Note: Descriptions are shown in the official language in which they were submitted.
BA~OK~l~D OF ~ I~VENTION
me presence of large deposits of oil shale in the Rocky
Mountain region of the U~ited States has given rise to extensive efforts
to develop methods of recovering shale oil from kerogen in the oil shale
deposits~ It should be noted that the term "oil shale" as used in the
industry is in fact a misnomer; it is neither shale nor does it contain oilO
It is a sedimentary formation comprising marlstone deposit with layers
containing an organic polymer called "kerogen", which upon heating
decomposes to produce liquid and gaseous products~ It is the formation
1~ containing kerogen that is called "oil shale" herein, and the liquid
hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil
shale which involve either first mining the kerogen bearing shale and pro-
cessing the shale on,the surface, or processing the shale in situ. The
latter approach is preferable from the standpoint of environmental impact
since the spen~ shale remains in place, reducing the chance of surface
contamination and the requirement for disposal of solid wastes.
The recovery o~ liquid and gaseous products from oil shale
deposits has been described in several patents, one of which is United States
Patent No. 3,661,423, issued May 9, 1972~ to Donald E. ~arrett, assigned
to the assignee of this application. This patent describes in situ
recovery of ~iquid and gaseous hydrocarbon materials from a subterranean
formation containing oil shale by fragmenting such formationto form a
stationary, fragmented permeable body or mass of formation particles
containing oil shale within the formation, referred to herein as in si-tu oil
shale retort. Hot retorting gases are passed through the in situ 0il shale
retort to convert kerogencontainedin the oil shale to liquid and gaseous
pro~ucts, thereby producing "retorted oil shale",
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One method of supplying hot retorting gases used for converting
kerogen contained in the oil shale, as described in United States Patent
No. 3,661~423, includes establishment o~ a combustion zone in the retort
and introduction of an oxygen containing retort inlet mixture into the
retort as an oxygen supplying gaseous combustion zone ~eed to advance the
combuStion zone downwardly through the retort. In the combustion zone
oxygen in the combustion zone feed is depleted by reaction with hot
carbonaceous materials to produce heat and combustion gas. By the
continued introduction of the retort and inlet mixture into the retort,
the combustion zone is advanced through the retort.
The combustion gas and the portion o~ the combustion zone feed
that does not take part in the combustion process pass through the frag-
mented mass in the retort on the advancing side of the combustion zone to
heat the oil shale in a retorting zone to a temperature sufficient to
produce kerogen decomposition, called retorting, in the oil shale to gaseou~
and liquid products including gaseous and liquid h~drocarbon~products and
to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler
oil shale fragments in the retort on the advancing side of the retorting
zoneO The liquid hydrocarbon products, together with w~ter produced
in or added to the retort, are withdrawn from the retort on the advancing
side of the retorting zoneO An off gas containing combustion gas generated
in the combustion zone, gaseous products produced in the retorting zone, gas
from carbonate decomposition, and an~ gaseous retort inlet mixture that does
not take part in the combustion process is also withdrawn from the retort
on the advancing side of the retorting zone. The products of retorting
are referred to herein as liquid and gaseous productsO
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The residual carbonaceous material in the retorted oil shale
can be used as fuel for advancing the combustion zone through the retorted
oil shale. When the residual carbonaceous material is heated to its spon-
taneous ignition temperature it reacts with oxygenO The portion of the
retort where the greater part of the oxygen in the retort inlet mixture
that reacts with residual carbonaceous material in ~etorted oil shale is
consumed is called the primary combustion zone. As the residual
carbonaceous material becomes depleted in the combustion process~ the
oxygen penetrates farther into the oil shale retort where it combines with
remaining unoxidized residual carbonaceous material, thereby causing the
combustion zone to advance through the fragmented mass containing oil shale.
The rate of retorting of the oil shale to liquid and gaseous
products is temperature dependent, with relatively slow retorting occurring
at 600 F, and relatively rapid retorting of the kerogen in oil shale
occurring at 950 F and higher temperatures. ~s the retorting of a segment
of the fragmented oil shale in the retorting zone progresses and less heat
is extracted from the gases passing through the segment, the combustion
gas heats the oil shale farther on the advancing side of the co~bustion
zone to retorting temperatures, thus adYancing the retorting zone on
the advancing side of the combustion ~oneO
It can be desirable to limit the oxygen content of the combustion
zone feed to about 15%~ ~t oxygen concentrations higher than about 15%,
high primary combustion zone temperatures resulting in fusion o~ the
oil shale can occur if a high volumetric flow rate of combustion zone feed
is provided. Thus to reduce the oxygen content of air, which is presently
the most economical source of oxygen, the air can be diluted with a portion
of off gas generated by retorting of oil shaleO However, it has been found
that when recycled off gas is used to dilute the air, the off gas from the
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retort can have a fuel value as low as about 45 BTU/SCF (British thermal
units per standard cubic foot), which can be insufficient to power a work
engineO
It is desirable to provide a method for retorting an in situ oil
shale retort such that the retort off gas generated during retorting ~
has sufficient fuel value for combustion in a stack or for use in power
generation in a work engine.
The introduction of a retort inlet mixture into the retort on the
trailing side of the combustion zone and the flowing of such gas therethrough
generally reduces the temperature of the fragmented permeable mass of
particles on the trailing side of the combustion 7one. When the retort
inlet mixture is introduced into~the retort at atmospheric temperature, the
fragmented permeable mass on the trailing side of the combustion zone can
have its temperature reduced to a temperature below the retorting temperature
of oil shale. This reduction in temperature terminates the retorting of
oil shale in unfragmented formation adjacent to such fragmented permeable
mass of particles, thereby reducing the reco~ery from the retortO
This reduction in temperature can also reduce the tamperature
of residual carbonaceous material in oil shale on the trailing side of the
primary combustion ~one to a temperature below the spontaneous ignition
temperature of such materials. Residual carbonaceous material so cooled
cannot be oxidi7ed to provide the energy required for the endothermic
retorting of oil shale, thereby requiring oxidation of organic materials
which otherwise could be retorted to yield hydrocarbon products.
Thus, it is desirable to provide a method for recovering liquid
and gaseous products from an in situ oil shale retort which yields off gas of
sufficient fuel value to operate a work engine and which gives high recovery
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of product.
SUM~ARY OF DHE INVENTION
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The invention provides a method for forming a gaseous primary com- :
bustion zone feed for advancing a primary combustion zone through a fragmented
permeable mass of formation particles containing oil shale in an in situ oil
shale retort, comprising the steps of: establishing a primary combustion
zone in the fragmented permeable mass; introducing air to the fragmented
permeable mass on the trailing side of the primary combustion zone for advanc_
ing the primary combustion zone through the fragmented permeable mass,
introducing fuel to the fragmented permeable mass for reaction with a portion
of the oxygen of the introduced air on the trailing side of the primary com-
bustion zone for forming a primary combustion zone feed comprising about
15% oxygen; and controlling *he composition of introduced fuel and the
proportion of the introduced fuel to the introduced air such that the
volume (STP) of the primary combustion zone feed on a dry basis is less than
the volume (STP) of the introduced airn
From another aspect, the invention provides a method of recovering
liquid and gaseous products from an in situ oil shale retort in a sub-
terranean formation containing oil shale, said in situ oil shale retort
containing a fragmented permeable mass of par*icles containing oil shale
and ha~Lng a primary combustion zone and a retorting zone advancing there-
through~ khe fragmented mass having gas flow paths therethrough, which com-
prises the steps of: introducing into the:in situ oil shale retort on the
trailing side of the primary combustion zone, a retort inlet mixture com-
prising fuel and oxygen supplying gas, controlling the composition of the
retort inlet mixture such that it contains sufficient oxygen to oxidize
the fuel for forming a secondary combustion zone and for forming a primary
combustion zone feed containing at least 10% by volume oxygen on a dry basis,
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such that the retort inlet mixture has a spontaneous ignition temperature
lower than the temperature of the primary combustion zone, and such that the
retort inlet mi~ture comprises sufficient fuel for maintaining the
temperature of the secondary combustion zone and the temperature of a
portion of the fragmented mass in a gas flow path between the primary and
secondary combustion zones abovethespontaneous ignition temperature of
residual carbonaceous material in retorted oil shale, passing the primary
combustion zone feed into the primary combustion zone for advancing the
primary combustion zone through the fragmented mass of particles and produce
primary combustion gas; passing said primary combustion gas and any
unreacted gaseous portion of the retort inlet mixture through a retorting
zone in the fragmented mass of particles on the advancing side of the
primary combustion zone whereby oil shale is retorted and gaseous and liquid
products are produced~ and withdrawing liquid products and retort off gas
comprisi.ng such gaseous products, primary combustion gas and any gaseous
unreacted portion of the retort inlet mixture from the in situ oil shale
retort on the advancing side of the retorting zone.
The secondary combustion zone can supply heat to unfragmented
formation adjacent the fragmented permeable mass of particles for retorting
oil shale in the unfragmented formationO
DRAWINGS
These and other features, aspects and advantages of the present
invention will become more apparent when considered with respect to the
following description, appended claims, and accompanying drawings where:
Figure 1 illustrates semi-schematically two in situ oil shale
retorts; and
Figure 2 illustrates the location of primary and secondary
combustion zones during in situ oil shale retorting according to principles
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of this invention, employing the in situ oil shale retorts illustrated in
~igure 1.
DESC N
This invention concerns an improved method for pro~uc~ng liquid
and gaseous products in an in situ oil shale retort having boundaries of
unfragmented formation referred to herein as "walls", and containing a
fragmented permeable mass of particlesO ~ primary combustion zone îs
advanced through the fragmented permeable mass of particles. A secondary
combustion zone is maintained on the trailing side of the primary combustion
zone by introducing a retort inlet mixture comprising fuel and oxygen to
a selected location in the fragmented mass on the trailing side of the
primary combustion zoneO The retort inlet mixture comprises an excess of
oxygen over that needed to oxidize the fuel. The secondary combustion zone
in the portion of the retort where fuel in the retort inlet mixture is
burnedO The secondary combustion zone supplies heat to the fragmented
permeable mass of particles on the trailing side of the combustion zone.
Preferably, sufficient heat is generated in the secondary combustion zone
to maintain the temperature of tha retort walls adjacent the secondary
combustion Pone at a temperature above the retorting temperature of oil
shaleO The advance of the primar~ combustion zone through the retort causes
retorting of oil shale and the production of liquid and gaseous productsO
This invention is described with reference to Figure 1, where
there is illustrated a plurality of retort walls or pillars lO~adjacent a
plurality of in situ oil shale retorts 12. A subterranean oil shale
formation is fragmented to form a stationary, fragmented, permeable mass 14
of formation particles containing oil shale. The pillars 10 comprise
formation containing oil shaleO The amount of the formation remaining as
pillars serving as retort walls can be as much as about 20% to about 4~%
of the portion of the total deposit containing the retorts 12.
The pillars remaining between the retorts prevent gas flow
between retorts. The oil shale in the pillars has little if any porosity
and permeabilityO However~ as kerogen in the oil shale is decomposed and the
retorting products leave the oil shale, the permeability and porosity
increase. Therefore, since retorting progresses into the pillar from the
heated retort~ retorting products including shale oil produced inside the
pillar 10 tend to move toward the retorts where the products pass through a
high temperature interfacial zone 16 at the retort wall 22 provided by the
pillars. In the movement of these products through this high temperature
interfacial zone, thermal cracking of the shale oil can occur, resulting in
the production of a light oil and gasO This gas and light oil mix with
gases such as hydrocarbons, hydrogen and carbon monoxide formed in the
primary combustion zone 32 and retorting zone 12 ~see Figure 2) during in
situ retorting of the fragmented permeable mass containing oil shale 14 in
the retorts 12 to form a retort off gas.
To retor~ oil shale in the pillars at elevations above the
primary combustion zOne~ this invention provides for establishment of a
seconda~y combustion zone on the trailing side of the primary combustion
7one. Sufficient heat can be generated in the secondary combustion zone to
maintain the temperature of retort walls adjacent the secondary combustion
zone at a ~emperature greater than the retorting temperature of oil shaleO
Preferably, this temperature is greater than about 900F and more preferably
greater than about 1000Fo It is particularly preferred to maintain the
retort walls adjacent the secondary combustion zone at a temperature higher
than about 1200F~ At the higher temperatures7 additional products are
recovered from the pillars and the fuel value of the retort off gas is
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enhanced. Therefore, preferably the secondar~ combustion zone is maintained
at a temperature above about 90GFa and more preferably at a temperature
higher than about 1200Fo
It has been calculated that a 900 F isotherm, iOe.~ a retorting
zone having a temperature of 900 F, will advance through a pillar at a rate
of about 1.34 inches per day when the surface of the pillar is maintained
a~ a temperature of 1200 F~
In the absence of the secondary combustion zone, gases introduced
into the retort on the trailing side of a primary combustion zone and the
flowing of such gases therethrough generally reduce the temperature of the
fragmented permeable mass of particles on the trailing side of the primary
combustion zone. When the gases are introduced into the retort at about
atmospheric temperature, a portion of the fragmented permeable mass on the
trailing side of the primary combustion zone can have its temperature reduced
to a temperature below the retorting temperature of oil shaleO The
reduction in temperature terminates the retorting of oil shale in the
pillars adjacent to such fragmented permeable mass of particles, thereby
reducing the recovery of shale oil from the retort. This reduction in
temperature also reduces the temperature of a portion of the solid
carbonaceous materials on the trailing side of the primary combustion zone
to a temperature below the spontaneous ignition tempera~ure of such
materials, thereby eliminating some of the residual carbonaceous material as
a sou~ce of energy for the endothermic retorting of oil}shale.
As shown in ~igure 2, the secondary combustion zone is
established and maintained in a top portion or region of the fragmented
permeable mass in the retort near a~ inlet 20 to the fragmented mass. This
is preferred because oil shale on the trailing side of the primary
combustion zone and oil shale in unfragmented formation adjacent t~le
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fragmented mass on the trailing side of the primary combustion zone are
maintained at an elevated temperature due to flowing gases passing from
the secondary combustion ~one. The closer the secondary combustion zone
is to the top of the fragmented mass, the greater the amount of oil shale
in the fragmented mass and oil shale in the walls maintained at an elevated
temperature, and therefore the greater the amount of kerogen available
for retorting and the greater the amount of residual carbonaceous material
available for oxidation~
To provide high temperatures at the wall. 22 of retorts 12 formed
by the adjacent pillars 10 for an increased or extended time period, this
invention provides for the establishment of a secondary combustion zone as
indicated at 24 in ~igure 20 Such a secondar~ combustion zone 2~ is
illustrated as being located near the top.of the retort 12 and can be
maintained at substantially the same location throughout the in situ
; retorting operation. In one embodiment, the secondary combustion zone is
initiated by injecting shale oil 26, produced during retorting, via a
sha.le oil sprayer 28, and introducing via on inlet 20 to a mixture of
air 31 and recycled off gas 18 also produced :in retorting, into the top
portion 30 of the retort as a retort inlet mixture~ Shale oil is removed as
product by means of an oil pumping line 29 as shown in ~igure lo O~f gas can
be withdrawn from the bottom of the retort by a blower (not shown~O Instead
of or in addition to recycled off gas, steam can be included in the retort
inlet mixture.
In the embodiment shown in Figure 2, only a portion of the off gas
from the bottom of the retort is recycled through the retortO
In the embodiment shown in ~igure 2, the injection rate of shale
oil into the top of the respective retorts 12 is decreased as the heating
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value or fuel value of recylced off gas increasesO When the heating value
of such off gas is sufficient for combustion in the secondary combustion zone
and for maintaining the temperature of the secondary combustion at a desired
temperature, the injection of the shale oil via a shale oil sprayer 28 into
the secondary combustion zone 24 near the top of the retort is discontinued.
~t this stage, the off gas can have a heating value from about 80 to about
100 BTU/SCF or higher. During the remainder of the retorting operation, the
secondary combustion zone can be maintained by burning only the off gas of
enhanced heating value in the presence of oxygen. The o~ygen can be pro-
lC vided by an oxygen supplying gas such as airO
~ n the embodiment of Figure 2, the secondary combustion zone
supplies a primar~ combustion zone feed gas comprising combustion or flue
gas together with oxygen at a high temperatureO However, it will be noted
that the primary combustion zone, indicated at 32, also supplies heat for
advancing the retorting zone~33'.~ ~s~the primary combustion zone 32 advances,
a zone of hot combusted oil shale on the trailing side of the primary
combustion zone grows continuously throughout the retorting process until
retorting is completed, while the secondary combustion 30ne can remain at
a subskantially constant location near the top of the retort. The upper
part of the fragme~ted mass, therefore, stays hot during all retortingO
This results in heating oil shale in the pillars and the
recovery of products therefrom. This also makes carbonaceous material in
the hot combusted shale on the trailing side of the primary combustion zone
available for reaction with oxygen passing through the region between the
secondary combustion zone and the primary combustion zone. ~ portion of
this carbonaceous material would not have been available for reaction with
oxygen if the temperature of a portion of the hot combusted oil shale had
been reduced by conducting gas at atmospheric temperature through the portion
of the fragmented mass cotltaining such hot combusted oil shale.
~ s used herein, the term "retorted oil shale" refers to oil shale
heated to a sufficient temperature to decompose kerogen in an environment
substantially free of free oxygen so as to produce liquid and gaseous
products and leave a solid carbonaceous residue. As used herein, the
term "combusted oil shale" referS to oil shale through which a primary
combustion zone has passed, the combusted oil shale having reduced carbon
content due to oxidation of such carbonaceous residue. An individual
particle containing oil shale can have a core of retorted oil shale and
an outer "shell" of combusted oil shale. Such can occur when oxygen has
diffused only part way through the particle during the time it is at an
elevated temperature and in contact with an oxygen supplying gas.
As used herein, ~he term "raw oil shale" refers to oil shale which has not
been subjected to processing for decomposing kerogen in the oil shale.
The oxygen concentration of effluent gas from the secondary
combustion zone can be depleted as it passes through the hot combusted oil
shale between the secondary combustion 7one and the primary combustion zone
when carbonaceous material remains in the particles. Therefore~ the
primary combustion zone feed which is passed into the primary combustion zone
32 is thought to be of substantially lower oxygen concentration than would
be the case in the absence of the secon~ary combustion zone 24 for the same
rate of heat input to the retorting zone. Therefore, the rate of advance
of the primary combustion zone is lower in the presence of the secondary
combustion 7one 24 than would be the case in the absence of a secondary
combustion zone.
It is thought that there is a desirable oxygen free en~ironment
adjacent the retort wall 22 of the retort when there is a secondary
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combustion zone 240 This is thought to be due to the oxygen concentration
of the primary combustion æone feed being substantially lower in the
presence of the secondary combustion zone 24 and to the limited amount of
oxygen in the gas moving through the retort adjacent the walls~ Both the
lower downward velocity of the primary combustion zone 32, and the lower
oxygen concentration of the feed to the primary combustion zone 32,
together with the maintenance of a high interfacial retort wall temperature,
provide substantial recovery of oil and gas from pillars 10 adjacent the
retort, as well as high recovery of shale oil from the fragmented permeable
mass in the retort. Additionally, as the products from the pillars enter
~he retort, they are conducted downwardly along the walls. ~ portion of the
product can contact oxygen being conducted along the same portion of the
retort. Therefore, product from the pillar entering the retort in excess
of the amount which is oxidized can be conducted along the wall and through
the retorting zone without being consumed.
~ primary combustion zcne can be established by any known method
such as, for example, a method described in the aforementioned United States
Patent No. 3,661,423. In establishing a primary combustion zone an ignition
mixture is introduced into the retort through a conduit and ignited in the
retort for heating oil shale to a sufficient temperature to sustain combus-
tion.
Once a self~sustaining primary combustion zone is formed, a
secondary combustion zone can be formed by introducing a retort inlet
mixture or feed into the retort on the trailing side of the primary com-
bustion zone. The retort feed contains fuel and more than sufficient
oxygen supplying gas for oxidation of the fuel. The retort feed has a
spontaneous ignition temperature less than the temperature in the primary
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combustion zone and preferably less than the temperature at the selected
location where it is introduced Thus, when introduced into the retort,
the fuel in the retort inlet mixture is oxidized by the oxygen in the retort
feed. The Oxidation of the fuel liberates heat and establishes a secondary
combustion ~one in the fragmentéd massJ The gaseous mixture resulting from
introducing the retort inlet mixture into the retort and burning the fuel
in the secondary combustion zone feed contains oxidation products of fuel
such as carbon dioxide and water vapor, nonreactive components of the
oxygen supplying gas such as nitrogen when air is the oxygen supplying gas,
1.0 carbon dioxide from decomposition of inorganic carbonates, and oxygen
contained in the retort feed beyond that required for oxidation of the fuel.
Heat from the seconda.ry combustion 7one is supplied to the fragmented mass
on the t.railing side of the primary combustion zone by the primary combus-
tion zone feedO The fragmented mass in a gas flow path between the
secondary combustion zone and the primary combustion zone is maintained at
an elevated temperature approaching the temperature of the secondary
combustion zone, i.eO, above about 900F.
Preferably the secondary combustion zone is maintained at a
temperature of at least aboutllSO F for thermal decom~osition of alkaline
earth metal carbonates present in oil shale at an appreciable rateO Such -
thermal decomposition results in release of carbon dioxide and formation of
the corresponding alkaline earth metal oxideO Oil shale contains appreciable
amounts of alkaline earth metal carbonates such as magnesium carbonate and
calcium carbona~e. Complete deco~position of calcium carbonate to carbon
dioxide and calcium oxide occurs at a temperature of about 1517 Fo
It is believed that carbon ~ioxide released by decomposition of
alkaline earth metal carbonates can react with residual carbonaceous
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material within a formation particle according to the reaction:
c ~ co2~3 2C0 (l)
The carbon monoxide therein formed can react with oxygen in the retort inlet
mixture according to the following reaction:
2CO + 2-~ 2 (2)
By these reactions, residual carbonaceous material in combusted oil shale
which might be left unrecovered can be oxidized with liberation of heat
which can be used for retorting oil shale in a retorting zone on the
advancing side of the primary combustion zone.
To cause the primary combustionzone to advance through the retort,
the rate of introduction of the retort feed into the retort is at least
sufficient to generate a primary combustion zone feed at a superficial
volumetric rate of 0.1 SCFM per square foot of cross-sectional area of the
fragmented permeable mass being retorted. Preferably the primary combus-
tion zone advances through the Pragmented mass at a rate of at least about
0.5 feet per day to produce hydrocarbon products at a sufficiently fast
rate to justify the capital investment required for retorting oil shaleO
~t rates of advancement higher than 2 feet per day of the primary combus- -
tion zone, hydrocarbon yield per ton of oil shale being retorted can be
adversely affected due to oxidation of hydrocarbon productsO Therefore,
prePerably the primary combus~tion is advanced through the fragmented mass
at a rate of up to about 2 feet per day to avoid significant yield lossesO
To cause the primary combustion zone to advance through the
retort at an economical rate of from about 0.5 to 2 feet per day, depending
on the kerogen content of the oil shale through which the primary combus-
tion zone is advancing, the retort feed is introduced into the retort at
a rate sufficient to generate from about 0.5 to about 1 SCFM of primary
combustion zone feed per square foot of the cross-sectional area of the
fragmented permeable mass being retorted. Introduction of retort inlet
mixture into the retort at a rate generating more than about 2 SCFM of
primary combustion zone feed per square foot of cross-sectional area may
result in a portion of the oxygen in the primary combustion zone feed being
carried through an established or desired primary combustion zone location
and into the retorting zone. In the retorting zone, such oxygen can burn
hydrocarbon products and unretorted carbonaceous material in the oil shale~
Therefore, it is preferred to introduce the retort feed into the retort at
a rate sufficient to generate less than about 2 SC~M of the primary combus
tion zone feed per square foot of cross-sectional area of the fragmented
permeable mass,
The oxygen concentration of the gaseous primary combustion zone
feed is preferably maintained greater than about 10% by volume on a dry
basis of the primary combustion zone feed to maintain the temperature in
the primary combustion zone at a temperature above the retorting temperature
of oil shale. Therefore, sufficient oxygen supplying gas is provided in the
retort feed to oxidize the fuel in the retort feed and produce a primary
combustion zone feed which contains at least 10% oxygen by volume on a dry
basis~ At an oxygen concentration greater than about 20% by volume on a
dry basis of the primary combustion zone feed, contact of the primary com-
bustion zone feed with regions of high concentration of carbonaceous
material in the retort can cause localized fusion of the fragmented mass of
oil shale particlesO Fusion of the fragmented mass can restrict the movement
of gases through the retort. Therefore, it is preferred to use a retort
inlet mixture having sufficient oxygen to form a gaseous primary combustion
zone feed having from about 10% to about 20% oxygen by volume on a dry basisO
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Maintenance of the oxygen concentration at less than about 15%
by volume on a dry basis o~ the gaseous primary combustion feed provides a
margin of safety to prevent fusion of the mass of particles. At an oxygen
concentration of at least 10% by volume on a dry basis of the gaseous
primary combustion zone feed, the maximum temperature in the primary
combustion ~one can readily be adjusted to a desired temperature above the
retorting temperature of the oil shaleO Therefore, the use of a retort
inlet mixture containing sufficient oxygen to form a gaseous primary
combustion zone feed having from about 10% to about 15% oxygen by volume
on a dry basis constitutes a preferred embodiment~
The concentration of oxygen in the retort inlet mixture depends
upon such factors as the volume of primary combustion zone feed desired to
be generated per square foot of cross_sectional area of the fragmented
permeable mass `being retorted, the desired temperature in the primary com-
bustion zone, and the amount of residual carbonaceous material left in the
shale after retorting. A lower concentration of oxygen is needed in the
primary combustion zone feed as the volumetric flow rate of the primary com-
bustion 7~one feed increases, as the desired temperature in the primary com-
bustion zone decreases, and as the concentration of residual carbonaceous
material in the retorted oil shale increasesO Conversely, a hi~her concen-
tration of oxygen is required in the primary combustion zone feed at lower
volumetric flow rates of the gaseous primary combustion zone feed, higher
desired primary combustion zone temperatures, and lower concentrations of
residual carbonaceous material.
The desired concentration of oxygen in the primary combustion
zone feed is dependent upon the concentration of residual carbonaceous
material in the retorted oil shale because the more carbonaceous material
present, the more heat which can be generated per unit volume of spentshale
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Thus, it is advantageous to know the concentration of carbonaceous material
available for combustion in various regions throughout the retort. Concen-
tration of carbonaceous material available for combustion can be estimated
by conducting assays of core samples taken at various regions and strata of
the retortO Generally, the higher the concentration of kerogen in a region
of a retort, the higher will be the concentration of residual carbonaceous
material for combustion in the retorted shale.
The fuel for the retort feed can be a gaseous fuel such as post-
retorting gas from an in situ oil shale retort, rich off gas having a
heating value of àt least about 80 BTU/SCF (British thermal units per
standard cubic foot) from an active in situ oil shale retort, butane,
propane~ natural gas, liquefied petroleum gas, or the like; a liquid fuel
such as shale oil, cruda petroleum oil, diesel fuel~ alcohol9 or the like:
a comminuted solid fuel such as coal; and mixtures thereof.
Post-retortin~ gas is gas generated during a post-retorting
operation. As used herein, the term "post-retorting operation"refers to a
period at the end of normal retorting operation; that is, it refers to a
period after a retorting zone has advanced through substantially all of the
fragmented permeable mass in the retort~
The retort inlet mixture requires a heating value of at least
about 22 BTU/SCF to maintain a secondary combustion zone having a temperature
of 1200 F~ i.e., a secondary combustion sufficiently ~t to efficiently
retort oil shale in unfragmented formation of the boundaries of the retort.
When air is the source of oxygen of the retort inlet mixture, the retort
inlet mixture must contain at least about two parts by vol~e air per one
part by volume retorting off gas to produce a primar~ combustion 7one feed
containing 10~ oxygen by volume. About two parts by volume air per part by
volume off gas are required rather than 1 part by volume air per 1 part by
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volume off gas because a portion of the oxygen of the retort inlet mixture
is consumed by reaction with combustible components of retorting off gasO
Therefore~ to have a secondary combustion zone having a temperature of
1200 F where retorting off gas is the fuel, the retorting off gas must have
a heating value of at least about 66 B~U~SCF.
When retorting off gas is used as the fuel of the retort inlet
mixture, preferably it has a heating value of at least about 80 BTU/SCF to
consistently maintain a secondary combustion zone having a temperature of
1200F. An off gas of such a high heating value has not been achieved by
conventional techniques~ where after establishment of a combustion zone in
the fragmented mass, retorting off gas is used to dilute introduced air.
According to the present invention, off gas having a heating value of 80
BTU/SCF is obtained by first operating a retort using a fuel other than off
gas in the retort inlet mixture. As retorting progresses, this results in
generation of off gas having a heating value of at least 80 BTU/SCF, which
can then be used as at least a portion of the fuel of the retort inlet
mixture.
Preferably, a liquid fuel is used in the retort inlet mixture
because liquid fuels are readily available and easily transportedO In
addition, liquid fuels have a lower volume per BTU than gaseous fuels,
Therefore, the pressure drop across the fragmented mass in a retort with a
liquid fuel is less than the pressure drop with a gaseous fuel supplying
an equivalent amount of heating value.
Low pressure drop across the fragmented mass is important inasmuch
as retorting may be continued for an extensive period of time. For example~
one experimental in situ retort a little over 80 feet high was retorted over
a period of 120 daysO
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If there is a high pressure drop along the length of the fragmented mass,
blowers or compressors used for inducing gas flow will operate at relatively
high presswre (for example, 5 psig), which requires appreciable more energy
for driving the blowers or compressors than if the pressure drop is relatively
low. The total energy requirements can be relatively high because of the long
time required f~r retortingO Higher pressure operation also can take a
greater capital expenditure for blowers or compressors, and some gas
leakage from the retort can occur, further reducing efficiency. . .
The preferred liquid fuel is shale oil which has been withdrawn
from an in situ oil shale retort because it is readily available and has
little, if any, value added by processing.
Oxygen for the retort inlet mixture can be provided by oxygen
supplying gases such as ai.r or air mixed with oxygen or air mixed with a
diluent to reduce the oxygen concentration of the mixture.
Beneficial effects can be obtained from the presence of water in
the primary combustion zone feed.
Beneficial effects of providing a retort inlet mixture containing
water vapor and oxygen are discussed in United States Patent No. 4,036,2990 .
Therefore, preferably the retort inlet mixture contains water
vapor and/or liquid water. To obtain significant beneficial effects of the
presence of water in the retort, preferably the primary combustion zone feed
contains at least about 10% by volume water vapor. In forming the retort
inlet mixture, allowance should be made for water resulting from oxidation
of any hydrogen containing compounds present in the retort inlet mixture,
connate water, and leakage of water into the retort from underground
aquifers.
The water in the retort inlet mixture can be a portion of water
withdrawn from an in situ oil shale retort. This is an advantageous use of
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such water since it can contain some hydrocarbon products of retorting and
inorganic materials and therefore could require treatment before release to
the environmentO When such water is used in the process, treatment is not
required~ Water containing impurities from other sources such as boiler
blow down and sewage can be used as the source of water.
In a preferred embodiment, fuel and an oxygen supplying gas are
substantially homogeneously mixed prior to introduction into the retort as a
retort feed. This can be accomplished by any number of methodsO For
example, when the fuel is a liquid, such as shale oil, the fuel can be dis-
persed in the oxygen supplying gas by means of a venturi gas/liquid
contactor or similar device. When the fuel is a gaseous fuel, the oxygen
supplying gas and gaseous fuel can be mixed by means of an injection nozzle.
Water or steam in the retort inlet mixture can also be homogeneousl~ mixed
with the fuel and the oxygen supplying gas prior to introduction to the
retort.
The gaseous primary combustion zone feed is formed and introduced
into the primary combustion zone at a rate sufficient to maintain the maxi-
mum temperature in the primary combustion zone at a temperature above the
retorting temperature of the oil shale and tG advance the primary combustion
zone through the in situ oil shale retort.
The upper limit on the temperature in the primary and secondary
combustion zones is determined by the fusion temperature of the oil shale,
which is about 2100F. The temperature in the primary and seconda~y combus-
tion zones preferably is maintained below abuut 1800F to provide a margin
of safety between the temperature in the primary and secondary combustion
zones and the fusion temperature of the oil shale. In this specification,
when temperature of a primary or secondary combustion zone is mentioned,
reference is being made to the maximum temperature in the combustion zoneO
.
Retorting of oil shale can be carried out with primary combustion
70ne temperatures as low as about 800 F. However, in order to have retorting
at an economically fast rate, it is preferred to maintain the primary combus-
tion ~one above about 900 Fr
In one embodiment of this invention, a plurality of in situ oil
shale retorts each containing a fragmented permeable mass of particles con-
taining oil shale, is formed with pillars of oil shale providing walls between
adjacent retorts. The amount of oil shale deposit remaining between retorts
is about 30% of the entire formation prepared for in situ retorting. The
kerogen assay of the fragmented shale in the retort is about the same as the
kerogen assay of the shale in the pillars adjacent to the retortsO
During the initial stage of retorting, wall temperatures of the
order of 900 F are developed at the interface of the fragmented permeable
mass at the top of the retort and the pillars~ Shale oil produced during the
initial stages of retorting is collected at the bottom of the retortO A por-
tion of the shale oil is mixed with air and is fed into the top of each
retortO This mixture of shale oil and air is ignited at the top of the
retorts for establishing a secondary combustion zone near the top oP the
retorts. This secondary combustion zone maintains wall temperatures at the
top of the retort between about 1000F and about 1200Fo The secondary com-
bustion zone can be identified by measuring the temperature and oxygen con-
centration of gases near the top of the retortO As the retorting continues,
both the primary combustion zone and the retorting 30ne move downward~
As retorting continues, the richness or heating value of the
retort off gas increasesO This may be due to an increase in the gas produc-
tion rate from the pillars. Such off gas is rich in hydrocarbons with high
heating value. When the heating value of the retort off gas is sufficient
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for combustion in the secondary combustion zone and maintaining the tempera-
ture in the secondary combustion zone at about 1200 F, the injection of the
shale oil is discontinued and only recycled off gas of sufficient heating
value is introduced into the retort and burned in the secondary combustion
zone therein, said secondary combustion zone being maintained thereafter
only by combustion of such recycled off gas Combustible retort off gas
from another retort can also be used as fuelO
Upon completion of the retorting operation, the amount of oil
recovered is about 85% to about g5% of the maximum oil recoverable from the
fragmented permeable mass of oil shale, exclusive of the oil which is
recycled and burned in the secondary combustion zone. Oil recovery from sub-
stantially the same grade oil shale which is retorted and without
establishing a secondary combustion zone with wall temperature of about 1000F
is substantially less, of the order of about 60% to about 80% of the maxi- -
mum oil recoverable from the fragmented permeable mass of oil shale.
Gases are passed from the secondary combustion zone into the
primary combustion zone at a rate sufficient to maintain the maximum
temperature in the primary combustion zone at a temperature abo~e the
retorting temperature of oil shale and to advance the primary combustion
zone through the in situ oil shale retort. In the primary combustion zone,
residual carbonaceous material in the retorted oil shale is believed to
be oxidized to yield carbon dioxide according to reactions (1) and (2)
above and the reaction:
C + 2-~ C2 (3)
Reactions {2) and (3), which are exothermic generate heat required for the
endother~ic retorting of kerogen in the oil shale in the retorting zone.
Carbon dioxide produced by carbonate decomposition within a large oil shale
_ 23 -
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particle can react with residual carbonaceous material contained therein by
reaction (l)o Also, carbon diox.ide generated by oxidation of fuel for
generation of a hot primary combustion zone feed can react with residual .
carbonaceous material contained in oil shale particles by reaction (l)o
When carbonaceous material in the retort is at a sufficiently
high temperature, water vapor can react by the water gas reaction:
H20 + C ~ H2 + CO
or by its equivalent:
n H20 + CnHn+2--~(n+l)H2 + nC0 (5)
The water gas reaction is bel;eved to occur when water contacts
carbonaceous material heated to a temperature above about 1200F.,It is
thought that the residual carbonaceous material remaining in retorted oil
shale is in a highly active form and the water gas reaction can occur at a
temperature as low as about 1000 to 1100 F~
Carbon monoxide generated by the water gas reaction can be
oxidized by oxygen in the combustion zone feed according to reaction (2)
and hydrogen generated by the water gas reaction can be oxidized according
to the reaction:
2H2 + 2-~ 2H2 (6)
Carbon monoxide can also be oxidized by the reaction: ' ,
CO ~ H2 ~1 C2 ~ H2 ( )
Although the water gas reaction is endothermic, reactions (2), ,
(3)~ and (6) are exothermic. The net result of reactions ~2), ¦4) and (6)
is the oxidation of carbon to carbon,dioxide with regeneration of thetwater
used in the water gas reaction by reaction (6)o
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Utilizing the water gas reaction for oxidation of residual
carbonaceous material is a reason for maintaining the secondary combustion
zone at a temperature of at least about 1150 F.
Oil shale is a poor heat conductor, and therefore, heat generated
in a regi.on in an in situ oil shale retort tends to remain within the region
and increase the temperature of oil shale within the regionO However, with
the method described herein, gases are moved through the primary combustion
zone in the direction of advancement of the primary combustion zone through
the retort. The gaseous mixture passing form the primary combustion zone
into the retorting zone contains combustion gas generated in the primary
combustion zone and any gaseous unreacted portion of the retort inlet
mixtureO This gas stream provides the heat required for the endothermic
retorting of the kerogen in the oil shale particles.
Retorting of the oil shale in the retorting æone produces gaseous
and liquid products such as carbon dioxide, carbon monoxide, hydrogen,
hydrogen sulfide, water liberated from the shale~ and h~drocarbons such as
methane, ethane, and shale oil. Raw oil shale on the advancing side of the
retorting zone is at the ambient temperature of the oil shale prior to
establishing the combustion zone in the retort and gradually increases in
temperature as the retorting zone approachesO Some of the raw oil shale is
below the dew point of gas on the advancing side of the retorti.ng zone.
Thus, water~ if any, intrvduced into the retort as part of the retort inlet
mixture and any water released from the oil shale can condense on raw
oil shale, Such condensed water percolates to the bottom of the fragmented
mass and is collected in the sump 20 as a portion of the liquid products.
Also collected in the sump are hydrocarbons produced in the retorting zone
which condense above ambient temperatureO The uncondensed gaseous products,
_ 25 -
combustion gas from the combustion zone, carbon dioxide from carbonate
decomposition, and any gaseous unreacted portion of the retort inlet mixture
are withdrawn from the bottom of the retort in the off gas stream. The off
gas stream can be saturated with water vapor.
A method of in situ retorting with a secondary combustion zone
has significant advantages compared to a method of retorting oil shale
without a secondary combustion zoneO Among these advantages is increased
yield of liquid hydrocarbons~ It is believed that enhanced yields can be
obtained because residual carbonaceous material in oil shale on the trailing
side of the primary combustion zone is maintained at a temperature higher
than its spontaneous ignition temperature. Thus, this residual carbonaceous
material can react with Gxygen in the retort inlet mixtureO Also, high -
temperatures in the combusted oil shale on ~he trailing side of the primary
combustion zone increases the diffusivitycfoxygen and water vapor into oil
shale, thereby resulting in oxidation of residual carbonaceous material in
large formation particles containing oil shale which otherwise might not
react with oxygen. Furthermore, carbon dioxide produced by carbonate
decomposition in large particles of `oil shale can react with residual
carbonaceous material by reaction ~1) and generate heat by reaction (2) to
provide additional heat for retorting of kerogen in the retorting zone~
Thus, enhanced yields of hydrocarbon products can be obtained because
residual carbonaceous material which otherwise might not have reacted with
oxygen is used to produce the energy required for retorting, thereby allowing
kerogen in the retorting zone to be retorted, rather than being oxidized to
generate energy required for retortingO
Also contributing to enhanced yields obtained by operating a
retort with a secondary combustion zone is that kerogen in unfragmented
formation adjacent the retort and kerogen bypassed by the retorting zone and
and primary combustion zone, such as kerogen in oil shale in the upper
corners of the retort, can produce hydrocarbon productsO This kerogen
otherwise might not have been retorted,
Another advantage of retorting with a secondary combustion zone
is enhancement of the fuel value or heating value of the retort off gas.
In retort operations utilizing a gaseous feed comprising air and recycled
retort off gas, and having a temperature about the same as ambient
temperature, the heating value of the retort off gas is relatively low, i.eO,
in the order of about 20 to 60 BTU/SCF on a dry basis, This retort off gas
is of marginal value, if usable at all, for use in a work engine to
generate power, and if it is used, it may be necessary to augment the retort
off gas with other combustible material. It is found that when the retort
inlet mixture contains fuel for establishing and maintaining a secondary
combustion zone, an off gas with a heating value of from about 50 to about
100 BTU/SCF or higher can be obtained. At such heating value the off gas
is satisfactory for combustion in a work engine such as a gas turbinen
Relatively high heating value off gas, i~eO, off gas having a heating value
of 80 B~U/SCF or higher, can be used as at least part of the fuel of the
retort inlet mixtureO
It is believed that this improvement in the heating value of the
off gas is attributable to the fact that inclusion of fuel in retort inlet
mi~ture results in introduction of less non-combustibles into the retort
than inclusion of recycled off gas in the retort inlet mixture. Both fuel
and recycled off gas can be used to reduce the oxidation concentration of air
to a value from a primary combustion 7one feed having an oxygen concentration
of 10 to 15% by volumeO Fuel does this primarily by consuming oxygen~
Retort off gas does this primarily by diluting the air with non-combustible
components of the off gas, off gas can contain more than 80% by volume
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carbon dioxide and nitrogen. Thus, use of fuel in the retort inlet mixture
in place of recycled off gas results in introduction of less non-combustibles
into the retort, and therefore off gas withdrawn from the retort contains
less non-combustibles. Furthermore, the bulk of water vapor produced from
oxidation of fuel of the retort inlet mixture does not appear in the off gas
from the retort, but instead is condensed on the shale on the advancing side
of the retorting zone and is withdrawn as liquid with the condensed hydro-
carbon products~ Condensation of the water vapor removes an inert diluent
from the off gas, enhancing its fuel value on a vol~etric dry basisO
An advantage of retorting with a secondary combustion zone where
the fuel is a liquid -fuel or a rich gaseous fuel of high heating value, such
as hydrogen, methane, propane, natural gas, or liquified petroleum gas, is
thatthere is less pressure drop across the fragmented mass -than when
retorting off gas is used in the retort inlet mQxture. This is because
when retorting off gas is used to dilute air of the retort inlet mixture,
the presence of non-combustible constitue~ts;of the retorting off gas results
in a primary combustion zone feed having a higher volumetric flow rate per
square foot of retort cross-sectional area than the volumetric flow rate
of the air introduced ko the retort. On the other hand, when the fuel of
the retort inlet mixture is a liquid fuel or a rich gaseous fuel~ the volume
at standard temperature and pressure (STP) of the primary combustion zone
feed on a dry basis is less than the volume (STP) of the introduced air.
This occurs because fuel of the retort inlet mixture reacts with oxygen of
the air to produce carbon dioxide, carbon monoxide and waterO Even on a wet
basis, the volume (STP) of the oxidation products of the fuel in combination
with the nonreacted components of the introduced air can be less than 10%
greater than the volume (STP) of the introduced air.
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Therefore, ùse of a liquid fuel and/or a rich gaseous fuel as
the fuel of the retort inlet mixture rather than recycled off gas results
in a lower volumetric flow rate of gas through the retort~ Thus, the
pressure drop across the retort is smaller, which reduces the size of blowers
required for retorting and reduces the energy cost associated with passing
gases through the fragmented mass.
In one embodiment of this invention, a secondary combustion zone
is established in a substantially square in situ oil shale retort having
sides measuring about 120 feet wide and having a height of about 270 feet
and containing a fragmented permeable mass of particles containing oil shale.
A primary combustion zone is established at the top of the retort and is
advanced downwardly through the retort, leaving a hot fragmented permeable
mass of particles on the trailing side of the primary combustion zone and
producing shale oil. While a portion of the fragmented permeable mass of
particles near the top of the retort is still at a temperature greater than
about 900F, a secondary combustion zone is established at the top of the
retort.
The secondary combustion zone is formed by introducing air at the
rate of 7,900 SCFM, water at the rate of 3.48 gallons per minute and shale
oil at the rate of 1~32 gallons per minute into the top of the retort. The
shale oil is shale oil produced in the retort or another retortO An atomizer
is used for mixing the shale oil and water with the air just prior to intro-
ducing the air into the retort. The shale oil is oxidi~ed when heated to
its spontaneous ignition temperature at the top of the retort to form the
secondary combustion zone~ The water vaporizes on being heated and is con-
ducted along with the gases resulting from the oxidation of the shale oil
and the remaining portions of the air into the primary combustion zone.
These gases are conducted into theprimary combustion zone at a superficial
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velocity of about 0~62 SCF~i per square foot o:E the fragmented mass being
retorted and with o~ygen and water vapor concentrations of about 140'~ and
10.2 percent by volume, respectively~ The net shale oil recovery from the
retort is about 72% of Fischer assayO
~ lthough this invention has been described in considerable
detail wi.th reference to certain versions thereof, other versions are within
the scope of this invention. For example, although the invention has been
described in terms of a single in situ oil shale retort containing a primary
combustion zone, a secondary combustion zone, and a retorting zone, it is
possible to practice this invention with two serially connected retorts~
The first retort can contain retorted oil shale and both combustion zonesO
'~he gases generated in the primary combustion zone of the first retort
would be passed to a second retort for retorting raw oil shale contained
therein.
In addition, although Figure 2 shows a retort where the combus-
tion and retorting zones are advancing downwardly through the retort, this
invention is also useful for retorts where the combustion and retorting zones .
are advancing upwardly or transverse to the verticalO
Furthermore, although the invention has been described with
water, the source of oxygen, and the fuel comprising a retort.inlet mixture
being introduced together and continuously into a retort, theSe three
components of the inlet mixture can be introduced intermittently and/or
independently into the retortO
Because of variations such as these, the spirit and scope of the
appended claims should not necessarily be limited to the description of the
preferred versions contained hereinu
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