Development of a microkinetic framework for analysis of biomass and its thermochemical conversion

Evan Terrell

A microkinetic framework for chemical process and reaction engineering is built upon a first-principles look at the molecular level of how reactants evolve into desired products and what pathways can be achieved to improve selectivity and efficiency. For the analysis of lignocellulosic biomass thermochemical conversion processes, which are among the leading technologies for the development of green fuels and chemicals, this requires a thorough, fundamental characterization of the underlying biomass macromolecular structure. This work begins with a review of the challenges and opportunities for this microkinetic analysis, with particular emphasis on lignin pyrolysis and solvolysis. This review highlights research on the current understanding of reaction kinetics and mass transport mechanisms and their intimate link to molecular structures of the participating lignin species. The next section depicts a detailed structural characterization of milled wood lignin using high-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). In this approach, lignin is directly characterized with FT-ICR MS and structural representations are proposed for detected oligomers based on the novel coupling of laboratory analysis results with a computational stochastic structure modeling technique. This coupled laboratory analysis-computational structure modeling approach is then extended in Chapter 3 to the analysis of biomass pyrolysis oils. These oils were characterized with FT-ICR MS and the structures of their oligomers were modeled using a reactive combinatorics-based expansion of unmodified cellulose, hemicellulose, and lignin oligomers. In this way, potential/reasonable reactive pathways to produce detected oligomers in biomass pyrolysis oils are suggested. The final technical chapter further extends the techniques from the previous chapters to the analysis of milled wood lignin vacuum pyrolysis. Based on FT-ICR MS analysis of lignin pyrolysis oligomers, structures are proposed based on their high degree of similarity with initial, pre-pyrolysis milled wood lignin. Construction of vaporization curves is also presented to highlight the distinct possibility of trimer and tetramer evaporation for given sets of pyrolysis temperature and pressure. An integrated pyrolysis model, which explicitly accounts for the effect of pressure on mass transport, is presented and shown to reproduce experimentally observed trends in lignin pyrolysis.

Development of a microkinetic framework for analysis of biomass and its thermochemical conversion

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