Correlation between Polycyclic Aromatic Hydrocarbon Concentration

Jun 5, 2017 - To check the relation between polycyclic aromatic hydrocarbons (PAHs) and particulate matter (PM), this study chose a low-low temperatur...
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Correlation between Polycyclic Aromatic Hydrocarbon Concentration and Particulate Matter during the Removal Process of a Low-Low Temperature Electrostatic Precipitator Jingwei Li,† Xiaodong Li,*,† Chenyang Zhou,† Min Li,† Shengyong Lu,† Jianhua Yan,† Zhifu Qi,‡ and Chunhui Shou‡ †

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China ‡ Zhejiang Energy Group R&D, Zhejiang Province, Hangzhou 310003, China ABSTRACT: To check the relation between polycyclic aromatic hydrocarbons (PAHs) and particulate matter (PM), this study chose a low-low temperature electrostatic precipitator (LLT-ESP) in an ultra-low-emission coal-fired power plant to study the mechanism at two operation conditions. Sixteen U.S Environmental Protection Agency PAHs in filterable and condensable PM were analyzed. The removal efficiency of the LLT-ESP for filterable PM2.5 was 98.8% and 98.4% at two different stages. The removal efficiency of the LLT-ESP for condensable PM was 77.1% at high temperature and 89.6% at low temperature. The boiler load markedly affected the emission of filterable PM, condensable PM, and particle-associated PAHs. The PAHs’ concentration distribution was related to the PM concentration distribution. The mass concentration of total PAHs and the proportion of 5ring and 6-ring PAHs in filterable PM2.5 increased significantly when the flue gas passed through the LLT-ESP. The 3- and 4-ring PAHs accounted for the main part of condensable PM in the flue gas at the inlet and outlet of the LLT-ESP. The PAHs distribution in filterable PM2.5 was not only influenced by the mass concentration of filterable PM2.5, but some other factors might also have had an effect. The 5- and 6-ring PAHs in FPM2.5 were more significantly related to mass concentration of filterable PM2.5 than were the 2-, 3-, and 4-ring PAHs. The coefficients of determination (COD, R2) between the PAH content and the concentration of condensable PM were close to 1. The mass concentration of CPM and the PAH contents are thus strongly correlated. The LLT-ESP had a greater influence on the PAHs distribution in the FPM than it did in the CPM.

1. INTRODUCTION Airborne particulate matter (PM), especially PM2.5, has caused serious environmental and health problems. Ambient PM contains a large number of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), heavy metals, acidic oxides, and dioxins.1−4 PAHs are a class of organic compounds that are generated by the incomplete combustion and pyrolysis of fossil fuels. PAHs are of great concern due to their teratogenicity, mutagenicity, and carcinogenicity.1,5 Previous studies have shown that coal-fired power plants are among the major sources for ambient PM2.5 and PM2.5‑10 and associated PAHs.6,7 According to Kong, most of the total ambient particleassociated PAHs were in PM2.5.6 Most studies focused on the correlation of PAHs in the atmospheric PM2.5. However, it is necessary to study the emission and correlation characteristics between PM2.5 and particle-associated PAHs from the coal-fired power plants for the sake of pollution control. For coal-fired power plants, the low-low temperature electrostatic precipitator (LLT-ESP) is one key piece of equipment for controlling PM2.5 emissions in ultra-lowemission technology.8−10 The LLT-ESP uses a gas−gas heat exchange (MGGH) to reduce the temperature of the inlet flue gas from approximately 140 °C to 90−100 °C, which is below the acid dew point. A lower inlet flue gas temperature leads to the condensation and adhesion of condensable compounds, such as PAHs and SO3. Furthermore, the dust resistivity and the flue gas viscosity were reduced in the LLT-ESP, and the PM © XXXX American Chemical Society

removal efficiency increased. Most of the studies about the LLT-ESP were focused on soot control and economic analysis. On the basis of the data obtained by Shou,11 the removal efficiency of the LLT-ESP for PM1 was 99.44%, which was much higher than that of traditional ESP. By studying the effect of the LLT-ESP on the soot emission in exhaust gas, Li12 confirmed that the LLT-ESP had reasonably good adaptability with different loads. Previous studies showed that the LLT-ESP performed well in scavenging particulate matter. The ESP eliminated most PM in the flue gas, but it is necessary to study the influence of the ESP on the particleassociated PAHs. Guerriero13 investigated the removal efficiency by ESP on PAHs and found that the removal efficiency of the ESP on the total PAH concentration was as low as 5.2% in iron ore sintering plants. In Lee’s14 study, the ESP used in medical waste incinerators (MWIs) with a mechanical grate had a 2.2% removal efficiency on HMW, and a 28.8% removal efficiency on HMW for the ESP used in a MWI with a fixed grate. Yan15 confirmed that the removal efficiency of the ESP on the PAHs was low and that the removal effects were related to both the operation conditions and the electric field. In the flue gas cooling process, the content of individual PAHs increased with the decrease of Received: January 14, 2017 Revised: May 5, 2017 Published: June 5, 2017 A

DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels particle granularity, as observed by Zhao.16 Funcke17 studied the influence of the electrostatic precipitator on the concentration of organic compounds in flue gas and found both the formation and the destruction of PAHs. The influence on PAHs by wet flue gas desulfurization (WFGD) and the wet electrostatic precipitator (WESP) in the ultra-low-emission coal-fired power plant was studied in another previous paper.8 However, few data are available on the correlation between PM and the PAH emission characteristics in the LLT-ESP. Primary PM emissions from coal-fired power plants include filterable PM (FPM) and condensable PM (CPM).18−20 In this study, filterable and condensable PM samples were collected at the inlet and outlet of the LLT-ESP in an ultra-low-emission coal-fired power plant. The removal characteristics for the filterable and condensable PM by the LLT-ESP were investigated when the LLT-ESP was operated at different temperatures and boiler load conditions. The filterable and condensable PM samples were collected to analyze the distribution characteristics of particle-associated PAHs. A correlation analysis between the particle-associated PAHs and PM was also conducted. The aims of this paper are to study (1) the PM removal performance and process features of the LLTESP; (2) the distribution characteristics of PAHs in the filterable and condensable PM2.5 in the flue gas in the LLTESP; and (3) the emission relationship between the particleassociated PAHs and PM emissions. The results could provide more information on the pollutant emission and control characteristics of the LLT-ESP by investigating the distribution profiles of PAHs and the equipment removal efficiency.

2.2. Sampling Methods and Devices. There were two sampling sites in this study. Site 1 was set at the inlet of the MGGH, before the LLT-ESP, and Site 2 was set at the outlet of the LLT-ESP. The sampling sites were located in the middle of the flue. The sampling points were set at the cross section of the flue at each sampling site. 2.2.1. Particulate Matter Sampling and Measurement. The sampling system is shown in Figure 1. For filterable PM (FPM),

Figure 1. Sketch of the sampling system: 1 - isokinetic nozzles; 2 stainless steel sampling probes; 3, 6 - hoses and hose connector; 4 impactor heater and heater controller; 5 - Dekati PM10 impactor; 7 condenser; 8 - recirculation pump; 9 - water dropout impinger; 10 modified Greenburg−Smith impinger (backup impinger); 11 condensable PM filter; 12 - moisture trap; 13 - silica gel trap; 14 sampler; 15 - water bath; 16 - ice bath.

each sampling site was sampled thrice. For condensable PM (CPM), each sampling site was sampled twice because the sample collection and pretreatment take time. At Site 1, the sampling time was set at 10 min for each sample due to the high concentration of dust in the flue gas. At Site 2, the sampling time was 1 h for each sample. The sampling probe and the Dekati PM10 impactor were heated to 130 °C, and the temperature was higher than the temperature of the flue gas. The elevated temperature helped minimize the influence of moisture. Before each sampling, the flue gas velocity was measured by a Pitot tube and a leak check was conducted. The sampling flow rate was 10 L/min, thus meeting the requirement of the Dekati PM10 impactor. The flue gas parameters (temperature, moisture, and O2 concentration) were measured according to the GB/T 16157-1996, China. Filterable PM sampling and measurement were done according to ISO 23210-2009. The Dekati PM10 impactor was used for filterable PM collection. In the Dekati PM10, the filterable PM was divided into four sections: ≥ 10 μm, 10−2.5 μm, 2.5−1 μm, and ≤ 1 μm. The impactor-specific 25 mm foil films (for PM: ≥ 10 μm, 10−2.5 μm, and 2.5−1 μm) and 47 mm polyester filters (for PM: ≤ 1 μm) were used for filterable PM collection and were fixed in collection plates. The foil films and polyester filters were purified and dried well. Before and after sampling, the foil films and polyester filters were conditioned and then weighed on an analytical balance. The analytical balance was a Sartorius BT25s, which is capable of weighing at least 0.01 mg. Condensable PM sampling and measurement were performed according to the U.S Environmental Protection Agency (EPA) Method 202. Condensable PM was collected in the condenser, connecting glassware, water dropout impinger, modified Greenburg− Smith impinger, and the condensable PM filter. A post-test nitrogen purge was conducted immediately after each sampling according to Method 202. The detailed sampling and measurement procedure was on the basis of the Method 202. 2.2.2. Particle-Associated PAHs Analysis. The PM samples collected in Dekati PM10 were gathered for PAHs analysis. The organic fraction of condensable PM samples was extracted by n-hexane with an ultrasonicator, and preserved for PAHs analysis. At Stage 1, the total mass of filterable PM2.5 at Site 1 was approximately 60 mg, and the total mass of filterable PM2.5‑10 at Site 1 was approximately 95 mg. The total mass of filterable PM2.5 at Site 2 was approximately 20 mg, and the total mass of filterable PM2.5‑10 was too small to fulfill the PAHs analysis.

2. MATERIALS AND METHODS 2.1. Facility. The studied coal-fired power plant was equipped with a 1000 MW ultrasupercritical pressure once-through operation boiler and was located in Zhejiang Province, China. The plant transformed to ultra-low-emissions status in 2014 with the implementation of the LLT-ESP, WESP, MGGH, and the reformation of the WFGD. The detailed layout of the air pollution control devices (APCDs) is shown in a previous study.8 The studied LLT-ESP includes a six-room four electric field.11 The inlet temperature of the flue gas in the LLT-ESP could be reduced to 87−100 °C with the operation of the MGGH. The sampling process involved two stages. At Stage 1 (3 days), the boiler operated at 1000 MW during sampling, and the operation temperature of the LLT-ESP was maintained at 100 °C. The sampling of filterable and condensable PM was conducted from 9:30 am to 5:00 pm. At Stage 2 (4 days), the boiler load was maintained at 500 MW during sampling. Due to the hot weather during the test, the operation temperature of the LLT-ESP could be reduced to less than 95 °C only in the evening. The operation temperature of the LLT-ESP was maintained at 95 ± 1 °C, and the sampling of filterable and condensable PM was conducted from 6:00 pm to 12:00 am. The properties of the coal blends are shown in Table 1.

Table 1. Properties of Coal Blends during the Test Stage 1

Stage 2

parameter

basis

value

value

moisture, % moisture, % ash, % volatile matter, % fixed carbon, % sulfur, % Qgr, MJ/kg Qnet, MJ/kg

as received dry as received as received as received as received as received as received

14.03 3.77 12.97 27.06 45.94 0.51 26.89 22.49

14.13 3.50 12.64 27.20 46.04 0.44 23.66 22.59

-

B

DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Mass Concentrations of Filterable and Condensable PM at Sampling Sites (mg/m3) FPM Stage 1 Stage 2

CPM

sites

PM2.5

PM10

total

organic

inorganic

total

Site Site Site Site

465.0 5.6 329.8 5.3

1218.1 6.9 905.5 6.2

5678.6 7.5 5472.6 7.1

100.4 19.4 235.8 19.1

42.3 13.2 99.9 15.7

142.7 32.6 335.7 34.8

1 2 1 2

Figure 2. Removal efficiency of the LLT-ESP for PM. At Stage 2, the total mass of filterable PM2.5 at Site 1 was approximately 48 mg, and the total mass of filterable PM2.5‑10 at Site 2 was approximately 75 mg. The total mass of filterable PM2.5 at Site 2 was nearly 20 mg, and the total mass of filterable PM2.5‑10 was too small to fulfill the PAHs analysis. Sixteen U.S. Environmental Protection Agency (EPA) PAHs were studied and analyzed: naphthalene (Nap, 2-rings), acenaphthylene (Acpy, 3-rings), acenaphthene (Ace, 3-rings), fluorene (Flu, 3-rings), phenanthrene (Phe, 3-rings), anthracene (Ant, 3-rings), fluoranthene (Fla, 4-rings), pyrene (Pye, 4-rings), chrysene (Chr, 4-rings), benz[a]anthracene (BaA, 4-rings), benzo[b]fluoranthene (BbF, 5rings), benzo[k]fluoranthene (BkF, 5-rings), benzo[a]pyrene (BaP, 5rings), dibenz[a,h]anthracene (DBA, 5-rings), indeno[1,2,3-cd]pyrene (IND, 6-rings), and benzo[ghi]perylene (BghiP, 6-rings). The determination process was conducted according to HJ 646-2013 (China). All PAHs samples were extracted by dichloromethane for 22 ± 2 h. The extracts were concentrated to less than 5 mL, and the solvent was transferred to n-hexane. In the purification procedure, silica gel columns (each sample, 10 g of silica gel baked at 130 °C for 16 h, with 2 cm Na2SO4 in the upper part) were used. The detailed purification procedure was shown in a previous study.8 The eluted solvent containing PAHs was concentrated to less than 1 mL. Mixed internal standards (naphthalene-d8, acenaphthene-d10, phenanthrened10, chrysene-d12, and perylene-d12, 10 μL, 40 ppm) were added to the solvent. Then, the solvent was added to 1 mL with n-hexane. A gaseous chromatograph coupled with a mass spectrometer (GCMS) system (Agilent 6890N GC/5975B inert XL MSD) was used for analyzing the PAHs. The chromatographic column was an HP-5MS (30 m × 0.25 mm × 0.25 μm). The injection volume was 1 μL. The inlet temperature was 250 °C, the ion source temperature was set to 230 °C, and the transmission line temperature was set to 280 °C. A temperature program was run from 50 °C (2 min hold) to 200 °C (1 min hold) at a rate of 15 °C min−1 and then to 300 °C (5 min hold) at a rate of 10 °C min−1. The quantification of PAHs was performed using the selective ion monitoring (SIM) mode. 2.2.3. Quality Control. During the PM sampling, reagent and field blanks for filterable and condensable PM were collected and measured. Duplicate tests were conducted for every experimental condition. The concentrations of the reagent blanks and filterable PM field blanks approached 0. The concentrations of the condensable PM field blanks

were nearly 10 times smaller than the concentrations of the condensable PM samples. During the PAHs analysis, the PAHs concentrations were calculated with a group of 6 concentration levels from 0.04 to 2 μg/mL. The calibration curves were linear in the concentration ranges with correlation coefficients (R2) in the range of 0.9943 (BKF) to 0.9993 (Nap). The limits of detection (LOD) of 16 detected PAHs were determined from the lowest standard in the calibration curve using the area of peak having a signal-to-noise of 3, which ranged from 0.04 to 0.35 μg/L. Surrogate standards (2-fluorobiphenyl and p-terphenyld14) were added to the samples prior to the extraction. The mean recovery rate of 16 detected PAHs ranged from 61% to 109%.

3. RESULTS AND DISCUSSION 3.1. Mass Concentrations and Removal Efficiency for PM. The average mass concentrations of filterable PM and condensable PM at Site 1 and Site 2 are shown in Table 2. For Stage 1, at Site 1, the mass concentrations ranged from 349.5− 573.9 mg/m3 and 822.6−1500.1 mg/m3 for filterable PM2.5 and PM10, respectively. The mass concentrations ranged from 80.7−121.6 mg/m3 and 30.4−47.1 mg/m3 for the organic and inorganic fractions of condensable PM, respectively. At Site 2, the mass concentrations ranged from 1.9−16.6 mg/m3 and 2.8−18.6 mg/m3 for filterable PM2.5 and PM10, respectively. The mass concentrations ranged from 14.8−27.4 mg/m3 and 7.8−20.3 mg/m3 for the organic and inorganic fractions of condensable PM. For Stage 2, at Site 1, the mass concentrations ranged from 321.6−416.7 mg/m3 and 897.3−1105.3 mg/m3 for filterable PM2.5 and PM10, respectively. The mass concentrations ranged from 212.5−259.1 mg/m3 and 62.3−137.6 mg/m3 for the organic and inorganic fractions of condensable PM. At Site 2, the mass concentrations ranged from 2.2−10.8 mg/m3 and 3.3−12.4 mg/m3 for filterable PM2.5 and PM10, respectively. The mass concentrations ranged from 9.2−30.5 mg/m3 and 7.8−20.2 mg/m3 for the organic and inorganic fractions of condensable PM. C

DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Concentration Distributions of 16 PAHs in PM (μg/m3) Stage 1

Stage 2

Site 1

Site 1

Site 1

Site 2

Site 2

Site 1

Site 1

Site 1

Site 2

Site 2

compounds

FPM2.5

CPM

FPM2.5‑10

FPM2.5

CPM

FPM2.5

CPM

FPM2.5‑10

FPM2.5

CPM

Nap Acpy Acp Flu Phe Ant Fla Pye BaA Chr BbF BKF BaP IND DBA BghiP Total

0.01 0.02 0.00 0.01 0.85 0.85 2.36 1.63 1.68 1.68 1.20 1.20 0.25 0.14 0.08 0.09 12.04

0.38 0.07 0.05 0.20 0.52 0.54 0.25 0.18 0.40 0.27 0.33 0.33 0.33 0.00 0.00 0.00 3.85

0.01 0.02 0.01 0.02 0.51 0.51 1.40 0.93 0.66 0.66 0.61 0.60 0.25 0.22 0.09 0.20 6.68

0.00 0.00 0.00 0.00 0.01 0.01 0.03 0.03 0.06 0.06 0.04 0.04 0.11 0.06 0.02 0.02 0.49

0.04 0.01 0.01 0.03 0.25 0.26 0.17 0.11 0.10 0.24 0.14 0.14 0.05 0.05 0.00 0.05 1.64

0.03 0.01 0.01 0.01 0.09 0.09 0.17 0.13 0.42 0.42 0.42 0.42 0.10 0.07 0.06 0.06 2.52

0.85 0.19 0.28 0.81 1.75 1.80 0.39 0.33 0.73 0.63 0.87 0.83 0.79 0.00 0.00 0.00 10.23

0.01 0.00 0.00 0.01 0.04 0.04 0.07 0.05 0.05 0.05 0.13 0.12 0.08 0.07 0.00 0.05 0.77

0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.04 0.04 0.15 0.15 0.06 0.02 0.01 0.01 0.49

0.04 0.01 0.01 0.04 0.09 0.10 0.11 0.08 0.07 0.05 0.06 0.06 0.06 0.00 0.00 0.00 0.78

The average concentrations of filterable PM2.5 and PM10 were much lower than the concentrations of the total filterable PM at Site 1. At Site 2, the average concentrations of total filterable PM were 7.5 and 7.1 mg/m3 at the two stages. At Site 2, the filterable PM2.5 and PM10 accounted for 74% and 92% in Stage 1, and they accounted for 74% and 87% in Stage 2. At Site 1, the concentrations of condensable PM were much lower than the filterable PM, and the mass concentration of total condensable PM in Stage 2 was higher than in Stage 1. The result indicated that the boiler load had an obvious influence on condensable PM emissions. The good combustion at higher boiler load reduced the condensable PM formation. Compared to filterable PM, the average concentrations of condensable PM at Site 2 were much higher, at 32.6 and 34.8 mg/m3 for the two stages. In two stages, condensable PM occupied 81% and 83% of the total PM at Site 2, respectively. The result implied that condensable PM contributed significantly to the total PM emissions. At Site 2, the proportions of the organic fractions in condensable PM were 59.5% and 54.8% in the two stages. The proportions of the organic fractions were higher than the inorganic fractions. The removal efficiency of the LLT-ESP for filterable and condensable PM is shown in Figure 2. The removal efficiency for filterable PM2.5, PM10, and total FPM at Stage 1 was 98.8, 99.8, and 99.9%, respectively. The removal efficiency for filterable PM2.5, PM10, and total FPM at Stage 2 was 98.4, 99.8, and 99.9%, respectively. In some researches, the removal efficiency of dry ESP for filterable PM2.5 was among 95− 99%.21,22 Liu23 studied emission characteristics of filterable PM2.5 from a coal-fired boiler; the removal efficiency of the ESP for filterable PM2.5 was only 90.6%, which was not effective comparing with the results in this study.The results indicated that the LLT-ESP had a good removal effect for the filterable PM, especially for filterable PM2.5 and PM10. The removal efficiency for condensable PM was 77.1% and 89.6%, respectively, in two stages, which was relatively lower than the removal efficiency for filterable PM. The removal effect for condensable PM in Stage 1 was lower than that in Stage 2. The results implied that the lower operation temperature of the LLT-ESP led to the greater condensation

and adhesion of the condensable compounds and that the LLTESP had a better removal effect for condensable PM. In this study, the removal effect of the LLT-ESP for the organic fractions were better than for the inorganic fractions in the two stages. 3.2. PAH Concentrations and Distribution Characteristics in PM. The PAHs’ average concentration distributions in PM are shown in Table 3. In Stage 1 and Stage 2, the concentrations of total PAHs in filterable PM2.5 (FPM2.5), filterable PM2.5‑10 (FPM2.5‑10), and CPM at Site 1 were much higher than those at Site 2. However, the PAH concentration distributions were different at Site 1 in the two stages. In Stage 1, the total concentrations of PAHs in FPM2.5 and FPM2.5‑10 at Site 1 were 12.04 and 6.68 μg/m3, respectively. In Stage 2, the total concentrations of PAHs in FPM2.5 and FPM2.5‑10 at Site 1 were 2.52 and 0.77 μg/m3, respectively. In Stage 1, the total concentration of PAHs in CPM at Site 1 was 3.85 μg/m3. The total concentration of PAHs in CPM at Site 1 was 10.23 μg/m3 in Stage 2. In Stage 1, the concentrations of PAHs in FPM2.5 and condensable PM at Site 2 were 0.49 and 1.64 μg/m3, respectively. Accordingly, in Stage 2, the concentrations of PAHs in FPM2.5 and condensable PM at Site 2 were 0.49 and 0.78 μg/m3, respectively. The total concentrations of PAHs in CPM at Site 1 in Stage 2 were approximately 2.7 times the concentrations of PAHs in CPM at Site 1 in Stage 1. Accordingly, the average concentration of CPM at Site 1 in Stage 2 was approximately 2.4 times the concentration of CPM at Site 1 in Stage 1, which could explain the PAHs concentration differences in the CPM. A similar phenomenon could be found in the PAH distributions in filterable PM. The PAHs’ concentration distributions were related to the PM concentration distributions. The results also indicated that the boiler load seriously affected the emissions particle-associated PAHs at the outlet of the boiler. In this study, a lower boiler load increased the proportions of PAHs in the CPM, and a higher boiler load led to higher fractions of PAHs in the FPM. The stacked percentages of individual PAHs from the 16 particle-associated PAHs from two sampling sites in two stages are shown in Figure 3. Comparing (a) and (b) in Figure 3, 3-, D

DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 3. Stacked percentages of individual PAHs among 16 particle-associated PAHs in 4 conditions: (a) at Site 1 in Stage 1; (b) at Site 2 in Stage 1; (c) at Site 1 in Stage 2; (d) at Site 2 in Stage 2.

more 5-ring and 6-ring PAHs in the LLT-ESP. Correspondingly, the removal efficiency of the PAHs in CPM was similar to that of the organic fractions of CPM in the two stages. It could be concluded that the LLT-ESP had a greater influence on the PAHs distributions in FPM than in CPM. Former studies showed quite low removal efficiencies for PAHs by the electrostatic precipitator in other emission sources, such as iron ore sintering plant and medical waste incinerators.13,14 However, the LLT-ESP showed quite a good removal effect for most particle-associated PAHs in this study; the LLT-ESP could eliminate particle-associated PAHs effectively in the flue gas. In Yin’s study, the removed effect of the electrostatic precipitator for PAHs was selective at different boiler loads.25 In this study, the removal efficiencies for BaP and IND in FPM2.5 were lower than 60%, and the removal efficiency for most 4-rings PAHs in CPM was lower than 45% in Stage 1. Correspondingly, the removal efficiency for most 5-rings in FPM2.5 was lower than 65% in Stage 2. The results also showed the selective removal effect of the LLT-ESP for PAHs at different boiler loads, due to the different operation conditions and distribution characteristics of particle-associated PAHs at two stages. 3.3. Correlation between the Particle-Associated PAHs Content and PM. A mass concentration correlation between the mass concentrations of particle-associated PAHs and PM was conducted. The total numbers of particle-

4-, and 5-ring PAHs accounted for the main proportion in FPM2.5 and FPM2.5‑10 at Site 1, and the proportion of 5- and 6ring PAHs increased significantly in FPM2.5 at Site 2. From (a) and (b) in Figure 3, the proportion of the 2-ring PAHs in CPM decreased, and that of 6-ring PAHs in CPM increased. From (c) and (d) in Figure 3, the PAHs distribution in the FPM2.5, FPM2.5‑10, and CPM in Stage 2 was different from that in Stage 1. The proportion of Fla and Pye in FPM2.5 and FPM2.5‑10 was much lower in (c) than in (a). The proportions of 4- and 5-ring PAHs in the CPM were higher in (c) than those in (a). Comparing (c) and (d), the proportion of 5-ring PAHs in FPM2.5 increased significantly. The proportion of 4ring PAHs in the CPM increased in (c) over that in (d). From (b) and (d), the PAHs distribution characteristics in FPM2.5 and CPM were in accordance with previous studies that suggested the lower ring PAHs were found preferentially in the gas phase, whereas the proportion of higher ring PAHs was much higher in the particle phase.24−26 For the LLT-ESP, the removal efficiency for PAHs in FPM2.5 and CPM in Stage 1 was 86% and 87%, respectively. Accordingly, the removal efficiency for PAHs in FPM2.5 and CPM in Stage 2 was 81% and 92%, respectively. The removal efficiency for PAHs in FPM2.5 was lower than the removal efficiency for FPM2.5 in the two stages. The result indicated that the PAHs mass concentration in FPM2.5 increased while the flue gas passed through the LLT-ESP. The FPM2.5 adsorbed E

DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels associated PAHs data (from Table 3) in linear fitting analysis for FPM2.5 & PAHs and CPM & PAH were 4, respectively. Figures 4 and 5 show the results of linear fitting analysis

concentrations of FPM2.5 in the LLT-ESP. However, the low R2 values indicated that more factors, not simply the mass concentration of FPM2.5, influenced the PAHs distributions in FPM2.5 as the flow gas passed through the LLT-ESP. The adsorption and condensation of gas phase PAHs, and the discharge process in the LLT-ESP, also affected the PAHs distributions in FPM2.5. For the correlation between the CPM and PAHs, the R2 values were 0.98698 for the T-PAHs, 0.98388 for the LMW, 0.97339 for the MMW, and 0.9854 for the HMW. The R2 values generally approached 1, thus showing a strong correlation between the mass concentrations of CPM and PAHs content. The results indicated that the LLT-ESP mainly influenced the mass concentrations of PAHs in CPM when the flue gas passed through the LLT-ESP. The concentrations and distributions of PAHs in CPM were strongly correlated with the concentration of CPM in the flow gas in the LLT-ESP. The correlation analysis results supported the above conclusion that the LLT-ESP had a greater influence on the PAHs distributions in the FPM than in the CPM. Considering that the proportion of 5-ring and 6-ring PAHs in FPM2.5 increased at Site 2 in both stages, it could also be concluded that some chemical reactions occurred, and some HMW generated and absorbed in FPM duo to the discharge process in the LLT-ESP. The emission factors of filterable and condensable PM, and the correlation between the particle-associated PAHs contents and PM obtained from the present study, might be helpful in understanding the removal effect of the LLT-ESP for different pollutants. However, the quantity of data in this study is limited. The removal mechanism on particle-associated PAHs by the LLT-ESP should be investigated in depth. More low-low temperature precipitators and operation conditions should be studied further.

Figure 4. Correlation between PAHs and filterable PM2.5.

4. CONCLUSIONS This study investigated the removal effect of the LLT-ESP for the filterable and condensable PM at two stages. The concentration and distribution characteristics of 16 U.S. EPA PAHs in the sampled filterable and condensable PM were investigated. The correlation between the particle-associated PAHs and PM was analyzed. The boiler load influenced the filterable PM, condensable PM, and PAHs emission characteristics. Condensable PM contributed significantly to the total PM emissions. The proportions of the organic fraction in condensable PM were higher than the proportions of the inorganic fraction at the outlet of the LLT-ESP in two stages. The LLT-ESP had a good removal effect for filterable PM, and the removal efficiency for condensable PM was relatively lower than filterable PM. The removal efficiency for filterable and condensable PM was higher when the LLT-ESP was operated at a lower temperature. The PAHs concentration and distribution characteristics in the filterable and condensable PM were different. The LLTESP markedly influenced the PAHs distributions in the filterable PM2.5. The proportions of 5- and 6-ring PAHs in the FPM2.5 increased significantly when the flue gas passed through the LLT-ESP. The 3- and 4-ring PAHs accounted for the main part in the CPM. A positive correlation between the mass concentrations of particle-associated PAHs and PM could be observed in this study. The mass concentrations of the CPM and the PAHs correlated significantly, with the R2 values approaching 1. The correlation analysis for filterable PM2.5 and PAHs indicated that the distribution of PAHs in filterable PM2.5

Figure 5. Correlation between PAHs and condensable PM.

between the mass concentrations of particle-associated PAHs and PM. In Figures 4 and 5, LMW means the lower molecular weight (2-ring and 3-ring PAHs), MMW means the middle molecular weight (4-ring PAHs), and HMW means the higher molecular weight (5-ring and 6-ring PAHs). Positive correlations can be observed between the mass concentrations of PAHs and PM. However, the coefficients of determination (COD, R2) for the PAHs & FPM2.5 and the PAHs & CPM were different, thus reflecting the correlation differences. The R2 values between the PAHs and FPM2.5 were generally lower than the R2 values between the PAHs and CPM. The result indicated that the mass concentrations of PAHs and CPM correlated more significantly than the mass concentration correlation between the PAHs and FPM2.5. For the correlation between FPM2.5 and the PAHs, the R2 values were 0.73404 for the total PAHs (T-PAHs), 0.70061 for the LMW, 0.69978 for the MMW, and 0.8414 for the HMW. It could be concluded that the mass concentrations of the HMW in FPM2.5 were more significantly related to the mass F

DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels was mainly related to the mass concentration of filterable PM2.5, but other factors also exerted an effect. The LLT-ESP had greater influence on the PAHs distributions in the FPM than in the CPM.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaodong Li: 0000-0002-5331-5968 Zhifu Qi: 0000-0003-3043-2894 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The research was supported by the Major State Basic Research Development Program of China (973 Program, No. 2011CB201500) and the Public Welfare Projects for Environmental Protection (No. 201209022).

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DOI: 10.1021/acs.energyfuels.7b00145 Energy Fuels XXXX, XXX, XXX−XXX