Mixed-Conducting Membranes for Hydrogen Production and Separation
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Mixed-Conducting Membranes for Hydrogen Production and Separation U. (Balu) Balachandran, Beihai Ma, Tae H. Lee, Sun-Ju Song, Ling Chen, and Stephen E. Dorris Energy Systems Division, Argonne National Laboratory, Argonne, IL, 60439 ABSTRACT Mixed-conducting oxides, possessing both ionic and electronic charge carriers, have found wide application in recent years in solid-state electrochemical devices that operate at high temperatures, e.g., solid-oxide fuel cells, batteries, and sensors. These materials also hold promise as dense ceramic membranes that separate gases such as oxygen and hydrogen from mixed-gas streams. We are developing Sr-Fe-Co oxide (SFC) as a membrane that selectively transports oxygen during partial oxidation of methane to syngas (mixture of CO and H2) because of SFC's high combined electronic and ionic conductivities. We have evaluated extruded tubes of SFC for conversion of methane to syngas in a reactor that was operated at ≈900°C[AE1]. Methane conversion efficiencies were >90%, and some of the reactor tubes were operated for >1000 h. We are also developing dense proton-conducting oxides to separate pure hydrogen from product streams that are generated during methane reforming and coal gasification. Hydrogen selectivity in these membranes is nearly 100%, because they are free of interconnected porosity. Although most studies of hydrogen separation membranes have focused on proton-conducting oxides by themselves, we have developed cermet (i.e., ceramic-metal composite) membranes in which metal powder is mixed with these oxides in order to increase their hydrogen permeability. Using several feed gas mixtures, we measured the nongalvanic hydrogen permeation rate, or flux, for the cermet membranes in the temperature range of 500-900°C. This rate varied linearly with the inverse of membrane thickness. The highest rate, ≈32 cm3(STP)/min-cm2, was measured at 900°C for an ≈15-µm-thick membrane on a porous support structure when 100% H2 at ambient pressure was used as the feed gas.
INTRODUCTION Hydrogen is expected to play a vital role in the transportation sector via fuel cell vehicles (FCVs) and in the distributed power generation market via stationary fuel cells because of concerns over global climate change. One of the crucial requirements for successfully introducing FCVs is a low-cost supply of hydrogen. At present, petroleum refining and the production of ammonia and methanol collectively consume ≈95% of all manufactured hydrogen in the U.S. Most of the demands for hydrogen are currently met by steam reforming of methane and naphtha reforming (mainly in refineries). In the first stage of steam reforming, methane is oxidized to form syngas (a mixture of hydrogen and carbon monoxide). The syngas is then subjected to a water-gas shift reaction, which converts carbon monoxide to additional hydrogen and carbon dioxide, and hydrogen is separated from this stream. Steam reforming is usually very energy- and capital-intensive, requiring high temperatures and pressures. Although direct part
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