Element and PAH constituents in the residues and liquid oil from biosludge
pyrolysis in an electrical thermal furnace
by
Hung-Lung Chiang*, Kuo-Hsiung Lin, Nina Lai, Zhu-Xin Shieh
*To whom correspondence should be addressed;
E-mail:hlchiang@mail.cmu.edu.tw;
Tel:
+886-4-22079685;
Fax:
+886-4-22079687
1
Element and PAH constituents in the residues and liquid oil from biosludge
pyrolysis in an electrical thermal furnace
Hung-Lung Chianga．Kuo-Hsiung Linb, Nina Laic, Zhu-Xin Shieh b
a
Department of Health Risk Management, China Medical University, Taichung,
Taiwan
b
Department of Environmental Engineering and Science, Fooyin University,
Kaohsiung, Taiwan
c Instrument
Department Center, National Cheng Kung University, Tainan, Taiwan
ABSTRACT
Biosludge can be pyrolyzed to produce liquid oil as an alternative fuel. The
content of five major elements, 22 trace elements and 16 PAHs was investigated
in oven-dried raw material, pyrolysis residues and pyrolysis liquid products.
Results indicated 39% carbon, 4.5% hydrogen, 4.2% nitrogen and 1.8% sulfur
were determined in oven dried biosludge. Biosludge pyrolysis, carried out at
temperatures from 400-800oC, corresponded to 34-14% weight in pyrolytic
residues, 32-50% weight in liquid products and 31-40% weight in the gas phase.
The carbon, hydrogen and nitrogen decreased and the sulfur content increased
with an increase in the pyrolysis temperature at 400-800oC. NaP (2 rings) and
AcPy (3 rings) were the major PAHs, contributing 86% of PAHs in oven-dried
biosludge. After pyrolysis, the PAH content increased with the increase of
pyrolysis temperature, which also results in a change in the PAH species profile.
In pyrolysis liquid oil, NaP, AcPy, Flu and PA were the major species, and the
2
content of the 16 PAHs ranged from 1.6 to 19 g/ml at pyrolysis temperatures
ranging from 400 to 800oC. Ca, Mg, Al, Fe and Zn were the dominant trace
elements in the raw material and the pyrolysis residues. In addition, low toxic
metal (Cd, V, Co, and Pb) content was found in the liquid oil, and its heat value
was 7800-9500 kcal/kg, which means it can be considered as an alternative fuel.
Keyword: sludge; metal; polycyclic aromatic hydrocarbons (PAHs); pyrolysis
1. Introduction
Biosludge is associated with biological wastewater treatment. The sludge is an
inevitable major byproduct, and its disposal is a complex environmental problem.
Sludge treatment can account for over half of the capital and operating costs of a
wastewater treatment facility (Environment Canada, 1987). Owing to land-use
limitations and odor problems, landfill and composting methods are not suitable
for final disposal of biosludge in metropolitan areas because farmland sludge
applications are limited by the uptake capacity of the soil, especially potential
pollution by heavy metals (Dumpleman et al., 1991). Incineration and pyrolysis
are better alternative methods to minimize, recycle and reuse biosludge.
Incineration can provide a large volume reduction and result in energy recovery
(Mininni et al., 1997; Cao and Pawłowski, 2012), while the residue can be used in
road surfacing, building material metal reclamation and biochar (Brunner, 1980;
Agrafioti et al., 2013).
Sludge disposal is an important issue for wastewater treatment plants, and
environmental problems and the reuse of residues and products after disposal
should be carefully considered. Biomass and sludge have been converted via
pyrolysis processes into usable resources, i.e., adsorbents or fuels that are
3
regarded as a renewable resource for sustainable development and energy
saving.
Incinerators, which operate at a combustion temperature of 800-900 K, yield
residues consisting of dust and ash that are similar to the residues from
incinerated municipal sewage sludge and must be landfilled. These methods are
far from satisfactory under current recycling philosophies.
Recent work has investigated the pyrolysis of sewage sludge in an inert
atmospheric environment where the organic matter was transformed into liquid oil
and gases both containing hydrocarbons under different temperature conditions
(Environment Canada, 1987; Bayer and Kutubuddin, 1982, 1984; Compbell and
Bridle, 1989; Boocock et al., 1992; Bahadur et al., 1995) The sewage
sludge/organic sludge can be converted into useful bioenergy in the form of oil
(Cao and Pawłowski, 2012; Sliva et al., 2012; Cao et al., 2013; Yang et al., 2013 ).
This method not only is economically sound but also reduces the pollutants
associated with sludge (Kim and Parker, 2008) to consider the yield and quality of
bio-oil as well as process efficiency and safety (Manara and Zabaniontou, 2012).
In addition, the biomass contains a high proportion of hydrocarbon compounds
that may be used as a secondary raw material. Although many studies have
investigated the reuse of sludge, most have focused on the characteristics of
pyrolytic residues, liquid products or exhaust gas.
Comprehensive and detailed
information, i.e., operation parameters of pyrolysis and the compositions of
residues, liquid products and pyrolytic gas, etc., are necessary for the reuse of
sludge.
In this work, biosludge was obtained from the wastewater treatment system of the
petrochemical industry and dried and pyrolyzed in a scaled-up electrical thermal
furnace. The oven-dried biosludge, pyrolysis residues and liquid oil were analyzed
4
for their element and PAH constituents. In addition, the organic compositions of
liquid oil were analyzed in this study. Baseline information about the raw material
and pyrolysis products was gathered to judge the recycle characteristics of
biosludge.
2. Experimental
2.1 Raw material
Biosludge samples were obtained from a petrochemical wastewater treatment
plant in Taiwan. The main products of the petrochemical industry include cumene,
phenol, acetone, bisphenol-A, cyclohexanone, -methyl styrene and maleic
anhydride. The biosludge contains primarily microorganisms, obtained from an
activated sludge process of wastewater treatment plants. In total, about 2000 kg
of biosludge cake was sampled for all experiments to avoid varying the biosludge
characteristics at different sampling times.
Due to the inherently high water
content of sludge, about 80%, it was necessary to obtain large sludge cakes to
ensure that there were sufficient sludge solids to produce liquid oil during
pyrolysis. Furthermore, based on the pre-experimental study, a variation of
10-20% was observed in C, N, H and S for the different batch samples, so a large
number of samples was needed to reduce uncertainty in this study.
The biosludge was stored at 4oC until it was pyrolyzed.
Raw biosludge
cakes had a total solids content of 12.3±1.2 wt% and a volatile solids
concentration of 68.4±4.9 wt%. Carbon, oxygen, hydrogen, nitrogen and sulfur
concentrations were 36.0, 27.4, 4.3, 4.2, and 2.4%, respectively, in the biosludge
cakes (oven-dried at 105oC).
5
2.2 Pyrolysis process
Pyrolysis was conducted in a cylinder electric furnace reactor (stainless steel
316L, ID: 300 mm, height: 450 mm, shown as Figure 1) associated with an
agitator to ensure complete heating of the sludge mixture. At the beginning of
each run, 10 kg of biosludge was put into the reactor, and the reactor was heated
to the designated temperature at a rate of 40 K min-1. The biosludge pyrolysis
experiments were conducted mainly at pyrolysis temperatures ranging from 400
to 800oC.
The temperature profile was taken into consideration during reactor
design to ensure that the temperature distribution was homogeneous. A
temperature variation from 20 to 60oC was observed, corresponding to
temperatures from 400 to 800oC.
This variation represented 5-10% of the
setting temperatures, which is in the acceptable range.
After pyrolysis, the reactor was cooled to room temperature before the pyrolysis
residue was removed for quantification and characterization. The biosludge was
weighed on an analytical balance (Mettler-Toledo, model EL16001G, Switzerland,
weighing limit is less than 0.1g), and the pyrolysis residue was weighed by the
Mettler-Toledo model JE3002GE (weight limit less than 0.01g), Switzerland.
2.3 Chemical compositions of pyrolytic residues
The nitrogen, carbon, hydrogen, and oxygen constituents were analyzed
with an element analyzer (Heraeus CHN-O Rapid Element analyzer, USA).
Sulfur and chloride concentrations were measured with the Tacussel Coloumax
78 (USA) element analyzer. Sulfanilic acid (NH2C6H4SO3H, Merck, USA) and
1-chloro-2,4-dinitrobenzene (ClC6H3(NO2)2, Sigma-Aldrich, USA) were used as
standards.
Biosludge pyrolysis residue was digested with a mixture of HNO3: HClO4:
6
HF in a 3:5:2 proportion.
A 20 ml acid mixture was mixed with the biosludge
residue in a Teflon-lined closed vessel and placed in a high-pressure digestion
oven at 170oC for 5 hrs. Then, the digested acid mixture was placed in the
Teflon vessel and heated at 90oC to concentrate the acid mixture to 0.2 ml. Finally,
the concentrated acid mixture was diluted to 25 ml by 2% nitric acid. The diluted
digested acid mixture was analyzed to determine the trace elements. A Perkin
Elmer OPTIMA 3000 ICP-AES was used to determine the Al, Ca, Fe, K, Mg, Na, S,
Co and Zn concentrations. Additionally, a SCIEX Elan Model 5000 ICP-MS
manufactured by Perkin-Elmer was employed to determine the As, Ba, Cd, Cr, Cu,
Mn, Ni, Pb, Sb, Se, Sr, Ag and V concentrations.
Although ICP-AES is not a
recognized method for determining sulfur, it was the same treatment and analysis
method as used with other elements in this work, and it is a convenient tool. In
addition, analysis was performed on five samples in duplicate for quality
assurance and quality control.
2.4 Chemical composition of biofuels
The pyrolysis liquid product was distilled to separate the oil from liquid. The
distillation temperature followed the standards for typical diesel fuel (ASTM, 1992)
and the literature on sewage sludge pyrolysis. The distillation ranges of the
hydrocarbon liquids of sewage sludge went from 62 (initial boiling point, IBP) to
335oC (95% recovery), and the typical diesel fuel temperatures corresponding to
the IBP ranged from 162-180oC, with 90% recovery from 316-326oC (Bahadur et
al., 1995). Results indicated the liquid oil could be recovered and separated from
the pyrolysis liquid at this temperature range.
The temperature program for distillation was as follows: heating to 120oC
(20oC min-1) for 10 min to remove water and then heating to 250oC (10o C min-1)
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for 15 min to collect liquid oils. After that, the temperature was increased to 350oC
(10oC min-1) for 10 min, which volatilized most of the liquid and left some char in
the bottom of the distillate bottle. Over 80% of the liquid oil was collected at
200-250oC. The composition of the pyrolysis oil was measured via gas
chromatography with mass spectrometry.
A gas chromatograph (GC) coupled with a mass spectrometer (MS) (GC-MS,
model QP-2010; Shimadzu, Japan) was used for the composition analysis of bulk
samples of biosludge pyrolysis oils.
A DB-5 column (0.25 mm  30 m, 0.25 m
film thickness, Agilent J&W) was used with the following temperature program
and conditions: 45oC was initially maintained for the sample inlet, and then the
temperature was raised to 270oC at a rate of 5oC per min, which was held for 15
min; injector and detector temperatures of 250oC; carrier gas nitrogen with
constant pressure of 37 kPa; detector dual FID; split ratio 1:10; and injection of
0.5 l with high pressure of 100 kPa.
GC peaks were identified by comparing the
MS fragmentation pattern and relative retention time with those of the reference
compounds available.
The relative proportions of the biosludge pyrolysis oil
constituents were expressed as percentages obtained by FID peak-area
normalization.
2.5 PAH analysis in residues and liquid oil
Sample
solid
residue
(10
g)
was
extracted
with
100
mL
of
acetone+dichloromethane (1:3, by volume) in an ultrasonic bath (XB14, Grant
Instruments Ltd.) at 251 °C. To avoid the loss of volatile compounds, an
appropriate airtight vial with low head space was used for the extraction
procedure. The supernatant was passed through a 0.45 μm PTFE filter after 30
min.
After extracting three times, the combined extract was concentrated on a
8
rotary evaporator (EYELA, Japan) equipped with a water bath held at 40°C, then
reduced to 1-2 mL. The extract was cleaned with solvent elution in a silica
column,
through
which
10
mL
of
n-hexane
and
50
n-hexane+dichloromethane (1:1, by volume) were respectively passed.
mL
of
Only the
part of the elution using n-hexane+dichloromethane, the effluent containing the
PAHs, was collected and evaporated just to dryness, then quantified to 2 mL for
analysis via gas chromatography (GC).
Sample liquid oil (10 mL) was mixed with 50 mL of dichloromethane,
vigorously shaken in an airtight bottle, and centrifuged, followed by filtering of the
supernatant. The remaining residue was extracted a total of three times. The
combined extract (3×50=150 mL) was passed through anhydrous Na 2SO4, then
further concentrated, cleaned and analyzed as described above.
The GC (HP 6890) was equipped with an HP-5MS capillary column (30 m at
0.32 mm i.d. with 0.25 μm film thickness) and connected to the MS (HP 5973 N).
To analyze the PAHs, the injector program was set to 280°C at splitless mode,
and the carrier gas (99.9995% helium) flow rate was held at 1.5 mL min -1. The
oven temperature program was 60°C for 1 min, 35°C min-1 to 170°C, 8°C min-1 to
210°C, and 4°C min-1 to 300°C, which was held for 15 min. A certified PAH
standard (Supeclo Inc., USA) was mixed with toluene to prepare the calibration
standard solutions. The 16 PAH species included Naphthalene (NaP),
Acenaphthylene (AcPy), Acenaphthene (Acp), Fluorene (Flu), Phenanthrene (PA),
Anthracene (Ant), Fluoranthene (FL), Pyrene (Pyr), Benzo(a)anthracene (BaA),
Chrysene (CHR), Benzo(b)fluoranthene (BbF), Benzo(k)fluoranthene (BkF),
Benzo(a)pyrene (BaP), Indeno(1,2,3-cd)pyrene (IND), Dibenzo(a,h)anthracene
(DBA), and Benzo(g,h,i)perylene (BghiP). The instrument detection limit (IDL) was
determined by analyzing standard solutions (near the estimated IDL) and
9
calculating the signal-to-noise ratio for each standard concentration.
Linear
regression of the signal-to-noise ratios against concentrations was then used to
determine the IDL (signal-to-noise ratio = 3). The sample that had the lower
content of PAHs was divided into the same size and then spiked with a low
concentration. The method detection limit (MDL) was determined by analyzing the
spiked samples (n = 7 replicates) and calculated by multiplying the standard
deviation of the resulting concentrations by a t-test value of 3.1 (Taylor, 1987). The
MDL for the 16 PAHs fell in the range 6.8- 28 ng/g and 5.7- 26.9 ng/mL for solid
residue and liquid oil, respectively.
Five concentrations of calibration standards were prepared by diluting the
mixed standard solution.
The PAH spike was performed for recovery analyses
based on quality assurance and quality control.
The average recoveries of the
PAHs from solid residues and liquid oils were 74% (NaP) to 98% (BkF) and 79%
(NaP) to 102% (BkF), respectively.
3. Results and discussion
3.1 Raw material
The oven-dried biosludge and the pyrolytic residues were analyzed for
carbon, hydrogen, nitrogen, sulfur, and chlorine content using an elemental
analyzer.
Figure 2 shows that the element contents in the oven-dried biosludge
were 38.72.56% carbon, 4.530.09% hydrogen, 4.200.32% nitrogen and
1.820.27% sulfur.
Trace element compositions in the pyrolysis residues of biosludge are
summarized in Table 1. Zn content was high in the dried raw material and
pyrolytic residues. The results obtained for Zn, Ni, and Cr content were in the
same range for both this study and the study of Dai and coworkers (2007), but Cu,
10
Pb and Cd were low and As was high in our study.
Some elemental
concentrations were toxic, i.e., Cd was 0.0003-0.0008 mg g-1, Cr was
0.026-0.071mg g-1, Pb was 0.0042-0.017 mg g-1, As was 0.0045-0.0135 mg g-1,
and Cu was 0.050-0.076 mg g-1.
3.2 Pyrolysis characteristics and residues
3.2.1 Pyrolysis mass distribution
Generally, the sludge cakes contained 75-80% water obtained from the
wastewater treatment plant of the petrochemical industry. The oven-dried
biosludge was pyrolyzed to gain the fraction of products in the gas phase, liquid
phase, and solid residue, which were 31-40, 32-50, and 34-14%, respectively, at
pyrolytic temperatures from 400-800oC (shown in Figure 3). High temperature
reduces the yield of residues and enhances the gas products.
The liquid product
reached a high yield at 600oC, after which increasing the temperature reduced the
yield of liquid product because liquids volatilize at high temperature. Previous
studies have indicated that many factors can affect pyrolysis conditions and
product characteristics, including feedstock type and composition, pressure,
reaction temperature, retention time and heating rate (Rulkens, 2008; Pokorna et
al., 2009; Zuo et al., 2012; Cao et al., 2013). Generally, a high heating rate was
favorable for the formation of oil and gas and a low char yield, whereas a low
heating rate at low temperatures enhanced the yield of char. Higher temperatures
and longer residence times promote gas production, whereas higher char yields
are obtained at lower temperature and slow heating (Vamvuka, 2011). In this
study, we addressed only the temperature effect on sludge pyrolysis and not the
effect of heating rate and retention time, could be a limitation of this work.
11
3.2.2 Pyrolysis residues
1. Major elements
In addition, for the pyrolytic residue of biosludge, carbon decreased from
36 to 22%, hydrogen decreased from 3.0 to 1.1%, nitrogen decreased from 4.0 to
1.7%, and sulfur increased from 2.5 to 3.6% when the pyrolysis temperature was
increased from 400 to 800oC.
Results were similar to the sewage sludge
composition in Piatkowski and Steinfeld’s (2010) investigation, except that sulfur
content was high in their study.
Chlorine was not detected (<0.01%) in any residue or biosludge.
The
results of elemental analysis showed 28-50% biosludge fractions. A large fraction
of biosludge could not be identified in this work, but based on another study (Shen
and Zhang, 2005), oxygen content was high in the biosolid. Therefore, most of the
unanalyzed elemental fraction could be oxygen.
The percentage of each
element in the pyrolysis residue, except for sulfur, was less than that of the
oven-dried biosludge. When pyrolytic temperatures increased, the elemental
content (C, H, and N) decreased in the residues. The results also revealed that
the amount of carbon, nitrogen, and hydrogen from the residues decreased when
the pyrolysis temperature was raised from 400 to 800oC, suggesting that the
carbon, nitrogen, and hydrogen were desorbed, but the sulfur was not. The
residual sulfur content increased as the pyrolysis temperature increased
throughout the experimental temperature range. This may be attributed to the fact
that the sulfur forms stable high-boiling-point compounds (due to the high bond
energy of sulfur compounds) with other pyrolysis products; therefore, sulfur
compounds could be volatilized at a higher temperature (Kinoshita, 1987;
Kuramochi et al., 2005) than the temperatures set in this study.
2. Trace elements
12
Table 1 reveals the trace element content in pyrolysis residues. The
analyzed trace element concentration was 108-128 mg/g. Ca, Mg, Al, Fe and Zn
were the dominant trace elements in the raw material and pyrolysis residues. In
addition, Zn, and Pb were volatilized at high temperature. The As and Cr content
were in the range of that reported by Rauckyte et al. (2006), but Cd and Pb were
lower than in their study.
In contrast to other elements in pyrolytic residues, Cd
concentration decreased because Cd was volatile at temperatures higher than
600oC (Nerín et al., 1999).
3.3 Chemical constituents of liquid oil
3.3.1. Major elements in liquid oil
Figure 3 shows the biosludge reuse process and mass fraction of the solid,
liquid and gas phases. Initially the sludge cake was dried at 105oC in the reactor.
At this step, over 50% by weight of the water vapor and volatile compounds was
volatilized. The biosludge pyrolysis, carried out at temperatures from 400-800oC,
corresponded to 34-14% weight in pyrolytic residues, 32-50% weight in liquid
products (water and crude oil) and 31-40% weight in the gas phase. Furthermore,
the liquid product was distilled to product oil. There was 4.2-7.8 wt% of oil based
on the original biosludge cake. Domínguez et al. investigated pyrolysis of sludge
by an electric furnace; in their results, char was 10%, aqueous was 59%, oil was
0.9% and gas was 30%.
The product yields were similar to those obtained in this
study, and the oil product was a little high. The average main elemental
components of biofuel were C (67-73%), H (6.6-9.3%), N (3.45-5.43%), and S
(0.4-3.3%) with the pyrolytic temperature at 400-800oC (shown as Figure 4). In
addition, the chlorine content was less than 0.01%.
increased slightly with an increase in pyrolytic temperature.
The carbon content
Hydrogen, nitrogen
13
and sulfur contents varied in the biofuel at different pyrolytic temperatures. The
heat values of liquid oils are 7800-9500 kcal/kg, similar to the values obtained with
H2O (10-20%) and diesel emulsion liquid (US Patent, 2003; 2006).
3.3.2 Organic constituents of liquid oil
The pyrolysis liquid consisted of aqueous (containing primarily water and
water-soluble organics including alcohols, ethers, aldehydes, carboxylic acid, etc.)
and oil fractions. Pyrolytic oils are complex mixtures of organic compounds with a
wide variety of chemical groups. The detailed compounds of biofuel are shown in
Figure 5, which was made using the percentage of area of the chromatographic
peaks.
Results indicated that 4-Hydroxy-4-methyl-2-pentanone (C6) (21-33%),
Nitrobenzyl alcohol (p + m) (C7) (15-23%), 4-Hydroxypentan-2-one (C5)
(1.3-3.5%), (Z)-7-Hexadecenal (C16) (1.3-13.6%), p-Cresol (C7) (1.7-4.0%),
Pentadecane
(C15)
(1.1-1.9%),
n-Nonadecanenitrile
(C19)
(2.2-2.7%),
3-Methylindole (C9) (1.0-4.2%), n-Heptadecanonitrile (C17) (1.1-6.3%), Cetyl
alcohol (C16) (1.6-2.6%), and n-Nonadecane(C19) (0.99-4.6%) were the major
components.
Based on the carbon number fraction of biofuel, C5-C9 accounts
for about 55-64%. In addition, the variation of the composition fraction of liquid
biofuel at various pyrolytic temperatures may be caused by the cracking and
reformulation of organic molecules at high temperature. Generally, pyrolysis of
sludge yielded liquid products that contained significant amounts of oxygenated
compounds and unacceptable amounts of nitrogen and possibly sulfur
(Karaylidirim et al., 2006). According to Table 2, 55-65% oxygenated compounds
(oxygenated aromatics compounds, ketones, alcohols etc.), 8-20% aliphatics
(n-alkanes, alkenes and branched hydrocarbons), and 10-15% nitrognated
compounds were the main components in liquid biofuel.
Although low aromatic
and polar contents are desired in fuels, they are very important as raw materials in
14
industry, and the proteins and lipids in sludge are responsible for the polar and
aromatic content in pyrolysis fuel.
Ideally, the components in oil should be
straight chain hydrocarbons, as these have a high heating value and lower
viscosity.
In this study, the biofuel contains a high portion of oxygenated
compounds and aliphatics, which are similar to the composition of fuel oil and can
be regarded as an alternative fuel after its oil quality is upgraded or it is blended in
a fuel mixture.
3.3.3 Trace element of liquid oil
Table 3 shows the element composition of liquid oil from the pyrolysis of
sludge under various pyrolysis temperatures. Sulfur was found at a significant
level in oil, with a range from 0.29-1.09 mg/g. The total elements ranged from 1.1
to 1.9 mg/g-sludge, which indicted that low heavy metals (such as Cd, V, Co and
Pb) transfer from sludge into liquid products during pyrolysis. In addition, few
studies in the literature have discussed the trace metal content in oil. Based on
heat value and low heavy metal content, the liquid oil could be regarded as an
alternative fuel after proper treatment to upgrade its quality.
3.4 PAH concentration in residues and oil
3.4.1 PAHs content in pyrolysis residues
Figure 6(a) shows the PAH concentration in the oven-dried sludge and pyrolysis
residues. The total PAH content (from 16 PAHS) was 0.20 g/g in oven-dried
biosludge and 0.21-1.0 g/g for pyrolysis residues associated with temperatures
in the 400-800oC range. NaP (2 rings) and AcPy (3 rings) were the major PAHs,
contributing 86% of PAHs in oven-dried biosludge. After pyrolysis, the PAH
content increased with the increase of pyrolysis temperature, resulting in a
change in the PAH species profile. At 400oC, NaP and AcPy were still the
15
dominant PAHs; but when the temperature was over 500oC, the main PAH
species were PA (3 rings) and Ant (3 rings). The content of high-ring PAHs ( 5
rings) was low in the raw material and residues. BaP (5 rings), IND (6 rings), DBA
(5 rings) and BghiP (6 rings) contents were lower than the method of detection
limit in the oven-dried sludge and residues. Xu et al.(2013) indicated that the 2- to
3-ring PAHs were the major species in the dewatering sewage sludge, and there
was almost no formation of PAHs with more than 4 rings at temperatures lower
than 450oC and reaction times less than 60 min. This finding is similar to that of
this work; that is, high-ring PAHs (> 4 rings) formed in the residues after
high-temperature pyrolysis due to their high formation energy (Wang and
Frenhlach, 1997).
The formation of PAHs can be explained by several reaction pathways. The
Diels–Alder reaction mechanism assumes that the pyrolysis of alkanes to produce
alkenes and dienes occurs via dehydrogenation at high temperature, cyclization
and subsequent aromatization producing the aromatic compound (Cypres, 1987;
Fairburn et al., 1990; Sánchez et al., 2009; Hu et al., 2014). In sludge pyrolysis,
the oxygenated compounds and aliphatic compounds suggest a Diels–Alder
reaction to form PAHs. However, other routes of formation for PAH formation
during combustion or pyrolysis have also been suggested. For example, a direct
combination of aromatic ring and H2 abstraction–C2H2 addition, and secondary
reaction of oxygenated compounds such as phenols, cresols and xylenols
atmoderate to high temperatures can produce PAHs via deoxygenation (Morf et
al., 2002; Hu et al., 2014). According to the H-abstraction-C2H2 addition
mechanism, the 4-phenanthryl radical could react via the addition of C2H2 to form
pyrene in a sooting acetylene flame (the reaction presented a large exothermicity
∆ Ho=-56 kcal/mol) (Wang and Frenhlach, 1997). High formation energy for
16
high-ring PAHs (>4 rings) could be one reasons that low-ring PAHs were
dominant at low temperature. Some studies have indicated that the condensation
of PAHs to form low-ring PAHs is the main mechanism during the pyrolysis
process at low temperature.
3.2.3 PAHs in liquid oil
Figure 6(b) shows the PAH concentration in liquid oil. In pyrolysis liquid oil, the
content of the 16 PAHs ranged from 1.6 to 19 g/ml, corresponding to
temperatures from 400 to 800oC. Results indicated that NaP, AcPy, Flu and PA
were the major species. When the pyrolysis temperature was up to 500oC, the
fraction of Flu and PA was over 10%. A low fraction of PAHs with more than four
rings was found in the liquid oil and in the oven-dried sludge, suggesting that
high-ring PAHs could be formed during the pyrolysis process. Some results
indicate that the formation of low-ring PAHs was the main mechanism during
biomass pyrolysis at low temperatures. In addition, when the pyrolysis
temperature is above 700oC, the product is predominantly composed of PAHs and
substituted PAHs (McGrath et al., 2003, 2007; Sánchez et al., 2013).
In this work, the total PAH content in raw sludge was about 0.2 μg/g and the 2and 3-ring PAHs (low-ring PAHs) contributed almost 98%. The content of
high-ring PAHs (more than 4 rings) was low in the raw material. BaP (5 rings),
IND (6 rings), DBA (5 rings) and BghiP (6 rings) contents were lower than the
method of detection limit in the raw sludge. The total PAH contents were 1.6-19
μg/g (the density of pyrolysis liquid product was close to 1g/mL) for pyrolysis liquid
product associated with temperatures in the 400-800oC range. They regularly
increased as the final temperatures increased from 400 to 800°C, and the most
abundant PAHs were still the low-ring PAHs (>90%).
17
The yields of pyrolysis liquid product were 32, 41, 47, 50 and 46%, respectively,
at pyrolysis temperatures from 400 to 800oC, so we can assume that the amount
of PAHs in pyrolysis liquid product is greater than that in raw sludge. This
indicates that a large amount of new low-ring PAHs were formed during the
pyrolysis sludge process.
In addition, the generated low-ring PAHs could be
discharged with the gas phase, rather than concentrated in the pyrolysis liquid
product. The 4-ring and 5-ring PAHs were formed in the pyrolysis liquid product
after high-temperature pyrolysis, and they contributed about 10% of the total
amount. The BaP was the only high-ring PAH formed during high-temperature
pyrolysis, and it was negligible when compared with the total content.
In addition,
though the total content of PAHs was the highest at a pyrolysis temperature of
800oC, the total amount of PAHs in pyrolysis liquid product reached the highest at
700oC (because its yield of pyrolysis liquid product was the highest, 50%). It
seems that the higher yield of pyrolysis liquid product (pyrolyzed from unit sludge
mass), the higher the amount of PAHs formed.
4. Conclusions
Biosludge can be reused to produce char residues, and liquid oil after pyrolysis in
a large-scale electric thermal furnace. The trace element content in pyrolysis
residues was 108-128 mg/g, and Ca, Mg, Al, Fe and Zn were the dominant trace
elements in the raw material and pyrolysis residues. NaP and AcPy were the
dominant PAHs in oven-dried sludge and pyrolysis residues at 400oC; however,
when the temperature was over 500oC, PA and Ant were the main PAH species in
pyrolysis residues. NaP, AcPy, Flu and PA were the major species in liquid oil.
High-ring PAHs (>4-ring PAHs) could be formed during the pyrolysis process at
high temperature, but their levels were low in residues and liquid oils. Sulfur
18
revealed a significant level in oil, which could come from the biosludge. Low
heavy metals were determined in oil, indicating that a trace of heavy metal
transferred
from
sludge
into
4-Hydroxy-4-methyl-2-pentanone,
the
liquid
nitrobenzyl
products
alcohol
during
(p
pyrolysis.
+
m),
4-hydroxypentan-2-one and (Z)-7-Hexadecenal were the main constituents, and
C5-C9 species accounted for over 50% fractions in liquid oil. The heat values of
liquid oil are 7800-9500 kcal/kg, suggesting that it could be used as an alternative
fuel if its quality is upgraded.
Acknowledgements
The authors express their sincere thanks to the National Science Council, Taiwan
(NSC-100-2221-E-039-005-MY2 and NSC-102-2221-E-039-002-MY3) for the
support.
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Table and Figure Captions
Table
Table 1 Trace element constituents (mg/g) of sludge and pyrolysis residues at
different temperatures
Table 2 Liquid oil constituents of sludge pyrolysis under different temperatures
Table 3 Trace element constituents (mg/g) of pyrolysis oils of sludge at different
temperatures
Figure
Figure 1 Schematic diagram of sludge pyrolysis reactor
Figure 2 Major elemental compositions of sludge and pyrolysis residues at
different temperatures
Figure 3 Mass fraction distribution of sludge pyrolysis at different temperatures
Figure 4 Major elemental compositions of liquid oil from sludge pyrolysis under
different pyrolysis temperatures
Figure 5 Typical liquid oil spectrum and constituents from sludge pyrolysis
Figure 6(a) PAH concentration in oven dried sludge and pyrolysis residues at
different pyrolysis temperatures
Figure 6 (b) PAH concentration in liquid oil at different pyrolysis temperatures
24