US 1954352 A
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April 10, 1934- F. L. DoRNBRooK Er AL 1,954,351?
APPARATUS FOR TREATING PULVERIZED FUEL SUCH AS COAL AND THE LIKE Filed MaIQh 14, 1932 2 Sheets-Sheet 1 will 65 b j INV NT i -mef ATTORNEY.
April l0, 1934.
F. DoRNBRvooK Er AL '1,954,352
APPARATUS FOR TREATING PULVERIZED FEL'ASUCH AS COAL AND THE LIKE Filed March 14, 1932-. 2 Sheets-Sheet 2 nun"n"n"nUnn"nl'nlnlnununl]I ununununununununununununu ATTORNEY lnununununlnunununl'nun ,n
lILlLlUlIlllIU Patented Apr. 10, 1934 UNITED STATES PATENT OFFICE APPARATUS FOR TREATIN G PULVERIZED FUEL SUCH AS COAL AND THE LIKE Application March 14, 1932, Serial No. 598,796
This invention relates to a new and improved apparatus for treating hydrocarbon-containing fuels such as coal and the like in powdered form, including the step of Ylow temperature carbonization for the recovery of coal gas and other products resulting from carbonization and a powdered coke fuel which may if desired, be fed directly to a combustion chamber without appreciable loss of heat from the carbonizing process and burned with highly satisfactory results.
The characterizing feature of this invention deals with an apparatus for the treatment of powdered hydrocarbon-containing fuel such as coal, including an oxidation pre-treatment and llsubsequent low temperature carbonization and in addition, special provision in mechanically handling and controlling the movement and condition of the fuel while being subjected to the pre-oxidation and carbonizing treatments resulte ingin the production of powdered coke and the elimination of all tendency to ycake or to in any wise clog the apparatus, and, for the first time, in a successful performance of this type of treatment.
Prior to this invention, low temperature distillation was known to be desirable Vin many in-` stances, and numerous attempts have been made to successfully perform low temperature carbonizat'ion with both lump and powdered fuels. With the increasing use of powdered coal, it was considered highly-desirable to endeavor to perfect an apparatus for carbonizing powdered coal for the recovery of coal gas and other products of distillation and with the aim of acquiring a powdered coke suitable for use in ordinary pulverized fuel furnaces.
The difficulty, however, which prior to the present invention, has blocked the successful developrnent of such apparatus flows from the 40 tendency of coal to cake under the temperature and treatment necessary for a satisfactory carbonizing action which has not only destroyed the possibility of recovering a powdered coke for fuel purposes but has without exception, caked in and clogged the apparatus to an extent which rendered the undertaking entirely impractical and unsuccessful from the commercial standpoint. It is this caking quality of coal thathas also prevented the development of a successful continuous treatment process of the type provided by this invention.
Among the methods and apparatus heretofore devised in an effort to successfully carbonize powdered coal with a resulting powdered coke is included a pre-heating and/or oxidizing treat- (Cl. 21R- 108) ment and whereas oxidation is known to free coal of its caking properties, the utilization of this known discovery has thus far baflled those skilled in the art insofar as the development of a commercial workable apparatus is concerned, which `would not suffer the defeat of ultimate caking failure.
Accordingly, it is an object of this invention to provide a new and improved apparatus which nection, means may be provided by which the,
powdered coke is fed While still hot from the carbonizing retort into a suitable combustion chamber such as a furnace used for any desired purpose such as for instance, in heating steam boilers of a generating plant or any other practical purpose.
It is a further object of this invention to provide an apparatus which is highly satisfactory in performance and is well adapted to economical and successful commercial installation whereby when used in conjunction'with a furnace, high efciency is realized both in the combustion of the powdered coke and generation of heat and in the recovery of the desirable products of low temperature carbonization.
It is a furtherobject of this invention to provide an apparatus for accomplishing the above stated objects which will utilize for treatment, powdered fuel such as coal or the like, and produce in addition to the by-prod'ucts of the carbonizing treatment, a coke fuel in powdered form which may be cooled and stored or lburned directly in its heated condition, but which in either case, may be satisfactorily handled and burned in combustion chambers now used for the combustion of powdered. coal.
As an advantage of this invention, the powdered coal and the resulting powdered coke may be conveyed and transferred in enclosed pipes either by gravity or by currents or drafts of air or gases, so that the entire apparatus may comprise an enclosed system which makes for a high degree of cleanliness and increased efcienoy and the avoidance of wastes and other losses incident to apparatus differing in this respect.
It is a further object of this invention to provide an apparatus in which pulverized, oxidized fuel is carbonized in a closed retort by heating with gases which are isolated from the space of the retort whereby the gas recovered is not diluted or rendered of inferior grade or lowered B. t. u. value, as would be occasioned when heating gases are brought into direct association with the fuel in the carbonizing retort.
It is a further object of this invention when used in combination with a combustion space of a furnace to remove the hydrogen from the fuel during the carbonizingy treatment, and to thereby enhance the heat value of the resulting powdered coke by preventing the formation of water vapor during combustion and eliminating the consequent loss of heat of vaporization to the stack. In the accomplishment of this object, most of the hydrogen passes off with the, gas in the carbonizing retort and does not enter the furnace, and it is found in practice, that the heat saved in this Way more than compensates for the losses due to radiation.
When the oxidizing and carbonizing apparatus is used in combination with the combustion space of a furnace, it is a further object to provide means for utilizing gaseous products of combustion taken from the combustion space as a source of heat for conducting both the oxidizing and carbonizing treatments and in the oxidizing treatment means for utilizing such gaseous products of combustion as a conveying gaseous medium for the powdered fuel. In this latter case the gas is ultimately returned to the combustion space for vthe conservation of all fine fuel particles which may remain suspended therein after the oxidized fuel has been separated therefrom. In the accomplishment of this object the gaseous products of combustion during oxidation serve as an inert atmosphere containing only the desired and regulated quantity of oxygen for proper oxidation of the entire mass with assurance against combustion during the oxidizing treatment.
One aspect of this invention is based upon a more thorough knowledge of the effect of preoxidation in the subsequent handling and condition of the coal in passing from the oxidizing apparatus to the carbonizer and through the carbonizer until the carbonizing rtreatment is completed.
The applicants have discovered that the results of an oxidizing treatment upon a given mass of ordinary pulverized coal, for a like period of exposure under the same conditions, differ widely insofar as the caking properties are concerned, for particles of different size, that is to say; between the particles of maximum size as compared with those of minimum size. A corresponding difference exists throughout the intermediate range of particle sizes. The period of oxidation treatment, temperature and other factors must be substantially the same for the entire mass as it is entirely impractical from the commercial standpoint to subdivide the mass of powdered coal into numerous batches, each of a particular size, and subject each bath to a different oxidizing treatment, each designed and selected especially for properly oxidizing the part there is no tendency to cake and the mass remains ticles of the particular batch.
It is furthermore impractical from the commercial standpoint to subject the entire batch to a prolonged oxidizing treatment of length sucient to thoroughly oxidize the largest particles, on account of the fact that such treatment will involve a temperature, quantity of oxygen, and treatment period productive of combustion of the more minute particles. Inasmuch as an oxidation pre-treatment which would cause combustion of the more minute particles cannot be employed, the outstanding difficulty of processes heretofore devised and tried, insofar as the applicants ,can determine, has been that of inadequate oxidation of the larger particles, resulting in the subsequent caking of the larger particles to an extent that the apparatus employed was clogged with caked coke and the process rendered inoperative.
Accordingly, the present invention contemplates an apparatus for accomplishing an oxidizing treatment which is substantially the same for the entire ymass so that it may be supposed if the applicants theory is correct, that some of the larger particles will be inadequately oxidized but with a handling of the entire mass in the oxidizer and in the carbonizer in such a manner that the smaller oxidized particles serve as a coating for the larger, less oxidized particles whereby to irnpart to the entire mass, the quality of behaving as a completely oxidized mass with the avoidanceA and entire elimination of caking tendencies during the carbonizing treatment.
As one aspect of the teaching of this invention, care should be taken to assure, during the grinding operation, the production of a sufciently large proportion of smaller particle sizes so that under the subsequent handling during oxidizing and carbonizing a treatment which may be referred to as an average oxidizing treatment may be practised with the result that the entire mass behaves as a completely oxidized mass. Of course, it is understood that in no case should the oxidizing treatment be increased either by the amount of heat, quantity of oxygen or treatment period, to the point where combustion will occur in the oxidizer.
According to the present invention the oxidizing treatment may be conducted in any suitable manner such as hereinafter outlined, provided however, that the apparatus is constructed so that after separation of the fuel particlesand the gaseous medium, the fuel particles are allowed to collect or settle as a static mass whereby the particles are at rest and free from disturbing or yagitating forces or movements' such as would operate to disassociate and separate the fine oxidized particles from their coating association on larger particles.
Furthermore, the same conditions must prevail in the carbonizing retort an/d accordingly, this apparatus involves means for carbonizing the fuel While existing in the condition of a relatively static mass free from disturbing forces or movements which would cause exposure to each other and to inner surfaces of the carbonizer of the larger partly oxidized particles. The movement of the mass through the carbonizer, essential for a continuous operation is accomplished by permitting the mass to slowly settle under the'action of gravity while the volatile material is being driven olf. Such movement does not disturb the condition of the mass insofar as the ne coating on larger particles is concerned and consequently entirely in powdered form whereby it is free to continue its slowly settling movement until it reaches the bottom where the fuel exists as coke dust. The coke dust can then be removed by any suitable method such as by means of a. conveyor and -as here disclosed, fed directly into the combustion space of a furnace. j
In addition -to the above stated objects, this apparatus contemplates various means which make for economy and commercial success and these will more readily appear to one skilled in the art. VAs the following description proceeds, however, additional vadvantages may be mentioned as follows:
'I'his apparatus is well adapted for use in electric generating plants having steam boilers equipped for the handling and burning of pulverized fuel and in such cases the apparatus may be conveniently placed between the pulverized fuel bunker or similar source of supply and the inlet to the combustion space without disturbing' -or seriously affecting the steam boiler installation.
Accordingly, the valuable volatile constituents of the lcoal which would otherwise be burned in the steam generating boiler for their heat value, may by such an installation, be separated from the residual coke dust and recovered as coal gas, coal tar and oils. The advantage of so doing will be appreciated when it is known that the com-- mercial value of coal gas, coal tar and the other by-products of coal distillation, is considerably in excess of their heating value. The present invention therefore is productive of the profit from the volatile products and at the same time provides an adequate and satisfactory fuel in the form of coke dust for combustion in the furnace, the result of which is an increased profit on the entire operation.
A further advantage is realized from the fact that the recovered by-products are produced by low temperature distillation treatment which is productive of a gas of great value and high in heating units. This is due to the fact that a low distillation temperature does not expose the coal gas tothermal decomposition or cracking such as might impair its` heating value per unit volume. The quality of the gas is also assured by the avoidance of heating gases in direct contact with the fuel in the carbonizer whereby only undiluted and clean products of distillation are recovered. Other details of the process such as the filtering of the gas through the pulverized coal, the maintenance of the carbonizing temperature, and so forth, assist in the production of clean gas and tar products withoutY danger of re-condensation in the carbonizing retort.
Likewise; tar of high value is produced by the low distillation temperature because high destructive temperatures in the carbonizing retort are thus avoided. The products of distillation when removed from the retort, may be recovered by condensation of the tars and oils and the collection of the gas in suitable containers, all in accordance with known and satisfactory practice.
In the drawings, one form of apparatus is shown which has proven to be highly successful in practice and well adapted for large scale commercial installation. i
` I'n order that the invention may be more readily understood, it will be described in connection with the disclosed apparatus but it should be understood that the invention is not to be limited by this illustrative description as the scope thereof will be pointed out in the appended claims.
The apparatus of this invention may be considered .in connection with applicants co-pending application Serial No. 525,940, filed March 28,
filed March 14th,'1932, which is a continuation in part thereof, and some of the novel features relating to the treatment of hydrocarbon-containing fuels shown and described but not specifically claimed herein form the subject matter of the before-mentioned co-pending applications.
In the drawings- Figure l is a side view, partly in vertical section of a form of apparatus by which the present invention may be practised and as installed in connection with a boiler-furnace;
Figure 2 is an enlarged elevational view in cross-section of the oxidizer and separator shown in Figure 1; and
Figure 3 is an enlarged plan view in cross-section taken on the line 3-3 of Figure 1 and looking in the direction of the arrows.
Construction of oxz'dz'eer and separator By referring to the drawings it will be noted that the apparatus of this invention comprises a combined oxidizer and separator, that is the pulverized coal is oxidized, separated and collected in a cylindrical container 60, which is constructed in the general form of the usual cyclone separator. lindrical chamber 61 which serves as a vestibule space for the tangential introduction of pulverized coal to be treated. The coal is fed into the oxidizer through pipe 62 which is connected with any suitable source of coal, such as an overhead bunker, hopper or if desired, directly with the pulverizing mill. The container is preferably cy,- lindrical in shape at its top portion and is provided with inclined conical walls 63 at the bottom portion terminating in an outlet pipe 64. The outlet pipe leads directly to the carbonizer 65 as shown in the drawings.
When this apparatus is used in connection with a furnace it is preferable to use flue gases for the oxygen and heat carrying medium which is fed into the oxidizer tangentially as at 66 through a pipe 67. By virtue of the tangential inlet, the gases are given a whirling motion into which the coal falls from the vestibule chamber 61. the action of the centrifugal force the coal is thrown outwardly against the walls of the oxidizer but only after the particles thereof have been exposed to the heating and oxidizing effect of the fiue gas. By virtue of this action, a. separation also occurs as the gases free from coal particles are caused to occupy the central portion of the container where they are withdrawn through an outlet pipe 68 which extends upwardly and thence downwardly and connects with a suction fan 69.
In order that more accurate control over the oxidizing action may be realized, provision is made for re-circulating a desired amount ofthe treated coal after it has fallen and collected at the bottom of the container for instance, as shown at '70. This means comprises an outlet pipe 71 which connects with an intake of a fan '72 which fan discharges upwardly through pipe '73 tanl Centrally of the top is a smaller cy- Due to comprises two star wheels 74 and 75. Each wheel is designed to occupy substantially the entire interior space of the outlet pipe whereby coal is fed to the carbonizing retort solely by rotation of the star wheels. To avoid leakage of coal gas past the seal a flue gas pressure equal to or slightly above the pressure in the carbonizer is maintained between the star wheels. This is accomplished by connecting a pipe '74', between the discharge of fan 69 and the space between the star wheels. Other means are possible for providing a gas-tight seal and for conveying coal from the oxidizer to the carbonizer.
As shown in Figure 1, iiue gas pipe 67 is in communication with flue gas pipes 7G and 77 respectively, both of which communicate with the flue gas passages from the furnace for withdrawing ue gases therefrom.
It will be noted, however, that these pipes take gases from different portions of the iiue gas passages and consequently at different temperatures whereby a mixture of gas may be fed to the oxidizer by adjustment of valves '78 and '79 respectively, to supply any desired temperature. Valve 78 is located in the pipe 80, which extends from the pipe 77 to pipe 67 at a point above Valve 81. Pipe '77 also conveys flue gas to pipe 82 which is here shown as a downward extension of pipe 67 and which ultimately leads to the heating elements in the carbonizer. Valve 81 serves to separate the upwardly flowing gas from the downwardly flowing gas but by means of these valves and valve 83 located in pipe 77, any desired temperature may be provided either in the oxidizer or the carbonizer. In addition, pipe 67 is provided with an air bleed pipe 84 prov`ded with a valve 85 by which additional air may be introduced into the flue gas.
Operation of oxidizer and separator In operating this apparatus, the coal is introduced centrally of the container as above described and the flue gas enters tangentially to establish a whirl'ng motion. The coal falls downwardly from the vestibule space 61 and upon entering the oxidizer, the coal and flue gas are set in rotary motion which thoroughly exposes the coal to the oxygen and the flue gas and causes oxidation to that degree which effectively destroys the caking property of the mass of coal. During the oxidizng treatment, the coal is continuously separated from the gas by centrifugal force as it is thrown outwardly toward the walls of the oxidizer and the gas travels inwardly and is discharged through the outlet pipe 68. The coal then moves by the action of gravity down the inclined walls at the lower portion of the container and collects at the bottom thereof for subsequent discharge into the retort. If greater oxidation is necessary, the re-circulating fan 72 is put in operation whereupon any desired portion of the coal may be again introduced at the top, and exposed a second time to the oxidizing treatment. This recirculation enables a wide variance n the degree of oxidation as it is possible if found necessary, to adjust the capacity of the re-circulating fan relative to the flow of coal through the oxidizer in a manner to assure re-circulation two, three or more times.
The re-circulation means has another important function which is that of withdrawing gas from the bottom of the separator which has been foundto improve the efficiency of the separator.
Furthermore, re-eirculation maintains more coal in the oxidizer and thus provides an increased mass of coal furnishing heat which enables the reduction in the amount of heating flue gas required for thorough oxidation. Reduction in the flue gas flow increases the time that coal is in the oxidizer and further improves theoxdizing action. This principle of separating the coal from the ue gases and re-circulating it is invaluable in reducing the size of the oxidizer and incidentally minimizing surface heat losses. To increase the time element by increasing the size of the oxidizer only works a hardship if re-erculation is not practised.
The time that the coal is in the oxidizer is proportional to the time that gas is also present, other conditions being equal. Therefore, re-circulating all the coal once, is equivalent to doubling the oxidizer size.
Published tests show that the rate of oxidation does not slacken until after the caking properties are destroyed. Therefore, having some coal particles pass to the carbonizer with only one passage through the oxidizer, is not undesirable since other particles may re-circulate more times than the average and are oxidized proportionally.
This apparatus likewise provides a collected mass. At the end of the oxidizing action the minute particles will associate themselves with or affix themselves to the larger particles and form a coating thereon and when collected as a mass at substantial rest in the base of the oxidizer and subsequently fed to the carbonizer, the desired conditions are maintained for assuring non-cakingf properties in the mass as a whole.
Oxidz'zing treatment For successful destruction of the caking properties of coal, the following conditions must be satisfied:
l. Temperature must be adequate.
2. Time of treatment must be sufficient.
3. Enough oxygen must be present.
4. Oxygen must be brought into thorough contact with coal.
Temperature accelerates the process markedly. It is common knowledge that weathering of coal reduces its caking properties. Yet years of exposure are necessary to totally destroy caking of most coking coals. Most chemical reactions are doubled with each 10 C. increase of temperature, which explains why the oxidizer described herein can be eiTective.
Temperature of the coal and flue gas mixture is normally regulated slightly under the initial distillation temperature of the coal beingprocessed. For most coals this temperature ranges between 500 and '700 F. The oxidation temperature employed may slightly exceed the initial distillation temperature to deliberately sacrice by-preduct yield in favor of more thorough oxidation. Since tests have shown that oxidation decreases tar yield and increases gas yield, thorough oxidation at high temperatures may be desirable in some cases.
With this invention the oxidizer serves as a preheaterfor the carbonizer, efficiently heating the coal nearly to its distillation temperature and i thus greatly reducing the size of the heating plates and the consumption of heat in the carbonizer. With the form of apparatus described, any desirable temperature can be obtained in the' oxidizer by proper proportioning of the hot and cooler flue gases. Some heating occurs due to the oxidizing in the oxidizer which serves to reduce the amount of hot gases utilized. The active swirling of coal and gases serves to maintain uniform temperature in the oxidizer.
wardly to the feeder.
'Iime of contact between coal and oxygen as obtained in the cyclone type of oxidizer is suificient for good results because the coal particles move relatively slowly to the walls and ,down- The coal is thus concentrated, and the weight of coal in the oxidizer at any one time will be considerably in'excess of the weight of flue vgas and coal without such concentration. This principle of concentration greatly'reduces the requisite size of the oxidizer. This concentration of coal in the oxidizer is especially true of the finer particles, for they travel most slowly to the walls. As above pointed out, experience has shown that the coarser particles of pulverized fuels cannotbe oxidized with practical success, and that only the nes need be oxidized to prevent caking of the mixture. A type of oxidizer that would retain the coarse particles at the expense of hurrying the fines would thus be less eicient. Non-caking fine particles surrounding or coating caking coarse particles prevent caking of the whole and allow maintenance of the mass in pulverized form throughout the carbonization process.
Preferential oxidation of simply the fines af fords attainment of relatively high tar yields if desired. It is known that oxidation of coal decreases tar yield, though with an increase of gas yield. In situations Where tar is more. valuable than gas, this special feature of obtaining noncaking properties by oxidizing only a fraction of the total coal, is valuable.
In some coals experiments show that, though oxidation increases gas yield at the expense of tar yield, the sum of heat units'in the two products is reduced'. When tar and gas are equally valuable, it is thus desirable to effect destruction of caking properties with minimum oxidation. Such is possible by the process of this invention.
By selecting larger oxldizers of the cyclone type or by increasing the recirculation of the coal,
the time which coarse and fine particles remain in the oxidizing atmosphere can be in, creased to obtain higher gas yield from carbonization. Increase of the ilue gas inlet pipe size reduces the whirling velocity of the gas in the oxidizer and decreases the centrifugal force causing slower coal movement to the walls and thus increases the time of oxidation.
vThis invention contemplates the use of all types of oxidizers and `separators so long as they satisfy the above statedrequirements. It is important that no flat surfaces on which coal may collect be present and that the separators have reasonably high efficiency so that too large a percentage of coal will not by-pass the carbonizer. Though the heat value of coal by-passing the carbonizer may be conserved in the furnace, high separation efiiciency is highly desirable because the most thoroughly oxidized and most non-caking particles, the fines, are otherwise lost to the furnace.
' Proper ,oxidation temperatures will depend upon the caking properties ofthe coal employed, the degree of oxidation desired to increase gas yield, and the amount of preheating wanted. The usual range is between 500 degrees Fahr. and 700 degrees Fahr., ,thoughunusual conditions may dictate more or less. To reduce the oxidizer and carbonizer investment cost, the use oftemperatures above which a small amount of volatile matter is lost, may be found desirable. The oxidizer is a much more efficient heater than is the carbonizer, and the volatile lost is gener` ally largely water. No heat units are lost since the discharge is burned in the furnace.
Oxidation with flue gas, as shown in this particular disclosure, is desirable because its -high efficiency of oxidation permits completion of the '380 reactions with a less percentage of necessary Oxy/- gen than in usual air. Fluegases prevent the possibility of explosion, since the rate of spontaneous heating in low oxygen mixtures is too slow to cause sudden increase of pressure. Hot flue gases are most readily obtained in a power plant, and in fact if their heat can be absorbed in other equipment than existing boilers, economizers, or air heaters, overall plant efciency is increased. yThis invention contemplates re-circulation of flue gases which in itself is not ordinarily more efficient, but since it reduces boiler duty, it does improve heat absorption efficiency.
In cases where iiue gases and transport air do not contain sumcient oxygen, room air or pr'eheated air can be added as above described. If only heated air ror other gases are needed, the plan does not preclude their use. This type of oxidizer may be used with various 'types of carbonizers or one or more oxidizers may be used for each carbonizer, depending upon length of' the carbonizei bin and resulting levelling of coal above the carbonizer plates. Small separators are more efficient mechanically than large ones, though the latter cost less, occupy less space and suffer smaller heat losses.
Construction of carbonizer The carbonizer comprises a container 65, which is of substantially the same shape as that of the apparatus previously described in applicants copending applications, including heating elements of substantially the same character but having specic means and mechanism for withdrawing the volatile gases distilled from the coal. 'Ihe upper portion of the container is provided with vertical walls whereas the base thereof, has tapering walls 101 as shown in Figure 1.
At a point intermediate the height of the container are a plurality of heating elements 102, spaced apart to provide slot-like spaces therebetween which heating elements are hollow and in general constructed as those previously described in said co-pending applications. They are preferably provided with internal partitions 103, providing a circuitous path for the heating gases passing therethrough. The heating gases enter each element through an inlet pipe 104 which are supplied with hot flue gas from header 105 received from pipe 82, taken from the combustion 130 space of the furnace as previously described.
At the opposite side of the elements at the top, the gases are withdrawn through pipes 106 which lead to header 107 which is in communication with pipe 68, leading to the suction fan 69.
In addition, the heating elements may be provided with fins 120, which project laterally from the fiat plate surfaces thereof and which are preferably formed of thin strips of steel'extending vertically and at right angles to the faces of 140 the heating plates. f
These Aplates may be placed at frequent intervals in order to assist the heat transfer efficiency.
In practice it is found that the heat transfer on the flue gas side of the plates is considerably 145 more favorable than that on the coal side and thus extension of the surface on the coal side is desirable.
The fins constitute a means of reducing the distance through which heat must be conveyed 15( ico through the coal. When constructed in the manner described, they project into the coal and contact coaly on both sides and by being made thin, they serve as little or no obstacle to the downward passage of the coal. The use of such fins improves the efficiency of the heater as it is found that the heat conduction through the ns is approximately 250`t'imes greater than heat conduction through pulverized coal.
It will be noted that the suction fan 69 also withdraws gases from the oxidizer and carbonizer through pipe 68 and that thereafter all of the gas is discharged through pipe 108 to a header 109, and thence through pipe 110 into the coke dust feed pipe 111. Coke dust feed pipe 111 discharges directly into the combustion space of the furnace whereby the coke dust is burned as the heating fuel. The coke dust is extracted from the bottom of the carbonizing container by any suitable means such as a screw conveyor 112, actuated by a pulley 113 rotated from any suitable source of power (not shown). A super-heater may be provided comprising one or more hot gas pipes 114, extending through the upper portion of the space in the carbonizer and connecting with the flue gas pipe 82 at their inlet ends and nue gas pipe 68 at their discharge ends, whereby hot flue gases maybe circulated therethrough at a temperature substantially equal to that of gas entering the heating elements of the carbonizer. This may or may not be employed as described, but its function is that of preventing the condensation of tar and oils on the carbonizer walls above the coal space and incidentally exerting a cracking action if desired, on the volatile gases of distillation for purposes hereinafter described.
The gases of distillation are taken off of the top through an outlet pipe 115 leading to a header 116 which conveys the gases to suitable cleaning and condensing apparatus (not shown) after which the separated constituents may be recovyered and stored in the usual way.
It is important to observe that the carbonizing container is adapted tobe kept constantly filled with oxidized, pulverized coal up to a point substantially as represented by the line 117 in Figure 1. As previously mentioned in connection with the collecting operation, the pulverized coal is thus allowed to accumulate as a mass which is free from disturbing forces or movements, such as would tend to separate the fine and large particles. To characterize this condition of the coal it is herein referred to as a static mass although it is to be understood that the mass is free to settle downwardly by the action of gravity as it undergoes carbonization.
The plate-like heating elements are disposed vertically so as to permit the free passage of the coal downwardly at av rate determined by the rate of extraction from the bottom by the screw conveyor 112. An important aspect of the apparatus is that the oxidized, pulverized coal exists in the form of a mass when exposed to the carbonizing treatment whereby all tendency to cake is eliminated.
By virtue of this treatment, the coal, after oxidation in the above described manner, behaves as a completely oxidized mass and thereby continues to exist in pulverized form and passes downwardly through the carbonizer Without caking and is finally extracted in the form of a coke dust.
Under thev influence of the screw conveyor, the coke dust is readily fed through the fuel inlet 111, into the combustion space of the furnace, It
is not essential to this invention to introduce the returned iiue gases into the inlet pipe 111, as is here shown, but this is found to assist in the conveying action and to cause the fuel to bev supplied in the combustion space in a desirable manner.
By returning the flue gases all of the fuel particles which may have remained suspended in the gases withdrawn from the oxidizer, are returned to the furnace with the consequent saving of their heat value.
Operation of carbonizer In operation, when the coal treating apparatus of this invention is used in connection with a furnace, the carbonizing container may be an existing structure or it may be built especially for the process. It is possible to use containers interchangeably for untreated and oxidized coal because by proper manipulation the change from one kind of coal to the other may be made without interfering with the boiler operation.
If the carbonizer is operating and coke is being fed to the furnace the change to the feeding of untreated coal may be made by gradually cooling the carbonizer plates, and then shutting off the supply of oxidized coal to the container and turning on the supply of untreated coal. If the carbonizer is not operating and untreated coal is being fed to the furnace the change to feeding coke dust may be made by turning on the supply of oxidized coal and when the untreated coal in the container has reached a level below the carbonizer heating elements, the hot flue gases may be turned on and the carbonizer heated to operating temperature. Slow variation of coal temperatures leaving the oxidizer and elimination of heat from the carbonizer plates can afford any degree of coal temperature to facilitate the changing of operations. The sole requirements of the container consist of sufficient size for installation of adequate heating surfaces and of its proper design to allow fairly uniform flow past these heating surfaces. Need of insulation is obvious.
The heating surfaces in the carbonizer may take any form commensurate with their efficient performance, reasonable cost, and proper maintenance. As shown in Figures 1 and 3, plates of envelope shape through which the heating gases maypass are preferable, but tubes or other types of heating surfaces may be employed. Carbon steel, stainless steel, calorized carbon steel, or other materials resistant to deterioration by high temperatures maybe employed. Carbon steel allows heating the coke dust to a temperature of 1,000 degrees F., and other metals, if used, may extend the limit to 1,500 degrees F.
Efficient heat transfer is the most important requirement of these heating surfaces. Short distance of heat travel from the heating surfaces to the center of coal strata heated is of major importance, since pulverized coal is a very poor conductor of heat. Approximate counterow of flue gas and coal is likewise important for maximum efficiency. The heating surfaces should not cause undue restriction to the downward ow of coal, and preferably should displace a minimum volume of coal in the container so that a maximum time of contact for heating may occur. In present average power plant practice, coal falls in pulverized fuel containers at the rate of about one foot per hour. Making the heating surfaces high and of small displacement of volume affords long time of contact.
Since the sustained strength of steels at high temperature is very low, it is important that the heating surfaces be of efficient design from the strength standpoint. Each element of the surfaces should have a high section modulus when considered as a horizontal beam so that maxir mum stresses causing sagging may be below the b widely and 'causes inefcient transfer.
reasonable creep limit of the material employed. Oxidation of the steel is low because of low oxygen content in the flue-gas and coal gas.
Applying fins on the plate-type surfaces as above* described is more efficient than applying them to tubes because in the former case all fins are parallel and the distance of heat transfer through the layers of coal is uniform. When used on tubes radially this same distance varies Since strength of these fins is not important they may be made of less expensive material than the heating platesv-r Slow heating of coal in the carbonizer is a special feature reducing possibility of caking. It is generally known that rapidity of heating increases the degree of caking. In the usual case Where plates four feet high are\employed ap'- proximately four hours continual, uniform heat-A ing is required to carbonize coal.
Continual motion throughout the carbonizing process further reduces tendency` of caking.
Upward passage of coal gas through the coal being carbonized is a further means vof insuring non-caking and uniform coal flow between the heating plates. This upward gas flowv prevents mechanical packing of the coal and causes a continual maintenance of a very fluid, low` density mixture of coal gas and coal. Thoroughly aerated pulverized coal weighs approximately 35 pounds per cubic foot, yet tamping of the container holding this coal allows reduction of the specic volume to the point that 55' ,pounds per cubic foot density occurs. When packed to this latter density, the coal is almost self-supporting in a vertical pipe and its downwardl flow is not dependable. However, an aerated coal weighing 35 pounds per cubic foot resembles water in its fluidity. This feature of maintaining the pulverized coal in an aerated state throughout its flow between the heating plates is very important towards success of the process.
In addition to the above results obtained by reason of the upward passage of coalgas through the coal being carbonized, it has been, found that this action is important in attainment of effective heating of the coal mass. The coal gas passlng upwardlyA therethrough carries with it considerable heat, and the heat so carried serves to heat the coal so that the coal is heated more effectively than it would be if heated merely by conduction alone from the envelope-like heating elements.
Coal dust separation from the coal gas occurs by subsidence, for the upward velocity of the coal gas is extremely low. .If the oxidized coal is introduced in the carbonizer without undue dust disturbanceexperience shows that coal gas drawn from thetop of the coal container is exceedingly clean.
If tar of maximum quantity and value is desired it will be obtained from this design, since immediately upon distillation of the coal it enters regions of no higher temperature. There is no cracking of the tar to inferior products. Though it is retained in the coal gas space above the coal for some appreciable period of time,
cracking at this point is minimum because of the temperature existing there, 'unless the superheater is used for cracking purposes as described later.
High B. t. u. gas ris obtained'from the/ process because there is little or no dilution by the heating flue g s and because 'there is no cracking to deteriorate the heating value.
If it is desired to obtain a high gas yield at the expense of the tar yield, super-heating surfaces can be installed in the coal gas space at the top of the container for the purpose of crackingpart of the tar vapors to gas. Tests have shown that exposure of these tarvapors to a temperat'ure of 1300 degrees F. foran Aappreciable period of time may halve tar yield and double gas yield. The time element is important' for this tar cracking and utilization ofthe large space in the top of the carbonizing container is desirable from this standpoint. These superheating surfaces will also have the m'erit of preventing condensation ofthe tars on walls or the top of the container especially lwhen starting and stopping.
V If the carbonizer discharges coke dust too hot fuel-treatment apparatus including the oxidizer and carbonizer Will be realized insofar as the production of the distilled gases and powdered coke is concerned when not used with a furnace. 'I'he apparatus is merely one which is peculiarly adapted with greater advantage for use with a furnace.
What is claimed is: i
1. In an, apparatus for treating pulverized fuel, acasing forming an oxidizing chamber, and a concentric fuel admission chamber of lesser diameter located at one end of the oxidizing chamber, means for introducing oxygencontaining heating gas tangentially into said oxidizing /chamber from the periphery-thereof to produce a vortex of gas within and advancing along said chamber, means for introducing pul- Verized fuel tangentially into said fuel admission chamber to produce a similarly swirling vortex advancing along said oxidizing chamber within the gas vortex vto cause the particles of fuel to move outwardly through the gas, and' means .for withdrawing the gas from said oxidizing chamber near the center of the vortex and remote from said gas and fuel introducing means. f
2. In an apparatus for treating pulverized fuel, a casing forming an oxidizing chamber, and a concentric fuel admission chamber of lesser diameter located at one end of the oxidizingv chamber, Imeans for introducing oxygen-containing heating gas tangentially into said oxidiz- CII move outwardly through the gas, means for withdrawing the gas from said oxidizing chamber near the center of the vortex and remote from said gas and fuel introducing means, and means for removing treated fuel from the lower portion of said oxidizing chamber remote from the zone of said vortices.
3. A combined oxidizing and carbonizing unit for pulverized fuel comprising an oxidizer, a superimposed fuel entrance chamber centrally at the top thereof, means for introducing fuel into said entrance chamber, means for introducing oxygen-containing, heating Agas into the upper portion of the oxidizer from the periphery thereof to produce a vortex of gas within the chamber, means for introducing pulverized fuel into said entrance chamber and into said vortex near the center of the oxidizer to cause the fuel particles to move outwardly through the gas, means forming a carbonizing chamber disposed adjacent said oxidizing chamber so as to receive the oxidized fuel from the lower portion of said oxidizing chamber and having a lower fuel outlet, means within said carbonizing chamber for nally carbonzing the fuel passing therethrough, and means for conducting the oxidizing gas from said oxidizing chamber to the fuel outlet from Said carbonizing chamber.
4. A combined oxidizing and carbonizing unit for pulverized fuel comprising an oxidizer, a superimposed fuel entrance chamber centrally at the top thereof, means for introducing fuel into said entrance chamber, means for introducing oxygen-containing, heating gas into the upper portion of the oxidizer from the periphery thereof to produce a vortex of gas within the chamber, means for introducing pulverized fuel into said entrance chamber and into said vortex near the center of the oxidizer to cause the fuel particles to move outwardly through the gas, means forming a carbonizing chamber disposed adjacent said oxidizing chamber so as to receive the oxidized fuel from the lower portion of said oxidizing chamber and having a lower fuel outlet, means for withdrawing desired amounts of treated fuel from said oxidizer and re-introducing it into said oxidizer for re-treat-ment, means Within said carbonizing chamber for nally carbonizing the fuel passing therethrough, and means for conducting the oxidizing gas from said oxidizing chamber to the fuel outlet from said carbonizing chamber.
5. A combined oxidizing and carbonizing unit for pulverized fuel comprising, means forming an oxidizing chamber having a fuel inlet and a heating and oxygen-containing, heating gas inlet, a carbonizer disposed adjacent said oxidizing chamber so as to receive the oxidized fucl therefrom and having a lower fuel outlet, said carbonizer comprising a retort adapted to contain a collected mass of fuel received from the oxidizing chamber and adapted to have said mass of fuel descend slowly by the action of gravity, enclosed heating elements disposed intermediate the height of the retort, said heating elements being spaced and relatively thin so that the free downward passage of said fuel therebetween and the transfer of heat relatively uniformly throughout the entire mass of fuel Within the region of said heating elements is accomplished, with the upward passage of coal gas through the fine fuel mass, whereby the fuel mass will absorb heat from said upwardly passing gas, and means for conducting the oxidizing gas from said oxidizing chamber to the fuel outlet from said carbonizing retort.
FREDERICK L. DORNBROOK. MONTROSE K. DREWRY.