Hydrothermal carbonization (HTC) thermochemically converts wet biomass to carbonaceous solids (hydrochar, HC) and aqueous and gaseous byproducts. Secondary char (SC) is an amorphous fraction of the solid HC formed from the re-polymerization of HTC intermediates present in the liquid phase that (re)condense on the solid HC. SC increases the thermal reactivity of HC, which may impact combustion. Removing or preventing the formation of SC may improve the combustion efficiency of the remaining primary char (PC). To date, we do not know which compounds present in the SC impact the fuel characteristics of the HC, nor the specific feedstock and process conditions responsible for SC formation. In this study, Cellulose, Hemicellulose, Lignin, and a representative plant mixture (Cellulose:Hemicellulose:Lignin=55:25:20) were hydrothermally carbonized at 220C and 250C for 1 hour. High-performance Liquid Chromatography analysis of the compounds present in the aqueous phases from HTC of all building blocks at 220C mainly contain carboxylic acid and furan, with phenol present in the lignin product. For the mixture, only phenol and one carboxylic acid were present in the aqueous phase. The solid HC was contacted with ethyl acetate to extract the SC from the PC. The SCs produced from both cellulose and lignin contain phenol, whereas the SC from hemicellulose contains only furan. The mixture’s SC is rich in phenol due to synergistic interaction among the biomass constituents. Increasing the HTC temperature to 250C reduces the concentrations of carboxylic acids and furans in the aqueous products of all three feedstocks and increases the concentration of phenol for lignin. At 250°C the aqueous product from the mixture HTC contains furan and phenol. Following extraction, the SC of cellulose and hemicellulose contain organic acids and 1, 3 – Cyclopentadione, while SC from lignin contains mostly phenol. The SC of the mixture contains mostly furan. Overall, at moderate temperature (220C), synergistic reactions between model compounds increase the concentration of phenol in SC of the mixture likely by promoting the decomposition of lignin. This finding can be used as a tool to optimize the composition of SC for better extraction and improvement in the fuel characteristics of the primary char.

Impact of biomass constituents and hydrothermal carbonization conditions on the formation, composition, and reactivity of secondary char

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