Drying and depolymerization technologies of Zhaotong lignite: A review

Fuel Processing Technology 186 (2019) 88–98

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Review

Drying and depolymerization technologies of Zhaotong lignite: A review ⁎

T

Zhan-Ku Li , Hong-Lei Yan, Jing-Chong Yan, Zhi-Cai Wang, Zhi-Ping Lei, Shi-Biao Ren, ⁎ Heng-Fu Shui School of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Coal Clean Conversion and High Valued Utilization, Anhui University of Technology, Ma'anshan 243002, Anhui, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Lignite Drying Thermal dissolution Pyrolysis Supercritical water gasification

Zhaotong lignite (ZL), a soft lignite from southwest of China, is an abundant coal resource. However, it has not been utilized on large scale due to its very high moisture content and low calorific value. Drying and depolymerization technologies of ZL for upgrading or producing valuable gases, value-added chemicals, and/or liquid fuels have been widely investigated in laboratories. This paper introduces geological setting and chemical structures of ZL. Drying and depolymerization technologies (including thermal dissolution, pyrolysis, and supercritical water gasification) of ZL are reviewed in detail. Different drying methods have different merits and demerits. Thermal dissolution (especially ethanolysis) of ZL under mild conditions can be used to produce oxygen-containing compounds. Pyrolysis of ZL follows the second-order kinetics and optimal temperatures for preparing tar and valuable gases are distinct. High yields of H2 and CH4 are generated from supercritical water gasification of ZL. In addition, future investigations on efficient utilization of ZL are advised. Although the topic of this review mainly focused on a specific lignite, the drying and depolymerization technologies are also applied to other low-rank coals, especially lignites.

1. Introduction Zhaotong basin is located at Zhaoyang District, Zhaotong City, Yunnan Province, in the southwest of China and covers an area of 230 km2. The basin has three lignite beds with a total reserve of ca. 8 billion ton lignite formed during Tertiary and Neogene [1]. Zhaotong lignite (ZL) is a soft lignite and its major maceral composition is humic [2]. ZL has attracted much attention because of its easy mineability and abundant deposit in China. As Table 1 shows [1,3–28], ZL has very high moisture content as received (ca. 50%–60%) and high oxygen content (ca. 20%–40%), resulting in low calorific value. Such characteristics of ZL extremely limit its industrial application. Drying of lignites is usually needed prior to long distance transportation or utilization. Various drying technologies such as thermal upgrading, hydrothermal dewatering (HTD), and mechanical thermal expression (MTE) have been developed [29,30]. Certainly, these methods were used to dry ZL for obtaining upgraded ZL. High yields of soluble portions rich in oxygen-containing compounds can be offered from thermal dissolution of ZL due to its high oxygen content. Pyrolysis of ZL can be used to produce valuable gases, value-added tar, and semichar. Supercritical water gasification (SCWG) of ZL was also investigated owing to no requirement of drying prior.

⁎

In this paper, structural features and drying of ZL are presented. Depolymerization technologies of ZL, including thermal dissolution, pyrolysis, and SCWG, are also reviewed. In addition, further researches on efficient utilization of ZL are proposed. 2. Structural features of organic matter in ZL Understanding the chemical structure of ZL is significantly important for developing efficient utilization processes of ZL. In our previous research [3], nuclear magnetic resonance and X-ray photoelectron spectrometer were used to characterize ZL for obtaining a deep insight into the chemical structure of ZL. Based on 13C nuclear magnetic resonance analysis (Table 2), ZL is mainly comprised of aliphatic (52.3%) and aromatic (42.2%) carbons and rich in methylene carbon. Each aromatic cluster contains 2 rings on average, which is the same as Xiaolongtan lignite [31] and Huolinhe lignite [32]. X-ray photoelectron spectrometric analysis exhibits that oxygen in ZL mainly exists in CeO moieties appearing in alcohols, phenols, and ethers and pyrrolic nitrogen is the most abundant in all the organic nitrogen structures, followed by quaternary nitrogen. Alkyl side chains on aromatic rings, alkylene bridges connecting aromatic rings, and condensed aromatic rings in ZL were investigated

Corresponding authors. E-mail addresses: [email protected] (Z.-K. Li), [email protected] (H.-F. Shui).

https://doi.org/10.1016/j.fuproc.2019.01.002 Received 16 November 2018; Received in revised form 31 December 2018; Accepted 3 January 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Proximate and ultimate analyses (wt%) and calorific value (MJ/kg) of ZL. Proximate analysis

Ultimate analysis (daf)

St,

Mar

Mad

Ad

VMdaf

C

H

N

O

59.0

14.7 11.6

31.1 31.8

24.8 21.0 19.4 17.0 23.0 24.0 14.7 25.9 20.1 21.4 29.1 23.0

7.5 27.3 1.9

15.7 22.6 22.0

56.9 53.6 59.2 58.7 59.1 62.8 56.5 63.6 60.3 62.8 63.1 59.1 55.6 55.0 61.6 56.4

52.5 65.8 61.7 63.2 57.2 55.8 54.1 66.6 66.6 64.0 67.2 59.8 66.9 57.2 60.8

3.3 4.1 5.3 8.3 4.9 4.9 4.6 4.0 4.0 6.1 8.3 4.9 5.1 4.6 5.4

1.0 2.0 1.8 2.1 2.1 1.1 2.5 1.8 1.8 2.0 2.1 1.4 1.6 1.7 1.7

> 41.8 > 26.2 > 30.2 > 25.5 > 33.8 > 36.3 > 38.2 > 25.7 > 25.7 > 26.2 > 21.7 > 33.3 > 25.6 > 32.9 > 31.1

56.1 52.3

9.6 31.8

33.5 50.1 58.0

32.7

d

a

Calorific value Measured

1.5 1.4 1.9 1.0 0.9 2.0 1.9 0.6 1.9 1.9 1.7 0.7 0.6 0.8 3.6 1.0

b

Ref. Calculated (daf)

6.8

16.5 13.4 16.0 9.9

13.4

18.7 25.7 22.7 25.1 21.1 20.7 20.0 23.2 23.2 23.8 26.2 21.9 24.0 20.9 22.5

c

[1] [3–9] [10,11] [12] [13] [14,15] [16] [17,18] [19] [20] [21] [22] [23] [24] [25–27] [28]

daf: dry and ash-free base; Mar: moisture (as received base); Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); VMdaf: volatile matter (dry and ash-free base); St, d: total sulfur (dry base). a By difference. b The basis is different. c Calculated calorific value = 1.3675 + 0.3137 C + 0.7009 H + 0.0318 O. Table 2 Carbon structural parameters in ZL determined by solid-state magnetic resonance [3].

13

C nuclear

Structural parameter

Value

Aromaticity index Aliphaticity index Ratio of carbonyl carbon Molar fraction of aromatic bridgehead carbon Average methylene chain length Substituted degree of aromatic ring

42.2% 52.3% 5.5% 0.22 2.4 0.6

by ruthenium-ion-catalyzed oxidation and subsequent analysis of the resulting products [4]. The results suggest that ZL is rich in alkylene (especially eCH2CH2e and eCH2CH2CH2e) bridged linkages. Such bridged linkages are liable to catalytic hydrocracking or pyrolysis. The carbon number in alkyl side chains on aromatic rings in ZL ranges form 8–28. 3. Drying of ZL Abundant oxygen-containing functional groups existing in lignites result in hydrophilicity and high moisture content, which significantly affects their utilization processes. To develop efficient drying technologies for lignites, it is imperative to understand lignite-water interactions. Water associated with lignites can be divided into five types: interior adsorption water, capillary water, adhesion water, interparticle water, and surface adsorption water. Some water can be easily removed using conventional approaches (e.g., centrifugation) and others must be evaporated by heating lignites to a relatively high temperature. In the past decades, various drying technologies for lignites have been developed [29,30]. Generally, drying technologies are classified into evaporative drying [33] and non-evaporative drying (e.g., HTD, MTE dewatering, and solvent extraction) based on drying method. Drying processes are always affected by three factors: (a) the parameters of the drying media (including temperature, pressure, velocity, and relative humidity); (b) the coal parameters (coal structure and particle size); and (c) the drying method [30,33]. Thermal upgrading, an evaporative dewatering method, is the simplest drying technology. Remarkable changes in physicochemical structures of lignites are caused by thermal upgrading, improving the

Fig. 1. Schematic diagram of fixed bed for hot N2 drying of ZL [20].

heating value and reducing the spontaneous combustion tendency [11]. Miao et al. [20] examined drying behaviors and kinetics of ZL and Shengli lignite with particle size of < 2 mm or < 13 mm at 150–300 °C using hot nitrogen at a flowing velocity of 160 mL/min in a fixed bed (Fig. 1). The results imply that temperature plays a key role in the drying process, which is consistent with Pusat's study for a Turkish lignite drying [34]. For example, the drying rates at 300 °C for ZL and Shengli lignite with particle size of < 2 mm are 13.6 and 4.4 times that at 150 °C, respectively, which indicates that temperature is more prominent for ZL than Shengli lignite. They constructed the residual moisture content prediction equation (R2 = 0.96) associated with temperature, initial moisture content, and drying time based on the 89

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processes such as K-Fuel®, continuous HTD, and hot water HTD have emerged on the commercial market [30]. MTE technology was developed in the mid-1990s and it combines mechanical energy with thermal energy, making the rapid dewatering possible [30]. The dewatering process involves four steps [44]: Putting a lignite into the sample chamber; heating the lignite to a set temperature; compressing to a constant pressure for mechanical dewatering; cooling the lignite with flash evaporation dewatering. Comparing with thermal upgrading and HTD, MTE can produce briquettes with a particle size of ca. 15 cm [10]. The moisture readsoption of MTE dried ZL was strongly inhibited owing to the much larger particle size of the dried ZL. The MTE dried ZL was more liable to spontaneous combustion than thermally upgraded ZL with the same residual moisture and particle size, while the larger particle size of MTE dried ZL made it more stable to spontaneous combustion [10,11]. Based on MTE process, a new dewatering method, i.e., vibration mechanical thermal expression (VMTE) process, was exploited by Zhang et al. [19]. They investigated the dewatering of ZL by a VMTE rig (Fig. 5) under different vibration forces (0–5 kN), temperatures (50–250 °C), and pressure (3.4–12.7 MPa). As displayed in Fig. 6, the moisture content of ZL almost linearly decreased from 1 to 0.19 with raising the temperature to about 200 °C. The dewatering efficiency of MTE process enhanced > 10% by exerting a vibration force of 5 kN. Most importantly, the time used for dewatering to the same residual moisture content by VMTE process is much shorter than by MTE process. For instance, the moisture content was reduced to 0.415 by VMTE process in 10 s, while the similar moisture content (0.408) was obtained in 30 s by MTE process. The results demonstrate that VMTE process exhibits great potential to improve the productivity of MTE process. The mechanism of vibration effect on VMTE process was discussed. Vibration could promote MTE process by compacting lignites. With the increase of vibration force lower than critical value, ZL became more compact and the total pore volume of ZL decreased, hence directly leading to the decrease of moisture content. One obstacle for HTD and MTE application in industrial is the treatment of waste water. The liquid phase separated after HTD or MTE processes contains both organic and inorganic matter [42]. The large volumes of organic, salty, and acidic-rich water make it difficult and cost-expensive for treatment. Investigations on treating the waste water were performed by several researchers and more work is needed [30]. For ZL drying in practice, the choice of the most suitable dryer or drying method is affected by the available sources of heat, the final moisture content, the achievable capacity range, and the invest and operational cost [30].

logarithmic model, which is different for drying kinetics of coarse lignite particles (20–50 mm) best fitted by Wang&Singh model [35] or modified Wang & Singh model [36]. Different from Shengli lignite with small change during the drying process, the volatile matters and carboxyl group of ZT slightly and significantly decreased, respectively, while the contact angle increased. In addition to temperature and particle size, bed height and velocity of drying gas also have influence on the drying process. Further study on the effects of these factors on ZL thermal upgrading can be properly designed referring to Design of Experiment [37] or adaptive-network-based fuzzy inference system method [38]. Although thermal upgrading has the aforementioned advantages, the merits of thermal upgrading would be limited to some extent by moisture readsorption due to a large number of crevices and holes existing on the upgraded lignite surface and high energy consumption. High energy cost could be saved using waste heat from a power plant's boiler exhaust gases [29,39]. An approach to prevent moisture readsorption was developed by Zhang et al. [12]. They evaluated moisture readsorption and combustion characteristics of ZL thermally upgraded with the addition of asphalt (0–10%) taking advantage of its good hydrophobicity. The results indicate that hydrophilic oxygen-containing functional groups (hydroxyl and carboxyl) and pore diameter of the upgraded ZL decreased with the increasing temperature, while pore volume and surface area increased owing to the coverage of some pores by asphalt. The moisture readsorption and spontaneous combustion tendency of the upgraded ZL decreased with the increasing temperature, but the effect of asphalt on the moisture readsorption and spontaneous combustion is temperature-dependent. In recent years, HTD is attracting more and more attention since it could save latent heat of vaporization and remarkably destroy oxygencontaining functional groups to prevent moisture readsorption [13–16,40,41]. After HTD at 250 and 300 °C, the inherent moisture and oxygen content of ZL significantly decreased, thus improving calorific value from 13.36 to 19.38 and 21.02 MJ/kg, respectively, which is more obvious than Yimin lignite [13]. In addition, HTD upgraded ZL exhibits difficulty in ignition but combust easily (Table 3), agreeing with reported results on HTD of other low-rank coals [42]. HTD of ZL also changed nitrogen and sulfur forms [17,18], e.g., pyrrolic nitrogen was transformed to pyridinic or quaternary nitrogen and some of triple bonds (eCN) were broken to form amides, and organic sulfur was partially converted to inorganic sulfur. Similar results were also found after HTD of other lignites such as Inner Mongolia and Yunnan lignites [43]. Such changes can reduce environmental effect to some extent during lignite utilization. HTD (150–300 °C) and thermal upgrading (200–500 °C) of ZL were comparatively investigated [15]. As a result, the main hydrophilic oxygen-containing functional groups (hydroxyl and carboxyl) were effectively removed with raising the temperature and the removal of hydroxyl groups is more obvious during HTD process (Fig. 2). The gel-like structure of ZL experienced violent shrinkages and collapses. As exhibited in Fig. 3, moisture readsorption ratio continuously decreased with raising the upgrading temperature under the synergistic effect of the physicochemical structure. At the same upgrading temperature, HTD is more efficient for preventing moisture readsorption of upgraded ZL than thermal upgrading. Additionally, a moisture holding capacity model to describe moisture readsorption of upgraded lignites was constructed. Most of researches on HTD of lignites were carried out in an autoclave (Fig. 4). In fact, several HTD

4. Thermal dissolution of ZL In the past decade, thermal dissolution of lignites has attracted more and more attention due to its some advantages, such as mild conditions and no consumption of catalysts and gaseous hydrogen [45]. Thermal dissolution of lignites can be used to produce HyperCoals (ash-free coals) and chemicals. The yields of thermally soluble portions from lignites strongly depend on the types of lignites and solvents. In our recent investigations [5,6], thermal dissolution behaviors of ZL with different solvents, including cyclohexane, methanol, and ethanol, at 300 °C in a 100 mL stainless autoclave were examined. The results show that ethanol (64.9%) is much more effective for thermally

Table 3 The influence of HTD on combustion characteristics of ZL [13]. Sample

Ignition temperature (°C)

Peak temperature (°C)

Burnout temperature (°C)

Maximum combustion rate (%/min)

Average combustion rate (%/min)

ZL HTD-250 HTD-300

287.3 293.8 313.3

368.3 345.8 335.0

536.0 579.0 588.0

−0.116 −0.146 −0.187

−0.0529 −0.0531 −0.0568

90

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Fig. 2. Changes of hydroxyl and carboxyl groups of ZL during HTD and thermal upgrading [15].

composition of the soluble portions. Gas chromatograph/mass spectrometer (GC/MS) was commonly used to characterize organic species in coal-derived liquids [54]. Most of the detected species in the ESP with GC/MS are oxygen-containing organic compounds, corresponding to the high oxygen content of ZL. Esters are the most abundant in the ESP, most of which are ethyl alkanoates [8]. The ethyl alkanoates should be produced via esterification of alkanoic acids with ethanol and/or transesterification. Phenolic compounds are the secondarily group component in the ESP. Although GC/MS has identified more than one hundred compounds in the ESP, it is limited to relatively volatile, thermally stable, and less polar species. Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS) proved to be a powerful tool for molecular characterization of extremely complex samples (e.g., petroleum [55,56], bio-oils [57,58], and coal-derived liquids [59,60]) due to its ultrahigh mass resolution (exceeding 200,000) and mass accuracy (< 1 ppm), which allows for baseline resolution of closely spaced isobaric species and distinct assignment of a unique elemental composition to each mass spectral peak. Polar species in complex mixtures can be selectively ionized and identified by combination of electrospray ionization and FTICRMS. The ESP was characterized with a 9.4 T ESI FTICRMS both in positive- and negative-ion modes for analyzing basic nitrogen species and acidic oxygen compounds, respectively [5,9]. Such identification facilitates the understanding of polar species in the ESP. In positive-ion mode [9] (Fig. 7 bottom), 86.4% of the compounds detected in the ESP are basic nitrogen species of classes N1Ox (x = 0–5) and N2Oy (y = 0–2) with double bond equivalent (DBE, i.e., double bonds plus rings) values of 0–14 and carbon numbers of 9–39. N1Ox class species should be mainly ascribed to pyridines and quinolines along with small amounts of amines, while N2Oy class species are predominantly in the form of alkaloids. The

dissolving ZL than methanol (24.3%) and cyclohexane (10.1%). Similar results were also observed for other lignites, such as Huolinguole lignite [46] and Xianfeng lignite [47]. The difference in yields of thermally soluble portions is ascribed to that cyclohexane is effective for thermally extracting inherent components in lignites without remarkably breaking the covalent bonds [48], while methanol and ethanol were involved in the thermal dissolution process, i.e., alkanolysis proceeded, resulting in the high yields [4,46,49]. The alkanolyses of lignite-related model compounds, including benzyloxybenzene, anisole, phenethoxybenzene, and oxydibenzene, were simulated by density functional theory [50]. The results indicate that the alkanolyses involve nucleophilic attack, hydrogen transfer, and bond cleavage, as shown in Scheme 1, and activation energies of ethanolysis are much lower than those of methanolysis, i.e., ethanolysis proceeds much more easily than methanolysis, which should be responsible for the higher yield of ethanol-soluble portion (ESP) than that of methanol-soluble portion. The lower activation energy of ethanolysis than that of methanolysis should be attributed to the stronger nucleophilicity of ethanol than that of methanol. Reactivities of the lignite-related model compounds toward alkanolysis decrease in the order: benzyloxybenzene > anisole > phenethoxybenzene > oxydibenzene. Alkali could enhance the nucleophilicity of alkanol, thus promoting the alkanolysis of lignites. Ethanolyzed residue of ZL was subjected to ethanolysis with NaOH at 300 °C [7,8]. The total yield of ESP is up to 88.8%, suggesting that most of the organic matter in ZL was converted to soluble species by the two-step ethanolysis. High yields of soluble portions from alkanolysis of other lignites with a alkali were also obtained [51–53]. Detailedly analyzing molecular composition of thermally soluble portions from ZL is very important for efficiently utilizing the resulting soluble portions and challenging owing to the highly complex

Fig. 3. Moisture readsorption of ZL after (a) HTD and (b) thermal upgrading [15]. 91

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Fig. 4. Schematic diagram of the HTD apparatus [15].

Fig. 5. Schematic diagram of the VMTE rig [19].

detection of basic nitrogen species could be responsible for the high quaternary nitrogen content from X-ray photoelectron spectrometric analysis [3]. The release of basic nitrogen species from ZL is probably caused by thermal- and solvent-induced destruction of noncovalent bonds in ZL. In negative-ion mode [5] (Fig. 7 top), the detected species almost are oxygen-containing compounds, especially acidic O1–O4 class species with DBE values of 1–12 and carbon numbers of 8–34. On the basis of DBE distributions, characteristic structures in the ESP are phenols, benzenepolyols, and benzoic acids with an aliphatic ring and alkyl chains. The detection of large amounts of phenolic compounds and the high ESP yield imply that ethanolysis can be considered as a promising approach for converting ZL into chemicals. Special attention should be paid to the consumption of ethanol during ethanolysis of ZL, the effect of moisture on the ethanolysis, and separation and purification of the ESP in the future. Fig. 6. Effects of temperature and pressure on residual moisture content of ZL during VMTE and MTE processes [19].

5. Pyrolysis of ZL lignite pyrolysis are usually volatile gases, tar, and semi-coke. Volatile gases can be used as fuel gas. Fuels and/or chemicals could be produced from tar. Semi-coke is commonly used to burn or gasify. A number of

Pyrolysis, involved in liquefaction, gasification, and combustion, is one of the most important coal conversion processes. In recent years, lignite pyrolysis is attracting more and more attention. The products of 92

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Scheme 1. Reaction mechanism for benzyloxybenzene methanolysis [50].

factors, e.g., final pyrolysis temperature, particle size, heating rate, and atmosphere, intensively affect the yields and compositions of pyrolysis products. Gas evolution characteristics from pyrolysis of virous coals were widely investigated and similar evolution profiles were obtained [21,61,62]. For ZL pyrolysis with a thermogravimeter/mass spectrometer [21], H2 release occurs at about 300 °C originated from the degradation of hydrogen-rich matrix. The amount of H2 release significantly increased with raising the temperature to above 600 °C and then decreased with further raising the temperature. The released H2 at above 600 °C could be ascribed to condensation of aromatic rings or decomposition of heterocyclic rings. CH4 evolution proceeds in a broad temperature range between 200 and 800 °C. CH4 produced in low temperature range (below 345 °C) comes from thermal desorption of adsorbed CH4 trapped in the macromolecular skeleton of ZL [63], while DBE

0

1

2

3

4

5

CH4 formed at high temperatures mainly origins from the cleavage of alkyl side chains. CO emission exhibits a bimodal distribution ranging from 200 to 800 °C with a maximum at 460 and 700 °C primarily due to decarbonyl and rupture of ether bonds below 500 °C and decomposition of phenolic hydroxyl and ring crack above 500 °C. CO2 release appears at low temperatures (approximately 200 °C) and reached to a peak at about 400 °C, which is correlated with decarboxylation reaction of ZL. Specific release volume and calorific value of gases from ZL pyrolysis in a tube furnace were also performed [22]. As Fig. 8 displays, release volume of pyrolysis gases increased by two times with raising temperature from 500 to 1000 °C. The relative percentage of H2 varied from 8.2% to 48.1% with raising temperature from 500 to 1000 °C. The relative percentage of CH4 slowly increased with raising temperature from 500 to 600 °C and then rapidly increased with further raising temperature to 1000 °C. In the pyrolysis process, gases firstly come from 6

7

8

9

N0 90.6%

30

11

12

13

14 ESI -

N1 9.4%

DBE < 4: fatty acids DBE > = 4: phenols, benzenepolyols, and aromatic acids

24

10

DBE > = 3: pyrroles DBE > = 6: indoles DBE > = 9: carbazoles

18

Relative abundannce (%)

12

6

0

O1

O2

20

O3

O4

O5

O6

N1

N1 69.0%

N1O1

N1O2

N1O3

N1O4 N0 13.6%

N2 17.4%

N1O5 ESI +

DBE < 4: amines DBE < 4: diamines esters, ketones, DBE > = 4: pyridines DBE > = 4: alkaloids ethers, and etc DBE > = 7: quinolines

16

12

8

4

0

N1

N1O1

N1O2

N1O3

N1O4

N1O5

N2

N2O1

N2O2

O2

O3

Class Fig. 7. Distributions of NmOn class species in the ESP from FTICRMS analysis [5,9]. 93

O4

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Fig. 8. Volume of released gases (left) and composition of released gases (right) at different temperatures from ZL pyrolysis [22]. Fig. 9. Schematic diagram of ZL pyrolysis (1. temperature controller; 2. carbonization furnace; 3. inlet of cooling water; 4. gas cooling device; 5. bottle for gas; 6. metering device; 7. U-shaped tube; 8. injection system; 9. ZL; 10. outlet of cooling water; 11. purge port of N2; 12. gas chromatography; 13. chromatography workstation) [23].

100

others bitumen

Content (wt.%)

80

water nitrogen compounds

60

oxygen compounds phenols

40

arenes alkenes cycloalkanes

20

alkanes

0 tar

< 170

170-210 210-270 270-300 300-340 distillate (oC)

Fig. 11. Compositions of the tar from ZL pyrolysis at 600 °C and its distillates [23].

Table 4 Pyrolysis kinetics parameters (n = 2) of ZL at different heating rates [23]. Fig. 10. Effect of temperature on the tar yield of ZL pyrolysis [23].

the breakage of weak bridges, then from the fracture of functional groups, and finally from the condensation reaction. The calorific value of gases from ZL pyrolysis dramatically increased from 7.16 to 16.71 MJ/Nm3 with raising temperature from 500 to 800 °C owing to significant increase of H2 and slow increase of CH4, and the optimal temperature for producing valuable gases from ZL pyrolysis is 700–800 °C. Distributions of tar and semi-char from ZL pyrolysis in a self-researched fixed bed reactor (Fig. 9) under different conditions were also examined [23]. The results suggest that smaller particle size and higher heating rate favor the tar yield. As displayed in Fig. 10, the tar yield firstly increased with raising temperature from 450 to 600 °C and then

Heating rate (oC/min)

Temperature (°C)

Pre-exponential factor (/min)

Activation energy (kJ/mol)

R2

5

300–400 400–540 540–600 300–400 400–530 530–600 300–400 400–530 530–600

235 37 24 933 782 519 994 6340 617

57.2 69.3 93.9 54.1 65.2 90.4 51.3 61.0 88.2

0.999 0.999 0.987 0.997 0.998 0.999 0.989 0.996 0.997

10

20

decreased with further raising temperature to 650 °C, which could be attributed to more secondary pyrolysis than tar production at 650 °C. In other words, the optimal temperature for the yield of tar from ZL pyrolysis is 600 °C. The tar yield from ZL pyrolysis (11.4%) at 600 °C is 94

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Fig. 14. Effect of temperature on gas yields and CGE for SCWG of ZL with or without Ru/CeO2-ZrO2 [26].

Fig. 12. Schematic diagram of ZL SCWG equipment (1. N2 cylinder; 2. NS 336 alloy autoclave; 3. furnace; 4. heat exchanger; 5. gas-liquid separator; 6. motor; 7. safety valve; 8. vent valve; 9. back pressure valve; 10. filter; 11. wet type flow meter; 12. gas bag) [25].

ethylmethylindenols) are predominately distributed in distillates < 210 °C and arenes (mainly polyalkylbenzenes) distributed in different distillates. The specific surface area of semi-char slightly changes below 550 °C (8.1–10.8 m2/g), but violently increased at 600 °C (34.0 m2/g) due to generation of considerable gases. Steam gasification reactivity (850 °C) of semi-char or char from ZL pyrolysis at 500–1000 °C was carried out [24]. The results imply that semi-char from ZL pyrolysis at 700 °C is most suitable for producing H2 and CO. Based on the model of Costs-Redfern, kinetics of ZL pyrolysis was also calculated [23]. The kinetic curves show a better linear correlation with the reaction orders of n = 2 than n = 1 and n = 3. Therefore, the kinetic equation of ZL pyrolysis can be expressed as dα/dt = Ae^(−E/RT)(1-α)^2, where α, A, E, R, and T denote conversion, pre-exponential factor, activation energy, Boltzmann constant, and Kelvin temperature, respectively. As Table 4 demonstrates, the activation energy of ZL pyrolysis varies from 51 to 94 kJ/mol, which is lower than that of Dongsheng lignite pyrolysis with the reaction order of n = 1 or n = 1.5 [65], and decreases with the increasing of heating rate but increases with the increasing of temperature. 6. SCWG of ZL Owing to the high moisture content and low calorific value of ZL (Table 1), it is improper to be gasified using conventional methods including moving gasification, fluidized gasification, and entrainedflow gasification. For high moisture feedstocks such as biomass and lignites, it was found to be an efficient approach to gasify these feedstocks in supercritical water avoiding extra drying pretreatment [66]. As a solvent, supercritical water (T > 374.3 °C, p > 22.1 MPa) possesses high diffusivity, low viscosity, and excellent solvency for organic compounds and gases, leading to a high gasification efficiency (GE). More importantly, supercritical water can reduce the activation energy of coal gasification. It is difficult to gasify coals due to chemical inertness of aromatic carbons in coals. Zhang et al. [67] examined the effect of supercritical water on coal pyrolysis and hydrogen generation using a combined ReaxFF and density functional theory method. They found that water clusters under supercritical state can weaken CeC bonds in aromatic rings of coals, significantly decreasing the cracking energy of the CeC bonds by 287.3 and 94.6 kJ/mol compared with those in coal pyrolysis and in coal pyrolysis in vapor state, respectively. Using the similar method, SCWG of anthracene also confirmed the aforementioned conclusion [68]. Relatively high H2 (74.8 mL/g ZL) and CH4 (55 mL/g ZL) yields were obtained from SCWG of ZL at 550 °C for 20 min in a NS336 anticorrosion alloy autoclave (Fig. 12) [25]. Comparing with pyrolysis, thiophene and SO2 formation was restricted during SCWG of ZL and

Fig. 13. Effect of temperature on gas yields, GE, and CGE (top), and gas composition (bottom) for SCWG of ZL with KOH [25].

higher than Xiaolongtan lignite (8.0%) and Inner Mongolia lignite (6.3%) but lower than Xianfeng lignite (13.7%), which is mainly influenced by aliphatic carbon content, CH2/CH3 ratio, and oxygen functional groups in lignites [64]. The crude tar from ZL pyrolysis contains 15% water and 85% tar. According to GC/MS analysis, a total of 139 organic compounds were identified in dehydrated tar, including 33.8% alkanes, 16.7% arenes, 12.0% phenols, and others. As Fig. 11 exhibits, phenols (mainly cresols, dimethylphenols, and 95

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Fig. 15. Possible reaction pathways for SCWG of ZL with Ru/CeO2-ZrO2 (heavy real lines represent the faster reaction rates than real lines and dash line) [27]. Table 5 Main results on drying and depolymerization technologies of ZL. Method

Condition

Main results

Drying

Thermal upgrading (150–500 °C)

Drying kinetics based on the logarithmic model; carboxyl obviously decreased lower than 300 °C while hydroxyl higher than 300 °C Hydroxyl and carboxyl significantly decreased; difficulty in ignition but combust easily; change nitrogen and sulfur forms Time very short (< 1 min); produce briquettes inhibiting moisture readsorption Ethanol much better than other solvents; ethanolysis proceeded; ESP (64%) rich in phenols and esters 600 °C suitable for tar formation (11.4%); 700–800 °C for gases (ca. 17 MJ/Nm3); second-order reaction; gas evolution behavior CGE 42.1% with KOH; CGE 86% with Ru/CeO2-ZrO2; high yields of H2 (655 mL/g) and CH4

HTD (< 300 °C)

Thermal dissolution Pyrolysis

MTE or VMTE (5 kN, 8.4 MPa, < 250 °C) 300 °C; methanol, thanol, cyclohexane 450–1000 °C

SCWG

500–600 °C; < 30 min

yields of gases (except CO) increased with raising temperature from 400 to 600 °C. GE and carbon gasification efficiency (CGE) dramatically increased from 14.4% to 91.4% and from 5.9% to 42.1%, respectively. The molar ratios of H2 and CH4 increased from 23.0% to 47.2% and from 12.2% to 18.7%, respectively. Since both water gas shift reaction and methanation reaction are exothermal, the equilibrium constants of the two reactions decrease with raising temperature and the equilibrium constant of methanation reaction reduces more than that of water gas shift reaction. On the other hand, the higher the temperature, the faster the reaction proceeds. Accordingly, elevated temperature favors more H2 production than CH4 during SCWG of coals. Although alkaline catalysts exhibit good catalytic activity for SCWG of coals, they are difficult to be recovered and they aggravate the corrosion of the reactor. To overcome these disadvantages, metal catalysts such as Ni-based and Ru-based catalysts were developed for SCWG of coals [26,71]. As demonstrated in Fig. 14, at the same temperature, the yields of H2 and CH4 from SCWG of ZL with Ru/CeO2-ZrO2 are obviously higher than that without Ru/CeO2-ZrO2 [26]. CGE and the yield of H2 reached up to 86% and 655.0 mL/g ZL, respectively, at 500 °C for 17 min with Ru/CeO2-ZrO2, which are nearly two times with KOH as catalyst [25], showing high catalytic activity of Ru/CeO2-ZrO2. The kinetics for SCWG of ZL with Ru/CeO2-ZrO2 was investigated based on pseudo-first-order reaction. The apparent activation energy

Wangjiata subbituminous coal [69]. To further enhance the yields of H2 and CH4, alkaline catalysts (e.g., Na2CO3, K2CO3, NaOH, and KOH) were generally applied to catalyze SCWG of coals [66,70]. Qu et al. [28] found that potassium displays better catalytic activity than sodium for SCWG of ZL, which is slightly different from alkali-catalyzed SCWG of Yimin lignite (K2CO3 ≈ KOH ≈ NaOH > Na2CO3) [70]. With adding KOH (10%), the yields of H2 and CH4 from SCWG of ZL under the same reaction conditions markedly increased to 321.8 and 115.0 mL/g ZL, respectively [25], implying that KOH remarkably promoted SCWG of ZL. The typical chemical reactions occurred during SCWG process include: (1) steam reforming reaction (CHxOy + (1 − y) H2O → CO + (x/2 + 1 − y)H2); (2) water gas shift reaction (CO + H2O ⇌ CO2 + H2(ΔH = − 41 kJ/mol)); (3) methanation reaction (CO + 3H2 ⇌ CH4 + H2O(ΔH = − 211 kJ/mol)). Although various mechanisms (e.g., oxygen transfer mechanism, free radical mechanism, electrochemical mechanism, and intermediate mechanism) for alkali catalyzed SCWG of coals were proposed, it is widely believed that alkalis can promote water gas shift reaction, leading to a lower yield of CO and a higher yield of H2. However, different alkalis have distinct effects on SCWG of coals. Therefore, further investigation on the mechanism of alkali catalyzed SCWG of coals using density functional theory is needed in the future. In addition to catalyst, temperature has the greatest effect on SCWG of coals. As Fig. 13 displays, the 96

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Acknowledgments

(430–500 °C) is 130 ± 26 kJ/mol. The catalyst reuse and stability were conducted at 500 °C for 32 min. The CGE decreased from 83.5% to 60.7% after the fourth run. Chemical state and particle size of Ru changed little after used, while some of CeO2 transformed into Ce(CO3) (OH) in supercritical water and the presence of ZrO2 can improve the stability of CeO2 to some extent. To well understand the SCWG of ZL with Ru/CeO2-ZrO2, liquid intermediates formed during the SCWG of ZL were characterized [27]. GC/MS analysis indicates that the liquid intermediates can be primarily classified into alkanes, alkenes, arenes, phenols, and other oxygencontaining organic compounds. The components slowly varied without adding the catalyst, while all the components (except arenes) sharply decreased with prolonging time and raising temperature. Arenes were regarded as the ‘last hurdle’ to come over for complete gasification of ZL in supercritical water. Based on the analysis of the liquid intermediates, simplified possible reaction pathways for SCWG of ZL with Ru/CeO2-ZrO2 were proposed, as shown in Fig. 15. To improve energy efficiency, a novel power generation system with integrated SCWG was proposed. The coal-electricity efficiency of this novel power generation system can reach up to 60% much easier than traditional thermo power generation system [72]. When CO2 capture was considered, a total thermal efficiency of 38.3% was obtained [73]. Owing to high moisture content of lignites, combining SCWG with power generation may be an alternative way for efficient utilization of lignites, certainly including ZL.

This work was subsidized by the National Key Research and Development Program of China (Grant 2018YFB0604600), the Natural Science Foundation of China (Grants 21776001, 21476002, 21875001, and 20108002), and the Natural Science Foundation of Anhui Provincial Department of Education (Grants KJ2018A0058 and KJ2018A0064). Authors are also appreciative for the financial support from the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource. References [1] S. Dai, Analysis on the use and processing direction of Zhaotong lignite, Chin. Coal 32 (2006) 47–57. [2] A. Wang, Y. Qin, F. Lan, Geochemical characteristics and microbial populations of Neogene brown coal from Zhaotong Basin, China, Environ. Earth Sci. 68 (2012) 1539–1544. [3] Z.K. Li, X.Y. Wei, H.L. Yan, Z.M. Zong, Insight into the structural features of Zhaotong lignite using multiple techniques, Fuel 153 (2015) 176–182. [4] Z.K. Li, X.Y. Wei, H.L. Yan, Z.M. 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7. Conclusions Main results on drying and depolymerization technologies are summarized in Table 5. In this review, advantages and disadvantages of different drying methods including thermal upgrading, HTD and MTE or VMTE for upgrading ZL are presented. The main hydrophilic oxygencontaining functional groups, i.e., hydroxyl and carboxyl, can be effectively removed from ZL by HTD at low temperatures and the time used for dewatering ZL to desired moisture content by MTE or VMTE is very short. However, the urgent problem for HTD and MTE or VMTE is the treatment of the resulting waste water. Large amounts of oxygencontaining compounds can be obtained from thermal dissolution (especially ethanolysis) of ZL. The optimal temperature for producing tar from ZL pyrolysis is 600 °C, while higher temperatures (700–800 °C) is required for providing valuable gases with calorific value of ca. 17 MJ/Nm3. The reaction order of n = 2 is suitable for describing ZL pyrolysis at 300–600 °C. SCWG of ZL with KOH or Ru/CeO2-ZrO2 could offer high yields of H2 and CH4 under mild conditions. The related mechanisms for SCWG of ZL were preliminarily discussed. Large-scale drying and depolymerization of ZL to valuable gases, value-added chemicals, and/or fuels needs further investigation, including a technoeconomic analysis of the aforementioned processes, treatment of waste water, design of dryers or reactors, integration of SCWG with thermo power generation, preparation of highly active catalysts for efficient depolymerization of ZL, and development of technologies for effective separation of the resulting liquid products. Nomenclature CGE DBE ESP FTICRMS GC/MS GE HTD MTE SCWG VMTE ZL

carbon gasification efficiency double bond equivalent ethanol-soluble portion from ZL Fourier transform ion cyclotron resonance mass spectrometer gas chromatograph/mass spectrometer gasification efficiency hydrothermal dewatering mechanical thermal expression supercritical water gasification vibration mechanical thermal expression Zhaotong lignite 97

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