US3472907A - Method of operating regenerative furnaces and apparatus therefor - Google Patents

Description

METHOD OF OPERATING REGENERATIVE FURNACES AND APPARATUS THEREFOR Clarencel Coberly, San Marino, Calif., yassignor. to

Union Carbide Corporation, a corporation of New v York Filed Aug. 31, 1965, Ser. No. 484,027 Int. Cl. C10g 3.7/14; C07c 7/00 U.S. Cl. 260-679 16 Claims ABSTRACT F THE DISCLOSURE This invention relates to a regenerative furnace and method of operating the same in the pyrolysis of hydrocarbons to produce acetylene and/or ethylene.

The regenerative furnace of this invention is generally "United States Patent O similar to that shown and described in lPatent No.

2,692,819 with the modifications described hereinafter.

As described in my Patent No. 2,945,905, in the usual operation of a conventional regenerative furnace for the production of acetylene and/or ethylene, certain amounts of oil and heavy tars are produced in the cracked gas during the usual cracking cycle. This is particularly true when a liquid, such as naphtha or gas oil, is employed as the feed stock and cracked during the cracking cycle. Such liquids usually contain fractions that are aromatic or naphthenic and which do not crack into a gaseous product under the usual conditions maintained in such a regenerative furnace, and produce some tars `and oils, having high dew points, which tend to condense and form a tarry deposit in the passages of the outer ends of the furnace if their temperature falls below about 300 C. As a result, it is necessary to operate the cracking cycle so as to maintain the exit temperature of the cracked gas above such dew point temperature of about 300 C. and preferably under conditions such that the exit temperature of the cracked gas is 400 C. or higher to avoid a rapid build-up of tar in the outer ends of the passages of the furnace.

A The effect of tar deposition and build-up in the passages of the furnace is very detrimental in that it reduces the size of the passages, which increases the pressure drop therethrough, and if such tar build-up is not uniform in all of the passages, which normally it is not, it substantially changes the cross-sectional distribution of gas llow therethrough and adversely affects the cracking reaction which is dependent on a close control of the. temperature and residence time of the gases in the passages. Variations in flow from passage to passage, due to an uneven tar build-up therein, causes overcrackingin some passages and sometimes undercracking in others, with an overall loss in cracking eiciency. v

Although, as pointed out above, such tar deposits can be minimized or avoided by operating the cracking cycle so as to maintain the exit temperature of the cracked gas above 400 C., doing so results in a loss of sensible heat from the furnace and requires a corresponding increase in fuel consumption to maintain the desired temperatures in the reaction zone of the furnace, and is therefore undesirable. Also, it normally requires twelve to twenty-four hours of normal operation of such a regenerative furnace to bring the exit temperature of the 3,472,907 `Patented Oct. 14, 1969 ice cracked gas up to 400 C., and during this initial heatup period the exit gas temperature normally will not be sufficiently high to avoid such tar deposit in the passages at the ends of the furnace during the cracking cycles, which is also undesirable.

` In the usual operation of a conventional furnace, such as shown in said Patent No. 2,692,819, it has been the practice to periodically eliminate the carbon and tar deposit in the outer ends of the ceramic checkers of the furnace by heating the same to a temperature above4 the ignition point of the deposited material, in the presence of excess air, to ignite the deposit, which then burns out substantially completely to leave the passages through the ceramic checkers sufficiently clean for efficient operation of the furnace. This is done by continuing the heating cycle through the furnace in one direction for a suicient length of time to carry the heat from the normally relatively narrow high temperature zone in thecentral portion of the furnace outwardly to the end of the furnace, and then reversing the heating cycle to similarly carry the heat from the central high temperature zone outwardly to the other end of the furnace. If the furnace is at operating temperature atthe start of the bum-out cycle it is not necessary to add fuel as the heat stored in the ceramic in the center section of the furnace is sufficient and the air ow will carry the heat to the end of the furnace. However, if the furnace has previously been operating under temperature conditions normal for the production of acetylene and/ or ethylene, it requires about 20 minutes for the heat to carry out to one end of the furnace to ignite the deposit at that end and about l0 minutes more to burn it out, and a similar 20 minute period of a reversed heating cycle to carry the heat out to the other end of the furnace and another 10 minutes to burn out the deposit at that end. Allowing a reasonable period to insure complete bum-out of each of the two deposits in a furnace, it requires about one hour for each furnace for such burn-out operation. Since such furnaces are usually operated in pairs, as generally illustrated and described in my Patent No. 2,956,864, the foregoing means that it requires about two hours to so clean up an installation. During such time, the production of acetylene and/or ethylene must be discontinued, which disrupts Vplant operation and adds to production costs. It has been normal practice in a commercial plant to perform such burn-out operation on a routine rbasis of approximately once a week.

A primary object of this invention is to provide an improved regenerative furnace and method of operating the same to avoid or minimize the disadvantages referred to above which have been usual in the operation of conventional regenerative furnaces in the manufacture of acetylene and/or ethylene. Stated generally, this is accomplished by periodically introducing a free flame into each outer end of the furnace, preferably at the beginning of a heating cycle, allowing it to continue to burn n' for a small fraction of the heating cycle and for a suffi- 4matically in each outer end of the furnace, without interrupting the normal operation of the furnace.

Another object of the invention is to form such free flame `by diverting at least a portion of the ffuel gas, normally used in the heating cycle, alternately to the outer ends of the furnace, where it is mixed with the air normally passed into the furnace during such heating cycle to form a flammable mixture, and igniting such mixture before it passes into the outer end of the checkers of the furnace.

Further objects of the invention are: to ignite such flammable mixture by means of an electric spark or arc; and to control such ignition with an automatic timing device and in phase with the valve which admits fuel gas into each outer end of the furnace.

Still another object of the invention is to operate such a furnace so as to form such a tar deposit deliberately during each cracking cycle of the furnace and then to burn out such deposit on a subsequent heating cycle to utilize such burn-out as an additional source of heat during the heating cycle.

Other objects, features, and advantages will appear from the following specification and the drawing, which are for the purpose of illustration only, and in which:

FIG. 1, in diagrammatic form, partly in section, shows the furnace of my invention and the piping and some of the auxiliary apparatus thereof;

FIGS. 2, 3, and 4 are diagrams illustrating certain of the steps in the practice of the method of the invention; and

FIG. 5 is an enlarged, fragmentary, sectional view showing an igniting device of the invention.

Referring to the drawing, FIG. l shows a furnace 10, having a steel shell 11 provided with a `heat insulating lining 12. Inside the lining 12 are three spaced regenerative masses of checkers 13, 14, and 15, providing a combustion space 16 between the masses 14 and 15 and a combustion space 17 between the masses 13 and 14. The regenerative masses 15 and 13, for convenience of description, are hereinafter referred to as the left-hand (LH) and right-hand (RH) end masses, respectively, and the regenerative mass 14 as the central mass. The RH mass 13 is provided with a plurality of passages 19 which extend longitudinally therethrough (shown in endview in FIG. 5), and the masses 14 and 15 have similar passages therethrough. The right-hand and left-hand ends of the furnace are provided with plenum chambers 20 and 21 respectively.

A fuel supply pipe 23 supplies fuel gas through a main valve 24 to a manifold 25 provided with a pair of threeway valves 26 and 27. The valve 26 has connected thereto a gas supply pipe 28 which leads to a plurality of vertically spaced nozzles 29 which extend into the combustion space 17 to supply fuel gas thereto. Similarly, the valve 27 has connected thereto a gas supply pipe 30 which leads to a plurality of vertically spaced nozzles 31 which extend into the combustion space 16 to supply fuel gas thereto. Actually, the supply pipes 28 and 30 are each in the form of a yoke straddling the furnace and each is provided with a set of such nozzles on each side of the furnace 10, it being understood that the supply pipe 28 has a second set of nozzles similar to the nozzles 31, on the near side of the furnace, and the supply pipe 30 has a second set of nozzles similar to the nozzles 29 on the far side of the furnace. Although for convenience only three such nozzles have been shown in each bank thereof, it is to be understood that conventionally additional vertically spaced nozzles are usually provided in each bank thereof.

The three-way valve 26 also has connected thereto a secondary gas supply pipe 33 which leads to a manifold 34 communicating with the RH plenum chamber 20. Similarly, the three-way valve 27 has connected thereto a secondary gas supply pipe 35 which leads to a manifold 36 communicating with the LH plenum chamber 21.

Connected between the manifolds 34 and 36 are: an air supply pipe 37 communicating with a source of air (not shown), and having valves 38 and 39 therein; an

exhaust pipe 41 leading to a stack 42 or other point of disposal, and having valves 43 and 44 therein; an off-gas pipe 45 adapted to convey cracked-gas from the furnace 10 to a suitable point of storage or use (not shown), and having valves 46 and 47 therein; and an in-gas pipe 48, having valves 49 and 50 therein, which is connected to a suitable mixer 51 to which is connected a diluent supply pipe 52 adapted to supply a diluent, such as steam, to the mixer and having a valve 53 therein, there also being connected to the mixer a feed stock supply pipe 54 adapted to supply a suitable hydrocarbon feed, such as, for example, propane, to the mixer, and having a valve 55 therein.

Also connected to the manifolds 34 and 36 are igniters 57 and 58, respectively, which are identical, the ignitor 57 being shown in detail in FIG. 5. The igniter 57 includes a nipple 59 connected to the manifold 34 and into which is fitted an electric sparking device 60, supplied with a high potential electric current, such as 5000 to 10,000 volts, by an electric cable 61. The sparking device 60 includes a metal collar 62 threaded into the nipple 59 and onto which tits a tubular collar 63, the outer end of which is threaded to receive a coupling 64 in which is mounted an insulator which houses the end of the cable 61 and the end of an ignition rod 66 electrically connected to the end of the cable and extending into the manifold 34. The collar 62 has a tubular extension 67 which surrounds the rod 66, the outer end being provided with perforations 63 to admit gas thereinto between the extension and the rod.

The three- way valves 26 and 27 are conventional solenoid-operated valves and are controlled by a conventional timer (not shown). Each of such valves may be closed to shut off any tiow of fuel gas therethrough, actuated to pass the full flow of fuel gas from the fuel supply pipe 23 to one of the combustion chambers 16 or 17, actuated to pass the full ow of fuel gas from the supply pipe 23 to one of the manifolds 34 or 36, or actuated to divide the flow of fuel gas between the plenum chamber 20 and the combustion chamber 17, or between the plenum chamber 21 and the combustion chamber' 16.

In operation, the regenerative masses are first heated up to operating temperatures by a preheating step, as generally described in my Patent No. 2,967,205. By such preheating step, for the production of acetylene the central mass 14 is heated to a temperature of about 2200 F. and the end masses 13 and 1S are heated to a temperature somewhat lower than 2200 F. adjacent to the combustion spaces 16 and 17 and to a much lower temperature, e.g., below 400 C. at their outer ends, with a fairly uniform temperature drop profile between the inner and outer end of each of the end masses.

Assuming that it is desired to start such normal operation of the furnace with a left-hand (LH) make step, the valving is adjusted to provide the connections illustrated in FIG. 4, in which valves 46, 50, 53, and 55 are opened and all other valves are closed. The hydrocarbon feed stock, such as propane, ows into the mixer 51 from the pipe 54, where it is mixed with a diluent, such as steam, from the pipe 52, to form the usual in-gas. The ingas then ows through the pipe 48 and the manifold 36 into the left-hand (LH) plenum chamber 21 from which bustion space 16, the central mass 14, the combustion space 17, and the RH mass 13, cracking taking place largely in the central mass 14, to form an off-gas containing the desired end product, e.g., acetylene and/or ethylene. The off-gas is quenched to a much lower temperature during its passage through the cooler RH end mass 13, and passes through the plenum chamber 20 into the manifold 34 and thence through the off-gas pipe 45 to storage or use. In a right-hand (RH) make step, some of such valving is reversed from that shown in FIG. 4 to convey in-gas into the RH plenum chamber 20, through the furnace from right to left, out through the LH plenum chamber 21, manifold 36, and off-gas pipe 45 to storage or use. Such make, or cracking, steps are generally conventional with a regenerative furnace of this general type. However, it is to be noted that this invention contemplates cooling the off-gas to an exit temperature below 400 C., which is normally below the dew point of tars in the olf-gas, whereas in the conventional operation of such a furnace the exit temperature of the olf-gas is maintained above the dew point of such tars to prevent their deposition in the furnace. In the preferred embodiment of this invention, such exit temperature of the off-gas is maintained below such dew point to deliberately foster the deposition of such tars in the furnace, which is an lalternative object of the invention.

At the conclusion of such a vLI-I make step, the LH mass lwill have been cooled considerably by the passage of relatively cool in-gas therethrough, the central mass 14 will have been cooled to about the minimum cracking temperature for the product desired, and the RH mass 13 will have been heated considerably by the passage of hot off-gas therethrough. For a subsequent RH make step, the temperature values in the various regenerative masses must be readjusted. Consequently, such a LH make step is followed by a LH heat step, as diagrammatically illustrated in FIG. 3, and designated as LH Heat-B. With the time cycle usually used this temperature change in one step lof the cycle is on the order of 100 F.

As illustrated in FIG. 3, in a LH heat step valves 26, 38, and 44 are opened and all other valves are closed. Air is conveyed through the air supply pipe 37 and the manifold 34 into the RH plenum chamber 20 and thence through the RH mass 13, cooling the same, and into the RH combustion chamber 17 where it mixes with fuel gas supplied thereto through thefuel supply pipe 28 and nozzles 29, to form a combustible mixture which automatically ignites and burns, the hot products of combustion passing therefrom through the central mass, from right to left, to heat it back up to cracking temperatures, and then through the LH combustion chamber 116, the LH regenerative mass 15, the LH plenum chamber 21, the manifold 36, and the exhaust pipe 41 to the stack 42 from which such products of combustion are discharged. Such a LH heat step is conventional. For a similar RH heat step, the valving is readjusted to pass air into the LH end of the furnace, through the LH mass 15 and into the LH combustion chamber 16 where it mixes with fuel gas introduced thereinto through the nozzles 31, ignites to form products of combustion which then pass left to right through the balance of the furnace and the manifold 34 to the stack 42, to reestablish the heat balance following a RH make step.

The novel features of the method of my invention, relate to the conventional heat steps previously described. In a preferred embodiment of the invention, at the beginning of a LH heat step the valving is set as illustrated in FIG. 2, in which the valves 38 and 44 are opened and the three-way valve 26 is adjusted to direct all fuel gas from the fuel supply line 23 through the supply pipe 33 to the manifold 34, so that such fuel gas mixes in the manifold with air therein, adjacent to the RH plenum chamber to form a flammable or combustible mixture. At the same time, electricity is momentarily supplied by the cable 61 to the ignition rod 66 and a spark or arc thereupon bridges between the rod and the tubular extension 67 (which is grounded through the manifold 34 and steel shell 11 of the furnace), to ignite the combustible mixture in the manifold. Such burning mixture passes through the plenum chamber 20 and into the passages 19 of the RH regenerative mass and therethrough. In its travel through such passages the burning combustible mixture ignites the carbon and tars which have deposited therein during a preceding make step or steps and the same iburnS or is burnt off, leaving the walls of the passages substantially clean of such undesirable deposits. This is an important feature of the invention.

The electrical impulse delivered through the cable 61 need only be momentary, as the spark therefrom immediately ignites the combustible mixture, which continues to burn, and the electrical impulse is then discontinued by suitable switching (not shown). A suitable timer (not shown) may be provided to synchronize automatically the opening of the valves 26, 38, and 44 with the supply of high voltage to igniter 57, and this is a further feature of the invention.

The step illustrated in FIG. 2, which may be referred to as a burn-out step, is continued for only a brief period. Normally, a conventional heating step, as generally illustrated in FIG. 3 and described above, requires about one minute. I have found that by using such a separate burn-out step, as illustrated in FIG. 2, for a pe-riod of l5 seconds or less, adequate burn-out is accomplished. At the conclusion of such burn-out step, the valving is reset as shown in FIG. 3 and the regular heating step then continues until the end of its normal period of time. However, if such burn-out has not been completed during the separate burn-out step, the carbon and tar deposits in the passages 19 continue to burn during the regular heating step, since air continues to flow' through such passages during the regular heating step, to eliminate such deposits, which is a further ,feature and advantage of my method. Such burn-out step is in fact a part of the main heating step, as the burning of the combustible mixture and the hot products of combustion therefrom transfers sensible heat to the walls of the passages 19 of the masses 13, 14, and 15 and serves to heat the same. Also, the burning of the carbon and tar deposits, as described above, furnishes additional heat, such carbon and tar deposits serving as fuel therefor, and this is a further feature and advantage of the invention in that it improves the overall heat eciency of the operation.

As an alternative to the method described above, the burn-out step and heating step may be combined, without departing from the spirit of the invention. In such alternative practice, the valving is as illustrated in FIG. 2, but the three-way valve 26 is adjusted to divide the supply of fuel gas between the pipes 28 and 33, so that a portion of the fuel gas passes to the plenum chamber 20, is there mixed with air and ignited as previously described to provide the burn-out feature, while at the same time the balance of the fuel gas supply is passed directly into the RH combustion space 17 through the pipe 28, where it mixes with the excess air from the plenum chamber 20 and the mixture ignites to provide the normal heating step previously described. After the brief period required for such burn-out, normally between 5 and l5 seconds, the three-way valve 26 is readjusted to shut off the iiow of fuel gas to the plenum chamber 20 through the supply pipe 33 and convey all of the fuel gas supply directly into the combustion space 17 through the pipe 28, after which the heat step continues normally as described above.

Although only a LH heat step following a LH make step, in which the ignited fuel gas and hot products of combustion pass from right to left in the furnace, has been described, it will be understood that a RH make step is followed by a similar, but reversed, RH heat step, in which the valving is reversed to put air in the lefthand end of the furnace, and fuel gas'in the left-hand plenum chamber 21 for the burn-out of the left-hand end of the furnace and fuel in the LH combustion space 16 for the normal RH heat step.

In the usual operation of a regenerative furnace of this general type, it is conventional to use solenoid-type valves throughout the system and to control their operation automatically by standard electric timers to automatically provide the desired sequence of heat and make steps in both directions through the furnace, and I contemplate the use of such an automatic timing in connection with this invention. In addition, as indicated above,

7 I also employ the same or other suitable timer equipment to control automatically the burn-out step described above, merely by controlling the sequence of operation of the three-way valves 26 and 2'7, and this is another object of the invention.

Although the burn-out step may be effected at the beginning of each heat step, and this is frequently desirable if the feed stock and operating conditions deposit any substantial carbon or tar during the normal make steps, if such deposit is light during any single make step, due to less stringent operating conditions, it is unnecessary to provide such burn-out step at the beginning of every heat step. Consequently, when conditions permit, my method is practiced by using the burn-out step at the beginning of a heat step, in each direction, only periodically during normal operation. I have found that under some favorable conditions of operation of the furnace, although the heat and make steps may be alternated with periods of one minute each, for example, the burn-out step need only be effected every l or 15 minutes, or even less frequently, to maintain the furnace adequately clean of carbon and tar deposits. Consequently, I do not desire to be limited to using the burn-out step with every heat step, but desire to include the use of such burn-out steps periodically at regular intervals during normal furnace operation, particularly with a timed sequence thereof correlated with the normal operation of the furnace, so that the entire operation is automatic.

An alternative embodiment of the invention is to add a differential pressure switch (not shown) which measures the pressure drop across the furnace from one plenum chamber to the other and which closes a control circuit any time this pressure differential becomes too great to bring the burn-out function into operation. This may either cut off as soon as the differential has been brought down or run for a predetermined period of time each time the differential reaches a point too high.

By the use of the method of this invention, the furnace can be kept adequately clean of carbon and tars at all times, with the result that the pressure drop through the passages 19 of the ceramic checkers remains substantially uniform and maintains the yield of cracked gas from the make steps at a maximum. This is done automatically and requires no manual control or attention during operation. The result is an improved efficiency of operation and also a reduction of nonproductive downtime and maintenance labor.

Also, although I have described a system in which the same fuel gas source is used to supply fuel for the burnout and normal heat steps, it will be understood that a separate fuel source may be employed to supply fuel to the plenum chambers 20 and 21 for the burn-out steps. In this case, the valving for the fuel supply for the normal heat steps and the valving for the fuel supply to the plenum chambers for the burn-out steps are separate and independent, although timed to operate in the sequences described above. By this means, the air-fuel ratios of each may be set independently at optimum values, which is sometimes advantageous and another feature of the invention.

While I have shown and described a preferred embodiment of this invention, it will be apparent to those skilled in the art that its novel features may be applied in other equivalent forms, and I do not desire to be limited to such specific embodiment but desire to be afforded the full scope of the following claims.

I claim:

1. A method of operating a regenerative furnace having regenerative masses divided into a pre-heated section and a quenching section with a heat input section therebetween, said masses having a plurality of longitudinal passages therethrough terminating at their outer ends in plenum chambers, which includes alternating heat and make steps through the furnace, each make step depositing undesirable foreign materials on the wall of the outer portions of the passages of one of the masses, including: a periodic burn-out of said deposits by alternately adding air and fuel into each of the plenum chambers, igniting the mixture of air and fuel at said plenum chambers, and passing the resulting burning mixture into and through the longitudinal passages of the adjacent mass from the outer end thereof and towards the other mass to ignite and burn out said undesirable deposits in said adjacent mass.

2. A method as set forth in claim 1, in which a burnout step follows each make step in each direction.

3. A method as set forth in claim 1, in which there is a burn-out step in each direction following a preselected number of make steps.

4. A method as set forth in claim 3, above, in which the burn-out step is a defined part of the heat step.

5. A method as set forth in claim 4, above, in which the burn-out is a part of a heat step after a preselected number of make steps.

6. A method as set forth in claim 1, above, measuring the pressure difference between plenum chambers and initiating a burn-out step when the differential pressure exceeds a predetermined value.

7. A method as set forth in claim 6, above, in which the burn-out step is initiated and then a preselected number of timed burn-out steps are performed.

8. A method of operating a regenerative furnace having at least three regenerative masses including two end masses and a central mass therebetween, said masses being spaced apart, each of the masses having a plurality of longitudinal passages therethrough, which includes alternating heat and make steps in each direction through the furnace, the make steps depositing undesirable foreign materials on the walls of the outer portions of the passages of the end masses, each heat step including passing an air stream through one of the outer masses from the outer end thereof towards the others of said masses and into the space between said one of said outer masses and said central mass Where it is mixed with a fuel gas to form a first combustible mixture which ignites and the products of combustion pass through the central mass and the other of said end masses, characterized by:

periodically and automatically, after a make step, mixing air and a fuel gas, to form a second combustible mixture, igniting said second mixture, and passing the same while ignited into the outer end of one of said end masses and therethrough towards said center mass to burn said foreign materials from the passages of said one mass and add heat thereto, and repeating the same through the other of said end masses in a reverse direction. 9. A method of operating a regenerative furnace having at least three regenerative masses including two end masses and a central mass therebetween, said masses being spaced apart, each of the masses having a plurality of longitudinal passages therethrough, which includes alternating heat and make steps in each direction through the furnace, the make steps depositing undesirable foreign materials on the walls of the outer portions of the passages of the end masses, each heat step including passing an air stream through one of the outer masses from the outer end thereof toward the others of said masses and into the space between said one of said outer masses and said central mass where it is mixed with a fuel gas to form a first combustible mixture which ignites and the products of combustion pass through the central mass and the other of said end masses, characterized by: at the beginning of a heat step, diverting at least a portion of said fuel gas to an end of the furnace and there mixing such portion with the air to form a second combustible mixture;

igniting said second combustible mixture and passing the same while ignited into the outer end of one of said end masses and therethrough towards said center mass to burn said foreign materials from the pasages of said one mass; and

repeating the same through the other of said end masses in a reverse direction.

10. A method of operating a regenerative furnace having at least three regenerative masses including two end masses and a central mass therebetween, said masses being spaced apart, each of the masses having a plurality of longitudinal passages therethrough, which includes alternating heat and make steps in each direction through the furnace, the make steps depositing undesirable foreign materials on the walls of the outer portions of the passages of the end masses, each heat step including passing an air stream through one of the outer masses from the outer end thereof toward the others of said masses and into the space between said one of said outer masses and said central mass where it is mixed with a fuel gas to form a first combustible mixture which ignites and the products of combustion pass through the central mass and the other of said end masses, characterized by:

at the beginning of a heat step, diverting at least a portion of said fuel gas to an end of the furnace and there mixing such portion with the air to form a second combustible mixture;

igniting said second combustible mixture and passing the same while ignited into the outer end of one of said end masses and therethrough towards said center mass to burn said foreign materials from the passages of said one mass;

discontinuing the diversion of such portion of fuel gas to the end of the furnace, and continuing the heat step to its normal conclusion; and

repeating the same through the other of said end masses in a reverse direction.

11. A method of operating a regenerative furnace having at least three regenerative masses including two end masses and a central mass therebetween, said masses being spaced apart, each of the masses having a plurality of longitudinal passages therethrough, which includes alter- Dating heat and makes steps in each direction through the furnace, and makes steps depositing undesirable foreign materials on the walls of the outer portions of the passages of the end masses, each heat step including passing an air stream through one of the outer masses from the outer end thereof toward the others of said masses and into the space between said one of said outer masses and said central mass where it is mixed with a fuel gas to form a first combustible mixture which ignites and the products of combustion pass through the central mass and the other of said end masses, characterized by:

at the beginning of a heat step, diverting at least a portion of said fuel gas to an end of the furnace and there mixing such portion with the air to form a second combustible mixture;

igniting said second combustible mixture and passing the same while ignited into the outer end of one of said end masses and therethrough towards said center mass to burn said foreign materials from the passages of said one mass;

discontinuing the diversion of such portion of fuel gas to the end of the furnace after only a minor portion of said heat step, and continuing the heat step to its normal conclusion; and

repeating the same through the other of said end masses in a reverse direction.

12. A method of operating a regenerative furnace having at least three regenerative masses including two end masses and a central mass therebetween, said masses being spaced apart, each of the masses having a plurallty of longitudinal passages therethrough, which includes alternating heat and make steps in each direction through the furnace, the make steps depositing undesirable foreign materials on the walls of the outer portions of the passages of the end masses, each heat step including passing an air stream through one of the outer masses from the outer end thereof toward the others of said masses and into the space between said one of said outer masses and said central mass where it is mixed with a fuel gas to form a rst combustible mixture which ignites and the products of combustion pass through the central mass and the other of said end masses, characterized by:

at the beginning of a heat step, diverting all of said fuel gas to an end of the furnace and there mixing the same with the air to form a second combustible mixture;

igniting said second combustible mixture and passing the same while ignited into the outer end of one of said end masses and therethrough towards said center mass to burn said foreign materials from the passages of said one mass; g

repeating the same through the other of said end masses in a reverse direction.

13. In a regenerative furnace having first and second end regenerative masses and a central regenerative mass therebetween, such masses being spaced apart to provide a first combustion space between said first and central masses and a second combustion space between said second and central masses, each of said masses having a plurality of longitudinal passages therethrough, the combination of:

a plenum chamber at each end of the furnace, each communicating with an outer end of one of said end masses;

means for conveying an in-gas to either of said plenum chambers;

means for conveying an off-gas from either of said plenum chambers;

means for conveying an exhaust gas from either of said plenum chambers;

means for conveying air to either of said plenum chambers;

means for conveying a fuel gas to either of said combustion spaces;

means for conveying a fuel gas to either of said plenum chambers; and

separate igniting means at each of said plenum chambers for creating a free flame therein.

14. In a regenerative furnace having first and second end regenerative masses and a central regenerative mass therebetween, such masses being spaced apart to provide a first combustion space between -said first and central masses and a second combustion space between said second and central masses, each of said masses having a plurality of longitudinal passages therethrough, the combination of:

a plenum chamber at each end of the furnace, each communicating with an outer end of one of said end masses;

means for conveying an in-gas to either of said plenum chambers;

means for conveying an off-gas from either of said plenum chambers;

means for conveying an exhaust gas from either of said plenum chambers;

means for conveying air to either of said plenum chambers;

means for conveying a fuel gas to either of said combustion spaces;

means for conveying a fuel gas to either of said plenum chambers;

separate igniting means at each of said plenum chambers for creating a free flame therein; and

timing means for automatically and periodically causing fuel gas and air to flow into each of said plenum chambers to form a combustible mixture and operating said igniting means to ignite said mixture.

15. A method according to claim 1 wherein the air is added for a time beyond the time during which the fuel is added whereby the ignition and burn-out of said undesirable deposits is continued with air alone.

16. In a regenerative furnace having a pair of masses spaced apart to form combustion space means therebe- 11 l?. tween, each of said masses having a plurality of longitudi- References Cited nal passages therethrough, the combination of: UNITED STATES PATENTS a plenum chamber at each end of the furnace7 each 3 communicating with an outer end of one of said 3109 697 6/1963 Kasbohn et al 260679 masses; 5 2,785,212 3/1957 Bagley 23-277 XR means for conveying an in-gas to either of said plenum Chambers; FOREIGN PATENTS means for conveying an off-gas from either of said 538,572 3/1957 Canada,

plenum chambers;

means for conveying an exhaust gas from either of said lo JAMES H. TAYMAN JR primary Exmmncl.

plenum chambers;

means for conveying air to either of said plenum cham- U S CL X R bers;

means for conveying a fuel gas to either of said plenum 23-277g 165-5; l96-122; 20224lg 208-48;

chambers; Y 1D 260-683 means for conveying a fuel gas to said combustion space means; and

separate igniting means at each of said plenum chambers for creating a free ame therein.

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