonization chamber. There are several reasons for this. Firstly, surface cooling of the briquettes, while the central region thereof is still hot, causes shrinkage cracks in the surface which develop into fissures during subsequent coking. Secondly, While hot and semiplastic, the briquettes withstand mechanical shock better than when cold, in which condition, they are relatively brittle. Thirdly, hot briquettes can be transferred with less breakage. Fourthly, the briquettes suffer less thermal shock when introduced into the carbonizer if they are at a temperature of about 655 F. Lastly, coking can be accomplished in a shorter time, using less fuel gas.

In a preferred embodiment of the present invention, a conventional conveyor transports the briquettes to the carbonizer. Such a conveyor is totally enclosed and preferably air tight. During the time the briquettes are on the conveyor, at atmosphere containing about 5 percent oxygen is passed through the enclosure, and the briquettes are again oxidized for a period of between 5 and minutes.

Heretofore, in order to form a hard, nonfusible shell on the preformed pieces, it has been proposed to rapidly heat the briquettes at the commencement of carbonization, drop the temperature after a short period of time, and finally complete the carbonization at a more nearly conventional rate of increase. Such methods have not been entirely satisfactory and, moreover, they require elaborate, expensive carbonizing apparatus.

In the present invention an oxidized shell is formed on the briquette by exposing the shape to an atmosphere containing oxygen at the temperature of forming; that is, at a temperature between about 660 F. and 840 F. and preferably between about 730 F. and 790 F. As mentioned previously, the oxygen concentration is not critical as long as it is low enough to preclude combustion. It is necessary, nevertheless, that the shapes not be allowed to accumulate in large masses in the presence of an oxygen bearing atmosphere, because such massing promotes combustion.

It is desirable to expose the shapes, while at this high temperature, to a relatively high concentration of oxygen, since only a thin, rigid, oxidized shell is desired. Oxidation of the shape to a considerable depth, or throughout the shape, produces weak, readily friable units. Coal which has been processed in accordance with the present invention up to this stage, has been oxidized to the extent desired without adverse effect on the properties of the coke. Briquettes which have a surface shell thickness of a few hundredths of an inch may show the imprint of one shape upon another during massing, but there is no tendency for them to fuse together.

As mentioned previously, subsequent to briquetting or formation in a suitable shape, the briquettes are carbonized in a furnace, such as a conventional shaft type furnace, where a stream of hot gases flow upwardly countercurrent to the downward movement of the briquettes. Of course, other suitable apparatus may be used if desired. The gas is usually generated by burning a part of the make-gas with a deficiency of air to provide a reducing atmosphere at about 2,300 E, or such a temperature as will bring the briquettes to a temperature of about 2000 F.

The advantage in the use of a shaft type furnace is to coordinate the hot gas flow with the rate of heat absorption by the briquettes and the height of the briquettes in the bed, so that a time of residence of about 2 hours will result in completely coked briquettes. The effluent gas escaping from the top of the carbonizer shaft furnace will be at about 800 F. In passing through the carbonizer, the sweet gas will be enriched by coal gas from the briquettes, which is expected to be equivalent to about 7,000 standard cubic feet per hour of 600 B.t.u. gas, based on the volatile matter in the briquettes. The carbonizing sweet gas will be heated by passing a stream of detarred gas through the briquette dry quenching space in order to heat it to about 1250 F. Thence the gases flow to a combustion chamber, along with enough air to raise the temperature to 2300 F. Such temperature is generally sufiicient to raise the briquettes to a final temperature of 2000 F. As mentioned previously, the first oxidation reduces the free swelling index to about 2.5, this being the desirable free swelling index before raising the temperature for devolatilization. In the heating, or devolatilizing step, the volatile matter should be reduced to about 25 percent, or less.

After carbonizing, the briquettes may be cooled below their ignition point by air, or they may be cooled by a water fog in an air stream, or in any other suitable manner.

It has been found that breakage of green briquettes is minimized if they are not allowed to cool after being discharged from the briquette machine or press. A semiplastic briquette is much less susceptible to mechanical shock when it is hot, than after cooling when it becomes relatively brittle. Thus, if the briquettes, while hot, are transferred to the coking chamber, there is less chance for breakage and, of course, the coking time is shorter and less heat is required for coking. If the briquettes coming from the press or the machine are allowed to cool to atmospheric temperature, they are often broken and initial cracks appear which are conductive to breakage in later handling. Thus, the surface cracks appear to be caused by shrinkage of the rapidly cooling surface layer, and the break is already initiated before the briquette is charged into the carbonizer.

In accordance with the present invention, partial devolatilization and some internal chemical rearrangement in the coal takes place during the heat treating stage, and the reaction is quite fast when temperature in the neighborhood of 750 F. is reached. The devolatilization rate, however, is practically independent of particle size within the size ranges involved in the method of the present invention. The rate of production, therefore, hinges or depends on the rate of heating, which, in turn, is limited by the heat carrying capacity of the gas at the allowable maximum temperature (fusion point of the coal) and the flow rate (about 1.2 ft./sec.).

It is to be noted that green briquettes have a crushing resistance within the range of -460 pounds, whereas briquettes which are carbonized in accordance with the method of the present invention have a crushing resistance in the range of 730-2250 pounds. The higher value of crushing resistance, of course, can be of significant importance for coke which has to support a heavy burden in a blast furnace.

Two different types of coal, type 0 and type PS, were tested for application of the present invention. It is to be clearly understood, however, that the invention is in no way limited by these examples.

Type coal, is a poorly coking coal having the typical Free swelling index Type PS coal is a good coking coal having the typical 6 The first size fraction test of type PS coal was oxidation run No. 33, the results of which are shown in the following table. In this run the bed of minus mesh coal analysis! was heated to 257 C. in pure nitrogen and oxy en,

8 Surface moisture perce11t 2 5 5 equlvalent to 8% by volume of the gas stream, was Inherent moisture d0 44 cut in at the time designated 0 minute.

BATCH OXIDATION RUN NO. 33

[Type PS Coal, Minus 60 Mesh Whole Coal, Fluidizing Gas 1,100 s.c.f.h. at 8% O4] 0 Minute (02 in) 40 Minutes Minutes Sample No. 2 Sample No. 4 Sample No. 6

wt. VM wt. VM wt. VT U.S.S. Mesh Size percent FSI percent percent FSI percent percent ."FSI percent Wt. of coal in Bed, Cumulative Oz Consumed, lb./1b.

Ash do 6.3-7.0 Batch oxidation run No. 34 Was a duplicate of run Volatile matter do 36-38 No. 33 but using type 0 coal. The results of run No. 34 Sulphur do 1.14-1.23 are shown in the following table.

0 Minute (0: in) 26 Minutes 58 Minutes Sample No. 2 Sample No. 5 Sample No. 9

Wt. VM Wt. VM Wt. percent FSI percent percent FSI percent percent USS. Mesh Size 0 .6 4 .4 36 .66 0 .6 3 .1 35 .07 0 .8 11.1 4.1 36.31 11.5 3.1 35.78 11.4 12 .3 4 .0 13 .0 2 .3 12.8 8 .6 3 .6 9 .5 1 .3 8 .5 5.0 2.5 4.5 1 .3 1 7 Whole coal by analysis 100 .0 4 .7 .4 Fluid bed Temp, C 225 Fluidizing gas, f.p.s 0.

Wt. of coal in bed, lbs Cumulative O3 Consumed, lb./1b. coal Subsequently, a batch of type 0 coal was oxidized to a analysis: free swelling index of 2.6 after which the oxygen was cut out of the fiuidizing gas and the temperature raised to 440 C. for analysis of the heat treating step. As in the Moisture "percent" 1.01.7 oxidation step, samples were taken at intervals and Ash do 4.5-6.8 5 screened into size fractions and the percent volatile mat- Volatile matter do 36.9-38.6 ter and the free swelling index were determined. The Sulphur do 0.9-1.6 results of test run No. 35 are shown in the following Free swelling index 8-10 table.

BATCH OXIDATION AND HEAT TREATMENT RUN NO. 35

[Type 0 Coal, Minus 35 Mesh Whole Coal, Fluidizing gas 1,008 s.c.f.h. (7.8% 02 Orsat) 1,000 s.c.f.h. Nitrogen] 0 Minute (0 in) 32 Minutes l Minutes Sample No. 2 Sample No. 6 Sample No. 9

Wt. VM Wt. VM Wt. VM U.S.S. Mesh Size percent FSI percent percent FSI percent percent FSI percent 20 x 35 3 3 36 .39 0.6 2.8 35 .0 2 .7 0.5 24.26 60 X 70.. .4 2.8 0 .5 x 140. .4 1.5 0.5 200 x 325 .4 1 .0 0 .5 Minus 325 .5 1 .0 Whole Coal by analysis .6 Fluid bed Temp,

Fluidizing Gas, f.p.s

Wt. Goal in Bed, lbs

Cumulative Oz Consumed Ib./lb. coaL l Oxidation completed at 32 minutes; heat treatment starts.

Comparative results of batch oxidation of type PS coal are given in the following table.

(b) drying the particulate coal to attain a surface moisture of about one percent;

BATCH OXIDATION RUN NO. 31, HEAT TREATMENT AND BRIQUETTING RUN NO.

[Type PS Coal, Minus 60 Mesh Whole Coal] Time, minutes 1 Oxidation completed; heat treatment starts.

2 Increased Fluidizing Gas from 610 to 800 s.c.f.h.; Oxygen concentration increased from The heat treatment and briquetting data, determined from test run No. 15, are shown in the following table.

TYPE PS GOAL, MINUS 60 MESH WHOLE COAL l Brlquetting started after 60 minutes time had elapsed. 2 Descri tion of briqnette Samples:

Samp e No. B-2, Good; swelled slightly on carbonization.

Sample No. B-3, Good; slight deformation on carbonization.

Sample No. B-4, Good; green briquettes brittle due to reduced volatile matter.

From these test data it is concluded that, to assure satisfactory briquettes, the volatile matter must be reduced to a value of or below, that the free swelling index of the oxidized coal must be reduced to a value in the range of 1.8 to 2.8. These conclusions are valid for both the poorly coking type 0 coal and the good coking type PS coal.

BRIQUETTE QUALITY Strength of briquettes INSIRON COMPRESSION TESTS [Green and carbonized Briquettes] Crushing Force, Pounds Green Carbonized Sample No. High Low Avg. High Low Avg.

l Crosshead movement, 0.05 inches/minute.

Density of briquettes The bulk density of green briquettes was 43 pounds per cubic foot and the bulk density of carbonized briquettes was 48.6 pounds per cubic foot. During carbonization, volatile matter equal to 21% by weight, referred to the green briquettes, was evolved.

It should be understood that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications and alterations may be made therein without departing from the scope of the invention as set forth in the appended claims.

I claim:

1. The method for producing metallurgical grade coke from swelling coals comprising the steps of:

(a) breaking the coal to a particulate size in the range of to 325 mesh screen;

(c) elutriating the particulate coal to remove fines smaller than 325 mesh;

(d) oxidizing said particulate coal in a stream of substantially pure nitrogen mixed with 5 percent oxygen, said coal being heated to and maintained at a temperature below the fusion point of the coal until the free swelling index of the coal is reduced to a value of about 2.5;

(e) heat treating the coal by raising its temperature to the temperature of maximum fluidity and reducing the volatile matter content thereof to a value of about 25 percent;

(f) forming the hot heat-treated coal into desired sized shapes;

(g) surface oxidizing the hot-formed shapes in an atmosphere containing 5 percent oxygen while the shapes are at a temperature in the range of 660 to 840 R;

(h) carbonizing the shapes by exposing them to a reducing atmosphere at about 2300 P. so as to heat the shapes to a temperature of about 2000 F.; and

(i) cooling the formed shapes by quenching the same in an inert gaseous environment.

2. The method for producing metallurgical grade coke from swelling coals comprising the steps of:

(a) drying said coal until the surface moisture is about one percent;

(b) crushing said coal to a particulate size in the range of 35 to 325 mesh screen;

(c) elutriating the particulate coal to remove substantially all minus 325 mesh particulate material;

(d) oxidizing said particulate coal until the free swelling index is reduced to a value of about 2.5;

(e) heating said particulate coal to a temperature of maximum fluidity until the volatile matter content is reduced to a value of about 25 percent;

(f) forming the heat treated coal into briquettes of desired shape and size while at the temperature of maximum fluidity;

(g) oxidizing the surface of said briquettes to a depth of about 0.010 inch;

(h) carbonizing said briquettes in a reducing gaseous environment; and

(i) cooling said briquettes by subjecting them to a fog spray of water.

3. In the method to produce metallurgical grade coke from swelling coals, the improvement comprising the steps of:

(a) oxidizing the coal until the free swelling index of said coal is about 2.5; and

(b) heat treating the coal until the volatile matter therein is reduced to a value of about 25%, prior to forming said coal into briquettes of desired shape and size.

4. In the method to produce metallurgical grade coke from swelling coals, the improvement comprising the steps of:

(a) oxidizing the coal until the free swelling index of said coal is about 2.5;

(b) heat treating the coal until the volatile matter therein is reduced to a value of about 25%, prior to forming said coal into briquettes of desired shape and size; and

(c) oxidizing the surface of hot devolatilized preformed coal to a depth of about 0.010 inch by maintaining the preforms at a temperature in the range of 660 F. to 840 F. and subjecting said preforms to an atmosphere containing about 5 percent oxygen, prior to carbonizing said preforms.

5. In the method to produce metallurgical grade coke from swelling coals, the improvement comprising the steps of:

(a) oxidizing the coal until the free swelling index of said coal is about 2.5;

(c) oxidizing the surface of the hot devolatilized preformed particulate coal to a depth of about 0.010 inch by subjecting the preforms to heat and an atmosphere containing more than one percent oxygen but less than the percentage of oxygen necessary to support combustion, prior to carbonizing said preforms.

References Cited UNITED STATES PATENTS 2,105,832 1/1938 Becker 201-9 2,805,189 9/1957 Williams 2019 2,861,028 11/1958 Jenkner 2019 3,018,227 1/1962 Baum et a1 20l9 3,140,985 7/1964 Schmalfeld 201--9 3,172,823 3/1965 John et a1 201--9 3,337,417 8/1967 Albright 201-9 3,355,363 11/1967 Gasior 2019 NORMAN YUDKOFF, Primary Examiner.

DAVID EDWARDS, Assistant Examiner.

US. Cl. X.R. 2018, 9, 39