Pathways for Production of CO2 and CO in Low-Temperature

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Pathways for Production of CO2 and CO in Low-Temperature Oxidation of Coal Haihui Wang, Bogdan Z. Dlugogorski,* and Eric M. Kennedy Process Safety and Environment Protection Research Group, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308, Australia Received April 18, 2002

Oxidation of a bituminous coal has been studied using an isothermal flow reactor operated at temperatures between 60 and 90 °C, and packed with coal particles smaller than 853 µm in diameter. The time-dependent rates of production of CO2 and CO during experiments were obtained simultaneously from the measurement of CO2 and CO concentration at the reactor’s exit, just after the onset of oxidation of the coal, using a dual-column micro gas chromatograph. The ratio of the rates of production of CO2 and CO was high at the beginning of an experiment, but decreased to reach a steady state after 10 hours or so. Experimental data indicated that this ratio, at steady state, depends on temperature, and is independent of the size of coal particles and oxygen concentration in the oxidizing medium. The latter discovery has led to formulation of a new mechanism for low-temperature oxidation of coal, which is consistent with the existing spectroscopic data. It is proposed that carbon oxides are produced via the direct burnoff reaction and the decomposition of stable oxygenated complexes, such as carboxyl and carbonyl species. The decomposition of the initial chemisorption intermediates liberates carbon dioxide. For the first time, the paper reports an estimate of the activation energy for the direct burnoff reaction, which ranges between 62.3 and 70.1 kJ/mol for the present coal. The proposed mechanism in conjunction with the activation energy for the direct burnoff reaction could be used for predicting the emission of CO2 and CO from low-temperature oxidation of coal in practical applications.

Introduction As a major heat source, low-temperature oxidation plays a vital role in self-heating and spontaneous combustion of coal.1-5 Coal oxidation is also believed to alter the coal’s physical and chemical properties, significantly influencing the quality of the coal as a fuel. Over the past hundred years, low-temperature oxidation of coal has been extensively studied by a large number of investigators. Relevant research work has focused on gaining insight into oxygen adsorption during coal oxidation and the formation of oxidation products.6-23 * Corresponding author. Fax: + 61 (0)2 4921 6920. E-mail: cgbzd@ alinga.newcastle.edu.au. (1) Schmidt, L. D. Changes in Coal During Storage. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; John Wiley & Sons: New York, 1945; pp 627-676. (2) Van Krevelen, D. W. Coal: Typology - Chemistry - Physics Constitution, 3rd ed.; Elsevier: Amsterdam, 1993; pp 627-658. (3) Berkowitz, N. The Chemistry of Coal; Elsevier: Amsterdam, 1985; pp 143-151. (4) Nelson, C. R. Coal Weathering: Chemical Processes and Pathways. In Chemistry of Coal Weathering; Nelson, C. R., Ed.; Elsevier: Amsterdam, 1989; pp 1-32. (5) Carras, J. N.; Young, B. C. Prog. Energy Combust. Sci. 1994, 20, 1-15. (6) Jones, R. E.; Townend, D. T. A. Nature 1945, 155, 424-425. (7) Jones, R. E.; Townend, D. T. A. J. Soc. Chem. Ind. 1949, 68, 197-201. (8) Carpenter, D. L.; Giddings, D. G. Fuel 1964, 43, 247-266. (9) Carpenter, D. L.; Sergeant, G. D. Fuel 1966, 45, 311-327. (10) Marinov, V. N. Fuel 1977, 56, 158-164. (11) Swann, P. D.; Evans, D. G. Fuel 1979, 58, 276-280. (12) Gethner, J. S. Appl. Spectrosc. 1987, 41, 50-63. (13) Gethner, J. S. Fuel 1987, 66, 1091-1096. (14) Kaji, R.; Hishinuma, Y.; Nakamura, Y. Fuel 1985, 64, 297302.

It has been reported that coal oxidation is influenced by several factors, of which temperature, coal particle size, and oxygen concentration in the oxidizing medium are recognized as the most important.2,8 Coal oxidation at low temperatures is a complicated process, involving a reaction sequence which consists of several steps.2-4,10-13,15,17,22,23 These steps include chemisorption of oxygen on the surfaces of the coal pores and the formation of unstable carbon-oxygen complexes; decomposition of unstable solid oxygenated intermediates to gaseous products and stable solid complexes; degradation of the stable complexes and generation of new active sites for coal oxidation following the decomposition of the solid complexes. Some investigators have also suggested, in addition to the chemisorption process, a parallel reaction pathway, (15) Itay, M.; Hill, C. R.; Glasser, D. Fuel Process. Technol. 1989, 21, 81-97. (16) Kelemen, S. R.; Freund, H. Energy Fuels 1990, 4, 165-171. (17) Clemens, A. H.; Matheson, T. W.; Rogers, D. E. Fuel 1991, 70, 215-221. (18) Krishnaswamy, S. K.; Bhat, S.; Gunn, R. D.; Agarwal, P. K. Fuel 1996, 75, 333-343. (19) Krishnaswamy, S. K.; Gunn, R. D.; Agarwal, P. K. Fuel 1996, 75, 344-352. (20) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Energy Fuels 1999, 13, 1173-1179. (21) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Fuel 1999, 78, 1073-1081. (22) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Fuel 2002, 81, 1913-1923. (23) Wang, H. Coal Oxidation at Low Temperatures: Oxidation Products, Reaction Mechanism and Kinetic Modelling. Ph.D. Thesis, The University of Newcastle, Australia, 2002.

10.1021/ef020095l CCC: $25.00 © 2003 American Chemical Society Published on Web 12/18/2002

CO2 and CO from Low-Temperature Oxidation of Coal

which results in direct formation of gaseous products.1,3,18-20,24-27 This so-called direct burnoff reaction is hypothesized to occur at particular sites in the coal matrix in a manner that is similar to the direct combustion reaction.24 It has been found that the gaseous oxidation products primarily include carbon dioxide, carbon monoxide, and water vapor.2,8,9,11,14,15 The initial chemisorbed unstable intermediates may consist of peroxygen and hydroperoxides, and the stable oxygenated products include carbonyl (-CdO) and carboxyl (-COOH) species present in coal aliphatic or aromatic structure, as identified by a number of researchers using methods including spectroscopic techniques.2,7,10-13,16,17 The major parameters which characterize low-temperature oxidation of coal include the rate of oxygen consumption and the rates of formation of carboncontaining gaseous products.2 It has been found that there is no direct stoichiometric relationship between the rate of formation of gaseous products and the rate of oxygen consumption during coal oxidation at low temperatures.8,11,19 The rate of oxygen consumption usually reflects the combined effects of mass transport (oxygen diffusion in coal pores) and the chemical kinetics of coal oxidation (including oxygen chemisorption).21 The rates of formation of gaseous products have been recognized as important parameters associated with the chemical kinetics of coal oxidation. Early investigators6-9 examined the production of CO2 during coal oxidation, by means of oxygen adsorption techniques. Carpenter and Giddings8 found that the amount of liberated carbon dioxide increases with temperature, for some types of coal during the initial 5 h of oxidation. Using an isothermal flow reactor, Kaji et al.14 observed a decrease in the formation rate of CO2 with time. Similar trends have been also reported by other researchers who studied coal oxidation at temperatures between 150 and 300 °C.26,27 The results presented by the current authors20 have demonstrated that the rate of formation of CO2 rapidly decreases during the initial stage of oxidation, with a progressive decrease that follows afterward. It has been also reported that the variation in the rate of formation of CO2 with time can be modeled by a combination of exponential-decay and constant terms.20 As carbon monoxide is a toxic gas, substantial attention has been paid to its formation in practical situations. However, relatively few studies reported in the literature involve detailed examination of CO liberated from coal oxidation at low temperatures.7,8,11,14,17 For example, Jones and Townend reported that CO is the major product from coal oxidation below 65 °C.7 Carpenter and Giddings8 observed an opposite trend, indicating that the amount of CO produced is significantly lower than that of CO2. This observation has been also confirmed by other investigators.11,14,17 Clemens et al.17 found no CO produced in the experiments with six types of New Zealand coals at temperatures below 60 (24) Jensen, E. J.; Melnyk, N.; Wood, J. C.; Berkowitz, N. Adv. Chem. Ser. 1966, 55, 621-642. (25) Kam, A. Y.; Hixson, A. N.; Perlmutter, D. D. Chem. Eng. Sci. 1976, 31, 815-819. (26) Kam, A. Y.; Hixson, A. N.; Perlmutter, D. D. Chem. Eng. Sci. 1976, 31, 821-834. (27) Karsner, G. G.; Perlmutter, D. D. Fuel 1982, 61, 29-34.

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°C. Similarly, Krishnaswamy et al.18,19 reported negligible CO production during the experiments with a fresh Wyoming subbituminous coal at temperatures between 27 and 57 °C. Kelemen and Freund16 also pointed out that the primary oxidation product is CO2 rather than CO, for the fresh subbituminous coal oxidizing at temperatures between 22 and 125 °C. These laboratory observations are supported by data from coal mines, which indicate that the concentration of carbon monoxide in mine air is in the order of 10 ppm under normal operating conditions. If the concentration of CO increases to hundreds of ppm or higher, it points to conditions of self-heating or spontaneous combustion in a mine.28-30 Detailed reaction pathways responsible for the generation of carbon-containing gaseous products have not been fully elucidated and remain controversial. Early work by Jones and Townend7 suggested that the decomposition of chemisorption intermediates primarily generates CO, and the liberation of CO2 comes from the decomposition of the carboxyl groups (-COOH). However, this conflicts with the experimental findings reported in the past decade,16-20 which indicate no detectable production of CO at lower temperatures. Using an FTIR spectrometer, Gethner12,13 observed that the production of CO2 and CO is a consequence of the decomposition of both carboxyl and carbonyl groups. More specifically, some other researchers17,22 concluded that the decomposition of the carboxyl groups leads to the formation of CO2 while CO is a result of the decomposition of carbonyl groups. The results reported in this paper aim at obtaining an improved understanding of the formation of gaseous products in coal oxidation, leading eventually to the development of reaction mechanisms for the formation of these products. We describe the results from systematic experiments carried out under various conditions to examine factors affecting the production rates of gaseous carbon oxides. Specifically, we focus on the analysis of the ratio of the rates of production of CO2 and CO. Finally, the reaction mechanism responsible for the formation of these products is discussed on the basis of the experimental findings. Experimental Section A bituminous coal, labeled WHML, obtained from a coal mine in the Hunter Valley region of the state of New South Wales, Australia, was crushed and sieved to produce samples of particle sizes of below 125, 125-353, 353-500, and 500853 µm, respectively. The proximate and ultimate analyses of this coal are given in Table 1. The coal samples had undergone oxidation of several hours prior to the current studies. Oxidation experiments were performed using an isothermal flow reactor. The experimental rig consisted of a cylindrical reactor body, a constant temperature enclosure, gas lines, and a micro gas chromatograph for quantification of evolving gas species. A custom designed oven provided a constant temperature environment around the reactor. Other details of the experimental apparatus have been reported elsewhere.20,23 (28) Graham, J. I. Trans. 1nst. Min. Eng. 1920-21, 60, 222-234. (29) Cudmore, J. F.; van den Broek, E. Analysis of Mine Gases by Gas Chromatography; Australian Coal Association (Res.) Ltd., New South Wales, Australia, 1964. (30) Cliff, D.; Rowlands, D.; Sleeman, J. Spontaneous Combustion in Australian Underground Coal Mines, SIMTARS, Queensland, 1996.

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Table 1. Proximate and Ultimate Analyses 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

The experimental procedure is briefly described as follows: A weighed coal sample was packed in the reactor and dried by purging pure nitrogen through the reactor for at least 18 h at 65 °C. Before each experiment, the oven temperature was set to a predetermined level, and the pressure in the reactor was maintained at 101 kPa. Each sample was preheated to the desired temperature in pure nitrogen for more than 2 h. The oxygen concentration of the gas stream was adjusted by setting the flow rate of both oxygen and nitrogen and keeping a total flow rate at 45 ( 0.5 cm3/min (NTP) through a bypass line. An experiment commenced by flowing the oxygen/nitrogen mixture into the reactor. The concentrations of CO2 and CO products in the gas stream at the reactor’s exit were measured by an on-line micro gas chromatograph (MTI P200), which was operated continuously but required at least 3 min for each gas analysis cycle. The micro gas chromatograph was equipped with Molesieve 5 Å and PoraPLOT U columns and two thermal conductivity detectors. The instrument was capable of detecting a concentration of gas species as low as a few ppm. The transient variation of oxygen concentration in gas stream was monitored by a paramagnetic oxygen analyzer (ADC 7000) with a nominal resolution of (0.005% in O2. A water trap was used for the collection of water vapor generated during oxidation. The sample mass was measured by an analytical electronic balance (HR-300) before and after each experiment with airtight plastic bags. The rates of production of carbon oxides were determined from the following equations:

RCO2 )

CCO2,o Vgas W

(1)

RCO )

CCO,o V W gas

(2)

where W denotes the dry mass of a coal sample, Vgas represents the flow rate of the gas stream, CCO2,o and CCO,o stand for carbon dioxide and carbon monoxide concentrations at the reactor’s exit, respectively. W was measured after the experiment was completed, to avoid opening the reactor after the drying process. Our recent observations indicated 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).23,31 The temperature in the reactor varied by less than (0.5 °C during an experiment. The experimental uncertainties for the rates of production of CO2 and CO were found to be in the order of (10-13 kmol/((kg coal) s).23 A mass balance, based on the measurement of the mass of coal and concentrations of reactants and products in the gas stream, was established to be accurate to within (0.2%, as calculated on the basis of the original mass of coal samples.23

Results and Discussion General Trends in Production of CO2 and CO during Coal Oxidation. Typical experimental results for the production rates of carbon oxides with time are shown in Figure 1. Experimental data covering the initial 15 (31) Jones, J. C. J. Fire Sci. 1996, 14, 159-166.

Figure 1. Variation in the rates of production of carbon oxides with time for a coal sample with particle size between 500 and 853 µm, oxidizing in a gas stream with an oxygen concentration of 19% and at temperature of 60 °C (a) and a coal sample with particle size between 125 and 353 µm, oxidizing in the same gas medium but at a temperature of 90 °C (b).

min were removed from the graphs, following preliminary experiments with glass beads that indicated dilution of the residual nitrogen in the reactor, which could mask any trends in the production of carbon oxides during this period. It is observed that the rate of production of CO2 decreases rapidly during the first few hours of each experiment. After this period, the rate of production of CO2 decreases at a reduced rate. The rate of production of CO also undergoes a progressive reduction with time. It has been acknowledged in the literature that the gradual accumulation of stable complexes at the surface of coal pores during coal oxidation is responsible for decay in the rates of production of CO2 and CO.8 However, the present results illustrate different trends in the formation of CO2 and CO, especially during the first few hours of experiments, indicating distinct reaction pathways responsible for the liberation of these products. Temperature has a pronounced effect on the formation rates of carbon oxides (Figure 2). Although all the curves illustrated in Figure 2 decrease with time, the production rates of carbon oxides are substantially higher when the oxidation temperature is increased. This trend is in agreement with the other experimental findings.8,14 As expected, higher temperatures enhance the rate of coal oxidation, resulting in a higher level of emission of these products. Experiments at varying oxygen concentrations in the gas medium show that a higher feed oxygen concentration leads to elevated formation rates of both carbon dioxide and carbon monoxide (Figure 3). The reactions responsible for the liberation of carbon oxides, via a



Figure 2. Formation rates of carbon oxides, for coal samples with particle size between 353 and 500 µm, oxidizing in the gas stream with an oxygen concentration of 73% at various temperatures.

Figure 3. Effect of oxygen concentration in the gas stream on the formation rates of carbon oxides for coal samples with particle size of less than 125 µm, oxidizing at a temperature of 70 °C.

chemisorption type mechanism, include thermal decomposition of unstable and stable oxygenated complexes.4,10-13,17 These reactions are essentially independent of oxygen concentration in the gas stream, provided that the oxygen chemisorption rate is limited by the availability of active sites (as is the case of a previously oxidized or weathered coal). Thus, the only explanation for this phenomenon is that the direct burnoff reaction occurs in parallel with chemisorption pathways. As suggested by Kam et al.,25,26 both zero- and first-order (in oxygen concentration) reactions may be present in the direct burnoff reaction for the current coal. Figure 4 demonstrates that the rates of production of CO2 and CO show no dependence on coal particle size. Some scatter is present in the experimental data for coal samples with various particle sizes, which may be attributed to the difference in oxidation history of these samples. The data in Figure 4 suggest that the formation of CO2 and CO is governed by chemical kinetics rather than by oxygen diffusion in coal pores. In contrast, experiments with samples of a fresh coal show that particle size has a significant effect on the formation rate of CO2, suggesting a different reaction regime of coal oxidation.20 For a fresh coal characterized by relatively fast oxygen adsorption on pore surfaces, oxygen transport in the coal pores may limit the supply of oxygen to the reactions proceeding in the oxidation process. However, for an oxidized coal, oxygen consumption is limited by the availability of the reaction sites

for oxygen adsorption. Thus, the oxidation process is no longer affected by oxygen transport. The effect of oxidation history on the production rates of carbon oxides was examined by executing experiments using the same sample type, but pre-oxidized for different periods of time. Figure 5 compares the experimental data for a coal sample after an initial oxidation (about 5 h) and after a long period of oxidation (several days in total). It is shown that for the significantly oxidized sample, the production rates of carbon oxides are at lower levels. It has been proposed that, coal preoxidation results in the formation of some unreactive oxygenated complexes, the so-called humic acids. These acids do not undergo thermal decomposition at low temperatures,3 leading to the deactivation of the reaction sites for oxygen adsorption and to a decrease in the concentration of the stable oxygenated complexes which are responsible for the liberation of carbon oxides. As a consequence, the rates of production of carbon oxides substantially decrease for significantly pre-oxidized coal. The Ratio of CO2/CO Production Rates. The ratio of CO2/CO production rates is obtained by dividing the molar formation rate of CO2 by that of CO. Figure 6 illustrates the ratio of the rates of formation of CO2 and CO with time under experimental conditions specified in the caption for Figure 1. This ratio has a rather high value at the beginning of the experiment, but decreases with time to attain a steady-state value after about 10 h. As illustrated in Figure 6b, at higher temperatures,

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Figure 4. Comparison of the formation rates of carbon oxides for coal samples of various particle sizes, oxidizing in the gas stream with an oxygen concentration of 19% and at a temperature of 70 °C.

Figure 5. Effect of oxidation history on the production rates of CO2 (a) and CO (b) for a coal sample with particle size between 500 and 853 µm, oxidizing at an oxygen concentration of 19% and a temperature of 90 °C.

the ratio of CO2/CO production rates decreases more rapidly than in the case at lower temperatures (Figure 6a). However, comparing Figure 6a and b, it is clear that it takes almost the same amount of time for both reactions to attain steady state. A plot of the ratios of CO2/CO production rates at various oxygen concentrations in the gas stream indicates that all curves converge to the same constant value, despite different behavior over the first 10 hours of each experiment (Figure 7). The same conclusion can be reached for the data obtained during experiments carried out for coal samples with various particle sizes (Figure 8). A constant ratio of CO2/CO production rates is also attained for coal samples pre-oxidized over different time periods, although it is evident that for the coal sample with a prolonged history of oxidation, this ratio reaches its steady-state value within the first hour of the experiment (Figure 9). The logarithm of the ratio of CO2/CO production rates at steady state for various experimental conditions are plotted as a function of the reciprocal temperature (Figure 10). Although some scattering of data is observed, the ratio of the rates of production of CO2 and CO evidently depends on temperature, but is independent of coal particle size and oxygen concentration in the gas stream (refer to Figures 7 and 8). This ratio decreases with an increase in temperature, and the trend is more significant at temperatures below 70 °C. It seems that the relationship between the logarithm of the ratio of CO2/CO production rates and the recipro-

cal temperature can be well fitted by two straight line segments, rather than a single straight line. The present trend in the temperature dependence of this ratio is in agreement with the findings reported by some previous investigators.8,15 However, Jones and Townend2,7 presented a lower ratio (0.77) at 65 °C and a higher ratio (1.67) at 125-150 °C, which conflicts with the current observations. Reaction Pathways for the Production of CO2 and CO. The dependence of the production rates of carbon oxides on oxygen concentration in gas stream suggests the presence of the direct burnoff reaction proceeding in the coal oxidation process. As argued by several investigators,1,3,18-20,25-27 this reaction produces both CO2 and CO at a constant molar ratio. This suggests that the reaction pathways for the liberation of gaseous products during coal oxidation include the direct burnoff reaction as well as the decomposition of unstable and stable oxygenated complexes. We propose that the decomposition of chemisorption complexes (i.e., the unstable oxygenated intermediates) primarily generates CO2 rather than CO. Consequently, during the initial stage of the experiment, CO2 mainly derives from the decomposition of the unstable chemisorption intermediates. As time progresses, accumulation of stable oxygenated complexes at the surface of coal pores retards the chemisorption process, resulting in a considerable decrease in CO2 production from the decomposition of unstable intermediates. This explains



Figure 8. Comparison of the ratio of CO2/CO production rates for coal sample mass of various particle sizes. Experimental conditions are those for runs illustrated in Figure 4.

Figure 6. Ratio of CO2/CO production rates as a function of time. Experimental conditions are the same as for the data in Figure 1. Figure 9. Comparison of the ratio of CO2/CO production rates for pre-oxidized coal samples under the same experimental conditions as shown in Figure 5.

Figure 7. Variation in the ratio of CO2/CO production rates at various oxygen concentrations in the gas stream. Experimental conditions have been reported in Figure 3.

the drop in the rate of emission of CO2 in the initial hours of the experiments, and would also explain why some investigators16,18,19 observed that the initial oxidation of coal led predominantly to the formation of CO2. The proposed reaction sequences proceeding during coal oxidation are shown in Figure 11. Carbon dioxide is suggested to be generated by (i) the direct burnoff reaction, (ii) the decomposition of the unstable chemisorbed intermediates, and (iii) the decomposition of the stable oxygenated complexes containing -COOH groups. Carbon monoxide is produced by two independent

Figure 10. Dependence of the ratio of CO2/CO production rates on temperature at steady state.

reactions: the direct burnoff reaction and the decomposition of the stable oxygenated complexes containing -CdO groups. It is known that the direct burnoff reaction is temperature dependent and is more significant at higher temperatures. In addition, the decomposition reaction becomes significant when the temperature exceeds a threshold value at around 70 °C.6,7 It is not surprising then that Clemens et al.17 reported no production of CO for coal oxidation at temperatures

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Figure 11. Proposed reaction pathways proceeding in low-temperature oxidation of coal.

below 60 °C and the present authors20 found negligible CO production in experiments conducted at 50 °C. With the deactivation of reaction sites for oxygen chemisorption, the thermal decomposition of stable oxygenated complexes becomes the limiting step during the later stage of the chemisorption process. At this stage, the emission of CO2 and CO occurs via the decomposition of stable oxygenated complexes and the direct burnoff reaction, which can be formulated by

RCO2 RCO

)

(1) (4) + RCO RCO 2 2 (5) R(1) CO + RCO

(3)

(1) and R(1) where RCO CO denote the production of CO2 and 2 (4) CO from the direct burnoff reaction, respectively. RCO 2 (5) and RCO represent the formation of carbon oxides from two independent decomposition reactions, responsible (4) for the production of CO2 (RCO ) and CO (R(5) CO), respec2 17,22 tively. The value of RCO2/RCO can be approximated by the (1) ratio of RCO /R(1) CO (stoichiometric relationship between 2 the production of CO2 and CO in the direct burnoff (4) and R(5) reaction), as the terms, RCO CO disappear, which 2 may occur after a very long reaction period.18,25-27 Usually, R(4)CO2 and R(5) CO are not zero, and the ratio of the rates of production of CO2 and CO exceeds the ratio (1) of RCO /R(1) CO. Rearrangement of eq 3 gives 2

RCO2 RCO

)

(1) /R(1) (RCO CO) 2 (1) 1 + (R(5) CO/RCO)

+

(4) (RCO /R(5) CO) 2 (5) 1 + (R(1) CO/RCO)

(4)

If the rate of thermal decomposition of the particular oxygenated complexes obeys the Arrhenius equation, it follows that (4) RCO 2

R(5) CO

∝ exp

(

)

E5 - E4 RT

(5)

where (E5 - E4) denotes the difference of the activation energies between two different thermal decomposition reactions, which is positive as has been demonstrated in another work;22 T represents the oxidation temperature; and R has the usual meaning of the universal gas constant.

Temperature dependence of the ratio of CO2/CO production rates illustrated in Figure 10 can be explained as follows. As argued by previous investi(1) gators,18,20,25-27 the term of R(5) CO/RCO may be much smaller than unity for a prolonged period of coal oxidation; this phenomenon is more significant for coal oxidation at higher temperatures. At elevated temperatures, the rate of deactivation of reactive sites for oxygen adsorption accelerates due to the accumulation of more stable oxygenated complexes at coal surface, which, in turn, results in a faster decrease in the concentrations of the oxygenated complexes responsible for the liberation of carbon oxides. At higher tempera(4) tures (above 70 °C), the value of RCO /R(5) CO decreases (eq 2 (5) (1) 5). As the term of RCO/RCO is far less than unity and consequently its reciprocal far exceeds unity, the ratio of RCO2/RCO is well approximated by the first term at the right side of eq 4. This means that the ratio of RCO2/ RCO is not particularly temperature sensitive at higher temperatures. However, at temperatures below 70 °C, (5) the term of R(1) CO/RCO is relatively small and the value of (5) (4) RCO2/RCO is high enough to necessitate the inclusion of the second term at the right side of eq 4 during the determination of the ratio of RCO2/RCO. In this case, the ratio reflects a stronger dependence on temperature, as shown in Figure 10. It seems that 70 °C is a critical temperature, not only for the thermal decomposition reactions, but also for the direct burnoff reaction. As a result, for coal oxidation at temperatures below 70 °C, the contribution of thermal decomposition reactions to the production of carbon oxides is almost comparable to that of the direct burnoff reaction. If it is true that the rates of production of carbon oxides for coal oxidation after a prolonged period of oxidation are essentially a result of the direct burnoff reaction, the activation energy for the direct burnoff reaction can be estimated on the basis of the experimental data of the formation rates of carbon oxides. Arrhenius plots of the last values of the rates of production of CO2 and CO for coal samples oxidizing at various experimental conditions are shown in Figure 12. The experimental data below 70 °C were not incorporated in the plots, as it was argued that the contribution of thermal decomposition to the production of CO2 and CO at temperatures below 70 °C may not be negligible, compared to that of the direct burnoff reaction.


Energy & Fuels, Vol. 17, No. 1, 2003 157 Table 2. Activation Energy for the Direct Burnoff Reaction Determined by the Rates of Production of CO2 and CO at Specific Experimental Conditions experimental conditions particle size (µm)

Pathways for Production of CO2 and CO in Low-Temperature

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