The Evolution of Char Surface Area along Pulverized CoalCombustion

Diego Alvarez* and Angeles G. Borrego

Instituto Nacional del Carbo´n, CSIC, P. O. Box 73, 33080 OViedo, Spain

ReceiVed NoVember 13, 2006. ReVised Manuscript ReceiVed January 18, 2007

Pulverized (36-75 µm) samples from three coals were fed to a drop tube furnace (DTF) operated at 1300°C under different O2/N2 mixtures. From the obtained set of char samples, one low burnout char from eachparent coal was re-entered into the DTF under varying gas compositions. The aim of this work is to monitorthe textural changes of the chars along pulverized coal combustion, with special care to isolate the variationsof surface area attributable to char combustion from those arising from early char/oxygen interactions duringthe pyrolysis stage. The burnout, helium density, and CO2 and BET surface areas of the combusted and refiredchars were determined, and the patterns of variation of the textural parameters were established for these twotypes of unburned material. It was found that the presence of oxygen in the vicinity of pyrolyzing coal particlesaffected the development of surface area of the newly formed chars and that this effect was both rank- andmaceral-dependent.

Introduction

The early stages of pulverized coal combustion comprise anumber of physicochemical processes of great complexity. First,the coal particles, on entering the boiler, undergo a suddenheating which provokes the release of the volatile fraction ofthe coal, and then both the volatiles and the carbon-rich charparticles resulting from the pyrolysis stage are consumed bythe oxygen present in the boiler. There is not a universalconsensus about the environmental conditions under which thesetwo processes take place simultaneously or consecutively,1,2 butit is well-known that the degree of overlap between pyrolysisand combustion depends on the amount of volatiles releasedby the coal particles, as these volatiles will, to varying extent,prevent the oxygen from reaching the surface of the particlesbefore the pyrolysis processes cease. The evolution of volatiles,in turn, mainly depends on the coal chemical composition andparticle size, as well as other process variables such as thetemperature and heating rate.3 For a given set of operatingconditions, the cloud of volatiles will more effectively screenthe oxygen the bigger the particle size (more volatiles per unitsurface area of the particles) and the lower the rank of the coal(more volatiles per unit volume of the coal). For the range ofparticle sizes prevailing in pulverized coal combustion (typically,less than 75µm diameter), it is generally assumed that the biggerparticles will pyrolyze in an essentially oxygen-free environ-ment, whereas for the smaller particles, the processes ofpyrolysis and combustion will take place with considerableoverlap. Much research work has been devoted to studying theinteractions of the oxygen with the char and the volatiles, inorder to elucidate whether the ignition takes place in ahomogeneous (gas-gas) or a heterogeneous phase (gas-solid),

as this is a critical issue for those involved in modeling work.4

Even when there is limited access of oxygen to the surface ofthe pyrolyzing coal particles, too low to promote their hetero-geneous ignition, it has been reported that some surfaceoxidation can still take place, which notably reduces the fluidityof the molten carbonaceous matter.5 This is attributed to theformation of oxygenated cross-links on the surface of theparticles, with the net effect of reducing the mobility of thepolyaromatic rings which make up the molten ground mass,and therefore preventing their rearrangement into larger units.6

Both the surface area and the intrinsic reactivity of theresolidified char shall be modified accordingly: more activesites shall remain in the char, and its surface area shall be higherthan in a char obtained in an oxygen-free environment.7,8

All these early interactions between char and oxygen cansubstantially modify the combustion behavior of a coal, as, laterin the combustion process, once the coal particles have evolvedtheir volatile matter and modified their morphology andchemical structure accordingly, the course of the carbon-oxygenreaction will be mainly conditioned by the surface area and/orchemical structure of the chars.9-11 The relative importance ofthese two features is dictated mainly by the combustiontemperature and the particle size of the char, and threecombustion regimes are defined according to the rate-limitingstep in the process: (I) the kinetics of the reaction, (II) the

* Corresponding author phone:+34 985119090; fax:+34 985297662;e-mail: diegoalv@incar.csic.es.

(1) Essenhigh, R. H.; Misra, M. K.; Shaw, D. W.Combust. Flame1989,77, 3-30.

(2) Wall, T. F.; Gupta, R. P.; Gururajan, V. S.; Zhang, D.Fuel 1991,70, 1011-1016.

(3) Fletcher, T. H.Combust. Flame1989, 78, 223-236.

(4) Smith, K. L.; Smoot, L. D.; Fletcher, T. H. InFundamentals of CoalCombustion for Clean and Efficient Use; Smoot, L. D., Ed.; Elsevier:Amsterdam, 1993; pp 131-293.

(5) Street, P. J.; Weight, R. P.; Lightman, P.Fuel 1969, 48, 343-365.(6) Alvarez, D.; Borrego, A. G.; Menendez, R.Procceedings of the 12th

International Conference on Coal Science; The Australian Institute ofEnergy: Toukley, New South Wales, Australia, 2003; p CD-6.

(7) Hurt, R. H.; Dudek, D. R.; Longwell, J. P.; Sarofim, A. F.Carbon1988, 26, 433-449.

(8) Gale, T. K.; Fletcher, T. H.; Bartholomew, C. H.Energy Fuels1995,9, 513-524.

(9) Zolin, A.; Jensen, A.; Dam-Johansen, K.Combust. Flame2001, 125,1341-1360.

(10) Bar-Ziv, E.; Kantorovich, I. I.Prog. Energy Combust. Sci.2001,27, 667-697.

(11) Feng, B.; Bhatia, S. K.Carbon2003, 41, 507-523.

1085Energy & Fuels2007,21, 1085-1091

10.1021/ef0605697 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 03/01/2007

diffusion of oxygen into the pore network of the char, and (III)the diffusion of oxygen across a boundary layer surroundingthe outer surface of the particles.12 Again, the identification ofthe combustion regime is essential for the modeling of thereaction.

As in any basic research work, the list of references providedhere cannot pretend to be comprehensive. Rather, an attemptwas made to select, from the vast literature existing in the field,those studies where the points raised in this paper are morespecifically highlighted. The present paper addresses the influ-ence of the pyrolysis environment on the texture and reactivityof the chars obtained, with an emphasis in the early interactionsbetween the newly formed char and the oxygen present in thegas phase. Three coals with different ranks and maceralcompositions were fed to a drop-tube furnace (DTF) operatedat 1300 °C and simulating the time-temperature historiesprevailing in pulverized coal combustion, and under oxygenconcentrations varying from substoichiometric to highly super-stoichiometric. Combustion chars with varying degrees ofburnout were obtained by different routes, and the evolution ofsurface area and reactivity along combustion was determinedfor the different combustion environments tested.

Experimental Section

Three steam coals have been selected for this study on the basisof their variable rank and maceral composition. A Medium volatilebituminous coal was obtained from the Fording River mine (FM)in British Columbia (Canada). This coal forms part of the largecoal measures available in the Elk Valley region, which producesboth metallurgical and thermal coals. A High volatile bituminouscoal was taken at Emma mine, in Puertollano (PT), one of themining localities in south-central Spain, and is mostly consumedin a power plant nearby. The third coal is named Bayswater (WA)and is a High volatile bituminous coal from New South Wales(Australia). The mine is part of the huge resources of Hunter Valleyfrom which most of the coal is sold overseas.

The coal characterization comprised (i) ultimate analyses per-formed using a LECO CHN600 for carbon, nitrogen, and hydrogen;a LECO SC132 for sulfur; and a LECO VTF900 for oxygen; (ii)proximate analyses carried out following the standard proceduresdescribed in UNE 32-019-84 for volatile matter and ISO-1171/1981 for ash contents; and (iii) petrographic analyses (maceral ISO7404-3, 1994, and random reflectance ISO 7404-5, 1994). Thechemical and petrographic characterization data of the three coalsselected for this study are given in Table 1. PT and WA are Highvolatile bituminous coals with the same rank, as indicated by theirvitrinite reflectance (0.66%) and different inertinite contents (21.4and 39.2 vol %, respectively). FM is a Medium volatile bituminouscoal with a high inertinite content (44.0 vol %).

The drop-tube furnace used for the combustion experiments hasbeen described elsewhere.13 Size-graded (36-75 µm) samples ofthe coals described above were used in the combustion experiments,

which were carried out at 1300°C, with a gas flow rate of 20 Lmin-1 and under different O2/N2 gas mixtures. Three differentstrategies were designed in order to obtain partly burnt chars atvarying burnouts. Figure 1 shows a sequential scheme of the charsamples obtained along this study. In the first series of experiments,the combustion tests were carried out under different oxygenconcentrations. The three coals were burnt under 0, 2.5, 5, 10, 15,and 21% O2 in N2. The samples thus obtained will hereafter bereferred to as XXB0, XXB2, XXB5, XXB10, XXB15, and XXB21,respectively, where XX is the corresponding parent coal, FM, WA,or PT. The second series consisted of a sequence of refiringexperiments where the coals were fed to the drop tube operatingunder 2.5% O2 in N2, and then the collected chars were successivelyre-entered in the reactor three more times using the same gascomposition. Subsamples obtained after every passage through thereactor were reserved for their characterization. The refired sampleswill be identified here as XXR2, XXR2-2, and XXR2-3 where,again, XX is to be substituted by FM, WA, or PT, the parent coals.The third series of combustion experiments also took the charsamples obtained from every coal under 2.5% O2 in N2 (XXB2) asa starting material, and these chars were subsequently refired under2.5, 5, 10, and 15% O2 in N2. These refired chars are denoted XXR2(equivalent to the first refired sample of series II), XXR5, XXR10,and XXR15. Up to eight runs in the DTF were necessary in orderto obtain a sufficient amount of the XXB2 samples for thesubsequent refiring experiments of series II and III.

The burnout of the samples was estimated using the ash-tracertechnique

Two widely used methods to determine the pore surface area ofcarbon from gas adsorption isotherms were applied in this study,using CO2 at 273 K and N2 at 77 K as adsorptives. The equipmentused was a Micromeritics ASAP 2020. Prior to gas adsorptionexperiments, the chars were heated under a vacuum at 5°C min-1

and holding temperatures of 90°C (1 h) and 350°C (4 h). CO2

adsorption isotherms were performed at 0°C at the interval ofpressure 0.035-0.0001 Torr, and the Dubinin-Radushkevich (D-R) equation14 was applied to the adsorption data. The Brunauer-

(12) Roberts P. T.; Morley C. InFundamentals of the Physical-Chemistryof PulVerised Fuel Combustion; Lahaye, J., Prado, G., Eds.; KluwerAcademic Publishers: Dordrecht, The Netherlands, 1987; NATO ASI Series137, pp 452-463.

(13) Milenkova, K. S.; Borrego, A. G.; Alvarez, D.; Xiberta, J.;Menendez, R.Fuel 2003, 82, 1883-1891.

(14) Dubinin, M.; Radushkevich, L.Proc. Acad. Sci. USSR1947, 55,331-335.

Table 1. Proximate, Ultimate and Petrographic Analysis of the Coalsa

wt % db wt % daf % vol %, mmf

coal ash VM C H N S O Rr V L I

PT 19.7 38.4 82.3 5.0 1.7 0.6 10.4 0.66 65.6 13.0 21.4WA 6.6 36.9 80.8 5.1 1.9 0.3 11.9 0.66 57.2 3.6 39.2FM 10.6 25.6 86.5 4.6 1.3 0.2 7.4 1.07 55.6 0.4 44.0

a db) dry basis; daf) dry, ash-free basis; mmf) mineral-matter-free basis; vol) volume; wt) weight; VM ) volatile matter; Rr) random reflectance;V ) vitrinite; L ) liptinite; I ) inertinite.

Figure 1. Diagram showing the various sets of char samples obtainedand the nomenclature used.

Conversion (%)) [1 - ( ashcoal

100- ashcoal)(100- ashchar

ashchar)] × 100

(1)

1086 Energy & Fuels, Vol. 21, No. 2, 2007 AlVarez and Borrego

Emmett-Teller (BET) theory was applied to the N2 adsorption datato obtain the surface area.15 These two methods can be regarded ascomplementary, given the difficulties of CO2 to fill large microporesand the slow diffusion of N2 in the small micropores,11 and thereforethe combination of both methods is considered to describe ap-propriately the surface area of micropores (CO2 D-R) andmesopores (N2 BET). As some of the samples contained largeamounts of mineral matter with different adsorption properties thanthe organic fraction, the isotherms were corrected for mineraleffects. An extensively burned char from every coal was ashed,and the corresponding CO2 and N2 isotherm was obtained andsubtracted from the corresponding sample isotherm before calculat-ing the surface area.

The isotherms were analyzed using the Micromeritics densityfunctional theory (DFT) software package DFT plus. The pore sizedistribution was obtained in the size range of 4-10 Å for CO2

adsorption and in the range 4-2500 Å for N2 adsorption.The helium densities of the chars were measured in a Micromer-

itics AccuPyc 1330. The raw data obtained were recalculated toan ash-free basis using 2.4 g cm-3 as the helium density of theashes. Due to the generally limited amount of sample available,combined with the diluting effect of the ashes, the helium densitiesof the high-burnout chars are subject to big uncertainties. Thus,the determination of the carbon density of a char with more than40 wt % ash is subject to a(5% error, which goes up to(26%for ash contents higher than 90 wt %. For this reason, the densitiesof high-ash samples will not be reported in this paper.

The apparent density (Fap) of the char material was estimatedthrough the pore volume (VP), as given by DFT, and the true density(FHe), using

This is not the apparent density of the char particles, considered asthe weight of the sample divided by the total volume enclosed bythe char particle outer surfaces, including the large devolatilizationvoids. Rather, the density values given by eq 2 refer to the volumeof char material plus the volume of pores smaller than 1µm indiameter. In this work, it has been preferred to use these densityvalues, on the basis of the observation of the morphologiescommonly encountered in bituminous coal chars, with porositiesdue to large devolatilization voids typically higher than 50%. Forsuch highly swollen particles, it is unlikely that the oxygen transferfrom the outer area of the particles to the surface of the char wallsthrough these large devolatilization pores might play an essentialrole in the control of the reaction. From the perspective of theoxygen molecules, the char material is seen as a solid with a truedensityFHe and a pore network formed of pores<1 µm with atotal volumeVp. It is the rate of penetration of oxygen into thispore network which defines the combustion regime, and it is thedensity defined in eq 2 which should remain constant (no oxygenpenetration, regime III) or change along combustion (reaction inthe pore surfaces, regimes I-II) depending on the rate-limiting stepof the combustion reaction.

Char reactivities were measured in a TG 7 Perkin-Elmerthermogravimetric apparatus in which approximately 13 mg of charwere heated under N2 at 25°C min-1 until 550 °C: after weightstabilization, the gas flow was switched to air and weight losseson combustion were isothermally recorded until a constant weightwas reached. The reactivity (ash-free basis) was calculated asR )1/mo (dm/dt), wheremo is the initial sample weight.

The observation of char samples was carried out on a ZEISS-DSM 942 scanning electron microscope (SEM). The particles weredispersed on graphite double-sided adhesive tape attached to analuminum stub, and a 20-nm-thick film of gold was deposited overthe samples in order to enhance the quality of the images.

Results and Discussion

While all the three series of combustion experiments wereaimed at obtaining chars with different degrees of burnout, thereis a substantial difference between series I and either series IIor III, and this is the fact that coal pyrolysis takes place undervarying gas compositions in series I, whereas both series II andIII make use of a pyrolysis char as a starting material, and thischar was always obtained under the same conditions (XXB2).It is well-known that the plastic behavior of coals duringpyrolysis is strongly influenced by the presence of oxygen,which can eventually reach the surface of the molten carbon-aceous matter and inhibit its plasticity.6 This is due to theformation of oxygenated groups on the particle surfaces, whichpromote the cross-linking of the char material and notably reduceits plastic properties. As a result, not only will the surface areaand porosity of the char change but also the size and arrange-ment of the polyaromatic units that form the char will bedifferent, and these two characteristics are critical to thecombustion behavior of the char. The oxidation of the charsurfaces mainly depends, among many other aspects, on theoxygen concentration in the gas and also on the amount ofvolatiles released by the pyrolyzing coal particles, as the cloudof volatiles surrounding the char will be preferentially consumedby the oxygen, thus preventing it from reaching the solid duringthe pyrolysis stage.

The SEM micrographs of Figure 2 illustrate the effect of thepresence of oxygen in the atmosphere of pyrolysis. Thephotograph in Figure 2A was taken from a PT char obtainedunder a N2 atmosphere, and it shows the smooth surface of apyrolyzed vitrinite region, with small spherical particles, identi-fied as soot, resulting from the recombination of volatiles inthe vicinity of the char particle. The photographs in Figure 2Band C show a similar region from the surface of a PT charparticle obtained under 2% O2 in N2, with less soot particles asa consequence of the competing effect of volatile combustion

(15) Brunauer, S.; Emmett, P.; Teller, E.J. Am. Chem. Soc.1938, 60,309-315.

Fap ) (VP + 1FHe

)-1(2)

Figure 2. SEM micrographs of vitrinite char from (A) PT coalpyrolyzed under N2, (B and C) PT coal burned under 2% O2 in N2, (D)FM coal pyrolyzed under N2, (E and F) FM coal burned under 2% O2

in N2.

The EVolution of Char Surface Area Energy & Fuels, Vol. 21, No. 2, 20071087

over recombination. Also, the char surface shows evidence ofheterogeneous reaction with the oxygen, as indicated by thepresence of small pores (20-50 nm). The same features can bedistinguished in the counterpart chars from FM (Figure 2D-F), and the inherently more ordered (anisotropic) arrangementof the carbonaceous matter in this char makes the surfacereaction more evident (Figure 2F) than in the case of PT char.

Bearing all this in mind, series II and, to some extent, seriesIII can be regarded as sequential experiments where a well-defined material (XXB2) is re-entered into the drop-tube furnacein order to obtain increased levels of burnout, whereas thecombustion chars obtained in series I were submitted toincreasingly drastic combustion conditions at the cost of alteringthe characteristics of the pyrolysis char formed in the initialstages. The latter series cannot thus be interpreted, strictlyspeaking, as a single char material burned at increasing levels.The differences observed among series I and series II-III charshave to be interpreted in terms of early interactions of the oxygenwith the pyrolyzing coal particles. The results obtained fromthe characterization of both sets of samples will be first shownseparately, and then a comparative study among them will bemade.

Series I Chars.The burnouts achieved by the coals after thecombustion experiments under different gas compositions aregiven in Figure 3. Despite the significant differences existingbetween the three coals selected, they all burned to comparableextents under any of the gas compositions tested. The relativelysmall differences found are in agreement with the well-knowntrends of low-rank coals (PT and WA) to being more reactivethan high-rank coals (FM) and of low-inertinite coals (PT) tobeing more reactive than high-inertinite coals (WA). However,if the differences in volatile yields upon pyrolysis of the parentcoals are taken into account, and bearing in mind that the ashtracer technique is an indicator of the fuel lost (by both pyrolysisand combustion) in the reactor, then it is likely that thedifferences in conversion strictly due to heterogeneous combus-tion of the coals could be even lower than those shown in Figure3. Everything considered, the combustion of the coals in thereactor seems to take place under conditions which minimizethe differences among the different fuels used, that is, nonregimeI conditions. This is important in that the textural transformationsof the chars will follow a similar course to those occurring infull-scale combustion. Additional evidence of diffusional controlin the DTF experiments carried out here will be given later.

Figure 4 (top) shows the variation of the BET and CO2 surfaceareas of the chars with the oxygen concentration in the DTF.For the three coals, the BET surface areas first increase withthe oxygen content of the gas and then decrease at high oxygenconcentrations. This trend is more pronounced for the low-rank,high-vitrinite coal PT, whereas the medium-rank coal FM, whichdisplays the lowest BET surface areas, seems to be the least

sensitive to the pyrolysis/combustion environment as regardsthe BET surface area. The mechanism that explains the increaseof mesopore surface area during combustion under regimes I-IIis described as micropore widening. Under regimes II-III, thegasification of the micropore mouths (partial enlargement) orthe opening of blocked mesopore channels would produce thesame effect of enlarged surface area.8 And the reduction ofsurface area at high burnout levels is attributed in either caseto the coalescence of pores.

It is striking that the BET surface areas of the PT chars morethan double those of the WA chars, although these coals onlydiffer in their inertinite contents, and even this difference is notparticularly high (18 vol % higher for WA coal).

The CO2 surface areas of these chars (Figure 4, bottom)change little with the increase of oxygen in the reactor, showingjust a slight decrease at 15% oxygen and above. Only PT coalchars display a noticeable maximum in micropore surface areaat 5% oxygen. Again, FM chars have the lowest microporesurface areas and PT the highest, but the differences betweenPT and WA are now less pronounced than those found for theBET surface areas. This indicates that PT chars have moresurface area located in the bigger micropores (those reachedby the N2) than WA chars, which is attributable to the higherplasticity of the vitrinite-derived chars compared with theinertinite-derived chars: highly plastic chars develop moredegassing vesicles and give rise to thinner char walls, therebyincreasing their surface-to-volume ratio and accordingly theirsurface area. It has already been mentioned that the differencein inertinite content between PT and WA coals is not particularlylarge (18 vol % more inertinite in WA than in PT). However,this difference is further enhanced by the preferential consump-tion of vitrinite- over inertinite-derived chars during pyrolysis,

Figure 3. Burnouts achieved by the coals under the different gascompositions used.

Figure 4. N2 BET (top) and CO2 D-R (bottom) specific surface areasof series i chars (2, PT chars;9, WA chars;(, FM chars).

1088 Energy & Fuels, Vol. 21, No. 2, 2007 AlVarez and Borrego

as it is well-known that, for a given coal rank, vitrinites havehigher volatile yields (lower char yields) upon pyrolysis thaninertinites.16

The maximum reactivities of the chars change with theoxygen concentration in the DTF as depicted in Figure 5a. Inagreement with literature results, for coals with the same rank,inertinite-derived chars (more frequent in WA) are less reactivethan vitrinite-derived chars (more frequent in PT), and for coalswith the same maceral composition, higher-ranked coal chars(FM) are also less reactive than low-rank coal chars (PT).17 Onthe other hand, the maximum reactivities of WA and FM charshardly change with the amount of oxygen injected in the DTF,whereas PT chars more than double their reactivities from thepyrolysis experiment under pure nitrogen to the combustionexperiment under air. As regards the intrinsic reactivities of thechars, expressed as the rate of weight loss per unit surface area(as measured by CO2 adsorption), their variations with theoxygen concentration in the DTF are given in Figure 5b, wherea common trend of increased char reactivity with the increaseof oxygen concentration can be observed for the three coals.This trend is particularly pronounced for PT chars, which,together with the general behavior of increased reactivity athigher burnout, suggests that the mineral ashes, more abundantin the high-burnout chars from WA and FM and in all the charsfrom the high-ash coal PT, might be playing a catalytic role inthe combustion of the chars at low temperatures (550°C).

Series II and Series III Chars. The burnouts achieved bythe three XXB2 chars after successive passages through the DTFare shown in Figure 6a. These burnouts were calculated withrespect to the chars obtained under 2% oxygen, and not with

respect to the parent coals. The rates of consumption of thechars are thus related with their reactivities to oxygen, and notwith the pyrolysis yields of the coals. After three passagesthrough the reactor, FMB2 char only achieved two-thirds ofthe burnout levels reached by WAB2 or PTB2. On the otherhand, the rates of consumption of WAB2 and FMB2 decreasein the third refiring experiment, as seen by the decrease in theslope of the corresponding curves, while the PTB2 sample showsan almost monotonic increase of char burnout with the residencetime (number of refiring experiments) in the DTF. The formerbehavior is described in the literature as the effect of chardeactivation along combustion and is attributed to a number ofmechanisms (annihilation of the active sites etc.), although thefinal reasons for this deactivation are still unclear.9 In order tobetter illustrate the variation of char reactivity with char burnout,Figure 6b plots the burnout achieved in every refiring experimentversus the cumulative char burnout with respect to the parentchars (XXB2). In this graph, it can be readily noticed how thecombustion rate of FMB2 char draws a maximum at a burnoutrate of∼30%. Assuming a similar trend for WAB2, this charseems to have already gone beyond its point of maximumreactivity, and its curve only shows a decreasing rate ofconsumption with the increase of char burnout. Finally, PTB2,as mentioned above, continuously increases its reactivity alongcombustion. If we again assume a similar pattern of increasedreactivity with burnout until char deactivation processes becomepredominant at high burnouts, the maximum reactivity of thischar would only be reached at very high burnouts.

The variations of BET and CO2 surface areas of the charsalong the series of refiring experiments are shown in the graphsof Figure 7. The BET surface areas (Figure 7a) change littlealong combustion, and the same can be said about the CO2

surface areas of WA and FM (Figure 7b). Such equilibriumcan be reached when the mechanisms of pore enlargement andthe opening of blocked channels are compensated by thecoalescence of both micro- and mesopores as the carbonaceousmaterial is consumed.8 However, WAB2 char shows a noticeabletrend of decreasing surface area as combustion proceeds. It hasalready been mentioned that WA coal is more heterogeneousthan PT. Given the close similarity existing between PT andWA, the different evolution of their respective textures has to

(16) Alonso, M. J. G.; Alvarez, D.; Borrego, A. G.; Mene´ndez, R.;Marban, G. Energy Fuels2001, 15, 413-428.

(17) Alvarez, D.; Borrego, A. G.; Menendez, R.; Bailey, J. G.EnergyFuels1998, 12, 849-855.

Figure 5. Maximum (a, top) and intrinsic (b, bottom) reactivities ofseries I chars (2, PT chars;9, WA chars;(, FM chars).

Figure 6. (a) Variation of char burnout with the number of passagesthrough the DTF. (b) Variation in the burnout achieved in each refiringexperiment, in relation with the cumulative burnout of the chars.

The EVolution of Char Surface Area Energy & Fuels, Vol. 21, No. 2, 20071089

be attributed to a preferential consumption of a high surfacearea char component.

The intrinsic reactivities of these char hardly changed alongthe series 5.0( 0.8 × 10-6, 2.0 ( 0.2 × 10-6, and 8.0( 0.8×10-7 for the PT, WA, and FM series, respectively.

The third series of combustion experiments, those consistingin refiring the XXB2 chars under increasing oxygen concentra-tions, was not carried out to completion due to excessive burnoutlevels and/or problems of sample availability. Thus, only WAR2,WAR10, and WAR15 were prepared from the parent charsample WAB2, and PTR2, PTR5, PTR10, and PTR15 wereprepared from the char PTB2. Series II and series III chars sharea common parent material, the corresponding char producedunder 2% oxygen in the DTF (XXB2). In the former series,high burnouts were achieved by successive refiring under 2%oxygen, whereas the latter produces high-burnout samplesthrough one single refiring, carried out under increasing oxygencontents. The study of the chars thus obtained reveals that thepattern of variation of their textural characteristics with theirburnouts is basically the same as that found for series II chars.This indicates that high oxygen levels in the environment ofcombustion do not modify the course of the reaction, but onlythe extent to which it proceeds. For the sake of brevity, therelevant data about series III chars are omitted in this section.A comparative study of the characteristics of burnt (series I)and refired (series II-III) samples will be carried out in thenext section, where series III samples will be described in thecontext of series I-II chars.

Comparison between Series I and Series II-III Chars.The CO2 and BET surface areas of all the chars studied hereare plotted in Figure 8 versus their char burnouts. In this figure,it can be readily noticed that the chars from series III lie in, orappear as credible extrapolations of, the trend lines drawn forthe corresponding series II chars. Also, for the three coalsstudied, the chars obtained under increasing oxygen concentra-tions always had higher surface areas than their refiredcounterparts. This difference was more pronounced for the BETsurface areas than for the CO2 surface areas, and as regards theparent coals, the discrepancy was higher for PT chars than forWA chars, whereas for FM chars, the difference in surface area

is only apparent in relative terms. It seems therefore that thepresence of high oxygen levels in the early stages of combustionpromotes the development of higher surface areas and that thiseffect is both rank- and maceral-dependent. Thus, higher-rankedvitrinites such as those present in FM coal are less reactive tothe environmental oxygen, and their plastic properties duringpyrolysis are hardly affected by surface oxidation, while thelower-ranked vitrinites of PT coal undergo a more extensiveoxidation during the early stages of pyrolysis. This earlyoxidation prevents the molten carbonaceous matter from fullydeveloping its plastic properties, which results in a lowerswelling and macroporosity, and a higher BET surface area. Infact, to obtain active carbons from plastic coals, it is commonpractice to implement a preoxidization stage in order to avoidthe rearrangement of the carbonaceous matter. On the otherhand, WA coal has more inertinite than PT, and this maceral isalso known to be less reactive to oxygen than the vitrinites ofthe same rank. This is why the decrease of surface area fromseries I to series II-III is more pronounced in the vitrinite-richer PT coal than in WA coal. This increase of surface areawith the oxygen concentration in the pyrolysis environmentcould also be inferred from the curves of Figure 4a, but in thatcase, the surface area developed by the chars was the result ofthe combined effect of pyrolysis and combustion, whereas inthe curves of Figure 8, the effect becomes apparent for charsburned by different routes but to comparable extents. As anexample, Table 2 lists the surface areas of selected burned andrefired char samples, all of them reacted by 60% under differentgaseous environments. In this table, it is summarized how theCO2 surface areas of the refired chars are only 15-25% lower

Figure 7. N2BET (a) and CO2D-R (b) specific surface areas of seriesII chars.

Figure 8. BET and CO2 specific surface areas of series I and seriesII-III chars. (a and d) PT chars, (b and e) WA chars, and (c and f) FMchars. Solid symbols: series I chars. Large void symbols: series IIchars. Small void symbols: series III chars.

Table 2. Comparison of the Specific Surface Areas of Chars withComparable Burnouts and Different Parent Coals and Combustion

Histories (Series I vs Series II-III)

sampleburnout

(%)

BET surfacearea

(m2 g-1)

CO2 surfacearea

(m2 g-1)

series I PTB5 63 364 502series II-III PTR2 59 150 415series I WAB5 60 168 384series II-III WAR2 60 42 296series I FMB10 59 60 191series II-III FMR2-3 62 26 163

1090 Energy & Fuels, Vol. 21, No. 2, 2007 AlVarez and Borrego

than those of their counterparts obtained in a single run.However, the BET surface areas of the chars changed dramati-cally depending on whether they had been prepared in a singlestep (series I) or in a refiring experiment (series II-III). Thus,from the data of Table 2, it can be noticed that the BET surfaceareas of the series I chars were 2-4 times higher than those ofseries II-III.

Density Variations along Combustion.Figure 9 shows thevariation of the apparent density, as defined in eq 2, of all thechars with their burnouts. The dashed line indicates thetheoretical variation under regime I conditions, where thecombustion would take place homogeneously in all the volumeof carbonaceous matter and the density would accordinglydecrease linearly with the burnout. The dotted line correspondsto a situation where the oxygen would not penetrate the charpore network at all (regime III). In the framework of these twolimit cases, it seems that the combustion conditions in the presentwork are closer to diffusion control than to chemical control. Ifit is borne in mind that the coal samples used here had adistribution of particle sizes (36-75 µm), the above-mentionedobservation cannot rule out the possibility that the lower-sizeend of the distribution could experience a combustion patterncloser to regime I conditions. In any case, both the relativelynarrow width of the particle size distribution and the smalldeviations of the experimental data from the dotted line in Figure9 (regime III) indicate that the contribution of the chemical

constraints to the control of the char combustion reaction mustbe in any case small. Only the PT chars from series I (solidtriangles in the figure) are plotted halfway between the twotheoretical trends, but it is unlikely that precisely the mostreactive of the chars studied here would burn under conditionsof chemical control, whereas, for instance, the less reactive FMchars would be consumed without chemical constraints. It hasalready been mentioned that the density determinations in thehigh-ash chars were subject to higher uncertainties, and thiscould well be the case for the three chars plotted at lowerdensities in Figure 9, namely, PTB5 (40% ash), PTB10 (52%),and PTB15 (87%).

Conclusions

From the study of a set of combustion chars of varyingburnouts, obtained from three coals of different ranks andmaceral compositions, the following conclusions can be drawn.

The mesopore surface area developed by coal chars largelydepends on the oxygen concentration in the vicinity of theinjection point, whereas the micropore surface area is hardlyaffected. The surface area of the obtained chars increases withthe oxygen concentration in the pyrolysis environment, and thiseffect is maintained along the subsequent combustion stage. Thishas implications for such industrial practices as fuel- or air-staging, where the injected coal faces varying gaseous environ-ments.

The surface area achieved by a coal char depends on itspetrographic characteristics (rank and maceral composition).

For the same level of burnout and thermal history, the surfacearea of a char can still notably vary depending on the rate ofsupply of oxygen which gave rise to that specific burnout.

The experimental data suggest that the various char samplesanalyzed had reached their burnout levels under diffusion-controlled conditions (regime II-III). No large discrepanciesshould be expected from extrapolation of these observations tofull-scale conditions.

Acknowledgment. The financial support of the Principality ofAsturias through the project PC03-04 and of the Spanish Ministryfor Education PSE 2-2005 is gratefully acknowledged.

EF0605697

Figure 9. The variation of apparent density of the chars with theirburnouts. Triangles, PT chars; squares, WA chars; diamonds, FM chars.Solid symbols, series I chars; void symbols, series II-III chars.

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