Energy & Fuels 1999, 13, 1173-1179
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Experimental Study on Low-Temperature Oxidation of an Australian Coal Haihui Wang, Bogdan Z. Dlugogorski,* and Eric M. Kennedy Industrial Safety and Environment Protection Group, Department of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia Received March 11, 1999
Low-temperature oxidation of an Australian coal is examined in an isothermal flow reactor, operated at atmospheric pressure and 50 °C. Strong dependence of the rates of consumption of oxygen and the formation of carbon dioxide are observed on coal particle size and oxygen concentration in the gas stream. Following an induction period, oxygen consumption proceeds at a quasi steady state rate indicating that this process is limited by oxygen diffusion in the pores of coal particles. The rate of carbon dioxide production decreases with time, due to the progressive reduction in the number of active sites at the internal surfaces of pores in coal particles. This points to the kinetic control of CO2 production. The apparent reaction order for oxygen consumption at quasi steady state varies between 0.63 for large and 0.41 for small particles, with the latter number providing the best estimate of the intrinsic order. Fitting the rate of CO2 production to the expression developed by Kam et al. for higher temperatures suggests that, under the present experimental conditions, CO2 is formed via parallel pathways of (i) direct oxidation of coal to CO2 and (ii) formation and subsequent decomposition of solid coal-oxygen complexes, with carbon dioxide produced mainly via the first pathway at long times. The results presented in this paper have applications to self-heating and spontaneous combustion of coal.
Introduction Coal oxidation at low temperatures poses a serious problem in coal-related industries. Exothermic reactions between coal and oxygen are the main source of heat released in stored coal, leading to self-heating, and eventually to spontaneous combustion in certain cases.1-5 During weathering, coal may lose up to 15% of its calorific value, and its properties, which are needed in processing of coal to final products, may be downgraded.3-5 For these reasons, an understanding of lowtemperature oxidation of coal is fundamentally important from both the safety and economic perspectives. In the last hundred years, a large number of publications have reported results from investigations of lowtemperature oxidation of coal. Several experimental methods1,4-6 have been developed to examine the characteristics of coal oxidation at low temperatures, includ* Corresponding author. Fax: + 61 (0) 2 4921 6920. E-mail:
[email protected]. (1) Kim, A. G. Laboratory Studies on Spontaneous Heating of Coal: A Summary of Information in the Literature, U.S. Department of Interior, Information Circular 8756, 1977; pp 1-13. (2) Schmal, D. Spontaneous Heating of Stored Coal. In Chemistry of Coal Weathering; Nelson, C. R., Ed., Elsevier: Amsterdam, 1989; pp 133-214. (3) Jones, R. E.; Townend, D. T. A. Nature 1945, 155, 424-425. (4) Van Krevelen, D. W. Coal: Typology-Chemistry-PhysicsConstitution; Elsevier: London, 1961; pp 238-262. (5) Carras, J. N.; Young, B. C. Prog. Energy Combust. Sci. 1994, 20, 1-15. (6) Jones, J. C.; Chiz, P. S.; Koh, R.; Matthew, J. Fuel 1996, 75, 1755-1757.
ing isothermal and adiabatic reactor techniques, and oxygen adsorption. It has been also reported that the main factors2,4 which influence the oxidation process involve oxygen and water vapor concentrations in the oxidizing medium, particle size, and the physical and chemical nature of the coal, such as its porosity and chemical composition, including chemically bonded water. Previous investigators3-5,7,8 argued that coal oxidation at temperatures below 100 °C proceeds in four steps: (1) diffusion of oxygen in coal pores and its adsorption by the active sites at the internal surface of coal pores; (2) generation of gaseous oxygenated products by direct interaction between oxygen and coal, and the formation of unstable carbon-oxygen complexes; (3) decomposition of the unstable solid oxygenated intermediates to gaseous products and stable solid complexes, which incorporate carboxyl, carbonyl, and other functional groups; (4) generation of new active sites for coal oxidation following the decomposition of the solid complexes. Steps 2-4 can also be illustrated as follows: (7) Swann, P. D.; Evans, D. G. Fuel 1979, 58, 276-280. (8) Gethner, J. S. Fuel 1987, 66, 1091-1096. (9) Sevenster, P. G. Fuel, London 1961, 40, 7-17. (10) Sevenster, P. G. Fuel, London 1961, 40, 18-32. (11) Carpenter, D. L.; Giddings, D. G. Fuel 1966, 45, 247-266. (12) Carpenter, D. L.; Sergeant, G. D. Fuel 1966, 45, 311-327. (13) Nordon P.; Young, B. C.; Bainbridge, N. W. Fuel 1979, 58, 443449. (14) Kaji, R.; Hishinuma, Y.; Nakamura, Y. Fuel 1985, 64, 297302.
10.1021/ef990040s CCC: $18.00 © 1999 American Chemical Society Published on Web 11/15/1999
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Low-temperature oxidation of coal is usually described by two parameters: the rate of oxygen consumption and the rate of formation of gaseous oxygenated products. The rate of oxygen consumption has been extensively used in the modeling of self-heating and spontaneous combustion of coal stockpiles.2,5 Current experimental findings4,5,9-16 indicate that oxygen consumption by coal at low temperatures is governed by pore diffusion and chemical kinetics, depending on the intrinsic reactivity of the coal itself and the capacity of coal pores to transport oxygen. When the particle size is large (more than 1 mm in diameter), coal oxidation is controlled by continuum diffusion, while for very fine particles the reaction regime becomes limited by Knudsen diffusion (for active coal) or chemical kinetics (for less-active coal).17 The rate of oxygen consumption shows a dependence on the particle size; however, in the case of the less-active coal the rate may be independent of the particle size.5,12,14,15,17 A few studies involved the examination of gaseous products from coal oxidation at low temperatures. Possible products include CO2, CO, and H2O. For coal oxidation at temperatures below 100 °C, it has been reported that the formation rate of CO2 is much higher than that of CO.11,14-16 Clemens et al.16 did not find any CO and water products when they examined the oxidation of six types of New Zealand coals at temperatures below 60 °C. There are some relevant studies available in the literature,18-21 which investigated coal oxidation at temperatures between 150 and 300 °C. In these studies, a decay in the formation rate of gaseous products with time was observed. Starting with the reaction sites theory, Kam and co-workers18,19 developed a model for coal oxidation at temperatures between 200 and 300 °C, which suggests an exponential decay of the formation rates of gaseous oxidation products with time. Also, Karsner and Perlmutter20,21 presented similar results for the formation rates of oxidation products, while examining coal oxidation at temperatures between 150 and 225 °C. (15) Itay, M.; Hill, C. R.; Glasser D. Fuel Process. Technol. 1989, 21, 81-97. (16) Clemens, A. H.; Matheson, T. W.; Rogers, D. E. Fuel 1991, 70, 215-221. (17) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Fuel 1999, 78, 1073-1081. (18) Kam, A. Y.; Hixson, A. N.; Perlmutter, D. D. Chem. Eng. Sci. 1976, 31, 815-819. (19) Kam, A. Y.; Hixson, A. N.; Perlmutter, D. D. Chem. Eng. Sci. 1976, 31, 821-834. (20) Karsner, G. G.; Perlmutter, D. D. Fuel 1982, 61, 29-34. (21) Karsner, G. G.; Perlmutter, D. D. Fuel 1982, 61, 35-43.
Wang et al.
In the present work, an isothermal reactor is used to investigate the characteristics of low-temperature oxidation of an Australian coal from the Hunter Valley region in the state of New South Wales. The experiments are performed at different oxygen concentrations, and for various coal particle sizes. The rates of oxygen consumption and the formation of gaseous oxidation products are measured, reported, and discussed here. The aim of this study is to further the understanding of the mechanisms of coal oxidation at low temperatures, and especially to obtain an estimate of the reaction order in the limit of small particle sizes. Experimental The proximate and ultimate analyses of the bituminous coal, used in the current study, are given in Table 1. The coal samples were crushed, ground, and sieved into particle classes of below 0.125, 0.125-0.353, 0.3530.500, 0.500-0.853, and 0.853-1.350 mm. During these processing operations, liquid nitrogen was poured on the coal to cool the coal and inhibit contact between coal and oxygen. The prepared coal samples were stored in a deep freezer at all times before use. Number mean particle sizes that characterize coal samples in each class were obtained from a Zeiss microscope, and are 0.06, 0.22, 0.42, 0.70, and 0.97 mm, respectively. A schematic diagram of the experimental setup is shown in Figure 1. This setup was designed to study coal oxidation at elevated pressures, although the results presented in this paper were obtained at the atmospheric condition. The apparatus consists of reactor body, constant temperature enclosure, gas lines, and relevant diagnostic instrumentation. Gases are introduced into the manifold from two compressed gas cylinders, through particulate filters using a pair of mass flow controllers, which can set the concentration of oxygen to any level. The gas mixture is preheated to the desired temperature, by heat exchange coils placed in the constant-temperature enclosure (not shown in Figure 1), before flowing into the reactor. Upon exiting the back-pressure valve, the gas mixture is cooled to ambient temperature by another group of heat exchange coils immersed in water (not shown in Figure 1). The constant-temperature enclosure consists of a custom-designed oven, which can provide constanttemperature control from ambient to 150 °C, with an accuracy of (0.5 °C. The reactor body is made of stainless steel tube (i.d. 38 mm, height 296 mm). A pair of porous disks with 20 µm pores are fixed at the upper and lower ends of the reactor, to contain the coal particles. An additional porous disk is placed at the
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Figure 1. Schematic diagram of major components of the experimental apparatus. Table 1. Proximate and Ultimate Analysis of the Coal Used in This Study proximate analysis (air-dry basis) moisture (%) ash (%) fixed carbon (%) volatile matter (%) total sulfur (%)
ultimate analysis (dry ash-free basis) 5.2 26.0 27.9 40.3 0.6
C (%) H (%) N (%) S (%) O (%) (by difference)
81.1 5.1 2.1 0.8 10.9
reactor inlet to distribute the incoming gases uniformly across the cross section of the reactor. Temperatures of the coal bed and reactor wall are monitored with a pair of sheathed thermocouples. The composition of gas products at the reactor’s exit is determined by a micro gas chromatograph (MTI P200), which is equipped with Molsieve 5 Å and PoraPLOT U columns, and is capable of detecting gas concentrations as low as 5 ppm. The transient variation of oxygen concentration in the gas stream is also monitored, to determine the starting time of each experiment. This is done by an on-line paramagnetic oxygen analyzer (ADC 7000) with repeatability of (0.05% O2. Under ideal conditions, the starting time of the experiment denotes the time when the oxygen-nitrogen mixture first comes in contact with coal particles. Practically, however, oxygen variation can be detected only by the oxygen analyzer located after the reactor. Thus, we consider the starting time of the experiment as the time when oxygen is first detected by the oxygen analyzer. Before each experiment, a sample of about 157 g was charged into the reactor. The temperature of the oven was set to 50 °C, and the pressure in the reactor was fixed at 101 kPa. Pure nitrogen gas was passed through the reactor for at least 18 h, to remove the coal moisture. When a sample was ready to commence an experiment,
the bypass line was switched on and the oxygen concentration of gas stream was adjusted by setting the flow rate of both oxygen and nitrogen and keeping a total flow rate of 45 ( 0.5 mL min-1 (NTP). The concentration of the inlet gas was verified with the oxygen analyzer. The experiment was started by flowing the oxygen and nitrogen mixture to the reactor. Under the present experimental conditions, we did not detect carbon monoxide or water on the gas chromatograph, carbon dioxide was the only product quantified. Carbon monoxide was below the detection limit, and water did not elute from the GC columns. However, water was produced during the oxidation process since we observed water condensation in the heat-exchange coils at the reactor exit. The coal samples were re-weighed after each experiment. The rates of oxygen consumption and CO2 production were calculated from the following equations:
RO2 )
(CO2,i - CO2,o) Vgas W
(1)
CCO2,o Vgas W
(2)
RCO2 )
where W denotes the dry mass of a coal sample, Vgas represents the flow rate of the gas stream, CO2,i and CO2,o imply oxygen concentration in the inlet and outlet gas streams, respectively, and CCO2,o is carbon dioxide concentration at the reactor exit. W was measured after the experiment was completed, to avoid opening the reactor after the drying process. Our recent observations indicate that the increase in the mass of coal undergoing oxidation, even for as long as 26 h, is less than a gram (less than 0.5% of the initial mass).
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Wang et al.
At this stage, the rate of oxygen consumption may be restricted by the rate of oxygen diffusion in pore branches (ordinary and Knudsen diffusion), or chemical kinetics (reactivity of reaction sites in coal pores to oxygen adsorption), as discussed by the present authors in a previous paper.17 Evidently, the current experimental data of steady-state oxygen consumption suggest diffusion control, especially in micropores. Also, Carpenter and co-workers11 reported a similar phenomenon in their experiments. Had the oxidation rate been controlled by chemical reaction, one would have observed a declining oxidation rate due to the depletion of active sites. Note that we expect the oxygen consumption rate to decline at very long time once the supply of active sites becomes small and starts controlling the oxidation process. For this reason, we use the adjective “quasi” to describe the observed steady state. Also, the experimental runs reported here were limited to 6 h in duration. The rate of oxygen consumption by coal particles in a coal bed can be quantified by the following expression:
RO2 ) Figure 2. Variation in the rates of oxygen consumption and carbon dioxide production with time, for coal oxidation under the following reaction conditions: (a) mean particle size 0.22 mm, inlet oxygen concentration 7.17 × 10-3 kmol m-3; (b) mean particle size 0.97 mm, inlet oxygen concentration 1.43 × 10-2 kmol m-3.
Results and Discussion General Trends in the Rate of Oxygen Consumption and CO2 Production. Typical experimental results illustrating the time-dependent rate of oxygen consumption and the rate of carbon dioxide production are shown in Figure 2. The experimental data covering the initial 15 min were removed from the graphs, on the basis of preliminary experiments with glass beads that indicated oxygen dispersion occurred in the reactor, and masking any trends in oxygen consumption during this period. Figure 2, parts a and b, demonstrates elevated rates of oxygen consumption at the initial stage of oxidation, followed by a drop. About 1 h into the experiment, the rate of oxygen consumption attains a quasi steady value, though fluctuating slightly. There is a progressive decrease of the rate of CO2 production with time, as measured at the reactor outlet. When water is removed from coal pores, the openings of coal pores at the external surface of the particles are unblocked, and a number of active sites for oxygen adsorption are recovered as well, including some active sites situated at the external surfaces of coal particles. When coal particles come in contact with oxygen, the active sites at the external surface of coal particles and the pores that have openings to the particle exterior become immediately accessible to oxygen adsorption. Initially, oxygen adsorption occurs in pores close to the external surface of coal particles. However, as time progresses, oxygen necessarily diffuses further into the pore branches or micropores, which have connections with the pores that originate at the external surface of coal particles, to reach the reaction sites in these pores.17
1
∑
dNO2
Wparticles dt
)
1
∑
∑
Wparticles effective pores
d2CO2 |x)0 dx2 (3)
πr2p‚De
where NO2 implies the number of moles of oxygen diffusing into a coal particle, x indicates the distance from the surface of a particle, De represents the effective diffusivity of oxygen into the internal pores of coal particles, and CO2 is the local oxygen concentration. Note that not all the pores in a particle are included in the summation. As argued by the authors in their previous work,17,22 only the effective pores play a role in coal oxidation. It is obvious that the magnitude of the rate of formation of carbon dioxide is far less than the rate of oxygen consumption. This distinction reflects different mechanisms responsible for these two phenomena occurring in the oxidation process. The oxygen consumption involves oxygen diffusion followed by physical and chemical adsorption processes. On the other hand, the gaseous oxidation products are formed along two reaction pathways, the direct burnoff reaction and the decomposition of unstable carbon-oxygen complexes, with both pathways liberating gaseous products. Subsequently, the gaseous oxidation products diffuse in the pores toward the outside surfaces of coal particles. Although there is no independent data to compare with the current results, the trend in the CO2 production rate agrees with the observations for a few types of coal at higher temperatures (200-300 °C).19-21 The diminishing rate of CO2 emission can be explained by the following argument. The unstable carbonoxygen complexes either decompose to form gaseous oxygenated products and new active sites, or transform to stable carbon-oxygen complexes. This mechanism is illustrated in a schematic diagram included in the (22) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Chemeca ’98, Proc. 26th Australian and New Zealand Chem. Eng. Conf., Port Douglas, paper 94, 1998; pp 1-9.
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Figure 3. Dependence of the rate of oxygen consumption on oxygen concentration in the gas stream, for coal samples with mean particle sizes: 0 0.06 mm, 4 0.22 mm, ] 0.70 mm.
Introduction. The new active sites enter into further reactions. However, during the oxidation, the pool of active sites becomes reduced due to the formation of stable carbon-oxygen complexes, which do not decompose at the experimental temperature of 50 °C.3,4 This leads to the declining rate of CO2 production, observed in the experiments. Apparent Reaction Order for Oxygen Consumption. There is a significant difference in the rate of oxygen consumption at quasi steady state, when coal particles are oxidized under various oxygen concentrations (Figure 3). Clearly, the rate of oxygen intake increases with oxygen concentration in the gas phase external to the coal particles, as was also acknowledged by some previous investigators, for example Carpenter and Giddings.11 Since oxygen consumption is dominated by diffusion in micropores, an increase of oxygen concentration in the gas phase enhances oxygen transport due to steeper oxygen concentration gradients in the pores, resulting in a higher rate of oxygen diffusion (refer to eq 1). According to the literature,2 the influence of oxygen concentration in the gas phase on the rate of oxygen consumption by coal follows a power-law relationship: n RO2 ∝ CO 2,i
(4)
where n represents an apparent reaction order of oxygen adsorption by coal particles. Figure 4 shows average values of the rate of oxygen consumption at quasi steady state versus the oxygen concentration plotted on a double-logarithm graph. The apparent reaction order is found by least-squares fitting of the straight lines to the experimental data. The apparent reaction order decreases from 0.63 to 0.41 with decreasing particle size. As the particle size is reduced, the oxygen diffusion flux into particles increases, resulting in a change of the controlling mechanisms for oxidation from diffusion to chemical reaction.12,17 This means that the reaction order of 0.41, corresponding to the small particles used in the present study, provides the best estimate of the intrinsic reaction order. From this perspective, note that the data on the reaction order reported in the literature often reflect apparent rather than intrinsic measurements.
Figure 4. Data from Figure 3 re-plotted to obtain an apparent reaction order for oxygen consumption.
Figure 5. Effect of particle size on the rate of oxygen consumption for coal samples oxidizing in gas medium with various oxygen concentrations: 4 7.17 × 10-3 kmol m-3, ] 1.43 × 10-2 kmol m-3, 0 2.76 × 10-2 kmol m-3.
Analysis of the Effect of Particle Size on Oxygen Consumption. Finer coal particles display a higher rate of oxygen consumption (Figure 5). This phenomenon can be explained by considering the effect of particle size on coal pore structure. As suggested in our previous publication,22 pores in a coal particle can be classified as effective pores that participate in coal oxidation and blind pores that play no role in oxidation process, since they are not accessible to oxygen. According to the Simons pore tree model,23 the number of pore trunks whose radii are between rp and rp + drp are determined by 4πa2ge(rp)drp, where ge(rp) is the pore size distribution function, given by
ge(rp) )
(1-φ)θ 2βπr3p
(5)
where θ is the porosity of a coal particle of size a, β is a constant, and φ is defined as the ratio of pore volume taken by blind pores to the total pore volume of a coal particle. Assuming that a coal sample consists of particles of the same size and taking De as a constant for all effective pores present at the surface of an arbitrary particle, eq 1 can be cast into the following form: (23) Simons, G. A. Prog. Energy Combust. Sci. 1987, 9, 269-290.
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RO2 )
d2CO2 W 1 ‚ ‚D |x)0‚ e W (4/3)πa3 F dx2 C
∫rr
Wang et al.
πr2p‚
max
min
(4πa2)ge(rp)drp (6) where FC is the density of coal particles, rmin and rmax denote the radii of the smallest and the largest pores in a particle, respectively. Note that rmax can be obtained from the following expression:22,23
rmax )
2(1-φ)1/3θ1/3 -γ ae 3KO
(7)
where KO is a constant, and γ is yet another constant associated with different coal types. Integrating eq 6, we obtain
(
)
d2CO2 3(1-φ)θ 2(1-φ)1/3θ1/3a 1 | ‚ ln RO2 ) ‚De x)0 FC 2βa dx2 3K0rmineγ
(8)
The term on right-hand side is a monotonically decreasing function of particle size a. This means that the sum of cross-sectional areas of all effective pores at the particle surface per outside surface area decreases with particle size, resulting in a higher rate of oxygen consumption for smaller particles. Equation 6 considers De as constant and does not account for a size distribution of coal particles within different classes. Actually, De is a function of the pore-size distribution, when the pore radius is less than a critical value.17,23 Despite this, eq 8 provides a good representation of the particle size effect on oxygen consumption. Rate of CO2 Production. The dependence of the rate of CO2 production with time on oxygen concentration in the gas phase is shown in Figure 6. Similarly to the rate of oxygen consumption, the rate of CO2 production decreases rapidly at the beginning of each experiment. After this initial period the rate of CO2 production decreases monotonically at a slower rate. Two parallel reaction pathways, occurring at the surface of coal pores, contribute to the production of carbon dioxide.3-5,7,8 These pathways include direct oxidation of coal to CO2 and the decomposition of solid oxygenated complexes. The rate of oxygen consumption is controlled by a diffusion process, so, a higher level of oxygen concentration in the gas phase enhances the oxygen transport into pores, and consequently promotes the chemical reactions, which lead to the liberation of gaseous oxidation products. Furthermore, the rate of formation of gaseous oxidation products demonstrates a dependence on particle size (Figure 7). This rate is primarily influenced by the variation of the internal surface area as a function of particle size, which is accessible to oxygen diffusion. The model developed by Kam et al.,18,19 for coal oxidation at temperatures between 200 and 300 °C, takes into account the contribution of the two parallel reaction pathways leading to the formation of gaseous oxidation products. In this model, the formation rate of carbon dioxide is given by a i a + (RCO - RCO ) e-kdt RCO2 ) RCO 2 2 2
Figure 6. Variation in the rate of CO2 production for coal samples with mean particle size of 0.70 mm and oxidizing in gas medium with various oxygen concentrations: ] 1.89 × 10-3 kmol m-3, O 3.78 × 10-3 kmol m-3, 0 1.43 × 10-2 kmol m-3, 3 2.76 × 10-2 kmol m-3.
(9)
Figure 7. Effect of particle size on the rate of CO2 production, for coal samples oxidizing in gas medium with 7.17 × 10-3 kmol m-3 of oxygen. Mean particle sizes: 3 0.06 mm, 0 0.22 mm, O 0.97 mm.
Figure 8. The experimental data for the rate of CO2 production fitting to the expression developed by Kam et al.
where RiCO2, RaCO2 represent the initial and asymptotic rates of CO2 production, respectively, and kd implies a reaction constant for the deactivation of the active sites. The asymptotic rate can be interpreted as the rate of CO2 production in a significantly oxidized coal. (See Figure 8.) Comparison of the experimental data with this equation is illustrated in Figure 7. The values of relevant parameters are given in Table 2. The reaction constant, kd, is found to be 0.17 h-1, which is in the same order of magnitude as that found by Kam and co-workers.19
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Conclusions
Table 2. Calculated Initial and Asymptotic Rates of CO2 Production experimental conditions mean size (mm)
0.70 0.06 0.22 0.97
}
i RCO 2
a RCO 2
oxygen concn (kmol m-3)
(kmol (kg coal)-1 s-1)
1.89 × 10-3 3.78 × 10-3 1.43 × 10-2 2.76 × 10-2
5.85 × 10-11 7.24 × 10-11 9.37 × 10-11 1.12 × 10-10
2.39 × 10-11 2.50 × 10-11 4.98 × 10-11 6.45 × 10-11
1.25 × 10-10 7.98 × 10-11 4.97 × 10-11
6.69 × 10-11 4.95 × 10-11 2.91 × 10-11
{
7.17 × 10-3
{
(kmol (kg coal)-1 s-1)
Clearly, the equation allows for a good description of the experimental data. This indicates that the two parallel reaction mechanisms in conjunction with the active sites theory provide an adequate explanation for the present results. However, this does not mean that the whole model proposed by Kam et al.18,19 can be directly applied to explain the low-temperature oxidation process. The model of Kam et al. does not consider the stability of the functional groups formed by decomposition of chemisorption complexes at temperatures below 70 °C. It is known that, when the temperature is below 70 °C, the liberation of the gaseous products is a consequence of the decomposition of the chemisorption complexes along the second reaction pathway. The deactivation of the active sites is due to the accumulation of the stable oxygenated complexes in the coal structure rather than the bimolecular interaction of the active sites, as assumed by Kam et al.18 Furthermore, the assumed stoichiometric relationship19 between the rate of oxygen consumption and the rate of formation of gaseous products, in the model of Kam et al. is not applicable to low-temperature oxidation of coal, because of the formation of solid oxygenated complexes at low temperatures. As discussed earlier in the paper, the decay of the formation rate of carbon dioxide results from the progressive decrease in the number of active sites (deactivation of the reaction sites) in the course of oxidation. Thus, at long reaction times, with the disappearance of active sites responsible for the formation of unstable solid oxygenated products, the gaseous products of coal oxidation at low temperatures are generated by the direct burnoff reaction.
Present experiments demonstrate that the initial high rate of oxygen consumption decreases rapidly to attain a quasi steady state. A progressive decrease is observed in the rate of formation of carbon dioxide during the experiment. It is suggested that oxygen adsorption is controlled by the rate of diffusion of oxygen molecules in micropores. A reduction in the number of active sites with time, because of the formation of stable solid complexes, is regarded as the main reason for the reduction in the rate of CO2 formation. The apparent reaction order for oxygen consumption tends to decrease with particle size. This indicates that the dependence of the rate of oxygen consumption on oxygen concentration in gas stream becomes less pronounced for smaller coal particles. With the reduction in particle size, the cross-sectional area of trunks of coal pores at the surface of the particles increases per surface area, leading to higher diffusion rates of oxygen into the coal pores and thus higher rates of coal oxidation. With the improved capability of coal pores to transport oxygen in finer particles, the reaction regime of coal oxidation can become controlled by chemical kinetics rather than pore diffusion. However, this transition is not observed in the present study. Variation in the rate of CO2 production with time is described well by an equation developed by Kam et al.18,19 This appears to support the conjuncture that during low-temperature oxidation of coal, the gaseous products are generated by two pathways: a direct reaction between coal and oxygen and the decomposition of solid oxygenated complexes. It is postulated that at very long times the gaseous products may be generated only via the burnoff pathway. This implies progressive deactivation of active sites, due to the formation of stable oxygenated complexes, at the surface of coal pores. Acknowledgment. This work was funded by the Australian Research Council. The authors extend their thanks to Mr. Phil Hocking of Phil Hocking Engineering Services Pty. Ltd. for providing coal samples used in this study. EF990040S
