Patent Publication Number: US-2013239481-A1

Title: Process for the preparation of gaseous synfuel

Description:
The present invention is related to a process for the preparation of a gaseous synfuel having a heating value in the same range as natural gas. 
     The invention is in particular useful when a carbonaceous feedstock is available and there is a need for producing a gaseous synfuel capable of replacing natural gas. 
     By conventional technology carbonaceous feed stock is gasified with oxygen in order to obtain a basically inert gas-free synthesis gas, which in several catalytic steps can be converted into substitute natural gas (SNG). 
     For use in smaller plants, oxygen is too expensive, and air-blown gasification is typically employed in these plants. 
     When gaseous SNG is produced by air-blown gasification, the problem arises that the SNG product, i.e. mainly methane, are introduced into the SNG product, said product containing large amounts of nitrogen having been contained in the air. Separation of CH4 from such a mixture is cost-intensive. 
     A mixture of nitrogen and dimethyl ether (DME) appears to be inter-changeable with base natural gas. Thus, a mixture of for instance 40% by volume of N 2  and 60% by volume of DME has the same fuel value (BTU) as natural gas. 
    
    
     The invention is based upon the above fact as well as on the finding that a high content of nitrogen in the synthesis gas mixture does not adversely effect a subsequent catalytic conversion of the gas to dimethyl ether. Furthermore, nitrogen and other inerts are much easier removed from DME than from SNG in order to obtain the desired BTU. As an advantage of the above findings, synthesis gas for the preparation of DME based synfuel for the replacement of natural gas may contain nitrogen and is manufacturable by less expensive methods comparable to that of synthesis gas for preparation of SNG. 
     Thus, a general embodiment of the invention is a process for the preparation of a gaseous dimethyl ether synfuel having a BTU value corresponding substantially to the BTU value of natural gas, said process comprising the steps of providing a synthesis gas comprising hydrogen, carbon monoxide and nitrogen; 
     catalytically converting the synthesis gas into a dimethyl ether raw product further comprising unconverted synthesis gas, the nitrogen and dissolved carbon dioxide; 
     cooling and condensing the dimethyl ether raw product with the dissolved carbon dioxide to a liquid and a gaseous phase with the unconverted synthesis gas and the nitrogen; 
     separating the liquid phase from the gaseous phase; 
     treating the liquid phase to remove the carbon dioxide from the dimethyl ether raw product and to obtain a purified dimethyl ether product; and 
     admixing and adjusting content of inert gas having a BTU value of about zero into the purified dimethyl ether raw product to obtain the gaseous dimethyl ether synfuel having a BTU value substantially equal to that of the BTU value of natural gas, wherein the synthesis gas is prepared by air-blown gasification of a carbonaceous material. 
     The carbonaceous material can be any solid material containing carbon, including coal and biomass. 
     The important carbon gasification reactions are 
       C+O 2 →CO 2  
 
       C+CO 2 →2CO
 
       C+H 2 O→CO+H 2  
 
       CO+H 2 O→CO 2 +H 2  
 
     The carbonaceous material is preferably gasified with air at a pressure of between 2 to 3 MPa, which is the typically employed maximum pressure at which the solid carbonaceous material is fed into the gasifier. 
     The gasification temperature is typically between 600° C. and 900° C. and below the ash agglomeration point. 
     In addition to carbon oxides and hydrogen, a raw synthesis gas obtained by air-blown gasification comprises tar and more than 30 mole % of inert gases, mainly nitrogen, and methane. 
     The content of methane is preferably removed from the raw gas by tubular steam reforming of the gas in accordance with the conventional steam reforming processes, wherein methane is steam reformed to hydrogen and carbon oxides. 
     Synthesis gas prepared by coal or biomass gasification has additionally a relatively high content of dust and tar when it is gasified in a fluid bed gasifier. Tar is formed in the gasifier and comprises a wide spectrum of organic compounds, which should be removed from the raw gas prior to conversion of the synthesis gas into DME in order to avoid fouling of the process equipment and a plugging and poisoning of the catalyst in the DME reactor. 
     Accordingly it is preferred to remove the dust in the raw synthesis gas from the gasifier by conventional means and to remove the tar by adiabatic steam reforming into methane, hydrogen and carbon oxides. 
     The reforming of tar is accomplished at temperatures from 650° C. to 900° C. in contact with a modified steam methane reforming catalyst known in the art, such as Ni on different supports, including Al 2 O 3 , ZrO 2 , TiO 2 , SiO 2 , or a Ni/MgO—CaO catalyst, or M-CeO 2 —SiO 2  , where M=Rh, Pt, Pd, Ru, Ni. 
     The raw gas from the gasifier contains in addition certain impurities, which have a poisonous effect on downstream catalysts being employed in the subsequent conversion of the tar and of the synthesis gas into DME. These impurities are particularly carbonyl sulphide, metal carbonyls, carbon disulphide, hydrogen sulphide, hydrogen cyanide, ammonia and arsenic and chlorine. 
     Thus, according to a further embodiment of the invention, the carbonyl sulphide, metal carbonyls, carbon disulphide, hydrogen sulphide, hydrogen cyanide, ammonia and arsenic and chlorine are removed from the raw gas by contacting the gas with a series of sorbents having a sufficient sorption activity for adsorption or absorption of the above mentioned impurities. These sorbents and the use thereof are described in European patent application no. 2 156 877 A, in which the raw gas is contacted in succession with a first purification agent comprising active carbon, with a second purification agent comprising alumina, with a third purification agent comprising zinc oxide, with a fourth purification agent comprising a zeolitic material and a fifth purification agent comprising zinc oxide and copper oxide. 
     The conversion of the nitrogen containing synthesis gas into dimethyl ether is carried out in one or more reactors in which the synthesis gas is catalytically converted into methanol, cf. equation (1), and dimethyl ether as shown in equation (2). The shift reaction also takes place and is shown in equation (3). 
       CO+2H 2 →CH 3 OH   (1)
 
       2CH 3 OH→CH 3 OCH 3 +H 2 O   (2)
 
       CO+H 2 O→CO 2 +H 2    (3)
 
     The catalysts active in the conversion of synthesis gas into methanol and dimethyl ether are well known in the art. 
     Maximum conversion of synthesis gas is obtained when dimethyl ether is prepared at a stoichiometric ratio between hydrogen and carbon monoxide equal to one. At ratios above or below this ratio one less dimethyl ether is prepared. At maximum conversion (H 2 /CO≈1), the overall reaction takes place essentially according to equation (4): 
       3H 2 +3CO→CH 3 OCH 3 +CO 2    (4)
 
     The carbon dioxide present in the synthesis gas and formed during the above reaction (3) is soluble in dimethyl ether. To obtain the dimethyl ether product with a required BTU value, it is necessary to remove the carbon dioxide. 
     In accordance with an embodiment of the invention, the effluent from the dimethyl ether synthesis is cooled, and the dimethyl ether raw product containing dissolved carbon dioxide, unconverted synthesis gas, methane and nitrogen is cooled and condensed to the liquid phase. The remaining gas phase containing the above-mentioned gaseous components is separated from the liquid phase to obtain partially purified dimethyl ether with carbon dioxide dissolved in the ether. 
     The separated dimethyl ether raw product is preferably washed in a scrubbing zone with a liquid solvent rich in potassium carbonate or amine. Thereby, the carbon dioxide is selectively absorbed in the liquid solvent. 
     The liquid solvent for the removal of carbon dioxide contains typically 20 to 40 wt % potassium carbonate. 
     The gaseous dimethyl ether synfuel prepared according to the invention can be used as pipeline gas in an existing natural gas distribution net. 
     In order to be useful as a gaseous synfuel for the replacement of natural gas or for mixing with natural gas, the BTU value of the purified dimethyl ether raw product must be adjusted by admixing inert compounds having a BTU of about zero into the product to adapt the BTU value to that of the natural gas. 
     Nitrogen and other inert diluents are suitable for use in the admixture into the purified dimethyl ether product and the adjustment of the BTU value of the product. 
     Nitrogen is easily available from many sources. Nitrogen gas is for instance produced by fractional distillation of liquid air, or by mechanical means using gaseous air and pressurized reverse osmosis or pressure swing adsorption.