Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Biomass pyrolysis oils, or bio-oils, are produced from the pyrolysis of lignocellulose and are considered a renewable source of carbon. Bio-oil contains many different compounds, comprising a wide array of different oxygen-containing functional groups. The relative concentrations of these compounds can be affected by process conditions that give rise to secondary effects caused by transport limitations or by the presence of catalysts, either in situ, as in the case of natural Alkali and Alkaline Earth Metals (AAEM), or ex situ, such as the hydrodeoxygenation of pyrolysis vapors using molybdenum oxide.
Analysis of the micropyrolyzer system calculated there to be negligible heat transfer limitations, however mass transfer limitations were apparent when an excessive sample weight (>800 μg) was applied. Secondary effects were found to be catalyzed by char and included the decomposition/dehydration of levoglucosan to low molecular weight products, furans, and dehydrated pyranose, as well as secondary char and gas formation. Levoglucosan yield was also diminished upon the addition of added AAEMs, specifically sodium chloride and calcium chloride. The diminishment was most severe in cellulose pyrolysis where the levoglucosan yield decreased by 85% when NaCl was added at a loading of 1.75 wt.%.
Utilizing bio-oil in the production of fuels or chemicals poses challenges due to the high oxygen content of bio-oil. Numerous efforts have been carried out to remove oxygen efficiently while minimizing the loss of carbon. Here, the hydrodeoxygenation of biomass pyrolysis vapors was conducted with a tandem micropyrolyzer system using low pressure hydrogen, a molybdenum oxide catalyst situated ex situ of the pyrolysis reactor, and cellulose, lignin, and corn stover feedstocks. The highly oxygenated pyrolysis vapors were effectively converted to hydrocarbons by the MoO3 catalyst. Oxygenated compounds were observed in the products from the first feed injection but not in succeeding injections. However, when the catalyst was pre-reduced to a more active state, the products were fully deoxygenated from the first injection. The products included mainly linear alkanes (C1 to C6) and monocyclic aromatics. Remarkably, the total hydrocarbon yield for each feedstock was as high as ~75-90 C% from the volatile carbon (excluding char).
Using different MoO3 catalyst loadings for cellulose hydrodeoxygenation found alkanes to dominate at higher loadings, while at lower loadings alkene yields were increased. However, at too low of a loading, the pyrolysis vapors were not totally deoxygenated. The hydrodeoxygenation of monooxygenate C4 compounds found hydroxyl groups to be the most readily reacted and ether linkages to be the most recalcitrant. In general, the reactivity towards deoxygenation of the tested oxygen-containing functional groups was observed to be C-OH > C=O > C-O-C. Several cellulose pyrolysis model compounds were tested, including methyl glyoxal, glycolaldehyde, furfural, 5-hydroxymethylfurfural, and levoglucosan, and found the same general trend to be present, except for levoglucosan, which was totally converted and did not yield any oxygenated species despite containing two ether linkages. Pyridine temperature programmed desorption studies found the acidity of MoO3 to greatly increase after reduction for 1 hr under H2 flow. The general reaction pathway was observed to include carbonyl/hydroxyl hydrogenation/dehydrogenation, alkene isomerization, and alkene hydrogenation.
Nolte, Michael, "In situ and ex situ catalysis in biomass fast pyrolysis" (2016). Graduate Theses and Dissertations. 15782.