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 2.8wt.% 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 2.2×10(4) to 3.0×10(4)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.