Oil shale retorting process with desulfurization of flue gas

A method is provided for retorting oil shale whereby full utilization of the heat energy available in the retorted shale and maximum desulfurization of the flue gas released to the atmosphere are simultaneously effected. Basically, the process comprises passing a crushed shale feed upwardly through preheating and retorting zones in a retort vessel wherein eduction of shale oil and product gases is achieved by direct heat exchange with a preheated, recycled portion of said product gases passed countercurrently to the shale feed, and then passing the retorted shale downwardly through combustion and cooling zones. Complete combustion of coke on the retorted shale in the combustion zone not only results in full utilization of the potential heat energy stored within the retorted shale but also in the production of gaseous sulfur components (mostly SO.sub.2) that chemically react with the alkaline components of the shale. Concurrent flow of gas and retorted shale in the combustion zone at temperatures between 900.degree. and 1670.degree. F permits the reaction between said SO.sub.2 and the alkaline components of the shale to proceed essentially to completion, thus desulfurizing the flue gas produced in said combustion zone.

BACKGROUND OF THE INVENTION 
This invention relates generally to a process for the treatment of 
oil-containing or oil-producing solids to extract fuel gases and liquid 
crude oil products therefrom. More particularly, the invention relates to 
a process for the retorting of oil shale to produce a high BTU product gas 
in addition to liquid shale oil and, at the same time, to recover as much 
heat energy from the retorted oil shale as is practicable while at the 
same time discharging a flue gas essentially free of sulfur compounds to 
the atmosphere. 
Vast deposits of oil shale, a sedimentary inorganic rock containing about 
35 weight-percent calcite (CaCO.sub.3), 15 weight-percent dolomite 
(MgCO.sub.3.CaCO.sub.3), and 10 weight-percent alkali metal salts are 
known to exist in the United States, especially in the Green River 
formation in Colorado, Utah, and Wyoming. The oil shale in these deposits 
contains between 5 and 35 weight-percent of hydrocarbons in a form known 
as kerogen. When pyrolized, this kerogen decomposes to produce crude shale 
oil vapors, which, upon condensation, become a valuable source of fuel. 
Several pyrolytic processes have heretofore been developed to produce crude 
shale oil from oil shale. One such process is shown in my previous U.S. 
Pat. No. 3,361,644, which is incorporated herein by reference. In this 
process oil shale is fed upwardly through a vertical retort by means of a 
reciprocating piston. The upwardly moving oil shale continuously exchanges 
heat with a downwardly flowing high-specific-heat, hydrocarbonaceous 
recycle gas introduced into the top of the retort at about 1200.degree. F. 
In the upper section of the retort (the pyrolysis zone), the hot recycle 
gas educes hydrogen and hydrocarbonaceous vapors from the oil shale. In 
the lower section (the preheating zone), the oil shale is preheated to 
pyrolysis temperatures by exchanging heat with the mixture of recycle gas 
and educed hydrocarbonaceous vapors plus hydrogen. Most of the heavier 
hydrocarbons condense in this lower section and are collected at the 
bottom of the retort as a product oil. The uncondensed gas is then passed 
through external condensing or demisting means to obtain more product oil. 
The remaining gases are then utilized as a product gas, a recycle gas as 
hereinbefore described, and a fuel gas to heat the recycle gas to the 
hereinbefore specified temperature of 1200.degree. F. 
The advantages of this process, especially in comparison to those processes 
wherein retorting heat is generated by combustion within the retort 
itself, and wherein a gas containing air is used as the 
combustion-eduction gas, are numerous. Firstly, the product gas is of high 
BTU content and is therefore suitable as a commercial fuel. Secondly, by 
using a high specific heat recycle gas, it is possible to educe more oil 
from the shale rock per volume of recycle gas utilized; thus, higher mass 
velocities of oil shale can be employed and no loss in yield is realized. 
Also, the use of a recycle gas containing essentially no oxygen avoids the 
oxidation and degradation of the shale oil product into gums, tars, etc. 
Furthermore, since the recycle gas is heated by means external to the 
retort, retorting temperature control difficulties (which usually result 
in excessive cracking of shale oil vapors in the retorting zone and the 
formation of clinkers which adversely affect the flow of the oil shale in 
the retort) are not encountered. Moreover, because of the better control 
of temperature in the retorting zone, the process can be so optimized that 
minimum heating rates, maximum oil yields, and a minimizing of the amount 
of coke left on the retorted shale can all be achieved. Lastly, no problem 
is encountered, as is common in prior art gas-upflow retorting processes, 
of refluxing of product oil in the preheating and eduction zones, with 
consequent loss of yield by polymerization into heavy residual fractions; 
instead, the condensed liquid product is continuously swept by gravity and 
gas flow away from the retorting zone. 
However, one disadvantage in the foregoing process resides in the use of a 
portion of the product gas as fuel for heating the recycle gas, rather 
than using the coke on the spent shale. This represents a loss in thermal 
efficiency and a wasting of potential heat energy. Prior art attempts to 
use the coke in the retorted shale to provide heat energy for heating the 
recycle gas usually result in other disadvantages. For example, in U.S. 
Pat. No. 3,503,869 to Haddad, the use of the coke in retorted shale as a 
source of fuel necessarily results in a dilution of the product gas with 
gaseous products of combustion; this produces a product gas of lower BTU 
content than that produced in processes having effective means for 
separating the recycle gas and the flue gas. Thus, a method is required 
which will utilize the potential heat energy of the coke on the retorted 
shale without also sacrificing the advantages obtained in my previously 
described process. 
In addition to the difficulties posed by the foregoing, the development of 
a practical shale oil recovery method is also hampered by the fact that 
the operation of present commercial processes results in the atmospheric 
discharge of flue gases containing excessive proportions of sulfur 
compounds sometimes in excess of 3000 ppmv total sulfur compounds. In the 
U.S.S.R., for example, one of the major impediments to the development of 
a successful shale oil recovery process is the difficulty in preventing 
the atmospheric discharge of sulfur compounds. (See Oil & Gas Journal, 
Vol. 73, No. 40, Oct. 6, 1975, pages 42-43). 
A review of present oil shale retorting techniques will reveal that the 
discharge of sulfur compounds therefrom is especially difficult to 
prevent. In processes wherein a portion of the product gas is utilized as 
a fuel to provide heat for retorting purposes, the H.sub.2 S normally 
present in said fuel is also burned and is hence discharged to the 
atmosphere as SO.sub.2. In those processes wherein the coke on the 
retorted shale is burned to provide direct heat for retorting, the 
operating conditions are usually such that only partial combustion of the 
coke is effected, this being necessary to prevent temperatures in the 
combustion zone from becoming excessive and thus causing clinkering and 
unnecessary cracking of the shale oil vapors. In so doing, however, 
H.sub.2 S, a contaminant which some air pollution regulations require to 
be discharged in concentrations no greater than about 10 ppmv, is released 
from the coke in substantial amounts and must be removed from the flue 
gases by means of costly sulfur recovery processes. On the other hand, in 
those processes in which the coke is fully combusted, the resulting flue 
gases may contain excessive amounts of SO.sub.2, another pollutant whose 
atmospheric discharge must be controlled. In Colorado, for example, 
SO.sub.2 is required to be discharged in concentrations no greater than 
150 ppmv, or no greater than 500 ppmv if the total amount of SO.sub.2 
discharged in one day is no greater than 5 tons. 
It is therefore one object of the present invention to provide an oil shale 
retorting process which significantly reduces atmospheric pollution caused 
by discharge of gaseous sulfur compounds. It is another object to provide 
a process combining the advantages of my process described in U.S. Pat. 
No. 3,361,644 with that of utilizing to the fullest extent possible the 
potential heat energy available in the coke in the retorted shale. It is 
yet another object to provide an oil shale retorting process whose overall 
efficiency is maximized by converting most of the kerogen in the oil shale 
to useful products of shale oil and undiluted, high BTU fuel gas, while 
the remainder is utilized to the fullest extent possible as a source of 
heat energy. It is another object to provide a method for continuously 
combusting essentially all the coke on the retorted shale traversing the 
combustion zone of an oil shale retorting process and, at the same time, 
continously removing from said combustion zone a flue gas that is 
essentially free of sulfur compounds. Other objects will appear to those 
skilled in the art from the specification and claims herein. 
SUMMARY OF THE INVENTION 
The present invention provides a novel oil shale retorting process which 
utilizes essentially all the potential heat energy available in the 
retorted shale, produces a flue gas containing no more than about 100 ppmv 
of total sulfur compounds, and produces a high BTU product gas and an 
essentially unoxidized product shale oil. 
One embodiment of the process of the invention involves firstly passing a 
crushed shale feed upwardly in a vertical retort wherein, by direct heat 
exchange with a countercurrently fed eduction gas, shale oil vapors and 
product gases are educed in a pyrolysis zone and the separated by the 
condensation of said shale oil vapors in a subjacent preheating zone, from 
which product gases and liquid shale oil product are collected. The 
eduction gas consists of a preheated portion of the product gas, and, 
since it therefore is of the same chemical makeup as the uncondensed 
product vapors, the final product gases are not diluted with N.sub.2, 
O.sub.2, or excessive quantities of CO and CO.sub.2 ; they thus retain 
their high BTU content. Also, because the eduction gas contains 
essentially no oxygen, the liquid shale oil product is collected 
unoxidized and essentially free of undesirable polymers, gums, and sludge. 
After being retorted, the shale is passed into a combustor wherein it 
gravitates successively through a combustion zone and a cooling zone. 
Eduction gases employed in the retort and gases present in the combustor 
are maintained separately from each other by means of a steam seal between 
the retort and the combustor, the upper portions of both of which are 
preferably maintained at an equal gas pressure. Separating the retort and 
combustor gases in this manner not only prevents the dilution of eduction 
gases with air and flue gases from the combustor or the loss of said 
eduction gases by passage into the combustor, but also makes it possible 
to utilize high temperatures in the combustion zone without also causing 
excessive cracking of shale oil vapors in the retort. An air/flue gas 
mixture utilized to support combustion in the combustion zone of the 
combustor is introduced at the top of the combustion zone, is then passed 
concurrently with the descendingly moving shale, and finally is removed as 
a flue gas at the bottom of said combustion zone. All coke available for 
combustion in the retorted shale is burned in the combustion zone, at a 
temperature between about 900.degree. and 167.degree. F., and essentially 
all of the gaseous sulfur components released in said combustion zone are 
in one or more forms, or converted to one or more forms, that react with 
the alkaline components of the shale traversing the combustion zone to 
produce stable inorganic salts.

DETAILED DESCRIPTION OF THE INVENTION 
Any of a large number of naturally occurring oil-producing solids can be 
used in this process. The characteristics of these materials are generally 
well known and hence need not be described in detail. For practical 
purposes however, the the raw shale should contain at least about 10, 
preferably at least 20, and usually between about 20 and about 80 gallons 
of oil per ton of raw shale by Fischer assay. The shale should be crushed 
to produce a raw shale feed having no particles greater than 6 inches and 
preferably none greater than 3 inches mean diameter. Average particle 
sizes of 1/8-inch to about 3 inches mean diameter are preferred. 
The process may best be understood by reference to FIG. 1 of the drawing. 
It will be understood, however, that for the sake of simplicity 
conventional pumps, compressors, level-control devices, and other 
equipment which form no part of the invention nor aid in its description 
have, for the most part, been omitted. 
Referring now to FIG. 1 of the drawing, raw crushed oil shale is fed at 2 
into hopper 4 of shale feeder 6 from which it is pumped upwardly into 
retort 8. The details of shale feeder 6 are described in more detail in my 
U.S. Pat. No. 3,361,644. The shale feed rate will, of course, vary 
considerably depending upon the size of the retort and the desired holding 
time therein. 
The raw shale passes upwardly through retort 8, traversing a lower 
preheating zone and an upper retorting (or pyrolysis) zone. Temperatures 
in the lower portion of the retort are sufficiently low to condense 
product oil vapors from the superjacent retorting zone. As the shale 
progresses upwardly through the retort its temperature is gradually 
increased to retorting levels by countercurrently flowing eduction gases 
comprising a preheated recycle portion of retort product gas from line 
100. This product gas, and hence also the recycle gas, are of high BTU 
content, generally between about 700 and 1000 BTU/Ft.sup.3, and also of 
high specific high, usually between about 14 and 18 BTU/mole/.degree.F. 
Eduction temperatures are conventional, usually in excess of about 
600.degree. F, and preferably between about 900.degree. and about 
1200.degree. F. Essentially all of the oil will have been educed from the 
shale by the time it reaches a temperature of about 900.degree. F. Gas 
temperatures above about 1300.degree. F in the eduction zone should not be 
exceeded since they result in excessive shale oil cracking. Other 
retorting conditions include shale residence times in excess of about 10 
minutes, usually about 30 minutes to about one hour, sufficient to educe 
the desired amount of oil at the selected retort temperatures. Shale feed 
rates usually exceed about 100, and are preferably between about 400 and 
about 2000 pounds per hour per square foot of cross-sectional area in the 
retort. These values refer to average cross-sectional areas in the tapered 
retort illustrated in the drawing. 
Pressure in retort 8 may be either subatmospheric, atmospheric, or 
superatmospheric. Retorting pressures normally exceed about 0.3 and are 
preferably between about 5 and about 1000 psia. The recycle gas is 
introduced via line 100 at a temperature and flow rate sufficient to heat 
the crushed shale to retorting temperatures. Heat transfer rates depend in 
large part on the flow rate, temperature, and heat capacity of this 
recycle gas. Flow rates of at least about 3000, generally at least about 
8000, and preferably between about 10,000 and about 20,000 SCF of recycle 
gas per ton of raw shale feed are employed. The temperature differential 
between the recycle gas and solids at the top of the retorting zone is 
usually between 10.degree. and 100.degree. F. Excessive temperature 
differentials, e.g., in excess of about 400.degree. F should be avoided. 
As the recycle gas from line 100 passes downwardly through retort 8, it 
continuously exchanges heat with the upwardly moving oil shale. In the 
upper portion of retort 8 oil contained within the oil shale is educed 
therefrom by pyrolysis, thereby producing shale oil vapors and fuel gases 
comprising such normally uncondensable gases as methane, hydrogen, ethane, 
etc. These shale oil vapors and fuel gases pass downwardly with the 
recycle gas, firstly into the lower portion of retort 8 wherein the cool 
oil shale condenses the shale oil vapors, and thence into a frusto-conical 
product disengagement zone 78. This disengagement zone comprises 
peripheral slots 80 through which liquid shale oil and product vapors flow 
into surrounding product collection tank 82. The liquid shale oil is 
withdrawn therefrom at a rate between about 5 and 60 gallons/ton of raw 
shale feed via line 84, while the aforementioned product vapors at a 
temperature between about 80.degree. and 300.degree. F are withdrawn via 
line 86. 
The product vapors are introduced into venturi scrubber 88 wherein a liquid 
scrubbing medium recirculating via line 90 is used to remove any remaining 
traces of water, shale oil vapors, and shale oil mist contained therein. 
The shale oil/water mixture so obtained in venturi scrubber 88 is directed 
by pump 102 via lines 104 and 106 to condenser 108 in which the shale 
oil/water mixture is cooled by indirect heat exchange with cold water to 
provide as much recirculating scrubbing medium via line 90 as necessary, 
the remainder being sent to appropriate shale oil/water separation 
facilities via line 110. An essentially mist-free product gas having both 
high BTU and high specific heat properties is obtained at a rate between 
about 11,000 and 21,000 SCF/ton of raw shale feed from venturi scrubber 
88 via line 92. A portion of this product gas is then sent to storage via 
line 94 while the remainder is recycled to retort 8 via line 96, 
compressor 98, lines 126, 128, and 139, preheater 24, and line 100. 
While the product vapors are being removed from retort 8 via line 86 and 
collected as a product gas via line 94, the retorted oil shale overflowing 
the top of retort 8 falls onto the inclined peripheral floor 10 of shroud 
12, which is affixed in fluid-tight fashion to the outer wall of the 
retort. The retort shale, now at a temperature between about 900.degree. 
and 1300.degree. F, preferably between about 900.degree. and 1100.degree. 
F, then gravitates down floor 10 through chute 14 into the top of vertical 
combustor 16, in which is maintained a combustion zone 18 and upper and 
lower ash-cooling zones 20 and 50. The retorted shale is essentially 
oil-free and, when the preferred operating conditions herein are utilized, 
will contain at least about 2%, usually between 3% and about 5%, and 
preferably at least 3% by weight of carbon as coke. (As used herein, the 
term coke refers to all the carbon-containing components remaining in the 
oil shale after retorting.) Usually, this coke contains at least about 0.5 
wt.%, usually between about 0.5 and 2.0 wt.% of sulfur. 
None of the recycle gas used to educe shale oil and product gas from the 
oil shale in retort 8 is allowed to pass with the retorted shale into 
combustor 16. Chute 14 is provided with a purge sealing gas, preferably 
saturated steam, from line 30 to keep the recycle gas in retort 8 separate 
from the air and flue gases used in combustor 16. This purge steam is 
introduced at a pressure preferably about 0.01 to 15 psi greater than that 
maintained in the upper sections of retort 8 and combustor 16, both of 
which upper sections are preferably maintained at some equal pressure 
between about 5 and 1000 psia. Thus, some of the steam so introduced 
travels up chute 14 countercurrently with the descending retorted shale 
and is withdrawn by suction via line 32 into the throat of the venturi in 
venturi scrubber 34. Typical steam rates of steam fed via line 30 are 
between about 10 and 50 pounds per ton of raw shale feed; between about 40 
and 60% of the steam so fed is recovered via line 32 while 40 to 60% 
commingles with the gases in the combustor. 
Air for the combustion of coke in combustor 16 is provided from line 22. It 
is preheated in heater 24 to between about 100.degree. and about 
800.degree. F and then diluted with a flue gas at between about 
200.degree. and 800.degree. F from line 26. The resultant mixture is fed 
through line 28 into the top of the combustion zone 18 at a temperature 
between about 100.degree. and 800.degree. F. The amount of the flue 
gas-air mixture which is introduced to the combustor 16 via line 28 is 
between about 12,000 and about 34,000 SCF/per ton of raw shale feed, of 
which about 40% to about 90% comprises air from line 22. 
The dilution of the air from line 22 with flue gas from line 26 as 
hereinbefore set forth is critical if external temperature control in 
combustor 16 is to be avoided. The peak temperature in the combustion zone 
should be maintained above about 900.degree. F, usually between about 
1200.degree. and 1670.degree. F, preferably between 1400.degree. and 
1650.degree. F, and more preferably still between 1400.degree. and 
1600.degree. F. Without dilution of the air with the flue gas from line 26 
(or some other source of inert gas) peak temperatures in the combustion 
zone 18 of combustor 16 can easily exceed 1670.degree. F, thereby 
resulting in the discharge via line 38 of a flue gas containing excessive 
proportions of SO.sub.2. 
Preferably, the air-flue gas feed rate and the feed rate of the retorted 
shale in the combustion zone are adjusted so that (a) unconsumed oxygen 
will be present in the flue gas leaving the combustion zone via line 38 
and (b) essentially all the coke contained within the retorted shale while 
is available for combustion is consumed. Generally speaking, at least 80 
wt.%, more usually at least 90 wt.%, of the coke in the retorted shale can 
be consumed under the preferred operating conditions herein, the remaining 
10 to 20 wt.% being so deeply embedded within the sedimentary rock itself 
that it is essentially incombustible. Normally, the design of the 
combustor should be such that shale feed rates in the combustion zone 18 
will be between about 300 and 800 pounds per hour per square foot of 
average cross-sectional area while residence times in the combustion zone 
18 will vary between 0.25 and 2 hours. Such feed rates and residence times 
will insure that maximum combustion of coke contained in the retorted 
shale is effected. 
The desulfurization of the flue gases produced in combustion zone 18 is 
believed to be accomplished herein by the chemical reaction of gaseous 
sulfur components produced in the combustion zone 18, such as SO.sub.2, 
SO.sub.3, etc., with the alkaline components of the retorted or 
decarbonized shale rock to produce stable inorganic salts. (The terms 
alkakline components and alkaline mineral components, as used herein, 
refer to those components of the retorted or decarbonized shale which 
react, or decompose under combustion zone conditions to components which 
react, with one or more gaseous sulfur components at elevated temperatures 
to produce a stable inorganic salt.) Although the invention is not 
intended to be limited to any particular theory, it is presumed that the 
CaCO.sub.3 and, to a lesser extent, the MgCO.sub.3 components of the 
retorted shale rock passing through the combustion zone decompose to CaO 
and MgO, respectively, and that these components react mainly with 
SO.sub.2 or SO.sub.2 and O.sub.2 to form one or more of the salts: 
CaSO.sub.3, CaSO.sub.4, MgSO.sub.3, and MgSO.sub.4. When shale from the 
Green River formation is being processed by the method herein described, 
essentially complete removal of the SO.sub.2 is easily achieved because of 
the relatively large amount of calcite and dolomite available in such 
shale. With such shales, complete desulfurization of the flue gases (i.e., 
to less than 100 ppmv of total sulfur compounds) is easily achieved. For 
other shales complete desulfurization of the flue gases will be achievable 
only if the ratio between the weight percent of calcite plus dolomite in 
the retorted shale to the weight percent of total sulfur in the retorted 
shale is at least 2:1, preferably at least 5:1. In the absence of other 
alkaline components in the raw shale which can chemically react with the 
SO.sub.2, or unless a substantial percentage of said total sulfur is 
present as a stable inorganic component, ratios of calcite plus dolomite 
to total sulfur less than a 2:1 ratio will produce only partial 
desulfurization. 
Desulfurization of the combustion zone flue gas by the process of the 
invention results in the discharge from said combustion zone of no more 
than about 5.0 SCF of total gaseous sulfur compounds per ton of raw shale 
feed. When the preferred combustion zone operating conditions hereinbefore 
recited are utilized, the flue gas leaving the combustion zone (i.e., 
before being combined with the gases utilized in the cooling zone in the 
manner to be shown hereinafter) will contain less than 100 ppmv of total 
gaseous sulfur compounds; this corresponds, on the average, to less than 
about 2.5 SCF of gaseous sulfur compounds discharged from the combustion 
zone per tone of raw shale feed. When conditions are chosen which tend to 
maximize the residence time of the retorted shale in the combustion zone 
maintained at a temperature less than 1600.degree. F, the flue gas 
discharged from said combustion zone will contain less than 50 ppmv of 
total sulfur compounds; this corresponds, on the average, to less than 
about 1.25 SCF of total sulfur compounds discharged per ton of raw shale 
feed. It should be noted, however, that for the range of combustion zone 
conditions given herein, the flue gas limitations of 100 ppmv and 50 ppmv 
of total gaseous sulfur compounds discharged correspond, respectively, to 
the discharge of between about 1.5 and 3.5 and about 0.75 and 1.75 SCF of 
said sulfur compounds per ton of raw shale feed. 
Under ideal conditions it is possible to produce a flue gas containing less 
than 10 ppmv of total sulfur compounds, which corresponds, on the average, 
to less than about 0.25 SCF of sulfur compounds discharged per tone of raw 
shale feed. As shown in the Example hereinafter, it is possible to produce 
a flue gas containing less than 10 ppmv of SO.sub.2. However, trace 
amounts of other sulfur compounds may also be present in the flue gas. COS 
may be present in proportions as high as 30 ppmv while mercaptans may be 
as high as 5 ppmv. The total concentration of other sulfur compounds, 
however, will be less than 5 ppmv. Hence, in accordance with this 
invention, the H.sub.2 S concentration will be less than 5 ppmv in the 
flue gas, usually less than 1 ppmv. 
It is an essential feature of the preferred embodiment of the invention 
that both the maximum desulfurization of the flue gas produced in the 
combustion zone 18, and the full releasing of the heat energy stored in 
the retorted shale be simultaneously effected. Incomplete combustion 
(i.e., with insufficient oxygen) of the coke usually results in the 
liberation of H.sub.2 S in the combustion zone 18, which H.sub.2 S being 
essentially unreactable with the alkaline shale ultimately must be 
discharged as an atmospheric pollutant with the flue gas. But complete 
combustion of the coke insures that essentially all gaseous sulfur 
compounds which were not relesed from the shale as SO.sub.2 will be 
converted to SO.sub.2, which SO.sub.2 is chemically reactable with the 
alkaline components of the shale under the conditions hereinbefore 
specified. Hence by fully releasing the heat energy stored within the 
retorted shale, it is insured that a completely desulfurized flue gas will 
be produced. 
(It should be noted that it is possible in non-preferred embodiments of the 
invention to discharge a completely desulfurized flue gas and not burn all 
the available coke on the retorted shale. For example, if the residence 
time for the retorted shale passing through the combustion zone is 
insufficient to allow for the full combustion of the available coke, the 
flue gas discharged from the combustion zone may still be completely 
desulfurized provided it contains at least some oxygen and provided the 
air/flue gas feed rate is such that the gaseous sulfur components have 
sufficient time to react with the alkaline components traversing the 
combustion zone.) 
Downflowing spent shale from combustion zone 18 gravitates at a rate 
between about 300 and 800 lbs/hr/ft.sup.2 of average crosssectional area 
through upper cooling zone 20, suitable shale residence times therein 
being between about 0.25 and 2 hours. The shale descending in this cooling 
zone is contacted with upwardly flowing flue gas introduced from line 36 
at a temperature between about 200.degree. and 500.degree. F and at a rate 
of 8,000 to 16,000 SCF/ton of raw shale feed. As this flue gas ascends to 
the interface of the upper cooling zone 20 and combustion zone 18, it 
exchanges heat with the descending shale ash by countercurrent heat 
exchange. Upon reaching the interface, it is combined with the hot flue 
gas produced in the combustion zone 18 and the resulting mixture is 
removed via line 38 to a cyclone separator 40 wherein fines are removed 
via line 42. 
To recover the heat from the combined flue gases treated in cyclone 
separator 40, these gases, usually at a temperature between about 
1400.degree. and 1650.degree. F when preferred operating conditions are 
utilized, are passed via line 44 to heater 24 wherein heat is recovered by 
indirect heat exchange for such purposes as preheating the air used in 
combustor 16, preheating the recycle gas used to educe oil from the oil 
shale in retort 8, and heating boiler water for the production of high 
pressure steam. A portion of the combined flue gases used in heater 24 is 
removed at a temperature between about 200.degree. and 800.degree. F via 
line 26 for the hereinbefore described purpose of diluting air to 
combustor 16. The remaining combined flue gases, after having had as much 
heat as practicable extracted therefrom, are divided into two portions, 
one to be sent to atmospheric discharge through lines 46 and 48 at a rate 
of about 15,000 to 35,000 SCF/ton of raw shale feed, and the other to be 
used via lines 46 and 36 as the gaseous cooling medium required in the 
upper cooling section 20 of combustor 16 as hereinbefore described. 
Optionally and preferably, coolant to lower cooling zone 50 is provided by 
a water seal, especially if the operating pressure of combustor 16 is less 
than about 25 psig. As shown in the drawing, a water level is maintained 
in lower cooling zone 50 and in inclined conduit 70. The shale ash 
entering lower cooling zone 50 must drop to the bottom of said zone before 
a drag chain conveyor 74 forces the shale to discharge via line 72. Thus, 
final cooling of the decarbonized shale ash is accomplished by direct heat 
exchange with water delivered from a makeup source via lines 52 and 54 
and/or with water recovered via line 68 from the purge steam employed as a 
sealing gas in chute 14. 
It is emphasized, however, that the water seal method for cooling in the 
lower cooling zone 50 is not critical. In the semi-arid regions wherein 
oil shale is found, it may not be economical to use water in this manner. 
Also, the pressures employed in combustor 16 may make the length of 
inclined conduit 70 prohibitively long. Hence, in these and other 
situations wherein it is not desired to use water for cooling decarbonized 
shale ash, the lower cooling zone 50 can be eliminated entirely. Under 
such circumstances, however, mechanical means should be employed 
substantially to prevent the 200.degree.-500.degree. F flue gas introduced 
into the combustor 16 via line 36 from escaping with the decarbonized 
shale ash via conduits 70 and 71. Conventional star feeders or lock 
systems comprising at least two valves separated by a large compartment 
can be used for this purpose. 
One method by which water is recovered from the purge steam in chute 14 for 
use in lower cooling zone 50 is by introducing the steam via line 32 into 
the throat of the venturi of venturi scrubber 34, said steam thus being 
absorbed and condensed by the cool water fed into the venturi through line 
56. Upon condensation of the steam in this manner, the non-condensable 
gases entrained with the purge steam (i.e., fuel gases and Co.sub.2 formed 
in chute 14 by the reaction of steam with coke in the retorted shale) are 
recovered as a product gas of moderately high BTU and specific heat 
content (200-600 BTU/Ft.sup.3 and 9-13 BTU/mole.degree.F, respectively), 
which product gas is recovered via line 58, normal recovery rates being 
between about 100 and 300 SCF/ton of raw shale feed. The heated water, 
however, which is produced by the mixture of purge steam and cool water, 
is passed via line 60 to a suitable pump 62 from which it is sent by line 
64 to condenser 66 wherein it is cooled by direct or indirect heat 
exchange with an external source of cold water. The cool water so produced 
is then utilized as the source of scrubbing medium for the venturi 
scrubber via line 56, and as a source of coolant for lower cooling zone 50 
of combustor 16 through lines 68 and 54. 
As noted above, a portion of the heat of the combined flue gases entering 
heater 24 can be utilized to heat boiler water to generate high pressure 
steam. This can be accomplished by directing boiler make-up water into 
steam drum 112 from line 124. The water in steam drum 112 is then drawn 
through line 114 by means of pump 116 to be sent by line 118 into heater 
24. After exchanging heat with the flue gases in heater 24, some of the 
water in line 118 is vaporized to pressurized steam, which, after being 
coolected in steam drum 112, is then passed via line 120 to a steam 
turbine 122, or other prime mover, for the generation of electrical power. 
A fossil fuel fired heater 76 is provided for start-up purposes. This 
heater is used to heat the recirculating recycle gas until retort 8, 
combustor 16, and heater 24 come up to operating temperatures at which 
time it is no longer utilized. 
It will be seen by those skilled in the art that the retorting process as 
above described integrates many of the advantages sought by the prior art 
but few, if any, of the disadvantages. For example, the use of concurrent 
flow of gas and shale in the combustion zone 18 provides, as is shown in 
U.S. Pat. No. 3,503,869 to Haddad, a method for effecting better control 
of temperature therein and for minimizing the pressure drop problems 
prevalent in most countercurrent processes due to the accumulation of 
fines. But unlike the process shown in the aforesaid patent, in the 
preferred embodiment of this invention essentially all the available coke 
within the retorted shale is necessarily conbusted so that as much heat 
energy as possible to recovered from the shale, and, simultaneously, the 
discharge of a flue gas essentially free of sulfur compounds is 
accomplished. 
It will also be seen that the over-all thermal efficiency of the process as 
described hereinbefore is maximized. Essentially all the kerogen in the 
oil shale is utilized. A portion of the kerogen is educed from the oil 
shale to produce an unoxidized liquid shale oil product, an undiluted high 
BTU product gas, and a moderately high BTU product gas; the remainder is 
used to provide heat for the process and/or for conversion into useful 
work. 
Further advantages which the art has been attempting to integrate into one 
process are also apparent. The eduction gas, being a recycled product gas, 
contains essentially no free oxygen with which educed oil can combine 
chemically, nor does it contain nitrogen or any more CO and CO.sub.2 than 
that which unavoidably is educed from the oil shale with the shale oil 
vapors; normally, the product gases produced by the process described 
hereinbefore will contain no more than 8% CO and 15% CO.sub.2 (by volume). 
Thus, this recycle gas is both of high specific heat capacity and of high 
BTU content, the former permitting increased shale feed rates, and the 
latter insuring that the product gases are of high BTU content. 
Additionally, none of the product gas is used as a fuel gas in the 
combustion zone, all the necessary fuel therein being supplied by the coke 
in the retorted shale. Refluxing of shale oil vapors in retort 8 is 
prevented by passing the eduction gas downwardly through retort 8, and, 
although high temperatures are utilized in the combustion zone 18, the 
excessive cracking of shale oil vapors usually concomitantly occurring 
therewith in the retort is prevented herein by effectively separating the 
gases produced or utilized in the combustor 16 from those produced or 
utilized in retort 8. Furthermore, water requirements for the process are 
not extreme, inasmuch as the major use of water in lower cooling zone 50 
is optional. Lastly, due to the complete combustion of coke in combustor 
16 enough heat is generated not only for retorting purposes but also for 
generating electrical power or for operating turbine drive units on pumps 
and compressors. 
The following Example is provided to show how the SO.sub.2 concentration in 
a combustion zone flue gas can be controlled by controlling the 
temperature in said combustion zone below 1670.degree. F. 
EXAMPLE 
Hot retorted shale, air, and a recycle flue gas were passed concurrently 
through a combustion zone of a combustor. The retorted shale was fed to 
said combustion zone at the rate of 0.82 ton/ton of raw shale fed to a 
conventional retort from which said retorted shale was obtained at a 
temperature of about 925.degree. F. The air feed rate was maintained at 
14,000 SCF/ton of raw shale feed. The recycle flue gas feed rate was 
varied between about 4,000 and 15,000 SCF/ton of raw shale feed so as to 
control the peak combustion zone temperature at any desired temperature 
between about 1500.degree. and 1750.degree. F. Both the air and recycle 
flue gas were fed at 300.degree. F. The retorted shale residence time in 
the combustion zone was maintained at about 1 hour. Essentially all the 
coke in said retorted shale was consumed. 
As shown in FIG. 2, as long as the peak combustion zone temperature was 
maintained below about 1640.degree. F, the SO.sub.2 concentrations (as 
measured by Drager tube and mass spectrometrical techniques) in the 
combustion zone flue gas was less than 25 ppmv, usually at or less than 10 
ppmv. These figures correspond to a maximum of 0.75 and 0.3 SCF of sulfur 
compounds discharged from said combustion zone per tone of raw shale feed. 
Until the peak combustion zone operating temperature reached 1670.degree. 
and 1680.degree. F, respectively, the SO.sub.2 concentration in the 
combustion zone flue gas was maintained below 50 ppmv and 100 ppmv. These 
figures correspond to a maximum of 1.5 and 3.0 SCF of sulfur compounds 
discharged from said combustion zone per ton of raw shale feed (assuming 
the maximum flue gas rate from said combustion zone is 30,000 SCF/tone of 
raw shale feed). 
H.sub.2 S concentration in the flue gas was always less than 1 ppmv, 
regardless of the peak temperature maintained in the combustin zone. 
Although the invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives, modifications, 
and variations will be apparent to those skilled in the art in light of 
the foregoing description. Accordingly, it is intended to embrace all such 
alternatives, modifications, and variations that fall within the spirit 
and scope of the appended claims.