ol

in

chn

Thermal analysisKineticsPyrolysisCombustion

impcribganurie

was calculated using Friedman, Starink, distributed activation energy model, and Ozawa iterative meth-ods. Results indicated that the thermal decompositions of three oil shales exhibit multiple reaction pro-

ly disce of enic mat

yearly [10]. Fushun Basin hosts the largest opencast coal and oilshale mine in Asia [11]. The annual shale oil production inFushun is about 350,000 t [10]. Compared with Huadian Basinand Fushun Basin, the yield of oil shale in Songliao Basin is usuallylower [12] although its reserve is largest; nonetheless, its oil shaleis hardly utilized. High temperature generally plays an important

ysis has specicactical procedure.erical simck under d

operating conditions [5]. The application of thermal analysishale pyrolysis and combustion is not a recent approachstudies in literature focus mainly on the inuences of experiatmosphere [18], thermal procedure [19,20], heating rate [1925],particle size [19], and data processing [1921,26,27] on oil shalepyrolysis and combustion kinetic results. Generally, the thermaland kinetic characteristics of different oil shales markedly varyfor its complex composition [1,2,14,21,24,28], except laboratoryoperations. The activation energy values even differ from differentcomputational methods [20,21,26]. Presently, most kinetic studies

Corresponding author at: College of Construction Engineering, Jilin University,Changchun 130021, PR China. Tel./fax: +86 431 88502066.

E-mail address: syh@jlu.edu.cn (Y. Sun).

Energy Conversion and Management 97 (2015) 374381

Contents lists availab

Energy Conversion

seeast regions such as Huadian Basin, Fushun Basin, and SongliaoBasin [8]. Huadian Basin hosts a remarkably high number of oilshale layers, and its oil shale belongs to the low ash, high oil yieldtype [9]. Currently, Huadian oil shale produces 50,000 t of oil

techniques [1417]. The non-isothermal analadvantages and results in close proximity to prThe kinetic triplets can be further used in nummodels to investigate the behavior of the feedstohttp://dx.doi.org/10.1016/j.enconman.2015.03.0070196-8904/ 2015 Elsevier Ltd. All rights reserved.ulationifferents to oil. Manymentalinorganic matrix [15]. In fact, oil shale reserves have been consid-ered part of the worlds oil reserves because high oil prices andnew technology enable them to be protably extracted andupgraded to usable products [6,7].

Chinas oil shale reserves are about 720 billion tons, in situ con-verted to 47.6 billion tons of shale oil, distributed mainly in north-

before exploration and launch of development projects [5,13],especially for oil shale in Songliao Basin.

Thermogravimetric/differential thermogravimetry (TG/DTG)analysis provides general information on the overall reactionkinetics and has been widely used in the evaluation of thermalcharacteristics of fossil fuels by isothermal and non-isothermal1. Introduction

The energy issue is an extensivenumber of concerns about the futuring. Oil shale is composed of organcesses, especially combustion process, and the diversity of organic and mineral compositions in oil shalehave considerable effects on the thermal behavior and kinetic characteristics of the three samples. Novelexploitation technology for oil shale in Songliao Basin needs to be proposed.

2015 Elsevier Ltd. All rights reserved.

ussed subject, and theergy supply is increas-erials distributed in an

role in the conventional utilization of oil shale regardless of ren-ing or burning, but the type and content of organic matters andminerals of oil shale vary signicantly in different basins [12].Therefore, research on pyrolysis and combustion behavior of eachoil shale should be conducted separately and comprehensivelyKeywords:Oil shale

to characterize the hydrocarbons and minerals in oil shale. Non-isothermal thermogravimetric analysiswas used to investigate the pyrolysis and combustion behaviors of oil shales. The activation energyThermal and kinetic characteristics of pyroil shales

Fengtian Bai, Youhong Sun , Yumin Liu, Qiang Li, MCollege of Construction Engineering, Jilin University, Changchun 130021, PR ChinaKey Laboratory of Ministry of Land and Resources on Complicated Conditions Drilling Te

a r t i c l e i n f o

Article history:Received 15 December 2014Accepted 1 March 2015Available online 5 April 2015

a b s t r a c t

High temperature plays anor burning. This paper desHuadian, Fushun, and Nonshale in Songliao Basin. Fo

journal homepage: www.elysis and combustion of three

gyi Guo

ology, Jilin University, Changchun 130021, PR China

ortant role in the conventional utilization of oil shale regardless of reninges the thermal and kinetic characteristics of pyrolysis and combustion ofoil shales from three northeast basins of China, particularly Nongan oilr transform-infrared spectroscopy and X-ray diffraction were performed

le at ScienceDirect

and Management

vier .com/locate /enconman

Songliao Basin is still rare.

nd MOil shale involves several steps and mechanisms during pyroly-sis/combustion, and its activation energy tends to vary as conver-sion increases [2123]. International Confederation for ThermalAnalysis and Calorimetry (ICTAC) Kinetics Committee [35] positsthat isoconversional methods are very suitable for complex kinet-ics analysis of fossil fuels because they depend on temperature andconversion degree without the necessity of assuming the reactionmodel. Thus, the new research on the thermal and kinetic charac-teristics of Huadian and Fushun oil shales using isoconversionalmethods will be necessary and signicant, which will provide afundamental and comparable viewpoints for samples.

To provide important information for the design and optimiza-tion of the comprehensive utilization project, the pyrolysis andcombustion behaviors of three oil shales from Huadian Basin,Fushun Basin, and Songliao Basin were investigated using non-isothermal TG method at different heating rates. Fourier trans-form-infrared (FTIR) spectroscopy and X-ray diffraction (XRD)were performed to characterize the hydrocarbons and mineralsin oil shale. The kinetic parameters were determined usingFriedman, Starink, distributed activation energy model (DAEM),and Ozawa iterative methods. The results were then discussed.

2. Materials and methods

2.1. Materialson pyrolysis and combustion of Huadian [2732] and Fushun[33,34] oil shales use model tting method and overall rst-orderreaction model. However, the thermal report of oil shale in

AbbreviationTG thermogravimetryDTG derivative thermogravimetryTrange temperature range, CTmax temperature of maximum mass loss takes place, CConst constant

SymbolsA pre-exponent factor, s1

E activation energy, kJ mol1Nomenclature

F. Bai et al. / Energy Conversion aThe oil shales used in this study were from Northeastern Chinaspecically Huadian (HD) of Huadian Basin, Fushun (FS) of FushunBasin, and Nongan (NA) of Songliao Basin. Raw oil shale sampleswere sampled, crushed, and sieved to a grain size of 00.088 mmand dried at 4550 C to constant mass according to the ASTMD2013-07 (USA) and GB 474-2008 (China) standards. The proxi-mate, elemental, and Fischer assay analysis results are summarizedin Table 1. The HD oil shale from the Huadian Basin is partly of highquality when the oil yield of the sample reached 19.69%, whereasthe FS sample afforded shale oil about 8.66%. The oil yield of theNA oil shale from the Songliao Basin is the lowest of the three.

2.2. Characterization of oil shale

The FTIR spectra of samples were taken in KBr pressed pelletson a Bruker IFS 66V/S FTIR spectrometer (Germany) in the mid-IR region of 4004000 cm1. The XRD measurements wererecorded on a Rigaku D/MAX 2550 diffractometer (Japan) with2.4. Kinetic methods

The non-isothermal kinetic study of combustion and pyrolysisprocess is extremely complex because of the presence of numerouscomponents and their parallel and consecutive reactions[21,22,28,31]. In solid fuel kinetic analysis, the rate of kinetic pro-cess is usually expressed as

dadt

kf a or dadT

Abexp E

RT

f a 1

The parameter activation energy (E), pre-exponential factor (A),and kinetic model function [f(a)] are kinetic triplet. Friedman,Starink, DAEM, and Ozawa iterative isoconversional kinetic meth-ods were used to obtain activation energies of combustion andCu Ka radiation. Multiple experiments were performed to ensurereproducibility.

2.3. TG analysis

TG analysis was performed using a Netzsch STA 449C thermalanalyzer system (Germany) at heating rates of 2, 5, 10, 20, and50 C min1 at temperature ranging from ambient temperature to900 C. Nitrogen was used as the purge gas for pyrolysis experi-ment, and high-purity air (oxygen:nitrogen = 21:79, >99.99%) wasused for combustion experiment. The ow rate was maintainedat 60 mL min1. The experiments were performed at least threetimes for each sample. The error of mass loss and temperaturewas less than 0.5 wt% and 1 C, respectively.

f a general expression of kinetic model functionT absolute temperature, KR universal gas constant, 8.314 J mol1 K1

t time of conversion, sa conversion, a m0 mT=m0 mf , mT = sample

mass at temperature T , m0 = values of initial weight,mf = values of total weight

b heating rate, C min1, b dT=dtk rate constant, k A expE=RT

anagement 97 (2015) 374381 375pyrolysis reactions.The equation of Friedman [36] is given as

ln bdadT

lnAf x E

RT2

The equation of Starink [37] method is given as

lnb

T1:92

Const 1:0008 E

RT3

The simplied DAEM equation [38] is as follows:

lnb

T2

ln AR

E

0:6075 E

RT4

Under the same conversion at different heating rates, theactivation energy can be determined by the slope of the regressionlines of ln b dadT

vs. 1=T for Friedman, lnb=T1:92 vs. 1=T for Starink

method and lnb=T2 vs. 1=T for DAEM method.Ozawa iterative procedure [39] is expressed as

This method nonetheless is very sensitive to experimental noise

e (M

Fig. 1. (a) FTIR spectra and (b) XRD patterns of HD, FS, and NA oil shales.

Table 2Characteristics peaks in FTIR spectrum of main compositions in oil shale [4].

Name Wavenumber (cm1)

Kaolinite 3700, 3655Montmorillonite 3630, 3420, 1630, 530, 470Interlayer water 3420Kerogen 2928, 2850, 1700, 1625, 1460, 10301040Calcite 2516, 1800, 1440, 873, 742, 716, 697Silicate 1870, 1440, 530, 470Quartz 800, 780Feldspar 742Siderite 1440, 1090

nd Mand tends to be numerically unstable [35]. According to ICTAC,the Starink method is an accurate equation; hence, it isrecommendable for use [35]. The DAEM is a multiple reactionmodel and is widely used in the pyrolysis of biomass, coal, andother thermally degradable materials. The E determined by theDAEM method is equal to that from KissingerAkahiraSunosemethod for the same regression lines. Ozawa iterative procedureas a numerical integration can produce an insignicant amountof error because it does not use the oversimplied temperatureintegral approximation [35].

3. Results and discussion

3.1. FTIR and XRD characterizations

The FTIR spectrum of each oil shale consists of stretchingand bending vibrations from the aliphatic and aromaticgroups of kerogen, which overlap with the peaks of mineralssuch as carbonates, quartz, and clay [4,31] (Fig. 1a, Table 2).Moreover, the intensities of aliphatic hydrocarbon stretchingbands (2928 cm1, 2850 cm1) in HD, FS, and NA oil shalesreciprocally increased similar with the oil yield obtainedby Fischer assay analysis. The XRD patterns (Fig. 1b) of threeoil shales support some of the FTIR results, especially thoseof inorganic minerals. The carbonates, quartz, and clay areprimary inorganic minerals in HD oil shale. The intensities oflnb

Hu

ln 0:00484AERGa

1:0516ERT

5

where u E=RT, Hu euQ4u=u2

0:00484e1:0516u, Q4u

u4 18u3 88u2 96uu4 20u3 120u2 240u 120.

First, suppose H(u) = 1 to estimate the initial value of activationenergy E1. Use E1 to calculate H(u). Then, from Eq. (5), calculate anew value E2 for the activation energy form the plot of ln(b/H(u)) vs. 1/T, replacing E1 with E2, and so on until the absolute

difference meets Ei Ei1 < 0:1 kJ mol1. The last value Ei is theexact value of the activation energy of the reaction.

The derivative-based Friedman method is the most accuratethan the integral methods because it uses no approximations.

Table 1Physical properties of HD, FS, and NA oil shales.

Samples Proximate analysis (wt%)

Volatiles Fixed carbon Ash Moisture Gross Caloric valu

HDad 39.34 3.75 56.91 3.26 13.07FSad 12.16 13.63 72.22 2.46 7.67NAar 9.70 1.58 89.34 5.98 2.88

376 F. Bai et al. / Energy Conversion amineral characteristic peaks in both FTIR and XRD of NA oilshale are stronger than those in HD and FS, especially thatof the calcite. Conversely, the characteristic peaks of kaolinite,montmorillonite, and siderite except calcite can be found easilyin XRD pattern in FS sample, indicating that clay and sideriteare dominant minerals in FS.

The basic principles of pyrolysis/combustion of oil shale may beunderstood by examining the overall pyrolysis/combustionbehavior of original oil shale and by considering the interactionbetween kerogen and inorganic matrix. Further, the mineral matterpresent in oil shale is believed to play an important role in thethermally induced catalytic alteration of kerogen during petroleumformation [13].Elemental analysis (wt%) Fischer assay analysis (wt%)

J kg1) C H N S Shale oil Gas Water Residue

29.23 4.28 0.61 4.92 19.69 6.38 4.98 68.9513.82 2.15 0.76 4.43 8.66 6.12 4.65 80.589.69 1.03 0.32 5.73 3.68 0.92 3.10 92.30

anagement 97 (2015) 3743813.2. Pyrolysis and combustion characteristics

The pyrolysis TG and DTG curves of three oil shales at differentheating rates are shown in Fig. 2. With various chemical andphysical compositions, the samples exhibit different thermaldecomposition patterns with increasing temperature [3].Although extremely complex processes contribute to the mass lossof oil shale, the three basic stages can be observed in the samples.The rst stage is below 260 C, and a small mass loss (2%) of mois-ture was observed, especially from clay minerals. The second stage

Pyrite 1164, 420

nd MF. Bai et al. / Energy Conversion ais mainly devolatilization of organic matter in oil shale from 260 Cto 650 C, and the mass loss was about 30% for HD, 21% for FS, and7.5% for NA oil shale (Table 3). This nding proved that the higherthe content of the organic matter, the higher the grade of oil shaleis [28]. Notably, the decomposition temperature of siderite isbetween 500 C and 600 C [40,41]. The third mass loss can beobserved above 600 C, a stage governed by the thermaldecomposition of inorganic minerals like carbonates and clay,among others. The mass loss of NA oil shale reached 16.44%(2 C min1) on the third stage, which is larger than that of HD(5.536.61%) and FS (3.244.24%) oil shale because calcite is adominant composition of NA oil shale.

Fig. 2. Pyrolysis TG and DTG curves of three oil shales at various heating rates: (a)HD oil shale, (b) FS oil shale, and (c) NA oil shale.Table 3TG/DTG reaction intervals, peak temperatures, and moss losses of oil shale pyrolysis.

Oilshale

b(C min1)

Second stages Third stages

Interval(C)

Tmaxa

(C)Loss(%)

Interval(C)

Tmax(C)

Loss(%)

HD 2 266542

429 30.08 541705

638 6.61

5 274556

447 30.10 556731

664 6.06

10 285586

460 30.69 585755

696 5.80

20 290599

471 31.07 599770

715 5.53

50 296621

489 30.58 621785

732 5.39

FS 2 282582

448 20.87 582809

752 3.24

5 295578

453 20.54 578829

736 3.92

10 305612

468 21.30 612841

758 3.91

20 315627

476 21.70 627857

775 4.10

50 325643

489 21.49 643863

792 4.24

NA 2 272545

412 6.79 545763

692 16.44

5 273554

424 6.96 554773

707 16.34

10 275590

436 7.55 590805

746 15.97

20 292613

454 7.89 613833

760 15.91

50 309 470 7.93 642 779 15.91

anagement 97 (2015) 374381 377In the presence of oxygen, except for the simple degradationprocess similar to the pyrolysis, a thermochemical process combin-ing thermal and oxidation effects develops simultaneously [2,4].From the combustion TG/DTG curves in Fig. 3, the combustion pro-cess of three oil shales can be also divided into three stages,namely, water evaporation (600 C). The mass loss of the second stage in the combustion pro-cess is larger than that in pyrolysis process, especially for HD oilshale (39%, Table 4). The third stage of HD (4.195.06%) and NA(15.5715.90%) in combustion is slightly smaller than in pyrolysis.The third stage of FS oil shale combustion can be neglected becausexed carbon and some inorganic minerals (clay) decompose inadvance because of oxygen, but the calcite decomposition is freefrom the atmosphere.

The HD oil shale exhibited noticeably lower thermal stabilitycompared with other oil shales, and its decomposition and com-bustion processes started early and completed quickly (Tables 3and 4). The overall mass losses of pyrolysis and combustion ofeach oil shale at different heating rates are almost the same, indi-cating that the heating rates have little effect on the total massloss of HD, FS, and NA oil shales. However, the Tmax of oil shalepyrolysis (Fig. 2, Table 3) was systematically moved from 429 Cat 2 C min1 to 489 C at 50 C min1 during the second stageand shifted from 638 C at 2 C min1 to 732 C at 50 C min1

during the third stage for HD oil shale, which corresponds to448489 C (second stage) and 752792 C (third stage) for FS,412470 C (second stage) and 692779 C (third stage) for NA.Similar phenomenon can also be found in the combustion pro-cesses of the three samples (Fig. 3 and Table 4), which is relatedto the mass transfer resistance and thermal hysteresis, and hasalso been observed in other origins of oil shale by Williams and

642 845

a The temperature at which the maximum mass loss rate reached.

nd M378 F. Bai et al. / Energy Conversion aAhmad [20], Kk and Senguler [21], Kk [22,24], Jankovic [23],and Al-Harahsheh et al. [25]. At low heating rate, oil shaleparticles are gradually heated, leading to an improved andeffective heat transfer to the inner portions. As heating rateincreases, the temperature gradient between surface and innerof sample particles becomes large, and the extent of diffusioncontrol also increases, strengthening the inertia effect ofdevolatilization [20,23], hence increasing the Tmax temperature.In addition, the combustion DTG curves uctuate as the conver-sion proceeds, showing multi sub-peaks in the second stage andbecoming more obvious as the heating rate increases, especiallyfor HD and FS oil shale.

Fig. 3. Combustion TG and DTG curves of three oil shales at various heating rates:(a) HD oil shale, (b) FS oil shale, and (c) NA oil shale.Table 4TG/DTG reaction intervals, peak temperatures, and moss losses of oil shalecombustion.

Oilshale

b(C min1)

Second stages Third stages

Interval(C)

Tmax(C)

Loss(%)

Interval(C)

Tmax(C)

Loss(%)

HD 2 193516

380 37.15 516698

651 4.85

5 202555

399 38.52 555716

673 5.06

10 232573

419 38.23 573729

694 4.73

20 249616

433 39.00 616783

730 4.78

50 258650

488 38.90 650798

742 4.19

FS 2 195640

491 25.31

5 210648

502 25.01

10 230677

508 25.17

20 249690

515 25.09

50 264723

519 25.02

NA 2 198509

341 9.10 509741

687 15.90

5 214544

369 9.09 544764

721 15.67

10 220556

376 9.10 556778

734 15.60

20 239572

389 9.29 572818

765 15.71

anagement 97 (2015) 3743813.3. Pyrolysis and combustion kinetic analysis

Fig. 4 illustrates the relationship between activation energy/temperature and conversion rate. The activation energy ofHD oil shale at the conversion rate of 0.10.74 stands for the sec-ond stage, whereas 0.740.9 is the third stage, which correspondsto 0.10.76 (second stage), 0.760.9 (third stage) for FS, 0.10.3(second stage), and 0.30.9 (third stage) for NA oil shale. The massloss ratio is related to the organics and minerals contents in thesamples.

The pyrolysis activation energies calculated by Starink, DAEM,and Ozawa iterative methods are much closer to, but smaller thanthat determined by Friedman, as shown in Fig. 4 and Table 5because of the different equation parameters, imprecision ofnumerical differentiation, or the assumptions from which thosemodels are based [35]. The activation energies vary greatly for dif-ferent oil shales and are unstable in the entire conversion. The lar-gest uctuation of activation energy always occurs at thewatershed between two stages, especially the conversion rate at0.30.4 in NA oil shale. The activation energy of HD oil shale slowlyincreases with the conversion rate at the second stage. The averagevalue is around 255 kJ mol1, and it exhibits a practically constantvalue around 350 kJ mol1 in the third stage. This nding is largerthan the results obtained by Liu et al. [28] using CoatsRedfernmethod, but nearly consistent with the experimental resultsgained by Wang et al. [32] using Friedman (237.6, 249.61, and242.67 kJ mol1). The activation energy of FS oil shale uctuatesaround 270 kJ mol1, which is slightly larger than the resultsreported by Li and Yue [34] (194250 kJ mol1). For NA oil shale,the E determined by DEAM rises sharply from 200 kJ mol1

(a = 0.1) to 419 kJ mol1 (a = 0.36) and rapidly declines to314 kJ mol1 (a = 0.42) and then stabilizes at 302 kJ mol1 in the

50 270602

425 9.48 602836

798 15.57

Abbreviations as in Table 3.

F. Bai et al. / Energy Conversion and Management 97 (2015) 374381 379third stage. The uctuation of activation energy indicates the mul-tiple reaction processes and mechanisms of oil shale pyrolysis.

The pyrolysis activation energy of HD oil shale in the secondstage is the smallest among the samples and that of NA oil shaleis the largest one. The differences in activation energy of the threesamples are related to the various contents of organic matter andinorganic minerals [3,23] because carbonates and clays have somecatalytic effects on the pyrolysis of oil shale, whereas silicates

Fig. 4. Ea and Ta curves of pyrolysis of three oil shales: (a) HD oil shale, (b) FS oilshale, and (c) NA oil shale.

Table 5Mean activation energy of oil shale samples (kJ mol1).

Sample Stages Pyrolysis

Friedman Starink DEAM Ozawa

HD II 265.41 253.77 253.50 253.75III 355.56 349.41 350.13 349.39

FS II 281.89 267.95 267.66 267.93III 388.70 372.16 371.78 372.12

NA II 247.06 236.86 236.57 236.84III 318.55 302.69 302.27 302.67acted as inhibitors in pyrolysis reactions [1,2]. Moreover, thehigher volatile content allows easier reaction.

Similar to pyrolysis, the combustion activation energiesobtained by Starink, DAEM, and Ozawa iterative methods are smal-ler than that from Friedman (Fig. 5). The values of E varied widelywith the extent of conversion both between and within the sam-ples. The conversions at 0.050.86 and 0.861 represent the secondand third combustion stages for HD oil shale, respectively, which

Combustion

iterative Friedman Starink DEAM Ozawa iterative

95.78 84.98 84.61 85.17248.83 235.55 235.11 235.56160.75 149.25 148.88 149.31

110.22 99.86 99.50 99.99261.48 245.46 244.99 245.47

Fig. 5. Ea and Ta curves of combustion of three oil shales: (a) HD oil shale, (b) FSoil shale, and (c) NA oil shale.

shale is difcult. The average combustion activation energies of

energy. New unconventional oil extraction technologies are

K, et al. Effect of demineralization and heating rate on the pyrolysis kinetics ofJordanian oil shales. Fuel Process Technol 2011;92:180511.

nd Mneeded because the conventional technologies to extract gas andoil from NA oil shale are economically unfeasible and ecologicallydevastating. Considering the large reserves and serious energy cri-sis, novel exploitation scheme and new research for oil shale inSongliao Basin need to be proposed and performed timely.

4. Conclusions

This research provided the thermal behavior of HD, FS, and NAoil shales from the northeast region of China, particularly the NAoil shale in Songliao Basin. The quality and grade of HD, FS, andNA oil shales reciprocally decreased. Given the distinct behaviorof oil shale, the TG curves of the three samples were divided intothree separate stages regardless of atmospheres. The thermaldecomposition of oil shale exhibited multiple reaction processes,especially the combustion process. Furthermore, the diversity oforganic and mineral compositions in oil shale led to different ther-mal behavior and kinetic parameters of the three samples. Amongthe three samples, the Huadian oil shale presented the smallestactivation energies both in pyrolysis and combustion processes,whereas the Nongan oil shale was the largest. The atmosphere alsohad some inuences on the mass loss and activation energy of thethree samples. The combustion of three oil shales was easier to betriggered than pyrolysis. The heating rates had little effect on thetotal mass loss of oil shale.

Considering the characteristics of NA oil shale together withindustrial technologies and cost, novel exploitation technologyand new research for oil shale in Songliao Basin need to be pro-posed. Moreover, a specic study to further investigate the actionpath of minerals in oil shale needs to be designed.

Acknowledgmentsthe third stage are both larger than in the second stage for HDand NA oil shales (Table 5) because of the decomposition of calciteand silicate in these two samples. Although comparing differentsamples from literature is difcult because of the different equip-ment and experiment parameters used, the E of HD oil shaleobtained from the present study is within the range of thosereported by Han et al. [29]. The activation energies of FS uctuateand vary in the range of 90255 kJ mol1 because of the clay andsiderite minerals in FS oil shale that decompose early together withthe organic combustion when the conversion is above 0.8. The uc-tuating Ea curves of the samples reveal the complex combustionprocesses and multiple reaction mechanisms of oil shale. Overall,the combustion activation energy of HD oil shale is the smallestamong the three samples and NA oil shale is the largest becauseof the difference of oil shale species in accordance with the pyroly-sis results.

Compared with the pyrolysis, the combustion of HD, FS, and NAoil shales is easily triggered, especially in the second stage, becauseof the exothermic reaction of organic oxidation. Yan et al. [2]reported that the resultant catalytic effect of minerals in oil shaleincreases the reactivity of organic matter during the pyrolysis pro-cess, and this promotion seems to be stronger in the presence ofoxygen [30].

The aforementioned results indicated that the NA oil shalebelongs to the high ash, low oil yield type with high activationcorrespond to 0.050.4 (second stage) and 0.41 (third stage) forNA oil shale. Distinguishing the second and third stages of FS oil

380 F. Bai et al. / Energy Conversion aThis work was supported by the National CooperativeInnovation Project on Chinese Potential Oil and Gas Resources(Grant No. OSR-06), the Science and Technology Project of the[26] Kk MV, Iscan AG. Oil shale kinetics by differential methods. J Therm AnalCalorim 2007;88:65761.

[27] L XS, Sun YH, Lu T, Bai FT, Viljanen M. An efcient and general analyticalapproach to modelling pyrolysis kinetics of oil shale. Fuel 2014;135:1827.

[28] Liu QQ, Han XX, Li QY, Huang YR, Jiang XM. TG-DSC analysis of pyrolysisprocess of two Chinese oil shales. J Therm Anal Calorim 2014;116:5117.

[29] Han XX, Jiang XM, Cui ZG. Thermal analysis studies on combustion mechanismDepartment of Jilin Province, China (Grant No. 20130302030SF),the Strategic Emerging Industry Development Projects of JilinProvince, China (Grant No. 2013Z050), and the Science andTechnology Development Project of Jilin Province, China (GrantNo. 20150520073JH).

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F. Bai et al. / Energy Conversion and Management 97 (2015) 374381 381

Thermal and kinetic characteristics of pyrolysis and combustion of three oil shales1 Introduction2 Materials and methods2.1 Materials2.2 Characterization of oil shale2.3 TG analysis2.4 Kinetic methods

3 Results and discussion3.1 FTIR and XRD characterizations3.2 Pyrolysis and combustion characteristics3.3 Pyrolysis and combustion kinetic analysis

4 ConclusionsAcknowledgmentsReferences