The challenges with thermal gasification of biomass to produce syngas include high transportation and drying costs of biomass and the need for high-temperature (700-1000 degrees C) gasifiers and conditioning of the produced gases. Some of these challenges can be addressed by first converting biomass into high-energy-density biochar before transportation and then hydrothermal gasification of biochar at the site of the Fischer-Tropsch plant for liquid fuel synthesis. In this work, a high-energy-density (>27 MJ/kg) biochar is first produced via hydrothermal carbonization of switchgrass at 300 degrees C, and then the biochar is gasified in hydrothermal medium at 400-650 degrees C. The carbon gasification efficiency in hydrothermal medium is much better than that in the thermal medium. For example, at 550 degrees C, only 5.9% carbon gasification was achieved in the thermal medium, as compared to 23.8% in hydrothermal medium. The addition of 25 wt % K2CO3 catalyst enhances the hydrothermal gasification to 43.8%. The gasification can be further enhanced if the biochar is passivated with a small amount of Ca(OH)(2) when producing from biomass. With the use of Ca(OH)(2) passivation during hydrothermal carbonization and the use of K2CO3 catalysis during hydrothermal gasification, a high carbon gasification efficiency of 75% is achieved at 600 degrees C, using short reaction times of 5 and 30 min, respectively. With the recycle of the alkali catalyst and passivation agent, the hydrothermal process can provide an attractive alternative to thermal gasification.