AU2009263401A1

AU2009263401A1 - Method and combustion catalyst for purifying carbon dioxide off-gas, and method for producing natural gas

Info

Publication number: AU2009263401A1
Application number: AU2009263401A
Authority: AU
Inventors: Tomoo Ikoma; Toshiyuki Nagai; Takanobu Sakurai; Takashi Sasaki; Takayoshi Yamabi
Original assignee: JGC Corp; Nikki Universal Co Ltd
Current assignee: JGC Corp; Nikki Universal Co Ltd
Priority date: 2008-06-23
Filing date: 2009-06-23
Publication date: 2009-12-30
Also published as: RU2010136325A; WO2009157434A1; JPWO2009157434A1

Description

DESCRIPTION METHOD AND COMBUSTION CATALYST FOR PURIFYING CARBON DIOXIDE OFF-GAS, AND METHOD FOR PRODUCING NATURAL GAS 5 TECHNICAL FIELD [0001] The present invention relates to a method and a combustion catalyst for purifying a gas containing carbon dioxide as a major component (hereafter, referred to as "carbon 10 dioxide off-gas"), and a method for producing natural gas. More specifically, the present invention relates to a purification method and a combustion catalyst for purification, the method in which the sulfur compounds, such as hydrogen sulfide (H 2 S) and mercaptan, and volatile organic compounds (hereafter, simply referred to as "VOC"), such as benzene, toluene and xylene, contained in the carbon dioxide off-gas exhausted from 15 natural gas, petroleum associated gas, or the like (hereafter, collectively abbreviated as "natural gas or the like") are oxidatively degraded by a combustion catalyst, thereby reducing the concentrations of sulfur compounds (excluding sulfur oxides) and VOC in the carbon dioxide off-gas. Priority is claimed on Japanese Patent Application No. 2008-162728, filed June 20 23, 2008, the content of which is incorporated herein by reference. BACKGROUND ART [0002] The VOC is a gas formed of the light components in organic solvents, petroleum 25 products or the like which are released into the atmosphere, and it is said to be the 2 substance responsible for the formation of photochemical smog and suspended particulate matter. For this reason, regulations for emission of VOC have been issued worldwide. In the conventional plants for natural gas or the like, off-gas is generated during 5 the step for purifying natural gas in which carbon dioxide and sulfur compounds contained in the natural gas is removed, and the generated off-gas contains hydrogen sulfide and VOC. VOC combustion catalysts have been available from various manufacturers. However, since impurities such as hydrogen sulfide are generated in the plants for natural gas or the like, technical barriers for the off-gas purification method 10 using a VOC combustion catalyst are high, and thus commercial processes therefor have not been established. Therefore, in the current situation, when releasing the above-mentioned off-gas into the atmosphere, the off-gas is subjected to a combustion treatment (direct firing) using a thermal incinerator under high temperature conditions at about 900*C, and is then 15 released into the atmosphere. However, in the current method for treating off-gas by the direct firing, since the off-gas generated at about normal temperature is heated to about 900*C and is then subjected to a combustion treatment, the treatment cost has been extremely high. [0003] 20 As a method for purifying an exhaust gas containing VOC, a method for purifying an exhaust gas containing organic silicon, VOC, carbon monoxide or the like by catalytic combustion has been disclosed (for example, refer to Patent Document 1). Although organic silicon has conventionally been considered to be a poisoning substance for combustion catalysts, in the method for purifying exhaust gas disclosed in Patent 3 Document 1, by using a noble-metal-supporting zeolite as a catalyst, it has become possible to purify an exhaust gas containing VOC for a long time. In the meantime, in the plants for natural gas or the like, examples of typical poisoning substance for combustion catalysts include sulfur compounds such as hydrogen sulfide. However, in 5 Patent Document 1, no study has been conducted on the resistance of combustion catalysts with respect to the sulfur compounds. [0004] As the combustion catalysts for VOC degradation, the following catalysts have been disclosed (for example, refer to Patent Document 2): (1) catalysts containing a 10 calcium salt, amorphous silica and a copper compound; (2) catalysts containing amorphous silica and a copper compound; (3) catalysts containing at least one of crystalline silica and amorphous silica, as well as a calcium salt and a copper compound; and (4) catalysts containing at least one of crystalline silica and amorphous silica, as well as a copper compound. Although noble metals such as platinum have been 15 conventionally used for the combustion treatment of VOC, by using the aforementioned combustion catalysts (1) to (4), it has become possible to achieve the same level of combustion performance as that when using a noble metal catalyst at a low cost. However, even in Patent Document 2, no study has been conducted on the resistance of combustion catalysts with respect to the sulfur compounds. 20 [0005] As a method for decomposing a low concentration of methane emitted from the plants for natural gas or the like or from the coal mines, a method has been disclosed, in which a mixed gas containing a low concentration of methane is preheated to a temperature so that a catalytic combustion can be conducted, and the low concentration 4 methane in the mixed gas is then decomposed by catalytic combustion (for example, refer to Patent Document 3). In this method, by using decomposed gas and a thermal storage medium for the preheating of mixed gas, the amount of fuel fed at preheating is reduced. Accordingly, methane can be released into the atmosphere as carbon dioxide 5 having a low global warming potential rather than being released as it is, as a result of which the amount of total emission of greenhouse gases can be reduced. In the plants for natural gas or the like, based on the emission regulations (environmental standards), the purification of VOC and sulfur compounds has been conducted. Meanwhile, in Patent Document 3, no study has been conducted on the poisoning of combustion catalysts by 10 the sulfur compounds included in the mixed gas which contains a low concentration of methane or on the decomposability of sulfur compounds and VOC. [0006] As a method for removing sulfur from a gas containing hydrogen sulfide, benzene, toluene, xylene or the like, a method has been disclosed in which at least one Claus 15 catalyst provided with a capability to hydrolyze the carbonyl sulfide (COS) and carbon disulfide (CS 2 ) originating from hydrocarbons and carbon dioxide is installed in a reactor in order to reduce the level of sulfur compounds leaking from the sulfur recovery unit (for example, refer to Patent Document 4). In this method, as a Claus catalyst, those having at least one material selected from alumina, titanium oxide and zirconia as a 20 support and at least one material selected from iron, cobalt, nickel, copper and vanadium as an active metal are used. However, in Patent Document 4, the Claus catalysts used in the sulfur recovery units have been the subjects of the invention, and no study has been conducted regarding the decomposability of sulfur compounds and VOC in the gas containing VOC and hydrogen sulfide and having carbon dioxide as a major component.

5 [0007] With respect to the use of an apparatus for acid gas removal (AGR) in order to remove the carbon dioxide in natural gas, a method has been disclosed in which the coabsorbed VOC is selectively evaporated and removed in a low pressure flash drum 5 installed in front of the stripping tower (regenerator) while controlling the temperature and pressure so that the acid gas does not evaporate (for example, refer to Patent Document 5). Because the VOC removed by this method is hypergolic fuel, it is either released into the atmosphere by direct combustion treatment or used effectively as fuel. Moreover, in this method, an amine absorption liquid from which the majority portions 10 of VOC have been removed and containing carbon dioxide and sulfur compounds is separated into a gas containing a trace amount of VOC, carbon dioxide and sulfur compounds, and an amine absorption liquid in the stripping tower. The obtained gas is either subjected to a direct combustion treatment or treated in a sulfur recovery unit in the later stage. When installing a sulfur recovery unit in the later stage, if carbon dioxide and 15 traces of VOC are directly fed to this sulfur recovery unit, carbonyl sulfide (COS) or carbon disulfide is produced, thereby causing the Claus catalyst to deteriorate. When a gas containing traces of VOC or hydrogen sulfide and having carbon dioxide as a major component which is emitted from the upper portion of an absorber is released into the atmosphere by installing a hydrogen sulfide enrichment unit in a stage prior to that of the 20 sulfur recovery unit, the gas having carbon dioxide as a major component needs to be subjected to a direct combustion treatment under the high temperature condition of about 900*C. Even when a sulfur recovery unit is not installed in the later stage, the gas needs to be subjected to a direct combustion treatment as described above. Therefore, the method disclosed in Patent Document 5 involves a direct combustion treatment for the 6 purification of VOC and sulfur compounds contained in the above-mentioned gas having carbon dioxide as a major component, and thus the method required a large amount of fuel consumption and extremely high treatment cost. [Prior-art Documents] 5 [Patent Document] [0008] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2003-290626 [Patent Document] 10 [0009] [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2003-126696 [Patent Document] [0010] 15 [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2006-312143 [Patent Document] [0011] [Patent Document 4] United States Patent Application, Publication No. 20 2004/0033192 [Patent Document] [0012] [Patent Document 5] United States Patent No. 6605138 7 DISCLOSURE OF INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0013] 5 In the past, before being released into the atmosphere, the carbon dioxide off-gas emitted from the natural gas purification process in the plants for natural gas or the like which contains sulfur compounds such as hydrogen sulfide, mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide, VOC or carbon monoxide, and having carbon dioxide as a major component was required to be purified by a thermal decomposition 10 process. For this reason, the gas having a temperature of about 40'C at the time of emission needs to be heated to about 900*C and to be subjected to a direct combustion treatment, and thus a high treatment cost and large amount of carbon dioxide emission associated with the large consumption of fuel have been a problem. Since a direct combustion system involves a high temperature processing, it is 15 necessary to use heat resistant materials for the combustor and heat exchanger therein, and thus an equipment cost has been a problem. In addition, in the direct combustion system, the production of thermal NOx generated by a treatment involving flames has also been a problem. Moreover, in a direct combustion system involving a heat recovery process, the 20 heat recovery is carried out by circulating flue gas not vertically but horizontally, and thus equipment requires a large occupying area which posed a problem for small plants and plants of floating production storage and offloading in terms of the plot arrangement. Furthermore, although VOC combustion catalysts are available from various manufacturers, since there is no catalyst available which is resistant to sulfur compounds 8 and is also capable of retaining high purification efficiency for VOC and sulfur compounds for a long time, no relevant process has been commercially established in the plants for natural gas or the like. [0014] 5 Objects of the present invention include the following. i) Reduction of the concentrations of sulfur compounds and VOC at lower temperatures as compared to the prior art by conducting an oxidative degradation on the carbon dioxide off-gas which is emitted from natural gas or the like and containing sulfur compounds and VOC using a combustion catalyst. 10 ii) Long term retention of high purification efficiency without the generation of thermal NOx. iii) Reduction of the amount of carbon dioxide emission associated with the large consumption of fuel, and lowering of the treatment cost. 15 MEANS FOR SOLVING THE PROBLEMS [0015] In a method for purifying carbon dioxide off-gas according to the present invention, a gas containing at least VOC and sulfur compounds of not less than 50 parts per million volume (ppmV) and not more than 10,000 ppmV, and having carbon dioxide 20 as a major component is fed to a catalytic combustor, and the VOC and sulfur compounds contained in the aforementioned gas having carbon dioxide as a major component are oxidatively degraded by conducting an oxidative degradation in the catalytic combustor using a combustion catalyst. The above combustion catalyst contains at least one metal oxide selected from the group consisting of zirconium oxide, 9 titanium oxide and silicon oxide, and at least one noble metal selected from the group consisting of platinum, palladium and iridium. Note that the concentration of sulfur compounds (excluding sulfur oxides) in the carbon dioxide off-gas following the oxidative degradation is 5 ppmV or less. 5 [0016] In the present invention, the following aspects can be selected and combined appropriately. The aforementioned gas having carbon dioxide as a major component may be a gas emitted during the purification of natural gas. 10 It is preferable that the reaction temperature for oxidative degradation process using the aforementioned combustion catalyst be at least 250*C and not more than 6504C. In addition, it is preferable that the pressure for oxidative degradation process using the aforementioned combustion catalyst be at least 0.01 MPa and not more than I MPa. 15 Moreover, it is preferable to breakdown the VOC contained in the aforementioned gas having carbon dioxide as a major component into carbon dioxide by an oxidative degradation process. Furthermore, the above VOC preferably contains at least one substance selected from benzene, toluene, and xylene. 20 [0017] By oxidatively degrading at least one substance selected from benzene, toluene, and xylene contained in the aforementioned gas having carbon dioxide as a major component, it is preferable to reduce the concentration of benzene to 10 ppmV or less, the concentration of toluene to 50 ppmV or less, and/or the concentration of xylene to 50 10 ppmV or less, which is contained in the aforementioned gas having carbon dioxide as a major component. [0018] In addition, it is preferable to preheat the aforementioned gas having carbon 5 dioxide as a major component and/or air and then to feed it to the aforementioned catalytic combustor. Moreover, it is preferable to partially or entirely carry out the above preheating process using at least one device selected from a combustion furnace, an electric line heater, a thermal storage medium and a heat exchanger exploiting the heat exchanged 10 with the gas having carbon dioxide as a major component following the oxidative degradation by the combustion catalyst. When exceeding the aforementioned upper limit of the reaction temperature due to the heat generated by the oxidative degradation process, it is preferable that the catalytic combustor have at least two catalytic combustion zones in which a combustion 15 catalyst is installed, and at least one material selected from a gas obtained following the oxidative degradation process having carbon dioxide as a major component, air and water, be fed between the catalytic combustion zones so as to cool the gas having carbon dioxide as a major component which is fed to the catalytic combustor. [0019] 20 It is also preferable to remove the mercury species contained in the above gas having carbon dioxide as a major component, and then to introduce the gas having carbon dioxide as a major component from which the mercury species are removed to the catalytic combustor.

11 The aforementioned gas having carbon dioxide as a major component is preferably a gas emitted from an acid gas removal apparatus, which brings an acid gas in the natural gas produced from a gas field into contact with a liquid solvent and thereby separates and recovers the acid gas. 5 Further, the aforementioned gas having carbon dioxide as a major component is preferably a gas that is emitted after the level of hydrogen sulfide therein is reduced by any one of the apparatuses among a hydrogen sulfide enrichment unit, sulfur recovery unit and tail gas treatment unit which is provided in a step which follows the acid gas removal apparatus. 10 [0020] The combustion catalyst with which the aforementioned catalytic combustor is filled is preferably provided with a base and a catalyst layer which is formed on the surface of the base and is composed of the aforementioned metal oxide and the aforementioned noble metal. 15 The base preferably has a structure of honeycomb, pellet or spherical. The base is preferably composed of a ceramic, a metal oxide or a metal alloy. The thickness of the catalyst layer is preferably not less than 10 .im and not more than 500 pm. The content of the noble metal is preferably, per unit catalyst-filled-volume, not 20 less than 0.1 g/L and not more than 10 g/L. The specific surface area of the metal oxide is preferably not less than 10 m 2 /g and not more than 300 m 2 /g.

12 When conducting an oxidative degradation process using the aforementioned combustion catalyst, the oxygen concentration in the gas having carbon dioxide as a major component is preferably not less than I volume% and not more than 15 volume%. [0021] 5 The combustion catalyst for the purification of carbon dioxide off-gas according to the present invention is a combustion catalyst for oxidatively degrading at least the VOC and sulfur compounds contained in the gas having carbon dioxide as a major component at a reaction temperature of not less than 250'C and not more than 650"C. The above combustion catalyst contains at least one metal oxide selected from the group 10 consisting of zirconium oxide, titanium oxide and silicon oxide, and at least one noble metal selected from the group consisting of platinum, palladium and iridium. [0022] It is preferable that the combustion catalyst be provided with a base and a catalyst layer which is formed on the surface of the base and is composed of the aforementioned 15 metal oxide and the aforementioned noble metal, and the base have a structure of honeycomb, pellet or spherical. It is preferable that the metal oxide be titanium oxide and the content of the noble metal be preferably, per unit catalyst-filled-volume, not less than 0.1 g/L and not more than 10 g/L. 20 When conducting an oxidative degradation process using the aforementioned combustion catalyst, the oxygen concentration in the gas having carbon dioxide as a major component is preferably not less than I volume% and not more than 15 volume%. [0023] 13 The combustion catalyst includes a base having a honeycomb structure provided with multiple air passages, a metal oxide layer formed on the inner surface of the air passages and composed of the metal oxide, and the noble metal deposited at least on the surface layer portion of the metal oxide layer at a density of not less than 0.1 mg/cm 2 and 5 not more than 10 mg/cm2 , and the base may be formed of a ceramic, a metal oxide or a metal alloy. [0024] The catalytic combustor includes a vessel having an inlet at one end and an outlet on the other, and a plurality of combustion catalyst units installed inside the vessel 10 between the inlet and the outlet with a certain interval between each other. Each of the combustion catalyst unit includes a base having a honeycomb structure provided with multiple air passages for passing the carbon dioxide off-gas through, a metal oxide layer formed on the inner surface of the air passages and composed of the metal oxide, and the noble metal deposited at least on the surface layer portion of the metal oxide layer at a 22 15 density of not less than 0.1 mg/cm and not more than 10 mg/cm 2 , and the base is formed of a ceramic, a metal oxide or a metal alloy. The base is formed of a ceramic, a metal oxide or a metal alloy, and the inner diameter of the air passage in the combustion catalyst unit close to the outlet may be greater than the inner diameter of the air passage in the combustion catalyst unit close to the inlet. 20 [0025] The method for producing natural gas according to the present invention includes a step for feeding raw natural gas to a slug catcher and thereby separating the raw natural gas into a liquid phase and a vapor phase using the slug catcher; a step for removing acid gas in which the carbon dioxide off-gas that has carbon dioxide as a major component 14 and contains VOC and sulfur compounds is separated from the vapor phase; a step for removing water in which the raw gas following the separation of carbon dioxide off-gas is cooled and the water condensed as a result is removed; a step for removing heavy components in which the raw gas following the separation of carbon dioxide off-gas is 5 subjected to fractional distillation in a distillation tower and thereby removing heavy hydrocarbons; and a step for purifying off-gas in which the carbon dioxide off-gas is treated by any one of the aforementioned methods for purifying carbon dioxide off-gas. EFFECT OF THE INVENTION 10 [0026] According to the method for purifying carbon dioxide off-gas of the present invention, the sulfur compounds contained in the carbon dioxide off-gas which cause highly toxic or irritating odor, such as hydrogen sulfide and mercaptan, can be purified and emitted as SOx. By using at least one metal oxide selected from the group consisting 15 of zirconium oxide, titanium oxide and silicon oxide as a combustion catalyst, the level of deterioration of the combustion catalyst caused by the sulfatization of SOx generated by the oxidative degradation process can be reduced. By using at least one noble metal selected from the group consisting of platinum, palladium and iridium as a combustion catalyst, the oxidation reaction of carbon dioxide off-gas can be proceeded in lower 20 temperatures than those in the conventional cases, since these noble metals exhibit a high level of oxidative activity in the low temperature region. The metal oxides such as zirconium oxide, titanium oxide and silicon oxide are hardly affected by the sulfur compounds with little deposition of sulfur thereon, and are thus stable with little changes in the shape thereof. Therefore, the combustion catalyst can maintain the initial catalyst 15 structure, and the performance for oxidative degradation can be maintained for a long time with little deterioration over time. Moreover, the carbon dioxide off-gas can be treated by oxidative degradation at a low cost without generating thermal NOx and with a low level of carbon dioxide emission. 5 [0027] In addition, by the use of the method for producing natural gas according to the present invention, natural gas can be produced at a low cost while efficiently treating the off-gas. 10 BRIEF DESCRIPTION OF THE DRAWINGS [0028] FIG. 1 is a schematic diagram showing an example of an apparatus for purifying carbon dioxide off-gas used in a first embodiment of a method for purifying carbon dioxide off-gas according to the present invention. 15 FIG. 2 is a schematic diagram showing a first example of a recovery unit for the carbon dioxide off-gas treated by the purification method according to the present invention. FIG. 3 is a schematic diagram showing a second example of a recovery unit for the carbon dioxide off-gas treated by the purification method according to the present 20 invention. FIG. 4 is a schematic diagram showing a third example of a recovery unit for the carbon dioxide off-gas treated by the purification method according to the present invention.

16 FIG. 5 is a schematic diagram showing an example of an apparatus for purifying carbon dioxide off-gas used in a second embodiment of the purification method according to the present invention. FIG. 6 is a schematic diagram showing an apparatus for purifying carbon dioxide 5 off-gas used in the conventional method for purifying carbon dioxide off-gas which employs a direct combustion system. FIG. 7 is a graph showing the relationship between the experimental value and calculated value for the reaction rate of benzene oxidation reaction using the combustion catalyst prepared in Example 2. 10 FIG. 8 is a graph showing the relationship between the reaction temperature and benzene conversion rate using the combustion catalyst prepared in Example 2. FIG. 9 is a schematic diagram showing a testing apparatus used in a performance test for combustion catalysts. FIG. 10 is a graph showing the relationship between the lengths of time lapsed 15 from the start of the reaction and benzene conversion rate using the combustion catalysts prepared in Example 2 and Comparative Examples 2 and 4. FIG. II is a flow diagram showing an embodiment of a method for producing natural gas according to the present invention. FIG. 12 is a longitudinal sectional view showing an example of a catalytic 20 combustor which can be used in the present invention. FIG. 13 is a perspective view showing a combustion catalyst unit used in the above catalytic combustor. BEST MODE FOR CARRYING OUT THE INVENTION 17 [0029] Embodiments of the present invention will be described below. However, unless specifically stated otherwise, the present invention is not limited to the following embodiments. 5 [0030] (1) First embodiment FIG. 1 is a schematic diagram showing an example of an apparatus for purifying carbon dioxide off-gas used in a first embodiment of a method for purifying carbon dioxide off-gas according to the present invention. 10 This apparatus for purifying carbon dioxide off-gas (hereafter, abbreviated as "purification apparatus") 10 includes a heater 11 for heating a gas having carbon dioxide as a major component (carbon dioxide off-gas) to a predetermined reaction temperature; a preheater 12 for preheating the carbon dioxide off-gas and/or air before introducing the gas to the heater 11; a catalytic combustor 13 in which a combustion catalyst for 15 conducting an oxidative degradation process on the carbon dioxide off-gas heated in the heater 11 is installed; and passages 14 to 26 for connecting these devices and making various gases to flow through. [0031] As the heater 11, devices such as a combustion furnace, an electric line heater, a 20 thermal storage medium and a heat exchanger exploiting the heat exchanged with the carbon dioxide off-gas following the oxidative degradation process by the combustion catalyst in the catalytic combustor 13. As the preheater 12, devices such as a combustion furnace, an electric line heater, a thermal storage medium and a heat exchanger exploiting the heat exchanged with the 18 carbon dioxide off-gas following the oxidative degradation process by the combustion catalyst in the catalytic combustor 13. [0032] For the combustion catalyst filled in the catalytic combustor 13, a combustion 5 catalyst having a base and a catalyst layer which is formed on the surface of the base and is composed of a metal oxide and a noble metal is used. An oxymeter (not shown) is provided partway along the passage 25 or passage 26 which is connected to the outlet of the catalytic combustor 13. [0033] 10 As the base, a structure of honeycomb, spherical, pellet or the like can be used. Examples of the material for this base include ceramics which exhibit excellent levels of heat resistance and base strength, such as cordierite (2MgO 2 -2Al 2 03-5SiO 2 ), glass fiber, zirconia oxides, titanium oxides, fecralloy and stainless, metal oxides and metal alloys. 15 The size of the base is not particularly limited, and is appropriately set depending on the amount of combustion catalyst required for one carbon dioxide off-gas treatment. [0034] When the base has a honeycomb structure, a catalyst layer composed of a metal oxide and a noble metal is formed on the inner wall surface of the cell of this honeycomb 20 structure. When the base has a pellet structure or a spherical structure, a catalyst layer composed of a metal oxide and a noble metal is formed on the outer surface of this base. [0035] 19 When adopting a honeycomb structure, the number of cells therein is preferably not less than 10 cpi 2 (i.e., the number of cells per 1 square inch) and not more than 1,000 cpi2 , and more preferably not less than 100 cpi2 and not more than 500 cpi2 When the number of cells in the honeycomb structure is less than 10 cpi 2 , the 5 total surface area of the catalyst layer provided on the inner wall surface of this cell reduces, which makes it impossible to efficiently treat the carbon dioxide off-gas by oxidative degradation using the combustion catalyst. When the number of cells in the honeycomb structure exceeds 1,000 cpi 2 , the size of the cells reduces, which makes it difficult to form the catalyst layers on the inner wall surface of the cells. Moreover, 10 because the level of pressure loss within the honeycomb structure also increases, as a result, the honeycomb structure may be damaged. [0036] FIGS. 12 and 13 are showing an example of a catalytic combustor which can be used in the present invention. The catalytic combustor 13 includes a circular or square 15 cylindrical vessel 212 in which an inlet 214 is formed at one end (the upper end in this example) while an outlet 216 is formed at the other end (the lower end). The inlet 214 is connected to the passage 24 whereas the outlet 216 is connected to the passage 25. The upper and lower portions of the vessel 212 are narrowed in a tapered shape towards the inlet 214 and outlet 216, respectively. Annular flanges 218 are formed in the upper and 20 lower ends of the vessel 212 so that pipes can be connected thereto. A plurality of combustion catalyst units 220A to 2201 are installed inside the body of the vessel 212 in the longitudinal direction with a certain interval between each other, and the off-gas fed from the inlet 214 passes through the combustion catalyst units 220A to 2201 in order 20 and is emitted from the outlet 216. Gaps are formed between the adjacent combustion catalyst units. When the base has a honeycomb structure, these gaps between the units are effective in rectifying the gas channeling. When a gas is fed to the honeycomb structure, 5 although the gas cannot diffuse in the direction perpendicular to the flow, the rectifying properties can be improved by providing the gaps between the units. In addition, a coolant supply passage which is not shown may be connected to each of these gaps. Further, by feeding at least one material selected from a gas obtained following the oxidative degradation process which contains carbon dioxide as a major 10 component, air and water through these coolant supply passages, the temperature of combustion catalyst units in the downstream side may be reduced to control the reaction conditions. The control may be a feedback control in which the valve of each coolant supply passage is automatically opened/closed depending on the output of a temperature sensor provided in the downstream of the catalytic combustor 13, thereby controlling the 15 amount of cooling fluid supply. [0037] Each of the combustion catalyst units 220A to 2201 has a plate shape with a constant thickness, and is configured so that rectangular parallelepiped catalyst blocks 222 as shown in FIG. 13 are arranged without any gaps in the horizontal direction. All 20 the catalyst blocks 222 are fixed to the inner wall of the vessel 212 by a supporting structure which is not shown. The catalyst block 222 has a base composed of a square frame portion 224 and a honeycomb structure portion 226 in which catalyst are arranged inside the frame portion 224 in a fine grid-like pattern. Although the honeycomb structure portion 226 in this example has a tetragonal grid pattern, the pattern may be a 21 hexagonal grid pattern, trigonal grid pattern, or a pattern in which circular holes are arranged. Inside each grid forms a cell (air passage) which retains a constant inner diameter from the upper end to the lower end, and the off-gas passes through homogeneously inside these cells. The cell density is not limited, although the 5 aforementioned range is preferable. The base material may be those described earlier. By using the catalyst block 222 that has such a honeycomb structure, the efficiency of catalytic reaction can be increased while suppressing the gas passage resistance. [0038] In the present embodiment, it is configured so that the inner diameter of the air 10 passage (cell) in the combustion catalyst unit close to the outlet 216 is greater than the inner diameter of the air passage in the combustion catalyst unit close to the inlet 214. More specifically, the cell inner diameters (passage cross sections) increase in 3 steps; i.e., those of the combustion catalyst units 220A to 220C, those of the combustion catalyst units 220D to 220F, and those of the combustion catalyst units 220G to 2201. 15 Because the cell inner diameters increase in such an order, there is an advantage in that even when the temperature of the off-gas increases due to the heat generated while the off-gas flows through from the inlet 214 to the outlet 216 and the gas superficial velocity also increases, the passage resistance can be suppressed to a low level. Since the combustible materials in the off-gas are combusted when the gas passes through the 20 combustion catalyst units 220A to 2201, the temperature of the off-gas increases, thereby increasing the gas superficial velocity. Hence, when the combustion catalyst units 220A to 2201 have the same cell inner diameter, a problem was discovered in that the passage resistance in the combustion catalyst units in the downstream side increases, and the passage resistance of the catalytic combustor 13 as a whole also increases.

22 [0039] In terms of the rate of change in the cell inner diameter, when the passage cross sections of each cells in the combustion catalyst unit 220A situated in the most upstream side are SI and the passage cross sections of each cells in the combustion catalyst unit 5 2201 situated in the most downstream side are S2, SI is preferably about I to 1/5 times as large as S2, and more preferably about I to 1/3 times as large as S2. Although the cell inner diameters (passage cross sections) in the present embodiment increase in 3 steps in the order of those of the combustion catalyst units 220A to 220C, those of the combustion catalyst units 220D to 220F, and those of the combustion catalyst units 220G 10 to 2201, the increase may take place in 2 steps or may take place in n steps (where n represents the total number of combustion catalyst units). Although the number of combustion catalyst units 220 was 9 in the present embodiment, the number is not limited. In general, the number of combustion catalyst units 220 is preferably about 1 to 30 in view of the cost. 15 [0040] On the surface of each combustion catalyst unit 220A to 2201 including the inner surface of each cell (air passage), a metal oxide layer composed of a metal oxide is formed. As the metal oxide, at least one metal oxide selected from the group consisting of zirconium oxide (ZrO 2 ), titanium oxide (TiO 2 ) and silicon oxide (SiO 2 ) can be used. 20 Titanium oxide or zirconium oxide is particularly preferable. [0041] A noble metal is deposited at least on the surface layer portion of the metal oxide layer at a density of not less than 0.1 mg/cm 2 and not more than 10 mg/cm 2 . As the noble metal, at least one metal selected from the group consisting of platinum (Pt), 23 palladium (Pd) and iridium (Ir) is preferable. Platinum is particularly preferable. When the density of noble metal is within the above range, excellent effects as a combustion catalyst can be achieved. It is more preferable that the deposition density of noble metal be not less than 0.001 mg/cm2 and not more than 0.1 mg/cm 2 . The performance as a 5 combustion catalyst is enhanced even more when the density is within this range. [0042] Examples of the combinations of noble metal and metal oxide include Pt/ZrO 2 , Pt/CeO 2 -ZrO 2 , Pt/TiO 2 , Pt/SiO 2 , Pd/ZrO 2 , Pd/CeO 2 -ZrO 2 , Pd/TiO 2 , Pd/SiO 2 , Ir/ZrO 2 , Ir/CeO 2 -ZrO 2 , Ir/TiO 2 and Ir/SiO 2 . In view of the performance and cost, the 10 combinations of Pt/ZrO 2 and Pt/TiO 2 are particularly preferable industrially. [0043] The thickness of the catalyst layer composed of such metal oxide and noble metal is preferably not less than 10 jim and not more than 500 pm, and more preferably not less than 20 pm and not more than 100 Rm. When the thickness of the catalyst layer is less 15 than 10 pm, it becomes difficult to efficiently disperse a noble metal on a support. When the noble metal is heterogeneously distributed, it becomes difficult to improve the efficiency of oxidative degradation process using the combustion catalyst. When the thickness of the catalyst layer exceeds 500 jim, not only the production cost increases, but also the level of pressure loss increases since the cell inner diameter is reduced. 20 [0044] The content of the noble metal (supported amount) is preferably not less than 0.1 g/L and not more than 10 g/L, and more preferably not less than 1 g/L and not more than 5 g/L. When the content of noble metal is less than 0.1 g/L, the catalytic activity is reduced, and thus it becomes necessary to increase the reaction temperature for the 24 oxidative degradation of carbon dioxide off-gas. As a result, the treatment cost increases. When the content of noble metal exceeds 10 g/L, although the catalytic activity is enhanced, the material cost increases, and thus it is not practical. [0045] 5 Rather than forming a dense film, the noble metal is preferably distributed on the surface and inside of the metal oxide as fine particles having a particle size of about 0.1 to 10 [tm. In terms of the noble metal distribution, the noble metal may be deposited only on the surface of the metal oxide layer, may be distributed all over the metal oxide layer in the thickness direction at an almost uniform concentration, or may be distributed 10 so that the concentration thereof increases as it approaches the surface side of the metal oxide layer in the thickness direction. However, in view of the cost and reaction efficiency, it is preferable that the noble metal is distributed so that the concentration thereof is higher in the surface side. [0046] 15 It is preferable to use a metal oxide having an average particle size of not less than 0.01 ptm and not more than 50 ptm, and more preferably not less than 0.5 jim and not more than 10 jim. When the average particle size of metal oxide is less than 0.01 gm, if made into slurry for the coating process, the level of viscosity increases, and thus it is not practical. When the average particle size of metal oxide exceeds 50 pm, particles are 20 precipitated, which makes it difficult to prepare homogeneous slurry. Accordingly, when using such slurry, it is easy to generate unevenness within the obtained coating layer, which makes it difficult to uniformly disperse a noble metal on the surface of a support composed of the metal oxide, and the total surface area of the noble metal exposed on the surface of this support is also reduced. As a result, the catalytic activity is reduced.

25 [0047] It is preferable to use a metal oxide having a BET specific surface area (hereafter, simply referred to as "specific surface area") of not less than 10 m 2 and not more than 300 m 2 , more preferably not less than 10 m 2 and not more than 100 m 2 . In other words, 5 the metal oxide layer is not a dense film, but rather in a porous state microscopically. When the specific surface area of metal oxide is less than 10 m 2 , it becomes impossible to improve the efficiency of oxidative degradation process using the combustion catalyst. Although it is possible to support a noble metal when the specific surface area of metal 2 oxide exceeds 300 m , since the level of heat resistance and water resistance reduces, as 10 well as that of the specific surface area due to the morphological changes caused by sulfur or the like, the noble metal is condensed or buried by deposition as a result. Accordingly, the level of catalytic activity reduces, and thus it is not practical. [0048] The form of combustion catalyst is not limited to the aforementioned honeycomb 15 structure, and may be a pellet structure or a spherical structure, or may be filled inside a column as amorphous particles with gaps therebetween. In terms of the base, metal oxide, type and concentration of noble metal, thickness or the like, the same as those described for the aforementioned honeycomb-type catalyst may be used. In this case, the average particle size of catalyst particles is preferably not less than 0.1 mm and not more 20 than 50 mm, and more preferably not less than 2 mm and not more than 20 mm. When the average particle size is less than 0.1 mm, although the contact efficiency between the carbon dioxide off-gas and catalyst is high, the level of pressure loss is increased. When the average particle size exceeds 50 mm, although the pressure loss can be suppressed to 26 a low level, the contact efficiency between the carbon dioxide off-gas and catalyst is reduced, and thus it is not practical. [0049] The method for producing the aforementioned combustion catalyst will be 5 described. A metal oxide, a sol (binder) of this metal oxide and a polar solvent are mixed using a mortar, a grinding machine, a kneader or the like, thereby preparing slurry containing a metal oxide. The mixing ratio in terms of mass ratio between the metal oxide and a sol (binder) of the metal oxide is preferably within a range from 95/5 to 30/70. 10 Although water (pure water) is optimal as a polar solvent, polar organic solvents including alcohols, such as methanol, ethanol and propanol; ethers, such as diethyl ether and tetrahydrofuran; esters, nitriles, amides and sulfoxides can also be used. [0050] The slurry containing a metal oxide is coated on the base surface (on the inner 15 wall surface in the case of a honeycomb structure, and on the outer surface in the case of a spherical structure or a pellet structure), and an excess portion of slurry is removed by an air blowing process. The base coated with slurry containing a metal oxide is dried at a temperature of not less than I 00*C and no more than 200'C using a dryer for at least I hour. 20 The obtained base is calcined at a temperature of not less than 400*C and not more than 600'C using a firing furnace for not less than 1 hour and not more than 8 hours, thereby forming a layer composed of the metal oxide on the base surface.

27 As the metal oxide, both organic salts and inorganic salts can be used. Examples of organic salts include acetates, acetylacetonates and cyan salts, and examples of inorganic salts include nitrates and chlorides. [0051] 5 An aqueous solution of a compound of noble metal (noble metal compound) and a polar solvent is mixed to prepare a solution of a noble metal compound having a predetermined concentration. As the noble metal compound, both organic salts and inorganic salts can be used. Examples of organic salts include acetates, acetylacetonates and cyan salts, and examples of inorganic salts include nitrates and chlorides. 10 The solution of a noble metal compound is coated on the surface of the metal oxide layer, and the solution is absorbed to the layer composed of a metal oxide and an excess portion of solution is removed by an air blowing process. The base coated with a solution of a noble metal compound is dried at a temperature of not less than 1 00*C and not more than 200*C using a dryer for at least 1 15 hour. The obtained base is calcined at a temperature of not less than 400*C and not more than 600*C using a firing furnace for at least 1 hour, and if necessary, further treatment is carried out in a hydrogen stream at a temperature of not less than 400*C and not more than 600*C, thereby yielding a combustion catalyst in which a catalyst layer 20 composed of a metal oxide and a noble metal is formed on the base surface. [0052] Next, the method for purifying carbon dioxide off-gas using the purification apparatus 10 will be described.

28 In the method for purifying carbon dioxide off-gas according to the present embodiment, the carbon dioxide off-gas is fed inside the purification apparatus 10 via a passage 14 while introducing an air for supporting firing (hereafter, abbreviated as "supporting air") inside the purification apparatus 10 from a passage 18. 5 [0053] The carbon dioxide off-gas is a gas that contains carbon dioxide as the major component thereof and also contains at least sulfur compounds, such as hydrogen sulfide

(H

2 S), mercaptan (R-SH, in which R represents an organic group), carbonyl sulfide (COS), carbon disulfide (CS 2 ) and sulfur dioxide (SO 2 ), of not less than 50 ppmV and 10 not more than 10,000 ppmV. Examples of the carbon dioxide off-gas include a gas emitted during the purification of natural gas in the plants for natural gas and petroleum associated gas or the like. The carbon dioxide off-gas contains VOCs other than the sulfur compounds such 15 as benzene, toluene and xylene, and may also contain carbon monoxide, methane, water, or the like. The VOC contained in the carbon dioxide off-gas contains at least one substance from benzene of 50 ppmV or more and 2,000 ppmV or less, toluene of 100 ppmV or more and 2,000 ppmV or less, and xylene of 100 ppmV or more and 2,000 ppmV or less. 20 [0054] The supporting air is used as an oxidizing agent for the burner fuel in the heater 11 and is subsequently used as an oxidizing agent for the carbon dioxide off-gas in the catalytic combustor 13. In the burner in the heater 11, it is essential that the supporting air contain oxygen at a higher concentration than in the catalytic combustor 13 for 29 achieving a stable level of combustion. In the catalytic combustor 13, the combustion process can be carried out at lower oxygen concentrations, as compared to those of the burner combustion conditions. Furthermore, in the heater 11, because the amount of heat kept by the flue gas can 5 be used for increasing the temperature of the carbon dioxide off-gas without any heat loss by mixing the flue gas with the carbon dioxide off-gas, the thermal efficiency is high. Therefore, the supporting gas is not only used as an oxidizing agent for the carbon dioxide off-gas, but is also effectively used as an oxidizing agent in the heater 11. [0055] 10 When the carbon dioxide off-gas is not preheated (that is, when p = 1), the carbon dioxide off-gas is fed to the heater 11 via a passage 16. When the carbon dioxide off-gas is preheated (that is, when p = 0), the carbon dioxide off-gas is fed to the preheater 12 via a passage 15, and after preheating the carbon dioxide off-gas to a predetermined temperature in the preheater 12, the carbon 15 dioxide off-gas is fed to the heater 11 via passages 17 and 16. The preheating of carbon dioxide off-gas in the preheater 12 is carried out in order to reduce the amount of energy consumption (fuel consumption) when the temperature of the carbon dioxide off-gas is increased in the heater 11 to a level, which is equal to or higher than the temperature for the oxidative degradation by the combustion catalyst in the catalytic combustor 13. 20 [0056] When the supporting air is not preheated (that is, when a = 1), the supporting air is fed to the heater 11 via a passage 20. When the supporting air is preheated (that is, when a = 0), the supporting air is fed to the preheater 12 via a passage 19, and after preheating the supporting air to a 30 predetermined temperature in the preheater 12, the supporting air is fed to the heater 11 via passages 21 and 20. The preheating of supporting air in the preheater 12 is carried out in order to reduce the amount of energy consumption (fuel consumption) when the temperature of the supporting air is increased in the heater 11 to a level, which is equal to 5 or higher than the temperature for the oxidative degradation by the combustion catalyst in the catalytic combustor 13. [0057] In the preheating of carbon dioxide off-gas and supporting air in the preheater 12, either one of carbon dioxide off-gas and supporting air may be preheated to a 10 predetermined temperature, or both of carbon dioxide off-gas and supporting air may be preheated to a predetermined temperature. In order to suppress the apparatus cost, the preheater 12 may be omitted. [0058] The preheating temperature of carbon dioxide off-gas and supporting air in the 15 preheater 12 is preferably at least 100*C and no more than 400*C. If the preheating temperature of carbon dioxide off-gas and supporting air is too low, the amount of energy consumption (fuel consumption) increases when the temperature of the carbon dioxide off-gas is increased in the heater I1 to a level, which is equal to or higher than the temperature for the oxidative degradation by the combustion catalyst. 20 [0059] In the present embodiment, it is preferable that the preheating of carbon dioxide off-gas and supporting air in the preheater 12 be partially or entirely carried out using at least one device selected from the above-mentioned devices. Among these devices, when a heat exchanger is used which exploits the heat exchanged with the carbon dioxide 31 off-gas following the oxidative degradation process by the combustion catalyst in the catalytic combustor 13, since there is no need to separately supply energy (heat) which is required for preheating, the thermal efficiency can be improved and the treatment cost can be reduced. 5 [0060] In the heater 11, the fuel gas fed inside the purification apparatus 10 via the passage 22 is combusted by the supporting air, and thereafter forms a flue gas. Subsequently, the flue gas is mixed with carbon dioxide off-gas, and the resulting mixed gas is then heated so that the temperature thereof is increased to a level (i.e., not less than 10 250*C and not more than 650*C), which is equal to or higher than the temperature for the oxidative degradation by the combustion catalyst in the catalytic combustor 13. More specifically, it is preferable to set the temperature of the mixed gas to not less than 350*C and not more than 500*C using the heater 11. [0061] 15 The fuel gas is a gas that contains methane, ethane, propane, n-butane, i-butane, or the like. [0062] The mixed gas whose temperature is increased to a predetermined temperature by the heater 11 is fed to the catalytic combustor 13 which is filled with the above 20 mentioned combustion catalyst, and by bringing the carbon dioxide off-gas into contact therewith, an oxidative degradation process by the combustion catalyst is conducted on the carbon dioxide off-gas, thereby converting the sulfur compounds contained in the carbon dioxide off-gas such as hydrogen sulfide, mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide into sulfur oxides (SOx). The VOCs contained in the carbon 32 dioxide off-gas such as benzene, toluene and xylene are converted to carbon dioxide by the oxidative degradation process. When the carbon dioxide off-gas contains carbon monoxide, the process also converts carbon monoxide to carbon dioxide. [0063] 5 In the oxidative degradation of carbon dioxide off-gas using the combustion catalyst, the reaction temperature is preferably not less than 250*C and not more than 650'C, and more preferably not less than 350*C and not more than 550*C. When the reaction temperature for the oxidative degradation process using the combustion catalyst is less than 250*C, the oxidative degradation reaction of sulfur compounds and VOCs 10 contained in the carbon dioxide off-gas does not proceed satisfactorily. On the other hand, when the reaction temperature for the oxidative degradation process using the combustion catalyst exceeds 650*C, the combustion catalyst deteriorates due to the heat, which makes it impossible to maintain the purification rate of carbon dioxide off-gas for a long time at a high level. Accordingly, it becomes necessary to change the material of 15 the catalytic combustor to a heat resistant material, which increases the material cost. Moreover, the fuel consumption required for the oxidative degradation process increases, which increases not only the treatment cost but also the amount of carbon dioxide emission. Furthermore, thermal NOx may be generated. In all the embodiments described herein, the difference between the initial temperature of the carbon dioxide off 20 gas when fed to the catalytic combustor and the final temperature of the carbon dioxide off-gas when leaving the catalytic combustor is preferably about 30 to 300C, and more preferably about 50 to 200C. When the difference is too great, the passage resistance increases, which causes some disadvantages, such as the need for changing pressure 33 conditions in the former processes or for installing a vacuum pump or a high chimney stack in the latter steps. [0064] In the oxidative degradation of carbon dioxide off-gas in the catalytic combustor 5 13, the pressure of carbon dioxide off-gas (i.e., the pressure of the carbon dioxide off-gas fed to the catalytic combustor 13) is preferably set to not less than 0.01 MPa and not more than 1 MPa, and more preferably not less than 0.05 MPa and not more than 0.15 MPa. When the pressure of carbon dioxide off-gas is set to not less than 0.2 MPa and not more than I MPa, the volume of carbon dioxide off-gas is reduced and the residence time 10 thereof in the catalytic combustor 13 is prolonged. As a result, the treatment efficiency improves and the power can also be recovered by a gas expansion engine before releasing the carbon dioxide off-gas into the atmosphere after the treatment, thereby reducing the treatment cost. When the pressure of carbon dioxide off-gas is less than 0.05 MPa, the volume of carbon dioxide off-gas is increased and the residence time 15 thereof in the catalytic combustor 13 is shortened, thereby reducing the treatment efficiency. When the pressure of carbon dioxide off-gas exceeds 1 MPa, it is necessary that the catalytic combustor 13 and other devices which is located upstream be pressure resistant vessels. As a result, the extent of adverse effects increases, for example, the apparatus cost increases, the power for compressing the supporting air or the like is 20 required, and the operation cost also increases. [0065] The pressure of carbon dioxide off-gas is appropriately adjusted depending on a unit for recovering carbon dioxide off-gas described later and a destination to which the treated carbon dioxide off-gas is released (emitted).

34 In the conventional method for purifying carbon dioxide off-gas which employs a direct combustion system, the treated carbon dioxide off-gas is released into the atmosphere. For this reason, the unit for recovering carbon dioxide off-gas is operated at a pressure which is made close to the atmospheric pressure as much as possible. 5 However, in some units for recovering carbon dioxide off-gas, an amine solution is flashed at a high pressure of 0.5 to I MPa without using an amine regenerator, or an amine regenerator is operated using an amine solution exhibiting a high level of heat resistance at high temperature and high pressure conditions of about 180'C and of 0.5 to I MPa. In this case, a carbon dioxide off-gas exhibiting a high pressure of 0.5 to I MPa 10 is generated. [0066] In general, because the gas volume reduces as the pressure increases, the gas residence time in a catalytic combustor is prolonged, and thus the size of a catalytic combustor can be reduced. Because the power can be recovered by a gas expansion 15 engine before releasing the high pressure gas into the atmosphere, the treatment cost can be reduced. Therefore, when releasing the treated carbon dioxide off-gas into the atmosphere, the pressure of the carbon dioxide off-gas in the catalytic combustor 13 is set to 0.01 to 1 MPa, which corresponds to the pressure of the carbon dioxide off-gas generated in the unit for recovering carbon dioxide off-gas. 20 [0067] By carrying out the oxidative degradation process using the combustion catalyst in the catalytic combustor 13, concentration of the sulfur compounds (excluding sulfur oxides) contained in the carbon dioxide off-gas is reduced down to 5 ppmV or less. In terms of the VOCs contained in the carbon dioxide off-gas such as benzene, toluene and 35 xylene, due to the oxidative degradation process, the benzene concentration is reduced down to 10 ppmV or less, the toluene concentration is reduced down to 50 ppmV or less, and the xylene concentration is reduced down to 50 ppmV or less. [0068] 5 As the amount of flow of the supporting air fed together with carbon dioxide off gas to the catalytic combustor 13 increases, the amount of fuel gas used for raising the gas temperature to the reaction temperature for the oxidative degradation process increases, and thus the thermal efficiency declines. Therefore, it is preferable to control the amount of supporting air fed to the catalytic combustor 13 so that the concentration of 10 oxygen contained in the carbon dioxide off-gas following the treatment which is measured by the oxymeter provided in the latter step of the catalytic combustor 13 is 0.5 to 15 volume%, and more preferably 0.5 to 5 volume%. [0069] The treated carbon dioxide off-gas emitted from the catalytic combustor 13 is fed 15 to the preheater 12 via a passage 25. When the preheater 12 is a heat exchanger, heat is exchanged in the heat exchanger between the treated carbon dioxide off-gas and either one of the untreated carbon dioxide off-gas and supporting air or both of the untreated carbon dioxide off-gas and supporting air, and the untreated carbon dioxide off-gas or supporting air is preheated to a predetermined temperature. 20 [0070] The treated carbon dioxide off-gas (including sulfur oxides (SOx)) which passed through the preheater 12 is emitted from the purification apparatus 10 via a passage 26. [0071] 36 In the method for purifying carbon dioxide off-gas according to the present embodiment, it is preferable to remove the mercury species contained in the carbon dioxide off-gas, such as organic mercury, ionic mercury, and simple substance of mercury, and then introduce the carbon dioxide off-gas from which mercury species are 5 removed to the catalytic combustor 13. The treatment for removing mercury species contained in the carbon dioxide off-gas is either conducted in the step prior to the introduction of carbon dioxide off-gas inside the purification apparatus 10 or conducted inside the purification apparatus 10 in the step prior to the introduction of carbon dioxide off-gas to the heater 11. 10 The treatment for removing mercury species contained in the carbon dioxide off gas is carried out by an adsorption treatment using activated carbon or the like. [0072] By conducting the oxidative degradation of carbon dioxide off-gas in the catalytic combustor 13 after removing the mercury species contained in the carbon dioxide off-gas 15 in advance as described above, it is possible to prevent the release of the mercury species contained in the carbon dioxide off-gas into the atmosphere and the adverse effects caused by the mercury species on the human bodies and ecosystems. [0073] Next, a method for recovering the carbon dioxide off-gas treated by the method 20 for purifying carbon dioxide off-gas according to the present invention will be described. Examples of the carbon dioxide off-gas treated by the method for purifying carbon dioxide off-gas according to the present invention include: (1) a gas emitted from an acid gas removal apparatus, which brings an acid gas in the natural gas produced from a gas field into contact with a liquid solvent and thereby separates and recovers the acid 37 gas; and (2) a gas that is emitted after most hydrogen sulfide therein is removed by any one of the apparatuses among a hydrogen sulfide enrichment unit, sulfur recovery unit and tail gas treatment unit which is provided in the step which follows the acid gas removal apparatus. 5 [0074] The method for recovering carbon dioxide off-gas from the natural gas produced from a gas field will be described by referring to FIG. 2. FIG. 2 is a schematic diagram showing a first example of a recovery unit for the carbon dioxide off-gas treated by the purification method according to the present invention. The recovery unit for the carbon 10 dioxide off-gas (hereafter, abbreviated as "recovery unit") 30 constitutes an acid gas removal (AGR) apparatus. In the method for recovering carbon dioxide off-gas using the recovery unit 30, the natural gas produced from a gas field is first fed to an absorber 31 via a passage 35. [0075] 15 In the absorber 31, an impurity gas contained in the natural gas having carbon dioxide as a major component and composed of hydrogen sulfide, trace amounts of sulfur compounds (including mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide), and hydrocarbons such as VOCs (including benzene, toluene and xylene) is selectively absorbed to a chemically absorbent or physically absorbent (hereafter, collectively 20 referred to as "absorbent") inside the absorber 31. The natural gas absorbed to the absorbent is emitted as a purified gas from the top of the absorber 31 via a passage 36, and is recovered as a product or in a separate treatment step. The absorbent which absorbed the impurity gas is collected from the bottom of the absorber 31. Thereafter, the absorbent may be fed to a flash drum 32 via a passage 38 37 after lowering the pressure thereof, or the pressure thereof may be lowered via the flash drum 32. In the flash drum 32, light hydrocarbons are recovered as a flash gas via a passage 38. 5 The absorbent from which light hydrocarbons are removed is collected from the bottom of the flash drum 32 and is fed to a regenerator 33 via a passage 39. In the regenerator 33, the absorbent is heated to a predetermined temperature so as to release the impurity gas therefrom as the carbon dioxide off-gas, and the released carbon dioxide off-gas is fed to a flash drum 34 via a passage 40. 10 The absorbent which released the carbon dioxide off-gas is collected from the bottom of the regenerator 33, and is fed to the absorber 31 via a passage 41 to be reused. A portion of the absorbent collected from the bottom of the regenerator 33 is fed to the regenerator 33 via a passage 42, and a treatment (heating) for releasing the carbon dioxide off-gas is conducted thereon again. 15 [0076] In the flash drum 34, the carbon dioxide off-gas is recovered as a flash gas. The recovered carbon dioxide off-gas is emitted from the flash drum 34 via a passage 43 and then fed to a purification apparatus as shown in FIG. 1. A trace amount of absorbent which is mixed in the carbon dioxide off-gas is 20 collected from the bottom of the flash drum 34 and is fed to the regenerator 33 via a passage 44. [0077] The method for recovering a gas that is emitted after most hydrogen sulfide therein is removed by any one of the apparatuses among a hydrogen sulfide enrichment 39 unit, sulfur recovery unit and tail gas treatment unit which is provided in the step which follows the acid gas removing apparatus shown in FIG. 2 as a carbon dioxide off-gas will be described by referring to FIGS. 3 and 4. FIG. 3 is a schematic diagram showing a second example of a recovery unit for 5 the carbon dioxide off-gas treated by the purification method according to the present invention. In FIG. 3, the same components as those in the recovery unit shown in FIG. 2 are given the same reference symbols and the descriptions therefor will be omitted. [0078] In the method for recovering carbon dioxide off-gas using a recovery unit 50, the 10 carbon dioxide off-gas A emitted from the recovery unit 30 is first fed to an absorber 51 of a hydrogen sulfide enrichment (i.e., acid gas enrichment (AGE)) unit via the passage 43, together with the gas from which water is removed after the tail gas treatment is conducted thereon. [0079] 15 In the absorber 51, an impurity gas contained in the carbon dioxide off-gas A and which is composed of a large amount of hydrogen sulfide, a small amount of carbon dioxide, sulfur compounds (such as mercaptan, carbonyl sulfide and carbon disulfide) and VOCs (such as benzene, toluene and xylene) is selectively absorbed to a chemically absorbent or physically absorbent (hereafter, collectively referred to as "absorbent") 20 inside the absorber 51. A carbon dioxide off-gas B containing a trace amounts of sulfur compounds (such as hydrogen sulfide, mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide), VOCs, carbon monoxide or the like is emitted from the top of the absorber 51 via a passage 56 and is fed to a purification apparatus as shown in FIG. 1.

40 The absorbent which absorbed the impurity gas is collected from the bottom of the absorber 51 and fed to a regenerator 52 of a hydrogen sulfide enrichment unit via a passage 57. [0080] 5 In the regenerator 52, the absorbent is heated to a predetermined temperature so as to release the impurity gas therefrom as the hydrogen sulfide enriched gas, and the released hydrogen sulfide enriched gas is fed to a flash drum 53 via a passage 58. The absorbent which released the hydrogen sulfide enriched gas is collected from the bottom of the regenerator 52, and is fed to the absorber 51 via a passage 59 to be 10 reused. A portion of the absorbent collected from the bottom of the regenerator 52 is fed to the regenerator tower 52 via a passage 60, and a treatment (heating) for releasing the hydrogen sulfide enriched gas is conducted thereon again. [0081] 15 In the flash drum 53, the hydrogen sulfide enriched gas is recovered as a flash gas. The recovered hydrogen sulfide enriched gas is emitted from the flash drum 53 via a passage 61 and then fed to a sulfur recovery unit (SRU) 54 constituted from a combustor and multiple Claus reactors. A trace amount of absorbent which is mixed in the hydrogen sulfide enriched gas 20 is collected from the bottom of the flash drum 53 and is fed to the regenerator 52 via a passage 62. [0082] In the sulfur recovery unit 54, due to the oxidative degradation of hydrogen sulfide enriched gas using the fuel gas and supporting air fed inside the sulfur recovery 41 unit 54 from the passage 63, most sulfur components are recovered as an elementary sulfur. A tail gas containing a trace amount of sulfur compounds (such as hydrogen sulfide, carbonyl sulfide, carbon disulfide and sulfur dioxide) and carbon dioxide and 5 having nitrogen as the major component thereof is emitted from the sulfur recovery unit 54 and is fed to a tail gas treatment (TGT) unit 55 constituted of a hydrogenation treatment reactor via a passage 66, together with the fuel gas and supporting air fed from a passage 64 and the hydrogen fed from a passage 65. [0083] 10 In the tail gas treatment unit 55, sulfur compounds other than hydrogen sulfide (such as carbonyl sulfide, carbon disulfide and sulfur dioxide) are subjected to a reduction treatment to form hydrogen sulfide. The obtained hydrogen sulfide is emitted from the tail gas treatment unit 55, and is fed to the absorber 51 via a passage 67 to be recycled. 15 The water contained in the hydrogen sulfide emitted from the tail gas treatment unit 55 is discharged from a discharge channel 68 provided partway along the passage 67. [0084] FIG. 4 is a schematic diagram showing a third example of a recovery unit for the carbon dioxide off-gas treated by the purification method according to the present 20 invention. In FIG. 4, the same components as those in the recovery unit shown in FIGS. 2 and 3 are given the same reference symbols and the descriptions therefor will be omitted. [0085] 42 In the method for recovering carbon dioxide off-gas using a recovery unit 70, the carbon dioxide off-gas A emitted from the recovery unit 30 is first fed to an absorber 51 of a hydrogen sulfide enrichment unit via the passage 43, together with the gas from which water is removed after the tail gas treatment is conducted thereon. 5 [0086] In the absorber 51, an impurity gas contained in the carbon dioxide off-gas A and which is composed of a large amount of hydrogen sulfide, a small amount of carbon dioxide, sulfur compounds (such as mercaptan, carbonyl sulfide and carbon disulfide) and VOCs (such as benzene, toluene and xylene) is selectively absorbed to a chemically 10 absorbent or physically absorbent (hereafter, collectively referred to as "absorbent") inside the absorber 51. A carbon dioxide off-gas B containing a trace amounts of sulfur compounds (such as hydrogen sulfide, mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide), VOCs, carbon monoxide or the like is emitted from the top of the absorber 51 and fed to a passage 66 via the passage 56. 15 The absorbent which absorbed the impurity gas is collected from the bottom of the absorber 51 and fed to the regenerator 52 of the hydrogen sulfide enrichment unit via the passage 57. [0087] In the regenerator 52, the absorbent is heated to a predetermined temperature so 20 as to release the impurity gas therefrom as the hydrogen sulfide enriched gas, and the released hydrogen sulfide enriched gas is fed to the flash drum 53 via the passage 58. The absorbent which released the hydrogen sulfide enriched gas is collected from the bottom of the regenerator 52, and is fed to the absorber 51 via the passage 59 to be reused.

43 A portion of the absorbent collected from the bottom of the regenerator 52 is fed to the regenerator 52 via the passage 60, and a treatment (heating) for releasing the hydrogen sulfide enriched gas is conducted thereon again. Furthermore, a portion of the absorbent collected from the bottom of the regenerator 52 is fed to an absorber 71 of the 5 tail gas treatment unit via a passage 74. [0088] In the flash drum 53, the hydrogen sulfide enriched gas is recovered as a flash gas. The recovered hydrogen sulfide enriched gas is emitted from the flash drum 53 via the passage 61 and then fed to the sulfur recovery unit 54. 10 A trace amount of absorbent which is mixed in the hydrogen sulfide enriched gas is collected from the bottom of the flash drum 53 and is fed to the regenerator 52 via the passage 62. [0089] In the sulfur recovery unit 54, due to the oxidative degradation of hydrogen 15 sulfide enriched gas using the fuel gas and supporting air fed inside the sulfur recovery unit 54 from the passage 63, most sulfur components are recovered as an elementary sulfur. A tail gas containing a trace amount of sulfur compounds (such as hydrogen sulfide, carbonyl sulfide, carbon disulfide and sulfur dioxide) and carbon dioxide and 20 having nitrogen as the major component thereof is emitted from the sulfur recovery unit 54 and is fed to the tail gas treatment unit 55 via the passage 66, together with the carbon dioxide off-gas B emitted from the top of the absorber 51, the fuel gas and supporting air fed from the passage 64 and the hydrogen fed from the passage 65. [0090] 44 In the tail gas treatment unit 55, the sulfur compounds contained in the tail gas and carbon dioxide off-gas B other than hydrogen sulfide (such as carbonyl sulfide, carbon disulfide and sulfur dioxide) are subjected to a reduction treatment to form hydrogen sulfide. The obtained tail gas and carbon dioxide off-gas B which contain 5 hydrogen sulfide are emitted from the tail gas treatment unit 55, and are fed to the absorber 71 of the tail gas treatment unit via the passage 67. The water contained in the tail gas and carbon dioxide off-gas B emitted from the tail gas treatment unit 55 is discharged from the discharge channel 68 provided partway along the passage 67. 10 [0091] In the absorber 71, the hydrogen sulfide contained in the tail gas and carbon dioxide off-gas B are selectively absorbed to a chemically absorbent or physically absorbent (hereafter, collectively referred to as "absorbent") inside the absorber 71. A carbon dioxide off-gas C containing a trace amounts of sulfur compounds (such as 15 hydrogen sulfide, mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide), VOCs, carbon monoxide or the like is emitted from the top of the absorber 71 via a passage 72 and is fed to a purification apparatus as shown in FIG. 1. The absorbent which absorbed the hydrogen sulfide is collected from the bottom of the absorber 71, and is fed to the regenerator 52 via a passage 73 to be recycled. 20 [0092] As described above, according to the method for purifying carbon dioxide off-gas of the present embodiment, the sulfur compounds contained in the carbon dioxide off-gas and causing highly toxic or irritating odor, such as hydrogen sulfide and mercaptan, can be purified and emitted as SOx, thereby making the concentration of the sulfur 45 compounds (excluding sulfur oxides) contained in the treated carbon dioxide off-gas to 5 ppmV or less. By using at least one metal oxide selected from the group consisting of zirconium oxide, titanium oxide and silicon oxide as a combustion catalyst, the level of 5 deterioration of the combustion catalyst caused by the sulfatization of SOx generated by the oxidative degradation process can be reduced. By using at least one noble metal selected from the group consisting of platinum, palladium and iridium as a combustion catalyst, the oxidation reaction of carbon dioxide off-gas can be proceeded in lower temperatures than those in the conventional cases, 10 since these noble metals exhibit a high level of oxidative activity in the low temperature region. [0093] Because the level of the carcinogenic VOCs contained in the carbon dioxide off gas such as benzene, toluene and xylene can be reduced by the oxidative degradation 15 process using the combustion catalyst, for example, the concentration of benzene to 10 ppmV or less, the concentration of toluene to 50 ppmV or less, and the concentration of xylene to 50 ppmV or less, the extent of adverse effects caused by the VOCs to human bodies can be reduced. The VOCs which are the substances responsible for the formation of photochemical smog and suspended particulate matter can be purified, and 20 thus the environmental loads can be reduced. Moreover, the carbon dioxide off-gas can be treated by oxidative degradation at a low cost without generating thermal NOx and with a low level of carbon dioxide emission. [0094] 46 Meanwhile, in the combustion reaction of benzene, toluene and xylene using a conventional Pt/Al 2

O

3 or Pd/AI20 3 -based catalyst, although sufficient performance for oxidative degradation can be achieved in a temperature region equal to or greater than 200*C when there is no sulfur compound present, the performance declines when the 5 sulfur compounds are present depending on the concentration of the compounds. In typical cases, when a sulfur compound is present for a short period of time or at a low concentration, an increase in the reaction temperature can compensate for the performance decline. However, when the concentration of the sulfur compounds is high, if a conventional Pt/Al 2 0 3 or Pd/Al 2 0 3 -based catalyst is used, A1 2 0 3 serving as a support 10 acts on sulfur to form A1 2

(SO

4

)

3 , thereby reducing the catalyst surface area. Since platinum or palladium aggregates when the catalyst surface area reduces, the degree of dispersion of these noble metals within the catalyst declines, thereby reducing the performance for oxidative degradation. As A1 2 0 3 is converted to A1 2

(SO

4

)

3 , it is possible that the bonding state of Pt-A1 2 0 3 may adversely affect the oxidation power of platinum, 15 although to a slight extent, this also becomes one of the causes for decline in the catalytic activity. [0095] Since a conventional Mn/CuO-based catalyst does not contain any noble metals and thus can be obtained at low cost, the amount of supported catalyst can be increased. 20 However, although the Mn/CuO-based catalyst exhibits a high performance for oxidative degradation if used under the condition where there is no sulfur compound present, if there is a sulfur compound or halogen-based substance present, the catalyst forms a Mn salt or Cu salt with these substances, which considerably declines the performance thereof for oxidative degradation.

47 [0096] On the other hand, with the Pt/TiO 2 -based catalyst used in the method for purifying carbon dioxide off-gas according to the present invention, since TiO 2 serving as a support is hardly affected due to the low level of sulfur deposition in the form of sulfur 5 compounds thereon, the catalyst is stable with little changes in the shape thereof. Therefore, the Pt/TiO 2 -based catalyst is a catalyst capable of maintaining the initial catalyst structure and exhibiting little deterioration in the performance for oxidative degradation over time. Likewise, SiO 2 and ZrO 2 also exhibit excellent durability with respect to sulfur, 10 and the combustion catalyst using these metal oxides can maintain the performance for oxidative degradation for a long time. [0097] (2) Second embodiment A second embodiment of the method for purifying carbon dioxide off-gas 15 according to the present invention will be described below by referring to the drawings. Incidentally, in the method for purifying carbon dioxide off-gas according to the present invention, the constitution of an apparatus for purifying carbon dioxide off-gas (hereafter, referred to as "purification apparatus") is appropriately determined depending on the concentration of combustible gas (such as methane) contained in the carbon 20 dioxide off-gas. When the concentration of combustible gas contained in the carbon dioxide off gas is within a range so that the reaction temperature can be controlled to not less than 250'C and not more than 650'C even after the heat generation associated with the oxidative degradation of carbon dioxide off-gas, a purification apparatus as shown in FIG.

48 1 is used which includes a catalytic combustor having one catalytic combustion zone in which a combustion catalyst is installed. When the concentration of combustible gas contained in the carbon dioxide off-gas is within a range so that the reaction temperature exceeds 650'C due to the heat generation associated with the oxidative degradation of 5 carbon dioxide off-gas, a purification apparatus is used which includes a catalytic combustor having at least two catalytic combustion zones in which a combustion catalyst is installed, and is constituted so that at least one material selected from the carbon dioxide off-gas obtained following the oxidative degradation process, air and water, is fed between the catalytic combustion zones so as to cool the'carbon dioxide off-gas fed 10 to the catalytic combustor. [0098] In the present embodiment, a purification apparatus which includes a catalytic combustor having two catalytic combustion zones is shown in FIG. 5, and the method for purifying carbon dioxide off-gas will be described by referring to this drawing. 15 FIG. 5 is a schematic diagram showing an example of an apparatus for purifying carbon dioxide off-gas used in the second embodiment of the method for purifying carbon dioxide off-gas according to the present invention. The purification apparatus 80 of carbon dioxide off-gas is mainly constituted of a heater 81 for heating the carbon dioxide off-gas to a predetermined reaction temperature; 20 a preheater 82 for preheating the carbon dioxide off-gas or air before being fed to the heater 81; a catalytic combustor 83 having two catalytic combustion zones 83a and 83b in which combustion catalyst is installed for conducting an oxidative degradation process on the carbon dioxide off-gas heated in the heater 81; and passages 84 to 97 for connecting these devices and passing various gases through.

49 [0099] As the heater 81, the same heater as the above-mentioned heater 11 is used. As the preheater 82, the same preheater as the above-mentioned preheater 12 is used. [0100] 5 The catalytic combustor 83 includes two catalytic combustion zones 83a and 83b, and a quenching fluid supply zone 83c disposed between these two catalytic combustion zones 83a and 83b for feeding air in order to cool the carbon dioxide off-gas so that the reaction temperature for the oxidative degradation process of carbon dioxide off-gas becomes within a predetermined temperature range, and the above-mentioned 10 combustion catalyst is installed in the catalytic combustion zones 83a and 83b. An oxymeter (not shown) is provided partway along a passage 95 which is connected to the outlet of the catalytic combustor 83. [0101] Next, the method for purifying carbon dioxide off-gas using the purification 15 apparatus 80 will be described. In the method for purifying carbon dioxide off-gas according to the present embodiment, the carbon dioxide off-gas is fed inside the purification apparatus 80 via a passage 84 while introducing a supporting air inside the purification apparatus 80 from a passage 88. 20 In the present embodiment, the concentrations of VOCs, such as benzene, toluene and xylene, and combustible gases, such as methane, which are contained in the carbon dioxide off-gas are high. [0102] 50 When the carbon dioxide off-gas is not preheated (that is, when p = 1), the carbon dioxide off-gas is fed to the heater 81 via a passage 86. When the carbon dioxide off-gas is preheated (that is, when p = 0), the carbon dioxide off-gas is fed to the preheater 82 via a passage 85, and after preheating the 5 carbon dioxide off-gas to a predetermined temperature in the preheater 82, the carbon dioxide off-gas is fed to the heater 81 via passages 87 and 86. The preheating of carbon dioxide off-gas in the preheater 82 is carried out in order to reduce the amount of energy consumption (fuel consumption) when the temperature of the carbon dioxide off-gas is increased in the heater 81 to a level, which is equal to or higher than the temperature for 10 the oxidative degradation by the combustion catalyst in the catalytic combustor 83. [0103] When the supporting air is not preheated (that is, when a = 1), the supporting air is fed to the heater 81 via a passage 90. When the supporting air is preheated (that is, when a = 0), the supporting air is 15 fed to the preheater 82 via a passage 89, and after preheating the supporting air to a predetermined temperature in the preheater 82, the supporting air is fed to the heater 81 via passages 91 and 90. The preheating of supporting air in the preheater 82 is carried out in order to reduce the amount of energy consumption (fuel consumption) when the temperature of the supporting air is increased in the heater 81 to a level, which is equal to 20 or higher than the temperature for the oxidative degradation by the combustion catalyst in the catalytic combustor 83. [0104] In the preheating of carbon dioxide off-gas and supporting air in the preheater 82, either one of carbon dioxide off-gas and supporting air may be preheated to a 51 predetermined temperature, or both of carbon dioxide off-gas and supporting air may be preheated to a predetermined temperature. In order to suppress the apparatus cost, the preheater 82 may be omitted. [0105] 5 The preheating temperature of carbon dioxide off-gas and supporting air in the preheater 82 is preferably at least 100*C and no more than 400*C. If the preheating temperature of carbon dioxide off-gas and supporting air is too low, the amount of energy consumption (fuel consumption) increases when the temperature of the carbon dioxide off-gas is increased in the heater 81 to a level, which is 10 equal to or higher than the temperature for the oxidative degradation by the combustion catalyst. [0106] In the heater 81, the fuel gas fed inside the purification apparatus 80 via the passage 92 is combusted by the supporting air, and thereafter forms a flue gas. 15 Subsequently, the flue gas is mixed with carbon dioxide off-gas, and the resulting mixed gas is then heated so that the temperature thereof is increased to a level (i.e., not less than 250*C and not more than 650*C), which is equal to or higher than the temperature for the oxidative degradation by the combustion catalyst in the catalytic combustor 83. More specifically, it is preferable to set the temperature of the mixed gas to not less than 350*C 20 and not more than 500*C using the heater 81. [0107] The mixed gas whose temperature is increased to a predetermined temperature by the heater 81 is fed to the catalytic combustor 83 which is filled with the above mentioned combustion catalyst, and by bringing the carbon dioxide off-gas into contact 52 therewith, an oxidative degradation process by the combustion catalyst is conducted on the carbon dioxide off-gas, thereby converting the sulfur compounds contained in the carbon dioxide off-gas such as hydrogen sulfide, mercaptan, carbonyl sulfide, carbon disulfide and sulfur dioxide into sulfur oxides (SOx). The VOCs contained in the carbon 5 dioxide off-gas such as benzene, toluene and xylene are converted to carbon dioxide by the oxidative degradation process. When the carbon dioxide off-gas contains carbon monoxide, the process also converts carbon monoxide to carbon dioxide. [0108] In the oxidative degradation of carbon dioxide off-gas using the combustion 10 catalyst, the reaction temperature is preferably not less than 250*C and not more than 650*C, and more preferably not less than 350*C and not more than 500'C. When the reaction temperature for the oxidative degradation process using the combustion catalyst is less than 250*C, the oxidative degradation reaction of sulfur compounds and VOCs contained in the carbon dioxide off-gas does not proceed 15 satisfactorily. On the other hand, when the reaction temperature for the oxidative degradation process using the combustion catalyst exceeds 650*C, the combustion catalyst deteriorates due to the heat, which makes it impossible to maintain the purification rate of carbon dioxide off-gas for a long time at a high level. Accordingly, it becomes necessary to change the material of the catalytic combustor to a heat resistant 20 material, which increases the material cost. Moreover, the fuel consumption required for the oxidative degradation process increases, which increases not only the treatment cost but also the amount of carbon dioxide emission. Furthermore, thermal NOx may be generated. [0109] 53 In the oxidative degradation of carbon dioxide off-gas in the catalytic combustor 83, the pressure of carbon dioxide off-gas (i.e., the pressure of the carbon dioxide off-gas fed to the catalytic combustor 83) is preferably set to not less than 0.01 MPa and not more than I MPa, and more preferably not less than 0.05 MPa and not more than 0.15 5 MPa. When the pressure of carbon dioxide off-gas is set to not less than 0.01 MPa and not more than 1 MPa, the volume of carbon dioxide off-gas is reduced and the residence time thereof in the catalytic combustor 83 is prolonged. As a result, the treatment efficiency improves and the power can also be recovered by a gas expansion engine 10 before releasing the carbon dioxide off-gas into the atmosphere after the treatment, thereby reducing the treatment cost. When the pressure of carbon dioxide off-gas is less than 0.01 MPa, the volume of carbon dioxide off-gas is increased and the residence time thereof in the catalytic combustor 83 is shortened, thereby reducing the treatment efficiency. When the pressure 15 of carbon dioxide off-gas exceeds 1 MPa, it is necessary that the catalytic combustor 83 and other devices which is located upstream be pressure resistant vessels. As a result, the extent of adverse effects increases, for example, the apparatus cost increases, the power for compressing the supporting air or the like is required, and the operation cost also increases. 20 [0110] In the present embodiment, because the concentration of combustible gases contained in the carbon dioxide off-gas is within a range so that the reaction temperature in the oxidative degradation of carbon dioxide off-gas exceeds 650*C, the supporting air 54 is fed as a quenching fluid to the quenching fluid supply zone 83c in the catalytic combustor 83 via a passage 97 which is branched from the midway of the passage 90. The carbon dioxide off-gas emitted from the outlet of the catalytic combustion zone 83a is cooled by the supporting air so as to suppress the temperature inside the 5 catalytic combustion zone 83b from increasing too much, as a result of which the reaction temperature in the oxidative degradation process is controlled so as to be kept within the above-mentioned temperature range. [0111] By carrying out the oxidative degradation process using the combustion catalyst 10 in the catalytic combustor 83, concentration of the sulfur compounds (excluding sulfur oxides) contained in the carbon dioxide off-gas is reduced down to 5 ppmV or less. In terms of the VOCs contained in the carbon dioxide off-gas such as benzene, toluene and xylene, due to the oxidative degradation process, the benzene concentration is reduced down to 10 ppmV or less, the toluene concentration is reduced down to 50 ppmV or less, 15 and the xylene concentration is reduced down to 50 ppmV or less. [0112] As the amount of flow of the supporting air fed together with carbon dioxide off gas to the catalytic combustor 83 increases, the amount of fuel gas used for raising the gas temperature to the reaction temperature for the oxidative degradation process 20 increases, and thus the thermal efficiency declines. Therefore, it is preferable to control the amount of supporting air fed to the catalytic combustor 83 so that the concentration of oxygen contained in the carbon dioxide off-gas following the treatment which is measured by the oxymeter provided in the latter step of the catalytic combustor 83 is 0.5 to 15 volume%, and more preferably 0.5 to 5 volume%.

55 (0113] The treated carbon dioxide off-gas emitted from the catalytic combustor 83 is fed to the preheater 82 via a passage 95.For example, when the preheater 82 is a heat exchanger, heat is exchanged in the heat exchanger between the treated carbon dioxide 5 off-gas and either one of the untreated carbon dioxide off-gas and supporting air or both of the untreated carbon dioxide off-gas and supporting air, and the untreated carbon dioxide off-gas or supporting air is preheated to a predetermined temperature. [0114] The treated carbon dioxide off-gas (including sulfur oxides (SOx)) which passed 10 through the preheater 82 is emitted from the purification apparatus 80 via a passage 96. [0115] Also in the method for purifying carbon dioxide off-gas according to the present embodiment, it is preferable to remove the mercury species contained in the carbon dioxide off-gas, such as organic mercury, ionic mercury, and simple substance of 15 mercury, and then feed the carbon dioxide off-gas from which mercury species are removed to the catalytic combustor 83. The treatment for removing mercury species contained in the carbon dioxide off gas is either conducted in the step prior to the introduction of carbon dioxide off-gas inside the purification apparatus 80 or conducted inside the purification apparatus 80 in 20 the step prior to the feed of carbon dioxide off-gas to the heater 81. The treatment for removing mercury species contained in the carbon dioxide off gas is carried out by an adsorption treatment using activated carbon or the like. (3) Embodiments of method for producing natural gas and production apparatus therefor 56 FIG. 11 is a flow diagram showing an embodiment of a method for producing natural gas according to the present invention. The method includes a liquid phase removal step 200 in which the raw natural gas collected from a well is first measured, and is then fed to a slug catcher, thereby separating the raw natural gas into a liquid 5 phase and a vapor phase using the slug catcher or the like. [0116] The vapor phase obtained by the liquid phase removal is transferred to an acid gas removal step 202 in which the carbon dioxide off-gas having carbon dioxide as a major component and containing VOCs and sulfur compounds is separated. The separation 10 method may be a conventionally known chemisorption method, physisorption method, or a combination thereof. The raw gas following the separation of carbon dioxide off-gas therefrom is transferred to a water removal step 204, and is cooled down to a temperature close to a level for forming a gas hydrate, and the water condensed as a result is removed. 15 [0117] The raw gas from which water is removed is transferred to a mercury removal step 206, and the concentration of mercury in the raw gas is reduced to about 0.1 to 0.01 pig/Nm 3 by an activated carbon adsorption method or the like. The obtained raw gas following the mercury removal is transferred to a heavy 20 component removal step 208 in which the raw gas is subjected to a fractional distillation in a plurality of distillation towers and the heavy hydrocarbons, such as pentane and heavier hydrocarbons, and the like are removed to yield a natural gas. Furthermore, the raw gas from which the heavy hydrocarbons are removed is transferred to a liquefaction step 210, cooled and compressed, and is then filled in a tank. The carbon dioxide off-gas 57 is transferred to an off-gas purification step 201, and the off-gas is purified by the method for purifying carbon dioxide off-gas according to any one of the aforementioned embodiments. [0118] 5 According to such a method for producing natural gas, cleaning of exhaust gas can be achieved while suppressing the amount of energy consumption, and thus the level of environmental loads due to the natural gas production can be reduced. EXAMPLES 10 [0119] The present invention will be described below in further detail using Examples and Comparative Examples. However, the present invention is not limited to the following Examples. [0120] 15 [Example 1] In the purification apparatus of carbon dioxide off-gas employing a catalytic combustion system as shown in FIG. 1, the oxidative degradation of carbon dioxide off gas was carried out by setting the conditions as a = 1 and p = 1, and using the carbon dioxide off-gas shown in Table 1, the supporting air shown in Table 2 and the fuel gas 20 shown in Table 3. As a catalyst, the catalyst prepared in Example 2 described later was used. The treatment conditions and results are shown in Table 4. [0121] [Comparative Example 1] 58 In the purification apparatus of carbon dioxide off-gas employing a direct combustion system as shown in FIG. 6, the oxidative degradation of carbon dioxide off gas was carried out by setting the conditions as a = I and P = 1, and using the carbon dioxide off-gas shown in Table 1, the supporting air shown in Table 2 and the fuel gas 5 shown in Table 3. The treatment conditions and results are shown in Table 4. [0122] The outline of a method for purifying carbon dioxide off-gas using the purification apparatus 100 shown in FIG. 6 will be described. 10 In the method for purifying carbon dioxide off-gas using the purification apparatus 100, the carbon dioxide off-gas was fed inside the purification apparatus 100 via a passage 104 while introducing a supporting air inside the purification apparatus 100 from a passage 108. [0123] 15 When the carbon dioxide off-gas was not preheated (that is, when p = 1), the carbon dioxide off-gas was fed to a combustor 101 via a passage 106. When the carbon dioxide off-gas was preheated (that is, when p = 0), the carbon dioxide off-gas was fed to a carbon dioxide off-gas preheater 103 via a passage 105, and after preheating the carbon dioxide off-gas to a predetermined temperature in the carbon dioxide off-gas preheater 20 103, the carbon dioxide off-gas was fed to the combustor 101 via passages 107 and 106. [0124] When the supporting air was not preheated (that is, when a = 1), the supporting air was fed to the combustor 101 via passages 110 and 111. When the supporting air was preheated (that is, when a = 0), the supporting air was fed to an air preheater 102 via a 59 passage 109, and after preheating the supporting air to a predetermined temperature in the air preheater 102, the supporting air was fed to the combustor 101 via the passage 111. [0125] Subsequently, together with a fuel gas fed inside the purification apparatus 100 5 via a passage 112, the carbon dioxide off-gas and supporting air fed to the combustor 101 were subjected to a direct combustion treatment in the combustor 101. In the oxidative degradation of carbon dioxide off-gas by this direct combustion process, the reaction temperature was set to 900'C. [0126] 10 The treated carbon dioxide off-gas emitted from the combustor 101 was fed to the air preheater 102 via a passage 113, and was then further fed to the carbon dioxide off gas preheater 103 via a passage 114. When the air preheater 102 and carbon dioxide off-gas preheater 103 were heat exchangers, heat was exchanged in the heat exchangers between the treated carbon 15 dioxide off-gas and the untreated supporting air or carbon dioxide off-gas, and the untreated supporting air or carbon dioxide off-gas was preheated to a predetermined temperature. [0127] The treated carbon dioxide off-gas (including sulfur oxides (SOx)) which passed 20 through the carbon dioxide off-gas preheater 103 was emitted from the purification apparatus 100 via a passage 115. [0128] 60 [Table 1] Temperature (*C) 40 Pressure (kg/cm 2 ) 1.40 Flow rate (Nm 3 /h) 100,000 Composition of carbon Carbon dioxide 92.4 oxide off-gas (volume%) Methane 0.1 Hydrogen sulfide 0.025 Benzene 0.025 Toluene 0.025 p-xylene 0.025 Water 7.4 [0129] [Table 2] Temperature (*C) 40 Pressure (kg/cm 2 ) 1.40 Composition of supporting Nitrogen 77.0 air (volume%) Oxygen 20.5 Water 2.5 5 [0130] [Table 3] Temperature (*C) 40 Pressure (kg/cm') 1.40 Composition of fuel gas Methane 84.0 (volume%) Ethane 10.0 Propane 4.0 n-butane 1.0 i-butane 1.0 [0131] 61 [Table 4] Example I Comparative Example I (catalytic combustion system) (direct combustion system) Combustor inlet temperature ("C) 400 Combustor outlet temperature (*C) 435 900 Amount of supporting air (Nm'/h) 31,000 125,000 Amount of fuel consumption (Nm 3 /h) 2,200 8,300 Air fuel ratio 1.09 1.25 Amount of CO 2 generated associated with 3,200 10,900 combustion (a) (Nm 3 /h) Conversion rate of CH 4 in CO 2 off-gas (%) 0 100 Conversion rate of H 2 S in CO 2 off-gas (%) 100 100 Conversion rate of benzene in CO 2 off-gas (%) 100 100 Conversion rate of toluene in CO 2 off-gas (%) 100 100 Conversion rate of p-xylene in CO 2 off-gas (%) 100 100 Amount ofCH 4 slip equivalent to CO 2 emission 2,100 0 (b) (Nm 3 /h) Amount of CO 2 in CO 2 off-gas (c) (Nm 3 /h) 92,400 92,400 (a) + (b) (Nm 3 /h) 5,300 10,900 (a) + (b) + (c) (Nm /h) 97,700 103,300 [0132] From the results shown in Table 4, while the amount of fuel consumption in 5 Comparative Example 1 employing a direct combustion system was 8,300 Nm 3 /h, the amount of fuel consumption in Example I employing a catalytic combustion system was 2,200 Nm 3 /h, and thus it was indicated that the amount of fuel consumption can be reduced by at least 70%. In Example 1, since the combustion treatment was carried out at a lower 10 temperature than in conventional cases, the combustibility of methane (CH 4 ) contained in the carbon dioxide off-gas was expected to deteriorate. Accordingly, in Example 1, by assuming the conversion rate of methane (having a global warming potential 21 times higher than that of carbon dioxide) was 0%, the effects of methane slip on the carbon dioxide emission were examined. By converting the amount of methane slip to the 15 equivalent amount of carbon dioxide, the amount of carbon dioxide emission associated with each combustion treatment was calculated (shown in Table 4 as "(a) + (b)"). While the amount of carbon dioxide emission in Comparative Example 1 employing a direct 62 combustion system was 10,900 Nm 3 /h, the amount of carbon dioxide emission in Example I employing a catalytic combustion system was 5,300 Nm 3 /h, and thus it was indicated that the amount of carbon dioxide emission can be reduced by at least 50%. Further, by taking the amount of carbon dioxide emission contained in the carbon 5 dioxide off-gas into consideration, the total amount of carbon dioxide emission associated with each combustion treatment was calculated (shown in Table 4 as "(a) + (b) + (c)"). While the amount of carbon dioxide emission in Comparative Example 1 employing a direct combustion system was 103,300 Nm 3 /h, the amount of carbon dioxide emission in Example 1 employing a catalytic combustion system was 97,700 Nm 3 /h, and 10 thus it was indicated that the amount of carbon dioxide emission can be reduced by 5%. [0133] [Example 2] 58.6 g of a titanium oxide powder (PC-500 (product name) in the form of anatase, manufactured by Millennium Inorganic Chemicals, Inc.), 138.0 g of titania sol (TA-15 15 (product name) manufactured by Nissan Chemical Industries, Ltd.) and 103.5 g of pure water were mixed, thereby preparing a slurry containing a titanium oxide powder. The slurry containing a titanium oxide powder was coated onto the inner wall surface of a cordierite honeycomb (400 cpi 2 , manufactured by NGK Insulators, Ltd.), and an excess portion of the slurry was removed by an air blowing process. 20 The cordierite honeycomb onto which the slurry containing a titanium oxide powder was coated was dried at 150'C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, thereby forming a layer in which the content of titanium oxide powder was 50 g/liter.

63 [0134] 88.89 g of an aqueous solution of dinitrodiamine platinum (having the platinum content of 4.5% by mass) and 211.11 g of pure water were mixed, thereby preparing an aqueous solution of platinum having a platinum content of 1.33% by mass. 5 The aqueous solution of platinum was coated onto the inner wall surface of a cordierite honeycomb in which a layer composed of titanium oxide powder was formed, and the aqueous solution of platinum was absorbed to the layer composed of titanium oxide powder while an excess portion of the solution was removed by an air blowing process so that the content of the aqueous solution of platinum within the layer composed 10 of titanium oxide powder became 150 g/liter. The cordierite honeycomb onto which the aqueous solution of platinum was coated was dried at 150*C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, and a Pt/TiO 2 catalyst layer in which the content of platinum was 2 g/liter was 15 subsequently formed by conducting a reduction treatment in a hydrogen atmosphere at 500*C for 2 hours, thereby yielding a combustion catalyst of Example 2. [0135] [Example 3] A layer in which the content of titanium oxide powder was 52 g/liter was formed 20 in the same manner as in Example 2. 30 g of an aqueous solution of palladium nitrate (having a palladium content of 10.0% by mass) and 270 g of pure water were mixed, thereby preparing an aqueous solution of palladium having a palladium content of 1.33% by mass.

64 The aqueous solution of palladium was coated onto the inner wall surface of a cordierite honeycomb in which a layer composed of titanium oxide powder was formed, and the aqueous solution of palladium was absorbed to the layer composed of titanium oxide powder while an excess portion of the solution was removed by an air blowing 5 process so that the content of the aqueous solution of palladium within the layer composed of titanium oxide powder became 150 g/liter. The cordierite honeycomb onto which the aqueous solution of palladium was coated was dried at 150*C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500'C by a firing furnace for 2 10 hours, and a Pd/TiO 2 catalyst layer in which the content of palladium was 2 g/liter was subsequently formed, thereby yielding a combustion catalyst of Example 3. [0136] [Example 4] 50.8 g of a zirconium oxide powder (RC-100 (product name) manufactured by 15 Daiichi Kigenso Kagaku Kogyo Co., Ltd.), 69.0 g of zirconia sol (NZS-30A (product name) manufactured by Nissan Chemical Industries, Ltd.) and 180.2 g of pure water were mixed, thereby preparing a slurry containing a zirconium oxide powder. The slurry containing a zirconium oxide powder was coated onto the inner wall surface of a cordierite honeycomb (400 cpi 2 , manufactured by NGK Insulators, Ltd.), and 20 an excess portion of the slurry was removed by an air blowing process. The cordierite honeycomb onto which the slurry containing a zirconium oxide powder was coated was dried at 150 0 C for 6 hours using a dryer.

65 The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, thereby forming a layer in which the content of zirconium oxide powder was 50 g/liter. [0137] 5 88.89 g of an aqueous solution of dinitrodiamine platinum (having the platinum content of 4.5% by mass) and 211.11 g of pure water were mixed, thereby preparing an aqueous solution of platinum having a platinum content of 1.33% by mass. The aqueous solution of platinum was coated onto the inner wall surface of a cordierite honeycomb in which a layer composed of zirconium oxide powder was formed, 10 and the aqueous solution of platinum was absorbed to the layer composed of zirconium oxide powder while an excess portion of the solution was removed by an air blowing process so that the content of the aqueous solution of platinum within the layer composed of zirconium oxide powder became 150 g/liter. The cordierite honeycomb onto which the aqueous solution of platinum was 15 coated was dried at 150*C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, and a Pt/ZrO 2 catalyst layer in which the content of platinum was 2 g/liter was subsequently formed by conducting a reduction treatment in a hydrogen atmosphere at 500*C for 2 hours, thereby yielding a combustion catalyst of Example 4. 20 [0138] [Comparative Example 2] 43.3 g of an aluminum oxide powder (NST-5 (product name) manufactured by Nikki-Universal Co., Ltd.), 138.0 g of alumina sol (A-10 (product name) manufactured 66 by Kawaken Fine Chemicals Co., Ltd.) and 76.7 g of pure water were mixed, thereby preparing a slurry containing an aluminum oxide powder. The slurry containing an aluminum oxide powder was coated onto the inner wall surface of a cordierite honeycomb (400 cpi 2 , manufactured by NGK Insulators, Ltd.), and 5 an excess portion of the slurry was removed by an air blowing process. The cordierite honeycomb onto which the slurry containing an aluminum oxide powder was coated was dried at 150'C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, thereby forming a layer in which the content of aluminum oxide powder was 50 10 g/liter. [0139] 88.89 g of an aqueous solution of dinitrodiamine platinum (having the platinum content of 4.5% by mass) and 211.11 g of pure water were mixed, thereby preparing an aqueous solution of platinum having a platinum content of 1.33% by mass. 15 The aqueous solution of platinum was coated onto the inner wall surface of a cordierite honeycomb in which a layer composed of aluminum oxide powder was formed, and the aqueous solution of platinum was absorbed to the layer composed of aluminum oxide powder while an excess portion of the solution was removed by an air blowing process so that the content of the aqueous solution of platinum within the layer composed 20 of aluminum oxide powder became 150 g/liter. The cordierite honeycomb onto which the aqueous solution of platinum was coated was dried at 150*C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, and a Pt/A 2

O

3 catalyst layer in which the content of platinum was 2 g/liter was 67 subsequently formed by conducting a reduction treatment in a hydrogen atmosphere at 500*C for 2 hours, thereby yielding a combustion catalyst of Comparative Example 2. [0140] [Comparative Example 3] 5 43.3 g of an aluminum oxide powder (NST-5 (product name) manufactured by Nikki-Universal Co., Ltd.), 138.0 g of alumina sol (A-10 (product name) manufactured by Kawaken Fine Chemicals Co., Ltd.) and 76.7 g of pure water were mixed, thereby preparing a slurry containing an aluminum oxide powder. The slurry containing an aluminum oxide powder was coated onto the inner wall 10 surface of a cordierite honeycomb (400 cpi 2 , manufactured by NGK Insulators, Ltd.), and an excess portion of the slurry was removed by an air blowing process. The cordierite honeycomb onto which the slurry containing an aluminum oxide powder was coated was dried at 150*C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 15 hours, thereby forming a layer in which the content of aluminum oxide powder was 50 g/liter. [0141] 30 g of an aqueous solution of palladium nitrate (having a palladium content of 10.0% by mass) and 270 g of pure water were mixed, thereby preparing an aqueous 20 solution of palladium having a palladium content of 1.33% by mass. The aqueous solution of palladium was coated onto the inner wall surface of a cordierite honeycomb in which a layer composed of aluminum oxide powder was formed, and the aqueous solution of palladium was absorbed to the layer composed of aluminum oxide powder while an excess portion of the solution was removed by an air blowing 68 process so that the content of the aqueous solution of palladium within the layer composed of aluminum oxide powder became 150 g/liter. The cordierite honeycomb onto which the aqueous solution of palladium was coated was dried at 150*C for 6 hours using a dryer. 5 The cordierite honeycomb was then calcined at 500*C by a firing furnace for 2 hours, and a Pd/Al 2 0 3 catalyst layer in which the content of palladium was 2 g/liter was subsequently formed, thereby yielding a combustion catalyst of Comparative Example 3. [0142] [Comparative Example 4] 10 84.0 g of a hopcalite powder as a Cu/Mn powder (N-840 (product name) manufactured by Sid-Chemie Catalysts Japan, Inc.), 148.5 g of silica sol (Snowtex C (product name) manufactured by Nissan Chemical Industries, Ltd.) and 67.5 g of pure water were mixed, thereby preparing a slurry containing a Cu/Mn powder. The slurry containing a Cu/Mn powder was coated onto the inner wall surface of 15 a cordierite honeycomb (400 cpi 2 , manufactured by NGK Insulators, Ltd.), and an excess portion of the slurry was removed by an air blowing process. The cordierite honeycomb onto which the slurry containing a Cu/Mn powder was coated was dried at 150*C for 6 hours using a dryer. The cordierite honeycomb was then calcined at 400'C by a firing furnace for 2 20 hours, and a Cu/Mn layer in which the content of Cu/Mn powder was 100 g/liter was subsequently formed, thereby yielding a combustion catalyst of Comparative Example 4. [0143] [Chemical reaction rate of benzene by combustion catalyst] 69 When hydrogen sulfide and benzene (C 6

H

6 ) were subjected to an oxidative degradation process using a catalytic combustor which included a combustion catalyst composed of the cordierite honeycomb prepared in Example 2 having a Pt/TiO 2 catalyst layer, the reaction rate for the oxidation reaction of benzene (C 6

H

6 + 15/202 -+ 6C O 2 + 5 3H 2 0) was expressed by the following Langmuir-Hinshelwood equation (equation (1)). [0144] [Equation 1] R = k-PC 6

H

6 "-(Ko 2 )0 L1+1b (1) I+ K H 2

S'PH

2 S + K 0 2 'P02 [0145] 10 Note that in the equation (1), R represents a reaction rate (mol/h/g-cat), k represents a reaction rate constant (mol/h/g-cat/Pa base) as a function of the temperature T (K), Pi represents a partial pressure (Pa) of component i, Ki represents an adsorption equilibrium constant (1/Pa) of component i as a function of the temperature T (K), and a and b represent orders of reaction. 15 As a result of expressing the reaction rate by the above equation (1), it was confirmed as shown in FIG. 7 that the experimental value and calculated value were almost identical for the reaction rate. [0146] [Sensitivity analysis of combustion catalyst by reaction rate equation] 20 When hydrogen sulfide and benzene (C 6

H

6 ) were subjected to an oxidative degradation process using a catalytic combustor which included a combustion catalyst composed of the cordierite honeycomb prepared in Example 2 having a Pt/TiO 2 catalyst layer at a gas hourly space velocity (GHSV) of 30,000 h-, a benzene concentration of 70 500 ppm and a pressure of the catalytic combustor of I atmospheric pressure, the relationship between the reaction temperature and benzene conversion rate (degradation rate) at the hydrogen sulfide concentration and oxygen concentration shown in Table 5 is shown in FIG. 8. 5 The benzene conversion rate was defined by the following formula. Conversion rate (%) = (1 - (benzene concentration in outlet side of catalytic combustor) / (benzene concentration in inlet side of catalytic combustor)) x 100 [0147] [Table 5] Hydrogen sulfide Oxygen concentration Benzene GHSV concentration (ppm) (volume%) concentration (ppm) (1/h) Analytical Ex. 1 0 5.0 500 30,000 Analytical Ex. 2 600 5.0 500 30,000 Analytical Ex. 3 2,000 5.0 500 30,000 Analytical Ex. 4 600 2.0 500 30,000 10 [0148] From the results of analytical examples 1 to 3 shown in FIG. 8, it was shown that although the oxidation reaction of benzene proceeded even at the reaction temperature of about 250*C when the concentration of hydrogen sulfide was 0 ppm, the reaction activity 15 declined when a trace amount of hydrogen sulfide was present. In other words, it was indicated that when the concentration of hydrogen sulfide was 600 ppm, in order to make the reaction proceed in the same manner as that when the concentration of hydrogen sulfide was 0 ppm, it was necessary to increase the reaction temperature by about 1 00*C. In addition, it was indicated that when the concentration of hydrogen sulfide was 2,000 20 ppm, in order to make the reaction proceed in the same manner as that when the concentration of hydrogen sulfide was 0 ppm, it was necessary to increase the reaction temperature by about 130'C.

71 From the results of analytical examples 2 and 4 shown in FIG. 8, it was shown that the reaction activity in the oxidation reaction of benzene declined when the oxygen concentration dropped from 5.0 volume% to 2.0 volume%. In other words, it was indicated that when the oxygen concentration was 2.0 volume%, in order to make the 5 oxidation reaction of benzene proceed in the same manner as that when the oxygen concentration was 5.0 volume%, it was necessary to increase the reaction temperature by about 20*C. From the results as described above, it was indicated that the oxidation reaction of benzene using a combustion catalyst composed of the cordierite honeycomb prepared in 10 Example 2 having a Pt/TiO 2 catalyst layer was adversely affected by the concentrations of hydrogen sulfide and oxygen which coexisted. It was indicated that in order to make the oxidation reaction of benzene proceed, it was necessary to set the reaction temperature to at least 250*C. [0149] 15 [Verification of upper limit for the reaction temperature of oxidative degradation process using a combustion catalyst] After exposing the combustion catalyst composed of the cordierite honeycomb prepared in Example 2 having a Pt/TiO 2 catalyst layer under a high temperature atmosphere as shown in Table 6, the oxidative degradation of benzene was conducted 20 with the reaction conditions shown in Table 7. The results are shown in Table 8. [0150] 72 [Table 6] Ambient temperature (*C) 500/550/600/650 Time (h) 4.0 Atmospheric gas air [0151] [Table 7] Benzene concentration (ppm) 1,000 Hydrogen sulfide concentration (ppm) 600 Water (steam) concentration (volume%) 2.5 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 73.1 GHSV (1/h) 30,000 Pressure (MPa) 0.1 Reaction temperature (*C) 400/450 5 [0152] [Table 8] Reaction temperature Ambient temperature (*C) (*C) 500 0 C 550 0 C 600*C 650 0 C 400 99.75% 97.86% 27.61% 17.33% 450 99.99% 99.99% 99.96% 98.90% [0153] 10 From the results shown in Table 8, it was indicated that when the combustion catalyst obtained in Example 2 was exposed to a high temperature atmosphere of 600*C, the reaction activity achieved at 400'C declined, and it was necessary to increase the reaction temperature as compared to the conditions prior to the exposure to the high temperature atmosphere. 15 From the results as described above, it was indicated that in the oxidation reaction of benzene using a combustion catalyst composed of the cordierite honeycomb prepared 73 in Example 2 having a Pt/TiO 2 catalyst layer, the upper limit for the reaction temperature was about 650*C in view of the stability to heat of the catalyst. [0154] [Performance test for combustion catalyst] 5 Using the combustion catalysts prepared in Examples 2 to 4 and Comparative Examples 2 to 4, the basic performance and life performance of the combustion catalysts and the relationship between the performance of combustion catalysts and the reaction conditions were tested. FIG. 9 is a schematic diagram showing a testing apparatus used in a performance 10 test for combustion catalysts. The performance test for combustion catalysts was conducted using this testing apparatus 120 by the following method. First, a new combustion catalyst 122 was installed inside a catalytic combustor 121 for each testing conditions, and a crushed quartz 123 was installed in the former step 15 thereof (that is, in the upstream side in the flow direction of a gas to be treated). [0155] While controlling the flow rate using a precision rotameter 125, carbon dioxide was fed from a gas cylinder 124 to the catalytic combustor 121. At the same time, while controlling the flow rate using a precision rotameter 127, air was fed from a gas cylinder 20 126 to the catalytic combustor 121. The carbon dioxide and air fed to the catalytic combustor 121 were heated to a predetermined temperature by electric furnaces 128 disposed in the periphery of the catalytic combustor 121.

74 The supply of water from a vessel 129 containing water to the catalytic combustor 121 was started by a rotary pump 130. [0156] While controlling the flow rate using a mass flow controller 131, air was fed from 5 a gas cylinder 126 to the VOC (benzene, toluene or p-xylene) inside a vessel 132, and the VOC was volatilized, thereby supplying a predetermined amount of VOC to the catalytic combustor 121. At the same time, while controlling the flow rate using a mass flow controller 135, a predetermined amount of hydrogen sulfide or nitrogen containing mercaptan was fed from a gas cylinder 134 to the catalytic combustor 121. 10 [0157] After a predetermined time, a Tedlar bag was installed at a sample collection port 136 provided in the outlet side of the catalytic combustor 121, and a sample of outlet gas was collected. After completion of the collection of outlet gas sample, a Tedlar bag was installed 15 at a sample collection port 137 provided in the inlet side of the catalytic combustor 121, and a sample of inlet gas was collected. [0158] With respect to the outlet gas sample and inlet gas sample, the benzene concentration, toluene concentration or p-xylene concentration was measured, together 20 with the concentration of hydrogen sulfide or mercaptan as well as the concentration of carbon monoxide, and the reaction rates for the oxidation reaction of these gases and the reaction products were analyzed. After collecting gas samples, the supply of gases other than air and carbon dioxide was stopped, and the measurements were terminated.

75 [0159] The concentration of benzene, toluene or p-xylene contained in the gas to be treated was measured by gas chromatography. For gas chromatography, the GC-14B instrument (manufactured by Shimadzu 5 Corporation) was used. As a detector, a flame ionization detector (FID) was used. The concentration of hydrogen sulfide, mercaptan or carbon monoxide contained in the gas to be treated was measured using a gas detector tube manufactured by Gastec Corporation. 10 [0160] The conversion rate (degradation rate) of benzene, toluene or p-xylene in the following performance test for combustion catalyst was defined by the formula shown below. Conversion rate (%)= (1 - (concentration of benzene, toluene or p-xylene in 15 outlet side of catalytic combustor) / (concentration of benzene, toluene or p-xylene in inlet side of catalytic combustor)) x 100 [0161] (1) Basic performance of combustion catalyst Using the combustion catalysts prepared in Examples 2 to 4 and Comparative 20 Examples 2 to 4, the oxidative degradation of carbon dioxide containing sulfur compounds and VOCs was conducted with the reaction conditions shown in Table 9. The benzene conversion rates are shown in Table 10. [0162] 76 [Table 9] Benzene concentration (ppm) 500 Hydrogen sulfide concentration (ppm) 500 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 70.6 GHSV (1/h) 25,000 Pressure (MPa) 0.1 Reaction temperature (*C) 350/400/450 [0163] [Table 10] Catalyst Reaction temperature (C) 350 0 C 400 0 C 450 0 C Ex. 2 Pt/TiO 2 99.44% 99.99% 99.99% Ex. 3 Pd/TiO 2 99.33% 99.53% 99.95% Ex. 4 Pt/ZrO 2 99.38% 99.72% 99.99% Comp. Ex. 2 Pt/Al 2 0 3 20.30% 21.20% Comp. Ex. 3 Pd/Al 2 0 3 4.75% 99.83% Comp. Ex. 4 Cu/Mn 11.87% 31.37% 5 [0164] From the results shown in Table 10, it was confirmed that the combustion catalysts prepared in Examples 2 to 4 were suitable as the catalysts for oxidatively degrading the carbon dioxide that contained sulfur compounds and VOCs. 10 On the other hand, it was confirmed that the combustion catalysts prepared in Comparative Examples 2 and 3 using aluminum oxide (A1 2 0 3 ) were sulfatized by sulfuric acid produced by the oxidation of sulfur compounds, and even when the same active metal species (platinum or palladium) was used, a stable performance in the oxidative degradation was not exhibited.

77 It was confirmed that since the combustion catalyst prepared in Comparative Example 4 was not using any noble metals, with the reaction conditions shown in Table 9, the reactivity thereof at a temperature of 350*C and 400'C was considerably low. [0165] 5 (2) Life performance of combustion catalyst Using the combustion catalysts prepared in Example 2 and Comparative Examples 2 and 4, the oxidative degradation of carbon dioxide containing sulfur compounds and VOCs was conducted with the reaction conditions shown in Table 11. The benzene conversion rates are shown in FIG. 10. 10 [0166] [Table 11] Benzene concentration (ppm) 500 Hydrogen sulfide concentration (ppm) 600 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 70.6 GHSV (1/h) 30,000 Pressure (MPa) 0.1 Reaction temperature (*C) 400 [0167] From the results shown in FIG. 10, it was confirmed that the combustion catalyst 15 prepared in Example 2 exhibited a long term activity as the catalyst for oxidatively degrading the carbon dioxide that contained sulfur compounds and VOCs. It was confirmed that the combustion catalyst prepared in Comparative Example 2 using aluminum oxide was gradually sulfatized by SOx produced by the oxidation of sulfur compounds, and even when the same active metal species (platinum) was used, the 78 activity declined rapidly and a stable performance in the oxidative degradation was not exhibited. It was confirmed that also with the combustion catalyst prepared in Comparative Example 4, the activity declined rapidly and a stable performance in the oxidative 5 degradation was not exhibited due to the adverse effects of SOx produced by the oxidation of sulfur compounds. [0168] (3) Relationship between the performance of combustion catalysts and the reaction conditions 10 (a) Reactivity when benzene, toluene or p-xylene was contained Using the combustion catalyst prepared in Example 2, the oxidative degradation of carbon dioxide containing hydrogen sulfide as well as benzene, toluene and p-xylene was conducted with the reaction conditions shown in Table 12. The conversion rates of benzene, toluene and p-xylene are shown in Table 13. 15 [0169] [Table 12] Concentration of benzene, toluene or p- 500 xylene (ppm) Hydrogen sulfide concentration (ppm) 600 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 70.6 GHSV (1/h) 30,000 Pressure (MPa) 0.1 Reaction temperature (*C) 300/350/400 [0170] 79 [Table 13] Reaction temperature 300 0 C 350 0 C 400 0 C Benzene 19.69% 97.30% 99.99% Toluene 15.95% 90.01% 99.98% p-xylene 10.02% 50.39% 99.99% [0171] From the results shown in Table 13, it was confirmed that the combustion catalyst 5 prepared in Example 2 exhibited almost the same level of performance in the oxidative degradation of benzene, toluene and p-xylene in the temperature range from 300 to 400 0 C. [0172] (b) Reactivity when sulfur compounds were contained 10 Using the combustion catalyst prepared in Example 2, the oxidative degradation of carbon dioxide containing hydrogen sulfide and mercaptan as well as benzene was conducted with the reaction conditions shown in Table 14. The benzene conversion rates are shown in Table 15. [0173] 15 [Table 14] Benzene concentration (ppm) 500 Concentration of hydrogen sulfide or mercaptan (ppm) 600 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 70.6 GHSV (1/h) 30,000 Pressure (MPa) 0.1 Reaction temperature (*C) 350/400 [0174] 80 [Table 15] Reaction temperature 350 0 C 400 0 C Mercaptan 98.20% 99.99% Hydrogen sulfide 97.30% 99.99% [0175] From the results shown in Table 15, it was confirmed that the combustion catalyst 5 prepared in Example 2 exhibited extremely high levels of performance in the oxidative degradation of benzene at a temperature of 350*C and 400*C, regardless of the type of sulfur compounds. [0176] (c) Effects of hydrogen sulfide concentration on reactivity 10 Using the combustion catalyst prepared in Example 2, the oxidative degradation of carbon dioxide containing hydrogen sulfide and benzene was conducted with the reaction conditions shown in Table 16. The benzene conversion rates are shown in Table 17. [0177] 15 [Table 16] Benzene concentration (ppm) 500 Hydrogen sulfide concentration (ppm) 0/1,000/2,000 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 70.6 GHSV (1/h) 30,000 Pressure (MPa) 0.1 Reaction temperature (*C) 380/400/420 [0178] 81 [Table 17] Hydrogen sulfide Reaction temperature concentration (ppm) 380 0 C 400 0 C 420 0 C 0 99.99% 99.99% 99.99% 1,000 99.67% 99.99% 99.99% 2,000 90.93% 99.82% 99.98% [0179] From the results shown in Table 17, it was confirmed that with the combustion 5 catalyst prepared in Example 2, although the reactivity in the oxidative degradation of benzene somewhat declined when the concentration of hydrogen sulfide increased, it was possible to improve the reactivity by increasing the reaction temperature. [0180] (d) Effects of oxygen concentration on reactivity 10 Using the combustion catalyst prepared in Example 2, the oxidative degradation of carbon dioxide containing hydrogen sulfide and benzene was conducted with the reaction conditions shown in Table 18. The benzene conversion rates are shown in Table 19. [0181] 15 [Table 18] Benzene concentration (ppm) 500 Hydrogen sulfide concentration (ppm) 600 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 2.0/5.0 Nitrogen concentration (volume%) 8.1/19.3 Carbon dioxide concentration (volume%) 81.8/70.6 GHSV (1/h) 30,000 Pressure (MPa) 0.1 Reaction temperature (*C) 350/375/400 [0182] 82 [Table 19] Oxygen concentration Reaction temperature (ppm) 350 0 C 375 0 C 400 0 C 2.0 44.92% 99.98% 99.99% 5.0 97.30% 99.99% 99.99% [0183] From the results shown in Table 19, it was confirmed that with the combustion 5 catalyst prepared in Example 2, it was possible to improve the reactivity in the oxidative degradation of benzene in the temperature range from 350 to 400 0 C by increasing the oxygen concentration. [0184] (e) Effects of platinum content in combustion catalyst on reactivity 10 Using the combustion catalyst prepared in Example 2, the oxidative degradation of carbon dioxide containing hydrogen sulfide and benzene was conducted with the reaction conditions shown in Table 20. The benzene conversion rates are shown in Table 21. [0185] 15 [Table 20] Platinum content (g/liter) 1.0/2.0/3.0 Benzene concentration (ppm) 500 Hydrogen sulfide concentration (ppm) 600 Water (steam) concentration (volume%) 5.0 Oxygen concentration (volume%) 5.0 Nitrogen concentration (volume%) 19.3 Carbon dioxide concentration (volume%) 70.6 GHSV (1/h) 25,000 Pressure (MPa) 0.1 Reaction temperature ( 0 C) 350/400 [0186] 83 [Table 21] Reaction temperature Platinum content (OC) 1.0 g/liter 2.0 g/liter 3.0 g/liter 350 98.46% 99.02% 99.54% 400 99.93% 99.98% 99.99% [0187] From the results shown in Table 21, it was confirmed that with the combustion 5 catalyst prepared in Example 2, it was possible to improve the reactivity in the oxidative degradation of benzene at a temperature of 350'C and 400*C by increasing the platinum content in the combustion catalyst. INDUSTRIAL APPLICABILITY 10 [0188] According to the method for purifying carbon dioxide off-gas of the present invention, the sulfur compounds contained in the carbon dioxide off-gas which cause highly toxic or irritating odor, such as hydrogen sulfide and mercaptan, can be purified and emitted as SOx, and thus the method is useful industrially. 15 DESCRIPTION OF THE REFERENCE SYMBOLS [0189] 10, 80: Apparatus for purifying carbon dioxide off-gas (purification apparatus) 11, 81: Heater 20 12, 82: Preheater 13, 83: Catalytic combustor 14 to 26, 84 to 97: Passages 84 212: Vessel 214: Inlet 216: Outlet 220A to 2201: Combustion catalyst unit

Claims

1. A method for purifying carbon dioxide off-gas which is a method for purifying carbon dioxide off-gas in which volatile organic compounds and sulfur compounds in a gas 5 having carbon dioxide as a major component thereof are oxidatively degraded, the method comprising: introducing a carbon dioxide off-gas containing at least a volatile organic compound and sulfur compounds of not less than 50 ppmV and not more than 10,000 ppmV and having carbon dioxide as a major component to a catalytic combustor; and 10 oxidatively degrading the volatile organic compound and the sulfur compound using a combustion catalyst in the catalytic combustor, wherein the combustion catalyst contains at least one metal oxide selected from the group consisting of zirconium oxide, titanium oxide and silicon oxide, and at least one noble metal selected from the group consisting of platinum, palladium and iridium, 15 and a concentration of the sulfur compounds (excluding sulfur oxides) in the gas following an oxidative degradation process is 5 ppmV or less.

2. The method for purifying carbon dioxide off-gas according to Claim 1, 20 wherein the gas having carbon dioxide as a major component and/or air is preheated and is then fed to the catalytic combustor.

3. The method for purifying carbon dioxide off-gas according to Claim 1, 86 wherein the catalytic combustor have at least two catalytic combustion zones in which a combustion catalyst is installed, and at least one material selected from a gas obtained following an oxidative degradation process having carbon dioxide as a major component, air and water, is fed 5 between the catalytic combustion zones so as to cool the gas having carbon dioxide as a major component which is fed to the catalytic combustor.

4. The method for purifying carbon dioxide off-gas according to Claim 1, wherein mercury species contained in the gas having carbon dioxide as a major 10 component are removed, and the gas having carbon dioxide as a major component and from which the mercury species are removed is fed to the catalytic combustor.

5. The method for purifying carbon dioxide off-gas according to Claim 1, 15 wherein the gas having carbon dioxide as a major component is a gas emitted from an acid gas removal apparatus, which brings an acid gas in natural gas produced from a gas field into contact with a liquid sorbent and thereby separates and recovers the acid gas. 20

6. The method for purifying carbon dioxide off-gas according to Claim 5, wherein the gas having carbon dioxide as a major component is a gas that is emitted after a level of hydrogen sulfide therein is reduced by any one of apparatuses among a hydrogen sulfide enrichment unit, a sulfur recovery unit and a tail gas treatment unit which is provided in a step which follows the acid gas removal apparatus. 87

7. The method for purifying carbon dioxide off-gas according to Claim 1, wherein the combustion catalyst with which the aforementioned catalytic combustor is filled is provided with a base and a catalyst layer which is formed on the 5 surface of the base and is composed of the metal oxide and the noble metal, and the specific surface area of the metal oxide is not less than10 m 2 /g and not more than 300 m 2 /g.

8. The method for purifying carbon dioxide off-gas according to Claim 1, 10 wherein the combustion catalyst includes a base having a honeycomb structure provided with multiple air passages, a metal oxide layer formed on the inner surface of the air passages and composed of the metal oxide, and the noble metal deposited at least on the surface layer portion of the metal oxide layer at a density of not less than 0.1 2 2 mg/cm 2 and not more than 10 mg/cm , and 15 the base is formed of a ceramic, a metal oxide or a metal alloy.

9. The method for purifying carbon dioxide off-gas according to Claim 1, wherein the catalytic combustor includes a vessel having an inlet at one end and an outlet on the other, and a plurality of combustion catalyst units installed inside the 20 vessel between the inlet and the outlet with a certain interval between each other, each of the combustion catalyst unit includes a base having a honeycomb structure provided with multiple air passages for passing the carbon dioxide off-gas through, a metal oxide layer formed on the inner surface of the air passages and composed of the metal oxide, and the noble metal deposited at least on the surface layer 88 portion of the metal oxide layer at a density of not less than 0.1 mg/cm 2 and not more than 10 mg/cm 2 , and the base is formed of a ceramic, a metal oxide or a metal alloy, and an inner diameter of the air passage in the combustion catalyst unit close to the outlet is greater than an inner diameter of the air passage in the combustion catalyst unit 5 close to the inlet.

10. A combustion catalyst for purifying carbon dioxide off-gas which is a combustion catalyst for oxidatively degrading at least volatile organic compounds and sulfur compounds contained in the gas having carbon dioxide as a major component at a 10 reaction temperature of not less than 250'C and not more than 650*C, the combustion catalyst comprising: at least one metal oxide selected from the group consisting of zirconium oxide, titanium oxide and silicon oxide; and at least one noble metal selected from the group consisting of platinum, palladium 15 and iridium.

11. The combustion catalyst for purifying carbon dioxide off-gas according to Claim 10, wherein the combustion catalyst includes a base and a catalyst layer which is formed on the surface of the base and is composed of the metal oxide and the noble metal, 20 and the base has a structure of honeycomb, pellet or spherical.

12. The combustion catalyst for purifying carbon dioxide off-gas according to Claim 11, 89 wherein the combustion catalyst includes a base having a honeycomb structure provided with multiple air passages, a metal oxide layer formed on the inner surface of the air passages and composed of the metal oxide, and the noble metal deposited at least on the surface layer portion of the metal oxide layer at a density of not less than 0.1 2 2 5 mg/cm and not more than 10 mg/cm , and the base is formed of a ceramic, a metal oxide or a metal alloy.

13. The combustion catalyst for purifying carbon dioxide off-gas according to Claim 11, wherein the catalytic combustor includes a vessel having an inlet at one end and 10 an outlet on the other, and a plurality of combustion catalyst units installed inside the vessel between the inlet and the outlet with a certain interval between each other, each of the combustion catalyst unit includes a base having a honeycomb structure provided with multiple air passages for passing the carbon dioxide off-gas through, a metal oxide layer formed on the inner surface of the air passages and 15 composed of the metal oxide, and the noble metal deposited at least on the surface layer portion of the metal oxide layer at a density of not less than 0.1 mg/cm2 and not more 2 than 10 mg/cm , and the base is formed of a ceramic, a metal oxide or a metal alloy, and an inner diameter of the air passage in the combustion catalyst unit close to the outlet is greater than an inner diameter of the air passage in the combustion catalyst unit 20 close to the inlet.

14. A method for producing natural gas which is a method for producing natural gas from raw natural gas, the method comprising: 90 a step in which the raw natural gas is fed to a slug catcher, thereby separating the raw natural gas into a liquid phase and a vapor phase using the slug catcher; an acid gas removal step in which a carbon dioxide off-gas having carbon dioxide as a major component and containing volatile organic compounds and sulfur compounds 5 is separated from the vapor phase; a water removal step in which raw gas following a separation of carbon dioxide off-gas therefrom is cooled down, and water condensed as a result is removed; a heavy component removal step in which raw gas following a water removal is subjected to a fractional distillation by a distillation tower to remove heavy hydrocarbons 10 and thereby to yield a natural gas; and an off-gas purification step in which the carbon dioxide off-gas is purified by the method for purifying carbon dioxide off-gas of Claim 1.