A process for controlling both fireside ash deposits and corrosion, and fouling, corrosion, and emissions due to SO₃ formation within a fossil-fuel-fired combustion system, such as a furnace forming part of an electrical power generating plant. A solution of a soluble magnesium compound, which can be derived from wastes, such as the bleed stream from the power plant's SO₂ scrubber, is injected into the combustion products within the furnace in the form of a fine spray and at a point at which the temperature is sufficiently high to produce submicron-size MgO particles. The SO₃ reacts with the MgO particles to form MgSO₄. Insoluble magnesium compounds can be added to the solution to produce larger (micron sized) MgO particles on thermal decomposition. The micron-sized MgO particles are deposited on furnace surfaces to reduce ash deposits and to reduce catalytic generation of SO₃. The boiler wastes can be reacted with other industrial process waste products to provide marketable chemicals.