US2416053A - Fuel burning method - Google Patents

Description

Feb. 1a, 1941.

P. R. GROSSMAN FUEL BURNING iamon mm: .Jan. 29', 1942 Fig. 1

3 Sheets-Sheet 1.

INVENTOR.

BY F ul 1?. Gmssman ATTORNEY- Patented Feb. 18, 1947 UNITED STATES PATENT OFFICE FUEL BURNING IHETHOD Paul R. Grossman, Cleveland, Ohio, assignor. to z The Babcock & Wilcox Company, Newark, N. J., a corporation of NewJersey Application January 29, 1942, Serlal.No..428,724

14 Claims; (Cl. 122-235) My discovery and invention relates in general to the construction. and operation of fluidheating apparatus, and more particularly, to the construction and operation of steam. generating units in which they heat is supplied by the combustion of. a pulverized slag-forming fuel.

In medium and high. capacity steam generating units of thetype described, the. convection: heated. surface usually includes a bank of relatively closely spaced small diameter steam superheating tubes adapted to superheat the steamto a relatively high temperature. installations it is desirable to position a bank. of steam generatingtubes in the path of the heating gases from the furnace chamber to the superheater" tubesto screen the superheater from In. such as low a value as possible to minimize the fan power requirements. For. this reasonit is cusradiation from the furnace chamber and reduce the gas temperature sufficiently that overheating of the super-heater tubes is avoidedwhile per-- mitting the desired high degree of superheat to be obtained. In. pulverized fuel fired installations of' this type, such boiler screen tubesare designed to-serve the additional. function of re ducing the temperature of slag particles in. a. molten or sticky condition suspended in the heating gases to a dry solid state or non-adhesive condition to thereby minimize'the deposition of turned gases can. mix with the higher temperature gases generatedby the burning fuel and thus reduce the average temperatureof the gas. stream leaving the furnace chamber. Due'to therelatively low temperature at which gases can be safely contacted with the operating parts of a fan or other gas circulating device, it has been necessary to withdraw the gases to be recirculated at a point. where their temperature has been reduced by contact with a major portion of the convection heatedv surface of the unit. Gas recirculating arrangements of this type re-- suit in a substantial increase in the fan power requirements of the unit and thus correspondingly increase the installation and operating costs of the unit.

It is widely recognized that in the designing .of convection heated fluid heating apparatus, the

flow path of the heating gases should be arrangedfor a maximum contact between the heating gases and the heat absorbing surface while maintaining the draft loss through the unit at tomary to provide a flow path: for the heating gases in. which flow influencing elements,- such as baflle and wall arrangements, tending to produce local .eddy currents in the'gas flow are eliminated as far as possible, so that a streamlined straight through flow. of the heating gases will takeplace.

Ihave. discovered howeverthat under certain circumstances factors inducing substantial eddy currents in the gas flow; are not only not harmful, butin-fact can. behighly'beneficial in. providing an inherent. recirculation of a large portion of the heatinggases at a location whereby the average temperatureof the gases leavingthe furnace chamber will. be reduced to a value at which safe wall temperatures for the superheater. tubes are insured'and, when the unit is flredwith aslag-forming fuel, the temperature of suspended slag particles. will be reduced below the fuelash softening temperature before contacting with the superheater tubes. I have discovered that a gas recirculation. effect of the character described can be enhanced and maintained by aspecial construction of the bank of heat absorbing tubes first contacted by the heating gases-leaving the furnace chamber, further by controlling thedirection and velocity of the furnace gases adjacentv the location at which the recirculated gases. enter thefurnace chamber, andstill. further-by controlling the. path of travel of. the-heating gases in the furnace chamber andapproaching the first bankof convection heated tubes; I

In utilizing my'discovery; the high temperature heating gases leaving the furnace chamber should be first directed across a bank of vertically arranged. fluid. heatingtubes constructed and arrangedto have a predetermined relation of effective heat absorbing" area, tube height in the gas flow passage; and gas pressure drop across the tube bank tendingv to establish a thermal by providing ahigh velocity for the ascending high temperature furnace gas stream adjacent the location where the recirculated gasesreturn to-the furnace chamber, and directed at an angle.

away from the lower portion of the first convection heated bank. Such high gas velocities will result in a lower static pressure condition at this point, and thus an additional draft effect tending'to increase the flow of gas reversely across the lower portionof the tube bank. In effect the high velocity 'gas stream will-act as a fluid. jet to produce entrainment of the recirculated 7 ized fuel entering that section and maintain the ture high velocity gas stream upwardly along the front wall and roof in the'upper portion of thei horizontal cross-section in the lower portion below furnace chamber, so that the inertia effect of these gases will naturally cause the hotter gases to concentrate along and flow across the upper portion of the first convection heated tube bank."

In the drawings: Fig. 1 is a sectional elevation of a pulverized fuel fired slag tap steam generating unit taken on the line of Fig. 2, and having a construction and arrangementof the furnace chamber and convection heated fluid heatingsurfaceproviding a recirculation of a substantial amount of heating gases to the furnace chamber;

Fig. 2 is a horizontalsection taken on 2-2 of Fig. 1; and 7 Fig. 3 is an enlarged view of a portion of the the line unit shown in Figs. 1 and 2 diagrammatically il.ustrating the gas flow therein.

The steam generating unit illustrated in the drawings utilizes the various features of mydiscovery, and comprises a vertically elongated furnace chamber I0 defined by a-vertical front wall I I, opposite sidewalls l2, and a short rear wall I3, all of which are provided with vertical water desired high temperature conditions therein. A slag discharge port 31 is providedin one of the side walls l2 at the floor level thereof through which molten slag depositing on the floor |4 may be discharged either continuously or intermittently. The remaining. portions of the furnace wall tubes are left bare to provide a large amount of radiant heat absorbing surface throughout the remaining heightof the furnace chamber.

With the described furnace wall construction the furnace chamber will be of substantial height and of rectangular horizontal cross-section, the

the rear v arch I6 being substantially greater than I the cross-section above that arch. The vertical the furnace chamber which opens into a gas flow tubes connected into the steam generating circuit of the'unit. The bottom of the furnace chamberis closed by a floor I4 and the top by a rearwardly inclined roof I5. The vertical'rear wall l 3sextends along only the lower part of thefurnace chamber, and is continued as a forwardly inclined "steam and water drum 25,'1ocated above the roof. Each of the side walls I2 of the furnace chamber is cooled by a row of water tubes 25 connected at their lower ends to a longitudinal header 2'! supplied by the drum 2| and at their upper ends to a header28 discharging to the drum 251% A I row of tubes 30 extends from the drum 2| along the remaining portion of the floor I4 and thence upwardly along the rear wall I3, arch I6, and vertical wall section I! to a small diameter transverse drum -3| invertical alignment with the drums 2| and 25. Downcomerpipes 32 externally connect the bottom of the drum 25 to the Upper and lower rows of pulverized coal burner nozzles 35 are downwardly inclined and arranged to discharge between the portions of the tubes 20 at the lower part of the front wall I I towardsthe rear wall I3. 'Combustionair is supplied through a windbox 36 surrounding the burner nozzles. The portions of the remaining furnace wall tubes from the floor level thereof to'a; level above the burner level are covered with metallic studs and refractory to promote combustion of the pulverpassage 40 of uniform width between the side walls l2. The passage 40 is further defined by a fiat roof 4| forming a continuation of the roof l5 and a rearwardly inclined floor 42 having a short vertical section 43 at its forward end connecting with the top of therear wall section I'I, so that the passage 4|i progres-sively decreases in height rearwardly of the unit. :m the passage 40 is 'arranged the, convection heated steam generating and superheating surface of the unit consisting of a bank of vertical steam generating tubes 44 and a. three-section pendant type superheater 45. The steam generating tubes 44 are connected at their-lower ends to the transverse header 3| and have their upper portions extending through the roof |5.and connected to the steam and water drum 25. The 'rearmost section ofthe superheater 45 is connected to the steam space of the.

' tion, passage 40; below the superheater is lined with block-covered water tubes 42"- having their lower ends connected to the drum 3| and their upper ends to a transverse header 41. As shown in Fig. 2, the superheater passage 51 does not extend the full width of the unit but is separated by a water cooled bailie 48extending parallel to the side wallls I2 fromthe front end of the s uperheater to the rear end of the unit, to provide a superheater by-pass passage 49- Rows of steam generating tubes. 50'are positioned in the by-pass fpassage with their lower ends connectedto the header 41 and their upper ends connected to an upper header and the steam and water drum 25. V The remaining portion of the gas flow path is.

formed by a vertical passage 5| extending downwardly at the rear end of the unit and occupied by a plurality of vertically spacedserially connected groups of horizontally arranged'multiple looped transverse economizertubes 52. The bailie 48 has an extension in the passage 5| to provide a continuation of the superheater andby-pass passages.

across the economizer tubes,and dampers 53 are positioned 'therebelow to regulate the new of the heating gases through these passages.- The gases on leaving the economizer tubes turnfabove an ash hopper 54 andflow-upwardly through a tubular air heater 55 and induced draft fan to'a suitable stack connection.. j a I I .1 Steam generating units of the general character described are usually designed to operate in the following manner:

Pulverized fuel and combustion air are intro- -duced through the lower portion of the front wall H and the pulverized fuel burned in suspension in the lower part of the furnace chamber Hi, the supply of combustion elements and-heat absorbing surface in the lower section of the furnace chamber being proportioned to maintain a normal mean temperature therein, particularly adjacent the floor l4, above the fuel ash fusion temperature, whereby the burning fuel will'be deposited on the floor 14 as'molten slag and withdrawn through the slag outlet 31. The gaseous'products of combustion and any ash particles in suspension therein are designed to pass upwardly through the furnace chamber, and are cooled during'their passage by the radiant heat absorption of the furnace wall tubes. Due to the furnace draft efiect'the high temperature heating gases and suspended ash particles move laterally in the upper part of the furnace into the gas passage 40. The temperature of the gases and suspended ash particles is re- 44 to a value designedly below the fuel ash fusion the ash particles separating from a 6 tion of the heating gases entering the tube bank One of these features is embodied in-thesteam generator illustrated, i. e., theconstruction and proportioning of thefirst bank of convection heated tubes in the gaspass .40, and particularly, the relative proportioning of the effective-heat absorbing area. thereof, tube height, andgas pressure drop'or draft loss thereacross. Bywayof example and notof limitation, the tube bank 44 is shown as composed of 95 tubes 4" O. D. arranged in flve'rows spaced 12" apart. The tubes in the two rearmost rows are spaced on 9'5 centers while the tubes in the forward rows are on 18", centers. The rearmost row is spacedji' from the adjacent row of superheater tubes. The effective length of the tubes in the bank, 1. e.,"from' the drum 3| to the superjacent roofportion is 29'6", the width ,duced by their flow across the bank of screen tubes of the furnace chamber in this plane being 2011. The expected performance of thisunitoperating at a'capacity of 345,300 lbs. of steam per hr. with a straight through flow and no recirculation of temperature before they contactwith the more closely spaced superheater tubes'in the superheater passage 41. With pulverized fuel-having the designed ash fusion temperature characteristics, the ash particles should be in a dry condislagging problems in the superheater. Any ash depositing on' the superheater tubes can therefore be blown off and down the inclined floor 42 to the furnace chamber. The gases flowing through the superheater passage'il or by-pass passage 49, or both, flow downwardly across the economizer tube banks 52 and thence upwardly through the tubular air heater 55 to the induced draft fan and stack connection. Under proper operating conditions therefore the ash in the fuel separated during combustion or during the subsequent pas sageof the heating gases will be removed as a molten slag through the slag outlet 31 at the furnace bottom or as a dry ash through the ash hopper'54, and the heating gases will flow through the gas passage 40 in a single pass flow over the tube bank 44, superheater tubes 45, economizer tubes 52 and air heater 55 successively. I When, however, pulverized fuel having a lower fuel ash fusion temperature than thatfor which the unit is designed is used, or when the rate of heat abculating a substantial portion of the heating gases leaving the furnace chamber after their temperature has been reduced by their passage across the boiler slag screen tubes 44,- without requiring special circulating fans or other mechanicaldevices for this purpose and without materially adding to the fan power requirements of the unit. The gas recirculation effect, and consequently the proportion of the gases recirculated, I have found to be dependent upon several features which Jointly contribute to the recirculation of a large proporcharacter described without resultgases tothe furnace chamber would have a. gas temperature leaving the'furnace chamber of 2320 F. and leaving the tube bank 44 of 2036 F. Under such conditions the gas pressure drop across the tube bank would be approximately .025 in. H2O.

tion on reaching the superheater and present no Thus an appreciable temperature reduction in the heating gases flowing across the tube bank would .40 I differential to exist.

occur, yet the draft loss would be of a low order. With a draft loss and heat absorption of this 'order, the temperature difference of the ases'at opposite sides of the tube bank 44in conjunction with the substantial height of the tube bank produces a draft differential at an elevation near the a lower end of the tubebank sufficiently great to establish and maintain a gas flow forwardly through the tube bank at this position. The forward flow, or recirculation, of the gases will con- .tinue as long as there is a tendency for this draft The described thermal siphon action thus effects a return-of some of the gases after being cooled by their'passage'through the tube bank 44;

Another feature of the illustrated construction which substantially promotes the gas recirculation-effect is the relative arrangement of the fuel burners 35 and rear furnace arch or roof I6, As diagrammatically illustrated in' Fig. 3, the fuel burners discharge downwardly and rearwardly. towardsthe'rear wall l3 so that the furnace gases generated will sweep upwardly along the rear wall and be directed forwardly and upwardly by the forwardly. inclined arch I6 and at a relatively high' velocity when leaving the forward end of the arch. A correspondingly low static pressure condition will thus be created in this area which will cause an additional draft effect on the gases fiowingforwardly across the lower part of the tube bank 44. The ascending furnace gas stream thus acts as. a fluid jet to produce entrainment of the recirculating gas flowing forwardly through the tube bank, as indicatedin Fig. 3. A mixing of the lower temperature recirculated gases with the ascending high temperature gases takes place, lowering the average temperature of the gases entering the furnace side of the tube bank. I The gas recirculation effect is also aided by the direction given to the gases on leaving the rear arch l6, 1. e., upwardly along the upper part of the front wall II and roof [5, The inertia of aaiaosa 7 g that these gases. naturally tend to enter the unper part of the tubejbank." I y and a total steam temperature of 900 F. High:

ve'locity thermocouple measurements :of .gas "tem-.

peraturestaken at different points around the tube bank 44' while the unit was-"operating at a capacity of approximately 350,000. lbs: perl hr. are

indicated on Fig. 3. The direction of gas .flowat different points in this area was also 'deter- I mined by test. There was found :toi'be a very substantial recirculation .of the gases, as 'indi- 'cated by the'arrowsinlFig, 3. Bycalculating the temperature drop through the upper portion of the tube bank .over which the gases were found to be flowing rearwardly with various weights of flue gas, it was .estimated from observations and calculations that approximately 100% of the furnace gases generated were being recircu lated at this load, Anadditionaliquantity of gas partly enters the tube .ban'k. and-then. returns to the furnace chamber with the recirculatedgases,

in front of the lower end ofthe tube bank relative to that at the lower rear part thereof. v 1 The described gas recirculation is highly belie-- ficial in regard to ash and slag accumulations.

The rearwall I 3, floor 14 and rear part of the lower side walls have slag deposited thereon in a fluid state. The slag fluid'zon'e extends to the chamber to bedeflacted at a relatively, high velocitypast ,and in a direction away from the- '25' accounting for the higher gas temperature values per .part of'the furnace chamber, and causing the gaseousproducts of combustion vflowing upwardly through the lower part of the furnace chamber to be deflected mainly. along the side of said furnace chamber opposite saidvtu'be bank.

g .2. Themethod of burningya fuel which c0m-.

prises introducing the fuel and combustion air into a furnace chamber and burning the same in the lower part of saidchamber, directing the gaseous products of combustion in the upper part of the furnace chamber intovcontactiwith a vertically'arranged bank of fluidheating tubes in the Y fumace chamber outlet and ofa height, draft I close and heat absorbing capacity "proportioned to establish and maintain a thermal siphon effectrproviding a circulation of a portion of the gaseous products of combustionin the furnace chamber outlet in a reverse direction-across the lower :portion'of said tube bank and into the upper part of-the furnace chamber, and causing the gaseous products of combustion flowing upwardly through the lower part of the furnace lower portion of the {tube bank toypromote the prises introducing a'stream of the fuel and combustion air into a vertically elongated furnace chamber and burning the same in suspension in front end of the arch is, as indicated in broken lines in Fig. 3, but the slag .fluid zone sharplyends .at the lower end of thevertical wall portion l7. The recirculated gases thus cause thesuspended slag particles to pass rapidly through the I the lower part of said chamber, directing the gaseous productsof combustion in the upper part of thefurnace chamber into contact witha vertically arranged bank of fluid heating tubes in the yfurnace chamber out-let. and of a height, draft sticky stage before reaching the convection heated surface. 'I'hatbare tube surface is maincumulations and that only dry ash deposited on the sloping floors below the'super'heater and over the header 3 I, from which it was swept back into.

the furnace chamberby the gases. 1

Fluid heating apparatus utilzing the described: features of my discovery will thus have an effective use ofthe furnace chamber volume and a mean heating gas temperaturefat the furnace: chamber outlet which will avoid overheating of burning the same in suspension inthe lowerpa'rt' of said chamber, directing the gaseous products of combustion in theupper part of thefurnace. chamber laterally into contact with a vertically arranged bank of fluid heating tubes in the furnace chamber outlet and of a substantial height; and sufl'iciently low draft loss and high heat absorbing capacity to establish and maintain at substantially the maximum designed continuous rate of operation a thermal siphon effect providing a circulation of a substantial portion of the aseous products of combustion in the furnace chamber outlet in a reverse direction across the 4 tained' clean by the downward sweep of the cooler jrecirculated gases. It was-also found that the lossv and [heat absorbing capacity proportioned to establish and maintain athermalsiphon effect 1 providing a circulation of a portionof the gaseous products of combustion .in the furnace chamber outlet in a reverse direction across the lower portion of saidtube bank and into the upper tube bank 44 was maintained clean of slag acpart of the furnace chambenand directing the part of the furnace chamber at a relatively high velocity and at an angle to the direction oi re-,

- circulated gas flow across the lower partof said tube bank and rev'ersely of the direction of entrance of the fuel and air stream to promote the' gas recirculation effect and mixing of. the recirculated gases.

'4. The method of burning a fuel which comprises introducing the fuel and combustion air into a vertically elongated; furnace chamber and burning the same in suspensionin the lower'part of saidv chamber; directingthe' gaseous products of combustion in the upper part :of'the furnace chamber into contact with a vertically arranged bank of fluid heating tubes of a substantial height andsufiiciently low draft loss and high heat ab sorbing capacity to establish and maintain a thermal siphon efie'ct'providin'g a circulation of a substantial portion ;of 'thegaseous products of combustion in-a reverse direction across the lower portion of said tube bank'and'i-nto'the upper part ofthe furnace chamber, and directin'gthe gaseous products of combustion leaving the lower part of the 'furnace chamber at a relatively high .velocity'and in a direction adjacent the lower portion of said tube bank reversely of the entering fueland combustion air and of "the gas flow across the upper part of said tube hank to promote the gas recirculation effect and mixing of the recirculated gases.

lower portion of said tube bank and into the up- 5. The method of burning a pulverized slag,-

forming fuel which comprises'introducing the fuel and combustion air into a furnace chamber and burning the same in suspension in the lower part of said chamber under a normal mean temperature therein above the fuel ash fusion temperature, removing molten slag depositing in the bottom of the furnace chamber, and directing the gaseous products of combustion in the upper part'of the furnace chamber laterally into contact with a vertically arranged bank of fluid heating tubes in the furnace chamber outlet and of a height, draft loss and heatabsorbing capacity proportioned to establish and maintain a thermal siphon effect providing a circulation of a major portion of the gaseous products of combustion at the maximum continuous designed capacity in the furnacechamber outlet in a reverse direction-across the lower portion of said tube bank and into the upper part of the furnace chamber sufficient to reduce the average gas temperature at the furnace chamber outlet below the fuel ash fusion temperature. v

6. The method of burning a pulverized slagforming fuel which comprises introducing a stream of the fuel and combustion air into a vertically elongated furnace chamber and burning the same in suspension in the lower part of and burning the same in suspension in the lower part of said chamber under a normal mean temperature therein above the fuel ash fusion tern perature, removing molten slag depositing in the bottom of the furnace chamber, directing the gaseous products of combustion in the upper part of the furnace chamber laterally into contact with a vertically arranged bank of fluid heating tubes in the furnace chamber outlet and having a substantialheight and high heat absorbingcapacity proportioned relative to draft loss therethrough' to establish and maintain a circulation of a portion of the gaseous products of combustionin the furnace chamber outlet in a reverse direction across the lower portion of said tube bani: and into the upper part of the furnace chamber, and causing the gaseousproducts of combustion flow ing upwardly through the lower part of the funnace chamber to be deflected at a relatively high velocity and in a direction away from the lower portion of said tube bank tending to promote the gas recirculation effect and mixing of he recirculated gases.

' 9. The method of burning a pulver zed slagforming fuel which comprises introducing the fuel and combustion air into a vertically elongated said chamber, directing the gaseous products of combustion in the upper part of thefurnace chamber laterally into contact with a vertically arranged bank of fluid heating tubes in the furnace chamber outlet and of a substantial height, low draft loss and heat absorbingcapacity proportioned to establish and maintain a circulation of a substantial portion of the gaseous products of combustion in the furnace chamber outlet in a reverse direction across the lower portion of said tube bank and into the upper part of the furnace chamber, and directing-the gaseous products of combustion leaving the lower part ofthe furnace chamber past the lower portion of said tube bank opposite to the directionof the entering stream of fuel and combus'tion'air to promote the gas recirculation'eifect and mixing of the recirculated gases. I

'7. The method of burning a pulverized slagforming fuel which comprises introducing the fuel and combustion air into a vertically elongated furnace chamber and burning the same in suspension in the lower part of said chamber under a normal mean temperature therein above the fuel ash fusion temperature, removing molten slag depositing in the bottom of the furnace chamber, directing the gaseous products of combustion in the upper part of the furnace chamber laterally into contact with a vertically arranged bank of fluid heating tubes in the furnace chamber outlet and of a substantial height and sufficiently low draft loss and high heat absorbing capacity to establish and maintain a thermal siphon effect providing a circulation of a portion of the gaseous products of combustion in the furnace chamber outlet in a reverse direction across the lower portion of said tube bank and into the upper part of the furnace chamber sufficient to reduce the average gas temperature at the furnace chamber outlet below the fuel ash fusion temperature, and causing the gaseous products of combustion flowing upwardly through the lower part of the furnace chamber to be deflected towards and along the side of said furnace chamber opposite said tube bank.

8. The method of burning a pulverized slagforming fuel which comprises introducing thev fuel and combustion air into a furnace chamber furnace chamber and burning the same in suspension in the lower part of said chamber and under a normal mean temperature therein above the fuel ash fusion temperature, removing molten slag depos ting in the bottom of the furnace chamber, directing the gaseous products of combustion in the upper part of the furnace chamber laterally into contact with a vertically arranged bank of fluid heatingtubes in the furnace chamber outlet and of a substantial height and sumciently low draft loss and high heat absorbing capacity to establish and mainta n a thermal siphon efiectproviding a circulation'of a substantial portion'ofthe gaseous products of combustion inthe furnace chamber outlet in a reverse direction across the lower portion of said tube bank, and into the upper part of the furnacev chamber suflicient to reduce the average gas'temperature at the furnace chamber outlet below the fuel ash softening temperature, and directing the gaseous products of combustion leaving the lower part of the furnace chamber at a relatively high velocity and in asdirection adjacent the lower portion of said tu e bank reversely of the direction of the entering stream of fuel and combustion air to prom'ote the gas recirculation effect and mixing of the recirculated gases.

, 10. In a steam generating unit having a vertically elongated furnace chamber and a heating gas outlet at one side of the upper portion thereof,

a lateral gas passage opening to said gas outlet, a vertically arranged bank of steam generating tubes in said gas passage having a substantial height, low pressure drop and high heat absorbing capacity, and a group of closely spaced steam superheater tubes in said gas passage spaced from said tube bank, the method of burning fuel in.

said unit which comprises burning the fuel in the lower part of said furnace chamber, d recting the furnace gases in the upper part of the furnace chamber across the upper part of said tube bank, and causing the furnace gases flowing upwardly through the lower part of the furnace chamber to be deflected in a diverging direction adjacent the lower portion of said tube bank tending to promote a flow of heating gases downwardly between said tube bank and superheater tubes and reversely across the lower portion of said tube bank into the furnace chamber.

' furnace gases flowing upwardly throughthe lower 11. In a steam generating unit havin a vet-1 tically elongated furnace chamber and a heating gas outlet at the rear side of the upper portion thereof, a lateral gas passageopening to said gas outlet, a vertically arranged bank of steamgenerating tubes in said gas'passage having a substantial height, low pressure'drop and high} heat'absorbingcapacity, and a group of closely spaced steam superheater tubes in said gas passage spaced rearwardly of saidtube bank, the

, methodof burning a pulverized slag-forming fuel insaid'unit which comprises burning the fuel" in suspension in the lower part of said furnace chamber under 'a normal mean temperature therein above the fuel ashfusion temperature;

removing molten slag depositing in the bottom of the furnace chambendirecting the furnace gases in the upper part of the furnace chamber across part of a vertically, elongated furnace chamber having'a gas outletin one side of they upper partv thereof, directing the gaseous-productsof come bustion in the upper part of the furnacecham-' ber laterally into contact with a vertically arranged bank of. fluid heating tubes across the furnace chamber gaswoutlet and of a height, I

' draft loss andheat absorbingcapacity proporthe upperpart of said tube bank, and causing the part of-the furnace chamber to be deflected in a directiontending to promote a flow of heating gases downwardly between said tube bank and superheater tubes and reversely across the lower portion ofsaid tube bank into the furnace chama ber to an extent sufiioient to reduce'the average tioned to establish and maintain a thermal siphon effect providing a circulation of a portionof the gaseous products ofcombustion in the furnace chamber outlet in a reverse direction, across the" lower portion of said tube bank and into the upper part of the furnace chamber,- and causing the gaseous products of combustion flowing up.- wardly through the lower part of they furnace chamber -tobe deflected atv a converging angle to the direction. of recirculated gas flow across the lower part of said tube bank ,to'promotethe gas recirculation effect-andmixing of the recirculated gases.

14.-The method of burning a fluid fuel con- .taining an incombustible ashwhich comprises in.-

temperature of the heating gases contacting withj said superheater tubes below the fuel ash soften-1 ingtemperature;

12..In a steam generating unit-having a vertically elongated furnace chamber and a heating; gas outlet at the rear side of the upper portion thereof, a lateralgas passage opening to said gas outlet, 'a vertically arranged bank of steam gen-l erating tubes in said gas passage having a sub-1 stantial height, low pressure drop and high heat absorbing capacity, and a group of closely spaced steam superheater tubes in said gas passage spaced rearwardly ofsaid tube bank, the method of buming a pulverized slag-forming fuel in said unit which comprises burning the-fuel in suspen-; sion inthe lower part of said furnace chamber,

under a normal mean t emperatu're therein. above the fuel'ash fusion temperature, removing molten slag depositing in the bottom of the furnace chamber, directing the furnace gases in the upper part'of the furnace chamber across the upper part of'saidtube bank, and directing the furnace;

gases leaving the lower part of the furnace chamber at a relatively high velocity and in a direction reversely of the direction of the entering stream 'of fuel and combustion air to promote a flow of heating gases downwardly betweensaid tube bank and superheater tubes and reversely across the lower portion of "said tube bank into the furnace chamber to an extent suflicient to; reduce the average temperature of the heating gases contacting with. said superheater tubes belowthe fuel ash softening temperature.

13. The method of burning fuel which com-v prises introducing and burning a fuel in the lower troducing andburning a fuelin the lower part. of a vertically elongated furnace chamber having a gas outlet in one side of the upper part thereof, removing (molten ash from the bottom of. the

furnace chamber, directingthe gaseous productsof combustion in the upper part ofthe furnace i chamber laterally into contact with'a vertically nace chamber to be deflected at a. converging angle to the direction of recirculated gas flow across the lower part of said tube bank to promote the gas recirculation effect and mixing of the recirculated gases. I I ,1 I 1 v PAUL R (EROSSIVLAN. REFERENCES men a The following references arev of record in the flle'of this patent:

' UNITED STATES PA'rEn'rs Number Name I Date 2,231,872 Bailey 8'; 8 ,1 Feb. .18, 1941 2,254,226 K0611 Sept. 2, 1941 2,252,061 Cassidy Aug, 12, 1941 2,263,433 Allah NOV. 16, 19 41 Certificate of Correction Patent No. 2,416,053. February 18, 1947;

PAUL R. GROSSMAN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 9, line 42, claim 6, before the word opposite insert in a direction; and that the said Letters Patent should be read with this correction thereinthat the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 28th day of September, A. D. 1948.

THOMAS F. MURPHY,

Assistant Uommissioner of Patents.

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