PURPOSE:To make it possible to suppress the quantity of soot and dust particularly in the case of using a heavy fuel in a high-load furnace or a small- size furnace by using an emulsified fuel as a secondary fuel. CONSTITUTION:Upon carrying out three-step combustion by successively disposing a primary burner 12, a secondary fuel nozzle 13 and a secondary air nozzle 14 on the rear stream side, an emulsified fuel prepared by mixing and agitating a fuel F and hot water W with each other by a mixer 22 as a fuel supplied to the secondary fuel nozzle 13, is used. Accordingly, the relation between the emulsification ratio of the secondary fuel and the quantity of soots and dirts generated upon the actual burning operation lies in that, as compared with the case where the entire fuel is emulsified, the increase in the heat loss can be suppressed, and a great suppression of the quantity of soot and dust generated becomes possible.
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