Apparatus for the recovery of oil from shale

An apparatus for the recovery of oil from shale is disclosed in which the shale travels through processing zones on a moving grate. Among the processing zones are a destructive distillation zone and a carbon combustion zone. A conduit is provided for recirculating gases to the carbon combustion zone.

BACKGROUND OF THE INVENTION 
The invention relates to a grate retort. In another aspect, the invention 
relates to retorting a material on a moving grate to recover hydrocarbon 
values. 
Various mineral materials, such as oil shale, lignite, and tar sands 
contain hydrocarbon values. Retorting is one manner in which the 
hydrocarbon values can be recovered from the minerals in a more usable 
liquid or gaseous form. One type of retorting system utilizes a traveling 
grate to transport the material through a retorting zone. The material is 
generally subjected to several other steps in addition to retorting for 
reasons of economy. Usually, residual carbonaceous material remaining on 
the residue from the retorting step will be burned off to form hot 
combustion gases and hot particulate residue. The combustion gases are 
conveyed to some other heat requiring step. The hot particulate residue is 
usually cooled prior to being discharged and the hot gases formed in 
cooling the residue are conveyed to some other heat requiring step. 
Recovery of the hydrocarbon values from the mineral matter as a high value 
product and maximizing exploitation of the residual energy value of the 
mineral matter is an area of extensive research. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide a process for recovering oil 
and energy from a mineral material containing hydrocarbon values with a 
high degree of efficiency. 
It is another object of this invention to provide an apparatus for 
recovering oil from a mineral material containing hydrocarbon values 
characterized by highly efficient energy recovery and conversion of the 
hydrocarbon values into oil. 
SUMMARY OF THE INVENTION 
The invention provides a process for recovering oil from a mineral material 
containing hydrocarbon values. The mineral material is carried on a grate 
sequentially through a destructive distillation zone and a carbon recovery 
zone which are defined in a housing. Hot gases are circulated through the 
material in the destructive distillation zone and combustible gases and 
oil are withdrawn. An oxygen-containing gas is introduced into the carbon 
recovery zone to burn off residual carbon. By maintaining the oxygen and 
combustible gas concentration in the carbon recovery zone at low values, 
carbon burnoff can be increased and the temperature of the effluent gases 
and the amount of carbonate decomposition can be controlled. In one 
aspect, the oxygen content of the oxygen-containing gas introduced into 
the carbon recovery zone is reduced by dilution with cooled combustion 
gases recovered from a preheating zone. 
In another aspect of the present invention there is provided an apparatus 
which comprises a housing and a movable grate positioned in the housing 
dividing it into an upper portion and a lower portion. The housing has an 
inlet for introducing particulate material into it and an outlet for 
exhausting the particulate material and the grate is coupled to a means 
for moving it through the housing. The housing is divided into zones along 
the path of the grate. Each of the zones is divided into an upper portion 
and a lower portion by the grate and has a fluid inlet in the upper 
portion and a fluid outlet in the lower portion. The housing is divided 
into at least a destructive distillation zone and a carbon combustion 
zone. The apparatus is characterized by a conduit means defining a flow 
path from the lower portion of the carbon combustion zone to the upper 
portion of the carbon combustion zone, and a means for cooling the 
contents of the conduit means.

DETAILED DESCRIPTION OF THE INVENTION 
According to certain aspects of the invention, a retort 2 comprises a 
housing 4 having an inlet 6 for introducing particulate material and an 
outlet 8 for exhausting particulate material. A movable grate 10 is 
positioned in the housing 4, dividing it into an upper portion 12 and a 
lower portion 14. The grate 10 is movable through the housing by a means 
11. The housing 4 is divided into physically separated zones along the 
path of the grate 10, for example, by a means 16. Each of the zones is 
divided into an upper portion and a lower portion corresponding to the 
upper portion 12 and lower portion 14 into which the housing is divided by 
the movable grate 10. The housing 4 is divided into at least a destructive 
distillation zone 18 and a carbon combustion zone 20. In one embodiment, 
the housing 4 is further divided into a preheating zone 22, a first heat 
recovery zone 24 and a second heat recovery zone 26. A fluid inlet is 
provided in the upper portion of each zone and a fluid outlet is provided 
in the lower portion. A dump zone 28 can be provided apart from the other 
zones which has the particulate material outlet in its lower portion. The 
apparatus is characterized by a means 30 separate from the housing for 
defining a flow path from a lower portion of the carbon combustion zone 20 
to the upper portion of the carbon combustion zone 20, said means 30 being 
routed into heat exchange relationship with means 32 for cooling its 
contents. 
The grate 10 is preferably circular and the housing 4 surrounding it is 
preferably generally toroidal in shape. The destructive distillation zone 
18 is preferably positioned between the preheat zone 22 and the carbon 
combustion zone 20. In one embodiment of the invention, the means 30 
defining a flow path between the lower portion of the carbon combustion 
zone 20 and the upper portion of the carbon combustion zone 20 comprises a 
first conduit means 34 connecting the lower portion of the carbon 
combustion zone 20 with the upper portion of the preheat zone 22 and a 
second conduit means 36 connecting the lower portion of the preheating 
zone 22 with the upper portion of the carbon combustion zone 20. In this 
arrangement, the preheating zone 22 forms at least a portion of the means 
32 for cooling the contents of the means 30 defining the flow path between 
the lower portion of the carbon recovery zone 20 and the upper portion of 
the carbon recovery zone 20. 
In a further embodiment, the cooling means 32 for cooling contents of means 
30 includes a cooler 38. Preferably, the cooler 38 is formed by an 
indirect heat exchanger, most preferably a steam generator. It can be 
conveniently located in or adjacent to the lower portion of the carbon 
combustion zone 20 and is connected with a source 40 of cooling fluid, 
usually water or steam. Steam can be withdrawn from the cooler 38 via line 
42. Flow of coolant from source 40 to the cooler 38 can be controlled by 
means of a temperature sensor 44 associated with the first conduit means 
34 between the cooler 38 and the preheat zone 22. A suitable temperature 
sensor 44 is a temperature recorder controller operable for producing a 
signal representative of the fluid temperature in the first conduit means 
34. A cool fluid feed line 46 connects the source 40 with the cooler 38 
and has a valve 48 positioned therein. A means 50 which can be pneumatic, 
hydraulic or electrical, for example, is associated with the temperature 
sensor 44 and the valve 48 for manipulating the valve 48 responsively to 
the signal from the temperature sensor 44, thereby controlling fluid flow 
to the cooler 38. 
In another aspect, the housing 4 can be provided with a tubular chute 52 
emptying into the upper portion of the preheat zone 22 and forming the 
inlet 6 for introducing particulate material into the housing 4. The chute 
52 has an upper end 54 positioned to receive particulate material from a 
source 56 of particulate material and a lower end 58 positioned so as to 
deposit particulate material on the movable grate 10. A particle feeder 60 
such as a rotary star valve or a screw feeder is positioned near an upper 
portion of the chute 52 to control the admission of particulate material 
into the chute 52. A valve or choke 62 throttles the exhaust of 
particulate material from the chute 52 onto the grate 10. Where the 
tubular chute 52 is used, the means 30 defining the flow path from the 
lower portion of the carbon combustion zone 20 to the upper portion of the 
carbon combustion zone 20 can further comprise a conduit means 63 
connecting an upper portion of the chute 52 with the upper portion of the 
carbon combustion zone 20. 
For further efficiency of heat recovery, the housing 4 can be divided by 
the means 16 to form the first heat recovery zone 24, the carbon 
combustion zone 20 being positioned between the first heat recovery zone 
24 and the destructive distillation zone 18. The fluid inlet in the upper 
portion of the first heat recovery zone 24 can be connected to a source of 
gaseous fluid 64 such as a gas/liquid separation zone 66. The gas/liquid 
separation zone 66 can be connected to the fluid outlet in the lower 
portion of the destructive distillation zone 18 by conduit means 68. The 
gas/liquid separation zone 66 is provided with a gas outlet 69 and a 
liquid outlet 70. A conduit means 72 connects the gas outlet 69 of the gas 
liquid separation means 66 with the fluid inlet in the upper portion of 
the first heat recovery zone 24. A conduit means 73 can connect the fluid 
outlet in the lower portion of the first heat recovery zone 24 with the 
fluid inlet in the upper portion of the destructive distillation zone 18. 
According to still further aspects of the present invention, there is 
provided an external combustor 74 which is connected by a conduit means 76 
to the fluid inlet in the upper portion of the destructive distillation 
zone 18. The housing 4 can be further divided by the means 16 to form the 
second heat recovery zone 26, the first heat recovery zone 24 being 
positioned between the second heat recovery zone 26 and the carbon 
combustion zone 20; the preheating zone 22, the destructive distillation 
zone 18, the carbon combustion zone 20, the first heat recovery zone 24 
and the second heat recovery zone 26 being serially arranged. The fluid 
outlet in the lower portion of the second heat recovery zone 26 can be 
connected to the external combustor 74 by a conduit means 78. A conduit 
means 80 connects the gas outlet 69 of the gas liquid separation zone 66 
with the fluid inlet in the upper portion of the second heat recovery zone 
26. Preferably, the conduit means 80 includes part of the conduit means 72 
and further comprises a condenser 82 having a vapor outlet 84 and a liquid 
outlet 86, the condenser 82 being connected to the conduit means 72 by a 
conduit means 88, the vapor outlet 84 of the condenser 82 being connected 
to the fluid inlet in the upper portion of the second heat recovery zone 
26 by a conduit means 90. 
A source of oxygen-containing gas 92 is connected to the external combustor 
74 by a conduit means 94. Preferably, the conduit means 94 includes a 
blower 95 and an indirect heat exchanger 96 such as a heater. A conduit 
means 98 connects the fluid outlet in the lower portion of the second heat 
recovery zone 26 with the heater to provide the working fluid. The conduit 
means 98 preferably includes a portion of the conduit means 78. The heater 
96 can discharge the working fluid to a vapor outlet line 100 from the 
vapor outlet of the condensor 82 via a line 102. 
A source of supplemental fuel 104 and a conduit means 106 connecting the 
source of supplemental fuel with the external combustor 74 is a desirable 
feature to assist in maintaining the unit in heat balance. The 
supplemental fuel usage can be controlled with a temperature sensor 108 
associated with the conduit means 76 connecting the external combustor 74 
with the destructive distillation zone 18. The temperature sensor 108 is 
operable for producing a signal representative of the fluid temperature in 
the conduit means 76, preferably a signal representative of the 
temperature of the hot gases entering the zone 18. A valve 110 is 
positioned in the conduit means 106 which connects the source of 
supplemental fuel with the external combustor 74. A means 112 is 
associated with the second temperature sensor 108 and the second valve 110 
for manipulating the second valve 110 responsively to the signal from the 
second temperature sensor 108. Preferably, the means 112 comprises a means 
114 associated with the conduit means 106 for determining the rate of flow 
therethrough. A suitable means 114 can be a flow recorder controller, for 
example. The flow recorder controller 114 produces a signal representative 
of the rate of fluid flow through the conduit means 106, biases it with 
the signal established by the temperature recorder controller 108 and 
manipulates the valve 110 responsively to the temperature in the conduit 
means 106. 
Preferably, the supplemental fuel introduced into the external combustor 74 
via conduit 106 and the primary fuel introduced into the external 
combustor 74 via the conduit means 78 are combusted with oxygen-containing 
gas introduced into the combustor 74 via the conduit means 94 at near 
stoichiometric conditions. Preferably, the conduit means 94 includes a 
first branch line 116 and a second branch line 118 each connecting to the 
combustor 74 and flow through the first branch line 116 is manipulated 
responsively to the flow through the conduit means 94 and the flow through 
the second branch line 118 is manipulated responsively to the flow through 
the conduit means 106. Control can be accomplished by a means 120 
associated with the conduit means 78 for establishing a signal 
representative of the rate of fluid flow through the conduit means 78 and 
a means 122 for receiving the signal from the means 120 and establishing a 
signal used to manipulate a valve 124 in the first branch line 116. A 
suitable means 120 comprises a flow recorder controller. A suitable means 
122 can comprise a ratio controller. The signal received by the valve 124 
from the ratio controller 122 can be electric, pneumatic or hydraulic in 
nature, for example. Similarly, a valve 126 positioned in the second 
branch line 118 receives a signal from a means 128 and is manipulated 
thereby to control the rate of fluid flow through the line 118. The means 
128 can comprise a ratio recorder controller, for example. The means 128 
is associated with the flow recorder controller 114 so as to receive a 
signal therefrom representative of the rate of fluid flow through the line 
106 and manipulate the valve 126 in response to the signal. 
For good operating results, it is preferred in one embodiment that each of 
the zones 18, 20, 22, 24 and 26 be operated at about the same pressure and 
utilize downflow of gases through the grate. By maintaining the zones at 
about equal pressure, cross flow between the zones can be drastically 
reduced. Preferably the means 16 dividing the housing in the zones is 
formed by a series of curtains or flaps 130 above the grate 10 and 
partitions or baffles 132 below the grate. A gas purge, such as an inert 
gas purge, can be utilized between the flaps, if desired, to cut down on 
cross flow. The further the zones are spaced apart by the means 16, the 
less gas leakage will occur between them. This type of seal, however, 
reduces throughput capacity of the device because active traveling grate 
area must be used to provide the seal space between the zones. The sides 
of the grate 10 can be liquid sealed, for example, to prevent gas 
channeling around the sides of the grate. In addition to the curtains and 
partitions above and beneath the traveling grate, each of the zones 18, 
20, 22, 24 and 26 has a means associated therewith so that the zones will 
be maintained at about equal pressure with respect to each other. To 
accomplish this, a pressure recorder controller 134 connected to a motor 
valve 136 and a blower 138 are associated in series with the fluid outlet 
of the preheating zone 22; a pressure recorder controller 140 connected to 
a motor valve 142 and a blower 144 are associated in series with the fluid 
outlet of the destructive distillation zone 18; a pressure recorder 
controller 146 connected to a motor valve 148 and a blower 150 are 
associated in series with the fluid outlet from the carbon recovery zone 
20; a pressure recorder controller 152 connected to a motor valve 154 and 
a blower 156 are associated in series with the fluid outlet of the first 
heat recovery zone 24; and a pressure recorder controller 157 connected to 
motor valve 158 and a blower 160 are associated in series with the fluid 
outlet of the second heat recovery zone 26. 
A means 162 is further provided for controlling the pressure in the 
preheating zone 22. The means 162 comprises a bypass line 164 which 
connects the conduit means 34 with the conduit means 36. The bypass line 
164 preferably has a cooler 166 associated therewith for recuperating at 
least a portion of the heat content of the stream carried by the line. If 
desired, the preheat zone 22 can be bypassed altogether by closing the 
valve 165. A valve 168, preferably a motor valve, is also positioned in 
the line 164 and is manipulated responsively to a signal from a means 170 
for establishing a signal representative of the pressure in the upper 
portion of the zone 22. A suitable means 170 comprises a pressure recorder 
controller. 
To further assist in pressure control of the zone 18, a bypass line 172 
connects the conduit means 73 with the conduit means 88, which connects 
the conduit means 72 with the condenser 82. A cooler 174 is preferably 
positioned in the line 172 to recuperate a portion of heat therefrom. A 
valve 176, preferably a motor valve, is positioned in the means 73 between 
the bypass line 172 and the zone 18 and is associated with a means 178 for 
establishing a signal representative of the pressure in the upper portion 
of the zone 18 so as to be manipulated thereby. The means 178 can comprise 
a pressure recorder controller, for example. Preferably, the conduit means 
73 empties into the conduit means 76 connecting the external combustor 74 
with the zone 18 to temper the hot gases entering the zone 18 and reduce 
the possibility of localized hot spots and excessive thermal cracking or 
carbonate decomposition. 
A branch line 180 off conduit means 94 connects the source of 
oxygen-containing gas 92 with the upper portion of the carbon recovery 
zone 20. The branch line 180 preferably has a valve, perferably a motor 
valve 182 associated therewith. A pressure sensor 184, such as a pressure 
recorder controller, is associated with the upper portion of the zone 20 
so as to generate a signal representative of the pressure in the upper 
portion of the zone 20. The valve 182 is connected to the pressure 
recorder controller 184 so as to be manipulated in response to the 
pressure in the zone 20. 
According to further aspects of the present invention, there is provided a 
process for recovering oil from a mineral material which contains 
hydrocarbon values. Generally the mineral material is selected from the 
group consisting of lignite, tar sands, and oil shale and preferably the 
mineral material comprises oil shale. Generally, the mineral material will 
be in particulate form having a particle size which is usually in the 
range of from about 0.1 to about 10 inches and preferably in the range of 
from about 0.2 to about 6 inches. The process is applicable to material 
which is carried on a grate sequentially through a destructive 
distillation zone or pyrolysis zone and a carbon recovery or combustion 
zone. A straight line or circular grate can be used. Preferably, a 
circular grate is used since the capital investment will be less. 
The process is characterized by a molecular oxygen-containing gas being 
circulated into the carbon combustion zone which contains between about 1 
and 12 percent by volume of molecular oxygen and has a heat of combustion 
of less than about 50 BTU/SCF. This stream can be formed by diluting air 
with combustion gases from the carbon combustion zone, preferably after 
they have been cooled. The hot gases can be cooled by circulating them 
through a preheating zone preceding the pyrolysis zone. Preferably, the 
hot gases and material in the preheating zone flow in countercurrent 
arrangement since intimate countercurrent contact provides for highly 
efficient heat transfer. However, the invention is also applicable to 
other methods of hot gas circulation through the preheating zone, such as 
down flow of the gases through the material on a grate. Generally 
speaking, the material in the preheating zone is heated to a temperature 
in the range of from about 300.degree. to about 800.degree. F., generally 
from about 400.degree. to 750.degree. F., and preferably in the range of 
from about 450.degree. to about 650.degree. F. It is important that the 
material in the preheating zone not be heated to a temperature which 
causes substantial hydrocarbon evolution because this would raise the 
energy content of the stream entering the carbon combustion zone. For 
example, preheating the material in the preheat zone to a temperature of 
about 550.degree. F. can be used to provide good results. Generally, the 
hot gases entering the preheat zone will be at a temperature in the range 
of from about 400.degree. F. to about 1400.degree. F., although it is more 
desirable that the upper temperature limit be maintained below a 
temperature which might result in substantial hydrocarbon evolution. For 
this reason, a temperature range from about 400.degree. to about 
900.degree. F. is expected to provide good results. Most preferably, the 
hot gases are at a temperature in the range of from about 500.degree. to 
about 750.degree. F. The time over which the preheating step is carried 
out will generally range from about 30 seconds up to 30 minutes or so. 
Longer time periods can be utilized if desired but will require a longer 
grate and are not expected to yield an appreciable advantage. Usually, the 
preheating step will be carried out over a time period in the range of 
from about 2 minutes to about 20 minutes. 
The gases withdrawn from the preheating zone will generally be 
substantially cooler than the gases which entered. Preferably, these gases 
contain little or no combustible component such as hydrogen, carbon 
monoxide, and light hydrocarbons such as methane, ethane and the like, and 
preferably have a heat of combustion of less than 25 BTU/SCF. Usually, the 
gases withdrawn from the preheating zone will be at a temperature in the 
range of from about 50.degree. F. to about 250.degree. F., preferably in 
the range of from about 70.degree. F. to about 150.degree. F. or such 
temperature as will provide suitable stack draft where all or a portion of 
the gases are to be exhausted. Most preferably, the gases withdrawn from 
the preheating zone will consist essentially of inert components which are 
unoxidizable or reducible under conditions found in the unit. A mixture of 
nitrogen, carbon dioxide, and water vapor is presently most preferred. 
Hot gases are circulated through the material in the destructive 
distillation zone. Pyrolysis and evolution of oil is significant at 
temperatures above 800.degree. F., so the pyrolysis zone is usually 
designed to heat the material to a temperature in the range of 800.degree. 
to say 1600.degree. F. Selectivity for oil is greater at the lower 
temperatures and selectivity for gases is greater at the higher 
temperatures in this range. Where oil is the desired product and the 
material comprises oil shale the destructive distillation zone will be 
operated to heat the mineral material, generally to a temperature in the 
range of 800.degree. F. to about 1200.degree. F., usually in the range of 
from about 850.degree. F. to about 1050.degree. F. Oil shale is preferably 
brought up to the desired temperature over a time period in the range of 
from about 0.5 minutes to about 50 minutes, preferably over a time period 
in the range of from about 1 to 10 minutes where it has been preheated to 
a temperature in the range of 500.degree. F. to 600.degree. F. The time 
period spent by the material in the destructive distillation zone will 
usually be in the range of from about 5 minutes to about 500 minutes, 
usually in the range of from about 10 to about 100 minutes. Flow of gases 
through the material on the grate is preferably downwardly so that oil 
which is drawn off can be collected with the assistance of gravity. In 
order to bring the material up to destructive distillation temperatures in 
a reasonable period of time, the hot gases introduced into the destructive 
distillation zone are usually at a temperature well above the temperature 
which is desirable to impart to the material. Where the hot gases are 
formed in an external combustor, they will usually comprise carbon 
dioxide, water, and nitrogen and generally be at a temperature in the 
range of from about 950.degree. to about 1950.degree. F., or metallurgical 
limits, usually in the range of from about 1100.degree. to about 
1600.degree. F., and preferably at a temperature in the range of from 
about 1200.degree. to about 1400.degree. F. The gases and liquids 
withdrawn from the destructive distillation zone will generally be at a 
temperature in the range of from about 500.degree. to 800.degree. F., 
usually in the range of from about 600.degree. to 700.degree. F. The oil 
and product combustible gases, usually termed process gases, are withdrawn 
from the destructive distillation zone and charged to a vapor/liquid 
separation zone for separation into desired product streams. 
A blended oxygen-containing gas is introduced into the carbon recovery 
zone. Preferably, the blended gases introduced into the carbon recovery 
zone consist substantially of nitrogen, carbon dioxide, and water vapor 
and have a free oxygen content in the range of from about 1 mole percent 
to about 10 mole percent, usually from about 2 mole percent to about 8 
mole percent and preferably in the range of from about 3 mole percent to 
about 7 mole percent. The oxygen-containing gas which is blended with the 
inert gases, such as withdrawn gases from the preheating zone, preferably 
is drawn from air. Where the air is preheated prior to being blended and 
introduced into the carbon recovery zone, it is preferably heated 
sufficiently to impart to the gas introduced into the carbon recovery zone 
a temperature in the range of from about 120.degree. F. to about 
1200.degree. F., usually in the range of from about 200.degree. F. to 
about 900.degree. F., preferably in the range of from about 250.degree. F. 
to about 750.degree. F. The oxygen reacts with the hot material entering 
the carbon recovery zone, burning the carbon and liberating heat. The 
material entering the carbon recovery zone is generally at the final 
retorting temperature, usually in the range of from about 900.degree. to 
about 1200.degree. F. The hot gases produced by the carbon combustion 
process are generally at only a slightly higher temperature, preferably 
below the temperature at which carbonate decomposition becomes 
substantial, which is about 1300.degree. F. Usually, the hot gases 
produced during carbon recovery will have a temperature in the range of 
from about 1000.degree. F. to about 1400.degree. F., usually in the range 
of from about 1050.degree. F. to about 1300.degree. F. Depending on the 
length of the carbon recovery zone and the preheated temperature of the 
combustion supporting gases introduced into the carbon recovery zone the 
material exiting carbon recovery on the grate will generally have cooled 
somewhat. Usually, however, the material will still contain considerable 
heat. For example, it will generally be at a temperature in the range of 
from about 500.degree. F. to 1000.degree. F., usually at a temperature in 
the range of from about 600.degree. F. to about 900.degree. F. 
At least a portion of the hot gases from the carbon recovery zone can 
desirably be introduced into the preheating zone for circulating through 
the material on the grate therein. Preferably, the hot gases from the 
carbon recovery zone are first cooled to form a tempered hot gas stream by 
passing the hot gases into indirect heat exchange relationship with a flow 
of cooling fluid. Usually, the cooling fluid will be water or steam and 
steam or superheated steam will be produced by cooling the hot gases from 
the combustion zone. Where it is desired to regulate or temper the 
temperature of the hot gas stream from carbon combustion a desirable 
control scheme comprises sensing the temperature of the hot gas stream 
after it has been tempered, generating a signal representative of the 
temperature, and manipulating the flow of cooling fluid responsively to 
the signal. A temperature recorder controller can be associated with the 
tempered hot gas stream to compare the signal representative of the 
measured temperature with a set point signal representative of the desired 
temperature, generating a comparison signal representative of the 
comparison and the flow of cooling fluid can be manipulated by motor valve 
responsively to the comparison signal. Whether or not the hot gases from 
the carbon combustion zone are tempered, they can be withdrawn and divided 
into a first portion and a second portion for the purposes of controlling 
the pressure in the preheating zone and maximizing heat recovery from the 
stream. A first portion of the hot gases from the carbon recovery zone can 
be introduced into the preheating zone. A signal representative of the gas 
pressure in the preheating zone can be generated. The flow of the second 
portion of gases from the carbon recovery zone can be manipulated 
responsively to the pressure signal. The heat from the second portion of 
gases can be recuperated by passing the second portion of hot gases from 
the carbon recovery zone into indirect heat exchange relationship with a 
flow of cooling fluid. These gases can be exhausted in the stack or at 
least a portion of them circulated back to the carbon recovery zone to 
reduce the oxygen concentration in the carbon recovery zone. 
In one arrangement, the mineral material is fed onto the moving grate from 
an enclosed chute. It can be preheated as it is fed down the chute and 
loaded onto the grate. In this embodiment the enclosed chute functions as 
the preheating zone. A first portion of the gases from the preheating zone 
can be withdrawn from an upper portion of the chute so that the material 
and the first portion of hot gases pass through the chute in 
countercurrent flow. A second portion of the gases can be withdrawn from 
the preheating zone, such as the lower portion thereof. Preferably, at 
least a portion of the first and/or second portion of gases withdrawn from 
the preheating zone are charged to the carbon recovery zone. Usually, 
these portions will be combined prior to being introduced into the carbon 
recovery zone. 
Preferably, the material from the carbon recovery zone is carried on the 
grate from the carbon recovery zone into a first heat recovery zone and 
gases are circulated through the material in the first heat recovery zone 
to recover a first portion of the heat therefrom. Preferably, at least a 
portion of the gases from the destructive distillation process are 
introduced into the first heat recovery zone for circulating through the 
material on the grate. These process gases can be withdrawn from the first 
heat recovery zone and recirculated to the destructive distillation zone 
since they generally will have been heated to a temperature higher than 
the temperature of the material exiting the preheating zone. Usually, 
where the combustible gases from the destructive distillation zone are 
passed through the first heat recovery zone they will be introduced at a 
temperature in the range of from about 100.degree. F. to about 500.degree. 
F., usually in the range of from about 100.degree. F. to about 300.degree. 
F. The temperature of the gases withdrawn from the first heat recovery 
zone will generally be in the range of from about 600.degree. F. to about 
1000.degree. F., usually in the range of from about 750.degree. F. to 
about 950.degree. F. If desired, the hot combustible gases from the first 
heat recovery zone can be added as diluent to the stream to enter the 
destructive distillation zone from the combustor. In this manner, the 
temperature in the external combustor can be maintained sufficiently high 
to provide for smooth combustion and the temperature of the gases 
introduced into the destructive distillation zone can be maintained 
sufficiently low so as to avoid excessive gas formation or exceeding the 
metallurgical temperature limits of equipment. 
The material is then preferably carried on the grate from the first heat 
recovery zone into a second heat recovery zone. In the second heat 
recovery zone, gases are circulated through the material to recover a 
second portion of the heat therefrom. Preferably, the gases circulated 
through the material in the second heat recovery zone are formed from a 
portion of the combustible or process gases from the destructive 
distillation zone. These gases will preferably enter the second heat 
recovery zone at a temperature in the range of from about 100.degree. F. 
to about 400.degree. F., preferably at a temperature in the range of from 
about 100.degree. F. to about 200.degree. F. and be preheated in the 
second heat recovery zone to a temperature in the range of from about 
200.degree. F. to about 750.degree. F., usually in the range of from about 
250.degree. F. to about 650.degree. F. The material on the grate entering 
the second heat recovery zone will usually be at a temperature in the 
range of from about 250.degree. F. to about 850.degree. F., usually in 
the range of from about 300.degree. F. to about 700.degree. F. The 
material exiting the second heat recovery zone can be discharged if 
desired and will usually be at a temperature in the range of from about 
150.degree. F. to about 350.degree. F. 
At least a portion of the process gases from the second heat recovery zone 
are preferably introduced into an external combustion zone where they are 
combined with an oxygen-containing gas to form hot combustion gases at 
least a portion of which can in turn be introduced into the destructive 
distillation zone for circulation through the material on the grate. If 
desired, a second portion of the hot combustible gases from the second 
heat recovery zone can be circulated into indirect heat exchange 
relationship with the oxygen-containing gas to be introduced into the 
external combustion zone. To provide a more flexible process, the flow of 
oxygen-containing gas can be split into two or more streams. The first 
flow of the oxygen-containing gas can be utilized to combust the process 
gas from the second heat recovery zone. A second portion of the 
oxygen-containing gas can be utilized to combust a supplemental flow of 
combustible fluid which can originate apart from the unit, natural gas, 
for example, to provide supplemental heat for the hot combustion gases to 
be circulated through the material in the destructive distillation zone. 
Where used, the combustion of process gas and supplemental gas can be 
controlled by sensing the flow rate of the first flow of combustible 
fluid, which can be the process gas flow for example, generating a first 
signal representative of the first flow rate, and manipulating the first 
flow of oxygen-containing gas responsively to the first signal. The flow 
rate of the second flow of combustible fluid, which can be the 
supplemental gas for example can be sensed and a second signal generated 
which is representative of the second flow rate, and the second flow of 
oxygen-containing gas manipulated responsively to the second signal. The 
temperature of the hot combustion gases produced by the external combustor 
can be regulated by sensing the temperatures of the combustion gases 
formed by combusting the first and second flows of combustible fluid with 
the first and second flows of oxygen-containing gas and establishing a 
third signal representative of the temperature, establishing a fourth 
signal representative of a predetermined relationship between the second 
signal (supplemental fuel) and the third signal and manipulating the flow 
rate of the second flow of combustible fluid responsively to the fourth 
signal. In a preferred embodiment, a temperature recorder controller 
connected to the combustion gas conduit immediately prior to the 
destructive distillation zone produces the signal which biases the signal 
from the flow recorder controller, manipulating the flow of gas through 
the supplemental feed line. 
The invention is illustrated by the following example. 
EXAMPLE 
A simple experiment was conducted to determine how the presence of 
combustible gas in the oxidant stream influences the flame front during 
carbon burnoff of spent shale. For each of three runs a 4-gram sample of 
retorted western oil shale was ground to 20-40 mesh and packed into a 1/2" 
I.D. transparent tube in four 1-gram portions separated by a thin layer of 
quartz chips. The bed void fraction was about 0.45 and volume was about 
4.4 cm.sup.3. The tube was placed in a furnace heated to 1200.degree. F. 
(650.degree. C.) and the oxidant stream, at about 1200.degree. F., 1 
atmosphere, and comprising 5.23 vol.% O.sub.2 in helium was flowed through 
the tube at about 150 cm.sup.3 /min to provide a residence time of 0.8 
seconds and a space velocity of 2000 hr.sup.-1. At various time intervals, 
the tube was pulled from the furnace and the progress of the flame front 
was visually estimated by the location of the bed discontinuity between 
the layers of quartz chips. The table below illustrates the effect of 
small amounts of methane on the advance of the combustion front. A similar 
effect was observed when the test was conducted with carbon monoxide as 
the combustible in the oxidant stream. 
TABLE 
______________________________________ 
Time Front Location, % of bed 
(min) 0 BTU/SCF Me 26 BTU/SCF Me 
55 BTU/SCF Me 
______________________________________ 
5 19 15 15 
10 32 28 -- 
15 -- 40 34 
20 60 50 -- 
25 -- -- 45 
30 91 70 -- 
35 100 -- 52 
40 -- 92 -- 
45 -- -- -- 
50 -- 97 -- 
60 -- -- 65 
92 -- -- 75 
180 -- -- 84 
______________________________________ 
At 26 BTU/SCF, the oxidant stream contained about 2.6 vol.% methane and the 
burning of residual carbon was markedly impaired. At 55 BTU/SCF the 
oxidant stream contained about 5.5 vol.% methane and carbon burnoff was 
seriously impaired.