US2966400A - Lignite processing method - Google Patents

Description

Dec. 27, 1960 H. e. LYKKEN LIGNITE PROCESSING METHOD JI'roRA/EYJ 2 Sheets-Sheet 1 I INVENTOR. HENRY 6. .yK/aw BY Filed Sept. 27, 1954 Dec. 27, 1960 H. G. LYKKEN LIGNITE PROCESSING METHOD Filed Sept. 27, 1954 FIG: E

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nited States Patent LIGNITE PROCESSING METHOD Henry G. Lykken, 4820 Sheridan Ave. S., Minneapolis, Minn.; Frances H. Lykken, administratrix of said Henry G. Lykken, deceased Filed Sept. 27, 1954, Ser. No. 458,458

8 Claims. (Cl. 44-1) This invention relates to improved methods and apparatus for treating and drying lignite and similar highmoisture content lignitic fuels.

This application is related to my application, Serial No. 52,008, filed September 30, 1948, and issued as Patent No. 2,610,115 on September 9, 1952; Serial No. 144,804, filed February 17, 1950, now abandoned; cpending applications, Serial No. 193,333, filed November 1, 1950, and issued as Patent No. 2,708,654 on May 17, 1955; Serial No. 308,400, filed September 8, 1952, now US. Patent No. 2,811,427; and Serial No. 437,751, filed June 18, 1954, now abandoned; the disclosures of some of which will be referred to in detail hereinafter and are incorporated herein by reference to the extent necessary for a fuller and more complete understanding of this invention.

Lignitic fuels normally contain from about thirty-five to forty percent moisture. When allowed to air dry, or when dried by mere application of heat, such fuels break downand disintegrate into high dust content materials. If the moisture content is reduced to below about ten to fifteen percent by ordinary drying, moisture will be reabsorbed. Lignitic coals, when so dried, are unstable and susceptible to spontaneous ignition during storage. Because of this, the use of lignitic fuels has been limited to those instances in which special precautions may be taken to store the materials in small quantities or where the material is protected to prevent spontaneous ignition.

The present invention has for its primary object the provision of a new and improved method and apparatus for producing stable moisture resisting and storable lignitic fuel wherein the lignite is gradually deprived of its free and combined moisture and rendered more activated and adsorptive while simultaneously permitted to adsorb and hold mineral hydrocarbon compounds by means of an efiicient two-stage processing operation.

It is a further object of this invention to provide a U-shaped treating kiln system comprising two parallel rotary units wherein the inlet portion of the kiln is heated with exhaust vapors from the direct fired downdraft or bottom gas outlet furnace at the outlet side of the kiln.

Another object of the invention is to provide an efficient method and apparatus for treating lignitic fuels in a two-stage operation, wherein heated exhaust vapors from the processing zone are recirculated to assist in preheating the fuel product and relatively cooled exhaust vapors from the preheating zone are recirculated to assist in cooling the finished fuel product.

Other objects of this invention will become apparent as the description proceeds.

To the accomplishment of the foregoing and related ends, this invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

The invention is illustrated by means of the drawings, wherein the same numerals are used to indicate corresponding parts and in which:

Figure 1 is a top plan view of the kiln of this invention, taken in section generally along the line 11 of Figure 3, showing the details of the interior of the kiln;

Figure 2 is a vertical section taken along the line 2-2 of Figure 1 and in the direction of the arrows; and

Figure 3 is a vertical section taken along the line 3-3 of Figure 1 and in the direction of the arrows.

Broadly stated, this invention comprises a process for treating lignitic fuels in which coarsely crushed and screened lignite is fed with a minor amount of fluidal hydrocarbon material into and through a rotary preheating kiln where a portion of the moisture is expelled, and then into and through a rotary processing kiln wherein the temperature of the lignite is raised to about 600 F. eliminating substantially all the free moisture, initiating the breakdown of the hydrolysed lignin with an accompanying liberation of combined water and carbon dioxide, agglomerating the fines and forming an impervious varnish-like coating on the lignite granules, and the resulting product is finally cooled; and apparatus for carrying out the process. More particularly, the invention comprises the utilization of heated steam and hydrocarbon vapors from the processing kiln to preheat the incoming raw lignite product and the recirculation of a portion of this exhaust steam cooled from contact with the incoming lignite, back into the product outlet to cool the heated product before it leaves the processing kiln. In order to thus utilize a portion of the exhaust steam as a constantly recirculating heat regulating and heat transfer media within the kiln system, the kiln consists of two parallel rotary units enclosed in a twin refractory furnace structure, one referred to as the preheating kiln and the other as the processing kiln. The two parallel kilns are so arranged as to provide a U-shaped path for the material being treated with the raw lignite inlet and the finished product outlet being at the same end of the system.

The processing kiln is suspended in a downdraft or bottom outlet furnace, fired with any suitable fuel, such as pulverized lignite, gas, oil or the like, the full length of the kiln if desired and necessary. The gas from the processing kiln furnace is withdrawn from behind its bridge wall at floor level into the furnace of the preheating kiln at its hot end and along its full length to a chimney outlet at the other end.

The preheating kiln furnace is also a downdraft furnace having spaced bafiies along the furnace the full length of the kiln to induce a continuous up-and-down flow of the hot furnace gases until they are finally withdrawn at the bottom of the cold end. It will be understood that the flow of hot furnace gases through the preheating kiln furnace is eountercurrent to the flow of lignitic fuel through the kiln, so that the hot or inlet end of the preheating kiln furnace corresponds to the outlet end of the kiln suspended within the furnace, and the cold or outlet end of the furnace corresponds to the inlet end of the kiln. Both kilns are especially designed to provide large heating surfaces as well as a rigid and rugged construction. Both kilns are supported at their ends, outside the furnace structure, for rotation on parallel horizontal axes.

The row lignite is first coarsely crushed and screened to pass a /s to one and one-half inch mesh for convenience in handling and rapid heat penetration. Although size is not critical, preferably the lignite is screened to pass a three-fourths inch mesh. The raw fuel is fed with a small percentage from about three to ten percent by weight, based on the finished product, of a,

fiuidal mineral hydrocarbon to the inlet end of the pre heating kiln. The hydrocarbon oil assists in picking up dust and fines which are fed into the kiln with the raw product and lubricates the flow through the kiln.

Lignite normally contains from about thirty-five to forty percent free moisture. From about twenty to twenty-five percent reduction of total free moisture may be achieved in the course of its passage through the preheating kiln with the flue gas going out at about 300 to 400 F. and the water vapor going off at about 220 F. As will be explained in more detail hereinafter, the lignite moves through the preheating kiln in countercurrent heat exchanging flow with super-heated exhaust vapors from the processing kiln. The preheated lignite is transferred from the outlet end of the preheating kiln to the inlet of the processing kiln. V

In the processing kiln the temperature of the lignite is raised to about 600 F. At this temperature, not only is all the free moisture eliminated, but breakdown of the hydrolysed lignin is initiated, liberating additional water and carbon dioxide. At this stage, in typical North Dakota, thirty-six percent free moisture lignite, about a forty percent total reduction in weight has occured.

The partial breakdown of hydrolignins, when terminated at 600 F., involves no loss of combustibles, and the extra fuel required to bring about this breakdown is a negligible percentage of the total fuel requirement. As will be noted later, this partial breakdown is a vital consideration in stabilizing the finished product.

The lignite is dried in this process in an atmosphere of superheated steam and superheated hydrocarbon vapors. Both the carbonized portion of the lignite (if any of it can actually be said to be wholly carbonized) and the partially broken down hydrolysed lignin are subjected to an intense activating reaction with the steam, followed by equally intense deactivating reaction with the hydrocarbons. While the nature and sequence of these reactions are not fully understood and applicant does not wish to be bound by any particular theory relating to them, it is known that the end product is a closed end bond, deactivated, chemically saturated material, not subject to firing in the processing, nor heating and firing in storage.

Although the hydrocarbon material may be added in amounts from about three to ten percent by weight, or even more, if it is desired to load the product with hydrocarbon, for a boiler plant fuel from about three to four percent hydrocarbon will suffice to produce a clean, dust free, bone dry and storable fuel. For domestic use and to make it more moisture repellent, the hydrocarbon content should be increased to about five to six percent to produce an impervious varnish-like coating on every particle. The varnish-like coating which develops in this process is believed to be made up not wholly of the additive or oil introduced with the lignite, but is a reaction product between the hydrocarbon and the breaking down hydrolysed lignin.

The finished product is discharged from the processing kiln adjacent to the raw lignite inlet of the preheating kiln. The inlet end of the preheating kiln also serves as outlet for the moisture removed from the product, which is discharged as steam.

As soon as the moisture is expelled from the lignitic fuel in the processing kiln and evaporated, the temperature of the charge rises very rapidly in an atmosphere that may be several hundred degrees hotter. Since, to limit the breakdown of the hydrolysed lignin, the temperature should be limited to about 600 F. (that is, between about 550 F. and 650 F.), means must be provided for easy control of this temperature. The temperature of thecharge should also be reduced as much as possible before it is discharged with recovery of as much as possible of its heat content. It is desirable, too, that the heating capacity of the hot end of the processing kiln be utilized as much as possible and carried ba'ck'to the vaporizing section. It is to these problems that this invention is principally directed.

A controlled amount of the exhaust steam from the inlet end of the preheating kiln, say about twenty to forty percent, or preferably about twenty-five to thirty percent as needs dictate, is transferred and forced into the product outlet end of the processing kiln to act as a constant recirculating heat regulating and heat transfer media within the kiln system. The thus introduced exhaust steam can be used to move the highest heat zone any desired distance from the discharge end of the processing kiln. The steam cools the product, and at the same time, it picks up the excess heat at the hot end of the kiln, serving as a means of controlling the temperature of the charge and carrying back a material percentage of the required heat to where it is needed. The steam, superheated as a result of its countercurrent passage in heat exchange relation with the hot product in the processing kiln, and commingled with some hydrocarbon vapors, is transferred at a temperature of about 600 F. to 1000 F. to the outlet end of the preheating kiln where it exchanges a substantial portion of its heat with the raw lignite product. If the finished product is to be stock piled, further cooling may be required. This may be accomplished on a chain grate using the excess spent steam as a coolant. The excess exhaust steam may also be utilized to preheat the raw lignite material toa limited extent on a moving chain grate before it is introduced to the preheating kiln. Any remaining exhaust steam may be vented to the atmosphere.

The hydrocarbons used in the process normally will be the heavier by-products of the oil refineries in the lignite ares. As explained in more detail in my copending applications, Serial No. 193,333, Serial No. 308,400 and Serial No. 437,751, in the relatively high temperatures in sections of the kiln and its atmosphere, of 800 F. to 900 F. or more, there will be a high cracking effect with liberation of lighter hydrocarbon fractions that may escape with the steam. As described in the copending applications, suitable means for collecting and condensing these vapors'can readily be provided. Experiments indicate, however, that a large percentage of these lighter fractions are adsorbed by the product, such that when using up to five to six percent of hydrocarbon additive, the loss of light hydrocarbons is negligible, even though no attempt is made to recover them. The mineral hydrocarbon used is ordinarily a low-grade petroleum fraction, such as road oil, tar, asphaltum, bunker oil, pitch and the like which are liquid at ordinary temperatures or can be liquified by slight heating. It will be understood that crude oil or higher-grade petroleum by-products may be used where they are readily available. The mineral hydrocarbon is applied to the lignite by spraying or dripping or the like before the lignite is introduced to the preheating kiln, or at the inlet end of the kiln itself. The constant tumbling and cascading action of the product within the kiln serves to distribute the hydrocarbon uniformly throughout the product.

If, for' any reason, or for some special objective or product such as a briquetting material, a heavier coating is desired, a second charge of from about tWo to ten percent of hydrocarbon material may be added to the almost finished product at the discharge end of the processing kiln,'a few feet back from the outlet.

Referring now to the drawings, and particularly to Figure 1, there is shown here a plan view of the kiln of this invention. The kiln is shown broken away through the center to avoid needless duplication of parts, although it is in fact of considerable length. For example, a kiln of desired capacity of fifty tons of product per hour from thirty-five to forty percent free moisture lignite, requiring a feed rate of about eighty tons of raw ligniterper hour, will have a lengthapproaching ."Ee'vehtyfet,"andthe'parallel rotaryunits willhave an inside diameter of about eight feet. The plan view is taken generally along the line 11 of Figure 3 and in the direction of the arrows, so that the upper or preheating kiln is shown with the top of the furnace re moved looking upon the top of the rotary kiln, and the lower or processing kiln is shown in section through the center of the kiln along its axis. The housings at both ends of the kiln system are shown broken away just sufficiently to show the interior structure.

The kiln comprises two parallel rotary units, a preheating kiln indicated generally at and a processing kiln indicated generally at 11, both enclosed in a twin refractory furnace structure indicated generally at 12. The kilns are supported at both ends and rotated by rotary drive wheels 14 positioned outside of the furnace housing. Sealing means 15 are provided to minimize loss of heat from the furnace around the apertures through which the inlet and outlet ends of the kilns extend outside the furnace. The inlet end of the preheating kiln is provided with a hopper 16 through which the raw lignite may be introduced into the kiln by any convenient feeding means. This end of the preheating kiln is provided with a covering hood or housing 17 to prevent free escape of exhaust steam from the kiln. A fan 18 is mounted on a shaft 19 journaled within the housing substantially coaxially with the kiln and driven from any outside power source for drawing steam from kiln 10.

A closed common housing 20 encloses the outlet end of preheating kiln 10 and the inlet end of processing kiln 11 to provide means for transferring hot vapors from the processing kiln to the preheating kiln, as well as to house a product conveyor. The apertures through which the ends of the kilns extend are closed by sealing means 21. Housing 20 is provided with a sloping floor 22 terminating in a central pit 24 from which the preheated product is lifted by a bucket elevator 25 or other similar conveying means to the top of elevator housing 26 which extends through the roof of housing 20. The preheated lignite is discharged from the elevator to the top of chute 27 and drops to the inlet of the processing kiln.

The outlet end of the processing kiln is provided with a housing 28 closed at the top, but opening at the bottom to a pit from which the finished product may be withdrawn. Housings 17 and 28 are in direct communication by means of a duct 29 joining the two housings to permit passage of exhaust steam from the preheating kiln to the outlet of the processing kiln. A fan 30 is positioned in housing 28 substantially coaxially with kiln 11 for drawing controlled small amounts of steam from housing 17 and introducing it as a blast into the kiln, assisted by an annular baflie or directing member 31. The remaining exhaust steam may be vented to the atmosphere or utilized to pre-preheat the raw lignite or further cool the finished product.

The twin furnace 12 is divided by a refractory wall 32 into two chambers, one for each of the kilns. The processing kiln furnace is directly fired from the fire box 34 by any suitable heating element, such as jets 35 from which pulverized lignite, gas, oil or other fuel and air are ejected. A plurality of fuel jets are provided along the length of the furnace to be used as needs dictate. A bafile 36 in the floor of the processing kiln furnace directs the hot gases from the flames upwardly and around kiln 11 and in a downdraft flow to an inlet trench 37 below the floor level 38 at one end of preheating kiln furnace chamber. A plurality of transverse bafiles extending the width of the preheating kiln furnace are suspended at spaced intervals from the roof of the furnace. The hot rising furnace gases from the processing kiln furnace chamber rise to the ceiling and upon encountering baffles 39 are deflected downwardly, thus passing through the length of the furnace in and up-and-down undulating flow. The gases exhaust through an outlet trench 40 to a duct 41 to a stack at the opposite end of the furnace.

The kilns 10 and 11 are similarly constructed, being adapted for'field assembly into strong rigidunits pro.- viding large heating surfaces. The kilns are formed from a plurality of elongated U-shaped channel members 45 joined together at their open ends by elongated plates 46 into a tubular structure. At spaced intervals strengthening ribs 47 are welded in the open ends of the channel members, and in the same plane a plurality of strengthening and heat-conducting fins 48 are welded on the outside of the rotary units between the channel members. A plurality of helical blade members 49 are spaced at intervals throughout the length of the kilns to assist the passage of the product through the kilns. The ends of the kilns extending through and beyond the furnace housing are plain tubular structures having the helical members on their inner walls.

In the operation of the kiln system of this invention, the processing kiln side of the furnace is fired directly from the fuel jets 35 into the fire box 34. Pulverized off-grade lignite is one fuel material which may be used. The lignite is crushed to about one-fourth inch mesh as pulverizer feed and passed through a pulverizer in which about forty to fifty percent of the required air for combustion is taken through the grinding chamber. Enough hot gas is withdrawn from the furnace and mixed with the air to heat it to about 700 F. to 800 F. entering the pulverizer. In this way, a flash drying takes place, which reduces grinding costs and greatly improves firing and furnace conditions. The hot furnace gases from the flames rise to the top of the processing kiln side of the furnace and back in a downdraft to the inlet trench of the preheating kiln side of the furnace. The temperature at the ceiling in the top of the processing side of the furnace will range from about 1600 F. to 2000 F., and the hot furnace gases will enter the preheating side of the furnace at about 1200 F. These hot gases rise to the top of the preheating side of the furnace and are directed back downwardly by bafl les 39 spaced along the top of the furnace. The result is an up-and-down undulating flow of hot furnace gases ranging in temperature from about 1200 F. at the inlet end to about 350 F. at the outlet end. The undulating flow of hot gases through the furnace is countercurrent to the flow of lignite through the kiln.

Raw untreated lignite is introduced into the inlet end of preheating kiln 10 along with a minor amount of hydrocarbon material. The lignite is introduced in an amount to fill the kiln from about one-third to two-thirds of its volume. The lignite particles fall into the pockets formed by U-shaped channels 45 and tumble and cascade through the horizontal kiln advancing by contact with spiral I blade members 49. At the outlet end of kiln 10, the lignite, preheated by contact with the hot kiln walls and the countercurrent flow of superheated steam and hydrocarbon vapors has lost about twenty to twenty-five percent of its total moisture and falls into pit 24 in the end housing 20. From here, the preheated lignite is lifted by bucket elevator 25 and discharged through chute 27 to the processing kiln 11. In the inlet end of the processing kiln, the lignite loses substantially all its free moisture, and, through partial breakdown of the hydrolysed ligin, a substantial portion of the combined water. When the water is driven oif, the temperature of the product rises rapidly. It is cooled in preparation for discharge from the kiln by exhaust steam from the preheating kiln. Fan 18 assists in drawing exhaust steam, cooled by passing in heat exchange relation with the raw product, into housing 17. A controlled amount of this steam is drawn through duct 29 to housing 28 and injected by fan 30 into kiln 11. Here, passing in countercurrent heat exchanging flow with the hot dried lignite product, the steam becomes superheated and passes along with hydrocarbon vapors through housing 20 into preheating kiln 10. The steam leaving kiln 11 has a temperature of about 600 F. to 1000" F. The finished prod- 7 net is discharged to a pit at the bottom ofhousing 28 and removed by anysuitable conveyor means.

The product of thisinvention is a clean, dust free, bone dry fuel which may bestored without danger of spontaneous ignition. The process achieves maximum utilization of the furnace gases by using exhaust gas from the processing kiln to preheat the product in the preheating kiln. Efiiciency is improved by channeling the gas from the firebox over the processing kiln and in a downdraft How to the bottom gas outlet to the preheatin'g kiln and then in an undulating flow along the length of that kiln. Temperature within the processing kiln is regulated by recirculating exhaust steam, and the heat transfer properties of the steam are used to carry back useful heat from the cooling finished product.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, but the appended claims should be construed as broadly as permissible in view of the prior art.

I claim as my invention:

1. A continuous process of producing a clean, dry, storable, dust-free lignite-mineral hydrocarbon fuel which comprises tumbling and cascading crushed natural lignite of particle size from about tol /2 inch meshand a minor amount from about three to ten percent by weight of the moisture-free product of a liquid hydrocarbon along a generally U -shaped horizontal path in sequence through a partially dehydrating preheating zone, a processing zone wherein the temperature israised to about 550 F. to 650 F. to substantially completely expel free moisture and initiate breakdown of hydrolysed lignins, and a subsequent cooling zone; said process being characterized by constant heat transfer and temperature regulation by continuously recirculating at least part of the steam generated from moisture expelled in the process in countercurrent heat exchanging flow with said lignite whereby the steam attains a temperature of from about 600 F. to 1000 F. in the processing zone to assist in preheating the raw lignite in the preheating zone, the steam is cooled to about 220 F. to 400 F. in thepreheating zone and at least part of this cooled steam is recirculated to assist in cooling the finished product in the cooling zone and is there reheated to assist in transferring heat back to and regulating temperatures in the processing zone before being superheated for recirculationto the preheating zone.

2. The process of claim 1 further characterized in that from about twenty to forty percent of the exhaust steam from the preheating zone is recirculated. I

3. The process of claim 1 further characterized in that an additional small amount of liquid hydrocarbon is admixed with the treated lignite product in the cooling zone.

4. A continuous process of producing a clean, dry, storable, dust-free lignite-mineral hydrocarbon fuel which comprises passing. crushed natural lignite and a minor amount of a liquid hydrocarbon in sequence through a partially dehydrating preheating zone, a processing zone wherein free moisture is substantially completely expelled and breakdown of hydrolignins is initiated and a cooling zone, said process being characterized by continuously recirculating at least part of the steam resulting from expelled moisture from the lignite in countercurrent heat exchanging flow with said lignite whereby superheated steam from the processing zone assists in preheating the raw lignite in the preheating zone, exhaust steam from the preheating zone assists in cooling the processed lignite in the cooling zone and waste steam from the cooling zone picks up and carries back heat, and serves to regulate temperatures in the processing zone, there becoming superheated for recirculation to the preheating zone.

5. The process of claim 4 further characterized in that an additional small amount of liquid hydrocarbon is admixed with the treated lignite product in the cooling zone.

6. A continuous process of producing a clean, dry storable, dust-free lignite-mineral hydrocarbon fuel which comprises introducing crushed natural lignite and a minor amount of a liquid hydrocarbon into an elongated preheating zone, passing said lignite and hydrocarbon through said zone in countercurrent heat exchanging flow with heated vapors to partially dehydrate said lignite and thence into and through one end of an elongated processing zone of higher temperature. to drive oif remaining free Water in said lignite and to initiate breakdown of hydrolysed lignin, and passing said lignite product through and out of the other end of said processing zone in countercurrent heat exchanging flow with exhaust vapors from the inlet end of said preheating zone to cool the product.

7. In a continuous process of producing a clean, dry, storable, dust-free lignite-mineral hydrocarbon fuel which consists in tumbling crushed natural lignite and a minor amount of a liquid hydrocarbon in sequence through a preheating zone, a dehydrating processing zone and a 'cooling zone, the improvement which consists in continuously recirculating at least part of the steam generated from the moisture from the lignite in countercurrent heat exchanging flow through each of said treating zones. 7

8. The process of claim 7 further characterized in that an additional small amount of liquid hydrocarbon is admixed with the treated lignite product in the cooling zone.

References Cited in the file of this patent UNITED STATES PATENTS 1,386,472 Urquhart Aug. 2, 1921 1,508,617 Schoch Sept. 16, 1924 1,817,228 Bliss Aug. 24, 1931 2,131,702 Berry Sept. 27, 1938 2,144,178 Blumrner Jan. 17, 1939 2,183,924 Schoch Dec. 19, 1939 2,610,115 Lykken Sept. 9, 1952 FOREIGN PATENTS 337,721 Great Britain Nov. 6, 1930

Claims (1)

1. A CONTINUOUS PROCESS OF PRODUCING A CLEAN, DRY, STORABLE, DUST-FREE LIGNITE-MINERAL HYDROCARBON FUEL WHICH COMPRISES TUMBLING AND CASCADING CRUSHED NATURAL LIGNITE OF PARTICLE SIZE FROM ABOUT 3/8 TO 1 1/2 INCH MESH AND A MINOR AMOUNT FROM ABOUT THREE TO TEN PERCENT BY WEIGHT OF THE MOISTURE-FREE PRODUCT OF A LIQUID HYDROCARBON ALONG A GENERALLY U-SHAPED HORIZONTAL PATH IN SEQUENCE THROUGH A PARTIALLY DEHYDRATING PREHEATING ZONE, A PROCESSING ZONE WHEREIN THE TEMPERATURE IS RAISED TO ABOUT 550*F. TO 650*F. TO SUBSTANTIALLY COMPLETELY EXPEL FREE MOISTURE AND INITIATE BREAKDOWN OF HYDROLYSED LIGNINS, AND A SUBSEQUENT COOLING ZONE, SAID PROCESS BEING CHARACTERIZED BY CONSTANT HEAT TRANSFER AND TEMPERATURE REGULATION BY CONTINUOUSLY RECIRCULATING AT LEAST PART OF THE STEAM GENERATED FROM MOISTURE EXPELLED IN THE PROCESS IN COUNTERCURRENT HEAT EXCHANGING FLOW WITH SAID LIGNITE WHEREBY THE STEAM ATTAINS A TEMPERATURE OF FROM ABOUT 600*F. TO 1000*F. IN THE PROCESSING ZONE TO ASSIST IN PREHEATING THE RAW LIGNITE IN THE PREHEATING ZONE, THE STEAM IS COOLED TO ABOUT 220*F. TO 400*F. IN THE PREHEATING ZONE AND AT LEAST PART OF THIS COOLED STEAM IS RECIRCULATED TO ASSIST IN COOLING FINISHED PRODUCT IN THE COOLING ZONE AND IS THERE REHEATED TO ASSIST IN TRANSFERRING HEAT BACK TO AND REGULATING TEMPERATURE IN THE PROCESSING ZONE BEFORE BEING SUPERHEATED FOR RECIRCULATION TO THE PREHEATING ZONE.

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