US3181488A - Apparatus for drying coal in bunkers - Google Patents

Description

May 4, 1965 R. c. ROE ETAL APPARATUS FOR DRYING COAL IN BUNKERS 5 Sheets-Sheet 1 Filed Feb. 23. 1961 NQQQQD Eukk hunt, EQkmSQSS INVEN TOR-i PA L PH C Po 5. By dbseullcnrczvsrzlm y 4, 1965 R. c. ROE ETAL 3,181,488

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BY JQSEPHL zcuravsrs/m Arm/Mam United States Patent 3,181,488 APPARATUS FOR DRYING COAL 1N BEJ'NKERS Ralph C. Roe, Tenafly, NJ., and .ioseph Lichtenstein, Bayside, N.Y., assignors to Burns and Roe, Inc., New York, N.Y., a corporation of New Jersey Filed Feb, 23, 1961, Ser. No. 91,171 6 Claims. (6!. 110-491) This invention relates to power plants in general, and more particularly to a method and apparatus for removing the surface moisture from coal in bunkers just prior to its being delivered to the furnace for combustion therein.

Those persons skilled in the art will readily appreciate that the difliculty experienced in the flow of coal through bunkers presents a very considerable problem. Thus, so called ratholing" or channeling and bridging frequently occur in the coal to interrupt its continuous flow to the furnace. It is well known in the art that this difficulty is caused by surface moisture on the coal, geometry of bunker design and, to a relatively minor extent, surface roughness on bunker walls, with moisture as the primary cause.

It has been found that even if the best bunker geometry is employed, it is most important to the continuous flow of the coal that surface moisture be eliminated; and in fact, the full extent of the problem presented by this moisture will only be appreciated when it is realized that if such moisture reaches the furnace, it has to be evaporated there and the heat required to accomplish this is wasted, thus reducing the efficiency of the plant.

Heretofore, in attempting to dry coal prior to firing it has been proposed to pass the hot gases of combustion through a jacket surrounding a coal hopper or bunker, or such gases have been passed through the coal itself in direct contact with the coal particles. Each of these methods is objectionable since, in the first case heating, and therefore drying, is not uniform, the coal nearest the bunker wall being heated while that in the center of the bunker receives little heat, if any. In the second case, passing the hot combustion gases through the coil has proven ineffective by virtue of the high moisture content of the gases themselves due to the release of water vapor upon combustion of the fuel. Also, some of the combustion gases such as sulphur dioxide in the presence of moisture are very corrosive at certain temperatures.

In essence, our invention resides in the recapture of otherwise wasted heat in the system, and the application of such heat to dry the coal as it moves through the bunkers, without any of the aforementioned difficulties or disadvantages. Thus, we utilize waste heat from the exhaust furnace gases, usually after they have passed the normal air heater, and transfer the heat thus recaptured, to the coal in the bunkers by means of heat transfer fluid.

According to one aspect of the invention, a gas-to-liquid type heat exchanger is placed in the gas exhaust of a furnace and is connected by piping and a pump to a series of spaced, vertically disposed, thin, hollow plates extending nearly the full height and length of each bunker, and the entire system is filled with a heat transfer liquid, such as water under low pressure, for example. The fluid is heated by the exhaust gases and pumped through the hollow plates contiguous with the coal in the bunkers, whereupon the heat iiows by convection and radiation to the adjacent coal. Since the coal flows slowly from the top to the bottom of the bunkers, there is a sufficient period of time in which to heat the surface moisture on the coal to the temperature necessary to evaporate it. Of course, a certain amount of air is required to be supplied, either by natural convection, or by being forced into the coal, to

carry the moisture from the surface of the coal to the atmosphere.

A further aspect of the invention involves a heat exchanger of the liquid-to-air type, positioned exterior of the bunkers whereby heat is transferred from the heat transfer liquid to air which is then forced, by means of a fan or blower, through a series of hollow air chambers spaced apart in the bunkers. Each chamber has suitable perforations so that the heated air passes therefrom up through the coal, by reason of the void spaces between the coal particles, and evaporates the surface moisture thereon, carrying it out of the bunkers in the form of increased humidity of the air.

Still a further aspect of the invention contemplates the direct heating of air by a gas-to-air heat exchanger in the path of flow of the furnace exhaust gases. The air, thus heated by the exhaust gases, is delivered to perforated air chambers in the bunkers from which it passes up through the coil, again to dry the same.

It is important in each of the several aspects of the invention alluded to that the air temperature within the bunkers be low enough to obviate the danger of spontaneous combustion; and in each case, but particularly where air is discharged in the bunkers, the velocity of the air through the bunkers must be small enough to insure that even the finest coal particles are not carried out into the atmosphere.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent constructions as do not depart from the spirit and scope of the invention.

A specific embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawings, forming a part of the specification, wherein:

FIG. 1 is a diagrammatic illustration of a system in accordance with the present invention;

FIG. 2 is a perspective view, partly broken away, of a bunker equipped with a hot water coal drying assembly;

FIG. 3 is a perspective detail view of a hot water heater unit broken to indicate greater length;

FIG. 4 is a diagrammatic illustration similar to FIG. 1, but showing a further embodiment of the present invention;

FIG. 5 is a perspective view, partly broken away, similar to FIG. 2, and showing a bunker equipped with a heated air coal drying assembly;

FIG. 6 is a perspective view of a heated air unit; and

FIG. 7 is a diagrammatic illustration of a modification of the embodiment illustrated in FIG. 4.

Referring now to the drawings, and particularly to FIG. 1 thereof, there is shown a system embodying the present concept and including a heat transfer unit 159 which preferably consists of a gas-to-liquid heat exchanger of the cast iron surface type positioned in the path of combustion gases from the furnace (not shown) to the stack 11. A suitable heat exchange liquid which may be water under a pressure of the order of about 250 lbs. per sq. in., to prevent its vaporization, is pumped through the heat transfer unit 10 by a pump 12 which is positioned in a line 14 equipped with valving where necessary to conduct the heated fluid to a series of take-01f pipes as at 15 for delivery through manifolds 16 to a group of thin, hollow chamber-s or plates 17 vertically positioned in spaced relation in coal bunkers 19. The heated fluid passes through the plates 17 and is returned through valved return manifolds 20 and return line 21 to the heat transfer unit 1 0 for recirculation.

The liquid system may be equipped with an expansion tank 22 to permit variations in fluid volume in the system, and with a make-up line .24 for maintaining the system suitably supplied with liquid.

As shown in FIGS. 2 and 3, the heated liquid flows from inlet manifolds 16 into the upper portions of the plates 17 and leaves theplates at the bottom thereof via manifold 21). The plates are provided with a series of horizontal, spaced baffles 25 staggered longitudinally relatively to one another. As shown in FIG. 3, these baffies are alternately connected to opposite end walls 26 of the plates 17 and each bafile extends the full width of its respective plate. There is thus provided a tortuous path for the heated liquid from end to end of each plate, each excursion bringing the liquid to a lower level, as indicated by the arrows in FIG. 3 for example, until it reaches the outlet manifold 24) which is connected to each plate beneath its lowermost baffle. Experience indicates that generally, the plates 17 should be maintained a minimum of 24 inches apart. It is also preferred that the top surfaces of the plates 17 be rounded as shown to present a streamlined surface to the downwardly flowing coal, the upper sloped surface of which is indicated by the reference numeral 27 (FIG. 2).

It will be appreciated that the heat'carried by the liquid to the plates 17 flows by both convection and radiation to the coal between the plates, and inasmuch as the coal flows slowly downwardly through the bunker, it is exposed to heat for a period of time quite sufficient for evaporation of its surface moisture. A certain amount of air is required as a vehicle to carry the moisture from the coal to the atmosphere. While natural convection may well suffice for this purpose, it is a part of the present concept to force air into the coal if desired in order better to remove the water vapor. For this reason, a series of air chambers 2 are positioned substantially coextensive with and beneath the plates 17, and while such chambers may be independent of the plates, they are illustrated as being constructed integral therewith. Atmospheric air is supplied to the chambers 29 through air line 30, and this air is discharged from the chambers in a manner and by means later to be described in detail. For present purposes, it is sufficient to understand that the air thus discharged in the bunkers passes upwardly through the coal particles to carry the water vapor off to atmosphere through vents 32 in the upper regions of the bunkers. While nine plates 17 have been shown in FIG. 2, it will be understood that any number may be employed as required in each specific instance, and it is of course preferred that the same number of chambers 29 be utilized.

Turning now to FIG. 4, there is illustrated a system similar to that already described, but with certain modifications. Thus, a heat transfer unit it? is' positioned ahead of the stack '11 and the pump 12 pumps a'heat'transfer liquid which may be water under pressure from the unit 10 through pipe 14 and returns same to the unit 10 throughthe pipe 21, as before. The expansion tank 22 and make-up line 24 are also part of the present system.

However, according to the embodiment of the invention illustrated in FIG. 4, the liquid heated by the exhaust combustion gases is circulated through a liquid-to-gas heat transfer unit 35 positioned between the pipes 14 and 21. Air under pressure supplied by a motor driven blower (not shown) is admitted to the unit 35 through air inlet 35 and leaves at hot air outlet 37 after having been heated by the liquid in the unit. The heated air passes through air lines 39, 40 and 41 to inlet manifolds 42 from which it is delivered to ,a series of elongate, hollow air chambers 44 disposed in spaced horizontal relation within the bunkers 19.

As shown in FIG. 5, the heated air is discharged from the chambers 44 and comes into direct contact with the coal, the .air moving upwardly through the bunkers, as indicated by the arrows; The heated air evaporates the surface moisture from the coal particles, carrying such moisture off .as increased humidity. Vents 45 are provided in the top of the bunkers to allow for the escape of the air to atmosphere.

It is of course, important that the coal be prevented from clogging the outlets of the chambers 44. Accordingly, as shown in FIG. 6, the bottom surface 43 of each of these chambers is provided with a plurality of perforations as at 46, and lower portions of the side and end walls 4-7 and 49, respectively, depend downwardly to provide a peripheral flange or skirt extending beyond the bottom surface 43. Thus, the coal passing downwardly around the chambers will not come into contact with the bottom surfaces thereof, sothat the heated air is free to escape into the voids in the coal and to move upwardly towards the vents 45. Again, the top surface of the chambers 44 are rounded or otherwise streamlined. The chambers 29 illustrated in FIG. 2 and referred to in accordance with an embodiment of the inventionaalready described may be constructed similarly to the chambers 44.

Referring finally to FIG. 7, there is shown a modification of the embodiment of the invention illustrated in FIG. 4. Thus, in this case, instead of the heat transfer units 10 and 35, a gas-to-air unit Si) is positioned in the stream of combustion gases just ahead of the stack 11. An air compressor 51 supplies atmospheric air under pressure to the unit 5%, and the compressed air, after being heated by the stack gases is conveyed by an air duct-52 to one or a number of bunkers via take-off ducts 54. The bunker-s 11 'are'identica'l with those already described and the heated air is discharged into the coal through a plurality of hollow air chambers '44, as shown and described in connection with FIGS. 4 to 6.

In a typical application of the present invention, 'consider a power plant generating a million pounds of steam per hour or approximately 10 Btu. per hour, and wherein 200,000 pounds per hour of coal enter the bunkers with average surface moisture equal to 8.72 percent of the weight of the coal. In this case, 17,450 pounds per hour of moisture enters the bunkers and must be removed' In accordance with the present contribution, air externally heated by means of warm water obtained in a heat transfer unit or economizer from the exhaust furnace gases in accordance with the modification illustrated in FIG. 4 for example, could effect a fuel saving of approximately 2 percent, and this without utilizing the warm air for furnace combustion. This saving alone is suflicient to pay for a substantial portion, if not all, of the required equipment, i.e., the heat transfer unit, air heater, motor, fan, circulating pump, piping and duct work. A preferred air velocity through the coal of about 19.6 feet per minute is effective to remove the moisture, and is so low that the air will notpick up even fine coal particles and, therefore, would not create a dust nuisance or loss of coal.

'From the foregoing description, it will be seen that we have contributed by our invention a novel system of drying coal in bunkers by utilizing the waste furnace gases and without any of the difliculties or disadvantages attendant upon prior systems known to us.

'We believe that the construction and operation of our novel system will now be understood and that the advantages of our invention will be fully appreciated by those persons skilled in theart.

We now claim? 1. In a system of the class described for removing surface moisture from coal in bunkers prior to its introduction into a furnace, a heat exchanger positioned in heat exchange relation to the furnace exhaust gases, means defining chambers respectively positioned within the bunkers immediately adjacent coal moving therethrough, and means directing air under pressure through said heat exchanger to absorb heat from said exhaust gases and thence into said chambers, said chambers having a plurality of apertures in the undersurfaces thereof for discharging heated air into the natural void spaces between coal particles passing through the bunkers to evaporate and carry olf as increased humidity surface moisture from the coal, and means venting the moisture laden air from the bunkers,

2. In a system of the class described for removing surface moisture from coal in bunkers prior to its introduction into a furnace, a heat exchanger positioned in heat exchange relation to the furnace exhaust gases, means defining chambers respectively positioned within the lower region of the bunkers immediately adjacent coal moving downwardly therethrough, means directing air under pressure through said heat exchanger to absorb heat from said exhaust gases and thence into said chambers, said chambers having a bottom surface formed with a plurality of downwardly facing apertures therein for dis charging heated air into the natural void spaces between coal particles passing through the bunkers to evaporate and carry off as increased humidity surface moisture from the coal, said chambers also having a peripheral epending skirt extending to a level below that of said bottom surface preventing the downwardly moving coal particles from interfering with the flow of air through said apertures, and means venting the moisture laden air from the bunkers.

3. In a system of the class described for removing surface moisture from coal in bunkers prior to its introduction into a furnace, means defining chambers positioned respectively in the bunkers for contact with coal particles moving downwardly therethrough, a heat exchanger positioned in heat exchange relation to the furnace exhaust gases, means directing a heat transfer liquid through said heat exchanger to heat the liquid, at second heat exchanger, means directing a flow of air in heat exchange relation to said heated liquid in said second heat exchanger and thence to said chambers, said chambers having a bottom surface formed with a plurality of downwardly facing apertures therein for discharging the heated air into the natural void spaces between coal particles passing through the bunkers to evaporate and carry off as increased humidity surface moisture from the coal, said chambers also having a peripheral skirt depending downwardly below the level of said apertures to prevent the downwardly moving coal particles from interfering with the flow of air through said apertures, and means venting the moisture laden air from the bunkers.

4. In a system of the class described including a furnace, coal bunkers for delivering coal to said furnace, and means for conducting exhaust gases from said furnace, the improvement that comprises a heat exchanger positioned in said exhaust gas conducting means and in heat exchange relation to the furnace exhaust gases to heat fluid therein, means defining a series of spaced thin fiat chambers positioned side-by-side within the bunkers and extending from adjacent the top to adjacent the bottom and crosswise thereof immediately adjacent coal moving therethrough, and means directing a heat transfer fluid through said heat exchanger and thence through said chambers to transfer heat from said chambers to the coal to evaporate surface moisture therefrom, a series of vertically spaced horizontal bafile plates Within each of said chambers providing a tortuous path for the heat exchange fluid as it moves through said chambers thus to permit same to transfer heat by radiation and con vection to the bunker contents to effect evaporation of surface moisture therefrom, perforated air chambers extending substantially coextensive with and: directly be low each of said chambers to provide a flow of air upwardly through the coal to carry oif the evaporated moisture and means at the top of the bunkers venting the moisture laden air.

5. A system according to claim 4, wherein the chambers are spaced at least twenty-four inches apart.

6. A system according to claim 4, wherein the air is admitted to the bunkers at a rate to establish an air velocity through the coal of about 19.6 feet per minute.

References Cited by the Examiner UNITED STATES PATENTS 1,608,699 11/26 Kreisinge-r -106 2,033,685 3/36 Coutant 110-15 X 2,200,379 5 40 Williams 3486 2,333,089 11/43 Burkhardt 34l77 2,458,412 1/49 Payne 34-168 X 2,552,254 5/51 Brunschwyler 34177 X 2,642,314 6/53 Dupasquier 239-566 X 2,706,343 4/55 Oholm 34-177X FOREIGN PATENTS 156,746 5/54 Australia. 15,790 11/01 Sweden.

NORMAN YUDKOFF, Primary Examiner. BENJAMIN BENDETT, Examiner,

Claims (1)

1. IN A SYSTEM OF THE CLASS DESCRIBED FOR REMOVING SURFACE MOISTURE FROM COAL IN BUNKERS PRIOR TO ITS INTRODUCTION INTO A FURNACE, A HEAT EXCHANGER POSITIONED IN HEAT EXCHANGE RELATION TO THE FURNACE EXHAUST GASES, MEANS DEFINING CHAMBERS RESPECTIVELY POSITIONED WITHIN THE BUNKERS IMMEDIATELY ADJACENT COAL MOVING THERETHROUGH, AND MEANS DIRECTING AIR UNDER PRESSURE THROUGH SAID HEAT EXCHANGER TO ABSORB HEAT FROM SAID EXHAUST GASES AND THENCE INTO SAID CHAMBERS, SAID CHAMBERS HAVING A PLURALITY OF APERTURES IN THE UNDERSURFACES THEREOF FOR DISCHARGING HEATED AIR INTO THE NATURAL VOID SPACES BETWEEN COAL PARTICLES PASSING THROUGH THE BUNKERS TO EVAPORATE COAL PARTICLES PASSING THROUGH THE BUNKERS TO MOISTURE FROM THE COAL, AND MEANS VENTING THE MOISTURE LADEN AIR FROM THE BUNKERS.

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