High Pressure Micro-Scale Studies of Fast-Hydropyrolysis and Catalytic Hydrodeoxygenation of Biomass and Related Model Compounds

Harshavardhan J Choudhari, Purdue University

Abstract

Fast pyrolysis of biomass followed by catalytic hydrodeoxygenation of bio-oil is considered a promising biomass conversion route to produce drop in hydrocarbon fuels. The H2Bioil process was proposed as an integrated high pressure fast hydropyrolysis and catalytic vapor phase hydrodeoxygenation (HDO) pathway for utilizing biomass to produce high energy density fuel. During fast hydropyrolysis biomass is rapidly heated to generate a complex mixture of compounds with high oxygen content (35–40 wt %). In the H2Bioil process, hydropyrolysis vapors are immediately upgraded via a downstream catalytic reactor to reduce the oxygen content and produce a high energy density bio-oil. In this dissertation, fast hydropyrolysis and inline catalytic hydrodeoxygenation studies were conducted in a micro pyrolyzer, with a unique modification, which allowed online sampling of biomass pyrolysis vapor products under high pressure hydrogen (up to 35 bar) directly into the gas chromatograph and mass spectrometer (GC-MS) for analysis. Identification and quantification of the entire range of vapor phase products from fast pyrolysis is essential to understand the governing mechanisms during pyrolysis as well as to design a suitable catalyst for downstream upgrading. Quantification of the pyrolysis and HDO products using the GC-MS accounted for > 90% of the starting mass from the cellulose, lignin, and biomass. The structure of native lignin differs from that of extracted lignin and therefore, well characterized synthetic guaiacyl (G) lignin model oligomers and a polymer were used to investigate β-O-4 bond scission under fast pyrolysis conditions. The effect of degree of polymerization (Dp) on char formation and pathways for β-O-4 bond scission were also investigated, with the char yield increasing with increase in Dp. The major monomeric product observed from β-O-4 bond scission was coniferyl alcohol, along with the presence of a significant proportion of dimers (19-70 wt %) in the product distribution. Vapor phase residence time studies revealed that these lignin-derived oligomers underwent secondary reactions in the vapor phase to form monomers, which increased in abundance with an increase in the residence time. These results conclusively showed, for the first time, the presence of a significant proportion of dimers (>19%), and possibly oligomers, along with monomers amongst the primary products from lignin pyrolysis. Similar, results were observed with cellulose pyrolysis products resolving the debate in literature about the nature of primary products from lignin and cellulose pyrolysis. Additionally, no deoxygenation was observed during cellulose and lignin fast pyrolysis experiments, in presence of hydrogen (up to 25bar), thereby showing the need for a downstream catalyst. We began with a study of HDO of the cellulose and lignin based model compounds, levoglucosan, and dihydroeugenol, over a series of supported PtMo catalysts. Complete deoxygenation was obtained for both levoglucosan (~72% C4+ hydrocarbons) and dihydroeugenol (98% C9 hydrocarbons) over a Pt-Mo/MWCNT catalyst at 100% conversion. Increasing the Mo:Pt (0:1–5:1) ratio was shown to favor the hydrodeoxygenation selectivity as well as decrease the extent of C-C bond cleavage, demonstrating the importance of Mo for oxygen removal. Reaction pathway studies were carried out with dihydroeugenol to demonstrate the role of Mo as an oxophilic promoter, which in conjunction with Pt improved the C-O bond scission selectivity. Based on these model compound studies, the 5%Pt2.5%Mo/MWCNT catalyst was tested to maximize C4+ hydrocarbon recovery from cellulose, xylan, lignin polymer and intact biomass. Hydrodeoxygenation of biomass pyrolysis products (poplar, pine, and maize) over the 5%Pt2.5%Mo/MWCNT catalyst gave >69% carbon yield to hydrocarbons, with >41% yield to liquid fuel range (C4+) hydrocarbons, at 300°C and 25 bar hydrogen pressure. Hydrogen pressure played a critical role in determining the hydrocarbon product distribution due to a significant impact on the degree of C-C scission. Decrease in the hydrogen pressure was shown to increase the degree of C-C scission, thereby decreasing the yield of liquid fuel range hydrocarbons by ~10 carbon wt %, within the pressure range of 1–25 bar. Studies with cellulose, xylan and lignin polymer 2 showed that cellulose and xylan fraction contributed to a greater extent toward C-C scission than lignin, primarily due to the aromatic structure of the lignin pyrolysis products. Decrease in the hydrogen pressure also resulted in an increase in the yield of aromatic hydrocarbons (up to ~14 carbon wt % yield), which were chiefly derived from the lignin fraction of the biomass. Hydrogen pressure is a critical parameter, which can be tuned to control the hydrocarbon product distribution based on the composition of the biomass and maximize the value of products. These trends were replicated in the continuous-flow cyclone-type fast-hydropyrolysis (FHP) reactor with a downstream vapor-phase catalytic HDO reactor. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Agrawal, Purdue University.

Subject Area

Chemistry|Chemical engineering

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