US2357302A

US2357302A - Method of and apparatus for burning fuel

Info

Publication number: US2357302A
Application number: US382263A
Authority: US
Inventors: Howard J Kerr; Fletcher James; George A Watts; Kooistra Lambert
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1941-03-07
Filing date: 1941-03-07
Publication date: 1944-09-05
Anticipated expiration: 1961-09-05

Description

Sept. 5, 1944.

H. J. KERR EIAL METHOD OF AND APPARATUS FOR BURNING FUEL Filed March 7, 1941 4 Sheets-Sheet 1 INVENTORS Howard J. K err, James Hefc/rer, BY George ,4.Wa zz:/Z0mberzM0/'stra ATTORNEY.

'Sept. 5, 1944. H. J. KERR ETIAL METHOD OF AND APPARATUS FOR BURNING FUEL Filed March 7, 1941 4 Sheets-Sheet 2 INVENTORS Howardl Kerr, James Fletch er, BY Gear 8/] Wait; f lambe/i Koo/515m ATTORNEY.

Sept. 5, 1944. H. J. KERR EIAL 2,357,302

METHOD OF AND APPARATUS FOR BURNING FUEL Filed March 7, 1941 4 Sheets-Sheet 5 s F! g. 5

To Expansion Chambers F 5 I J INVENTORS j Howard .7. A err, limes F/ezc/ver, 54 54 5 BY George/4. Waits f Zomba/t Koo/56m WATTORNEY.

H. J. KERR EI'AL 2,357,302

METHOD OF AND APPARATUS FOR BURNING FUEL 4 Sheets-Sheet 4 Sept. 5, 1944.

Filed March 7, 1941 INVENTORS Howard J Ker/,- -Jome: flats/ref,

BY Ge rqeAM mzs I lumber! A oo/stra Q A ORNEY.

Patented Sept. 5, 1944 UNITED V STATES, PATENT OFFICE METHOD OF AND APPARATUS FOR BURNING FUEL Howard J. Kerr. Westfield. N. 1., and James Fletcher, Akron, George A. Watts, Barberton, and Lambert Kooistra, Akron. Ohio. assignors to The Babcock & Wilcox Company, Newark, N. J., a corporation of New Jersey Application March I, 1941, Serial No. 382,263

The present invention relates in general to a method of and apparatus for burning ash-con of in a inolten condition .over a relatively wide range of furnace operation.

The various features of novelty which characterize the invention are a pointed out. with particularity in the claims annexed to and forming a part of this specification. standing of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of th invention.

Of the drawings:

Fig. 1 is an elevation of an experimental test installation constructed and operating in accordance with this invention; r-ziFig. 2 is a plan view of the apparatus shown in Fig. 3 is an enlarged view of the cyclone fur-- the lines 6-6 and 'I-'I respectively of Fig. 5; and

Fig. 8 is an enlarged view of a portion of the furnace wall shown in Fig 5.

In the experimental installation illustrated, a cyclone furnace constructed as hereinafter described has associated therewith suitable auxiliary apparatus for supplying controlled amounts of fuel and air to the furnace. The auxiliary apparatus includes an air compressor having an inlet pipe l2 and a discharge pipe I3 leading to a water heated air heater l capable of heating the air supplied to a relatively high temperature. The preheated air leaves the air heater through a damper controlled air duct l6 and is divided into primary, secondary, and tertiary air streams which enter the cyclone furnace as hereinafter described.

While various kinds of liquid. gaseous and solid fuels can be burned in the cyclone furnace illus- For a better undersemi-bituminous coals having an ash fusion temperature below 2800 F. and reduced by crushing or pulverizati'on to an aggregate or mixture having a particle size not over Solid fuel of this character has been referred to as granular" or granulated" fuel. The fines in the mixture passing through a 200-mesh screen should be between 3% and The minimum volatile content of the coal may vary considerably, ranging, for example, from 20% .for a coalhaving an ash fusion temperature of 2350 F. to for a coal with an ash fusion temperature of 2700 F. A certain percentage of fine in the mixture is desirable to aid ignition and promote combustion of the entering fuel, but an excessive amount is undesirable as the amount of ash leaving the furnace as fly ash is proportional- The larger the size of the coal particles, the less the amount of fly ash, but the higher the percentage of coarse particles, the higher the air velocity required to keep the particles in motion in the furnace. A considerattion of all of the factors involved make a coarse fuel mixture most desirable. For example, a desirable mixture for bituminous coals having about 11% moisture, 16% ash, 39% volatiles, and a heat value of 10,300 13. t; u. per pound as fired, would be 98-100% through a 4-mesh screen, 40-50% through 30-mesh, 10-18 through mesh, and 6-10% through a 200-mesh screen.

In the experimental installation illustrated, coal of the described character is supplied to a bucket elevator for delivery to a bin l8 above the furnace level. The bin supplies a coal feeder I! which discharges the coal at; a regulable rate into a discharge pipe 2|. The lower end of the pipe 2| extends concentrically into the rounded end of a damper controlled primary 'air pipe 22, which is connected to the duct l6, as shown in Figs. 3 and 4. The lower end of the pipe 2| opens concentrically into the upper end of a pipe 23 connected to the bottom of the air pipe 22. with this arrangement the primary air stream sweeps around the lower end of the pipe 2| at a high velocity and mixes with the descending coal stream, creating an intimate mixture of the air and coal particles in suspension in the pipe 23. The lower portion of the air duct It extends laterally to the cyclone furnace l0, the lateral section tapering in width towards the furnace and terminating in a relatively narrow vertically elongated nozzle secprimary air-coal spout 28- positioned in spaced relation within the tapering end of the duct IS. The spout 28 tapers in width towards its discharge end which i adjacent to the discharge end of the duct nozzle section 24. An extension arm 29 on the flange 21 permits the spout 28 to be manually shifted laterally about the axis of the flange 27 relative to the duct nozzle section.

In accordance with the invention, the cyclone furnance i is of axially elongated substantially cylindrical form and arranged with itsaxis vertical. The furnace has a substantially cylindrical casing 30 with a central gas discharge opening in its top and a slag outlet in its bottom. The furnace chamber 31 is defined by refractory faced fluid cooled walls enclosed by the casing and consisting of a single water tube 32, or a plurality of serially connected superposed tube coil sections, arranged in a single coil having tube portions bent to provide the various openings required in the furnace chamber. The inner side of the tube portions in the boundary walls of the furnance chamber have metallic studs 34 thereon, as shown in Fig. 8, covered by a layer of suitable refractory 85, such as plastic chrome ore. A layer of heat insulation material 36 is arranged between the tube coil and casing 30. The refractory-faced circumferential wall of the furnace chamber is made as far as possible of uniform circular cross-section to avoid interference with the flow of solids and gases through the chamber.v This is especially important because of the helical flow path taken by the solids and gases upwardly along the circumferential wall of the chamber in accordance with this invention.

The combustion air and fuel inlet openings are advantageously located in the circumferential wall of the furnace chamber at positions where there will be minimum interference with the whirling movement of the main body of gases moving upwardly in the chamber and the downward passage of slag in and discharge from the chamber. As illustrated in Figs. and 6, adjacent parallel tube portions in the circumferential wall are bent into spaced overlapping groups of 180 bends 38 to define a narrow vertically elongated inlet port 89 therebetween which is tangentially arranged to the outer end of a section 40 of the circumferential wall shaped in the form of an involute curve extending 360 before meeting the furnace chamber diameter. The bottom of the inlet 39 is arranged a sufilcient distance above the slag outlet of the furnace chamber to avoid excessive cooling of the slag on the furnace bottom by the entering air and fuel stream. The nozzle section 24 of the duct [5 and the air and fuel spout 28 extend into the inlet 89, as shown in Fig. 6.

With the described arrangement a narrow vertically elongated stream of intimately mixed preheated primary air and reduced fuel particles will be discharged at a relatively high velocity from the fuel spout 28 through the inlet 39. The entering primary air-fuel stream will be completely surrounded in the arrangement shown by a stream of preheated secondary air entering at a high velocity through the nozzle section 25- The combined fuel and air streams sweep horizontally along and in contact with the inner refractory faced surface of the circumferential wall, the particular involute configuration of this portion of the wall causing the moving stream to be wholly within the-minimum diameter of the furnace chamber after completing substantially one revolution, so as to minimize interference with the incoming streams from the inlet 39. The furnace draft and the displacing action of the subsequently entering fuel and air from the inlet 39 causes the combined stream to follow a helical fiow path upwardly along the circumferential wall after their initial circuit of the furnace chamber is completed.

A tertiary air supply isdelivered to the furnace chamber from the air duct l8 through a damper controlled branch connection 41 and an air inlet 4| located in the circumferential wall at a position above and angularly spaced from the inlet 29. The inlet ll is formed in the same manner as the inlet 28 by spaced groups of overlapping bent tube portions 42 and a. tapered nozzle section 42 at the end 01' the branch duct 48 therebetween. The inlet II is narrower and of less height than the inlet 38, and also arranged tangentially to the outer end of a section 44 of the circumferential wall in the shape of an involute curve simflar to that of the wall section as shown in Fig. '7. The tertiary air port "is angularly spaced approximately 120 in advance of the inlet 39 relative to the direction of rotation of the gases in the furnace chamber. Some of the tube portions in the section 40 are bent to form an inclined inspection hole 45 in the circumferential wall.

The circular bottom or fioor of the furnace chamber is of concave refractory, except for a downwardlytapering extension-of the tube coil at its periphery which is studded and covered with a layer of suitable refractory. The refractory floor construction tends to increase the floor temperature and thus aid in maintaining the slag thereon in a fluid condition. The floor has a central downwardly flaring slag discharge opening 52 therein defined by a metal cone 5! and leading to a closed slag pit (not shown). The slag opening 5| can be closed or opened when desired by manually positioning or manipulating a tapered plug or gig 53 mounted on the end of an operating bar 54 which is fulcrumed on a supporting bar 54 when the plug is in the slag 40 hole.

The tube coil is continued at its upper end in a spiral to define an annular flat top section 55 and a depending concentric downwardly tapering throat 58 which forms an upwardly flaring gas outlet 51 from the furnace chamber. The tube portions defining the throat are studded on both sides and covered with a layer of refractory. The throat extends downwardly a substantial distance into the furnace chamber, terminating with a minimum diameter about one-third that of the furnace chamber at a level above the top of the tertiary air inlet 4|. The described throat construction results in the formation of a downwardly flaring annular pocket 58 at the upper end of the circumferential wall. open at its lower side and due to which the whirling furnace gases entering the pocket will be contracted radially and have their direction of movement axially reversed before reaching the gas outlet 51.

The throat is extended upwardly above the furnace roof in a straight fluid cooled section formed by a bare tube coil 6|. The upper end of the coil 6| is connected to the lower end of the throat coil, as indicated in Fig. 5. The lower end of the coil 8| and of the peripheral fioor coil thus form opposite ends of a steam generating section, water being supplied at the bottom and a mixture of steam and water discharged at the top. This circuit can be connected into the circulation system of a steam generator for which the cyclone furnace forms a source of heat. In the experimental illustration illustrated however, the furnace gases leaving the throat pass out through a stack arranged above and forming a continuation of the throat extension. The hot and removal of a high percentage of the recoverable ash content of the fuel in a molten condition while in the combustion zone. The cyclone furnace construction described is especially designed and particularly adapted for carrying out this fuel burning method, more particularly described hereinafter.

The total air for combustion is delivered to the air heater by the compressor II and preheatcd to a relatively high temperature. The preheated air passes into the furnace as separate high velocity streams of primary, secondary and tertiary air, the high temperature given to the air acting to speed up ignition of the entering fuel, and the high velocity and tangential entry of the air maintaining the desired centrifugal effect. The total air supply is directly proportioned to the amount of fuel supplied to the furnace, the fuel-air ratio maintained being such that the total air supplied' is notmore than and preferably less than 15%, in'excess of the theoretical combustion air requirements. The air-fuel ratio may be varied to some extent, a lower excess air ratio being desirable at high fuel rates to increase the adiabat c furnace temperature and facilitate slag tapping.

About 40% of the air supplied is used as primary or carrier air, which intimately mixes with the coarse fuel mixture from the pipe 2| and discharges through the spout 28. The vertically elongated primary air-fuel stream is normally surrounded by a high velocity stream of secondary air from the air nozzle 24 as it enters the furnace and the combined streams flow as a single stream horizontally along and in scrubbing contact with the involute curved wall of the furnace. At starting, an auxiliary oil or gas burner is inserted through one of the furnace wall openings and after a short period of operation user to ignite the solid fuel entering the furnace chamber. The air-fuel stream flows at a high velocity along the involute curved circumferential wall section 40 and the exposure of the fuel particles to the high furnace temperature causes them to pass rapidly through the several stages of combustion. Due to the location of the furnace gas outlet and the continuous tangential entry of air and fuel to the furnace, the burning fuel stream moves upwardly along the circumferential wall in a helical path.

The rapid combustion of the fuel particles results in an early release of the ash content thereof, and due to the centrifugal effect thereon, the sep arated ash is deposited on the furnace walls, and particularly the circumferential wall, resulting in the formation of a thin layer or film of molten ash or slag which adheres to the refractory surface of the walls and provides a sticky surface to which fuel particles in the whirling fuel-air stream will adhere and be burned thereon. the scrubbing action of the contacting gaseous stream aiding the rapid combustion of the particles.

The combustion of the fuel particles in suspension and on the walls is expedited by the introduction of the high temperature tertiary air through the port 4| (ill in a narrow high velocity 15 stream moving in the same direction as and gradually merging with the rapidly rotating air-fuel stream. The tertiary air stream intimately mixes with the rising whirling stream of burning fuel, air and products of combustion and passes upwardly therewith in the helical path of flow. Combustion of the remaining fuel particles in suspension approaches completion as the rotating burning stream reaches the upper end of the furnace chamber. With the described method of fuel and air admission to the furnace chamber and the furnace chamber construction described, the normal mean temperature therein can be easily maintained over a relatively wide range of operation substantially above the fuel ash fusion temperature.

Due to the location and configuration of the throat 56 the whirling stream is forced to move inwardly and downwardly to reach the gas outlet 51. This relatively abrupt reversal in direction of axial movement of the burning stream results in a further mixing of air and unburned combustible, effecting completecombustion of substantially all of the remaining fuel particles. This change in direction also results in an ash or slag particles in suspension being thrown out of the whirling stream and deposited on the furnace roof or outer side of the throat. The incompletely burned fuel particles will be retained in the annular pocket, either partly embedded in the slag layer on the walls thereof or moving around in the gases therein. The particles are thus agitated and scrubbed by the gases until all of the combustible is consumed and the ash content released.

The slag coating on the furnace walls rapidly reaches an equilibrium thickness which is dependent upon the velocity of the contacting furnace gases, the rate of heat absorption in the furnace walls, the ash fusion temperature, and the furnace chamber temperature. Additional slag deposited on the walls will flow downwardly thereon to the floor 50, the slag collecting on the floor discharging through the slag outlet 52.

With a cyclone furnace construction of the character described, i. e. with the furnace gas outlet at its upper end andthe slag outlet in the bottom. and the fuel and air for combustion entering horizontally in tangential streams, it is essential that the slag flowing down the furnace walls and on the floor should not interfere with the entering fuel and air streams. For this reason, the inlet port 39 is located a suflicient distance above the slag hole to avoid any slag accumulation on the furnace floor reaching the inlet if the slag hole should be closed. Any slag depositing in the inlet port would cause the flow resistance therethrough to increase and correspondingly reduce the air flow. Such accumulations in the port or on the circumferential wall would also tend to disrupt the air and fuel flow path in the furnace and destroy the desired high velocity whirl. With the slag flow mainly down the minimum diameter portions of the circumferential wall and the air and fuel streams entering the furnace chamber tangentially to an involute wall sector extending substantially 360, the entering streams have an opportunity of gradually merging with the whirling stream in the furnace chamher. top and downwardly along the inner side of the incoming streams in a shell-like formation, is minimized. The division of the combustion air into a stream of primary and secondary air en- The formation of slag eyebrows over the mary air, 2850 lbs. of

I had a velocity B. t. u. per cu.

tering with the fuel, and a stream of tertiary air entering at a level substantially above the other stream, aids in maintaining the helical flow path of the furnace gases and suspended particles upwardly through the furnace chamber. It has been found that the temperature in the lower part of the furnace chamber and slag temperature on the furnace bottom can be increased when desired to increase the fluidity of the slag on the bottom by reducing or cutting ofl entirely the secondary air supply, so that most or all of the combustion air would then be supplied to the furnace chamber by the primary and tertiary air streams.

The relative proportions of the furnace parts, and particularly of the height of the furnace chamber, height and diameter of the throat, and dimensions of the inlet ports 39 and ll, relative to the furnace chamber diameter, play an important part in the operating characteristics of a furnace of this type, as described and claimed in the copending application of Erwin G. Bailey -et al., Serial No. 382,262, filed March 7, 1941.

By way of example and not of limitation, test runs of the. experimental installation illustrated gave the following values which indicate representative conditions maintained in accordance with the present method. The fuel burned was Ohio No. 8 coal, reduced to the following size:

Per cent Through A" mesh. 99.2 Through #16 mesh 75.2 Through #50 mesh 34.4 Through #100 mesh 17.6 Through #200 mesh 6.8 A proximate analysis of the coal showed Per cent Volatile matter 38.2 Fixed carbon 49.4 Ash 12.4 Moisture as fired 5.0

The heat content of the coal per pound as fired was 12,600 B. t. u. At a rating of 1650 lbs. of coal per hour, the total air flow was 17,500 lbs. per hour, which was divided into 7650 lbs. of prisecondary air, and 7000 lbs. of tertiary air per hour. The air temperature leaving the air heater was 445 F., and the furnace temperature maintained was approximately 3000 F. The primary air velocity was 34,800 ft. per min., and the tertiaryair velocity 25,900 ft. per min. The gases entering the stack of 30,500 ft. per min. The excess air was 17.4%, with a heat release of 680,000 ft. of furnace volume per hour. The net loss of fuel due to carryover was 0.63%. A gas analysis of the stack gases showed 15.1% CO2, 3.2% Oz, and CO. About 80% of the recoverable ash content of the fuel was recovered in the furnace and removed through the slag outlet in the furnace bottom and found to be free of combustible.

In another test run the same coal was burned at the rate of 919 lbs. per hour. In this run the total air flow was 10,500 lbs. per hour divided between 5600 lbs. primary air and 4900 lbs. tertiary air, no secondary air being used. The primary air velocity was 25,350 ft. per min. and the tertiary air velocity 14,800 ft. per min. The gas velocity at the stack entrance was 17,500 ft. per min. The air supply was equivalent to 21.4% excess air. The net loss due to carryover was 0.83% and the reat release rate 381,000 B. t. u.

per cu. ft. of furnace volume per hour. An

consequence.

analysis of the flue gases showed 14.5% CO2, 3.8% 02, and 0.0% CO. In both of the test runs described the furnace chamber was relatively clear, with no slag or coke accumulations of any No difficulty was had in tapping the slag throughout substantially the entire runs.

While in accordance with the provisions of the statutes we have illustrated and described herein the best embodiment of the invention now known to us, those skilled in the art will understand that changes may be made in the method and apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

We claim:

1. The process of burning an ash-containing solid fuel in a furnace chamber of substantially circular cross-section having a gas outlet at its upper end and a slag outlet at its lower end which comprises introducing the fuel in suspension in a stream of primary air at a high velocity directly into the lower part of the furnace chamber at a point tangentially'to the circumferential wall thereof, burning the fuel therein to ,maintain a normal mean temperature in the furnace chamber above the fuel ash fusion temperature, introducing air for combustion in a high velocity stream tangentially to the circumferential wall of the furnace chamber between the point of fuel entry therein and the chamber gas outlet, causing the fuel and air so introduced to move upwardly through the chamber in a helical path along the circumferential wall of sufllcient length to cause the release of ash in the fuel therein and the deposition of slag on the circumferential wall sufficient to form a sticky surface thereon to which fuel particles adhere the contacting gases, and withdrawing slag separated in the furnace chamber in a molten condition through the slag outlet.

2. The process of burning an ash-containing solid fuel at high rates of heat release in a furnace chamber of substantially circular cross-section having a gas outlet at its upper end and a slag outlet at its lower end which comprises introducing all of the fuel in suspension in a single stream of air at a high velocity directly into the lower part of the furnace chamber at a point tangentially to the circumferential wall thereof, burning the fuel therein to maintain a normal mean temperature in the furnace chamber above the fuel ash fusion temperature, introducing the remaining air for combustion in a single high velocity stream tangentially to the circumferential wall of the furnace chamber between the point of fuel entry therein and the chamber gas outlet, causing the fuel and air so introduced to move upwardly through the chamber in a helical path along the circumferential wall of sufficient length to cause the release of a high percentage of the ash in the fuel therein and the deposition of slag on the circumferential wall sufficient to form a sticky surface thereon to which fuel particles adhere and are scrubbed by the contacting gases, and withdrawing slag separated in the furnace chamber in a molten condition through the slag outlet. 7

3. The process of burning an ash-containing solid fuel at high rates of heat release in a furnace chamber of substantially circular cross-section having a gas outlet at its upper end and a slag and are scrubbed by mary air at a high velocity directly into the lower partwof the furnace chamber at a point tangentially to the circumferential wall thereof, burning the fuel therein to maintain a normal mean temperature in the furnace chamber above the fuel ash fusion temperature, introducing air for combustion in a high velocity stream tangentially to the circumferential wall of the furnace chamber between the point of fuel entry therein and the chamber gas outlet, causing the fuel and air so introduced to move upwardly through the chamber in a helical path along the circumferential wall of sufllcient length to cause the release of the ash in the fuel therein and the deposition of a layer of slag on the circumferential wall sufficient to form a sticky surface thereon to which fuel particles adhere and are scrubbed by the contacting gases, causing the ascending gases to be deflected at the upper end of the furnace chamber inwardly and downwardly before entering said gas outlet, and withdrawing the ash separated in the furnace chamber in a molten condition through the slag outlet.

4. The process of burning an ash-containing solid fuel at high rates of heat release in a vertically arranged substantially cylindrical furnace chamber having a gas outlet at its upper end and a slag outlet at .its lower end which comprises continuously introducing all of the fuel in a reduced condition in suspension in a stream of primary air at a high velocity directly into the lower part of the furnace chamber at a point tangentially arranged relative to an involute curved portion of the circumferential wall thereof, introducing air for combustion in a high velocitystream tangentially to an involute curved portion of the circumferential wall of the furnace chamber at a level between the point of fuel entry therein and the chamber gas outlet, causing the fuel and air so introduced to move upwardly through the chamber in a helical path along the circumferential wall of sufllcient length to cause the release of a high percentage of the ash in the fuel therein and the deposition of a layer of slag on the circumferential wall suflicient to form a sticky surface to which fuel particles adhere and are scrubbed by the contacting gases, and withdrawing slag separated in the furnace chamber in a molten condition through the slag outlet.

5. The process of burning bituminous and semibituminous coals at high rates of .heat release in a vertically arranged substantially cylindrical furnace chamber having a gas outlet at its upper end and a slag outlet at its lower end which comprises continuously introducing all of the fuel in a reduced condition in suspension in a horizontal stream of air at a high velocity directly into the lower part of the furnace chamber tangentially to the circumferential wall thereof while maintaining a normal mean temperature in the furnace chamber above the fuel ash fusion temperature, continuously introducing the remaining air for combustion in a single high velocity stream at a point tangentially to the circumferential wall of the furnace chamber and between the point of fuel entry therein and the chamber gas outlet, causing the fuel and air so introduced to move upwardly through the cham ber in a helical path along the circumferential wall of suflicient length to cause the release of a high percentage of the ash in the fuel therein and the deposition of slag on the circumferential wall sufiicient to form a sticky surface to which fuel particles adhere and are scrubbed by the contacting gases, causing the ascending gases to be deflected at the upper end of the furnace chamber inwardly and downwardly before entering said gas outlet, and withdrawing slag separated in the furnace chamber in a molten con dition through the slag outlet.

6. The process of burning bituminous and semi-bituminous coals at high rates of heat release in a vertically arranged substantially cylindrical furnace chamber having a gas outlet in its upper end and a slag outlet in its bottom which comprises continuously introducing all of the fuel in a reduced condition in suspension in a single high velocity stream of preheated air directly into the lower part of the furnace chamber at a point tangentially to an involute curved portion of the circumferential wall thereof and in a horizontal direction, burning the fuel therein to maintain a normal mean temperature in the furnace chamber above the fuel ash fusion temperature, continuously introducing the remaining preheated air for combustion in a single high velocity stream tangentially to an involute portion of the circumferential wall of the furnace chamber at a point between the point of fuel entry therein and the chamber gas outlet and in the same angular direction as the fuel stream, causing the fuel and air so introduced to move upwardly through the chamber in a helical path along the circumferential wall of sufficient length to cause the release of substantially all of the ash in the fuel therein and the deposition of a layer of slag on the circumferential wall, causing the ascending streams to be deflected at the upper end of the furnace chamber inwardly and downwardly before reaching the furnace gas outlet, and withdrawing the ash separated in the furnace chamber in a molten condition through said slag outlet.

'7. lflpparatus for burning a slag-forming fuel which comprises a combustion chamber of substantially circular cross-section arranged with its axis substantially vertical and definedby walls having an inner exposed refractory surface and fluid cooling means pioportioned to permit the maintenance of said refractory under a normal mean temperature in said combustion chamber above the fuel ash fusion temperature, means for introducing a high velocity stream of primary air and slag-forming fuel in suspension into the lower part of said combustion chamber at a point tangentially to the circumferential wall of said chamber, a gas outlet at the upper end of said combustion chamber, an air inlet arranged tangentially to said circumferential wall at a location intermediate the point of fuel entry and said gas outlet, means for introducing 'air for combustion at a high velocity through said air inlet in the same angular direction as the primary airfuel stream and so as to move upwardly in a helical path along said circumferential wall, and a slag outlet at the bottom of said combustion chamber.

8. Apparatus for burning a slag-forming fuel which comprises a substantially cylindrical'combustion chamber arranged with its axis substantially vertical, the circumferential wall of said chamber having upper and lower involute curved wall sections, each of said involute curved wall sections having a horizontal cross-sectional area enlarged relative to the horizontal cross-sectional area of the superjacent wall section, means for introducing a high velocity stream of primary air and slag-forming fuel in suspension into said combustion chamber at a point at the outer end of, said lower involute curved wall section, a gas outlet at the upper end of said chamber, an air inlet arranged at the outer end of said upper inwhite curved wall section at a location intermediate the point of fuel entry and said gas outlet, means for introdu g air for combustion at a high velocity through said air inlet in the same angular direction as the primar air-fuel stream and so as to move upwardly in-a helical path along said circumferential wall, and a slag outlet at the bottom of said combustion chamber below the point of fuel entry.

9. Apparatus for burning a slag-forming fuel which comprises a combustion chamber of substantially circular cross-section arranged with its axis substantially vertical and defined by walls having an inner exposed refractory surface and fluid cooling means proportioned to permit the maintenance of said refractory surface under a normal mean temperature in said combustion chamber above the fuel ash fusion temperature. means for introducing a high velocity stream of primary air and slag-forming fuel in suspension into the lower part of said combustion chamber at a point tangentially to the circumferential wall of said chamberand in a direction producing a helical path of travel thereof upwardly along said circumferential wall, a fluid cooled wall at the upper end of said combustion chamber including a downwardly projecting throat forming a gas outlet surrounded by an annular pocket, an air inlet arranged tangentially to said circumferential wall at a location between the point of fuel entry and said gas outlet, and a slag outlet in the lower part of said combustion chamber below the bottom of said fuel inlet.

10. Apparatus for burning a slag-forming fuel which comprises a substantially cylindrical com-- bustion chamber arranged with its axis substantially vertical and defined by walls having an inner exposed refractory surface and fluid cooling means proportioned to permit the maintenance of said refractory surface under a normal mean temperature in said combustion chamber above the fuel ash fusion temperature, the circumferential wall of said chamber having upper and lower involute curved circumferential wall portions, means for introducing a high velocity stream of primary air and slag-forming fuel in suspension into said combustion chamber at a point tangentially to said lower involute curved wall portion and in a direction producing a helical path of travel thereof upwardly along said circumferential wall, a gas outlet at the upper end of said combustion chamber, an air inlet arranged tangentially to said upper involute-curved wall portion at a location between the point of fuel entry and said gas outlet and angularly spaced from the point of fuel entry, means for introducing a high velocity air stream through said air inlet in the same angular direction as the fuel stream, and a slag outlet at the bottom of said combustion chamber below the point of fuel entry.

11. Apparatus for burning a slag-forming fuel which comprises a substantially cylindrical combustion chamber arranged with its axis substantially vertical and defined by walls having an inner exposed refractory surface and fluid cooling means proportioned to permit the maintenance of said refractory surface under a normal mean temperature in said combustion chamber above the fuel ash fusion temperature, means for introducing a high velocity stream of primary air and slag-forming fuel in suspension into said combustion chamber at a point tangential to an involute-shaped portion of the circumferential wall of said chamber and in a direction producing a helical path of travel thereof upwardly along said circumferential wall, a fluid cooled wall at the upper end of said combustion chamber including a downwardly projecting fluid cooled throat forming a gas outlet, an air inlet arranged tangentially to an involute-shaped portion of said circumferential wall at a location between the point of fuel entry and said gas outlet, a refractory faced bottom for said chamber below the point of fuel entry, and a slag outlet centrally arranged in the bottom of said combustion chamber.

12. Apparatus for burning a slag-forming fuel which comprises a substantially cylindrical combustion chamber arranged with its axis substantially vertical and defined by walls having an inner exposed refractory surface and fluid cooling means proportioned to permit the maintenance of said refractory surface under a normal mean temperature in said combustion chamber above the fuel ash fusion temperature, means for introducing a high velocity stream of air and slag-forming fuel in suspension into said combus- 1 tion chamber including a narrow vertically elongated fuel inlet arranged tangentially to the outer end of an involute-shaped portion of the circumferential wall of said chamber, a fluid cooled wall at the upper end of said combustion chamber including a downwardly projecting fluid cooled throat forming a gas outlet flaring towards its upper end and surrounded by an annular pocket, an air inlet arranged tangentially to the outer end of an involute-shaped portion of said circumferential wall at a location between the point of fuel entry and said gas outlet and angularly spaced from said fuel inlet, and a slag outlet at the bottom of said combustion chamber below the bottom of said fuel inlet.

13. The process of burning an ash-containing granular fuel in a vertically arranged combustion chamber of substantially. circular cross-section having a fuel inlet opening at its lower end and a gas outlet in its upper end, which comprises introducing a high velocity stream of air and fuel in suspension through said fuel inlet opening into the combustion chamber so as to whirl about a vertical axis and move upwardly along the circumferential wall thereof while burning the fuel to maintain a normal mean temperature in the chamber above the fuel ash fusion temperature, introducing combustion air in a stream entering at a high velocity tangentially to the circumferential wall of the combustion chamber intermediate the fuel inlet and gas outlet thereof and in the same angular direction as the whirling streamof fuel and air, causing the fuel and air streams so introduced to merge and move upwardly in the combustion chamber towards the gas outlet through a helical path along the circumferential wall of sufficient length to cause substantially