Comparison of hydrothermal carbonization and torrefaction of azolla biomass: Analysis of the solid products

The effect of torrefaction and hydrothermal carbonization has been studied on Azolla filiculoides, which is a potential aquatic biomass feedstock for renewable energy sources. Hydrochar and torrefied azolla samples were prepared at three temperatures (260, 280, 300 °C) with 15 min residence time. The thermal behavior and the volatile content of the raw and thermally treated biomass samples were determined by thermogravimetric analysis, while the detailed composition of the volatile contents was analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The major pyrolysis products were phenolic compounds, monocyclic and condensed aromatic hydrocarbons, nitrogen heterocycles, nitriles, carbohydrate decomposition products, phytosterols, and fatty acid derivatives. Principal component analysis on Py-GC/MS data revealed that the pyrolyzates of the samples differed significantly depending on the temperature and type of pretreatment. Hydrothermal carbonization resulted in more severe decomposition at each temperature than torrefaction. The volatile content of torrefied azolla decreased gradually with torrefaction temperature, while 280 °C was a sufficient temperature to remove the majority of the volatile content during HTC. The amounts of phytosterols decreased to the highest extent during both treatments. The thermally less stable carbohydrate components of azolla were decomposed during torrefaction, while hydrothermal carbonization affected other constituents of the biomass (e.g., proteins), as well. During pyrolysis, the formation of aromatic compounds (phenol, benzene, pyridine, and pyrrole derivatives) was more favored from hydrochars than from torrefied azolla. © 2020 The Authors