Hydrothermal carbonization of loblolly pine: reaction chemistry and water balance

Hydrothermal carbonization (HTC) is a thermochemical process to convert lignocellulosic biomass into lignite-like HTC biochar. In this study, chemical reactions occurring during a relatively short HTC reaction time are discussed (5–30 min), and reaction mechanisms are examined at temperatures between 200 and 260 °C. Solid HTC biochar products were analyzed by attenuated total reflectance (ATR)/Fourier transform infrared spectroscopy (FTIR), elemental analysis, and gas chromatography-mass spectrometry (GC-MS), while liquid products were analyzed with GC-MS and ion chromatography (IC) to predict the reaction schemes. HTC reactions for whole biomass (loblolly pine) were proposed in the context of HTC reactions for individual biomass fractions. Hydrolysis, dehydration, and decarboxylation reactions are the major reactions of HTC, though condensation, polymerization, and aromatization also occur. An experimental procedure was developed to determine the net water production, a balance between consumption by hydrolysis reactions and production by dehydration reactions. Net production of water is evaluated. At lower HTC temperature (200 °C), water was consumed. However, at higher HTC temperatures, water was produced and the production increases with increasing reaction time.