AU2013254955B2

AU2013254955B2 - Apparatus and method for producing synthetic natural gas

Info

Publication number: AU2013254955B2
Application number: AU2013254955A
Authority: AU
Inventors: Joon Hyun Baik; Suk Hwan Kang; Jin Ho Kim; Kyong Tae Kim; Su Han Kim; Dong Jun Koh; Young Don Yoo
Original assignee: Research Institute of Industrial Science and Technology RIST
Current assignee: Research Institute of Industrial Science and Technology RIST
Priority date: 2012-11-15
Filing date: 2013-11-08
Publication date: 2016-04-14
Anticipated expiration: 2033-11-08
Also published as: CN103820182B; CN103820182A; AU2013254955A1; KR101328697B1

Abstract

There are provided a method and apparatus for producing synthetic natural gas. The method includes a synthetic gas preparation process, a desulfurization process, a methane synthesis and water-gas shift process, a moisture and carbon dioxide removal process, a primary methane synthesis process, and a secondary methane synthesis process. The apparatus includes an isothermal reactor, a condenser, a carbon dioxide removing device, a primary adiabatic reactor, and a secondary adiabatic reactor. According to the method and apparatus, the yield of methane synthesis can be increased, and economical and efficient processes can be performed because a recirculation process and a water-gas shift process are not additionally performed. 48526131 (GFIMatters) P95408.AU 8/11/13 uJIF 00 co > LUu o cr a 0m coo N ~LUW~ 0) ( 0-cc CO (D) cnu2CD 0 O <W 0JUjC < C 0 w 2 : r Cl) CI T)=c- U =D 2: ~F-0 on [0 rwZC ClO 0) <-Er0 L= a P -u Cr~r a-0) (9D

Description

- 1 APPARATUS AND METHOD FOR PRODUCING SYNTHETIC NATURAL GAS CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of Korean Patent Application No. 10-2012-0129674 filed on November 15th, 2012, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. BACKGROUND [0002] The present disclosure relates to an apparatus and method for producing synthetic natural gas. [0003] More particularly, the present disclosure relates to a method for producing synthetic natural gas by synthesizing methane using a synthetic gas containing carbon monoxide, carbon dioxide and hydrogen, the synthetic gas being obtained from byproducts of the petrochemical industry or through the gasification of coal or biomass. [0004] FIG. 1 is a schematic flowchart for explaining a method of producing synthetic natural gas in the related art. Referring to FIG. 1, synthetic gas containing CO, CO 2 , and H 2 is prepared in a coal gasification process S100, and after cooling the synthetic gas and removing dust from the synthetic gas in a heat and dust collecting process S110, the synthetic gas is supplied to a water-gas shift reactor to perform a water-gas shift reaction process S120. Generally, the synthetic gas prepared through the coal gasification process S100 has an H 2 /CO ratio of 0.5 to 1. 4852613_ (GHMatters) P95408.AU 8/11/:3 - 2 However, after the water-gas shift reaction process S120, the H 2 /CO ratio of the synthetic gas is increased to about 3. Then, the synthetic gas undergoes a desulfurization and carbon dioxide removal process 130 and a methane synthesis process S140. (00051 The synthetic natural gas having methane as a main component may be produced using the synthetic gas through reactions expressed by chemical formulas 1 and 2 and reaction heat values below. CO + 3H 2 - CH 4 + H 2 0 (reaction heat: 206 kJ/mol) (1) C02 + 4H 2 - CH 4 + 2H20 (reaction heat: 165 kJ/mol) (2) [0006] Due to moisture generated by the reaction between CO and H2, a water-gas shift reaction occurs as a side reaction of a methane synthesis reaction as shown in chemical formula 3 below, and C02 generated in the water-gas shift reaction participates in the methane synthesis reaction expressed by chemical formula 2. CO + H20 - H 2 + C02 (reaction heat: 41.1 kJ/mol) (3) [0007] Thermodynamically, the methane synthesis reaction becomes more active as pressure increases, but the yield of the methane synthesis reaction decreases as reaction temperatures increase. In the methane synthesis reaction, a Ni-containing catalyst is usually used, and a method of efficiently dissipating and controlling a large amount of reaction heat is a core synthesis technique. Particularly, 4B52613_1 (GHMutters) P95408.AU 8/11/13 -3 since the lifespan of general Ni-containing catalysts is reduced at temperatures of 700 0 C or higher because of a sintering phenomenon, temperature control may be important. Even Ni-containing catalysts improved in heat resistance cannot resist a high temperature of 8000C or higher, and thus, it may be important to control the temperature of a catalyst layer. [0008] Therefore, in general commercial processes, a plurality of adiabatic reactors are used to increase the methane content of a final product, and gas discharged from some of the adiabatic reactors is recirculated to inlets of the adiabatic reactors to mix the gas with inflow gas and supply the mixture back the adiabatic reactors. In this way, the concentrations of CO and H 2 of the inflow gas are reduced, and reaction heat is reduced. [00091 FIG. 2 is a schematic view illustrating an apparatus for producing synthetic natural gas in the related art. The apparatus uses adiabatic reactors. In the apparatus, about 70% of gas discharged from a first adiabatic reactor 201 is recirculated as recirculation gas 102 to an inlet of the first adiabatic reactor 201 using a recirculation compressor 204 to control reaction heat in the first adiabatic reactor 201 by the effect of dilution, and 30% of the gas discharged from the first adiabatic reactor 201 is sequentially supplied to a second adiabatic reactor 202 and a third adiabatic reactor 203. In this way, the yield of methane synthesis is increased. If the temperature of the first 4852613.1 (GHMatters) P95408.AU 8/11/13 - 4 adiabatic reactor 201 is maintained at 700 0 C or lower by using recirculation gas 102, gas discharged from the first adiabatic reactor 201 may have a methane concentration of about 50%. At least three adiabatic reactors may be required to obtain gas having a methane concentration of 90% or greater, and about five adiabatic reactors may be required to obtain gas having a methane concentration of 97% or greater. In a technique disclosed in Korean Patent Application No. 10-2010-0042266 (filed on May 6th, 2010), synthetic gas is supplied to first and second adiabatic reactors, and some of a product discharged from the first adiabatic reactor is recirculated back to the first adiabatic reactor to control reaction heat. In the disclosed technique, four adiabatic reactors are used to obtain gas having a methane concentration of 90% or greater. [0010] However, according to such adiabatic reactor combination technology, high equipment costs are required to recirculate high-temperature, high-pressure gas, and a plurality of adiabatic reactors have to be used. SUMMARY [0011] An aspect of the present disclosure may provide an economical and efficient method for producing synthetic natural gas. In the method, synthetic gas prepared by coal gasification is supplied to an isothermal reactor together with steam, without adjusting the H 2 /CO ratio of the synthetic gas through a water-gas shift reaction, so as to 48526131 (GHMatters) P95408AU 8/11/13 - 5 simultaneously cause a methane synthesis reaction and a water-gas shift reaction while controlling the reaction temperature by the effects of cooling the isothermal reactor and dilution by the steam. Thus, owing to the omission of gas recirculation and a water-gas shift reaction, synthetic natural gas may be produced economically and efficiently. [0012] An aspect of the present disclosure may also provide an apparatus for producing synthetic natural gas including an isothermal reactor in which a methane synthesis reaction and a water-gas shift reaction are simultaneously performed to increase the yield of methane synthesis. In addition, steam is supplied to the isothermal reactor to control the reaction temperature without gas recirculation and water-gas shift equipment. Thus, the apparatus may produce synthetic natural gas economically and efficiently. [0013] According to an aspect of the present disclosure, a method for producing synthetic natural gas includes: performing a synthetic gas preparation process to prepare synthetic gas containing carbon monoxide, carbon dioxide, and hydrogen by reacting coal or biomass with oxygen, hydrogen, or a mixture gas thereof; performing a desulfurization process to collect heat from the synthetic gas and remove impurities and hydrogen sulfide from the synthetic gas; performing a methane synthesis and water-gas shift process to simultaneously cause a methane synthesis reaction and a water-gas shift reaction by supplying the synthetic gas to an isothermal reactor together with steam 4852613_1 (GHMauers) P96408.AU 8/11/13 - 6 after the desulfurization process; performing a moisture and carbon dioxide removal process to remove moisture and carbon dioxide from gas discharged from the isothermal reactor by cooling the gas; performing a primary methane synthesis process to synthesize methane by supplying the gas to a primary adiabatic reactor after moisture and carbon dioxide are removed from the gas; and performing a secondary methane synthesis process to synthesize methane by supplying gas discharged from the primary adiabatic reactor to a secondary adiabatic reactor. [0014] The synthetic gas may have an H 2 /CO ratio in a range of 0.5 to 1.5. [00151 In the methane synthesis and water-gas shift process, the synthetic gas and steam may be supplied to the isothermal reactor at a volumetric ratio of 1:0.4 to 1.5. [0016] The method may further include generating electricity by supplying supersaturated steam to a steam turbine, wherein the supersaturated steam may be produced by saturating steam with reaction heat in the isothermal reactor and allowing the saturated steam to absorb heat from the gas discharged from the primary adiabatic reactor. [0017] The gas supplied to the primary adiabatic reactor after moisture and carbon dioxide are removed from the gas may have a temperature in a range of 2500C to 350 0 C. [00181 According to another aspect of the present 48526131 (GHMatters) P95408.AU 8/11/13 - 7 disclosure, an apparatus for producing synthetic natural gas includes: an isothermal reactor to which steam and synthetic gas are supplied to simultaneously cause a methane synthesis reaction and a water-gas shift reaction, the synthetic gas being produced through a coal gasification reaction and including carbon monoxide, carbon dioxide, and hydrogen; a condenser configured to remove moisture from gas discharged from the isothermal reactor; a carbon dioxide removing device configured to remove carbon dioxide from the gas discharged from the isothermal reactor; a primary adiabatic reactor to which the gas from which moisture and carbon dioxide are removed is supplied to cause a methane synthesis reaction; and a secondary adiabatic reactor to which gas discharged from the primary adiabatic reactor is supplied to cause a methane synthesis reaction. [0019] The gas supplied to the primary adiabatic reactor after moisture and carbon dioxide are removed from the gas has a temperature in a range of 2500C to 350 0 C. [00201 The apparatus may further include a cooling water supply tank configured to supply cooling water to the isothermal reactor and collect saturated steam generated by reaction heat in the isothermal reactor. [00211 The apparatus may further include a heat exchanger in which steam saturated with reaction heat of the isothermal reactor is allowed to exchange heat with the gas discharged from the primary adiabatic reactor. 4852613.1 (GHMaters) P95408.AU 8/11/13 - 8 [0022] The apparatus may further include a condenser configured to remove moisture from gas discharged from the secondary adiabatic reactor. BRIEF DESCRIPTION OF DRAWINGS 10023] The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a schematic flowchart for explaining a method for producing synthetic natural gas in the related art; FIG. 2 is a schematic view illustrating an apparatus for producing synthetic natural gas in the related art; FIG. 3 is a schematic flowchart for explaining a method for producing synthetic natural gas according to an exemplary embodiment of the present disclosure; and FIG. 4 is a schematic view illustrating an apparatus for producing synthetic natural gas according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION [00241 Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. The disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set 4852613_1 (GHMatters) P95408.AU 8/11/13 - 9 forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. In the drawings, the shapes and dimensions of elements may be exaggerated for clarity. [0025] FIG. 3 is a schematic flowchart for explaining a method for producing synthetic natural gas according to an exemplary embodiment of the present disclosure, and FIG. 4 is a schematic view illustrating an apparatus for producing synthetic natural gas according to an exemplary embodiment of the present disclosure. The exemplary embodiments of the present disclosure will now be described with reference to FIGS. 3 and 4. [0026] According to the method and apparatus of the embodiments of the present disclosure, synthetic natural gas 306 is produced by supplying synthetic gas 301 having a low

H

2 /CO ratio to an isothermal reactor 401 to simultaneously cause a methane synthesis reaction and a water-gas shift reaction, and supplying steam to the isothermal reactor 401 to control reaction temperature. According to the exemplary embodiments of the present disclosure, a water-gas shift process for controlling the ratio of H2/CO and a gas recirculation process for controlling a reaction temperature may not be additionally performed, and thus processes may be simplified. In addition, owing to the isothermal reactor 401, the yield of methane synthesis may be increased. 48526131 (GHMaLLers) F95408.AU 8/11/13 - 10 [0027] According to the embodiment of the present disclosure, the method for producing synthetic natural gas includes: performing a synthetic gas preparation process S200 to prepare synthetic gas 301 containing carbon monoxide, carbon dioxide, and hydrogen by reacting coal or biomass with oxygen and/or hydrogen; performing a desulfurization process S210 to collect heat from the synthetic gas 301 and remove impurities and hydrogen sulfide from the synthetic gas 301; performing a methane synthesis and water-gas shift process S220 to simultaneously cause a methane synthesis reaction and a water-gas shift reaction by supplying the synthetic gas 301 to the isothermal reactor 401 together with steam 302 after the desulfurization process S210; performing a moisture and carbon dioxide removal process S230 to remove moisture 304 and carbon dioxide 305 from gas discharged from the isothermal reactor 401 by cooling the gas; performing a primary methane synthesis process S240 to synthesize methane by supplying the gas to a primary adiabatic reactor 407 after moisture 304 and carbon dioxide 305 have been removed from the gas; and performing a secondary methane synthesis process S250 to synthesize methane by supplying gas discharged from the primary adiabatic reactor 407 to a secondary adiabatic reactor 408. [00282 In the synthetic gas preparing process S200, coal or biomass is reacted with oxygen and/or hydrogen to prepare synthetic gas 301 containing carbon monoxide, carbon 48526]3_1 (GHMatters) P95408.AU 8/11/MJ - 11 dioxide, and hydrogen. Since the prepared synthetic gas 301 has a temperature of 900 0 C or higher and contains sulfur and other impurities, the prepared synthetic gas 301 may not yet be suitable for producing synthetic natural gas. Therefore, in the desulfurization process S210, heat is collected from the synthetic gas 301 to adjust the temperature of the synthetic gas 301 to be to 500 0 C or less, and sulfur and other impurities are removed from the synthetic gas 301. In the desulfurization process S210, a heat exchanger, a dust collecting device, and a desulfurizing device may be used (not shown in FIG. 4). Types of the heat exchanger, the dust collecting device, and the desulfurizing device are not limited. For example, a heat exchanger, a dust collecting device, and a desulfurizing device that can be easily operated by those of ordinary skill in the related art may be used. After the desulfurization process S210, the synthetic gas 301 may have an H 2 /CO ratio in the range of 0.5 to 1.5 because the synthetic gas 301 is not subjected to an additional water-gas shift reaction. However, the H 2 /CO ratio of the synthetic gas 301 is not limited thereto. [0029] Thereafter, in the methane synthesis and water-gas shift process S220, the synthetic gas 301 is mixed with steam 302 and is supplied to the isothermal reactor 401 to simultaneously cause a methane synthesis reaction and a water-gas shift reaction. The isothermal reactor 401 may be a shell-tube or annular reactor. However, the isothermal reactor 401 is not limited thereto. For example, any 4852613_1 (GHMatters) P95408AU 8/11/13 - 12 isothermal reactor that can be easily operated by those of ordinary skill in the related art may be used as the isothermal reactor 401. The isothermal reactor 401 may contain a Ni-containing catalyst to increase the yield of the methane synthesis reaction. However, the isothermal reactor 401 is not limited thereto. For example, any methane synthesis catalyst that can be easily used by those of ordinary skill in the related art may be used. [0030) Cooling water 308 is supplied from a cooling water supply tank 409 to the isothermal reactor 401 to cool the isothermal reactor 401. If the isothermal reactor 401 is not cooled with cooling water 308, the temperature of the isothermal reactor 401 may be about 7000C. However, if the isothermal reactor 401 is cooled with cooling water 308, the temperature of the isothermal reactor 401 may range from 270 0 C to 370 0 C. Since the reaction temperature of the isothermal reactor 401 is lower than the reaction temperature of an adiabatic reactor by about 300 0 C or more, the isothermal reactor 401 is effective in increasing the yield of methane synthesis. [0031] In the methane synthesis and water-gas shift process S220, the volumetric ratio of synthetic gas 301:steam 302 supplied to the isothermal reactor 401 may be 1:0.4 to 1.5. The steam 302 is a reactant of the water-gas shift reaction and dilutes the synthetic gas 301 to suppress reaction heat. If the volumetric ratio of the steam 302/synthetic gas 301 supplied to the isothermal reactor 401 is less than 0.4, 4852613.1 (GHMaters) P95408AU 8/11/13 - 13 reaction heat may not be sufficiently suppressed, and thus a reaction temperature may be increased to shorten the lifespan of the catalyst. If the volumetric ratio of the steam 302/synthetic gas 301 supplied to the isothermal reactor 401 is greater than 1.5, the synthetic gas 301 may be excessively diluted, and thus the yield of methane synthesis may be lowered. [00321 In the moisture and carbon dioxide removal process S230, moisture 304 and carbon dioxide 305 are removed from gas discharged from the isothermal reactor 401. As a result of the methane synthesis reaction and the water-gas shift reaction, gas discharged from the isothermal reactor 401 contains methane, moisture 304, and carbon dioxide 305. Therefore, after the moisture 304 and the carbon dioxide 305 have been removed from the gas, the gas may be supplied to the primary adiabatic reactor 407 to increase the yield of methane synthesis. [0033] To this end, gas discharged from the isothermal reactor 401 may be allowed to pass through a heat exchanger 402 to cool the gas to a temperature of 40CC to 1000C, and then the gas may be allowed to pass through a condenser 404 to condense and remove moisture 304. After moisture 304 is removed from the gas, the gas may be allowed to pass through a carbon dioxide removing device 405 to remove carbon dioxide 305 from the gas. The efficiency of removing moisture 304 and carbon dioxide 305 in the moisture and carbon dioxide removal process S230 is not limited. For 485213_1 (GHMatters) P95408.AU 8/11/13 - 14 example, about 80% to 98% of moisture 304 and carbon dioxide 305 contained in the gas discharged from the isothermal reactor 401 may be removed. [0034] Since the gas is cooled in the moisture and carbon dioxide removal process S230, the gas may be heated by a heating unit 406 to a temperature suitable for a methane synthesis reaction before the gas is supplied to the primary adiabatic reactor 407. The temperature may be 250 0 C to 3500C. However, the temperature is not limited thereto. The heating unit 406 may be a heat exchanger. However, the heating unit 406 is not limited to a heat exchanger. For example, any heating unit that can be easily used by those of ordinary skill in the related art may be used. [00351 In the primary methane synthesis process S240, the gas from which moisture 304 and carbon dioxide 305 have been removed is supplied to the primary adiabatic reactor 407. Methane is additionally synthesized in the primary adiabatic reactor 407, and the methane concentration of gas discharged from the primary adiabatic reactor 407 may be about 90 vol% to about 95 vol% based on the total volume of the gas. [00361 The gas discharged from the primary adiabatic reactor 407 is supplied to the secondary adiabatic reactor 408 to increase the yield of methane synthesis. For example, the methane concentration of gas discharged from the secondary adiabatic reactor 408 may be about 97 vol% or greater. [0037) The gas discharged from the secondary adiabatic 4852613_1 (GHMatters) P95408.AU 8/11/13 - 15 reactor 408 may be cooled in a heat exchanger 402', and moisture 304' may be removed from the gas in a condenser 4041. In this way, the gas may be separated as synthetic natural gas (SNG) 306. [00381 Cooling water 308 supplied to the isothermal reactor 401 is heated by the isothermal reactor 401 and discharged back to the cooling water supply tank 409 as saturated steam 309. The saturated steam 309 is supplied to a heat exchanger 403 through a transportation path such as a pipe, and as the saturated steam 309 passes through the heat exchanger 403, the saturated steam 309 is heated by gas discharged from the primary adiabatic reactor 407 and is turned into supersaturated steam 307. In the embodiment of the present disclosure, the method for producing synthetic natural gas may further include generating electricity by supplying the supersaturated steam 307 to a steam turbine. [0039] According to the embodiment of the present disclosure, the apparatus for producing synthetic natural gas includes: the isothermal reactor 401 to which steam 302 and synthetic gas 301 are supplied to simultaneously cause a methane synthesis reaction and a water-gas shift reaction, the synthetic gas 301 being produced through a coal gasification reaction and containing carbon monoxide, carbon dioxide, and hydrogen; the condenser 404 configured to remove moisture 304 from gas discharged from the isothermal reactor 401; the carbon dioxide removing device 405 configured to remove carbon dioxide 305 from the gas 4852613_1 (GHMatters) P95408.AU 8/11/13 - 16 discharged from the isothermal reactor 401; the primary adiabatic reactor 407 to which the gas from which moisture 304 and carbon dioxide 305 have been removed is supplied to cause a methane synthesis reaction; and the secondary adiabatic reactor 408 to which gas discharged from the primary adiabatic reactor 407 is supplied to cause a methane synthesis reaction. [0040] Before the synthetic gas 301 produced through a coal gasification reaction and containing carbon monoxide, carbon dioxide, and hydrogen is supplied to the isothermal reactor 401, the synthetic gas 301 may be allowed to pass through a heat exchanger, a dust collecting device, and a desulfurizing device (not shown in FIG. 4) to cool the synthetic gas 301 and remove dust and sulfur from the synthetic gas 301. [0041] The synthetic gas 301 produced through a coal gasification reaction and containing carbon monoxide, carbon dioxide, and hydrogen is mixed with steam 302 and is then supplied to the isothermal reactor 401. The isothermal reactor 401 may be a shell-tube or annular reactor. However, the isothermal reactor 401 is not limited thereto. For example, any isothermal reactor that can be easily operated by those of ordinary skill in the related art may be used as the isothermal reactor 401. The isothermal reactor 401 may contain a Ni-containing catalyst to increase the yield of methane synthesis. However, the isothermal reactor 401 is not limited thereto. For example, any 4852613_1 (GHMutters) F95408.AU 8/11/13 - 17 methane synthesis catalyst that can be easily used by those of ordinary skill in the related art may be used. [00421 The apparatus for producing synthetic natural gas may further include the cooling water supply tank 409 to supply cooling water 308 to the isothermal reactor 401 and collect saturated steam 309 generated by the reaction heat in the isothermal reactor 401. Cooling water 308 is supplied from the cooling water supply tank 409 to the isothermal reactor 401 to cool the isothermal reactor 401. If the isothermal reactor 401 is not cooled with cooling water 308, the temperature of the isothermal reactor 401 may be about 7000C. However, if the isothermal reactor 401 is cooled with cooling water 308, the temperature of the isothermal reactor 401 may range from 2700C to 370 0 C. Since the reaction temperature of the isothermal reactor 401 is lower than the reaction temperature of an adiabatic reactor by about 300 0 C or more, the isothermal reactor 401 is effective in increasing the yield of methane synthesis. [00431 Moisture 304 and carbon dioxide 305 are removed from gas discharged from the isothermal reactor 401 in a moisture and carbon dioxide removal process S230. As a result of the methane synthesis reaction and the water-gas shift reaction, the gas discharged from the isothermal reactor 401 contains methane, moisture 304, and carbon dioxide 305. Therefore, after the moisture 304 and the carbon dioxide 305 have been removed from the gas, the gas may be supplied to a methane synthesis device to increase the yield of methane synthesis. 48526131 (GHMatters) F95408.AU 8/11/13 - 18 [0044] To this end, the gas discharged from the isothermal reactor 401 may be allowed to pass through the heat exchanger 402 to cool the gas to a temperature of 400C to 1000C, and then the gas may be allowed to pass through the condenser 404 to condense and remove moisture 304. After moisture 304 is removed from the gas, the gas may be allowed to pass through the carbon dioxide removing device 405 to remove carbon dioxide 305 from the gas. [0045] The efficiency of removing moisture 304 and carbon dioxide 305 in the moisture and carbon dioxide removal process S230 is not limited. For example, about 80% to 98% of moisture 304 and carbon dioxide 305 contained in the gas discharged from the isothermal reactor 401 may be removed. [0046] When the gas is supplied to the primary adiabatic reactor 407 after moisture 304 and carbon dioxide 305 have been removed from the gas, the temperature of the gas may be within the range of 250 0 C to 350 0 C. The temperature range includes a reaction start temperature of methane synthesis. The gas cooled in the moisture and carbon dioxide removal process S230 may be heated using the heating unit 406 to adjust the temperature of the gas to the temperature range. The heating unit 406 may be a heat exchanger. However, the heating unit 406 is not limited to being a heat exchanger. For example, any heating unit 406 that can be easily used by those of ordinary skill in the related art may be used. [0047] The gas heated by the heating unit 406 is supplied to 48526131 (GHMauers) P95408.AU 8/11/13 - 19 the primary adiabatic reactor 407 to additionally synthesize methane. The methane concentration of gas discharged from the primary adiabatic reactor 407 may be 90 vol% to 95 vol% based on the total volume of the gas. The gas discharged from the primary adiabatic reactor 407 is supplied to the secondary adiabatic reactor 408 to increase the yield of methane synthesis. For example, the methane concentration of gas discharged from the secondary adiabatic reactor 408 may be about 97 vol% or greater. The apparatus for producing synthetic natural gas may further include the condenser 404' to remove moisture 304' from the gas discharged from the secondary adiabatic reactor 408, and thus synthetic natural gas (SNG) 306 having a higher methane concentration may be obtained. [0048] Cooling water 308 supplied to the isothermal reactor 401 is heated by the isothermal reactor 401 and discharged back to the cooling water supply tank 409 as saturated steam 309. The apparatus for producing synthetic natural gas may further include the heat exchanger 403 in which the saturated steam 309 generated by the reaction heat in the isothermal reactor 401 exchanges heat with the gas discharged from the primary adiabatic reactor 407. The saturated steam 309 may be supplied to the heat exchanger 403 through a transportation path such as a pipe, and as the saturated steam 309 passes through the heat exchanger 403, the saturated steam 309 may be heated by the gas discharged from the primary adiabatic reactor 407 and may be turned 48526131 (GHMatters) P95408.AU 8/11/13 - 20 into supersaturated steam 307. The supersaturated steam 307 may be supplied to a steam turbine to generate electricity. [0049] As set forth above, according to the method and apparatus for producing synthetic natural gas of the exemplary embodiments of the present disclosure, the yield of methane synthesis can be increased, while economical and efficient processes can be performed because a recirculation process and a water-gas shift process are not additionally performed. [00501 While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the spirit and scope of the present disclosure as defined by the appended claims. [00511 In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. [0052] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any 48526131 (GHMattuers) P95408.AU 8/11/13 - 21 other country. 48526131 (GHMatters) P95408.AU 8/11/13

Claims

1. A method for producing synthetic natural gas, the method comprising: performing a synthetic gas preparation process to prepare synthetic gas comprising carbon monoxide, carbon dioxide, and hydrogen by reacting coal or biomass with oxygen, hydrogen, or a mixture gas thereof; performing a desulfurization process to collect heat from the synthetic gas and remove impurities and hydrogen sulfide from the synthetic gas; performing a methane synthesis and water-gas shift process to simultaneously cause a methane synthesis reaction and a water-gas shift reaction by supplying the synthetic gas to an isothermal reactor together with steam after the desulfurization process; performing a moisture and carbon dioxide removal process to remove moisture and carbon dioxide from gas discharged from the isothermal reactor by cooling the gas; performing a primary methane synthesis process to synthesize methane by supplying the gas to a primary adiabatic reactor after moisture and carbon dioxide are removed from the gas; and performing a secondary methane synthesis process to synthesize methane by supplying gas discharged from the primary adiabatic reactor to a secondary adiabatic reactor; with the proviso that the method excludes gas recirculation into the isothermal reactor. 74651541 (GHMatters) P95408.AU 29/02/16 - 23

2. The method of claim 1, wherein the synthetic gas has an H 2 /CO ratio in a range of 0.5 to 1.5.

3. The method of claim 1 or 2, wherein in the methane synthesis and water-gas shift process, the synthetic gas and steam are supplied to the isothermal reactor at a volumetric ratio of 1:0.4 to 1.5.

4. The method of any one of claims 1 to 3, further comprising generating electricity by supplying supersaturated steam to a steam turbine, wherein the supersaturated steam is produced by saturating steam with reaction heat in the isothermal reactor and allowing the saturated steam to absorb heat from the gas discharged from the primary adiabatic reactor.

5. The method of any one of claims 1 to 4, wherein the gas supplied to the primary adiabatic reactor after moisture and carbon dioxide are removed from the gas has a temperature in a range of 250 0 C to 350 0 C.

6. An apparatus for producing synthetic natural gas, the apparatus comprising: an isothermal reactor to which steam and synthetic gas are supplied to simultaneously cause a methane synthesis reaction and a water-gas shift reaction, the synthetic gas being produced through a coal gasification reaction and comprising carbon monoxide, carbon dioxide, and hydrogen; a condenser configured to remove moisture from gas discharged from the isothermal reactor; 74651541 (GHMatters) P95408.AU 29/02/16 - 24 a carbon dioxide removing device configured to remove carbon dioxide from the gas discharged from the isothermal reactor; a primary adiabatic reactor to which the gas from which moisture and carbon dioxide are removed is supplied to cause a methane synthesis reaction; and a secondary adiabatic reactor to which gas discharged from the primary adiabatic reactor is supplied to cause a methane synthesis reaction; with the proviso that the apparatus excludes gas recirculation into the isothermal reactor.

7. The apparatus of claim 6, wherein the gas supplied to the primary adiabatic reactor after moisture and carbon dioxide are removed from the gas has a temperature in a range of 250 0 C to 350 0 C.

8. The apparatus of claim 6 or 7, further comprising a cooling water supply tank configured to supply cooling water to the isothermal reactor and collect saturated steam generated by reaction heat in the isothermal reactor.

9. The apparatus of any one of claims 6 to 8, further comprising a heat exchanger in which steam saturated with reaction heat of the isothermal reactor is allowed to exchange heat with the gas discharged from the primary adiabatic reactor.

10. The apparatus of any one of claims 6 to 9, further comprising a condenser configured to remove moisture from 74651541 (GHMatters) P95408.AU 29/02/16 - 25 gas discharged from the secondary adiabatic reactor.

11. Synthetic natural gas produced by the method of any one of claims 1 to 5. 74651541 (GHMatters) P95408.AU 29/02/16