TECHNICAL FIELD 
     The present invention relates to a system to and a method for producing gasoline or dimethyl ether, and more specifically relates to a system to and a method for producing gasoline or dimethyl ether from natural gas via methanol. 
     BACKGROUND ART 
     In synthesizing methanol from natural gas, in most cases, natural gas is steam-reformed, then reformed gas containing hydrogen and carbon monoxide is generated, and methanol is then synthesized from the reformed gas. In a synthetic plant for producing methanol in the above-described manner, steam and heat required within the plant are provided by heat recovery from flue gas generated by the steam reforming and heat recovery from reformed gas and also by using heat of reaction resulting from the synthesis of methanol so that the so-called self-balance is secured at the stage of designing of the system. 
     On the other hand, Japanese Patent Publication (B2) No. H04-51596 discloses a method for synthesizing gasoline from methanol via dimethyl ether (DME). A reaction for synthesizing DME from methanol is an exothermic reaction, and the heat of reaction is 231 kcal equivalent to 1 kg of methanol. In addition, a reaction for synthesizing gasoline from methanol via DME is also an exothermic reaction, and the heat of reaction is 416 kcal equivalent to 1 kg of methanol. 
     BACKGROUND LITERATURE 
     Patent Literature 
     Patent Literature 1: Japanese Patent Publication (B2) No. H04-51596 
     DISCLOSURE OF INVENTION  
     Problem to be Solved by Invention 
     In producing DME or gasoline from natural gas in a methanol synthesis plant in which a synthesis column for synthesizing DME or gasoline from methanol as discussed in Japanese Patent Publication (B2) No. H04-51596 is installed, energy becomes excessive compared with the self-balance of energy secured in a conventional methanol synthesis plants because the reaction for synthesizing DME or gasoline is an exothermic reaction, as described above. However, a problem may arise in that exothermic energy generated by a reaction for synthesizing DME or gasoline may be of no use within the plant and therefore will be wasted. 
     Further, in methanol synthesis plants, a distillation operation for removing moisture from methanol is performed in the production process, but the heat used for distillation may be in excess because a distillation operation like this is not necessary for synthesizing DME and gasoline from methanol. 
     In order to solve the above-described problem, the purpose of the invention is to provide a system or a method for producing gasoline or DME from natural gas via methanol, which is capable, in producing gasoline or DME from natural gas via methanol, of effectively using exothermic energy generated by the synthesis of gasoline or DME and heat generated and remaining because distillation of methanol is unnecessary without having to waste it. 
     Means for Solving the Problem 
     In an aspect, the present invention provides a system for producing gasoline or DME from natural gas via methanol includes: a steam-reforming device for steam-reforming natural gas to generate reformed gas; a methanol synthesis device for synthesizing methanol from the reformed gas generated by the steam reforming device; a gasoline or DME synthesis device for synthesizing gasoline or DME from the methanol synthesized by the methanol synthesis device; and at least one device selected from the group consisting of: a pre-reforming device for pre-reforming the natural gas prior to the steam-reforming of the natural gas; a carbon dioxide recovery device for recovering carbon dioxide from flue gas of the steam reforming device; and an air preheating device for preheating combustion air to be supplied to the steam-reforming device by using the gasoline or DME synthesis device, wherein an overall energy balance of the system is constructed by using heat recovery from the flue gas generated in the steam-reforming device, heat recovery from the reformed gas, synthesis heat generated in the methanol synthesis device, synthesis heat generated in the gasoline or dimethyl ether synthesis device, heat recovered in the pre-reforming device if selected, heat recovered in the carbon dioxide recovery device during recovery of carbon dioxide if selected, and heat recovered in the air preheat device if selected. 
     In another aspect, the present invention provides a method for producing gasoline or DME from natural gas via methanol includes: a steam-reforming step of steam-reforming natural gas to generate reformed gas; a methanol synthesis step of synthesizing methanol from the reformed gas generated in the steam-reforming step; a gasoline or DME synthesis step of synthesizing gasoline or DME from the methanol synthesized in the methanol synthesis step; and at least one step selected from the group consisting of: a pre-reforming step of pre-reforming the natural gas prior to the steam-reforming of the natural gas; a carbon dioxide recovery step of recovering carbon dioxide from flue gas of the steam-reforming step; and an air preheating step of preheating combustion air to be supplied to the steam-reforming step by using synthesis heat generated in the synthesis of gasoline or DME, wherein an overall energy balance of a system for carrying out the method is constructed by using heat recovery from the flue gas generated in the steam-reforming step, heat recovery from the reformed gas, synthesis heat generated in the methanol synthesis step, synthesis heat generated in the gasoline or dimethyl ether synthesis step, heat recovered in the pre-reforming step if selected, heat recovered in the carbon dioxide recovery step during recovery of carbon dioxide if selected, and heat recovered in the air preheating step if selected. 
     Advantageous Effects of Invention 
     As described above, according to the present invention, at least one is selected from the group consisting of: the pre-reforming for pre-reforming natural gas prior to the steam-reforming of natural gas; the carbon dioxide recovering for recovering carbon dioxide from flue gas generated in the steam-reforming; and the air preheating for preheating combustion air to be supplied to the steam-reforming by using synthesis heat generated in the synthesis of gasoline or DME, and in addition, an overall energy balance of the system is constructed by using heat recovery from flue gas generated in the steam-reforming, heat recovery from the reformed gas, synthesis heat generated in the synthesis of methanol, synthesis heat generated in the synthesis of gasoline or DME, and the heat recovered in the pre-reforming, carbon dioxide recovering, or air preheating, respectively, if selected, and thereby the exothermic energy generated in the synthesis of gasoline or DME can be effectively used without wasting it. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an embodiment of a system for producing gasoline from natural gas via methanol according to the present invention. 
         FIG. 2  is a schematic view showing another embodiment of a system for producing gasoline from natural gas via methanol according to the present invention. 
         FIG. 3  is a schematic view showing still another embodiment of a system for producing gasoline from natural gas via methanol according to the present invention. 
         FIG. 4  is a schematic view showing an embodiment of the system for producing gasoline from natural gas via methanol according to the present invention, which is a combination of the embodiments illustrated in  FIGS. 1 to 3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinbelow, an embodiment of a system and a method for producing gasoline from natural gas via methanol according to the present invention will be described with reference to the accompanying drawings. 
     As shown in  FIG. 1 , the system according to the present embodiment mainly includes a steam reformer  10 , which is configured to generate reformed gas by steam-reforming natural gas, a methanol synthesis column  20 , which is configured to synthesize methanol from the reformed gas generated by the steam reformer, a gasoline synthesis column  30 , which is configured to synthesize gasoline from the methanol synthesized by the methanol synthesis column, and a pre-reformer  40 , which is configured to pre-reform natural gas before the natural gas is steam-reformed. 
     The steam reformer  10  primarily includes a reaction tube  11  for steam reforming, a burning portion  12  disposed around the reaction tube  11 , a waste heat recovery portion  15 , which is configured to recover waste heat of the flue gas generated in the burning portion  12 , and a stack  16 , which is configured to release the flue gas to the atmosphere after waste heat has been recovered therefrom. The reaction tube  11 , which includes a steam reforming catalyst charged inside the tube, is a device for generating hydrogen, carbon monoxide, and carbon dioxide from natural gas containing methane as its main ingredient by carrying out the following reactions. For the steam reforming catalyst, known catalysts such as a nickel-based catalyst can be used, for example.
 
CH 4 +H 2 O→3H 2 +CO   (1)
 
CO+H 2 O→H 2 +CO 2    (2)
 
     A material supply line  13  for supplying a material  1 , which includes natural gas and steam, is connected to an inlet of the reaction tube  11 . The burning portion  12  includes a combustion burner (not shown) for heating the reaction tube  11 , and a fuel supply line  14  for supplying a fuel  2  such as natural gas is connected to the combustion burner. A reformed gas supply line  18  is connected to an outlet of the reaction tube  11 , which is a line for supplying reformed gas containing hydrogen, carbon monoxide, and carbon dioxide generated by the steam reforming reaction as its main ingredients to a methanol synthesis column  20 . 
     A pre-reformer  40  is a device configured to pre-reform C2 or higher hydrocarbons contained in the natural gas such as ethane, primarily, into C1 hydrocarbons such as methane, carbon monoxide, or hydrogen. The pre-reformer  40  includes a pre-reforming catalyst charged inside the tube. For the pre-reforming catalyst, known catalysts such as a nickel-based catalyst can be used. 
     The pre-reformer  40  is disposed on the upstream side of the steam reformer  10  in the direction of supply of the material, more specifically, in the material supply line  13 . In the material supply line  13 , a first flue gas-material heat exchanger  41 , which is configured to preheat the material  1  with flue gas from the waste heat recovery portion  15 , is provided on the further upstream side of the pre-reformer  40  in the direction of supply of the material, and a second flue gas-material heat exchanger  42 , which is configured to preheat the material that has been pre-reformed by the pre-reformer  40  with the flue gas from the waste heat recovery portion  15 , is provided on the downstream side of the pre-reformer  40  in the direction of supply of the material. 
     In other words, the waste heat recovery portion  15  of the steam reformer  10  includes the second flue gas-material heat exchanger  42  and the first flue gas-material heat exchanger  41  described above, and also a flue gas-steam heat exchanger  17 , disposed in order of the flow of the flue gas from the burning portion  12  to the stack  16 . The flue gas-steam heat exchanger  17  is a device for obtaining steam or heat to be used within the system, and is configured to recover heat from the flue gas and obtain high-pressure steam by heating boiler water and the like with the flue gas flowing inside the waste heat recovery portion  15 . 
     Similarly, the reformed gas supply line  18  is provided with a reformed gas-steam heat exchanger  19 , which is provided in order to obtain steam or heat to be used within the system. The reformed gas-steam heat exchanger  19  is a device configured to obtain high-pressure steam and recover heat from the reformed gas by heating boiler water and the like by using the reformed gas. 
     The methanol synthesis column  20  is a device configured to synthesize methanol from reformed gas by running the following reactions.
 
CO+2H 2 →CH 3 OH   (3)
 
CO 2 +3H 2 →CH 3 OH+H 2 O   (4)
 
     The methanol synthesis column  20  includes a methanol synthesis catalyst charged inside the tube. For the methanol synthesis catalyst, known catalysts such as a copper-based catalyst can be used. A methanol supply line  22  is connected to methanol synthesis column  20 , which is a line for supplying methanol synthesized by the methanol synthesis column  20  to the gasoline synthesis column  30 . Note that in addition to the synthesized methanol, liquid crude methanol containing water, which is a byproduct of the reaction of Formula 4, flows in the methanol supply line  22 . 
     The gasoline synthesis column  30  is a device which is configured to synthesize gasoline from methanol by running the reactions of the following Formulae.
 
2CH 3 OH→CH 3 OCH 3 +H 2 O   (5)
 
1/2nCH 3 OCH 3 →(CH 2 ) n+1/2nH 2 O   (6)
 
     As described above, methanol is synthesized by the gasoline synthesis reaction expressed in Formula 3 into gasoline via the dimethyl ether (i.e., DME) synthesis reaction expressed by Formula 5. In the gasoline synthesis column  30 , two types of catalysts including the DME synthesis catalyst and the gasoline synthesis catalyst are provided in two stages so that two reactions can be run in stages. For the DME synthesis catalyst, known catalysts such as an aluminosilicate type zeolite-based catalyst can be used, for example. In addition, for the gasoline synthesis catalyst, known catalysts such as an aluminosilicate type zeolite-based catalyst can be used. 
     A gasoline supply line  32  is connected with the gasoline synthesis column  30 , which is a line for supplying gasoline synthesized by the gasoline synthesis column  30  to storage facilities (not shown). Note that although the example illustrated in  FIG. 1  includes the gasoline synthesis column  30 , a DME synthesis column, which is configured to produce DME by running the reactions up to the DME synthesis reaction of Formula 5, can be provided instead of the gasoline synthesis column  30 . 
     According to the above-described configuration, the fuel  2  such as natural gas is first supplied to the burning portion  12  of the steam reformer  10  via the fuel supply line  14 . In the burning portion  12 , the fuel  2  is burned together with air, and the reaction tube  11  is heated to a temperature ranging from about 800° C. to about 900° C. The flue gas containing carbon dioxide generated in the burning portion  12  flows into the waste heat recovery portion  15 . 
     On the other hand, the material  1  containing natural gas and steam is heated by the first flue gas-material heat exchanger  41  of the waste heat recovery portion  15  of the steam reformer  10  to a temperature ranging from about 450° C. to about 550° C., and then the heated material  1  is supplied to the pre-reformer  40 . In the pre-reformer  40 , C2 or higher hydrocarbons contained in the material  1 , such as ethane, is pre-reformed into methane and the like. The pre-reformed gas is heated by the second flue gas-material heat exchanger  42  again to a temperature ranging from about 600° C. to about 700° C., and then it is supplied to the reaction tube  11  of the steam reformer  10  via the material supply line  13 . 
     After the material  1  is heated serially by the second flue gas-material heat exchanger  42  and the first flue gas-material heat exchanger  41  of the waste heat recovery portion  15  as described above and heat is recovered by the flue gas-steam heat exchanger  17  by heating boiler water or the like, the flue gas containing carbon dioxide generated in the burning portion  12 , which has the temperature of about 1,000° C., is released from the stack  16  to the atmosphere. 
     In the reaction tube  11  of the steam reformer  10 , the material  1  is steam-reformed by the reactions of Formulae 1 and 2 and converted into reformed gas containing hydrogen, carbon monoxide, and carbon dioxide as its main ingredients. Before the reformed gas is supplied to the methanol synthesis column  20  via the reformed gas supply line  18 , heat is recovered by the reformed gas-steam heat exchanger  19  by heating boiler water or the like. 
     In the methanol synthesis column  20 , methanol is synthesized from the reformed gas by the reactions of Formulae 3 and 4. The methanol synthesis reactions are exothermic reactions. The temperature of the reformed gas is controlled by the reformed gas-steam heat exchanger  19  to the range of about 160° C. to about 200° C., which is suitable for synthesis of methanol. Methanol synthesized by the methanol synthesis column  20  is supplied to the gasoline synthesis column  30  via the methanol supply line  22  as crude methanol containing water. 
     In the gasoline synthesis column  30 , gasoline is synthesized from methanol by the reactions of Formulae 5 and 6. The gasoline synthesis reactions are an exothermic reaction. In addition, because water is generated as a byproduct in the reaction of Formula 6, the crude methanol may contain water, and it is therefore not necessary to provide the methanol supply line  22  for supplying methanol to the gasoline synthesis column  30  with a purification device for removing water by distilling crude methanol, which is necessary in a conventional methanol synthesis plant. 
     In the present embodiment, as described above, and differently from conventional methanol synthesis plants, the gasoline synthesis column  30  is provided in which exothermic reactions are run and thermal energy is generated, and in addition, it becomes unnecessary to provide a methanol distillation column which consumes energy, and thereby the amount of supply of the fuel  2  to the burning portion  12  of the steam reformer  10  can be reduced, although excessive energy is generated, by providing the pre-reformer  40  to heat the material at locations across the pre-reformer  40  by using the first and the second flue gas-material heat exchangers  41  and  42  of the waste heat recovery portion  15  of the steam reformer  10 . In addition, in the waste heat recovery portion  15  of the steam reformer  10 , the recovered heat decreases because the first and the second flue gas-material heat exchangers  41  and  42  are provided; however, the system can be designed so that the energy of the entire system can be self-balanced by using the exothermic energy generated in the gasoline synthesis column  30  to compensate for the decreased heat. 
     Next, an embodiment illustrated in  FIG. 2  will be described. A system of the present embodiment primarily includes the steam reformer  10 , the methanol synthesis column  20 , the gasoline synthesis column  30 , and a CO 2  recovery device  50 , which is configured to remove CO 2  from flue gas of the steam reformer. Note that the same configurations as those of the system illustrated in  FIG. 1  are provided with the same reference symbols, and detailed descriptions thereof will not be repeated here. 
     The CO 2  recovery device  50  is a device configured to absorb and remove carbon dioxide from flue gas by bringing CO 2  absorbing liquid into gas-liquid contact with the flue gas that flows in the waste heat recovery portion  15  of the steam reformer  10 . The CO 2  recovery device  50  is disposed on the flue gas downstream side of the flue gas-steam heat exchanger  17 . Note that an absorbing liquid regeneration device (not shown) is added to the CO 2  recovery device  50 . The absorbing liquid regeneration device is a device configured to obtain carbon dioxide gas as well as regenerate the CO 2  absorbing liquid by separating carbon dioxide from the CO 2  absorbing liquid which has absorbed carbon dioxide. 
     The CO 2  recovery device  50  is provided with a CO 2  supply line  51  for supplying the recovered carbon dioxide gas to the methanol synthesis column  20  to reuse it as a material of the reaction expressed by Formula 4 mentioned above, which is run in the methanol synthesis column  20 . 
     With the above-described configuration, first, the fuel  2  such as natural gas is supplied to the burning portion  12  of the steam reformer  10  via the fuel supply line  14 . In the burning portion  12 , the fuel  2  is burned together with air, and the reaction tube  11  is heated to a temperature ranging from about 800° C. to about 900° C. After boiler water or the like is heated by the flue gas-steam heat exchanger  17  of the waste heat recovery portion  15  to recover heat and CO 2  is removed by the CO 2  recovery device  50 , the flue gas containing carbon dioxide generated in the burning portion  12 , which has the temperature of about 1,000° C., is released from the stack  16  to the atmosphere. 
     On the other hand, the material  1  containing natural gas and steam is supplied to the reaction tube  11  of the steam reformer  10  via the material supply line  13 . In the reaction tube  11  of the steam reformer  10 , the material  1  is converted by a steam reforming reaction into reformed gas. After heat is recovered by heating boiled water or the like by the reformed gas-steam heat exchanger  19 , the reformed gas is supplied to the methanol synthesis column  20  via the reformed gas feed line  18 . In addition, carbon dioxide recovered by the CO 2  recovery device  50  is also supplied to the methanol synthesis column  20  via the CO 2  supply line  51 . 
     In the methanol synthesis column  20 , methanol is synthesized from the reformed gas and the carbon dioxide gas by running the reactions of Formulae 3 and 4. By adding the carbon dioxide gas, excessive hydrogen contained in the reformed gas can be converted into methanol, and as a result, the production of methanol can be increased. In addition, because the methanol synthesis reactions are an exothermic reaction, the exothermic energy generated in the methanol synthesis column  20  increases as the production of methanol increases. Methanol synthesized by the methanol synthesis column  20  is supplied to the gasoline synthesis column  30  via the methanol supply line  22  as crude methanol containing water. 
     In the gasoline synthesis column  30 , gasoline is synthesized from methanol by the reactions of Formulae 5 and 6. Because the supply of methanol increases, the production of gasoline also increases, and the exothermic energy generated in the gasoline synthesis column  30  also increases in accordance with the increase in the production because the gasoline synthesis reactions are an exothermic reaction. 
     In the present embodiment, as described above and differently from conventional methanol synthesis plants, the gasoline synthesis column  30  is provided in which exothermic reactions are run and thermal energy is generated, and in addition, it becomes unnecessary to provide a methanol distillation column which consumes energy, and thereby the system can be designed, although excessive energy is generated, so that the energy of the entire system can be self-balanced by providing the CO 2  recovery device  50  and the absorbing liquid regeneration device (not shown) that consume energy. In addition, the production of gasoline in the gasoline synthesis column  30  can be increased by supplying carbon dioxide recovered by the CO 2  recovery device  50  to the methanol synthesis column  20  together with the reformed gas. 
     An embodiment illustrated in  FIG. 3  will be described. A system of the present embodiment primarily includes the steam reformer  10 , the methanol synthesis column  20 , the gasoline synthesis column  30 , and an the air preheater  60 , which is configured to preheat combustion air to be supplied to the burning portion of the steam reformer. Note that the same configurations as those of the system illustrated in  FIGS. 1 and 2  are provided with the same reference symbols, and detailed descriptions thereof will not be repeated here. 
     The air preheater  60  includes a fan  63  for feeding combustion air, a flue gas-combustion air heat exchanger  62 , which is configured to preheat combustion air with the flue gas that flows in the waste heat recovery portion  15  of the steam reformer  10 , a combustion air introduction line  61  for introducing the preheated combustion air into the gasoline synthesis column  30  with the synthesis heat generated in the gasoline synthesis column  30  in order to further heat the preheated combustion air, and a combustion air supply line  64  for supplying the combustion air heated with the synthesis heat to the burning portion  12  of the steam reformer  10 . The flue gas-combustion air heat exchanger  62  is disposed on the flue gas downstream side of the flue gas-steam heat exchanger  17 . 
     Means for heating combustion air with the heat of reaction generated in the gasoline synthesis column  30  is not limited to specific means, but for example, the combustion air can be heated with steam obtained by heating boiler water with the heat of reaction generated in the gasoline synthesis column  30 . Alternatively, heat can be exchanged between the DME synthesis catalyst in the gasoline synthesis column  30  or the reaction tube (not shown) charged with the gasoline synthesis catalyst and the combustion air. 
     According to the above-described configuration, the fuel  2  such as natural gas is first supplied to the burning portion  12  of the steam reformer  10  via the fuel supply line  14 . In the burning portion  12 , the fuel  2  is burned together with air, and the reaction tube  11  is heated to a temperature ranging from about 800° C. to about 900° C. After boiler water or the like is heated by the flue gas-steam heat exchanger  17  of the waste heat recovery portion  15  to recover heat and CO 2  is removed by the CO 2  recovery device  50 , the flue gas containing carbon dioxide generated in the burning portion  12 , which has the temperature of about 1,000° C., is cooled to a temperature ranging from about 300° C. to about 400° C. Then, after the combustion air from the fan  63  is heated by the flue gas-combustion air heat exchanger  62 , the flue gas is released from the stack  16 . 
     On the other hand, the material  1  containing natural gas and steam is supplied to the reaction tube  11  of the steam reformer  10  via the material supply line  13 . In the reaction tube  11  of the steam reformer  10 , the material  1  is converted into reformed gas by a steam reforming reaction. After heat is recovered by heating boiled water or the like by using the reformed gas-steam heat exchanger  19 , the reformed gas is supplied to the methanol synthesis column  20  via the reformed gas feed line  18 . 
     In the methanol synthesis column  20 , methanol is synthesized from the reformed gas and carbon dioxide gas. Methanol synthesized by the methanol synthesis column  20  is supplied to the gasoline synthesis column  30  via the methanol supply line  22  as crude methanol containing water. 
     In the gasoline synthesis column  30 , gasoline is synthesized from methanol by the reactions of Formulae 5 and 6. The synthesis reaction from methanol to DME run in the gasoline synthesis column  30  is an exothermic reaction, and its heat of reaction is 185 kcal equivalent to 1 kg of methanol. In addition, the gasoline synthesis reaction is also an exothermic reaction, and its heat of reaction is 231 kcal equivalent to 1 kg of methanol. Therefore, in synthesizing gasoline from methanol, the heat of reaction is 416 kcal equivalent to 1 kg of methanol. The combustion air introduced from the combustion air inlet line  61  is heated by using this heat of reaction. 
     With respect to the condition of the DME synthesis reaction performed by the gasoline synthesis column  30 , it is preferable that the temperature range from 250° C. to 300° C. In addition, for the condition of the gasoline synthesis reaction, it is preferable that the temperature range from 380° C. to 450° C. Therefore, the combustion air can be heated up to the range of about 300° C. to about 380° C. 
     The combustion air heated by the gasoline synthesis column  30  is supplied to the burning portion  13  of the steam reformer  10  via the combustion air supply line  64  together with the fuel  2 . Because the combustion air is heated as described above, the supply of the fuel  2  to the burning portion  13  can be reduced. 
     In the present embodiment, as described above and differently from conventional methanol synthesis plants, the gasoline synthesis column  30  is provided in which exothermic reactions are run and thermal energy is generated, and in addition, it becomes unnecessary to provide a methanol distillation column which consumes energy, and thereby the system can be designed, although excessive energy is generated, so that the energy of the entire system can be self-balanced because the supply of the fuel  2  to the steam reformer  10  can be reduced by preheating the combustion air in the steam reformer  10  and preheating the combustion air by using the exothermic energy generated in the gasoline synthesis column  30 . 
     An embodiment illustrated in  FIG. 4  will be described. A system of the present embodiment is a combination of all the embodiments illustrated in  FIGS. 1 to 3 . More specifically, the system primarily includes the steam reformer  10 , the methanol synthesis column  20 , the gasoline synthesis column  30 , the pre-reformer  40 , the CO 2  recovery device  50 , and the air preheater  60 , which is configured to preheat air to be supplied to the burning portion of the steam reformer. Note that the same configurations as those of the system illustrated in  FIGS. 1 to 3  are provided with the same reference symbols, and detailed descriptions thereof will not be repeated here. 
     The components of the waste heat recovery portion  15  of the steam reformer  10  are disposed in the following order from the flue gas upstream side, i.e., the second flue gas-material heat exchanger  42 , the first flue gas-material heat exchanger  41 , the flue gas-steam heat exchanger  17 , the flue gas-combustion air heat exchanger  62 , and the CO 2  recovery device  50 . 
     According to the above-described configuration, the fuel  2  such as natural gas is first supplied to the burning portion  12  of the steam reformer  10  via the fuel supply line  14 . In the burning portion  12 , the fuel  2  is burned together with air, and the reaction tube  11  is heated to a temperature ranging from about 800° C. to about 900° C. After the material is heated by the second flue gas-material heat exchanger  42  and cooled to a temperature ranging from about 450° C. to about 550° C. and the material is heated by the first flue gas-material heat exchanger  41 , the flue gas containing carbon dioxide generated in the burning portion  12 , which has the temperature of about 1,000° C., is cooled to a temperature ranging from about 600° C. to about 700° C. Next, boiler water or the like is heated by the flue gas-steam heat exchanger  17  of the waste heat recovery portion  15 , cooled to a temperature ranging from about 300° C. to about 400° C., and then heat is recovered by heating the combustion air by using the flue gas-combustion air heat exchanger  62 . Then, after CO 2  is removed by the CO 2  recovery device  50 , the flue gas is released from the stack  16  to the atmosphere. 
     On the other hand, the material  1  containing natural gas and steam is supplied to the reaction tube  11  of the steam reformer  10  via the material supply line  13 . In the reaction tube  11  of the steam reformer  10 , the material  1  is converted by a steam reforming reaction into reformed gas. After heat is recovered by heating boiled water or the like by using the reformed gas-steam heat exchanger  19 , the reformed gas is supplied to the methanol synthesis column  20  via the reformed gas feed line  18 . In addition, carbon dioxide recovered by the CO 2  recovery device  50  is also supplied to the methanol synthesis column  20  via the CO 2  supply line  51 . 
     In the methanol synthesis column  20 , methanol is synthesized from the reformed gas and carbon dioxide gas by running the reactions of Formulae 3 and 4. By adding the carbon dioxide gas, the production of the methanol and exothermic energy can be increased in the methanol synthesis column  20 . Methanol synthesized by the methanol synthesis column  20  is supplied to the gasoline synthesis column  30  via the methanol supply line  22  as crude methanol containing water. 
     In the gasoline synthesis column  30 , gasoline is synthesized from methanol by running the reactions of Formulae 5 and 6. Because the supply of methanol increases, the production of gasoline and the exothermic energy can be increased in the gasoline synthesis column  30 . In the gasoline synthesis column  30 , the combustion air introduced from combustion air inlet line  61  is heated by the heat of reaction. 
     In the present embodiment, as described above and differently from conventional methanol synthesis plants, the gasoline synthesis column  30  is provided in which exothermic reactions are run and thermal energy is generated, and in addition, it becomes unnecessary to provide a methanol distillation column which consumes energy, and thereby the amount of supply of the fuel  2  to the steam reformer  10  can be reduced, although excessive energy is generated, by providing the pre-reformer  40 , the CO 2  recovery device  50  and the absorbing liquid regeneration device (not shown), and the air preheater  60  which preheats combustion air by using the heat of reaction of the gasoline synthesis column  30  and by heating the material at locations across the pre-reformer  40  by using the first and the second flue gas-material heat exchangers  41  and  42  of the waste heat recovery portion  15  of the steam reformer  10 , and the supply of the fuel  2  to the steam reformer  10  can also be reduced by preheating the combustion air. In addition, in the waste heat recovery portion  15  of the steam reformer  10 , the recovered heat decreases because the first and the second flue gas-material heat exchangers  41  and  42  are provided, however, the system can be designed so that the energy of the entire system can be self-balanced because the exothermic energy generated in the gasoline synthesis column  30  can be used to compensate for the decreased heat. The production of gasoline in the gasoline synthesis column  30  can be increased by supplying carbon dioxide recovered by the CO 2  recovery device  50  to the methanol synthesis column  20  together with the reformed gas. Further, the supply of the fuel  2  can be reduced by converting the entire carbon dioxide gas or a part thereof recovered by the CO 2  recovery device  50  into carbon monoxide gas and by supplying it to the burning portion  12  of the steam reformer  10  together with the fuel  2 , which also enables self-balancing of the system. 
     EXAMPLES 
     Simulation of energy balance was carried out for the respective embodiments illustrated in  FIGS. 1 to 3 , respective embodiments including a combination of two of the embodiments illustrated in  FIGS. 1 to 3 , and the embodiment illustrated in  FIG. 4 , which includes all the embodiments illustrated in  FIGS. 1 to 3 . The results are shown in Table 1. Note that the simulation was carried out for the case in which the daily production of methanol was 2,500 tons. For the conditions of both the material and the fuel, natural gas was used. In addition, for comparison, results of a conventional example in which methanol is synthesized from natural gas and those of a reference example in which gasoline or DME is synthesized from natural gas via methanol are also shown in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                   
                   
                   
                 Reference 
                 Reference 
               
               
                   
                   
                 Conventional 
                 Example 1 
                 Example 2 
               
               
                   
                   
                 Example 
                 Synthesis of 
                 Synthesis 
               
               
                   
                   
                 Synthesis of 
                 DME via 
                 of gasoline via 
               
               
                   
                   
                 methanol 
                 methanol 
                 methanol 
               
               
                   
               
               
                 Methanol 
                 (T/D) 
                 2,500 
                 2,500 
                 2,500 
               
               
                 DME 
                 (T/D) 
                 — 
                 1,797 
                 — 
               
               
                 Gasoline 
                 (barrel/D) 
                 — 
                 — 
                 8,000 
               
               
                 LPG 
                 (barrel/D) 
                 — 
                 — 
                 1,030 
               
               
                 Material 
                 (10 6  kcal/H) 
                 733.5 
                 733.5 
                 733.5 
               
               
                 Fuel 
                 (10 6  kcal/H) 
                 370.8 
                 370.8 
                 370.8 
               
               
                 Process steam 
                 (T/H) 
                 211 
                 211 
                 211 
               
               
                 Process heat recovery 
                 (T/H) 
                 176.1 
                 176.1 
                 176.1 
               
               
                 Flue gas heat recovery 
                 (10 6  kcal/H) 
                 127.2 
                 127.2 
                 127.2 
               
               
                 Methanol synthesis heat 
                 (10 6  kcal/H) 
                 44.4 
                 44.4 
                 44.4 
               
               
                 Methanol distillation heat 
                 (10 6  kcal/H) 
                 109.5 
                 — 
                 — 
               
               
                 DME synthesis heat 
                 (10 6  kcal/H) 
                 — 
                 19.6 
                 — 
               
               
                 MTG synthesis heat 
                 (10 6  kcal/H) 
                 — 
                 — 
                 43.3 
               
               
                 CO 2  recovery amount 
                 (T/H) 
                 — 
                 — 
                 — 
               
               
                 Heat recovered by CO 2  recovery 
                 (10 6  kcal/H) 
                 — 
                 — 
                 — 
               
               
                 Heat recovered by pre-reforming 
                 (10 6  kcal/H) 
                 — 
                 — 
                 — 
               
               
                 Heat recovered from flue gas of synthesis 
                 (10 6  kcal/H) 
                 — 
                 — 
                 — 
               
               
                 of MTG 
               
               
                 CO 2  compression energy 
                 (kw) 
                 — 
                 — 
                 — 
               
               
                 Auxiliary boiler 
                 (10 6  kcal/H) 
                 18.1 
                 18.1 
                 18.1 
               
               
                 Residual energy 
                 (10 6  kcal/H) 
                 0 
                 129.1 
                 152.8 
               
               
                   
               
             
          
           
               
                   
                   
                   
                   
                 Example 3 
               
               
                   
                   
                 Example 1 
                 Example 2 
                 Synthesis of 
               
               
                   
                   
                 Synthesis of 
                 Synthesis of 
                 gasoline via 
               
               
                   
                   
                 gasoline via 
                 gasoline via 
                 methanol + 
               
               
                   
                   
                 methanol + 
                 methanol + 
                 MTG heat 
               
               
                   
                   
                 pre-reforming 
                 CO 2  recovery 
                 recovery 
               
               
                   
               
               
                 Methanol 
                 (T/D) 
                 2,500 
                 3.375 
                 2,500 
               
               
                 DME 
                 (T/D) 
                 — 
                 — 
                 — 
               
               
                 Gasoline 
                 (barrel/D) 
                 8,000 
                 10,800 
                 8,000 
               
               
                 LPG 
                 (barrel/D) 
                 1,030 
                 1,417 
                 1,030 
               
               
                 Material 
                 (10 6  kcal/H) 
                 733.5 
                 733.5 
                 733.5 
               
               
                 Fuel 
                 (10 6  kcal/H) 
                 296.6 
                 370.8 
                 359.4 
               
               
                 Process steam 
                 (T/H) 
                 211 
                 211 
                 211 
               
               
                 Process heat recovery 
                 (T/H) 
                 176.1 
                 176.1 
                 176.1 
               
               
                 Flue gas heat recovery 
                 (10 6  kcal/H) 
                 53.0 
                 127.2 
                 127.2 
               
               
                 Methanol synthesis heat 
                 (10 6  kcal/H) 
                 44.4 
                 59.9 
                 44.4 
               
               
                 Methanol distillation heat 
                 (10 6  kcal/H) 
                 — 
                 — 
                 — 
               
               
                 DME synthesis heat 
                 (10 6  kcal/H) 
                 — 
                 — 
                 — 
               
               
                 MTG synthesis heat 
                 (10 6  kcal/H) 
                 43.3 
                 58.5 
                 43.3 
               
               
                 CO 2  recovery amount 
                 (T/H) 
                 — 
                 1,205 
                 — 
               
               
                 Heat recovered by CO 2  recovery 
                 (10 6  kcal/H) 
                 — 
                 32.0 
                 — 
               
               
                 Heat recovered by pre-reforming 
                 (10 6  kcal/H) 
                 74.2 
                 — 
                 — 
               
               
                 Heat recovered from flue gas of synthesis 
                 (10 6  kcal/H) 
                 — 
                 — 
                 11.4 
               
               
                 of MTG 
               
               
                 CO 2  compression energy 
                 (kw) 
                 — 
                 7,000 
                 — 
               
               
                 Auxiliary boiler 
                 (10 6  kcal/H) 
                 18.1 
                 0 
                 18.1 
               
               
                 Residual energy 
                 (10 6  kcal/H) 
                 60.5 
                 133.4 
                 123.3 
               
               
                   
               
             
          
           
               
                   
                   
                 Example 4 
                 Example 5 
               
               
                   
                   
                 Synthesis of gasoline via 
                 Synthesis of gasoline via 
               
               
                   
                   
                 methanol + 
                 methanol + 
               
               
                   
                   
                 CO 2  recovery + 
                 CO 2  recovery + 
               
               
                   
                   
                 pre-reforming 
                 MTG heat recovery 
               
               
                   
               
               
                 Methanol 
                 (T/D) 
                 3.375 
                 3.375 
               
               
                 DME 
                 (T/D) 
                 — 
                 — 
               
               
                 Gasoline 
                 (barrel/D) 
                 10,800 
                 10,800 
               
               
                 LPG 
                 (barrel/D) 
                 1,417 
                 1,417 
               
               
                 Material 
                 (10 6  kcal/H) 
                 733.5 
                 733.5 
               
               
                 Fuel 
                 (10 6  kcal/H) 
                 296.6 
                 359.4 
               
               
                 Process steam 
                 (T/H) 
                 211 
                 211 
               
               
                 Process heat recovery 
                 (T/H) 
                 176.1 
                 176.1 
               
               
                 Flue gas heat recovery 
                 (10 6  kcal/H) 
                 53.0 
                 115.8 
               
               
                 Methanol synthesis heat 
                 (10 6  kcal/H) 
                 59.9 
                 59.9 
               
               
                 Methanol distillation heat 
                 (10 6  kcal/H) 
                 — 
                 — 
               
               
                 DME synthesis heat 
                 (10 6  kcal/H) 
                 — 
                 — 
               
               
                 MTG synthesis heat 
                 (10 6  kcal/H) 
                 58.5 
                 58.5 
               
               
                 CO 2  recovery amount 
                 (T/H) 
                 1,205 
                 1,205 
               
               
                 Heat recovered by CO 2  recovery 
                 (10 6  kcal/H) 
                 32.0 
                 32.0 
               
               
                 Heat recovered by pre-reforming 
                 (10 6  kcal/H) 
                 74.2 
                 — 
               
               
                 Heat recovered from flue gas of synthesis 
                 (10 6  kcal/H) 
                 — 
                 11.4 
               
               
                 of MTG 
               
               
                 CO 2  compression energy 
                 (kw) 
                 7,000 
                 7,000 
               
               
                 Auxiliary boiler 
                 (10 6  kcal/H) 
                 0 
                 0 
               
               
                 Residual energy 
                 (10 6  kcal/H) 
                 47.9 
                 110.7 
               
               
                   
               
               
                   
                   
                   
                 Example 7 
               
               
                   
                   
                 Example 6 
                 Synthesis of gasoline via 
               
               
                   
                   
                 Synthesis of gasoline via 
                 methanol + 
               
               
                   
                   
                 methanol + 
                 CO 2  recovery + 
               
               
                   
                   
                 pre-reforming + 
                 pre-reforming + 
               
               
                   
                   
                 MTG heat recovery 
                 MTG heat recovery 
               
               
                   
               
               
                 Methanol 
                 (T/D) 
                 2,500 
                 3.375 
               
               
                 DME 
                 (T/D) 
                 — 
                 — 
               
               
                 Gasoline 
                 (barrel/D) 
                 8,000 
                 10,800 
               
               
                 LPG 
                 (barrel/D) 
                 1,030 
                 1,417 
               
               
                 Material 
                 (10 6  kcal/H) 
                 733.5 
                 733.5 
               
               
                 Fuel 
                 (10 6  kcal/H) 
                 2852 
                 285.2 
               
               
                 Process steam 
                 (T/H) 
                 211 
                 211 
               
               
                 Process heat recovery 
                 (T/H) 
                 176.1 
                 176.1 
               
               
                 Flue gas heat recovery 
                 (10 6  kcal/H) 
                 53.0 
                 41.6 
               
               
                 Methanol synthesis heat 
                 (10 6  kcal/H) 
                 44.4 
                 59.9 
               
               
                 Methanol distillation heat 
                 (10 6  kcal/H) 
                 — 
                 — 
               
               
                 DME synthesis heat 
                 (10 6  kcal/H) 
                 — 
                 — 
               
               
                 MTG synthesis heat 
                 (10 6  kcal/H) 
                 43.3 
                 58.5 
               
               
                 CO 2  recovery amount 
                 (T/H) 
                 — 
                 1,205 
               
               
                 Heat recovered by CO 2  recovery 
                 (10 6  kcal/H) 
                 — 
                 32.0 
               
               
                 Heat recovered by pre-reforming 
                 (10 6  kcal/H) 
                 74.2 
                 74.2 
               
               
                 Heat recovered from flue gas of synthesis 
                 (10 6  kcal/H) 
                 11.4 
                 11.4 
               
               
                 of MTG 
               
               
                 CO 2  compression energy 
                 (kw) 
                 — 
                 7,000 
               
               
                 Auxiliary boiler 
                 (10 6  kcal/H) 
                 18.1 
                 0 
               
               
                 Residual energy 
                 (10 6  kcal/H) 
                 49.1 
                 36.5 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, in the Conventional Example for synthesizing methanol, the residual energy was 0 kcal/H for the entire system, and the self-balance was achieved. On the other hand, in the Reference Examples 1 and 2 for synthesizing DME or gasoline via methanol, the synthesis heat generated in the synthesis of DME or gasoline (MTG) increased and methanol distillation heat became unnecessary, and accordingly, excessive energy was generated. 
     In Example 1 in which the pre-reformer configured to carry out pre-reforming was provided, the supply of fuel and the flue gas heat recovery amount in the steam reformer decreased, and accordingly, better self-balance was achieved compared with Reference Examples 1 and 2. In addition, in Example 2 in which the CO 2  recovery device was provided, CO 2  recovery heat became necessary and the production of gasoline increased, and accordingly, better self-balance was achieved compared with Reference Examples 1 and 2. In Example 3 in which combustion air was used to recover gasoline synthesis heat (MTG heat), the supply of fuel in the steam reformer and the amount of recovered flue gas decreased, and accordingly, better self-balance was achieved compared with Reference Examples 1 and 2. Similarly in Examples 4 to 7, which are a combination of the above-described Examples, more remarkably better self-balance was achieved compared with Reference Examples 1 and 2. 
     DESCRIPTION OF REFERENCE NUMERALS 
       10 : Steam reformer 
       11 : Reaction tube 
       12 : Burning portion 
       13 : Material supply line 
       14 : Fuel supply line 
       15 : Waste heat recovery portion 
       16 : Stack 
       17 : Flue gas-steam heat exchanger 
       18 : Reformed gas supply line 
       19 : Reformed gas heat exchanger 
       20 : Methanol synthesis column 
       22 : Methanol supply line 
       30 : Gasoline synthesis column 
       32 : Gasoline supply line 
       40 : Pre-reformer 
       41 : First flue gas-material heat exchanger 
       42 : Second flue gas-material heat exchanger 
       50 : CO 2  recovery device 
       51 : CO 2  supply line 
       60 : Air preheater 
       61 : Combustion air inlet line 
       62 : Flue gas-combustion air heat exchanger 
       63 : Fan 
       64 : Combustion air supply line
A system or method for producing gasoline or dimethyl ether from natural gas via methanol includes: steam-reforming natural gas to generate reformed gas; synthesizing methanol from the reformed gas; synthesizing gasoline or dimethyl ether from the methanol; and at least one is selected from the group consisting of: pre-reforming natural gas prior to the steam-reforming; recovering carbon dioxide from flue gas generated in the steam-reforming; and preheating combustion air to be supplied to the steam-reforming by using synthesis heat generated in the synthesis of gasoline or dimethyl ether. In addition, an overall energy balance of the system is constructed by using heat recovery from flue gas generated in the steam-reforming, heat recovery from the reformed gas, synthesis heat generated in the synthesis of methanol, synthesis heat generated in the synthesis of gasoline or DME, and heat recovered in the pre-reforming, carbon dioxide recovering, or air preheating, respectively if selected.