Application of organic geochemistry to the characterization of hydrochar and biochar: Insights into composition and optimization

Hydrothermal carbonization has garnered significant attention in recent studies due to potential applications of the hydrochar produced, which overlap with those of torrefied biochar, including soil improvement, carbon sequestration, and energy production. However, unlike the relatively uniform properties of biochar, hydrochar characteristics can vary greatly depending on the feedstock used. Advanced characterization techniques are essential to better understand these materials and enhance their practical applications. Research on hydrochar requires a multi-faceted approach, including molecular-level studies, to optimize production processes and identify specific applications. In this regard, analytical methods commonly used in organic geochemistry could play a crucial role in deepening our knowledge of hydrochar and biochar. Thus, this study evaluates the properties and key characteristics of five types of hydrochar and a reference biochar using a range of advanced analytical techniques. Among these, HAWK® pyrolysis with flame ionization and infra-red detectors (Py-FID/IRD) — typically employed in petroleum geochemistry — has proven to be a rapid and effective method for assessing some key properties and facilitating quick comparisons. Additionally, thermal desorption and pyrolysis analysis followed by gas chromatography coupled with mass spectrometry (TD-Py-GC-MS) provided detailed molecular characterizations of the hydrochars. These findings demonstrate that traditional techniques used in fossil fuel and environmental geochemistry are equally applicable in this field. Consequently, these techniques offer valuable insights into hydrochar composition, addressing critical knowledge gaps and leveraging advanced geochemical methods to assist in optimizing carbonaceous materials for practical applications including production of biofuels and platform chemicals for bio-refining. Highlights • Hydrothermal carbonization of “real world” waste materials at pilot plant scale. • Hydrochars characterized using traditional organic geochemical techniques. • Pyrolysis-FID/IRD and thermodesorption-pyrolysis-GC-MS especially useful. • Lipid-rich food waste hydrochars suggest biodiesel potential. • Lignocellulosic waste hydrochars suggest potential as platform chemical sources.