Hydrothermal carbonization of waste protein powder for production of energy-dense hydrochars

Abstract The hydrothermal carbonization (HTC) of waste protein powder (WPP) was investigated as a sustainable strategy to transform underutilized protein-rich residues into solid biofuels, contributing to waste-to-energy transitions and circular resource management. Process parameters, including WPP-to-water ratio, temperature, and residence time were systematically varied to evaluate their effects on hydrochar yield and quality. Higher WPP-to-water ratios consistently enhanced hydrochar yields by reducing organic matter dissolution. The maximum yield (23.27 wt%) was achieved at 200 °C, while higher temperatures promoted volatilization and decomposition, reducing yields. With the exception of 1.5 h, extending residence time generally increased hydrochar yields, suggesting that recombination reactions favored solid recovery at longer durations. Compositional and structural characterization confirmed significant upgrading of WPP into energy-dense carbon materials. Compared to raw WPP, hydrochars exhibited reduced volatile matter, higher fixed carbon, and increased ash content. The high carbon contents and higher heating values (30.92 and 37.17 MJ/kg) made the produced hydrochars comparable to bituminous coal. Hydrochars showed enhanced aromaticity, amorphous carbon structures, and thermal stability with increasing HTC severity. The aqueous phase was dominated by nitrogen-containing compounds and ketones.