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Energy and the Environment
Dual Roles of Nano-Sulfide in Efficient Removal of Elemental Mercury from Coal Combustion Flue Gas within a Wide Temperature Range Jiexia Zhao, Hailong Li, Zequn Yang, Lei Zhu, Mingguang Zhang, Yong Feng, Wenqi Qu, Jianping Yang, and Kaimin Shih Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04340 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018
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Environmental Science & Technology
Dual Roles of Nano-Sulfide in Efficient Removal of Elemental Mercury from Coal Combustion Flue Gas within a Wide Temperature Range
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Jiexia Zhaoa, Hailong Lia *, Zequn Yangb, Lei Zhua, Mingguang Zhanga, Yong Fengb, Wenqi Qua,
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Jianping Yanga, Kaimin Shihb
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a. School of Energy Science and Engineering, Central South University, Changsha, 410083, China
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b. Department of Civil Engineering, The University of Hong Kong, Hong Kong, Hong Kong SAR,
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China
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Revision Submitted to Environmental Science & Technology
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*To whom correspondence should be addressed:
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TEL: +86-18670016725
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E-mail:
[email protected] 21
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ABSTRACT: Nanostructured zinc sulfide (Nano-ZnS) has been demonstrated to be an efficient
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adsorbent for removal of elemental mercury (Hg0). However, the Hg0 removal performance
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deteriorates once the flue gas temperature deviates from the optimal temperature of 180 °C. In this
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study, ultraviolet (UV) light, which is generally generated through corona discharge in electrostatic
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precipitators (ESPs), was adopted to enhance Hg0 removal by Nano-ZnS. With the UV irradiation,
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Nano-ZnS exhibited excellent performance in Hg0 removal within a much wide temperature range
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from room temperature to 240 °C. A Hg0 removal efficiency of 99% was achieved at 60 °C even
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under extremely adverse conditions, i.e. gas flow with an extremely high gas hourly space velocity
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but without hydrogen chloride. At low temperatures, Hg0 was mainly oxidized by superoxide
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radicals (·O2-) and hydroxyl radicals (·OH) generated by UV photostimulation to form mercuric
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oxide (HgO). At high temperatures, most Hg0 was immobilized as mercuric sulfide (HgS), as both
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the enhanced chemisorption and the accelerated transformation of HgO to HgS facilitated the
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formation of HgS. Compared with commercial activated carbon, injection of Nano-ZnS can utilize
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the UV in ESPs to warrant a higher Hg0 removal efficiency within a much wider temperature range.
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KEYWORDS: Sulfide mineral; Adsorption; Photocatalytic oxidation; Mercury; Flue gas
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Environmental Science & Technology
INTRODUCTION
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As a hazardous atmospheric pollutant, mercury causes seriously damages to the environment and
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human health due to its volatility, persistence, and neurotoxic through the biological accumulation. 1
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Minamata Convention on Mercury came into force from August 2017, which requests the
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contracting nations including China and the United States to take actions to reduce mercury emission.
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atmosphere globally.
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extraordinarily exigent, and has great potential in reducing mercury emission.
Coal combustion was identified as the largest anthropogenic source of mercury emissions to the 3
Thus, the abatement of mercury emission from coal combustion flue gas is
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Various technologies were studied and used to limit mercury emission from coal combustion flue
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gas. 4, 5 The effectiveness of the technology largely depended on the nature of mercury species in the
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coal-fired flue gas.
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particulate-bound mercury (HgP) and oxidized mercury (Hg2+) are easy to be controlled using
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existing air pollution control devices (APCDs), 7 whereas elemental mercury (Hg0) as the dominant
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form is difficult to be captured or collected due to its volatility, insolubility and chemical stability. 8,
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controlling mercury emission from coal combustion. 10 However, ACI still suffers its high operating
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cost and narrow applicable temperature range. 11, 12 In addition, the increase of carbon content after
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injection of activated carbons prevents the reuse of fly ash as a concrete raw material. 13-15 More than
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that, carbon materials are benificial to mercury methylation in landfills 16, 17 where we dump most of
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coal fly ash wastes.
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drawbacks related to carbon materials, and has been demonstrated in our former studies to be an
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efficient adsorbent for capturing Hg0 from coal combustion flue gas.
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chemisorbed over the semi-conductive Nano-ZnS to generate stable mercuric sulfide (HgS), and the
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optimal temperature for this process was found to be around 180 °C.
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Among the three basic forms of mercury in coal combustion exhaust,
To date, activated carbon injection (ACI) is identified as the best available technology for
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Nano-structured zinc sulfide (Nano-ZnS) can overcome most of the
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Hg0 was identified to be
However, the chemisoption
Environmental Science & Technology
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of Hg0 on Nano-ZnS was limited at lower temperatures. Not only that, higher temperatures
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accelerated the decomposition of HgS, and hence inhibited the immobilization of Hg0 on the
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Nano-ZnS. As the flue gas temperature fluctuates with operating conditions, industrial applications
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therefore request the Nano-ZnS to be efficient within a wide temperature range. Photocatalytic oxidation of Hg0 and its subsequent immobilization as mercuric oxide (HgO) offer
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a way to remove Hg0 from flue gas at low temperatures.
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photocatalytic performances were believed to be almost independent on the temperature.
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Moreover, the higher decompostion temperature for HgO compared to that for HgS also may be
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benificial for Hg0 immobilization at higher temperatures.
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would probably enhance the performance of Hg0 removal by the Nano-ZnS at temperatures far away
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from the optimal adsorption temperature. As the band-gap energies for ZnS nanoparticles is
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3.60-3.80 eV,
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precipitators (ESPs), in which the corona discharge could offer stable UV irradiation. 29 Therefore, it
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is reasonable to believe that in addition to efficient chemisorption of Hg0, the Nano-ZnS injected
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before ESPs could further utilize UV to achieve a higher Hg0 removal efficiency.
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For many photocatalysts, their 22-24
Therefore, photocatalytic oxidation
ultraviolet (UV, λ
