The invention relates to a process for firing an industrial furnace, in particular for electricity generation, wherein coal or cokes together with a secondary fuel comprising cellulose and plastic, in the form of pellets of a size larger than about 3 mm thickness, and having a caloric value of about 16 GJ/ton or more is ground to a powder wherein about 95 wt % or more has a particle size smaller than 2 mm and wherein the d50 of the particle size distribution is between about 5 and about 100 μm, wherein the powder is injected in the flame of the furnace. In this process the grinding is performed in a roller mill or ball mill, and the amount of pellets used together with the coals preferably is about 3 wt % or more, relative to the coal.