Exploration of the Combined Action Mechanism of Desulfurization and

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Cite This: Energy Fuels 2017, 31, 13248−13258

Exploration of the Combined Action Mechanism of Desulfurization and Ash Removal in the Process of Coal Desulfurization by Microwave with Peroxyacetic Acid Longfei Tang,†,‡ Songjiang Chen,†,‡ Shiwei Wang,†,‡ Xiuxiang Tao,*,†,‡ Huan He,*,†,‡ and Huidong Fan†,‡ †

Key Laboratory of Coal Processing & Efficient Utilization, Ministry of Education and ‡School of Chemical Engineering & Technology, China University of Mining & Technology, Xuzhou 221116, PR China ABSTRACT: The combined effect of desulfurization and ash removal in the process of coal desulfurization by microwave with peroxyacetic acid was determined by evaluating variations in minerals and sulfur forms using X-ray diffraction and X-ray photoelectron spectroscopy. Dielectric properties of minerals or organic sulfur functional groups in coal were studied utilizing a vector network analyzer in the 100 MHz−6.5 GHz frequency range. Results showed that the trend of ash reduction was similar to that of desulfurization with the treatment time. The main minerals removed were kaolin, montmorillonite, and pyrite under microwave with peroxyacetic acid. The influences of minerals on the dielectric properties of pure coal were higher than those of organic sulfur containing groups. Peak values of the complex permittivity imaginary part (ε″) and magnetic loss tangent (tan δ) for pyrite or montmorillonite were significantly higher than those of the other model materials. In addition, it is found that the dielectric property curves of the mixed samples contained the characteristic peaks of the corresponding minerals or organic sulfur compounds. Thus, the sulfur-containing substances of coal could be selectively removed with heating selectivity under the microwave of a particular frequency. These findings confirmed that the cause of the combined effect was the difference of dielectric properties between minerals and organic sulfur-containing groups.

1. INTRODUCTION

P is the power of absorption (W); f is the applying frequency (Hz); E is the electric field intensity (V/m); ε0 is vacuum absorption dielectric constant; ε″ is the complex permittivity imaginary part. As can be seen from the eq 1, the power of absorption for coal is in proportion to the imaginary part of complex dielectric constant. In the alternating electric field, the proportion of the electric energy converted into heat energy by the dielectric accounting for the electric field energy stored through the dielectric polarization reflects the loss degree of the electric energy converted into heat energy by the dielectric, generally, which is represented by loss tangent (tan δ):16,17

Among the primary coal desulfurization methods, including physical, physicochemical, chemical, and microbial desulfurization, chemical-related desulfurization is considered to be the most effective methods for both inorganic and organic sulfur of coal,1,2 and effective desulfurization technologies could be achieved by combining microwave field.3,4 With the aid of a microwave field, selective heating and the special function of polarizable molecules could be realized,5,6 the reaction yield could be increased, and some difficult reactions could be promoted to carry out.7−9 In this method, sulfur of coal was removed through utilizing the response differences of sulfur components to microwave field.5,10−12 Coal is a kind of nonhomogeneous mixture consisted with various components of different dielectric constant, whose sulfur-containing components are a good microwave absorber. Therefore, the microwave irradiation could be selectively absorbed by the various sulfur-containing components in coal and transformed into heat energy.6,13−15 Thus, investigating the dielectric properties of the sulfur-containing components becomes the foundation of studying coal desulfurization by microwave irradiation. Microwave desulfurization of coal is not only related to the method used but also closely related to the physical and chemical properties of the sulfur-containing components in coal. The relationship between coal and microwave energy can be expressed as the following equation:6 2

P = 2πε0fE ε ′′

tan δ = ε ′′/ε′

The tan δ is the loss tangent; the ε′ is the complex permittivity real part, which represents the ability of polarization. The greater value of ε′ for a dielectric is, the stronger the coupling effect of the dielectric and microwave becomes. Researches in this field were mainly concentrated on optimizing the parameters of desulfurization conditions, determining the variation of the sulfur forms in the desulfurization process18−22 and the dielectric properties of coal. Their results showed that the removal of organic sulfur was more difficult than that of inorganic sulfur in coal by microwave.23,24 Model sulfur compounds studies revealed that the desulfurization process was effective with aliphatic type of compounds by electron-transfer process, while thiophene and Received: July 19, 2017 Revised: November 1, 2017 Published: November 22, 2017

(1) © 2017 American Chemical Society

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DOI: 10.1021/acs.energyfuels.7b02112 Energy Fuels 2017, 31, 13248−13258

Article

Energy & Fuels Table 1. Proximate, Ultimate, and Sulfur Speciation Analysis of Coal proximate analysis (wt % ad) XY GX QD

ultimate analysis (wt % daf)

sulfur distribution (wt % ad)

Ma

A

V

FC

Cb

H

Oc

N

Std

So

Sp

Ss

0.68 0.41 0.30

9.76 9.64 0.93

18.41 19.73 25.36

71.15 70.22 73.41

81.32 88.66 86.14

6.16 4.86 6.78

8.81 2.91 5.32

1.02 1.12 1.41

2.69 2.45 0.35

2.35 2.00 0.33

0.26 0.42 0.00

0.08 0.03 0.02

a

In proximate analysis, M, A, V and FC represent the moisture content in coal, the ash content in coal, the volatile matter content in coal and the fixed carbon content in coal, respectively (air-dry base). bIn ultimate analysis, C, H, N, and O represent the carbon content, the hydrogen content, the nitrogen content and the oxygen content in coal, respectively (dry ashfree base). cBy difference. dIn sulfur distribution, St is the total sulfur content in coal. Ss, Sp and So are sulfatic sulfur content, pyritic sulfur content and organic sulfur content in coal, respectively (dry ash free base).

Figure 1. Structures of the organic sulfur-containing compounds.

2. MATERIALS AND METHODS

condensed thiophene-type compounds remained unaffected due to their higher stability.25,26 It was found that the ε″ of pyrite in coal was much larger than that of pure coal (excluding minerals), and the heating rate of the former was about 9 times of the latter.27,28 The desulfurization of coal by microwave irradiation at the frequency of 915 MHz was better than 2450 MHz.29 Variable frequency microwave heating, 2450 MHz initially and 915 MHz thereafter, contributed to the large-scale coal processing.30 The dielectric properties of different coal types exhibited substantial differences with increasing frequency beyond the W-band.31 With increasing frequency from 75 to 110 GHz, the complex dielectric constant of bituminous and anthracite decreased considerably, and the dielectric properties of coal samples were strongly dependent on its moisture content.32 The microwave penetration depth was reduced continuously with increasing temperature, while the microwave absorption capability of the coal was improved at high temperatures.33 The volatiles release of coal during pyrolysis led to an increase in the density of delocalized electrons as temperature above 500 °C.34 The dielectric properties of chars were strongly dependent on the crystallite structure, coal rank, carbon layer stacking, and heat-treatment temperature.35,36 The interior moisture content could cause coal dielectric anisotropy.37 However, the effect laws of various minerals or organic sulfur groups in coal on the dielectric properties and desulfurization with microwave of coal were rarely reported. The objective of this study was to identify the combined effect of coal desulfurization and ash removal with microwave and auxiliaries. The removal degrees of sulfur and ash contents with treatment time were analyzed. Changes in minerals and sulfur forms after treatment were determined by XRD and XPS. Instead of the sulfur group in coal, model materials (pure minerals and organic sulfur-containing model compounds) were selected as research subjects. To quantify the regularity, the dielectric properties of pure coal (low sulfur and low ash), model materials, and their mixtures were determined using the transmission-reflection method.

2.1. Materials. A pair of high-sulfur coking coal samples used in this study were collected from Xiyu (XY) and Guxian (GX) coal mine in Shanxi province, China. Representative samples were dried in a vacuum-drying oven and then ground and sieved to the size of montmorillonite > kaolinite > quartz, which suggested that the

ability to absorb microwave energy and convert them into heat energy reduced according to the above order. So the microwave energy could be converted into the heat energy easily by pyrite, and its heating speed was the fastest under the microwave field. The values of tan δ followed the sequence: montmorillonite > pyrite > kaolinite > quartz, which indicated that the efficiency of montmorillonite to convert the microwave energy stored by polarization into heat energy was the highest. This is because that the computational formula of tan δ is tan δ = ε″/ε′, a relative value. For these four minerals, the conversion efficiency of microwave energy stored of montmorillonite was the highest under the microwave field. Although the ε″ sizes of pyrite was the highest, its ε′ was also the maximum, resulting in its efficiency of converting microwave energy stored into heat energy, slightly less than that of montmorillonite. For the frequency of the peaks, the ε″ and tan δ of the minerals had their own characteristics dielectric relaxation frequency. Thus, the selective heating of these minerals could be realized by adjusting the frequency of microwave. For their peak values, the ε″ sizes of pyrite and montmorillonite were both greater than 1 in the test frequency range, while that of the 13251


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Figure 5. Dielectric response characteristics of montmorillonite, quartz, and QD coal with the addition of these two minerals.

Table 2. Ε′′ and tan δ peaks of Minerals ε′′

minerals pyrite

kaolinite montmorillonite quartz

of QD coal, suggesting that the storage capacity of microwave energy for minerals was significantly higher than that of QD coal in microwave field. The ε′ values of the mixed samples increased with the quantity of minerals added in the QD coal, which indicated that the ability of storage microwave for coal could be improved by adding minerals of high ε′ sizes. The values of ε″ and tan δ for quartz were less than those of the QD coal, while those of the other three minerals were higher than those of the QD coal. For quartz and kaolinite, the ε″ and tan δ variations of the mixed samples were small with increasing mineral amounts in the range of 100 MHz−4 GHz frequency. However, in the frequency range of 4−6.5 GHz, their ε″ and tan δ were significantly elevated when these two minerals were added according to mass percentage of 3%. It indicated that if these two minerals were added into coal, the ability of coal to convert microwave energy into heat energy was enhanced in the frequency range of 4−6.5 GHz. For the montmorillonite and pyrite, due to their ε″ and tan δ sizes being significantly greater than those of QD coal, the ε″ and tan δ values of mixed samples increased significantly with the added mass percentage of these two minerals increasing.

tan δ

frequency range (GHz)

peak position (GHz)

peak value

peak position (GHz)

peak value

1.0−1.1 3.3−3.4 4.2−4.4 5.2−5.4 3.6−4.0 − 1.0−1.6 4.3−4.5

1.044 3.332 4.292 5.236 3.892 − 1.284 4.324

3.680 3.036 3.067 2.986 0.265 − 0.045 0.039

1.092 3.380 4.356 5.380 3.892 − 1.284 4.324

0.192 0.177 0.179 0.186 0.042 − 0.010 0.009

two other minerals were less than 0.5. The heating rate of pyrite and montmorillonite was the fastest under the microwave field with the test frequency range. 3.3. Dielectric Properties Analysis of the QD Coal with Minerals. In the test frequency range of 100 MHz-6.5 GHz, the ε′ values of four minerals were significantly higher than that 13252


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Figure 6. Dielectric response characteristics of the mixture of QD coal and sulfether.

structure and polarization of test material, while the ε″ indicates the hysteresis of the electric displacement and the response of polarization intensity to electric field.43 Considering the QD coal and dibenzyl sulfide varying widely in the ε′, the polarization of the mixture of the QD coal and sulfurcontaining compounds changed little after mixed. However, in the process of impregnating model compounds onto the QD coal, model compound molecules interacted with macromolecular structures of coal, which changed the electron distribution and the dipole of these two components.44 Thus, the relaxation phenomenon of mixtures in an alternating electric field were more than the sum of that of the two components. As diphenyl disulfide mixed with the QD coal according to mass percentages of 3% and 6%, the ε′ of the mixture almost unchanged except for the highest percentage of diphenyl disulfide added. For the ε″ and tan δ, their sizes were between those of the QD coal sample and diphenyl disulfide and showed the characteristic peaks of the QD coal sample and dibenzyl sulfide.

Therefore, if the coal contained these two minerals, the ability of converting microwave energy into heat energy would be improved significantly, and the microwave heating speed of coal would be enhanced. In summary, it is inferred that the capability of orientation polarization and storage microwave of coal would be improved when the coal contained minerals of greater ε′. As the coal contained minerals with greater ε″ and tan δ, the heating rate of coal would be also improved significantly under the microwave field. After these minerals were mixed with the QD coal, it is found that the characteristic peaks of ε″ and tan δ for the added minerals appeared in the dielectric properties curves of the corresponding mixtures, which was conducive to achieve the selective heating and removal of minerals in coal. 3.4. Effect of Sulfether on Dielectric Properties of Coal. As shown in Figure 6, the ε′ of the mixture of QD coal and dibenzyl sulfide decreased with dibenzyl sulfide quantity. The addition of dibenzyl sulfide increased the ε″ and tan δ value of the mixture in the 4.5 GHz-6.5 GHz range, but the ε″ and tan δ changed little below 4.5 GHz with increasing dibenzyl sulfide content. The ε′ is mainly determined by the molecular 13253


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Figure 7. Dielectric response characteristics of QD coal containing sulfoxide or sulfone.

3.5. Effect of Sulfoxide and Sulfone on Dielectric Properties of Coal. Dielectric properties of the QD coal containing sulfoxide and sulfone are shown in Figure 7. It is observed that the ε′, ε″, and tan δ of diphenyl sulfoxide were all less than those of QD coal, and the dielectric properties variation tendencies of the mixture were similar to that of diphenyl sulfoxide curve. With the adding increasing quantities sulfoxide, the ε′ decreased first and then increased, while the ε″ and tan δ almost did not change. In addition, the characteristic peaks of the mixture and diphenyl sulfoxide were exactly the same in the curves of the ε′, ε″, and tan δ. For the QD coal containing diphenyl sulfone, its ε′ decreased continuously, but values of ε″ and tan δ first increased and then decreased. Moreover, the ε″ and tan δ of the mixed samples also showed the characteristic peaks of QD coal and diphenyl sulfone in the range of 100 MHz−4.9 GHz. The ε″ and tan δ characteristic peaks of diphenyl sulfone at 6 GHz were not present in the ε″ and tan δ curves of mixed samples, which was due to the limitation of the test frequency range according to the Figure 7. In addition, the ε″ and tan δ values of the mixed

samples or diphenyl sulfone were greater than those of the QD coal sample in the range of 4.9−6.5 GHz. 3.6. Effect of Thiophene and Thiophene Sulfone on Dielectric Properties of Coal. The dielectric properties of the QD coal containing thiophene and thiophene sulfone are presented in Figure 8. It is found that the dielectric properties of dibenzothiophene were all less than those of the QD coal. When added mass percentage was 3% or 6%, the ε′ of mixed samples and QD coal were basically the same. The ε′ sizes of mixed samples decreased significantly as the mass percentage of dibenzothiophene increased to 10%. However, with the increase of dibenzothiophene content, the overall sizes of the ε″ and tan δ for mixed samples kept almost unchanged compared to the QD coal. Only some changes in their height of the peak value were noticed. The characteristic peaks of mixed samples that coincided with the QD coal were weakened, but those that coincided with dibenzothiophene were strengthened. The ε′, ε″, and tan δ values of dibenzothiophene sulfone were less than that of the QD coal in the frequency range of 100 MHz−4.9 GHz and larger than those of QD coal in the 13254


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Figure 8. Dielectric response characteristics of QD coal containing thiophene or thiophene sulfone.

dipole polarization, which result in the friction between molecules and the production of heat energy from electromagnetic energy.49 Polar oxygen-containing functional groups exist in coal, including oxhydryl, carboxyl, and carbonyl groups, which could be selectively effected by microwave.50 The influence degree of different organic sulfur model compounds on the dielectric properties of QD coal follows the order: sulfone or sulfoxide > thiophene > disulfide > sulfether. The ε′, ε″, and tan δ characteristic peaks of organic sulfur compounds were present in the dielectric properties curves of the corresponding mixed samples, suggesting that the selective heating could be achieved for the different types of organic sulfur containing groups in coal. 3.7. Relationships of Ash Removal and Dielectric Properties. According to Figure 3, the diffraction light absorption intensity of four main minerals detected all declined after treatment. It could be found that the diffraction light absorption peaks of montmorillonite and pyrite were both very small in the treated coal samples, while peaks of quartz changed little, which was consistent with dielectric properties analysis results of these minerals. The ε″ and tan δ sizes of pyrite and

range of 4.9−6.5 GHz. For the QD coal containing dibenzothiophene sulfone, its ε′ value first increased and then decreased and was greater than that of QD coal or dibenzothiophene sulfone. The ε″ and tan δ characteristic peaks of dibenzothiophene sulfones appeared in the dielectric characteristic curve of mixture samples. According to the above analysis, it is observed that the dielectric properties of organic sulfur containing compounds were mostly less than those of the QD coal, and the peak positions of their ε″ and tan δ were totally different. It is due to the fact that QD coal is a complex mixture of many pores and polar groups, which results in the strong response of coal to microwave field. Coal could absorb water vapor from the atmosphere, resulting in the increase of moisture content, and the dielectric constant of coal increased with the moisture content.45,46 The adsorption characteristics of coal affected by its pore structure, so the dielectric properties are affected the pore volume and porosity of coal samples.47,48 As solid materials with polar molecules or groups are irradiated by microwave, polar molecules or groups undergo high-frequency rotation with an alternating electromagnetic field due to their 13255


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Figure 9. S 2p XPS fitting spectrum of the raw coal and the desulfurized coal samples.

Figures 9 and 10. After coal samples were irradiated for 90 s with the 2.45 GHz frequency and 800 W microwave, contents

montmorillonite were both greater than those of the two other minerals. Thus, the heating rate of pyrite and montmorillonite was fast under the microwave field, resulting in their obvious removal effects. The contents decreasing of kaolin and montmorillonite in coal was due to the fact that the aluminum silicate of coal decomposed in the acidic environment after microwave irradiation. In addition, the pyrite in coal was removed by oxidation to soluble sulfates under the microwave with peroxyacetic acid. The proposed mechanism of the pyrite coal oxidization in the HAc−H2O2 system followed: hydroxyl cations produced from the deformation of peroxyacetic acid attack the pyrite, and iron ions and sulfate ions are produced at the end of the process.9,51,52 This procedure can be simplified by reaction eqs 3−5. However, the reduction of quartz might because that the ash adhered to the surface of the coal particles were washed away with microwave irradiation based on its weak response of the microwave field and activity of reaction with additives. CH3COOH + H 2O2 → CH3COOOH + H 2O

(3)

CH3COOOH + H+ → CH3COOH + OH+

(4)

Figure 10. Changes of sulfur forms contents before and after treatment.

of sulfether, thiophene, and sulfate decreased, and the removal degree of thioether was the highest, while the contents of sulfoxide and sulfone increased obviously. Based on the dielectric properties analysis results of organic sulfur compounds, the difference in the ε″ and tan δ sizes of sulfide and thiophene were little, while the values of ε″ and tan δ for disulfide or sulfoxide and sulfone of sulfether were higher than those of thiophene. In addition, thiophene was of stable

2FeS2 + H 2O + 15OH+ → 2Fe3 + + 4SO24 − + 17H+ (5)

3.8. Relationships of Desulfurization and Dielectric Properties. The organic sulfur mainly existed in the forms of thiophene and sulfether in the raw coal samples, as shown in 13256


Energy & Fuels ring structures and its C−S bond energies were larger than those of sulfide or disulfide.5 Thus, compared to thiophene, these sulfide and disulfide and their sulfoxide or sulfone were apt to remove by C−S bond cleavage under the microwave field. Thiophene was difficult to completely remove by C−S bond breaking, and most of it was oxidized to sulfoxide or sulfone. Although the ε″ and tan δ sizes of sulfoxide and sulfone were much greater than those of other organic sulfur model compounds, their contents increased obviously after treatment. According to the previous research results,10 sulfether and thiophene were oxidized into the sulfoxide, sulfone, or sulfate in the processing of the microwave treatment with assistants. Furthermore, oxidation products of thiophene were also difficult to remove.25,26,53 For the XY raw coal with high organic sulfur content, the ratio of thiophene in organic sulfur was more than 50%, resulting in its unsatisfactory desulfurization degree. However, the ratio of sulfether in organic sulfur was more than that of thiophene in the GX raw coal, which was consistent with previous results of high sulfur-removal degree.

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ACKNOWLEDGMENTS

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REFERENCES

This study was supported by the Fundamental Research Funds for the Central Universities (grant no. 2017BSCXA17) and the Research and Innovation Project for College Graduates of Jiangsu Province (grant no. KYCX17_1518).

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4. CONCLUSIONS (1) The variation tendency of desulfurization and ash reduced were the same during the coal treated by microwave with peroxyacetic acid. The content of montmorillonite and pyrite in samples decreased significantly. This was due to the fact that the aluminum silicate decomposed in the acidic environment and the pyrite was oxidized into soluble sulfates under the microwave with peroxyacetic acid. (2) Compared with the organic sulfur groups in coal, the minerals in coal had more influence on the dielectric properties of coal. The effects of pyrite and montmorillonite on the desulfurization of coal were the greatest. It was due to the fact that the abilities of pyrite and montmorillonite to convert microwave energy into heat energy was much higher than those of other minerals. (3) According to the peak values of ε″ and tan δ, it is inferred that the heating rate of coal containing pyrite or montmorillonite would be enhanced substantially under the microwave field with the tested frequency range. Moreover, it was found that the characteristic peaks of the minerals or organic sulfur compounds were present in the dielectric properties curves of the corresponding mixed samples, which was conducive to achieving the selective removal of sulfur-containing substances in coal with heating-selectivity effectively under the microwave of a particular frequency.

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AUTHOR INFORMATION

Corresponding Authors

*(X. Tao) E-mail: [email protected]. Phone: +86-51683591057. *(H. He) E-mail: [email protected]. Phone: +8615150002965. ORCID

Xiuxiang Tao: 0000-0001-8706-8275 Notes

The authors declare no competing financial interest. 13257


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