Hydrothermal Carbonization and Torrefaction of Kenaf, Rice Husk, Corncob, and Wood Chip: Characteristics and Differences of Hydrochar and Torrefied Char

The characteristics of biochar vary widely depending on the type of biomass and thermochemical conversion method. In this study, four types of biomass (kenaf, rice husk, corncob, and wood chips) were subjected to hydrothermal carbonization and torrefaction at 220 °C, 260 °C, and 300 °C for 30 min. The acquired biochars showed significant differences in the type of reaction and biomass. At each temperature, the decomposition of volatiles was more severe in hydrochar (HC) than in torrefied char (TC). The mass yields of HC were 44.30–61.63 wt.% (220 °C), 20.89–37.04 wt.% (260 °C), and 12.59–29.19 wt.% (300 °C), whereas the mass yields of TC were 94.73–97.69 wt.% (220 °C), 87.19–95.04 wt.% (260 °C), and 68.22–80.78 wt.% (300 °C). The elemental and thermal characteristics of TC changed gradually as the reaction temperature increased, and the characteristics of HC were enhanced rapidly. Wood chip biochar that was reacted at 300 °C showed the highest heating values of 28.77 MJ/kg (HC) and 21.09 MJ/kg (TC). The results of chemical analyses showed that hydrothermal carbonization strongly affected the cleavage of inter- and intra-molecular carbon bonds in cellulose and hemicellulose. In contrast, torrefaction removed the thermally fragile moisture and hemicellulose content from biomass.

Graphical Abstract