US3337417A

US3337417A - Coal carbonization process

Info

Publication number: US3337417A
Application number: US146947A
Authority: US
Inventors: Charles W Albright
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1961-10-23
Filing date: 1961-10-23
Publication date: 1967-08-22
Anticipated expiration: 1984-08-22

Description

1967 c. w. ALBRIGHT COAL CARBONIZATION. PROCESS Filgd Oct. 25. 1961 GAS and Tar RECOVERY DH M m A A 3 W m 5 5 N E G on M 4 w,

a A MI I M HW WWW AL? INVENTOR. CHARLES W. ALBRIGHT BY 9%, A. J 1

ATTORNEY United States Patent York Filed Oct. 23, 1961, Ser. No. 146,947 7 Claims. (Cl. 201-9) This invention relates to an improved process for the carbonization and destructive distillation of agglomerating coals. More particularly it relates to an improved method of pretreating agglomerating coals prior to carbonizing said coal by heating a gas-fluidized suspension of said coal at an elevated temperature.

Heating of coal at elevated temperatures in order to remove volatile components therefrom and thereby effect carbonization of the coal is well known. One method of doing this is by heating a gas-fluidized bed or stream of finely-divided coal particles. A major obstacle to the successful operation of such a process presents itself, however, because of the tendency of most coal particles to agglomerate when heated at the temperatures required to effect carbonization. The formation of these agglomerates drastically interferes with the maintenance of the fluidized bed or stream, and usually makes it impossible to maintain fluidization.

One means proposed for preventing agglomeration is to mildly oxidize the coal particles prior to carbonization by heating them to an elevated temperature in the presence of a fluidizing, oxygen-containing gas. Such procedure requires from about to 40 minutes of heating and results in the loss of large quantities of volatile materials from the coal, sometimes as much as fifty percent of the entire volatile content. This tends to make such procedure commercially unattractive.

It has now been discovered, in accordance with the instant invention, that the time required to effect oxidation can be dramatically cut to 2 to 4 seconds if the coal is first preheated to an elevated temperature in the absence of oxygen and then oxidized in a standpipe with a similarly preheated oxygen-containing gas. As a result of such procedure, the amount of volatile material lost during the oxidation step is lessened to a negligible amount.

Carbonization is accomplished according to the process of the instant invention by rapidly heating finely-divided particles of agglomerating coal in the absence of oxygen to an elevated temperature above the ignition temperature of the coal by continuously conducting a stream of the gas-fluidized coal in dense phase flow through a tubularly-shaped heating zone; oxidizing the heated coal in a standpipe with a measured, predetermined amount of a preheated oxygen-containing gas; heating the oxidized coal in a fluidized state at a temperature sufficiently elevated to cause pyrolysis of the coal; and separating the mixture of vapor and devolatilized coal produced by the pyrolysis. By dense phase is meant a concentration of solids in fluidizing gas of from about pounds to about 35 pounds of solids per cubic foot of gas.

According to the process of the instant invention, agglomerating coal finer than mesh, preferably finer than 40 mesh, is first heated rapidly in the absence of oxygen to a temperature of from about 390 C. to about 420 C. Heating is accomplished in the shortest possible time by continuously conducting a stream of the gas-fluidized coal in dense phase through a tubular heat exchanger. Such a heat exchanger provides a very favorable ratio of heating surface to internal volume, and heating of the moving coal particles can be accomplished by such means in less than 15 seconds, sometimes in less than 8 seconds. In any event, the heating period should not be permitted to exceed 60 seconds or excessive devolatilization and ag- 3,337,417 Patented Aug. 22, 1967 glomeration of the coal will result. Any inert gas, i.e. any non-oxidizing gas, can be used as the fluidizing gas, e.g. flue gas, fuel gas, nitrogen, hydrogen, and the like.

The heated coal is then mildly oxidized by contacting it with an oxygen-containing gas in a standpipe. The oxygen-containing gas is preferably maintained at a temperature substantially equal to the temperature of the coal, i.e. from about 390 C. to about 420 C. The amount of oxygen employed should be sufiicient to oxidize the coal sufficiently to prevent agglomeration, which will, of course, depend upon the particular type of coal employed. For most coals, the oxygen should be employed in an amount suflicient to provide about two to about three percent by weight based on the weight of the coal. However, no more oxygen than is needed to oxidize the coal sufficiently to prevent agglomeration should be employed or the yield of volatile products will be lowered.

Oxidation is accomplished by passing the heated coal and oxygen-containing gas upwardly through a standpipe. The coal is entrained in the gas and carried through the standpipe with it. A superficial gas velocity greater than the free fall velocity of the largest coal particle is employed so that each coal particle makes only one pass through the standpipe. The amount of oxygen in the oxygen-containing gas and the superficial gas velocity are so controlled that the oxygen is substantially consumed and the coal leaves the standpipe substantially free of oxygen. Oxidation can be accomplished within two to four seconds by this technique. By minimizing the oxidation period in this manner, the amount of oxygen consumed, and the amount of volatile material lost during this step, can be lessened to a negligible amount.

The oxidized coal is then pyrolyzed, employing known dense phase fluidizing techniques, by heating a gasfluidized bed or stream of the coal. Preferably, the oxidized coal is separated from any devolatilization products and gases present before being pyrolyzed. For most coals pyrolysis can be conveniently accomplished at temperatures of from about 500 C. to about 700 C., preferably from about 500 C. to about 550 C. The higher the temperature, the shorter the time required to devolatilize the coal and effect carbonization. At temperatures of about 500 C., nearly complete volatilization of the desired liquid products is effected in about 10 minutes. At temperatures of about 700 C., the pyrolysis period can be reduced to about 4 seconds. Longer pyrolysis periods than those specified cause increased degradation of the liquid products present into less desirable gaseous products.

The volatile product recovered by the carbonization process of the instant invention contains a liquid fraction and a gaseous fraction. The liquid product is obtained in a higher yield than is obtainable by prior art processes which employ an oxidation step because of the short oxidation period employed in the instant process. The gaseous product obtained has a heating value of 1000 Btu/cu. ft. or greater. The gaseous products obtained by most other carbonization processes have a lower heating value. Thus the carbonization process of the instant invention is considerably more attractive commercially than prior art processes.

The manner in which the invention is carried out will be more fully understood from the following description when read with reference to the accompanying drawing which represents a diagrammatic sketch or flow sheet of apparatus suitable for carrying out the process.

In FIGURE 1 is shown a heat exchanger 10 wherein pulverized coal is rapidly heated in the absence of oxygen to a temperature of from about 390 C. to about 420 C. Heating is accomplished by any convenient means, e.g.

. by means of radiant heat or a hot flue gas which enters the heat exchanger through line 14 and leaves through line 15. The pulverized coal, fluidized by an inert gas, enters heat exchanger in a dense phase flow through line 11 from a pressurized feed hopper, not shown, passes through coils 12, and leaves the heat exchanger through line 13. The heated coal is picked up at the bottom of insulated standpipe 16 by an oxygen-containing gas from line '17, supplied by a compressor, not shown. The coal is entrained in the gas and carried with it through the standpipe 16 int-o transfer line 18. The oxygen-containing gas is preferably at substantially the same temperature as the coal, i.e. from about 390 C. to about 400 C. The superficial gas velocity is greater than the free fall velocity of the largest coal particle so that each coal particle makes only one pass through the pipe. A gas velocity of five feet per second is satisfactory when employing 40 mesh coal since 40 mesh coal has a free fall velocity of about two feet per second. The finer coal particles, with their greater surface area (per unit of weight) and lower free fall velocity, are in the oxidation zone for a shorter time than the coarser particles. This tends to level out the degree of oxidation of each particle to a nearly equal degree.

The oxygen in the oxygen-containing gas is substantially consumed in standpipe 16. Any excess oxygen, together with the remainder of the gas and any devolatilization products, are separated from the oxidized coal in stripper 19. The coal leaves standpipe 16 via transfer line 18 and enters stripper 19 where it is picked up by a heated stream of an inert gas from line 20, supplied by a compressor, not shown, or preferably by superheated steam. The coal is fluidized and stripped of volatiles present in stripper 19 by the heated gas stream. The volatiles and gas are vented through line 21 where they can be recovered and separated. The fluidized coal passes from stripper 19 through line 22 to heat exchanger 23 where it passes through coils 24. The coal is heated in the heat exchanger to the desired carbonization temperature by any convenient means, e.g. by means of radiant heat or a hot flue gas which enters the heat exchanger thrqugh line 25 and leaves through line 26.

The coal which is now at carbonization temperature leaves the heat exchanger via line 27 and enters carbonizer 28 where carbonization is effected, usually at a temperature of from about 500 C. to about 700 C. A heated stream of an inert gas, supplied by a compressor, not shown, or preferably superheated steam, is added to the bottom of carbonizer 28 via line 29 to fluidize the coal and strip it of devolatilization products. The devolatilization products and gas pass through overhead line 30,

I which is equipped with a cyclone separator for removal of entrained char particles, and are recovered. Gas and tar products may be separated in a gas and tar recovery system 31. These products are useful as raw materials in the chemical industry, e.g. as a source of olefins.

The hot devolatilized char is recovered via line 32. The char may be burned in a boiler 33 to produce steam without further treatment and without wasting its specific heat.

Example I Pulverized particles of Pittsburgh No. 8 coal, 100 percent passing 40 mesh, was fluidized with nitrogen gas and fed in dense phase (30 pounds of coal per cubic foot) at a rate of 128 pounds per hour through coal carbonization apparatus suitable for carrying out the process of the instant invention. The coal was fed through tubing having an inner diameter of 0.186 inch to a hairpin heating coil having fifty feet of tubing with an inner diameter of 0.186 inch and one hundred feet of tubing with an inner diameter of 0.311 inch. The coal was heated in the coil at an average contact time of about 8 seconds to a temperature of about 400 C., and then fed through an iron pipe side downcomer (20 feet long and 0.5 inch in inner diameter) to the bottom of an iron pipe side riser (20 feet long and 2 inches in inner diameter). The coal was picked up at the bottom of the riser with hot air at about 400 C., supplied from a compressor at a superficial gas velocity of about 5 feet per second. The air and coal rose through the riser together, with substantially all the oxygen present being consumed in oxidizing the coal. The coal which emerged from the top of the riser was substantially free of oxygen and was passed through a hairpin heating coil one hundred feet long and 0.541 inch in inner diameter. The coal was heated to a temperature of 500 C. in the coil at an average contact time of about 5 seconds, and then fed to a tank 6 feet tall and 7.25 inches in outer diameter where superheated steam at a temperature of about 500 C. was passed up through the tank to fiuidize the coal in the tank and strip it of devolatilization products. Each particle was devolatilized for an average time of about ten minutes. The steam and devolatilization products were vented through an overhead line. Entrained char particles were removed therefrom by means of a cyclone separator, and the gas and tar products were recovered and separated into their components. Data on the liquid and gaseous recovery products of this experiment are shown in Table A below. The total run took 3 hours.

Example II Example I was repeated using a rate of 122 pounds per hour and a carbonization temperature of 550 C. Data on the liquid and gaseous products obtained are compared with those of Example I in Table A below:

TABLE A Example I Example II Time liquid collected, minutes 130 Total tar collected, gallons per ton c 32. 5 30. 5 Tar boiling 200 0., lb./ton 37. 4 47. 5 Tar boiling ZOO-260 0., lb./ton 24. 2 20. 5 Tar boiling 260325 0., lb./ton 27. 7 22. 5 Tar boiling 325 0., lb./t0n 202. 7 183. 5 Percent tat boiling 260 C 21.0 24. 7 Percent volatiles in feed 40 40.0 Percent volatiles remaining in char 16 9.13

Gas composition, volume percent:

9. 7 15. 4 36. 4 38.1 12.9 10. 4 4.1 3. 2 3. 4 2.6 2. 2 1. 9 11.6 12.0 C0; 19.7 16.4 Volume, cubic feet per ton l, 500 2, 680

What is claimed is:

1. In a process for carbonizing agglomerating coal employing a gas fluidized technique, the improvement which comprises treating said coal prior to carbonization by rapidly heating it in the absence of oxygen to a temperature of from about 390 C. to about 420 C. by continuously conducting a stream of finely-divided particles of said coal fluidized with an inert gas in dense phase flow through a hot tubular heat exchanger; mildly oxidizing the coal to an extent necessary to prevent agglomeration of the coal at pyrolysis temperatures by passing the heated coal and an oxygen-containing gas upwardly through a standpipe, the oxygen-containing gas entering the standpipe being at a temperature substantially equal to the temperature of the heated coal entering the standpipe and having a superficial gas velocity greater than the free fall velocity of the largest coal particle, the amount of oxygen in the gas and the superficial gas velocity being so controlled that the oxygen is substantially consumed and the coal leaves the standpipe substantially free of oxygen; and then immediately carbonizing the heated, oxidized coal.

2. A process as in claim 1 wherein the coal is finer than 20 mesh.

3. A process as in claim 2 wherein the oxygen-containing gas employed is air.

4. A process as in claim 2 where the oxygen-containing gas employed is oxygen.

5. In a process for carbonizing agglomerating coal employing a gas-fluidized technique, the improvement which comprises treating said coal prior to carbonization by rapidly heating it in the absence of oxygen to a temperature of from about 390 C. to about 420 C. by continuously conducting a stream of finely-divided particles of said coal of finer than 20 mesh fluidized with an inert gas in dense phase flow through a hot tubular heat exchanger; mildly oxidizing the coal to an extent necessary to prevent agglomeration of the coal at pyrolysis temperatures by passing the heated coal and an oxygencontaining gas upwardly through a standpipe, the oxygencontaining gas entering the standpipe being at a temperature substantially equal to the temperature of the heated coal entering the standpipe and having a superficial gas velocity greater than the free fall velocity of the largest coal particle, the amount of oxygen in the gas and the superficial gas velocity being so controlled that the oxygen is substantially consumed and the gas leaves the standpipe substantially free of oxygen; and then separating the oxidized coal from any devolatilization products and gases present and immediately carbonizing the heated, oxidized coal.

6. A process as in claim 5 Where the oxygen-containing gas employed is air.

7. A process as in claim 5 where the oxygen-containing gas employed is oxygen.

References Cited UNITED STATES PATENTS 1,805,109 5/1931 Runge et a1. 20227 2,560,478 7/ 1951 Roetheli 202--27 2,586,703 2/1952 Odell 202-27 2,815,316 12/1957 Kruppa et al. 202-25 3,032,477 5/1962 Nathan 201-31 X 3,047,472 7/1962 Gorin et al 201-9 3,070,515 12/ 1962 Sylvander 201-9 3,094,467 6/1963 Kruppa 201-9 3,140,242 7/ 1964 Work et a1 20131 X FOREIGN PATENTS 757,083 9/1956 Great Britain.

MORRIS O. WOLK, Primary Examiner. I. ZATARGA, Assistant Examiner.

Claims

1. IN A PROCESS FOR CARBONIZING AGGLOMERATING COAL EMPLOYING A GAS FLUIDIZED TECHNIQUE, THE IMPROVEMENT WHICH COMPRISES TREATING SAID COAL PRIOR TO CARBONIZATION BY RAPIDLY HEATING IT IN THE ABSENCE OF OXYGEN TO A TEMPERATURE OF FROM ABOUT 390*C. TO ABOUT 420*C. BY CONTINUOUSLY CONDUCTING A STREAM OF FINELY-DIVIDED PARTICLES OF SAID COAL FLUIDIZED WITH AN INERT GAS IN DENSE PHASE FLOW THROUGH A HOT TUBULAR HEAT EXCHANGER; MILDLY OXIDIZING THE COAL TO AN EXTENT NECESSARY TO PREVENT AGGLOMERATION OF THE COAL AT PYROLYSIS TEMPERATURES BY PASSING THE HEATED COAL AND AN OXYGEN-CONTAINING GAS UPWARDLY THROUGH A STANDPIPE, THE OXYGEN-CONTAINING GAS ENTERING THE STANDPIPE BEING AT A TEMPERATURE SUBSTANTIALLY EQUAL TO THE TEMPERATURE OF THE HEATED COAL ENTERING THE STANDPIPE AND HAVING A SUPERFICIAL GAS VELOCITY GREATER THAN THE FREE FALL VELOCITY OF THE LARGEST COAL PARTICLE, THE AMOUNT OF OXYGEN IN THE GAS AND THE SUPERFICIAL GAS VELOCITY BEING SO CONTROLLED THAT THE OXYGEN IS SUBSTANTIALLY CONSUMED AND THE