Waste Management 123 (2021) 88–96

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Waste Management

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Char derived from sewage sludge of hydrothermal carbonization andsupercritical water gasification: Comparison of the properties of twochars

https://doi.org/10.1016/j.wasman.2021.01.0270956-053X/� 2021 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Jiangsu Collaborative Innovation Center of Atmo-spheric Environment and Equipment Technology (CICAEET), Jiangsu Key Laboratoryof Atmospheric Environment Monitoring and Pollution Control, School of Environ-mental Science and Engineering, Nanjing University of Information Science &Technology, Nanjing 210044, China.

E-mail address: wangchenyu@nuist.edu.cn (C. Wang).

Chenyu Wang a,b,⇑, Wei Zhu b, Xihui Fan b,c

a Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Jiangsu Key Laboratory of Atmospheric Environment Monitoringand Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, ChinabCollege of Environment, Hohai University, Nanjing, Jiangsu 210024, ChinacNanjing Environment Group Co. LTD, Nanjing, Jiangsu 210026, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 September 2020Revised 18 January 2021Accepted 19 January 2021Available online 8 February 2021

Keywords:CharSewage SludgeHydrothermal CarbonizationSupercritical Water Gasification

Supercritical water gasification (SCWG) is considered a promising technology for sewage sludge (SS)treatment and utilization; however, char produced by a side reaction has become a bottleneck inSCWG. In this study, SS and its model compound (10% humic acid) were treated in an autoclave bySCWG at 400 �C for 30 min and by hydrothermal carbonization (HTC) at 250 �C for 300 min. The charyield was 15.4% in SCWG and 41.3% in HTC. The chars were characterized by scanning electron micro-scopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer–Emmett–Teller(BET) analysis, and elemental analysis. By comparing the properties the char produced by SCWG andthe hydrochar produced by HTC, which has been considered a valuable product, the feasibility of usingchar as an additional product in SCWG was explored. Compared with the char produced by HTC, the chargenerated in SCWG exhibits a lower BET specific surface area (8.257 and 15.782 m2/g) and combustionactivity, a higher proportion of small pores (with pore sizes of 1–2 nm), and greater thermal stability.The formation pathway of the two types of chars is related to both dehydration and aromatization; decar-boxylation also occurs in char formation during SCWG. Humus was proved to be related to char formationduring the SCWG of SS based on experimental results obtained with the model compound. This work pro-vides insights needed to guide follow-up treatments or utilization of the char produced during the SCWGof SS.

� 2021 Elsevier Ltd. All rights reserved.

1. Introduction

The treatment and disposal of sewage sludge (SS) has becomean important topic in the environmental protection field. Landfill-ing, incineration, and land application are current common meth-ods for sewage disposal (Chen et al., 2019), reuse in building andconstruction material is also widely studied (Min et al., 2021;Palmieri et al., 2019); however, these methods have some limita-tions because of the two special properties of SS: high moisturecontent and organic pollution risk.

Supercritical water gasification (SCWG) is a potential technol-ogy for SS treatment. Under the condition of high temperatureand pressure, the water in SS will transforms into supercriticalwater (SCW), which becomes an ideal reaction medium. Theorganic matter in SS decomposes into small-molecule organic mat-ter and transforms into syngas after a series of chemical reactionsin SCW. Pre-drying of wet SS can be avoided in SCWG, and theorganic pollutants can be converted into clean energy. Therefore,SCWG is not limited by the high moisture content and organic pol-lution risk of SS.

Since the first proposal of SCWG by Modell (1985), systematicworks on SCWG reaction control, the associated reaction mecha-nism, catalyst selection, and process improvement have beenreported. However, numerous technical difficulties in the SCWGof real biomass remain unresolved; this restricts the further appli-cation of this technology. The formation of char via side reactionduring SCWG, which leads to an increase in the gasification effi-

C. Wang, W. Zhu and X. Fan Waste Management 123 (2021) 88–96

ciency, deactivation of the catalyst, and reactor blockage, is animportant bottleneck.

Numerous scholars have reported the phenomenon of char for-mation in SCWG or other hydrothermal conversion processes.Gong et al. (2018) found char and tar deposited on the reactor dur-ing the SCWG of landfill leachate at 625 �C for 30 min. Lu et al.(2010) studied the catalytic performance of Ni on different sup-ports for the SCWG of glucose; the catalyst was also deactivatedbecause of the deposition of char. In our previous study (Wanget al., 2018), it was also found that char is generated during theSCWG of SS, which leads to a decrease in catalyst stability.

Some researchers have also explored the formation mechanismof char. Kruse and Gawlik (2003) identified the compounds fromthe decomposition of biomass via SCWG; they noted that ring com-pounds tended to form in subcritical water. These ring compounds,especially 5-hydroxymethylfurfural (5-HMF), are believed to trig-ger the polymerization reaction responsible for the formation ofbyproducts such as char. Chuntanapum and Matsumura investi-gated char formation in SCWG using 5-HMF (Chuntanapum et al.,2008) and glucose (Chuntanapum and Matsumura, 2010) as modelcompounds to simplify the system. The reaction of 5-HMF alone inSCWG did not produce any detectable char. Chuntanapum andMatsumura thus speculated that the side reaction between 5-HMF and other glucose decomposition products was responsiblefor char formation. Müller and Vogel (2012) measured the amountof char generated in the hydrothermal conversion of glucose, phe-nol, hydroquinone, and ethylene glycol to determine the precur-sors of char. Phenol and hydroquinone were ruled out asprecursors for the formation of char in their study.

The organic composition of SS is more complex than that ofplant biomass. In a previous study (Wang et al., 2020), we treatedfive model compounds (i.e., humic acid, glutamic acid, glycerol,guaiacol, and glucose) representing the main organic componentsin SS using hydrothermal gasification at 300–400 �C. Given the sta-tistical data for the organic matter content in SS, we consideredhumus the most likely precursor of char in the SCWG of SS. Gonget al. (2017) tested humic acid in SCW at 325–600 �C, 24 MPaand 30 min, and also observed the char formation.

To avoid the adverse effects caused by char, char yield can beinhibited using certain methods. Matsumura et al. (2018) reportedthat the char formed via radical reactions in SCWG could be inhib-ited by the addition of an organic acid radical scavenger. Moreover,char yield can be suppressed by adding alkaline additives and oxi-dants as inhibitors in SCWG(Wang et al., 2021). We studied theinfluence of reaction parameters on char yield in our previous work(Wang et al., 2019) and found that increasing the reaction temper-ature and heating rate reduced the char yield in the SCWG of SS.However, the use of additional additives or a change in the reactionconditions leads to an increase in operating costs; in addition, theenvironmental risks of the products generated when additives areused also need to be determined. Actually, hydrothermal processescan be directed in different directions through appropriate manip-ulation of temperature and pressure. Gas, liquid products, andbyproducts are formed in all cases but with substantially differentproduct distributions. Syngas is the main product of SCWG, andchar is formed as a solid byproduct resulting from inappropriatereaction conditions or special reactant properties, which lead to adecrease in the gas yield in SCWG. However, in milder hydrother-mal processes such as hydrothermal carbonization (HTC), most ofthe products are distributed in the solid phase, a char called hydro-char (or biochar). Hydrochar can be used as a wastewater treat-ment material and in other applications after appropriatemodification and activation. Hydrochar also can be used as feed-stock for thermal processes. For example, Shen et al. (2018) mixedthe hydrochar obtained from the HTC of SS with waste leaves asfeedstock for co-gasification, and obtained a gasification efficiency

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of 55.1–70.7%. He et al. (2020) mixed hydrochar with threedifferent-rank coals, and investigated the pyrolytic behaviors,kinetics, and gas releasing characteristics of the co-pyrolysis of coaland hydrochar; further, they determined the feasibility ofhydrochar/coal. Therefore, we proposed reducing the char yieldin SCWG while reusing it as a target product like hydrochar pro-duced via HTC. On the basis of the aforementioned analysis, SSwas treated by HTC and SCWG in the present study, where an in-depth comparison of the properties of char produced in SCWGand HTC was carried out, to evaluate whether the char producedin SCWG has potential utilization. Since the char produced byHTC has been proved to have certain utilization value, the utiliza-tion potential of char generated by SCWG can be evaluated bycomparison, and thereby explore the possibility of reusing charproduced in SCWG.

2. Material and methods

2.1. Material

SS was collected from a wastewater treatment plant (WWTP) inNanjing, China. The SS sample was stored in a refrigerator at a tem-perature less than 4 �C. The basic properties of the SS are given inTable 1. Prior to the experiment, raw wet SS was dried by freeze-drying and then ground into a uniform powder to prevent anuneven SS composition from affecting the experiment results.

Because humus is the main precursor of char formation duringthe SCWG of SS (Wang et al., 2020), humic acid was selected as themodel compound of humus and was also tested in the presentstudy. The various chemical agents used in the experiment werepurchased from Aladdin Chemistry Co., Ltd., and Sinopharm Chem-ical Reagent Co., Ltd.; all reagents were American Chemical Societygrade.

2.2. Experimental apparatuses and procedures

SCWG and HTC experiments were both conducted in a stainlesssteel autoclave with a volume of 100 mL; the experimental proce-dure is shown in Fig. 1. First, a dry SS sample was mixed with dis-tilled water to a moisture content of 80 wt%, which is the generalmoisture content of SS after preliminary dewatering in the WWTP;the wet SS sample was the placed in the reactor. When humic acidwas tested, the concentration was 10 wt%, the determination ofhumic acid concentration was based on the results of the investiga-tion of humus content of SS (Wang et al., 2020). SCWG and HTCwere performed at 400 �C for 30 min and at 250 �C for 300 min,respectively. The SCWG reaction condition chosen in this studywas above the critical point, but char was still formation under thiscondition. According to the results of He et al.(2019), the reactionconditions of HTC were determined. The reaction pressure wasset to the predetermined value by adjusting the amount of distilledwater in the autoclave; the amount of distilled water was 20 mL inthis experiment. According to the IAPWA-IF97 thermodynamicmodel of water and steam (Wagner et al., 2000), the pressureswere 4.0 and 26.4 MPa (above the critical-point pressure of22.1 MPa) at 250 and 400 �C, respectively. After the reaction, theautoclave was removed from the oven and cooled to room temper-ature under cold water.

The autoclave was opened to release the gas-phase product;solid-phase and liquid-phase products were separated by vacuumfiltration. Dichloromethane (DCM) was used to rinse the residueand reactor walls to remove the oil-phase products remaining onthe solid products and reactor. The solid product was collected ina beaker. The suspended solid in the beaker was oven-dried at65 �C overnight and weighed to obtain the weight of chars. The

Table 1Properties of dry sewage sludge and chars.

Sample VMa (%) Asha (%) Elemental analysis (%) HHVc

(MJ/kg)CharYield(%)

SurfaceArea(m2/g)

Cdafb C H N S Ob

Dry SS 53.20 ±2.61

46.80 ±2.61

45.92 ±1.05

23.43 ±0.56

4.40 ±0.09

3.90 ±0.02

0.62 ±0.01

20.85 ±0.46

10.65 ±0.14

– –

Humicacid

71.00 ±0.78

29.00 ±0.78

45.00 ±1.66

31.95 ±1.18

2.70 ±0.25

0.78 ±0.03

0.28 ±0.02

35.29 ±1.48

8.41 ± 1.03 – –

sC-SCW 15.89 ±1.11

84.11 ±1.11

60.60 ±3.08

9.63 ± 0.49 1.83 ±0.02

1.17 ±0.04

0.02 ±0.00

3.24 ± 0.43 5.34 ± 0.21 15.4 ± 3.1 8.257

sC-HTC 30.61 ±4.11

69.39 ±4.11

55.01 ±2.12

16.84 ±0.65

2.45 ±0.01

1.88 ±0.01

0.30 ±0.02

9.14 ± 0.67 7.66 ± 0.33 41.3 ± 3.9 15.782

hC-SCW 69.90 ±0.88

30.10 ±0.88

48.80 ±0.31

34.11 ±0.22

1.82 ±0.03

0.83 ±0.05

0.14 ±0.01

33.00 ±0.21

8.27 ± 0.15 58.4 ± 1.2 –

hC-HTC 69.27 ±1.06

30.73 ±1.06

46.90 ±0.59

32.49 ±0.41

2.53 ±0.02

0.83 ±0.02

0.28 ±0.01

33.14 ±0.40

8.73 ± 0.24 72.9 ± 2.8 –

dDetermined by Dulong equation: HHV = 0.3393mC + 1.443(mH � mO/8) + 0.0927mS + 0.01494mN, where m is the mass fraction of each respective element in the samplesa Dry basis, VM: Volatile matterb Dry ash-free basis

Fig. 1. Schematic of the experimental procedure.

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chars were labeled as sC and hC in accordance with different feed-stock, where sC indicates that the feedstock was SS and hC indi-cates that the feedstock was the model compound humic acid. Inaddition, suffixes such as SCW and HTC were used to indicate dif-ferent hydrothermal processes. For example, ‘‘sC-SCW” denotes thechar derived from SS in SCWG.

Because SS contains a large amount of organic matter, to avoidthe influence of the high ash content on char yields, the daf basisproposed by Cao et al. (2019) was used to represent the char yield.The daf basis yield of char was calculated as follows:

charyield ¼ MS � 1� AshSð ÞMSS � 1� AshSSð Þ ð1Þ

where MS and MSS are the mass of dry solid product after reactionand the mass of the SS sample, respectively, and AshS and AshSS

represent the ash contents of the solid product and SS sample,respectively.

2.3. Product analysis

The method used to measure the volatile matter content in theSS sample and char were based on the standard method D1762-84published by ASTM International (ASTM, 2013). Scanning electronmicroscopy (SEM; Gemini 330, ZEISS, Germany) was used toobserve the surface structure of the char. N2 adsorption–desorp-tion isotherms of char were recorded in an automatic surface areaand porosity analyzer (Micromeritics, ASAP2020, USA); sampleswere degassed at 200 �C for 8 h prior to the measurements. Specific

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surface area was determined using the Brunauer–Emmett–Teller(BET) method. An elemental analyzer (Vario EL III, Elementar, Ger-many) was used to measure the elemental composition (C, H, N, S)of the SS sample and char, and the oxygen content was calculatedby difference. An Fourier transform infrared (FTIR) spectrometer(Alpha, Bruker, Germany) was used to determine the functionalgroups present in the samples and feedstock. Thermogravimetricanalysis (TGA; TGA5500; TA Instruments, USA) was used to studythe thermal stability and combustion behavior of char, with a heat-ing rate of 10 �C/min and N2 flow rate of 50 mL/min. In order todetermine the variability of the analysis results, two parallel sam-ples were analyzed in TGA and SEM analysis, while three parallelsamples were tested at same time in other characterization.

3. Results

3.1. Char yields and HHV

The char yields in SCWG and HTC are given in Table 1, all exper-iments were conducted at least three times, until the standarddeviation was within ± 5%. The char yields were 15.4% and 41.3%after SCWG and HTC, respectively. The char yield of SCWG wasmuch lower—only 37% of that of HTC. Organic matter in the bio-mass was distributed more in the gas and liquid phases after SCWGthan after HTC because of the harsher reaction conditions of SCWG.Solid products tend to form at lower reaction temperatures, lowerpressures, and longer retention times (Wang et al., 2019). The charyields in both HTC and SCWG tend to decrease with an increase in

Fig. 2. N2 adsorption–desorption isotherms and pore size distribution of (a) sC-SCW and (b) sC-HTC.

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the reaction temperature. However, in the temperature range ofHTC, the char yield is always above 40% (Cao et al., 2019). The obvi-ous difference in char yield between the two hydrothermal pro-cesses leads to the difference in the target products.

It was found that HHV of sC-HTC decreased after HTC. The par-ticular change in HHV was due to reduction of the carbon contentin sC-HTC, which may be related to the reaction temperature ofHTC, that in this study was higher than in other cases. VMwas con-verted into gas and TOC in aqueous product when HTC was per-formed at 250 �C. In other words, the loss of VM resulted inreduction of HHV. Carbon content is generally expected to increaseby HTC of other biomass, but high ash content of SS may beanother reason for reduction of carbon content. The carbon contentbased on ash-free is given in Table 1. It can be seen that the carboncontent actually increased without considering ash. Similar con-clusions are also reported in other studies. He et al. (2020) studiedthe HTC of SS at 200 �C, the carbon content decreased from 36.7% inSS to 32.5% in the char. Cao et al. (2019) investigated the HTC ofbiogas digestate at 170–250 �C, and also found the carbon contentdecreased from 34.3% in feedstock to 32.3% in the solid product.Compared with these studies, the reduction of carbon content inthis study was more obvious. Because of the difference of wastewater source, the organic matter composition of SS is quite differ-ent, which may be an important reason for the difference in results.In the future, it is necessary to expand the SS samples size to verifythe conclusion.

3.2. Pore structure and surface morphology

Surface properties strongly influence the subsequent utilizationof chars, and the formation mechanism of chars can be deduced bycharacterizing their surface properties. N2 adsorption–desorptionexperiments and SEM observations were used to explore the porecharacteristics and surface morphology, respectively, of the twokinds of chars.

The N2 adsorption–desorption isotherms and pore size distribu-tion of the two types of chars are shown in Fig. 2. sC-SCW and sC-HTC both exhibit typical type IV(a) sorption isotherms, which arecharacteristic of mesoporous (2 nm < pore diameter lessthan 50 nm) and nonporous adsorbents. This interpretation is con-sistent with their pore size distribution curves, which show poresize distributions of 1–22 nm in both cases. Some differencesbetween the two kinds of chars were also observed. The sC-HTCis mesoporous with a pore size in the range of 6–22 nm and mostof its pores are larger than 10 nm. By contrast, sC-SCW containsnumerous microporous with pore sizes of 1–2 nm. As shown inTable 1, the BET specific surface area of sC-SCW and sC-HTC was8.257 m2/g and 15.782 m2/g, respectively. The BET specific surfacearea of the char obtained by HTC was nearly double that of the charobtained by SCWG. The BET specific surface area of the chardepends on the feedstock, reaction process, and whether it hasbeen activated. Compared with SCWG, HTC was advantageous forproducing a char with a higher BET specific surface area.

Fig. 3 (a) and 3(b) show the SEM images of sC-SCW and sC-HTC,respectively. The surface morphologies of the two kinds of charswere similar, exhibiting porous clustered aggregates. However,an obvious mesoporous structure is observed in sC-HTC, and alarge number of microporous structures with a pore diameterof ~ 1 nm are observed in sC-SCW, consistent with the N2 adsorp-tion–desorption results. The char morphology is also related to thetransformation and volatilization of organic matter in feedstock,the reaction conditions of SCWG are harsher than those of HTC.Therefore, organic components which are difficult to react underHTC conditions will be prone to decompose in SCWG; this is theprobable reason for the difference of morphology between thetwo kinds of chars.

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SCWG and HTC experiments were also carried out with humicacid as the model compound for humus, because humus is consid-ered a potential precursor of char formation (Wang et al., 2020).The humic acid tested in this experiment was a brownish-blacksolid, and the content of fulvic acid (a kinds of organic acid withlow molecular weight in humic acid) was more than 90%. TheSEM images of the char derived from humic acid are shown inFig. 3 (c) and (d). The surface morphology of hC-SCW is similarto that of sC-SCW; however, the pore size of the char derived fromhumic acid is slightly larger, approximately 2–3 nm. On the onehand, this result confirms that the humus in SS is indeed an impor-tant precursor for the formation of char during SCWG; on the otherhand, it also suggests that other organic components in SS may alsoparticipate in the formation of char. Sugar in SS, like glucose, hasbeen found to form microspherical char during SCWG (Wanget al., 2020), which may fill the pores formed by the dehydrationof humic substances, and the different char morphology formedby various organic components leads to a final morphology of charproduced by real SS. hC-HTC was present as dispersed particles,and a regular polyhedral structure was observed, indicating that

Fig. 3. SEM image of (a) sC-SCW, (b) sC-HTC, (c) hC-SCW, and (d) hC-HTC.

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other types of organic components strongly influence the forma-tion of char in HTC. However, the role of specific organic compo-nents needs to be studied further.

Fig. 4. Van Krevelen diagram of sewage sludge and chars.

3.3. Elemental composition and functional groups

The formation pathways and functional-group compositions ofthe two types of char were analyzed through elemental analysisand FTIR. The results of the proximate and elemental analyses ofsC-SCW and sC-HTC are shown in Table 1. The VM content of theraw SS was 53.2%; part of the organic matter was transformed intogas and aqueous phases during the hydrothermal process. The VMcontent in sC-SCW was reduced to 15.89%, indicating that morethan 70% organic matter in SS was decomposed after SCWG. ForsC-HTC, the VM content was 30.61% and the degradation ratewas approximately 42%. The difference in VM content was theresult of a harsher reaction temperature and pressure in SCWG,which makes organic matter decompose and transform morequickly and thoroughly. Even though the reaction time of HTCwas several times that of SCWG, the organic matter with a highmolecular weight or stable structure was still difficult to decom-pose. The HHV of the sC-SCW (5.34 MJ/kg) was slightly lower thanthat of the sC-HTC (7.66 MJ/kg), which is also attributed to a morethorough degradation of organic matter. In addition to the differ-ence in HHV, there may be some difference in the organic compo-sition of the two kinds of char, resulting in the differentcombustion characteristics. This aspect will be discussed furtherin section 3.4.

To explore the formation pathway of chars further, we con-structed the Van Krevelen diagram of SS and chars based on ele-mental analysis results (Fig. 4). The formation pathways of thechars produced via SCWG and HTC differ. Decarboxylation is the

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main pathway for char formation during SCWG, followed by adehydration reaction. This result is somewhat different from ourprevious experimental results; the contribution of decarboxylationto char formation was not observed in previous experiments withmodel compounds of SS. The different experimental resultsbetween model compounds and real SS may be due to the experi-ments of the model compounds being conducted only for a singlecompound; that is, the organic components in SS may interact,thereby affecting the formation of char. In addition, the organicmatter in SS is more complex than that in the model compound,

Table 2FTIR peak assignments for chars derived from SS and humic acid.

Char derivedfrom

Peaks(cm�1)

Assignments

SS 1640 –C = O in ketone and amide groups1535 –C = O in carboxylic groups1450 –C = C stretching in aromatic ring carbons1010 –C–O–R in aliphatic ethers or alcohol –C–O

stretchingHumic Acid 3300 –OH stretching vibrations in hydroxyl or

carboxyl groups1560 –C = C ring polynuclear aromatic1380 stretching of –C–O in alcohols1010 –C–O–R in aliphatic ethers or alcohol –C–O

stretching

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resulting in additional reaction pathways. For the formation of sC-HTC, a dehydration reaction was the main pathway, consistentwith the results of He et al. (2013). In terms of elemental compo-sition, sC-HTC is similar to lignite, which is a low-rank coal withH/C and O/C atomic ratios in the range of 0.80–1.30 and 0.20–0.38, respectively. Therefore, sC-HTC with a similar element com-position can be mixed with lignite for combustion.

The FTIR spectra of SS and the two kinds of chars are shown inFig. 5 (a) and their peaks are given in Table 2. The functional groupsof the sC-SCW and sC-HTc are similar to those of SS. Comparedwith the spectrum of SS, those of the chars show weaker absorp-tion peaks at 1,640 and 1,535 cm�1. The peak at 1,640 cm�1 isattributed to a stretching vibration of –C = O in ketone and amidegroups (Silva et al., 2012). The peak at 1,535 cm�1 is attributed tothe asymmetric stretching of –C = O in carboxylic groups (Li et al.,2011), a reduction in the relative intensity of chars in these twopeaks indicated that decarboxylation occurred, consistent withthe conclusion from the Van Krevelen diagram. A weak peak at1,450 cm�1 is observed in the spectra of the chars; this peak is

Fig. 5. FTIR spectra of char derived from (a) sewage sludge and (b) humic acid.

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related to –C = C stretching in aromatic ring carbons (He et al.,2013), revealing the formation of polycyclic aromatic hydrocar-bons (PAHs) during SCWG. Gong et al. (2016) investigated thechanges in the PAH concentration and distribution in 10 differenttypes of SS and gasified residues. They found that the total PAHconcentrations were higher in residues after SCWG than in theraw SS and that the PAHs produced were mainly in the solid resi-dues, which accounted for more than 95% of the total concentra-tions. Therefore, the char produced in SCWG may risk persistentorganic pollutants pollution, which needs follow-up attention.The intensity of the peak at 1,010 cm�1, which is assigned to –C–O–R in aliphatic ethers or alcohol –C–O stretching, increases inthe spectra of the chars (özçimen and Ersoy-Meriçboyu, 2010),likely because of the dehydration of alcohols.

The FTIR spectra of humic acid and chars derived from humicacid are shown in Fig. 5 (b) to further understand the formationmechanism of chars. The peak at 3,300 cm�1 was due to –OHstretching vibrations in hydroxyl or carboxyl groups (Gong et al.,2017), and the intensity of this peak decreased in the spectra ofthe chars because of the dehydration of humic acid, whichoccurred in both HTC and SCWG. The peak at 1,560 cm�1 is attrib-uted to the stretching of a –C = C polynuclear aromatic ring (Fooet al., 2016), the peak at 1,380 cm�1 is attributed to the stretchingof –C–O in alcohols (Srinivasan et al., 2005), and the peak at1,010 cm�1 is assigned to –C–O–R in aliphatic ethers or alcohol –C–O stretching (özçimen and Ersoy-Meriçboyu, 2010). The changesin the FTIR spectrum of the humic acid group were similar to thechanges in the spectrum of the SS group: both showed that arom-atization and dehydration reactions occurred during char forma-tion. These results again demonstrate that humus plays animportant role in char formation. However, decarboxylation wasnot observed in humic acid by FTIR analysis. This result is consis-tent with our previous work, further confirming that the decar-boxylation reaction of nonhumic substances occurs inhydrothermal processes. Gong et al. (2017) tested humic acid at325–600 �C in subcritical water and SCW, and the FTIR analysisof humic acid and char was also carried out, the FTIR spectra areconsistent with those in our study. According to Gong et al.’s result,the FTIR spectra of char produced by humic acid did not changewith the increase of temperature.

3.4. Thermal stability and combustion behavior

The thermogravimetric analysis (TGA) results of sC-SCW andsC-HTC are shown in Fig. 6. TGA was used to determine the thermalstability and combustion behavior of the two kinds of chars. TheTGA results were similar to those observed by Milan and Anuragfor the hydrochar derived from centrifuged SS (Malhotra andGarg, 2020); three evident peaks were observed in DTG curve ofchar. Milan and Anurag pointed out the peaks were attributed to

Fig. 6. TG-DTG curves of chars. (a): sC-SCW; (b): sC-HTC.

C. Wang, W. Zhu and X. Fan Waste Management 123 (2021) 88–96

the mass loss of moisture content, volatile matter and fixed carbon,respectively. Because of the differences in reaction conditions andfeedstock, the first peak was not detected in some studies (Liuet al., 2020; Zheng et al., 2020), but the second and third peakswere detected. Referring to the methods of He et al. (2013) andWang et al. (2009), four characteristic temperatures (i.e., T0, Ti,Tm, and Tb) were determined from the thermogravimetry–derivative thermogravimetry (TG–DTG) curves to describe the combus-tion behavior of chars well, as shown in Table 3. The DTG curvesshow a first higher peak when the test temperature is below150 �C. The weight loss of char in this stage was mainly due toits higher moisture content. After this stage, the VM of chars beginto decompose gradually; the starting temperature correspondingto point B is T0. When the weight-loss rate reached the maximumvalue (i.e., (dw/dt)max), the temperature was Tm, corresponding topoint A. In the late stage of TGA, the weight-loss rate graduallydecreased and become stable, which is regarded as the end of com-bustion, and the temperature was the burnout temperature Tb. Inaddition, the ignition temperature Ti refers to the temperature atwhich fuels start to burn, which is determined by points A and B.Point C can be derived by crossing the tangent with the TG curveat point A with the horizontal tangent to point B. Ti is the corre-sponding temperature at point C.

Both the Tb and Ti of sC-SCW are higher than those of sC-HTC,indicating that the char produced by SCWG exhibits better thermalstability than that produced by HTC. If sC-SCW is regarded as abyproduct, it will deposit onto the wall of the reactor or the surface

Table 3Characteristic temperatures, residues, and comprehensive combustibility indexes (S) of th

Sample Ti (�C) Tb (�C) Residues (%)

sC-SCW 275 550 89.4sC-HTC 260 530 75.4

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of the heterogeneous catalyst, which may lead to reactor blockageor catalyst deactivation. Greater stability under thermal conditionsmeans that these problems are more difficult to solve by heating.

A parameter called the combustion characteristic factor (S) canbe used as a criterion for fuel combustion performance (He et al.,2013). It is defined in eqn (2), and the value of S is directly propor-tional to the combustion activity of chars.

S ¼ ðdw=dtÞmaxðdw=dtÞmean

T2i Tb

ð2Þ

where (dw/dt)mean is the average weight-loss rate (%/min).The S values of the chars are given in Table 3, the S values of sC-

SCW and sC-HTC were 0.071 � 10�8 and 0.501 � 10�8, respec-tively. The S value of sC-HTC was more than seven times that ofsC-SCW. In terms of weight-loss rate, both the maximum weight-loss rate and average weight-loss rate of sC-HTC were more thantwice those of sC-SCW. This result shows that the combustionactivity of the char produced by HTC was much greater than thatproduced by SCWG, which is attributed to the higher VM contenin sC-HTC making part of it more conducive to combustion. Thiscan also be confirmed from the residual amount of sample afterthe TGA test, as given in Table 3. The residual content of sC-HTCwas 75.4%, whereas that of sC-SCW was 89.4%.

4. Discussion

The properties of the chars produced by SCWG and HTC weresystematically compared in this study, and certain differences inproperties were observed between the two kinds of chars. Com-pared with the hydrochar produced by HTC, the char producedby SCWG has a lower BET specific surface area, lower combustionactivity, higher proportion of small pore pores, and greater thermalstability.

At present, hydrochar produced by HTC has been found to bevaluable in many fields; it has been used for gas or water cleaning(Kruse and Dahmen, 2018), CO2 sequestration (Li et al., 2016), H2

storage (Zhao et al., 2018), and electrode materials for batteries(Wang et al., 2013). Therefore, a comparison of the properties ofthe two chars can provide a reference for the subsequent utiliza-tion of the char generated by SCWG. Because of the lack of specificsurface area and combustion activity of the char obtained fromSCWG, its resource utilization potential is not as good as that ofchar obtained from HTC. Notably, however, the target products ofthe two processes differ. Hydrochar is the main target product ofHTC, whereas syngas is the main target product of SCWG. Charcan only be considered a potential additional product in SCWG.When the recovery benefit of the char produced by SCWG isgreater than the negative impact it may cause, it is more worth-while to consider the char recovery instead of char reduction orinhibition.

In this study, the properties of char produced in SCWG weredetermined. The next step considering the conclusions is to reflecton how to reuse char as an additional product. Although the per-formance of char exhibits certain deficiencies, precedents still existfor its application. Parsa et al. (2019) obtained two kinds of charsderived from marine macroalgae in hydrothermal liquefaction at350 �C for 15 min. The BET specific surface areas were 2.11 and6.45 m2/g, which are still slightly lower than the BET specific sur-

e chars.

(dw/dt)max (%/min) (dw/dt)mean (%/min) S (�10�8)

�0.230 �0.128 0.071�0.602 �0.298 0.501

C. Wang, W. Zhu and X. Fan Waste Management 123 (2021) 88–96

face area of sC-SCW. However, these two chars still have applica-tion potential in water treatment as adsorbents, where the maxi-mum adsorption capacity for aqueous cations can reach 226 mg/g. Lu et al. (2012) prepared sludge-derived biochar (SDBC) viapyrolysis, and studied the lead sorption capacity of SDBC to deter-mine whether treatment of acid mine drainage containing metalswith SDBC was feasible. We found that SDBC could effectivelyremove Pb2+ with capacities of 16.11, 20.11, 24.80, and30.88 mg/g at initial pH values of 2, 3, 4, and 5, respectively. Caoet al. (2009) obtained char from dairy manure at 200 and 350 �Cand found that it exhibited better Pb2+ adsorption efficiency thancommercial activated carbon. In addition, the properties of charcan be improved through activation to achieve a considerablespecific surface area and pore structure. Jiang et al. (2018) usedswine manure digestate as a feedstock to obtain biochar viaoxygen-limiting pyrolysis. Its specific surface area was17.070 m2/g, which is similar to that of the char in the presentstudy. However, after modification by adding HCl, NH3�H2O, andKMnO4, its specific surface area was substantially increased to186.516–207.016 m2/g, and it exhibited good adsorption capacityfor metals and antibiotics. Referring to these experimental resultsand considering the known properties of sC-SCW, we conclude thatthe char produced by SCWG has certain utilization value.

In terms of the formation pathway of chars in HTC and SCWG, adehydration reaction and aromatization reaction were involved inboth processes. The formation of char in SCWG also involves decar-boxylation and is more closely related to the precursor humus. Thedifference in reaction conditions between SCWG and HTC is animportant reason for the difference in the char formation pathway.The ion product of water increases with increase in reaction tem-perature but decreases rapidly when the temperature exceedsthe critical point of water (i.e., 375 �C, 22.1 MPa). The free-radical reaction is promoted in SCW, and the yield of gas productsincreases substantially. In this experiment, the char yield of HTCwas much higher than that of SCWG. The char produced in HTCexhibited an elemental composition close to that of lignite, andthe oxygen content in this char decreased because of waterelimination.

5. Conclusion

In this study, the properties of chars produced by SCWG andHTC were investigated and compared to evaluate the feasibilityof reusing char produced by SCWG. In terms of pore characteristics,the BET specific surface area of the char produced by SCWG was8.257 m2/g, which was lower than that of the char produced byHTC (15.782 m2/g). In addition, there were more microporous witha diameter less than 2 nm in the char produced by SCWG, whereasthe main pore diameter was in the range of 6–22 nm in the charproduced by HTC. According to the TGA results, the char producedby SCWG was more stable under thermal conditions, resulting inlow VM content, its combustion activity was also weaker than thatof the char produced by HTC. The combustibility index of sC-HTC(0.501 � 10�8) was more than seven times that of sC-SCWG(0.071 � 10�8). In view of these properties and lower char yield(15.4%, compared with 41.3% in HTC process), the char generatedby SCWG is more suitable as an additional product during syngasproduction. To improve the overall value of SCWG products, a sub-sequent activation steps are needed to obtain better performance.

Dehydration and aromatization were involved in the formationof the two kinds of chars. Decarboxylation involving char forma-tion in SCWG was not found in previous experiments with modelcompounds. Through SEM observations and FTIR analysis, wedemonstrated that the char formation during SCWG is closelyrelated to humus.

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This work may provide a better understanding of the char prop-erties and formation mechanism in the SCWG process. On thisbasis, we will continue to explore its application in various aspects.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgements

This research was supported by the Startup Foundation forIntroducing Talent of NUIST (Grant No. 2020r097) and NationalScience and Technology Major Project of the Ministry of Scienceand Technology of China (2017ZX07603-003-04).

References

ASTM, 2013. Standard Test Method for Chemical Analysis of Wood Charcoal.American Society for Testing Materials.

Cao, X., Ma, L., Gao, B., Harris, W., 2009. Dairy-Manure Derived Biochar EffectivelySorbs Lead and Atrazine. Environ. Sci. Technol. 43, 3285–3291.

Cao, Z., Jung, D., Olszewski, M.P., Arauzo, P.J., Kruse, A., 2019. Hydrothermalcarbonization of biogas digestate: Effect of digestate origin and processconditions. Waste Manage. 100, 138–150.

Chen, G.Y., Wang, X.T., Li, J., Yan, B.B., Wang, Y., Wu, X., Velichkova, R., Cheng, Z.J.,Ma, W.C., 2019. Environmental, energy, and economic analysis of integratedtreatment of municipal solid waste and sewage sludge: A case study in China.Sci. Total Environ. 647, 1433–1443.

Chuntanapum, A., Matsumura, Y., 2010. Char formation mechanism in supercriticalwater gasification process: a study of model compounds. Ind. Eng. Chem. Res.49, 4055–4062.

Chuntanapum, A., Yong, T.-L.-K., Miyake, S., Matsumura, Y., 2008. Behavior of 5-HMF in subcritical and supercritical water. Ind. Eng. Chem. Res. 47, 2956–2962.

Foo, G.S., Rogers, A.K., Yung, M.M., Sievers, C., 2016. Steric Effect and Evolution ofSurface Species in the Hydrodeoxygenation of Bio-Oil Model Compounds overPt/HBEA. ACS Catal. 6, 1292–1307.

Gong, M., Nanda, S., Romero, M.J., Zhu, W., Kozinski, J.A., 2017. Subcritical andsupercritical water gasification of humic acid as a model compound of humicsubstances in sewage sludge. The Journal of Supercritical Fluids 119, 130–138.

Gong, M., Zhu, W., Zhang, H., Su, Y., Fan, Y., 2016. Polycyclic aromatic hydrocarbonformation from gasification of sewage sludge in supercritical water: Theconcentration distribution and effect of sludge properties. The Journal ofSupercritical Fluids 113, 112–118.

Gong, Y., Lu, J., Jiang, W., Liu, S., Wang, W., Li, A., 2018. Gasification of landfillleachate in supercritical water: Effects on hydrogen yield and tar formation. Int.J. Hydrogen Energy 43, 22827–22837.

He, C., Giannis, A., Wang, J.-Y., 2013. Conversion of sewage sludge to clean solid fuelusing hydrothermal carbonization: Hydrochar fuel characteristics andcombustion behavior. Appl. Energy 111, 257–266.

He, C., Tang, C., Liu, W., Dai, L., Qiu, R., 2020. Co-pyrolysis of sewage sludge andhydrochar with coals: Pyrolytic behaviors and kinetics analysis using TG-FTIRand a discrete distributed activation energy model. Energy Convers. Manage.203.

He, C., Zhang, Z., Ge, C., Liu, W., Tang, Y., Zhuang, X., Qiu, R., 2019. Synergistic effectof hydrothermal co-carbonization of sewage sludge with fruit and agriculturalwastes on hydrochar fuel quality and combustion behavior. Waste Manage. 100,171–181.

Jiang, B., Lin, Y., Mbog, J.C., 2018. Biochar derived from swine manure digestate andapplied on the removals of heavy metals and antibiotics. Bioresour. Technol.270, 603–611.

Kruse, A., Dahmen, N., 2018. Hydrothermal biomass conversion: Quo vadis?. TheJournal of Supercritical Fluids 134, 114–123.

Kruse, A., Gawlik, A., 2003. Biomass conversion in water at 330–410 ℃ and 30–50MPa. Identification of key compounds for indicating different chemical reactionpathways. Ind. Eng. Chem. Res. 42, 267–279.

Li, M., Li, W., Liu, S., 2011. Hydrothermal synthesis, characterization, and KOHactivation of carbon spheres from glucose. Carbohydr. Res. 346, 999–1004.

Li, Y., Li, D., Rao, Y., Zhao, X., Wu, M., 2016. Superior CO2, CH4, and H2 uptakes overultrahigh-surface-area carbon spheres prepared from sustainable biomass-derived char by CO2 activation. Carbon 105, 454–462.

Liu, X., Zhai, Y., Li, S., Wang, B., Wang, T., Liu, Y., Qiu, Z., Li, C., 2020. Hydrothermalcarbonization of sewage sludge: Effect of feed-water pH on hydrochar’sphysicochemical properties, organic component and thermal behavior. J.Hazard. Mater. 388.

Lu, H., Zhang, W., Yang, Y., Huang, X., Wang, S., Qiu, R., 2012. Relative distribution ofPb2+ sorption mechanisms by sludge-derived biochar. Water Res. 46, 854–862.

C. Wang, W. Zhu and X. Fan Waste Management 123 (2021) 88–96

Lu, Y., Li, S., Guo, L., Zhang, X., 2010. Hydrogen production by biomass gasification insupercritical water over Ni/cAl2O3 and Ni/CeO2-cAl2O3 catalysts. Int. J.Hydrogen Energy 35, 7161–7168.

Malhotra, M., Garg, A., 2020. Hydrothermal carbonization of centrifuged sewagesludge: Determination of resource recovery from liquid fraction and thermalbehaviour of hydrochar. Waste Manage. 117, 114–123.

Matsumura, Y., Goto, S., Takase, Y., Inoue, S., Inoue, T., Kawa, Y., Noguchi, T.,Tanigawa, H., 2018. Suppression of Radical Char Production in SupercriticalWater Gasification by Addition of Organic Acid Radical Scavenger. Energy Fuels32, 9568–9571.

Min, F., Wang, X., Li, M., Ni, Y., Al-qadhi, E., Zhang, J., 2021. Preparation of highporosity and high strength ceramisites from municipal sludge using starch andCaCO3 as a combined pore-forming agent. J. Mater. Civ. Eng. 33, 04020502.

Modell, M., 1985. Gasification and Liquefaction of Forest Products in SupercriticalWater. Fundamentals of Thermochemical Biomass Conversion, 95–119.

Müller, J.B., Vogel, F., 2012. Tar and coke formation during hydrothermal processingof glycerol and glucose. Influence of temperature, residence time and feedconcentration. The Journal of Supercritical Fluids 70, 126–136.

özçimen, D., Ersoy-Meriçboyu, A., 2010. Characterization of biochar and bio-oilsamples obtained from carbonization of various biomass materials. RenewableEnergy 35, 1319–1324.

Palmieri, S., Cipolletta, G., Pastore, C., Giosue, C., Akyol, C., Eusebi, A.L., Frison, N.,Tittarelli, F., Fatone, F., 2019. Pilot scale cellulose recovery from sewage sludgeand reuse in building and construction material. Waste Manage. 100, 208–218.

Parsa, M., Nourani, M., Baghdadi, M., Hosseinzadeh, M., Pejman, M., 2019. Biocharsderived from marine macroalgae as a mesoporous by-product of hydrothermalliquefaction process: Characterization and application in wastewatertreatment. Journal of Water. Process Engineering 32.

Shen, Y., He, C., Chen, X., Lapkin, A.A., Xiao, W., Wang, C.-H., 2018. Nitrogen Removaland Energy Recovery from Sewage Sludge by Combined HydrothermalPretreatment and CO2 Gasification. ACS Sustainable Chem. Eng. 6, 16629–16636.

Silva, J.d.O., Rodrigues Filho, G., Meireles, C.d.S., Ribeiro, S.D., Vieira, J.G., da Silva, C.V., Cerqueira, D.A., 2012. Thermal analysis and FTIR studies of sewage sludgeproduced in treatment plants. The case of sludge in the city of Uberlandia-MG,Brazil. Thermochim Acta 528, 72–75.

96

Srinivasan, S., Gunasekaran, S., Ponnambalam, U., Savarianandam, A.,Gnanaprakasam, S., Natarajan, S., 2005. Spectroscopic and thermodynamicanalysis of enolic form of 3-oxo-L-gulofuranolactone. Indian J. Pure Appl. Phys.43, 459–462.

Wagner, W., Cooper, J.R., Dittmann, A., Kijima, J., Kretzschmar, H.J., Kruse, A., Mares,R., Oguchi, K., Sato, H., Stocker, I., Sifner, O., Takaishi, Y., Tanishita, I.,Trubenbach, J., Willkommen, T., 2000. The IAPWS industrial formulation 1997for the thermodynamic properties of water and steam. Journal of Engineeringfor Gas Turbines and Power-Transactions of the Asme 122, 150–182.

Wang, C., Fan, Y., Hornung, U., Zhu, W., Dahmen, N., 2020. Char and tar formationduring hydrothermal treatment of sewage sludge in subcritical andsupercritical water: Effect of organic matter composition and experimentswith model compounds. J. Cleaner Prod. 242, 118586.

Wang, C., Wang, F., Yang, Q., Liang, R., 2009. Thermogravimetric studies of thebehavior of wheat straw with added coal during combustion. BiomassBioenergy 33, 50–56.

Wang, C., Wu, C., Hornung, U., Zhu, W., Dahmen, N., 2021. Suppression of tar andchar formation in supercritical water gasification of sewage sludge by additiveaddition. Chemosphere 262, 128412.

Wang, C., Zhu, W., Chen, C., Zhang, H., Lin, N., Su, Y., 2018. Influence of reactionconditions on the catalytic activity of a nickel during the supercritical watergasification of dewatered sewage sludge. The Journal of Supercritical Fluids 140,356–363.

Wang, C., Zhu, W., Zhang, H., Chen, C., Fan, X., Su, Y., 2019. Char and tar formationduring hydrothermal gasification of dewatered sewage sludge in subcritical andsupercritical water: Influence of reaction parameters and lumped reactionkinetics. Waste Manage. 100, 57–65.

Wang, L., Schnepp, Z., Titirici, M.M., 2013. Rice husk-derived carbon anodes forlithium ion batteries. J. Mater. Chem. A 1, 5269–5273.

Zhao, X., Becker, G.C., Faweya, N., Correa, C.R., Yang, S., Xie, X., Kruse, A., 2018.Fertilizer and activated carbon production by hydrothermal carbonization ofdigestate. Biomass Convers. Biorefin. 8, 423–436.

Zheng, X., Chen, W., Ying, Z., Jiang, Z., Ye, Y., Wang, B., Feng, Y., Dou, B., 2020.Structure-Reactivity Correlations in Pyrolysis and Gasification of Sewage SludgeDerived Hydrochar: Effect of Hydrothermal Carbonization. Energy Fuels 34,1965–1976.