Abstract: The system comprises a boiler (1), main burner groups (2F, 2E, 2D, 2C) located on different levels or zones through which pulverized solid fuel is injected into the boiler (1) and main mills (3F, 3E, 3D, 3C) for solid fuel, where each of them is connected to one of the main burner groups (2F, 2E, 2D, 2C) to which a flow of solid fuel is directed. As a complement, it incorporates a substitution mill (3B) intended to operate only on the main burner group (2F, 2E, 2D, 2C) whose main associated mill (3F, 3E, 3D, 3C) has stopped, as well as a support mill (3A) which operates constantly and directs an additional flow of solid fuel towards the main burner group or groups (2F, 2E, 2D, 2C) selected, supplementing the flow of solid fuel provided by the main mills (3F, 3E, 3D, 3C).