Emissions of Polycyclic Aromatic Hydrocarbons from Batch Hot Mix

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Research Emissions of Polycyclic Aromatic Hydrocarbons from Batch Hot Mix Asphalt Plants WEN-JHY LEE,† WEN-HUI CHAO,† MINLIANG SHIH,‡ CHENG-HSIEN TSAI,§ THOMAS JENG-HO CHEN,| AND P E R N G - J Y T S A I * ,⊥ Department of Environmental Engineering, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan, Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, 60 Eer-Jen Road, Sec. 1, Jen-Te, Tainan 717, Taiwan, Department of Chemical Engineering, National Kaohsiung University of Applied Science, 415 Chien-Kung Road, Kaohsiung 807, Taiwan, Department of Occupational Safety and Health, Chang Jung Christian University, 396 Sec. 1, Chang-Jung Road, Kway-Jen, Tainan 711, Taiwan, and Department of Environmental and Occupational Health, Medical College, National Cheng Kung University, 138 Sheng-Li Road, Tainan 70428, Taiwan

This study was set out to assess the characteristics of polycyclic aromatic hydrocarbon (PAH) emissions from batch hot mix asphalt (HMA) plants and PAH removal efficiencies associated with their installed air pollution control devices. Field samplings were conducted on six randomly selected batch HMA plants. For each selected plant, stack flue gas samples were collected from both stacks of the batch mixer (n ) 5) and the preheating boiler (n ) 5), respectively. PAH samples were also collected from the field to assess PAHs that were directly emitted from the discharging chute (n ) 3). To assess PAH removal efficiencies of the installed air pollution control devices, PAH contents in both cyclone fly ash (n)3) and bag filter fly ash (n ) 3) were analyzed. Results show that the total PAH concentration (mean; RSD) in the stack flue gas of the batch mixer (354 µg/Nm3; 78.5%) was higher than that emitted from the discharging chute (107 µg/Nm3; 70.1%) and that in the stack flue gas of the preheating boiler (83.7 µg/Nm3; 77.6%). But the total BaPeq concentration of that emitted from the discharging chute (0.950 µg/Nm3; 84.4%) was higher than contained in the stack flue gas of the batch mixer (0.629 µg/Nm3; 86.8%) and the stack flue gas of the preheating boiler () 0.112 µg/Nm3; 80.3%). The mean total PAH emission factor for all selected batch mix plants () 139 mg/ton‚product) was much higher than that reported by U.S. EPA for the drum mix asphalt plant (range ) 11.8-79.0 mg/ton‚product). We found the overall removal efficiency of the installed air pollution control * Corresponding author phone: +886-6-2088390; fax: +886-62088391; e-mail: [email protected]. † Department of Environmental Engineering, National Cheng Kung University. ‡ Chia-Nan University of Pharmacy and Science. § National Kaohsiung University of Applied Science. | Chang Jung Christian University. ⊥ Medical College, National Cheng Kung University. 5274

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devices (i.e., cyclone + bag filter) on total PAHs and total BaPeq were 22.1% and 93.7%, respectively. This implies that the installed air pollution control devices, although they have a very limited effect on the removal of total PAHs, do significantly reduce the carcinogenic potencies associated with PAH emissions from batch HMA plants.

Introduction Asphalt (CAS Registry No. 8052-42-4) is the product of the nondestructive distillation of crude oil in petroleum refining. Because of its adhesive properties, durability, flexibility, water resistance, and ability to form strong cohesive mixtures with mineral aggregates, it is widely used in hot mix asphalt (HMA) industries for producing paving materials (1). The operating procedures associated with the production of HMA paving materials can be classified into three categories: (a) drum mix, (b) continuous mix, and (c) batch mix (2). In the United States, ∼90% of HMA plants adopt the drum mix operation, while the batch mix and continuous mix account only for ∼10% and 90%) during the combustion process. The above results further confirmed the results obtained from this study could be theoretically plausible. We found that PAH contents in the stack flue gas of the batch mixer, stack flue gas of preheating boiler, and that emitted from the discharging chute were consistently dominated by LM-PAHs (accounting for 94.5%, 99.5%, and 80.7% of total PAHs, respectively) (see Figure 1). The above results were not so surprising since either thermal mixing or combustion were involved in these processes. But it should be noted that PAH homologue distributions of LM-PAHs: MM-PAHs:HM-PAHs for the stack flue gas of the batch mixer () 94.5%:2.54%:2.94%) were quite different from that emitted from the discharging chute () 80.7%:2.68%:16.7%); even both shared the same pollution source (i.e., batch mixer) (see Figure 1). Here, it should be noted that the direction of the airflow emitted from the discharging chute (i.e., moved downward) was quite different from that of the stack flue gas (i.e., moved upward). Because both MM- and HM-PAHs were heavier than LM-PAHs, and hence it can be expected that the fractions of MM- and HM-PAHs contained in the stack flue gas were lower than that emitted from the discharging chute due to the gravitational effect. For the same reason, it can be found that the fraction of LM-PAHs contained in

TABLE 3. Emission Concentrations of PAHs in the Stack Flue Gas of the Batch Mixer (n ) 30), Stack Flue Gas of the Preheating Boiler (n ) 30), and That Emitted Directly from the Discharging Chute (n ) 18) batch mixer

preheating boiler

discharging chute

compound

mean (µg/Nm3)

range (µg/Nm3)

RSD (%)

mean (µg/Nm3)

range (µg/Nm3)

RSD (%)

mean (µg/Nm3)

range (µg/Nm3)

RSD (%)

Nap AcPy Acp Flu Ant PA FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC BghiP COR total PAHs LM-PAHs MM-PAHs HM-PAHs total BaPeq

308 12.7 3.38 5.94 2.50 1.79 3.52 4.41 7.99 1.04 1.38 0.132 0.082 0.176 0.083 0.069 0.076 0.039 0.113 0.188 0.114 354 334 8.97 10.4 0.629

72.8-830 0.105-39.5 0.314-8.99 0.618-28.2 0.007-11.2 0.017-7.41 0.736-14.8 0.200-18.1 0.011-41.9 0.011-3.33 0.029-4.80 ND-0.506 ND-0.395 ND-0.791 ND-0.443 ND-0.342 ND-0.545 ND-0.176 ND-0.521 ND-0.945 ND-1.02 90.5-928 96.8-906 6.54-13.1 5.48-16.8 0.242-0.987

81.7 85.5 69.9 94.6 95.6 145 99.4 107 145 110 107 113 117 122 161 152 134 122 136 131 129 78.5 79.8 67.9 90.5 86.8

76.3 0.540 0.467 1.93 4.02 0.028 0.195 0.067 ND 0.026 0.005 0.009 ND 0.009 0.004 0.006 0.015 0.020 ND 0.019 0.004 83.7 83.3 0.288 0.091 0.112

19.7-180 0.168-1.03 0.164-1.02 0.025-3.57 2.80-5.41 0.022-0.037 0.187-0.207 0.060-0.074 ND 0.019-0.036 ND-0.017 ND-0.020 ND ND-0.021 ND-0.019 ND-0.019 ND-0.039 ND-0.052 ND ND-0.055 ND-0.016 28.6-190 34.6-206 0.156-0.398 0.076-0.135 0.068-0.213

83.1 61.6 72.0 68.3 24.0 22.3 39.4 79.4

59.8 11.3 1.95 8.18 4.30 0.906 0.387 2.16 12.2 0.323 0.514 0.235 0.103 0.397 0.284 0.265 0.353 0.461 2.42 0.162 0.505 107 86.4 2.87 17.9 0.950

26.8-156 1.03-27.4 0.075-4.21 0.923-24.5 2.31-7.43 0.765-1.856 0.098-0.987 0.890-8.67 6.83-26.3 0.065-1.267 0.062-2.86 0.091-0.913 ND-0.421 ND-1.45 ND-1.34 ND-1.30 ND-1.56 ND-2.34 0.789-7.96 ND-0.876 ND-2.31 32.1-167 36.7-178 0.897-7.03 7.67-29.8 0.358-1.320

89.8 78.2 78.1 68.3 40.5 69.8 78.2 91.2 67.5 98.8 101 89.0 88.2 103 112 102 90.5 102 92.5 99.6 128 70.1 69.8 77.2 88.6 84.4

the stack flue gas was much higher than that emitted from the discharging chute. In addition, it also should be noted that PAHs with higher molecular weights are known with higher carcinogenic potencies. Based on this, it can be found that total BaPeq concentrations for that emitted from the discharging chute (mean ) 0.950 µg/Nm3; RSD ) 84.4%) were higher than that exhausted from the stack flue gas of the batch mixer (mean ) 0.629 µg/Nm3; RSD ) 86.8%). The high total BaPeq concentrations emitted from the discharging chute indicate the importance of assessing PAH exposures for workers in batch HMA plants in the future. Finally, we also found that PAH homologue distributions of the stack flue gas of the preheating boiler (LM-PAHs:MM-PAHs:HMPAHs ) 99.5%:0.34%:0.011%) were significantly different from that of the other two emission sources. This was mainly due to the intrinsic differences in their involved industrial processes (i.e., heavy oil combustion vs asphalt hot mixing process). On the other hand, we also found that PAH homologue distributions of the stack flue gas of the preheating boiler were quite similar to that contained in the stack flue gas of boilers fueled with heavy oil (21).

FIGURE 1. PAH homologue distributions of the three investigated emission sources in batch mix plants.

23.7 147 79.8 101 138 122 89.6 110 111 127 77.6 86.3 56.2 78.7 80.3

Importance of PAH Emissions from Batch Mix Plants. Table 4 shows the mean emission rates of the batch mixer, preheating boiler, and discharging chute of the investigated batch mix plants. For total PAHs, this study yielded mean emission rates 125 mg/min (RSD ) 78.5%), 0.837 mg/min (RSD ) 77.6%), and 9.64 mg/min (RSD ) 70.1%) for the batch mixer, preheating boiler, and discharging chute, respectively. The above results were not so surprising since the batch mixer had higher mean total PAHs concentration and flow rate () 354 µg/nm3 and 353 m3/min) than that of the preheating boiler () 83.7 µg/nm3 and 10.0 m3/min) and the discharging chute () 107 µg/nm3 and 89.9 m3/min) (see Tables 1 and 3). In the United States, PAH emissions from HMA manufacturing were simply focused on the drum mix process. This is mainly because ∼90% HMA products were manufactured by drum mix plants (2). But it should be noted that HMA plants with the batch mix design are widely used in many other countries because of the simplicity of the involved manufacturing processes. In Taiwan, all HMA products are manufactured through the batch mix process. Table 5 shows the calculated emission factors for the investigated batch mix plants. For total PAHs, mean emission factors of 128, 0.860, and 9.90 mg/ton‚product were found for the batch mixer, preheating boiler, and discharging chute, respectively. Because PAH emissions from batch mix plants have never been reported before, PAH emissions from the drum mix process were adapted in this study for comparisons (Table 5). We found that the PAH emission factor (i.e., batch mixer + preheating boiler + discharging chute) obtained from this study for batch mix plants () 139 mg/ton‚product) was much higher than that reported by U.S. EPA (range ) 11.8-79.0 mg/ton‚product) for drum mix plants. Yet, it is true that the values reported by U,S, EPA did not include the emission from the discharging chute. Nevertheless, the emission factor of total PAHs obtained from this study for the batch mixer ()128 mg/ton‚product) was still much higher than that reported by U.S. EPA for the entire drum mix plant. Moreover, we also found that the mean total BaPeq emission factor obtained from this study for the batch mix plant () 0.317 mg/ton‚product) was also higher than that for the drum mix VOL. 38, NO. 20, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 4. Emission Rates of PAHs from the Stacks of the Batch Mixer (n ) 30), Stacks of the Preheating Boiler (n ) 30), and That Emitted Directly from the Discharging Chute (n ) 18) batch mixer

preheating boiler

discharging chute

compound

mean (mg/min)

range (mg/min)

RSD (%)

mean (mg/min)

range (mg/min)

RSD (%)

mean (mg/min)

range (mg/min)

RSD (%)

Nap AcPy Acp Flu Ant PA FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC BghiP COR total PAHs LM-PAHs MM-PAHs HM-PAHs total BaPeq

109 4.48 1.19 2.10 0.883 0.632 1.24 1.55 2.82 0.367 0.487 0.047 0.029 0.062 0.029 0.024 0.027 0.014 0.040 0.066 0.040 125 118 3.17 3.69 0.222

25.7-293 0.037-13.9 0.111-3.17 0.218-9.95 0.002-3.95 0.006-2.62 0.260-5.22 0.071-6.39 0.004-14.8 0.004-1.18 0.010-1.69 ND-0.179 ND-0.139 ND-0.279 ND-0.156 ND-0.121 ND-0.192 ND-0.062 ND-0.184 ND-0.334 ND-0.360 31.9-328 34.2-320 2.31-4.62 1.93-5.93 0.085-0.348

81.7 85.5 69.9 94.6 95.6 145 99.4 107 145 110 107 113 117 122 161 152 134 122 136 131 129 78.5 79.8 67.9 90.5 86.8

0.763 0.005 0.005 0.019 0.040 0.000 0.002 0.001 ND 0.000 0.000 0.000 ND 0.000 0.000 0.000 0.000 0.000 ND 0.000 0.000 0.837 0.833 0.003 0.001 0.001

0.197-1.80 0.002-0.010 0.002-0.010 0.000-0.036 0.028-0.054 0.000-0.000 0.002-0.002 0.001-0.001

83.1 61.6 72.0 68.3 24.0 22.3 39.4 79.4

0.000-0.000 ND-0.000 ND-0.000

23.7 147 79.8

0.000-0.000 0.000-0.000 0.000-0.000 0.000-0.000 0.000-0.001

101 138 122 89.6 110

0.000-0.001 0.000-0.000 0.286-1.900 0.346-2.06 0.002-0.004 0.001-0.001 0.001-0.002

111 127 77.6 86.3 56.2 78.7 80.3

5.37 1.01 0.175 0.736 0.386 0.081 0.035 0.194 1.10 0.029 0.046 0.021 0.009 0.036 0.026 0.024 0.032 0.041 0.218 0.015 0.045 9.63 7.77 0.258 1.61 0.085

2.41-14.0 0.093-2.46 0.007-0.378 0.083-2.20 0.208-0.668 0.069-0.167 0.009-0.089 0.080-0.779 0.614-2.36 0.006-0.114 0.006-0.257 0.008-0.082 ND-0.038 ND-0.130 ND-0.120 ND-0.117 ND-0.140 ND-0.210 0.071-0.716 ND-0.079 ND-0.208 2.89-15.0 3.30-16.0 0.081-0.632 0.690-2.68 0.032-0.119

89.8 78.2 78.1 68.3 40.5 69.8 78.2 91.2 67.5 98.8 101 89.0 88.2 103 112 102 90.5 102 92.5 99.6 128 70.1 69.8 77.2 88.6 84.4

TABLE 5. Emission Factors of PAHs Obtained from This Study for the Batch Mixer, Preheating Boiler, and Discharging Chute of the Selected Batch Mix Plants and that Reported by U.S. EPA for Drum Mix Plants (unit ) mg/ton‚products) drum mix planta-c

batch mix plant compound

batch mixer

preheating boiler

discharging chute

total

U.S. EPA (1998)a

U.S. EPA (1998)b

U.S. EPA (1994)a

U.S. EPA (1994)c

U.S. EPA (1994)a

Nap AcPy Acp Flu Ant PA FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC BghiP COR total PAHs LM-PAHs MM-PAHs HM-PAHs total BaPeq

112 4.61 1.23 2.15 0.907 0.649 1.28 1.60 2.90 0.377 0.500 0.048 0.030 0.064 0.030 0.025 0.028 0.014 0.041 0.068 0.041 128 121 3.25 3.79 0.228

0.784 0.006 0.005 0.020 0.041 0.000 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.860 0.856 0.003 0.001 0.001

5.52 1.04 0.180 0.756 0.397 0.084 0.036 0.199 1.13 0.030 0.048 0.022 0.010 0.037 0.026 0.024 0.033 0.043 0.224 0.015 0.047 9.90 7.98 0.265 1.65 0.088

118 5.65 1.41 2.93 1.34 0.733 1.31 1.80 4.02 0.407 0.548 0.070 0.039 0.101 0.056 0.049 0.060 0.057 0.265 0.083 0.088 139 130 3.52 5.44 0.317

21.0 0.430 0.620 0.980 1.60 0.150 0.160 0.031 NAd 0.0023 0.0031 0.0023 0.0012 NAd NAd NAd NAd NAd NAd NAd NAd 25.0 24.8 0.193 0.007 0.027

22.0 NAd NAd NAd NAd 18.0 12.0 27.0 NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd 79.0 40.0 39.0 0.000 0.241

27.0 0.680 0.950 1.34 1.90 0.260 0.255 0.037 NAd 0.0023 0.0031 0.0023 NAd NAd NAd NAd NAd NAd NAd NAd NAd 32.4 32.1 0.294 0.005 0.035

22.0 NAd NAd NAd 18.0 NAd 12.0 27.0 NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd NAd 79.0 40.0 39.0 0.000 0.079

9.50 1.60 0.290 0.330 NAd 0.044 0.022 0.024 NAd NAd NAd 0.011 0.012 NAd NAd NAd NAd NAd NAd NAd NAd 11.8 11.8 0.046 0.023 0.015

a Fuel used for the preheating boiler: natural gas. heavy oil. d NA: not available.

b

Fuel used for the preheating boiler: combustion oil. c Fuel used for the preheating boiler:

plant (range ) 0.015-0.241 mg/ton‚product) (Table 5). The above results clearly indicate that PAH emissions from batch mix plants could be quite significant, especially for those countries with batch mixers widely used for HMA productions. PAH Removal Efficiencies of Air Pollution Control Devices Used in Batch Mix Plants. Table 6 shows PAH contents in both cyclone fly ash and bag filter fly ash for those samples collected from the six studied batch mix plants. 5278

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We found that the total PAHs content in cyclone fly ash (mean ) 2800 ng/g; RSD ) 42.4%) was significantly lower than that contained in bag filter fly ash () 4900 ng/g; RSD ) 46.8%). It is known that particulate matters separated by cyclones were coarser than those filtered by bag filters. In principle, a particulate matter with a smaller particle size would result in a greater PAH adsorption surface area. Based on this, it is not so surprising to see that total PAHs content in cyclone fly ash was significantly lower than that contained in bag

TABLE 6. PAH Contents Contained in the Cyclone Fly Ash (n ) 18) and Bag Filter Fly Ash (n ) 18) and PAH Removal Efficiencies for both Cyclone and Bag Filter Installed on the Batch Mixer cyclone fly ash

bag filter fly ash

removal efficiency (%)

compound

mean (ng/g)

range (ng/g)

RSD (%)

mean (ng/g)

range (ng/g)

RSD (%)

cyclone

bag filter

overall

Nap AcPy Acp Flu Ant PA FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC BghiP COR total PAHs LM-PAHs MM-PAHs HM-PAHs total BaPeq

179 59.4 62.0 137 152 138 139 131 180 172 184 167 141 118 75.3 92.6 119 70.4 156 197 129 2800 728 442 1630 212

113-206 26.0-84.3 45.8-71.9 122-145 121-173 101-162 128-149 122-139 138-210 145-189 102-267 122-189 101-176 78.2-154 45.3-112 71.1-123 67.9-152 33.2-82.3 89.9-201 143-242 79.9-168 2030-3670 453-867 346-501 960-2370 139-278

33.5 54.9 27.0 11.9 30.5 49.8 21.7 18.1 57.0 38.1 76.8 37.5 51.7 60.1 69.7 53.0 53.4 44.4 78.9 40.0 56.1 42.4 41.3 38.9 60.4 51.2

344 152 121 170 171 160 230 271 290 225 237 270 227 184 155 290 377 276 232 265 256 4900 1120 726 3060 548

210-508 63-259 78.5-155 134-220 143-209 101-234 145-302 223-324 129-438 112-294 155-302 223-321 156-299 111-255 101-223 156-396 234-450 180-379 187-276 167-345 176-323 3400-6320 689-1970 465-967 2010-4690 345-678

69.4 66.4 48.9 41.6 38.5 53.8 51.6 48.3 67.9 48.7 564 45.5 56.3 54.1 59.8 65.3 63.9 76.4 43.9 52.3 49.8 46.8 43.6 56.3 64.7 61.6

1.02 6.96 19.4 24.3 39.1 42.0 29.8 24.7 22.8 48.1 46.5 51.0 51.2 50.7 44.8 35.4 35.2 30.7 53.0 55.0 45.9 11.1 3.64 32.6 41.2 37.9

1.13 10.9 26.8 22.6 41.2 47.8 40.1 38.6 27.1 68.9 63.7 95.4 96.6 91.4 95.0 97.7 98.1 98.6 95.5 93.5 95.8 12.4 3.31 45.3 75.0 89.9

2.14 17.1 41.0 41.5 64.2 69.7 57.9 53.7 43.7 83.8 80.6 97.8 98.3 95.8 97.2 98.5 98.7 99.1 97.9 97.1 97.7 22.1 6.8 63.1 85.3 93.7

filter fly ash. Moreover, we found PAH contents in both cyclone and bag filter fly ash were consistently lower than that contained in ESP fly ash for samples collected from the two medical waste incinerators in our previous research work () 13800 ng/g and 47000 ng/g, respectively) (6). Obviously, the above differences could be due to the intrinsic differences in their involved industrial processes (i.e., asphalt hot mixing process vs medical waste incineration). We also found that the resultant PAH homologue distribution for cyclone fly ash (LM-PAHs:MM-PAHs:HM-PAHs ) 26.0%:15.8%:58.2%) was quite similar to that for bag filter fly ash () 22.8%:14.8%:62.3%). This indicates that PAH compositions in both cyclone and bag filter fly ash could be quite similar. Because of this, it is not so surprising to see that the mean total BaPeq content in bag filter fly ash (mean ) 548 ng/g; RSD ) 61.6%) was higher than that in cyclone fly ash (mean ) 212 ng/g; RSD ) 51.2%), which was consistent with what we found on total PAHs contents contained in both types of fly ash. In this study, all studied batch mixers were known with the same type of air pollution control device (i.e., a cyclone and bag filter installed in series). Therefore, PAH removal efficiencies for both cyclone and bag filter can be determined according to the following

ηc ) [CcQc/(CcQc + CbQb + CsQsTb)] × 100% ηb ) [CbQb/(CbQb + CsQsTb)] × 100% ηo ) [(CcQc + CbQb)/(CcQc + CbQb+ CsQsTb)] × 100% where ηc is the PAH removal efficiency for cyclone (%), ηb is the PAH removal efficiency for bag filter (%), ηo is the overall PAH removal efficiency (i.e., cyclone + bag filter) (%), Cc is the PAH contents in cyclone fly ash (ng/kg), Qc is the fly ash collecting rate for cyclone (kg/batch), Cb is the PAH contents in bag filter fly ash (ng/kg), Qb is the fly ash collecting rate for bag filter (kg/batch), Cs is the PAH concentration contained in the stack flue gas (ng/Nm3), Qs is the stack flue gas flowrate rate for the batch mixer (Nm3/min), and Tb is the detention time for each batch (min/batch).

As shown in Table 6, the removal efficiencies of MMPAHs and HM-PAHs for cyclone () 32.6% and 41.2%, respectively) were consistently lower than those corresponding for bag filter () 45.3% and 75.0%, respectively). The above results can be explained by the following: (a) most MM- and HM-PAHs contained in PAHs were presented in the form of the particle phase, and (b) the particle removal efficiency for the bag filter was higher than that for the cyclone. Because most total BaPeq were mainly contributed by both MM- and HM-PAHs, and hence the removal efficiency of total BaPeq for cyclone () 37.9%) was also lower than that for bag filter ()89.9%). On the other hand, it should be noted that both control devices of the cyclone and bag filter were designated for the removal of particulates. Since LM-PAHs were mainly presented in the form of gaseous phase, hence we found that the resultant LM-PAHs removal efficiencies for both cyclone () 3.64%) and bag filter () 3.31%) were quite low in common. Yet, it is true that fly ash collected by both cyclone and bag filter might act as an adsorbent during the manufacturing process. On the other hand, LM-PAHs carried by particles and collected in any of the control devices may subsequently be released in gaseous form after particle collection. In this study, although both control devices were installed for the removal of particulates, the low LM-PAHs removal efficiency still suggests that the extent associated with the adsorption of LM-PAHs could be slightly higher than the extent that was associated with the release of LM-PAHs from any of the control devices. We found that the overall removal efficiency of the installed air pollution control devices on total PAHs ()22.1%) was much lower than that on total BaPeq ()93.7%). In principle, the above results were quite consistent with the results obtained from our previously study conducted on two medical waste incinerators (total PAHs ) 15.2% and 15.4%, respectively; total BaPeq ) 83.6% and 84.2%, respectively) (6). Therefore, it is concluded that the installed air pollution control devices, although having a very limited effect on the removal of total PAHs, do significantly reduce the carcinogenic potencies associated with PAH emissions from batch hot mix asphalt plants. Here, it should be noted that both cyclone and bag filter are also widely used in drum mix VOL. 38, NO. 20, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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plants for the removal of particulates (2). Considering the involved processing materials in both batch mixer and drum mixer were quite similar, our results suggest that both control devices could be also helpful for reducing the emission of carcinogenic potencies arising from drum mix plants. Finally, it should be noted that PAH emissions from the discharging chute were not properly controlled in all selected plants. In particular, the mean total BaPeq concentration emitted from the discharging chute () 0.950 µg/Nm3) was much higher than that exhausted from the stack flue gas of the batch mixer () 0.629 µg/Nm3) (Table 3). This study suggests that the installation of a local exhaust hood on top of the discharging chute might provide an effective way to reduce the carcinogenic potencies arising from PAH emissions from the batch mixer to the workplace atmosphere.

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Received for review December 23, 2003. Revised manuscript received July 21, 2004. Accepted July 29, 2004. ES035455D