Low-Temperature Selective Catalytic Reduction of NO with Urea Supported on Pitch-Based Spherical Activated Carbon

Urea as a reducing agent supported on pitch-based spherical activated carbon (PSAC) was studied for NO reduction at low temperatures (30-90 degrees C). The results showed that PSAC with 8 wt % urea loading exhibited high activity in the selective catalytic reduction (SCR) of NO at 30 degrees C. The SCR activity decreased markedly when urea loading was increased above 8 wt % due to pore plugging, which restricted the adsorption of gas phase reactants on PSAC, although the NOx removal period was extended. A low reaction temperature was favorable for NO reduction on account of the increased NO adsorption on PSAC. It was found that the SCR activity was improved by increasing NO or O-2 concentration in the feed gas, owing to the enhanced NO oxidation by O-2 to NO2, which was then reduced by urea to form N-2. Increasing space velocity not only decreased the SCR activity but also shortened the NOx removal period. More than 85% NOx conversion for 55 h could be achieved over PSAC with 8 wt % urea loading at 30 degrees C under the conditions of 500 ppmv NO, 21 vol % O-2, and a space velocity of 2000 h(-1). Furthermore, PSAC showed a superior hydrodynamic property, and the pressure drop ratio of PSAC to a commercial granule activated carbon with the equivalent particle size was about 35% with the apparent air flow velocity in a range of 0.12 similar to 0.51 m/s.