An energy efficient process for converting biomass into a higher carbon content, high energy density slurry. Water and biomass are mixed at a temperature and under a pressure that are much lower than in prior processes, but under a non-oxidative gas, which enables a stable slurry to be obtained containing up to 60% solids by weight, 20 - 40% carbon by weight, in the slurry. The temperature is nominally about 200°C under non-oxidative gas pressure of about 150 psi, conditions that are substantially less stringent than those required by the prior art.