Reduction of Polycyclic Aromatic Hydrocarbon Emission by Porous

Jan 4, 2016 - Department of Chemistry and Chemical Engineering, Niigata University, 2-8050 Ikarashi, Niigata, Niigata 950-2181, Japan. ABSTRACT: In th...
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Reduction of Polycyclic Aromatic Hydrocarbon Emission by Porous Alumina Bed Material during Sewage Sludge Incineration Linbo Qin,† Jun Han,*,† Yiqiu Zhan,† Wangsheng Chen,† and Heejoon Kim‡ †

College of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan, Hubei 430081, People’s Republic of China ‡ Department of Chemistry and Chemical Engineering, Niigata University, 2-8050 Ikarashi, Niigata, Niigata 950-2181, Japan ABSTRACT: In this paper, porous alumina was used as an alternative bed material to reduce polycyclic aromatic hydrocarbon (PAH) emission and improve the combustion efficiency during sewage sludge combustion in a fluidized bed combustor (FBC). To discover the reduction mechanism, the Brunauer−Emmett−Teller (BET) surface area, critical fluidized velocity, and heattransfer coefficient of three bed materials (silica sand, alumina sand, and porous alumina sand) were characterized. In comparison to the conventional silica bed material, the reduction efficiencies of PAH emission and PAH total toxic equivalent (TEQ) by porous alumina bed material under 850 °C were 52.0 and 97.7%, respectively. Porous alumina bed material had more BET surface area than that of the conventional silica sand, which could adsorb gaseous hydrocarbon and prolong the residence time of hydrocarbon in the diluted zone of the FBC. At the same time, it was well-known that gaseous hydrocarbons were a precursor of PAHs. Thus, PAH formation was suppressed. Moreover, the low heat-transfer coefficient would decrease the heat transmission rate between the bed materials and sewage sludge, which caused the sewage sludge decomposition rate in porous alumina bed material to be lower than that of silica bed material. Hence, less gaseous hydrocarbons and PAHs were formed. In addition, alumina bed material may inhibit agglomeration and enhance the fluidization quality of a fluidized bed incinerator. The above mechanism may account for reducing PAH formation by porous alumina bed material. Chang et al.7 reported that the destruction removal efficiencies (DREs) of PAHs by the prepared and commercial V2O5−WO3 catalysts were 64 and 72%, respectively. However, catalyst decomposition was limited by its durability because the catalyst was easily poisoned by heavy metals in flue gas. Zhou et al.12 found that PAHs could be removed effectively from flue gas using in-duct activated carbon injection, and DREs of PAHs were about 76−91%. However, the operation cost of treating flue gas by active carbon injection was as high as 0.004 RMB/ m3. According to the PAH formation mechanism, PAHs are mainly formed by two main routes, namely, pyrolysis and pyrosynthesis.13,14 Hence, hydrocarbons are one of the PAH precursors. Shimizu et al.15,16 discovered that porous alumina could adsorb some hydrocarbons during plastic combustion in a fluidized bed combustor (FBC) and decrease hydrocarbon emission. Thus, the porous alumina bed material can also suppress PAH formation.2 The advantages of porous alumina bed material reducing PAHs are no additional equipment needed and no changing the operation parameters of the FBC. In this paper, the comparison of PAH emission under porous alumina, alumina, and conventional silica bed materials was conducted. To understand the mechanism of reducing PAHs by porous alumina, the Brunauer−Emmett−Teller (BET) surface area, critical fluidization velocity, and heat-transfer coefficient of three bed materials (silica sand, alumina sand, and porous alumina sand) were characterized.

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are known as a group of environmental organic pollutants that are harmful to the environment and human health. Most PAHs are suspected to cause cancer, birth defects, and a wide variety of other health problems in humans.1 Hence, the United States Environmental Protection Agency (U.S. EPA) defined 16 PAHs as priority pollutants.2 Owing to their carcinogenicity, the World Health Organization (WHO) and many countries have prescribed the PAH emission standards.3 It was estimated that the global total annual emission of 16 PAHs in 2007 was 530 000 tons. Among them, south Asia (87 000 tons), east Asia (111 000 tons), and southeast Asia (52 000 tons) were the regions with the highest PAH emission densities, contributing half of the global total PAH emissions.4 China is considered to have the highest amount of PAH emissions of any other country, contributing approximately 22% of the total global PAH emissions.5 PAHs are mainly generated from straw burning, coal combustion, coking, and municipal solid waste (MSW) incineration. Xu et al.6 reported that the emission of 16 PAH species was around 25 300 tons in 2003, and the related contributions of the straw burning, coal combustion, and coking industry were in the sequence as 60, 20, and 16%. The complete removal of PAHs from flue gas of the incinerator or combustor is impossible; they can only be controlled. At present, the main PAH control methods are plasma,1,7 catalytic decomposition,8,9 adsorption,10−12 and optimizing the operational parameters.13,14 Lin et al.1 carried out experiments of reducing PAH emission by plasma and found that the total PAH-reduced efficiency was 20.7% when the optimum voltage of the plasma was 3200 V. © 2016 American Chemical Society

Received: November 2, 2015 Revised: January 2, 2016 Published: January 4, 2016 544

DOI: 10.1021/acs.energyfuels.5b01942 Energy Fuels 2016, 30, 544−550

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Energy & Fuels Table 1. Characteristics of Sewage Sludge ultimate analysis (%, dry and ash free)

proximate analysis (%, air dried) Mad

Aad

Cdaf

Hdaf

Odaf

Ndaf

Sdaf

22.91

4.06

68.20

4.04

0.79 2.25 45.53 ash composition (g kg−1, on dry basis)

Vad

FCad

lower heating value (MJ/kg)

45.39

6.83

9.06

Na

K

Si

Al

Fe

Ca

Mg

S

P

Mn

others

6.3

25.8

485.1

137.3

73.6

40.8

39.5

25.6

78.6

1.4

86

Figure 1. PAH contents in sewage sludge and ash. heat-transfer coefficients of silica, alumina, and porous alumina sand were measured according to the method reported by Franke et al.17 2.2. Methods. The FBC used in this experiment was described in detail in our previous paper.18 In brief, the combustor was made of silica with a 1.2 m vertical length and a 0.04 m internal diameter, as shown in Figure 2. The combustor was electrically heated, and its temperature was automatically controlled by a proportional−integral− derivative (PID) controller and monitored by two thermocouples. The

2. MATERIALS AND METHODS 2.1. Materials. The sewage sludge used as fuel was sampled from a wastewater treatment plant in Wuhan, China. Prior to use, sewage sludge was dried, milled, and sieved to less than 200−300 μm. The properties of the sewage sludge were summarized in Table 1. Figure 1 presented the PAH content in sewage sludge to be about 3.354 mg/kg. NaP, Flu, Pha, and BaA were the main PAHs, and their contents were 0.810, 0.32, 0.38, and 0.76 mg/kg, respectively. In this experiment, porous alumina, alumina, and silica sand were used as bed materials. To keep the similar fluidized properties, the bed materials with similar critical fluidization velocity were selected and the critical fluidization velocities of porous alumina, alumina, and silica sand were 0.062, 0.057, and 0.063 m/s. The density, diameter distribution, and BET surface area of the bed materials were listed in Table 2. The true density and apparent density were determined according to GB/T 23561.02-2009 and GB/T 23561.03-2009. The particle diameter distribution of the bed materials was analyzed by a laser particle size analyzer (Winner2308, China), and the BET surface area of the bed materials was measured by a Micromeritics surface area and porosity analyzer (ASAP 2020, Norcross, GA). In addition, the

Table 2. Properties of Bed Materials Used in the Tests

name alumina silica porous alumina

specific surface area (m2/g)

average diameter (μm)

bulk density (kg/m3)

true density (kg/m3)

4.403 1.718 177.9

100 150 500

830 1070 780

3200 2600 3200

Figure 2. Diagram of the FBC and sampling equipment. 545

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Energy & Fuels Table 3. TEF of 16 Kinds of PAHs components TEF components TEF

Nap

AcPy

AcP

Flu

PhA

AnT

FluA

Pyr

0.001 BaA

0.001 Chr

0.001 BbF

0.001 BkF

0.001 BaP

0.01 DbA

0.001 BghiP

0.001 InP

0.1

0.01

0.1

0.1

1

1

0.01

0.1

experimental temperatures were 750−950 °C. The total flow rate of air was 7 L/min, and the stoichiometric ratio of air was kept at 1.2 during the experiment. PAHs were isokinetically sampled by a sampling system according to U.S. EPA Method 23, as described in detail in our previous paper.13,14,18 Before the PAH sampling system, a cyclone separator was used to remove the particles above 10 μm. In the sampling system, a cooling tube was applied to condense PAHs of light molecular weight, followed by XAD-2 resin used to capture PAHs. Other gaseous PAHs were absorbed by dichloromethane solution. All samples containing PAHs were treated by Soxhlet extraction, Kuderna−Danish (K−D) concentration, and purification with the activated silica gel and analyzed by high-performance liquid chromatography (HPLC, Agilent Corp., Santa Clara, CA). The analysis procedure of PAHs was also described in our previous paper.13,14,18 In this study, each run was performed 3 times and the average data were used. DRE was defined as eq 1 DRE =

C1 − C2 C1

0.348, and 0.563 mg/m3, respectively, while they cannot be detected when porous alumina was used as the bed material. Figure 3 indicated the influence of bed materials on PAH distribution during sewage sludge incineration. The contents of three-, four-, and five-ring PAHs were decreased from 1.007, 1.775, and 1.347 mg/m3 to 0.621, 0.369, and 0.286 mg/m3, respectively, when silica sand was replaced by alumina bed material at 750 °C, and the DREs of the three-, four-, and fivering PAHs using alumina bed material were 38.3, 79.2 and 78.8%, respectively (in Table 5). Contrary to other PAHs, the two- and six-ring PAH contents were increased using alumina bed material. As for porous alumina bed material, two-, three-, four-, five-, and six-ring PAH contents were lower than those of silica bed material. Table 5 indicated that the DREs of the two-, three-, four-, five-, and six-ring PAHs were 59.4, 68.8, 91.9, 92.7, 93.8, and 79.2%, respectively. It was postulated that there were three mechanisms that caused the above phenomenon. First, the BET surface areas of silica, alumina, and porous alumina were 1.718, 4.403, and 177.94 m2/g, respectively. Porous alumina bed material had more BET area than that of the conventional silica sand. Hence, porous alumina bed material could capture more gaseous hydrocarbons released from sewage sludge,15,16 which reduced hydrocarbon in the diluted and bubbled zones of the FBC. According to the PAH formation mechanism, gaseous hydrocarbon is the precursor of PAHs.20 Thus, porous alumina bed material inhibits the PAH formation as a result of its adsorption effects. The second mechanism was the difference of the heat-transfer coefficient. The heat-transfer coefficients of silica, alumina, and porous alumina sand were 700, 520, and 450 W m−2 K−1. The low heat-transfer coefficient would decrease the heat transmission between the bed materials and sewage sludge, which caused the temperature increase rate of sewage sludge in porous alumna bed material to be lower than that in the conventional silica sand at the moment of feeding sewage sludge into the FBC. Thus, initial hydrocarbon released from the fuel particle in the pyrolysis stage was decreased, which also accounted for the decrease of the PAH formation rate. Lastly, alkaline metals, such as Na and K in sewage sludge were easily sintered with conventional silica sand bed material and deteriorated the fluidization quality of a fluidized bed incinerator, while alumna bed material may inhibit agglomeration and enhance the fluidization quality of a fluidized bed incinerator.21,22 Thus, PAH formation could be suppressed by porous alumina bed material. Figure 4 presented the bed materials to also have a significant effect on PAH TEQ. At 750 °C, total PAH TEQ under silica, alumina, and porous alumina were 729, 121, and 14.6 μg of TEQ/m3, respectively. In comparison to the conventional silica sand bed materials, the reduction efficiencies of PAH TEQ by alumina and porous alumina at 750 °C are 83.3 and 98.0%, respectively. In the optimum temperature (850 °C), PAH TEQ under silica, alumina, and porous alumina bed materials was in the sequence of 233, 65.9, and 5.34 μg of TEQ/m3. Figure 5 demonstrated that the reduction efficiencies of PAH TEQ by

(1)

where C1 was the amount of PAHs or PAH total toxic equivalent (TEQ) concentration in the case of silica bed material and C2 is the amount of PAHs or PAH TEQ concentration in the case of alumina or porous alumina bed materials. The toxic equivalent factor (TEF) is generally used to assess the toxicity of PAHs from the health-risk assessment view based on the most toxic PAHs (BaP). The U.S. EPA specified the TEF of 16 PAHs, as indicated in Table 3.19 In brief, the PAH TEQ concentration could be calculated by eqs 2 and 3. TEQ i = C PAHi × TEFPAHi

(2)

TEQi was the TEQ concentration of the ith PAHs (μg of TEQ/m3), CPAHi was the content of the ith PAHs (μg of TEQ/m3), and TEFPAHi was the TEF of the ith PAHs. The total TEQ concentration of 16 PAHs specified by the U.S. EPA was calculated as

TEQ =

∑ (TEQ i)

(3)

TEQ was the total TEQ concentration of the 16 specified PAHs (μg of TEQ/m3).

3. RESULTS AND DISCUSSION 3.1. Influence of Bed Material on PAHs and PAH TEQ. Table 4 indicated the influence of bed materials on PAH distribution during sewage sludge incineration. When the reaction temperature was 750 °C, the total PAH emission under porous alumina, alumina, and silica sand bed materials was 1.241, 3.276, and 5.968 mg/m3, respectively. In comparison to the conventional silica sand bed material, it was found that PAH reduction efficiency by porous alumina and alumina bed materials was 79.2 and 45.1%, respectively. Moreover, the morphology of PAHs was also influenced by the bed materials, as shown in Table 4. For example, the contents of Flu, FluA, Pyr, BaA, Chr, BbF, BkF, BaP, and InP were obviously decreased when conventional silica sand was replaced by alumina bed material. Especially, the contents of Flu, Pyr, BaA, BbF, and BaP in silica bed material were 0.554, 0.552, 0.495, 546

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5.968 3.276 1.241 0.307 0.023 0.019 nd 0.338 nd nd nd nd 0.563 0.092 nd 0.437 0.128 0.097

Figure 3. PAH content under different bed materials at 750−950 °C.

0.374 0.073 0.059 0.553 nd nd

0.495 nd nd

0.348 0.067 nd

nd refers to not detected.

silica alumina porous alumina

1.532 1.639 0.666

0.061 0.208 0.202

0.071 0.098 nd

0.554 0.088 nd

0.220 0.055 0.028

0.100 0.172 0.085

0.353 0.296 0.085

alumina and porous alumina at 850 °C are 71.7 and 97.7%, respectively. Figure 6 indicated the influence of bed materials on grouped PAH distribution during sewage sludge incineration at 750− 950 °C. As listed in Table 3, TEF of five rings is very high (TEF of BaP is 1.0). Hence, five-ring PAHs were the main contributor for PAH TEQ. At 750 °C, five-ring PAH TEQ under silica, alumina, and porous alumina were in the sequence of 641, 111, and 10.0 μg of TEQ/m3, which accounts for 87.93, 91.73, and 68.50% of the total PAH TEQ, respectively (as in Figure 6). At the same time, PAH concentration reduction efficiency is similar (as shown in Table 5). The reduction efficiencies of the five-ring PAH TEQ by alumina and porous alumina at 750 °C were 82.7 and 98.4%, as shown in Table 6. The values indicated that the five-ring PAHs were the main contributor in reducing the PAH TEQ. At 850 °C, it can be seen that the bed materials also had the most significant influence on five-ring PAH TEQ. In comparison to silica bed material, the reduction efficiencies of five-ring PAH TEQ by alumina and porous alumina bed materials were 66 and 98%, respectively. 3.2. Influence of the Reaction Temperature on PAHs and PAH TEQ. The influence of the temperature on PAH emission can also be found in Figure 3. In general, the PAH concentration was decreased with the reaction temperature and then increased when the reaction temperature was above the optimum reaction temperature. For example, the total PAH concentration under 750, 800, 850, 900, and 950 °C in the case of conventional silica sand bed material was 5.968, 2.926, 2.031, 3.314, and 3.730 mg/m3, respectively. When the conventional

a

InP BghiP DbA BaP BkF BbF Chr BaA Pyr FluA AnT PhA Flu AcP AcPy Nap

Table 4. PAH Concentration under Three Bed Materials at 750 °C (mg/m3)

a

total

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Energy & Fuels Table 5. Effect of Bed Materials on PAH Destruction Efficiency under Different Temperatures PAH destruction efficiency (%) temperature (°C)

bed material

two ring

three ring

four ring

five ring

six ring

total

750

alumina porous alumina alumina porous alumina alumina porous alumina alumina porous alumina alumina porous alumina

−6.95 59.4 65.7 18.0 44.4 −13.7 55.0 29.0 69.3 35.2

38.3 68.8 −87.0 −32.8 −27.3 21.4 56.7 41.2 26.2 30.1

79.2 91.9 81.0 91.4 78.0 92.0 60.5 93.8 50.9 85.8

78.8 92.7 78.1 95.5 48.3 89.8 71.1 88.7 70.8 86.7

−17.6 93.8 96.0 100 94.5 100 92.7 85.1 91.7 81.9

45.1 79.2 60.9 58.8 48.6 52.0 63.6 64.5 60.9 63.0

800 850 900 950

Figure 4. Effect of bed materials on PAH TEQ at 750−950 °C.

Figure 5. PAH TEQ destruction efficiency using alumina and porous alumina.

silica sand bed material was replaced by porous alumina, the total PAH concentration under 750, 800, 850, 900, and 950 °C was in the sequence of 1.241, 1.206, 0.976, 1.175, and 1.382 mg/m3, respectively. Hence, the optimum temperature was 850 °C, which had also been proven by other researchers.21−23 As for the grouped PAHs, it can be seen that the concentrations of two-, three-, four-, five-, and six-ring PAHs in the case of conventional silica sand and alumina bed materials were generally decreased with the reaction temperature and then increased when the reaction temperature was above the optimum reaction temperature. As for the porous alumina bed material, the four-, five-, and six-ring PAHs were first decreased and then increased as the reaction temperature increased from 750 to 950 °C, while the variations of two- and three-ring PAHs were independent of the reaction temperature. According to formation mechanisms, PAHs were mainly formed by the pyrolytic decomposition as a result of incomplete combustion before 850 °C. It was well-known that the increase of the temperature can promote the combustion efficiency and the hydrocarbon concentration will

Figure 6. PAH TEQ under different bed materials at 750−950 °C.

decrease with the increase of the temperature. Thus, PAH emission is also decreased with the increase of the reaction temperature when the temperature is below 850 °C. When the temperature was above 850 °C, more PAHs could be formed because the synthesis formation was the main contributor for the PAH formation.18,20,23,24 Figure 6 also stated the influence of the temperature on PAH TEQ under different bed materials. Similar to the PAH concentration, the PAH TEQ was also first decreased and then increased with the increasing reaction temperature. PAH TEQ under 750, 800, 850, 850, 900, and 950 °C in the case of silica bed material was 729, 430, 233, 488, and 559 μg of TEQ/m3, respectively. When porous alumina was used as bed material, PAH TEQ under 750, 800, 850, 850, 900, and 950 °C was in the sequence of 14.6, 5.45, 5.34, 15.3, and 18.9 μg of TEQ/m3, respectively. Thus, the optimum temperature for controlling 548

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Energy & Fuels Table 6. Effect of Bed Materials on PAH TEQ Destruction Efficiency under Different Temperatures PAH TEQ destruction efficiency (%) temperature (°C)

bed material

two ring

three ring

four ring

five ring

six ring

total

750

alumina porous alumina alumina porous alumina alumina porous alumina alumina porous alumina alumina porous alumina

−6.95 59.4 36.2 18.0 44.4 −13.7 55.2 29.4 69.3 35.2

−13.6 43.4 −103 25.9 −120 −10.7 70.7 59.5 61.9 82.1

98.1 98.7 95.4 96.9 98.0 98.4 91.3 98.5 88.6 97.8

82.7 98.4 90.0 99.0 66.0 98.1 79.5 97.8 91.0 97.9

81.5 93.7 94.0 100 93.0 100 92.7 85.1 91.8 81.9

83.3 98.0 90.2 98.7 71.7 97.7 81.0 96.9 90.8 96.6

800 850 900 950

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PAH TEQ under silica and porous alumina bed materials was 850 °C. As for alumina bed material, PAH TEQ under 750, 800, 850, 850, 900, and 950 °C was 121, 42.4, 65.9, 93.1, and 51.7 μg of TEQ/m3, respectively, and the minimum PAH TEQ occurred at 800 °C. Similar to the total PAH TEQ, the optimum temperatures for reducing five-ring PAH TEQ under silica, alumina, and porous alumina bed materials were 850, 800, and 850 °C, respectively.

4. CONCLUSION In this paper, the reductions of PAHs or PAH TEQ concentration by porous alumina bed material during sewage sludge incineration were investigated in a FBC. The following conclusions have been drawn: In comparison to conventional silica bed material, the reduction efficiencies of PAHs and PAH TEQ by porous alumina bed material under 850 °C were 52.0 and 97.7%, respectively. It was postulated that there were three mechanisms that resulted in reducing PAH formation. First, porous alumina bed material could adsorb gaseous hydrocarbons (a precursor of PAHs) and prolong the residence time of hydrocarbons in the bed material, which could suppress PAH formation. Second, the lower heat-transfer coefficient of the porous alumina bed material would decrease the heat transmission rate between the bed material and sewage sludge, which would decrease the sewage sludge decomposition rate in porous material and cause less gaseous hydrocarbon or PAH formation. Lastly, alumna bed material may inhibit agglomeration and enhance the fluidization quality of a fluidized bed incinerator, which may result in decreasing PAH formation.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partly supported by the National Natural Science Foundation of China (51576146 and 51476118) and the Natural Science Foundation of Hubei Province (2014CFA030).

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