An injection system for solid particles comprises a conveying hopper (11) located at an upstream location (1), a fluidizing device (21) for fluidizing the solid particles at the outlet of the conveying hopper (11) and forming a solid-gas flow, a pneumatic conveying line (15) for conveying the solid-gas flow from the fluidizing device (21) to a downstream location (2) and a static distribution device (17) with a plurality of injection lines (19), connected thereto. An upstream flow control system controls the mass flow rate in the pneumatic conveying line (15) at the upstream location (1) by controlling the opening of an upstream flow control valve (35) responsive to the solid material mass flow measured in the pneumatic conveying line (15) at the upstream location (1). A downstream flow control system controls the mass flow rate in the pneumatic conveying line (15) at the downstream location (2) by controlling the opening of a downstream flow control valve (51, 79i) responsive to the instantaneous mass flow rate sensed by a main downstream mass flow rate sensor (53).