US2776825A - Pebble furnaces and method of heating pebbles - Google Patents

Description

R. R. Goms 2,776,825

PEBBLE FURNACES AND METHOD 0F HEATING PEBBLES 2 Shets-Shee;

Jan. 8, 1957 Filed June 5, 1952 INVENTOR. R. R. GOINS ATTORNEYS /f/f/ ////U/ n 1 l Q Jan. 8, 1957 R. R. Goms 2,775,825

PEBBLE FURNACES AND METHOD 0F HEATING PEBBLES Filed June 5 1952 2 SheetS-Sheet 2 FIG. 4.

F /G. 3. JNVENToR. R. R. GOINS ATTORNEYS United States Patent O AND METHOD OF HEATING PEBBLES Application .lune 5, 1952, Serial No. 291,884 16 Claims. (ci. l26a- 19) PEBBLE FURN ACES This invention relates to improved pebble heater apparatus and to improved methods of heating pebbles.

This is a continuation-in-part of application Serial No. 90,921, filed May 2, 1949, now Patent No. 2,635,990.

Pebble heat-exchangers are finding increasing favor in heating gases, such as air, nitrogen, etc., and in effecting various chemical processes in the vapor phase at elevated temperatures, especially hydrocarbon conversion processes. A conventional pebble heat-exchanger unit comprises a pair of vertically aligned pebble heat-exchangers connected by an axially positioned throat therebetween so as to provide for gravity ow of a contiguous stream of heat-exchange pebbles through the heat-exchangers. The pebbles in the upper heat-exchanger, or furnace, are contacted with hot combustion gas (or other hot gas) formed either in burners adjacent the base of the upper heat-exchanger or directly in the pebble mass itself. ln this manner, the pebbles are heated to any suitable temperature above the required heating or reaction temperature in the lower heat-exchanger. The hot pebbles then gravitate to the lower chamber where they are contacted with the feed gas to be heated, treated, and/or reacted. The lower heat-exchanger usually has a funnel-shaped bottom converging to a pebble conduit which leads to the bottom of a pebble elevator or lift, usually of the bucket type, but which may be of the screw or air-lift type. The elevator transfers the pebbles to a pebble chute above the level of the top of the upper heat-exchanger and this pebble chute or conduit leads into the upper portion of the upper heat-exchanger. This arrangement provides for continuously heating one section of a gravitating mass of pebbles and simultaneously continuously heating a gas in the lower section of the gravitating mass of pebbles and returning the cooled pebbles from the lower section of the gravitating mass to the upper section for repeating the cycle.

Pebbles utilized in the process and apparatus with which this invention is concerned are compact, spherical units consisting of alumina, mullite, zirconia, thoria, periclase, magnesia, high temperature alloys, such as Monel and Inconel, and in some cases where temperature requirements are not too severe, metals such as iron, nickel, chromia, etc. ln some processes, pebbles which will withstand temperatures upwards of 3000? F. are required and several good pebbles which function at these temperatures have been developed, one of the most suitable being mullite-alumina. Pebbles range in size from about 1/s to l" in diameter, but usually are within the range of 1A to s/s.

One of the problems involved in pebble furnace operation and design is in avoiding the overheating and too rapid heating of pebbles with resulting thermal shock and fracturing of the pebbles which renders them unfit for pebble heater use. The avoidance of subjecting pebbles to severe thermalv shock poses a difficult problem in pebble heater operation. The problem of bringing the pebble bed up to operating temperature after a 2,776,825 Patented Jan. 8, 1957 ICC shutdown or at any other time when the pebbles in the lower section of the furnace have dropped in temperature considerably below operating temperature without severe thermal shock is particularly difficult. The hot combustion gas at temperatures of'upwards of 2500 F. when contacting relatively cold pebbles, subjects them to extremely severe thermal shockv and causes the fracturing of a portion of the pebbles.

Another problem of pebble furnace design is in designing the furnace so as to obtain uniform pebble flow therethrough. Uniform pebble flow in the pebble heating chamber is important because without it, over-heating of some of the pebbles will occur while some sections of the gravitating pebble mass will be underheated. Conventional design provides a cylindrical chamber with a hopper or funnel-shaped bottom which directs the flow of pebbles into a relatively narrow throat leading to the lower heat-exchanger. This corresponds to the general shape of the lower heat-exchanger also. It is found that pebble flow in this type of vessel is non-uniform through a considerable portion of the Vessel. Stagnant flow areas exist chiefly around the juncture of the conical bottom and the cylindrical sides of the vessel, while extremely rapid flow areas are found around the axis of the vessel. This difficulty of obtaining non-uniform flow of contact material in a cylindrical vessel is emphasized in the U. S. Patent 2,430,669, to John A. Crowley. While the patent is not concerned with pebble heatexchanger operation but rather with the flow of irregular particulate contact material, the flow problems are similar.

Another difficulty encountered in pebble heater operation and design lies in obtaining uniform gas flow upwardly through all sections of the heat-exchanger chamber. Without uniform gas flow, it is obvious that there will be a lack of uniform heating, and especially treating or reacting of the gas in the lower chamber. In the conversion of hydrocarbons, particularly, uniform gas ow is paramount in order to control the exact contact time required in most conversion processes and especially processes which are effected at the high temperatures provided in pebble heater processes. It has been found that over-contacting or soaking of the hydrocarbon in the heat-exchanger results in complete cracking to coke and deposition of the same on both the pebbles and the interior of the chamber which has sometimes resulted in shutting.y down of the unit due to clogging of the pebble passageways with fragments of coke from the walls of the exchanger or agglomeration of the pebbles with carbon.

The principal object of the present invention is to provide a method and. apparatus which substantially reduces thermal shock involved in heating the pebbles in a pebble furnace up to operating temperature after shutdowns and other interruptions and in effecting other required changes iny pebble temperatures. It is also an object of the invention to provide uniform pebble and gas flow in a pebble heater system. Another object of the invention is to provide uniform contact between gas and pebbles in a pebble heat-exchanger. A further object of the invention is to provide uniform gas-pebble contact with a minimum of pressure drop through `the pebble bed. A stillV further object is to provide a means and method of introducing secondary air into the combustion gas being fed to the pebble bed in a pebble furnace. Other objects of the inventionl will become apparent from a consideration of the accompanying disclosure.

The instant invention provides a novel means and method for regulatingthe temperature of the heating gas in a pebble furnace wherein secondary air is admixed with the combustion gas' intermediate the burner (or burners) and the ports. or gas distributing means which feed the heating gas to the lower section of the pebble bed.

The invention also provides a pebble furnace design which requires relatively short horizontal gas ow through the bed and therefore permits the gas to traverse the whole horizontal cross-section of the bed on its way upwardly through the heating chamber. It has been found that it is advantageous to introduce the hot pebbles from a pebble furnace into the lower heat-exchange chamber through a plurality of pebble inlets around the periphery of the heat-exchange chamber. This method of introducing pebbles to the lower chamber provides a more uniform pebble bed surface and therefore more uniform flow of pebbles and of gas to be heated or reacted in the lower bed with resultant uniformity of heating and/ or conversion of the gas being treated.

The furnace of the invention in one modification provides an axial combustion chamber below the floor of the heating chamber and a series of tangential, generally lateral or horizontal passageways leading into the combustion chamber for introducing and mixing secondary air with the combustion gas in any desired proportion so as to regulate the temperature of the mixture in accordance with the temperature of the pebbles in the bottom of the heating chamber and with the desired change in the pebble temperature. These tangential passageways or conduits communicate with an annular space between the metal shell of the furnace and the refractory surrounding the combustion chamber which has a suitable air supply means so as to provide the required secondary air. This arrangement or design reduces heat losses from the combustion chamber and aids in preventing overheating of the refractories. Another advantage of the structure is found in the utilization of a single burner and combustion chamber to supply a plurality of ports and gas distributing members with the hot combustion gas mixture required for heating the pebbles. A second modification utilizes a separate burner for each gas distributing member with one or more secondary air inlet conduits leading into each combustion chamber.

In order to improve the ow of pebbles through the heat-exchanger, the present design provides a relatively narrow pebble column above each pebble outlet. It has been found that in order to obtain any degree of uniformity of pebble flow, even in the upper section of the bed, the height of the bed must be at least one and onehalf times the horizontal width of the bed in a vessel where the pebble outlet is positioned at the center of the bottom. By effectively dividing up the gravitating pebble mass into several narrow columns with a separate pebble outlet for each, the effective height of the column is increased and its width decreased, and it is therefore found possible to utilize each column as a uniformly gravitating mass for contact with the contacting gas admitted at points near the bottom of the column. The uniformly gravitating section of ythe pebble bed in the apparatus of the invention includes approximately the upper seven-eighths of the bed. The high narrow columns of flowing pebbles provide maximum uniformity of flow without `cutting down the effective width of the entire pebble bed and, hence, the capacity of the unit. The etfective narrowing of the pebble column is combined with the injection of contacting gas at points closer to the center of the column than is provided in conventional pebble heat-exchangers.

In order to provide a more complete understanding of the invention, reference is made to the drawing of which Figure 1 is an elevation, partly in section, on line 1 1 of Figure 2, showing, somewhat diagrammatically, a pebble furnace with a subjacent gas heating and/or reaction chamber; Figure 2 is a horizontal cross-section of the furnace of Figure l taken on the line 2-2; Figure 3 is a horizontal cross-section through the combustion chamber of the furnace of Figure l taken on the line 4 3-3 thereof; and Figure 4 is a cross-section of u partial section of a combustion chamber and its gas distributing means in accordance with one modification of thc invention.

Figure l shows a pebble furnace 1t) in axial alignment with a subjacent gas heating or reaction chamber 1l. The furnace and heater may have a common shell 12 which is lined with suitable refractory materials 13 and 14 in two or more layers. The inner refractories in the lower section of the furnace and around the combustion chamber as well as in the upper section of the gas heating chamber are constructed of super refractories which will withstand extremely high temperatures.

Pebbles are fed into the furnace from an elevator 16 through chute 17 and pebble inlet conduit 18. The upper end of pebble inlet conduit 18 functions as a combustion gas stack or flue. An expansion of this pebble conduit where it joins the top of the furnace cooperates with a pebble distributor 19 in passing the pebbles to the distributor and allowing a portion of the combustion gas from the furnace to bypass the pebble distributor through an annular space 21 and to pass thru the incoming pebbles and out conduit 18. A stack 22 in the top of the furnace carries otf the remainder of the combustion gas. With a stack damper in stack 22 (not shown) this design permits removal of fines from the system by entrainment in the flue gas withdrawn thru 18.

The function of the pebble distributor is to deliver the incoming pebbles rather uniformly over the top of the pebble bed so as to provide more uniform distribution and more uniform flow of pebbles. The distributor may have any suitable number of arms or delivery spouts. The `modlcation in Figure l, shows a hollowl center column 23 having ports 24 in its lower end and a burner 26, supplied by one or more conduits 30, at its base. This modi'cation is desirable in furnaces of rather large diameter so as to improve the uniformity of pebble flow and gas flow therein. It is particularly advantageous to use a center column with combustion gas distributing means therein as well as a combustion gas source in furnaces having four or more gas distributing sections or elements around the periphery of the furnace wall and an equal number of pebble outlet conduits. However, it is not essential to have a center column in relatively small furnaces and the center column can be solid or hollow and imperforate where distribution of combustion gas from the axis of the heating chamber is not essential. A floor 27 separates the furnace into a heating chamber 28 and a combustion chamber 29. A plurality of pebble conduits 31 extend downwardly thru the floor of the furnace and feed hot pebbles into gas heating chamber 32. Each pebble outlet conduit cooperates at its upper end with a funnel or cone shaped depression 33 in the combustion chamber oor so as to expedite the How of pebbles out of the pebble column above the funnel or cone and its adjacent area.

A series of gas distributing sections 34 are spaced uniformly around the oor of the heating chamber and serve to introduce the combustion gas mixture to the bottom of the pebble bed. These gas distributing sec-- tions are made up of a vertical wall of refractory brick checker work in the form of an arc of a circle projecting inwardly from the refractory wall of the heating chamber. A downwardly and inwardly sloping refractory roof or cap mortised into the wall of the heating chamber expeditcs the flow of pebbles over the gas distributing section and prevents the passage of pebbles into the port 37 which conducts gas to the distributing section. These ports 37 communicate through a series of conduits 38 with combustion chamber 29 and provide means for passing the combustion gas-air mixture from the combustion-mixing chamber 29 to the gas distributing section 34.

The number of gas distributing sections required in a pebble furnace depends upon the diameter of the furhace and on, Whether or not a cente combustion column is used and the size thereof as well as upon the uniformity of gas distribution desired in the pebble heating chamber. At least three gas distributing sections are desirable in most furnaces at least four gas distributing sec tions, are advantageous.

The heater of Figure l utilizes a series of secondary air inlets 39, which are preferably tangentially arranged as shown in Figure 3, and may be advantageously inclined upwardly toward the combustion chamber outlet end so as to aid inimparting spiral gas flow and mixing of the secondary air with the combustion gas as it passes upwardly through the combustion chamber to outlet ports or conduits 38. Air inlet conduits 39 communicate with annular manifold space 41 surrounding the combustion chamber refractory inside shell 12. One or a plurality of air inlet conduits 42 serve to supply air to the manifold.

The refractory wall of the furnace above manifold 41 is supported by an annular' steel plate 43 welded to the shell and supported by steel angle braces 44 spaced around the bottom of plate 43 at suitable intervals. Other suitable means may be used for supporting the refractory wall of the furnace.

In the modification of the invention shown in Figure l, a single burner 46 in the bottom of combustion chamber 29 is utilized to supply the hot combustion gas for the furnace. Lines 47 and 48 supply air and fuel, respectively, to burner 46.

Pebble outlet conduits 31 extend directly into gas heating chamber 32 which is formed by the refractory wall of the unit extending down from the furnace, by the roof or dome 49, and by conical bottom 51. A gas takeoff 50 in dome 49 serves to remove product gas. The height of the gas heating or reaction chamber may be 'adjusted to suit the type of process to be effected in the apparatus. The gaseous mixture fed to the heating charnber or reactor is passed throughv line 52 to manifold space 53 formed between the conical bottom of the chamber and the bottom of shell 12. The feed gas passes from manifold space 53 through a series of gas distributing conduits 54 and inverted troughs 56 which form, a gas distributor in the bottom of chamber 32. Pebble outlet conduit 5'/ passes pebbles from the reaction chamber to a downwardly sloping chute leading into an elevator 16 which delivers pebbles to chtite 17 at the top of the :apparatus. The elevator may be a bucket type, a gas lift, or any other suitable means for transferring pebbles from the bottom to the top of the apparatus.

Figure 2 shows the floor plan of the pebble heating chamber of the furnace. The modification shown utilizes four gas distributing sections 34 and a corresponding number of pebble outlet conduits 31 passing through the floor of the chamber. rThe arcuate wall of the gas distributing section is preferably made up of a continuous refractory 5b supported by tapered refractory bricks 58. By utilizing a. continuous refractory slab such as 59, refractory bricks 58 can be spaced rather far apart so as to provide maximum open space for gas distribution. By utilizing this type of construction the vertical face of the gas distributor may contain as much :as 70 percent open space. Of` course, a plurality of continuous refractories 59 with the requiredz separating and supporting bricks may be utilized in each distributor to obtain the necessary open space for gas distribution. Figure 2 bear the same numeral designations as those shown in Figure l and further description `of the figure is believed unnecessary.

Figure 3 is a cross section of the furnace taken on line 3 3 of Figure 1 and shows a. plan of the secondary air supply and distribution system in relation yto thev combustion and mixing chamber 29. This section of the apparatusshows to `advantage the tangential arrangement of Vsecondary air inlet conduits 39 in relation to combustion chamberv 29, manifold space 41, and pebble outlet The other elements of e conduits 31. The tangential arrangement of the secondary air inlets, while improving the mixing of the air and combustion gas, is not` essential to the invention. Introduction of the air radially is also feasible and within the scope of the invention. It is also feasible to introduce the secondary `air directly from a source outside of the unit.

Figure 4 shows a modification of the gas distributing structure which `may be utilized in the furnace yof Figure l, or with the combustion chamber structure shown in Figure 4. Likewise, the gas distributing structure of Figure l may be utilized with the burner and secondary air arrangement of Figure 4. The structure of Figure 4 utilizes a gas distributing section 61 which is a vertical section of a cone cut by fa cylinder, the cylinder being the inner wall of the pebble heating chamber. Section 61 may also be an oblique section of a cylinder cut by the cylindrical chamber. A sufficient number of radially disposed inwardly and downward sloping holes or ports 62 in element 6l serve to distribute gas to the lower section of the pebble bed in the pebble heating chamber. lt is advantageous in obtaining uniform gas distribution to position the openings 62 in elem-ent 61 and the open spaces in the gas distributing section 34 of Figure 1 radially with respect to the port under the gas distributor. In this way gas is directed outwardly over the whole area of the pebble bed surrounding the gas distributor so as to result in more uniform heating than would otherwise be provided. In the modification shown in Figure 4 a burner 63 positioned in a housing 60 directs combustion gas into a combustion chamber 64'which opens into the oor 27 of the furnace thereby providing a passageway for combustion gas from the combustion chamber to the distributing section 61. One or more secondary air supply lines or conduits 66 open into. combustion chamber 64 and supply air thereto for mixing with and tempering the combustion gas as desired.

The apparatus o-f the invention has numerous advantages over conventional pebble furnaces and pebble heater arrangements. T he structure of Figure 1 is compact and relatively simple so as to render the apparatus comparatively easy to manufacture and assemble. The combustion chamber and the secondary air supply system are incorporated inside the shell so as to simplify the shell structure. The pebble furnace can be operated at practically any temperature since the burner uses av pre-mix and the secondary air is used as a diluent to reduce temperature. This is particularly advantageous at start-up from cold condition in order to prevent thermal shock to both the refractories and the pebbles. Temperature of the heating gas at start-up can be held to SOO-350 orv 400 F. until the refractories are warmed up approximately to the gas temperature at which time the gas temperature can be gradually increased over a consider-l able period of time by decreasing the proportion of secondary air in the gas mixture either by decreasing .the amount of secondary air introduced into the combustion chamber or by increasing the amount of combustion gas. The secondary `air distributor and manifold are also integral parts of the furnace incorporated within the shell. Tubes imbedded within the castA refractory carry the secondary air to the combustion chamber. The bottom of the heating chamber slopes toward the pebble outlets to prevent stagnant area-s of the pebble bed with resulting non-uniformity of heating. The extension of the gas distributing section only a short distance up the side of the furnace and the sloping roof thereon make it possible to utilize the full diameter of the chamber for heating the pebbles while still obtaining relatively uniform pebble flow through the bed. The elongated character of pebble conduits 31 is advantageous in preventing gas flow from either pebble heating chamber 28 to gas'heating chamber 32 or vice versa. In this manner these pebble conduits function as valves to prevent gas flow in either direction when filled with pebbles, as they of necessity are, during pebble heater operation.

The effective narrowing of the pebble bed provided by the invention is obvious from a consideration of the designs shown in the various figures of the drawing, and this is effective in reducing the horizontal distance which the combustion gas must travel in order to traverse the entire cross-section of the pebble bed. This horizontal distance is shortened by application of the invention to a pebble heat-exchanger of any given diameter.

Pebble heat-exchangers of the type with which this invention is concerned have utility in a wide variety of heat-exchange processes which require the heating, treating, and/or reaction of a gaseous feed. The application of M. O. Kilpatrick, Serial No. 761, 696, tiled July 17, 1947, now abandoned, describes the detailed operation of a pebble heat-exchanger unit in the conversion of hydrocarbons. The application of C. W. Perry, Serial No. 677,357, filed June 17, 1946, now Patent No. 2,596,507, shows the use of pebble heat-exchangers in the synthesis of HCN from NH3 and carbon-containing gases. The application of Sam P. Robinson, Serial No. 665,673, filed April 29, 1946, now Patent No. 2,551,905, illustrates the use of pebble heat-exchanger apparatus in the desulfurization of gases, particularly hydrocarbon gases. The use of pebble heat-exchangers in the synthesis of carbon disulfide from hydrocarbon and sulfur-containing gases is disclosed in the application of Sam P. Robinson, Serial No. 651,293, filed March l, 1946, now abandoned. The heating of air for high temperature use in the fixation of atmospheric nitrogen in a pebble heat-exchanger is disclosed in the application of Sam P. Robinson, Serial No. 767,300, filed August 7, 1947, now abandoned. The process of this last application illustrates the use of a heating gas other than combustion gas for heating the pebbles in the upper heat-exchanger. In the process referred to, air heated in the lower heat-exchanger is mixed with fuel and burned in a high temperature furnace to produce a temperature of approximately 4000 F. so as to effect the fixation of nitrogen, and, in order to prevent decomposition of the product, the hot effluent is irnmediately passed through the upper heat-exchanger in contact with relatively cold pebbles, thereby heating the pebbles and quenching the product effluent from the reaction in the furnace. The hot pebbles gravitate to the lower heat-exchanger and are there contacted with atmospheric air so as to pre-heat the air for the fixation process. The apparatus of the invention i-s readily adaptable to the nitrogen fixation process by introducing the gas to be quenched directly to the ports 37 of the apparutus.

The instant invention is applicable to the processes which the applications just referred to relate. Since the pebble heat-exchanger disclosed provides uniform pebble flow and gas flow, it is particularly applicable to the con version of hydrocarbons and to chemical processes which require short, specific reaction times. ln hydrocarbon conversion processes involving dehydrogenation and cracking to specific products without over-cracking, the present invention offers particular utility.

As will be apparent to those skilled in the art, various modifications of the invention, in addition to those disclosed, are within the scope and spirit of the disclosure and claims.

I claim:

l. A pebble furnace for heating a gravitating compact bed of pebbles, comprising a refractory-lined upright closed cylindrical vessel having gas outlet and pebble inlet means in its top; a lioor in said furnace intermediate the top and bottom thereof forming a pebble heating chamber between said floor and said pebble inlet means, said fioor having at least three upwardly extending ports therein adjacent the vertical wall of said vessel and being imperforate except for said ports and the hereinafter described pebble outlet conduits; means for supplying hot combustion gas to each of said ports including burner means, combustion chamber means communicating with said burner means, and conduit means connecting said combustion chamber means witheach of said ports; a series of separate inwardly convex gas distributing sections enclosing the outlet ends of said ports and extending only a portion of the height of said chamber, said sections being perforate and adapted to pass combustion gas into said chamber from said ports; conduit means intermediate said burner and said ports for introducing secondary air into the combustion gas passing thru said ports; and at least three pebble outlet conduits extending thru said fioor, spaced alternately with respect to said gas distributing sections and intermediate the wall and the center of said chamber.

2. The furnace of claim l in which said gas distributing sections are in the form of a vertical segment of a cone cut by a cylinder with the base at the port and the apex on the wall of said vessel.

3. The furnace of claim 1 in which said gas distributing sections are in the form of an upright wall of refractory checkerwork having an inwardly and downwardly sloping imperforate roof mortised into the wall of said vessel.

4. A pebble furnace for heating a gravitating cornpact bed of pebbles, comprising a refractory-lined upright closed cylindrical vessel having gas outlet and pebble inlet means in its top; a oor in said furnace intermediate the top and bottom thereof forming a pebble heating chamber between said oor and said pebble inlet means, said floor having at least three upwardly extending ports therein adjacent the vertical wall of said vessel; an axial refractory-walled combustion chamber below said floor having a burner in its lower section; tubes for combustion gas leading from said combustion chamber to each of said ports; conduit means leading into said combustion chamber intermediate said burner and said tubes for introducing secondary air to said combustion chamber; a separate inwardly convex gas distributing section enclosing each of said ports and extending only a portion of the height of said chamber, said section being perforate so as to pass combustion gas into said chamber while avoiding passage of pebbles to said port; a pebble outlet conduit in said fioor intermediate each adjacent pair of gas distributing sections and intermediate the wall of said vessel and the axis thereof extending downwardly through the wall of said axial refractory-walled combustion chamber to an expanded pebble chamber; and a funnel-shaped depression in said floor around each said pebble outlet for facilitating pebble fiow out of said chamber.

5. The furnace of claim 4 in which said conduit means includes a series of tangentially and symmetrically dis posed passageways leading into said combustion chamber thru the refractory wall thereof from an air source.

6. The furnace of claim 5 including an annular open space between said refractory wall and the outside wall of said vessel and at least one opening thru said outside wall into said open space for introducing air and providing said air source.

7. The furnace of claim l including a hollow upright cylindrical refractory column disposed axially on said floor and extending into the upper section of said chamber; a burner in said hollow column for providing cornbustion gas; and downwardly radially directed ports in the wall of said column in the lower section thereof for passing combustion gas from said column to said chamber.

8. The furnace of claim l including a combustion chamber leading into each of said ports thru the wall of said vessel; a burner in the outer end of said combustion chamber; and a secondary air inlet in said combustion chamber intermediate said burner and said port.

9. The method of tiring a pebble furnace containing a compact bed of pebbles which comprises burning a fuel so as to produce combustion gas; mixing with said combustion gas a secondary air stream so as to decrease the temperature of the mixture, thereafter passing said mixture upwardly thru said bed of pebbles in direct heatexchange therewith so as to heat same with less thermal shock than would occur without the mixing step.

10. The method of tiring a pebble furnace containing a compact cylindrical bed of pebbles which comprises burning a combustible mixture of oxygen and fuel in an axial cylindrical combustion zone below said bed; introducing secondary air tangentially to said combustion zone so as to form an intimate mixture of air and combustion gas of lower temperature than said combustion gas and of higher temperature than said pebbles; passing said mixture to a series of uniformly spaced distributing Zones around the outer periphery of the lower section of the said bed; distributing said mixture radially outwardly and laterally from each of said distributing zones into the lower section of said bed; thereafter, passing `said mixture upwardly thru said bed in -direct heat-exchange with the pebbles therein so as to heat same to the desired temperature.

1l. The combination of the pebble furnace of claim 4 with a subjacent axially disposed refractory lined heatexchange chamber in which the pebble outlet conduits open into the upper end of said heat-exchange chamber and serve as pebble feeder conduits thereto, said heatexchange chamber having pebble outlet means and gas inlet and distribution means in its lower end and gas takeoff means in its upper end.

12. The furnace of claim l in which said gas distributing sections are in the form of an oblique section of a cylinder fitting the inside wall of the furnace.

13. The furnace of claim 4 wherein said burner is upwardly directed and positioned at the axis of said combustion chamber.

14. A method of bringing a cool bed of pebbles in a pebble heating chamber up to operating temperature with a minimum of thermal shock to the pebbles which comprises burning a combustible mixture of oxygen and fuel in an axial combustion zone below said bed; introducing secondary air tangentially to said combustion Zone so as to form an intimate mixture of air and combustion gas initially at a temperature in the range of 300 to 400 F.; passing said mixture to a series of uniformly spaced distributing zones around the outer periphery of the lower section of said bed; distributing said mixture radially outwardly and laterally from each of said distributing zones into the lower section of said bed; thereafter, passing said mixture upwardly through said bed in direct heat-exchange with the pebbles therein so as to` heat same to the desired temperature; and gradually increasing the temperature of said mixture by increasing the proportion of combustion gas therein as 'the temperature of said pebble bed is increased, until the desired operating temperature is reached.

l5. A method of bringing a cool bed of pebbles in a pebble heating chamber up to operating temperature with a minimum of thermal shock rto the pebbles which comprises burning a combustible mixture of oxygen and fuel in a combustion zone outside of said bed; introducing a secondary air to said combustion Zone so as to form an intimate mixture of air and combustion gas initially at a temperature in the range of 300 to 400 F.; passing said mixture to a series of uniformly spaced distributing zones around the outer periphery of the power section of said bed; distributing said mixture radially outwardly and laterally from each of saiddistributing zones into the lower section of said bed; thereafter, passing said mixture upwardly through said bed in direct heat-exchange with the pebbles therein so as to heat same to the desired temperature; and gradually increasing the temperature of said mixture by increasing the proportion of combustion gas therein as the temperature of said pebble bed is increased, until the desired operating temperature is reached.

16. A method of bringing a cool bed of pebbles in a pebble heating chamber up to operating temperature with a minimum of thermal shock to the pebbles which comprises burning a combustible mixture of oxygen and fuel in a plurality of combustion zones positioned around the periphery of a lower section of a bed of pebbles; introducing secondary air into each of said combustion zones so -as to form an intimate mixture of air and combustion gas initially at a temperature in the range of 300 to 400 F.; passing said mixture to a series of uniformly spaced distributing Zones around the outer periphery of the lower section of said bed; distributing said mixture radially outwardly and laterally from each of said distributing Zones into the lower section of said bed; thereafter, passing said mixture upwardly through said bed in direct heat-exchange with the pebbles therein so as to heat same to the desired temperature; and gradually increasing the temperature of said mixture by increasing the proportion of combustion gas therein as the temperature of said pebble bed is increased, until the desired operating temperature is reached.

References Cited in the file of this patent UNITED STATESl PATENTS 2,272,108 Bradley Feb. 3, 1942 2,285,718 Isaacson June 9, 1942 2,505,257 Quigg Apr. 25, 1950 2,534,625 Robinson Dec. 19, 1950 2,536,436 Goins Jan. 2, 1951 2,563,323 Grossman Aug. 7, ll 2,635,950 Robinson Apr. 21, 1953 2,635,990 Goins Apr. 21, 1953

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