Performance of silica-supported copper oxide sorbents for SO x/NO x-removal from flue gas : II. Selective catalytic reduction of nitric oxide by ammonia

Abstract

The selective catalytic reduction (SCR) of nitric oxide by ammonia was studied for silica-supported copper oxide particles to be used as a sorbent/catalyst in a continuous process for the simultaneous removal of SO x and NO x from flue gases. The SCR-behaviour was determined as a function of the sulphation degree, i.e. the fraction of copper oxide converted to copper sulphate, at temperatures ranging from 20 to 450°C. Up to 350°C, the fresh catalyst with 0% CuSO 4 showed a high selectivity towards production of nitrogen and water by the reaction of nitric oxide with ammonia and oxygen. At higher temperature, nitric oxide removal efficiencies decreased due to the oxidation of ammonia by oxygen. With an increase of the sulphation degree, the maximum temperature for selective catalytic reduction of nitric oxide gradually increased up to 420°C for a sulphation degree of 80%. In addition, the maximum nitric oxide removal efficiency increased as well. After regeneration of catalyst particles with a sulphation degree of 80%, realised by reduction with hydrogen and subsequent re-oxidation, the catalytic behaviour was similar to that of fresh catalyst particles with a sulphation degree of 5%. This is ascribed to the formation of some Cu 2S during the reduction, which is oxidised to CuSO 4 in the subsequent oxidation step. Since the selectivity towards the reduction of nitric oxide with ammonia is maintained up to about 375°C, a temperature which is very suitable for SO x removal as well, the silica-supported CuO investigated can be applied as a sorbent/catalyst for the simultaneous removal of SO x and NO x from flue gases. The reaction rate constants for SO x and NO x removal appeared to be of the same order of magnitude provided that the reduced sorbent/catalyst enters the absorber directly, i.e. without a separate pre-oxidation.