Diffusion of Solvents in Coals: 2. Measurement of DiffusionCoefficients of Pyridine in Cayirhan Lignite

Meryem Seferinogˇlu†,‡ and Yuda Yu¨rum*,§

Department of Chemistry, Hacettepe UniVersity, 06532 Beytepe, Ankara, Turkey, andFaculty of Engineering and Natural Sciences, Sabanci UniVersity, 34956 Tuzla, Istanbul, Turkey

ReceiVed December 1, 2005. ReVised Manuscript ReceiVed March 10, 2006

The aim of this study is to measure the diffusion coefficients of pyridine in Turkish C¸ ayirhan lignite(C: 57.1 wt %, dmmf) at temperatures about 20-27 °C and determine the type of transport mechanism ofdiffusion. The raw coal sample was demineralized with HCl and HF by standard methods, and the raw anddemineralized coal samples were extracted with pyridine. To investigate the diffusion of pyridine vapor incoal samples, the mass of pyridine uptake per mass of coal sample (Mt/M∞) was calculated as a function oftime. The diffusion coefficients were measured from the slope of graphs ofMt/M∞ versust1/2. The diffusioncoefficient of pyridine in the raw coal increased from 10.0× 10-15 to 11.9× 10-15 m2/s when the temperaturewas elevated from 21.1 to 26.9°C, respectively. The diffusion coefficients of pyridine raw coal and of thosetreated with HCl and HF were 11.9× 10-15, 4.3× 10-15 , and 4.8× 10-15 m2/s at 26.9°C, respectively. Thestudies in the present work on pyridine vapor diffusion in raw coals have generally indicated that the diffusionobeyed the Fickian diffusion mechanism the temperatures 20.0-27.0 °C. Generally, the diffusion exponentvalues increased when the temperature elevated from 20.0 to 27.0°C, but this raise placed the diffusion ofpyridine between the Fickian diffusion and Case II diffusion mechanisms.

Introduction

Diffusion of solvents through coal is the main limiting stepof many coal processes; therefore, it is necessary to understandthe mechanism and kinetics of solvent diffusion into the coalmatrix.1 Coal has a cross-linked three-dimensional network gelstructure in which different physical forces such as hydrogenbonds, London, and van der Waals forces exist. In addition totheseπ-π and charge-transfer interactions intermolecular forcestake place between coal molecules. The solvent molecules mustbreak these intermolecular forces and penetrate the solid coal.2

The dynamics of solvent swelling of the macromolecular systemcan provide significant knowledge about the structure of thematerial itself and the interaction between the penetrant andmacromolecular material. It may be possible to identify thethermodynamic glassy and rubbery states of the macromolecularsystem. If the system is in the glassy state, it can be determinedwhether the diffusion of the solvent is controlled by Fickiandiffusion and/or by relaxation of the macromolecular system.

It is known that coals are glassy, strained, macromolecularsolids.3 The optical anisotropy of Illinois No. 6 coal viewed ina thin section through a polarizing microscope was associatedwith the existence of strain in a glassy system.4 If the coal was

swollen with a good solvent such as pyridine, noncovalent bondssuch as hydrogen bonds between coal-coal molecules couldbe removed, and hence the coal could convert to rubberystate.4,5

Glassy coals and rubbery coals are different materials. In theglassy state, all large molecular motions are restricted althoughsegmental motion may still be exhibited. Diffusion rates arevery low because diffusion through the macromolecule allowsthe passage of the diffusing molecule. As the temperature isincreased vibrational motions also increase, the macromolecularunits move apart, and the density of the whole materialdecreases. At some critical small range of temperature, there isnot only sufficient space for rotations and inter- and intrachainmovements, but also sufficient thermal energy.6 In a rubberysolid, molecular motion is similar to that in a non-cross-linkedpolymer solution of the same composition.

Diffusional limitations are of concern in virtually all typesof coal processing. It is generally necessary to diffuse reactantsinto coal, and/or products or moisture out of coal particles atsome point in any process. The studies of solvents diffusionhave generally indicated that diffusion in coals is similar in manyrespects to the diffusion of solvents through glassy polymers.7

The similarities in structure between coal and glassy polymershave led to the application of theories of solvent diffusionbehavior of polymers to coals. In the modeling of solventdiffusion in coal, it is necessary to assume that the particles arespherical, isotropic, nonporous, and identical. These assumptionsare major approximations. In particular, coals are heterogeneouswith different physical and chemical properties. It possible to

* Corresponding author. Phone: 90 216 4839512. Fax: 90 216 4839550.E-mail: yyurum@sabanciuniv.edu.

† Hacettepe University.‡ Present address. Directorate of Customs and Customs Enforcement,

Ankara Center Laboratory, Behic¸bey, Ankara, Turkey.§ Sabanci University.(1) Yurum, Y. Clean Utilization of Coal, Coal Structure and ReactiVity

and EnVironmental Aspects; NATO Advanced Study Institute Series C;Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol. 370.

(2) Sharma, D. K.; Giri. C. C.Fuel 2000, 79, 577.(3) Larsen, J. W. InClean Utilization of Coal; Yurum, Y., Ed.; NATO

Advanced Study Institute Series C; Kluwer Academic Publishers: Dor-drecht, The Netherlands, 1992; Vol. 370, p 2.

(4) Brenner, D.Fuel 1985, 64, 167.(5) Nomura, S.; Thomas, K. M.Fuel 1998, 77, 829.(6) Peppas, N. A.; Lucht, M. L.Chem. Eng. Commun.1985, 37, 333.(7) Otake, Y.; Suuberg, F. M.Energy Fuels1997, 11, 1155.

1150 Energy & Fuels2006,20, 1150-1156

10.1021/ef050399i CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 04/06/2006

express the diffusional solvent penetration in terms of the generalequation:8

whereMt is the amount of solvent diffused in the macromo-lecular structure at timet, M∞ is the amount of solvent diffusedat steady state,k is a constant that depends on structuralcharacteristic of the system, andn is an exponent characteristicof the mode of transport of the solvent in the macromolecularstructure. Whenn ) 0.5, the diffusion is Fickian and describesa system in which the process is controlled by the diffusioncoefficient. When n ) 1.0, case II transport occurs andcharacterizes the moving-boundary phenomenon. In the case IImechanism, the velocity of a sharp advancing front betweenthe inner glassy core and the outer swollen rubbery materialcontrols the process. When the values ofn are between 0.5 and1.0, this indicates anomalous transport and is characterized bya situation where the diffusional and structural relaxation energyare comparable.9

The solution of Fick’s second law of diffusion in sphericalsystems gives10 the following equation:

where Mt and M∞ represent the amount of solvent diffusedentering the spheres with radiusa, at timest and steady state,respectively.D is the coefficient of diffusion of the solvent.Neglecting the contribution of the term 3Dt/a2, since the absolutevalue of 3Dt/a2 is much smaller than 6[Dt/πa2], the value ofDis found from the slope of a plot ofMt/M∞ versust1/2.

The activation energy of diffusion is calculated using theequation11

whereD0 is a temperature-independent pre-exponential (m2/s),Ea is the activation energy for diffusion,R is the gas constant,andT is temperature. The activation energy was calculated fromthe slope of the straight line of the graph lnD versus 1/T.

The aim of the present study is to measure the diffusioncoefficients in a temperature range of 21.0-27.0 °C, todetermine the effect of demineralization and extraction processeson the solvent diffusion into coal matrix, and to determine thetype of transport mechanism of diffusion of pyridine in TurkishCayirhan lignite.

Experimental Section

Turkish Cayirhan lignite with a carbon content of 51.7 wt %(dmmf) was used in the study. Analysis of the C¸ ayirhan lignite ispresented in Table 1. The coal sample was ground under a nitrogenatmosphere to 60 mesh ASTM and stored under nitrogen. The coalsample was Soxhlet-extracted with toluene/ethyl alcohol solventcouple (1:1) at its atmospheric boiling point to separate the resinsof coal and was dried in a vacuum oven at 50°C for 24 h under anitrogen atmosphere. This sample is called raw coal throughoutthe text of the present work. The raw coal samples were deminer-

alized with HCl and HF by standard methods.13 A volume of 2 Lof 6 N HCl was added to 200 g of coal. The slurry was stirred for24 h under a nitrogen atmosphere, and then it was filtered andwashed with distilled water until the filtrate became neutral.Consecutively, 1.6 L of distilled aqueous (40%) HF was added toHCl-washed coal, and the mixture was stirred for about 24 h undera nitrogen atmosphere. After filtering, the demineralized coal waswashed with 1 L of distilled water and dried at 50°C for 24 hunder vacuum.

The raw and demineralized coal samples were extracted withpyridine exhaustively in a Soxhlet extractor under a nitrogenatmosphere until the color of solvent in the sidearm of the extractorbecame colorless. After extracting the coal exhaustively with thecarbon disulfide (CS2)/N-methyl-2-pyrrolidinone (NMP) mixedsolvent, Takahashi et al.14 washed the residue with acetone toremove CS2 and retain NMP. A similar technique was employedin the present study; the extracted samples were stirred with 500mL of ethanol (98%, by volume) in a flask for 24 h and dried ina vacuum oven at 60°C to remove pyridine retained in the poresof the coal.

Particle size distributions of raw and pyridine-extracted coalsamples were determined with a set of sieves of 800, 400, 200, 63,and 40µm mesh size, and the average radii of the samples werecalculated.

An adiabatic isothermal setup,15 designed and manufactured inour laboratories and made from Plexiglas, which contained a heaterand digital temperature control system, an electronic digital balanceof 0.001-g accuracy, and a beaker filled with pyridine, was used inthe diffusion experiments. At the start of the experiment, about0.25 g of coal sample was evenly distributed in a Petri dish andthe initial weight of the coal was recorded. The temperature of theexperiment was set up, the system was closed and flushed withnitrogen, and the weight increase due to pyridine uptake wasrecorded until a constant weight was attained. The equilibrium timeto reach a constant weight changed between about 3600 and 2500min depending on the temperature set at the start of the experiment,which varied from 21.0 to 27.0°C, respectively.

The extent of swelling of original and treated lignite samples inpyridine was measured according to the methods given by Larsen3

and Liotta et al.16 Approximately 100 mg of a coal sample wasplaced in a 6-mm o.d. tube and centrifuged for 5 min at 3500 rev/min. The height of the sample was measured ash1. Excess pyridine(∼1 mL) was added to the tube, the contents of the tube were mixed,the tube was centrifuged after 24 h, and the height of the sample

(8) Ritger, P. L.; Peppas, N. A.Fuel 1987, 66, 1379.(9) Ndaji, F. E.; Thomas, K. M.Fuel 1993, 72, 1525.(10) Crank, J.Mathematics of Diffusion; Oxford University Press:

London, 1970.(11) Callister, W. D., Jr.Material Science and Engineering, 2nd ed.;

Wiley: New York, 1991.

(12) Mazumdar, B. K.Fuel 1999, 78, 1097.(13) Yurum, Y.; Kramer, R.; Levy, M.Thermochim. Acta1985, 94, 285.(14) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M.Energy Fuels2001,

15, 141.(15) Seferinogˇlu, M. Ph.D. Thesis. Hacettepe University, Ankara, Turkey,

1999.(16) Liotta, R.; Rose, K.; Hippo, E.J. Org. Chem.1981, 46, 277.

Mt

M∞) ktn (1)

Mt

M∞) 6[Dt/πa2]1/2 - 3Dt

a2(2)

D ) D0 e-Ea/RT (3)

Table 1. Proximate and Ultimate Analyses of C¸ ayirhan Lignite

proximate analysismoisture, % 1.8mineral matter, %, dry 22.0volatile matter, %, dmmf 30.9fixed carbon, %, dmmf 45.3

ultimate analysis, %, dmmfcarbon 57.1hydrogen 4.5nitrogen 1.8sulfur (total) 5.7oxygen (by difference) 30.9H/C 0.96O/C 6.5carbon aromaticity,a fa 0.6

a The carbon aromaticity (fa) for the Cayirhan coal sample was calculatedusing an equation that was defined for low rank coal by Mazumdar.fa )1.20-0.617 H/C (Mazumdar, 1999).12

Measuring Diffusion Coefficients in C¸ ayirhan Lignite Energy & Fuels, Vol. 20, No. 3, 20061151

in the tube (h2) was measured. The volumetric swelling ratio wascalculated asQv ) h2/h1.

Results and Discussion

Particle Size Distribution. Particle size distributions of theraw, acid-washed, raw-pyridine-extracted and acid-washed/pyridine-extracted coal samples are presented in Figures 1 and2, respectively. Particle size of the raw coal showed a maximumat 200µm. This fraction constituted about 70% (by weight) ofthe whole sample. In the case of HCl- and HCl/HF-washedsamples, particle size shifted toward a slightly smaller particlesize to maximal at 100µm (Figure 1). Some bigger particles ofcoal presumably contained higher contents of mineral matterwhich was soluble in HCl and in HF solutions and thoseminerals were washed off during acid treatment and thereforea size reduction was observed. Krzesin´ska17 claimed that HClacid removes oxide, carbonates (such as calcite), exchangeablecations, and monosulfides from coal structure and affects thephysical structure of coal. Similarly, HF acid removes silicateminerals.17,18

Particle size distribution of pyridine-extracted raw and acid-washed coal samples showed a similar trend (Figure 2).Pyridine-extracted raw and acid-washed coal formed a samplethat was composed of approximately 30% (by weight) of thewhole sample and showed a maximum particle size at 200µm.This indicated that pyridine extraction was not effective tochange the particle size of C¸ ayirhan lignite.

Diffusion Coefficients. A Cayirhan lignite sample with anaverage particle radius that changed in the range of 5.9× 10-5

to 10.9× 10-5 m was used. The coal particles were assumedto be of spherical shape which had completely similar features.

In all experiments, the pyridine uptake of C¸ ayirhan lignite wasrecorded until equilibrium was reached. A graph ofMt/M∞versust1/2 for pyridine diffusion in raw C¸ ayirhan lignite at 21.0°C is presented in Figure 3. To determine the slope of the linearportion of a similar graph, a new graph that contained ap-proximately 60% of the data from the start of the experimentwas reconstructed.6,9

This type of graph for the same experiment is given in Figure4. The diffusion coefficients were measured from the slope ofsuch graphs for all the samples. Figure 5 gives the change ofthe measured diffusion coefficients of pyridine in raw and acid-treated C¸ ayirhan lignite with temperature. Increasing temperatureled to an increase in diffusion coefficients. This situation mightbe attributed to an increase of the concentration of pyridinevapor when temperature was increased. Therefore, the pyridinemolecules diffused into the coal matrix at a faster rate. Inprevious reports,19,20 it was observed also that there was a(17) Krzesinska, M.Energy Fuels1997, 11, 686.

(18) Davidson, R.Models of Occurrence of Trace Elements in Coal;IEA Coal Research CCC/36; The Clean Coal Centre: London, June 2000. (19) Wargadalam, V. J.; Norinaga, K.; Lino, M.Fuel 2002, 81, 1403.

Figure 1. Particle size distribution of raw, HCl-, and HCl/HF-washedCayirhan lignite samples.

Figure 2. Particle size distribution of the raw/pyridine-extracted, HCl-washed/pyridine-extracted, and HCl/HF-washed/pyridine-extractedCayirhan lignite.

Figure 3. Mt/M∞ versust1/2 for the pyridine diffusion in raw C¸ ayirhanlignite at 21.1°C.

Figure 4. Mt/M∞ versust1/2 for the pyridine diffusion in raw C¸ ayirhanlignite at 21.1°C with 60% of the data from the start of the experiment.

Figure 5. Change of diffusion coefficients of pyridine in raw and acid-washed C¸ ayirhan lignite with temperature.], Raw coal.0, HCl-washedcoal. 4, HCl/HF-washed coal.

1152 Energy & Fuels, Vol. 20, No. 3, 2006 Seferinogˇ lu and Yurum

tendency toward an increase in diffusion with increasingtemperature. It is suggested that the final extent of swelling wasnot sensitive to temperature, but the rate of swelling increasedsignificantly with increasing temperature.20

Sharma and Giri2 studied the kinetics of diffusion ofN-methyl-2-pyrolidone (NMP) swelling of coal. It was alsofound that the rate of swelling of coal in NMP was faster athigher temperature. It is proposed that the probable reason forthis phenomenon at higher temperatures is the increase of kineticenergy of the solvent molecules, and hence, the moleculesdiffuse into the coal matrix at a faster rate.

It was observed in the present work that the diffusioncoefficient of pyridine in the raw coal sample slightly increasedfrom 1.0 × 10-14 to 1.2 × 10-14 m2/s when the temperaturewas elevated from 21.1 to 26.9°C (Figure 5). All of the acid-treated coal samples also showed a similar trend. In the case ofthe HCl-washed coal samples, the diffusion coefficients rangedbetween 3.8× 10-15 and 4.3× 10-15 m2/s with an increase intemperature. With regard to HCl/HF-treated coal samples, thediffusion coefficients changed from 3.0× 10-15 to 4.8× 10-15

m2/s when temperature was elevated from 22.6 to 25.6°C.The diffusion of pyridine in the raw coal was rather high

compared to those of acid-treated coal samples. With respectto this result, demineralization of the raw coal might beresponsible for a reduction in the pyridine uptake of thedemineralized coal sample. It seemed that the minerals in coalenhanced the pyridine diffusion. It is possible that pyridineinteracted preferentially and strongly with the minerals in coal.Pyridine is known to be an organic Lewis base and hydrogenbond acceptor stronger than any other nitrogen-containingcompound. It is also well-known that lignite and lower rankcoals are highly oxygenated.21 Hence, it is thought that thenoncovalent interaction such as hydrogen bonds would bebetween pyridine molecules and mineral components in coal.As the concentration of minerals that are capable of forminghydrogen bonds increases, the number of bonded pyridinemolecules can be increased. Therefore, diffusion of pyridine inraw coal must have increased. When the coal was treated withHCl and HF acids, HCl removed carbonate and ion-exchange-able minerals, while HF removed silicate and clay minerals fromthe coal. It was estimated that pyridine molecules could interactwith carbonate and/or ion-exchangeable mineral matter and withsilicate and/or clay minerals on the original coal surface. Whenthese minerals were removed by acid treatment, the interactionbetween pyridine molecules and minerals might be reduced. Thatmight be the possible reason for the lower values of the diffusioncoefficients of pyridine in acid-treated coal samples comparedto those in raw coal. Another reason for the reduction of thediffusion of pyridine in acid-treated coal samples might be thecollapse of the physical structure; the total porosity of sub-bituminous coals is about 25-30%.22 Using the dependence ofcoal porosity on coal rank, one can deduce that the lignite usedin the present work was also a highly porous material. Porosityas well as defects (e.g., cracks) associated with minerals canaffect measurements of diffusion parameters. The removal ofminerals from coal may cause the collapse of the structure anddecrease of number of cracks resulting in decrease of observeddiffusion.

Some organic bases such as alcohols and amines have bothan unshared electron pair and a hydrogen atom to donate in the

interactions with a surface. Those adsorbates may form hydrogenbonds to the surface by accepting or donating a hydrogen atom.Some adsorbates interact preferentially and strongly with coalmineral matter.23 Glass and Larsen23 reported that alcohol,amines, and pyridine molecules had more exothermic specificadsorption heats on the original coal (17% ash) compared tothat of demineralized (2% ash) coal. Pyridine, alcohols, andamines were found to interact more strongly with carbonate and/or ion-exchangeable minerals and with silicate or clay mineralson the original coal surface from the IGC data. It was proposedthat pyridine, alcohol, and amines have additional interactionswith the mineral matter-containing coals.23

Figure 6 shows the change of the diffusion coefficients ofpyridine in raw and pyridine-extracted C¸ ayirhan lignite withtemperature. The diffusion coefficients of pyridine in all of thepyridine-extracted coal samples seemed to be less, comparedto that of the raw coals. Moreover, extraction of raw coal withpyridine decreased the pyridine diffusion in the coal very much.Similar results were also obtained by Seferinogˇlu and Yurum24

for Elbistan lignite.The diffusion coefficients of pyridine in pyridine-extracted

raw coal increased from 2.9× 10-15 to 4.1× 10-15 m2/s whentemperature was elevated from 22.7 to 26.7°C, respectively.Although pyridine extraction might have increased the surfacearea, it appears that the porosity created by pyridine extractionwas not effective to enhance the pyridine diffusion. The reasonfor this result might be the relaxation of the coal during itsextraction with pyridine which created irreversible solventswelling of coal. It is well-known that removing pyridine fromcoal after extraction is very difficult, and some pyridine canremain in the coal structure even after drying under vacuum attheir boiling points.7,25 Therefore, one possible reason for thereduced diffusion of pyridine into coal might be the presenceof some amounts of pyridine that remained after pyridineextraction of the coal that prevented the diffusion of extraamounts of pyridine into the coal matrix.

Yurum et al.26 reported that pre-extraction of Zonguldakbituminous coal with pyridine caused a sharp decrease inpyrolysis yield and the initial glass transition temperatureincreased compared with that of untreated coal. The reason forthis result was that small amounts of pyridine remained sorbedin the dried coal after pyridine extraction. In the pyridine

(20) Otake, Y.; Suuberg, E. M.Fuel 1989, 68, 1609.-(21) Sugano, M.; Mashimo, K.; Wainai, T.Fuel 1999, 78, 945.-(22) van Krevelen, D. W.Coal: Typology, Physics, Chemistry, Constitu-

tion, 3rd ed.; Elsevier: Amsterdam, 1993; p 197.

(23) Glass, A. S.; Larsen, J. W.Energy Fuels1994, 8, 629.(24) Seferinogˇlu, M.; Yurum, Y. Energy Fuel2001, 15, 135.(25) Nishioka, M.Fuel 1993, 72, 997.(26) Yurum, Y.; Karabakan, A. K.; Altuntas¸, N. Energy Fuels1991, 5,

701.

Figure 6. Change of the diffusion coefficients of pyridine in raw andpyridine-extracted C¸ ayirhan lignite with temperature.], Raw coal.0,Raw coal/pyridine extracted.4, HCl-washed coal/pyridine extracted.O, HCl/HF-washed coal/pyridine extracted.

Measuring Diffusion Coefficients in C¸ ayirhan Lignite Energy & Fuels, Vol. 20, No. 3, 20061153

extraction process, while pyridine swells the solid coal matrixit also diffuses into the micropores of the solid coal.

Pyridine breaks the intermolecular forces between coalmolecules such as hydrogen-bonding, van der Waals andLondon forces, andπ-π and charge-transfer interactions andthen diffuses out with coal molecules soluble in pyridine duringextraction. It was estimated that the noncovalent association suchas hydrogen bonds could occur between pyridine and coalmolecules during pyridine extraction,27 and these proportionallycould reduce the diffusion coefficient of pyridine in pyridine-extracted raw coal.

In the event of acid-treated/pyridine-extracted coal samples,diffusion coefficients were found to be higher than those ofpyridine-extracted raw coal. Diffusion coefficients of pyridinein HCl-washed/pyridine-extracted coals increased from 4.1×10-15 to 5.3× 10-15 m2/s as the temperature was raised from22.3 to 25.8 °C. In the case of HCl/HF-washed/pyridine-extracted coal samples, diffusion coefficients have also beenobserved to increase from 5.3× 10-15 to 6.4× 10-15 m2/s whenthe temperature was increased from 22.0 to 26.6°C, respec-tively.

The situation in the event of acid-treated coal samples is fardifferent than that for pyridine-extracted coal samples. In thefirst step, coal was treated with HCl and then with HF acids. Inthe second step, the acid-treated coal sample was extracted withpyridine. When the coal was treated with HCl and HF acids,the metal cations (such as K+, Na+, Mg2+, Ca2+, Fe3+, and Al3+)which were known to be the major elements in coal mineralstructure were removed from coal, and they were replaced withhydrogen ions to form-COOH groups.21 When acid-treatedcoal was in contact with pyridine, the pyridine molecules whichare basic in chemical nature reacted with these carboxylic groupsto form pyridine-coal hydrogen bonds. The number of bondedpyridine molecules would be increased while the concentrationof carboxylic groups created by acid treatment was increased.This may enhance the diffusion of pyridine in the coal. It wasestimated that the concentration of newly formed carboxylicgroups was probably the driving force for the pyridine diffusionin the acid-treated coals. Further, removal of divalent cationswhich had contributed to cross-link density of coal could havealso added to formation of a more relaxed structure which wouldallow higher diffusion rates of pyridine.

When the diffusion coefficient of pyridine in HCl-washedcoal was compared with that of HCl/HF-washed/pyridine-extracted coal, it was found that pyridine diffusion in HCl/HF-washed coal was slightly higher than that of only HCl-washedcoal. This result might be also concerned with producing thenewly nascent carboxylic groups during HCl/HF washing ofcoal.

Consequently, in all of the samples, the diffusion coefficientsof pyridine increased linearly with an increase in the temper-ature. It has also been found that the mineral matter that existedin raw coal was effective to increase the rates of diffusion ofpyridine in coals. When the coal was treated with HCl/HF acids,new carboxylic groups probably formed after pyridine-extractedcoal samples. As the concentration of the carboxylic groupsincreased, the diffusion coefficients of pyridine in pyridine-extracted coal probably increased.

The equilibrium swelling values of coal samples in pyridinewere determined for raw, acid-treated, and pyridine-extractedcoals in the present study and are presented in Table 2. Theswelling ratios slightly decreased from 1.6 in the case of the

raw coal to 1.3 in the case of pyridine-extracted coal. This resultwas confirmed with diffusion coefficient values obtained forraw and extracted coal samples. The swelling values providedevidence that it formed a newly nascent noncovalent interactionsuch as a hydrogen bond between pyridine and coal duringpyridine extraction. It would appear that this effect wasresponsible for reducing the swelling value and diffusioncoefficient of pyridine in coal.

The swelling ratios increased to 1.8 for only demineralizedcoals and to 1.6 and 1.9 for HCl- and HCl/HF-washed/pyridine-extracted coals, respectively. The removal of minerals from coalalmost certainly increased the surface area and the number ofcarboxylic groups in structure of coal samples and the amountby which coal samples were swollen. It seemed that this effectincreased the diffusion rates for acid-washed/pyridine-extractedcoal samples and pyridine-extracted raw coal. The removal ofminerals from coals caused increases in the swelling values butconversely decreased the diffusion constants of pyridine in coal.The reason for this phenomenon might be enhanced absorptionof pyridine in minerals in raw coal.

Activation Energy of Diffusion of Pyridine in Coal. In thepresent study, the activation energy of diffusion for all coalsamples was calculated by the slope,-EA/R, of the straight lineof the plot of lnD versus 1/T. The calculated activation energyvalues for raw and treated coals are given in Table 2. It wasobserved that the values of activation energy for diffusionprocess all fall in the range from 22.5 to 75.1 kJ/mol. Theseresults are supported by a previous observation28 that theactivation barrier is associated with the breakage of an internalelectron donor-acceptor (such as hydrogen bonding) interaction.It has been suggested that a solvent will disrupt only those coal-coal hydrogen bonds whose bond strengths were lower thanthose of coal-solvent hydrogen bonds.

The activation energies measured are in accord with the valuesof diffusion coefficients of pyridine for raw and treated C¸ ayirhanlignite. The activation energy might be thought of as the energyrequired to initiate the diffusive motion of 1 mol of the penetrantmolecules. A large activation energy results in a relatively smalldiffusion coefficient. It was observed in the present work thatthe diffusion coefficient of pyridine in raw coal was the biggestcompared to the others. The activation energy of pyridinediffusion in the raw coal was the smallest as 22.5 kJ/mol (Table2). In the case of acid-treated coals, activation energy increasedfrom 22.5 to 33.1 kJ/mol for HCl-washed and to 75.1 kJ/molfor HCl/HF-washed coal, in conformity with decreased diffusioncoefficients of pyridine in these coal samples. These resultsprovided evidence that pyridine molecules interact preferentiallyand more strongly with the inorganic component than with theorganic component of the coal surface.

(27) Larsen, J. W.; Flower, R. A.; Hall, P. J.Energy Fuels1997, 11,998. (28) Otake, Y.; Suuberg, F. M.Fuel 1998, 77, 901.

Table 2. Solvent-Swelling Ratios and Activation Energy of Diffusionof Pyridine in Coal Samples

sample Q

activationenergy ofdiffusion,EA, kJ/mol

raw Cayirhan lignite 1.6 22.5HCl-washed C¸ ayirhan lignite 1.8 33.1HCl/HF-washed C¸ ayirhan lignite 1.8 75.1raw Cayirhan lignite/pyridine-extracted 1.3 65.4HCl-washed C¸ ayirhan

lignite/pyridine-extracted1.6 56.0

HCl/HF-washed C¸ ayirhanlignite/pyridine-extracted

1.9 28.6

1154 Energy & Fuels, Vol. 20, No. 3, 2006 Seferinogˇ lu and Yurum

It was observed that activation energy increased to 65.4 kJ/mol for pyridine-extracted raw coal contrary to its raw coal.This behavior was thought to be caused by the removal ofpyridine from the swollen coal by the refluxing treatment, withconsequential rearrangement, reassociation, and shrinking of thecoal structure to a conformation of lower free energy containingmore noncovalent interaction after pyridine extraction.9 Pyridine,because of its strong basicity, is capable of breaking nearly allhydrogen bonds in coal. Therefore, pyridine extraction disruptsthe hydrogen bonds in the coals and causes swelling. Ndaji andThomas9 reported that the swelling of the extracted coal inpyridine is markedly lower than that for the coal, indicatingsome modification of the macromolecular structure (e.g.,covalent cross linking andπ-π interactions). They suggestedthat the extraction process may cause decomposition, leadingto the formation of cross-links or the collapse of the structurebecause the removal of soluble material leads to the formationof strongπ-π interactions which act as effective noncovalentcross links.

When the activation energy of pyridine diffusion in pyridine-extracted coals is compared with each other, the value ofactivation energy decreased to 65.4, 56.0, and 28.6 kJ/mol forthe raw/pyridine-extracted, HCl/pyridine-extracted, and HCl/HF/pyridine-extracted samples, respectively (Table 2). Theseresults are in agreement with increasing diffusion coefficientsof pyridine in these coals. The biggest diffusion coefficient wasmeasured for HCl/HF-washed/pyridine-extracted coal samples,and this was parallel with the lowest activation energy ofdiffusion of this sample (22.6 kJ/mol). These results might beassociated with the formation of carboxylic groups during acidtreatment of coal. It can be declared that the value of activation

energy of diffusion of pyridine in acid-treated coal decreasesas the concentration of carboxylic groups was increased.

Type of Transport of Pyridine in Coal Structure. Tables3 and 4 present the diffusion rate constants, diffusion exponents,and transport mechanisms of pyridine in coal samples.R2 valuesin all of the experiments were equal to or greater than 0.99,indicating a linear relationship between ln(Mt/M∞) and ln t.Using this fact, it seemed that diffusion of pyridine in coalsamples can be approximated with a first-order rate law for allof the coals studied.

The values ofk andn are constants for particular diffusionsystems. The diffusion rate constant may only be viewed as aproportionality constant in the empirical rate law and does notclearly represent either the diffusion coefficient or the relaxationconstant for the coal. The unit ofk depends on the value ofn,but the values are consistent witht in minutes. Therefore, valuesof k obtained at two different temperatures cannot be directlycompared unless the values ofn are the same at the twotemperatures.7,28

It might be better to compare the chain relaxation times. Thechain relaxation time is the reciprocal of diffusion rate constantk obtained from analysis using eq 1 at constantn value (n )1). For the coal samples in the present work, the relaxation timeis of the order of 1100-4900 s. The relaxation time for Elbistanlignite with 53.0% C content obtained in our previous studywas of the order of 1800-6000 s.22 It must be stated that thesevalues obtained for C¸ ayirhan and Elbistan lignite are much lowerthan those measured for coals with C contents in the range from70.0 to 94.0%, 33 000 to 200 000 s, respectively.8 The effectof transport rates has also been studied by Peppas and Lucht.6

It was observed that the diffusion rate of pyridine in coals

Table 3. Diffusion Rate Constants, Diffusion Exponent, and Transport Mechanisms of Pyridine in Raw and Acid-Washed Coal Samples

sample T, °C k, s-1 n R2 transport mechanism

raw Cayirhan lignite 21.1 3.62× 10-4 0.52 0.9991 Fickian-anomalous22.3 3.51× 10-4 0.53 0.9965 Fickian-anomalous23.5 3.15× 10-4 0.54 0.9985 Fickian-anomalous25.3 2.11× 10-4 0.60 0.9937 Fickian-anomalous26.9 4.22× 10-4 0.50 0.9961 Fickian

HCl-washed C¸ ayirhan lignite 22.6 4.36× 10-4 0.51 0.9991 Fickian-anomalous23.5 2.67× 10-4 0.58 0.9950 Fickian-anomalous24.1 4.52× 10-4 0.51 0.9985 Fickian-anomalous24.7 4.21× 10-4 0.52 0.9992 Fickian-anomalous25.6 4.96× 10-4 0.50 0.9990 Fickian

HCl/HF-washed C¸ ayirhan lignite 21.6 3.02× 10-4 0.52 0.9958 Fickian-anomalous22.6 8.51× 10-4 0.39 0.9873 Fickian24.7 3.35× 10-4 0.54 0.9959 Fickian-anomalous25.1 3.04× 10-4 0.55 0.9967 Fickian-anomalous26.5 3.48× 10-4 0.54 0.9975 Fickian-anomalous

Table 4. Diffusion Rate Constants, Diffusion Exponent, and Transport Mechanisms of Pyridine in Raw/Pyridine-Extracted and Acid-Washed/Pyridine-Extracted Coal Samples

sample T, °C k, s-1 n R2 transport mechanism

raw Cayirhan lignite/pyridine-extracted 22.7 2.86× 10-4 0.55 0.9982 Fickian-anomalous23.5 2.63× 10-4 0.56 0.9943 Fickian-anomalous23.9 4.44× 10-4 0.49 0.9971 Fickian24.4 3.12× 10-4 0.56 0.9956 Fickian-anomalous26.7 3.80× 10-4 0.53 0.9978 Fickian-anomalous

HCl-washed C¸ ayirhan lignite/pyridine-extracted 22.3 6.22× 10-4 0.46 0.9880 Fickian22.6 2.67× 10-4 0.60 0.9942 Fickian-anomalous24.4 2.07× 10-4 0.62 0.9956 Fickian-anomalous25.8 2.24× 10-4 0.60 0.9968 Fickian-anomalous

HCl/HF-washed C¸ ayirhan lignite/pyridine-extracted 22.0 3.11× 10-4 0.54 0.9911 Fickian-anomalous22.7 4.19× 10-4 0.50 0.9969 Fickian24.0 4.22× 10-4 0.50 0.9981 Fickian24.8 3.00× 10-4 0.55 0.9964 Fickian-anomalous26.6 3.91× 10-4 0.52 0.9957 Fickian-anomalous

Measuring Diffusion Coefficients in C¸ ayirhan Lignite Energy & Fuels, Vol. 20, No. 3, 20061155

increased while the C content of coals was increasing. It wasfound that the value of the relaxation constant depended on theC content of coal and was on the order of 10-4.6 Thus, thetransport mechanism of pyridine in the macromolecular coalnetwork of Cayirhan lignite may be considered as nonrelaxationcontrolled. It was concluded that pyridine diffusion in C¸ ayirhanlignite that contained a macromolecule smaller than thatcompared to coals of higher ranks obeyed a mechanism closeto Fickian.

Tables 3 and 4 show the variation of diffusion exponents todetermine the mechanism of pyridine diffusion in the coals. Ingeneral, the values ofn changed when the temperature wasincreased, but the diffusion mechanism was not changed. Otakeand Suuberg7 reported that the values ofn are reasonablyconstant over the temperature range of interest (10-60 °C).

The diffusion exponent,n, was calculated to fall in the rangefrom 0.39 to 0.60 for raw and acid-treated coal samples. In caseof pyridine-extracted raw and acid-treated coal samples, thediffusion exponent values were observed to change in the rangeof 0.46-0.60. These values remained generally between 0.5 forFickian diffusion and 1.0 for relaxation (case II) controlleddiffusion. These results indicated that diffusion of pyridinethrough Cayirhan lignite was controlled by an anomalous (non-Fickian) diffusion mechanism. In general, diffusion mechanismsfor solvent trough coals have been shown to usually varybetween the extremes that are Fickian and relaxation controlleddiffusion.7,29

The n values measured in the present work were nearer tothe values for Fickian diffusion control than the values forrelaxation control. The anomalous diffusion coefficients in theliterature usually are in the range of 0.5-1.0.2,9 Therefore, itwill not be erroneous to claim that the diffusion mechanism ofpyridine in a low rank coal was with a process somewherebetween Fickian and relaxation controlled.

Conclusions

The acid treatments of coal created a fraction of reducedparticle size due to removal of mineral contents from coal. Inall of the samples, the diffusion coefficients of pyridine increasedlinearly with an increase in the temperature. The diffusion ofraw coal was observed to be higher, compared to those of thetreated coal samples. It seemed that the mineral contents of coalwere much more effective to increase the rates of diffusion ofpyridine in coals. Extraction of raw coal with pyridine decreasedthe pyridine diffusion into coal matrix. But in the case of acid-washed/pyridine-extracted coal samples, the diffusion coef-ficients of pyridine were observed to increase. The formationof new carboxylic acid groups in the acid-washed sampleenhanced diffusion of pyridine.

It was observed that, while the diffusion of pyridine in coalincreased, the activation energy gradually decreased. It mightbe noted that the coals with higher rates of pyridine diffusiongenerally exhibited lower activation energy for the diffusionprocess. The smaller diffusion coefficients were encounteredwith the coal samples which were extracted with pyridine. Itseemed that the diffusion coefficient decreased with irreversiblepyridine swelling of coal, as a result of structural variationsincreasing the activation energy of diffusion of pyridine to thecoal.

The values of diffusion exponents were found to vary in therange of 0.39-0.62 for raw, acid-treated, and pyridine-extractedcoal samples. It was also observed that then values were closerto Fickian transport mechanism boundaries than that of case IItransport mechanism. Therefore, it was assumed that thediffusion of pyridine in the low rank C¸ ayirhan lignite wasgoverned generally by Fickian mechanism or an intermediatecase of Fickian anomalous mechanism. Furthermore, the tem-perature did not changed diffusion mechanism at the rangestudied in the present work.

EF050399I(29) Hall, P. J.; Thomas, M. K.; Marsh, H.Fuel 1992, 71, 1271.

1156 Energy & Fuels, Vol. 20, No. 3, 2006 Seferinogˇ lu and Yurum