Thermal Transformation of Carbon and Oxygen-Containing Organic Compounds in Sewage Sludge During Pyrolysis Treatment

Organic carbon (C) and oxygen (O) contained in sewage sludge strongly impact its thermal behavior during pyrolysis treatment. This study was aimed at getting an insight into the decomposition mechanism of organic compounds containing C and O during sludge pyrolysis using thermo-gravimetric Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis-gas-chromatography/mass spectrometry (Py-GC/MS) and helpfully improving energy conversion of sewage sludge. The temperature domains of transformation were determined and indications of the main compounds produced during each stage were obtained. Results showed that the volatile compounds that evolved after sludge pyrolysis were mainly distributed into six groups: alkenes, aromatic hydrocarbons, alcohols, aldehydes, phenols and carboxylic acids. Comparison in thermal behavior and composition of the evolved volatile compounds were observed. In the low temperature stage (<350 ◦C), compounds containing O–C=O accounted for the highest proportion in the evolved gas (55%). Over 350 ◦C, the production of C=C, –OH, and –C6H5 compounds gradually increased; but little was found of compounds containing O–C=O. Above 550 ◦C, as thermal chemical reaction involving oxygen-containing groups enhanced, compounds containing O–C=O and –OH tended to disappear, and an increasing amount of macromolecular polycyclic aromatic hydrocarbon was formed. Finally, the thermal transformation pathways of the oxygen and carbon-containing compounds were proposed.


Introduction
The growing number of municipal wastewater treatment plants in China are producing ever-increasing amounts of wastewater sewage sludge as a byproduct. It is reported that the annual generation of wet sewage sludge in China was about 40.9 million tons in 2017, with a growth rate of 8-10% per-year [1]. To deal with the huge amount of sludge, its reutilization in an environmentally friendly way is increasingly becoming an urgent issue [2][3][4]. By weight, sewage sludge contains approximately 50% organic content; with the main components being hemicelluloses, cellulose, lignin, lipids, and protein. This gives sewage sludge an interesting potential to be used as a biomass resource to be converted to energy.
Pyrolysis has been attracting increasing worldwide attention due to its economic and environmentally acceptable method of sewage sludge disposal. It has large potential for the disposal

Thermo-Gravimetric Fourier Transform Infrared (TG-FTIR) Spectroscopy
To better understand the pyrolysis behavior of organic compounds containing carbon and oxygen, TG-FTIR experiments of sewage sludge were performed using a Diamond Thermogravimetric Analysis and Differential Thermal Analysis (TG/DTA) (PerkinElmer, US) coupled with Fourier transform infrared measurements (Nicolet 6700, US). The experimental conditions selected in this study were based on some previous literatures [23,24]. Temperature precision of the TG was ±0.5 • C with a microbalance sensitivity of ±0.1 µg. Pure N 2 was used as the agent gas, with a 20 mL (STP)/min flow rate. Sample weight was determined to be 10 ± 0.5 mg to minimize heating transfer limitations.
In our pyrolysis experiments, each sample was heated from 25 • C to 850 • C with a 15 • C/min heating rate. The volatiles released during this process were detected online by FTIR spectrometer, with the ability to record spectra in the range of 4000 to 400 cm −1 . FTIR analysis was carried out with 4 cm −1 resolution and 8 scans per sampling, starting when the sample temperature reached 100 • C. The transfer line and detector were kept at a constant temperature of 225 • C to prevent condensation. To eliminate background noise, blank tests were done before runs using samples. Interpretation of FTIR spectra was made by comparing the results to the National Institute of Standards and Technology (NIST) atomic spectra database.

X-ray photoelectron spectroscopy (XPS) Analysis
Surface elemental composition and chemical species of the dried sewage sludge were characterized by XPS. XPS analysis was carried out using a VG Scientific ESCALab220i-XL spectrometer of SHIMADZU-KRATOS Japan, equipped with an Al Ka monochromator X-ray source. The C1s peak at 284.6 eV was used as binding energy (BE) reference. Quantitative analysis of relative contents of organic groups is based on the linear correlation of signal intensity and element content.

Pyrolysis-Gas-Chromatography/Mass Spectrometry (Py-GC/MS) Technology
Py-GC/MS experiments were performed to investigate the distribution of organic products from sludge pyrolysis. The experimental conditions selected in this study were based on some previous literatures [15,23,24]. A CDS Pyroprobe 5000 HP pyrolyser connected to a gas chromatography/mass spectrometry (Agilent Technologies 5977A MSD) was employed to conduct these tests. All of the samples were weighed at 1 ± 0.1 mg. Pyrolysis was carried out at 250 • C, 350 • C, 550 • C, 650 • C, 750 • C, and 850 • C. The temperatures of oven and transfer line were both maintained at 300 • C. The temperature of the GC setup equipped with a HP-5MScapillary column (30 m × 0.25 mm × 0.25 µm) and its interface was held at 300 • C. The GC oven temperature was first set at 50 • C for a duration Energies 2019, 12, 2258 4 of 13 of 2.25 min. After that, the oven was heated at a rate of 15 • C/min until it reached 300 • C. Finally, the GC oven temperature was kept on 300 • C for 10 min. Pure nitrogen was used as agent gas of GC/MS with a flow rate of 1.1 mL (STP)/min and a 1:20 split ratio. Mass spectra detection covered the range of 35-500 amu. The compounds were determined by comparing the spectral reading to the NIST database library. All of the experiments mentioned above including the ultimate analysis, the proximate analysis, the yield of the resulted bio-oil, TG-FTIR analysis, and Py-GC/MS analysis were performed at least twice to ensure the repeatability of the results. Figure 1 shows the results from the scanning of sewage sludge by XPS analysis. It was found that the most abundant elements on the surface of sludge were C and O. Small amounts of N, Si and Al were also detected. In this study, semi-quantitative analysis was done on the basis of total content of C, O, N, Si and Al. H element was excluded from the analysis. Presence of the C1s and O1s peaks also indicated that the sludge samples contained considerable organic carbon and oxygen-containing chemical groups.

Distribution of Organic Carbon and Oxygen-Containing Groups in Sewage Sludge
According to the results obtained by resolving overlapping peak and some previous reports [25][26][27][28], the C1s spectrum could be broken down into four peaks, as shown in Figure 2a. The electronic binding energies of the C1s spectrum was concentrated between 282 and 292 eV; indicating that the detectable organic carbon-containing chemical groups included C-C, C-O, C=O and O-C=O. The large peak at 285.0 eV belonged to C-C bond, and its proportion was 69.33%. of the total area. The C-O bond also was one of the main carbon-containing groups, and its proportion is 19.81% of total area. The peak of 287.6 eV belonged to the C=O bond with an area proportion of 6.62%. This type of bond might belong to compounds such as aldehydes, ketones, esters, carboxylic acids, and carboxylic acid derivatives. The small peak at 289.0 eV belonged to the O-C=O bond, with the lowest area proportion, at only 4.24%.
Oxygen is the second most plentiful element in sludge, after carbon. The study of oxygen-containing chemical groups is also essential to better understand the thermochemical conversion during sludge pyrolysis. Figure 2b shows the fitting results of O1s spectrum. Oxygen-containing groups can be fitted into four kinds of functional groups: inorganic oxygen such as Al 2 O 3 /SiO 2 (530.4 eV); organic oxygen such C-O (531.4 eV) in alcohols and ethers; C=O (532.8 eV) in aldehydes, ketones and esters; and O-C=O (533.7 eV). C-O was the main species of organic oxygen in sludge, with an area proportion of 56.92%. The area proportions of C=O and O-C=O were 22.63% and 16.44%, respectively. Inorganic oxygen content had the least proportion, at only 4% area.
Energies 2018, 11, x FOR PEER REVIEW 4 of 14 of 35-500 amu. The compounds were determined by comparing the spectral reading to the NIST database library. All of the experiments mentioned above including the ultimate analysis, the proximate analysis, the yield of the resulted bio-oil, TG-FTIR analysis, and Py-GC/MS analysis were performed at least twice to ensure the repeatability of the results. Figure 1 shows the results from the scanning of sewage sludge by XPS analysis. It was found that the most abundant elements on the surface of sludge were C and O. Small amounts of N, Si and Al were also detected. In this study, semi-quantitative analysis was done on the basis of total content of C, O, N, Si and Al. H element was excluded from the analysis. Presence of the C1s and O1s peaks also indicated that the sludge samples contained considerable organic carbon and oxygen-containing chemical groups.

Distribution of Organic Carbon and Oxygen-Containing Groups in Sewage Sludge
According to the results obtained by resolving overlapping peak and some previous reports [25][26][27][28], the C1s spectrum could be broken down into four peaks, as shown in Figure 2a. The electronic binding energies of the C1s spectrum was concentrated between 282 and 292 eV; indicating that the detectable organic carbon-containing chemical groups included C-C, C-O, C = O and O-C = O. The large peak at 285.0 eV belonged to C-C bond, and its proportion was 69.33%. of the total area. The C-O bond also was one of the main carbon-containing groups, and its proportion is 19.81% of total area. The peak of 287.6 eV belonged to the C = O bond with an area proportion of 6.62%. This type of bond might belong to compounds such as aldehydes, ketones, esters, carboxylic acids, and carboxylic acid derivatives. The small peak at 289.0 eV belonged to the O-C = O bond, with the lowest area proportion, at only 4.24%.
Oxygen is the second most plentiful element in sludge, after carbon. The study of oxygencontaining chemical groups is also essential to better understand the thermochemical conversion during sludge pyrolysis. Figure 2b shows the fitting results of O1s spectrum. Oxygen-containing groups can be fitted into four kinds of functional groups: inorganic oxygen such as Al2O3/SiO2 (530.

Thermo-Gravimetric Fourier Transform Infrared Spectroscopy (TG-FTIR) Analysis
TG-FTIR was used for online detection of the gas-phase products that occurred during sludge pyrolysis. The 3D infrared spectrum of the volatiles presented in Figure 4 shows the relationship between wave number, absorbance, and temperature. It was found that a non-conspicuous absorption peak

Thermo-Gravimetric Fourier Transform Infrared Spectroscopy (TG-FTIR) Analysis
TG-FTIR was used for online detection of the gas-phase products that occurred during sludge pyrolysis. Figure 3 showed that the TG and DTG profiles. From the TG curves, a plateau could be observed before 200 • C. The weight of sewage sludge decreases sharply between 200 • C and 300 • C.
After 300 • C, the loss rate of weight tends to decrease up until at 500 • C. The splitting of the DTG curves is supposed to be due to the decomposition of organic matter. The decomposition of the sludge sample can, therefore, be divided into three stages The first stage below 200 • C resulted in about 5.18% weight loss from moisture evaporation and dehydration process. The sludge started to decompose at about 200 • C and the process continued in a wide temperature range. The second stage from 200 to 500 • C was the main decomposition stage with a 46.77% weight loss due to severe decomposition of organic structure. The third stage above 500 • C mainly saw the carbonization process that resulted in a 5.56% weigh loss.

Thermo-Gravimetric Fourier Transform Infrared Spectroscopy (TG-FTIR) Analysis
TG-FTIR was used for online detection of the gas-phase products that occurred during sludge pyrolysis.  The 3D infrared spectrum of the volatiles presented in Figure 4 shows the relationship between wave number, absorbance, and temperature. It was found that a non-conspicuous absorption peak The 3D infrared spectrum of the volatiles presented in Figure 4 shows the relationship between wave number, absorbance, and temperature. It was found that a non-conspicuous absorption peak appeared at the range of 3800-3500 cm −1 , which was caused by the stretching vibration of -OH, indicating the presence of a small amount of H 2 O. Adsorption peak at the range of 3012-2940 cm −1 was related to C-C and C=C antisymmetric stretching vibration, indicating that a small number of aliphatic hydrocarbons were thermally released. The C=O stretching absorbance peaks appearing in 1900-1750 cm −1 were representative of carbonyls, including aldehyde, ketone and carboxylic acids compounds [29]. Skeleton C=C vibration absorption peaks of benzene rings was detected at the  [30]. Meanwhile, the pyrolysis of sludge also produced other gas compounds with small molecular structure, such as CO 2 (2400-2250cm −1 or 680-660cm −1 ) and CO (2200-2100cm −1 ). FTIR did not detect diatomic gases such as H 2 and N 2 due to the absence of dipole moment changes incurred by these gases [31]. appeared at the range of 3800-3500 cm , which was caused by the stretching vibration of -OH, indicating the presence of a small amount of H2O. Adsorption peak at the range of 3012-2940 cm −1 was related to C-C and C = C antisymmetric stretching vibration, indicating that a small number of aliphatic hydrocarbons were thermally released. The C = O stretching absorbance peaks appearing in 1900-1750 cm −1 were representative of carbonyls, including aldehyde, ketone and carboxylic acids compounds [29]. Skeleton C = C vibration absorption peaks of benzene rings was detected at the range of 1600-1440 cm −1 , indicating that various forms of benzene compounds exist in the sludge.
FTIR did not detect diatomic gases such as H2 and N2 due to the absence of dipole moment changes incurred by these gases [31]. The FTIR analysis of the evolved gas from TG measurements offered detailed information on the thermal behavior of sludge. However, the thermal degradation of sludge is a complex process that produces a variety of compounds. To better understand the evolution of main carbon and oxygen-containing chemical groups during sludge pyrolysis, the variation in the absorbance of OH, -C6H5, C-C, C = C, C-O, C = O, CO, CO2 with temperature was further characterized. The results are presented in Figure 5 (a, b, c, d).
From Figure 5a, it can be seen that -OH structure is mainly generated in 85-450 °C. In the second stage of pyrolysis, -OH is mainly derived from the release of crystal water in sludge. The absorbance intensity of -OH peaks at 288 °C was from the decomposition of oxygenates and formation of compounds containing hydroxyl functional groups [32]. -C6H5 structures are mostly present in benzene and its derivatives, and is generated at two temperature ranges: 67-500 °C and 762-850 °C. The highest spectral intensity of -C6H5 was achieved at 340 °C, mainly due to the volatilization of aromatic hydrocarbons in the feed and the cleaving of aromatic hydrocarbons with side chains.
During the third stage of pyrolysis (762-850 °C), the spectral intensity of -C6H5 increased with temperature. This is caused by the polycondensation of polycyclic aromatic hydrocarbons and hydrogenated aromatic hydrocarbons. In addition, organic macromolecules (cellulose, lignin, etc.) in sludge could undergo thermal cracking and form compounds containing -C6H5 structure.
It can be observed from Figure 5b that C-C was mainly produced in the second stage (195-336 °C) and the third stage (336-700 °C), with peaks occurring at around 304 °C and 570 °C, respectively. C = C structure mainly exist in the methyl(-CH3) and methylene(-CH2) groups of aliphatic and alkane bonds [33]. The unsaturated C = C structure was detected with the existence of two absorption peaks The FTIR analysis of the evolved gas from TG measurements offered detailed information on the thermal behavior of sludge. However, the thermal degradation of sludge is a complex process that produces a variety of compounds. To better understand the evolution of main carbon and oxygen-containing chemical groups during sludge pyrolysis, the variation in the absorbance of OH, -C 6 H 5 , C-C, C=C, C-O, C=O, CO, CO 2 with temperature was further characterized. The results are presented in Figure 5a-d.
From Figure 5a, it can be seen that -OH structure is mainly generated in 85-450 • C. In the second stage of pyrolysis, -OH is mainly derived from the release of crystal water in sludge. The absorbance intensity of -OH peaks at 288 • C was from the decomposition of oxygenates and formation of compounds containing hydroxyl functional groups [32]. -C 6 H 5 structures are mostly present in benzene and its derivatives, and is generated at two temperature ranges: 67-500 • C and 762-850 • C. The highest spectral intensity of -C 6 H 5 was achieved at 340 • C, mainly due to the volatilization of aromatic hydrocarbons in the feed and the cleaving of aromatic hydrocarbons with side chains.
During the third stage of pyrolysis (762-850 • C), the spectral intensity of -C 6 H 5 increased with temperature. This is caused by the polycondensation of polycyclic aromatic hydrocarbons and hydrogenated aromatic hydrocarbons. In addition, organic macromolecules (cellulose, lignin, etc.) in sludge could undergo thermal cracking and form compounds containing -C 6 H 5 structure.
It can be observed from Figure 5b that C-C was mainly produced in the second stage (195-336 • C) and the third stage (336-700 • C), with peaks occurring at around 304 • C and 570 • C, respectively. C=C structure mainly exist in the methyl(-CH 3 ) and methylene(-CH 2 ) groups of aliphatic and alkane bonds [33]. The unsaturated C=C structure was detected with the existence of two absorption peaks at 195-368 • C and 368-673 • C. It was noted that the spectral intensity of the C=C structure tend to slowly increase from 673 • C to 850 • C, which might be related to the dehydrocyclization of aliphatic hydrocarbon molecules and further aromatization reactions at high temperatures.
From Figure 5c, a single absorption peak was observed in 30-850 • C. It appeared to correspond with C-O and C=O groups which peaked at 310 • C and 295 • C, respectively. A previous study reported that C-O and C=O groups mainly derived from the pyrolysis of cellulose, hemicellulose, and lignin in sludge [23]. Above 310 • C, the absorbance of C-O and C=O groups decreased as temperature increased. This phenomena might be related to the reduction of carboxylic acids, esters and other compounds. Figure 5d shows the spectral intensities of CO 2 and CO in the evolved gas at different temperatures. It was reported that CO 2 mostly originated from the degradation of oxygen-containing groups such as carboxyl, carbonyl and oxygen-containing heterocyclic [34]. CO 2 was one of the key components in the gas-phase product at the early stage of sludge pyrolysis. It could be observed that CO 2 peaked in the ranges 167-582 • C and 582-725 • C. The first peak occurred at 332 • C, primarily from to the decomposition of the carboxyl group with low thermal stability. The second peak appeared at 627 • C and was attributed to the decomposition of oxygen-containing heterocyclic ring and the carboxyl group with high thermal stability. In the 725-850 • C range, the spectral intensity of CO 2 increased with the rise in temperature, possibly due to the increase of polycondensation reaction of macromolecular aromatic rings derived from sludge that produced more CO 2.
slowly increase from 673 °C to 850 °C, which might be related to the dehydrocyclization of aliphatic hydrocarbon molecules and further aromatization reactions at high temperatures.
From Figure 5c, a single absorption peak was observed in 30-850 °C. It appeared to correspond with C-O and C = O groups which peaked at 310 °C and 295 °C, respectively. A previous study reported that C-O and C = O groups mainly derived from the pyrolysis of cellulose, hemicellulose, and lignin in sludge [23]. Above 310 °C, the absorbance of C-O and C = O groups decreased as temperature increased. This phenomena might be related to the reduction of carboxylic acids, esters and other compounds. Figure 5d shows the spectral intensities of CO2 and CO in the evolved gas at different temperatures. It was reported that CO2 mostly originated from the degradation of oxygen-containing groups such as carboxyl, carbonyl and oxygen-containing heterocyclic [34]. CO2 was one of the key components in the gas-phase product at the early stage of sludge pyrolysis. It could be observed that CO2 peaked in the ranges 167-582 °C and 582-725 °C. The first peak occurred at 332 °C, primarily from to the decomposition of the carboxyl group with low thermal stability. The second peak appeared at 627 °C and was attributed to the decomposition of oxygen-containing heterocyclic ring and the carboxyl group with high thermal stability. In the 725-850 °C range, the spectral intensity of CO2 increased with the rise in temperature, possibly due to the increase of polycondensation reaction of macromolecular aromatic rings derived from sludge that produced more CO2.

Py-GC/MS Analysis
Based on the TG analysis in section 3.2, the decomposition process was completed before the pyrolysis temperature reached 850 °C, which was taken as the analysis temperature range for Py-GC/MS tests. There are many kinds of organic components in the gas phase products from the 250 to 850 °C pyrolysis process. The typical ion chromatograms above 350 °C are more than 100 peaks, and the temperature is obviously affected. Over 90% of the volatile compounds was identified by the summation of the peak areas of individual compound. The identified compounds are those with the

Py-GC/MS Analysis
Based on the TG analysis in Section 3.2, the decomposition process was completed before the pyrolysis temperature reached 850 • C, which was taken as the analysis temperature range for Py-GC/MS tests. There are many kinds of organic components in the gas phase products from the 250 to 850 • C pyrolysis process. The typical ion chromatograms above 350 • C are more than 100 peaks, and the temperature is obviously affected. Over 90% of the volatile compounds was identified by the summation of the peak areas of individual compound. The identified compounds are those with the peak area matching degree larger than 70% and significant peak area larger than 0.1% when compared to compounds listed in the NIST library. Table 2 showed that the composition and peak area of the identified organic compounds in the evolved gas at different temperature. The experiments were performed at least twice, and the results were repeatable. It can be seen that the organic compounds in evolved gas mainly include six groups: alkenes, aromatic hydrocarbons, alcohols, aldehydes, phenols, and carboxylic acids by their functional groups. Figure 6a-f further showed the classification and peak-area percentage of the different types of organic compounds. Due to the lack of standard mixture for the evolved gas, it was difficult to calibrate the MS detectors. This constraint limited the quantitative analysis of volatile product obtained. Therefore, an approach based on peak area value was adopted to represent the concentration of each compound identified and enable a direct comparison on the transformation of oxygen and carbon-containing compounds in this work [15]. At the low temperature pyrolysis (<350 • C), the volatile compounds containing O-C=O accounted for the highest proportion of the total detected substances, at about 53.27%. These compounds were mainly produced by the thermal degradation of cellulose and hemicellulose, to produce much high-grade mono-fatty acids including palmitic acid and palmitoleic acid. -OH was also generated into the evolved volatile product from the thermal release of a large number of alcohol hydroxyl groups Energies 2019, 12, 2258 9 of 13 containing R-OH; such as DL-2-aminopropanol, cholesterol-3-ol and cholesterol. In this stage, there were little trace of compounds containing -CHO, C=C and -C 6 H 5 found.
In the middle temperature pyrolysis process (350-550 • C), production of compounds with O-C=O gradually decreased as temperature increased. This is caused by the carboxylic acid destruction and hydrogen bond breaking, which produce a large amount of carbon dioxide. The percentage of peak area belonging to compounds with R-OH and Ar-OH functional groups was found to increase. Over 350 • C, compounds with C=C and -C 6 H 5 began to form from chain-breaking and dehydrogenation of long-chain aliphatic hydrocarbons or aromatic hydrocarbons with side-chain containing C-C and C-H. It was observed that a large number of compounds containing benzene ring (-C 6 H 5 ) were formed by the decomposition of cellulose, hemicellulose, and lignin from the sludge. The compounds containing -CHO was found in less than 1% peak area at 550 • C.
Energies 2018, 11, x FOR PEER REVIEW 10 of 14 550 °C did the peak area of compounds with Ar-OH show a trend of decreasing first, then rising and decreasing.

Pyrolysis Evolution Pathway of Organic Carbon and Oxygen-Containing Compounds
Active groups such as aromatic groups, phenolic hydroxyl groups, alcoholic hydroxyl groups, or carbon-based conjugated double bonds mainly exist in the form of organic molecular structures which are linked by C-C and C-O bonds. These structures can produce a large amount of carboxylic acid and its derivatives during pyrolysis at medium and low temperatures, and form macromolecular Above 550 • C, carboxylic acid compounds containing O-C=O and compounds containing R-OH began to disappear due to the propagation of dehydration, decarboxylation, and decarbonylation reactions as temperature increased. No O-C=O and R-OH was detected above 750 • C and 650 • C, respectively. The peak area of compounds containing -CHO increased from 0.31% to 2.51% at 550-750 • C due to the production of large amounts of trimethyl butyraldehyde. Conversely, the peak area of olefin containing C=C rapidly decreased after 650 • C and disappeared entirely at 850 • C. In the low temperature pyrolysis process, Ar-OH was not easily cracked and substituted. Only above 550 • C did the peak area of compounds with Ar-OH show a trend of decreasing first, then rising and decreasing.

Pyrolysis Evolution Pathway of Organic Carbon and Oxygen-Containing Compounds
Active groups such as aromatic groups, phenolic hydroxyl groups, alcoholic hydroxyl groups, or carbon-based conjugated double bonds mainly exist in the form of organic molecular structures which are linked by C-C and C-O bonds. These structures can produce a large amount of carboxylic acid and its derivatives during pyrolysis at medium and low temperatures, and form macromolecular aromatic compounds at high temperatures, which lowers the quality of pyrolysis oil. Meanwhile, some small molecular gases such as CO, CO 2 , H 2 , and CH 4 are produced. The results presented in Sections 3.2 and 3.3 showed that the thermal decomposition of most organic components in sludge began above 200 • C. Small molecular carbohydrates mainly decomposed at 200-300 • C. Most cellulose and lignin are pyrolyzed in the temperature range of 300-350 • C. Protein and lipid are pyrolyzed in the wide temperature range of 200-600 • C. Because of the overlapping pyrolysis temperature range, cross-linking reactions between different compounds could occur, which makes the pyrolysis mechanism more complex.
On the basis of the aforementioned results and discussions and previous reports [36][37][38][39], this study proposed the transformation pathways of relevant reaction routes for carbon and oxygen-containing compounds in sludge pyrolysis process. These pathways are depicted in Figure 7. At the lower temperature pyrolysis stage (200-350 • C), macromolecule proteins break their peptide bonds to form small molecules containing amino chains, including aliphatic amines, cyclic amines and aromatic amines. The lignin and cellulose molecules form monomers mainly by the breaking of C-O-C and C-C bonds. Lignin mostly forms three kinds of alcohol monomers (p-coumaryl, coniferyl, and sinapyl alcohols) [39]. Lipid is mainly pyrolyzed to form hydroxy-containing glycerol, cholesterol, and alkanoic acids containing O-C=O. At higher temperatures (350-850 • C), acidic sites with small molecules and amino chains can promote the cleavage of streptamine to form olefins and NH 3 . Cyclic and aromatic amines are prone to dehydrate, decarboxylate, and deaminate to form aromatic compounds [40]. Cellulose monomers further break C-O to form short-chain molecules until they are converted into glucose. Lignin monomers deoxidize directly to form small hydrocarbons or aromatic compounds by condensation reaction. Moreover, carboxylic acids can be decarbonylated to alcohols, decarboxylated to long-chain alkanes, and dehydroxylated to aldehydes. Some aldehydes are decarbonylated to long-chain alkanes, while small amounts of alkanes can be cyclized and further aromatize to form polycyclic aromatic hydrocarbons.
NH3. Cyclic and aromatic amines are prone to dehydrate, decarboxylate, and deaminate to form aromatic compounds [40]. Cellulose monomers further break C-O to form short-chain molecules until they are converted into glucose. Lignin monomers deoxidize directly to form small hydrocarbons or aromatic compounds by condensation reaction. Moreover, carboxylic acids can be decarbonylated to alcohols, decarboxylated to long-chain alkanes, and dehydroxylated to aldehydes. Some aldehydes are decarbonylated to long-chain alkanes, while small amounts of alkanes can be cyclized and further aromatize to form polycyclic aromatic hydrocarbons.

Conclusions
This work has investigated the redistribution and transformation pathway of C-and O-containing organic compounds during sludge pyrolysis, providing an understanding of the pyrolysis mechanism. Sludge was found to decompose in a wide temperature range, starting at about 200 • C and peaking in the 200-550 • C range. Based on the types of carbon and oxygen-containing functional groups, the distribution of the evolved volatile compounds after sludge pyrolysis were mainly divided into six groups: alkenes, aromatic hydrocarbons, alcohols, aldehydes, phenols and carboxylic acids. Compounds containing -OH were generated in 85-450 • C. Benzene and its derivatives containing -C 6 H 5 , were mainly generated at the two temperature ranges: 67-500 • C and 762-850 • C. C=C structure mainly exist in the methyl (-CH 3 ) and methylene (-CH 2 ) groups of aliphatic and alkane bonds, which were produced in the 195-336 • C and 336-700 • C ranges. The C-O and C=O groups peaked at 310 • C and 295 • C. CO 2 was one of the main components in the gas-phase product at the early stage of sludge pyrolysis, which peaked in the 167-582 • C and 582-725 • C ranges. Temperature has an important role in the pyrolysis evolution of the functional groups. Over 550 • C, as the thermal chemical reaction of oxygen-containing groups enhanced, compounds containing O-C=O and -OH tended to disappear. From temperature of 550 • C and upwards, it was also observed that an increasing amount of macromolecular polycyclic aromatic hydrocarbon was formed through condensation and aromatization. The results obtained in the study will contribute to understanding of the pyrolysis mechanism of sludge and hence to the determination of the appropriate reaction conditions, improving the quality of bio-oil, and increasing the yield of the target products.