US2495763A - Coking process - Google Patents

Description

A. R. POWELL Jan; 3l', 1950 COKING PROCESS 2 Sheets-Sheet 1 Filed Aug. 23, 1945 INVENTOR. HL'FR o .R ou..

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Jan. 3l, 1950 .Filed Aug. 23, 1945 A. R. POWELL COKING PROCESS 2 sheets-sheet 2 EPANsxoN Ps'zsssusze Las/salma /bh-f/ 1 Houns A Fran CHARGING co 1 z 5 AL 5 e 7 25159: 4. 7b z l Houzs AFTER. Omg-,ga

oo 4zum Persson: Las/smsen z i Eig 72/ -o Hooks AFTER. CHARGING I o 1 2 a 4 .s e 7 wlTNESsEs: lNvENToR if@ ATTORNEY Patented Jan. 3l, 1950 como rnooEss Alfred R. Powell, Pittsburgh, Pa., aor to Koppers Company, Inc., Pittsburgh, Pa., a corporation of Delaware Application August 23, i945, Serial No. 612,16@

7 Claims. l(ibi. 202-29) The present invention relates to the coking of coals in by-product coke ovens, and has reference, more particularly, to novel procedure for the utilization of expanding coals and expanding coal mixtures in such coke ovens.

In the colring industry, coals and coal mix tures are considered expanding if they exert, during the process of coking, such pressures against the walls of the oven chamber that they either damage the oven walls, or aiect deleteriously their useful life. The problem of this expension of coal charges during `colring has long been known in the industry and has been subjected to much study to alleviate the condition so that coals otherwise of economic value could be used in spite of their expanding characteris tic. The development of the modern horizontallyuelongated coke oven and of high temperature coking, and the enhanced productive ca pacity of coke ovens obtainable thereby, has increased the importance of the problem since such higher rates oi coking develop higher expansion pressures.

Methods for reducing coal expansion pressures by altering the general physicaland chemical characteristics of the selected coal or coal mixtures have been relatively unsuccessful for they also deleteriously affect the quality of the coke produced therefrom; and in many important instances the expanding characteristic of the selected coal or coal mixture is so pronounced that such known expedients are ineffectual. A major cause of the abovementioned deterioration in coke quality appears to result from elimination of pressure that is exhibited during the general course of coking and is an important factor in the production of a high quality coke having stable physical characteristics.

It is now recognized that during the coking of coals, a plastic seam, comprising coal in plastic state and gases evolving therefrom, is formed by the heat which is directed into the coal and that the plastic seam uniformly travels in the direction of coking. it has further been found that in the customary horizontally-elongated coke oven wherein heat is directed from at least two oppositely adjacent surfaces of the containing oven. the plastic seams approach each other toward the center oi' an oven charge, entrap the gases produced during carbonization, and thereby exert pressures during the nal stages of the coking period. These pressures appear to be greatest when the coal orcoal mixtures exhibit plastic seams of high viscosity Aduring coking. Substantial elimination, or reduction to within safe limits, of these peak pressures during final coking without elimination of the aforementioned milder general pressures is, therefore, a primary object of the present invention.

Another object of the present invention is the provision of method and means whereby coal and coal mixtures that are ordinarily expanding can be coked in customary commercial coke ovens without the aforesaid expansion or resultant damage to the coke oven Walls.

Yet another object of the invention is the proa vision of a method ior reducing expansion pressure of coal and coal mixtures during ook without reduction in the coke quality.

A further object of the invention is the provi= sion of method and means for the reduction of the expansion of coal or coal mixtures during coking without substantial reduction in the operating capacity of employed coke ovens.

A still further object of the invention is to provide a method of and apparatus for increasing the rate of production oi coke while controlling the expansion of coal without putting deleterious pressure on the coke oven walls.

The invention has for further objects such other operative advantages or results as may be found to obtain in the processes or apparatus hereinafter described or claimed.

According to the present invention, coal or a mixture of coals that exhibit in the latter part of their high temperature coking period an abrupt increase in pressure against the walls of a coking chamber are provided with a stratum of solid fuel' in parallel disposition along one surface of the enclosed coal charge and in the plane of a surface adjacently opposite to a heating surface of the said coking chamber whereby the continuous plastic zone is broken at the said stratum and the retention of gases of carbonication within the plastic envelope is thereby avoided. The stratum can constitute an entire and uniform layer in the plane of one surface of the coal charge, or it can be a series of broken layers constituting only a portion oi any particular plane oi the charge, or it can be a single layer not entirely covering one surface of the charge, or it may be one or more layers in one or more heating planes of the oven charge. The stratum or layer can comprise; an inert material through which gases can readily permeate during coking of which one example is coke breeze; a coal, which during coking has a high plasticity and is thereforereadily permeable by carbonization gases: or a coal or coal mixtures which have been treated to reduce the viscosity or the homo- Vgeneity of their plastic seams. Such a treatment may be secured by the oxidation of the coal or its decrease in bulk density by addition of free moisture, so likewise to make more penetrable the said stratum. The coal or coal mixture employed in the solid fuel stratum can be the same as that comprising the major portion of the coal charge except that it shall be treated as above or have, for example, a lower bulk density owing to a different degree of pulverization. The stratum of solid fuel, when disposed vertically at the end of the oven chamber, can be kept out of contact of any flued walls that are at coking temperatures by, for example, blocking oir the end nues whereby a further factor to prevent formation of a gas-sealing plastic seam in the stratum will be introduced.

The solid-fuel stratum which is provided performs not only the described function of failing to close the plastic envelope but also acts as a heat insulator to inhibit the formation of a plastic seam in parallel disposition beneath it.

The method of this invention permits an ini crease in the rate of coking that can be employed without encountering excessive coal expansion pressures as in those instances where in order to coke an expanding coal mixture a low coking rate -was heretofore necessarily used. 'Ihus the novel method has the advantage of permitting higher coking rates, and hence increased productive cyapacity of coke ovens.

'I'he accompanying drawings forming a part of the specification show a preferred apparatus in which the invention may be embodied and practiced:

Fig. 1 is a diagrammatic, elevational view in vertical section taken crosswise of the length of a coke-oven battery and longitudinally of a cokeoven chamber'through the center of a coal charge therein, and showing the coal charge partially coked.

Fig. 2 is a fragmental, elevational view in vertical section of coke-oven chambers and their accompanying ues and crossovers taken longitudinally of the coke-oven battery and at right angles to the horizontal extension of the individual ovens, and showing `one oven chamber as charged and another oven at an intermediate period during coking, and illustrating a stratum at the top of the ovens to prevent the development of high pressure in the oven.

Fig. 3 lis a. fragmental, elevational view of that same portion of a coke-oven battery that is shown in Fig. 2 and showing alsogone of the coke ovens'immediately after charging and the other at an intermediate period in the course of coking, but with each oven charge containing a layer of a coal which produces during coking a pervious plastic seam, and

Figs. 4, 5, and 6v are curves showing coal expansion pressures plotted against coking time of an expanding coal mixture which was coked according to standard practice in the instance of Fig. 4, and according to the methods oi' the invention in the instances of Figs. 5 and 6.

The same characters of reference designate the same parts in each of the views of the drawings.v

Referring now to the drawings, Fig. 1 shows a charge of coal I at a period in its coking cycle soon after the oven was charged. The coking of the said coal charge as shown in the drawing has proceeded to a point at which an envelope of coke has been formed about the charge and along the flued heating walls I 2 shown more clearly in Figs. 2 and 3. Coking has proceeded so that a layer of coke has been formed adiacent to the sole I4 of the oven, to the oven doors I8 at the pusher side Il and coke side 20, and -over the top of the oven charge. Although it is not shown in Fig. 1, a layer of coke at the period of coking illustrated in the gure, has also been formed adjacent to the said ilued walls I2 in parallel disposition longitudinally along both sides of the oven charge. These coke layers are shown more clearly in oven chamber in Figs. 2 and 3, in which oven chamber there is also shown a partially coked charge of coal. The said oven chamber containing the coal charge I0 comprises also a gas space 23,which extends longi- 15 tudinally across the whole oven charge to permitl the travel of carbonization gases to a standpipe 24 and the escape of the gases therethrough.

The oven is also equipped .with .three charging holes 26 through which the coal is charged to the oven chamber. Crossover ilues 28 are shown in the masonry oven roof 30 in Figs. 1, 2 and 3.

As can be seen from the above description of the character of a coal charge after 'a brief period of coking-such coking proceeds substantially from all directions toward the center of the oven charge at a more rapid rate from the adjacent heated walls I2 and ata lesser rate from the 'sole I4, the oven doors I6 and the oven roof 30. Such process of coking, therefore, produces a substantially impervious plastic envelope about the interior uncoked coal.

Because of the lower rate of coking at those portions of the coal charge immediately adjacent to the doors and roof of the coke oven, the carbonization gases that are rst emitted frori the inner side of the plastic seams are relatively free to flow from the center of the oven .eventually to the free space above the charge; but toward the end of the coking period a uniform plastic seam has also been formed at the planes of the less rapidly coked surfaces and thus carbonization gases under high pressure are enclosed within the envelope that is formed by the joined plastic seams. Measurements of this pressure have determined that it may be of the order of forty pounds per square inch when an expanding, low-volatile coal is employed, and when the oven charge is nearly entirely coked.

In Fig. 2, there is shown one preferred embodiment of the present invention whereby the formation of the' hereinbefore described plastic envelope isprevented. In this instance, a layer of coke breeze is added to the coal charge after the coal has been leveled oif in the oven so that a layer approximately no greater in thickness than the width of an oven chamber is formed across the -top of the oven charge. Fig. 2 shows two oven chambers, the aforesaid chamber 20, and an adjacent oven chamber 32, each of which is formed by twol longitudinally extending iiued masonry walls lI2 and oven sole I4 and a masonry roof 30. The ovens are also supplied with regenerators 34, air and waste heat ports 36 and 38 respectively, and with fuel gas-burner nozzles 40. As is the well known practice in the heating of reversible, regenerative coke ovens, air is passed through heated checkerwork in the said regenerators 34 and is flowed therefrom through the air ports 36 into on flues 42 wherein it intermingles with, 7o and burns fuel gas introduced through the said burner 4II. The burning mixed gases tlow through crossover ducts 2l, shown also in cross section in Fig. 1, and descend on the opposite side of an oven chamber through an oiT' nue 42. The waste heat ilows from the latter said nue through ducts 3l` into another set oi!Av regenerators lll*- wherein they heat checker-brick contained Y therein.

The described improved technology i'if'j'xilrizi'g.Y coal provides uniformity of heating andthe de-jV livery of heat from all sides of the oven chamber thus ensuring the formation of the described be formed cannot be sealed as an impervious envelope because of the absence of a plastic seam in the non-coking layer. A layer lil of coke breeze, such as is shown in the Fig. 2. is particulariy eii'ective to permit escape of gases from the underlying coal charge for the said breeze is substantially uninfluenced by the coking heat and is itself iinely divided and especially pervious. This layer 50 of coke breeze can be introduced into the oven chamber either through the said charging holes 26 after the oven has been charged with coal, or else it can be extruded from the leveling ram, not shown, that is introduced through a leveling door 52 shown in Fig. 1 and passes across the entire top of the oven charge lil. By this second method of using the leveling ram also as a charging mechanism, the layer of breeze can readily be distributed with uniform thickness over the entiretop of the oven-charge iii..

The gas space 23 ordinarily provided at the top oi all oven charges to permit the describedA thermore, the said layer 5@ is itself so relatively pervious to the passage oi carbonization gases therethrough that it would not interfere markediy with the escape oi carbonization gases from the oven even though the layer occupied a substantial vportion o the gas space 23;

Although, as it has been hereinabove described, it is relatively simple to distribute the coke breeze layer 5@ uniformly across the entire top of the oven charge, it is not necessary that the said layer be a complete covering for the oven charge i0 but rather it can be distributed as partially covering layers here and there or it can be a single 4layer not entirely covering the top of the charge. Any distribution of a, pervious, inert material which will interrupt the formation of a plastice envelope and be of sufilcient thickness that heat radiated from the wall adjaoently opposite thereto is absorbed therein and prevented from passing through the said layer and forming a plastic seam in the coal beneath it, will serve toI allow an escape of gases from between the plastic seams iormed'in the coking coal and thus prevent the high coal expansion pressures which would otherwise occur.

The coal charge 64 which is shown in oven chamber 20 in Fig. `2 reveals the iniluence of the layer of breeze E0 that it superimposed on the oven charge. Here it can be seen that a plastic seam 58 has been formed along the entire inner surface of a formed coke layer 60, has been joined at the bottom, but has been kept open at the top for egress of gas therefrom by the said coke breeze layer. The condition of the oven charge, here vpictorially represented, corresponds to the test conditions reported in the curve of Fig. 5 after several hours oi' coking.

- `Fig.g3comprises a 'fragmental section 'o1a .j -ccke .oven showing parts iioenncaljte vmamie-1 .x82 isshown asit would appearimmediately 'afterf f charging vand an adjacentovn charge isshown as it 'iwuld appear' afteroolring .has s proceeded forisoxn'e time.; 'At thetop:ofthe chargezinI each?l of vthe'ovensis disposed.a.layerj6,3'of, high volav tile coal, or ,whatcou1d alsobe a layer of coal of vlofwer bulk 'density than ,that oi the oven charge as a whole. Here again as shown in the oven charge 84 a plastic'seam 66 has been formed along the inner face of a layer of coke which is disposed in parallel relationship immediatelyI adjacent to the heating walls of the oven chamber. The coking has proceeded for a sufficient length of time that coke has also been formed along the sole of the oven and the plastic seams that are parallel to each oi the heating ilues i2 are joined at the inner face of the coal adjacentv to the oven sole. The coal layer resting on the partially ccked ovencharge 64 has, during the process of coking, formed a partially coked .layer 63A, which, however, has remained relatively pervious to the passage of gases therethrough as a result of low viscosity of its plastic seam and of a less dense distribution of the fused coal particles therein.

Such a coal layer as the layer 63 or the partially coked layer 63A can be made of lower bulk density, and though being of the same coal as that comprising the maior portion of the coalcharge, will perform its function as an escape valve for the emission o carbcnization gases. The formation of a layer of lower bulk density can be accomplished either by charging this layer as particles of largerV size than that comprising the main body of the coke-charge, or by charging a layer of coal having a higher moisture content (a specific example of Which'is hereinafter described and is illustrated in Fig. 6), or by spraying the superimposed coal layer with water after charging. .A high-rank, high-volatile coal can also perform the function of releasing gases irom the plastic seam since its plastic layer is of lower viscosity than that ex-rl hibited by expanding coals and will, therefore, permit the bubbling of carbonization gases there through.

-The embodiment illustrated in Fig. 3 is an especially preferred one of invention when it is necessary to maintain a high productive capacity of the coke oven since, oi course, the coal of the layers 63 and 63A also is coked during the process of coking, and therefore doesv not substantially increase breeze content or require the recirculation of breeze through the coking apparatus.

Fig. 4 illustrates by a curve the expansion pressure developed during coking of a mixture of two expanding coals. The said curve is plotted as pressures exerted by the expanding coal as the ordinate against the time of coking plotted as the abscissa. A line 10 originating at a point on the ordinate and drawn parallel to the abscissa indicates an arbitrary limit of about two pounds per square inch pressure above which value coke voven operators, on the basis of experience, believe that coking pressures are deleterious to the oven structure. During coking some expansion within the limit should be exerted so that coke of good quality wiil be formed. It is therefore preferred that in operation to form high quality coke, any coal or coal mixture exert during coking a pressure or series of pressures within the prescribed limits of from stratum has been distributed over the top of the oven charge. 'As shown, the peak expansion pressure has been eliminated and the-pressure exerted'by the coking coal never has extended into that area defining dangerous expansion pressures. conditions as nearly identical as possible to those of the test reported in the curve of Fig. 4.

Example 1 To provide a specific example of the pressures exerted by an expanding coal mixture, 80 percent of an expanding coal which alone exhibits a maximum expansion pressure of 3.6 pounds per These coking tests were made under- 8 coking by a charge of the same coal mixture al that which exhibited the peak pressure of approximately 4.5 pounds in Fig. 4 never at any time exceeded the said arbitrary limit of safe expansion pressure of two pounds per square inch.

In order to prove that it was not simply an effect of removing a portion of a charge of an expanding coal mixture, the test was repeated with the major difference that no breeze was charged upon the top of the charge and -this space of a depth of about six inches was left empty. A pressure peak of 5.75 pounds per square inch was obtained, thus clearly demonstrating that it was not merely the absence of coal from the top of the charge but the presence of a material which would function both to insulate one plane or a part of one plane of the coal from coking heat so that no continuous plastic seam in that plant will be formed or its formation be greatly inhibited and to form, if any, a plastic seam of such high plasticity or non-uniformity that it will permit ready passage of carbonization gases therethrough.

square inch was mixed with 20 percent of anv expanding coal which alone exhibits a pressure of 7.6 pounds per square inch and the resultant admixture was charged to a test coke oven. The said oven contained one movable wall, the pres- -sures against which were indicated during the course of coking by suitable apparatus. The coal mixture as charged had a pulverization of 85 percent through a one-eighth inch squarehole screen and a bulk density of 51.0 pounds per cubic foot. The average iiue temperature during coking was 2481 F. The curve of pressures exhibited during coking is shown in Fig. 4. As can be seen from an inspection of the curve, the expansion pressure remained within safe limits until the final period of coking when the plastic seams met and formed a plastic envelope. The higher pressures then exhibited show numerous peaks and valleys which presumably are caused by the sudden escapeof carbonization gases which had been entrapped within the plastic envelope. Soon after maximum pressure is exhibited, the plastic layers forming the envelope become coked, form ssures, and thus permit the relatively rapid escape ofthe gases of carbonization. It has in fact been observed in industrial coking practice that, where mildly expanding coal was employed, a sharp increase in the ow of gases from an oven occurs .shortly before the end of the coking period thus indicating the sudden breaching'of a plastic envelope.

Breaks in the curve such as are shown in Fig.

4^ have also been induced by puncturing the in Example 1, and in the same proportion of 80 y percent of one and 20 percent of the other, and having a bulk density of 50.0 pounds per cubic foot, a moisture of 1 percent and a pulverization of 83.3 percent through a one-eighth inch squareholed screen, was charged into the said movablewalled test oven.l r y Coke breeze was thereafter charged upon the coal and leveled to form a layer approximately six inches in depth. Coking proceeded with an average iiue temperature of 2533 F.

The pressures exerted by the coal charge are shown in the curve of Fig. 5.v It can readily be observed that the'pressures developed during Example 3 /In this example an -20 mixture of the above coals having a bulk density of 51.3 pounds per cubic foot and a moisture content of 1 percent was charged into the said test oven. Thereafter wet coal of the same coal mixture and containing 9 percent by weight of water was charged on top of the drier, first-charged coal, and was thereafter leveled to form a layer approximately ten inches in depthl over the Atop of the charge. The average iiue temperature during coking was 2480 F., the same that is as the coking temperature in Example 1.

The expansion pressures exhibited by the above charge during coking are shown by the curve of Fig. 6. It is especially to be noted that although a minor peak of iinal pressure was formed, the excessive moisture in the top layer of the coal and its resultantly low bulk density provided a plastic seam too tenuously cohesive or too weakly viscous to restrain excessive pressure of carbonization gases.

In another example, a coal charge comprising the 80-20 coal mixture aforesaid was covered with a ten-inch layer of three-inch sizes of the one of these coals having a maximum expansion of 3.6 pounds per square inch. The overall bulk densityl was 49.3 pounds per cubic foot. The charge was coked in the aforementioned test oven for 7 .1/2 hours at an average flue temperature of 2427 F. The maximum pressure exhibited was 1.6 pounds per square inch.

Although in the hereinbefore described Figs. 2 and 3 the strata there illustrated are shown to be rectangular in cross-section, the stratum of invention need not be so and equally good results have been obtained when the stratum was trian- 'gular in cross-section with the apex pointing down or, in another instance, when the crosssection of the stratum roughly conformed to the cross-section of an inverted truncated cone.

'I'he invention as hereinabove set forth is embodied in particular form and manner but may be variously embodied within 'the scope of the claims hereinafter made.

I claim: Y

l'. In a process of carbonizing coal in an externally-heated horizontally-elongated intermittent ,coke oven having side walls which are susceptible of injury when subjected to pressure' in 76 excess of a normal value. the method of preventing injury to the walls of said oven during carbonization of coal of a type which when carbonized in the usual manner causes pressures in excess oi' said normal value to be exerted against the oven side walls during the latter part of the carbonizing period because oi' the formation in the coal-charge during the latter part of the carbonizing period of a substantially uninterrupted plastic zone surrounding and enclosing the central portion of said coal-charge, said plastic zone being so highly impermeable to passage of gas therethrough as to restrict escape of gas emitted from the portion of the coal-charge within said plastic zone to such a degree as to causea relatively high pressure to be built up within said plastic zone, said method comprising charging said oven with coal of said type which when carbonized in the usual manner causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the carbonizing period, and maintaining in said coal-charge during the latter portion of the carbonizing period a relatively permeable path for the escape of gas from the interior of said coalcharge by providing over at least a substantial part of the above-identied coal-charge a porous layer of granular material of such composition that during exposure of said covering layer to the temperatures encountered in said oven said covering layer remains substantially more permeable to ow of gas therethrough than the plastic zones in said coal-charge, said covering layer being of such thickness as to prevent heat radiated from the roof of said oven and from the side walls of the upper portion of said oven from developing a. highly impermeable plastic zone across the top of said coal-charge.

2. In a process of carbonizing coal in an externally-heated horizontally-elongated intermittent coke oven having side walls which are susceptible of injury when subjected to pressure in excess of a normal value, the method of preventinginjury to the walls of said oven during carbonization of coal of a type which when carbonized in the usual manner causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the carbonizing period because of the formation in the coal-charge during the latter part of the carbonizing period of a substantially uninterrupted plastic zone surrounding and enclosing the central portion of said coal-charge, said plastic zone being so highly impermeable to passage of gas therethrough as to restrict escape of gas emitted from the portion of the coal-charge within said plastic zone to such a degree as to cause a relatively high pressure to be built up within said plastic zone, said method comprising charging said oven with coal of said type which when carbonized in the usual manner causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the carbonizing period. and preventing the formation in said coal-charge during the latter portion of the carbonizing period of an uninterrupted plastic zone by covering at least a substantial part of the above-identified coal-charge with an insulating blanket comprising a layer of coke breeze oi' such thickness as to prevent heat radiated from the roof of said oven and from the side walls of the upper portion of said oven from developing a highly impermeable plastic zone across the top of said coal-charge.

3. In a process of carbonizing coal in an externally-heated horizontally-elongated intermittent coke oven having side walls which are sustion of the coal-charge within said plasticv zone to suena degree as to cause a relatively high pressure to be built up within said plastic zone with resultant forcing of said coal-'charge against the oven side walls at pressures in excess of said normal value, said method comprising providing over at least a substantial part of a charge of coal of the above-identifiedv type a porous covering layer of granular material of substantially different composition and characteristics than said coal-charge, said covering layer being oi' such thickness as to prevent heat radiated from the roof and side walls of said oven from causing the formation of a highly impermeable plastic zone across the top of said coal-charge, the composition and characteristics of the material of said covering layer being such that during exposure of said covering layer to the temperatures encountered in said oven during carbonizing of a coal-charge, said material remains substantially more permeable to flow of gas therethrough than the plastic zone formed in said coalcharge.

4. A method of carbonizing in an externallyheated horizontally-elongated intermittent coke oven having side walls which are susceptible oi injury when subjected to pressure in excess of a normal value coal of a type which when carbonized in the usual manner has created therein during the latter part of the coking period a substantially continuous plastic zone which substantially completely encloses the central portion of the coal-charge and restricts the escape of gas emitted from the portion of said coal-charge within said plastic zone and thus causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the coking period, said method comprising charging said oven with coal of said type which when carbonized in the usual manner causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the coking period and providing over at least a substantial part of the above-identified coalcharge a porous layer of granular material, the material of said covering layer being of such composition that substantially throughout the period required to carbonize said coal-charge the material of said covering layer remains substantially more permeable to gases emitted from said coalcharge than the plastic zone formed in said coalcharge, said covering layer being of such thickness as to prevent heat radiated from the roof of said oven and from the upper portions of the side walls of said oven from developing in the upper portion of said coal-charge a plastic zone which can cooperate with the plastic zones developed in the lower portions of said coal-charge to restrict the escape of gases from the portion of the coal-charge within said plastic zone, whereby the gases evolved from the central portion of said coal-charge during carbonization of said coal-charge may readily escape without building yfaces of a coal-charge in said oven are exposed to relatively severe temperature conditions and the end and top faces of a coal-charge in said oven are exposed to substantially less severe temperature conditions than the side faces of said coalcharge, the method of preventing injury to the side walls of said oven during carbonization of coal of a type which when carbonized in the usual manner causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the carbonizing period because of the formation of the coalcharge during the latter part of the carbonizing period -oi' a substantially uninterrupted plastic lone surrounding and enclosing the central portion of said coal-charge, said plastic zone being so highly impermeable to passage of gas therethrough as to restrict escape of gas emitted from the portion of the coal-charge within said plastic zone to such degree as to cause a relatively high pressure to be built up within said plastic zone, said method comprising charging said oven with coal of said type which when carbonized in the usual manner causes pressures in excess of said normal value to be exerted against the oven side walls during the latter part of the carbonizing period, and preventing the formation in said coalcharge during the latter portion of the carbonizing period of an uninterrupted plastic zone by providing over at least a substantial part of one oi' the faces of said coal-charge which is subjected to said less severe temperature conditions a porous layer o'f granular material of such thickness as to prevent heat radiated from' the portion of the oven adjacent said layer of material from penetrating through said layer of material and developing a highly impermeable plastic zone across the face of the coal-charge abutting said layer of granular material, said porous layer of granular material being of such composition that 5 during exposure of said material to the temperaf' Numbery ture conditions encountered in the pQrtion of the oven in 'which said material is located said material remains relatively permeable and oiiers substantially less resistance to flow of gas therethrough than the plastic zone formed in said coalcharge. A

6. In a method of carbonizing coal in an externally heated horizontally elongated coke oven having side walls which are susceptible of injury when subjected to pressure in excess of 2 lbs. per square inch, the method of preventing injury to the side walls during carbonization bf an expanding coal of the type which when carbonized in an oven in the usual manner has created therein during the coking period a plastic condition along with the generation of gas and vapors which cause pressures in excess of 2 lbs. per square inch to be exerted against the side walls of the oven.

said method comprising: charging the oven with said expanding coal to fill a major portion of the oven, driving heat through opposite sides of the coal body to make it plastic and coke it within the temperature range for high temperature coking and providing during the carbonization period a covering for a surface of the coal body consisting of a porous layer of granular material of such composition and thickness as to provide a structure of the coal being carbonized and the material thereon to permit the ready escape therethrough of gas generated by the coal carbonization without developing a pressure in excess of 2 lbs'. per square inch against the oven walls during. the coking period. g

7. The method deiined in claim 6 in which the porous layer of granular material is placed on the top of the coal body in the oven to a thickness oi at least six inches.

ALFRED R. POWELL.

REFERENCES CITED The following references are of record in the ille of this patent:

'UNITED STATES PA'I'ENTS Name Date Korting Apr. 26, 1910 Fitz Oct. 29, 1935 FOREIGN PATENTS Country Date Great Britain C. A. Dec. 5. 1929 Number

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