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Studies on mercury adsorption species and equilibrium on activated carbon surface Qiang Zhou, Yufeng Duan, Mingming Chen, Meng Liu, and Ping Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02699 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017
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Energy & Fuels
Studies on mercury adsorption species and equilibrium
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on activated carbon surface
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Qiang Zhoua,b, Yufeng Duanb, Mingming Chenb, Meng Liub, and Ping Lua
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a
Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China
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b
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of
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Energy and Environment, Southeast University, Nanjing 210096, China
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ABSTRACT： ：Mercury adsorption capability of raw activated carbon (R-AC) and NH4Br modified
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activated carbon (NH4Br-AC) was evaluated in a fixed-bed reactor. Effect of inlet Hg0 concentration
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and flue gas components on mercury adsorption was investigated. The mercury adsorption species on
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sorbent surface was analyzed by Programmed Temperature Desorption (TPD) method. Finally,
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mercury adsorption equilibrium of R-AC and NH4Br-AC was explored. Mercury adsorption results
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show that the mercury adsorption capability of NH4Br-AC is significantly larger than that of R-AC and
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increased with higher inlet Hg0 concentration. O2 and NO promotes mercury adsorption on NH4Br-AC,
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but SO2 inhibits the mercury adsorption. The TPD result shows that mercury adsorption on R-AC is
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mainly physical in nature enhanced by chemisorption with the product of HgO. Mercury adsorption on
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NH4Br-AC is in the major form of chemisorption with the product of HgBr2. O2 has little effect on the
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mercury adsorption mechanism. SO2 reduces the generation of HgBr2, but promotes little of HgS
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generation on NH4Br-AC. NO significantly improves the generation of Hg2(NO3)2 on NH4Br-AC.
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Adsorption equilibrium analysis shows that Langmuir and Temkin equations are more suitable to
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describe the mercury adsorption on R-AC, indicating the uniformity of R-AC surface. Mercury
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adsorption on NH4Br-AC surface can be described well by Freundlich equation, illustrating the
*Correspondence author. Tel.: 86+025-85481253, E-mail address:
[email protected] (Qiang Zhou)
ACS Paragon Plus Environment
Energy & Fuels
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non-uniformity of NH4Br-AC surface.
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KEY WORDS: Activated carbon, Mercury adsorption, Mercury desorption, Adsorption species,
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Adsorption equilibrium
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1. Introduction
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Mercury emission has attracted worldwide concern, because mercury is one of environmentally toxic
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metals with the feature of volatility, bioaccumulation and persistence [1-3]. It is well known that coal
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combustion is major anthropogenic source of mercury emission [4-6]. Therefore, worldwide scientists
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and governments are devoting significant efforts to reducing mercury emission from coal-fired power
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plants. It is generally agreed that mercury existing in coal-fired flue gas includes elemental mercury
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(Hg0), oxidized mercury (Hg2+) and particle-bound mercury (HgP) [7-9]. HgP can be captured with fly
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ash through Fabric Filters (FFs) or Electrostatic Precipitators (ESPs), and part of Hg2+ may be removed
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if the power plant is equipped with wet flue gas desulfurization (WFGD) device [10,11]. However, due
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to the high volatility and insolubility of Hg0, it is difficult to remove the Hg0 from flue gas by
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conventional pollution control devices [12,13]. The mercury emitted into the atmosphere can form
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methyl mercury, which is seriously harmful to the environment. [14].Therefore, in order to reduce
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mercury emission from coal-fired power plant, it is necessary to develop other advanced control
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technologies to reduce mercury emission.
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Activated carbon injection (ACI) upstream of FF or ESP has been considered as the most effective
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and promising technology for reducing mercury emission from power plants [15,16]. In order to further
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improve the ACI technology, many mercury removal sorbents have been developed. To date,
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brominated activated carbon has been confirmed to be an effective sorbent [17,18], but finding out the
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mercury adsorption mechanism of brominated activated carbon still be a challenge. Sasmaz et al [19]
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Energy & Fuels
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utilized extended X-ray absorption fine structure (EXAFS) and X-ray photoelectron spectroscopy (XPS)
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spectroscopy methods to analyze the binding mechanism of Hg0 adsorption on brominated powder
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activated carbon surface and found that gas-phase Hg0 was oxidized by the brominated carbon and
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coordinated to two Br atoms. Zhou et al [20] applied adsorption kinetic and thermodynamic methods to
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analyze the Hg0 adsorption on brominated activated carbon and found that Hg0 adsorption on
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brominated activated carbon was spontaneous and needed more activated energy compared to that of
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raw activated carbon because of the enhancement of chemisorption. Liu et al [21] used first principles
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quantum mechanical theoretical methods which based on density functional theory to study the
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mercury–bromine adsorption mechanism on carbonaceous surface and found that the bromine atom
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had a positive effect on Hg0 adsorption resulted by the transfer of charge. The Hg-Br adsorption on
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carbonaceous surface was highly thermally stable than that of HgBr2. Rupp et al [22] studied the
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interaction between Hg0 and other flue gas components including SO2 and NOx on brominated AC
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fibers. It was found that NO and NO2 were helpful to promote Hg0 oxidation on brominated AC fiber,
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but SO2 inhibited the Hg0 adsorption. SO2 and NOx in flue gas can lead to the Br deactivation on the
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brominated AC fiber surface. To date, although researches on mercury adsorption by brominated
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activated carbon have been widely conducted, the high complexity mercury adsorption procedure leads
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to the limited understanding of mercury adsorption mechanism and the demand of further research.
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The Programmed Temperature Desorption (TPD) method is an effective method for identifying
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mercury compounds species on solid samples including iron-based sorbents [23], fly ash [24,25] and
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WFGD gypsum [26]. It takes advantage of the fact that the order of mercury compounds desorption
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temperature is HgCl2/HgBr2
