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Characterization of bio-oil and biochar from high temperature pyrolysis of sewage sludge ab
ab
ab
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Hongmei Chen , Yunbo Zhai , Bibo Xu , Bobin Xiang , Lu Zhu , Lei Qiu , Xiaoting Liu , ab
Caiting Li
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& Guangming Zeng
a
College of Environmental Science and Engineering, Hunan University, Changsha 410082, P. R. China b
Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, P. R. China Accepted author version posted online: 07 Aug 2014.
To cite this article: Hongmei Chen, Yunbo Zhai, Bibo Xu, Bobin Xiang, Lu Zhu, Lei Qiu, Xiaoting Liu, Caiting Li & Guangming Zeng (2014): Characterization of bio-oil and biochar from high temperature pyrolysis of sewage sludge, Environmental Technology, DOI: 10.1080/09593330.2014.952343 To link to this article: http://dx.doi.org/10.1080/09593330.2014.952343
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Publisher: Taylor & Francis Journal: Environmental Technology DOI: 10.1080/09593330.2014.952343
Characterization of bio-oil and biochar from high temperature pyrolysis of sewage sludge
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College of Environmental Science and Engineering, Hunan University, Changsha 410082, P. R. China
Key Laboratory of Environmental Biology and Pollution Control (Hunan University),
an
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Ministry of Education, Changsha 410082, P. R. China
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Xiaoting Liua,b, Caiting Lia,b, Guangming Zenga,b
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Hongmei Chena,b, Yunbo Zhaia,b,* , Bibo Xua,b，Bobin Xianga,b, Lu Zhua,b, Lei Qiua,b,
*
Corresponding Author. Tel.+86 731 8882 2829,Fax. +86 731 8882 2829. E-mail Address: [email protected], [email protected] (Y.B. ZHAI) 1
2
The influence of temperature (550-850 oC) on the characteristics of bio-oil and
3
biochar from pyrolysis of sewage sludge in a horizontal tube reactor was investigated.
4
Results showed that when the pyrolysis temperature increased from 550 oC to 850 oC,
5
the yield of bio-oil decreased from 26.16% (dry ash free basis) to 20.78% (dry ash
6
free basis). Main components of bio-oil were phenols, esters, cholests, ketones,
7
amides, indoles, and nitriles. Besides, the elevated heating rate of 25 oC/min was
8
demonstrated to favor the complete combustion of bio-oil. Moreover, caused by the
9
increase of temperature, the yield of biochar decreased from 54.9wt% to 50.6wt%,
10
BET surface area increased from 48.51 to 81.28 m2/g. Furthermore, pH was increased
11
from 5.93 of sewage sludge to 7.15 – 8.96 of biochar. The negative ζ-potential was
12
also strengthened (-13.87- -11.30 mV) and principal functional groups on the surface
13
of biochar were –OH, C=O, C=C, –NO2, S=O.
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Keywords: Bio-oil; Biochar; Combustion characteristic; Pyrolysis; Sewage sludge
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Abstract
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Sewage sludge, an organic byproduct formed from the wastewater treatment process,
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is being considered as a feedstock for the renewable fuel biorefining industry. Two
18
main conversion technologies are being cultivated with the aim of finding the best
19
yielding process with maximum fuel potential: thermochemical and biological.
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Pyrolysis is a promising thermochemical process for producing liquid fuel and biochar
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[1-2]. This process is convenient, effective, and environmentally sustainable [3].
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1. Introduction
Liquid fuels originated from the pyrolysis process and intended for energy use
23
are also known as bio-oil. Bio-oil is a promising renewable fuel for heat and power
24
generation. However, since bio-oil containing different families of compounds,
25
mainly aliphatic, aromatic, and polycyclic aromatic hydrocarbons [4], it is
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non-homogenous, dark, bad-smelling, and sticky [5]. Bio-oil also has the potential to
27
be used as fuel for transportation [6] and stationary engines [7]. Among various
28
reaction conditions of pyrolysis, temperature is demonstrated to be the main
29
parameter that directly influences the properties of bio-oil [8]. Generally, lower
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temperature conversion processes in the temperature range between 275 oC and 500
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C have been used to produce bio-oil from sewage sludge [9]. Bio-oil production in
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the high temperature range (> 500 oC) is considered to be inappropriate for the high
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rate of bio-oil production.
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Considering the application of bio-oil as a substitute for fuel, the combustion
35
characteristics are also important. Herein, optimizing the combustion conditions is
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one way to upgrade combustion performance and emissions. Moloodi et al. [10]
3
38
as ethanol blending on the combustion characteristics of bio-oil in a pilot stabilized
39
spray burner. In addition to the investigation conducted in the stabilized spray burner,
40
thermogravimetic analysis (TGA) has also been utilized as a tool to monitor the
41
different stages of droplet evaporation and combustion [11]. Moreover, TGA is
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considered to act as a good indicator of the solid residue forming or coking potential
43
of droplets undergoing combustion when performed under either nitrogen or air
44
atmosphere [12]. However, there is limited information on the relationship between
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combustion conditions and performance of bio-oil produced by pyrolysis of sewage
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sludge. At these points, it is necessary to have a full scale study on the composition of
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bio-oil obtained at various reaction conditions and investigate the combustion
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characteristics.
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investigated the effects of bio-oil properties (solids, ash, and water contents) as well
Biochar is another primary product from the pyrolysis of sewage sludge. An
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increasing interest in the beneficial application of biochar has opened up
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multidisciplinary areas for science and engineering. Converting sewage sludge into
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biochar and its following application as soil remediation have been proposed as one of
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the best ways to mitigate climate change by sequestering carbon in soil [13]. Many
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attentions have focused on the chemical and physical characteristics of pyrolysis
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products from wood [14], pitch pine [15], poultry litter [16], and swine manure [27].
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Pyrolysis method, temperature, activation process and feedstock type are the main
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factors influencing chemical and physical properties of biochar [18]. Therefore, it is
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possible to identify the pyrolysis process from a perspective of determining the
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follow-up application of biochar. A lot of investigations on characteristics of biochar
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from sewage sludge; whereas these studies are mainly focused on structural changes
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of biochar produced at low temperature pyrolysis (300-500 oC) or characteristics for
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some specific applications [19-20]. The biochar applied as adsorbent or soil conditioner is obtained from either high
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or low temperature pyrolysis, Ahmad et al. pointed out that [18] the biochar produced
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at relatively high temperature favored the adsorption of organic contaminants;
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whereas the biochar obtained at low temperatures were more suitable for removing
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inorganic / polar organic contaminants. In previous investigations, pyrolysis of
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biomass for production of bio-oil was commonly conducted at low temperature. It is
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the double benefit to obtain a relatively high bio-oil yield while preparing a biochar
70
with superior performance. However, insufficient consideration has actually been
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given to the characterization of both the bio-oil and biochar. Moreover, most of these
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existing papers only considered the product yield and chemical properties; the
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analysis of the combustion properties of the bio-oil is rarely.
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In light of the previous researches, the primary objective of this study was to
investigate the potential of recovering energy from pyrolysis of SS. In addition,
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effects of temperatures on products yields, composition of bio-oil, and characteristics
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of biochar were also studied. Furthermore, the combustion characteristics of bio-oil in
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the air at different heating rates were investigated with (TGA).
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2. Experimental
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2.1. Materials preparation and pyrolysis
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The feedstock used in this study was dewatered sewage sludge (SS) obtained from the
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second municipal wastewater treatment plant in Changsha, China. Before the
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experiments, SS was air-dried at 105 oC for 24 h to completely remove the water and
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then crushed smaller than 60 mesh. Pyrolysis was performed in a horizontal tubular
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furnace. The experimental schematic diagram of the pyrolysis apparatus is illustrated
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in Fig. 1. Fig. 1 is here
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In each experiment, 20 g of raw dry SS was placed into the quartz tube carefully.
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Then the tube was placed inside the electric furnace. To maintain an inert atmosphere
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during the whole process, nitrogen (99.99%) at a rate of 200 mL/min was used as the
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purge gas. There was no movement of sludge particles. The temperature increased
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from the room temperature to the desired temperature of 550 oC, 650 oC, 750 oC and
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850 oC at a constant heating rate of 25 oC/min, and then held at the preset temperature
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for the desired retention time of 120 min. The bio-oil was condensated in the
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condensate retriever with ice bath, the composite gas was stored by the gas collector,
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and the biochar was left in the quartz tube. After each experiment, bio-oil in the
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condensate retriever was washed with dichloromethane and then separated by rotary
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evaporation. The biochar was collected and weighed carefully and then kept in the
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sample bag before analysis. The detailed experimental procedure could also find in
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the previous studies [1]. The samples are named as B - X, where B represents the
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sample, X is the temperature. Each experiment was conducted three times, and the
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result presented was the mean value.
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2.2. Analysis of SS
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The primary analysis (TGA, STA 409 model, NETZSCH, Germany) and elemental
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analysis (2400 Series II CHNS Analyzer, PerkinElmer, USA) of the sludge sample are
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presented in Table 1. Table 1 is here
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2.3. Analysis of bio-oil
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2.3.1 Gas chromatography-mass spectrometer (GC-MS)
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The composition of bio-oil were analyzed by a gas chromatograph (GC) equipped
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with a mass spectrometer (MS) detector (GC-MS, QP2010, SHIMADZU, column:
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Rtx-5MS, 30 m×0.25 mm×0.25 µm). The solvent of the bio-oil was ethanol (GR), and
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the carrier gas was helium kept at a rate of 5 mL/min. The temperature of interface
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was 250 oC and the ion source was kept at 230 oC. The temperature program of the
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column was as follows: 40 oC (hold for 2 min), up to 190 oC (12 oC /min, hold for 1
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min), up to 290 oC (4 oC /min, hold for 20 min), up to 320 oC (20 oC /min, hold for 5
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min). The MS system was operated in the full scan mode with a mass range from m/z
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50 to 700. Chromatograms of the compounds in the oil were compared with the
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standard compounds in the NITST mass spectral data library, which generated all the
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information available of the compounds, including their formulas, structures and
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molecular weights. The area for each compound (defined by the percentage of the
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compound’s chromatographic area out of the total) was calculated from the total ion
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chromatogram (TIC).
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2.3.2 Combustion characteristics
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and differential thermal analysis (DTA) (TGA, STA 409 model, NETZSCH, Germany)
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were used to test the combustion characteristics of the oils in the dry air at different
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heating rates of 15 oC/min, 20 oC/min, and 25 oC/min. In each experiment, the sample
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of 20 mg was spread uniformly on the bottom of the crucible made of alumina and
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heated from 25 oC to 850 oC under an air flow of 50 mL /min.
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2.4 Analysis of biochar
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2.4.1. pH, ζ-potential value and electric conductivity
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The pH on the surface of the biochar was measured with a glass electrode (Leici,
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pHS-3C, Shanghai scientific instrument co., LTD) in the same way as previous studies
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[8, 21]. In a typical measurement, 0.5 g of biochar was added into 50 mL of ultrapure
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water and left still to stabilize in a centrifuge tube for 2 days until the pH of the slurry
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stabilized. The pH was measured three times for each biochar sample, and a mean
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value was obtained.
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Thermo-gravimetric analysis (TG), differential thermo-gravimetric analysis (DTG)
The ζ-potential value, electric conductivity (EC) and electrophoretic mobility (μe)
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of the biochar particles were determined with a Nano-Zeta-Sizer (Malvern
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Instruments, U.S.) as described elsewhere [22-23]. In each measurement, 0.15 g of
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fresh biochar was added to a plastic bottle containing 65 mL of freshly distilled water.
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Each bottle was hand-shaken periodically for 24 h and then left still to stabilize for 1 h
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before measurements. All measurements were repeated three times, and a mean value
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was obtained for each sample.
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2.4.2. BET Surface Area
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Micromeritics ASAP 2020 Surface Area and Porosity Analyzer (Micromeritics
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Instrument Corporation, USA). Samples were initially degassed at 293 K for 24 h.
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The nitrogen adsorption/desorption isotherms were measured at 77 K. The specific
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surface area was calculated by Brunauer-Emmet-Teller (BET) equation [24]. The
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mesopore
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Barrett-Joyner-Halenda (BJH) theory [25].
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2.4.3. FTIR
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Functional groups on the surface of biochar were measured with KBr pellets
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containing 1% of each sample by Fourier Transform Infrared Spectroscopy (FTIR)
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(FTIR-8400S Spectrometer, USA). These samples of particle sizes < 45μm were first
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dried overnight at 105 oC before the spectra were generated. The resulting spectra
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were normalized to the peak in the region between 4000 and 400 cm-1 with 100 scans
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per sample. A previously recorded background spectrum of KBr was subtracted from
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the spectrum of each sample.
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3. Results and Discussion
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3.1. Yields of bio-oil and biochar
were
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The specific surface areas of B-550, B-650, B-750, and B-850 were measured using a
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Since the bio-oil was originated from the organic components in SS, and the ash
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content in SS was higher than the common biomass, the oil yield was calculated based
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on the dry ash free basis, and the biochar yield was on the basis the initial weight of
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feedstock. As shown in Table 2, the increase in pyrolysis temperature was
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demonstrated to contribute to the decrease of bio-oil yield by accelerating the crack of
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the vapors and formation of gases [26]. In addition, the yield of biochar was also
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decreased but not much. Since the pyrolysis temperature was high, most pyrolytic
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volatiles had already reacted at a lower temperature and resulted in a relatively small
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biochar yield variation was found [15]. Furthermore, it is easy to conclude that with
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the increase of temperature, there was more volatile substances generated and
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conduced to the decrease of the total weight.
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Table 2. is here
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3.2. Characterization of the bio-oils
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3.2.1 Composition of bio-oils
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The GC-MS analysis results of the bio-oil obtained from pyrolysis of SS at
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temperatures of 550 oC, 650 oC, 750 oC, and 850 oC with retention time of 120 min
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are presented in Table 3. Bio-oil was demonstrated to be a complex mixture which
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consists of organic compounds from a wide variety of chemical groups, and the
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compositional differences were insignificant among the bio-oils originating from
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different temperatures. Main components of bio-oil could be classified into phenols
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(e.g., phenol, 2(4)-methyl-phenol, 4-ethyl-phenol), esters (e.g., hexadecanoic acid
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methyl ester and hexadecanoic acid ethyl ester), cholests (e.g., cholest-2(4)-ene and
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cholesta-3,5-diene),
ketones
(e.g.,
2-piperidinone),
amides
(e.g.,
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(Z)-9-octadecenamide and tetradecanamide), indoles (e.g., 3-methyl-1H-indole and
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indole), nitriles (e.g., hexadecanenitrile and benzyl nitrile). The result was in line with
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Cao et al [27]. The content of amides was quite higher than that of nitrile, and the
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content of hexadecanamide was 8.95% in bio-oil of B-550. However, it was
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palmitonitrile that pointed out to be the most abundant one in their study. It might
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attribute to the difference of reaction degree. Table 3 is here 3.2.2 Combustion of bio-oil in air with different heating rates
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As the combustion characteristics of biomass previously investigated [28-29],
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combustion characteristics of bio-oil in the air were investigated with TGA and DTA
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experiments. Fig.2 presents the (a) TG-DTG, and (b) DTA curves of combustion of
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B-550 at different heating rates of 15 oC/min, 20 oC/min and 25 oC/min. Fig. 2. is here
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As shown in Fig. 2 (a), combustion of bio-oil could be divided into five stages.
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For the heating rate of 15 oC/min, the first region (100-150 oC) observed was
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corresponding to the moisture release. Release of volatile organic compounds
203
(150-220 oC) happened in the second region. In the third region (220- 340 oC), release
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of semi-volatiles and their ignition occurred. The rapid burning and formation of the
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secondary char generated in the fourth region (340- 480 oC). Lastly, rapid loss of mass
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immediately slowed in the fifth region (480- 630 oC) and the partly carbonized residue
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burning slowly. Fig. 2 (a) presents that TG curves of the three different heating rates
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exhibited very similar patterns. In addition, the increased heating rate contributed to a
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more complete combustion, residues for heating rates of 15 oC /min, 20 oC/min and 25
210
o
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rate could contribute to better heat penetration inside the bio-oil and allowed to
212
determine these conditions in anticipation, which made it possible to program the
C/min were 7.60wt%, 5.39wt% and 2.28wt%, respectively. The increased heating
11
combustion in a relevant and effective way [28]. However, with the increase of heating rate, DTG curves were enhanced and
215
postponed at the fifth stage. The enhancement of the curve could due to the decreased
216
combustion activation energy with the increase of heating rate. Moreover, with the
217
increase of heating rate, the initial combustion temperature and burnout temperature
218
were increased and then resulted in the DTG curves lagged behind.
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The DTA curves could effectively illustrate the endothermic and exothermic
220
stage of bio-oil in the combustion. As shown in Fig.2 (b), at the heating rate of 15
221
o
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of volatile organic compounds in the second stage was also endothermic though the
223
heat adsorbed was reduced. The last three stages were all exothermic and the heat
224
given off increased with the proceeding of the combustion. Endothermic and
225
exothermic stages presented in Fig. 2 (b) were consisted with the mass loss shown in
226
Fig. 2 (a). Besides, as the heating rate increased from 15 to 25 oC/min, the
227
endothermic stage lagged behind
228
3.3. Characterization of biochar
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3.3.1 Surface properties of biochar
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As presented in Table 4, the untreated SS with pH of 5.93 was weakly acidic,
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while biochar showed basic property with pH ranged from 7.75 (B-750) to 8.96
232
(B-650) irrespective of the pyrolysis conditions. The basic property of biochar was
233
explained by the removal of acidic functional groups that were not stable at high
234
temperatures [30]. Besides, the organic components in SS decomposed at the high
12
236
also be responsible for the increase in pH [31]. The increased pH caused by thermal
237
treatments was demonstrated to be effective in the treatment of some organic
238
hydrocarbons and facilitated the adsorption of H2S and SO2 [32]. Moreover, it may
239
also benefit for the reclamation of acid soil.
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pyrolysis temperature, the alkali salts that separated from the organic materials could
The ζ-potential values of SS and biochar were negative. Negatively charged
241
biochar were found to be preferable for the adsorption of cations [33]. Pyrolysis
242
decreased the EC from 0.350 mS/cm of the untreated SS to 0.090 mS/cm of the
243
sample B-850, and a similar trend was noted in the study of Méndez et al. [34].
244
Electrolytes added in the soil were found to affect the flocculation of soil and
245
sensitivity of crops [35].
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Table 4 is here
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The BET surface area (SBET), total pore volume (Vtotal), micropore volume
248
(Vmico), mesopore volume (Vmeso) and average pore width (Dave) of four representative
249
biochar B-550, B-650, B-750, and B-850 are also listed in Table 4. Both SBET and
250
Vtotal were temperature dependent. When the temperature increased from 550 to 650
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C, SBET increased remarkably by 32.77 m2/g. However, when the temperature further
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increased to 850 oC, the SBET and Vtotal only slightly increased, and the change in Vmico
253
was also insignificant. The overactivity caused by elevated temperatures might lead to
254
the sintering of micropores. The removal of organic compounds from the pores by
255
thermal treatment contributed to the enhancement of SBET, Vmeso and Vtotal. These
256
trends are in consistent with previous studies [19, 36]. The SBET result obtained by Lu
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258
with the SBET consequence (647.4 m2/g) by Chen et al. [38] obtained at B-550, the
259
SBET of B-550 in this paper was quite low. This could be explained by the fact that in
260
the study by Chen et al., the pyrolysis was activated by impregnating SS with 5 mol /L
261
ZnCl2, which had a positive effect on the increase of SBET. In addition, the product
262
was washed with 3 mol/L HCl and rinsed with deionized water, which also favored
263
the increase in SBET.
264
3.3.2 FTIR Analysis
265
Main functional groups of –OH, C=O, C=C, –NO2, S=O were found on the surface of
266
biochar as shown in Fig. 3. The insignificantly varied position of peaks suggesting
267
that functional groups on the biochar were about the same; whereas, the intensity of
268
peaks was significantly different, indicating the contents of functional groups on the
269
surface of sample were varied.
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et al. [37] at 850 oC was 85 m2/g, which almost the same as ours. However, compared
Fig. 3. is here
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As shown in the figure, the 4000-2500 cm-1 stretching vibration was assigned to
272
functional groups of X-H (X= C, N, O, S). The strong and broad peak displayed on
273
the spectrum near 3450 cm-1 was assigned to the associated O-H [39], indicating the
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existence of alcohols or carboxylic acids. Furthermore, there was a strong and sharp
275
peak at 2350 cm-1, which was assigned to CO2 stretching vibration. Peaks between
276
2000 and 1500 cm-1 were C=C stretching vibration area. The 1770-1600 cm-1 band
277
indicated the existence of C=O [40]. Because of the effect of the C=O, peaks were
278
shifted toward lower wave numbers of 1667-1640 cm-1, where C=C groups of olefins
14
compounds were found. The peak at 1510 cm-1 was considered to be the -NO2
280
functional group. Near the end of the spectrum band, peaks at 1100-860 cm-1 were
281
related to S=O, and peaks at 830-650 cm-1 were assigned to aromatic structures,
282
skeletal vibrations, amine and amide groups [17]. The peaks corresponding to carbonyl group on biochar were considered to be
284
relatively high in intensity, which might facilitate the adsorption of aromatic
285
compounds through the formation of an electron acceptor-donor complex [36].
286
Besides, acidic oxygen functional groups were considered to be beneficial to the
287
adsorption of metal ions in aqueous solutions [31].
288
4. Conclusion
289
Pyrolysis of sewage sludge at high temperatures favored the enhancement of surface
290
properties of biochar; whereas, the bio-oil yield was decreased. Composition of
291
bio-oil insignificant varied with the variation of temperature. As regards the
292
combustion profile of bio-oil in the air, increased heating rate contributed to more
293
complete combustion. Moreover, with the increase of heating rate, the initial
294
combustion temperature and burnout temperature were increased. This paper provides
295
beneficial information to the application of bio-oil and biochar; whereas the focus of
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the present research is on the properties of products, profound investigation of
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applying the biochar as soil remediation or adsorbents for removal of pollutants
298
should be conducted in the following study.
299
Acknowledgements
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This work was supported by Hunan Provincial Natural Science Foundation of
15
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(NCET-12-0169). The Scientific and Technological Project of Hunan Province in
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China (2013FJ4040 ) and the Scientific Research Foundation for Returned Scholars.
304
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Table 1 Primary analysis and ultimate analysis of the raw sewage sludge
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an us cr
Ultimate analysis
Ma (wt%)
VMb (wt%)
Ash (wt%)
FCc (wt%)
HHVd (MJ/kg)
C(wt%)
H(wt%)
N(wt%)
Oe(wt%)
S(wt%)
1.670
52.220
46.110
1.840
13.040
28.940
4.480
4.230
13.920
0.650
a: moisture;
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b: volatile matter;
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c: fixed carbon;
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d: higher heating value;
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e: calculated by mass difference.
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Primary analysis
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Table 2 Yields of bio-oil and biochar at different pyrolysis temperatures Sample
B-550
B-650
B-750
B-850
Bio-oil (%daf.a)
26.16
23.25
21.29
20.78
Biochar (wt%)
54.90
54.45
50.00
50.60
a: dry ash free basis
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Table 3 Main composition of B-550, B-650, B-750 and B-850
430
Compounds
Formula
B-550b
8.41 8.47 9.47 9.64 9.96 10.34 10.39 10.67 11.02 11.28 12.75 13.90 19.65
Phenol 3-hexanol 2(4)-methyl-phenol 2-pyrrolidinone 1-methyl-2,5-pyrrolidinedione 9-octadecenamide,(Z)2,5-pyrrolidinedione Benzyl nitrile 4-ethyl-phenol 2-piperidinone Indole 3-methyl-1H-indole 1,2-benzenedicarboxylic acid bis (2-methylpropyl) ester Cyclododecane Hexadecanenitrile Hecadecanoic acid methyl ester Pentadecanenitrile Glycyl-L-proline Tetradecanamide Hexadecanoic acid ethyl ester Heptadecanenitrile N,N-dimethyl-octanamide 1,2-benzenedicarboxylic acid diisooctyl ester Cholest-5-en-3-ol (3.beta)Cholest-2 (4)-ene cholesta-3,5-diene Hexadecanoic acid tetradecyl
C6H6O C6H14O C7H8O C4H7NO C5H7NO2 C18H35NO C4H5NO2 C8H7N C8H10O C5H9NO C8H7N C9H9N C16H22O4
30.17 30.77 31.33 32.15
B-750
B-850
3.04 1.17 5.74 0.41 nd 6.87 0.23 0.31 0.85 nd 3.84 0.50 nd
2.62 ndc 5.99 0.99 0.32 4.37 1.32 0.67 1.04 0.38 2.96 2.20 2.47
2.65 nd 5.98 1.15 0.31 5.16 1.27 0.87 1.05 0.36 2.97 2.36 3.61
3.15 nd 7.02 0.76 0.34 5.19 0.68 0.82 1.79 0.29 3.51 2.96 2.02
C12H24 C15H31CN C17H34O2 C14H29CN C17H12N2O3 C14H29NO C18H36O2 C16H33CN C10H21NO C24H38O
nd 2.06 nd nd 2.05 1.10 nd 2.02 nd 3.48
1.34 5.18 nd nd 1.40 0.30 nd 1.74 0.71 0.69
0.67 3.34 nd nd 2.28 1.52 nd 1.49 0.55 1.01
nd 3.12 0.30 1.19 0.92 0.58 0.44 1.92 nd 0.86
C36H62O2 C27H46 C19H28 C30H60O2
1.28 10.99 3.15 2.43
1.09 9.01 2.56 1.90
0.65 10.25 2.48 1.82
3.11 11.74 3.31 0.59
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B-650
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19.96 20.19 20.48 20.79 20.88 21.10 21.32 22.74 24.51 27.68
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RT a (min)
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23
ester Hexadecanamide
32.62 431
a
432
b
433
c
C16H33NO
8.95
6.83
7.78
7.59
:retention time : relative composition by area (%)
: nd=not detected
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Table 4 pH, ζ-potential, electric conductivity, and electro-phoretic mobility of SS and
437
biochars SS
B-550
B-650
B-750
B-850
pH
5.93
8.53
8.96
7.75
8.73
ζ-potential (mV)
-7.29
-13.87
-12.17
-13.87
-11.30
EC a (mS/cm)
0.35
0.08
0.10
0.11
0.09
Mob. b (m2/Vs×10-8)
-0.57
-1.09
-0.96
-1.09
-0.89
48.51
72.50
73.28
81.28
Vmico (cm3/g)
0.006
0.005
0.009
0.006
Vmeso (cm3/g)
0.065
0.088
0.075
0.097
Vtotal (cm3/g)
0.097
0.125
0.136
0.146
6.82
6.59
5.96
5.91
pt
a
439
b
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Dave (nm) 438
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SBET (m2/g)
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Characteristics
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: Electric conductivity
: Electro-phoretic mobility
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440 441 442 443 444
24
445
Figure captions
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Fig. 1. The experimental schematic diagram of the pyrolysis apparatus
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Fig. 2. Combustion characteristics of B-550 in the air at heating rates of 15 oC/min, 20
448
o
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Fig. 3. FTIR spectra of (a) B-550, (b) B-650, (c) B-750, and (d) B-850
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C/min, and 25 oC/min: (a) TG-DTG, (b) DTA
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Fig. 1
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Figure 2
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Figure 3
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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Characterization of bio-oil and biochar from high temperature pyrolysis of sewage sludge ab
ab
ab
ab
ab
ab
ab
Hongmei Chen , Yunbo Zhai , Bibo Xu , Bobin Xiang , Lu Zhu , Lei Qiu , Xiaoting Liu , ab
Caiting Li
ab
& Guangming Zeng
a
College of Environmental Science and Engineering, Hunan University, Changsha 410082, P. R. China b
Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, P. R. China Accepted author version posted online: 07 Aug 2014.
To cite this article: Hongmei Chen, Yunbo Zhai, Bibo Xu, Bobin Xiang, Lu Zhu, Lei Qiu, Xiaoting Liu, Caiting Li & Guangming Zeng (2014): Characterization of bio-oil and biochar from high temperature pyrolysis of sewage sludge, Environmental Technology, DOI: 10.1080/09593330.2014.952343 To link to this article: http://dx.doi.org/10.1080/09593330.2014.952343
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Publisher: Taylor & Francis Journal: Environmental Technology DOI: 10.1080/09593330.2014.952343
Characterization of bio-oil and biochar from high temperature pyrolysis of sewage sludge
us c
College of Environmental Science and Engineering, Hunan University, Changsha 410082, P. R. China
Key Laboratory of Environmental Biology and Pollution Control (Hunan University),
an
b
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M
Ministry of Education, Changsha 410082, P. R. China
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a
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Xiaoting Liua,b, Caiting Lia,b, Guangming Zenga,b
t
Hongmei Chena,b, Yunbo Zhaia,b,* , Bibo Xua,b，Bobin Xianga,b, Lu Zhua,b, Lei Qiua,b,
*
Corresponding Author. Tel.+86 731 8882 2829,Fax. +86 731 8882 2829. E-mail Address: [email protected], [email protected] (Y.B. ZHAI) 1
2
The influence of temperature (550-850 oC) on the characteristics of bio-oil and
3
biochar from pyrolysis of sewage sludge in a horizontal tube reactor was investigated.
4
Results showed that when the pyrolysis temperature increased from 550 oC to 850 oC,
5
the yield of bio-oil decreased from 26.16% (dry ash free basis) to 20.78% (dry ash
6
free basis). Main components of bio-oil were phenols, esters, cholests, ketones,
7
amides, indoles, and nitriles. Besides, the elevated heating rate of 25 oC/min was
8
demonstrated to favor the complete combustion of bio-oil. Moreover, caused by the
9
increase of temperature, the yield of biochar decreased from 54.9wt% to 50.6wt%,
10
BET surface area increased from 48.51 to 81.28 m2/g. Furthermore, pH was increased
11
from 5.93 of sewage sludge to 7.15 – 8.96 of biochar. The negative ζ-potential was
12
also strengthened (-13.87- -11.30 mV) and principal functional groups on the surface
13
of biochar were –OH, C=O, C=C, –NO2, S=O.
14
Keywords: Bio-oil; Biochar; Combustion characteristic; Pyrolysis; Sewage sludge
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Abstract
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2
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Sewage sludge, an organic byproduct formed from the wastewater treatment process,
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is being considered as a feedstock for the renewable fuel biorefining industry. Two
18
main conversion technologies are being cultivated with the aim of finding the best
19
yielding process with maximum fuel potential: thermochemical and biological.
20
Pyrolysis is a promising thermochemical process for producing liquid fuel and biochar
21
[1-2]. This process is convenient, effective, and environmentally sustainable [3].
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1. Introduction
Liquid fuels originated from the pyrolysis process and intended for energy use
23
are also known as bio-oil. Bio-oil is a promising renewable fuel for heat and power
24
generation. However, since bio-oil containing different families of compounds,
25
mainly aliphatic, aromatic, and polycyclic aromatic hydrocarbons [4], it is
26
non-homogenous, dark, bad-smelling, and sticky [5]. Bio-oil also has the potential to
27
be used as fuel for transportation [6] and stationary engines [7]. Among various
28
reaction conditions of pyrolysis, temperature is demonstrated to be the main
29
parameter that directly influences the properties of bio-oil [8]. Generally, lower
30
temperature conversion processes in the temperature range between 275 oC and 500
31
o
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C have been used to produce bio-oil from sewage sludge [9]. Bio-oil production in
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the high temperature range (> 500 oC) is considered to be inappropriate for the high
33
rate of bio-oil production.
34
Considering the application of bio-oil as a substitute for fuel, the combustion
35
characteristics are also important. Herein, optimizing the combustion conditions is
36
one way to upgrade combustion performance and emissions. Moloodi et al. [10]
3
38
as ethanol blending on the combustion characteristics of bio-oil in a pilot stabilized
39
spray burner. In addition to the investigation conducted in the stabilized spray burner,
40
thermogravimetic analysis (TGA) has also been utilized as a tool to monitor the
41
different stages of droplet evaporation and combustion [11]. Moreover, TGA is
42
considered to act as a good indicator of the solid residue forming or coking potential
43
of droplets undergoing combustion when performed under either nitrogen or air
44
atmosphere [12]. However, there is limited information on the relationship between
45
combustion conditions and performance of bio-oil produced by pyrolysis of sewage
46
sludge. At these points, it is necessary to have a full scale study on the composition of
47
bio-oil obtained at various reaction conditions and investigate the combustion
48
characteristics.
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investigated the effects of bio-oil properties (solids, ash, and water contents) as well
Biochar is another primary product from the pyrolysis of sewage sludge. An
50
increasing interest in the beneficial application of biochar has opened up
51
multidisciplinary areas for science and engineering. Converting sewage sludge into
52
biochar and its following application as soil remediation have been proposed as one of
53
the best ways to mitigate climate change by sequestering carbon in soil [13]. Many
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attentions have focused on the chemical and physical characteristics of pyrolysis
55
products from wood [14], pitch pine [15], poultry litter [16], and swine manure [27].
56
Pyrolysis method, temperature, activation process and feedstock type are the main
57
factors influencing chemical and physical properties of biochar [18]. Therefore, it is
58
possible to identify the pyrolysis process from a perspective of determining the
4
follow-up application of biochar. A lot of investigations on characteristics of biochar
60
from sewage sludge; whereas these studies are mainly focused on structural changes
61
of biochar produced at low temperature pyrolysis (300-500 oC) or characteristics for
62
some specific applications [19-20]. The biochar applied as adsorbent or soil conditioner is obtained from either high
64
or low temperature pyrolysis, Ahmad et al. pointed out that [18] the biochar produced
65
at relatively high temperature favored the adsorption of organic contaminants;
66
whereas the biochar obtained at low temperatures were more suitable for removing
67
inorganic / polar organic contaminants. In previous investigations, pyrolysis of
68
biomass for production of bio-oil was commonly conducted at low temperature. It is
69
the double benefit to obtain a relatively high bio-oil yield while preparing a biochar
70
with superior performance. However, insufficient consideration has actually been
71
given to the characterization of both the bio-oil and biochar. Moreover, most of these
72
existing papers only considered the product yield and chemical properties; the
73
analysis of the combustion properties of the bio-oil is rarely.
75
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In light of the previous researches, the primary objective of this study was to
investigate the potential of recovering energy from pyrolysis of SS. In addition,
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76
effects of temperatures on products yields, composition of bio-oil, and characteristics
77
of biochar were also studied. Furthermore, the combustion characteristics of bio-oil in
78
the air at different heating rates were investigated with (TGA).
79
2. Experimental
80
2.1. Materials preparation and pyrolysis
5
The feedstock used in this study was dewatered sewage sludge (SS) obtained from the
82
second municipal wastewater treatment plant in Changsha, China. Before the
83
experiments, SS was air-dried at 105 oC for 24 h to completely remove the water and
84
then crushed smaller than 60 mesh. Pyrolysis was performed in a horizontal tubular
85
furnace. The experimental schematic diagram of the pyrolysis apparatus is illustrated
86
in Fig. 1. Fig. 1 is here
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87
In each experiment, 20 g of raw dry SS was placed into the quartz tube carefully.
89
Then the tube was placed inside the electric furnace. To maintain an inert atmosphere
90
during the whole process, nitrogen (99.99%) at a rate of 200 mL/min was used as the
91
purge gas. There was no movement of sludge particles. The temperature increased
92
from the room temperature to the desired temperature of 550 oC, 650 oC, 750 oC and
93
850 oC at a constant heating rate of 25 oC/min, and then held at the preset temperature
94
for the desired retention time of 120 min. The bio-oil was condensated in the
95
condensate retriever with ice bath, the composite gas was stored by the gas collector,
96
and the biochar was left in the quartz tube. After each experiment, bio-oil in the
97
condensate retriever was washed with dichloromethane and then separated by rotary
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98
evaporation. The biochar was collected and weighed carefully and then kept in the
99
sample bag before analysis. The detailed experimental procedure could also find in
100
the previous studies [1]. The samples are named as B - X, where B represents the
101
sample, X is the temperature. Each experiment was conducted three times, and the
102
result presented was the mean value.
6
103
2.2. Analysis of SS
104
The primary analysis (TGA, STA 409 model, NETZSCH, Germany) and elemental
105
analysis (2400 Series II CHNS Analyzer, PerkinElmer, USA) of the sludge sample are
106
presented in Table 1. Table 1 is here
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2.3. Analysis of bio-oil
109
2.3.1 Gas chromatography-mass spectrometer (GC-MS)
110
The composition of bio-oil were analyzed by a gas chromatograph (GC) equipped
111
with a mass spectrometer (MS) detector (GC-MS, QP2010, SHIMADZU, column:
112
Rtx-5MS, 30 m×0.25 mm×0.25 µm). The solvent of the bio-oil was ethanol (GR), and
113
the carrier gas was helium kept at a rate of 5 mL/min. The temperature of interface
114
was 250 oC and the ion source was kept at 230 oC. The temperature program of the
115
column was as follows: 40 oC (hold for 2 min), up to 190 oC (12 oC /min, hold for 1
116
min), up to 290 oC (4 oC /min, hold for 20 min), up to 320 oC (20 oC /min, hold for 5
117
min). The MS system was operated in the full scan mode with a mass range from m/z
118
50 to 700. Chromatograms of the compounds in the oil were compared with the
119
standard compounds in the NITST mass spectral data library, which generated all the
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information available of the compounds, including their formulas, structures and
121
molecular weights. The area for each compound (defined by the percentage of the
122
compound’s chromatographic area out of the total) was calculated from the total ion
123
chromatogram (TIC).
124
2.3.2 Combustion characteristics
7
126
and differential thermal analysis (DTA) (TGA, STA 409 model, NETZSCH, Germany)
127
were used to test the combustion characteristics of the oils in the dry air at different
128
heating rates of 15 oC/min, 20 oC/min, and 25 oC/min. In each experiment, the sample
129
of 20 mg was spread uniformly on the bottom of the crucible made of alumina and
130
heated from 25 oC to 850 oC under an air flow of 50 mL /min.
131
2.4 Analysis of biochar
132
2.4.1. pH, ζ-potential value and electric conductivity
133
The pH on the surface of the biochar was measured with a glass electrode (Leici,
134
pHS-3C, Shanghai scientific instrument co., LTD) in the same way as previous studies
135
[8, 21]. In a typical measurement, 0.5 g of biochar was added into 50 mL of ultrapure
136
water and left still to stabilize in a centrifuge tube for 2 days until the pH of the slurry
137
stabilized. The pH was measured three times for each biochar sample, and a mean
138
value was obtained.
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Thermo-gravimetric analysis (TG), differential thermo-gravimetric analysis (DTG)
The ζ-potential value, electric conductivity (EC) and electrophoretic mobility (μe)
140
of the biochar particles were determined with a Nano-Zeta-Sizer (Malvern
141
Instruments, U.S.) as described elsewhere [22-23]. In each measurement, 0.15 g of
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fresh biochar was added to a plastic bottle containing 65 mL of freshly distilled water.
143
Each bottle was hand-shaken periodically for 24 h and then left still to stabilize for 1 h
144
before measurements. All measurements were repeated three times, and a mean value
145
was obtained for each sample.
146
2.4.2. BET Surface Area
8
148
Micromeritics ASAP 2020 Surface Area and Porosity Analyzer (Micromeritics
149
Instrument Corporation, USA). Samples were initially degassed at 293 K for 24 h.
150
The nitrogen adsorption/desorption isotherms were measured at 77 K. The specific
151
surface area was calculated by Brunauer-Emmet-Teller (BET) equation [24]. The
152
mesopore
153
Barrett-Joyner-Halenda (BJH) theory [25].
154
2.4.3. FTIR
155
Functional groups on the surface of biochar were measured with KBr pellets
156
containing 1% of each sample by Fourier Transform Infrared Spectroscopy (FTIR)
157
(FTIR-8400S Spectrometer, USA). These samples of particle sizes < 45μm were first
158
dried overnight at 105 oC before the spectra were generated. The resulting spectra
159
were normalized to the peak in the region between 4000 and 400 cm-1 with 100 scans
160
per sample. A previously recorded background spectrum of KBr was subtracted from
161
the spectrum of each sample.
162
3. Results and Discussion
163
3.1. Yields of bio-oil and biochar
were
calculated
based
on
the
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The specific surface areas of B-550, B-650, B-750, and B-850 were measured using a
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Since the bio-oil was originated from the organic components in SS, and the ash
165
content in SS was higher than the common biomass, the oil yield was calculated based
166
on the dry ash free basis, and the biochar yield was on the basis the initial weight of
167
feedstock. As shown in Table 2, the increase in pyrolysis temperature was
168
demonstrated to contribute to the decrease of bio-oil yield by accelerating the crack of
9
the vapors and formation of gases [26]. In addition, the yield of biochar was also
170
decreased but not much. Since the pyrolysis temperature was high, most pyrolytic
171
volatiles had already reacted at a lower temperature and resulted in a relatively small
172
biochar yield variation was found [15]. Furthermore, it is easy to conclude that with
173
the increase of temperature, there was more volatile substances generated and
174
conduced to the decrease of the total weight.
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175
3.2. Characterization of the bio-oils
177
3.2.1 Composition of bio-oils
178
The GC-MS analysis results of the bio-oil obtained from pyrolysis of SS at
179
temperatures of 550 oC, 650 oC, 750 oC, and 850 oC with retention time of 120 min
180
are presented in Table 3. Bio-oil was demonstrated to be a complex mixture which
181
consists of organic compounds from a wide variety of chemical groups, and the
182
compositional differences were insignificant among the bio-oils originating from
183
different temperatures. Main components of bio-oil could be classified into phenols
184
(e.g., phenol, 2(4)-methyl-phenol, 4-ethyl-phenol), esters (e.g., hexadecanoic acid
185
methyl ester and hexadecanoic acid ethyl ester), cholests (e.g., cholest-2(4)-ene and
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cholesta-3,5-diene),
ketones
(e.g.,
2-piperidinone),
amides
(e.g.,
187
(Z)-9-octadecenamide and tetradecanamide), indoles (e.g., 3-methyl-1H-indole and
188
indole), nitriles (e.g., hexadecanenitrile and benzyl nitrile). The result was in line with
189
Cao et al [27]. The content of amides was quite higher than that of nitrile, and the
190
content of hexadecanamide was 8.95% in bio-oil of B-550. However, it was
10
191
palmitonitrile that pointed out to be the most abundant one in their study. It might
192
attribute to the difference of reaction degree. Table 3 is here 3.2.2 Combustion of bio-oil in air with different heating rates
195
As the combustion characteristics of biomass previously investigated [28-29],
196
combustion characteristics of bio-oil in the air were investigated with TGA and DTA
197
experiments. Fig.2 presents the (a) TG-DTG, and (b) DTA curves of combustion of
198
B-550 at different heating rates of 15 oC/min, 20 oC/min and 25 oC/min. Fig. 2. is here
an
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194
As shown in Fig. 2 (a), combustion of bio-oil could be divided into five stages.
201
For the heating rate of 15 oC/min, the first region (100-150 oC) observed was
202
corresponding to the moisture release. Release of volatile organic compounds
203
(150-220 oC) happened in the second region. In the third region (220- 340 oC), release
204
of semi-volatiles and their ignition occurred. The rapid burning and formation of the
205
secondary char generated in the fourth region (340- 480 oC). Lastly, rapid loss of mass
206
immediately slowed in the fifth region (480- 630 oC) and the partly carbonized residue
207
burning slowly. Fig. 2 (a) presents that TG curves of the three different heating rates
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208
exhibited very similar patterns. In addition, the increased heating rate contributed to a
209
more complete combustion, residues for heating rates of 15 oC /min, 20 oC/min and 25
210
o
211
rate could contribute to better heat penetration inside the bio-oil and allowed to
212
determine these conditions in anticipation, which made it possible to program the
C/min were 7.60wt%, 5.39wt% and 2.28wt%, respectively. The increased heating
11
combustion in a relevant and effective way [28]. However, with the increase of heating rate, DTG curves were enhanced and
215
postponed at the fifth stage. The enhancement of the curve could due to the decreased
216
combustion activation energy with the increase of heating rate. Moreover, with the
217
increase of heating rate, the initial combustion temperature and burnout temperature
218
were increased and then resulted in the DTG curves lagged behind.
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The DTA curves could effectively illustrate the endothermic and exothermic
220
stage of bio-oil in the combustion. As shown in Fig.2 (b), at the heating rate of 15
221
o
222
of volatile organic compounds in the second stage was also endothermic though the
223
heat adsorbed was reduced. The last three stages were all exothermic and the heat
224
given off increased with the proceeding of the combustion. Endothermic and
225
exothermic stages presented in Fig. 2 (b) were consisted with the mass loss shown in
226
Fig. 2 (a). Besides, as the heating rate increased from 15 to 25 oC/min, the
227
endothermic stage lagged behind
228
3.3. Characterization of biochar
229
3.3.1 Surface properties of biochar
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230
As presented in Table 4, the untreated SS with pH of 5.93 was weakly acidic,
231
while biochar showed basic property with pH ranged from 7.75 (B-750) to 8.96
232
(B-650) irrespective of the pyrolysis conditions. The basic property of biochar was
233
explained by the removal of acidic functional groups that were not stable at high
234
temperatures [30]. Besides, the organic components in SS decomposed at the high
12
236
also be responsible for the increase in pH [31]. The increased pH caused by thermal
237
treatments was demonstrated to be effective in the treatment of some organic
238
hydrocarbons and facilitated the adsorption of H2S and SO2 [32]. Moreover, it may
239
also benefit for the reclamation of acid soil.
t
pyrolysis temperature, the alkali salts that separated from the organic materials could
The ζ-potential values of SS and biochar were negative. Negatively charged
241
biochar were found to be preferable for the adsorption of cations [33]. Pyrolysis
242
decreased the EC from 0.350 mS/cm of the untreated SS to 0.090 mS/cm of the
243
sample B-850, and a similar trend was noted in the study of Méndez et al. [34].
244
Electrolytes added in the soil were found to affect the flocculation of soil and
245
sensitivity of crops [35].
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240
Table 4 is here
246
The BET surface area (SBET), total pore volume (Vtotal), micropore volume
248
(Vmico), mesopore volume (Vmeso) and average pore width (Dave) of four representative
249
biochar B-550, B-650, B-750, and B-850 are also listed in Table 4. Both SBET and
250
Vtotal were temperature dependent. When the temperature increased from 550 to 650
251
o
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C, SBET increased remarkably by 32.77 m2/g. However, when the temperature further
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252
increased to 850 oC, the SBET and Vtotal only slightly increased, and the change in Vmico
253
was also insignificant. The overactivity caused by elevated temperatures might lead to
254
the sintering of micropores. The removal of organic compounds from the pores by
255
thermal treatment contributed to the enhancement of SBET, Vmeso and Vtotal. These
256
trends are in consistent with previous studies [19, 36]. The SBET result obtained by Lu
13
258
with the SBET consequence (647.4 m2/g) by Chen et al. [38] obtained at B-550, the
259
SBET of B-550 in this paper was quite low. This could be explained by the fact that in
260
the study by Chen et al., the pyrolysis was activated by impregnating SS with 5 mol /L
261
ZnCl2, which had a positive effect on the increase of SBET. In addition, the product
262
was washed with 3 mol/L HCl and rinsed with deionized water, which also favored
263
the increase in SBET.
264
3.3.2 FTIR Analysis
265
Main functional groups of –OH, C=O, C=C, –NO2, S=O were found on the surface of
266
biochar as shown in Fig. 3. The insignificantly varied position of peaks suggesting
267
that functional groups on the biochar were about the same; whereas, the intensity of
268
peaks was significantly different, indicating the contents of functional groups on the
269
surface of sample were varied.
ed
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et al. [37] at 850 oC was 85 m2/g, which almost the same as ours. However, compared
Fig. 3. is here
pt
270
As shown in the figure, the 4000-2500 cm-1 stretching vibration was assigned to
272
functional groups of X-H (X= C, N, O, S). The strong and broad peak displayed on
273
the spectrum near 3450 cm-1 was assigned to the associated O-H [39], indicating the
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257
274
existence of alcohols or carboxylic acids. Furthermore, there was a strong and sharp
275
peak at 2350 cm-1, which was assigned to CO2 stretching vibration. Peaks between
276
2000 and 1500 cm-1 were C=C stretching vibration area. The 1770-1600 cm-1 band
277
indicated the existence of C=O [40]. Because of the effect of the C=O, peaks were
278
shifted toward lower wave numbers of 1667-1640 cm-1, where C=C groups of olefins
14
compounds were found. The peak at 1510 cm-1 was considered to be the -NO2
280
functional group. Near the end of the spectrum band, peaks at 1100-860 cm-1 were
281
related to S=O, and peaks at 830-650 cm-1 were assigned to aromatic structures,
282
skeletal vibrations, amine and amide groups [17]. The peaks corresponding to carbonyl group on biochar were considered to be
284
relatively high in intensity, which might facilitate the adsorption of aromatic
285
compounds through the formation of an electron acceptor-donor complex [36].
286
Besides, acidic oxygen functional groups were considered to be beneficial to the
287
adsorption of metal ions in aqueous solutions [31].
288
4. Conclusion
289
Pyrolysis of sewage sludge at high temperatures favored the enhancement of surface
290
properties of biochar; whereas, the bio-oil yield was decreased. Composition of
291
bio-oil insignificant varied with the variation of temperature. As regards the
292
combustion profile of bio-oil in the air, increased heating rate contributed to more
293
complete combustion. Moreover, with the increase of heating rate, the initial
294
combustion temperature and burnout temperature were increased. This paper provides
295
beneficial information to the application of bio-oil and biochar; whereas the focus of
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the present research is on the properties of products, profound investigation of
297
applying the biochar as soil remediation or adsorbents for removal of pollutants
298
should be conducted in the following study.
299
Acknowledgements
300
This work was supported by Hunan Provincial Natural Science Foundation of
15
302
(NCET-12-0169). The Scientific and Technological Project of Hunan Province in
303
China (2013FJ4040 ) and the Scientific Research Foundation for Returned Scholars.
304
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during pyrolysis, Fuel 74(1995), pp. 344-348.
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the activated carbon made from sewage sludge, Waste Manage. 22(2002), pp.
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755-760. [39] Q.B. Wen, C.T. Li, Z.H. Cai, W. Zhang, H.L. Gao, L.J. Chen, Study on activated
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Table 1 Primary analysis and ultimate analysis of the raw sewage sludge
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an us cr
Ultimate analysis
Ma (wt%)
VMb (wt%)
Ash (wt%)
FCc (wt%)
HHVd (MJ/kg)
C(wt%)
H(wt%)
N(wt%)
Oe(wt%)
S(wt%)
1.670
52.220
46.110
1.840
13.040
28.940
4.480
4.230
13.920
0.650
a: moisture;
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b: volatile matter;
423
c: fixed carbon;
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d: higher heating value;
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e: calculated by mass difference.
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Primary analysis
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Table 2 Yields of bio-oil and biochar at different pyrolysis temperatures Sample
B-550
B-650
B-750
B-850
Bio-oil (%daf.a)
26.16
23.25
21.29
20.78
Biochar (wt%)
54.90
54.45
50.00
50.60
a: dry ash free basis
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Table 3 Main composition of B-550, B-650, B-750 and B-850
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Compounds
Formula
B-550b
8.41 8.47 9.47 9.64 9.96 10.34 10.39 10.67 11.02 11.28 12.75 13.90 19.65
Phenol 3-hexanol 2(4)-methyl-phenol 2-pyrrolidinone 1-methyl-2,5-pyrrolidinedione 9-octadecenamide,(Z)2,5-pyrrolidinedione Benzyl nitrile 4-ethyl-phenol 2-piperidinone Indole 3-methyl-1H-indole 1,2-benzenedicarboxylic acid bis (2-methylpropyl) ester Cyclododecane Hexadecanenitrile Hecadecanoic acid methyl ester Pentadecanenitrile Glycyl-L-proline Tetradecanamide Hexadecanoic acid ethyl ester Heptadecanenitrile N,N-dimethyl-octanamide 1,2-benzenedicarboxylic acid diisooctyl ester Cholest-5-en-3-ol (3.beta)Cholest-2 (4)-ene cholesta-3,5-diene Hexadecanoic acid tetradecyl
C6H6O C6H14O C7H8O C4H7NO C5H7NO2 C18H35NO C4H5NO2 C8H7N C8H10O C5H9NO C8H7N C9H9N C16H22O4
30.17 30.77 31.33 32.15
B-750
B-850
3.04 1.17 5.74 0.41 nd 6.87 0.23 0.31 0.85 nd 3.84 0.50 nd
2.62 ndc 5.99 0.99 0.32 4.37 1.32 0.67 1.04 0.38 2.96 2.20 2.47
2.65 nd 5.98 1.15 0.31 5.16 1.27 0.87 1.05 0.36 2.97 2.36 3.61
3.15 nd 7.02 0.76 0.34 5.19 0.68 0.82 1.79 0.29 3.51 2.96 2.02
C12H24 C15H31CN C17H34O2 C14H29CN C17H12N2O3 C14H29NO C18H36O2 C16H33CN C10H21NO C24H38O
nd 2.06 nd nd 2.05 1.10 nd 2.02 nd 3.48
1.34 5.18 nd nd 1.40 0.30 nd 1.74 0.71 0.69
0.67 3.34 nd nd 2.28 1.52 nd 1.49 0.55 1.01
nd 3.12 0.30 1.19 0.92 0.58 0.44 1.92 nd 0.86
C36H62O2 C27H46 C19H28 C30H60O2
1.28 10.99 3.15 2.43
1.09 9.01 2.56 1.90
0.65 10.25 2.48 1.82
3.11 11.74 3.31 0.59
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19.96 20.19 20.48 20.79 20.88 21.10 21.32 22.74 24.51 27.68
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RT a (min)
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23
ester Hexadecanamide
32.62 431
a
432
b
433
c
C16H33NO
8.95
6.83
7.78
7.59
:retention time : relative composition by area (%)
: nd=not detected
t
434
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Table 4 pH, ζ-potential, electric conductivity, and electro-phoretic mobility of SS and
437
biochars SS
B-550
B-650
B-750
B-850
pH
5.93
8.53
8.96
7.75
8.73
ζ-potential (mV)
-7.29
-13.87
-12.17
-13.87
-11.30
EC a (mS/cm)
0.35
0.08
0.10
0.11
0.09
Mob. b (m2/Vs×10-8)
-0.57
-1.09
-0.96
-1.09
-0.89
48.51
72.50
73.28
81.28
Vmico (cm3/g)
0.006
0.005
0.009
0.006
Vmeso (cm3/g)
0.065
0.088
0.075
0.097
Vtotal (cm3/g)
0.097
0.125
0.136
0.146
6.82
6.59
5.96
5.91
pt
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Characteristics
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: Electric conductivity
: Electro-phoretic mobility
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440 441 442 443 444
24
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Figure captions
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Fig. 1. The experimental schematic diagram of the pyrolysis apparatus
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Fig. 2. Combustion characteristics of B-550 in the air at heating rates of 15 oC/min, 20
448
o
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Fig. 3. FTIR spectra of (a) B-550, (b) B-650, (c) B-750, and (d) B-850
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C/min, and 25 oC/min: (a) TG-DTG, (b) DTA
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Fig. 1
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Figure 2
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Figure 3
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