Aqueous phase recirculation during hydrothermal carbonization of microalgae and soybean straw: A comparison study

Aqueous phase (AP) is the wastewater (byproduct) produced during hydrothermal treatment of biomass. This study aims to investigate the effect of AP recirculation on hydrochar from different types of biomass. Therefore, Chlorella (CL) and soybean straw (SS) were hydrothermally carbonized (HTC) at 220 °C for 240 min with AP recirculation for four cycles, respectively. The yield of hydrochar was increased from 20.5% to 26.7% for CL and from 47.7% to 54.7% for SS. More carbon distribution in hydrochar was observed, which increased the energy recovery from biomass by HTC by 10%-12%. Most fuel properties such as elemental compositions of the hydrochar were only slightly influenced by the AP recirculation. Furthermore, AP was used for algal cultivation, and the maximum biomass density of ~1.2 (inoculum optical density 680 _nm of ~ 0.2) was obtained in CL AP without recirculation and in SS AP with three times of recirculation, respectively.