Abstract: A new method for denitration by ammonia of exhaust gas containing arsenic compounds which degrades a denitrating catalyst used is disclosed as well as a method and an apparatus, based on the same principle as the above method, for regenerating an already degraded catalyst. These effects are achieved, without taking the catalyst out of the system, by adjusting the humidity and temperature of the atmosphere surrounding the catalyst filled in the denitration reactor with simple and economical means so that arsenic compounds accumulated on the catalyst surface will diffuse into the catalyst reducing their concentration on the surface, which is important for the catalytic action.

Abstract: Here is provided an adsorbent for adsorbing and removing an arsenic compound which becomes a catalyst poison in a selective contact reduction process for removing nitrogen oxides (NOx) from a combustion exhaust gas by the use of an ammonia as a reducing agent and a denitrating catalyst.The adsorbent of the present invention comprises a material in which the total volume of pores is 0.2 to 0.7 cc/g and the volume of the pores having a pore diameter of 300 .ANG. or more is 10% or more with respect to the total pore volume, and the material is a specific element, its oxide, an ion-exchanged zeolite or the like.In addition, the present invention is directed to a method for removing the arsenic compound from the combustion exhaust gas by injecting the adsorbent into the flow of the gas on the upstream side of the denitrating catalyst.