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Energy & Fuek 1994,8, 1002-1003
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Role of Glass Transitions in Determining Enthalpies of Air Oxidation in North Dakota Lignite Peter J. Hall,* Alexander J. Mackinnon, and Fanor Mondragont Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street Glasgow GI l X L , Scotland Received March 4, 1994 The effects of self-heatingand spontaneous combustion are well-known in low-rank coals.' There have been a large number of studies which have investigated chemical changes for coala following low-temperature oxidation and weathering; Davidson2has given a comprehensive review of this work. Other attempts to manage the problem of spontaneous combustion have adopted an engineering approach by investigationof heat flow around and through coal piles. Besides this, there have been a number of thermodynamicinvestigationsof coal oxidation,generally using flow calorimetry.3 By comparison, there have been relatively few investigations of the influence of the macromolecular structure on coal oxidation, or of the effects of oxidation on the macromolecular structure of coal. Recently, Mondragon et aL4 have shown how oxidation alters the structure of a Colombian subbituminous coal, h a g & Mondragon et ala4demonstrated how oxidation at 423 K for 4 h changes swelling behavior in pyridine. They also demonstrated that this oxidationchanges the intensity of glass transitions observed for the coal at 475 K. Mackinnon et al.6 have recently reported the existence of glass transitions for a wide range of coals based on evidence from differential scanning calorimetry and low frequency dielectric loss. These transitions were similar to those observed for polypyrrole and were believed to represent a significant change in the structure. These transitions were only observed after an endothermic thermal relaxation process. Below the transition, coal is in aglassy state and diffusion of gases and liquids is slow. In the more rubbery state above thetransition,diffusionmaybeexpectsdtobefaster. There have been no studies of this to date and the objective of the present Communication is to observe the oxidation behavior of dried, thermally relaxed North Dakota lignite below and above its glass transition. A Mettler DSC 30 system was used and a detailed experimental procedure has been presented elsewhere.6 Essential details are as follows. Temperatures are accurate to i0.5 K. Enthalpy calibration was by integrating the melting endotherm of an Indium standard supplied by t Department of Chemistry, University of Antioquia, Medellin, Colombia. (1)G h d , R. Fuel 1986,6S, 1042. (2) Davidson, €2. Natural Oxidationof Coal;IFAPublications: London,
1990. (3) Taraba, B.; Dobal, V.; Cap, K.; Haraeta, M. Fuel 1988,67,768. (4) Mondragon, F.; Jaramillo, A.; Quintero,G.; Mackinnon, A. J.; Hall,
P. J. Submitted to Fuel, 1994. (5) Mackinnon, A. J.; Antxuetegi, M. M.; Hall, P. J. Fuel 1994,79,113. (6) Larsen, J. W.;Wemett, P. C. Energy Fuela, 1988,2, 719.
0887-0624/94/250&1002$04.50/0
Heat flow (W/g) -Ov2
-".T
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Temperature (K) Figure 1. Differential calorimetry at 10K min-l under nitrogen on North D a k o t a b t ethat had been dried,heat treated to 523 K, and then quenched to room temperature.
Mettler. It was estimated that enthalpies were accurate to f0.05 J/g. Standard aluminum pans were used with two pinholes to allow evaporation of water and the typical sample size was 10mg. DSC was performed at 10 K/min, although no difference was noticed in the results if the heating rate was varied between 0.5 and 30 K/min. A nitrogen carrier was used. North Dakota lignite (NDL) was obtained from the Argonne Premium Coal Sample program. The experimental procedure was as follows: Samples of lignite were weighed into an aluminum pan and the coal was dried for 30 min in the DSC cell under N2 at 383 K. The sample was cooled to 303 K and reweighed, and DSC was performed to a temperature of 523 K at a rate of 10K/min. Following this, the sample was quenched to 303 K at a nominal quenching rate of 100 K/min and reweighed. DSC was re-run to a temperature of 473 K at a rate of 10 K min-l. The sample was then quenched to 303 K. The dried, thermally relaxed lignite was then heated to the oxidation temperature under nitrogen at a rate of 10 K min-l. When the heat flow had stabilized, the nitrogen carrier was changed to air and any changes in the heat flow, representing endothermic or exothermic reactions, were observed. The procedure of thermal relaxation followed by oxidation was performed for seven different oxidation temperatures between 348 and 473 K. Figure 1 shows DSC for dried and thermally relaxed NDL. A glass transition, similar to that observed by Mackinnon et al.,s can be observed at 423 K. The temperature of the transition depends strongly of the rate Q 1994 American
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Energy & Fuels, Vol. 8, No. 4, 1994 1003
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of cooling following the thermal relaxation (as for most polymers). The intensity is much greater than for higher rank coals. Also, the temperature difference between onset and end of transition is less than for higher rank coals. We have not completed any systematic study of the effects of coal rank on these transition parameters and they are presented merely as observations here. All of the isothermal oxidation studies exhibited exothermic reactions. A typical result, at 423 K, is shown in Figure 2. To ensure that the exotherm was not an experimental feature of changing from one carrier was to another, blank experiments using sand and empty pans were performed. These blank experiments showed no endotherms or exotherms and hence we are certain that the exotherms during oxidation are due to oxygen/coal interactions. Integration under the curve gave the overall enthalpy of oxidation. The times for the completion of the oxidation, i.e., the time for the heat flow to return to its normal baseline, were also noted. The variation of enthalpy of oxidation with temperature of oxidation is shown in Figure 3. It can be seen that the enthalpies increase with temperature in a nonuniform way. Below 423 K the enthalpies increase almost linearly a t a rate of 0.48 J g-1 K-l. Above 423 K the enthalpies of oxidation increase at a much greater rate. There was no systematic variation for the time for the completion of the oxidation process with temperature. This may be because diffusion of oxygen through the pinholes in the lids of the aluminum pans determines the rate of oxidation. If more detailed information on this is required the experimental procedure may have to be changed. A better method may be to perform the oxidation in a thermogravimetric apparatus. Comparison of Figures 1 and 3 shows that the temperature of the glass transition coincides with the temperature at which the enthalpies of oxidation suddenly increase. This suggests that the glass transition is important to determining the extent of oxidation.
340 360 380 400 420 440 460 480 500
Oxidation Temperature (K)
Figure 3. Variation of enthalpy of oxidation in air for thermally relaxed North Dakota lignite with temperature of oxidation.
As mentioned, our experimental apparatus prohibits us obtaining explicit information concerning the kinetics of oxidation and a physical explanation of the phenomenon is necessarily somewhat speculative. As mentioned, diffusion in glasses is severely restricted; Larsen has shown how molecular size determines accessibility to the macromolecular structure of coals? Therefore, below the glass transition there may be regions of the coal structure that are inaccessible to oxygen in any reasonable time scale. Accessibility therefore may be considerably enhanced above the glass transition. A simpler explanation could be that the enthalpy of reaction with temperature at constant pressure is given by ACp of the products and reactants. This would be expected to be dominated by the C, of the coal. Therefore, the sharp increase in (6(AH)/6T), at the glass transition is consistent with a sharp increase in the heat capacity of the coal above TB' We believe an understanding of the phase behavior of coals may be in some part important to an understanding of self-heating and spontaneous combustion. As a working hypothesis we believe that in the initial stages of spontaneous combustion there is a relatively slow buildup of heat due to oxidation. Eventually, there may be enough energy to thermally relax the coal structure. When the coal is in a rubbery state, the rate of heating would be expected to increase significantly, thus giving rise to spontaneous combustion. Such a two-stage process may go some way to explaining the notoriously unpredictable nature of spontaneous combustion. Acknowledgment. This work was funded by SERC grant GR/H18821. The authors are grateful to the referees for helpful comments.
