Effect of hydrothermal dewatering on the pyrolysis characteristics of Chinese low-rank coals

Applied Thermal Engineering 141 (2018) 70–78

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Research Paper

Effect of hydrothermal dewatering on the pyrolysis characteristics of Chinese low-rank coals

T

⁎

Lichao Gea,b, , Hongcui Fengc, Chang Xua, Yanwei Zhangb, Zhihua Wangb a

College of Energy and Electrical Engineering, Hohai University, Nanjing 211100, China State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China c HuaTian Nanjing Engineering & Technology Corporation, MCC, 210019, China b

H I GH L IG H T S

lignites with various ranks are upgraded through hydrothermal dewatering. • Typical Krevelen diagrams show the artificial carbonization reactions process. • van coals exhibit weak pyrolysis characteristics similar to bituminous coal. • Upgraded from stable and saturated components (p-xylene, phenol, CH ) increase. • Products • Products from unstable and unsaturated components (CO , CO, formic acid) decrease. 4

2

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrothermal dewatering Low-rank coal Pyrolysis Volatile matter Thermal stability

This paper describes the effect of the hydrothermal dewatering (HTD) on the pyrolysis characteristics of lowrank coals (LRCs). The effect of HTD on the characteristics of LRCs was explored based on scanning electronic microscopy and N2 adsorption analyses. The pyrolysis behaviors and gas products were determined using a TGFTIR instrument. The results show that the crosslink structure and overall gel structure were broken. The pore structure expanded to the micropore region, and the surface area and total pore volume initially increased and then decreased as the treatment temperature increased. The TG-DTG results confirmed that the pyrolysis process moved towards the high-temperature and bituminous coal region. The characteristic parameters of pyrolysis indicated that the thermal stability of the coal structure was improved. According to the TG-FTIR results, the amount of released gas products from stable and saturated components (such as p-xylene, phenol and CH4) increased, whereas the amount of released gas products from unstable and unsaturated components (such as CO2, CO and formic acid) decreased. The unstable structure and components of the LRCs were decomposed and transformed, and a stable structure and phase were created. Moreover, pyrolysis activity declined, and thermal stability improved.

1. Introduction Coal accounts for 39.3% of the fuel used for global electricity generation [1]. Coal use is particularly prevalent in China, which is the world’s largest producer and consumer of coal [2]. Coal supplies 67.5% of China’s primary energy and more than 75% of China’s electricity [3]. However, the country's coal resource endowment conditions are poor, and its reserves of low-rank coals (LRCs) total approximately 190.3 billion tons (41.18% of the total coal reserve) [4]. The utilization of LRCs (including lignite, brown coals and sub-bituminous coals) is expected to become increasingly important. Unfortunately, the inherent limitations of LRCs, such as high moisture and oxygen contents, high ⁎

transportation costs, high CO2 emissions, low calorific values, and high propensities for spontaneous combustion, greatly restrict their largescale application [5]. Thus, upgrading LRCs by increasing their energy value is of considerable importance, and dewatering is the first critical step in this process [6]. Hydrothermal dewatering (HTD, wet torrefaction, hot compressed water, or hot water pretreatment) is a popular research topic. HTD is a non-evaporative method that effectively removes water from coal and improves the chemical structure of LRCs [7]. The HTD process has been developed for almost a century, with the original aim of simulating the mechanisms of natural coalification in the laboratory [8,9]. This method involves a thermochemical conversion process at temperatures

Corresponding author at: College of Energy and Electrical Engineering, Hohai University, Nanjing 211100, China. E-mail address: [email protected] (L. Ge).

https://doi.org/10.1016/j.applthermaleng.2018.05.098 Received 23 February 2018; Received in revised form 13 May 2018; Accepted 26 May 2018 Available online 26 May 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.

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coal-producing areas in China: Zhao Tong (ZT), Yi Min (YM), and Zhun Dong (ZD). These LRCs formed during different geological ages, namely, the Tertiary, Late Jurassic, and Early or Middle Jurassic, respectively. Da Tong (DT) bituminous coal was used for comparison as an HRC. The raw coals were crushed and sieved through a 2.5-mm mesh before the upgrading process. The raw and upgraded coal samples were milled for 30 s in a ring mill and were then sieved through a 74μm mesh for analysis.

typically between 200 and 350 °C, and the vessel pressure is autogenously generated to stop water from evaporating [10]. Many chemical reactions, such as hydrolysis, dehydration, deoxygenation, decarboxylation, demethanation, polymerization, and aromatization, can occur during this process, and artificial carbonization can result in the modification of LRC characteristics [11]. Recent investigations into upgrading LRCs with HTD have generally focused on (1) the effects of HTD conditions on the properties of the treated coal, including the temperature, residence time, dry coal/water ratio, and so on [12–14]; (2) the properties of gas-, liquid-, and solidphase products [15–18]; (3) the disposal of wastewater produced from HTD [19–22]; (4) the effect of the HTD treatment of LRC on its slurryability [23–26]; (5) the thermal conversion and utilization of upgraded coal, including liquefaction [27,28], pyrolysis, gasification, combustion [29–33], caking and coking [34,35]; (6) pollutant migration and emission characteristics during the HTD process, such as trace elements [36], sulfur [37], nitrogen [38,39], and ash [40]; and (7) the upgrading mechanism and simulation of HTD for LRCs [7,41]. Previous studies mainly focused on the effects of HTD related to changes in physicochemical properties and aimed to enhance the performance of LRCs in the preparation of coal water slurry. A few studies have focused on the effects on thermal conversion behaviors and the utilization of upgraded LRCs. Umar et al. [31,32] studied the changes in the combustion characteristics of three Indonesian LRCs with upgraded brown coal (UBC), hot water drying (HWD) and steam drying (SD). They found that the ignition temperature of the upgraded coals increased and that the maximum combustion rate increased significantly. After upgrading, the combustion peak of volatile matter decreased slightly, whereas the char combustion process was obviously strengthened. In addition, the combustion characteristics of the HWD and SD processes were superior to those of the UBC process because the former were conducted at lower temperature and pressure. The author has studied the influence of HTD on the gasification and combustion characteristics of LRCs [30,33], and the results suggest that the process indeed changes the physicochemical properties of LRCs, improving their coal rank, delaying thermal conversion processes towards the high-temperature region, increasing the activation energy of the process and upgrading the characteristics of LRCs to levels comparable to those of high-rank coals (HRCs). Pyrolysis is the first essential step in the coal conversion process and has an important effect on coal gasification and combustion. Volatile releases during the pyrolysis stage control the ignition behavior, flame temperature and combustion stability. Thus, the effects of HTD on the pyrolysis characteristics of LRCs should be studied in detail. Liu et al. [29] studied the effects of HTD on the pyrolysis kinetics and CH4 release characteristic of two lignites. They divided the thermogravimetric process into three stages. The results showed that the initial temperatures of each pyrolysis stage of HTD products were higher than those of raw coals. The temperatures at which CH4 appeared during the pyrolysis process increased. The activation energy also increased after HTD upgrading, indicated that thermal stability was improved. However, volatile matter contains many species, and changes in their release before and after the HTD process is also important for thermal conversion behaviors. In this paper, three typical LRCs with different geological ages were hydrothermally dewatered and upgraded at 250 and 300 °C. The effects of the HTD temperature on the pyrolysis performance and volatile release characteristics of the upgraded coals and on the coal composition, microscopic morphology and pore size distribution were studied. The kinetics were also measured and modeled using the Coats–Redfern integral method.

2.2. HTD upgrading process The HTD process was performed in a bench-scale hydrothermal reaction system (WeiBa WHFS-2). The system included a 2-L cylindrical autoclave with a maximum pressure of 25 MPa and maximum temperature of 350 °C. The end HTD temperatures in this study were 250 °C and 300 °C, and the detailed process description was shown in a previous work [33]. All upgraded coals were abbreviated based on the coal type initials and treatment temperature, such as “ZT-250” and “ZT300”. 2.3. Scanning electronic microscopy (SEM) The surface morphologies of the raw and upgraded coals were observed using a SEM (SIRON-100) at a voltage of 25 kV with various amplifier times. 2.4. N2 adsorption The pore structures, including the Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size distribution, of the raw and upgraded coals were obtained through N2 adsorption measurements using a commercial instrument (ASAP 2010, American Micromeritics Co. Ltd.). Each coal sample (approximately 0.2 g) was placed in a sealed glass tube and then degassed at 250 °C for 4 h in a nitrogen stream. After degassing of the coal samples, the glass tube was placed in the instrument, and measurements were performed. The surface area was calculated using the BET equation, and the Barret–Joyner–Halenda (BJH) model was used to determine the pore distribution and volume. 2.5. Thermogravimetric and Fourier transform infrared spectroscopy (TGFTIR) A Mettler-Toledo TGA/SDTA 851e thermo-balance coupled with a Nicolet NEXUS 670 FTIR was used to study the pyrolysis and devolatilization characteristics of raw and upgraded coals. Specifically, 10 mg of dried coal sample was placed in an aluminum crucible and heated at 15 °C/min within the temperature range of 40–1000 °C under a steady nitrogen flow of 60 ml/min. Volatiles released during coal pyrolysis were rapidly transported into the FTIR gas cell by pure nitrogen. The Teflon tube and FTIR gas cell were preheated to 180 °C before each experiment to prevent the condensation of volatiles. A deuterated triglycine sulfate pyroelectric detector was used to detect gaseous products because of its rapid response and low noise. The spectrum scope ranged from 400 to 4000 cm−1 with a resolution of 4 cm−1. Volatile release characteristics were studied based on the analysis of FTIR spectra, which were obtained online during TG runs. TG analysis coupled with FTIR has been widely applied to study the compounds that evolve during the pyrolysis of coal. Such analyses provide continuous important information regarding devolatilization, including the identification of major volatile species and the typical temperature range of release. Detailed methods and operations can be found in [42,43]. In this study, CO2, CO, CH4 and formic acid were chosen as light components in volatile matter, and the heavy components chosen included p-xylene and phenol, which are representative of benzenecontaining species and tar. Their standard spectra and main characteristic absorption bands are given in Fig. 1 and Table 1 [43,44].

2. Experimental 2.1. Coal sample The typical LRCs selected for this study were from the three largest 71

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Absorption

5 4

1793.7

2360.86

1.4

Hydrogenation Oxidation

Reduction

ZT-RAW ZT-250 ZT-300 YM-RAW YM-250 YM-300 ZD-RAW ZD-250 ZD-300 DT-RAW

Dehydrogenation 1.2

P-xylene

H/C molar ratio

6

1.6

1183.76

2179.21

2934.39 3013.97 Phenol

Formic acid

3 CH4

2

1.0

Sub-bituminous

Lignite

0.8 0.6

Bituminous Decarboxylation

0.4

CO

1 0

Demethanation

CO2

0.0 0.0

4000

3200

2400

1600

800

Functional group

Vibrations

CO2

2360.86 671.76

C]O C]O

Stretching

CO

2179.21 2114.41

CeO CeO

Stretching

CH4

3013.97 1301.65

CeH HeCeH

Stretching Bending

Formic acid

2959.86 1793.7 1758.98 1120.52 1086.27 627.95

CeH C]O

Stretching Stretching

C]O

Stretching

CeH

Stretching

3024.82 2934.39 2882.46 1517.83 793.26 477.63

CeH

Stretching

C]C CeH

In-plane bending Stretching

OeH C]C

Stretching In-plane bending

OeH

In-plane bending

CeO(H) CeH

Stretching Stretching

Phenol

3652.01 1602.1 1499.01 1336.93 1257.9 1183.76 746.81 496.78

0.3

demonstrates that the coal rank increased from lignite to sub-bituminous and bituminous coal as the O/C and H/C ratios decreased. During the HTD process, three major reactions occurred along a straight line: the dehydration reaction was expressed by moving in the lower-left direction; decarboxylation was expressed by moving in the upper-left direction; and the demethanation reaction was expressed by moving in the lower- and slightly right direction [8]. Heat and pressure changes can disrupt the colloidal nature of LRCs during HTD, resulting in solid products with low equilibrium moisture contents. The extensive loss of oxygen also led to decreases in the equilibrium moisture contents of the solid products. Moreover, oxygen from the LRCs was removed by reduction (loss of oxygen) and dehydration reactions. HTD causes the dehydration, reduction and decarboxylation of raw coals to liquid and gas products [16].

Table 1 Characteristic absorption bands and corresponding vibration types of species. Absorption bands/(cm−1)

0.2

Fig. 2. H/C and O/C molar ratios on van Krevelen diagrams of raw and upgraded LRCs.

Fig. 1. Standard spectra of the investigated species and their characteristic absorption bands.

Species

0.1

O/C molar ratio

Wavenumber (cm-1)

P-Xylene

Dehydration

Anthracite

0.2

3.2. Effect of upgrading on coal surface morphology As shown in Fig. 3 [45], the surfaces and edges of raw coal particles gradually changed from muddy to clear as the coal rank increased from ZT to ZD. Young lignite usually has more obvious plant fiber characteristics. After upgrading, the relatively large and flat surfaces of coal particles ruptured and broke into pieces due to the release of vapor and gaseous products and the effects of relatively high temperature and pressure. The breakage and aggregation effects during the HTD upgrading process produced many small particles, and the overall gel structure of the coal began to be destroyed. These phenomena were more obvious when the treatment temperature increased. After upgrading, the coal structure became denser, with increased brittleness and hardness, and the upgraded coal was easier to crush. These characteristics reflect an improvement in coal rank because anthracite is harder than lignite and is thus easier to crush. Due to the long coalification time and the effects of high temperature and pressure conditions, the HRC structure was more compact and denser than that of the LRC, and graphitization characteristics were more obvious. As the coal grindability improved, the power consumption required for grinding decreased, and the particle size distribution of the crushed coal widened.

3. Results and discussion 3.1. Effect of upgrading on coal composition The coal composition of raw and upgraded coals was described in a previous study [33]. Studies have focused on analyzing changes in the O/C and H/C atomic ratios to understand the chemical transformations that occur during the HTD process. In Fig. 2, the O/C and H/C atomic ratios of raw and upgraded coals are plotted on the coal band. As the HTD process progressed and the treatment temperature increased, the H/C and O/C atomic ratios of the solid products decreased. Artificial carbonization was performed throughout the HTD process; thus, C increased, and O and H decreased. Additionally, volatile matter, including hydroxyl groups and carboxyl groups (namely, the hydrophilic groups), decreased due to decomposition [26]. The O/C and H/C ratios are closely related to the rank of the coals. The van Krevelen band

3.3. Effect of upgrading on pore structure The structural model of coal pores is generally classified as follows [46]: micropore, with a pore size < 2 nm, mesopore, with a pore size range of 2–50 nm, and macropore, with a pore size > 50 nm. As shown in Fig. 4, the pore structure of the upgraded coals expanded to the micropore region, and more distribution peaks appeared from 1 to 10 nm (namely, micro- to mesopores) compared with raw coal. The 72

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Raw coal

HTD-250

HTD-300

ZT

YM

ZD

Fig. 3. Results for the effect of upgrading on the surface morphology.

dV/dlogD (ml/(g*nm))

0.04

Table 2 Results for the effect of upgrading on pore structure.

ZT-RAW HTD-250 HTD-300

Condition

BET pore surface area/ (m2/g)

BJH desorption pore volume/(cm3/g)

BJH desorption average pore diameter/nm

ZT

RAW HTD-250 HTD-300

0.08 19.89 15.83

0.0070 0.0363 0.0272

52.39 6.44 6.52

ZD

RAW HTD-250 HTD-300

2.28 4.85 4.41

0.0039 0.0120 0.0112

25.68 17.55 18.09

0.02

0.01

0.00

10

changes in the pore structure caused by HTD were mainly attributed to the following three effects. (1) The destruction of active groups, such as methylene, methoxyl and carboxyl groups, induced the formation of micropores and the expansion of the pore structure. (2) Moisture release and the effects of increased temperature caused a pressure gradient between the inside and outside of the coal particle, resulting in damage to the pore structure. (3) Tar produced at high temperatures was able to remain in the pores, thus blocking some pores. When the treatment temperature was below 250 °C, the effect of (1) played a major role. Notably, the pore structure developed, and the surface area and total volume increased. However, when the treatment temperature further increased to 300 °C, the effects of (2) and (3) were more obvious. Therefore, as the temperature increased from 250 °C to 300 °C, the surface area and total volume decreased due to the damaging effects. Wu et al. [41] studied the effects of pore structure using mercury intrusion porosimetry. N2 adsorption and mercury intrusion porosimetry are widely used to characterize the pore structure of lignite for micro- to mesopores and meso- to macropores, respectively. The pore structures of their raw and upgraded coals were mainly macropores with an average pore size range of 50–400 nm. They found that the macropores of lignite decreased with increasing HTD temperature, whereas the mesopores were enriched compared to raw coal. The disintegration of macropore structures was attributed to the shrinkage forces originating from the hydrothermal treatment on the coal surface. The jet flow pressure opened and cross-linked the blind and closed

1 00

Pore size (nm) (a) ZT coal

ZD-RAW HTD-250 HTD-300

0.010

dV/dlogD (ml/(g*nm))

Coal

0.03

0.008 0.006 0.004 0.002 0.000 10

100

Pore size (nm) (b) ZD coal Fig. 4. Results for the effect of upgrading on the pore size distribution.

73

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0.000

90

-0.005

Weight (%)

-1

Pyrolysis rate (min )

100

80

70

ZT-RAW HTD-250 HTD-300 DT-RAW

60

-0.010

ZT-RAW HTD-250 HTD-300 DT-RAW

-0.015

-0.020

50

-0.025

200

400

600

800

1000

200

400

600

800

1000

0.000

100

-0.005

Weight (%)

-1

Pyrolysis rate (min )

90

80

YM-RAW HTD-250 HTD-300 DT-RAW

70

-0.010

-0.015

YM-RAW HTD-250 HTD-300 DT-RAW

-0.020

-0.025

200

400

600

800

1000

200

600

800

1000

0.000

100

-0.005

-1

Pyrolysis rate (min )

90

Weight (%)

400

80

ZD-RAW HTD-250 HTD-300 DT-RAW

70

200

-0.010

ZD-RAW HTD-250 HTD-300 DT-RAW

-0.015

400

600

800

1000

200

400

600

800

1000

Fig. 5. Results for the effect of upgrading on coal pyrolysis.

3.4. Effect of upgrading on pyrolysis behavior

pores, resulting in the development of mesopores. As shown in Table 2 [45], after HTD upgrading, the BET surface area and total volume increased simultaneously with a decrease in pore size. However, when the end temperature increased from 250 °C to 300 °C, the surface area and volume decreased, and the average pore diameter increased. In addition, the changes caused by the HTD treatment were more obvious for ZT lignite due to its relatively low coal rank. For example, the BET surface area changed from 0.08 m2/g (ZT) to 19.89 m2/g (ZT-250) and 15.83 m2/g (ZT-300), but from 2.28 m2/g (ZD) to only 4.85 m2/g (ZD-250) and 4.41 m2/g (ZD-300).

Fig. 5 shows the pyrolysis TG and DTG curves of raw and upgraded coals. After upgrading, the pyrolysis curves moved towards the hightemperature region and were similar to those of DT bituminous coal. The entire coal thermal conversion process was delayed due to the increase in the stable composition of the coal structure and improvement in coal rank. The thermal stability of the coal structure was enhanced. The TG curves of final weight loss during the pyrolysis process were reduced for the upgraded coals, and the DTG curves of the weight loss 74

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Table 3 Results for the effect of upgrading on pyrolysis characteristic parameters. Coal

Condition

Start temperature/ °C

Peak temperature/ °C

Finish temperature/°C

Half width of peak/°C

Maximum pyrolysis rate/(%/min)

Average pyrolysis rate/(%/min)

Final weight loss/%

Da * 1011/ (min−2 K−3)

ZT

RAW HTD-250 HTD-300

202 214 220

448 448 449

628 628 628

179 146 116

−2.451 −2.140 −2.102

−1.203 −1.054 −0.853

47.19 42.63 39.41

1.82 1.61 1.56

YM

RAW HTD-250 HTD-300

260 270 300

445 453 453

684 687 690

125 134 131

−2.270 −2.113 −1.938

−0.988 −0.961 −0.901

37.00 36.48 33.97

1.55 1.24 0.98

ZD

RAW HTD-250 HTD-300

279 280 281

445 446 462

772 780 800

257 377 395

−1.480 −1.145 −1.053

−0.800 −0.735 −0.685

33.49 30.44 28.62

0.37 0.18 0.14

DT

RAW

340

469

740

131

−1.661

−0.762

26.14

0.61

a

D: comprehensive pyrolysis characteristic parameter.

on the thermal conversion characteristics was more obvious in low-rank samples, such as ZT lignite, than in high-rank samples. The pyrolysis characteristic parameters derived from the TG-DTG curves can be used to evaluate thermal performance during the pyrolysis process [42,49], including the start temperature (Ts), finish temperature (Tf), peak temperature (Tmax), maximum pyrolysis rate (Rmax), average pyrolysis rate (namely the average value between the start and finish temperatures, Rmean), half width of the peak (temperature interval when R/Rmax = 1/2, ΔT1/2) and final weight loss (Δm). A comprehensive pyrolysis characteristic parameter (D), defined in Eq. (1), was used to compare the pyrolysis characteristics of different coal samples. The coals with high D values exhibited better pyrolysis performance.

Table 4 Results for the effect of upgrading on the kinetics parameters of coal pyrolysis. Coal

Condition

Activation energy/ (kJ/mol)

Pre-exponential factor/min−1

Correlation coefficient

ZT

RAW HTD-250 HTD-300

36.87 45.20 45.27

5.1E+1 2.1E+2 1.7E+2

0.9989 0.9941 0.9868

YM

RAW HTD-250 HTD-300

53.20 58.77 62.33

4.1E+2 2.0E+3 2.0E+3

0.9961 0.9975 0.9792

ZD

RAW HTD-250 HTD-300

54.14 61.02 72.40

8.1E+2 1.2E+3 1.9E+4

0.9984 0.9851 0.9988

DT

RAW

99.69

1.2E+6

0.9981

0.10

CH4

H2O

CO2CO

D=

1000 °C

797 °C

0.06

0.04

448 °C 105 °C

0.00 4000

3200

2400

1600

(1)

As shown in Table 3, the upgrading process increased Ts, Tf and Tmax, and decreased Rmax, Rmean and Δm. During the HTD process, the active materials were reduced or converted into inactive components, and the unstable composition was modified to a stable composition. All these changes resulted in the contraction of the coal structure and increases in the density and coal rank, which consequently decreased the coal activity in the thermal conversion process. The upgraded products exhibited pyrolysis characteristics similar to those of HRC, indicating enhanced thermal stability. Moreover, parameter D decreased after the upgrading process, reflecting a reduction in pyrolysis reactivity. The abovementioned trends were more obvious when the treatment temperature increased to 300 °C. In addition, pyrolysis kinetics were investigated in this study, and the Coats–Redfern integral method was adopted [43]. As shown in Table 4, the activation energy (E) increased after the HTD process, from 36.87 kJ/mol (ZT) to 45.20 kJ/mol (ZT-250) and 45.27 kJ/mol (ZT300), from 53.20 kJ/mol (YM) to 58.77 kJ/mol (YM-250) and 62.33 kJ/ mol (YM-300), and from 54.14 kJ/mol (ZD) to 61.02 kJ/mol (ZD-250) and 72.40 kJ/mol (ZD-300). The hydrolysis of O-containing structures, such as ethers, esters, and carboxylates, during the pretreatment in the presence of steam plays an important role in the fate of these O-containing structures during pretreatment and the subsequent pyrolysis. These changes can lead to variations in the pyrolysis behavior of the lignite [48]. The presence of water likely reduced cross-linking during the thermal processing of LRCs. During the HTD process, water removed as liquid may dissolve and leach out some water soluble inorganic and hydrophilic organics containing oxygen functional groups with the waste water [47], thereby reducing the quantities of catalytic ion constituents and active oxygen-containing functional groups in the upgraded solid products [50]. These changes may, to a certain extent, reduce the thermal conversion reactivity. In addition, along with the improvement in coal rank, pyrolysis reactivity weakened after the HTD upgrading process.

H2O

0.08

0.02

Rmax ∗Rmean Ts ∗Tmax ∗ΔT1/2

800

Fig. 6. IR spectra at different temperature stages during coal pyrolysis.

peak also decreased, indicating that the volatile matter decreased or was converted to a stable composition after upgrading. These results might indicate that the HTD can dissociate the aggregated structure of coal and create a less aggregated structure [34,35]. During the HTD process, water can participate in one or more roles with existing oxygen functional groups in lignite: as a catalyst, reactant, and solvent in the aquathermolysis chemistry at the treatment temperature [47]. Steam is able to depolymerize the coal macromolecular network by cleaving weak covalent bonds, such as ether linkages that are thermally stable but hydrothermally unstable at elevated temperatures (> 250 °C) [48]. These effects were more obvious when the treatment temperature increased to 300 °C. Similar to the above results, the effect of upgrading 75

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ZT-RAW HTD-250 HTD-300 DT-RAW

12

Absorption (10-3)

Absorption

0.06

0.04

0.02

ZT-RAW HTD-250 HTD-300 DT-RAW

8

4

0

0.00 200

400

600

800

1000

200

Temperature (°C)

400

(a) CO2

800

1000

(d) CH4

ZT-RAW HTD-250 HTD-300 DT-RAW

12

Absorption (10-3)

Absorption (10-3)

12

600

Temperature (°C)

8

4

ZT-RAW HTD-250 HTD-300 DT-RAW

8

4

0

0 200

400

600

800

200

1000

400

600

800

1000

Temperature (°C)

Temperature (°C)

(e) P-xylene

(b) CO 2.5 ZT-RAW HTD-250 HTD-300 DT-RAW

Absorption (10-3)

2.0

-3

Absorption (10 )

3

ZT-RAW HTD-250 HTD-300 DT-RAW

1.5

2

1.0

1

0.5 0.0

0 200

400

600

800

200

1000

400

600

800

1000

Temperature (°C)

Temperature (°C)

(c) Formic acid

(f) Phenol

Fig. 7. Results for the effect of upgrading on volatile release characteristics.

groups containing CO were very stable; thus, CO was released at high temperatures [51]. The volatile release profiles throughout the coal pyrolysis are shown in Fig. 7. Notably, the release of volatiles was relatively concentrated, except for that of CO. After HTD upgrading, the release peak of each component was delayed to the high-temperature region and in the bituminous coal direction, consistent with previous results. It is generally believed that CO2 is derived from carboxyl and other oxygen-containing compounds in coal that easily decompose above 200 °C. The amount of CO2 released gradually decreased above 600 °C, and in this stage, CO2 was mainly decomposed from ether, quinone and stable oxygen-containing heterocycles. In contrast to CO2, the temperature at which CO release peaked was approximately 700–900 °C. CO was mainly produced in four ways: as part of the phenolic hydroxyl group, which is the

3.5. Effect of upgrading on release of volatiles As shown in Fig. 6, the IR absorption peak of CO2 was the largest and most obvious, and those of H2O, CH4, CO and p-xylene were also obvious. By contrast, the formic acid and phenol peaks were relatively weak due to their low volatile contents. Before the temperature reached 150 °C, moisture and adsorbed gas were released from the coal. As the temperature increased, the active components in the lignite began to decompose. By the time the peak temperature of 448 °C was reached, the amounts of released volatiles had reached their maximums, except for CO. Then, the release of CO began to increase, and the release of other substances gradually decreased. The release of CO reached a maximum at 797 °C and then slightly decreased, and the process had not completed when the end temperature was reached. The functional 76

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by the National Natural Science Foundation of China (Grant No. 51706059), the Fundamental Research Funds for the Central Universities of China (Grant No. 2018B24914).

most stable among the oxygen-containing functional groups and decomposes in the form of CO above 700 °C; as part of ether bonds, which break to produce CO above 800 °C [52]; due to carbonyl fracture at temperatures higher than 400 °C; and due to breaking and release of oxygen-containing heterocycles at temperatures above 700 °C. Formic acid is produced from the cleavage of oligosaccharides. CH4 is mainly produced by the cleavage of aliphatic chains, aromatic side chains, oxymethylene and polymethanes, which contain methyl functional groups [53]. p-Xylene and phenol, benzene ring-containing macromolecules that are mainly produced by the degradation and cracking of coal macromolecules, are formed by the polycondensation of aromatic structures and hydrogenated aromatic structures in coal. Previous FTIR analysis results [33] suggest that the macromolecular structure of coal becomes more dense and stable after HTD upgrading. Additionally, active carboxyl, carbonyl and ether groups and unstablephase aliphatic compounds and oligosaccharides are decomposed. Notably, water acts as a reactant and the reaction medium for hydrolysis, depolymerization, dehydration and decarboxylation reactions [54]. The proton-catalyzed mechanism, direct nucleophilic attack mechanism, hydroxide ion catalyzed mechanism, and radical mechanism play important roles in the conversion of LRCs in a hydrothermal medium [55]. Hydrothermal carbonization is defined as the combined dehydration and decarboxylation of a fuel to increase its carbon content with the objective of achieving a higher heating value [56]. The formation of carbon-rich solids through the hydrothermal carbonization of saccharides is the consequence of dehydration, condensation, polymerization and aromatization reactions. The processes reduce the amounts of CO2, CO and formic acid released. In addition, unsaturated methylene and aromatic side chains will break and generate saturated methyl. The stable-phase components of the aromatic structure will increase in quantity, and the macromolecular structure of the coal will shrink and reorganize, thus increasing the amounts of methane and the macromolecular substances p-xylene and phenol during the pyrolysis process. The release of each component was close to that for DT bituminous coal, which indicates that the macromolecular structure of lignite became more stable, the quantity of active components in the volatile matter decreased, and the amount of macromolecular material increased. After HTD upgrading, the pyrolysis and volatile release characteristics of lignite trended toward those of HRC, and the coal rank improved.

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4. Conclusion HTD is an effective dewatering and upgrading process that can decrease the O/C and H/C ratios of coal and improve the coal rank. After upgrading, the coal particles were broken into pieces, and many small coal fragments were produced. The pore structure expanded to the micropore region, and more distribution peaks appeared. The BET surface area and total pore volume initially increased but then slightly decreased when the treatment temperature increased to 300 °C. Due to the improvement in coal rank, the thermal conversion process of the pyrolysis reactions moved towards the high-temperature region. The start, finish and peak temperatures increased; the maximum and average pyrolysis rates and final weight loss decreased; and the comprehensive pyrolysis characteristic parameter decreased. Moreover, the thermal stability of the coal structure improved, and the pyrolysis activation energy increased after upgrading. Among the volatile materials released, the release of low-activity molecules (CO2, CO and formic acid) decreased, and the release of benzene-ring containing macromolecules with aromatic structures (p-xylene and phenol) increased. Moreover, due to the conversion of unsaturated bonds to saturated bonds, the release of CH4 increased. Acknowledgements The authors gratefully acknowledge the financial support provided 77

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