Release and Transformation Characteristics of Arsenic during Coal Pyrolysis and Gasification

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Energy Procedia 142 Energy Procedia 00(2017) (2017)3332–3337 000–000 www.elsevier.com/locate/procedia

9tth Internatioonal Conferrence on Ap pplied Energgy, ICAE20 017, 21-24 August A 201 7, Cardiff, UK U

Release annd Transsformatiion Charracteristiics of Arrsenic duuring Co oal The 15th International Symposium on District Heating and Cooling Pyrollysis andd Gasificcation a,b Assessing Xue theelifeasibility of using heat demand-outdoor a,b,,,***, Yueqia a,b, Haifeng a,b Chena,b, ang Qinthe Liua,b temperature function forChemical a long-term district heat demand forecast Key C Engine ast nd Key Laboratory Laboratory of of Coal Coal Gasifica Gasification tion and and Energy Energy Chemical C Engineeering eering of of Ministry Ministryyy of of Education,Ea Education,Ea ast China China Univers Universsity sity of of Science Science an an nd aa

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Technology 7, Technologyy, y, P.O. P.O. Box Box 272, 272, Shanghai Shanghai 200237 200237 7, PR PR China China a,b,c a a bS c c 2 Shan nghai and nology, Shan nghai Engineering Engineeringgg Research Research Cente Centeer er of of Coal Coal Gasific Gasificcation, cation, East East Chin Chinna na University University of of Science Science S and Techn Techn nology, P.O. P.O. Box Boxxx 272, 272, Shanghai Shanghai 200237, 200237, 2 PR China PR C C China a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

I. Andrić

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre

Abst Absttract tract

Pyro ments nous nducted urnace Pyrolysis lysis and and gasifi gasifiication ication experim experim ments of of bitumin bitumin nous coal coal (Yunn (Yunnnan nan Zhenxiong) Zhenxiong))) have have been been con con nducted in in aa hig higgh gh frequency frequency fu fu urnace Abstract tor tion nt grated react tor to to investiga investigaate ate the the distinctio distinctioon on of of release release aand nd transformati transformati tion of of arsenic arsenic at at two two differen differen nt stages stages decoup decouppled pled from from integ integ grated react gasif fication. The re esults indicate t that the release e rate of arsenic c changes sign ificantly with increasing i temp perature and ho olding gasiffication. The reesults indicate that t the releasee rate of arsenicc changes significantly with increasing i tempperature and ho olding District heating networks arecom commonly addressed in Escape the literature as oneac of the most solutions decreasing the time during gasifica ation mpared pyr ee of for release arsen during pyro olysis, time during gasifica ation when when com mpared with with pyrrolysis. rolysis. Escape of volatiles volatiles acccounts ccounts for the theeffective release of of arsennic nic for during pyro olysis, greenhouse gas emissions from the sector. These systems require high investments are mel returned the heat the promotion promotion of migration migration d building gasificat tion is is attributed to the the carbon minerals mel lting. through Arsenic in in char while conversion and ee the nn of dduring during gasificat tion attributed dd to carbon ddwhich minerals lting. Arsenic nn char while conversion and sales. theysis changed climate and building policies, heat demand the future could decrease, deriv ved from pyroly and ation mainly as ms bound dues sulfid des, nn of ee form deriv ved Due from to pyroly ysis and gasific gasific ationconditions mainly pr prresents resents as form form msrenovation bound to to resi resi dues and and sulfid des, and andinfraction fraction of extractable extractable form prolonging the investment return period. of ar senic from gasi ified char is hig gher than that fr rom pyrolysis ch har. of arsenic from gasiified char is higgher than that frrom pyrolysis chhar. main ofrs. paper isby to assess the feasibility of using the heat demand – outdoor temperature function for heat demand 017 The Author Published yy Elsevier Ltd. © 20 017 Thescope Author rs.this Published by Elsevier Ltd. ©The 20 ©forecast. 2017 The Authors. Published by Ltd. The district of Alvalade, in Lisbon (Portugal), used onal as aConferenc case study. district is consisted of 665 Peer-review under rresponsibility responsibility off Elsevier committee of th thhe he 9th 9thwas Internati onal Conferenc ce on onThe Applied E Energy. thelocated scientific Peer-review under r o the scientific committee of Internati ce Applied E Energy. Peer-review under responsibility of the scientific of the 9th International Conference Applied high) Energy. buildings that vary in both construction periodcommittee and typology. Three weather scenarios (low,onmedium, and three district Keyw words: pyrol gasification n; release transformati renovation scenarios developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Keyw words: coal; coal; pyrollysis; lysis;were gasification n; arsenic; arsenic; release and and transformatiion ion compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1. In ntroduction 1.(the In ntroduction scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The of slope on du average within the rangeisofone 3.8% up to 8%so per decade, that to the The T emission ooof arsenic aatmosphere atmosphere uring util ee of ources leading gg to environm mental The T value emission of coefficient arsenic to to increased a du uring coal coal utillization lization is one of primary primary so ources leading tocorresponds environm mental decrease inThis the number of heating hours of 22-139h during the heating season (depending on forms the combination of weather and pollu ution. tra ace element w with relatively y high volatil lity and toxici ity vaporizes in of h hazardous gas s pollu ution. This traace element with w relatively y high volatillity and toxiciity vaporizes in forms of hhazardous gass and and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the spec cies to fine partic during coal cc conversi ion, has hh given ri to disord ders ecolo ogical spec cies bound bound tooo The finevalues particcles cles during coal conversi ion, which which has given rise se for to the disord ders of of ecolo ogical coupled scenarios). suggested could be used to modify the function parameters scenarios considered, and envi ironment[1-2] . Gasification n is a key tech hnology of cle ean and efficie ent utilization ch provides cr rucial of coal, whic envi ironment[1-2] . Gasification n is a key tech hnology of cle ean and efficie ent utilization ch provides cr rucial of coal, whic improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and ** Corresponding C aut Corresponding C autthor. thor. Tel.: Tel.: +86-21 +86-211-64250734; 1-64250734; fax: fax: +86-21-6425131 +86-21-642513112. 12. Cooling. E-mail E address: E-mail E address: cx [email protected] [email protected]

Keywords: Heat demand; Forecast; Climate change -6102 © 1876--6102 © 2017 2017 Th Thhe he Authors. Authors. Publis Publisshed shed by by Elsevier Elsevier Ltd. Ltd. 1876Peer-r ed Peer-rreview review under under resp respponsibility ponsibility of of the theee scientific scientific comm committee ittee of of the the 9th 9th In Innternational nternational Conf Confference ference on on Applie Applie ed Energy. Energy. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.466

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basis for developments of coal-derived chemicals, fuel cell, integrated gasification combined cycle(IGCC) and other process industries[3]. Coupled to combustion and gasification as elementary stage, pyrolysis is one of feasible ways to obtain clean fuel by coal pretreatment[4,5]. However, purification of syngas during gasification is far more complicated than combustion, and gaseous elements could condense on fine particles in syngas emitted to the atmosphere[6]. Nowadays a majority of studies concentrate on volatility with changing temperature as well as retention mechanisms of gaseous species during combustion. Specific chemical forms of arsenic are limited to thermodynamic equilibrium calculation[7,8]. Besides, studies on retention mechanisms of gaseous arsenic with great toxicity have also been carried out to investigate its interactions with ash compositions or adsorbents such as activated carbon, calsium-based compounds, other metallic compounds etc.[9-12]. Decoupling pyrolysis from integrated gasification, this paper focuses on the transformation of occurrence form and migration path of arsenic during different stages of gasification reaction. Effects of temperature, minerals transformation and carbon conversion on the release and transformation of arsenic during pyrolysis and gasification were investigated in a high frequency furnace, which provided more details for studies on release and transformation of arsenic at various stages of industrial gasification. 2. Experimental 2.1. Materials Bituminous coal collected from Yunnan Zhenxiong(YNZX) with particle size between 0.074-0.125 mm was used as raw material. Proximate and ultimate analysis of YNZX and mineral composition of coal ash were presented in our previous study[13]. 2.2. Coal pyrolysis and gasification experiments Rapid pyrolysis and gasification experiments of YNZX were conducted in a high frequency furnace reactor, which was used in our previous study[13]. Ar with flow rate of 250 mL/min inlet from the top of quartz tube provides an inert atmosphere for pyrolysis process, and CO2 with flow rate of 300 mL/min flowed from the side edge of the reactor works as gasification agent. (0.8±0.01) g YNZX was weighed and placed in crucible surrounded by heating coil. The setting temperature was between 900-1100 °C with holding time of 20 min under inert condition. After pyrolysis, chars were gasified under CO2 atmosphere for 30 min. 2.3. Quantitative determination and species analysis of arsenic Microwave digestion method was adopted to digest solid samples. (0.1000±0.05) g sample was weighed each time.The content of arsenic was measured by hydrogen-generation atomic absorption spectroscopy according to GB/T 3058-2008. Based on speciation analysis method of elements established by Tessier and Querol et al.[14,15], this paper divides occurrence form of arsenic into compounds bound to sulfides and residues, exchangeable and organic ones. Occurrence form of arsenic in raw coal was analyzed in our previous work[13]. 3. Results and discussion 3.1. Release behavior of arsenic The release rates of pyrolysis and gasification were calculated according to Chen[16], which were as follows:

 X RP

C0  CP (1  wP )  100% C0

(1)

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The release rate of gasification was calculated by difference:

 X R G

CP (1  wP )  CG (1  wP ,G ) C0

100%

(2)

Where wp and wP, G represent weight loss during pyrolysis and integrated gasification respectively, %; C0 is the carbon content of raw coal, %; CP and CG represent the carbon content of pyrolysis and gasified char respectively, %. 80

80

60

60

80

pyrolysis+gasification pyrolysis gasification

70

Release rate of As/%

pyrolysis+gasification pyrolysis gasification

Release rate of As/ %

Release rate of As/ %

100

60

40

40 (a)

20 900

1000 Temperature/ °C

50

20 0

1100

40

(b) 0

15 30 45 Gasification time/min

60

30 55

(c) 60

65

70 75 80 85 Conversion rate/%

90

95

Fig.1. Release rate of arsenic versus temperature and gasification time. (a) effects of temperature; (b) effects of gasification time; (c) effects of carbon conversion rate during gasification.

Q Q Q Q

Intensity

Q (b) K

10

Q

O

M M O

O T T

1100 °C

TO

1000 °C

O T T

TO

900 °C

Q Q K

20

C P P 30

Q 40 50 2-Theta(°)

Q

(c)

Intensity

(a)

Q Q Q Q SMSMQ M Q Q M M (d)

60

70

80

10

Q

Q Q SM MQ M Q Q S MSMQ M Q Q

P

1000 °C-60 min 1000 °C-30 min

20

Q 30

1000 °C-15 min 1100 °C-30 min 1000 °C-30 min

Q 40 50 2-Theta(°)

900 °C-30 min 60

70

80

Fig. 2. Mineral composition of raw coal and chars. Q-Quartz(SiO2); M-Mullite(xAl2O3·ySiO2); P-Pyrite(FeS2); S-Sillimanite(Al2SiO5); CCalcite(CaCO3); K-Kaolinite(Al2(Si2O5)(OH)4); T-Troilite(FeS); O-Oldhamite(CaS). (a): chars derived from pyrolysis; (b): raw coal; (c): chars derived from gasification at 1000 °C; (d): chars derived from gasification for 30 min.

As shown in Fig. 1(a), comparatively speaking, temperature does not have much effect on the volatility of arsenic during pyrolsis, while release species vary with changing temperature. Arsenic may exist in forms of elementary state and hydride like As4(g), As2(g) and AsH3(g) at temperature below 500 °C, while As2O3(g) and As4O6(g) are inclined to form above 500 °C[17,18]. Fig. 2(a) shows that the diffraction intensity of troilite and oldhamite gradually decreases with increasing temperature and the diffraction peaks disappear at 1100 °C. Combining Fig. 2(b) and the fact that arsenic is mainly associated with sulfide mineral in coal, it could be deduced that arsenic releases along with the decomposition of sulfide mineral at specifical temperature. Fig. 1(a) indicates that most arsenic tends to volatilize in the process of gasification compared with pyrolysis. Thermodynamic equilibrium calculation demonstrates that when the temperature is higher than 1000 °C, a large number of arsenic release to the gas phase in the form of AsO(g)[19]. Fig. 1(b) shows that arsenic releases at early stage of gasification and release rate of arsenic increases more slowly after 15 min. As shown in Fig. 2(c) and Fig. 2(d), diffraction peaks of mullite and sillimanite appear with increasing temperature and gasification time and there

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are only peaks of quartz (SiO2) when reaching 60 min at 1000 °C. The above release behaviors of arsenic may lies in the melting and transformation of minerals like aluminosilicate, leading to the capture of arsenate in char[20]. 3.2. Speciation transformation of arsenic Fraction of different occurrence forms of arsenic during pyrolysis and gasification was calculated by the following equations: CP  Asi  (1  X R  P )

CP  i  100% C0 i

CG  Asi  (1  X R  P ,G )

(3)

CG  i  100% C0  i

(4)

Where C0-i is the fraction of each occurrence form of arsenic in raw coal, μg/g; CP-i and CG-i represent the fraction of each occurrence form of arsenic in pyrolysis and gasified char respectively, μg/g.

80

P-pyrolysis

(a) P

G

62.6 48.8

P

G-gasification G

52.4 30.5

P

100

G

58.4 52.8

60 40 20 0

19.4 36.2

900

27.2 60.5

1000 Temperature/°C Residual

21.1 34.2

1100 Bound to organic

80 Fraction of As/%

Fraction of As/%

100

Pyro-zone

(b)

Gasification-zone

54.5

66.8

68.5

30.5

43.2

52.4

84.3

43.4

28.5

22.3

60.1

41.5

27.2

2.4

60 40 20 0

0min

5min 10min 20min 35min 50min 80min Holding time/min

Bound to sulfides

Ion exchangeable

Fig. 3. Different occurrence forms of arsenic versus temperature and holding time

As shown in Fig. 3(a), generally, the value during gasification is much higher than that during pyrolysis, indicating that arsenic tends to have greatly stronger chemical mobility during gasification. Moreover, species of arsenic bound to sulfides and residues are the dominant forms in chars derived from pyrolysis and gasification. The fraction of residual species during gasification which reaches its maximum value at 1100 °C is lower than that during pyrolysis. While the fraction of arsenic bound to sulfides obtained from pyrolysis is higher than that from gasification. Consequently, it could be deduced that arsenic in form of sulfides is inclined to decompose and transform to residual species in the proceeding from pyrolysis to gasification, resulting from complex reactions of minerals which leads to the capture of arsenic trapped in mineral crystal lattice. Moreover, fraction of organic arsenic during pyrolysis is higher than that during gasification, implying that the binding energy of chemical bonds of C-As and S-As in species of arsenic generating from gasification is relatively weaker[21]. It could be observed from Fig. 3(b) that there mainly is the transformation of arsenic bound to sulfides and residues during pyrolysis. The fraction of residual species of arsenic increases gradually with increasing holding time during gasification, while the fraction of arsenic bound to sulfides has a radically different trend. It is worth noting that the fraction of exchangeable species in gasified char is much higher, which may lie in the formation of soluble arsenate via the oxidation of arsenic during gasification. As shown in Fig. 4(a), contrary to pyrolysis, there is a significantly negative correlation between the fraction of organic arsenic and content of carbon during gasification. It could be deduced that the consumption of carbon results

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in the decrease of arsenic in form of adsorption state, leading to the transformation to organic species. Basically, the fraction of arsenic bound to sulfides is positively correlated to the content of sulfur both during pyrolysis and gasification according to Fig. 4(b), which demonstrates that the transformation of arsenic existing in form of sulfides is along with the release of sulfur. Pyrolysis Gasification

(a)

14

60 Fraction of As bound to sulfides/%

Fraction of As bound to organics/%

16 12 10 8 6 4 2 0 20

30

40

50 60 Content of C/%

70

Pyrolysis Gasification

(b)

50 40 30 20 10 0 1.2

80

1.4

1.6

1.8 2.0 2.2 Content of S/%

2.4

2.6

Fig. 4. Fraction of arsenic bound to organics and sulfides versus content of carbon and sulfur

3.3. Release and transformation path of arsenic Release

As-Organic

Over 500°C

Release

As2O3(s)

⇄ 

As-Sulfides

CO2

As4O6(g) Over 1000°C

As-Exchangeable As-Residual

Ca, Al, Si, Fe

Pyro-zone

Arsenate AsO(g)

Al, Si

AsAluminosilicate

C-Residual Adsorption

As-C

Gasification- zone

Fig. 5. Release and transformation path of arsenic during pyrolysis and gasification

On the basis of several studies[17,18-21] and experimental results in this paper, release and transformation path of arsenic during pyrolysis and gasification is summarized in Fig. 5. The extractable arsenic release to the atmosphere in form of As4O6(g) above 500 °C and part of this gaseous species transform to As2O3(s). A portion of arsenic may be adsorbed by Al、Si or react with Ca，Fe and other mineral compositions, leading to the increase of residual species. During gasification, AsO(g) tends to be the most stable form of arsenic when temperature is higher than 1000 °C. Part of this compound could be adsorbed by residual carbon and aluminosilicate, while other portion of that transforms to soluble arsenate by oxidation of reaction. 4. Conclusion

The release rate of arsenic in the process of gasification increases more significantly with increasing temperature and holding time compared with pyrolysis. The slowing down of the release of arsenic may be attributed to the retention by aluminosilicate with the proceeding of gasification. Escape of volatiles accounts for the release of

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arsenic during pyrolysis, while the promotion of release and transformation during gasification lies in carbon conversion and minerals melting. Arsenic tends to have greatly stronger chemical mobility during gasification, and species of arsenic bound to sulfides and residues are the dominant forms in chars derived from these two processes. The binding energy of C-As is inclined to reduce during gasification, resulting in the decomposition and transformation of organic species. Acknowledgements

The research was supported by the National Key Research and Development Project (2016YFB0600400-02), the Fundamental Research Funds for the Central Universities(222201718003). References [1] Swaine DJ. Trace elements in coal. Butterworth-Heinemann, 1990. [2] Vejahati F, Xu Z, Gupta R. Trace elements in coal: Associations with coal and minerals and their behavior during coal utilization-A review. Fuel, 2010, 89(4): 904-911. [3] Wang FC, Yu GS, Gong X, Liu HF, Wang YF, Liang QF. Research and development of large-scale coal gasification technology. Chem Ind Eng Prog, 2009, 28(2): 173-180. [4] Gryglewicz G. Effectiveness of high temperature pyrolysis in sulfur removal from coal. Fuel Process Technol, 1996, 46(3): 217-226. [5] Nern C, Domeno C, Moliner R, Lazaro MJ, Suelves I, Valderrama J. Behaviour of different industrial waste oils in a pyrolysis process: metals distribution and valuable products. J Anal Appl Pyrol, 2000, 55(2): 171-183. [6] Yoshiie R, Taya Y, Ichiyanagi T, Ueki Y, Naruse I. Emissions of particles and trace elements from coal gasification. Fuel, 2013, 108: 6772. [7] Roy B, Choo WL, Bhattacharya S. Prediction of distribution of trace elements under oxy-fuel combustion condition using Victorian brown coals. Fuel, 2013, 114: 135-142. [8] Frandsen F, Dam-Johansen K, Rasmussen P. Trace elements from combustion and gasification of coal‒an equilibrium approach. Prog Energ Combust, 1994, 20(2): 115-138. [9] López-Antón MA, Díaz-Somoano M, Fierro JLG, Martinez-Tarazona RM. Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons. Fuel Process Technol, 2007, 88(8): 799-805. [10] Sterling RO, Helble JJ. Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates. Chemosphere, 2003, 51(10): 1111-1119. [11] Wang CB, Zhang Y, Liu HM. Experimental and Mechanism Study of Gas-Phase Arsenic Adsorption Over Fe2O3/gamma-Al2O3 Sorbent in Oxy-Fuel Combustion Flue Gas. Ind Eng Chem Res, 2016, 55(40): 10656-10663. [12] Liu H, Pan WP, Wang C, Zhang Y. Volatilization of Arsenic During Coal Combustion Based on Isothermal Thermogravimetric Analysis at 600-1500° C. Energ Fuel, 2016, 30(8): 6790-6798. [13] Qin YQ, Chen, XL, Chen HD, Liu HF. Effects of adding CaO on the release and transformation of aesenic and sulfur during coal pyrolysis. J Fuel Chem Technol, 2017, 45(2):147-156. [14] Tessier A, Campbell PGC, Bisson M. Sequential extraction procedure for the speciation of particulate trace metals. Anal Chem, 1979, 51(7): 844-851. [15] Querol X, Juan R, Lopez-Soler A, Fernandez-Turiel J, Ruiz CR. Mobility of trace elements from coal and combustion wastes. Fuel, 1996, 75(7): 821-838. [16] Chen H, Chen X, Qiao Z, Liu H. Release and transformation behavior of Cl during pyrolysis of torrefied rice straw[J]. Fuel, 2016, 183: 145-154. [17] Lu H, Chen H, Li W, Li B. Transformation of arsenic in Yima coal during fluidized-bed pyrolysis. Fuel, 2004, 83(6): 645-650. [18] Schulman JH, Schumb WC. The Polymorphism of Arsenious Oxide1. J Am Chem Soc, 1943, 65(5): 878-883. [19] Dıaz-Somoano M, Martınez-Tarazona MR. Trace element evaporation during coal gasification based on a thermodynamic equilibrium calculation approach. Fuel, 2003, 82(2): 137-145. [20] Zeng T, Sarofim AF, Senior CL. Vaporization of arsenic, selenium and antimony during coal combustion. Combust Flame, 2001, 126(3): 1714-1724. [21] Tian C, Gupta R, Zhao Y, Zhang J. Release behaviors of arsenic in fine particles generated from a typical high-arsenic coal at a high temperature. Energ Fuel, 2016, 30(8): 6201-6209.