Abstract: A moisture-containing viscous secondary fuel is combusted in the chamber of an industrial furnace by comminuting the secondary fuel to a particle size of 5-50 mm and propelling the particles at an initial speed of 1-10 m/sec. onto a layer of intensely burning primary fuel, such as solid refuse, coal or wood. The particles are propelled from a level at 0.5-2 m above the burning layer and in such a way that the length of their flight spans is 0.2-2 m. This insures that the particles retain moisture during travel in the combustion chamber and do not agglomerate prior and/or subsequent to contacting the burning layer of primary fuel. The rate of admission of comminuted secondary fuel is regulated in dependency on changes of temperature in the chamber, in dependency on changes of the CO.sub.2 or O.sub.2 content of combustion products and/or in dependency on variations of a parameter (e.g., length) of the burning layer of primary fuel.