Investigation on the Occurrences and Interactions of Corrosive

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Investigation on the Occurrences and Interactions of Corrosive Species during Pyrolysis of Zhundong Coal Using SSNMR and HTXRD Xiongchao Lin,*,† Yuanping Yang,† Xujun Chen,‡ Caihong Wang,† Jun-ichiro Hayashi,§ and Yonggang Wang† †

School of Chemical & Environmental Engineering, China University of Mining and Technology (Beijing), D11 Xueyuan Road, Haidian District, Beijing 100083, P.R. China ‡ Department of Chemical Engineering, Curtin University, Perth, Western Australia 6840, Australia § Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga Koen, Kasuga 861-8580, Japan ABSTRACT: The utilization of a typical Chinese low-rank coal (Zhundong coal) usually gives rise to severe fouling and slagging in equipment due to its excessively high content of sodium-bearing corrosive substances. This study systematically clarifies the occurrences and transformation mechanism of corrosive materials during pyrolysis of Zhongdong coal by combining solid-state nuclear magnetic resonance (SSNMR), in situ high-temperature X-ray diffraction (HT-XRD), and Factsage simulation. For the first time, the experimental evidence in this study shows that the corrosive elements demonstrated distinct forms in coal and could be significantly varied by thermal treatment. Specifically, homogeneously distributed Na ions could mutually transform between inorganic and organic-bounded form under the influence of ionic force, resulting in its elutable feature. During pyrolysis, Na was successively transformed to be inorganic form and completely volatilized above 800 °C, thus diversifying Na-related fouling propensity in various pyrolysis stages. The Cl was unlikely to entirely exist as inorganic form; nevertheless it was strongly restrained by functional groups of coal matrix. The organic Cl-containing functional groups was gradually decomposed to volatile Cl at pyrolysis temperature higher than 500 °C, whereas the inorganic Cl was more stable and possibly exposed on the surface of char particle. In situ analysis further revealed that the formation of aerosol by the diffused corrosive elements was the key step leading to the deposition. More importantly, Factsage thermodynamic calculation demonstrates that the sequential release of Cl as well as S, and their interactions with Na are the prerequisite and essential factor governing the generation of low-temperatureeutectic and subsequently initial corrosion. techniques.13−16 However, such sample pretreatments are technically complicate and would frequently change the initial occurrences of the active elements in coal. Therefore, the original properties of active elements in coal are still ambiguous and thus need to be further clarified.17,18 Fortunately, the advanced solid-state nuclear magnetic resonance (SSNMR) have been extensively used in characterizing coal samples recently,19 such as assessing the carbon and hydrogen features in coal20 and capturing high resolution of half-integer nuclei, consequently enabling the analysis of trace element in coal with amorphous nature.21 Particularly, as a nondestructive approach, 23 Na- and 35Cl-NMR analyses could integrally present the detailed information on the spatial correlations among the corrosive elements in coal. The transformation of corrosive elements is usually dependent on a specific conversion route. Because pyrolysis is the first stage of coal thermal conversion, the investigation on the diffusion of corrosive elements during coal pyrolysis is important for the control of corrosion and deposition during its industrial utilization. Recently, the staged coal utilization (i.e., combining coal pyrolysis at low temperature with

1. INTRODUCTION Zhundong coalfield in the east of Junggar Basin, Xinjiang province, China, is the largest intact coalfield in the world, with more than 390 billion tons of technically recoverable coal reserve.1 Typical coal from the Zhundong field has high contents of alkali metals,2 and therefore, its utilization would practically give rise to severe fouling,3,4 thus posing a potential threat to stable system operation and making thermal conversion unfeasible. To control the fouling problem, the pretreatment on raw coal, e.g., water elution, solvent extraction, steam treatment, etc., is eagerly expected to remove a more or less portion of the corrosive substances from coal.5−7 Nevertheless, the understanding on the occurrence and transformation characteristics of those corrosive elements is fundamentally important. The occurrences of active elements in coal and their evolutionary features have been tentatively determined by a variety of conventional techniques, such as energy-dispersive X-ray fluorescence spectrometry (EDXRF),8 ion chromatography,9,10 cold vapor molecular absorption spectrometry,11 and electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICPMS).12 Inevitably, a series of sample pretreatments, i.e., alkaline fusion, pyrohydrolysis, microwaveinduced combustion or oxygen bomb combustion, are accordingly required prior to the analyses using these © XXXX American Chemical Society

Received: February 26, 2018 Revised: April 1, 2018 Published: April 2, 2018 A

DOI: 10.1021/acs.energyfuels.8b00661 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Proximate and Ultimate Analysis of Coal and Ash Samplesa proximate analysis (wt %)

a

Mad

FCad

Vad

11.78

51.14

30.02

ultimate analysis (wt %) Aad

Cdaf.

Hdaf.

Ndaf.

Sdaf.

Odaf. (diff.)

7.06 73.44 3.66 Elemental Compositions of Ash (wt%)

1.37

0.16

21.37

SiO2

CaO

Al2O3

Cl

Fe2O3

Na2O

SO3

MgO

TiO2

SrO

K2O

Total

32.16±0.23

23.18±0.21

18.68±0.19

6.61±0.21

6.39±0.12

5.71±0.12

3.35±0.09

2.43±0.08

0.74±0.04

0.30±0.02

0.18±0.01

99.73±0.23

Abbreviations: ad., air-dry basis; daf., dry and ash free; diff., by different. Ash was prepared according to GB T 1574−2007.

Figure 1. Schematic diagram of pressurized fixed bed pyrolysis/gasification reactor.

strategic guidance on minimizing the fouling and slagging propensity during their utilization were proposed.

subsequent coal char gasification/combustion at high temperature) is under development and capable of removing most corrosive maters.22,23 During pyrolysis, a majority of corrosive elements would be evolved into the gas phase and condensed downward following the gas stream, thus preliminarily separated the corrosive substances and mitigate the fouling and slagging in the subsequent combustion/gasification stage. Previous researches mainly focused on the transformation of alkali and alkaline earth metals by examining the forms of those elements after pyrolysis of low rank coal;24−27 however, the mechanism on the interactions between the corrosive elements (such as Na, Cl, as well as S elements) under in situ conditions is still unclear due to the technical limitation. Thus, appropriate analysis technologies are necessary to investigate on original and in situ occurrences of corrosive elements. To some extent, in situ high-temperature X-ray diffraction (HT-XRD), an advanced analytical technique for characterizing inorganic matter in coal, could achieve the in situ analysis of minerals without recrystallization and decomposition of coal samples.28 Moreover, solid−liquid phase transformation could also be monitored by HT-XRD, enabling the detection on the fouling propensity induced by low-temperature eutectic.29 Overall, the present study innovatively elucidated the original occurrences, especially the in situ transformation behavior and mechanism on the interactions among the corrosive substances during pyrolysis of Zhundong coal by combining the SSNMR, in situ HT-XRD and Factsagesimulation. The sequential removal of corrosive substances in high-basic coals and the

2. EXPERIMENTAL SECTION 2.1. Coal Sample. As-mined coal from Zhundong coalfield was dried at 60 °C for 48 h, pulverized, and then sieved to 0.9−2.0 mm. Properties the coal samples are summarized in Table 1. The ultimate analysis was performed with an analyzer (Vario EL III, Elementar Analysensysteme GmbH). Every coal was ashed by gentle combustion at 500 °C, and resulting ash was analyzed by X-ray fluorescence spectrometry (XRF; The Thermo Scientific, ESCALAB 250Xi). 2.2. Coal Pyrolysis. Each coal sample was pyrolyzed in a pressurized atmosphere of N2 (purity ≥99.99%) at pressure of 1.0 MPa in a fixed-bed reactor that is schematically shown in Figure 1. More details of the reactor were reported elsewhere.2 Briefly, the fixed bed of the coal with a mass of about 70 g was heated up to 1000 °C at a heating rate of 50 °C/min, and further heated at 1000 °C for 60 min. The sampling probes (Al2O3 with purity >99.9%; 26 × 5.5 × 1 mm) were inserted into the reactor at the prescribed vertical positions for monitoring deposition behavior, and the instant temperature under different pyrolysis temperatures were measured by thermocouples. 2.3. Coal and Char Characterization. 2.3.1. Occurrence of Na and Cl. In order to classify the occurrence of Na in coal, the sample was packed into a column (φ 2.5 cm) after ground and sieved to 700 °C) after the decomposition of relevant minerals, as exhibited in Figure 8e−i. Possibly, the formation temperature of calcium aluminosilicate was significantly lowered because of the interactions among those high active alkali and halogen substances.2 3.3. Interactions among Corrosive Substances during Pyrolysis. The formation of aerosol following the release of the corrosive elements from coal matrix and minerals was the key step leading to the deposition. Because the condensation and deposition behaviors of fouling matters were governed by the coexisting of Na/K, Cl, S, as well as Ca/Mg, etc., in the aerosol, a systematic investigation into the elemental interactions during thermal conversion is supposed to further illustrate the deposition mechanism. A schematic mechanism on the release and transformation behavior of corrosive species in various occurrence modes, as well as the correlation between transformation behavior and deposition characteristic of those corrosive species are presented in Figure 9. Some typical surface images, representatives of the elemental composition of the deposition substances at specific areas (the windward side) of probe are depicted in Figure 9i−vii with the elemental distributions examined by the SEM-EDX analysis. SEM-EDX microanalyses show that after raw coal pyrolysis at 1000 °C, the deposited particles contained large amount of NaCl cubic crystal (Figure 9i), whereas only residual carbon

Figure 8. In situ HT-XRD patterns of coal ash (as-prepared at 500 °C) at (a) 500, (b) 550, (c) 600, (d) 650, (e) 700, (f) 750, (g) 800, (h) 900, and (i) 1000 °C. Peaks were Q, quartz-SiO2 (PDF#74-1811); H, halite-K0.2Na0.8Cl (PDF No. 26-0918); C, calcite-CaCO3 (PDF No. 72-1937); A, anhydrite-CaSO4 (PDF No. 74-2421); F, fluorapatite,syn-Ca5(PO4)3F (PDF No. 15-0876); Cc: calcium chloride phosphideCa2ClP (PDF No. 32-0156); M, mayenite-Ca12Al14O33 (PDF No. 481882); Cs, calcium silicate-Ca2SiO4 (PDF No. 23-1042); G, gehleniteCa2Al2SiO7 (PDF No. 73-2041).

shifted to the peak representing calcium chloride phosphide (Ca2ClP) due to the transformation of halite to calcium chloride phosphide, demonstrating the interactions between those highly active substances. Meanwhile, the phase transformation might occur along with the decomposition of minerals, and in this process, the minerals with similar crystal structure or local configuration successively experienced mutual F

DOI: 10.1021/acs.energyfuels.8b00661 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 9. SEM-EDX images of deposition morphologies of (i) raw coal; (ii) raw coal after acid-elution; (iii) acid-eluted coal loading with CH3COONa; (iv) acid-eluted coal loading with CH3COONa and FeS2; (v) acid-eluted coal loading with CH3COONa and PVC; (vi) acid-eluted coal loading with CH3COONa and NH4Cl; and (vii) acid-eluted coal loading with CH3COONa, FeS2, PVC and NH4Cl under pyrolysis temperature 1000 °C and pressure of 1.0 MPa.

Figure 10. FactSage evaluation of interactive transformation behavior of active materials as a function of temperature, (a) simulated coal loading with CH3COONa and FeS2, (b) simulated coal loading with CH3COONa and PVC, (c) simulated coal loading with CH3COONa and NH4Cl, and (d) simulated coal loading with CH3COONa, FeS2, PVC, and NH4Cl. G

DOI: 10.1021/acs.energyfuels.8b00661 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

phase transformation and the formation of low-temperature eutectic (Figure 10 d). For an instance, the phase transformation temperature of Cl- and S-bearing substances (such as FeCl2 and FeS2, etc.) decreased to ca. 300 °C; especially, the phase transformation temperature of NaCl (s) reduced to ca. 400 °C from ca. 600 °C. Such interactions significantly enhanced the diffusion of fouling elements, and consequently led to the aggregated deposition of low-temperature eutectic on the initial layer. Indeed, as shown in Figure 9vii, abundant NaCl crystal and Fe−Na−Cl complex were observed on the deposit probe. Summarily, the interactions among the released Cl, S, and Na ions are the key factors governing the generation of low-temperature eutectic and the initial corrosion during Zhundong coal pyrolysis.

was found on the probe after pyrolysis of acid-eluted raw coal (Figure 9ii) since most of the active minerals remained in coal would be released at 1000 °C. Interestingly, the coal after acidelution and CH3COONa-loading generated less Na-bearing deposits than that of raw coal, as depicted in Figure 9iii. Though the organic Na could have been decomposed at lower temperature, fouling precursor caused by the condensation of Na vapor would be formed only in the presence of other inorganic species. Minerals in coal could generate oxides in regions where appropriate elemental compositions were available, and then alkali and alkaline earth metal oxides could be preferably combined with other metals to form the low-temperature eutectics at considerably lower temperature than theoretically required.40 In addition, sulfur is recognized as one of the most corrosive elements in coal and its interaction with other elements apparently occurred during the pyrolysis of the sample loaded with both CH3COONa and FeS2. As presented in Figure 9iv, abundant granular and cubic Na−S−O bearing particles were observed on the depositing surface. Notably, different occurrences of halogen and sulfur in coal show distinct transformation behaviors. Therefore, the forms and activities of those corrosive elements should be taken into account in order to predict the transformation and equilibrium status in fouling aerosol. On the basis of the experimental results, thermodynamic calculations were conducted from 200 to 1200 °C, as shown in Figure 10. The thermodynamic model reveals that the FeS2 transforms to FeS below 500 °C under the pyrolysis condition (as seen in Figure 10a). Meanwhile, H2S generated from FeS2 tends to be combined with Na atom in the gaseous phase.41 The formation of Na−S−O substantially reduced the formation temperature of Na−Si−Al−O low-temperature eutectic, resulting in the aggravated fouling propensity. Halogens, especially Cl, are considered as the key element inducing corrosion and fouling during coal conversion; and organic Cl is more active than inorganic one under the pyrolysis condition.42 To simulate the interactions between organic Na and Cl during pyrolysis, the acid-washed coal sample was pyrolyzed after the loading of CH3COONa and PVC (representing the organic Cl). As shown in Figure 9 v, in agreement with the thermodynamic calculations (Figure 10 b), NaCl is the main deposit on the probe. However, the temperature for the transformation of NaCl (ca. 600−800 °C) was higher than that of Na2S (ca. 450− 500 °C). This implies that the condensation temperature of such aerosol formed by the fine particles and inorganic species in gaseous phase is also a critical factor affecting deposit. In addition, NH4Cl with easily evaporable Cl was added to investigate the interactions between organic Na and evaporable Cl. Surprisingly, less NaCl was observed in the deposit though Cl-bearing matter was detected (Figure 9vi), indicating that Na+ and NH4+ might compete to combine with Cl in the aerosol. Compared with NaCl, NH4Cl is much easier to decompose rather than condense at lower temperature. In other words, the formation of NH4Cl will locally prevent the generation of NaCl, thus mitigating the initial deposit of corrosive substances (Figure 10c). Because these active materials coexisted in the coal matrix, acid-washed coal samples loaded with CH3COONa, FeS2, PVC, and NH4Cl were used to elucidate the potential influence of corrosive substances on the deposition behavior. Factsage thermodynamic calculation shows that the mutual interaction of Cl- and S-bearing substances play important role in the

4. CONCLUSIONS The following conclusions can be drawn based on the experimental data: (1) Homogeneously distributed Na ions could mutually transform between inorganic and organic form under the influence of ionic force. Na in char was successively transformed at different pyrolysis temperatures, thus diversifying the Na-related fouling propensity in various pyrolysis stages; (2) The content of organic Cl gradually decreased after pyrolysis up to 500 °C due to the decomposition of Clcontaining functional groups and the generation of inorganic Cl, which was more stable and gradually exposed on the surface of coal matrix; (3) The formation of aerosol was the key step leading to the deposition, following the release of the corrosive elements. More importantly, the interactions among the released Cl, S, and Na are the prerequisite and essential factor governing the generation of low-temperature eutectic and leading to the initial corrosion.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-6233-9907. ORCID

Xiongchao Lin: 0000-0003-1370-7059 Jun-ichiro Hayashi: 0000-0001-5068-4015 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Key Research and Development Program (Grant 2016YFB060030301) and Yue Qi Young Scholar Project, China University of Mining & Technology, Beijing. The authors express their deep gratitude to Mr. Jinchang Liu and Ms. Keiko Ideta in Prof. Seongho Yoon’s and Jin Miyawaki’s research group in Kyushu University, Japan, for their help on NMR analysis.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.8b00661 Energy Fuels XXXX, XXX, XXX−XXX