PURPOSE: To recover the energy retained by a reactive gas by using a turbine by interposing a function to control furnace top pressure by a bypass valve and performing automatically change-over between the functions to control the furnace top pressure for a gas scrubber and a turbine.

CONSTITUTION: A reactive gas scrubber 5, an expansion turbine 9 and a bypass valve 12 provided in a gas flow passage bypassing said turbine are provided to control the top pressure of a reaction furnace 3. A slight difference is given between the set furnace top pressures indicated by a furnace top pressure setter 13 to the respective control mechanisms for said scrubber 5 and valve 12 in such chemical reaction system. The competence for function participating in the control of the furnace top pressure is changed over while both control functions are kept operating.