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Valorization of sewage sludge through catalytic sub- and supercritical water gasification Mi Yan a, Dwi Hantoko a, Ekkachai Kanchanatip a, b, Rendong Zheng c, Yingjie Zhong a, Ishrat Mubeen a, * a b c
Institute of Energy and Power Engineering, Zhejiang University of Technology, Hangzhou, 310014, China Center of Excellence in Environmental Catalyst and Adsorption, Faculty of Engineering, Thammasat University, Pathumthani, 12120, Thailand Hangzhou Linjiang Environmental Energy Co. Ltd., Hangzhou, 311222, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2019 Received in revised form 31 December 2019 Accepted 6 January 2020 Available online xxx
Sub- and supercritical water gasification is applied to recover energy from sewage sludge in a batch reactor. The effects of reaction temperature and water-soluble additives as catalysts on gasification were examined. The resultant products, including syngas, hydrochar and liquid residues were characterized. The rise of temperature without the presence of catalysts increased the yield of H2 (0.06 (350 C) to 1.91 mol/kg (450 C) and enhanced the gasification efficiency (1.29e19.61%), and decreased total organic carbon (TOC) by 68.50% in liquid residue. The changes in product distribution and characteristics of hydrochar and liquid residue implied that the organic matters in sewage sludge were dissolved and hydrolyzed in sub- and supercritical water, resulting in the production of syngas. The catalytic effect of different catalysts in relation to the H2 gas yield was in the following order: KOH > NaOH > Na2CO3 z K2CO3. In the case of catalytic supercritical water gasification at 400 C, the highest molar fraction (37.28%) and yield of H2 (1.60 mol/kg) were obtained in the presence of KOH. Furthermore, the scanning electron microscopy (SEM) analysis indicated that a conversion and dissolution of the organic matters in sewage sludge to liquid and gas, produced a porous, fragmented structure and disintegrated surface of hydrochar. © 2020 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Sewage sludge Supercritical water gasification Alkali catalyst Total organic carbon Hydrochar
1. Introduction Rapid growing of human population and urbanization together with the decline of fossil fuels are pursuing to look for alternative fuels to fulfill the energy demand. Recently, sewage sludge resulting from wastewater treatment processes have gained much attention due to its potential energy recovery. Sewage sludge is a heterogeneous mixture of organic and inorganic materials, consisting of carbohydrates, lipids, proteins and high ash content, including heavy metals that are harmful to the environment [1]. In China, the production of sewage sludge was approximately 30 million tons in 2015 and this will exceed 60e90 million tons by 2020 [2]. Therefore, more attention has been paid to the utilization of sewage sludge, not only due to the challenge in energy demand but also because of the environmental issues related to conventional sewage sludge treatment. Conventional treatment of sewage sludge
* Corresponding author. E-mail address:
[email protected] (I. Mubeen).
such as landfill, composting, land application and incineration may generate secondary pollution (e.g., odor, leachate, NOx and SOx emission) to the water and air [3]. Supercritical water gasification (SCWG) seems to be a more suitable technique to deal with sewage sludge which has high moisture content (80e90 wt%). Sewage sludge can be gasified without prior drying into high value syngas (mainly H2, CH4, CO2 and CO) under supercritical water (374.3 C and 22.1 MPa). Owing to the special properties of supercritical water (e.g., high diffusivity, low viscosity, low density, and low dielectric constant), water becomes excellent solvent which can improve the solubility of organic substances and gases, thus the homogenous condition for gasification can be achieved [4e6]. The high reactivity of supercritical water can enhances the decomposition rate of polymeric structure of biomasses or wastes and increases the H2 yield in syngas [7]. In addition, minimal resistance to inter-phase mass transfer exists during SCWG [8]. A wide variety of feedstock materials (containing lignocellulose, fatty acids, proteins and lipids) can be gasified in sub- and
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supercritical water [5]. SCWG was also found to give significant environmental merits on the treatment of municipal and animal wastes, e.g., odor elimination, BOD removal, and pathogen kill [9,10]. Moreover, a number of researchers have also applied SCWG to treat polluted wastewater, thus the treated wastewater may be safely released to the environment with low COD and TOC levels [11e13]. According to the previous work, there are three operating temperature ranges for SCWG, namely subcritical water gasification (<374 C), low temperature SCWG (374550 C), and high temperature SCWG (550e700 C) [14]. No catalyst is required for complete SCWG at high temperature, leading to the production of H2-rich syngas. Meanwhile, the methane-rich syngas would be produced at lower SCWG temperature (374e550 C) and the addition of catalyst is necessary to complete SCWG [15]. Numerous studies have shown that the use of both homogenous alkali metal catalysts (e.g., KOH, NaOH, K2CO3, and Na2CO3) and heterogeneous metal-based catalysts (e.g., Ni, Ru, Rh, Cr, Pt, Zn, Pd, Ti, and Mo) can significantly improve the SCWG reactions [5,12,16]. Among them, the alkali metal catalysts are widely used in batch or continuous SCWG system since they can dissolve in water and further enhance water-gas shift reaction, resulting in higher H2 yield and gasification efficiency [16e20]. They are relatively low cost compared with metal-based catalysts. Additionally, alkaline assisted supercritical water process plays a significant role on the stabilization of heavy metals in medical waste, thus the leachability of heavy metals can be reduced [21]. Muangrat et al. [16,20] found that the addition of alkali catalysts promoted hydrothermal gasification of food processing waste and improved the H2 yield by accelerating water-gas shift reaction via the formation of formate salts. Schmieder et al. [17] gasified model and real biomass at 600 C and 25 MPa in the presence of KOH or K2CO3 and produced H2-rich gas with CO2 as the main carbon compound. Xu et al. [22] investigated gasification of glycine and glucose as model compounds of protein and fat in sewage sludge, respectively, at 380e500 C and 25 MPa with and without Na2CO3 catalyst. They reported that Na2CO3 catalyst increased the H2 yield and COD destruction. In general, their reports showed that alkali-catalyzed SCWG was feasible for converting wet biomass or organic wastes containing carbohydrates, proteins, or fats to H2-rich syngas. The aim of this paper is to investigate the thermal decomposition of sewage sludge in sub- and supercritical water in a batch reactor. Compared to previous researches, in this study simultaneously all products (gas, solid and liquid) were collected for the
measurement. The comprehensive characterization of all the products was carried out to understand the organic matters conversion. The experimental results obtained in this work will be helpful to demonstrate the suitability and feasibility of specific organic material as a feedstock for SCWG. A comprehensive characterization of sewage sludge and its hydrochar was performed for a comparative evaluation of their physicochemical properties. In addition, the potential energy recovery of SCWG of sewage sludge in the form of syngas was also investigated. The effect of temperature (350e450 C) on the product distribution, gas yield and gas composition was assessed. Furthermore, detailed attention was also given to explore the effect of different alkali catalysts during SCWG of sewage sludge at low temperature (400 C). It is expected that SCWG can be an alternative technology to solve the problem on the disposal of sewage sludge as a solid waste from the wastewater treatment plant. 2. Materials and methods 2.1. Sewage sludge Sewage sludge was obtained from a wastewater treatment plant, Hangzhou, Zhejiang Province, China. Prior to the experiment, sewage sludge was dried in oven at 105 C and then grounded and sieved through a 100 mm sieve. The characteristics of sewage sludge are presented in Table 1. The higher heating value (HHV) of sewage sludge and its hydrochar was calculated according to empirical equation as proposed by Sheng and Azevedo [23]. Potassium hydroxide (KOH, purity: 99.99%) and sodium hydroxide (NaOH, purity: 99.99%) were purchased from Hangzhou Xiaoshan Chemical Reagent Factory, while potassium carbonate (K2CO3, purity: 99.99%) and sodium carbonate (Na2CO3, purity: 99.99%) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd, and Taicang Meida Reagent Co., Ltd, respectively. 2.2. Experimental procedure The SCWG experiment was carried out in a 500 ml Hastelloy batch reactor which can operate up to 500 C and 30 MPa. The detailed schematic diagram of the system could be found in our previous works [12,24]. The reactor was equipped with PIDcontrolled electric furnace, stirrer, water cooling system and pressure relief valve. Temperature and pressure were measured using
Table 1 Proximate and ultimate analyses of sewage sludge and its hydrochar. Parameter
Sewage sludge
Proximate analysis (wt%-air dry basis) Moisture 3.67 Fixed carbon 0.78 Volatile matter 23.97 Ash 71.57 Ultimate analysis (wt%-dry basis) C 14.13 H 0.81 Oa 10.49 N 2.32 S 0.67 Atomic ratio H/C 0.69 O/C 0.56 N/C 0.14 HHV (MJ/kg) 3.97 LHV (MJ/kg) 3.72
SS-350
SS-400
SS-450
0.82 1.52 5.16 92.5
0.29 1.04 5.52 93.15
0.46 1.31 5.01 93.23
6.06 0.03 0.37 0.53 0.51
5.34 0.04 0.66 0.41 0.40
4.83 0.03 1.10 0.41 0.41
0.06 0.05 0.08 0.57 0.54
0.08 0.09 0.07 0.35 0.34
0.08 0.17 0.07 0.21 0.19
a O (wt%) ¼ 100 (C þ H þ N þ S þ ash). Note: the heating value was calculated according to the empirical equation [23].
Fig. 1. Product distribution of SCWG at different temperatures.
Please cite this article as: M. Yan et al., Valorization of sewage sludge through catalytic sub- and supercritical water gasification, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2020.01.004
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Fig. 2. Effect of temperature on gas yield.
Fig. 4. Effect of temperature on CGE, CE, and HE.
K-type thermocouple and pressure gauge. In each experiment, the feedstock including 108 g deionized water and 12 g sewage sludge was loaded into the reactor. Then, the gas tightness of the system was ensured. Before the reaction, the reactor was purged with high purity nitrogen to ensure an oxygen-free condition. Afterwards, the reactor was heated to a set temperature and pressure at about critical point. Once the set temperature was reached, the reaction was maintained for 60 min under continuous stirring at 80 rpm. After the completion, a cooling water was used to cool down the reactor to room temperature before sampling the produced gas for further analysis. The volume of produced gas was measured by the displacement of saturated NaHCO3 solution. Subsequently, the reactor was opened to collect the liquid and hydrochar. The hydrochar was separated using a vacuum pump through 0.45 mm PVDF membrane and then oven-dried overnight at 105 C.
packed column with the length of 2 m and internal diameter of 3 mm. The carrier gas was high purity helium (99.99%) with the flow rate of 30 ml/min. The temperature of the injection port, column, and TCD were set at 80 C. TOC Analyzer (Shimadzu TOC-V CPN, Japan) was used to analyze total organic carbon (TOC) of the liquid residue. Functional groups of sewage sludge and hydrochar were characterized by fourier transform infrared (FTIR) spectroscopy (Nicolet 6700 FT-IR spectrometer, Thermo Scientific, USA). The surface morphology of sewage sludge and hydrochar was examined on a Hitachi Se4700II scanning electron microscope (SEM). Thermogravimetric analysis (TGA) of sewage sludge and its hydrochar was conducted in a TGA analyzer (TA Instruments SDT 2960, USA). The mineral composition of sewage sludge was analyzed by X-ray fluorescence spectrometer (ARL ADVANT’X Series, Thermo Scientific, USA).
2.3. Analytical procedure The composition of produced syngas was analyzed using gas chromatograph (Fuli GC-9790, China) equipped with a thermal conductivity detector (TCD) and carbon molecular sieve TDX-01
Fig. 3. Effect of temperature on syngas composition.
2.4. Parametric study Several parameters such as syngas composition, total syngas yield, cold gas efficiency (CGE), carbon efficiency (CE), and hydrogen efficiency (HE) were used to evaluate the performance of SCWG of sewage sludge. The total syngas yield, CGE, CE and HE were calculated by using Eq. (14), respectively.
Fig. 5. Effect of alkali catalysts on gas yield at 400 C.
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Fig. 7. Effect of alkali catalysts on CGE, CE and HE at 400 C.
Fig. 6. Effect of alkali catalysts on syngas composition at 400 C.
Total syngas yield ð
mol total moles of syngas Þ¼ kg total mass of sludge
(1)
total volume of syngas LHVsyngas 100 mass of sludge LHVsludge
(2)
CE ð%Þ ¼
mole of carbon atom in syngas ðmolÞ 100 mole of carbon atom in sludge ðmolÞ
(3)
HE ð%Þ ¼
mole of hydrogen atom in syngas ðmolÞ 100 mole of hydrogen atom in sludge ðmolÞ
(4)
CGE ð%Þ ¼
LHVsyngas ¼
X yi LHVi
(5)
where: yi , LHVsyngas (MJ/Nm3), and LHVsludge (MJ/kg) stand for mole fraction of an individual gas, the lower heating value of syngas and, the lower heating value of sewage sludge, respectively. The LHV of syngas was estimated based on the mole fraction of a certain gas and heating value of the individual gases in syngas (LHVi), as mentioned in Eq. (5).
compared to 1.93%, 8.31% and 4.03% at 350 C, respectively (Fig. 4). Therefore, it can be concluded that higher reaction temperature promotes the production of H2-rich syngas production and enhance the gasification efficiency. Among the other chemical reactions involved in SCWG, steam reforming (Eqs. (6)e(8)), water-gas shift reaction (Eq. (9)), and methanation (Eq. 10 and 11), are taken into account in the change in syngas composition [24]. Steam reforming reaction (Eqs. (6)e(8)) is greatly endothermic which require energy to complete the reaction toward the products. Based on Le Chatelier principle, steam reforming reaction would be promoted to produce syngas with high H2 content with the rise of temperature [25]. On the other hand, increase in temperature inhibited water-gas shift reaction owing to mildly exothermic nature. The rate of water-gas shift reaction slowed down as temperature increases [26]. It can be seen that the fraction of H2 obviously increased with increasing temperature, particularly when the temperature was higher than subcritical condition (350 C) (Fig. 3). It suggested that the steam reforming reactions (Eqs. (6)e(8)) and water-gas shift reaction (Eq. (9)) altered to be dominant, which contributed to produce more H2 in syngas. Meanwhile, the increase of CH4 mole fraction was due to the consumption of CO and CO2 by methanation reactions (Eq. 10 and 11). Steam reforming reactions
3. Results and discussion
CxHyOz þ (x-z)H2O / xCO þ (x-z þ y/2)H2 Endothermic
(6)
3.1. Effect of temperature and alkali catalysts on gas production
CxHyOz þ (2x-z)H2O / xCO2 þ (2x-z þ y/2)H2 Endothermic
(7)
The effect of temperature and alkali catalysts on gas production was evaluated in term of syngas yield, syngas composition and SCWG efficiency. The increase in temperature from 350 C to 450 C led to a rise in the syngas yield, while relatively higher solid product (hydrochar) was achieved at lower temperature (10.20 g), which gradually decreased with increasing temperature; 8.36 and 8.04 g were achieved at 400 and 450 C, respectively and Fig. 1 shows the products distribution. It implied that organic matters in sewage sludge were mostly dissolved and hydrolyzed during SCWG resulting in the production of syngas. As the temperature raised from 350 C to 450 C, the H2 yield increased from 0.06 to 1.91 mol/kg, respectively (Fig. 2). Accordingly, the total gas yield increased from 1.04 mol/kg (350 C) to 4.86 mol/kg (600 C). The H2 concentration significantly increased with the increase of temperature from 6.00% (350 C) to 39.27% (450 C), as shown in Fig. 3. Higher CGE, CE, HE were 31.82%, 25.06% and 90.97% at 450 C,
C þ H2O / CO þ H2 DH ¼ þ131 kJ/mol
(8)
Water-gas shift reaction CO þ H2O 4 CO2 þ H2 DH ¼ 42 kJ/mol
(9)
Methanation reaction CO þ 3H2 4 CH4 þ H2O DH ¼ 206 kJ/mol
(10)
CO2 þ 4H2 4 CH4 þ 2H2O DH ¼ 165 kJ/mol
(11)
Moreover, higher reaction temperature assisted the free-radical reactions, thus the ring-cleavage reactions can be enhanced to produce hydrocarbons with low molecular weight, and leading to
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the production of syngas [25]. The free-radical mechanisms could facilitate the dissolution of organic compounds in sewage sludge and further decomposition of their intermediates to gases [24]. The effect of four different alkali catalysts including two carbonate catalysts (K2CO3 and Na2CO3) and two hydroxide catalysts (KOH and NaOH) on the SCWG performance was studied at 400 C, 10 wt% sludge concentration, and 6 wt% catalyst loading. Previous study suggested that the addition of 6 wt% of alkali catalyst optimally sufficient for sewage sludge gasification experiment [27]. An increase in gas yield was observed in the presence of alkali catalysts, especially H2 yield (Fig. 5). This result was similar to those obtained from previous studies, which revealed that alkali catalyst could enhance the water-gas shift reaction (Eq. (9)) through formate salts such as sodium formate and potassium formate as intermediate products [16,18,19,28]. Additionally, the alkali salts could also disrupt flocs and cells in the sludge, resulting in the release of inner organic matters and increase the amount of soluble organic matter in the water [29,30]. Afterwards, the dissolved organic matters were further gasified in supercritical water to produce syngas. Among all the catalysts used, KOH catalyst was more effective to improve the H2 yield (1.60 mol/kg) which was around 37% of the total produced syngas (Fig. 6). Compared to noncatalytic SCWG, H2 yield increased by ~28% and ~26% with the addition of KOH and NaOH, respectively. This result indicated that the catalytic effect of Kþ was larger than that of Naþ. Likewise, the addition of KOH and NaOH increased the CGE and HE, caused by the increase of H2 yield in comparison with non-catalytic SCWG (Fig. 7). It has been reported that complete gasification of model and real organic wastes in supercritical water can be achieved in the presence of KOH as a catalyst at 600 C and 25 MPa, resulted in H2-rich syngas containing CO2 as the main carbon compound [17]. The addition of KOH not only enhanced the breakage of chemical bonds to produce low-molecular weight gases, but also promoted the gasification reaction by altering the pore structure [31]. The use of strong base catalysts could promote the degradation of lignin in biomass into smaller monomers [32]. Furthermore, previous observations showed that the NaOH catalyst played two important roles during SCWG; firstly, decomposition of biomass to gasifiable intermediates; secondly, the enhancement of water-gas shift reaction to produce H2 by capturing CO2 to form metal carbonates [20,33]. The presence of KOH and NaOH in SCWG played a role on the CO2 fraction in syngas (Fig. 6). According to Eq. (12) and Eq. (13), the addition of hydroxides of alkali catalysts might react with CO2 and further formed metal carbonates. Two mol of MOH could fix up to 1 mol of CO2 during the reaction. However, the removal of CO2 was basically responsible for shifting the water gas shift reaction to enhance H2 yield. In other words, the metal carbonates could not undergo the reaction for capturing CO2, it could only be a CO2-exchange instead of CO2 capture [16]. It can be further confirmed by the high CO2 fraction in syngas when carbonate alkali catalysts
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Fig. 8. FTIR analysis of sewage sludge and hydrochars at different SCWG temperature.
(K2CO3 and Na2CO3) were used (Fig. 6). The higher CO2 yield lead to the higher CE compared to those data obtained from SCWG with the addition of hydroxide alkali catalysts (Fig. 7). 2MOH þ CO2 4 M2CO3 þ H2O
(12)
M2CO3 þ CO2 þ H2O 4 2MHCO3
(13)
where: M ¼ Na or K. In the case of carbonate alkali catalysts, H2 yield of 1.44 mol/kg was obtained from SCWG of sewage sludge with the addition of Na2CO3 and K2CO3, respectively. Those two carbonate catalysts enhanced H2 yield via water-gas shift reaction and produced formate salt (HCOOM) as an intermediate product (Eq. (14) - (15)). Furthermore, the format salt reacted with water to produce H2 and salt bicarbonate (MHCO3) (Eq. (16)). Afterwards, CO2 and M2CO3 will be released by thermal decomposition of salt carbonate (Eq. (17)). It can be seen from the increase in H2 and CO2 yield when K2CO3 and Na2CO3 were applied as catalyst, as shown in Fig. 5. M2CO3 þ H2O / MHCO3 þ MOH
(14)
MOH þ CO / HCOOM
(15)
HCOOM þ H2O / MHCO3 þ H2
(16)
2MHCO3 / H2O þ M2CO3 þ CO2
(17)
From the obtained results, the effectiveness of catalysts with respect to the H2 yield decreased in order of KOH > NaOH > Na2CO3 z K2CO3. Similar sequence on the catalytic activity was also observed from other researchers, showing that
Table 2 Mineral composition of sewage sludge and selected biomasses. Ash compounds
Sewage sludge
Wood ash [42]
Empty fruit bunch [43]
Wheat straw [39]
SiO2 Al2O3 Fe2O3 K2O Na2O CaO MgO TiO2
46.84% 21.73% 6.15% 1.50% 0.39% 19.61% 2.71% 1.07%
11.07% 2.54% 10.52% 7.67% 0.00% 51.95% 15.52% 0.74%
44.84% 1.07% 1.98% 24.72% 4.81% 12.59% 9.87% 0.12%
68.27% 0.91% 0.57% 19.26% 0.57% 8.32% 2.05% 0.05%
Note: Data is presented in weight percentage (wt%) with modification (normalized to 100%).
Please cite this article as: M. Yan et al., Valorization of sewage sludge through catalytic sub- and supercritical water gasification, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2020.01.004
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Fig. 9. (a) Thermogravimetric and (b) differential thermogravimetric analyses of sewage sludge and hydrochars derived at 350e450 C.
metal hydroxides produced higher H2 gas yield than metal carbonates [18]. It has been reported that hydrocyl radicals (,OH) from KOH was very reactive in supercritical water, which had strong oxidability and could enhance the gasification of aromatics of lignite [34]. In addition, higher absorption capacity of hydroxide alkali catalysts than carbonate alkali catalysts resulted the different inflection point in the yield of CO2 [27]. Muangrat et al. [16] revealed the presence of hydroxide alkali catalysts provided more highly basic medium which could increase the production of H2 yield due to a higher possibility of formate salt formation. The different catalytic effects might also be due to the different solubility, different dissociation constant and density and different molecular weights among these four selected alkali catalysts in supercritical water. 3.2. Effect of temperature on hydrochar The physicochemical properties, FTIR analysis, thermogravimetic analysis and SEM analysis were conducted to evaluate the effect of temperature on hydrochar. The proximate and ultimate analyses of sewage sludge and its hydrochars are presented in Table 1. The hydrochars produced at 350, 400, and 450 C are denoted as SS-350, SS-400, and SS-450, respectively. Compared to raw sewage sludge, the moisture content and volatile matter of the hydrochars decreased after SCWG process, while fixed carbon and ash content apparently increased. However, no clear trend was
observed with the increase of temperature. It might be explained by the high ash content in sewage sludge (71.57%) which gave adverse effect for the functionalization of carbonaceous materials, as mentioned by Zhang et al. [35]. However, a sufficient amount of mineral content in ash could provide some catalytic effects on the performance of gasification [36]. The increased ash content was primarily due to the decomposition and transformation of organic matters through different reactions, such as dehydration, hydrolysis, deamination, decarboxylation and gasification [23]. On the other hand, a significant effect of temperature on the increase of fixed carbon in hydrochar was found during SCWG of lignocellulosic biomass [19,37]. The stable and fixed carbon structure was formed in hydrochar because the removal of volatile matter at high temperatures. Except O, all of the elements including C, H, N, and S were found to decrease when the temperature increased. There was nearly 57% and 66% decrease in fixed carbon content in the hydrochar at 350 C (SS-350) and 450 C (SS-450) compared to raw sewage sludge, respectively. It indicated that the major organic carbon in sewage sludge was converted into syngas. Likewise, some organic carbon was dissolved in the liquid residue. The atomic ratios of hydrochar were reduced and significantly lower than that of sewage sludge owing to several reactions such as dehydration, decarboxylation, dehydrogenation, and decarbonylation of organic components. The mineral composition of sewage sludge and other biomasses is shown in Table 2. Considerable quantities of SiO2 (46.84%), Al2O3 (21.73%), CaO (19.61%), and Fe2O3 (6.15%) were found in sewage sludge. Sewage sludge is produced by wastewater treatment plant which receives polluted wastewater from several sources including domestic community, industrial, hospital, and agricultural activities. This various mixture of components generates sewage sludge with fairly heterogeneous in its organic and inorganic constituent. The major portion of SiO2 in sewage sludge used in this work might be caused by the longer rainy season in Hangzhou which carry more sand and soil in the wastewater. Although the excess ash content gives adverse effects to the gasification process, the presence of some inorganic species such as alkali and alkaline earth metals and iron in ash could provide some catalytic effects on the gasification process [38,39]. In contrast, other inorganic species such as silica, alumina, and phosphates could inhibit the gasification by lowering the reactivity of char and blocking the catalyst [40,41]. Fourier Transform Infrared spectroscopy (FTIR) spectra of the sewage sludge and its hydrochar is depicted in Fig. 8. The spectra of sewage sludge in the region 32003600 cm1 represented OeH and NeH functional groups and suggested the existence of several compounds such as water, alcohols, carboxylic acids, and amines/amides [44,45]. The gradual shrinking of the peak at 3355 cm1 in hydrochar was observed with the increase of temperature. It indicated the loss of oxygenated compounds, e.g. alcohols, phenols, ethers, and acids caused by the dissolution and hydrolysis during SCWG. The peak appeared at 2930 cm1 corresponded to the aliphatic chain (CeH alkanes) which indicated the presence of lipids in sewage sludge [46]. When the temperature increased from 350 C to 450 C, the intensity of the peaks of CeH gradually decreased which might be attributed to the decomposition of aliphatic hydrocarbons. The rupturing of aliphatic chains raised the yield of methane and some C2 hydrocarbons in the gas product [47]. The existence of protein might be confirmed by the presence of amide I band and amide II band. These two bands existed at 1540 cm1 and 1652 cm1 which represented the C]O stretching vibration and NeH bending, respectively [48]. The intensity of amide I greatly decreased, while amide II almost disappeared in the hydrochar from SCWG at 450 C. These results implied that the
Please cite this article as: M. Yan et al., Valorization of sewage sludge through catalytic sub- and supercritical water gasification, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2020.01.004
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Fig. 10. Scanning electron microscopy analysis of sewage sludge and hydrochar derived at 350e450 C.
Fig. 12. Effect of alkali catalysts on total organic carbon in the liquid residue at 400 C. Fig. 11. Effect of temperature on total organic carbon removal in the liquid residue without catalyst addition.
dissolution and hydrolysis of proteins occurred and nearly completed at 450 C. A remarkable reduction of the C]O stretching vibrations in the amide I band led to the formation of CO and CO2 [49]. The CeO stretching vibrations between 1000 cm1 and 1200 cm1 indicated the presence of cellulose [50]. In addition, the existence of peaks between 650 cm1 and 900 cm1 represented the aromatic compounds (CHaromatic), and they would be further cracked and reformed to produce H2 [47,49]. The similarity between FTIR spectra of sludge and hydrochars between 400 cm1 and 1200 cm1 revealed that SCWG have a minimum effect on these constituents. Polysaccharides (mainly from bacterial cell walls/plants, or from paper-derived cellulose) or SieO bonds (main content of the sludge ash) corresponding to the stretching bands between 800 cm1 and 1300 cm1 and remained less effected during SCWG. The thermogravimetric analysis of sewage sludge and its
hydrochar is illustrated in Fig. 9. The thermal decomposition of sewage sludge initially started at around ~80 C, and the maximum weight loss occurred at 225e450 C (Fig. 9a). The initial weight loss was due to the loss of moisture in sewage sludge, while no weight loss was noticed in hydrochar at this temperature. It can be confirmed by the low moisture content in hydrochar obtained from different SCWG temperature (Table 1). A notable weight loss between 100 C and 240 C was observed, which indicated the release of volatile matter from sewage sludge. In general, sewage sludge contains bacterial constituents (nucleic acid, lipids, proteins and carbohydrates) and their derivative products, undigested organic material (cellulose and hemicellulose), and inorganic materials [47]. Therefore, maximum weight loss for sewage sludge occurred between 225 C until 450 C, which might be attributed to the degradation of hemicellulose and cellulose at the temperature ranges of 200e300 C and 250e350 C, respectively, while lignin degraded at 200e500 C [50]. It has also been reported that lipids were decomposed at 300 C, while proteins would be decomposed
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at the temperature ranges of 360e525 C [51]. The TGA curves for all the hydrochars showed higher thermal stability compared to raw sewage sludge. While raw sewage sludge thermally decomposed at 225 C, the hydrochar SS-350 started decomposing at 325 C. The thermal stability of hydrochars increased with the increase of temperature. However, almost no weight loss was apparently observed in the hydrochar SS-400 and SS-450. The trend of final weight loss was; sewage sludge (23%) > SS-350 (10%) > SS-400 z SS-450 (3%). The remaining of the materials including sewage sludge and its hydrochar were mainly inorganic compounds in the form of ash. In decomposition of lignocellulose biomass which had lower in ash content and the high thermal stability of hydrochar was due to its stronger CeC and CeH bonds [19]. The rate of decomposition, dehydrogenation and aromatization of organic compounds also affected weight loss in raw material at higher temperature [52]. Fig. 9b shows the differential weight loss for sewage sludge and its hydrochar. Similar to TGA curves, sewage sludge showed remarkable DTA peaks between 225 and 450 C, which were due to the decomposition of hemicellulose, cellulose, and lipid compounds. Nanda et al. [52] have reported similar observations from the thermogravimetric analysis of horse manure and its biochars. Parabolic patterns were also noticed for hydrochar SS-350, but not for SS-400 and SS-450. These patterns of hydrochars overlapped a wide range of temperatures indicating less weight loss, which also confirmed by TGA curves (Fig. 9a). Fig. 10 shows the surface morphology of sewage sludge and its hydrochars obtained from different SCWG temperatures. The results suggested that hydrochar derived at 400 C and 450 C (i.e., SS-400 and SS-450) were highly porous compared to sewage sludge, whereas no significant difference was observed in hydrochar SS-350. It indicated the organic matters in sewage sludge were dissolved and hydrolyzed during SCWG and further reacted to form gas and other products, resulting in the fragmented and disintegrated surface of hydrochar. The loosened char structure in hydrochar became more obvious with increasing temperature. Hydrochar produced at 450 C (SS-450) demonstrated more porous, cracked and broken surface structure. Therefore, higher temperature led to the higher conversion of organic matters to syngas and produced hydrochar with lower structure and high porosity. 3.3. Effect of temperature and alkali catalyst on liquid residue As shown in Fig. 11, it was observed that the total organic carbon remaining in liquid residue significantly decreased with increasing temperature from 350 to 450 C. It indicated that gasification of hydrolyzed products was occurred in supercritical water. As a result, the total gas yield increased with the increase of temperature. TOC concentration reduced by 68.50% from 2751 mg/L to 866 mg/L when the temperature increased to 450 C and this decrease in concentration was accordance with the increase of total gas yield (Fig. 11) and also the FTIR spectra and SEM characteristics of hydrochar respectively. This observation was in a good agreement with the study by Chen et al. [44]. In addition, higher temperature was found to have more significant effect on the TOC concentration than longer reaction time [53]. The effect of the addition of alkali catalysts on the TOC of liquid residue was also studied at 400 C and is presented in Fig. 12. The addition of K2CO3 and KOH exhibited lower TOC concentration compared with the case without catalyst addition, in which K2CO3 performed the best for TOC removal. However, TOC concentration was found to increase with the addition of NaOH and Na2CO3. This behavior might be due to the formation of some intermediate compounds (i.e sodium format and sodium acetate) during the reaction [20,53].
4. Conclusions In this work, gasification of sewage sludge in sub- and supercritical water was carried out in a batch reactor. The effect of reaction temperature (350e450 C) and water-soluble catalysts (Na2CO3, K2CO3, NaOH and KOH) on gas yield, gas composition and gasification efficiency were investigated. In the case of syngas production, the increase of temperature and the addition of alkali catalyst improved the yield of H2 and enhanced the gasification efficiency. The organic matter in sewage sludge was dissolved and hydrolyzed during SCWG, leading to the production of H2-rich syngas. The results also showed that the order of catalyst effectiveness in the favor of H2 production was in the following order; KOH > NaOH > Na2CO3 z K2CO3. Compared to carbonates catalysts, hydroxides catalysts were more effective in improving the H2 yield through water-gas shift reaction by intermediate formation of format salts. The FTIR spectra for sewage sludge and hydrochars revealed that the dissolution and hydrolysis of organic matters that mainly composed of carbohydrates, lipids and proteins occurred during gasification process. The TGA analysis showed lower weight loss in hydrochar at higher temperature, resulting in higher thermal stability in the hydrochar. In addition, hydrochar produced at higher temperature exhibited more porous, cracked, loosened, and broken surface structure. The experimental results and observation in this research might help for understanding on the thermal decomposition of sewage sludge which could assist many aspects of its disposal. The overall findings suggested that sewage sludge can be an option for catalytic SCWG to produce valuable energy in the form H2-rich syngas. The utilization of sewage sludge in SCWG could contribute a win-win scenario, i.e., waste treatment and disposal and energy recovery. Authors contribution Mi Yan and Dwi Hantoko contributed equally to this report. Acknowledgements This research work has been financially supported by the International Cooperation Project of Zhejiang Province (2019C04026) and National Natural Science Foundation of China (Grant No. 51976196). Dwi Hantoko gratefully acknowledges the Zhejiang Province, China for the Zhejiang Provincial scholarship. References [1] S.S.A. Syed-Hassan, Y. Wang, S. Hu, S. Su, J. Xiang, Thermochemical processing of sewage sludge to energy and fuel: fundamentals, challenges and considerations, Renew. Sustain. Energy Rev. 80 (2017) 888e913. [2] C. Zheng, X. Ma, Z. Yao, X. Chen, The properties and combustion behaviors of hydrochars derived from co-hydrothermal carbonization of sewage sludge and food waste, Bioresour. Technol. 285 (2019) 121347. [3] H. Spliethoff, W. Scheurer, K.R.G. Hein, Effect of Co-combustion of sewage sludge and biomass on emissions and heavy metals behaviour, Process Saf. Environ. Prot. 78 (2000) 33e39. [4] N. Akiya, P.E. Savage, Roles of water for chemical reactions in hightemperature water, Chem. Rev. 102 (2002) 2725e2750. [5] C. Rodriguez Correa, A. Kruse, Supercritical water gasification of biomass for hydrogen production e Review, J. Supercrit. Fluids 133 (2018) 573e590. [6] J.A. Onwudili, P.T. Williams, Catalytic supercritical water gasification of plastics with supported RuO2: a potential solution to hydrocarbonsewater pollution problem, Process Saf. Environ. Prot. 102 (2016) 140e149. [7] E. Gasafi, M.-Y. Reinecke, A. Kruse, L. Schebek, Economic analysis of sewage sludge gasification in supercritical water for hydrogen production, Biomass Bioenergy 32 (2008) 1085e1096. [8] J. Louw, C.E. Schwarz, A.J. Burger, Catalytic supercritical water gasification of primary paper sludge using a homogeneous and heterogeneous catalyst: experimental vs thermodynamic equilibrium results, Bioresour. Technol. 201 (2016) 111e120.
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Please cite this article as: M. Yan et al., Valorization of sewage sludge through catalytic sub- and supercritical water gasification, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2020.01.004