Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the recovery of the energy
in gases produced in an in-situ combustion process for the pro-
duction of oil from underground carbonaceous deposit~ and more
particularly from underground deposits of oil and oil shale.
One method for increasing the production of heavy
crude oils of high viscosity from underground formations is
the in-situ combustion prQceSS. In that proces~, air is
injected at a high pressure throu~h an injection well into
the underground formation containing the heavy oil. The oil
in the formation adjacent the injection well is ignited by
any of several known procedures such as the procedure disclosed
in U. S. Patent No. 3,172,472 of F. M. Smith. Injection of air
is continued after ignition to burn part of the oil in the
formation and to increase the pressure in the formation adjacent
the injection well and thereby drive oil in the formation toward
a production well spaced from the injection well. A typical
in-si~u combustion process is described in U. S. Patent No.
2,771,951 of Simm. The heat released by combustion of some of
the oil in the formation heats the formation and oil whereby
the viscosity of the oil i~ greatly reduced by the high tempera-
ture, cracking of the oil, and by solution in the oil of low
molecular weight hydrocarbons formed by the cracking. The
reduced viscosity and the pressure of the injected gases cause
the oil to flow throuyh the underground reservoir to a production
well.
During in-situ combustion processes, the combustion
front at which oil in the formation is hurned does not move
radially outward at a uniform rate in all directions.
Usually the oil reservoir will vary in permeability and oil
saturation from one location to another between the injection
and production wells. Some of the injected air fingers through
the formation toward a production well and combustion oc~urs at
the boundaries of the fingers. There may be a breakthrough
of combustion products in the nature of a flue ~as long before
the production of oil by the in-situ process is completed.
-2-
, .
Volatile constituents in the oil, or formed by cracking o
th~ oil, are entrained in the injected air or flue gases and
carried by them ts:~ the production well~ All o~ these factors
contribute toward variations in the composition of the gas
5 produced from time to time during an in-situ combustion for
the production of oil ~rom a reservoir. Such variations may
re~ult in periodic increases of 100 percent or more in the
heating value of the gas producedf
The fluids produced at the production well are
separated into liquid petroleum products which are delivered
to storage or a delivery line and gaseous products. The
gaseous products customarily have been vented ~o the atmosphere.
The gaseous products, hereina~ter referred to as LHV gas, from
in-situ combustion contain low concentrations of methane and
C2-C6 hydrocarbons, as well as nitrogen, carbon dioxide, sulfur
compounds such as hydrogen sulide, mercaptans and carbonyl
sulfide, and in 50me instances a small amount of carbon monoxida
and traces of oxygen. Those gaseous products constitute low
heating value fuel capabIe of supplying a subst~ntial part of
the en rgy required to compress the air for in~ection into the
su~surace formation at the injection well. The shortage of
natural gas make~ it important that the energy in the products
from an in-sltu combustion process be ~ully utllized. Moreover,
tightening of l~w9 relating to pollution of the atmosphere has
place~ stringent limitations on the amount of carbon monoxide,
the sulfur compounds most freguently present in the gaseous
products, and hydrocarbon~ other than methane that may be dis-
charged into the atmosphere.
U. S. Patent No. 3,113,620 of Hemminger describes a
single well in-situ combustion process in which a cavity filled
3--
with rubble is formed in a subsurface oil shale deposit b~
means of a nuclear explosion. An in-situ combustion process
in the cavity is then conducted to remove oil from the rock,
aid in draining the oil into a pool in the bottom of the cavity,
and force the oil up th~ well to th~ surface. The composition
of the gases produced with the oil difers from the composition
of gas produced in a conventional in-situ combustion process in
an oil reservoir. Because o the diferent composition of the
gas, Hemminger is able to burn the off-gas directly in a flame
combustor of a gas turbine used to drive an air compressor.
U. S. Patent No. 2,44~,096 of Wheeler, ~r. describes
a process for the recovery of power from gas discharged from a
regenerator in a fluidized catalytic cracking process. The hot
gases from the regenerator are first passed countercurrently to
a nonvolatile oil in a scrubber whereby a small amount of hydro~
carbons is entrained in the gas. The gas with entrained
hydrocarbons is passed through a catalytic oxidizer for burn
ing ~he hydrocarbons and the ~ombustion products are delivered
either to a turbine or direGt power recovery or to a steam
generator.
U. S. Patent No. 2, 8591~54 of Grey describes a power
recovery system for a blast furnace in which blast urnace gas
is compressed, burned, and then expanded in a turbine to provide
energy ~for compressing air used in the blast furnaceO The com-
bustible material in the blast furnace gas is largely carbonmonoxide and hydrogen. Those gases are more readily ignited
and burned in dilute mixtures with inert gases than is methane.
The blast furnace gas, which typically has a heating value
above 90 btu/scf, is burned by Grey in flame-type combustors
for release of the thermal energy.
--4
'
.. .. .
3~P~
U. S. Patent No. 3~g28,9~1 of Pfeferle describes
the catalytic combustion o fuels to drive gas turbines. rrhe
method of Pfefferle conducts catalytic oxidations at a
temperature in the range of 1700 F. to 3~00~ F., preferably
in the range of 2000 P. to 3000 F., described by Pfe~ferle a~
the autoignition range. That temperature is high enough to
initiate thermal combustion, but not high enou~h to c~use
subs~antial formation of nitrogen oxides. Combustion in the
P~eferle method is primarily thermal combustion. Air at
least stoichiometrically e~uivalent to the fuel is used to
complete the oxidation. When the combustion is used for gas
turbine operations, the weight ratio of air to ~uel is far above
the stoichlometric ratio and ranges ~om about 30:1 to 200 or .
more to 1. Because the composition o the ~uel is constant, thP
excess of oxygen in the air-fuel mixture does not cause wide
variations in the temperature of the combustion products.
Because oil shale is impermeable, in-situ combustion
processes for the recuvery of oil from oil shale require that
permeability of the shale be established befoxe the in-situ
combustion is begun~ It is pref~rred that the permeability be
established by rubbli2ing shale in a selected portion of the
shale deposit to form an underground retort. Combustion of
shale in the retort to release shalP oil is preferably accom-
plished at low pressures to avoid leakage of gas to adjacent
retorts that are being rubblize~. Gases discharged from
retorts for in-siku combu~tion processes for shale oil pro-
duction are usually, there~ore~ at pressuxes too low for
delivery directly into gas turbines or power recovery.
Typical in-situ combustion processes for the recovery of oil
from oil shale are descri~ed in U. S. Patent No. 2,780,449 o
Fisher et al and U. S. Patent No. 3,0~1,776 of Van Poollen.
--5~
Summary of the Invention
This invention resides in a system for the recovery
of power from and reduction of pollutants in LHV gas discharged
from production wells of in-situ combustion processes for the
production of oil. The LHV gas, after separating rom liquid
products produced from the well, i~ mixed with a limited
quantity of air~ preheate~ to a tempexature exceeding approxi~
mately 400 F., and delivered into a cata]ytic combustion
chamber in which combustibles in the LHV gas are burned.
The amount of air mixed with the L~V ga~ is less than the
stoichiometric equivalent of the combustibles in the L~V ga~
and thereby limits the amo~nt of combustion and the maximum
temperature that may occur in the combustion chamber. In a
preferred embodiment, the I.HV gas is burned in a primary and
a secondary catalytic combustion chamber with approximately
fifty percent of the total air introduced into each combustion
chamber. Hot gases discharged frvm khe primary ca$alytic
combustion chamber~are mixed with the air for the sacondary
combustion and~then cooled to a temperature suitable for
introduction into the secondary catalytic combustion chamber~
; The cooled ga~es are delivered into the secondary catalytic
combustion chamber for combustion of combustible material
remaining in the gas. Preferably, the combustion products
discharged from the second combustion chamb~r are expanded in
2S a turbine to generate power ~or compression of air used in the
in-situ combustion process.
Brief Description o~ the Drawin~s
Figure 1 of the drawings is a diagrammatic flow sheet
of a preferred embodiment of this invention as used for the
recovery of power rom LHV gas discharged from an in-situ
combustion system ~or the reco~ery of oil from an underground
oil reservoir.
--5
Figure 2 of the drawing~ i~ a diagrammatic 10w
sheet of an embodiment of this invention in which the combusti~n
products generate steam that is used to drive a turbine.
Fi~ure 3 is a chart showing tne varia~ion in ~he
S heating value of c3ases produced in an in-situ combustion
process for the recovery of oil from an oil reservoix over a
two and one-half year period.
Description o~ Preferred Embodiment
Referring to Figure 1 of the drawing~, a subsurface
formation 10 containing crude oil, u~ually of hiyh density and
viscosity, is penetrated by a production well 12 and an injection
well 14 spaced from the production well. Flu~ds produced from
the production well 12 are delivered through a line 16 into a
separator 18 in which the produced LHV gas is separated from
liquids produced through well 12. The liquids are discharged
from the lower end of the separator 18 into a delivery line 20
and the ~HV gas is discharged from the top of separator 18
into a feed line 22. It will usually be desirable to pass the
gas from the separator 18 through a suitable gas clean-up, not
shown, to remove solid particulates, catalyt poisons, or other
undesirable constituente before delivery into line 22. A 10w
controller 23 maintains a constant flow rate of the LHV gas to
the turbine as hereinafter described.
Typical hydrocarbon concentrations in the LHV gas
range fxom about 1 to 8 per cent by volume. The hydrocarbons
are principally methane; the concentration of C2-C~ hydrocarbons
usually belng less than 2 percent. In the concentrations
present in the LHV gas, stable combustion of methane and the
other low molecular weiyht hydrocarbons produced in in situ
production is obtained only in the presence of a catalyst.
The heating value of the LHV yas produced from in-situ
--7--
combustion in oil reservoirs may range from S to 80 BTU/scf,
and ordinarily will be in the range of 40 to 70 BTU/scf,
a range in which the process is particularly useful. On some
occasions, usually fox short periods, the heating value may
rise to 100 BTU/scE. LEIV gas having a heating value above
lS BTU/scf can be burned in a catalytic combustor without an
external source of heat. If other sources of heat are available
to provide additional preheat, LHV gas having a heating value
as low as 5 BTU/scf can be oxidized in catalytic combustion
chambers.
For most affective use in driving a gas turbine to
compress air used in the in-situ combustion process, for example,
the gases dischaxged from the sepaxator 18 should be at a pres-
sure of at least 75 psig. I~ the gas is at lower pressure,
i
part of the energy is used in compressing the LHV gas to a pres-
sure high enough to drive a turbine, however, LHV gas at lower
pressures may contain substantial energy which can be useful in
supplying heat for heater-treaters and other oil field equipment
or for developing power by supplying heat to generate steam at
:
a pressure high enough to drive a turbine. The pressure in the
production wells of an in-situ combustion process usually range
from slightly above atmospheric pressuxe to 800 psig. The
pressure is dependent, at Jeast in part on the depth of the
formation in which the combustion occurs. Pressures higher than
800 psig in the production wells can be used but such high
pressures suffer the disadvantage of high costs for compressing
the air injected into the underground formation; however, much
- of the energy used in compressing the air can be recovered.
The gas in line 2~ is mixed with air from a line 24
and, during startup, passed through a heater 26. A fuel, such
..
3~
as LPG, supplied through line 28 is burned in heater 26 to raise
the temperature of the mixkure of LHV gas and air to a tempera-
ture at which oxidation will occur on contact with the oxidation
catalyst hereinafter described. The heater 26 is used only during
S startup, and after oxidation of the LHV gas commences, heat~r 26
is bypassed by flowing the air from line ~ through bypa.ss 30.
The mixture o~ air and LHV gas discharged from
heater 26 is delivered through line 32 into a heat exchanger
34. No hea* kransfer to the mixture of air and ~HV gas
occurs in heat exchanger 34 until after combustion has begun
in khe catalytic combustion chamber as hereinafter descri~ed.
After pa~sing through h~at exchanger 34, the mixture o air
and LHV gas is delivered through line 36 into a primary
catalytic combustion chamber 38. The temperature of the gas
delivered to the combustion chamber 38 should be in the range
of approximately 400 to 800 F. whereby oxidation will be
i~ltiated on contact of the LHV gas with the catalyst.
Usually, the~inlet temperature will be in the upper part of
that:temperature range.
In a preferred catalytic oxidation chamber, platinum
is deposited on the surfaces of a commercially available ceramic
honeycomb-type catalyst suppoxt arranged in primary combustion
chamber 38 in a series of transverse slabs 39 sepa.rated from
one another by ~ree spaces 40. The free spaces between the slabs
are designed:to allow equalization o~ the temperature of the
gases between the slabs and thereby minimize development of hot
spots; however, satisfactory operation has been obtained wikh
adjacent slabs touching. The honeycomh-type of catalyst support
.
~is advantageous in allowing high throughputs with a low pressure
drop through the catalyst. ~n a typical combustion chamber
there may be 10 to 20 or even more slabs 39. ~ catalyst that
can be used is described in U. S. Patent No~ 3,870,455 of Hindin~
~h~3~J~
The arrangement a~d catalyst shown are preferred for use in
this invention; however, this invention is not restricted to
the use of a particular catalyst. Other oxida~ion catalysts
supported on other supports can be used. For example, catalysts
containing cobalt or lanthanum as the cata]ytically active
ingredients s~pported on honeycomb or on sponge type supports
can be used. Other catalysts that can be used are described in
U. S. Patent No. 3,565,830 of Keith et al. Oxidation catalysts
used at temperatures higher than ~000 Fo are described in U. S.
Patent No. 3,928,961 of Pfefferle.
The gases are discharged from combustion chamber 38
at a maxim~m temperature preferably of approximately 1600 F.
through line 41 and mixed with additional air supplied through
line 42. The maximum temperature is limited by the highest
temperature that the catalyst can withstand over long operating
periods without deterioration. Preferred oxidation catalysts for
this invention in which combustion products are passed through
a gas turbine that consist of platinum on a ceramic support can
withstand higher temperatures than 1600 F. for short periods
~0 but their life lS shortened by continued use at the higher tem-
peratures. As previously indicated, the amount of air supplied
through line 42 preferably is approximately one-half of the
total amount o~ air supplied ~or oxiclation of the LHV gas.
Addltion of air through line 42 will ordinarily result in a
temperature drop of approximately 200 F. in the gases in
line 41. The gases are deli~ered throuyh line 41 into heat
exchanger 34 and passed therein in indirect heat exchange with
the initial mixture of air and LHV gas delivered to the hea~
exchanger through line 32. ~he mixture o air and partially
oxidized LHV gas i~ discharged from heat exchanger 34 at a
temperature preferably o~ 600 F. to 300 Fo and delivered
via line 44 into a secondary catalytic combustion chamber 46.
Secondary catalytic combustion chamber 46 may be identical to
-10~
3~ g
primary c~talytic combustion chamber 38. Oxidation of combus-
tibles in the partially oxidized LHV gas delivered ~rom heat
exchanger 34 occurs in secondary combustion chamber 46. By
limiting -the total air flow into the primary and secondary
catalytic combustion chambers to an amount slightly ~e.g. 5 .
percent) less than the s$oichiometric equivalent of an LHV
gas heating value that would cause the maximum permissible
temperature rise,excessive temperatures in the catalytic
combustion chambers are. avoi-led even though the composition
of the L~IV may vary widely.
Heat released in the secondary combustion chamber
may increase the temperature o~ the gases to, ~or example,
approximately 1600 F. at the outlet line 48 rom the secondary
combustion chamber. Usually, it will be desirable to cool the
gases in line 48 to reduce the temperature of the gases to a
temperature that provides a margin of safety below the maximum
operating temperature o~ a gas turbine 52 into which the gas is
delivered. Some turbines are capable of withstandiny tem-
peratures up to about ~000 ~.; however, it is preferred to
~ reduce the temperature of the gas~s to 1400 F. to 1600 F.
by dilution with air before delivery into the turbine. The
desired cooling may be accomplished by introducing cooling air
into line ~8 from line 50. The temperature at which the com-
bustion products leave the secondary combustion chamber is far
below the temperature required to ignite those partially burned
gases o low heating value in the absence of a catalyst;
consequently, there is no danger of further combustion with
resultant development of excessive temperatures occurxing on
mixing air from line 50 with the hot gases in line 48 even
though the hot gases usually include unburned combustibles.
The air introduced through line 50 is, therefore, cooling air
rather than combustion air.
Referring to Figure 3 of the drawings, it will be
noted that the heating value of the LHV gas produced during
a typical in-situ combustion process varies substantially.
Most of the time during the 2-l/2 year operation shown in
Figure 3, the LHV gas produced by the in-situ process had a
heating value above about 55 BTUJsc~; however, the heating
value was frequently in the range of 70 BTV/scf and on one
occasion increased to over lO0 B~U/scf. At other times, the
heating value dropped below 50 BTU/scf. The wide swings in LHV
gas composition introduce problems of controlli.ng the temperature
to prevent excessive temperatures that would damage the catalyst
or the gas turbine and still operate ~he gas turbine under opti-
mum conditions. To control the maximum temperature that may be
attained and also maintain a uniform temperature~ the catalyt.ic
comhustion chambers are operated with less alr than the stoich-
iome$ric equivalent of the combustibles in the LHV gas. In this
manner r the amount of air supplied to the catalytic combustion
chambers establishes the maximum temperature rise in the
catalytic combustion cha~bers and r consequently, the maximum
; 20 ~emperature that is attained. If the maximum permissible tem-
perature in the catalytic combustion chamber i.s 1600 F., the
amount of air mixed with the cold LHV gas entering the heat
exchanger 34 will be such that when the oxygen in that air is
completely consumed in burning combustibles in the LHV gas, the
temperature in the combustion chamber will not exceed 160~ F.
If oxidation catalysts having an acceptabl~ life when used at
higher temperatures to burn LHV gas should be used, and ~he
~eating value of the LHV gas should be high enough to raise the
temperature of the:combustion products abo~e the maximum
operating temperature of the catalyst, the amount of air delivered
tc the combustion chambers could be increased and thereby raise
-12-
d3
the maximum temperakure that might be reached in the com-
bustion chambers. The amount of air would be less than the
stoichiometric equivalent of the LHV ~as to maintain the
desired temperature control.
Assuming that the mixture of LHV gas and air in line
32 to the preheater 34 is at a temperature of 175 F. and the
maximum permissible catalyst temperature is 1600 F., the
maximum total amount of air mixed with the I.HV gas entering
the primary combustion chamber 38 and with the hot combustion
products discharged from that combustion chamber can contain
oxygen stoichiometrically equivalent to the combustibles in LHV
gas having a heating value of approximately 50 BTU/scf. If the
heating value of the LHV gas should be as shown in Figure 3,
that amount of air would be less than stoichiometrically equiva
lent dur1ng approximately 90 percent of the time. Preferably,
approximately fifty per~cent of the total amount of combustion air
is introduced into the system by mixing with the LHV gas before
delivery into the primary:combustion chamber and the remainder
is mixed with the products of combustion from the primary
combustion chamber before delivery to the secondary combustion
chamber.
While controlling the rate of air a~mixture with the
LHV gas introduced into the primary and into the secondary
catalytic combustion chambers at a rate that is less than the
2S stoichiometric equivalent of the normal LHV gas composition
limits the maximum temperature rise in the combustion chambers
and is effective in protecti.ng the catalyst from temperature
rises that might:~otherwise result ~rom increases in the heating
value of the LHV~gas, precise con~rol to maintai.n a constant
mass flow rat and temperature of the gas delivered to the
turbine is desirable for most efi.cient operation of the turbine.
-13-
3r~
Flow controllex ~3 in line 22 from separator 18 maintains the
desired rate of ~low of LHV gas into the system. Since the
rate of flow of air to the combustion chambers is constant,
a drop in the temperature of gases discharg~d Erom the secondary
catalytic combustion chamber indicates that the combustible
materials in the LHV gas are less than the stoichiometric
equivalent o~ the combustion air delivered into the systemO
The desired temperature can be obtained with li-ttle change in
the mass 10w rate by introducing an auxiliary uel into the
system. A temperature sensor 53 in line 48 adjacent the outlet
end of secondary combustion chamber 46 can bs connected to
actuate a flow control valve 55 in an auxiliary fuel line 57.
If the temperature in line 48 should drop, the temperature
sensor 53 would actuate flow control valve 55 to enrich the
mixture delivered to the secondary combustion chamber 460 To
provide stable operation, the controls will inject sufficient
auxiliary fuel to increase the heating value of the LHV gas
delivered to the co~bustion chambers an amount such that the
air is slightly less than the stoichiomet.ric equivalent of the
total combustibles in the LHV gas and auxi.liary fuel. Alterna-
tively, a hydrocarbon analyzer can be installed to withdraw a
sample from line 22 and determine the hydrocarbon content, and,
hence, the heating value, of the LHV gas. A signal generated
by the hydrocarbon analyzer can be used to contxol valve 55.
Clearly, the auxiliary fuel can be introduced into the system
at any point upstream of the secondary catalyti.c combustion
chamber, such a~ into line 22 Usually, the auxiliary fuel
will be natural gas or LPG. The heating values of natural gas
and LPG are high and the concentration of inerts is low;
consequently, a desired increase in the temperature of the
combustion products can be o~kaine~ with a negligible change
in the mas.s 10w rate to the turbine.
-14-
~33~
The hot gases are expanded in the turbine to sub-
stantially atmospheric pressure~ and the gases from the turbine
are diseharged to the atmosphere. I necessary, gases discharged
from the turbine can be passed through a suitable scrubber to
remove contaminants before being discharged into the atmosph~re.
The gases discharged from turbine 52 will be hot and could be
passed through a heat exchanger to supply heat for process or
other use, i desired.
Energy produced by turbine 52 is used to compress
air used in the combustion of the LMV gas and in the in-situ
combustion process. In ths embodiment illustrated, the turbine
is shown as directly connected to drive a low pressure compressor
54 which compresses air to a pressure exceeding the operating
pressure of the catalytic combustion chamber 38 sufficiently ~or
easy control of air flow into the system. That air is delivered
in~o a header 56 to which line.s 24, 42 and 50 are connected or
delivery of aix as needed for the combustion of th~ LH~ gas and
subsequent cooling of the combustion products from the secondary
con~ustion chamber~
Turbine 5~ also drives a high-pressure compressor 58
that compresses air for use in the in-situ combustion process.
Air compressed in compressor 54 in excess of the air required
for combustion o~ the LHV gas and cooling is bled from
compressor 54 and delivered to compressor 58 through a line 59.
~5 Line 59 is provided with flow control means indicated by valve
61 to control the flow of bleed air to compressor 58. In a
preferred arrangement, ~he turbine-type compressor 58 compresses
air to an intermediate pressure of ~he order of 250-300 psig,
and that air is d~livered through line 60 to a cen rifugal or
reciprocating compressor 62 for ~urther compression to a
pressure typically in the range of 2,000 psig to 3~000 psig.
~15-
The pressure to which the air is compressed by compressor 62
will depend upon the parti.cular in situ combustion process
and will vary with several parameters of ~he in-si~u combus~ion
process such as the depth oE the subsurface formation into
which the air is to be injected, the viscosity of the oil, ~he
permeability of the subsur~ace formation, the rate of in~ection
of air into the formation, and the desired pres~ure at the
production well. Air from compressor 62 is delivered through
line 64 into injection well 14. In an alternative arrangement,
air compressor 58 is constructed to allow withdrawal of air
from an intermediate stage of the compressor at a low pressure
for use in combustion of the LHV gas. With that arrangement,
low pressure compressor 54 will not be necessary.
Whil~ supplying air at a rate that is less than the
stoichiometric equivalent of the combustibles in the LHV gas
causes some loss of the potential enexgy available in the com-
bustibles in the LHV gas, the importance of such loss is minimized
by the gain in the compressed air that is made available for use
in the in-situ combustlon process. If air were delivered to the
ca~alytic combustion chambers at a rate equivalent to the stoich-
iometric equi~alent of an LHV gas having a heating value of 70
BTU/scf, for example, and the maximum permissibla temperature in
the gas turblne remained at 1~00 F., it would be necessary to
cool the gas discharged from the secondary combustion chamber
before delivery to the turbine. I~ air from compressor 54 should
be used to dilute the combustion products and thereby cool them
to the desired *emperature for delivery *o the gas turbine, such
air would reduce the amount of air delivered from compressor 54
to compressor 58 for use in the in-situ combustion process.
It is an important advantage of this invention that
the desired safeguards against temperature and flow rate
~3~ 3
variations and excessive temperatures can be obtained while
complying with the usual antipollu~ion regulations.
Hydrogen sulfide and the higher molecular weight
hydrocarbons are preferentially, as compared to methane,
oxidized in the catalytic combust:ion chambers. The unoxiaized
combustible material discharged from the secondary combustion
chamber because of the deficiency in air i.s almost entirely
methane, a gas that usually is nok subject to antipollution
regulations.
As an example of the preferential oxidation of the
hydrocarbons having a higher molecular weight than methane,
a hydrocarbon gas-air mixture was passed in contact with a
platinum catalyst deposited on a ceramic support at an inlet
temperature of 840 F. and a maximum temperature of 1430 F.
The hydro~arbons were mixed with about 50 percent oF the
stoichiometric air, thereby giving a mixture with a severe
- deflciency in air.~ The composition of the hydrocarbons,
the outlet composition, and the percent conversion of each
: of the hydrocarbons are set forth below:
TABLE I
H~drocarbon Conversion
Inlet Composition Outlet Compositlon % Converslon
Methane 1.09% 0.97% 11.00
Ethane 0.22% 0.15% 31.8
Propane 0-34% 0.19% 44.1
i Butane 0.06~ 0.03% 50~0
n Butane 0.22% 0.10~ 54,5
i Pentane 0.06% 0% 100.0
n Pentane 0.08~ 0% 100.0
Similar tests were made with larger equivalence
ratios (ratio of amount of air supplied to the amount of air
17-
stoichiometrically equivalent to the comhustibles in the
gas). The results are presented in Tables II and II:
TABLE II
ydrocarbon Conversion
Inlet Composition Outlet Composition % Conversion
Methane 3.54% 0~47% 87
~thane 0.39% 0% 100
Propane 0.32% 0% 100
i-Butane 0.11~ 0~ 100
n-Butane Q.24~ 0% 100
i-Pentane 0O14% 0% 100
n-Pentane 0.16% 0% 100
Equivalence Ratio = 0.96
TABLE III
Hydrocarbon Conversion
Inlet Composition Outlet Composition ~ Conversion
Methane ~ 3.62% 2.23% 38~
Ethane 0.40% ; ~ 0.16% 60%
Propane 0.28% 0.06% 79~
i-Butane 0.09% 0~ 100%
n-Butane 0.18% 0~ 100%
i-Pentane 0.08% 0~ 100~
n-Pentane 0.07% o% 100%
~ Equlvalence Ratio = 0.60
It is apparent from the results shown in Ta~les I, II
:
and III that even with a deficiency of air more severe than
would normally be caused by a surge in combusti~les in the LHV gas,
preferential removal of the heaviest hydrocarbons from the LHV
gas was accomplished in the catalytic combustion chamber.
Combustion of methane lagged far behind the combustion of
other hydrocarbons. ~s the equivalence ratio increased to the
-18-
3~
range normall~ used, the hydrocarbons other than methane are
virtually completely consumed. This invention is/ therefore,
particularly advantageous when the fuel available varies in
heating value and con~ists of a mixture o~ combustibles that
includes methane as one of the combus~ible constituents.
An important ad~antage of the process of this inven-
tion in utilizing LHV gas from in-situ combustion processes
from which the LHV gas is produced at pressures, preferably
exceeding 75 psig, high enough to drive a gas turbine, is
that the utilization of energy in the LHV gas is more efficient
than in gas of higher heating value. If fuels of high heating
value such as natural gas are used in gas turbines, the air to
fuel ratio is high, such as 40 to 1 or higher, to dilute the
combustion products and thereby avoid temperatures higher than
the turbine blades can withstand. A substantial portion of the
energy produced by the turbine is used to compress the diluent
air for mixture with the hot gases before delivery into the
turbine. In contrast, the LHV gas contains inert gases that
will serve as a diluent to help avoid excessive temperatures in
the turbine, and it is necessary to compress only very little
dilution air above that required for burning the combustibles
in the LHV gas.
The temperature rise that occurs on burning a 50
BTU/scf ~HV gas with s-tolchiometric air is approximately
1425 F. Because it is necessary that the temperature of the
LHV~air mixture delivered to the ca~alyst be at least 40G F.,
and preferably in the range of 600 F. to 800 F., burning a
50 BTU gas in a single stage would cause catalyst -temperatures
exceeding 1600 F~ Thereforel a single-stage, self-supporting
LHV catalytic combustion process in which the combustibles in
a 50 BTU/scf LHV gas are consumed is possible only if the
--19--
33~;v ~
catalyst can withstand prolonged exposure to temperatures of
1825 F. and higher, and preferably higher than 2025 F. ~he
term self-supporting is used to designate a process in which
the hot combustio~ products from the catalytic combustion
chamber heat the LHV gas-air mixture to 400 F. or higher.
Single-stage catalytic combustion with a catalyst limited to
a maximum temperature of 1600 F. is limited to burning
combustibles to liberats about 33 BTU/scf of the L~V gas.
Such an operation would suffer a disadvantage, as contrasted
to ~he pre~erred two-stage operation, of lower turbine efficiency
resulting from the lower temperature of the gases delivered to
the turbine if the hot gases from the combustion chamber are
used to heat the L~V gas-air mixture to 400 F. or higher.
In a single stage operation uti~izing a catalyst limited tv a
maximum temperature of 1600 F., the amount o air mi~ed with
the LHV gas would be limited to the stoichiometric equivalent
of 33 BTU/scf L~ gas to protect the catalyst from excessive
temperatures resulting from periodic increases in he heating
value of the LHV gas composition such a~ occur in in-situ
combustion processes. The concept of protection against varia
tions in fuel composition by limiting the oxygen supplied to
the combustion chamber can, therefore, be used in a single-stage
operation. A two-stage catalytic combustion process is preferred
for L~IV gas having a heating value of 40-80 BTU/scf, however
to recover a larger part of the energy available in the LHV gas
and to operate the tuxbine more efficiently.
If two-stage catalytic combustion of the type described
with respect to Figure 1 is used to burn gases consistently
having heating values higher than about 80 BTU/scf, it will be
necessary to discard a substantial amount of the heating value
of the gas in the form of unburned hydrocarbons leaving the
secondary combustion chamber.
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~ ~ ~ 3 3~D
If the normal heating value of the gas from the
in-situ combustion process should be consist~ntly above
approximately 70 BTU/scf, for example ranging upwardly from
70 to 80 BTU/scf, and the maximum permissible catalyst and
turbine temperatures are appxoximately 1600 F., it is desirable
to use a third catalytic combustion chamber to reduce the loss
of energy in the unburned hydrocarbons. It will then be
necessary to cool the gases discharged from the secondary
catalytic combustion chamber before delivering the gases to the
third combustion chamber. Such cooling can be obtained by
passing gases discharged from the second combustion chamber,
preferably after mixing with air for the third combustion stage,
through a boiler in heat exchange with water to generate steam.
Steam generated in the boiler can be used as process steam or to
drive a turbine. A preferred method for recovering additional
energy from LHV gas having a heating value normally exeeding
about 70 BTU is to poSitiQn a tertiary catalytic combustion
~hamber 66 downstream of the gas turbine 52. The gases dis-
~ charged from the turbine are delivered to the third stage
combustion chamber at a temperature in the range of 400 to 800 F.
Air is dellvered to the tertiary ombustion chamber 66 through a
line 67 from the compressor 54 and the mixture passed in contact
with an oxidation catalyst in the combustion chamber to oxidize
hydrocarbons xemaining in the gas discharged from the turbine.
Valves in lines 67 and 59 allow control of the amount of air
delivered to each of tertiary catalytic combustion chamber 66 and
compressor 58. The hot gases from tertiary combustion chamber 66
are delivered to a boiler 68 for the generation of steam. In the
event three combustion chambers are used, the delivery of air to
the fixst two combustion stages is controlled to operate the
21-
3~
turbine at optimum conditions. The third combustion stage
could operate at stoichiometric conditions or with an excess
of air if the heating value of the gas i8 such that excessiv~
catalyst temperaturess are not developed ln the third sta~e.
The gas turbine 52 operates with le~s than sto:ichiom~tric air
at all times.
The embodiment o~ the invention illustrated in
Figure 2 is or use in those situations where the LHV gas
discharged from the in-situ process is not at a high enough
pressure for efficient use in driving a gas turbine. The
low pressure LHV gas may, for example, be produced in an in-situ
com~ustion process for recovery of oil fxom a shallow under-
ground reservoir at a low pressure or in the in situ combustion
of oil shale. The LHV gas is delivered through a pipe line 70
to a suitable gas cleanup unit 7~ for xemoval of solid particles
or other components that may interfere with the subsequent
catalytic oxidation of the gas. The gas cleanllp unit 72 may be
in the form of a suitable separator or screening or washing
apparatus. LHV ga~ from the gas cleanup unit 72 is delivered to
a preheater 74 similar to preheater 34 in the embodiment illus-
trated in Figure 1, for preheating to a temperature of 400 F. to
800 F. Preheated LHV gas discharged from the preheater 74 is
mixed with combustion air supplied by a suitable blower or com-
pressor 76 and delivered into a primary catalytic combustion
chamber at a temperature o 400 F. to 800 F. Catalytic com-
bustion chamber 78 is preferably similar to catalytic COmbU~tiGII
chamber 38 illustrated in Figure 1.
The hot combustion products from the primary catalytic
combustion chamber 78 are deli~ered into preheater 74 for indirect
heat transfer with the LHV gas. The combustion products from
preheater 74 are mixed with additional combustion air from
-22-
blower 80, which results in a temperature in the range of 400 F.
to 800 F., and delivered into a secondary catalytic combustion
chamber 82. As in the embodiment illustrated in Figure l, air
for oxidation in the secondary chamber also can be mixed with
S the partially burned gases rom the primary combustion chamber
before those gases are delivered to preheater 74.
Instead of delivering the combustion products from
secondary catalytic combustion chamber 82 to a gas turbine,
as in ~he embodiment illustrated in Figure 1, the combustion
I0 products are delivered to a boilex 84 fo~ generating steam.
Because the combustion products from the secondary catalytic
combustion chamber 82 are not delivered to a turbine, they are
not cooled hefore delivery to the boiler 84. The high tem-
perature of the combu~tion products discharged rom the
secondary catalytic co~bustion chamber allows generation of
steam at a high pressure suitable for driving a steam turbine.
Steam from the boilsr 84 is delivered through line 86 to a
steam tyrbine 88 which lS preferably connected to a generator 90
to generate electricity~to drive air compressors for the in-situ
combustion process. The steam turbine may, alternati~ely, be
directly connected to compressors. Steam discharged from
turbine 88 Ls delivered to a condenser 92 and the condensed
steam passed through suitable water treatment 94 which includes
a pump for returning the condensed water to boiler 84. ~s an
alternative to the generation of steam as illustrated in
Figure 2, low pressure off gases rom in-situ combustion of oil
shale, for example, could be used in the system o E'igure l modi-
fied to utiliæe a portion of the energy generated by turbine 52
to compress the LHV gas to a pressure high enough to drive the
turbine.
3~i~3
The temperatures to which the LHV gas is preheated
and combustion products from the primary catalytic combustion
chamber are cooled in preheater 74 are designed to avoid
excessively high temperatures which could damage the catalyst
in combu~tion chambers 78 and 82. As in the embodiment
illustrated in Figure 1, control of the maximum temperatures
is obtained by control of the amount of air mixed with the
LHV gas before delivery into the catalytic combustion chambers
to prevent overheating of the catalytic combustion chambers even
though there should be a substantial i~crease in the concentration
of combustibles in the LHV gas delivered through pipe 70.
The amount of air supplied by blower 76 is preferably approxi-
mately one-half the stoichiometric equivalent o the quantity
of combustibles that will give tha maximum permissible tem-
perature use; and an equal amount of air is supplied by blower 80for the oxidation in the secondary combustion chamber 82.
The method and apparatus herein described are highly
advan~ageous or the recovery of power from LHV gas from in-situ
combustion process~s for the recovery of oi~ because of the
variations from time to time in the heating value of the gas.
While the method is most useful for the recovery of power from
the gas produced at elevaked pressures in in-situ combustion
processes for the recovery of oil from oil or tar sand reservoirs,
it is also useful;in reco~ery of power from low-pressure off-gases
from the in-situ combustion of oil shale in those instances when
such off-gases cannot be burned in a flame co~ustor because of
its low heating value. The limitation of ~he amount of air mixed
with the LHV gas tG the stoi~hiometric equivalent of the normal
LHV gas provides protection for both the catalyst and the turbine
against wide variations that occur in the composition o the LHV
gas. This invention is particularly valuable when methane is one
of the combustible constituents of the LHV gas.
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