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Journal of Analytical and Applied Pyrolysis 95 (2012) 164–170

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h o me page: www.elsev ier .com/ locate / jaap

tructural changes in coal chars after pressurized pyrolysis

rancisco Jiménez, Fanor Mondragón, Diana López ∗

nstitute of Chemistry, University of Antioquia, A.A. 1226, Medellín, Colombia

r t i c l e i n f o

rticle history:eceived 27 September 2011ccepted 3 February 2012vailable online 10 February 2012

eywords:

a b s t r a c t

The aim of this work was to evaluate the effect of pressure on the structural properties and subsequentreactivity of coal chars. Pyrolysis reactions were carried out in a fixed bed reactor by varying the pressureup to 2.0 MPa. Two coal samples with a substantial difference in the swelling index were used for the anal-ysis. Pyrolysis experiments were carried out at 800 ◦C for 30 min after heating the sample at a constant rateof 20 ◦C/min and some samples were pyrolyzed at 900 ◦C and 0.1 MPa for comparison. Structural analysis

igh pressure pyrolysisaman spectroscopyoal reactivity

of the coal chars was performed using Raman microscopy; this characterization was complemented byscanning electron microscopy analysis, gas adsorption and reactivity towards molecular oxygen in a ther-mogravimetric equipment. Characteristic Raman bands of coal chars exhibited significant changes from0.1 to 0.5 MPa, after this pressure no significant changes were observed with pressure increments. Thepyrolysis pressure showed to have an influence in the ordering of the carbonaceous structures throughthe deconvoluted Raman spectra.

. Introduction

Modern thermal coal conversion processes use pressures higherhan 0.2 MPa. High pressure is mandatory in order to improve thefficiency of the conversion process, to facilitate the gas clean-ng units operation and to enhance the CO2 capture and furthertorage. Therefore, it is of general interest to assess the effect ofressure on the physical and chemical transformations of coal dur-

ng the stages involved in the thermal conversion of this fuel forower generation [1]. Thermal coal conversion processes start withhe devolatilization of the material, at this stage, the evolution ofolatile components leaves a carbonaceous residue whose reactiv-ty towards subsequent gasification reactions is highly influencedy the structural characteristics induced by the pyrolysis conditions2,3].

Coal char morphology and structure have been extensivelytudied, mainly at atmospheric pressure [4,5]. Experimental databtained at different pressures of pyrolysis have shown variableasification and oxidation reactivities, which vary according to theharacteristics of the parent coal. Overall, there have been observa-ions that char reactivity changes with variations in the pyrolysisressure [6]. These results are thought to be associated with atructure–reactivity relationship, the most frequent descriptor for

uch relationship has been the total surface area of the coal charhich is known to be in some cases affected by pyrolysis pres-

ure [7]. However, reactivity is a local process and something more

∗ Corresponding author. Tel.: +57 4 2196613; fax: +57 4 2196565.E-mail address: dplope@gmail.com (D. López).

165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.jaap.2012.02.003

© 2012 Elsevier B.V. All rights reserved.

specific than the total surface area should lead the reactivity ofthe carbonaceous material. The detailed description of reactivitychanges is associated with the identification of structural details atthe level of aggregates and micro-crystals or even in the molecularstructure of the coal char [8,9].

Raman spectroscopy is a high resolution photonic techniquewhich provides within few seconds, chemical and structural infor-mation of organic and inorganic materials, which is very useful intheir identification. Raman spectroscopy is based on the analysis ofscattered light by a material when a beam of monochromatic lightstrikes on it. A small portion of the light is inelastically scatteredwhich undergoes a frequency change that is characteristic of thesample and independent of the incident light frequency. Recentlypublished results have tried to correlate carbon structure, analyzedby Raman spectroscopy, with reactivity at atmospheric pressure[10–12], but experimental data on high pressure pyrolysis withmicro-structural characterization is still limited.

The potential of Raman spectroscopy in providing informa-tion about the carbon skeleton structure of coal/char at molecularlevel has long been realized, even for carbons without any signif-icant XRD-detectable crystalline structure [13]. Temperature hasbeen recognized as the main structure director among pyroly-sis conditions. Chabalala and coworkers showed a Raman studyof the char evolution in the range from 300 ◦C to 1000 ◦C [14],the work of Zaida and coworkers extended the thermal treat-ment up to 2600 ◦C [10]. Raman spectra of the coal chars formed

at 600 ◦C already presented well-defined carbonaceous structuresthat allowed semi-quantitative analysis of the pyrolysis tempera-ture effects [14,15]. Regarding the extend of char crystallinity, XRDhas shown well defined stacking height and radial spread of coal

F. Jiménez et al. / Journal of Analytical and A

Table 1Properties of coal samples.

Sample Venecia Titiribí

Proximate analysis (%)Moisture 6.7 1.9Volatile matter 44.4 37.8Fixed carbon 45.3 51.2Ash 3.6 9.1

Elemental analysis (%)C 67.2 78.1H 5.0 5.4N 1.9 1.9S 0.6 0.7O (by difference) 25.4 13.9

Maceral composition (%)Vitrinite 77.6 80.7Liptinite 10.5 12.1Inertinite 3.7 4.1

Ash composition analysis (%)K2O 1.2 1.3Na2O 3.5 0.5MgO 4.7 3.2CaO 10.5 15.8SiO2 38.7 37.2Al2O3 19.8 24.5Fe2O3 7.1 6.5

cardi

cpmesa(pp

2

2

tfttsntwh

2

tTm

TiO2 1.2 0.9

Swelling index 0.0 2.0

hars obtained at 600 ◦C and 900 ◦C, however without much vari-bility [16]. Low temperature characterization of the coal chars isequired to get further insight of the structure and reactivity depen-ence, especially for fluidized bed gasification technologies where,

n general, the reactor temperature does not overcome 900 ◦C [17].Raman spectroscopy was used in this work to determine

hanges in the coal char structural properties induced by theyrolysis pressure; particular attention was paid to possibleorphological differences between two materials with differ-

nt indexes of swelling. The information obtained through thispectroscopic technique was complemented by SEM analysis, gasdsorption and reactivity in air by thermogravimetric analysisTGA). Experiments were carried out mainly at 800 ◦C due toressurized fluidized bed reactor requirements. However, a com-arative study was done with some coal chars obtained at 900 ◦C.

. Experimental

.1. Coal samples

Venecia and Titiribí coal samples were collected directly fromhe coal mine. After grinding, the −60 + 80 (ASTM standard mesh)raction was chosen for the pyrolysis experiments. Table 1 showshe proximate and elemental analysis, the maceral composition andhe main inorganic components of the high temperature ash. Bothamples are low rank coals with high vitrinite content with no sig-ificant difference in the ash composition. It is important to noticehat the Titiribí sample has a value of 2 for the swelling index whichill be reflected in the softening of the sample during the thermaleating.

.2. Pyrolysis experiments

Coal samples were pyrolyzed in a high pressure fixed bed reac-or under inert atmosphere at pressures between 0.1 and 2.0 MPa.he reactor used in these experiments is part of a pressurized ther-ogravimetric analyzer CAHN TG-2151 (HPTGA). The temperature

pplied Pyrolysis 95 (2012) 164–170 165

controller allows a maximum of 900 ◦C with heating rates up to25 ◦C/min in pressurized conditions. For this work the HPTGA reac-tor was separated from the microbalance and adapted to carry outthe pressurized pyrolysis experiments. A system of tar condenserswas implemented in two stages. The first one, for heavy tars, had acooling coil with flowing water at 0 ◦C. The second stage was a con-denser for light volatile compounds, cooled with liquid nitrogen.The scheme of the experimental set-up can be seen in Fig. 1.

Pyrolysis experiments were conducted at pressures of 0.1, 0.5,1.0, 1.5 and 2.0 MPa. About 0.2 g of the coal sample was placed in thecenter of the reactor in a cylindrical sample pan, made of stainlesssteel wire mesh, number 200 of the ASTM mesh series. A thermo-couple (K type) for temperature control was located at 0.5 cm belowthe sample pan (see Fig. 1). Once the sample was in place, the reac-tor was sealed and the whole system purged for about 20 min with anitrogen flow of 0.2 l/min. After the purge, the reactor was pressur-ized with a nitrogen gas flow of 1.5 l/min, once the pressure reachedthe specified value, the gas flow was lowered to 0.2 l/min and thefurnace heating was started. For all pyrolysis experiments the tem-perature was risen from ambient to final temperature at 20 ◦C/min,with an isothermal time of 30 min.

2.3. Characterization and reactivity

The char samples from the pressurized pyrolysis tests wereanalyzed by Raman spectroscopy, using a standard HeNe laser of20 mW, polarization 500:1 and with a wavelength of 632.8 nm.Spectra were taken randomly in each sample. All spectra were sub-jected to a baseline correction. Then, they were normalized andaveraged with the available tools for spectroscopy data analysisincluded in the Origin® software. The structural characterization ofthe coal chars was complemented by SEM analysis and gas adsorp-tion. Reactivity tests of the chars were carried out at atmosphericpressure in a TGA with air under a non-isothermal heating rate of20 ◦C/min.

3. Results and discussion

3.1. Morphology of char particles

Morphology of the coal chars obtained at different pressureswas assessed with the SEM technique. SEM micrographs of somerepresentative samples are shown in Fig. 2. Particles of Venecia coalpyrolyzed at 0.1 MPa had flat surfaces and angular edges; there islittle fragmentation and the particle surface did not show evidenceof any macroporosity development in relation with the micrographscale. Samples obtained at high pressure showed great heterogene-ity; it is noticeable that the presence of particles with some smoothfaces and others with porosity development localized in slits. Thisfeature may be related to the distribution of macerals concentrateswhich are known to have different behaviors in pyrolysis even inthe same sample.

Titiribí coal char exhibited completely different features in rela-tion to the Venecia coal char. Upon heating, Titiribí coal particlessoftened and released volatiles trapped inside the molten “coal”and therefore causing swelling of the coal particles. The pressurehas a marked effect during this process. SEM images of parti-cles pyrolyzed at 0.1 MPa showed rounded surfaces. This featuremarkedly increases with the increment of the pyrolysis pressurealong with the appearance of large holes which are originated bythe evacuation of gaseous substances when the sample is in its

fluid state. The holes acquire a definite shape in the resolidifica-tion process. These cavities and evacuation holes are bigger withthe increasing of the pyrolysis pressure. Features observed in theSEM images for the softening coal are in good agreement with the

166 F. Jiménez et al. / Journal of Analytical and Applied Pyrolysis 95 (2012) 164–170

using

c[

macippcss

Fig. 1. Scheme of the experimental set-up for pressurized pyrolysis

onventional classification of particles after pressurized pyrolysis7,18].

The gross morphological characteristics assessed by SEMicroscopy clearly show differences between the samples and

mong different pyrolysis pressures. These differences are not veryonsistent with the N2-BET surface area of the samples. As shownn Fig. 3, surface area does not seem to be affected by the pyrolysisressure in both samples. This result could be associated with the

resence of secondary pyrolysis and tars re-adsorption over theoal char, causing a porosity blockage due to the deposition of aecondary char [19,20]. These processes are favored by high pres-ures and they would severely affect the porosity evolution during

Fig. 2. SEM micrographs

a modified high pressure vessel of a thermogravimetric equipment.

devolatilization, causing very low surface areas as the ones shownby the samples of this investigation.

3.2. 3.2 Raman spectroscopy

Fig. 4 shows that pyrolysis temperature between 800 ◦C and900 ◦C does not make an important difference in the resolutionof the Raman spectrum of the coal samples pyrolyzed at 1.0 MPa.

Raman spectra of the coal chars obtained at different pressures andat 800 ◦C are shown in Fig. 5. Characteristic bands assigned to thepresence of carbonaceous structures, in the first order spectrum,are clearly present in the chars of Venecia and Titiribí coals. The

of the coal chars.

F. Jiménez et al. / Journal of Analytical and Applied Pyrolysis 95 (2012) 164–170 167

Pressure (MPa)

2.01.51.00.50.0

BE

T S

urf

ace a

rea (

m2

/g)

0.0

1.0

2.0

3.0Venecia

Titiribí

bdtceattt1a

Ft

nte

nsit

y

0.8

1.0

1.2

0.1

0.5

1.0

1.5

2.0

TitiribíPressure MPa

Raman shift (cm-1

)

18001600140012001000800

No

rmalized

in

ten

sit

y

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1

0.5

1.0

1.5

2.0

VeneciaPressure MPa

a

b

Fig. 3. BET surface areas as a function of pyrolysis pressure.

and around 1360 cm−1 is known as the D band. It is considered aescriptor of the defects in graphite crystals such as the presence ofetrahedrally bonded carbons [21], however, for highly disorderedarbon compounds like coal chars, D band are thought to be gen-rated by aromatic clusters of no less than 6 rings [13]. The bandround 1580 cm−1 is named as the G band, its presence is related tohe energy of sp2 bonds and is assigned to normal graphitic struc-ures [22,23]. Coal chars used in this research exhibit a Raman band

hat barely pronounces as a shoulder in the D band, located at about180 cm−1. It has been related to the presence of Caromatic–Calkyl,romatic (aliphatic) ethers, C–C on hydroaromatic rings and C–H

Raman shift (cm-1

)

Raman shift (cm-1

)

18001600140012001000800

No

rma

lize

d in

ten

sit

y

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

800 ºC

900 ºC

Venecia (1.0 MPa)

18001600140012001000800

No

rmalized

in

ten

sit

y

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

800 ºC

900 ºC

Titiribí (1.0 MPa)

a

b

ig. 4. Raman spectra of the coal chars pyrolyzed at 1.0 MPa at different pyrolysisemperature.

Raman shift (cm-1 )

18001600140012001000800

No

rmalized

i

-0.2

0.0

0.2

0.4

0.6

Fig. 5. Raman spectra of the coal chars produced at different pressures at 800 ◦C for30 min.

on aromatic rings [13]. The broadening in the Raman spectra ofour coal chars gives rise to a pronounced valley between G andD bands. This feature is considered a response to the presence ofsmall aromatic ring systems and related structures very commonin amorphous carbon [13].

Fig. 5 shows that there is no specific variation in Raman inten-sity or wavelength number for the G and D bands in each sample.However, there are differences in the height of the valley betweenthe G and D bands. The observed bands have a quite large ampli-tude, but it is clear a variation in the breath at the half height withthe change of pyrolysis pressure. Characteristic bands for Titiribícoal chars have wider band-widths and higher heights of the val-ley than Venecia coal char. This implies a major ordering of carbonstructures in Venecia coal with pyrolysis pressure. The disorder inTitiribí coal could be induced by the softening and subsequent reso-lidification of the coal melt during pyrolysis, generating a broaderdistribution of carbonaceous structures in the solid residue. Thiscomparison is shown in Fig. 6 for samples pyrolyzed at 2.0 MPa.

The raw data from the Raman spectra show that the incrementof pressure from 0.5 MPa to 2.0 MPa in the pyrolysis experimentsdoes not induce evident structural changes in the carbonaceousresidue for either of the samples. Only the coal chars preparedat the pressure of 0.1 MPa exhibits a significant difference in theRaman spectrum in relation to the samples prepared at higher pres-

sures. Even after baseline correction and normalization of Ramanspectra, it is difficult to extract micro-structure information fromthe data due to the wide range (from 800 cm−1 to 1800 cm−1) of

168 F. Jiménez et al. / Journal of Analytical and Applied Pyrolysis 95 (2012) 164–170

Raman shift (cm-1

)

18001600140012001000800

No

rma

lize

d in

ten

sit

y

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

Titiribí

Venecia

2.0 MPa

Ratc

ps1vstvwbpww

mgrbiat

2.01.51.00.50.0

In

ten

sit

y r

ati

o (

I D/I

G)

0.95

1.00

1.05

1.10

Venecia

Titiribí

Fig. 6. Raman spectra of the chars obtained at 2.0 MPa at 800 ◦C.

aman shifts in the spectrum. Deconvolution of the Raman spectrallows the identification of masked features under the curve andheir relation with different carbonaceous structures in the coalhar [8,13,24].

The Raman spectra were subjected to a mathematical fittingrocess and deconvoluted into four peaks, a typical deconvolutedpectrum is depicted in Fig. 7. Fitted peaks are located around 1180,350, 1550, and 1600 cm−1. Peak R1 (1180 cm−1) is attributed toibration modes of sp2–sp3 or C C and C C bonds in polyene liketructures [25]. Peak D* (1350 cm−1) and G* (1600 cm−1) are relatedo the G and D bands of graphite like materials as described pre-iously. The valley created by the overlapping of D* and G* bandsas fitted by peak R2; this region of the spectrum is considered to

e generated by a wide distribution of carbon structures like off-lane defects, organic molecules, fragments or functional groupsith sp2 bonding [26]. The R-squared of the mathematical fittingas in the order of 0.999.

The peak intensity ratio between bands D and G (ID/IG) is com-only used to characterize carbonaceous structures of partially

raphitized materials [27]. Fig. 8 shows the variation of the ID/IGatio as a function to the pyrolysis pressure. There is a smallut clear trend in the reduction of the ID/IG ratio as pressure is

ncreased. This behavior is more pronounced for Venecia coal char,nd overall it could indicate a trend towards more ordered struc-ures for this sample as the pyrolysis pressure increases.

Raman shift (1/cm)

18001600140012001000800

Inte

nsit

y

0

5000

10000

15000

Deconvoluted peaks

Raw spectrum

Fitted spectrum

G*D*

R1

R2

Fig. 7. Deconvoluted Raman spectrum.

Pressure (MPa)

Fig. 8. ID/IG ratio of the char as a function of pyrolysis pressure.

The ID/IG relation with carbon graphitization is arguable forhighly disordered coal chars; its decrease in these kinds of materi-als is hardly related with the removal of defects that originate the Dband. Peak area fractions are gaining more attention for the descrip-tion of the carbonaceous structures distribution [22,24] than theintensity, which is known to be affected by technical aspects dur-ing the spectra collection. Fig. 9 shows the area fractions of peaksR1, D*, R2 and G* with increasing pyrolysis pressure. Both sam-ples exhibit quite similar trends in the distribution of carbonaceousstructures, the fraction due to aromatic clusters with no less than6 rings [8], D*, accounts for nearly the 43% of the carbonaceouscomposition. Peak area of R2 band decreases for Titiribí coal charswith increasing pressure while G* increases; these pair of peaksseems to compensate each other keeping the net trend invariantwith pyrolysis pressure. R1 band peaks are associated with theconcept of active sites for gasification reactions. It gathers mainlylinking bonds between aromatic rings and aliphatic chains [28],this band did not show appreciable changes with pyrolysis pres-sure. The deconvoluted spectra reveal structural modifications ofthe coal chars upon increasing pyrolysis pressure.

3.3. Coal chars reactivity

Coal chars reactivity in combustion was evaluated using ther-mogravimetric analysis (TGA), specific reactivity versus conversionprofiles are presented in Fig. 7. Carbon conversion was calculated as

follows: (m0 − mt)/(m0 − mf) [11], where m0 refers to the coal charinitial mass and mf is the mass of the residue after the reaction, mt

is the mass in each time in the TGA data. The specific reactivity isexpressed as 1/mt × (dmt/dt).

Pyrolysis pressure (MPa)

21,510,50,1

Pe

ak

(R

1,D

*,R

2,G

*) a

rea

fra

cti

on

0,0

0,2

0,4

0,6

0,8

1,0

Venecia

Titiribí

R1

D*

R2

G*

Fig. 9. Area fractions of deconvoluted peaks as a function of pyrolysis pressure.

F. Jiménez et al. / Journal of Analytical and Applied Pyrolysis 95 (2012) 164–170 169

Conversion

1,00,90,80,70,60,50,40,30,20,10,0

Specific

Reactiviy

(1/s

)

0,0

0,2

0,4

0,6

0,8

0.1

0.5

1.0

1.5

2.0

Pressure (MPa)

Venecia

Conversion

1,00,90,80,70,60,50,40,30,20,10,0

Sp

ecific

Re

activiy

(1

/s)

0,0

0,2

0,4

0,6

0,8

0.1

0.5

1.0

2.0

Pressure (MPa)

Titiribí

pcttritcmaTRTr

ttotcp[lhIa2oa

Pyrolysis pressure (MPa)

2,01,51,00,50,0

"O

nset"

te

mp

era

ture

(ºC

)

540

560

580

600

620

640

660

680

700

Valley h

eig

ht

(IV)

0,40

0,42

0,44

0,46

0,48

0,50

0,52

0,54

0,56

0,58Grey: Onset temperatureWhite: Valley height

Circle: TitiribíTriangle: Venecia

References

Fig. 10. Coal chars reactivity in air in TGA.

Fig. 10 shows no significant difference in air reactivity with theyrolysis pressure in each group of coal chars. For each set of coalhars, TGA profiles suggest that there is no relation between struc-ure changes observed in the deconvoluted Raman spectra and inhe SEM micrographs while increasing pyrolysis pressure with theate of combustion of the coal chars. Apparently, the informationn Fig. 10 implies that pyrolysis pressure does not impact the dis-ribution or concentration of active sites that participate in theombustion reaction even with slight changes in the carbonaceousicro-structure as revealed in the changes of ID/IG ratio (Fig. 8),

nd band peaks areas (Fig. 9). Overall, reactivity profiles show thatitiribí chars exhibits a higher combustion rate than Venecia chars.aman spectra could be related with this feature in the way thatitiribí coal generated wider bands, describing a less ordered mate-ial which in general has been considered to be more reactive.

A more detailed analysis of the TGA data (see Fig. 11) shows thathe onset (ignition) temperature for the non-isothermal combus-ion of the coal chars follows the same trend of the valley intensitybserved in the Raman spectra. The onset temperature describeshe moment of ignition and reflects the thermal behavior of thearbon structures in coal chars. The calculation of the onset tem-erature was done as described in the work of Li and coworkers29]. Fig. 11 shows that onset temperatures of Titiribí chars areower than these for Venecia char; this is in accordance with theigher reactivity that Titiribí chars exhibited, as shown in Fig. 10.

ncrease in pyrolysis pressure from 0.1 to 0.5 MPa seems to have

n influence in ignition temperature causing an increase of about6 ◦C for Titiribí char and 22 ◦C for Venecia char. Further increasef pressure did not cause any change in the ignition temperaturend the difference between the two samples remained constant at

Fig. 11. On-set temperature of the pressurized chars in combustion and relatedvalley height.

about 86 ◦C. The ignition temperature increment in the first changeof pressure can be due to a reduction in reactivity of the char whichcan be related to the decrease in the intensity of the valley, createdby the overlapping of bands G and D in the Raman spectra.

4. Conclusion

A pressurized fixed bed reactor was used to evaluate the pres-sure effect on the pyrolysis of two coal samples. The coal chars forboth samples after the pressurized pyrolysis showed considerablesurface features in SEM micrographs. Titiribí coal, with a swellingindex of 2.0, generated swollen particles with evacuation holes andporosity that increased with the pyrolysis pressure.

Raman spectroscopy of the coal chars showed slight variationsin the carbonaceous structures among the sample pyrolyzed at0.1 MPa and all the samples obtained at high pressure. There wasno apparent difference in the raw Raman spectra for the coal charsrecovered between 0.5 and 2.0 MPa for either coal sample. Coalchars from Titiribí sample had Raman peaks with higher ampli-tude at the half-height, implying low definition and regularity inthe carbonaceous structures with regard to Venecia coal char.

The small differences in the structural ordering between thesamples may be due to the fluid stage and subsequent resolidifi-cation of Titiribí coal. However, structural changes due to pyrolysispressure slightly affect the coal chars IG/ID ratios and area fractionsof deconvoluted peaks.

Observed differences in morphology and Raman structuralchanges do not appear to have a meaningful impact in the reactiv-ity in air of the coal chars obtained at high pressure. The reactivityof the chars seems to be a more global process not influenced bythe micro-structure of carbon. However, the ignition temperaturechanged inversely with the trend of spectra valley (overlappingbetween band D and G) as pyrolysis pressure increased.

Acknowledgments

The authors thank ISAGEN, COLCIENCIAS and University ofAntioquia for financing the project No 1118-06-1753. F.J. speciallythanks financial support during the doctoral research. Authorsthanks the University of Antioquia for the financing of the “Sosteni-bilidad” program 2011–2012.

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