Overcoming limitations of hydrothermal carbonization of biomass: secondary char extraction enhances solid fuel and soil amendment applications

Abstract Hydrothermal carbonization (HTC) thermochemically upcycles agro-industrial waste into a two-phase hydrochar (HC) consisting of primary char (PC) and amorphous secondary char (SC). SC limits the use of HC; its furan groups are phytotoxic, and the SC is thermally reactive, making it unsuitable for many direct combustion applications. Apple pomace (AP) was carbonized at 175, 200, and 250 °C, and SC was extracted using an organic solvent. The SC showed particularly high concentrations of 5-hydroxymethylfurfural (5-HMF). While such concentrations may inhibit germination if HC is used as a soil amendment, 5-HMF is an important biorefinery compound and is recoverable via solvent extraction. The resulting extracted hydrochar (called PC) has a higher surface area, fixed carbon content, and increased bioavailability of key nutrients than the as-carbonized HC. The PC is also more thermally stable than HC, particularly at 250 °C carbonization, where the SC contains considerable oxygenated compounds. HTC at 250 °C likely breaks down more lignin in the AP, resulting in more phenols in the SC, which are more reactive. At lower temperatures, we observe more phenols (comprising up to 96% of SC for HTC at 200 °C), which suggests that it may be possible to engineer SC to yield lucrative biorefinery intermediates from high-sugar biomasses such as AP. While the extraction of HC offers the potential to valorize SC as a biofuel, it also yields a more stable PC for use as a soil amendment and solid fuel.