PURPOSE: To enable effective utilization of waste lime stone and to reduce an amount of lime stone, by a method wherein a crusher is connected to the discharge port of a waste desulfurizing agent, a molding extruder, to which the lime stone piping of the crusher and a different fuel feed pipe are connected, is connected to the combustion chamber of a boiler body through a reduction pipe.

CONSTITUTION: Waste lime stone 15, which is modified by desulfurization and gathered at an upper layer higher than an inflow port 24a, of lime stone 15 in a desulfurizing chamber 10, flows over the inflow port 24a, and discharged through an extraction cylinder 24 to a crusher 25. By crushing the waste lime stone by means of the crusher 25, plaster being an outer hull is crushed to expose quick lime present at the interior, and is fed to a molding extruder 27 through a fuel feed pipe 28. In this case, since waste plastic, lime stone, fuel such as coal are fed through a fuel feed pipe 29 into the moulding extruder 27, the quick lime and the fuel are kneaded, and the kneaded substance is extruded in a pelletform through an extrusion port. The kneaded substance of the quick lime and the fuel is returned through a reduction pipe 30 to a combustion chamber 9, the fuel is burnt, and the quick lime is used for desulfurization of combustion gas.