Distribution of trace elements during coal Gasification:The effect of upgrading method

Distribution of trace elements during coal Gasification:The effect of upgrading method

Accepted Manuscript Distribution of Trace Elements during Coal Gasification:The Effect of Upgrading Method Guanyi Chen, Yunan Sun, Beibei Yan, Ruilei...

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Accepted Manuscript Distribution of Trace Elements during Coal Gasification:The Effect of Upgrading Method

Guanyi Chen, Yunan Sun, Beibei Yan, Ruilei Yang, Bin Liu, Zhanjun Cheng, Wenchao Ma PII:

S0959-6526(18)31096-5

DOI:

10.1016/j.jclepro.2018.04.077

Reference:

JCLP 12658

To appear in:

Journal of Cleaner Production

Received Date:

08 November 2017

Revised Date:

01 March 2018

Accepted Date:

09 April 2018

Please cite this article as: Guanyi Chen, Yunan Sun, Beibei Yan, Ruilei Yang, Bin Liu, Zhanjun Cheng, Wenchao Ma, Distribution of Trace Elements during Coal Gasification:The Effect of Upgrading Method, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.04.077

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

5910 words in total

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Distribution of Trace Elements during Coal Gasification:The Effect of

3

Upgrading Method

4

Guanyi Chena,b,c, Yunan Suna, Beibei Yana,d*, Ruilei Yanga, Bin Liue, Zhanjun Chenga,c,

5

Wenchao Maa,c

6

a

School of Environmental Science and Engineering/State Key Lab of Engines, Tianjin University, Tianjin 300072, China

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b

School of Science, Tibet University, No.36 Jiangsu Street, Lhasa 850012, Tibet Autonomous Region, China

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c

Tianjin Engineering Center of Biomass-derived Gas/Oil Technology, Tianjin300072, China

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d

Key laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education, School of

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Mechanical Engineering, Tianjin 300072, China

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e

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Abstract:

Qingdao Institute for Ocean Engineering of Tianjin University, Qingdao 266237, China

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A combination of upgrading and gasification process seems quite promising to cleanly utilize low-

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rank coal (LRC). The distribution of trace elements in this process has rarely been researched. In this

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study, four upgrading methods for LRC (evaporative drying, pyrolysis upgrading, microwave drying and

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hydrothermal dewatering (HTD)) were used and then the upgraded coal (UC) was gasified with

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investigation on the distribution and emissions of 13 trace elements ( beryllium, vanadium, chromium,

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manganese, cobalt, nickel, copper, zinc, arsenic, selenium, cadmium, mercury and lead). The relative

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enrichment factors (RE) were introduced to quantitatively compare four different upgrading methods.

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The results show that gasification of UC helps controlling trace element of LRC by reducing its contents

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in feedstock and enhancing the ash enrichment. Different upgrading methods have their specific effects

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on different trace elements, respectively. Microwave drying presents to be best for most trace elements

ACCEPTED MANUSCRIPT 23

enriched to ash, in particular for beryllium, vanadium, chromium, manganese, cobalt, copper. Majority

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of 13 elements could reduce roughly 50% of the content through microwave drying method, and the

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reduction of lead reaches 72.5%. It seems the upgrading is quite effective in controlling emissions of

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trace elements from coal gasification.

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Key words: Low-rank coal; upgrading method; trace elements; gasification; enrichment.

1. Introduction

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Coal is the primary source of energy in China at present and will continuously dominate the energy

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sector in a long term (Li et al., 2010; Liu et al., 2016). Low-rank coal (LRC), including lignite and sub-

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bituminous coal with high moisture and ash contents (Hassan Katalambula, 2009), is quite abundant and

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greatly exploited, and therefore has been paid enormous attention (Jangam et al., 2011). High moisture

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and ash contents of LRC usually lead to low heating value, more transportation cost and serious emission

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problem. Different utilization methods of LRC are thus extensively researched, among which upgrading

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and gasification techniques present to be quite promising.

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Evaporative drying, pyrolysis upgrading, microwave drying and hydrothermal dewatering are

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widely used in coal industrial application (Amy C. Merdes, 1998; Graham, 2008; Hassan Katalambula,

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2009; Jangam et al., 2011; Min Wang, 2000; Nikolopoulos et al., 2015; Wu et al., 2015; Xu and Xiong,

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2013). Through these upgrading methods, the moisture and ash contents of LRC could be significantly

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reduced, and even some minerals or related harmful constituents could be partly removed (Hassan

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Katalambula, 2009; Vassilev et al., 2001; Wu et al., 2015). Different with upgrading of coal, gasification

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technology can transfer the energy carrier from solid phase into gaseous phase, resulting in subsequent

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clean combustion (Wen et al., 2016). Gasification of UC produces gas product (fuel gas) with higher

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heating value and less pollutants emission compared with gasification of LRC.

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Trace elements, which are usually less than 100μg/g in coal (Schweinfurth, 2003; Sia and Abdullah,

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2011), include Hg, Mn, As, Se, Pb, etc. Some toxic trace elements such as heavy metals could make great

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harm to the environment with very small amount. Moreover, the huge consumption of coal in China

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makes the amount of trace elements no more a negligible number (Dai et al., 2012; Zhu et al., 2016).

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Therefore, study on trace elements behavior (distribution and emissions) in coal combustion and

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gasification has aroused growing interest, focusing on distribution of trace elements during coal

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combustion in power plant (Bhangare et al., 2011; Tang et al., 2013), the classification of trace elements

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based on their volatility (Bunt and Waanders, 2008, 2009, 2010), emissions of trace elements from coal

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gasification (L. B. Clark, 1992; Yoshiie et al., 2013) and thermodynamic study on behavior of trace

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elements in fluidized bed gasification (Konttinen et al., 2013). With gasification of UC becoming a

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promising utilization method of LRC, the distribution of trace elements during this process is supposed

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to be better understood, so that the utilization process of LRC could cause less environmental impact.

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While according to publications we can reach, few efforts have been made in this field.

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13 common toxic trace elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd, Hg and Pb ) with

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various volatility were chosen to be studied in this work with reference to the above-mentioned research.

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Considering the effect of upgrading methods on distribution of trace elements during gasification of UC

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is quite few in the literatures, our work seems interestingly innovative. The variation of trace element

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content, and its enrichment in ash through UC gasification is studied here. In addition, the various effects

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of different upgrading methods (evaporative drying, pyrolysis upgrading, microwave drying and

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hydrothermal dewatering (HTD)) on distribution of trace elements during UC gasification are

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investigated.

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2. Material and Methods

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2.1 Materials preparation

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LRC was obtained from a thermal power plant in Tianjin, China. After milled and sieved through a

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150μm sieve, the LRC sample was stored in sealed bags and stored bags in an airtight container before

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being used. Table 1 shows four upgrading methods and experiment conditions. There are a few rules

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when determining upgrading methods in this study: (1) Upgrading methods widely used in industry scale

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are preferred, for which evaporative drying method was chosen; (2) Upgrading methods which are too

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simple are not considered, for which coal briquetting method was not chosen; (3) Upgrading methods

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with promising prospect or extensive research were preferred, for which pyrolysis upgrading, microwave

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drying and hydrothermal dewatering were chosen (Amy C. Merdes, 1998; Graham, 2008; Hassan

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Katalambula, 2009; Jangam et al., 2011; Min Wang, 2000; Nikolopoulos et al., 2015; Wu et al., 2015;

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Xu and Xiong, 2013). The experiment condition of each upgrading method was chosen in a proper range

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(Kang et al., 2013; Min Wang, 2000; Shi, Study on the characteristic of lignite upgrading through hot

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gas and microwave dryness; Skodras et al., 2015; Wu et al., 2015; Zou, 2013). A fixed bed was used in

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evaporative drying process of LRC for better accessibility. The reactor was heated to 200℃ at a heating

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rate of 10℃/min, and kept for an hour. Later the reactor was cool down to the room temperature naturally,

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the evaporative upgraded coal was removed into sealed bags labeled UC-1 and was kept dry. A tube

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furnace was used for pyrolysis upgrade with nitrogen atmosphere heated to 480℃ at a heating rate of

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15℃/min which was continued for 1 hour, and then the pyrolysate was cooled to the room temperature

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naturally. Then the semi-coke gotten from the quartz boat was stored in sealed bags labeled UC-2 and

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stored bags in an airtight container before being used. Microwave drying was performed with a

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microwave tube furnace. With a nitrogen atmosphere of 650ml/min, LRC samples were put into a quartz

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tube of the microwave oven and was heated to 200℃ by 600W microwave power. The power of heat

ACCEPTED MANUSCRIPT 89

preserved with 400W to keep drying for 10min. Upgraded coal samples were dried in a drying oven at

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60℃ for 24h, and stored in sealed bags which were labeled as UC-3 and stored bags in an airtight

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container before being used. A 100ml cylindrical autoclave (Parr Instrument Company-4848) equipped

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with an automatic temperature and stirred controller was used for HTD upgrade. Almost 13 grams of

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LRC and 39ml distilled water were mixed into the coal/water slurry. Under initial 4MPa N2 atmosphere,

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the reactor was heated to 320℃ automatically (Wu et al., 2015). After keeping at 320℃ and 12MPa for

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1hour, the autoclave was cooled down to the room temperature naturally. Qualitative filter paper was

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used to separate the solid and liquid products. HTD upgrade coal samples were preserved after being

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dried and kept in sealing bags labeled UC-4 for later use. Each sample was stored in sealing bags under

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air dry basis.

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Table 1

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Upgrading methods and experiment conditions

Upgrading methods

1

2

3

4

Evaporative

Pyrolysis

Microwave

Hydrothermal

drying

upgrading

drying

dewatering

10g

15g

30g

13gLRC+39ml LRC

distilled water 200℃, 1h,

480℃, 1h,

200℃, 10min, 320℃, 1h, Initial

Upgrading conditions

200ml/min

300ml/min

650ml/min 4MPa nitrogen

nitrogen

nitrogen

nitrogen

Upgraded coal (UC)

UC-1

UC-2

UC-3

UC-4

Gasification ash of UC (A)

A1

A2

A3

A4

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2.2 Coal gasification

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The gasification of LRC and UC were performed in a tube furnace at 950℃ using carbon dioxide

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as gasifying agent (Kale et al., 2014; Veca and Adrover, 2014). Firstly, the tube furnace was heated to

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950℃ from the room temperature at a heating rate of 15℃/min and 0.5g coal samples was prepared at

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one end of the tube out of the furnace. Then coal sample was pushed into the middle of the reactor and

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gasified for about one hour in 600ml/min CO2. The ash samples of each coal sample were collected from

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the quartz boat after gasification. The ash of LRC was labeled A0 and A1-A4 corresponded to the ash of

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UC1-UC4, respectively. Gas product was not collected for investigation for both resource limitation and

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difficulty in implementing.

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2.3 Analytical test

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Table 2 shows the ultimate and proximate analysis results of LRC and four types of UC, which

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including moisture content (Mad), ash content (Aad), volatile matter content (Vad) , fixed carbon content

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(FCad) and contents of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S) and net heating

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value (Qnet,ad) in air dry basis. Proximate and ultimate analysis of samples were conducted on

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thermogravimetric analyzer (SDT-Q600, DSC-TGA) and elemental analyzer (Vario Micro cube,

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Elementar), respectively. Ash and oxygen content were calculated by difference. The Qnet,ad of LRC and

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4 types of UC were calculated by two empirical formulae as formula (1) - (2), and the average was used

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for evaluation of Qnet,ad (Hu Jun et al., Identifying and Appraising of Coal Calorific Experience Formula).

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All prepared materials were stored in room atmosphere prior to being used.

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Qnet,ad=34814-24.7Aad-382.2Vad-563.0Mad

(1)

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Qnet,ad=33323-417.8Aad+251.8Vad-362.9Mad

(2)

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The mineral composition of LRC and UC was studied using X-ray Diffract-meter (XRD), which

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was conducted on D/MAX-2500 (Rigaku, Japan) by using Cu Ka radiation and a step-scan pattern

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(scanning range: 2θ, 5°-60°; scanning speed: 4°/min ).

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The concentrations of the trace elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd, Hg and Pb )

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in each coal or ash sample (LRC and four types of UC) were determined by ICP-MS (Agilent 7700x)

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with microwave digestion pretreatment. HNO3 was chosen to make microwave digestion pretreatment

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for convenience and feasibility. A microwave digestion instrument (PyNN-CEM MARS HACKER) with

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a temperature of 230℃ was used. 0.1g coal or gasification ash sample was digested in 9ml HNO3 each

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time. The mixture sample was heated up from room temperature to 230℃ and kept for 30min. After

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digestion, the digestion solution was evaporated on a heating oven with 60℃ until the solution became

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0.5-1.0ml. Then each sample was cooled down to the room temperature and diluted to 100ml volume.

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After filtration, trace element content in these clear liquid samples was detected by ICP-MS with

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detection limits of 10-9 (Oboirien et al., 2014). Each sample has been detected for three times in the same

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way, and the average of each test was used to improve accuracy.

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Table 2

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Ultimate and proximate analysis results of LRC and UC. Coal

Ultimate analysis (wt%)

Net calorific value

Proximate analysis (wt%)

samples

Cad

Had

Oad*

Nad

Sad

Qnet,ad(MJ/kg)

Mad

Aad*

Vad

FCad

LRC

60.41

3.87

8.50

0.77

0.40

26.80

2.61

23.44

24.90

49.05

UC-1

64.17

3.71

15.20

0.79

0.31

28.89

2.18

13.65

29.14

55.03

UC-2

67.66

2.62

10.47

0.82

0.55

29.69

0.85

17.03

14.72

67.40

UC-3

65.33

3.93

15.18

0.78

0.42

29.45

1.10

13.27

29.44

56.19

UC-4

61.92

3.54

10.51

0.90

0.49

27.87

1.61

21.05

23.67

53.68

ACCEPTED MANUSCRIPT 138

*Calculated

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3. Results and discussion

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3.1 The distribution of trace elements

by balance; ad: air dry basis.

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The XRD pattern of LRC, 4 types of UC and their ash samples were showed in Fig. 1 and Fig. 2,

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respectively. Vejahati et al. (Vejahati et al., 2010) made a review on associations of coal with minerals,

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and pointed out that the coal-mineral associations play an important role in the formation of fine particle

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and in condensation of trace elements in ash particles. As shown in Fig. 1, the peak of CaCO3 in UC-4 is

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hard to find, and the peak of CaCO3 in UC-3 is significantly higher than others. The peak of kaolinite in

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UC-2 is smaller than that in others. Thus, kaolinite and CaCO3 was changed when using different

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upgrading methods. Fig. 2 shows little difference among ashes of LRC and UC, however, concentration

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of kaolinite is somewhat different. Clark and Sloss (L. B. Clark, 1992) presented the modes of occurrence

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of trace elements in coal could be associated with mineral parts with chemically bound, physically bound

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or discrete minerals. Querol et al. (Querol et al., 1995) have given several affinities of minerals and trace

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elements, which shows that: Cr, Cu, and V have association with clay minerals and feldspars such as

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kaolinite or montmorillonite; As, Cd, Co, Hg, Ni, Pb, Se, Zn and Cu have association with iron sulfides

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such as pyrite or sphalerite; Co and Mn have association with carbonates such as calcite or dolomite; Ni,

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Cu and Pb show inorganic affinity in clay minerals and sulfides mineral phases. It implies that the

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variations in mineral formation and microstructure by upgrading methods might cause influences on the

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content and morphology of trace elements, and even change the distribution of trace elements during UC

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gasification.

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158 159

Fig. 1. XRD patterns of LRC and 4 UCs

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(K: kaolinite/Al2(SiO5)(OH)4; Q: quartz/SiO2; C: calcite/CaCO3)

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Fig. 2. XRD patterns of 5 ash samples

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(K: kaolinite/Al2(SiO5)(OH)4; Q: quartz/SiO2; A: Anorthite/CaAl2Si2O8)

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The contents of 13 trace elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd, Hg and Pb) in coal

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and its ash samples were listed in Table 3. A0 represents the ash from LRC gasification, and A1-A4

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represents the ash from UC1-UC4 gasification, respectively. Because of the reduction of moisture and

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ash content by upgrading method, the variety of trace elements content in coal is fuzzy. The ratio of trace

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element content in UC/ content in LRC was used to show the variation of content in feedstock in Fig. 3

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(a). The contents of Be, V, Cu, Zn, As and Se reduced during evaporative drying. The contents of V, Co,

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Cu, As, Se, Cd and Pb reduced during pyrolysis upgrading. The contents of Be, V, Cr, Mn, Co, Ni, Cu,

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As, Se, Cd and Pb reduced during microwave drying. The contents of V, As, Se and Cd reduced during

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HTD. Overall, microwave drying was best in reducing contents of trace elements, followed by pyrolysis

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upgrading. Majority of these elements could reduce roughly 50% of the content through microwave

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drying method, and the reduction of Cu, As and Pb reaches 62.1%, 63.4% and 72.5% respectively. Trace

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element content in UC gasification ash/ content in A0 was used to show the variation of content in ash

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in Fig. 3 (b). Almost all of these 13 trace elements’ contents in ash of UCs are much higher than those in

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ash of LRC except for Cu and Cd in A3 as well as As and Cd in A4. If the content of one element in UC

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was less than that in LRC, meanwhile its content in UC gasification ash was greater than that in A0, the

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upgrading method could be considered as effective on controlling trace element’s vaporization during

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gasification based on the conservation of mass. Therefore, it can be inferred that the volatilization of V

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and Se can be reduced through all these 4 upgrading methods and the volatilization of As can be reduced

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through evaporative drying, pyrolysis upgrading and microwave drying. Microwave drying presents to

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perform best in reducing volatilization of 9 elements except Cu, Zn, Cd and Hg. Nevertheless, the

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analysis is not completed by relying solely on the contents of trace elements in coal and its ash samples.

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Table 3

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Trace elements contents of coal and gasification ash (A) samples Contents LRC

A0

UC-1

A1

UC-2

A2

UC-3

A3

UC-4

A4

Bead

1.17

3.59

0.98

6.77

1.20

6.16

0.64

4.97

1.27

4.43

Vad

21.05

51.10

12.09

66.23

15.31

57.21

8.93

53.64

21.01

53.96

Crad

28.48

67.96

34.81

184.72

39.89

139.31

17.36

101.61

35.54

87.17

(μg/g)

ACCEPTED MANUSCRIPT

187

ad:

Mnad

136.93

387.70

145.74

802.74

177.07

686.76

88.46

564.93

158.05

403.80

Coad

6.58

23.70

7.02

41.83

6.42

37.86

4.07

37.24

6.96

27.55

Niad

13.61

44.56

16.31

90.90

15.00

69.64

7.84

57.23

47.70

325.55

Cuad

11.08

26.36

7.65

43.07

7.96

32.43

4.20

24.34

13.22

35.29

Znad

43.57

37.65

41.35

129.58

77.96

87.17

57.19

44.94

50.00

47.44

Asad

1.61

4.03

1.07

6.56

1.45

5.42

0.59

4.17

0.95

2.39

Sead

0.51

0.74

0.36

1.00

0.39

0.89

0.32

0.82

0.45

0.82

Cdad

4.25

1.03

5.26

18.44

2.12

1.07

1.84

0.79

4.23

0.99

Hgad

0.38

0.05

0.72

0.31

1.64

0.18

1.25

0.18

0.39

0.16

Pbad

10.43

1.16

22.87

24.19

6.65

1.28

2.87

1.80

15.04

1.66

air dry basis.

188 189

(a)Variation of content in feedstocka

(b) Variation of content in ashb

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Fig. 3. Variation of trace element content through upgrading methods

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(a: trace element content in UC/ content in LRC; b: trace element content in UC gasification ash/ content in A0)

192

3.2 Enrichment behavior of trace elements

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In order to make quantitative analysis for trace elements, their enrichment condition in ash was

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evaluated by the relative enrichment factor (RE). The RE of each trace element in ash was calculated

ACCEPTED MANUSCRIPT 195

based on formula (3) proposed by Meij (Meij, 1994).

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RE = (Ci, a / Ci, coal)*((%ash) coal/100)

(3)

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Ci,a : the content of i element in ash;

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Ci,coal : the content of i element in coal;

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(%ash)coal : the ash content in feedstock (including UC and LRC for gasification).

200

The greater the RE, the more trace elements enriched in the ash and fewer vented with fuel gas

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(Yoshiie et al., 2013). For instance, if the RE of element i in the ash is larger, the element i has a larger

202

trend to enrich in ash; otherwise, it means element i is more likely to vent with fuel gas. The RE of 13

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trace elements in 5 ash samples collected from LRC and 4 types of UC gasification were shown in Table

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4. A0 represents the ash of LRC gasification, and A1-A4 represents the ash of UC1-UC4 gasification.

205

Table 4

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RE of trace elements in ash of five coal samples during gasification RE

A0

A1

A2

A3

A4

Be

0.719

0.943

0.874

1.030

0.734

V

0.569

0.748

0.636

0.797

0.540

Cr

0.559

0.724

0.595

0.776

0.516

Mn

0.664

0.752

0.661

0.847

0.538

Co

0.844

0.814

1.004

1.214

0.833

Ni

0.767

0.761

0.791

0.968

1.436

Cu

0.558

0.769

0.694

0.769

0.562

Zn

0.203

0.428

0.190

0.104

0.200

As

0.587

0.837

0.637

0.938

0.529

ACCEPTED MANUSCRIPT

Se

0.340

0.379

0.389

0.340

0.383

Cd

0.057

0.479

0.086

0.057

0.049

Hg

0.031

0.059

0.019

0.019

0.086

Pb

0.026

0.144

0.033

0.083

0.023

207 208

Fig. 4. RE variation of trace elements through upgrading method

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RE variation ratio (RE of trace element in UC gasification ash/ RE in A0) was calculated, and ln(RE

210

variation ratio) was used to show the various effect of four upgrading methods in Fig. 4. Fig. 4 shows

211

that most RE of trace elements in ash of UC was greater than in ash of LRC. It can be concluded that

212

through upgrading process, trace elements could mostly enrich in ash rather than vent with fuel gas. For

213

a detailed comparison between each UC and LRC, it could be found that through evaporative drying, the

214

enrichment of almost all 13 trace elements, except for Co and Ni, could become strong. RE of Zn and

215

Hg in ash of UC-2 was weaker, and other trace elements in UC-2 showed better enrichment to ash than

216

in LRC. Also enrichment of Zn and Hg in ash still did not work through microwave drying, which is in

217

a similar situation with UC-2; HTD did not work well for most elements, but as Hg was hard to enrich

218

in ash. HTD had an outstanding effect on Hg as well as Ni.

ACCEPTED MANUSCRIPT 219

In order to better process the large data in an efficient and specific way, dynamic clustering analysis

220

was used to analyze the effect of different upgrading methods on these trace elements. Cluster analysis

221

is a data mining process which consists in dividing the samples into groups (clusters) based on

222

information found within the data which describes these samples and its relationships (Tan et al., 2006).

223

Samples belonging to the same cluster must show a similarity pattern among them while being as

224

dissimilar as possible from samples associated to other clusters (Maione et al., 2018). Dynamic clustering

225

analysis is one of the most widely used cluster analysis methods, which is efficient to deal with samples

226

of high complexity, and without the need for compound specific, predefined parameters (Andersen et al.,

227

2016). The whole analysis process was conducted with R software in this research. When setting the

228

analyzing parameters, grouping type of 7 was found optimal. In the initial grouping results, Cd, Hg, Pb,

229

Ni and Zn were in separate groups, respectively. To cluster these elements by their suitable upgrading

230

methods, the groups were rearranged as shown in Table 5. The RE variation ratio of Cd in UC-1, Hg in

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UC-4 and Pb in UC-1/UC-3 were significantly higher than others. The RE variation ratio of Cd in UC-1

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can reach 8.40, and the RE variation ratio of Hg in UC-4 can reach 2.77. The RE variation ratio of Pb in

233

UC-1 and UC-3 can reach 5.54 and 3.19, respectively. The RE variation ratio of others are almost less

234

than 2.00. Above all, evaporative drying, pyrolysis upgrading and microwave drying all work well on

235

most trace element. Evaporative drying method worked better for Cu, Zn, Cd and Pb. The pyrolysis

236

upgrading method worked better for Se. The microwave drying method worked better for Be, V, Cr, Mn,

237

Co, Cu, As. Thus, microwave drying seems most prominent.

238

Table 5

239

Trace elements grouping and their characteristics through cluster analysis Group

Trace elements

Characteristics

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the difference of sensibility to four upgrading methods are relatively little I

Be, Co, Cu, Se than others much more sensitive to the evaporative drying method and microwave

II

Pb, V, Cr, Mn, As drying method

III

Ni, Hg

much more sensitive to the HTD method

IV

Cd, Zn

much more sensitive to the evaporative drying method

240

The remaining distribution of these trace elements was also compared. As shown in Fig. 5 where

241

these elements were re-ranked by their RE values from large to small, upgrading processes didn’t change

242

their remaining distribution circumstances. Grouped by dynamic clustering analysis method, these 13

243

elements could be separated into four groups, depending on the RE of them in A0 and Aa (average value

244

of A1-A4). Group A: Co, Ni, Be, these three elements could remain more in ash compared to others;

245

Group B: Mn, As, V, Cr, Cu, these five elements showed medium volatility; Group C: Se and Zn were

246

more likely emitted with fuel gas than above elements; Group D: Cd, Hg, Pb were highly volatile. From

247

group A to D, the volatility of elements increases, and elements can easily be emitted with fuel gas than

248

remain in the ash.

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249 250

Fig. 5. RE of trace elements in A0 and Aa

251

(Aa: average RE of trace elements among A1-A4)

252

Through the comparison between RE in A0 and Aa as showed in Fig. 5, upgrading method

253

obviously plays a role of making more toxic trace elements remain in the ash than vent with fuel gas

254

during gasification. Trace elements have association with minerals (mainly in ash) in coal. With the ash

255

removal from coal during upgrading method, the concentrations of related trace elements would change.

256

Wang M. et al. (Min Wang, 2000) and Merdes AC. et al. (Amy C. Merdes, 1998) found that the mercury

257

could be partly removed from coal through mild pyrolysis. To the volatile elements Cd, Pb and Hg in

258

Group D, evaporative drying works well on their volatilization. As the semi-volatile elements Zn and Se

259

in Group C, upgrading method could mostly effect on the enrichment of Se in ash, but only evaporative

260

drying works well on Zn. For elements in Group A and Group B (Co, Ni, Be, Mn, As, V, Cr and Cu)

261

except Co and Ni, the effects of upgrading method on their RE value are microwave drying, evaporative

262

drying, pyrolysis upgrading and HTD in the order of decrease. As for Co, microwave drying and

263

pyrolysis upgrading works well on it. For Ni, HTD did an outstanding effect on it and followed by

264

microwave drying. For restraining trace elements’ volatilization, evaporative drying and microwave

ACCEPTED MANUSCRIPT 265

drying are better. It is obvious that upgrading methods did a positive effect on restraining the

266

volatilization of trace elements. Meanwhile, it can be assumed that, to any toxic trace element, there can

267

be a suitable upgrade method to get the element removed from gaseous phase or make it mostly remain

268

in the ash.

269

3.3 Comparison of upgrading methods

270

It could be definitely confirmed that upgrading method was conducive to efficiently utilize coal

271

(Graham, 2008; Nikolopoulos et al., 2015; Wu et al., 2015; Xu and Xiong, 2013) as well as restrain

272

volatilization of trace elements. In general, each upgrading method has its advantages and disadvantages.

273

HTD could effectively restrain Hg from venting with the fuel gas and increase the RE of Hg in ash by

274

179.95%, but little effects were founded on other trace elements. Moreover, the operation of HTD was

275

complicated due to release of slurry and leaching process. As high temperature and pressure upgrading,

276

extensive production of UC through HTD should be carefully considered. Pyrolysis upgrading works

277

well on the inhibition of trace elements’ volatilization and can produce semi coke with heating value

278

increasing nearly 25%, nevertheless it would cost more energy owing to high temperature pyrolysis

279

condition. Evaporative drying and microwave drying are almost equivalent in improving coal quality.

280

These two methods are both suitable for extensive production in terms of energy consumption and

281

operation convenience. The results showed that microwave drying seems to be best among four methods

282

on both reducing trace element content in feedstock and enhancing their enrichment in ash. As heating

283

up to 200℃ in only few seconds, microwave drying can be more efficient. Also, it is considered as a

284

cleaner technology. Nikolopoulos, et al. have reviewed the advantages and limitations of microwave

285

drying (Nikolopoulos et al., 2015). The advantages of microwave drying are volumetric heating, faster

286

drying rates and reduction of impurities. However, there are still several limitations such as hot spots

ACCEPTED MANUSCRIPT 287

resulting from the presence of impurities, fire hazards due to high dielectric losses and high cost when

288

handling large amounts. The above comparison was just from the perspective of trace element

289

volatilization. When it comes to practical industrial application, economy and other factors need to be

290

considered as well.

291

4. Conclusions and future concern

292

Upgrading of LRC can enhance feedstock quality for gasification and influence the distribution of

293

trace elements in two ways. On the one hand, the upgrading method could reduce the contents of some

294

trace elements in coal, leading to lower trace elements emission. On the other hand, some trace elements

295

are more likely to be enriched in the ash after upgrading treatments, making it easier to remove them

296

from the fuel gas product. It should be mentioned that different upgrading methods have their specific

297

effects on different trace elements, respectively.

298

In respect of the reduction of trace element in feedstock, the evaporative drying method reduced the

299

contents of Be, V, Cu, Zn, As and Se. The pyrolysis upgrading method reduced the contents of V, Co,

300

Cu, As, Se, Cd and Pb. The microwave drying method reduced the contents of Be, V, Cr, Mn, Co, Ni,

301

Cu, As, Se, Cd and Pb. The HTD method reduced the contents of V, As, Se and Cd. Overall, microwave

302

drying was the best in reducing contents of trace elements in coal. Majority of 13 elements could reduce

303

roughly 50% of the content through microwave drying method, and the reduction of Cu, As and Pb

304

reaches 62.1%, 63.4% and 72.5% respectively.

305

The RE variation ratio of Cd in UC-1, Hg in UC-4 and Pb in UC-1/UC-3 were significantly

306

higher than others which are almost less than 2.00. The RE variation ratio of Cd in UC-1 can reach

307

8.40, and the RE variation ratio of Hg in UC-4 can reach 2.77. The RE variation ratio of Pb in UC-1 and

308

UC-3 can reach 5.54 and 3.19, respectively. RE calculation shows evaporative drying method worked

ACCEPTED MANUSCRIPT 309

better for Cu, Zn, Cd and Pb. The microwave drying method worked better for Be, V, Cr, Mn, Co, Cu,

310

As. The pyrolysis upgrading method worked better for Se. The HTD method worked better for Ni and

311

Hg. So evaporative drying and microwave drying covered larger ranges of trace elements.

312

Considering trace elements controlling capability only, microwave drying seemed to be more

313

efficient, which can both reduce trace elements contents in feedstock and enrich them in gasification ash.

314

While for controlling of some specific trace elements, the other methods could perform as well. Future

315

work will address trace element transformation during upgrading and their impact on downstream

316

gasification.

317

Acknowledgments:

318

This work was financially supported by the National Science & Technology Pillar Program

319

[2014BAC26B04] and the National Natural Science Foundation of China through Project [51406134].

320

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ACCEPTED MANUSCRIPT Highlights 

Upgrading of low-rank coal could enhance its quality for gasification and reduce the contents of some trace elements in the coal.



Microwave drying was the best in reducing contents of trace elements in coal, followed by pyrolysis upgrading.



Upgrading technology could promote the enrichment of trace elements in ash.



Among 4 upgrading methods, upgraded coal obtained from evaporative drying and microwave drying could remain more trace elements in ash.