Hydrothermal Carbonization: Reactions and Water Production

Hydrothermal carbonization (HTC) is a promising thermochemical conversion process for lignocellulosic biomass to offer value-added products. Biomass is not required to be dry , as the treatment is actually conducted in hot water. The process is done at temperatures between 200 °C and 280 °C , and the pressure is high enough to maintain liquid water. HTC converts lignocellulosic biomass to solid bio-carbon , aqueous phase chemicals , and gases. In the decomposition of lignocellulosic biomass , knowledge of the product yields and water production is very important. In this study chemical reactions occurring during the relatively short HTC reaction time are discussed. Analytical techniques include characterization of solid and liquid HTC products through ATR/FTIR , ultimate analysis , GC-MS , and IC. HTC reactions for loblolly pine are proposed in the context of HTC reactions for individual biomass components. Based on the results , dehydration and decarboxylation reactions are proposed to be the major reactions of HTC after hydrolysis , though condensation , polymerization , and aromatization also occur simultaneously. An experimental procedure  was developed to quantify dehydration rate and the production or consumption of water. Experiments were performed with loblolly pine at 200 °C , 230 °C , and 260 °C for 5 , 15 , and 30 min reaction times. For the first time , we present quantitative results showing yield of water by dehydration and hydrolysis reactions. Detailed analysis shows that water used for initial hydrolysis at 200 °C cannot be recovered with a 30 min reaction time. However , dehydration and water production increases with both HTC temperature and reaction time. With longer reaction times , the conversion rate of biomass to high carbon content char apparently slows down within 5 to 10 min , with side reactions occurring in the liquid phase during the rest of the reaction time. In general , the solid phase hydrolysis reactions are much faster than the aqueous phase dehydration reactions.