US2638684A

US2638684A - Process for heat-treating combustible solids

Info

Publication number: US2638684A
Application number: US154473A
Authority: US
Inventors: Walfred W Jukkola
Original assignee: Dorr Co
Current assignee: Dorr Co
Priority date: 1950-04-07
Filing date: 1950-04-07
Publication date: 1953-05-19
Anticipated expiration: 1970-05-19

Description

May 19, 1953 Filed April 7, 1950 GOAL DRYING ZONE AIR PREHEATING ZONE F l G. l.

WET

FEED

W. W. JUKKOLA PROCESS FOR HEAT-TREATING COMBUSTIBLE SOLIDS DRY COAL RECOVERY 3 Sheets-Sheet l FINE COAL RECOVERY GOARSE GOAL RECOVERY ASH INVENTOR:

WALFRED W. JUKKOLA,

ATTORNEY y '9, 1953 w. w. JUKKOLA 2,638,684

' PROCESS FOR HEAT-TREATING COMBUSTIBLE SOLIDS Filed April 7, 1950 :5 Sheets-Sheet. s

WET

FEED

MATERI COARSE INERT RESIDUAL BED PARTICLES UNDERGO- ING TREAT- MENT F l G. 3.

INVENTOR:

BY W Mel-W ATTORNEY the mine.

Patented May 19, 1953 PROCESS FOR HEAT-TREATING COMBUSTIBLE SOLIDS WValfrcd W. J ukkola, Westport, Conn., assignor to The Dorr Company, Stamford, Conn, a corporation of Delaware Application April 7, 1950, Serial No. 154,473

8 Claims.

This invention relates to a method of drying moistened or damp combustible solid materials by drying them under conditions whereby the materials to be dried are maintained in a fluidlike state. More specifically, this invention relates to a method and apparatus for drying coal whereby the coal fines areburned under pressure to furnish the heat necessary to dry the coal.

In modern coal or ore treatment operations, it is customary to subject raw material being recovered from the veins to an upgrading operation whereby the valuable constituents of the mined material are separated from rock, slate or other gangue. These upgrading processes are usually conducted in a liquid medium wherein the heavy materials sink and the lighter constituents float, thus efiecting the separation. Coal thus recovered from such an operation is necessarily saturated with water and/or other liquids. Coal is also subjected to various types of washings to clean it, remove slag, etc. and is also upgraded by flotation processes. All of these operations leave the coal with a considerable moisture content which causes solidification of coal cars in freezing weather, and increases shipping costs, and decreases B. t. u. value per ton.

It is thus the principal object of this invention to discover a method of drying these coals or ores or other combustible solids whilethey are in the solids-fluidization state, so that the aforementioned difficulties will be overcome. It is a further object to efiect a separation of finer particles of coal or ore from the larger pieces so that these fine particles will not be present in the finished product, thus decreasing the amount of dusting present when handling the material. It is yet another object to discover a method of utilizing the latent heat values present in these fine particles of coal or ore so that the heat may be used to dry the desired material and also to eliminate the disposal problem attendant to the fine particles.

In connection with this object, it is necessary to discover a means of burning the fine coal in a pressurized vessel which must operate at aback pressure of more than two or three inches of water. i

As a broad object of the invention it might be said that it is to use coal or ore in some fashion to dry the wet material, prlncipaly because thematerial itself when used as fuel would normally be the most available source of heat directly at This invention accomplishes the above Objects,

2 and others which are disclosed in the later description, by carrying out the drying process in a reactor in which the coal or ore to be dried is maintained in a fluidized condition. If coal is maintained in this condition, gases to maintain this condition are forced upwardly through the fluidized bed and by adjusting the velocity of these fluidizing gases, the size and amount of the particles which are entrained in the gas stream can be controlled Thus a classification can be effected in the reactor. The larger particles remain behind in the bed and are subjected to drying. The dried finer particles of coal are carried along in the gas stream and removed from the main body of the coal. These finer particles, by this invention, are separated from the gas stream by any sort of conventional solidsdiminishing equipment, such as a cyclone or a settling chamber. As an alternative, an inert bed may be fluidized in the drying zone and the entire quantity'of coal to be dried swept out by the gas stream. A classification into a main body of coal and a fine fraction is then accomplished in a plurality of cyclones in series.

The fine solids are collected and the necessary amount is supplied to a heat generating station.

The heat generating station is another furnace in which there is also a bed of solids being maintained in the fluidized condition. This bed is to be an inert bed, that is, it is composed of solid material which does not react with the fiuidizing gases and which remains substantially constant, and which is composed of particles whose size is significantly large so as not to be carried out by the upward gas stream but are of such size range that it can be fluidized. These particles will generally be larger than that :of the fine particles of materialwhich were recovered from the drying furnace. The inert bed is maintained in a fluidized condition and while in this state, the fine solids previously recovered are supplied to the bed. Air or other oxygen bearing gas is used to fiuidize the bed. The fine material being supplied to the bed burns when it contacts the fluidizing .air and serves to preheat the air to a high temperature, and also to heat the inert material to a corresponding temperature. The heated inert particles serve as a large heat reservoir in which nearly perfect heat exchange is accomplished. The fine particles thus burn almost instantaneously upon reaching the bed. The inert material also serves to hold the particles for a longer residence time than theywould normally have if supplied alone into the gas stream. An important feature of carrying out the burning process as described in this invention is that this is a process which will operate under a back pressure of O to p. s. i. or higher and thus the pressurized hot gases are immediately available for supplying to any unit which requires gases in this condition. At this point, the process is partially distinguished from the usual burner in that in the usual unit, additional compression equipment is needed if a pressurized gas must be supplied.

The thus heated air then is conducted to the previously mentioned drying furnace, where it serves the threefold purpose of drying the coal which is in that furnace, classifying "the coal the-rein so that the fine particles are removed from the larger particles, and finally of fiuidizing the materials in this furnace. "Finished dry coal is recovered from this drying furnace. If no inert bed is used, the majority will be recovered from the bed directly, with some being recovered from the cyclones or other-separation system. If an inert bed is used, the major .partwill be recovered from the cyclones.

Reverting now to. the. use of the terms fluidize, fluidized-solids and fluidized bed, it is to be noted that these terms are used interchangeably in the art. to'wdesignate a type" ofrdense-suspension wherein finely-dividedzisolids particles are dispersed in'an. upwardly 'movingstream of gas. When a: gas passes upwardly: through a mass of finely-divided T solids particles three phenomena may occur; At very low spacevelocities, say of the order of less: than..0.5. feet :per second, the gas permeateslanddiifuses up through the solid mass; without: imparting any." apparent motion to the particles; The gas velocity through the solid mass is always higher :thanthe space veloc- -ity but the latter term is used in the art for convenience; it is the velocitythe gases would have if they flowed u through anunobstructed passage having a free' cross-sectional area equal to that occupied by the solid mass of particles. At

"very high superficial velocities, e. g. ofthe order of 50 feet per-second, thezgasrstream'picks up the particles'and.. entrains'them'with the gas stream, thus formingthe typical dilute gas-solid suspension or:dispersion; asptypified by dusty air At intermediate spacer velocities,- xanother phenomenonoccursri AtJSpace velocities rangin b tween about 0.5 and '5...() feet per second, the gas stream suspends the solid particles larger in size than about. 250-500; microns as a fluid bed. A fluidized bed is a'very dense suspension of solids in a gas; the solids, content. may vary from 10 pounds to over 100 pounds per cubic foot depending upon the nature of the pa-rticles and the gas velocity. The particles in a fluidized bed are in turbulent, zig-zagmotion and in appearance the fluidized bed resembles a. boiling liquid, it presents a fluid-like level and theparticles therein flow under fluistatic head.- But more importantly, from theviewpointrof conducting chemical reactions, is the high. heat capacity .and rapid heat transfer withinafiuidizedbed. These qualities result in a very high degree of temperature uniformity throughout a fluidized bed; so much so that a fluidized bed may be characterized as thermally-homogeneous.

While the use of fluidized beds has been-prominent in the petroleum refining industry, especially in catalytic cracking, they have not been adapted to many other arts and the abovedescription has been incorporated inthis specification in order to distinguisha fluidized bed-from.

Figure 1 is an idealized view of a solids-fluid- -izationreactor showing the respective positions of the three essential zones when contained in one reactor.

' Figure 2 shows a two stage embodiment reactor and all its accompanying units in detail.

-- 11*"igure 3 is an enlarged view of the drying zone showing the wet coal being supplied and the dry coal being entrained.

More specifically in Figure 2 there is shown reactor. collectively designated 2 ll, comprising top :plate 25, outer shell. 34,-andbottom conedplate 50:, and-supported by structural members 6 h The reactor in the embodiment shown inthese -draw .ings isdi-vided intotwoz-zones-namely-the heat generating zone Aand the drying zone B. The

heat generating zone A is lined'with insulating brick 39 andfirebrickd!) and M. At the lower portion of heat producing zone A is located the @apertured constriction-plate dl-which serves to support-the bed ofesolids to be treated in zone A. Thispiate has a number of apertures 53., passing completelytherethrcugh-and serving to conduct fluidizinggases through the plate andinto bed 43. Gasfor fluidizingthe respective beds is introduced through-pipe M controlled by valve 48, passes into-lwindbox 5i and thenceupwardly through the constriction plate .52. -In the bottom of windbox 5! there is vprovided a clean-out port .9 from which any line "solids. which percolate downwardly through. the plate may. beremoved.

Drying zone B in this embodiment is located directly above zone A and is. enclosed in the unitary shell .34. The bed in thiszone is supported by constriction plate l'l. containing apertures iii, The. heated gas uprising from .zone A. passes through the constriction .plate and into bed 36 of zone B where it serves to dry, classify and fluidize the particles in this bed.

In operating this .reactorthe. wet material to be dried is furnished to hopper 2| which supplies the material to screw-feed means 24 encased in shell 23 and powered by motor 22.' The wet feed is supplied first to the drying zone B where it enters at a pointsomewhat above the level 12 of bed 36. In falling downwardly to .reachbed 38 the wet feed is first contacted with hot fluidizing gases and is given an opportunity to become partially dried. The location of .the feed is not critical, but is helpful. Upon reaching bed 36 it becomes fluidized and dried by the uprising gases. 9- Bed 36- iscomposed. principally of an inert material such as ceramics, metals, metal oxides, or a coarse fraction ofthe materials being dried. This serves as the heat reservoir to which the incoming wetfeed is supplied. The

velocity of theincoming gases is so adjusted that these inert materials are fluidized while simultaneously the majority of :particles contained in the incoming coal are entrained. They are carried upwardly out of bed- 36.into freeboard area 73-, from which they pass upwardly into pipe 26 and into primary cyclone separator or settling chamber C. In this separator C, also wet slurries, so-calledfixed-beds and dilute dis- 75 .designatedzl, all but the fines areremoved from now in a finished condition. A

The fine materials are carried along through the gas stream and these so removed particles pass downwardly through pipe 32 controlled by valve 14 into storage hopper 65 where they are pipe 23 into secondary cyclone separator D, also designated 29. In this separator the fines are collected and the clean gas passes upwardly throughpipe 30 controlled by valve 3| to exhaust. The fines collected in separator 29 pass downwardly through pipe 33 through fitting 6| to hopper 63. If more fines are collected than is desired, then valve 62 may be closed down, thus diverting the fines and any ash which they might "contain through pipe 60 controlled by valve 59 into storage hopper 58. Thus dry finished fines will be recovered in storage hopper 58 and dry finished coarse materials will be recovered in storage hopper 65. a

The fines which are collected in hopper 63 are used to furnish the necessary heat for conducting the drying operation. These fines are supplied to star feeder'54 empowered by motor 56. This star feeder supplies the fines to the gas stream passing through conduit 8|, controlled by valve 80.

They are picked up by this gas stream and delivered to bed 43 located in heat generating zone A. By the use of this standpipe feed with compressed air conveying system the danger of flash back is eliminated. This is accomplished by maintaining the carrier air velocity in the coal feed nozzles at rates considerably above the fiame propagation rate. The bed in zone A is comprised principally of some fluidized inert material of grain-size considerably coarser than the incoming fines being supplied by star-feeder 54. The fines are entrapped in this fluidized inert bed for a few moments and they are caused a to burn therein by supplying oxygen through constriction plate 52. Upon burning therein they serve to heat up this incoming gas which then passes upwardly into drying zone B. In order to control the temperature of the now heated uprising gases, an air bleed-in pipe controlled by valve I6 is supplied. 'To this pipe there may be furnished cool gases which will blend with the hot uprising gases to give a controlled temperature to the gas whichis being used for drying material in zone B. It is also possible to control the heated gas temperature and bed temperature to prevent fusion by supplying excess air to the inert combustion bed. Any ash which might build up in zone A and which is not entrained by the gas stream may be removed from zone A by spill pipe 42 and the rate of take-off controlled by valve 11. This ash will go into storage hopper '45 from whence it is either discarded or subjected to further use beyond the scope invention. r

Any particles in zone B which are too large to be fluidized by the uprisinggas may be inof this termittently 0r continuously withdrawn, as conditions require, through spill pipe 3'! controlled by valve 38. These particles represent dry finished product and are hence blended with the coarse dried product in storage hopper 65 EXAMPLE 1 This unit has been found useful in drying Pennsylvania anthracite coal containing to moisture. Since completely dry coal dusts severely, drying operations are usually controlled to leave a residual moisture content of 3% to 5%. It has been foundhowever thatit is possi ble to reduce the moisture content to j 0%, if

I desired.

The screen analysis of the were used is as follows. l I C'oal used as inert bed'inj drying compartment constituents} which Percent Qum.+

I Mesh Mesh Sand used as the inert bed in thecocl burner Percent Mesh Cum.+

Fine coal burned in combustion chamber SAMPLE A Percent Mesh SAMPLE B Mesh gg i The reactor was operated at a feed rate of 50 tons of the above wet feed per hour and at this rate, under the conditions which I shall describe,

36 tons of fines were required to be supplied per day to the heat generating zone A in order to accomplish the necessary drying. The reactor was operated with the heat generating zone at a temperature of 1850 F. and the drying zone was operated at a temperature of 200 F. The stack gas coming from the drying zone was 200 F.

Another advantage of this inert bed coal burner is the high burning efficiency. The'use of the inert bed system eliminates the loss of fuel due to sifting through the conventional grates. My tests have indicated thatpractically no unburnt coal is carried out by the stack gases leaving the zone A. Tests indicated combustion hot gases from the pressurized burnings-operaration. may also. be. used-:for: low ,temperaturecalcination operations suchas;. -decomp.osition of .trona dehydration. of hydroxides, .etc,, in .addition to dryingwet materials.

EXAMPLE. 2

V A further use of this invention is the drying of 5 therein in afluidized condition,.sup,plying mate- .-.rial. to be treated toithe, heat treating. zone-sup- ..plying fiuidizing .gas to. the zone at a velocity .suificient to. .fluidize the..inert material. inthe bed 8 -I-:claim: 1:...Th6. continuousprocess of heattreating finely divided solidcomhustible materials whichcomprises establishing andmaintaining a-heat; treating-zone co-ntainingan inert material as abed any combustible materials from which it is possi able to s.eparate...the.fines.andburn them inde- .'pendent of thedrying of the main'body of ma- ;..terial.

. An example of-such-material, in'addition to z-Icoal, would be. pyriticmaterials which have. come,v from a. flotation cel1.- -Thesematerialsare not loand .entrain .thematerials being treated but insuflicient to entrain the inertmaterial', separating .entrainedwoarse. materials .andentrained. fine ..materials.. from-the entraining gas-at a point functionally remote from theheat treatingzone l5.- and.:functionallyremote from each other; establishingandmaintaining a heat generatingzone 'only wet but contain flotation agents as a contaminant: If it is-desired to dry'the-pyritic concentrates before shipping them to a smelter or forother treatment, or if it is desired to partially.

heat the pyritic concentrates in order to decompose the flotation agents, then the process of this .invention is'useful: The-pyriticfinesare separated, passed into. the combustion bed and burned; The heated gases then pass into the.

; drying bed. to dry the mainbody ofwthe ore.

In employing the apparatus ofthis invention, the source of the fine materials is notnecessarily to be confined to those which are recovered from the drying zone but may also include any fine materials which are combustible, so as to supply heat for drying. In commercial applications, this Will frequently include stockpiled fine material. It is also within the ambit of this invention ...that itbecarriedout in a..plurality ofreactors.

which need not be located one above the other or contained-Within the .sameshell. .Thus .any

. operations above outlined which arecurrently using oil etc. as a source of heat, can be adapted to this process.byinstallinga.separate heat generating unit to consume the fine combustible materials and supply hot gas to the drying unit, or for other operations.

I In the description of the-drawings, a unit was shown inivhich thedrying is performed using an inert bed and collecting the dried material from containing .an.winert..material as a bed -therein in a ,fluidizedcondition,--supplying-an Oxy en bearing gas to said zone at fiaidizing velocities, supplying saidseparated entrained, fine. particles to ..theiinert.fluidized bed in said-zone; burning the fine particles. toyield a heatedgaasupplying theheatedgas to-the-heat treati-ng zoneand discharging treated. combustible material from-the .heat treating zone.

. 27 The. process. according to claim. 1, wherein the temperature ofthe drying :zone is maintained in the range between 165 .and250 F. 3.,The. process according to .claim 1, wherein the temperature. of the heat-generating'zone is maintained: in. the range betweene1500 and 2100 F.

. 4. .The process according to.- claim 1,:Wherein the inert. material of the bed in theheat treating zone isone of the group comprising coalpceramic balls, gravel andsand.

. 5.;The.-processaccording toclaim l,-wherein the. inert material of the-bed in the heat: generating zone is one ot-the group comprising sand, rcemmlcwballs, rave pulverized binders and metal oxides.

6. .The continuous 1 process for drying --finelydivided combustible solids which comprises the steps. I of establishingand maintaining a bed of such. solids in a dryingzone lay-feeding such solids thereto. and removing. them. therefrom, mainthe cyclones, or. other separatingmeans. This operation can be carried out-equally aswell, a1- though not as economically, by fluidizing the mataining solids. of the bedxa-s a. turbulentlymobihzed .iluidized .bed bypassing therethrough, at a velocity-suificient to both entrainfine-solids-and terial to be dried directly in bed 36,.and using a gas velocity low enough. so that the solid entrainment is kept to a minimum. This will be determined by the material being treated or dried and by the screen analysis of this material.

- It is far more'advant-ageous' to use =an-inert bed because the coarseinert material serves. to

I culties was about 17%. However, with the use of an inert bed and increasing the space rate to 12 ft./sec'.', it is possible to treat coal with 35% moisture. 'The inert bedalso permits an increase in the unit capacity of the unit several fold. One unit whichwas tested was increased in capacity from -..2.5 tons/sq. it./24 hrs. using no inert bed to a capacity of 25.0.tons/sq. Lit/24 hours, when using aninert'bed. 1

As an example, in the dry-* zoneabed of finely-divided non-combustible solids, maintaining such solids as a turbulently mobllized .fluidized bedby passing therethrough an uprismgwstreameof free oxygen bearing gas at of the drying-zone, to. maintain drying-tempera fiuidize, larger... solids -but. insufficient to. entrain such larger jso1ids,..an uprising stream of fluid- .izing gas initially. having asensible heatcontent sufiicient to maintain solids. of the bed. at solids drying temperatures, dryingsolids in the bed, discharging gases. containing entrained fine. solids from the zone'whereby dried fine solidsv areseparated from larger solids and removed from the zone, and discharging dried larger solids from the -zone;- characterized in that the drying zone is maintained at solids drying temperatures by establishing and maintaining in a combustion fluidizing-velocities, supplying :dried fine combustible solids to; therifiuidized bed to pass therethrough, combusting such fine solids during their pas-sage throughthefluidized bed to produce a sensible-heat carrying gas, and supplying such heated carrying gas. uprisingly through the solids tures therein as-well as to serve as a quantity of the fluidizinggas supplied to the drying zone.

7. -The process according to claim 6 wherein the combustible materials as initiallysupplied to the bed of the drying zone comprise wet coal.

8. The process according to claim 6 wherein the combustible materials as initially supplied to the bed of the drying zone comprise wet pyritic materials.

WALFRED W. JUKKOLA.

References Cited in the file of this patent UNITED STATES PATENTS Number Date 10 Number Name Date 2,465,464 Meyer Mar. 29, 1949 2,528,098 White Oct. 31, 1950 2,534,728 Nelson et a1. Dec. 19, 1950 2,567,959 Mun'day Sept. 18, 1951 2,573,906 Huff Nov. 6, 1951 FOREIGN PATENTS Number Country Date 286,404 Great Britain Mar. 8, 1928 OTHER REFERENCES FIuid-Solid Air Sizer and Dryer, by C. S. Wall and W. J. Ash; Ind. and Eng. Chemistry, vol. 41, N0. 6, June 1949, pages 1247 to 1249.