Article pubs.acs.org/est
Sulfur Transformation during Microwave and Conventional Pyrolysis of Sewage Sludge Jun Zhang,*,† Wei Zuo,† Yu Tian,*,† Lin Chen,‡ Linlin Yin,† and Jie Zhang† †
State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China ‡ College of Environment, Hohai University, Nanjing 210098, China S Supporting Information *
ABSTRACT: The sulfur distributions and evolution of sulfurcontaining compounds in the char, tar and gas fractions were investigated during the microwave and conventional pyrolysis of sewage sludge. Increased accumulation of sulfur in the char and less production of H2S were obtained from microwave pyrolysis at higher temperatures (500−800 °C). Three similar conversion pathways were identified for the formation of H2S during microwave and conventional pyrolysis. The cracking of unstable mercaptan structure in the sludge contributed to the release of H2S below 300 °C. The decomposition of aliphatic-S compounds in the tars led to the formation of H2S (300−500 °C). The thermal decomposition of aromatic-S compounds in the tars generated H2S from 500 to 800 °C. However, the secondary decomposition of thiophene-S compounds took place only in conventional pyrolysis above 700 °C. Comparing the H2S contributions from microwave and conventional pyrolysis, the significant increase of H2S yields in conventional pyrolysis was mainly attributed to the decomposition of aromatic-S (increasing by 10.4%) and thiophene-S compounds (11.3%). Further investigation on the inhibition mechanism of H2S formation during microwave pyrolysis confirmed that, with the special heating characteristics and relative shorter residence time, microwave pyrolysis promoted the retention of H2S on CaO and inhibited the secondary cracking of thiophene-S compounds at higher temperatures.
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At temperatures above 650 °C, the major product of pyrolysis is biosyngas, in which H2 and CO accounted for 53−72 vol % of total gas products.10,11 One of the major issues in the production of biosyngas during the sludge pyrolysis is the formation of sulfur-containing gases. The sulfur content in sewage sludge was reported to be higher than 1 wt % (daf).12,13 The sulfur-containing gases might be converted into SOx compounds, contributing to the severe photochemical smog and acid rain pollutions. Therefore, removal of sulfur-containing gases is an issue of particular concern. Many efforts have been made to study the behavior of sulfur compounds and sulfur removal during the pyrolysis process.14−16 It was reported that pyrite in the coal would decompose to sulfide and nascent sulfur capturing hydrogen to form H2S during the pyrolysis.15,17 The decomposition of aliphatic sulfur in the coal was considered to be responsible for the release of
INTRODUCTION One of the main environmental problems is the safe disposal of the huge amount of sewage sludge that is produced every day in wastewater treatment plants.1,2 The sludge production is anticipated to be about 300 million tons (dry solids, DS) each year with the increasing rate of 5−10% in China.2 Several methods are used for treating or disposing sludge, including aerobic or anaerobic digestion, land-filling, agricultural application and incineration. However, most of these methods result to subsequent problems, such as necessary secondary pollution and treatment, or high cost consumption. Consequently, it is of great importance to find an economically and environmentally acceptable alternatives of sludge dispose in recent times.3,4 Recently, there has been increasing concerns about the thermo-chemical conversion of sludge into biofuels for energy recovery through pyrolysis and gasification technology.5 Through gasification, sludge can be transformed into high-quality biosyngas.6,7 Apart from gasification, pyrolysis also seems to be a promising technology for converting sewage sludge into biosyngas (noncondensable), biotar (condensable volatiles) and biochar residue in the absence of air.8,9 © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
July 28, 2016 November 13, 2016 December 2, 2016 December 2, 2016 DOI: 10.1021/acs.est.6b03784 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 1. Characteristics of Raw Sludgea proximate analysis (wt %) M Ad Vd 78 42 55 Ash Analysis (Expressed as Wt % of Metal Oxides) SiO2 Al2O3 Fe2O3 P2O5 CaO 26.40 8.64 8.30 6.13 5.12 a
ultimate analysis (wt %, daf) FCd 3 K2O 1.62
C 30.94 TiO2 0.65
ZnO 0.14
H 4.77 CuO 0.03
N 4.61
O 20.56
S 1.07
SrO 0.03
M, moisture content; A, ash content; V, volatile content; FC, fixed carbon; d, dried basis; daf, dried and ash-free basis.
filtered and reconditioned with distilled water until the pH of the aqueous sludge suspension reached pH 7. Finally, the remaining solid sample was dried under vacuum at 50 °C for 48 h to obtain the demineralized sludge. In the case of CaO loaded samples, the demineralized sludge obtained by the above procedure was first mixed with the distilled water to form the sludge solution, and then CaO was added to the sludge solution stirring for 24 h to ensure uniformity of the sample. Based on the CaO fraction in the dried sludge (2 wt %), the CaO/demineralized sludge ratio of 2% was chosen. The remaining mixture was dried in a vacuum at 106 °C for 48 h. Pyrolysis Procedure. The pyrolysis experiments were carried out separately in a multimode-microwave cavity oven and in an electrical furnace. In the case of microwave pyrolysis, details of the experimental setup have been described previously.28 In brief, batch experiments were conducted to collect the pyrolysis char, tar and gas products at the end of each experiment for analysis. The sludge samples were placed in a quartz reactor inside the microwave cavity. The samples were heated from room temperature to a final temperature of 100−800 °C in an increment of 100 °C per minute. After the setup of required temperatures, the samples were subjected to microwave radiation. The sulfur conversion of volatile matters might be greatly influenced by the residence time in the pyrolysis process. Thus, the evolution of pyrolysis volatile contents in solid with residence times was determined in previous experiments. No significant volatiles were observed in the solid residue when the pyrolysis was operated after 8 min for all the experiments. Therefore, all the samples subjected to microwave radiation were maintained for 8 min in the microwave experiments. It is noted that sewage sludge has a high transparency to microwave. It was necessary to mix it with microwave receptor for reaching the required temperatures during the pyrolysis. Activated carbon (1 × 1 mm) was selected as microwave receptor and its optimal dosage in the dry sludge was also determined.25 In each experiment, the activated carbon (ca. 2.5 g) was homogeneously mixed with 24 g dried sludge (moisture of 5%) before microwave heating. After the experiments, activated carbon could be recovered from the solid residue of sludge pyrolysis due to their different grain sizes. To ensure an inert atmosphere, Argon (Ar) was injected into the system with a constant flow rate of 10 L/min for 20 min and then taken off before the commencement of the experiment. The microwave generator was turned off after the arrival of designated reaction time. The carrier gas was reinjected into the system to purge out the residual gas for 20 min. The sludge samples were pyrolyzed in an electric furnace using the same quartz reactor, Ar flow conditions (10 L/min for 20 min), amount of sludge-activated carbon (24−2.5 g) as in the case of microwave pyrolysis. To perform pyrolysis under the most similar conditions in both pyrolysis processes, the samples were heated from room temperature to a final temperature of
H2S at temperatures below 500 °C.18,19 Additionally, pyrolysis of sewage sludge to obtain sludge-derived adsorbents has been demonstrated to be efficient for H2S removal at room temperature. This remarkable effect was mainly attributed to the oxides of iron and calcium in the sludge.13,20−22 It should be noted that the sulfur structures in the sewage sludge are quite different from those of coal. Pyrite is identified as the major sulfur-containing compound in coal,15,17 while none of pyrite is found in sewage sludge.23 Besides, the relative contents of sulfur-containing compounds in the coal are different from those in the sludge.23,24 Thus, it is considered that the sulfur transformation during the pyrolysis of sewage sludge is different from that of coal. In our previous study,25 a microwave heating reactor (MHR) was designed to introduce microwave in the sewage sludge pyrolysis. By using this reactor, a biosyngas with high hydrogen contents (40%) was obtained in a few minutes. Moreover, the pretest results of this study showed that the H2S yields obtained from microwave pyrolysis were decreased significantly compared with those from conventional pyrolysis. Thus, special attention was paid to the reasons contributing to the difference of H2S yields during the pyrolysis process. Consequently, the present study was undertaken to (1) compare the distribution and evolution of sulfur in the char, tar and gas fractions from microwave and conventional pyrolysis of sewage sludge; (2) propose the sulfur conversion pathways in relation to H2S during the two pyrolysis processes; (3) insight into the inhibition mechanism of H2S formation induced by microwave pyrolysis.
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MATERIALS AND METHODS Sewage Sludge. The raw sludge was obtained from an urban wastewater treatment plant (UWWTP) in Harbin, China, in which the dewatering of sludge was conducted using a belt filter press and cationic polymeric flocculants were used for sludge conditioning. The sludge had a moisture content of 78%, an ash content of 42% (dry basis) and a volatile matter content of 55%. The main chemical properties of raw sludge were given in the Table 1. The raw sludges were dried at 106 °C for 24 h and then stored in airtight containers until they were cooled to room temperature. After cooling, the dried sludge samples were sieved to obtain a particle size of 106−150 μm. The dried samples were subjected to microwave pyrolysis for each experiment. Comparing with the raw sludge, the characteristics of the dried sludge did not change too much with the exception of moisture content decreasing from 78% to 5%. Demineralized and CaO Loaded Sewage Sludge. The preparation of demineralized sludge was according to the methods described previously.26,27 The brief introduction of this process was given below: First, the dried raw sludge was agitated with 2 L of 6 N HCl in a water bath at 60 °C for 1 h, after which the solution was decanted. The sludge residue was then washed with distilled water and treated with 800 mL of 48% HF at 60 °C for 2 h. After this treatment, the residue was B
DOI: 10.1021/acs.est.6b03784 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology 100−800 °C, being kept at the final pyrolysis temperature for 8 min. After the setup of required temperatures, the samples were subjected to electric pyrolysis. For both pyrolysis experiments, the volatile substances evolved from the sludge pyrolysis passed through a number of dichloromethane-containing condensers placed in ice bathes. The tar products would be recovered from the condensers by evaporating the dichloromethane solvent in a water-bath at 60 °C for 24 h and then retrieved. The residual chars in the reactors were collected and stored in airtight containers until they were cooled to room temperature. The noncondensable gases were collected in Tedlar sample bags with a polypropylene fitting for sampling. Thermogravimetric Experiments. Compared to the heating time of conventional pyrolysis (usually hours), a clear advantage of microwave pyrolysis was the shorter heating time (several minutes) to achieve the final temperature. The studies on microwave pyrolysis had been widely investigated recently,29−32 while the TGA conducted in the microwave pyrolysis has not been reported yet. This might be related to the different natures of heat transfer induced by the conventional and microwave pyrolysis. The mass loss versus temperature curve in the microwave pyrolysis system was obtained by the batch experiments. Briefly, approximate 12 g of the dried sludge samples were used for each experiment. After the arrival of the desired temperature, the microwave generator was turned off. The residual chars in the reactors were collected and stored in airtight containers until they were cooled to room temperature. The mass loss was then calculated from the weight difference of each sample before and after the experiment. The conventional pyrolysis process was characterized in a thermogravimetric analyzer (TGD 9600S, ULVAC-RIKO), which was conducted at the highest heating rate of 100 °C/min from ambient temperature to desired temperatures. The comparison between the mass loss curve of microwave pyrolysis and a TG curve at heating rate of 100 °C/min was shown in Figure 1. It was seen that the mass loss in the
(C, H, N, and S) of the raw sludge, tar and char samples were measured in an Elemental Analyzer (Americas Vario EL III). The sulfur content in the gas was represented by H2S−S. H2S gas was analyzed in a HP 5890 series II gas chromatograph fitted with a TCD detector. An HP 3 FT Molecular Sieve 13 × 45/60 column was used. The oven temperature was set at 70 °C and the carrier gas flow (H2) was 20 mL/min. The gas samples (1 mL) were injected into the gas chromatograph and the injector temperature was 120 °C. The TCD was calibrated with a standard gas mixture at periodic intervals. XPS analysis of the char-S compounds was performed according to the previous methods.26 The areas of peaks reflected the relative contents of different components, which were normalized from dividing the peak area values by the total SS-S content for the semiquantitative analysis. GC-MS was conducted for analysis of the tar-S products.28,32 The chromatographic peaks were identified from the NIST mass spectral data library. The changes in the tar compositions relating to the pyrolysis temperatures were normalized from dividing the peak area values by the concentrations of the pyrolysis tar compounds. The contents of the individual compounds were calculated using mixed standard solution of coal tar with different concentrations (J&K Scientific Ltd.). The microstructures of chars obtained from microwave and conventional pyrolysis were investigated using a scanning electron microscope (SEM, S-4700, HITACHI). The types of the crystalline phases were characterized by X-ray with Cu Kα radiation (XRD: P|max-γβ, Rigaku, Japan). All of the experiments were repeated 3 times, and the average values of these results were taken as final results. The experimental errors were not greater than 5%.
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RESULTS AND DISCUSSION Sulfur Distributions in the Char, Tar, and Gas Products. The sulfur distributions in the char, tar, and gas produced from the microwave pyrolysis of sewage sludge are shown in Figure 2a. The results indicated that the increasing temperature in this stage mainly promoted the char-S (decreasing by 39.6%) cracking into tar-S (increasing by 31.2%) compounds at the temperatures below 500 °C. The similar conclusions were observed during the pyrolysis of coals.33,34 It was reported that the decomposition of sulfur in the char mainly contributed to the release of volatile sulfur compounds in the tar below 500 °C. When the temperature increased from 500 to 800 °C, the remarkable increase in the char-S yields (6.0%) was obtained accompanied by the decreasing trend of tar-S (7.4%) yields. Besides, the gas-S yields increased by 1.4% at this stage. The sulfur distributions became almost constants above 800 °C. The tar-S and gas-S distributions from the conventional pyrolysis showed similar trends with those from microwave pyrolysis at temperatures from 20 to 800 °C (Figure 2b). The char-S yields from both pyrolysis processes had a tendency to decrease at temperatures of 20−500 °C. Contrary to the increase of char-S yields from microwave pyrolysis, the monotonous decline of char-S yields (decreasing from 48.8% to 43.0%) was observed from conventional pyrolysis in the temperature range of 500−800 °C. Compared the sulfur distributions from microwave pyrolysis to conventional pyrolysis, it was found that more char-S and less gas-S were generated at higher temperatures (500−800 °C). Thus, microwave pyrolysis seemed as a more advanced process compared with conventional pyrolysis for the more effective S-retaining in the char.
Figure 1. Mass loss curve for microwave pyrolysis and TG curve of dried sludge at 100 °C/min.
microwave pyrolysis process was faster in comparison with the conventional pyrolysis system. In microwave pyrolysis the samples were heated directly and uniformly, leading to the body heating inside and outside synchronously. Thus, a relative shorter time was needed to achieve the high temperature necessary for pyrolysis in the microwave pyrolysis. Analytical Methods. The yields of char and tar were calculated from the weight of each fraction, while the gas yields were evaluated by difference. The elemental compositions C
DOI: 10.1021/acs.est.6b03784 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
same temperature range. It was reported that the thermal cracking of char-S compounds, especially the aliphatic sulfur compounds, tended to release volatile sulfur compounds below 500 °C.34 Thus, the thermal decomposition of aliphatic mercaptan (S1) might contribute to the formation of tar-S compounds at temperatures of 300−500 °C. Besides, the decline of inorganic sulfide at this stage might be related to the increase of sulfate in the sludge during the pyrolysis. In sludge materials, part of the sulfides could get oxidized to sulfite/sulfate during the heat treatment by the oxygen/hydroxyl present on the surface of sludge materials. This was in agreement with the above experimental results in this study. With the temperature increasing from 500 to 800 °C, large amounts of inorganic sulfide and organic sulfur (thiophene) were produced during the microwave pyrolysis. The S 2p spectra results from conventional pyrolysis are shown in Table 3. At temperatures below 500 °C, the similar tendency of sulfur transformation in the chars was observed for both pyrolysis process. With the temperature increasing from 500 to 800 °C, there were two large differences in the sulfur conversion between microwave and conventional pyrolysis. It was observed that a lower increment of inorganic sulfide (2.1%) yields was achieved in the conventional pyrolysis compared with that (6.5%) of microwave pyrolysis. Apart from this difference, the significant decline of thiophene yields was found in the conventional pyrolysis at temperatures from 700 to 800 °C. The results indicated that the stable structure of thiophene-S was susceptible to decomposition under the conventional pyrolysis atmosphere. Changes on the Sulfur-Containing Compounds in the Tars. During microwave and conventional pyrolysis of sewage sludge, the evolution of tar-S compositions (% normalized peak areas) is investigated by GC-MS. The effect of temperature on the tar-S compositions from microwave pyrolysis is shown in Table 4. It was seen that the pyrolysis tars were mainly composed of the aliphatic-S compounds, aromatic-S compounds and thiophene-S compounds. Below 300 °C, the above sulfur-containing compounds appeared to be insignificant. At the temperatures of 300 to 500 °C, the main changes corresponded to the significant decline of aliphatic-S contents (from 16.97% to 10.95%), which were susceptible to the easy breakage of weak C−S bond in the organic sulfur structure. With the temperature increasing from 500 to 800 °C, the formation of thiophene-S substances in the tars (increasing by 7.33%) took place accompanying the cracking reactions of aromatic-S (decreasing by 9.30%) and aliphatic-S (decreasing by 4.42%) compounds. The tar constituents derived from the conventional pyrolysis are presented in Table 5. The trends for the tar-S transformation from the conventional pyrolysis were consistent with
Figure 2. Sulfur distributions in the char, tar and gas products from the microwave pyrolysis (a) and conventional pyrolysis (b).
Evolution of Sulfur-Containing Compounds in the Chars. The evolution of sulfur-containing compounds in the raw sludge and chars produced from microwave pyrolysis is investigated by XPS (Table 2). Classification of the binding energies of different S species was based on the literatures.23,37 Six peaks, including the mercaptan (S1, 162.6ev), inorganic sulfide (S2, 163.0ev), thiophene (S3, 164.0ev), sulfoxide (S4, 166.0ev), sulfone (S5, 168.5ev), and sulfate (S6, 170.8ev) were contained in the S 2p spectrum of raw sewage sludge with the relative contents of 41.1%, 8%, 15.6%, 9.8%, 5.4%, and 20.1%, respectively. At the temperatures below 300 °C, it was found that the mercaptan contents decreased by 5.3%, while the contents of other sulfur forms did not change significantly. The unstable mercaptan structure was partly subjected to decomposition after heat treatment, which was the reason for the decline of mercaptan contents during the pyrolysis process. When the temperature increased from 300 to 500 °C, the sharp decline of mercaptan contents (25.1%) were found in the chars, meanwhile the contents of inorganic sulfide decreased by 1.8% in the experiments. It was interesting to find that the contents of sulfate increased by 1.2% under the
Table 2. Normalized Relative Intensities (%) of XPS S 2p Peaks at Different Temperatures from the Microwave Pyrolysisa total-S (%) S1 S2 S3 S4 S5 S6 a
raw sludge (20 °C) 100 41.1 8.0 15.6 9.8 5.4 20.1
± ± ± ± ± ± ±
0.3 0.5 0.3 0.4 0.1 0.2 0.4
300 °C 93.5 35.8 6.7 15.5 9.8 5.4 20.3
± ± ± ± ± ± ±
0.2 0.3 0.2 0.2 0.1 0.2 0.2
400 °C 84.9 26.5 6.3 16.2 9.7 5.5 20.7
± ± ± ± ± ± ±
500 °C
0.4 0.1 0.6 0.4 0.3 0.4 0.7
70.5 10.7 6.2 17.1 9.7 5.3 21.5
± ± ± ± ± ± ±
0.3 0.5 0.1 0.6 0.1 0.3 0.5
600 °C 70.7 6.4 6.3 20.0 9.8 5.4 22.8
± ± ± ± ± ± ±
0.7 0.5 0.4 1.2 0.8 0.5 0.6
700 °C 75.8 4.7 10.4 21.8 9.6 5.4 23.9
± ± ± ± ± ± ±
0.3 0.2 0.3 0.7 0.2 0.1 0.3
800 °C 78.1 4.8 12.7 21.3 9.7 5.5 24.1
± ± ± ± ± ± ±
0.3 0.4 0.2 0.6 0.1 0.3 0.4
S1, mercaptan; S2, inorganic sulfide; S3, thiophene; S4, sulfoxide; S5, sulfone; S6, sulfate. D
DOI: 10.1021/acs.est.6b03784 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
Table 3. Normalized Relative Intensities (%) of XPS S 2p Peaks at Different Temperatures from the Conventional Pyrolysisa total-S (%) S1 S2 S3 S4 S5 S6 a
raw sludge (20 °C) 100 41.1 8.0 15.6 9.8 5.4 20.1
± ± ± ± ± ± ±
0.4 0.6 0.1 0.3 0.1 0.3 0.7
300 °C 93.1 35.6 6.6 15.6 9.8 5.4 20.1
± ± ± ± ± ± ±
0.3 0.4 0.2 0.4 0.3 0.1 0.2
400 °C 82.5 23.2 6.4 16.4 9.9 5.5 21.1
± ± ± ± ± ± ±
500 °C
0.3 0.3 0.1 0.3 0.2 0.3 0.9
77.5 17.3 6.4 17.3 9.7 5.6 21.2
± ± ± ± ± ± ±
600 °C
0.2 0.1 0.5 0.2 0.2 0.4 0.3
75.6 11.8 7.0 19.7 9.9 5.4 21.8
± ± ± ± ± ± ±
0.4 0.3 0.4 0.9 0.2 0.1 0.7
700 °C 71.0 4.5 7.2 21.9 9.8 5.3 22.3
± ± ± ± ± ± ±
800 °C
0.3 0.6 0.2 0.4 0.1 0.4 0.4
69.8 4.4 8.5 18.3 9.7 5.4 23.5
± ± ± ± ± ± ±
0.4 0.2 0.3 0.7 0.3 0.2 0.5
S1, mercaptan; S2, inorganic sulfide; S3, thiophene; S4, sulfoxide; S5, sulfone; S6, sulfate.
Table 4. Pyrolysis Compounds for Tar Products Evolved during Microwave Pyrolysis of Sewage Sludge at Different Temperatures (Normalized Peak Areas, AN) temperature (°C) Tar-S compounds
300
400
500
600
700
800
aliphatic-S compounds heptane, 1-(ethenylthio)4-fluorothiophenol S-(isopropoxythiocarbonyl)thiohydroxylamine chloromethyl thiocyanate hydrazinecarbothioamide 6-hydroxy-8-mercaptopurine 2-methyl-5,5-diphenyl-4-(methylthio)imidazole aromatic-S compounds thiophene-3-carbonitrile, 5-acetyl-4-amino-2-methylthio4(1H)-pyrimidinone, 2-(propylthio) 4(1H)-pyrimidinone, 2-(ethylthio)4(1H)-pyrimidinone, 2-(butylthio)4-cholesten-3-thiol ethanone, 2-(2-benzothiazolylthio)-1-(3,5-dimethylpyrazolyl)benzenesulfinic acid, 4-chloro2-pyrrolidinethione thiazolo[3,2-a]benzimidazol-3(2H)-one, 2-(2-fluoro-5-nitrobenzylideno)pentanoic acid, 5-(3-benzoylaminothien-2-yl)-3-methylthiophene-S compounds 7-methylbenzo[b]thiophene 2-methylbenzo[b]thiophene dimethylbenzo[b]thiophene trimethyldihydrobenzo[b]thiophene naphtho[2,3-b]thiophene benzo[b]naphtha[2,1-d]thiophene
16.97 2.43 4.63 2.82 2.24 2.26 1.61 0.98 4.84 2.38 1.16 0.13 0.10 0.20 0.09 0.13 0.04 0.14 0.47 2.47 1.57 1.41 1.13 0.35 0.12 0.89
12.97 2.43 1.63 2.82 3.24 2.28 2.61 0.98 4. 84 2.38 1.16 0.13 0.10 0.20 0.09 0.13 0.04 0.14 0.47 2.47 1.57 1.41 1.13 0.35 0.12 0.89
10.95 1.43 0.63 2.82 1.24 2.28 1.61 0.98 13.26 2.24 1.61 0.86 0.25 0.33 0.84 0.91 0.88 1.43 0.91 3.53 3.24 1.34 2.42 0.36 0.46 2.37
8.24 1.38 1.06 2.87 0.54 0.82 0.08 0.11 8.43 1.24 0.61 0.36 0.25 0.33 0.34 0.41 0.38 0.43 0.91 8.91 3.24 1.34 2.42 0.36 0.46 2.37
6.86 1.38 1.06 2.87 0.54 0.82 0.08 0.11 6.26 1.24 0.61 0.36 0.25 0.33 0.34 0.41 0.38 0.43 0.91 10.04 3.24 1.34 2.42 0.36 0.46 2.37
6.57 0.23 0.21 0.24 0.28 0.01 3.96 1.12 0.68 0.41 0.13 0.02 0.21 0.02 0.26 0.19 0.42 10.86 2.82 0.66 1.12 2.04 0.15 2.27
products might result in the formation of H2S at higher temperatures (500−800 °C). The small difference in the H2S yields between the two pyrolysis processes was found at temperatures below 500 °C. However, the H2S concentrations from conventional pyrolysis exhibited about twice those from microwave pyrolysis in the temperatures ranging from 500 to 800 °C (Table 6). From Table 5, it was noted that the thiophene-S declined significantly in the conventional pyrolysis at 700−800 °C. Meanwhile, no changes were found for the thiophene-S contents in the microwave pyrolysis. Thus, the stable thiophene-S might be one reason for the formation of H2S gas in the conventional pyrolysis at higher temperatures. Sulfur Transformation in Relation to H2S during the Pyrolysis. Based on the above analysis, the sulfur transformation in relation to H2S from the microwave pyrolysis of sewage sludge is proposed as shown in Figure 3. The formation of H2S from the sludge-S might be via the following three conversion pathways during the pyrolysis. At the temperatures
those in the microwave pyrolysis in the temperature ranges of 300−700 °C. At temperatures above 700 °C, the sharp decline of thiophene-S compounds was observed in the conventional pyrolysis. The results indicated that in conventional pyrolysis, the thiophene-S in the tar like thiophene-S in the char was susceptible to decomposition at higher temperatures. Changes on the Yields of Gaseous Sulfur Compounds. H2S was found to be the most abundant compound in the sulfurcontaining gases. The change in H2S yields (% of sludge-S) as a function of temperature from microwave pyrolysis of sewage sludge is given in Table 6. It was seen that the H2S emission increased significantly at temperatures from 100 to 500 °C and then increased slightly to the maximum values from 500 to 800 °C. From Table 4 the sharp decreases in aliphatic-S and aromatic-S contents were observed at temperatures of 300− 500 °C and 500−800 °C, respectively. It was inferred that the unstable aliphatic-S compounds in the tars might be responsible for the formation of H2S at low temperatures (
