Effects of Dewatering on the Pyrolysis and Gasification Reactivity of

Oct 18, 2006 - Attempts to produce blast furnace coke from Victorian brown coal. 3. Hydrothermally dewatered and acid washed coal as a blast furnace c...
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Energy & Fuels 2007, 21, 399-404

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Effects of Dewatering on the Pyrolysis and Gasification Reactivity of Victorian Brown Coal† Cai Zeng,*,‡ Sam Clayton,‡ Hongwei Wu,‡,§ Jun-ichiro Hayashi,| and Chun-Zhu Li‡ CooperatiVe Research Centre for Clean Power from Lignite, Department of Chemical Engineering, Monash UniVersity, PO Box 36, Victoria 3800, Australia, and Centre for AdVanced Research of Energy ConVersion Materials, Hokkaido UniVersity, N13-W8, Kita-ku, Sapporo 060-8628, Japan ReceiVed August 20, 2006. ReVised Manuscript ReceiVed September 4, 2006

This study aims to investigate the effects of dewatering on the primary pyrolysis behavior and gasification reactivity of Victorian brown coal. Dewatered brown coal samples were prepared by dewatering/drying of the brown coal in steam and by means of mechanical thermal expression (MTE). The primary pyrolysis behavior and gasification reactivity of these dewatered coal/char samples were evaluated in a wire-mesh reactor and in a thermogravimetric analyzer (TGA), respectively. It was found that changes in the physical structure of the brown coal and the loss of NaCl during dewatering at low temperatures did not significantly affect the char and tar yields. However, dewatering/drying at temperatures > 250 °C not only reduced the volatile yield but also shifted the normalized derivative thermogravimetric (DTG) peak to a higher temperature during pyrolysis in the TGA under constant heating rates. The analysis of the pyrolysis kinetics of dewatered brown coal samples using a distributed activation energy model indicates that dewatering/drying at elevated temperatures partially degraded the brown coal structure, leading to stronger covalent bonds formed during pyrolysis that require higher temperature to break down. It was also found that changes in pore structure and chemical structure in the brown coal during dewatering had little effect on the char reactivity. However, the loss of inorganics (mainly NaCl) during dewatering led to decreases in char reactivity.

Introduction Coal is and will continue to be an important energy source in the world in the foreseeable future. Coal gasification is gaining increasing acceptance as an environmentally friendly way to generate electricity, heat, and other high-value chemicals. Coal gasification is particularly suitable for low-rank coals such as Victorian brown coal due to their high gasification reactivity.1,2 However, in order to achieve high thermal efficiency for reducing greenhouse gas emissions, dewatering will have to be an integral part of future advanced gasification-based technologies using the brown coal of high moisture content (up to 70 wt %).3,4 A better understanding of the effects of dewatering/ drying on the pyrolysis and gasification reactivity of brown coal will be essential for the successful commissioning of efficient † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * Corresponding author. Present address: Coal Polygeneration Technologies Lab, GE China Technology Center, 1800 Cailun Road, Zhangjiang High-tech Park, Pudong, Shanghai, 201203, China. E-mail: Cai.Zeng@ ge.com. ‡ Monash University. § Department of Chemical Engineering, Curtin University of Technology, GPO, Box U1987, Perth WA 6845, Australia. | Hokkaido University. (1) Li, C.-Z. Introduction. In AdVances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Oxford, UK, 2004; Chapter 1, pp 1-10. (2) Tomita, A.; Ohtsuka, Y. Gasification and combustion of brown coal. In AdVances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Oxford, UK, 2004; Chapter 5, pp 223-285. (3) Allardice, D. J. The water in brown coal. In The Science of Victorian Brown Coal; Durie, R. A., Ed.; Butterworth-Heinemann: Oxford, UK, 1991; Chapter 3, pp 103-150. (4) Allardice, D. J.; Chaffee, A. L.; Jackson, W. R.; Marshall, M. Water in brown coal and its removal. In AdVances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Oxford, UK, 2004; Chapter 3, pp 85-133.

dewatering technologies and advanced gasification-based technologies using dewatered brown coal. Dewatering can be carried out physically by applying force, thermally by applying heat, or by combining both force and heat in a single process such as mechanical thermal expression (MTE).3-6 Depending on the dewatering conditions, significant changes to the brown coal structure and properties may take place in at least three ways during the dewatering processes.3-9 First, the chemical structure of the organic matter in the brown coal is likely to be altered. The oxygen-containing functional groups in the brown coal are very susceptible to thermal pretreatment during dewatering. It was reported that the thermal decomposition of carboxyl groups in Victorian brown coal could commence at temperatures as low as 150 °C.10 Dewatering/ drying is often carried out with coal particles being surrounded by steam or hot water, which is able to depolymerize the coal macromolecular network by cleaving weak covalent bonds such as ether linkages that are thermally stable but hydrothermally (5) McIntosh, M. J.; Huynh, D. Q. In Pre-drying of high moisture content Australian brown coal for power generation. Proceedings of the Coal Prep and Aggregate Processing Conference, Lexington, Kentucky, May 2005. (6) Huynh, D. Q.; McIntosh, M. J. In Technologies for reducing greenhouse gases emissions for lignite. Proceedings of the 2nd Annual Conference Optimising Capacity and Efficiency of Coal Fired Power Stations, Sydney, Australia, September 2004. (7) Favas, G.; Chaffee, A. L. In Steam drying-changes in lignite structure at different drying temperatures. Proceedings of 18th Annual International Pittsburgh Coal Conference, Newcastle, Australia, December 2001. (8) Bongers, G. D.; Jackson, W. R.; Woskoboenko, F. Fuel Process. Technol. 1998, 57, 41. (9) Bongers, G. D.; Jackson, W. R.; Woskoboenko, F. Fuel Process. Technol. 2000, 64, 13. (10) Jones, J. C. Pyrolysis. In The Science of Victorian brown coal; Durie, R. A., Ed.; Butterworth-Heinemann: Oxford, UK, 1991; Chapter 9, pp 465516.

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unstable at temperatures > 250 °C.11-16 Second, the physical structure of the brown coal is likely to be altered. A high content of oxygen (around 22-26%) is present in Victorian brown coal in a wide variety of functional groups. The presence of oxygencontaining functional groups makes noncovalent bonding (especially hydrogen bonding) an important feature of brown coal structure. The changes in the concentration and strength distribution of noncovalent bonds due to the chemical changes mentioned above can lead to the rearrangement of the macromolecular structure of the coal. Furthermore, significant reduction in pore volumes, particularly the macropore structure of the brown coal, occurs when external mechanical forces are applied during the MTE process.4,17-19 Third, the chemical forms and concentrations of inherent alkali and alkaline earth metallic (AAEM) species are also likely to be altered. For example, it was found that the MTE process also beneficially removed significant amounts of Na (up to 40% and mainly in the form of NaCl) and relatively small amounts of Ca, Mg, and K.17,18,20 The reduction of these AAEM species can reduce serious fouling and slagging problems in the current brown coal fired power stations and in the future plants using fluidized-bed-based gasification technologies. Little work has been done to evaluate how the changes in physicochemical properties of brown coal during dewatering would affect the subsequent pyrolysis and gasification reactivity. This study aims to investigate the fundamental mechanisms underlying the possible effects of dewatering on the pyrolysis and gasification reactivity of Victorian brown coal. MTEdewatered and steam-treated brown coal samples were used for this purpose. A wire-mesh reactor (WMR) capable of multistep heating and minimizing extra-particle secondary reactions of the evolved volatiles was used to evaluate the primary pyrolysis behavior of dewatered brown coal samples. The pyrolysis kinetics of dewatered brown coal samples were also measured in a thermogravimetric analyzer (TGA) and modeled using a distributed activation energy model (DAEM). The char gasification reactivity was evaluated using a TGA at 400 °C in air. Experimental Description Coal Samples. Two “as-mined” Loy Yang brown coal samples from the Latrobe Valley, Victoria, Australia, were used to prepare the MTE-dewatered brown coal samples and the steam-treated brown coal samples,7 respectively, in this study. Their properties are shown in Table 1. Sample A and sample B were used to prepare MTE-dewatered brown coal samples and steam-treated brown coal, respectively. MTE experiments were conducted using an “axial” batch MTE cell previously described.17-19 The detailed operation of this MTE (11) Siskin, M.; Katritzky, A. R. Science 1991, 254, 231. (12) Siskin, M.; Katritzky, A. R.; Balasubramanian, M. Fuel 1993, 72, 1435. (13) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494. (14) Khan, M. R.; Chen, W. Y.; Suuberg, E. Energy Fuels 1989, 3, 223. (15) Kashimura, N.; Hayashi, J.-i.; Chiba, T. Fuel 2004, 83, 353. (16) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III Energy Fuels 1997, 11, 1278. (17) Favas, G.; Kealy, T.; Chaffee, A. L.; Tiu, C. In Effect of kneading coal prior to mechanical thermal expression processing. Proceedings of the 18th Annual International Pittsburgh Coal Conference, Newcastle, Australia, December 2001. (18) Kealy, T.; Favas, G.; Chaffee, A. L.; Tiu, C. In Lignite upgrading by mechanical-thermal expression (MTE). Proceedings of the 6th World Congress of Chemical Engineering, Melbourne, Australia, September 2001. (19) Guo, J. Hydrothermal-mechanical dewatering of brown coal. Ph.D. Thesis, Monash University, Melbourne, Australia, 2002. (20) Qi, Y.; Chaffee, A. L. Proceedings of the 11th International Conference on Coal Science. San Francisco, CA, Sep 30-Oct 5, 2001.

Zeng et al. Table 1. Properties of the Loy Yang Brown Coal Samples Used in This Study coal sample

sample A

sample B

moisture content, wt % (wb) ash, wt % (db) volatile matter, wt % (db) C, wt % (daf) H, wt % (daf) N, wt % (daf) S, wt % (daf) O (by diff.), wt % (daf)

66.4 3.4 50.6 68.3 5.8 0.7 0.5 24.7

59.8 1.0 51.5 69.0 4.7 0.6 0.3 25.4

unit can be found elsewhere.17-19 Briefly, 100 g of raw coal was placed in the cell, filled with approximately 100 g of distilled water, sealed, and heated up to the desired temperature (100, 150, 180, and 200 °C). The temperature was maintained for 20 min to ensure that the heat was well distributed within the brown coal bed. An “Instron 5569” universal machine then pressed the sample until the desired effective pressing pressure (2, 6, 12, and 18 MPa with the consideration of saturated steam vapor pressures at the processing temperatures) was reached. The effective pressing pressure is the net pressure between applied mechanical pressure from the machine and saturated steam vapor pressure. After treatment, the sample was allowed to cool to room temperature and was collected for weight measurement. Care was taken to minimize evaporation during the cooling period. Some MTE-dewatered brown coal samples were also prepared under the conditions of 6 MPa of effective pressing pressure with a pressing temperature from 230 °C up to 300 °C. However, these samples were only held for 10 min at the operating temperature due to the difficulty of sealing the cell at high steam saturation pressures and increased dewatering temperatures. The experimental procedures for the preparation of the set of steam-treated coal samples can be found elsewhere.7 Briefly, 25 g of as-mined Loy Yang brown coal was steam-treated at saturation pressure for a range of temperatures (130-350 °C) in a batch autoclave for 30 min. The MTE-dewatered and steam-treated brown coal samples were each milled using an agate pestle motor and sieved to a obtain sample with particle sizes from 106 to 150 µm for the subsequent pyrolysis and reactivity measurement experiments. Pyrolysis. A wire-mesh reactor similar to that described by Gibbins and co-workers21,22 and Li and co-workers23,24 was used for pyrolysis. The operation procedures for the wire-mesh reactor and quantification of pyrolysis yields (char and tar yields) were outlined previously.25,26 The TGA capable of multistep heating was also used for measuring the pyrolysis kinetics. In order to minimize buoyancy effects, about 5 mg of sample was used in each pyrolysis experiment. The sample was held at ambient temperature for 60 min to achieve an oxygen-free environment in a stream of nitrogen (purity > 99.999%) at a flow rate of 50 mL min-1. The sample was then heated up at 20 °C min-1 from ambient to 105 °C and held for 30 min to remove the inherent moisture ( 250 °C, MTE dewatering increased the char yield. It should be noted that increasing processing temperature during the MTE process also enhanced the loss of inorganic (mainly water-soluble inorganics such as NaCl) and organic matters. The loss of NaCl during the dewatering was believed to have negligible effects on the pyrolysis yield. In fact, heating a set of NaCl-loaded brown coal at 1000 °C s-1 to 900 °C with 0 s of holding time was not found to significantly affect the pyrolysis yields.32 The increased loss of organic matters at elevated MTE processing temperatures is also supported by the decrease in the atomic ratio of O/C of MTE-dewatered brown coal samples at temperatures > 250 °C, as shown in Figure 3, indicating the enhanced thermal decomposition and loss of oxygen-containing functional groups during MTE treatment at elevated temperatures.

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Figure 3. Effects of processing temperature on the O/C atomic ratio of the MTE-dewatered brown coal samples.

By considering the loss of organic matter during MTE dewatering, the pyrolysis yields in Figure 2a were converted into the pyrolysis yields shown on the basis of coal before MTE dewatering in Figure 2b. The data in Figure 2b indicate that MTE dewatering (temperatures up to 300 °C) had negligible effect on the overall char yields; while tar yield slightly decreased at elevated temperatures (>250 °C). Figure 2b also shows similar yields of the steam-treated coal samples.32 It is clear that the dewatering temperature experienced by coal is more important than the way the coal was treated (MTE versus pretreatment in steam) in governing the pyrolysis behavior of the resultant coal. The decrease in tar yield at >250 °C was mainly due to the hydrolysis and/or decomposition of carboxylates and other oxygen-containing functional groups (such as esters and ethers), which enhanced the thermal decomposition of the brown coal during dewatering in the presence of highpressure steam or hot water and enhanced the cross-linking reactions during the subsequent pyrolysis of the dewatered brown coal.32 Both of these types of reactions could lead to decreases in the observed tar yield. In fact, UV-fluorescence spectroscopy of tars also revealed that at temperatures < 350 °C the decease in tar yield was mainly due to the loss of aliphatic and oxygen-containing functional groups from the tar precursors.32 However, the dewatering at temperatures > 350 °C could result in significant decreases in the yields of large aromatic ring systems mainly as a result of major changes in the coal structure during dewatering, which enhanced the cross-linking reactions during subsequent pyrolysis.32 Kinetic Analysis of Dewatered Brown Coal Samples. Figure 4 shows the effective volatile yields of steam-treated and MTE-dewatered brown coal samples pyrolyzed at 50 °C min-1 to 700 °C with 0 s of holding time in the TGA. As expected, the effective volatile yield decreased with increasing pretreatment temperature because the thermal decomposition during MTE or steam treatment resulted in the loss of some volatiles. The DTGs of steam-treated and MTE-dewatered brown coal samples are shown in Figure 5a and b, respectively. To facilitate comparison, the DTGs of MTE-dewatered and steam-treated brown coal samples were normalized by dividing by the initial weight (dry basis) of the coal sample used in the experiment. It is noted that pretreatment temperatures > 250 °C not only reduced the normalized DTG peak intensity but also shifted the peak to higher temperatures, indicating a change (32) Zeng, C.; Favas, G.; Wu, H.; Chaffee, A. L.; Hayashi, J-i.; Li, C.Z. Energy Fuels 2006, 20, 281.

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Figure 4. Effects of pretreatment temperature on the effective volatile yield of steam-treated brown coal samples and MTE-dewatered brown coal samples.

Figure 5. Normalized DTG of (a) steam-treated coal samples; (b) MTE-dewatered brown coal samples pyrolyzed at 50 °C min-1 to 700 °C in the TGA.

in the brown coal structure. The loss of coal organic matter during pretreatment, which was likely associated with the coal matrix via weak bonds, appeared to mainly affect the pyrolysisnormalized DTG at temperatures lower than 500 °C. The pyrolysis kinetics of these steam-treated and MTEdewatered coal samples under three constant heating rates were further analyzed using the integral method of DAEM. Figure 6a shows the f(E) of the set of steam-treated coal samples. The intensities of f(E) curves at high activation energies gradually increased with increasing steam pretreatment temperature. For the steam pretreatment at 350 °C, another peak appeared at 320 kJ mol-1. As f(E) dE represents the fraction of potential volatile loss which has an activation energy between E and E + dE,33,34 the data in Figure 6a seem to indicate that much stronger

Effects of Dewatering on Victorian Brown Coal

Figure 6. Plots of the f(E) curves for (a) steam-pretreated brown coal samples and (b) MTE-dewatered brown coal samples.

covalent bonds and/or their precursors have been formed during the pyrolysis of the coal steam-treated at elevated temperatures. A similar trend in f(E) was also found for the MTE-dewatered brown coal samples prepared under a constant effective processing pressure of 6 MPa as shown in Figure 6b. It is known that the presence of high-pressure steam or hot water during dewatering can not only reduce the cross-linking but also cleave the weak covalent bonds such as ether linkages. Therefore, it is unlikely that the stronger covalent bonds as indicated from Figure 6 were formed during dewatering. For the same reasoning outlined earlier,15,32 it is possible that highpressure steam or hot water hydrolyzed the coal macromolecular network during dewatering, resulting in increased hydroxyls in the dewatered coal samples. During pyrolysis, the dehydration among hydroxyls would form new cross-links which are stronger bonds that might require higher temperature to break down. The description given here seems to provide a plausible explanation of why the intensity of f(E) curves at high activation energies gradually increased with increasing dewatering temperatures. Effects of Dewatering on the Char Gasification Reactivity. Figure 7 shows the reactivities of chars from the pyrolysis of the MTE-dewatered brown coal samples in the wire-mesh reactor. The data in Figure 7a and b show a slight decrease in the char reactivity of the MTE-dewatered brown coal samples compared with that of the raw coal. However, varying MTE processing conditions did not significantly affect the char reactivity. Similar results were also found for the reactivity of chars from the pyrolysis of the MTE-dewatered brown coal samples in the TGA at 40 °C min-1 to 400 °C with 5 min of holding time. Previous work17-19 found that increasing processing temperature and applied mechanical pressure over the similar (33) Anthony, D. B.; Howard, J. B. AICHE J. 1976, 22, 625. (34) Hayashi, J-i.; Miura, K. Pyrolysis of Victorian brown coal. In AdVances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Oxford, UK, 2004; Chapter 4, pp 134-222.

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Figure 7. Specific char reactivity in air at 400 °C as a function of (a) MTE processing temperature under constant effective processing pressure of 6 MPa and (b) MTE effective processing pressure under constant temperature of 150 °C. The chars were prepared from the pyrolysis of MTE-dewatered brown coal samples at 1000 °C s-1 to 900 °C with 0 s of holding time in the wire-mesh reactor.

ranges to this study significantly reduced the internal pore structure of the MTE-dewatered brown coal samples. Therefore, the data in Figure 7 clearly indicate that the physical changes in the brown coal during the MTE process under the conditions studied have not taken place to such a significant extent as to alter the char gasification reactivity of the MTE-dewatered brown coal samples. The decrease in char reactivity from raw coal to MTEdewatered brown coals may be due to the removal of AAEM species and/or structural changes during dewatering. Further experiments were carried out to investigate whether the changes in coal structure, particularly the loss of functional groups, during dewatering would affect the reactivity of char from the pyrolysis of thermally pretreated coal samples at a higher temperature. The raw coal was thermally preheated in helium at a range of pretreatment temperatures from 100 to 450 °C with 30 min of holding time in the wire-mesh reactor. The coal sample was then further in situ heated up to 900 °C at a heating rate of 1000 °C s-1 with 0 s of holding time. The char reactivities from the pyrolysis of thermally pretreated raw coal samples are shown in Figure 8. It can be seen that preheating of the brown coal up to 450 °C with 30 min of holding time did not significantly affect the char reactivity, indicating that the changes in chemical structure at low pretreatment temperatures had negligible effects on the char reactivity. It was generally believed that the strong affinity between Na+ and Cl- loaded into char would not enable Na to interact with char to form the necessary catalytically active species during gasification.35 However, a recent study29 has revealed that it is (35) Takarada, T.; Nabatame, T.; Ohtsuka, Y.; Tomita, A. Energy Fuels 1987, 1, 308.

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Figure 8. Effects of pretreatment temperature on the specific char reactivity in air at 400 °C. The char was prepared from the in-situ pyrolysis of the thermally pretreated Loy Yang brown coal sample at 1000 °C s-1 to 900 °C with 0 s of holding time in the wire-mesh reactor.

the chemical forms of catalysts in char rather than those in coal that govern the catalytic reactivity of the catalysts during gasification. In other words, by controlling the conditions to facilitate the preferential release of Cl, Na in the form of NaCl in coal could become an active catalyst during char gasification. It is known that AAEM species would not volatilize during the oxidation of char with air at 400 °C during the reactivity measurement.36 On the other hand, during char oxidation at 400 °C in air, the Cl in the char could be preferentially released to allow Na to interact favorably with char.37 Therefore, the higher Na content in the char (even if the Na is in the form of NaCl in the char) will result in higher char gasification reactivity. As significant amounts of Na in the coal are removed (up to 40% and mainly in the form of NaCl) during the MTE process,17-20 it is reasonable to conclude that the decrease in the char reactivity from raw coal to MTE-dewatered brown coal samples is mainly due to the loss of Na during dewatering. It should be noted that the Na concentration (mainly NaCl) in the brown coal was diluted when 100 g of distilled water was mixed with 100 g of brown coal to prepare a water-coal slurry to be used in the MTE process (see the Experimental Description section). With this dilution water, the differences in the amount of water removed among different MTE conditions were relatively small. Consequently, the removal of additional water from the brown coal with intensifying MTE conditions would not lead to significant decrease in the level of Na between the MTEdewatered brown coal samples. The similar content of inorganics in the MTE-dewatered brown coal samples might be the reason for the similarity in the reactivity of chars from the MTEdewatered brown coal samples as shown in Figure 7. Conclusions The main conclusions from this study are as follows: • MTE dewatering under externally applied pressure up to 18 MPa at 150 °C did not significantly affect the pyrolysis (36) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427. (37) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143.

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yields. Thus, it can be concluded that the changes in pore structure of the brown coal associated with dewatering have negligible effects on the pyrolysis yields. • Dewatering at temperatures > 250 °C enhanced the loss of inorganic and organic matter, resulting in a decrease in tar yield. Moreover, dewatering/drying at >250 °C not only reduced the volatile yield but also shifted the normalized DTG peak to a higher temperature during pyrolysis in the TGA under constant heating rates. The analysis of the pyrolysis kinetics of dewatered brown coal samples using a distributed activation energy model indicates that dewatering/drying at elevated temperatures partially degraded the brown coal structure, leading to stronger covalent bonds formed during pyrolysis that require higher temperature to break down. • Under the range of conditions studied, varying MTE conditions did not significantly alter the char reactivity, indicating that the physical changes in pore structure during dewatering have little effect on char reactivity. • Thermal pretreatment of the raw coal up to 450 °C with 30 min of holding time did not significantly affect the char reactivity, indicating that the changes in chemical structure at dewatering temperatures have negligible effects on the reactivity of the char from the pyrolysis of pretreated coal samples at higher temperature such as 900 °C. It is believed that char reactivity decreased with increasing dewatering/drying temperature, mainly due to the loss of water-soluble inorganics (e.g., NaCl) during dewatering.

Acknowledgment. The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for Clean Power from Lignite, which is established and supported under the Australian government’s Cooperative Research Centres program. The authors also gratefully thank Dr. George Favas and Dr. Alan L. Chaffee for the provision of the steam-treated brown coal samples. The assistance and discussion during the preparation of MTE-dewatered brown coal samples from Professor Carlos Tiu, Dr. Scholes Oliver, and Mr. Ross Ellingham are also gratefully appreciated.

Nomenclature a ) constant heating rate (°C min-1) E ) activation energy (kJ mol-1) f(E) ) distribution function of activation energy (mol kJ-1) k0 ) frequency factor (s-1) R ) gas constant, 8.3145 (J mol-1 K-1) Rc ) specific reactivity normalized to mass at time t ) reaction time (s) T ) temperature (°C) V ) amount of volatile yield evolved up to time t (kg kg-1) V* ) effective volatile yield at 700 °C by heating at 50 °C min-1 (kg kg-1) Wc ) weight of the char at any given time during reaction with air (mg) EF060404Y