US1912002A - Process of making carbonized fuel briquettes - Google Patents

Description

May 30, 1933. H. o. LOEBELL PROCESS OF MAKING CARBONIZED FUEL BRIQUETTES Filed Sept. '12, 1929 2 Sheets-Sheet l INVENTOR HEN RY O. LOE BELL ms ATTORNEY Patented May 30, 1933 UNITED STATES PATENT OFFICE HENRY O. LOEBELL, OF MALIBA, NEW YORK, ASSIGNOR TO COMBUSTION UTILITIES CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF MAINE PROCESS OF MAKING CARBONIZED FUEL BRIQUETTES Application filed September 12, 1929. Serial No. 392,272;

This invention relates to processes for the production offuel briquettes, and more especially it concerns the productlon of such briquettes from mixtures containing carbonaceous materials particularly those of relatively low volatile content such as anthracite coal, low volatile bituminous coal, Dubbss pressure still coke residues, coke breeze, and the like, together with suitable bituminous binder materials or cementing agents. It is of especial utility in connec tion with pressure briquetting and carbonization treatments concurrently applied to the fuel mixture being treated. The invention is applicable to processes involving the concurrent carbonization and briquetting of fuel mixtures in relatively thin layers under controlled pressure, such as that disclosed in applicants co-pending application Serial N 0. 347,802, filed March 18, 1929, although it is obviously susceptible of being carried out in other manners and with different types of apparatus than those disclosed in the said application.

,In the past, many attempts have been made to briquette relatively non-coking fuels of low volatile content such as anthracite coal and similar solid fuels, and particularly anthracite waste materials such as culm, which is produced in large quantities in connection with the usual methods employed "for mining the coal. In certain of the prior known processes, the finely divided anthracite culm was -mixed with a high volatile coking coal, or with a pitch or tar and the resultant plastic mixture was then shaped under pressure as by extrusion through cooled dies under high pressures, as for example, in the neighborhood of 50 to 20,000 pounds per square inch, at temperatures at which the mixture is just sufliciently plastic to be capable of being shaped in the briquetting machine. The fuel briquettes thus prepared were in some cases further heattreated, in some instances at temperatures below that at which the coking components of the mixture tend to become plastic and to be partially carbonized, the purpose being to remove a certain proportion of the volatile components of the mixture so as to reduce the smoking of the fuel during combustion of the same when burned under conditions of natural draft for domestic purposes.

The type of apparatus required for carrying out the said prior known processes isquetting apparatus frequently results due to the abrasive action of the coke at the high pressures used. After baking the preformed briquettes at temperatures substantially below the low temperature carbonization range, the volatile constituents thereof frequently have not been removed in sulficient amounts to yield uniform, satisfactory smokeless briquettes for domestic purposes.

Among the principal objects of the present invention are to provide an improved process for utilizing a high melting pitch in the production of an easily ignitable homogeneous carbonized fuel briquette suitable as a domestic fuel; to provide in an improved manner for simultaneously briquetting and carbonizing a relatively thin layer of a fuel mixture containing a substantially non-coking, low volatile coal and a normally solid bituminous binder material; to provide an improved process for concurrently briquetting and carbonizing mixtures of anthracite coal and bituminous material in which at least a portion of the bituminous material is a relatively highmelting point pitch; and to provide in a process for carbonizinga fuel mixture containing anthracite coal and a bituminous material such as a high melting point pitch, the step of controlling the volatile content of the said fuel mixture to give upon carbonization a. free burning readily ignitable briquette, suitable as a domestic fuel. These objects and others will appear'more fully in connection with the following disclosure.

The invention comprises in its broadest scope the preparation of fuel briquettes having the necessary properties renderin them suitable as domestic fuel from finely ivided mixtures containing certain non coking or low coking, low volatile fuels such as anthracite culm, hydrocarbon still residues and the like intimately mixed with suitable amounts of bituminous material, preferably containing a pitchy substance of determinate volatile content, and having a meltin point above 300 F., and thereafter,-pre erably after arranging the said fuel mixture in a relatively thin layer,exposing the said layer of fuel to carbonization at temperatures for example within the low temperature carbonization range, while exerting upon the fuel a relatively low degree of pressure preferably mechanically applied. The fuel mixture to be briquetted should preferably be dried in any well known manner prior to the carbonization.

The bituminous material forming an essential component of the fuel mixture to be briquetted may also comprise a high or a low volatile coking coal or mixtures of the same with or without a substantial proportion of a low temperature coke.

The high melting pitch may be intimately intermixed with the other components of the fuel mixture either in molten condition or in the solid condition in the cold, followed in either instance by grinding in the cold. Cold mixing and grinding is generally preferred as apparently yielding a stronger, tougher briquette than that obtained with hot mixing. It also permits a more intimate mixture of the various solid components and facilitates the interdiifusion thereof during the coking process. I

It has been discovered that high melting point pitches and particularly those having melting points in the range of from 300 to 400 F. and above, and derived from petroleum or coal tar or their equivalents, as by suitable distillation processes, may be substituted for all or a portion .of the coking coal so as to produce an excellent car bonized briquette under the conditions of carrying out the present process, particularly when used in amounts varying from 15 to 30 per cent of the fuel mixture being treated. Such pitches have volatile contents as high as 60%, while they still retain marked coking properties at temperaturesin the neighborhood of 900 to 1000 F. and above. They are normally solid and can if desired be ulverized and mixed dry with the noncoking coal and the other components of the mixture to be briquetted. When the high melting point pitch is used alone as the bituminous component of the fuel mixture, the pitch should preferably contain 40% to 60% of volatile matter. The most satisfactory pitches for use in the process should meet the following approximate specification:

Melting point above 340 F.

Volatile content below 50%.

Asphaltic content 60 to 30%.

Distillation temperature above 840 F. Such pitches are usually distilled at a liquid temperature of 825 F. or above. While the amounts of these high melting point pitches which will satisfactorily bind mixtures containing noncoking fuels such as anthracite culm of a size which will pass through 40 mesh so as to make an excellent carbonized briquette is somewhat variable, depending upon the character of the fuel mixture and of the treatment to which it is subjected in the process, 15% to 30% of the pitch is ordinarily required to obtain the best results in briquetting anthracite culm of a size less than 40 mesh.

While carbonizing heat is preferably applied to each of the respective surfaces of the thin layer of the fuel being processed, from heat sources respectively above and below the same, good results are obtained by applying the heat to the layer of fuel from one direction only. The carbonizing heat may be applied either prior to the application of ,the relatively low pressure necessary for forming the briquettes or simultaneously therewith. Any heat applied to the layer of fuel prior to the application of pressure thereto may be suflicient to raise the temperature of the fuel layer to a point where the mixture becomes more or less plastic, such as for example, a temperature of 750 F. to 850 F. or above,although this preheating treatment may be limited to the use of temperatures considerably below that mentioned.

The carrying surface, upon which the thin layer of fuel is to be deposited, is generally preheated by suitable means to temperatures in the neighborhood of 750 F. or thereabove, immediately prior to the deposit thereon of the fuel mixture. A temperature of 850 F. for this carrying surface is very satisfactory. This apparently results in the rapid formation of a thin 'film or coating of carbonized material in contact with the said carrying surface while allowing free escape ground to as nearly a uniform size as possible in order to avoid any tendency toward heterogeneity of structure of the resultant product. Such uniformity of particle size is not essential however. Preferably the various ingredients are ground to a size less ,than 20 or 30 mesh, and thereafter they are mixed together and the mixture ground further if necessary until approximately thereof will pass through a 40 mesh sieve. As a general rule the quality of the briquettes improves with the degree of fineness of grinding of the fuel mixture until the 40 mesh size material is reached. When the materials used are too finely divided, there is a tendency for the briquettes prepared therefrom to be slightly weaker than those made from somewhat larger particle sizes,- but to be close-grained and homogeneous. In such instances the briquettes may if desired be strengthened by slightly increasing the amount of high melting pitch employed.

The accompanying drawings show one form of apparatus disclosed in my said copending application 347,802 and adapted for carrying out the invention, in which Fig. 1 is a horizontal cross-section of a carbonizing and briquetting apparatus taken along the lines 11 of F 2, parts being broken away; Fig. 2 is a transverse crosssection throu h the same on the line 22 of Fig. 1; ig. 3 is a transverse section through a portion of the rotor and associated briquetting conveyor; and Fig. 4 is a perspective view of one of the conveyor links.

Referring more particularly to the drawings, numeral 10 designates a closed fluidtight, horizontally-disposed carbonizing retort suitably lined with refractory material and having a heating jacket 12 as shown. A hollow cylindrical rotor or drum 13 of heat resistant metal or alloy extends transversely of the retort with its intermediate portion disposed within the same and'having its ends extending through the opposite retort side walls. The rotor is rotatably supported at its'ends in any suitable manner as upon rollers coacting with annular tracks secured to the rotors outer surface. Annular stufiing boxes or the like are normally associated with the rotor for preventing escape from around the rotor of the volatile products formed in the carbonization chamber or retort 10.

An idler roll or rotor 15, preferably of smaller diameter than the rotor 13, of heat resistant metal or alloy has its horizontal axis arranged parallel to that of the latter. The rotor 15 is supported within the retort 10, having its axle 17 rotatably mounted in end bearings 19 19, the latter being carried upon brackets 21 21 and adapted for sliding --movement thereon for adjustment towards and away from the rotor 13. Each of the end bearings 19 has co-operatively associated therewith one of a pair of members 20, one end of each of the latter of which is adapted for sliding movement through an end wall of retort 11. A compression s ring 22 surrounds the portion of each mem er 20 outside of the retort, being operatively interposed between the retort wall and an adjusting nut 23 carried by the threaded end of the member 20.

An endless conveyor belt 25 composed of mold sections or links 27 are disposed within the retort and surround both the rotors 13 and 15 in the manner shown, and an upper reach thereof passes beneath a deflecting roll 29, the latter of which serves to reverse the curvature of the belt and thus to discharge the carbonized briquettes from the mold sections 27. The associated links 27 making up the conveyor 25 are interconnected by means of a series of pins or rods 28,this construction being more clearly shown in Fig. 2 and Fig. 3. By the said arrangement, the conveyor belt forms a plurality of associated mold sections in all positions excepting where the curvature of adjacent links is reversed' at points adjacent the roll 29, at which points adjacent links cooperate to effeet the discharge of the carbonized fuel from the molds.

A trough 31 receives the briquettes thus discharged, and a conveyor 33 therein conducts the briquettes through an air lock or the like to a point of discharge. One marginal portion 35 of the trough 31 is adapted to function for removing carbonized bri quettes from the surface of rotor 13 when such is necessary and for directing them into the trough 31.

A raw carbonizable fuel conduit 37 extends transversely across the retort adjacent that portion of the lower reach of the conveyor near the rotor 13, and has therein a longitudinal opening 39 for permitting a uniform flow of fuel on to the lower reach of the conveyor 25. Avertically adjustable guide member or scraper {11, associated with the conduit 37, permits adjustment of the depth of fuel deposited upon the conveyor.

he cylindrical surfaces of the rotors 13 and 15 have therein transverse grooves 50 60. Additional heat for the carbonization may be supplied by heat transferred through the heating jacket 12.

An outlet 64 is provided for conveying the carbonization vapors and gases from the 'rethereby into contact with the rotor 13, the

BOT

latter of which is maintained at a temperature adapted to carbonize the said fuel mixture at temperatures within the low temperature carbonization range during the time that it passes in contact with the rotor 15 '13. Pressure to be applied upon the fuel mixture during this carbonization is adjusted by means of the compression springs 22, or the deflecting roll 29 may be adjustable vertically to function as a belt tightener to serve this purpose. With the apparatus shown, it will be observed that the pressure exerted upon the fuel layer gradually increases during the early stages of the carbonization.

Where a suitable apparatus such as that disclosed in my said copending application is employed, the process may be carried out continuously. In such instances, a thin layer of the desired fuel mixture is continuously and uniformly deposited upon one of the moving sections of a hot briquetting moldcarrying belt, which may or may not. have been preheated 'in the manner hereinbefore described to temperatures in neighborhood of 750 F., or thereabove, immediately prior to the deposition of the said mixture thereon. The free surface of the moving layer of material thus deposited on the highly heated mold section is thereafter exposed to heat radiated thereto from an internallyheated rotor acting as the complementary mold section, to bring the said material to a temperature approximating that required for rendering it plastic prior to the actual contact thereof with the said complementary mold section. The material" is thereafter brought into contact with the heated rotor and moves with the latter through a portion of its path while a low pressure is exerted upon the fuel layer thus enclosed between the rotor and belt. The pressure is yieldingly applied to the fuel layer throughout 'the remainder of the carbonization step.

The briquettes are thus moved continuously along a curved, heated path until suitably carbonized, and are subsequently relieved of the said pressure and are discharged from the apparatus.

The following examples will serve to illustrate certain preferred embodiments of the invention, but they are not to be considered in any sense as limiting the scope thereof as clearly defined in the accompanying claims.-

Ewwmple 1.A fuel mixture comprising 55% of anthracite culm and 45% of Washington coal (a high volatile cokin coal) was ground to less than 30 mesh. T e mixture was thereafter subjected to a combined briquetting-and carbonization operation in a continuous machine of the rotary carbonizing type disclosed in the aforementioned co-pending application. The mold section or support for the thin layer of fuel mixture was heated to a temperature above 850 F. immediately prior to placing the fuel thereon. The fuel layer was then moved to bring its opposite surface into contact with a 1'0- tor or second mold section held at a temperature of approximately 1050 F., and the fuel was heated at this temperature for a period of about 40 minutes, under a yielding pressure of about 100 pounds per square foot. A surcharge of v employed. The briquettes thus formed and carbonized had a specific gravity of 1.07, were homogeneous, and of very satisfactory texture. They possessed a; high resistance to crushing forces, to impact and to abrasion. They were very readily i nitable and burned freely and uniformly w en ignited. Similarly, briquettes suitable for use as domestic fuel have been prepared from mixtures of anthracite culm and bituminous coking coal in amounts varying from 80% of anthracite and 20% of coking coal, to 50% of anthracite culm and 50% of coking coal, employing temperatures in the neighborhood of from 900 to 1100 F. and carbonizing periods of 40 minutes and more, and under pressures of from 1 to 12 pounds per square inch and higher. The anthracite culm may be ground to the same size as that of the bituminous fuel employed, or it may be somewhat coarser than the latter.

- Example 2.A mixture composed of 80% anthracite culm, 10% Washington high volatile coking coal and 10% Skelly prepared coal), all ground to less than 40 mesh, was supported in a thin layer on the surface of one member of a set of cooperating briquetting mold sections and was carbonized after receiving a preheat treatment for eight minutes at an average temperature of around 1000 F. The carbonization was thereafter completed by heating the mixture for 45 minutes,-one surface of the fuel layer being exposed to temperatures of approximately 1110 F., the other surface being heated during the same time to an-average temperature'of around 930 F. A pressure of apapplied upon the fuel layer followin the preheat treatment,and a surcharge o of the mixture over that required to fill the mold sections was employed. The resultant carbonized briquettes were very satisfactory proximately 250 pounds per square foot was The Skelly coal employed above is a product obtained from a low melting point petroleum pitch which has been reduced by distillation as to have a melting point of approximately 350 F.

Tars or relatively low melting pitches, such as those preparecLby the distillation of hydrocarbon oils and the like at temperatures around 450 F. are not entirely satisfactory for use in the concurrent briquetting and carbonization operation here disclosed, due to the tendency of the briquettes to stick in the molds under the conditions of operation employed. On the other hand when the reduction of such tars and pitches has been carried to a point where their melting points are within the range of from 250 to 400 F. and higher,and particularly around 350 F.,excellent briquettes can be prepared from fuel mixtures containing such pitches and noncoking fuels of low volatile content such as anthracite coal, prepared according to the present process.

Similarly, carbonized briquettes can be prepared by the simultaneous carbonization and briquetting of fuel mixtures consisting solely of a noncoking or low coking carbonaceous material such as anthracite coal and a relativelyhigh melting point pitch: Such a mixture containing 70% anthracite culm and 30% of an asphaltic base oil itch distilled at 875 F. and having a v0 atile content of 18.2% and a melting point above 340 F. was carbonized at temperatures around 1050 F. while under a pressure of about 250 pounds per square foot. Very satisfactory briquettes were obtained both with regard to external appearance, homogeneity, physical strength and combustability. In like manner substantially any low grade still residue such as is obtained inthe distillation of petroleum oils, tar oils, and the like may be successfully treated, as by distillation to reduce its volatile content to the desired degree,and to make a pitch having the desired melting .point (250 to 400 F. or above), and which still retains suitable coking properties. Such pitch can, when cool, be readily pulverized and serves satisfactorily as a binder in the present process. Preferably the pitch employed should have a volatile content of less than 60% in order to prevent sticking of the briquette to the molds, and to prevent the development of a porous internal structure within the briquette.

Among other suitable distillation products may be mentioned the pressure still bottoms recovered as residues from the pressure distillation of crude petroleum oil, and which has been further reduced by distillation at a liquid temperature of 860 F. to yield a pitch of 340 F. melting point. Those residuums which have been distilled to temperatures near their coking temperatures are quite suitable for use in briquetting anthracite fines and the like. For example, a pitch having a melting point of 322 F. and a V.C.M. of 50.6-prepared by distilling low temperature tar until 80% had been distilled off,was intimately mixed with anthracite culm in the proportion of 80% of the culm to 20% of the pitch. The mixture was concurrently carbonized and briquetted at an average temperature of 1030 F. for a period of 45 minutes under a yielding pressure of 250 pounds per square foot. Briquettes prepared from this fuel mixture under substantially the conditions given were more readily ignitable than anthracite coal, and burned much more freely than the latter when ignited at temperatures in the neighborhood of 8001-000 F. They are highly resistant to abrasion and possess strength to resist impact and crushing forces definitely greater than the briquettes of this type on the market, insofar as the applicant is aware.

Moreover it is desirable'in many instances particularly where the only coking bituminous material used is a high melting pitch to control the process by so selecting the bituminous componentsof the fuel mixture as to give the latter a volatile content of around 8% prior to the carbonization treatment; The character of the volatile in the treated pitch is such as to be largely held by the fuel during the carbonization and yield a carbonized product of satisfactory volatile content and readily ignitable.

In instances where the fuel mixture being carbonized has too high a percentage of volatile matter and a high coking index, as where the fuel mixture is too rich in volatile, coking bituminous materials,any tendency of the carbonized product to stick in the molds may be completely neutralized by the addition to the oil pitch or to the fuel mixture of small amount,for instance approximately 2%,of paraffin or its equivalent prior to the processing of the fuel mixture. The-addition of the small amount of parafiin apparently does not reduce the strength of' the briquette nor otherwise change its desirable properties. The :tendency toward sticking of the briquettes in the molds may be prevented also by a suitable preheating treatment of the mold surpreferably at temperaturesbelow the coking temperature. Moreover, for the same purpose,."suitable amounts of low temperature char and the like, such as that made in the present processor by other low temperature carbonization processes, may be incorporated into the fuel mixture prior to the carbonization and briquetting operations. The low temperature char apparently is capable of absorbing at least substantial portions of the coking ingredients of the fuel mixture and it prevents such ingredients from exerting a cementing action upon thehparts of the apparatus in contact therewit Where the step of preheating the fuel mixture with flue gases or air is employed, the time required for carbonizing the briquettes may be substantially reduced by moving the preheated fuel directly into the high temperature carbonizing zone.

In instances where the fuel mixture employed consists only of anthracite culm and high melting point pitch the fuel supporting and carrying surface is preferably not preheated before chargin the fuel thereon. In such modifications of the process the layer of the material being carbonized is preferably heated at both surfaces simultaneously.

By practicing the invention in accordance spalling characteristics of the latter and sub-' stantially eliminated. The carbonized fuel burns with a smokeless flame under suitable conditions of ventilation, and will retain its shape in the fire until completely consumed.

The binder employed being composed en-' tirely of combustible material, does not add substantially to the ash content of the anthracite fuel briquetted, while it has a melting point sufficiently high to permit the briquette to hold its shape in the fire, until the combustion thereof has been effected.

By the term relatively high melting point pitch and similar expressions in the specification and claims, I refer to pitches obtained from any source which have been so processed as to have melting points of 300 F. or above.

The term low volatile fuel as employed in the claims is intended to cover not only low volatile bituminous coal but to include as well anthracite coal, coke breeze, carboncontaining still residues from the distillation of hydrocarbons, coal or similar carbonaceous materials, either as recovered from the still or after treatment to modify or reduce the volatile content thereof, and other similar natural or artificially-prepared low volatile fuels and mixtures thereof.

The invention is susceptible of modifica-- 1. The process of producing a carbonized fuel briquette suitable for use as a domestic fuel, which comprises carbonizing within the low temperature carbonization temperature range a finely divided fuel mixture arranged in a thin layer and containing a low volatile fuel and a high melting point pitch having a melting point at least as high as 300 F., while concurrently forming the said layer being subjected to such heat treatment into a plurality of separate bodies and maintaining such-bodies under a low mechanical pressure throughout the carbonization.

2. The process as defined in claim 1 in which the said pitch employed has a melting point about 325 F. and a volatile content of not more than 60%.

3. The process as defined in claim 1 in which the fuel mixture being carbonized comprises anthracite culm and a pitch having a melting point above 300 F.

4. The process of producing a carbonized fuel briquette containing anthracite coal and suitable as a domestic fuel which comprises the steps of carbonizing a thin layer of a finely divided fuel mixture containing anthracite culm and solid bituminous material at temperatures within the low temperature carbonization range while concurrently forming the said layer into a plurality of separate bodies, and subjecting such bodies to a low mechanical pressure at some stage during the said heating step, the said solid bituminous material comprising a pitch havin a melting point at least as high as 300 5. The process of producing a carbonized fuel briquette suitable as a domestic fuel which comprises the step of depositing upon a surface, heated to a temperature of approximately 850 F., a thin layer of a fuel mixture containing anthracite culm and solid bituminous material, flowing additional heat from a separate source into the fuel layer from the surface thereof opposite that in contact with the said highly heated surface and thereby carbonizing the said fuel layer at a temperature within the low temperature carbonization range while concurrently forming the said heated layer into a plurality of separate briquette-shaped bodies and subjecting the latter to a low pressure throughout the carbonization.

6. The process of producing a carbonized fuel briquette suitable as a domestic fuel which comprises forming in a thin layer a finely divided fuel mixture containing anthracite clum and a solid bituminous material, including a pitch having a melting point at least as high as 300 F., moving the said thin layer while exposing one surface thereof to heat radiated thereto from a heat radiating surface adjacentthereto for heating the surface of the fuel to a temperature within the low temperature carbonization tion of the time of such carbonization.

7 The continuous process of producing a carbonized fuel briquette suitable as a doniestic fuel which comprises continuously forming a thin layer of a finely divided fuel mixture containing anthracite culm and a solid bituminous material upon a heat conducting surface heated to a temperature within the low temperature carbonization range, moving the said thin layer to bring its free surface into contact with a second heated surface held at a temperature within the low temperature carbonization range thereby carbonizing the fuel mixture under a low pressure maintained throughout the carbonization by compressing the mixture between the said surfaces, and concurrently forming the said layer into a plurality of separate briquette-shaped bodies during such movement.

8. A process of producing a carbonized fuel suitable for domestic use which comprises the step of carbonizing at temperatures above 800 F. and within the low temperature carbonization range a fuel mixture containing a carbonaceous component having a low volatile content and a pitch having a melting point at least as high as 300 F., and maintaining a low. pressure upon the fuel mixture substantially throughout the carbonization,

9. The process of producing a carbonized fuel briquette adapted for use as a domestic fuel, which comprises carbonizing a thin layer of a finely-divided fuel mixture containing a low volatile solid fuel and a pitch having a melting point at least as high as 300 F. at temperatures within the low temperature carbonization range by heat applied from two opposite surfaces of the said thin layer, and concurrently forming the said layer undergoing heat treatment into a plurality of separate bodies while maintaining a low mechanical pressure upon such bodies throughout the carbonization.

l0.-The process as defined in claim 9, in which the said fuel mixture contains sufiicient of the said high melting pitch to give the fuel mixture, prior to carbonization, a volatile content of around 8%.

11. 'The process of producing a carbonized fuel briquette suitable for use as a domestic fuel, which comprises heating and carbonizing a thin layer of a fuel mixture containing at least 50% of a low volatile coal and a. high melting point pitch having a volatile content of less than 60% and a melting point of at least 300 F., while maintaining the said fuel layer throughout the heat treatment under a low, mechanical pressure.

12. The process of producing a carbonized fuel briquette adapted for use as a domestic fuel, which comprises depositing upon a highly heated surface a thin layer of a fuel mixture containing a low Volatile solid fuel and a pitch having a melting point of at least 300 F., and which can be made plastic by heating, radiating heat to the free surface of the said layer thereby rendering the fuel mixture plastic, subsequently conductin heat to the said fuel layer in amount su cient to heat the latter to a temperature within the low temperature carbonization range thereby carbonizing the same while concurrently exerting a low pressure upon the layer, and maintaining the said pressure throughout the carbonization, irrespective of volume changes in the fuel mixture being heated.

In testimony whereof I affix my signature.

HENRY O. LOEBELL.

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