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Improving the removal of particles and trace elements from coal-fired power plants by combining a wet phase transition agglomerator with wet electrostatic precipitator
Accepted Manuscript Improving the removal of particles and trace elements from coal-fired power plants by combining a wet phase transition agglomerator with wet electrostatic precipitator Ruijie Cao, Houzhang Tan, Yingying Xiong, Hrvoje Mikulčić, Milan Vujanović, Xuebin Wang, Neven Duić PII:
S0959-6526(17)30971-X
DOI:
10.1016/j.jclepro.2017.05.046
Reference:
JCLP 9581
To appear in:
Journal of Cleaner Production
Received Date: 23 January 2017 Revised Date:
2 May 2017
Accepted Date: 8 May 2017
Please cite this article as: Cao R, Tan H, Xiong Y, Mikulčić H, Vujanović M, Wang X, Duić N, Improving the removal of particles and trace elements from coal-fired power plants by combining a wet phase transition agglomerator with wet electrostatic precipitator, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.05.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights A novel equipment named wet phase transition agglomerator (WPTA) was proposed.
A system of WPTA and wet electrostatic precipitator was commercially applied.
The removal efficiency of particles was significantly improved by the WPTA.
The WPTA improved the performance of the system for trace elements removal.
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Improving the Removal of Particles and Trace Elements from Coal-fired Power Plants by Combining a Wet Phase Transition Agglomerator with Wet Electrostatic Precipitator
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Ruijie Cao a, Houzhang Tan a,*, Yingying Xiong b, Hrvoje Mikulčić c, Milan Vujanović c, Xuebin Wang a, Neven Duić c a MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China b Department of Power and Engineering, Shanxi University, Taiyuan, Shanxi, 030006, China c Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lučića 5, 10000 Zagreb, Croatia *Corresponding author: [email protected]
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Abstract
A novel technology for flue gas pre-treatment in the phase transition process is proposed in this paper to better remove the fine particles and trace elements from coal-fired power plants. Wet removal, Brownian diffusion, diffusiophoresis, thermophoresis and disturbed pipe flow
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occurring in the phase transition process were taken into consideration during the development of the technology. An item of equipment called a wet phase transition agglomerator (WPTA) was developed based on the aforementioned technology. The WPTA
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and the wet electrostatic precipitator (WESP) constituted a wet dust removal system (WDRS),
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which was used at a 660 MW ultra-supercritical unit in China as a demonstration project. The particles at the inlet and outlet of the WDRS, as well as the wastewater from the system, were Abbreviations WPTA Wet Phase Transition Agglomerator WESP Wet Electrostatic Precipitator WDRS Wet Dust Removal System TSP Total Suspended Particulates TEs Trace Elements FRP Fiber Reinforced Polymer PSDs Particle Size Distributions
sampled to investigate the performance of the WDRS. The results indicate that the WDRS 1
ACCEPTED MANUSCRIPT helps to bring about ultra-low emissions of particles from the coal-fired power plant, keeping the level of particle emission below 5 mg/m3 under all measurement conditions. It was found that the collection efficiency of the WESP increased significantly with an increasing applied
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voltage. The performance of the WDRS can be further improved by the WPTA, for the removal efficiency of total suspended particulates at boiler operating loads of 90% and 75% rose 4.01 and 3.17 percentage points, respectively. Moreover, the removal efficiency of PM1
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increased from 68.67% to 83.61% with the WPTA running at a load of 90%. The TEs removal
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was also found to be enhanced by the WPTA. The masses of Hg, As and Mn carried by the wastewater per hour with the WPTA running increased 4.2, 2.8 and 1.5 times, respectively, over values when the WPTA was turned off. Keywords
1
Introduction
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phase transition; fine particle; growth; agglomeration; trace element; simultaneous removal
Coal combustion is a major source of atmospheric pollutants in China (Huang et al., 2014;
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Li et al., 2016; Lyu et al., 2016). Coal accounts for 63.7% of China’s primary energy
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consumption, and 43% of the coal is consumed in thermal power plants. It is of great importance for environmental management to reduce the emission of pollutants from coal-fired power plants. Fine particles and trace elements (TEs), which cause severe harm to both human health and the environment (Song et al., 2016), are of particular concern. According to the Action Plan on Coal-fired Power Emission Reduction and Upgrading released by the Chinese government in 2015, coal-fired power units with the capacities over 300 MW are all forced to reach the particle emission limitation of 10 mg/m3 by 2020. What’s 2
ACCEPTED MANUSCRIPT more, power groups in China have been focusing on the research and practice of ultra-low emissions these years. The particle emission concentration bellow 5 mg/m3 is pursued, or even lower.
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The traditional flue gas cleaning technologies of the coal-fired power plants in China are selective catalytic reduction (SCR), dry electrostatic precipitator (ESP) and wet flue gas desulfurization (WFGD), all of which have had remarkable influence on the emission
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characteristics of particles from coal burning. Field measurements found that the
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concentrations of fine particles, especially PM1, were elevated due to the formation of (NH4)2SO4 or NH4HSO4 when the flue gas passed through the SCR (Li et al., 2015). Due to the existence of back corona and re-entrainment, as well as the difference in charging characteristics of particles in different size ranges, dry ESPs cannot effectively capture fine
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particles, especially particles in the size range of 0.1-1 µm (Jaworek et al., 2007; Wang et al., 2014). Moreover, the WFGD would produce new particles from the entrainment of desulfurization slurry droplets containing gypsum and unreacted calcium carbonate (Meij and
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Te Winkel, 2004). Fine particles cannot be controlled effectively by the use of aforementioned
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technologies, and the particle emission concentration is generally over 15 mg/m3 in China. Some new technologies, including the low-low temperature ESP (Wang et al., 2015), the composite electrostatic-bag precipitator (Zhang et al., 2013) and the wet electrostatic precipitator (WESP) (Yang et al., 2016), have been applied to further reduce particle emission. Considering the formation of fine particles in the wet desulfurization units located downstream of the dust collectors, WESP installed ahead of the stack is more effective in attaining a better result of particle emission reduction. Back-corona discharge and dust 3
ACCEPTED MANUSCRIPT re-entrainment can be avoided because the discharge and collection electrodes in the WESP are cleaned continuously by the water spray instead of rapping in the dry ESP. The water spray also enhances particle collection. Hence, the WESP effectively captures particles
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penetrating the upstream pollution control devices. However, it is reported that the collection efficiency of the WESP for particles in the size range of 0.1-1 µm is unsatisfying due to the poor charging capacity of those particles (Du et al., 2016; Wang et al., 2016). Furthermore,
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Lin et al. (2010) reported that the performance of the WESP declined with an increasing
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particle load. In contrast to the lab-scale and pilot-scale studies above, the field measurements conducted by Xu et al. (2016) indicated that the WESP showed a high removal efficiency for particles in the size range of 0.1-1 µm, and the removal efficiency was relatively lower for particles smaller than 0.1 µm. Removal efficiencies less than zero were observed even for
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coarse particles.
It can be seen that the removal performance of the WESP is closely related to the particle size. Although the WESP has a high collection efficiency in removing the total suspended
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particulates (TSP), the effective capture of particles in each size range cannot be realized by
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the WESP alone. Methods for flue gas pre-treatment have been proposed by researchers, and particle growth by condensation was considered as one of the most promising preconditioning techniques for particle collection (Yoshida et al., 1976). The research on the growth of fine particles by condensation has been conducted for decades. It was reported by Schauer et al. (1951) that the condensation of water vapor on particles is an effective method for their growth. Yoshida et al. (1976) reported that the growth rate of fine particles in a supersaturated atmosphere of water vapor was very rapid, and even the hydrophobic particles grew well. 4
ACCEPTED MANUSCRIPT Experiments on the growth of fine particles and simultaneous removal of SO3 were also carried out (Wu et al., 2016). Steam was added to achieve a supersaturated atmosphere in the extensive lab-scale and pilot-scale studies described above. However, in the wet phase
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transition agglomerator (WPTA) developed by our group, the flue gas is cooled to achieve a supersaturated atmosphere, given that the water vapor downstream of the WFGD is under or near the saturation condition. Collision and subsequent agglomeration will happen among
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particles during their growth in the supersaturated atmosphere, due to such factors as
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Brownian diffusion, diffusiophoresis and thermophoresis (Leong, 1984) of particles, as well as disturbed pipe flow. The main body of the WPTA can also capture part of the particles. The WPTA improves the performance of the WESP by facilitating the growth, agglomeration and partial capture of particles in advance.
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TEs emission characteristics are closely related to their vaporization behavior, and three basic classes can be broadly defined with the measure of volatility (Ratafia-Brown, 1994; Xu et al., 2004). Class
: TEs which are approximately equally distributed between the bottom
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ash and fly ash or show no significant enrichment or depletion in the fly ash. Class
: TEs
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which are enriched in the fly ash and depleted in the bottom ash, or show increasing enrichment with decreasing fly ash particle size. Class
: TEs which are volatized, but are not
enriched on the fly ash. They are emitted fully in the vapor phase. Hence, TEs emission can be controlled by removing both gaseous TEs and the TEs in particles. According to the Emission Standard of Air Pollutants for Thermal Power Plants of China (GB 13223-2011), the emission limitation for mercury and its compounds is 0.03 mg/m3, which is not as strict as that in America. The coal-fired power plants with the SCR, ESP and 5
ACCEPTED MANUSCRIPT WFGD shall be adequate to the mercury emission limitation in China, but not that in America. What’s more, the evaluation conducted by Hui et al. (Hui et al., 2017) indicated that the emission limitation for mercury in GB 13223-2011 was not tough enough to meet the
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requirement in the Action Plan for Prevention and Control of Atmospheric Pollution (implemented by the Chinese government in 2013). Thus, the Chinese emission limitation for mercury shall be stricter in the near future.
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There has been no commercial-scale pollution control device for the removal of TEs alone.
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It has been reported that TEs could be captured by the WESP (Zhao et al., 2016; Zhu et al., 2016). Moreover, the results of the measurements in this article indicated that the removal of TEs was markedly elevated when the WPTA was on.
In the following sections, the structure and operating principles of the WPTA is introduced,
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and this equipment working with the WESP constitutes a wet dust removal system (WDRS). The impact of WPTA on the performance of the WDRS for particles and TEs removal is
study.
Description of the WDRS
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evaluated, and the particles removal characteristics of the WESP are also discussed in this
2.1 Introduction to the WPTA The WPTA was designed for flue gas pre-treatment to improve the performance of the WDRS. As shown in Fig. 1b and 1c, the main body of the WPTA consists of multiple modules for heat transfer and dust removal, which makes it convenient to repair and replace any individual module which is out of order. Fluoroplastic pipes, fixed by pore plates, are arranged in a honeycomb structure in each module. Twenty-seven modules in total are 6
ACCEPTED MANUSCRIPT involved in the WPTA of the demonstration project, and the flow area for flue gas of the WPTA is 12.96 m in width and 25.46 m in length. Besides the main body, the WPTA consists
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of a circulating water system, a cooling system, and a water charging system.
Fig. 1. (a) Sketch of the WDRS, as well as the objects of the (b) module and (c) main body of WPTA
depicts the growth, agglomeration and capture of fine particles. When flue gases
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Fig. 2
enter the WPTA, they will undergo three stages: condensation of water vapor, particles agglomeration and capture of particles. During the first stage, the flue gas is cooled when
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passing through the WPTA, and the walls of the water-cooling pipes as well as particles in the
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flue gas will act as the condensation nucleus. The subsequent water film forms on the walls of the water-cooling pipes and will be broken up into large masses of droplets and fog-drops by the flue gas and the gravity of water. Additionally, the growth of particles by condensation also exists in this part. During the second stage, the thermophoresis of particles, caused by the temperature gradient existing between the flue gas and the water-cooling pipes, prompts them to move towards the walls of the pipes. The diffusiophoresis of particles, an analogous phenomenon occurring in the concentration gradient and due to the condensation of water 7
ACCEPTED MANUSCRIPT vapor on large particles and pipes, also leads fine particles to move towards the larger ones and the pipes. Together with the Brownian diffusion of particles and the disturbed pipe flow, thermophoresis and diffusiophoresis promote the chaotic movement of particles, which
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facilitates the agglomeration of the particles through increased frequency of particle collisions. During the last stage, particles will be partially captured by the pipes and the droplets through such factors as inertial impaction, interception, Brownian diffusion, thermophoresis, and
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diffusiophoresis. Moreover, the water film on the pipe walls will carry the captured particles
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to the wastewater tank, thus achieving self-cleaning by the pipes.
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Fig. 2. Operating principles of WPTA
As shown in Fig. 2
, large masses of droplets formed in the WPTA will capture the
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gaseous Hg2+. The gaseous Hg2+ will also be absorbed by the fog-drops, turning the gaseous species into liquid-phase particles which would experience the analogous process of growth, agglomeration and capture described above. Particles, droplets and fog-drops downstream of the WPTA will enter the WESP. The WESP will capture these more easily because of their growth in size and the enhancement in the charging capacity of the solid-phase particles. 8
ACCEPTED MANUSCRIPT 2.2 Collaboration of the WPTA and the WESP The WESP in the demonstration project has four electrostatic fields, and the flexible fabric and barbed wire are used for the collection electrode and discharge electrode, respectively.
Table 1 Technical Parameters of the WPTA and WESP Items
Units 3
m /h m/s Pa mm
Pa vol%
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Pressure drop of flue gas Effective height of electrodes Number of channels Channel width Collection area Dimensions Gas temperature at outlet Pressure at outlet Humidity at outlet
Pa mm mm m2 m
Pa vol%
2,780,666 <3 <100 about 2 600 57 800 13.5
<250 6,500 1,568 420×420 17,122.56 28.56×13.29×21.30 55 500 9
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Values
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Full load flue gas flow (hot) Flow velocity of flue gas Pressure drop of flue gas Temperature drop of flue gas Active height Gas temperature at inlet Pressure at inlet Humidity at inlet
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WPTA
Parameters
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Primary information on the WPTA as well as the WESP is listed in Table 1.
The average flow velocity of flue gas is close to 3 m/s at full load in the flow area of the
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WDRS. The flow velocity of flue gas must be uniform to obtain the best performance of the WESP. The experiment in our unpublished studies indicated that the WPTA straightened the flue gas flow well, and it could replace the flow straightening grid to be installed at the inlet of the WESP, as shown in Fig. 1a. A drainage system consisting of fiber reinforced polymer (FRP) pipes, wastewater tanks and drainage pumps, is shared by the WPTA and the WESP. Together with the spray water of the WESP, the water recovered by the WPTA goes into wastewater tanks through the FRP pipes. The demonstration project of the WDRS was 9
ACCEPTED MANUSCRIPT implemented at a 660 MW ultra-supercritical unit, and the WDRS was installed in the vertical duct between the WFGD tower and the stack. 3
Experimental methods
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The field measurements were carried out at the 660 MW ultra-supercritical unit. Measurement of particles was conducted at the inlet and outlet of the WDRS at the boiler operating loads of 90% (600 MW) and 75% (500 MW), as shown in Table 2. The study on the
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performance of WESP was conducted by particle sampling at the inlet and outlet of WDRS with the WPTA off.
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Table 2 Particle sampling conditions
Sampling sites
Applied voltage of WESP (kV)
Status of WPTA
Study on WESP
75%
Study on WPTA
75%
WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet
35 45 60 60 60 60
off off off on off on
90%
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Load
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The wastewater was sampled from the FRP pipes when the boiler was operated at 70% (460 MW) of full load. The quality of the feed-coal in the measurements is listed in Table 3.
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The values were obtained by the methods in national standards of China (GB/T 211-2007, GB/T 212-2008, GB/T 214-2007, GB/T 213-2008, and GB/T 476-2008). Table 3
The quality of the feed-coal in the measurements Moisture (wt%) 21.27
a
Ash (wt%) 12.86
a
Total sulfur (wt%) 0.71
a
Volatile (wt%) 45.38
Qnet (kJ/g)
a
18.79a
Note: aThe values were given on received basis. bThe value was given on dry ash-free basis.
The Dekati low pressure impactor (DLPI) sampling system was applied to particle sampling, the schematic diagram of which is shown in Fig. 3. An isokinetic sampling nozzle 10
ACCEPTED MANUSCRIPT was selected at the entrance of the sampling probe, and a stream of flue gas (about 10 L/min) was first extracted from the center of the duct through a heating probe and then introduced into the DLPI sampling system for particle separation and collection. The DLPI sampling
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system was composed mainly of a DLPI, a vacuum pressure gauge, and a vacuum pump. The data acquisition (DAQ) was set for the temperature and pressure monitoring at the inlet of the DLPI, as well as the temperature control of the sampling system. Considering the low
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concentration of coarse particles at the sampling sites, the cyclone upstream of the DLPI was
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omitted. Particles were fractionated into 13 stages according to their aerodynamic diameter and collected on the aluminum foils in the DLPI. The mass of the collected particles was obtained by weighing the aluminum foils before and after the experiments with a microbalance. The sample volume of flue gas was normalized to normalization condition
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(273.15 K, 101,325 Pa, 0 vol% humidity and 6 vol% O2), and the results were calculated with the normalized sample volume in this paper. At least three parallel samplings were conducted
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for each condition, and the average values were reported.
Fig. 3. Schematic diagram of the particle sampling system
The wastewater was sampled with the WPTA off and on, respectively. The applied voltage 11
ACCEPTED MANUSCRIPT of the WESP was set to 60 kV, and all the operating parameters remained unchanged in the period of wastewater sampling. The sample was collected 30 minutes later after the status of the WPTA was changed, and the whole sampling process lasted for 120 minutes. The
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concentration of Hg, As was obtained by atomic fluorescence spectroscopy (AFS), and the concentration of other TEs was obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES). Results and Discussion
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4.1 Particle Size Distributions (PSDs) at the Inlet and Outlet of the WESP
Fig. 4 shows the PSDs of the particles at the inlet and outlet of the WESP at the applied voltages of 35 kV, 45 kV and 60 kV. Particle sampling at the WESP applied voltage of 60kV was not on the same day as that at other applied voltages, as shown in Table 2, denoted by
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dash dot lines. Although the particles at the inlet of WESP were sampled under the same condition, there was still a little difference in the feed-coal properties, and a more significant fluctuation occurred in the sampling data at the applied voltages of 35 kV and 45 kV. These
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two reasons caused the obvious difference at 0.04 µm in the PSDs. The influence of the
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applied voltage on the collection efficiency of the WESP is shown in Fig. 5. The mass concentration of TSP at the inlet of the WESP was 15.716 mg/m3 (for the applied voltages of 35 kV and 45 kV) and 15.752 mg/m3 (for the applied voltage of 60 kV). Ultrafine particles and coarse particles shared a large proportion of the particles at the inlet of the WESP, and peaked at ~0.03 µm (for ultrafine particles), ~2 µm and ~6 µm (for coarse particles), respectively (see Fig. 4a). It can be seen in Fig. 5 that the applied voltage played an important role in the collection efficiency of the WESP, especially for particles with 12
ACCEPTED MANUSCRIPT aerodynamic diameters of <0.3 µm and >3 µm (see Fig. 4b) because of their sensitivity to the electrostatic field via diffusion charging and field charging. The collection efficiency of the WESP for TSP increased significantly with an increase in the applied voltage (74.06%, 82.86%
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and 91.97% for 35 kV, 45 kV and 60 kV, respectively). When the applied voltage increased from 35 kV to 45 kV and 60kV, the mass concentration of TSP decreased from 4.077 mg/m3 to 2.693 mg/m3 and 1.265 mg/m3. As for the particles with aerodynamic diameters of <0.3 µm
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and >3 µm, the collection efficiency increased from 70.67% and 68.84% to 94.42% and
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92.38%, respectively, when the applied voltage increased from 35 kV to 60 kV. The collection efficiency of large particles (>3 µm) is lower than that of fine particles (<0.3 µm), which was caused by the large particle generation. When the flue gas entered the WDRS, some slurry and gypsum droplets were entrained out of the WFGD, and the pipes of WPTA causing
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disturbed flow promoted the agglomeration of droplets, facilitating the generation of large particles after the droplets were evaporated. Xu et al. (2016) observed the similar results in
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their research.
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Fig. 4. PSDs of particles at the (a) inlet and (b) outlet of the WESP. Particle sampling at the WESP applied voltage of 60kV was on a different day from that of the other applied voltages, denoted by dash dot lines.
Fig. 5. The collection efficiency of WESP for TSP at the applied voltages of 35 kV, 45kV and 60 kV
Besides the applied voltage, the number of running electrostatic fields in the WESP markedly influenced the performance of the WESP for particles removal, as shown in Fig. 6b. The removal efficiency of particles with aerodynamic diameters of <0.3 µm decreased from 49.68% to 1.10% when one of the four electrostatic fields was off, and the removal efficiency 14
ACCEPTED MANUSCRIPT of PM1 (particles with an aerodynamic diameter of ≤1 µm), PM2.5 (particles with an aerodynamic diameter of ≤2.5 µm) and TSP decreased from 68.67%, 82.75% and 88.30% to 43.63%, 69.82% and 82.80%, respectively.
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To sum up, both the number of running electrostatic fields and the applied voltage had a significant influence on the performance of the WESP for particles removal. The WESP reached its greatest collection efficiency when all electrostatic fields were on and the applied
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voltage exceeded 60 kV. However, the operating expense of the WESP is much higher with
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the applied voltage of 60 kV. The collection efficiency of the WESP for PM1, PM2.5 and TSP decreased by 8.13, 5.86 and 9.11 percentage points, respectively, when the applied voltage decreased from 60 kV to 45 kV. Similarly, the collection efficiency decreased by 11.37, 7.13 and 8.8 percentage points when the applied voltage decreased from 45 kV to 35 kV.
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Furthermore, the performance of the WESP is reported as decreasing when the particle load increases. Therefore, the WPTA was set to further elevate the performance of the WESP, especially for low applied voltage, making it more flexible to a change in the unit load and the
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concentration of particles downstream of the WFGD.
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4.2 Influence of the WPTA on the performance of the WDRS for particles removal Comparison of the PSDs of the particles at the inlet and outlet of the WDRS is shown in Fig. 6a. Fig. 6b and 6c present the PSDs of the particles at the outlet of the WDRS with the WPTA off and on, respectively, at boiler operating loads of 90% and 75%. The applied voltage of the WESP was set to 60 kV, and all the operating parameters remained unchanged during the period of particle sampling. The performance of the WDRS for particle removal is excellent (see Fig. 6a), and the WPTA could significantly improve the collection efficiency of 15
ACCEPTED MANUSCRIPT the WDRS for particles (see Fig. 6b and 6c). From Fig. 6b, it can be seen that the emission concentration of the particles with aerodynamic diameters of <0.3 µm and >3 µm was reduced markedly at the load of 90% when the WPTA was on, compared with the concentration when
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the WPTA was turned off. The removal efficiency of the particles with aerodynamic diameters of <0.3 µm and >3 µm increased from 49.68% and 94.15% to 76.04% and 95.60%, respectively. The emission concentration of particles in all size ranges, especially particles
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with an aerodynamic diameter of >0.3 µm, was reduced significantly by the WPTA at boiler
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operating load of 75%, as shown in Fig. 6c. The removal efficiency of particles (>0.3 µm) increased from 91.69% to 94.96%. What’s more, the removal efficiency of particles with aerodynamic diameters of <0.3 µm at a 75% load increased from 94.51% to 96.75%, rising
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2.24 percentage points.
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Fig. 6. (a) Comparison of PSDs of the particles at the inlet and outlet of the WDRS, as well as the PSDs of particles at the outlet of the WDRS with the WPTA off and on at (b) 90% load and (c) 75% load.
The emission concentration of particles in different size ranges and the fractional collection efficiency of the WDRS with the WPTA off and on, at load rates of 90% and 75%, respectively, are listed in Table 4. The mass concentrations of TSP at the inlet of the WDRS were 13.586 mg/m3 (at load rate of 90%) and 15.752 mg/m3 (at load rate of 75%). When the boiler operating load was at 90%, the collection efficiency of the WDRS for PM1, PM2.5 and 17
ACCEPTED MANUSCRIPT TSP rose 14.94, 4.94 and 4.01 percentage points, respectively, with the WPTA on. The collection efficiency rates of the WDRS for PM1, PM2.5 and TSP were all over 90% at load of 75%. However, they rose a further 2.48, 3.32 and 3.17 percentage points by the use of the
Table 4
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WPTA.
Concentration of PM1, PM2.5 and TSP at the outlet and the fractional collection efficiency of WDRS PM1 3
mg/m
%
mg/m
%
0.987 0.516 0.311 0.203
68.67 83.61 92.83 95.31
1.071 0.764 0.717 0.433
82.75 87.69 91.63 94.95
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500 MW
WPTA off WPTA on WPTA off WPTA on
TSP
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600 MW
PM2.5 3
3
mg/m
%
1.589 1.044 1.265 0.766
88.30 92.31 91.97 95.14
As mentioned above, the growth, agglomeration and partial capture of particles existed in the WPTA when the flue gas passed through it. And these processes contributed to the particle capture in the WESP, improving the performance of the WDRS in particle removal.
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4.3 TEs Removal by the WDRS
The analysis showed that the Hg, As and Mn had higher concentrations than other TEs in ,
and
,
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the wastewater. They were studied as the representative of the TEs in Class
respectively, in the present study. The content of Hg, As and Mn in the wastewater is listed in
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Table 5. It is noteworthy that a lot of water is recovered with the WPTA on. Measurements showed that the mass rate of wastewater with the WPTA off and on was 2 t/h and 14 t/h, respectively. The masses of Hg, As, and Mn carried by the wastewater per hour are calculated by adopting a wastewater density of 1×103 kg/m3, neglecting the influence of impurities, and the results are shown in Fig. 7. It can be seen that the WPTA could further improve the performance of the WDRS for TEs removal. With the WPTA on, the masses of Hg, As, and Mn carried by the wastewater per hour were 4.2, 2.8 and 1.5 times more than those when the 18
ACCEPTED MANUSCRIPT WPTA was off. The removal mechanisms of Hg, As and Mn by the WPTA are different. Table 5 The TEs content of the wastewater Hg (mg/L)
As (mg/L)
Mn (mg/L)
WPTA on WPTA off
0.0658 0.0889
0.00249 0.00456
0.056 0.159
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Elements
Fig. 7. The mass of Hg, As and Mn carried by the wastewater per hour
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The mercury in flue gas exists with a small proportion of Hg in particles and a large proportion of Hg in the gas phase (mainly in Hg0). It was reported that the WFGD exhibited
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an excellent performance for Hg2+ removal. Given the transformation of Hg0 to Hg2+ in SCR and the removal of Hg in particles by dust removal devices, the emission of Hg is well
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controlled (Zhu et al., 2016). However, it is indicated that the removal of Hg2+ in the WFGD is insufficient (see Fig. 7), and the WPTA can improve the collection efficiency of the WDRS for mercury by enhancing the removal of Hg2+ and the Hg in particles. As for TEs in Class Ⅱ and Ⅰ(e.g., As and Mn), the WPTA improved the removal efficiency of these by enhancing the removal of particles. 5
Conclusions The WPTA developed by our group has been applied at a 660 MW ultra-supercritical unit 19
ACCEPTED MANUSCRIPT in China as a demonstration project, in which WPTA worked with the WESP to act as a new dust removal system, a wet dust removal system (WDRS). Field measurements were carried out to investigate the performance of the WDRS at the power plant mentioned above. The
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results indicated that the WDRS was adequate to the particle emission limitation in China, and further reduced the mercury emission. The specific conclusions from the demonstration study are as follows.
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(1) The applied voltage played an important role in the collection efficiency of WESP,
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especially for particles with aerodynamic diameters of <0.3 µm and >3 µm. When the applied voltage increased from 35 kV to 45 kV and 60 kV, the collection efficiency of WESP for TSP increased from 74.06% to 82.86% and 91.97%, respectively. The number of running electrostatic fields also markedly affected the WESP performance.
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(2) The WPTA can significantly improve the performance of the WDRS. Compared with the situations where only WESP was operated, the collection efficiency of the WDRS for PM1, PM2.5 and TSP at load of 90% increased from 68.67%, 82.75% and 88.30% to
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83.61%, 87.69% and 92.31%, respectively, when the WPTA was put into operation; and
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when at 75% of the full load, the collection efficiency for those particles increased from 92.83%, 91.63% and 91.97% to 95.31%, 94.95% and 95.14%, respectively. (3) The WPTA can obviously enhance the TEs removal. With the WPTA on, masses of Hg, As, and Mn carried by the wastewater per hour were 4.2, 2.8 and 1.5 times more than those when the WPTA was turned off.
Acknowledgements 20
ACCEPTED MANUSCRIPT This study was supported by the National Natural Science Foundation of China (Nos. 91544108 and 51376147), and the National Key Research Program of China
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(No.2016YFB0601500；No.2016YFB0600605).
Reference
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Du, Q., Su, L., Dong, H., Gao, J., Zhao, Z., Lv, D., Wu, S., 2016. The experimental study of a water-saving wet electrostatic precipitator for removing fine particles. J. Electrostat. 81, 42-47. Huang, R.-J., Zhang, Y., Bozzetti, C., Ho, K.-F., Cao, J.-J., Han, Y., Daellenbach, K.R., Slowik, J.G., Platt, S.M., Canonaco, F., 2014. High secondary aerosol contribution to particulate pollution during haze events in China. Nature. 514, 218-222. Hui, M.L., Zhang, L., Wang, S.X., Cai, S.Y., Zhao, B., 2017. Evaluation of co-benefits on atmospheric mercury emission control for coal combustion in China and future projection. Acta Sci Circumstantiae. 37, 11-22. In Chinese. Jaworek, A., Krupa, A., Czech, T., 2007. Modern electrostatic devices and methods for exhaust gas cleaning: A brief review. J. Electrostat. 65, 133-155. Leong, K., 1984. Thermophoresis and diffusiophoresis of large aerosol particles of different shapes. J. Aerosol. Sci. 15, 511-517. Li, W., Shao, L., Zhang, D., Ro, C.-U., Hu, M., Bi, X., Geng, H., Matsuki, A., Niu, H., Chen, J., 2016. A review of single aerosol particle studies in the atmosphere of East Asia: morphology, mixing state, source, and heterogeneous reactions. J. Clean. Prod. 112, 1330-1349. Li, Z., Jiang, J., Ma, Z., Wang, S., Duan, L., 2015. Effect of selective catalytic reduction (SCR) on fine particle emission from two coal-fired power plants in China. Atmos. Environ. 120, 227-233. Lin, G.-Y., Tsai, C.-J., Chen, S.-C., Chen, T.-M., Li, S.-N., 2010. An efficient single-stage wet electrostatic precipitator for fine and nanosized particle control. Aerosol. Sci. Tech. 44, 38-45. Lyu, W., Li, Y., Guan, D., Zhao, H., Zhang, Q., Liu, Z., 2016. Driving forces of Chinese primary air pollution emissions: an index decomposition analysis. J. Clean. Prod. 133, 136-144. Meij, R., Te Winkel, B., 2004. The emissions and environmental impact of PM10 and trace elements from a modern coal-fired power plant equipped with ESP and wet FGD. Fuel. Process. Technol. 85, 641-656. Ratafia-Brown, J.A., 1994. Overview of trace element partitioning in flames and furnaces of utility coal-fired boilers. Fuel. Process. Technol. 39, 139-157. Schauer, P., 1951. Removal of submicron aerosol particles from moving gas stream. Ind. Eng. Chem. 43, 1532-1538. Song, Y., Wang, X., Maher, B.A., Li, F., Xu, C., Liu, X., Sun, X., Zhang, Z., 2016. The spatial-temporal characteristics and health impacts of ambient fine particulate matter in China. J. Clean. Prod. 112, 1312-1318. Wang, A., Song, Q., Tu, G., Wang, H., Yue, Y., Yao, Q., 2014. Influence of flue gas cleaning system on characteristics of PM2.5 emission from coal-fired power plants. Int. J. Coal. Sci. Technol. 1, 4-12. Wang, C., Liu, X., Li, D., Si, J., Zhao, B., Xu, M., 2015. Measurement of particulate matter and trace elements from a coal-fired power plant with electrostatic precipitators equipped the low temperature economizer. P. Combust. Inst. 35, 2793-2800. 21
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Wang, X., Chang, J., Xu, C., Wang, P., Cui, L., Ma, C., 2016. Electrical characteristics of electrostatic precipitator with a wet membrane-based collecting electrode. J. Electrostat. 80, 85-94. Wu, H., Pan, D., Xiong, G., Jiang, Y., Yang, L., Yang, B., Peng, Z., Hong, G., 2016. The abatement of fine particles from desulfurized flue gas by heterogeneous vapor condensation coupling two impinging streams. Chem. Eng. Process: Process Intensification. 108, 174-180. Xu, M., Yan, R., Zheng, C., Qiao, Y., Han, J., Sheng, C., 2004. Status of trace element emission in a coal combustion process: a review. Fuel. Process. Technol. 85, 215-237. Xu, Y., Liu, X., Cui, J., Chen, D., Xu, M., Pan, S., Zhang, K., Gao, X., 2016. Field measurements on the emission and removal of PM2.5 from Coal-Fired power stations: 4. PM removal performance of wet electrostatic precipitators. Energ. Fuel. 30, 7465-7473. Yang, Z., Zheng, C., Chang, Q., Wan, Y., Wang, Y., Gao, X., Cen, K., 2016. Fine Particle Migration and Collection in a Wet Electrostatic Precipitator. J. Air. Waste. Manage. 67, 498-506. Yoshida, T., Kousaka, Y., Okuyama, K., 1976. Growth of aerosol particles by condensation. Ind. Eng. Chem. Fund. 15, 37-41. Zhang, Y.S., Wang, C.Y., Wang, L., Cheng, J.W., Qiao, K., 2013. Control Technology of PM2.5 in Coal-Fired Power Plant and Experiment on Dedusting Performance of Typical Filters to PM2.5, Adv. Mater. Res. Trans Tech Publ, pp. 2135-2139. Zhao, S., Duan, Y., Tan, H., Liu, M., Wang, X., Wu, L., Wang, C., Lv, J., Yao, T., She, M., 2016. Migration and Emission Characteristics of Trace Elements in a 660 MW Coal-Fired Power Plant of China. Energ. Fuel. 30, 5937-5944. Zhu, C., Tian, H., Cheng, K., Liu, K., Wang, K., Hua, S., Gao, J., Zhou, J., 2016. Potentials of whole process control of heavy metals emissions from coal-fired power plants in China. J. Clean. Prod. 114, 343-351.
22
S0959-6526(17)30971-X
DOI:
10.1016/j.jclepro.2017.05.046
Reference:
JCLP 9581
To appear in:
Journal of Cleaner Production
Received Date: 23 January 2017 Revised Date:
2 May 2017
Accepted Date: 8 May 2017
Please cite this article as: Cao R, Tan H, Xiong Y, Mikulčić H, Vujanović M, Wang X, Duić N, Improving the removal of particles and trace elements from coal-fired power plants by combining a wet phase transition agglomerator with wet electrostatic precipitator, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.05.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights A novel equipment named wet phase transition agglomerator (WPTA) was proposed.
A system of WPTA and wet electrostatic precipitator was commercially applied.
The removal efficiency of particles was significantly improved by the WPTA.
The WPTA improved the performance of the system for trace elements removal.
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ACCEPTED MANUSCRIPT
Improving the Removal of Particles and Trace Elements from Coal-fired Power Plants by Combining a Wet Phase Transition Agglomerator with Wet Electrostatic Precipitator
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Ruijie Cao a, Houzhang Tan a,*, Yingying Xiong b, Hrvoje Mikulčić c, Milan Vujanović c, Xuebin Wang a, Neven Duić c a MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China b Department of Power and Engineering, Shanxi University, Taiyuan, Shanxi, 030006, China c Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lučića 5, 10000 Zagreb, Croatia *Corresponding author: [email protected]
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Abstract
A novel technology for flue gas pre-treatment in the phase transition process is proposed in this paper to better remove the fine particles and trace elements from coal-fired power plants. Wet removal, Brownian diffusion, diffusiophoresis, thermophoresis and disturbed pipe flow
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occurring in the phase transition process were taken into consideration during the development of the technology. An item of equipment called a wet phase transition agglomerator (WPTA) was developed based on the aforementioned technology. The WPTA
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and the wet electrostatic precipitator (WESP) constituted a wet dust removal system (WDRS),
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which was used at a 660 MW ultra-supercritical unit in China as a demonstration project. The particles at the inlet and outlet of the WDRS, as well as the wastewater from the system, were Abbreviations WPTA Wet Phase Transition Agglomerator WESP Wet Electrostatic Precipitator WDRS Wet Dust Removal System TSP Total Suspended Particulates TEs Trace Elements FRP Fiber Reinforced Polymer PSDs Particle Size Distributions
sampled to investigate the performance of the WDRS. The results indicate that the WDRS 1
ACCEPTED MANUSCRIPT helps to bring about ultra-low emissions of particles from the coal-fired power plant, keeping the level of particle emission below 5 mg/m3 under all measurement conditions. It was found that the collection efficiency of the WESP increased significantly with an increasing applied
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voltage. The performance of the WDRS can be further improved by the WPTA, for the removal efficiency of total suspended particulates at boiler operating loads of 90% and 75% rose 4.01 and 3.17 percentage points, respectively. Moreover, the removal efficiency of PM1
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increased from 68.67% to 83.61% with the WPTA running at a load of 90%. The TEs removal
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was also found to be enhanced by the WPTA. The masses of Hg, As and Mn carried by the wastewater per hour with the WPTA running increased 4.2, 2.8 and 1.5 times, respectively, over values when the WPTA was turned off. Keywords
1
Introduction
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phase transition; fine particle; growth; agglomeration; trace element; simultaneous removal
Coal combustion is a major source of atmospheric pollutants in China (Huang et al., 2014;
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Li et al., 2016; Lyu et al., 2016). Coal accounts for 63.7% of China’s primary energy
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consumption, and 43% of the coal is consumed in thermal power plants. It is of great importance for environmental management to reduce the emission of pollutants from coal-fired power plants. Fine particles and trace elements (TEs), which cause severe harm to both human health and the environment (Song et al., 2016), are of particular concern. According to the Action Plan on Coal-fired Power Emission Reduction and Upgrading released by the Chinese government in 2015, coal-fired power units with the capacities over 300 MW are all forced to reach the particle emission limitation of 10 mg/m3 by 2020. What’s 2
ACCEPTED MANUSCRIPT more, power groups in China have been focusing on the research and practice of ultra-low emissions these years. The particle emission concentration bellow 5 mg/m3 is pursued, or even lower.
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The traditional flue gas cleaning technologies of the coal-fired power plants in China are selective catalytic reduction (SCR), dry electrostatic precipitator (ESP) and wet flue gas desulfurization (WFGD), all of which have had remarkable influence on the emission
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characteristics of particles from coal burning. Field measurements found that the
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concentrations of fine particles, especially PM1, were elevated due to the formation of (NH4)2SO4 or NH4HSO4 when the flue gas passed through the SCR (Li et al., 2015). Due to the existence of back corona and re-entrainment, as well as the difference in charging characteristics of particles in different size ranges, dry ESPs cannot effectively capture fine
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particles, especially particles in the size range of 0.1-1 µm (Jaworek et al., 2007; Wang et al., 2014). Moreover, the WFGD would produce new particles from the entrainment of desulfurization slurry droplets containing gypsum and unreacted calcium carbonate (Meij and
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Te Winkel, 2004). Fine particles cannot be controlled effectively by the use of aforementioned
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technologies, and the particle emission concentration is generally over 15 mg/m3 in China. Some new technologies, including the low-low temperature ESP (Wang et al., 2015), the composite electrostatic-bag precipitator (Zhang et al., 2013) and the wet electrostatic precipitator (WESP) (Yang et al., 2016), have been applied to further reduce particle emission. Considering the formation of fine particles in the wet desulfurization units located downstream of the dust collectors, WESP installed ahead of the stack is more effective in attaining a better result of particle emission reduction. Back-corona discharge and dust 3
ACCEPTED MANUSCRIPT re-entrainment can be avoided because the discharge and collection electrodes in the WESP are cleaned continuously by the water spray instead of rapping in the dry ESP. The water spray also enhances particle collection. Hence, the WESP effectively captures particles
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penetrating the upstream pollution control devices. However, it is reported that the collection efficiency of the WESP for particles in the size range of 0.1-1 µm is unsatisfying due to the poor charging capacity of those particles (Du et al., 2016; Wang et al., 2016). Furthermore,
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Lin et al. (2010) reported that the performance of the WESP declined with an increasing
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particle load. In contrast to the lab-scale and pilot-scale studies above, the field measurements conducted by Xu et al. (2016) indicated that the WESP showed a high removal efficiency for particles in the size range of 0.1-1 µm, and the removal efficiency was relatively lower for particles smaller than 0.1 µm. Removal efficiencies less than zero were observed even for
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coarse particles.
It can be seen that the removal performance of the WESP is closely related to the particle size. Although the WESP has a high collection efficiency in removing the total suspended
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particulates (TSP), the effective capture of particles in each size range cannot be realized by
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the WESP alone. Methods for flue gas pre-treatment have been proposed by researchers, and particle growth by condensation was considered as one of the most promising preconditioning techniques for particle collection (Yoshida et al., 1976). The research on the growth of fine particles by condensation has been conducted for decades. It was reported by Schauer et al. (1951) that the condensation of water vapor on particles is an effective method for their growth. Yoshida et al. (1976) reported that the growth rate of fine particles in a supersaturated atmosphere of water vapor was very rapid, and even the hydrophobic particles grew well. 4
ACCEPTED MANUSCRIPT Experiments on the growth of fine particles and simultaneous removal of SO3 were also carried out (Wu et al., 2016). Steam was added to achieve a supersaturated atmosphere in the extensive lab-scale and pilot-scale studies described above. However, in the wet phase
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transition agglomerator (WPTA) developed by our group, the flue gas is cooled to achieve a supersaturated atmosphere, given that the water vapor downstream of the WFGD is under or near the saturation condition. Collision and subsequent agglomeration will happen among
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particles during their growth in the supersaturated atmosphere, due to such factors as
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Brownian diffusion, diffusiophoresis and thermophoresis (Leong, 1984) of particles, as well as disturbed pipe flow. The main body of the WPTA can also capture part of the particles. The WPTA improves the performance of the WESP by facilitating the growth, agglomeration and partial capture of particles in advance.
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TEs emission characteristics are closely related to their vaporization behavior, and three basic classes can be broadly defined with the measure of volatility (Ratafia-Brown, 1994; Xu et al., 2004). Class
: TEs which are approximately equally distributed between the bottom
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ash and fly ash or show no significant enrichment or depletion in the fly ash. Class
: TEs
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which are enriched in the fly ash and depleted in the bottom ash, or show increasing enrichment with decreasing fly ash particle size. Class
: TEs which are volatized, but are not
enriched on the fly ash. They are emitted fully in the vapor phase. Hence, TEs emission can be controlled by removing both gaseous TEs and the TEs in particles. According to the Emission Standard of Air Pollutants for Thermal Power Plants of China (GB 13223-2011), the emission limitation for mercury and its compounds is 0.03 mg/m3, which is not as strict as that in America. The coal-fired power plants with the SCR, ESP and 5
ACCEPTED MANUSCRIPT WFGD shall be adequate to the mercury emission limitation in China, but not that in America. What’s more, the evaluation conducted by Hui et al. (Hui et al., 2017) indicated that the emission limitation for mercury in GB 13223-2011 was not tough enough to meet the
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requirement in the Action Plan for Prevention and Control of Atmospheric Pollution (implemented by the Chinese government in 2013). Thus, the Chinese emission limitation for mercury shall be stricter in the near future.
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There has been no commercial-scale pollution control device for the removal of TEs alone.
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It has been reported that TEs could be captured by the WESP (Zhao et al., 2016; Zhu et al., 2016). Moreover, the results of the measurements in this article indicated that the removal of TEs was markedly elevated when the WPTA was on.
In the following sections, the structure and operating principles of the WPTA is introduced,
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and this equipment working with the WESP constitutes a wet dust removal system (WDRS). The impact of WPTA on the performance of the WDRS for particles and TEs removal is
study.
Description of the WDRS
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2
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evaluated, and the particles removal characteristics of the WESP are also discussed in this
2.1 Introduction to the WPTA The WPTA was designed for flue gas pre-treatment to improve the performance of the WDRS. As shown in Fig. 1b and 1c, the main body of the WPTA consists of multiple modules for heat transfer and dust removal, which makes it convenient to repair and replace any individual module which is out of order. Fluoroplastic pipes, fixed by pore plates, are arranged in a honeycomb structure in each module. Twenty-seven modules in total are 6
ACCEPTED MANUSCRIPT involved in the WPTA of the demonstration project, and the flow area for flue gas of the WPTA is 12.96 m in width and 25.46 m in length. Besides the main body, the WPTA consists
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of a circulating water system, a cooling system, and a water charging system.
Fig. 1. (a) Sketch of the WDRS, as well as the objects of the (b) module and (c) main body of WPTA
depicts the growth, agglomeration and capture of fine particles. When flue gases
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Fig. 2
enter the WPTA, they will undergo three stages: condensation of water vapor, particles agglomeration and capture of particles. During the first stage, the flue gas is cooled when
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passing through the WPTA, and the walls of the water-cooling pipes as well as particles in the
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flue gas will act as the condensation nucleus. The subsequent water film forms on the walls of the water-cooling pipes and will be broken up into large masses of droplets and fog-drops by the flue gas and the gravity of water. Additionally, the growth of particles by condensation also exists in this part. During the second stage, the thermophoresis of particles, caused by the temperature gradient existing between the flue gas and the water-cooling pipes, prompts them to move towards the walls of the pipes. The diffusiophoresis of particles, an analogous phenomenon occurring in the concentration gradient and due to the condensation of water 7
ACCEPTED MANUSCRIPT vapor on large particles and pipes, also leads fine particles to move towards the larger ones and the pipes. Together with the Brownian diffusion of particles and the disturbed pipe flow, thermophoresis and diffusiophoresis promote the chaotic movement of particles, which
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facilitates the agglomeration of the particles through increased frequency of particle collisions. During the last stage, particles will be partially captured by the pipes and the droplets through such factors as inertial impaction, interception, Brownian diffusion, thermophoresis, and
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diffusiophoresis. Moreover, the water film on the pipe walls will carry the captured particles
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to the wastewater tank, thus achieving self-cleaning by the pipes.
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Fig. 2. Operating principles of WPTA
As shown in Fig. 2
, large masses of droplets formed in the WPTA will capture the
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gaseous Hg2+. The gaseous Hg2+ will also be absorbed by the fog-drops, turning the gaseous species into liquid-phase particles which would experience the analogous process of growth, agglomeration and capture described above. Particles, droplets and fog-drops downstream of the WPTA will enter the WESP. The WESP will capture these more easily because of their growth in size and the enhancement in the charging capacity of the solid-phase particles. 8
ACCEPTED MANUSCRIPT 2.2 Collaboration of the WPTA and the WESP The WESP in the demonstration project has four electrostatic fields, and the flexible fabric and barbed wire are used for the collection electrode and discharge electrode, respectively.
Table 1 Technical Parameters of the WPTA and WESP Items
Units 3
m /h m/s Pa mm
Pa vol%
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Pressure drop of flue gas Effective height of electrodes Number of channels Channel width Collection area Dimensions Gas temperature at outlet Pressure at outlet Humidity at outlet
Pa mm mm m2 m
Pa vol%
2,780,666 <3 <100 about 2 600 57 800 13.5
<250 6,500 1,568 420×420 17,122.56 28.56×13.29×21.30 55 500 9
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Values
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Full load flue gas flow (hot) Flow velocity of flue gas Pressure drop of flue gas Temperature drop of flue gas Active height Gas temperature at inlet Pressure at inlet Humidity at inlet
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WPTA
Parameters
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Primary information on the WPTA as well as the WESP is listed in Table 1.
The average flow velocity of flue gas is close to 3 m/s at full load in the flow area of the
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WDRS. The flow velocity of flue gas must be uniform to obtain the best performance of the WESP. The experiment in our unpublished studies indicated that the WPTA straightened the flue gas flow well, and it could replace the flow straightening grid to be installed at the inlet of the WESP, as shown in Fig. 1a. A drainage system consisting of fiber reinforced polymer (FRP) pipes, wastewater tanks and drainage pumps, is shared by the WPTA and the WESP. Together with the spray water of the WESP, the water recovered by the WPTA goes into wastewater tanks through the FRP pipes. The demonstration project of the WDRS was 9
ACCEPTED MANUSCRIPT implemented at a 660 MW ultra-supercritical unit, and the WDRS was installed in the vertical duct between the WFGD tower and the stack. 3
Experimental methods
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The field measurements were carried out at the 660 MW ultra-supercritical unit. Measurement of particles was conducted at the inlet and outlet of the WDRS at the boiler operating loads of 90% (600 MW) and 75% (500 MW), as shown in Table 2. The study on the
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performance of WESP was conducted by particle sampling at the inlet and outlet of WDRS with the WPTA off.
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Table 2 Particle sampling conditions
Sampling sites
Applied voltage of WESP (kV)
Status of WPTA
Study on WESP
75%
Study on WPTA
75%
WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet WDRS inlet and outlet
35 45 60 60 60 60
off off off on off on
90%
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Load
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The wastewater was sampled from the FRP pipes when the boiler was operated at 70% (460 MW) of full load. The quality of the feed-coal in the measurements is listed in Table 3.
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The values were obtained by the methods in national standards of China (GB/T 211-2007, GB/T 212-2008, GB/T 214-2007, GB/T 213-2008, and GB/T 476-2008). Table 3
The quality of the feed-coal in the measurements Moisture (wt%) 21.27
a
Ash (wt%) 12.86
a
Total sulfur (wt%) 0.71
a
Volatile (wt%) 45.38
Qnet (kJ/g)
a
18.79a
Note: aThe values were given on received basis. bThe value was given on dry ash-free basis.
The Dekati low pressure impactor (DLPI) sampling system was applied to particle sampling, the schematic diagram of which is shown in Fig. 3. An isokinetic sampling nozzle 10
ACCEPTED MANUSCRIPT was selected at the entrance of the sampling probe, and a stream of flue gas (about 10 L/min) was first extracted from the center of the duct through a heating probe and then introduced into the DLPI sampling system for particle separation and collection. The DLPI sampling
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system was composed mainly of a DLPI, a vacuum pressure gauge, and a vacuum pump. The data acquisition (DAQ) was set for the temperature and pressure monitoring at the inlet of the DLPI, as well as the temperature control of the sampling system. Considering the low
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concentration of coarse particles at the sampling sites, the cyclone upstream of the DLPI was
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omitted. Particles were fractionated into 13 stages according to their aerodynamic diameter and collected on the aluminum foils in the DLPI. The mass of the collected particles was obtained by weighing the aluminum foils before and after the experiments with a microbalance. The sample volume of flue gas was normalized to normalization condition
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(273.15 K, 101,325 Pa, 0 vol% humidity and 6 vol% O2), and the results were calculated with the normalized sample volume in this paper. At least three parallel samplings were conducted
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for each condition, and the average values were reported.
Fig. 3. Schematic diagram of the particle sampling system
The wastewater was sampled with the WPTA off and on, respectively. The applied voltage 11
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concentration of Hg, As was obtained by atomic fluorescence spectroscopy (AFS), and the concentration of other TEs was obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES). Results and Discussion
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4.1 Particle Size Distributions (PSDs) at the Inlet and Outlet of the WESP
Fig. 4 shows the PSDs of the particles at the inlet and outlet of the WESP at the applied voltages of 35 kV, 45 kV and 60 kV. Particle sampling at the WESP applied voltage of 60kV was not on the same day as that at other applied voltages, as shown in Table 2, denoted by
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dash dot lines. Although the particles at the inlet of WESP were sampled under the same condition, there was still a little difference in the feed-coal properties, and a more significant fluctuation occurred in the sampling data at the applied voltages of 35 kV and 45 kV. These
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two reasons caused the obvious difference at 0.04 µm in the PSDs. The influence of the
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applied voltage on the collection efficiency of the WESP is shown in Fig. 5. The mass concentration of TSP at the inlet of the WESP was 15.716 mg/m3 (for the applied voltages of 35 kV and 45 kV) and 15.752 mg/m3 (for the applied voltage of 60 kV). Ultrafine particles and coarse particles shared a large proportion of the particles at the inlet of the WESP, and peaked at ~0.03 µm (for ultrafine particles), ~2 µm and ~6 µm (for coarse particles), respectively (see Fig. 4a). It can be seen in Fig. 5 that the applied voltage played an important role in the collection efficiency of the WESP, especially for particles with 12
ACCEPTED MANUSCRIPT aerodynamic diameters of <0.3 µm and >3 µm (see Fig. 4b) because of their sensitivity to the electrostatic field via diffusion charging and field charging. The collection efficiency of the WESP for TSP increased significantly with an increase in the applied voltage (74.06%, 82.86%
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and 91.97% for 35 kV, 45 kV and 60 kV, respectively). When the applied voltage increased from 35 kV to 45 kV and 60kV, the mass concentration of TSP decreased from 4.077 mg/m3 to 2.693 mg/m3 and 1.265 mg/m3. As for the particles with aerodynamic diameters of <0.3 µm
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and >3 µm, the collection efficiency increased from 70.67% and 68.84% to 94.42% and
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92.38%, respectively, when the applied voltage increased from 35 kV to 60 kV. The collection efficiency of large particles (>3 µm) is lower than that of fine particles (<0.3 µm), which was caused by the large particle generation. When the flue gas entered the WDRS, some slurry and gypsum droplets were entrained out of the WFGD, and the pipes of WPTA causing
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disturbed flow promoted the agglomeration of droplets, facilitating the generation of large particles after the droplets were evaporated. Xu et al. (2016) observed the similar results in
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their research.
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Fig. 4. PSDs of particles at the (a) inlet and (b) outlet of the WESP. Particle sampling at the WESP applied voltage of 60kV was on a different day from that of the other applied voltages, denoted by dash dot lines.
Fig. 5. The collection efficiency of WESP for TSP at the applied voltages of 35 kV, 45kV and 60 kV
Besides the applied voltage, the number of running electrostatic fields in the WESP markedly influenced the performance of the WESP for particles removal, as shown in Fig. 6b. The removal efficiency of particles with aerodynamic diameters of <0.3 µm decreased from 49.68% to 1.10% when one of the four electrostatic fields was off, and the removal efficiency 14
ACCEPTED MANUSCRIPT of PM1 (particles with an aerodynamic diameter of ≤1 µm), PM2.5 (particles with an aerodynamic diameter of ≤2.5 µm) and TSP decreased from 68.67%, 82.75% and 88.30% to 43.63%, 69.82% and 82.80%, respectively.
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To sum up, both the number of running electrostatic fields and the applied voltage had a significant influence on the performance of the WESP for particles removal. The WESP reached its greatest collection efficiency when all electrostatic fields were on and the applied
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voltage exceeded 60 kV. However, the operating expense of the WESP is much higher with
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the applied voltage of 60 kV. The collection efficiency of the WESP for PM1, PM2.5 and TSP decreased by 8.13, 5.86 and 9.11 percentage points, respectively, when the applied voltage decreased from 60 kV to 45 kV. Similarly, the collection efficiency decreased by 11.37, 7.13 and 8.8 percentage points when the applied voltage decreased from 45 kV to 35 kV.
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Furthermore, the performance of the WESP is reported as decreasing when the particle load increases. Therefore, the WPTA was set to further elevate the performance of the WESP, especially for low applied voltage, making it more flexible to a change in the unit load and the
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concentration of particles downstream of the WFGD.
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4.2 Influence of the WPTA on the performance of the WDRS for particles removal Comparison of the PSDs of the particles at the inlet and outlet of the WDRS is shown in Fig. 6a. Fig. 6b and 6c present the PSDs of the particles at the outlet of the WDRS with the WPTA off and on, respectively, at boiler operating loads of 90% and 75%. The applied voltage of the WESP was set to 60 kV, and all the operating parameters remained unchanged during the period of particle sampling. The performance of the WDRS for particle removal is excellent (see Fig. 6a), and the WPTA could significantly improve the collection efficiency of 15
ACCEPTED MANUSCRIPT the WDRS for particles (see Fig. 6b and 6c). From Fig. 6b, it can be seen that the emission concentration of the particles with aerodynamic diameters of <0.3 µm and >3 µm was reduced markedly at the load of 90% when the WPTA was on, compared with the concentration when
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the WPTA was turned off. The removal efficiency of the particles with aerodynamic diameters of <0.3 µm and >3 µm increased from 49.68% and 94.15% to 76.04% and 95.60%, respectively. The emission concentration of particles in all size ranges, especially particles
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with an aerodynamic diameter of >0.3 µm, was reduced significantly by the WPTA at boiler
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operating load of 75%, as shown in Fig. 6c. The removal efficiency of particles (>0.3 µm) increased from 91.69% to 94.96%. What’s more, the removal efficiency of particles with aerodynamic diameters of <0.3 µm at a 75% load increased from 94.51% to 96.75%, rising
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2.24 percentage points.
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Fig. 6. (a) Comparison of PSDs of the particles at the inlet and outlet of the WDRS, as well as the PSDs of particles at the outlet of the WDRS with the WPTA off and on at (b) 90% load and (c) 75% load.
The emission concentration of particles in different size ranges and the fractional collection efficiency of the WDRS with the WPTA off and on, at load rates of 90% and 75%, respectively, are listed in Table 4. The mass concentrations of TSP at the inlet of the WDRS were 13.586 mg/m3 (at load rate of 90%) and 15.752 mg/m3 (at load rate of 75%). When the boiler operating load was at 90%, the collection efficiency of the WDRS for PM1, PM2.5 and 17
ACCEPTED MANUSCRIPT TSP rose 14.94, 4.94 and 4.01 percentage points, respectively, with the WPTA on. The collection efficiency rates of the WDRS for PM1, PM2.5 and TSP were all over 90% at load of 75%. However, they rose a further 2.48, 3.32 and 3.17 percentage points by the use of the
Table 4
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WPTA.
Concentration of PM1, PM2.5 and TSP at the outlet and the fractional collection efficiency of WDRS PM1 3
mg/m
%
mg/m
%
0.987 0.516 0.311 0.203
68.67 83.61 92.83 95.31
1.071 0.764 0.717 0.433
82.75 87.69 91.63 94.95
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500 MW
WPTA off WPTA on WPTA off WPTA on
TSP
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600 MW
PM2.5 3
3
mg/m
%
1.589 1.044 1.265 0.766
88.30 92.31 91.97 95.14
As mentioned above, the growth, agglomeration and partial capture of particles existed in the WPTA when the flue gas passed through it. And these processes contributed to the particle capture in the WESP, improving the performance of the WDRS in particle removal.
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4.3 TEs Removal by the WDRS
The analysis showed that the Hg, As and Mn had higher concentrations than other TEs in ,
and
,
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the wastewater. They were studied as the representative of the TEs in Class
respectively, in the present study. The content of Hg, As and Mn in the wastewater is listed in
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Table 5. It is noteworthy that a lot of water is recovered with the WPTA on. Measurements showed that the mass rate of wastewater with the WPTA off and on was 2 t/h and 14 t/h, respectively. The masses of Hg, As, and Mn carried by the wastewater per hour are calculated by adopting a wastewater density of 1×103 kg/m3, neglecting the influence of impurities, and the results are shown in Fig. 7. It can be seen that the WPTA could further improve the performance of the WDRS for TEs removal. With the WPTA on, the masses of Hg, As, and Mn carried by the wastewater per hour were 4.2, 2.8 and 1.5 times more than those when the 18
ACCEPTED MANUSCRIPT WPTA was off. The removal mechanisms of Hg, As and Mn by the WPTA are different. Table 5 The TEs content of the wastewater Hg (mg/L)
As (mg/L)
Mn (mg/L)
WPTA on WPTA off
0.0658 0.0889
0.00249 0.00456
0.056 0.159
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Elements
Fig. 7. The mass of Hg, As and Mn carried by the wastewater per hour
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The mercury in flue gas exists with a small proportion of Hg in particles and a large proportion of Hg in the gas phase (mainly in Hg0). It was reported that the WFGD exhibited
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an excellent performance for Hg2+ removal. Given the transformation of Hg0 to Hg2+ in SCR and the removal of Hg in particles by dust removal devices, the emission of Hg is well
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controlled (Zhu et al., 2016). However, it is indicated that the removal of Hg2+ in the WFGD is insufficient (see Fig. 7), and the WPTA can improve the collection efficiency of the WDRS for mercury by enhancing the removal of Hg2+ and the Hg in particles. As for TEs in Class Ⅱ and Ⅰ(e.g., As and Mn), the WPTA improved the removal efficiency of these by enhancing the removal of particles. 5
Conclusions The WPTA developed by our group has been applied at a 660 MW ultra-supercritical unit 19
ACCEPTED MANUSCRIPT in China as a demonstration project, in which WPTA worked with the WESP to act as a new dust removal system, a wet dust removal system (WDRS). Field measurements were carried out to investigate the performance of the WDRS at the power plant mentioned above. The
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results indicated that the WDRS was adequate to the particle emission limitation in China, and further reduced the mercury emission. The specific conclusions from the demonstration study are as follows.
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(1) The applied voltage played an important role in the collection efficiency of WESP,
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especially for particles with aerodynamic diameters of <0.3 µm and >3 µm. When the applied voltage increased from 35 kV to 45 kV and 60 kV, the collection efficiency of WESP for TSP increased from 74.06% to 82.86% and 91.97%, respectively. The number of running electrostatic fields also markedly affected the WESP performance.
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(2) The WPTA can significantly improve the performance of the WDRS. Compared with the situations where only WESP was operated, the collection efficiency of the WDRS for PM1, PM2.5 and TSP at load of 90% increased from 68.67%, 82.75% and 88.30% to
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83.61%, 87.69% and 92.31%, respectively, when the WPTA was put into operation; and
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when at 75% of the full load, the collection efficiency for those particles increased from 92.83%, 91.63% and 91.97% to 95.31%, 94.95% and 95.14%, respectively. (3) The WPTA can obviously enhance the TEs removal. With the WPTA on, masses of Hg, As, and Mn carried by the wastewater per hour were 4.2, 2.8 and 1.5 times more than those when the WPTA was turned off.
Acknowledgements 20
ACCEPTED MANUSCRIPT This study was supported by the National Natural Science Foundation of China (Nos. 91544108 and 51376147), and the National Key Research Program of China
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(No.2016YFB0601500；No.2016YFB0600605).
Reference
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Du, Q., Su, L., Dong, H., Gao, J., Zhao, Z., Lv, D., Wu, S., 2016. The experimental study of a water-saving wet electrostatic precipitator for removing fine particles. J. Electrostat. 81, 42-47. Huang, R.-J., Zhang, Y., Bozzetti, C., Ho, K.-F., Cao, J.-J., Han, Y., Daellenbach, K.R., Slowik, J.G., Platt, S.M., Canonaco, F., 2014. High secondary aerosol contribution to particulate pollution during haze events in China. Nature. 514, 218-222. Hui, M.L., Zhang, L., Wang, S.X., Cai, S.Y., Zhao, B., 2017. Evaluation of co-benefits on atmospheric mercury emission control for coal combustion in China and future projection. Acta Sci Circumstantiae. 37, 11-22. In Chinese. Jaworek, A., Krupa, A., Czech, T., 2007. Modern electrostatic devices and methods for exhaust gas cleaning: A brief review. J. Electrostat. 65, 133-155. Leong, K., 1984. Thermophoresis and diffusiophoresis of large aerosol particles of different shapes. J. Aerosol. Sci. 15, 511-517. Li, W., Shao, L., Zhang, D., Ro, C.-U., Hu, M., Bi, X., Geng, H., Matsuki, A., Niu, H., Chen, J., 2016. A review of single aerosol particle studies in the atmosphere of East Asia: morphology, mixing state, source, and heterogeneous reactions. J. Clean. Prod. 112, 1330-1349. Li, Z., Jiang, J., Ma, Z., Wang, S., Duan, L., 2015. Effect of selective catalytic reduction (SCR) on fine particle emission from two coal-fired power plants in China. Atmos. Environ. 120, 227-233. Lin, G.-Y., Tsai, C.-J., Chen, S.-C., Chen, T.-M., Li, S.-N., 2010. An efficient single-stage wet electrostatic precipitator for fine and nanosized particle control. Aerosol. Sci. Tech. 44, 38-45. Lyu, W., Li, Y., Guan, D., Zhao, H., Zhang, Q., Liu, Z., 2016. Driving forces of Chinese primary air pollution emissions: an index decomposition analysis. J. Clean. Prod. 133, 136-144. Meij, R., Te Winkel, B., 2004. The emissions and environmental impact of PM10 and trace elements from a modern coal-fired power plant equipped with ESP and wet FGD. Fuel. Process. Technol. 85, 641-656. Ratafia-Brown, J.A., 1994. Overview of trace element partitioning in flames and furnaces of utility coal-fired boilers. Fuel. Process. Technol. 39, 139-157. Schauer, P., 1951. Removal of submicron aerosol particles from moving gas stream. Ind. Eng. Chem. 43, 1532-1538. Song, Y., Wang, X., Maher, B.A., Li, F., Xu, C., Liu, X., Sun, X., Zhang, Z., 2016. The spatial-temporal characteristics and health impacts of ambient fine particulate matter in China. J. Clean. Prod. 112, 1312-1318. Wang, A., Song, Q., Tu, G., Wang, H., Yue, Y., Yao, Q., 2014. Influence of flue gas cleaning system on characteristics of PM2.5 emission from coal-fired power plants. Int. J. Coal. Sci. Technol. 1, 4-12. Wang, C., Liu, X., Li, D., Si, J., Zhao, B., Xu, M., 2015. Measurement of particulate matter and trace elements from a coal-fired power plant with electrostatic precipitators equipped the low temperature economizer. P. Combust. Inst. 35, 2793-2800. 21
ACCEPTED MANUSCRIPT
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TE D
M AN U
SC
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Wang, X., Chang, J., Xu, C., Wang, P., Cui, L., Ma, C., 2016. Electrical characteristics of electrostatic precipitator with a wet membrane-based collecting electrode. J. Electrostat. 80, 85-94. Wu, H., Pan, D., Xiong, G., Jiang, Y., Yang, L., Yang, B., Peng, Z., Hong, G., 2016. The abatement of fine particles from desulfurized flue gas by heterogeneous vapor condensation coupling two impinging streams. Chem. Eng. Process: Process Intensification. 108, 174-180. Xu, M., Yan, R., Zheng, C., Qiao, Y., Han, J., Sheng, C., 2004. Status of trace element emission in a coal combustion process: a review. Fuel. Process. Technol. 85, 215-237. Xu, Y., Liu, X., Cui, J., Chen, D., Xu, M., Pan, S., Zhang, K., Gao, X., 2016. Field measurements on the emission and removal of PM2.5 from Coal-Fired power stations: 4. PM removal performance of wet electrostatic precipitators. Energ. Fuel. 30, 7465-7473. Yang, Z., Zheng, C., Chang, Q., Wan, Y., Wang, Y., Gao, X., Cen, K., 2016. Fine Particle Migration and Collection in a Wet Electrostatic Precipitator. J. Air. Waste. Manage. 67, 498-506. Yoshida, T., Kousaka, Y., Okuyama, K., 1976. Growth of aerosol particles by condensation. Ind. Eng. Chem. Fund. 15, 37-41. Zhang, Y.S., Wang, C.Y., Wang, L., Cheng, J.W., Qiao, K., 2013. Control Technology of PM2.5 in Coal-Fired Power Plant and Experiment on Dedusting Performance of Typical Filters to PM2.5, Adv. Mater. Res. Trans Tech Publ, pp. 2135-2139. Zhao, S., Duan, Y., Tan, H., Liu, M., Wang, X., Wu, L., Wang, C., Lv, J., Yao, T., She, M., 2016. Migration and Emission Characteristics of Trace Elements in a 660 MW Coal-Fired Power Plant of China. Energ. Fuel. 30, 5937-5944. Zhu, C., Tian, H., Cheng, K., Liu, K., Wang, K., Hua, S., Gao, J., Zhou, J., 2016. Potentials of whole process control of heavy metals emissions from coal-fired power plants in China. J. Clean. Prod. 114, 343-351.
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