NOVELTY - The method comprises supplying fresh biomass, preheating the supplied fresh biomass, hydrolyzing the preheated biomass, cooling the hydrolyzed biomass, and discharging the cooled biomass. The supplied biomass is preheated in two steps. The preheating step includes mixing a part of the supplied biomass with a part of the hydrolyzed biomass and bringing the part of the supplied biomass into heat-exchanging contact with a prewarming or preheating medium. The prewarming or preheating medium comprises hydrolyzed biomass.    USE - The method is useful for thermal digestion of pumpable biomass (claimed) such as sewage sludge, a paste-like material or a biodegradable material.    ADVANTAGE - The method is capable of thermally and efficiently digesting the pumpable biomass with less effort.    DETAILED DESCRIPTION - The method comprises supplying fresh biomass, preheating the supplied fresh biomass, hydrolyzing the preheated biomass, cooling the hydrolyzed biomass, and discharging the cooled biomass. The supplied biomass is preheated in two steps. The preheating step includes mixing a part of the supplied biomass with a part of the hydrolyzed biomass and bringing the part of the supplied biomass into heat-exchanging contact with a prewarming or preheating medium. The prewarming or preheating medium comprises hydrolyzed biomass, or is present in heat-exchanging contact with the part of the hydrolyzed biomass. The entire hydrolyzed biomass is mixed with the supplied fresh biomass. The supplied fresh biomass has a viscosity, which is a magnitude higher than the viscosity of the hydrolyzed biomass. The method further comprises adding flocculent to the fresh biomass, and separating a biomass mixture formed during the preheating step. A part of the hydrolyzed biomass is entrained by the preheated fresh biomass during the separation step. The fresh biomass and the hydrolyzed biomass are mixed such that the fresh biomass is heated. The biomass mixture is separated by a screening process. The part of the hydrolyzed biomass is fed back and mixed with the supplied fresh biomass before the preheating step. The hydrolyzed biomass is further cooled by brought into heat-exchanging contact with a cooling medium. The fresh biomass is brought into heat-exchanging contact with the preheating medium. A single medium is used as the cooling medium for the hydrolyzed biomass, as the preheating medium for the fresh biomass, as pre-cooling medium for the hydrolyzed biomass and as heating medium for the supplied biomass. The supplied biomass is brought into heat-exchanging contact with a heating medium. The hydrolyzed biomass is pre-cooled before the step of mixing by brought into heat-exchanging contact with a pre-cooling medium. An INDEPENDENT CLAIM is included for an installation for thermal digestion of pumpable biomass.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an installation for thermal digestion of pumpable biomass.    Supplying unit (3)    Reactor (4)    Preheating unit (11)    Heating unit (12)    Buffer. (21)