Gasification performance of biowaste-derived hydrochar: The properties of products and the conversion processes

Fuel 260 (2020) 116320

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Full Length Article

Gasification performance of biowaste-derived hydrochar: The properties of products and the conversion processes

T

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Xiuzheng Zhuanga,c, Yanpei Songa,c, Hao Zhanb, Xiuli Yina, , Chuangzhi Wua,c a Key Laboratory of Renewable Energy, CAS, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China b State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Protection and Resources Utilization, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Industrial biowastes Hydrothermal carbonization Gasification behaviors Reactivity

Hydrothermal carbonization (HTC) coupled with gasification is recognized as an effective approach to produce hydrogen-rich syngas from biowastes, but only a few studies concerning the gasification capacity of various biowaste-derived hydrochars have been published yet. In this work, three types of biowaste (i.e., lignocellulosic, non-lignocellulosic and ash-rich types) were selected for the HTC experiments, and the subsequent gasification of hydrochars was carried out to investigate its thermogravimetric curves and the properties of products (i.e., syngas, tar and biochar). The results found that HTC improves the gasification efficiency in terms of the syngas quality and the conversion degree of sample. The concentrations of H2 and CH4 in syngas were increased, while the gasified tar from hydrochars was concurrently reduced to half of its original value from biowastes under similar conditions. Furthermore, as HTC progressed, the gasification period and dynamic kinetics of samples exhibited slightly different, but the variation in the conversion process of gasification was similar; biowaste diversity in components is the reason for the former, while the developed aromatic structures in hydrochars is the explanation for the latter. These findings can not only provide a comprehensive knowledge on the gasification conversion of hydrochar, but also give a referential observation for designing, optimizing as well as scaling up the thermochemical conversion of industrial biowastes.

1. Introduction Biowastes, a type of organic waste originated from the industrial activities, are rising rapidly nowadays because of the accelerating process of industrialization [1,2]. In fact, the disposal of biowastes is one of the biggest challenges worldwide, and much efforts have been put on its suitable and harmless treatment until nowadays. Through gasification application, biowastes can be used in combined heat and power production, which is applied as an effective technology for the sustainable development [3,4]; however, prior to directly utilize biowaste as a feedstock for gasification, Tremel et al. [5] indicated that the original biowastes are not acceptable for established gasifiers which are basically designed for the coal or coke. Meanwhile, Mau et al. [6] and Zhan et al. [7] stated that the direct gasification of biowastes still facing technological difficulties and is easy to cause serious operational and environmental problems. For instance, the high content of nitrogen, sulfur and chlorine in biowastes result in the emission of gaseous

⁎

pollutant during gasification [7–9]; the stink and unstable nature of biowastes limit its store and transportation, while the high moisture content requires an expensive step (i.e., pre-drying) before gasification [10,11]. Tar formation from excessive volatiles and the lack of fuel uniformity are another two intractable problems encountered when gasifying biowastes [3,12]. For these reasons, to properly manage industrial biowastes, a pretreatment ahead of the gasification process is necessary, which is expected to serve as an upgrading process for not only the successful application in industrial scale but also the better performance in gasification. To the best of our knowledge, hydrothermal carbonization (HTC), also known as wet torrefaction, is one of the approach that developed as an ideal pretreatment for biowastes, especially for the wet types. Initially, HTC is reported by Bergius in 1913 with the purpose of upgrading low-rank coal in the present of water [10,13], and it has been recently reported as an artificial process concentrating on simulating the natural coalification of biomass under mild conditions [1,11]. This

Corresponding author. E-mail address: [email protected] (X. Yin).

https://doi.org/10.1016/j.fuel.2019.116320 Received 15 June 2019; Received in revised form 21 August 2019; Accepted 28 September 2019 Available online 16 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 The properties of industrial biowastes. Samples

HTW PMW SS

Proximate analysis (wt%, db)

Ultimate analysis (wt%, db)

HHV (MJ/kg)

VM

FC

Ash

C

H

S

N

O

69.38 78.28 39.89

17.23 14.04 3.51

13.39 7.68 56.60

45.08 44.78 21.57

5.91 6.16 3.67

0.26 0.55 0.51

2.61 7.43 3.43

32.75 33.40 14.22

19.37 19.06 9.67

Component analysis (wt%, daf)

Ash analysis (expressed as wt% of metal oxides)

Protein

Carbohydrate

Lipid

SiO2

Al2O3

CaO

Fe2O3

P2O5

K2O

MgO

13.02 35.49 34.35

82.74 61.12 57.79

4.24 3.39 7.86

21.98 0.39 42.40

7.92 0.14 22.34

20.78 22.64 5.24

4.82 0.50 8.04

4.56 30.86 14.56

7.64 19.15 2.45

7.66 3.62 2.53

Note: VM, volatile matters; A, ash; FC, fixed carbon; O (oxygen) was calculated by difference based on dry base; HHV, higher heating value; component analysis is determined on dry ash-free base.

well as ash-rich types, which corresponds to the feedstock of herb tea waste (HTW), penicillin mycelial waste (PMW) and sewage sludge (SS) in this study, respectively. The HTC experiment of different biowastes were carried out at first; subsequently, the gasification experiments of different hydrochars were performed via a tubular furnace (to explore the properties of gasification products) and a thermogravimetric analyzer (to explore the conversion process of gasification). On the whole, the specific objectives can be divided into two parts: 1) the influences of HTC on the gasification efficiency and the properties of gasification products (i.e., syngas, tar and solid phase; 2) the thermal conversion and the kinetic analysis of hydrochars during gasification process.

artificial process of coalification is capable of converting biowastes into coal-like fuels (i.e. hydrochar) which have significant advantages on grinding, handling, transport and storage; compared to the original biowastes, hydrochar is a far preferable feedstock for the subsequent applications aimed at energy production [2,8,14]. He and his coworkers [1] also confirmed that the HTC process is beneficial for providing a upgraded fuel without consuming exceeded energy and cost on dewatering; at the same time, the operational and environmental hazard caused by the direct gasification of biowastes can be minimized by the removal of harmful substances and the homogenization of carbonaceous structures during hydrothermal process [2,9]. HTC coupled with gasification technology is thus assumed as an effective and environmental-friendly approach to produce hydrogen-rich syngas from biowastes. Recently, relevant studies have gradually emerged and offer useful information for the combined system of HTC and gasification. Gai et al. [14] found that the gasification of hydrothermal biowastes results in a higher yield of syngas, which indicates an superior gasification efficiency of hydrochar to that of original biowastes. Chen et al. [15] employed HTC to pretreat bamboo and produce hydrochar being suitable for gasifier; their revealed that the upgraded bamboo have better effects on the temperature distribution in heat sink. In addition, the formation of tar is remarkably reduced by gasifying hydrochars, while the gasification degree have also proved to be increased after HTC [4,14,16]. As we commonly known, gasification is actually a chemically controlled process where the reactions are mutually occurred on solid particles, through external and internal diffusion [17]; so it can be deduced that the improvement in gasification performance mentioned above is primarily ascribed to the structural and compositional alterations of biowastes during HTC. However, the differences in biowaste components maybe an interrelated factor affecting the gasification reactivity of hydrochars because each component follows a unique route during HTC process and therefore leads to the different structural features in hydrochars to some extent [11]. So far, most of the available reports usually focus on the single biowaste with specific purpose (such as the dewatering of sewage sludge via HTC), and the influence of biowaste diversity on the HTC coupled with gasification system is unfortunately lacking. Biowaste diversity deserves a great consideration since they behave differently from each other during the HTC pretreatment and subsequent gasification process, which is essential in bridging the gap from those potential resources to the alternative renewable fuel. Consequently, the present work aims at providing a general understanding on the feasibility of utilizing different types of biowaste for energy production, so the selection of feedstock is dependent on the diversity of industrial processes and the major composition. According to previous literatures [11,13], industrial biowastes can be mainly classified into three categories: lignocellulosic, non-lignocellulosic as

2. Material and methods 2.1. Biowaste samples Given this work, HTW was supplied by a traditional Chinese medicine enterprise (Guangdong Province), while PMW and SS were obtained from a pharmaceutical enterprise (Hebei Province) and a largescale wastewater treatment plant (Guangdong Province), respectively. Prior to the analyses and experiments, all of them were required by drying, grinding and sieving to harvest target samples with the uniform particle size ranged from 0 to 300 μm. The results relevant to the fundamental properties of industrial biowastes are listed in Table 1, including the proximate, ultimate and composition analysis. 2.2. Experimental procedure 2.2.1. Hydrothermal carbonization (HTC) The HTC experiments of industrial biowaste were performed via a 250 ml bench-top autoclave reactor (SLM250, Senlang Co. Ltd, China), of which the schematic diagram is depicted in Fig. 1(a). Detailed information for the operational processes of HTC had been reported elsewhere [9,11]. In brief, each reactant was premixed to achieve a slurry density of 10% w/w by means of blending 10 g of oven-dried sample with 100 ml of pure water in reaction vessel; then, the reactor was sealed and flushed with high purity argon (99.999%) for 10 min to create an oxygen-free atmosphere. Terminal temperatures for HTC ranged from 150 °C to 240 °C with the regular intervals of 30 °C in this study, while the heating rate, holding time and stirring speed were fixed at 5 °C/min, 30 min and 300 rpm, respectively, functioning as an avoidance for the secondary interferences. Once the reaction period had elapsed, the reactor was rapidly quenched to ambient temperature and the hydrothermal slurry inside was filtered to obtain the solid product, namely hydrochar. Finally, the dry (105 °C, 24 h) and reground (0–300 μm) hydrochars for the following gasification were labeled as “HTW/PMW/SS-XXX”, where the former “HTW/PMW/SS” and the latter “XXX” represents the type of biowaste and the hydrothermal 2

Fuel 260 (2020) 116320

X. Zhuang, et al. Mass flow meter

Volumetric gas meter

Temperature probe

Motor

Pressure Gauge

rubber plug Cotton filter

Venting

Glass bead

Temperature Probe

Ice/water mixture Desiccant bead

horizontal tubular quartz reactor Three-way control value

Reactor tube Heating furnace

Tar trap system

Steam injector Gasification processes A

Ar

Hydrothermal reactor

V

T/

Control panel

A

Ar

V

T/

Conversion ratio/ kinetic analysis

Tar phase

GC/MS

Gas phase

Gas chromatography

Control panel

(a) Hydrothermal carbonization

(b) Steam gasification Fig. 1. The schematic diagram of experimental setups.

2.3. Physicochemical analysis

temperatures, respectively.

2.3.1. Fundamental properties The ultimate and proximate analyses of raw materials and hydrochars were conducted in an elemental analyzer (Vario EL cube, Elementar analyse, Germany) and a muffle furnace (MXX1100-30, shmicrox Co., Ltd., China), respectively. Both the fraction of fixed carbon (FC) and oxygen (O) were calculated by differences. Additionally, the higher heating value (HHV) of solid samples was measured by a calorimetric bomb (IKA C2000, Germany), while the energy density was assessed via the quotient of HHV in hydrochar and that in raw material. Non-condensable gases were monitored by a gas chromatograph (GC, Agilent, GC490) which equipped with two thermal conductivity detectors (TCDs). One of the columns in TCDs was Molsieve5A (120 °C) that mainly served for the measurement of H2, CO and CH4, while the another column in TCDs was PoraPLOT Q (80 °C) that used for analyzing CO2, C2H2, C2H4 and C2H6. The lower heating value of gaseous products (LHVg, MJ/Nm3) was evaluated according to the volume percentage of H2, CO and CH4, whose equation was descripted as Eq. (1) below:

2.2.2. Steam gasification Steam gasification of samples was performed by using a bench-scale high-temperature fixed bed system, including a horizontal tube reactor, a steam injector and a tar trap system [7]. Herein, the tubular reactor (44 mm in inner diameter, 1200 mm in length) was heated by an electric furnace equipped with a control panel that govern the terminal temperature and the heating rate. In addition, as could be seen from the schematic diagram depicted in Fig. 1(b), the upstream of tubular reactor attached with an inert gas line and steam injection kit, while the downstream was followed by a tar trap system which also functioned as a gas purification unit. Specifically, distilled water was fed by a peristaltic bomb, and the steam was mainly generated in the connection (approximately 200–300 °C) between injector and reactor. The tar trap system was comprised of a cold trap and a bubbling absorption lines with four washing bottles. In this study, the middle bottles (the second and third) filled with 200 ml dichloromethane were immersed in the ice/water mixture for the sake of effectively collecting tar product, while the first (empty) and the fourth (with desiccant bead) bottles were set for preventing the inverse liquid flow and purifying the gaseous product, respectively. All of these parts were directly connected with tubular reactor in a flexible seal way, and the connection segment was preheated by an electrical heating belt to avoid volatiles condensation during transferring. All runs of the steam gasification were carried out in a fast mode and based on the below procedure: the quartz boat loaded with 1 g of biowaste or hydrochar was placed in the up zone of tubular reactor at first. Afterwards, H2O (every 0.5 g per min) carried by the high purity argon with a flow rate of 100 ml/min was continuously supplied into the reactor to create a humid atmosphere during the heating program of tubular reactor from 25 °C to 900 °C. When the terminal temperature became stable and the air inside was completely replaced, the quartz boat was promptly pushed to the heating zone (the middle of reactor) by a rod and hold for 30 min. Within the gasification period, the condensable phase in gas flow was trapped by dichloromethane in the washing bottles. Gasified tar was condensed by naturally drying in the fume hood below 55 °C; the obtained tar was then stored at 4 °C prior to the next analyses. Meanwhile, the non-condensable syngas was gathered by a sampling bag during the entire reaction stage, followed by the analysis of gaseous composition. At the end of the gasification process, electric power was turned off and the quartz boat was pulled back to the up zone of tubular reactor waiting to cool down and weight.

LHVg = 10.8 × H2 + 12.6 × CO2 + 35.8 × CH4

(1)

The components in tar phase were analyzed via GC–MS analysis that was operated on a Thermo Trace 1300-ISQ QD (Thermo Fisher Scientific, USA) with a TriPlus RSH™ auto sampler. A fixed flow of highly purified helium at 1.0 ml/min was functioned as carrier gas and passed through a nonpolar TG-5MS capillary column (30 m × 0.25 mm × 0.25 μm). The oven temperature program was initiated at 40 °C for 3 min before heating to 180 °C at 10 °C/min, where was followed by an another increase up to 280 °C at 5 °C/min and a balance at 280 °C for 10 min. Furthermore, the temperature of transfer line, ion source and auxiliary in the MS detector were set at 300 °C, 280 °C and 260 °C, respectively. Data acquisition from GC–MS was scanned through a range of 30–550 m/z (1600 mua s−1), and a solvent delay of 5 min was adopted in this study. Finally, the obtained mass spectra were compared with those from the National Institute of Standards and Technology (NIST) database, thereby identifying the specific components and its related fraction in tar. 2.3.2. Thermogravimertic analysis (TGA) In attempt to further analyzing the gasification performance, the conversion processes of raw and hydrothermally carbonized biowastes were investigated by a thermogravimetric analyzer (SDT 650, TA Instruments Co. Ltd, U.S.A.) in the presence of CO2 atmosphere. In each test, approximately 5 mg of sample was loaded in a 150 μl alumina crucible; then the crucible with sample was subsequently placed back to the analyzer chamber. Non-isothermal conditions were employed for 3

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X. Zhuang, et al.

21

60 50

18 1.36

1.2

1.02

1.06

1.14

1.0 0.8

150

180

210

80

O

34.79%

60

H

6.96%

20

C

120

60

24

40

22

20

20

1.2 1.0

HHV/ (MJ/kg)

26

1.4

1.53

1.24 1.12 150

180

210

150

180

6.85%

6.99%

62.54%

68.40%

210

240

FC

15.36%

16.04%

16.58%

19.15%

VM

68.85%

65.81%

63.40%

58.38%

Ash

15.79%

18.15%

20.02%

22.47%

150

180

210

240

80 60 40 20 0

120

N 80 60

0

240

7.36%

7.64%

O

34.33%

H

7.26%

31.59%

7.24%

6.74%

20.61%

16.55%

C

120

Hydrothermal temperature/ °C

7.03%

7.14% 7.15%

40 20

Hydrothermal temperature/ °C

PMW-based hydrochar

100

Element proportion/ %

Energy density

Yield/ wt%

80

1.44

58.61%

54.90%

22.12%

Hydrothermal temperature/ °C

Hydrothermal temperature/ °C

1.6

6.89%

27.61%

40

0

240

31.61%

53.35%

50.20%

150

180

64.46%

69.17%

210

100

proximate proportion/ %

1.4

100

N proximate proportion/ %

70

100

Element proportion/ %

Yield/ wt%

Energy density

24

80

HHV/ (MJ/kg)

HTW-based hydrochar 90

14.35%

15.89%

17.15%

18.94%

VM

76.94%

73.61%

68.24%

63.87%

Ash

8.71%

10.50%

14.61%

17.19%

150

180

210

240

60 40 20 0

240

FC 80

120

Hydrothermal temperature/ °C

Hydrothermal temperature/ °C

75

10 9

70

8

Energy density

65 1.2

1.12

1.1

1.0

0.87

0.83

N 80 60

O

6.15%

6.38%

27.16%

33.56%

5.98% 16.66% 9.41%

9.89%

8.27%

H

8.37%

40 20

5.89% 11.05%

C

50.38%

120

150

57.07%

66.37%

71.53%

100

proximate proportion/ %

11

HHV/ (MJ/kg)

80

Element proportion/ %

Yield/ wt%

SS-based hydrochar 100

80

FC VM

31.57%

Ash

65.37%

19.66%

16.83%

72.50%

77.91%

81.00%

180

210

240

24.70%

60 40 20

0.8 150

180

210

Hydrothermal temperature/ °C

240

0

180

210

240

Hydrothermal temperature/ °C

0

120

150

Hydrothermal temperature/ °C

Fig. 2. The fundamental properties of biowastes and its hydrochars (note: the results of element analysis were based on dry ash-free base).

where mi and mf indicate the initial and final mass of sample at each individual stage, respectively; mt is the instantaneous mass of sample during conversion process.

the gasification process, which followed an uncomplicated program of heating from 25 to 1200 °C at a rate of 10 °C/min. Carbon dioxide was continuously supplied at a steady rate of 150 ml/min throughout the overall process of gasification; at the same time, the loss of mass and its loss rate were recorded under the dynamic conditions. All experiments were repeated twice to ensure the great reliability and reproducibility. Moreover, there is well established that the entire gasification processes includes a preliminary devolatilization (for organic matter) and a following gasification stage (for char) [18]; the former is identified as the precursor step of gasification [8,19]. As a result, several characteristic parameters, i.e., the initial temperature (Ti), the final temperature (Tf), the temperature of maximum conversion rate (Tm), the maximum conversion rate ((dα/dt)max), the mean conversion rate ((dα/dt)mean) and the conversion degree (α), were all evaluated for two independent stages to fully understand the gasification performance of biowastederived hydrochars. Among them, the α of sample in the devolatilization or gasification stages was calculated as Eq. (2) below:

α = (mi − mt )/(mi − mf )

2.3.3. Kinetic models Frist of all, it needs to point out that the kinetic analysis only focuses on the second stage (i.e., the char gasification) in this study. Based on our best knowledge, all existing kinetic models can be divided into two groups: a theoretical model and a semi-empirical one [20]. Volumetric model is widely used for the kinetic analysis, which assumes that the gasified agents are uniformly distributed on both outside and inside of the particle surface; hence, gasified agents react with sample at all active sites [21,22]. Substantially, the volumetric model is a pseudofirst order kinetic model which has proved to be sufficient enough for the thermal conversion of biomass or organic wastes [1,13]. The kinetic parameters of CO2 gasification can be descripted as Eq. (3) according to the Arrhenius formula [23]:

(2) 4

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dα A E ⎞·(1 − α ) = ·exp ⎛− dT β ⎝ RT ⎠

form), while the C content is accumulated via the carbonization [25]. Above reactions occur cooperatively during HTC process and reduce the atomic ratios of H/C and O/C significantly, in turn, developing the structural network and upgrading the energy density [13,26]. Furthermore, the variations in the elemental compositions correspond exactly to the results of proximate analysis. The loss of volatile matter (VM) in hydrochars (HTW: 69.38–58.38 wt%; PMW: 78.28–63.87 wt%; SS; 39.89–16.83 wt%) confirms the devolatilization process of HTC by the aforementioned reactions (i.e. carboxylation, dehydration and carbonization) during HTC; part of the released VM is converted to the gaseous products, while the remaining is dissolved into the liquid phase and further contributed to the formation of additional fixed carbon (FC) in higher hydrothermal temperatures [9,27]. Interestingly, He et al. [28] stated that the VM is probably not transformed to the FC directly, but tends to form the heavy oil which subsequently deposits on the surface of hydrochars and thus raise the FC content in proximate results. Finally, the excessive loss of VM and the retained minerals in hydrochars give rise to the progressive increase of ash content. Once again, the large amount of ash in SS-derived hydrochars explains its inferior energy density, albeit the upgrading process for SS did occur to some extent.

(3)

where E and A represent the activation energy (kJ·mol−1) and pre-exponential factor (min−1) of the reaction, respectively; α, which is descripted above, is the thermal conversion of char at kelvin temperature T (K); R is the constant coefficient of universal gas (8.314 J·K−1·mol−1), while β indicates the fixed heating rate of 10 °C/min. Moreover, Eq. (3) is rearranged in an integral form as:

G (α ) =

∫0

α

dα = (1 − α )

∫T

T

0

A E ⎞ dT exp ⎛− β ⎝ RT ⎠

(4)

Further using the Coats-Redfern method to integrated Eq. (4), exhibiting as:

G (α ) AR 2RT ⎤ E ⎛1 − ⎞ − In ⎡ 2 ⎤ = In ⎡ ⎢ E ⎠⎥ RT ⎣ T ⎦ ⎣ βE ⎝ ⎦

(5)

In general, the term 2RT/E is far less than 1 as the given temperatures in gasification stage, and Eq. (5) is thus simplified as:

G (α ) −In (1 − α ) ⎤ AR E In ⎡ 2 ⎤ = In ⎡ = In − 2 T T βE RT ⎦ ⎣ ⎦ ⎣

(6)

2

Consequently, a straight line of In[G(α)/T ] versus 1/T is obtained. The E and A can be calculated by linear fitting with the slope-E/R and intercept In(AR/βE), respectively. The values with the highest determination coefficient (R2) is selected.

3.2. The gasification efficiency of hydrochars 3.2.1. Gas products The steam gasification of samples was carried out in a tubular reactor before and after the HTC pretreatment, and the conversion efficiency of HTC coupled with gasification technology was investigated by analyzing the characteristics of gas, tar and char products. As can be seen in Fig. 3, the non-condensable gaseous components were comprised of carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), hydrogen (H2) and several light hydrocarbons (C2-C4) [4,14]. Among them, as the primary component in non-condensable gases, the concentration of H2 (based on the amount of total gas) became even more predominant over other after HTC, increasing from 44.2% (HTW), 40.0% (PMW) and 49.6% (SS) to 56.6%, 53.1%, and 56.3% at 240 °C, respectively. Previous studies indicated that the gasification of biowastes under steam atmosphere proceeds in two consecutive stages: an initial step related to the decomposition of volatiles (at 200–600 °C) and a following step included the conversion of char and gaseous substances (over 600 °C) [29–31]. For this reason, the formation of H2 during steam gasification is accordingly grouped into two steps according to the temperature range. Part of H2 is formed as the cracking or reforming of volatile matters (3.1) in the preliminary step, while the rest of H2 originates from the secondary reaction of steam-tar reforming (3.2), char gasification (3.3) as well as water-gas shift (3.4) at the higher temperatures [29,30]. In fact, Gai et al. [14,17] found that the secondary reactions contribute a more importing part in producing H2 during gasification process; thus, it can be deduced that the higher H2 content produced by hydrochars is caused by the change in carbonaceous structures that is relevant to the secondary stage of gasification, although the devolatilization process in HTC slightly decreases the formation of H2 in the first step. On one hand, the surface area and pore volume of hydrochars were obviously improved by hydrothermal pretreatment, which accelerated the diffusion of steam and tar on the particle sites and consequently promotes both the water-gas shift reaction and the in situ reforming of tar [4]. The SEM images from Gai et al. [17] support this explanation because the fragmentized structure of hydrochars enhances the porous access for steam reforming. On the another hand, the carbonaceous residue in hydrochars was constantly accumulated during HTC, thereby raising the reactant for both char gasification and water-gas shift reactions [30]. Additionally, alkalineearth metals such as K, Na, Ca, and Mg were also enriched in the solid phase after HTC, especially for SS whose ash content reached the highest value, playing an positive role in catalyzing the production of H2 via the steam gasification and tar reforming [4].

3. Results and discussion 3.1. Fundamental properties of hydrochar Fig. 2 displays the yield of biowaste-derived hydrochars under different HTC temperatures; as expect, a steady reduction of yield was observed by elevating the hydrothermal temperature in HTC process, dropping from 84.08 wt% (HTW-150), 71.16 wt% (PMW-150) and 79.75 wt% (SS-150) to 53.5 wt%, 23.3 wt% and 68.0 wt% at 240 °C, respectively. Both Peng et al. [24] and Smith et al. [25] believed that the downtrend of yield with temperature is mainly attributed to the thermal decomposition of organic matters into the liquid and gaseous phase via hydrolysis, dehydration, and decarboxylation during HTC. In addition, the another noteworthy finding in Fig. 2 is the upgrading effect of HTC on biowastes, which is in accord with the previous studies reported by Ma et al. [26]. A significant uptrend in the higher heating value (HHV) was observed after HTC, and the energy density was correspondingly increasing, especially for HTW and PMW whose HHV jumped up to 23.25 and 25.75 MJ/kg at a hydrothermal temperature of 240 °C, respectively. This result is caused by the conversion of lowenergy chemical bonds toward the bonds with high-energy state during hydrothermal process, which is beneficial for the subsequent gasification in terms of efficiency and reactivity [1,11]. However, SS might have an obstacle for its upgrading via HTC pretreatment because the conversion of chemical bonds from “low-energy state” to “high-energy state” in SS is affected by their limited organic matter and abundant unreactive ash. Consequently, the HHV and the energy density in SSderived hydrochars were almost constant or decreased slightly [13]. The elemental and proximate results visualized in Fig. 2 also support the energy densification during HTC process. With regard to its elemental compositions, the content of nitrogen decreased steadily after HTC, while that of sulfur followed the similar trends but could be negligible in Fig. 2 as all samples contained a low sulfur content (HTW: 0.26 wt%; PMW: 0.55 wt%; SS: 0.51 wt%). The carbon (C) content in hydrochars treated at 240 °C got a remarkably increase to 24.6%, 37.8%, and 41.9% for HTW, PMW and SS, respectively; on the contrary, that of oxygen (O) and hydrogen (H) gradually decreased as the HTC progressed. The decreased in the O and H contents are mainly owing to the reactions of decarboxylation (in CO2 form) and dehydration (in H2O 5

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60% 40% 20% 0%

78.8

78.9

77.7

76.4

76.7

10.1

10.3

9.86

8.99

9.04

19.2

17.9

16.5

13.8

12.3

10.0

8.84

11.7

12.0

36.4

34.8

Raw

150

11.5

38.1

180

40.7

210

43.4

80% 60% 40% 20% 0%

240

100%

10.0

HTW PMW SS

98%

9.5

96% 94% 92%

83.3

83.2

12.8

11.6

11.1

20.7

16.0

14.9

13.4

13.7

14.2

39.3

37.7

33.8

Raw

150

180

79.7

77.8

11.7

9.58

16.1

13.1

10.9

37.5

210

9.71

41.3

80% 60% 40% 20% 0%

240

SS-based sample

40.1 4.8 8.3 5.3

32.0

26.8

22.2

6.3 4.2

5.5

19.9

16.4

14.3

12.1

10.8

Raw

150

180

210

240

19.2

4.1

Hydrothermal temperature/ °C

HTW PMW SS

9.0 8.5 8.0

90% 88%

84.5

Hydrothermal temperature/ °C

LHVg/ (MJ/Nm3)

Conversion degree/ % dry ash-free basis

Hydrothermal temperature/ °C

100%

PMW-based sample

Gas compositions/ Vol.% dry basis

80%

100%

HTW-based sample

Gas compositions/ Vol.% dry basis

Gas compositions/ Vol.% dry basis

100%

Raw

150

180

210

240

7.5

Hydrothermal temperature/ °C

Raw

150

180

210

240

Hydrothermal temperature/ °C C -C 2 4

CO

2

CO

CH

4

H 2

gas yield

Fig. 3. The characteristic of gas products from hydrochar gasification in tubular reactor.

CxHyOz → tar + permanent gases(CO,CO2,H2,CH4) + H2O

(3.1)

CxHy + 2xH2O → 2x + y/2)H2 + xCO2

(3.2)

C + H2O ⇋ CO + H2

(3.3)

CO + H2O ⇋ CO2 + H2

(3.4)

hydrochar are also illustrated in Fig. 3. With the increased severity of HTC conditions, the LHVg of syngas steady climbed from 7.9 MJ/Nm3 (HTW), 8.2 MJ/Nm3 (PMW) and 8.4 MJ/Nm3 (SS) to 9.1 MJ/Nm3, 9.5 MJ/Nm3 and 9.6 MJ/Nm3 at a hydrothermal temperature of 240 °C, respectively. This improvement in LHVg coincides with the results of gas composition mentioned above, which is achieved by the increase in H2 and the decrease in CO2 [4,17]. At the same time, a better conversion degree is obtained when gasifying hydrochars; the improved gasification efficiency confirms that HTC benefits in the production of hydrogen-rich syngas by ameliorating the carbonaceous structures in hydrochar for steam gasification [14,17].

Regarding CH4 whose concentration occupied the minimum part in all samples, the contribution of CH4 toward gaseous products could be negligible, even though the CH4 content from samples slightly increased (via (3.5)) after treated by HTC. In comparison to the uptrend of H2 and CH4, the content of CO, CO2 and C2-C4 followed a reverse trend in hydrochars during gasification process. This downtrend can be explained by referring to the devolatilization process of HTC [12,31]. All of them started from a relative high value in raw materials and then reduced gradually with the increase of HTC temperatures, CO2 and CO in particular (follows a reaction pattern of (3.6) and (3.7), respectively). As a consequence, it can be concluded that HTC is favorable to remove the oxygen-containing functional groups in biowaste matrix for the subsequent gasification, which avoids the formation of useless gaseous components [30,31]. The removal of oxygen-containing functional groups also suppresses the consumption of hydrogen [12]; thus, the production of CO and CO2 maintains at a lower fraction in syngas derived from hydrochars, whereas that of H2 is opposite. C + 2H2 ⇋ CH4

(3.5)

C + CO2 ⇋ 2CO

(3.6)

CH4 + H2O ⇋ CO + 3H2

(3.7)

3.2.2. Tar products As we commonly known, tar is a black and sticky material arising potential risks on the system malfunction during industrial operation; tar is therefore considered as one of the biggest problems in the commercialization of syngas via gasification application. Many studies define tar as a generic (unspecific) term for complex organic compounds that exists in the gaseous products (excluding light hydrocarbons) of gasification [3]. Unfortunately, less of them so far have discussed the effects of HTC on the gasified tar. In this study, Fig. 4 compares the differences in tar compositions derived from various feedstock and hydrochars; herein, the tar yields are exhibited on mass basis by weighting the tar products after steam gasification. Under the similar gasification conditions, a reduction of tar yield was easily observed for all biowastes as a function of increased HTC temperatures, which is similar to the variation of volatile matters. The maximum yields of gasified tar were 8.98 wt%, 11.10 wt%, and 5.76 wt% for HTW, PMW, and SS, respectively, whereas the minimal value appeared in the hydrochars pretreated by 240 °C that reached 3.22 wt% (HTW-240), 5.27 wt% (PMW-240), and 2.37 wt% (SS-240). Actually, the decrease in gasified tar is caused by the removal of VM during HTC, which inhibited the production of harmful tar in the subsequent gasification application [3]; this phenomenon can also be found in the torrefied biowaste via dry torrefaction. Gai et al. [14,17] and Feng et al. [4]

In general, the yield of total gas (based on db) underwent an apparent downtrend after HTC, but the extents were different from each biowaste. This result is mostly due to the hydrolysis of organic matters in HTC pretreatment; however, the quality of syngas derived from hydrochars was proved to be enhanced. The evolution of lower heating value (LHVg) in syngas and the conversion degree (based on daf) of 6

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4.6

10 Tar yield/ wt%

7.2

7.4

9.9

100 15.3 80

28.6 44.3

61.0 73.4

8

74.8

77.5

90.0

60 80.1

61.5

21.2 7.4

6 25.4

17.8 4

HTW PMW SS

2 Raw

6.6 12.3 150

180

210

Hydrothermal temperature/ °C

240

7.9

5.0

12.4

6.0 8.0

5.9

17.6

40

26.4

20 10.7

12.0

8.1

180 240 40 80 SS HTW HT PMW PMW- PMWSS-2 SS-1 HT Mono-aromatic hydrocarbons (MAHs) Poly-aromatic hydrocarbons (PAHs) Nitrogenous compounds Aliphatic hydrocarbons Oxygenous compounds 0 W-18

0 W-24

Tar composition/ %

12

0

Fig. 4. The characteristic of tar products from hydrochar gasification in tubular reactor.

PMW: 6.4 wt%–16.9 wt%; SS: 54.2 wt%–78.5 wt%). So far, the application of gasified biochar has been paid special attention for the sake of a sustainable and comprehensive development of gasification process. Many scholars have envisioned the feasibility of biochar as a basic material with various capabilities [32], such as 1) the catalysts for tar removal or biodiesel production, 2) the additives for anaerobic digestion or re-gasification, 3) the capacitor for direct carbon fuel cells, and 4) the adsorbent for soil or water amendment. Moreover, gasified biochar can even be used as the precursor of activated carbon characterized by sufficient surface area and larger porosity [33]; but the difficulties still exist due to the weakness of biochar in terms of physicochemical and electrochemical properties. The possible improvements of HTC on the biochar properties require further investigation when considering HTC coupled with gasification technology.

supported this results by concluding that with an increase in pretreatment severity, the yield of tar reduces whereas the mass fraction of char and syngas increases. In addition to the yield of gasified tar, the specific components in tar calculated from GC–MS results are compared in Fig. 4; overall, these components can be classified into five types [4]: 1) mono-aromatic hydrocarbons (MAHs); 2) poly-aromatic hydrocarbons (PAHs); 3) nitrogenous compounds; 4) aliphatic hydrocarbons, and 5) oxygenous compounds. To be accurate, the gasified tar obtained in this study may belong to the tertiary tars according to the conclusions from Horvat et al. [3] Primary and secondary tars undergo rearrangement and are converted into tertiary tars (> 800 °C) via the condensation reaction, which results in the purely aromatic compounds. As a consequence, it is reasonable that the sum of aromatic types in tar was dominant and occupied over 50% of the total composition in almost all samples. The content of PAHs accounted for a much higher proportion when compared to MAHs, and its proportion in tar experienced a rapid growth with the increase of HTC temperatures; after HTC, the PAHs increased significantly from 73.4% (HTW), 61.0% (PMW) and 28.6% (SS) to 90.0% (HTW-240), 80.1% (PMW-240) and 61.5% (SS-240). Herein, HTW possesses the maximum content of PAHs due to the exist of lignin that reacts as the major precursor for aromatic derivatives [11]; PMW ranks the second because the abundant organic matter in it can also provide the intermediates for aromatic derivative during the HTC as well as the gasification processes [7,13]. However, PAHs in SS-derived tar was relatively lower than that in HTW and PMW, which is probably caused by the limited organic substance in SS. Two explanations are responsible for the growth of PAHs [3,4]: (1) the aromatization degree of biowastes is strongly developed under hydrothermal environment, which results in releasing more aromatic compounds in the following gasification directly; (2) the further evolution of aromatic matrix from lower to higher aromaticity occurs during gasification process (via cyclopentadienyl recombination and Diels-Alder reaction). Consequently, the increase of aromatic compounds was identified in gasified tar, PAHs in particular. The variation of PAHs also coincides with the development of primary or secondary tars toward the tertiary tar as the polyaromatic structures are superior in thermal stability. On the contrary, other compositions (i.e., aliphatic hydrocarbons, nitrogenous and oxygenous compounds) in gasified tar were all found to be at a lower level after HTC, which indicates a good agreement with the decrease of VM in hydrochars.

3.3. Gasification behavior of hydrochar 3.3.1. Thermogravimetric analysis The CO2 gasification of samples following a non-isothermal procedure was performed in a thermogravimetric analyzer to clarify the influences of HTC on the subsequent gasification conversion, which includes the thermogravimetric processes, the gasification period and the kinetic parameters. Generally, the gasification of samples under CO2 atmosphere is a complex process as it involves several chemical reactions with mutual effects [30]. The whole conversion process is classified into two consecutive steps as discussed above: a devolatilization step within 200–600 °C and a gasification step between 600 °C and 1200 °C in this study. Each step serves as different functions during the conversion process. Bach et al. [18] stated that the former step mainly concentrates on producing char in an oxygen-starved environment; subsequently, the partial oxidation of carbonaceous residue occurs in the latter step, leading to the remarkable production of syngas. Therefore, the discussions of thermogravimetric process are also divided into two parts for the better understandings of gasification performance, but an emphasis is on the latter step of gasification. This emphasis is because the devolatilization of biowastes under oxygenstarved atmosphere shows a great similarity with that in the pyrolysis process, which is fully elucidated in other works [11,13]. Fig. 5 presents useful information in the mass loss and the conversion curves of CO2 gasification of raw and hydrothermally treated biowastes within the temperature range of 50–1200 °C. Several characteristic parameters associated with the conversion curves are extracted and summarized in Table 2. Based on the Fig. 5(a), (d), and (g), it is visible that all biowastes and hydrochars suffered from two sharp and major stage of mass loss during the whole conversion process,

3.2.3. Char products Biochar is another essential product from gasification, whose yield experienced a stable uptrend after HTC pretreatment due to the enrichment of ash content in hydrochars (HTW: 12.3 wt%–20.0 wt%; 7

Fuel 260 (2020) 116320

X. Zhuang, et al.

Fig. 5. The gasification curves for biowastes and its hydrochars.

Table 2 Characteristic parameters of biowaste-based hydrochars during gasification processes. Sample

HTW HTW-150 HTW-180 HTW-210 HTW-240 PMW PMW-150 PMW-180 PMW-210 PMW-240 SS SS-150 SS-180 SS-210 SS-240

Ti (°C)

Tm (°C)

Tf (°C)

(dα/dt)max × 103 (s−1)

(dα/dt)mean × 103 (s−1)

D-step

G-step

D-step

G-step

D-step

G-step

D-step

G-step

D-step

G-step

185 195 200 195 198 183 185 188 190 188 176 179 172 188 184

630 640 645 658 664 785 790 799 853 865 702 728 745 758 764

343 360 363 368 369 291 305 317 367 373 286 340 349 354 450

832 788 815 840 846 901 911 943 1011 1009 866 862 883 886 878

550 560 595 610 624 534 548 556 598 613 616 617 631 633 629

912 910 923 975 1034 948 961 980 1121 1164 955 982 991 1113 1158

1.271 1.408 1.256 1.307 0.922 0.739 0.816 0.921 0.644 0.521 0.258 0.226 0171 0.123 0.108

0.250 0.259 0.251 0.276 0.331 0.604 0.583 0.727 0.449 0.456 0.066 0.072 0.067 0.073 0.071

0.309 0.311 0.306 0.282 0.255 0.298 0.301 0.275 0.236 0.203 0.123 0.096 0.075 0.066 0.064

0.151 0.147 0.153 0.151 0.146 0.184 0.207 0.185 0.135 0.120 0.023 0.024 0.023 0.025 0.024

Note: D-step, devolatilization step; G-step, gasification step; Ti, initial temperature; Tm, corresponding temperature of the peak conversion rate; Tf, final temperature; dα/dtmax, maximum value of conversion rate; dα/dt mean, mean value of conversion rate.

8

Fuel 260 (2020) 116320

PMW-derived hydrochar

SS-derived hydrochar

0

21

0

18

0

15

0

Ra w

HTW-derived hydrochar

24

Hydrothermal temperature/ °C

X. Zhuang, et al.

24

28

32

36

40

10

15

20

25

30

20

25

30

35

40

Gasification period/ min Fig. 6. The specific gasification period of biowaste-derived hydrochar. -12

-12

HTW-derived hydrochars

SS-derived hydrochars

PMW-derived hydrochars

-16

In[-ln(1-Į)/T2]

-14

-14

In[-ln(1-Į)/T2]

In[-ln(1-Į)/T2]

-14

-16 PMW, y=-37886x+18.18, R2=0.9932

SS, y=-16505x+0.023, R2=0.9715

HTW-150, y=-17620x+2.23, R2=0.9986

PMW-150, y=-39119x+19.14, R2=0.9957

SS-150, y=-17590x+1.07, R2=0.9877

2

HTW-180, y=-18474x+3.08, R =0.9979

2

PMW-210, y=-11864x-6.26, R2=0.9936

2

0.9

PMW-240, y=-12519x-5.31, R =0.9825

1.0

1.1

0.80

0.85

(1/T)î10-3/ .

activitation energy/ (.-/mol)

SS-210, y=-16472x-0.283, R2=0.9736

2

HTW-240, y=-14508x-1.21, R =0.9931

0.8

SS-180, y=-16930x+0.473, R2=0.9921

PMW-180, y=-30341x+10.6, R =0.9715

-18

HTW-210, y=-15611x+0.001, R2=0.9953

-18

-16

HTW, y=-17188x+1.65, R2=0.9944

SS-240, y=-16561x--0.142, R2=0.9867

-18 0.90

0.95

0.85

0.90

(1/T)î10-3/ .

1.00

1.05

HTW-derived hydrochars PMW-derived hydrochars SS-derived hydrochars

325.24

314.98

0.95

(1/T)î10-3/ .

300

252.26 200

142.90

137.22

146.49

146.24

153.59

140.76

129.79

0

128.63 98.64

100

Raw

140

160

180

200

120.62

220

104.08

129.37

240

Hydrothermal temperatures/ °C

Fig. 7. The kinetic results and the activation energy of biowaste-derived hydrochars during gasification process.

above conclusions in Section 3.2.1. In addition, the curves of conversion rate versus the heating temperature within devolatilization (Fig. 5(b), (e), and (h)) and gasification stage (Fig. 5(c), (f), and (i)) are concurrently visualized, along with the conversion degree (α) shown on its upper right. Following the conversion curves between 200 and 600 °C, it can be deduced that the samples are converted into char via different pathways as the differences in conversion rates and the shape of conversion curves; however, several similarity can also be found. Despite the differences of original components (i.e., lignocellulose and non-lignocellulose) in sample, all of them are converted into a carbonaceous material with similar structures after the devolatilization step and then gasified to form syngas in higher temperatures (> 700 °C) in the gasification step [22,34]. Accordingly, the conversion peak within the gasification step usually appears in a single broad peak (without shoulder or tail), which is featured by the gasification of produced char [20]. With the increase of HTC temperatures, the gasification onset temperature (Ti in G-step) increased slightly but the gasification process was generally prolonged due to the significant rise of Tf in G-step (shown at Table 2). Meanwhile, the peak corresponded to the maximum conversion rate within

which is consistent with the division of devolatilization and gasification stage. A total mass loss reached 93.5% (HTW), 92.3% (PMW) and 41.7% (SS) for raw biowastes; nevertheless, the decomposition of VM during HTC pretreatment reduced the conversion content in CO2 gasification, reflecting in the decrease of mass loss in hydrochars (HTW: 93.4–86.5%; PMW: 91.9–79.5%; SS: 34.7–25.8%). It is noteworthy that the inferior conversion degree of SS-derived hydrochars in gasification step is possibly caused by the limited FC and the abundant ash in its original material, even though the alkali and alkaline earth metals in ash may catalyze the gasification process [25]. Furthermore, regarding the specific stage of conversion curves in samples, the variations of mass loss were different from each other after treated at different HTC temperatures. During the devolatilization stage, the mass losses of HTW, PMW and SS decreased from 69.4 wt%, 68.2 wt% and 31.8 wt% to 56.0 wt%, 52.1 wt% and 17.3 wt% at 240 °C, respectively, whereas these of samples in the gasification stage were found in an opposite trend [18,20,34]. The hydrolysis of organic matter and the further polymerization of soluble intermediates are the reasons for these change, which demonstrates the advantage of HTC on supplying high quality materials for the gasification application and confirms the 9

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to the original one [21]. Furthermore, the activation energy within the gasification step is usually higher than that in devolatilization step because the secondary char produced from the devolatilization stage of gasification contains less vulnerable VM [18,22]. On-going conversion of sample toward char with anthracite- or graphite-like structures during hydrothermal process is another reason for this change [36]. For SS, the value of E varied around 128.6–146.2 kJ·mol−1, suggesting that the HTC severity has slight influence on these SS as its higher ash content. Accordingly, the variation of A in hydrochars was similar to that of E, which ranged from 4.33 × 10–4.02 × 103 min−1 for HTW, 2.27 × 10−1-8.02 × 1010 for PMW and 8.89–5.13 × 102 for SS.

gasification step was shifted to a higher temperature after HTC, increasing from 832 °C (HTW), 901 °C (PMW) and 866 °C (SS) to 846 °C, 1009 °C and 878 °C at 240 °C, respectively. These changes can be ascribed to the improved aromaticity during HTC process, which delays the period and the peak of gasification in hydrochars [13,22]. Moreover, referring to the maximum and average value of gasification rate, an adverse trend is observed for PMW- and HTW-derived hydrochars. The former was gasified at a rate of 0.604 × 103·s−1 before HTC, which increased to 0.727 × 103·s−1 at PMW-180 but suddenly reduced to 0.456 × 103·s−1 at PMW-240; in contrast, the gasification rate of HTW maintained a gradual growth in response to the increasing HTC temperature, from 0.250 × 103·s−1 ((dα/dt)max) and 0.151 × 103·s−1 ((dα/dt)mean) to 0.331 × 103·s−1 and 0.146 × 103·s−1, respectively. Uptrends in here indicate the development of porosity by means of the opening of closed pores, the creation of new pores, and the enlargement of existent pores, whereas the downtrend for PMW-derived hydrochars at over 180 °C may be due to the synergistic reaction between amino acid and reduced sugar during HTC that cause the block and collapse of pores on surface structures [11]. Lastly, the lowest value of gasification rate was found in SS and its derived hydrochars (without obvious variation after HTC); this result is probably owing to the significant amount of unreactive ash and the limited FC.

4. Conclusion In summary, biowaste-derived hydrochars have similarity in the gasification process, which supports that the solid fuel obtained from HTW pretreatment are homogeneous, despite of the different components in feedstock. Specifically, HTC removed most of VM and formed additional FC with the increased severity, thereby improving the fuel properties of biowaste. Meanwhile, this conversion of “biowaste-tofuel” promotes the following gasification and gives rise to the similar changes in several parts: 1) the quality of syngas produced by hydrochars was improved and the H2 concentration in syngas increased significantly; 2) the formation of gasified tar from hydrochar could be reduced to less than half of the original value from biowaste. Additionally, the developed aromaticity in hydrochars led to the growth of aromatic compounds in gasified tar, PAHs in particular; 3) the gasification period of sample was prolonged after HTC, while the peak of conversion rate was shifted to a higher temperature as the increase of aromaticity during HTC process. These results can offer a detailed observation on the utilization of biowastes by HTC coupled with gasification technology and provide referential information for the designing, optimizing and even scaling up the thermochemical conversion processes.

3.3.2. Gasification period The time required by the stage of char gasification is defined as the gasification period, which was prolonged as HTC progressed. It is obvious in Fig. 6 that the gasification period of biowastes increased from 28.2 min (HTW), 16.3 min (PMW) and 24.6 min (SS) to 40.0 min, 29.9 min and 39.4 min for HTW-240, PMW-240 and SS-240, respectively. Two explanations are cited for this change [13,35]: 1) HTC can lead to the significant cross-linking and polymerization of biowastes, and the active sites in hydrochars are thus deeper hid as the more ordered and condensed structures in hydrochar; 2) FC is converted by VM and keeps accumulating during the hydrothermal process, which prolongs the gasification step because of the increased reactant. Such evolution in hydrochars may not be favor of accelerating the gasification process, but it can improve the gaseous quality due to the increased carbonaceous contents. Additionally, the increase of reaction period in each temperature interval can be used to differentiate the types of biowaste; it is because the soluble intermediates derived from the components in biowaste are the major reactant for further aromatization or polymerization process during HTC, which means that the hydrolysis of component affects the growth of aromaticity to some extent. Smith et al. [25] demonstrated that hemicellulose in HTW is easy to hydrolyze at 180 °C, while both cellulose and lignin are not hydrothermally degraded until 210 °C. In comparison, protein and polysaccharide in PMW and SS are hydrolyzed significantly around 180 °C [9,13]. Therefore, it is reasonable that a remarkable increase of gasification period occurred between 210 °C and 240 °C in HTW, while that of PMW and SS was within 180–210 °C.

Acknowledgements The authors thank the National Key Research and Development Program of China (NO. 2016YFE0203300), the National Natural Science Foundation of China (NO. 51661145022), the Science and Technology Program of Guangdong Province (NO. 2018A050506068) and the China Scholarship Council (NO. 201804910597) for financial support to this work. References [1] He C, Giannis A, Wang JY. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior. Appl Energy 2013;111(11):257–66. [2] Zhao PT, Shen YF, Ge SF, Chen ZQ, Yoshikawa K. Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment. Appl Energy 2014;131:345–67. [3] Horvat A, Kwapinska M, Xue G, Rabou LPLM, Pandey DS, Kwapinski W, et al. Tars from fluidized bed gasification of raw and torrefied Miscanthus × giganteus. Energy Fuel 2016;30(7):5693–704. [4] Feng YH, Yu TC, Ma KY, Xu GL, Hu YY, Che DZ. Effect of hydrothermal temperature on the steam gasification performance of sewage sludge: syngas quality and tar formation. Energy Fuel 2018;32(6):6834–8. [5] Tremel A, Stemann J, Herrmann M, Erlach B, Spliethoff H. Entrained flow gasification of biocoal from hydrothermal carbonization. Fuel 2012;102:396–403. [6] Mau V, Gross A. Energy conversion and gas emissions from production and combustion of poultry-litter-derived hydrochar and biochar. Appl Energy 2018;213:510–9. [7] Zhan H, Zhuang X, Song Y, Yin X, Wu C. Insights into the evolution of fuel-N to NOx precursors during pyrolysis of N-rich nonlignocellulosic biomass. Appl Energy 2018;219:20–33. [8] Gunarathne DS, Mueller A, Fleck S, Kolb T, Chmielewski JK, Yang WH, et al. Gasification characteristics of hydrothermal carbonized biomass in an updraft pilotscale gasifier. Energy Fuel 2014;28(3):1992–2002. [9] Zhuang X, Zhan H, Huang Y, Song Y, Yin X, Wu C. Denitrification and desulphurization of industrial biowastes via hydrothermal modification. Bioresour Technol

3.3.3. Kinetic analysis The kinetic analysis of samples is shown in Fig. 7; herein, the value of x-coordinate corresponds to the temperature range of gasification step [23]. Following the Arrhenius formula, activation energy (E) and pre-exponential factor (A) were calculated from the TGA data under an non-isothermal procedure. The values of R2 for all samples were acceptable (> 0.9715), indicating that the volumetric model captures the gasification process properly. The E of HTW laid in the range of 120.6–154.4 kJ·mol−1, while that of PMW was between 98.6 and 325.2 kJ·mol−1. Both of them climbed slightly from 142.9 kJ·mol−1 (HTW) and 315.0 kJ·mol−1 (PMW) to 153.6 kJ·mol−1 (HTW-180) and 325.4 kJ·mol−1 (PMW-150) at first, followed by a sharp decrease to 120.6 kJ·mol−1 and 104.1 kJ·mol−1 at a HTC temperature of 240 °C, respectively. This decrease in E is an evidence for the lower reaction resistance of gasification in hydrothermally treated biowaste compared 10

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