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Energy & Fuels 1999, 13, 748-755
Effects of Heating Rate and Ion-Exchangeable Cations on the Pyrolysis Yields from a Victorian Brown Coal Chirag Sathe, Yanyuan Pang, and Chun-Zhu Li* CRC for New Technologies for Power Generation from Low-Rank Coal, Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia Received November 3, 1998
A Victorian low-rank coal (Loy Yang) was acid-washed and ion-exchanged with Na and Ca to prepare the H-form, Na-form, and Ca-form coal samples. Two more H-form samples were also prepared by rewashing the Na-form and Ca-form samples with acid. These coal samples were pyrolyzed in a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimized. The ion-exchanged coal samples were found to give very different tar yields from those of the raw coal samples. While the tar yields from the pyrolysis of the raw and H-form coal samples were observed to be very sensitive to changes in heating rate, the tar yields from the Ca-form and Na-form samples showed little heating rate sensitivity. Unlike higher rank coals studied previously, the tar yields from the pyrolysis of the raw coal and the H-form coal samples at 600 °C were found to increase much more than the corresponding increases in the total volatile yields as the heating rate was increased from 1 to 2000 K s-1. Reexchanging Na in the Na-form sample and Ca in the Ca-form sample with H confirmed the effects of Na and Ca but also suggested that the irreversible structural changes taking place during ion-exchange, possibly including the loss of humic materials and the physical reconfiguration of the macromolecular network, should also be considered to ascertain the effects of ion-exchangeable cations during pyrolysis. The heating rate sensitivity of pyrolysis yields is believed to be at least partly related to the presence of carboxyl/carboxylate groups and other bulky substitution groups in the coal as well as the rapid pressure buildup within the particles. The major roles of ion-exchangeable cations during pyrolysis are also discussed in the paper.
Introduction Victorian brown coals in the Latrobe Valley, Victoria, Australia, form a family of low-rank coals with unique physical and chemical properties. The energy from these brown coals has long been the mainstay of the Victorian economy, providing low-cost electricity to the state. As the initial steps of many current and potential thermal processes for utilization of these brown coals, pyrolysis of the brown coals has been the subject of many studies, and excellent comprehensive reviews may be found in the literature.1 For example, pyrolyzing a Loy Yang coal sample in a fluidized-bed reactor, Tyler2 found the composition and yields of tar and total volatiles to change significantly with temperature. The tar yields from the pyrolysis experiments in the fluidized-bed reactor were found to be much higher than those from the standard Fischer Assay. Brockway and Stacy3,4 found that about one-half of the total volatiles from the pyrolysis in a wire-mesh reactor consisted of simple (1) Jones, J. C. Pyrolysis. In The Science of Victorian Brown Coal Durie, R. A., Ed.; Butterworth-Heinemann: Oxford, U.K., 1991; Chapter 9. (2) Tyler, R. J. Fuel 1979, 58, 680-686. (3) Brockway, D. J.; Stacy, W. O. Methods for the determination of devolatilisation products from brown coal using mass spectrometry. Report No. SO/82/52; State Electricity Commission of Victoria: Australia, 1982. (4) Brockway, D. J.; Stacy, W. O. Devolatilisation of Victorian brown coal. Report No. SO/82/53; State Electricity Commission of Victoria: Australia, 1982.
gaseous oxygen-containing compounds. Cliff and coworkers5 obtained similar results when a Yallourn coal sample was pyrolyzed in a fluidized-bed reactor and in a shock tube reactor, although a different temperature dependence was noted apparently due to the vast difference in residence times used in two reactors. Yeasmin and co-workers6 recently studied the product distribution from the pyrolysis of a Yallourn coal sample in a pressurized drop-tube furnace. The effects of residence time were found to depend strongly on other reaction conditions, such as temperature and pressure. Although the ash yields from this unique family of low-rank coals are generally very low (usually < 2 wt %), the ash-forming constituents in the coals are mostly in the form of carboxylates. The metallic species such as Na, Mg, and Ca in the coals can be easily removed by acid-washing and ion-exchanged back to coal mainly as carboxylates. As an important part of the brown coal, the roles of these cations during pyrolysis have been the subject of many studies.1 For example, Tyler and Schafer7 studied the pyrolysis behavior of ion-exchanged brown coal samples in a fluidized-bed reactor and found that both tar and volatile yields decreased with cation (5) Cliff, D. I.; Doolan, K. R.; Mackie, J. C.; Tyler, R. J. Fuel 1984, 63, 394-400. (6) Yeasmin, H.; Mathews, J. F.; Ouyang, S. Proceedings of the 14th Annual International Pittsburgh Coal Conference (CD-ROM), Taiyuan, China, Sept. 23-27, 1997. (7) Tyler, R. J.; Schafer, H. N. S. Fuel 1980, 59, 487-494.
10.1021/ef980240o CCC: $18.00 © 1999 American Chemical Society Published on Web 03/31/1999
Pyrolysis Yields from a Victorian Brown Coal
Energy & Fuels, Vol. 13, No. 3, 1999 749
Table 1. Properties of Loy Yang Raw Coal Sample Studied ash, wt % (db)
volatile matter, wt %
C, wt %
H, wt %
N, wt %
S (total), wt %
Cl, wt %
O (by diff), wt %
1.0
51.5
68.5
4.8
0.55
0.32
0.11
25.7
content. Wornat and Nelson8,9 found that the presence of cations could have profound effects on the volatile composition. Vernaglia and co-workers10 recently found the aromatic/aliphatic composition of tar to change with the type of cation (H, Na, and Ca). Despite the intensive studies carried out in the past, the thermal decomposition mechanisms and kinetics during the pyrolysis of these coals still remain imperfectly understood. The roles of ion-exchangeable cations during pyrolysis are still unclear. This is partly due to the fact that some important experimental factors influencing the thermal decomposition have not been studied separately. For example, heating rate, peak temperature, holding time at peak temperature, etc., cannot be varied independently over a wide range for many reactors used in previous studies. Furthermore, the determination of primary pyrolysis yields, essential for the understanding of reaction mechanisms and kinetics during pyrolysis, is often complicated by the secondary reactions of the evolved volatiles. Considering the perceived high reactivities of the primary pyrolysis products from these low-rank coals, the elimination of the extraparticle secondary reactions is particularly important for the better understanding of mechanisms and kinetics of thermal decomposition of these low-rank coals. In particular, to the best of our knowledge, there has been no systematic report on the combined effects of heating rate and ion-exchangeable cations on the pyrolysis yields from these Victorian low-rank coals. The purpose of this study is to experimentally investigate the effects of pyrolysis conditions (particularly heating rate) and the presence of ion-exchangeable cations (H, Na, and Ca) on the product distribution from the pyrolysis of a Loy Yang coal sample. A wire-mesh reactor is used where each parameter can be varied independently while the extraparticle secondary reactions of volatiles are minimized. Experimental Section Coal Preparation. 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 (
