There is provided a process for operating a coal-fired furnace to generate heat. The process has the steps of a) providing the coal to the furnace and b) combusting the coal in the presence of a first slag-reducing ingredient and a second slag-reducing ingredient in amounts effective to reduce slag formation in the furnace. In one embodiment, the first slag-reducing ingredient is one or more oxygenated magnesium compounds and the second slag-reducing ingredient is selected from the group consisting of one or more oxygenated calcium compounds, one or more oxygenated silicon compounds, and combinations thereof. In another embodiment, the first slag-reducing ingredient is one or more oxygenated silicon compounds, and wherein the second slag-reducing ingredient is one or more oxygenated aluminum compounds. There are also provided methods for reducing slag formation in a coal-fired furnace. There are also provided methods for treating coal. There are also treated coals.