PURPOSE: To reduce loss of energy and time and to prevent an adverse influence due to the high temperature of a container from being exercised, by a method wherein a powdery material, such as peat, processed within a pressure vessel, is discharged from the vessel to a spot under an atmospheric pressure in a condition to hold a pressure in the pressure vessel.

CONSTITUTION: A high pressure steam pipe 2 is connected to a reactor 1, and fed peat slurry is dehydrated by means of temperature and a pressure. A peat slurry feed pipe 12 is connected to an inlet 3 of the reactor 1 through a valve 10 and a pump 11, and a discharge part 30 for the processed powdery material is connected to an outlet 4. A drain line 47 for dehydrated and separated water is installed to the lower part of the reactor 1. The discharge part 30 comprises a plunger 33, having an opening 31 connected to the reactor 1, reciprocating within a cylinder 32, and opening and closing the opening 31 during said reciprocation, and a pressure seal part 37 which is connected to the outlet end of the cylinder 32, and includes a duct 34, a throttle 35, and a sluice valve 36.