A method for reducing the cost of emission control by utilizing a combination of low cost, combustion and post-combustion processes to eliminate emissions of sulfur and nitrogen oxides, and trace metals, including mercury, and trace dioxins and furans. These processes are applied sequentially to the combustion flow train of an air-cooled, slagging combustor-boiler, and include two groups of process steps. One group is implemented in the combustor and the other in post combustion zones.