Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition

Catalytic steam reforming of condensable vapors (i.e. bio-oils) derived from pyrolysis of biomass is a technically viable process for hydrogen production. In this study the aqueous fraction of bio-oil, generated from fast pyrolysis, was catalytically steam reformed at 825 and 875 degrees C, high space velocity (up to 126,000 h(-1)) and low residence time (26 ms). Using a fixed-bed micro-reactor interfaced with a molecular beam mass spectrometer (MBMS), a variety of research and commercial nickel-based catalysts were tested. The catalysts were prepared by impregnation of an alpha-Al2O3 support with nickel and additives. Since the main constraint in reforming bio-oils is catalyst deactivation caused by carbon deposition, two strategies were applied to improve the performance of the catalysts. The first approach aimed at enhancing steam adsorption to facilitate the partial oxidation, i.e. gasification of coke precursors. The second one attempted to slow down the surface reactions leading to the formation of the coke precursors due to cracking, deoxygenation, and dehydration of adsorbed intermediates. Magnesium and lanthanum were used as support modifiers to enhance steam adsorption while cobalt and chromium additives were applied to reduce coke formation reactions. The cobalt-promoted nickel and chromium-promoted nickel supported on MgO-La2O3-alpha-Al2O3 catalysts showed the best results in the laboratory tests. At the reaction conditions progressive catalyst deactivation was observed leading to a decrease in the yields of hydrogen and carbon dioxide and an increase in carbon monoxide. The loss of activity also resulted in the formation of higher amounts of methane, benzene and other aromatic compounds. Commercial catalysts that were developed for steam reforming of natural gas and crude oil fractions proved to be more efficient for hydrogen production from bio-oil than most of the research catalysts mainly due to the higher water-gas shift activity. (C) 2000 Elsevier Science B.V. All rights reserved.