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

Abstract The effect of two process water (PW) recirculation strategies after 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, which are residence time between 2 and 4 h and temperature in the range of 200–220 °C. The composition of the hydrochars reveals that operating conditions have a more significant effect than PW recirculation. The composition of the liquid produced by HTC with PW recirculation 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 also has been done. The main components of the liquid phase are lactic, formic, acetic, levulinic, and propionic acid as well as 5-hydroxymethylfurfural, that affect the surface structure of the hydrochars.