U.S. patent application number 16/078620 was filed with the patent office on 2019-02-28 for combustion system.
This patent application is currently assigned to Renaissance Energy Research Corporation. The applicant listed for this patent is RENAISSANCE ENERGY RESEARCH CORPORATION. Invention is credited to Nobuaki HANAI, Hideaki MATSUO, Osamu OKADA, Peng YAN.
Application Number | 20190060826 16/078620 |
Document ID | / |
Family ID | 59790311 |
Filed Date | 2019-02-28 |
United States Patent
Application |
20190060826 |
Kind Code |
A1 |
OKADA; Osamu ; et
al. |
February 28, 2019 |
COMBUSTION SYSTEM
Abstract
A biogas combustion system that obtains a stable output and
saves energy is realized. A combustion system comprises a
separation portion 14 that removes carbon dioxide from a treatment
target gas containing a mixture gas containing methane as a main
component and containing carbon dioxide to obtain methane gas of a
high purity in which at least a content of carbon dioxide has been
reduced, and a combustion portion 15 that combusts the methane gas.
The separation portion 14 includes a first treatment chamber 11 and
a second treatment chamber 12 separated from each other by a
separation membrane 13 therebetween. The separation membrane 13
selectively allows the carbon dioxide in the treatment target gas
supplied to the first treatment chamber 11 to pass therethrough to
the second treatment chamber 12 to obtain a first separation gas
having a higher methane purity than the treatment target gas in the
first treatment chamber 11 and a second separation gas containing
the carbon dioxide in the treatment target gas in the second
treatment chamber 12.
Inventors: |
OKADA; Osamu; (Kyoto,
JP) ; HANAI; Nobuaki; (Kyoto, JP) ; YAN;
Peng; (Kyoto, JP) ; MATSUO; Hideaki; (Kyoto,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RENAISSANCE ENERGY RESEARCH CORPORATION |
Kyoto-shi, Kyoto |
|
JP |
|
|
Assignee: |
Renaissance Energy Research
Corporation
Kyoto-shi, Kyoto
JP
|
Family ID: |
59790311 |
Appl. No.: |
16/078620 |
Filed: |
February 17, 2017 |
PCT Filed: |
February 17, 2017 |
PCT NO: |
PCT/JP2017/005881 |
371 Date: |
August 21, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 69/00 20130101;
F23K 5/007 20130101; Y02E 50/343 20130101; B01D 2257/504 20130101;
B01D 2257/80 20130101; Y02C 20/40 20200801; B01D 53/229 20130101;
B01D 2257/55 20130101; B01D 53/268 20130101; B01D 2258/05 20130101;
Y02E 50/30 20130101; B01D 2256/245 20130101; B01D 2325/20 20130101;
F23K 5/00 20130101; B01D 69/10 20130101; B01D 53/228 20130101; Y02C
10/10 20130101; B01D 53/22 20130101; B01D 69/06 20130101; B01D
2257/304 20130101; B01D 69/08 20130101 |
International
Class: |
B01D 53/22 20060101
B01D053/22; B01D 69/08 20060101 B01D069/08; F23K 5/00 20060101
F23K005/00; B01D 69/10 20060101 B01D069/10; B01D 53/26 20060101
B01D053/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2016 |
JP |
PCT/JP2016/057453 |
Claims
1. A combustion system comprising: a separation portion that
removes carbon dioxide from a treatment target gas containing a
mixture gas containing methane as a main component and containing
carbon dioxide to obtain methane gas of a high purity in which at
least a content of carbon dioxide has been reduced; and a
combustion portion that combusts the methane gas, wherein the
separation portion includes a first treatment chamber and a second
treatment chamber separated from each other by a separation
membrane therebetween, wherein the separation membrane selectively
allows the carbon dioxide in the treatment target gas supplied to
the first treatment chamber to pass therethrough to the second
treatment chamber to obtain a first separation gas having a higher
methane purity than the treatment target gas in the first treatment
chamber and a second separation gas containing the carbon dioxide
in the treatment target gas in the second treatment chamber, and
wherein the separation membrane is a facilitated transport membrane
to which a carrier that selectively reacts with carbon dioxide is
added.
2. (canceled)
3. The combustion system according to claim 1, comprising a water
vapor supply portion that supplies water vapor to the first
treatment chamber, wherein the mixture gas containing the water
vapor supplied by the water vapor supply portion is supplied to the
first treatment chamber as the treatment target gas.
4. The combustion system according to claim 3, wherein the water
vapor supply portion supplies, to the first treatment chamber,
water vapor generated by heating water by heat exchange with a
high-temperature exhaust gas generated by a combustion of methane
by the combustion portion.
5. The combustion system according to claim 3, wherein the water
vapor supply portion supplies water vapor contained in an exhaust
gas generated by the combustion of methane by the combustion
portion to the first treatment chamber.
6. The combustion system according to claim 3, further comprising
an exhaust gas supply portion that mixes the mixture gas with at
least part of an exhaust gas containing carbon dioxide and water
vapor generated by the combustion of methane by the combustion
portion and supplies the mixture gas mixed with the at least part
of the exhaust gas to the first treatment chamber as the treatment
target gas.
7. The combustion system according to claim 3, further comprising a
water vapor removing portion that removes water vapor from the
first separation gas and supplies the first separation gas from
which water vapor has been removed to the combustion portion.
8. The combustion system according to claim 7, wherein the water
vapor supply portion supplies the water vapor removed by the water
vapor removing portion to the first treatment chamber.
9. The combustion system according to claim 1, further comprising a
sweep gas supply portion that supplies a sweep gas to the second
treatment chamber.
10. The combustion system according to claim 9, wherein the sweep
gas contains water vapor, and the water vapor supply portion
supplies the water vapor contained in the sweep gas to the sweep
gas supply portion.
11. The combustion system according to claim 7, further comprising
a sweep gas supply portion that supplies a sweep gas containing
water vapor to the second treatment chamber, wherein the sweep gas
supply portion supplies the sweep gas containing the water vapor
removed by the water vapor removing portion to the second treatment
chamber.
12. The combustion system according to claim 9, wherein the sweep
gas supply portion supplies, to the second treatment chamber, water
vapor generated by heating water by heat exchange with a
high-temperature exhaust gas generated by the combustion of methane
by the combustion portion.
13. The combustion system according to claim 9, wherein the sweep
gas supply portion supplies water vapor contained in an exhaust gas
generated by the combustion of methane by the combustion portion to
the second treatment chamber.
14. The combustion system according to claim 1, wherein the mixture
gas contains a gas derived from a biogas generated by methane
fermentation of organic matter.
15. The combustion system according to claim 14, further comprising
a desulfurization apparatus including a super-high desulfurization
catalyst to remove a sulfur component contained in the gas derived
from the biogas.
Description
TECHNICAL FIELD
[0001] The present invention relates to a combustion system for
obtaining energy by combusting a gas containing carbon dioxide and
methane as a main component such as a biogas obtained by methane
fermentation of organic matter such as biomass and organic
waste.
BACKGROUND ART
[0002] In recent years, it has drawn attention to utilize, as a new
energy source, a biogas obtained by methane fermentation of organic
waste such as biomass and sewage sludge. The biogas is used as a
substitute for fossil fuel, and is used for power generation,
boiler, and the like.
[0003] Biogas generally contains about 40% of carbon dioxide in
addition to methane, although the content varies depending on the
production conditions (fermentation conditions) of the methane gas.
The biogas also contains a very small amount of sulfur compounds
such as siloxane and hydrogen sulfide, and this must be removed at
the time of use.
[0004] For example, digested gases from sewage sludge treated in
relatively large facilities contain sulfur compounds such as
hydrogen sulfide (HO and many impurities such as siloxane. Also, in
the case of biogas obtained by methane fermentation of livestock
excrement, food waste, and the like which is supposed for small
facilities, there are various trace components (oil, trace
elements: V, Pb, Cl, etc., ethane, propane, dienes, benzene,
toluene, etc.) depending on difference in individual facilities and
gas production conditions, and the amounts and concentrations
thereof may be different. These trace impurities affect the
performance and durability of gas engines.
[0005] In a biogas engine, carbon dioxide in the biogas causes
decrease in output and thermal efficiency of the gas engine.
Further, engine adjustment work is required depending on the
content and degree of variation of the composition of the
biogas.
[0006] The output and thermal efficiency of a biogas engine that
uses a mixture gas of methane and carbon dioxide as a fuel greatly
decreases as the carbon dioxide concentration increases. For
example, when a mixture gas containing 40% of CO.sub.2 is used, the
engine output and thermal efficiency of the gas engine decreases
40% and 14%, respectively, as compared with a methane fuel of a
purity of 100%. This means that only a 60 kW output will be
obtained even with a 100 kW natural gas engine, and on the
contrary, if a 100 kW output is required, a natural gas engine of
about 170 kW is needed. As a result, since the facility cost of the
engine is almost proportional to the output thereof, the facility
cost increases by 70%. Also, since the thermal efficiency decreases
by 14%, the fuel cost increases by about 16% as compared with a
natural gas engine.
[0007] For this reason, both the initial cost and the running cost
of an engine for biogas have been high cost.
[0008] Patent Document 1 describes a biogas power generation
apparatus that controls the total number of gas engines to be
driven and driving of a surplus gas combustion apparatus in
accordance with the pressure with which a biogas is supplied to the
engines.
[0009] Patent Document 2 describes a power generation method in
which carbon dioxide in a digested gas obtained by methane
fermentation of organic matter such as biomass and organic waste is
absorbed and separated using an alkali absorbing liquid and methane
gas of a high purity is supplied to an engine.
[0010] Patent Document 3 describes, regarding a gas engine using a
gas whose property changes during operation, such as biogas, as a
fuel, a control method of a gas engine of a premixing type in which
engine misfire or combustion abnormality is prevented by correcting
and controlling an air-fuel ratio when the temperature of an
exhaust gas is not within a preset range.
[0011] Patent Document 4 and Patent Document 5 relate to combustion
control of an engine using a mixture gas of biogas and city gas
(natural gas) as a fuel, and decrease in output of the engine is
suppressed such that a biogas, whose heat generation amount
changes, can be stably combusted, by respectively adjusting the
oxygen concentration in an exhaust gas in Patent Document 4 and
adjusting the mixture ratio of the biogas in accordance with the
temperature of an exhaust gas in Patent Document 5.
PRIOR ART DOCUMENTS
Patent Documents
[0012] Patent Document 1: Japanese Patent Application Publication
NO. 2010-209706
[0013] Patent Document 2: Japanese Patent Application Publication
NO. 2002-275482
[0014] Patent Document 3: Japanese Patent Application Publication
NO. 2012-13011
[0015] Patent Document 4: Japanese Patent Application Publication
NO. 2012-242011
[0016] Patent Document 5: Japanese Patent Application Publication
NO. 2013-163984
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0017] As described above, the amount of generation and the
components of biogas is not stable depending on the conditions
(fermentation conditions) at the time of generation of methane gas.
Therefore, when a biogas is used as a fuel, combustion becomes
unstable and a stable output sometimes cannot be obtained.
[0018] In order to obtain a stable output, a mechanism for
controlling and adjusting the engine in accordance with the
increase in size of the engine and change in the gas composition is
required, and this has led to increase in the cost of the engine.
In addition, the engine adjustment work that is complicated and
requires a long time have been a major burden/risk factor for users
and manufacturers.
[0019] In order to solve the above problem, if a gas obtained by
removing a carbon dioxide component from the biogas is supplied to
the engine, the quality of the gas becomes closer to ordinary
natural gas, and thus it can be considered that adjustment becomes
no longer required and cost reduction becomes possible. However,
existing chemical absorption method, high-pressure water absorption
method, PSA, etc. all require expensive large-scale equipment and
energy, resulting in impairment of environmental merit of using
biogas.
[0020] As an example, in the case of converting a sewage digested
gas into methane gas of a high purity by the high-pressure water
absorption method and using the methane gas as a fuel for natural
gas vehicles etc., the cost of refining facilities for processing
660 Nm.sup.3/h of digested gas is reported to be about 1.53 billion
yen. Since the digested gas of 660 Nm.sup.3/h corresponds to the
rated fuel of a 2000 kW biogas engine, and the price of the 2000 kW
biogas engine itself is currently about 400 million yen, CO.sub.2
removal by the existing high-pressure water absorption method will
make the cost of the engine about five times. In the high-pressure
water absorption method, energy of about 1 kW is consumed per 1
Nm.sup.3 of CO.sub.2 in order to remove CO.sub.2, and this energy
consumption corresponds to about 15% of the energy obtained by
combustion of methane of biogas. In this way, the energy required
for CO.sub.2 removal by an existing water absorption method cancels
the efficiency improvement effect of the gas engine that the
CO.sub.2 removal has.
[0021] Therefore, it is an object of the present invention to, even
in the case of using an engine that uses a gas containing methane
as a main component and containing carbon dioxide such as a biogas
as a fuel gas, provide a combustion system with which a stable
output can be obtained without necessity of complicated engine
adjustment work, which does not require a process that consumes a
large energy for removing carbon dioxide, and thus with which the
environmental merit of using biogas can be enjoyed.
Means for Solving the Problem
[0022] In order to achieve the above-described object, as a first
feature, a combustion system according to the present invention
comprises
[0023] a separation portion that removes carbon dioxide from a
treatment target gas containing a mixture gas containing methane as
a main component and containing carbon dioxide to obtain methane
gas of a high purity in which at least a content of carbon dioxide
has been reduced, and
[0024] a combustion portion that combusts the methane gas.
[0025] The separation portion includes a first treatment chamber
and a second treatment chamber separated from each other by a
separation membrane therebetween.
[0026] The separation membrane selectively allows the carbon
dioxide in the treatment target gas supplied to the first treatment
chamber to pass therethrough to the second treatment chamber to
obtain a first separation gas having a higher methane purity than
the treatment target gas in the first treatment chamber and a
second separation gas containing the carbon dioxide in the
treatment target gas in the second treatment chamber.
[0027] The combustion system according to the present invention
having the first feature preferably further has a second feature in
which the separation membrane is a facilitated transport membrane
to which a carrier that selectively reacts with carbon dioxide is
added.
[0028] The combustion system according to the present invention
having the second feature preferably further has a third feature in
which the combustion system comprises a water vapor supply portion
that supplies water vapor to the first treatment chamber, and the
mixture gas containing the water vapor supplied by the water vapor
supply portion is supplied to the first treatment chamber as the
treatment target gas.
[0029] The combustion system according to the present invention
having the third feature preferably further has a fourth feature in
which the water vapor supply portion supplies, to the first
treatment chamber, water vapor generated by heating water by heat
exchange with a high-temperature exhaust gas generated by a
combustion of methane by the combustion portion.
[0030] The combustion system according to the present invention
having the third feature or the fourth feature preferably further
has a fifth feature in which the water vapor supply portion
supplies water vapor contained in an exhaust gas generated by the
combustion of methane by the combustion portion to the first
treatment chamber.
[0031] The combustion system according to the present invention
having any one of the third to fifth features preferably further
has a sixth feature in which the combustion system comprises an
exhaust gas supply portion that mixes the mixture gas with at least
part of an exhaust gas containing carbon dioxide and water vapor
generated by the combustion of methane by the combustion portion
and supplies the mixture gas mixed with the at least part of the
exhaust gas to the first treatment chamber as the treatment target
gas.
[0032] The combustion system according to the present invention
having any one of the third to sixth features preferably further
has a seventh feature in which the combustion system comprises a
water vapor removing portion that removes water vapor from the
first separation gas and supplies the first separation gas from
which water vapor has been removed to the combustion portion.
[0033] The combustion system according to the present invention
having the seventh feature preferably further has an eighth feature
in which the water vapor supply portion supplies the water vapor
removed by the water vapor removing portion to the first treatment
chamber.
[0034] The combustion system according to the present invention
having any one of the second to eighth features preferably further
has a ninth feature in which the combustion system comprises a
sweep gas supply portion that supplies a sweep gas to the second
treatment chamber.
[0035] The combustion system according to the present invention
having the ninth feature preferably further has a tenth feature in
which the sweep gas contains water vapor, and
[0036] the water vapor supply portion supplies the water vapor
contained in the sweep gas to the sweep gas supply portion.
[0037] The combustion system according to the present invention
having the eighth feature preferably further has an eleventh
feature in which the combustion system comprises a sweep gas supply
portion that supplies a sweep gas containing water vapor to the
second treatment chamber, and
[0038] the sweep gas supply portion supplies the sweep gas
containing the water vapor removed by the water vapor removing
portion to the second treatment chamber.
[0039] The combustion system according to the present invention
having any one of the ninth to eleventh features preferably further
has a twelfth feature in which the sweep gas supply portion
supplies, to the second treatment chamber, water vapor generated by
heating water by heat exchange with a high-temperature exhaust gas
generated by the combustion of methane by the combustion
portion.
[0040] The combustion system according to the present invention
having any one of the ninth to twelfth features preferably further
has a thirteenth feature in which the sweep gas supply portion
supplies water vapor contained in an exhaust gas generated by the
combustion of methane by the combustion portion to the second
treatment chamber.
[0041] Further, in the combustion system according to the present
invention having any one of the ninth to thirteenth features, the
mixture gas may be preferably a gas derived from a biogas generated
by methane fermentation of organic matter. Furthermore, preferably,
in the case where the separation membrane is a facilitated
transport membrane to which a carrier that selectively reacts with
carbon dioxide is added, the combustion system comprises a
desulfurization apparatus including a super-high desulfurization
catalyst to remove a sulfur component contained in the gas derived
from the biogas.
Effect of the Invention
[0042] The combustion system of the present invention has a
configuration in which the separation membrane is used to remove
carbon dioxide contained in the biogas or the like and to supply
methane gas of a high purity to a combustion chamber. Thus, even in
the case of using an engine using a gas containing methane as a
main component and containing carbon dioxide such as biogas as a
fuel gas, it is possible to obtain a stable output without
requiring a complicated engine adjustment work. Here, as the
separation membrane for removing carbon dioxide, a facilitated
transport membrane to which a carrier that selectively reacts with
carbon dioxide is added can be suitably used. Furthermore, the
removed carbon dioxide can be recovered and reused for various
industrial applications.
[0043] Removal of carbon dioxide by permeation through the above
separation membrane (facilitated transport membrane) requires a
large membrane area to obtain a separation gas of a high purity,
but requires less energy than a process that consumes large energy
for removal of carbon dioxide such as the high pressure water
absorption method, and thus can maximize the environmental merit of
using a biogas.
[0044] Engines that uses a gas containing methane gas as a main
component and containing carbon dioxide as a fuel gas include, for
example, as disclosed in Patent Documents 3 to 5, engines that
detect a combustion state in a combustion chamber by a method such
as measuring the temperature of exhaust gas and control the
air-fuel ratio or the mixture ratio of the fuel gas on the basis of
the combustion state, and engines that detect the combustibility
(methane purity) of the fuel gas and perform control of increasing
the pressure of the fuel gas in accordance with the combustibility
and supplying the fuel gas to the combustion chamber. However,
according to the combustion system of the present invention, as a
result of removing carbon dioxide by using a separation membrane,
such complicated control is not required and a sensor for detecting
the combustion state or the combustibility of the fuel gas is
neither required. Therefore, a low cost engine with a simple
configuration can be used. A general-purpose inexpensive natural
gas engine can be used.
[0045] Meanwhile, in the case of using a facilitated transport
membrane, the presence of moisture is indispensable for obtaining a
high permeation rate. Therefore, by allowing a gas obtained by
mixing a water vapor gas with a biogas to pass through the
facilitated transport membrane, carbon dioxide gas can be allowed
to pass through with high selectivity even in a high temperature
environment. As a result, the water vapor gas is mixed in the
separation gas, but the water vapor gas is easily removed by
cooling or using another selectively permeable membrane.
[0046] Further, the water vapor to be mixed with the biogas can
also be separated from a mixture gas of water vapor and carbon
dioxide discharged by combustion of methane gas and be reused.
Further, it is also possible to recover and reuse the carbon
dioxide contained in the exhaust gas via the separation membrane,
and it is possible to reduce the environmental burden by not
discharging carbon dioxide to the external environment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a schematic diagram showing a configuration of a
main part of a combustion system according to an embodiment of the
present invention.
[0048] FIG. 2 is a schematic diagram showing a configuration of a
main part of a combustion system according to an embodiment of the
present invention.
[0049] FIG. 3 is a schematic diagram showing a configuration of a
main part of a combustion system according to an embodiment of the
present invention.
[0050] FIG. 4 is a schematic diagram showing a configuration of a
main part of a combustion system according to an embodiment of the
present invention.
[0051] FIG. 5 is a schematic diagram showing a configuration of a
main part of a combustion system according to an embodiment of the
present invention.
[0052] FIG. 6 is a schematic diagram showing a configuration of a
main part of a combustion system according to an embodiment of the
present invention.
[0053] FIG. 7 is a table showing conditions with which membrane
performance of a separation membrane is evaluated and evaluation
results thereof.
DESCRIPTION OF EMBODIMENT
[0054] Embodiments of the present invention will be described in
detail below.
[0055] FIG. 1 is a schematic diagram showing a configuration of a
main part of a combustion system 1 according to an embodiment of
the present invention. An arrow in FIG. 1 indicates a flow path and
direction in which a gas flows in a simplified manner, and chemical
formulae shown in FIG. 1 indicate main components included in the
gas conceptually flowing in the arrow direction in the figure.
Description of 3-way valves, mixing valves, etc. required in the
gas flow path is omitted. The same applies to configuration
diagrams of main parts of the combustion system to be described
later. In addition, in each main part configuration diagram, the
same constituent elements are denoted by the same reference
symbols, and description thereof may be omitted in some cases.
[0056] The combustion system 1 includes a separation portion 14 and
a combustion portion 15. The separation portion 14 removes carbon
dioxide from a treatment target gas containing, as a component, a
mixture gas containing methane as a main component and containing
carbon dioxide, and separates methane gas of a high purity having
at least a reduced content of carbon dioxide from the treatment
target gas. Then, the combustion portion 15 combusts the methane
gas of a high purity obtained by the separation portion 14. The
combustion portion 15 is, for example, a combustion chamber of a
gas engine or a gas turbine and is provided to convert thermal
energy resulting from a combustion reaction of methane gas into
energy such as kinetic energy or electric power.
[0057] The separation portion 14 has two treatment chambers 11 and
12 separated by a separation membrane 13. A mixture gas containing
a component derived from biogas is supplied as a treatment target
gas to the treatment chamber 11 (first treatment chamber) via a gas
flow path 21. Although the mixture gas is a gas containing methane
gas as a main component and containing carbon dioxide, impurities
such as hydrogen sulfide and siloxane among components derived from
biogas are removed in advance by using an existing desulfurization
apparatus, a siloxane removal apparatus of an activated carbon
adsorption system, etc.
[0058] As the desulfurization apparatus, a wet desulfurization
method using an absorption liquid or an adsorption desulfurization
method using a sulfur adsorption material such as zinc oxide or
iron oxide can be used. Also, sulfur can be completely removed to
the ppb level or less by using a copper zinc-based super-high
desulfurization catalyst. Particularly when using a facilitated
transport membrane for the separation membrane 13, depending on the
type of carrier used and the concentration thereof, the facilitated
transport membrane may be influenced by hydrogen sulfide, so it is
preferable to use a super-high desulfurization catalyst.
[0059] The separation membrane 13 has a function of selectively
allowing carbon dioxide gas contained in the treatment target gas
to pass through to the treatment chamber 12 (second treatment
chamber) side with a permeability higher than the permeability of
methane gas. As a result, the purity of carbon dioxide in the gas
in the treatment chamber 11 decreases, and the purity of methane
gas increases. In contrast, the purity of the carbon dioxide in the
gas in the treatment chamber 12 increases.
[0060] Here, the purity of the gas refers to the molar
concentration ratio of the gas component to the total gas (that is,
equal to the ratio of the partial pressure of the gas). This also
applies to the following description.
[0061] The separation membrane 13 is preferably composed of a
facilitated transport membrane. The facilitated transport membrane
is a membrane formed by adding a carrier that selectively reacts
with a specific gas molecule (here, carbon dioxide), for example,
in a gel membrane. The specific structure of the facilitated
transport membrane will be described later.
[0062] In the CO.sub.2 facilitated transport membrane described
above, the permeation rate of CO.sub.2 is facilitated because
CO.sub.2 permeates as a reaction product with the carrier in
addition to physical permeation by the dissolution/diffusion
mechanism. In contrast, gases such as N.sub.2, CH.sub.4, and
H.sub.2 that do not react with the carrier only permeates through
the dissolution/diffusion mechanism, and therefore the separation
factor of CO.sub.2 to these gases is extremely large. The same
applies to inert gases such as Ar and He, and, since these gases do
not react with the carrier, the permeability thereof is extremely
small compared with CO.sub.2. Furthermore, since the energy
generated during the reaction between CO.sub.2 and carrier is used
as energy for the carrier to release CO.sub.2, there is no need to
supply energy from the outside, which is essentially an energy
saving process.
[0063] Here, "carrier" is substance having an effect of increasing
the permeation rate of a specific gas when the substance is
contained in a membrane.
[0064] Since the facilitated transport membrane not only has a high
energy-saving effect and but also extremely compact, and a CO.sub.2
separation/recovery process of much lower cost than existing
chemical absorption methods and a further expensive pressure swing
adsorption (PSA) method can be configured if the membrane can be
mass produced, this membrane can be applied to recovery of CO.sub.2
from power generation exhaust gas, iron-making exhaust gas, cement
exhaust gas, etc. in addition to a decarbonation process, and
further to a next-generation energy process such as a coal to
liquids (CTL: production of liquid fuel from coal) field and to
small chemical plants and facilities to which the existing
decarbonation cannot be applied, and therefore CO.sub.2 can be
easily separated and recovered. Therefore, this membrane is
expected to be a great contribution to a low-carbon society.
[0065] By the separation membrane 13, the treatment target gas
containing methane gas and carbon dioxide gas is separated into a
first separation gas in the treatment chamber 11, which is higher
in purity of methane gas and lower in purity of carbon dioxide gas
than the treatment target gas, and a second separation gas in the
treatment chamber 12. The first separation gas is sent to the
combustion portion 15 through a gas flow path 23, a water vapor
removing portion 16, and a gas flow path 24, and the methane gas is
used for combustion. Meanwhile, the second separation gas contains
a large amount of carbon dioxide and can be recovered and reused
for various industrial applications.
[0066] Meanwhile, in the case where the separation membrane 13 is a
facilitated transport membrane and there is no moisture in the
separation membrane 13, the permeation rate of carbon dioxide is
generally very small, and therefore moisture in the membrane is
indispensable for obtaining a high permeation rate. One method of
keeping moisture in the separation membrane 13 is to constitute the
gel layer with a highly water retentive hydrogel. This makes it
possible to keep moisture in the membrane as much as possible even
at a high temperature at which moisture in a separation function
layer decreases, and it is possible to realize high selective
permeation performance at a high temperature of, for example,
100.degree. C. or higher. In addition, when the treatment target
gas contains moisture (water vapor), it is preferable to supply the
treatment target gas into the treatment chamber 11 while keeping
the moisture without removing the moisture.
[0067] As another method of keeping the moisture in the separation
membrane 13, preferably, a water vapor gas (steam) may be further
mixed with a mixture gas containing methane gas and carbon dioxide
gas, and the mixture gas containing water vapor may be introduced
into the treatment chamber 11 as the treatment target gas. For this
purpose, a water vapor supply portion 17 is provided in the
combustion system 1. In the present embodiment, the water vapor gas
supplied from the water vapor supply portion 17 is mixed with a
mixture gas containing methane gas and carbon dioxide gas, and the
mixture gas containing methane, carbon dioxide, and water vapor is
supplied to the treatment chamber 11 of the separation portion 14
through the gas flow path 21.
[0068] The relative humidity of the treatment target gas containing
water vapor is preferably 30% to 100% and more preferably 40% to
100%.
[0069] The treatment target gas containing water vapor may be
pressurized and heated. By increasing the pressure, it is possible
to increase the partial pressure difference of the carbon dioxide
gas, which is the driving force of permeation, and to increase the
permeation amount of carbon dioxide. In addition, increasing the
partial pressure of steam by increasing the pressure also has an
effect of increasing the relative humidity which is lowered by
increasing the temperature. In view of the energy required for
increasing the pressure, the pressure in the case of increasing the
pressure is preferably 200 kPa (A) to 1000 kPa (A) and more
preferably 400 kPa (A) to 1000 kPa (A). Although the temperature
may be about room temperature, since the permeation performance of
carbon dioxide tends to increase with temperature, the temperature
is preferably 60.degree. C. to 130.degree. C. and more preferably
from 80.degree. C. to 120.degree. C.
[0070] However, in the case of the configuration in which the water
vapor supply portion 17 is provided as described above, the first
separation gas is a gas containing methane gas and water vapor
although the purity of carbon dioxide is low. When transferring the
first separation gas to the combustion portion 15, it is preferable
that the water vapor is removed.
[0071] Therefore, in the present embodiment, a water vapor removing
portion 16 is provided between the treatment chamber 11 and the
combustion chamber of the combustion portion 15, the water vapor
removing portion 16 removes the water vapor mixed in by the water
vapor supply portion 17 from the first separation gas, and the
methane gas of a high purity from which the water vapor has been
removed is supplied to the combustion portion 15. As the water
vapor removing portion 16, a known configuration such as a
configuration using a condenser or a configuration using a water
vapor permeable membrane such as a perfluoro-based membrane (or a
perfluorosulfonic acid-based membrane) can be used. For example, in
the case of using a water vapor permeable membrane, since the water
vapor gas is recovered in a gaseous state (in a state having latent
heat) rather than in a cooled liquid state, at least a part of the
removed water vapor gas can be returned as it is to the water vapor
supply portion 17 to be reused as the water vapor gas to be mixed
with the treatment target gas. As the water vapor permeable
membrane, the facilitated transport membrane described above can
also be used. In this case, the facilitated transport membrane may
be made of a material different or the same material from or as
that of the separation membrane 13. An example of a water vapor
selective permeable membrane using a facilitated transport membrane
is disclosed in WO 2012/014900.
[0072] The water vapor removed by the water vapor removing portion
16 can be supplied to the water vapor supply portion 17 via a gas
flow path 25 to be added to the treatment target gas.
[0073] The method of supplying water vapor by the water vapor
supply portion 17 is not limited to the method of utilizing the
water vapor removed by the water vapor removing portion 16.
Although it consumes additional energy, the water vapor may be
generated by heating water. In this case, energy saving can be
expected by using a high-temperature exhaust gas generated by
combustion of methane in the combustion portion 15 to heat the
water by heat exchange with the high-temperature exhaust gas to
generate water vapor. As will be described later, it is also
possible to reuse the water vapor contained in the exhaust gas
after the combustion reaction of methane.
[0074] It is preferable to flow a sweep gas in the treatment
chamber 12 in order to lower the partial pressure of the carbon
dioxide on the permeation side to obtain the partial pressure
difference serving as the driving force for selective permeation.
The sweep gas is supplied from a gas flow path 22 (sweep gas supply
portion). The sweep gas preferably contains water vapor gas. In the
present embodiment, the water vapor supply portion 17 supplies the
treatment target gas to which the water vapor is added to the
treatment chamber 11, and also supplies the water vapor to the
treatment chamber 12 such that the sweep gas contains water vapor.
Regarding the water vapor contained in the sweep gas, energy saving
can be expected by generating the water vapor by heating water by
heat exchange with a high-temperature exhaust gas generated by
combustion of methane similarly to the water vapor supplied to the
treatment chamber 11. Further, as will be described later, it is
also possible to reuse the water vapor contained in the exhaust gas
after the combustion reaction of methane.
[0075] By supplying the water vapor gas serving as the sweep gas to
the treatment chamber 12, it is possible to reduce the partial
pressure difference of the water vapor gas between the supply side
(the treatment chamber 11) and the permeation side (the treatment
chamber 12), to reduce the permeation amount of the water vapor gas
in the treatment target gas, and thus to suppress the decrease in
the relative humidity of the treatment target gas. Further, as the
recovery rate of CO.sub.2 is higher, the proportion of the water
vapor gas on the permeation side becomes smaller, so that the
relative humidity of the gas (second separation gas) in the
treatment chamber 12 becomes lower. However, the decrease in the
relative humidity can be suppressed by increasing the flow rate of
the water vapor gas contained in the sweep gas. However, in the
case of using water vapor as a sweep gas, it is necessary to
perform control such that the pressure on the permeation side is
equal to or lower than the saturated vapor pressure at the
temperature at which the water vapor is used. That is, when water
vapor gas alone is used as the sweep gas under a temperature
condition lower than 100.degree. C., the permeation side needs to
be depressurized.
[0076] In this way, by supplying the treatment target gas
containing methane and carbon dioxide to the treatment chamber 11
and allowing the carbon dioxide in the treatment target gas to pass
through the separation membrane 13 with a higher permeability than
that of methane, methane gas of a high purity hardly containing
carbon dioxide can be supplied to the combustion portion 15. As a
result, by incorporating a gas engine as the combustion portion 15
of the combustion system 1, the gas engine can obtain a stable
output without the need for complicated engine adjustment work even
when biogas is used as the fuel, and miniaturization and increase
in output can be expected.
[0077] In addition, since complicated and high-grade engine control
such as control of the air-fuel ratio of the fuel gas according to
the combustion state of the combustion chamber or pressurizing
control of the supplied fuel gas according to the combustibility
(methane purity) of the fuel gas is not necessary, a gas engine of
a simpler configuration can be used, and therefore cost reduction
can be expected. It becomes possible to use a general-purpose
inexpensive natural gas engine.
[0078] Further, in the combustion system 1, by using the CO.sub.2
selective permeable membrane, carbon dioxide can be removed without
consuming large energy, which saves energy, and it is possible to
enjoy the environmental merit of using biogas without impairing the
merit.
Second Embodiment
[0079] FIGS. 2 to 6 schematically show other configuration examples
of the combustion system of the present invention. Combustion
systems 2 to 6 shown in FIGS. 2 to 6 enable reuse of the exhaust
gas generated by the combustion reaction of methane in the
combustion portion 15.
[0080] The combustion reaction of methane produces water vapor and
carbon dioxide. By using the water vapor gas as a sweep gas to be
supplied to the second treatment chamber 12 of the separation
portion or mixing the water vapor gas with the treatment target
gas, it becomes possible to keep moisture in the separation
membrane even under a high temperature condition as described above
and obtain a high permeability. Meanwhile, by removing the carbon
dioxide gas by the separation membrane, the burden exerted on the
environment by the exhaust gas can be reduced. Also, by increasing
the purity, various industrial applications become possible.
[0081] In the combustion system 2 shown in FIG. 2, the exhaust gas
generated in the combustion portion 15 is mixed with water vapor
from the water vapor supply portion 17 and supplied as a sweep gas.
Accordingly, the water vapor contained in the exhaust gas is
effectively utilized. However, since the exhaust gas contains
carbon dioxide, it is necessary to adjust the mixing ratio and flow
rate of water vapor such that the partial pressure of carbon
dioxide in the sweep gas does not exceed the partial pressure of
carbon dioxide in the treatment target gas.
[0082] When it is assumed that the ratio of nitrogen and oxygen in
the air is 4:1 and all oxygen in the air taken into the combustion
chamber is used for combustion of methane without excess or
deficiency, the composition of the exhaust gas after methane
combustion is CO.sub.2:H.sub.2O:N.sub.2=1:2:8. When the exhaust gas
is used as a sweep gas for the facilitated transport membrane, it
is necessary to supply a pressurized exhaust gas to the treatment
chamber 12 in order to obtain the preferable relative humidity
described above. However, the pressurization of the exhaust gas
also increases the partial pressure of carbon dioxide contained in
the exhaust gas, and there is a possibility that the driving force
necessary for selective permeation of carbon dioxide decreases. For
this reason, in general, water vapor is separately added to the
sweep gas in order to obtain high selectivity when using the
exhaust gas as a sweep gas for the facilitated transport membrane.
However, since selective permeation membranes other than the
facilitated transport membrane (for example, a CO.sub.2 separation
membrane utilizing a dissolution/diffusion mechanism) do not
require moisture for membrane permeation, an effect as a sweep gas
can be expected by just directly introducing the exhaust gas on the
permeation side.
[0083] In the combustion system 3 shown in FIG. 3, a water vapor
separation portion 18 is provided in a flow path 26 through which
the exhaust gas from the combustion portion flows. The water vapor
separation portion 18 separates water vapor contained in the
exhaust gas. The separated water vapor can be mixed with the sweep
gas or the treatment target gas through the water vapor supply
portion 17. Similarly to the water vapor removing portion 16, a
known configuration including a water vapor permeable membrane can
be used for the water vapor separation portion 18. Further, a
facilitated transport membrane can also be used for the water vapor
separation portion. Meanwhile, gas containing carbon dioxide and
nitrogen remaining after the separation of the water vapor can also
be used as the sweep gas to be supplied to the treatment chamber 12
similarly to the combustion system of FIG. 2 (although not
illustrated).
[0084] In the combustion system 4 shown in FIG. 4, the exhaust gas
from the combustion portion 15 flows into a treatment chamber
(supply side) 31 of a separation portion 34 that is provided with a
separation membrane 33 (CO.sub.2 facilitated transport membrane)
and different from the separation portion 14. Although components
of the exhaust gas include nitrogen gas derived from taking in
oxygen necessary for combustion from the air, by supplying the
exhaust gas to the treatment chamber 31 of the separation portion
34, gas containing carbon dioxide and water vapor from which
nitrogen has been removed can be obtained in a treatment chamber 32
on the permeation side. This gas can be used for various industrial
applications as carbon dioxide gas of a high purity by removing a
water vapor component. A sweep gas can flow into the treatment
chamber 32. Water vapor is preferred as the sweep gas. The water
vapor supply portion 17 can supply water vapor to be mixed with the
exhaust gas in the treatment chamber 31 to obtain a high carbon
dioxide permeation rate and water vapor to be supplied into the
treatment chamber 32 as the sweep gas.
[0085] The combustion system 5 shown in FIG. 5 is the same as the
combustion system 4 of FIG. 4 in that the separation portion 34 for
removing nitrogen in the exhaust gas is provided. The gas obtained
by the nitrogen separation and containing carbon dioxide and water
vapor is mixed with a biogas by an exhaust gas supply portion 19
and the mixture gas is supplied to the supply side (treatment
chamber 11) of the separation portion 14 as the treatment target
gas. As a result, carbon dioxide in the exhaust gas selectively
permeates the separation membrane 33 of the separation portion 34,
further selectively permeates the separation membrane 13 of the
separation portion 14, and is recovered as the second separation
gas in the treatment chamber 12. The recovered carbon dioxide gas
can be used for various industrial applications as carbon dioxide
gas of a high purity after removal of water vapor.
[0086] The combustion system shown in FIG. 6 has a configuration in
which the treatment chamber 12 of the separation portion 14 and the
treatment chamber 32 of the separation portion 34, to both of which
the sweep gas is supplied, in the configuration of FIG. 4 are
integrated. In place of the separation portions 14 and 34, a
separation portion 35 is provided. The separation portion 35 is
divided into three treatment chambers by the separation membranes
13 and 33. A mixture gas of biogas and water vapor supplied from
the water vapor supply portion 17 is supplied as the treatment
target gas to a treatment chamber 36 separated by the separation
membrane 13. A mixture gas obtained by mixing the water vapor
supplied from the water vapor supply portion 17 with the exhaust
gas after combustion of methane is supplied to a treatment chamber
38 separated by the separation membrane 33. A water vapor gas as a
sweep gas is supplied to a treatment chamber 37 separated by both
of the separation membrane 13 and the separation membrane 33, the
carbon dioxide gas contained in the biogas is selectively allowed
to pass through the separation membrane 13 from the treatment
chamber 36 to the treatment chamber 37, and the carbon dioxide gas
in the exhaust gas is selectively allowed to pass through the
separation membrane 33 from the treatment chamber 38 to the
treatment chamber 37. According to this configuration, both carbon
dioxide in the exhaust gas and carbon dioxide in the biogas can be
recovered in the treatment chamber 37 and reused as carbon dioxide
gas of a high purity.
[0087] As described above, in the combustion system shown in FIGS.
2 to 6, the combustion portion 15 can reuse a water vapor gas or
carbon dioxide gas in the exhaust gas generated by the combustion
reaction of methane.
[0088] In particular, in the combustion systems shown in FIGS. 4 to
6, a configuration in which the carbon dioxide gas generated by
combustion is recovered through the separation membrane 13 or 33
such that carbon dioxide is not discharged to the external
environment can be employed, and the environmental burden can be
reduced.
[0089] Hereinafter, the configuration and production method of the
separation membrane 13 (33) will be specifically described.
[0090] <Membrane Structure>
[0091] The separation membranes 13 and 33 are CO.sub.2 facilitated
transport membranes, and as described above, have a structure in
which carriers that selectively react with CO.sub.2 are contained
in a gel membrane. Examples of the CO.sub.2 carrier include
carbonates and bicarbonates of alkali metals such as cesium
carbonate and cesium bicarbonate, and rubidium carbonate and
rubidium bicarbonate. Likewise, hydroxides of alkali metals such as
cesium hydroxide and rubidium hydroxide can be referred to as
equivalents because these also react with carbon dioxide to produce
carbonate and bicarbonate. In addition, amino acids such as
2,3-diaminopropionic acid salt (DAPA) and glycine are known to
exhibit high CO.sub.2 selective permeation performance.
[0092] More specifically, the CO.sub.2 facilitated transport
membrane may be formed by supporting a gel layer containing the
carriers in the gel membrane with a hydrophilic or hydrophobic
porous membrane. Examples of a membrane material constituting the
gel membrane include polyvinyl alcohol (PVA) membranes, polyacrylic
acid (PAA) membranes, and polyvinyl alcohol-polyacrylic acid
(PVA/PAA) salt copolymer membranes. Here, the polyvinyl
alcohol-polyacrylic acid salt copolymer may be sometimes referred
to as a polyvinyl alcohol-polyacrylic acid copolymer by one skilled
in the art.
[0093] It is known that the CO.sub.2 facilitated transport membrane
having the above-described configuration exhibits high CO.sub.2
selective permeation performance.
[0094] However, such the permeation rate of carbon dioxide through
such a CO.sub.2 facilitated transport membrane is very small in the
case where no moisture is in the membrane, and moisture in the
membrane is indispensable for obtaining a high permeation rate.
Therefore, it is preferable that the gel membrane is a hydrogel
membrane. By constituting the gel membrane by a highly water
retaining hydrogel membrane, it is possible to keep moisture in the
membrane as much as possible even in an environment where moisture
in the gel membrane is reduced (for example, at high temperature of
100.degree. C. or higher), and high CO.sub.2 permeance can be
realized. In the above example, the polyvinyl alcohol-polyacrylic
acid (PVA/PAA) salt copolymer membrane and the polyacrylic acid
membrane are hydrogel membranes.
[0095] The hydrogel is a three-dimensional network structure formed
by crosslinking a hydrophilic polymer by chemical crosslinking or
physical crosslinking, and has a property of swelling by absorbing
water.
[0096] Further, a catalyst for accelerating the reaction between
the CO.sub.2 carrier and CO.sub.2 may be contained in the membrane.
As such a catalyst, it is preferable to include carbonic anhydrase
and an oxo acid compound, and it is particularly preferable to
include an oxo acid compound of at least one element selected from
Group 14 elements, Group 15 elements, and Group 16 elements.
Alternatively, it is preferable that the catalyst contains at least
one of a telluric acid compound, a selenious acid compound, an
arsenious acid compound, and an orthosilicic acid compound.
[0097] In the present embodiment, the CO.sub.2 facilitated
transport membrane 13 (33) is composed of a gel membrane composed
of a hydrogel containing carbon dioxide carriers and a porous
membrane supporting the gel membrane. Incidentally, the membrane
structure of the CO.sub.2 facilitated transport membrane is not
limited to this specific example. For example, a structure in which
a gel membrane containing carriers is formed on the outer
peripheral side surface or the inner peripheral side surface of a
cylindrical porous support body may be employed.
[0098] <Membrane Production Method>
[0099] Hereinafter, a method of producing the CO.sub.2 facilitated
transport membrane (separation membranes 13 and 33) will be
described.
[0100] First, a cast solution composed of an aqueous solution
containing a PVA/PAA salt copolymer, a CO.sub.2 carrier (here,
Cs.sub.2CO.sub.3), and a CO.sub.2 hydration reaction catalyst is
prepared (Step 1). More specifically, 2 g of a polyvinyl
alcohol-polyacrylic acid (PVA/PAA) salt copolymer (for example, SS
gel manufactured by Sumitomo Seika Chemicals Co., Ltd.), 4.67 g of
cesium carbonate, and 0.025 times of potassium tellurite with
respect to the cesium carbonate in terms of molar number are added
to 80 g of water and stirred until dissolved to obtain a cast
solution. Next, the cast solution obtained in Step 1 is cast on a
PTFE porous membrane with an applicator (Step 2). Thereafter, the
cast solution is caused to gel by drying to form a gel layer (Step
3).
[0101] <Performance Evaluation Results>
[0102] Results of evaluation of selective permeability of carbon
dioxide of the CO.sub.2 facilitated transport membrane formed by
the above-described production method are shown below.
[0103] The separation membrane 13 is obtained by using the
above-described Cs.sub.2CO.sub.3 as the CO.sub.2 carrier
constituting the CO.sub.2 facilitated transport membrane and by
adding the CO.sub.2 carrier to a hydrogel containing a PVA/PAA salt
copolymer of a hydrogel membrane as a main component and by
supporting the CO.sub.2 carrier added membrane with a hydrophobic
PTFE porous membrane.
[0104] In the evaluation results shown below, for the sake of
convenience of the evaluation experiment, instead of evaluating the
selective permeation performance of CO.sub.2 with respect to
CH.sub.4, methane was substituted by nitrogen, and the selective
permeation performance of CO.sub.2 with respect to N.sub.2 in a
mixture gas containing nitrogen and carbon dioxide is evaluated. As
described above, since CH.sub.4 and N.sub.2 do not react with
carriers in the CO.sub.2 facilitated transport membrane, the
permeability thereof is extremely small as compared with that of
CO.sub.2. In fact, an experiment using a mixture gas containing
three components of CH.sub.4, N.sub.2 and steam (H.sub.2O) was
conducted on the above-described separation membrane, and as a
result, the ratio of CH.sub.4 permeance to N.sub.2 permeance was
0.74. Therefore, in the following simulation example, a value
obtained by multiplying the N.sub.2 permeance by 0.74 is adopted as
the CH.sub.4 permeance in the evaluation condition of the
membrane.
[0105] Results of evaluation of the separation membrane 13
performed on the above-described membrane in three conditions in
which the temperature and the pressure (total pressure) on the
supply side (the treatment chamber 11 side) are kept constant and
the relative humidity of the treatment target gas and the sweep gas
is changed are shown.
[0106] First, the treatment target gas was a mixture gas containing
nitrogen (instead of methane), carbon dioxide, and water vapor as
described above. At this time, the treatment temperature and the
total pressure of the treatment target gas were kept constant at
110.degree. C. and 900 kPa, and the partial pressure of the water
vapor gas to be supplied to the treatment chamber 11 was changed.
Meanwhile, considering a general biogas composition
(CO.sub.2:CH.sub.4=4:6), the partial pressures of nitrogen and
carbon dioxide were respectively changed such that the composition
ratio (partial pressure ratio) of carbon dioxide and nitrogen not
considering water vapor maintained CO.sub.2:N.sub.2=4:6. The sweep
gas is a water vapor gas or a mixture gas of water vapor and Ar,
the partial pressure of the water vapor gas is set to be the same
as that of the treatment target gas, and in the conditions 1 and 2,
an Ar gas was added to the sweep gas such that the total pressure
was 100 kPa (atmospheric pressure).
[0107] FIG. 7 shows the temperature, the pressure, composition
ratio (partial pressure ratio), and relative humidity of the
treatment target gas and sweep gas, and the evaluation results of
the CO.sub.2 permeance and the N.sub.2 permeance under each
evaluation condition. In an evaluation condition 1, the N.sub.2
permeance being "equal to or lower than the GC detection limit"
means that the concentration of N.sub.2 in the second separation
gas that has passed through the separation membrane 13 was too low
to detect N.sub.2 by gas chromatography to calculate the permeance
thereof. In this case, the N.sub.2 permeance is estimated to be at
most 1.37.times.10.sup.-8 [mol/m.sup.2skPa].
[0108] The selectivity of CO.sub.2 over N.sub.2 (CH.sub.4) can be
expressed as the ratio of CO.sub.2 permeance over N.sub.2
(CH.sub.4) permeance. From FIG. 7, it can be seen that the CO.sub.2
facilitated transport membrane has CO.sub.2/N.sub.2 selectivity
larger than 500. Therefore, regarding CO.sub.2/CH.sub.4
selectivity, equivalent selective performance is achieved.
[0109] It is noteworthy that in the evaluation conditions 1 to 3,
the higher the relative humidity is, the higher the CO.sub.2
permeance is. Such humidity dependence is considered to be a
feature of the facilitated transport membrane. The facilitated
transport membrane has very high CO.sub.2 permeance and
selectivity, particularly in the high humidity region, as compared
with other separation membranes (separation membrane of
dissolution/diffusion mechanism, etc.).
[0110] <Required Membrane Area>
[0111] On the basis of the membrane performance evaluation results
of the evaluation conditions 1 to 3 described above, a membrane
area required for the methane concentration (purity) on the first
separation gas exit side (near the gas flow path 23) to be 90% or
higher was calculated, and the results thereof are shown. In the
evaluation of the required membrane area, the composition of the
treatment target gas, the composition of the sweep gas, and the
membrane permeation performance of the evaluation conditions 1 to 3
described above were input in a simulator, and the minimum membrane
area in which the methane concentration (purity) was 90% or higher
was determined while changing the membrane area and the flow rate
of the sweep gas. For CH.sub.4 permeance, as described above, a
value obtained by multiplying N.sub.2 permeance by 0.74 was
adopted. However, in the evaluation condition 1, since N.sub.2
permeance is equal to or lower than the GC detection limit, a value
obtained by multiplying N.sub.2 permeance in the evaluation
condition 3 by 0.74 was adopted as the CH.sub.4 permeance in the
evaluation condition 1 (therefore, actual CH.sub.4 permeance is
considered to be lower than this). The flow rate of the treatment
target gas (excluding water vapor) to be supplied to the treatment
chamber 11 was set to 330 Nm.sup.3/h.
[0112] In addition, the CO.sub.2 permeance was set to a constant
value (value shown in FIG. 7) regardless of a region of the
membrane. However, the facilitated transport membrane has a feature
that the CO.sub.2 permeance is higher when the CO.sub.2 partial
pressure difference between the supply side (the treatment chamber
11 side) and the permeation side (the treatment chamber 12 side) is
lower (see, for example, JP 2015-223893). Therefore, in fact, the
CO.sub.2 permeance at a position closer to the exit side (near the
flow path 23) of the membrane starting from the entrance side (near
the flow path 21) of the membrane is higher due to the distribution
of the CO.sub.2 partial pressure on the membrane in the treatment
chamber 11. Therefore, in the case of using a facilitated transport
membrane, it is considered that the membrane area that is actually
necessary can be smaller than the calculated value.
[0113] As a result of the calculation, the required membrane area
was 575 m.sup.2 in the case of the evaluation condition 1, and 250
m.sup.2 in the case of the evaluation condition 3. Although this is
relatively a large area, this is possible enough to realize as a
combustion system for combusting biogas by combining a plurality of
membrane modules.
[0114] Therefore, according to the combustion system of the present
invention, by removing the carbon dioxide contained in the biogas
via the CO.sub.2 separation membrane and supplying the methane gas
of a high purity after the removal to the combustion chamber, it is
possible to realize a combustion system that can maximize the
environmental merit of using the biogas, save energy, and obtain a
stable output.
Other Embodiments
[0115] Other embodiments will be described below.
[0116] <1> In the combustion systems 1 to 6 of the above
embodiment, the separation membranes (CO.sub.2 facilitated
transport membranes) 13 and 33 are flat membranes, but the present
invention is not necessarily limited to this, and may be applied to
a membrane having a curved surface shape or a hollow fiber shape
having a gel layer containing carriers on the inner side surface or
the outer side surface of a cylindrical porous membrane. Likewise,
the present invention does not depend on the arrangement of the
treatment chambers in respective treatment portions, and a
configuration in which a plurality of coaxial cylindrical treatment
chambers are separated by a CO.sub.2 facilitated transport membrane
or a permeable membrane and a configuration in which treatment
chambers are arranged in series in the extending direction of a
center axis can be considered.
[0117] <2> In the above embodiment, a gel membrane made of a
polyvinyl alcohol-polyacrylic acid salt copolymer is used as a
material of the CO.sub.2 facilitated transport membrane. However,
this is only an example, and a similar hydrophilic polymer that
exerts CO.sub.2 selective separation performance can be adopted.
Also, the CO.sub.2 carrier is not limited to the materials
mentioned in the embodiment, and other material membranes may be
adopted as long as the material membranes have desired CO.sub.2
selective permeation performance.
[0118] <3> Although water vapor is used as the sweep gas in
the above embodiment, the sweep gas flowing into the treatment
chamber 12 of the separation portion 14, the treatment chamber 32
of the separation portion 34, or the treatment chamber 37 of the
separation portion 35 is not limited to water vapor. For example,
the sweep gas may contain gas components such as nitrogen gas and
argon gas. However, since the gas component is contained in the
second separation gas, when considering reuse of the carbon dioxide
gas in the second separation gas, an additional step of separating
the gas component is required. Further, although it is possible to
use a mixture gas containing a gas component other than water vapor
as the sweep gas to be supplied to the treatment chamber 32 in the
combustion system 5 shown in FIG. 5, since this gas component is
mixed with the biogas and supplied to the treatment chamber 11 in
circulation, a step of removing the gas component is required at a
stage before the combustion portion 15.
[0119] In this respect, the sweep gas flowing into the treatment
chambers 12, 32 (FIG. 4), and 37 is preferably a gas that can be
easily separated from the carbon dioxide gas, and a water vapor gas
is preferred considering reuse of the carbon dioxide gas in the
second separation gas. Similarly, the sweep gas flowing into the
treatment chamber 32 of FIG. 5 is preferably a gas that can be
easily separated from methane gas and carbon dioxide gas, and a
water vapor gas is preferable. It is also possible to mix a water
vapor gas with a part of the second separation gas in the treatment
chambers 12 and 37, a part of the permeated gas in the treatment
chamber 32, or a part of the exhaust gas after the combustion of
methane, and to reuse the mixture gas as the sweep gas. However,
since the mixture gas contains carbon dioxide, it is necessary to
adjust the mixing ratio of the water vapor gas such that the
partial pressure of carbon dioxide in the sweep gas is lower than
the partial pressure of carbon dioxide in the treatment target
gas.
[0120] <4> In addition, although the combustion systems 1 to
6 respectively shown in FIGS. 1 to 6 are mentioned as examples of
the configuration of the combustion system in the embodiment
described above, the present invention is not limited to these
specific configurations, and one skilled in the art can easily
configure a different combustion system by appropriately combining
part or all of the configurations of the combustion systems 1 to 6
within a range that is not contradictory as a whole. It can be said
that such configurations that is suggested by the combustion
systems 1 to 6 are also disclosed in this specification.
INDUSTRIAL APPLICABILITY
[0121] The present invention can be applied to a combustion system
that uses, as a fuel, a mixture gas including carbon dioxide gas in
combustible gas such as a biogas obtained by methane fermentation
of organic matter, and by supplying the mixture gas from which
carbon dioxide has been removed by a separation membrane to the
combustion chamber, the present invention can be used as a
combustion system that maximizes the environmental merit of using a
biogas, saves energy, and obtains a stable output.
DESCRIPTION OF SYMBOLS
[0122] 1-6 combustion system [0123] 14 separation portion [0124] 11
first treatment chamber [0125] 12 second treatment chamber [0126]
13 separation membrane [0127] 15 combustion portion [0128] 16 water
vapor removing portion [0129] 17 water vapor supply portion [0130]
18 water vapor separation portion [0131] 19 exhaust gas supply
portion [0132] 21-26 gas flow path [0133] 34, 35 separation portion
[0134] 31, 32, 36-38 treatment chamber [0135] 33 separation
membrane
* * * * *