Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Gayfree Barney Ellison

Second Advisor

Veronica Bierbaum

Third Advisor

John Daily

Abstract

This dissertation describes experiments performed to study the thermal decomposition of biomass from a molecular viewpoint. The structure of biomass consists of three major parts: cellulose, hemicellulose and lignin. Thermochemical conversion of biomass, specifically pyrolysis and gasification, yields a complex mixture of light gases, condensable vapors and aromatic tars. The goal for the gasification of biomass is to maximize the production of syngas (CO and H2 ) and minimize the production of aromatic tars. This thesis provides thermochemical information particularly related to cellulose decomposition.

The current technology for the conversion of biomass to biofuels is hindered by the lack of fundamental knowledge concerning detailed mechanisms and kinetic parameters that govern the process. In order to approach this problem, this work provides such information for furan, furfural, acetaldehyde and propionaldehyde, known intermediates in the pyrolysis of cellulose.

The thermal decomposition of the aforementioned biomass molecules was formed in a microtubular reactor with pressures of 75-100 torr and up to temperatures of 1700 K corresponding to residence times of roughly 30-100 μs in the heated reactor. The biomass molecules were entrained in the carrier gases He or Ar and passed through the reactor. The thermal decomposition of the molecules occurs during transit through the heated reactor and products are cooled upon expansion into a vacuum chamber. The pyrolysis product beam was interrogated by three unique schemes: Photoionization Time of Flight Mass Spectroscopy (PIMS) using 10.5 eV light, Matrix Isolation Infrared (IR) Spectroscopy and PIMS using tunable iv vacuum ultraviolet (VUV) radiation at the chemical dynamics beamline of the Advanced Light Source located at Lawrence Berkley National Laboratory in Berkley, CA. Unlike previous studies of biomass decomposition, these experiments were able to identify the initial pyrolysis products.

The first half of this thesis will deal with the thermal decomposition pathways and kinetics of furan and furfural. Earlier G2(MP2) electronic structure calculations predicted that furan will thermally decompose to acetylene, ketene, carbon monoxide, and propyne at lower temperatures. At higher temperatures, these calculations forecast that propargyl radical could result. We see all these products as well as the formation of aromatic hydrocarbons at higher concentrations. This is the first study to show radicals present in biomass decomposition. Thermal decomposition of furfural generates furan and thus follows the same mechanistic pathways as described above.

The second half of this manuscript details the thermal decomposition of acetaldehyde and three isotopologues CH3CDO, CD3CHO and CD3CDO as well as benzaldehyde. As thermal decomposition products of CH3CHO, we have identified CH3 (PIMS), CO (IR, PIMS), H (PIMS), H2 (PIMS), CH2CO (IR, PIMS), CH2=CHOH (IR, PIMS) and HCCH (IR, PIMS). The mechanism for decomposition of benzaldehyde is analogous to that of furfural with appropriate products.

The results in this thesis have reveled detailed mechanisms in the pyrolysis of biomass. These mechanisms can serve as the foundation for examining the thermal decomposition of biomass from a molecular perceptive. The conversion mechanisms that were observed will aid in the overall design of future gasifiers that produce clean syngas.

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