Adsorbents for the adsorption purification of NO_x in the ore-sintering flue gas

Nitrogen oxides (NO_x) are major air pollutants produced by fuel combustion that cause adverse effects on the environment and human health. The deep purification of NO_x from flue gases has become a worldwide issue. In China, a rigorous ultra-low emission standard (ULES) of NO_x ≤ 50 mg·m^–3 has been implemented in the power and steel industries in recent years. NO₂ is a valuable chemical feedstock that is worthy of being recycled from flue gas. Adsorption is a promising technology that can achieve deep purification and resource recovery of NO_x from industrial flue gas, in which a high-performance NO_x adsorbent plays a key role. However, a systematic understanding of NO_x adsorbents for practical applications is still lacking. This study compares and analyzes the NO_x adsorption and desorption performances of typical practical adsorbents including zeolites, metal oxides, and silica-alumina gels based on the practical need for both NO_x purification efficacy and material thermal stability. NO_x adsorption capacities, breakthrough curves, uptake curves, and temperature-programmed desorption (TPD) curves were also measured. Results show that compared to Fe–Mn–Ce and 13X as competitive adsorbents, H-ZSM-5_25 showed NO_x deep purification (purification efficiency close to 100% before adsorption breakthrough), great NO_x adsorption capacity (0.206 mmol·g^–1), and high NO₂ concentration ratio in desorption, which are likely due to its high NO catalytic oxidation rate and comparable NO₂ physical adsorption rate. Regarding the desorption characteristics, H-ZSM-5_25 showed a bimodal TPD desorption peak with a lower desorption temperature (400–470 K) in the low-temperature region. Meanwhile, NO₂ is the primary NO_x component in the desorbed gas (adsorption-desorption enrichment ratio of NO₂ being up to 57), which can easily be recovered using the liquefaction method. Furthermore, by comparing the adsorption performances on the H-ZSM-5 with different silica-to-alumina ratios, the NO_x adsorption was found to decrease (from 0.706 to 0.206 mmol·g^–1 for H-ZSM-5_25 and from 0.454 to 0.127 mmol·g^–1 for H-ZSM-5_38) with increasing temperature (298–398 K). The dependence of the NO₂ adsorption on the temperature was more significant for H-ZSM-5_25 compared to H-ZSM-5_38. Compared to H-ZSM-5_38, H-ZSM-5_25 with a lower silica-to-alumina ratio (consequently, more cation sites) rendered greater NO oxidation performance, a potentially higher NO₂ adsorption capacity, and a greater decreasing trend of the adsorption capacity with increasing temperature. Results of adsorption kinetic experiments showed that the NO_x mass transfer parameters on H-ZSM-5_25 were lower than those on H-ZSM-5_38 with a smaller primary micro pore channel. Results of the current work can provide technical references for economic flue gas denitrification.