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Monochlorinated to Octachlorinated Polychlorinated Dibenzo#p#dioxin and Dibenzofuran Emissions in Sintering Fly Ash from Multiple-field Electrostatic Precipitators Mengjing Wang, Qianqian Li, Wenbin Liu, Mingliang Fang, and Ying Han Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04848 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
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Monochlorinated to Octachlorinated Polychlorinated Dibenzo‑p‑dioxin and
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Dibenzofuran Emissions in Sintering Fly Ash from Multiple-field Electrostatic
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Precipitators
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Mengjing Wang†‡, Qianqian Li†§, Wenbin Liu*†§, Mingliang Fang‡, Ying Han†
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†
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Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing
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100085, China;
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‡
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50 Nanyang Avenue, Singapore 639789, Singapore;
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research
School of Civil and Environmental Engineering, Nanyang Technological University,
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§
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*Corresponding
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+86-10-62923563; E-mail:
[email protected].
University of Chinese Academy of Sciences, Beijing 100049, China. author:
Dr.
Wenbin
Liu;
Phone:
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TOC art.
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+86-10-62849356;
Fax:
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ABSTRACT
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Polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) emissions in fly ash
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from multiple-field electrostatic precipitators in different sized sintering plants were
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studied. The monochlorinated–trichlorinated and tetrachlorinated–octachlorinated
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PCDD/F concentrations were higher for small plants (90 m2) than for medium (91–
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180 m2) and large (>180 m2) plants. The PCDD/F concentrations and less-chlorinated
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PCDD/F contributions to the total PCDD/F concentrations increased as the fly ash
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particle size decreased moving through the precipitator stages; the abundance of
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monochlorinated–trichlorinated PCDD/F congeners and homologues also increased.
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The ash particle size and surface area can be directly used to indicate
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monochlorinated–trichlorinated PCDD/Fs and toxic equivalents (TEQs). Previously
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ignored PCDD/F emissions in discarded fly ash were identified. Estimated total
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monochlorinated–trichlorinated PCDD/F and TEQ emissions in discarded fly ash
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were 155 kg and 1.979 kg TEQ, respectively, in 2003–2014, and the ratio between
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annual PCDD/F emissions in discarded fly ash and flue gases has gradually increased.
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Reductions in monochlorinated–trichlorinated PCDD/F emitted in flue gas and fly ash
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in 2003–2014 were 28 and 40 kg, respectively, because of the phasing out of
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small-scale plants. Reductions in TEQs emitted in flue gas and fly ash in 2003–2014
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were 7476 and 180 g TEQ, respectively.
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INTRODUCTION
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Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs
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and PCDFs, together abbreviated PCDD/Fs) have been studied extensively because
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they can cause adverse health effects. Industrial activities have historically been the
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most important sources of PCDD/Fs.1-7 Less-chlorinated PCDD/Fs (mono-, di-, and
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tri- (Mo, Di, and Tr)) have recently attracted attention because higher, less-chlorinated
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PCDD/F levels have been found in certain environmental matrices, are toxic,8-12 and
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could be precursors of 2,3,7,8-chlorinated PCDD/Fs involved in thermal processes.8,9
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It is therefore important to gain a full picture of MoCDD/F–TrCDD/F emissions and
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environmental behaviors.
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Sintering is the most important single industrial source of PCDD/Fs in China,
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being responsible for about 34.8% of total PCDD/F emissions to the atmosphere in
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flue gases in 2013. In our previous study, an intensive investigation was carried out to
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estimate and characterize the emission levels and profiles of MoCDD/Fs to octa- (O)
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CDD/F in flue gas from Chinese sintering plants.13 However, there are still no
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available data or knowledge regarding emission levels and profiles of MoCDD/Fs to
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OCDD/F emitted in fly ash from sintering to date.
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The amount of sintering fly ash produced in China has increased dramatically,
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and more fly ash is produced during sintering than during waste incineration.14 It has
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been estimated that 163 kt of fly ash (4.6% of total industrial fly ash emissions) was
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emitted from Chinese sintering plants in 2015.15,16 Serially connected ESPs are used
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to capture large amounts of fine particulate matter from flue gases to meet Chinese 3
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emission standard GB 28662-2012, which is 50 mg m−3. PCDD/F emissions to the
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atmosphere have been decreased by using ESPs to remove fly ash particles, but the
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PCDD/Fs in the particles were not actually destroyed but mostly transferred to the fly
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ash. PCDD/F emissions in flue gases may therefore accurately represent emissions to
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the ambient air while not accurately reflecting the amounts and patterns of PCDD/Fs
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formed during the sintering processes. The PCDD/F concentrations and profiles in fly
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ash may be more representative than the concentrations and profiles in the flue gases
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when studying the unintentional formation of PCDD/Fs and underlying mechanisms
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involved. Furthermore, it is widely recognized that fly ash can promote PCDD/F
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formation during industrial thermal processes through heterogeneous reactions
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because fly ash contains large amounts of carbonous residue and catalytic
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elements.17,18
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The fly ash particle size captured decreases strongly moving through serially
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connected ESPs. No systematic research has been performed on the link between the
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properties (e.g., particle size distribution, surface area, and total organic carbon (TOC)
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and chloride contents) of fly ash collected by serially connected ESPs and the
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amounts and profiles of MoCDD/Fs to OCDD/F formed during sintering. Performing
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such research will improve our understanding of the concentrations and profiles
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involved in PCDD/F formation during sintering because previous research has
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focused on mixed fly ash.
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The release of PCDD/Fs in fly ash produced during sintering processes was not
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addressed in the Chinese national implementation plans for the Stockholm 4
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Convention on Persistent Organic Pollutants in 2003 and 2012 because almost all fly
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ash produced by sintering plants was assumed to be recycled. In fact, approximately
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95% of the fly ash captured in the initial ESP stages is recycled by adding it to the raw
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materials, and the fly ash collected in the last ESP stage is sent to landfill or
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stockpiled. The emission of PCDD/Fs from discarded fly ash was, however,
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neglected.
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Decreasing CO2, SO2, NOx, and particulate emissions will require advanced
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technologies to be promoted and outdated facilities to be closed.19,20 These measures
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could also indirectly decrease PCDD/F emissions. The PCDD/F emission decreases
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that could be achieved by phasing out obsolete facilities in the most energy-intensive
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industries (cement, coke, iron, power, and steel production, all of which are major
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sources of PCDD/F emissions in China) have been estimated.20-23 However, the
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PCCD/F emission decreases that could be achieved by closing old sintering plants and
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improving sintering processes are unclear and have not been estimated.
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To address these gaps in knowledge, 87 fly ash samples from serially connected
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ESPs in 24 typical sintering plants of different sizes in this study were collected and
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analyzed to allow the MoCDD/F–OCDD/F concentrations and profiles emitted to be
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determined, emission factors to be derived, and the relationships between PCDD/F
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concentrations and ash properties to be investigated.
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EXPERIMENTAL SECTION
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Sintering and sampling. Sintering is a pretreatment step in the iron production
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process. Sintering involves heating fine particles of metal ores to cause them to 5
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agglomerate. Approximately 40% of all iron ore sintering (more than in any other
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country) occurs in China.13 The Chinese government is attempting to decrease energy
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use by closing less efficient and obsolete sintering facilities. Sintering plants
