Fuel 130 (2014) 92–99
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Formation of PAHs during the pyrolysis of dry sewage sludge Qianjin Dai a, Xuguang Jiang a,⇑, Yunfan Jiang b, Yuqi Jin a, Fei Wang a, Yong Chi a, Jianhua Yan a a b
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, PR China Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
16 PAHs were numerously formed
during pyrolysis of dry sewage sludge. Temperature, sample mass, residence time and gas flow all had influences. PAHs yielded most at 950 °C while TEQ was highest at 1150 °C. PAHs formation was favoured in fuel rich condition. PAHs in different residence time of 2–16 s was highest at 10 s.
a r t i c l e
i n f o
Article history: Received 4 February 2014 Received in revised form 19 March 2014 Accepted 8 April 2014 Available online 20 April 2014 Keywords: Sewage sludge Pyrolysis Temperature Sample mass Gas residence time
a b s t r a c t The formation of the 16 polycyclic-aromatic hydrocarbons (PAHs), characterized by USEPA as priority pollutants, was studied during the pyrolysis of dry sewage sludge in a wide range of operation conditions, i.e., reaction temperature (250–1250 °C), sample mass (0.1–4 g), gas residence time (2–16 s) and gas flow (50–1000 mL/min). The pyrolysis was conducted in a tubular reactor with well-controlled temperature profile and carrier gas flow. The yield of PAHs strongly increased with temperature, reaching a maximum value at 950 °C, while the toxicity equivalent (TEQ) became highest at 1150 °C. The sample mass also influenced upon both the PAHs concentration and the TEQ, with the lowest output found for 1 g sample; rising residence time of the gas flow inflates the PAHs formed and their TEQ up to 10 s, and then they decreased slightly. Raising the gas flow from 50 to 100 mL/min moderately decreases the concentration of PAHs, yet sharp declines their TEQ. To evaluate any influences of the experimental parameters the fractional distribution of the total PAH amount and the TEQ of the 16 PAHs, as well as their subtotal grouped by ring number, were separately displayed. The present results may be valuable for assessing sewage sludge pyrolysis as thermal treatment method and also for studying the formation and destruction mechanism of PAHs during sludge pyrolysis. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a group of ubiquitous organic contaminants drawing great public concern for their ⇑ Corresponding author. Tel.: +86 571 87952775; fax: +86 571 87952438. E-mail address:
[email protected] (X. Jiang). http://dx.doi.org/10.1016/j.fuel.2014.04.017 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
potential carcinogenic and mutagenic characteristics and harmful implication for human health [1,2], which become more serious still in winter [3]. PAHs are also precursors of PM (particulate matter), formed in locally fuel rich areas [4] and a great issue in China. The global distribution of PAHs is also influenced by the thermal treatment of biomass and as chemically stable pollutants. PAHs also accumulated in the atmosphere regardless of long-range
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transport [5]. Thus PAHs emissions should be reduced to protect the environment. PAHs have been extensively studied in the pyrolysis of coal [6–11], wood [12], wastes tires [13] and other biomass: gas [14], tar [15], char [16] analysis and the formation mechanism of PAHs have also been discussed in some detail [17,18]. Catechol [19], propyne [20], and 1,3-butadiene [21] were also submitted to pyrolysis separately or together to investigate PAHs emissions. However, the formation of PAHs depends much on their source and the reaction conditions [22]. PAHs emission is highly influenced by the pyrolysis operating conditions (temperature in pyrolysis of terpenes [23,24], acetylene [4], sample mass, gas residence time, gas flow); however the subject is still causing some controversy [4]. Sewage sludge is raising great concerns in China for its rapidly increasing amount and concomitant risks for human health and the environment [25]. Pyrolysis of sewage sludge shows great advantage over combustion in terms of low cost in pollutant control [26,27]. Yet, the formation of PAHs during sewage sludge pyrolysis is of great concern [28]. In the pyrolysis of sewage sludge PAHs were usually discussed but mostly for qualitative analysis [29] and general discussion [15,22]. Some parameters, such as temperature, residence time have been reviewed [30]. Other parameters such as sample mass, gas residence time and gas flow are rarely discussed in literature. In this study, the 16 PAHs, characterized by USEPA as priority pollutants were analyzed in the gas effluent evolving during sewage sludge pyrolysis in a tubular reactor. The parameters, temperature, sample mass, gas residence time and gas flow were all varied to investigate their influence on PAHs emission. The formation and destruction mechanism of PAHs are discussed. The results may also be useful in designing sewage sludge disposal for energy recovery and environmental protection. 2. Experimental 2.1. Reagents and materials Sewage sludge was collected from the wastewater treatment plant of Shi Dong Kou in Shanghai, China. The sludge underwent aerobic digestion and mechanical dehydration before sampling. The wet sewage sludge with moisture of 84.2 (wt.%) was dried in a fluidized bed at 85 °C. The dry sewage sludge with moisture of 6.24% was then ground, sieved (80–200 mesh), and stored in brown bottles. Proximate and ultimate analysis results on air dry basis are presented in Table 1. The values of Mad, Aad, Vad and Fcad were determined according to method GB/T212-2008 of China. The elementary analysis of Cad and Had were determined by the Liebig method (ISO 625-1996). The content of Nad was determined by the semimicro Kjeldahl method (ISO 333-1996). The content of Sad was determined by the IR spectrometry (ISO 19579-2006). The content of Clad was determined by combustion-hydrolysis/ion chromatography (IC) Method. The content of Oad was determined by
difference. Dichloromethane (DCM), hexane and copper powder were obtained from ShanghaiHuShi Corporation. 2.2. Experimental study All experiments were performed in a horizontal high temperature resistant (up to 1400 °C) quartz tubular reactor (diameter 20 mm, length 600 mm) (Fig. 1). The furnace was heated to the target temperature and then maintained at this temperature (±2 °C). Approximately 1 g (±0.01 g) of dried sewage sludge was evenly loaded into a quartz crucible (width 10 mm, length 100 mm) and then the crucible was introduced into (the right side in Fig. 1) a horizontal quartz reactor tube, at a temperature below 100 °C. The gas outlet was connected to a set of sampling apparatus, composed of a tube with XAD-2 resin and two bottles of DCM, after purging the reactor for 10 min with nitrogen at a flow rate of 3 L/min to avoid potential impacts of oxygen. The flow rate of nitrogen carrier gas is adjusted to 0.2 L/min, sufficient to prevent the pyrolysis gas generated from accumulating in the tube and at the same time not to affecting the temperature of the sample too much. Then, with a quartz stick the crucible is pushed to the centre of the reactor and pyrolysis starts. The generated gases passed through the sampling apparatus, consisting of a XAD-2 resin column and a set of DCM scrubbing solutions contained in ice baths, to absorb PAHs and other non-condensed compounds from in the gaseous effluent. After each pyrolysis run of 30 min the furnace was turned off and the sample was allowed to cool in an inert atmosphere for 30 min almost down to room temperature. The residue was then weighted and saved. The resin and DCM were saved and analyzed for PAHs. Temperature was measured by a K type thermocouple and the temperature distribution profile in the reactor at 850 °C (Fig. 1) was shown to be representative. It was observed that a temperature close to set point is reached on the whole. Distance values in Fig. 1 range from 50 to 350 mm, while the outlet point and inlet point remain below 200 °C for all pyrolysis runs. Pyrolysis runs with different temperature, ranging from 250 to 1250 °C, were conducted by adjusting the furnace temperature to target temperature. The sample mass loaded was varied (mass of 0.1 g, 0.4 g, 1 g, 2 g, 4 g) to investigate the influence of sample mass. The pyrolysis gas residence time was controlled between 2 and 16 s by adjusting the sample location from 2 to 16 cm (Fig. 1), while the gas flow rate was calculated to 1.06 cm/s when a flow of 200 mL/min nitrogen was induced. The gas flow rate was adjusted from 50 to 1000 mL/min using a rotameter. The 24 test conditions are all listed in Table 2. 2.3. PAHs analysis The EPA method 8100 was taken as reference, with slight modification in the PAHs analysis. Soxhlet extraction according to EPA method 3540C was used as extraction procedure to guarantee good contact between solid sample and solvent [31]. Some 2 g cleaned
Table 1 Proximate and ultimate analysis of sewage sludge (mass%). Mad
Aad
Vad
Fcad
Cad
Had
Nad
Sad
Oad
Qbad (kJ/kg)
6.24
43.05
44.99
5.72
25.97
3.49
4.49
0.81
15.95
11 580
ad% = air dry basis. M = moisture content. A = ash content. V = volatile matter content. Fc = fixed carbon content. C, H, N, S, O, Cl = the elements composing the combustible fraction. Qb = net calorific value.
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Fig. 1. Schematic view of the experimental set-up showing the axial temperature distribution for the experiment at 850 °C.
Table 2 Experimental conditions. Run
Variable
Nominal temperature (°C)
Sample mass (g)
Gas residence time (s)
P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11
Temperature
250 350 450 550 650 750 850 950 1050 1150 1250
1
10
200
P12 P13 P14 P15
Sample mass
850
0.1 0.4 2 4
10
200
P16 P17 P18 P19 P20
Gas residence time
850
1
2 4 8 12 16
200
P21 P22 P23 P24
Gas flow
850
1
10
50 100 400 1000
copper powder was mixed with the sewage sludge before extraction to eliminate sulfur according to EPA method 3660B. Silica gel clean-up was adopted according to EPA method 3630C while rota-evaporation was used to concentrate the sample solution. The 16 PAHs characterized by USEPA as priority pollutants were determined using GC (Varian CP-3800, equipped with a 30 m * 0.25 mm * 0.25um DB-5MS capillary)/MS (1200 L) in a selected ion monitoring mode (SIM). The carrier gas was He (1 mL/min) while the injection mode was splitless (270 °C, 1 lL). The temperature program started at 50 °C, held for 1 min then the temperature
Gas flow (mL/min)
was raised to 300 °C at a rate of 20 °C/min and held there for 7 min. The transfer line temperature was set at 300 °C and the Electron impact (EI) source at 270 °C. The PAHs were determined using the external standard method. A standard mixture of the 16 EPA-PAHs was supplied by SIGMA–ALDRICH (PAHs concentration of 2000 lg/mL, and a solvent mixture of methylene chloride: benzene, v:v = 1:1). The PAHs concentrations were calculated with a group of 5 concentration levels ranging from 0.05 lg/mL to 1 lg/mL with hexane as solvent. The GC/MS SIM profile programmed in the MS and the concentration of external standards calculated using the
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Q. Dai et al. / Fuel 130 (2014) 92–99 Table 3 The working curve used to quantity PAHs with GC/MS. PAHs
TEF
SIM MS window time (min) RT (min) 0.05 lg/mL 0.1 lg/mL 0.2 lg/mL 0.4 lg/mL 1 lg/mL Correlation coefficients Sewage sludge (mg/kg)
Nap Acpy Acp Flu Phe Ant Fla Pyr BaA CHR BbF BkF BaP IND DBA BghiP
0.001 0.001 0.001 0.001 0.001 0.01 0.001 0.001 0.1 0.01 0.1 0.1 1 0.1 1 0.01
128 152 154 166 178 178 202 202 228 228 252 252 252 276 302 276
5–7 7–8.5 8.5–9.5 9.5–11 11–12.5 12.5–14 14–16
16–22
6.256 8.141 8.353 8.985 10.168 10.231 11.623 11.899 13.357 13.401 14.840 14.886 15.392 17.832 17.905 18.544
0.048 0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.048 0.049 0.049 0.049 0.050 0.049 0.049
0.105 0.104 0.104 0.103 0.104 0.105 0.103 0.103 0.109 0.103 0.103 0.104 0.105 0.101 0.104 0.102
working curve is given in Table 3. The linear fitting of the resultant calibration curves shows good correlation coefficients between 0.9986 and 0.9997 for each compound. The gas sample included XAD-2 resin and 250 mL DCM. The XAD-2 resin was extracted using 250 mL DCM, and then concentrated to 1 mL to transform the solvent to hexane; after the cleanup procedure, the sample was concentrated and fixed to 10 mL using hexane. Then 1 lL of the fixed sample was introduced to GC/MS analysis. The detection limit during GC/MS analysis was 0.001 lg/mL for each of the 16 PAHs. Considering a sample mass of 1 g and the fixed solvent volume, the detection limit for the PAHs yield was 0.01 mg PAHs per 1 kg dry sewage sludge. The original content of PAHs was detected by using the same method with XAD-2 resin: 20 g sewage sludge (dry basis) was extracted. The results were listed in Table 3. 3. Results and discussion The toxic equivalent factor (TEF) was defined in order to characterize the carcinogenic properties of PAH more precisely, comparing every compound with the BaP, which has the highest value of TEF (Table 3) [32]. The carcinogenic equivalence (KE), referred to as toxic equivalent (TEQ), is a parameter that determines the inhalative carcinogenic potential caused by airborne-PAH. It can be determined using the TEF concept. These TEQ-values were also determined in our work to evaluate the influence of different operation conditions on the harmful health effect of the gaseous effluent of sewage sludge pyrolysis, and to characterize the real toxicity. The 16 EPA-PAHs are classified into high molar weight (HMW) PAHs with five and six rings (BbF, BkF, BaP, IND, DAB, BghiP), middle molar weight (MMW) PAHs with four rings (Fla, Pyr, BaA, CHR), and low molar weight (LMW) PAHs with two and three rings (Nap, Acpy, Acp, Flu, Phe, Ant) [30]. Results were expressed as mg PAHs generated during the pyrolysis of 1 kg dry sewage sludge. The PAHs in the original sewage sludge totaled 0.251 mg/kg, a rather low value for sewage sludge, since it varied from 0.096 to 7.718 mg/kg for nine samples from Tunisia [33]. The values of the PAHs generated in different conditions are displayed in the supplementary material (Tables 4–7 and Figs. 6–17 classified according to ring number and PAHs distribution profiles). 3.1. Influence of the temperature on total PAHs and ring number Temperature is the most investigated parameter in PAHs formation. The sample mass, gas residence time and gas flow were separately fixed to 1 g, 10 s and 200 mL/min when temperature
0.213 0.209 0.209 0.209 0.211 0.208 0.212 0.212 0.207 0.198 0.213 0.207 0.207 0.202 0.201 0.205
0.390 0.389 0.385 0.387 0.388 0.378 0.385 0.380 0.382 0.406 0.373 0.384 0.380 0.381 0.385 0.380
0.946 0.970 0.978 0.979 0.964 0.994 0.972 0.984 0.999 0.945 0.996 0.987 0.991 1.026 1.008 1.014
0.999390 0.999754 0.999656 0.999713 0.999654 0.999192 0.999560 0.999298 0.999293 0.999058 0.998630 0.999625 0.999381 0.999008 0.999533 0.999121
0.0125 0.0025 0.0130 0.1640 0.0040 0.0045 0.0055 ND 0.009 0.0090 0.0090 0.0095 0.0010 ND 0.0025 0.0050
was varied from 250 °C to 1250 °C. PAHs yield first strongly increased when temperature was raised while the total concentration peaked at 950 °C, and the total TEQ-value culminated at 1150 °C (Fig. 2). At low pyrolysis temperatures of 250–550 °C, PAHs seem still to be influenced by their original content (Table 3) in sludge, with mainly Nap, Acp, Flu present. In our previous TG experiments sewage sludge pyrolysis began at temperatures lower than 200 °C [34]. Thus PAHs began to be released already at 250 °C. The decomposition of biodegradable materials, dead bacteria and non-biodegradable polymers in temperature region of 170–500 °C all may contribute to the release of PAHs from the sample already at temperature below 550 °C. When the temperature surpassed 650 °C, PAHs yields increased greatly, all along with the temperature rising to 850 °C, which is consistent with other research of PAHs in pyrolysis, where PAHs (mainly Nap, Flu, Phe, Fla, BghiP) increased with temperature rising from 600 to 900 °C for polyethylene pyrolysis [35]. Between 850 °C and 950 °C, all PAHs but Acpy and Flu further rose. The decline in Acpy and Flu was also observed in acetylene pyrolysis [4] for relatively low temperature, which may indicated that Acpy and Flu have relatively low stability. When temperature raised further from 950 to 1050 °C, most LMW PAHs (Nap, Flu, Phe, Ant) decreased, while most HMW PAHs (BbF, BkF, BaP, IND, BghiP) amplified, still leading to a total drop. This may be explained by the rupture of aromatic rings in LMW PAHs above 950 °C [36]. As an alternative, HMW PAHs may also be pyro-synthesized from lower rings [37] in the competition between formation and destruction of PAHs, while HMW may be easier to be synthesized than decomposed at this atmosphere of high concentration of LMW, which promoted the chemical equilibrium to HMW PAHs synthesis more than rupture, regardless of reaction acceleration caused by the temperature rise. When the temperature augmented from 1050 °C to 1150 °C, all PAHs but Acp, Flu, Pyr, BaP, IND decreased, continuing a total diminution at rising temperature. When temperature rose to 1250 °C, all PAH-species were heading towards destruction. From the above analysis, most PAHs show similar trends of peaking at a specific temperature (except for Acpy, Flu); Nap, Phe, Ant, BaA, CHR, DBA peaked at 950 °C, Fla, BbF, BkF, BghiP at 1050 °C, and Acp, Pyr, BaP, IND at 1150 °C. The difference of peak temperature presumably represented their relative stability in the complicated thermal reactions. In the pyrolysis of polyvinyl chloride a high peak of PAHs (BaA, BbF, BaP, DBA, Chr, BkF, INd) yield was at 850 °C [38]. In the pyrolysis of polyphenolic compounds the LMW PAHs exhibited a maximum at 870 °C [39]. In the pyrolysis of acetylene some light
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Fig. 2. Influence of the temperature on total PAHs and their TEQ.
PAHs-priority (Nap, Flu, Ant, BaA, Acp) were diminished from 1073 K to 1473 K while there was a maximum for other heavier PAHs at 1173 K, attributed to the competition between formation and destruction reactions of PAH [40], the less reactive and porous characteristic of soot particles formed at higher temperature also leads to fewer PAHs adsorbed [41]. To sum up, the controversy between different conclusions [4] induced may be due to their different substances and atmospheres which to some degree changed the stability of different PAHs, although they have the same molecular structure. When PAHs are classified by their ring number (Fig. 6), the trend is clear in our work: the 2-ring and 3-ring PAHs peaked at 950 °C, the 4-ring and 5-ring PAHs peaked at 1050 °C, and the 6-ring PAHs peaked at 1150 °C. Thus it can be deduced that PAHs with more rings are decomposed at higher temperature. In the 16 PAHs distribution profile (Fig. 7), Flu dominated at temperatures lower than 750 °C, which indicated that the same formation mechanism prevails below 750 °C. The Nap faction began to amplify greatly to attain nearly 30% at 850 °C and then stay unchanged till 1300 °C. The fraction distribution is determined by competitive reactions, as described above. It was observed that Nap also dominated the PAHs in the pyrolysis of acetylene [40], and of bio-fuel [42]. TEQ profile peaked at 1150 °C, although the concentration culminated already at 950 °C. This is due to the LMW PAHs, with low TEF, which were decomposed more than the HMW PAHs with high TEF-value, when temperature rose from 950 °C to 1150 °C. The gain in TEQ between 850 °C and 950 °C in our research (Fig. 2) was also observed in the pyrolysis of acetylene, due to the formation of heavier PAHs with higher toxic potential [40]. In the PAHs TEQ distribution profile (Fig. 8), Flu dominated TEQ at temperatures lower than 650 °C, due to its high yield; BaP greatly amplified its proportion at 850 °C and dominated at 1150 °C, leading to the high peak of total TEQ at 1150 °C. The turning point in the distribution profiles was 750 °C for both concentration and TEQ distribution (Figs. 7 and 8).
3.2. Influence of the sample mass on the total PAHs and ring number distribution The temperature, gas residence time and gas flow were separately fixed to 850 °C, 10 s and 200 mL/min when sample mass was inspected from 0.1 g to 4 g. When the sample mass increased from 0.1 g to 1 g, the total PAHs and TEQ declined; however, when the sample mass increased from 1 g to 4 g, PAHs augmented slightly (Fig. 3). When the 16 PAHs are considered separately, they show different trends. When the sample mass was raised from 0.1 g to 4 g Nap, Phe, Fla, BaA, CHR, BbF all declined, and only Flu and IND amplified; moreover, Acpy, Acp and Ant peaked at 1 g, Pyr and BkF peaked at 0.4 g, whereas BaP and DBA were lowest at 1 g. When the influence of sample mass on PAHs yield was considered, higher yield at less sample mass was observed by comparing different operating conditions conducted by former researchers, while gas effluent during coal pyrolysis yielded around 100 mg/kg PAHs at 0.5 g sample mass [6] comparing to around 1000 mg/kg PAHs at 0.001 g sample mass [7] ignoring their different apparatus. The sample mass represents the concentration of fuel, which can lead to extent variation of the pyrolysis reactions. Less fuel led to more contact between the pyrolysis products and the nitrogen flow, therefore more complete pyrolysis of heavier molecule to light hydrocarbon including the 16 PAHs, Nap being the most prominent. However the obvious growth of Flu, when sample mass increased, may indicate that Flu tends to form at fuel rich atmosphere, where primary pyrolysis rather than secondary pyrolysis was favored. It was confirmed when domination of Flu at low temperature (Fig. 7) was considered. It was noted that Ant was proliferated at 1 g. When PAHs are classified by their ring number (Fig. 9), 2-ring, 4-ring, 5-ring were diminished while 3-ring 6-ring PAHs grew when the sample mass increased due to Flu and IND’s special behavior. From 16 PAHs distribution profile (Fig. 10), Nap, Flu and Phe dominated (80%) the concentration at different mass sample, while Nap decreased and Flu increased when the sample mass was raised from 0.1 g to 4 g.
Q. Dai et al. / Fuel 130 (2014) 92–99
97
Fig. 3. Influence of the sample mass on total PAHs and their TEQ at 850 °C.
When the TEQ distribution is considered (Fig. 11), BaP, Fla and DBA yield most but not dominant.
3.3. Influence of the gas residence time on total PAHs and ring number The temperature, sample mass and gas flow were separately fixed to 850 °C, 1 g and 200 mL/min when gas residence time was inspected from 2 s to 16 s. Residence time may be a very important issue, largely determining the conversion of small hydrocarbons into PAHs [4]. Amplified residence time of pyrolysis vapors in the hot zone of the reactor will promote secondary reactions including PAHs formation [43].
When the residence time grew from 2 s to 16 s, the amount of PAHs rose until 10 s and then declined slightly (Fig. 4). This is consistent with the observation that PAHs rose as the residence time grew from 1.5 to 3.5 s at 1123 K and 1173 K in acetylene pyrolysis [4]. In our study, 10 s may be a turning point at 850 °C, where destruction of PAHs exceeds formation of PAHs. All of 16 PAHs, except for Acy and BghiP peaked at 10 s, while Acy peaked at 12 s and BghiP was not detected. Thus the conclusion of all 16 PAHs peaking at around 10 s may be deduced at 850 °C. However the trend would differ at different temperature [4]. When PAHs are classified by their ring number (Fig. 12), all PAHs peaked at 10 s. The 3-ring PAHs are prominent, for their high quantity. It is noted that Ant proliferated at 10 s.
Fig. 4. Influence of the gas residence time on total PAHs and their TEQ at 850 °C.
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Fig. 5. Influence of the gas flow on total PAHs and their TEQ at 850 °C.
In the 16 PAHs distribution profile (Fig. 13), Nap, Flu and Phe dominated (80%) the concentration at different residence time, while the profiles tendency are not clear. When the TEQ distribution is considered (Fig. 14), BaP, Fla and DBA yield most but not dominant.
3.4. Influence of the gas flow on total PAHs and ring number The temperature, sample mass and gas residence time were separately fixed to 850 °C, 1 g and 10 s when gas flow was inspected from 50 mL/min to 1000 mL/min. Gas flow changes the residence time and also the gas phase concentrations. In our study a gas flow of 50, 100, 200, 400, 1000 mL/min corresponds with a residence time of 37.7 s, 18.8 s, 9.4 s, 4.7 s, 1.9 s separately. When the gas flow was raised from 50 to 100 mL/min, the residence time dropped and fuel concentration also reduced by a factor 2. The resulting PAHs amounts decreased by 22.17% while their TEQ dwindled by 74.77%. The sharp decrease was strongest for high ring-number PAHs of DBA, BaP, which have a high TEF of 1. This shows that a long residence time combined with high concentration of fuel can lead to the synthesis of high TEF PAHs. While longer residence time (over 10 s) leads to slightly lessening of BaP and DBA (Table 6), high concentration appears to promote their formation: BaP and DBA augmented, when sample mass was raised from 1 g to 4 g (Fig. 5). Particularly BghiP was only detected at 50 mL/min. However, most of the other PAHs diminished due to the drop in residence time, for decreasing fuel concentration from 4 g to1 g leads to their increase. When the gas flow rose from 100 to 1000 mL/min, the residence time dropped from 18.8 s to 1.9 s and the fuel concentration further reduced, by one order of magnitude. The total PAHs and their TEQ show a slight decline. Most PAHs (Nap, Acpy, Acp, Phe, Fla, Pyr, BaA, CHR, BbF, BaP, IND, BDA) decreased due to the lower gas residence time since they had trend of increase when fuel concentration was reduced from 4 g to1 g (Fig. 3). Flu was special since it shows a rising trend when the gas flow was raised. It can also be concluded that the trends of PAHs for sample mass and flow gas are roughly opposite, just like for combustion conditions the air–fuel ratio is import for PAHs formation.
When PAHs are classified by their ring number (Fig. 15), all ring PAHs have a descending trend when the gas flow was raised from 50 to 1000 mL/min, except for the 3-ring PAHs, due to the special behavior of Flu. The Nap, Flu and Phe dominated (80%) the 16 PAHs distribution profile at different gas flow, except for 50 mL/min, where largely high ring PAHs are formed (Fig. 16). When the TEQ distribution is considered (Fig. 17), BaP, Fla and DBA yield most however not dominant.
4. Conclusions In the investigation into the influence of the operating conditions, such as reaction temperature, sample mass, gas residence time and gas flow, yield of most PAHs had similar trend according to different conditions. When temperature was raised, PAHs increased extensively at 850 °C and decreased considerably at 1250 °C. PAHs yielded most at 950 °C while TEQ was highest at 1150 °C. The trend was an integration of different formation and destruction temperature of PAHs different ring. When sample mass was raised from 0.1 g to 4 g, PAHs yields decreased from 1288.00 mg/kg(0.1 g) to 868.22 mg/kg(1 g) and then increased slightly due to the special growth of Flu. Flu tended to be formed at fuel rich atmosphere, where primary pyrolysis rather than secondary pyrolysis was favored. When residence time of gas flow was raised from 2 s to 16 s, PAHs and their TEQ increased before 10 s, and then decreased slightly. 10 s may be a special residence time for reactions to destroy PAHs molecular in sewage sludge pyrolysis. The rise of gas flow from 50 to 100 mL/min led to moderate decrease of concentration and sharp decrease of TEQ while increase from 100 to 1000 mL/min led to slight fluctuation. The extremely high TEQ for 50 mL/min was due to formation of PAHs with 5-ring and 6-ring. The trend of PAHs for sample mass and flow gas are roughly reverse, for they were together related to the air– fuel ratio. When the comprehensive comparisons of these parameters were considered, Temperature obviously has the most influence while sample mass may has the least influence on the PAHs formation.
Q. Dai et al. / Fuel 130 (2014) 92–99
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