In Situ Catalytic Upgrading of Coal Pyrolysis Tar over Carbon-Based

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In-situ catalytic upgrading of coal pyrolysis tar over carbonbased catalysts coupled with CO2 reforming of methane Mingyi Wang, Lijun Jin, Yang Li, Jiaofei Wang, Xiaoyu Yang, and Haoquan Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01950 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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Energy & Fuels

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In-situ catalytic upgrading of coal pyrolysis tar over carbon-based

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catalysts coupled with CO2 reforming of methane

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Mingyi Wang, Lijun Jin, Yang Li, Jiaofei Wang, Xiaoyu Yang, Haoquan Hu*

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State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of

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Chemical Engineering, Dalian University of Technology, Dalian 116024, China

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Abstract

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In-situ catalytic upgrading of tar from the integrated process of CO2 reforming of methane

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with coal pyrolysis (CRMP) was investigated over carbon-based catalysts to increase light tar

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yield. The results showed that the light tar (boiling point < 360 °C) content and light tar yield

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increase when char, modified char (MC) or activated carbon (AC) were used as the catalysts. The

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effectiveness of tar upgrading was closely related with the properties of carbon catalysts.

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Compared with char or MC, AC showed a better catalytic effect in upgrading tar due to its higher

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disorder structure and specific surface area. Compared with non-upgrading tar, light tar content of

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above 88% was obtained and light tar yield was increased by 45.0% on AC catalyst at 650 oC. The

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contents of four light fractions increase, along with the obvious decrease of the content of pitch.

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2

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catalysts in upgrading process catalyze heavy tar cracking and CO2 reforming of methane reaction

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simultaneously, resulting in their interaction and high content and yield of light tar. In addition, the

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AC catalyst amount and flowrates of CO2 and methane affect the tar upgrading.

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Keywords: coal pyrolysis; tar; catalytic upgrading; CO2 reforming of methane; activated carbon

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1. Introduction

H NMR spectra analysis of the tar obtained by isotopic trace suggested that carbon-based

* Corresponding author. Tel. & Fax: +86-411-84986157 E-mail: [email protected] (Haoquan Hu)

ACS Paragon Plus Environment

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Pyrolysis is considered as an effective and clean technology for comprehensive utilization of

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low rank coal. Coal tar, gases and high heating-value char can be obtained via mild conversion of

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coal. Because coal has low molar ratio of hydrogen to carbon, the resultant tar from pyrolysis

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process is limited 1, and always has large amount of heavy fractions (boiling point > 360 °C)

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These heavy tar fractions may foul and/or block downstream apparatus and pipelines because of

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high corrosion and viscosity. Therefore, it’s necessary to upgrade heavy components into more

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high-value light fractions.

2-4

.

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Coal tar upgrading is a process to improve the H/C molar ratio of tar. Therefore,

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decarburization or hydrogenation of coal tar should be taken to improve light tar yield. Besides

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thermal cracking, catalytic upgrading of coal pyrolysis volatiles, which presents an alternative

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method to convert heavy fractions into light liquid fractions and more gases, has being widely

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studied

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reduce heavy tar component. Han et al.

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bituminous coal pyrolysis tar through catalytic upgrading over Co-, Ni-, Cu-, Zn- modified char.

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They found that the upgrading process led to decrease of tar yield, but higher yield and fraction of

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light tar. Li et al.

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upgrading of biomass pyrolysis vapors. They found Fe/HZSM-5 and Zr/HZSM-5 demonstrated

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better performance in the catalytic process than HZSM-5. Compared with metal catalysts, some

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carbon catalysts seem to be more appropriate for industrial application because of cheap starting

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material and easy to get 11-14. Some properties, such as the disorder structure, specific surface area,

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and sulfur resistance ability of carbon-based catalysts are beneficial to tar upgrading. Gilbert et al.

44

15

5-10

. Deng et al. 5 utilized catalytic cracking ability of olivine and Co-modified olivine to

10

8

focused on improving the quality of a Chinese

investigated the effects of Fe‐, Zr‐, and Co-modified zeolites on catalytic

investigated the catalytic upgrading effect of hot char at different temperatures. When the


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temperature increases from 500 oC to 800 oC, the heavy fraction content of biomass pyrolysis tar

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declines from 18.4 wt.% to 8.0 wt.%. Zeng et al.

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adsorb and disperse pyrolysis tar, and their higher specific surface area could prolong the resident

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time of tar contacting with catalysts, and enhance the cracking of tar. Jin et al. 17 also investigated

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the adsorption and catalytic function of carbon-based catalysts to pyrolysis tar, and they believed

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that defects and specific surface area of carbon-based catalysts played the primary role for

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upgrading tar.

16

thought carbon-based catalysts were able to

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Compared with thermal or catalytic cracking in N2 or other inert atmospheres, upgrading

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under H2 or other reactive gases could be an effective approach to improve coal tar quality and

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quantity by in-situ hydrogenation of coal tar. Chareonpanich et al.

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cracking of the pyrolysis products of Mae-Moh lignite coal in a two-stage reactor. They found 15

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wt.% of aromatic fractions was obtained under H2 atmosphere, which was about 3 times of that

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under N2 atmosphere at 600 °C, and the hydrocarbon yields from the upgrading of coal volatile

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over HY zeolite was approximately 3 wt.% (daf) more than that without catalyst. Takarada et al. 19

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investigated a subbituminous coal pyrolysis under H2 atmosphere in a pressurized fluidized bed.

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The light aromatic hydrocarbons yield was about 2.4 wt.% over Co-Mo/A12O3 catalyst at

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pyrolysis conditions of 580 °C and 1.0 MPa of H2, which was 2 times higher than that under He

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atmosphere. It is thought that some active H species from H2 could combine with the cracked

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fragments from tar, resulting in the increased content and yield of light tar in tar upgrading.

64

18

studied secondary catalytic

Our previous studies found that tar yield could be greatly improved when CO2 reforming of 20-22

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methane (CRM) was coupled with traditional coal pyrolysis (CRMP)

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that the active species like •CHx and •H from the activated methane take part in the formation of


. The analyses showed

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tar via combination of radicals from coal pyrolysis. As known, in coal tar upgrading, tar is also

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cracked into the fragments or radicals, which provides a possibility to combine with the radicals

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like •CHx and •H as in coal pyrolysis. However, to our best knowledge, few researches were

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reported on using methane as hydrogen source in catalytic upgrading of tar.

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In this paper, tar from CRMP process was in-situ upgraded over carbon-based catalysts based

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on two following aspects. On the one hand, high tar yield but relative high heavy component

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content can be obtained by CRMP; on the other hand, the unreacted CH4/CO2 in CRMP provide

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an opportunity to study the effect of CH4/CO2 on tar composition, expecting to provide a new

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method for tar upgrading. The role of carbon-based catalysts in tar upgrading was discussed and

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the effect of process conditions was also investigated.

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2. Experimental

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2.1. Coal and catalyst samples

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Buliangou (BLG) subbituminous coal, which was from Inner Mongolia of China, was used in

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this work. Before the pyrolysis experiment, coal sample was ground and sieved to the particle size

81

of below 80 mesh (< 0.178 mm), then was dried under vacuum at 65 oC for 24 h.

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Char, modified char (MC), and activated carbon (AC) were selected as tar upgrading

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catalysts in this work. In detail, pyrolysis char from CRMP at 650 oC was directly used as char

84

catalyst. MC was prepared by physical activation of char in steam at 800 oC for 3 h, and AC was

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obtained by chemical activation of the char sample. Char was first physically mixed with KOH in

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a mass ratio of 1:2, and then carbonized in N2 of 110 mL/min. The mixture of char and KOH was

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heated from 25 oC to 800 oC at a rate of 5 oC /min. After staying at the final temperature for 3 h

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and cooling down, the mixture was treated by a sequence of washing, filtrating and drying. Table 1


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Energy & Fuels

shows the proximate and ultimate analyses of BLG, char, MC and AC.

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In CRMP process, a commercial Ni/Al2O3 catalyst was used for CRM. The detailed

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components of the catalyst are listed in Table 2. After crushing and sieving, the catalyst with a

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particle size of 20-40 mesh was reduced by H2 at 650 °C for 3 h before use.

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2.2. Experimental apparatus and procedure

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In-situ catalytic upgrading of coal pyrolysis tar under N2 or CO2 reforming of methane

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atmosphere was performed in a vertical fixed-bed reactor, as shown in Fig. 1. In each experiment,

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the reactor was filled with Ni/Al2O3 reforming catalyst (1 g) for CRM, coal sample (5 g) and

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carbon-based upgrading catalyst (1 g) from top to bottom separated by quartz wool.

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N2 of 300 mL/min was used to purge the reactor for approximately 3 min to remove the air.

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Because the volume of reactor was about 45 mL (inner diameter of 14 mm and length of 290 mm),

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which was far less than the volume of N2, and there wasn’t any O2 being detected in the output

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gases by GC as shown in Fig. 5, so it could be ensured that all air in the reactor has been removed.

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After that, the mixture of methane (120 mL/min), CO2 (120 mL/min) and N2 (60 mL/min) was

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injected into the reactor before the pyrolysis experiment. The reactor was heated to the setting

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temperature (550-750 °C) in 10 min through a moving preheated furnace, and maintained for 30

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min. The gaseous products from CRMP entered into the catalytic upgrading layer. Liquid products

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and non-condensable gases were collected by a cold trap (-15 oC) and an aluminum foil bag,

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respectively. The char and spent catalyst were collected from the reactor after the experiment.

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For comparison, the tar from CRMP process was obtained at the same conditions as above

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except that no carbon-based catalysts were used.

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2.3. Characterization of catalyst


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N2 adsorption was used to measure the textural properties of char, MC and AC catalysts in a

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physical adsorption apparatus (ASAP 2420) at -196 oC. The specific surface area and pore volume

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were calculated through Brunauer-Emmett-Teller and Barrett-Joyner-Halenda methods, and the

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micropore volume (Vmic) was obtained by t-plot method. The textural properties of carbon-based

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catalysts were given in Table 3. Structural defects of carbon-based catalysts were measured by

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Raman spectra (DXR Microscope) and its specific parameter was shown in our previous

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research17.

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2.4. Analysis of pyrolysis products

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In accordance with ASTM D95-05e1 (2005), toluene was used as solvent to separate water

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and tar. Components analysis of tar was performed by simulated distillation GC (SCION 456-GC

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with CP-SimDist column) according to ASTM 2887. Before analysis, the tar was dissolved in CS2,

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and then Na2SO4 was used to absorb water in the tar. After the Na2SO4 was removed by filtration,

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a rotary evaporator was used to concentrate the tar with CS2. The boiling point ranges of tar

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fractions are listed in Table 4. The light tar is defined as the fractions that the boilong point is

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below 360 oC.

126 127

Dry and ash-free basis tar yield (Ytar) and light tar yield (Ylight tar) were calculated by the following equations: Ytar 

128 129

Wtar  100% Wo  (1  Aad  Mad )

Ylighttar  Ytar  w1%

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where, Wtar stands for the pyrolysis tar weight; W0, Aad and Mad represent the weight, ash content

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and moisture content of the coal sample, respectively; w1% represents the light tar content. Three

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equivalent experiments were repeated at least to get the average values of tar and gas yields, and


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the experimental relative error was within ± 2%, indicating good reproducibility. 2

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H NMR (400 MHz) analysis of the tar was measured by a Bruker Avance II 400 NMR

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spectrometer. Gas chromatography (GC7890Ⅱ) was employed to analyze pyrolysis gases, as

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described in our previous research 17.

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3. Results and discussion

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3.1. Catalytic upgrading effect of carbon-based catalysts on coal tar in CRMP

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3.1.1. Tar and light tar yields

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Fig. 2 presents total tar yield, light tar content and yield in coal pyrolysis with or without

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upgrading carbon-based catalysts. As shown in Fig. 2a, all the tar yields first increase with the

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increasing temperature up to 650 °C and then decline no matter whether carbon-based catalysts

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were used or not. However, compared with coal pyrolysis without upgrading catalyst, tar yield has

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a slight decrease from 15.2 wt.% to 15.1 wt.% over char catalyst, to 14.4 wt.% over MC and to

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13.1 wt.% over AC at 650 oC, indicating that the upgrading catalysts promote part of tar

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macromolecules cracking into light tar and gases. Compared with catalytic upgrading process

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under N2 atmosphere in our previous studies17, tar yield decreases more slightly when CH4 and

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CO2 added, indicating CRM process may happen over AC, which will be discussed in Section

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3.2.2.

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Fig. 2b illustrated the pyrolysis temperature effect on light tar content. Obviously, high

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pyrolysis temperature resulted in a slight decrease of light tar content. It is thought that high

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pyrolysis temperature promotes the cracking of coal, and some bulk macromolecules were formed.

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However, compared with coal pyrolysis tar alone, light fraction content of the upgraded tar is

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higher, which is closely related with the catalysts. At 650 °C, the light tar content increases from


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52.8 wt.% without upgrading to 54.6 wt.% over char, and further increases to 62.4 wt.% over MC.

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Han et al. 8 and Zeng et al. 16 believed that the activity for converting heavy fractions into light tar

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components over char catalyst was ascribed to the inherent transition metals, alkali and alkaline

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earth metals in char. In comparison, AC as upgrading catalyst can evidently improve light tar

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content, and the content of light fractions is 88.2 wt.% at 650 °C, suggesting that AC is beneficial

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to tar upgrading, which could be attributed to its more surface defects and large surface area.

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As seen from Table 3, the surface area of AC (1533 m2/g) is markedly higher than 758 m2/g

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of MC and 11 m2/g of pyrolysis char catalyst. Larger specific surface area can provide more active

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sites and increase retention time for heavy tar cracking 17, 23. Raman spectra of three carbon-based

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catalysts are compared in Fig. 3. The G band at around 1600 cm−1 is ascribed to ordered structure,

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and the D band at around 1350 cm−1 is assigned to the defects or disordered structure 24. ID/IG, the

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intensity ratio of D band to G band, is usually used to determine quantifying defect

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upgrading process, the defects and disordered structure in carbon-based catalysts can provide

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active sites

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centers for decomposition and CO2 reforming of methane

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significantly higher than MC and char, so it maybe another explanation for better tar upgrading

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effect of AC. The char catalyst exhibits poor upgrading behaviors, and light tar yield has a slight

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increase from 8.0 wt.% to 8.3 wt.% (Fig. 2c). MC and AC exhibit better catalytic upgrading effect

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than char catalyst, and the light tar yields are 9.0 wt.% and 11.6 wt.%, respectively, despite the

174

decreasing total tar yield, which increased by 12.5% and 45.0% compared to that without

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upgrading catalyst. Therefore, tar quality in CRMP can be enhanced when carbon-based catalysts

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were used in upgrading stage.

25

. In the

17

. At the same time, structural defects are also one of the most important active 23

. It can be seen that ID/IG of AC is


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3.1.2. Tar fractions

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To further acquire the information of tar components before and after upgrading tar and

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speculate the upgrading mechanism, the non-upgrading and upgrading tar obtained at 650 °C was

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analyzed by simulated distillation. As shown in Fig. 4, compared with the non-upgrading pyrolysis

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tar, almost similar components are obtained when char is used, which is quite different from those

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over other two catalysts, especially AC catalyst. The upgrading tar obtained by AC catalyst has

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high contents of light oil, phenol oil, naphthalene oil and wash oil, which remarkably increase by

184

120%, 161%, 84% and 50%, respectively. And the content of pitch is 11.8 wt.%, which decreases

185

by 75%, further confirming the conversion of coal pitch to light components.

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3.1.3. Gas composition

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During the upgrading tar, some small molecules are simultaneously produced along with

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catalytic cracking of heavy tar. Therefore, the analysis of gas products is helpful to understand

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upgrading mechanism of coal tar. The gas yield is calculated by subtracting the volume of the inlet

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gas from that of the outlet gas. The produced gases are mainly from three aspects: CO2 reforming

191

of methane, coal pyrolysis and tar upgrading process. Fig.5a illustrates the gas yields during the

192

whole process when different carbon catalysts are used. We can see that gas yield apparently

193

increases with reaction temperature whether carbon-based catalysts are added or not, but the

194

increment strongly depends on the carbon catalysts. When the char catalyst was used, almost

195

similar gas yield to that without upgrading was obtained, indicating poor upgrading performance

196

of char catalyst, which is accordant with the change trend of light tar content. As to the AC or

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modified char catalysts, the difference in gas yield between upgrading with non-upgrading became

198

more remarkable with the increasing temperature, which likely come from their different ability of


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tar cracking and catalytic reforming of CH4 over carbon catalysts. With the increasing temperature,

200

more macromolecules (like pitch) were converted to the light tar or gases over the catalysts.

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Owing to high catalytic performances in tar upgrading, the gas yield over AC catalyst is the

202

highest.

203

From the variation of specific gas component products at 650 oC in Fig. 5b, it can be found

204

that yields of CH4 and CO2 are minus, which is ascribed to the reaction CO2 reforming of methane

205

on Ni/Al2O3 before coal pyrolysis. Compared with non-upgrading process, however, the CH4 and

206

CO2 yields further decrease when carbon-based catalysts are used for tar upgrading, and yields of

207

H2 and CO increase, especially on the MC and AC catalysts. Generally, CH4 is mainly produced

208

by the cleavage of aliphatic hydrocarbons and aliphatic side chains in aromatic heterocyclic

209

structures or ethers

210

carboxyl groups at below 300 °C, and decomposition of stable ether, carbonates and

211

oxygen-bearing heterocycles at higher temperature. Once thermal or catalytic cracking of coal tar

212

happens, the yield of CH4 and CO2 should increase in theory. However, the fact of less CH4 and

213

CO2 in the outlet of upgrading process than those without upgrading suggests that some unreacted

214

CH4/CO2 in CRMP probably react again on the MC and AC catalysts during tar upgrading. The

215

obviously higher yields of H2 and CO in outlet gases over MC and AC than those without catalyst

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further confirm the speculation. In addition, the decomposition of the heterocycle structures 27 and

217

phenolic groups, and the condensation of aromatic structures probably also contribute to higher H2

218

and CO yields.

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3.2. Role of AC in upgrading the tar from CRMP

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17, 26

. CO2 is generated through decomposition of aliphatic and aromatic

The variations of yield and content of light tar and gas products indicated that carbon-based


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catalysts can decrease the content of heavy components, upgrade the pyrolysis tar and promote

222

CRM. To further figure out carbon-based catalysts’ role for tar upgrading in CRMP, AC with

223

better upgrading performance was chosen as the catalyst to investigate the function of

224

carbon-based catalysts.

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3.2.1. Effect of CRM on tar formation without upgrading catalyst

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Upgrading atmosphere will influence the tar formation and properties of the resultant tar. In

227

the process of upgrading tar under CRM atmosphere, the reaction of CO2 reforming of methane

228

may exert some functions through participating in the tar formation and upgrading tar. Therefore,

229

to clarify the role of CRM in the whole upgrading process clearly, coal pyrolysis under CRM or

230

N2 atmosphere without carbon-based catalyst were first investigated.

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As shown in Fig. 6a, tar yield under CRM was obviously higher than that under N2

232

atmosphere despite the similar change trend of tar yield with pyrolysis temperature, and the

233

difference in tar yield increases with pyrolysis temperature. Compared with that under N2, tar

234

yield under CRM reaches almost 1.4 times at 750 oC, which is in accordance with our previous

235

studies

236

stabilize the free radicals from coal pyrolysis 20. Because high reaction temperature is profitable to

237

CRM process and the formation of •H and •CHx, the difference of pyrolysis tar yield between

238

CRM and N2 atmospheres became more pronounced. Nevertheless, the interaction between •CHx

239

with free radicals from coal cracking resulted in a slight decrease of light tar content (Fig. 6b). At

240

650 °C, about 53 wt.% light tar content was obtained. Owing to high tar yield, as a result, light tar

241

yield under CRM increases to 8.0 wt.% from 6.5 wt.% under N2 at 650 °C, as shown in Fig. 6c.

242

Therefore, pyrolysis under CRM is profitable to improve the yields of total tar and light tar.

20, 22

. It is believed that the •H and •CHx generated from CRM over Ni/Al2O3 could


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3.2.2. Role of AC in tar upgrading in the mixture of CH4/CO2 The above results and our previous studies

20, 22, 28

showed that the active species formed

245

from CRM participate in the tar formation during the coal pyrolysis. The tar from CRMP process

246

was remarkably upgraded and light tar yield and contents were also enhanced when AC catalyst

247

was used. However, it is confused whether CH4 and CO2 also participated in the upgrading

248

process. To make it clear and ascertain the role of AC catalyst, the tar upgrading was investigated

249

under the mixture of CH4 and CO2 without reforming catalyst (abbreviated as MG) and N2 with or

250

without AC or SiO2 as the catalyst. In this section, experiment conditions are the same as those in

251

Section 2.2 except no Ni/Al2O3 reforming catalyst.

252

Fig. 7 gives the tar yield, light tar content and yield in different processes. Clearly, tar yield,

253

light tar content and yield under MG at different temperatures are almost the same as those under

254

N2 when no upgrading catalyst is used, indicating that the mixture gas of CO2 and CH4 has almost

255

no effect on coal pyrolysis in the investigated temperatures. When SiO2 is utilized in the

256

upgrading zone, no obvious changes in tar yield, light tar content and light tar yield happened

257

compared with no upgrading catalyst, indicative of no catalytic upgrading performance of SiO2,

258

which is accordant with our previous results 17.

259

When AC was used as the upgrading catalyst, total tar yield decreased no matter in MG or N2

260

atmosphere (Fig. 7a), suggesting its better ability to the decomposition of heavy compositions. In

261

comparison, tar yield in the mixture gas of CO2 and CH4 was slight higher than that in N2, and the

262

difference of the tar yield gradually became obvious with the increasing temperature. It is

263

speculated that the mixture gas of CO2 and CH4 could participate in the upgrading process.

264

Different from almost the same content of light tar over SiO2 as that under MG atmosphere


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265

without catalyst (Fig. 7b), the higher light tar content over AC suggested that the active sites for

266

tar upgrading are mainly from AC catalyst. As known, carbon catalyst can catalyze CO2 reforming

267

of methane despite its relative lower conversion than Ni catalyst

268

reaction can be accelerated at high temperature owing to the endothermic process. Some radicals,

269

like •H and •CHx, will be produced in CRM over AC. Therefore, it is speculated that these radicals

270

may combine with the radicals from catalytic cracking of tar to avoid excessive decomposition of

271

tar. Moreover, their interaction between •H and •CHx with those from tar cracking results in high

272

tar yield and light tar yield, and became more remarkable with the temperature.

23, 29, 30

, and the reforming

273

To confirm the deduction, the isotopic trace technique is used. CH4 was replaced by CD4 in

274

MG and the upgraded tar was analyzed by 2H NMR. According to the chemical shift, the total

275

signals are divided into two parts. Aromatic deuterons and aliphatic deuterons are absorbed in

276

6.3-9.3 ppm region and 0.5-4.3 ppm region, respectively 21. As seen from Fig. 8, it can be found

277

that there are no peaks of tar in MG without AC except a solvent peak at 4.6 ppm, indicating CD4

278

doesn’t take part in the formation of tar. The solvent peak is mainly from the trace amounts of

279

deuterium in CH2Cl2. However, when AC was added as the upgrading catalyst, the deuterium

280

amplitude in upgraded tar is obvious. Moreover, the peaks are almost at 0.5-4.3 ppm, which is

281

assigned to aliphatic deuterons. It is analyzed that some active species (like •D and •CDx)

282

generated from the reforming reaction are produced over carbon catalyst, and these •D and •CDx

283

participate in upgrading process via stabilizing the radicals from catalytic cracking of pyrolysis tar.

284

Based on the above-mentioned discussion, we can easily make conclusion that the role of AC in

285

this upgrading process is not only to improve catalytic cracking of heavy tar, but also to catalyze

286

CRM reaction in upgrading process.


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

287

3.3. Influence of process conditions on tar upgrading over AC catalyst

288

3.3.1. Weight ratio of AC to coal

289

The above discussion shows that AC catalyst can provide some active sites for tar cracking

290

and CO2 reforming of CH4, so the amount of catalysts will influence the tar upgrading. To confirm

291

it, the experiments with different AC amount were carried out. As shown in Fig. 9, with the weight

292

ratio of AC to coal increasing from 0 to 0.2, light tar content increases significantly from 52.8 wt.%

293

to 88.2 wt.%, accompanying the decline of total tar yield from 15.2% to 13.1 wt.%. However,

294

when further increasing AC amount, the enhancement of light tar content became slowly,

295

suggesting that most of heavy components in tar volatiles have been decomposed into light tar or

296

gases, and the increase of light tar content is mainly from the CO2 reforming of CH4 on AC

297

catalyst. About 11.6 wt.% yield of light tar was obtained at the weight ratio of AC to coal being

298

0.2, and kept almost constant when the weight ratio exceeds 0.2.

299

3.3.2. Gas flowrate

300

Gas flowrate will affect the residence time of gaseous tar in pyrolysis zone and upgrading

301

zone as well as the upgrading catalyst. It is necessary to investigate the influence of flowrate on

302

the upgrading of tar from CRMP when AC is used as the catalyst. Fig. 10 compares the tar yield,

303

light tar content and yield under different gas flowrate. The flowrate greatly affected the tar yield

304

and the content of light tar, i.e. high flowrate results in the increase of tar yield and decrease of

305

light tar content. These can be explained that the increase of flowrate promotes the formation of

306

more •CHx radicals, and the interaction between pyrolysis radicals and •CHx radicals, resulting in

307

high tar yield. But residence time of pyrolysis volatiles on the upgrading catalyst AC is shorten at

308

high flowrate, so the tar upgrading effect is weakened, leading to the decrease of light tar content


Page 14 of 32

Page 15 of 32

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

309

from 90.9 wt.% to 85.8 wt.% when flowrate increases from 100 mL/min to 400 mL/min. The

310

highest yield of light tar can be acquired at the flowrate of 300 mL/min.

311

4. Conclusions

312

In-situ catalytic upgrading of coal tar from CRMP was performed over char, MC and AC

313

catalysts. The upgraded tar quality obviously suffered the influence of catalysts. Compared with

314

char and MC, AC catalyst exhibits better upgrading effect due to its more disordered structure,

315

higher surface area and pore volume. When AC is used, the light tar content and yield are both

316

remarkably enhanced, and the light tar yield is 45.0% more than that in the non-upgrading tar at

317

650 oC. The content of light oil, phenol oil, naphthalene oil and wash oil increase along with the

318

obvious decrease of the pitch content. 2H NMR spectra analysis of upgraded tar suggested both tar

319

cracking and CRM can take place over AC catalyst, and the increasing yield of light tar is related

320

with the participation of •H and •CHx generated from CO2 reforming of methane over upgrading

321

catalyst during the tar upgrading process. The mass ratio of AC to coal and gas flowrate influence

322

the tar upgrading effect and the optimal conditions are 0.2 of the mass ratio and 300 mL/min of

323

the gas mixture.

324

Acknowledgements

325

This work was performed with the support of the National Natural Science Foundation of

326

China (No.U1510101, U1503194), and the National Key R&D Program of China

327

(No.2016YFB0600301).

328

References

329

(1) Li, Z.; Xu, S.; Wei, Z.; Liu, S. Fuel 2007, 86, 353-359.

330

(2) Wang, J.; Lu, X.; Yao, J.; Lin, W.; Cui, L. Industrial & Engineering Chemistry Research


Energy & Fuels

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2005, 44, 463-470.

332

(3) Liang, P.; Wang, Z.; Bi, J. Fuel Processing Technology 2007, 88, 23-28.

333

(4) Qu, X.; Liang, P.; Wang, Z.; Zhang, R.; Sun, D.; Gong, X.; Gan, Z.; Bi, J. Chemical

334 335 336 337 338

Engineering & Technology 2011, 34, 61-68. (5) Deng, J.; Li, W.; Li, X.; Yu, C.; Feng, J.; Guo. X. Journal of Fuel Chemistry & Technology 2013, 41, 937-942. (6) Cao, J.; Shi, P.; Zhao, X.; Wei, X.; Takarada, T. Fuel Processing Technology 2014, 123, 34-40.

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(7) Li, G.; Yan, L.; Zhao, R.; Li, F. Fuel 2014, 130, 154-159.

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(8) Han, J.; Wang, X.; Yue, J.; Gao, S.; Xu, G. Fuel Processing Technology 2014, 122, 98-106.

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(9) Veses, A.; Puértolas, B.; Callén, M. S.; García, T. Microporous and Mesoporous Materials

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2015, 209, 189-196.

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(10) Li, P.; Li, D.; Yang, H.; Wang, X.; Chen, H. Energy & Fuels 2016, 30, 3004-3013.

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(11) Hosokai, S.; Norinaga, K.; Kimura, T.; Nakano, M.; Li, C.; Hayashi, J. Energy & Fuels

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2011, 25, 5387-5393.

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(12) El-Rub, Z. A.; Bramer, E. A.; Brem, G. Fuel 2008, 87, 2243-2252.

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(13) Zhang, L. X.; Matsuhara, T.; Kudo, S.; Hayashi, J. I.; Norinaga, K. Fuel 2013, 112,

348 349 350 351 352

681-686. (14) Li, X.; Ma, J.; Li, L.; Li, B.; Feng, J.; Turmel, W.; Li, W. Fuel Processing Technology 2016, 143, 79-85. (15) Gilbert, P.; Ryu, C.; Sharifi, V.; Swithenbank, J. Bioresource Technology 2009, 100, 6045-51.


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(16) Zeng, X.; Wang, Y.; Yu, J.; Wu, S.; Han, J.; Xu, S.; Xu, G. Energy & Fuels 2011, 25, 5242-5249. (17) Jin, L.; Bai, X.; Li, Y.; Dong, C.; Hu, H.; Li, X. Fuel Processing Technology 2016, 147, 41-46. (18) Chareonpanich, M.; Boonfueng, T.; Limtrakul, J. Fuel Processing Technology 2002, 79, 171-179. (19) Takarada, T.; Tonishi, T.; Fusegawa, Y.; Morishita, K.; Nakagawa, N.; Kato, K. Fuel 1993, 72, 921-926.

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(20) Wang, P.; Jin, L.; Liu, J.; Zhu, S.; Hu, H. Energy & Fuels 2010, 24, 4402-4407.

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(21) Wang, P.; Jin, L.; Liu, J.; Zhu, S.; Hu, H. Fuel 2013, 104, 14-21.

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(22) Liu, J.; Hu, H.; Jin, L.; Wang, P.; Zhu, S. Fuel Processing Technology 2010, 91, 419-423.

364

(23) Fidalgo, B.; Menédez, J. Á. Chinese Journal of Catalysis 2011, 32, 207-216.

365

(24) Sheng, C. Fuel 2007, 86, 2316-2324.

366

(25) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Nano Letters 2010,

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10, 751-758.

368

(26) Mráziková, J.; Sindler, S.; Veverka, L.; Macák, J. Fuel 1986, 65, 342-345.

369

(27) Hodek, W.; Kirschstein, J.; Heek, K. H. V. Fuel 1991, 70, 424-428.

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(28) He, X.; Jin, L.; Wang, D.; Zhao, Y.; Zhu, S.; Hu, H. Energy & Fuels 2011, 25, 4036-4042.

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(29) Bermúdez, J. M.; Arenillas, A.; Menéndez, J. A. International Journal of Hydrogen

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Energy 2011, 36, 13361-13368. (30) Bermúdez, J. M.; Fidalgo, B.; Arenillas, A.; Menéndez, J. A. Fuel 2010, 89, 2897-2902.

374


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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 18 of 32

375 Table 1 Proximate and ultimate analyses of BLG coal and carbon-based catalysts

376

Proximate analysis (wt. %)

Sample BLG coal Char MC AC 377

*

Ultimate analysis (wt.% daf)

Mad

Ad

Vdaf

C

H

N

S

O*

2.11 1.06 1.08 1.22

15.37 20.50 20.68 12.51

37.68 9.09 4.21 3.64

77.50 92.13 95.40 96.15

4.62 2.29 1.65 0.98

1.27 1.79 1.57 1.46

0.84 0.50 0.46 0.52

15.77 3.29 0.92 0.89

By difference

378


Page 19 of 32

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

379 380

Table 2 XRF analysis of an industrial Ni/Al2O3 catalyst (wt.%) Composition

Al2O3

NiO

CaO

La2O3

SiO2

Na2O

Fe2O3

Content (wt.%)

60.1

27.4

6.5

4.5

0.7

0.5

0.3

381 382


Energy & Fuels

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Page 20 of 32

383 384 Sample Char MC AC

Table 3 Textural properties of carbon-based catalysts Smic Sext Vt SBET (m2/g) (m2/g) (m2/g) (cm3/g) 11 5 6 / 758 426 332 0.39 1533 1086 447 0.75

385 386


Vmic (cm3/g) / 0.19 0.48

Dave (nm) 3.61 2.08 2.11

Page 21 of 32

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

387 Table 4 Boiling points range for classifying tar fractions

388

Light tar

Tar fractions

Boiling point range（oC）

Light oil Phenol oil Naphthalene oil Wash oil Anthracene oil

< 170 170-210 210-230 230-300 300-360

Pitch

> 360

389 390


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

391

Figure Captions

392

Fig. 1 Schematic of the experimental apparatus

393

Fig. 2 Tar yield (a), light tar content (b) and light tar yield (c) over different catalysts at different

394

temperatures under CRM atmosphere

395

Fig. 3 Raman spectra of three carbon-based catalysts

396

Fig. 4 Effect of upgrading catalyst on the components of tar obtained at 650 oC under CRM

397 398 399 400 401

atmosphere Fig. 5 Effect of upgrading catalyst on gas yield at different temperature (a) and variation of gas components obtained at 650 oC (b) under CRM atmosphere Fig. 6 Tar yield (a), light tar content (b) and light tar yield (c) in coal pyrolysis under CRM and N2 atmospheres

402

Fig. 7 Tar yield (a), light tar content (b) and light tar yield (c) in different pyrolysis processes

403

Fig. 8 2H NMR spectra of tar from coal pyrolysis in MG with and without AC at 650 oC

404

Fig. 9 Tar yield, light tar content and yield obtained with different weight ratio of AC to coal at

405 406

650 oC Fig. 10 Tar yield, light tar content and yield obtained in different gas mixture flowrate at 650 oC

407


Page 22 of 32

Page 23 of 32

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

408 409 410 411

Fig. 1 Schematic of the experimental apparatus


Energy & Fuels

a

16

Tar yield (wt.% daf)

15 14 13 without catalyst Char MC AC

12 11 10

550

600 650 700 Temperature (oC)

750

550


750

550


750

412

Light tar content (wt.%)

b 100 80 60 40 20

413

c

14

Light tar yield (wt.% daf)

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 24 of 32

12 10 8 6

414 415 416 417 418

Fig. 2 Tar yield (a), light tar content (b) and light tar yield (c) over different catalysts at different temperatures under CRM atmosphere


Page 25 of 32

G

D AC

Intensity (a.u.)

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

MC

Char 3000

419

2500

2000 1500 1000 Wavenumber (cm-1)

500

420 421

Fig. 3 Raman spectra of three carbon-based catalysts

422


Energy & Fuels

50 40 Content (wt.%)

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

30

without catalyst Char MC AC

20 10 0

423

Page 26 of 32

oi l o il o il l o il t oi l Ligh Pheno thalene Washhracene h A nt Na p

P it c

h

424 425 426

Fig. 4 Effect of upgrading catalyst on the components of tar obtained at 650 oC under CRM atmosphere

427


Page 27 of 32

a1200


400 Volume (mL/g daf)

800 600 400

200 0

-200

200 0

428

b 600


1000 Gas yield (mL/g daf)

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

550


-400

750

CH4

CO2

H2

CO

C2-C3

429 430

Fig. 5 Effect of upgrading catalyst on gas yield at different temperature (a) and variation of gas

431

components obtained at 650 oC (b) under CRM atmosphere

432 433


Energy & Fuels

a

16


15 14 13 12 N2

11 10

CRM 550


750

550


750

550


750

434

b

60

Light tar content (%)

58 56 54 52 50 48

435

c

9

Light tar yield (wt.% daf)

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 32

8 7 6 5

436 437 438 439 440

Fig. 6 Tar yield (a), light tar content (b) and light tar yield (c) in coal pyrolysis under CRM and N2 atmospheres


Page 29 of 32

a

20


16

MG

MG+SiO2

N2

MG+AC

N2+AC

12 8 4 0

550

441


750


750


750


b 100 80 60 40 20 0

550

442

c Light tar yield (wt.% daf)

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

10 8 6 4 2 0

443

550

444 445

Fig. 7 Tar yield (a), light tar content (b) and light tar yield (c) in different pyrolysis processes

446 447


Energy & Fuels

Aliphatic deuteriums Intensity (a.u.)

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 32

Aromatic deuteriums with AC

without AC 10 9

448 449 450

8

7

6 5 4 (ppm)

3

2

1

0

Fig. 8 2H NMR spectra of tar from coal pyrolysis in MG with and without AC at 650 oC


Page 31 of 32

451 16

100

14

90

12

80

10 8 6

452

70 Tar yield Light tar yield Light tar content 0.0

0.1 0.2 0.3 0.4 Weight ratio of AC to coal

60


Yield (wt.% daf)

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

50

453 454 455 456

Fig. 9 Tar yield, light tar content and yield obtained with different weight ratio of AC to coal at 650 oC


Energy & Fuels

90

14

80 12 70 10 8

457

Tar yield Light tar yield Light tar content 100 200 300 400 Flow rate of gas mixture (mL/min)

60


100

16

Yield (wt.% daf)

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 32

50

458 459

Fig. 10 Tar yield, light tar content and yield obtained in different gas mixture flowrate at 650 oC

460


In Situ Catalytic Upgrading of Coal Pyrolysis Tar over Carbon-Based

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