Sewage sludge was collected from the Gangneung wastewater treatment facility [6]. Previously, we optimized the process of lipid extraction from sewage sludge for biodiesel production [6]. To save on organic solvents, we changed the extraction conditions; lipids were extracted twice from dried sewage sludge by incubating 8 mL/g-sludge with chloroform (99%; Wako, Japan) and methanol (99% purity, Showa, Japan) in a ratio of 1:2 (v/v) for 4 h at 70°C. The sewage sludge obtained after lipid extraction was saccharified to obtain sugars for bioethanol production by a treatment with acid (5.9% (v/v) at 120°C for 85 min), followed by a treatment with 500 μL of an enzymatic solution comprised of Celluclast (Novozyme, Denmark) and Viscozyme (Novozyme, Denmark) in a ratio of 1:2 (v/v) at pH 4.8 and 50°C for 12 h [15]. The remaining sewage sludge after lipid and sugar extraction was dried at 105°C for 24 h and used to obtain bio-oil by pyrolysis in this study. Carbohydrate content was determined by using the phenol-sulfuric acid method [16]. Crude protein content was determined by the Kjeldahl method using the Kjeldahl system (Buchi, Flawil, Switzerland). The moisture content of the sewage sludge was measured according to ASTM E 1756 [17]. The ash content of sewage sludge was measured according to ASTM E 1755 [17]. The C, H, N, and S content of sewage sludge were determined using an elemental analyzer (Flash EA1112, CE Instrument, UK), and the O content was estimated by subtracting the sum of C, H, N, and S content from 100% [17–18].

Thermogravimetric analysis (TGA) of the sewage sludge sample (12.0 ± 1.0 mg) was carried out using TGA equipment (Q50 Instrument, New Castle, USA). Nitrogen was used as the carrier gas at a flow rate of 25 mL/min. The temperature was elevated from 25 to 700°C at heating rates of 5, 10, 15, and 20°C/min. The differential rate of conversion, dX/dt, was determined by differentiating the conversion by time at different temperatures and heating rates.

The pyrolysis of the sewage sludge sample was carried out in a micro-tubing reactor that was tightly closed. The body of the apparatus consisted of a stainless-steel reactor that was 110 mm in length and 22 mm in diameter. The pyrolysis system (Fig. 1) consisted of a salt bath, temperature controller, mechanical stirrer, and micro-tubing reactor with an inner volume of approximately [18]. In each experiment, the tubing reactor was loaded with 2.0 g of the sewage sludge sample and placed into a molten salt bath 40 cm³. A molten salt bath, which has excellent heat transfer properties, was prepared with eutectic salt of KNO₃ (59%) and Ca(NO₃)₂ (41%) that had been pre-heated to the desired reaction temperature of 370, 380 or 390 ± 1°C. These pyrolysis temperatures were selected based on data from the differential thermogravimetric analysis (DTG) curve. Various reaction times ranging from 1 to 5 min were tested at each reaction temperature. After the desired reaction time at the giving temperature was reached, the reactor was immediately removed from the molten salt bath and cooled to the ambient temperature using cold water. After cooling for 60 min, the reactor was opened by removing the valve assembly to release the gas, and the oil and char fractions were collected [19]. The pyrolysis product distribution was determined by weighing the gas, oil and char products. The gas yield was obtained by weighing the micro-tubing reactor before and after the release of gas. The mixture of char and bio-oil were separated into bio-oil (acetone-soluble) and char (acetone-insoluble) using a micro filter paper (pore size: 0.45 um), with a vacuum pump after solvent extraction with acetone. After separated, the char were dried in an oven at 90°C for 60 min and cooled to the ambient temperature in exsiccator. After that, the char yield was obtained by weighing the sample before and after reaction. Each experiment was repeated three times, and the average data were used for estimating yields. The product yields were calculated according to the following equations [18].

The analysis of bio-oil composition was performed using GC-MS (7890A, Agilent, USA) with a capillary column of HP-5MS (30 m × 0.25 mm × 0.25 μm). Highly pure helium was used as a carrier gas with a flow rate of 1.0 mL/min. The temperature of the GC injector was kept at 280°C and the injection volume was 1 μL. The GC oven was programmed from an initial temperature of 40°C to 200°C with a heating rate 5°C/min; the temperature was maintained for 5 min, and then increased to 300°C at a heating rate of 10°C/min. This temperature was maintained for a duration of 5 min [18].

Kinetic parameter estimation was carried out through a nonlinear least-squares regression of the experimental data using Sigma Plot (Ver.10, Systat software, Inc, USA). The reaction rate constants k₁, k₂, and k₃ that were obtained for gas, liquid, and solid products at three different reaction temperatures are shown in Table 3.

The composition of raw sewage sludge (RSS), sludge after lipid extraction (SALE), and sludge after lipid and sugar extraction (SALSE) were measured and compared, as presented in Table 1. RSS consisted of 14.5% lipids, 8.9% carbohydrates, 42.8% proteins, 32.1% ash, and 1.7% moisture. After lipid extraction for biodiesel production, the compositions of lipids, carbohydrates, proteins, ash, and moisture were 2.6%, 8.9%, 52.2%, 35.3%, and 1.0%, respectively. Notably, the lipid content was significantly decreased from 14.5% to 2.6% while carbohydrates and proteins were barely extracted. The residual sewage sludge was further utilized for sugar extraction using acid pretreatment, followed by enzyme pretreatment for bioethanol production. After lipid and sugar extraction, the residual sewage sludge mainly contained 56.1% proteins, 2.3% lipids, 1.0% carbohydrates, 38.9% ash, and 1.7% moisture. To achieve the complete utilization of sewage sludge, SALSE was pyrolyzed to obtain valuable bio-oil.

Table 2 shows that the C, H, O, N, and S content of SALSE was 47.8%, 6.5%, 38.7%, 5.6%, and 1.5%, respectively. According to the Demirbas method [18, 20], the higher heating value (HHV) of biomass could be calculated using the following equation [8, 18]:

Here, [C], [H], [O], and [N] represent the concentrations (wt%) of carbon, hydrogen, oxygen, and nitrogen, respectively. The HHV is strongly dependent on the carbon and oxygen content of the feedstock; high carbon and low oxygen content result in an increase in the HHV. The HHV of SALSE was 18.51 MJ kg⁻¹, which was 35.7% and 20.2% higher than those of sewage sludge in studies by Ma et al. [21] (13.64 MJ kg⁻¹) and Calvo et al. [22] (15.4 MJ kg⁻¹), and higher than that of the microalgae sample (12.11 MJ kg⁻¹) [23], indicating its feasibility as a source of energy [24].

The TGA of SALSE can be expressed as a function of the conversion factor X, which is defined as follows [23–24]:

Here, W₀ is the initial mass of SALSE, W is the mass of pyrolytic SALSE at time t, and W_∞ is the final residual mass after TGA. The degree of conversion versus temperature of the sewage sludge in TGA at different heating rates of 5, 10, 15, and 20°C/min are shown in Fig. 2(a). The formation of a peak at a temperature lower than 200°C could be attributed to the vaporization of moisture attached to the sample surface [8, 25–26]. However, in this study, a few peaks appeared below 200°C because SALSE was well dried, which was consistent with the fact that the moisture content of SALSE was as low as 1.68%. The conversion curves at different heating rates of 5, 10, 15, and 20°C/min showed a similar behavior, and the height of the curves started to increase rapidly in the temperature range of 200 to 400°C. However, the conversion curves were shifted toward the right with an increase in the heating rate, which was because of a higher instantaneous thermal energy introduced into the reaction system at a higher heating rate, resulting in a higher conversion rate [23].

The DTG curves of SALSE at different heating rates are shown in Fig. 2(b). The DTG curve at each heating rate has two peaks between 200 and 550°C (peaks were around 337.0 and 379.3°C), indicating the decomposition of the main components (lipids, carbohydrates, and proteins) in the SALSE [27]. The figure shows that the maximum rate of conversion or decomposition tends to increase with an increase in the heating rate, possibly because a high heating rate provides more energy for heat transfer into the sewage samples [28]. The maximum points of the DTG curves occurred at 284, 301, 317, and 334°C at the heating rates of 5, 10, 15, and 20°C/min, respectively. Previous studies report that the peaks between 200 and 350°C could be attributed to the decomposition of carbohydrates, lipids, and proteins, while those below 200°C could be attributed to the vaporization of water and some very light volatile compounds [25, 27, 29–30].

Studies regarding TGA graphs contribute to the enhancement of knowledge of the kinetics of this thermal process, and therefore, to the establishment of the optimal operational conditions for effective utilization of sewage sludge [31]. The TGA graphs were analyzed to determine the Arrhenius parameters (activation energy and pre-exponential factor) of sewage sludge. The activation energy is the minimum energy required to start a reaction, and a reaction with low activation energy takes place more easily than a reaction with high activation energy [32]. The pre-exponential factor, also known as the frequency factor, indicates the number of collisions between two molecules. The differential method was used to determine the kinetic parameters of pyrolysis from the thermogravimetric data [14, 23–24]. The rate of conversion, dX/dt, during thermal decomposition is expressed as [28],

The temperature-dependent reaction rate constant k is expressed by the Arrhenius equation;

Here, A, Ea, R, and T represent the pre-exponential factor, activation energy, universal gas constant, and temperature, respectively. The temperature-independent conversion function, f(x), is expressed as,

Substituting Eq. (7) and (8) into Eq. (6) and taking the natural logarithm, the above equation yields

Here, A, n, T, and R represent the pre-exponential factor (s⁻¹), reaction order, pyrolysis temperature (K), and gas constant (8.314 J/mol⁻¹K⁻¹), respectively.

The activation energy, E (kJ/mol), based on Eq. (9), can be determined from the slope of -E/R based on a plot of 1/T vs ln(dX/dt). The intercept ln(A.X_n) was obtained from Eq. (9) for each conversion, from 10 to 90%. The pre-exponential factor (A) could be obtained by assuming that the order of reaction (n) is 0, 1, or 2 [17]. Table 2 shows that the activation energies had similar values of 67.0, 71.9, and 81.7 kJ mol⁻¹ at conversions of 10, 20 and 30%, respectively, until a conversion of 40%, where the activation energies increased to 101.7 kJ mol⁻¹. This may be due to the decomposition of carbohydrates [17, 23]. After that, the activation energy decreased slowly with a 40% to 70% increase in the conversions. When the conversions increased from 70% to 90%, the activation energy increased sharply because of protein degradation [1, 17].

The results in this study are similar to those reported by Li et al. [7], where the activation energies and pre-exponential factors of sewage sludge were in the range of 59.6–150.3 kJ mol⁻¹ and 1.6 × 10⁷ – 6.0 × 10¹¹ min⁻¹, respectively. When compared with the activation energy for the pyrolysis of palm fiber (192.7 to 689.0 kJ/mol for the conversion of 10 to 90%), SALSE could be degraded much more easily than palm fiber [24]. Table 2 shows the pre-exponential factors calculated from Eq. (9), assuming the reaction to be of the 0^th, 1^st or 2^nd order. The pre-exponential factors increased with an increase in the heating rate. Most of the decomposition occurred at the conversions from 20% to 80%, where the pre-exponential factors were in the range of 10⁰–10⁵ s⁻¹.

The effect of the temperature (370, 380, and 390°C) and reaction time (1 to 5 min) used for pyrolysis on the product yield was investigated. Fig. 3 shows the yield of bio-oil, gas, and char as a function of the pyrolysis conditions for SALSE. The yields of bio-oil and gaseous products were increased with an increase in the reaction temperature and reaction time. The bio-oil yield increased from 8.8 to 30.9% at 370°C from 1 to 5 min and increased to a maximum of 10.1 to 33.3% at 390°C from 1 to 5 min. The gas yield increased from 3.9 to 16.6% at 370°C from 1 to 5 min and increased to a maximum of 2.48 to 17.7% at 390°C from 1 to 5 min. The higher bio-oil yield at higher temperatures is mainly attributed to the enhancement of devolatilization reactions at higher temperatures, because of more energy being available to break the bonds of organic compounds in the sludge [33]. However, the proportion of solids decreased with an increase in the reaction temperature and reaction time. The char yield of SALSE was decreased from 100 to 56.0% at a reaction temperature of 390°C and reaction time of 5 min, while a char yield of 87.3% was obtained at 370°C in 1 min.