Functional Mechanism of Inorganic Sodium on the Structure and

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Functional Mechanism of Inorganic Sodium on the Structure and Reactivity of Zhundong Chars during Pyrolysis Hao Tang, Jun Xu, Zejun Dai, Liangping Zhang, Yi Sun, Wei Liu, Mohamed Elsayed Mostafa, Sheng Su, Song Hu, Yi Wang, Kai Xu, Anchao Zhang, and Jun Xiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02253 • Publication Date (Web): 04 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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TITLE PAGE

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a. Title of the paper

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Functional Mechanism of Inorganic Sodium on the Structure and Reactivity of Zhundong Chars

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during Pyrolysis

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b. Authors and affiliations

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Hao Tanga, Jun Xua, Zejun Daia, Liangping Zhanga, Yi Suna, Wei Liua, Mohamed Elsayed Mostafaa, Sheng Sua*, Song Hua, Yi Wanga, Kai Xua, Anchao Zhangb, Jun Xianga*

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a

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

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University of Science and Technology, Wuhan, 430074, China

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b

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China

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c. Corresponding author

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State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

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School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 454000,

University of Science and Technology, Wuhan, 430074, China

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Phone: (+86) 27-87542417-8313; Fax: (+86) 27-87545526

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E-mail:[email protected]

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[email protected]

(Jun Xiang) (Sheng Su)

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ACS Paragon Plus Environment

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ABSTRACT

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The reactivity of Zhundong chars is remarkably affected by the high sodium

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content of their parent Zhundong coal. The functional mechanism of inorganic sodium

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(NaCl) on the structure and reactivity of Zhundong chars was investigated in this

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study. Chars were prepared in a single-bed reactor under different pyrolysis

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temperatures and durations. Preliminary experimental results showed that inorganic

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sodium has a dual effect on the char yield of pyrolyzed Zhundong coal: inorganic

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sodium can decrease char yield at high pyrolysis temperatures of ≥ 600 ℃ but

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increase char yield at the low pyrolysis temperature of 400 ℃. Nitrogen adsorption

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technique and scanning electron microscopy were utilized to identify the effects of

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inorganic sodium on the physical structure of Zhundong chars. The results showed

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that inorganic sodium affects pore structure detrimentally, inhibits the growth of char

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vesicles, and enables the formation of smooth char surfaces. Fourier transform

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infrared spectroscopy and Raman spectroscopy were used to identify the effect of

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inorganic sodium on the chemical structure of Zhundong chars and to investigate the

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homogenous and heterogeneous NaCl–char interactions that occur during pyrolysis.

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The results showed that homogeneous and heterogeneous NaCl–char interactions both

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can affected the char’s chemical structure. Homogeneous NaCl(s)–char interactions

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accelerate the decomposition of O-containing functional groups and the formation of

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new Na-containing carboxylic groups. Heterogeneous NaCl(g)–char interactions

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accelerate the decomposition of functional groups and increase the ratio of small

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aromatic ring systems to large aromatic ring systems in the char. Thermogravimetric


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analysis revealed that inorganic sodium has a catalytic effect on the combustion

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reactivity of Zhundong chars. Finally, the catalytic mechanism of inorganic sodium on

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the reactivity of Zhundong chars was proposed.

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Key words: Zhundong coal; inorganic sodium; char structure; char reactivity;

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catalytic mechanism

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1. Introduction

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The utilization of coal has received considerable attention from Chinese

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researchers given that coal remains the main energy source in China. Zhundong coal

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has theoretical and practical significance for China’s energy consumption: the

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estimated coal reserves (164 Gt) of Zhundong Coalfield in Xinjiang Province, China,

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can meet the national total coal requirement for a long period of time1. Moreover,

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Zhundong coal is more environmentally friendly than other types of coal because of

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its extremely low ash, sulfur, and nitrogen contents. However, the Zhundong Coalfield

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is an oceanic coalfield; thus, Zhundong coal has high sodium content that is

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dominated by water-soluble inorganic sodium2, which can cause serious economic

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and safety problems, such as fouling, slagging, and bed agglomeration3, 4. In addition,

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inorganic sodium can accelerate the formation of fine particles5-8. Therefore, the

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effects of inorganic sodium on the utilization of Zhundong coal should be explored.

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Pyrolysis is the primary reaction in coal thermochemical conversion. The

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structure and reactivity of pyrolyzed char determine the subsequent gasification and

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combustion directly; thus, investigating the functional mechanism of inorganic

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sodium on the char structure and reactivity is critical. The functional mechanisms of

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sodium salts on char structure have been extensively investigated over the past few


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decades. Xu et al.9 investigated the effect of various inorganic sodium species on char

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structure. They found that inorganic sodium can inhibit the smoothing of char surfaces

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and the graphitization of chars. Sheng et al.10 investigated char structure through

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Raman spectroscopy and found that the presence of inorganic sodium in chars

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marginally affects the evolution of the average char microstructure. Guo et al.11

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utilized Fourier transform infrared spectroscopy (FTIR) to investigate the effects of

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NaOH and Na2CO3 on the functional groups of char during alkali lignin pyrolysis and

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gasification. They reported that the reactivity of char is determined by its structure;

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thus, structure and reactivity are correlated10, 12-14. Li et al.15 found that the reactivity

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of char decreases with increasing reaction temperature because the crystal structure of

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the inorganic component undergoes transformation. Quyn et al.16 found that inhibiting

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the combination of Cl and Na favors the reactivity of chars. Unfortunately, despite

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previous efforts, some uncertainties still exist. For example, the effects of Na species

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on the physical structure of pyrolyzed chars have received minimal attention. The

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comprehensive transformation and catalytic mechanism of inorganic sodium in

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Zhundong coal during thermal conversion (e.g., pyrolysis, combustion, and

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gasification) remains unclear.

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Moreover, Li et al.17-19 studied the effect of volatile–char interactions on sodium

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volatilization and char structure during pyrolysis. They found that these interactions

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can drastically enhance the volatilization of AAEM species and affect the char

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structure. On the other hand, most part of inorganic sodium were released into gas

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phase directly during pyrolysis20-22, which implied the existence of interactions


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between inorganic sodium and pyrolyzed chars. However, few studies have mentioned

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the effect of gaseous inorganic sodium on the structure and reactivity of chars.

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In addition, different sample preparation methods have been utilized in studies

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on the catalytic effect of alkali metals. The non-standardized preparation methods of

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different studies can affect sample character to some extent. Many studies have

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investigated the effects of the catalysis of alkali metals through leaching methods,

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which involve the direct leaching of a portion of the alkali metals23, 24. However,

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leaching methods can also influence the other inherent mineral content, such as

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calcium- and potassium-containing salts, of the coals. Thus, it is hard to distinguish

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the effect between alkali metals and other mineral content. In other studies, samples

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were prepared by loading catalysts on acid-washed (H-form) samples that do not

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contain inherent minerals9, 15, 25. Moreover, acid-washed and raw coals have similar

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chemical structures26,

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catalysts-loading method is suitable to investigate the effect of catalysts of alkali

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metals.

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. Therefore, the combination of acid-washed and

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The purpose of this study is to clarify the functional mechanism of inorganic

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sodium on the structure and reactivity of chars. To achieve this purpose, the effects of

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inorganic sodium on the structure and reactivity of pyrolyzed Zhundong chars were

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investigated. In this work, coal samples were prepared through acid washing and

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NaCl impregnation methods. Chars were prepared in a single-bed reactor under

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different pyrolysis temperatures or durations. The influence of inorganic sodium salts

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was illustrated by comparing chars produced from NaCl-loaded coal with those


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produced from acid-washed coal. The effects of inorganic sodium on the physical

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structure of Zhundong chars were measured through N2 adsorption technique and

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Scanning Electron Microscopy (SEM). A pyrolysis experiment on homogenous and

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heterogeneous NaCl–char interactions was performed in a single/double-bed reactor

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to show the effects of homogenous and heterogeneous inorganic sodium on the

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chemical structure of Zhundong char. The combustion reactivity of Zhundong chars

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was determined through thermogravimetric analysis to reveal the effect of inorganic

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sodium on the combustion reactivity of char. Finally, the detailed catalytic mechanism

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of inorganic sodium on the reactivity of Zhundong char was proposed. This work

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aimed to obtain a better understanding of the effects of coal sodium content on coal

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pyrolysis to improve the utilization of high-sodium coal.

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2. Experimental procedure

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2.1. Sample preparation

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Three kinds of samples were prepared. Zhundong coal was treated as RAW coal.

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Acid-washed coal (AW coal) and NaCl-containing coal (NaCl-loaded coal) were then

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obtained in sequence from the RAW coal. The RAW coal was ground in a mill and

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sieved to a particle diameter of 74–105 µm, then dried for 48 h at 30 ℃. AW coal was

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prepared by immersing RAW coal in hydrochloric acid (1 mol/L) with magnetic

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stirring for 24 h. After filtering and repeated washes with deionized water, the sample

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was dried for 48 h at 30 ℃. NaCl-loaded coal was prepared by mixing a known

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amount of NaCl with AW coal–water slurry. The mixed coal–water slurry was placed

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in rotary evaporators and dried under vacuum at room temperature. The NaCl loading

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rate was expressed as the weight percentages of added NaCl in NaCl-loaded coal,


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which was 5% (air-dry basis). All the prepared samples were placed in a drying bottle

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before being further analyzed. The proximate and ultimate analysis of the RAW and

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AW coals are shown in Table 1.

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For ash composition analysis, Zhundong coal was heated from room temperature

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to 500 ℃ at a slow heating rate in a muffle furnace, and then kept at this temperature

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for 1 h. The low-temperature ash samples were analyzed through X-ray fluorescence

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(EAGLE III- EDAX Inc.). The main inorganic mineral content of Zhundong coal is

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shown in Table 2. It shows that Zhundong coal ash has high calcium and sodium

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content. Given the relatively low potassium content of Zhundong coal, this paper will

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not discuss the effects of potassium.

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2.2 Sodium salt and anion content of Zhundong coal

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The modes of the occurrence of sodium in Zhundong coal were identified. The

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widely used method of sequential chemical extraction was employed2, 28 . Briefly,

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RAW coal was sequentially extracted with deionized water, ammonium acetate (1

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mol/L), and hydrochloric acid (1 mol/L) with magnetic stirring for 24 h (40 ℃) and a

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solid-to-liquid ratio of 1 g solid sample to 50 mL solution. Solid insoluble sodium was

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obtained through microwave digestion after hydrochloric acid extraction. Finally, the

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sodium contents of abstraction solutions and digestion liquid were analyzed with an

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ICP-MS (PerkinElmer ELAN DRC-e). In this work, the concentrations of the main

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anions (Cl−, NO3−, SO42−) in the deionized water extract were analyzed through ion

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chromatography (881 Compact IC pro) with Na2CO3/NaHCO3 as the buffer solution.

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2.3 Char preparation


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A schematic of the experimental setup is shown in Figure 1. Chars were prepared

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in a single-bed reactor at different pyrolysis temperature and durations, as shown in

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Figure 1(a). For each experiment, approximately 1 ± 0.01g coal sample was thinly

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spread on a quartz boat. The reactor was heated from room temperature to a preset

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temperature (400, 600, 800, and 1000 ℃). Then, the quartz reactor was sparged with

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N2 at a flow rate of 500 mL/min to blowout existing oxygen. Then, the quartz boat

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was quickly placed in the center of the quartz reactor using a horizontal rod and kept

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in the reactor usually for 1 h. After the sample was held for the preset time, it was

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removed, and cooled under N2 atmosphere. Chars prepared from RAW, AW, and

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NaCl-loaded coal were denoted as RAW, AW, and NaCl-loaded char, respectively.

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Each pyrolysis experiment was repeated at least three times to ensure accuracy.

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NaCl-loaded chars from a single-bed reactor (Figure 1(a)) were defined as the

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product of homogeneous NaCl(s)–char interactions. This study also investigated the

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effects of heterogeneous NaCl(g)–char interactions on char structure. The special

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double-bed reactor is shown in Figure 1(b). The samples were heated in a vertical

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furnace as described in our previous Refs.29, 30. First, the AW coal sample (1 ± 0.01g)

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and a known amount of NaCl (0.05 g or 0.10 g) were placed in the sublayer and upper

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layer of a double-bed reactor, respectively. Then, the remaining O2 was replaced with

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N2 (1 L/min), and the reactor was quickly placed into the constant temperature zone

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after the temperature was maintained at 1000 ℃. After heating the sample for 20 min,

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the sample was quickly removed from the furnace and cooled down under N2

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atmosphere. All the chars were collected and stored in a drying box.


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2.4 Sample characterization

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The Brunauer–Emmett–Teller (BET) surface areas of the samples were

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characterized by N2 adsorption technique using Micrometritics ASAP 2020. The BET

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surface area of samples was calculated using the multilayer adsorption model

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developed by Brunauer, Emmett, and Teller. The surface morphology of the samples

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was quantified by field emission scanning electron microscopy (FEI-Quanta 650).

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Information on the functional groups of samples was recorded on a VERTEX 70

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FTIR spectrometer. Samples were mixed with KBr at the same mass ratio of 1:100

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(coal sample to KBr). The mixture was then ground to powder with particle sizes of

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less than 2 µm. Each time, 60 mg of mixture was pressed to a pellet under 30,000

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N/cm. The measuring range for FTIR spectroscopy was 4000–400 cm−1, and its

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resolution and scan number were 4 cm−1 and 32, respectively.

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The Raman spectra of the Samples were obtained with a micro-Raman

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spectrometer (Jobin Yvon Lab RAMHR800) equipped with a Nd:YAG laser (λ0 = 532

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nm). A highly sensitive Peltier-cooled CCD detector was used to collect the Raman

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signals. The peak-fit processing was performed for first-order Raman spectrum (800–

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1800 cm−1) in accordance with peak-fit methods described in Refs.29-32. The band

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assignments are summarized in Table S1. In this study, Gr (1540 cm−1), Vl (1465

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cm−1), and Vr (1380 cm−1) collectively represent the typical structures in amorphous

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carbon, particularly small aromatic ring systems (three to five fused benzene rings).

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The D band is mainly attributed to large aromatic ring systems ( ≥ 6 fused benzene

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rings) in the Samples. Hence, the ratio between the Raman band areas of the (Gr + Vl


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+ Vr) bands and the D band (I(Gr+Vl+Vr)/ID) can reflect the ratio of small aromatic ring

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systems to large aromatic ring systems in the Samples.

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The combustion reactivity of samples was studied through TGA (PerkinElmer

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STA8000). The gas flow was 100 mL/min (air) for each experiment. Approximately

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6.0 mg of sample was placed in a TG balance, then heated from room temperature to

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105 ℃ and held for 20 min to remove moisture. Finally, the sample was heated from

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105 ℃ to 1000 ℃ at a heating rate of 10 ℃/min and maintained for 20 min to

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completely burn out. In this study, char reactivity was characterized using the widely

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adopted curve method for the determination of ignition and burnout temperature33.

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3. Results and discussion

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3.1 Sodium salts in RAW coal

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The modes of occurrence of sodium are shown in Figure 2. Water-soluble

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sodium predominates Zhundong coal and contributes 71.1% of its total sodium

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content. The fraction of organic NH4AC-soluble sodium accounts for only 16.6% of

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the total sodium content of Zhundong coal. The fraction of HCl-soluble sodium and

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insoluble silicate/aluminosilicate sodium is low and is only 12.3%. As seen in Figure

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2, Zhundong coal has high Cl− and SO42− contents (500 and 2000 µg/g) but low NO3−

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content (65 µg/g), implying that NaCl and Na2SO4 are the main components of the

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inorganic sodium in Zhundong coal. In this work, NaCl is regarded as the main

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inorganic sodium component given its higher catalytic effect than Na2SO49.

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3.2 Effect of sample preparation methods on coal structure

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The parameters of pore structure were measured and are presented in Table 3.

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The results show that RAW coal has low BET surface area (0.86 m2/g) and cumulative


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pore volume (0.005cm3/g), implying that Zhundong coal is not microporous or that

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the pores within these chars are extremely small and occluded. Moreover, coals

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produced through acid washing have pores with high BET surface area (SBET) and

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cumulative pore volume (Vc) because this method enabled the leaching of minerals

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from the sample. As shown in Table 3, the NaCl impregnation method decreases the

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SBET of coals as a likely result of pore blocking by NaCl. Therefore, acid washing can

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effectively remove minerals and increase the SBET of coal. By contrast, the NaCl

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impregnation method blocks the pores of the coal.

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Figure S1 shows the FTIR spectra of RAW, AW and NaCl-loaded coals. The

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shape of the infrared spectrum of AW coal is similar to that of RAW coal at the band

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of 400 to 4000 cm-1, which implies that acid washing has limited effect on chemical

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structure of coal. However, the order of the intensity bands near 1403 cm−1 of the

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three samples is RAW coal>AW coal=NaCl-loaded coal, and the bands near 1403

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cm−1 are attributed to the symmetric stretching vibration of two 1.5-grade C–Os27, 34

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related to the amount of carboxylate groups. This result implies that the AW coal loses

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ion clusters upon acid treatment because of conversion of carboxylate groups to

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carboxylic acid groups, proving that acid washing can substitute hydrogen ions for

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alkaline anion exchange membranes (AAEMS) cations and produces more carboxyl

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groups. From Figure S1, it can be seen that the shape of the infrared spectrum of

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NaCl-loaded coal is similar with that of AW coal. This result implies that the NaCl

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impregnation method is a physical loading process that does not affect the functional

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groups of the coal.


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3.3 Effect of inorganic sodium on char yield

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Char yield as a function of pyrolysis temperature is shown in Figure 3. Char

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yield is calculated on a dry ash-free basis to exclude the effect of variation in ash

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content and to directly investigate the pyrolysis of pure organic matter in the three

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samples.

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As shown in Figure 3, the char yield of AW coal is lower than that of RAW coal,

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which implies that acid washing can promote the devolatilization during Zhundong

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coal pyrolysis. It is mainly due to the fact that CM–AAEMS (CM represents the coal

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matrix) are broken and hydrogen ions are substituted for AAEMS cations to form

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CM–H during acid washing coal, and the newly formed CM–H bonds are not as

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stable as CM–AAEMS bonds at high temperatures and are easily broken35, thus

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releasing high amounts of tar or gas. Moreover, NaCl-loaded coal has higher char

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yield than AW coal when both were pyrolyzed at 400–600 ℃. This finding likely

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resulted from the following: 1) the blockage of AW pores by NaCl decreased the

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surface area of the AW coal or 2) the volatilization of NaCl is endothermic, thus

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inhibiting the devolatilization of pyrolyzed Zhundong coal. However, high amounts of

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activated sodium will form in the pyrolytic coal/char particles with increasing

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pyrolysis temperature16, thus increasing the likelihood of subsequent thermal cracking

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reactions. This finding provides a plausible explanation for the lower char yield of

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NaCl-loaded coal than that of AW coal under high pyrolysis temperatures ( ≥ 600 ℃).

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Therefore, inorganic sodium has a dual effect on the char yield of pyrolyzed

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Zhundong coal: inorganic sodium can increase the char yield of Zhundong coal under

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low pyrolysis temperatures but decrease char yield under high pyrolysis temperatures


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( ≥ 600 ℃).

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3.4 Effect of inorganic sodium on the physical structure of Zhundong chars

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The BET surface areas of the various chars from the pyrolysis of Zhundong

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coal are presented in Figure 4. Zhundong char has a lower surface area than other

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chars29 with a maximum value of 7 m2/g. This result likely occurred because

275

Zhundong chars are not microporous or have extremely small and dead-ended pores.

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The BET surface area of RAW char increases with increased pyrolysis temperature.

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This result implies that the amount of bubbles and pores in the char will increase with

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the release of volatiles, thus increasing surface area. However, when the temperature

279

exceeds 600 ℃, superficial matter in the RAW char can form a plastic material and

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form many closed pores due to secondary melting36. Therefore, the BET surface area

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of the RAW char decreases under pyrolysis temperatures of 600 °C–1000 ℃.

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As shown in Figure 4, the BET surface area of NaCl-loaded char pyrolyzed under

283

600 °C–800 ℃ has a sharply decreasing trend, implying that inorganic sodium inhibits

284

the formation of new pores or that inorganic sodium blocks pores. However, the BET

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surface area of the NaCl-loaded char increases with temperature when pyrolysis

286

temperature exceeds 800 ℃ compared with that of AW char, which implies the

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formation of pore structures with the release of inorganic sodium at high temperatures.

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Therefore, the decrease of BET surface area of NaCl-loaded chars most probably is

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attribute to the blockage of inorganic sodium under 600 ℃–800 ℃. With the increase

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of pyrolysis temperature ( ≥ 800 ℃), the blocked pores in the char are opened because

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of the volatilization of inorganic sodium, and the effect of inorganic sodium on pore


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structure becomes small.

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Figure 5 shows the effects of inorganic sodium on the surface morphology of

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chars. AW coal has a compact microscopic structure (Figure 5(a)). Some NaCl

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particles are present on the surface of the NaCl-loaded coal (Figure 5(b)). Figure 5(c)

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shows that AW char (600 ℃) have softened, melted, and formed vesicles. However,

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the NaCl-loaded char has a smooth structure (Figure 5(d)) and fewer vesicles than AW

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coal. This result implies that the reaction of NaCl with the coal/char matrix inhibits

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melting and vesicle formation. As pyrolysis temperature increased, the majority of the

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vesicles of AW chars and NaCl-loaded chars disappeared because the char underwent

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secondary melting. Compared with AW char, some pores of NaCl-loaded char are

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blocked when both were pyrolyzed at 1000 ℃ (Figure 5(e–f)). This result should be

303

attributed to the transfer of part of inorganic sodium to silicates or aluminosilicates,

304

which can block char pores under high pyrolysis temperatures. However, compared

305

with the formation of silicates or aluminosilicates, more inorganic sodium will

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released directly into gas phase37, so the formation of pore structures is more

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significant than the blockage of pores.

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3.5. Effect of inorganic sodium on the chemical structure of Zhundong chars

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3.5.1 Analysis of homogeneous NaCl(s)–char interactions

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The FTIR spectra (900–1800 cm−1) of chars at different pyrolysis temperatures is

311

baseline corrected and shown in Figure 6. The peak between 950 and 1350 cm−1

312

reflects the information of O-containing functional groups in the char, such as all C–O

313

single bond stretching vibrations and O–H in-plane deformation vibrations, and that at

314

1650–1800 cm−1 indicated C=O bonds29, 34. The IR intensity of the RAW char is


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315

greater than that of AW char at the band of 950-1350 cm−1, indicating that acid

316

washing can accelerate the decomposition of O-containing functional groups. This

317

result is mainly due to the fact that the substitution of hydrogen ions for AAEMS

318

cations and form CM–H bonds during acid washing, and CM–H bonds are easily

319

broken; thus, the O-containing functional groups of AW coal easily decompose. As

320

shown in Figure 6(b) and Figure 6(c), The IR intensity of the AW char is greater than

321

that of NaCl-loaded char at the band of 950–1350 cm−1. This result implies that

322

inorganic sodium can accelerate the decomposition of the O-containing functional

323

groups of Zhundong coal during pyrolysis and provides a plausible explanation for the

324

catalytic effects of inorganic sodium on coal pyrolysis. The bands near 1403 cm−1 are

325

caused by the symmetric stretching vibration of two 1.5-grade C–Os in the

326

carboxylate groups24. The higher intensity of the band at 1403 cm−1 of NaCl-loaded

327

chars than that of AW chars indicates that inorganic sodium increases the ionic

328

clusters of chars because of the formation of carboxylate groups34. This finding

329

occurred because inorganic sodium transforms into Na-containing carboxylic groups

330

during pyrolysis2, 22. Figure 6(b) and Figure 6(c) show that the band at 1650–1800

331

cm−1 in the spectra of AW chars are more intense than those in the spectra of

332

NaCl-loaded chars. This result implies that inorganic sodium can accelerate the

333

decomposition of C=O bonds. The chars exhibited a high degree of graphitization

334

under high temperature; thus, the difference of IR intensity (950-1800cm-1) decreased

335

between the AW and NaCl-loaded coal chars when the pyrolysis temperature

336

exceeded 800 ℃.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

337

The effects of inorganic sodium on Zhundong chars depend on the duration and

338

temperature of pyrolysis. Figure 7 shows the FT-IR spectra of AW char and

339

NaCl-loaded chars that have been pyrolyzed for different durations (5, 10, and 20 min)

340

at 1000 ℃. When the duration of pyrolysis exceeds 20 min, the difference between the

341

AW and NaCl-loaded chars decreased given the increased degree of graphitization. As

342

seen in Figure 7, the IR intensity of the AW char is greater than that of NaCl-loaded

343

char at the band of 950–1350 cm−1 when being prepared for less than 20 min,

344

indicating that inorganic sodium facilitates the decomposition of O-functional groups.

345

Similar to the results presented in the previous section, inorganic sodium also affects

346

C=O (1600–1800 cm−1). The band at 1403 cm−1 in the IR spectrum of the

347

NaCl-loaded char becomes more intense with prolonged pyrolysis duration, implying

348

the formation of carboxylate groups.

349

3.5.2 Analysis of heterogeneous NaCl(g)–char interactions

350

The effect of gaseous NaCl(g) on the chemical structure of the chars was

351

analyzed through Raman spectroscopy. Figure S2 shows an example of curve-fitting.

352

The example shows that the Raman spectrum is well fitted with this method, with all

353

other spectra showing a similar satisfactory degree of fitting. The total band area as a

354

function of sodium content is shown in Figure 8. The total Raman area represents the

355

light absorptivity and Raman scatter of the chars. O-containing functional groups can

356

increase Raman intensity given the resonance effect between oxygen and aromatic

357

ring systems

358

content, implying that inorganic sodium can accelerate the decomposition of the

359

O-containing functional groups of char. This result is consistent with the conclusion

30, 32

. The total Raman band area decreases with increasing NaCl(g)


Page 16 of 40

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360

on homogeneous NaCl(s)–char interactions. As shown in Figure 8, NaCl(g)–char

361

interactions increase the I(Gr+Vl+Vr)/ID ratio implying that gaseous inorganic sodium can

362

increase the ratio of small aromatic ring systems to large aromatic ring systems to

363

some extent. The results suggest that inorganic sodium may inhibit the polymerization

364

and condensation reaction of chars to form large aromatic ring systems; thus, the ratio

365

of small aromatic ring systems to large aromatic ring systems increases.

366

3.6 Effect of inorganic sodium on the combustion reactivity of Zhundong chars

367

Figure 9 shows the igniton/burnout temperature of pyrolyzed chars (TG/DTG

368

shown in Figure S3). It shows that NaCl-loaded char has a lower ignition temperature

369

than AW char. This result implies that inorganic sodium has a catalytic effect on the

370

combustion reactivity of char. RAW char has the lowest ignition temperature among

371

all of the char samples given its high inherent mineral matter content, particularly

372

organic sodium, which has a better catalytic effect than inorganic sodium. However,

373

with increasing pyrolysis temperature, the difference in ignition temperature among

374

the chars decreases because of the increased degree of graphitization and the release

375

of sodium at high temperature. The effect of inorganic sodium on burnout temperature

376

(Figure 9(b)) is similar to that on ignition temperature. The burnout temperature of

377

NaCl-loaded char is lower than that of RAW char when both were pyrolyzed at

378

temperatures that exceed 600 ℃. This result indicates that inorganic sodium has better

379

catalytic effect on pyrolyzed chars at high temperature.

380

Char structure can also determine char reactivity. Specifically, a high I(Gr+Vl+Vr)/ID

381

ratio and BET surface area of char implies the high char reactivity30, 34. According to

382

the previous analysis of char structure, inorganic sodium increase the I(Gr+Vl+Vr)/ID


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

383

ratio, which implies inorganic sodium is in favor of char reactivity. On the other hand,

384

inorganic sodium reduce the BET surface area of char, which is detrimental to char

385

reactivity. From the result of char reactivity by TGA, it can be seen that inorganic

386

sodium improve the reactivity significantly. Therefore, The effect of chemical

387

structure is more significant that physical structure on char reactivity.

388

3.7 Catalytic mechanism of inorganic sodium

389

The catalytic mechanism of inorganic sodium during pyrolysis is illustrated in

390

Figure 10. The migration of inorganic sodium during pyrolysis is complex. Briefly,

391

part of the inorganic sodium will be directly released into the gas phase, and another

392

part of inorganic sodium will react with the coal/char matrix and transfer into

393

insoluble sodium and organic sodium2, 20.

394

The transformation behavior of inorganic sodium is the key to the catalytic

395

mechanism of inorganic sodium. The fact that homogeneous inorganic sodium has

396

catalytic effect on pyrolyzed chars implies that inorganic sodium can react with the

397

carboxylic acid groups within the char to form organic sodium as reaction R120, 35.

398

NaCl(s) + RCOOH → RCOONa + HCl ↑

(R1)

399

Moreover, heterogeneous (gaseous) inorganic sodium also shows the similar

400

catalytic effect with homogeneous inorganic sodium, which accelerates the

401

decomposition of the O-containing functional groups of char and increase I(Gr+Vl+Vr)/ID

402

ratio. It reflects that NaCl(g)–char interaction can affect the char structure directly.

403

Meanwhile, inorganic sodium can also be captured by chars through both physical

404

adsorption and chemical fixation39, 40, which increase the formation of organic sodium.

405

Therefore, heterogeneous (gaseous) inorganic sodium can also react with the


Page 19 of 40

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Energy & Fuels

406

carboxylic acid groups to form organic sodium as reaction R2. Two possible pathways

407

of R1 and R2 represent homogeneous NaCl(s)–char and heterogeneous NaCl(g)–char

408

interactions.

409

NaCl(g) + RCOOH → RCOONa + HCl ↑

(R2)

410

RCOO–Na (R represents radicals from benzene and naphthalene) will then

411

undergo a series of reaction during pyrolysis. During the primary pyrolysis stage, the

412

newly formed RCOO–Na continues to decompose. This reaction is accompanied by

413

the release of CO2 as shown in reaction R320, 35.

414

RCOO − Na → (R − Na) + CO 2 ↑

(R3)

415

The newly formed R–Na is attached to the coal/char matrix, which is unstable

416

enough at high temperatures and thus may be broken again to generate free radical

417

sites with the release of gas20, 35, 41.

418

(R - Na) → ( - R) + Na(g) ↑

(R4)

419

( − R) → ( − R' ) + gas ↑

(R5)

420

Some sodium atoms may be released into the gas phase, which is the main

421

precursor of aerosol7. Stable R–Na bonds may form again through recombination as

422

in reaction R635.

423

- R' + Na → (R' − Na)

(R6)

424

The reaction pathway (R3–R6) reflects the repeated bond formation and bond

425

breakage of R–Na. These processes increase the density of free radical sites in the

426

pyrolyzed Zhundong char38. This finding provides a reasonable explanation for the

427

transformation of inorganic sodium into catalysts (R–Na) for the subsequent char


Energy & Fuels

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428

Page 20 of 40

pyrolysis, thus accelerating the decomposition of O-containing functional groups.

429

The formation of R–Na and the repeated formation and breakage of bonds in the

430

cracking of tar precursors also significantly decreased the number of large aromatic

431

ring systems in the pyrolysis of Zhundong coal. Polymerization and condensation of

432

the char can also be inhibited because the presence of inorganic sodium increases the

433

probability of subsequent thermal cracking reactions at high temperatures. This

434

provides a plausible explanation for how inorganic sodium can increase the ratio of

435

small aromatic ring systems to large aromatic ring systems.

436

According to the reaction schemes outlined above, high numbers of RCOO–Na

437

will form with increasing inorganic sodium content. However, RCOO–Na is also

438

transferred to Na-containing free radicals (–COONa) because of the breakage of C–C

439

bonds with increasing temperature, thus leading to the formation of silicates or

440

aluminosilicates with ash as shown in reactions R7 and R817, 42.

441

RCOO − Na → R + −COONa

442

( −COONa) + Al 2 O 3 + SiO 2 ... → sodium silicates/ aluminosol icates

(R7) (R8)

443

Therefore, a portion of inorganic sodium favors the formation of silicates or

444

aluminosilicates, which is detrimental to the pore structure. On the other hand, the

445

formation of R–Na will increase the density of free radical sites in the Zhundong char,

446

thus affecting the decomposition of functional groups during primary pyrolysis and

447

increasing the ratio of small aromatic ring systems to large aromatic ring systems in

448

secondary pyrolysis. Therefore, inorganic sodium improves the char reactivity mainly

449

because of the chemical structure, which has a more significant effect than physical


Page 21 of 40

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Energy & Fuels

450

structure on char reactivity.

451

4. Conclusions

452

This study investigated the functional mechanism of inorganic sodium on the

453

structure and reactivity of Zhundong chars. The effects of inorganic sodium on the

454

char yield, physical structure, chemical structure and combustion reactivity of

455

Zhundong chars during pyrolysis were investigated. Finally, the catalytic mechanism

456

was proposed. The specific conclusions are as follows:

457

(1) Inorganic sodium has a dual effect on the char yield of pyrolyzed Zhundong

458

coal. Inorganic sodium can increase the char yield of Zhundong coal under

459

low pyrolysis temperatures but decrease char yield under high pyrolysis

460

temperatures ( ≥ 600 ℃).

461

(2) The BET analysis of AW and NaCl-loaded chars shows that inorganic sodium

462

is detrimental to the physical structure of Zhundong char and reduces the BET

463

surface area through pore blockage. The SEM analysis of AW and

464

NaCl-loaded chars shows that inorganic sodium can inhibit the growth of

465

vesicles and promote the formation of smooth surfaces because of interactions

466

between inorganic sodium and the coal/char substrate.

467

(3) Homogeneous NaCl(s)–char interactions show that inorganic sodium can

468

accelerate the decomposition of O-containing functional groups during

469

pyrolysis and inorganic sodium favors the formation of carboxylate groups

470

(RCOO–Na). Heterogeneous NaCl(g)–char interactions show that gaseous

471

inorganic sodium can also accelerate the decomposition of O-containing

472

functional groups and increase the ratio of small aromatic ring systems to

473

large aromatic ring systems in the char.

474

(4) The detailed catalytic mechanism of inorganic sodium is proposed. The

475

formation of silicates or aluminosilicate is detrimental to the pore structure

476

and reactivity of the char. However, the formation of activated sodium (R–Na)

477

and repeated bond formation and breakage provide additional free radical

478

sites in the pyrolyzed char, thus affecting the decomposition of functional

479

groups and secondary pyrolysis. Inorganic sodium improves the char

480

reactivity, which is mainly determined by chemical structure.

481 482

5. Acknowledgements The authors greatly appreciate the financial support for this research from


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

483

National Key R&D program of China (No.2017YFB0601802), the National Science

484

Foundation of China (NO.51576086, 51576081), the Science and Technology Project

485

of China Huadian Corporation (2017). We also acknowledge the extended help from

486

the Analytical and Testing Center of Huazhong University of Science and Technology.

487


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Energy & Fuels

488

Table list

489

Table 1 Proximate and ultimate analysis of coal samples.

490

Table 2 Composition of Zhundong coal ash prepared at 500 ℃.

491

Table 3 Pore structure parameters of the samples.

492 493


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

494

Table 1 Coal samples RAW coal AW coal

495

Page 24 of 40

a.

ultimate analysis (wt.%, daf)

proximate analysis (wt.%, ad) a

N

C

H

S

O

0.46 0.32

61.45 61.78

4.29 4.15

0.41 0.25

33.39 33.50

M

V

A

FC

14.34 5.5

24.91 30.85

6.85 4.51

53.91 59.14

By difference

496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524


Page 25 of 40

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Energy & Fuels

525

Table 2 Samples (wt. %)

CaO

MgO

Na2O

K2O

Fe2O3

Al2O3

SiO2

TiO2

SO3

RAW coal

20.29

5.36

5.91

0.48

10.6

12.13

28.83

0.66

13.93

526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

567

Page 26 of 40

Table 3 Samples

SBET(m2/g)

Vc(cm3/g)

RAW coal AW coal NaCl-loaded coal

0.86 7.34 3.45

0.005 0.016 0.010

568 569 570 571 572 573 574


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Energy & Fuels

575

Figure captions:

576

Figure 1. Schematic of the pyrolysis installation: (a) single-bed reactor and (b) double-bed reactor.

577

Figure 2. Modes of the occurrence of sodium and main anions in Zhundong coal

578

Figure 3. Char yield as a function of pyrolysis temperature

579

Figure 4. Effect of pyrolysis temperature on the BET surface area of char

580

Figure 5. SEM micrographs of Zhundong coal and chars obtained at different pyrolysis

581

temperatures: (a) AW coal, (b) NaCl-loaded coal, (c) AW char pyrolyzed at 600 ℃, (d)

582

NaCl-loaded char pyrolyzed at 600 ℃, (e) AW char pyrolyzed at 1000 ℃, and (f) NaCl-loaded char

583

pyrolyzed at 1000 ℃

584

Figure 6. FTIR spectra of chars pyrolyzed at different temperatures: (a) RAW char, (b) AW char,

585

and (c) NaCl-loaded char

586

Figure 7. FTIR spectra of chars pyrolyzed at 1000 ℃ for (a) 5 min, (b) 10 min, and (c) 20 min

587

Figure 8. Raman band area ratio I(Gr+Vl+Vr)/ID and total band area as a function of NaCl(g) content

588

Figure 9. Reactivity index of the chars prepared at different pyrolysis temperature: (a) ignition

589

temperature and (b) burnout temperature

590

Figure 10. Illustration of the catalytic mechanism of inorganic sodium during coal pyrolysis

591 592


Energy & Fuels

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593

Figure 1

594

595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613


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614

Figure 2

615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

635 636

Figure 3

637 638


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Energy & Fuels

639

Figure 4

640 641 642 643 644 645 646 647 648 649 650 651 652 653 654


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

655 656

Figure 5

657

658

659 660


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Energy & Fuels

661

Figure 6

662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

681

Figure 7

682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710


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Energy & Fuels

711

Figure 8

712 713 714 715 716 717 718 719 720 721 722


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

723

Figure 9

724

725 726 727


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Energy & Fuels

728 729

Figure 10

730 731 732 733 734 735 736 737 738 739


Energy & Fuels

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References

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Functional Mechanism of Inorganic Sodium on the Structure and

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