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.

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.

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.