APPARATUS AND METHOD FOR PRODUCING SYNTHETIC NATURAL GAS USING LOW H 2 /CO RATIO SYNTHESIS GAS CROSS-REFERENCE TO RELATED APPLICATION 5 [0001] This application claims the benefit of Korean Patent Application No. 10-2012-0153905 filed on Dec 26th, 2012, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 10 BACKGROUND [0002] The present disclosure relates to an apparatus and method for producing synthetic natural gas (SNG) using synthesis gas having a low H 2 /CO concentration ratio. [0003] Since coal is abundant throughout the world as compared 15 with other fuel resources, a large amount of research has been conducted globally to convert coal into SNG, a relatively clean fuel. [0004] Synthesis gas mainly containing carbon monoxide and hydrogen may be obtained from raw materials such as coal or a 20 biomass containing a large amount of carbon through a gasification process, a dust collection process, a purification process, or a C02 removal process. Then, SNG may be obtained by performing a methane synthesis process on such synthesis gas. [0005] Methods of producing SNG from coal include a direct 25 method in which methane is directly converted from coal by Page 1 reacting hydrogen or high-temperature steam with coal, and an indirect method in which synthesis gas is produced through a reaction between coal and oxygen and is converted into SNG through a methane synthesis process. 5 [0006] FIG. 1 is a schematic view illustrating a typical indirect method in the related art. SNG 12 may be produced from coal 1 (a raw material) by processing the coal 1 through processes such as a gasification process 4, a dust collection process 6, a water-gas shift process 7 in which the 10 concentration ratio of hydrogen and carbon monoxide is controlled, an acid gas (sulfur compounds) and carbon dioxide removal process 8, and a methane synthesis process. [Related Art Document] (Patent Document 1) Korean Patent Laid-open Publication No. 15 10-2004-0015790 (Patent Document 2) U.S. Patent Publication No. 2009/0173081 (Patent Document 3) U.S. Patent Publication No. 2009/0264542 [0007] Referring to FIG. 1, in the gasification process 4, coal 1 is supplied together with oxygen 2, and organic substances 20 contained in the coal are converted into synthesis gas while inorganic substances such as ashes and slag 3 are continuously removed. After the synthesis gas is processed through a heat collection process 5 and the dust collection process 6, the concentration ratio of H 2 :CO in the synthesis gas is adjusted 25 to be 3:1 in the water-gas shift process 7, and sulfur compounds Page 2 and carbon dioxide 10 are removed from the synthesis gas in the acid gas and carbon dioxide removal process 8. The synthesis gas of which the hydrogen and carbon monoxide concentration is 99% or greater is converted into methane in a methane synthesis 5 process 9, and the methane is processed through a drying and compression process 11 to obtain SNG 12. [0008] In most SNG processes of the related art, the concentration ratio of H2/CO is adjusted to be 3.0 or greater through a water-gas shift process, and most C02 is removed 10 together with sulfur compounds through an acid gas treatment process. Thereafter, synthesis gas is supplied to respective methane synthesis reactors to control the reaction temperatures of the methane synthesis reactors, or a certain amount of gas discharged from first or second methane synthesis reactors is 15 compressed and recirculated to control the reaction temperatures of the first or second methane synthesis reactors. In this manner, deactivation of a catalyst is suppressed. [0009] However, if the concentration ratio of H 2 /CO is not controlled or the temperature of a reactor is maintained at a 20 temperature equal to or greater than about 650'C to 7000C, a nickel-containing catalyst may be deactivated in an SNG process generally having a process pressure of 20 atm, and thus the high concentration SNG may not be produced. [0010] In addition, since costs of compressors used for 25 recirculation in many processes account for several tens of Page 3 percent of total equipment costs in SNG synthesis processes, an improved SNG synthesis method not using compressors is required. [0010a] Throughout this specification the word "comprise", or 5 variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 10 [0010b] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present 15 disclosure as it existed before the priority date of each claim of this application. SUMARY [0011] Aspects of the present disclosure provide an apparatus 20 and method for producing synthetic natural gas (SNG) by supplying a synthesis gas having a low H 2 /CO concentration ratio to a reactor together with steam for controlling the interior temperature of the reactor and increasing the concentration of the SNG without a recirculation process owing to a water-gas 25 shift reaction and a methane synthesis reaction simultaneously Page 4 occurring in the reactor. [0012] According to an aspect of the present disclosure, an apparatus for producing SNG includes: a primary SNG synthesis device including a methane synthesis catalyst, the primary SNG 5 synthesis device being configured to receive steam and synthesis gas containing H 2 , CO, and C02 and discharge methane-containing gas produced through a methane synthesis reaction; and a secondary SNG synthesis device configured to receive the methane-containing gas and discharge SNG produced 10 through a methane synthesis reaction, wherein the synthesis gas has a H 2 /CO concentration ratio of 0.6 to 0.95. [0013] The primary SNG synthesis device may include two to four reactors. [0014] The steam may be supplied to the respective reactors 15 of the primary SNG synthesis device. [0015] The apparatus may further include a heat exchanger configured to exchange heat with the methane-containing gas discharged from the primary SNG synthesis device so as to generate steam. 20 [0016] According to another aspect of the present disclosure, a method for producing SNG includes: performing a primary SNG synthesis process in which synthesis gas containing H 2 , CO, and C02 is supplied together with steam to simultaneously cause a water-gas shift reaction and a methane synthesis reaction and 25 discharging a methane-containing gas; and performing a Page 5 secondary SNG synthesis process in which the methane-containing gas is supplied for producing SNG through a methane synthesis reaction and discharging the SNG, wherein the synthesis gas supplied in the primary SNG synthesis process has a H 2 /CO 5 concentration ratio of 0.6 to 0.95, and wherein a volumetric ratio of steam and carbon monoxide supplied in the primary SNG synthesis process is in a range of 2 to 5. [0017] The primary SNG synthesis process may be performed two to four times. 10 [0018] [0019] [0020] The primary SNG synthesis process may be performed at a reaction temperature of 3000C to 680 0 C. [0021] The secondary SNG synthesis process may be performed at a reaction temperature of 250'C to 350*C. 15 BRIEF DESCRIPTION OF DRAWINGS [0022] 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 20 conjunction with the accompanying drawings, in which: Page 6 FIG. 1 is a schematic flowchart illustrating a typical indirect method of the related art; FIG. 2 is a schematic view illustrating an apparatus for producing synthetic natural gas (SNG) according to an exemplary 5 embodiment of the present disclosure. FIG. 3 is a schematic flowchart for explaining a method for producing SNG according to an exemplary embodiment of the present disclosure; FIG. 4 is a graph schematically showing temperature 10 variations in a primary SNG synthesis device according to the volumetric ratio of steam and carbon monoxide; FIG. 5 is a graph schematically showing the conversion percentage of carbon monoxide in the primary SNG synthesis device in Example 3; and 15 FIG. 6 is a graph schematically showing the conversion Page 6a percentage of carbon monoxide in the primary SNG synthesis device in Example 4. DETAILED DESCRIPTION 5 [0023] 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 forth herein. 10 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. [00241 FIG. 2 is a schematic view illustrating an apparatus for producing synthetic natural gas (SNG) according to an 15 exemplary embodiment of the present disclosure, and FIG. 3 is a schematic view illustrating a method for producing SNG according to an exemplary embodiment of the present disclosure. Embodiments of the present disclosure will now be described with reference to FIGS. 2 and 3. 20 [0025] The apparatus of the exemplary embodiment of the present disclosure is provided to produce SNG using synthesis gas 10 having a low H 2 /CO concentration ratio. In detail, the apparatus includes: a primary SNG synthesis device including a methane synthesis catalyst, the primary SNG synthesis device 25 being configured to receive synthesis gas 10 containing H 2 , CO, Page 7 and CO 2 together with steam 20 and discharge methane-containing gas produced through a methane synthesis reaction; and a secondary SNG synthesis device 180 configured to receive the methane-containing gas and discharge SNG 80 produced through 5 a methane synthesis reaction, wherein the synthesis gas 10 has a H 2 /CO concentration ratio of 0.6 to 0.95. [0026] The method of the exemplary embodiment of the present disclosure is provided to produce SNG using synthesis gas 10 having a low H 2 /CO concentration ratio. In detail, the method 10 includes: performing a primary SNG synthesis process in which synthesis gas 10 containing H 2 , CO, and CO 2 is supplied together with steam 20 to simultaneously cause a water-gas shift reaction and a methane synthesis reaction and discharging a methane-containing gas; and performing a secondary SNG 15 synthesis process in which the methane-containing gas is supplied for producing SNG 80 through a methane synthesis reaction and discharging the SNG 80, wherein the synthesis gas 10 supplied in the primary SNG synthesis process has a H 2 /CO concentration ratio of 0.6 to 0.95. 20 [0027] The apparatus may include the primary SNG synthesis device to receive synthesis gas 10 and synthesize methane. The synthesis gas 10 may be produced through a coal gasification process and may have a low H 2 /CO concentration ratio because the synthesis gas 10 is not subjected to a water-gas shift 25 process. For example, the H 2 /CO concentration ratio of the Page 8 synthesis gas 10 may be in the range of 0.6 to 0.95. However, the embodiment is not limited thereto. Therefore, if the synthesis gas 10 having a low H 2 /CO concentration ratio is supplied to the primary SNG synthesis device, a water-gas shift 5 reaction and a methane synthesis reaction may occur simultaneously as a primary SNG synthesis process. Owing to the water-gas shift reaction and the methane synthesis reaction, methane-containing gas may be discharged from the primary SNG synthesis device. 10 [0028] In addition, the volumetric ratio of steam 20 and carbon monoxide supplied to the primary SNG synthesis device in the primary SNG synthesis process may be within the range of 2 to 5. If the volumetric ratio of steam 20 and carbon monoxide is less than 2, coking may occur together with a water-gas shift 15 reaction and a methane synthesis reaction, which may make the methane synthesis catalyst inactive and increase the pressure of a reactor, to decrease the yield of methane synthesis. If the volumetric ratio of steam 20 and carbon monoxide is greater than 5, due to excessive steam 20, the competitiveness of the 20 primary SNG synthesis process may be lowered in terms of economical aspects thereof. [0029] The primary SNG synthesis device may include a plurality of SNG synthesis reactors connected in series. The number of reactors is not limited. For example, the number of the 25 reactors may range from two to four. An adiabatic reactor may Page 9 be additionally connected to increase the selectivity and yield of a final methane product. [0030] Steam 20 may be supplied to the primary SNG synthesis device together with synthesis gas 10. In this case, steam 20 5 may be supplied to each of the reactors of the primary SNG synthesis device. In the related art, methane-containing gas discharged from the reactors is recirculated to increase the yield of methane synthesis and control a reaction temperature. However, recirculation equipment is relatively expansive. 10 According to the embodiment of the present disclosure, the temperatures of the reactors are controlled by supplying steam 20 to each of the reactors, and thus costs necessary for recirculation equipment may be saved to improve economical aspects. Steam 20 may be generated through a heat exchange 15 process using methane-containing gas discharged from the reactors of the primary SNG synthesis device. For this, in the current embodiment, the SNG producing apparatus may further include a heat exchange 140 configured to exchange heat with methane-containing gas discharged from the reactors of the 20 primary SNG synthesis device. [0031] The reaction temperature of the primary SNG synthesis device, that is, the reaction temperature of the primary SNG synthesisprocessrmaybe adjusted tobe within the range of 300'C to 680*C by supplying steam 20. However, the current embodiment 25 is not limited thereto. As shown in FIG. 2, if the primary SNG Page 10 synthesis device of the SNG producing apparatus includes first to three reactors 110, 120, and 130 (three reactors), the reaction temperatures of the first to three reactors may be adjusted to be within in the ranges of 3000C to 6800C, 3000C 5 to 510'C, and 3000C to 3650C, respectively. However, the current embodiment is not limited thereto. If the reaction temperatures of the reactors of the primary SNG synthesis device are outside of the above ranges, the methane synthesis catalyst may undergo coking or sintering to cause deactivation of the 10 catalyst and a reverse water-gas shift reaction, and as a result, the conversion percentage and yield of methane synthesis may be reduced. [0032) Methane-containing gas discharged from the primary SNG synthesis device may be treated to remove water 90 and carbon 15 dioxide 100 therefrom, before the methane-containing gas is supplied to the secondary SNG synthesis device 180, so as to increase the yield in methane synthesis. Then, the methane-containing gas may be compressed to a pressure suitable for being supplied through a natural gas pipe. For this, the 20 SNG producing apparatus of the current embodiment may further include a condenser 150, a carbon dioxide removal device 160, and a compressor 170. The compressor 170 may compress the methane-containing gas to a pressure of 50 atm to 70 atm, and then the methane-containing gas may be supplied to the secondary 25 SNG synthesis device 180. However, the current embodiment is Page 11 not limited thereto. [0033] The methane-containing gas may be supplied to the secondary SNG synthesis device 180 to perform the secondary SNG synthesis process and thus to produce SNG 80 through a methane 5 synthesis reaction. The methane concentration of the SNG 80 may be 95% or greater. However, the current embodiment is not limited thereto. In addition, a condenser 150 may remove moisture from the SNG 80 discharged from the secondary SNG synthesis device. 10 [0034] Hereinafter, examples of the present disclosure will be described. However, the present disclosure is not limited to examples. Example 1 [0035] SNG was produced using the SNG producing apparatus of 15 the embodiment of the present disclosure. Synthesis gas having an H 2 /CO concentration ratio of 0.93 was supplied at a rate of 4,905 Nm 3 per hour to the first reactor of the primary SNG synthesis device. The amounts and concentrations of materials at each device and reactor are shown in Table 1 by using reference 20 numerals of FIG. 2 to indicate flows of the materials. [Table 1] Page 12 jm/ 1 30 1 40_ 50 60 70 so8 Nm1/h M% Nm 3 /h Mol% Nm 3 /h Mol% Nm 3 /h Mol% NmO/h Mol% NRAh Mo1% Nm 3 /h Mol% 14 243 4.95 244 4.95 657 14.00 982 27.54 1098 34.01 1142 90.47 1042 97.50 Co 1840 37.51 1849 37.51 454 9.67 52 1.44 2 0.07 2 0.17 0 0.00 :O 1090 22.22 1096 22.22 2086 44.45 2069 58,02 2033 63.01 21 1.67 4 0.37 12 1717 35.01 1726 35.02 1481 31.56 448 12.57 79 2.44 82 6.501 8 0,72 a 15 0.32 15 0.30 15 0.32 15 0.42 15 0.47 15 1.19 15 1.40 120 0 4701 4138 4316 4520 1 3 Dry 4905 4930 4693 3567 3227 1263 1069 rotal 4905 100 9631 100 8832 100 7883 100 7747 100 1263 100 1072 100 {0036] As shown in Table 1, the methane concentration of methane-containing gas discharged from the reactors of the 5 primary SNG synthesis device was 14.00 Mol%, 27.54 Mol%, and 34.1 Mol%, respectively, and after removing water and carbon dioxide from the methane-containing gas and compressing the methane-containing gas, the methane concentration of the methane-containing gas was increased to 90.47 Mol%. The 10 methane concentration of final SNG produced by the secondary SNG synthesis device and dewatered was 97.50 Mol%. That is, SNG having a methane concentration of 95% or greater may be produced using the SNG producing apparatus of the embodiment of the present disclosure. 15 Example 2 [0037] Synthesis gas (H 2 /CO concentration ratio = 0.93) including 2 g of nickel-containing catalyst (by Sud-Chemie) and having a carbon dioxide concentration of 22% was used to produce Page 13 SNG at 20 atm and 3600C. The synthesis gas was supplied to a reactor while decreasing the volumetric ratio of steam and carbon monoxide in the synthesis gas in the order of 4.0, 2.5, and 2.0, and the interior temperature of the reactor was 5 measured. The measured temperature values are shown in FIG. 4. [0038] As shown in FIG. 4, when the volumetric ratio of steam and carbon monoxide of the synthesis gas was 2.0, the interior temperature of the reactor was maintained at 360 0 C. In the 10 condition, a water-gas shift reaction and a methane synthesis reaction occurred, and the interior temperature of the reactor was maintained at 360'C owing to reaction heat. However, since the volumetric ratio of steam and carbon monoxide was less than 2.0, during the water-gas shift reaction and the methane 15 synthesis reaction, the catalyst was deactivated because of coking, and as a result, the interior pressure of the reactor was increased. Therefore, it may preferable that the volumetric ratio of steam and carbon monoxide be 2. 0 or greater. In addition, if the H 2 /CO concentration ratio of the synthesis 20 gas is greater than 0.93, the ratio of steam and carbon monoxide may be decreased. In FIG. 4, TC-1 to TC-5 refer to temperatures measured from upper catalyst layers to lower catalyst layers in the reactor, and the intervals of the measurements were 20 mm. 25 Example 3 Page 14 [0039] SNG was produced in the same conditions as those of Example 2 except that the volumetric ratio of steam and carbon monoxide was 2.5 and the space velocity was maintained at 7,000 ml/gcat-h. The percentages of conversion of carbon monoxide 5 to carbon dioxide and methane in the primary SNG synthesis device were measured as shown in FIG. 5. [0040] As shown in FIG. 5, a water-gas shift reaction and a methane synthesis reaction continued stably for 42 hours. The conversion percentage of carbon monoxide was 100%: 48. 7% of the 10 carbon monoxide was converted into carbon dioxide, and 51.3% of the carbon monoxide was converted into methane. Example 4 [0041] SNG was produced in the same conditions as those of 15 Example 2 except for the following conditions: 1 g of nickel-containing catalyst (by Sud-Chemie) was used; the volumetric ratio of steam and carbon monoxide was 2.5; and the space velocity was maintained at 32,500 ml/gcat-h. The percentages of conversion of carbon monoxide to carbon dioxide 20 and methane in the primary SNG synthesis device were measured as shown in FIG. 6. [0042] As shown in FIG. 6, a water-gas shift reaction and a methane synthesis reaction continued stably for 620 hours. The conversion percentage of carbon monoxide was 96.59%: 48.24% of 25 the carbonmonoxide was converted into carbon dioxide, and 48.35% Page 15 of the carbon monoxide was converted into methane. 100431 As set forth above, according to the exemplary embodiments of the present disclosure, in the apparatus and method for producing SNG, synthesis gas having a low H 2 /CO 5 concentration ratio is supplied in the primary SNG synthesis process to simultaneously cause a water-soluble phenoxy resin and a methane synthesis reaction. Therefore, an additional water-gas shift process may not be performed. In addition, since steam is supplied in the primary SNG synthesis process, 10 a recirculation process for controlling reaction temperature maynotbe performed, and thus SNGmaybe producedwith lowcosts. [0044] While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations can be made without 15 departing from the spirit and scope of the present disclosure as defined by the appended claims. Page 16