U.S. patent number 3,925,190 [Application Number 05/492,447] was granted by the patent office on 1975-12-09 for preheating oil shale prior to pyrolysis thereof.
This patent grant is currently assigned to The Oil Shale Corporation. Invention is credited to George C. Kane, Kenneth D. Van Zanten, John A. Whitcombe.
United States Patent |
3,925,190 |
Whitcombe , et al. |
December 9, 1975 |
Preheating oil shale prior to pyrolysis thereof
Abstract
Oil shale is preheated to temperatures in the order of
400.degree.F to 650.degree.F prior to the pyrolysis thereof in a
retort to produce vaporous hydrocarbonaceous products therefrom by
entraining the oil shale with hot flue gases in a series of at
least two dilute phase fluidized beds having an incineration zone
interposed upstream of the final fluidized bed for reheating flue
gases threfrom, and for combusting oil shale fines and hydrocarbon
vapors evolved during the preheating operation and entrained in the
flue gases prior to discharge to the atmosphere.
Inventors: |
Whitcombe; John A. (Los
Angeles, CA), Van Zanten; Kenneth D. (Littleton, CO),
Kane; George C. (Pacific Palisades, CA) |
Assignee: |
The Oil Shale Corporation (Los
Angeles, CA)
|
Family
ID: |
23956292 |
Appl.
No.: |
05/492,447 |
Filed: |
July 29, 1974 |
Current U.S.
Class: |
208/409; 201/12;
208/427; 432/14; 201/41; 208/951; 432/15; 34/371 |
Current CPC
Class: |
C10G
1/02 (20130101); Y10S 208/951 (20130101) |
Current International
Class: |
C10G
1/00 (20060101); C10G 1/02 (20060101); C10G
001/02 () |
Field of
Search: |
;208/11 ;201/12-15
;196/120 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gantz; Delbert E.
Assistant Examiner: Hellwege; James W.
Attorney, Agent or Firm: Brumbaugh, Graves, Donohue &
Raymond
Claims
What is claimed is:
1. A process for preheating oil shale to a temperature in the order
of 400.degree.F to 650.degree.F prior to the pyrolysis thereof in a
retort which comprises heating the oil shale in a series of at
least two dilute phase fluidized beds by entraining partially
preheated oil shale in the final dilute phase fluidized bed with
hot flue gas to provide oil shale preheated to between about
400.degree.F and 650.degree.F for introduction into the retort and
partially cooled flue gas containing entrained oil shale fines and
hydrocarbon vapors, passing the partially cooled flue gas through
an incineration zone and incinerating in the presence of combustion
products the entrained oil shale fines and hydrocarbon vapors
emanating from the final dilute phase fluidized bed to provide a
portion of the fuel requirement to reheat the flue gas, cooling the
flue gas from the incineration zone and entraining crushed raw oil
shale in a first dilute phase fluidized bed with the cooled flue
gas from the incineration zone to provide partially preheated oil
shale for subsequent introduction into the final dilute phase
fluidized bed and cooled flue gas.
2. The process as defined in claim 1 wherein the entrained oil
shale fines and hydrocarbon vapors are incinerated at a temperature
in excess of about 1300.degree.F.
3. The process as defined in claim 2 wherein the residence time of
the entrained oil shale fines and hydrocarbon vapors in the
incineration zone is between about 0.3 second and 1.0 second.
4. The process as defined in claim 1 wherein the flue gas from the
incineration zone is cooled to a temperature below about
900.degree.F.
5. The process as defined in claim 1 comprising the further step of
entraining partially preheated oil shale from the first dilute
phase fluidized bed in a second dilute phase fluidized bed with the
cooled flue gas from the incineration zone to provide partially
preheated oil shale for subsequent introduction into the final
dilute phase fluidized bed and partially cooled flue gas for
introduction into the first dilute phase fluidized bed.
6. The proces as defined in claim 5 comprising the further steps of
passing the partially cooled flue gas from the second dilute phase
fluidized bed through a second incineration zone so as to partially
reheat the flue gas and combust entrained hydrocarbon vapors and
oil shale fines emanating from the dilute phase fluidized beds
downstream of the first dilute phase fluidized bed prior to cooling
and introducing the partially reheated flue gas into the first
dilute phase fluidized bed.
7. The process as defined in claim 1 wherein the oil shale feed to
the final dilute phase fluidized bed is preheated to a temperature
of about 350.degree.F.
8. The process as defined in claim 1 wherein the residence time of
the flue gas in the incineration zone is between about 0.5 second
and 1.0 second at an incineration zone temperature of between about
1400.degree.F and about 1500.degree.F.
Description
BACKGROUND OF THE INVENTION
The pyrolysis of oil shale by solid-to-solid heat transfer
techniques to convert the kerogen content of oil shale into oil and
other gaseous hydrocarbons is well known as exemplified by the
disclosures in U.S. Pat. Nos. 3,265,608 and 3,691,056. In these
prior art processes, oil shale is preheated and thereafter
pyrolyzed by solid-to-solid heat transfer contact with
heat-carrying bodies to produce shale oil and other effluent
vapors. Upon completion of the kerogen conversion, the
heat-carrying bodies are recycled through a reheating zone for
further use in pyrolyzing additional preheated oil shale.
In general, the process herein described is an improvement in the
oil shale preheating procedure disclosed in the above-cited prior
art patents which utilizes the residual sensible heat of the flue
gases previously employed in the process to heat the heat-carrying
bodies. In the above-cited U.S. Pat. No. 3,265,608, the transfer of
heat from the hot flue gases to the crushed shale is accomplished
by entraining the shale with flue gases heated to about
1100.degree.F in a gas lift line, i.e., dilute phase fluidized bed.
The shale is thereby preheated to a temperature of about
300.degree.F. In the preheat zone, the raw shale and hot flue gases
pass co-currently up the lift line during the heat transfer
operation. At the top of the gas lift line, the partially cooled
flue gas is separated from the shale and vented to the atmosphere
at a temperature substantially higher than the temperature to which
the raw shale is preheated prior to being pyrolyzed. While this
preheat procedure conserves a portion of the waste heat contained
in the ball heater flue gas, it has not been completely
satisfactory when utilized to achieve higher levels of preheating,
that is, above about 300.degree.F to 400.degree.F. While the method
of preheating shale is not described in detail in U.S. Pat. No.
3,691,056, the process herein described may be used to advantage to
obtain high levels of preheat.
It is generally known that the conversion of kerogen into shale oil
and other hydrocarbonaceous vapor does not readily occur at
temperatures less than about 500.degree.F, although some of the
very fine raw shale may be prematurely pyrolyzed when contacted
with very hot flue gases. Additionally, oil shale usually contains
some bitumen which can be partially vaporized during the preheat
operation. Because both these situations are likely to occur when
crushed oil shade is preheated to temperatures as high as
600.degree.F prior to entering the pyrolysis zone, these vaporized
hydrocarbons may be lost to the recovery system by being discharged
directly to the atmosphere at relatively high temperatures. Such a
situation is not only economically inefficient, but it is also
ecologically undesirable.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to
provide a process for preheating crushed oil shale prior to the
retorting thereof in a pyrolysis zone without significantly
effecting premature pyrolysis of a majority of the oil shale prior
to the retorting operation.
A further object of the invention is to provide a process for
recovering the heating value of oil shale fines and pyrolyzing the
remaining portion of crushed oil shale so as to recover
substantially 100% of the hydrocarbons contained in the oil
shale.
A still further object of the invention is to provide for
combusting oil shale fines which essentially eliminates the fines
as a potential source of hydrocarbon emissions.
Another object of the present invention is to provide a process for
preheating oil shale prior to the pyrolysis thereof which will
efficiently recover a substantial portion of the heating value of
the hydrocarbons generated during the preheat operation as fuel
within the preheat system and thereby minimize the emission of
hydrocarbons to the atmosphere.
These and other objectives are accomplished according to the
present invention by preheating crushed raw oil shale in a series
of at least two dilute phase fluidized beds, i.e. gas lift lines or
lift pipes, having an incineration zone interposed therebetween
through which the flue gas from the final preheat zone is passed
prior to contacting and preheating additional oil shale to a
temperature between that of the initial raw shale temperature and
the final preheat temperature prior to introduction of the
preheated shale into the pyrolysis zone. When oil shale is
preheated in this manner, it is possible to heat the shale to a
temperature in the range of from about 400.degree.F to about
650.degree.F without emitting significant amounts of hydrocarbons
to the atmosphere as the result of premature pyrolysis of the oil
shale during the preheating operation.
The preheat procedure of this invention provides for a gradual
heating of the oil shale through the use of a series of dilute
phase fluidized beds which are each operated at different flue gas
temperatures. Dilute phase fluidized beds are used to obtain rapid
heat transfer from flue gas to a solid with a fairly wide range of
particle size distribution. Because of this range of particle size,
the finer particles are heated much more rapidly than the larger
particles. For this reason, it is important to control the flue gas
temperature at the inlet to the lift line to minimize premature
pyrolysis and hydrocarbon generation. It has been found that the
major portion of the hydrocarbons evolved during the preheat
operation occurs when the shale (Green River Formation) is heated
to temperatures in excess of 350.degree.F. At temperatures below
350.degree.F, hydrocarbon concentrations in the flue gas discharged
to the atmosphere, resulting primarily from partial vaporization of
the bitumen content of the shale, have been shown to be less than
100 ppm. At preheat temperatures in the order of 500.degree.F to
600.degree.F, hydrocarbon concentrations in the flue gas are
generally in the order of 500 to 1000 ppm. These higher
concentrations are caused by partial pyrolysis of the kerogen in
addition to vaporization of the bitumen. From an environmental
standpoint, the emission of excessive amounts of hydrocarbons is
undesirable. However, if the generation of large amounts of
hydrocarbons can be effectively controlled and the heat content
thereof utilized, it is advantageous in terms of both conserving
the natural resource and improving project economics by generating
limited amounts for use as a process fuel. Tests, which are
described in greater detail below, indicate that as much as 5 to
15% of heat required to preheat oil shale from ambient temperature
to about 550.degree.F can be supplied by the oil shale fines and
hydrocarbons evolved during the preheat operation.
Accordingly, this process provides for a gradual preheating of the
whole oil shale to a temperature of about 350.degree.F without the
generation of significant concentrations of hydrocarbons, the
further preheating of the whole oil shale to between 400.degree.F
and 650.degree.F with the attendant generation of hydrocarbons, and
the recovery of the heating value of these evolved hydrocarbons as
well as the heating value of oil shale fines which are entrained in
the flue gas. The preheat procedures of this invention utilize the
final stage of dilute phase fluidized bed preheating to generate
hydrocarbons and entrain these hydrocarbons, together with the oil
shale fines, in the flue gas stream which is then passed to an
incineration zone wherein the heating value of these materials is
recovered. It has been found that the evolved hydrocarbons and the
entrained oil shale fines can be efficiently burned at temperatures
between about 1300.degree.F and 1500.degree.F. Higher temperatures
could be utilized if increased levels of carbonate decomposition
can be tolerated. It has also been found that virtually complete
incineration can be accomplished in about 0.3 second to 1.0 second
at these temperatures. The hot flue gas from the incineration zone
is partially cooled by conventional heat recovery techniques and is
used to preheat cooler raw shale in upstream (shale side) preheat
stages. The temperature of the flue gas which contacts the oil
shale in the upstream preheat stages must be low enough, usually
less than about 800.degree.F to 900.degree.F, to avoid generating
and discharging significant amounts of hydrocarbons to the
atmosphere.
Two stages of preheat with one intermediate flue gas incineration
and reheating operation adequately accomplish the objectives
discussed above; however, three stages of preheat utilizing a
single flue gas incineration zone located between the second and
third stages are preferred. Two interstage incineration zones may
also be advantageously utilized wherein one incineration zone is
located between the first and second preheat zones and the other
incineration zone is located between the second and third preheat
zones.
DESCRIPTION OF THE PROCESS
Oil shale crushed to nominal 1/2-inch size is fed from the crusher
and introduced into a first dilute phase fluidized bed preheat zone
wherein the shale is contacted with flue gas at a temperature
sufficient to partially preheat the shale and remove the free
moisture. The flue gas discharged from this first stage is passed
to the atmosphere after passing through a dust removal apparatus,
such as a bag filter, electrostatic precipitator, web scrubber, or
the like. In the higher temperature preheat zones located
downstream of the first preheat zone, the oil shale is heated to
increasingly higher temperatures by contacting the shale with
hotter flue gases from the heat-carrying body reheating zone. The
preheated oil shale from the final preheat zone is passed to the
pyrolysis zone. The partially cooled flue gases from the preheat
zones located downstream (gas side) of the final preheat zone are
passed through one or more interstage incineration zones wherein
the flue gases are reheated by incinerating a mixture of fuel,
entrained hydrocarbons and raw shale fines.
Temperature control of the flue gases fed to the various preheat
zones may be accomplished by blending additional cool, fresh air or
flue gas with the primary hot flue gas stream or by passing the
flue gases through heat exchangers.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The process hereinafter described in connection with the
accompanying drawing utilizes a combination of three dilute phase
fluidized bed preheat zones with one interstage incineration zone
located between the second and third (final) preheat zones to
preheat oil shale from about 50.degree.F to a temperature of about
500.degree.F to 600.degree.F prior to the pyrolysis thereof. The
drawing is a process flow diagram of the preferred embodiment of
the invention described in combination with the pyrolysis process
described in the above-cited U.S. Pat. No. 3,265,608. The following
Table 1 present a typical screen analysis for the nominal 1/2-inch
raw shale to which this embodiment is directed.
TABLE 1 ______________________________________ Nominal 1/2-inch Raw
Shale Screen Analysis Size % Retained % Passing
______________________________________ 0.500 inches 0 100.0 0.371
inches 10.3 89.7 4 mesh 38.5 51.2 8 mesh 21.7 29.5 20 mesh 17.2
12.3 40 mesh 5.5 6.8 80 mesh 3.8 3.0 -80 mesh 3.0 --
______________________________________
With reference to the embodiment as illustrated in the drawing,
freshly crushed oil shale (including fines and moisture) is
continuously charged at about 50.degree.F to lift line or first
preheat zone 10 at a rate of about 450 tons per hour wherein it is
contacted and transported with relatively warm flue gas at about
475.degree.F fed from line 11. The flue gas to the first preheat
zone 10 from line 11 is a blend of the flue gas having a
temperature of about 375.degree.F from a second preheat zone or
lift line 12 fed via line 13 with the flue gas having a temperature
of about 1400.degree.F from the interstage incinerator-recuperator
24 fed via line 16. In the first preheat zone 10, the oil shale is
preheated to about 200.degree.F while being transported to
accumulator 17 and cyclone separator 18 wherein oil shale particles
are separated from flue gas. The partially preheated oil shale is
then transferred by gravity feed to the second preheat zone 12 via
lines 19 and 20 at a rate of about 450 tons per hour to be further
preheated therein. The flue gas having a temperature of about
225.degree.F from cyclone separator 18 and containing oil shale
dust and water is passed through a wet scrubber (not shown) and
then vented to the atmosphere at a low temperature of about
125.degree.F. Approximately 30 gallons per minute of water are
removed from the shale in this first preheat zone 10 which do not
have to be carried through the downstream preheating and retorting
zones and ultimately removed from the oil product.
In the lift line or second preheat zone 12, the oil shale is
further preheated to about 350.degree.F while being transported by
hot flue gas having an entry temperature of about 800.degree.F. The
hot flue gas in the second preheat zone 12 is fed thereto via line
21 and consists of the flue gas from the third preheat zone 22
having an exit gas temperature of about 575.degree.F which is fed
via line 23 to interstage incinerator-recuperator 24 wherein it is
reheated to a temperature of about 1400.degree.F by the combustion
of hydrocarbons evolved during the final preheat operation, oil
shale fines, and supplementary fuel. The 1400.degree.F flue gas is
cooled in a heat recovery zone 24 d to about 800.degree.F before it
enters the second preheat zone 12. The shale is separated from the
flue gas in accumulator 25 and cyclone separator 26 and fed via
lines 27 and 28 to the third preheat zone 22. Some additional water
vapor is removed in the second preheat zone 12.
The partially preheated oil shale at about 350.degree.F is
introduced into the third preheat zone 22 wherein it is contacted
and transported by flue gas introduced at about 1200.degree.F to
about 1400.degree.F via line 29 so as to uniformly preheat the
entire raw shale stream to about 550.degree.F without effecting
significant pyrolysis thereof prior to the retorting in pyrolyzer
30. The flue gas to the final preheat zone 22 comprises a blend of
the ball heater flue gas fed via line 15 which may be cooled if
necessary for process control purposes by adding quench air at
about 100.degree.F to 120.degree.F via line 31. The preheated oil
shale is separated from the flue gas in accumulator 32 and cyclone
separator 33 and thereafter fed to the retort 30 via lines 34 and
35 wherein the pyrolysis of the oil shale is effected by contact
with hot ceramic bodies fed from the ball heater 14 via line
36.
The incinerator-recuperator 24 generally comprises a combustion
zone 24 a in which air and fuel are burned, a mixing zone 24 b in
which the flue gas from line 23 is mixed with combustion products
from zone 24 a, an incineration zone 24 c wherein the oil shale
fines and hydrocarbons from the third preheat stage 22 are
incinerated, and a heat recovery zone 24 d for cooling the flue gas
to about 800.degree.F prior to exiting via line 21. The heat
recovery zone 24 d may incorporate conventional air heat exchangers
and/or waste heat boilers for steam generation.
In the ball heater 14, ceramic balls are introduced from ball
elevator 37 via line 38 into the heating chamber 39 wherein the
balls are contacted with hot flue gas from a combustion chamber 40.
Air and fuel are supplied via lines 41 and 42, respectively, to an
atomizing type burner (not shown) located in the combustion chamber
40 wherein the mixture is burned to provide hot flue gas. As the
ceramic balls and flue gas are cocurrently drawn downward through
the ball heater, the balls are heated while the flue gas is cooled.
The hot flue gas is withdrawn from the ball heater 14 via
disengaging zone 43 and line 44 for use in the raw shale preheat
zone 22. The heated balls are withdrawn from ball heating zone 39
via line 36 and fed to pyrolyzer 30.
In the rotating pyrolysis drum 30, the hot ceramic balls from the
ball heater 14 are contacted with the raw shale preheated to about
550.degree.F to effect the pyrolysis thereof. The balls and oil
shale pass co-currently through the pyrolyzer 30 where the heat of
the balls is imparted to the oil shale with the production of an
effluent hydrocarbonaceous shale oil vapor and processed shale
solids. The effluent vapor, processed shale solids and cooled balls
exit from pyrolysis drum 30 through tunnel 45 and pass to a
rotating trommel screen 46 having openings therein such that the
comminuted processed shale solids pass through while the passage of
the large size balls is precluded. The effluent vapor is removed
from vapor dome 47 and passed via line 48 to a recovery section
(not shown). Substantially all the processed shale solids pass
through trommel screen 46 and into processed shale solids
accumulator 49 from which they are removed via line 50 and passed
to a processed shale solids disposal area. Cooled balls are removed
from accumulator 51 by means of line 52, passed to ball elevator 37
and returned to the ball heater 14 via line 38 to be reheated
therein.
In the above description of the preferred embodiment, the
temperatures of the various flue gas and solid streams are
exemplary of one set of preheat conditions that have been found to
be desirable when processing a specific type and grade of oil
shale. These conditions have been found applicable to oil shale
from the Mahogany Zone of the Piceance Basin located in
northwestern Colorado. Mahogany Zone oil shale usually contains 30
to 40 gallons of recoverable liquid hydrocarbons per ton of ore
processed. If either lower or higher grades of oil shale are
processed, the flue gas and solids temperatures in each of the
respective lift lines may have to be altered in order to optimize
the system. Thus, when preliminary tests on a particular grade of
oil shale indicate that it is desirable to either increase or
decrease the degree of preheat in any one of the three stages of
preheat for the purpose of altering atmospheric hydrocarbon
emissions and/or fuel supply to the preheat zone, the preheat
conditions may be altered without departing from the basic concept
herein disclosed.
Tests have been performed at a plant capable of processing 1000
tons per day of Mahogany Zone oil shale to determine the amount of
hydrocarbons generated under specified preheat conditions for each
of the three stages of preheat. From these tests, it has been
determined that in preheating oil shale (crushed to minus 1/2-inch)
from 50.degree.F to 500.degree.F in a threestage system, an average
of 400 to 700 ppm of hydrocarbons is generated. Of this amount, 400
to 500 ppm of hydrocarbons are generated in the final stage of
preheat, and 75 to 100 ppm of hydrocarbons are generated in the
first two stages. In these tests, the lift lines were operated at
flue gas inlet temperatures of approximately 500.degree.F,
900.degree.F and 1000.degree.F, respectively, while the shale was
heated to successively higher temperatures of approximately
200.degree.F, 350.degree.F, and 500.degree.F, respectively, in each
of the three stages of preheat.
Another series of tests was performed to determine the conditions
required to substantially reduce the hydrocarbon content of the
flue gas discharged from the third lift line, and the approximate
particle size and organic carbon content of the oil shale fines
entrained in the flue gas. Table 2 presents an average particle
size analysis of oil shale fines which entered the incineration
zone from the third preheat stage.
TABLE 2 ______________________________________ Particle Size
Analysis of Oil Shale Fines Entering Incineration Zone Particle
Size Percent by Weight (microns) Greater than Size
______________________________________ 40 0.1 30 0.2 20 0.6 15 1.2
10 2.0 8 3.3 5 11.6 3 50.1 1 92.4 0.5 97.4 0.1 100.0
______________________________________
Table 3 presents analyses of the organic carbon content of oil
shale fines which were obtained from isokinetic samples taken at
the inlet and outlet of the combustion chamber while operating an
incineration zone at temperatures in the range of from
1330.degree.F to 1425.degree.F. These data illustrate that the
fines contain organic carbon which can be burned to recover its
heating value.
TABLE 3 ______________________________________ Organic Carbon
Content of Entrained Oil Shale Fines Fines Fines Organic Combustion
Organic as Carbon Content, lb/hr Chamber Carbon % of (Based on 450
TPH Sample Location wt % Feed Shale Feed)
______________________________________ Inlet 16.05 0.36 536 Outlet
0.23 8 Inlet 5.91 0.39 214 Outlet 0.10 4
______________________________________
Table 4 presents the results of 17 tests which were performed to
determine the amount of hydrocarbons generated in the third preheat
stage and the operating conditions necessary to effect a
substantial reduction in hydrocarbon content of the flue gas
passing through the incineration zone. The hydrocarbons to the
incinerator in this series of tests are considerably higher than
indicated in the tests described above because of differences in
the mechanical size and configuration of the plant. These tests
were performed in a 24-ton per day pilot plant.
TABLE 4 ______________________________________ Hydrocarbon
Reduction with Incineration Zone Between Second and Third Preheat
Stages Hydrocarbon Incineration Zone Samples Incineration Residence
Inlet Outlet Zone Temp. Time HC HC Efficiency .degree.F Sec ppm ppm
% ______________________________________ 1220 0.31 2129 475 71.2
1260 0.29 2129 346 79.3 1275 0.39 913 47 93.2 1290 0.41 1021 12
98.4 1325 0.48 751 9 98.5 1330 0.47 1040 11 98.6 1345 0.48 648 3
99.4 1350 0.33 1214 9 99.0 1355 0.32 1632 17 98.6 1360 0.50 890 12
98.2 1375 0.41 3846 2 99.9 1385 0.55 986 11 98.4 1390 0.49 1019 2
99.7 1420 0.50 508 3 99.2 1425 0.52 1230 6 99.4 1435 0.52 1226 7
99.7 1440 0.53 1005 4 99.5
______________________________________
From the above three tables, it is evident that a significant
amount of very fine oil shale is entrained in the flue gas stream
which can be used as a source of fuel for the preheat system that
would otherwise not be recovered. In addition, it is apparent from
Table 4 that the interposition of an incineration zone between the
second and third preheat stages is very effective in reducing the
hydrocarbon content of the flue gas when the incineration zone is
operated at a temperature in excess of about 1300.degree.F, and
preferable at about 1400.degree.F, at residence times in the order
of at least 0.5 seconds.
From the above description of the improved process of the
invention, it will be apparent that the process provides a
procedure for retorting or pyrolyzing oil shale which utilizes the
whole shale, including the fine material, to recover substantially
100% of the recoverable hydrocarbons either as a liquid and gaseous
product or as fuel in the preheat system. Moreover, the process
provides for the preheating of raw oil shale to temperatures in the
order of 400.degree.F to 650.degree.F without significantly
effecting pyrolysis of a major portion of the shale prior to the
retorting or pyrolysis operation. Furthermore, the process provides
increased heat economy by discharging low temperature, rather than
high temperature, flue gas to the atmosphere.
The process, in addition, provides for substantial elimination of
hydrocarbon emissions by combusting hydrocarbon vapors that are
derived primarily by vaporization of bitumen contained in the
shale. The use of a single preheat lift line to obtain these higher
preheat temperatures is not practical because the fresh raw shale
would be contacted by high temperature flue gases, i.e.,
1200.degree.F to 1400.degree.F which would result in severe
hydrocarbon emissions. In addition, if a single preheat lift line
were operated in the manner just described, the flue gas discharge
temperature would necessarily be in the order of 600.degree.F, and
would obviously contain substantial amounts of unburned
hydrocarbons.
It will be appreciated that various modifications and changes may
be made in the process of the invention by those skilled in the art
without departing from the essence thereof. Therefore, the
invention is to be limited only within the scope of the appended
claims.
* * * * *