Abstract: A process for enhancing the fuel form, raising the energy content, and lowering the level of impurities of low rank coals and/or carbonaceous wastes, like Municipal Solid Waste (MSW), Refuse Derived Fuel (RDF), and sewage sludge, by providing the low rank fuel, carbonaceous waste, or mixtures thereof as a slurry in water of processable viscosity. This feed slurry is heated under pressure, usually in the presence of an alkali, to a temperature at which a significant physical and molecular rearrangement occurs, characterized by the splitting off of a substantial proportion of the oxygen bound in the low rank coal or carbonaceous waste as carbon dioxide. At these conditions, solid particles in the feed slurry lose much of their fibrous and hydrophilic character, and are broken up into smaller particles of char, resulting in a slurry of dramatically improved rheology, i.e., capable of a much higher solids concentration (or energy density) at processable viscosity.