Effects of hydrothermal treatment temperature and residence time on characteristics and combustion behaviors of green waste

• The hydrothermal treatment of green waste was studied.
• Effects of temperature and time on product characteristics were evaluated.
• The energy density and combustion performance of hydrochar was improved.
• The optimum reaction temperature and residence time of were selected.
• The activation energy of the main combustion process was calculated.

The hydrothermal treatment of green waste was investigated to elucidate the effect of different temperature and residence time on the combustion behaviors and properties of hydrochar. The higher heating value of the hydrochar after the hydrothermal treatment was enhanced by 1.00–1.58 times (energy content per mass). The combustion performance was also significantly improved. The reaction temperature of 210 °C was the most suitable temperature of hydrochar production compared with different temperature. It was found that the energy recovery efficiency was decreased with the residence time increased. The oxygen/carbon atomic ratio indicated that the upgrading process converted the green waste near to clean solid fuel with high energy density. The thermal gravimetric analysis showed that the temperature and residence time had a significant impact on combustion behavior and activation energy of hydrochar. The consecutive first order reaction model could well describe the combustion process.