High-temperature ethanol steam reforming in PdCu membrane reactor

Abstract

Membrane-assisted steam reforming of ethanol is an intriguing option for on-site H2 production from biomass in refueling stations for automotive fuel cells. The energy consumption for H2 compression and thus fuel costs can be significantly reduced if this process could deliver H2 at elevated pressures. Thermodynamic analyses show that reforming temperatures above 823 K are needed for attaining H2 partial pressures that will enable operation without sweep gas and allow appreciable compression energy savings. Reforming of a 6:1 steam/ethanol mixture was practically driven to completion at 873 K and 1.3 MPa employing thin, supported Pd and PdCu membranes without sweep gas. Both H2 yield and H2 recovery factor remained stable during 10 day continuous testing of these membranes. The H2 yield reached nearly 94% with ca. 92% H2 separated by the PdCu membrane, for example. However, purity of the permeated H2 declined from about 97% to nearly 91% in the Pd membrane reformer whereas it remained steady at 98% when using the PdCu membrane. In addition, the twice as high single gas H2 permeation rate of the Pd membrane was reduced by ca. 85% during reforming due to inhibition by unstable carbon compounds while that of the PdCu membrane was diminished by less than 60%. Hence, the relative H2 permeability of the two membranes was virtually inverted under reforming conditions. In consequence, PdCu membranes are clearly the better option for integration into high-temperature ethanol steam reformers.