Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

James B. Michael

Second Advisor

Terrence R. Meyer

Abstract

Fast pyrolysis of biomass has the potential for high-yield production of valuable fuels and chemicals from renewable biological and agricultural waste feedstocks. Optimization of this technology is dependent on developing a deeper understanding of the complex transport and kinetic phenomena which drive product formation. In this dissertation, direct, time-resolved imaging of feedstock degradation and product formation mechanisms for the pyrolysis of whole biomass and several of its components has been made inside an optically accessible pyrolysis reactor. The reactor thermal and transport characterization allows the determination of dominant mechanisms of pyrolysis product formation in the multiphase reacting environment. This novel investigation of biomass pyrolysis simultaneously captures relevant transport and kinetic phenomena including melt, agglomeration, ejection, evaporation and condensation throughout the pyrolysis of solid biomass feedstocks and the surrounding reactive environment.

A novel pyrolysis reactor was developed in order to provide four-sided optical access to probe near-particle surface phenomena. The reactor and pyrolysis conditions were characterized using multiple techniques in order to better understand the relevant transport and kinetic limitations from which to interpret the imaging results. Condensed-phase products were analyzed via gas chromatography/mass spectrometry (GC/MS) to verify product compositions obtained using the optically accessible reactor match those observed in other small-scale test reactors. Acetone planar laser-induced fluorescence (PLIF) temperature measurements and multi-point thermocouple mapping provide a detailed thermal profile of the reaction environment and the convectively driven transport near the reaction filament. A line heat source conductivity measurement was developed and used to characterize the biomass feedstock effective conductivity in order to understand heat transport through the biomass sample from the heated filament strip. These studies indicate non-isothermal pyrolysis conditions with both heat and mass transfer limitations which must be accounted for in the interpretation of results - as is common in many pyrolysis reactors.

Planar Mie scattering of product condensation across a well characterized thermal boundary layer allows for time resolved tracking of product formation through distinct condensation bands which are attributed to unique classes of compounds. Correlating the time-resolved condensation scattering signal to simultaneous color micro-scale imaging of feedstock morphological changes allows for developing an understanding of the physical and chemical mechanisms which drive product formation during the pyrolysis of biomass. Dominant transport appears to occur via evaporation and condensation/re-polymerization reactions with minimal contribution from ejection of large aerosols/droplets. In order to elucidate the nature and timescales of the transport of evaporated products away from the reacting biomass sample, a planar fluorescence imaging technique was utilized coincident with planar Mie scattering imaging in order to track the product stream prior to and after Mie scattering signal is apparent from chemical condensation in the thermal boundary layer. Three excitation wavelengths (532 nm, 355nm, 266nm) generated from a 10 Hz Nd:YAG laser were used to probe the volume directly above the pyrolyzing biomass. Differences in the timescales of product/intermediate formation were observed and correlated with the primary observed condensed phase products via Mie scattering. In order to further explore the nature of the primary aerosols and droplets from biomass fast pyrolysis, products were collected directly above the reacting biomass sample and electron and fluorescence microscopy were used to explore the condensed products. Through time-resolved observation of fast pyrolysis of whole red oak, cellulose, and lignin, comparisons among the mechanisms and timescales of thermal degradation and product formation have been made. Single component studies may aid in building a comprehensive understanding of whole biomass pyrolysis but the application of these results must be framed within the context of the complex physicochemical characteristics unique to each feedstock and the specific reaction and transport limitations for a given system. Observations such as those presented in this dissertation indicate that predictive modeling efforts should incorporate the full complexity of biomass pyrolysis and include dynamics of the bulk systems in addition to pure kinetic results. For the pyrolysis regime utilized in this study where kinetic and transport effects both contribute, particle size, degree of polymerization, molecular complexity of the pseudo-components and feedstock compositional and structural variations were shown to influence the phenomena governing the conversion process.

Copyright Owner

Jordan A. Tiarks

Language

en

File Format

application/pdf

File Size

134 pages

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