Abstract: The present invention provides a method of operating a solid fuel fired boiler comprising introducing a solid fuel and an iron-bearing material into the boiler. The solid fuel is at least partially combusted in the boiler to produce an ash slag, wherein the ash fusion temperature characteristics (i.e., one or more of the IDT, ST, HT, and FT) of the ash slag are different than the ash fusion temperature characteristics of the ash slag that would result on combustion of the solid fuel alone. The method of the present invention is particularly applicable to slag tap boilers, including cyclone-type boilers. These boilers are, typically, designed to operate with a liquid ash slag.