PURPOSE: To be able to discharge molten ash easily even at the low load time by installing an upper burner at the upper part and a lower burner at the lower part, whose tips are directed downward respectively in the condition that a primary burning chamber is divided into the upper and lower parts.

CONSTITUTION: The inner space of a primary burning chamber 1 is divided into two parts of the upper part 2 and lower parts 3 and an upper burner 4 and lower burner 5 whose tips are directed downward are installed at the upper part 2 and lower part 3 respectively. Molten ash 'b' produced in the chamber 1 flows along an inner wall to reach an outlet hopper 8. At the high load time of a boiler, the whole chamber 1 is set in high temp. by using the upper 4 and lower 5 burners, however, at the low load time, only the lower burner 5 is used because of reduced burning quantity. At the low load time, the lower part of the chamber 1 is able to be kept in high temp. to prevent sticking or slow movement of molten ash and smooth operation is able to be obtained.