MOLECULAR PATHWAY ANALYSIS OF BIOCRUDE FORMATION IN HYDROTHERMAL LIQUEFACTION

Leclerc, Heather

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MOLECULAR PATHWAY ANALYSIS OF BIOCRUDE FORMATION IN HYDROTHERMAL LIQUEFACTION

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Hydrothermal liquefaction (HTL) is a process utilized to convert high water content feedstocks into a usable biocrude oil at temperatures between 250 – 350 °C and pressures in excess of 100 bar. Despite HTL’s prevalence as a research topic since the turn of the century, the majority of research until recently focused on improving oil yields from single-source algal feeds. In recent years, the focus has shifted towards waste-based feedstocks to address feedstock availability and prohibitive cost issues plaguing early HTL research. Food, yard, and wood wastes from municipal solid waste (MSW) account for 223 million tons of waste annually. These energy-dense waste feeds have the potential to produce spatially distributed and usable biocrude oil across the United States. Despite research interest in this area, hydrothermal liquefaction has yet to realize commercialization due to insufficient biocrude yields and a lack of fundamental understanding regarding heteroatom distribution and subsequent removal. Due to the complexity of waste feedstocks as well as the severe reaction conditions, the chemistry of waste HTL is poorly understood and colloquially referred to as a black-box process. To uncover the governing chemical pathways towards biocrude production from waste, experimental research coupled with advanced analytical chemistry techniques is required. Mass spectrometry (MS) is an analytical technique used to measure the mass-to-charge ratio (m/z) of individual molecules. The most common MS technique is gas chromatography mass spectrometry (GC-MS), which measures the volatile portion of a sample (boiling point ≤ 325 °C). In typical bio-oil samples from waste HTL, less than 40% of the biocrude can be analyzed using GC-MS. In addition to GC-MS, further high-resolution mass spectrometry techniques are available with increased resolution and the ability to distinguish between mass-to-charge ratio changes on the order of parts per billion. One such technique, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) allows for this ppb resolution due to the use of a superconducting magnet at strengths of 9 – 21 Tesla. In this work, a detailed chemical understanding and pathway analysis is developed to describe the effect of feedstock composition and catalysts on biocrude formation and heteroatom distributions. HTL chemical pathways are experimentally assessed through batch reactions and subsequent mass spectrometry analysis and further confirmed using density functional theory and kinetic model simulations. Research included in this thesis utilizes analytical and computational chemistry to determine the effects of feedstock composition, catalysts, and pretreatment conditions on resultant hydrothermal liquefaction molecular pathways. Enhanced chemical knowledge of biocrude formation pathways was then applied to determine sub-mechanisms for real-world waste feedstocks.

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