Fast Hydrothermal Liquefaction: Processing Conditions, Product Characterization, and Kinetic Modeling.

Abstract

This dissertation describes the study of processing conditions for the hydrothermal liquefaction (HTL) of microalgae, which resulted in the development and subsequent examination of fast HTL, a variation of the HTL process that produces similar biocrude yields in a fraction of the time necessary for isothermal HTL. Application of the fast HTL process to bacteria and yeast biomass (in addition to other algae species) successfully produced biocrude, establishing fast HTL as a robust biomass conversion process. Experiments probing different reaction conditions and biomass loadings indicate that heating rate, temperature, time, and the fraction of the reactor volume occupied by liquid water have a significant effect on product formation from fast HTL, while slurry solid content does not. Fast HTL of microalgal slurries in reactors with different loaded volume fractions yields biocrude and aqueous-phase products of significantly different composition, as identified via elemental analysis and molecular characterization using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). Biocrude products of less desirable composition were obtained in greater quantities than those with more desirable composition, indicating the existence of trade-offs between product yield and quality for different HTL reaction conditions.

The aforementioned results warranted a more comprehensive study of both fast and isothermal HTL reaction conditions, especially at low conversion of algal biomass. Systematic evaluation of fast and isothermal HTL reaction conditions at both low and high liquid water-occupied volumes informed the formulation of a modified HTL reaction network, including a novel pathway for physical disruption of algal cells. This network enabled calculation of pathway kinetic parameters using MATLAB®. These kinetic parameters are physically realistic and enable calculation of product yields that accurately match those observed experimentally. This model captures observed trends for all products from HTL at both low and high algal cell conversion, including the effects of heating rate.