Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal

Fuel 79 (2000) 427–438 www.elsevier.com/locate/fuel

Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal C.-Z. Li*, C. Sathe, J.R. Kershaw, Y. Pang CRC for New Technologies for Power Generation from Low-Rank Coal, Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia

Abstract H-, Na- and Ca-form coal samples were prepared from a sample of Loy Yang brown coal and pyrolysed in a wire-mesh reactor. The tars were characterised with UV-absorption and UV-fluorescence spectroscopies. Increases in heating rate (1 to 2000 K s 21) and temperature (up to 7008C) were found to facilitate the release of larger (“equivalently” larger than naphthalene) aromatic ring systems from coal during pyrolysis. The presence of alkali and alkaline earth metallic (AAEM) species in the coal samples greatly hindered the release of the larger aromatic ring systems during pyrolysis. The AAEM species also reduced the effects of heating rate on the release on aromatic ring systems at lower temperatures. However, the hindering effect was not proportional to the contents of AAEM species in the coal. In addition, the ionexchange processes caused irreversible changes to coal structure. Significant proportions of the AAEM species in the coal samples were volatilised during pyrolysis even at temperatures as low as 3008C. The volatilisation of AAEM species was not sensitive to changes in heating rate but was intensified with increasing temperature. The monovalent species (Na) was always volatilised to a much larger extent that the divalent species (Mg and Ca) under similar pyrolysis conditions. At high temperatures (900–12008C), the drastic volatilisation of Na (up to 80%) and of Ca (up to 40%) was accompanied by the increases in tar yield during the pyrolysis of the Na-form and Ca-form samples. The fates and roles of the AAEM species during pyrolysis are thought to be related to their transformation during pyrolysis. The AAEM species might have been involved in a repeated bond-forming and bond-breaking process between the AAEM species and the coal/char matrix. During this process, tar precursors were repeatedly linked to the coal/char matrix and were thermally cracked. Some of the more aliphatic components and/or smaller aromatic ring systems in a tar precursor were cracked to gas and some of the larger aromatic ring systems were charred. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Brown coal; Pyrolysis; UV-absorption spectroscopy; UV-fluorescence spectroscopy; Ion-exchangeable cations; Alkali and alkaline earth metals in coal; Aromatic ring systems

1. Introduction One of the most prominent features of the Victorian brown coals is the presence of significant amounts of alkali and alkaline earth metals (AAEM) associated with the carboxylic and phenolic functionalities in the coal structure [1–3]. These AAEM metallic cations in the brown coals can be removed by washing with acid. Individual cations (e.g. Na 1 or Ca 21) can then be ion-exchanged onto the coal structure [4]. Although the AAEM species generally account for less than 1% of the raw coal, they play very important roles in the utilisation of the brown coals [3–5]. For example, they are largely responsible for the particular fouling/slagging problems encountered during the pulverised-fuel combustion of the brown coals [3]. When the brown coals are used in the future to generate electricity * Corresponding author. Tel.: 161-3-9905-9623; fax: 161-3-9905-5686. E-mail address: [email protected] (C.-Z. Li).

with advanced technologies such as advanced pressurised fluidised-bed combustion (APFBC), the roles of the AAEM species are two-fold. The volatilisation of the AAEM species, either during pyrolysis or during gasification/ combustion, will probably cause severe problems for the operation of gas turbines due to the corrosion/erosion of the turbine blades. On the other hand, if these metallic species are retained in the char after pyrolysis, they can act as good catalysts for the subsequent gasification/ combustion of the char. Therefore, the future success of the potentially highly efficient and environmentally friendly technologies such as APFBC will to a large extent depend on our understanding on the fates and roles of these AAEM species under the conditions pertinent to those in the APFBC process. A large number of researchers have examined the roles of these AAEM metallic species and experimental conditions on the yields of light hydrocarbons [1,6–11] oxygencontaining species [8,10–14], char [6,7,10,11,14,15] and

0016-2361/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00178-7

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Table 1 Properties of the Loy Yang raw coal sample studied Ash (wt%, db) 1.0

Volatile matter (wt% daf) 51.5

C (wt% daf) 68.5

H (wt% daf) 4.8

tar [1,7,8,12,14,16] from the pyrolysis of brown coals using a variety of reactors. Experimental conditions such as heating rate, peak temperature and pressure can all have profound effects on the pyrolysis behaviour of brown coals. Tar samples from the pyrolysis of brown coals/ lignites have been characterised with Fourier-transform infrared (FTIR) spectroscopy [17], nuclear magnetic resonance (NMR) spectroscopy [18,19] and high performance liquid chromatography (HPLC) [20–22]. Most researchers have found that the presence of AAEM cations in the coal substrate tends to cause depression in both tar and total volatile yields. The AAEM cations in coal also seem to change the aromatic/aliphatic composition of the resulting tars [13,17,20]. A number of researchers [1,12,23,24] have attributed the reduction in tar yield by divalent cations to the cross-linking effects of the divalent ions, which serve as virtual bonds between (carboxylic) groups in the coal structure. A recent study [25] using proton magnetic resonance thermal analysis (PMRTA) has suggested that the tar yield reduction by both monovalent (Na 1) and divalent (Ca 21 or Ba 21) cations is associated with the changes in coal matrix density (compacting) and in maximum fusibility (becoming more rigid) during pyrolysis. Despite the great efforts made so far, the roles of AAEM species during the pyrolysis of brown coals still remain poorly understood. Very little is known about the combined effects of heating rate and the presence of the AAEM species in the coal structure on the pyrolysis behaviour of brown coals. Although it is known that a significant proportion of the AAEM species is volatilised during pyrolysis [26–28], little exists in the literature regarding the relationship between the release of volatiles and that of AAEM species. The purpose of this study is to experimentally investigate the fates and roles of AAEM species in a Victorian brown coal (Loy Yang) during pyrolysis in a wire-mesh reactor, including the combined effects of heating rate and the presence of AAEM species on the release of aromatic ring systems as well as the volatilisation of AAEM species themselves during pyrolysis. Table 2 Contents of alkali and alkaline earth metals in the Loy Yang coal samples studied

Na, % (db) K, % (db) Mg, % (db) Ca, % (db) a

Raw coal

Na-form

H-form-2

Ca-form

H-form-3

0.128 0.012 0.058 0.034

2.80 n/d 0.001 n/d

0.010 0.008 0.005 0.007

0.017 n/d a 0.001 3.27

0.008 0.010 0.007 0.010

n/d: not detected.

N (wt% daf) 0.55

S (total) (wt% daf) 0.32

Cl (wt% daf) 0.11

O (by diff.) (wt% daf) 25.7

2. Experimental 2.1. Coal samples A sample of “as-mined” coal was obtained from the Loy Yang field in the Latrobe Valley, Victoria, Australia. The sample was partially dried at low temperature (,358C) and then pulverised and sieved to obtain a fraction sample of particle sizes between 106 and 150 mm. The properties of the “raw” coal sample (2150 1 100 mm fraction) are given in Table 1. The raw coal sample was stirred in 0.1 M sulphuric acid for about 16 h under nitrogen atmosphere and then washed with double distilled water to prepare an H-form coal sample (H-form-1). The Na-form and Ca-form samples were prepared by adding the H-form-1 coal sample into the solutions of sodium acetate and calcium acetate respectively. The mixtures were kept under nitrogen atmosphere and stirred intermittently. During the ion-exchange process, the pH values of the mixtures were adjusted intermittently to around 8.3 by the addition of sodium hydroxide. A significant amount of humic material was lost during the ionexchange process with Na 1. Some humic material was also lost during the preparation of Ca-form coal sample albeit to a much lesser extent than during the preparation of Na-form coal sample. As was noted in a previous paper [29], in addition to the loss of some humic materials, the ion-exchange processes might have also caused significant changes to coal structure. This will also be further discussed below. Another two samples, H-form-2 and H-form-3, were prepared by re-washing the Na-form and Ca-form samples with acid respectively. The contents of Na, K, Ca and Mg of these samples are given in Table 2. 2.2. Pyrolysis Pyrolysis experiments were carried out in a wire-mesh reactor similar to that described by Gibbins and co-workers [30,31] and Li and co-workers [32–34]. Briefly, less than 10 mg of coal sample was sandwiched between two layers of wire-mesh and heated electrically with alternating current. A continuous stream of helium (Ultra High Purity grade, .99.999%) was passed continuously through the sample-laden wire-mesh sample holder at 0.1 m s 21 (calculated at ambient conditions) to sweep the evolving volatiles into a liquid-N2-cooled tar trap [32–34] which was packed with Teflon. Total volatile yield was determined by weighing the sample-laden wire-mesh sample holder before and after heating. Prior to a pyrolysis experiment, the coal sample to be used was air-dried in order to overcome the weighing problems due to the rapid take-up of moisture in

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the air that would take place if a dry coal sample was used. The weighing of the char sample after pyrolysis was also carried out after the char sample was considered to have reached equilibrium with ambient air. The moisture contents of the coal and the char were considered in the calculation of pyrolysis yields. At the end of a pyrolysis experiment, tar was recovered by washing the tar trap with HPLC grade CHCl3:CH3OH (80:20) [32–34]. Tar yield was then determined [29] through quantifying concentration of tar in the solution.

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Retention of an AAEM species in char was calculated by comparing the content of the AAEM species in the char with that in the raw substrate coal sample, considering the weight loss of the coal sample during pyrolysis.

3. Results and discussion

2.3. Characterisation of tar

3.1. Effects of temperature, heating rate and AAEM species on the release of aromatic ring systems during pyrolysis at lower temperatures

UV-fluorescence spectra were recorded with a Perkin– Elmer LS50B luminescence spectrometer. The tar solution obtained by washing the tar trap (see above) was diluted with SpectrosoL grade CH3OH (BDH) to a concentration of the order of magnitude of 10 26 g ml 21 where the effects of self-absorption were minimal. A visible cell of 1 cm light pathlength was used. Slit widths for both excitation and emission monochromators were 3.0 nm. Spectra were recorded using a scan speed of 200 nm min 21. The instrument features the automatic correction of source intensity variation. The emission intensity was not further corrected. Spectra in Figs. 1–5 and those in Figs. 7 and 8 were recorded on different occasions using different detectors. Thus, these spectra may not be compared directly. UVabsorption spectra were recorded with a GBC 918 double beam UV/VIS spectrometer. UV–Visible cells of 20 mm light pathlength were used. Both the fluorescence intensity and absorbance signals were multiplied by a factor

A fuller account of the tar and total volatile yields from the pyrolysis of raw, H-form-1, Na-form, Ca-form, H-form2 and H-form-3 Loy Yang coal samples at temperatures up to 900 or 10008C has been given elsewhere [29]. For the raw coal and H-form coal samples, the increases in heating rate resulted in large increases in tar yield while the corresponding increases in total volatile yield, if any, were small [29]. The presence of AAEM cations in coal tended to reduce the tar (and to a lesser extent the total volatile) yields, and, more importantly, reduced the heating rate sensitivity of the tar yield [29]. As was discussed elsewhere [29], the loss of the humic materials might have caused the widening of pores due to the extraction of the humic materials originally present in the pores. It was also possible that some humic materials, extracted out of the pores but not lost during filtration, might also have blocked some pores at the external particle surface when the samples were dried. However, more importantly, the ion-exchange processes, particularly

f ˆ

tar yield …wt%; db† concentration of tar in the solution with which measurement was carried out

in order to facilitate comparison of the signals of different tars on the basis of “per g of substrate coal”. 2.4. Quantification of AAEM species in coal/char samples Less than 4 mg of coal/char sample was ashed in O2 in a thermogravimetric analyser–differential thermal analyser (TGA–DTA). Care was taken to control the conditions in the TGA to ensure that particles were not ignited and that all organic matters were fully oxidised. The final ashing temperature was 6008C with 30 min holding time. At the end of a TGA ashing experiment, the ash sample together with the Pt crucible was placed in a Teflon vial for acid digestion with a hot mixture of HNO3:HF (1:1) solution for at least 16 h. HNO3 and HF were then evaporated on a hot plate and the residue was re-dissolved in 20 mM CH3SO3H. Quantification of Na, K, Mg and Ca was carried out using a Dionex DX 500 ion chromatograph with a suppressed conductivity detection system. Separation was carried out on a Dionex CS12 column using 20 mM CH3SO3H aqueous solution as eluent.

with divalent ions (e.g. Ca 11), might have caused changes to the coal macromolecular network [25,29] due to the ionic forces. The same sets of tar samples have been characterised with UV-absorption and UV-fluorescence spectroscopies in this study. Fig. 1 shows the synchronous spectra of the tars produced from the pyrolysis of the Loy Yang raw coal with a heating rate of 100 K s 21. A constant energy difference of 22800 cm 21 between the emission and excitation monochromators was used in recording these spectra. Similar trends were also observed when a constant energy difference of 21400 cm 21 was used. In agreement with the previous UV-fluorescence spectroscopic study [35] on a set of tars from the pyrolysis of a set of rank-ordered coals in a wire-mesh reactor, the synchronous spectra in Fig. 1 show two broad peaks centred around 340 and 380 nm respectively. The tars from the pyrolysis of the same raw coal are seen to become progressively more fluorescent (Fig. 1a) with increasing temperature, although the shapes of the spectra do not seem to change very much. The fluorescence intensities expressed on the basis of “per g of

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Fig. 1. Effect of temperature on the constant energy (22800 cm 21) synchronous spectra of tars from the pyrolysis of Loy Yang raw coal at 100 K s 21. Holding time: 10 s. (a) Fluorescence intensity on the basis of the same tar concentration; (b) fluorescence intensity on the basis of unit weight substrate coal.

coal” (Fig. 1b) are also seen to increase with increasing temperature. Although UV-fluorescence spectroscopy has been shown to be a useful tool in studying the structural features of coalderived liquids [35–39], caution must be exercised in interpreting the UV-fluorescence spectra of coal-derived samples. Firstly, the fluorescence quantum yields of individual aromatic ring systems vary significantly. The presence of substitutional groups may also affect, to some limited extent, both the position and quantum yield of the aromatic ring system. Secondly, the intensity and shape of fluorescence spectra of individual polycyclic aromatic compounds are solvent dependent [40]. It has been found [32] that fluorescence intensity of a tar sample in CH3OH tends to be higher than that of the same tar in CHCl3 if the wavelength of the synchronous spectra is shorter than about 350 nm. Thirdly, excimers may form. This includes the intermolecular excimers as well as intramolecular excimers that are more important for the tar molecules of relatively high

molecular masses [35]. Fourthly, fluorescence emission of a component may be absorbed by another component, i.e. the masking of one component by another [41,42]. Fifthly, energy transfer may take place from an excited aromatic ring system to another aromatic ring system, including the energy transfer from an excimer. The masking and the energy transfer would seem to produce similar effects. This means that the smaller aromatic ring systems can act as sensitisers for the larger ones, particularly if they are within the same tar molecule. Energy transfer from a smaller aromatic ring system to a larger aromatic ring system seems to be an important factor influencing the fluorescence properties of tars [35]. Because of this nature of energy transfer (or masking effect), the fluorescence properties of the smaller aromatic ring systems in tar can be significantly affected by the presence of larger aromatic ring systems in the same tar molecule. On the other hand, if the larger aromatic ring systems are excited at longer wavelengths, their fluorescence properties will be little affected by the presence of smaller aromatic ring systems in the same tar molecule. In another word, the synchronous spectra at longer wavelengths are much less affected by energy transfer than at shorter wavelengths [35]. As the tars investigated (Fig. 1) were all from the pyrolysis of the same Loy Yang raw coal substrate with minimal secondary extraparticle reactions, it may be expected that the types of aromatic ring systems in the tars would be very similar although their relative proportion might vary with pyrolysis conditions. Therefore, the synchronous fluorescence intensity, particularly at longer wavelengths, can be considered as a semi-quantitative indication of the concentration of (larger) aromatic ring systems. The spectra in Fig. 1a thus appear to suggest that the concentrations of the larger aromatic rings (“equivalently” larger than naphthalene) in the tar increased with increasing temperature. The spectra in Fig. 1b show that increasing temperature from 500 to 6008C has resulted in a large increase in fluorescence intensities, indicating the further release of the larger aromatic ring systems. This is in line with the relatively large increase in tar yield observed with increasing temperature from 500 to 6008C [29]. Further increase in temperature, particularly over 8008C, up to 10008C did not seem to have significant effects on the release of the larger aromatic ring systems that can be seen with the synchronous spectra. The effect of peak pyrolysis temperature shown in Fig. 1 was also seen with the tars from the pyrolysis of the same raw coal using a heating rate of 1, 1000 or 2000 K s 21. Furthermore, similar experiments carried out with the tars from the pyrolysis of the coal samples ion-exchanged with H 1, Na 1 and Ca 21 have all generally shown that increasing temperature from 700 to 10008C resulted in little further release of larger aromatic ring systems if the same heating rate, i.e. either 1 or 1000 K s 21, was used. Further discussion on the data obtained up to 12008C will be given later in the paper. Fig. 2 shows the synchronous spectra of tars produced

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only if they are within a tar molecule of relatively low volatility. Secondly, the smaller aromatic ring systems are not seen with the synchronous spectroscopy in this study (see above). Due to their low absorptivity, their contribution to the absorption spectra is also minimal. Therefore, the spectra in Fig. 2 provide little information about the smaller aromatic ring systems. The spectra in Fig. 2 are thus interpreted as to indicate that the “yields” of larger aromatic ring systems are much higher at higher heating rates than at lower heating rate. In another word, higher heating rates have facilitated the release of aromatic ring systems, particularly the relatively large ones, confirming the conclusions inferred from the tar and total volatile yields of these experiments reported earlier [29]. As the total volatile yields from the pyrolysis of raw coal are not very sensitive to heating rate [29], some part of the tar precursors which would have been released as tar at high heating rates (e.g. 1000 K s 21) must have been thermally cracked through repeated bond-forming and bond-breaking processes involving substitutional groups or smaller aromatic ring systems [29]. The net result is the release of the more aliphatic materials together with the smaller aromatic ring systems as gases. During this repeated bond-forming and bond-breaking process, some of more aromatic material and/or larger aromatic ring systems were charred inside the particle. Similar effects of heating rate were also observed with synchronous spectroscopy of tars from the raw coal pyrolysed at temperatures higher than 6008C. The presence of AAEM cations in the coal substrates has

Fig. 2. Effect of heating rate on the constant energy (22800 cm 21) synchronous spectra and UV absorption spectra of tars from the pyrolysis of Loy Yang raw coal at a peak temperature of 6008C with 10 s holding time. (a) Fluorescence intensity and absorbance on the basis of the same tar concentration; (b) fluorescence intensity and absorbance on the basis of unit weight coal substrate.

from the pyrolysis of the raw coal at 6008C as a function of heating rate. The concentrations of the larger aromatic ring systems are much higher in the tar produced at 1 K s 21 than those prepared at higher heating rates (Fig. 2a). However, when the intensity is expressed on the basis of “per g of coal”, the spectra in Fig. 2b show that the fluorescence intensities of the tars prepared at high heating rates are much higher than at slow (1 K s 21) heating rate. The observations made with UV-fluorescence spectroscopy have also been largely confirmed with UV-absorption spectroscopy, as is also shown in Fig. 2. It is worth pointing out here that the experimental techniques used in the present study are strongly biased towards the larger aromatic ring systems. Firstly, the tar collection procedure is thought to have resulted in the loss of the majority of the isolated single aromatic ring systems (e.g. benzene and toluene). Single aromatic ring systems are present in the final tar samples

Fig. 3. Effect of the presence of AAEM cations in substrate on the synchronous (22800 cm 21) spectra of the tar prepared by heating the substrate at 1000 K s 21 to 6008C with 10 s holding time.

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played an important role during pyrolysis in terms of the release of aromatic ring systems. Fig. 3 shows the synchronous spectra of the tars from the pyrolysis of various forms of the same Loy Yang coal. Due to the loss of humic materials and the changes in the spatial configuration of the macromolecular network during the ion-exchange processes with Na 1 and Ca 21 [29], the effects of Na and Ca should only be assessed by comparing the spectra of the tars from Na-form and Ca-form with those of the tars from H-form-2 and H-form-3, respectively. Similarly, the effects of the inorganics in the raw coal should be assessed by comparing the spectrum of the tar from the raw coal with that of the tar from the H-form-1 coal sample. Clearly, the presence of AAEM species such as Na, Mg and Ca in the coal substrates has decreased the “yields” of larger aromatic ring systems. The Na and Ca contents in the Na-form and Ca-form coal samples are much higher than the total contents of “inorganics” (Na, Mg, Fe, etc) in the raw coal, as is shown in Table 2. It is surprising to note that the difference between the “yields” of larger aromatic ring systems from the Na-form (or Ca-form) and the H-form-2 (or H-form-3) samples is not much bigger than that between the “yields” from the raw coal and H-form-1 sample (Fig. 3). In another word, the effect of AAEM species on the release of larger aromatic ring systems is not proportional to their contents in the unpyrolysed substrate coal samples. Further work is being carried out to understand the effects of the concentration of AAEM species on the release of aromatic ring systems. The presence of AAEM species in the unpyrolysed substrate has also changed the effects of heating rate on the release of aromatic ring systems during the pyrolysis of these coal samples. Like the raw coal (Fig. 2), H-form samples have generally shown sensitivities to heating rate, particularly at low temperatures. As is shown in Fig. 4 for H-form-1 sample, both the fluorescence and absorption spectra indicate that the release of aromatic ring systems is favoured by the increased heating rate. On the contrary, the Ca-form sample from the same raw coal showed little sensitivity to changes in heating rate, as is shown in Fig. 5. Similar experiments on Na-form coal sample were difficult as tar yield was negligible [29] at 6008C when the sample was heated at 1 K s 21. The trends reported here on the release of aromatic ring systems are in line with the trends in tar yield reported elsewhere [29].

Fig. 4. Effect of heating rate on the constant energy (22800 cm 21) synchronous spectra and UV absorption spectra of tars from the pyrolysis of Hform-1 sample at a peak temperature of 6008C with 10 s holding time.

require that the partition of the AAEM species in the gasphase and in the solid-phase during pyrolysis be quantified. Some preliminary studies [43,44] have suggested that the AAEM species in char may not be extracted directly with acids and that char samples should be ashed before acids are

3.2. Volatilisation of AAEM species and release of volatiles at higher temperatures The results presented above and elsewhere [29] clearly show that the presence of AAEM cations in the coal substrate has profound effects on the release of volatiles during pyrolysis. This implies that the AAEM cations in the substrate coal have taken part in the thermal decomposition reactions during pyrolysis. A better understanding of the roles of AAEM species on the release of volatiles would

Fig. 5. Effect of heating rate on the constant energy (22800 cm 21) synchronous spectra of tars from the pyrolysis of Ca-form sample at a peak temperature of 6008C with 10 s holding time.

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used to bring the AAEM species into solutions for subsequent quantification. Experiments were carried out in this study to investigate the choice of optimum ashing conditions in a TGA (also see Experimental). Fig. 6a and b shows the effects of ashing temperature in O2 in TGA on the loss of Na and Mg. It is clear that no discernible volatilisation of Na and Mg can be seen at temperatures lower than 6008C, although a larger scatter exists in the data for Mg due to the low content of Mg in the raw coal and the broader peak of Mg in ion chromatogram. Direct acid-digestion of the raw coal gave the same results within experimental error, further confirming that minimal amounts of Na or Mg were volatilised during ashing in O2 in the TGA. While temperatures lower than 6008C might still be enough for the complete oxidation of organic matter in the raw coal, a temperature of 6008C with 30 min holding time has been chosen as the optimum ashing condition for coal and char samples, considering that the reactivity of high-temperature chars may be lower than that of the raw coal. The TGA was so programmed that no ignition took place during ashing. Data in Fig. 6c and d show that significant proportions of Na and Mg in the raw coal have been volatilised during pyrolysis. It is however surprising to note that around 10% of Na and Mg were volatilised during pyrolysis at temperatures as low as 3008C when the total volatile yield was only about 10% [29]. During the course of this study,

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Olsson and co-workers [45] also reported the volatilisation of alkali metallic species during the pyrolysis of biomass at temperatures as low as 1808C. Because virtually no Na or Mg were seen to become volatilised even at much higher temperature of 6008C during ashing in the TGA (Fig. 6a and b), the volatilisation of Na and Mg shown in Fig. 6c and 6d must have taken place during the pyrolysis in the wire-mesh reactor and not during ashing in the TGA. This conclusion has been further confirmed by carrying out pyrolysis experiments using a trap totally made of Teflon. The trap was also cooled with liquid N2 during experiments. At the end of a pyrolysis experiment, the inner surface of the trap where volatiles normally condense was washed with 20 mM CH3SO3H (the eluent for ion chromatography in this study). Experiments at 3008C, where tar evolution was negligible, seemed to suggest that the Na (as well as Ca and Mg) volatilised from the coal particles during pyrolysis was found in the trap. There are two possible reasons for the difference between the volatilisation of AAEM species during pyrolysis in the wire-mesh reactor and the virtual total retention of AAEM species during combustion in the TGA: 1. the minimisation of extraparticle reactions during pyrolysis in the wire-mesh reactor has played an important role for the volatilisation of AAEM species, and/or

Fig. 6. Effects of ashing temperature (a) and (b) in O2 in a TGA and pyrolysis temperature (c) and (d) in He in a wire-mesh reactor on the volatilisation of AAEM species from the Loy Yang raw coal.

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2. the atmosphere surrounding the particle is critical for the volatilisation of AAEM species. Although the amount of coal sample used in the ashing experiments in TGA was generally less than 4 mg and coal particles were uniformly distributed in the Pt crucible as a thin layer, the AAEM species, having left a particle, could still react with the neighbouring particles. However, in the case of pyrolysis experiments in the wire-mesh reactor, the use of carrier gas would mean that the AAEM species were swept into the trap as soon as they were released from the pyrolysing char particles. However, this possibility may not be the dominating reason for the difference between the retention of AAEM species during pyrolysis in the wiremesh reactor and that during combustion in the TGA. This is because the volatilisation of AAEM species is not sensitive to changes in heating rate (see below), which in turn suggests that even intraparticle reactions do not seem to play an important role for the volatilisation of AAEM species. With the very same reasoning, it is believed that the possible entrainment of AAEM species by evolving volatiles, particularly at 1000 K s 21, is not the major reason for the volatilisation of AAEM species during pyrolysis. It therefore seems that the difference between the atmosphere surrounding the particles in the TGA during ashing (O2) and that in the wire-mesh reactor during pyrolysis (He) accounts for the corresponding difference in the volatilisation of AAEM species. An oxidising atmosphere has greatly helped to retain the AAEM species in the solid phase (also see below). In contrast to the release of larger aromatic ring systems (Fig. 2), the volatilisation of Na and Mg from the raw Loy Yang coal was not sensitive to changes in heating rate from 1 to 1000 K s 21 (Fig. 6). Temperature seemed to be the most important experimental factor controlling the volatilisation of Na and Mg; increasing temperature led to drastic increases in the volatilisation extent of both monovalent AAEM species (Na) and divalent AAEM species (Mg). At 12008C, about 70% of Na and over 35% of Mg in the raw coal were volatilised during pyrolysis. The volatilisation of Ca from the Ca-form coal sample also did not show any heating rate sensitivity, as is shown in Fig. 7a. Pyrolysing the Ca-form sample at 1 K s 21 or at 1000 K s 21 has led to the same extent of Ca volatilisation over the temperature range studied (Fig. 7a). The volatilisation of Ca seemed to show some distinct temperature regions. Between 3008C and 6008C, the increases in temperature did not seem to cause much further increases in the volatilisation of Ca. Between 6008C and 9008C, the increases in temperature caused corresponding increases in the volatilisation of Ca. Increasing temperature from 9008C to 10008C did not seem to enhance the volatilisation of Ca. However, the volatilisation of Ca was seen to accelerate at temperatures higher than 10008C. It is interesting to note that the tar yield and, to a lesser extent, char yield from the Ca-form coal (Fig. 7a) followed similar trends to the volatilisation of Ca over the temperatures between 6008C and

12008C. The synchronous spectra in Fig. 7b show the corresponding increase in the “yield” of larger aromatic ring systems with increasing temperature from 6008C to 7008C. Although the tar yield almost doubled from 10008C to 12008C, the corresponding increase in the “yield” of larger aromatic ring systems as seen with the synchronous spectroscopy did not seem to double. This may mean that the structural features of the tar have changed with increasing temperature; for example, the volatilised Ca may be associated with the tar and leads to changes in the fluorescent properties of the tar. The similarities among the spectra (Fig. 7b) of tars from the pyrolysis of the Ca-form sample from 700 up to 12008C do not seem to support this possibility. Therefore, the spectra in Fig. 7b are thought to indicate that the increases in the “yields” of larger aromatic ring systems are relatively small from 1100 to 12008C. Thus, the increases in tar yield from 1000 to 12008C are mainly due to the survival of (alkyl) substitutional groups and the smaller aromatic ring systems as a part of tar. The preliminary UV-absorption measurement on the tar samples does indicate a significant increase in, although not doubling, absorbance when the pyrolysis temperature was increased from 1100 to 12008C. Taken together, the data indicate that the pyrolysis at higher temperatures at 1000 K s 21 has somehow reduced the intraparticle cracking of the tar precursors. This will be further discussed below. In the case of Na-form coal sample, the tar yield and the volatilisation of Na (Fig. 8a) were all seen to increase rapidly at temperatures higher than 9008C. As was in the case of Ca-form coal sample, the drastic increases in tar yield were not accompanied by the corresponding increases in the “yields” of larger aromatic ring systems (Fig. 8b). On the contrary, the fluorescence intensity decreased slightly with increasing pyrolysis temperature from 1000 to 12008C. Preliminary UV-absorption measurement showed similar decreases in absorption intensity. The exact reasons for the apparent decreases remain unclear. Nevertheless, it is believed that, as was in the case of Ca-form sample, the volatilisation of Na might has reduced the intraparticle cracking of tar precursors which would otherwise lead to the release of more aliphatic tar precursors as light gaseous species. 3.3. Fates and roles of AAEM species during pyrolysis The introduction of ion-exchangeable cations, particularly Ca 21, is believed to increase the matrix densities of the coal [25]. However, we believe that the roles of AAEM species (Na, Mg and Ca) during the pyrolysis of the ionexchanged samples mainly have their origin in their transformation during pyrolysis. It is known that the carboxylates in coal will undergo decomposition at relatively low temperatures (even lower than 3008C, see Ref. [4]). It is speculated that, during this process in the absence of oxygen, some AAEM species associated with the –COO 2 groups at the outer particle surface may leave the particle

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435

Fig. 7. (a) Effect of temperature on the release of aromatic ring systems and the volatilisation of Ca during the pyrolysis of Ca-form coal sample at 1 K s 21 (W) and 1000 K s 21 (A). Corresponding tar (K) and char (S) yields from the pyrolysis at 1000 K s 21 are also shown for comparison. Holding time: 10 s for peak temperatures up to 10008C or 5 s for peak temperatures at 1100 and 12008C. (b) Effect of temperature on the constant energy (22800 cm 21) synchronous spectra of the resulting tars.

together with the –COO group if the bond between the –COO 2 group and the coal matrix was broken first. It should be pointed out here that the –COONa or (COO)2Ca groups in coal may behave quite differently from those in the simpler salts such as sodium acetate. The –COONa groups in coal are isolated from each other, whereas the –COONa groups in sodium acetate are bonded together through ionic force in a crystal. In the presence of oxygen such as during oxidation in TGA in this study, these AAEM species in coal become the active centres for the chemisorption of O2, leading to the formation of surface complexes. These surface complexes will bond the AAEM species tightly to the coal/ char matrix. The reaction scheme given here seems to be in agreement with the experimental data presented in Fig. 6, which indicates certain degrees of volatilisation of Na and Mg during pyrolysis at temperatures lower than 6008C and almost total retention during oxidation in TGA.

With the release of CO2, Ca originally associated with a –COO group in coal matrix may still be bonded to coal/ char matrix (–CM), …–COO–Ca–OOC–† 1 …–CM† ˆ …–COO–Ca–CM† 1 CO2 …1† …–COO–Ca–CM† 1 …–CM† ˆ …CM–Ca–CM† 1 CO2

…2†

or for Na, …–COO–Na† 1 …–CM† ˆ …CM–Na† 1 CO2

…3†

continuously serving as a virtual cross-linking point. For those AAEM species retained in the coal/char matrix which still contains significant amounts of substitutional groups, particularly oxygen-containing groups (e.g. phenolic groups), the AAEM species are likely to be bonded to these oxygen-containing groups. As the

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Fig. 8. (a) Effect of temperature on the release of aromatic ring systems and the volatilisation of Na during the pyrolysis of Na-form coal sample at 1000 K s 21. Corresponding tar (K) and char (S) yields are also shown for comparison. Holding time: 10 s for peak temperatures up to 10008C or 5 s for peak temperatures at 1100 and 12008C. (b) Effect of temperature on the constant energy (22800 cm 21) synchronous spectra of the resulting tars.

temperature is further increased, the newly formed Ca– CM or Na–CM bonds are not very stable and will be broken again to generate free radical sites …CM–Ca–CM† ˆ …–CM† 1 …–Ca–CM†

…4†

…–Ca–CM† ˆ …–CM† 1 Ca

…5†

…CM–Na† ˆ …–CM† 1 Na

…6†

…–CM† ˆ …–CM 0 † 1 gas

…7†

together with the release of oxygen-containing species or aliphatic materials. Some of the AAEM species may leave the particle. The AAEM species are also likely to be highly reactive and will recombine with the free radicals

generated to form more stable bond Ca–CM or Na–CM: …–CM 0 † 1 …–Ca–CM† ˆ …CM 0 –Ca–CM†

…8†

…–CM 0 † 1 Na ˆ …CM 0 –Na†

…9†

From reactions (4)–(6), the formation of a free Na involves the breakage of one CM–Na bond, whereas the formation of a free Ca requires the simultaneous breakage of two CM–Ca bonds. Our experimental data in Figs. 6–8 do show that Na was volatilised to a much larger extent than Ca or Mg under the same experimental conditions. The reaction schemes outlined above would imply that the CM–Na or CM–Ca bonds become progressively stronger with increasing temperature. Experiments indeed showed that the chemical forms of the AAEM species

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changed with increasing temperature. While the Na and Mg in chars prepared from the raw Loy Yang coal at lower pyrolysis temperatures could still be extracted with acids, the proportion of acid-extractable Mg in the char appeared to decrease drastically with increasing temperature, in broad agreement with the literature [26,43,44]. For the release of organic volatiles, the net effect of the repeated Ca–CM or Na–CM bond-forming and bondbreaking process described above is that tar precursor fragments are repeatedly linked to the char matrix through reactions involving free radicals and get thermally cracked. During this thermal cracking process, the more aliphatic parts in a tar fragment, including the smaller aromatic ring systems, will be released as gas. Some more aromatic units, especially the larger aromatic ring systems, may eventually become a part of char. Because the volatilisation of AAEM species (Figs. 6c,d and 7a) is not sensitive to the changes in heating rate, the Ca–CM and Na–CM bond stabilities are mainly affected by temperature. This in turn is in agreement with the fact that the “yields” of larger aromatic ring systems from the Ca-form coal sample are not sensitive to changes in heating rate (Fig. 5). With increasing temperature, the Ca–CM or Na–CM bond-forming or bond-breaking process will be progressively intensified, leading to high extents of volatilisation of AAEM species (Figs. 6–8). Particularly, pyrolysis at higher temperatures would result in the release of the majority of oxygen-containing species, reducing the sites for the AAEM to connect with the char matrix. The schemes outlined here do not exclude the possibility for the AAEM species to be released together with a tar fragment when the fragment is small enough to be released as volatiles. In fact, further studies are planned to ascertain the forms of AAEM species in the volatiles. When the coal particles are heated to very high temperatures (at 1000 K s 21 to 1000 or 12008C in Figs. 7 and 8), the majority of the polar groups and substitutional groups in the coal/char matrix would be released within a very short period of time. The volatilisation of AAEM species is thus intensified. With the volatilisation of AAEM species, the cross-linking density in the pyrolysing coal/char matrix due to the AAEM cations diminishes rapidly. The repeated bond-forming and bond-breaking process mentioned above is thus greatly shortened. The net result is that the tar precursors can now be released as a big fragment without loosing the more aliphatic components and/or the smaller aromatic ring systems, leading to the increased tar yield at high temperatures (Figs. 7a and 8a).

4. Conclusions 1. The release of the larger aromatic ring systems (“equivalently” larger than naphthalene) is affected by both experimental conditions (eg. heating rate and temperature) and the presence of AAEM species in the coal

2.

3.

4.

5.

437

samples. At temperatures between 600 and 9008C, the effect of heating rate (1–2000 K s 21) was stronger than that of peak temperature. The presence of AAEM species in the coal samples greatly hindered the release of the larger aromatic ring systems during pyrolysis. The AAEM species also reduced the effect of heating rate on the release of larger aromatic ring systems. However, the hindering effect was not proportional to the contents of AAEM species in the coal. In addition, the ion-exchange processes caused irreversible changes to coal structure. Significant proportions of the AAEM species in both raw coal and ion-exchanged coal samples were volatilised during pyrolysis even at temperatures as low as 3008C. The volatilisation of AAEM species was mainly determined by the bonding strength between the AAEM species and the coal/char matrix. Thus, temperature was the main experimental factor determining the volatilisation of the AAEM species. The effect of heating rate on the volatilisation of the AAEM species was not discernible within experimental error. The monovalent species (Na) was always volatilised to a much larger extent that the divalent species (Mg and Ca) under similar pyrolysis conditions. At high temperatures (900–12008C), the drastic volatilisation of Na (up to 80%) from the Na-form sample and of Ca (up to 40%) from the Ca-form sample was accompanied by larger increases in tar yield during the pyrolysis of the Na-form and Ca-form samples. The increases in tar yield were mainly due to the survival of the more aliphatic components and/or the smaller aromatic ring systems as a part of tar rather than as gas. The fates and roles of the AAEM species during pyrolysis are thought to be related to their transformation during pyrolysis. The AAEM species might have been involved in a repeated bond-forming and bond-breaking process between the AAEM species and the coal/char matrix. During this process, tar precursors were repeatedly linked to the coal/char matrix and were thermally cracked. Some of the more aliphatic components and/or smaller aromatic ring systems in a tar precursor were cracked to gas and some of the larger aromatic ring systems were charred.

Acknowledgements The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for Elean Power from Lignite, which is established and supported under the Australian Government’s Cooperative Research Centres program. The authors also gratefully thank Dr J.R. Gibbins of Imperial College for his help in setting up the reactor system, Dr P.J. Redlich of Monash University for his help in preparing the ion-exchanged samples, and Dr Albert Mau for the access to the luminescence spectrometer.

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