Emission Characteristics of Heavy Metals and Their Behavior During

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Emission Characteristics of Heavy Metals and Their Behavior During Coking Processes Ling Mu, Lin Peng,* Xiaofeng Liu, Huiling Bai, Chongfang Song, Ying Wang, and Zhen Li College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China S Supporting Information *

ABSTRACT: Besides organic pollutants, coke production generates emissions of toxic heavy metals. However, intensive studies on heavy metal emissions from the coking industry are still very scarce. The current work focuses on assessing the emission characteristics of heavy metals and their behavior during coking. Simultaneous sampling of coal, coke, residues from air pollution control devices (APCD), effluent from coke quenching, and fly ash from different processes before and after APCD has been performed. The total heavy metal concentration in the flue gas from coke pushing (CP) was significantly higher than that from coal charging (CC) and combustion of coke oven gases (CG). Emission factors of heavy metals for CP and CC were 378.692 and 42.783 μg/kg, respectively. During coking, the heavy metals that were contained in the feedstock coal showed different partitioning patterns. For example, Cu, Zn, As, Pb, and Cr were obviously concentrated in the inlet fly ash compared to the coke; among these metals Cu, As, and Cr were concentrated in the outlet fly ash, whereas Zn and Pb were distributed equally between the outlet fly ash and APCD residue. Ni, Co, Cd, Fe, and V were partitioned equally between the inlet fly ash and the coke. Understanding the behavior of heavy metals during coking processes is helpful for the effective control of these heavy metals and the assessment of the potential impact of their emissions on the environment.

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combustion of coke oven gas in the battery flues (CG). Previous studies of coking emissions have primarily focused on organic compounds,15−17 and little information about the emission characteristics of heavy metals from coking is available. Moreover, during the utilization of coal, such as in combustion and coking, heavy metals can redistribute in the bottom ash, the fly ash, and the gaseous phase.18 The phenomena involved in heavy metal partitioning are important because they determine how and where heavy metals are emitted. The behavior of heavy metals during coal combustion has been subject to intensive studies by numerous researches groups.19−25 It has been reported that partitioning of heavy metals among various combustion products depends on such factors as elemental volatility,23 initial concentration and the occurrence mode of metals in coal, design and operation conditions of the combustor, air pollution control devices,22 and the amount of chlorine in the flue gas.26 Because of this, the fate of heavy metals during coking might be different from that of coal combustion because of the differences in technological conditions involved in these two processes. The

INTRODUCTION Heavy metals have been associated with heart rate variability1 and increased cancer risk in people after long-term exposures in the environment.2 Source identification and quantification of heavy metals is one of the key steps for controlling their emission and effectively reducing further environmental exposure to them. Emission characteristics of heavy metals from coal combustion,3−8 transportation,9−11 and waste incineration12−14 have been studied extensively in recent years. However, investigations on unintentional release of heavy metals from potential sources and their fate during manufacturing processes are not sufficient for establishing an inventory of heavy metal emissions or for assessing the potential of the metals for emissions to the environment. Coking is an important coal conversion process. During coking, the prepared coal is charged into the oven and is then subjected to external heating to approximately 1000 °C in an oxygen-free atmosphere. The gases and hydrocarbons that evolve during the thermal distillation are sent to the byproduct plant for recovery and then combusted in the battery flues to heat the coal. At the end of the coking cycle, incandescent coke is pushed from the oven into a quench car, which carries the coke to a quench tower, where the coke is deluged with water to prevent it from burning after exposure to air. During these processes, heavy metals can be released from coal charging (CC), coke pushing (CP), coke quenching (CQ), and © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6425

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effective control of heavy metal emissions requires an understanding of their fate through the various coal conversion processes and downstream air pollution control devices (APCD).18 Although the distribution of several heavy metals (Hg, As, Pb, and Cr) in product and byproduct has been investigated,27 the real behavior of heavy metals during coking is not known because of the difficulties in sampling and complexities in coke production. The main objectives of the present study were to determine (a) the emission characteristics of heavy metals from the main sources involved in coke production and (b) the behavior of heavy metals during the coking process. To this end, the contents of heavy metals in the feedstock (coal), pyrolysis products (coke), APCD residues, effluent from coke quenching, inlet/outlet fly ash of APCD for CC and CP and fly ash for CG from a typical coke plant were determined. The outcome of this study is expected to provide a heavy metal inventory for further research.

Figure 1. Heavy metal sampling sites (A−I) during the coking process.

quench tower (site B), the coke (site C), fly ash from the combustion stack for CG (site D), inlet and outlet fly ash of the APCD for CC (sites E and F, respectively) and CP (site G and H, respectively), and ash from the APCD collector hopper (APCD residues, site I). Samplings of solid and liquid samples were carried out corresponding to the sampling of stack flue gas. All the experiments for each sampling point were repeated at least three times to make sure that the results were reproducible. For solid material sampling (at sites A, C, and I), 100−150 kg of feedstock coal, coke and APCD residues were collected from five randomly selected locations at each site for each sample. To obtain representative samples, these had to be quartered and ground into particles of sizes less than 154 μm for each type of sample.13 In addition, the samples (200 mL for each sample) of wastewater from quenching were collected using glass bottles (pretreated with 10% nitric acid, rinsed with distilled water). Fly ash samples were collected by an isokinetic sampling system (TH-880F, Tianhong Intelligent Instrument Plant of Wuhan, China) consisting of a coaxial dust sampling probe with a filter holder, a pump, a calculating box and a gas flow meter. The maximum gas-sampling rate was 80 L/min. The velocity of flue gas was calculated by the difference between total pressure and static pressure. The nozzle size of the coaxial dust collection tube was adjusted according to the flue gas velocity. Behind the nozzle, tube silica glass fiber filters (Whatman GF25 × 90 mm) were placed to collect fly ash and heavy metals. The operating conditions of this equipment are presented in detail in the Supporting Information. Particle sampling of each flue stack was conducted by U.S.EPA Method 5.28 Proper stack position was selected, and then the sampling probe was put into the stack sampling point. About 2−5 min was required for each run according to the actual operating conditions of CC and CP, and each sample includes at least three runs. The weights of particulate matter collected on the filters were determined by weighing the filters before and after sampling under the same conditions (25 °C, 50% RH, 48 h). Chemical Analysis. Each filter sample (after being broken into pieces) was placed in a Teflon vessel and extracted initially with concentrated acid solutions (0.5 mL HNO3 and 1 mL HF). Second, it was placed into a stainless steel vessel, which was then covered and then put into an oven for 24 h at 190 °C (total digestion). After cooling to room temperature, the sample in the Teflon vessel was boiled to dryness on a hot plate (190 °C). Five mL HNO3 was added to the vessel for another

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EXPERIMENTAL SECTION The Coke Plant and Its Operating Condition. The whole study was conducted on a typical coke plant in China with a daily output of 1728 tons, and the coking time and temperature were 36 h and 1270 °C, respectively. The height of the coke oven is 4.3 m. The coal used for coking in this plant was a blend of high sulfur coal (30%), coking coal (35%) and lean coal (35%). The technique of stamp charging was applied in coal charging, in which the prepared coal was tamped into a large briquette before charging into the ovens. A bag filter (BF) was applied for the purpose of removing particulate matter from the stack flue gas from charging and coke oven pushing. For this selected plant, the coke oven gas after recovery of chemical products was combusted to heat the coal, and we found that no APCD was installed for the combustion stack. Wet quenching was used for preventing the coke from burning after exposure to air. The basic information is listed in Table 1, and detailed descriptions of the production operation for the selected coke plant can be found in the Supporting Information. Sample Collection. Samples were collected from the nine streams (A−I) during the coking process as shown in Figure 1. The streams included coal (site A), wastewater from the Table 1. Specified Operational Conditions of the Selected Coke Planta facility type of coke oven height of ovens (m) feedstock (kg batch−1) composition of raw coal technique of coal charging operating time per year (h) air pollution control device flow rate of the stack flue gas (m3/s) coking time (h) coking temp. (°C) way of coke quenching daily output (ton)

background information JL4350D 4.3 22000 high sulfur coal (30%), coking coal (35%) and lean coal (35%) stamp charging 8760 bag filter 159.62, 147.41, and 343.66 for CC, CP and CG 36 1270 wet quenching 1728

a

CC: emission from coke oven charging. CP: emission from coke oven pushing. CG: emission from combustion of coke oven gas. 6426

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on the PM as the flue gas cools during pushing. Considering this behavior, it is not so surprising to see that the total heavy metal concentration in the stack flue gas for CP was significantly higher than it was for CC and CG (P < 0.05). This distribution of heavy metals in the stack flue gases (CP > CC≈CG) was quite different from that of organic compounds (CC > CG > CP)36 because of their different generative mechanisms during coking. Thus, to effectively control the pollutants from coking, the emphasis of pollution control for heavy metals and organic compounds should be placed on different production procedures. Cu, Zn, As, Pb, Cr, Ni, and Co were found to be the most abundant species, and the sum of these seven elements accounted for an average 98.74%, 99.31%, and 97.05% of the measured metals for CC, CP, and CG, respectively. These heavy metals were major components of raw coal (see Table 3), and the higher concentrations in the flue gas were predominantly attributed to their volatilization during coking. With the exception of Cd and Fe, heavy metal concentrations in fly ash from coke production were higher than those from a coal-fired power plant37 despite the lower content of heavy metals in coal used for coking. The relatively high heavy metal emissions from coking can be attributed to the different operating conditions and the effect of APCD. Distribution of Heavy Metals and Their Behavior during Coking. The heavy metal contents in coal collected at Site A may provide the inherent metal information in the feedstock materials. Co, Ni, and Zn were found to be the most abundant metals in the coal (Table 3). It has been reported that there are five major genetic types to account for the enrichment of heavy metals in coal based on Chinese coal geology, including source-rock controlled type, sedimentation-controlled type, magmatic-controlled type, fault-controlled type and groundwater-controlled type.38 Therefore, the predominance of Co, Ni, and Zn in the coal used in this coke plant was attributed to one or more of the factors mentioned above. Furthermore, it should also be noted that although these elements dominated in the coal used for coking, they occur in concentrations lower than those used in power plants because of their different compositions (i.e., blending coal vs high ash and low sulfur bituminous coal).37 Compared with the feedstock coal, the coke formed in the destructive distillation had lower heavy metal concentrations. Besides bottom ash (coke), fly ash was an important emission stream during the coal conversion. Heavy metal contents in the fly ash and APCD residues can be used to directly examine the effect of APCD and indirectly explore the metal concentration effect on fly ash.33 As shown in Table 3, all the heavy metal contents (except Cr, Ni, Co, and Cd) in the outlet fly ash of APCD for CC were significantly lower than those for CP (shared the same BF with CP), corresponding to the distribution of heavy metal concentrations (Table 2, P < 0.05). On the other hand, the metal contents (except Fe) for CC were significantly higher than those for CG, despite the fact that CC had lower concentrations (P < 0.05). It is known that the stack flue gas from CC was treated with BF, and it is expected that the mean particle sizes of the fly ash for CC are smaller than those for CG because of the high efficiency of larger particles by BF. Since heavy metals concentrated more on smaller particles because of their greater surface area per unit weight,39 the higher content of heavy metals for CC can be explained. In addition, we have found that Zn and Pb had higher content in the APCD residues, and the ratios of APCD

treatment and kept at 130 °C for 3 h. The solution was cooled and transferred to a clean plastic bottle. The extracted solution (3 mL) was transferred into a Teflon vial, and finally it was diluted to 50 mL with distilled water. This sample digestion method has been successfully used in several earlier studies.29,30 Heavy metals in solid samples were also extracted into acid solution using the same procedure, and the wastewater samples were acidified to pH < 2 with 10% HNO3 after filtration.31 Chemical analysis was performed by inductively coupled plasma-mass spectrometry (ICP-AES) (Thermo Electron),32−35 and 10 heavy metals (Cu, Zn, As, Pb, Cr, Ni, Co, Cd, Fe, and V) were analyzed. The measuring conditions in the determination of these heavy metals by ICP-AES are provided in the Supporting Information. To ensure the accuracy and reliability of this analysis, two standard reference materials (Soil Standard Series: GBW 07408 for ocher and water NIST 1640) were digested and analyzed by the same procedure, and the recovered values (85.9−101%) for all the target elements fell into the range of the certified values. The recovery efficiency was 92.4−107.5% when solutions with known heavy metal concentrations were used. Background contamination was routinely monitored by using operational blanks, which were processed simultaneously with the field samples. Reproducibility was examined using standard solutions of 1 and 10 ppm. If the difference between the standards and the calibrated values was more than 5%, the apparatus was recalibrated. The relative standard deviations of duplicate analyses were within 0.30−2.86%, below the control level of 5%.

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RESULTS AND DISCUSSION Heavy Metal Concentrations in the Stack Flue Gases. It can be seen from Table 2 that the concentrations of total Table 2. Average Concentrations of Heavy Metals in the Stack Flue Gas from Coal Charging, Coke Pushing and Combustion of Coke Oven Gas (μg/m3, n = 3)a coal charging

a

coke pushing

combustion of coke oven gas

heavy metals

mean

sd

mean

sd

mean

sd

Cu Zn As Pb Cr Ni Co Cd Fe V sum

12.094 0.600 1.697 0.532 25.336 2.817 5.997 0.060 0.003 nd 49.136

2.133 1.040 1.492 0.802 9.438 3.202 5.110 0.013 0.002 nd 16.723

249.023 212.845 76.747 102.201 226.713 14.181 53.730 0.344 0.695 5.469 941.948

5.199 5.736 1.691 2.050 3.176 1.979 6.871 0.370 0.303 2.374 124.305

12.251 0.633 2.298 0.224 25.512 1.451 8.962 0.114 0.113 1.333 52.891

0.805 1.096 0.428 0.388 5.450 2.513 10.760 0.164 0.068 2.308 12.091

sd: Standard deviation; nd: not detectable

heavy metals in the stack flue gas for CP (941.948 μg/m3) were 19.17- and 17.81-fold higher than for CC (49.136 μg/m3) and CG (52.891 μg/m3). It is known that when the incandescent coke is pushed from the oven into a quench car, substantial amounts of granular coke, which contains some heavy metals, is emitted because of the collision. In addition, CP is a significant source of particulate matter (PM) emissions, and some volatile elements exiting in the coke oven gas may condense or adsorb 6427

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Table 3. Average Contents of Heavy Metals in the Feedstock Coal, Coke Products, Fly Ash, APCD Residues and Effluent from Quenching in the Coke Plant (μg/g, n = 3)a heavy metals

a

Zn

As

Pb

Ni

Co

Cd

Fe

V

coal

mean sd

Cu 5.66 1.83

22.41 4.85

3.89 1.55

9.19 3.61

Cr 9.81 2.94

23.10 4.46

51.46 9.90

0.85 0.23

0.79 0.17

13.50 5.18

coke

mean sd

4.94 0.72

27.37 2.16

2.41 0.59

1.84 0.24

3.42 0.44

15.70 2.30

14.97 1.12

1.26 0.14

0.92 0.18

14.30 3.03

fly ash from coal chargingb

mean sd

431.86 31.32

137.79 22.38

47.58 16.90

29.96 9.85

900.60 108.72

88.88 18.75

220.21 49.94

2.12 0.42

0.11 0.07

nd

fly ash from coke pushingb

mean sd

1055.67 44.69

996.68 86.41

328.67 18.19

430.93 75.25

1028.20 116.04

74.87 20.60

296.90 58.03

1.88 0.59

2.86 0.71

22.53 4.02

fly ash from combustion stack

mean sd

94.02 27.26

7.53 5.31

18.38 13.10

0.87 0.60

179.79 42.62

5.66 2.22

93.32 37.08

0.75 0.51

0.68 0.04

5.20 2.65

APCD residues

mean sd

17.62 3.00

1059.00 62.06

5.22 2.19

256.70 51.90

54.95 7.61

5.27 1.70

nd

1.28 0.19

0.90 0.31

12.77 1.20

effluent from coke quenching, mg/L

mean sd

0.23 0.08

0.15 0.06

0.41 0.11

18.24 1.10

0.14 0.03

0.10 0.03

0.01 0.01

15.60 1.75

0.83 0.26

0.01 0.01

nd: Not detectable. bRefer to the fly ash after APCD.

residues to inlet fly ash for these two heavy metals were higher than for other metals (see Figure 2), indicating that the BF employed in this coke plant exhibited good efficiencies for Zn and Pb capture.

compared with other emission streams (Table 3), heavy metals in the wastewater from quenching were not taken into account. According to Figure 2, two classes of partitioning behavior are observable: (1) Class I, including Cu, Zn, As, Pb,and Cr, which were significantly concentrated in the inlet fly ash of APCD compared to the coke (Xinlet/Xcoke > 10); (2) Class II, including Ni, Co, Cd, Fe,and V, which were partitioned equally between the inlet fly ash and the coke to some extent (Xinlet/Xcoke < 4). The different behaviors of these elements during coking mainly depend on their volatility. Most Class I elements have relatively low boiling points; they are volatilized during coking but condense or become adsorbed on the fly ash downstream. However, most Class II elements have relatively high boiling points, except Cd; they are not volatilized during coking but instead form a melt of rather uniform composition that both becomes fly ash and is contained in the coke. It is worthwhile to note that although Cd is semivolatile, it behaves as if it has a high boiling point (Class II). Xu et al.24 indicated that the occurrence modes of heavy metals in coal affected their behavior to a great extent, and this could be the main reason for the specific behavior of Cd observed in the present study. Furthermore, it should also be noted that although Fe was classified into Class II in this study, its concentration in the wastewater from quenching was higher than that of other heavy metals in Class II. This behavior might be related to the erosion of devices used for quenching in addition to the transfer of Fe contained in the coke, resulting in the higher content of Fe in the wastewater. For the heavy metals in Class I, a further partitioning of the flue gas stream occurs in the BF, which effectively removes larger fly ash particles but is less efficient for finer particles. For Cu, As, and Cr, the ratios of outlet to inlet fly ash were, respectively, 42-, 36-, and 18-fold higher than those of APCD residues to inlet fly ash, indicating that the contents of Cu, As and Cr in outlet fly ash were higher than those in APCD residues. These observations demonstrate that these heavy metals in the inlet fly ash, after passing though the BF, were

Figure 2. Average concentration ratios of heavy metals from various emission streams during the coking process (n = 3). Error bars indicate standard errors.

At the end of the coking cycle, the coke produced should be deluged with water to prevent it from burning after exposure to air. The concentrations of Pb (18.24 mg/L) and Fe (15.60 mg/ L) in quenching effluent were much higher than those of other metals, suggesting higher solubility in the wastewater from coke quenching. The dominance of Pb has also been reported in wet scrubber effluent from animal carcass incineration.33 To obtain a better understanding of heavy metal behavior, concentration ratios of the heavy metals in the main streams during coking are shown in Figure 2. In view of the negligible amount of heavy metals (except Fe) contained in the effluent 6428

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significantly concentrated in the downstream outlet fly ash rather than APCD residues. In contrast to these metals, Zn and Pb were distributed equally between the outlet fly ash and APCD residue (the ratio of Xoutlet/Xinlet close to that of Xresidue/ Xinlet). This phenomenon indicated that different transformation mechanisms, such as adsorption, condensation and chemical transformation,24 dominated in the partitioning behaviors of these heavy metals.20 Since condensation and adsorption are surface phenomena, the concentrations of condensed metals should be inversely proportional to particle size.19 Based on these considerations, the enrichment behavior of Cu, As, and Cr in smaller particles (outlet fly ash) was mainly attributed to condensation and adsorption, and a complicated process affected by multiple mechanisms could result in the behavior of Zn and Pb during coking. Compared with coal combustion, coke production is more complex in that it integrates some different processes with associated stacks. Figure 3 shows the concentration ratios of

basis of the mass of heavy metals emitted per unit fuel consumed. The EF of heavy metals obtained in this study are calculated by the following equation:16 EF = (flow rate × concentration)/feeding rate. Table 4 shows the calculated Table 4. Average Emission Factors of Heavy Metals Obtained from This Study for Coke Oven Charging and Pushing and Those Reported by the U.S. EPA (μg/kg coal, n = 3)a this study charging coal

U.S. EPA coke pushing

heavy metals

mean

sd

mean

sd

coke pushing

Cu Zn As Pb Cr Ni Co Cd Fe V total

10.530 0.523 1.478 0.463 22.060 2.453 5.221 0.052 0.003 nd 42.783

2.133 1.040 1.492 0.802 9.438 3.202 5.110 0.013 0.002 nd 16.723

100.115 85.570 30.855 41.088 91.145 5.701 21.601 0.138 0.280 2.199 378.692

5.199 5.736 1.691 2.050 3.176 1.979 6.871 0.370 0.303 2.374 124.305

3.83 17.40 4.69 7.65 2.49 5.60 0.58 0.08

a

nd: Not detectable.

emission factors for this investigated coke plant. For total heavy metals, mean emission factors of 42.783 and 378.692 μg/kg of coal charged were obtained for CC and CP, respectively. We also found that the emission factor of each heavy metal for CP was higher than that reported by the U.S. EPA,40 mainly because of the different analytical procedures. For example, elemental analysis was performed by the EPA only on the residual material following solvent (methylene chloride) extraction instead of on the original samples, as done in the present study. In addition, the differences in the process and conditions of coke pushing and collection efficiency of APCD can also result in large variations in emission factors. To obtain accurate estimates of heavy metal emissions from coke production, further investigations are urgently needed.

Figure 3. Average concentration ratios of heavy metals in outlet fly ash to inlet fly ash of air pollution control device for coal charging and coke pushing (n = 3). Error bars indicate standard errors.

heavy metals in the outlet to inlet fly ash for CC and CP. It can be seen that the concentration ratios for Zn, As, Pb, and Cd tend to be higher in the outlet fly ash for CP than for CC, whereas the converse trend occurred in Cr, Ni and Co, even though both groups shared the same BF (see Figure 1). During the process of CC, incomplete combustion takes place when the coal charged contacts the high temperature oven walls. It can be seen in Table 2 that Cr, Ni, and Co existed in the coal with higher contents, and thus their higher content in smaller ashes for CC may be due to the incomplete combustion of coal during charging. Finally, it should be pointed out that heavy metals in the gaseous phase are not involved in this investigation owing to their very small proportion (less than 2%) of the coal-bound heavy metal emissions21 and that therefore more studies on the fate of heavy metals during coking, including those in the vapor phase and especially Hg and Se, are needed in the future. Emission Factors of Heavy Metals. Emission factors (EF) have been used for different applications, since they allow an easy estimation of emission rate and concentration of emitted pollutants,12 nevertheless, to our knowledge few studies to date have reported the emission factors of heavy metals from coking. In general, EF can be expressed on the

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ASSOCIATED CONTENT

S Supporting Information *

The coke plants, technological descriptions of the sampler and ICP-AES, experiment times and the result of every experiment. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 3516010799; fax: +86 3516010192; e-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the R&D Special Fund for Public Welfare Industry of China (Grant 200809027) and the National Science Foundation of China (Grant 41173002). 6429

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