Hydrothermal carbonization of Miscanthus × giganteus: Structural and fuel properties of hydrochars and organic profile with the ecotoxicological assessment of the liquid phase

In this study, hydrothermal carbonization (HTC) was employed for thermochemical conversion of energy crop Miscanthus × giganteus GREEF et DEU. The effect of process temperatures, in the range between 180 and 220 °C, on the relevant characteristics of the obtained products, hydrochars and process waters, was investigated. The obtained results showed that the HTC promotes fuel properties and energy density of the solids regarding the feedstock. Furthermore, temperature governs the lowering of the volatiles, ash, and moisture in hydrochars, making its potential use as solid fuels more beneficial than the miscanthus. FT-IR spectroscopy and thermal analysis confirmed degradation of hemicellulose at temperatures above 200 °C, and an increase of the total content of cellulose and lignin in the hydrochars. In general, hydrochar obtained at 220 °C exhibited the best combustion characteristics and is, therefore, the most suitable for use as a solid biofuel. However, in the residual liquids, some amounts of fluorene, phenanthrene, fluoranthene, and pyrene were detected. The polluting potential, visible through the growth of TOC, COD and BOD values, of the analyzed process waters, increases with the rise in carbonization temperature. The TOC values (5.8–9.9 gC L−1) were on average lower than those reported for organic wastewaters (>10 gC L−1). The proportion of hydrocarbons in the process water increases with the increase in the carbonization temperature from 2.92 to 20.9%. Consequently, bioassays with Vibrio fischeri showed a relatively high toxicity of the liquid phase, where a concentration of about 1% was causing bacteria inhibition of 50%.