Energy & Fuels 2003, 17, 929-933
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Low-Temperature Oxidation of Bituminous Coal and the Influence of Moisture Alan L. McCutcheon* and Michael A. Wilson Deans Unit, University of Western Sydney, Hawkesbury Campus Building H4, Locked Bag 1797, Penrith South DC NSW 1797, Australia Received November 19, 2002
The oxidation of three bituminous coals, ranging from low rank (84.3 wt % carbon) to high rank (88.3 wt % carbon) in the presence and absence of water vapor, has been studied using X-ray photoelectron spectroscopy (XPS) and gravimetric techniques. The effects of wetting the coal surface with liquid water, following oxidation with humid air, have also been studied. On oxidation, the sample weight increased as time increased up to at least 40 h. XPS measurements show that, when the coal was oxidized in the presence of dry oxygen, only minor amounts of the CdO groups were formed; however, the amount of these and the C-O groups increased when the coal surface was wetted. The water-vapor adsorption properties of unoxidized coals and both types of oxidized coals were also studied. Oxidation with dry oxygen caused a decrease in watervapor uptake, compared to that observed for an unoxidized coal. However, if the surface was wetted, the surface water catalyzed both oxygen and water uptake. Thus, two types of water have been recognized, and only that which is delivered by surface wetting is capable of catalysis. Catalytic water is probably chemisorbed and other water is physically adsorbed.
Introduction Oxidation of coal in stockpiles can significantly affect its storage and its potential use. For example, heat generated within coal stockpiles due to oxidation can increase the temperature, typically to 50 °C, depending on the coal type and the ambient conditions, and this situation can cause self-ignition. Even oxidized coals that do not reach this temperature have limited use. Small amounts of additional oxygen functionality reduce the capacity of a coal to form mesophase and, hence, strong cokes.1,2 When coals are oxidized by atmospheric oxygen, Wang et al.3 observed a high initial rate of oxygen consumption, which then decreased rapidly to attain a quasi steady state. It was suggested that the oxidation rate is controlled by the diffusion rate of oxygen molecules through the micropores (diameter of pores < 2 nm) and decreases as the number of available sites is reduced over time. It has been shown4,5 in this process that there is an increase in the surface density of hydrophilic sites such as CdO and C-O-R. For example, Rossi et al.4 used X-ray photoelectron spectroscopy (XPS) to show that a coal exposed to water vapor and oxygen for 20 days had an increase in surface oxygen content, from 25.51 at. % for the fresh coal sample to 28.33 at. %. * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Fredericks, P. M.; Warbrooke, P.; Wilson, M. A. Org. Geochem. 1983, 5, 89-98. (2) Van Krevelen, D. W. Coal, 3rd ed. Elsevier: Amsterdam, The Netherlands, 1993. (3) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Energy Fuels 1999, 13, 1173-1179. (4) Rossi, P. F.; Vettor, A.; Milana, G.; Busca, G. Colloids Surf. 1991, 54, 297-311. (5) Groszek, A. J.; Templer, C. E. Fuel 1988, 67, 1658-1661.
Iskhakov6 reported that the aforementioned water was important in increasing oxidation, and this effect was dependent on the equilibrated water content,7 up to a critical value beyond which the oxygen absorption rate decreases. It is well-known that different rank coals adsorb different amounts of water;2 therefore, the critical moisture content is also rank dependent, and, hence, the effects of water are also rank dependent. One explanation for the critical water content7 is that two types of water exist, namely, chemically bound and free water. It has been suggested that chemically bound water activates the oxidation process and free water retards it.7 Below the critical water content, chemically bound water is forming; above it, only free water is left on the surface. This explanation seems reasonable, because it has been well established that there is a relationship between the density of hydrophilic sites on the coal surface and the amount of water adsorbed at low watervapor pressures, i.e., below the critical value. Allardice and Evans8 reported a correlation between monolayer water coverage and the number of hydrophilic functional groups on the coal surface. They found that, as coal samples decrease in rank and the density of the hydrophilic sites increases, the monolayer water coverage on the coal had a corresponding increase. Thus, it might be concluded that chemically bound water is associated with hydrophilic functional groups through hydrogen bonds and free water adsorbed at other sites is represented by weaker, noncovalent interactions. (6) Iskhakov, Kh. A. Solid Fuel Chem. 1990, 24 (2), 17-21. (7) Panaseiko, S. P. Solid Fuel Chem. 1974b, 8, 26-30. (8) Allardice, D. J.; Evans, D. G. Fuel 1971, 50, 236-252.
10.1021/ef0202788 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/15/2003
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Energy & Fuels, Vol. 17, No. 4, 2003
McCutcheon and Wilson
Figure 1. Experimental procedure for the oxidation and analysis of the coal samples used for this study.
Elsewhere,9 we have shown, by molecular orbital calculations, that water physically binds to carbonaceous surfaces in a less specific way than in chemisorption and, for chemisorption, one hydrogen atom is stretched away from the oxygen atom in the transition state. The energy of this transition state would be considerably lowered by the presence of oxygen at the surface, because the other hydrogen atom would be bonded, thereby aiding adsorption. This observation also makes it likely, therefore, that there is a relationship between water uptake and the C-O or related structures on the coal surface. A simple method that can be used to follow this processing employs a high-precision balance, combined with XPS analysis. Coal that has been exposed separately to dry oxygen, humid air, and humid air followed by exposure to surface water could be studied to estimate the differences in the C-O functional groups and examine the role of water. By then measuring the water-vapor adsorption on the coal samples before and after oxidation, any effect of the increase in the number of O-C functional groups (hydrophilic sites) on the coal surface could be measured by difference. This approach is used here. Experimental Section Coals. The coal samples used for this study were high- and low-rank bituminous coals obtained from the Moura seam (coal A) and Goonyella seams (coals B and C) in Queensland, Australia and stored immediately under nitrogen. The coal samples were crushed and coal particles
