Article pubs.acs.org/est
Investigation on the Evolution of N‑Containing Organic Compounds during Pyrolysis of Sewage Sludge Ke Tian, Wu-Jun Liu, Ting-Ting Qian, Hong Jiang,* and Han-Qing Yu Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China S Supporting Information *
ABSTRACT: Pyrolysis is an emerging technology for the disposal of huge amounts of sewage sludge. However, the thermochemical decomposition mechanism of organic compounds in sludge is still unclear. We adopt a novel online TG-FTIR-MS technology to investigate the pyrolysis of sludge. The sludge samples were pyrolyzed from 150 to 800 °C with heating rates of 10, 50, and 200 K min−1. We found for the first time that the heating rate of pyrolysis can significantly change the species of liquid organic compounds produced, but cannot change the gaseous species produced under the same conditions. The contents of produced gas and liquid compounds, most of which were produced at 293−383 °C, are influenced by both the heating rate and temperature of pyrolysis. The results also showed that heterocyclic-N, amine-N, and nitrile-N compounds are obtained from the decomposition of N-compounds in sludge, such as pyrrolic-N, protein-N, amine-N, and pyridinic-N. Heterocyclic-N compounds are the dominant N-containing products, which can be due to the thermochemical decomposition of pyridine-N and pyrrole-N, whereas fewer amine-N compounds are produced during the pyrolysis. A mechanism for the decomposition of N-containing compounds in sludge is proposed based on the obtained data.
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INTRODUCTION The activated sludge method has been used to treat wastewater since the 1910s, due to its low cost and excellent adaptability for various wastewaters.1,2 However, the treatment of excess sludge remains a thorny issue to date.3−5 For example, in the European Union, nearly 10 million tons of dry sludge have been produced in 2005, and the European Environmental Agency has identified it as “a future waste problem”.6−8 Traditional options for sludge reduction include landfilling, composting, thermo-chemical treatment, and land application following anaerobic and aerobic digestion;9 among these, the thermo-chemical treatment (e.g., pyrolysis and combustion) is an efficient and most widely used method.10 Pyrolysis of sludge is a promising and proven technology because it is often carried out at relatively low temperatures and under anoxic conditions, thereby precluding the production of highly toxic persistent organic compounds (e.g., dioxin) and particulate matter (e.g., PM2.5 and PM10), and the simultaneous immobilization of heavy metals in the carbonaceous matrix.11−14 Furthermore, gases and oils produced in the pyrolysis of sludge can be used as fuels and chemical feedstocks.15−17 Transformation and migration behaviors of elements in sludge during pyrolysis are closely related to the occurrence of second-pollution. Several researchers have investigated the pyrolysis behavior of different components of sludge and potential secondary pollutants, such as heavy metals,18 chlorinated pollutants,7 polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxin, and dibenzofuran (PCDD/Fs).13,19 However, the circulation of nitrogen in the © XXXX American Chemical Society
environment is more important because it is mainly responsible for the eutrophication of water columns.20−23 Only a few reports have investigated the fate of N-containing components of sludge during pyrolysis. These N-containing compounds are decomposed to different gaseous, liquid, and solid compounds. Cao et al.24 concluded that NH3 is the predominant part in nitrogenous gases under fast pyrolysis of sludge, its yield increases with increasing pyrolytic temperature and decreasing sweeping gas flow rate. Zhang et al.25 reported that NH3 and HCN were the main parts in nitrogenous gases during microwave pyrolysis of sludge, and the pyrolysis temperature is a significant factor in the transformation of nitrogen. The natures of the ultimate N-containing products are usually determined by the pyrolysis conditions, such as temperature and heating rate.26−28 Thus, tracing the fate of N during sludge pyrolysis is critical for controlling the N-species and diminishing the potential second-pollution. Several analytical methods have been developed to probe the pyrolysis behavior of sludge. Fonts et al.26 analyzed the composition of the pyrolytic liquids by GC-MS and GC-FID. Ischia et al.29 investigated the sludge pyrolysis process using thermogravimetry (TG) coupled to mass spectrometry (MS) and gas chromatography (GC), i.e., TG/MS or TG/GC/MS. Shao et al.30 studied the pyrolysis characteristics, kinetics, and Received: May 5, 2014 Revised: August 18, 2014 Accepted: August 20, 2014
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heating. A helium carrier gas (50 mL min−1) was used during pyrolysis. Data Analysis. Data obtained by TG/DTG were analyzed by Pyris software (PerkinElmer, Inc., U.K.). 3D infrared spectrum data were analyzed using Spectrum TimeBase Version 3.1.0 (PerkinElmer, Inc., U.K.), and TurboMass Ver 6.0.0 (PerkinElmer, Inc., U.S.A.) was employed for analyses of mass spectra.
gas production properties of sludge pyrolysis process by TG coupled to Fourier transform infrared analyses (TG/FTIR). These integrated means of analyses deepen our understanding of the sludge pyrolysis. However, to elucidate the mechanism underpinning the sludge pyrolysis, especially the fate of Ncontaining components, an online, integrated TG-FTIR-MS method can provide more information, thereby leading to more efficient and accurate analyses. Herein, we report the first integrated TG-FTIR-MS online approach to monitor the migration and transformation of pollutants in sludge. We analyze the production of NOx and N2O precursors (HNCO, HCN, NO, NH3), N-containing organic compounds, and small-molecule hydrocarbons during the pyrolysis of sludge. We also investigate the effects of the heating rate and temperature, and propose migration and transformation mechanisms based on the gathered data. The results from this study can provide valuable information and insight into controlled conversion of N-containing components in sludge pyrolysis.
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RESULTS AND DISCUSSION Characteristics of Sewage Sludge. The chemical characteristics of the used sludge are listed in Supporting Information (SI) Table S1. The nitrogen content of sludge is 4.9 wt %, which is similar to other sludges.9,27 After pyrolysis, the N content as well as the contents of C, H, and O of the residual char decreases significantly, suggesting that the release of N is accompanied by the release of C, H, and O. XPS spectrum of pyrolytic char shows that the protein-N (nitrogen contained in the proteins) and amine-N (nitrogen contained in the amines) in sludge have decomposed completely, and pyrrolic-N (nitrogen contained in pyrroles) and pyridinic-N (nitrogen contained in the pyridines) have partly decomposed, while NO3− is present (SI Figure S1). XPS survey can be used to analyze the chemical nature of N and other elements on the surface of sludge. N 1s spectra of the raw sludge (Figure 1a) can be decomposed into four peaks at 399.0 eV (pyrrolic-N), 399.5 eV (protein-N), 399.9 eV (amineN), and 400.4 eV (pyridinic-N) with the relative contents of 25.1, 22.3, 29.3, and 23.4 atom %, respectively. Protein-N is originated in proteins, amine-N derived from saccharides, and pyrrolic-N or pyridinic-N are from the decomposition products of nucleic acids.30 Inorganic ammonium-N is not detected in XPS spectra, which can be ascribed to its low content. XPS analysis shows that N is distributed equally among the four Ncontaining chemical species, which can adopt different thermochemical decomposition behaviors during pyrolysis. C 1s spectra are provided in SI Figure S2. Besides the N, C, H, and O, P and some metals (e.g., Ca, Mg, Al, Fe, Na, K, and Si) are also found in the sludge (Figure 1b), which can catalyze certain transformations during pyrolysis of sludge.6,31−33 TG Analysis. TG analysis (TGA) can determine the optimum temperature range for sludge pyrolysis. More importantly, when the results from TGA are combined with the results of analysis by FTIR and MS, an accurate plan for reducing the formation of harmful N-containing air pollutants during the pyrolysis of sewage sludge can be formulated. Sewage sludge is primarily pyrolyzed in TG equipment from room temperature to 900 °C with different heating rates under helium atmosphere. TG/DTG profiles during the pyrolysis of sewage sludge as a function of temperature are shown in Figure 2. 50.8, 51.6, and 52.1 wt % residues remain after pyrolysis of the sludge at 900 °C at heating rates of 10, 50, and 200 K min−1, respectively. This indicates that the heating rate, a critical factor influencing pyrolysis, does not have an obvious effect on the degree of decomposition of sludge; however, the heating rate can still influence decomposition pathways of sludge. Temperature, not heating rate, has a significant effect on the degree of sludge decomposition. Minor loss in mass at temperatures lower than 150 °C can be attributed to the release of moisture.34 Significant weight loss is observed in the temperature range from 150 to 550 °C, which can attributed to the decomposition of the sludge.35 With a further increase in
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EXPERIMENTAL SECTION Sample Collection and Characterization. Sewage sludge used in this study was collected from Wangtang Wastewater Treatment Plant located in Hefei, China. Raw sludge from the anaerobic digestion process was first dewatered by air-drying, then dried at 80 °C for 24 h and stored in airtight bags until it was cooled to room temperature. The cooled, dried sludge was crushed and filtered through a sieve (60 mesh). Finally, the filtered samples were stored in refrigerator until further use. Elemental composition (C, H, N, and O) of sludge was determined with an elemental analyzer (VARIO EL III, Elementar, Germany). X-ray photoelectron spectroscopy (XPS) measurements of the sludge were carried out with Xray photoelectron spectrometer (ESCALAB 250, Thermo-VG Scientific Inc., U.K.) equipped with a monochromatized Al Kα radiation (1486.6 eV). The measurements were carried out in a Constant Analyzer Energy (CAE) mode with 30 eV pass energy for survey spectra and 20 eV for high resolution spectra. The XPS peaks were matched into subcomponents using a Gaussian (80%)−Lorentzian (20%) curve-fitting program (XPSPEAK 4.1), with a Shirley type background. TG-FTIR-MS Analysis of Sewage Pyrolysis. Simultaneous TG, FTIR, and MS data were obtained using a TG analyzer coupled with a Fourier transform infrared spectrometer outfitted with a gas cell and mass spectrometer (TL-9000). Gas products released from TG analyzer were directly collected in the gas cell and analyzed immediately using the FTIR spectrometer. FTIR spectra (4000−450 cm−1; resolution = 2 cm−1) of the gaseous products were acquired continuously with the baseline amended. After being analyzed by FTIR, the gas products were immediately swept to the mass spectrometer. The entire flow-path of gas was insulated (wrapped with heating wire) to prevent any condensation of volatile decomposition products. Dried sludge (∼20 mg) was heated from room temperature to 900 °C in the TG apparatus with heating rates of 10, 50, and 200 K min−1. The gas products released from TG analyzer were directly transferred to the FTIR spectrometer and then to the mass spectrometer, as described above. The data record time of mass spectrometer was initiated 10 min prior to the TG analyzer and FTIR detector. Generated N-containing gaseous products were determined over the temperature range of B
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Figure 1. (a) N 1s XPS spectrum of sewage sludge and (b) XPS survey spectra of sewage sludge.
temperature, there is only a marginal weight loss, which is due to the deep decomposition of some sludge components.30 The decomposition of sludge (150−550 °C) could be divided into two stages in the DTG curve: (1) ∼15 wt % weight loss occurs at between 150 and 350 °C, which is predominantly due to decomposition of biodegradable matter, and (2) 25 wt % weight loss occurs in the temperature range from 350 to 550 °C, which pertains to the degradation of dead bacteria.30 The two temperatures representing maximum decomposition (Tmax1 and Tmax2) are 293 and 338 °C, 301 and 353 °C, and 338 and 383 °C when the rates of heating are 10, 50, and 200 K min−1, respectively (SI Table S2). With an increase in the heating rate, the value of Tmax shifts to a higher temperature. The peak height (or reaction rate) is also increased by 2 orders of magnitude when the heating rate is raised from 10 and 50 K min−1 to 200 K min−1. These changes are caused by the delayed decomposition kinetics of sewage sludge.36 Gaseous Small-Molecule Compounds. TG analysis shows that the heating rate can change the Tmax, which suggests that the decomposition pathway of sludge is different at different heating rates. To further trace the transformation of N during pyrolysis, gaseous compounds produced in TG equipment were analyzed by online FTIR. Generally, the sludge was pyrolzed at ∼Tmax to obtain more volatiles and minimum residue during sludge minimization. Volatiles can be separated
Figure 2. TG/DTG profiles of sewage sludge at different heating rates: (a) 200, (b) 50, and (c) 10 K min−1.
into pyrolytic bio-oil and gas-mixture by condensation; the gases can be analyzed by FTIR.37,38 We investigated the FTIR spectra while pyrolyzing sludge at different heating rates and four different temperatures, i.e., 150 °C (initial decomposition), Tmax1, Tmax2, and 800 °C (final decomposition). The 3D infrared spectra compiled over the entire temperature range of pyrolysis show that the volatiles generated at different heating rates give rise to IR absorptions at similar wavenumbers (Figure 3 and SI Figure S3). It indicates that while the heating rate significantly alters the yield of gases, it has minimal influence on the composition of produced gases. C
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Figure 3. 3D infrared spectrum of gaseous compounds released during sewage sludge pyrolysis at (a) 200 K min−1, (b) 50 K min−1, and (c) 10 K min−1.
Highest yields of gaseous compounds are generated in the temperature range from 300 to 500 °C with all the three heating rates (Figures 3a−c), which is in agreement the results from TGA. FTIR spectra at four specific temperatures during pyrolysis with heating rates of 200, 50, and 10 K min−1 are shown in Figure 4. Spectra I and IV are obtained at 800 and 150 °C, respectively, while spectra II and III describe the gases at Tmax1 and Tmax2, respectively. Absorbance peaks at 4000−3400 cm−1 and 1460−1380 cm−1 are assigned to the stretching and rotational vibrations of functional group of H2O. A sharp doublet peak at 2400−2250 cm−1 and a low intensity peak at 780−600 cm−1 are due to the stretching vibrations of CO2. CO has a peak at 2250−2000 cm−1, and the characteristic peak at 3100−2800 cm−1 is attributed to the stretching vibrations of saturated CH bonds from CH4. Peaks at 1820−1660 cm−1 are related to acids, ketones, or aldehydes.39−41 At temperatures below 200 °C, only the peaks pertaining to H2O are observed. At ∼Tmax1, absorption peaks due to CO2, CH4, acids, ketones, and aldehydes are clearly observed and their intensity increases with further increase in the temperature. This can result from the decomposition of easily reduced oxygen-containing organic compounds in sewage sludge. At Tmax2, the production of CO2 and volatile organic compounds like acids, ketones, and aldehydes is gradually reduced, whereas the production of CO is increased, suggesting that some refractory organics, such as dead bacteria, have begun to decompose.30 Spectra I and IV
Figure 4. FTIR spectra of pyrolysis products derived from sewage sludge at different temperatures, (I) 800 °C, (II) Tmax2, (III) Tmax1, and (IV) 150 °C, with different heating rates.
in Figure 4 show that the number and intensity of peaks decrease with elevation of heating rate, suggesting that a high heating rate can suppress the decomposition of sludge during initial and final stages of pyrolysis.42−44 In other words, sludge is prone to decomposition in a narrow temperature range with D
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N-containing matter in sludge than lower heating rate. A more rapid increase in the contents (percentages) of oxygen-free (Ofree) SNCs (e.g., NH3 and HCN) than O-containing SNCs (e.g., HNCO and NO) at a higher heating rate suggests that the N-containing fragments produced by nitrile-N and heterocyclicN compounds tend to combine with ·CH or ·H radicals, rather than ·OH radicals.48,49 Given that the O-free SNCs are condensed more easily than the O-containing SNCs, the release of SNCs can bring second-pollution to atmospheric environment, it is, therefore, beneficial to pyrolyze the sludge with a higher heating rate. In addition, we find that the promotion of heating rate to 200 K min−1 leads to a shift of the peaks of SNCs to a higher temperature, which suggests that a decrease in the pyrolytic temperature can efficiently suppress the production of SNCs during the sludge pyrolysis. N-Containing Macromolecule Compounds Analysis. MNCs can be more accurately characterized by MS. Thus, the volatiles that pass through FTIR, enter into the online MS detector ultimately. The data show that the major MNCs can be categorized as heterocyclic-N, amine-N, and nitrile-N compounds (Figures 6 and SI Figures S4−S6). However, unlike the SNCs, the species of MNCs produced are dependent on the heating rates. Additionally, MNCs begin to form at 300−350 °C, when the heating rate is 10 K min−1 or 50 K min−1, and at 350−550 °C, with the heating rate is 200 K min−1. Among MNCs, heterocyclic-N MNCs, due to their high activation energy, are more stable than amine-N and nitrile-N MNCs.27 When the heating rate is 10 K min−1, fewer heterocyclic-N MNCs are produced, which include pyrrole (m/z 67), pyridine (m/z 79), aminopyridine (m/z 94), and vinylpyridine (m/z 105). At a heating rate of 50 K min−1, the production of heterocyclic-N species starts at ∼Tmax1 and its production reaches a maximum value at ∼Tmax2; the production trend matches the release curve of HNCO and the initial part of HCN release curve (Figure 5). At 200 K min−1, heterocyclicN MNCs are observed at temperatures >550 °C, which is in agreement with previously reported results.15,28,50 HeterocyclicN MNCs are more likely to have their roots in the thermochemical decomposition of pyridine-N and pyrrole-N in sewage sludge. Obviously, such heterocyclic-N MNCs increase significantly with increase in the heating rate. Similar phenomena are observed in the mass spectra of amine-N and nitile-N MNCs (SI Figures S4 and S5). It must be stated here that fast temperature changes at higher heating rates and short retention times hinder subsequent or extended decompositions of MNCs; this is consistent with the routine observation in pyrolysis, namely, that fast pyrolysis produces more bio-oil, while slow pyrolysis produces more gases. Some detected MNCs characterized by comparisons with samples in the MS database are listed in SI Table S4. Notably, only three amine-N MNCs are detected during pyrolysis, suggesting that the production of SNCs through the further decomposition of amine-N MNCs is not a main pathway. Exceptionally, the amounts of 2-(1-methyl-1H-pyrrol-2-yl) (m/ z 120, heterocyclic-N) acetonitrile and furosemide-methelute (m/z 256, heterocyclic-N) decrease while the heating rate increases from 50 to 200 K min−1. This can be due to the radical reactions, which need appropriate heating rate and temperature (SI Figure S7). Mechanism of Migration and Transformation of Nitrogen in Sludge. On the basis of the TG-FTIR-MS analysis of the intermediates, the mechanisms of migrations and
higher heating rate, whereas it is decomposed over wide temperature range with lower heating rate. N-containing compounds produced during sludge decomposition at normal pressure and temperature include gaseous and liquid products, which are denoted as small molecules (SNCs) and macromolecule nitrogen compounds (MNCs) here, respectively. SNCs are detected by their characteristic IR bands (Figure 5). HNCO has the characteristic IR band at 2250
Figure 5. Nitrogen species release curves. N-containing gaseous compounds released from sewage sludge when pyrolyzed with different heating rates: 10, 50, and 200 K min−1.
cm−1, which is assigned to CN bending vibration. Similarly, NO is represented by the band at 1900 cm−1 (NO stretching vibration), NH3 has the band at 966 cm−1 (NH bending vibration), and HCN has the band at 714 cm−1 (CH bending vibration). Concentrations of the SNCs are determined based on the integral values and percentage of their release curves.40,45−47 During the pyrolysis of sewage sludge, HNCO and NH3 are predominantly formed at 350−550 °C. HNCO is the product of decomposition of furosemide-methelute, (9Hcarbazol-9-yl) methanol, and N-(2,2-dichloro-1-hydroxyethyl) pivalamide, which are the major pyrolytic products of sludge.26,28 At higher temperatures, minor quantities of HNCO can be produced by metal oxide catalyzed thermal cracking of the N-containing char.45 The onset of NH3 release is probably ascribed to decomposition of ammonium salts and amine-N, while predominantly generated NH3 is attributed to the pyrolysis of protein-N.27 Initial NO formation is observed at 150 °C, and its production is high between 300 and 550 °C, which can be attributed to the reaction of N-containing compounds and OH radicals generated on the cleavage of C OH bonds (Figure 3). It is interesting to note that HCN release curve has a bimodal shape. The first peak is observed at 430 °C due to the direct pyrolysis of nitrile-N or heterocyclic-N in sludge (dehydrogenation and polymerization of amine-N results in the formation of the nitrile-N and heterocyclic-N compounds27), whereas the later peak at 750 °C is ascribed to the secondary thermal cracking of protein-N. SI Table S3 shows that more SNCs (integral value) are produced when the heating rate is higher, indicating that the higher heating rate is more favorable for the decomposition of E
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Figure 6. Mass spectra of heterocyclic-N compounds produced during the sewage sludge pyrolysis under different heating rates: 10, 50, and 200 K min−1.
straightforward. In the temperature range from 300 to 550 °C, protein-N is decomposed first to amine-N, and is then transferred to heterocyclic-N with the simultaneous release of NH3. When the temperature is >550 °C, the heterocyclic-N is decomposed and HCN is released. Nitrile-N, another intermediate of protein-N, is decomposed with HNCO release. As the heating rate determines the retention time that can influence the reaction between intermediates, the heating rate is
transformations of nitrogen in sludge during pyrolysis with different heating rates and temperatures are proposed (Figure 7). N of sludge is contained in pyrrolic-N, protein-N, amine-N, and pyridinic-N. Pyrrolic-N and pyridinic-N are decomposed to heterocyclic-N at temperatures >300 °C and are accompanied by the release of HCN. Amine-N is transformed to Ncontaining compounds (not amino) along with the release of NH3 at 300 °C. The decomposition of Protein-N is not F
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Figure 7. Mechanisms of migration and transformation of nitrogen during sewage sludge pyrolysis.
yields, will offer a new scientific insight into the reduction of the emission of NOx and other nitrogen pollutants during the pyrolysis process. In addition, on the basis of the mechanisms for transformation and migration of nitrogen during the pyrolysis process, we may find an effective approach to control the decomposition behavior of the nitrogen compounds in the sludge.
an important factor in the transformation of N. When the heating rate is 10 K min−1, we can only detect few N-species (e.g., aniline, aminopyridine, benzonitrile, methylbenzonitrile, pyrrole, pyridine, aminopyridine, and vinylpyridine), whereas when the heating rate is increase to 50 or 200 K min−1, more than 30 N-species are detected by MS in TG-FTIR-MS (number of N-species generated at 200 K min−1 is greater than those at 50 K min−1). Environmental Implications. Pyrolysis of huge amounts of sludge can reduce the sludge volume and recover valuable chemicals. This study shows that the heating rate and temperature of pyrolysis are critical factors. A higher heating rate (200 K min−1) and medium temperature (400−550 °C) are favorable for the complete decomposition of sludge and production of large quantities of liquid compounds. Some value-added organic compounds can be harvested from the biooil by means of separation or refinery. However, nitrogen is one of the main elements in the sludge, the investigation on the evolution of N-containing compounds (e.g, NH3, HCN, HCNO, and organic nitrogen compounds) and their relative
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ASSOCIATED CONTENT
S Supporting Information *
Elemental composition of sewage sludge and char obtained in pyrolysis (Table S1); thermal parameters of sewage sludge at different heating rate (Table S2); yields and conversion of nitrogen from sewage sludge under different heating rates (Table S3); mass results of MNCs during the sewage sludge pyrolysis (Table S4); XPS spectra of char (Figure S1); C 1s XPS spectrum of sewage sludge (Figure S2); 3D infrared spectrum of gaseous compounds during sewage sludge pyrolysis (Figure S3); mass spectra of amine-N compounds produced during the sewage sludge pyrolysis under different G
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(14) Takeda, N.; Hiraoka, M. Combined process of pyrolysis and combustion for sludge disposal. Environ. Sci. Technol. 1976, 10, 1147− 1150. (15) Domínguez, A.; Menéndez, J.; Inguanzo, M.; Pis, J. Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating. Biores. Technol. 2006, 97, 1185− 1193. (16) Domínguez, A.; Fernández, Y.; Fidalgo, B.; Pis, J.; Menéndez, J. Bio-syngas production with low concentrations of CO2 and CH4 from microwave-induced pyrolysis of wet and dried sewage sludge. Chemosphere 2008, 70, 397−403. (17) Ding, H. S.; Jiang, H. Self-heating co-pyrolysis of excessive activated sludge with waste biomass: Energy balance and sludge reduction. Bioresour. Technol. 2013, 133, 16−22. (18) Kistler, R. C.; Widmer, F.; Brunner, P. H. Behavior of chromium, nickel, copper, zinc, cadmium, mercury, and lead during the pyrolysis of sewage sludge. Environ. Sci. Technol. 1987, 21, 704− 708. (19) Zhang, G.; Hai, J.; Ren, M.; Zhang, S.; Cheng, J.; Yang, Z. Emission, mass balance, and distribution characteristics of PCDD/Fs and heavy metals during cocombustion of sewage sludge and coal in power plants. Environ. Sci. Technol. 2013, 47, 2123−2130. (20) Ryther, J. H.; Dunstan, W. M. Nitrogen, phosphorus, and eutrophication in the coastal marine environment. Science 1971, 171, 1008−1013. (21) Channar, A. G.; Rind, A. M.; Mastoi, G. M.; Almani, K. F.; Lashari, K. H.; Memon, R. A.; Qurishi, M. A.; Mahar, N. Comparative study of water quality of manchhar lake with drinking water quality standard of world health organization. Am. J. Environ. Prot. 2014, 3, 68−72. (22) Chen, N.; Wu, J.; Chen, Z.; Lu, T.; Wang, L. Spatial-temporal variation of dissolved N2 and denitrification in an agricultural river network, southeast China. Agric. Eco. Environ. 2014, 189, 1−10. (23) Bennion, H.; Juggins, S.; Anderson, N. J. Predicting epilimnetic phosphorus concentrations using an improved diatom-based transfer function and its application to lake eutrophication management. Environ. Sci. Technol. 1996, 30, 2004−2007. (24) Cao, J.-P.; Li, L.-Y.; Morishita, K.; Xiao, X.-B.; Zhao, X.-Y.; Wei, X.-Y.; Takarada, T. Nitrogen transformations during fast pyrolysis of sewage sludge. Fuel 2013, 104, 1−6. (25) Zhang, J.; Tian, Y.; Zhu, J.; Zuo, W.; Yin, L. Characterization of nitrogen transformation during microwave-induced pyrolysis of sewage sludge. J.Anal. Appl. Pyrolysis 2014, 105, 335−341. (26) Fonts, I.; Azuara, M.; Lázaro, L.; Gea, G.; Murillo, M. Gas chromatography study of sewage sludge pyrolysis liquids obtained at different operational conditions in a fluidized bed. Ind. Eng. Chem. Res. 2009, 48, 5907−5915. (27) Tian, Y.; Zhang, J.; Zuo, W.; Chen, L.; Cui, Y.; Tan, T. Nitrogen conversion in relation to NH3 and HCN during microwave pyrolysis of sewage sludge. Environ. Sci. Technol. 2013, 47, 3498−3505. (28) Fullana, A.; Conesa, J. A.; Font, R.; Martín-Gullón, I. Pyrolysis of sewage sludge: Nitrogenated compounds and pretreatment effects. J. Anal. Appl. Pyrolysis 2003, 68, 561−575. (29) Ischia, M.; Maschio, R. D.; Grigiante, M.; Baratieri, M. Clay− sewage sludge co-pyrolysis. A TG−MS and Py−GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge. Waste Manage. 2011, 31, 71−77. (30) Shao, J.; Yan, R.; Chen, H.; Wang, B.; Lee, D. H.; Liang, D. T. Pyrolysis characteristics and kinetics of sewage sludge by thermogravimetry fourier transform infrared analysis. Energy Fuel. 2007, 22, 38−45. (31) Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J. Am. Chem. Soc. 2012, 134,, 9082−9085. (32) Misra, M. K.; Ragland, K. W.; Baker, A. J. Wood ash composition as a function of furnace temperature. Biomass Bioenerg. 1993, 4, 103−116.
heating rates (Figure S4); mass spectra of nitrile-N compounds produced during the sewage sludge pyrolysis under different heating rates (Figure S5); total ion current (TIC) chromatographs of the evolved gases at different heating rates (Figure S6); and radical reactions (Figure S7). This material is available via the Internet at http://pubs.acs.org.This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-551-63607482; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (No. 2012ZX07103-001), National Key Technology R&D Program of the Ministry of Science and Technology (2012BAJ08B00), and Shanghai Tongji Gao Tingyao Environmental Science and Technology Development Foundation (STGEF).
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REFERENCES
(1) Tabata, S.-i.; Suzuki, T.; Hamamoto, Y.; Hayakawa, N.; Watanabe, K. Activated sludge method. In Google Patents: 1987. (2) Hashimoto, S.; Furukawa, K. Immobilization of activated sludge by PVA−boric acid method. Biotechnol. Bioeng. 1987, 30, 52−59. (3) Hassani, A.; Nejaei, A.; Torabian, A. Excess sludge minimization in conventional activated sludge pilot plant by three chemical matters. Int. J. Environ. Res. 2011, 5, 981−988. (4) Brdjanovic, D.; Slamet, A.; Van Loosdrecht, M.; Hooijmans, C.; Alaerts, G.; Heijnen, J. Impact of excessive aeration on biological phosphorus removal from wastewater. Water Res. 1998, 32, 200−208. (5) Cai, M.; Liu, J.; Wei, Y. Enhanced biohydrogen production from sewage sludge with alkaline pretreatment. Environ. Sci. Technol. 2004, 38, 3195−3202. (6) Nomeda, S.; Valdas, P.; Chen, S.-Y.; Lin, J.-G. Variations of metal distribution in sewage sludge composting. Waste Manage. 2008, 28, 1637−1644. (7) Fullana, A.; Conesa, J. A.; Font, R.; Sidhu, S. Formation and destruction of chlorinated pollutants during sewage sludge incineration. Environ. Sci. Technol. 2004, 38, 2953−2958. (8) Xu, H.; He, P.; Wang, G.; Shao, L.; Lee, D. Anaerobic storage as a pretreatment for enhanced biodegradability of dewatered sewage sludge. Biores. Technol. 2011, 102, 667−671. (9) Fytili, D.; Zabaniotou, A. Utilization of sewage sludge in EU application of old and new methodsA review. Renew. Sust. Energy Rev. 2008, 12, 116−140. (10) Cao, Y.; Pawłowski, A. Sewage sludge-to-energy approaches based on anaerobic digestion and pyrolysis: Brief overview and energy efficiency assessment. Renew. Sust. Energy Rev. 2012, 16, 1657−1665. (11) Eskicioglu, C.; Prorot, A.; Marin, J.; Droste, R. L.; Kennedy, K. J. Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion. Water Res. 2008, 42, 4674−4682. (12) Bougrier, C.; Delgenes, J.-P.; Carrere, H. Combination of thermal treatments and anaerobic digestion to reduce sewage sludge quantity and improve biogas yield. Process Saf. Environ. Prot. 2006, 84, 280−284. (13) Chang, S.-S.; Lee, W.-J.; Wang, L.-C.; Chang-Chien, G.-P.; Wu, C.-Y. Energy recovery and emissions of PBDD/Fs and PBDEs from co-combustion of woodchip and wastewater sludge in an industrial boiler. Environ. Sci. Technol. 2013, 47, 12600−12606. H
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(33) Nakhaei Pour, A.; Shahri, S. M. K.; Bozorgzadeh, H. R.; Zamani, Y.; Tavasoli, A.; Marvast, M. A. Effect of Mg, La, and Ca promoters on the structure and catalytic behavior of iron-based catalysts in Fischer− Tropsch synthesis. Appl. Catal. A: Gen. 2008, 348, 201−208. (34) Urban, D. L.; Antal, M. J., Jr Study of the kinetics of sewage sludge pyrolysis using DSC and TGA. Fuel 1982, 61, 799−806. (35) Font, R.; Fullana, A.; Conesa, J.; Llavador, F. Analysis of the pyrolysis and combustion of different sewage sludges by TG. J.Anal. Appl. Pyrolysis 2001, 58, 927−941. (36) Chang, C.-Y.; Shie, J.-L.; Lin, J.-P.; Wu, C.-H.; Lee, D.-J.; Chang, C.-F. Major products obtained from the pyrolysis of oil sludge. Energy Fuel. 2000, 14, 1176−1183. (37) Becidan, M.; Skreiberg, Ø.; Hustad, J. E. NOx and N2O precursors (NH3 and HCN) in pyrolysis of biomass residues. Energy Fuel. 2007, 21, 1173−1180. (38) Ledesma, E. B.; Li, C.-Z.; Nelson, P. F.; Mackie, J. C. Release of HCN, NH3, and HNCO from the thermal gas-phase cracking of coal pyrolysis tars. Energy Fuel 1998, 12, 536−541. (39) Gu, X.; Ma, X.; Li, L.; Liu, C.; Cheng, K.; Li, Z. Pyrolysis of poplar wood sawdust by TG-FTIR and Py-GC/MS. J.Anal. Appl. Pyrolysis 2013, 102, 16−23. (40) Magalhaes, W.; Job, A.; Ferreira, C.; da Silva, H. Pyrolysis and combustion of pulp mill lime sludge. J.Anal. Appl. Pyrolysis 2008, 82, 298−303. (41) De Jong, W.; Di Nola, G.; Venneker, B.; Spliethoff, H.; Wójtowicz, M. TG-FTIR pyrolysis of coal and secondary biomass fuels: Determination of pyrolysis kinetic parameters for main species and NOx precursors. Fuel 2007, 86, 2367−2376. (42) Babu, B. Biomass pyrolysis: A state-of-the-art review. Biof. Biopr. Bioref. 2008, 2, 393−414. (43) Bridgwater, A. V. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenerg. 2012, 38, 68−94. (44) White, J. E.; Catallo, W. J.; Legendre, B. L. Biomass pyrolysis kinetics: A comparative critical review with relevant agricultural residue case studies. J. Anal. Appl. Pyrolysis 2011, 91, 1−33. (45) Ren, Q.; Zhao, C. NOx and N2O precursors from biomass pyrolysis: Nitrogen transformation from amino acid. Environ. Sci. Technol. 2012, 46, 4236−4240. (46) Ren, Q.; Zhao, C. NOx and N2O precursors from biomass pyrolysis: Role of cellulose, hemicellulose and lignin. Environ. Sci. Technol. 2013, 47, 8955−8961. (47) Ren, Q.; Zhao, C. NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: Interaction between amino acid and mineral matter. Appl. Energy 2013, 112, 170−174. (48) Tian, F.-J.; Li, B.-Q.; Chen, Y.; Li, C.-Z. Formation of NOx precursors during the pyrolysis of coal and biomass. Part V. Pyrolysis of a sewage sludge. Fuel 2002, 81, 2203−2208. (49) Chang, L.; Xie, Z.; Xie, K.-C.; Pratt, K. C.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z. Formation of NOx precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the formation of NH3 and HCN. Fuel 2003, 82, 1159−1166. (50) Inguanzo, M.; Domınguez, A.; Menéndez, J.; Blanco, C.; Pis, J. On the pyrolysis of sewage sludge: The influence of pyrolysis conditions on solid, liquid and gas fractions. J. Anal. Appl. Pyrolysis 2002, 63, 209−222.
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Investigation on the Evolution of N‑Containing Organic Compounds during Pyrolysis of Sewage Sludge Ke Tian, Wu-Jun Liu, Ting-Ting Qian, Hong Jiang,* and Han-Qing Yu Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China S Supporting Information *
ABSTRACT: Pyrolysis is an emerging technology for the disposal of huge amounts of sewage sludge. However, the thermochemical decomposition mechanism of organic compounds in sludge is still unclear. We adopt a novel online TG-FTIR-MS technology to investigate the pyrolysis of sludge. The sludge samples were pyrolyzed from 150 to 800 °C with heating rates of 10, 50, and 200 K min−1. We found for the first time that the heating rate of pyrolysis can significantly change the species of liquid organic compounds produced, but cannot change the gaseous species produced under the same conditions. The contents of produced gas and liquid compounds, most of which were produced at 293−383 °C, are influenced by both the heating rate and temperature of pyrolysis. The results also showed that heterocyclic-N, amine-N, and nitrile-N compounds are obtained from the decomposition of N-compounds in sludge, such as pyrrolic-N, protein-N, amine-N, and pyridinic-N. Heterocyclic-N compounds are the dominant N-containing products, which can be due to the thermochemical decomposition of pyridine-N and pyrrole-N, whereas fewer amine-N compounds are produced during the pyrolysis. A mechanism for the decomposition of N-containing compounds in sludge is proposed based on the obtained data.
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INTRODUCTION The activated sludge method has been used to treat wastewater since the 1910s, due to its low cost and excellent adaptability for various wastewaters.1,2 However, the treatment of excess sludge remains a thorny issue to date.3−5 For example, in the European Union, nearly 10 million tons of dry sludge have been produced in 2005, and the European Environmental Agency has identified it as “a future waste problem”.6−8 Traditional options for sludge reduction include landfilling, composting, thermo-chemical treatment, and land application following anaerobic and aerobic digestion;9 among these, the thermo-chemical treatment (e.g., pyrolysis and combustion) is an efficient and most widely used method.10 Pyrolysis of sludge is a promising and proven technology because it is often carried out at relatively low temperatures and under anoxic conditions, thereby precluding the production of highly toxic persistent organic compounds (e.g., dioxin) and particulate matter (e.g., PM2.5 and PM10), and the simultaneous immobilization of heavy metals in the carbonaceous matrix.11−14 Furthermore, gases and oils produced in the pyrolysis of sludge can be used as fuels and chemical feedstocks.15−17 Transformation and migration behaviors of elements in sludge during pyrolysis are closely related to the occurrence of second-pollution. Several researchers have investigated the pyrolysis behavior of different components of sludge and potential secondary pollutants, such as heavy metals,18 chlorinated pollutants,7 polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxin, and dibenzofuran (PCDD/Fs).13,19 However, the circulation of nitrogen in the © XXXX American Chemical Society
environment is more important because it is mainly responsible for the eutrophication of water columns.20−23 Only a few reports have investigated the fate of N-containing components of sludge during pyrolysis. These N-containing compounds are decomposed to different gaseous, liquid, and solid compounds. Cao et al.24 concluded that NH3 is the predominant part in nitrogenous gases under fast pyrolysis of sludge, its yield increases with increasing pyrolytic temperature and decreasing sweeping gas flow rate. Zhang et al.25 reported that NH3 and HCN were the main parts in nitrogenous gases during microwave pyrolysis of sludge, and the pyrolysis temperature is a significant factor in the transformation of nitrogen. The natures of the ultimate N-containing products are usually determined by the pyrolysis conditions, such as temperature and heating rate.26−28 Thus, tracing the fate of N during sludge pyrolysis is critical for controlling the N-species and diminishing the potential second-pollution. Several analytical methods have been developed to probe the pyrolysis behavior of sludge. Fonts et al.26 analyzed the composition of the pyrolytic liquids by GC-MS and GC-FID. Ischia et al.29 investigated the sludge pyrolysis process using thermogravimetry (TG) coupled to mass spectrometry (MS) and gas chromatography (GC), i.e., TG/MS or TG/GC/MS. Shao et al.30 studied the pyrolysis characteristics, kinetics, and Received: May 5, 2014 Revised: August 18, 2014 Accepted: August 20, 2014
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heating. A helium carrier gas (50 mL min−1) was used during pyrolysis. Data Analysis. Data obtained by TG/DTG were analyzed by Pyris software (PerkinElmer, Inc., U.K.). 3D infrared spectrum data were analyzed using Spectrum TimeBase Version 3.1.0 (PerkinElmer, Inc., U.K.), and TurboMass Ver 6.0.0 (PerkinElmer, Inc., U.S.A.) was employed for analyses of mass spectra.
gas production properties of sludge pyrolysis process by TG coupled to Fourier transform infrared analyses (TG/FTIR). These integrated means of analyses deepen our understanding of the sludge pyrolysis. However, to elucidate the mechanism underpinning the sludge pyrolysis, especially the fate of Ncontaining components, an online, integrated TG-FTIR-MS method can provide more information, thereby leading to more efficient and accurate analyses. Herein, we report the first integrated TG-FTIR-MS online approach to monitor the migration and transformation of pollutants in sludge. We analyze the production of NOx and N2O precursors (HNCO, HCN, NO, NH3), N-containing organic compounds, and small-molecule hydrocarbons during the pyrolysis of sludge. We also investigate the effects of the heating rate and temperature, and propose migration and transformation mechanisms based on the gathered data. The results from this study can provide valuable information and insight into controlled conversion of N-containing components in sludge pyrolysis.
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RESULTS AND DISCUSSION Characteristics of Sewage Sludge. The chemical characteristics of the used sludge are listed in Supporting Information (SI) Table S1. The nitrogen content of sludge is 4.9 wt %, which is similar to other sludges.9,27 After pyrolysis, the N content as well as the contents of C, H, and O of the residual char decreases significantly, suggesting that the release of N is accompanied by the release of C, H, and O. XPS spectrum of pyrolytic char shows that the protein-N (nitrogen contained in the proteins) and amine-N (nitrogen contained in the amines) in sludge have decomposed completely, and pyrrolic-N (nitrogen contained in pyrroles) and pyridinic-N (nitrogen contained in the pyridines) have partly decomposed, while NO3− is present (SI Figure S1). XPS survey can be used to analyze the chemical nature of N and other elements on the surface of sludge. N 1s spectra of the raw sludge (Figure 1a) can be decomposed into four peaks at 399.0 eV (pyrrolic-N), 399.5 eV (protein-N), 399.9 eV (amineN), and 400.4 eV (pyridinic-N) with the relative contents of 25.1, 22.3, 29.3, and 23.4 atom %, respectively. Protein-N is originated in proteins, amine-N derived from saccharides, and pyrrolic-N or pyridinic-N are from the decomposition products of nucleic acids.30 Inorganic ammonium-N is not detected in XPS spectra, which can be ascribed to its low content. XPS analysis shows that N is distributed equally among the four Ncontaining chemical species, which can adopt different thermochemical decomposition behaviors during pyrolysis. C 1s spectra are provided in SI Figure S2. Besides the N, C, H, and O, P and some metals (e.g., Ca, Mg, Al, Fe, Na, K, and Si) are also found in the sludge (Figure 1b), which can catalyze certain transformations during pyrolysis of sludge.6,31−33 TG Analysis. TG analysis (TGA) can determine the optimum temperature range for sludge pyrolysis. More importantly, when the results from TGA are combined with the results of analysis by FTIR and MS, an accurate plan for reducing the formation of harmful N-containing air pollutants during the pyrolysis of sewage sludge can be formulated. Sewage sludge is primarily pyrolyzed in TG equipment from room temperature to 900 °C with different heating rates under helium atmosphere. TG/DTG profiles during the pyrolysis of sewage sludge as a function of temperature are shown in Figure 2. 50.8, 51.6, and 52.1 wt % residues remain after pyrolysis of the sludge at 900 °C at heating rates of 10, 50, and 200 K min−1, respectively. This indicates that the heating rate, a critical factor influencing pyrolysis, does not have an obvious effect on the degree of decomposition of sludge; however, the heating rate can still influence decomposition pathways of sludge. Temperature, not heating rate, has a significant effect on the degree of sludge decomposition. Minor loss in mass at temperatures lower than 150 °C can be attributed to the release of moisture.34 Significant weight loss is observed in the temperature range from 150 to 550 °C, which can attributed to the decomposition of the sludge.35 With a further increase in
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EXPERIMENTAL SECTION Sample Collection and Characterization. Sewage sludge used in this study was collected from Wangtang Wastewater Treatment Plant located in Hefei, China. Raw sludge from the anaerobic digestion process was first dewatered by air-drying, then dried at 80 °C for 24 h and stored in airtight bags until it was cooled to room temperature. The cooled, dried sludge was crushed and filtered through a sieve (60 mesh). Finally, the filtered samples were stored in refrigerator until further use. Elemental composition (C, H, N, and O) of sludge was determined with an elemental analyzer (VARIO EL III, Elementar, Germany). X-ray photoelectron spectroscopy (XPS) measurements of the sludge were carried out with Xray photoelectron spectrometer (ESCALAB 250, Thermo-VG Scientific Inc., U.K.) equipped with a monochromatized Al Kα radiation (1486.6 eV). The measurements were carried out in a Constant Analyzer Energy (CAE) mode with 30 eV pass energy for survey spectra and 20 eV for high resolution spectra. The XPS peaks were matched into subcomponents using a Gaussian (80%)−Lorentzian (20%) curve-fitting program (XPSPEAK 4.1), with a Shirley type background. TG-FTIR-MS Analysis of Sewage Pyrolysis. Simultaneous TG, FTIR, and MS data were obtained using a TG analyzer coupled with a Fourier transform infrared spectrometer outfitted with a gas cell and mass spectrometer (TL-9000). Gas products released from TG analyzer were directly collected in the gas cell and analyzed immediately using the FTIR spectrometer. FTIR spectra (4000−450 cm−1; resolution = 2 cm−1) of the gaseous products were acquired continuously with the baseline amended. After being analyzed by FTIR, the gas products were immediately swept to the mass spectrometer. The entire flow-path of gas was insulated (wrapped with heating wire) to prevent any condensation of volatile decomposition products. Dried sludge (∼20 mg) was heated from room temperature to 900 °C in the TG apparatus with heating rates of 10, 50, and 200 K min−1. The gas products released from TG analyzer were directly transferred to the FTIR spectrometer and then to the mass spectrometer, as described above. The data record time of mass spectrometer was initiated 10 min prior to the TG analyzer and FTIR detector. Generated N-containing gaseous products were determined over the temperature range of B
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Figure 1. (a) N 1s XPS spectrum of sewage sludge and (b) XPS survey spectra of sewage sludge.
temperature, there is only a marginal weight loss, which is due to the deep decomposition of some sludge components.30 The decomposition of sludge (150−550 °C) could be divided into two stages in the DTG curve: (1) ∼15 wt % weight loss occurs at between 150 and 350 °C, which is predominantly due to decomposition of biodegradable matter, and (2) 25 wt % weight loss occurs in the temperature range from 350 to 550 °C, which pertains to the degradation of dead bacteria.30 The two temperatures representing maximum decomposition (Tmax1 and Tmax2) are 293 and 338 °C, 301 and 353 °C, and 338 and 383 °C when the rates of heating are 10, 50, and 200 K min−1, respectively (SI Table S2). With an increase in the heating rate, the value of Tmax shifts to a higher temperature. The peak height (or reaction rate) is also increased by 2 orders of magnitude when the heating rate is raised from 10 and 50 K min−1 to 200 K min−1. These changes are caused by the delayed decomposition kinetics of sewage sludge.36 Gaseous Small-Molecule Compounds. TG analysis shows that the heating rate can change the Tmax, which suggests that the decomposition pathway of sludge is different at different heating rates. To further trace the transformation of N during pyrolysis, gaseous compounds produced in TG equipment were analyzed by online FTIR. Generally, the sludge was pyrolzed at ∼Tmax to obtain more volatiles and minimum residue during sludge minimization. Volatiles can be separated
Figure 2. TG/DTG profiles of sewage sludge at different heating rates: (a) 200, (b) 50, and (c) 10 K min−1.
into pyrolytic bio-oil and gas-mixture by condensation; the gases can be analyzed by FTIR.37,38 We investigated the FTIR spectra while pyrolyzing sludge at different heating rates and four different temperatures, i.e., 150 °C (initial decomposition), Tmax1, Tmax2, and 800 °C (final decomposition). The 3D infrared spectra compiled over the entire temperature range of pyrolysis show that the volatiles generated at different heating rates give rise to IR absorptions at similar wavenumbers (Figure 3 and SI Figure S3). It indicates that while the heating rate significantly alters the yield of gases, it has minimal influence on the composition of produced gases. C
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Figure 3. 3D infrared spectrum of gaseous compounds released during sewage sludge pyrolysis at (a) 200 K min−1, (b) 50 K min−1, and (c) 10 K min−1.
Highest yields of gaseous compounds are generated in the temperature range from 300 to 500 °C with all the three heating rates (Figures 3a−c), which is in agreement the results from TGA. FTIR spectra at four specific temperatures during pyrolysis with heating rates of 200, 50, and 10 K min−1 are shown in Figure 4. Spectra I and IV are obtained at 800 and 150 °C, respectively, while spectra II and III describe the gases at Tmax1 and Tmax2, respectively. Absorbance peaks at 4000−3400 cm−1 and 1460−1380 cm−1 are assigned to the stretching and rotational vibrations of functional group of H2O. A sharp doublet peak at 2400−2250 cm−1 and a low intensity peak at 780−600 cm−1 are due to the stretching vibrations of CO2. CO has a peak at 2250−2000 cm−1, and the characteristic peak at 3100−2800 cm−1 is attributed to the stretching vibrations of saturated CH bonds from CH4. Peaks at 1820−1660 cm−1 are related to acids, ketones, or aldehydes.39−41 At temperatures below 200 °C, only the peaks pertaining to H2O are observed. At ∼Tmax1, absorption peaks due to CO2, CH4, acids, ketones, and aldehydes are clearly observed and their intensity increases with further increase in the temperature. This can result from the decomposition of easily reduced oxygen-containing organic compounds in sewage sludge. At Tmax2, the production of CO2 and volatile organic compounds like acids, ketones, and aldehydes is gradually reduced, whereas the production of CO is increased, suggesting that some refractory organics, such as dead bacteria, have begun to decompose.30 Spectra I and IV
Figure 4. FTIR spectra of pyrolysis products derived from sewage sludge at different temperatures, (I) 800 °C, (II) Tmax2, (III) Tmax1, and (IV) 150 °C, with different heating rates.
in Figure 4 show that the number and intensity of peaks decrease with elevation of heating rate, suggesting that a high heating rate can suppress the decomposition of sludge during initial and final stages of pyrolysis.42−44 In other words, sludge is prone to decomposition in a narrow temperature range with D
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N-containing matter in sludge than lower heating rate. A more rapid increase in the contents (percentages) of oxygen-free (Ofree) SNCs (e.g., NH3 and HCN) than O-containing SNCs (e.g., HNCO and NO) at a higher heating rate suggests that the N-containing fragments produced by nitrile-N and heterocyclicN compounds tend to combine with ·CH or ·H radicals, rather than ·OH radicals.48,49 Given that the O-free SNCs are condensed more easily than the O-containing SNCs, the release of SNCs can bring second-pollution to atmospheric environment, it is, therefore, beneficial to pyrolyze the sludge with a higher heating rate. In addition, we find that the promotion of heating rate to 200 K min−1 leads to a shift of the peaks of SNCs to a higher temperature, which suggests that a decrease in the pyrolytic temperature can efficiently suppress the production of SNCs during the sludge pyrolysis. N-Containing Macromolecule Compounds Analysis. MNCs can be more accurately characterized by MS. Thus, the volatiles that pass through FTIR, enter into the online MS detector ultimately. The data show that the major MNCs can be categorized as heterocyclic-N, amine-N, and nitrile-N compounds (Figures 6 and SI Figures S4−S6). However, unlike the SNCs, the species of MNCs produced are dependent on the heating rates. Additionally, MNCs begin to form at 300−350 °C, when the heating rate is 10 K min−1 or 50 K min−1, and at 350−550 °C, with the heating rate is 200 K min−1. Among MNCs, heterocyclic-N MNCs, due to their high activation energy, are more stable than amine-N and nitrile-N MNCs.27 When the heating rate is 10 K min−1, fewer heterocyclic-N MNCs are produced, which include pyrrole (m/z 67), pyridine (m/z 79), aminopyridine (m/z 94), and vinylpyridine (m/z 105). At a heating rate of 50 K min−1, the production of heterocyclic-N species starts at ∼Tmax1 and its production reaches a maximum value at ∼Tmax2; the production trend matches the release curve of HNCO and the initial part of HCN release curve (Figure 5). At 200 K min−1, heterocyclicN MNCs are observed at temperatures >550 °C, which is in agreement with previously reported results.15,28,50 HeterocyclicN MNCs are more likely to have their roots in the thermochemical decomposition of pyridine-N and pyrrole-N in sewage sludge. Obviously, such heterocyclic-N MNCs increase significantly with increase in the heating rate. Similar phenomena are observed in the mass spectra of amine-N and nitile-N MNCs (SI Figures S4 and S5). It must be stated here that fast temperature changes at higher heating rates and short retention times hinder subsequent or extended decompositions of MNCs; this is consistent with the routine observation in pyrolysis, namely, that fast pyrolysis produces more bio-oil, while slow pyrolysis produces more gases. Some detected MNCs characterized by comparisons with samples in the MS database are listed in SI Table S4. Notably, only three amine-N MNCs are detected during pyrolysis, suggesting that the production of SNCs through the further decomposition of amine-N MNCs is not a main pathway. Exceptionally, the amounts of 2-(1-methyl-1H-pyrrol-2-yl) (m/ z 120, heterocyclic-N) acetonitrile and furosemide-methelute (m/z 256, heterocyclic-N) decrease while the heating rate increases from 50 to 200 K min−1. This can be due to the radical reactions, which need appropriate heating rate and temperature (SI Figure S7). Mechanism of Migration and Transformation of Nitrogen in Sludge. On the basis of the TG-FTIR-MS analysis of the intermediates, the mechanisms of migrations and
higher heating rate, whereas it is decomposed over wide temperature range with lower heating rate. N-containing compounds produced during sludge decomposition at normal pressure and temperature include gaseous and liquid products, which are denoted as small molecules (SNCs) and macromolecule nitrogen compounds (MNCs) here, respectively. SNCs are detected by their characteristic IR bands (Figure 5). HNCO has the characteristic IR band at 2250
Figure 5. Nitrogen species release curves. N-containing gaseous compounds released from sewage sludge when pyrolyzed with different heating rates: 10, 50, and 200 K min−1.
cm−1, which is assigned to CN bending vibration. Similarly, NO is represented by the band at 1900 cm−1 (NO stretching vibration), NH3 has the band at 966 cm−1 (NH bending vibration), and HCN has the band at 714 cm−1 (CH bending vibration). Concentrations of the SNCs are determined based on the integral values and percentage of their release curves.40,45−47 During the pyrolysis of sewage sludge, HNCO and NH3 are predominantly formed at 350−550 °C. HNCO is the product of decomposition of furosemide-methelute, (9Hcarbazol-9-yl) methanol, and N-(2,2-dichloro-1-hydroxyethyl) pivalamide, which are the major pyrolytic products of sludge.26,28 At higher temperatures, minor quantities of HNCO can be produced by metal oxide catalyzed thermal cracking of the N-containing char.45 The onset of NH3 release is probably ascribed to decomposition of ammonium salts and amine-N, while predominantly generated NH3 is attributed to the pyrolysis of protein-N.27 Initial NO formation is observed at 150 °C, and its production is high between 300 and 550 °C, which can be attributed to the reaction of N-containing compounds and OH radicals generated on the cleavage of C OH bonds (Figure 3). It is interesting to note that HCN release curve has a bimodal shape. The first peak is observed at 430 °C due to the direct pyrolysis of nitrile-N or heterocyclic-N in sludge (dehydrogenation and polymerization of amine-N results in the formation of the nitrile-N and heterocyclic-N compounds27), whereas the later peak at 750 °C is ascribed to the secondary thermal cracking of protein-N. SI Table S3 shows that more SNCs (integral value) are produced when the heating rate is higher, indicating that the higher heating rate is more favorable for the decomposition of E
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Figure 6. Mass spectra of heterocyclic-N compounds produced during the sewage sludge pyrolysis under different heating rates: 10, 50, and 200 K min−1.
straightforward. In the temperature range from 300 to 550 °C, protein-N is decomposed first to amine-N, and is then transferred to heterocyclic-N with the simultaneous release of NH3. When the temperature is >550 °C, the heterocyclic-N is decomposed and HCN is released. Nitrile-N, another intermediate of protein-N, is decomposed with HNCO release. As the heating rate determines the retention time that can influence the reaction between intermediates, the heating rate is
transformations of nitrogen in sludge during pyrolysis with different heating rates and temperatures are proposed (Figure 7). N of sludge is contained in pyrrolic-N, protein-N, amine-N, and pyridinic-N. Pyrrolic-N and pyridinic-N are decomposed to heterocyclic-N at temperatures >300 °C and are accompanied by the release of HCN. Amine-N is transformed to Ncontaining compounds (not amino) along with the release of NH3 at 300 °C. The decomposition of Protein-N is not F
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Figure 7. Mechanisms of migration and transformation of nitrogen during sewage sludge pyrolysis.
yields, will offer a new scientific insight into the reduction of the emission of NOx and other nitrogen pollutants during the pyrolysis process. In addition, on the basis of the mechanisms for transformation and migration of nitrogen during the pyrolysis process, we may find an effective approach to control the decomposition behavior of the nitrogen compounds in the sludge.
an important factor in the transformation of N. When the heating rate is 10 K min−1, we can only detect few N-species (e.g., aniline, aminopyridine, benzonitrile, methylbenzonitrile, pyrrole, pyridine, aminopyridine, and vinylpyridine), whereas when the heating rate is increase to 50 or 200 K min−1, more than 30 N-species are detected by MS in TG-FTIR-MS (number of N-species generated at 200 K min−1 is greater than those at 50 K min−1). Environmental Implications. Pyrolysis of huge amounts of sludge can reduce the sludge volume and recover valuable chemicals. This study shows that the heating rate and temperature of pyrolysis are critical factors. A higher heating rate (200 K min−1) and medium temperature (400−550 °C) are favorable for the complete decomposition of sludge and production of large quantities of liquid compounds. Some value-added organic compounds can be harvested from the biooil by means of separation or refinery. However, nitrogen is one of the main elements in the sludge, the investigation on the evolution of N-containing compounds (e.g, NH3, HCN, HCNO, and organic nitrogen compounds) and their relative
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ASSOCIATED CONTENT
S Supporting Information *
Elemental composition of sewage sludge and char obtained in pyrolysis (Table S1); thermal parameters of sewage sludge at different heating rate (Table S2); yields and conversion of nitrogen from sewage sludge under different heating rates (Table S3); mass results of MNCs during the sewage sludge pyrolysis (Table S4); XPS spectra of char (Figure S1); C 1s XPS spectrum of sewage sludge (Figure S2); 3D infrared spectrum of gaseous compounds during sewage sludge pyrolysis (Figure S3); mass spectra of amine-N compounds produced during the sewage sludge pyrolysis under different G
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Environmental Science & Technology
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heating rates (Figure S4); mass spectra of nitrile-N compounds produced during the sewage sludge pyrolysis under different heating rates (Figure S5); total ion current (TIC) chromatographs of the evolved gases at different heating rates (Figure S6); and radical reactions (Figure S7). This material is available via the Internet at http://pubs.acs.org.This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-551-63607482; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (No. 2012ZX07103-001), National Key Technology R&D Program of the Ministry of Science and Technology (2012BAJ08B00), and Shanghai Tongji Gao Tingyao Environmental Science and Technology Development Foundation (STGEF).
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