Hydrothermal Liquefaction of Microalgae and Other Microorganisms: Developing a Kinetic Model.

Abstract

Microalgae are rapid-growing photosynthetic organisms, capable of capturing carbon emissions and treating wastewaters, that can be used as a feedstock for biofuels. Hydrothermal liquefaction (HTL), a high-temperature (> 250 °C) and high-pressure process (> 4 MPa), can convert wet biomass such as microalga into an energy-dense biocrude. We examined how feedstock composition and operating conditions affected the yields and characteristics of each product fraction. We investigated the HTL of Nannochloropsis sp. at different reaction temperatures (250 - 400 °C), batch-holding times (10 - 90 min), water densities (0.3 - 0.5 g/mL), biomass loadings (5 - 35 wt %), and headspace composition (He, air). From the experiments with Nannochloropsis sp. we elucidated a reaction network and derived a kinetic model of HTL. We used the kinetic model predict the yields of product fractions from the HTL of Nannochloropsis sp. We hydrothermally treated Chlorella protothecoides and Scenedesmus sp. to expand the model to other microalgae with different biochemical compositions. We incorporated the results into the model to create a unified kinetic model that correlated the yields of the product fractions based on the feedstock composition, residence time, and reaction temperature. We investigated other microorganisms such as Escherichia coli, Pseudomonas putida, Bacillus subtilis, and Saccharomyces cerevisiae as potential feedstocks for HTL. More than half of the chemical energy in the biomass could be recovered in the biocrude product fraction. These results demonstrate the feasibility of applying HTL to produce high yields of biocrude from bacteria and yeast that are high in protein and low in lipids when compared against some microalgae. Such microorganisms could serve as a renewable feedstock for biofuels as well.