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摘要: 吸附法是有望同时实现烟气NOx超低排放深度净化与资源化的关键技术,高效NOx吸附剂是其核心关键,然而目前针对满足应用需求的NOx吸附剂仍缺乏系统认识。本文基于烟气NOx净化效率及材料热稳定性实际需求,分析挑选了沸石、金属氧化物、硅铝胶等代表性吸附剂,研究了NOx在各吸附剂上的吸附穿透、吸附量、程序升温脱附等关键特性,结合吸附剂孔道特性对比发现,中低硅H-ZSM-5沸石兼具较高NOx净化深度、NOx吸附量、较低脱附温度且可获得更易于资源化的NOx解吸气,因而可作为优选NOx吸附剂。进一步地,随着吸附温度升高,硅铝比(w(SiO2)/w(Al2O3) )为25、38的H-ZSM-5的NOx吸附量均降低,其中低硅H-ZSM-5的NOx吸附量较高,但吸附传质系数较低。本文可为烟气NOx吸附净化的效益环保技术提供指导。Abstract: Nitrogen oxides (NOx) are major air pollutants produced by fuel combustion that cause adverse effects on the environment and human health. The deep purification of NOx from flue gases has become a worldwide issue. In China, a rigorous ultra-low emission standard (ULES) of NOx ≤ 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 NOx from industrial flue gas, in which a high-performance NOx adsorbent plays a key role. However, a systematic understanding of NOx adsorbents for practical applications is still lacking. This study compares and analyzes the NOx adsorption and desorption performances of typical practical adsorbents including zeolites, metal oxides, and silica-alumina gels based on the practical need for both NOx purification efficacy and material thermal stability. NOx 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 NOx deep purification (purification efficiency close to 100% before adsorption breakthrough), great NOx 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 NOx 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 NOx 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 NOx 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.
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Key words:
- flue gas purification /
- nitrogen oxide /
- adsorbent /
- adsorption /
- recovery
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表 1 吸附剂的物理参数及NOx吸附量
Table 1. Physical parameters and NOx adsorption capacity of adsorbents
Adsorbents Brunauer–Emmett–Teller surface area/(m2·g–1) Pore volume/
(cm3·g–1)Primary micropore channel/nm NOx adsorption capacity/(mmol·g–1) H-ZSM-5_25 353 0.32 0.68 0.206 H-ZSM-5_38 337 0.30 0.72 0.127 H-ZSM-5_198 397 0.43 0.72 0.013 13X 393 0.48 1.00 0.181 NaY 537 0.38 1.15 0.027 Si–Al gel 463 0.43 1.17 0.016 Fe–Mn–Ce 99 0.09 4.58 0.200 表 2 LDF模型拟合参数
Table 2. Linear driving force model fitting parameters
Adsorbents R2 –ki C H-ZSM-5_25 0.998 –1.14 × 10–4± 3.79 × 10–8 0.08 ± 5.83 × 10–4 H-ZSM-5_38 0.995 –2.19 × 10–4 ± 1.32 × 10–7 0.05 ± 0.01 -
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