Hydrothermal carbonization (HTC) and liquefaction (HTL) are potential green routes to convert biomass waste into valuable products, such as liquid and solid fuels, bio-based chemicals, and advanced carbon-based solid materials. The inherent complexity behind these thermochemical conversions arises from feedstock heterogeneity, concurrent and sequential reactions occurring during processing, and the unique role water plays as both a reaction medium and catalyst. Understanding the effects of those factors on carbon distribution and the quality of products produced from HTC and HTL of model feedstocks helps researchers optimize the operating conditions to control reaction endpoints and product distributions. To examine a range of HTC and HTL conditions, model compounds (i.e., cellulose and lignin) and wood chips were hydrothermally processed at 220-320 °C and 0-2 hours, Results show that higher reaction temperature and longer reaction time lead to a progressive decarboxylation and aromatization of the hydrochar, which indicate an increment and decrement of the carbon and oxygen contents, respectively, and improve the overall thermal stability of the hydrochar according to the thermogravimetric analysis (TGA). The TGA results also reveal the re-polymerization of the aqueous organics into secondary char that causes an overall high solid oxidative reactivity at low temperatures. Results of HPLC analysis show that the chemical compositions of the aqueous products vary depending on the substrate and operating conditions. For example, cellulose and wood chips show a predominance of carboxylic acids, phenols, and furans, such as 5-HMF that reach their maximum concentration between 0.5 and 1 hour. Finally, the power-law model fitted into the experimental results of carbon yield in all products allows the numerical calculation of kinetic parameters, such as reaction rate constants and Arrhenius pre-exponential and activation energy, to explain the lumped reaction scheme assumed in this study. Calculations show that carbon distributions generally follow the first-order kinetic model except for the formation of secondary char that follows a higher-order kinetic model.

The distribution of carbon products in the hydrothermal conversion of lignin–cellulosic biomass

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