The conversion of a wet biomass feedstock to a hydrochar via hydrothermal carbonization (HTC) involves series of deoxygenation, dehydration, and decarboxylation reactions that intensify as the severity of the carbonization process increases. Hydrochar forms via two pathways: (1) <i>solid-solid conversion</i>, in which the hydrochar maintains some structure and morphology of the parent biomass, and (2) <i>aqueous phase degradation</i> followed by polymerization of organic molecules into a solid phase. The literature calls hydrochar formed via pathway (1) “primary char”. “Secondary char” is used for the amorphous solid formed following pathway (2). It is often comprised of tarry, aromatic compounds that may be phytotoxic when hydrochars are used as soil amendments, or pose health risks when hydrochars are employed as adsorbents for water treatment. The separation of the primary and secondary char through solvent extraction provides a potential solution for these issues. This work demonstrates how solvent extraction can be used to valorize secondary char from agricultural biomasses into a liquid fuel (as analyzed via GC-MS) with varying composition depending on HTC conditions. The primary char was investigated as a water treatment material and soil amendment and shown to have an increased adsorption capacity for a model organic compound relative to that of the hydrochar. As a soil amendment, the primary char led to less germination inhibition and fewer high molecular weight organics present compared to the untreated hydrochar.

Extracting value-added compounds and improving water treatment capabilities of hydrochar through secondary char separation

ACS Fall 2021

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