Method and apparatus for separating at least one harmful substance such as SO.sub.2, HCl or NO.sub.x where x is 1 or 2, from combustion exhaust gases containing the same. The exhaust gases are contacted with at least one particulate adsorbent which, at a release temperature below 400.degree. C., releases at least one of water, ammonia, or carbon dioxide, for reaction with the harmful substances. This reactant leaves the adsorbent in activated condition. Thus, the reactant and activated adsorbent serve to remove harmful substances from the flowing exhaust gas.