Forming mechanism of coke microparticles from polymerization of aqueous organics during hydrothermal carbonization process of biomass

Corn stalk was selected as a typical biomass, and the growth mechanism of coke microparticles was studied. The coke microparticles that formed from through the polymerization of aqueous organics during the hydrothermal carbonization (HTC) showed a considerable effect on the mass yield and physicochemical properties of the hydrochar. Results indicated that the average particle size of the coke microparticles rapidly increased to 6.25 μm within the initial 2 h of HTC. The lower C/O atomic ratio of the surface layer of the coke microparticles than that of the core, suggested that the core had a higher carbonization degree than the shell. On the basis of various measurements, the coke microparticles were inferred to have formed through the two stages of micronuclei formation and growth. Dissolved organics, including furfurals, furans, and phenols polymerized each other into aromatized macromolecules, and then formed solid-phase micronuclei. Subsequently, the oxygen containing groups of the micronuclei formed bonded with aqueous organic molecules, as a result, the micronuclei grew up into coke microparticles. Meanwhile, the carbonization degree of the coke microparticles was enhanced due to continuous intra- and inter-molecular dehydration and decarboxylation reactions occurred between hydroxyl and carboxyl groups.