Formaldehyde was used as a polymerization agent to perform hydrothermal carbonization of black liquor for solid fuel production from 220 to 285 degrees 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 x 10(4) to 3.0 x 10(4) kJ/kg, while the HHV of hydrochar-F produced at 285 degrees 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. (C) 2012 Elsevier Ltd. All rights reserved.