Physicochemical characterization of hydrochar from polyurethane foam waste: Assessing the role of processing parameters

Polyurethane foams are widely used polymers. Their end-of-life is largely managed through landfilling and incineration, causing environmental concerns and resource loss. Hydrothermal carbonization and hydrothermal liquefaction have recently emerged as promising thermochemical approaches to valorise polyurethanes waste into value added products. This study presents a systematic investigation of the effects of operating parameters on hydrochars (HCs) yields and properties, using a rigid polyurethane foam as feedstock. Product yields were rationalized using the Severity Value (SV) parameter and the obtained HCs were characterized by ultimate analysis, FTIR, SEM and thermogravimetric analysis. An increase in SV led to a reduction of HCs yields, whereas the effect of the feedstock to solvent ratio exhibited only a minor influence. Ultimate analysis revealed decreased H/C and O/C ratios in all HCs, while the nitrogen content increased in all samples compared to PUR, suggesting their potential application as nitrogen-doped carbon materials. All produced HCs exhibited HHVs higher than that of PUR (27.59 MJ/kg), indicating energy densification. The calculated combustion parameters indicated that HCs exhibit lower reactivity and a slower, more controlled combustion process compared to PUR. These results together with their positioning in the coal-like regions of the Van Krevelen diagram, suggest their potential use as solid fuels. The liquid phases recovered for each test were rich in value-added compounds, including aromatic heterocycles. Overall, these findings offer valuable insights for advancing circular economy strategies in PUR waste management, highlighting the potential of HTC and HTL to simultaneously produce energy-dense solid fuels and value-added platform chemicals. • Hydrothermal treatments were applied to recycle rigid polyurethane foam waste. • The operating parameters strongly influence hydrochar yields and properties • An inverse relationship is observed between severity values and hydrochar yields • Hydrochar has potential applications as nitrogen doped material and solid fuel • Organic phase is rich in compounds with potential applications as chemicals