LAUSR

Abstract Hydrothermal carbonization (HTC) was used to convert macrophyte Potamogeton lucens into solid biofuel-hydrochar . HTC was carried out in a high pressure reactor with and without chemical additives under subcritical temperatures (240–320 °C), residence times (10–40 min) and biomass to water dilution ratios varying from 1:4 to 1:12. Hydrochar produced in HTC with chemical additives viz. KOH, Na 2 CO 3 and acetic acid had improvement in higher heating value (HHV) and energy density (E d ) in comparison to non-additive HTC from level of sub-bituminous coal to bituminous coal. As reflected by Van Krevelen diagram, HTC with additives had also improvement in H/C and O/C atomic ratios. Enhancement in HHV by utilization of additives followed order; acetic acid > Na 2 CO 3  > KOH. Hydrochar yield of 39% obtained in non additive process decreased to 36% (both with KOH, Na 2 CO 3 ) and 37% (with acetic acid). However at same time E d was enhanced from a value of 1.69 in non-additive HTC to 1.72, 1.76 and 1.84 with KOH, Na 2 CO 3 and acetic acid respectively. The hydrochar produced has thus huge potential to serve as an alternate fuel. The study also expresses yield (Y), HHV, E d and energy yield (E y ) in terms of temperature, residence time and biomass to water dilution ratio by linear and nonlinear laws.