Structure-reactivity relationships of biowaste-derived hydrochar on subsequent pyrolysis and gasification performance

Energy Conversion and Management 199 (2019) 112014

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Structure-reactivity relationships of biowaste-derived hydrochar on subsequent pyrolysis and gasification performance

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Xiuzheng Zhuanga,c, Hao Zhanb, Yanpei Songa,c, 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 Structural evolution Pyrolysis reactivity Gasification reactivity

Hydrothermal carbonization (HTC) is emerged as a potential technology to convert wet biowastes into clean solid fuels with significant advantages, which means that the insight into the relationships between HTC pretreatment and subsequent thermochemical utilizations is of important. In this study, industrial biowastes, including lignocellulosic, non-lignocellulosic and ash-rich types, were selected for HTC experiment under different temperatures. Except for the fuel properties of hydrochar, the evolution in carbonaceous structures was analyzed and compared to that of coals with different ranks (i.e., lignite, bitumite and anthracite); furthermore, these changes were used to establish a correlation with the reactivity of pyrolysis and gasification processes. The results found HTC not only upgraded the fuel quality of feedstock but could also develop their aromatic structures, although each biowaste contained different components. Such improvement simulated the development of coals from low to high ranks because the carbonaceous structure in hydrochars was gradually changed to that of bitumite or even anthracite when HTC temperature increased from 120 °C to 300 °C. TG analysis demonstrated that both of the pyrolysis and gasification reactivity of hydrochar were in a generally negative correlation with HTC temperatures, but the extent and the specific relationship differentiated from each other due to the various components in biowaste. These findings are believed to contribute an essential part in bridging the gap from a theoretical potential energy source to the sustainable development of an alternative renewable fuel.

1. Introduction The disposal of industrial biowastes has been widely recognized as one of the biggest challenges nowadays, especially for the developing countries where the industrialization stage is still proceeding. To properly normalize the recycling of wastes, three basic principles are proposed as a standard to be obeyed by various industrial activities, consisting of quantitative minimization, harmless treatment, and effective utilization [1–4]. It thus requires to develop an effective and sustainable approach with dual advantages on the alleviation of environmental pressure and the utilization of potential energy. Over the years, thermochemical technologies, such as pyrolysis and gasification, have emerged and attract much attentions from academic and industrial scientists owing to its functions of converting carbonaceous materials into syngas, bio-oil or biochar with the superior quality [5–7]. Pyrolysis and gasification processes usually follow two similar stages at ⁎

the beginning (i.e., a drying stage of losing moisture at 100–200 °C, and a devolatilization stage of releasing volatiles and forming char at 200–600 °C), but they can be differentiated from each other according to the subsequent stage of the conversion of volatiles and chars at temperatures higher than 600 °C [8,9]. For pyrolysis, the thermal cracking of volatiles and chars occur under an oxygen-free environment, resulting in the formation of several products (i.e., pyrolytic gas, oil and char) depending on the pyrolytic conditions [8,10]; compared to pyrolysis, volatiles are reformed and chars are reacted with gasified agents (CO2, H2O, Air, etc.) during the gasification process, which give rise to a syngas mainly contained H2 and CO [9]. However, on the practical basis, the pyrolysis is usually recognized as the devolatilization stage in gasification because both of them aim at producing char for gasification [7,9]. Recently, Cha et al. [5], Wang et al. [6] and You et al. [7] have published relevant reviews that focus on clarifying the feasibility of utilizing organic wastes via pyrolysis and gasification

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

https://doi.org/10.1016/j.enconman.2019.112014 Received 2 July 2019; Received in revised form 28 August 2019; Accepted 29 August 2019 Available online 12 September 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. The experimental framework and design.

sulfur and chlorine are previously dissolved in processed water during HTC and exist in an easy-use form waiting for recycling (i.e., algae cultivation or anaerobic fermentation) [16,20]. More importantly, Erlach et al. [13] reported that the carbonized biowastes have a better effect on the temperature distribution in gasifier and are thus more suitable for industrial application, which is supported by the findings from Chen et al [21] and Tremel et al. [14]. As a consequence, HTC coupled with pyrolysis or gasification is a promising technology to utilize biowastes for energy production. Unfortunately, despite many studies associated with HTC can be found in published papers, of which the focus is mostly on the effects of operational parameters or the fuel properties of hydrochar, only a few of them are relevant to the correlation between HTC and subsequent pyrolysis or gasification applications. To our knowledge, the carbonaceous structures in biowaste are changed after HTC, which definitely affects the thermochemical reactivity of sample on the following processes (i.e., pyrolysis and gasification) [17,18]. Regarding pyrolysis, it is the preliminary step for gasification or can be used as a standalone platform to produce bio-fuel; the reactivity and the product distribution of pyrolysis are strongly dependent on the aromatic networks of feedstock [22]. For another side, the conversion of char from devolatilization stage is the rate-determining step for gasification, which indicates that the structural features of carbonaceous matter in char, such as aromatization degree, internal surface area or active sites, are the main inter-related factor to the gasification reactivity [23,24]. Given the close relationship between HTC pretreatment and subsequent pyrolysis/gasification, several methods have been used to evaluate the thermochemical reactivity of sample (i.e., fixed carbon [25], X50 in conversion degree [26], ID/IG in Raman analysis [27] and atomic ratio H/C [28]), but none of them can provide a comprehensive explanation up to now; some of them are even contrary to each other [25,26]. But at least, it is well accepted that the evolution of carbonaceous structures in biowastes is the key as the coalification process of sample occurs during HTC pretreatment [29]. An evaluable index related to these changes needs to be identified and correlated with the following applications so that the influence of HTC on the thermochemical reactivity of pyrolysis or gasification can be clarified, which is of essential to the sustainable proceeding of industrial activity.

technologies; but they also mentioned that the industrial application of pyrolysis and gasification still encounters technological difficulties because of the inferior quality of raw materials. The problems of the insufficient efficiencies, the serious emission of pollutant and the incompatibility on established equipment should be taken into consideration when applying pyrolysis or gasification to industrial biowastes in practice. Specifically, the formation of tar and gaseous precursor (i.e., NH3, HCN and SO2) from excessive volatiles cause the operational problems in power generation as well as product purification [11,12], and the handing of fusible ash in biowaste is another difficulty faced by industrial facilities [13,14]. In addition, biowaste diversity suggests that there is no perfect method for the utilization of biowastes once for all. Some of them may be suitable to produce bio-oil through fast pyrolysis, while the another may be favorable of gasifying for syngas because of their different components and properties [5,6,15]. This characteristic provides various potential for the utilization of biowaste, but it also reduces the efficiency of centralized disposal at the same time. To properly address the above problems, hydrothermal carbonization (HTC), also known as wet torrefaction, is adopted as an ideal pretreatment for biowastes, those with high moisture in particular. Compared to dry torrefaction, HTC uses milder conditions while employing water as a reaction medium, mainly under moderate temperatures (120–300 °C), a short holding time (5–60 min) and autogenous pressure (2–10 MPa); actually, HTC is substantially considered as a thermochemical process that simulating the natural coalification of biomass under controlled conditions [2,16,17]. In our previous studies, we elaborated the mechanism of this artificial process and supposed that such changes in carbonaceous structure may facilitate the technology difficulties on subsequent pyrolysis and gasification application [17,18]. He et al. [16] and Gunrathne et al. [19] found that HTC not only improve the energy density of biowastes but also ease the difficulties in grinding, pelletizing, handling, transport and storage operations, which therefore leads to a far preferable feedstock for subsequent applications aimed at energy production. The significant expense for dewatering prior to the pyrolysis or gasification is reduced because the dewaterability of biowastes is improved during HTC pretreatment; the gaseous pollutions from NOX and SO2 are decreased as most of nitrogen,

2

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hydrothermal temperatures.

In this study, biowaste diversity was reached by selecting three typical samples: herb tea waste (HTW, lignocellulosic type), penicillin mycelial waste (PMW, non-lignocellulosic type) and sewage sludge (SS, ash-rich type). Coals with different ranks (i.e. lignite, bitumite and anthracite) were used to compared with hydrochars in order to verify the evolutions of carbonaceous structure during HTC process. The specific framework and experimental design in the present work are visualized in Fig. 1, which contains three specific objectives: 1) investigating the changes of carbonaceous structures in biowastes under various temperatures, and then comparing the structural features of hydrochars to that of coals with different maturity degrees; 2) analyzing the thermochemical reactivity of hydrochars during the pyrolysis and gasification processes; 3) establishing correlations between HTC pretreatment and subsequent pyrolysis/gasification for different biowastes.

2.3. Physicochemical analysis 2.3.1. Fundamental properties The ultimate analysis of samples was performed in an elemental analyzer (Vario EL cube, Elementar analyse, Germany), while the proximate analysis was conducted in a muffle furnace (MXX1100-30, shmicrox Co., Ltd, China) according to the American Standard for Testing and Materials (ASTM) methods. Both the fraction of fixed carbon (FC) and oxygen (O) were calculated by differences. The atomic ratio of oxygen/carbon (O/C) and hydrogen/oxygen (H/C) were further calculated to estimate the coalification process of HTC based on VanKrevelen diagram [16,18], and the variation of HHV in samples was measured by a calorimetric bomb (IKA C2000, Germany) according to ASTM D2015 standard method. In addition, the structural information of samples was explored with the help of Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman Spectra (Raman) and Nuclear Magnetic Resonance Spectroscopy (NMR). For the FTIR, hydrochar (10 mg) and KBr (200 mg) were ground together and pressed into slice ahead of measuring on a Bruker Vertex 70 spectrophotometer in the range of 4000–400 cm−1 with a resolution of 4 cm−1. XRD (PANalytical, X’Pert PRO, Netherlands) and Raman (LabRAM HR800-LS55, Horiba Jobin Yvon, France) technologies are two complementary approaches functioned in analyzing the carbon micro-structures of samples, whose experimental conditions were described in our previous work [17]. The recorded diffractogram (from XRD) and spectra (from Raman) were calibrated by a curved baseline, and the overlap hump within target area was fitted into several Gaussian peaks via OriginPro 2018 software. Meanwhile, the solid-state 13C NMR was carried out via a Bruker Avance III 300WB NMR spectrometer (Germany) for the semi-quantitative compositional information of carbon structures in sample; the specific conditions for 13C NMR were shown as below: 1) a frequency of 75 MHz; 2) a rotor spinning speed of 6500 Hz; 3) a contact time of 2 ms; 4) a recycle delay of 5 s, and 5) a scan number of 5012. The relative 13C intensity distribution could be divided into five chemical shift regions based on previous studies [17,30]: 1) alkyl C (0–43 ppm); 2) OCH3/NCH (43–60 ppm); 3) carbohydrate C (60–110 ppm); 4) aromatic C (110–160 ppm), including aromatic eCeH (110–125 ppm), eCeC (125–145 ppm) and eCeO (145–160 ppm); 5) eCOO/NeC]O (160–190 ppm).

2. Material and methods 2.1. Sample preparation HTW, PMW and SS were collected separately from a traditional Chinese medicine enterprise (Guangdong Province), a pharmaceutical enterprise (Hebei Province) and a large-scale wastewater treatment plant (Guangdong Province), while the coals with different maturity degrees were obtained from a coal yard in Shanxi Province. All biowastes and coals were directly dried at 105 °C for 24 h after collection, followed by a series of pretreatment (i.e., grinding, sieving and redrying) to gain the target samples prior to experiment and analysis. The basic properties of samples are listed in Table 1, including the higher heating value (HHV), the proximate and ultimate compositions. 2.2. Hydrothermal carbonization In HTC experiment, each biowaste was blended with the deionized water at a mass ratio of 1:10 (10 g of sample and 100 ml of deionized water). The mixed slurry was then loaded into a stainless reactor with a volume of 250 ml (shown in Fig. S1), and the reactor was sealed before heating by an electric heater. Argon gas (Ar, 99.999%) was used to provide an inert atmosphere via an inlet pipe in the bottom of reactor and an outlet pipe in the top. Operational temperatures ranged from 120 °C to 300 °C with regular intervals of 60 °C in this study, while the heating rate, the reaction time and the rotation speed of magnetic stirrer were set as 5 °C/min, 30 min and 300 rpm, respectively, to avoid secondary interferences. Once the HTC process had elapsed, the reactor was removed from the electric heater and cooled down to ambient temperature rapidly through a fan. The hydrothermal slurry was subsequently taken out from the reactor and filtered by 0.45 μm filter membrane to separate the solid phase (hydrochar) from the processed water (hydrolysate). Finally, the obtained hydrochars were dried, ground and sieved again to reach the uniform particle size of 0–300 μm before analyses, with an individual label of “HTW/PMW/SS-120/180/ 240/300” for each one. The former “HTW/PMW/SS” indicates the type of biowaste, while the latter “120/180/240/300” represents the

2.3.2. Thermogravimetric analysis (TGA) TGA is recognized as an effective tool to investigate the thermal degradation of solid sample. In this work, a TG analyzer (SDT 650, TA Instruments Co. Ltd, USA) was employed and operated in different procedures for the pyrolysis and gasification processes separately. During the pyrolysis stage, about 5 mg sample was heated from 25 °C to 900 °C at a heating rate of 10 °C/min. High-purity nitrogen (N2, 99.999%) was supplied as carrier gas in this stage to create an oxygenfree environment for thermal decomposition. Several characteristic

Table 1 The properties of industrial biowastes. Samples

Biowastes

Coals

Proximate analysis (wt%, db)

Ultimate analysis (wt%, db)

HHV (MJ/kg)

VM

FC

Ash

C

H

S

N

O

HTW PMW SS Lignite Bitumite Anthracite

69.4 78.3 39.9 47.3 28.2 10.7

17.2 14.0 3.5 45.3 57.9 73.3

13.4 7.7 56.6 7.4 13.9 16.0

45.1 44.8 21.6 63.8 70.4 75.9

5.9 6.2 3.7 4.9 4.2 2.9

0.3 0.6 0.5 0.5 0.7 0.4

2.6 7.4 3.4 0.9 1.2 1.0

Note: VM, volatile matters; FC, fixed carbon; O (oxygen) was calculated by difference based on dry basis; HHV, higher heating value. 3

32.7 33.3 14.2 22.5 9.6 3.8

19.37 19.06 9.67 21.42 28.12 27.23

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stability of hydrochar and its evolution in chemical structures. As shown in Fig. 2, there was a steady decline in the yield of hydrochars when elevating the reaction temperature up to 300 °C in the HTC process; their yield decreased from 91.7 wt% to 34.1 wt% for HTW, 69.0 wt % to 16.9 wt% for PMW and 87.3 wt% to 67.2 wt% for SS. Similar results had been observed by Mursito et al. [32] and Yang et al. [33] who performing HTC on typical peat and bamboo, respectively. This decline can be attributed to the devolatilization process of HTC, where several typical reactions, such as hydrolysis, dehydration and decarboxylation, occurred [18,29]. However, the hydrochars from HTW were obtained with a higher yield than that of PMW, which indicates that the lignocellulosic components are superior to non-lignocellulose in terms of thermal stability. Generally, hemicellulose in HTW is easy to hydrolyze at 180 °C, while both cellulose and lignin are not hydrothermally degraded until 210 °C [6,34]; in comparison, protein and polysaccharide in PMW are hydrolyzed significantly even below 180 °C [20]. For SS whose hydrochar yield seem to be the highest, most of them are actually the ash contents. If only taking ash-free basis into account, the yield of organic matters in both PMW and SS were lower than that of HTW, especially at 180 °C (PMW: 37.5 wt%; SS: 54.5 wt%; HTW: 67.1 wt%). The modest yield of organic matters in SS (relatively higher than that of PMW but less than that of HTW) is probably owing to the reason that SS contains both proteins and fibers which are characterized by PMW (non-lignocellulosic type) and HTW (lignocellulosic type), respectively. Meanwhile, the results of proximate and ultimate analysis are presented in Table 2; the preliminary comparison of coalification degree between coals and biowaste-derived hydrochars is also exhibited in Table 2, which was evaluated by means of the atomic ratio O/C and H/ C in Van-Krevelen diagram. As expect, the volatile matter (VM) was significantly removed from biowastes following two reaction routes during HTC process [17,18]: 1) the conversion of VM to soluble intermediates in liquid phase or light gases in gaseous phase; 2) the conversion of VM to additional fixed carbon (FC) in solid phase. Thus, the content of FC in hydrochars was accordingly increased to 28.6 wt%, 27.5 wt% and 4.2 wt% for HTW-300, PMW-300 and SS-300, respectively. Although part of the ash content was washed under hydrothermal environment, the relative increase of the ash in hydrochars was observed along with HTC progress due to the excess loss of VM and the retained minerals [16]. Additionally, by raising the hydrothermal temperature, the oxygen (O) and hydrogen (H) contents in hydrochars gradually decreased, escaping in the forms of CO2 and H2O from sample surface. This release of CO2 and H2O could be intuitively observed from their SEM images (in Fig. S2), where a smoosh surface with little cavities for raw materials and a cracked surface with higher porosity for hydrochars were found. The content of carbon (C) was on its rise,

parameters were extracted from the pyrolysis curves to evaluate the influence of HTC on thermal decomposition, including the total mass loss, the initial temperature (Ti), the final temperature (Tf), the temperature of maximum loss rate (Tm), the maximum loss rate ((dw/ dt)max) and the mean loss rate ((dw/dt)mean). Furthermore, the comprehensive pyrolysis index (CPI) was introduced to quantitatively assess the pyrolysis reactivity of biowaste-derived hydrochars according to the parameters mentioned above, whose formula was described as follows [31]:

CPI = [(dw / dt )max × (dw / dt )mean]/(Ti2 × Tf )

(1)

where Ti and Tf are not the physical properties of a fuel but the evaluations for different samples to compare their reactivity under the same reaction-off procedure or operating conditions, i.e., sample mass, heating rate and reaction atmosphere. Gasification process was followed by the pyrolysis stage after temperature reached 900 °C and lasted for 10 min. The isothermal gasification of sample was initiated by switching the carrier gas from N2 to CO2 containing gas flow at a concentration of 60 vol% in N2. At this stage, the mass loss was monitored and recorded against the reaction time until no mass loss occurred. The time corresponding to CO2 injection was defined as the initial time (t0), while the mass of sample at t0 was the initial mass for gasification. Carbon conversion (xt), instantaneous gasification reactivity (rt, min−1) and average gasification reactivity (ra, min−1) were calculated by the following formulas, respectively [9,26,27]:

xt =

wi − wt wi − wash

−1 dw dx t · t = wi − wash dt dt

rt =

(2)

(3)

t

ra =

∫t0 rt dt t

(4)

where wi indicates the initial mass of sample at gasification stage, while wt and wash stand for the mass at time t and the mass of ash left after the thermogravimetric procedure. All experiments were repeated at least twice to ensure reliability and reproducibility. 3. Results and discussion 3.1. The characteristics of hydrochars HTC temperature is the essential factor affecting the thermal

Fig. 2. The yield of hydrochars under different hydrothermal temperatures. 4

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Table 2 Fundamental properties of hydrochars derived from HTW, PMW and SS. Sample

Proximate analysis (wt%, db)

Ultimate analysis (wt%, db)

Van-Krevelen

HHV (MJ/kg)

Ash

VM

FC

C

H

O

N

S

O/C

H/C

HTW

120 °C 180 °C 240 °C 300 °C

15.0 17.2 19.6 26.7

67.9 64.1 58.5 44.7

17.1 18.7 21.9 28.6

45.9 47.0 53.2 60.2

5.8 5.6 5.5 5.0

30.6 28.0 19.8 6.4

2.5 2.1 1.8 1.6

0.2 0.1 0.1 0.1

0.50 0.45 0.28 0.08

1.52 1.43 1.24 1.00

19.5 20.4 22.1 26.2

PWM

120 °C 180 °C 240 °C 300 °C

8.2 9.3 16.1 20.2

76.0 69.1 58.2 52.3

15.8 21.6 25.7 27.5

45.5 49.1 56.8 59.9

6.2 6.0 5.9 5.7

32.9 28.7 14.9 8.6

6.7 6.4 5.9 5.3

0.5 0.5 0.4 0.3

0.54 0.44 0.20 0.11

1.64 1.47 1.25 1.14

21.2 23.6 27.4 29.2

SS

120 °C 180 °C 240 °C 300 °C

61.6 68.6 77.0 80.6

34.9 28.1 19.3 15.2

3.5 3.3 3.7 4.2

17.5 13.7 9.7 8.8

3.4 2.5 1.8 1.6

14.4 12.9 10.0 7.8

2.6 1.9 1.2 0.9

0.5 0.4 0.3 0.3

0.62 0.71 0.77 0.66

2.33 2.19 2.23 2.18

10.7 10.5 8.3 7.9

Note: VM, volatile matters; FC, fixed carbon; O (oxygen) was calculated by difference based on dry basis; HHV, higher heating value.

evaluative index for the coalification process of HTC could be found. Within the 3600–2800 cm−1 region of FTIR spectra (in Fig. 3(a)), the strong and board vibration of OeH bond (3600–3000 cm−1) was visible in lignite, indicating a mixture of inter- and intra-molecular hydroxyl groups (i.e. secondary hydrogen bonds in carbohydrates, aliphatic and phenolic hydrogen bonds in lignin) [37,38]. Another two stretching vibrations at 2920 cm−1 and 2850 cm−1 were ascribed to CeH bonds that are most likely identified as aliphatic methyl groups [18,38]. However, these characteristic peaks became weaker with increasing coal maturity or even eliminated in anthracite. The intensity of different oxygen-containing functional groups between 1750 and 1000 cm−1 (i.e. C]O vibration in 1720–1650 cm−1, CeOeC vibration in 1290–1250 cm−1 and CeO vibration in 1150–1050 cm−1) also underwent similar decline and obeyed the order of lignite > bitumite > anthracite. One of the possible explanation for this variation is that most of organic matters retained in lignite result in a large amount of chemical bonds with “low-energy state”; as the enhancement of coal maturity, a conversion of “low-energy state” to “high-energy state” occurs in the chemical structures of coals, which is arrived by the cost of oxygen-containing functional groups [18]. The decrease in oxygen-containing functionalities is in accordance with the ultimate results. In addition to FTIR analysis, complementary techniques included Raman and XRD analyses were conducted, because the Raman bands can be correlated with the characteristic parameters obtained from XRD to provide comprehensive information on the structural features of coals [17,39]. Fig. 3 (b) shows the total Raman peak areas and two

except for SS whose carbon content still kept on slightly reducing after HTC. The higher ash in SS is one of the reasons for the decreased carbon content, suggesting that the carbonization of SS may not be effective as that for HTW and PMW [34]. Overall, these variations in the elemental compositions corresponded to the changes in HHV and confirmed the coalification process of HTC because of the upgraded quality. Following the reduction of the atomic ratio O/C and H/C in Van-Krevelen diagram, it can be preliminarily deduced that the coalification process of HTC is governed by the dehydration and decarboxylation, while demethanation take a relatively small effect [35,36]. The location of both HTW- and PMW-derived hydrochars in Van-Krevelen diagram attached nearly to the region of bitumite whose atomic ratio of O/C and H/C were 0.0–0.2 and 0.5–1.0, respectively [18,35]. This result implies that not only the carbon contents but also the carbon chemical structures in hydrochar are developed toward that of coals, which definitely have an influence on their thermal reactivity. SS was excluded from the VanKrevelen diagram as its higher ash content weakened the coalification process during HTC. 3.2. The comparison of carbon chemical structures 3.2.1. Coals To our best knowledge, coals originate from biomass via a series of reactions under natural strata, and the coalification degree of coals mainly depends on their carbonaceous structures. The differences in three coals with low, medium and high maturity degree were thus investigated through the FTIR, XRD and Raman analyses; so that an

Fig. 3. The spectra of (a) FTIR, (b) Raman and (c) XRD for three ranks of coals. (Note: the quotient of π-band and γ-band intensity defined as the coal maturity, and the fraction of π-band area in total area stood for the aromatization degree of coals). 5

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major bands (i.e. the G and D bands) between 1800 cm−1 and 1000 cm−1; the total peak area of coals from high to low could be ranked as lignite, bitumite and anthracite, which is affected by the Raman scattering ability and the light absorptivity [40]. For coals whose XRD results did not show significant graphite structures, the G band at 1595 cm−1 indicates the aromatic ring breathing rather than E22g fundamental vibration for graphite, while the D band at 1395 cm−1 represents aromatic clusters more than 6 rings with little contribution from “defects” [17]. Consequently, the increase of ID/IG from lignite to anthracite explains the development of aromatics systems, particularly the medium-to-large aromatic ring systems [40]. The X-ray diffraction profiles of coals in Fig. 3(c) reveal the intensity of diffracted beam as a function of Bragg angle (2θ, in degrees). Their high background intensities suggest the highly disordered structures in the form of amorphous carbon which plays an important role during the thermal conversion. Two peaks at around 20° (γ-band) and 26° (π-band) are accepted as the indications for saturated structures (i.e. aliphatic long or side chains) and interlayer between aromatic rings, respectively [17]. Thus, the shift of dominant peak from aliphatic carbon at 20° to aromatic carbon at 26° demonstrates the increased aromaticity in coals with higher ranks, which supports the findings from Raman analysis. Above results verify that coals develop on the basis of aromatic nucleus which serve as the constitutional units and is connected by oxygencontaining functional groups, alkyl side chain or other bridge bonds [39]. Coals with a higher maturity usually contain complex aromatic structures and hence show a higher degree of aromatization, which means the aromaticity can be considered as an index for this study to evaluate the coalification process of HTC.

the conversion mechanism of non-lignocellulosic components by assuming a poly-furan core with additional N-heterocyclic units instead of a benzene network. The temperature range of specific aromatic carbon can also be used to differentiate the type of biowastes. When HTC temperature was lower than 180 °C, the aromaticity of hydrochars climbed slowly because this stage functions as generating the soluble intermediates for the following poly-condensation. Subsequently, a jump from 12.96% to 40.04% for PMW and 15.25% to 42.04% for SS were observed between 180 °C and 240 °C, while the similar jump for HTW was within 240–300 °C. This distinction is achieved by the better thermal stability of lignocellulosic components than that of non-lignocellulose [17,18]; hence, an obvious increase in aromaticity could be observed for PMW and SS ahead of HTW because the abundant intermediates from non-lignocellulosic and ash-rich biowastes are formed at relatively lower temperatures. 3.2.2.2. Raman analysis. Fig. 5 compares the Raman spectra between hydrochars and provides more valuable information in describing the presence of aromatic structures, accompanied with the identification of specific Raman peaks in Table S1. As mentioned above (in Section 3.2.1), the total peak area between 1800 and 1000 cm−1 facilitates the determination of overall Raman intensity. The detectable intensity is governed by two factors associated with the hydrochar properties: the intrinsic Raman scattering ability and the light absorption; herein, the light absorption is proved to be superior to the scattering ability in terms of its effect on observed Raman intensity [17]. Similar to the results from Xin et al. [40], the total peak area in hydrochars constantly decreased due to the improved light absorptivity after HTC. The increasing aromaticity in hydrochars elevates their light absorbing ability, thereby reducing the Raman intensity. Since the electron-rich functional groups appear evidently in Raman intensity, the removal of oxygen-containing groups from biowastes leads to the decrease of Raman intensity in hydrochars. Nevertheless, the Raman signals for HTW, PMW and PMW-120 were undetectable, which can be explained by the possible degradation of samples under strong laser beam. At the same time, the Raman spectra of hydrochars shown in Fig. 5 mainly contained two overlaps with the maximum intensity at around 1590 cm−1 (G band) and 1350 cm−1 (D band), respectively. Three additional peaks (namely Gr at 1540 cm−1, Vl at 1465 cm−1 and Vr at 1380 cm−1) were further separated by curve-fitting to make up this “gap” between the G and D bands, which involves the semi-circle breathing of aromatic rings more than 2 but less than 6 rings [17]. The area of Gr, Vl and Vr bands always lumps together and virtually treats together as one band when discussion. The last S band at 1185 cm−1 indicates the sp3-rich structures originated from alkyl-aryl structures or methyl carbon grafted to the aromatic rings, which is used as a brief measure for the crosslinking density and the substitution groups in hydrochars [36]. As a result, the discussion of Raman spectra concentrates on four bands (i.e., G, Gr + Vl + Vr, D and S band), enabling to gain an insight into the aromatic features of hydrochars as follows: 1) the reduction of ID/IG reflects the growth of aromatic systems in hydrochars rather than the improved graphitization degree. This change is because biowaste-derived hydrochar is usually a disorder polymer, and their size of aromatic clusters is far from the size required for forming graphite crystals [40]. Also, it should be noted that the variation of ID/ IG in hydrochars is opposite to that of coals, but both of them support the aromatization process with increasing coalification degree regardless of during HTC process (for hydrochar) or on natural environment (for coal); the decreased ID/IG ratio in hydrochars stands for the development of small aromatic systems, while the increased ID/IG ratio in coals prefer to the development of medium-to-large aromatic systems [17]. This result is confirmed by the 13C NMR spectra of hydrochars in Fig. 4 and that of coals in Fig. S3, where the aromatic carbon in coals (lignite: 55.41%; bitumite: 79.78%; anthracite: 91.24%) was larger than that in hydrochars; 2) When the temperatures exceeded 240 °C, the ratio of ID/IG had leveled off, whereas that of ID/I(Gr+Vl+Vr) kept

3.2.2. Biowaste-derived hydrochars 3.2.2.1. 13C NMR analysis. 13C NMR, a semi-quantitative technique for constitutional information, was adopted to visualize the dynamics in carbon-containing functional groups of samples and the corresponding aromaticity degree. The distribution of carbonaceous structures was identified according to the previous publications [18,30]; herein, the variation of aromatic structures was the primary content in this study, including the peaks of aromatic carbons bound to hydrogen (110–125 ppm), aromatic non-oxygenated carbon (125–145 ppm) and aromatic oxygenated carbon (145–160 ppm). As can be seen from Fig. 4, the carbon resonances of hydrochars in aromatic region (110–160 ppm) became progressively broader and stronger after hydrothermal process, albeit a significant distinction exists on the major components of biowaste. By increasing the HTC temperature, aromatic carbons were formed via the on-going polymerization of soluble intermediates derived from the conversion of carbohydrate carbon [17,34]. The aromatic CeC in hydrochars, compared to aromatic CeH and CeO, contributed a more important role to the total aromatic carbons, which is in consistent with Yao et al. [41]. Afterwards, insoluble aromatic clusters with higher aromaticity were gradually enlarged due to the accumulation of aromatic intermediates [18,29]. It is interesting that the soluble intermediates from both lignocellulosic and non-lignocellulosic components can support the formation of aromatic structures in hydrochars; during HTC process, the aromaticity of samples increased from 4.35% (HTW), 7.89% (PMW) and 6.64% (SS) to 53.75%, 50.27% and 43.94% at 300 °C, respectively. However, the differences caused by various components in biowastes varied the constitution of aromatic structures and the conversion pathways. Several studies have demonstrated that the aromatic carbons most likely exist in the forms of aryl C and oxygen-containing or nitrogen-containing aryl C structures [30,41,42]; the former is believed to be the consequence of aromatic clusters derived from the lignocellulosic components, while the latter suggests the formation of furans and nitrogen-heterocyclic intermediates from polysaccharide and protein. It is in clear agreement with Zhuang et al. [17], whose conclusion stated that the aromatization of HTW follows a nucleation growth process built up by phenol units; in comparison, Paneque et al. [42] elucidated 6

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Fig. 4. The 13C NMR spectra and the aromaticity variation of biowaste-derived hydrochars under various HTC temperatures. (Note: R1, alkyl C; R2, OCH3/NCH; R3, carbohydrate C; R4, aromatic C; R5, eCOO/NeC]O).

against pyrolysis temperature from 25 °C to 900 °C. Several characteristic parameters related to this process are summarized in Table S2 for a reference. Overall, it is clear from Fig. 6 and Table S2 that three types of biowaste-derived hydrochars have similarity in pyrolysis process, but also differentiate from each other to some extent. According to the TG curves, the major pyrolysis stage occurred in the temperatures ranged from approximately 200–550 °C for HTW and PMW, and 150–600 °C for SS. The total mass loss of biowaste was negatively correlated with the increase in HTC temperatures, decreasing from 68.08 wt% (HTW), 74.63 wt% (PMW) and 38.87 wt% (SS) to 50.64 wt%, 42.23 wt% and 18.19 wt% at 300 °C, respectively. This variation results from the enrichment of ash content in hydrochars and gives evidence for the devolatilization process during HTC [22,43]. If taking no account of the drying period below 150 °C, the DTG curves of biowastes in this region is mainly characterized by one sharp peak at around 335 °C for HTW and 280 °C for PMW and SS, which corresponds to the decomposition of specific components in raw materials. Lignocellulosic components, such as cellulose and lignin, are responsible for the thermal degradation of HTW at higher temperatures [6,43], while carbohydrate, protein and lipid are considered for the decomposition of PMW and SS [44].

growing rapidly with the increasing temperatures, ranging from 0.66 to 1.28, 1.07–2.14 and 0.55–1.52 for HTW, PMW and SS, respectively. Zhuang et al. [17] reported that the ID/I(Gr+Vl+Vr) can be taken as an indication for the relative proportions between the large aromatic systems (> 6 rings) and the small or medium systems with 2–8 aromatic rings; consequently, the increase of ID/I(Gr+Vl+Vr) implies the enlargement of aromatic ring systems during HTC process [17,40]; 3) the ratio of IS/IG slightly reduced as the temperature further increased, suggesting a gradual transformation of crosslinking structures to aromatic one. Such decline in IS/IG ratio may be the result of dehydrogenation, demethylation or condensation among small aromatics [40].

3.3. Relationship between structural features and thermal reactivity 3.3.1. Pyrolysis behavior and reactivity The TG and DTG curves obtained from the pyrolysis processes of untreated biowastes and hydrochars are illustrated in Fig. 6, which were operated under N2 atmosphere and a controlled rate of 10 °C/min. These curves exhibit the mass loss and the time derivative of mass loss 7

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Fig. 5. The Raman spectra of biowaste-derived hydrochars under various HTC temperatures.

increased aromaticity exerts the same impact on the pyrolysis rate as the crosslinking reactions of non-covalent bonds (such as hydrogen bonds between oxygen-containing groups) also suppress the release of volatile matter during pyrolysis process [28]; 2) for PMW, similar trend was observed, except for PMW-120 which showed a higher reactivity with respect to pyrolysis. The presence of abundant intermediates with low molecular weights (hydrolyzed at lower temperatures < 150 °C), such as oligosaccharides from carbohydrates and oligopeptide from protein, are the reason for the improved reactivity [8]. These intermediates are inferior in term of thermal stability and thus cause a better CPI in PMW-120 when compared to raw biowaste. Further increasing HTC temperatures, the polymerization of small intermediates into aromatic clusters becomes dominant, which results in the variation of CPI back into the negative linear relationship with aromaticity at higher temperatures; 3) for SS, the pyrolysis reactivity of hydrochars had an exponential correlation with aromaticity instead of a linear relationship. It showed that the CPI was promptly decreasing at the early stage and tended to be gentle afterward. This variation can be possibly explained by the excessive ash content enriched in SS-derived hydrochar at temperature over 180 °C, which weakens the effects of aromaticity on pyrolysis reactivity [18,34]. In addition, the CPI of coals reduced in a similar linear relationship with the increasing aromaticity (shown in Fig. S4), which indicates that such structure-reactivity model has universality for various samples. Li et al. [27] used both the aromaticity and H/C molar ratio as individual indexes to establish the correlation with CPI, but the results were contradicted with each other. It seems unreasonable and incompatible to the present study because the effects of aromaticity and H/C molar ratio on pyrolysis reactivity, according to the 13C NMR and Van-Krevelen diagram, should be coincident with each other. Both aromaticity and H/C molar ratio of sample can substantially reflect the aromatic structures in same way. Takagi et al. [28] reported that the reactivity of pyrolysis reactions is fundamentally relevant to the aromatic structures with main chains; consequently, the growth of aromatic clusters, which is built up by the main chain with an aromatic core, are the major reason for the general decrease in the pyrolysis reactivity of hydrochars

Hydrochars, by contrast, are different to biowastes as the DTG curves of hydrochars contained a relatively strong peak at around 450 °C with a shoulder on the left, especially for those derived at temperatures over 240 °C. Apparently, HTC removes the volatiles in biowastes and thus weakens the peak at lower pyrolytic temperatures, but HTC concurrently generates carbonaceous components with better thermal stability that leads to the emergence of peak at higher pyrolytic temperatures [18]. This result once again demonstrates the increasing aromaticity of biowastes during HTC process, which coincides with the conclusion reported by He et al. [16,30] and Zhao et al. [2,36]. The comprehensive pyrolysis index (CPI) was introduced and calculated to quantitatively describe the reactivity of coals and hydrochars during pyrolysis process. Higher CPI commonly suggests the rapid and easy proceeding of pyrolysis reaction, whereas the lower one represents the opposite way. As shown in Fig. 6, the CPI of coals decreased substantially from 3.99 × 10-8 at lignite to 0.63 × 10-8 at bitumite and further to 0.03 × 10-8 at anthracite; this reduction in CPI indicates that the development of coalification degree may be a possible obstacle for pyrolysis [28]. Similar observation can be noted for the hydrochars (except for PMW-120), whose pyrolysis reactivity was less than that of raw materials as the gradual decrease in CPI along with the increasing HTC temperatures. Two changes in hydrochars are considered as the reasons [8,22,26]: 1) the removal of volatile matter; 2) the formation of aromatic structures. Detailed discussion is made in the next section together with the relationship between the structural features and the pyrolysis reactivity. 3.3.2. Structure-reactivity relationship for pyrolysis Representation of the structure-reactivity relationship is evident when comparing the CPI and the aromaticity variation obtained from 13 C NMR spectra, which is depicted in Fig. 7. Basically, the CPI of three biowastes are all in a negative correlation with the increased aromaticity after HTC, but the decreased extent is affected by different ways: 1) for HTW, its CPI declined monotonically with the increased aromaticity. On one hand, the removal of volatiles from HTC process decreases the pyrolysis rate in hydrochars; on the other hand, the 8

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Fig. 6. The TG-DTG curves and the pyrolysis reactivity of biowaste-derived hydrochars.

3.3.3. Gasification behavior and reactivity The mass loss curves under 900 °C and CO2 atmosphere were used to calculate the gasification conversion of samples versus reaction time and the instantaneous rate of gasification at different extents of conversion. As plotted in Fig. 8, the gasification periods required to achieve 95% conversion were 12.7 min, 14.1 min and 44.6 min for HTW, PMW and SS, respectively; the periods were slowly prolonged to 34.1 min

(with increased HTC temperatures) and coals (with different ranks) [17]. However, one thing worth noting that the side chains originated from various components may complicate the pyrolysis reactivity to some extent, thereby making the specific mathematical relationships slightly different for each sample.

Fig. 7. The relationship between CPI and aromaticity for biowaste-derived hydrochars. 9

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Fig. 8. The gasification behavior and reactivity of biowaste-derived hydrochars.

may be caused by [25,43]: 1) the blockage or collapse of pores; 2) the volatilization of catalytic metals (especially potassium); 3) the enrichment of non-volatile inorganic elements; or 4) the consumption of residual carbon. Due to the differences in biowaste properties, one or several reasons mentioned above are functioned individually or cooperatively during the gasification process, which leads to the diversity in shape and extent. The average gasification reactivity of biowaste-derived hydrochars is given in the bottom of Fig. 8, following with a comparison to that of coals. Some scholars reported that the gasification reactivity of coals become worse as the coalification degree increased, supporting the results in this study (lignite: 0.91 × 10−1 min−1; bitumite: 0.06 × 10−1 min−1; anthracite: 0.03 × 10−1 min−1) [24–27]. Similar to coals, with the increase of hydrothermal temperatures from 120 °C to 300 °C, the average reactivity of HTW- and SS-derived hydrochars reduced from 0.175 min−1 and 0.047 min−1 to 0.034 min−1 and 0.032 min−1, respectively, while that of PMW-derived hydrochars elevated first but followed by a decrease when temperature exceeded 180 °C. These results confirm the variation of instantaneous reactivity between biowastes, and these changes can also be ascribed to the control of active sites and structural features of hydrochars [31]. It is worthy to note that a much lower average reactivity for SS was observed when compared to HTW and PMW, which is probable the

(HTW-300), 97.1 min (PMW-300) and 45.9 min (SS-300) after HTC. It suggests that the gasification period varies with the type of biowastes and the HTC severity, which puts a significant influence on the industrial process [22,31,43]. The most notable difference between biowastes is that HTC at lower temperatures (< 180 °C) seem to accelerate the gasification process of PMW, whose periods and rates followed an order of PMW-180 > PMW-120 > PMW > PMW-240 > PMW-300. This trend may be related to its properties: the inferior thermal stability to HTW, and the less ash content than that of SS. Compared to PMW, the instantaneous rates of HTW and SS were found to be commonly higher than that of their derived hydrochars, which were steadily reducing with a similar shape when increasing HTC temperature to 300 °C. From the shape of instantaneous reactivity, the gasification process of HTW and SS can be grouped into two stages, while that of PMW contains one more stage for the plateau between about 10% and 75% [31]. In the early period of gasification, the rates increased rapidly with the proceeding of gasification conversion below 10%. Possible processes below may result in such increase to the conversion rates of gasification [24,45]: 1) opening of the closed pores; 2) creation of the new pores, and 3) enlargement of the existent pores. These changes in porous structure facilitate the increase of surface area, thereby accelerating the instantaneous rates of gasification. Subsequently, the steady decrease in gasification rates at higher values of the char conversion 10

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consequence of less char generated from the limited organic matter in SS during the pyrolysis process.

[27]. According to previous studies, the pore structures and the specific area surface are commonly improved after pyrolysis stage, which are supposed to accelerate the gasification process by offering more porous sites for the surface reaction with CO2 [9,23]; interestingly, a general reduction in the gasification reactivity of hydrochars (except for the PMW-120 and PMW-180) was observed with the increasing HTC temperatures, indicating that there is another factor governing the gasification process of hydrochars compared to the porous structures. This trend also coincides on the conclusion of Fan et al. [24] where the authors performed HTC on different coals with the subsequent gasification. Ulbrich et al. [25] stated that the active sites in hydrochars are relevant to not only the porous features but also the carbonaceous structures, i.e., carbon sites, edges and dislocations, while Wang et al. [31] suggested that the condensed coal-like structures in hydrochars may be unfavorable for the gasification process. It is thus deduced that the decreases in average gasification reactivity are attributed to a loss of available active sites for CO2 absorption along with the increased aromaticity in higher HTC temperatures; the higher aromaticity is, the lower the gasification reactivity can be. Meanwhile, Li et al. [27] found that the aromaticity of sample also increases during the pyrolysis stage, although the increased extents of aromaticity in pyrolytic char is less than that in hydrochars. This finding makes the direct relationship between the aromaticity of hydrochars and gasification reactivity inappropriate, but a preliminary conclusion that the development of aromaticity after HTC pretreatment have a negative and dominant influence on the gasification reactivity can still be made. In addition, other secondary factors, such as the content and composition of inorganic matters in ash, definitely complicate the influence of HTC pretreatment on gasification process. Taking SS for an example, the abundant alkali and alkaline earth metals (AAEMs) in ash are beneficial for char gasification [27], which may be responsible for the slight decrease in the reactivity of SS-derived hydrochars at higher aromaticity because much of ash are enriched in SS-240 and SS-300. Furthermore, the gasification reactivity of PMW-derived hydrochars formed at lower HTC temperatures increased at first, which indicates that there is another factor controlling the gasification reactivity rather than aromaticity at this temperature range. In our opinions, the less volatile-char interactions at PMW-120 and PMW-180 may be relevant, but more investigation should be carried out in the further.

3.3.4. Structure-reactivity relationship for gasification Prior to the discussion, it is important to emphasize again that the gasification process of sample mainly involves two sequential stages, i.e., devolatilization stage and gasification stage [45]; hereinto, the devolatilization stage before gasification can be accepted as the pyrolysis stage in the present TG procedure because they serve as the same role. The former stage functions at removing volatiles from sample and forming char, so that the pure char can react with oxidizing agent in the gasification stage to provide a syngas comprising of H2, CO, CO2 and CH4 [8–10]. Several parallel reactions relevant to the production of syngas, such as the Bourdouard reaction (C + CO2 → 2CO), water gas reaction (C + H2O → H2 + CO) or hydrogasification reaction (C + 2H2 → CH4), mainly use pyrolytic char rather than organic matters as a reactant [31]. Thus, both on the practical and theoretical basis, it is always necessary to pass through a pyrolysis stage to accomplish the gasification of char, and pyrolysis is one aspects of gasification; for this reason, the changes of structural feature in sample during pyrolysis stage is inevitable, which disables us to build a certain and direct relationship between HTC pretreatment and gasification stage. In Fig. 9, it is known that HTC pretreatment directly affects the pyrolysis reactivity of samples, whose correlation between aromaticity and CPI was elaborated in Section 3.3.2; but if considering the pyrolysis stage as a part of gasification process, the indirect effects of HTC pretreatment on the gasification stage can be deduced to some extent as the regular trends in the average gasification reactivity of hydrochars were found in Section 3.3.3. As can be seen from Fig. 9, although the general correlation of HTC temperatures with average gasification reactivity showed similar trend to that with pyrolysis reactivity, the reasons for such phenomenon are different. Gasification process is actually a chemically controlled surface reaction, which undergoes three consecutive steps including the chemical adsorption to char surface, the surface reactions and the desorption of syngas from char surface [46]. As a sufficient CO2 gas flow of 100 ml/min−1 was used in the TG analysis, the second and third steps are more likely the rate limiting step affecting the gasification reactivity of hydrochars, where involves the formation of carbonoxygen complexes on active sites via absorbing oxygen atom of CO2

Fig. 9. The influence of HTC temperatures on the CPI and average gasification reactivity of biowastes. 11

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4. Conclusion [5]

In summary, the fuel quality of biowaste is upgraded as HTC progressed, which reflects in their proximate and ultimate results. Most of volatiles are removed due to the devolatilization process of HTC, whereas the additional fixed carbon is enriched in solid phase via the conversion of volatile matter. These changes correspond to the decreases in the atomic ratio of O/C and H/C after HTC, leading to the upgrading toward the region of lignite or even bitumite in Van-Krevelen diagram and the increase in HHV (HTW: 19.4 to 26.2 MJ/kg; PMW: 19.1 to 29.2 MJ/kg; SS: 9.7 to 10.7 MJ/kg). Additionally, the chemical structure of biowastes is converted to a coal-like structure as the aromatization degree of HTW, PMW and SS grow from 4.35%, 7.89%, and 6.64% to 53.75%, 50.27% and 43.94% at a HTC temperature of 300 °C, respectively. The increased ID/I(Gr+Vl+Vr) further confirms the enlargement of aromatic ring systems during HTC, which is coincided with the development of coals from low to high ranks. Regarding the relationship between HTC pretreatment and subsequent thermal utilization, both of the pyrolysis and gasification reactivities are affected by HTC in a generally negative way, but the extent and the specific relation differentiate from each other due to the differences in major components. For pyrolysis process, the maximum DTG peak for hydrochars is shifted to higher temperatures and becomes weaker with the increased HTC temperatures, while the CPI is decreased following with the evolution of aromatic structures. On the other hand, the gasification period of hydrochars is prolonged significantly after HTC, accompanied with the reduction of average gasification reactivity. This result can be explained by several reasons, i.e., the blockage or collapse of pores, the volatilization of catalytic metals (especially K), the enrichment of non-volatile inorganic elements or the consumption of residual carbon. These findings can not only provide an insight into the pyrolysis and gasification application of biowaste-derived hydrochars, but also give a referential observation for designing, optimizing as well as scaling up the thermochemical utilization of industrial biowastes.

[6] [7]

[8] [9]

[10] [11]

[12]

[13] [14] [15]

[16]

[17]

[18]

[19]

[20]

[21]

Declaration of Competing Interest

[22]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[23]

[24]

Acknowledgements [25]

The authors thank the National Natural Science Foundation of China (No. 51676195), the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA21060600), and the Science and Technology Program of Guangdong Province (No. 2018A050506068) for financial support to this work.

[26]

[27]

Appendix A. Supplementary data

[28] [29]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.enconman.2019.112014.

[30]

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