U.S. patent application number 10/313876 was filed with the patent office on 2003-07-24 for oxy-fuel combustion process.
Invention is credited to Acharya, Divyanshu R., Clarke, Richard H., Fitch, Frank R., Lin, Jerry Y.S., MacLean, Donald L., Ramachandran, Ramakrishnan, Ramprasad, Narayanan, Tamhankar, Satish S., Zeng, Yongxian.
Application Number | 20030138747 10/313876 |
Document ID | / |
Family ID | 27502069 |
Filed Date | 2003-07-24 |
United States Patent
Application |
20030138747 |
Kind Code |
A1 |
Zeng, Yongxian ; et
al. |
July 24, 2003 |
Oxy-fuel combustion process
Abstract
Production of oxygen-enriched gas streams is disclosed herein.
Air streams contact an oxygen-selective mixed conductor
particularly a perovskite material whereby oxygen is retained or
adsorbed on the perovskite and can be employed in a variety of
processes such as in combusting a fuel gas, heat recovery and
boiler related operations.
Inventors: |
Zeng, Yongxian; (North
Plainfield, NJ) ; Acharya, Divyanshu R.;
(Bridgewater, NJ) ; Tamhankar, Satish S.; (Scotch
Plains, NJ) ; Ramprasad, Narayanan; (Hillsborough,
NJ) ; Ramachandran, Ramakrishnan; (Allendale, NJ)
; Fitch, Frank R.; (Bedminster, NJ) ; MacLean,
Donald L.; (Clinton, NJ) ; Lin, Jerry Y.S.;
(Cincinnati, OH) ; Clarke, Richard H.; (Abingdon,
GB) |
Correspondence
Address: |
Philip H. Von Neida
The BOC Group, Inc.
Intellectual Property Dept.
100 Mountain Ave.
Murray Hill
NJ
07974
US
|
Family ID: |
27502069 |
Appl. No.: |
10/313876 |
Filed: |
December 6, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60347268 |
Jan 10, 2002 |
|
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60346582 |
Jan 8, 2002 |
|
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60346597 |
Jan 8, 2002 |
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Current U.S.
Class: |
431/2 |
Current CPC
Class: |
B01D 2253/10 20130101;
Y02P 20/10 20151101; C01B 2210/0046 20130101; F23L 2900/07001
20130101; F23C 2202/30 20130101; Y02E 20/344 20130101; B01D 53/04
20130101; B01D 2257/102 20130101; B01D 2256/12 20130101; Y02E 20/34
20130101; Y02E 20/32 20130101; Y02P 20/124 20151101; F23C 9/00
20130101; F23L 7/007 20130101; B01D 2259/40001 20130101; C01B
13/0259 20130101; B01D 53/047 20130101; Y02E 20/322 20130101 |
Class at
Publication: |
431/2 |
International
Class: |
F23B 001/00 |
Claims
Having thus described the invention, what we claim is:
1. A method of combusting a fuel gas in a combustion zone
comprising the steps of: (a) feeding into said combustion zone said
fuel gas; (b) feeding into said combustion zone an oxygen-enriched
gas from an oxygen retention system; (c) combusting said fuel gas;
and (d) recovering and recycling the combustion exhaust gas from
said combustion zone to said oxygen retention system.
2. The method as claimed in claim 1 wherein said oxygen retention
system contains a ceramic adsorbent.
3. The method as claimed in claim 2 wherein said ceramic adsorbent
is an oxygen-selective mixed conductor.
4. The method as claimed in claim 3 wherein said oxygen-selective
mixed conductor is a perovskite type ceramic having the structural
formula A.sub.1-xM.sub.xBO.sub.3-.delta..
5. The method as claimed in claim 4 wherein A is a rare earth ion,
M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe or
mixtures of these; x varies from greater than 0 to about 1; and
.delta. is the deviation from stoichiometric composition resulting
from the substitution of Sr, Ca and Ba for rare earth ions.
6. The process as claimed in claim 5 wherein x varies from about
0.1 to about 1.
7. The process as claimed in claim 4 wherein A is La, Y or mixtures
of these; M is Sr, Ca or mixtures of these; and B is Co, Fe or
mixtures of these.
8. The process as claimed in claim 4 wherein x is about 0.2 to
1.
9. The method as claimed in claim 1 wherein said oxygen-enriched
gas is delivered at temperatures greater than 150.degree. C.
10. The method as claimed in claim 1 wherein said oxygen-enriched
gas is produced at pressures of about 1 to about 20 bar.
11. The method as claimed in claim 1 wherein said oxygen retention
system produces oxygen-enriched gas through a two-step process of
retention and purge.
12. The method as claimed in claim 11 wherein oxygen is adsorbed
from an oxygen-containing feed gas stream.
13. The method as claimed in claim 12 wherein nitrogen is removed
from said retention system
14. The method as claimed in claim 1 wherein high purity nitrogen
is produced as a by-product during the oxygen retention step.
15. The method as claimed in claim 1 wherein said oxygen retention
system comprises two or more adsorbent beds.
16. A method for producing oxygen-enriched gas for use in a
combustion zone comprising the steps: (a) feeding air to a
retention system; (b) retaining oxygen from said air onto an
oxygen-selective mixed conductor; (c) removing nitrogen from said
retention system; (d) feeding oxygen-enriched gas to said
combustion zone; (e) combusting a fuel gas in the presence of said
oxygen-enriched gas; and (f) feeding the exhaust gas from said
combustion zone to said retention system.
17. The method as claimed in claim 16 wherein said method is
cyclical.
18. The method as claimed in claim 16 wherein a portion of said
exhaust gas from step (f) is withdrawn.
19. The method as claimed in claim 18 wherein CO.sub.2 is recovered
from said exhaust gas.
20. The method as claimed in claim 16 wherein said oxygen retention
system contains a ceramic adsorbent.
21. The method as claimed in claim 20 wherein said ceramic
adsorbent is an oxygen-selective mixed conductor.
22. The method as claimed in claim 21 wherein said oxygen-selective
mixed conductor is a perovskite type ceramic having the structural
formula A.sub.1-xM.sub.xBO.sub.3-.delta..
23. The method as claimed in claim 22 wherein A is a rare earth
ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe
or mixtures of these; x varies from greater than 0 to about 1; and
.delta. is the deviation from stoichiometric composition resulting
from the substitution of Sr, Ca and Ba for rare earth ions.
24. The process as claimed in claim 23 wherein x varies from about
0.1 to about 1.
25. The process as claimed in claim 24 wherein A is La, Y or
mixtures of these; M is Sr, Ca or mixtures of these; and B is Co,
Fe or mixtures of these.
26. The process as claimed in claim 25 wherein x is about 0.2 to
1.
27. The method as claimed in claim 16 wherein said oxygen-enriched
gas is produced at temperatures greater than 300.degree. C.
28. The method as claimed in claim 16 wherein said oxygen-enriched
gas is produced at pressures of about 1 to about 20 bar.
29. The method as claimed in claim 16 wherein said oxygen retention
system produces oxygen-enriched gas through a two-step process of
retention and purge.
30. The method as claimed in claim 29 wherein oxygen is adsorbed
from an oxygen-containing feed gas stream.
31. The method as claimed in claim 30 wherein nitrogen is purged
from said retention system.
32. The method as claimed in claim 31 wherein said oxygen retention
system comprises two or more adsorbent beds.
33. A method for combusting a gas stream and recovering heat from
said combustion comprising the steps: (a) passing an air gas stream
into a retention system containing an oxygen-conducting ceramic;
(b) retaining oxygen from said air gas stream onto said
oxygen-conducting ceramic; (c) passing a combustible gas over said
oxygen-conducting ceramic whereby said combustible gas combusts in
the presence of the retained oxygen producing carbon dioxide,
H.sub.2O and heat; and (d) recovering said carbon dioxide, H.sub.2O
and heat in the form of super-heated steam.
34. The method as claimed in claim 33 wherein said retention system
is a circulating fluidized bed reactor.
35. The method as claimed in claim 33 wherein a fuel stream is
passed over said oxygen-conductive ceramic in step (c).
36. The method as claimed in claim 33 wherein said fuel stream
comprises CH.sub.4, H.sub.2, CO, C.sub.2H.sub.4, C.sub.2H.sub.6 and
mixtures thereof.
37. The method as claimed in claim 33 wherein said ceramic
adsorbent is an oxygen-selective mixed conductor.
38. The method as claimed in claim 34 wherein said oxygen-selective
mixed conductor is a perovskite type ceramic having the structural
formula A.sub.1-xM.sub.xBO.sub.3-.delta..
39. The method as claimed in claim 38 wherein A is a rare earth
ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe
or mixtures of these; x varies from greater than 0 to about 1; and
.delta. is the deviation from stoichiometric composition resulting
from the substitution of Sr, Ca and Ba for rare earth ions.
40. The process as claimed in claim 39 wherein x varies from about
0.1 to about 1.
41. The process as claimed in claim 40 wherein A is La, Y or
mixtures of these; M is Sr, Ca or mixtures of these; and B is Co,
Fe or mixtures of these.
42. The process as claimed in claim 40 wherein x is about 0.2 to
1.
43. A method of operating a boiler to generate heat comprising the
steps: (a) passing air over an oxygen-conducting perovskite in a
reactor system and retaining oxygen on said oxygen-conducting
perovskite; (b) passing the effluent gas from said boiler to said
oxygen-conducting perovskite; and (c) feeding a gas stream
containing oxygen to said boiler with a fuel gas wherein said gas
stream combusts in said boiler to fuel said boiler.
44. The process as claimed in claim 43 wherein said process is
cyclic.
45. The method as claimed in claim 43 wherein said ceramic
adsorbent is an oxygen-selective mixed conductor.
46. The method as claimed in claim 44 wherein said oxygen-selective
mixed conductor is a perovskite type ceramic having the structural
formula A.sub.1-xM.sub.xBO.sub.3-.delta..
47. The method as claimed in claim 46 wherein A is a rare earth
ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe
or mixtures of these; x varies from greater than 0 to about 1; and
.delta. is the deviation from stoichiometric composition resulting
from the substitution of Sr, Ca and Ba for rare earth ions.
48. The process as claimed in claim 47 wherein x varies from about
0.1 to about 1.
49. The process as claimed in claim 48 wherein A is La, Y or
mixtures of these; M is Sr, Ca or mixtures of these; and B is Co,
Fe or mixtures of these.
50. The process as claimed in claim 48 wherein x is about 0.2 to
1.51. A method of converting a feed gas to a product gas in a
cyclical process comprising the steps: (a) introducing said feed
gas containing an oxidant into a first reactor, wherein said first
reactor contains a catalyst contained between inert materials
having heat transfer properties disposed at each end of said first
reactor, and said first reactor having an opening at both ends
wherein at least one heat exchanger with channels is connected to
said openings of said first reactor; wherein said first feed gas is
preheated by heat transfer with said product gas in said heat
exchanger prior to introducing said first feed gas into said first
reactor; (b) withdrawing a first product gas from said first
reactor; (c) introducing a second flow of said feed gas into a
second reactor, wherein said second reactor contains a catalyst
contained between inert materials disposed at each end of said
second reactor and said second reactor having an opening at both
ends wherein at least one heat exchanger with channels is connected
to said openings of said second reactor; wherein said second feed
gas is preheated by heat transfer with said product gas in said
heat exchanger prior to introducing said second feed gas into said
second reactor; (d) withdrawing a second product gas from said
second reactor; (e) diverting said first feed gas flow into said
second reactor thereby forming said first product gas and diverting
said second feed gas flow into said first reactor thereby forming
said second product gas.
52. The method as claimed in claim 51 wherein said feed gas is a
reducing gas.
53. The method as claimed in clam 52 wherein said reducing gas is
natural gas.
54. The method as claimed in claim 51 wherein said first product
gas and said second product gas are the same gas.
55. The method as claimed in claim 51 wherein said product gas is a
mixture of carbon monoxide and hydrogen.
56. The method as claimed in claim 51 wherein said catalyst is a
perovskite type mixed conductor.
Description
[0001] This application claims priority from Provisional U.S.
Patent Applications 60/346,582 filed Jan. 8, 2002; 60/346,597 filed
Jan. 8, 2002; and 60/347,268 filed Jan. 10, 2002.
BACKGROUND OF THE INVENTION
[0002] The primary purpose of combustion processes is to generate
heat. In a power plant or in an industrial boiler system, the heat
is utilized to generate high pressure steam which in turn may be
used to provide process heating or may be used to produce
electricity. Most conventional combustion processes utilize air as
a source of oxygen. The presence of nitrogen in air does not
benefit the combustion process and may even create problems. For
example, nitrogen will react with oxygen at combustion temperatures
forming nitrogen oxides (NOx), an undesirable pollutant. In many
cases, the products of combustion must be treated to reduce
nitrogen oxide emissions below environmentally acceptable limits.
Moreover, the presence of nitrogen increases the flue gas volume
which in turn increases the heat losses and decreases the thermal
efficiency of the combustion process. Additionally, high nitrogen
content in the flue gas may make it unattractive to capture
CO.sub.2 either as a product or for sequestration. With the current
emphasis on CO.sub.2 sequestration to alleviate harmful effects of
global warming, it is critical to develop processes which will
enable CO.sub.2 capture in a cost effective way.
[0003] One way to eliminate nitrogen from the combustion exhaust or
flue gas is to use pure oxygen in the combustion process instead of
air. However, combustion with oxygen generates very high
temperatures and therefore some of the flue gas produced must be
recycled to moderate temperatures. This in turn dilutes the oxygen
content to about 27% (remaining .about.73% is CO.sub.2 and water)
and maintains the flame temperature to the same value. While such a
scheme would eliminate the problems associated with nitrogen, the
cost of oxygen at present is too high to make it economically
attractive.
[0004] Production of oxygen-enriched gas stream using ion transport
ceramic membrane is discussed in U.S. Pat. No. 5,888,272 which
discloses a process for separating a feed gas stream into an
oxygen-enriched gas stream which is used in a combustor and an
oxygen-depleted gas stream. The feed gas stream is compressed, and
oxygen is separated from the compressed feed gas stream using an
ion transport module including an ion transport membrane having a
retentate side and a permeate side. The permeate side of the ion
transport membrane is purged with at least a portion of a
combustion product gas stream obtained from the combustion in the
combustor of the gas stream exiting the permeate side of the ion
transport module. The disadvantages of this method of oxygen
production are the high cost of fabrication of the membrane and the
difficulty in producing membrane structures that are leak-proof.
Also, oxygen recovery is typically low in membrane units.
[0005] The present invention is based on the use of
high-temperature, oxygen-selective ceramic materials made in
particulate form to produce a substantially nitrogen-free oxygen
stream suitable for oxy-fuel application, and may provide an
attractive option to reduce oxygen cost. Such systems utilize
either pressure swing or temperature swing mode since the oxygen
retention capacity of the ceramic material is strongly dependent on
temperature and pressure. The process normally operates at
temperatures greater than 300.degree. C. and offers several
advantages, including high oxygen capacity and large oxygen
selectivity. A key advantage of this process is that it uses the
oxygen-selective material in conventional pellet form in fixed bed
reactors, which can be designed using traditional methods. Thus,
the process can be commercially adopted more easily compared to the
membrane based process mentioned above, which requires special
fabrication, sealing and assembly procedures, and is known to have
several issues in this regard. An additional advantage of the fixed
bed, ceramic-based system is that it can directly produce an oxygen
containing stream, substantially free of nitrogen, with the oxygen
concentration suitable for oxy-fuel application. This is unlike
conventional processes, such as cryogenic air separation method,
which first produce high purity oxygen, and require subsequent
dilution to get the required oxygen concentration.
[0006] The present invention is aimed at reducing the cost of
oxygen by producing substantially nitrogen-free oxygen containing
stream suitable for combustion processes. It relates to the use of
a high-temperature, oxygen generation system to produce an
oxygen-containing stream, substantially free of nitrogen. More
particularly, it describes the use of an oxygen-selective ceramic
material to separate oxygen from an air stream to produce an oxygen
containing stream which can be employed in an industrial boiler or
fired heater or in other combustion based processes as an oxygen
source instead of air.
SUMMARY OF THE INVENTION
[0007] The present invention provides for a method for producing an
oxygen stream for use in an industrial boiler or fired heater. A
process is described wherein a part of the flue gas from the
boiler, primarily containing water vapor and CO.sub.2, is used to
sweep a reactor containing oxygen-saturated high temperature
oxygen-selective ceramic material (e.g., perovskite) to produce an
oxygen-containing stream. The oxygen-containing stream is fed to
the boiler along with a fuel, which is burned in the boiler to
generate heat. The oxygen-depleted ceramic material is saturated
with oxygen by exposing it to air in a cyclic fashion. Therefore,
the process for operating the ceramic system consists of at least
two steps in each cycle of the cyclic operation. In the first step,
an air stream is introduced into the reactor containing the high
temperature oxygen-selective ceramic material, which selectively
retains oxygen. In the second step, a portion of the flue gas from
the boiler is fed into the reactor to purge out at least a part of
the oxygen from the ceramic material, so that the material becomes
oxygen-depleted. The oxygen retention step is exothermic while the
oxygen removal step is endothermic. The overall process is
thermo-neutral, in principle; however, some heat loss will occur,
which needs to be compensated.
[0008] In one embodiment of the process, the boiler is operated
under slightly under-oxidized conditions, so that the flue gas
contains no oxygen, but contains a small amount of CO and H.sub.2.
The CO+H.sub.2 is burned in the reactor with a portion of the
oxygen retained in the ceramic material to generate heat required
to sustain the cyclic operation of the reactor.
[0009] In another embodiment of the process, the boiler is operated
under conditions such that the fuel is completely burned, and a
small amount of excess 02 is present in the flue gas (typically
.about.0.5-5.0 vol. %). In this case, the recycled flue gas is fed
to the reactor along with the addition of a small amount of
suitable fuel (CO, H.sub.2, CH.sub.4, etc. or a combination
thereof), in the amount at least sufficient to react with the
excess oxygen present in the flue gas. The combustion catalyst may
be combined with the oxygen-selective ceramic material in the same
reactor, as a layer at the entrance. Also, a layer of perovskite
can act as a combustion catalyst. This combustion generates heat
necessary for the cyclic process. The amount of fuel gas added is
adjusted so as to generate sufficient heat. Any excess fuel added
reacts with the oxygen stored in the ceramic material. If higher
temperature results due to the combustion, it helps extract more
oxygen from the ceramic material since the amount of oxygen
retained in the ceramic material generally decreases with
increasing temperature.
[0010] Optionally the flue gas can be passed through an additional
reactor to which a controlled amount of fuel gas is added. The
reactor may contain a catalyst, such as a supported noble metal
catalyst. The oxygen is consumed in this reactor by reaction with
the added fuel. As described above, a portion of the resulting gas,
after heat recovery, is then fed to the reactor for generating the
oxygen-containing gas stream.
[0011] If high temperature valves are used, the hot flue gas from
the boiler can be fed directly into the reactor. When low
temperature valves are used, the hot flue gas from the boiler is
first passed through a heat exchanger to recover the heat and to
generate steam as a useful product, before it is fed to the
reactor. The portion of the flue gas, which is not recycled may be
used to capture CO2 from it after separating water and other
impurities.
[0012] In another embodiment of the process, the oxygen-containing
gas leaving the reactor is cooled to separate the water in the
stream by condensation, thereby increasing the concentration of
oxygen in the stream returning to the boiler. The increased oxygen
concentration may provide more flexibility in the operation of the
boiler.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic representation of a boiler and the
ceramic oxygen generation system as practiced by the present
invention.
[0014] FIG. 2 is a schematic representation of the ceramic oxygen
generation system for oxyfuel application as practiced in the
present invention.
[0015] FIG. 3 is a schematic representation of a ceramic oxygen
generation system with steam purge as practiced in the present
invention.
[0016] FIG. 4 is a schematic representation of ceramic oxygen
generation reactor showing the layer arrangement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] FIG. 1 is a schematic embodiment of a boiler or fired heater
and an oxygen generation ceramic system B. Contained therein in B
is the oxygen-selective ceramic material. Line 10 carries fuel gas
to the boiler A. The fuel can be selected from the group consisting
of CH.sub.4, H.sub.2, CO, C.sub.2H.sub.4, C.sub.2H.sub.6 and
mixtures thereof or can be coal, char or other solids as well as
various refinery waste streams, fuel oils, etc. or any suitable
combustible material. The combustion exhaust gas or flue gas, which
consists primarily of carbon dioxide and water vapor, exits
combustion/heat recovery zone A through line 12. A part of the
combustion exhaust gas is directed through line 14 to the oxygen
generation system B. Compressed air enters the oxygen generation
system through line 20. Oxygen lean stream containing mainly
nitrogen, up to 98%, exits the oxygen generation system through
line 22. Oxygen from the air is retained onto the oxygen-selective
ceramic material. The combustion exhaust gas enters the system B,
removes this oxygen and regenerates the ceramic material. The gas
leaves through line 18 as substantially nitrogen-free oxygen rich
gas and enters the boiler A whereby combustion can occur anew.
[0018] The ceramic system primarily comprises at least 2 reactors
filled with high temperature oxygen-selective ceramic material,
such as perovskite material, and an inert ceramic material for
internal heat exchange, optional multi-pass heat exchangers and
switchover valves. The process is cyclic and may be compared to a
pressure swing retention process. Briefly, air is passed into first
bed where oxygen is preferentially retained onto the material and
oxygen lean stream is withdrawn from the top of the bed. Once the
material becomes at least in part saturated with oxygen, the
operation is transferred to another vessel. The first bed is now
purged with the combustion exhaust gas or recycled flue gas, which
removes at least part of the oxygen and as a result also
regenerates the material. Minimum two reactors are required to
ensure continuous operation.
[0019] Turning now to FIG. 2, air is compressed, and, after passing
through multi-pass heat exchanger, will pass through one of the
beds, which contains high temperature oxygen-selective ceramic
material, such as perovskite material. Oxygen will be retained on
the perovskite and nitrogen will leave the bed as effluent. This
effluent gas stream will then pass up again through one of the
multi-pass heat exchangers and will leave the cyclic system. While
one bed is undergoing the air step, the second perovskite bed which
is already partially saturated with oxygen is purged with the
recycled flue gas stream. Like air, the recycled flue gas also
passes through a multi-pass heat exchanger before passing through
the perovskite bed. As the recycled flue gas passes through the
bed, it picks up the oxygen stored on the perovskite and also
regenerates the perovskite. The oxygen rich gas then leaves the bed
through the multi-pass exchanger, exchanging heat with the incoming
recycled flue gas.
[0020] FIG. 2 is described here with bed B on air step and bed A on
recycled flue gas or regeneration step. Air is first compressed to
the desired pressure using air blower E. The compressed air is fed
to the multi-pass heat exchanger G through valve V5. Valve V6 is
closed during this step. Air is heated in exchanger G by exchanging
heat with the returning oxygen-lean stream 16. The heated air, 14,
is fed to the perovskite bed B. The oxygen-lean stream, 15, exits
bed B, exchanges heat with incoming air in exchanger G and then
leaves the system through valve V8 as stream 20.
[0021] Recycled flue gas from the boiler system is first cooled in
cooler C and then compressed in blower D prior to feeding it into
multi-pass heat exchanger F through valve V1. Once heated, it
passes through bed A, which is saturated with oxygen. The oxygen
rich stream, 35, leaves the bed from the bottom, passes through the
exchanger F and into the buffer tank H through valve V3.
[0022] A typical valve sequence is given in the table below:
1 Duration Bed A Bed B Valves Step Sec Feed Feed V1 V2 V3 V4 V5 V6
V7 V8 1 30 Air Flue open close open Close open Close open Close Gas
2 30 Flue Air close open close Open close Open close Open Gas
[0023] The present invention can be integrated with a boiler or
fired heater in several ways with an objective of improving the
efficiency. In one embodiment of this process, the boiler is
operated under slightly under-oxidized conditions so that the flue
gas contains no oxygen but contains a small amount of carbon
monoxide and hydrogen. The carbon monoxide and hydrogen are burned
in the perovskite reactor to generate heat required to sustain and
improve the cyclical operation of the perovskite reactor.
[0024] Alternatively, the boiler is operated under conditions such
that the fuel is completely burned and a small amount of excess
oxygen is present in the flue gas, typically about 0.5 volume %. In
this case, the recycle flue gas is fed to the perovskite reactor
along with the addition of a small amount of a suitable fuel such
as carbon monoxide, hydrogen, methane or a combination thereof in
an amount at least sufficient to react with the excess oxygen
present in the flue gas (stream 50 in FIG. 2). This combustion
generates heat necessary for the cyclical process. The amount of
fuel gas added is adjusted so as to generate sufficient heat. Any
excess fuel added reacts with the oxygen stored on the perovskite.
If higher temperature results due to the combustion, it helps
extract more oxygen from the perovskite.
[0025] Alternatively yet, the boiler is operated under conditions
of excess oxygen to assure complete combustion of all the fuel. In
this case, the flue gas can contain up to 5% by volume oxygen. This
flue gas is passed through an optional reactor to which a
controlled amount of fuel gas as described above is added. The
reactor may contain a catalyst such as a supported noble metal
catalyst. The oxygen is consumed in this reactor by a reaction with
the added fuel gas. As described above, a portion of the resulting
gas after heat recovery is then fed to the perovskite reactor for
generating the oxygen-containing gas stream. The combustion
catalyst can be separate or may be combined with the perovskite in
the same reactor, as a layer at the entrance to the reactor. Also,
a layer of perovskite can act as a combustion catalyst.
[0026] Alternatively, the oxygen-containing gas leaving the
perovskite reactor is cooled to separate the water in the stream as
condensate thereby increasing the concentration of oxygen in the
stream returning to the boiler. The increased oxygen concentration
may be beneficial to the boiler operation and may provide more
flexibility to the operation of the boiler. An extension of this
scheme is to use steam only as a regeneration gas as shown in FIG.
3. The main advantage of this scheme is that oxygen can be produced
in any concentration by cooling the oxygen-rich stream and
condensing the steam out. Since the process still operates at low
pressure, only low-pressure steam is necessary. The availability of
low-pressure steam is usually not a problem as schemes presented
here are integrated as part of an overall boiler or power
plant.
[0027] In one embodiment, water is removed from the recycled flue
gas before it enters the ceramic oxygen generation system so that
it consists of mainly CO.sub.2. It has been discovered that when
the purge gas in the oxygen extraction step is CO.sub.2, the amount
of oxygen recovered from the ceramic bed is higher compared to
other gases such as N.sub.2 or steam. This is believed to be due to
exothermic retention of CO.sub.2 on the ceramic material leading to
greater oxygen release.
[0028] The schemes presented in FIGS. 2 and 3 are based on partial
pressure swing process i.e. the driving force for extraction of
stored oxygen is provided by the difference in partial pressure of
oxygen between the oxygen retention and extraction steps. The
pressure to which the air is compressed is mainly determined by the
required concentration of oxygen in the oxygen-rich stream.
According to the invention, air is fed at a pressure of 15-400
psia, preferably 15-100 psia, and more preferably 20-40 psia, and
the recycled flue gas at 0.1-200 psia, preferably 8-50 psia, and
more preferably 10-30 psia, so that the pressure difference between
the two streams at the entrance to the reactor is maintained
between 5 and 20 psi.
[0029] The schemes presented here relate to the concepts employed
in ensuring efficient heat management. For example, one aspect of
the invention provides for the use of inert materials for
regenerative heat transfer in cyclic catalytic processes. The
reactor configuration with inert materials is shown in FIG. 4. In
particular, such regenerative heat transfer is used in conjunction
with at least one external heat exchanger to achieve the desired
heat transfer for the overall process. Through heat exchange with
these inert materials, temperatures of hot gas streams exiting a
reactor can be significantly reduced, e.g., to below about
900.degree. C., and preferably as low as about 500.degree. C. Such
a reduced gas stream temperature allows use of low-cost
construction materials, and results in corresponding cost
reduction, as well as an increased operating life of the external
heat exchanger required for additional heat transfer.
[0030] While such a heat transfer scheme is generally applicable to
any cyclic process, it is particularly well-suited for processes
with relatively high operating temperatures, e.g., about
250.degree. C. or higher, where the unavailability of switchover
valves for high temperature operation necessitates that all hot gas
streams be effectively cooled so that standard valves can be
employed. Furthermore, it is also well-suited to cyclic processes
with relatively short cycle times, such as those in which the
heating and cooling times are below about a minute, e.g., between
about 15 to about 60 seconds.
[0031] According to embodiments of the invention, multi-pass
compact heat exchangers are used to carry out supplemental heat
transfer from hot gas streams. These include two external heat
exchangers, which operate on cyclic duty in synchronization with
the cyclic operation of the reactors. The heat exchange is further
complemented with the internal regenerative heat exchange using
inert layers of ceramic material. The external heat exchangers
allow heat exchange between the inlet and outlet of the same
streams, for example air and waste nitrogen stream or recycled flue
gas and oxygen-rich streams. On the other hand, internal
regenerative heat exchange allows heat exchange between two
different streams, for example air and oxygen rich stream and waste
nitrogen and recycled flue gas. This heat exchange philosophy also
allows the use of low temperature switchover valves and enhances
the reliability of the cyclic process.
[0032] The multi-pass exchangers, which are a part of the compact
heat exchanger family, offer significant thermal advantages over
conventional shell and tube exchangers. They are available
commercially and may be employed for pressures as high as 2000 bar
and temperatures as high as 800.degree. C. A detailed review of
compact heat exchangers can be found in an article by V. V.
Wadekar, in CEP, December 2000, which is herein incorporated by
reference. For high temperature applications, these heat exchangers
are typically fabricated from stainless steel or other alloys.
[0033] While multi-pass exchangers are integral part of the schemes
presented here, it may also be possible to adjust process
parameters to complete all heat exchange using inert materials
placed inside the reactor. This will eliminate the need for
external heat exchange. On the other hand, it is also possible to
carry out all heat exchange in heat exchangers thereby eliminating
the need for inert layers within the reactor vessels.
[0034] One characteristic of cyclic processes is the possibility of
contamination of the desired product stream with impurities as a
result of vessel voids. For the present case, this means that the
oxygen rich stream may get contaminated with nitrogen present in
the voids at the end of the oxygen retention step. In order to
avoid this, an additional step may be introduced. In this step the
reactor will be rinsed with steam after the oxygen retention step.
This will remove any nitrogen that may be present in the voids. The
reactor now can be purged with the combustion exhaust gas or flue
gas.
[0035] The oxygen-selective ceramic materials are typically
oxygen-selective mixed conductors, which exhibit both high
electronic and oxygen ionic conductivities at elevated temperature.
Examples of these mixed conductors are perovskite-type oxides,
CeO.sub.2-based oxides, Bi.sub.2O.sub.3-based oxides,
ZrO.sub.2-based oxides, and brownmillerite oxides. In order to
further enhance its electronic conductivity and catalytic activity
for oxygen ionization, some metal phase can be added into the
ceramic material to form a ceramic-metal composite. The metals can
be selected from Cu, Ni, Fe, Pt, Pd, Rh and Ag.
[0036] In general, the oxygen-selective ceramic materials retain
oxygen through conduction of oxygen ions and filling up the oxygen
vacancies in its bulk phase. The oxygen retention capacity usually
increases with increasing oxygen partial pressure and decreasing
temperature. Therefore, the retention and release of oxygen into
and from the ceramic material during retention and release steps
perform efficiently in that the oxygen partial pressure during the
retention step is much higher than that in the release step.
[0037] In a preferred embodiment, the at least one oxygen-selective
ceramic material comprises an oxygen-selective mixed ionic and
electronic conductor. In a more preferred embodiment, the
oxygen-selective ceramic material comprises a perovskite-type
ceramic having the structural formula
A.sub.1-xM.sub.xBO.sub.3-.delta., where A is an ion of a metal of
Groups 3a and 3b of the periodic table of elements or mixtures
thereof; M is an ion of a metal of Groups 1a and 2a of the periodic
table or mixtures thereof; B is an ion of a d-block transition
metal of the periodic table or mixtures thereof; x varies from
>0 to 1; and .delta. is the deviation from stoichiometric
composition resulting from the substitution of ions of metals of M
for ions of metals of A.
[0038] In a more preferred embodiment, the at least one
oxygen-selective ceramic material is a perovskite-type ceramic and
x varies from about 0.1 to 1.
[0039] In another more preferred embodiment, the at least one
oxygen-selective ceramic material is a perovskite-type ceramic and
A is one or more f-block lanthanides. In a more preferred
embodiment, A is La, Y, Sm or mixtures thereof.
[0040] In another more preferred embodiment, the at least one
oxygen-selective ceramic material is a perovskite-type ceramic and
M is at least one metal of Group 2a of the periodic table of
elements. In a more preferred embodiment M is Sr, Ca, Ba or
mixtures thereof.
[0041] In another more preferred embodiment, the at least one
oxygen-selective ceramic material is a perovskite-type ceramic and
B is Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn or mixtures thereof. In a
more preferred embodiment, B is V, Fe, Ni, Cu or mixtures
thereof.
[0042] In another more preferred embodiment, the at least one
oxygen-selective ceramic material is a perovskite-type ceramic and
x is about 0.2 to 1.
[0043] In another more preferred embodiment, the at least one
oxygen-selective ceramic material is a perovskite-type ceramic and
A is La, Y, Sm or mixtures thereof, M is Sr, Ca or mixtures
thereof, and B is V, Fe, Ni, Cu or mixtures thereof.
[0044] In another embodiment, the at least one oxygen-selective
ceramic material conductor is a member selected from the group
consisting of (1) ceramic substances selected from the group
consisting of Bi.sub.2O.sub.3, ZrO.sub.2, CeO.sub.2, ThO.sub.2,
HfO.sub.2 and mixtures thereof, the ceramic substances being doped
with CaO, rare earth metal oxides or mixtures of these; (2)
brownmillerite oxides; and (3) mixtures of these.
[0045] In another embodiment, the at least one oxygen-selective
ceramic material conductor is at least one ceramic substance
selected from the group consisting of Bi.sub.2O.sub.3, ZrO.sub.2,
CeO.sub.2, ThO.sub.2, HfO.sub.2 and mixtures of these, and the at
least one ceramic substance is doped with a rare earth metal oxide
selected from the group consisting of Y.sub.2O.sub.3,
Nb.sub.2O.sub.3, Sm.sub.2O.sub.3, Gd.sub.2O.sub.3 and mixtures of
these.
EXAMPLES
Example 1
[0046] Preparation of
La.sub.0.2Sr.sub.0.8Co.sub.0.6Fe.sub.0.4O.sub.3-.del- ta.
Perovskite Powder
[0047] The powder of perovskite-type oxide was prepared first by
mixing of corresponding metal oxides or hydroxides and then
repeated steps of sintering, ball-milling and filtration for three
times. The temperatures in 3 sintering steps were, respectively,
900.degree. C., 950.degree. C. and 1000.degree. C., and the
sintering time was 8 hours. The first sintering was conducted right
after dry-mixing of La.sub.2O.sub.3, Sr(OH).sub.2.8H.sub.2O,
Ni.sub.2O.sub.3, Co.sub.2O.sub.3 and Fe.sub.2O.sub.3. The ball the
material was carried out with grinding media and water after each
sintering. The solid was collected by filtration after ball
milling. The filtration cake was dried at 100.degree. C. for
overnight before it was subjected to the next sintering. After the
last ball-milling, the dried filtration cake was crushed and ground
into fine powder. The final powder had a perovskite-type phase
structure.
Example 2
[0048] Fabrication of
La.sub.0.2Sr.sub.0.8Co.sub.0.6Fe.sub.0.4O.sub.3-.del- ta.
perovskite extrudates
[0049] The perovskite-type oxide powder made in Example 1 was
transformed into a slurry after addition of about 5 wt %
hydroxyethyl cellulose and 14.5 wt % water. Thus obtained slurry
was aged overnight before it was loaded into an extruder and
transformed into extrudates (3 mm in diameter and 4 mm in length).
The extrudates were dried in an oven at 90.degree. C. for about 2
hr, and then calcined at 600.degree. C. for 5 hr. The extrudates
were finally sintered at 1050.degree. C. for 8 h. The final
extrudates were porous and mechanically strong.
Example 3
[0050] The extrudates made in Example 2 were packed in a tubular
reactor made of high temperature metal alloy. The reactor was
designed in such a way that the gas streams of air, CO.sub.2 and
steam could be fed into the reactor from either the top end or the
bottom end of the reactor as required. Mass flow controllers
controlled the flow rates of the gas streams. The reactor
temperature and valves were controlled with PLC. The product and
waste streams during purge and retention steps were collected in a
tank, and their average compositions were analyzed with a gas
analyzer and a GC. In the experiment, the reactor temperature was
controlled at 825.degree. C. An air stream at 7.6 slpm and a
CO.sub.2 stream at 4.7 slpm were alternately fed into the reactor
for every 30 seconds in a counter-current fashion. The reactor
pressures were kept at 23.7 psia and 18.7 psia respectively during
air and CO.sub.2 steps. During the last 2 seconds of the air step,
the reactor pressure decreased from 23.7 psia to 18.7 psia. The
average product composition during CO.sub.2 step was: 27.8%
O.sub.2, 67.1% CO2 and 7.4% N2, while the waste stream generated
during air step contained 2.3% O2, 12.5% CO2 and 83.5% N2. This
demonstrates that an oxygen-rich stream containing primarily CO2
and O2 can be produced with the process disclosed in this
invention
Example 4
[0051] In this experiment, an air stream at 7.6 slpm and a stream
of CO.sub.2+steam mixture at 4.5 slpm were alternately fed into the
reactor described in Example 3 for every 30 seconds in a
counter-current fashion. The reactor pressures were kept at 23.7
psia and 18.7 psia respectively during air and CO.sub.2+steam
steps. The average product composition (on a dry basis) during
CO.sub.2+steam step was: 40.8% O.sub.2, 44.5% CO2 and 14.7% N2,
while the waste stream generated during air step contained 3.7%
O.sub.2, 11.4% CO2 and 84.9% N2. This result indicates that an
oxygen-rich stream can be produced with a mixture of CO2 and steam
as purge gas using the process disclosed in this invention.
Example 5
[0052] In this experiment, an air stream at 7.6 slpm and a stream
of steam at 6.2 slpm were alternately fed into the reactor
described in Example 3 for every 30 seconds in a counter-current
fashion. The reactor pressures were kept at 23.7 psia and 18.7 psia
respectively during air and steam steps. The average product
composition (on a dry basis) during steam step was: 70.4% O.sub.2,
29.6% N2, while the waste stream generated during air step
contained 0.3% O2 and 99.7% N2 (with trace amount other non-oxygen
gases). This result showed that an oxygen-rich stream can be
produced with stream as purge gas using the process disclosed in
this invention.
2TABLE 1 Summary of the results in Examples 3-5 Example Product
Waste Stream # Flow O.sub.2 % CO.sub.2 % H.sub.2O % N.sub.2 % Flow
O.sub.2 % CO.sub.2 % N.sub.2 % 3 Dry 5.36 27.8 67.1 0 7.4 7.02 2.3
12.5 83.4 wet 5.36 27.8 67.1 0 7.4 7.02 2.3 12.5 83.4 4 Dry 3.82
40.8 44.5 0 14.7 5.75 3.7 11.4 84.9 wet 6.06 25.7 28.1 37.0 9.3
5.75 3.7 11.4 84.9 5 Dry 3.37 70.3 0 0 29.6 6.99 0.3 0 99.7 wet
9.59 24.7 0 64.9 10.4 6.99 0.3 0 99.7
[0053] Table 1 summarizes the results in Examples 3-5 and compares
the product compositions on the wet basis, i.e. including the steam
in the product stream. As shown, O.sub.2 concentration in the
product on the wet basis increases with increasing CO.sub.2
concentration in the purge gas, indicating that CO.sub.2 has
stronger regeneration capability than steam. As noted in the
examples, there was some amount of nitrogen still presented in the
product stream due to the void space in the reactor. This nitrogen
can be easily eliminated from the void space by an additional step
between the air and the purge gas steps.
[0054] While this invention has been described with respect to
particular embodiments thereof, it is apparent that numerous other
forms and modifications of the invention will be obvious to those
skilled in the art. The appended claims in this invention generally
should be construed to cover all such obvious forms and
modifications which are within the true spirit and scope of the
present invention.
* * * * *