Pyrolysis Characteristics and Evolution of Char Structure during

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Pyrolysis characteristic and evolution of char structure during pulverized coal pyrolysis in drop tube furnace: influence of temperature Qian Li, Zhihua Wang, Yong He, Qiang Sun, Yanwei Zhang, Sunel Kumar, Kang Zhang, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00002 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Energy & Fuels

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Title

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Pyrolysis characteristic and evolution of char structure during pulverized coal pyrolysis in drop

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tube furnace: influence of temperature

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Changed from the title ‘effects of pyrolysis temperature on pyrolysis behaviors of pulverized coal in drop tube furnace’

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Authors

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Qian Li, Zhihua Wang*, Yong He, Qiang Sun, Yanwei Zhang, Sunel Kumar, Kang Zhang, Kefa Cen

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Corresponding author

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Zhihua Wang

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State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38, Zheda Road, Hangzhou

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310027, P.R. China

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Tel.: +86 571 8795 3162

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Fax: +86 571 8795 1616

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E-mail address: [email protected] (Z.H. Wang)

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ACS Paragon Plus Environment

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Pyrolysis characteristic and evolution of char structure during pulverized

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coal pyrolysis in drop tube furnace: influence of temperature

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Qian Li, Zhihua Wang*, Yong He, Qiang Sun, Yanwei Zhang, Sunel Kumar, Kang Zhang, Kefa Cen

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State Key Laboratory of Clean Energy Utilization, Zhejiang University, 310027, Hangzhou, China

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∗

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Email address: [email protected]

Corresponding author: Tel.: +86 571 87953162; Fax: +86 571 87951616.

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Abstract: The aim of the presented work was to investigate the pyrolysis behavior of pulverized coal in

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drop tube furnace (DTF), two typical Chinese coals Shenhua bituminous and Pingzhuang lignite were

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used to conduct the pyrolysis experiment in DTF under the temperature of 600~1000oC (100oC

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increments). Fourier transform infrared analysis technique was used to analyze the functional groups in

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pyrolysis coal char, and pyrolysis gas was measured online by an online flue gas analyzer. The results

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showed that pyrolysis in DTF was also a dehydration upgrading process, increasing temperature caused

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more volatiles released from coal. There were more H2 and CO, but less CO2 and CH4 in pyrolysis gas

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when the temperature was rising. Compared to bituminous, lignite was more sensitive to the changes of

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coal pyrolysis temperature, more valuable combustible gases could be obtained from lignite in DTF with

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less thermal power cost. From infrared analysis results, after pyrolysis process in DTF, aromatic structure

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in coal char increased, apparent aromaticity and coal rank improved, unsaturated bond in coal decreased,

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coal char structure became more stable and mature. Because of the hydrophilic oxygen-containing groups

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was lost after pyrolysis in DTF, the dried coal char had a good ability for reducing moisture reabsorption.

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Keywords: pyrolysis, drop tube furnace, infrared, coal char

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1. Introduction

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Coal is not only an important energy but also a valuable resource. Referenced by the Statistical

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Review of World Energy from British Petroleum (BP) [1], in the consumption of world primary energy,

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coal is still the second largest energy in the world, ranking only second to petroleum. In China, where

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coal is the main energy source of this country, although the renewable energy keep quickly increasing,

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coal still accounts for 64% of China’s primary energy consumption in the year of 2015. The state of art

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combustion technology like Integrated Gasification Combined Cycle (IGCC) and Ultra Super Critical

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(USC) have achieved higher power generation efficiency (>45%) and lower emission with advanced air

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pollution control devices (Particle Matter methoxyl (–OCH3) [33]. CO formed from

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The variation trend of gaseous products component in drop tube furnace is similar to the results

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obtained by many other researchers [37, 38]. However, it should be noted that, compared to other

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pyrolysis method, like fixed bed furnace, the production of CO and CH4 shows an opposite trend when

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pyrolysis temperature increases. In other words, increasing temperature is conducive to the formation of

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more CO and less CH4 in drop tube furnace, but less CO and more CH4 in fixed bed furnace. The major

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factor which makes difference in two pyrolysis method is the difference of heating rate. A theory has been

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found [39] that higher heating rate, which is more than 104 K/S in drop tube furnace, causes a higher free


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radical concentration at high temperature in a shorter period of heating time. The chemical bonds in

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methoxyl, which forms CH4, can be broken at a high heating rate, which causes more CO production in

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drop tube furnace compared to fixed bed furnace.

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3.2 Changes in chemical structure

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After pyrolysis in DTF, the changes of the oxygen-containing functional group in coal char can be

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expressed by the ratio of hydroxyl-oxygen/ether-group-oxygen (Ahy/Aet) and the ratio of

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carboxyl-oxygen/aromatic-hydrocarbon (Ac=o/Aar), the results are shown in Figure 6(a) and 6(b). It can be

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seen from the figure that the values of Ahy/Aet and Ac=o/Aar showed a reduction for both Shenhua

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bituminous and Pingzhuang lignite after pyrolysis in DTF, and these values continuously decreased when

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rising pyrolysis temperature. For Shenhua coal, the value of Ahy/Aet in raw coal was 5.5, it became 5.0

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after 600oC pyrolysis, and it decreased continuously to 2.0 when temperature raised to 1000oC. Ac=o/Aar

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was 0.8 in raw coal, it reduced to a half (0.4) after 600oC pyrolysis, and Ac=o/Aar decreased gradually

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when the temperature was rising. Similar to Shenhua coal, the value of Ahy/Aet and Ac=o/Aar of

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Pingzhuang coal were 6.6 and 0.60, they decreased to 4.0 and 0.19 after 600oC pyrolysis, they also

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decreased continuously when the temperature was rising and were only 1.6 and 0.03 at 1000oC pyrolysis

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condition.

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Organic oxygen existed as an oxygen-containing functional group in coal has great influence on

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properties of coal. Polar oxygen-containing groups in coal like hydroxyl and carboxyl are typical

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hydrophilic groups, they can form a hydrogen bond with water molecules, so moisture is retained in the

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coal molecules. After high-temperature pyrolysis in DTF, the hydrophilic groups in coal char reduced,

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because of (i) the hydroxyl condensation reaction to form H2O or ether oxygen bond and (ii) breakage of

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acid, ketone and alcohol functional groups [40]. Therefore, the moisture which can be constrained in the


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coal char will be less, the char showed a good hydrophobicity. What’s more, the reduction of hydrophilic

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groups is an irreversible process, the char reabsorption ability to water is much lower compared to raw

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coal. For Pingzhuang lignite, the moisture content in raw coal was 22.38%, it is better for long-distance

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transport and clean utilization of coal to conduct pyrolysis process. Ahy/Aet and Ac=o/Aar decreased with

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the increasing of pyrolysis temperature, which indicated that pyrolysis in DTF caused a transformation

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from hydroxyl-oxygen and carboxyl-oxygen to more stable ether-group-oxygen, the aromatic

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hydrocarbon in coal increased comparatively, aromaticity also increased.

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The transformation of aliphatic structure in char can be expressed by the ratio of methyl (–CH3) and

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methylene (–CH2), the result is shown in Figure 6(c). It can be seen from the figure that CH3/CH2 showed

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an ascending trend after pyrolysis in DTF and with the increasing pyrolysis temperature. Comparing to

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the common ascending trend of Shenhua coal, the ascending trend of Pingzhuang coal was more obvious

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and violent, the value of CH3/CH2 in 1000oC pyrolysis char was almost 3~4 times than that in raw coal.

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During pyrolysis process, the amount of methyl increased, methylene decreased comparatively, it was

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because high-activation hydrogen atoms had been generated due to the deacidification and

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dihydroxylation effects in pyrolysis process. These hydrogen atoms can attack some cyclic structure, and

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add into some unsaturated position, forming the saturated methyl [31]. Thus, the amount of methyl

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increases. Furthermore, under the high-temperature pyrolysis condition in DTF, high-strength thermal

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power will break those chemical bonds with low-activation-energy and good-reacting-activation, such as

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methylene bridge bond, causing the reduction of methylene. Only need a fewer energy, the methylene in

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low-quality coals like lignite can be cracked. During pyrolysis process, the transformation from aromatic

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methylene to benzene structure also caused the reduction of methylene.

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The CH3/CH2 appears as major composition during the pyrolysis process which indicates that methyl

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increased, methylene decreased, methylene bridge bonds cracked, condensation degree of coal was higher,


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saturated aliphatic branches were more, alkyl branches were shortened. The results suggested that coal

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structure moved forward to a direction with higher condensation degree and higher saturability after

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pyrolysis in DTF[20].

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The

changes

of

aromatic

structures

can

be

expressed

by

the

ratio

of

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aromatic-hydrocarbon/aliphatic-hydrocarbon (Aar/Aal), this ratio defines the maturity of organic material

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in coal [28], and the result is shown in Figure 6(d). It can be seen in the figure that the content of aromatic

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hydrocarbon increased after pyrolysis in DTF, and it continuously increased with the increasing of

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pyrolysis temperature, which indicated that the increasing of pyrolysis temperature was conducive to the

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transformation from aliphatic structures to aromatic structures. This transformation is due to the pyrolytic

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removal of peripheral groups attached to the aromatic groups during the high-heating-rate pyrolysis

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process in DTF [41]. The stability of coal was closely related to the contents of aromatic hydrocarbon and

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aliphatic hydrocarbon in coal, higher content of aromatic hydrocarbon and less aliphatic hydrocarbon

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branches indicated a more compact spatial structure of coal molecule, higher stability, and a higher coal

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rank. For both two coal, pyrolysis process occurs with increasing of pyrolysis temperature resulting that

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Aar/Aal was increasing all the time. It can be concluded that pyrolysis in DTF enhanced the stability of

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coal char structure, improved coal rank, and coal char structure was more stable due to a higher pyrolysis

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temperature.

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The apparent aromaticities of raw coal and coal char were calculated and the result shown in Figure

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7. The apparent aromaticity means the maturity of the coal structure. It can be seen in the figure that the

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aromaticities of Shenhua and Pingzhuang raw coal were 0.7 and 0.6, respectively. The aromaticity in

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Shenhua raw coal was a little bit higher than Pingzhuang raw coal. After pyrolysis in DTF, the

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aromaticities of both two coals had a growth, and it increased continuously with the increasing of

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pyrolysis temperature, but the aromaticity of Pingzhuang coal was more sensitive to the variation of


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pyrolysis temperature. It may be because that there was a more aromatic structure in Shenhua raw coal

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than in Pingzhuang raw coal, which caused a more stable coal structure of Shenhua coal, thus, Shenhua

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coal has a stronger resistance to the thermal power in high-temperature pyrolysis.

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Synthesize the above FTIR analysis results of coal structures and coal constituents, it can be

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concluded that the pyrolysis process in DTF had similar effects for Shenhua bituminous and Pingzhuang

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lignite, aromatization process of coal molecule structure took place, aliphatic branches were shorter, the

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amount of methylene decreased, the content of aromatic structure increased, the apparent aromaticity

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improved, coal rank also improved, active oxygen-containing functional groups like carboxyl and

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hydroxyl decreased, which was conducive to reduce hydrophilicity and the ability to constrain moisture, it

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also reduced the reabsorption to moisture.

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4. Conclusion

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In this paper, Shenhua bituminous and Pingzhuang lignite were used to conduct the pyrolysis

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experiments in DTF under pyrolysis temperatures of 600~1000oC, the analysis of the products coal char

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and pyrolysis gas was discussed. After pyrolysis process in DTF, moisture in Pingzhuang lignite had been

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removed significantly. Meanwhile, hydrophilic oxygen-containing groups like carboxyl and hydroxyl

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were also removed, which indicates that pyrolysis process in DTF is also a dehydration upgrading process

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for lignite, and it also can avoid moisture reabsorption.

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With the increasing of pyrolysis temperature, the production of H2 and CO increased, but CO2 and

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CH4 decreased, which caused a reduction of the heat value of pyrolysis gas. Due to the release of volatiles

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gases, the element of carbon, hydrogen, and oxygen in coal char also decreased compared to raw coal.

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After pyrolysis process in DTF, methylene bridge bonds cracked, there were more saturated aliphatic

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branches in coal char, the condensation degree and saturability of coal char were higher after DTF


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pyrolysis and increased with the rise of pyrolysis temperature. Meanwhile, there was a more aromatic

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structure in coal char, the apparent aromaticity of coal char also increased with temperature, which

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indicated that DTF pyrolysis is also enhanced the stability of coal char structure, improved coal rank.

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Acknowledgements

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This work was supported by the National Natural Science Foundation of China (No.51422605 and

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51390491) and Zhejiang Provincial Natural Science Foundation (LR16E060001).

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Table captions

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Table 1 Proximate and ultimate analysis of raw coal.

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Table 2 Bands assignments observed on FTIR spectra.

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Table 3 Proximate and ultimate analysis of coal char.

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Figure captions

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Figure 1 Schematic of DTF pyrolysis installation

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Figure 2 FTIR spectrum of Shenhua and Pingzhuang Coal

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Figure 3 Results of peak separation of Shenhua raw coal

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Figure 4 Component of pyrolysis gas of Shenhua and Pingzhuang coal under different temperature in

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DTF

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Figure 5 Typical secondary reactions generating H2 and CH4

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Figure 6 Changes of chemical structure after pyrolysis in DTF.

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Figure 7 Changes of aromaticity after pyrolysis in DTF.

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Table 1

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Proximate and ultimate analysis of raw coal. Coal

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Shenhua

Pingzhuang

Huainan

Babaoshan

Moisture(%,ar)

1.11

22.38

1.74

10.58

Ash(%,d)

25.11

11.03

18.50

16.28

Volatiles(%,d)

27.76

40.53

28.11

38.87

C(%,d)

58.92

63.58

71.12

58.95

H(%,d)

3.39

4.57

4.47

3.69

N(%,d)

0.87

1.24

1.10

1.50

St(%,d)

0.48

0.60

0.61

1.44

O(%,d)

11.23

18.98

4.20

18.15

Qnet(J/g,ad)

22914

20299

26598

21123

474

Note: Qnet, lower heating value; ar, on as-received basis; ad, on air-dried basis; d, on dry basis; St, total sulphur; Oxygen

475

is calculated by difference.

476 477 478


Page 25 of 33

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

479 480

Energy & Fuels

Table 2 Bands assignments observed on FTIR spectra Section

Wavenumers/cm-1

hydroxyl 3600~3000 cm-1

3625~3200 3300 3090~3030

aliphatic structures 3000~2700 cm-1

2954 2921 2895 2874 2849

oxygen-containing functional groups and aromatic structures 1800~1000 cm-1

1772 1703 1650 1618~1502 1458 1437 1410 1377 1350 1274 1222 1195~1168 1138 1094 1036

low-wavenumber aromatic structures and ash 900~700 cm-1

873 860~750 820 758

Assignment hydrogen-bond interactions stretching –OH, –NH, –OH(phenolic), OH(proxidic) (C–H)ar (stretching)

–

asym.RCH3 asym.R2CH2 R3CH sym.RCH3 sym.R2CH2 aryl esters carboxyl acids conjugated C=O aromatic C=C δ as. CH3–, CH2– Aromatic C=C δ as. CH–(CH3); δ OH δ s. CH3–Ar, R δ s. CH2–C=O υ C–O in aryl ethers υ C–O and δ OH, phenoxy structures, ethers υ C–O phenols, ethers υ C–O, tert, alcohols, ethers υ C–O sec. alcohols alkyl ethers, Si–O substituted benzene ring with isolated hydrogen aromatic rocking HCC angular system of condensed ring benzene ring orto-substituted and meta-substituted

481 482


Energy & Fuels

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

483 484

Page 26 of 33

Table 3 Proximate and ultimate analysis of coal char Proximate Analysis /wt,%

Ultimate Analysis /wt,%

Mad

Ad

Vd

FCd

Qnet,d J/g

SH600

0.93

33.94

20.89

45.17

20299

51.64

2.55

0.95

0.48

10.44

26.83

SH700

1.47

34.63

17.63

47.74

20563

52.56

2.08

0.86

0.48

9.39

28.30

SH800

1.16

35.47

14.90

49.63

19811

52.96

1.85

0.79

0.38

8.54

30.00

SH900

1.10

37.87

12.76

49.37

18777

51.49

1.30

0.52

0.46

8.37

34.43

SH1000 PZ600 PZ700 PZ800 PZ900 PZ1000

1.04 6.11 5.29 3.81 3.93 3.02

39.07 15.10 16.82 18.44 19.68 19.87

11.66 19.71 13.06 7.71 4.69 3.50

49.27 65.14 70.12 73.84 75.63 76.63

19646 25797 26809 26883 26161 26651

50.40 70.42 72.74 73.75 74.29 74.23

1.21 3.01 2.42 1.81 1.29 1.08

0.50 1.38 1.47 1.27 1.21 1.21

0.44 0.42 0.48 0.49 0.59 0.65

8.38 9.62 6.08 4.24 2.93 2.96

36.44 27.17 34.42 40.19 43.95 44.49

Sample ID

Cd

Hd

Nd

St,d

Od

Mass loss/%

485

Note: Qnet, lower heating value; ad, on air-dried basis; d, on dry basis; St, total sulphur; Oxygen is calculated by

486

difference; coal and char are referenced by their abbreviated name and pyrolysis temperature (e.g.,SH600 and PZ600)..

487 488


Page 27 of 33

Feedstock M N?

Bottle gas

Reactor 4000

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

Energy & Fuels

Control cabinet ON/OFF

Flue Gas Analyzer

489 490 491 492

Char colleted chamber

Figure 1 Schematic of DTF pyrolysis installation


Energy & Fuels

0.15

Raw Coal o 600 C o 700 C o 800 C o 900 C o 1000 C

0.22

Raw Coal o 600 C o 700 C o 800 C o 900 C o 1000 C

0.20

Absorbance

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

Page 28 of 33

0.20 0.18 0.16 0.14 0.12

0.10

0.10 0.08 0.06

0.05

0.04 0.02 0.00

0.00 -0.02 0

500

1000

1500

2000

2500

-1

3000

3500

4000

4500

0

500

1000

Wavenumbers /cm

1500

2000

2500

-1

3000

Wavenumbers /cm

(a) Shenhua coal

(b) Pingzhuang coal

Figure 2 FTIR spectrum of Shenhua and Pingzhuang Coal 493


3500

4000

4500

Page 29 of 33

0.04

0.10

Original Curve Fitting Curve


0.02

Intensity

Intensity

0.05

0.00

3700

0.00

3600

3500

3400

3300

3200

3100

Wave Numbers /cm-1

3000

2900

3050

3000

2950

2900

2850

2800

2750

2700

2650

-1

Wave Numbers /cm

(a) SHraw 3600~3000

(b) SHraw 3000~2700

0.15

0.02

Original Curve


0.10

Fitting Curve

0.01

Intensity

Intensity

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

Energy & Fuels

0.05

0.00

0.00

1800

1600

1400

1200

1000

950

-1

Wave Numbers /cm

(c) SHraw 1800~1000

900

850

800

(d) SHraw 900~700

Figure 3 Results of peak separation of Shenhua raw coal 494


750 -1

Wave Numbers /cm

700

650

Energy & Fuels

495 60

50

H2

H2

CO CO2 CH4

40 30 20 10

CH4

40 30 20 10

0

0 600

496 497 498 499

CO CO2

50

Concentration /%

60

Concentration /%

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

Page 30 of 33

700

800

900

1000

600

700

800

900 o

1000

Pyrolysis Temperature / C Pyrolysis Temperature /oC (a) Shenhua coal (b) Pingzhuang coal Figure 4 Component of pyrolysis gas of Shenhua and Pingzhuang coal under different temperature in DTF


Page 31 of 33

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

Energy & Fuels

+ 3H2 (a) Aromatization Reaction

+ H2 (b) Derectly Cleavage Reaction

+ C4H 8

+ 2H2

(c) Condensation Reaction

CH3 + H2

500 501 502

+ CH4

(d) Hydrogenation Reaction

Figure 5 Typical secondary reactions generating H2 and CH4


Energy & Fuels

7

Shenhua Coal Pingzhuang Coal


0.8

6 0.6

Ac=o/Aar

Ahy/Aet

5

4

3

0.4

0.2

2 0.0

1

Raw coal

600

700

900

800

Raw Coal

1000

600

800

700

900

1000

Pyrolysis Temperature /oC


(a)Ahy/Aet

(b)Ac=o/Aar

1.4 4.0


1.2


3.5 3.0

1.0

Aar/Aal

2.5

CH3/CH2

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

Page 32 of 33

0.8

2.0 1.5

0.6

1.0

0.4

0.2

0.5 0.0

Raw Coal

600

700

800


900

1000

Raw Coal

600

700

(c)CH3/CH2 (d)Aar/Aal Figure 6 Changes in chemical structure after pyrolysis in DTF 503 504


800


900

1000

Page 33 of 33

1.0


0.8

Aromaticity

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

Energy & Fuels

0.6

0.4

0.2 Raw coal

600

700

800

900

1000

o

505 506

Pyrolysis Temperature / C

Figure 7 Changes of aromaticity after pyrolysis in DTF


Pyrolysis Characteristics and Evolution of Char Structure during

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