(523h) Overcoming Engineering Challenges for the Hydrothermal Liquefaction of Cost-Advantaged Algal Feedstocks

Hydrothermal liquefaction (HTL) is a carbon-efficient process for upgrading biomass feedstocks to renewable fuel products. Microalgae have long been considered an ideal feedstock for HTL because of its production potential to meet national demands for renewable fuels [1]. At current estimates, the cost of mass-cultivated algae contributes 50% of the total production cost for every gasoline gallon equivalent of produced fuel [2]. To reduce the overall production costs and minimize the fuel selling price, low-cost, or cost-advantaged algal feedstocks for HTL processing need to be identified and assessed. Nutraceutical industries that produce algal-based products, remediation services that use algae for wastewater treatment or pollution management, and environmental services that collect harmful algal blooms for disposal all generate cost-advantaged algal alternatives. The varied sources each present unique engineering challenges for hydrothermal processing.

In this work we assessed cost-advantaged micro- and macroalgae feedstocks. The feedstocks varied compositionally, being depleted of energetic compounds (e.g., low lipids) or consisting of considerably high ash (>25 wt %). The feeds also varied in physical structure, the macroalgae requiring effective physical formatting to create a concentrated and pumpable slurry. Harvesting flocculants also altered the slurry characteristics of some feeds. For each feed we tested various methods to create a pumpable slurry, manage ash solids, and valorize high-value components. We also assessed blending strategies to improve flowability of the feedstock slurry and to boost the overall biocrude yield. Both single-step and multi-step or sequential HTL processes [3] were utilized to fractionate the components of the algae feeds to recover valuable byproducts, such as polysaccharides, or to concentrate the slurries for more cost-effective high-temperature processing. For each feedstock we determined a unique strategy and valorization pathway to maximize the biocrude yield and simplify the pre-processing steps.

[1] Jones et al., PNNL-23227 Technical Report, (2014)

[2] Zhu et al., Algal Res., 51 (2020) 102053

[3] Zhu et al., PNNL-29861 Technical Report, (2020)