Acid treatment for enhancing Hg0 removal efficiency of chlorine-loaded biochar: mechanism and kinetic analysis

Abstract

Adsorbents modified solely with chlorine have limited effectiveness in removing mercury at high temperatures. This study aims to investigate the influence of various acid (HNO₃, H₂SO₄, and H₂O₂) loadings on the removal efficiency of mercury from NH₄Cl-modified adsorbents. The objective is to develop rice straw carbon adsorbents that are both more efficient and cost-effective. The experiments were conducted on a fixed bed experimental platform, with SEM and BET to observe the physical property changes of the modified char samples. XPS analysis was employed to analyze the effects of oxygen, chlorine, and sulfur functional groups. Additionally, a kinetic model was used to investigate the interaction mechanism between the adsorbent and mercury. The findings demonstrate that co-modification surpasses the use of NH₄Cl alone, with the combination of NH₄Cl and HNO₃ yielding the best results. Co-modification enhances the development of a more refined and compact pore structure on the char surface, promoting the physical adsorption of mercury. Moreover, an increased presence of chlorine and oxygen functional groups is observed on the char surface, particularly in the NH₄Cl and HNO₃ co-modified samples, further enhancing the chemical adsorption capacity of the char. The results from the kinetic analysis support this conclusion. Furthermore, the adsorption process of Hg⁰ relies on both external mass transfer and chemical adsorption, with the chemical adsorption process playing a more significant role as the controlling factor.

Keywords: Acid chlorine co-modification; Functional group; Kinetic analysis; Mercury adsorption; Oxy-fuel combustion; Rice straw char.