Continuous Pyrolysis of Sewage Sludge in a Screw-Feeding Reactor

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Continuous pyrolysis of sewage sludge in a screw-feeding reactor: products characterization and ecological risk assessment of heavy metals. Ningbo Gao, Cui Quan, Baoling Liu, Zongyang Li, Chunfei Wu, and Aimin Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03112 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Continuous pyrolysis of sewage sludge in a screw-feeding reactor: products characterization and ecological risk assessment of heavy metals

Ningbo Gao†, Cui Quan*,†, Baoling Liu‡, Zongyang Li†, Chunfei Wu§, Aimin Li‡ †

‡

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, China, 710049.

School of Environmental Science and Technology, Dalian University of Technology, Dalian, China, 116024 §

School of Engineering and Computer Science, University of Hull, Hull, HU6 7RX, UK

ABSTRACT: Pyrolysis of dried sewage sludge is regarded as an effective treatment method as well as a promising technology for energy/fuel production. In this work, thermo-chemical conversion of dried sewage sludge in a continuous reactor at different pyrolysis temperatures (400–800 °C) and solid residence time (6–46 min) was conducted. The pyrolysis products obtained using different pyrolysis conditions were extensively investigated. It is indicated that high pyrolysis temperature (>700 °C) and long solid residence time (> 23 min) could enhance secondary reactions and decrease the yield of bio-oil. The maximum yield of bio-oil of 16.69% was achieved at reaction temperature of 700 °C and solid residence time of 23 min. The FTIR and GC-MS analyses of the bio-oil obtained at optimum condition indicated that it contained large amounts of phenols and esters. H2 and CO2 were the main components of pyrolysis gas, with a total amount that exceeded 52.18%. The characteristics of the char, including elemental

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composition, functional groups and combustion property were investigated by XRF, FTIR and TG. FTIR analysis of the char showed that the main functional groups are M-X, C-O and C-H. The volatile content in the char decreased with an increase of pyrolysis temperature, whilst it increased with an increase of solid residence time. Heavy metals distribution in pyrolysis products of dried sewage sludge were investigated by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The results showed that most of the heavy metals remained in char. In addition, the potential ecological risk assessment of heavy metals was assessed.

KEYWORDS: Dried sewage sludge; Pyrolysis; Char; Ecological risk assessment

1. INTRODUCTION Sewage sludge, a major by-product generated from the biological process of wastewater treatment plant, has raised an increasing concern due to its large production and potential risks for human health and environment.1 Although traditional disposal methods, e.g. landfill, composting and simple incineration, could treat sewage sludge, facing the strict environmental regulations, the treatment of sewage sludge is becoming technically problematic and more costly because of its hazardous composition, high moisture content and lower heating values. After drying the sewage sludge, the heating value increases to a greater extent. Therefore, alternative technologies for the treatment of dried sewage sludge are required. Thermo-chemical technologies such as pyrolysis, gasification, liquefaction and hydrogenation are considered as effective methods to convert sewage sludge into fuel, energy and chemical feedstock.2 In particular, pyrolysis of sewage sludge is regarded as a very promising alternative to the applications of landfill and farmland.3 Pyrolysis is a thermal decomposition of organic material


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in the absence of oxygen. The main pyrolysis products are pyrolysis gas, bio-oil and char. Pyrolysis gas can be used as feedstock for chemical industry or for the generation of heat and electricity. Pyrolysis oil can be used as a fuel for boiler. It could also be upgraded to transport fuel.4 Unlike incineration, sewage sludge pyrolysis usually produces a large fraction of char (about 50% of the weight of raw material). The pyrolysis char derived from sewage sludge can be utilized as solid fuel for heat production or can be applied to adsorb heavy metals or organic pollutants. Additionally, pyrolysis char has been studied as an inexpensive catalyst or catalytic support and has been used as an excellent soil amendment.5 Therefore, the method of pyrolysis for the sewage sludge disposal is a technology that not only meets the ecological requirements of sustainable development, but implies economic and social responsibilities as well.6 During pyrolysis of sewage sludge, several factors such as pyrolysis temperature, reactor type and heating rate have obvious influence on product characteristics and distribution.7-10 Optimization of process conditions for the pyrolysis of sewage sludge has been carried out using various techniques including thermo-gravimetric analysis, slow pyrolysis, fast pyrolysis and microwave-assisted pyrolysis.11 Gao et al.12 reported a maximum tar yield for dried sewage sludge pyrolysis (46.14%) at 550 °C in a tubular pyrolyzer, when a temperature range between 450-650 °C was investigated. Jaramillo et al.13 investigated pyrolysis products of anaerobically digested sewage sludge in a fluidized bed reactor and reported the following conclusion: a) below 500 °C, the main products produced were char, CO2 and H2O, b) when temperature reached above 600 °C, CO and H2 were dominant. Fan et al.14 observed fast pyrolysis of sewage sludge collected from Shanghai in a curie-point pyrolyzer at four predetermined temperatures (315, 485, 650, and 764 °C). It was found that increasing the pyrolysis temperature increased the content of carboxylic acids and monoaromatics but decreased the amounts of long-chain


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aliphatic amides and nitriles. In addition, Alvarez et al.15 proposed that a conical spouted bed reactor was suitable for maximizing liquid yield (77 wt.% at 500 °C) because the reactor produced a low residence time of volatiles. Xu et al.16 proposed that temperature of 450 °C, retention time of 75 min and heating rate of 3 °C/min were the optimum combination of pyrolysis parameters in terms of iodine value and surface area of pyrolysis char. As a consequence of the uses of different technologies, conditions and raw materials, the pyrolysis behavior of sewage sludge in literature differs substantially. Most of the researches were carried out on batch experiments. A continuous pyrolysis of sludge might have more advantages for potential commercial application. To optimize the technique of sewage sludge pyrolysis and have a better understanding of the influence of different pyrolysis conditions on pyrolysis products, in-depth investigations and a systematic study about pyrolysis of sewage sludge are still needed, which are also important for the practical applications of the pyrolysis technology for sewage sludge management. One of the advantages of sewage sludge pyrolysis is the evaporation of heavy metals can be largely prohibited, because a lower reaction temperature was required for the pyrolysis process compared to the incineration of sewage sludge. Thus, the emission of heavy metals into the atmosphere causing severe environmental and health problems can be greatly minimized.17 However, in considering the utilization or disposal of pyrolysis char produced from sewage sludge, the toxicity of pyrolysis char is a critical issue. Recently, researchers have focused on the studies of physical and chemical properties of char, produced from the pyrolysis of sewage sludge, under different conditions.12,18 For example, Lu et al.19 investigated the characteristics of heavy metals in biochar derived from sewage sludge at temperatures between 300 and 700 °C; it was reported that the enrichment of heavy metals in the biochar was enhanced with the increase


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of pyrolysis temperature. Jin et al.20 proposed that most of heavy metals existed in pyrolysis char after at 600 °C, resulted in a significant decline in bioavailability, leading to a very low ecological risk of biochar exposed to the environment. In particular, pyrolysis temperature has shown a significant influence on the physical and chemical properties of pyrolysis char. However, there are few comprehensive and specialized analyses of the products composition as well as the ecological risk assessment for the products produced from the pyrolysis of sewage sludge. In this paper, pyrolysis of sewage sludge was carried out using a continuous screw-feeding reactor. The influences of pyrolysis temperature and solid residence time on the distribution and properties of pyrolysis products were investigated. Various characterization methods, such as elemental analyzer, fourier transform infrared spectroscopy (FTIR), gas chromatography-mass spectroscopy (GC-MS), thermo-gravimetric (TG), x-ray fluorescence (XRF), inductively coupled plasma atomic emission spectrometry (ICP-AES) were applied to analyze the properties of the pyrolysis products. Furthermore, the distribution of heavy metals during pyrolysis process and the potential ecological risk assessment of heavy metals related to the sewage sludge char were studied. Detailed analysis of the results will permit the optimization of sewage sludge pyrolysis process.

2. MATERIALS AND METHODS 2.1. Materials. Dried sewage sludge used in this study was obtained from a sewage sludge drying plant in Dalian, China. The property of dried sewage sludge is given in Table1.

Table 1. The Property of Dried Sewage Sludge Used in This Study


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Proximate analysis (wt.%, air dried)

Ultimate analysis (wt.%, air dried)

M

C

V

A

FC

H

N

Oa

Higher heating value (HHV) (MJ/kg)

6.30 a

54.06

31.84

7.80

35.16

5.44

5.61

15.64

14.76

By difference.

2.2. Experimental Equipment and Procedure. In this work, pyrolysis experiments were carried out in a spiral continuous pyrolysis reactor (as indicated in Figure 1). The pyrolysis reactor is composed of a spiral screw (internal diameter = 40 mm and length = 1200 mm), electrical heating furnace, condensate system and product collection system. A K-type thermocouple was used to control the temperature of the reactor. A detailed description of the pyrolysis reactor was given in the previous work.12

Figure 1. Schematic diagram of the screw reactor assembly.

Pyrolysis of dried sewage sludge at different pyrolysis temperature and solid residence time was investigated. Pyrolysis temperature was varied from 500 to 800 °C with a solid residence time of 23 min. When the solid residence time was investigated at 6, 18, 23 and 46 min,


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respectively, the pyrolysis temperature was maintained at 700 °C. Feeding of the raw material was 4.35, 13.04, 23.61 and 100 g min-1 when the residence time was 46, 23, 18 and 6 min, respectively. Before each experiment, N2 was used to purge the reactor system to obtain an oxygen-free atmosphere. When the temperature of tubular reactor reached the desired value, N2 was stopped and the dried sewage sludge was fed into the reactor at a corresponding feeding rate controlled by the frequency of the motor. Once the reaction reached a steady state after 30–40 min of each run, gas samples were collected every 5 min and analyzed by gas chromatograph (Agilent, Micro GC490). The liquid and solid products were collected and weighed when the pyrolysis reactor was cooled down to room temperature.

2.3. Product Analysis. Ultimate and proximate analysis of raw material and chars were tested by automatic proximate analyzer (SDTGA5000, Sundy, China) and elemental analyzer (VarioEL III, Germany), respectively. The calorific value of the sample was measured by an oxygen bomb calorimeter. Combustion behavior of the raw material and the chars was investigated by TG analysis (EXSTAR 6000 TG/DTA 6300, Japan). In each run, approximately 16 mg of the sample was placed in an alumina crucible. The sample was heated from ambient temperature to 900 °C at a heating rate of 20 °C min-1 and then held at 900 °C for 10 min under air atmosphere. The functional groups in bio-oil and char were detected with a FTIR analyzer (Bruker EQUINOX 55, Germany). Bio-oil samples were extracted by dichloromethane (CH2Cl2), an organic solvent to remove water before FTIR analysis, and the pyrolysis chars were dried and sieved prior to the FTIR analysis. In addition, the composition of the bio-oil was measured by GC-MS (Agilent HP6890/5973MS, USA).


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The inorganic elements contained in the char were determined by X-ray fluorescence spectrometer (Shimadzu XRF-1800, Japan). The heavy metal contents in the raw material, biooil and char were detected by an inductively coupled plasma atomic emission spectrometry (ICPAES) (PerkinElmer Optima 2000DV, USA). 3. RESULTS AND DISCUSSION 3.1. Yield Distribution. Figures 2a and 2b show the influence of solid residence time and pyrolysis temperature on product distribution of dried sewage sludge pyrolysis, respectively. The gas, char and oil yield can be defined by Equation 1. Yi =

mi m0

(1)

where, Yi is the yield rate, %; mi is the mass of products, kg; i represents the mass of gas, char and oil produced in pyrolysis, kg and m0 is the initial mass of dry sludge, kg. As shown in Figure 2a, when the solid residence time increased from 6 to 46 min, the char yield was decreased from 65.20 to 55.55%, while the gas yield increased from 16.49 to 25.69%. With the increase of solid residence time, the bio-oil yield was obviously increased from 13.63% at 6 min to 16.69% at 23 min, and then slightly declined to 15.81% at 46 min. It should be noted that when the solid residence time was greater than 23 min, the char yield changed only slightly, indicating that the influence of solid residence time on the char yield was negligible when the residence time was over 23 min. It has been reported that longer residence time promoted the conversion of raw material and the formation of gaseous products during pyrolysis process.2 Gheorghe et al.21 reported that the yield of char was reduced slightly from 30.5 to 28.7%, when the residence time was increased from 5 to 30 min during pyrolysis of cherry sawdust. Even though the yield of pyrolysis char decreased slightly with the rise of the residence time from 23


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to 46 min, only slight enhancement of the conversion of sewage sludge was obtained. It was therefore suggested that the solid residence time of 23 min was taken as an optimum value. (a) 100

(b)

100

90

90

80

80

70

70

60

Yield / %

Yield / %

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50 40

60 50 40

30

30

20

20

10

10

0

0 6

18

23

46

Solid Residence Time / min

400

500

600

700

o

800

Pyrolysis Temperature / C

Figure 2. Effect of (a) solid residence time and (b) pyrolysis temperature on products distribution. Figure 2b presents the product distribution from the pyrolysis of dried sewage sludge at different pyrolysis temperature. The gas yield increased from 10.49 to 29.28%, while the char yield decreased from 74.92 to 56.98% with the increase of pyrolysis temperature from 500 to 800 °C. The variation of char yield was consistent with the results reported by Zhang et al.22 for a microwave-induced pyrolysis of sewage sludge. It is worth noting that the yield of char slightly decreased when the pyrolysis temperature increased from 700 to 800 °C, indicating that the majority of volatiles had been released at 700 °C23. Trinh et al.3 investigated the influence of pyrolysis temperature on sewage sludge product distribution using a centrifugal reactor. They reported that the char yield was significantly reduced from 55% to 25% when the pyrolysis temperature was increased from 475 to 575 °C. In addition, they also reported that only slight reduction of char yield was obtained (25% to 22%) when the pyrolysis temperature was


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increased from 575 to 625 °C. It is suggested that the major release of volatiles in terms of pyrolysis temperature was related to the nature of raw sewage sludge during pyrolysis. In this work, char yield was greater than 55% for all experiments. This might be due to the high ash content in the dried sewage sludge. As indicated in Figure 2b, the yield of bio-oil gradually increased from 11.97 to 17.51% between 400 and 700 °C, and then decreased to 11.79% when the temperature was further raised to 800 °C. The maximum bio-oil yield of 16.69% was obtained at 700 °C. When the pyrolysis temperature was lower than 700 °C, more large-molecule organic chemicals in the sludge were released as volatiles with the increase of temperature, and the rate of bio-oil formation was faster than secondary reactions, thus the bio-oil yield increased with the increase of temperature. Between 400 and 700 °C, the pyrolysis product yield was consistent with the results reported by Yuan et al.24, who investigated the pyrolysis of municipal sludge. However, most large molecule compounds were decomposed into small molecules such as CO, CH4, H2, CO2 and CmHn, and the secondary reactions of gaseous bio-oil were enhanced when the reaction temperature was higher than 800 °C. This leads to a decrease in bio-oil yield and an increase in gas yield for reaction temperature greater than 800 °C. 3.2. Gas Composition. The effect of solid residence time and pyrolysis temperature on the composition of gas product is shown in Figure 3. The main gases under different pyrolysis conditions were H2 and CO2 with a total content over 52.18%. In all cases, C2-C3 concentration was the least among the detected gases.


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H2

CO

CH4

CO2

C2 ~ C3

(a)

80

Gas composition / vol%

40

Gas composition / vol%

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30

20

(b)

H2

CO

CH4

CO2

C2 ~ C3

60

40

20

10 0

6

18

23

46

400

500

Solid Residence Time / min

600

o

700

800

Temperature / C

Figure 3. Effect of (a) solid residence time and (b) pyrolysis temperature on gas composition. As shown in Figure 3a, H2 concentration sharply increased from 30.65 to 37.94%, when the solid residence time increased from 6 to 46 min. Meanwhile, the solid residence time shows an obvious influence on the concentrations of CO2 and CO, which were around 14.24-30.04% and 12.70-21.59%, respectively. It is to be noted that the concentration of C2-C3 increased from 8.68% at 6 min to 14.18% at 23 min and then reduced to the value of 8.35% at 46 min. From Figure 3(a), it is suggested that the variation of CH4 concentration was relatively stable between 15.50 and 17.91%, indicating that the solid residence time has little influence on the concentration of CO. From Figure 3b, it is observed that the gas product derived at higher pyrolysis temperature has higher concentrations of H2, CH4 and CO, which could be due to the enhanced secondary thermal decomposition reactions of the primary volatiles occurred at higher temperature.25 However, the influence of pyrolysis temperature ranging from 500 to 800 °C on the concentration of H2 seems to be minor compared with that obtained at 400 °C. The concentration of CO2 decreased significantly from 79.28 to 21.26% as the pyrolysis temperature increased.


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This may be resulted from the secondary reaction of bio-oil and the further formation of char through polymerization.12

3.3. Bio-Oil Characteristics. 3.3.1. Composition of Bio-Oil. GC-MS was used to quantitatively identify the chemical composition of the bio-oil samples obtained at different pyrolysis conditions, and the results are shown in Table 2. The chemical composition of the bio-oil is classified into the following groups: halohydrocarbon, phenols, alkenes, alcohols, nitriles, acids, esters, siloxanes, alkanes, amides, ketones and aromatic compounds. In comparison to the composition of bio-oil obtained at 700 °C, the bio-oil produced at 500 °C was more complex with the eleven classes of compounds identified indicating that the cracking of large-molecule compounds were promoted at higher reaction temperature”. Among these compounds listed in Table 2, halohydrocarbons, nitriles, acids, siloxanes, alkanes, amides and ketones are abundant in bio-oil obtained at pyrolysis temperature of 500 °C. As shown in Table 2, the bio-oil obtained at 700 °C contains larger amounts of phenols, which might be resulted from the pyrolysis of polysaccharides and proteins into phenols at high pyrolysis temperature25. The absence of alkanes at higher pyrolysis temperature could be explained by cracking and thermal dehydrogenation of alkanes 26. Table 2. Chemical Composition of Bio-Oil from Pyrolysis of Dried Sewage Sludge Yield / % Compounds 500 °C, 23 min

700 °C, 23 min

700 °C, 6 min

halohydrocarbon

20.44

9.24

10.79

phenols

7.00

10.67

10.08

alkenes

3.53

5.01

5.68


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alcohols

-

1.54

2.22

nitriles

5.47

-

1.36

acids

20.13

-

8.36

esters

9.18

30.31

1.06

siloxanes

6.37

2.93

44.59

alkanes

2.33

-

-

amides

9.93

1.53

2.65

ketone

6.40

-

-

aromatic compounds

9.21

38.74

13.20

The bio-oil produced at different solid residence time shows some similar chemical composition such as halohydrocarbon, phenols, alkenes, alcohols, and amides with the yield variation ranging from 0.59 to 1.55%. In addition, the bio-oil obtained at solid residence time of 23 min contains more esters and aromatic compounds as well as less siloxanes when compared to the bio-oil obtained at solid residence time of 6 min. This suggests that a longer solid residence time was in favor of the production of esters and aromatic compounds. It could be due to the fact that aromatic compounds were generated from secondary reactions such as DielsAlder reaction and the cyclization of alkenes.27 Among these compounds listed in Table 2, nitriles and acids are abundant in bio-oil obtained at the solid residence time of 6 min. A reasonable explanation could be that there was insufficient solid residence time to the thermal cracking of macromolecular organic compounds within the raw material when the solid residence time was short.


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3.3.2. Functional Groups in the Bio-Oil. In order to determine the functional groups distribution in the liquid products obtained at different pyrolysis conditions, FTIR analysis was applied and the resultant FTIR spectra of bio-oil are presented in Figure 4. The bands centered at 2920 and 2850 cm-1 correspond to the aliphatic groups such as -CH3 and -CH2, together with a peak at 1450 cm-1 caused by the C-H deformation, which demonstrate the presence of alkyl aromatic and alkenes compounds in the liquid products and the raw material.28

o

500 C, 23min o 700 C, 23min o 700 C, 6min Raw materail

-1

1650cm

-1

2850cm

-1

560cm

-1

1550cm

-1

1030cm -1

2230cm

-1

-1

3230cm -1 2920cm 4000

3500

3000

2500 2000 -1 Wavenumber / cm

1450cm -1 1720cm -1 1270cm 750cm-1 1500

1000

500

Figure 4. FTIR spectrum of bio-oil at different pyrolysis conditions.

The absorbance band of M-X stretching vibration at 560 cm-1 is assigned to the presence of inorganic and organic halogens compounds.29 The band in 1030 cm-1 related to the stretching of OH vibration might be attributed to the presence of mineral and silicate compounds, and hydrocarbon in the dried sewage sludge.30 The bands at 3230 cm-1, 1650 cm-1, 1550 cm-1 are typical of N-H stretching, amide I band and amide Ⅱ band, respectively, indicating the presence of proteins in the raw material.25 The − C ≡ N stretching vibration band at 2230 cm-1 indicates the


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presence of nitriles, as shown in the FTIR spectra of liquid products. The band at 1720 cm-1 was caused by the C-O stretching vibration of fatty acids.31 Absorption peak of 1270 cm-1 is attributed to the C-O-C stretching vibration of ester and phenol. The peaks of spectra in liquid products disappeared completely compared to the raw material, especially for the bands at 560, 1030 and 1550 cm-1. It is suggested to be the decomposition of inorganic and organic compounds such as halogens, mineral compounds, hydrocarbon and proteins during the pyrolysis process. The decreased intensity of peak at 3275 cm-1 observed in Figure 4 indicated that the organic NH and OH groups were greatly unstable at rising pyrolysis temperature.32 The O-H stretching vibration indicates the presence of alcohols, carboxylic, phenols and water. The FTIR analysis suggested that the intensity of spectra in bio-oil obtained at different reaction temperatures are similar. However, significant influence of solid residence time on the intensity of FTIR spectra of bio-oil was observed. Furthermore, the presence of main function groups including alcohols, carboxylic, phenols are consistent with the results obtained from GC-MS analysis of bio-oil.

3.4. Bio-char characteristics. 3.4.1. Elemental Composition of Char. In order to investigate the evolution behavior of element during pyrolysis process, XRF was used to analyze the elemental composition in the dried sewage sludge and its derived char obtained at different pyrolysis conditions. The results of XRF analysis are shown in Table 3. The ratio of the elemental content in the char to that in the raw material (Cchar / Craw material) is defined as accumulation factor. When the accumulation factor is greater than 1, the elemental content in the char was higher than that in the raw material. When the accumulation factor is less than 1, it indicates that these elements evaporated into the


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pyrolysis gas. In addition, the accumulation factor of zero suggests that the elements were completely evaporated during the pyrolysis process. Based on the elemental composition, it is observed that both the raw material and the char have a high content of SiO2 and P2O5 (>16%). The values of accumulation factor for SiO2, Al2O3, MgO and Na2O were higher than 1.00, suggesting that the accumulation of these elements occurred during the pyrolysis process. I, Y2O3, Rb2O and Br were detected in the raw material. However, they could not be detected in the char, revealing that these elements were completely evaporated during the pyrolysis process.

Table 3. The Elemental Composition of Raw Material and Char Detected by XRF Content / % Element Raw material Char (700 °C, 23 min)

Accumulation factor

SiO2

22.42

25.76

1.15

P2O5

16.66

16.64

1.00

Fe2O3

11.99

10.82

0.90

Al2O3

9.92

11.42

1.15

CaO

8.78

8.67

0.99

SO3

8.64

5.72

0.66

CuO

5.85

4.92

0.84

K2O

4.61

4.67

1.01

MgO

2.95

4.02

1.36

ZnO

1.71

1.30

0.76

SnO2

1.54

1.49

0.97

TiO2

1.20

1.17

0.97

MnO

0.90

0.83

0.93


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Na2O

0.78

0.88

1.14

Cl

0.62

0.52

0.84

Cr2O3

0.43

0.40

0.93

BaO

0.31

0.30

0.98

ZrO2

0.17

0.14

0.86

NiO

0.15

0.13

0.86

SrO

0.13

0.11

0.86

PbO

0.07

0.05

0.79

Co2O3

0.03

0.03

1.00

I

0.06

0.00

0.00

Y2O3

0.04

0.00

0.00

Rb2O

0.02

0.00

0.00

Br

0.02

0.00

0.00

3.4.2 Functional groups in the char. The distribution of functional groups in the pyrolysis chars has been investigated by FTIR. The FTIR spectra of the dried sewage sludge and the chars produced at different pyrolysis conditions are presented in Figure 5. The band centered at 560 cm-1 is related to M-X stretching vibrations of inorganic and organic halogens compounds.29 The absorption band around 1038 cm-1 corresponds to C-O bonds, indicating the presence of stable C-O function groups.33 The band at 1425-1461 cm-1 is related to the deformation vibration of CH, revealing that the presence of lignin and carbohydrates.34 The above FTIR bands remained similar for the char samples obtained from the pyrolysis of sewage sludge at different pyrolysis conditions, suggesting that lignin and carbohydrates contained in the raw sewage sludge were difficult to be completely decomposed at the conditions studied in this work.


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Energy & Fuels

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Page 18 of 33

o

700 C,46min

o

700 C,6min

o

800 C,23min

o

700 C,23min

o

600 C,23min

o

500 C,23min

Raw materail

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm )

Figure 5. FTIR spectra of the pyrolysis char and the raw material.

In Figure 5, the band at 1640-1650 cm-1 corresponds to the stretching of conjugated C=C and COO-.35 The stretching of asymmetric (2930 cm-1) and symmetric (2850 cm-1) C-H indicates the presence of aliphatic functional group.12 The absorption of aliphatic groups disappears when the pyrolysis temperature was above 600 °C. This might be due to the break of C-H bond in alkyl compounds.36 The absorption band at 3200-3300 cm-1 could be due to the N-H stretching vibration, indicating the presence of amine and amide groups.22 Similar FTIR spectra are observed for the chars produced at solid residence time of 6 and 46 min and pyrolysis temperature of 700 °C (Figure 5), proposing that the solid residence time was not a key factor affecting the distribution of functional groups in the pyrolysis chars.


18

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Energy & Fuels

3.4.3. Proximate and Ultimate Analysis of Char. The proximate and ultimate analysis of the pyrolysis char obtained at different pyrolysis conditions are shown in Table 4, along with the HHV of each sample. The property of the dried sewage sludge is also presented in Table 4. As shown in Table 4, the content of volatiles in the pyrolysis char is lower than that in the raw material. This could be attributed to the degradation of volatile matter during the pyrolysis process. The ash content of the pyrolysis char was increased from 50.87 to 64.36% and the content of fixed carbon was increased from 22.57 to 26.81% with the increase of pyrolysis temperature from 500 to 800 °C, while the volatile matter content was decreased from 26.56 to 8.83%. This is consistent with the results reported by Trinh et al.3 that the content of ash and fixed carbon increased, while the volatile matter decreased with an increase in pyrolysis temperature. When the solid residence time was increased from 6 to 46 min, the ash content of the char was decreased with the increase of volatile matter, indicating that much longer solid residence time was not beneficial for the pyrolysis process in terms of the conversion of raw material. This might be caused by the polymerization reactions and the adsorption of heavy hydrocarbons on the surface of the solid char.

Table 4. Proximate and Ultimate Analysis of Pyrolysis Char (Wt.%, Dry) Proximate analysis

Ultimate analysis

Samples Vd

Pyrolysis temperature / °C

C

54.06 31.85

7.80

35.16

500

26.56 50.87

22.57 35.09

3.41 4.79 5.84

13.17

600

18.13 56.81

25.06 31.07

2.25 3.82 6.05

11.76

700

9.46

25.89 26.85

1.25 3.97 3.28

9.20

64.65

H

N

Ob

FCd

Sewage sludge

Ad

HHV (MJ kg-1)

5.44 5.61 21.94 14.76


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Energy & Fuels

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Solid residence time / min b

Page 20 of 33

800

8.83

64.36

26.81 26.65

1.15 3.84 4.00

8.97

6

8.89

69.25

21.86 26.33

0.83 3.58 0.01

9.28

23

9.46

64.65

25.89 26.85

1.25 3.97 3.28

9.20

46

11.23 64.23

24.53 26.53

1.69 4.22 3.33

9.20

By difference.

Ultimate analysis of the bio-char obtained from different conditions was carried out and the results are shown in Table 4. The total content of carbon, hydrogen, nitrogen and oxygen in the char is lower than that of the raw material. The content of carbon, hydrogen or the HHV of the char decreases with an increase in pyrolysis temperature of 500 to 800 °C. This might be due to the dehydrogenation, condensation (aromatization), and the gradual enhancement of secondary cracking reactions.25 The contents of carbon, hydrogen and nitrogen reached the maximum values of 35.09, 3.41 and 4.79%, respectively, at the solid residence time of 23 min and the pyrolysis temperature of 500 °C. From Table 4, it seems that the pyrolysis temperature have an obvious influence on the element distribution of the pyrolysis chars. However, the solid residence time of 6 to 46 min showed negligible influence on the elemental distribution of the produced pyrolysis chars. The maximum calorific value of the char obtained at pyrolysis temperature of 500 °C and solid residence time of 23 min was 13.17 MJ kg-1. The calorific value of the char obtained at different solid residence time is approximately 9.2 MJ kg-1, demonstrating that the solid residence time has little influence on the calorific value of the pyrolysis chars. This is consistent with the influence of the solid residence time on the elemental distribution of the chars.


20

Page 21 of 33

3.4.4. Combustion Behaviors of the Char. Figure 6 shows TG and Derivative thermogravimetric (DTG) profiles of the combustion of the raw material and the chars obtained at different pyrolysis conditions. A residual mass of 34.2% was left after combustion of the raw material, and this is consistent with the ash content (~32%) of the raw material shown in Table 1. It seems that the total weight loss of the char is closely associated with the pyrolysis conditions. For example, the weight loss of the char was reduced from 65.8 to 59.9% when the pyrolysis temperature was increased from 500 to 700 °C. The lowest weight loss (35.6%) was observed for the char obtained at pyrolysis temperature of 700 °C and solid residence time of 6 min. The ash content of the char obtained at solid residence time of 6 min was greater than that obtained at solid residence time of 23 min. In addition, the variation trend of weight loss measured by TG analysis is consistent with the results obtained from the proximate analysis (Table 4). 100

80 70

14 12 10

60 8

50 40

DTG / %/min

Raw material o 500 C, 23min o 700 C, 23min o 700 C, 6min

90

TG / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6

30

4

20 2

10 0 100

200

300

400

500 o

600

700

800

0 900

Temperature / C

Figure 6. TG and DTG curves of the raw material and chars under air atmosphere. As shown in Figure 6 about the DTG results, the raw material shows three weight loss peaks at approximately 94, 316 and 460 °C, respectively. The first region on the DTG curve is in the temperature range of 20-142 °C with a weight loss of 5.45% and was mainly due to the evaporation of moisture. The second region is from 142 to 379 °C with a weight loss about


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Page 22 of 33

36.22%, which might be due to the decomposition of organic polymers such as protein, carbohydrates and lipids.12,22,37 The third region is observed between 379 and 520 °C corresponding to the oxidation of char after the volatiles were removed from the samples. As shown in Figure 6, the DTG curve of the char obtained at pyrolysis temperature of 500 °C and solid residence time of 23 min is similar to that of the raw material. Compared to the DTG curve of the char obtained at 500 °C, the char obtained at 700 °C shows two distinct peaks. It seems that the second region peak in relation to the decomposition of volatile organic compounds is not obvious for the pyrolysis char produced at 700 °C, since most of the volatiles have already been evolved during pyrolysis process when the pyrolysis temperature reached 700 °C. The proximate analysis in Table 4 also indicates that less volatiles were remained in the char produced at 700 °C compared to the char produced at 500 °C. Based on the TG and DTG results, three stages are identified in the TG curves for the combustion of the biochars. As shown in Table 5, the first stage for the TGA combustion of the char is assigned within the temperature range from 20 to 200 °C, representing the release of water and light volatile organic compounds. The second stage for the TGA curve is observed between 200 and 330 °C, which was attributed to the decomposition of organic matter remained in the sample. The third stage for the TGA curve in the temperature range from 330 to 530 °C was due to oxidation of char remaining after the volatiles were removed from the samples. Table 5. Weight Loss Stages of the Raw Material and Chars during TG Analysis under Air Atmosphere Temperature interval (°C)

Mass loss (%)

Samples Stage I Raw material

Stage II

Stage III Stage I Stage II Stage III

20-142 142-379 379-520

5.45

36.22

22.37


22

Page 23 of 33

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Energy & Fuels

500 °C, 23 min 20-199 199-332 332-515

21.58

10.59

25.91

Char 700 °C, 23 min 20-182 182-326 326-532

10.75

2.39

27.00

4.69

2.09

26.27

700 °C , 6 min

20-180 180-328 328-500

3.5. Distribution and Potential Ecological Risk Assessment of Heavy Metals. 3.5.1 Distribution of Heavy Metal during Pyrolysis. The analysis of heavy metals distribution in dried sewage sludge pyrolysis process is shown in Table 6. The pyrolysis products mentioned in Table 6 were produced at pyrolysis temperature of 700°C and solid residence time of 23 min. The proportion in Table 6 was calculated by Equation 2. It is seen that the raw material contained some trace heavy metals, such as Cu, Zn, Mn, Cr, Ni, Pb and Cd. In the dried sewage sludge, the content of Cu, Zn and Mn were high and in the region of 11220-1925 mg kg1

, whereas the content of Cr, Ni and Pb were in the range of 533 to 113 mg kg-1. The

concentration of Cu (11220 mg kg-1) and Cd (2 mg kg-1) were found to be the highest and lowest among the detected heavy metals in the raw material. The concentration of heavy metals in the char was found to be higher when compared with the concentration of heavy metals in the raw material. This might be due to the accumulation effect during the pyrolysis process. The result agrees with the work reported by Werle et al.38 who investigated the gasification of dried sewage sludge. Furthermore, the maximum heavy metal content of Cu in the char can be reached at 15200 mg kg-1. The bio-oil has low contents of heavy metals (0-129 mg kg-1), with Cu as the most abundant heavy metal.

Table 6. Heavy Metals Distribution during Pyrolysis Process


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Content / mg kg-1 Elements

Page 24 of 33

Proportion / % Bio-gas +

Raw material

Bio-char

Bio-oil

Bio-char

Bio-oil Mass loss

Cu

11220

15200

129

77.88

0.19

21.93

Zn

2331

2996

38

73.89

0.27

25.84

Mn

1925

2388

33

71.32

0.29

28.40

Cr

533

680

10

73.35

0.31

26.34

Ni

248

337

5

78.12

0.34

21.54

Pb

114

134

4

67.58

0.59

31.84

Cd

2

2

0

57.49

0.00

42.51

Proportion =

Cproduct x Yproduct

(2)

C raw material

It can be observed from the results of proportion in Table 6 that most of the heavy metals were accumulated in the char, and the bio-oil contained a low level of heavy metals during the pyrolysis of sewage sludge. The tendency of proportion of heavy metals in char and bio-oil was similar with the results reported by Trinh et al.3, although the pyrolysis temperature was different. Additionally, the proportion of Cd represented the maximum value in gas and the mass loss of 42.51%, indicating that a high fraction of Cd was probably transferred into gaseous product. It is reported that the condensation of Cd species could be significant at pyrolysis temperature of lower than 380 °C39 The proportion of heavy metals in the chars was between 57.49 and 78.12%, indicating a good stability of the above mentioned heavy metals during


24

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Energy & Fuels

pyrolysis.24 The proportion of heavy metals in the char and the bio-oil reached at the maximum values of 78.12 and 0.59%, respectively.

3.5.2. Potential Ecological Risk assessment of Heavy Metals. The potential ecological risk index (RI) was studied to assess the pollution degree of heavy metals presented in the pyrolysis chars for the potential application in farmland. The value of RI was calculated by the following Equations 3-5 provided by Chabukdhara et al.40

Cif =

Ci Cn

(3)

E ir = Tri C if

(4) i

RI = ∑ E r

(5)

where Cif is the single-metal pollution factor, Ci is the concentration of heavy metal in char, Cn is the reference value of heavy metal. E ir and Tri are the potential ecological risk index and toxic factor values of individual heavy metal, respectively. The single-metal pollution factor Cif is the ratio of the concentrations of the heavy metal to the reference value of heavy metal obtained from the limited value of pollution concentration in the Chinese Standard of “Soil environmental quality standard for agricultural land (GB 156181995)”. The introduction of the standard is to ensure the production of agro forestry and the normal growth of plant. The potential ecological index of a heavy metal (Er) is obtained by multiplying the single-metal pollution factor Cif with the toxic factor (Tr) of individual heavy metal. The potential ecological RI of biochar is the sum of the potential ecological index of each


25

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Page 26 of 33

heavy metal present in the biochar. The significance of the Cf, Er and RI along with their risk potential is shown in Table 7.

Table 7. Indices for the Ecological Risk Assessment41 Cf

Metal contamination

Potential ecological risk

Er

RI

Sludge/biochar contamination

Cf≤1

Clean

Er≤40

Low

RI≤150

Low

1

Continuous Pyrolysis of Sewage Sludge in a Screw-Feeding Reactor

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