US1828839A - Furnace and method of operating the same - Google Patents

Description

ocnz?, 1931. C, E, HAWKE 1,828,839

FURNACE AND METHOD OF OPERATING THE SAME Filed Jan. 5, 1928 2Sheets-Sheet l l VIENTOZ Oct. 27, 1931. c. E. HAwKE FURNACE AND METHOD OF OPERATING THE SME Filed Jan. 5, 1928 2Sheets-Sheet 2 Patented Oct. 2?, 1931 4entran STATES PATENT ori-ica CLARENCE E. HAWXE, 0F METUCI'ENI', NEW JERSEY, ASSIGNOR T0 TIE-E CARBOEUNDUM COMPAN Y, 0F NIAGARA FALLS, EW YORK, A CORPORATION 0F SYLVANIA FURNAOE 4,AND METHQD 0I' OPERA'II'IN G THE SAME Continuation of application tiled lay 3, 1923, Serial No. 636,146. This application filed January 5, 1928.

Serial No. 244,734, and in Canada .Tune 1.5, 192,6.

This invention relates to furnaces and more particularly to the construction of furnace linings2 and constitutes a continuationof my apphcation Serial No. 636,146 filed :May 2, i923.

There is any increasing tendency to operate furnaces at extremely high rates of combustion. boiler furnaces are quite generally operated at from 300 to 500% of their rating. Whereas in old type furnaces the rate of combustion ranged generally between 15,000 to possibly 30,000 B. t. u./cu. ft./hi.,l the rate of combustion according to present practice is frequently materially above these values.

One serious dicu'lt increased rates of comgustion is the rapidity with which the brickwork of the furnace is destroyed. At these temperatures the slagorfused ash' flows over the refractory walls, reacts with them, and erodes them. Inthe burning of injected fuels, especially owdered coal, the destructive action at igh rates of combustion is articularly severe.

If high rates of com ustion are to bese- 'cured and the efficient burning of fuel is to be obtained, there should be a minimum excess of air supplied to the combustion chamber over that required -theoretically for complete combustion of the fuel, as the excess of 4air decreases the furnacetemperature and retards combustion. It is also desirable that the air be preheated, as the rate of reaction between the air and fuel increases v'ery rapidly with the increase in the temperature of the air. p

Iuleretofore furnace walls have generally been made of a -re clay refractory, which refractory is a relatively good heat insulator.l

To decrease the destructive influences within the furnace, the llame temperature is depressed by the. introduction of an excessive amount of air, which of course means a lower rate of combustion and a greater amount of heat carried out of the furnace in In' line with this general practicel that arises with the theform of a larger volume of ue gases.

According to the present invention I 'provide a fur'nace and a method of operating the same according to .which the 'slag or molten" ash is congealed as a thin protective film' on the exposed face of the refractory. This congealed film of4 slag of course does not erode the refractorybecause there .is no washing action againstthe exposed face of the refractory. Moreover, it prevents the hot furnace ases, fuel part1cles and catalytic materials rom coming directly into contact with the a and suiciently below. the ametemperature to cause the skin or slag immediately adj am centvv the furnace wallv to substantially congeal.

In order todo this Iuse a refractory which -is a relatively good conductor of heat-in place of 'a refractory which is a' relatively good heat insulator, By reason of thehigh thermal conductivity of the refractory employed the heat can be carried away from the expose face of the wall at a high rate. The amount of heat which can be carried away increases with the increase in the temperature drop .between the exposed face and the unexposed face of the wall or furnace lining.

' According to my invention, a cooling fluid, peferably air, is circulated back of the exposed face of the wall and therateof air circulation is varied as the rate of combustion in the furnace is varied. As the vrate of combustionin the furnace-is increased, the amount of v heat that has to be removed through the refractory to form the protective layer of slag increases, and the amount of air is therefore increased. l

In order to obtain the method-of operation against the unexposed surface ing which I have outlined, a heat'insulating refractory such as fire'clay cannot be used because the heat will not go through it lfast enou h. Thus a fire clay wall which is suiiicient y thick to have mechanical stability is such a good insulator of heat that it cannot be used for the practice of my method; No matter how fast or how much air is circulated of` a fire cla wal1,'the heat-cannot be removed fast enougli to cause the formation of the protective layer ofv ash on the exposed face of the refractory. Therefore my invention contem lates the use of a fairly good conductor o ,heat for the lining of the furnace chamber rather than theuse of a highly insulating material. In order to conserve the heat which is carried through the wall, the invention contemplates that the air used for cooling the wall shall be returned to the furnace chamber for supporting combustion therein. The air is thus preheated, and as stated above,

of the air increases the rate of combustion. On the other hand, the slag or ash which accumulates on the inside of the furnace-is a relatively poor conductor of heat. The.

protective film which forms on the surface of the refractory retards the conduction ofheat to. the refractory so that outside the rotecting film. the slag or molten ash ows freely over the protecting film. The protecting film thus automatically maintains itself relatively thin, but is always sufficiently thick to afford protection to the refractory vsall itself against the washing of the molten s ag.

As an example of a refractory that may be employed, the invention contemplates the use of silicon carbide refractories of high thermal conductivity. The thermal conductivity of silicon carbide` is many times that of fire clay, whereas the former is considered a heat conductor, and the latter is considered a heat insulator.

My inventionmay be'readily understood by reference to the accompanyin drawings which illustrate embodiments o? my invention and in which vFigure I, represents diagrammatically a cross-section through a boiler provided with la powdered coal burning furnace and conn structed in accordance with the present in: vention;

Figure 2 isa detail sectional view in the plane of; line II-II of Figure l;

Figure 3 is a detail view vshowing a slight modification of the burner and air intake; and

Fi re 4 is a section through a mechanically-stoked boiler furnace embodying my invention.

It will be understood that the constructions shown in the accompanying drawings are merely illustrative'of the invention, and that the invention is not confined to the particular type or design of apparatus illustrated, being the preheatapplicable to combustion chambers in. furnaces of various types and designs.

The arrangement shown in Figure 1 is a typical powdered coal burning installation applied lto a boiler furnace. In this figure, 2 is the pulverizer fan for feeding powdered coal from a hopper 3 through a feeder pipe 4 into the combustion space 5 of the furnace, `designated generally as 6. The boilers are indicated generally as 7. The wall of the furnace has an inner or lining portion 8 and an outer wall portion 9, and in the wall between the inner portion 8 and the outer portion 9 is an air circulating space or passage l0. The inner wall or lining is formed of a refractory material having a high co-eficient of thermal conductivity as compared with fire clay, as for instance, a good grade of silicon carbide refractory. Air is supplied to the space 10 by a fan 11 which forces the air through, a pipe 12`into an openin at 13 communicating with the s ace 10. The air which is forced through t e space 10 4is discharged into a pipe 14v that communicates with a chamber 15 surrounding the turbulent burner 16 at the end of the pipe 4, the arrangement being such that the air which is. forced through the space 10 between thesilicon carbide walls charged with the fuel into the combustion chamber. The position of the deiecting cone 18 of the burner may be adjusted by moving the adjusting member 17, thus controlling the shape and position of the flame. Air may, if desired, be introduced through member` 17'.

In the modification shown in Figure 3 the outer 'shell of the furnace is designated 9 and the-inner wall of silicon carbide is designated 8 Between the two wall portions is a space 10 and there are one or more openings 16' from which air may flow from the space 10 into the combustion chamber. With this arrangement the air which circulates between the inner silicon carbide Wall and the outer shell of the furnace is drawn vdirectly into the combustion space without the use of pipes. ,The opening 16" may be adjacent or remote from'a fuel feedin pipe, such as 4.

In the operation of the urnace, the fuel is burned in the usual manner. In order to secure mostl favorable conditions of combustion and prevent chilling of the iiame, the- 8 should be u temperature of the inner walls maintained -at 400 C. or above. This condition must be maintained over a considerable area of combustion space. At the same time the walls must not be allowed to become too` hot. The effect of heat plus the ash is to destroy the walls 'by softening and erosion.

4With the presentl mvention air is forced through the cooling space at a higher velocity than when t-he furnace is operating under a light load.` Due to the good heat conducting properties possessed by silicon carbide, the heat can be conducted away from the exposed surface of the walls to t-he extent necessary to maintain the temperature of the exposed face below the congealing point of the ash and slag so that ash and slag coming into contact with these walls will be cooled enough to pre- Diff vent softening and erosion of the walls by the slag. At the same time the temperature of the walls m'ay be kept sufficiently high-to prevent the flame from being chilled by contact therewith, so that eiiicient conditions of combustion are maintained. Variation in the wall temperature may be controlled by varying t-he volume or velocity 0f air or other cooling medium circulating therethrough. This may be done, for instance,by suitably varying the speed of the fan which circulates air through the cooling passages.

Generally speaking, the cooling of a silicon carbide wall 4.1/2 inches thick or less is much more nearly proportional to the velocity of the cooling air over the surfaces than it is with `fire clay or other poorly conducting refractory walls of similar thickness. Where all of the air used in combustion is ulled or forced through furnace walls so t at it is preheated, a method of regulation is secured. As the temperature of the furnace goes up, the rate of combustion is increased. This means that an increased amount of air must be suppliedto the combustion chamber so that there is an increased flow of air through the space l0 and the walls are cooled at a higher rate by reason of the greater volume of air Howing through the walls. The sys'- tem is, under these conditions, nearly `self regulating.

The following lfigures secured on actual Walls of fire clay and of ,'silicon carbide with the furnace in each case held at 1300 C. clearly demonstrate the practicability of temperature control with a silicon carbide Wall or a wall composed of other suitable refractorics, and the lack of such possibility of control with lire clay even though the test wall of fire clay was only half the thickness required for commercial installations and hence was more amenable to control. The figures hereinafter given are for silicon car bide and clay walls having a thickness of 41/2 inches, whereas a clay wall in order to be commercially practical must have a thickness of nine inches or greater.

Cooling air 'velocity Across wall-feet per sec.; 10 20 30 40 50 60 Temp. hot tace of wall- SlC 1207 1170 1140 1112 1097 1090 Clay 1268 1287 1265 1263 1259 1252 Rise in temp. of cooling air- SC 187 120 80 57 46 39 Clay 57 37 23 15 11 l0 The degree to which the Wall is cooled is evidently roughly dependent upon the rate atpwhich the heat passes through it. Measurements made with air flowing at the rate of forty feet per second as an example show the following sie SiC Clay Ratio-- Clay B.t.u./sq.f0./hr.znmugnwa11 32,100 5,100 0.a

e'rence in C. tween furnace temp. and that of hot tace of wall 188 37 5. 1

This means that the cooling ofthe clay wall mlght be as efectlve as the coohng of the silicon carblde wall 1f its th1ckness were reduced to such a point that the rate of heat transmission became equal to that of the silicon carbide. From the foregoing figures it is apparent that the fire clay wall could only be 1/6.3 times as thick. While the foregoing test data indicates that silicon carbide-has a thermal conductivity 6.3 times that of fire clay, this ligure' will vary according to the density of the silicon carbide wall, the amount of bonding material, and according to the Way in which the brick is made. A silicon carbide brick of high density has been found to possess a thermal conductivity nine times'that of fire clay. Assuming a thermal conductivity of nine as compared With fire clay, walls of equal thickness formed of different` refractories would possess the following properties:

Mag-

nesite Clay Thickness l Same. Same. Thermal conductivity Transverse str. at 1350 C Crushing str. at 1,350" C Resistance to fused ash Resistance to abrasion 3 Resistance to spalling Fusion point 2,240o C;

Fused- Aho. Clay SiC Thickness Thermal conductivity Transverse str. at 1,350 C. Crushing str. at 1,350 C. 9, Resistance to fused ash Resistance to abrasion Resistance to spelling 9 1. 25 0. 75 Fusion point 2,240e C.

(Decomposes) l Same. 200

From the foregoing it becomes apparent that silicon carbide possesses a high mechanical strength and resistance to shock as compared with other refractories. Itssuperior ability to permit control of the 'temperature of the face of the wall has also been shown. Not only this, but silicon carbide radiates a much greater amount of heat than -ire clay.

The combination of these factors permits a 6 silicon carbide wall' to be used satisfactorily,

the wall beine'v mechanically sound .and the controlled coo ing keeping the temperature of the wall below the point at which the slag will erode it and yet above that at which the flame is extinguished. The greatly increased preheating of the air with silicon carbide is also of great advantage where the air which has been reheated is used in supporting combustion or the reason that each approximately' l 100o C. rise in temperature of the yair substantially doubles the rate of reaction of the 'air and the fuel. i c

It will therefore be seen that a ventilated or cooled silcon carbide wall wherein 'there is lforced circulation of the cooling fluid to secure a controlled temperature condition at the exposed surface' of the wall is peculiarly suited for the burning of injected fuels at a high rate per cubic foot of combustion space per hour. Attempts have previously been made to avoid the necessity of cooling the walls by keeping them out of contact with the flame. This entails the necessity for ve large furnaces in order to keep the walls su ficlently remote from the flame and is not satisfactory because the remoteness of the Walls fails to cause a turbulence in the combustionspace which has been found extremely beneficial to the improvement of combustion.

With my invention the furnace volume may be kept at relatively small values without the walls being destroyed by flame or slag impingement, but rather being protected by a layer of congealed slag; nor is the use of large percentages of excess air essential as would otherwise be the case, the above protection permitting operation with excess air or less.

It will therefore be seen thatin the burning the use of a refractory provided with my controlled means for cooling the refractory gives rise to conditions which are most favorable to 5 periods of time and most favorable to eilicient combustion. As a specific example, a furnace operated at a temperature in excess of 1400 C. may be cooled, according to the present invention, to a point where the temperature of the exposed face of the lining is decreased more than 100 C. below this vfurnace temperature. This would be entirely impractical with a fire clay wall.

Although peculiarly suited to the burning of injected fuels my invention is not confined to furnaces for the burning of fuels of this class. Thecontrol of the temperature at the inner face of the furnace wall which is possible with the practice of the present invenof injected fuels, particularly powdered coal,

sustained operation at a high rating over long o tion makes it also highly satisfactory in the burning o f other fuels.

In Figure 4 there is shown an apphcation of the resent invention to a. mechanicallystoked urnace for the burning of solid fuel. In this view 20 designates the furnace generally, 21 the tuyre blocks and 22 the mechanical stoker. The walls of the furnace are formed generally of fire clay, but the ortions at the sides and vrear of the fuel be are of silicon carbide. The silicon carbide walls are designated 23 and in back of these walls is a Ventilating space 24. Air is circulated 'through thel space 24 back of the silicon car'- bide walls 23 in the direction of the arrows '80 by means of a positive draft venting device 25 which draws the air from the space -24 through the pipe 26. The 'preheated air is 'discharged from the positive draft venting -the heat away from the walls at the proper rate, the temperature of the -slag and ash in' immediate Contact with the walls cali be kept just below the melting point. On the other hand the heat will not be conducted away from the wall at such a Irate as to materially cool this portion of the fuel bed.

Itl is therefore evident that the present inventon may be applied to furnaces of vari- .l ous types where control of the temperature of the refractory yface or furnace lining is necessary or desirable.

By reason of the high mechanical strength of silicon carbide at elevated temperatures,.110 I have found that for most installations the thickness of the ventilated wall need not exceed 4% inches whereas a fire clay wall for practical purposes in commercial installations must' be nine inches thick orY Greater in 115 an7 height up to thirty feet. By reducing the thickness of the refractory wall better temperature control is possible.

While I am aware that silicon carbide refractories have been extensively used, and the high heat conducting properties ofsilicon carbide are known, it has not heretofore been proposed to make any practical use of silicon carbide refractories in furnaces in combination with a means whereby an accurate control of temperature conditions atA the face of the refractory can be secured. A refractory, to be practical in the carrying out' of my invention, should have a thermal conductivity in excess of 0.006 cal/cmB/C./sec 130 Fire clay 1 thereof, and means providing a is rated at .004-silica ordinarily rated at .005+ or fused alumina between .008 and .010; fused magnesium oxide between .009 and .011, and silicon carbide is rated between .038 and .029. It will thusbe seen that silicon carbide particularly is many tmes more thermallyconductive than fire c av.'

While I have illustrated and described certain specific embodiments of my invention, it will be understood that this is merely by way of illustration and that the invention may be applied to various types of furnaces and combustion units, various changes and modifications being contemplated under the scope of the following claims:

l. In a furnace, a refractoryv wall subject toslag erosion formed of a silicon carbide refractory, said wall having a closed air circulating passage back of the ex osed face ferced flow of air through said passage by means of which the rate of air circulation may be varied to vary the cooling of the wall and protect it from slag erosion.

2. In a furnace, a wall subject to slag erosion having an inner lining portion formed of a heat-conducting non-metallic refractor substance the thermal conductivity of which is many times that of a fire clayV wall of equal. thickness and an outer portion, said wall having a' closed air. circulating passage therein between the inner and outer portions thereof, and means for inducing a forced circulation of aithrough the passage to cool the exposed face of the wall and heat the air.

3. In a furnace, a wall subject to slag erosion formed of a heat-conducting non-metallic refractory substance the thermal conductivity of which is many times that of a fire clay wall of equal thickness, said wall having a closed air circulatingpassage therein back of the exposed face thereof, means for inducing a forced circulation of air through the passage to cool the exposed face of the wall and heat the air, the air so heated into the furnace for supporting combustion therein. Y

4. In a furnace, a wa'll subject to exposure to molten slag, said wall comprising'in combination an inner lining portion of non-metallic refractory which has 'a thermal conductivity in excess of .006 cal/cmS/sec./C.`and an outer portion spaced from said inner lining to provide an air passage between the two, and means forvariably circulating cooling airthrough the passage in contact with the unexposed surface of said inner lining portlon.

5. In a furnace, a wall exposed to slag ero# sion, said wall comprising an inner lining portion of a non-metallic refractory less than nine inches thick with a thermal conductivity in excess of .006 cal/cm3/sec./C. and an and means for conducting,

Outer wall portion spaced from said inner' linin portion to provide l an air passage there etween, and means for forcing a variable current of cooling air through thepassage in contact with the unexposed sur ace of the inner lining in order that the rate of cooling of the lining adjacent to the combustion chamber may be varied in accordance with the rate of combustion in the furnace.

6. In a furnace, a' combustion chamber having a wall subject to slag erosion, said wall comprising an inner lining portion composed plrincipally of a silicon carbide refractory w ich has a thermal conductivity in excess of.` .006 cal/cms/sec./C. and an outer portion spaced from said inner lining to provide an a1r passage therebetween, and means for forcing a variable volume of air through said passage, 4the air coming into Contact with the unexposed surface of said inner hnmg.

7. In a furnace, a combustion chamber having a wall subject to slag er'osion, said wall comprising an inner lining portion composed princlpally of a silicon carbide refractorywhich has a thermal conductivity in excess of .006 cal/cm"/sec./C. and an outer portion spaced from said inner limng to provide a closed air passage between the two,l

means for forcing a variable Avolume of air through the passage, the air coming int`o contact with the unexposed surface of said inner lining, and means for conducting the air so heated into the combustion chamber.

8. In the operation of a furnace having a combustion chamber with a lining of a nonmet'allic refractory materialcomposed prin,

cipally of silicon carbide to retard oxidation of the silicon carbideby the furnace gases and revent slag erosion thereof, the method which comprises the steps of removing heat from the refractory lining ata rate sufficient t'o maintain the temperature of the exposed face of the lining below the temperature atr which the slag in the furnace is fluid, whereby a thin protecting film of slag is formed over theexposed face of the lining, by means of a current of air circulated against an unexposed face of the lining, and varying the flow of air t'o compensate for variations in furnace temperature or variations in the melting point of the slag.

9. In the operation of a combustion chamber having a non-'metallic lining formed of a refractory whose thermal conductivity is in excess of .006/cal/cms/sec./C., the steps which comprise the removal of heat from an'unexposed surface of said lining by the forced circulation of a current of air against said unexposed surface and at a rate suflicient to maintain said lining below the flame temperature in the furnacev to congeal a thin protective layer of ash over the exposed surface thereof, but at a rate insufficient to cool the exposed face of the lining below the kindling' temperature of the injected fuel,-

and varying the amount of air circulated directly with the rate of combustion of the fuel, whereby the removal of heat varies dieciily with the rate of combustion of the 10. In the operation of a combustion chamber having a non-metallic lining formed of a. refractory Whose thermal conductivity is in excess of .006 cal/cmS/sec/OC., the steps which comprise the removal of heat from an unexposed surface of said lining by the forced circulation of a current of air against said unexposed surface'and at a rate suflicient .to maintain said lining below the flame temperature in the furnace to congeal a thin protective layerof ash over the exposed surface of said lining, but' at a rate insuflicient to cool the exposed face of the linin ,below the kindling temperature of the llnjected fuel, varying the amount of air circulated directly with the rate of combustion of the fuel, whereby the removal of heat varies directly with the rate of combustion of the fuel, and utilizing the air so circulated in he1 combustion chamber for burning the In testimony whereof I have hereunto set my hand.

CLARENCE E. I-IAWKE.

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