A seal mechanism for a gate valve (3) operable between a solid fuel feed system (1) and a combustion space (3), such as a gas generator combustion chamber, the fuel feed system comprising a hopper (1) having at least its bottom end portion shaped substantially cylindrical, whereby gate valve (3) controls the discharge opening of the hopper bottom end, and the open/closed positions of the gate valve being controllable by means of a gate valve actuator mechanism (5, 6) in such a fashion that a seal is provided between the bottom end (4) of hopper (1) and gate valve plate (3). The invention is imple­mented by way of providing the lower edge of hopper (1) with a sealing member (4) whose edge facing the gate valve plate (3) is annular and is sharpened substantially over its entire circumferential length.