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
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ACCEPTED MANUSCRIPT 1
5910 words in total
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Distribution of Trace Elements during Coal Gasification:The Effect of
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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
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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
ACCEPTED MANUSCRIPT 67
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
ACCEPTED MANUSCRIPT 123
was conducted on D/MAX-2500 (Rigaku, Japan) by using Cu Ka radiation and a step-scan pattern
124
(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
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*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,
142
respectively. Vejahati et al. (Vejahati et al., 2010) made a review on associations of coal with minerals,
143
and pointed out that the coal-mineral associations play an important role in the formation of fine particle
144
and in condensation of trace elements in ash particles. As shown in Fig. 1, the peak of CaCO3 in UC-4 is
145
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
151
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
153
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)
161 162
Fig. 2. XRD patterns of 5 ash samples
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(K: kaolinite/Al2(SiO5)(OH)4; Q: quartz/SiO2; A: Anorthite/CaAl2Si2O8)
164
The contents of 13 trace elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd, Hg and Pb) in coal
165
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
167
ash content by upgrading method, the variety of trace elements content in coal is fuzzy. The ratio of trace
168
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,
ACCEPTED MANUSCRIPT 170
Cu, As, Se, Cd and Pb reduced during pyrolysis upgrading. The contents of Be, V, Cr, Mn, Co, Ni, Cu,
171
As, Se, Cd and Pb reduced during microwave drying. The contents of V, As, Se and Cd reduced during
172
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
175
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
178
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
194
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).
196
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;
199
(%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
201
(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
203
trace elements in 5 ash samples collected from LRC and 4 types of UC gasification were shown in Table
204
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
209
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
231
UC-4 and Pb in UC-1/UC-3 were significantly higher than others. The RE variation ratio of Cd in UC-1
232
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
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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
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Trace elements grouping and their characteristics through cluster analysis Group
Trace elements
Characteristics
ACCEPTED MANUSCRIPT
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
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The remaining distribution of these trace elements was also compared. As shown in Fig. 5 where
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these elements were re-ranked by their RE values from large to small, upgrading processes didn’t change
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their remaining distribution circumstances. Grouped by dynamic clustering analysis method, these 13
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elements could be separated into four groups, depending on the RE of them in A0 and Aa (average value
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of A1-A4). Group A: Co, Ni, Be, these three elements could remain more in ash compared to others;
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Group B: Mn, As, V, Cr, Cu, these five elements showed medium volatility; Group C: Se and Zn were
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more likely emitted with fuel gas than above elements; Group D: Cd, Hg, Pb were highly volatile. From
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group A to D, the volatility of elements increases, and elements can easily be emitted with fuel gas than
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remain in the ash.
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Fig. 5. RE of trace elements in A0 and Aa
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(Aa: average RE of trace elements among A1-A4)
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Through the comparison between RE in A0 and Aa as showed in Fig. 5, upgrading method
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obviously plays a role of making more toxic trace elements remain in the ash than vent with fuel gas
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during gasification. Trace elements have association with minerals (mainly in ash) in coal. With the ash
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removal from coal during upgrading method, the concentrations of related trace elements would change.
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Wang M. et al. (Min Wang, 2000) and Merdes AC. et al. (Amy C. Merdes, 1998) found that the mercury
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could be partly removed from coal through mild pyrolysis. To the volatile elements Cd, Pb and Hg in
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Group D, evaporative drying works well on their volatilization. As the semi-volatile elements Zn and Se
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in Group C, upgrading method could mostly effect on the enrichment of Se in ash, but only evaporative
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drying works well on Zn. For elements in Group A and Group B (Co, Ni, Be, Mn, As, V, Cr and Cu)
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except Co and Ni, the effects of upgrading method on their RE value are microwave drying, evaporative
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drying, pyrolysis upgrading and HTD in the order of decrease. As for Co, microwave drying and
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pyrolysis upgrading works well on it. For Ni, HTD did an outstanding effect on it and followed by
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microwave drying. For restraining trace elements’ volatilization, evaporative drying and microwave
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drying are better. It is obvious that upgrading methods did a positive effect on restraining the
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volatilization of trace elements. Meanwhile, it can be assumed that, to any toxic trace element, there can
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be a suitable upgrade method to get the element removed from gaseous phase or make it mostly remain
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in the ash.
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3.3 Comparison of upgrading methods
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It could be definitely confirmed that upgrading method was conducive to efficiently utilize coal
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(Graham, 2008; Nikolopoulos et al., 2015; Wu et al., 2015; Xu and Xiong, 2013) as well as restrain
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volatilization of trace elements. In general, each upgrading method has its advantages and disadvantages.
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HTD could effectively restrain Hg from venting with the fuel gas and increase the RE of Hg in ash by
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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,
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extensive production of UC through HTD should be carefully considered. Pyrolysis upgrading works
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well on the inhibition of trace elements’ volatilization and can produce semi coke with heating value
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increasing nearly 25%, nevertheless it would cost more energy owing to high temperature pyrolysis
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condition. Evaporative drying and microwave drying are almost equivalent in improving coal quality.
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These two methods are both suitable for extensive production in terms of energy consumption and
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operation convenience. The results showed that microwave drying seems to be best among four methods
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on both reducing trace element content in feedstock and enhancing their enrichment in ash. As heating
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up to 200℃ in only few seconds, microwave drying can be more efficient. Also, it is considered as a
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cleaner technology. Nikolopoulos, et al. have reviewed the advantages and limitations of microwave
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drying (Nikolopoulos et al., 2015). The advantages of microwave drying are volumetric heating, faster
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drying rates and reduction of impurities. However, there are still several limitations such as hot spots
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resulting from the presence of impurities, fire hazards due to high dielectric losses and high cost when
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handling large amounts. The above comparison was just from the perspective of trace element
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volatilization. When it comes to practical industrial application, economy and other factors need to be
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considered as well.
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4. Conclusions and future concern
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Upgrading of LRC can enhance feedstock quality for gasification and influence the distribution of
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trace elements in two ways. On the one hand, the upgrading method could reduce the contents of some
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trace elements in coal, leading to lower trace elements emission. On the other hand, some trace elements
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are more likely to be enriched in the ash after upgrading treatments, making it easier to remove them
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from the fuel gas product. It should be mentioned that different upgrading methods have their specific
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effects on different trace elements, respectively.
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In respect of the reduction of trace element in feedstock, the evaporative drying method reduced the
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contents of Be, V, Cu, Zn, As and Se. The pyrolysis upgrading method reduced the contents of V, Co,
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Cu, As, Se, Cd and Pb. The microwave drying method reduced the contents of Be, V, Cr, Mn, Co, Ni,
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Cu, As, Se, Cd and Pb. The HTD method reduced the contents of V, As, Se and Cd. Overall, microwave
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drying was the best in reducing contents of trace elements in coal. Majority of 13 elements could reduce
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roughly 50% of the content through microwave drying method, and the reduction of Cu, As and Pb
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reaches 62.1%, 63.4% and 72.5% respectively.
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The RE variation ratio of Cd in UC-1, Hg in UC-4 and Pb in UC-1/UC-3 were significantly
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higher than others which are almost less than 2.00. The RE variation ratio of Cd in UC-1 can reach
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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
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UC-3 can reach 5.54 and 3.19, respectively. RE calculation shows evaporative drying method worked
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better for Cu, Zn, Cd and Pb. The microwave drying method worked better for Be, V, Cr, Mn, Co, Cu,
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As. The pyrolysis upgrading method worked better for Se. The HTD method worked better for Ni and
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Hg. So evaporative drying and microwave drying covered larger ranges of trace elements.
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Considering trace elements controlling capability only, microwave drying seemed to be more
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efficient, which can both reduce trace elements contents in feedstock and enrich them in gasification ash.
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While for controlling of some specific trace elements, the other methods could perform as well. Future
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work will address trace element transformation during upgrading and their impact on downstream
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gasification.
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Acknowledgments:
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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].
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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.