Abstract: The present application provides a method of treating flue gas in a duct with an injection system, the flue gas comprising an acid gas and/or one or more metal components, the injection system comprising at least one injection nozzle in communication with an air supply and a supply of sorbent; wherein the method comprises supplying air and sorbent through the nozzle to the duct such that the penetration of the sorbent into the duct is represented by the formula: Y=(Dna(pnVn2/pfVf2)0.5(x/Dn)0.33)/Df where ‘Y’ is the fraction of duct penetration depth of the sorbent when the duct length is ‘x’, ‘Dn’ is the diameter of the nozzle, Df is the depth of the duct, ‘pn’ and ‘pf’ are the densities of the air supply and flue gas, respectively Vn? and ‘Vf’ are the velocities of the air exiting the nozzle and the flue gas, respectively, and where ‘a’ is between 0.3 and 1.0 and where Y is between 0.3 and 0.8.