A method of treating a coal combustion flue gas, which includes injecting a molecular halogen or thermolabile molecular halogen precursor, such as calcium hypochlorite, able to decompose to form molecular halogen at flue gas temperature. The molecular halogen converts elemental mercury to mercuric halide, which is adsorbable by alkaline solids such as subbituminous or lignite coal ash, alkali fused bituminous coal ash, and dry flue gas desulphurization solids, capturable in whole or part by electrostatic precipitators (ESPs), baghouses (BHs), and fabric filters (FFs), with or without subsequent adsorption by a liquid such as a flue gas desulphurization scrubbing liquor.