PURPOSE: To keep the discharged slug in a flowable state by connecting the formed-gas flue being downstream of the cyclone provided through a thermal cracking chamber and a leading flue to the space below the slug discharge exit through a bypass pipe.

CONSTITUTION: The space below the slug discharge exit 10 formed on the bottom of a slug formation chamber 1 is connected to a flue 9 which leads the produced gas from a cyclone provided through a thermal cracking chamber 4 and a leading flue 6 to purification equipment through a bypass pipe 14. The pressure of the gas in the flue 9 from the cyclone 7 differs from the pressure just below the slug discharge exit 10 by about 100mmH2O because of the pressure loss across the cyclone 7. Therefore, the gas in the slug formation chamber 1 is sucked into the slug discharge exit 10 by connecting them through the bypass pipe 14, sent through the bypass 14, and enters the flue 9 at downstream side of the cyclone 7.