An injection system for solid particles comprising: a conveying hopper located at an upstream location; a fluidizing device for fluidizing the solid particles at the outlet of said conveying hopper and forming a solid-gas flow; a pneumatic conveying line for conveying said solid-gas flow from said fluidizing device to a downstream location, said pneumatic conveying line including at said downstream location a static distribution device with a plurality of injection lines connected thereto; and an upstream flow control system including: an upstream flow control valve arranged in said pneumatic conveying line at said upstream location; and an upstream mass flow rate determination means capable of measuring a solid material mass flow in said pneumatic conveying line at said upstream location; said upstream control system being capable of controlling the mass flow rate in said pneumatic conveying line at said upstream location by controlling the opening of said upstream flow control valve responsive to said solid material mass flow measured in said pneumatic conveying line at said upstream location; characterized by a downstream flow control system including: at least one downstream flow control valve (51, 79i) arranged in said pneumatic conveying line at said downstream location upstream of said static distribution device; and a main downstream mass flow rate sensor arranged in said pneumatic conveying line at said downstream location upstream of said static distribution device, said downstream control system being capable of controlling the mass flow rate in said pneumatic conveying line at said downstream location by controlling the opening of said at least one downstream flow control valve (51, 79i) responsive to said instantaneous mass flow rate sensed by said main downstream mass flow rate sensor. a conveying hopper located at an upstream location; a fluidizing device for fluidizing the solid particles at the outlet of said conveying hopper and forming a solid-gas flow; a pneumatic conveying line for conveying said solid-gas flow from said fluidizing device to a downstream location, said pneumatic conveying line including at said downstream location a static distribution device with a plurality of injection lines connected thereto; and an upstream flow control system including: an upstream flow control valve arranged in said pneumatic conveying line at said upstream location; and an upstream mass flow rate determination means capable of measuring a solid material mass flow in said pneumatic conveying line at said upstream location; an upstream flow control valve arranged in said pneumatic conveying line at said upstream location; and an upstream mass flow rate determination means capable of measuring a solid material mass flow in said pneumatic conveying line at said upstream location; said upstream control system being capable of controlling the mass flow rate in said pneumatic conveying line at said upstream location by controlling the opening of said upstream flow control valve responsive to said solid material mass flow measured in said pneumatic conveying line at said upstream location; characterized by a downstream flow control system including: at least one downstream flow control valve (51, 79i) arranged in said pneumatic conveying line at said downstream location upstream of said static distribution device; and a main downstream mass flow rate sensor arranged in said pneumatic conveying line at said downstream location upstream of said static distribution device, at least one downstream flow control valve (51, 79i) arranged in said pneumatic conveying line at said downstream location upstream of said static distribution device; and a main downstream mass flow rate sensor arranged in said pneumatic conveying line at said downstream location upstream of said static distribution device, said downstream control system being capable of controlling the mass flow rate in said pneumatic conveying line at said downstream location by controlling the opening of said at least one downstream flow control valve (51, 79i) responsive to said instantaneous mass flow rate sensed by said main downstream mass flow rate sensor.