Low-temperature selective catalytic reduction of NO₂ with urea supported on pitch-based spherical activated carbon

Urea as reducing agent supported on pitch-based spherical activated carbon (PSAC) was studied for NO₂ reduction at low temperatures (30~90℃). The results showed that increasing urea loading from 8% to 30% raised the reaction probabilities of selective catalytic reduction (SCR) of NO₂ by urea, which resulted in significant increase of the SCR activity; moreover, the NO_x removal period was extended. In the temperature range of 30~90℃, high reaction temperature was unfavorable for NO_x removal on account of the decreased NO₂ adsorption on PSAC.It was found that the SCR activity was improved by increasing NO₂ or O₂ concentration in the feed gas. However, further increase in O₂ concentration above 9% made a weak contribution to the improvement of the SCR activity. Decreasing space velocity not only increased the SCR activity but also prolonged the NO_x removal period. More than 85% NO_x conversion for 49 h could be achieved over PSAC with 8% urea loading at 30℃ under the condition of 0.05% NO₂, 21% O₂ and a space velocity of 2000h^-1.