Abstract: A gaseous substance such as a flue gas or a combustion gas contains an adsorbable impurity which is capable of undergoing an exothermic reaction in the presence of oxygen. The gaseous substance is passed into an adsorber via an inlet conduit. In the adsorber, the impurity in the gaseous substance is adsorbed therefrom. The purified gaseous substance is withdrawn from the adsorber therefrom. The purified gaseous substance is withdrawn from the adsorber via an outlet conduit and conveyed to an exhaust stack. The adsorption of the impurity, as well as the reaction thereof with oxygen in the adsorber, liberate heat. The liberated heat is conveyed from the adsorber by the gaseous substance which is being purified and, accordingly, no undue temperature increase, which may harm or even cause combustion of the adsorbent, occurs in the adsorber.