US3759795A - Calciner preheater - Google Patents

Abstract

A METHOD IS DESCRIBED FOR PREHEATING COKE IN A CALCINING PROCESS TO A TEMPERATURE BETWEEN ABOUT 600* TO 1000* F., PREFERABLY TO BETWEEN 850* AND 1000*F. THIS PREHEATING AVOIDS SHATTERING OF THE COKE INTO UNDESIRABLE SMALL PARTICLES IN THE CALCINING KILN. THE HOT COMBUSTION GASES FROM THE KILN ARE ADMIXED WITH AIR TO BURN ALL VOLATILE COMBUSTIBLE MATERIAL CONTAINED IN THE GASES AND A PORTION OF THE RESULTANT HOT GASES IS ADMIXED WITH COOLER GAS TO OBTAIN A HEATING GAS FOR THE PREHEATER HAVING A TEMPERATURE BETWEEN ABOUT 800* AND 1250*F. THIS HEATING GAS IS SUPPLIED TO THE PREHEATER IN SUFFICIENT VOLUME TO PREHEAT THE COKE TO ABOVE ABOUT 600*F., PREFERABLY ABOVE 850*F.

Description

sept. 18, 1973 R K QLNER ETAL 3,759,795

' CALCINER-PREHEATER Filed July l5, 1971 Sheets-Sheet 1 mm2; |Vx! BY 7E F ATTORNEY Sept. 18, 1973 R. K. QLIVER ET AL CALCINER --PREHEATER 2 Sheets-Sheet 2 Filed July 15, 1971 MR m@ Y V//a E .mi m 7., m E N a 7 Y B United States Patent O1 liceN 3,759,795 CALCINER-PREHEATER Robert K. Oliver, San Luis Obispo, and George L. Ford,

Arroyo Grand, Calif., assgnors to Union Oil Company of California, Los Angeles, Calif.

Filed July 15, 1971, Ser. No. 163,003 Int. Cl. Cb 49/04, 53/00 U.S. Cl. 201-25 8 Claims ABSTRACT OF THE DISCLOSURE A method is described for preheating coke in a calcining process to a temperature between about 600 to 1000 F., preferably to between 850 and 1000 F. This preheating avoids shattering of the coke into undesirable small particles in the calcining kiln. The hot combustion gases from the kiln are admixed with air to burn all volatile combustible material contained in the gases and a portion of the resultant hot gases is admixed with cooler gas to obtain a heating gas for the preheater having a temperature between about 800 and 1250o F. This heating gas is supplied to the preheater in sufficient volume to preheat the coke to above about 600 F., preferably above 850 F.

DESCRIPTION OF THE INVENTION The invention relates to improvements in coke calcination and, in particular, to improvements in preheating of the coke feed prior to the coke calcination.

Petroleum coke, such as produced by delayed coking operations, contains substantial amounts of volatile combustible material, generally from about 10 to 15 weight percent. This volatile material renders the coke unsuited for electrode manufacture and similar uses. It is common practice, therefore, to remove the volatile combustible material by calcining the coke. The calcination is commonly performed by passing the coke through a rotary kiln in contact with hot combustion gases to raise the temperature of the coke suticiently to reduce its volatile combustible material content to less than about l percent. Calcination temperatures required to do this typically range from about 2000 to 2600n F.

Many petroleum cokes when introduced into a calcining kiln tend to rupture and shatter into small particles, a large percentage of which are too small for subsequent use. It has been found that this tendency can be minimized by preheating the coke to a temperature greater than about 600 F. and preferably between about 850 and 1000 F. in the absence of an oxidizing atmosphere prior to its introduction into the kiln. The preheating treatment has been found to be effective in limiting the shattering of the coke. A limited modification of this treatment has been in commercial use for several years; however, this use has been plagued with operating diiculties and, heretofore, has limited the maximum temperature to which the coke is preheated to less than about 850 F. and has also limited the coke rate through the kiln to less than its design capacity.

Initial attempts to preheat the coke were made in a refractory lined preheater which similar to that designed for use with rotary lime burning kilns. The extended use of such a preheater for coke was found to be fraught with diculties, resulting from the combustibility of the coke and its volatiles, from air leaks into the preheater which caused |combustion tand explosions, and from the coking tendency of volatile matter released from the coke during preheating.

In the process, the coke is heated to the desired temperature by passing hot gases through the preheater in the absence of any oxygen. These hot gases are generated in an afterburner where the gas effluent from the kiln is 3,759,795 Patented Sept. 18, 1973 contacted with air to burn the volatiles released in the calcination step. A portion of the resulting hot com bustion gases from the afterburner are passed to the preheater and the remainder are discharged into a vent stack. These gases, after contacting the coke in the preheater are recycled, part to the preheated inlet and the remainder to the afterburner, with the amount and proportion of the recycled gases being varied to control the temperature of the gases introduced into the preheater. The design and construction of a preheater that can function with gases laden with cokable volatile material has, until recently, plagued the commercial use of coke preheating.

The prior successful employment of a coke preheating step prior to calcination required that the afterburner combustion gases, which are circulated through the preheater by a fan, do not contain any significant amount of coking precursors. This requirement has, heretofore, limited the operation since the tempearture of the gas to the preheater must be less than that temperature which would volatilize a significant amount, e.g., about 1 percent, of volatile combustible :material from the raw coke. This limits the maximum temperature to which the coke can be preheated to less than about 850 F. and the maximum benets of preheating are not achieved.

The higher, more desirable preheat temperatures, which are above 850 F., are attained by this invention which utilizes a preheater afterburner to eliminate coke precusors from the gaseous eftiuent of the preheater. The combustible matter of these gases is eliminated by combustion, the gases are preferably cooled, repressured by a circulation fan and are returned in part to the kiln afterburner and in part ,to the preheater inlet to control the inlet gas temperature to the preheater. The immediate combustion of the combustible matter from the gaseous effluent avoids the coke deposits previously experienced on the equipment during attempts to operate the preheater at the aforementioned high temperatures.

The invention will be described with reference to the following figures of which:

FIG. l is a side view of the preheater, afterburner and calciner and shows the coke and. circulating gas flow; and

FIG. 2 is a cross-sectional view of the preheater taken along line 2-2 of FIG. l.

Referring now to FIG. l, the coke treatment facilities are shown as comprising a coke preheater 10, the preheater afterburner 11; coke calcining kiln 12 and the kiln afterburner 14. Raw petroleum coke, which is received from a -delayed 4coking unit of a petroleum refinery, is classified into a suitable size range for calcining and preheating. Particles which are so small that they will result in an excessive pressure drop in the gas flow through the preheater are separated. Typically, particles having diameters of Vs inch or less are separated and, preferably, particles having diameters of 1A inch or less are separated. The particles having the larger, more desirable sizes are thus separated from the liner particles and are discharged on to conveyor 16. The coke is transported by conveyor 16 to the top of the preheater where it is discharged into distributor 1S.. This distributor com prises a conduit which narrows at its base and splits into two smaller conduits 20 and 22. A gate 24 is pivotably supported to move between positions which divert the coke flow into one orthe other of the conduits 20` and 22. Each of the smaller conduits is connected to the top of the preheater through serially connected first and second feed traps 26 and 28 and slide gate 30. The feed traps provide a double lock chamber through which the coke may be transferred without allowing air to enter the preheater. These chambers are separated by trap doors 23 and 27 that are supported inside the chambers and are pivotable between open and closed positions. Movement of the doors is effected by air pressure as described in greater detail hereinafter with reference to FIG. 2. The slide gates are horizontal, impermeable plates which, when closed, slide over the entrances to the preheater and seal the preheater from the feed traps. Each slide gate is biased with a closing force such as a pneumatic ram actuated by air pressure. The slide gates are open when filling the preheater and in the event of a power failure, or a failure in the air pressure to gates 23 and 27, these slide gates 30 will close from the air pressure available in an air reservoir, thereby retaining the preheater isolated from the atmosphere.

The preheater 10, which is shown in greater detail in FIG. 2, comprises vessel 32 which contains a gravitating bed of coke solids. Baffie means, internally positioned in preheater 10 distribute the coke to a heat transferring section at the base of the preheater Where the coke is contacted by hot combustion gases introduced therein through conduit 36'. The hot gases are distributed throughout the coke bed in the heat transferring section by a gas distributing means. The gases pass through the coke bed and are collected within the preheater by a baled collecting means and are finally removed from preheater 10 through conduit 34.

The coke solids are discharged from the preheater by hydraulically actuated rams which force the coke into discharge hopper means 38 and conduit 40. Conduit 40 discharges the coke directly into the rotary kiln 12. The ram mechanisms for forcing the coke into discharge hopper 38 are shown in end view together with coke collection means for retaining any coke which spills behind the rams during their forward motion and which is pulled from the bed when the rams are retracted. This backspill coke is collected in hoppers 37 which are connected by inclined conduit 39 to standpipe 41. This standpipe discharges the coke into a screw conveyor 43 which has a water seal to prevent air leakage into the preheater. The screw conveyor thereafter transfers the coke particles to conduit 40 and kiln 12 through line 45.

The coke is calcined in kiln 12 by contacting the coke therein with hot combustion gases. The kiln is rotated at a velocity to achieve a residence time of the coke in the kiln of about 50 to about 58 minutes, preferably about 54 minutes. The kiln combustion gases are generated by burner 42 which is supplied with air from fan 44 and fuel such as methane through line 46. Air is directly injected into kiln 12 through auxiliary fan 48 which maintains a sufficient amount of air within the kiln to burn a portion of the volatile combustible material released by the coke during calcination. The amount of air injected into the kiln by fan 48 is also controlled so as to maintain tb'e gas temperature exiting the kiln between about l000 and l200 F.

The coke is discharged from the revolving kiln through a stationary hood 50 maintained at the rear of the kiln. A seal is maintained between hood 50 and kiln 12 and cooled by circulating cooling air through jacket 54 which encircles the seal. The cooling air is supplied to cooling jacket 54 by fan 56. The hood is a refractory lined housing which encompasses the end of kiln 12 and receives the calcined coke exiting the kiln drum. The bottom of the hood is connected to a rotary coke cooler through conduit 52 to allow coke to discharge directly from kiln 12 through hood 50 and conduit 52 into the coke cooler, not shown.

The hot gases exit kiln 12 through opening 60 at the coke inlet end of kiln 12 and discharge directly into afterburner 14. The afterburner 14 is a stationary, refractory lined vessel with one end thereof 'connected to the coke inlet end of kiln 12. As the gases enter this vessel from the kiln they pass over dam means 58 which is an annular baille formed of refractory bricks and peripherally mounted within afterburner 14 and extending radially inwardly a suficient distance to shield the steel tail ring of kiln 12. This refractory dam shields kiln 12,

and, in particular, tail ring 62 of kiln 12 from radiation of the gases in afterburner 14 and, thereby, avoids warping of this steel ring and resultant failure of its refractory lining.

The hot gases in afterburner 14 are contacted with air that is forced therein through several tangential jets circularly located around the afterburner. A circular plenum formed by bussel Iring 68 encompasses the afterburner and bears tangential jets 69 through which air is forced by fan 66 and connecting conduit 64. The air, entering tangentially into the afterburner, creates vortices within the chamber and improves intermixing of the gaseous constituents. A sufficient amount of air is supplied to the afterburner to insure nearly complete combustion of all combustible material in the gases. The resultant combustion generally raises the temperature of the gases to about 1500 F.

A portion of the gases in afterburner 14 is discharged through damper means 70 into a refractory lined, pyrolytic scrubber 72. The damper means 70 comprises a refractory lined plate which, when fully inserted into the afterburner, isolates the afterburner from the pyrolytic scrubber. The amount of gases vented to the scrubber can be adjusted by simply raising or lowering damper means 70. Air is injected into the scrubber 72 from line 74 and fan 76. Line 74 passes through preheater 10 so as to cool the internal preheater baffles and raise the temperature of the air passing therethrough to about 150 F. -An excess of air is injected into the scrubber so that all of the unburned volatile material is completely combusted along with any coke particles which may be entrained in the exiting afterburner gas stream. The purified gases are discharged into the base of exhaust stack 78 through a refractory lined breeching 80.

The remainder of the afterburner gases is pulled through large diameter duct 36 to the preheater. A butterily type damper 82 is disposed within duct 32 to provide means for isolating the preheater 10 from the afterburner 12 when shutdown of the facilities is necessary. After the hot gases contact the raw coke in the preheater, they are removed through duct 34. When the coke is preheated to temperatures in excess of about 800 F., volatile cornbustible matter is evolved. At preheat temperatures from 800 to about l000 F., these volatiles can comprise up to about 3 weight percent of the coke, and considerable coke deposition can be expected on equipment exposed to the gaseous eilluent from the preheater.

To avoid diflculties with coke deposition, the effluent is passed to the preheater afterburner 11 which is a refractory lined vessel. Air is introduced into. the afterburner through line 13 from air fan 76. This air ow is controlled by valve 15. The volatiles ignite and burn in vessel 11 and -unburned carbon and the resultant hot gases are discharged into a refractory lined cyclone 9 to collect the carbon, then the unburned carbon collected in the cyclone is discharged back to the kiln through line 23. Water can be sprayed into the gas stream from line 19 and spray heads 21 to cool the gases to a temperature less than about 750 F., preferably less than 450 F., before the vapors contact fan 84. The amount of preheater gases that are circulated by fan 84 comprises about 3000 to 4000 pounds per hour per ton of raw coke. A slide gate 86 is maintained within duct 17 to isolate the preheater and its afterburner from the kiln afterburner 14 during shutdown. The circulating fan discharges the gases to two locations; a portion is passed directly into the afterburner through line 88 which enters at the side of the afterburner so that particles in the afterburner' cannot fall into the line when the fan is shut down, and the raminder of the gases is discharged through line 90 into duct 36 with the relative amounts and portions of the preheater gases being controlled by damper 92.

The damper 92 is adjusted so that the amount of gas recycled to the preheater is sucient to maintain the temperature of the gas entering the preheater between about 850 and 1200" F., sufficient to preheat the coke to a temperature from 800 to about 1000 F. The temperature of these gases will vary somewhat, depending on the coke source and type which is to be preheated.

Successful operation of the preheater requires that all gas circulation lines and dampers be sized for minimum iloW resistance. The preheater is designed for minimum pressure drop through the coke bed at the aforementioned flow rates, e.g., about 12 to 18 inches of water. The pressure drop through line 17 to the fan is about 1 to 3 inches of water.

The preheater is illustrated more detailed in FIG. 2 which is a cross-sectional view taken along line 2--2 of FIG. 1 and displays the internal components of the preheater. An A-shaped baille 100 longitudinally traverses the lower center of the preheater and extends from one side of the preheater to the other. The baille 100 is supported within the preheater by refractory lined support columns 101 which vertically extend from the preheater base 118 and connect with each leg of the baille. Cooling tubes 102 pass through the center of each support column 101 and connect with each leg of the baille. These cooling tubes provide a means for cooling the baille and supports during operation of the preheater. The baille is protected by refractory bricks 104 which line the lower half of the baille exposed to the hot circulating gases. A triangular passageway 103 passes through the preheater and upper portion of baille `100 and communicates with the atmosphere. This pasageway is iluid-tightly sealed from the internals of the preheater so as to prevent air from entering the preheater. A cooling line 106 is longitudinally disposed within triangular passageway 103 and communicates with the upper portion of baille 100. This line provides a cooling means for the upper parts of the baille by circulating air therethrough from an externally located fan shown as 76 in FIG. l. The air passes through the baille and is removed through line 74 shown in FIG. l which discharges the heated air into pyrolytic scrubber 72.

A gas distributor 108 is disposed immediately beneath A-shaped baille 100 and longitudinally extends the length of the baille and preheater 10. The distributor comprises a permeable conduit with one end sealed against one side of the preheater wall and the other end extending through the opposite side of the preheater and connecting to gas conduit 36. The hot combustion gases from the afterburner enter the preheater through the gas distributor 108 and pass through the coke around the base of the A-shaped baille 100.

The gas, after contacting the coke around the base of baille 100, is collected by a bailled collecting means located above the gas distributor 1018 and removed from the preheater by conduit 34. The collecting lmeans comprises baille plates 110 which are longitudinally disposed along each side of the preheater wall above A-shaped conduit 100. Each baille plate is inclined downwardly towards baille 100 to form a longitudinally disposed inverted V-shaped enclosure between the baille plate and wall of the preheater. An adjustable short skirt 112 on each baille plate extends downwardly from the lower edge of the baille plate towards the legs of the A-shaped baille 100 and constricts the area between the baille 110 and the legs of baille 100. A permeable duct 114 is located under each lbaille plate 110 within the V-shaped enclosure and extends longitudinally the length of the baille. One end of each permeable duct 114 is connected to eilluent conduit 34 through a common header7 not shown, and the other end terminates at the wall of the preheater. The baille plates 110 as well as the skirts 112 are refractory lined for protection from the hot circulating gases.

The preheater is equipped with hydraulic rams which push the particulate coke between the legs of baille 100 and into a discharge hopper 38. The hydraulic rams 116 extend through wall 3K2 on each side of the preheater near the refractory lined preheater base 118. Each ram is comprised of a hydraulic cylinder 120 and associated piston, housing seal 124, push rod 126 and ram piston 130. The seal 124 is a rubber sleeve that is secured at its opposite ends to the hydraulic cylinder 122 and to the outside Wall of hopper 37 so that air is precluded from leaking through the sliding joint around rod 126. The rams 116 are disposed at a slight angle from horizontal so that when ram piston is extended it moves downwardly along slide plate 132 towards the center of the preheater and discharge hopper 38. The actuation cylinders 122 as well as seal 124 are located outside the preheater walls with push rods 126 extending through spill hole 134 in preheater wall 32 to the ram piston 130 within the preheater. The lower portion of the preheater is lined with refractory bricks 136. The ram pistons 130 are stainless steel.

During the forward motion of ram 116, a portion of the particulate coke falls behind ram piston 130, between the back of the piston and the preheater wall. When the piston is retracted, the ram discharges this coke through hole 134. Hoppers 37 are connected "to the outside of the preheater around ram 116 to collect the coke forced through hole 134 during the retraction period. The hoppers are sealed around holes 134 so as to prevent. air from entering the preheater. The hoppers are connected to an inclined conduit 39 and standpipe 41 shown in FIG. 1.

Coke is introduced into the preheater through a set of first and second feed traps 26 and. 28 and slide gate 30. The feed traps when connected in series, as shown in FIG. 2, provide a double lock chamber through which the coke can be transferred to the preheater without allowing any signicant amount of air to enter the preheater. Each feed trap comprises a rectangular chamber which has a large cross-sectional area near the top and which tapers inwardly to a smaller cross-section near the bottom of the trap. The feed conduit 20 projects into the top of iirst trap 26 for a short distance and is completely encompassed by chamber 140. The tapered bottom of the trap projects into the top of the second trap 28 and is similarly encompassed by chamber 142 of feed trap 23. The bottom of feed trap 28 discharges into the top of the preheater through slide gate 30. A trap door 23 closes against the bottom of conduit 20 and seals chamber 140 from the conduit. Similarly, trap door 27 closes against the bottom of feed trap 26 and seals chamber 142 from chamber 140. These trap doors 23 and 27 are respectively connected by crank means 148 and 150 to pneumatically actuate cylinders 152 and 154 so that when the pistons are retracted in these cylinders, the trap doors are opened, allowing communication between the preheater and conduit 20. These trap doors are sequentially operated so that when trap door 27 is closed and chamber 142 is isolated, trap door 23 can be opened and chamber 140 can be filled with coke. When trap door 23 is closed, trap door 27 is opened, thereby allowing the coke in chamber 140 to empty into the preheater. With the sequential operation of the trap doors, the coke can be delivered to the preheater with a minimum inilux of air.

As previously mentioned, slide gate 30 is disposed between feed trap 28 and preheater 10 to provide means for isolating the preheater from the feed traps. Slide gate 30 comprises a horizontal impermeable plate 156 which, when closed, seals the entrance to preheater 10. The slide gate 30 is actuated with a pneumatic cylinder and piston assembly which is supplied with air pressure from an air reservoir. Any failure in the actuating pressure which results in a simultaneous opening of both trap doors 144 and 146 also results in an automatic closing of slide gate 30, thereby maintaining preheater 10 isolated from the atmosphere.

ln operation, particulate coke having a mean diameter greater than 1A inch and a size range from 1A to about 5 inches is conveyed to the preheater and discharged into feed conduit 18. Sequential operation of dividing gate 24 and feed traps 26 and 28 iills preheater 10 with coke.

The filling procedure continues automatically until the preheater is completely filled and the coke contacts level indicator 160 which interrputs the filling procedure. The coke flows downwardly within preheater 10 through the constricted area between bal-lies 100 and 110 and forms a compact bed of solids beneath skirted baie 110. The coke is forced under the legs of A-shaped baf'lie 100 towards the center of the preheater by rams 116. Simultaneously with the flow of coke through the preheater, hot combustion gases are introduced into the preheater from gas distributor 108. These gases pass around the bottom of the A-shaped baille and up through the coke bed to gas collector 114. During the time the coke is preheated, generally between about 35 and 50 minutes, the coke temperature is raised from ambient to approximately 700 to 1000 F. and almost all of the water contained within the raw coke (between about 8 and 12 weight percent) is vaporized and removed with the effluent gas.

The coke forced to the center of the preheater by rams 116 falls into hopper 38, through conduit 40 to kiln 14. During the retraction period of the pneumatic rams a small portion of particulate coke is trapped behind the retracting ram 130 and forced into discharge hoppers 37.

l The coke in the hopper 37 falls through conduit 41 by gravity to screw conveyor 43. The screw conveyor has a Water seal of from 4 to about 12 inches water to prevent any introduction of oxygen into the preheating apparatus. The coke is transported from the screw conveyor and discharged through conduit 40 to kiln 14. The coke after dropping into the kiln contacts hot combustion gases which exit from the kiln at about 1000-l200 F. The coke is calcined within the rotary kiln for approximately 50 to 60 minutes at which time the coke temperature increases to about 2000 F. to 2400" F. At these conditions almost all of the volatile combustible materials have been driven out of the coke and are partially combusted in the kiln. The amount of volatile material combusted in the kiln is controlled by the amount of air introduced into the kiln from fan 48 which is operated to control the temperature of the combustion gases exiting the kiln at the aforestated range and, preferably, at about 1000 F. The calcined coke falls from kiln 14 through hood 50 and conduit 52 into a forced draft, rotary cooler where the coke particles are water cooled to about 300 F.

The combustion gases exiting from the kiln have temperatures which range from 800 to about 1200 F. These gases are passed directly into afterbunrer 12. In the afterburner, the gases are mixed with air and a portion of the unburned volatile material released from the coke in the kiln is burned. The combustion in the afterburner raises the temperature of the gases to about 1500 to 2000 F. A portion of the resulting hot gases is then pulled into pyrolytic scrubber 72 where it mixes with an excess of warmed air and the mixture burned to remove any remaining volatiles and entrained coke particles in the gases.

In a typical installation for use with a kiln having a coke capacity of l5 tons calcined coke per hour, the kiln is 9 feet in diameter and 160 feet long. The kiln is inclined at an angle to provide a residence time of about 54 minutes. The kiln afterburner is 30 feet long and has a diameter 0f approximately 91/2 feet. The preheater is 8 of about 3.67 feet. The circulating fan 84 has an Operating capacity of about 28,000 cubic feet per minute at 1000 r.p.m.

Although we have illustrated the present invention in connection with specific embodiments thereof, it is not intended that the illustration set forth herein shall be regarded as limitations on the scope of the invention, but rather, it is intended that the invention be defined by the steps set forth in the claims and their equivalents.

We claim:

1. In the method of preheating and calcining particulate petroleum coke to produce calcined coke in a preheater and kiln that are maintained at subatmospheric pressure, wherein hot gases from calcination of said coke in a calciner kiln are mixed with air and combusted in an afterburner to obtain combustion gas having a temperature of from l500 to 2000 F., and a portion of the combustion gas is introduced into a preheater vessel containing said petroleum coke and passed therethrough in direct contact with said coke to heat said coke to a temperature about 800 F. to 1000 F. and wherein effluent gases laden with combustible material are withdrawn from said preheater, the improvement which comprises: burning the combustible material in said effluent gases immediately after their removal from said preheater by admixing a gas containing elemental oxygen therewith, thereafter repressuring the resultant gases and admixing a portion thereof with said combustion gases being introduced into said preheater.

2. The method defined in claim 1 wherein air is supplied to the inlet of said kiln to control the temperature of said gases from the kiln between about 800 and l200 F.

3. The method defined in claim 1 wherein from 10 to percent of the eiuent gases from said preheater are recycled and mixed with said portion of said combustion gases from the afterburner.

4. The method defined in claim 3 wherein the proportion of said combustion gases and effluent is controlled to preheat the coke in said preheater to a temperature from 800 to about 1000 F.

5. The method of claim 1 wherein said step of burning of combustible material is performed by passing said eluent gases to a preheater afterburner and introducing air thereto.

6. The method of claim 1 wherein said resultant gases are cooled to a temperature below about 750 F. prior to repressuring.

7. The method dened in claim 6 wherein said cooling is performed by quenching said gases with water.

8. The method defined in claim 1 wherein the resultant gases from said burning step contained unburned carbon particles and are passed through a cyclone separator to remove said carbon particles before repressuring.

References Cited UNITED STATES PATENTS 1,564,730 12/1925 Walden 201-25 1,551,956 9/1925 Hubmann 20l-36y X 2,710,280 `6/1955 Borch 201--27 X 1,491,894 4/1924 Atkinson 110-14 X 2,687,992 8/1954 Leffer 20'1--27 X NORMAN YUDKOFF, Primary Examiner D. EDWARDS, Assistant Examiner U.S. C1. X.R.

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