Dry methods of flue gas cleaning are usually based on calcium hydroxide or sodium bicarbonate (NaHCO3), which is “in situ” decomposed to active soda (Na2CO3). Together with acid gases also emissions of volatile heavy metals (e.g., mercury and some Zn-compounds) have to be reduced. Removal of both main forms of mercury (Hg0 and Hg2+) from the flue gas is dependent on chemical reactions and sorption processes at the temperatures below approx. 300 °C. In this theoretical study, attention was concentrated on the thermodynamics of gas–solid reactions (de-HCl and desulfurization), reactions of Hg/HgCl2 vapours and Zn-compounds, with soda and Ca(OH)2 in the presence of acid gases, and on effects of elemental sulfur and HBr addition. Influence of sorbents/additives and conditions (temperature, gas phase composition) on the equilibrium concentrations of gaseous and solid species was studied at temperatures 400–560 K (127–287 °C). Two types of alkaline solid sorbents have been theoretically analyzed: Na2CO3 and Ca(OH)2. The thermodynamic analyses involved systems with computations of gas/solid phase composition on a basis of assumption of solid phases with unit activities (stable, basic state). Application of the minimization of Gibbs energy for the chosen systems was used for the determination of equilibrium composition of gas and solid phases.