Effects of water recycling in hydrothermal carbonization of loblolly pine



Hydrothermal carbonization (HTC) is a process to densify, homogenize, and stabilize diverse biomass feedstocks. The water requirements of HTC need to be assessed to determine commercial feasibility. This current research work focuses on the effects on HTC of using recycled process water for multiple process cycles. Loblolly pine was treated in hot, compressed water at 200, 230, and 260°C for 5 min with a 5:1 water:biomass mass ratio. Liquid product was separated and recycled for reuse in HTC, nine cycles at 200 and 230°C and five cycles at 260°C. The solid products (biocarbon) were characterized by their mass yields, higher heating values (HHVs), and equilibrium moisture content (EMC), whereas in the liquid samples, total organic carbon (TOC) content and pH were determined. With successive recycling, biocarbon mass yield increases by 5–10% above the yield recorded for the initial cycle at each temperature investigated, whereas biocarbon HHV is essentially unchanged. The aqueous TOC is increasingly concentrated with the number of cycles and reaches an equilibrium. The EMC results suggest that the effects of recycling process water on hydrophobicity of the biocarbon are negligible. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 1309–1315, 2014