Modifications to the surface chemistry of low-rank coal-based carbon catalysts to improve flue gas nitric oxide removal

The effectiveness of carbons as low-temperature selective catalytic reduction (SCR) catalysts will depend upon their physical and chemical properties. Surface functional groups containing oxygen are closely related to the catalytic activity of carbons. These groups are expected to change the interaction between the carbon surface and the reactants through a variation in adsorption and reaction characteristics. This paper presents a more detailed study of the effects of either gas-phase sulfuric acid or oxygen oxidation treatments on the catalytic NO reduction by low-rank coal-based carbon catalysts. Raw and treated carbons were characterized by N2 and CO2 surface areas, TPD and ash content. NO removal capacity of carbons was determined by passing a flow containing NO, H2O, O2, NH3 and N2 through a fixed bed of carbon at 150°C and 4 s of residence time, the effluent concentration being monitored continuously during the reaction. The effects of varying the type and conditions of the treatment on the physicochemical features of carbons were studied. The gas-phase sulfuric acid treatment (corresponding to a first step SO2 removal) markedly enhanced carbon activities for NO removal. On the contrary, oxygen oxidation enhanced NO removal capacity of chars to a lower extent. Therefore, the carbons studied could be used in a combined SO2/NO removal process, because the use and regeneration of the carbon in the first step is beneficial for the performance in the second one.