The use of Processing Water (PW) on the Hydrothermal Carbonization (HTC) of tunisian pine cones was investigated under different temperature (200 and 240 °C) and residence time conditions (3 and 20 h). Recirculating PW improved the HC (hydrochar) solid yield, without being detrimental to the carbon content of the HC. The heating value of the HCs, that with a single reaction already involved a rise of this parameter in the range 22.1–55.8% as compared to the raw material, was further increased up to 24.9–58.6% for cyclic runs. This was attributed to enhanced secondary char formation by reusing liquid, although this effect depended on the severity of previous HTC and was affected by both temperature and time.

The HCs showed an increment on C and especially on N for cyclic runs. Also, thermal degradation peaks during HC TGA/DTA analyses showed a broader temperature range decomposition upon pyrolysis when PW was recirculated. The modifications of HC surface morphologies and functional groups was also investigated and associated to related reaction mechanisms.