Effect of chemical activation of an activated carbon using zinc chloride on elemental mercury adsorption

Abstract

Hg⁰ adsorption experiments with different kinds of activated carbons (ACs) were conducted in nitrogen environment to study the effects of ACs with different surface characteristics on adsorption. Three kinds of ACs prepared by steam activation did not adsorb elemental mercury (Hg⁰) in nitrogen environment, while another kind of AC prepared by chemical activation using zinc chloride (ZnCl₂) showed significant Hg⁰ adsorption capability in the same experimental conditions because Hg⁰ was oxidized by the oxidative elements on the surface of AC. ACs without oxidative elements did not adsorb Hg⁰ through physical adsorption in nitrogen environment. It was therefore concluded that Hg⁰ adsorption by AC was a chemical adsorption process.

Introduction

Environmental mercury levels have increased considerably in recent years. Even regions without significant emissions, such as the Arctic, are affected by the transcontinental transport of mercury [1], [2], [3]. Coal-fired power plants are one of the largest anthropogenic sources of mercury emission to the atmosphere [4], [5]. Currently, there is no perfect method that can be applied broadly to control mercury emissions [6]. Injection of powder activated carbon (AC) upstream of a particulate control device is one of the most promising near-term gaseous mercury control technologies [7].

The adsorption process of mercury is truly complicated in the coal-fired flue gas. A number of studies had been made to understand the mercury adsorbing mechanism of AC [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. For example, sulfur trioxide was found recently to have a strong inhibition effect on reducing Hg⁰ adsorption [18]. With the presence of NO₂, Hg⁰ was catalytically oxidized on the surface to generate the nonvolatile nitrate Hg(NO₃)₂, which was attached to the basic sites on the carbon. Adsorption of Hg⁰ by AC was decided by many factors, such as AC's surface characteristic, temperature, and composition of the flue gas.

Surface functional groups (SFGs) on AC surface are important factors for mercury adsorption by AC. Recent studies had their attentions on modifying the SFGs by sulfur (S) and bromine (Br) impregnations to improve mercury adsorption, and Br-impregnated ACs showed the most promising results. Some other work on adsorption of mercury by Cl-impregnated ACs was also reported [19], [20]. A better understanding of the effect of Cl-containing SFGs on mercury adsorption is required to accelerate the development of AC injection as a cost-effective method for reducing mercury emissions from coal-fired power plants.

In this paper, the effect of chemical activation of an AC using zinc chloride on improving elemental mercury adsorption was studied. All the Hg⁰ adsorption experiments with different kinds of ACs were conducted in nitrogen (N₂) environment. Thus, the effect of different surface characteristics of ACs on Hg⁰ adsorption was studied without the influence of flue gas composition. It would be helpful for distinguishing the influence of physical process from chemical one on Hg⁰ adsorption by ACs.