Comparison of structural characteristics and combustibility analysis about hydrochar and pyrochar.

The structure characteristics and combustibility of pyrochar and hydrochar were compared using scanning electron microscopy, nitrogen adsorption, Fourier transform infrared spectrum, Raman spectrum and thermogravimetry. The random pore model was applied to analyze the combustion process of chars. With the temperature increasing, analysis of physical structure revealed that the pore structure of hydrochar was more developed than that of pyrochar. Meanwhile, the hydrothermal process had an advantage over pyrolysis in terms of removing oxygen-containing functional groups and improving the coal rank, mainly attributed to the existence of subcritical water. In addition, the high determination coefficient of random pore model indicated that the model could accurately obtain the kinetic parameters. The activation energies calculated of hydrochars were higher than that of pyrochars, indicating that carbon in hydrochar had a more structural stability. The pyrochar obtained after 220 °C was less combustible than hydrochar due to its severe pore collapse during the combustion process.