TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Process: The process is carried out continuously in a mixed fixed-bed reactor. The device allows the supply of existing fuel, which are included in the resulting product stream and exhibits a boiling point of 25 degrees C or less at normal pressure, to the reactor container and then the reactor container with fuel is heated. Preferred Components: The device allows a continuous reaction control. The device is a fixed bed reactor. The reactor container is a reaction tube which is inclined so that the liquid components of the product stream can flow by gravity into a collecting container.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The wood ash is produced by combustion of wood at atmospheric pressure. The reaction is carried out at a pressure of 0.1-2, preferably 0.8-1.2 bar. The reaction is carried out in the presence of steam or an inert gas including nitrogen or argon. The resulting product stream of existing fuels with a boiling point of 25 degrees C or less at normal pressure are used for the heating of biomass and carbonate-containing solids. The process is carried out continuously by an inert gas flow through the mixture of biomass and the carbonate-containing solid during the reaction. Preferred components: The wood ash has a bulk density of 0.01-0.5, preferably 0.2-0.4 kg/l. The biomass is aerobic or anaerobic sludge, raw sewage sludge, digested sludge, algae, animal meal, meat bone meal, rapeseed cake and fats and fatty acids and/or soaps. The biomass has a carbon content of 55-80, preferably 64-77 wt.%. The carbonate-containing solid is a mixture consisting of wood ash and an alkali- or alkaline earth metal carbonate in a ratio 10-1 to 1-1. The weight ratio of biomass contained carbon to carbonate is 10-1 and 1-1, preferably 6-1 and 2-1. EXAMPLE - Material portion of air-dry sludge (with carbon (24 %), hydrogen (4.3%), nitrogen (3.7%), phosphorus (0.7%), moisture (8.6%) and residue on ignition (46.4%)) (250 g) was mixed with wood ash (250.3 g) at room temperature. After filling the reaction apparatus, the mixture was heated at a heating rate of 1.8 degrees C/minute in the absence of air to 400 plus minus 20 degrees C for 1.5 hours. Finally the fuel was obtained.