High temperature methanation with molten salt-based catalyst systems

#This invention relates to catalytic methanation or hydrogenation of carbon oxides to methane and to a catalyst system for effecting same. More particularly, this invention is directed to a methanation process carried out at high temperatures and a molten metal salt-based heterogeneous catalyst system which is uniquely suited for such high temperature methanation.

#Catalytic methanation is a well-known reaction which is widely employed in the chemical and energy providing industries. Probably its most widespread current and potential application is in the treatment of the gaseous effluent from the gasification or partial oxidation of carbonaceous fuels with oxygen and/or water, e.g., steam-hydrocarbon reforming and partial combustion of liquid and solid carbonaceous fuels, to produce a hydrogen-rich gas for chemical synthesis, e.g., ammonia manufacture, or petroleum refining, e.g., catalytic hydrocracking and hydrogenation, or to form a methane-rich gas having high BTU value and low CO content for use in residential and industrial heating or power generation. In the former case, the gasification or partial oxidation effluent, which typically contains substantial quantities of H.sub.2, CO, CO.sub.2 and H.sub.2 O as well as N.sub.2 in cases where air is used as the oxidant source, is generally subject to a process known as the carbon-monoxide shift-conversion reaction prior to catalytic methanation. In this case the CO-shift reaction converts a substantial quantity of the CO present to H.sub.2 and CO.sub.2 by reaction with H.sub.2 O in the presence of a catalyst and the primary purpose of catalytic methanation is to remove small quantities of CO which remain in the hydrogen-rich product gas by conversion to methane in order to avoid poisoning of downstream processing catalysts. In the latter case, i.e., conversion of partial oxidation effluent gas to methane-rich gas, the gasification or partial oxidation effluent gas is subject to CO-shift to obtain the appropriate ratio of H.sub.2 to CO (usually 3 to 1) and the CO shift product gas is then subject to catalytic methanation for conversion of carbon oxides and hydrogen contained therein to methane. In either case, the CO-shift effluent gas is subject to an intermediate processing step to remove sulfurous materials in cases where a sulfur-containing carbonaceous fuel feedstock is employed since all commercially used methanation catalysts are highly sensitive to poisoning by sulfur compounds.

#Because of the increasing demand for a high BTU, clean gas as an energy source in the United States and the acknowledged decreasing and finite nature of natural gas reserves in the United States as well as happenings on the world scene which make energy self-sufficiency desirable or even essential, there has been a dramatic increase in interest in the manufacture of a clean, high BTU gas energy source which will meet pipeline standards by synthetic means from alternative carbonaceous resources such as coal or heavy hydrocarbons. Many of the more attractive synthetic approaches which have been proposed are based on gasification or partial combustion of the carbonaceous material, and, as indicated above, include catalytic methanation as part of the integrated process scheme to upgrade the BTU value of the product gas to a level acceptable for pipeline gas applications. CO and H.sub.2 have heating values of about 300 BTU/ft.sup.3 whereas pipeline natural gas has a value above 1000 BTU/ft.sup.3. While a number of metallic species are known to be active and selective methanation catalysts including, inter alia, nickel, ruthenium, cobalt, iron and molydenum, their application to the manufacture of high BTU or pipeline gas has been less than satisfactory for several reasons which relate to the physical form of the catalyst employed and/or the nature of the methanation reaction, itself.