Diverse methods of utilization and the corresponding, related modifications, of linear type, Variable Gas Atomization (VGA) nozzles and lances are described. Therein is joined the mixing and gas atomization of chemicals that react to form active products that further react with harmful flue gas constituents of combustion so as to enable their effective collection. One version combines, in a lance assembly of nozzles, the injection of reactants, in the form of finely divided dry solids conveyed in a carrier air stream, and the fine atomization of water, aqueous solutions or slurries. Atomization of such streams is employed to effectively wet the surfaces of the dry particles and, thereby, promote the rapid sorption of gaseous constituents of flue gas being treated. Another version simultaneously mixes and atomizes reacting chemicals to produce, and release into the flue gas stream, strongly oxidizing, transient, species of their reaction products. The simultaneous atomization and chemical reaction enables the effective capture of targeted flue gas constituents. An accompanying flue gas duct modification is described providing means for cost effective, high-performance system operation of linear VGA nozzle-lance arrays, as components of the various subject flue gas cleaning processes. The combination of linear VGA nozzle-lances and, as needed, flue gas duct enhancement, is intended to provide an economical and technically practicable means of retrofitting existing fossil fueled boilers of a wide range of capacities to meet future emission limits for SO2, SO3, NOx and mercury.