coal29

ARTICLE IN PRESS

0360-1285/$ - se

doi:10.1016/j.pe

�CorrespondE-mail addr

Please cite th

thermoproper

Progress in Energy and Combustion Science ] (]]]]) ]]]–]]]

www.elsevier.com/locate/pecs

Formation of the structure of chars during devolatilization ofpulverized coal and its thermoproperties: A review

Jianglong Yua, John A. Lucasb, Terry F. Wallb,�

aFaculty of Energy and Power Engineering, Shenyang Institute of Aeronautical Engineering, 52 Huanghe Bei Avenue,

Shenyang, 110034, PR ChinabDepartment of Chemical Engineering, School of Engineering, University of Newcastle, Callaghan, NSW 2308, Australia

Received 21 July 2005; accepted 25 July 2006

Abstract

The paper provides an overview of current studies on the behaviour of coal during devolatilization, especially the

experimental studies and modelling efforts on the formation of char structure of bituminous coals in the open literature.

Coal is the most abundant fossil fuel in the world. It dominates the energy supply in the future and plays an increasing role

particularly in the developing countries. Coal utilization processes such as combustion or gasification generally involve

several steps: i.e., the devolatilization of organic materials, homogeneous reactions of volatile matter with the reactant

gases and heterogeneous reactions of chars with the reactant gases. The devolatilization process exerts its influence

throughout the life of the solid particles from the injection to the burnout, therefore is the most important step which needs

to be understood. When volatile matter is generated, the physical structure of a char changes significantly during the

devolatilization, some accompanying a particle’s swelling. The complexity of a char’s structure lies in the facts that the

structure of a char itself is highly heterogenous inside an individual particle and between different particles and

the chemistry of a char is strongly dependent on the raw coal properties. A char’s structure is strongly dependent on the

heating conditions such as temperature, heating rate and pressure. Understanding the swelling of coal and the formation of

char’s pore structure during the devolatilization of pulverized coal is essential to the development of advanced coal

utilization technologies. During combustion and gasification of pulverized coal, the behaviour of individual particles

differs markedly due to the variation of their maceral composition. Particles with different maceral constituents generate

different types of char structure. The structure of a char has a significant impact on its subsequent heterogeneous reactions

and ash formation. The review also covers the most recent studies carried out by the authors, including the experimental

observations of the thermoplastic behaviour of individual coal particles from the density fractions using a single-particle

reactor, the experimental analysis on chars prepared in a drop tube furnace using the density-separated coal samples, the

development of a mathematical model for the formation of char’s pore structure based on a simplified multi-bubble

mechanism and the investigation on the effect of pressure on char formation in a pressurized entrained-flow reactor.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Char structure; Devolatilization; Pulverized coal; Coal swelling; Thermoproperty

e front matter r 2006 Elsevier Ltd. All rights reserved.

cs.2006.07.003

ing author. Tel.: +612 49 216179; fax: +61 2 49 216920.

ess: [email protected] (T.F. Wall).

is article as: Jianglong Yu et al., Formation of the structure of chars during devolatilization of pulverized coal and its

ties: A review, Progress in Energy and Combustion Science (2006), doi:10.1016/j.pecs.2006.07.003

www.elsevier.com/locate/pecs

dx.doi.org/10.1016/j.pecs.2006.07.003

mailto:[email protected]

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESSJ. Yu et al. / Progress in Energy and Combustion Science ] (]]]]) ]]]–]]]2

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Coal and its heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Devolatilization of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1. Mechanism of coal pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.2. Relation between the yield of products and heating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Please cite t

thermoprop

3.2.1. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.2. Effect of heating rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2.3. Effect of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.4. Effect of particle size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3. Devolatilization models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.4. Changes in physical properties of coal during heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. Formation of char structure from pulverized coal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1. Importance of char structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2. Classification of char structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.1. Pore system in coal and char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.2. Classification of char structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.3. Relation between char structure and coal properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.4. Relation between char structure and heating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.4.1. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.4.2. Effect of heating rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.4.3. Effect of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.4.4. Char characteristics and gas environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.5. The relationship between char structure and coal swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.6. Studies on transient coal swelling using single-particle reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.7. Application of density-separation techniques in studies of char structure . . . . . . . . . . . . . . . . . . . . . . . 23

5. Modelling the swelling and the formation of char’s structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

1. Introduction

Coal is the most abundant fossil fuel in the world[1]. Worldwide, coal continues to dominate theenergy supply in the future and play an increasingrole particularly in developing countries. Coalutilization processes such as combustion or gasifica-tion generally involve several steps [2]: i.e., thedevolatilization of organic materials leaving charbehind, homogeneous reactions of volatile matterwith the reactant gases and heterogeneous reactionsof the char with the reactant gases during which ashis formed. The devolatilization process exerts itsinfluence throughout the life of the solid particlesfrom the injection to burnout [3], therefore is themost important step which needs to be considered inmodelling coal combustion [4]. While volatilematter is generated during the devolatilization, thephysical structure of a char changes significantly,some accompanied by the particle’s swelling [5]. The

his article as: Jianglong Yu et al., Formation of the struc

erties: A review, Progress in Energy and Combustion Sci

complexity of char’s structure lies in the facts thatthe structure itself is highly heterogenous within oneindividual particle and between different particles;the chemistry of a char is strongly dependent on theraw coal properties and a char’s structure is stronglydependent on heating conditions such as thetemperature, heating rate and pressure. Under-standing of the swelling of coal and the formationof a char’s pore structure during the pyrolysis ofpulverized coal is crucial to the development ofadvanced technologies for coal utilization. Duringthe combustion and gasification of pulverized coal,the behaviour of individual particles in a given coalvaries markedly due to the variation of theirmaceral composition. Particles with different mac-eral constituents generate different types of charstructure [6]. The char structure has a significantinfluence on the subsequent char reactions and ashformation [7–12]. These have attracted wide re-search interests over the past decades. This paper

ture of chars during devolatilization of pulverized coal and its

ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 1. Hydrogen content (a) and reflectance (b) of macerals as a

function of coal rank [14,19].

J. Yu et al. / Progress in Energy and Combustion Science ] (]]]]) ]]]–]]] 3

provides an overview of current studies on thebehaviour of coal pyrolysis, particularly the experi-mental studies and modelling efforts on the forma-tion of char’s structure of bituminous coals in theopen literature.

2. Coal and its heterogeneity

As a sediment rock, coal varies remarkably in itschemical and physical properties depending on itsmaturity and geological environment of the coalifi-cation. In standard systems, coal is generallyclassified by its rank with fixed carbon contentand calorific value as the major indicators [13–16]and is termed as lignite, sub-bituminous coal,bituminous coal and anthracite. The major trans-formations in coal’s properties with increasing thecoal rank can be summarized as [17]: (1) a drop inmoisture and a marked decreases in the oxygencontent due to the loss of hydroxyl, carbonyl andcarboxyl groups; (2) a removal of aliphatic andalicyclic groups which causes an important reduc-tion in the volatile matter content with a parallelincrease in the aromaticity during the bituminouscoal stage; (3) the anthracite stage is characterizedby a rapid fall of hydrogen (H2) content and astrong increase in both the reflectance and theoptical anisotropy. An increase in the aromaticity ofcoal with increasing the rank was reported byWhitehurst et al. [18]. The aromatic carbon contentincreases from 40% to 50% for sub-bituminous coalto over 90% for anthracite. Owing to the changes inits properties, the behaviour of a coal of differentrank varies drastically during combustion andgasification [2].

Coal is well known as a heterogenous substancewith a mixture of organic material (i.e., the coalmatrix) and inorganic material (i.e., the mineralmatter) [14,19,20]. Macroscopically, coal has apronounced banded feature recognized as ‘bright’,‘predominantly bright’ or ‘dull’ in the appearance[14,21]. The identifiable ‘banded components’ aretermed as lithotypes. Microscopically, the organicmaterial of coal consists of complex maceralconstituents classified as three groups, i.e., liptinite,vitrinite and inertinite [19,21–23]. Since differentmacerals are derived from different original planttissues or coalified in different geological environ-ment, there are remarkable distinctions in theirchemical and physical properties between differentmaceral groups. In general, the volatile mattercontent, H2 content and H/C ratio appear in the

Please cite this article as: Jianglong Yu et al., Formation of the struc

thermoproperties: A review, Progress in Energy and Combustion Sci

order: liptinite4vitrinite4inertinite. However, thereflectance changes in the order: liptiniteovitrini-teoinertinite. With increasing the coal rank, thechange in its properties such as chemical composi-tion and reflectance of each maceral group followsits own path, as shown in Fig. 1(a) and (b). Formost of the world coals, vitrinite group is the mostabundant constituent. There is a tendency that thereflectance of vitrinite is used as the indicator of coalrank [24]. For pulverized coal, remarkable varia-tions in the maceral composition exist betweenindividual particles from the same coal [6,7]. During


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


heating, particles with different maceral constituentsbehave differently including the swelling, the yieldof volatile matter, char structure, the reactivity andash chemistry. Therefore, the microscopic hetero-geneity of coal has attracted wide scientific interestsin pulverized coal combustion and gasification [6].However, the variation in the property of thedifferent maceral groups diminishes at high rank.Therefore, it may be expected that maceral compo-nents of different groups from high rank coals maybehave similarly during combustion [25,26].

The chemical structure, i.e., the macromolecularnetwork of a coal is extremely important in terms ofits devolatilization behaviour [16]. In the meantime,coal has a complex pore structure system whichplays an important role during pyrolysis or gasifica-tion [27]. Upon heating, the pore structure of a coalchanges dramatically. The physical structure of thesolid residue generated during devolatilization issignificantly different from that of the feed coal. Forsoftening coals (in bituminous rank), the originalpore structures of a coal may be blocked due to thehigh fluidity when it is heated [16]. The charstructure is therefore determined by the devolatili-zation process (e.g., thermoplastic properties of thecoal and heating conditions) rather than by itsoriginal pore structure.

3. Devolatilization of coal

Coal devolatilization is referred to as pyrolysiswhen coal is devolatilized in an inert gas. Devola-tilization and pyrolysis are usually not distinguishedfrom each other due to the similar behaviour of coalin the two processes in terms of char chemistry andthe composition of volatile matter [28]. Thebehaviour of coal during pyrolysis under differentheating conditions has been extensively studied andreviewed over the past few decades [3,5,6,14,28–38].During heating, coal particles experience verycomplex physical and chemical transformationswhile yielding volatile matter and generating solidresidues. Early review provided summaries of thekinetics of coal pyrolysis [31]. The experimentalprocesses, kinetic rates and mechanisms of coalpyrolysis were reviewed by Solomon et al. [36].Products resulting from the coal pyrolysis includegases, tar and solid char. The definition of pyrolysisproducts has also been summarized. Gas compo-nents can be defined straightforward, includingsome sulphur- or nitrogen-containing gases. Tar isdefined as room temperature condensable compo-



nents which separate from the solid products duringpyrolysis. The remaining solid is char whichincludes extractables of large molecular weights.

Experimental investigations involve a variety ofeither captive or non-captive techniques such aswire mesh reactors or heating grids (screens), droptube furnaces (DTFs) or gas flow reactors, TGA orfluidized bed reactors. A summary of experimentalconditions for different techniques has been pro-vided by Solomon et al. [36] and Gavalas [33]. Apartfrom the particle size used in the different experi-mental techniques, heating conditions such as theheating rate, temperature and pressure of differentreactors vary significantly. Therefore, one should bevery careful when comparing the experimental databy different researchers.

3.1. Mechanism of coal pyrolysis

The mechanism of pyrolysis has been summarizedin the literature [5,16,31,33,36,38]. Pyrolysis reac-tions are complex, broadly involving bond break-ing, vaporization and condensation or cross-linking,accompanying changes in the density of aliphaticgroup and aromaticity. Saxena [38] summarizedthat pyrolysis reactions commence with the ruptureof weak bonds. Since a minimum amount of energyis required to overcome the C–C bond energy,pyrolysis reactions do not commence until thetemperature is close to 670K. The C–C bonds atthe bridge between the ring systems are muchweaker than other C–C bonds, particularly the onesin the aromatic ring structures. Therefore, thepyrolysis begins with the cracking of bridgesbetween the ring systems, resulting in the formationof free radical groups such as –CH2, –O–, etc. Thesefree radicals are highly reactive and readily combinein the gas phase to produce the aliphatics (mainlymethane, CH4) and water. Since the polynucleararomatic compounds diffuse slowly even at hightemperatures, they start to condense with theelimination of H2. The ultimate product due to thecondensation reaction is coke/char. In addition, athigh temperature, CO is also produced through thecracking of heterocyclic oxygen groups. The follow-ing typical reactions take place in stages as thetemperature is increased [38].

ture of chars during devolatili

ence (2006), doi:10.1016/j.pecs.

Cracking:
R–CH2–R0-R–R0+–CH2.
Saturation:
–CH2+2H0-CH4,
–OH+H0-H2O.
Tar production: –R–CH2+H0-R–CH3.
zation of pulverized coal and its

2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESSJ. Yu et al. / Progress in Energy and Combustion Science ] (]]]]) ]]]–]]] 5

Condensation (cross-linking) reactions:

R2OHþH2R! R2RþH2O:

R2HþH2R0 ! R2R0 ðcokeÞ þH2:

The R radical is obtained from benzene, naphtha-lene or phenantherene, etc. Oxides of carbon areproduced by the following reaction:

R2COOH! R2Hþ CO2:

Obviously, H2 is a key elemental composition interms of pyrolysis reactions, particularly for theevolution of tars [38]. H2 in coal is used up partly toproduce hydrocarbons and water and partly isliberated as molecular H2. The hydro-aromatic H2

is consumed in three different reactions, i.e., (i) insaturating the OH and O radicals to produce water;(ii) by the CH2 radicals to produce the aliphatics;and (iii) in saturating the larger radicals to producetar molecules. The aromatically bonded H2 isliberated as molecular H2 during the condensationof aromatic nuclei to produce char. During gasifica-tion process, the objective is to use the H2 in coal asefficiently as possible by devolatilization. But thestructure of coal is such that it preferentially evolvesH2 as chemical water and light aliphatics depletingthe remaining carbon of much needed H2 duringpyrolysis. Hence pyrolysis always produces char dueto the inefficient use of the intrinsic H2. Excess H2

during hydro-pyrolysis significantly enhances theproduction of tars and hydrocarbons [5,31,37].

During pyrolysis, Td is commonly identified asthe active thermal decomposition temperature ofcoal beyond which massive weight loss takes place.Significant thermally induced structural changesmay occur without generating major amounts ofvolatile matter below Td [14]. These structurechanges have important association with the pyr-olysis reactions occurring later on. The overalldecomposition process is therefore viewed as threesuccessive stages: (i) limited thermal alterations ofthe original molecular structures (mostly by con-densation reactions) at temperatures below Td; (ii)active decomposition leading to the generation anddischarge of the bulk of volatile matter, primarily inthe form of tars and light oils between Td and�820K; and (iii) secondary degasification resultingin the formation and evolution of a variety ofhydrocarbon gases, elemental H2 and oxides ofcarbon over an extended temperature range above�820K.



An important concept in coal pyrolysis may bethe ‘functional group’ [18]. Gavalas [33] summar-ized functional groups and their roles in the thermaldecomposition of coal. The reactivity of coal inpyrolysis can be characterized by the followingfunctional groups, i.e., the aromatic nuclei, hydro-aromatic structure, alkyl chains, alkyl bridges andoxygen containing groups. The thermal reactionssuch as bond dissociation, H2 abstraction and H2

addition were also described accordingly.Nine steps of the pyrolysis reactions were

proposed to interpret the evolution of volatilematter by Solomon et al. [36]. As coal is heated,three processes occur in the temperature range of470–670K. These processes are the disruption of H2

bonds (Step1), the vaporization and the transport ofthe non-covalently bonded ‘molecular phase’ (Step2) and low-temperature cross-linking in coals withmore than 10% oxygen (Step 3) which coincideswith CO2 or H2O evolution. During primarypyrolysis, the weakest bridges can break by produ-cing molecular fragments (depolymerization) (Step4). The fragments abstract H2 from the hydro-aromatics or aliphatics, thus increasing the aromaticH2 concentration (Step 5). These fragments will bereleased as tar if they are small enough to vaporizeand be transported out of the char particle (Step 6)under conventional pyrolysis conditions and do notundergo moderate temperature cross-linking reac-tions before escaping from the particle. Themoderate temperature cross-linking reactions (Step7) are slightly slower than the bridge breakingreactions and appear to correlate with CH4 evolu-tion. The other event during primary pyrolysis is thedecomposition of functional groups to release gases(Step 8), mainly CO2, light aliphatic gases and someCH4 and H2O. The release of CH4, CO2 and H2Omay produce cross-linking, e.g., CH4 by a substitu-tion reaction in which the attachments of a largermolecular weight release the methyl group, CO2 bycondensation after a radical is formed on the ringwhen a carboxyl is removed, and H2O by thecondensation of two –OH groups or an –OH groupand a –COOH group to produce an either link. Thecross-linking is important to determine the releaseof tar and the visco-elastic properties of the char.The primary pyrolysis completes when the dona-table H2s from hydro-aromatic or aliphatic portionof the coal are depleted. During secondarypyrolysis, there is additional gas formation(Step 8), i.e., CH4 evolving from methyl groups,HCN from ring nitrogen compounds and CO from


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


ether links. And finally H2 evolves by ringcondensation (Step 9).

Another comprehensive reaction mechanism ofpyrolysis was proposed by Juntgen [39,40] andoverviewed by Smith [16]. This mechanism includesthe desorption of water at 390K, distillation of themobile phase beginning at 520–620K forming analiphatic tar, the formation of tar and gases by thedegradation of the macromolecule beginning up to620K and the formation of char by condensationreaction with simultaneous evolution of H2 and COat high temperature. This overall picture providesinterpretations to some low heating rate experi-mental data.

Pyrolysis of lignite is comprised of five principaldevolatilization phases according to the study bySuuberg et al. [41]. Pyrolysis behaviour of softeningcoals has been described as three stages [42], asillustrated in Fig. 2. Coal undergoes a reduction ofH2 bonding during stage I. Sufficient labile bondbreaking of the macromolecular network alsooccurs to form primary gas and liquid componentswhich are often referred to as metaplast. Themetaplast is generally recognized to be responsiblefor the fluidity of coal upon heating [43–47]. Bondbreaking competes with bond stabilization to formchar during stage I. Much of the material that isassociated with the mobile phase and extractablewith suitable solvents is released in this stage. StageII is featured by bulk evolution of tar which isconsidered to be the low-molecular-weight compo-nent of metaplast. The remaining high-molecular-weight components in the metaplast re-attach to thechar structure by cross-linking reactions. In FG-DVC model [36,48,49], tar forms as a result of thecombined depolymerization and vaporization pro-cesses. The formation of gases is correlated with thedecomposition of the functional groups. Thus coalswith different functional group compositions willbehave differently during pyrolysis. During stage

Fig. 2. Pyrolysis process o



III, char evolves CO and H2 while continuing tocross-link with further ring condensation.

Fig. 3 shows the experimental results of evolutionrate and cumulative weight loss of volatile species ofa bituminous coal analysed on a TG-FTIR [36].During the pyrolysis at slow heating rates(o1K s�1) [16,38], the occluded carbon dioxide(CO2) and CH4 are driven off at about 470K.Above this temperature, internal condensationoccurs among the macromolecular structure oflow-rank coals with the evolution of CO2 andwater. In the range 470–770K, CH4 begins to evolvewith its higher homologues and olefins; most of theoxygen in coal structure is eliminated as water andoxides of carbon. The decomposition of bothnitrogen structure and organic sulphur speciesbegins in this temperature range. The evolution ofH2 begins at 670–770K with a critical point atabout 970K characterized by a rapid evolution ofH2 and carbon monoxide (CO). In the temperaturerange 770–970K, the volume of gases such as H2,CO, CH4 and nitrogen increases with increasing thetemperature while most hydrocarbons decrease. Tarformation begins at around 570–670K with amaximum yield occurring at approximately770–820K. For some bituminous coals, the tarevolution rate curves often show a small peak orshoulder at a lower temperature before the mainpeak due to the non-covalently bonded guestmolecules [16]. The main peak is due to the releaseof coal fragments during the break-up of themacromolecular coal structure by bonds breaking,evaporation and transport. The character andcomposition of tars vary with the temperature andcoal type.

The proportion of the gases and tars vary widelywith coal rank [16,50,51]. In general, low-rank coalsgenerate a high yield of gases and a low yield of tar.High volatile bituminous coals generate a high yieldof tar and a moderate yield of gases while high-rank

f softening coal [42].


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 4. Effect of temperature on weight losses during pyrolysis at

different residence times [31].

Fig. 3. Evolution rates and cumulative yields of Illinois No. 6 coal during pyrolysis, from TG-FTIR analysis [36].


coals generate a moderate or low yield of tar and alow yield of gases.

3.2. Relation between the yield of products and

heating conditions

In practice, devolatilization of coal is a kineticprocess. Experimental data show that the yield ofpyrolysis products is strongly dependent on heatingconditions, e.g., the temperature, heating rate,particle size, pressure and the presence of H2

[37,41,52–62] and have been extensively reviewedin literature [5,14,31,33,36,38].

3.2.1. Effect of temperature

The trend that the amount of ultimate weight lossof a coal during pyrolysis increases with increasingthe peak temperature observed by earlier work[37,52,54,55] has continuously been confirmed bylater investigations [56,57,59,61]. Most of the aboveobservations were conducted at high heating rates.The residence time needs to be considered whencomparing the data. In the early review, Anthony etal. [31] summarized the effect of temperature onweight losses at different residence times bycomparing the data from different workers, asshown in Fig. 4. The weight losses at differenttemperatures were normalized with the observedweight loss at 1270K as 100%. A later review byHoward [5] provided additional informative datasuggesting further devolatilization occurs beyond



1270K on short residence time based on resultsfrom US coals by a number of investigators. Datafrom the DTF [56,61] showing an increase in theweight loss when the furnace temperature increasedfrom 1070 to 1670K also supported this point.Additional information of the yields of pyrolysisproducts, e.g. H2O, CO, CO2, tar and H2 from a


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


bituminous coal with increasing the temperaturewere illustrated by Gavalas [33] based on theexperimental data for the Ohio No. 2 coalinvestigated by Solomon et al. [55]. An increase inthe yield of tar from bituminous coal has beenreported [59] as the peak temperature increasesbelow 1100K during pyrolysis at a heating rate of1000K s�1 using a heating-screen reactor duringinvestigating the effect of pressure. Fig. 5 presentsthe yield of products from a sub-bituminous coalmeasured by Suuberg [37] at a heating rate of1000K s�1.

3.2.2. Effect of heating rate

The effect of heating rate on weight losses duringpyrolysis have been proved to be significant in thepast studies [5,14,31,37,41,52,54,58]. Anthony[31,54] investigated and compared the yields ofvolatile matter from a bituminous coal in a widerange of heating rates using different type ofreactors such as crucibles, wire-mesh reactors andelectrical strip furnace and found that when theheating rate increased from 1 to 104K s�1, the totalyield of volatile matter increased around 11%. Ahigh heating rate may shift the pyrolysis reactions to

Fig. 5. Product yield from Pittsburgh Seam bituminous coal

during pyrolysis when heated to different peak temperatures [37].



a much higher temperature range. Therefore, thedevolatilization takes place at a much higher rate[5,37]. A high heating rate results in a moreextensive thermal fragmentation of coal’s moleculestructure and suppresses secondary reactions [14]and the loss of fixed carbon [52]. Critical observa-tions are the excess yield of volatile matter over theproximate volatile matter content at high heatingrates, leading to a Q factor (i.e., the ratio of the yieldof volatile matter during devolatilization to thevolatile matter content by standard proximateanalysis) greater than 1 due to some of the fixedcarbon being carried into the gas phase [52]. Anexperimental comparison of the yield of volatilematter to the proximate volatile matter content wasprovided by Howard [5]. The Q value ranges from1.3 to 1.5 for weakly swelling coals and from 1.4 toabove 1.8 for highly swelling coals [53]. This effecthas been the strongest for the medium rank coals,i.e. the bituminous coal where the yield of tarexhibits the maximum [5]. Other experimentalevidences also show that higher heating rates resultin higher a weight loss during devolatilization,particularly for intermediate rank coals, as pre-sented in Fig. 6 [63]. It may be concluded from thecurrent experimental data that excess volatile yieldis primarily due to a higher yield of tar at when theheating rate increases. In the meantime, the max-imum rate of devolatilization increases almostlinearly with increasing the heating rate for somecoals [64,65] because the pyrolysis reactions takeplace at a higher temperature range. However, theexperimental data that bear on this mater are notwithout ambiguity [14]. Some of the data contain a

30

40

50

60

-1 0 31 2 64 5

Dev

olat

iliza

tion

, wt%

Heating rate (lg, F/sec)

Fig. 6. The effect of heating rate on the weight loss during

devolatilization of a sub-bituminous coal [14,63].


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


mixture of the effects of the temperature and theheating rate. Some experimental data also showed Q

factors less than 1 at high heating rates [5] possiblydue to the incomplete pyrolysis of the coal.

Fig. 7. The yields of volatile species vs. pressure during the

pyrolysis of Pittsburgh No. 8 coal at 1270K [37].

3.2.3. Effect of pressure

The effect of pressure on the behaviour of coalduring devolatilization have attracted wide interestsin the recent years because of the development ofpressurized facilities for coal utilization such asIGCC [66,67] and pressurized fluidized bed combus-tion (PFBC) [68]. Wall et al. [11,12] provided arecent review on the impact of pressure on a varietyof aspects of coal reactions during pulverized coalconversion. In general, pressure significantly influ-ences the yield of volatile matter, coal particleswelling and the structure of the char residue. Thisfurther influences the char reaction rate [9] and theash formation mechanism [10]. A pronouncedreduction of the total weight loss and the yield oftar at elevated pressures and high temperatures hasbeen observed using different types of reactors[34,37,54]. The early investigations carried out usingthe Pittsburgh bituminous coal [54] showed that thetotal yield of volatile matter decreased withincreasing the pressure and the effect was moredistinguishable at high temperatures. Measurementson Pittsburgh No. 8 coal by Suuberg et al. [37]revealed that when the pressure increased the totalyield of volatile matter and the yield of tar decreasedwhile the total gas production increased, as is shownin Fig. 7. The published data regarding the pressureeffect on the total yield of volatile matter undervarious conditions have been summarized in theliterature [69], as is shown in Fig. 8.

It has been clear that high pressure suppresses theformation and release of tar and shifts the molecularweight of tar to a lighter fraction. On theother hand, high pressure promotes secondaryreactions, hence increases the total yield of lightgases. Because tar is the predominant product ofvolatile matter, the total yield of volatile matterdecrease significantly when pressure is increased[5,14]. In general, the effect of pressure is to increasethe yields of solid and gases at the expense of tarproduction [65]. The effect of secondary reactionsmay not be significant for non-plastic coals accord-ing to Lewellen’s study [70] on the Montana lignite.His results showed that total yield of volatile matterdid not vary over a wide range of pressures andheating rates.



3.2.4. Effect of particle size

Very little experimental data have been accumu-lated regarding the effect of particle size of coalduring devolatilization [5]. Badzioch et al. [53]found that particle size had no significant effect onthe weight loss based on rather limited observationof coals with the mean size of 20, 40 and 60 mm andattributed the results to that the heating rate of theparticle was controlled mainly by the heating rate ofthe carrier gas, so that the coarser particles heatedonly at slightly lower rates than the fine particles.Mathews and co-workers [71] concluded that theparticle size dependence of the measured volatilematter after correction for effect of mineral matteris due almost entirely to differences in maceralcomposition based on the observations of two purebituminous coal vitrinite in the size range of 60–400US mesh (37–250 mm). However, Anthony [54,70]found the yields of volatile matter somewhat fellwith increasing particles size over the size range53–1000 mm for Pittsburgh Seam bituminous coalduring pyrolysis. Gavalas [33] observed significant


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 8. The yield of volatile matter as a function of pressure [69].

J. Yu et al. / Progress in Energy and Combustion Science ] (]]]]) ]]]–]]]10

particle size effect during the pyrolysis of a sub-bituminous coal. The yield of gases showed asubstantial increase with increasing the particle sizewhile the yield of tar generally decreased. Thisobservation is also consistent with Suuberg’s results[37]. The effect of particle size on the total yield ofvolatile matter may be interpreted by the masstransfer and secondary reactions. Larger particlesizes restrain the transport of volatile matter out ofa particle and promote secondary reactions whichincrease the yield of light gases and decrease theyield of tar. Overall, the effect of particle size issimilar to that of pressure [5].

3.3. Devolatilization models

The kinetics of devolatilization deals with howfast the volatile matter evolves and is dischargedfrom the coal under certain conditions [14]. Anearlier review by Gavalas [33] summarized thekinetic modelling efforts on describing coal pyr-olysis including the first order reaction [5,31], thecompeting reactions [41,72] and the detailed Che-mical Models [33]. The first-order reaction modelsdescribe the total weight loss using one set of kineticrates, i.e., the rate constant k and the activationenergy E. The yield of individual volatile matter isalso described with several independent first-orderreactions. The competing reactions are based on theexperimental observations showing a negative cor-



relation between the ultimate yields of tar and gaseswhich are generated by the competition betweenpurely chemical steps or secondary reactions inconjunction with mass transfer limitations. Thedetailed chemical model developed by Gavalas et al.[33] describes the pyrolysis on the basis of functionalgroups and their elementary reactions. The distrib-uted activation energy model has been widelyapplied to predict the overall conversion and theyield of given components during coal reactions anda simple method was developed to estimate f(E) andk0(E) from experimental data [73,74]. The modelwas employed [75] to investigate the pyrolysis andgasification reactions of hydrothermally and super-critically upgraded low-rank coal, and their resultssuggested that some changes in the rate-controllingstep or in the physical structure of coal might havetaken place during gasification.

Many studies have suggested that the chemicalstructure of coal can be considered as a macro-molecular network to which the concepts of cross-linked polymers may be applied [58]. A number ofresearchers have applied statistical methods topredict how the network would behave when coalis subjected to thermally induced bridge-breaking,cross-linking and mass transport processes and havedeveloped network models with different features.The macromolecular network models are the func-tional group-depolymerization, vaporization andcross-linking (FG-DVC) model [42,48,49,76,77],


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


the distributed-energy chain (FLASHCHAIN)model [78–80] and the chemical percolation modelfor devolatilization (CPD) [50,51,81–83]. Networkmodels of coal thermal decomposition approximat-ing the breaking-down of the macromolecularnetwork have demonstrated success in modellingthe devolatilization behaviour of coal [16,49,69].The network code for devolatilization has beenincorporated in the modelling of coal combustion toimprove the design of combustion plants [4].

3.4. Changes in physical properties of coal during

heating

When coal is subjected to heating, two physicalproperties, i.e., the viscosity and pore structure ofcoal during the plastic stage are extremely impor-tant because they govern the rate of mass transport[33] and further determine the yield of volatilematter. Some coals such as coking coals developsignificant fluidity [14,84] and may be considered tobehave as a Newtonian fluid [85]. These coals arealso referred to as softening or plastic coals whichare in the medium rank. The thermoplastic proper-ties of a coal determine the size and pore distribu-tion of char particles [3]. During devolatilization,coal particles swell to different extents and thereforegenerate solid residues with different physicalstructures. Another important physical propertywhich is not yet understood is the surface tension ofcoal during the plastic stage [46].

The mechanism and experimental measurementsof the thermo-plasticity of coal has been a subject ofdiscussion for long time because of its importance incoke production [84]. Thermoplastic phenomena areobviously associated with thermal decompositionsof coal. However, the fluidity is very difficult to becorrelated to other individual associated plasticphenomena such as dilatation, softening, contrac-tion or swelling [14]. Crossing-linking reactions arecritically important to the development of fluidityand tar formation [16,86]. The occurrence of low-temperature cross-linking reactions provides inter-pretations to the absence of fluidity in lignites[86,87]. Some evidences show that the fluidity of acoal has a direct correlation to its cross-linkingdensity during heating. Different theories for soft-ening such as physical melting, thermo-bitumen andphysico-chemical process are hypothesized to inter-pret the thermoplastic behaviour of softening coal[14,65]. The metaplast pre-exists in coal and thatdeveloped during thermal decomposition are re-



sponsible for coal’s thermo-plasticity during heating[19,43,46,47,88]. The thermal decomposition of coalupon heating is therefore conceptually simplified asa two-step reaction, i.e., ‘coal-metaplast-coke’[41,44,48]. A number of viscosity models have beendeveloped based on the metaplast theory to predictcoal’s viscosity which is generally a function ofmetaplast fraction under different heating condi-tions [47,88–90]. On the basis of the macromole-cular theory, Lynch et al. [91] described thetransient nature of thermo-plasticity of bituminouscoals as three overlapping processes, i.e., a physicalmobilization of aromatic-rich structure, thermo-chemical decomposition of the macromolecularstructure and a rapid condensation to produce arigid semi-coke.

The thermoplastic properties of coal are compli-cated functions of coal properties such as the rankand petrographic compositions as well as pyrolysisconditions such as the heating rate, particle size andpressure [65]. The petrographic composition is acritical property which governs the thermoplasticbehaviour of a coal [19,92–95]. Sung [96] concludedthat the plasticity of bituminous coal was primarilyattributed to the presence of exinite and vitrinitemaceral components. This coincides with earlierobservations by van Krevelen et al. [19,44]. In-ertinites exhibit little plasticity upon heating atconventional heating rate of 3Kmin�1 whereasexinites become extremely fluid while vitrinites takean intermediate position. Both vitrinites and ex-inites develop the highest fluidity at the mediumcoal rank where the carbon content of vitrinites isaround 86–88% (as shown in Fig. 9). An additiverelation is found when vitrinites and exinites aremixed. Inertinites have a strong suppressing effecton the fluidity of a coal [19]. Nomura and Kidena[93–95,97] studied the nature of plastic phenomenaof vitrinite- and inertinite-rich fractions of twobituminous coals using a number of techniques.They reported that the inertinite-rich fractionsexhibited little fluidity in the plastic temperaturerange. 13C NMR analysis suggests they had a largersize of aromatic clusters, a less amount of sub-stituents (alkyl- and oxygen functional groups) onaromatic rings and a higher density of cross-linkingthan the vitrinite-rich fractions. The vitrinite-richfractions of the same coal exhibited a higher fluiditydue to relatively higher a concentration of aliphaticchains and bridges and branched aliphatic moietiesand alicyclic parts, along with a low aromaticity andmuch more transferable H2.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


Heating rate has a crucial impact on the coalfluidity [16,44,98]. At slow heating rates(o10Kmin�1), both the maximum fluidity and thetemperature range of the plastic region increase withincreasing the heating rate [19], as shown inFig. 10(a). Fong et al. [88] developed a fastplastometer to measure the viscosity of coal underrapid heating conditions (40–800K s�1). The resultsdemonstrated significant impacts of heating rates on

Fig. 10. Fluidity of coal during plastic stage as a function of heating

Fig. 9. Maximum Gieselar plasticity of vitrinites and exinites as a

function of carbon content (with the heating rate 3Kmin�1) [19].



the temperature range, the duration of the plasticregion and the temperature of the maximumfluidity, as shown in Fig. 10(b). It may be notedthat the maximum fluidity decreased with increasingthe heating rate. This implies that the coal plasticitywill not increase infinitely when the heating rate isincreased. An optimal value of the heating rate forthe maximum fluidity may exist. Smith [16] suggeststhat plastic properties become more pronounced athigher heating rates up to a point. If the heating ratebecomes too high, coals cannot plasticize or fluidizebecause the cross-linking reaction temperature isattained and the cross-linking reactions are initiatedbefore diffusion processes can manifest a fluid-likebehaviour. However, the heating rate range for theoptimum fluidity is not clear. On the other hand,low rank coals (lignites) which do not exhibitfluidity at conventional heating rates may developa fluidity if they are heated at extremely highheating rates (42� 104K s�1) when the low tem-perature cross-linking becomes significantly reduced[16,99]. Chan et al. [98] studied the thermo-plasticbehaviour of a range of coals under the pressurizedcarbonization conditions and found that, at slowheating rates (�20Kmin�1), the plastometry torquedecreased with increasing the heating rate for allcoals investigated while the effect of pressure ismore complicated. It may be noted that therheological properties of coal may also play an

rate: (a) slow heating rates [19], and (b) fast heating rates [88].


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 11. Gieseler fluidity as a function of pressure in N2 gas

[65,101].


important role in coal’s thermoplastic properties atfast heating rates [64,65,100].

An increased Gieseler fluidity under elevatedpressures at the heating rate of 3Kmin�1 has beenobserved by Lancet et al. [101] and the effectappears to be stronger in the low-pressure range upto 1.5MPa, as presented in Fig. 11. Although Fonget al. [88] reported a decrease in the fluidity underthe pressure of 3.5MPa at 350K s�1 and anenhanced repolymerization reactions resulting inan increased resolidification rate were believed to beresponsible for the decrease in the fluidity underelevated pressures, it is generally expected thatpressure has a strong enhancement to the fluidity ofcoal during devolatilization.

4. Formation of char structure from pulverized coal

The change in the internal structure of a char isone of the most important issues during coaldevolatilization and is closely associated with theswelling phenomenon of coal during plastic stage.The extent to which the pore structure changes isdependent on the coal type and is strongly affectedby the conditions under which coal is devolatilized.

4.1. Importance of char structure

The importance of char’s structure arises from thefollowing three aspects:

(i)

Ple

the

The change of the internal structure of a charduring pyrolysis determines the mass transportof the volatile matter. During devolatilization,

ase cite this article as: Jianglong Yu et al., Formation of the structure

rmoproperties: A review, Progress in Energy and Combustion Science (

the pore openings of a softening coal will beblocked at the onset of the plastic stage due tothe high fluidity contributed by the metaplast.Therefore, the volatile matter will be trapped inthe coal particles to form bubbles. The releaseof volatile matter and the ultimate structure ofa char will be largely determined by thebehaviour of the bubbles [16,46,89] rather thanby the original pore structure of the feed coal[70]. According to the multi-bubble mechanism[46], the volatile matter is transported throughthe movements of bubbles during the plasticstage. When a large number of bubbles exist inthe coal fluid, the volatile matter diffuses intobubbles instead of diffusing directly out of theparticle’s surface. This causes the growth ofbubbles and swelling of the coal particle. Whenbubbles reach the surface of the particle, theyovercome the force balance to burst and releasethe volatile matter. This mechanism has beengenerally accepted to interpret the swellingphenomena of bituminous coals during heating.The observation by Yu et al. [102] conductedon a single particle reactor confirmed that thegrowth and rupture of bubbles was thedominant phenomenon during the plastic stage.

(ii)
The ultimate structure of a char plays asignificant role in its reactivity during thesubsequent char oxidation to devolatilization,particularly during char gasification [9,103].Based upon the analysis on three Britishbituminous coals, Koranyi [104] found that agood correlation exists between a char’s reac-tivity and its micro-porosity. Hurt et al. [105]studied the roles of micro-porosity and con-cluded that CO2 gasification reactions tookplace primarily outside the micro-pore network,i.e., on the surfaces of larger pores. Thedistribution of pores determines the diffusionof the reactants in the particle which is often therate-limiting step for the char’s oxidation [3].The morphology of the char formed duringpyrolysis will affect the overall combustionefficiency [106,107]. Hampartsoumian et al.[108] studied the effect of the porous structureof char on its rate of gasification and proposedan estimated effectiveness factor during inves-tigating the influence of the pore diffusionunder different conditions. Menendez et al.[109] listed the most important char character-istics with increasing combustion temperaturesuccessively as follows: (i) the surface area and
of chars during devolatilization of pulverized coal and its

2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


Ple

the

the surface structure, i.e., the total surfacewhich may be accessible to the reacting gases;(ii) the porosity which influences the rate ofaccess of the reacting gas to the surface; and(iii) the size of a char particle. These parametersare crucial in modelling of pf combustions andgasification. Highly porous chars have experi-enced much more extensive devolatilizationduring heating resulting in a loss of its mostmaterials. Therefore, during the char oxidation,these chars will burnout at a rather early stagecompared to solid ones, even they burn at asimilar rate [10,11].

(iii)
Char structure has a significant impact on ashformation and pollutant emission during coalconversion. Porous char particles are easilyfragmented during combustion. Kantorovich etal. [110] investigated the role of pore structurein the fragmentation of highly porous charparticles and claimed that the reason for localfragmentation under non-uniform oxidation isthe increase in the local macro-porosity. Thetendency of fragmentation of different types ofchars has a major impact on the chemistry ofthe final ash particles, therefore is crucial to theperformance of a furnace [10,111]. Kang et al.[112] studied the effect of char’s structure onash formation during pf combustion andconcluded that the fragmentation of a charinduced by macro-pores can influence theparticle size distribution (PSD) of the final ashdue to less occurrence of the ash coalescence.Using a pressurized drop tube furnace (PDTF),Wu et al. [10,113] reported that finer ashparticles were generated under elevated pres-sures due to a higher yield of highly porouschars. The textural properties (i.e., the sizedistribution of pores and the active surfacearea) of a char from bituminous coals are alsofound to have some important effect on theNOx emissions [114].
4.2. Classification of char structure

4.2.1. Pore system in coal and char

The pore structure is a rather complex system andhighly heterogeneous in terms of their dimensionsand spatial configurations in both coal and char.Pores are described to have a random spatialdistribution in the carbon matrix with linkagesand intersections existing between each other [27].

ase cite this article as: Jianglong Yu et al., Formation of the struc

rmoproperties: A review, Progress in Energy and Combustion Sci

The pore size of a porous solid system is generallyexpressed by either the diameter of the opening(assuming cylindrical) or the width of the slit [115].Pores with a diameter (or slits with a width) lessthan 20 Angstrom units (A) are referred to as micro-pores. Pores with a diameter larger than 500 A arereferred to as macro-pores. Those with dimensionsin between 20 and 500 A are meso-pores [27,115].This description applies to both parent coal andchar, although the configuration of the porestructure of a char from softening coals afterdevolatilization is significantly different from thatof the feed coal. As the major parameters of a char’sstructure, the porosity and the surface area areusually of the most concern. Micro-pores accountfor the major portion (490%) of the internalsurface area and consequently provide most of thesurface reaction sites [9,116]. On the contrary, themeso-pores and macro-pores which are importantfor the mass transport of gaseous phases accountfor the major portion of total pore volume [117].

A number of techniques are available to measurethe porosity and the surface area of coal and charsincluding the mercury intrusion porosimetry (MIP)and gas adsorption [115]. In recent years, thecharacterization of a char’s structure extensivelyinvolves image-processing techniques [6,107,109,111,118–125] mainly due to their practical applica-tions in describing char’s morphological para-meters. Image-processing techniques can provide anumber of geometric parameters of individual charparticles such as mean particle diameter, two-dimensional macro-porosity and particle sphericity[6]. These parameters are crucial in the charstructure classification systems to determine thestructure type of individual chars.

4.2.2. Classification of char structure

Extensive efforts have been made in the pastdecades to classify morphologically complicatedchar structures [25,26,103,106,109,118,119,124–130]. These classification systems largely relyon image-processing techniques to obtain charmorphological parameters including macro-poros-ity, the wall thickness, particle shape, etc. [6,9,10,129]. Benfell [6] provided a recent summary ofdifferent char classification systems after Clokeet al. [129], as shown in Table 1. These systemsare based upon a combination of the structuralparameters, e.g., the external dimensions of parti-cles, macro-porosity, size distribution of macro-pores, wall thickness and anisotropy. The detailed


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Table

1

Summary

ofvariouscharmorphologyclassificationsystem

s,after

Benfell[6,129]

Chartype

AB

CD

EF

GH

IJ

Jones

etal.

(1985)

Cenosphere

Cenosphere

Honeycomb

Unfused

Unfused

Unfused

Okaet

al.

(1987)

Thin-w

alled

balloon

Thick-w

alled

balloon

Network

Unfused

block

Unfused

block

Skeleton

Micro-

disrupted

Goodarziand

Vleeskens

(1988)

Thin-w

alled

cenosphere

Thick-w

alled

cenosphere

Cenosphere

withribs

Non-reacted

Fusinite

Fragment

Bailey

etal.

(1990)

Tenuisphere

Crassisphere

Tenuinetwork

Mesosphere

Inertoid

Solid

Fusinoid

Mixed

porous

Mixed

dense

Fragment

Menendez

etal.

(1993)

Cenosphere

(TypeI)

Cenosphere

(TypeI)

Network

(Type

II)

Network

(TypeII)

Solid(Type

III)

Solid(Type

III)

Vleeskenset

al.

(1993)

Plastic/porous

Plastic/porous

Plastic/porous

Plastic/

porous

Inertoid

Inertoid

Fusinoid

Rosenberget

al.(1996)

Tenuisphere

Crassisphere

Penuinetwork

Variable

density

type

Inertoid

Fusinoid/

solid

Fusinoid/

solid

Variable

density

type

Variable

density

type


Please cite this article as: Jianglong Yu et al., Formation of the structure of chars during devolatilization of pulverized coal and its

thermoproperties: A review, Progress in Energy and Combustion Science (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Table 2

Summary of the three-fold char structure classification system by Bailey and Benfell [118,125], after Benfell [6] and Liu [9]

Char groups Group I Group II Group III

Char subtypes Cenosphere tenuisphere,

tenuinetwork

Crassisphere, crassinetwork,

mesosphere, mixed porous

(mixed dense)

Inertoid, solid, fusinoid (mixed

dense)

Char particle shape Spheroidal Spheroidal to irregular Subspheroidal, rectangular or

irregular

Porosity 480% 450% �50%

Pore shape Spheroidal Variable Spheroidal to elongate and

angular

Wall thickness o5mm Variable 45 mmDominant maceral components Vitrinite Vitrinite and inertinite Inertinite

Swelling ratio 41.3 o1.0 o0.9

Fig. 12. The yields of volatile matter of maceral components as a

function of coal rank [19,21].


description of the characteristics of each char groupcan be found in the literature [6,129].

A three-group classification system suggested byBenfell and Bailey [6,125] based upon Bailey’searlier work [118] has demonstrated its practicabil-ity in the assessment of combustion characteristicsof chars [6,7,118,125] and has been adopted by anumber researchers [7,9,111,113,131]. The charac-teristics of the three groups of chars are presented inTable 2. The feasible parameters to define a char’sstructure type are porosity, sphericity and wallthickness because they can be easily quantifiedthrough image analysis.

4.3. Relation between char structure and coal

properties

Previous studies show that the structure of a charis strongly dependent on coal rank and is certainlyassociated with thermoplastic properties of the coalduring heating. Low rank coals such as lignitesusually generate network-type to solid char struc-ture. As the rank increases up to low volatilebituminous rank, there is an increase in coal’saromaticity and fusibility [118]. Therefore, theproportion of network-type chars decreases whilethe proportion of cenospheric chars increases[6,106,118,129,130]. Further increase in the coalrank results in an increase in the wall thickness ofchars while the population of thin-walled ceno-spheric char decreases [103,128].

The maceral composition of coal plays a domi-nant role in the morphology of char during the



devolatilization [6,26,118,124,129], although vitri-nites and inertinites from coals ranking above semi-anthracites may be expected to behave similarlyduring combustion [25,26]. Vitrinite-containingparticles from bituminous coals commonly producecenospheric chars while the inertinite produces ahigher proportion of relatively solid chars with alow porosity [6,9,25,26,106,118,122,124,132].

The difference in the morphology of a char fromdifferent maceral components may be attributed tothe difference in their thermo-plasticity and theextent to which a coal is devolatilized [19,44]. Thethermo-plasticity of macerals as a function of coal


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 13. The porosity and swelling ratio as a function of heating

rate [142,143].


rank has been shown in Fig. 9. Fig. 12 demonstratesthe yields of volatile matter of different macerals asa function of the coal rank. Significant differencesexist in the yields between different macerals inmedium to low rank coals [19,21]. The presence ofinertinites significantly reduces the thermo-plasticityof a coal during heating [19] and promotes theformation of thick-walled cenospheric chars[133–135]. Inertinites can also make major con-tribution to the population of cenospheric charswhen suitably fusible under certain heating condi-tions [6,118].

At conventional heating rates, it is possible tocorrelate a char’s structure to the maceral composi-tion of coal under the assumption that porous charsare derived from liptinites and vitrinites whichdevelop fluidity and experience a large extent ofdevolatilization. An empirical equation [9] has beenproposed to correlate the population of Group Ichars to vitrinite (including liptinite) content andpressure based on the experimental results:

nGrpI ð%Þ ¼ 0:6� Ptþ 0:53� vitrþ 37, (1)

where nGrpI is the number percentage of Group Ichar, Pt is the total pyrolysis pressure (atm), andvitr is the vitrinite (including liptinite) content (%).The correlation has been applied in predicting ashformation [136].

4.4. Relation between char structure and heating

conditions

For softening coals, the formation of differenttypes of char structures is closely associated withtheir thermoplastic behaviour such as fluidity andswelling during heating. Therefore, the heatingconditions that influence the thermo-plasticity of acoal will strongly influence its char morphology.

4.4.1. Effect of temperature

The temperature history plays a significant roleduring the evolution of a char’s morphology[32,57,59,88,118,124,137,138]. As the temperatureincreases, sub-bituminous coals produce lessamount of heavy-density and thick-walled chars[118]. Similar observations by Griffin et al. [59]showed that char morphology changed to structureswith larger central pores and network voids at anincreased temperature. A decrease of the charmicro-porosity at a higher temperature at 1770Kwas measured compared to chars from the samecoal prepared at 1270K [32]. This is consistent with



the observation by Jenkins et al. [139] whichindicated that the intrinsic reactivity of chardecreased when the pyrolysis temperature increased.Lewellen [70] measured the surface area of charsfrom softening coal with increasing the temperatureand found that a sharp decrease of accessiblesurface area started at the softening points. Theporosity of chars from non-plastic coal increasedsteadily with increasing the carbonization tempera-ture.

4.4.2. Effect of heating rate

Limited data by Cai et al. [140] showed that charsprepared at fast heating rates contained moremicro-pores and meso-pores and had a greaterinternal surface area, resulting in a higher reactivity.Fast heating rates lead to a greater anisotropy in achar [141]. Gale et al. [142,143] compared theirresults to others [144] and found that the overallporosity and swelling ratio of char increased withincreasing the heating rate up to 103K s�1, thenlevelled off. A further increase in the heating rateabove 2� 104K s�1 resulted in a decreased porosityand swelling, as shown in Fig. 13. This is attributedto the faster generation rate of volatile matter thanthe relaxation time for expanding the char particle.The temperature gradient in a particle at a very fastheating rate may also take some effect.

4.4.3. Effect of pressure

There have been wide research interests in theeffect of pressure on char structure. The develop-ment of efficient power generation technologies


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

0.2

0.4

0.6

0.8

1.0

0.4 0.8 1.2 1.6

Mac

ro-p

oro

sity

(%

)

0.4

0.6

0.8

1.0

Par

ticl

e sp

her

icit

y

Porosity

Sphericity

0.0

Pressure (MPa)

Fig. 15. Macro-porosity of chars from a DTF as a function of

pressure [113].


such as PFBC and integrated gasification combinedcycle (IGCC) [6,9,10,66,68] provide several potentialadvantages over the conventional coal firing pro-cesses including an increased coal throughput, areduced pollutant emission and an enhancement ofcoal reactions. Recent previous work on coalpyrolysis [34,57,59,62,125,140,145–147], coal swel-ling and char structure [6,10,125,148–151] and charreactivity [7,140,147,152–154] have revealed that thepressure has a critical impact on coal swellingduring the devolatilization. In the meantime, thereactivity of a char is changed at high pressures andthe pressure significantly influences the ash forma-tion through its effect on the structure of chars. Theeffect of pressure on ash formation and coalreactions has been recently reviewed by Wall et al.[11,12].

A high swelling ratio of chars under elevatedpressures under a variety of heating conditions[6,59,111,125,146–150] implies that a higher poros-ity may result under pressurized pyrolysis condi-tions. The opponent factors, i.e., the decrease in theyield of tar and the increase in the yield of gases andfluidity result in a maximum swelling ratio at anoptimum pressure range, as is show in Fig. 14 [65].A measurement by Wu et al. [111] on chars collectedfrom a PDTF showed that both the porosity and the

Fig. 14. Swelling ratio as a function of pressure [65].

Fig. 16. Char structure types of an Australian bituminous coal

generated at different pressures [10].



sphericity of a char increased with increasing thepressure up to 1.5MPa, as shown in Fig. 15. Thiswas accompanied by the increased swelling ratio atthe same pressure range. Gadiou et al. [154]investigated the influence of pressure on thestructure and reactivity of millimetre sized singlecoal particles using a laser heating reactor andconcluded that their results are consistent with theresults of Wu et al. [111] in terms of the swelling.

Results from Australian bituminous coals andmaceral concentration coal samples [6,9,10], asshown in Figs. 16 and 17, indicate that a pressureincreases the overall proportion of Group I chars.When the pressure increases from 0.5 to 1.5MPa,the population of Group I chars increases from 38%


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


to 72%. Chars with different structures tend tobehave differently during the subsequent charcombustion or gasification. Group I chars, due totheir high porosity, are more readily fragmentedleading to the formation of finer ash particles [10].Wu et al. [10] found that a highly porous foam charstructure tended to evolve during pyrolysis when thepressure increased in a PDTF. A mechanism for theevolution of the foam structure through bubblegeneration was also proposed, as shown in Fig. 18.However, this process has not been quantitativelymodelled in their study. Their experimental resultssuggest that the population of chars with a foamstructure appears to dominate the char sample. Thechar morphology under SEM (Fig. 19) is alsodistinctive from chars produced on an ordinary

Fig. 18. The evolution of a highly porous foam char structure under p

[10].

Fig. 17. Percentages of Groups I, II and III chars for an

inertinite-concentration sample as a function of the pressure

(prepared at 1570K in a PDTF) [6].



DTF. More recently, Yu et al. [155] compared thestructure and morphology of chars produced on apressurized entrained flow reactor (PEFR) to thechars produced on an ordinary DTF, as shown inFig. 20. It has been found that the surface texture ofa char from a PEFR is similar to that of a char froma PDTF, but very different from that of a charproduced from an ordinary DTF. This clearlyindicates that the pressure plays the dominant rolein the evolution of a char’s structure and morphol-ogy. It has also been found that the population ofporous chars increased substantially. The physicalcharacter and gasification reactivity of chars pro-duced under different pressures were investigated byMatsuoka and co-workers [151]. They claimed thatthe film material was preferentially gasified com-pared to the skeleton carbon. They also found thatporous char was more graphitic therefore lessreactive during gasification than a dense char.

ressurized conditions during devolatilization of a pulverized coal

Fig. 19. SEM image of a char particle produced in a PDTF [10].


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 20. Comparison of surface textures of a char from the PEFR with the char from a DTF: (a) a char from PEFR (20 bar), and (b) a

char from DTF (1 bar).

Fig. 21. Typical swelling curve measured by dilatometer of a

caking coal [14].


4.4.4. Char characteristics and gas environment

Some comparisons have been made in theliterature between the char morphologies in inertgas and in oxidizing environments. Bailey et al. [118]found that a solid residue from early combustionstage had the morphology similar to chars frompyrolysis produced at comparable temperatures,although the proportion of different types of charsmay vary. Under combustion conditions at aburnout level of 50–60wt%, the vitrinite-dominatedmicro-lithotypes generate crassisphere and tenui-sphere chars [124]. Inertinites fuse more readily incombustion than in pyrolysis, producing a largerquantity of tenuisphere chars [6,118]. Chars col-lected from industrial furnaces also demonstratethat inertinites may fuse more readily in boilers[133,134]. This may be due to a higher the heatingrate in industrial boilers than in laboratory scalereactors such as a DTF or a wire mesh reactor.Fletcher [156] suggested that the difference in theswelling behaviour during devolatilization experi-ments from char combustion experiments is not dueto the presence of oxygen, but because of theheating rate or post-flame gas species other thanoxygen. The presence of H2 during pyrolysissignificantly increases the yield of tar and thefluidity of coal [37,54,110], therefore may change achar’s morphology significantly due to the strongassociation between char structure and the thermo-plastic properties and the evolution of volatilematter [110,148]. The solid residues from inertinitesare less susceptible to a high-temperature loss ofreactivity than vitrinites [133,134]. Overall, a charfrom pyrolysis is suitable for the study of char’smorphologies developed under early-stage combus-tion conditions [6,118].



4.5. The relationship between char structure and coal

swelling

The ultimate structure of a char from softeningcoals is strongly associated with its swelling historyduring plastic stage [5,14]. The extent to which coalswells determines not only the particle size of a char,but also its porosity and wall thickness. Apparently,the more a particle swells, the more porous thestructure will be. The swelling ratio may beexpressed as the volume difference between the charand the initial coal [14,149] or by comparing thediameter of the char particle to that of the initialparticle of the feed coal (d/d0) [89,157]. The latterone has been widely used in pulverized coalcombustions [3,6,9,58,89,111,131,150].

Fig. 21 shows the typical volume change of acaking coal measured by a dilatometer [14] at a slowheating rate of 1–5Kmin�1. It demonstrates that a


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 22. Swelling of two bituminous coals as a function of

heating rate [65].


caking coal follows quite complex swelling historyduring which contraction, dilatation and resolidifi-cation take place at different stage. The finalswelling ratio is referred to as the ratio of thevolume of a char particle to the original coalparticle. The dilatation is referred to as themaximum volume of the particle to its minimumvolume at the contraction. The dilatation can bemuch higher than the swelling for a caking coalwhich develops a significant fluidity during heating.The transient physical structure change may not bereflected by the ultimate structure of the solidresidue. The maximum porosity and swelling ratioof a coal particle during devolatilization may bemuch higher than that measured from the final charat the completion of the devolatilization.

The final swelling ratio of a char from softeningcoal is dependent on the coal type and the maceralcomposition is the determining factor [6,158]. At aheating rate between 102 and 104K s�1 [89], typicalswelling of particles from Pittsburgh No. 8 coalexhibit three types: �10% shows virtually no sign offluidity, hence no swelling; the majority (�80%)experience swelling of about the same magnitude;the other �10% exhibits behaviour that can beassociated with a much lower viscosity than themajority. These three types of behaviour areassociated with maceral compositions, i.e., inerti-nites, vitrinites and exinites [89]. Recent studies [6,9]showed that the Group I chars had a larger particlesize compared to solid chars. Cenospheric charshave the largest swelling and the highest porosityduring devolatilization. Vitrinites and liptinitesdevelop fluidity during heating. Therefore, theyare the major compositions contributing to theswelling of coal and the population of porous chars.

Similar to a char’s structure, the swelling ratio ofcoal is strongly influenced by heating conditionsthrough their effects on the fluidity and the ratedevolatilization. When heating rate is increased,both swelling ratio and maximum devolatilizationrate increase, as is shown in Fig. 22 [65]. The peakswelling and porosity appear at heating rate rangeof 2–7� 104K s�1 [142,159], as shown in Fig. 13.This heating rate range is believed to be thetransition value between the heating rate of DTFsand that of a flat-flame burner [142]. However,Solomon et al. [3] predicted a different heating raterange for the maximum swelling ratio after which asharp drop in swelling took place during thepyrolysis of Illinois No.6 coal. Khan et al. [149]found that swelling may not be sensitive to the



heating rate under pressurized conditions. Thechange in the thermoproperties especially thefluidity of coal in presence of H2 will significantlyinfluence the swelling of coal and further the charstructure [110,148]. In the presence of H2, moreporous chars will be generated.

The effect of pressure on the swelling of a coal hasbeen found to be significant under a variety ofconditions [6,59,111,141,149,160,161]. An optimumpressure range exists for the maximum increase inthe swelling when the pressure increases, as shownin Fig. 14. Khan and Jenkins [149] investigated theswelling behaviour of 12 bituminous coals (fromLVB to HVB) under elevated pressures with aparticle size of 355� 710 mm, among which ninepresented an increased swelling with increasing thepressure. Solomon and Fletcher [3] compared themodel predictions to the experimental results[146,150] on swelling of Illinois No. 6 coal andfound that the peak appeared at 0.5–1.5MPa, asshown in Fig. 23. That more volatile matter istrapped inside the coal particle due to the elevatedexternal pressure has been believed to be the reasonfor the increase of the swelling. However, otherfactors such as viscous forces, the extent ofsecondary reactions and surface tension are ob-viously doing some work [3]. Wu et al. [10,111]reported an increased swelling of Australian bitu-minous coals under elevated pressures and attrib-uted the formation of finer ash to the increasedpopulation of highly porous Group I chars.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 24. The burning time as a function of the maximum particle

expansion rate [169].

0.0

1.0

2.0

3.0

4.0

0 100 200 300 400 500

Sw

ellin

g (

St/

S0)

0

200

400

600

800

1000

Tem

per

atu

re(°

C)

Swelling

Temperature

Time (ms)

Fig. 25. Transient swelling of a coal particle from Goonyella coal

at a fast heating rate using laser heating [160], laser in-

tensity ¼ 2.22MWm�2, particle size 149mm.

0.0

1.0

2.0

3.0

4.0

0 10 20 30 40

Pressure (atm)

Sw

ellin

g r

atio

(r/

r 0)

ExperimentalPredicted

Fig. 23. Experimental results and model predictions on swelling

ratio of Illinois No. 6 coal as a function of pressure (after

Solomon and Fletcher [3]).


4.6. Studies on transient coal swelling using single-

particle reactors

In single-particle experiments, the laser heatingtechnique has been extensively employed due to itsunique capacity of simulating the flame heatingfluxes in conventional and high intensity combus-tion [160,162], particularly in investigations on thecombustion characteristics [163–168]. For instance,Shen et al. [169] investigated the combustionbehaviour of single pulverized coal particles andfound a good correlation between the maximumparticle expansion rate, (Vmax�V0)/V0, and theburning time. The burning time decreased linearlywith increasing the maximum expansion rate, asshown in Fig. 24.

By using a CO2 laser heating apparatus, Gaoet al. [170] observed the transition of the surfacestructure of coal particles and concluded that therewas an optimum range of heating rate for theincrease in coal’s plasticity. In their further studies[160], the swelling and fluidity of single coalparticles heated with a CO2 laser were evaluatedusing image analysis. They linked the bubblebehaviour to the coal plastic properties. Fig. 25shows the transient swelling of a particle fromGoonyella coal recorded using a high-speed camera[160]. Johnson et al. [171] studied the pyrolysis ofsingle particles using several UK coals at a heatingrate of 100–1500K s�1. The onset of the melting andswelling was recorded. Their results showed that themaximum rate of devolatilization and the yield ofhydrocarbon gases appeared within the same



temperature range (700–1000K) for a vigorousswelling.

Yu et al. [172] conducted experiments on acaptive single-particle reactor using a density-separated Australian coal at a heating rate of10–200K s�1 under the atmospheric pressure. Re-markable variations in the swelling took placebetween particles from different density fractionsof the same coal. The light-density fraction ex-hibited a higher swelling and more intense bubbling,as is shown in Fig. 26. The images in Fig. 26(a)indicate that the particle grows larger during theplastic stage due to the growth of the bubbles whilethe particle shrinks as the bubbles burst. This


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 26. Transient swelling behaviour of the observed particles from different density-fraction samples of an Australian coal at the heating

rate of 100K s�1: (a) F1.25; (b) S1.30–F1.35; and (c) S1.35–F1.50, scale bar—130mm [172].


process takes place repeatedly until the particleresolidifies. At the later plastic stage, the particlelooks almost transparent when the particle swells toa very large extent. This implies that the particle ishighly porous and the outer liquid shell is very thin.The swelling and bubbling decrease significantly asthe coal density increases. The particle in Fig. 26(c)exhibits very little change in its morphology. Theresults of petrographic analysis showed that theliptinites and vitrinites contents decreased whilethe ash content increased with increasing the coaldensity. Therefore, particles observed in the case offigure (a) and (b) are most likely liptinites orvitrinites particles while the particle in figure (c)contains inertinites or mineral matter. Inertinitesexhibit little fluidity at conventional heating rates.Ash grains remain solid at the pyrolysis temperatureand suppress the apparent fluidity of the wholeparticle. Fig. 27 presents the transient swelling



ratios quantified through image analysis [172]. Alarge swelling ratio is obtained for the particle fromthe lightest density fraction (F1.25) and the swellingdecreases drastically as the coal density increases.The maximum swelling ratio of the particle fromF1.25 fraction is 3.2 while the final swelling ratio isaround 2.0. This implies that the swelling ratio ofchars from a DTF is not able to reflect the transientswelling and the char’s structure change during theintermediate stage of the devolatilization process.

4.7. Application of density-separation techniques in

studies of char structure

The fact that different structures are derived fromdifferent pulverized coal particles is attributed to thevariations in their maceral composition [6,9,122]. Ingeneral, particles containing liptinites or vitrinitesgenerate a porous char while those containing


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

1.5

1

0.5

2.5

2

3.5

3

1 2 3 4 5 6 7

Time (s)

Sw

ellin

g r

atio

(d/d

0)

F1.25

F1.35

F1.50

Fig. 27. Transient swelling ratio of the observed particles from

three density fractions of an Australian coal at heating rate

100K s�1 [172].


inertinites generate relatively dense char structures.Therefore, samples of selective maceral concentra-tions are very useful in investigating the formationof char’s structure during pyrolysis. This can beachieved by sampling from the different coal facesselectively [6–8,125]. An empirical correlation be-tween char’s structure and the vitrinite concentra-tion has been established recently [6]. The reactivityof maceral concentrates was also investigated[32,173].

Density-separation techniques such as the sink-float technique and centrifugation have been exten-sively employed to separate coal macerals [174,175].Cloke et al. [120] characterized density-separatedcoal fraction samples using a FT-IR and petro-graphic analysis. It was concluded that the Aar/Aal

ratios increased for heavier fractions, indicating achange in the aromaticity when the coal density wasincreased. Kawashima et al. [176] used the NMRtechnique to characterize the density-separatedcomponents during pyrolysis. They found that thelighter components underwent a greater structuralchange than the heavier components did. Theelimination of aliphatic side chains took place to agreater extent in lower rank coals than in a highrank coal and the content of aliphatic moieties ineach maceral group determines its reactivity. Gilfil-lan et al. [122,132] examined the structure andreactivity of density-separated coal fractions of sixworld coals. The morphological analysis of the charclearly indicated that thin-walled chars were pro-duced from the light density fractions while theproportion decreased with increasing the density.



The chemical structure investigations using a FT-IRand 13C NMR showed that aromaticity increasedfor heavier density fractions with the inertinite-richfractions having the highest values. During adensity-separation process, the particle size isusually reduced to below 38 mm (10�6m) [122] bycrushing down the raw coal. The raw coal particlesizes are even reduced to several microns to achieveeffective liberation of the maceral components.Minerals are usually removed prior to the densityseparation [175] through acid washing.

During investigating the heterogeneity of largepulverized coal particles, Gibbins et al. [177] foundsome synergic effect during the devolatilization andsuggested that the ability of using relatively puremacerals (from hand-picking or by micronizing,demineralization and density separation) to repre-sent actual pf combustion behaviour may belimited. In their study, a wide range of particlemineral content was also observed and the mineralswere generally well dispersed and frequently asso-ciated with the inertinite maceral. By measuring thechar intrinsic reactivity, they suggested that miner-als may act as a catalyst for char oxidation,although the effect may be small. Instead, somedecrease in the combustion rate of a particle duringthe later stage of a char burnout took place due tothe increased resistance to oxygen transport andheat losses caused by a large quantity of mineralmatter admixed with organic materials. It is there-fore suggested that both the inertinite content andmineral matter should be included to explain thepoor burnout of chars in some cases.

Yu et al. [131] conducted a systematic study onthe chemistry of chars prepared on an DTF usingdensity fractions of an Australian coal which wasnot subjected to a demineralization. The densityseparation was carried out using ZnCl2 aqueoussolutions. Petrographic analysis indicated that thelight density fractions contained high liptinites andvitrinites while the heavy density fractions had ahigh content of inertinites and mineral matters.SEM images of the chars from DTF are shown inFigs. 28 and 29. The results indicate that the light-density fractions (i.e., F1.25 and F1.30) generatedvery porous char particles with a cenosphericstructure. A cenospheric char has the configurationof a single central void surrounded by a thin outershell (�5 mm) [123,126]. In these chars, the popula-tion of Group I chars were higher than 70%. Charsfrom the heavy-density fractions (i.e., F1.50 andS1.50) had a rather solid structure with a low


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 28. SEM images of cross-sections of chars prepared in a DTF at 1570K using density-fraction samples of an Australian coal: (a)

F1.25; (b) S1.30–F1.35; (c) S1.35–F1.50; and (d) S1.50 (I—group I chars, II—group II chars, III—group III chars, SG—solid carbon

particle, SM—solid particle with included mineral grains, SC—small cenospheric char particle, M—excluded mineral grains, after Yu et

al. [131]).

0

20

40

60

80

Group I Group II Group III

Char type

Per

cen

tag

e (%

)

F1.25F1.30F1.35F1.50S1.50

100

Fig. 29. The char type distribution of chars from the density

fractions of an Australian coal, as prepared in a DTF at 1570K

[131].




porosity. Over 80% particles are Group III chars.Solid particles were either relatively pure carbonparticles (marked as ‘SG’) or carbon particles withsome included ash grains (marked as ‘SM’). In theS1.50 fraction, some excluded pure mineral grainswere also observed (marked as ‘M’). The char fromthe medium-density fraction (i.e., F1.35) containeda mixture of different types of chars with amoderate porosity and wall thickness. A conclusionhas been drawn that both maceral composition andmineral matter are playing roles in the formation ofchar structure. Yu et al. [131] also provided asystematic analysis on the surface morphology ofchar particles produced on a DTF using density-fraction samples. Table 3 lists the typical morphol-ogies of group A to H and their characteristics andoccurrence. Littlejohn et al. [127] carried out a


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Table

3

Typicalmorphologyofcharparticlespreparedin

aDTF

under

SEM

[131]

Morphology

AB

CD

EF

FH

Image

Occurrence

Low-density

samples(F1.25

andF1.30)

Low-density

samples(F1.25

andF1.30)

Medium-to

high-

density

samples

(F1.35,F1.50

andS1.50)

High-density

samples(F1.50

andS1.50)

Low-density

samples(F1.25

andF1.30)

Medium-and

low-density

samples(F1.35

andF1.30)

Low-density

sample

(F1.25of

coalA)

Heavy-density

samples(F1.50

andS1.50)

Description

Poruscharwith

oneblow

hole,

andflow

patterns

onsurface

Poruscharwith

closedsurface

andflow

patterns

Charparticles

withpore

openingonthe

surface

Solidchar

particleswith

cracksonthe

surface

Smallandround

particles

Charparticles

withmultiple

component

Charparticlesof

highconversion

Solidchar

particles




dx.doi.org/10.1016/j.pecs.2006.07.003


similar study. The particles from heavy densityfractions develop little fluidity and generate muchless amount of volatile matter. The volatile matter isreleased through the open pores or cracks on theparticle’s surface. These particles will generatemostly group C, D, or H chars. Little change inthe morphology takes place for these particlesduring pyrolysis. Some particles may develop localfluidity so that they have some parts are liquidized,although the whole particle does not soften or swell.These particles are most likely to generate group C

or F chars. Particles that develop high fluidityduring pyrolysis may have three pathways:

�

P

th

A particle may break up to form several smallliquid drops during the plastic stage due to a veryhigh fluidity and intense generation of volatilematter. The small particles continue to undergopyrolysis and swelling. These particles generategroup E or B chars.
� Some particles may swell to a large extent to
form a cenospheric char. During this process,bubbles may rupture. However, the holes maynot be closed again due to the intense release ofvolatile matter through the holes. These particleswill generate group A chars with a large blow-hole on their surface.
� The particle may swell but the bubbles do not
rupture or the blow hole is closed again after thebubble rupture. A group B char is then gener-ated.

A coal particle with a mixture of two differentmaceral components may generate a group F char.group G chars evolve from coal particles consistedof liptinites. These particles experienced a highextent of swelling and a high conversion duringpyrolysis. Overall, light-density fractions generategroups, A, B, E or G chars while heavy-densityfractions produce groups, C, D or H chars.

5. Modelling the swelling and the formation of char’s

structure

Significant modelling efforts have been made todeal with the physical aspects of coal pyrolysis, i.e.the swelling and a char’s structure. Early studieshave been focused on providing mechanistic inter-pretations to the mass transport and swellingphenomena when coal is heated [46,70,96]. Themode of transport of volatile matter for a plasticcoal differs radically from a non-plastic coal

lease cite this article as: Jianglong Yu et al., Formation of the struc

ermoproperties: A review, Progress in Energy and Combustion Sci

[46,70,96]. Instead of using pore transport mechan-ism, Lewellen [70] proposed the concept of volatilematter transport via gas bubbles and used thismechanism to model the secondary reactions duringpyrolysis of coal. Based on a number of assump-tions, the physics of bubble generation, bubblegrowth and destruction was described. Sung [96]employed this mechanism to interpret the swellingphenomena of a coal during pyrolysis.

A multi-bubble mechanism for the mass transportof volatile matter for plastic coal during devolati-lization was proposed [46,90], as illustrated inFig. 30. According to this mechanism, the metaplastis generated through a physical melting (those pre-existing in the parent coal) and thermal decomposi-tions upon heating. The primary gases and char arealso formed simultaneously. The further decom-position of the metaplast generates tars, secondarygases and chars. Bubbles are initiated from macro-pores of the parent coal at the onset of the plasticstage or through nucleation mechanism. Volatilematter (i.e., tars and gases) may be transported intobubbles or to the surface of a particle throughdiffusion. However, time for a direct diffusion ofgases to the particle’s surface is several magnitudeslonger than that into the internal bubbles. There-fore, the physics of multi-bubble mechanism issummarized as: the volatile matter is carried bybubbles; the volatile matter is released throughbubble movements rather than by the directdiffusion to the particle’s surface; the swelling of aparticle is caused by the growth of bubbles due tothe generation of volatile matter; the rupture ofbubbles at the particle’s surface is determined by aforce balance for which the viscous force is a majorterm.

The change of the number of bubbles is describedas Eq. (2):

dnj

dt¼ Kj�1nj�1n1 � Kjnjn1

þ1

2

XN�j

i¼2

Pi;j�ininj�i �XN

i¼2

Pi;jninj � Ejnj, ð2Þ

where nj is the number of the bubbles with size j, andN is the total number of bubbles. Ej is the escapingrate of bubbles, and Kj and Pi,j are the kinetic ratesfor the growth and coalescence of bubbles, respec-tively. This equation has a similar form to that fordescribing the behaviour of a colloid. Consideringthe uncertainty of the size distribution of bubbles atthe onset of a plastic stage, this equation is a rather


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

Fig. 30. The multi-bubble mechanism for mass transport of volatile matter in a plastic coal during devolatilization [46,90].


complicated and un-applicable. A number ofsimplified cases have been thereby considered,including the extreme case that the bubbles areuniformly spatially distributed with a uniform size[46]. The growth of a bubble is caused by thegeneration of volatile matter. The growth rate of abubble determines the bubble escaping rate andcoalescence rate. The volatile matter builds up theinternal pressure and is restrained by the externalpressure, surface tension and viscous force. Theequation describing the growth rate of a bubbleessentially describes the force balance, as shown inEq. (3) [46]:

Pb � P� 2s=rb

r¼ rb

dr

dtþ

3

2

dr

dt

� �2

þ4mrrb

dr

dt, (3)

where dr/dt is the rate of a bubble’s growth, rb is thebubble’s radius, m is the viscosity, Pb is the internalpressure, P is the external pressure, s is the surfacetension and r is the density of coal.

A single-bubble model has been proposed bySolomon et al. [89] which is incorporated with theFG-DVC model [42,76] to predict a coal’s swelling.In this model, the viscosity is calculated from anempirical model by Solomon et al. [47]. The single-bubble model has been employed by Sheng et al.[178] to the transient swelling and morphology



changes of a char. The physics of the single-bubblemodel is based upon the assumptions that theliquidized coal particle forms a single centric voidsurrounded by a porous outer fluid shell. Volatilematter is released through both the rupture ofbubbles and the direct diffusion of volatile mattersto the particle’s surface. The rupture of a bubbletakes place when the internal pressure builds up andreaches the following criterion [89]:

1:5r3bðPb � PÞ

r3p � r3b� P4Swc, (4)

where rb and rp are the radius of the bubble and theparticle, respectively, Swc is the critical wall stress,Pb is the internal pressure and P is the externalpressure. The diffusion term of the volatile matterthrough the porous shell is expressed as [89,178]

4pDvCb

�ð1=rb � 1=rpÞ, (5)

where Dv is the diffusivity of volatile matter in theporous liquid and Cb is the molar concentration ofvolatile matter in the bubble. DL is used inSolomon’s work [89] while the effective diffusivityterm, De, has been used by Sheng et al. [178].Sheng et al. also compared their model predictionswith the experimental measurements in the litera-ture [160].


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


Yu et al. [179,180] combined the multi-bubblemechanism and the single-bubble model and pro-posed a simplified mechanism for the evolution ofchar’s structure of softening coal during pyrolysis.According to this mechanism, devolatilization isdivided into three stages, i.e., the pre-plastic stage(m4mc, ToTs), the plastic stage (mpmc,

TspTpTd) and the resolidified stage (m4mc,

T4Td). This division is based on the viscosity ofcoal calculated from the volume fraction of themetaplast during heating. The critical viscosity, mc,is from 4� 104 to 2� 105 Pa s. The plastic stage isthe key step for the evolution of a char’s structure.The change in the number of bubbles, the forcebalance and mass balance all need to be formulatedcarefully to describe the behaviour of a coal particleduring the plastic stage.

As illustrated in Fig. 31 [179], a large number ofbubbles with a uniform size and spatial distributionexist in the molten coal particle at the onset of theplastic stage. The bubbles originate from thosemacro-pores which have survived the initial soft-

Fig. 31. The evolution of char structure of so



ening. The initial number density of bubbles at theonset of the plastic stage is estimated at 1013 g–1 coalaccording to Oh [90]. Bubbles grow by the genera-tion of volatile matter during the decomposition ofa coal and diffusion of the volatile species into abubble. When bubbles rupture at the surface of theparticle, the gas inside those bubbles is released. Thegrowth of these bubbles results in the particle’sswelling. In the meantime the internal physicalstructure of the particle changes to a differentextent. The direct diffusion of volatile matter to thesurface of the particle is also considered when acenospheric char has formed. The ultimate structureof a char depends on when the coal particleresolidifies. As such, a dense char (Group III), afoam structure (Groups II or III) or a cenosphericchar (Group I) may evolve, depending on theproperties of the coal. An important feature of thismodel is its capacity of predicting the distribution ofa char’s structure under different heating conditionsin combination with the density-separation techni-que. Results in Fig. 32 demonstrated that the model

ftening coal during coal pyrolysis [179].


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

ARTICLE IN PRESS

0

10

20

30

40

50

60

(a)

(b)

Prediction

Experimental

Wei

ght l

oss

(wt.%

)

0.0

0.5

1.0

1.5

Por

osity

(%

)S

wel

ling

ratio

(d/

d0)

Prediction

Malvern

Image Analysis

0

20

40

60

80

(c)

Density fraction

Experimental

Prediction

Experimental

Prediction

Group I Group II Group III

0

10

20

30

40

50

60

(d)

Char type

Cha

r di

strib

utio

n (%

)

S1.50F1.50F1.35F1.30F1.25

Fig. 32. Comparison of the model’s prediction with measure-

ments on chars prepared from density fractions of an Australian

coal in a DTF [179]: (a) weight loss; (b) swelling ratio; (c)

porosity; and (d) distribution of char structures.


predicts experimental trends for the behaviour of abituminous coal during rapid heating, including theswelling, the porosity and the distribution of charstructures. The decrease in the weight losses withincreasing coal’s density by both predictions andmeasurements is apparent in Fig. 32(a). Fig. 32(b)shows that the swelling ratio predicted by the modeland the measurement using a Malvern Sizer andthrough SEM image analysis on chars from theDTF decreases as coal density increases. However,the model predicts a higher swelling ratio than themeasured value for the light fractions. Duringexperiments, when the lightest density fraction coalsample was devolatilized at high heating rates in gasflow reactors, some particles may break up duringthe plastic stage due to a high fluidity and a highrate of devolatilization. Although the fragmentedliquid particles continue to swell, the final swellingratio of these chars may be much smaller thanparticles not experiencing fragmentation. This hasnot been included in the multi-bubble model.Fig. 32(c) demonstrates a good agreement of thepredicted porosity of chars with measurements onchars from the DTF. Corresponding to a decrease inthe swelling ratio, the porosity decreases remark-ably as coal density increases. The porosity of a charis one of the major parameters determining itsstructure. Therefore, the decrease in the porosityvirtually leads to a change in the char’s structure.Group I chars have a porosity above 80%, Group IIchars have a porosity of 50–80% while Group IIIchars have a porosity below 50%. The modelpredicts different types of structures for chars fromdifferent density fractions by considering eachdensity fraction as a homogeneous material. Incombination with the yield of each density fractionfrom a density separation, the distribution of charstructures of the full coal was obtained. Fig. 32(d)compares the model’s predictions with the measure-ments on chars from the DTF. The results agreereasonably well.

6. Concluding remarks

In summary, the extensive experimental andmodelling efforts made in the past have significantlyadvanced our understanding of the mechanism andkinetics of the formation of char structure duringdevolatilization of pulverized coal. The mechanismof the evolution of volatile matter during pyrolysishas been well established. Network models fordevolatilization have been developed to provide



dx.doi.org/10.1016/j.pecs.2006.07.003


quantitative predictions to the yields of volatilespecies. The swelling and structure of a char havebeen studied extensively using a variety of experi-mental techniques and have been classified usingdifferent systems based on morphological para-meters of chars. Limited experimental observationson the transient swelling behaviour of individualcoal particles during devolatilization improved ourfundamental understanding of coal’s thermoplasticbehaviour. Mathematical models have been devel-oped based on bubble transport mechanism topredict the transient swelling behaviour of softeningcoals and provide a potential capability in describ-ing the evolution of char structure.

Acknowledgements

The authors acknowledge the support providedby the Cooperative Research Centre for Coal inSustainable Development (CCSD), which is fundedin part by the CRC Program of Australia, to carryout the research mentioned in this paper. Thanksare also due to Dr. Vladmir Strezov, Dr. Guisu Liu,Dr. Hongwei Wu and Dr. Katharine E. Benfell forhelpful discussion and communicating their infor-mation in this field. The authors also acknowledgethe support from the National Basic ResearchProgram of China (2005CB221203).

References

[1] Elliott MA, Yohe GR. The coal industry and coal research

and development in perspective. In: Elliott MA, editor.

Chemistry of coal utilization. New York: Wiley-Inter-

science; 1981. p. 1–52.

[2] Smoot LD, Pratt DT. Pulverized-coal combustion and

gasification: theory and applications for continuous flow

processes. New York: Plenum Press; 1979.

[3] Solomon PR, Fletcher TH. Impact of coal pyrolysis on

combustion. In: Proceedings of the 25th international

combustion symposium, 1994. p. 463–74.

[4] Williams A, Backreedy R, Habib R, Jones JM, Pourka-

shanian M. Modelling coal combustion: the current

position. Fuel 2002;81:605–18.

[5] Howard JB. Fundamentals of coal pyrolysis and hydro-

pyrolysis. In: Elliott MA, editor. Chemistry of coal

utilization. New York: Willey; 1981.

[6] Benfell KE. Assessment of char morphology in high

pressure pyrolysis and combustion. P.h.D thesis, Depart-

ment of Geology, University of Newcastle, 2001.

[7] Benfell KE, Liu G-S, Roberts DG, Harris DJ, Lucas JA,

Bailey JG, et al. Modelling char combustion: the influence

of parent coal petrography and pyrolysis pressure on the

structure and intrinsic reactivity of its char. Proc Combust

Inst 2000;28:2233–41.



[8] Wall TF, Tate AG, Bailey JG, Jenness LG, Mitchell RE,

Hurt RH. The temperature, burning rates and

char character of pulverized coal particles prepared

from maceral concentrates. In: Proceedings of the

24th international combustion symposium, 1992.

p. 1207–215.

[9] Liu G-S. Mathematical modelling of coal char reactivity in

a pressurised entrained flow gasifier. PhD thesis, Depart-

ment of Chemical Engineering, University of Newcastle

(N.S.W.), Australia, 1999.

[10] Wu H. Ash formation during pulverised coal combustion

and gasification at pressure. Ph.D. thesis, Department of

Chemical Engineering, University of Newcastle, Australia,

2000.

[11] Wall TF, Liu GS, Wu H-W, Roberts DG, Benfell KE,

Gupta S, et al. The effect of pressure on coal reactions

during pulverized coal combustion and gasification. Prog

Energy Combust Sci 2002;28:405–33.

[12] Wall TF, Yu J, Wu H, Liu G, Lucas JA, Harris D. Effects

of pressure on ash formation during pulverized coal

combustion and gasification. Preprints of Symposia—

American Chemical Society, Division of Fuel Chemistry,

vol. 47, 2002. p. 801–06.

[13] Averitt P. Coal resources. In: Elliott MA, editor. Chemistry

of coal utilization. New York: Wiley; 1981. p. 57.

[14] Berkowitz N. The chemistry of coal. Amsterdam, New

York: Elsevier; 1985.

[15] Smoot LD, Smith PJ. Coal combustion and gasification.

New York: Plenum Press; 1985.

[16] Smith KL. The structure and reaction processes of coal.

New York: Plenum Press; 1994.

[17] Borrego AG, Marban G, Alonso MJG, Alvarez D,

Menendez R. Maceral effects in the determination of

proximate volatiles in coals. Energy & Fuel 2000;14:

117–26.

[18] Whitehurst DD. A primer on the chemistry and constitu-

tion of coal. In: Gould RF, editor. Organic chemistry of

coal. Washington: American Chemical Society; 1978.

p. 1–35.

[19] Van Krevelen DW. Coal, typology–chemistry–physics–-

constitution. Amsterdam, Oxford, New York: Elsevier;

1981.

[20] Neavel CR. Origin, petrography, and classification of coal.

In: Elliott MA, editor. Chemistry of coal utilization. Wiley-

Interscience: New York; 1981. p. 91–158.

[21] van Krevelen DW. Coal—typology, physics, chemistry,

constitution, 3rd, completely rev. ed. Amsterdam, New

York: Elsevier; 1993.

[22] International Committee for Coal and Organic Petrology

(ICCP). The new inertinite classification (ICCP System

1994). Fuel 2001;80:459–71.

[23] International Committee for Coal and Organic Petrology

(ICCP). The new vitrinite classification (ICCP System

1994). Fuel 1998;77:349–58.

[24] Bailey J. Coal and its petrography-in CCSD course on coal,

2002.

[25] Jones RB, Morley C, McCourt CB. Maceral effects on the

morphology and combustion of coal char. Proc—Int Conf

Coal Sci 1985:669–72.

[26] Jones RB, McCourt CB, Morley C, King K. Maceral and

rank influences on the morphology of coal char. Fuel

1985;64:1460–7.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


[27] Simons GA. The role of pore structure in coal pyrolysis and

gasification. Prog Energy Combust Sci 1983;9:269–90.

[28] Hayhurst AN, Lawrence AD. The devolatilization of coal

and a comparison of chars produced in oxidizing and inert

atmospheres in fluidized beds. Combust Flame 1995;100:

591–604.

[29] Alonso MJG, Borrego AG, Alvarez D, Menendez R.

Pyrolysis behaviour of pulverized coals at different

temperatures. Fuel 1999;78:1501–13.

[30] Alonso MJG, Alvarez D, Borrego AG, Menendez R,

Marban G. Systematic effects of coal rank and type on the

kinetics of coal pyrolysis. Energy & Fuel 2001;15:413–28.

[31] Anthony DB, Howard JB. Pyrolysis and hydropyrolysis.

AIChE J 1976;22:625.

[32] Cai HY, Megaritis A, Messenbock R, Dix M, Dugwell DR,

Kandiyoti R. Pyrolysis of coal maceral concentrates under

pf-combustion conditions (I)—changes in volatile release

and char combustibility as a function of rank. Fuel

1998;77:1273–82.

[33] Gavalas GR. Coal pyrolysis. Amsterdam, Oxford, New

York: Elsevier Scientific publishing company; 1982.

[34] Mill CJ. Pyrolysis of fine coal particle at high heating rate

and pressure. PhD thesis, School of Chemical Engineering

and Industrial Chemistry, University of New South Wales,

Australia, 2001.

[35] Serio MA, Solomon PR, Yu ZZ, Bassilakis R, Markham

JR, Klapheke JG. The effect of rank on coal

pyrolysis kinetics. Preprints of Papers—American Chemi-

cal Society, Division of Fuel Chemistry, vol. 34, 1989.

p. 1324–328.

[36] Solomon PR, Serio MA, Suuberg EM. Coal pyrolysis:

experiments, kinetic rates and mechanisms. Prog Energy

Combust Sci 1992;18:133–220.

[37] Suuberg EM. Rapid pyrolysis and hydropyrolysis of coal.

Sc. D. thesis, Department of Chemical Engineering,

Massachusetts Institute of Technology, 1977.

[38] Saxena SC. Devolatilizatin and combustion characteristics

of coal particles. Prog Energy Combust Sci 1990;16:55–94.

[39] Juntgen H. Coal characterization in relation to coal

combustion. Erd Koh Ep 1987;40:153–65.

[40] Juntgen H. Review of the kinetics of pyrolysis and

hydropyrolysis in relation to the chemical constitution of

coal. Fuel 1984;63:731–7.

[41] Suuberg EM, Peters WA, Howard JB. Product composi-

tions and formation kinetics in rapid pyrolysis of pulverized

coal—implications for combustion. In: Proceedings of the

17th international combustion symposium, vol. 17, 1979.

p. 117–30.

[42] Solomon PR, Hamblen DG, Carangelo RM, Serio MA,

Deshpande GV. General model of coal devolatilization.

Energy & Fuel 1988;2:405–22.

[43] Fitzgerald D. Viscoelastic properties of coal during

carbonization. Fuel 1957;36:389–94.

[44] van Krevelen DW, Huntjens FJ, Dormans HNM. Chemi-

cal structure and properties of coal. XVI. Plastic behavior

on heating. Fuel 1956;35:462–75.

[45] van Krevelen DW. Physical characteristics and chemical

structure of bituminous coal. Brennstoff-Chem 1953;34:

167–82.

[46] Oh MS. Softening coal pyrolysis. Department of Chemical

Engineering, Massachusetts Institute of Technology; 1985.



[47] Solomon PR, Best PE, Yu ZZ, Charpenay S. An empirical

model for coal fluidity based on macromelecular network

pyrolysis model. Energy & Fuel 1992;6:143–54.


Deshpande GV. Models of tar formation during coal

devolatilization. Combust Flame 1988;71:137–48.

[49] Solomon PR, Hamblen DG, Yu ZZ, Serio MA. Network

models of coal thermal decomposition. Fuel 1990;69:754–8.

[50] Fletcher TH, Kerstein AR, Pugmire RJ, Solum M, Grant

DM. A chemical percolation model for coal devolatiliza-

tion: milestone report, 1992: 72.

[51] Fletcher TH, Kerstein AR, Pugmire RJ, Solum MS, Grant

DM. Chemical percolation model for devolatilization. 3.

Direct use of carbon-13 NMR data to predict effects of coal

type. Energy & Fuel 1992;6:414–31.

[52] Kimber GM, Gray MD. Rapid devolatilization of small

coal particles. Combust Flame 1967;11:360–2.

[53] Badzioch S, Hawksley PGW. Kinetics of thermal decom-

position of pulverized coal particles. Ind Eng Chem,

Process Des Dev 1970;9:521–30.

[54] Anthony DB. Rapid devolatilization and hydrogasification

of pulverized coal. Sc.D. thesis, Chemical Engineering, M.

I. T., USA, 1974.

[55] Solomon PR. United Technologies Research Center Ro-

port, 1977.

[56] Jamaluddin AS, Truelove JS, Wall TF. Devolatilization of

bituminous coals at medium to high heating rates.

Combust Flame 1986;63:329–37.

[57] Cai HY, Guell AJ, Dugwell DR, Kandiyoti R. Heteroatom

distribution in pyrolysis products as a function of heating

rate and pressure. Fuel 1993;72:321–7.

[58] Solomon PR, Fletcher TH, Pugmire RJ. Progress in coal

pyrolysis. Fuel 1993;72:587–97.

[59] Griffin TP, Howard JB, Peters WA. Pressure and

temperature effects on bituminous coal pyrolysis: experi-

mental observations and a transient lumped-parameter

model. Fuel 1994;73:591–601.

[60] Yeasmin H, Mathews JF, Ouyang S. Kinetics of Yallourn

brown coal devolatilization at elevated pressure in an

entrained flow reactor. In: Proceedings—annual interna-

tional Pittsburgh coal conference, Pittsburgh, 1997. p. S20/

6–S20/8.

[61] Yeasmin H, Mathews JF, Ouyang S. Rapid devolatilization

of Yallourn brown coal at high pressures and temperatures.

Fuel 1998;78:11–24.

[62] Mill CJ, Harris DJ, Stubington JF. Pyrolysis of fine coal

particles at high heating rates and high pressure. In: Eighth

Australian coal science conference proceedings, Sydney,

Australia, 1998. p. 151–6.

[63] Eddinger RT, Friedman LD, Rau E. Devolatilization of

coal in a transport reactor. Fuel 1966;45:245–52.

[64] Khan MR. Thermoplastic properties of coal pyrolyzed at

elevated pressures: influence of experimental variables,

inorganic additives and pre-oxidation. PhD thesis, Chemi-

cal Engineering, Pennsylvania State University, The US,

1985.

[65] Khan MR, Jenkins RG. Influence of weathering and low-

temperature oxidation on the thermoplastic properties of

coal. In: Nelson CR, editor. Chemistry of coal weathering.

Amsterdam: Elsevier; 1989. p. 109.

[66] Takematsu T, Maude C. Coal gasification for IGCC power

generation. London: Gemini House; 1991.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


[67] Harris D. Ash formation during high-pressure gasification

in a reducing atmosphere, CCSD project 3.1 Report, 2001.

[68] Wang A, L.T., Stubington JF. Characterisation of Aus-

tralian black coal for pressurised fluidized bed combustion.

In: Eighth Australian coal science conference proceedings,

Sydney, Australia. 1998. p. 261–6.

[69] Shan G. A comprehensive model for coal devolatilisation.

PhD thesis, Deparmant of Chemical Engineering, Uni-

versity of Newcastle (NSW), Australia, 2000.

[70] Lewellen PC. Product decomposition effects in coal

pyrolysis. MS thesis, Chemical Engineering, Massachusetts

Institute of Technology, USA, 1975.

[71] Mathews JP, Hatcher PG, Scaroni AW. Particle size

dependence of coal volatile matter: is there a non-

maceral-related effect? Fuel 1997;76:359–62.

[72] Solomon PR, Colket MB. Coal devolatilization. In:

Proceedings of the 17th international combustion sympo-

sium, vol. 17, 1979. p. 131–43.

[73] Miura K. A new and simple method to estimate f(E) and

k0(E) in the distributed activation energy model from three

sets of experimental data. Energy & Fuel 1995;9:302–7.

[74] Miura K, Maki T. A simple method for estimating f(E) and

k0(E) in the distributed activation energy model. Energy &

Fuel 1998;12:864–9.

[75] Liu X, Li B, Miura K. Analysis of pyrolysis and

gasification reactions of hydrothermally and supercritically

upgraded low-rank coal by using a new distributed

activation energy model. Fuel Process Technol 2001;69:

1–12.


Deshpande GV. General model of coal devolatilization.

ACS Div Fuel Chem Prepr 1987;32:83.

[77] Serio MA, Solomon PR, Yu ZZ, Deshpande GV. An

improved model of coal devolatilization. In: International

conference on coal science proceedings, Tokyo, Japan,

1989. p. 209.

[78] Niksa S, Kerstein AR. The distributed-energy chain model

for rapid coal devolatilization kinetics. Part I: formulation.

Combust Flame 1986;66:95–109.

[79] Niksa S. The distributed-energy chain model for rapid coal

devolatilization kinetics. Part II: transient weight loss

correlations. Combust Flame 1986;66:111–9.

[80] Niksa S, Kerstein AR. On the role of macromolecular

configuration in rapid coal devolatilization. Fuel 1986;66:

1389–99.

[81] Grant DM, Pugmire RJ, Fletcher TH, Kerstein AR. A

chemical model of coal devolatilization using percolation

lattice statistics. Preprints of Papers—American Chemical

Society, Division of Fuel Chemistry, vol. 33, 1988. p. 322–32.

[82] Grant DM, Pugmire RJ, Fletcher TH, Kerstein AR.

Chemical model of coal devolatilization using percolation

lattice statistics. Energy & Fuel 1989;3:175–86.

[83] Fletcher TH, Kerstein AR, Pugmire RJ, Grant DM.

Chemical percolation model for devolatilization. 2. Tem-

perature and heating rate effects on product yields. Energy

& Fuel 1990;4:54–60.

[84] Habermehl D, Orywal F, Beyer H-D. Plastic properties of

coal. In: Elliott MA, editor. Chemistry of coal utilization.

New York: Wiley-Interscience; 1981. p. 317–424.

[85] Attar A. Bubble nucleation in viscous material due to gas

formation by a chemical reaction: application to coal

pyrolysis. AIChE J 1978;24:106–15.



[86] Suuberg EM, Unger PE, Larsen JW. The relation between

tar and extractables formation and crosslinking during coal

pyrolysis. Preprints of Papers—American Chemical

Society, Division of Fuel Chemistry, vol. 30, 1985.

p. 291–96.

[87] Suuberg EM, D L, Larsen JW. Low temperature cross-

linking during coal pyrolysis. Fuel 1985;64:1668–9.

[88] Fong WS, Khalil YF, Peters WA, Howard JB. Plastic

behaviour of coal under rapid-heating high-temperature

conditions. Fuel 1986;65:195–201.

[89] Solomon PR, Serio MA, Hamblen DG. Measurement and

modelling of advanced coal conversion process. Final

report, DOE/MC/23075, 1993. p. 463–74.

[90] Oh MS, Peters WA, Howard JB. An experimental and

modelling study of softening coal pyrolysis. AIChE

J 1989;35:775–92.

[91] Lynch LM, Webster DS, Sakurovs R, Barton WA, Maher

TP. The molecular basis of coal thermoplasticity. Fuel

1988;67:579–83.

[92] Nomura M, Kidena K, Hiro M, Katsuyama M, Murata S.

The nature of plasticity phenomena of coal. In: 18th

proceedings—annual international Pittsburgh coal confer-

ence, 2001. p. 2776–781.

[93] Nomura M, Kidena K, Hiro M, Murata S. Mechanistic

study on the plastic phenomena of coal. Energy & Fuel

2000;14:904–9.

[94] Kidena K, Murata S, Nomura M. Investigation on coal

plasticity: correlation of the plasticity and a TGA-derived

parameter. Energy & Fuel 1998;12:782–7.

[95] Kidena K, Katsuyama M, Murata S, Nomura M, Chikada

T. Study on plasticity of maceral concentrates in terms of

their structural features. Energy & Fuel 2002;16:1231–8.

[96] Sung WF. The study of the swelling property of bituminous

coal. MS thesis, Department of Chemical Engineering,

Massachusetts Institute of Technology, 1977.

[97] Nomura M, Artok L, Murata S, Yamamoto A, Hama H,

Gao H, et al. Structural evaluation of Zao Zhuang Coal.

Energy & Fuel 1998;12:512–23.

[98] Chan ML, Parkyns ND, Thomas KM. Thermoplastic

behavior of coal and its relationship to coke structure

during high-pressure carbonization. Fuel 1991;70:447–53.

[99] Solomon PR, Serio MA, Despande GV, Kroo E. Cross-

linking reactions during coal conversion. Energy & Fuel

1990;4:42–54.

[100] Waters PL. Rheological properties of coal during the early

stage of thermal softening. Fuel 1962;41:3.

[101] Lancet MS, Sim FA. The effect of pressure and gas

composition on the fluidity of Pittsburgh No. 8 coal.

Am Chem Soc, Div Fuel Chem Prepr Pap 1981;26:

167–73.

[102] Yu J, Strezov V, Lucas JA, Liu G, Wall TF. A single

particle reactor for investigating the thermoplastic beha-

viour of Australian coals during pyrolysis. In: Eighteenth

annual international Pittsburgh coal conference proceed-

ings, Newcastle, Australia, 2001.

[103] Alvarez D, Menendez R, Fuente E, Vleeskens J. Study of

the influence of coal rank on char structure and behavior.

In: Seventh conference proceedings—international confer-

ence on coal science, 1993. p. 77–80.

[104] Koranyi AD. The relationship between specific reactivity

and the pore structure of coal chars during gasification.

Carbon 1989;22:55–61.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


[105] Hurt RH, Sarofim AF, Longwell JP. The role of micro-

porous surface area in the gasification of chars from a sub-

bituminous coal. Fuel 1991;70:1079–82.

[106] Oka N, Murayama T, Matsuoka H, Yamada S, Yamada T,

Shinozaki S, et al. The influence of rank and maceral

composition on ignition and char burnout of pulverized

coal. Fuel Process Technol 1987;15:213–24.

[107] Lester E, Cloke M, Allen M. Char characterization using

image analysis techniques. Energy & Fuel 1996;10:696–703.

[108] Hampartsoumian E, Pourkashanian M, Trangmar DT,

Williams A. Effect of the porous structure of char on the

rate of gasification. Preprints of Papers—American Che-

mical Society, Division of Fuel Chemistry, vol. 34, 1989. p.

212–9.

[109] Menendez R, Vleeskens JM, Marsh H. The use of scanning

electron microscopy for classification of coal chars during

combustion. Fuel 1993;72:611–7.

[110] Kaiho M, Toda Y. Change in thermoplastic properties of

coal under pressure of various gases. Fuel 1979;58:397–8.

[111] Wu HW, Bryant G, Benfell K, Wall T. An experimental

study on the effect of system pressure on char structure of

an Australian bituminous coal. Energy & Fuel 2000;14:

282–90.

[112] Kang SG, Sarofim AF, Beer JM. Effect of char structure on

residual ash formation during pulverized coal combustion.

In: In: Proceedings of the 24th international combustion

symposium, 1992. p. 1153–159.

[113] Wu H, Bryant G, Wall T. The effect of pressure on ash

formation during pulverized coal combustion. Energy &

Fuel 2000;14:745–50.

[114] Arenillas A, Rubiera F, Pis JJ, Jones JM, Williams A. The

effect of the textural properties of bituminous coal chars on

NO emissions. Fuel 1999;78:1779–85.

[115] Webb PA, Orr C. Analytical methods in fine particle

technology, 1st ed. Norcross, USA: Micromeritics Instru-

ment Corporation, USA; 1997.

[116] Dutta S, Wen CY, Belt RJ. Reactivity of coal and char. 1.

In a carbon dioxide atmosphere. Ind Eng Chem Process

Des Dev 1977;16:20–7.

[117] Laurendeau NM. Heterogeneous kinetics of coal char

gasification and combustion. Prog Energy Combust Sci

1978;4:221–70.

[118] Bailey JG, Tate A, Diessel CFK, Wall TF. A char

morphology system with applications to coal combustion.

Fuel 1990;69:225–39.

[119] Bend SL, Edwards IAS, Marsh H. The influence of rank

upon char morphology and combustion. Fuel 1992;71:

493–501.

[120] Cloke M, Gilfillan A, Lester E. The characterization of

coals and density separated coal fractions using FTIR and

manual and automated petrographic analysis. Fuel

1997;76:1289–96.

[121] Cloke M, Lester E, Allen M, Miles NJ. Automated maceral

analysis using fluorescence microscopy and image analysis.

Fuel 1995;74:659–69.

[122] Gilfillan A, Lester E, Cloke M, Snape C. The structure and

reactivity of density separated coal fractions. Fuel

1999;78:1639–44.

[123] Bailey JG. The origin of unburnt combustibles in coal. PhD

thesis, Department of Geology, University of Newcastle,

1993.



[124] Rosenberg P, Petersen HI, Sorensen HS, Thomsen E,

Guvad C. Combustion char morphology related to

combustion temperature and coal petrography. Fuel

1996;75:1071–82.

[125] Benfell KE, Bailey JG. Comparison of combustion and

high pressure pyrolysis chars from Asutralian black coals.

In: Eighth Australian coal science conference proceedings,

Sydney, Australia, 1998. p. 157–62.

[126] Lightman P, Street PJ. Microscopical examination

of heat treated pulverized coal particles. Fuel 1968;47:

7–28.

[127] Littlejohn RF. Pulverized-fuel combustion: swelling under

rapid heating. J Inst Fuel 1967;40:128.

[128] Alvarez D, Borrego AG, Menendez R. Unbiased methods

for the morphological description of char structures. Fuel

1997;76:1241–8.

[129] Cloke M, Lester E. Characterization of coals for combus-

tion using petrographic analysis: a review. Fuel 1994;73:

315–20.

[130] Vleeskens JM, Menendez RM, Roos CM, Thomas CG.

Combustion in the burnout stage: the fate of inertinite.

Fuel Process Technol 1993;36:91–9.

[131] Yu JL, Lucas J, Strezov V, Wall T. Swelling and char

structures from density fractions of pulverized coal. Energy

& Fuel 2003;17:1160–74.

[132] Gilfillan A, Lester E, Cloke M. The reactivity of density-

separated coal fractions. DGMK Tagungsber 1997;9702:

239–42.

[133] Hurt RH, Davis KA, Yang NYC, Headley TJ, Mitchell

GD. Residual carbon from pulverized-coal-fired boilers. 2.

Morphology and physicochemical properties. Fuel

1995;74:1297–306.

[134] Hurt RH, Gibbins JR. Residual carbon from pulverized

coal fired boilers: 1. Size distribution and combustion

reactivity. Fuel 1995;74:471–80.

[135] Tsai SC, Scaroni AW. The structural changes of bitumi-

nous coal particles during the initial stages of pulverized-

coal combustion. Fuel 1987;66:200–6.

[136] Yan L. CCSEM analysis of minerals in pulverised coal and

ash formation modelling. PhD thesis, Department of

Chemical Engineering, University of Newcastle (NSW),

Australia, 2000.

[137] Alonso MJG, Borrego AG, Alvarez D, Menendez R. A

reactivity study of chars obtained at different temperatures

in relation to their petrographic characteristics. Fuel

Process Technol 2001;69:257–72.

[138] Alonso MJG, Borrego AG, Alvarez D, Parra JB, Menen-

dez R. Influence of pyrolysis temperature on char optical

texture and reactivity. J Anal Appl Pyrolysis 2001;58–59:

887–909.

[139] Jenkins RG, Nandi SP, Walker Jr PL. Intrinsic reactivity

and temperature. Fuel 1973;52:288–93.

[140] Cai HY, Megaritis A, Messenbock R, Vasanthakumar L,

Dugwell DR, Kandiyoti R. Combustion reactivity and

morphological change in coal chars-effect of pyrolysis

temperature, heating rate and pressure. Fuel 1996;75:

15–24.

[141] Chan ML, Thomas KM. An investigation of coal–pitch

interactions and their effect on thermoplastic properties at

elevated pressure. Fuel 1994;75:335–9.

[142] Gale TK, Bartholomew CH, Fletcher TH. Decreases in the

swelling and porosity of bituminous coals during devola-


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


tilization at high heating rates. Combust Flame 1995;100:

94–100.

[143] Gale TK, Bartholomew CH, Fletcher TH. Comparison of

reactivity and physical structure of chars prepared under

different pyrolysis conditions, i.e. temperature, gas atmo-

sphere, and heating rate. In: Seventh conference proceed-

ings—international conference on coal science, 1993.

p. 17–20.

[144] Zygourakis K. Effect of pyrolysis conditions on the

macropore structure of coal-derived chars. Energy & Fuel

1993;7:33–41.

[145] Megaritis A, Messenbock RC, Chatzakis IN, Dugwell DR,

Kandiyoti R. High-pressure pyrolysis and CO2-gasification

of coal maceral concentrates: conversions and char

combustion reactivities. Fuel 1999;78:871–82.

[146] Lee CW, Jenkins RG, Schobert HH. Mechanisms

and kinetics of rapid, elevated pressure pyrolysis of

Illinois No. 6 bituminous coal. Energy & Fuel 1991;5:

547–55.

[147] Lee CW. Structure and reactivity of char from elevated

pressure pyrolysis of Illinois No. 6 bituminous coal. Energy

& Fuel 1992;6:40–7.

[148] Khan MR, Walker PL, Jenkins RG. Swelling and plastic

properties of coal devolatilized at elevated pressures of

hydrogen and helium. Influences of added iron oxides. Fuel

1988;67:693–9.

[149] Khan MR, Jenkins RG. Swelling and plastic properties

of coal devolatilized at elevated pressures: an

examination of the influences of coal type. Fuel 1986;65:

725–31.

[150] Lee CW, Scaroni AW, Jenkins RG. Effect of pressure on

the devolatilization and swelling behaviour of a softening

coal during rapid heating. Fuel 1991;70:957–65.

[151] Matsuoka K, Akiho H, Xu W, Gupta R, Wall T, Tomita

A. The physical character of coal char formed during rapid

pyrolysis at high pressure. Fuel 2005;84:63–9.

[152] Roberts DG. Intrinsic reaction kinetics of coal chars with

oxygen, carbon dioxide and steam at elevated pressures. Ph

D thesis, Department of Chemical Engineering, University

of Newcastle, 2000.

[153] Liu G-S, Tate AG, Bryant GW, Wall TF. Mathematical

modelling of coal char reactivity with CO2 at high pressures

and temperatures. Fuel 2000;79:1145–54.

[154] Gadiou R, Bouzidi Y, Prado G. The devolatilisation of

millimetre sized coal particles at high heating rate: the

influence of pressure on the structure and reactivity of the

char. Fuel 2002;81:2121–30.

[155] Yu J, Harris D, Lucas JA, Roberts D, Wall TF. Effects of

pressure on char formation during pyrolysis of pulverised

coal. Energy & Fuel 2004;18:1346–53.

[156] Fletcher TH. Swelling properties of coal chars during rapid

pyrolysis and combustion. Fuel 1993;72:1485–95.

[157] Field MA, Gill DW, Morgan BB, Hawksley PGW.

Combustion of pulverised coal. Banbury, England: The

British Coal Utilisation Research Association; 1967.

[158] Gentzis T, Hirosue H, Sakaki T. Effect of rank and

petrographic composition on the swelling behavior of coals.

Energy sources 1996;18(2):131–41.

[159] Gale TK, Bartholomew CH, Fletcher TH. Effects of

pyrolysis heating rate on intrinsic reactivities of coal chars.

Energy & Fuel 1996;10:675–766.



[160] Gao H, Murata S, Nomura M, Ishigaki M, Qu M, Tokuda

M. Experimental observation and image analysis

for evaluation of swelling and fluidity of single coal

particles heated with CO2 laser. Energy & Fuel 1997;11:

730–8.

[161] Liu G-S, Wu H, Benfell KE, Lucas JA, Wall TF. The effect

of char structure on burnout during pulverized coal

combustion at pressure. In: 16th Proceedings—

annual international Pittsburgh coal conference, 1999,

p. 817–26.

[162] Maswadeh W, Tripathi A, Arnold NS, DuBow J,

Meuzelaar HLC. High speed, two-wavelength radiation

thermometry of single micro particles during CO2 laser

heating. J Anal Appl Pyrolysis 1994;28:55–70.

[163] Thomas CG, Gosnell ME, Gawronski E, Nicholls PM.

Single particle coal combustion reactivity using a nano-

balance. Proc—Annu Int Pittsburgh Coal Conf 1994;11:

282–7.

[164] Shen F, Inada T, Yamamoto K, Iwanaga Y. Combustion

characteristics of single [particle] pulverized coal. Tetsu to

Hagane 1994;80:1–6.

[165] Zhang DK. Laser-induced ignition of pulverized fuel

particles. Combust Flame 1992;90:134–42.

[166] Phuoc TX, Maloney DJ. Laser pyrolysis of single coal

particles in an electrodynamic balance. In: Proceedings of

the 22th international combustion symposium, 1989.

p. 125–34.

[167] Phuoc TX, Mathur MP, Ekmann JM. An experimental and

numerical study of high-energy Nd-Yag laser ignition of

coals. Conference proceedings—seventh international con-

ference on coal science, 1993. p. 47–50.

[168] Chen J. Combustion behaviour of single maceral particles.

PhD thesis, Chemical Engineering, University of Queens-

land, 2002.

[169] Shen F, Inada T, Yamamoto K, Iwanaga Y. Combustion

behavior of single pulverized coal. The first international

congress of science and technology of ironmaking, Sendai,

1994. p. 559–64.

[170] Gao H, Murata S, Nomura M, Ishigaki M, Tokuda M.

Preliminary surface structure transition of coal particles

during prepyrolysis with CO2 laser. Energy & Fuel

1996;10:1227–34.

[171] Johnson GR, Murdoch P, Williams A. A study of the

mechanism of the rapid pyrolysis of single particles of coal.

Fuel 1988;67:834–42.

[172] Yu JL, Strezov V, Lucas J, Wall T. Swelling behaviour of

individual coal particles in the single particle reactor. Fuel

2003;82:1977–87.

[173] Cai HY, Megaritis A, Messenbock R, Vasanthakumar L,

Dugwell DR, Kandiyoti R. Pyrolysis of coal maceral

concentrates under Pf-combustion conditions (II)—

changes in heteroatom partitioning as a function of rank.

Fuel 1998;77:1283–9.

[174] Dyrkacz GR, Horwitz EP. Separation of coal macerals.

Fuel 1982;61:3–12.

[175] Dyrkacz GR, Bloomquist CAA, Ruscic L. High-resolution

density variations of coal macerals. Fuel 1984;63:1367–73.

[176] Kawashima H, Yamashita Y, Saito I. Studies on structural

changes of coal density-separated components during

pyrolysis by means of solid-state 13C NMR spectra. J Anal

Appl Pyrolysis 2000;53:35–50.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003


[177] Gibbins JR, Beeley TJ, Crelling JC, Scott AC, Skorupska

NM, Williamson J. Observations of heterogeneity in

large pulverized coal particles. Energy & Fuel 1999;13:

592–601.

[178] Sheng C, Azevedo JLT. Modelling the evolution of particle

morphology during coal devolatilization. Proc Combust

Inst 2000;28:2225–32.



[179] Yu JL, Lucas J, Wall T, Liu G, Sheng CD. Modelling the

development of char structure during the rapid heating of

pulverized coal. Combust Flame 2004;136:519–32.

[180] Yu JL, Lucas SJ, Liu GS, Wall T. A mechanistic study on

char structure evolution during coal devolatilization-

experiments and model predictions. Proc Combust Inst

2003;29:467–73.


ence (2006), doi:10.1016/j.pecs.2006.07.003

dx.doi.org/10.1016/j.pecs.2006.07.003

coal29

Documents