US2536098A - Coal coking by cyclically circulated hot inert gases - Google Patents

Description

P. H. ROYSTER Jan. 2, 1951 GOAL COKING BY CYCLICALLY CIRCULATED HOT INERT GASES Filed Jan. 15, 1944 I ll.

A TTOR/VE) termed on combustion" and on carrier."

Patented'Jan. 2, 1951 coal. come at? crcmcmr cmcum'mn nor mea'r GASES Percy H. Router, Bethesda, Md. Application January 13, 1944, Serial No. 518,158

This invention relates to a process for carbonizing coal which consists in forcing a highly pre- 7 heated stream of non-oxidizing gas, containing essentially H2, and N2, to pass rapidly downwardly through an enclosed, thermally insulated, stationary bed of coal particles, whereby to distill off the volatile content, to eliminate the major portion of its sulphur and to produce, as a prin: cipal product, a low-volatile residue, suitable for use as a domestic, industrial and metallurgical fuel.

The object of the invention is to provide means for coking the various types of'coal ranging from anthracite and Bank A, B and C bituminous and Banks A, B andC sub-bituminous to lignite and at the extreme peat.

Before indicating in detail the application of the present process to the several examples of industrial practice, it will be convenient, first, to describe the operating features of the invention in the following general'terms: I

The accompanying drawing shows an elevation of apparatus suitable for. illustrating the present invention.

The apparatus shown, partially diagrammatically, partially in section, in the accompanying figure may be used to illustrate the procedure fol- 8 Claims. (01.202-16) bed l3, traversing the interstitial openings resl dent between the several particles of coal in bed [8, passing through grate-bars into it and exhausting by way of discharging duct I! which is clamped removably to open valve l8. The gas, contaminated with volatile matter from the coal, is stripped of contaminants by passage through condenser l9, tar and -oil scrubber and electrostatic precipitator 2l and discharges into reserlowed in carrying out a calcining or carbonizing step. Cover-plate 43 and refractory block 44 at the top of reaction chamber ii are removed. vA charge of coal is deposited in carbonizer ii to form a thermally insulated, rigidly constrained, substantially immovable assemblage of gastraversible solid particles I3, supported on the most any composition, under impetus of blower 2, driven by motor I, is forced to flow alternately through each of the regenerative stoves 8 and 8a in repetitive succession. Both of these stoves are subjected, in succession, to two steps which are Reversing valve 4, oriented in the position shown in the figure, permits the'carrier gas, leaving blower 2, to flow through open valves 3 and 5, into space 6 below the grate-bars supporting the heated bed of pebbles or refractory particles 1. After transit through 1, the carrier gas, heated by contact with the pebbles, discharges into open space 9; flows through the refractory-lined,- water-cooled, open valve l0; pursues passage through insulated conduit l I, and discharges into open space l2 in reactor l5, above bed l3. This hot carrier gas flows forcibly downwardly through voir 24. The carrier, freed thus from dust, fume, mist and condensible gases is fed ,by way of conduit to the, inlet of blower 2 for re-circulation.

,Any desired portion of the carrier may be difuel valve to burner 29, mixes with the draftair and combustion is eifected abovethe refractory bed in 8a, flowing through it thereafter and heating it thereby. The cold products of combustion discharge through open valve 5 and reversing valve 4, exhausting through chimney 34.

Hot carrier. valve Ilia is closed. At suitably chosen intervals,-e. g.,- 20 minutes, the functional roles of stoves '8 and 8a are reversed, 8a being placed 'on carrier and 8 "on combustion. Valves 28 and 30 are closed, steam is admitted by opening valve 38 whereby products of combustion are purged out through chimney 34 whereafter "is closed. Valve Illa is opened, reversing valve 4 rotated 90-degrees, diverting the carrier gas into the bottom of 80. Valve I8 is closed; stove 8 is purged with steam from 31; 3| and 32 are open, and fuel through 3| mixes with draftair from blower 21 in burner 33. Combustion takes place in open space 9, products of combustion flowing downwardly through bed 1 into space 8 through open valve 5. then through reversing valve 4 and out chimney 34.

When the calcination of the solid charge in reis opened briefly to purge the system of carrier' I gas, and hydraulic cylinder 40 isoperated to lower the hopper bottom of i5.

Usually thecharred solid mass l3 followsthe grate-bars l4 spilling out and promptly emptying reactor I5. At times when a highly coking,

low volatile, expanding coal is calcined, it will happen that "stickers" occur, e. g., the mass l3 will hang to the brick lining of IS. In such cases,

" refractory block 44 and cover plate 43 are removed ;tained gas-tight by the water seal 4|.

If it should chance that one desires to add carbon to the bed l3 at the end of the carbonizing operation, the cooling step just described is modified somewhat. A substantial amount of a thermally unstable carbonaceous material is added to the carrier gas, and the carrier loaded with this unstable additive is blown into the bed [3. stable carrier and unstable additive through the hot bed is to cool the bed and, at the same time, crack the additive whereby carbon is formed, is

' entrapped by, and is deposited in, bed l3. The

additive may be introduced into the carrier in the form of gas, vapor, liquid or'solid. The carrier gas blown into the bed in this cooling step may be at atmospheric temperature or several hundred degrees hotter e. g., above the condensing point of the additive, but preferably below its range of active decomposition. When natural The effect of blowing such a mixture of.

gas, refinery gases, ornon-condensible gases derived from the carbonizing operation itself are used as additive, heating the carrier in the coolant step above atmospheric temperature is not necessary.

A convenient additive and one which is frequently employed in the present process is oil or tar produced in the process.

The present process provides: (A) Positive propulsion of the carrier gas under pressure from a blower and maintaining the desired gas fiow independent of the drop in pressure in the bed, (B) pre-heating the carrier gas regeneratively by passage through refractories which are heated in a separate operation, (C) maintaining the bed under mechanical constraint to constitute an immovable mass and to prevent any change in its geometrical conformation. (D) establishing a narrow thermal wave in the stationary mass of particles, propelling this wave, by forced convection, through the mass causing a rapid transit of the carbonizing zone through the mass whereby each solid particle is carried through its decomposition range at a high rate and immediately thereafter quenching the moving gas to a low temperature in so short a time that little decomposition of volatile takes place, (E) confining the zone of active decomposition to such a restricted zone that only a few layers of the solid particles are subjected to decomposition at any one time.

' and CO 32.7%.

Example 1.In the carbonization of a highvolatile class B sub-bituminous coal, typical of large reserves in the western part of the U. a charge of 230 net tons (N. T., 2000 lbs.) of this coal, at 60 F., screened through inch and on inch, is poured through the top opening of reactor I 5 when cover plate 43 and refractory cu. ft.

When the coal in the present example is heated, it exhibits an initial deformation at 630 F. (To) termed the pre-plastic point. At 756 (T1), rapid deformation takes place, volatile matter is given off energetically and the stability of the solid-is impaired. The temperature Tl is termed the lower plastic limit." At 820 (T2), the "upper plastic limit, the lumps of coal resolidify.

A gas suitable for use as the carrier in the present example can begenerated as follows: At the start of carbonizing operations, stove B is heated to about 2500 F., the heating being continued until the temperature in 6 is above 300. All valves are closed except 5a, 60a, l8 and 22. Steam is introduced through conduit 34 and passes upwardly through stove 8a, being heated thereby to 2500. As this hot steam flows into I2 and downwardly through IS, the coal is heated and, by reaction with solid carbon, the carrier is converted into equal volumes of H2 and CO which discharges through open valve 22. Flow-of steam, at the rate of 300 lbs. per minute, is continued for 10 minutes, whereafter valve 23 is opened and the 50-50 mixture of H2 and CO (termed syn thesis gas) is re-circulated by blower 2 at a steady rate of 65,000 standard cu. ft. per minute.

In passing through the incandescent refractories in stoves 8 and 8a the hydrocarbons are de composed into carbon and H2. In passing through bed 13, CO2 is converted into CO by reaction with hot carbon. The volume of the whole circuit, including the scrubber system, is 130,000 The carrier gas therefore requires two minutes to complete the circuit. In one carbon izing operation, the gas re-circulates about twenty-five times. The composition of the carrier gas changes progressively. After ten passages around the circuit the gas analysis reads: H2 58.5, C0 41.5%. After continued recirculation, the analysis reaches a final value of Hz 67.3

In order to maintain the constant total volume of carrier gas, it is found necessary continuously to bleed, through valve 22, some two thousand cu. ft. per minute.

When steam is not available, one may readily enough use air for the production of a suitable carrier gas. Starting with the stoves heated to 2500 F., blower 2, taking in air through open valve 22, forces 65,000 std. cu.- ft. per min. through the system. The air heated in 8 and 8a alternately passes through bed l2 and reacts with the hot carbon to produce 78,700 cu. ft. per minute of gas analyzing CO 34.48% and N2 65.52. This gas, re-circulated by blower 2, passing around the circuit, exhibits a progressive change in composition. After .20 passages around the circuit the carrier gas analyzes: H2 45.2, C0 29.1 and N2 25.1%. Its final analysis, after continued recirculation, is: H2 51.5, C0 28.4 and N2 20.5%. After 20 minutes of re-circulation, its analysis is: H: 24.89, CO 31.04, N: 44.07%. After 20 passages around the circuit, the carrier gas analyzes: H2 45.25, CO 29.10, N2 25.65%. At the end of the carbonizing step, the circulating gas has the composition: Hz 51.08, CO 28.42, N 20.50%.

It is seen from the above that the gas selected for re-circulation is not a matter of permanent importance. Whatever its original composition, it promptly becomes thermally stable and chemically unreactive with respect to bed [3. No matter what thermal decomposition it may be initially liable to, decomposition will rapidly convert it into a stable gas. It may be chemically reactive at the start, but reaction will result in only non-reactive products.

' In the present example after 04- minutes of carrier gas circulation the carbonizing wave will have reached the grate bars l4. All valves are closed except 22, 39, and 5a. Reversing valve 4 is oriented into the position shown in the draw- .ing. A flow' of 65,000 std. cu. ft. per min. of carrier, flowing through valve 22, is forced by blower 2 to enter space It through open valve 39. The gas, at 60 F., flows upwardly through l3 establishing a cooling wave therein. This "reverse flow of the carrier is continued 65 minutes until the bed I3 is completely cooled to 60 F.

Valves 39 and Na are closed. Steam, through open valve 36, is purged through open valve 30 until the combustible carrier has been swept from Hydraulic cylinder 40 is operated to lower the bottom hopper of IS. The calcined residue, lacking in agglutinizing power, freely follows the gratebars and is readily removed. Cylinder 40 is elevated and the hopper bottom restored to position. The next batch of 240 tons of coal is charged into the carbonizer.

Example 2.-A bituminous coal has the following proximate analysis: moisture 2.53,. V. M. 36.78, F. C. 56.34, ash 4.35%. Its ultimate analysis is written: C 78.86, H 5.43, O 8.95, N 1.44, S 0.97, ash 4.35.

In carbonizing this coal, it is helpful to separate the run-of-mine material into a suitable number of screen sizes. A carbonizer designed to treat one of the finer sizes passing through 54, inch screen and caught on a inch screen will have an average inside diameter of 34 ft. A charge weighing 73'net tons will form a bed 3 feet deep. Carrier gas preheated to 1800 F. is blown downwardly through this bed, at a rate of 36,000 standard cu. ft. per minute.

The thermal wave established in i3 exhibits -a maximum temperature gradient of 1070 F. per inch. This wave travels downwardly through l3 at the rate of 1.05 inches per minute. The maximum rate at which the coal particles are heated is 1120 F. per minute (18.7 per second). When heated, the coal particlessuifer incipient deformation at 637 F. (T) at which primary volatile matter (P. V. M.) begins to be evolved at a noticeable rate. Rapid softening of the lumps-commences at 747 F. (T1) which is termed the intumesceht point, i. e., the lower limit of the plastic range. As soon as T1 is exceeded, the coal is converted from a solid to viscous, tarry mass of pseudo-liquid which is undergoing rapid thermal decomposition during its formation. When the temperature reaches 837, F. (T2), the coal substance re-solidifies.

The plastic range (Ta-T1) is 90 F. (837--747) The plastic zone is 0.084 inch wide' (90/1070). The time of passage of each particle through its plastic range is 4.8 seconds (90/l8.7) The heating is rapid; the coking zone is restricted, being less than one coal particle wide. The time during which the volatile matter is at a temperature of active instability is short and is swept away with great rapidity (51.5 inches per second) being quenched immediately upon evolution at a cooling rate of 55,000 F. per second. The coke has an analysis: fixed carbon 90.62, S 0.18 and ash 7.86%. I

Exhaust gas, stripped in i9, 20 and 2! of oil, liquor; tar, H23, and -NH3, and saturated with water vapor at 60 F., flows from 2| throughout the 34 minutes of the calcining step at a flow-rate of 39,800 std. cu. ft. per minute. After passing 5 stoves, the unstable hydrocarbons are cracked,

producing 107 pounds of carbon per minute, about half of which is entrapped on the pebbles in the stove. The C0: and H20 in the re-circulating gas reacts with this entrapped carbon on the stove 1o pebbles to produce CO and H2, consuming 33 pounds of carbon per minute. Due to the conversion of CO: and H 0 into CO and Hz, the volume of the gas is increased to 36,000 std. cu. ft. per minute.

The mass of hot coke remanent therein is rel2. Cover plate 43 and block 44 are removed.

moved at 1800 F. and quenched in the usual fashion.

The high volatile coal described above does not produce a particularly strong metallurgical coke.

It mechanical properties can be improved by cooling bed l3 in situ. This is done by forcing a carrier gas (H: 82, C0 17. N 1%) taken from the gas holder (not shown in. the figure) through open valves 22 and 23 into saturator chamber 24. This 25 chamber is heated by steam coils and is partially filled with a tar fraction resulting from the distillation of the tar produced in this process. The temperature is adjusted to distill out 65 gallons per minute of tar into the 35,000 cu. ft. per minute of carrier gas which is drawn into'the inlet of blower 2.v All valves are closed except 22, 23, 39, I0 and 5. Reversing valve 4 is oriented 90 from v the position shown in the figure. Thecarrier gas,

loaded with tar vapor, passes through open valve 39 into open space It and thereafter upwardly through bed l3. A cooling wave is established initially located at grate-bars [4. The characteristics, theory, shape and motion of the thermal waves established in a ifiass of broken solids when 40 a non-reactive heating gas is forced therethrough have been fully described in detail by T. E. W. Schumann, Journal, Franklin Institute, vol. 208, 1929, p. 305, by C. C. Furnas, Transactions, American Institute of Chemical Engineers, vol. 24, 1930,

5 pp. 142-193, U. S. Bureau of Mines Bulletin 361,

. 1932, as well as in applicant's own U. S. Patent 1,940,371 (1933) and' in applicant's co-pending application, Serial No. 460,658, now Patent 2,470,578. I

The tar vapor during its passage through the the decreased caking of the carbonaceous residue can be rectified to any required extent by returning to the bed any selected amount of the hydrocarbon volatil matter removed.

The question whether (1) to push hot coke from the carbonizer and quench after removal, or (2) to cool it in the carbonizer is a question of economy. In (1), the capacity and the yield of tar per ton of coal is high. In (2),,the yield of coke per ton of coal is increased but the tonnage produced per day is decreased. Decision as to procedure (1) or (2) will depend upon the relative market value of coke and tar.

It is observed that the sulphur in the coke (0.18%) is remarkably low. Loss of sulphur from through the gas cleaning system I9, 20 and 2|, the coke is inevitable when the carrier gas exassaoee' hibits such a high concentration of hydrogen (81.98%) If provision is made in the gas cleaning system to remove Hits from the re -circulating carrier gas, the sulphur content of the coke will become insignificant. At elevated temperature s reaction between H: and all forms of sulphur in the coal is remarkably complete. For example, if the sulphur is largely pyritic, the reaction between H: and FeS proceeds to such an extent that with a K38 content of the entering carrier as high as 4% by volume, the coke sulphur will be 0.32%. If the H28 in the carrier is maintained under 1%, the coke sulphur will be less than 0.1%. The reaction between H: and organic sulphur is even more effective than in the case of pyritic sulphur. The above description applies to quarter-inch screen-size of coal. It should be understood that in practical cases, a battery of several carbonizers will be used, each designed to treat the several screen sizes into which the run-of-the-mine is screened. y

In the operation described above, the rate of blowing was rather low.

It will perhaps be helpful to the operator to direct his attention to the following physical laws:

(1) The maximum thermal gradient in the bed is directly proportional to the difference in temperature between the incoming gas and the solid. I

The maximum thermal gradient is inversely proportional to the average diameter of the particle.

The maximum thermal gradient is independent of the rate of fiow of the gas.

The velocity of propagation of the thermal wave in the bed is equal to FC'p/Ms where F i the lbs. of gas per minute crossing a plane in the bed orthogonal to the direction of gas flow, Cp is the specific heat of the gas- (B. t. u./lb./ F.), M is the lbs. of solids enclosed between two planes one inch apart, both planes orthogonal to the direction of gas flow, and s is the specific heat of the solid. The velocity is given thus in inches per minute. The velocity of the wave is independent temperature, pressure, particle-size and the shape of the bed. 7 Due to the increase in entropy of the system resulting from the transfer of heat from a high to a low temperature, both in a heating wave and in a cooling wave, the temperature gradients in the wave-front decrease continuously as the wave is caused to travel surface substantially opposed to an upper free surface; causing a thermally stable non-oxidizing gas consisting essentially of C0, m and Na and containing minimum concentrations of the oxl-' dizing constituents H20, CO2 and Or, at a temperature superheated at least 1150 F. above the temperature of deformation of the coal particles, to enter an enclosed, gas-permeable thermally insulated empty space positioned above and im-- mediately contiguous tothe said upper surface; forcibly propelling the gas to pass downwardly through the bed along uniform lines of fluid flow substantially orthogonal to the said upper and lower surfaces; discharging the gas from the lower surface of the bed into an enclosed gas-permeable empty space positioned below and immediately contiguous to the said lower surface; discarding from the discharged gas a fractional volume substantially equal to the volumetric increase of gas resulting from the decomposition of the coal; reheating the remaining volume of gas to the said coking temperature; and recirculating the reheated gas through the bed of coal.

2. In the process described in claim 1, that improvement which comprises condensing and stripping from the discharged gas the condensible decomposition products evolved during the coking of the coal before discarding; reheating and re-circulating the said gas.

3. In the process described in claim 1, that improvement which comprises forming the said mass of coal into a bed having an average diameter greater than its vertical height.

4. In the process described in claim 1. that improvement which comprises screening the coal into a plurality of sized fractions and forming I the fractions into 'a number of beds equal to the number of fractions and proportioning the dimensions of the several beds with respect to diameter and height in conformity with the respective particle sizes therein, assigning large diamthrough the bed, the rate of decrease of the It is frequently convenient, for example, in coking coal, to place a layer of coke produced in a prior operation in immediate contact with the grate-bars I4 and to charge the batch of coal on top of this coke layer. When this is done, it is possible to force the coking wave completely through the coal without materially over-heating eters' and small vertical heights to the beds containing the smaller particle sizes and smaller diameters and greater vertical heights'to the beds containing the larger particle sizes, in a manner adapted to avoid excessive pressure drops in forcing the carrier gas through the bed.

5. In the process described in claim 4, that improvement which comprises maintaining the H: content of the carrier gas at entrance into the bed at least as great as two-thirds by volume, controlling the temperature of the gas above 1800 F. and concurrently removing from the said discharged gas the major portion of the H28 formed by reaction between the H2 content of the carrier gas and the sulphur contained in the coal.

T2 of the coal, wherein the said superheat is at least eleven-times as great as the temperature of the said plastic zone between its upper plastic limit T2 and its lower plastic limit T3.

7. The process of coking coal which comprises,

as a nut step, establishing an enclosed. thermally insulated, immobile mass oi initially unheated particles of coal to form a bed of solids delimited between a lower bounding surface substantially opposed to an upper free surface, causing a non-oxidizing gas consisting essentially of CO, H: and N2. and containing minimum concentrations of the oxidizing constituents H2O, CO: and at an elevated temperature T1 suillciently high to cause substantially complete thermal destruction of hydrocarbons into carbon and hydrogen to enter an enclosed, gas-permeable, thermally insulated empty space positioned above and immediately contiguous to the said upper surface; forcibly propelling the gas to pass downwardly through the bed, discharging the gas from the lower surrace of the bed into an enclosed gas permeable empty space positioned below and immediately contiguous to the said lower surface; condensing and stripping from the gas the condensible products evolved during the coking oi the coal; discarding from the discharged gas a fractional volume substantially equal to the volumetric increase oi the gas resulting from the decompositlon oi the coal; reheating the remaining volume of the gas to the temperature T1 and recirculating the reheated gas through the bed of coal: as a second step, mixing a thermally unstable carbonaceous material in controlled amount with a second non-oxidizing carrier gas consisting essentially of CO, H: andN: and containing minimum concentrations of the oxidizing constituente 1120,. CO: and 0:; forcing the resulting.

mixture at an entrant temperature T: below the broadly extended, gas-permeable,

decomposition temperature of the said unstable carbonaceous material through the said bed while initially at the temperature T1, thereby cooling the bed and concurrently thermally decomposing the added carbonaceous component intoa non-condensible gas eiiiuent with the carrier gas and into a solid carbonaceous decomposition 5 product adherent to the solids in the bed and cementing the latter into a mechanically strong, coherent mass; cooling the gasljexhausting from the bed and recirculating it through the said bed: and removing the bed of carbonized carbonaceous o solids as a, dual product when the cooling eii'ect of the said second step is substantially complete.

8. The process described in claim 7, wherein the temperature T1 is at least ashigh as 1800 1".

PERCY H. ROYSTER.

aarnnsscas crran The following references are of record in the tile of this patent: I

a UNITED STATES PATENTS Number Name Date Re. 1,605 Howarth Jan. '19 1864 21,805 Atwood Oct. '19, 1858 670,047 Westman Mar. 19, 1901 680,784 Knox Aug. 20, 1901 705,213 Danbe July 22, 1002 768,230 Knox Aug. 23, 1904 900,891 Loamis May 2, 1011 1,365,123 Thomas Jan. 11, 1221 1,814,463 Trent July 14. 1931 1,840,403 Loebell Jan. '12, 1932 2,121,733 Cottrell June 21, 1038 FOREIGN PATENTS,-

Number Country Date 211,033 Great Britain Ieb. 14. 1024 374,923 Great Britain June 13, 1032 114.824

Australia Mar. 3, 1942

Claims (1)

1. THE PROCESS OF COKING COAL WHICH COMPRISES ESTABLISHING AN ENCLOSED, THERMALLY INSULATED, IMMOBILE MASS OF INITIALLY UNHEATED PARTICLES OF COAL TO FORM A BED OF SOLID DELIMITED BETWEEN A BROADLY EXTENDED GAS-PERMEABLE LOWER BOUNDING SURFACE SUBSTANTIALLY OPPOSED TO AN UPPER FREE SURFACE; CAUSING A THERMALLY STABLE NON-OXIDIZING GAS CONSISTING ESSENTIALLY OF CO, H2 AND N2 AND CONTAINING MINIMUM CONCENTRATIONS OF THE OXIDIZING CONSTITUENTS H20, CO2 AND 02, AT A TEMPERATURE SUPERHEATED AT LEAST 1150* F. ABOVE THE TEMPERATURE OF DEFORMATION OF THE COAL PARTICLES, TO ENTER AN ENCOLOSED, GAS-PERMEABLE THERMALLY INSULATED EMPTY SPACE POSITIONED ABOVE AND IMMEDIATELY CONTIGUOUS TO THE SAID UPPER SURFACE; FORCIBLY PROPELLING THE GAS TO PASS DOWNWARDLY THROUGH THE BED ALONG UNIFORM LINES OF FLUID FLOW SUBSTANTIALLY ORTHOGONAL TO THE SAID UPPER AND LOWER SURFACES; DISCHARGING THE GAS FROM THE LOWER SURFACE OF THE BED INTO AN ENCLOSED GAS-PERMEABLE EMPTY SPACE POSITIONED BELOW AND IMMEDIATELY

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