Environ. Sci. Technol. 2009, 43, 5049–5053
Novel Fluidized Bed Reactor for Integrated NOx AdsorptionReduction with Hydrocarbons TERRIS T. YANG AND HSIAOTAO T. BI* Department of Chemical & Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada
Received January 9, 2009. Revised manuscript received May 6, 2009. Accepted May 12, 2009.
In order to avoid the negative impact of excessive oxygen in the combustion flue gases on the selectivity of most hydrocarbon selective catalytic reduction (HC-SCR) catalysts, an integrated NOx adsorption-reduction process has been proposed in this study for the treatment of flue gases under lean burn conditions by decoupling the adsorption and reduction into two different zones. The hypothesis has been validated in a novel internal circulating fluidized bed (ICFB) reactor using Fe/ZSM-5 as the catalyst and propylene as the reducing agent. Effects of propylene to the NOx molar ratio, flue gas oxygen concentration, and gas velocity on NOx conversion were studied using simulated flue gases. The results showed that increasing the ratio of HC:NO improved the reduction performance of Fe/ZSM-5 in the ICFB reactor. NOx conversion decreased with an increasing flue gas flow velocity in the annulus UA but increased with an increasing reductant gas flow velocity in the draft tube UD. The NOx adsorption ratio decreased with increasing UA. In most cases, NOx conversion was higher than the adsorption ratio due to the relatively poor adsorption performance of the catalyst. Fe/ZSM-5 showed a promising reduction performance and a strong inhibiting ability on the negative impact of excessive O2 in the ICFB reactor, proving that such an ICFB reactor possessed the ability to overcome the negative impact of excessive O2 in the flue gas using Fe/ZSM-5 as the deNOx catalyst.
Introduction The selective catalytic reduction of nitrogen oxides by hydrocarbons (HC-SCR) has been proven effective in the treatment of model flue gases (1-4). HC-SCR in an oxygenrich atmosphere has attracted considerable attention as an alternative using ammonia or urea derivates (5, 6). As a key component of the flue gas under lean burn conditions, excess oxygen in the flue gas imposed a significant impact on the performance of HC-SCR (2, 3). One of the main NO reduction mechanisms states that NO is first oxidized to NO2, followed by the reduction by the reducing agent (2). The presence of O2 in the flue gas is thus essential for the HC-SCR process. This mechanism appears to agree with many experimental findings that the presence of a small amount of O2 (e.g.,
