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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*,† †

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo 454000, China S Supporting Information *

ABSTRACT: The reactivity of Zhundong chars is remarkably affected by the high sodium content of their parent Zhundong coal. The functional mechanism of inorganic sodium (NaCl) on the structure and reactivity of Zhundong chars was investigated in this study. Chars were prepared in a single-bed reactor under different pyrolysis temperatures and durations. Preliminary experimental results showed that inorganic sodium has a dual effect on the char yield of pyrolyzed Zhundong coal: inorganic sodium can decrease char yield at high pyrolysis temperatures of ≥600 °C but increase char yield at the low pyrolysis temperature of 400 °C. Nitrogen adsorption technique and scanning electron microscopy were utilized to identify the effects of inorganic sodium on the physical structure of Zhundong chars. The results showed that inorganic sodium affects pore structure detrimentally, inhibits the growth of char vesicles, and enables the formation of smooth char surfaces. Fourier transform infrared spectroscopy and Raman spectroscopy were used to identify the effect of inorganic sodium on the chemical structure of Zhundong chars and to investigate the homogeneous and heterogeneous NaCl−char interactions that occur during pyrolysis. The results showed that homogeneous and heterogeneous NaCl−char interactions both can affect the char’s chemical structure. Homogeneous NaCl(s)−char interactions accelerate the decomposition of O-containing functional groups and the formation of new Na-containing carboxylic groups. Heterogeneous NaCl(g)−char interactions accelerate the decomposition of functional groups and increase the ratio of small aromatic ring systems to large aromatic ring systems in the char. Thermogravimetric analysis revealed that inorganic sodium has a catalytic effect on the combustion reactivity of Zhundong chars. Finally, the catalytic mechanism of inorganic sodium on the reactivity of Zhundong chars was proposed.

1. INTRODUCTION The utilization of coal has received considerable attention from Chinese researchers given that coal remains the main energy source in China. Zhundong coal has theoretical and practical significance for China’s energy consumption: the estimated coal reserves (164 Gt) of Zhundong Coalfield in Xinjiang Province, China, can meet the national total coal requirement for a long period of time.1 Moreover, Zhundong coal is more environmentally friendly than other types of coal because of its extremely low ash, sulfur, and nitrogen contents. However, the Zhundong Coalfield is an oceanic coalfield; thus, Zhundong coal has high sodium content that is dominated by watersoluble inorganic sodium,2 which can cause serious economic and safety problems, such as fouling, slagging, and bed agglomeration.3,4 In addition, inorganic sodium can accelerate the formation of fine particles.5−8 Therefore, the effects of inorganic sodium on the utilization of Zhundong coal should be explored. Pyrolysis is the primary reaction in coal thermochemical conversion. The structure and reactivity of pyrolyzed char determine the subsequent gasification and combustion directly; thus, investigating the functional mechanism of inorganic sodium on the char structure and reactivity is critical. The functional mechanisms of sodium salts on char structure have been extensively investigated over the past few decades. Xu et al.9 investigated the effect of various inorganic sodium species © XXXX American Chemical Society

on char structure. They found that inorganic sodium can inhibit the smoothing of char surfaces and the graphitization of chars. Sheng et al.10 investigated char structure through Raman spectroscopy and found that the presence of inorganic sodium in chars marginally affects the evolution of the average char microstructure. Guo et al.11 utilized Fourier transform infrared spectroscopy (FTIR) to investigate the effects of NaOH and Na2CO3 on the functional groups of char during alkali lignin pyrolysis and gasification. They reported that the reactivity of char is determined by its structure; thus, structure and reactivity are correlated.10,12−14 Li et al.15 found that the reactivity of char decreases with increasing reaction temperature because the crystal structure of the inorganic component undergoes transformation. Quyn et al.16 found that inhibiting the combination of Cl and Na favors the reactivity of chars. Unfortunately, despite previous efforts, some uncertainties still exist. For example, the effects of Na species on the physical structure of pyrolyzed chars have received minimal attention. The comprehensive transformation and catalytic mechanism of inorganic sodium in Zhundong coal during thermal conversion (e.g., pyrolysis, combustion, and gasification) remains unclear. Received: August 1, 2017 Revised: August 31, 2017 Published: September 4, 2017 A

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Proximate and Ultimate Analysis of Coal Samples ultimate analysis (wt %, daf)

a

proximate analysis (wt %, ad)

coal samples

N

C

H

S

Oa

M

V

A

FC

RAW coal AW coal

0.46 0.32

61.45 61.78

4.29 4.15

0.41 0.25

33.39 33.50

14.34 5.5

24.91 30.85

6.85 4.51

53.91 59.14

By difference.

Table 2. Composition of Zhundong Coal Ash Prepared at 500 °C 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

Moreover, Li et al.17−19 studied the effect of volatile−char interactions on sodium volatilization and char structure during pyrolysis. They found that these interactions can drastically enhance the volatilization of AAEM species and affect the char structure. On the other hand, most of the inorganic sodium was released into the gas phase directly during pyrolysis,20−22 which implied the existence of interactions between inorganic sodium and pyrolyzed chars. However, few studies have mentioned the effect of gaseous inorganic sodium on the structure and reactivity of chars. In addition, different sample preparation methods have been utilized in studies on the catalytic effect of alkali metals. The nonstandardized preparation methods of different studies can affect sample character to some extent. Many studies have investigated the effects of the catalysis of alkali metals through leaching methods, which involve the direct leaching of a portion of the alkali metals.23,24 However, leaching methods can also influence the other inherent mineral content, such as calciumand potassium-containing salts, of the coals. Thus, it is hard to distinguish the effect between alkali metals and other mineral content. In other studies, samples were prepared by loading catalysts on acid-washed (H-form) samples that do not contain inherent minerals.9,15,25 Moreover, acid-washed and raw coals have similar chemical structures.26,27 Therefore, the combination of acid-washed and catalyst-loading methods is suitable to investigate the effect of catalysts of alkali metals. The purpose of this study is to clarify the functional mechanism of inorganic sodium on the structure and reactivity of chars. To achieve this purpose, the effects of inorganic sodium on the structure and reactivity of pyrolyzed Zhundong chars were investigated. In this work, coal samples were prepared through acid washing and NaCl impregnation methods. Chars were prepared in a single-bed reactor under different pyrolysis temperatures or durations. The influence of inorganic sodium salts was illustrated by comparing chars produced from NaCl-loaded coal with those produced from acid-washed coal. The effects of inorganic sodium on the physical structure of Zhundong chars were measured through N2 adsorption technique and scanning electron microscopy (SEM). A pyrolysis experiment on homogeneous and heterogeneous NaCl−char interactions was performed in a single/double-bed reactor to show the effects of homogeneous and heterogeneous inorganic sodium on the chemical structure of Zhundong char. The combustion reactivity of Zhundong chars was determined through thermogravimetric analysis to reveal the effect of inorganic sodium on the combustion reactivity of char. Finally, the detailed catalytic mechanism of inorganic sodium on the reactivity of Zhundong char was proposed. This work aimed to obtain a better understanding of

the effects of coal sodium content on coal pyrolysis to improve the utilization of high-sodium coal.

2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation. Three kinds of samples were prepared. Zhundong coal was treated as RAW coal. Acid-washed coal (AW coal) and NaCl-containing coal (NaCl-loaded coal) were then obtained in sequence from the RAW coal. The RAW coal was ground in a mill and sieved to a particle diameter of 74−105 μm, then dried for 48 h at 30 °C. AW coal was prepared by immersing RAW coal in hydrochloric acid (1 mol/L) with magnetic stirring for 24 h. After filtering and repeated washes with deionized water, the sample was dried for 48 h at 30 °C. NaCl-loaded coal was prepared by mixing a known amount of NaCl with AW coal−water slurry. The mixed coal−water slurry was placed in rotary evaporators and dried under vacuum at room temperature. The NaCl loading rate was expressed as the weight percentages of added NaCl in NaCl-loaded coal, which was 5% (airdry basis). All the prepared samples were placed in a drying bottle before being further analyzed. The proximate and ultimate analysis of the RAW and AW coals are shown in Table 1. For ash composition analysis, Zhundong coal was heated from room temperature to 500 °C at a slow heating rate in a muffle furnace and then kept at this temperature for 1 h. The low-temperature ash samples were analyzed through X-ray fluorescence (EAGLE III- EDAX Inc.). The main inorganic mineral content of Zhundong coal is shown in Table 2. It shows that Zhundong coal ash has high calcium and sodium content. Given the relatively low potassium content of Zhundong coal, this paper will not discuss the effects of potassium. 2.2. Sodium Salt and Anion Content of Zhundong Coal. The modes of the occurrence of sodium in Zhundong coal were identified. The widely used method of sequential chemical extraction was employed.2,28 Briefly, RAW coal was sequentially extracted with deionized water, ammonium acetate (1 mol/L), and hydrochloric acid (1 mol/L) with magnetic stirring for 24 h (40 °C) and a solid-to-liquid ratio of 1 g of solid sample to 50 mL of solution. Solid insoluble sodium was obtained through microwave digestion after hydrochloric acid extraction. Finally, the sodium contents of abstraction solutions and digestion liquid were analyzed with an ICP-MS (PerkinElmer ELAN DRC-e). In this work, the concentrations of the main anions (Cl−, NO3−, SO42−) in the deionized water extract were analyzed through ion chromatography (881 Compact IC pro) with Na2CO3/ NaHCO3 as the buffer solution. 2.3. Char Preparation. A schematic of the experimental setup is shown in Figure 1. Chars were prepared in a single-bed reactor at different pyrolysis temperature and durations, as shown in Figure 1(a). For each experiment, approximately 1 ± 0.01 g of coal sample was thinly spread on a quartz boat. The reactor was heated from room temperature to a preset temperature (400, 600, 800, and 1000 °C). Then, the quartz reactor was sparged with N2 at a flow rate of 500 mL/min to blowout existing oxygen. Then, the quartz boat was quickly placed in the center of the quartz reactor using a horizontal rod and kept in the reactor usually for 1 h. After the sample was held for the preset time, it was removed and cooled under N2 atmosphere. Chars prepared from RAW, AW, and NaCl-loaded coal were denoted as B

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

The combustion reactivity of samples was studied through TGA (PerkinElmer STA8000). The gas flow was 100 mL/min (air) for each experiment. Approximately 6.0 mg of sample was placed in a TG balance, then heated from room temperature to 105 °C, and held for 20 min to remove moisture. Finally, the sample was heated from 105 to 1000 °C at a heating rate of 10 °C/min and maintained for 20 min to completely burn out. In this study, char reactivity was characterized using the widely adopted curve method for the determination of ignition and burnout temperature.33

3. RESULTS AND DISCUSSION 3.1. Sodium Salts in RAW Coal. The modes of occurrence of sodium are shown in Figure 2. Water-soluble sodium

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

RAW, AW, and NaCl-loaded char, respectively. Each pyrolysis experiment was repeated at least three times to ensure accuracy. NaCl-loaded chars from a single-bed reactor (Figure 1(a)) were defined as the product of homogeneous NaCl(s)−char interactions. This study also investigated the effects of heterogeneous NaCl(g)− char interactions on char structure. The special double-bed reactor is shown in Figure 1(b). The samples were heated in a vertical furnace as described in our previous refs 29 and 30. First, the AW coal sample (1 ± 0.01 g) and a known amount of NaCl (0.05 or 0.10 g) were placed in the sublayer and upper layer of a double-bed reactor, respectively. Then, the remaining O2 was replaced with N2 (1 L/min), and the reactor was quickly placed into the constant temperature zone after the temperature was maintained at 1000 °C. After heating the sample for 20 min, the sample was quickly removed from the furnace and cooled under N2 atmosphere. All the chars were collected and stored in a drying box. 2.4. Sample Characterization. The Brunauer−Emmett−Teller (BET) surface areas of the samples were characterized by N2 adsorption technique using Micrometritics ASAP 2020. The BET surface area of samples was calculated using the multilayer adsorption model developed by Brunauer, Emmett, and Teller. The surface morphology of the samples was quantified by field emission scanning electron microscopy (FEI-Quanta 650). Information on the functional groups of samples was recorded on a VERTEX 70 FTIR spectrometer. Samples were mixed with KBr at the same mass ratio of 1:100 (coal sample to KBr). The mixture was then ground to powder with particle sizes of less than 2 μm. Each time, 60 mg of mixture was pressed to a pellet under 30 000 N/cm. The measuring range for FTIR spectroscopy was 4000−400 cm−1, and its resolution and scan number were 4 cm−1 and 32, respectively. The Raman spectra of the samples were obtained with a microRaman spectrometer (Jobin Yvon Lab RAMHR800) equipped with a Nd:YAG laser (λ0 = 532 nm). A highly sensitive Peltier-cooled CCD detector was used to collect the Raman signals. The peak-fit processing was performed for the first-order Raman spectrum (800−1800 cm−1) in accordance with peak-fit methods described in refs 29−32. The band assignments are summarized in Table S1. In this study, Gr (1540 cm−1), Vl (1465 cm−1), and Vr (1380 cm−1) collectively represent the typical structures in amorphous carbon, particularly small aromatic ring systems (three to five fused benzene rings). The D band is mainly attributed to large aromatic ring systems (≥6 fused benzene rings) in the samples. Hence, the ratio between the Raman band areas of the (Gr + Vl + Vr) bands and the D band (I(Gr+Vl+Vr)/ID) can reflect the ratio of small aromatic ring systems to large aromatic ring systems in the samples.

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

predominates Zhundong coal and contributes 71.1% of its total sodium content. The fraction of organic NH4AC-soluble sodium accounts for only 16.6% of the total sodium content of Zhundong coal. The fraction of HCl-soluble sodium and insoluble silicate/aluminosilicate sodium is low and is only 12.3%. As seen in Figure 2, Zhundong coal has high Cl− and SO42− contents (500 and 2000 μg/g) but low NO3− content (65 μg/g), implying that NaCl and Na2SO4 are the main components of the inorganic sodium in Zhundong coal. In this work, NaCl is regarded as the main inorganic sodium component given its higher catalytic effect than Na2SO49. 3.2. Effect of Sample Preparation Methods on Coal Structure. The parameters of pore structure of coal samples were measured and are presented in Table 3. The results show that RAW coal has low BET surface area (0.86 m2/g) and cumulative pore volume (0.005 cm3/g), implying that Zhundong coal is not microporous or that the pores within these chars are extremely small and occluded. Moreover, coals produced through acid washing have pores with high BET surface area (SBET) and cumulative pore volume (Vc) because Table 3. Pore Structure Parameters of the Samples

C

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 DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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pyrolysis temperature,16 thus increasing the likelihood of subsequent thermal cracking reactions. This finding provides a plausible explanation for the lower char yield of NaCl-loaded coal than that of AW coal under high pyrolysis temperatures (≥600 °C). Therefore, inorganic sodium has a dual effect on the char yield of pyrolyzed Zhundong coal: inorganic sodium can increase the char yield of Zhundong coal under low pyrolysis temperatures but decrease char yield under high pyrolysis temperatures (≥600 °C). 3.4. Effect of Inorganic Sodium on the Physical Structure of Zhundong Chars. The BET surface areas of the various chars from the pyrolysis of Zhundong coal are presented in Figure 4. Zhundong char has a lower surface area

this method enabled the leaching of minerals from the sample. As shown in Table 3, the NaCl impregnation method decreases the SBET of coals as a likely result of pore blocking by NaCl. Therefore, acid washing can effectively remove minerals and increase the SBET of coal. By contrast, the NaCl impregnation method blocks the pores of the coal. Figure S1 shows the FTIR spectra of RAW, AW, and NaClloaded coals. The shape of the infrared spectrum of AW coal is similar to that of RAW coal at the band of 400−4000 cm−1, which implies that acid washing has limited effect on chemical structure of coal. However, the order of the intensity bands near 1403 cm−1 of the three samples is RAW coal > AW coal = NaCl-loaded coal, and the bands near 1403 cm−1 are attributed to the symmetric stretching vibration of two 1.5-grade C− Os27,34 related to the amount of carboxylate groups. This result implies that the AW coal loses ion clusters upon acid treatment because of conversion of carboxylate groups to carboxylic acid groups, proving that acid washing can substitute hydrogen ions for alkaline anion exchange membrane (AAEMS) cations and produces more carboxyl groups. From Figure S1, it can be seen that the shape of the infrared spectrum of NaCl-loaded coal is similar to that of AW coal. This result implies that the NaCl impregnation method is a physical loading process that does not affect the functional groups of the coal. 3.3. Effect of Inorganic Sodium on Char Yield. Char yield as a function of pyrolysis temperature is shown in Figure 3. Char yield is calculated on a dry ash-free basis to exclude the

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

than other chars29 with a maximum value of 7 m2/g. This result likely occurred because Zhundong chars are not microporous or have extremely small and dead-ended pores. The BET surface area of RAW char increases with increased pyrolysis temperature. This result implies that the amount of bubbles and pores in the char will increase with the release of volatiles, thus increasing surface area. However, when the temperature exceeds 600 °C, superficial matter in the RAW char can form a plastic material and form many closed pores due to secondary melting.36 Therefore, the BET surface area of the RAW char decreases under pyrolysis temperatures of 600 °C−1000 °C. As shown in Figure 4, the BET surface area of NaCl-loaded char pyrolyzed under 600−800 °C has a sharply decreasing trend, implying that inorganic sodium inhibits the formation of new pores or that inorganic sodium blocks pores. However, the BET surface area of the NaCl-loaded char increases with temperature when pyrolysis temperature exceeds 800 °C compared with that of AW char, which implies the formation of pore structures with the release of inorganic sodium at high temperatures. Therefore, the decrease of BET surface area of NaCl-loaded chars most probably is attributed to the blockage of inorganic sodium under 600−800 °C. With the increase of pyrolysis temperature (≥800 °C), the blocked pores in the char are opened because of the volatilization of inorganic sodium, and the effect of inorganic sodium on pore structure becomes small. Figure 5 shows the effects of inorganic sodium on the surface morphology of chars. AW coal has a compact microscopic structure (Figure 5(a)). Some NaCl particles are present on the surface of the NaCl-loaded coal (Figure 5(b)). Figure 5(c) shows that AW char (600 °C) has softened, melted, and formed vesicles. However, the NaCl-loaded char has a smoother structure (Figure 5(d)) and fewer vesicles than AW char. This

Figure 3. Char yield as a function of pyrolysis temperature.

effect of variation in ash content and to directly investigate the pyrolysis of pure organic matter in the three samples. As shown in Figure 3, the char yield of AW coal is lower than that of RAW coal, which implies that acid washing can promote the devolatilization during Zhundong coal pyrolysis. It is mainly due to the fact that CM−AAEMS (CM represents the coal matrix) are broken, and hydrogen ions are substituted for AAEMS cations to form CM−H during acid washing coal. The newly formed CM−H bonds are not as stable as CM−AAEMS bonds at high temperatures and are easily broken,35 thus releasing high amounts of tar or gas. Moreover, NaCl-loaded coal has higher char yield than AW coal when both were pyrolyzed at 400−600 °C. This finding likely resulted from the following: (1) the blockage of AW pores by NaCl decreased the surface area of the AW coal or (2) the volatilization of NaCl is endothermic, thus inhibiting the devolatilization of pyrolyzed Zhundong coal. However, high amounts of activated sodium will form in the pyrolytic coal/char particles with increasing D

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. SEM micrographs of Zhundong coal and chars obtained at different pyrolysis temperatures: (a) AW coal, (b) NaCl-loaded coal, (c) AW char pyrolyzed at 600 °C, (d) NaCl-loaded char pyrolyzed at 600 °C, (e) AW char pyrolyzed at 1000 °C, and (f) NaCl-loaded char pyrolyzed at 1000 °C.

result implies that the reaction of NaCl with the coal/char matrix inhibits melting and vesicle formation. As pyrolysis temperature increased, the majority of the vesicles of AW chars and NaCl-loaded chars disappeared because the char underwent secondary melting. Compared with AW char, some pores of NaCl-loaded char are blocked when both were pyrolyzed at 1000 °C (Figure 5(e,f)). This result should be attributed to the transfer of part of inorganic sodium to silicates or aluminosilicates, which can block char pores under high pyrolysis temperatures. However, compared with the formation of silicates or aluminosilicates, more inorganic sodium will be released directly into the gas phase,37 so the formation of pore structures is more significant than the blockage of pores. 3.5. Effect of Inorganic Sodium on the Chemical Structure of Zhundong Chars. 3.5.1. Analysis of Homogeneous NaCl(s)−Char Interactions. The FTIR spectra (900− 1800 cm−1) of chars at different pyrolysis temperatures is baseline corrected and shown in Figure 6. The peak between 950 and 1350 cm−1 reflects the information on O-containing functional groups in the char, such as all C−O single bond stretching vibrations and O−H in-plane deformation vibrations, and that at 1650−1800 cm−1 indicated CO bonds.29,34 The IR intensity of the RAW char is greater than that of AW char at the band of 950−1350 cm−1, indicating that acid washing can accelerate the decomposition of O-containing functional groups. This result is mainly due to the fact that the substitution of hydrogen ions for AAEMS cations forms CM−H bonds during acid washing, and CM−H bonds are

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

easily broken; thus, the O-containing functional groups of AW coal easily decompose. As shown in Figure 6(b) and Figure 6(c), the IR intensity of the AW char is greater than that of NaCl-loaded char at the band of 950−1350 cm−1. This result implies that inorganic sodium can accelerate the decomposition E

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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

of the O-containing functional groups of Zhundong coal during pyrolysis and provides a plausible explanation for the catalytic effects of inorganic sodium on coal pyrolysis. The bands near 1403 cm−1 are caused by the symmetric stretching vibration of two 1.5-grade C−Os in the carboxylate groups.24 The higher intensity of the band at 1403 cm−1 of NaCl-loaded chars than that of AW chars indicates that inorganic sodium increases the ionic clusters of chars because of the formation of carboxylate groups.34 This finding occurred because inorganic sodium transforms into Na-containing carboxylic groups during pyrolysis.2,22 Figure 6(b) and Figure 6(c) show that the bands at 1650−1800 cm−1 in the spectra of AW chars are more intense than those in the spectra of NaCl-loaded chars. This result implies that inorganic sodium can accelerate the decomposition of CO bonds. The chars exhibited a high degree of graphitization under high temperature; thus, the difference of IR intensity (950−1800 cm−1) decreased between the AW and NaCl-loaded coal chars when the pyrolysis temperature exceeded 800 °C. The effects of inorganic sodium on Zhundong chars depend on the duration and temperature of pyrolysis. Figure 7 shows the FTIR spectra of AW and NaCl-loaded chars that have been pyrolyzed for different durations (5, 10, and 20 min) at 1000 °C. When the duration of pyrolysis exceeds 20 min, the difference between the AW and NaCl-loaded chars decreased given the increased degree of graphitization. As seen in Figure 7, the IR intensity of the AW char is greater than that of NaClloaded char at the band of 950−1350 cm−1 when being prepared for less than 20 min, indicating that inorganic sodium facilitates the decomposition of O-functional groups. Similar to the results presented in the previous section, inorganic sodium also affects CO (1600−1800 cm−1). The band at 1403 cm−1 in the IR spectrum of the NaCl-loaded char becomes more intense with prolonged pyrolysis duration, implying the formation of carboxylate groups. 3.5.2. Analysis of Heterogeneous NaCl(g)−Char Interactions. The effect of gaseous NaCl(g) on the chemical structure of the chars was analyzed through Raman spectroscopy. Figure S2 shows an example of curve fitting. The example shows that the Raman spectrum is well fitted with this method, with all other spectra showing a similar satisfactory degree of fitting. The total band area as a function of sodium content is shown in Figure 8. The total Raman area represents the light

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

absorptivity and Raman scatter of the chars. O-containing functional groups can increase Raman intensity given the resonance effect between oxygen and aromatic ring systems.30,32 The total Raman band area decreases with increasing NaCl(g) content, implying that inorganic sodium can accelerate the decomposition of the O-containing functional groups of char. This result is consistent with the conclusion on homogeneous NaCl(s)−char interactions. As shown in Figure 8, NaCl(g)−char interactions increase the I(Gr+Vl+Vr)/ID ratio, implying that gaseous inorganic sodium can increase the ratio of small aromatic ring systems to large aromatic ring systems to some extent. The results suggest that inorganic sodium may inhibit the polymerization and condensation reaction of chars to form large aromatic ring systems; thus, the ratio of small aromatic ring systems to large aromatic ring systems increases. 3.6. Effect of Inorganic Sodium on the Combustion Reactivity of Zhundong Chars. Figure 9 shows the ignition/ burnout temperature of pyrolyzed chars (TG/DTG shown in Figure S3). It shows that NaCl-loaded char has a lower ignition temperature than AW char. This result implies that inorganic sodium has a catalytic effect on the combustion reactivity of char. RAW char has the lowest ignition temperature among all of the char samples given its high inherent mineral matter content, particularly organic sodium, which has a better catalytic effect than inorganic sodium. However, with increasing pyrolysis temperature, the difference in ignition temperature among the chars decreases because of the increased degree of F

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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Moreover, heterogeneous (gaseous) inorganic sodium also shows a similar catalytic effect with homogeneous inorganic sodium, which accelerates the decomposition of the Ocontaining functional groups of char and increases I(Gr+Vl+Vr)/ ID ratio. It reflects that the NaCl(g)−char interaction can affect the char structure directly. Meanwhile, inorganic sodium can also be captured by chars through both physical adsorption and chemical fixation,39,40 which increase the formation of organic sodium. Therefore, heterogeneous (gaseous) inorganic sodium can also react with the carboxylic acid groups to form organic sodium as reaction R2. Two possible pathways of R1 and R2 represent homogeneous NaCl(s)−char and heterogeneous NaCl(g)−char interactions. NaCl(g) + RCOOH → RCOONa + HCl↑

(R2)

RCOO−Na (R represents radicals from benzene and naphthalene) will then undergo a series of reactions during pyrolysis. During the primary pyrolysis stage, the newly formed RCOO−Na continues to decompose. This reaction is accompanied by the release of CO2 as shown in reaction R3.20,35 RCOO−Na → (R−Na) + CO2↑

The newly formed R−Na is attached to the coal/char matrix, which is unstable enough at high temperatures and thus may be broken again to generate free radical sites with the release of gas.20,35,41

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

graphitization and the release of sodium at high temperature. The effect of inorganic sodium on burnout temperature (Figure 9(b)) is similar to that on ignition temperature. The burnout temperature of NaCl-loaded char is lower than that of RAW char when both were pyrolyzed at temperatures that exceed 600 °C. This result indicates that inorganic sodium has better catalytic effect on pyrolyzed chars at high temperature. Char structure can also determine char reactivity. Specifically, a high I(Gr+Vl+Vr)/ID ratio and BET surface area of char imply high char reactivity.30,34 According to the previous analysis of char structure, inorganic sodium increases the I(Gr+Vl+Vr)/ID ratio, which implies inorganic sodium is in favor of char reactivity. On the other hand, inorganic sodium reduces the BET surface area of char, which is detrimental to char reactivity. From the result of char reactivity by TGA, it can be seen that inorganic sodium improves the reactivity significantly. Therefore, the effect of chemical structure is more significant than physical structure on char reactivity. 3.7. Catalytic Mechanism of Inorganic Sodium. The catalytic mechanism of inorganic sodium during pyrolysis is illustrated in Figure 10. The migration of inorganic sodium during pyrolysis is complex. Briefly, part of the inorganic sodium will be directly released into the gas phase, and another part of inorganic sodium will react with the coal/char matrix and transfer into insoluble sodium and organic sodium.2,20 The transformation behavior of inorganic sodium is the key to the catalytic mechanism of inorganic sodium. The fact that homogeneous inorganic sodium has a catalytic effect on pyrolyzed chars implies that inorganic sodium can react with the carboxylic acid groups within the char to form organic sodium as reaction R1.20,35 NaCl(s) + RCOOH → RCOONa + HCl↑

(R3)

(R−Na) → (−R) + Na(g)↑

(R4)

( −R) → (− R′) + gas↑

(R5)

Some sodium atoms may be released into the gas phase, which is the main precursor of aerosol.7 Stable R−Na bonds may form again through recombination as in reaction R6.35 −R′ + Na → (R′−Na)

(R6)

The reaction pathway (R3−R6) reflects the repeated bond formation and bond breakage of R−Na. These processes increase the density of free radical sites in the pyrolyzed Zhundong char. 38 This finding provides a reasonable explanation for the transformation of inorganic sodium into catalysts (R−Na) for the subsequent char pyrolysis, thus accelerating the decomposition of O-containing functional groups. The formation of R−Na and the repeated formation and breakage of bonds in the cracking of tar precursors also significantly decreased the number of large aromatic ring systems in the pyrolysis of Zhundong coal. Polymerization and condensation of the char can also be inhibited because the presence of inorganic sodium increases the probability of subsequent thermal cracking reactions at high temperatures. This provides a plausible explanation for how inorganic sodium can increase the ratio of small aromatic ring systems to large aromatic ring systems. According to the reaction schemes outlined above, high numbers of RCOO−Na will form with increasing inorganic sodium content. However, RCOO−Na is also transferred to Na-containing free radicals (−COONa) because of the breakage of C−C bonds with increasing temperature, thus leading to the formation of silicates or aluminosilicates with ash as shown in reactions R7 and R8.17,42

(R1)

RCOO−Na → R + − COONa G

(R7) DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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

(2) The BET analysis of AW and NaCl-loaded chars shows that inorganic sodium is detrimental to the physical structure of Zhundong char and reduces the BET surface area through pore blockage. The SEM analysis of AW and NaCl-loaded chars shows that inorganic sodium can inhibit the growth of vesicles and promote the formation of smooth surfaces because of interactions between inorganic sodium and the coal/char substrate. (3) Homogeneous NaCl(s)−char interactions show that inorganic sodium can accelerate the decomposition of O-containing functional groups during pyrolysis, and inorganic sodium favors the formation of carboxylate groups (RCOO−Na). Heterogeneous NaCl(g)−char interactions show that gaseous inorganic sodium can also accelerate the decomposition of O-containing functional groups and increase the ratio of small aromatic ring systems to large aromatic ring systems in the char. (4) The detailed catalytic mechanism of inorganic sodium is proposed. The formation of silicates or aluminosilicate is detrimental to the pore structure and reactivity of the char. However, the formation of activated sodium (R− Na) and repeated bond formation and breakage provide additional free radical sites in the pyrolyzed char, thus affecting the decomposition of functional groups and secondary pyrolysis. Inorganic sodium improves the char reactivity, which is mainly determined by chemical structure.

( −COONa) + Al 2O3 + SiO2 ... → sodium silicates/alumino silicates

(R8)

Therefore, a portion of inorganic sodium favors the formation of silicates or aluminosilicates, which is detrimental to the pore structure. On the other hand, the formation of R− Na will increase the density of free radical sites in the Zhundong char, thus affecting the decomposition of functional groups during primary pyrolysis and increasing the ratio of small aromatic ring systems to large aromatic ring systems in secondary pyrolysis. Therefore, inorganic sodium improves the char reactivity mainly because of the chemical structure, which has a more significant effect than physical structure on char reactivity.

4. CONCLUSIONS This study investigated the functional mechanism of inorganic sodium on the structure and reactivity of Zhundong chars. The effects of inorganic sodium on the char yield, physical structure, chemical structure, and combustion reactivity of Zhundong chars during pyrolysis were investigated. Finally, the catalytic mechanism was proposed. The specific conclusions are as follows: (1) Inorganic sodium has a dual effect on the char yield of pyrolyzed Zhundong coal. Inorganic sodium can increase the char yield of Zhundong coal under low pyrolysis temperatures but decrease char yield under high pyrolysis temperatures (≥600 °C). H

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels



(10) SHENG, C. Char structure characterised by Raman spectroscopy and its correlations with combustion reactivity. Fuel 2007, 86 (15), 2316−2324. (11) Guo, D.; Wu, S.; Liu, B.; Yin, X.; Yang, Q. Catalytic effects of NaOH and Na2CO3 additives on alkali lignin pyrolysis and gasification. Appl. Energy 2012, 95, 22−30. (12) Chen, C.; Wang, J.; Liu, W.; Zhang, S.; Yin, J.; Luo, G.; Yao, H. Effect of pyrolysis conditions on the char gasification with mixtures of CO2 and H2O. Proc. Combust. Inst. 2013, 34 (2), 2453−2460. (13) Xu, J.; Tang, H.; Su, S.; Liu, J.; Han, H.; Zhang, L.; Xu, K.; Wang, Y.; Hu, S.; Zhou, Y. Micro-Raman spectroscopy study of 32 kinds of Chinese coals: second-order Raman spectrum and its correlations with coal property. Energy Fuels 2017, 31 (8), 7884−7893. (14) Li, Q.; Wang, Z.; He, Y.; Sun, Q.; Zhang, Y.; Kumar, S.; Zhang, K.; Cen, K. Pyrolysis Characteristics and Evolution of Char Structure during Pulverized Coal Pyrolysis in Drop Tube Furnace: Influence of Temperature. Energy Fuels 2017, 31 (5), 4799−4807. (15) Quyn, D. M.; Wu, H.; Li, C. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples. Fuel 2002, 81 (2), 143−149. (16) Quyn, D. M.; Wu, H. W.; Hayashi, J.; Li, C. Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity. Fuel 2003, 82 (5), 587−593. (17) Li, C. Importance of volatile−char interactions during the pyrolysis and gasification of low-rank fuels − A review. Fuel 2013, 112, 609−623. (18) Zhang, S.; Min, Z.; Tay, H.; Asadullah, M.; Li, C. Effects of volatile−char interactions on the evolution of char structure during the gasification of Victorian brown coal in steam. Fuel 2011, 90 (4), 1529−1535. (19) Wu, H. W.; Li, X. J.; Hayashi, J.; Chiba, T.; Li, C. Z. Effects of volatile-char interactions on the reactivity of chars from NaCl-loaded Loy Yang brown coal. Fuel 2005, 84 (10), 1221−1228. (20) Li, R.; Chen, Q.; Zhang, H. Detailed Investigation on Sodium (Na) Species Release and Transformation Mechanism during Pyrolysis and Char Gasification of High-Na Zhundong Coal. Energy Fuels 2017, 31 (6), 5902−5912. (21) Wang, Z.; Liu, Y.; Whiddon, R.; Wan, K.; He, Y.; Xia, J.; Cen, K. Measurement of atomic sodium release during pyrolysis and combustion of sodium-enriched Zhundong coal pellet. Combust. Flame 2017, 176, 429−438. (22) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. The release of water-bound and organic sodium from Loy Yang coal during the combustion of single particles in a flat flame. Combust. Flame 2011, 158 (6), 1181−1192. (23) Zhang, Z.; Zhu, M.; Zhang, Y.; Setyawan, H. Y.; Li, J.; Zhang, D. Ignition and combustion characteristics of single particles of Zhundong lignite: Effect of water and acid washing. Proc. Combust. Inst. 2017, 36 (2), 2139−2146. (24) Wang, C.; Liu, Y.; Jin, X.; Che, D. Effect of water washing on reactivities and NOx emission of Zhundong coals. J. Energy Inst. 2016, 89 (4), 636−647. (25) Wu, L.; Qiao, Y.; Gui, B.; Wang, C.; Xu, J.; Yao, H.; Xu, M. Effects of Chemical Forms of Alkali and Alkaline Earth Metallic Species on the Char Ignition Temperature of a Loy Yang Coal under O2 /N2 Atmosphere. Energy Fuels 2012, 26 (1), 112−117. (26) Zhao, Y.; Qiu, P.; Chen, G.; Pei, J.; Sun, S.; Liu, L.; Liu, H. Selective enrichment of chemical structure during first grinding of Zhundong coal and its effect on pyrolysis reactivity. Fuel 2017, 189, 46−56. (27) Zhao, Y.; Liu, L.; Qiu, P.; Xie, X.; Chen, X.; Lin, D.; Sun, S. Impacts of chemical fractionation on Zhundong coal’s chemical structure and pyrolysis reactivity. Fuel Process. Technol. 2017, 155, 144−152.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02253.



Table S1 and Figures S1−S3 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone: (+86) 27-87542417-8313. Fax: (+86) 27-87545526. E-mail: [email protected] (Jun Xiang). *E-mail: [email protected] (Sheng Su). ORCID

Sheng Su: 0000-0003-3523-8222 Anchao Zhang: 0000-0002-0704-6736 Jun Xiang: 0000-0002-0627-1528 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS



REFERENCES

The authors greatly appreciate the financial support for this research from National Key R&D program of China (No.2017YFB0601802), the National Science Foundation of China (NO.51576086, 51576081), and the Science and Technology Project of China Huadian Corporation (2017). We also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.

(1) Zhou, J.; Zhuang, X.; Alastuey, A.; Querol, X.; Li, J. Geochemistry and mineralogy of coal in the recently explored Zhundong large coal field in the Junggar basin, Xinjiang province, China. Int. J. Coal Geol. 2010, 82 (1−2), 51−67. (2) Wang, C. A.; Jin, X.; Wang, Y.; Yan, Y.; Cui, J.; Liu, Y.; Che, D. Release and Transformation of Sodium during Pyrolysis of Zhundong Coals. Energy Fuels 2015, 29 (1), 78−85. (3) Wei, B.; Tan, H.; Wang, Y.; Wang, X.; Yang, T.; Ruan, R. Investigation of characteristics and formation mechanisms of deposits on different positions in full-scale boiler burning high alkali coal. Appl. Therm. Eng. 2017, 119, 449−458. (4) Wang, X.; Xu, Z.; Wei, B.; Zhang, L.; Tan, H.; Yang, T.; Mikulcic, H.; Duic, N. The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: A study from ash evaporating to condensing. Appl. Therm. Eng. 2015, 80, 150− 159. (5) Gao, Q.; Li, S.; Yuan, Y.; Zhang, Y.; Yao, Q. Ultrafine particulate matter formation in the early stage of pulverized coal combustion of high-sodium lignite. Fuel 2015, 158, 224−231. (6) Gao, Q.; Li, S.; Yang, M.; Biswas, P.; Yao, Q. Measurement and numerical simulation of ultrafine particle size distribution in the early stage of high-sodium lignite combustion. Proc. Combust. Inst. 2017, 36 (2), 2083−2090. (7) Xiao, Z.; Shang, T.; Zhuo, J.; Yao, Q. Study on the mechanisms of ultrafine particle formation during high-sodium coal combustion in a flat-flame burner. Fuel 2016, 181, 1257−1264. (8) Li, G.; Li, S.; Huang, Q.; Yao, Q. Fine particulate formation and ash deposition during pulverized coal combustion of high-sodium lignite in a down-fired furnace. Fuel 2015, 143, 430−437. (9) Xu, L.; Liu, H.; Fang, H.; Gao, J.; Wu, S. Effects of various inorganic sodium salts present in Zhundong coal on the char characteristics. Fuel 2017, 203, 120−127. I

DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (28) Li, G.; Wang, C.; Yan, Y.; Jin, X.; Liu, Y.; Che, D. Release and transformation of sodium during combustion of Zhundong coals. J. Energy Inst. 2016, 89 (1), 48−56. (29) Xu, J.; Su, S.; Sun, Z.; Si, N.; Qing, M.; Liu, L.; Hu, S.; Wang, Y.; Xiang, J. Effects of H2O Gasification Reaction on the Characteristics of Chars under Oxy-Fuel Combustion Conditions with Wet Recycle. Energy Fuels 2016, 30 (11), 9071−9079. (30) Xu, J.; Su, S.; Sun, Z.; Qing, M.; Xiong, Z.; Wang, Y.; Jiang, L.; Hu, S.; Xiang, J. Effects of steam and CO2 on the characteristics of chars during devolatilization in oxy-steam combustion process. Appl. Energy 2016, 182, 20−28. (31) Su, S.; Song, Y.; Wang, Y.; Li, T.; Hu, S.; Xiang, J.; Li, C. Effects of CO2 and heating rate on the characteristics of chars prepared in CO2 and N2 atm. Fuel 2015, 142, 243−249. (32) LI, X.; LI, C. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam. Fuel 2006, 85 (10−11), 1518−1525. (33) Huang, X.; Jiang, X.; Han, X.; Wang, H. Combustion Characteristics of Fine- and Micro-pulverized Coal in the Mixture of O2/CO2. Energy Fuels 2008, 22 (6), 3756−3762. (34) Gomez-Serrano, V.; Fern Ndez-Gonz Lez, M. C.; CuerdaCorrea, E. M.; Mac As-Garc A, A.; Alexandre-Franco, M. F.; RojasCervantes, M. L. Physico-chemical properties of low-rank coals. Powder Technol. 2004, 148 (1), 38−42. (35) Li, C. Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal. Fuel 2007, 86 (12−13), 1664−1683. (36) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X. H.; Chan, W. G.; Hajaligol, M. R. Characterization of chars from pyrolysis of lignin. Fuel 2004, 83 (11−12), 1469−1482. (37) Wei, X.; Huang, J.; Liu, T.; Fang, Y.; Wang, Y. Transformation of alkali metals during pyrolysis and gasification of a lignite. Energy Fuels 2008, 22 (3), 1840−1844. (38) Kosminski, A.; Ross, D. P.; Agnew, J. B. Influence of gas environment on reactions between sodium and silicon minerals during gasification of low-rank coal. Fuel Process. Technol. 2006, 87 (11), 953− 962. (39) Kosminski, A.; Ross, D. P.; Agnew, J. B. Reactions between sodium and kaolin during gasification of a low-rank coal. Fuel Process. Technol. 2006, 87 (12), 1051−1062. (40) Erickson, T. A.; Ludlow, D. K.; Benson, S. A. Interaction of sodium, sulfur, and silica during coal combustion. Energy Fuels 1991, 5 (4), 539−547. (41) Li, C. Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal. Fuel 2000, 79 (3), 427−438. (42) Zhang, H.; Guo, X.; Zhu, Z. Effect of temperature on gasification performance and sodium transformation of Zhundong coal. Fuel 2017, 189, 301−311.

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DOI: 10.1021/acs.energyfuels.7b02253 Energy Fuels XXXX, XXX, XXX−XXX