Liquefaction of Sewage Sludge To Produce Bio-oil in Different Organic

Jul 22, 2019 - Liquefaction is a new method of producing crude oil from sewage sludge. ... The process can even produce crude oil, which takes million...
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Liquefaction of Sewage Sludge To Produce Bio-oil in Different Organic Solvents with In Situ Hydrogenation Rundong Li,*,†,‡ Wenchao Teng,† Yanlong Li,‡ and Enhui Liu‡ †

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China Key Laboratory of Clean Energy Liaoning Province, Shenyang Aerospace University, Shenyang, Liaoning 110136, People’s Republic of China

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ABSTRACT: Liquefaction is a new method of producing crude oil from sewage sludge. The effect of in situ hydrogenation on sewage sludge liquefaction characteristics using different solvents was studied in this research. The highest bio-oil yields were 42.3% in ethanol at 360 °C, 43.1% in acetone at 360 °C, and 26.3% in methanol at 280 °C. Esters were the main components of bio-oils in ethanol and methanol, which comprised 43.55% ethyl esters and 61.05% methyl esters, respectively. N components originating from protein constituted the main heterocyclic compounds and amides. The oxygen content decreased as the temperature increased, and the highest higher heating values for ethanol, acetone, and methanol at 360 °C were 29.08, 29.53, and 27.95 MJ/kg, respectively. The differential scanning calorimetry−thermogravimetry−derivative thermogravimetry results illuminated that the bio-oil obtained from methanol was more stable than the others and the main small-molecule components decomposed before the temperature reached 373.5 °C. The highest weight loss rates were 4.31, 5.07, and 5.12%/min at 238.8, 233.6, and 260.5 °C, respectively.

1. INTRODUCTION The sewage treatment capacity increased dramatically with the rapid development of the economy and the construction of urbanization. Sewage sludge (SS) is a byproduct of sewage treatment plants and is rich in many organic components [protein (∼40%), saccharides (∼14%), esters (∼10%), and lignin (∼17%)].1−4 However, SS also contains heavy metals, pathogenic bacteria, virus, and other microorganisms that limit its direct application. Traditional SS treatment methods include landfilling, incineration, and composting.5 However, these methods require a large area and produce secondary pollutants. Therefore, the prospects of using these methods are becoming increasingly questioned. Identifying sustainable, clean, economical, and efficient methods for the treatment and utilization of SS has become the focus of the research area of SS treatment. Under the conditions of SS liquefaction, extracellular polymeric substances are cracked, the cell walls and cell membranes are destroyed, and organic matter, such as intracellular proteins, lipids, carbohydrates, and nucleic acids, is released by dissolution.6 These organic substances enter the liquid phase and hydrolyze into higher fatty acids and amino acids, even small-molecular fatty acids, polysaccharides, monosaccharides, glycerol, and carbon dioxide at high temperatures.7,8 Currently, the hydrothermal treatment of SS has a good effect on improving the dewatering performance and biodegradability of sludge and is often used in deep dewatering and anaerobic digestion pretreatment technologies.9 Hydrothermal liquefaction (HTL) is a new method that imitates the geological conditions of the early Earth to produce bio-oil at high temperatures and high pressures within a short time. The process can even produce crude oil, which takes millions of years to achieve in nature. HTL is a © XXXX American Chemical Society

thermochemical process that converts biomass into crude oil at sub- and supercritical conditions in water or an organic solvent media.10 Researchers have recently focused on studying the HTL behavior of waste biomass, such as lignocellulose, algae, and lipids.11−15 During HTL, macromolecular organic compounds are decomposed into small-molecular compounds by thermochemical conversion and then integrated into biofuels through further reactions. Owing to the high contents of moisture and organic matter in SS, it is a suitable raw material for bio-oil production by HTL. Under hydrothermal conditions, the organic matter in SS is decomposed into biooil, gas, and solid residue.16,17 During SS HTL processing, almost 60% of carbon in SS is converted to bio-oil;3 therefore, the products from SS liquefaction could provide a high-energydensity liquid fuel. Sludge liquefaction is a promising technology to convert organic components to viscous, black liquid fuels in sub- or supercritical fluids at high temperature and pressure in an inert or reducing atmosphere.18,19 Owing to its low price and wide availability, water can be regarded as a clean and sustainable solvent for HTL. However, its relatively harsh conditions (high temperature, high pressure, large amount of energy, and confined equipment) are required in engineering to achieve supercritical conditions;20 meanwhile, the nitrogen (N) and oxygen (O) contents are relatively high in bio-oil. N of bio-oil produced from SS is 9%, while that of crude oil is only 0.5%. An excessive N content would increase the emission of nitrogen oxides during SS bio-oil combustion processing.3 The O content of the bio-oil produced from SS reaches up to 9%.21 The oxygen compounds in bio-oil are concentrated in gum and Received: May 6, 2019 Revised: July 7, 2019

A

DOI: 10.1021/acs.energyfuels.9b01434 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels asphalt, which would decrease the higher heating value (HHV) of bio-oil. Meanwhile, HTL generates an aqueous-phase byproduct when water is used as a solvent and decreases the bio-oil yield.22 Furthermore, a large number of cycloalkanes, aromatic hydrocarbons, and other macromolecular compounds existed in the bio-oil produced from SS by HTL, which increases the viscosity of bio-oil.23−25 The bio-oil produced from the supercritical liquefaction of SS typically contains a large number of organic compounds, including nitrogenous heterocyclic compounds, organic acids, esters, ketones, phenols, alcohols, hydrocarbons, and various complex functional compounds.25 The composition of the biooil is affected by various factors, such as the raw material compositions, solvents, residence time, and operating conditions.19,26 Recently, researchers have used organic solvents instead of water to study the liquefaction of SS, owing to their relatively mild reaction conditions.7,10,27 Many studies have been conducted on the liquefaction of SS in different organic solvents to identify their effect on bio-oil. Most of the research indicated that the liquid products produced from SS sub-/ supercritical water included fatty acids, fatty acid esters, nitrogenated compounds, and their derivatives.21,28−30 Wang et al. found that, after the hydrothermal treatment of SS, large numbers of volatile organic chemicals (VOCs) mainly containing aldehydes, alkanes, and volatile fatty acids were enriched in solid-phase hydrothermal carbon.6 Huynh et al. reported that the content of fatty acids in bio-oil is high when it was produced using water. However, when methanol was added during liquefaction processing, the main component of the bio-oil was fatty acid methyl ester, which exhibited similar characteristics to diesel.31 The reduction in the organic acid content also weakened the acid value and corrosivity of bio-oil, providing a new concept for conducting sludge liquefaction to produce bio-oil. Leng et al. indicated that the bio-oil was composed of esters when produced by sub-/supercritical ethanol or methanol.32 The methyl esters could be completely extracted in diesel. In this study, dried SS underwent liquefaction within the temperature range of 280−360 °C with a solid concentration of 0.1 g/mL and 30 min to study the influence of organic solvents on liquefaction products of SS. The organic solvent was used to serve as an in situ hydrogen source to guide the liquefaction processes for SS and determine the optimum solvent and temperature for improving the sludge bio-oil yield and quality. The formation path and reaction mechanism of bio-oil production were studied to guide the liquefaction processes for SS.

Table 1. Primary Characteristic Analyses and Chemical Compositions of SS proximate and ultimate analyses (wt %) Mad Aad Vad FCada Cad Had Nad Sad Oadb

6.70 35.71 50.52 7.07 31.50 5.02 5.30 1.20 21.27

element (mg/kg) Cu Zn Pb Cr Ni As K Na Al Fe Ca Mg Si

main chemical composition (wt %, as 105 °C dried)

366.98 1304.60 75.56 245.86 84.62 31.60 4065.35 2939.11 10084.59 16806.75 14002.96 2744.57 2722.42

SiO2 Al2O3 P2O5 CaO Fe2O3 SO3 K2O MgO TiO2 Na2O ZnO MnO CuO

29.36 26.18 15.75 8.44 7.94 6.07 2.57 1.99 0.90 0.37 0.23 0.13 0.08

a FCad = 100% − Mad − Aad − Vad. bOad = 100% − Cad − Had − Nad − Sad − Aad.

methanol were fed into the reactor. The autoclave reactor was sealed and received nitrogen 3 times to create an inert atmosphere. Following this, the air inlet and outlet were closed and a heater was installed to heat the reactor to the required experimental temperature (280−360 °C) at an average heating rate of 5 °C/min with magnetic stirring at 200 rpm/min. After the reaction and cooling, the solid− liquid mixtures in the reactor was poured into a beaker and the remaining solid−liquid mixtures in the autoclave reactor were washed with ethanol and acetone. The collected suspensions underwent vacuum filtration to separate the mixtures, and the solid products were air-dried in an oven to a constant weight to obtain a solid residue. The liquid fraction was evaporated at 60 and 82 °C to remove ethanol and acetone, respectively, and the residual black liquid was collected as the bio-oil. 2.3. Analytical Methods. The liquefaction conversion ratio of SS and the yields of products were calculated as follows:

ij m yz liquefaction conversion ratio (wt %) = jjj1 − SR zzz × 100% j mSS z{ k m bio‐oil yield (wt %) = bio‐oil × 100% mSS

solid residue yield (wt %) =

mSR × 100% mSS

(1) (2) (3)

gas yield (wt %) = 100% − bio‐oil yield (wt %) − solid residue yield (wt %)

(4)

where mSR, mbio‑oil, and mSS are the masses of the solid residue, bio-oil, and dried SS, respectively. The HHV was calculated from the elemental analysis results.

2. MATERIALS AND METHODS 2.1. Materials. The SS samples were mechanical watering sludge that had been collected from waste water treatment in Shenyang, Liaoning, China, dried to a constant weight in an oven at 105 °C for 48 h, then sieved to a particle size of 100 mesh, and stored in zip-lock bags. The chemical reagents used in the experiments, including alcohol, acetone, and methanol, were analytical reagents purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., China. A summary of the proximate, ultimate, and chemical compositions of SS is presented in Table 1. 2.2. Liquefaction of SS and Separation of Products. A 500 mL autoclave-reactor designed to have a maximum pressure of 35 MPa at 500 °C with a magnetically coupled mechanical mixer was applied to conduct the experimental results. Figure 1 is the detailed experimental scheme and methods for the liquefaction treatment of SS. In the experiments, 15 g of SS and 150 mL of ethanol, acetone, or

HHV (MJ/kg) = −1.3675 + (0.3137C + 0.7009H + 0.0318O) (5) Approximately 3 mg of the bio-oil was placed in the tin capsules and then injected into a CHNS/O elemental analyzer (CHNS ≤ 0.1%, Eurovector EA 3000, Italy) to analyze the elemental compositions of the bio-oil. The O content was calculated by difference. The analysis was performed 3 times to reduce the errors. The chemical components of the bio-oil were analyzed using gas chromatography−mass spectrometry (GC−MS, QP5050, Japan) for separation. The carrying gas was helium at a flow rate of 1 mL/min. The HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) temperature was set at 50 °C for 3 min, then heated to 280 °C at a heating rate of 5 °C/min, and maintained for 15 min. MS scanned the B

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Figure 1. Experimental scheme and methods for the liquefaction treatment of SS. region from 20 to 500 amu. The obtained ion fragmentation spectra were identified by the spectra of compounds supplied by the National Institute of Standards and Technology (NIST) database. The detailed GC−MS operating procedure can be found in the study by Le et al.13 The main functional groups of bio-oils were identified by Nicolet iS50 Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet, Waltham, MA, U.S.A.) within the wavenumber range from 4000 to 500 cm−1 at a resolution of 4 cm−1. The pyrolysis behavior of the bio-oil was measured by differential scanning calorimetry (DSC) and thermogravimetry (TG) analysis configuration (DSC−TG, Netzsch STA, 449F3, Selb, Germany). The baseline was calibrated by calibration sets before each experiment. Approximately 30 mg of the bio-oil was poured into a platinum crucible, placed into the analysis equipment, and then gradually heated from 30 to 900 °C at a heating rate of 10 °C/min. The protective gas and purge gas were helium at flow rates of 50 and 20 mL/min, respectively. Figure 2. DSC−TG−derivative thermogravimetry (DTG) thermal analysis of SS.

3. RESULTS AND DISCUSSION 3.1. SS Characterization. The characteristics of the dried SS are shown in Table 1 and Figure 2. According to the proximate analysis, the main components of the SS were volatile matter and ash. The percentage of volatile matter in the SS reached 50.52%, indicating that SS contains a large amount of organic matter and could be considered as an ideal material for liquefaction. Heavy metals, alkali metals, alkaline earth metals, and other inorganic elements, such as P, S, and N, were other main components of SS. The inorganic elements in SS could enter the bio-oil and reduce its quality of bio-oil. 3.2. Distribution of Liquefaction Products at Different Liquefaction Conditions. At the same temperature, the conversion ratio of SS with the different solvents follows the order of ethanol > methanol > acetone. The conversion ratios in acetone were always lower than those of other solvents at the same temperature. The polarity value of acetone (56.1) is lower than those of ethanol (65.4) and methanol (64.5), which decrease the decomposition of organic components.34 The effect of different solvents and temperatures on the distributions of SS liquefaction products is illustrated in Figure

3. The distribution of liquefaction products varied among different solvents. With an increasing reaction temperature, the liquefaction solid residue yields first decreased and then gradually stabilized with the three different liquefaction solvents. SS liquefaction could be divided into three stages: (1) decomposition of SS macromolecules into smaller molecules, such as lipids, carbohydrates, and proteins, (2) transformation of small molecules into active and unstable small fragments by cracking, dehydration, decarboxylation, and deamination, and (3) integration of small-molecule active fragments integrated into new compounds by condensation, cyclization, and polymerization.10 The bio-oil yield was highest at 360 °C, reaching 42.3 and 43.1% in ethanol and acetone solvent systems, respectively. At low temperatures, lipids were decomposed into small molecules and transformed into bio-oil by polycondensation. Moreover, proteins were present in the liquid phases as free radicals, forming oil by nucleophilic, electrophilic, and elimination reactions. When the temperature C

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Figure 3. Distribution of liquefaction products obtained from SS with different solvents at different temperatures.

increased, the organic components in the SS decomposed more violently and the secondary cleavage reaction became active. Bio-oil was produced from free radicals by esterification, cyclization, and polymerization. Ethanol has good solvent properties, and its capacity to supply hydrogen under subsupercritical conditions promoted the reaction of free radicals to form bio-oil.33−35 When methanol was used as a solvent, temperatures had little effect on the bio-oil yield, which was approximately 23%. As the temperature and pressure gradually increased, the carbon chains of the macromolecular proteins in SS began to break, resulting in the formation of macromolecular oils. However, as the temperatures and pressures continued to increase, methanol reached the supercritical conditions and the increased solubility of SS caused the heterogeneous reaction to become a single-phase reaction that resulted in the uniform dispersal of SS in methanol. This caused the remaining ester substances in the SS to rapidly dissolve and promoted the formation of some intermediate products, such as phenolic compounds. These compounds have lower boiling and freezing points and are easy to convert to gas phases. The reaction processes of SS and methanol under supercritical conditions involved repolymerization and polycondensation. The volatiles in SS were first released, generating a large amount of non-condensable and condensable gas, and the bio-oil was then formed by cracking, deoxidizing, and cyclizing. The bio-oil yield decreased with the temperature further increasing. The intermediate products and bio-oil produced by the esterification of methanol and SS may undergo two changes: the production of macromolecular insoluble substances produced by condensation and cyclization and the cracking of some light oil to form low-volatility smallmolecular substances with a low boiling point, such as H2, CH4 and CO2,35 resulting in a decreasing tendency for the yield of bio-oil. 3.3. Characteristics of Bio-oil Affected by Solvent Types. 3.3.1. Main Functional Group Analysis of Bio-oil. The functional groups of the bio-oils with different solvents at 340 °C are presented in Figure 4. The O−H stretching vibration at 3200−3600 cm−1 was attributed to the phenols and alcohols. The absorption peaks of the C−Hn stretching

Figure 4. FTIR spectrum of bio-oils obtained from SS in different liquefaction solvents.

vibration at 2800−3000 cm−1 were caused by alkanes. The stretching vibration between 1660 and 1760 cm−1 was caused by CO, implying the presence of esters, ketones, aldehydes, amine, etc. The CC stretching vibration centered at 1628 cm−1 indicated the existence of an aromatic skeletal structure. The complex weak symmetric and antisymmetric stretching vibrations at 1100−1200 cm−1 from C−O−C indicated the presence of large numbers of esters in the bio-oil samples. The strong absorbance peaks, observed at 935 cm−1, were due to the out-of-plane bending vibration from C−O−H. However, the intensity of these peaks was lower in the bio-oil produced from ethanol. This change indicated that ethanol effectively promoted the conversion during SS liquefaction processing. In addition, the absorbance peaks at 700−800 cm−1 were ascribed to the rocking vibration of C−H2 in bio-oil samples. 3.3.2. Ultimate and Heating Value Analyses of Bio-oil. C, H, O, N, and S can be concentrated and enriched in bio-oils, which were obtained through SS liquefaction. The ultimate D

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Energy & Fuels Table 2. Elemental Compositions and HHV of Bio-oils Obtained under Different Liquefaction Conditions elemental composition (%) solvent SS CH3CH2OH

CH3COCH3 CH3OH

temperature (°C)

N

C

H

S

O

O/C

H/C

HHV (MJ/kg)

280 300 320 340 360 360 360

5.30 7.01 6.92 6.73 7.56 6.78 5.96 7.28

31.50 68.01 69.56 71.10 73.25 74.59 76.83 71.33

5.02 9.48 9.43 9.37 9.49 9.72 9.35 9.36

1.20 0.00 0.52 0.55 0.59 1.52 0.000 0.00

21.27 15.50 13.57 12.26 9.10 7.40 7.86 12.03

0.51 0.17 0.15 0.13 0.09 0.07 0.08 0.13

1.91 1.67 1.63 1.58 1.56 1.57 1.46 1.57

12.71 27.10 27.50 27.89 28.55 29.08 29.53 27.95

and heating value analyses of bio-oils and effects of different temperatures and solvents are presented in Table 2. C and O were 31.50 and 21.27%, respectively. A low proportion of C and a high proportion of O contents result in dried SS with a relatively low HHV (12.708 MJ/kg), limiting the potential for applying SS as a fuel. The C content of bio-oil (68.01− 74.59%) significantly exceeds the C content of SS (31.50%). Moreover, high temperatures promoted the enrichment of C in bio-oil. In comparison to SS, the H content almost doubled and the O content notably decreased after liquefaction treatment within the temperature range of 280−360 °C when ethanol was used. The same results were observed for acetone and methanol. These results were caused by the enrichment of H and revealed that H resource formation and hydrogenation occurred during SS liquefaction, which resulted in the removal of O by H in the bio-oil to the gas or light oil phase and formed stable C−H bonds.36 This indicates that O was effectively removed after in situ hydrogenation by organic solvents, which potentially reduced the number of acids, aldehydes, and ketones and improved the HHV of bio-oil (27.10−29.53 MJ/kg) compared to that of SS (12.71 MJ/kg). N in bio-oil samples originated from the proteins in the SS, and its level remained stable (9.37−9.72%). An excessive N content would cause the emission of nitrogen oxides during bio-oil application, causing environmental pollution. Figure 5 compares the coalification band of SS and its liquefaction products to those of lignite, sub-bituminous coal, bituminous coal, and anthracite. Coalification was affected by

the pressures, temperatures, and time. The atomic ratios of hydrogen/carbon (H/C) and oxygen/carbon (O/C) from coalification were used to compare the hydrogenation, decarboxylation, and deoxygenation levels.14,37 As shown in Figure 5, the values of H/C and O/C significantly decreased by liquefaction with different solvents, which could be attributed to the dehydration, dehydrogenation, decarboxylation, and esterification reactions. The H resource originated from the H donor of the solvents. The occurrence of dehydration could be explained by the removal of hydroxyl groups from alcohols and phenols or esterification, which occurs at a relatively low temperature, decreasing the number of oxygen-containing functional groups and resulting in an O/ C low ratio. Decarboxylation was caused by the cracking of long-chain carboxylic acids by the high temperature and pressure. During liquefaction processing, formic acid was decomposed and large amounts of CO2, CO, and H2O were produced. Similar results have been reported in previous studies.14 The values of H/C and O/C could effectively indicate the fuel combustion characteristics. Solid fuels, such as coal, which has favorable combustion characteristics, have high H/C and atomic O/C low ratios. However, H/C atomic ratios of the produced bio-oils (1.46−1.57) were much higher than those of coals (0.3−0.85), and their O/C atomic ratios (0.07− 0.13) were between those of anthracite (0.02) and subbituminous coal (0.22). These results suggested that the biooils had a high HHV and can be considered as a good fuel. 3.3.3. Main Chemical Composition Analysis of Bio-oil. The main chemical components of the bio-oils (Figure 6) obtained at 360 °C with different solvents and product distributions were analyzed by GC−MS. It should be noted that the GC−MS results cannot provide the actual contents of the chemical components but can provide their distributions. As shown in Figure 6a, the bio-oils contained phenols, esters, acids, ketones, aldehyde, ether, hydrocarbon, alcohol, heterocyclic compounds, and N compounds and the liquefaction solvents significantly affected the chemical composition of the products. The bio-oils produced with different solvents contained large numbers of heterocyclic and N-containing compounds (16.38−28.15%). N mainly originated from the protein in SS.2 Other than the heterocyclic and N-containing compounds, acids, alcohol, and esters were also the main components of bio-oil with pure water as a solvent.5 However, the acids (23.12%) and alcohol (15.2%) in bio-oil produced using water decreased to 0.46 and 3.25% when ethanol was used, respectively. Similar results were observed for methanol. Furthermore, the ester content increased from 13.75 to 45.16 and 61.47% when ethanol and methanol were used, respectively. This indicates that the ethanol and methanol organic solvents promoted the esterification reaction between

Figure 5. Coalification band of bio-oils and SS in comparison to lignite, sub-bituminous coal, bituminous coal, and anthracite. E

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of saturated and unsaturated fatty acid esters increased from 3.1 to 16.19 and 19.60% and from 9.6 to 28.97 and 41.89%, respectively. In addition, ethanol and methanol promoted the formation of ethyl esters (43.55%) and methyl esters (61.05%), respectively. While the ester contents clearly increased when ethanol and methanol were used, the total amount of saturated fatty acid esters could be further improved. This suggests that more active H should be provided to promote the conversion of long-chain fatty acids via esterification and transesterification or enhance the conversion of unsaturated fatty acids to saturated fatty acids in alcohol solvent systems to achieve the feasible biodiesel production through SS liquefaction technology. The effects of different solvents on the distributions of nitrogenated compounds in bio-oils are illustrated in Figure 6c. The N compounds in bio-oil produced by pure water were heterocyclic compounds (3.32%), amides (9.31%), and other N compounds (6.23%). The content of heterocyclic compounds increased to 12.21, 18.24, and 12.6% when the bio-oil was produced with ethanol, acetone, and methanol, respectively. The amide contents remained stable, excluding that of bio-oil obtained with methanol. The contents of other N compounds in bio-oil obtained with ethanol, acetone, and methanol were notably lower than those of bio-oil obtained with pure water, indicating that some N compounds were converted to heterocyclic compounds or decomposed to gas, such as N2 and NH3. A high concentration of N compounds would reduce the quality of bio-oil. Therefore, studies on the removal of N compounds should be carried out to increase the application potential of bio-oil. 3.3.4. Formation Path and Reaction Mechanism of Bio-oil during SS Liquefaction. Considering the main chemical compositions from GC−MS results and possible liquefaction mechanism described in the literature,38,39 the possible formation path and reaction mechanism of bio-oil obtained in this study are presented in Figure 7. As shown in Figure 7a, the main organic components of SS are proteins, saccharides, and lipids, which were broke down to amino acids (glycine, glutamic acid, alanine, arginine, and valine), sugars (glucose, reducing sugar, and D-fructose), glycerol, and fatty acids via the hydrolysis reaction. Amines and fatty acids were produced as a result of amino acid deamination and decarboxylation reactions. Fatty acids or low carbon acid underwent decarboxylation or esterification reactions to form hydrocarbons (1-hexadecene, 1-eicosene, etc.) or aliphatic esters (diethyl succinate, ethyl laurate, ethyl myristate, ethyl palmitate, etc.), respectively. Fatty acids or low carbon acid could also undergo a cyclization reaction to form benzene compounds (benzofuran, 2,4-dimethylquinolin, 4-propylpyridine, etc.) or react with amine through an acylation reaction to form amide [N-(1-oxo-indan-5-yl)-acetamide and stearamide]. Reducing sugar or sugar underwent degradation to form cyclic oxygenates or a decomposition reaction to form furfural derivatives or low carbon acids. Amino acids also could react with monosaccharides through the Maillard reaction to form N-heterocyclic compounds, such as 1-methyl-2-pyrrolidinone, 2,3,5,6-pyridinetetramine, 2-piperidone, 3-methylindole, etc. CO2, CO, and NH3 produced from decarboxylation and deamination reactions thus increased the gas yield. Hydrocarbons, esters, amides, acids, benzene compounds, and N-heterocyclic compounds were regarded as bio-oil. Figure 7b was the typical components of bio-oils in ethanol, acetone, and methanol as solvents. The main reaction

Figure 6. Main components of bio-oils obtained in different solvents: (a) main components of bio-oils, (b) distribution of esters in bio-oils, and (c) distribution of N compounds.

alcohols and acids in the supercritical system. When acetone was used as a solvent, the ketone content was much higher than that when other solvents were used. Ketones were generated from the decomposition of carbohydrates, which were unstable under the liquefaction conditions. Figure 6b shows that the bio-oils obtained with ethanol and methanol were rich in esters. Fatty acid esters are an important parameter for evaluating biodiesel. As shown in Figure 6b, esters can be divided into unsaturated fatty acid esters and saturated fatty acid esters. The percentage of main fatty acid esters was 12.7% in bio-oil obtained from pure water: 2.3% was methyl esters, and 9.2% was ethyl esters. Ester significantly increased when ethanol and methanol were used. The contents F

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Figure 7. Possible formation path and reaction mechanism of bio-oil obtained in this study: (1) hydrolysis, (2) deamination, (3) decarboxylation, (4) decomposition, (5) esterification, (6) Maillard reaction, (7) cyclization, and (8) acylation. (a) Possible formation path and reaction mechanism of bio-oil. (b) Main components of bio-oil in different solvents.

pathways and mechanism were similar to those described in Figure 7a. The main reactions were hydrolysis, decarboxylation, ammonolysis, esterification, cyclization, Maillard reaction, and ketonization. The esterification reaction was enhanced as a result of the presence of ethanol and methanol. Ethyl esters, such as diethyl succinate, ethyl phenylacetate, ethyl laurate, ethyl 9-hexadecenoate, ethyl margarate, etc., were the main esters in ethanol. Methyl esters, such as dimethyl succinate, methyl phenylacetate, undecanoic acid methyl ester, 1,4-benzenedicarboxylic dimethyl ester, methyl cis-9-tetradecenoate, tetracosanoic acid methyl ester, etc., were the main esters in methanol. The keto acid decarboxylation reaction was enhanced by ketonization in acetone to form ketones, such as 2-methyl-2-penten-4-one, 2-pentadecanone, 3,4-diaminoanisole, 2-phenosyphenol, 3-methyl-2-cyclohexen-1-one, 2,5-di-

methylacetophenone, 2,5,8-trimethyl-1-tetralone, 5-phenyl-2pentanone, etc. 3.3.5. Analysis of the Pyrolysis Characteristics of Bio-oils by TG−DTG−DSC. The pyrolysis characteristics of bio-oils obtained using different solvents at 360 °C for 40 min were analyzed by DSC−TG, and the results are shown in Figure 8 and Table 3. The temperature significantly affected the stability of bio-oils, and they could not be completely decomposed at 900 °C. The amount of residue was 20.77, 11.23, and 3.07% for bio-oil produced with ethanol, acetone, and methanol, respectively. This indicates that the bio-oil produced using methanol had high small-molecule components, which are easily decomposed, and the bio-oil produced using ethanol had high macromolecular components, which require higher temperatures for decomposition. In addition, the repolymeriG

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Energy & Fuels

temperature increased, the TG curves descended gradually slowly, particularly above 500 °C. The epitaxial starting temperature (TE‑S) and epitaxial finishing temperature (TE‑F) with the best repeatability could be used to indicate the stability of bio-oils. On the basis of TE‑S and TE‑F, the bio-oil (TE‑S of 180.8 °C) obtained in methanol was more stable than those obtained in ethanol and acetone (TE‑S of 137.8 and 147.5 °C, respectively). The weight losses of bio-oils obtained in ethanol, acetone, and methanol between TE‑S and TE‑F were 66.77, 76.80, and 73.38%, respectively, indicating that most weight loss occurred in this stage. Figure 8b presents the thermogravimetric results obtained through DSC, which were caused by the endothermic decomposition and volatilization of bio-oils. Bio-oil decomposition could be described as the decomposition of organic components to CO, CO2, CHm, H2, and NH3 at a high temperature. A clear endothermic peak of the decomposition of small-molecular compounds or evaporation of water can be found at 35−130 °C. Furthermore, the main components of bio-oils (such as phenols, acids, alcohols, esters, and alkenes) began to decompose, and 70−80% of mass was lost between 130 and 400 °C. During this stage, the gas (such as CO, CO2, or CH4) may be reformed and absorbed.3 As the temperature further increased, some small endothermic areas could be observed on the DSC curves, which were caused by the decomposition of macromolecular components or repolymerization of the small-molecular compounds in the oil.34

4. CONCLUSION The bio-oils produced by ethanol, acetone, and methanol via the in situ hydrogen method were outstanding, because their yield and quality were improved. The H/C and O/C ratios in bio-oils decreased in comparison to those of SS, indicating that dehydration, dehydrogenation, decarboxylation, and esterification reactions occurred during SS liquefaction with different organic solvents. According to the FTIR and GC−MS results, esters, phenols, hydrocarbons, and N-containing heterocyclic compounds were the main products in the bio-oils obtained using ethanol and methanol, while a large amount of acetone existed in the bio-oil obtained from acetone. The values of H/ C and O/C indicated that O was removed by liquefaction, and the temperature significantly affected O removal, causing a higher HHV at 360 °C. The thermal stability of the bio-oil obtained in methanol was higher than that produced in ethanol and methanol, and 70−90% of the bio-oils were decomposed below 400 °C. Considering the bio-oil yield, HHV, and cost of solvents, ethanol was the best solvent for SS liquefaction among ethanol, acetone, and methanol.

Figure 8. DSC−TG−DTG curves of the bio-oils obtained at 360 °C.

Table 3. Distillate Range Percentage (%) of Bio-oils Obtained in Different Solvents at 360 °C solvent distillate range (°C)

carbon number

ethanol (%)

acetone (%)

methanol (%)

30−100 100−200 200−300 300−400 400−500 >500 remaining ash

C1−C9 C5−C16 C20−C25 C20−C50 C20−C50 >C50

0.27 20.50 32.09 17.97 7.08 1.32 20.77

0.2 19.58 40.49 20.93 6.84 0.72 11.23

0.59 15.09 44.71 27.61 7.85 1.02 3.07

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zation reactions lead to the formation of tar and coke during liquefaction processing, causing high residue percentages for bio-oil produced with ethanol and methanol. Thermogravimetric analysis (TGA) was used to assess the thermal stability of bio-oils. As shown in Figure 8a and Table 3, the bio-oil exhibited unique thermal stability with three stages. In the first stage of 30−100 °C, the bio-oil decomposed slowly as a result of the removal of moisture. The bio-oils obtained with ethanol, acetone, and methanol mainly decomposed within the temperature range of 200−400 °C, which could be regarded as the second stage, with the maximum of weight loss rates of 4.31, 5.07, and 5.12%/min at 238.8, 233.6, and 260.5 °C, respectively. In the final stage (>400 °C), as the

AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-024-89728889. Fax: 86-024-89724558. Email: [email protected]. ORCID

Rundong Li: 0000-0002-8669-5397 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was fully supported by the National Natural Science Foundation of China (51876131). Rundong Li extends deep gratitude to his family for their encouragement, H

DOI: 10.1021/acs.energyfuels.9b01434 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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his supervisor Professor Rundong Li for guidance, his coauthors for their effort that allowed him to finish this research, and the reviewers and editors for their valuable suggestions.

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NOMENCLATURE SS = sewage sludge HTL = hydrothermal liquefaction HHV = higher heating value H/C = hydrogen/carbon ratio O/C = oxygen/carbon ratio FTIR = Fourier transform infrared spectroscopy GC−MS = gas chromatography−mass spectrometry To = temperature for the onset of weight loss TE‑S = epitaxial starting temperature T5% = temperature at 5% weight loss T50% = temperature at 50% weight loss TE‑F = epitaxial finishing temperature TF = temperature at the end of weight loss Tmax = temperature corresponding to the maximum mass loss rate DTGmax = maximum weight loss rate

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DOI: 10.1021/acs.energyfuels.9b01434 Energy Fuels XXXX, XXX, XXX−XXX