PURPOSE: To prevent the lowering of pH of ash treating water by transferring a part of scattered coal ashes arrested by a coal ash arrester provided in a flue or the like to a water-immersion type ash hopper, upon driving a coal-burning combustion device having at the furnace bottom the water-immersion type ash hopper in which ash treating water circulates.

CONSTITUTION: In order to cause the device to carry out the pH adjusting operation of ash treating water due to the injection of scattered coal ashes and an operation to prevent contact with circulated combustion waste gas due to suspention in correspondence to the pH of the ash treating water, it is advisable to provide a pH meter 20 concurrently used for a ash water transfer control valve 21 in the water-immersion type ash hopper 2 and an ash water transfer pipe 25, respectively and connect them to each other, and further an ash air transfer control valve 23 in an ash air transfer pipe 24. For instance, if the ash water transfer control valve 21 is set to operate when the pH is lowered to a predetermined value, and the ash air transfer control valve 23 is set to operated at every predetermined time, the lowering of the pH of the ash treating water can be prevented constantly, automatically and stably.