It is disclosed a system for feeding a ligno-cellulosic feedstock (50) to a pressurized vessel (200) having a vapor or gas phase of at least one fluid maintaining a vessel pressure greater than or equal to 1 bar. The pressurized vessel comprises a feedstock inlet (230), a feedstock outlet (220), a top and a bottom relative to the force of gravity, wherein the feedstock outlet is closer to the bottom of the pressurized vessel than the feedstock inlet. The feeding system further comprises a plug forming system (100) comprising a plug forming device (105) located in a plug forming chamber (130), the chamber having a first end (110B) and a second end (110A), said first end having a first end operating pressure which is lower than the vessel pressure and said second end being connected to the feedstock inlet and having a second end operating pressure which is greater than the first operating pressure. Said plug forming device is capable of receiving the ligno-cellulosic feedstock at the first end pressure; advancing the ligno-cellulosic feedstock through the plug forming chamber; and forming a plug from the ligno-cellulosic feedstock which prevents the passage of the vapor or gas phase from the pressurized vessel. The feeding system further comprises a sealing head located inside the pressurized vessel, which is movable at a seal opening speed between a sealing position located at the feedstock inlet and a rest position, wherein the sealing head in the sealing position isolates the pressurized vessel from the plug forming chamber. The seal opening speed is fast enough so that the sealing head disengages from the feedstock before or at the moment the sealing head reaches the rest position. The sealing head in the rest position is located at a distance far enough away from the feedstock inlet (sealing head) so that the feedstock entering the pressurized vessel does not contact the sealing head.