polycyclic aromatic hydrocarbons formation in particulate
POLYCYCLIC AROMATIC HYDROCARBONS FORMATION IN PARTICULATE MATTER DURING SEWAGE SLUDGE PYROLYSIS J.C. WANG*, Y.Q. LIAO*, Q. Xu*, J.H. KO*, * School of Environment and Energy, Shenzhen Graduate School, Peking University, Shenzhen, China
SUMMARY: In this study, the formation of particular mater (PM) during sludge pyrolysis and selected 16 polycyclic aromatic hydrocarbons (PAHs) in the PM have been investigated. PM was collected during sludge pyrolysis with various temperature ranges (100-400 oC, 400-600 oC, 600-800 oC, 800-1000 oC) and holding times (0-0.5, 0.5-1.0, 1.0-1.5, and 1.5-2.0 hours) at 1000oC using a PM impactor equipped with a quartz filter. A total 79 mg (2,641 mg PM/kg sludge) of PM was produced during the pyrolysis. The average PM generation rate ranged from 0.47 mg/min to 0.58 mg/min during heating from 100 to 1000 oC but decreased during extending holding time at 1000 oC. Total PAHs in PM collected during the pyrolysis was 5.53 μg PHAs/mg PM. However, the concentrations of PAH species varied. Among the measured PAHs, Pyr showed the highest concentration and Flu showed the lowest concentration in the collected PM. In addition, the emission rates of PAH species in PM varied during heating and holding temperature at 1000 oC. Overall, PAHs in PM increased with rising temperature from 100-1000 o C except low molecular weight (LMW) PAHs. Middle molecular weight (MMW) and high molecular weight (HMW) PAHs showed a peak generation rates during extending holding time at 1000 oC indicating that extending holding time at high temperature provided favorable reaction environments to produce PAHs.
1. INTRODUCTION A total of 32.34 million tons (dry weight) of sewage sludge was produced at China in 2014 (Buys et al., 2014). Sewage sludge is rich in organic matters containing heavy metals, microorganisms, and pathogens (Fytili and Zabaniotou, 2008). Pyrolysis gas contains components with high heating value, such as CH4, CO, and H2 as well as air pollutants including NOx, NH3, H2S, hydrocarbons, tar, and particulate matter (PM), etc. (Beneroso et al., 2015; Chianese et al.; Lv et al., 2016). In particular, PM emission is an important issue during thermal treatment of sewage sludge (Zhang et al., 2008). Because of the higher ash content in dry sludge (30-70 wt%), PM emission in sewage sludge combustion is more severe compared to those from other biomass combustion (Thipkhunthod et al., 2005). Ash can significantly contribute to PM emissions because particles are mainly produced by ash vaporization during thermal treatment and condensation (Takuwa et al., 2006). Many researchers have focused on PM emissions and characterized PM during sewage sludge incineration (Han et al., 2015; Ninomiya et al., 2004; Zhang et al., 2008). In contrast, sludge pyrolysis studies have rarely focused on PM emission characteristics in sewage sludge pyrolysis. Dewa et al. (2016) reported the particle size distribution of PM during ethylene pyrolysis was greatly influenced by holding time and pyrolysis Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
temperature. They concluded that PM nucleation rates increased with elevating temperatures (above 1540 K), resulting in enhancing PM formation. Pyrolysis temperature and holding time also affect biochar characteristics due to the release of volatiles as well as the formation and volatilization of intermediate melts (Lehmann and Joseph, 2009). Along with PM formation, polycyclic aromatic hydrocarbons (PAHs) emissions is a concern during thermal decompositions of biomass. The combination of PAH and PM increased the carcinogenic and mutagenic effects (Kok et al., 2006; Sánchez et al., 2012). It is known that PAHs are formed during biomass pyrolysis (Conesa et al., 2008). PAHs formation in different pyrolysis products (i.e. biochar, bio-oil, and biogas) during sewage sludge pyrolysis has been reported. Zielinska and Oleszczuk (2016) studied PAHs in biochars derived from various sewage sludge. PAHs contents in sewage sludge-derived biochars was 2-3 times lower than those in the fed sewage sludge. Hu et al. (Hu et al., 2014) characterized the PAHs in the pyrolysis bio-oil of sewage sludge and most of the selected PAHs were enriched in the pyrolysis bio-oil, with concentrations up to 48.9 mg/kg. Dai et al. (2014, 2015) measured that PAHs were over 1200 mg PAHs/kg sludge in gaseous phase during sludge pyrolysis at 850 oC (with a gas flow of 50 ml/min. They concluded that temperature, sample mass, holding time, particle size, and gas flow were important factors on PAHs formation in flue gas. PM can be a carrier for PAHs because of the large specific surface area and porous structure. However, studies on the formation of PM during sludge pyrolysis and the distribution of PAHs in PM were rare. In this study, PM emission during sludge pyrolysis was measured with different pyrolysis temperatures and PAHs in collected PM were characterized.
2. MATERIALS AND METHOD 2.1 Sewage sludge Sewage sludge, mechanically centrifuged to dewater, was obtained from a municipal wastewater treatment plant (WWTP) located in Shenzhen, China. The dewatered sludge cake was collected and then oven-dried for 24 h at 105 oC. Dry sludge samples were ground and then sieved. The physicochemical properties of the prepared sludge samples are presented in Table 1, including approximate analysis (GB/T 17664-1999) and ash composition (ICP-OES, Agilent, USA). 2.2 Sewage sludge pyrolysis As illustrated in Figure 1, pyrolysis was carried out using a horizontal furnace (Sanli Inc., China). In order to maintain an oxygen-free atmosphere, N2 (99.99% V/V) was purged into the furnace at a flow rate of 400 ml/min for 15 min before heating. In each run, a 30 g sludge sample was added into a crucible. The temperature of the furnace was held at 100oC for 20 min to remove absorbed moisture in the sample and then increased with a rate of 10oC /min up to 1000 o C. Pyrolysis temperature was held for 2 hours at 1000 oC. A cooling system was installed to cool down flue gas and to remove the condensable tar in the flue gas. The flow rate was fixed at 400 ml/min. 2.3 PM sampling A gas pump and a PM impactor were installed at the outlet of the cooling system to collect PM during sewage sludge pyrolysis. The flow rate of sampling gas was fixed at 1 l/min by a mass flowmeter controller (MFC). A quartz filter (Ø47 mm, Whatman Inc.) was installed in the impactor
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to collect suspended PM. Before installing, the quartz filter was roasted in a muffle furnace at 550oC for five hours to minimize contamination. The quartz filters were weighed before and after sampling PM. The quartz filter of the impactor was replaced at each desired temperature range (100-400 oC, 400-600 oC, 600-800 oC and 800-1000 oC) and holding time ranges (0-0.5 h, 0.5-1.0 h, 1.0-1.5 h and 1.5-2 h at 1000 oC). Table 1 Physicochemical properties of the raw sewage sludge Parameter Approximate analysis Moisture Volatile matter Ash Fixed carbon Ash composition Al2O3 SiO2 P 2O 5 Fe2O3 CaO K 2O TiO2 SO3 ZnO MnO CuO Cr2O3 PbO
Unit
Value
a
78.0 55.8 33.7 10.5
b
11.26 9.65 5.30 2.57 1.92 0.83 0.24 0.23 0.18 0.03 0.01 0.01 0.004
% b % b % b,c % % b % b % b % b % b % b % b % b % b % b % b % b %
a
: on as-received basis : on dry basis c : determined by the difference (Fixed carbon = 100% - Volatile matter - Ash) b
Figure 1. Schematic diagram of sewage sludge pyrolysis and PM collection system: (1) nitrogen gas cylinder, (2) mass flowmeter controller, (3) sewage sludge sample, (4) tube for dilution gas, (5) thermal couples, (6) crucible, (7) water bath, (8) condensing tube, (9) mass flowmeter, (10)
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cooling water, (11) PM impactor, (12) mass flowmeter controller, and (13) gas pump.
2.4 PAHs extraction and analysis PAHs in PM on a quartz filter were extracted and analyzed following Chinese standard method for determining the PAHs in PM from exhaust gas (HJ-646 2013). A quartz filter collected PM was size-reduced using a pair of ceramic scissors. The size-reduced quartz filter was added into an extraction solution (n-hexane: ether= 9:1 v/v) and sonicated for 20 minutes to extract PAHs. The supernatant was separated by centrifugation. The sonication-extraction step was repeated three times. The collected supernatant (45 ml) was concentrated to less than 1 ml by flushing N2 and purified with a PAHs purification column (Poly-sery MIP-PAHs SPE Cartridge, CNW, China). The final volume of each purified PAHs sample was adjusted to 1 ml using n-hexane. PAHs in the extract was analyzed with a GC (7890B, Agilent)/MS (5977B, Aglient) equipped with an HP-5MS capillary column (30 m×0.25 mm×0.25 mm). The oven temperature was held at 70 oC for 2 min, increased to 320 oC with a rate of 10 oC /min and held for 5.5 min. High-purity helium was used as a carrier gas. A standard mixture of the 16 EPA-PAHs was purchased from J&K Scientific (PAHs concentration of 2000 μg/ml, and a solvent mixture of dichloromethane: benzene, v: v= 1: 1) to determine the concentrations. The selected PAHs were listed in Table 2. Also, Table 2 shows the initial PAH concentrations in sewage sludge. The 16 EPA-PAHs are classified into high molecular weight (HMW) PAHs with five and six rings (BbF, BkF, BaP, IND, DAB, BghiP), middle molecular weight (MMW) PAHs with four rings (FL, Pyr, BaA, CHR), and low molecular weight (LMW) PAHs with two and three rings (Nap, AcPy, Acp, Flu, PA, Ant) (Zhao et al., 2016). Table 2. Selected PAHs for the analysis PAHs
Abbr.
Formula
Molecular weight
Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo [a] anthracene Chrysene Benzo [b] fluoranthene Benzo [k] fluoranthene Benzo [a] pyrene Indeno [1, 2, 3-cd] pyrene Dibenzo [a, h] anthracene Benzo [g, h, i] perylene
Nap AcPy Acp Flu PA Ant FL Pyr BaA CHR BbF BkF BaP IND DBA BghiP
C10H8 C12H8 C12H10 C13H10 C14H10 C14H10 C16H10 C16H10 C18H12 C18H12 C20H12 C20H12 C20H12 C22H12 C22H12 C22H14
128 152 154 166 178 178 202 202 228 228 252 252 252 276 278 276
*
Vapor pressure o (kPa, 25 C) -2 1.1×10 -3 3.9×10 -3 2.1×10 -5 8.7×10 -6 2.3×10 -6 3.6×10 -7 6.5×10 -6 3.1×10 -8 1.5×10 -10 5.7×10 -8 6.7×10 -8 2.1×10 -10 7.3×10 -11 10 -11 1.3×10 -11 1.3×10
Sewage sludge (mg/kg) 1.8067 0.7231 1.5144 0.9256 1.1457 1.0445 0.8719 0.9321 0.8154 * ND 1.3455 1.1864 0.9676 ND ND 1.2371
Not detected
2.5 QA/QC In order to quantify procedural recoveries, a known volume of surrogate solutions was added to samples prior to extraction. The recoveries of the spiked solutions were 92 ± 27% for PAHs.
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Since the recoveries were an acceptable range, analysis concentrations in samples were not surrogate-corrected. Detailed quality assurance/quality control (QA/QC) procedures for analysis have been described elsewhere (Cao et al., 2011). Method blanks, spiked blanks, and sample duplicates were routinely analyzed along with the regular samples. Blank concentrations were below 3% of the target value. The reagent blank has been subtracted from the values of concentration of the samples. 2.6 Thermogravimetric analysis Thermogravimetric analysis for sludge pyrolysis was conducted with a thermogravimetric analyzer (TGA-50H, Shimadzu, Japan). A known mass of dry sludge was used for each pyrolysis run in duplicate. Each sample was heated to 100 oC (with a heating rate of 10 K/min) and retained at the temperature for 20 min to remove adsorbed moisture, and thenheated to 1000 oC with 2 hour holding time. N2 gas (99.99%) was purged into TGA at a flow rate of 40 mL/min during pyrolysis.
3. RESULTS AND DISCUSSION 3.1 Impact of pyrolysis temperature on PM formation Figure 2 shows the average PM generation rate (ug/min) over temperature. A total 79 mg (2,641 mg PM/kg sludge) of PM was produced during heating from 100 to 1000 oC and extending holding time at 1000 oC for 2 hours. During the heating period (0-90 min), the average PM production rate also increased slightly as shown in Figure 2. The average PM generation rate ranged from 0.47 mg/min to 0.58 mg/min during heating from 100 to 1000 oC. However, PM generation rate decreased quickly with extending holding time at 1000 oC and reduced to 0.10 mg/min at the holding time rage of 1.5-2 hour (180-210 min in Figure 2). PM could be generated through the nucleation or condensation on the pyrolysis-generated gaseous precursors such as sulfuric acid, nitric acid and organic compounds (Yu et al., 2017).
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0.8
1200 PM generation rate temperature 1000
0.4 600
o
800
temperature ( C)
PM (ug/min)
0.6
400
0.2
200 0.0 0
50
100
150
200
Time (min)
Figure 2. PM generation rate and pyrolysis temperature over time.
On each TGA and mass loss rate (DTG) curves in Figure 3, three major thermal degradation areas were observed below 400 oC, 400-600 oC, and around 800-1000 oC. Comparing with the total mass loss during pyrolysis, seventy-eight percent of mass loss occurred below 600 oC. Generally, it is known that the low-temperature pyrolysis region (600 oC), the inorganic fraction of sewage sludge can be thermally stimulated to decompose, like carbonates and sulfides (Karayildirim et al., 2006). PM formation was promoted at high temperature range. For example, even though the mass loss occurred 13.2% between 70 min (800 oC) and 120 min (0.5 hour holding at 1000 oC), about 36% of PM was generated at this temperature range. This indicates that the increasing temperature favored the nucleation process and accelerated the formation of PM (Dewa et al., 2016).
12
0.000
1200
-0.001
1000
mass DTG temperature
8
-0.003
-0.004
DTG (mg/sec)
mass (mg)
-0.002 10
800
600
o
14
temperature ( C)
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400 6
-0.005 200
4
-0.006 0
50
100
150
200
time (min)
Figure 3. TGA and DTG curves during heating from 100 oC to 1000 oC and 2 hours holding at 1000 oC. The total operating time is 210 minutes. 3.2 Influence of temperature on the formation of PAHs in PM Total PAHs in PM collected during the 210-minute operation was measured to be 437.8 μg (5.53 μg PHAs/mg PM). However, PAH concentration in PM varied with PAH species as shown in Figure 4. Among the selected PAHs, FL, Pyr, BaA, BaP, and IND were included in a high emission group in the range from 0.52 μg/mg PM to 0.91 μg/mg PM. CHR, BbF, BkF, and BghiP belonged to the middle emission group with the range from 0.28 μg/mg PM to 0.36 μg/mg PM. The rest of PAHs including Nap, AcPy, Acp, Flu, PA, Ant, and DAB were categorized as the low emission group below 0.14 μg/mg PM. In the collected PM, Pyr showed the highest emission among PAHs with four rings and so did BaP among PAHs with five rings. However, LMW PAHs indicated relatively low emission with PM. The emission rates of PAH species in PM varied with temperature as shown in Figure 5. Each PAH specie emission with PM also varied during heating and holding temperature at 1000 oC. It was reported that high temperature favored the formation and release of PAHs (Llamas et al., 2017). During heating, PAHs concentration based on PM mass at 100-400 oC, 400-600 oC, 600-800 oC and 800-1000 oC was 0.55 μg PAHs/mg PM, 0.88 μg PAHs/mg PM, 0.89 μg PAHs/mg PM and 2.25 μg PAHs/mg PM, respectively.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
PAHs in PM (ug PAH/mg PM)
1.0
0.8
0.6
0.4
0.2
BghiP
DBA
IND
BaP
BkF
BbF
CHR
BaA
Pyr
FL
Ant
PA
Flu
Acp
AcPy
Nap
0.0
PAHs Figure 4. PHAs in collected PM during pyrolysis.
However, the generation rates of PAH species were varied with temperature as shown in Figure 5. Overall, PAHs in PM increased with rising temperature from 100-1000 oC except Nap, AcPy, Acp, Flu, PA, and FL. Nap, however, decreased with the temperature rising. AcPy, Acp, Flu, PA and Fla showed peaks concentrations at 400-600 oC. The decline in Nap, AcPy, Acp, Flu, PA and FL was also observed in acetylene pyrolysis at 800-1150 oC (Sánchez et al., 2012). These results of LMW PAHs (Nap, AcPy, Acp, Flu, PA) were different from those in Dai et al.’s work (2014). They observed that Nap, Flu and PA were dominant PAH species in gaseous phase in the range between 400 oC and 1300 oC. The difference may be due to that the Nap, AcPy, Acp, Flu, PA and Fla were presented in gaseous phase more than in PM (Peng et al., 2016b). In contrast to LMW, MMW and HMW PAHs showed a peak generation rates with extending holding time at 1000 oC (between 90min and 180min). MMW and HMW PAHs increased at high temperature might be precursors of PM (Sánchez et al., 2012). This result indicated that extending holding time at high temperature provided favorable reaction environment of volatile matter to produce PAHs as consequences of extended reaction time. Also, the extended holding time aided the formation of PM and the adsorption of PAHs on PM. The similar phenomenon was found in Dandajeh et al (2017)'s study.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
0.20
1200
0.10
1000 800 600
temperature (oC)
0.15 PAHs (ug/min)
(a)
Nap AcPy Acp Flu PA Ant temp.
400
0.05
200 0.00 0.20
1200 1000 800
0.10
600
o temperature ( C)
0.15 PAHs (ug/min)
(b)
FL Pyr BaA CHR
400
0.05
200 0.00 0.20
1200 1000 800
0.10
600
o temperature ( C)
0.15 PAHs (ug/min)
(c)
BbF BkF BaP IND DBA BghiP
400
0.05
200 0.00 0
50
100
150
200
time (min)
Figure 5. PAHs emission in PM during the heating and 2-hour holding at 1000 oC. (a) LMW PAHs, (b) MMW PAHs, (c) HMW PAHs.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
4. CONCLUSIONS In this study, the formation of particular mater (PM) and the levels of PAHs in the PM during sludge pyrolysis were investigated. A total 79 mg (2,641 mg PM/kg sludge) of PM was produced during heating from 100 to 1000 oC and extending holding time for 2 hours at 1000 oC. The average PM generation rate ranged from 0.47 mg/min to 0.58 mg/min during heating from 100 to 1000 oC but decreased quickly with extending holding time at 1000 oC Total PAHs of PM collected during the operation was 5.53 μg PHAs/mg PM. Among the measured PAHs, Pyr showed the highest emission and Flu showed the lowest emission in the collected PM. In addition, the emission rates of PAH species in PM varied during heating and holding temperature at 1000 oC. Overall, PAHs in PM increased with rising temperature from 100-1000 oC except LMW PAHs. In contrast, MMW and HMW PAHs showed a peak generation rate during extending holding time at 1000 oC indicating that extending holding time at high temperature provided favorable reaction environment to produce PAHs. The study results provide further understanding the emission characteristics of PM and PAHs in PM during sludge pyrolysis.
AKNOWLEDGEMENTS This research was supported by the Shenzhen government of China with Grant No. JCYJ20150616145013931.
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Lehmann J. and Joseph S. (2009). Biochar for Environment Management Science and Technology. Science and Technology; Earthscan., vol. 25, 15801-15811. Li J., Wang C., Gao X., Zhao F., Zhao K., Fan X., Zhang G., Zhang W. and Gao Z. (2016). Alkaline and alkaline earth salts of 5-ferrocenyl-1 H -tetrazole. Synthesis, characterization and catalytic effects on thermal decomposition of energetic compounds. POLYHEDRON., vol. 106, 58-64. Li K., Lei J., Yuan G., Weerachanchai P., Wang J. Y., Zhao J. and Yang Y. (2017). Fe-, Ti-, Zrand Al-pillared clays for efficient catalytic pyrolysis of mixed plastics. CHEM ENG J. Llamas A., Al-Lal A., García-Martínez M., Ortega M. F., Llamas J. F., Lapuerta M. and Canoira L. (2017). Polycyclic Aromatic Hydrocarbons (PAHs) produced in the combustion of fatty acid alkyl esters from different feedstocks: Quantification, statistical analysis and mechanisms of formation. SCI TOTAL ENVIRON., vol. 586, 446-456. Lu G. Q., Low J. C. F., Liu C. Y. and Lua A. C. (1995). Surface area development of sewage sludge during pyrolysis. FUEL., vol. 74, 344-348. Lv D., Zhu T., Liu R., Lv Q., Sun Y., Wang H., Liu Y. and Zhang F. (2016). Effects of co-processing sewage sludge in cement kiln on NOx, NH3 and PAHs emissions. CHEMOSPHERE., vol. 159, 595-601. Malińska K., Zabochnicka- Wi Tek M., Cáceres R. and Marfà O. (2016). The effect of precomposted sewage sludge mixture amended with biochar on the growth and reproduction of Eisenia fetida during laboratory vermicomposting. ECOL ENG., vol. 90, 35-41. Messina L. I. G., Bonelli P. R. and Cukierman A. L. (2017). In-situ catalytic pyrolysis of peanut shells using modified natural zeolite. FUEL PROCESS TECHNOL., vol. 159, 160-167. Ninomiya Y., Zhang L., Sakano T., Kanaoka C. and Masui M. (2004). Transformation of mineral and emission of particulate matters during co-combustion of coal with sewage sludge. FUEL., vol. 83, 751-764. Peng N., Li Y., Liu Z., Liu T. and Gai C. (2016). Emission, distribution and toxicity of polycyclic aromatic hydrocarbons (PAHs) during municipal solid waste (MSW) and coal co-combustion. SCI TOTAL ENVIRON., vol. 565, 1201-1207. Sánchez N. E., Callejas A., Millera A., Bilbao R. and Alzueta M. U. (2012). Formation of PAH and soot during acetylene pyrolysis at different gas residence times and reaction temperatures. ENERGY., vol. 43, 30-36. Takuwa T., Mkilaha I. S. N. and Naruse I. (2006). Mechanisms of fine particulates formation with alkali metal compounds during coal combustion. FUEL., vol. 85, 671-678. Thipkhunthod P., Meeyoo V., Rangsunvigit P., Kitiyanan B., Siemanond K. and Rirksomboon T. (2005). Predicting the heating value of sewage sludges in Thailand from proximate and ultimate analyses. FUEL., vol. 84, 849-857. Vu H. P. T., Tien V. N., Vigneswaran S. and Huu H. N. (2016). A review on sludge dewatering indices. WATER SCI TECHNOL., vol. 74, 1-16. Wang K., Zheng Y., Zhu X., Brewer C. E. and Brown R. C. (2017). Ex-situ catalytic pyrolysis of wastewater sewage sludge - A micro-pyrolysis study. BIORESOURCE TECHNOL., vol. 232, 229-234. Yu Z., Liscinsky D. S., Fortner E. C., Yacovitch T. I., Croteau P., Herndon S. C. and Richard M. L. (2017). Evaluation of PM emissions from two in-service gas turbine general aviation aircraft engines. ATMOS ENVIRON. Yuan H., Lu T., Wang Y., Chen Y. and Lei T. (2016). Sewage sludge biochar: Nutrient composition and its effect on the leaching of soil nutrients. GEODERMA., vol. 267, 17-23.
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Yuan H., Lu T., Wang Y., Huang H. and Chen Y. (2014). Influence of pyrolysis temperature and holding time on properties of biochar derived from medicinal herb (radix isatidis) residue and its effect on soil CO 2 emission. Journal of Analytical & Applied Pyrolysis., vol. 110, 277-284. Zhang H., Yan X., Cai B., Zhang Y., Liu J. and Ao H. (2015). The effects of aged refuse and sewage sludge on landfill CH4 oxidation and N2O emissions: Roles of moisture content and temperature. ECOL ENG., vol. 74, 345-350. Zhang L., Masui M., Mizukoshi H., Ninomiya Y., Koketsu J. and Kanaoka C. (2008). Properties of water-soluble and insoluble particulate matter emitted from dewatered sewage sludge incineration in a pilot-scale ash melting furnace. FUEL., vol. 87, 964-973. Zhang X., Sarmah A. K., Bolan N. S., He L., Lin X., Che L., Tang C. and Wang H. (2016). Effect of aging process on adsorption of diethyl phthalate in soils amended with bamboo biochar. CHEMOSPHERE., vol. 142, 28-34. Zhao O. Y., Feng S. D., Zhang X. N., Jia H. B., Shi W. and Yang Z. X. (2016). Starch-enhanced degradation of HMW PAHs by Fusarium sp. in an aged polluted soil from a coal mining area. CHEMOSPHERE. Zielińska A. and Oleszczuk P. (2016). Effect of pyrolysis temperatures on freely dissolved polycyclic aromatic hydrocarbon (PAH) concentrations in sewage sludge-derived biochars. CHEMOSPHERE., vol. 153, 68-74.
SUMMARY: In this study, the formation of particular mater (PM) during sludge pyrolysis and selected 16 polycyclic aromatic hydrocarbons (PAHs) in the PM have been investigated. PM was collected during sludge pyrolysis with various temperature ranges (100-400 oC, 400-600 oC, 600-800 oC, 800-1000 oC) and holding times (0-0.5, 0.5-1.0, 1.0-1.5, and 1.5-2.0 hours) at 1000oC using a PM impactor equipped with a quartz filter. A total 79 mg (2,641 mg PM/kg sludge) of PM was produced during the pyrolysis. The average PM generation rate ranged from 0.47 mg/min to 0.58 mg/min during heating from 100 to 1000 oC but decreased during extending holding time at 1000 oC. Total PAHs in PM collected during the pyrolysis was 5.53 μg PHAs/mg PM. However, the concentrations of PAH species varied. Among the measured PAHs, Pyr showed the highest concentration and Flu showed the lowest concentration in the collected PM. In addition, the emission rates of PAH species in PM varied during heating and holding temperature at 1000 oC. Overall, PAHs in PM increased with rising temperature from 100-1000 o C except low molecular weight (LMW) PAHs. Middle molecular weight (MMW) and high molecular weight (HMW) PAHs showed a peak generation rates during extending holding time at 1000 oC indicating that extending holding time at high temperature provided favorable reaction environments to produce PAHs.
1. INTRODUCTION A total of 32.34 million tons (dry weight) of sewage sludge was produced at China in 2014 (Buys et al., 2014). Sewage sludge is rich in organic matters containing heavy metals, microorganisms, and pathogens (Fytili and Zabaniotou, 2008). Pyrolysis gas contains components with high heating value, such as CH4, CO, and H2 as well as air pollutants including NOx, NH3, H2S, hydrocarbons, tar, and particulate matter (PM), etc. (Beneroso et al., 2015; Chianese et al.; Lv et al., 2016). In particular, PM emission is an important issue during thermal treatment of sewage sludge (Zhang et al., 2008). Because of the higher ash content in dry sludge (30-70 wt%), PM emission in sewage sludge combustion is more severe compared to those from other biomass combustion (Thipkhunthod et al., 2005). Ash can significantly contribute to PM emissions because particles are mainly produced by ash vaporization during thermal treatment and condensation (Takuwa et al., 2006). Many researchers have focused on PM emissions and characterized PM during sewage sludge incineration (Han et al., 2015; Ninomiya et al., 2004; Zhang et al., 2008). In contrast, sludge pyrolysis studies have rarely focused on PM emission characteristics in sewage sludge pyrolysis. Dewa et al. (2016) reported the particle size distribution of PM during ethylene pyrolysis was greatly influenced by holding time and pyrolysis Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
temperature. They concluded that PM nucleation rates increased with elevating temperatures (above 1540 K), resulting in enhancing PM formation. Pyrolysis temperature and holding time also affect biochar characteristics due to the release of volatiles as well as the formation and volatilization of intermediate melts (Lehmann and Joseph, 2009). Along with PM formation, polycyclic aromatic hydrocarbons (PAHs) emissions is a concern during thermal decompositions of biomass. The combination of PAH and PM increased the carcinogenic and mutagenic effects (Kok et al., 2006; Sánchez et al., 2012). It is known that PAHs are formed during biomass pyrolysis (Conesa et al., 2008). PAHs formation in different pyrolysis products (i.e. biochar, bio-oil, and biogas) during sewage sludge pyrolysis has been reported. Zielinska and Oleszczuk (2016) studied PAHs in biochars derived from various sewage sludge. PAHs contents in sewage sludge-derived biochars was 2-3 times lower than those in the fed sewage sludge. Hu et al. (Hu et al., 2014) characterized the PAHs in the pyrolysis bio-oil of sewage sludge and most of the selected PAHs were enriched in the pyrolysis bio-oil, with concentrations up to 48.9 mg/kg. Dai et al. (2014, 2015) measured that PAHs were over 1200 mg PAHs/kg sludge in gaseous phase during sludge pyrolysis at 850 oC (with a gas flow of 50 ml/min. They concluded that temperature, sample mass, holding time, particle size, and gas flow were important factors on PAHs formation in flue gas. PM can be a carrier for PAHs because of the large specific surface area and porous structure. However, studies on the formation of PM during sludge pyrolysis and the distribution of PAHs in PM were rare. In this study, PM emission during sludge pyrolysis was measured with different pyrolysis temperatures and PAHs in collected PM were characterized.
2. MATERIALS AND METHOD 2.1 Sewage sludge Sewage sludge, mechanically centrifuged to dewater, was obtained from a municipal wastewater treatment plant (WWTP) located in Shenzhen, China. The dewatered sludge cake was collected and then oven-dried for 24 h at 105 oC. Dry sludge samples were ground and then sieved. The physicochemical properties of the prepared sludge samples are presented in Table 1, including approximate analysis (GB/T 17664-1999) and ash composition (ICP-OES, Agilent, USA). 2.2 Sewage sludge pyrolysis As illustrated in Figure 1, pyrolysis was carried out using a horizontal furnace (Sanli Inc., China). In order to maintain an oxygen-free atmosphere, N2 (99.99% V/V) was purged into the furnace at a flow rate of 400 ml/min for 15 min before heating. In each run, a 30 g sludge sample was added into a crucible. The temperature of the furnace was held at 100oC for 20 min to remove absorbed moisture in the sample and then increased with a rate of 10oC /min up to 1000 o C. Pyrolysis temperature was held for 2 hours at 1000 oC. A cooling system was installed to cool down flue gas and to remove the condensable tar in the flue gas. The flow rate was fixed at 400 ml/min. 2.3 PM sampling A gas pump and a PM impactor were installed at the outlet of the cooling system to collect PM during sewage sludge pyrolysis. The flow rate of sampling gas was fixed at 1 l/min by a mass flowmeter controller (MFC). A quartz filter (Ø47 mm, Whatman Inc.) was installed in the impactor
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to collect suspended PM. Before installing, the quartz filter was roasted in a muffle furnace at 550oC for five hours to minimize contamination. The quartz filters were weighed before and after sampling PM. The quartz filter of the impactor was replaced at each desired temperature range (100-400 oC, 400-600 oC, 600-800 oC and 800-1000 oC) and holding time ranges (0-0.5 h, 0.5-1.0 h, 1.0-1.5 h and 1.5-2 h at 1000 oC). Table 1 Physicochemical properties of the raw sewage sludge Parameter Approximate analysis Moisture Volatile matter Ash Fixed carbon Ash composition Al2O3 SiO2 P 2O 5 Fe2O3 CaO K 2O TiO2 SO3 ZnO MnO CuO Cr2O3 PbO
Unit
Value
a
78.0 55.8 33.7 10.5
b
11.26 9.65 5.30 2.57 1.92 0.83 0.24 0.23 0.18 0.03 0.01 0.01 0.004
% b % b % b,c % % b % b % b % b % b % b % b % b % b % b % b % b %
a
: on as-received basis : on dry basis c : determined by the difference (Fixed carbon = 100% - Volatile matter - Ash) b
Figure 1. Schematic diagram of sewage sludge pyrolysis and PM collection system: (1) nitrogen gas cylinder, (2) mass flowmeter controller, (3) sewage sludge sample, (4) tube for dilution gas, (5) thermal couples, (6) crucible, (7) water bath, (8) condensing tube, (9) mass flowmeter, (10)
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cooling water, (11) PM impactor, (12) mass flowmeter controller, and (13) gas pump.
2.4 PAHs extraction and analysis PAHs in PM on a quartz filter were extracted and analyzed following Chinese standard method for determining the PAHs in PM from exhaust gas (HJ-646 2013). A quartz filter collected PM was size-reduced using a pair of ceramic scissors. The size-reduced quartz filter was added into an extraction solution (n-hexane: ether= 9:1 v/v) and sonicated for 20 minutes to extract PAHs. The supernatant was separated by centrifugation. The sonication-extraction step was repeated three times. The collected supernatant (45 ml) was concentrated to less than 1 ml by flushing N2 and purified with a PAHs purification column (Poly-sery MIP-PAHs SPE Cartridge, CNW, China). The final volume of each purified PAHs sample was adjusted to 1 ml using n-hexane. PAHs in the extract was analyzed with a GC (7890B, Agilent)/MS (5977B, Aglient) equipped with an HP-5MS capillary column (30 m×0.25 mm×0.25 mm). The oven temperature was held at 70 oC for 2 min, increased to 320 oC with a rate of 10 oC /min and held for 5.5 min. High-purity helium was used as a carrier gas. A standard mixture of the 16 EPA-PAHs was purchased from J&K Scientific (PAHs concentration of 2000 μg/ml, and a solvent mixture of dichloromethane: benzene, v: v= 1: 1) to determine the concentrations. The selected PAHs were listed in Table 2. Also, Table 2 shows the initial PAH concentrations in sewage sludge. The 16 EPA-PAHs are classified into high molecular weight (HMW) PAHs with five and six rings (BbF, BkF, BaP, IND, DAB, BghiP), middle molecular weight (MMW) PAHs with four rings (FL, Pyr, BaA, CHR), and low molecular weight (LMW) PAHs with two and three rings (Nap, AcPy, Acp, Flu, PA, Ant) (Zhao et al., 2016). Table 2. Selected PAHs for the analysis PAHs
Abbr.
Formula
Molecular weight
Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo [a] anthracene Chrysene Benzo [b] fluoranthene Benzo [k] fluoranthene Benzo [a] pyrene Indeno [1, 2, 3-cd] pyrene Dibenzo [a, h] anthracene Benzo [g, h, i] perylene
Nap AcPy Acp Flu PA Ant FL Pyr BaA CHR BbF BkF BaP IND DBA BghiP
C10H8 C12H8 C12H10 C13H10 C14H10 C14H10 C16H10 C16H10 C18H12 C18H12 C20H12 C20H12 C20H12 C22H12 C22H12 C22H14
128 152 154 166 178 178 202 202 228 228 252 252 252 276 278 276
*
Vapor pressure o (kPa, 25 C) -2 1.1×10 -3 3.9×10 -3 2.1×10 -5 8.7×10 -6 2.3×10 -6 3.6×10 -7 6.5×10 -6 3.1×10 -8 1.5×10 -10 5.7×10 -8 6.7×10 -8 2.1×10 -10 7.3×10 -11 10 -11 1.3×10 -11 1.3×10
Sewage sludge (mg/kg) 1.8067 0.7231 1.5144 0.9256 1.1457 1.0445 0.8719 0.9321 0.8154 * ND 1.3455 1.1864 0.9676 ND ND 1.2371
Not detected
2.5 QA/QC In order to quantify procedural recoveries, a known volume of surrogate solutions was added to samples prior to extraction. The recoveries of the spiked solutions were 92 ± 27% for PAHs.
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Since the recoveries were an acceptable range, analysis concentrations in samples were not surrogate-corrected. Detailed quality assurance/quality control (QA/QC) procedures for analysis have been described elsewhere (Cao et al., 2011). Method blanks, spiked blanks, and sample duplicates were routinely analyzed along with the regular samples. Blank concentrations were below 3% of the target value. The reagent blank has been subtracted from the values of concentration of the samples. 2.6 Thermogravimetric analysis Thermogravimetric analysis for sludge pyrolysis was conducted with a thermogravimetric analyzer (TGA-50H, Shimadzu, Japan). A known mass of dry sludge was used for each pyrolysis run in duplicate. Each sample was heated to 100 oC (with a heating rate of 10 K/min) and retained at the temperature for 20 min to remove adsorbed moisture, and thenheated to 1000 oC with 2 hour holding time. N2 gas (99.99%) was purged into TGA at a flow rate of 40 mL/min during pyrolysis.
3. RESULTS AND DISCUSSION 3.1 Impact of pyrolysis temperature on PM formation Figure 2 shows the average PM generation rate (ug/min) over temperature. A total 79 mg (2,641 mg PM/kg sludge) of PM was produced during heating from 100 to 1000 oC and extending holding time at 1000 oC for 2 hours. During the heating period (0-90 min), the average PM production rate also increased slightly as shown in Figure 2. The average PM generation rate ranged from 0.47 mg/min to 0.58 mg/min during heating from 100 to 1000 oC. However, PM generation rate decreased quickly with extending holding time at 1000 oC and reduced to 0.10 mg/min at the holding time rage of 1.5-2 hour (180-210 min in Figure 2). PM could be generated through the nucleation or condensation on the pyrolysis-generated gaseous precursors such as sulfuric acid, nitric acid and organic compounds (Yu et al., 2017).
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0.8
1200 PM generation rate temperature 1000
0.4 600
o
800
temperature ( C)
PM (ug/min)
0.6
400
0.2
200 0.0 0
50
100
150
200
Time (min)
Figure 2. PM generation rate and pyrolysis temperature over time.
On each TGA and mass loss rate (DTG) curves in Figure 3, three major thermal degradation areas were observed below 400 oC, 400-600 oC, and around 800-1000 oC. Comparing with the total mass loss during pyrolysis, seventy-eight percent of mass loss occurred below 600 oC. Generally, it is known that the low-temperature pyrolysis region (600 oC), the inorganic fraction of sewage sludge can be thermally stimulated to decompose, like carbonates and sulfides (Karayildirim et al., 2006). PM formation was promoted at high temperature range. For example, even though the mass loss occurred 13.2% between 70 min (800 oC) and 120 min (0.5 hour holding at 1000 oC), about 36% of PM was generated at this temperature range. This indicates that the increasing temperature favored the nucleation process and accelerated the formation of PM (Dewa et al., 2016).
12
0.000
1200
-0.001
1000
mass DTG temperature
8
-0.003
-0.004
DTG (mg/sec)
mass (mg)
-0.002 10
800
600
o
14
temperature ( C)
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400 6
-0.005 200
4
-0.006 0
50
100
150
200
time (min)
Figure 3. TGA and DTG curves during heating from 100 oC to 1000 oC and 2 hours holding at 1000 oC. The total operating time is 210 minutes. 3.2 Influence of temperature on the formation of PAHs in PM Total PAHs in PM collected during the 210-minute operation was measured to be 437.8 μg (5.53 μg PHAs/mg PM). However, PAH concentration in PM varied with PAH species as shown in Figure 4. Among the selected PAHs, FL, Pyr, BaA, BaP, and IND were included in a high emission group in the range from 0.52 μg/mg PM to 0.91 μg/mg PM. CHR, BbF, BkF, and BghiP belonged to the middle emission group with the range from 0.28 μg/mg PM to 0.36 μg/mg PM. The rest of PAHs including Nap, AcPy, Acp, Flu, PA, Ant, and DAB were categorized as the low emission group below 0.14 μg/mg PM. In the collected PM, Pyr showed the highest emission among PAHs with four rings and so did BaP among PAHs with five rings. However, LMW PAHs indicated relatively low emission with PM. The emission rates of PAH species in PM varied with temperature as shown in Figure 5. Each PAH specie emission with PM also varied during heating and holding temperature at 1000 oC. It was reported that high temperature favored the formation and release of PAHs (Llamas et al., 2017). During heating, PAHs concentration based on PM mass at 100-400 oC, 400-600 oC, 600-800 oC and 800-1000 oC was 0.55 μg PAHs/mg PM, 0.88 μg PAHs/mg PM, 0.89 μg PAHs/mg PM and 2.25 μg PAHs/mg PM, respectively.
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PAHs in PM (ug PAH/mg PM)
1.0
0.8
0.6
0.4
0.2
BghiP
DBA
IND
BaP
BkF
BbF
CHR
BaA
Pyr
FL
Ant
PA
Flu
Acp
AcPy
Nap
0.0
PAHs Figure 4. PHAs in collected PM during pyrolysis.
However, the generation rates of PAH species were varied with temperature as shown in Figure 5. Overall, PAHs in PM increased with rising temperature from 100-1000 oC except Nap, AcPy, Acp, Flu, PA, and FL. Nap, however, decreased with the temperature rising. AcPy, Acp, Flu, PA and Fla showed peaks concentrations at 400-600 oC. The decline in Nap, AcPy, Acp, Flu, PA and FL was also observed in acetylene pyrolysis at 800-1150 oC (Sánchez et al., 2012). These results of LMW PAHs (Nap, AcPy, Acp, Flu, PA) were different from those in Dai et al.’s work (2014). They observed that Nap, Flu and PA were dominant PAH species in gaseous phase in the range between 400 oC and 1300 oC. The difference may be due to that the Nap, AcPy, Acp, Flu, PA and Fla were presented in gaseous phase more than in PM (Peng et al., 2016b). In contrast to LMW, MMW and HMW PAHs showed a peak generation rates with extending holding time at 1000 oC (between 90min and 180min). MMW and HMW PAHs increased at high temperature might be precursors of PM (Sánchez et al., 2012). This result indicated that extending holding time at high temperature provided favorable reaction environment of volatile matter to produce PAHs as consequences of extended reaction time. Also, the extended holding time aided the formation of PM and the adsorption of PAHs on PM. The similar phenomenon was found in Dandajeh et al (2017)'s study.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
0.20
1200
0.10
1000 800 600
temperature (oC)
0.15 PAHs (ug/min)
(a)
Nap AcPy Acp Flu PA Ant temp.
400
0.05
200 0.00 0.20
1200 1000 800
0.10
600
o temperature ( C)
0.15 PAHs (ug/min)
(b)
FL Pyr BaA CHR
400
0.05
200 0.00 0.20
1200 1000 800
0.10
600
o temperature ( C)
0.15 PAHs (ug/min)
(c)
BbF BkF BaP IND DBA BghiP
400
0.05
200 0.00 0
50
100
150
200
time (min)
Figure 5. PAHs emission in PM during the heating and 2-hour holding at 1000 oC. (a) LMW PAHs, (b) MMW PAHs, (c) HMW PAHs.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
4. CONCLUSIONS In this study, the formation of particular mater (PM) and the levels of PAHs in the PM during sludge pyrolysis were investigated. A total 79 mg (2,641 mg PM/kg sludge) of PM was produced during heating from 100 to 1000 oC and extending holding time for 2 hours at 1000 oC. The average PM generation rate ranged from 0.47 mg/min to 0.58 mg/min during heating from 100 to 1000 oC but decreased quickly with extending holding time at 1000 oC Total PAHs of PM collected during the operation was 5.53 μg PHAs/mg PM. Among the measured PAHs, Pyr showed the highest emission and Flu showed the lowest emission in the collected PM. In addition, the emission rates of PAH species in PM varied during heating and holding temperature at 1000 oC. Overall, PAHs in PM increased with rising temperature from 100-1000 oC except LMW PAHs. In contrast, MMW and HMW PAHs showed a peak generation rate during extending holding time at 1000 oC indicating that extending holding time at high temperature provided favorable reaction environment to produce PAHs. The study results provide further understanding the emission characteristics of PM and PAHs in PM during sludge pyrolysis.
AKNOWLEDGEMENTS This research was supported by the Shenzhen government of China with Grant No. JCYJ20150616145013931.
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