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Fuel 86 (2007) 1664–1683
Review
Some recent advances in the understanding of the pyrolysisand gasification behaviour of Victorian brown coal q
Chun-Zhu Li *
CRC for Clean Power from Lignite, Department of Chemical Engineering, P.O. Box 36, Monash University, VIC. 3800, Australia
Received 26 July 2006; received in revised form 5 January 2007; accepted 9 January 2007Available online 6 February 2007
Abstract
The brown coal in the Latrobe Valley, Victoria, Australia, has many unique structural features and properties. The brown coal has avery low ash yield and contains highly dispersed alkali and alkaline earth metallic (AAEM) species, either as carboxylates forming part ofits organic matter or as NaCl dissolved in its moisture. Owing to its unique structural features and properties, the brown coal behaves verydifferently from many other solid fuels such as biomass, bituminous coals and anthracites. For example, the highly reactive nature of itsvolatiles, the vulnerable nature of its nascent char and the presence of finely distributed AAEM species mean that the volatile–char inter-actions, a common phenomenon in all gasifiers, especially in the fluidised-bed gasifiers, would influence almost every aspect of its pyrolysisand gasification behaviour. Some recent progress in the understanding of the pyrolysis and gasification behaviour of Victorian brown coalwill be reviewed in this paper. After a brief account of the effects of AAEM species on the pyrolysis yields, the factors influencing the vol-atilisation of AAEM species will be summarised. This will be followed by the discussion of the factors influencing the reactivity of browncoal char and the catalytic reforming/cleaning of volatiles and gasification products by char-supported catalysts. The effects of dewatering/drying on the pyrolysis behaviour of Victorian brown coal and the conversion of pollutant-forming elements will be mentioned briefly. Theprogress in the fundamental understanding of the pyrolysis and gasification behaviour of Victorian brown coal has laid solid foundation forthe further development of advanced gasification technologies for the clean and efficient utilisation of this cheap but important resource.� 2007 Elsevier Ltd. All rights reserved.
Keywords: Pyrolysis; Gasification; Victorian brown coal; Volatile–char interactions
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16652. Some special features of Victorian brown coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16653. Fates and roles of AAEM species during pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1666
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3.1. Effects of AAEM species on the pyrolysis yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16663.2. Volatilisation of AAEM species during pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1670
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3.2.1. Factor 1. Gas atmosphere surrounding the particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16703.2.2. Factor 2. Particle time–temperature history. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16703.2.3. Factor 3. Chemical form and valence of AAEM species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16703.2.4. Factor 4. Sweeping/carrier gas velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16703.2.5. Factor 5. Volatile–char interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16713.2.6. Factor 6. ‘‘Age’’ of char and the presence of other inorganic species in coal . . . . . . . . . . . . . . . . . . . . . . 1673
see front matter � 2007 Elsevier Ltd. All rights reserved.
uel.2007.01.008
in part at the 2005 International Conference on Coal Science and Technology, 9–14 October 2005, Okinawa, Japan.3 9905 9623; fax: +61 3 9905 5686.ress: [email protected]
C.-Z. Li / Fuel 86 (2007) 1664–1683 1665
4. Factors influencing the gasification reactivity of char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673
4.1. Effects of the chemical form of catalyst in char on char reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16744.2. Effects of catalyst concentration on char reactivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16744.3. Effects of catalyst dispersion on char reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16744.4. Effects of the physico-chemical structure/properties of char on char reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . 16754.4.1. Evidence for importance of char structure to reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16754.4.2. Information on char structure during pyrolysis and gasification from FT-Raman spectroscopy . . . . . . . . . 1676
4.5. Effects of volatile–char interactions on char reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16784.6. Nascent char reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1679
5. Catalytic reforming of volatiles and gasification products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1679
5.1. Catalytic destruction/reforming of volatiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16795.2. Further catalytic conversion/cleaning of gasification products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16795.2.1. Brown coal as a special raw material for catalyst preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16795.2.2. Char-supported catalysts for further conversion/cleaning of gasification products. . . . . . . . . . . . . . . . . . . 1679
6. Effects of dewatering/drying on the pyrolysis and gasification behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16807. Conversion of pollutant-forming elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16818. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1681
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1682References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1682
1. Introduction
The increasing world population and the continuousimprovement in living standards have led to and will con-tinue to lead to drastic increases in the demand for energy.Among the mix of energy supply within the foreseeablefuture, coal will continue to be an important energy sourcein many parts of the world. In the meantime, the public hasbecome increasingly aware about the possible environmen-tal impacts of coal utilisation. To secure cheap energy sup-ply and to protect environment, coal scientists andtechnologists have made great efforts to develop new tech-nologies for cheaper energy with less environmentalimpacts, especially reduced emissions of greenhouse gasesand other air pollutants. Coal gasification has particularlybeen considered as one of the most important and versatileclean coal technologies for such purposes as the generationof electricity and the production of chemicals, liquid fuelsand hydrogen. Indeed, the past 10–15 years have seenmajor progress in coal science and technology, particularlyin the understanding of pyrolysis and gasification behav-iour of various coals.
Gasification is particularly suitable for the utilisation oflow-rank coals, such as Victorian brown coal, due to thehigh gasification reactivity of these coals. This paper aimsto review some recent progresses made in the understand-ing of the pyrolysis and gasification behaviour of Victorianbrown coal, focusing on the reactions involving the inor-ganic and organic matter in Victorian brown coal duringpyrolysis and gasification. In particular, the presence offinely dispersed alkali and alkaline earth metallic (AAEM)species in the brown coal affects every aspect of the coal’sbehaviour and thus deserves some special attention. Thisreview will begin with an overview of some special featuresof Victorian brown coal. After a brief account of the effects
of AAEM species on the pyrolysis yields, the factors influ-encing the volatilisation of AAEM species will be summa-rised. This will be followed by the discussion of the factorsinfluencing the reactivity of brown coal char and the cata-lytic reforming/cleaning of volatiles and gasification prod-ucts by char-supported catalysts. The discussion onreactivity also includes the characterisation of char. Theeffects of dewatering/drying on the pyrolysis behaviour ofVictorian brown coal and the conversion of pollutant-forming elements will be mentioned briefly.
A review like this is bound to be either too long orincomplete and not without bias towards the author’sown work. A thorough review of the gasification technol-ogy development is beyond the scope of this paper. A moredetailed and general review of the recent advances in thescience of Victorian brown coal may be found elsewhere[1].
2. Some special features of Victorian brown coal
The brown coal in the Latrobe Valley, Victoria, Austra-lia, is of low rank and Tertiary age. Victorian brown coalhas also been loosely termed as lignite, possibly due tothe name of a research centre ‘‘CRC for Clean Power fromLignite’’ in Australia. Almost every aspect of the propertiesof Victorian brown coal is unique when compared withthose of other solid fuels such as biomass, peat, bituminouscoal and anthracite [2]. The efficient and environmentallyfriendly utilisation of the Victorian brown coal resourcemust consider its special structural features and properties.Among various unique features of Victorian brown coal,three deserve special mention in relation to its pyrolysisand gasification characteristics.
Firstly, as a class of low rank coal, the oxygen contentof Victorian brown coal is high and ranges from about
1666 C.-Z. Li / Fuel 86 (2007) 1664–1683
22–27 wt% (daf), depending on the lithotype [2]. The oxy-gen in the brown coal exists in a wide variety of functionalgroups. Of particular importance are the acidic functionalgroups. As will be discussed below, carboxyl and pheno-lic groups are largely responsible for the dispersion of somemetallic species at atomic/molecular scale in the coal. Thehighly reactive nature of these O-containing functionalgroups also means that they are involved in various reac-tions taking place during pyrolysis, affecting the propertiesof pyrolysis products – volatiles and char. For example, thechars from Victorian brown coal would be very differentfrom those from black coals, partly due to these O-contain-ing functional groups and partly due to the arrangementand nature of aliphatic and aromatic structural units inthese coals.
Secondly, the Victorian brown coal features the presenceof AAEM species and iron. Much of the inherent ash-forming species is well dispersed, e.g. at an atomic/molecu-lar scale, in the brown coal. The AAEM species (mainlyNa, Mg and Ca) exist in Victorian brown coal mainly intwo forms: as ion-exchangeable cations associated withthe carboxyl groups forming part of the organic coal sub-stance [3] or as soluble salts (mainly NaCl) associated withthe moisture in the brown coal [2–4]. An XAFS study [5]showed the absence of organic chlorine in the Victorianbrown coal: all chlorine, likely to be associated with Na+,exists in the as-mined Victorian brown coal (containingmore than 60 wt% of water) in a form similar to that ofchloride anions in an aqueous solution.
These finely dispersed ash-forming species would behavevery differently from the discrete minerals normally foundin black coals. Therefore, although the Victorian browncoal normally has very low ash yields (less than 1–2 wt%db), being so low as to render the traditional ash tra-cer method for weight loss determination invalid [6], thepresence of these AAEM species (and other ‘‘volatile’’ inor-ganic species) has significant impacts on almost all routesof Victorian brown coal utilisation. In a conventional pfcombustion system, the AAEM species in the brown coalare believed to be largely responsible for the ash-relatedproblems such as slagging and fouling [7]. In a fluidised-bed combustion system, the AAEM species in the browncoal are also likely to cause bed agglomeration, leadingto the defluidisation of the fluidised bed [8]. In an advancedgasification/reforming-based power generation system, theAAEM species are one of the major reasons that cause theerosion/corrosion of the turbine components [9]. However,these AAEM species can act as excellent catalysts for thesubsequent char gasification and combustion reactions[10], particularly considering that these inorganic speciesare likely to be very finely dispersed in the char matrix(not as discrete minerals). Therefore, an understanding ofthe mechanisms, which control the volatilisation of theAAEM species during pyrolysis and their catalytic effectsfor char gasification as well as volatile reforming, is essen-tial to the utilisation of the brown coal with gasificationtechnologies. As in the early days of brown coal research
(e.g. see Ref. [3]), a significant portion of recent researchefforts has continuously been focused on the understandingof the behaviour of these species during pyrolysis and gas-ification. Consequently, a significant portion of this paperis devoted to the behaviour of AAEM species during thepyrolysis and gasification of the brown coal.
Iron is also strongly associated with the acidic O-con-taining functional groups in coal in a similar manner tothe AAEM species [2,3]. Nickel and other transition metalscan also be easily introduced into the brown coal throughion-exchange (or impregnation/precipitation) and exist inthe finely (atomic scale) dispersed form [2,3,11–13]. Uponheat treatment, these finely dispersed species agglomerateinto small (nano) particles [14]. Therefore, the Victorianbrown coal, with its low ash yield and abundant pore struc-ture both in coal and char, can act as a cheap raw materialfor the production of specialty carbon materials, includingcarbon-supported nano-catalysts with many special fea-tures [15]. This will be reviewed later in the paper.
Thirdly, the moisture content of ‘‘as-mined’’ Victorianbrown coal can be as high as 66–70 wt%. While some waterin the brown coal has properties similar to that of normalliquid water, some water molecules are very closely boundto the O-containing functional groups in the macromolec-ular network of the brown coal [12,16] and become anindispensable part of the brown coal structure. Removingwater from Victorian brown coal in an efficient manner isan integral part of any advanced clean coal technologyfor the efficient utilisation of the brown coal [1,16]. How-ever, very significant physico-chemical changes could takeplace to the structure and properties of Victorian browncoal, depending on dewatering/drying conditions. Evenfor the existing boilers, the combustion of dewateredbrown coal may be very different from that of wet browncoal if one just simply considers the reduced gas flow andincreased combustion flame temperature as a result ofdewatering/drying [17]. Therefore, understanding the pos-sible effects, if any, of dewatering/drying on the pyrolysisand gasification behaviour is clearly important to the devel-opment of advanced brown coal gasification technologies.The recent progress in this area will be briefly summarisedlater.
3. Fates and roles of AAEM species during pyrolysis
3.1. Effects of AAEM species on the pyrolysis yields
It has long been known and reviewed [3,18,19] that theion-exchangeable cations as carboxylates in the browncoal, particularly Na+, Mg2+ and Ca2+, play importantroles during the pyrolysis of Victorian brown coal, affectingthe formation rates and final yields of light hydrocarbons[20–24], oxygen-containing species [23,25], char [21,22]and tar [22,23,26] (and/or aromatic ring systems [27–29]).Experimental conditions such as heating rate, peak temper-ature and pressure can all have profound effects on thepyrolysis behaviour of brown coals using a variety of reac-
C.-Z. Li / Fuel 86 (2007) 1664–1683 1667
tors. Tar samples from the pyrolysis of brown coals (lig-nites) have been characterised with Fourier-transforminfrared (FTIR) spectroscopy [30], nuclear magnetic reso-nance (NMR) spectroscopy [31], high performance liquidchromatography (HPLC) [32,33] and UV–vis absorptionand fluorescence spectroscopies [28,29,33,34]. In particular,the UV–vis absorption and fluorescence spectroscopicstudies [28,29,33,34] have shown some insights into the rel-ative ease with which the smaller and larger aromatic ringsystems (likely having lower and higher co-ordinationnumbers in coal respectively) are released during pyrolysis.
The effects of ion-exchangeable AAEM species on thetotal volatile yields depend on the holding time at the peaktemperature and other conditions, diminishing withincreasing holding time. This is largely due to the rather ali-phatic nature of tar (precursors) [28,29,33,34] which, ifretained in the solid phase, cracks to form mainly gasesduring pyrolysis. Therefore, the ion-exchangeable AAEMspecies exert more influence on the kinetics of the releaseof total volatiles than on the ultimate total volatile yields.Caution must be exercised in evaluating the effects ofAAEM species on the total volatile yields reported in theliterature.
The rather fragile nature of brown coal tar, at leastpartly due to its high contents of aliphatic (substitutional)and aromatic rings of lower co-ordination numbers[28,29,33,34] has meant that the observed tar yields (andother pyrolysis yields) are very sensitive to experimentalconditions, explaining some discrepancies in the literature.A wire-mesh reactor capable of minimising the secondaryextra-particle reactions of volatiles and varying holdingtime at milliseconds intervals has great advantages in thefundamental understanding of the pyrolysis behaviour ofbrown coal [27,28]. Unlike higher rank coals, the tar yieldsfrom the raw coal and the acid-washed samples could
Tar Volatiles0
10
20
30
40
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60H-form-1
Tar Volatiles
Yie
lds,
wt%
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coal
(d
b)
0
10
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60Raw coal
Tar Volatiles0
10
20
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60 Na-form
Tar Volatiles0
10
20
30
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60H-form-2
1 K s-1 1000 K s-1
H-
Tar0
10
20
30
40
50
60Ca
Tar0
10
20
30
40
50
60
Fig. 1. Effects of heating rate and the presence of cations on the tar and total vat a peak temperature of 600 �C (modified from Ref. [27]. Reprinted with permiSociety). The Loy Yang raw coal was firstly washed with an acid to prepareprepared by ion-exchanging the H-form-1 sample with Na+ and Ca2+ respectH-form-2 and H-form-3 samples respectively.
increase much more than the corresponding total volatileyields with increasing heating rate from 1 to 1000 K s�1
[27,28] as is shown in Fig. 1 by using the data at 600 �Cas an example.
Most researchers have found that the presence ofAAEM cations in the coal substrate tends to cause depres-sion in the yields of tar and/or large aromatic ring systems.The ion-exchangeable AAEM species in coal also seem tochange the aromatic/aliphatic composition of the resultingtars [25,30,32]. A study [35] using proton magnetic reso-nance thermal analysis (PMRTA) believed that the taryield reduction by both monovalent (Na+) and divalent(Ca2+ or Ba2+) cations was associated with the cation-induced changes in coal matrix density (compacting) andin maximum fusibility (becoming more rigid) during pyro-lysis. This physical reconfiguration of the macromolecularnetwork as a result of ion exchanging may also lead tothe increased entanglements, serving as virtual cross-link-ing points in the coal matrix and thus affecting the pyroly-sis yields. Indeed, recent studies [27,28] have demonstratedthat, while AAEM species might be ion-exchanged into thebrown coal and then removed from the brown coal ‘‘revers-ibly’’ by washing with an acid (i.e. exchanging with H+),the physical configuration of the macromolecular networkcould not be ‘‘restored’’ completely when the brown coalwas washed with an acid. As is shown in Fig. 1, the pyro-lysis yields, particularly the tar yield which is more sensitiveto changes in coal property or pyrolysis conditions, of H-form-1 sample (from the acid-washing of raw coal) differedfrom those of H-form-2 and H-form-3 samples (from theacid-washing of Na+- and Ca2+-exchanged samples),mainly due to the changes in the physical configuration.Some humic materials were lost during the ion-exchangingof H-form-1 to prepare Na/Ca-form coal samples (seeFig. 1 for more details about the sample preparation),
form-3
Volatiles
-form
Volatiles
Raw coal
H-form-1 H-form-3H-form-2
Ca-formNa-form
Aci
d-w
ashi
ng
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ashi
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ashi
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Ion-exchanging
olatile yields from the pyrolysis of various forms of Loy Yang coal samplesssion from Energy & Fuels 1999;13(3):748–755. � 1999 American Chemicalthe H-form-1 sample. The Na-form and Ca-form coal samples were thenively. Washing the Na-form and Ca-form samples with an acid gave the
1668 C.-Z. Li / Fuel 86 (2007) 1664–1683
partly responsible for the difference between H-form-1 andH-form-2/H-form-3. It is clear that the effects of Na+ canonly be ascertained from the comparison (Fig. 1) betweenthe yields of the Na-form coal and the H-form-2 (not H-form-1) coal and those of Ca++ between the yields of theCa-form coal and the H-form-3 (not H-form-3) coal[27,28].
A number of researchers (e.g. Refs. [30,36]) have attrib-uted the reduction in the tar yield by divalent cations to thecross-linking effects of the divalent ions, bringing the (car-boxylic) groups closer in the coal structure. However, theunderstanding of the roles of monovalent Na+ in theNa+-exchanged coal is less straightforward because, unlikedivalent Ca2+ or Ba2+, Na+ itself does not bring muchadditional direct cross-linking to the coal structure exceptfrom some increases in the ionic forces in the coal struc-ture. In fact, ion-exchanged Na+ in brown coal is at leastas effective as (or more effective than) Ca2+ in reducingthe tar yields (e.g. see Fig. 1). In addition to the changesin the structure of coal substrate, reactions taking placeduring pyrolysis must be considered.
It has recently been pointed out [27,28] that the roles ofAAEM cations (Ca2+ and Na+) during the pyrolysis of theion-exchanged samples are largely related to the transfor-mation of AAEM species during pyrolysis. It should bepointed out that the –COONa or (–COO)2Ca groups incoal may behave distinctly differently from those in thesimpler salts such as sodium acetate [28]. The –COONagroups in coal are evenly distributed and thus isolated fromeach other, whereas the –COONa groups in sodium acetateare bonded together through ionic force in a crystal.
It is known that the carboxylates in coal will undergodecomposition at relatively low temperatures. With therelease of CO2, Ca originally associated with –COO groupsin coal matrix may still be bonded to coal/char matrix(–CM) [27,28],
ð–COO–Ca–OOC–Þ þ ð–CMÞ ¼ ð–COO–Ca–CMÞ þ CO2
ð1Þð–COO–Ca–CMÞ þ ð–CMÞ ¼ ðCM–Ca–CMÞ þ CO2 ð2Þ
or for Na,
ð–COO–NaÞ þ ð–CMÞ ¼ ðCM–NaÞ þ CO2 ð3Þ
continuously serving as a cross-linking point. In the abovereactions, CM–X (X = Na, Ca or Mg) simply representsthe bonding between the AAEM species and the pyrolysingcoal/char matrix, without specifying the nature of thebonds. At lower temperatures, the char would still containsignificant amounts of oxygen-containing groups, servingas the sites to bind AAEM species. The newly formedCa–CM or Na–CM bonds may not be stable enough athigher temperatures and thus may be broken again to gen-erate free radical sites
ðCM–Ca–CMÞ ¼ ð–CMÞ þ ð–Ca–CMÞ ð4Þð–Ca–CMÞ ¼ ð–CMÞ þ Ca ð5Þ
ðCM–NaÞ ¼ ð–CMÞ þNa ð6Þð–CMÞ ¼ ð–CM0Þ þ gas ð7Þ
together with the release of oxygen-containing species (e.g.as COx) or aliphatic materials (e.g. as CH4) [27,28]. Someof the AAEM species may leave the particle (Reactions(5) and (6)). More stable bonds Ca–CM or Na–CM mayalso form through recombination reactions
ð–CM0Þ þ ð–Ca–CMÞ ¼ ðCM0–Ca–CMÞ ð8Þð–CM0Þ þNa ¼ ðCM0–NaÞ ð9Þ
According to the reaction schemes outlined above, theCM–Na or CM–Ca bonds, continuously being brokenand re-formed, would become progressively stronger withincreasing temperature [27,28]. Experiments indeed showedthat the physico-chemical forms of the AAEM specieschanged with increasing temperature. While the Na andMg in chars at lower pyrolysis temperatures could still beextracted with acids, the proportion of acid-extractableMg in the char appeared to decrease drastically withincreasing temperature [28,37].
The net effect of the repeated Ca–CM or Na–CM bond-forming and bond-breaking processes described above isthat tar precursor fragments (e.g. –CM or –CM 0 in theabove reactions) are repeatedly linked to the char matrixthrough reactions involving free radicals and get thermallycracked [27,28]. During this thermal cracking process, themore aliphatic parts in a tar fragment, possibly includingsome smaller aromatic ring systems, will be released asgas. Some more aromatic units, especially the larger aro-matic ring systems, may eventually become a part of char.The high aliphatic nature of the tar (and thus tar precur-sors) [28,29,34] means that the amount of char formeddue to the thermal cracking of tar precursors at high tem-perature would be small or even not enough to show as asignificant increase in the observed char yield. This pro-vides a plausible explanation for the less profound effectsof AAEM species on the ultimate char yield than on thetar yield for a given temperature.
The repeated CM–AAEM bond-forming and bond-breaking process also means increased concentration offree radical sites in the pyrolysing coal/char particles, pro-viding more chance for recombination (cross-linking) andsubsequent thermal cracking reactions. This appears tobe a plausible explanation for the larger effects of Na thanthose of Ca/Mg on pyrolysis yields (e.g. see Fig. 1) becauseCM–Na may be less stable than CM–Ca.
The presence of ion-exchangeable AAEM species, andthus the repeated bond-forming and bond-breaking forthe intraparticle cracking of tar precursors as outlinedabove, also greatly reduces the heating rate sensitivity ofthe yields of tar (Fig. 1) and large aromatic ring systemsfrom the pyrolysis of Victorian brown coal [27–29].
The effects of pressure on the pyrolysis yields of Victo-rian brown coal are very different from those of high rank
Fig. 2. Effects of pressure on tar yield during the pyrolysis of Loy Yangbrown coal [39] in a wire-mesh reactor by heating at 1000 K s�1 to 600,700 or 900 �C with 10 s holding time. Reprinted from Fuel [39], Sathe, C.,Hayashi, J-i., Li, C.-Z., Release of volatiles from the pyrolysis of aVictorian lignite at elevated pressures, 1171–1178. � 2002, with permissionfrom Elsevier.
C.-Z. Li / Fuel 86 (2007) 1664–1683 1669
coals, as is shown in Fig. 2. For a typical bituminous coal(e.g. Linby) that melts upon heating, its tar yield decreasedmonotonically with increasing pressure [38]. However,depending on the peak temperature (e.g. 700 �C), a mini-mum tar yield around 6–11 atm was observed with increas-ing pressure during the pyrolysis of Loy Yang brown coalat 1000 K s�1 in a wire-mesh reactor [39]. This is related tothe fact that Victorian brown coal has abundant pore
structure and melts to an extremely limited extent duringpyrolysis. The slow bulk diffusion flow driven by concen-tration gradient and the rapid forced flow/convection dri-ven by pressure gradient are important mechanisms forthe release of volatile precursors [39]. The former tendsto increase the residence time and thus the extent of ther-mal cracking of volatile precursors inside the particle toreduce the observed tar yield while the latter has the oppo-site effect [39]. The relative importance of these two pro-cesses changes with pressure, with the former being moreimportant at relatively low pressure and the latter beingmore important at relatively high pressure, to result in aminimum in tar yield with increasing pressure [39].
The introduction of ion-exchangeable cations into thebrown coal plays a dominant role, more important thanthe external pressure, in the formation and release of vola-tiles [40] during the pyrolysis of brown coal at elevatedpressures. As was discussed above, the introduction of cat-ions into the brown coal results in drastic changes to thebrown coal structure and controls (normally slows down)the formation of volatile precursors. The rate and theamount of volatile precursor formation, largely altered bythe introduction of AAEM cations into the brown coalstructure, determine the dominant mechanism of masstransfer for the release of the volatile precursors. The taryields from Na+- and Ca2+-exchanged samples were notvery sensitive to changes in the heating rate and pressureup to 11 bars, distinctly different from the behaviour ofthe raw or acid-washed coal samples [40]. At 20 bar, thetar yields from the Na+-exchanged sample nearly doubledwhereas from the Ca2+-exchanged sample nearly halvedcompared to those respective values at 1 bar [40]. In thecase of Na-form coal containing high concentration ofNa, it is speculated [40] that monovalent Na, easily to beattached and detached to coal/char matrix, has catalysedthe intra-particle reforming of volatile precursors for theformation of light species (gases), including H radicals tostabilise tar precursors. The light gases in turn would leadto the pressure build up in the particle and, based on thediscussion given above, thus the increases in tar yield athigh pressures (e.g. 20 bar). However, the divalent Ca inthe Ca-form coal has much lower catalytic activity thanNa [40]. The increases in pressure could only enhance thegradual cracking of volatile precursors for furtherdecreases in tar yield.
Compared with ion-exchangeable Na+, Na+ as NaCl inthe brown coal has received little attention, despite the factthat NaCl may account for as much as half of the Na in thebrown coal. The pyrolysis of NaCl-loaded Loy Yangbrown coal samples in a fluidised-bed/fixed-bed [41] andin a TGA [42] showed that NaCl has little effects on thetotal volatile yields from the pyrolysis of the brown coal.A more recent study [43] on the pyrolysis of a set ofNaCl-loaded Loy Yang brown coal samples in a wire-meshreactor under high (1000 K s�1) heating rate conditionsshowed that the presence of NaCl had little effects on thetar and char yields.
1670 C.-Z. Li / Fuel 86 (2007) 1664–1683
3.2. Volatilisation of AAEM species during pyrolysis
The above discussion shows that the AAEM species inbrown coal take part in complicated reactions during pyro-lysis. As well as influencing the pyrolysis yields, the AAEMspecies can also be easily volatilised. For example, it isoften agreed that NaCl in the brown coal and other solidfuels may be volatilised during pyrolysis and combustion[8,44–47]. However, contradicting results have often beenreported in the literature. While some believed [8,44] thatNa was not vaporised as molecular NaCl, others believed[45,47] that the direct vaporisation of NaCl could takeplace during combustion or gasification. The contradic-tions reflect the fact that many factors, some of which havenot been noticed/addressed before, affect the volatilisationof AAEM species from brown coal during pyrolysis.
A series of recent studies [28,41,48–53] have significantlycontributed to the understanding of the volatilisation ofAAEM species in brown coal during pyrolysis. An arrayof reactors such as a wire-mesh reactor, a thermogravimet-ric analyser (TGA), a fixed-bed reactor, a one-stage flui-dised-bed/fixed-bed reactor and a two-stage fluidised-bed/fixed-bed reactor were used in these studies. Each reactor,having its specific configuration, has been designed/usedspecially to probe a specific aspect of the volatilisation ofAAEM species during pyrolysis. The combination of thesereactors has covered a wide range of key experimental con-ditions including heating rate, temperature, gas atmospherecomposition and pressure surrounding the particles, allow-ing for the investigation of the effects of each individualparameter on the volatilisation of AAEM species. Themain factors influencing the volatilisation of AAEM spe-cies from brown coal during pyrolysis are summarisedbriefly as follows.
3.2.1. Factor 1. Gas atmosphere surrounding the particles
An inert or reducing atmosphere surrounding a heatedparticle would favour the release of AAEM species. Whilethe volatilisation of AAEM species during the oxidation inO2 at a slow heating rate in a TGA was negligible, the vol-atilisation was significant during the pyrolysis in helium,even at temperatures as low as 300–500 �C [28]. The totalretention of AAEM species during the oxidation in O2
was attributed to the involvement of AAEM species inthe oxidation reactions as catalysts and/or adsorption sites,both of which tended to bond the AAEM species in coal/char continuously [28]. The total retention of AAEM spe-cies in O2 has been the foundation for the determinationof AAEM species in coal or char by a well establishedmethod [28]: coal/char is firstly ashed without ignitionand then acid-digested to transfer AAEM species in coal/char into solution for quantification e.g. using an ionchromatograph.
The non-negligible (e.g. 10–15%) volatilisation ofAAEM species at a rather low temperature (e.g. 300 �C)[28] adds evidence to the belief that the well-isolatedAAEM carboxylates in coal behave very differently from
those in the simple (crystallised) organic salts such assodium acetate. It is believed [48] that the AAEM specieswere volatilised at the low temperatures in the form of lightcarboxylates such as formates and acetates formed fromthe breakdown of coal macromolecular network.
3.2.2. Factor 2. Particle time–temperature historyIn the absence of volatile–char interactions (see below),
e.g. during the pyrolysis experiments using a wire-meshreactor, the peak pyrolysis temperature was the dominantfactor [28] influencing the volatilisation of AAEM species:the thermal stability of the chemical bonds between theAAEM species and char matrix [see the discussion onReactions (5) and (6) given above] changes mainly withtemperature. In particular, regardless of heating rate, pyro-lysis at higher temperatures would result in the release ofthe majority of oxygen-containing species, reducing thesites for the AAEM to connect with the char matrix. Littleeffects of heating rate have been observed on the volatilisa-tion of AAEM species during the pyrolysis experiment in awire-mesh reactor [28].
3.2.3. Factor 3. Chemical form and valence of AAEM speciesDuring the pyrolysis of the Loy Yang raw coal and a
NaCl-loaded Loy Yang coal sample in the one-stage flui-dised-bed/fixed-bed reactor, the volatilisation of Na andthat of Cl showed different trends and/or magnitudes withincreasing temperature for both fast and slow heating rateexperiments [41], as is summarised in Fig. 3. These dataprovided strong evidence to the debate in the literature[8,44,45,47], indicating that Na and Cl in brown coal werenot mainly volatilised as NaCl but individually.
The pyrolysis of the NaCl-loaded coal, the Na-form andthe Ca-form coals in the one-stage fluidised-bed/fixed bedreactor confirmed the relative ease of Na and Ca volatilisa-tion at elevated temperatures (e.g. around 800–900 �C) [48]:Na (as NaCl) > Na (as –COONa)� Ca [as (–COO)2Ca].Monovalent Na species have always tended to show highervolatilities than the divalent Ca and Mg species (even dur-ing the volatile–char interactions, see below).
3.2.4. Factor 4. Sweeping/carrier gas velocity
The sweeping/carrier gas velocity may have two differentpossible effects on the observed volatilisation of AAEMspecies during pyrolysis and gasification. The first effectconcerns the minimisation of the re-condensation of thevolatilised AAEM species back to the parent char particles.This is because the volatilised AAEM species in the gasphase are likely to be very reactive, particularly at hightemperatures. A higher sweeping gas velocity passingthrough a sample bed would help to reduce the re-conden-sation of the evolved AAEM species on char. This shouldbe an important consideration in laboratory reactor design[41]. Although clear/direct experimental evidence appearsto be lacking for the Victorian brown coal, clear evidencehas very recently been gained for the pyrolysis of a pinesaw dust [54]. Net release of AAEM species from a fixed
Rel
ease
of
Cl,
wt%
of
coal
(d
b)
0.00
0.02
0.04
0.06
0.08
0.10
Vo
lati
lisat
ion
of
Na,
%0
20
40
60
80
100
Temperature, °C200 300 400 500 600 700 800 900
0.00
0.10
0.20
0.30
0.40
0.50
0
20
40
60
80
100
Raw coalclosed symbols: fast heating rateopen symbols: slow heating rate
NaCl-loaded (0.9%)fast heating rate
Fig. 3. Volatilisation of Na and Cl during the pyrolysis of Loy Yang raw coal and NaCl-loaded (loading level: 0.9 wt%) coal samples in the one-stagefluidised-bed/fixed-bed reactor when the coal particles were heated up rapidly (>103 K s�1) or slowly (�10 K min�1) (based on the data in Ref. [41]).During each experiment at the fast heating rate, about 1 g of coal was fed into the reactor within 10 min and the reactor was held for 15 min at theindicated peak temperature before it was lifted out of the furnace immediately.
C.-Z. Li / Fuel 86 (2007) 1664–1683 1671
bed of char would only be possible with a forced flow of Hepassing through the bed [54]. Even so, the release ofAAEM species was significantly influenced by the bedheight due to the continuous desorption and re-adsorptionof AAEM (e.g. K) species [54].
Similarly, although the effects of particle size on theobserved volatilisation of AAEM species may be easilyconceivable (the need for AAEM species to diffuse/trans-port through the abundant pore systems of char), there isstill a general lack of experimental evidence. From theabove discussion, it is clear that the reactor configurationin terms of hydrodynamics surrounding the pyrolysing par-ticle would be an important factor to consider in under-standing the experimental results on the volatilisation ofAAEM species.
The second effect of sweeping/carrier gas velocity is thepossible detachment and entrainment of AAEM speciesfrom char particles. During the gasification of brown coalchar in CO2 in a fluidised-bed/fixed-bed reactor, the phys-ical entrainment of AAEM-containing species (believed tobe CaCO3) by the gas passing through the bed was believedto be an important route for the loss of AAEM species [53],which intensified with increasing conversion.
3.2.5. Factor 5. Volatile–char interactions
In an industrial gasifier, especially a fluidised-bed gas-ifier, char particles are in constant direct contact with thevolatiles as well as the products from the reforming andthermal cracking of volatiles. There are strong interactionsbetween volatiles and char, particularly considering thehighly reactive nature of volatiles and the vulnerable struc-ture of nascent char from brown coal. However, the effectsof volatile–char interactions have not been fully realised inthe past literature.
In fact, the volatile–char interactions have been found tobe one of the most important factors influencing the vola-tilisation of AAEM species [41,48–53] and other behaviourof brown coal that will be discussed below. The effects ofvolatile–char interactions on the volatilisation of AAEMspecies become immediately apparent when the resultsfrom a wire-mesh reactor [28] and a one-stage fluidised-bed/fixed-bed reactor [41] are compared in Fig. 4. In theabsence of volatile–char interactions in the wire-mesh reac-tor [27,28], up to about 70% of Na in coal was retained inthe char at 900 �C (Fig. 4). However, in the presence ofstrong interactions between char and the thermallycracking volatiles in the fluidised-bed/fixed-bed reactor([41,48,49], also Fig. 4), little Na is retained in the char atthe same temperature. The decreases in the Na retentionwith temperature follow a qualitatively similar trend forthe decreases in tar yield (due to thermal cracking) in a sim-ilar fluidised-bed reactor [32,33,49,55], providing evidencefor the importance of the volatile–char interactions onthe volatilisation of Na.
A special two-stage fluidised-bed/fixed-bed reactor wasdeveloped [49] to study the importance of volatile–charinteractions, as is shown in Fig. 5. Different from theone-stage fluidised-bed/fixed-bed reactor (Fig. 4), anotherfrit (Frit 2 in Fig. 5) was installed in the freeboard that sep-arates the reactor into two stages. An AAEM-rich coal canbe pyrolysed firstly in the upper stage to prepare anAAEM-containing char. An AAEM-free coal (acid-washedor H-form coal) can then be fed into the lower part of thereactor to generate volatiles, which then pass through Frit2 to interact with the AAEM-containing char. When aNaCl-loaded Loy Yang brown coal was pyrolysed in theupper stage and its char subsequently interacted with thevolatiles from the H-form coal, very significant portion of
Temperature, °C200 400 600 800
Ren
tio
n o
f N
a, %
0
20
40
60
80
100
Wire-mesh reactorF-B/F-B reactor
Raw coalsfast heating rates
Fluidising gas
Water-cooledprobe
Quartz frits
Coal particles
Sandbed
Volatiles
Charparticles
a b
Fig. 4. Comparison of the volatilisation of Na during the pyrolysis of Loy Yang brown coal in a wire-mesh reactor (1000 K s�1) [28] and a one-stagefluidised-bed/fixed-bed reactor (103–104 K s�1) [41]. In a wire-mesh reactor, there is little volatile–char interaction: the volatiles released from a particlehave minimised interactions with the parent char. In a fluidised-bed/fixed-bed reactor shown above [41], a large proportion of char particles formed fromthe coal fed into the reactor earlier would be elutriated out of the sand bed and form a thin bed underneath the frit in the freeboard, through which thevolatiles formed from the coal fed later must pass through to interact with the char. The char in the sand bed would also interact with the nascent volatilesjust released from the pyrolysing coal particles in the bed. Reactor schematic diagram. Reprinted from Fuel [41], Quyn, D.M., Wu, H., Li, C.-Z.,Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I.Volatilisation of Na and Cl from a set of NaCl loaded samples, 143–149. � 2002, with permission from Elsevier.
Volatile-char interaction time, min0 5 10 15 20
Ret
enti
on
, %(b
ased
on
char
at 0
min
)
40
60
80
100
120
NaMgCa
Ret
enti
on
, %Water-cooledprobe
Frit 3
Fluidising gas
H-formcoal
Sandbed
Volatiles
Charparticles
NaCl-loadedcoal
Frit 2
Frit 1
Frit 3Frit 2
Frit 1
a
b
Fig. 5. (a) Two-stage fluidised-bed/fixed-bed reactor. (b) Effects of volatile–char interactions on the volatilisation of AAEM species. Modified from Ref.[49]. The pyrolysis conditions for the AAEM-rich coal in the upper stage can be very different from and independent of those for the AAEM-free coal inthe lower stage in the two-stage fluidised-bed/fixed-bed reactor. Reprinted from Fuel [49], Quyn, D.M., Wu, H.i., Li, C.-Z., Volatilisation and catalyticeffects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part III. The importance of theinteractions between volatiles and char at high temperature, 1033–1039. � 2002, with permission from Elsevier.
1672 C.-Z. Li / Fuel 86 (2007) 1664–1683
Na in the char was volatilised, volatilisation intensifyingwith volatile–char interaction time, as is shown in Fig. 5.
The essence of volatile–char interactions is the interac-tions of H or other radicals with char [49], during whichthe radicals ‘‘replace’’ the AAEM species bonded to thechar for the release of the AAEM species, as is representedby the following simplifying reaction:
H ðor RÞ þ CM–Na ¼ H ðor RÞ–CMþNa ð10Þ
In the absence of H (or R) radicals, the direct breakdownof a CM–Na bond or a CM–Ca bond (Reactions (5) and(6)) would require much higher temperature for the furtherrelease of Na or Ca, which was observed experimentally totake place at temperatures higher than 900 or 1000 �C [28].
The effects of volatile–char interactions, dominated byradical reactions, are un-surprisingly a strong function oftemperature. A typical example of the effects of volatile–char interactions as a function of temperature is shown in
Na
rete
nti
on
, %
0
20
40
60
80
100
NBFWBF
Ca
rete
nti
on
, %
0
20
40
60
80
100
(A), Na-form (B), Ca-form
With volatile-char interactionsWithout volatile-char interactions
Temperature, °C
500 600 700 800 900
Wei
gh
t lo
ss, %
25
30
35
40
45
50
55
Temperature, °C
500 600 700 800 900
Wei
gh
t lo
ss, %
25
30
35
40
45
50
55
Fig. 6. Changes in weight loss and Na/Ca retention as a function of temperature in the presence and absence of volatile–char interactions during thepyrolysis of ion-exchanged Loy Yang coal samples in the two-stage fluidised-bed/fixed-bed reactor (based on the data in Ref. [51]).
C.-Z. Li / Fuel 86 (2007) 1664–1683 1673
Fig. 6. With increasing temperature above 600 �C, the for-mation of soot from the volatiles reduced and the apparentweight loss increased, signalling the intensification of vola-tile thermal cracking to produce H and other radicals [51].The corresponding retention of Na in char decreased [51].
While the volatile–char interactions led to significantvolatilisation of monovalent Na, the interactions have littleeffects on the volatilisation of divalent Ca/Mg under theexperimental conditions investigated so far (e.g. <950 �C)(Figs. 5 and 6). This reflects the fact that Ca/Mg wouldbe bonded to the char much more strongly (e.g. at two sitesin the char) than Na (e.g. at one site).
In addition to the extra-particle volatile–char interac-tions, the intra-particle volatile–char interactions also havegreat effects on the volatilisation of AAEM species duringpyrolysis. The intra-particle thermal cracking of volatileprecursors (including the products of nascent char thermalcracking) at elevated temperature, greatly affected by pres-sure, can drastically enhance the volatilisation of AAEMspecies during pyrolysis, particularly at elevated pressures[50].
3.2.6. Factor 6. ‘‘Age’’ of char and the presence of other
inorganic species in coal
The extent of the effects of volatile–char interactions onthe volatilisation of AAEM species (e.g. Na) is also affectedby the ‘‘age’’ of the char [49]. The AAEM species (e.g. Na)
in an ‘‘aged’’ char prepared in a slow heating rate experi-ment are much less likely to be volatilised by the vola-tile–char interactions than those in a ‘‘nascent’’ charprepared in situ in a fast heating rate experiment [49]. Thisobservation is in good agreement with the above discussionthat the repeating bond-forming and bond-breaking pro-cess [Reactions (4)–(9)], taking place during the holdingat elevated temperature and thus thermal cracking of char,lead to more stable CM–Na bonds. Clearly, the displace-ment of Na by radicals [Reaction (10)] would be more dif-ficult for a thermally stable CM–Na bond than for anunstable CM–Na bond.
As was stated above, the Victorian brown coal featuresan extremely low ash yield. In particular, the silicon con-tent is normally very low. Therefore, the dominant reac-tions involving Na are those with organic matter. Thereis still a lack of knowledge about the effects of volatile–charinteractions on the volatilisation of Na in the presence ofother minerals such as silica; it is conceivable that the for-mation of sodium silicates is likely to inhibit the volatilisa-tion of Na.
4. Factors influencing the gasification reactivity of char
Both char-forming conditions and coal properties canaffect the gasification reactivity of char. Many reviewshave been published about the factors influencing the char
1674 C.-Z. Li / Fuel 86 (2007) 1664–1683
reactivity (e.g. [56–58]). The inherent inorganic species inbrown coal are likely to have much bigger effects on its charreactivity than those in higher rank coals [56,59]. The mainfactors influencing the reactivity of char from brown coalare the concentration (on the pore surface) of catalysts,the chemical forms of catalysts, the distribution of cata-lysts, the physico-chemical structure/properties of charand the presence of inhibiting agents. Some of these factorsare often inter-related and can be affected by the char-forming conditions. After all, there is always a question ifthe char–O2 and char–H2O reactions would follow thesame reaction pathway. Clearly, a central topic of the dis-cussion is still the transformation and volatilisation ofAAEM species during pyrolysis and gasification, which isinter-related to and inter-influenced by the changes in charstructure.
4.1. Effects of the chemical form of catalyst in char on char
reactivity
It has long been known that the catalytic activity ofAAEM species depends on its chemical form. The anionsin the Na-containing salts loaded into char can have impor-tant influence on the catalytic activity of Na during chargasification [60]. When a Na-containing compound isloaded into a char, the Na loaded in the form of NaClwould not at all be as catalytically effective as the Naloaded in some other forms such as Na2CO3 [10,60]. Thestrong affinity between Na+ and Cl� would not allowNa+ to interact favourably with char to form the neces-sary catalytically active species [61], particularly at lowtemperatures.
Attempts [61,62] have therefore been made to convertthe alkali halides in coal into active alkali catalysts, forexample, by using NH3 or Ca(OH)2 solutions to removechlorine and exchange sodium into the coal substratebefore pyrolysis or gasification. In this case, NaCl in thecoal substrate can be transformed into an active catalystfor gasification in steam [61,62]. Such pre-treatment, whileacademically interesting and providing fundamental under-standing about catalysis during gasification, would havelimited practical application due to the cost associated withsuch pre-treatment.
A more recent study [63] has found that NaCl in thecoal could become an active catalyst for the gasificationof the char if the pyrolysis conditions (e.g. slow heating rateand/or minimising the volatile–char interactions, see Fig. 3about the retention of Cl during pyrolysis) favoured thepreferential release of Cl [41]. Therefore, it is not the chem-ical form of AAEM species in coal but that of AAEM spe-cies in char that determines the catalytic effects of theAAEM species during char gasification. Upon pyrolysis/devolatilisation, both Na as NaCl and Na as carboxylatesin brown coal can be converted into effective catalysts forchar gasification [63]. The transformation of AAEM spe-cies during pyrolysis (and initial gasification) governs theircatalytic effects during gasification.
4.2. Effects of catalyst concentration on char reactivity
While it is generally expected that char reactivity wouldincrease with catalyst concentration up to some extent, theexact relationship between the two parameters, as will bediscussed below, may not always be simple and linear.Under certain experimental conditions, the char reactivityhas been observed to increase with increasing catalyst con-centration. A typical example [42] is shown in Fig. 7 usingthe plots of specific reactivity (weight loss rate per unitmass of gasifying char at any time) versus char conversionlevel. At lower char conversions, all reactivity data fellinto one single line. At higher char conversion levels,the char reactivity data fell into different lines althoughthe char reactivity still increased almost linearly with theNa concentration in char for each given char conversionlevel.
Very high concentrations of AAEM species have alsobeen used. When a slurry of Ca(OH)2 and brown coalwas heated to elevated temperature and pressure (e.g.690 �C and 30 MPa) [64], a product gas rich in hydrogencould be produced with little liquid (tar) production whileCO2 is also fixed. Indeed, the HyPr-Ring process [65–68]has the potential to produce high quality gas (rich in H2)with calcium absorbent being continuously regenerated,with the production of CO2 in high concentrations, benefi-cial for subsequent CO2 sequestration.
4.3. Effects of catalyst dispersion on char reactivity
A catalyst can only be active for gasification if it is onthe char (pore) surface and accessible to the gasifyingagent(s). As was stated above, in addition to NaCl, Naexists in the Victorian brown coal as sodium carboxylatesand thus is dispersed at an atomic/molecular scale in thecoal matrix. Upon pyrolysis at low temperatures (Reac-tions (1)–(3)), Na is likely to be bonded to the O-containingstructures present in the char. This means that Na is stillwell dispersed in the char matrix, even if not all at anatomic/molecular scale. Some Na inside char matrix wouldnot be accessible to the gasifying agents. Increasing pyroly-sis temperature would result in decreases in the oxygencontent of char and ordering of char structure, ‘‘forcing’’Na out of the char matrix onto the pore surface or evenvolatilised. In fact, it was believed that the limited holdingcapacity for Na by char, responsible for the volatilisationof Na during pyrolysis, was an important reason for theexistence of catalyst loading saturation level (LSL) [42].The discussion given here is best illustrated with the cata-lytic activity of the ion-exchanged Na in brown coal wherethe changes in Na dispersion have been observed to corre-late with char reactivity. As is shown in Fig. 8, the reactiv-ity of chars from the Na-form Loy Yang brown coal wasobserved [63] to increase with increasing pyrolysis temper-ature from 500 to 700 �C. This was due to the ‘‘migration’’of Na out of char matrix onto pore surface with increasingpyrolysis temperature, which has clearly outweighed the
Char conversion0.0 0.2 0.4 0.6 0.8 1.0
Sp
ecif
ic r
eact
ivit
y, m
in-1
0.00
0.05
0.10
0.15
0.20
0.25
700°C
500°C
900°C
5.5% Na
9.0% Na
Fig. 8. Specific reactivities of the chars measured in air in TGA at 400 �C[63]. The chars were prepared from the pyrolysis of the Na-form Loy Yangcoal at the temperature indicated in the fluidised-bed/fixed-bed reactorunder the fast heating conditions. The points in different reactivity curvesconnected by the dashed lines have the same overall Na concentrationindicated. Reprinted from Fuel [63], Quyn, D.M., Wu, H., Hayashi, J-i.,Li, C.-Z. Volatilisation and catalytic effects of alkali and alkaline brownearth metallic species during the pyrolysis and gasification of Victorianbrown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na incoal on char reactivity, 587–593. � 2003, with permission from Elsevier.
Na concentration in char, %0 2 4 6 8 10 12 14
Sp
ecif
ic r
eact
ivit
y, m
in-1
0.00
0.05
0.10
0.15
0.20
10%20%30%40%50%60%70%
80%85%90%
Char conversion level
Fig. 7. Specific char reactivity in air at 400 �C as a function of Na concentration in the reacting char. The chars were prepared from the pyrolysis of theNaCl-loaded Loy Yang coal samples at 900 �C (based on the data in Ref. [42]).
C.-Z. Li / Fuel 86 (2007) 1664–1683 1675
negative effects of char structure ordering on reactivity withincreasing pyrolysis temperature. As expected, the reactiv-
ity of char decreased with further increases in pyrolysistemperature from 700 to 900 �C.
The data in Fig. 7 indicate a change in the slope at about1.6 wt% Na concentration: the catalytic activity of Nachanges with the Na concentration in char. This is believedto be due to the changes in the form of Na in char.Increased Na concentration at the pore surface has pro-moted the formation of Na clusters or alike, which aremore active than the finely dispersed Na-containing species[69–73].
Clearly, the most important factor influencing the cata-lyst dispersion is the char structure itself. This will be dis-cussed below in more detail.
4.4. Effects of the physico-chemical structure/properties of
char on char reactivity
4.4.1. Evidence for importance of char structure to reactivity
The physico-chemical structure/properties of char couldaffect the char reactivity in two ways. Firstly, it is the siteson char where the gasification reaction takes place. Thechar structure could directly determine the ease with whichthe carbon (and other) atoms are gasified/removed duringgasification. Secondly, the catalyst must interact with thechar to exert its catalytic activity and the interaction wouldbe affected by the structure of char. For example, the dis-persion of catalyst on the char would change with suchchar structure/properties as the ordering of aromatic ringsand the presence of surface functional groups.
The importance of char structure to its reactivity hasbeen a topic of debate. For example, when K2CO3 was
1676 C.-Z. Li / Fuel 86 (2007) 1664–1683
loaded into a set of relatively well-ordered carbon materialsto the same K/C atomic ratio, very similar gasificationrates were observed [74]. These observations would seemto indicate that the structural features of char play only aminor role compared with the catalyst. On the contrary,some other literature data show that the char structure isas important as catalysts for char reactivity. During thegasification of 16 coals in steam [75] with a 5 wt% loadingof Ca(OH)2 into the coals, the brown coal char exhibitedmuch higher gasification rate than the bituminous coalchars prepared under the same conditions. These observa-tions would thus imply that the structure of char is asimportant as the catalyst loading to the observed charreactivity.
Two causes may contribute to the apparent discrep-ancy on the importance of char structure to its reactivityreported in the literature [42]. The first cause is that thechanges in char structure and/or catalytic activity ofAAEM species during the course of char conversion haveoften been neglected in many past studies. The char reac-tion rate at a chosen char conversion level (e.g. at 5%,10% or 50%) has often been correlated with the initial load-ing of catalysts in the coal or char [74–76]. Such a correla-tion would have neglected the heterogeneous nature of coalchar by assuming the invariability of reaction rate (or rela-tive trend among chars) with char conversion. The secondcause is that no special attention has always been paid tothe consideration of the transformation/volatilisation ofcatalytic species loaded into coal/char during pyrolysis orupon heating [77] prior to reactivity measurement, whichclearly affected the measured reactivity. When the AAEMcatalytic species were volatilised before reactivity measure-ment, the exact concentrations of AAEM species in thegasifying char would have been unknown unless they hadbeen specially quantified (this was seldom done in the paststudies). It is fair to state that the correct assessment of theimportance of char structure relies on knowing the accu-rate concentration of catalyst in the gasifying char.
Knowing the exact concentrations of catalysts in thegasifying char, more recent studies [42,52] have obtainedclear evidence about the importance of char structure onits reactivity. As was shown in Fig. 7, the relationshipbetween char reactivity and Na concentration in char chan-ged with char conversion level. The linear relationshipobserved at each char conversion level meant that otherpossible causes such as changes in catalyst dispersion/agglomeration and catalyst sintering were unlikely impor-tant factors influencing the char reactivity. In other words,for a given char structure (i.e. at a given conversion startingfrom the same char), the catalyst is the most important fac-tor governing the char reactivity. However, for chars at dif-ferent conversion levels (i.e. of different char structure dueto the heterogeneity of char structure), both catalyst andchar structure are important to its reactivity. The morereactive part of char (e.g. consisting of smaller aromaticring systems) would tend to be consumed preferentially atlow char conversions. The nature of reactive sites at low
conversion levels appears to be similar; the apparent acti-vation energy calculation and the kinetic compensationeffect support this conclusion [52]. At high char conversionlevels, the less reactive nature of the char residue appears tobe the dominant factor for the observed reactivity. Thedata such as those in Fig. 7 clearly show the importanceof char structure to its reactivity.
Comparison of the literature data on the char–O2 reac-tion and char–H2O reaction (e.g. those in Refs. [52,78])appears to indicate that the char structure may play a dif-ferent role for these two types of reactions. The first-orderreaction kinetics has been successful in describing somechar–H2O reactions [78]. However, the first-order reactionkinetics does not appear to apply to the char–O2 reactionbecause the specific reactivity (weight loss per unit massof gasifying char) changed with conversion during thechar–O2 reactions [51–53,63]. It appears to suggest thatthe char–O2 and char–H2O reactions may not follow thesame reaction route. This will be further discussed below.
4.4.2. Information on char structure during pyrolysis and
gasification from FT-Raman spectroscopy
It has become clear that a clear understanding of thedirect effects (the relative ease to remove its carbon andother atoms) of char structure on reactivity as well as theindirect effects (via changes in catalyst dispersion) of charstructure on reactivity would require the direct quantifica-tion of char structural features. However, the understand-ing of the char structure has been a challenging taskbecause the solid char is not suitable for analysis by manycommon analytical techniques. Observation of char undera scanning electron microscope (SEM) or a transmissionelectron microscope (TEM), while providing insightfulinformation about char morphology and formation of spe-cial carbon structure [79], does not provide detailed quan-titative information about char structure. X-ray diffraction(XRD), while providing useful quantitative information forwell-order carbon materials, has very limited applicationfor chars from brown coal because the XRD of brown coalchars does not show clear peaks due to the lack of well-ori-ented carbon structure in the chars [78,80].
The potential of Raman spectroscopy in providinginformation about the carbon skeleton structure of coal/char has long been realised. In fact, more importantly,Raman spectroscopy has the potential to probe the charstructure at molecular level before any significant XRD-detectable crystalline structure is developed. It thereforeshould be more powerful in probing the char structure thanXRD. Some pioneer attempts have been reported in the lit-erature [81–86]. However, as Krevelen [87] pointed out in1993 ‘‘up to now, Raman spectroscopy did not provideessential contributions to our knowledge of coal constitu-tion’’. In these past studies, the Raman spectral character-istics, mainly the so-called G (‘‘graphite’’) and D (‘‘defect’’)bands, were used to investigate the coal/char structure, lar-gely following the correlation established by Tuinstra andKoenig in 1970 [88]. Lasers in visible range, such as the
C.-Z. Li / Fuel 86 (2007) 1664–1683 1677
Ar+ laser at 488 and 515 nm and the HeNe laser at 623 nm,were often used, which easily excited strong fluorescenceemission, distorting Raman spectra and resulting in diffi-culties in structural quantification [89]. In fact, recordinghigh quality Raman spectra of highly absorbing coal/charhas always been a big challenge. The possible degrada-tion/oxidation of coal/char heated by the strong laserbeam, which is worsened when Raman microscopy isapplied [89], adds additional difficulty to the use of Ramanspectroscopy in the study of char structure.
Some progress has recently been made in developing aRaman-spectroscopy-based methodology to characterisethe structural features of brown coal chars [80,90–92].The progress is made in two aspects: the acquisition ofhigh-quality spectra and the interpretation of the deconvo-luted spectra. In addition to the use of a NIR laser at1064 nm in FT-Raman spectroscopy to minimise the fluo-rescence interference to the Raman signal, finely groundchar sample was mixed with KBr. With KBr acting as effec-tive heat-dissipating medium, the heating up of charsample during the acquisition of a Raman spectrum wasprevented. Using a relatively low laser power (50–100 mW), very high quality Raman spectra of chars wereobtained by adding 500–1000 scans. A typical example ofRaman spectra of char samples is shown in Fig. 9.
The high signal-to-noise ratios of such spectra as thatshown in Fig. 9 enable the detailed analysis of these spec-tra, e.g. by curve-fitting/deconvolution to gain insights intothe structural features of the char. As is shown in Fig. 9and explained in details elsewhere [80,90], the Raman spec-
Raman s
140016001800
Inte
nsi
ty, a
rb. u
nit
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
G
VL
GL
GR
VR
Fig. 9. A typical FT-Raman spectrum of char and its spectral deconvolution700 �C. Reprinted from Fuel [80], Li, X., Hayashi, J-i., Li, C.-Z. FT-Raman speVictorian brown coal, 1700–1707. � 2006, with permission from Elsevier.
trum was deconvoluted into 10 bands. The G band centredat 1590 cm�1 mainly represents the aromatic ring quadrantbreathing with little contribution from graphitic structures[80,90]. The Gr (1540 cm�1), Vl (1465 cm�1) and Vr
(1380 cm�1) bands, making up the ‘‘overlap’’ between theG and D bands, mainly represent the semi-circle breathingof aromatic rings and/or typical structures in amorphouscarbon, especially smaller aromatic ring systems. They rep-resent important information about char structure buthave been largely ignored or improperly considered in thepast studies. The D (1300 cm�1) band represents mainlyaromatics with not less than 6 rings rather than ‘‘defects’’.The S (1185 cm�1) band, showing together with Sl band asthe shoulder of the D band, mainly represents Caromatic–Calkyl, aromatic (aliphatic) ethers, C–C on hydroaromaticrings, hexagonal diamond carbon sp3 and C–H on aro-matic rings.
The spectral deconvolution scheme shown in Fig. 9 hasbeen based on the trials with chars from coals of differentrank and biomass as well as the spectra of model com-pounds [80]. The areas of GR, VL and VR bands are alwayslumped together and virtually treated as one band repre-senting the overlap between the G and D bands. In prac-tice, the discussion of Raman data was mainly focusedon four groups/bands (G, GR + VL + VR, D and S).
These studies [80,90–92] showed that the brown coalchars had very different features from the highly orderedcarbon materials such as graphite. The traditionalRaman-based methodology for the highly ordered carbonmaterials, largely based on the G and D bands, could not
hift, cm-1
80010001200
D
S
R
SR
SL
[80]. The char was from the pyrolysis of H-form Loy Yang brown coal atctroscopic study of the evolution of char structure during the pyrolysis of a
(A). in air
Char conversion, %0 20 40 60 80 100
I D/I (G
r+V
l+V
r)
0.3
0.6
0.9
1.2
1.5
1.8
H-form
Na-form
(B). in 15% steam
Coal-C conversion, %40 50 60 70 80 90 100
0.3
0.6
0.9
1.2
1.5
1.8
H-form
Na-form
Fig. 10. The ratio of band peak areas ID=IðGrþVlþVrÞ during the gasificationof chars from the H-form and Na-form Loy Yang brown coal. (a) Thechars were prepared under pyrolysis conditions in a fixed-bed reactor at5 K min�1 to 900 �C and then gasified in air at 400 �C in a TGA [91].(b) The chars were prepared in 15% steam at 900 �C in a fluidised-bed/fixed-bed reactor (see Fig. 4) under fast heating rate conditions (>103–104 K s�1) while the carbon conversion was determined using a massspectrometer [92]. Based on the data in Refs. [91,92].
1678 C.-Z. Li / Fuel 86 (2007) 1664–1683
be applied to the study of such highly disordered materi-als as brown coal chars. The Raman signals of browncoal char are largely dominated by its aromatic ringssystems.
The Raman spectroscopic data have provided importantinformation about the char structural changes during pyro-lysis [80,90] and during the char–O2 and char–H2O reac-tions [91,92]. Fig. 10 shows the changes in the peak arearatio ID=IðGrþVlþVrÞ during the gasification of chars fromLoy Yang brown coals. The peak area ratio broadlyreflects the ratio between the large aromatic ring systems(>6 rings) and the aromatic ring systems typically foundin amorphous carbon (2–8 or more fused benzene rings)[91,92]. During gasification in air at 400 �C or in 15% steamat 900 �C where the mass transfer effects are minimised, thesmaller ring systems are preferentially released. However,the presence of a catalyst (e.g. Na and Ca) in both casesaltered the reaction pathways: the Na and Ca catalyststended to make the removal of aromatic ring systems lessselective [91,92], e.g. by comparing the chars from H-formcoal and those from the Na-form coal (Fig. 10). The pref-erential consumption of smaller ring systems, coupled withthe loss of O-containing functional groups, would affect thedispersion of AAEM catalysts in the char.
Examination of the relative intensity of S band (repre-senting the cross-linking) indicated [91,92] that the gasifica-tion in air at 400 �C tended to consume these S-bandstructures selectively, loosening the char structure withthe progress of gasification. However, this was not the caseduring gasification in steam. These studies have providedcircumstantial evidence that the char–O2 and char–H2Oreactions may not follow the same pathway.
The use of Raman spectroscopy in the characterisationof char structural features clearly warrants further investi-gation, including the possible relationship/correlation
between Raman-derived parameters and other conven-tional parameters for char characterisation.
4.5. Effects of volatile–char interactions on char reactivity
The interactions between volatiles and char have stronginfluence on reactivity in a number of ways:
1. As was discussed above, the volatile–char interactionscould result in significant volatilisation of AAEM(mainly Na) species [59,51] and thus direct reductionin char reactivity [51–53].
2. As the volatilisation of Na due to the volatile–char inter-actions not only originated from Na on the char poresurface but also from Na inside the char matrix, Namust have been mobilised by the volatile–char interac-tions. This means that H (and/or other) radicals musthave penetrated into the char structure during the vola-tile–char interactions. Thus, the dispersion of Na in charcould be altered by the volatile–char interactions (evenbefore/without observing Na volatilisation), affectingthe char reactivity.
3. As Na was originally bonded to the char before the vol-atile–char interactions, the volatilisation of Na meansthat the char structure must have also been altered,affecting the char reactivity. Direct evidence has beenobtained on the changes in char structure due to the vol-atile–char interactions. After the volatile–char interac-tions, the reactivity of the chars from the acid-washedLoy Yang coal indeed decreased [51,52], reflecting thefact that the char structure was altered as no AAEMspecies was present in those chars.
4. During the volatile–char interactions, the char was sur-rounded by (H) radicals generated by the thermal crack-ing/reforming of volatiles, likely to inhibit the chargasification reaction. A recent study [93] showed thatthe volatile–char interactions in a fluidised-bed reactorcould practically inhibit the char gasification in steamand terminate the char gasification at a conversion levelranging from 62% to 85% on the coal carbon basis,depending upon temperature. A typical example isshown in Fig. 11. Volatiles were adsorbed onto the charsurface dissociatively and/or thermally cracked in thegas phase, donating hydrogen to free carbon sites muchmore significantly than H2 to inhibit the chargasification.
During the gasification of Loy Yang brown coal in afluidised-bed/fixed-bed reactor, the volatile–char interac-tions would cause the volatilisation of Na and changes inchar structure. Experiments showed that the gasificationrate of char subsequent to the in situ volatile–char interac-tions did not always increase with temperature from 800 to900 �C [92].
It is clear that the volatile–char interactions must be animportant factor to be considered in the design of anyindustrial gasifiers.
0 0.1 0.2 0.3 0.4 0.5 0.6
0.04
0.08
0.12
0.16
0.2
0.021
0.218
1173 K
Cum
ulat
ive
amou
nt o
f pro
duct
s, m
ol-C
Cumulative amount of coal fed into fluidized-bed, mol-C
Char (cyclone)
Char (fluidized-bed)
0
Fig. 11. Effects of volatile–char interactions on the gasification of charfrom Loy Yang coal [93]. The total amount of char in the fluidised bed andin the cyclone increased linearly with the amount of coal beingcontinuously fed into the fluidised-bed, indicating the limiting conversionof coal. Reprinted from Fuel [93], Bayarsaikhan, B., Sonoyama, N.,Hosokai, S., Shimada, T., Hayashi, J.-i., Li, C.-Z., et al. Inhibition ofsteam gasification of char by volatiles in a fluidized bed under continuousfeeding of a brown coal, 340–349. � 2006, with permission from Elsevier.
C.-Z. Li / Fuel 86 (2007) 1664–1683 1679
4.6. Nascent char reactivity
Minimising the volatile–char interactions, a wire-meshreactor provides an ideal tool to measure the gasificationreactivity of ‘‘nascent’’ char. A recent study [94] on thepyrolysis and gasification of Loy Yang brown coal indi-cated that the initial/nascent char gasification in CO2 tookplace at a much higher rate than the ‘‘aged/stable’’ char atthe same temperature. The high gasification rate of the nas-cent char depended strongly on its reactivity of the thermalcracking, which generated radicals to be attacked by CO2
and lasted only for a short period of time.
5. Catalytic reforming of volatiles and gasification products
5.1. Catalytic destruction/reforming of volatiles
In addition to catalysing the char gasification, theAAEM species also catalyse the reforming of volatiles[95–97]. In the absence of char, catalysed and non-cataly-sed steam reforming together with water–gas-shift reactionproduced active hydrogen species such as hydrogen radi-cals, which suppressed not only soot formation but alsofurther progress of steam reforming as the tar-derived frag-ments were stabilised. Thus, simultaneous decreases in thetar and soot yields could not be achieved [97]. In the pres-ence of char particles, the AAEM species on the char sur-face played essential roles in eliminating tar through steamreforming while Na in the vapour phase suppressed soot
formation [97]. These results are in agreement with theobservation of soot formation and destruction [51] duringthe volatile–char interactions shown in Fig. 6. These resultsfurther demonstrate the importance of the volatile–charinteractions in a commercial gasifier. In addition to thechanges in char structure and reactivity as discussed above,the volatile–char interactions also facilitate the reformingof volatiles in the presence of gasifying agent(s).
5.2. Further catalytic conversion/cleaning of gasification
products
5.2.1. Brown coal as a special raw material for catalyst
preparation
The O-containing (acidic) functional groups in Victorianbrown coal confer the coal with ion-exchanging capacityand ability to disperse inorganic salts within the coalmatrix [2,98,99]. For example, the catalytically active spe-cies may be ion-exchanged or impregnated into the rawbrown coal for these species to be well dispersed even atmolecular/atomic scales. Upon pyrolysis or partial gasifica-tion, the finely dispersed species would agglomerate intofine (nano) particles depending upon reaction conditionsand loading level/ratio. This special property of the browncoal has been used to prepare special char-supported cata-lysts, including transition metals, or adsorbents [100–102].
Some concerted efforts have been made to understandthe transformation of transition metals loaded into thebrown coal upon thermal treatment [11,12,103–108]. Anextended X-ray absorption fine structure (EXAFS) spec-troscopic study [11] revealed that ion-exchanged nickelexists in brown coal by coordinating with 6 oxygen atoms.Upon pyrolysis, nickel would agglomerates into fine parti-cles at 650–750 K depending upon the nickel loading level.The agglomeration of nickel was observed during bothpyrolysis and gasification [11,12,104–106].
Iron ion-exchanged or impregnated into the brown coalalso undergoes significant transformation during pyrolysisand gasification [109–113]. a-Fe, c-Fe and magnetite(Fe3O4) have been observed [112].
5.2.2. Char-supported catalysts for further conversion/
cleaning of gasification products
Compared with the conventional supported catalysts,the char-supported catalysts/adsorbents prepared usingbrown coal would be cheap and without catalyst disposalproblems. At the end of service life, the char-supported cat-alyst can be disposed of simply by gasifying/burning thecatalyst, during which the energy value of the char supportcan also be recovered.
The pyrolysis of Fe-loaded Victorian brown coal pro-duces nano-sized iron catalyst for the in situ destructionof N-containing species such as NH3 [111]. More recently,such Fe catalysts have been studied [114,115] for hot-gascleaning to destroy NH3 in the gasification product gas.Chars prepared from the Ca-exchanged brown coal havebeen found as effective H2S adsorbents for hot-gas cleaning
Moving-bed unit
Coal+O2-enriched
air+H2O
Adsorbents
Gasifier
Char
Gas-solidseparation(Cyclone)
Gasifier
Deactivated char
CO+H2
CO2 Capture
Catalyst Char
Feloaded-
Coal
Ash
Catalystpreparation
unit
Moving-bed
reactor
Gasseparation H2
Flue gas recycle
CO2 + H2
Fig. 12. A schematic diagram of a conceptual process for the hot-gas cleaning and catalytic production of hydrogen based on the gasification of browncoal using the char-supported nano-Fe catalyst in a non-isothermal moving bed [15]. Reprinted with permission from Process Safety and Environmental
Protection (Trans IChemE, Part B), 2006, 84, pp. 125–130. � 2006. IChemE.
1680 C.-Z. Li / Fuel 86 (2007) 1664–1683
[116,117]. Zinc ferrite (ZnFe2O4) supported on Yallournbrown coal char has also been tested for the effectiveremoval of H2S as a part of hot-gas cleaning [101]. Char-supported nano-iron catalysts [15] have recently beenfound to be effective for catalysing the water–gas-shift reac-tion, potentially useful for the large-scale production ofhydrogen based on the gasification/reforming of carbona-ceous fuels such as coal, biomass and natural gas.
The principle of a novel process based on the char-sup-ported nano iron catalyst for the hot-gas cleaning and sub-sequent production of hydrogen from the gasification ofcoal or biomass has recently been outlined [15], as is shownin Fig. 12. In this proposed process, Victorian brown coaland Fe-containing material such as FeCl3, which is a wastefrom iron and steel industry, are the main feedstock to pre-pare the catalyst. The hot and ‘‘dirty’’ product gas from thegasifier, containing tarry materials, alkali, N-/S-containingspecies (e.g. NH3 and H2S) and un-reacted steam, comesinto direct contact with the char-supported nano Fe cata-lyst in the high-temperature part of the non-isothermalmoving bed. The partial gasification of char catalyst bydirect contact with hot gas converts some latent thermalenergy of the product gas into chemical energy. This repre-sents an important advantage to the overall processefficiencies.
The N-containing species in the gasification product gas,particularly NH3, will be converted into N2 by iron[114,115] in the catalyst. H2S in the gasification productgas will also likely be caught by the catalyst to form FeSor CaS2 [116,117], which when returned to the gasifier willreact again with the in-bed adsorbent to enhance its chanceof removal as ash. With decreasing temperature along themoving bed, the tarry material and AAEM species willdeposit on catalyst. When the catalyst is sent to the maingasifier, the tar condensed on the catalyst will be gasified.
Similarly, the AAEM species condensed on the catalyst willhave one more chance to become gasifier ash as the char-supported catalyst is gasified in the main gasifier. There-fore, the tarry materials, AAEM species and N-/S-contain-ing species on the deactivated catalyst would all have theirroutes to leave the main gasifier as (CO/H2/N2 and ashcomponents) without being accumulated in the gasificationsystem.
The water–gas-shift reaction would be catalysed by thefresh catalyst supported on the char in the moving-bedreactor at low temperature.
The counter-current arrangement (Fig. 12) would havegreat potential for removing particulates and tar as wellas for managing the catalyst deactivation. Therefore, themoving-bed reactor carries out several integrated functionsof chemical heat pumping, hot gas cleaning, pollutant (par-ticularly NH3 and H2S) abatement and water–gas-shiftreaction. Clearly, further investigation is warranted toimprove the chemical formulation of the catalyst (e.g. toinclude species other than Fe) as well as its physicalstructure.
6. Effects of dewatering/drying on the pyrolysis and
gasification behaviour
Significant changes could take place to the physical andchemical structure of Victorian brown coal during dewater-ing/drying. When dewatering is carried out under highpressure (e.g. during mechanical thermal expression), apart of the pore structure in the brown coal will bedestroyed [16]. Dewatering/drying at elevated temperaturescan also result in the decomposition or hydrolysis of O-containing functional groups [16,43,118]. Some NaCl dis-solved in the moisture in coal can also be removed in theliquid water [16]. Due to these reactions, the water from
C.-Z. Li / Fuel 86 (2007) 1664–1683 1681
the non-evaporative dewatering of brown coal would con-tain high concentrations of organic and inorganic species.This wastewater must be treated before discharge into envi-ronment [119]. A Ni/carbon catalyst shows promise inremoving the organics in the wastewater together withenergy recovery [120].
A fundamental understanding of the effects of dewater-ing/drying on the pyrolysis and gasification behaviour willneed to investigate the effects of thermal treatment, theeffects of the reactions involving water during dewatering,the effects of NaCl removal and the effects of pressure (porestructure) separately. As was stated above, the presence ofNaCl in the brown coal has little effects on the pyrolysisyields from the brown coal at least under fast heating rateconditions [43]. Therefore, the removal of some NaCl aloneduring dewatering is unlikely to alter the organic volatileformation process greatly although there would be lessNa in the vapour phase and in the char during the subse-quent pyrolysis and gasification.
Pyrolysis of Victorian brown coal was carried out [118]in a wire-mesh reactor capable of multi-stage heating andholding to simulate the time–temperature history duringsome dewatering processes. The removal of the inherentmoisture (<10 wt%) from coal, including the freezableand non-freezable moisture, had little effects on the tarand char yields during the subsequent in situ pyrolysis ofthe dried coal at 900 �C even at 1000 K s�1. The hydrogenbonds between the moisture and the O-containing func-tional groups in the brown coal differ from the hydrogenbonds among the O-containing functional groups in theireffects on pyrolysis yields [118].
The balance of experimental evidence [43,118] suggeststhat dewatering at temperatures lower than 250 �C wouldhave small (but not zero) effects on the subsequent pyroly-sis behaviour of the dewatered brown coal, although thefates and roles of NaCl during pyrolysis and gasificationshould never be forgotten. Clearly, further study will stillbe warranted to fully assess the behaviour of dewatered/dried brown coal during pyrolysis and gasification.
Utilisation of dewatered/dried brown coal in the existingboilers would represent a significant fuel change. Signifi-cant efforts are made to understand the possible opera-tional problems associated with such fuel change [17].
7. Conversion of pollutant-forming elements
The main pollutant-forming elements in Victorianbrown coal are nitrogen, sulphur and chlorine. Little atten-tion is paid to the heavy metals in the brown coal. Adetailed review of the conversion of coal–N and coal–Sduring the pyrolysis, gasification and combustion of Victo-rian brown coal has recently been presented elsewhere [121]by the current author and will not be repeated here. Thereview [121] also included a summary of related studieson the pyrolysis of a large number of N-containing modelcompounds. The interactions between volatiles and charplay an important role in the conversion of coal–N during
pyrolysis and gasification [121]. A more recent study [122]provides further detailed insights into the effects of vola-tile–char interaction on the fates of coal–N in the presenceof O2.
Effects of pressure on the formation of HCN and NH3
during the pyrolysis and gasification of Loy Yang browncoal in steam were recently investigated [123] using apressurised drop-tube/fixed-bed reactor. The NH3 yieldincreased with increasing pressure during both pyrolysisand gasification. On the other hand, the HCN yield duringpyrolysis showed little sensitivity to changes in pressurealthough the HCN yield during gasification in steamshowed some increases with increasing pressure. Theseresults confirmed the general theoretical frameworkpresented before [121]: H radicals are instrumental forthe formation of NH3 and HCN in the solid phase. Thedirect conversion, either through hydrogenation or hydro-lysis, of HCN into NH3 on char surface during the pyroly-sis and gasification of brown coal at low temperature (e.g.<800 �C) is not an important route of NH3 formation. Ashpresent in the gasifiers would greatly influence the finalfates of coal–N during pyrolysis and gasification [124].
The information on the conversion of coal–Cl duringthe pyrolysis and gasification of Victorian brown coalseems relatively scarce. The data in Fig. 3 show that therelease of coal–Cl can be complicated by the volatile–charinteractions [41]. When the fluidised-bed/fixed-bed reactorwas operated at the fast heating rate mode, the reactor con-figuration encouraged the interactions of char with vola-tiles passing through the char bed. As a result, the datain Fig. 3 indicate that the released Cl, likely as HCl, couldeasily bond back to the char, causing decreases in therelease of Cl with increasing temperature. The Cl bondedback to char would be released again at very high temper-atures, reflecting the not-so-strong char–Cl bonds. A morerecent study [125] further showed that HCl reacted withbrown coal char at 500 �C to form several types of Cl func-tional forms.
8. Conclusions
Significant progress has been made towards a betterunderstanding of the pyrolysis and gasification behaviourof Victorian brown coal. Victorian brown coal behavesvery differently from other solid fuels such as biomassand bituminous coals, owing to its unique structural fea-tures especially the presence of high contents of oxygenin a wide variety of functional groups and the presenceof alkali and alkaline earth metallic species finely dispersedat an atomic/molecular scale in the brown coal. The highlyreactive nature of its volatiles and the vulnerable nature ofits nascent char mean that the volatile–char interactions, acommon phenomenon in all gasifiers (especially the flui-dised-bed gasifiers), would influence almost every aspectof its pyrolysis and gasification behaviour. The progressin the fundamental understanding of the pyrolysis and gas-ification behaviour of Victorian brown coal has laid solid
1682 C.-Z. Li / Fuel 86 (2007) 1664–1683
foundation for the further development of advanced gasifi-cation technologies for the clean and efficient utilisation ofthis cheap but important resource. Victorian brown coalhas been and will continue to be an important energysource in the foreseeable future. Its unique structure andbehaviour will continue to attract the attention of coal sci-entists and technologists.
Acknowledgements
The author gratefully acknowledges the financial andother support received from the Cooperative ResearchCentre (CRC) for Clean Power from Lignite (establishedand supported under the Australian Government’s CRCprogram) and the New Energy and Industrial TechnologyDevelopment Organisation (NEDO) in Japan.
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