Solid fuel production by hydrothermal carbonization of black liquor

Formaldehyde was used as a polymerization agent to perform hydrothermal carbonization of black liquor for solid fuel production from 220 to 285 °C. Compared to hydrochar prepared without formaldehyde, hydrochar produced in the presence of a 2.8 wt.% formaldehyde solution (hydrochar-F) had 1.27–2.13 times higher yield, 1.02–1.36 times higher heating value (HHV), 1.20–2.31 times higher C recovery efficiency, 1.20–2.44 times higher total energy recovery efficiency, 0.51–0.64 times lower sulfur content, and 0.48–0.89 times lower ash content. The HHV of hydrochar-Fs ranged from 2.2 × 104 to 3.0 × 104 kJ/kg, while the HHV of hydrochar-F produced at 285 °C was 1.90 times greater than that of the raw material (black liquor solid). These considerable improvements indicated that formaldehyde was an effective additive in hydrothermal carbonization of black liquor.