US3644193A

US3644193A - Process and apparatus for the recovery of oil from shale by indirect heating

Info

Publication number: US3644193A
Application number: US56037*A
Authority: US
Inventors: Ralph W Weggel; William A Blann
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1970-05-11
Filing date: 1970-05-11
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22

Abstract

A system is disclosed for recovering oil from oil-bearing shale rock in which shale is placed on a traveling grate and transported through a preheating zone; a retorting zone where oil is educted from shale; a combustion zone where residual carbon in the shale after the oil is removed, is burned; and a cooling zone. A first gas stream, which may be air, is passed through shale that has traveled on the grate at least to where combustion takes place. This air is thereby heated, and then used to heat a heat-transfer media such as alumina balls from which a second and oxygen-free gas stream extracts the heat. The second gas stream, heated indirectly by the first gas stream but not contaminated with combustion gases from the first gas stream, passes through the shale in the retorting zone to educt oil from the shale and then through the shale in the preheating zone where the educted oil is condensed and becomes suspended as a stable mist in the second gas stream from which the oil may be mechanically separated.

Description

United States Patent Weggel et al. Feb. 22, 1972 [54] PROCESS AND APPARATUS FOR THE 2,560,767 7/1951 Huff ..201/29 RECOVERY OF OIL FROM SHALE BY 3,020,209 2/ 1962 Culbertson et a1 INDIRECT HEATING 3,442,789 5/1969 Zimmerman 3,464,892 9/1969 Bennett ..202/98 [72] Inventors: Ralph W. Weggel, Wauwatosa; William A.

man, Mamtowoc, both of Primary Examiner-Curtis R. Davis [73] Assignee; Wham M f t i Company Attorney-Arthur M. Streich, Robert B. Benson and John P.

Milwaukee, Wis. [22] Filed: May 11, 1970 57 A T [21] APPL 56,037 A system is disclosed for recovering oil from oil-bearing shale Related Us. Application Data rock in which shale is placed on a traveling grate and transported through a preheating zone; a retorting zone where oil 15 [63] Continu iQlL i 5611 NOV. 1967, educted from shale; a combustion zone where residual carbon abandonedin the shale after the oil is removed, is burned; and a cooling zone. A first gas stream, which may be air, is passed through U.S. hale that has traveled on the grate at least to where com. 201/32 202/117 bustion takes place. This air is thereby heated, and then used 1] Int. Cl. ..Cl0b 53/06 to heat a heamransfer media Such as alumina balls f which [58] Field of Search ..208/1 1;201/3 2,29, 27, 16, a Second and oxygemfree gas Stream extracts the heat The 201/ 202/117 98 second gas stream, heated indirectly by the first gas stream but not contaminated with combustion gases from the first gas [56] References Cited stream, passes through the shale in the retorting zone to educt UNITED STATES PATENTS oil from the shale and then through the shale in the preheating zone where the educted oil 18 condensed and becomes Ban suspended as a stable mist in the second gas tream from Roystet the may be mechanically eparated 2,812,288 11/1957 Lankford et al. ...201/29 2,406,810 9/ 1946 Day ..208/11 3 Claims, 2 Drawing Figures 69 6| NON-CONDENSED GASES 72 1 53 53 MECHANICAL +2,000 E S PARATDR 7| FRACTlOgNNbR m gg 2g v AlR AND COMBUSTION PRODUCTS 6 67 WASTE GASES 2oo- 250F. 54 I 3L c 4 1,060 -|,20ol=. 2 I? l6 SHALE wens/mus ll gamma ANDOILCONDENSIW (on. EDUCTING) PROCESS AND APPARATUS FORTHE RECOVERY OF OIL FROM SI-IALE BY INDIRECT HEATING This application is a continuation of Ser. No. 836,038, filed Nov. 28, 1967, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to processes and apparatus for the recovery of oil from carbonaceous materials such as oil shale of the character found in the State of Colorado. In particular, this invention is directed to a process which may be performed advantageously with a horizontal traveling grate and a heattran'sfer device indirectly transferring heat from one gas stream to another gas stream, both of which pass through material on the grate, but without comingling of the two gas streams.

2. Description of the Prior Art United States Bureau of Mines Bulletin 635 published in 1966 states that 53 companies were producing oil from coal and shale in 1860 but the discovery of liquid oil in the United States in 1859 soon ended that industry in the United States. This bulletin tells about the rapid rise in consumption of petroleum products that eventually resulted in a revived interest in searching for an oil shale retorting system that would be satisfactory from a standpoint of economics, operability and the character of oil produced.

The Bureau of Mines bulletin states that oil shale retorts maybe divided into four general classes based on the method of heat application:

fluidized bed. Bureau of Mines hot-solids-contact, Aspect), TOSCO shule by introduction of hot solids into the retorting bed.

Fu'rther'information relating to these prior art systems may be found in the Bulletin and references cited therein.

An index of patents, issued by the United States and other nations, relating to the mining and retorting of oil shale and the recovery of its products, has been published by the United States Bureau of Mines as Bulletin 468, in 1948 (650 pages). A three-part supplement identified as Bulletin 574 was published in 1958 (Part I, 134 pages; Part II, 75 pages; and Part III, 62 pages).

As will be explained later, the present invention combines features of Class II and Class III systems for operation with a novel apparatus including a horizontal traveling grate. Certain of the prior art patents provide disclosures useful in describing the evolution of the present invention and these will now be specifically discussed.

U.S. Pat. No. 1,317,514 of 1919 discloses a Class II method applied to a horizontal traveling grate machine. In this operation combustion gases make contact with the shale, and oil gases driven off from the shale are contaminated with combustion gases and products.

British Patent No. 278,740 describes a Class III method applied to a horizontal traveling grate machine. In this operation shale is retorted on the grate by a previously heatedinert gas (steam) which thereby avoids contaminating oil with combustion gases and products.

British Patent No. 278,694 (same priority day as British Pat. No. 278,740) discloses a method that combines features of a Class H and a Class III method applied to a horizontal traveling grate machine. In this system shale is retorted with a heated gas to drive off the oil and then the shale is transported into a chamber where it is burned to provide heat for the system. The combustion gases transfer heat to the retorting gas by means of a tubular surface heat exchanger within the traveling grate housing.

U.S. Pat. No. 2,269,025 of 1942 is similar to British Pat. No. 278,694, but calls for retorting gases to pass horizontally through-the shale rather than vertically as disclosed in British Pat. No. 278,694.

In the combination Class II and Class III systems disclosed in British Pat. No. 278,694 and U.S. Pat. No. 2,269,025 the tubular heat exchanger is exposed, on both the inner and outer surfaces of heat exchanger tubes, to the buildup of scale or other deposits that tend to interfere with and reduce the desired gas flow through and around the tubes and heat transfer rate through the tubes.

SUMMARY OF THE INVENTION The objects of the present invention are directed to a search for a method and apparatus for the recovery of oil from oilbearing material, which is satisfactory from the standpoints of economics, operability, and character of the oil recovered; and toachieve to the greatest extent possible nine requirements that have'been defined by the U.S. Bureau of Mines and set forth in Bulletin 735 (page 6). The nine requirements for a desirable system there set forth are the following:

1. It should be continuous.

2. It should have a high feed rate per unit cross-sectional area.

3. It should have high oil recovery efficiency.

4. It should require a low capital investment, and possess a high operating time factor with low operating costs.

5. 1t shouldbe thermally self-sufficient; that is, all heat and energy requirements should be supplied without burning any of the product oil.

6. It should be amenable to enlargement into high-tonnage retorts rather than to a multiplicity of small units.

7. It should require little or no water because the Green River oil-shale deposits are located in an arid region of the State of Colorado.

8. It should be capable of efficiently processing oil shale of a reasonably wide range of particle sizes to minimize crushing and screening.

9. it should be mechanically simple, easily operable.

According to a preferred practice of the present invention oil-bearing shale material is charged to a horizontal traveling grate and transported through four zones. The first zone is a material-preheatingand oil-condensing zone; the second a retorting zone in which the material is heated to educt oil from the material; the third a combustion zone in which residual carbon in the'shale, after the oil is removed, is burned; and the fourth a material-cooling zone. A first gas stream, which may be air, is passed through material in either the combustion zone or cooling zone and thereby becomes heated. This first gas stream, which then contains combustion gases and suspended products of combustion, is passed into contact with a plurality of discrete bodies of a heat transfer media such as alumina. The heated bodies are then moved out of contact with the heated first gas stream and into contact with a second stream of relatively cool inert gases. The inert gases are heated as the bodies of the heat transfer media are cooled. The second gas stream of inert gases is therefore heated indirectly by the first gas stream via the heat-transfer bodies and without comingling of the two gas streams. The second gas stream heated in the aforesaid manner then passes through the shale in the retorting zone to heat the shale to oil-educting temperature', and the gases which now include eductcd oil then pass through the'shale material in the preheating and condensing zone where the educted oil is condensed to a mist. The second stream may then be delivered to a suitable mechanical device for separating the oil from uncondensed gas from the retorting operation. A portion of the uncondensed gas may be recirculated as the second gas stream to make contact with the bodies of heat-transfer media, and another portion of the uncondensed gas may be burned to supply added heat to the first gas stream prior to the first gas stream making contact with the bodies of heat-transfer media.

Other features and objects of the invention that have been attained will appear from the more detailed description to follow with reference to an embodiment of the present invention shown in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 of the accompanying drawing shows diagrammatically a side elevation, partly in section, an apparatus according to the present invention; and

FIG. 2 is a view taken along line IIII in FIG. 1 and viewing the structure in the direction indicated by arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a gas permeable traveling grate assembly l defines a loop with an upper strand 2 supported between head and tail shafts 3 and 4 for movement in a generally horizontal path in the direction indicated by arrows. A hood assembly is arranged over the upper strand 2 and baffles 11, 12, 13 and 14 divide the interior of the hood assembly 10 into

chambers

15, 16, 17 and 18.

Windboxes

19, 20, 21 and 22 are arranged beneath upper strand 2 and in vertical alignment with the

chambers

15, 16, 17 and 18, respectively.

A heat-transfer device 25, which may be spaced apart from grate assembly 1, in indicated in FIG. 1 and FIG. 2 as comprising a gas permeable horizontal annular revolving grate 26. The annular grate 26 is supported for rotation about a central vertical axis by suitable means which may be for example a plurality of support rollers 27 (only one of which is shown) arranged to engage a flange portion 28 and carry the revolving annular grate 26. The grate 26 may be revolved by a motor 29 driving a pinion gear 30 engaging the outer flange 28 of the annular grate 26. A stationary gas-confining structure 31 is arranged above the annular revolving grate 26 and a stationary gas-confining structure 32 is arranged below the annular revolving grate 26. Baffies 33 in both gas-confining

structures

31, 32 divide the device 25 into a first semiannular section 37 and a second semiannular section 38. A plurality of discrete alumina balls 39 are arranged in a bed of uniform depth on the entire upper annular surface of the annular revolving grate 26.

A first gas stream conveying system connects the first grate assembly 1 to the first section 37 of the heat-transfer device 25 and a second gas stream conveying system connects the second section 38 of the heat-transfer device 25 to the first grate assembly 1. The first gas stream conveying system includes a first conduit 41 for delivering ambient air from a blower 40 to the windbox 22. A second conduit 42 is provided for delivering heated air to the upper stationary gas confining structure 31 over the first section 37 of the heat-transfer device 25. Gas flow communication from the first conduit 41 to the second conduit 42 (through heated material on strand 2 to heat this first gas stream) may be established in one of several ways which will now be described. As shown in solid lines in FIG. 1, a conduit 43 provides an exit from chamber 18 to a blower 44 and then towindbox 21. The communication between the first conduit 41 and the second conduit 42 is thereby a path established through material on strand 2 in chamber 18 and then through material in chamber 17 to conduit 42. A second possible path from the first conduit 41 to the second conduit 42 may be established by a conduit represented by the broken line 45 and a third possible path may be established by a conduit represented by the broken line 46. More details of this first gas conveying system will appear later in this description but this description has now proceeded sufficiently to disclose how a gas flow from the first conduit 41 through material on the first traveling grate assembly 1 and through the second conduit 42 provides a means for transferring heat from material on the first traveling grate assembly 1 to the alumina balls 39 on the second (and annular) grate 26 and in the first section 37 of the device 25.

The second gas stream conveying system, as previously stated, connects the second section 38 of the device 25 to the first grate assembly 1. This system includes a third conduit 53 connected to structure 31 for delivering an inert gas over grate 26 in the second section 38, and a fourth conduit 54 connected to structure 32 for exiting the inert gas from below grate 26 in the second section 38 after the inert gas has passed downwardly through the bed of alumina balls 39 on the annular revolving grate 26. The fourth conduit 54 is connected to chamber 16 to discharge therein the second gas stream which has become heated as it passed through the alumina balls 39 on the revolving annular grate 26 and in the second section 38. A fifth conduit 55 is connected to chamber 15 to provide an exit therefrom and gas flow communication is established from the fourth conduit 54 through the upper strand 2 of the grate assembly 1 to the fifth conduit 55, preferably by a conduit 56, blower 57 and conduit 58 connecting windbox 20 to windbox 19. Enough of the second gas stream conveying system has been thus far described to disclose how a gas flow from the third conduit 53 passes through section 38 of the device 25, chamber 16 and then chamber 15 to provide a means for transferring heat from the balls 39 on the annular revolving grate 26, to the grate assembly 1.

Several more structural features are shown in the drawing that make the apparatus particularly adaptable to operate in a manner to perform the process of the present invention. Such structural features are believed to be best understood when described with reference to such an operation. The following described operation will therefore include a description of such structures and features of the apparatus that are shown in the drawing but have not thus far been described.

To operate an apparatus such as shown in the drawing to perform the process of the present invention, shale rock that has been crushed to pieces smaller than about 2 inches but not smaller than one-quarter inch, are charged to a feed hopper 60 shown in FIG. 1. The feed hopper 60 discharges the crushed shale on top of the upper strand 2 of the first grate assembly l which is driven by means (not shown) to move the grate in the direction shown by arrows. The grate 1 carries the shale through the

chambers

15, 16, 17 and 18, which in the practice of the present invention define a downstream material flow sequence comprising a shale-preheating and oil-condensing zone (in chamber 15); a retorting and oil-educting zone (in chamber 16); a combustion zone (in chamber 17); and a cooling zone (in chamber 18).

The shale material charged to the grate assembly 1 is ignited and burned as it moves through the combustion zone in chamber 17. The shale that has been burned in chamber 17 passes into the cooling zone in chamber 18 where ambient air at perhaps F. is blown by blower 40 through the first conduit 41 upwardly through windbox 22, strand 2, and into chamber 18, to cool the material thereon to about 200 F. and preheat the air to about 875-l,000 F. The preheated air passes from chamber 18 into conduit 43. This preheated air is then drawn in by blower 44 and blown upwardly through windbox 21 and strand 2 to support combustion in chamber 17. A mixture of oxidizing gases with combustion gases and combustion products exits through the second conduit 42 at about l,O60-l,200 F. Some noncondensed combustible gases obtained in a manner that will be described later are delivered through a conduit 61 to a combustion chamber 62 where they are burned to produce high-temperature combustion gases which are discharged from the combustion chamber 62 through a conduit 63 to mix with the gases in conduit 42 to deliver a mixture at over 2,000 F. to structure 31 over section 37 and revolving grate 26. The revolving grate 26, as previously stated, is charged with a bed of discrete bodies of heat transfer media, preferably alumina balls 39.

duit 65 at about 800-1,200 F. and blows the gases through a conduit 66 for recirculation through a conduit represented by broken lines 67, to combustion chamber 62 or other heat recovery operations such as a waste heat boiler (not shown) or up a stack 68 to the atmosphere.

The alumina balls 39 that have been heated in the aforementioned manner are carried by the revolving grate 26 out of section 37 and into section 38. Relatively cool inert gases (about 100250 F.) are delivered by conduit 53 to structure 31 over section 38 and revolving grate 26. These gases are blown downwardly (by a fan 69) through the alumina balls 39 that have been previously heated in section 37. This second gas stream is thereby heated to about 1,200 F. and passes through conduit 54 to the retorting and oil-educting zone in chamber 16. Fan 57 draws these gases downwardly through the shale on strand 2 in chamber 16 and heats the shale to at least slightly above oil-educting temperature which may be expected to be about 800 F. The gases drawn from windbox 20 at slightly above educting-condensing temperature are blown through conduit 58, windbox l9, and through the shale on strand 2 in chamber 15. The oil educted from the shale in the retorting chamber 16 is condensed to a stable mist in chamber while preheating raw shale from hopper 60. Oil mist and noncondensed gases exit from chamber 15 through conduit 55 at about 200-250 F. and are delivered to a mechanical oilgas separator 70. Separators suitable for this use are known to this-art and an example of a patent disclosing such a device is US. Pat. No. 2,386,196 of 1945. Liquid oil from the shale separated from noncondensed gases may be led off through a conduit 71 to a fractionator (not shown). The noncondensed and inert gases from the separator 70 may be drawn off through a conduit 72 with part of this flow going to waste or auxiliary uses (not shown) and part being drawn by the fan 69 into conduit 53 with a portion thereof being delivered to section 38 of the device 25 and a portion delivered through conduit 61 to the combustion chamber 62 in the manner and for the purposes previously described.

With the described apparatus according to the present invention and operated in the aforesaid manner to perform the process according to the present invention, combustion of shale to burn residual carbon after oil has been educted therefrom, indirectly, through the alumina balls 39 in the device 25, supply the heat to retort the shale and educt the oil therefrom without the first gas stream (containing oxygen and perhaps combustion products and gases) comingling with the second (inert) gas stream that retorts the shale to educt the oil. The heat-transfer media, i.e., the alumina balls 39 can be easily cleaned when necessary, and since the balls 39 are at rest on revolving grate 26 breakage of balls 39 is minimized. Therefore the manner in which this is accomplished avoids disadvantages associated with prior art approaches and meets to a satisfactory degree the previously set forth requirements for a commercially practicable operation.

From the foregoing detailed description an apparatus and operation according to a process of the present invention, it has been shown how the objects of the invention have been attained in a preferred manner. However, modifications to (for example, described temperature ranges) and equivalents of the disclosed'concepts such as readily occur to those skilled in the art are intended to be included within the scope of this invention. Thus, the scope of this invention is intended to be limited solely by the scope of the claims such as areor may hereafter be appended hereto.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the recovery of oil from oil-bearing material comprising the steps of:

A; charging the oil-bearing material to a traveling grate to form a burden thereon;

B. moving the grate with the burden in a path defining a downstream direction and through a material-preheating and oil-condensing zone, a retorting and oil-educting zone, a material combustion zone, and a material residue cooling zone;

C. passing a first gas stream through a portion of the burden that has traveled at least to the combustion zone to transfer heat from the burden to the first gas stream;

D. passing the heated first gas stream into contact with a horizontally disposed annular bed of discrete bodies of heat-transfer media to cool the first gas stream and heat the bodies;

E. rotating the annular bed of bodies about a central vertical axis with individual bodies in said bed being at rest relative to each other, to move the heated bodies out of the first gas stream;

F. passing a second and inert gas stream into contact with the heated bodies to transfer heat from the bodies to the second gas stream;

G. passing the heated second gas stream through the burden in the retorting zone to elevate the temperature of the burden to an oil-educting temperature to drive off oil as a gas from the burden material; and

H. passing the second gas stream containing the educted oil through the burden in the preheating and condensing zone to preheat the burden material and cool the second gas stream to condensing temperature to condense educted oil to a mist in the second gas stream that can be separated from the second gas stream after the second gas stream has passed through the burden material in the preheating and condensing zone.

2. A process according to claim 1 in which the second and inert gas stream is noncondensed gas educted from the burden material after condensed oil mist has been separated therefrom and a portion of said noncondensed gas is burned and the combustion gases thereof are mixed with said first gas stream to additionally raise the temperature of the first gas stream before the first gas passes into contact with the bodies of heat-transfer media.

3. An apparatus for the recovery of oil from oil-bearing material comprising:

A. a gas permeable traveling grate assembly defining a loop with an upper strand supported for material-transporting movement in a straight generally horizontal path,

a. a plurality of wind boxes within the loop and beneath the upper strand, and

b. hood structures over said upper strand defining a series of chambers arranged in a sequence and in a direction relative to upper strand travel to define a downstream path, and said hood structures comprising at least a material-preheating and gas-condensing chamber, a material retorting chamber, a material combustion chamber, and a material residue cooling chamber;

B. a gas permeable horizontal annular revolving grate,

a. a plurality of discrete bodies of heat-transfer media forming an annular gas permeable bed on top of said annular grate,

b. means for rotating said annular grate about a central vertical axis to carry said discrete bodies around a circular path,

c. stationary gas-confining housing structure above and below said annular grate enclosing said annular grate and bed of heat-transfer bodies, and

d. baffle structure within said housing structure above and below said grate and dividing said housing structure into a first and a second section;

C. gas conveying means including a. first conduit means connected to a windbox of said plurality selected to provide gas flow communication through said upper strand of said straight grate to at least one of said chambers in said downstream direction from said retorting chamber;

b. second conduit means connected on a first end thereof to said hood structure over said straight grate at a location establishing a gas flow communication through third conduit means for delivering inert gas to said second section of said annular grate housing and into heat-extracting contact with said bodies in said second section of said annular grate housing;

. fourth conduit means connected on a first end thereof to said second section of said annular grate housing to exit the inert gas therefrom, said fourth conduit being connected on a second end thereof to said retorting chamber;

e. fifth conduit means connected to said mineral-preheating and gas-condensing chamber and establishing gas flow communications through said upper strand of said straight grate and with said retorting chamber, whereby a first gas stream containing heat from combustion on said straight grate heats the bodies of media in the first section of said annular grate housing and a second gas stream extracts heat from the bodies of media in the second section of said annular grate housing and transmits such heat to the retorting chamber over said straight grate, and without commingling of the first and second gas streams; and

d. a second combustion chamber for burning combustible material to produce heated combustion gases, and a conduit connecting said combustion chamber to said second conduit means to supply additional heat to said bodies in said first section of the annular grate housing.

Claims

2. A process according to claim 1 in which the second and inert gas stream is noncondensed gas educted from the burden material after condensed oil mist has been separated therefrom and a portion of said noncondensed gas is burned and the combustion gases thereof are mixed with said first gas stream to additionally raise the temperature of the first gas stream before the first gas passes into contact with the bodies of heat-transfer media.
3. An apparatus for the recovery of oil from oil-bearing material comprising: A. a gas permeable traveling grate assembly defining a loop with an upper strand supported for material-transporting movement in a straight generally horizontal path, a. a plurality of wind boxes within the loop and beneath the upper strand, and b. hood structures over said upper strand defining a series of chambers arranged in a sequence and in a direction relative to upper strand travel to define a downstream path, and said hood structures comprising at least a material-preheating and gas-condensing chamber, a material retorting chamber, a material combustion chamber, and a material residue cooling chamber; B. a gas permeable horizontal annular revolving grate, a. a plurality of discrete bodies of heat-transfer media forming an annular gas permeable bed on top of said annular grate, b. means for rotating said annular grate about a central vertical axis to carry said discrete bodies around a circular path, c. stationary gas-confining housing structure above and below said annular grate enclosing said annular grate and bed of heat-transfer bodies, and d. baffle structure within said housing structure above and below said grate and dividing said housing structure into a first and a second section; C. gas conveying means including a. first conduit means connected to a windbox of said plurality selected to provide gas flow communication through said upper strand of said straight grate to at least one of said chambers in said downstream direction from said retorting chamber; b. second conduit means connected on a first end thereof to said hood structure over said straight grate at a location establishing a gas flow communication through said upper strand and with said first conduit means, a second end of said second conduit means being connected to said first section of said housing structure enclosing said annular grate to direct a gas flow from said first conduit means into heat-transmitting contact with said bodies in said first section of said housing structure enclosing said annular grate; c. third conduit means for delivering inert gas to said second section of said annular grate housing and into heat-extracting contact with said bodies in said second section of said annular grate housing; d. fourth conduit means connected on a first end thereof to said second section of said annular grate housing to exit the inert gas therefrom, said fourth conduit being connected on a second end thereof to said retorting chamber; e. fifth conduit means connected to said mineral-preheating and gas-condensing chamber and establishing gas flow communications through said upper strand of said straight grate and with said retorting chamber, whereby a first gas stream containing heat from combustion on said straight grate heats the bodies of media in the first section of said annular grate housing and a second gas stream extracts heat from the bodies of media in the second section of said annular grate housing and transmits such heat tO the retorting chamber over said straight grate, and without commingling of the first and second gas streams; and d. a second combustion chamber for burning combustible material to produce heated combustion gases, and a conduit connecting said combustion chamber to said second conduit means to supply additional heat to said bodies in said first section of the annular grate housing.