Temperature Influence and Distribution in Three ... - ACS Publications

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Temperature Influence and Distribution in Three Phases of PAHs in Wet Sewage Sludge Pyrolysis Using Conventional and Microwave Heating Qianjin Dai,† Xuguang Jiang,*,† Yunfan Jiang,‡ Yuqi Jin,† Fei Wang,† Yong Chi,† Jianhua Yan,† and Aihua Xu§ †

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡ Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China § Zhongkezhongmao Environment Protection Thermoelectric Company, Limited, Cixi 315300, People’s Republic of China ABSTRACT: Pyrolysis of wet sewage sludge with a moisture of 86.47 wt % was conducted using electric heating (conventional pyrolysis, CP) and microwave heating (microwave pyrolysis, MWP) separately on two sets of batch reactors. Sixteen polycyclic aromatic hydrocarbons (PAHs) characterized by United States Environmental Protection Agency (U. S. EPA) as priority pollutants were determined by gas chromatography/mass spectroscopy (GC/MS) in gas, liquid, and solid phases of pyrolysis products to get detailed transmission characteristics of PAHs in sewage sludge pyrolysis. Temperatures of 400−900 °C and microwave power of 400−900 W were investigated. Production yields in MWP and CP behaved similarly for different power in MWP and for different temperatures in CP, whereas liquid yielded more than 75% of the total weight for the high content of water in wet sewage sludge. Liquid yield went through a high peak at 500 W for MWP and 700 °C for CP. Total yield of PAHs had trend of increase in MWP, whereas it peaked at 700 °C in CP. Naphthalene, fluorene, phenanthrene, and anthracene contributed most of the yield change, which can be explained by integration of the secondary pyrolysis of solid, decomposition of PAHs and synthesis of gas in different temperature. CP yielded more PAHs than MWP, which is more obvious in gas fraction than oil fraction. Distillation effect had great influence on PAHs distribution between gas and oil fraction. PAHs yield in solid for MWP decreased with increase of power, which might be attributed to the special effect of microwave on the microscope characteristic of solid, whereas PAHs in solid for CP were even lower and would cause fewer environmental problems. The results also indicated “nonthermal” impacts of microwave on the process.

1. INTRODUCTION

sludge pyrolysis discussed for temperature influence and distribution in three phases were not in literature. PAHs in pyrolysis have been studied extensively in coal,25 wood,26 and wastes27,28 for gas,29 tar, and char30 analysis, and the formation mechanism of PAHs and soot have also been discussed in detail elsewhere.31,32 Catechol,33 propyne,34 and 1,3-butadiene were also pyrolyzed separately and together to investigate PAHs emission. However, the formation of PAHs depends much on the source and reaction condition,24 and sewage sludge may behave differently as a novel fuel. PAHs, with effects of carcinogenic and mutagenic, are precursors of soot formation, which accelerated haze problems globally.35 Thus, pyrolysis of sewage sludge needs thorough study for its pollutant emissions. In this study, the 16 PAHs characterized by the United States Environmental Protection Agency (U. S. EPA) as priority pollutants were detected in the gas, char, and tar generated during sewage sludge pyrolysis using MWP. CP was also investigated to have a comparable sight into the “nonthermal” impacts of microwave on the process. The yield of each phase and PAHs distribution at different temperatures were calculated

Sewage sludge is raising more and more concerns due to its rapidly increasing amount and potential risks for human health and the environment.1 The respectable heating value of dry sewage sludge (around 13 MJ/kg) compared with wood (around 18 MJ/kg) and the capability to produce various chemicals indicated sewage sludge being a substitute of fossil fuel.2,3 Incineration is commonly used to recover the energy in sewage sludge for its volume reduction and complete destruction of organic pollutants. But the characteristics of the fuel is hardly made of use, and incineration also brings some environmental problems.4 Pyrolysis including conventional pyrolysis (CP) and microwave pyrolysis (MWP) of sewage sludge was reported to obtain high quality biosyngas,5 oil,6 and solid with properties of adsorbents,7 which can maximize its value and, meanwhile, have advantages in pollutant control. Pyrolysis of sewage sludge was reported for the yield study,8−10 biosyngas production,11 oil recovery,6,12,13 char characterization,14,15 economic evaluation,16 and also for environment discussion such as heavy metals,17,18 nitrogen product,19,20 and dioxins.21,22 Polycyclic aromatic hydrocarbons (PAHs) in pyrolysis tar of sewage sludge were generally discussed, but mostly for qualitative analysis6 or general component discussion in MWP23 and CP.24 PAHs in sewage © 2014 American Chemical Society

Received: February 10, 2014 Revised: April 9, 2014 Published: April 16, 2014 3317

dx.doi.org/10.1021/ef5003638 | Energy Fuels 2014, 28, 3317−3325

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Table 1. Proximate and Ultimate Analysis of Sewage Sludge on Air Dry Basis (Mass %) Ma

Ab

Vc

Fcd

Ce

He

Ne

Se

Oe

Cle

Qb (kJ/kg)f

6.50

40.95

46.76

5.79

28.44

4.17

4.89

1.25

13.67

0.13

12 691

M = moisture content, ±0.30%. bA = ash content, ±0.50%. cV = volatile matter content, ±0.80%. dFc = fixed carbon content. eC, H, N, S, O, Cl = the elements composing the combustible fraction, C (±0.5%), H (±0.15%), N (±0.08%), S (±0.05%), Cl (±0.01%). fQb = net calorific value, ±120 kJ/kg. a

set of grounded thermocouples was inserted into the beaker of sludge to record the temperature. The gas outlet was connected to the sampling apparatus (Figure 1c) after purging the reactor with nitrogen at a flow rate of 3 L min−1 for 10 min. Afterward, the flow rate of nitrogen is reduced to 0.2 L min−1, the targeted input power (frequency of 2450 MHz) was fixed and pyrolysis starts. The gases generated by pyrolysis passed through a condenser, and the condensed liquid was collected in a flask. Noncondensed gases passed through a XAD-2 resin column and a set of DCM scrubbing solutions in ice baths to absorb PAHs compounds. After each pyrolysis run of 60 min, the microwave power was turned off and the sample was allowed to cool down to room temperature in an inert atmosphere in 30 min. The residue, condensed product, resin, and DCM (gas phase) were respectively saved and were separately analyzed for PAHs in solid, liquid, and gas phases. Vapor was mainly released at around 100 °C, the maintain time of which differed greatly from 5 to 15 min. Colored liquid was mainly liberated later, and the process appeared to finish in 30 min or less. Compared with CP, MWP proceeds faster, related to its higher final temperature of sludge sample. 2.4. PAHs Analysis. The Environmental Protection Agency (EPA) method 8100 was taken as reference with a slight modification in PAHs analysis. Soxhlet extraction according to EPA method 3540C was used as extraction procedure to guarantee good contact between the solid sample and solvent.37 Some 2 g of cleaned copper powder was mixed with sewage sludge before extraction to eliminate sulfur according to EPA method 3660B for origin sewage sludge on dry basis. Silica gel cleanup was adopted according to EPA method 3630C. Rotaevaporation was used to concentrate the sample solution. The 16 PAHs characterized by U. S. EPA as priority pollutants were determined using gas chromatography (GC, Varian CP-3800, equipped with a 30 m × 0.25 mm × 0.25 um DB-5MS capillary) and mass spectroscopy (MS, Varian 1200L) in a selected ion monitoring mode (SIM). The carrier gas was He (1 mL/min), and the injection mode was splitless (270 °C, 1 uL). Temperature program started at 50 °C held for 1 min, then raised at 20 °C/min to 300 °C and held for 7 min. The transfer line temperature was set at 300 °C and the electron impact (EI) source at 270 °C. PAHs were determined using the external standard method. A standard mixture of 16 EPAPAHs was supplied by Sigma-Aldrich (PAHs concentration of 2000 μg/mL, and a solvent mixture of methylene chloride:benzene, v:v = 1:1). The PAHs concentrations were calculated with a group of five concentration levels ranging from 0.05 to 1 μg/mL with solvent of hexane. The GC/MS SIM profile programmed in the MS is listed in Table 2. The linear fitting of resultant calibration curves for each compound in external standards shows good correlation coefficients between 0.9986 and 0.9997 for each compound. The gas, liquid, and solid samples collected from pyrolysis of sewage sludge were separately treated to calculate the concentrate of each sample in the working curve properly. The solid sample was extracted for 24 h using 250 mL of DCM and then concentrated to 1 mL to transform the solvent to hexane; after the cleanup procedure, the sample was concentrated and fixed to 10 mL using hexane. As with the liquid sample, 50 mL of DCM was mixed and shacked with the liquid that contains mainly water and oil to extract PAHs, the extraction was repeated to ensure high yield. The DCM (100 mL) was concentrated to 1 mL to transform the solvent to hexane; after the cleanup procedure, the sample was concentrate and fixed to 50 mL using hexane. The gas sample included XAD-2 resin and 250 mL of DCM. The XAD-2 resin was extracted using the 250 mL of DCM and then concentrated to 1 drop to transform the solvent to hexane; after the

to help with understanding production of PAHs and mechanism of their transport, which will help achieving our goal of waste disposal environmental friendly.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Sewage sludge was obtained from the wastewater treatment plant of Shanghai Zhuyuan, China. The sludge underwent aerobic digestion and mechanical dehydration. Proximate and ultimate analysis results on an air-dry basis are presented in Table 1. M, A, V and FC were determined according to GB/T212-2008 of China. The contents of C and H were determined according to GB/T 476-2008 of China. The content of N was determined according to GB/T 19227-2008 of China. The content of S was determined according to GB/T 214-2007 of China. The content of Cl was determined by combustion−hydrolysis/ion chromatography (IC) method (GB\T3558-1996). The content of O was determined by difference. The net calorific value (Qb) was determined by GB/ T213-2008 of China. The total moisture of wet sewage sludge before being air-dried is 86.47 ± 0.5 wt %. Active carbon granules (diameter 2 mm, length 4 mm) were obtained from LanTian Corporation. Dichloromethane (DCM), hexane, and copper powder were obtained from Shanghai HuShi Corporation. 2.2. Conventional Pyrolysis. The furnace (Figure 1a) was heated to the target temperature and then maintained at its temperature (±2 °C). Approximately 50 g (±0.1 g) of wet sewage sludge was loaded into a quartz crucible (width 50 mm, length 150 mm) and introduced into the left side of a horizontal quartz reactor tube (diameter 60 mm, length 600 mm), where temperature was still limited to 100 °C. The gas outlet was connected to the sampling apparatus (Figure 1c) after purging the reactor for 10 min with nitrogen at a flow rate of 3 L/min to avoid potential impacts of oxygen. The flow rate of nitrogen carrier gas is adjusted to 0.2 L/min, sufficient to prevent the pyrolysis gas generated from accumulating in the tube and at the same time not to affect the temperature of the sample. Then the crucible is pushed to the center of the reactor with a quartz stick and pyrolysis starts. The generated gases passed through a water-cooled condenser and a flask to collect the condensing liquid. The uncondensed gases were cooled below 80 °C and passed through a XAD-2 resin column and a set of DCM scrubbing solutions in ice baths, which could absorb PAHs and other noncondensed compounds in the gas effluent. After each pyrolysis run of 60 min, the furnace was turned off and the sample was allowed to cool down to room temperature in an inert atmosphere in 30 min. The residue, condensed product, resin, and DCM (gas phase) were respectively saved and were separately analyzed for PAHs in solid, liquid, and gas phases. As soon as the crucible was pushed to the center of the reactor during pyrolysis, vapor was released and eventually condensed. Colored liquid was liberated mainly about 15 min later, and the pyrolysis process appears to be finished in 50 min or less. 2.3. Microwave Pyrolysis. A set of multimode microwave cavity oven (Figure 1b) was used for MWP. Raw sewage sludge was homogeneously blended with active carbon granules by a weight ratio of 5:1 to absorb enough microwave energy to attain the temperatures required for pyrolysis.36 Approximately 60 g (±0.1 g) of well-prepared sample was loaded in a quartz beaker (diameter 40 mm, height 80 mm), which is transparent for microwaves. The beaker was supported vertically in the quartz tube (width 50 mm, length 150 mm). The quartz tube was fixed vertically to allow liquid products to flow out in case they might condense on the inner surface of the quartz tube, and also, the sludge sample was situated within the multimode resonant microwave cavity where MWP could be achieved using a waveguide. A 3318


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method, the same with solid char, while 20 g sewage sludge in dry basis was extracted. The results are listed in Table 1. The detection limit was 0.001 μg/mL for the 16 PAHs during GC/ MS analysis. Considering sample mass of 50 g and the fixed solvent volume, the detection limit for the solid, liquid, and gas sample were respectively 0.0002, 0.001, and 0.002 on the basis of micrograms of PAHs per 1 g of wet sewage sludge. The toxic equivalent factor (TEF) was defined in order to characterize the carcinogenic properties of PAH more precisely, comparing every compound with the BaP, which has the highest value of TEF (Table 2). Toxic equivalent quantity (TEQ) was calculated on the basis of TEF to determine the inhalative carcinogenic potential These TEQ-values were also determined in our work to evaluate the influence of different operation conditions on the harmful health effect of the gaseous effluent of sewage sludge pyrolysis and to characterize the real toxicity.

3. RESULTS AND DISCUSSION 3.1. Product Yields. Product yields in three phases (gas, solid, and liquid) under different conditions during MWP and CP are shown in Figure 2. Solid and liquid were weighted in experiment, whereas gas yield was obtained by deducing solid and liquid fraction (by percent) from 100%. Liquid accounted for more than 75% of the yield in each condition; thus, yields were displayed from 70% to 100% in Figure 2 to facilitate the analysis. Temperature in MWP reached 469 °C (400 W), 616 °C (500 W), 665 °C (600 W), 733 °C (700 W), 789 °C (800 W), 899 °C (900 W) for different power. The experimental errors were not greater than 2% with respect to the initial mass. The solid yield continually decreased from 7.09% (400 °C) to 5.84% (800 °C) and 5.46% (900 °C) when temperature increased for CP, which could be attributed to the devolatilization of the solid hydrocarbons and the partial gasification of the carbonaceous residues in the char at high temperature.38 Also, the solid yields have a decreasing trend but are highest at 400 W (7.48%) and lowest at 800 W (4.78%) for MWP, the fluctuation may be owing to the more complicated interaction among products in MWP, self-gasificaion included.39 Ash, fixed carbon, and volatiles were separately 5.54%, 0.78%, 6.33% in wet sewage sludge as received (calculated from Table 1 concerning the moisture content of 86.47%). Solid yield in CP (900 °C) and MWP (800 W and 900 W) surpassed the ash content, which means component in the ash might be decomposed at these conditions. All solid yields were smaller than the sum of ash, fixed carbon, and volatiles, which indicated that pyrolysis of volatile components took place in all conditions. However, the pyrolysis of volatile and fixed carbon could not be divided by comparing solid yield because the drying, pyrolysis, and partial gasification of the raw sludge took place in the same process, and they interacted with each other to produce more hydrogen gas.40 The gas yield in CP decreased along with temperature rising from 400 °C (13.00%) to 700 °C (0.76%) and then increased when the temperature rose to 900 °C (8.66%), which can be explained by the secondary cracking of liquid tar when temperature surpassed 700 °C.38 Gas yield in MWP have a similar trend but achieve its lowest at 500 W, which had the final temperature of 616 °C. However, it should be noted that gas yield at 500 W (3.66%) was much larger than at 700 °C (0.76%), which may be explained by the prominent biosyngas production from oil introduced by microwave heating,5 it was proved when comparing liquid yield of 500 W (91.23%) with 700 °C (93.24%).

Figure 1. Schematic figure of systems of the experiment. (a) Conventional pyrolysis: (1) pusher; (2) carrier gas inlet; (3) electric furnace; (4) quartz tube; (5) quartz crucible; (6) heating jacket; (7) gas outlet. (b) Microwave pyrolysis: (1) thermocouple; (2) gas inlet; (3) quartz tube; (4) microwave oven; (5) waveguide (a channel which directs the microwaves from the magnetron to the chamber); (6) quartz beaker; (7) gas outlet. (c) Gas sampling apparatus: (1) condenser; (2) flask; (3) XAD-2 resin; (4) dichloromethane; (5) ice bath.

cleanup procedure, the sample was concentrated and fixed to 100 mL using hexane. Then, 0.1 μL of the fixed sample was introduced to GC/ MS analysis. The origin content of PAHs was detected using the 3319


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Table 2. Working Curve Used to Quantity PAHs with GC/MS

a

PAHs

TEF

SIM

naphthalene (Nap) acenaphthylene (Acpy) acenaphthene (Acp) fluorene (Flu) phenanthrene (Phe) anthracene (Ant) fluoranthene (Fla) pyrene (Pyr) benzo[a]anthracene (BaA) chrysene (CHR) benzo[b]fluoranthene (BbF) benzo[k]fluoranthene (BkF) benzo[a]pyrene (BaP) indeno[1,2,3-cd]pyrene (IND) dibenzo[a,h]anthracene (DBA) benzo[g,h,i]perylene (BghiP)

0.001 0.001 0.001 0.001 0.001 0.01 0.001 0.001 0.1 0.01 0.1 0.1 1 0.1 1 0.01

128 152 154 166 178 178 202 202 228 228 252 252 252 276 302 276

MS window time (min) 5.07.0 7.0−8.5 8.5−9.5 9.511.0 11.0−12.5 12.514.0 14.016.0

16.022.0

RT (min)

correlation coefficients

sewage sludge (mg/kg)

6.256 8.141 8.353 8.985 10.168 10.231 11.623 11.899 13.357 13.401 14.840 14.886 15.392 17.832 17.905 18.544

0.999 390 0.999 754 0.999 656 0.999 713 0.999 654 0.999 192 0.999 560 0.999 298 0.999 293 0.999 058 0.998 630 0.999 625 0.999 381 0.999 008 0.999 533 0.999 121

1.745 0.005 0.013 0.640 0.330 0.005 0.075 0.015 0.010 0.065 0.025 0.010 0.001 NDa 0.003 0.005

ND: not detected.

Figure 2. Product yield in three phases under different conditions during MWP and CP.

Figure 3. Total PAHs in pyrolysis.

The liquid yields in conditions of 400 W (85.51%), 400 °C (79.91%), 500 °C (84.45%), 900 °C (85.88%) were lower than the initial moisture content of sewage sludge (86.47%). The loss of these water fractions indicated that a certain amount of the water released from the sample reacted with other pyrolysis products before leaving the reactor, giving rise to noncondensable gases,6 which can be also proved by the prominent yield of gas in these four conditions (Figure 2). Liquid yield in CP increased along with temperature rising from 400 °C (79.91%) to 700 °C (93.24%) and then decreased when temperature rose to 900 °C (85.88%), which can be also explained by the secondary cracking of liquid tar when the temperature surpassed 700 °C.38 Liquid yield in MWP have a similar trend but peaked at 500 W, which got the final temperature of 616 °C. 3.2. Total PAHs in MWP and CP. The original PAHs are listed in Table 2 and had a PAHs content of 2.946 mg/kg on dry basis. When moisture of 86.47% was considered, it was 0.399 mg/kg on received basis. The total PAHs detected in three phases are displayed in Figure 3. The PAHs yield were much larger than the PAHs content in original sewage sludge of 0.399 mg/kg; thus, the original content is not to be discussed. It was also proved that PAHs mainly came from complex chemical reactions of coal pyrolysis rather than from free PAHs

in the raw coals.41 Power in MWP and temperature in CP are proven to strongly affect the total PAHs production (Figure 3). When microwave power increased from 400 to 900 W, the total PAHs production increased gently from 7.259 to 31.753 mg/kg. The trend of increase is well lined to the final temperature (with coefficient of 0.90), indicating that temperature was the main factor in PAHs formation in MWP. It is interesting that it is better lined with microwave power (with coefficient of 0.94). The rapid increase of PAHs at 500 W (81%) and 700 W (75%) was noted for their temperature of 616 and 733 °C, the former can be attributed to the oil yield increase at 500 W (Figure 2.), whereas the latter got the temperature (733 °C) that was often peaked for PAHs yield in sewage sludge pyrolysis.42 In the CP condition, when the temperature rose from 400 to 500 °C, PAHs decreased by 38% along with the decrease of Nap by 69%, whereas Flu increased by 157%. In a previous qualitative study of oil composition, 450 °C was observed to get a maximum yield of 2−3 rings PAHs.43 In our study, synthesis of Nap to PAHs with higher ring numbers around 500 °C explained the decrease of PAHs well. When the temperature kept rising, PAHs went through a high peak at 700 °C, where oil also yielded most. When the four main PAHs that are thought to mostly influence the total yield were presented 3320


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(Figure 4), it is clear that the peak of PAHs yield at 700 °C can be mostly contributed to Flu and Ant formation. When the

the short reaction zone in MWP compared with that of CP led to short reaction time, which made much difference. 3.3. PAHs in Liquid Tar. PAHs were presented in liquid phase for fraction of 70.34% (400 W) to 96.76% (900 W) for MWP and 70.34% (400 °C) to 96.34% (900 °C), which were calculated from Table 3. The yield of PAHs in liquid phase can be mostly explained by the total amount. Table 3 lists the 16 PAHs in liquid phase. All compounds except for Bbf, Bkf, and Bap were detected. PAHs in liquid have the same trend with the total yield. When each PAH is concerned separately, Nap, Acpy, Flu, Phe, Ant, Fla, Pyr, CHR, and DBA roughly follow the increasing trend in spite of their great difference in quantity of MWP and CP, whereas IND and BghiP peaked at about 600 °C in both MWP and CP. Many researchers agreed on the overall trend of increase along with increasing temperature, and peaks for some components were also noted.44 The peak temperature was 800 °C for pyrolysis of coal and 950 °C for pyrolysis of catechol.45 From TEQ analysis of the PAHs in liquid (Figure 5), TEQ of these bio-oil samples were dominated by Nap, Flu, Ant, which

Figure 4. Four main PAHs in CP.

temperature rose to 800 °C, Flu and Ant may be decomposed to lighter molecules, maybe Nap and Phe for their increase, leading to the total decrease of PAHs yield. From the view of three-phase distribution, PAHs yielded mostly in the liquid, which contained water and oil evolved during pyrolysis. PAHs are negligible in solid except for condition of 400 W for MWP. PAHs in the gas phase are negligible in MWP and 400 °C for CP. However, its proportion increased along with the temperature and peaked at 800 °C. The total PAHs yield in CP had a high peak, whereas MWP did not; this may be due to characteristics of the reactor (Figure 1). CP had a region of high temperature allowing the sample to go through secondary pyrolysis, which may decompose PAHs. MWP only had a region of heat preservation with the help of a heating jacket, which cannot decompose PAHs anyway. Briefly,

Figure 5. TEQ of PAHs in liquid tar using MWP and CP.

Table 3. PAHs Liquid Tar Using MWP and CPa

a

PAHs

TEF

400 W

500 W

600 W

700 W

800 W

900 W

400 °C

500 °C

600 °C

700 °C

800 °C

900 °C

Nap Acpy Acp Flu Phe Ant Fla Pyr BaA CHR BbF BkF BaP IND DBA BghiP SUM TEQ

0.001 0.001 0.001 0.001 0.001 0.01 0.001 0.001 0.1 0.01 0.1 0.1 1 0.1 1 0.01

2.099 0.036 0.044 0.975 1.488 0.155 0.169 0.135 ND ND ND ND ND 0.001 0.004 ND 5.106 0.011

5.199 0.113 0.074 2.797 3.196 0.201 0.250 0.072 ND ND ND ND ND 0.001 ND 0.040 11.943 0.014

4.443 0.092 0.049 3.270 3.005 0.227 0.270 0.099 ND 0.083 ND ND ND 0.002 ND 0.041 11.581 0.015

8.815 0.180 ND 5.300 4.989 0.323 0.528 0.191 0.016 0.064 ND ND ND 0.002 0.005 ND 20.413 0.031

15.320 0.237 ND 5.791 5.414 0.321 0.624 0.351 ND 0.128 ND ND ND 0.001 0.011 ND 28.198 0.043

13.930 0.294 ND 7.999 7.120 0.326 0.660 0.287 ND 0.096 ND ND ND 0.002 0.009 ND 30.723 0.044

26.430 0.340 0.263 4.575 5.251 ND 0.398 0.084 ND ND ND ND ND 0.002 ND 0.054 37.397 0.038

6.944 0.104 0.112 12.330 1.947 ND 0.355 0.124 ND ND ND ND ND 0.005 0.015 0.191 22.127 0.039

12.880 0.252 ND 17.890 2.163 2.323 0.297 0.069 ND ND ND ND ND 0.036 ND 0.344 36.254 0.064

45.030 0.614 0.603 49.510 9.850 10.540 0.918 0.460 ND 0.067 ND ND ND 0.015 0.022 0.252 117.881 0.239

51.740 0.314 0.497 11.880 13.090 1.162 1.537 0.551 ND 0.096 ND ND ND 0.004 ND 0.078 80.949 0.093

42.910 0.094 0.158 4.059 7.549 0.565 0.715 0.195 ND 0.159 ND ND ND 0.002 0.015 ND 56.421 0.078

In mg/kg. ND: not detected. 3321


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Figure 6. Yield and TEQ of PAHs in gas effluent using MWP and CP.

Figure 7. Yield and TEQ of PAHs in solid char using MWP and CP.

production yield which indicated similar pyrolysis process. The abundant of LMW can be attributed to the strength of cracking and dehydrogenation in microwave pyrolysis, which was indicated as “self-gasification” to reduce high molecular weight PAHs6 and elevate syngas quality.5 Composition analysis of liquids obtained from sewage sludge pyrolysis using fluidized bed between 450 and 650 °C showed significant change at 650 °C due to secondary reactions of the primary pyrolysis products, whereas PAHs kept increasing along with temperature.46 It was also observed in our study when temperature arrived 700 °C. It can be explained by secondary pyrolysis of gaseous and solid, which led to synthesis of more oil and significant change in three phase production

are PAHs of 2−3 rings, and are usually considered low molecular weight (LMW) PAHs.23 Nap and Flu were in large quantity while Ant got high TEF of 0.01 (Table 2). It is noted that PAHs with high molecular weight (HMW) including BbF, BkF, BaP, IND, DBA, and BghiP were very low produced. The highest amount of HMW PAHs was 0.043 mg/kg at 600 W for MWP, which was only 11% of the highest amount at 600 °C for CP, 0.380 mg/kg, as can be calculated from Table 3. Bio-oils from induction-heating pyrolysis of food-processing sewage sludge were report to have similar distributions of PAHs, with LMW the most abundant.23 In our study, the similar distribution of PAHs in liquid (Figure 5) for 700, 800, and 900 W may be explained by the similar three phase 3322


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yield. The increase of liquid yield also caused the liquid fraction to dissolve more PAHs, especially Nap and Flu at 700 °C (Table 3). 3.4. PAHs in Gas. PAHs in gas fractions could be entirely condensed to the liquid fraction if substantially cooled. However, the gas sampling apparatus can simulate common condense procedure in pyrolysis and help to evaluate the gas emission in pyrolysis process. Yield and TEQ of PAHs in gas effluent of sewage sludge pyrolysis using MWP and CP were displayed in Figure 6. It is noted that the vertical ordinate displayed up to 1 mg/kg in MWP and 25 mg/kg in CP. Variation tends of PAHs in the gas along with the power and temperature were just similar to the oil. However, the compositions of PAHs in gas were somewhere different from that of liquid. Flu was dominant in most conditions with high temperature. This may be due to its physical properties of solution equilibrium between organic gas and liquid. PAHs yield in gas were lower in 400 °C than 500 °C, which may be due to the low temperature that causes the gas to be more completely condensed. The decrease in gas yield (Figure 2) caused fewer PAHs to be blown out, so the increase trend of PAHs along with temperature was not as evident as in the liquid fraction. This effect was clear when the temperature increased from 600 to 700 °C. The highest peak for PAHs yield in CP also moved from 700 to 800 °C. The 800 °C temperature definitely led to decomposition of PAHs for the total PAHs (Figure 3), and yet an increase of PAHs in gas at 800 °C was probably due to more prominent evaporation of PAHs from liquid phase because the total yield did not change immensely from 700 to 800 °C (Figure 3). This effect of distillation was also important in PCDD/Fs transmission in sewage sludge pyrolysis.21 3.5. PAHs in Solid Char. PAHs in solid char were predicted to have similar patterns. However, the results were interesting even though their yield in the solid phase were very low. PAHs in solid using MWP underwent a decrease from 1.705 mg/kg (400 W) to 0.016 mg/kg (900 W) when power increased, the extremely low content at 900 W may be due to vitrification at high temperature.47 PAHs in solid using CP underwent a high peak at 600 °C (0.084 mg/kg) and two low peaks at 500 °C (0.006 mg/kg) and 700 °C (0.016 mg/kg) (Figure 7). Regarding to the original content of 0.399 mg/kg in wet sewage sludge, microwave with power of 400 and 500 W gave a rise in PAHs content in solid phase. The high peak at 600 °C (0.084 mg/kg) was 21% of the original content. A detail study of PAHs in grass and wood biochars found great increase of PAHs at 400 and 500 °C, which was due to a low heat treatment temperature (HTT) of solid phase formation mechanism commonly used for industrial biochar production.30 The relatively high PAHs quantity at low microwave power may be due to special microscope characteristic generated by microwave heating, whereas adsorbents prepared in MWP often behave better than that in CP.48 BaP was highest at 400 W in MWP (0.011 mg/kg), which is far below the Chinese legislation limit value (3 mg/kg) for agriculture use (GB 189182002). Solid char in CP yield even less PAHs, which is no longer a environmental problem for solid disposal.49 It is noted that PAHs in solid phase may have relationship with its ability of adsorption or specific surface area characteristics. 3.6. PAHs Distribution in MWP and CP. The distribution of PAHs in MWP and CP was displayed in Figure 8. PAHs in the liquid accounted for more than 75% of the yield in each

Figure 8. PAHs distribution in three phases for MWP and CP.

condition; thus, yield were displayed from 70% to 100% in Figure 8 to facilitate the analysis. The distribution was a complex process including formation of PAHs from solid (pyrolysis) and gas (synthesis, including the well-known Diels− Alder reaction), composition of PAHs (below 800 °C), and distillation that dominate the distribution between gas and oil. The distillation effect can mainly refer to the production yield in three phases (Figure 2), and the formation and decomposition of PAHs can refer to yield of PAHs in the three phases. PAHs distribution fraction in solid decreased along with increase of power (MWP) and temperature (CP), and PAHs distribution fraction in gas and liquid were influenced greatly by three phases yield (Figure 2) and temperature variation from our study. The main trends of PAHs fractions in gas were a decrease before 500 W for MWP and 700 °C for CP, and then after a increase due to increase of gas yield (Figure 2). More detail mechanism will be proposed with more data on the composition of the three phases in our later work.

4. CONCLUSIONS Wet sewage sludge was pyrolyzed using CP and MWP, and PAHs were determined during the two processes. Product yield and PAHs yield in gas, liquid, and solid phase of pyrolysis product were investigated and the transmission characteristics were discussed. Production yield in MWP and CP behave similarly for different power in MWP and for different temperature in CP, whereas liquid yielded more than 75% of the total weight for the high content of water in wet sewage sludge. Liquid yield went through a high peak at 500 W for MWP and 700 °C for CP. Thus, 500 W (616 °C) of MWP played the role of rebound instead of 700 °C in condition of CP. Total yield of PAHs in MWP experienced a slow increase, whereas that in CP experienced a peak at 700 °C. Nap, Flu, Phe, and Ant contributed most of the yield change, which can be explained by secondary pyrolysis of solid, decomposition of PAHs, and synthesis of gas in different temperature. CP yielded more PAHs than MWP, which is more obvious in gas fraction than oil fraction. Distillation effect had great influence on PAHs distribution between gas and oil fraction. PAHs yield in solid for MWP decreased with increase of power, which might be attributed to the special effect of microwave on the microscope characteristic of solid. PAHs in solid for CP were even lower and will cause fewer environmental problems. 3323


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AUTHOR INFORMATION

Corresponding Author

*X. Jiang. Tel.: +86 571 87952775. Fax: +86 571 87952438. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports are acknowledged by the National Basic Research Program of China (Grant 2011CB201500), the National High Technology Research and Development Program (863 Program) of China (Grant 2012AA063505), the Special Fund for National Environmental Protection Public Welfare Program (Grant 201209023-4), the Program of Introducing Talents of Discipline to University (Grant B08026), and the National Key Technology Research and Development Program of China (No. 2012BAB09B01)

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