Process for retorting oil shale mixtures with added carbonaceous material

A process for retorting oil shale, especially lean oil shale, which comprises heating oil shale in a retort zone in the absence of substantial molecular oxygen to evolve hydrocarbonaceous vapors therefrom, thereby producing retorted shale; burning the retorted shale with added carbonaceous material in a combustion zone in the presence of an amount of added combustion catalyst sufficient to increase the rate of combustion of said added carbonaceous material in said combustion zone; and transferring heat from the combustion zone to heat said lean shale.

BACKGROUND OF THE INVENTION 
This invention relates to a process for retorting oil shale-coal mixtures 
and more particularly to a process for retorting mixtures of lean oil 
shale and coal. 
Oil shale is a fine-grained, sedimentary rock which contains an organic 
material known as kerogen. Upon heating, kerogen decomposes to yield 
liquid oil, gases and residual carbon. The kerogen content varies with 
different geological formations, and some shale does not yield sufficient 
quantities of oil to economically justify its recovery. Unfortunately, 
some of the lower grades of oil shale must be mined in order to reach the 
richer deposits. Unless the oil can be extracted from these leaner 
resources, the overall costs of extraction will escalate needlessly. 
Thermally efficient retorting systems use the energy of the residual 
carbonaceous material on the retorted shale for process heat. For example, 
the residual carbon may be burned to heat circulating solid heat carriers 
such as ceramic balls or particles, sand or spent shale. Alternatively, 
hot flue gases generated from the combustion can be used for direct or 
indirect heating of the raw shale. 
The amount of carbonaceous residue remaining on the shale mineral structure 
after retorting is dependent upon various factors. At the temperatures 
required for commercial retorting, the primary factor is the grade or 
richness of the raw shale, with lower grades having proportionately less 
residue. For oil shales yielding less than about 0.13 liters of shale oil 
per kilogram of oil shale (30 gallons per ton), the quantity of organic 
residue in the retorted shale is insufficient to supply the total heat 
required for retorting the raw shale, when directly mixed in the preferred 
ratios of spent shale to raw shale. 
It has been proposed to add supplementary carbonaceous material to the 
retorted shale in order to generate all or substantially all of the heat 
needed for heating the raw shale in the retorting zone. See, for example, 
U.S. Pat. Nos. 4,058,205; 3,939,057; 2,589,109; and the publication 
"Development of the Lurgi-Ruhr Gas Retort for the Distillation of Oil 
Shale," Lurgi Mineraloltechnik GMBH, Frankfurt (Main), October 1973, Page 
11, Paragraph 5. 
A particularly advantageous process for retorting oil shale and other 
similar materials is described in U.S. Pat. No. 4,199,432, issued Apr. 22, 
1980 to Tamm et al, which is incorporated herein in its entirety by 
reference. In this process, fresh oil shale particles are passed into an 
upper section of a vertically-elongated retort and downwardly therethrough 
in the presence of hot heat carrier particles to heat the fresh oil shale 
particles to retorting temperatures sufficiently high to drive off 
hydrocarbonaceous materials, which are removed from the upper portion of 
the retort. A heated, nonoxidizing gas, e.g., recycled product gas, flue 
gas, nitrogen, or steam, is passed upwardly through the retort at a 
velocity of between about 1 to 5 feet per second. The size of the fresh 
oil shale particles and the heat carrier particles include particles which 
are fluidizable at the gas velocity and particles which are nonfluidizable 
at the gas velocity. The shale particles are passed downwardly through the 
retort at a rate providing a residence time for substantially complete 
retorting of the particles. The retort contains internals to substantially 
limit backmixing and slugging of the particles. 
The retorted particles contain residual carbon. These particles are passed 
to a combustion zone where they are combusted with an oxygen-containing 
gas to heat the retorted particles along with any inert or spent shale 
particles present. The heated particles can then be recycled as heat 
carriers to the retort to provide process heat for retorting fresh shale 
particles. The combustion zone can be a liftpipe or an entrained bed 
reactor wherein the entrained particles are rapidly heated to combust 
residual carbonaceous material. To minimize the height of the liftpipe 
combustor, it is desirable to have a low combustor residence time of both 
gas and solids, e.g., less than about 4 seconds, preferably 1 to 2 
seconds. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved process for oil 
shale retorting which involves the addition of supplemental carbonaceous 
material to provide heat for retorting shale. It is a further object to 
provide improvement in the process of retorting mixtures of low-grade oil 
shale and other carbonaceous materials wherein a portion of the retorting 
heat requirement is met by burning the supplementary carbonaceous 
material. These and other objectives are met according to this invention 
in a process for retorting oil shale, especially lean oil shale, 
comprising: 
(a) heating said oil shale in a retort zone in the absence of substantial 
molecular oxygen to evolve hydrocarbonaceous vapors therefrom, thereby 
producing retorted shale; 
(b) burning said retorted shale with added carbonaceous material in a 
combustion zone in the presence of an amount of added combustion catalyst 
sufficient to increase the rate of combustion of said added carbonaceous 
material in said combustion zone; and 
(c) transferring heat from the combustion zone to heat the oil shale. 
The added carbonaceous material can be coal, including anthracite, 
bituminous or sub-bituminous lignite, peat, oils such as petroleum, heavy 
petroleum fractions, retorted shale oil, coal-derived oils, wood chips, 
sawdust, coke, tar, or devolatilized coal, e.g., coal char, or any other 
carbonaceous fuel. The "added combustion catalyst" is defined as catalytic 
material in addition to any material already naturally present in the oil 
shale or carbonaceous feed to the process. The heat can be transferred 
from the combustion zone to the retort zone by passing to the retort 
spent, combusted shale particles, which can contain ash from the added 
carbonaceous material. Alternately, some or all of the heat can be 
transferred indirectly by passing a fluid, e.g., flue gas, from the 
combustor to the retort.

DETAILED DESCRIPTION OF THE INVENTION 
As used herein, the term "oil shale" refers to fine-grained, sedimentary 
inorganic material which is predominantly clay, carbonates and silicates 
in conjunction with organic matter composed of carbon, hydrogen, sulfur, 
oxygen, and nitrogen and called "kerogen." 
The term "retorted shale," as used herein, refers to oil shale from which 
most or essentially all of the volatilizable hydrocarbons have been 
removed and which may still contain carbonaceous residue. 
The term "spent shale," as used herein, refers to retorted shale from which 
a substantial portion of the carbonaceous residue has been removed, for 
example, by combustion in a combustion zone. 
The term "shale oil" refers to the hydrocarbonaceous material volatilized 
from the oil shale during retorting. 
The term "devolatilized coal," as used herein, refers to coal from which a 
substantial portion of the volatilizable hydrocarbons have been removed, 
e.g., char. 
The terms "condensed," "noncondensable," "normally gaseous," or "normally 
liquid," are relative to the condition of the subject material at a 
temperature of 25.degree. C. (77.degree. F.) and at a pressure of one 
atmosphere. Particle size, unless otherwise indicated, is measured with 
respect to Tyler Standard Sieve sizes. 
According to this invention, it has been found that the combustion rate of 
retorted oil shale is significantly more rapid than the combustion rates 
of many other carbonaceous materials such as coal, shale oil and petroleum 
coke, and heavy oils, etc. Consequently, when retorted oil shale is 
combusted in the presence of the supplementary carbonaceous materials, 
e.g., carbonaceous materials not already present in the raw oil shale 
feed, a residence time must be provided which is greater than that 
necessary for combusting the retorted shale, e.g., by employing a larger 
combustion zone. The problem of differential combustion rates is 
particularly serious when retorted shale and added carbonaceous material 
are combusted in an entrained bed combustor which operates with a very low 
residence time, e.g., on the order of a few seconds such as that described 
in the aforementioned U.S. Pat. No. 4,199,432. 
FIG. 1 depicts the relative combustion kinetics of retorted oil shale, 
containing 3% coke, and five other carbonaceous materials, as shown in 
Table 1. 
TABLE 1 
______________________________________ 
Material Description 
______________________________________ 
A 510.degree. C. + Shale Oil Coke 
37% Coke Yield 
B Illinois #2 Bituminous Coal 
C Platinum-Promoted FCC Coke 
D Spent Shale with 13% Deposited 
Coke from a 230.degree. C. + Petroleum VGO 
E Noonan Lignite 
______________________________________ 
The combustion tests were performed in a packed bed reactor under 
essentially isothermal conditions. The reactor contained a removable 
screen with glass wool packing. The packing supported a thin bed of the 
carbonaceous particles, mixed with inert alundum particles. In each 
experiment, the tube was heated up under a flow of inert gas and then was 
switched to a oxygen-containing gas for the combustion. The alundum 
particles served to keep the burning sample essentially isothermal. 
Combustion rates were determined by monitoring carbon dioxide production. 
In Sample B (bituminous coal), it is assumed that some volatilization 
occurred during the heat-up period, but this volatilization is ignored for 
purposes of the experiment. As shown in FIG. 1, the retorted oil shale had 
a markedly higher combustion rate than the other carbonaceous materials. 
It is believed that the high combustion rate of the retorted shale is the 
result of catalysis by noncombustible mineral matter present in the shale, 
especially calcium carbonates. 
According to this invention, a combustion catalyst is added to the shale 
retorting process so that retorted shale and supplemental carbonaceous 
material are burned in the combustion zone in the presence of the added 
catalyst. The added catalyst need be present only in an amount sufficient 
to increase the rate of combustion of the supplemental carbonaceous 
material in the combustion zone. Preferably, the added catalyst is 
sufficiently active and is present in a sufficient amount to increase the 
combustion rate of the added carbonaceous material to near or above the 
combustion rate of the retorted shale. 
The combustion catalyst added according to this invention can be any of the 
catalytically-active metals which enhance combustion of carbonaceous 
materials such as catalytically-active metals or catalytically-active 
metal compounds, e.g., oxides, carbonates or hydroxides of metals selected 
from Groups Ia, IIa, and the transition elements as described in the 
Periodic Table of the Elements, Handbook of Chemistry and Physics, 
Chemical Rubber Co. (1964). Of course, the combustion catalyst should 
preferably be inexpensive enough to be discarded because separation of 
used catalyst from large quantities of combustion materials would be very 
costly. Most preferred combustion catalysts are carbonates, hydroxides or 
oxides of alkali or alkaline earth metals, especially carbonates and 
hydroxides of sodium, potassium, calcium or magnesium. 
The combustion catalyst can be added directly to the combustion zone, or 
can be added with the supplementary carbonaceous material to the process, 
or can be added anywhere else in the process, so long as the combustion 
catalyst is present and active in the combustion zone. Preferably, the 
supplementary carbonaceous material is contacted with the combustion 
catalyst prior to addition to the process, e.g., to the retort zone or to 
the combustion zone. When a solid supplementary carbonaceous material, 
such as coal, is added, the coal can be pretreated by contacting it with a 
solution or suspension of the combustion catalyst, for example, in an 
aqueous solution. The treatment of coal with inorganic agents prior to 
combustion is described in U.S. Pat. No. 4,148,613, "Process for Preparing 
Sulfur-Containing Coal or Lignite For Combustion," which is incorporated 
herein in its entirety by reference. 
Preferably, the catalyst is added in as small quantities as possible so 
that it can be discarded with the spent shale. The minimum effective 
amount for any given catalytic agent will depend upon the particular 
carbonaceous material added and upon the particular catalyst, and can be 
determined by routine testing. For example, when K.sub.2 CO.sub.3 or 
Na.sub.2 CO.sub.3 is used as a combustion catalyst with bituminous coal, 
about 1 to 20 pounds of catalyst per ton of coal should be sufficient to 
significantly enhance combustion. When K.sub.2 CO.sub.3 or Na.sub.2 
CO.sub.3 is used with added retorted shale oil, about 0.25 to 5 pounds of 
catalyst per barrel of added shale oil should be sufficient. 
A suitable impregnant solution for coal and other solid carbonaceous 
materials can be a 1 to 20 weight percent solution of Na.sub.2 CO.sub.3, 
K.sub.2 CO.sub.3, NaOH or KOH dissolved or suspended in water. 
The following specific embodiments demonstrate the addition of coal as a 
solid supplemental carbonaceous material and retorted shale oil as a 
liquid supplemental carbonaceous material. These embodiments are intended 
to be illustrative only and can be modified in various ways by one skilled 
in the shale retorting art without departing from the scope of this 
invention. 
SPECIFIC EMBODIMENT 1 
Referring to FIG. 2, raw coal is introduced through line 6 and an aqueous 
solution of sodium carbonate is added through line 4 to mixing zone 8 
wherein the coal is impregnated with the sodium carbonate. The 
catalyst-treated coal, raw oil shale particles, and hot spent shale 
particles are introduced through lines 10, 12 and 14, respectively, into 
an upper portion of a vertically-elongated retort 16 and pass downwardly 
therethrough. A stripping gas substantially free of molecular oxygen 
(e.g., steam) is introduced via line 18 to a lower portion of retort 16 
and is passed upwardly through the retort, fluidizing a portion of the 
shale and coal particles. Hydrocarbonaceous materials retorted from the 
raw oil shale and coal particles, stripping gas, and entrained fines are 
withdrawn overhead from an upper portion of retort 16 through line 20. The 
entrained fines are separated in zone 22 from the hydrocarbonaceous 
material and stripping gas, and the fines pass via line 24 to a lower 
portion of combustor 26. Alternatively, the entrained fines could be 
returned to a lower section of retort 16 if desired. The effluent retorted 
shale particles and coal char particles are removed from a lower portion 
of retort 16 through line 30 and also pass to the lower portion of the 
combustor 26. The retorted hydrocarbonaceous materials and stripping gas 
pass from zone 22 through line 28 for downstream processing to the 
ultimate product such as fuel oil, diesel, gasoline, and jet fuels. 
Air is introduced into a lower portion of combustor 26 through line 32 and 
provides oxygen to burn the organic residue on the effluent retorted shale 
particles, the coal char and fines. The combustor heats the previously 
retorted shale which is removed with the flue gas from an upper portion of 
the combustor through line 34 and passes to separation zone 36. A portion 
of the spent shale, preferably larger than about 200 mesh in size, is 
recycled from separation zone 36 through line 14 to retort 16 to provide 
process heat. Hot flue gas, fly ash, and the excess spent shale fines pass 
from separation zone 36 through lines 38 and 40, respectively. 
The maximum particle size for the solids introduced in the top of the 
retort 16 is at or below about 21/2 mesh (Tyler Standard Sieve size). 
Particle sizes in this range are easily produced by conventional means 
such as cage mills, or jaw or gyratory crushers. Crushing operations may 
be conducted to produce a maximum particle size, but little or no control 
is effected over the smaller sizes produced. This is particularly true in 
regard to shale which tends to cleave into wedge-shaped fragments. 
The temperature of the spent shale introduced to the retort via line 14 
will normally be in the range of 600.degree. C. to 800.degree. C., 
depending upon the selected operating ratio of heat carrier to shale. The 
fresh shale may be introduced at ambient temperature or preheated if 
desired to reduce the heat transfer required between fresh shale and heat 
carrier. The temperature at the top of the retort should be maintained 
within the broad range of 450.degree. C. to 540.degree. C. and is 
preferably maintained in the range of 480.degree. C. to 510.degree. C. 
The weight ratio of spent shale heat carrier to raw oil shale and coal may 
be varied to approximately 1.5:1 to 8:1, with a preferred weight ratio in 
the range of 2.0:1 to 3:1. It has been observed that some loss in product 
yield occurs at the higher weight ratios of spent shale to fresh shale. It 
is believed that the cause of such loss is due to increased adsorption of 
the retorted hydrocarbonaceous vapor by larger quantities of spent shale. 
Furthermore, attrition of the spent shale, which is an actual consequence 
of retorting and combustion of the shale, occurs to such an extent that 
high recycle ratios cannot be achieved with spent shale alone. If it is 
desired to operate at higher weight ratios of heat carrier to fresh shale, 
an auxiliary attrition resistant material such as sand may be substituted 
as part or all of the heat carrier. 
The mass flow rate of fresh oil shale and coal to the retort should be 
maintained between 4,900 kg/hr-m.sup.2 and 29,300 kg/hr-m.sup.2, and 
preferably between 9,800 kg/hr-m.sup.2 and 19,600 kg/hr-m.sup.2. Thus, in 
accordance with the broader recycled heat carrier weight ratios stated 
above, total solids mass rate will range from approximately 12,200 
kg/hr-m.sup.2 to 261,000 kg/hr-m.sup.2. 
A stripping gas, preferably steam, is introduced via line 18 into a lower 
portion of the retort and passes upwardly through the vessel in 
countercurrent flow to the downwardly moving solids. The flow rate of the 
stripping gas should be maintained so as to produce a superficial gas 
velocity at the bottom of the vessel in the range of approximately 30 cm 
per second to 150 cm per second, with a preferred superficial velocity in 
the range of 30 cm per second to 60 cm per second. Stripping gas may be 
comprised of steam, recycled product gas, hydrogen, or any inert gas. It 
is particularly important that the stripping gas be essentially free of 
molecular oxygen to prevent product combustion within the retort. 
The stripping gas will fluidize those solids having a minimum fluidization 
velocity less than the velocity of the stripping gas. Those particles 
having a fluidization velocity greater than the gas velocity will pass 
downwardly through the retort generally at a faster rate than the 
fluidized particles. Limiting the maximum bubble size and the vertical 
backmixing of the downwardly moving solids produces stable, substantially 
plugflow conditions through the retort volume. True plugflow, wherein 
there is little or no vertical backmixing of solids, allows much higher 
conversion levels of kerogen to vaporized hydrocarbonaceous material and 
cannot be obtained, for example, in a fluidized bed retort with gross top 
to bottom mixing. In conventional fluidized beds, as in stirred-type tank 
reactors, the product stream removed approximates the average conditions 
in the conventional reaction zone. Thus, in such processes, partially 
retorted material is necessarily removed with the product stream, 
resulting either in reduced product yield or a larger reactor volume 
giving much longer average particle residence times. Maintaining 
substantially plugflow conditions by substantially limiting top to bottom 
mixing of solids allows one to operate the process on a continuous basis 
with a much greater control of the residence time of individual particles. 
The use of means for limiting substantial vertical backmixing of solids 
also permits a substantial reduction in size of the retort required for a 
given mass throughput so that the chances for removing partially retorted 
solids with the retorted solids are reduced. Means for limiting backmixing 
and limiting the maximum bubble size, indicated by numeral 42, may be 
generally described as barriers, baffles, dispersers or flow 
redistributors or may, for example, include spaced horizontal perforated 
plates, bars, screens, packing, or other suitable internals. A preferred 
baffle system is described in copending U.S. patent application Ser. No. 
145,290, filed Apr. 30, 1980, for "Baffle System For Staged Turbulent Bed" 
by Spars et al, which is incorporated herein in its entirety by reference. 
Gross vertical backmixing should be avoided, but highly localized mixing is 
desirable for purposes of heat transfer in that it increases the degree of 
contacting between the solids and the solids and gases. The degree of 
backmixing is, of course, dependent on many factors but is primarily 
dependent on the particular internals or packing disposed within the 
retort. 
The product effluent stream comprised of hydrocarbonaceous material, 
admixed with the stripping gas, is removed from the upper portion of the 
retort by conventional means through line 20 and passes to the separation 
zone 22. The product effluent stream will contain entrained fines and it 
is preferred that said fines be separated from the remainder of the stream 
at this juncture. The separation can be effected by any suitable or 
conventional means such as cyclones, pebble beds, and/or electrostatic 
precipitators. Preferably, the fines which are separated from the product 
effluent stream pass via line 24 to the combustor 26. The product effluent 
passes from the separation zone via line 20 to appropriate downstream 
processing to make the final products. 
As the raw oil shale is pyrolyzed in the retort, the kerogen is decomposed 
and driven off in the vapor state, leaving an organic residue on the 
mineral structure. The amount of carbonaceous matter remaining is 
dependent upon various factors. At the temperatures required for 
commercial retorting, the primary factor is the grade or richness of the 
raw oil shale. When Green River kerogen is pyrolyzed at 500.degree. C., it 
yields approximately 62 weight percent oil, 13 weight percent gas, 8 
weight percent water, and 17 weight percent carbon residue. The lower 
grade shales will have proportionately less organic residue. For oil 
shales yielding less than about 0.13 liters per kilogram, the quantity of 
organic residue can be insufficient to supply the total heat requirement 
for retorting the raw shale when directly mixed in the preferred ratios of 
2:1 to 3:1 of spent shale to raw shale and coal after combustion of the 
carbon residue. 
The coal char comprised primarily of carbon and ash provides additional 
energy required to heat the spent shale. While combustor 26 may be of 
conventional design, it is preferred that it be a dilute-phase, liftpipe 
combustor of the type described in copending application Ser. No. 246,555, 
filed Mar. 23, 1981 in the name of Bertelsen, entitled "Process for 
Burning Retorted Shale and Improved Combustor," incorporated herein in its 
entirety by reference. Air is injected into the lower portion of the 
combustor via line 36 and the organic residue on the shale and the coal 
char is burned. Since the coal char contains sulfur, the combustion 
thereof results in the formation of SO.sub.2. The SO.sub.2 reacts with 
calcium and magnesium oxides produced by the decomposition of the 
respective carbonates in the shale minerals. Preferably, the combustion 
zone contains a sufficient ratio of retorted shale to coal to absorb 
substantially all of the SO.sub.x in the absence of added combustion 
catalyst. If the raw shale does not have sufficient initial carbonate 
content to absorb the SO.sub.2, additional carbonates or oxides, for 
example, may be admixed with the feed shale. If the carbonates or oxides 
are also to function as combustion catalysts, they should preferably be 
added in excess, at least 5% excess of the amount needed to absorb 
substantially all of said SO.sub.x. 
The combustion heats the retorted shale to a temperature in the range of 
600.degree. C. to 800.degree. C. and the hot shale, coal, fly ash and flue 
gas are removed from the upper portion of the combustor via line 34 and 
pass to separation zone 36. A portion of the hot spent shale is recycled 
via line 14 to provide heat for the retort. Preferably, said recycled 
shale is classified to remove substantially all of the -200 mesh shale 
fines and coal ash prior to introduction to the retort in order to 
minimize entrained fines carry-over in the effluent product vapor. Hot 
flue gases are removed from the separation zone via line 38 and excess 
spent solids are passed from the zone via line 40. The clean flue gas 
and/or spent solids passing from zone 36 via lines 38 and 40 may be used 
to provide heat for steam generation or for heating process streams. An 
alternative solution is to introduce the catalyst-treated crushed raw coal 
directly to the bottom of the lift combustor. This approach burns the 
volatile content of the coal along with the fixed carbon content or char 
of the coal, but increases the total fresh shale retorting capacity. Such 
a route may be preferable with caking coals or low volatile coals. 
The following amounts of coal, on a moisture- and ash-free basis, are 
required for heat balancing the retort for the specified shale grades at a 
weight ratio of spent shale to fresh shale of 2.5:1. 
TABLE 2 
______________________________________ 
Shale Grade 
Coal Fed To Combustor 
Coal Fed To Retort 
(liter/kg) 
(kg coal/kg shale) 
(kg coal/kg shale) 
______________________________________ 
0.083 0.013 0.022 
0.104 0.007 0.012 
0.126 0.001 0.002 
______________________________________ 
SPECIFIC EMBODIMENT 2 
Referring to FIG. 3, the system of FIG. 2 is modified to provide for 
retorted shale oil as a supplemental carbonaceous material to the 
combustor. The components of FIG. 3 common to FIG. 2 are shown with the 
same reference numerals and operate in the same manner. 
Product effluent from separator 22 passes through line 28 to distillation 
column 52. The distillation column preferably comprises a multitray 
fractionation tower which separates the retorted shale oil product into 
the desired boiling range materials, for example, gas and water vapor 44, 
naphtha 46, kerosene 48 and a heavy shale oil bottoms fraction 50. The 
heavy bottoms fraction removed via line 50 will normally boil above about 
465.degree. C. to above about 555.degree. C. and substantially all the 
remaining fines will be concentrated in this fraction. The heavy fraction 
will normally also contain the more refractory components of the whole 
shale oil, thus facilitating downstream treatment of the latter products. 
In addition to removing the fines which would poison or plug downstream 
catalytic units, alternatively the retort vapors could be partially 
condensed with fines present and the heavy oil produced. 
The heavy bottoms fraction 50 taken from column 52 is pumped or fed by 
conventional means to the bottom of the combustor 26 to provide additional 
energy required to heat the spent shale. Oxidation catalyst is added 
through line 52 through line 50 and prior to entering the combustor 26. Of 
course, the particular recycle rate of material through line 50 will 
depend upon the grade of the shale and can be determined or calculated in 
a straight-forward manner. 
It is contemplated that workers in the art will be able to carry out the 
present invention in a number of embodiments which do not depart from the 
spirit and scope of the invention. Such embodiments are considered as 
equivalents to those disclosed and claimed herein. While the primary 
utility of the process of this invention is for the retorting of lean oil 
shale, it will be useful for any oil shale retorting process where 
supplemental carbonaceous material is added, for example, to conveniently 
process a coal resource located in the proximity of an oil shale retorting 
complex.