Effects of process water recirculation on yields and quality of hydrochar from hydrothermal carbonization process of rice husk

Hydrothermal carbonization (HTC) is promising to convert moist lignocellulosic biomass into advanced fuels. However, it also generates a large amount of process wastewater (PW) which can't be discharged directly due to its protentional pollution to the environment. This study aims to investigate the effects of PW recycling on the hydrochar performance. Rice husk (RH) was hydrothermally carbonized under the temperature of 220 ℃ for 60 min and PW was recycled 5 times. In the first 3 times recycle, the yield and high heating value (HHV) of hydrochar was improved from 68.51% to 76.62%, 19.71 MJ/kg to 21.97MJ/kg, respectively, and the energetic recovery efficiency (ERE) was highly increased from 72.17% to 89.72%. Functional group analysis by titration shows that PW recycling promoted the formation of oxygen-containing functional groups. The decrease of H/C and O/C ratio shows that PW recirculation promoted dehydration and decarboxylation reactions, thus improving the thermal stability of generated hydrochars. The adsorption performance of hydrochar was evaluated by removal of malachite green (MG). The results imply that the PW recirculation could also improve adsorption capacity of hydrochar for MG. This study provides a new insight for the treatment of PW from HTC of lignocellulosic biomasses.