Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Brent H. Shanks
Robert C. Brown
Bio-oils from fast pyrolysis of lignocellulosic biomass can be phase separated into a heavy water insoluble portion and a light water-soluble portion. The water-rich portion contains mainly carboxylic acids, carbohydrates, aldehydes, ketones and alcohols. This highly oxygenated water-rich portion was evaluated for production of the renewable hydrogen required for the upgrading reactions. There are certain challenges introduced due to the complexity of these mixtures. Catalyst deactivation by coking and the formation of carbon deposits are major limitations although the specific causes were previously unidentified. A bio-oil fractionating system can separate heavier components from the light-end components. This light-end fraction has shown to be better suited for hydrogen production via steam reforming at moderate temperatures generating mainly hydrogen and carbon dioxide. Due to their chemical instability the bio-oils showed evident aging leading to decreased hydrogen production potential when stored for long periods of time.
Model compounds representing the water-soluble components we compared in controlled tests to find troublesome species in terms of resistance to reaction and carbon deposition tendency. Experiments were performed under kinetic control conditions at low conversions to reveal reaction characteristics while avoiding thermodynamics and transport limitations. It was found that levoglucosan, acetic acid, and furfural were the species with the highest limitation in terms of carbon deposition leading to decreased hydrogen production and lowcatalyst stability. Levoglucosan was found to decompose more easily leading to carbon deposition even in the absence of a catalyst. Acetic acid and furfural were then found to tend to coke over the catalyst but where mostly thermally stable. Acetic acid was found to be one of the most abundant and troublesome compounds in water-soluble bio-oil.
Magnesium and Cobalt modified Nickel/Alumina steam reforming catalysts were tested at 460 and 650yC using acetic acid as a probe molecule. These temperatures corresponded respectively to reforming regimes where no coke removal by steam was observed and where coke removal happened at an accelerated rate as determined by temperature program oxidation. The addition of Magnesium as support modifier led to reduced coke accumulation by promoting coke gasification at 650yC, but at 460yC a different trend was observed where coke removal rate was not prevalent. A supported bimetallic Ni-Co catalyst showed a coke reduction effect at 460yC, but at 650yC it seemed to promote coke formation. When comparing the results of supported and unsupported Ni-Co catalysts the presence of an Alumina support was appeared to be necessary to achieve high hydrogen formation with decreased coking. The data suggest that a synergistic effect exists between the Ni-Co and the Alumina support, where the latter may enhance water activation contributing to the reduced coking for the bimetallic catalyst.
Pedro J. Ortiz-toral
Ortiz-toral, Pedro J., "Steam reforming of water-soluble fast pyrolysis bio-oil: Studies on bio-oil composition effect, carbon deposition, and catalyst modifications" (2011). Graduate Theses and Dissertations. 11965.