US1925132A - Combustion of coal - Google Patents

Description

Sept. 5, 1933.

C. M. BUCK COMBUSTION OF COAL Filed Aug. 16 1952 Fa x 4 Patented Sept. 5, 1933 UNITED STATES PAITENT OFFICE Application August 16, 1932. Serial No. 629,063

4 Claims. (Cl. 110104) This invention relates to the combustion of coal and is primarily concerned with improvements in the art of burning pulverized coal.

The nature, objectsand advantages of the 5 invention can best be' understood after a consideration of the following comments with respect to various characteristics of coal or coals suitable for burning in a finely comminuted condition as vwell as the methods or procedures at present in more or less common commercial use.

For several reasons which need not be discussed, pulverized coal is almost universally milled from various soft coals such as bituminous coals. In view of the above, the following description refers more particularly to such soft coals and their treatment and burning in accordance with the novel method hereinafter more fully disclosed. 4

As is well recognized by many engineers and .authorities on combustion of carbonaceous maserials, coal, as it is heated from normal atmospheric temperatures up to and through combustion temperatures, undergoes various physical and chemical changes, some phases of which take place between very accurately determinable points in the temperature scale. The temperatures at which such changes occur are dif-'- ferent with diiferent coals, but these may be determined by observation of the coal itself or analysis of certain gases and the like which may be given off.

Thus, with some soft coals, for example, bituminous from the Pittsburgh district, treatment at temperatures up to approximately the boiling point of water, for example, 212 R, will result only in a drying operation which usually involves mere elimination of surface moisture, i. e., moisture or water carried on the surface of the pieces of coal and in the cracks and crevices thereof. In addition, all coals include in this range will serve the purpose of reducing the moisture content so that a given weight of coal has a considerably higher calorific value than before the heat treatment, these temperatures, for any given coal, are considerably above those at which exothermic combustion begins,

i. e., combustion or burning without the external application of heat. In' other words, at some point in this range the coal actually burns and produces suflicient heat in and of itself to continue its thermal decomposition and exothermic oxidation. I

In addition to the foregoing, some coal drying DIOCQSSGSlIll/GIVG heating the coal as by means of an inert gas which is at a temperature somewhat above the boiling point of water, but it is generally recognized that for drying purposes, the temperature of the coil itself, i. e., the mass or bulk of the coal lumps or pieces should not be raised substantially beyond the boiling point of water, this temperature usually being maintained materially below 250 F. Heating of the mass or bulk of the coal throughout, however, should not be confused with surface heating thereof intended only to dry up surface moisture, and while drying processes quite frequently employ gases at a temperature considerably above the boiling point of water, the drying processes are so designed as to avoid heating of the coal throughout to such high temperature, as by relatively rapid flow of the coal through the heating chamber.

There are, of course, additional processes involving heating of coal, such as those employed in the production of coke and other products, which necessitate destructive distillation or exothermic oxidation even beyond that referred to above in connection with known processes for increasing the calorific value of a given weight of coal by eliminating combined moisture. Such processes, however, also produce material thermal decomposition or oxidation.

In accordance with the more usual method of procedure where it is desired to burn finely comminuted or pulverized coal insuspension, some soft coal, for example, bituminous, is heat treated to remove surface moisture, then pulverized, and ultimately fed'to the furnace through burner nozzles, a blast of air being provided to carry the fine particles in suspension, either from a point of storage or more or less directly from the Dulverizing mill.

The foregoing procedure which has become commonly accepted, presents a number of very serious disadvantages, among which the following are mentioned.

Since coal, dried and pulverized in the manner above referred to, when combined with sufficient air for purposes of transportation in suspension, forms an extremely explosive mixture, apparently due, at least in part, to the presence in the coal of free active gases, ordinarily imprisoned or occluded, which have been released during pulverization, the handling of it and use thereof in power plants and the like is very dangerous indeed. In addition, even when being stored in bins, the increase in the exposed surface area in the coal, as a result of pulverization, quite frequently results in autogenous combustion.

A still further characteristic of coal treated and pulverized in accordance with prior practice is an extremely high rate of flame propagation when carbureted with material quantities of air. In view of the characteristics referred to, especially the high rate of flame propagation evidently resulting from the presence-of free active gases, the amount of air which is mixed with the' coal and used to transport the same into the combustion space, in accordance with prior practice, represents only a small fraction of the total amount necessary to complete combustion of the fuel. This small percentage of air, furthermore, has very high linear velocity for the purpose of avoiding burning back of the flame into the burner nozzles. In other words, in accordance with the commonly accepted method of procedure, it is essential to employ a very high velocity air blast to carry the pulverized coal into the combustion space. The additional air necessary to complete combustion of the fuel is admitted secondarily into the combustion chamber independently of the burner nozzles, by natural or forced draft. As a result of the high velocity fuel admission, with only a fraction of the total air necessary, the overall dimensions of the combustion space must be relatively great in order to ensure complete combustion of the fuel before entry of the fuel and flame stream into the furnace off-take.

Another disadvantage arises from rapid and serious erosion of the refractories employed, as well as of metallic elements, such as tubes with which the combustion chamber may be lined. Still further, since a major portion of the combustion air is admitted secondarily, through openings spaced from the burner nozzle and usually spaced along the path of travel of the fuel and flame stream, the mixture of the air with the powdered. coal is very irregular.

Because of improper or irregular admixture of the coal and air and because of certain characteristics of coal more fully considered hereinafter, the common practice of burning powdered fuel requires admission of substantially more air into the furnace than is actually necessary for complete combustion, with the result that the size of the combustion space is unnecessarily large for this reason also. Slagging difficulties at the bottom of the furnace have also been encountered.

Turning now to the present invention, it

I should be noted that, in general, the objects are involved in eliminating, or at least, ameliorating the foregoing difficulties and disadvantages.

More specifically the'present invention has in view a method for the burning or combustion of coal involving feeding of substantially all of the air necessary for complete: combustion of the coal with the coal as-it is delivered or discharged into the combustion space.

A further object of considerable importance is involved in certain features of my process or method which make possible complete combustion of the fuel in the combustion chamber with less air than has been required heretofore.

I have found that by heating the coal throughout, preferably prior to comminution or pulverization, up to a temperature materially above the boiling opint of water, but not beyond the point at which active exothermic oxidation or thermal decomposition commences, certain changes, principally physical, are brohght about. For bituminous coals which I refer to by way of example,

,I have found that temperatures between approximately 250 F. and 480 F. are suitable, and I preferably effect this heat treatment either through the medium of, or in the presence of, inert and non-combustible gases, such as stack gases discharged from a furnace.

Since the physical and chemical characteristics of coal vary considerably, the point at which exothermic combustion commences should be determined in each instance. This may be accomplished in a variety of ways, as by observing the coal undergoing treatment and noticing at what temperature certain changes in color are brought about, these changes probably being caused by the formation of oily substances on the surface of the coal. Another indication may be obtained by observing at what temperature the treated material smokes when discharged from the heating chamber (containing the inert gases) into the atmosphere.

While the present method involves heating of the coal, within the temperature range indicated, throughout the entire mass or body of the coal, I preferably terminate the period of heat treatment (especially where the temperature employed approaches the point of active thermal decomposition) very quickly, if not instantly, after the coal has been heated throughout. With this in mind, in accordance with the preferred procedure, the lump coal is first crushed to pieces approximating about one-quarter inch in diameter, and although my heat treatment may be employed with coal which has already been pulverized, I find it especially desirable to follow the method indicated as preferable since the coal may much more readily be pulverized after the treatment. One reason for the improvement in pulverization which results is the fact that the coal is expanded and becomes quite porous, the amount of expansion usually being in the neighborhod of about 10%. v

The foregoing heat treatment, of course, ac-

complishes removal of surface moisture which i has been heretofore brought about by the usual drying processes, but the major importance of my method results from certain other changes which take place.

The treatment in the above range, although it does not remove much of the combined moisture, does result in'the liberation of certain occluded gases, for example, methane, ethane, carbon dioxide, carbon monoxide, nitrogen, and possibly others in small quantities, some of which are highly inflammable. Moreover, as above brought out, the coal expands and becomes porous, and, as a result, it absorbs or. picks up substantial quantities of the inert gases in the presence of which the treatment is effected. It also seems clear from such tests as it has been possible to make that certain chemical affinities are also satisfied.

In any event, whatever may be the physical or chemical causes, I have found that the reactions and physical changes which are brought about during this phase of combustion of the coal, i. e., that portion of the endothermic phase which extends up to the point at which exothermic orthermal decomposition commences, provide a fuel which still retains a material percentage of combined moisture (by way of example, with some bituminous coals about 7% of moisture), and which, when mixed with substantially all the air necessary to complete its combustion, i. e., carry on the combustion through its exothermic phase, burns in a greatly improved and much more eficient manner.

While the endothermic phase of combustion (that is, the. phase during which externally applied heat may be absorbed by the coal) extends beyond the temperature at which thermal decomposition or active combustion sets in, by carrying on or completing the combustion process up to the point of active oxidation (at which point the reactions in the coal itself produce suihcient heat to create a tendency for the coal to carry on and complete its burning without the application of external heat), and, subsequent to. the completion of combustion to the extent referred to, mixing the fuel with the remaining air necessary, I am enabled to introduce the fuel at relatively low velocity as compared with prior practice without the flame burning back, principally because of the absence from the coal of any free active gas sufficient. to produce a self propagating flammable mixture with air, and further to utilize a combustion space of materially reduced size as compared with that heretofore required.

The carrying on of combustion in its several phases partly externally of the furnace and partly after introduction of the fuel into the combustion space, therefore, makes possible this use of a reduced space to burn a given quantity of fuel. In other words, I introduce the fuel and air at relatively low velocity and by virtue of this fact materially reduce erosion of refractories and the like, and, additionally, provide for com-= pletion of combustion of the fuel during a relatively short fuel and flame stream travel.

of the coal in the furnace are carried out in the combustion space itself, this procedure necessitating the feeding of excess quantities of air, by

following my procedure, a reduced quantity of air per pound of fuel may be fed therewith into the combustion space and still produce efficient and complete combustion.

Thus my improved method adapts the burning of pulverized coal to uses and conditions to which it has notbeen suitable heretofore, as in locomotives, the combustion chambers of which must, of necessity, be of relatively restricted dimensions.

Other advantages of the present method will now be apparent. For instance, since I feed all or substantially all of the air necessary for combustion in the furnace space with the fuel as it is delivered or discharged into the furnace, I am enabled to obtain greatly improved admixture of the fuel and air as will be apparent when this procedure is contrasted with prior methods according to which the major part of the combustion air is introduced into the furnace secondarily through spaced openings in the walls of the chamber. Moreover, since I introduc'ethe fuel in a relatively slowly moving current of air consid- -erably less power is necessary to create the air draft for a given" volume or quantity thereof, this being of very material importance in view of the fact that the power which would be necessary to.

create a blast of air of sumcient volume to complete combustion of the fuel at velocities necessarily employed with ordinary raw pulverized coal would be entirely impractical.

Since pulverized coal feeders adapted to deliver all of the air necessary to complete combustion are not of common knowledge or readily available, the present application is accompanied by a drawing illustrating a feeder suitable to carry out the purposes I have .in mind. As brought out more fully below the drawing also includes a showing of coal heating and pulverizing equipment such as that referred to above. In this drawing:

Fig. l is a vertical cross sectional view of the feeder;

Fig. 2 is a side elevational view of the apparatus shown in Fig. l, with portions thereof broken away in order to disclose others lying behind; and Figure 3 is a diagrammatic showing of coal crushing, heating and pulverizing equipment of a type suitable to carry on certain of the process steps above referred to. i

Referring first to Figure v3, as hereinbefore crushed to pieces approximating about id" in diameter, this being accomplished by delivering the lump coal through hopper A to any suitable crusher B. The crushed coal may be delivered to the retort or heating chamber C through a conduit D and, upon discharge of the coal from suitable for effecting the treatment. The coal may be conveyed. away from the pulverizer F through a conduit G and finally, of course, the coal is delivered to a feeder mechanism such'as that shown in es 1 and 2 and described below.

As before mentioned the coal may be heated in the chamber C by flue gases discharged from furnace H through connection I. A discharge or vent pipe J may serve to carry the gases away from the chamber C.

ing 4. The hopper has a discharge throat 5 in which I preferably mount a churning or heater wheel 6, the throat being slotted as at 7 to provide for the delivery of a relatively thin and wide sheet or stream of powdered coal into the sub h the feeder illustrated I employ a hopper or bin 3 adapted to be charged'through the openjacent and associated fan casing indicated -generally by the numeral 8.

The fan device includes a fan rotor 9 formed is spaced a relatively great distance from the periphery of the fan at the point indicated at 12, from which point the casing progressively ap- 5 proaches the fan periphery to the point 13. A

screened air inlet is provided peripherally of the fan as indicated at 14 and a discharge conduit 15 for delivering the fuel to the combustion chamber is extended away from the fan housing between the portion of the wall designated by the numeral 13 and an additional wall portion orpart 16, the latter being of relatively small dimention circumferentially of the fan and disposed in close proximity to the periphery of the fan blades. vWith this arrangement, as seen in the drawing, the air inlet 14 and the discharge conduit 15 are disposed relatively close to each other at the same side of the fan, there being only a relatively small wall part 16 interposed therebetween.

The air inlet opening, the width of the feed conduit 15 and the fuel discharge slot 7, are all preferably substantially equal in width to the axial length of the fan. If desired, elements 1'? in the nature of arcuate bars may be disposed toward the lower end of the bin 3 so as to prevent arching of the fuel therein, and packing of the fuel, especially in the lower portion of the bin, is further avoided by the use of aerating tubes 18.

In operation, therefore, the fuel, in being delivered downwardly through the bin and past the feeder wheel 6, is discharged into the fan housing in a thin but wide sheet, andas the fan rotates in the direction indicated by the arrow this fuel is entrained in the air drawn in through the inlet 14 and blown around the volute-like fan casing to the point of tangential discharge into the duct 15. As the fuel and air advance through the progressively narrowing volute-like fan passage, centrifugal force, acting on fuel and air disposed between adjacent fan blades, throws the mixture outwardly against the casing from which it is again reflected or deflected" toward the fan rotor and picked up. This operation is repeated until the point of discharge is reached, at which time very thorough and complete admixture of the fuel and air is provided. Upon discharge of the air and fuel from between thefan blades under the action of centrifugal force, a negative or reduced pressure is produced and this condition is maintained until the spaces between the blades move upwardly beyond the wall portion 16. In view of the fact that the wall portion 16 is of relatively small arcuate dimension, and further since it is disposed in close proximity to the periphery of the fan blades, a very substantial flow of air inwardly through the inlet opening 14 is produced and the incoming air, as above brought out, mixes with the descending fuel in the manner described.

By this feeder device I am enabled to thoroughly admix all or substantially all of the air necessary for combustion with the fuel and to discharge this mixture into the duct 15 to be conveyed thereby into the furnace combustion space.

The arrangement of the several parts of the device, furthermore, are such as to deliver the necessarily large quantity of air at the desired relatively low velocity.

While other feeder devices may be developed to accomplish the purpose I have in mind, I prefer the foregoing embodiment, especially since the structure thereof, having a fan, a fuel inlet slot, an air opening and a discharge conduit, all of substantially equal width, provides for uniform and thorough admixture throughout the entire width of a sheet even of relatively great width.

In conclusion, I desire to point out that by following the method or procedure herein disclosed,

the exposed surface area of the fuel is materially increased, not only as a result of theexpansion of the fuel but further since there is a substantial increase in ultrafines and pulverization to a much finer state is made possible, and when the fuel is delivered into the furnace it is ready for immediate gasification and active combustion. It is, therefore, of especial advantage'to feed substantially all of the air necessary to complete its combustion along with the fuel at the time of its admission.

The present application is a continuation in part of my copending application Serial No. 503,- 169, filed December 18, 1930, issued January 10th, 1933 asPatent No. 1,893,857.

I claim:

1. The method of burning coal of the softer grades which includes making external application of heat to the coal until its temperature is raised throughout its mass to a point, between about 250 F. and 480 F., approximating but not above the point at which active or exothermic combustion of the particular coal commences to render the coal substantially non-explosive, pulverizing the coal, and feeding the heat treated and pulverized coal to the combustion space of a furnacein suspension in substantially all the air necessary to complete its exothermic phase of combustion.

2. The method of burning coal of the softer grades which includes making external applica-' tion of heat to the coal until its temperature is raised throughout its mass to a point between approximately 250 F. and 480 F. to carry on certain phases of its endothermic combustion to render the coal substantially non-explosive, pulverizing the coal, and feeding the heat treated and pulverized coal to the combustion space of a furnace in suspension in substantially all the air necessary to complete its exothermic phase of combustion.

3. The method of burning coal of the softer grades which includes making external application of heat to the coal in the presence of inert gases until its temperature is raised throughout its mass to a point between approximately 250 F. and 480 F. to carry on certain phases of its endothermic combustion to render the coal substantially non-explosive, pulverizing the coal, and feeding the heat treated and pulverized coal to the combustion space of a furnace in suspension in substantially all the air necessary to complete its exothermic phase of combustion.

4. The method of burning, coal of the softer grades which includes making external application of heat to the coal in the presence of inert gases to carry on certain phases of the endothermic combustion of the coal, the application of heat being suflicient to bring thetemperature of the body or mass of the coal to a point between about 250 F. and 480 F. and in the neighborhood of but not above the point at which thermal decomposition or exothermic oxidation of the particular coal comnfences to render the heated coal substantially non-explosive, pulverizing the coal, and feeding the pulverized and partially burnt coal into the combustion chamber of a furnace in suspension in at least a major portion of the air necessary to complete its exothermic phase of combustion.

CHARLES M. BUCK.

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