Abstract:

In order to clearly describe the whole process of Hg⁰ adsorption by HBr-modified fly ash (HBr-FA), an adsorption kinetic model was established including diffusion and surface adsorption and oxidation based on the dynamic adsorption experiment on a fixed-bed column. The key influence factor of the adsorption and oxidation process of Hg⁰ was discussed, and the dynamic behavior of Hg⁰ in the diffusion region and surface active site was investigated. The model takes into account the processes of axial backmixing, internal and external diffusion, and intrinsic kinetics, which hence can fit well with the experimental breakthrough results with the corresponding kinetic parameters. The sensitivity analyses indicate that axial dispersion, gas film diffusion, and inner efficiency diffusion have dominant impacts on Hg⁰ adsorption, and the model is more sensitive to the gas film diffusivity. Furthermore, the increasing of initial Hg⁰ concentration is beneficial to decreasing the diffusion resistance and promoting the Hg⁰ adsorption-oxidation due to the improvement of mass transfer driving force. The external diffusion resistance can be reduced with the increase of inlet flow rate and bed thickness. In addition, the Hg⁰ concentration profiles inside the particle can be obtained from the model, which shows the external diffusion resistance has greater influence at the initial stage of Hg⁰ adsorption and oxidation by HBr-FA.

Key words: HBr-modified fly ash, mercury, adsorption and oxidation, kinetics, external diffusion, fixed-bed

摘要：

为清晰描述HBr改性飞灰（HBr-FA）对Hg⁰吸附的全过程，基于固定床动态吸附实验，在同时考虑扩散和表面吸附氧化的条件下，建立了固定床中HBr-FA对Hg⁰的吸附动力学模型，讨论了Hg⁰吸附氧化过程的关键影响因素，探究了Hg⁰在扩散区域和表面活性位上的动力学行为。结果表明：该模型综合考虑了轴向返混、内外扩散和本征动力学过程，能够较好地拟合实验穿透曲线，并可求得相应的动力学参数，对气膜扩散系数更加敏感；初始Hg⁰浓度的提高可提供更高的能量克服扩散阻力，适当提高流量和床层厚度可减小外扩散阻力，进而表现出更优越的表观吸附氧化效果；该模型还可得到床层内和颗粒内（扩散和动力学区域）的Hg⁰浓度分布，发现在吸附初期，外扩散阻力对改性飞灰吸附氧化Hg⁰的过程影响更大。

关键词: HBr改性飞灰, 汞, 吸附氧化, 动力学, 外扩散, 固定床