Effects of temperature, time and acidity of hydrothermal carbonization on the hydrochar properties and nitrogen recovery from corn stover

Corn stover is hydrothermally carbonized at different temperatures (180 °C, 200 °C, 220 °C, 240 °C and 260 °C) for 4 h, for different residence times (1 h, 2 h, 4 h, 12 h and 24 h) at 220 °C, or by adding acid (1% and 2%) at 220 °C for 4 h. The yields, elemental analysis, functional group analysis, molecular composition and microstructure of those hydrochars are characterized, and the nitrogen recovery in the by-products is analysed to further understand the elemental transformation in the progress of hydrothermal carbonization. The results indicate that a higher temperature, longer residence time or acid addition can cause lower yields and simultaneously reduce the H/C (0.76–1.33) and O/C (0.19–0.68) ratios of the hydrochars by 13.1%–50.5% and 9.3–73.4% compared with feedstock, respectively. Py-GC/MS analyses showed that furan compounds decreasing or disappearing and phenolic compounds increasing significantly promotes the aromaticity of hydrochar during the hydrothermal carbonization, especially under severe conditions. The SEM images of hydrochars show that the formation of carbon spheres is the cause of changes in the surface morphology of hydrochar. The nitrogen recovery efficiency increased with the reaction severity, so that the liquid by-product from corn stovers by hydrothermal carbonization may be used as a fertilizer due to its abundant nutrients. This study provides an in-depth understanding of the corn stover-derived hydrochar, which is of great significance for the potential application of hydrochar.