Assessment of the effects of process water recirculation on the surface chemistry and morphology of hydrochar

The effect of two process water (PW) recirculation strategies after the hydrothermal carbonization (HTC) of brewers spent grains (BSG) is evaluated with the focus on the hydrochar characteristics. The HTC process has been carried out under different operating conditions; residence time between 2 - 4 h and temperature in the range of 200 - 220 °C. The hydrochars composition reveals that operating conditions have a more significant effect than PW recirculation on the hydrochar composition. The composition of the liquid produced at HTC plus PW recirculation process is essentially controlled by the operating temperature, for instance, the total organic carbon (TOC) in the PW changes in the narrow range of 200 - 220 °C. A detailed analysis of PW has also been done. The main components of the liquid phase are lactic, formic, acetic, levulinic, and propionic acid and 5-hydroxymethylfurfural, that affect the surface structure of the hydrochars.