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Study on the Volatiles and Kinetic of In-situ Catalytic Pyrolysis of Swelling Low Rank Coal Chao He, Xiaojian Min, Huaan Zheng, Yingjie Fan, Qiuxiang Yao, Dan Zhang, Xing Tang, Chong Wan, Ming Sun, Xiaoxun Ma, and Charles Q Jia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02952 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Graphical Abstract for the Manuscript 60x25mm (600 x 600 DPI)

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Study on the Volatiles and Kinetic of In-situ Catalytic Pyrolysis of Swelling Low Rank Coal Chao He1, Xiaojian Min2, Huaan Zheng2, Yingjie Fan2, Qiuxiang Yao2, Dan zhang1, Xing Tang1, Chong Wan1, Ming Sun1*, Xiaoxun Ma1*, and Charles Q. Jia3 1. School of Chemical Engineering, Northwest University, International Scientific and Technological Cooperation Base for Clean Utilization of Hydrocarbon Resources, Chemical Engineering Research Center of the Ministry of Education for Advance Use Technology of Shanbei Energy, Shaanxi Research Center of Engineering Technology for Clean Coal Conversion, Collaborative Innovation Center for Development of Energy and Chemical Industry in Northern Shaanxi, Xi’an 710069, Shaanxi, China 2. Shaanxi Coal Chemical Industry Technology Research Institute Co., Ltd., Xi’an, 710070, Shaanxi, China 3. Department of Chemical Engineering and Applied Chemistry University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5

ABSTRACT: A new method combined solvent swelling with in-situ catalytic effect of metal ions was developed and introduced in coal pyrolysis to increase the coal conversion and the tar yield as well as to improve the quality of the tar products. Low rank coal of Shendong coal from China was used to investigate the effect of demineralization, swelling and in-situ catalytic on pyrolysis reactivity and kinetic characteristics, yield distribution of products and the tar composition. The experiments were performed using thermogravimetric analyzer/fourier 1


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transform infrared spectrometer (TG-FTIR), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and fixed-bed reactor to examine the pyrolysis behavior of raw coal, demineralized coal, methanol swelling demineralized coal and methanol swelling with metal ions (Ca2+, Cu2+ and Co2+) in-situ impregnated coal, respectively. The results showed that coal conversion could be promoted by pretreatment of solvent swelling and in-situ impregnated Cu2+ and Co2+ ions, respectively. The gas evolution results of FTIR indicated that the in-situ loading of Cu2+ and Co2+ ions had catalytic effect on the evolution of CO2, CH4 and aromatics of the swollen coal. The tar yield of demineralized coal was improved by the methanol swelling pretreatment. With the in-situ loading of Cu2+ and Co2+ ions, the tar yield of swelling coal further increased by 16.80% and 28.75%. The composition of tar analyzed by Py-GC/MS indicated that methanol swelling increased the relative content of the acidic compounds and also had positive effect on the yields of PCX (phenol, cresol, xylenol). The in-situ loading of metal ions increased the relative content of the aromatic compounds, but had different effect on the formation of BTXN (benzene, toluene, xylene, and naphthalene). The Cu2+ and Co2+ ions had catalytic effect on phenols decomposition during coal pyrolysis, resulting in a decrease of the relative content of the acidic compounds dramatically. The kinetic results showed that the in-situ impregnation of Ca2+, Cu2+ and Co2+ ions into the swollen coal could result in a decrease of activation energy and preexponential factor at the corresponding temperature range of the first and the second prolysis stage. In addition, a possible mechanism on in-situ catalytic pyrolysis of swelling coal was discussed and proposed based on the evolution and composition of the evolved species investigated during pyrolysis. The impregnation of the metal ions may catalyze the primary reactions and secondary reactions during coal pyrolysis.

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1. INTRODUCTION Low rank coals (i.e., lignite/brown coal and subbituminous coals) with large reserves in China constitute a significant resource for both energy and chemical feedstock 1. The coal consumption proportion continued to hold the first place in primary energy, even though the total coal consumption fell in China in recently years. It is imperative that low rank coals should be used efficiently and environmentally friendly. However, inefficient use of the coal has already made a large contribution to environmental pollution in the last few decades 2. As a simple and effective method and an important aspect of the clean coal technology, coal pyrolysis occurs among almost all coal thermal conversion progresses such as gasification, liquefaction, carbonization, and combustion 3–6. Moreover, high added-value chemicals, especially aromatic compounds, can also be separated from coal tar, so it is significant to obtain high tar yield in coal pyrolysis. In addition, the current coal pyrolysis technology is still immature for its low quality of tar and low yield of tar. A good understanding of influencing factors on the formation and distribution of those products and how to improve the yield of those products during coal pyrolysis is of vital importance. Previous studies have shown that many factors affect the process and product distribution of the coal pyrolysis, including the coal rank, organic macerals, content and composition of the minerals 7, 8 which are the self properties of the coal. Meanwhile, heating rate, the final pyrolysis temperature, atmosphere and the pressure 9–11 which are the process conditions of the coal pyrolysis were all studied, as these factors will assist in the selection of the process parameters for coal-oil and specific compound production. However, the properties of coal structure related to chemical reactivity have also received much attention among coal researchers in pyrolysis processes. As known, coal consists of a large three-dimensional cross-linked macromolecular 3


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network of polynuclear aromatic clusters connected by relatively strong bonds 12. Within this network there are appreciable amounts of lower molecular weight species, whose degree of mobility varies as they are physically trapped in closed pores or weakly bonded in some way to the network 13. Considering the structural properties of coal, swelling pretreatment of the coal has received much attention of many researchers in order to make the coal thermal conversion under mild conditions. In the presence of swelling solvents, coal molecules dissociate, rearrange and reassociate in lower free energy conformations, probably in a different molecular structure 14

. Swelling solvents can break weaker bonds and combine the effect of creating macropores in

coal structure, which decreases diffusional limitations, with the formation of active sites as the result of breaking some bonds 15, 16. Coal swelling can also reduce cross-linking reaction in the process of pyrolysis, inhibit the formation of CO2 and H2O 17, 18, increase the yield of the pyrolysis tar and change the composition of the product 19, 20. In order to control the pyrolysis process and the composition and distribution of the product, catalysts also attract much attention of the coal researchers. In recent studies, the researches of the catalysts of coal pyrolysis are focused on transition metal catalysts, molecular sieve or their mixture, alkali or alkali earth metal and iron ore 5, 21, 22, since the application of the catalyst can significantly increase the coal pyrolysis conversion and improve the quality of the coal pyrolysis products. Coals are often demineralized before researches in order to eliminate the effects of inherent minerals or investigate the effect of demineralization on the pyrolysis. The purpose of demineralization is to remove mineral matter without influencing the organic structure. The effect of demineralization on the effect of coal pyrolysis is also controversial 5, 7. Different inherent mineral matter may have different effects on the coal pyrolysis because of the different composition of the minerals in the coal structure. In 4


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these studies, metal salts, ore, or molecular sieve was added to the coal or acid washed coal directly with mechanical mixing or impregnation method to investigate the effect of individual element on the pyrolysis of the coal, in this way, three kinds of typical metal element of the alkali, alkali earth and transition metal have been found to be different chemical activities 23, such as Fe 24 and Co 23 which were investigated by direct impregnation method. It is suggested that Fe can increase the gas production and Co can increase the yield of tar, etc. during the process of coal pyrolysis. It is well known that three kinds of typical metal salts of the alkali, alkali earth and transition metal salts have different chemical activities 25. Considering the effect of solvent swelling on the coal structure and the catalytic effect of some metal salts on the coal pyrolysis as well as the swelling solvent for the dissolution of metal salts, the characteristics of the demineralized coal swollen and impregnated synchronously by the swelling solvent with metal salts dissolved were investigated during the pyrolysis. Moreover, little research has been reported on the relationship between demineralized swelling coal with metal in-situ impregnated and the process characteristics, product distribution and composition as well as the kinetics of pyrolysis. In this study, low rank coal, Shendong subbituminous coal (SD) was first through demineralization, afterwards the demineralized coal was swollen with methanol and swollen with metal ions (Ca2+, Cu2+ and Co2+) in-situ impregnated synchronously in the methanol, respectively. The main objective of this study was to investigate the effect of demineralization, swelling and in-situ catalytic on pyrolysis reactivity and kinetic characteristics, yield distribution of products and the tar composition. A new method combined solvent swelling with in-situ catalytic effect of metal ions was developed and introduced to increase the coal conversion and

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the tar yield as well as to improve the quality of the tar products. Furthermore, a possible mechanism on in-situ catalytic pyrolysis of swelling coal was discussed and proposed. 2. EXPERIMENTAL DETAILS

Figure 1. The flow chart of the experiment. 2.1. Preparation of Samples. SD used in this study was subjected to different pretreatments. Figure 1 shows the flow chart of the experiment. The SD was crushed, ground, sieved to particle size of SDDE > SD-DE-ME > SD-DE-ME-Ca > SD-DE-ME-Co > SD-DE-ME-Cu. These indicated that the procedure of demineralization, swelling and swelling with metal ions loaded synchronously could make the maximum weight loss rate -(dW/dt)Max decrease during the pyrolysis. 3.2. Releasing of small-molecule gases analysis. TG analysis coupled with FTIR is a necessary and valuable tool as it provides real-time important information of evolved volatiles from the devolatilization or the decomposition of experimental materials with the increasing of the temperature according to absorbance at a specific wave number of the FTIR spectrum 22. In addition, the tendency of yields of the gas volatiles can also be obtained. Table 3. Band assignment of typical FTIR spectra for light species during pyrolysis of the coal samples 9, 33, 34

Wave number (cm-1)

Assignment

3014

CH4

2240-2060

CO

2400-2240, 780-560

CO2

4000-3500

O-H

3100-3025

aromatic C-H

3010-2800

aliphatic C-H

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800 °C CO2

Absorbance

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aromatic hydrocarbons CO

CH4

H2O

aromatic Aliphatic hydrocarbons hydrocarbons

CO2 600 °C

400 °C 200 °C

500

1000

1500

2000

2500

3000

3500

4000

4500

-1

Wavenumber (cm )

Figure 5. FTIR spectra of gaseous products released from SD-DE during pyrolysis at different temperature. Figure 5 depicts the variation of FTIR spectra of gaseous products during pyrolysis at different temperature using SD-DE as an example. Based on the band assignment of typical FTIR spectra in Table 3, the major gaseous products during pyrolysis were CH4, CO, CO2, H2O, aromatic compounds, aliphatic compounds etc. In order to investigate the evolution of the gaseous products with the increasing of the temperature during pyrolysis, the most typical intense bands of CO2, CO, CH4, aromatic compounds and aliphatic compounds, 2358 cm-1, 2182 cm-1, 3014 cm-1, 3060 cm-1 and 2937 cm-1, were chosen as their characteristic absorption wave numbers, respectively 9, 35.

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Figure 6. FTIR evolution profiles of gaseous products during pyrolysis of SD and its samples with different pretreatments.

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As shown in Figure 6a, FTIR spectra of CO2 versus temperature profiles are depicted. According to the composition of the coal and previous studies 35, CO2 is released mainly from the cracking and reforming of carbonyl and carboxyl functional groups of the coal. The CO2 absorbance of all coal samples reached a maximum, followed by a slow continuous decrease till 800 °C except that of SD. Because of the decomposition of carbonate in the SD, a second evolution peak of CO2 appeared at about 700 °C which was consistent with the DTG curve of SD at 700 °C. For SD-DE, the second evolution peak of CO2 did not appear which was due to the demineralization. The evolution of CO2 absorbance were the same for SD-DE, SD-DE-ME, and SD-DE-ME-Ca which started at 200 °C and reached a maximum at 478 °C, followed by a slow continuous decrease till 800 °C. These indicated that the methanol swelling and methanol swelling with Ca2+ loaded synchronously did not affect the cracking and reforming of carbonyl and carboxyl functional groups. However, methanol swelling with Cu2+ and Co2+ loaded synchronously changed the process of cracking and reforming of carbonyl and carboxyl functional groups. The peak temperature at maximum absorbance of CO2 for SD-DE-ME-Cu and SD-DE-ME-Co dropped to 371°C and 449 °C, respectively compared with that of SD-DE-ME, which indicated that Cu2+ and Co2+ had catalytic effect on the cracking and reforming of carbonyl and carboxyl functional groups during the coal pyrolysis. The CO formation is due to the break of C-O-C and C=O functional groups 33. Meanwhile, the reduction reaction of CO2 and coke at high temperature (C+CO2=2CO) may also cause emission of CO 36. Figure 6b shows the FTIR absorbance of CO as the function of temperature. It suggested that the CO emission of all the coal samples during pyrolysis started at about 400 °C and then reached the maximum followed by a slow continuous decrease. The temperature at maximum absorbance of CO for SD-DE, SD-DE-ME, and SD-DE-ME-Cu were all at 664 °C 19


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which indicated that the effect of methanol swelling and the Cu2+ on the CO emission were not obvious. However, the Ca2+ and Co2+ showed different effects on the emission of CO. The Co2+ had catalytic effect on the emission of CO as the temperature at maximum absorbance of which dropped to 637 °C, while Ca2+ had an opposite effect and the temperature of which at maximum absorbance increased to 753 °C, compared with that of SD-DE-ME. The aromatic hydrocarbons are due to the break of the macromolecular structure of coal and the amount of hydrogen free radicals 37 during the pyrolysis. As shown in Figure 6d, the aromatic hydrocarbons emission of SD, SD-DE and SD-DE-ME showed the same trend which started at around 400 °C and then reached the maximum at 511 °C followed by a continuous decrease till 800 °C. These suggested that demineralization and methanol swelling did not have obvious effect on the emission characteristic of aromatic hydrocarbons during the pyrolysis. On the contrary, the Ca2+, Cu2+ and Co2+ all showed the catalytic effect on the aromatic hydrocarbons emission. The peak temperature at maximum absorbance of aromatic hydrocarbons dropped to 496 °C and 494 °C respectively, compared with that of SD-DE-ME at 511 °C. The aromatic hydrocarbons emission process of SD-DE-ME-Cu was changed dramatically during the pyrolysis, of which two peaks appeared in the emission profiles at 434 °C and 488 °C, respectively, indicating the catalytic effect of Cu2+ on the aromatic hydrocarbons emission compared with that of SD-DE-ME at 511 °C. The aliphatic hydrocarbons are due to the break of the macromolecular structure of coal 37. CH4 mainly comes from the pyrolysis of aliphatic hydrocarbons, aliphatic side chains of aromatic hydrocarbons and -O-CH3 38. As shown in Figure 6c and e, the evolution profiles of CH4 and aliphatic hydrocarbons presented Gaussian distribution. The evolution profiles of CH4 were the same for all coal samples except SD-DE-ME-Cu and SD-DE-ME-Co, as the peak 20


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temperature of SD-DE-ME-Cu and SD-DE-ME-Co at maximum absorbance of CH4 dropped to 474 °C and 472 °C respectively, compared with that of other coal samples at 483 °C. These indicated that the Cu2+ and Co2+ had catalytic effect on the CH4 emission. For the aliphatic hydrocarbons, all the coal samples had the same maximum emission peak temperature at 483 °C which suggested that the demineralization, methanol swelling and methanol swelling with metal ions loaded synchronously did not affect the characteristic temperature of aliphatic hydrocarbons emission during pyrolysis. Small emission peaks of CH4 and aliphatic hydrocarbons were observed at around 250 °C in both Figure 6c and e during the pyrolysis of the methanol swelling and methanol swelling with metal ions loaded coal samples. These can be attributed to the methanol swelling effect, which makes the coal structure loosing, breaks weaker bonds and promotes the emission of small molecules of the coal at relative low temperature during pyrolysis 15–17. 3.3. Fixed-bed pyrolysis and Py-GC/MS. 3.3.1. The product distribution by fixedbed pyrolysis. In order to investigate the effect of the different pretreatments on the products distribution, pyrolysis operations were performed in a fixed-bed reactor for all the coal samples at the final temperature of 650 °C. The results are showed in Figure 7. It suggested that the water and tar yield obtained from the SD-DE fraction dropped to 7.42% and 4.70%, respectively, when compared to that of the SD fraction (8.04% and 5.50%) during pyrolysis. However the gas yield of SD-DE pyrolysis increased by about 1.39% from 13.62 wt% (SD) to 15.01 wt% (SD-DE). These results were consistent with that of other researchers 7, 39. The reduction of the tar yield

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Figure 7. Yields analysis of tar, water, gas, and char produced by pyrolysis of different coal samples at the final temperature of 650 oC. after demineralization can be explained by hydrogen radical transfer occurring between the liquid and solid phases during pyrolysis. The tar formed during the decomposition and cracking of the coal matrix was closely related to the hydrogen radical transfer reactions. In addition, inorganic matters of coal had strong interaction with the free radicals of the coal during pyolysis 40–42

, which could promote hydrogen radical transfer, thus stabilizing the free radicals formed

during pyrolysis, causing increased tar precursor release and minimized polycondensation reactions, thus forming more tar products 43. In the absence of inorganic matters, the free radicals were not effectively capped and stabilized, thus the free radicals recombined to form char 43. Sert et al. 44 determined that the decreasing amounts of liquid products and increasing in gas yields after acid treatment with HCl/HF could also be attributed to more cracking of liquid products. The increasing in gas yield due to acid washing was due to the fact that the minerals are nonporous, thus the removal of mineral matter was believed to increase its porosity. Ye et al. 45 also

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found that there were more pores in acid-washed coal and surface cracks occurred due to a large number of defects and channels formed in the coal after removal of minerals. The tar and gas yield obtained from the SD-DE-ME increased about 1.45% and 0.71%, respectively, compared with that of SD-DE. In contrast, the water and the char yields decreased when compared to that of SD-DE during pyrolysis. These indicated that methanol swelling could affect the releasing behavior of the gas and tar during the pyrolysis. When exposed to solvent, the non-covalent interactions between functional groups of the coal could be broken by the solvent such as hydrogen bond 20, 46. The non-covalent interactions between functional groups were closely related to the cross-link reactions. Thus the solvent rearranged the structure of the coal and behaved differently during pyrolysis. Meanwhile, solvent could also cause changes in the physical structure of macromolecular network 47, which could not only break the noncovalent bonds but also expand the molecular network structure, therefore small and large molecules became liable to escape through the macromolecular network with the less severity char formation. In addition, the mobility of small molecules could suppress the repolymerization reactions of these molecules within the network. Xie et al. determined that swelling pretreatment could also induce an improvement in coal reactivity 48. Thus more tar and gas products might be formed after the swelling pretreatment. Three selected metal ions were in-situ impregnated synchronously into the SD-DE, using methanol as solvent. From Figure 7, it suggested that different metal ions had different effects on the product distribution of the coal samples during pyrolysis. The char yields of samples with metal ions impregnated decreased when compared to that of SD-DE-ME. However, all the water yields from the metal ions loaded samples increased. The tar yields could increase by the Cu2+ and Co2+ but decrease by Ca2+. The tar yield obtained from the SD-DE-ME-Ca fraction 23


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descended by about 1.05% from 8.87% (SD-DE-ME) to 7.82 % (SD-DE-ME-Ca), which indicated that Ca2+ could inhibit the formation of tar during pyrolysis. As is well known, the catalytic effect of calcium in tar reduction has been reported by many researchers 49–51. Tar reduction has been reported to take place due to secondary tar cracking and repolymerization, catalyzed by the calcium additive 31, thereby inhibiting the releasing of tar. Meanwhile, calcium may provide a nascent cross-link site in coal. This allows for coordination of groups such as carboxyls and hydroxyls which are prone to such reactions. These groups would otherwise coordinate with water, through hydrogen bonds 43. The tar yield increased by 16.8% with the insitu loading of Cu2+ ion, when compared to that of SD-DE-ME. Meanwhile, the tar yield could also increased by 28.75% with the in-situ loading of Co2+ (SD-DE-ME-Co). Obviously, there is a big difference between calcium and copper as well as cobalt, for the calcium is the alkaline earth metal and copper and cobalt are the transition metals. Since transition metal ions have a full or partial empty d-orbit, they can be chemically adsorbed by the oxygen containing groups including carboxyl groups and hydroxyl groups or the π-bonds of unsaturated hydrocarbons functional groups 52. Thus the interaction between intermolecular of the coal can be reduced effectively, promoting the pyrolysis of coal and the generation of more volatiles. 3.3.2. Chemical composition of pyrolysis tar by Py-GC/MS. Coal tar which consists of a lot of substances is closely related to pyrolysis process. Furthermore, a good understanding of the composition of coal tar is very important for understanding the mechanism of the coal pyrolysis. Figure 8 and 9 presented the total ion chromatograms and the categorized composition results of coal tar from all the coal samples, respectively. It indicated that the main organic species released from pyrolysis of all the coal samples were similar. All detected and identified compounds were categorized in aromatics, aliphatics, acidic compounds, oxygenated compounds 24


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(O-compounds), nitrogen compounds (N-compounds), sulfur compounds (S-compounds) and unknown compounds (confidence is less than 60%).

Figure 8. Py-GC/MS total ion chromatograms of light condensable fractions during different coal samples pyrolysis.

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Figure 9. Group composition analysis of light condensable fractions during different coal samples pyrolysis. In view of the results in Figure 9, for all coal samples, aromatic compounds accounted for the largest part of coal tar, followed by acidic compounds, aliphatic compounds, and oxygenated compounds. Compared to SD, the relative content of aromatic compounds obtained from the tar of SD-DE increased by up to about 6.90% but the relative content of aliphatic compounds decreased by about 6%. The variation of relative content of acidic compounds and oxygenated compounds were not obvious when compared to that of SD. Obviously, the relative content of the acidic compounds obtained from the tar of SD-DE-ME increased by 1.60% compared with that of SD-DE and reached to 31.50% which was the highest content among the samples during pyrolysis. Instead the relative content of aromatic compounds decreased by about 1.50% compared to that of SD-DE. Compared with the relative content of aromatic compounds of the tar of SD-DE-ME, the relative content of the aromatic compounds of tar from the Ca2+, Cu2+ and Co2+ loaded coal samples increased by about 23.30%, 13.40% and 21.10%, respectively. On the 26


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contrary, the relative content of the acidic compounds of tar from the Ca2+, Cu2+ and Co2+ loaded coal samples decreased dramatically by 27.20%, 22.90% and 26.90%, respectively. Meanwhile the aliphatic compounds of tar from all the Ca2+, Cu2+ and Co2+ loaded coal samples also decreased. In contrast, the relative content of the oxygenated compounds of the tar from all the metal ions loaded coal samples varied differently. The content of tar from the Cu2+ and Co2+ loaded coal samples increased compared with that of SD-DE-ME, but the variation for the Ca2+ loaded coal sample was opposite. Considering the decreased yield of the coal tar and the dramatically decreased relative content of acidic compounds from the Ca2+ loaded coal sample, the increased relative content of the aromatic compounds may be explained by the decomposition of phenols which is part of the acidic compounds. Since calcium has good catalytic activity on the decomposition of phenols to light aromatic hydrocarbons 53. For the Cu2+ and Co2+ loaded coal samples, the increased relative content of the aromatic compounds may be due to the higher conversion of the coal which characterized by less char production and also the decomposition of phenols to light aromatic hydrocarbons. Less char production implies that the coal sample tends to produce more volatiles rather than remains as char 54. 3.3.3. The analysis of BTXN and PCX formation. Light aromatics, such as benzene, toluene, xylene, and naphthalene (BTXN), are important industrial chemicals, widely used as fuel additives, solvents, and the feedstock for production of pesticides, plastic products, and synthetic fibers 55. The nature and quantity of BTXN are depicted in Figure 10 expressed as integrated intensity per mg coal. Compared with SD, the amount of benzene, toluene, xylene, and naphthalene released from SD-DE increased by 9.40%, 6.03%, 2.14% and 5.91%, respectively. A study on the yield of BTXN was investigated by Moliner et al. 56, 57, who also 27


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Figure 10. Variation of BTXN during pyrolysis of different coal samples at 650 oC. demonstrated that the relative content of BTXN was increased after demineralization. The similar phenomenon was also observed for the formation of BTXN of SD-DE-ME when compared to that of SD-DE. Solvent swelling can expand the molecular network structure, therefore small and large molecules became liable to escape during the pyrolysis 58 and the number of protons with different degrees of mobility also increased by swelling 59 which all favored for the formation of BTXN. These indicated that the pretreatment of demineralization and methanol swelling could promote the formation of BTXN. However, the phenomenon of BTXN formation was different for the different metal ions insitu impregnated coal samples. Compared with SD-DE-ME, the Ca2+ had a positive impact on the formation of toluene, xylene and naphthalene, but the effect of which was not obvious on the formation of benzene. In contrast, the effect of Co2+ and Cu2+ on the formation of benzene was obvious, which the content of benzene increased by 18.44% and 9.53%, respectively. The similar phenomenon of positive promoting effect on the formation of naphthalene during the pyrolysis was also observed. However, the content of xylene released from the Co2+ and Cu2+ in-situ 28


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impregnated coal samples decreased by 24.65% and 11.68%, respectively, when compared with that of SD-DE-ME. These suggested that Co2+ and Cu2+ in-situ impregnated had a negative impact on the formation of xylene. The Co2+ and Cu2+ showed different effects on the formation of toluene from each other. Compared with that of SD-DE-ME, the Co2+ had promoting effect on the formation of toluene. In contrast, the Cu2+ showed the opposite effect which might make the content of toluene descend. The metal ions loaded can catalyze the cracking of polycyclic aromatic hydrocarbons and phenolic compounds to generate light aromatics, such as benzene and toluene which has been reported by other researchers 31, 53.

Figure 11.Variation of PCX during pyrolysis of different coal samples at 650 oC. As a kind of widely used and high added value chemical products, phenolic compounds (phenol, cresol, xylenol and catechol) are the main components of coal tar. Figure 11 shows that the yields of phenol, cresol, xylenol (PCX) and catechol for the coal samples with different pretreatments during the pyrolysis. The yield of PCX after demineralization decreased by 15.20%, 10.20% and 21.70%, respectively when compared to that of SD. However, through methanol swelling the yield of PCX increased dramatically by 8.65% 14.11% and 85.63% 29


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compared to that of SD-DE. These indicated that demineralization and methanol swelling had different effects on the formation of the PCX. After the in-situ impregnation of Ca2+, Cu2+ and Co2+, the amount of cresol and xylenol dramatically decreased during pyrolysis. For example, xylenol decreased by about 75.53%, 86.04% and 83.14% during SD-DE-ME-Ca, SD-DE-ME-Cu and SD-DE-ME-Co pyrolysis. In contrast to cresol and xylenol, the amount of phenol increased by 6.56% in the in-situ loading of Ca2+. Whereas the Cu2+ and Co2+ had the negative effect on the formation of phenol which made the amount of phenol descend during pyrolysis compared to that of SD-DE-ME. An obvious phenomenon showed that the catechol only occurred and was detected during SD-DE and SD-DE-ME pyrolysis but was not detected in the presence of metal species including inherent minerals and the metal ions loaded. The results showed that the inherent minerals and the metal ions loaded had positive effects on the decomposition of catechol, which was also consistent with the finding of Bai et al. 31. As stated above, these suggested that inherent minerals and the in-situ loading of metal ions had catalytic effect on phenols formation and decomposition during coal pyrolysis. Some metal species may crack phenols into light aromatic hydrocarbons during pyrolysis 53. 3.4. Prolysis kinetic analysis. Determining the pyrolysis kinetic parameters related to the coal samples is important for better understanding their pyrolysis behavior. As was discussed in part 3.1, the variation of weight loss range and net weight loss from 200 °C to 800 °C were mainly investigated. All the kinetic parameters of coal pyrolysis, including activation energy and pre-exponential factor of coal pyrolysis were determined during this temperature range. The characteristic temperatures of pyrolysis are shown in Table 4. The beginning temperature of coal pyrolysis T0 is defined as temperature at x=5% and the end temperature Tf is that at x=85% 28.

30


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Figure 12. Plot of ln[-ln(1-x)/T2] vs 1/T of SD-DE pyrolysis calculated by one-step integral method.

Figure 13. Plot of ln[-ln(1-x)/T2] vs 1/T of SD-DE pyrolysis calculated by three-step integral method. As shown in Figure 12, according to the calculation, the typical plot of ln[-ln(1-x)/T2] versus 1/T was plotted, using SD-DE as an example which indicated that the reaction of coal pyrolysis cannot be described by one consecutive first order reaction. Obviously, it seems to be three 31


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independent first order reactions at different temperature regions. So we tried three independent first order reactions to describe the process of SD-DE pyrolysis, which means that Eq. (5) is applied separately to the three stages. To do so, the conversion x was recalculated for each stage reaction. A typical result was shown in Figure 13. From the slope of and intercept of each regression line, the value of Ea and A can be obtained for different stages. Table 4 showed the kinetic parameters determined by this method and the pyrolysis characteristic temperatures of all samples. The kinetic parameters are calculated in the range from x=5% to 85%, which represents the main reaction region 28. The good correlation coefficient suggests that three independent first order reaction model fits the experimental data very well. T0 and Tf are related to the difficulty with which the reaction proceeds 60.The T0 and Tf corresponding to conversion range were presented in Table 4. For the SD, the demineralization resulted in T0 increased by about 8 °C and Tf decreased by about 13 °C. The swelling made T0 of SD-DE descend by 12 °C but has no effect on Tf. The in-situ loading of Ca2+, Cu2+ and Co2+ ions made T0 of SD-DE-ME descend by 29 °C, 35 °C, and 22 °C, respectively, but made Tf increase by 5 °C and 3 °C except Co2+ which matter Tf descend by 6 °C. From Table 4, it can be seen that the characteristic temperatures, activation energy and preexponential factor are different for all the coal samples. These suggest that different pretreatments have different effects on the course and mechanism of coal pyrolysis. Obviously, the activation energy in second pyrolysis stage are higher than the first and third stage because of the decomposition and cracking of main coal macromolecules, for all the coal samples. Reaction with high activation energy needs a high temperature or a long reaction time 60. For coal pyrolysis, high activation energy means that the reaction needs more energy from the surroundings. The activation energy obtained by using this method was between 50 and 200 kJ 32


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mol-1, which was similar with the kinetic results reported by other researchers 28, 61. The activation energy from SD-DE increased at all three stages because of demineralization. In contrast, the swelling made the activation energy of SD-DE descend in the first and the second stage. Compared to the SD-DE-ME, the in-situ loading of Ca2+, Cu2+ and Co2+ ions could result in a decrease of activation energy and pre-exponential factor at the corresponding temperature range of the first and the second stage. However the effect of in-situ loading of Ca2+, Cu2+ and Co2+ ions on the activation energy and pre-exponential factor in the third stage were different. The in-situ loading of Ca2+ made activation energy and pre-exponential factor of SD-DE-ME descend. On the contrary, the in-situ loading of Cu2+ and Co2+ had the opposite effect. Considering the main pyrolysis stage which was corresponding to the second temperature range, the catalytic capability by the in-situ loading ions was ranked as such: Cu2+ > Co2+ > Ca2+. The change of activation energy indicated that different metal ions had different catalytic characteristics at different temperature regions.

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Table 4. Kinetic parameters and characteristic temperatures of coal pyrolysis Temp

Conversion range Ea (kJ mol-1)

Sample o

-Ra

( C)

(%)

294-430

5-21

82.41

1.63×106

0.9941

430-513

21-52

196.15

4.77×1013

0.9920

513-679

52-85

103.11

6.19×105

0.9902

302-430

5-21

90.15

6.89×106

0.9947

430-513

21-53

197.02

5.61×1013

0.9907

513-666

53-85

112.45

2.79×106

0.9901

290-430

5-23

80.64

1.26×106

0.9945

430-513

23-54

195.39

4.32×1013

0.9904

513-666

54-85

113.19

3.06×106

0.9903

261-430

5-28

57.05

1.64×104

0.9906

430-513

28-55

193.31

2.97×1013

0.9913

510-671

55-85

104.42

8.40×105

0.9902

SD-DE-ME-Cu 255-430

5-37

61.40

3.90×104

0.9905

430-513

37-56

176.59

1.87×1012

0.9904

513-669

56-85

119.28

6.46×106

0.9903

SD-DE-ME-Co 268-430

5-29

61.42

3.50×104

0.9908

430-513

29-56

180.11

3.50×1012

0.9901

513-660

56-85

114.11

3.87×106

0.9906

SD

SD-DE

SD-DE-ME

SD-DE-ME-Ca

a

A (min-1)

R, correlation coefficient.

3.5. Possible mechanism of in-situ catalytic pyrolysis. Coal pyrolysis consists of two sets of reactions: primary devolatilization reactions and subsequent secondary gas phase 34


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reactions. The former reactions consist of radical formation reactions, polymerizationcondensation reactions, radical recombination reactions, hydrogen addition reactions, etc., and the latter reactions are decomposition reactions of the volatile products produced through the primary reactions 58. A schematic diagram of thermal reaction mechanism was also proposed by Tromp 58. However, another new schematic diagram of corrected coal thermal reaction mechanism was proposed by Liu 62 recently, who reported that char was not all derived from the recombination reactions of volatile free radicals. The coal char contains free radicals and its free radical concentration increase with the increased degree of pyrolysis conversion, which is consistent with the findings of many researchers 62–64.

Figure 14. Schematic diagram of the swelling coal in-situ catalytic pyrolysis mechanism. 35


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During the formation of the coal volatiles, part of covalent bond of the coal structure reorganized before radical formation reactions, resulting in larger molecular weight char and smaller molecular weight volatiles. Obviously, the effects of metal compounds which played an important role during the formation of the pyrolysis products were not under consideration in the schematic diagrams of pyrolysis mechanism proposed by both Tromp 58 and Liu 62. According to the findings of many researchers 23, 28, lots of metal species had catalytic effect on the formation of the products during pyrolysis. As a widely used coal pretreatment method, swelling pretreatment can disrupt some non-covalent interactions such as hydrogen bonds, electron donoracceptor interaction, van der Waals force, etc. of the coal molecules, relax the network structure, produce a less aggregated structure and cause the expansion of the pore structure of the coal, improving the internal hydrogen donation ability of the hydrogen rich species in the coal 13, 58. Meanwhile, the swelling of coal also facilitates the impregnation of catalysts and the diffusion of reagents towards the reactive sites of coal as well as the dispersion of catalysts on the surface of the coal pores 65. Many researchers believed that the cations were associated with acidic functional groups, such as carboxyl groups or phenolic groups, in coal when incipient wetness impregnation was performed 53. The results were also consistent with the CO2 evolution change of the metal loaded coal samples during the pyrolysis. What is more, the non-covalent associations such as hydrogen bonds between solvent and coal molecules can also be reduced when the cations are associated with acidic functional groups 66. Based on the TG results and the kinetic analysis, the in-situ impregnation of the metal ions not only changed the quantity of the volatile species, but also matter the activation energy and preexponential factor descend at the primary reaction stage. The in-situ loading of Cu2+ and Co2+ ions had the catalytic effect on the evolution of CO2, CH4 and aromatics, the evolution curves of 36


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which moved to the low temperature region. In addition, all above results from Py-GC/MS and fixed-bed experiments showed that the composition and the yield of tar released from the metal ions impregnated samples were all changed. According to the stated above, because of the possible association between the cations and the acidic functional groups, the effect of swelling and the in-situ impregnation of metal ions on the change of coal structure and the non-covalent associations in coal can alter the initial formation of the free radicals during coal thermal decomposition, resulting in a change of the composition and the quality of the volatile products produced through the primary reactions. In addition, the residence time of free radicals may be prolonged during the diffusion from the inside of the coal to the out space because of the pore structure of demineralization. In this case, the large coal fragment free radicals may be stablized by other small free radicals, leading to the formation of char during the radical formation reactions. In other words, the coal conversion and the tar yield as well as the tar composition can be changed. The remaining of the metal ions in the char may also have inhibiting effect on the polymerization and the condensation reactions of the initial volatile products, resulting in a higher tar and gas yield. Furthermore, the coal char contains free radicals and its free radical concentration increase with the rising degree of pyrolysis conversion. Some large coal fragment free radicals may be catalyzed and cracked into small molecule radicals to form tar or gas because of acidity of some transition metal ions, resulting in a lower yield of char. Since the tar composition of volatile products contains a number of condensed aromatic compounds with negative π-electron cloud, the remaining cations may come into contact with condensed aromatic compounds of the primary products and affect the π-electron cloud’s stability of condensed aromatic compounds, resulting in the breaking of the C-C and C-H and a low char yield during pyrolysis 67. It is speculated that the remaining cations may catalyze those secondary reactions of 37


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primary products to change the tar and gas yield and composition. Therefore, as stated above, a method combined the catalytic effect of metal ions with solvent swelling was developed and introduced to control the primary devolatilization reactions and the secondary gas phase reactions. Meanwhile this method can increase the coal conversion and the tar yield as well as to improve the quality of the tar products. In addition, a possible in-situ catalytic pyrolysis reaction mechanism was also proposed in Figure 14. 4. CONCLUSION Pyrolysis of raw coal, demineralized coal, and methanol swelling demineralized coal with or without metal ions in-situ loaded were carried out using TG-FTIR, Py-GC/MS and fixed-bed reactor to understand the effect of demineralization, swelling and in-situ metal ions catalyst on pyrolysis reactivity and kinetic characteristics, yield distribution of products and the tar composition. A new method combined solvent swelling with in-situ catalytic effect of metal ions was set up. Furthermore, a possible mechanism on in-situ catalytic pyrolysis of swelling coal was discussed and proposed. A summary of the results obtained in this study is as follows: (1) Methanol swelling pretreatment improved the final conversion of demineralized coal. The in-situ loading of Cu2+ and Co2+ ions further promoted conversion of methanol swollen coal. The in-situ loading of Cu2+ ion dramatically changed the pyrolysis process of the swelling coal dramatically. The evolution of CO2, CH4 and light aromatics were also affected with the in-situ impregnation of Cu2+ and Co2+ ions during pyrolysis. (2) The tar yield of demineralized coal has increased by methanol swelling pretreatment and has also further increased by the in-situ loading of Cu2+ and Co2+ ions. The in-situ impregnation of Ca2+, Cu2+ and Co2+ may result in an increase of the relative content of the aromatic compounds from the tar but was not favorable for the formation of the 38


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

acidic compounds, when compared to that of the methanol swollen coal. (3) The pyrolysis process of all samples studied can be described by a three-step independent first order kinetic model. The in-situ loading of Ca2+, Cu2+ and Co2+ ions on the swelling coal can result in a decrease of activation energy in the first and the second coal pyrolysis stage and the catalytic capability by the three in-situ loading ions was in the order of Cu2+ > Co2+ > Ca2+. (4) The in-situ impregnation of metal ions could not only change the composition and the quality of the volatile products produced in the primary reactions, but also may catalyze the secondary reactions of the products obtained from primary reactions and change the yield and composition of the tar and gas. AUTHOR INFORMATION Corresponding Author *

Tel.: +86 029 88302633.

To whom correspondence should be addressed. E-mail: [email protected]; [email protected] ORCID Ming Sun: 0000-0003-1005-9738 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financed by the project supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. 21536009; Grant No. 21776229), Science and Technology Plan Projects of Shaanxi Province (2017ZDCXL-GY-10-03), Foundation of 39


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