Solid fuel combustion in thermal power plants, which use sulfur-rich fuels, poses the problem of SO2 removal from waste gases. In addition, purification of huge amounts of gas with low SO2 concentration is necessary. A maximum absorption rate could decrease the size of required big apparatuses. Gas purification from low-concentrated SO2 in physical and chemical absorption is analyzed theoretically. A convection-diffusion type of models is applied for creating average concentration models and for quantitative description of the absorption processes. Theoretical analysis of methods and devices for waste gas purification from SO2 by CaCO3 suspension indicates that the process is practically physical absorption.